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Evaluation of polyaniline in corrosion protection and microwave initiated free radical catalyzed polymerizations: Polystyrene

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A Bell & Howell Information Company
300 North Zeeb Road. Ann Arbor. Ml 48106-1346 USA
313/761-4700 800/521-0600
EVALUATION OF POLYANILINE IN CORROSION PROTECTION AND
MICROWAVE INITIATED FREE RADICAL CATALYZED POLYMERIZATIONS:
POLYSTYRENE
by
SRINIVAS PRAVIN SITARAM, 1966-
A DISSERTATION
Presented to the Faculty of the Graduate School of the
UNIVERSITY OF MISSOURI-ROLLA
In Partial Fulfillment of the Requirements for the Degree
DOCTOR OF PHILOSOPHY
in
T7080
193 pages
CHEMISTRY
1995
OA
J
A
s O. Stoflcr, Myimx
Thomas J. O'Keefe
arvest L. Collier
''Ju csk&b?—
Oliver K. Manuel
Nicholas C. Morosoff
OMI Number: 9611867
OMI Microform 9611867
Copyright 1996, by OMI Company. All rights reserved.
This microform edition is protected against unauthorized
copying under Title 17, United States Code.
UMI
300 North Zeeb Road
Ann Arbor, MI 48103
© 1995
SRINIVAS PRAVIN SITARAM
AND JAMES OSBER STOFFER
ALL RIGHTS RESERVED
Ill
PUBLICATION THESIS OPTION
Chapter VI of this dissertation has been prepared in the style utilized by the Journal
of Applied Polymer Science. Pages 132-163 have been submitted to this journal. An
appendix has been added for purposes normal to thesis writing.
iv
ABSTRACT
Considerable interest has been generated in the research of potential applications of
Intrinsically Conducting Polymers (ICP's). One of the areas identified has been its use in
corrosion inhibition. The first part of this dissertation (Chapters 1 through 5) focuses on the
use of ICP's (polyaniline to be specific) as possible corrosion inhibitive substances. These
materials could be considered as safe and non-toxic alternatives to chromates in corrosion
protection. This study evaluates the use of VERSICON (polyaniline manufactured by
Allied Signal) and PANDA (a polyaniline synthesized by Monsanto Co.,) in corrosion
inhibitive formulations. These samples were tested in a neutral salt spray environment, the
corrosion inhibition was evaluated, and compared with control samples incorporating
commercially available corrosion inhibitors. Evaluations of the coatings in salt spray were
compared to DC polarization. A good correlation between the salt spray data and the
accelerated DC polarization was obtained. Polyaniline was also electrochemically deposited
and evaluated for corrosion protection on aluminum.
The second part of this dissertation (Chapter 6) investigates the use of microwave
radiation as a tool for polymerization processes. The microwave initiated free radical
catalyzed bulk and solution polymerization of styrene using azobisisobutyronitrile (AIBN)
is reported. The effect of the power of microwave radiation and initiator concentration on
percent conversion and molecular weights is discussed. The effect of microwave radiation
on polymer stability is reported. The process is compared against conventional bulk thermal
polymerization.
v
ACKNOWLEDGEMENTS
I owe my sincere thanks to Dr. James StofFer for his guidance, moral and financial
support through the course of this research. My thanks to Dr. Thomas O'Keefe for his
guidance through the course of this study. I would like to thank Dr. Frank Blum, Dr. Harvest
Collier, Dr. Oliver Manuel and Dr. Nicholas Morosoff for consenting to be on my
committee, and for their support and encouragement. I would like to thank Dr. Patrick
Kinlen and the Monsanto Co., Mr. Robert Johnson and the McDonnell Douglas Co., for their
advice and financial support. My thanks to Dr. Pu Yu for his valuable input and for running
the DC Polarization experiments for this study. I cannot overlook the contribution of Lin
Fang and Eric Morris in this work. I would like to the thank the staff at Chemistry and
Materials Research Center for making life so much more easier for me. I owe my sincere
thanks to the Department of Chemistry and the Materials Research Center for supporting me
financially through the course of my graduate study.
I would like to express my thanks to Raj Singh for his encouragement and support
and all my friends and colleagues in Rolla who made my stay here enjoyable. I would like
to express my gratitude to my brother and sister-in-law Dr. and Mrs. Prasad and my in-laws
Dr. and Mrs. T. S. Nagaraj.
My thanks to my wife Jyothi for her love, support,
encouragement and belief in me. Finally, I would like to express my heartfelt gratitude to
my parents Mr. and Mrs. Sitaram, without whom this dissertation would have never been
possible.
vi
TABLE OF CONTENTS
Page
PUBLICATION THESIS OPTION
iii
ABSTRACT
iv
ACKNOWLEDGEMENTS
v
LIST OF ILLUSTRATIONS
xiii
LIST OF TABLES
xvii
SECTION
I.
INTRODUCTION
1
II.
CONDUCTING POLYMERS AND CORROSION
4
III.
EXPERIMENTAL DETAILS
16
A.
TEST METHODS AND APPARATUS
16
B.
MATERIALS USED IN THE STUDY
17
C.
VERSICON POLYANILINE COATINGS
19
1.
Paint Preparation
19
a. Preparation of the Epoxy/Polyaniline Dispersion
19
b. Formulations Tested in Salt Spray Cabinet
19
i.
PA-10AmineB/PA-30AmineB
19
ii. PA-10 AmineG/PA-30 AmineG
19
iii. PA-10AmideB/PA-30AmideB
20
iv. PA-10 AmideG/PA-30 AmideG
20
v. PA-10 AnhG/PA-30AnhG
21
vii
c. Evaluation of Substrates
21
d. Evaluation of the Polyaniline base in Corrosion
Protection
22
i.
D.
IV.
Preparation of Polyaniline base
22
ii. Preparation of the Polyaniline base
dispersion in EPON 828
22
iii. Formulations incorporating the Polyaniline
base dispersion
22
MONSANTO POLYANILINE COATINGS
23
1.
Synthesis of Monsanto Polyaniline (PANDA)
23
2.
Formulations with PANDA blends
23
3.
PANDA as a base coat primer
25
E.
CONTROL FORMULATIONS
26
F.
COMPARISON OF SALT SPRAY RESULTS OF
POLYANILINE WITH OTHER COMMERCIALLY
AVAILABLE CORROSION INHIBITORS AND
CONTROL SAMPLES
27
G.
ELECTROCHEMICAL
STUDIES
IMPEDANCE
SPECTROSCOPY
27
H.
CHARACTERIZATION OF POLYANILINE FILMS
28
I.
ANILINE ANODIZATION PROCESS
29
1.
Variation in the time of Anodization
30
2.
Variation in concentration of H2S04
30
3.
Variation in concentration of Aniline
30
RESULTS AND DISCUSSION
44
A.
VERSICON POLYANILINE COATINGS
44
1.
44
Formulations Tested in Salt Spray Cabinet
viii
2.
3.
B.
a. PA-lOAmideG
44
b. PA-30AmideG
44
c. PA-lOAnhG
45
d. PA-30AmineB
45
e. PA-lOAmineB
45
f.
46
PA-30AnhG
g. PA-30AmideB
46
h. PA-30AmineG
46
i.
PA-lOAmineG
46
j.
PA-lOAmideB
47
k. Summary of Corrosion Results
47
Evaluation of Substrates
47
a. PA-30AmideB
47
b. PA-30AmineB
48
c. Summary of Results
48
Evaluation of PANI base (DePANI, dedoped
VERSICON PANI) in Corrosion Protection
48
a. DePANI30Amine
48
b. DePANI30Amide
49
c. Summary of Results
49
MONSANTO POLYANILINE COATINGS
49
1.
Corrosion Protection of PANDA blends
49
a. PANDA-30AmineB
50
b. PANDA-30AmideB
50
ix
2.
3.
C.
c. PANDA-30AroG.
50
d. PANDA-30ACMEG
51
e. PANDA-30ACPUG
51
PANDA as a base coat primer
51
a. PANDA as a base coat primer with Aroflint top coat
51
b. PANDA as a base coat primer with clear
Acrylic/Melamine top coat
52
c. PANDA as a base coat primer with clear
Acrylic/Polyurethanetop coat
52
Summary of Results
52
CONTROL FORMULATIONS
53
1.
Control-Amine
53
2.
Control-Amide
53
3.
Control-Aroflint
54
4.
Control Acrylic/Melamine system
54
5.
Control Acrylic/Polyurethane system
54
D.
SUMMARY OF SALT FOG DATA
55
E.
COMMERCIAL INHIBITOR FORMULATIONS
55
1.
Formulations tested in the Salt Spray Cabinet
55
a. Moly-White ZNP (15% by weight) dispersed in
Epoxy/Polyamide system
56
b. Halox SZP-391 (15% by weight) dispersed in Epoxy/
Polyamide system
56
c. Busan 11-M1 (15% by weight) dispersed in Epoxy/
Polyamide system
56
X
2.
F.
G.
d. Nalzin2 (15% by weight) dispersed in Epoxy/
Polyamide system
57
e. Control Epoxy/Polyamide
57
f.
57
Summary of Results
Comparison of Salt Spray Results of Polyaniline
with other commercially available corrosion
inhibitors and control samples
58
ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY
STUDIES
58
CHARACTERIZATION OF POLYANILINE FILMS
60
1.
X-RayDiflraction
60
2.
SEMAnalysis
61
H.
OTHER MISCELLANEOUS OBSERVATIONS
62
I.
ANILINE ANODIZATION PROCESS
63
1.
Optimization of Anodization Parameters
63
a. Variation in Time
63
b. Variation of the acid concentration
64
c. Variation in concentration of Aniline
64
d. Summary of Results
65
Anodization and study of the effect of the
posttreatmentprocedures
65
a. Anodization followed by 15 minute hot water seal
65
b. Anodization, hot water seal, followed by dip in
5-10% sulfuric acid
66
c. Anodization followed by 15 minute seal in hot
dilute sulfuric acid
66
2.
xi
3.
4.
V.
d. Anodization, hot water seal, followed by dip in
10%PTSA
67
e. Anodization, hot water seal, followed by
dipinDNNSA
67
f.
67
Anodization, no seal
Corrosion Studies ofElectrochemically deposited
Polyaniline
67
a. Salt Spray Evaluations
67
b. Electrochemical Evaluations of the Aniline
AnodizedPanels
68
Surface Characterization
69
CONCLUSIONS
127
BIBLIOGRAPHY
VI.
129
MICROWAVE INITIATED FREE RADICAL CATALYZED
POLYMERIZATIONS: POLYSTYRENE
132
SYNOPSIS
133
INTRODUCTION AND OBJECTIVE
133
EXPERIMENTAL
137
The Microwave Oven
137
Reagents
137
Experimental Setup
138
Experiments
138
1. Investigation of Free Radical Initiators
138
2. Profile of the Polymerization as a function of time..139
3. Effect of Power on Percent Conversion
139
xii
4. Effect of Initiator Concentration on Percent
Conversion
139
5. Effect of Power and Initiator Concentration
onMolecular Weight
139
6. Effect of Solvent on the Microwave
Polymerization
140
7. Comparison of Microwave and Thermal
Polymerization
140
8. Effect of Microwave Radiation on Polymer
Stability
140
RESULTS AND DISCUSSION
1. Investigation of Free Radical Initiators
141
141
2. Profile of the Polymerization as a function of time..143
VITA
3. Effect ofPower on Percent Conversion
143
4. Effect of Initiator Concentration on Percent
Conversion
144
5. Effect of Power and Initiator Concentration
onMolecular Weight
144
6. Effect of Solvent on the Microwave
Polymerization
145
7. Comparison of Microwave and Thermal
Polymerization
146
8. Effect of Microwave Radiation on Polymer
Stability
146
CONCLUSIONS
147
REFERENCES
148
APPENDIX
164
173
xiii
LIST OF ILLUSTRATIONS
Page
PART I.
EVALUATION OF POLYANDLINE IN CORROSION PROTECTION
I.
Illustration of a Passivated Metal in an ambient environment
14
2..
Electroactive coating designed for passivation of metal in an ambient
environment
15
3.
Experimental Setup for the Aniline Anodization Process
43
4.
DC Polarization Curves for the Control Aroflint System with
an artificially drilled pit
94
AC Impedance Curves for the Control Aroflint System with
an artificially drilled pit
95
AC Impedance Curves for the Control Aroflint System with
an artificially drilled pit
96
DC Polarization Curves for the Pure PANDA System with
an artificially drilled pit
97
AC Impedance Curves for the Pure PANDA System with
an artificially drilled pit
98
AC Impedance Curves for the pure PANDA System with
an artificially drilled pit
99
5.
6.
7.
8.
9.
10.
II.
12.
13.
DC Polarization Curves for the PANDA dispersed in the Aroflint System
with an artificially drilled pit
100
AC Impedance Curves for the PANDA dispersed in the Aroflint System
with an artificially drilled pit
101
AC Impedance Curves for the PANDA dispersed in the Aroflint System
with an artificially drilled pit
102
DC Polarization Curves for the PANDA base coat with the clear
Aroflint top coat system with an artificially drilled pit
103
xiv
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
AC Impedance Curves for the PANDA base coat with the clear
Aroflint top coat system with an artificially drilled pit
104
AC Impedance Curves for the PANDA base coat with clear Aroflint
top coat system with an artificially drilled pit
105
DC Polarization Curves for the PANDA dispersed in the
Acrylic/Melamine System with an artificially drilled pit
106
AC Impedance Curves for the PANDA dispersed in the
Acrylic/Melamine System with an artificially drilled pit
107
AC Impedance Curves for the PANDA dispersed in the
Acrylic/Melamine System with an artificially drilled
pit
108
DC Polarization Curves for the PANDA base coat with the clear
Acrylic/Melamine top coat system with an artificially drilled pit
109
AC Impedance Curves for the PANDA base coat with the clear
Acrylic/Melamine top coat system with an artificially drilled pit
110
AC Impedance Curves for the PANDA base coat with the clear
Acry l i c / M e l a m i n e t o p c o a t s y s t e m w i t h a n a r t i f i c i a l l y d r i l l e d p i t
Ill
DC Polarization Curves for the Moly-White ZNP dispersed in the
Epoxy/Polyamide System with an artificially drilled pit
112
AC Impedance Curves for the Moly-White ZNP dispersed in the
Epoxy/Polyamide System with an artificially drilled pit
113
AC Impedance Curves for the Moly-White ZNP dispersed in the
Epoxy/Polyamide System with an artificially drilled pit
114
X-Ray Diffraction Scan for the substrate coated with the
Control Aroflint System after 500 hours exposure to salt spray
115
X-Ray Diffraction Scan for the substrate coated with the PANDA
base coat with the clear Aroflint top coat system after 500 hours
exposure to salt spray
116
X-Ray Diffraction Scan for the substrate coated with PANDA
dispersed in the Aroflint System after 500 hours exposure to salt spray
117
SEM Micrographs of the substrate coated with the Control
Aroflint System after after 500 hours exposure to salt spray
118
XV
29.
SEM Micrographs of the substrate coated with the PANDA dispersed
in the Aroflint System after 500 hours exposure to salt spray
119
SEM Micrographs of the substrate coated with the PANDA base coat
with the clear Aroflint top coat system after 500 hours exposure to salt spray
120
DC Polarization Curves for Aniline Anodized panels compared with
the Sulfuric Acid Anodized panels
121
32.
SEM Micrograph for the Aniline Anodized hot water sealed panel
122
33.
SEM Micrograph for the Aniline Anodized Sulfuric Acid sealed panel
122
34.
SEM Micrograph for the Aniline Anodized hot water sealed panel followed
by a 10 minute dip in a solution of p-toluene sulfonic acid
123
35.
SEM Micrograph of the Aniline Anodized unsealed panel
123
36.
SEM Micrograph for the Control Sulfuric Acid Anodized panel
124
37.
X-Ray Diffraction Scan for the Polyaniline produced during the
Aniline Anodization Process
125
X-Ray Diffraction Scan for the Chemically Synthesized Polyaniline
(VERSICON)
126
30.
31.
38.
PART II.
MICROWAVE INITIATED FREE RADICAL CATALYZED
POLYMERIZATIONS. POLYSTYRENE
1.
Mechanism of Microwave Heating
155
2.
Experimental Setup
156
3.
Polymerization Procedure
157
4.
Temperature profile of Styrene in the microwave oven in the
absence of an initiator
158
5.
Temperature profile of a solution of Styrene and AIBN in a microwave oven
159
6.
Profile of the polymerization as a function of time
160
7.
Variable Output of Microwave Energy
161
xvi
8.
9.
Effect of initiator concentration on the molecular weights of the
polymer obtained from the microwave polymerization process
162
Comparison of the temperature profiles of the samples undergoing
microwave and thermal polymerization
163
APPENDIX
1.
The Microwave Oven
172
xvii
LIST OF TABLES
Page
PART I.
EVALUATION OF POLYANILINE IN CORROSION PROTECTION
I.
Rating of Failure at Scribe
32
II.
Formulations for the VERSICON Polyaniline dispersions in EPON 828
and list of formulations incorporating VERSICON Polyaniline
dispersions
33
III.
Formulations for PA-10 AmineB and PA-30 AmineB
34
IV.
Formulations for PA-10 AmineG and PA-30 AmineG
34
,V.
Formulations for PA-10 AmideB and PA-30 AmideB
35
VI.
Formulations for PA-10 AmideG and PA-30 AmideG
35
VII.
Formulations for PA-10 AnhG and PA-30 AnhG
36
VIII.
Formulation for the preparation of the dispersion of Polyaniline
base in EPON 828
36
IX.
Formulations incorporating the Polyaniline base dispersion
37
X.
Formulations incorporating PANDA
38
XI.
Formulations for PANDA in Acrylic/Polyurethane and
Acrylic/Melamine Systems
39
Formulations for the clear Aroflint top coat applied on the
Polyaniline (PANDA) base coat
39
XIII.
Control Formulations
40
XIV.
Chemical Composition of Commercial Inhibitive Pigments
41
XV.
Preparation of the dispersions of the commercially available
Corrosion Inhibitive Pigments in EPON 828
41
Formulations incorporating Commercial Inhibitive Pigments
41
XII.
XVI.
xviii
XVII.
Bath Make-up for the Time Variation experiment
41
XVIII.
Parameters for the Aniline Anodization Process
42
XIX.
Corrosion results ofPA-10 AmideG.
71
XX.
Corrosion results of PA-30 AmideG
71
XXI.
Corrosion results of PA-10 AnhG
72
XXII.
Corrosion results of PA-30 AmineB
72
XXIII.
Corrosion results ofPA-10 AmineB
73
XXIV.
Corrosion results of PA-30 AnhG
73
XXV.
Corrosion results of PA-30 AmideB
74
XXVI.
Corrosion results of PA-30 AmineG
74
XXVII.
Corrosion results of PA-10 AmineG
74
XXVIII.
Corrosion results ofPA-10 AmideB
75
XXIX.
Evaluation of Substrates coated with PA-30 AmideB formulation
75
XXX.
Evaluation of Substrates coated with PA-30 AmineB formulation
76
XXXI.
Corrosion results for DePANI30 Amine
77
XXXII.
Corrosion results for DePANI30 Amide
77
XXXIII.
Corrosion results for PANDA-30 AmineB
78
XXXIV.
Corrosion results for PANDA-30 AmideB
78
XXXV.
Corrosion results for PANDA-30 AroG
79
XXXVI.
Corrosion results for PANDA-30 ACMEG
79
XXXVII. Corrosion results for PANDA-30 ACPUG
80
XXXVIII. Corrosion results for system with PANDA base coat with
clear Aroflint top coat
80
xix
XXXIX.
Corrosion results for system with PANDA base coat with
clear Acrylic/Melamine top coat
81
Corrosion results for system with PANDA base coat with
clear Acrylic/Polyurethane top coat
81
XLI.
Corrosion results for Control-Amine
82
XLII.
Corrosion results for Control-Amide
82
XLIII.
Corrosion results for Control-Aroflint
83
XLIV.
Corrosion results for Control Acrylic/Melamine system
83
XLV.
Corrosion results for Control clear Acrylic/Polyurethane system
84
XLVI.
Summary of Corrosion results for Coatings incorporating Polyaniline
85
XLVII.
Corrosion results for Moly-White ZNP dispersed in Epoxy/Polyamide system...86
XLVIII.
Corrosion results for Halox SZP-391 dispersed in Epoxy/Polyamide system
86
XLIX.
Corrosion results for Busan 11-M1 dispersed in
Epoxy/Polyamide system
87
L.
Corrosion results for Nalzin 2 dispersed in Epoxy/Polyamide system
87
LI.
Corrosion results for Control Epoxy/Polyamide system
88
LII.
Summary of Corrosion results for Polyaniline Coatings
vs Control and Commercial Corrosion Inhibitive Pigments on Steel
89
LIII.
Comparison of the DC Polarization data and Salt Fog data
90
LIV.
X-Ray Fluorescence results for Fe Loading Test
90
LV.
Effect of variation of acid concentration in the Aniline Anodization bath
91
LVI.
Effect of variation of aniline concentration in the Aniline Anodization bath
91
LVII.
Corrosion results for Aniline Anodized Panels
92
LVIII.
Corrosion results for Aniline Anodized Panels top coated
with BASF e-coat system
93
XL.
XX
PART II. MICROWAVE INITIATED FREE RADICAL CATALYZED
POLYMERIZATIONS: POLYSTYRENE
I.
Evaluation of different free radical initiators when exposed
to microwave radiation
150
II.
Effect of the Power of Microwave Radiation on Molecular Weights
151
III.
Effect of Initiator Concentration on Molecular Weights
151
IV.
Comparison of the Bulk Microwave and Thermal Polymerization Process
152
V.
Comparison of the Solution Microwave and Thermal Polymerization Process..153
VI.
Effect of Microwave Radiation on Polymer Stability
154
I. INTRODUCTION
Conducting polymers were rediscovered in the late 70's. The use of conducting
polymers gave material scientists the advantages of combining the important electronic and
optical properties of semiconductors and metals with the mechanical properties and ease of
processing associated with polymers[l]. Interest in the study of conducting polymers has
increased dramatically over the past fifteen years[2,3]. The advantages of conducting
polymers are light weight, low cost and more recently ease of processing[4], The electrical
conductivity in these polymers is considered to be intermediate between semiconductors and
metals.
The conductivity range is 1 -1000 S/cm. These properties make conducting
materials ideal candidates for use in electronic and switching devices, rechargeable batteries,
sensors and corrosion inhibitors.
Intrinsically conducting polymers (ICP's) have been identified as novel corrosion
inhibiting coatings for metals[5].
Intrinsically conducting polymers are defined as
"polymers that conduct electric currents without the addition of conductive (inorganic)
substances"[6], Among all the conductive polymers studied so far, polyaniline is the first
to achieve commercial availability.
Polyaniline is the oldest known synthetic organic conducting polymer. It was first
reported in 1862, by H. Letheby in the Journal of The Chemical Society. There was little
interest in polyaniline until the late 70's when there was a sudden burst in research in
polyanilines[7], Polyaniline has been used in rechargeable organic batteries, electrochromic
display devices, modified electrodes and as a microelectronic device[8], Polyanilines are
relatively easy to synthesize and can be prepared either chemically or electrochemically[9].
2
Standard coating systems for metal protection incorporate more than one type of
coating system[5]. A common first step in any coating process is usually the application of
a corrosion inhibitive metal oxide such as chromate or molybdate on the surface of the
substrate. The next step is the application of a primer which contains corrosion inhibitors
that cathodically protect the metal substrate, for example - application of a zinc rich primer.
The final step is usually the application of a barrier coating which shields the metal from the
environment. The use of oxides such as chromates is highly effective, but environmental
hazards and pending regulations are restricting their use in paint coating systems.
The focus of this dissertation is on the use of ICP's as potential corrosion inhibitive
substances.
ICP's such as polyaniline, polypyrrole etc., conduct electrons due to the
presence of conjugated double bonds. The high electron mobility in conducting polymers
is due to the fact that in a conjugated system of electrons, considerable delocalization of the
electrons is achieved along or between the polymer chains[9].
ICP's are believed to passivate metals by forming a protective oxide on the surface
of the metal, thereby reducing corrosion[5].
Unlike the heavy metal oxides such as
chromates, the protective oxide layer formed by ICP's is non-toxic and poses no threat to the
environment. DeBerry was the first to indicate the use of a conducting polymer in corrosion
inhibition[10]. One of the key features that he suggested in his study, was the possibility of
substrate protection where defects in the coatings existed. The basis for this argument was
that, since ICP's are conductive, the entire coating would act to passivate any area of the
exposed metal[5].
The objective of this study is to evaluate the use of VERSICON polyaniline (a
commercially available polyaniline) and PANDA (a polyaniline synthesized by Monsanto)
3
in corrosion inhibitive formulations. These samples were tested in a neutral salt spray
environment and the corrosion inhibition was evaluated and compared with control and
samples incorporating commercially available corrosion inhibitors. Some of the coatings
evaluated were also tested electrochemically to establish a trend between the salt spray
results and electrochemical results. Polyaniline was also electrochemically synthesized and
evaluated for corrosion protection on aluminum.
The dissertation is divided into the following sections. The first section presents the
introduction and the objectives. The second section describes a literature survey of the
studies on the use of ICP's as corrosion inhibitors. Chapter 3 outlines the experimental and
formulation details for the coatings studied. Chapter 4 discusses the results obtained. The
conclusions obtained and a possible mechanism for the process are presented in chapter 5.
4
H. CONDUCTING POLYMERS AND CORROSION
"Intrinsically conducting polymers"(ICP) are defined as "polymers that conduct
electric currents without the addition of conductive (inorganic) substances"[6], Among all
the conductive polymers studied so far, polyaniline is the first to achieve commercial
availability.
ICP's are technologically important because of the following properties.
Polyaniline is considered to be a synthetic metal. While its specific conductivity and the
temperature dependence of its conductivity are considered semi-metallic, properties such as
thermopower fall in the metallic region. The mechanism of conductivity in polyaniline is
electronic in nature, which is analogous to the mechanism in metals. Polyaniline can be
solution processable, a very important characteristic not available with most other
conducting polymers. Polyaniline is green in its doped form and can change color when
exposed to a variety of substances. The 'doping' process creates charge carriers by adding
electrons to the conduction band or removing electrons from the valence band of the
polymer. Doping can be carried out either with a gaseous dopant, with the dopant in
solution, or by electrochemical oxidation or reduction. Polyaniline is a reactive polymer,
but as a metal it is more noble than iron or copper. It can undergo reproducible chemical or
electrolytic "switching" between the different conducting states of the polymer.
Considerable interest has been generated in the research of potential applications for
polyaniline. Some of the possible applications foreseen for this polymer are EMI shielding,
preparation of dispersions of polyaniline in organic solvents to produce transparent ultra-thin
layers on different substrates, electrically conductive surface coatings, electronic functional
layers, corrosion protection, sensors, membranes for gas separation, transparent electrodes,
5
smart windows and electrochromic displays. The following section describes a review of
the literature on the research of different groups on the use of polyaniline as a probable
corrosion inhibitor.
DeBerry and Viehback (1984) were among the first to report the possibility of using
polyaniline as an electroactive coating on active/passive metals[10]. This electroactive
coating, they claimed, maintained stainless steel in the passive state potential region. The
passive film behaved like a very non-ideal semi-conductor with respect to charge transfer
mediation between the metal' and the electroactive film. Anodically deposited polyaniline
films were able to protect steels in a sulfuric acid medium by maintaining the metal in the
passive state and repassivating the damaged areas.
In another paper[l 1] DeBerry discussed the modification of the electrochemical and
corrosion behavior of steel using an electroactive coating. He used polyaniline as the
primary electroactive material in preparing coatings for 400 series stainless steels. The
advantage of polyaniline was its effective use in acidic environments. Hydrogen ion is the
preferred counter-ion for polyaniline since it maintains its electoneutrality during electron
transfer. He reported rates of corrosion of 3.1 x 104 jim/yr for bare stainless steel and less
than 25 |im/yr for polyaniline coated stainless steel specimens.
The coatings were initially
blue-black in color when removed from the deposition bath. On placing the sample in an
acid environment the coating color changed to green. The open circuit voltage decreased
from 0.6V to 0.2V when the color change was observed[l 1]. As the electrode failed, the
color change became a lighter green and became transparent before becoming active.
Extended exposure led to disbonding due to the evolution of hydrogen. Bare stainless steel
specimens became active in minutes when placed at open circuit voltage in an acid solution.
6
Polyaniline coated samples remained passive for several hours before becoming active.
Corrosion measurements of polyaniline coated steel panels in a solution containing 0.2M
NaCl and 0.2M H2S04 showed unusual oscillations in the open circuit potential. When the
solution was changed to only 0.2M H2S04, the oscillations stopped. The interpretation given
was that for the coated electrode, the passive film periodically underwent partial breakdown
in the "acidic chloride" solution. The passive layer was then reformed with the dopant acid
of the polyaniline. Visual observation of this effect was the change in color of the electrode.
The color of the electrode changed from clear to pale green. After repassivation, the color
changed from green to dark green. He thus showed, that stainless steel electrodes coated
with thin films of polyaniline remained passive for long periods of time in acid solutions.
N. Ahmad et al. [12] reported that steels coated with the emeraldine base form of polyaniline
had the capability of anodically passivating steels in certain corrosive environments.
Smyrl et al. [13,14] describe a method (Figures 1 and 2) to stabilize the potential well
of the metal in the passive region when it is covered with an oxide film using electroactive
coatings. They demonstrate the feasibility of using electroactive coatings in corrosion
protection. They proposed that the redox polymer film would keep the potential of the metal
surface (covered with the oxide layer) between the active and trans-passive potential regions.
The authors electrochemically deposited poly(3-methylthiophene) on Ti/Ti02 substrates by
anodic electropolymerization. This coating was shown to control the potential of the metal
in the passive region. The film did not remain active if the film was reduced. For this
process to work, galvanic coupling was necessary to hold the potential of the metal in the
passive region. The authors mention that oxygen reduction on the film is necessary to hold
the potential in the required region. Introduction of platinum particles in the conductive
7
polymer film improved the oxygen reduction kinetics. The oxygen reduction was fast
enough to maintain the passivation current and keep the metal passive, thereby protecting
it from corrosion. The authors, however, state that the films of poly(3-methyl thiophene)
were not stable enough for long term use. Thus, the role of the conducting polymer film was
to 'poise' the potential of the oxide-covered metal in the passive region. The conducting
polymer film does this by maintaining a current between the passive surface and the
reduction reaction occurring on the surface of the polymer film. However, the film cannot
remain active if the polymer is reduced. This is compensated by the oxygen reduction
reaction. The oxygen reduction was catalyzed by incorporating platinum particles in the
polymer film.
Thompson et al. [15] in a joint research effort involving NASA and the Los Alamos
National Laboratory reported the use of corrosion protective coatings using electrically
conducting polymers for metal surfaces. Electrically conductive polymer coatings were
developed which imparted corrosion resistance to saline and acidic environments. They
claimed these coatings imparted corrosion resistance to the metal where scratches existed
in the protective coating. The team of researchers studied several conducting polymers such
as polyaniline, poly(3-hexyl thiophene), poly(3-thienyl methylacetate) and poly(3-thienyl
ethylacetate). Of the different polymers studied, polyaniline was found to have the best
overall properties and hence was selected for further study. Polyaniline was synthesized by
the chemical polymerization of aniline in the presence of an oxidizing agent like ammonium
persulfate. The emeraldine base was applied from solution onto steel panels and then doped
with dopants such as p-toluene sulfonic acid, tetracyanoethylene (TCNE) and zinc nitrate.
After doping, a coating of epoxy cured with an amine hardener was applied on top of the
8
polyaniline coating. This coating they claim had the necessary 'electronic environment' and
'coating toughness and resistance' to harsh environmental conditions. Corrosion testing was
performed in 3.5% aerated NaCl and 0.1M hydrochloric acid solutions. Along with the
above specimens they also tested a series of specimens without any conductive polymer
present. Samples with the conductive polymer base coat and epoxy top coat (unscribed)
showed no corrosion in 3.5% NaCl solution after twelve weeks. The control sample
however showed a significant amount of corrosion. The samples with the conductive
polymer base coat and epoxy top coat (scribed) immersed in aerated hydrochloric acid
solution showed no corrosion after twelve weeks. The scratched surface still appeared to be
'shiny' after twelve weeks of exposure. On site testing of polyaniline primers top coated
with polyurethane coatings indicated that there was no visual change in the coating
containing the polyaniline base coat. The control sample without the polyaniline treatment
showed significant corrosion.
The authors go on to recommend these coatings for
applications on equipment exposed to heat, sunlight, saline environments and other outdoor
exposure concerns, coatings for bridges, rebar in concrete, underground storage tanks, and
the automotive industry.
Beck[16] in his paper described a method of electrochemically depositing
polypyrrole on substrates such as aluminum and steel. One of the possible applications he
proposed was corrosion protective coatings. He proposed that the heteroaromatic structures
in conducting polymers allowed their use in corrosion inhibition. According to him, the
advantages of using these materials in corrosion inhibition were flexibility, insolubility and
adhesion to the substrates.
Troch-Nagels et al. [17] electrochemically deposited conducting polymers on mild
9
steel by the electropoiymerization. Their aim was to coat mild steel with a conducting
polymer which would retard electrochemical corrosion of the metal by introducing another
electrochemical reaction occurring at the surface of the film. The polymers studied were
polyaniline and polypyrrole. Several electrolytes were studied for the electrochemical
synthesis of polyaniline on mild steel. In basic solutions, brown films were obtained which
were non-homogeneous. The films were insulating and did not change when dipped in
sulfuric acid. This phenomenon was also observed when the synthesis was carried out in
neutral solutions. In an acid solution such as 20 vol % CH3OH, 0.13M H2S04 and 0.3M
aniline, the films obtained were black and powdery.
The aniline oxidation started at
800mV/SCE. Below 800mV/SCE, no films were obtained and above 1400mV/SCE, the
films did not have good properties. The optimum current density was between 0.05 to 0.3
A dm"2. The lowest values of current density were found to give the best films. Since, the
sulfuric acid solution did not give them the results they expected, the solution was changed
to a nitric acid solution (0.1M HN03, 0.3M aniline). They found that with this solution, the
aniline oxidation occurred at the same potential as for the sulfate solution and that the
dissolution of the substrate decreased. The properties of films obtained in nitric acid
solutions were summarized as follows:
1) The films were powdery even after curing at 170° C for 10 minutes.
2) SEM micrographs showed good surface structure.
3) Films were brittle
4) Conductivities were low.
5) Substrate dissolution was decreased, but only marginally.
6) The films obtained were insulating, therefore painting by e-coat was not possible.
They concluded that polyaniline films did not meet the requirements for mild steel
protection. They also concluded that polyaniline did not increase the corrosion resistance
of the substrate. Their results using polypyrrole indicated that it was a better candidate than
polyaniline and films obtained in neutral solutions were conductive enough to allow
anaphoretic painting. The corrosion resistance of the substrates was substantially increased.
The adhesion and mechanical properties remained a problem. Sekine et al. [18] also found
that electrochemically deposited polyaniline was not very useful in corrosion protection.
Another study by Sathiyanarayan[19] showed that soluble poly(ethoxyaniline) acted to
reduce the corrosion of iron in a IN hydrochloric acid solution. Geskin[20] was the first to
report a successful electrochemical synthesis of polyaniline. The substrate he used was
nickel. In the absence of polyaniline, the bare metal electrode dissolved rapidly in an acid
solution. This process was slowed down when a polyaniline/nickel electrode was used.
Wessling[21] in his paper in Advanced Materials (1994) had a different approach to
coating the metals with polyaniline.
Previous studies had all tried to incorporated
polyaniline electrochemically. His objective was to coat metals such as iron, steel and
stainless steel non-electrochemically by using polyaniline dispersions or polyaniline
containing lacquers.
His initial experiments showed some improvement in corrosion
protection, but the effects observed in a practical test such as salt spray were not totally
convincing.
The polyaniline used in his studies was manufactured by Allied Signal
Company under the trademark of VERSICON. After suitable pretreatment, the polyaniline
coating process was applied by a dip process. The coating was done in a series of steps to
obtain the appropriate film thickness. The coating process was repeated at least five times
(after complete drying of the previous coating step) to a maximum of twenty times to
11
increase thickness. The corrosion potential measurements were made electrochemically in
1M NaCl solution.
He observed a significant and reproducible shift of the corrosion
potential and he also observed a reduction in the specific corrosion current or corrosion rate.
Visual observations indicated a change in the optical appearance of the metal surface after
removal of the polyaniline coating. While the original metal appeared shiny, after coating
with polyaniline, the metal surface appeared light to dark grey, mat and spotted.
The mechanism of the passivation of iron by polyaniline proposed was as follows:
The first step of the interaction between polyaniline and iron was an etching step. The
polyaniline removes the first few layers of iron and dirt. The fresh iron layer formed then
is coated with an oxide layer. The experimental evidence for this was the elemental analysis
performed on the pure iron, iron freshly treated with polyaniline and the passivated areas.
The pure iron showed no oxygen content, while the freshly etched areas showed some
oxygen and the passivated areas showed large amounts of oxygen. He therefore concluded
that proper coating of metals with pure polyaniline can lead to a significant shift of corrosion
potential to decrease the corrosion rate of the metal.
Lu, Elsenbaumer and Wessling[22] in another study, evaluated the anti-corrosion
behavior of polyaniline exposed to dilute acid and salt solutions.
Mild steel specimens
when coated with polyaniline and overcoated with an epoxy barrier paint showed a reduction
in the corrosion rates of about 1000 orders of magnitude in 0.1N HC1 and 10 orders of
magnitude in 3.5% NaCl. Neutralized polyaniline (VERSICON) was applied to the substrate
from a solution in dimethyl propylene urea or N-methyl pyrollidone. The neutral polyaniline
was then doped in a aqueous solution of p-toluene sulfonic acid for 4 to 24 hours. The
samples were then top coated with a epoxy system. A scribe/damage was simulated by
drilling a precise hole in the specimen. A control epoxy, a neutral polyaniline coating top
coated with epoxy and a doped polyaniline coating top coated with epoxy were studied.
Visual and electrochemical results indicated polyaniline affected the corrosion process. Both
doped and neutral polyanilines exhibited corrosion protection. In the acid medium, the
doped polyaniline sample showed greater shift, while in the NaCl medium, the neutral
sample showed a greater shift in corrosion potential. The samples in the acid medium
showed a shift to a more noble potential while the NaCl samples showed shifts to a more
active potential.
Corrosion rate measurements showed a decrease in corrosion rate for the samples
treated with polyaniline. The effect on the corrosion rate reduction for the doped polyaniline
was greater in the acid solution, and the neutral polyaniline had a greater rate reduction in
the NaCl solutions. The results mentioned above were for the scribed samples. In samples
where the metal surface was not exposed, the doped polyaniline exhibited better corrosion
protection than the neutral system. The results provided indicate significant corrosion
protection provided to scratched areas galvanically coupled to polyaniline coatings and that
the extent of protection depended upon both forms of polyaniline (doped, undoped) as well
as the nature of the corrosion environment. The data presented indicate that the polyaniline
coating caused the metal to form a passive oxide layer on the surface in an acid environment.
However, this effect was not observed in NaCl solutions. XPS and Auger studies indicated
the passive oxide layer was composed of Y-Fe203 on the outside with a 3Fe, O layer
sandwiched between the passive oxide layer and the nascent metal.
Jasty and Epstien reported that the neutral emeraldine base form of polyaniline
passivated all forms of iron when exposed to corrosive environments[23]. The passivation
13
was found to occur by the formation of a thin passive oxide layer consisting mainly of
hematite. They found that the emeraldine base passivated the surface of iron even when
applied as an undercoat. However, they found that the emeraldine hydrochloride does not
provide effective corrosion protection for iron.
Wei et al. in another study performed a series of electrochemical corrosion
measurements on polyaniline coated steel specimens[24]. It was found that the base form
of polyaniline offered better corrosion protection in aqueous NaCl solution. They observed
an increase in the corrosion potential and corrosion resistance and a decrease in the corrosion
current when compared to the uncoated specimen. They found that the polyaniline base with
a zinc nitrate treatment followed by an epoxy top coat exhibited the best overall protection.
14
Protective Oxide Film
wwmmsm
MMM:
tiimiii
Active
Trans-passive
Passive
*-(+)
Figure 1. Illustration of a Passivated Metal in an ambient environment[13, 14]
^y/y/y/y/y/y/y/y/y^
•'.'. •/.•.'.
'.*• •/•
•/• •.*• '/• •
•vvVv/vvvvvvvvV/^vv^v'.vvvvv.
•.*,• '•••'•.• *•V•.V *..•*.V ••• *•V •V
wmmmm
mmmmm
Charged Conductive
Polymer Film
Protective Oxide Film
AvvV-^v.*.v.,iv;.v;.v;.v;.^v.'.vy..
•yty.-yi?iytytytytytytytytytyt
Figure 2. Electroactive coating designed for passivation
of metal in an ambient environment13, 14]
16
III. EXPERIMENTAL DETAILS
This chapter outlines the experimental details involved in the evaluation of
polyaniline (PANI) as a corrosion inhibitive substance. This chapter includes the different
formulations and test procedures involved in this study.
Section A deals with the
experimental details for the polyaniline coatings studied for steel and aluminum. Section B
outlines the experimental details for the procedure followed for the aniline anodization
process.
A.
TEST METHODS AND APPARATUS
The corrosion evaluations of the different coatings were performed according to the
methods described in ASTM methods B 117-90 and D 1654 - 92[25, 26]. The first method
(B 117 - 90) outlines the conditions for the salt spray testing. The apparatus used for the salt
spray testing consists of a fog chamber, a salt solution reservoir, a distilled water reservoir
and a supply of compressed air. The specimens were prepared according to the test method.
The metallic specimens were first wiped with a solvent such as methanol or xylene using lint
free Chemwipes. The panels were coated with a draw-down blade immediately after the
solvent cleaning step. After the curing cycle was complete, the edges of the metal panel was
sealed using a electroplater tape from Acutape Co. This was done to prevent corrosion in
the bare, uncoated areas. The panels were then scribed using a carbide tip, pencil type tool
(D 1654 - 92). In order to ensure that the scribe went through the coating a beeping device
was used. The scribing tool and the metal specimen were incorporated in a circuit device
which set off an alarm when the scribing tool contacted the metal panel. These panels were
then placed in the salt fog cabinet at random locations and checked periodically. The
specimens were placed in the cabinet on a plastic base at an angle of 15° in such a way that
the scribe was facing the principal direction of the flow of fog through the chamber. The salt
solution was prepared by dissolving 5 parts by weight of sodium chloride (purchased from
Fisher Co.; manufactured by Malinckrodt, ACS grade) in 95 parts of water. The pH of the
salt solution was maintained between 6.5 - 7.2. The temperature in the salt fog cabinet was
maintained at 35°C. Four 100 ml cylinders were placed at different locations in the cabinet
to collect the condensate of the salt fog. These cylinders were equipped with 10 cm diameter
funnels. This was done to monitor the collection rate of the salt solution during the course
of the test. The collection rate was maintained between 1.0 - 2.0 ml of solution per hour
based on an average run of at least 16 hours. The pH of this collected solution was measured
regularly to monitor the operation of the cabinet. After removal of the specimens from the
salt spray cabinet, the specimens were first rinsed with distilled water. The specimens were
then subjected to visual examination. The coatings were then stripped using a 50:50 mixture
of phenol and methylene chloride. These specimens were then washed with xylene and the
rating of the failure at the scribe was made. After completion of this evaluation, a clear
epoxy top coat was applied on the surface to preserve the specimens from further corrosion.
The rating of failure from the scribe is given in Table 1.
B.
MATERIALS USED IN THE STUDY
The Epoxy resin EPON 828 and the polyamide curing agent EPICURE 3125 were
obtained from Shell Chemical Company. EPON 828 is an undiluted clear di-functional
bisphenol A/epichlorohydrin derived liquid epoxy resin having an epoxide equivalent weight
18
of 185-192[27], The polyamine curing agent, ANCAMINE 1693 was obtained from Air
Products. The polyamide curing agent HY825 was obtained from Ciba Geigy. AROFLINT
202-A6X-60 and AROFLINT 303-X-90 were obtained from Reichhold. Aroflint 303 is an
oxirane modified triglyceride supplied in xylene solution and Aroflint 202 is high acid value
polyester[28], ACRYLOID AU 608X, a acrylic polyol was obtained from the Rohm and
Haas company[29],
DESMODUR N-75, an aliphatic polyisocyanate resin based on
hexamethylene diisocyanate was obtained from the Bayer Company[30]. CYMEL 303 is
a commercial grade of hexamethoxy-methymelamine and was obtained from Cyanamid[31],
DABCO DMEA (dimethylethanolamine) catalyst was obtained from Air Products.
p-Toluene sulfonic acid was obtained from Fisher Co., and CYCAT 4040 was
obtained from Cytec Industries.
Moly-White ZNP, a commercial corrosion inhibiting
pigment, was obtained from the Sherwin Williams Company. Busanl 1-M1 was obtained
from Buckman Laboratories, Halox SZP-391 was obtained from Halox Chemicals and
Nalzin 2 was obtained from the National Lead Company. Aniline was obtained from the
Aldrich Company.
All these materials were used as received
and without any
modifications.
The polyaniline, VERSICON was obtained from Allied Signal and was sieved before
incorporating them in the films. The product was sieved between 100^ and 37// sieves. The
powder obtained between these two sieves was collected and used. All formulations in this
study are using the sieved polyaniline. The films were prepared on steel and aluminum
panels obtained from the Q Panel Company. The panels were first cleaned using a solvent
such as methanol and xylene. The coatings were applied on the panels immediately after the
cleaning procedure.
C.
VERSICON POLYANILINE COATINGS
1.
Paint Preparation
a.
Preparation of the Epoxy/Polyaniline Dispersion. The appropriate amount of
epoxy resin was first weighed. The amount of polyaniline in the formulations was based on
the total amount of epoxy resin in the formulations. The polyaniline was then slowly added
to the epoxy under continuous stirring at 800 rpm.
After complete addition of the
polyaniline, the speed was increased to 1800 rpm and was stirred for 15 minutes before
being discharged. In the case of the dispersion for the 30% polyaniline, the dispersion
became very viscous before all the polyaniline could be added. Hence, the addition was
stopped after 27.5% polyaniline was added.
A large amount of the epoxy/polyaniline
dispersion was first prepared and then used for all the formulations. Table 2 lists the
epoxy/polyaniline dispersions prepared.
b.
Formulations Tested in Salt Spray Cabinet
i.
PA-10 AmineB/PA-30 AmineB. Table 3 lists the formulations for PA-10 AmideB
and PA-30 AmideB. The required amount of the base dispersion (1 or 2 respectively)
was weighed. To this was added the required amount of the polyamine curing agent
(Ancamine 1693). After thorough mixing, the films were drawn down on the steel and
aluminum panels using a 6 MIL draw down-blade. These films were then cured in an oven
at 50°C overnight.
ii.
PA-10 AmineG/PA-30 AmineG. Table 4 lists the formulations for PA-10 AmineG
and PA-30 AmineG. Draw-downs of the formulations listed above (PA-10 AmineB and
PA-30 AmineB) showed that the polyaniline turned blue after the addition of the amine.
This was due to the dedoping of the polyaniline by the amine curative. To prevent this,
the amine was first neutralized with an acid. p-Toluene sulfonic acid was used as the
neutralizing acid. In all the cases, the amount of acid was 30% based on the number of
equivalents of the amine. The required amount of the polyamine curing agent was weighed.
The acid was then added and allowed to react with amine. After an exotherm, the mixture
cooled back to ambient temperature. The base dispersion (1 or 2 respectively) was then
added to this mixture. After thorough mixing, the panels were drawn in the same way as
described in the previous formulations. The panels were then cured in an oven at 120°C for
an hour.
iii.
PA-10 AmideB/PA-30 AmideB. Table 5 lists the formulations for PA-10 AmideB
and PA-30 AmideB. The required amount of the base dispersion (1 or 2 respectively)
was weighed. The required amount of the polyamide curing agent was added to the
dispersion.
In the case of the dispersion with 27.5% polyaniline, a solvent mixture was
added before the addition of the curing agent. This was done to achieve a workable
viscosity. After good mixing, the panels were drawn down on the steel and aluminum panels
using a 6 MIL draw-down blade. These films were then cured in an oven at 50°C overnight.
iv.
PA-10 AmideG/PA-30 AmideG. Table 6 lists the formulations for PA-10 AmideG
and PA-30 AmideG. The required amount of the polyamide curing agent was weighed. To
this was added the required amount of CYCAT 4040 catalyst. CYCAT 4040 is a 40%
solution of p-toluene sulfonic acid in isopropanol. The amount of acid used to neutralize the
21
amide was again 30% based on the number of equivalents of the amide present. After good
mixing, the base dispersion (1 or 2 respectively) was added. After efficient mixing, the films
were prepared and cured at 120° C for an hour.
v.
PA-10 AnhG/PA-30 AnhG. Table 7 lists the formulations for PA-10 AnhG and
PA-30 AnhG. This system was designed such that no excess acid was required to neutralize
the basic amine. The "required amount of the base dispersion (1 or 2 respectively) was
weighed. The AROFLINT 202, an acid rich polyester resin was added to this dispersion and
stirred. The panels were drawn down as described above and the panels were cured in an
oven at 80 °C for an hour.
c.
Evaluation of Substrates. Ten different formulations incorporating the VERSICON
PANI had been evaluated. The salt fog results indicated the following:
The blue form of Polyaniline seemed to have a better protective action than the
green form.
•
A higher loading of PANI was desirable.
According to these observations two formulations from the earlier set of ten were
selected for further evaluation. These were the PA-30 AmineB and the PA-30 AmideB. The
formulations for these are listed in Tables 3 and 5 respectively. Specimens were prepared
using the above two formulations and applied on the following substrates: untreated
aluminum, anodized aluminum, chromated aluminum, untreated steel and phosphated steel.
These panels were coated with the above formulations and cured at 50°C for seven days
before salt spray evaluation.
Five panels of each of the substrate were subjected to
evaluation in the salt spray cabinet. The panels were scribed using a carbide tip scribing tool
before place in the salt fog cabinet. The panels were checked every 100 hours and were
removed after 500 hours of testing. After removal the films were stripped using a 50:50
mixture of phenol and methylene chloride. The scribe area was then evaluated according to
the ASTM D-1654 method. The extent of corrosion was based on the mean creepage from
the scribe.
d.
Evaluation of the Polyaniline base in Corrosion Protection
i.
Preparation of Polvaniline base. One liter of Ammonium hydroxide solution (3%
w/w) was prepared in distilled water. 31.53 grams of the VERSICON polyaniline was added
to the solution and stirred for 2 hours. The solution was then vacuum filtered.
The
polyaniline base was then washed with successive 100 ml volumes of distilled water,
methanol and diethyl ether. The material was then dried at room temperature.
ii.
Preparation of the Polvaniline base dispersion in EPON 828. The formulation for the
preparation of the dispersion is given in Table 8. The EPON 828 was weighed into a beaker.
The polyaniline base was added in small increments with stirring at 400 rpm.
After
complete addition the mixture was dispersed at 1800 rpm for 15 minutes. The mixture was
allowed to cool overnight before further processing.
iii.
Formulations incorporating the Polvaniline base dispersion.
Table 9 lists the
formulations for incorporating the polyaniline base dispersion. The two systems selected
for these formulations were the amine cured system and the amide cured system. The
substrates selected for this evaluation were untreated aluminum and untreated steel. The
formulations were applied on solvent cleaned panels of these substrates and cured at 50 °C
for seven days before salt spray evaluations. As in the earlier studies these panels were
scribed before testing. The panels were checked every 100 hours and removed from the salt
spray after 500 hours. These panels were stripped using a 50:50 mixture of phenol and
methylene chloride and then evaluated as per the ASTM D-1654 method. The extent of
corrosion was based on the mean creepage from the scribe.
D.
MONSANTO POLYANILINE COATINGS
1.
Synthesis of Monsanto Polyaniiine (PANDA). Non-electrochemical synthetic
routes to polyaniiine involves the oxidation of aniline in the presence of a suitable acid. This
polyaniiine was synthesized by Monsanto and characterized by the laboratory at
Monsanto[32], The synthesis involved the oxidation of aniline with ammonium persulfate
and doped with an organic acid DNNSA (dinonylnaphthalene sulfonic acid). The other acids
such as hydrochloric acid, sulfuric acid, p-toluene sulfonic acid yield products which are
insoluble and difficult to process. The use of DNNSA in the synthesis produces a very fine
dispersion/solution of polyaniiine which was easily processable in a solvent such as xylene.
A glass slide coated with the polyaniiine was viewed under an optical microscope and it
revealed no visible particles.
The film was hydrophobic in nature.
It also exhibited
reasonable good adhesion (rating 4 on a scale of 0 to 5) using the ASTM cross hatch tape
test.(ASTM D3359)[33],
2.
Formulations with PANDA blends. PANDA is the polyaniiine synthesized by
Monsanto Co. The difference between PANDA and the VERSICON PANI is the doping
agent used during the synthesis. The doping agent used in PANDA is Dinonyl naphthalene
sulfonic acid (DNNSA). This material is a very fine dispersion / solution and hence could
be easily mixed in with the EPON 828.
The earlier experiments had indicated that the
higher loadings were preferred, hence all the experiments were done using a 30% loading
of PANDA. Three systems were tested. These were the 1) Amine cured system 2)
Polyamide cured system and 3) Anhydride cured system.
Table 10 describes the formulation for the systems evaluated. In each of the systems,
the order of addition is the order in which the materials are listed. The EPON 828 was
weighed. The PANDA was then added to the EPON and mixed till it was uniformly
distributed. The solvent was then added and the mixing was continued. The curing agent
was then added to the system. The substrates tested were untreated aluminum and untreated
steel. Ten panels of each of the formulations were prepared and cured at 50° C for seven
days. Five panels were then selected, scribed with the carbide scribing tool and placed in
the salt spray cabinet. The panels were checked every 100 hours and were removed after
500 hours of testing. These panels were stripped using a 50:50 mixture of phenol and
methylene chloride and then evaluated as per the ASTM D-1654 method. The extent of
corrosion was based on the mean creepage from the scribe. The films obtained using the
EPON 828 / Aroflint 202 system (PANDA-30 AnhG) were extremely brittle and cracked on
scribing.
Other systems were investigated, which exhibited desirable film properties and in
which the PANDA stayed green and . One of these was dodecenyl succinic anhydride. This
system required a cure of 100 °C for one hour followed by curing for another one hour
at 150°C. The catalyst used for this system was Henkel's Versamide EH-50. In the absence
of the PANDA, the films were cured and these films did not crack on scribing. However,
on addition of PANDA to the system, severe flocculation was observed and the distribution
of PANDA in the cured film was affected.
Other catalysts were investigated. These were 2-ethyl-4-methyl imidazole (EMI-24)
and DABCO DMEA catalyst. While the DABCO at higher loadings cured at lower
temperatures, the films obtained were again brittle and thus could not be used for further
studies. Therefore, the green formulations were prepared using the Aroflint 303/ Aroflint
202 systems. Aroflint 303 is an oxirane modified triglyceride and Aroflint 202 is a high acid
value polyester. This system cured at 50 °C and the films were flexible and did not crack
during scribing.
PANDA was also tested in acrylic/melamine formulations and acrylic/polyurethane
systems. The formulations are described in Table 11. Ten panels of each of the formulations
were prepared and cured at 50 °C for seven days. Five panels were then selected, scribed
with the carbide scribing tool and placed in the salt spray cabinet. The panels were checked
every 100 hours and were removed after 500 hours of testing. These panels were stripped
using a 50:50 mixture of phenol and methylene chloride and then evaluated as per the ASTM
D-1654 method. The extent of corrosion was based on the mean creepage from the scribe.
3.
PANDA as a base coat primer. The first two steps described the use of PANDA
in the form of a blend with other polymer systems. This section describes the use of
PANDA as a base coat primer for steel panels. The PANDA was synthesized and supplied
as a 50% dispersion/solution in xylene. The steel panels were first prepared as described
26
earlier. The PANDA was then applied onto these steel panels using a 4 MIL blade. The
panels were then heated at 100°C for 1 hour. The panels were then top coated with clear
Aroflint systems These were then cured at 50 °C for seven days before salt spray testing.
Table 12 describes the formulation for the clear Aroflint system applied on the pure PANDA
base coat. Severe cratering was observed when the clear coat was applied. This was
overcome by the addition of BYK 306 (BYK-Chemie), a surface tension modifier added to
prevent formation of craters.
E.
CONTROL FORMULATIONS
A series of control specimens were also tested in the salt fog cabinet. The control
specimens were prepared without any polyaniline or other corrosion inhibitors present in the
system. The systems tested were the control Epoxy/Amide, control Epoxy/amine, control
Aroflint, control Acrylic /melamine and control acrylic/polyurethane. The formulations for
these systems are described in Table 13.
Ten panels of each of the formulations were prepared and cured at 50°C for seven
days. Five panels were then selected, scribed with the carbide scribing tool and placed in
the salt spray cabinet. The panels were checked every 100 hours for corrosion and were
removed after 500 hours of testing.
After the 500 hour exposure in the salt spray cabinet, the panels were removed and
the coatings were stripped using a 50:50 mixture of phenol and methylene chloride and then
evaluated as per the ASTM D-1654 method. The extent of corrosion after 500 hours of
exposure in the salt spray cabinet was based on the mean creepage observed in the specimen
from the scribe as described in Section A.
27
F.
COMPARISON OF SALT SPRAY RESULTS OF POLYANILINE WITH
OTHER COMMERCIALLY AVAILABLE CORROSION INHIBITORS AND
CONTROL SAMPLES
A series of formulations were prepared incorporating commercially available
inorganic corrosion inhibiting pigments. The commercial pigments selected were Molywhite ZNP (Sherwin Williams Co.), Halox SZP-391 (Halox Chemical Company), Busan
11M1 (Buckman Laboratories) and Nalzin 2 (National Lead Co.) The chemical nature of
each of these pigments is given in Table 14. This information was obtained from the
technical
literature for these pigments.
Based on the technical literature available a
dispersion of each of the pigments (15% by weight) was prepared in EPON 828 which are
listed in Table 15.
These dispersions were then mixed with the curing agent. The curing agent used for
this study was Shell Chemical's Epicure 3125. Table 16 lists the formulations for the
dispersion with the curing agent. Five samples of each type of pigment was applied on steel
panels using a 4 MIL draw-down blade. These panels were then cured at 50°C for seven
days. The panels were removed from the oven and the film thicknesses were measured using
a "Positector". The edges were taped and panels were scribed. The samples were then
placed at random locations in the salt fog cabinet and tested for 500 hours. The extent of
corrosion and the corrosion rating was performed as described in Section A.
G.
ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY STUDIES
The formulations evaluated in the salt fog cabinet were also tested electrochemically
using Electrochemical Impedance Spectroscopy. The samples were taped with Electroplater
Tape and cut to give a circle with an area of 1 cm2 exposed. The samples were tested in a
5% potassium sulfate solution at 35°C and at open circuit potential (OPV) with agitation in
a 800 ml jacketed beaker for both scribed and unscribed specimens. OPV is the potential
reading when the sample is connected in the circuit without application of any external
potential. It is the test of the film as it is in a rest condition. The potential was determined
using a reference electrode (Mercury/Mercurous sulfate) with a platinum mesh counter
electrode.
The
AC
impedance
measurements
were
made
using
a
273A
potentiostat/galvanostat and a Solatron 1255A Frequency Response analyzer. Two areas
were exposed to form an area of 1 cm2 area. One area was used for the tests on the
unscribed area, while the other area exposed was scribed and tested. To control the defect
of the scribe area, a pit was artificially drilled into the coating using a 5/64" drill to make a
hemispherical pit of about 0.03 cm2 area.
The DC polarization curves or the cyclic
voltammetry tests were started from about open circuit potential (OPE) to the anodic
direction to about 1.1 V vs SHE (Standard Hydrogen Electrode) and then scanned back. The
scan rate was set to 1 mV/s. The AC impedance tests were performed by holding the
potential at OPE and consecutively run 3 times. The frequency range was from 100 KHz to
0.005 Hz. The AC signal amplitude was 10 mV.
H.
CHARACTERIZATION OF POLYANILINE FILMS
The three systems that were characterized, were the Aroflint control, PANDA
dispersed in the AROFLINT and PANDA base coat with a clear Aroflint top coat. The
formulations for these systems are described in Tables 10, 11 and 12. After 500 hours of
exposure in the salt spray chamber, the panels were washed with distilled water and dried
29
in a stream of compressed air. These panels were then cut near the scribe area with a band
saw into 1cm X 1cm specimens. The films were then stripped using a 50:50 phenol,
methylene chloride mixture, washed with xylene and dried.
These specimens were
submitted for Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) studies.
I.
ANILINE ANODIZATION PROCESS
This section describes the procedure for incorporating aniline in the anodizing
process. The following general procedure was followed for the anodizing process. The
aluminum panels used in this study were 2024 series aluminum supplied by McDonnell
Douglas Corporation. These 3x6" panels were cut into 1x3 " specimens. These panels were
then first cleaned using acetone. The panel was then cleaned using an alkaline cleaner. The
alkaline cleaner used was the Turco 4215S. The solution was made according to the
McDonnell Douglas specifications and the operating temperature of the bath was maintained
at ~ 65°C. The panels were placed in the alkaline cleaner bath for 15 minutes. The panel
was then rinsed with cold tap water. It was then placed in the deoxidizing bath. The
deoxidizing bath was prepared using the Amchem deoxidizer No. 7, sulfuric acid and water.
The procedures for the degreasing and the deoxidizing steps were supplied by the
McDonnell Douglas Company. The panel was immersed in the deoxidizing bath at room
temperature for 3 minutes. The panel was then rinsed with cold water and then subjected to
the anodizing process.
The following parameters were varied for the anodizing process: anodization time;
acid concentration in the bath; aniline concentration in the bath; and anodization
temperature.
1.
Variation in the time of Anodization. The concentration of the aniline and sulfuric
acid in the anodize bath is described in Table 17. The anodization time was varied from 10
minutes to 30 minutes. The current was maintained at 0.3 amperes and the voltage at 20V
using a standard power supply apparatus.
2.
Variation in concentration of H2S04. The concentration of the acid was varied
from 5% to 40% by volume. The concentration of the aniline in the bath was maintained at
1 Molar. The voltage was adjusted to produce a current of 0.3 amperes. All samples were
anodized for 15 minutes.
3.
Variation in concentration of Aniline. The concentration of the aniline in the bath
was varied from 0.1M to 1.0M. The concentration of the acid in the bath was maintained
at 10% by volume. The voltage was adjusted to produce a current of 0.3 amperes. All
samples were anodized for 15 minutes.
Based on the results obtained for the experiments, a set of parameters were fixed for
the aniline anodization process. The parameters for the aniline anodization process are listed
in Table 18. The experimental setup used for the anodization process is described in Figure
3. The films obtained by this process were then characterized using SEM and X-Ray
diffraction techniques. Control anodized panels were prepared under identical conditions.
The bath make up consisted of a solution of 20% sulfuric acid in distilled water.
Pretreatment and the post treatment of these samples were identical to the aniline anodization
process.
Samples were prepared according to the procedure described above.
Several
variations were attempted in the post treatment processes. The variations in the post
treatment process were:
31
1) 30 minute anodization, followed by 15 minute hot water seal.
2) 30 minute anodization, hot water seal, followed by dip in 5 -10% sulfuric acid.
3) 30 minute anodization, followed by 15 minute seal in hot dilute sulfuric acid.
4) 30 minute anodization, hot water seal, followed by dip in 10% PTSA.
5) 30 minute anodization, hot water seal, followed by dip in DNNSA.
6) 30 minute anodization, no seal.
7) 30 minute anodization, no seal, followed by dip in 10% PTSA.
Samples obtained from 1, 3, 6 and 7 were submitted for SEM evaluations. These
samples were also tested using polarization measurements. These were compared against
the control anodized panels.
Polyaniline was also electrochemically synthesized as follows. A solution similar
to the one used for the aniline anodize process was prepared. The cathode and the anode
used for the experiment was lead. This resulted in the formation of the polyaniline powder
in solution. The solution was then filtered and the polymer was isolated. The polymer was
characterized using X-ray Diffraction.
Table I. Rating of Failure at Scribe
Representative Mean
Creepage from Scribe
Millimeters
Number
Zero
10
Over 0 to 0.5
9
Over 0.5 to 1.0
8
Over 1.0 to 2.0
7
Over 2.0 to 3.0
6
Over 3.0 to 5.0
5
Over 5.0 to 7.0
4
Over 7.0 to 10.0
3
Over 10.0 to 13.0
2
Over 13.0 to 16.0
1
Over 16.0 to more
0
Rating
Table II. Formulations for the VERSICON Polyaniline dispersions in
EPON 828 and list of formulations incorporating
VERSICON Polyaniline dispersions
Formulation
EPON 828
Sieved
Polyaniline
Dispersion 1
727.2
72.8
Dispersion 2
615.4
169.2
Formulation
Curing agent
Percentage
Color
PA-10 AmideG
Epicure 3125
10.0
Green
PA-30 AmideG
Epicure 3125
27.5
Green
PA-10 AnhG
Aroflint 202
10.0
Green
PA-30 AmineB
Ancamine 1693
27.5
Blue
PA-10 AmineB
Ancamine 1693
10.0
Blue
PA-30 AnhG
Aroflint 202
27.5
Green
PA-30 AmideB
HY 825
27.5
Blue
PA-30 AmineG
Ancamine 1693
27.5
Green
PA-10 AmineG
Ancamine 1693
10.0
Green
PA-10 AmideB
HY 825
10.0
Blue
Table III. Formulations for PA-10 AmineB and PA-30 AmineB
Formulation
PA-10 AmineB
Materials
Wt%
Drawdown
blade
PANI Dispersion 1
65.1
4 MIL
Ancamine 1693
34.9
blade
TOTAL
PA-30 AmineB
100.0
PANI Dispersion 2
52.6
Ancamine 1693
24.4
Xylene
23.0
TOTAL
6 MIL
blade
100.0
Table IV. Formulations for PA-10 AmineG and PA-30 AmineG
Formulation
PA-10 AmineG
Materials
Wt%
Drawdown
Blade
PANI Dispersion 1
54.9
4 MIL
PTSA
15.8
Ancamine 1693
29.4
TOTAL
PA-30 AmineG
Blade
100.0
PANI Dispersion 2
45.3
PTSA
11.2
4 MIL
Ancamine 1693
20.9
Blade
Solvent mixture
22.6
TOTAL
100.0
Table V. Formulations for PA-10 AmideB and PA-30 AmideB
Formulation
PA-10 AmideB
PA-30 AmideB
Materials
Wt%
Drawdown
Blade
PANI Dispersion 1
48.8
4 MIL
Epicure 3125
26.7
Propyl Propasol
24.5
TOTAL
100.0
PANI Dispersion 2
40.1
Epicure 3125
19.1
Propyl Propasol
40.8
TOTAL
100.0
Blade
4 MIL
Blade
Table VI. Formulations for PA-10 AmideG and PA-30 AmideG
Formulation
PA-10 AmideG
Materials
Wt%
Drawdown
Blade
PANI Dispersion 1
44.9
4 MIL
CYCAT 4040
30.6
Epicure 3125
24.5
TOTAL
PA-30 AmideG
100.0
PANI Dispersion 2
48.5
CYCAT 4040
28.6
Epicure 3125
22.9
TOTAL
Blade
100.0
4 MIL
Blade
Table VII. Formulations for PA-10 AnhG and PA-30 AnhG
Formulation
Materials
Wt%
Drawdown
Blade
PA-10 AnhG
PANI Dispersion 1
42.3
4 MIL
Aroflint 202
57.7
Blade
TOTAL
PA-30 AnhG
100.0
PANI Dispersion 2
45.9
4 MIL
Aroflint 202
54.1
Blade
TOTAL
100.0
Table VIII. Formulations for the preparation of the dispersion of
Polyaniline base in EPON 828
Materials
Grams
EPON 828
100.0
PANI base
27.5
Table IX. Formulations incorporating the Polyaniline base dispersion
Formulation
DePANI30 Amine
Materials
Wt%
Drawdown
Blade
PANI BASE dispersion
53.4
6 MIL
Xylene
21.8
Ancamine 1693
24.8
TOTAL
DePANI30 Amide
100.0
PANI BASE dispersion
40.1
Propyl Propasol
40.8
Epicure 3125
19.1
TOTAL
Blade
100.0
6 MIL
Blade
Table X. Formulations incorporating PANDA
Formulation
PANDA-30 AmineB
Materials
Wt%
EPON 828
44.5
61% PANDA
21.8
6 MIL
Xylene
11.5
Blade
Ancamine 1693
22.2
TOTAL
PANDA-30 AmideB
100.0
EPON 828
42.1
61% PANDA
20.7
4 MIL
Propyl Propasol
11.9
Blade
Epicure 3125
25.3
TOTAL
PANDA-30 AnhG
100.0
EPON 828
36.5
61% PANDA
17.9
Aroflint 202
45.6
TOTAL
4 MIL
Blade
100.0
Aroflint 303
PANDA-30 AroG
Drawdown
Blade
4 MIL
61% PANDA
Aroflint 202
TOTAL
Blade
100.0
Table XI. Formulations for PANDA in Acrylic/Polyurethane and
Acrylic/Melamine Systems
Formulation
PANDA-30 ACMEG
Materials
Wt%
Drawdown
Blade
Acryloid 608U
63.9
4 MIL
48.5% PANDA
23.8
Cymel 303
12.3
TOTAL
PANDA-30 ACPUG
Blade
100.0
Acryloid 608U
60.2
48.5% PANDA
22.4
4 MIL
Desmophen N-75
17.2
Blade
Dibutyl Tin Laurate
0.2
TOTAL
100.0
Table XII. Formulations for the clear Aroflint topcoat applied on the
Polyaniline (PANDA) base coat
Formulation
Materials
Wt%
Drawdown
Blade
Clear Aroflint
Aroflint 303
45.6
4 MIL
Aroflint 202
54.3
BYK306
0.1
TOTAL
100.0
Topcoat
Blade
Table XIII. Control Formulations
Formulation
Materials
Wt%
Drawdown
Blade
Control Amine
EPON 828
62.8
4 MIL
Ancamine 1693
37.2
Blade
TOTAL
Control Amide
EPON 828
56.2
Propyl Propasol
10.0
Epicure 3125
33.8
TOTAL
Control Aroflint
Acrylic/Melamine
4 MIL
Blade
100.0
Aroflint 303
45.6
4 MIL
Aroflint 202
54.4
Blade
TOTAL
Control
100.0
100.0
Acryloid 608U
82.8
Cycat 4040
1.4
Cymel 303
15.8
TOTAL
Blade
100.0
Control
Acryloid 608U
77.6
Acrylic/Polyurethane
Dibutyl tin laurate
0.4
Desmophen N-75
22.0
TOTAL
4 MIL
100.0
4 MIL
Blade
41
Table XIV. Chemical Composition of Commercial Inhibitive Pigments
Pigment
Chemical
Composition
MOLY-WHITE ZNP
Basic Zinc Phosphomolybdate
BUSAN 11-Ml
Modified Barium Metaborate Monohydrate
NALZIN-2
Zinc Hydroxy Phosphite
HALOX
Calcium Strontium Zinc Phosphosilicate
Table XV. Preparation of the dispersions of the commercially available
Corrosion Inhibitive Pigments in EPON 828
Materials
Wt (%)
EPON 828
86.7
Pigment
13.0
Nuosperse 657
0.3
Table XVI. Formulations incorporating Commercial Inhibitive Pigments
Materials
Wt (%)
Dispersion
59.2
Epicure 3125
30.8
Propyl Propasol
10.0
Table XVII. Bath Make-up for the Time Variation experiment
Materials
Concentration
conc. H2S04
10% by volume
Aniline
1 Molar
Distilled water
Table XVIII. Parameters for the Aniline Anodization Process
Pretreatment
Solvent wipe- Acetone
Alkaline cleaner - 15 minutes
Deoxidizer - 3 minutes
Anodization
Aniline Concentration - 0.75M
Sulfuric Acid Concentration - 20% by volume
Anodization time - 30 minutes
Current density - 12 A/ft2
Agitation - Magnetic stirring
Water - Distilled
Aluminum alloy - 2024
Panel Size - 2" x 3"
Cathode - Lead
Area Cathode/Area Anode - 1/3
Post Treatment
Water Rinse - Distilled
Water Seal Temperature - 65 - 70°C
Water seal pH - Neutral
Water Seal Time - 15 minutes
Drying - Oil Free Compressed Air
1.
Bath consisting of aniline, sulfuric acid &distilled water
2.
Lead cathodes
3.
Aluminum 2024 alloy anode
4.
Inlet for cooling water
5.
Outlet for cooling water
Figure 3. Experimental Setup for the Aniline Anodization Process
44
IV. RESULTS AND DISCUSSION
A.
VERSICON POLYANMNE COATINGS
1.
Formulations Tested in Salt Spray Cabinet.
Ten different formulations
incorporating two different loadings of polyaniline were investigated. The polyaniline used
was the VERSICON polyaniline. The dopant acid in VERSICON polyaniline is p-toluene
sulfonic acid. The initial color of this material is dark green. On reaction with a base, the
polyaniline changes color from green to blue. In order to investigate whether the green or
blue form would be more effective in corrosion protection, formulations were developed
with the epoxy systems in order for the polyaniline to stay green. The coatings were applied
on virgin untreated steel and aluminum panels.
a.
PA-IO AmideG. The polyamide curing agent in this system reacted with the
polyaniline and converted it to a blue form. In order to prevent this, the amide was
neutralized with an excess amount of the acid and then the polyaniline was incorporated in
the system. This prevented the color change. Panels were prepared and evaluated in the salt
fog cabinet. Table 19 presents the corrosion results of PA-10 AmideG. The results obtained
for PA-10 AmideG indicated that this formulation was not suitable for corrosion protection.
The excess PTSA added could be a detriment in its use in corrosion protective coatings.
b.
PA-30 AmideG. The difference between the formulation listed above and the
present is the loading of polyaniline. Here again the polyamide curing agent was first
neutralized with PTSA and then blended with dispersion of epoxy and polyaniline. Table
20 describes the corrosion results for PA-30 AmideG. The salt spray results for this
formulation showed accelerated corrosion, probably due to the excess PTSA in the
formulation. The excess PTSA is apparently detrimental to the corrosion protection process.
c.
PA-10 AnhG. This system was an epoxy system cured with a acid rich polyester.
Alien the polyaniline dispersion was blended with the polyester resin Aroflint 202, no color
change was observed. Table 21 describes the corrosion results for PA-10 AnhG. The films
obtained contained large areas of discontinuities and this related as poor protection in the salt
fog cabinet.
d.
PA-30 AmineB.
No modifications were made with curing agents for this
formulation. The system showed poor adhesion to aluminum substrates (Table 22) while the
adhesion to steel substrates was satisfactory. Failure on the aluminum substrates can be
attributed to poor adhesion. Very slight rust formation was observed on the steel specimens
at the end of the 48 hour period. Some corrosion was observed in the scribe area. The extent
of corrosion was less than that for the AmideG, AmineG and the AnhG formulations.
e.
PA-10 AmineB. Table 23 describes the corrosion results for PA-10 AmineB
formulation. This formulation has a lower loading of polyaniline (10%). The corrosion
protection of this formulation was lower than that observed for PA-30 AmineB. The amount
of corrosion observed in 500 hours was not as severe as that observed for the AmineG,
AmideG or AnhG formulations. This coating also exhibited poor adhesion to aluminum.
The poor corrosion protection observed could be attributed to poor adhesion.
f.
PA-30 AnhG. Table 24 describes the corrosion results obtained for PA-30 AnhG.
This system was green in color after curing. The results for this system indicated that the
corrosion was observed after the first 48 hours. The extent of corrosion was not as severe
as that observed for the AmideG and AmineG, but was greater than that observed for the
AmineB and AmideB systems.
g.
PA-30 AmideB. Table 25 describes the corrosion results obtained for the system
PA-30 AmideB. After 500 hours of exposure in the salt fog cabinet, this system exhibited
the least amount of corrosion as compared to the other systems studied. This system worked
satisfactorily for steel substrates but exhibited poor adhesion on aluminum substrates over
a prolonged exposure in the salt fog cabinet.
h.
PA-30 AmineG. Table 26 describes the formulation for PA-30 AmineG. In this
system, excess PTSA was added to maintain the green form of polyaniline. The data
obtained after 500 hours of exposure in the salt fog cabinet indicated that this system
exhibited poor corrosion protection. Severe corrosion was observed in the specimens
exposed to 500 hours of salt spray. The poor performance of this coating could be attributed
to the presence of the excess PTSA in the system. The excess PTSA, as observed earlier
is detrimental in corrosion protection.
i.
PA-10 AmineG. Table 27 describes the corrosion results obtained for PA-10
AmineG. In this system, excess PTSA was added to maintain the green form of polyaniline.
The data obtained after 500 hours of exposure in the salt fog cabinet indicated that this
47
system exhibited poor corrosion protection. The excess PTSA in the system was detrimental
for corrosion protection.
j.
PA-10 AmideB. Table 28 describes the corrosion results obtained for PA-10
AmideB. The results indicated some corrosion after 500 hours of exposure. The extent of
corrosion was not as severe as the AmideG, AmineG or the AnhG formulations. The extent
of corrosion was greater than the PA-30 AmineB and the PA-30 AmideB formulations.
k.
Summary of Corrosion Results. Ten different formulations were studied initially.
All these samples were evaluated in the salt fog cabinet for a period of 500 hours each. The
study indicated that the addition of the excess PTSA to the formulation caused an
acceleration in the rate of corrosion. The study also indicated that the amine and the amide
blue formulations exhibit better corrosion protection than the green formulations. It also
indicated that the formulations containing a higher loading of polyaniline, exhibited better
corrosion protection.
2.
Evaluation of Substrates
a.
PA-30 AmideB. Table 29 describes the corrosion results for the panels in the salt
fog cabinet after a period of 500 hours. Results for this formulation indicated that there was
not much difference in the performance of the coating on the phosphated steel and the
untreated steel substrate. The corrosion protection was comparable for both the substrates.
For the aluminum substrates there was a significant difference in the performance of
the coating. The coating on the untreated aluminum showed poor adhesion and therefore had
48
a lower ability to protect. The chromated aluminum panels exhibited the best corrosion
protection, while the anodized aluminum panels exhibited, good protection. The extent of
protection was lower than the chromated aluminum substrate.
b.
PA-30 AmineB. Table 30 describes the results for the amine cured formulation. The
results are for the panels in the salt spray cabinet after a period of 500 hours. The results for
this formulation indicate very poor adhesion for all of the substrates considered. Compared
to the amide cured formulation, the amine cured formulation exhibited lower protection.
c.
Summary of Results. Evaluation of the substrates did not show any significant
differences for steel. Coatings on the chromated aluminum showed the best corrosion
protection, the anodized specimens were better for adhesion and the untreated aluminum
exhibited the least protection. PA-30 AmideB and PA-30 AmineB showed improved
corrosion protection compared to untreated, uncoated aluminum.
3.
Evaluation of PANI base (DePANI, dedoped VERSICON PANI) in Corrosion
Protection
a.
DePANI30 Amine. The results described in Table 31 are the corrosion results for
the panels in the salt spray cabinet after a period of 500 hours. The films obtained after
curing were gritty. During the preparation of the specimens for testing, it was observed that
the reaction was not as rapid as the VERSICON PANI. This proved the earlier theory that
the curing agent was actually dedoping the polyaniline and the polyaniline base was being
formed in situ. However, the free acid remained in the film and accelerated corrosion.
b.
DePANI30 Amide. Table 32 describes the corrosion results obtained for the
formulation incorporating the polyaniline base in the epoxy resin, cured with a polyamide
curative. The results obtained for this formulation were similar to the DePANI30 Amine
formulation. Corrosion was observed in steel and aluminum substrates after 500 hours of
exposure. The polyaniline base had no protective action for either the steel or the aluminum
substrates.
c.
Summary of Results. The polyaniline base did not show any corrosion protection.
Comparison of the corrosion rating for the doped and the base forms of the polyaniline
indicates that the doped form has a better protective action than the base form of polyaniline.
However, when the doped form was incorporated in the epoxy system cured with the amine
or the amide curing agents, the neutralization of the polyaniline took place converting it from
the green to the blue form. When this neutralization takes place, the acid released is trapped
in the system and could cause some corrosion to occur.
B.
MONSANTO POLYANILINE COATINGS
1.
Corrosion Protection of PANDA blends. PANDA is the polyaniline synthesized
by Monsanto Co. The difference between PANDA and the VERSICON polyaniline is the
doping agent used during the synthesis.
The doping agent used in the VERSICON
polyaniline is PTSA. The doping agent used in PANDA is Dinonyl naphthalene sulfonic
acid (DNNSA). PANDA is soluble in xylene and hence could be easily mixed in with the
EPON 828. The earlier experiments had indicated that the higher loadings were preferred,
hence all the experiments were done using a 30% loading of PANDA. The three systems
50
tested were the Amine cured system, Polyamide cured system, and the Anhydride cured
system.
a.
PANDA-30 AmineB. Table 33 describes the results for the panels after exposure
in the salt spray cabinet for 500 hours. During the preparation of these specimens, it was
observed that the color of the PANDA changed from green to blue. Also, the reaction was
very rapid and required thinning with solvents to apply the coating. As it can be observed
from the table, the adhesion of the films was better on the steel than the aluminum substrate.
Corrosion was observed in the scribe area and some blistering was observed in the film.
b.
PANDA-30 AmideB. Table 34 shows the results for the panels after exposure in the
salt spray cabinet for 500 hours. As it can be seen in the table, the formulation did not
protect in the scribe. Corrosion was obtained both on steel as well as aluminum panels. The
films showed no lifting for the amide cured formulations. The coating on steel showed some
blistering at the end of the 500 hour period.
c.
PANDA-30 AroG. Table 35 describes the results obtained for this formulation. As
it was mentioned earlier, this formulation incorporates the Aroflint 303 / Aroflint 202
system. Aroflint 303 is an oxirane modified triglyceride and Aroflint 202 is a high acid
value polyester.
The PANDA stays green in this formulation.
The mixture had a
considerably long pot life and the preparation of the specimens caused no problems. This
coating system indicated a better protection for the aluminum substrate than the steel
substrate.
d.
PANDA-30 ACMEG. Table 36 describes the salt spray results obtained for the
formulation incorporating PANDA in an acrylic melamine system after 500 hours of
exposure. The results indicate again that the polyaniline blended into the formulations did
not show any considerable protection against corrosion of the substrate. Corrosion was
observed in the scribe area along with blistering.
e.
PANDA-30 ACPUG. Table 37 describes the salt spray results for the formulation
incorporating PANDA in an acrylic polyurethane system after 500 hours of exposure. The
results for this system indicated that the polyaniline blended in the system did not improve
its protection against corrosion.
2.
PANDA as a base coat primer
a.
PANDA as a base coat primer with Aroflint top coat. Table 38 describes the
corrosion results for the system containing PANDA as a base coat with a clear Aroflint top
coat after 500 hours of exposure. Small amounts or rust was observed in the scribe area in
the first 100 hours. After 500 hours there was only a small amount of corrosion in the
specimen. Very near the scribe area color changes were observed from dark green/black to
blue. No blistering or lifting of the film was observed. On removal of the film, the surface
of the steel panel appeared to have a dull gray appearance to it. There was no creepage from
the scribe area. This system exhibited the best corrosion protection compared to all the
specimens studied. This result indicated that polyaniline needs to be present in a large
concentration on the substrate. Blending of polyaniline with other polymers reduces the
activity of polyaniline at the substrate and thus corrosion protection was not observed.
b.
PANDA as a base coat primer with clear Acrylic/Melamine top coat. Table 39
describes the corrosion results for the system containing PANDA as a base coat with a clear
acrylic/melamine top coat after 500 hours of exposure. For this system, the top coat was
brittle, and it chipped off during scribing. One panel was tested in the salt fog chamber.
Some rust formation was observed in the scribe area after the first 100 hours of exposure.
After 500 hours of exposure, there was a small amount of corrosion was observed in the
scribe. There was no visible color change observed after the 500 hour exposure. There were
no blister formation or under film corrosion. Upon removal of the coating, very little
corrosion was observed. The surface of the steel had a dull grey appearance.
c.
PANDA as a base coat primer with clear Acrylic/Polyurethane top coat. Table
40 describes the corrosion results obtained for the system with polyaniline as the base coat
with a clear top coat of an acrylic polyurethane system after salt spray exposure for 500
hours. Small amounts of rust was observed in the scribe area in the first 100 hours of the
test. At the end of the 500 hour exposure, corrosion was observed in the scribe area. The
extent of corrosion was lesser than the control system but was greater than the PANDA base
coat plus Aroflint clear top coat system. Some blistering was observed. Upon removal of
the coating, the substrate had a dull gray appearance. There were areas of discoloration.
Over the period of the test a color change was observed in the base coat. The color went
from a dark green to a yellowish green/brown.
3.
Summary of Results. The study for the PANDA polyaniline indicated that the
system did not work well when blended in the formulations. Incorporation of PANDA in
conventional formulations did not show any significant protection. However, when PANDA
was applied as a base coat followed by a top coat of some kind, significant improvement in
corrosion protection is observed. The PANDA base coat with Aroflint top coat indicated
only very small amounts of corrosion at the end of a 500 hour exposure period. On removal
of the coating, very little rust was observed in the scribe area. The substrate had a dull
grayish appearance and this could be the formation of a passive oxide, which prevents
further corrosion. This observation of a formation of a grayish appearance of the substrate
after the coating was removed, has been observed in the literature by Wessling[21], In the
paper, he describes this grayish coating to be a passive iron oxide formed on the surface.
C.
CONTROL FORMULATIONS
Specimens were prepared without any PANI / PANDA present in the system. This
was done to check if the PANI provided any corrosion protection.
1.
Control-Amine. Table 41 describes the results for the panels after 500 hours of
exposure. The results for the amine formulation indicate that this formulation is not suitable
for use on aluminum substrates. The corrosion rating for this formulation was comparable
that for the formulations containing PANDA or VERSICON polyanilines. However, the
corrosion rating for this formulation was much lower than that obtained for the system, when
PANDA was used as the base coat primer.
2.
Control-Amide. Table 42 describes the results for the panels after 500 hours of
exposure. The amide cured formulations did not exhibit the problems of the amine
54
formulations. The corrosion rating for this formulation was not much different than that for
the formulations containing PANDA or VERSICON polyanilines. The corrosion was,
however, greater than that observed when PANDA was used as the base coat.
3.
Control-Aroflint. Table 43 describes the results for this formulation. Severe
corrosion was observed on the steel specimen. No blistering was observed, but severe under
film corrosion was observed in this sample. On aluminum substrates, the extent of corrosion
was lesser. The corrosion protection of this system was much lower than when the PANDA
was blended in this system. There was a significant improvement in protection, when
PANDA was applied as a base coat followed by a clear coat of the AROFLINT system.
4.
Control Acrylic/Melamine system.
Table 44 describes the corrosion results
obtained for the control acrylic melamine system after 500 hours of exposure. The results
indicate severe corrosion was observed for the control acrylic melamine system. Severe
corrosion was observed in the scribe area in the first 100 hours with severe blistering along
the film.
5.
Control Acrylic/Polyurethane system. Table 45 describes the corrosion results for
the control acrylic/polyurethane system after 500 hours exposure to salt spray. The results
indicated that corrosion was observed in the first 100 hours of exposure. Severe corrosion
was observed at the end of the 500 hour exposure period. Some blistering was also
observed. The corrosion evaluation for this coating indicated that the PANDA base coat
enhanced the protective properties of the Acrylic/Melamine coating.
D.
SUMMARY OF SALT FOG DATA
Table 46 summarizes the results obtained for the salt fog testing performed on the
coatings incorporating polyaniline. The three different types of polyaniline were studied.
These were the VERSICON polyaniline, the polyaniline base and PANDA (Monsanto's
polyaniline). The results indicate that the polyaniline does not have much of an effect on the
corrosion protection of steel when blended with another polymer. The corrosion protection
was either comparable or lower than that observed for the control samples. However, when
the PANDA was applied as a base coat, significant protection was observed for steel. This
effect was tested on three different systems. For all the three systems, the results indicate
that when PANDA was applied as a base coat, the protection was significantly better than
when the PANDA was blended into the system. Also, for all the three systems with the
PANDA base coat, the substrate had a grayish appearance when the coating was removed.
This was not the case for the PANDA blend systems or the control samples. This indicated
that the PANDA was passivating the surface by forming a passive oxide on the surface and
the PANDA helped in maintaining this oxide, thereby preventing further corrosion. This
phenomenon of the formation of the passive oxide was observed by Wessling[21].
E.
COMMERCIAL INHIBITOR FORMULATIONS
1.
Formulations Tested in the Salt Spray Cabinet.
Specimens coated with
formulations incorporating the commercially available corrosion inhibitive pigments were
exposed to 500 hours of salt fog and evaluated similar to the evaluations performed for
polyaniline. The commercial pigments selected were Moly-White ZNP, Halox SZP 391,
Busan 11M1 andNalzin 2. These commercial corrosion inhibitive pigments were dispersed
56
in EPON 828 and the dispersion was cured using Epicure 3125, a polyamide curing agent
from Shell Chemical Company.
a.
Moly-White ZNP (15% by weight) dispersed in Epoxy/Polyamide system. Table
47 describes the corrosion results obtained for salt spray tests of a coating containing 15%
Moly-White in an epoxy/polyamide system after exposure for 500 hours. Only a slight
amount of corrosion was observed in the scribe after the 500 hours of exposure in the salt
spray cabinet. No blistering was observed in the coating. The film of one specimen exposed
to the salt spray disbonding after removal from the cabinet.
b.
Halox SZP-391 (15% by weight) dispersed in Epoxy/Polyamide system. Table
48 describes the corrosion results obtained for the salt spray tests of a coating containing
15% Halox SZP-391 in an epoxy/polyamide system after exposure for 500 hours. Corrosion
was observed in the scribe area after 100 hours of exposure. Corrosion was observed after
500 hours with some under film corrosion present. Some disbonding occurred after removal
from the salt spray cabinet.
c.
Busan 11-M1 (15% by weight) dispersed in Epoxy/Polyamide system. Table 49
describes the corrosion results obtained for the salt spray tests of a coating containing 15%
Busan11-M1 in an epoxy/polyamide system after exposure for 500 hours. Corrosion was
present throughout the panel. Blisters were observed throughout the panel and the corrosion
in the scribe area was also greater than the previous two specimens. During the preparation
of the dispersion of the pigment in EPON 828, it was observed that Busan 11-M1 was the
57
hardest pigment to disperse. The poor performance of this pigment could be attributed to
the fact that the pigment was not uniformly dispersed in the system.
d.
Nalzin2 (15% by weight) dispersed in Epoxy/Polyamide system. Table 50
describes the corrosion results obtained for the salt spray tests of a coating containing 15%
Nalzin 2 in an epoxy/polyamide system after exposure for 500 hours. The results indicate
that corrosion was observed in the scribe area. Some blistering was observed in the
specimen during the test in the salt spray cabinet.
e.
Control Epoxy/Polyamide. Table 51 describes the corrosion results obtained for
the control epoxy/polyamide system after exposure to the salt spray cabinet for 500 hours.
The results indicate that the corrosion inhibitors present in the system does improve the
protection. Corrosion was observed in the scribe area and blistering and under film
corrosion was observed.
f.
Summary of Results. The results for the salt spray tests on the commercial
corrosion inhibitors indicate that these inhibitors contributed to corrosion protection. The
extent of corrosion depended on the chemical nature of the pigment. Among the inhibitors
studied, the Moly-White exhibited the best protective action, while the Halox pigment
showed the least protective action. The order of the protective action was Moly-White ZNP
> Nalzin2 > Busanll-Ml > Halox SZP-391.
On comparison of the results of the
commercial inhibitors with the PANDA base coated specimens, it is evident that the PANDA
base coat system with the clear top coat exhibits better corrosion protection.
58
2.
Comparison of Salt Spray Results of Polyaniline with other commercially
available corrosion inhibitors and control samples. The salt spray results (Table
52) indicate that the performance of the polyaniline blends in some of the formulations were
comparable with the performance of the commercially available corrosion inhibitors. The
degree of corrosion protection was comparable with that obtained for Nalzin2, Halox and
Busanl 1-M1. However, the performance of the Moly-White ZNP, was slightly better than
the performance of the polyaniline blends. When polyaniline was applied as a base coat with
a clear top coat, there was a significant improvement in performance. The system with the
PANDA base coat with the Aroflint clear coat exhibited the best corrosion protection
compared to all the other sample studied.
F.
ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY STUDIES
The DC polarization curves and the AC impedance curves for the Aroflint control
sample with a pit immediately after immersion in 5% K2S04 are given in Figures 4, 5 and
6 respectively. The corrosion current observed is 10 mA and the potential at which this
current is observed is about 0.9V. The DC polarization curves indicates that the film has a
very poor protective action. Severe corrosion is to be expected. The curves do not indicated
any form of passivation and this sample is not expected to perform well in a corrosive
environment.
Figures 7, 8 and 9 are the DC polarization curves for the pure PANDA system with
a pit immediately after immersion in 5% K2S04 solution. The DC polarization curves for
this particular sample indicate that the current increases as the potential is increased and
then there is a drop in the current. The potential at which this drop is observed is the
59
passivation potential. When the scan is reversed the current stays at zero. For this particular
sample, the passivation potential is observed at about 0.75 V and the corrosion current
observed is about 0.01 mA.
Figures 10, 11 and 12 are the DC polarization curves for the PANDA dispersed in
the Aroflint system with a pit immediately after immersion in 5% K2S04 solution. The DC
polarization curves for this particular sample indicate that the current increases as the
potential is increased and then there is a drop in the current. For this particular sample the
passivation potential is observed at about 0.75 V and the corrosion current is about 0.001
mA.
Figures 13,14 and 15 are the DC polarization curves for the PANDA base coat with
a clear Aroflint top coat system with a pit immediately after immersion in 5% K2S04
solution. DC polarization curves for this sample indicate that the current increases as the
potential is increased and then there is a drop in the current. For this sample the passivation
potential was observed at about 0.5 V and the corrosion current is about 0.0001 mA.
Figures 16, 17 and 18 are the DC polarization curves for the PANDA dispersed in
the Acrylic/melamine system with a pit immediately after immersion in 5% K2S04 solution.
The DC polarization curves for this particular sample indicate that the current increases as
the potential is increased and then there is a drop in the current. For this particular sample
the passivation potential is observed at about 1.05 V and the corrosion current is about 10
mA.
Figures 19,20 and 21 are the DC polarization curves for the PANDA base coat with
the acrylic/melamine clear top coat system with a pit immediately after immersion in 5%
K2S04 solution. The passivation potential is observed at about 0.7 V and the corrosion
current is about 0.0001 mA.
Figures 22,23 and 24 are the DC polarization curves for the Moly-White dispersed
in the epoxy/polyamide system with a pit immediately after immersion in
5% K2S04
solution. The passivation potential is observed at about 0.65 V and the corrosion current is
about 0.001 mA.
Table 53 describes the summary of the results obtained using DC polarization curves
and the salt spray data for the same specimens. The data obtained from the DC polarization
curves indicated that a low passivation potential and a low corrosion current is necessary for
a coating to perform as a corrosion inhibitive coating. Among the specimens studied, the
sample with the PANDA base coat and the Aroflint clear top coat should exhibit the best
performance in the salt spray test. This was the trend observed from the results obtained.
The data also indicated that the polyaniline worked better as a corrosion inhibitive system
when applied as a base coat rather than blending it in a polymer system. The corrosion
current for the specimen with the PANDA base coat indicated a 10 fold reduction as
compared to the PANDA dispersed in the Aroflint system.
G.
CHARACTERIZATION OF POLYANILINE FILMS
From the salt fog tests it was evident that the system containing the polyaniline base
coat with the Aroflint top coat had the least amount of corrosion after 500 hours of exposure.
Three systems were tested once again in the salt fog cabinet. These three systems were the
Aroflint control, PANDA dispersed in the Aroflint and PANDA base coat with a clear
Aroflint top coat. These specimens were analyzed using SEM and XRD.
1.
X-Ray Diffraction. The X-ray diffraction patterns obtained are given in Figures 25,
61
26 and 27. Sample 1 is the control Aroflint sample. The x-ray diffraction pattern indicates
the presence of an iron oxide. It was difficult to identify the exact nature of the oxide. Also,
the pattern indicates that the oxide was not well formed and not completely crystalline. This
could be due to the fact that certain areas on the specimen were covered with the corrosion
product while certain areas were unaffected. For sample 2, the PANDA base coat with the
Aroflint top coat, it was observed that a grayish oxide covered the entire surface to the panel
and was uniform visually. The x-ray diffraction pattern indicated a well formed oxide
probably Fe^. The pattern showed sharp peaks and these corresponded to that for Fe203.
Again for the specimen coated with PANDA dispersed in Aroflint a similar pattern to
Sample 1 was observed and the oxide could not be exactly identified. This implies the
polyaniline base coat helps in protecting the substrate by the formation of a protective oxide
such as Fe203 and the polyaniline helps in maintaining a thin layer of the passive oxide on
the substrate.
2.
SEM Analysis. Figures 28, 29 and 30 are the scans obtained for the three different
specimens studied. The figures for the control Aroflint and the PANDA dispersed in
Aroflint seem to be similar with large discontinuities present on the surface.
These
discontinuities are probably the corrosion product obtained during the salt fog exposure. The
EDS spectrums for the control and the PANDA dispersed in Aroflint seem to be similar.
The scans for the specimen base coated with PANDA with a clear top coat indicate that
surface morphology is different from that observed for the control and the PANDA blend
samples. This could be due to the formation of the uniform passive oxide layer on the
surface.
H.
OTHER MISCELLANEOUS OBSERVATIONS
In order to observe if the polyaniline (PANDA) interacted with the substrate, a
sample of Iron was placed in a 50% solution of PANDA for 9 days. The solution was then
analyzed using X-Ray fluorescence to observe if any metal had dissolved in the solution.
Table 54 describes the results obtained for this study. It is evident from the table that iron
is active in a solution of PANDA. PANDA is a polyaniline with DNNSA as a dopant and
is a fine dispersion / solution in xylene. Further tests were performed in which Fe chunks
were immersed in xylene and DNNSA (Nacure 1051). The observations indicate that the
dopant in polyaniline makes the Fe active when it is in contact with a solution of the
polyaniline.
A very thin coating (0.5 MILS) of polyaniline was sprayed on steel and aluminum
substrates. A coating of the Aroflint was applied on the polyaniline coated panels. These
samples were cured for 7 days at 50°C. These panels were scribed and tested in the salt fog
cabinet and checked after 24 and 100 hours. The polyaniline coating for these systems were
green in color. At the end of the 24 hour period, it was observed that the color of the
polyaniline near the scribe area had turned blue. After 100 hours the area that had changed
color had increased. After about 100 hours, the polyaniline had completely turned blue
underneath the clear coating. After this period, the panel started corroding more rapidly.
For the aluminum panel, a color change was observed, but it was not as dramatic as that
observed for steel. A small area near the scribe had changed from green to blue, but the
majority of the panel was green even after a period of 100 hours. This indicates that the iron
is more active with polyaniline and therefore needs a thicker coating of polyaniline on the
surface. The aluminum is however, not as active as iron in polyaniline.
I.
ANILINE ANODIZATION PROCESS
1.
Optimization of Anodization Parameters
a.
Variation in Time. The time variation experiment was performed in the same bath.
As the aniline was added to the bath, a white colored solid was formed which slowly
dissolved to form a amber colored liquid. Beyond 0.75M, the solid did not dissolve in the
solution. The quality of the aniline solution used was important. In a previous experiment
relatively old and exposed aniline was used, which produced a amber colored solution,
Freshly purchased aniline, which was not exposed to the atmosphere produced a clear and
colorless solution which did not foam during the anodization procedure. The first specimen
was anodized for 10 minutes, the second for 15 minutes, the third for 20 minutes and the last
for 30 minutes. It was also observed that as the time was increased, the intensity of the blue
color increased. This was true for the 10, 15 and the 20 minute samples. But, for the 30
minute sample however, the intensity decreased. The cathode had a brown coating on it
which had very poor adhesion. In order to prevent this, a fresh bath was made up and the
anodizing was carried out for 30 minutes. The color of the sample from the fresh bath was
more intense than the 20 minute anodized sample. The cathode was then placed in a cathode
bag and the experiment was repeated. Again, a smooth and uniform blue coating was
observed. This indicated that the use of a cathode bag gave a more uniform coating, as it
prevented deposition on the cathode. The film thickness for the 15 minute sample was
between 0.1 to 0.2 MILS, while the thickness for the 30 minute sample was 0.2 MILS. The
thickness of the coating does not seem to change appreciably as the time for anodizing
increases, but the intensity of the blue color increases with time. When the aluminum
cathode was replaced with a lead cathode, no brown deposit was obtained at the cathode.
b.
Variation of the acid concentration. It was observed that for the 5 and 10% acid
solutions, the current was lower than that for the 20% and 40% solutions. Also, at higher
currents, the coating did not appear to be uniform and smooth. Therefore for the 20%
solution, the voltage was adjusted to 1IV to produce a current of 0.3 amperes. The intensity
of the polyaniline coating seemed to increase with increasing acid concentration up to about
25% after which non-uniform coatings were obtained. The 20% solution under the above
mentioned conditions produced an intense blue coating. The film thickness of these coatings
are reported in Table 55. It was observed that increasing the acid concentration, increased
the film thickness to a certain extent, but it levels off after about 10%. However the intensity
of the blue coating increases with increasing acid concentration.
The salt spray data
suggested that the 20% acid concentration specimen had lower corrosion product and pitting
compared to the other specimens.
c.
Variation in concentration of Aniline. As the aniline was added to the bath, a
white colored solid was formed which slowly dissolved to form a clear colorless liquid. The
amount of the white solid formed increased with increasing aniline concentration. Beyond
0.75M, the solid did not dissolve in the solution. The foaming of the bath increased with
increasing aniline concentration. This seemed to be related to the amount of solid particles
floating in the bath. As the concentration of the bath was increased the intensity of the
coating increased. The 0.1M solution had the lowest intensity and it increased as the
concentration was increased. The intensity seemed to level off after about 0.5M. The results
are described in Table 56. The 0.75M sample was sealed in an acid bath instead of the hot
water bath. The coating remained green after sealing, but on exposing it to the atmosphere
65
overnight, the color changed back to blue, but it was not as uniform as the coating produced
with regular hot water seal. The film thickness of the coatings do not seem to again be
affected by the concentration of the aniline. However, the intensity of the blue coating
increases with increasing aniline concentration until it levels off. The acid seal sample
seems to have a lower film thickness than the other samples. This may be due to the fact that
some of the coating is washed off during the acid seal.
d.
Summary of Results. Increase in the time of the anodization process increases the
intensity of the color of the coating. However, the thickness increased marginally and
leveled off after 15 minutes of anodization.
Increasing the acid concentration in the
anodization bath increased the intensity of the coating produced. The thickness of the
coating seems to be comparable for all the specimens studied.
Increasing the aniline
concentration increased the intensity of the coating, but the film thicknesses were
comparable. The acid sealed samples exhibited lower film thickness than the hot water
sealed samples. The temperature of the solution was also varied. The best coatings were
obtained at 30°C. As the temperature was increased, no deposition was obtained.
2.
Anodization and study of the effect of the post treatment procedures
a.
Anodization followed by 15 minute hot water seal. The anodization process
yielded green colored specimens. When these panels were sealed in hot water, the panels
changed color to a deep blue. These coatings obtained exhibited good adhesion to the
substrate and did not rub off easily.
atmosphere.
The coatings were stable when exposed to the
b.
Anodization, hot water seal, followed by dip in 5-10% sulfuric acid. The
anodized hot water sealed panels yielded blue colored specimens. When these samples were
dipped in 5% or 10% sulfuric acid, the color of the panels immediately changed to green.
The intensity of the green color diminished with time. The color change observed was blue
to green to yellowish green. The specimen was stable in the acid solution for about 15
minutes before gas bubbles began to evolve. The panel was removed and washed with
distilled water. The color of the panel was a pale yellowish green. On washing the panel
with distilled water, the green coating was washed away with the stream of distilled water.
This indicated the treatment in sulfuric acid degraded or dissolved the film. Further washing
of the panel reverted the color back to blue, indicating the dopant was being washed away.
The intensity of the blue was lower than the intensity of the panel after aniline anodization
and water sealed.
c.
Anodization followed by 15 minute seal in hot dilute sulfuric acid. The panel
after anodization was green in color. This panel was sealed in a 5% sulfuric acid solution
at 70°C. The color of the panel changed from the dark green to light yellowish green in a
minute. The intensity of the green coating decreased rapidly. The panel was taken out of
the solution and washed with distilled water. On washing with distilled water, the coating
appeared to wash away indicating the coating was destroyed. The panel changed color from
green to blue when exposed to the atmosphere, indicating that the dopant was being washed
away. References were obtained in the literature[34,35], which state that polyaniline is
soluble in sulfuric acid and other strong acids. Thus the decrease in the intensity of the color
of the coating could be due to the dissolution of polyaniline on the surface by sulfuric acid.
d.
Anodization, hot water seal, followed by dip in 10% PTSA. After the anodization
and the hot water seal the blue panel was dipped in a solution of 10% PTSA. The color of
the coating changed from blue to a bluish green color. No further color change was
observed. The color of the panel stayed the same bluish green after dipping the panel for 30
minutes. The polymer film did not wash away like the earlier specimens and the color
remained green for a longer time (~ 24 hours).
e.
Anodization, hot water seal, followed by dip in DNNSA. After the anodization
and the hot water seal process, the blue panel was dipped in a 50% solution of
Dinonylnaphthalene sulfonic acid in butyl cellosolve. The color of the panel changed from
blue to a green. No further color change was observed. The polymer film seemed to be
more stable to washing with distilled water. The green color obtained was stable and did not
revert back to blue for over 24 hours.
f.
Anodization, no seal. The panel after the anodization step was green in color. This
panel was not sealed in hot water. The panel changed in color from green to blue slowly.
In about an hour the panel turned completely blue in color. When such a specimen was
dipped in a 10% solution of PTSA, the panel reverted to a green color. This green coating
was not stable enough and washed away when treated with distilled water. A similar
observation occurred when the panel was sealed in a hot PTSA solution.
3.
Corrosion Studies of £!ectrochemically deposited Polyaniline
a.
Salt Spray Evaluations. Aniline anodized panels and plain anodized panels were
evaluated in the salt spray cabinet. Both scribed and unscribed specimens were tested. It
was observed that the aniline anodized panels exhibited good performance after the first
100 hours. However, at the end of 336 hours, there was severe discoloration and pitting
observed on the surface and the extent of corrosion was greater than that observed for the
plain anodized panels. This was true for both the scribed and unscribed specimens. A few
of the aniline anodized panels were coated at McDonnell Douglas with the BASF E-Coat
system. These were scribed and compared against the same E-coat system on thin sulfuric
anodized aluminum panels.
Table 57 and 58 describe the corrosion results obtained for these specimens. The
table indicates again that a top coat is necessary for obtaining good protection for a
polyaniline system. The electrochemically polyaniline was found to be marginally inferior
when compared with the plain anodized panels, when the samples had a top coat.
The
performance of the uncoated anodized panels seem to change as a function of time. At lower
exposure times, the aniline anodized specimens seemed to do better, while over a longer
period of time, the plain sulfuric acid anodized specimens exhibited better corrosion
protection.
b.
Electrochemical Evaluations of the Aniline Anodized Panels. The specimens
tested in the salt fog cabinet were also tested using DC polarization. The samples tested
were the plain sulfuric acid anodized, aniline anodized water seal, aniline anodized water
seal with a 10 minute dip in 10% PTSA solution and aniline anodization no seal. The DC
polarization curves were obtained immediately after immersion in 5% NaCl solution. The
curves indicated that for the specimens immediately after immersion, the aniline anodized
69
panels indicated better protection than the plain sulfuric acid anodized panels. The panel
which was aniline anodized with water seal followed by a 10 minute dip in PTSA and the
panel which was not sealed exhibited poorer protection than the other two specimens. Figure
31 are the curves obtained for the DC polarization experiment.
4.
Surface Characterization. Scanning electron micrographs were obtained for the
following samples: 1) Aniline Anodized Water Sealed; 2) Aniline Anodized water Sealed
followed by PTSA dip for 10 minutes; 3) Aniline Anodized no seal; 4) Aniline Anodized
H2S04 sealed; 5) Plain Anodized panels.
The SEM micrograph for the aniline anodized hot water sealed panel (Figure 32)
shows a granular structure with a few pores. The SEM micrograph for the acid sealed panel
(Figure 33) however shows a loss of the granular structure, indicating that the type of sealing
process affects the morphology of the coating. The SEM studies on the specimens indicate
different morphologies for the hot water sealed samples and the acid sealed samples. The
SEM micrograph for the aniline anodized, water sealed followed by 10 minute dip in PTSA,
and the aniline anodize, no seal specimens in PTSA solution (Figure 34 and Figure 35)
depicts a similar morphology as observed for the aniline anodize water sealed specimen.
Figure 36 is the SEM micrograph for the sulfuric acid anodized control specimen. The
results along with visual observations indicate that polyaniline is being formed on the
surface of the aluminum substrate. The morphology indicates the formation of a granular
structure with some pores present on the surface.
The powder obtained from the electrochemical polymerization of the solution was
submitted for X-ray diffraction studies. This sample was compared against the chemically
70
synthesized VERSICON polyaniline. Figure 37 and 38 are the X-ray diffraction patterns
obtained for these two specimens.
These data suggests that the electrochemically
synthesized polyaniline is has a more crystalline structure and has a more oriented structure.
The data for the chemically synthesized polyaniline suggested it was more amorphous. The
peaks were very broad and did not show any sharp peaks usually indicative of a crystalline
substance. The difference in the X-ray diffraction data for the chemically synthesized
polyaniline (VERSICON) and the electrochemically synthesized polyaniline suggests that
the two systems are different structurally.
Table XIX. Corrosion results of PA-10 AmideG
Substrate
Visual Observation after 500 hours
Untreated
Steel
Corrosion was observed in 48 hours.
Observed along the scribe and all over the
film. Severe corrosion observed in 500 hours
in all the specimens tested.
Untreated
Aluminum
Discoloration was observed in the scribe in 48
hours. Severe corrosion in 500 hours in all
the specimens tested.
Table XX. Corrosion results of PA-30 AmideG
Substrate
Visual Observation after 500 hours
Untreated
Steel
Corrosion was observed in 48 hours.
Observed along the scribe and all over the
film. Severe corrosion observed in 500 hours
in all the specimens tested.
Untreated
Aluminum
Severe discoloration was observed in the
scribe in 48 hours. Severe corrosion in 500
hours in all the specimens tested.
Table XXI. Corrosion results of PA-lOAnhG
Substrate
Untreated
Steel
Untreated
Aluminum
Visual Observation after 500 hours
Slight corrosion observed after 46 hours.
Corrosion was observed in 500 hours in all
the specimens tested. The corrosion however
was not as severe as the AmideG & the
AmineG.
Slight discoloration was observed along the
scribe in 46 hours. Severe corrosion was
observed in 500 hours in all the specimens
tested.
Table XXII. Corrosion results of PA-30 AmineB
Substrate
Visual Observation after 500 hours
Untreated
Steel
Slight corrosion was observed after 48 hours.
Some corrosion was observed, but not as
severe as the AmineG, AmideG or AnhG
formulations.
Untreated
Aluminum
Lifting of film due to poor adhesion. Test
continued for 500 hours. Corrosion was
observed under the film in all the specimens
tested.
Table XXIII. Corrosion results of PA-10 AmineB
Substrate
Visual Observation after 500 hours
Untreated
Steel
Slight corrosion was observed after 48 hours.
Some corrosion after 500 hours, but not as
severe as the AmineG, AmideG or AnhG
formulations.
Untreated
Aluminum
Lifting of film due to poor adhesion. Test
continued for 500 hours. Corrosion was
observed under the film in all the specimens
tested.
Table XXIV. Corrosion results of PA-30 AnhG
Substrate
Visual Observation after 500 hours
Untreated
Steel
Corrosion observed along the scribe after 48 hours.
The extent of corrosion after 500 hours was not as
severe as the AmideG and AmineG but was greater
than the AmideB & AmineB
Untreated
Aluminum
Slight discoloration after 48 hours. The trend in
extent of corrosion after 500 hours was similar to
the trend observed for steel. Some lifting of the
films were also observed.
Table XXV. Corrosion results of PA-30 AmideB
Substrate
Visual Observation after 500 hours
Untreated
Steel
Very slight corrosion after 48 hours. This
seemed to have the least amount of corrosion
after 500 hours.
Untreated
Aluminum
Not much evidence of corrosion after 48
hours. However, the films exhibited poor
adhesion after a prolonged exposure.
Table XXVI. Corrosion results of PA-30 AmineG
Substrate
Visual Observation after 500 hours
Untreated
Steel
Corrosion observed in 48 hours. Severe
corrosion observed in 500 hours.
Untreated
Aluminum
Discoloration observed after 48 hours.
Corrosion severe after 500 hours. Some
lifting of the film observed.
Table XXVII. Corrosion results for PA-10 AmineG
Substrate
Visual Observation after 500 hours
Untreated
Steel
Corrosion observed in 48 hours. Severe
corrosion observed in 500 hours.
Untreated
Aluminum
Discoloration of metal observed in 48 hours.
Severe corrosion observed in 500 hours.
75
Table XXVIII. Corrosion results of PA-10 AmideB
Substrate
Visual Observation after 500 hours
Untreated
Steel
Slight corrosion observed in 48 hours. Corrosion
observed in 500 hours, but not as severe as the
AmideG and AmineG formulations.
Untreated
Aluminum
Very slight corrosion observed in 48 hours. Lifting
of film observed during exposure.
Table XXIX. Evaluation of Substrates coated with PA-30 AmideB formulation
Mean Creepage
from scribe (mm)
Rating of
Failure
D-1654A
2.0-3.0
6-7
2.0-2.5
6
o
N(
t
Visual Observation
Substrate
7
(after 500 hours)
Untreated
Steel
Corrosion was observed in the
scribe after 100 hours. There were
blisters present close to the scribe
at the end of 500 hours.
Phosphated
Steel
Corrosion was observed in the
scribe after 100 hours. There were
no blisters present in the film after
500 hours.
Untreated
Aluminum
Slight lifting was observed along
the scribe. Some white product
was seen in the scribe at the end of
500 hours.
Anodized
Aluminum
No lifting of the film was
observed. Some white product
was seen in the scribe after 500
hours.
Chromated
Aluminum
No lifting of the film observed.
Some white product was seen in
the scribe after 500 hours
0.375 - 0.725
8-9
0.0-0.6
9-10
76
Table XXX. Evaluation of Substrates coated with PA-30 AmineB formulation
Visual Observation
Substrate
Mean Creepage
from scribe (mm)
(after 500 hours)
Phosphated
Steel
Untreated
Aluminum
Anodized
Aluminum
Chromated
Aluminum
m
l
o
Untreated
Steel
Corrosion was observed in the
scribe after 100 hours. There were
blisters present near the scribe
after 500 hours in salt spray.
Rating of
Failure
D-1654A
5
Corrosion was observed in the
scribe after 100 hours. Lifting of
the film was observed at the end of
the 500 hours and there was severe
corrosion.
7.0- 10.0
3
Lifting of the film was observed
after 100 hours. The film had
completely lifted at the end of 500
hours. White corrosion product
was observed
2.4-3.0
6
Slight lifting was observed after
100 hours. The film had lifted
after 500 hours, but the extent of
corrosion was not as severe as the
untreated aluminum
1.5-2.3
7-6
Slight lifting of the film after 100
hours. After 500 hours there was
lifting of the film due to poor
adhesion. The scribe area showed
very little corrosion.
0.0-0.5
9
77
Table XXXI. Corrosion results for DePANDO Amine
Substrate
Untreated
Steel
Untreated
Aluminum
Visual Observation
(after 500 hours)
Corrosion in the scribe was
observed after first 100 hours. The
film had lifted at the end of the 500
hour period and the scribe area had
been completely corroded. It was
not possible to make any
measurements for the specimens.
Lifting of the film was observed
after 100 hours. The scribe area
showed significant corrosion at the
end of the 500 hour period.
Mean Creepage
from scribe (mm)
Rating of
Failure
D-1654A
Not possible to
make any
measurements
<2
3.5-4.5
5
Table XXXII. Corrosion results for DePANDO Amide
Untreated
Aluminum
Mean Creepage
from scribe (mm)
Rating of
Failure
D-1654 A
Corrosion in the scribe was
observed after 100 hours of
testing. The film showed no lifting
but blistering was observed in
certain areas.
4.5-5.5
5-4
No lifting of the film was observed
after 100 hours. Some white
product was observed in the scribe
area after 500 hours.
o
Untreated
Steel
Visual Observation
(after 500 hours)
b
t
Substrate
5
78
Table XXXIII. Corrosion results for PANDA-30 AmineB
Substrate
Untreated
Steel
Untreated
Aluminum
Visual Observation
(after 500 hours)
Mean Creepage
from scribe (mm)
Rating of
Failure
D-1654A
Corrosion in the scribe was
observed after 100 hours of
testing. The film showed no lifting
but blistering was observed in
certain areas.
1.0-1.5
7
4.5 - 5.5
5
Slight lifting of the film was
observed after 100 hours.
Considerable lifting of the film
was observed near the scribe.
Scribe area showed considerable
corrosion.
Table XXXIV. Corrosion results for PANDA-30 AmideB
Substrate
Untreated
Steel
Untreated
Aluminum
Visual Observation
(after 500 hours)
Mean Creepage
from scribe (mm)
Rating of
Failure
D-1654 A
Corrosion in the scribe was
observed after 100 hours of
testing. Corrosion with blistering
was observed at the end of the 500
hour period
1.0-1.5
7
No lifting of the film was observed
after 100 hours. Some white
produce was observed in the
scribed area at the end of the 500
hour period.
1.0
8-7
79
Table XXXV. Corrosion results for PANDA-30 AroG
Corrosion in the scribe was
observed after 100 hours of
testing. Severe corrosion was
observed in the scribe area with
blistering.
Not possible to
make any
measurements
No lifting of the film was observed
after 100 hours. Not much
corrosion observed in the scribe.
One or two blisters observed near
the scribe area.
o
Untreated
Aluminum
Mean Creepage
from scribe (mm)
1
Untreated
Steel
Visual Observation
(after 500 hours)
0
L/i
Substrate
Rating of
Failure
D-1654A
<2
8
Table XXXVI. Corrosion results for PANDA-30 ACMEG
Substrate
Untreated
Steel
Visual Observation
(after 500 hours)
Corrosion observed in the scribe
area after the first 100 hours. The
panels showed significant
corrosion after the 500 hour
exposure in the salt fog cabinet.
Blistering was observed with a few
large blisters. No visible color
change was observed.
Mean Creepage
from the scribe
(mm)
Rating of
Failure
D-1654 A
3.4
5
2.9
6
80
Table XXXVII. Corrosion results for PANDA-30 ACPUG
Substrate
Untreated
Steel
Visual Observation
(after 500 hours)
Conbsion was observed in the
scribe area in the first 100 hours of
the test. Significant corrosion was
observed in the scribe area at the
end of the 500 hour exposure.
Some blistering was also observed.
Mean Creepage
from the scribe
(mm)
Rating of
Failure
D-1654 A
2.4
6
2.4
6
Table XXXVIII. Corrosion results for system with PANDA base coat with clear Aroflint
top coat
Substrate
Untreated
Steel
Visual Observation after 500
hours
Very little corrosion rust was
observed in the scribe area in the
first 100 hours. After 500 hours
there was only a small amount of
corrosion in the specimen. The
substrate underneath the coating
had a dull grey appearance
Mean Creepage
from the scribe
(mm)
Rating of
Failure
D-1654 A
0 - 0.25
9+
0-0.25
9+
81
Table XXXIX. Corrosion results for system with PANDA base coat with clear
Acrylic/Melamine top coat
Untreated
Steel
Visual Observation after 500
hours
Some corrosion was observed in
the scribe area after the first 100
hours of exposure. After 500
hours of exposure, there was a
small amount of corrosion was
observed in the scribe. There were
no blister formation or under film
corrosion. Upon removal of the
coating, very little corrosion was
observed. The surface of the steel
had a dull gray appearance.
Mean Creepage
from the scribe
(mm)
0
1
O
Substrate
Rating of
Failure
D-1654A
9
Table XL. Corrosion results for system with PANDA base coat with clear
Acrylic/Polyurethane top coat
Substrate
Untreated
Steel
Visual Observation
(after 500 hours)
Small amounts of rust was
observed in the scribe area in the
first 100 hours of the test. At the
end of the 500 hour exposure,
corrosion was observed in the
scribe area. Some blistering was
observed. Upon removal of the
coating, the substrate had a dull
gray appearance.
Mean Creepage
from the scribe
(mm)
Rating of
Failure
D-1654 A
0.25
9
0.25
(without taking
the areas of
discoloration into
account)
9
82
Table XLI. Corrosion results for Control-Amine
Substrate
Visual Observation
(after 500 hours)
Untreated
Steel
Corrosion in the scribe was
observed after 100 hours of
testing. Significant corrosion
observed in 500 hours
Untreated
Aluminum
Lifting of the film was observed
after the first 100 hours of testing.
Severe corrosion was observed in
the scribe area at the end of the
500 hour period.
Mean Creepage
from scribe (mm)
Rating of
Failure
D-1654A
1.0-2.0
7
Not possible to
make any
measurements.
<2
Table XLII. Corrosion results for Control-Amide
Substrate
Visual Observation
(after 500 hours)
Untreated
Steel
Corrosion in the scribe was
observed after 100 hours of
testing. Significant corrosion
observed after 500 hours.
Untreated
Aluminum
No lifting of the film was observed
after the first 100 hours of testing.
Corrosion was observed in the
scribe area at the end of the 500
hour period.
Mean Creepage
from scribe (mm)
Rating of
Failure
D-1654 A
1.0-2.0
7
2.0-3.0
6
83
Table XLIII. Corrosion results for Control-Aroflint
Substrate
Visual Observation
(after 500 hours)
Mean Creepage
from scribe (mm)
Untreated
Steel
Corrosion was observed in the
scribe after the first 100 hours.
Severe corrosion observed after
500 hours.
Not possible to
make any
measurements.
0
Untreated
Aluminum
Corrosion observed in the scribe
after the first 100 hours.
1.0-2.0
7
Rating of
Failure
D-1654 A
Table XLIV. Corrosion results for Control Acrylic/Melamine system
Substrate
Untreated
Steel
Visual Observation
(after 500 hours)
Severe corrosion was observed in
the scribe area. Severe blistering
was observed with corrosion
occurring away from the scribe
area.
Mean Creepage
from the scribe
(mm)
Rating of
Failure
D-1654 A
10.0- 11.0
2
84
Table XLV. Corrosion results for Control clear Acrylic/Polyurethane system
Substrate
Untreated
Steel
Visual Observation (after 500
hours)
Mean Creepage
from the scribe
(mm)
Rating of
Failure
D-1654A
Corrosion observed in the first
100 hours of the test. Severe
corrosion was observed in the
scribe area at the end of the 500
hours of exposure. Some
blistering was observed.
2.1
6
2.0
6
Table XLVI. Summary of Corrosion results for Coatings incorporating Polyaniline
Substrate
Untreated
Steel
Untreated
Aluminum
Formulation
Rating of Failure
D-1654 A
PA-30 AmideB
PA-30 AmineB
DePANI Amide
DePANI Amine
PANDA-30 AmideB
PANDA-30 AmineB
PANDA-30 AroG
PANDA-30 ACMEG
PANDA-30 ACPUG
PANBC+AROTC
PANBC+METC
PANBC+PUTC
Control Amide
Control Amine
Control Aro
Control ACME
Control ACPU
7
5
5
<2
7
7
5
6
6
9+
9
9
7
7
0
2
6
PA-30 AmideB
PA-30 AmineB
DePANI Amide
DePANI Amine
PANDA-30 AmideB
PANDA-30 AmineB
PANDA-30 AroG
Control Amide
Control Amine
Control Aro
7
6
5
5
8
5
8
6
<2
7
86
Table XLVII. Corrosion results for Moly-White ZNP
dispersed in Epoxy/Polyamide system
Substrate
Untreated
Steel
Visual Observation after 500
hours
Slight corrosion was observed in
the scribe area after 100 hours of
exposure. Slight amount of
corrosion was observed in the
scribe area after 500 hours of
exposure. No blistering was
observed. One panel indicated
disbonding after removal from the
salt spray cabinet.
Mean Creepage
from the scribe
(mm)
Rating of
Failure
D-1654 A
1.0
8
1.0
8
Table XLVIII. Corrosion results for Halox SZP-391
dispersed in Epoxy/Polyamide system
Substrate
Untreated
Steel
Visual Observation after 500
hours
Corrosion was observed in the
scribe area after 100 hours of
exposure. Corrosion was observed
after 500 hours exposure with
some underfilm corrosion present.
Some disbonding occurred after
the test period.
Mean Creepage
from the scribe
(mm)
Rating of
Failure
D-1654 A
2.3-3.0
6
87
Table XLIX. Corrosion results for Busanl 1-M1
dispersed in Epoxy/Polyamide system
Substrate
Untreated
Steel
Visual Observation after 500
hours
Corrosion was present throughout
the panel. Blisters were observed
throughout the panel and the
corrosion in the scribe area was
greater than the specimens
containing the Halox and MolyWhite pigments.
Mean Creepage
from the scribe
(mm)
Rating of
Failure
D-1654A
1.4
7
1.9
7
Table L. Corrosion results for Nalzin 2 dispersed in Epoxy/Polyamide system
Substrate
Visual Observation after 500
hours
Mean Creepage
from the scribe
(mm)
Rating of
Failure
D-1654 A
Untreated
Corrosion was observed in the
scribe area after 500 hours. A
small amount of blistering was
observed in the specimen.
1.1
7
1.1
7
Steel
88
Table LI. Corrosion results for Control Epoxy/Polyamide system
Substrate
Untreated
Steel
Visual Observation after 500
hours
Corrosion was observed in the
scribe area after the first 100 hours
of exposure. Corrosion in the
scribe area was observed after 500
hours in salt spray. Some
blistering and underfilm corrosion
was observed.
Mean Creepage
from the scribe
(mm)
Rating of
Failure
D-16S4A
2.7
6
2.0
6
Table LII. Summary of Corrosion results for Polyaniline Coatings vs Control and
Commercial Corrosion Inhibitive Pigments on Steel
Formulation
Rating of Failure
D-1654A
PA-30 AmideB
7
PA-30 AmineB
5
DePANI Amide
5
DePANI Amine
<2
Untreated
PANDA-30 AmideB
7
Steel
PANDA-30 AmineB
7
PANDA-30 AroG
5
PANDA-30 ACMEG
6
PANDA-30 ACPUG
6
PANBC+AROTC
9
PANBC+METC
9
PANBC+PUTC
9
Control Amide
6
Control Amine
7
Control Aro
0
Control ACME
2
Control ACPU
6
Moly-White in Epoxy
8
Nalzin2 in Epoxy
7
Busanl 1-M1 in Epoxy
7
Halox in Epoxy
6
Substrate
90
Table LIII. Comparison of the DC Polarization data and Salt Fog data
Sample
Description
Passivation
Potential (V)
vs SHE
Log i (mA)
Rating of
Failure
D-1654A
Control Aroflint
0.9
10
0
Pure PANDA
0.75
0.01
PANDA in Aroflint
0.75
0.001
5
PANDA base coat with
Aroflint clear top coat
0.5
0.0001
9
PANDA in
Acrylic/Melamine
1.05
10
6
0.7V
0.0001
9
0.65V
0.001
8
PANDA base coat with
Acrylic/Melamine
top coat
Moly-White in
Epoxy/polyamide
Table LIV. X-Ray Fluorescence results for Fe Loading Test
Sample
Average X-ray Counts
Fe (g/1)
Ancamine 1693 (blank)
5503
0.000
Ancamine 1693
5604
0.004
PANDA (blank)
4096
0.000
PANDA
30089
0.752
Epon 828 (blank)
5560
0.000
Epon 828
5705
0.004
Xylene (blank)
5809
0.000
Xylene
5949
0.001
DNNSA (blank) [64 hours]
4908
0.000
DNNSA
64076
3.277
91
Table LV. Effect of variation of acid concentration in the Aniline Anodization bath
Cone,
(volume %)
Film
Thickness
(MILS)
Salt Spray Data after
120 hours
0.1
White product observed in the scribed area after
exposure. Pitting was observed under the
microscope. The color or the specimen had faded
and had almost become colorless. The unscribed
sample also showed pitting.
10
0.2
White product was also observed in the scribe, but
the extent was lower than the 5% sample. The color
of the sample had changed to a light pink. The
unscribed sample also showed lesser pitting than the
5% sample.
20
0.2
30
0.2
5
This specimen had the least white product and
pitting. The color of the specimen had changed to
pink. The unscribed sample showed some pitting
but it was lower than the 5 & 10% samples.
Non- uniform coatings were obtained
Non-uniform coatings were obtained
40
Table LVI. Effect of variation of aniline concentration in the Aniline Anodization bath
Concentration
(molar)
Film Thickness
(MILS)
0.1
0.25
0.5
0.75
1.0
0.22
0.20
0.20
0.10
0.20
Table LVII. Corrosion results for Aniline Anodized Panels
Sample
# hours in
Salt Spray
Visual Observation
336
No corrosion observed in the first 100 hours of the
test. The color of the panel had changed from blue
to purple. After 336 hours of testing, severe
discoloration was observed on the panels and white
corrosion product was also observed.
Aniline
Anodized
Unscribed
336
Slight corrosion observed in the first 100 hours of
the test. The color of the panel had changed from
blue to purple. After 336 hours of testing, severe
discoloration was observed on the panels and white
corrosion product was also observed.
Sulfuric Acid
Anodize
Unscribed
336
Aniline
Anodized
Unscribed
Sulfuric Acid
Anodize
Scribed
336
No corrosion / white product was observed in first
100 hours of the test. At the end of the 336 hour
period there was a small amount of corrosion with
some pits present on the specimen.
Corrosion / white product was observed in the
scribe area after the first 100 hours of the test. At
the end of the 336 hour period there was a small
amount of corrosion with some pits present on the
specimen.
93
Table LVIII. Corrosion results for Aniline Anodized Panels top coated
with BASF e-coat system
Sample
Aniline
Anodized+
BASF E-coat
Sulfuric Acid
Anodized +
BASF E-coat
# hours in
Salt Spray
1000
1000
Visual Observation
No corrosion was observed in the first 100 hours of
the test. Corrosion with very slight blistering near
the scribe was observed after 500 hours of test. The
blistering did not increase with time. At the end of
the 1000 hour period, the coating was removed and
inspected. There were a few pits near the scribe and
the color of the coating underneath was blue, unlike
the purple color changed observed for the uncoated
specimen.
Slight amount of corrosion was observed in the first
100 hours. No blisters were observed on the film.
At the end of the test period, there were no pits
observed near the scribe. The plain anodized panels
exhibited slightly better protection than the aniline
anodized panels.
60
i
1
1
1
1
1
1000
r
VAROF-PB
"Aroflint contror wilh drilled pit, tested In 5% KjS04
50
with agitation, 35°C
Sea i rate: 1 mV/sec, low to high
Bet ire long term Immersion
100
[44~U
40
10
30
O
CO
20
0.1
*
/
10
0.01
0.001
0
-10
-0.5
3
>
j
i
«
0.0
«
«
»
JL
0.5
0.0001
1.0
Potential (vs. SHE)
Figure 4. DC Polarization Curves for the Control Aroflint System with an artificially drilled pit
VO
2000
AROF-PB
Aroflint clear with drilled pit
Beforetong term Immersion
In 5* KjSO,. 35°C.SBr
1500
Tested QOPE (vs. SHE)
—•— lit tun (OPE - -0.324V)
—2nd nm(OPE • -0.384V)
-a—3idiun (OPE • -0.417V)
1000 -
I 500 -
-500
500
1000
1500
ZRe (n
2000
2500
3000
3500
cm2)
Figure 5. AC Impedance Curves for the Control Aroflint System with an artificially drilled pit
i
J
1
[-—i
1
1
1
1
1
1
1
1
1
r
AROF-PB
Aroflint clear with drilled pit
Before long term immeiston
3.5
In 5% KjS04, 35°C, Stir
Tested @ OPE (vs. SHE)
-•-1st run (OPE =-0.324V)
-O—2nd run (OPE = -0.384V)
—A— 3rd run (OPE = -0.417V)
3.0
2.5
2.0
1.5
I
I
X
-1
0
1
2
3
4
5
6
1
0
1
2
3
4
5
6
70
60
50
40
30
20
10
0
-10
Log co (2jr Hz)
Figure 6. AC Impedance Curves for the Control
Aroflint System with an artificially drilled pit
20
100
. VPPAN-PB
Pure panda wilh drilled pit, lesled in 5% K2SO,
' wilh agitation, 35°C
- Scan rate = 1 mV/sec
~ Before long term immersion
^—
<
E
0.1
CO
c.
QJ
Q
c
2
0.01
0.001
0.0001
0.00001
-0.5
0.0
0.5
1.0
Potential (vs. SHE)
Figure 7. DC Polarization Curves for the Pure PANDA System with an artificially drilled pit
VO
--J
4.0X103
iiiiiiiii
PPAN-PB
Pure panda with drilledpit
Before long termImmersion
In 5*KjSOj. 35°C. Stir
Tested fi OPE (vs. SHE)
—•—1st run (OPE »-0.330V)
—2nd run(OPE - -0.417V)
—A—3rd run (OPE - -0 400V)
3.0x10
2.0X10J T
ITo
G.
M 1.0X103
M
-1 0x103 ^''1''' *'' *'1'11'1 *'! 1'1 *
0.0
1.0X103
2.0X103
3.0X103
»# i
4.0X103
... i~
5.0X103
6.0X103
7.0X103
ZRe (fi cm2)
Figure 8. AC Impedance Curves for the Pure PANDA System with an artificially drilled pit
00
99
1
1
1
1
1
I
1
1
I
PPAN-PB
1
1
1
1
Pure panda with drilled pit
Before long term immersion
-
-
in 5% KjS04, 35°C, Stir
E
o
G
r
Tested® OPE (vs. SHE)
-•—1st run (OPE =-0.330V)
—o—2nd run (OPE = -0.417V)
—A—3rd run .(OPE =-0.400V)
•• • •
i
—
-
N
CD
O
-
.
1
,
I
,
I
,
-I
1
1
1
Log CO (2TT HZ)
Figure 9. AC Impedance Curves for the pure
PANDA System with an artificially drilled pit
1
1
1
'
20
18
16
1
1
1
r
-I
i
1
1
1
1
100
VAPAN-PB
Aro<panda-twitti drilled pit, tested in 5% K2S04
with agitation, 35°C
Scan late = 1 mVfsec
Before long term Immersion
10
14
12
0.1
10
O
(Q
8
0.01
I
6
0.001
4
2
0.0001
0
-2
-I
I
I
|
I
J
0.0
I
_J
I
I
'
0.5
'
'
0.00001
'
1.0
Potential (vs. SHE)
10.
Polarization Curves for the PANDA dispersed in the Aroflint System with an artificially drilled pit
4000
APAN-PB
Aiotpanda-1 virilh dtllledpit
Bofor* long t«mImnwralon
In 5* KjSO,. 35°C,Stir
3000
Taslod Q OPE(vs. SHE)
—•—1stnin (OPE--0.3S1V)
-2nd tun(OPE - -0.410V)
—4—3rd run (OPE- -O.^ 11V)
2000
1=
o
a.
M
N
1000
-1000
1000
2000
3000
ZRe (n
4000
5000
6000
7000
cm2)
Figure 11. AC Impedance Curves for the PANDA dispersed in the Aroflint System with an artificially drilled pit
102
APAN-PB
Aro+panda-1 with drilled pit
Before long term immersion
in 5% KjS04, 35°C, Stir
Tested @ OPE (vs. SHE)
-1st run (OPE =-0.351V)
2nd run (OPE =-0.419V)
—A—3rd run (OPE = -0.411V)
1
2
3
Log <a (27t Hz)
Figure 12. AC Impedance Curves for the PANDA dispersed
in the Aroflint System with an artificially drilled pit
I
I
I
I
1
1
VPAC-PB
PanBC-arocclc-5 with drilled pit, tested in 5% K2S04
with agitation, 35°C
Sean rate = 1 mV/sec
Before long term Immersion
<
E
tn
tz
<u
Q
4-»
c
<D
L—
3
o
0.001
0.0001
0.00001
0.0
0.5
Potential (vs. SHE)
Figure 13. DC Polarization Curves for the PANDA base coat with the clear Aroflint top coat system with an artificially drilled
I
8000
I
-i—i—i—|—i—i—i—i—|—i—i—i—i—|—i—r-
T
-i—i—i—i—|—i—i—i-
. PAC-PB
Panbc-arocdc-5 with diifiodpit
8«for* long term Immftrakm
• In S% KjSO^.35°C, Stir
. Tested © OPE (vs. SHE)
run (OPE"-0.332V)
—O— 2nd ran (OPE - -0.433V)
. — A—3,d(un (OPE--0.410V)
—•—HI
6000
T=
5 4000
a,
E
N
2000
V
-i
2000
i
i j- _L
4000
_i
6000
i
i
*
_L
8000
• • • i
10000
J
I
1
• • I
12000
L.
14000
Z e (o cm2)
R
Figure 14. AC Impedance Curves for the PANDA base coat with the clear Aroflint top coat system with an artificially drilled pit
105
I
>
PAC-PB
Panbc-arocctc-5 with drilled pit
Before long term immersion
in 5% KjS04, 35°C, SUr
Tested @ OPE (vs. SHE)
-•—1st run (OPE =-0.332V)
2nd run (OPE = -0.433V)
—A— 3rd run (OPE = -0.410V)
Log co (271 Hz)
Figure 15. AC Impedance Curves for the PANDA base coat with
clear Aroflint top coat system with an artificially drilled pit
,
18
16
1
1
1
1
1
n
1
1
1
1
r
100
VAMP-PB
b Acmepan-2 with drilled pit, tested in 5% K2S04
with agitation, 35°C
Scan rate = 1 mV/sec
'Before tong term Immersion
10
14
12
0.1
10
8
0.01
3
6
0.001
4
2
0.0001
0
-2
-I
«
•
•
*
x
0.0
_l
I
L.
X
0.5
-1
'
1
1
•
0.00001
1.0
Potential (vs. SHE)
Curves for the PANDA dispersed in the Acrylic/Melamine System with an artificially drilled pit
3500
3000
2500
AMP-PB
Acmapan>2 vrilh drilledpK
B*for* longUrm imm«rsk>n
ln5*KJSO).35°C.Sdi
Tested O OPE (vs.SHE)
—•—1st din (OPE • -0.361V)
2nd lun (OPE--0.431V)
—a— Jtdiun (OPE--O.«3v)
2000
1=5
1500
G.
E
KT 1000
500
0
-500
1000
3000
2000
ZRe (Q
4000
5000
cm2)
Figure 17. AC Impedance Curves for the PANDA dispersed in the Acrylic/Melamine System with an artificially drilled
108
i
1
1
AMP-PB
Acmepan-2 with drilled pit
Before long term immersion
AAAA
in 5% KjS04, 3S°C, Stir
Tested @ OPE (vs. SHE)
—•—1st run (OPE =-0.361V)
2nd run (OPE =-0.431V)
A- 3rd run (OPE =-0.433V)
ID
50
o
40
Qj
cn
10
1
1
2
3
Log co (2B Hz)
Figure 18. AC Impedance Curves for the PANDA dispersed in the
Acrylic/Melamine System with an artificially drilled pit
20
100
- VPANT-FB
Panda acrmel topcoat with drilled pit, tested in 5% K2SO,
2 with agitation, 35°C
- Scan fate = 1 mV/sec
I" Before long term immersion
<
E
e
<J>
Q
—^—"
12
10
0.01
0.001
0.0001
0.00001
-0.5
0.0
0.5
1.0
Potential (vs. SHE)
Figure 19. DC Polarization Curves for the PANDA base coat with the
clear Acrylic/Melamine top coat system with an artificially drilled pit
~
*sO
4000 I—I—I—I—I—|—I—I—I—R—-R
PANT-PB
. Panda scrmel (opcoat with diilledpit
Before long termImnwslon
In 5% KjS04. 35®C.SUr
*
3000 -
Tested @ OPE (vs. SHE)
1attun lOPE*.O.3M\0
-0~2nd run(OPE - -0 413V)
-A—3rd run (OPE- 0.42 IV)
2000
TE
o
a.
E 1000
N
-1000 I-*—«0
1000
2000
3000
ZRe(n
4000
5000
6000
cm2)
Figure 20. AC Impedance Curves for the PANDA base coat with the
clear Acrylic/Melamine top coat system with an artificially drilled pit
7000
Ill
1—I—'—i
PANT-PB
Panda acnnel topcoat with drilled pit
Before long term immersion
in 5% KjS04, 3S°C, SUr
Tested @ OPE (vs. SHE)
—•—1st run (OPE =-0.384V)
2nd run (OPE =-0.413V)
-A—3rd run (OPE =-0.421V)
1
2
3
Log CD (2JC HZ)
Figure 21. AC Impedance Curves for the PANDA base coat with the
clear Acrylic/Melamine top coat system with an artificially drilled pit
1
i
I
I
1
I
100
VMW9-PB
MW-9 with drilled pit, tested in 5% K,S04
_ with agitation, 35°C
Scan rate: 1 mV/sec, low to high
Before long term Immersion
0.001
0.0001
.00001
0.0
0.5
Potential (vs. SHE)
Figure 22. DC Polarization Curves for the Moly-White ZNP dispersed in the Epoxy/Polyamide System with an artificially drilled
5000
MW9-PB
MW-9 wflh drilled pit
Beta® long lerm Immersion
ln5*K2SO).35°C.Stll
4000
T«st*d Q OPE (vs. SHE)
—triiun <OPE«-0.3B3V)
—2nd run (OPE •-0.417V)
-A— Stdiun (OPE"-0.416V)
3000
1
2000
G,
E
NJ
1000
0
-1000
0
1000
2000
3000
4000
5000
6000
7000
8000
ZRe (Q cm2)
Figure 23. AC Impedance Curves for the Moly-White ZNP dispersed in the Epoxy/Polyamide System with an artificially drilled pit
114
1
I
1
I
I
1
'
1
'
1
'
1
•'— I—
MW9-PB
MW-9 with drilled pit
Before long term immersion
In 5% KjS04, 35°C, Stir
—
Jk_
•
Tested @ OPE (vs. SHE)
— •— 1st run (OPE = -0.383V)
—o—2nd run (OPE =-0.417V)
—A—3rd run (OPE =-0.416V)
E
o
g,
-
•
NT
D)
O
1
CD
,
1
,
I
,
I
,
I
.
I
.
I
cn
cu
50
Q)
40
0
30
1
2
3
Log co (2n Hz)
Figure 24. AC Impedance Curves for the Moly-White ZNP dispersed
in the Epoxy/Polyamide System with an artificially drilled pit
.
FN:PR1Q.RD
DATE:09'06<-95
CPS 8.838
4146.0
ID:SAMPLE 1
TIME:14:53
PT:0.30000
4.436
2.976
2.252
1.823
STEP:0.03000
1.541
1.343
SCINTAG'USA
IJL: 1.54060
1.138
1.089*
3731.43316.8
- 80
2302.2-
- 70
2487.6-
- 60
2073.0
1658.41243.8"
- 30
829.2"
414.6• ^0* ' " '
Figure 25. X-Ray Diffraction Scan for the substrate coated with the Control Aroflint System after 500 hours exposure to salt spray
FN-.PR2Q.RD
DATE:09/06/95
CPS 8.?38
7286.0
ID:SAMPLE 2
TIME:15:08
PT:0.30000
4.436
2.976
2.252
i
1.823
STEP:0.03000
1.1541
1 .343
SCINTAG/USA
UL:1.54060
1.198
1.089}{
6557.45828.85100.24371.63643.02914.4"
2185.81457.2728.6-
Figure 26. X-Ray Diffraction Scan for the substrate coated with the PANDA base coat
with the clear Aroflint top coat system after 500 hours exposure to salt spray
' 100
FN:PR3Q.RD
DATE:09/'06<'95
CPS 8.
7863.0
ID:SAMPLE 3
TIME:15:2?
PT:0.30000
4.436
I
2.976
2.252
1.823
1
SCINTAG-'USA
UL:1.54060
STEP:0.03000
1 .541
1 .343
I
1 .198
I
7076.76290.45504.1
4717.83931.53145.22358.91572.6786.3-
Figure 27. X-Ray Diffraction Scan for the substrate coated with PANDA
dispersed in the Aroflint System after 500 hours exposure to salt spray
089 %
100
118
Figure 28. SEM Micrographs of the substrate coated with the
Control Aroflint System after 500 hours exposure to salt spray
119
Figure 29. SEM Micrographs of the substrate coated with the PANDA
dispersed in the Aroflint System after 500 hours exposure to salt sorav
120
Figure 30. SEM Micrographs of the substrate coated with the PANDA base coat
with the clear Aroflint top coat system after 500 hours exposure to salt spray
Aniline Anodization, No Seal/Dipped in PTSA
60-
Aniline Anodization, Hot Water Sealed/Dipped in PTSA
60
Aniline Anodization, No Seal
Aniline Anodization, Hot Water Sealed
Sulfuric Acid Anodized Aluminum
. y.i'?
40-
40
20-
-0.5
0.0
0.5
Potential vs. SCE (V)
Figure 31. DC Polarization Curves for Aniline Anodized panels compared with the Sulfuric Acid Anodized panels
Figure 32. SEM Micrograph for the Aniline Anodized hot water sealed panel
Figure 33. SEM Micrograph for the Aniline Anodized Sulfuric Acid sealed panel
123
Figure 34. SEM Micrograph for the Aniline Anodized hot water sealed panel
followed by a 10 minute dip in a solution of p-toluene sulfonic acid
Figure 35. SEM Micrograph of the Aniline Anodized unsealed panel
124
Figure 36. SEM Micrograph for the Control Sulfuric Acid Anodized panel
FN:PR5.RD
DATE:09^12^95
CPS 1?-66
1000.0
ID:ELECTR/CHEM'SYNT/POL
TIME:10:30
PT:1.00000
8.838
—'
5.901
4.436
1
1
3.559
4
2.976
STEP:0.03000
SCINTAG'USA
UL:1.54060
2.562
2.252
2.013
1
1
1
1.823*
H00
900.0-
- 90
800.0-
- 80
700.0-
- 70
600.0-
- 60
500.0-
- 50
400.0-
- 40
300.0-
- 30
200.0-
20
100.00.0-
10
50
Figure 37. X-Ray Diffraction Scan for the Polyaniline produced during the Aniline Anodization Process
FN:PR4.RD
DATE-.09''12/95
PT:1.00000
STEP:0.03000
SCINTFLG/USFL
UL:1.54060
1024.0-
100
921.6-
- 90
819.2"
- 80
716.8"
- 70
614.4-
- 60
512
- 50
409.6-
- 40
307.2-
- 30
- 20
102.4"
0.0
15
50
Figure 38. X-Ray Diffraction Scan for the Chemically Synthesized Polyaniline (VERSICON)
127
V. CONCLUSIONS
The effect of polyaniline on the corrosion protection of steel in neutral salt spray has
been evaluated. The polyanilines studied were the VERSICON polyaniline, VERSICON
polyaniline base and PANDA (Monsanto's polyaniline). The results indicate that PANDA
exhibits significant improvement in the corrosion protection of steel when applied as a base
coat followed by a clear top coat of a conventional coating system. Three different types of
top coats were evaluated. All three systems, exhibited better corrosion protection than the
conventional systems studied. However, the PANDA or the VERSICON did not exhibit
significant protection when blended into conventional formulations.
The corrosion
protection was either comparable or lower than observed for the control samples or the
formulations containing the commercial corrosion inhibitors. For all the systems, with the
PANDA base coat, some corrosion was observed in the scribe. The extent of corrosion was
significantly lower than that observed for the control samples. The substrate had a grayish
appearance, after the coating was removed, after 500 hours exposure in the salt spray
cabinet. This was not observed for the formulations in which the polyaniline was blended
in, or the conventional systems. X-Ray diffraction studies indicated that this grayish layer
on the substrate was Fe203, indicating that the steel substrate was passivated. This is in
agreement with observations made by other research groups.
DC polarization studies were performed on some of the specimens studied in the salt
spray chamber. On comparing the curves obtained from the DC/polarization and the salt
spray results concluded that a low passivation potential and a low corrosion current is an
indication of a good protective coating. Based on this conclusion, the system with the
128
PANDA base coat and clear top coat performed better than either the control, or the PANDA
blend system. Thus, DC polarization studies can be used as accelerated method to evaluate
corrosion. These studies correlate well with the salt spray data. The PANDA base coat
system showed a significant reduction in corrosion when compared to control Aroflint
system. The polyaniline coating on the steel substrate is a promising candidate for the
corrosion protection of steel. The polyaniline dissolves a some of iron from the substrate.
The dissolved iron then reacts with the ambient oxygen and water to form a protective oxide
layer, which then shuts off the substrate from further dissolution and corrosion. The
polyaniline maintains this passive oxide layer formed and thereby protects the substrate from
further corrosion.
Polyaniline was electrochemically deposited (Aniline Anodization Process) on
aluminum. Parameters influencing the anodization process were studied and a good adherent
coating was developed. Salt spray studies of the aniline anodized panels indicated that they
exhibited good performance in the first 100 hours of exposure to salt spray. On prolonged
exposure, the plain anodized panels performed better. When these systems were top coated,
the performance of the aniline anodized panels improved significantly. However, the aniline
anodized panels with a top coat were marginally inferior in performance when compared
with plain anodized panels with a top coat. The DC polarization curves for the aniline
anodized panels and the anodized panels after immediate immersion in an electrolyte,
indicated that the performance of the aniline anodized panels would be better than the plain
anodized panels. Surface characterization studies indicated a granular structure for the
polyaniline deposited on the surface.
X-Ray diffraction studies indicated that the
electrochemically synthesized polyaniline was more crystalline than VERSICON.
129
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MICROWAVE INITIATED FREE RADICAL CATALYZED
POLYMERIZATIONS: POLYSTYRENE
Srinivas P. Sitaram & James O. Stoffer
Department of Chemistry & Materials Research Center
University of Missouri-Rolla, Rolla, MO 65401
133
Microwave Initiated Free Radical Catalyzed Polymerizations:
Polystyrene
Srinivas P. Sitaram & James O. Stoffer
Department of Chemistry & Materials Research Center
University of Missouri-Rolla, Rolla, MO 65401
SYNOPSIS
This study investigates the use of microwave radiation as a tool for polymerization
processes. The microwave initiated free radical catalyzed bulk and solution polymerization
of styrene using azobisisobutyronitrile (AIBN) as the initiator is reported. The interaction
of microwave radiation with a series of free radical initiators has been studied and discussed.
The effect of the power of microwave radiation and the effect of the initiator concentration
on the percent conversion and molecular weight was studied. The effect of microwave
radiation on polymer stability is discussed. The process is compared with conventional bulk
thermal polymerization. The results of the solution polymerization of styrene are also
presented.
INTRODUCTION AND OBJECTIVE
A number of methods are available for the initiation of free radical polymerizations that
include, thermal decomposition of organic peroxides or hydroperoxides or azo or diazo
compounds and photochemical or radiation induced processes. Our research group has been
studying the ultrasonically initiated free radical catalyzed polymerizations of several
systems.1*'18
Microwaves have been used very successfully in food processing. Other applications
134
include biological, medical and agricultural applications, plastic fabrication and processing,
drying of coatings and other products, rubber products, desulfiirization and waste treatment
and microwave plasmas.2 Microwave applications are unique because its heating effect
depends on the dielectric properties of the material.3 While in thermal heating the heat is
generated externally and heat transfer takes place through a process of conduction,
convection and radiation, the heat due to microwave radiation is generated in the material
itself.
The microwave region of the electromagnetic spectrum is between the infrared
radiation and radio frequencies.
This corresponds to a wavelength of 1cm to lm or
frequencies of 30GHz to 300MHz respectively.
Certain regions of this spectrum are
restricted for RADAR and telecommunications use. In order for domestic and industrial
applications not to interfere with these frequencies, the manufacturers of commercial
microwave equipment are required to operate at 2.45GHz or 900MHz. Domestic microwave
ovens generally operate at 2.45GHz.4
Any organic or inorganic material with a low molecular weight and a high dipole
moment will interact effectively with microwaves at 2.45GHz. Water, methanol and ethanol
are examples of solvents used commonly in studies.4 Non-polar solvents such as benzene,
petroleum ethers and carbon tetrachloride have very low dielectric loss and therefore do not
couple efficiently with microwaves. The addition of small amounts of polar solvents to nonpolar solvents leads to dramatic coupling effects.
Several references are available for the use of microwave, radiation in polymerization
studies in the literature. These include cure of polymers such as polyurethane systems5,
epoxy curing systems, urethane elastomers and polyester resins.6 The studies have shown
135
that microwave heating is usually more energy efficient than conventional heating. The use
of microwave radiation seems to enhance the kinetics of an ideal pseudo first order solution
imidization reaction over conventional treatments.7 Microwave radiation has been used to
study the polymerization of epoxy copper composites.8
Microwave processing of
functionalized PEK has shown 10-20 fold enhancements in the curing rate over
conventional thermal curing.9 The mechanical properties of the microwave cured epoxy
resin was comparable to those of the conventional method of curing.10 The rate of crosslinking in samples cured by microwave radiation is higher than the rate of conventionally
cured samples. At higher temperatures however, the reverse seems to occur.11 Microwave
radiation has shown to be comparable to classical heating in the activation of the crosslinking of epoxy resins with diaminodiphenylmethane.12 Microwave curing has been shown
to have a shorter curing time, a lower curing temperature and a better degree of cure as
compared to conventional heating in the curing of thick polymer film pastes.13
Microwaves provide a feature to the processing of nonmetals: volumetric heating.3
In conventional heating, only the surface of the substrate is heated and the heat from the
substrate is transferred to the material through a process of conduction, convection and
radiation. The mechanism of energy transfer in microwave heating is due to the electric
dipolar coupling of the microwave radiation to the permanent dipole moments in the
material. Therefore microwave heating has also been referred to as dielectric heating.7 This
causes heating of the polar molecules while the non-polar molecules are heated only
indirectly.3 Thus, the requirement of efficient microwave heating is the presence of polar
molecules.
The mechanism of microwave heating is represented in Figure l.14 In the
absence of an electromagnetic field, there is a random arrangement of the molecules. When
136
the field is switched on, the polar molecules align themselves in the direction of the field.
Since the electromagnetic radiation changes direction rapidly, the polar molecules 'flip' back
and forth and heat is generated due to 'molecular friction'.14
Very few references are available in the literature that deal with the use of microwave
power in initiating free radical polymerization of vinyl monomers. One of them deals with
the polymerization of Hydroxymethylmethacrylate (HEMA).15 Studies have shown that
radical polymerization of HEMA can be activated at 2.45GHz. These studies have shown
that the radical polymerization of HEMA can be carried out with microwave radiation in the
absence of an initiator and that monomers with polar groups would undergo such a
polymerization. This present paper is continuation of the work done on the microwave
polymerization of styrene.16'17 The objective of this study is to investigate the use of
microwave power to initiate the free radical polymerization of vinyl compounds - styrene
to be specific. The dipole moment of styrene is extremely small (less than 0.2).18 The
absence of a dipole moment for styrene implies that it would not effectively interact with
microwave radiation and therefore will not experience dielectric heating. Thus, it would be
necessary to find an initiator that would interact with styrene and break apart to form free
radicals. These initiator free radicals would then interact with styrene to initiate classical
free radical polymerization.
This study investigates the use of microwave radiation as a tool for polymerization
processes. The microwave initiated free radical catalyzed bulk & solution polymerization
of styrene using azobisisobutyronitrile (AEBN) as the initiator is reported. The interaction
of microwave radiation with a series of free radical initiators has been studied and discussed.
The effect of the power of microwave radiation on the percent conversion and molecular
137
weight and the effect of the initiator concentration on percent conversion and molecular
weight is reported. The effect of microwave radiation on polymer stability is discussed. The
process is compared with a conventional bulk thermal polymerization. The results of the
solution polymerization of styrene are also presented.
EXPERIMENTAL
The Microwave Oven
A Sears Kenmore domestic microwave oven was used without any modifications in all the
experiments. The output power of the microwave oven was rated at 800W. The oven
operated at a frequency of 2.45GHz. The oven was equipped with the facility to vary the
power. The process of varying power in domestic microwave ovens is described in a later
section of the paper. All studies were carried out in 20mL or 50mL polypropylene beakers
in the absence of any microwave absorbing substances. The initial temperature for all the
reactions was 25°C.
Reagents
Styrene was purchased from Aldrich & Co., and was used after vacuum distillation to
remove the inhibitors present. Freshly distilled styrene was used in all the experiments. The
initiator azobisisobutyronitrile (AIBN) was purchased from Fisher & Co. Benzoyl peroxide,
t-butyl hydroperoxide, t-butyl peroxide, cumene hydroperoxide, dicumene peroxide,
commercial polystyrene and poly(methyl methacrylate) were all purchased from Aldrich.
t-Butyl peroxybenzoate and t-amyl peroxybenzoate were obtained from Elf Atochem. All
initiators were used as received and without further purification.
138
Experimental Setup
The experimental setup is described in Figure 2. The beaker containing the reaction mixture
was placed on an inverted polypropylene container. This was done to prevent heat transfer
from the turntable to the container. The polymerization procedure is depicted in Figure 3.
The sample size for almost all the experiments was 20 grams. The reaction mixture at 25 °C
was subjected to 2 minutes of microwave radiation. The solution could not be exposed to
radiation greater than two minutes because severe exotherm set in and the reaction was
uncontrollable. The reaction vial was removed from the oven and the temperature was
measured. The reaction vial was then quenched in an ice bath to 25 °C. This process of
heating and quenching was repeated as often as desired. The polymer was then precipitated
in a large excess of methanol. The product was then filtered and dried in a vacuum oven at
40°C overnight. The polymer was characterized using FT-IR, NMR and Gel Permeation
Chromatography (GPC).
Experiments
1. Investigation of Free Radical Initiators. Solutions of 5 mole % of the various free radical
initiators were prepared in freshly distilled styrene. These solutions were then individually
subjected to microwave radiation of maximum power (800W)
for 2 minutes.
The
temperature of the reaction mixture was noted and the mixture cooled back to 25 °C. The
process was repeated at least 5 times and the average maximum temperature was recorded.
The formation of the polymer was detected by dissolving a small amount of the reaction
mixture in an excess of methanol. A haze or precipitate in methanol indicated polymer
formation.
139
2. Profile of the Polymerization as a function of time. A solution of 5 mole % was prepared
in styrene. The solution was then subjected to microwave radiation at maximum power
(800W) for 2 minutes. The temperature of the solution was then measured and quenched to
25 °C. At the end of 10 minutes of radiation, the polymer was precipitated in a large excess
of methanol. The product was then filtered and dried to a constant weight. Similarly,
products for 20, 30 and 40 minutes of microwave radiation were obtained.
3. Effect of Power on Percent Conversion. Solutions of 5 mole % AIBN in styrene were
prepared. These solutions were then individually subjected to microwave radiation of
varying power for a period of two minutes each. The temperature of the reaction mixture
was recorded at the end to the two minute period and the mixture was cooled back to 25 °C.
The entire process was repeated 20 times. The polymer was precipitated in a large excess
of methanol. The product was then filtered, dried and weighed to a constant weight.
4. Effect of Initiator Concentration on Percent Conversion. Solutions of varying initiator
concentrations were prepared. Each of these solutions were then subjected to microwave
radiation as described above. The product was then precipitated in a large excess of
methanol, filtered, dried and weighed to a constant weight. In a different experiment the
solutions were subjected to microwave radiation for 15 pulses of 2 minutes each (30
minutes) and then processed as described in steps 2 and 3.
5. Effect of Power and Initiator Concentration on Molecular Weight. The molecular weight
analysis was performed using GPC according to the ASTM method D3536 01.
6. Effect of Solvent on the Microwave PoNTiSfrtration
A SO**? sokiiioit of symw r.~
ethylene giycoi moaooKinyi ether acetate was prepared AlBN 0 mole
was added to
this solution. This reaccon mixture was then subjected to microwave radiation as described
in steps 2 and 3. Percent conversions and molecular weights wvre determined tor this
process.
7. Comparison of Microwave and Thermal Polymerization. Solutions of 5 mole % AlBN
in styrene were prepared. The microwave polymerizations were carried out as described in
steps 2 and 3. Two sets of experiments were performed. One reaction mixture was
subjected to 20 pulses of microwave radiation of 1 minute each. The other set was exposed
to 20 pulses of microwave radiation of 2 minutes each. The average temperature was
recorded in both cases. Thermal polymerizations were carried out these temperatures
respectively for 20 and 40 minutes. The % conversions and molecular weights were
compared. A similar experiment was performed for the solution polymerization
1 lore
again, the average temperature for the sample subjected to microwave radiation was
recorded. A similar sample was subjected to thermal polymerization,
8.
Effect of Microwave Radiation on Polvmer Stability.
Solutions of commercial
polystyrene and poly(methyl methacrylate) (20% by weight) were prepared in toluene. Ten
grams of this solution was subjected to microwave radiation as such. This solution was then
dissolved in a large excess of methanol to precipitate the polymer. AlBN was then dissolved
in another 10 grams of this solution and this mixture was also subjected to microwave
radiation. The polymer was separated by dissolving the solution in a large excess of
141
methanol. A third sample of the polymer solution was dissolved in a large excess of
methanol without exposing it to any radiation. The three samples were then dried and
analyzed using GPC. This process was repeated for the PMMA solutions.
RESULTS AND DISCUSSION
1. Investigation of Free Radical Initiators. Table 1 describes the interaction of the different
free radical initiators with microwave radiation. The free radical initiators studied were tbutyl hydroperoxide, t-butyl peroxide, cumene hydroperoxide, dicumene peroxide, dicumene
peroxide, benzoyl peroxide, t-butyl peroxybenzoate, t-amyl peroxybenzoate and AIBN.
These were compared against a styrene without any initiator. In the absence of any initiator,
the temperature rise obtained for styrene was 5°C i.e. the solution went from 25°C to 30°C.
When a small amount of the reaction mixture was dissolved in methanol, the solution
remained clear with no haze, indicating that there was no polymer formation. Comparing
the first two initiators, t-butyl hydroperoxide and t-butyl peroxide, it was observed that the
temperature rise was small but it was greater for the t-butyl hydroperoxide. A similar trend
was obtained in the case of cumene hydroperoxide and dicumene peroxide. There was no
polymer formation observed in any of the four cases. On comparison of the chemical
structure for the four initiators, it can be implied that the presence of asymmetry or polar
character in the molecular structure of the initiator is necessary for efficient interaction with
microwave radiation. The same trend was observed in the case of benzoyl peroxide, t-butyl
peroxybenzoate and t-amyl peroxybenzoate. The temperature rise observed in the case of
benzoyl peroxide was lower compared to t-butyl peroxybenzoate and t-amyl peroxybenzoate.
Again, on comparison of the three structures, the benzoyl peroxide has a symmetrical
142
structure, while the perbenzoates have asymmetrical structures. The temperatures for the
perbenzoate solutions in styrene rose to 81.5°C and 84.0°C for t-butyl and t-amyl
peroxybenzoates respectively. Slight polymer formation was observed in the case of benzoyl
peroxide. Significant polymer formation was observed in the case of the perbenzoates. This
study indicates that the presence of the carbonyl group in the initiator molecule would make
it useful as a microwave absorbing initiator. The solution containing AIBN increased in
temperature from 25 °C to 70°C indicating that it could be used as an initiator suitable for
use in microwave polymerization studies.
Figure 4 describes the temperature profile of styrene in the absence of any initiator
when exposed to microwave radiation.
As it can be observed from the figure, the
temperature rise observed for styrene as such was very small (25 - -40 °C) even after 5
minutes of continuous microwave radiation. The temperature rise in 2 minutes was only
about 3°C. Figure 5 describes the temperature profile of a 5 mole % solution of AIBN in
styrene. In this case, the solution was exposed to microwave radiation only for a period of
2 minutes. This process was repeated several times. The average temperature was recorded.
The sample was then cooled to 25°C. The solution could not be exposed to radiation greater
than two minutes because severe exotherm set in and the reaction was uncontrollable. On
comparison of the two temperature profiles, one can observe the effect of the addition of the
polar initiator to the solution. When the same concentration of AIBN in toluene was
subjected to microwave radiation, the temperature rise observed (25 - ~70°C) was similar
to that observed for the solution of AIBN in styrene. This indicates that the AIBN is
absorbing the microwave radiation. This absorption causes it to heat up and it then transfers
the heat to the solution.
2. Profile of the Polymerization as a function of time. Figure 6 describes the profile of the
polymerization as a function of time. The concentration of AIBN in the solution was 5 mole
%. As mentioned in the procedure, 5 mole % solutions of AIBN in styrene was subjected
to microwave radiation (800 Watts - maximum power) for 10 minutes, 20 minutes, 30
minutes and 40 minutes respectively. Figure 6 is a plot of the lnfMo/M,], versus time in
minutes. Mo is the concentration of the monomer at time t = 0 and M, is the concentration
of the monomer at time t. M, was determined by the amount of polymer formation at time
t. As indicated by figure 6, the percent conversion increases with increasing time of
microwave radiation.
3. Effect of Power on Percent Conversion. The power of microwave radiation was varied
from 30% (315 Watts) to 100% or maximum power (800 Watts). Figure 7 describes the
variable output of microwave energy in a domestic microwave oven. Variable output in a
microwave oven can be produced by switching the magnetron on and off according to a duty
cycle. For example in a 800 Watts oven, which has a 30 second duty cycle, it can be made
to deliver an average of 400 Watts (50%) by switching the magnetron on and off for every
15 seconds. This implies that the variation in power for an unmodified microwave oven is
merely a variation of the time the solution is exposed to microwave energy.
The
concentration of the solution exposed to radiation was 5 mole % AIBN in styrene. As the
power of the microwave radiation was increased, an increase in the percent conversion was
observed. The percent conversion ranged from 4% for the 30% power to 32% power for
maximum power. This can be explained by the fact that increasing the power of radiation
causes the solution to absorb an increased amount of radiation. This increase corresponded
144
with an increase in the temperature of the reaction mixture. An increase in temperature
results in an increase in the free radicals generated, thereby increasing the percent
conversion.
4. Effect of Initiator Concentration on Percent Conversion. The AIBN concentration was
varied from 1 mole % to 5 mole %. The power of the radiation in all the cases was 800
Watts. All the solutions were exposed for a period of 40 minutes in 2 minute cycles. The
percent conversion varied from 7% for 3 mole % initiator concentration to 32% for 5 mole
% initiator concentration. The amount of polymer produced for 1 mole % initiator was small
and hence could not be determined.
Increasing the initiator concentration produced an increase in the temperature of the
solution after radiation.
This increase in temperature resulted in an increase in the
concentration of the free radicals generated. From kinetics of free radical polymerization19,
the rate of polymerization is directly proportional to the initiator concentration. The rate of
polymerization, R,, could be represented as the percent conversion. Thus agreement with
kinetics of free radical polymerization was observed in this case.
Increasing initiator
concentration resulted in a greater yield of the polymer.
5. Effect of Power and Initiator Concentration on Molecular Weight. Table 2 describes the
results obtained for the effect of the power of microwave radiation on the molecular weights
of the polymer produced.
As the power of the radiation was increased, an increase in the temperature of the
solution after radiation was observed. This resulted in the increase in the concentration of
145
the free radicals generated in the system. From conventional free radical kinetics19 it is
known that Rp is inversely proportional to the degree of polymerization and hence the
molecular weight. Thus with an increase in power a decrease in the molecular weights was
observed. The polydispersities in all the above cases was between 1.8- 2.0.
Table 3 describes the effect of initiator concentration on the molecular weights of the
polymer obtained. Increasing the initiator concentration decreased the molecular weights.
The logarithm of the weight average molecular weights were plotted against the logarithm
of the initiator concentration. This is represented in Figure 8. On fitting the data points to
the equation of the straight line, it was found that for the microwave polymerization of
styrene with AIBN, the molecular weights were inversely proportional to the square of the
initiator concentration.
From conventional free radical polymerization kinetics19, the molecular weights
or degree of polymerization, is inversely proportional to the square root of the initiator
concentration.
This deviation in the proportionality between microwave and thermal
polymerizations can be explained by the fact that conventional free radical polymerizations
are carried out under isothermal conditions, while in this case it was not possible to maintain
isothermal conditions.
Mmw « 1 / [I]2
M„ - 1 / [I]0 5
6. Effect of Solvent on the Microwave Polymerization. The addition of 20% solvent (by
weight) dramatically affected the heating profiles of the solutions. Table 5 describes the
effect of solvent on the polymerization. Initially the experiment was performed using 5 mole
146
% AIBN in the solution. This resulted in an exotherm, and the polymerization proceeded
rapidly and could not be controlled. Hence, the concentration of the initiator was reduced
to 1 mole %. The average temperature rise obtained in this case was 78.0°C (for 1 mole %
initiator solution in styrene and solvent) versus 70 °C (5 mole % initiator solution in styrene)
7.
Comparison of Microwave and Thermal Polymerization.
Table 4 describes the
comparison of the results obtained for both microwave and thermal polymerizations. Table
5 describes the comparison of the results obtained for the effect of the solvent on both the
microwave and thermal polymerization. Figure 9 describes the temperature profile of the
samples undergoing thermal and microwave polymerizations. As it can be observed from
the figure, the sample undergoing microwave radiation attains the temperature more rapidly
than the sample undergoing thermal polymerization. The microwave sample heats up to
70° C in about two minutes, but it takes longer for the sample undergoing thermal
polymerization to achieve the same temperature.
Thus, the results indicate that the
molecular weights and % conversions are comparable for both the microwave and thermal
polymerizations. However, the temperature profiles indicate that the sample subjected to
microwave radiation absorbs energy more efficiently and rapidly.
8. Effect of Microwave Radiation on Polvmer Stability. Table 6 describes the effect of
microwave radiation on polymer stability. This test was performed to see if microwave
radiation had any depolymerizing effect on polymers.
The polymers studied were
polystyrene and poly(methyl methacrylate) (PMMA). The table indicates that microwave
radiation had no measurable effect on the molecular weights of the polymers before and after
147
exposure to microwave radiation. The study therefore indicated that microwave radiation
had no depolymerizing effect on the polymers studied.
CONCLUSIONS
We have synthesized polystyrene by the microwave radiation of styrene in the presence of
a suitable microwave absorbing catalyst.
AIBN, t-butyl peroxybenzoate, t-amyl
peroxybenzoate are suitable microwave absorbing initiators. Comparison of the structures
studied, indicate that polar asymmetry increases the ability of the material to absorb
microwave radiation. Percent conversion is directly proportional to the power of radiation
and the initiator concentration. Molecular weights of the polymer produced are inversely
proportional to the power of the radiation and the initiator concentration. Comparison of the
microwave and thermal polymerization processes indicate that they are indistinguishable.
This seems to be the case for both bulk and solution polymerization processes.
The
temperature profiles indicate a more rapid absorption of energy for the microwave
polymerization process. Microwave radiation seems to have to no depolymerizing action
on the polymeric materials studied. Thus, studies have indicate that the polymerization
process occurs by the absorption of the microwave radiation by the initiator molecules. This
energy absorption causes the initiator molecules to heat up causing an increase in the
temperature of the solution. This heat then causes the initiator molecules to break down into
radicals that initiate the polymerization process.
148
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149
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J. 0. Staffer and S. P. Sitaram, Polymer Materials Science and Engineering Preprints,
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J. O. Staffer and S.P. Sitaram, Polymer Materials Science and Engineering Preprints,
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C. P. Smyth, Dielectric Behavior and Structure, Mcgraw Hill Book Co., New York,
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150
Table I. Evaluation of different free radical initiators
when exposed to microwave radiation
Soln. of
Styrene
+ initiator
Chemical
Structure
No Initiator
t-Butyl
Hydroperoxide
t-Butyl
Peroxide
CH3
1
H3C—<P—0—0—H
Avg. Temp
after
microwave
radiation
Polymer
Formation
30.0
No
38.0
No
33.0
No
45.0
No
36.5
No
CH3
CH3
CH3
H3C—<P—O— O-P- CH3
CH3
CH3
CH3
Cumene
Hydroperoxide
Dicumene
Peroxide
<0^RO-O~H
CH3
CH3
CH3
CH3
CH3
Slight
Benzoyl
Peroxide
o-t—to
42.0
Polymer
Formation
t-Butyl
Perbenzoate
0
CH3
^_C-0-0-P-CH3
81.5
Yes
84.0
Yes
70.0
Yes
CH3
t-Amyl
Perbenzoate
FF
FH3
^_
_
_
-P-C
<
C 0 0
2H5
CH3
CH3
AIBN
CH3
H3C—<P—N=N—
CN
CH3
CN
Table n. EfTect of the Power of Microwave Radiation on Molecular Weights
Power
Level (%)
Power
(Watts)
Avg.Max.
Temp
(°C)
Weight Average
Molecular Weight (Mw)
30
315
50.5
52,800
70
595
60.0
35,200
100
800
70.0
20,600
Table HI. Effect of Initiator Concentration on Molecular Weights
Initiator Cone.
(Mole %)
Weight Average
Molecular Weight (Mw)
3.0
52,000
4.0
43,600
5.0
20,600
152
Table IV. Comparison of the Bulk Microwave and
Thermal Polymerization Process
MICROWAVE
THERMAL
Initiator conc. (mole %)
5.0
5.0
Power (Watts)
800
Total Time
(min)
Average Temperature (°C)
% conversion
Number Average
Molecular Weights (Mn)
a.
20.0
(20 pulses, 1 min ea.)
20.0
b.
40.0
(20 pulses, 2 min ea.)
40.0
a.
50.0
50.0
b.
70.0
70.0
a.
2.7
1.2
b.
34.2
31.7
a.
29900
35000
b.
12000
12100
Conditions:
Microwave Polymerization:
Styrene - 20.0 gms
Cone AIBN - 5.0 mole %
Initial Temp - 25°C
Thermal Polymerization:
Styrene - 20.0 gms
Cone AIBN - 5.0 mole %
Initial Temp - 25 °C
(20 gms of the reaction mixture was used in the above experiment. The reaction
vials were heated for the time mentioned and then quenched to 25 °C before the next
pulse of radiation.)
153
Table V. Comparison of the Solution Microwave and
Thermal Polymerization Process
MICROWAVE
THERMAL
Initiator conc. (mole %)
1.0
1.0
Power (Watts)
800
—
Solvent
Ethylene glycol
monomethyl ether acetate
Ethylene glycol
monomethyl ether acetate
Concentration Styrene (%)
80.0
80.0
Total Time (min)
30.0
(15 pulses, 2 min ea.)
30.0
Average Temp (°C)
78.0
75.0
% conversion
15.8
16.5
Weight average
Molecular weights (MJ
29,000
27,000
Conditions:
Microwave Polymerization:
Sample size - 20.0 gins
Cone AIBN - 1.0 mole%
Initial Temp - 25°C
(20 gms of the reaction mixture was used in the above experiment. The reaction
vials were subjected to microwave radiation and then quenched to 25 °C before the
next pulse of radiation.)
Thermal Polymerization:
Sample size - 20.0 gms
Cone AIBN - 1.0 mole%
Initial Temp - 25°C
(20 gms of the reaction mixture was used in the above experiment. The reaction
vials were heated in a water bath to the desired temperature for the specified
amount of time.)
154
Table VI. Effect of Microwave Radiation on Polymer Stability
Molecular weight (M„)
COMMERCIAL
POLYSTYRENE
COMMERCIAL
PMMA
Samples as delivered
99000
103000
After microwave radiation
101000
108000
After microwave radiation
in the presence of AIBN
110000
109000
155
Random arrangement of polar molecules
in the absence of an Electrostatic field.
Alignment of polar molecules
in the presence of an Electrostatic field.
Polar molecules "flip" back & forth
as the direction of the field changes and
"heat" is generated due to "Molecular Friction".
Figure 1. Mechanism of Microwave Heating
Microwave
Oven
MWR
Styrene
+
Initiator
•••
•••
•••
Polypropylene
Container
Turntable
Polypropylene
Reaction Vial
Figure 2. Experimental Setup
STYRENE
+
A. I. B. N
•
25°C
2 MINUTES
MWR
T
MEASURE
TEMPERATURE
QUENCH/COOL
TO 25°C
REPEAT PROCESS
20 TIMES
COOL & PRECIPITATE
POLYMER IN A LARGE
EXCESS OF METHANOL
FILTER & DRY IN A
VACUUM OVEN AT 40°
OVERNIGHT
Figure 3. Polymerization Procedure
158
100
Q
o
w
&
a.
H
0
1
2
3
4
TIMEflVHN)
5
6
Conditions:
Mass of styrene - lOg
Initial temp - 25°C
Power - 800W
Figure 4. Temperature profile of Styrene in the
microwave oven in the absence of an initiator
(Reaction mixture cooled to 25°C each temperature measurement. The sample was first
subjected to microwave radiation for 1 min and the temperature measured. This was then
quenched to 25°C. The reaction mixture was then subjected to microwave radiation for 2
minutes and the temperature measured and so on.)
159
0
2
4
6
8
10
TIME(MIN)
12
14
16
Conditions:
Mass of Styrene - lOg
Cone AIBN - 5 JO mole %
Initial Temp - 25°C
Power - 800W
Figure 5. Temperature profile of a solution of
Styrene and AIBN in a microwave oven
(The reaction vials were subjected to microwave radiation for 2 minutes and then quenched
to 25°C before the next pulse of radiation.)
160
0.5
0.4
2
03
:
E
0.1 -
o.o
0
10
20
30
40
50
Time (min)
Conditions:
Mass of Styrene - lO.Og
AIBN Cone • 5.0 mole %
Initial Temp - 25°C
Power of Microwave Radiation - 800W
Figure 6. Profile of the polymerization as a function of time
The samples were subjected to microwave radiation pulses of 2 minutes and cooled to 25°C
before the next pulse. Depending on the time of exposure, the samples were subjected to
microwave radiation of a total of 10 minutes (5 pulses), 20 minutes (10 pulses), 30 minutes
(15 pulses) and 40 minutes (20 pulses).
OUTPUT
POWER
ON, OFF TIME ON
VARIABLE POWER SWITCH
ON
800 W
(100 %)
<
30s
>
640 W
(80 %)
ON
25s
OFF
5s
480 W
(60 %)
ON
19s
OFF
lis
320 W
(40 %)
ON
13s
OFF
17s
240 W
(30 %)
ON
10s
OFF
20s
96 W
(12 %)
ON
5s
OFF
25s
Figure 7. Variable Output of Microwave Energy
13
y = 8.51 - 2.01x RA2 = 0.998
12
11
10
-1.8
•1.6
-1.4
-1.2
-1.0
-0.8
Ln I
Conditions:
Mass of styrene - 50.0g
A.I.B.N Cone varied from 5.0 mole% to 2 mole%
Initial temp - 25 degrees C
Power of Microwave Radiation - 800W
Figure 8. Effect of initiator concentration on the molecular weights
of the polymer obtained from the microwave polymerization process
100
MICROWAVE POLYMERIZATION
s
oU
a
B
u
H
THERMAL POLYMERIZATION
40300
5
10
20
15
Time (min)
25
30
35
Conditions:
Microwave Polymerization:
Cone Styrene (weight %) - 80.0
Solvent - Ethylene glycol monomethyl ether acetate
Initial Temp - 25°C
Average Final Temp - 78.0°C
Thermal Polymerization:
Cone Styrene (weight %)• 80.0
Solvent - Ethylene glycol monomethyl ether acetate
Initial Temp - 25°C
Average Final Temp - 75.0°C
Figure 9. Comparison of the temperature profiles of the samples
undergoing microwave and thermal polymerization
164
APPENDIX
PRINCIPLE OF MICROWAVE HEATING
Microwaves provide a unique feature to the processing of non metals: volumetric
heating.[l] In conventional heating only the surface of the substrate is heated and the heat
from the substrate is transferred to the material through a process of conduction, convection
and radiation.
The mechanism of energy transfer in microwave heating is due to the electric
dipolar coupling of the microwave radiation to the permanent dipole moments in the
material. Hence microwave heating has also been referred to as dielectric heating.[2]
Thus, in dielectric heating, the polar molecules interact with the high frequency field. This
causes heating of the polar molecules while the non polar molecules are heated only
indirectly.[3]
A.
THEORY OF MICROWAVES
The microwave region of the electromagnetic spectrum is between the infra-red radiation
and radio frequencies. This corresponds to a wavelength of 1cm to lm or frequencies of
30GHz to 300MHz respectively. Certain regions of this spectrum are restricted for
RADAR and telecommunications use. In order for domestic and industrial applications not
to interfere with these frequencies, the manufacturers of commercial microwave equipment
are required to operate at either 12.2cm (2.45GHz) or 33.3cm (900MHz). Domestic
microwave ovens generally operate at 2.45GHz.
Microwave applications can be broadly classified into two types: Microwave
spectroscopy and Microwave dielectric heating. In microwave spectroscopy, molecules in
the gas phase are studied. These interact with the microwave frequencies and show many
sharp bands in the frequency range 3-60GHz. Thus, microwave spectroscopy provides a
very good technique for fingerprinting molecules in the gas phase and has also been used to
165
confirm the presence of many molecules in outer space.5 In liquids and solids, this is not
possible because the molecules are not free enough to rotate independently. Here, the
spectra are too broad to be observed and the microwave dielectric loss heating effects
become significant.
A material can be heated by exposing it to high frequency electromagnetic waves.
This occurs due to the fact that the electric field exerts a force on the charged particles. If
the charged particles present in a system can move freely through it, a current will be
induced. However, if the charged particles are bound to certain regions, they will not
move until a counter force balances them.This counter force is referred to as the dielectric
polarization. Both dielectric polarization and conduction are sources of microwave heating.
The microwave heating effect depends on the frequency as well as the power applied. This
effect does not create well spaced quantized energy states, but is more of a bulk
phenomenon.
1. Dielectric Polarization. The ability of an electric field to polarize charges in a
material and the inability of the polarization to adjust to rapid changes in the field, is one of
the two main sources of microwave dielectric heating. The total polarization in an electric
field is the sum of the individual components. These components are Electronic, Atomic,
Dipolar and Interfacial polarization.[3] This sum or these components can be represented
by the following equation.
a
t
= a
e
+a
a
+ a. + a
d
i
(1)
v
'
where
at = Total polarization.
ae = Electronic Polarization - this occurs due to the realignment of
the electrons around a specific nuclei,
166
aa = Atomic Polarization - this occurs due to the relative displacements of the
nuclei due to an unequal distribution of charges within a molecule,
ad = Dipolar Polarization - this occurs due to the orientation of the
permanent dipoles by the electric field and
a. = Interfacial polarization - this occurs when there is a build up of charges at
interfaces.
In the presence of an oscillating electric field, such as the field associated with
electromagnetic radiation, the way the material responds, depends on how fast or how
slowly the orientation and disorientation phenomena takes place relative to the frequency of
radiation. Electronic and Atomic polarization and depolarization (a and a respectively)
6
&
take place much faster than the frequency of microwave radiation. Hence, these effects do
not contribute to the dielectric heating phenomenon. However, the time scales for
polarization and depolarization phenomena for permanent dipoles (ad) and for some
interfacial processes (a.) are comparable to microwave frequencies. Hence, these
contribute to the dielectric heating phenomenon [3].
2.
Dipolar Polarization. Liquid water is a very good example of a dipolar
molecule. Dipolar polarization in water occurs due to its dipole moment. At low
frequencies, the response time of the dipoles in the molecule is much faster than the time
the electric field takes to change direction. In this case, the dipolar polarization keeps in
phase with the electric field. The field is therefore able to provide enough energy to make
the molecules rotate into alignment. Some of the energy is transferred to the random
167
motion each time a dipole is knocked out of alignment and then realigned. This transfer of
energy is quite small and therefore the temperature of the molecule does not rise
appreciably. However, if the electric field oscillates rapidly, the response time of the
dipoles in the molecule is slower than the rate at which the electric field changes direction.
Here, the dipoles do not rotate and no energy is absorbed. Therefore, the water does not
heat up.
In the microwave range of frequencies, the rate at which the field changes is about
the same as the response time of the dipoles in the molecule. The dipoles are therefore
rotated due to the forces they experience. But, the resulting polarization lags behind the
changes in the electric field. This lag indicates that water is absorbing energy from the field
to adjust to changes in the field, and is therefore heated up.
Two parameters define the dielectric properties of materials: e' and e". e' - This
is the dielectric constant. It describes the ability of the molecule to be polarized by the
electric field. At low frequencies this value reaches a maximum and is the maximum
amount of energy that can be stored in a material,
e" - This is the dielectric loss. It
measures the efficiency with which the energy of the electromagnetic radiation can be
converted to heat. The dielectric loss goes through a maximum as the dielectric constant
decreases.
The ratio of the dielectric loss and the dielectric constant e'Ve' is defined as tanS
and is the ability of the material to convert electromagnetic energy to heat energy at a given
frequency and temperature.[3] Higher the loss and lower the dielectric constant, higher is
the ability of the material to absorb microwave radiation. The time taken for the dipoles in a
water molecule to become polarized and depolarized depends on the relaxation time
constant. The time constants for electronic and atomic polarizations are much faster than
10"9 s. Hence these mechanisms do not contribute to the dielectric heating effects.
168
The dielectric properties of distilled water reaches a maximum at a frequency of
about 20GHz. However, commercial microwave ovens operate at a much lower frequency
of 2.45GHz. This is due to the fact that at higher frequencies, the microwave radiation is
absorbed only by the surface and not the interior. Hence, very little penetration occurs.
This is the reason why, 2.45GHz is used as at this frequency optimum penetration occurs.
3.
Conduction Losses. Conduction losses occur when one has a conducting
phase dispersed in a non conducting phase. As the concentration of the conducting phase
increases it interacts more intensively with the non conducting phase. This situation can be
modelled by the use of the two layer capacitor model considered by Maxwell and
Wagner.[3]
The total Maxwell - Wagner effect is seen to be a composite which is produced
from areas differing in dielectric constants and conductivity. This effect is more prominent
for highly conducting liquids such as those containing large amounts of salts. In such
cases, the conductive loss effects are much larger than the dipolar relaxation effects.
Thus, in the microwave heating of liquids, the temperature rise is determined by its
dielectric loss, and the specific heat capacity as well as the strength of the applied field.
The following equation represents this dependency:
(8T/8t) = constant xe"xfxE2r m g
P xC
(2)
p
The strength of the applied field E2rms is related to the applied power. Greater the power,
greater is the E2rms and therefore greater is the heating rate for a given material.
B. THE MICROWAVE OVEN
Figure 1 represents a pictorial representation of a microwave oven.[3] The
magnetron is the device that generates the microwave radiation in a microwave oven. A
169
magnetron is a thermionic diode which is made up of an anode and a directly heated
cathode. As the temperature of the cathode increases, electrons are given out and are
attracted towards the anode. The anode is made up an even number of cavities, each of
which acts as a tuned circuit. The gap across the end of each cavity behaves as a
capacitance. The anode is therefore made up of a series of circuits which are tuned to
oscillate at a specific frequency or its overtones. A very strong magnetic field is induced
axially through the anode assembly. This magnetic field bends the path of the electrons as
they travel from the cathode to the anode. As the deflected electrons pass through the
cavity gaps they induce a small charge into the tuned circuit. This results in the oscillation
of the cavity. Alternate cavities are connected by two small wire straps. This ensures the
correct phase relationship. The process of oscillation continues until the oscillation has
achieved a sufficiently high amplitude. It is then taken off the anode via the antenna. Of
the 1200 W of electrical line power that is used by the magnetron, around 600 W is
converted into electromagnetic energy. The remainder is converted into heat. This must be
dissipated through air or water cooling.
Variable power in domestic microwave ovens can be produced by switching the
magnetron on and off according to a duty cycle. For example if one has a 600 W oven,
which has a 30 s duty cycle, it can be made to deliver an average of 300 W of power by
switching the magnetron on and off every 15 s. Large duty cycles are not desirable in
chemical applications where samples may cool dramatically between switching steps. This
is therefore one factor one has to consider while adapting an oven for chemical
applications.
A waveguide is a rectangular channel made of sheet metal. It has reflective walls
which allow the transmission of the microwave radiation to the microwave cavity or the
applicator. The minimum frequency which can be propagated is related to the dimensions
of the rectangular cross section through the expression "c/ f = 2d"5 where c is the speed of
170
light, f the cut-off frequency and d the larger of the dimensions of the rectangular section of
the waveguide.
The walls of the microwave oven are reflective. This is necessary to prevent
leakage of radiation and also to increase the efficiency of the oven. There is seldom a
perfect match between the frequency used and the resonant frequency of the load.
Therefore, if the energy is reflected by the walls, absorbance is increased because of the
energy passes through the sample more often and can be partially absorbed each time it
passes through the sample. This effect can be particularly important if the sample
absorbing the radiation is dimensionally small. If too much energy is reflected back into
the waveguide, it can damage the magnetron. Most commercial ovens are protected by an
automatic cut-off. Some of them are also protected by a circulator which directs reflected
radiation into a dummy load. While working with small loads, poorly absorbing loads or
at high powers it is preferable to have a dummy load such as a beaker of water in the cavity
which can absorb the excess energy.
It is necessary for the all the areas in the oven to receive energy uniformly. This is
ensured by using a mode stirrer. Most microwave ovens are supplied with a turntable
which ensures that the average field experienced is the same in almost all directions. In the
processing of liquids or solids which are poor absorbers of microwave energy, the multimode ovens are no longer effective. A more expensive experimental set up has to be
used.[3] This utilizes a single mode resonant cavity that is tuned to the characteristics of
the material.
171
REFERENCES
1.
Peter L. Jones, Energy World, January 1991, 10-13
2.
D. A. Lewis, J. D. Summers, T. C. Ward and J. E. McGrath, Journal of Polymer
Science, Part A: Polymer Chemistry, 1992, Vol 30, 1647-
3.
1653.
D. Michael P. Mingos and David R. Baghurst, Chemical Society Reviews, 1991,
Vol 20, No. 1, 1 - 47.
172
ISi !!: ! l!
1
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
3
Micowave Cavity
Turntable
Door Lock Z& Safety Lock Mechanisms
H.T.Transfoimer
H.T. Capacitor
Magnetron
Air Blower
Waveguide
Magnetron Antenna
Cooling Air Output
Cavity Lamp
Wire Mesh to prevent leakage
Figure 1. The Microwave Oven
173
VITA
Srinivas Pravin Sitaram was born to Mr. and Mrs. S. U. Sitaram on February 17,
1966. He obtained his primary and secondary education in Bombay, India. He received his
Bachelor of Science degree in Chemistry, and his Bachelor of Science (Technology) in Paint
Technology from the University of Bombay, Bombay, India in 1986 and 1989 respectively.
He was employed as a Technical Officer at Nerolac Paints Ltd. He has been enrolled as a
Doctor of Philosophy candidate in the Chemistry Department at the University of MissouriRolla since August 1990.
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