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Thermal degradation of polyvinylchloride in phenolic solvents II.

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Die Angewandte Makromolekulare Chemie 24 (1972) 35-49 (Nr. 318)
From the Department of Chemistry
Indian Institute of Technology,
Hauz Khas, New Delhi-29
Thermal Degradation of Polyvinylchloride
in Phenolic Solvents I1
By I. K. VARMA
and S. S. GROVER
(Eingegangen am 18. August 1971)
SUMMARY:
The thermal degradation of polyvinylchloride in 2-ally1 phenol, 2-allyl-6-methyl
phenol and 2-methoxy-4-ally1 phenol (eugenol) has been investigated in an atmosphere of nitrogen in the temperature range 188-218°C. The reaction is very slow
in eugenol. The activation energy was lower in eugenol(40kcal/mole) than in 2-ally1
phenol (47 kcal/mole). The reaction is autocatalytic. The rate of dehydrochlorination in ethylbenzoate or methylsalicylate is suppressed by addition of small quantities of eugenol. The results have been explained on the bases of a free radical mechanism for the dehydrochlorination reaction.
ZUSAMMENFASSUNG :
Die Chlorwasserstoffabspaltung aus Polyvinylchlorid wurde in 2-Allylphenol,
2-illlyl-6-methylphenol und 2-Methoxy-4-allylphenol (Eugenol) unter Stickstoff
im Bereich von 188-218°C untersucht. I n Eugenol war die Reaktion langsam. I n
Eugenol war die Aktivierungsenergie niedriger (40 kcal/mole) als in 2-Allylphenol
(47 kcal/mole) ; bei 2-Allyl-6-methylphenol lagen die Werte dazwischen. Die Reaktioii ist autokatalytisch. Die Chlorwasserstoffabspaltung wird in khylbenzoat
und Methylsalizylat durch kleine Zusatze von Eugenol verlangsamt . Die Ergebnisse
lassen sich durch einen Radikal-Mechanismus fur die Chlorwasserstoffabspaltung
erklaren.
Phenolic compounds have been used as antioxidant stabilisers for polyvinylchloride1 (PVC) and t,he efficiency of such stabilisation is found t o depend on
the structure of phenolz. Our previous work has indicated t h a t in nitrogen a t mosphere, the dehydrochlorination of PVC in phenolic solvents is influenced
by polar as well a s steric factors and could be explained by the participation
of free radicals in the reaction3. There is some evidence which supports the
participation of free radicals in the degradation of PVC in vacuum and inert
atmosphere4-16. However, a n ionic mechanism has been given t o account for
the autocatalytic nature of the dehydrochlorination in bulk17, a six membered
35
I. K . VARMAand S. S. GROVER
transition state has been suggested in presence of certain solvents18 and a
molecular mechanism has been proposed on an analogy with model compounds19.
Whatever may be the mechanism for thermal dehydrochlorination, there
is a general agreement that the reaction is initiated by the rupture of labile
carbon-chlorine bonds associated with unsaturated groups, e. g. a t the chain
ends or with tertiary chloride#. I n the free radical chain reaction, the chlorine
atoms are believed to be the chain carriers6 and the chain is propagated by
abstraction of hydrogen atoms preferentially from allylic positions. If the
degradation of PVC is carried out in the presence of phenolic solvents containing allylic substituents then these allylic group should compete with the
polymer chain for the chlorine atoms. One should, therefore, expect a considerable decrease in the rate of dehydrochlorination in presence of such solvents
if a free radical chain mechanism is operating. So far no such work has been
reported in the literature and, therefore, we took up a systematic study with
2-ally1 phenol, 2-allyl-6-methyl phenol and eugenol (2-methoxy-4-ally1phenol)
to investigate the mechanism of dehydrochlorination.
Experimental
Polyvinylchloride (PVC) was the commercially available material (CSI make)
prepared by Calico Chemicals Ltd., India, and was purified by the method reported
earlier3.
Solvents
Methyl salicylate (BDH)and ethyl benzoate (Bush) were purified by distillation
phenol) (B. P. C.) was distilled under nitrobefore use. Eugenol (2-methoxy-4-ally1
gen 2 or 3 times till a colourless distillate was collected. The receiver was covered
with a black paper and the solvent was stored at low temperature. Fresh solvent
was distilled after three days.
2-ally1phenol and 2-allyl-6-met,hylphenol were prepared from ally1 chloride and
phenol (BDH) or o-cresol (BDH) by first preparing the corresponding ether and
then subjecting it to CLAISEN rearrangement as given in literaturezo.The apparatus for carrying out the degradations in nitrogen has been reported earlier3.
Results
The present investigations were limited to the temperature range 188-218 "C.
The effect of temperature on the dehydrochlorination of PVC in 2-ally1 phenol
is shown in Fig. 1. Similar results were obtained with 2-allyl-6-methyl phenol.
There is an obvious increase in the evolution of HC1 with time and this is more
predominant a t 194°C and above. I n case of eugenol, however, the autoacceleration is not so obvious a t lower temperatures (i. e. 188 and 194"C), and a t
36
Thermal Degradation of Polyvinylchloride in Phenolic Solvents 11
Q
218OC
4D 208OC
0 194oc
0 188OC
100
150
Time in minutes
Fig. 1.
The effect of temperature on the degradation of polyvinylchloridein 2-ally1
phenol, conc. = 83 5 2 81-l. Axis has been shifted t o avoid superposition
of lines.
208°C the rate of dehydrochlorination is constant for first 100 minutes and
then increases (Fig. 2). At still higher temperature autoacceleration is limited
to a short initial period and is followed by steady rate later on. The effect of
substituting the ally1 phenols in the benzene ring on the evolution of HC1 is
obvious from Fig. 3, where the amount of HC1 evolved is plotted as a function
of tiine in all the three solvents.
37
I. K. VARMA
and S. S. GROVER
8E
0 218OC,
Conc. = 76.0 g1-l
208OC,
Conc. = 79.0 gl-"
194'C,
Conc. = 79.0 gl-'
0 188OC,
Conc. = 850 gl-'
@
6E
n
0
U
x
0
G
44
0
u
Q
+.-0
v)
-a,
2
22
yy
)
I
I
50
Fig. 2.
I
100
Time in minutes
s
150
The effect of temperature on the degradation of PVC in eugenol.
The effect of concentration of the polymer on the degradation was investigated by varying the concentration between 35 gl-1 and 120 gl-1. The rate was
directly proportional to the concentration of the polymer. A plot of logarithm
of the rate against the logarithm of the polymer concentration in eugenol is
shown in Fig. 4. There is a variation in the order of the reaction, which increases
from w 0.5 to 1.13 as the temperature of the degradation is increased.
38
Thermal Degraddion of Polyvinylchloride in Phenolic Solvents II
0 2 - allyl phenol
45.7!
@
2 - allyl 6 - methyl phenol
0
Eugenol
2
!
2
30.1
0
u
c
3
v)
Y
2
-
15.2t
0
Fig. 3.
50
100
Time in minutes
150
The effect of substituents on the degradation of PVC in allyl phenols at
218"C, cone. = 84
1 gl.-l Axis has been shifted to avoid the superposition of lines.
The rates of dehydrochlorination of PVC a t different temperature in 2-ally1
phenol and 2-allyl-6-methyl phenol were calculated by taking slopes a t different
times (0-50 minutes (X), 60-100 minutes (Y) and 110-150 minutes (Z)) of the
acid evolved vs. time curves. During this time the conversion is not very high
and the maximum is about 14% in case of 2-ally1 phenol a t 218°C after 150
minutes of degradation. The first order rate constants were, therefore, calculated by dividing the rate of dehydrochlorination with the initial concentration
39
I. K. VARMA
and S. S. GROVER
0 218'C
8 208'C
0 194'C
0 188'C
1:.
0.E
-
u
v)
al
7
al
-
0.4:
al
c
m
1
m
J
0
+
a
0.c
- 1.5:
I
I
b
1.65
.1.80
1.95
Log (concentration) gl
Fig. 4.
40
-'
The dependence of the rate of dehydrochlorination on PVC a t temperatures from 188°C to 218°C in eugenol.
Thermal Degradation of Polyvinylchloride in Phenolic Solvents 11
of polymer and these are reported in Table 1, along with that of eugenol where
only one value is reported a t lower temperature because the autoacceleration
was not significant. However, a t 208°C two values are mentioned, one is above
150 minutes of heating time. The value of 218°C in eugenol is taken from the
steady rate, i. e. after 70 minutes of heating.
The activation energy was calculated by plotting the average value of the
rate constant a t each temperature and different reaction times in all the solvents (Fig. 5 ) . The rates have a temperature dependence of 47 & 1 kcal/mole,
42 f 1.5 kcal/mole and 40 f 1 kcal/mole in 2-ally1 phenol, 2-allyl-6-methyl
phenol and eugenol respectively.
Reciprocal of absolute temperature
Fig. 5.
*
TO3
Activation energy diagram for the dehydrochlorination of PVC in eugenol
9 and 2-allyl-6-methylphenol (X = 0-50 minutes; Y = 60-100 minutes;
Z = 110-150 minutes).
Since the rate of dehydrochlorination is smallest in case of eugenol, further
the inhibitory action of this compound for PVC dehydrochlorination in ethyl
benzoate and methyl salicylate was investigated. The addition of eugenol in
b0t.h the solvents decreases the rate of dehydrochlorination considerably
41
2-methoxy-4-ally1phenol
(eugenol)
2-ally1- 6-methyl phenol
2-ally1 phenol
Solvent
0- 50 (X)
60-100 (Y)
110-150(Z)
0- 50
60-100
110-150
0-100
150-300
Time interval in minutes
194OC
1.49
3.64
4.41
1.48
2.72
4.06
1.48
188°C
0.81
1.28
1.81
0.79
1.39
1.69
0.49
7.15
14.25
21.47
6.61
9.52
12.25
3.16
5.08
208 "C
First order rate constant
sec-1 x 106
Table 1. Rate constants for the dehydrochlorination of PVC in various solvents at different temperatures.
15.02
26.43
51.29
15.60
22.24
27.55
9.07
-
218°C
0
t?
YJ
?
I"@
Pm
H
Thermal Degradation of Polyvinylchloride in Phenolic Solvents I1
(Fig. 6). The fraction of unretarded rate vs. the mole ratio of [eugenol]/[VC] is
plotted in Fig. 7. Eugenol is very effective in retarding the rate of dehydro-
0 Methyl salicylate.
Polymer conc.
= 62.8 gl
Eugenol 2.5% v / v
Polymer conc.
= 61.3 g1-l
Polymer conc.
= 69.7 91-’
@
0 Eugenol 50%
V/V
Time in minutes
Fig. 6.
The effect of addition of eugenol on the dehydrochlorination of PVC in
methyl salicylate a t 208°C. Axis has been shifted to avoid superposition
of lines.
43
I. K . VARMA
and S. S. GROVER
0
1.0
Methyl salicylate
Ethyl benzoate
0.9
w
c
0.8
-0
-E
2
m
0.7
\c
0
c
0
__
c
p
06
L
0.5
0.4L
0
Fig. 7.
I
I
I
0.5
1.0
1.5
Retardation of dehydrochlorination of PVC by eugenol.
Solvent
Temp.
"C
First order rate const. sec-1 x l o 6
Conc. of eugenol
v/v
0
44
I
2.5
1
5
1
10
I
25
1
50
T h e r m a l Degradation of Polyvinylchloride i n Phenolic Solvents I 1
Few experiments were also performed with eugenol to account for the autoacceleration observed in the case of these allylic phenols. Such autoacceleration
was not observed in our previous work3 with salicylates, phenols and cresols.
The possibilities that exist in case of these allylic compounds on which one can
explain the autoacceleration are (a) catalysis by HC1, (b) formation of some
intermediate compounds on heating of these allylic compounds for a long time
which might cause a n acceleration, (c)formation of an intermediate by reaction
with HC1 and allyl phenols which can catalyse the reaction. It has been reported
that dehydrochlorination of bulk PVC samples is affected by HC1 catalysis
even in those cases where the volatile reaction products are being continuously
reinoved from the system13 because the diffusion could account for accumulation of certain amount of HC1 in the samples211 221 23.
However, one can eliminate the catalysis by HC1 on the grounds that in
solution state the diffusion should not be difficult and the problem which
exists in bulk may not be existing here.
To ascertain the possibilities (b) and (c) few experiments were performed by
heating eugenol a t 208°C in nitrogen atmosphere for 180 minutes and then
subjecting PVC to degrade in this eugenol (sample A). I n another experiment
(s:tmple B) a slow stream of HC1 gas was bubbled through eugenol along with
nitrogen a t 208 "C for 180 minutes, and then excess of HC1 gas dissolved in the
eugenol was removed by bubbling a stream of nitrogen gas for two hours till the
evolution of HCl gas ceased (checked by passing the gases through conductivit,y
water containing methyl orange as an indicator). PVC was degraded in this
eugenol. I n Fig. 8 the results obtained by degradation of PVC in this eugenol
are shown, while in Table 3 the dependence of the rate of dehydrochlorination
011 pretreatment of' eugenol is given.
This clearly shows that eugenol when treated with HC1 undergoes some change
to give a compound which catalyses the dehydrochlorination. Further experiments are being done to characterise this compound.
Discussion
It is obvious that the rate of dehydrochlorination of PVC is considerably
reduced in the presence of solvents containing allyl groups in phenolic compounds. The rate is smallest in eugcnol (2-methoxy-4-ally1 phenol) and maximum in 2-ally1 phenol. Introduction of a methyl group at position 6 reduces
the rate of dehydrochlorination showing that steric factors play a significant
role. The activation energy is again higher in 2-ally1 phenol and lowest in eugenol. If one compares the various solvents used for PVQ? 18, 159 16 the rate of
clehydrochlorination is the smallest in these allyl substituted phenols. But if
45
I. K. VARMA
and S. S. GROVER
Time in minutes
Fig. 8.
Effect of treatment of eugenol on the thermal degradation of polyvinylchloride @ eugenol, 0 eugenol heated a t 208°C under nitrogen for 3
hours; 0 eugenol heated with HCI gas a t 208°C under nitrogen for 3 hours.
Polymer conc. = 53 f 2 gl.-1 Axis has been shifted to avoid superposition of lines.
one looks a t the activation energies these are higher in all these solvents than
those investigated earlier. This variation could be explained on the basis of
chain transfer which is possible. The free radical mechanism for PVC degradation can be written as:
Initation : The radical R. abstracts hydrogen
R.4--CH2-CHCl-CH2-CHCl--+ -CH-CHCl-CHz-CHCl-CH=CH-CHz-CHCl-CH-CHCI-CH~-CHC~Propagation :
C1- -CH=CH-CHZ-CHCl-CH=CH-~H-CHCI-
+
--+
-CH=CH-eH-CHCl-CH=CH-CH=CH-
+ RH
+ C1.
+ HCl
+ C1.
Termination :
May be caused by cross bond formation or combination of chlorine atoms.
46
B
A
Sample
1.72
2.73
4.56
6.13
d HCl/dt moles of
acid 1-1 sec-1 x 106
0.045
0.049
0.083
0.077
Moles of acid
g-1 sec-1 x 106
Initial rate up to 120 minutes
38.0
55.2
55.1
79.6
Polymer conc.
gl-1
3.34
4.89
4.56
6.13
d HCl/dt moles of
acid 1-1 sec-1 x 106
0.088
0.089
0.083
0.077
moles of acid
g-1 sec-1 x 106
Final rate 150-300 minutes
2
0
2B
0,
ri..
s
b
R
--4.
I. K. VARMA
and S. S. GROVER
I n the presence of solvents containing -OH and -CH2--CH=CH2
two possibilities for chain transfer reaction exist :
1) Reaction of chlorine atoms with phenolic OH-groups:
groups
The allyl group may be a t ortho or para position.
The phenoxy radical will be stabilised by resonance and it can either combine
with a polymer radical or lose a hydrogen atom from allylic position, and so
the rate of dehydrochloripation will be less because the reactive chain carrier
has been replaced by a less reactive one.
2) Reaction of chlorine atoms with allylic groups:
If only reaction ( 1 ) is taking place then one would expect more or less similar
rates in o-cresol and 2-ally1 phenol. On the contrary the rate of dehydrochlorination is much less in the latter. So both reactions must be contributing to some
extent towards the stability of intermediate radicals. Allylic substitution becomes significant a t higher temperatures24, so probably a t higher temperature
reaction (2) is more important. This may also account for the higher energy of
activation observed in these solvents.
The lower rates in eugenol as compared to 2-allyl-6-methyl phenol can be
explained if one assumes that reaction (1) is suppressed to certain extent.
There is evidence for the existence of intramolecular hydrogen bonding in eugenol25. Such bonding will reduce the reactivity of phenolic OH-groups in eugenol.
Also the allyl group is in para position with respect to OH and perhaps some
stabilisation may take place in the following way:
OH
*OH
Such structures will be difficult to form a t ortho position because of crowding,
the groups may not remain planar.
48
Thermal Degradation of Polyvinylchloride in Phenolic Solvents 11
K. S. MINSKER,
G. T. FEDOSEYEVA,
and I. K. PAKHOMOVA,
Vysokomol. Soyedin.,
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and S. S. GROVER,
Angew. Makromol. Chem. 7 (1969) 29.
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and L. H. WARTMAN,
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and B. G. ACHHAMMER,
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and Y. TRAMBOUZE,
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and D. NEIL, Makromol. Chem. 117 (1968) 265.
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and D. F. FENTON,
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and M. CARENZA,
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and H. M. SHARPE,
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and I. K. VARMA,
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and G. F. GRANT,Eur. Polymer J. 4 (1968) 521.
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and M. ONOZUKA,
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and D. BRAUN,
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and E. M. VOROSHIN,
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1
49
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