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The effect of the solvent in the postchlorination of PVC. Chlorination studies in dimethylformamide

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Die Angewandte Makromobkulure Chemie 19 (1971) 103-111 ( N r . 251)
From the University of Helsinki,
Department of Wood and Polymer Chemistry,
Malminkatu 20, Helsinki 10, Finland
The Effect of the Solvent in the Postchlorination of PVC
Chlorination Studies in Dimethylformamide
By VAINOA. ERA*
(Eingegangen am 28. Oktober 1970)
SUMMARY:
Chlorination of PVC has been studied in DMF solution a t varying temperatures.
It was found that PVC may be chlorinated to 58,2 yo chlorine content in temperature range of 25-50 "C.Dehydrohalogenation of the polymers was noted at higher
temperatures. In IR-spectra carbonyl compounds were found as the oxidation
products of PVC. The evaluation of the thermal stability of the polymers was
carried out using DSC-calorimetry.The results show that the chlorinated product
possesses poorer thermal stability than that of the original PVC. The chlorination
mechanism is discussed and the aprotic nature of DMF was proposed to be the
influencing factor in the reaction.
ZUSAMMENFAGSUNG :
Die Chlorierung von PVC in DMF bei verschiedenen Temperaturen wurde untersucht. Es wurde festgestellt, daD PVC bis zu 58,2 YoChlorgehalt bei 25-50 "C chloriert werden kann. Die Dehydrochlorierungvon PVC wurde bei hoheren Temperaturen beobachtet. In IR-Spektren wurden Carbonylgruppen als Oxydations-Produkte von PVC beobachtet. Die Thermogramme zeigen, daB das chlorierte Produkt
eine schlechtere thermische Stabilitat hat als das urspriingliche PVC. Der Mechanismus der Chlorierung wird diskutiert und die aprotische Natur von DMF wird
als wirksamer Faktor in der Reaktion vorgeschlagen.
Introduction
The pos@,hlorination of PVC is carried out mainly in chlorinated hydrocarbons as in tetrachlorethanel. I n patent literature an example is mentioned
about the chlorination method, where dimethylformamide has been used as a
chlorinating solvent2. Dimethylformamide, H CO N(CH3)2, DMF, is a strong
d
are
solvent into which polymers such as polyurethanes a ~ polyvinylchloride
soluble. DMF is being used further as a solvent alone or in mixtures in the
- -
*
Kuusitie I 0 A 14, Helsinki, Finland.
V. A. ERA
chlorination of organic compounds3, because of its inertness towards chlorine
especially a t lower temperatures. I n the following experimental part the
effect of DMF in the postchlorination of PVC is studied.
Experimental
Chlorination studies
The chlorination of DMF and 1 yo solution of PVC in DMF was carried out at
temperatures of 25 ",50 " and 75 "C. Samples were taken from the solution after
definite intervals from which free chlorine and hydrogen chloride were determined
by means of titration with N/10 thiosulphate and 1N sodium hydroxide solutions
in a Multi Dosimat titrimeter. Chlorinated polymers were precipitated with water,
washed acid-free, separated by centrifuging and dried.
Characterization of polymer s a m p h
The original PVC-polymer was of commercial suspension grade, Ravinil R 100
(ANIC). The molecular weight characteristics were: K-value = 65 corresponding
to molecular weights M, = 140 000 and M, = 55 0004.
Chlorine contents of the polymers were determined according to PARR'S
methods.
Infrared analyses were carried out with a Perkin-Elmer 457 spectrophotometer.
In sample preparations the KBr-pellet technique waa used.
Viscosimetric measurements were carried out in cyclohexanone solution at
30 "CS.
Thermal analyses were carried out using Perkin-Elmer Differential Scanning
Calorimeter. Determinations were made in nitrogen, sample weights 6.5 mg, scan
speed 4 "C/min.
Results
Chlorination
The reaction between dimethylformamide and chlorine can be followed by
means of changes in chlorine and hydrogen chloride content of the solution
(Fig. 1 and 2). It can be seen that the solvent is rather inert towards chlorine
a t 25 "C. At higher temperatures, however, chlorine reacts vigorously with
DMF, which can be observed on the basis of increasing HC1 concentration in
the solution. At 75 "C no free chlorine can be detected. The same tendency
can be seen in PVC-solutions (Fig. 3 and 4).
Chlorine content of the polymers
It can be assumed, that in the reaction between chlorine and PVC 1,2dichlorethylene structures are formed7. In the dehydrohalogenation of PVC
104
Postchlorination of PVC
lot
A
25OC
Q
5OoC
8--.
ci
m0bS
6-
4-
2Q
1
I
I
I
1
2
3
Reaction Time ( h d
Fig. 1. Chlorination of DMF. Chlorine content of the solution versus time.
A 25
'c
0 50 OC
El
75oc
Reaction Time (hrd
Fig. 2. Chlorination of D M F . Hydrogen chloride content of the solution versus
time.
polyene groups (CH=CH) are obtained. On the basis of the chlorine content
the amount of various structural units per hundred monomer units can
be calculated7. The chlorine content of the original PVC was 55.9 % on which
the calculations were based. Results are shown in Table 1.
105
V. A. ERii
2
1
3
Reaction Time.(hrs)
Fig. 3.
Chlorination of PVC in DMF. Chlorine content of the solution versus time.
A 25T
0 SO'C
E 75 ' c
l o t
1
1
2
3
Reaction Time (hrd
Fig. 4.
Chlorination of PVC in DMF. Hydrogen chloride content of the solution
versus time.
Viscosity measurements
In Fig. 5 the intrinsic viscosities of the chlorinated polymers are plotted
versus chlorine content and a linear correlation is obtained. When the dehydrohalogenation occurs (Cl-content < 55.9) spreading of results can be
observed.
106
Postchlorination of P V C
Table 1. Chlorine contents and the amount of structural units of the polymers,
calculated according to'.
Temperature
25
25
25
25
50
50
50
75
75
75
1T;i
1
amount of structural units (yo)
Chlorine
content (%) CHz-CHCl
CHCI-CHC1
CH=CH
0.5
1
2
3
1
2
3
1
2
3
I
56.4
58.2
58.0
57.5
58.2
55.6
54.0
55.5
54.5
54.6
I
98
92
93
95
92
98
92
98
94
94
0,920
[?I
0900
0.880
4860
0,840
0,820
0.W5
Fig. 5. Intrinsic viscosities of the chlorinated PVC versus chlorine content.
Infrared abmrptim sgwctra
In the original PVC sample the following characteristic absorption bands
can be detected: 2967, 2920, 2949, 2820, 1427, 1330, 1250, 1197, 1096, 963,
833, 693, 636, 615 cm-1 8. The chlorinated polymers show the following additional bands a t the wavelengths 1730 cm-1, 1770 cm-1, and 1035 cm-1. Simi107
V. A. ERA
lar absorption bands can be found in the oxidized polyethylene productsg,
which indicate the presence of carbonyl groups. If the ratio of the absorbance
of carbonyl and methylene groups CO/CHz is presented as a function of the
chlorine content it can be shown that the ratio of carbonyl group decreases
when the degree of chlorination increases (Fig. 6). I n degraded PVC (Cl < 56 yo)
this ratio is directly related to the degree of dehydrohalogenation.
Fig. 6.
The ratio of the absorbances of carbonyl and methylene groups versus
chlorine content.
Thermal stability
Thermal changes in PVC can be indicated conveniently by means of DSCcalorimetrylo. It can be seen in thermograms (Fig. 7) that the endothermic
changes of the chlorinated are steeper a t 80-90 "C, than those of the original
PVC. The shape of the curves 2 and 3 is identical up to 220 "C, after which
differences in the endothermic behaviour can be noted. The exothermic de:
gradation of the chlorinated PVC (58.0yo Cl) takes place at a lower temperature (245 "C), than that of the original PVC (275 "C).
Discussion
It is learned from the experimental part that PVC may be chlorinated to
58.2 yo chlorine content in 1 yo DMF solution under relatively mild reaction
108
Postchlorination of P V C
L
Y
E
t
f
0,
YI
"
B
t
f
0,
YI
1
,
I
I
100
I
I
I
I
I
I
150
200
250
300
I
Temperature OC
1
Fig. 7 . DSC-thermogramsof PVC and chlorinated products.
1. Original PVC,
2. Chlorinated PVC, 58.0 yo C1,
3. Chlorinated PVC, 57.5 Yo C1.
conditions. The same degree of chlorination has been achieved in 1 yo tetrachlorethane solution a t 50 "C using a radical forming compound as an initiat0r7. The amount of HC1 formed in PVC-solution a t 50 "C is smaller than that
formed in the solvent respectively (Fig. 2 and 4). This finding points to the
retarding effect of PVC in the chlorination reaction which was also observed
in tetrachlorethane solution7. I n technical processes the chlorine content of
64-66 yo in PVC can be obtained when the reaction is promoted by means of
UV-lightll, free radicals12, or thermallylz.
On the basis of the former results it can be stated, that the solvent has a
specific effect in the postchlorination of PVC. Presumably in a good solvent as
in DMF where chains of the macromolecules appear in the shape of loose conformations the reaction with chlorine occurs more easily than in poor solvents
such as in tetrachlorethane where the more compact conformation is present.
It is probable that PVC can be chlorinated in DMF also to higher chlorine
content using such promoting agents as are normally employed in the manufacture of postchlorinated PVC.
It has been proposed that when the chlorine reacts with CHz-groups the
process can be assumed t o take place according to the substitution mechanism
as follows:
109
V. A. ERA
[-c-c-c-cH H 771
l C1
l H
l lC1 lH ]
-
I -n
HCI
[
-c-c-c-cc1 c1 c1 c1
j
n
This view is supported by FUCHS
and LOUIS^^. The dehydrohalogenation of
shown by FIERZ-DAVID
and ZOLLINQER~~
to occur in dioxane.
LUTHERAU
and PETIThave observed the same effect in DMF solution and
indicated that this reaction is autocatalytic in naturels. The autocatalytic
effect of DMF in the dehydrohalogenation of PVC may be explained on the
basis of the aprotic nature of the solventla. This type of solvent is able t o split
hydrogen chloride from the chlorinated hydrocarbons as follows :
Pvc has been
BI
+ R----CH&HzX
R--CH=CHz
+ BH+ + X- .
Halogen-ions act in solvents as strong nucleophilic agents solvating cations.
The basic halogen-ion activates the elimination-reaction according to E2mechanism and hydrogen containing compounds such as H Halz- (DMF)zH+
and (DMF) H Hal are formed. The solvent will prevent the proton to add
back to the reaction product olefin. The former mechanism is also supported
by our results. A similar mechanism has been proposed by HALMOS
and
MONAESIin their studies on the dehydrochlorination of PVC in DMF17.
The increase of the rate of carbonyl groups in the dehydrohalogenated
PVC products is shown in IR-spectra (Fig. 6) which indicates that the degraded
polymer is sensitive to oxidation. It can be assumed, that the oxidized products are formed during the precipitation and washing stages while the polymers are in contact with the air.
The poorer thermal stability observed in the chlorinated products (Fig. 7)
can be explained to be due to the structural changes caused by the oxidation20
as well as by the chain rupture of the polymer.
It is evident that the intrinsic viscosities of the chlorinated PVC decreases
linearly as a function of the degree of chlorination (Fig. 5 ) . The same trend has
also been observed by R ~ N B Y
The
~ ~anomalies
.
of the intrinsic viscosities of
110
Postchlorination of PVG
the degraded PVC (Fig. 5) may be assumed to be derived from several reactions which occur besides the dehydrohalogenation namely the formation of
copolymers between polyene groups, DIELS-ADLER
reaction and splitting of
HC1 between the adjacent polymer chains22.
Acknowledgement
LINDBERG
for most valuable
The author wishes t o thank Professor J. JOHAN
advice and discussions. Thanks are also due t o OY KESKUSLABORATORIO
The FINNISH PULPAND PAPER
RESEARCH
INSTITUTE
for the use of laboratory
facilities. To Mr. K. SODERSTROM
the author is indebted for technical assistance. Financial support from the FOUNDATION
OF NESTE OY in Finland is
gratefully acknowledged.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
C. R. RUEBENSAAL,
Mod. Plastics 25 (1948) 143; Bericht PB 77673, US-Handelsministerium; Kunststoffe 39 (1949) 51.
DDR P. 48540, Oct. 5, 1966.
HOUBEN-WEYL,
,,Methoden der Organischen Chemie", G. Thieme, Stuttgart
1963, Vol. 5/3, p. 709;
G. A. R. MATTHEWSand R. B. PEARSON,
Plastics 28 (1963) 307.
ASTM D 1303-55.
ASTM D 1243-66.
H. KALTWASSER
and W. KLOSE,Plash u. Kautschuk 13 (1966) 583.
S. KRIMM
and C. Y. LIANQ,
J. Polymer. Sci. 22 (1956) 95.
HUMMEL/SCHOLL,
Atlas der Kunststoff-Analyse, C. Hanser , Miinchen 1968,
.
Vol. I, Part 1, p. 126.
V. ERA and A. NYBERU,
Kemian Teollisuus 26 (1969) 9, 705.
Fr. P. 1. 444330 (July 1, 1966), B. F. Goodrich Co.
D.R.P. 596911, Fr.P. 755048, Ital. P. 324432, E. P. 401200, I. G. Farbenindustrie.
W. FUCHS
and D. LOUIS,Makromolekulare Chem. 22 (1957) 1.
H. E. FIERZ-DAVID
and H. ZOLLINUER,Helv. chim. Acta 28 (1945) 455.
H. LUTHERAU
and J. PETIT,C. R. hebd. Seances Acad. Sci. 268, Ser. C (1969)
584.
16
17
A. J. PARKER,Quart. Rev. (Chem. Soc., London) 16 (1962) 163.
M. HALMOS
and T. MONAESI, Acta Univ. Szegediensis, Acta physica. chem. 6
(1960) 99.
18
19
20
21
22
J. LE BRASand A. DELALANDE,
Les Derives Chimiques da Caotuchonc Natural, 1950, p. 267.
C. HAVENS,N.B.S. Circ. 525 (1953) 121.
F. CHEVASSUSand R. DE BROUTELLES,
The Stabilization of Polyvinyl Chloride,
Edward Arnold, London, 1963.
J. PETERSEN
and B. R ~ N B Y
Makromolekulare
,
Chem. 102 (1967) 83.
D. DRUESEDOW
and C. F. GIBBS, Mod. Plastics 30 (1953) 123.
111
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