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Indentification and characterisation of high impact polystyrenes. II. Impact index

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Die Angewandte Makromolekulure Chemie 19 (1971) 113-120 ( N r . 114)
From the Laboratoire d’Applications, Aquitaine Organic0
USINE DE MONT, 64 MONT, France
Identification and Characterisation of
High Impact Polystyrenes
II. Impact Index
By K. V. C. RAO*
(Eingegangen am 5 . Januar 1971)
SUMMARY:
The optical density of high impact polystyrene (HIPS) solution in carbon tetrachloride (0.5 %) a t 320 mp is found to be indicative of the improvement in impact
resistance of the HIPS sample. The percentage grafting has been determined in
the HIPS samples by solvent extraction method, and the dependency of impact
resistance on the percentage grafting is compared with that of optical density and
impact resistance. From the results it appears that the optical densities are as indicative as the percentage grafting, of the impact strength. For the ease of calculations the values of optical densities are expressed as whole numbers and are referred as impact indices.
ZUSAMMENFASSUNG :
Die Extinktion von 0,5-prOZ. Losungen von hoch schlagzahem Polystyrol (HIPS)
in Tetrachlorkohlenstoff bei 320mp sind ein MaB fiir die Schlagzahigkeit der
HIPS-Proben. Der Pfropfungsgrad der HIPS-Proben wurde durch Extraktionsversuche bestimmt. Die Abhiingigkeit der Schlagziihigkeit vom Pfropfungsgrad wurde
mit der Abhlingigkeit der Schlagziihigkeit von den Extinktionen der genannten
Losungen verglichen. Dabei wurde gefunden, daB die Extinktion ebenso wie der
Pfropfungsgrad ein Ma13 fur die Schlagziihigkeit ist. Die Werte fiir die optischen
Dichten wurden deshalb in ganzen Zahlen ausgedruckt und als Schlag-Indizes bezeichnet .
Introahction
T h a t Polystyrene when modified with rubber by grafting technique or by
blending exhibits superior mechanical properties, is now well realized in principles as well a s in practice. However, the grafted high impact polystyrene
(HIPS) has assumed more importance over t h e blended one and is fastly re-
*
Present address : Indian Institute of Petroleum, Dehra Dun, INDIA.
113
K. V. C. RAO
placing the latterl. C. B. BUCKNALL
has reviewed the contribution of grafted
elastomer of HIPS to impact resistancez. Various methods like solvent extraction3, turbidometric titration4 and degradation analysis5 have widely been
used for the determination of grafting but all of them involve laborious procedures. Since unsaturation of polybutadiene is not much effected by grafting,
it also will not be indicative of the extent of graftings.
An easy method based on ultra-violet spectroscopy has been evolved and
can be successfully used to estimate the extent of impact resistance when
polybutadiene is used as an elastomer in HIPS. This method is reported in this
communication.
Experimental
About 0.25 g of HIPS is accurately weighed, dissolved in carbon-tetrachloride
and made up to the volume of 50 ml in a standard flask. The grafted part of the
polymer remains suspended as gel. Different concentrations are prepared by dilution with carbon-tetrachlorideand the percentage absorbance is measured a t 320 mp
on JOBIN
and YVON,monophase spectrophotometerfor eacfi of them. Care is taken
to keep the gel suspended while measurements are taken. Similarly absorbance is
measured for physical mixtures of polystyrene (PS) and polybutadiene (PB) in
carbon tetrachloride keeping total polymer concentration 0.5 yo and varying the
PB content in PS-PB mixture from 2.5 to 10 Yo. It has been observed that the
optical densities do not vary much for various HIPS solutions. Hence, a correction is made to account for the free polystyrene and free polybutadiene (if any)
in the HIPS sample as follows :
Optical density = log
= log
100
100 - (X - 5.5)
100
(105.5 - X)
where X is the percentage absorbance of HIPS solution and the correction factor
is 5.5 which is the average absorbance of the physical mixture containing PS and
PB.
The LAMBERT-BEER'S law is verified a t the wave length 320 mp and is given
Fig. 5. The absorption spectra of PB, PS, HIPS and a physical mixture in carbon
tetrachloride solution have been given in Figs. 1, 2, 3 and 4 respectively.
A known amount of the HIPS sample is dissolved in ethyl acetate and the solution is centrifuged to separate the clear solution of ethyl acetate containing free PS,
from the insoluble gel and elastomer. The gel is washed with fresh aliquotes of
ethylacetate to extract the traces of free PS left if any. Free PS present in ethyl
acetate solution and in ethylacetate washings is determined by precipating PS
using methanol. PS is separated and is dried in a vacuum oven a t 50 "C to constant weight. The gel is also dried in the vacuum oven to constant weight, and
the percentage of gel present in HIPS is calculated and is expressed as gel content.
114
High Impact Polyatyrene
Y
12-
f
m
10-
w
0 -
E
rt
Fig. 1. Absorption spectrum of
polybutadiene (0.5 yo in carbon
tetrachloride).
64-
2-
0
250
270
290
310
330
350
310
330
3 0
), (mp)
r
40 -
Y
30-
z
m
5m
Fig. 2. Absorption spectrum of
polystyrene (0.5 Yo in carbon tetrachloride).
20-
10-
0
250
270
290
Results and Discussion
Table 1 lists the percentage absorbances of PB, (0.05 %); PS (0.5 %), HIPS
sample H (0.5 yo)and the physical mixture PB and PS. It is evident that
HIPS solution exhibits maximum absorption at the wave length 320 mp. The
excess absorbance is attributed to the quality and quantity of the gel present.
Inhibitors, plasticizers etc. do not show any absorbance at the wave length
320 my which is arbitrarily chosen within the range 310-350 mp where the
absorbance is very sensitive to the presence of HIPS in solution.
The optical densities, along with the values of percentage PB, gel content
and impact strength (expressed in kpcmlcmz) are given in Table 2.
115
K . V. C. RAO
Fig. 3. Absorption spectrum of
high impact polystyrene (0.5 yo in
carbon tetrachloride).
50
40
30
-
r
~
Lu
0
4
0"
:m:
20-
4
10-
0,
250
I
270
290
310
330
I
Fig. 4. Absorption spectrum of
physical mixture of PB and PS
(0.5 yo in carbon tetrachloride).
? 10
It is well know that the impact strength of PS can be improved by
incorporating an elastomer either by blending or by grafting PS to elastomer
molecules. I n the later case the quantity of elastomer required to give the improved impact strength is much less, compared to the elastomer quantityin a
blended sample t o get the same impact strength. During prepolymerization
stage the grafted elastomer is dispersed as micro gel in polystyrene phase7
when the polymerization is completed the grafted PS chains act like binding
chains between the elastomer particles and continuous PS phase, thus preventing the propagation of fracture during impact. I n the absence of such binding
chains of PS, the elastomer and PS phases exist separately as the two polymers are incompatible. Thus in addition t o molecular weight, molecular
weight distribution of PS phase, and the quantity and quality of the elastomer,
116
High Impact Polystyrene
*
g 0.3
k
P
w
2
4
2
0 2
I-
0
0.1
Fig. 5 . Verification of LAMBERT
BEER’S
law at 320 mp.
007.
0
0.04
0.06
0.08
X CONC. ( W T . f VOL.)
(
10
the extent of grafting has major contribution towards impact resistance. On
the other hand the higher the grafted elastomer in the HIPS, the higher will
be the gel content. I n addition to this, the size of the grafted micro gel
particles dispersed in PS phase has greater effect on the impact strength.
DELANDe t al. and B. W. BENDER^^ 7 have concluded that the optimum size
of the gel particle sho,uld be between 1 p to 10 p. While measuring the optical
densities of HIPS, as described, the size of the dispersed gel particle and quantity of the gel content in the sample are accounted for.
Knowing the P B content of the sample and the gel content, the excess PS
attached to P B is calculated. Defining the percentage grafting as the amount
of PS found on elastomer after grafting, out of 100 g of PS formed from sty-
Table
1.
No.
1
Optical densities of single components and mixtures compared with an
impact polystyrene (H).
1
Sample
Polybutadiene
(0.05
2
3
1
Optical
density
3.7
0.0165
3.3
0.0145
6.0
0.0265
29.0
0.1485
%)
Polystyrene
(0.5
1
Percentage
absorbance
a t 320mp
%)
Mixture of PB +PS
Yo)
(0.5 Yo)
(0.5
4
H
117
K. V. C. RAO
Table 2. Content of polybutadiene, content of microgel, optical density* and impact strength of different HIPS samples.
No.
11
sE; -
Ref.
*
1
A
2
3
4
5
6
7
B
c
D
E
F
G
8
9
10
11
12
13
14
15
H
1
J
K
L
M
N
Percen€t'T;utadiene
(PB)
6.5
5.75
6.5
5.9
7.5
3.4
4.75
6.8
4.5
2.6
2.6
6.5
6.0
6.8
PS
crvstal
Gel
content
(% by
Wt.)
8.05
21.81
18.69
20.82
26.40
10.00
14.18
20.26
12.95
6.85
3.00
19.50
11.26
24.35
-
Percentage
Graft-
Optical
Density
Im-
PWt
Index
ing
1.55
16.06
12.19
14.92
18.90
6.60
9.43
13.46
10.45
4.25
0.40
13.00
5.26
17.55
-
Impact
strength
CEMP Method
(kpcmlcmz)
The optical density is calculated from the formula 0. D.= log
**
11.5
18.0
14.0
17.5
19.0
15.0
16.0
18.0
13.0
11.0
9.0
15.0
1 .o
17.0
6.0
0.037
37.0
0.137
137.0
0.056
56.0
0.093
93.0
0.127
127.0
0.070
70.0
0.090
90.0
0.1485
148.5
0.036
36.0
0.025
25.0
0.017
17.0
0.065
65.0
0.004
4.0
0.093
93.0
0.0145** 14.5
100
(105.5 -A)
.
In caae of PS crystal the formula used is 0. D.= log )*::ol(
*
rene, the percentage grafting in all the samples studied is calculated and the
results are given in Table 2.
I n commercial samples the percentage grafting alone can indicate the improvement in impact strength but in the products obtained during the development of the process the percentage grafting or the gel content fail toindicate
the impact strength. Thus it seems that the quantity of the gel alone is not
enough to get an idea of the improvement of impact resistance, but a knowledge of the quality of the gel is also essential. The optical densities of dilute
solutions of various HIPS samples in carbon tetrachloride have been expressed
as whole numbers for the easy correlation as
v = lo00 x optical density
118
High Impact Polystyrene
where v is a number depending upon the quality and quantity of the gel
present. As the magnitude of v gives an idea of the improvement in impact
resistance, it is referred as i m p a t index. From Table 2 it results that the
higher the impact index, the better the impact resistance. Fig. 6 a) is the
plot of percentage grafting versus impact resistance of the sample and 6 b ) is
the plot of impact index versus impact resistance. The scattering of points in
6b) is much less compared to that in 6a) indicating that the impact index is
also as reliable parameter as percentage grafting if not better.
From the samples F, G and H and C, I and J (Table 2) it is evident
that the impact index varies correspondingly with percentage grafting and
percentage P B present. The products F and G are M e r e n t grade HIPS samples obtained from the product H. Similarly I and J are different grades of
the HIPS product C. The variation of impact index is in agreement with
variation of gel content and P B content.
151
101
51
0
5
10
I5
IMPACT STRENGHT (kpcm/cm')
Fig. 6.
Plots of:
a) percentage grafting vs. impact strength,
b) Impact index
YS. impact strength.
119
K. V. C. RAO
While a close resemblance has been shown to exist between the impact
index and impact resistance, its correlation with other mechanical properties needs a correction in the observed properties for the presence of excess
homopolymer, PS. No correlation studies between impact index and other
mechanical properties is attempted a t this stage, as it is felt the data are,
however, scanty.
A mathematical approach to the relation existing between impact index
and impact resistance is being worked out which will be reported in near future.
The author wishes to acknowledge with thanks the help rendered by Mr.
KUNESH,Mr. GLASSMAN,and Mr. K.K.KAuLfor carrying out this work.
Thanks are also due t o the authorities of AQUITAINE
ORGANICOfor the facilities and to Dr. S. K. BHATNAUAR
and Dr. M. G. KRISHNA
for their suggestions.
The author sincerely acknowledges with thanks the suggestions and comments made by Prof. KENZEN.
1
D. L. DELAND,J. R. PURDEN
and D. P. SHEWMAN,
Chem. Engng. Progr. 63/7
2
C. B. BUCKNALL,
British Plastics 4 0 / l l (1967) 118.
M. CLASSENand G. SMETS,
J. Polymer Sci. 8 (1952) 289.
J. A. BLANCHETTE
and L. W. NEILSON,
J. Polymer Sci. 20 (1956) 317.
I. M. KOLTHOFF
and T. S. LEE,J. Polymer Sci. 2 (1947) 199.
J. R. CROMPTON
and V. W. REID,J. Polymer Sci. A 1 (1963) 347.
B. W. BENDER,
J. appl. Polymer Sci. 9 (1965) 2887.
(1967) 118.
3
.4
5
6
7
120
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