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Influence of plasticizing on the rate of thermal dehydrochlorination of poly (vinyl chloride).

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Die Angewandte Makromokkulare Chernie 9 (1969) 186-189 (Nr. 129)
From the Deutsches Kunststoff-Institut, Darmstadt
Kurzmitteilung
Influence of Plasticizing on the Rate of Thermal
Dehydrochlorination of Poly (Vinyl Chloride)
By I. MILLAN*and D. BRAUN
(Ehgegangen am 12. September 1969)
Introduction
The thermal degradation of poly (vinyl chloride), PVC, has been the subject
of many investigations. However, little has been published on the thermal
dehydrochlorination of plasticized PVC, and the influence of various plasticizers
on the rate of degradationl92.
The principal function of a plasticizer in PVC is to interpose itself between
the polymer chains and to lower the attractive forces between the polymer
molecules ; the intermolecular forces between the plasticizer molecules and
between the plasticizer and the polymer molecules are also important. As a
consequence of all these forces, modifications in the thermal stability of plasticized PVC related to both the nature and the percentage of the plasticizer can
be expected.
This work deals with the degradation of plasticized PVC in the presence of
different amounts of tricresyl phosphate (TKP), dibutyl phthalate (DBP) and
Dioctyl sebacate (DOS).
Experimental
Materials
The poly (vinyl chloride) used was a suspension PVC supplied by the BASF. This
polymer was free of any additive.
* Present address: Instituto de PlBsticos, Calle Juan de la Cierva, 3, Madrid-6.
1.86
Dehydrochlorination of Poly (Vinyl Chloride)
Plasticizers were :
Palatinol C (Dibutyl phthalate) (bp. 34OOC) supplied by BASF,
Edenol 888 (Dioctyl sebacate) (bp. 248-250 at 4 mm) supplied by Dehydag,
Disflamoll TKP (Tricresyl phosphate) (bp. 265 at 10 mm) supplied by Bayer.
The plasticizers were purified by vacuum distillation.
Preparation of foils
Mixtures of PVC and plasticizer were first calendered at 1 5 0 T for 5 min. From
the calendered products, foils were obtained by pressing at l6OoC for 2 min. under
a pressure of 150 kg/cm2.
The weight percentages of plasticizer used were: 16.6, 23.0, 28.1,33.2, and 37.6 yo.
The corresponding molar ratios are :
tricresyl phosphate : 0.0339, 0.0486,0.0676, 0.0862, and 0.1023 moles per primary
mole of PVC.
Dibutyl phthalate: 0.045, 0.0675,0.0895,0.111, and 0.134 molesperprimary mole
of PVC.
Dioctyl sebacate: 0.0292, 0.0438, 0.0584, 0.0728, and 0.0876 moles per primary
mole of PVC.
The thickness of the foils was about 0.3-0.4 mm.
Degradation of foils
Apparatus
The apparatus used has been described by D. BRATJNand M. THALLMAIER3. It
consists of a tube heated by means of a silicone oil thermostat. The nitrogen is introduced through the bottom of the reaction tube after having been heated to the
reaction temperature in a spiral tube coiled around the reaction vessel. The finely
cut plasticized polymer sample containing 1OOmg of pure PVC is placed into a
small open glass vessel a t the bottom of the reaction tube. The split-off hydrogen
chloride is carried over into a conductivity cell by the nitrogen, where the HC1 is
absorbed in water and the rise in conductivity is .memured.
Several experiments were also carried out with different sample sizes of the polymer and with only calendered PVC in order to detect any dependence of the rate of
degradation on the particle size. Within the experimentalerror no effect of the thickness and size of the foils was observed.
Results and Discussion
The initial rate of dehydrochlorination of PVC was found to be constant
regardless of plasticizer content as shown in Fig. 1 for the degradation of foils
plasticized with TKP. It is also independent of the size ofthe polymer specimens
over a reasonable range. Experiments carried out with calendered and not
pressed foils showed no appreciable differences in the rates of degradation.
The rates of dehydrochlorination determined from the slopes of the straight
lines are plotted against the molar ratios of the plasticizers in Fig. 2.
187
I. MILLANand D. BRAUN
1
2
3.4
5
T h e lhrs. 1
Fig. 1. Thermal degradation of PVC plasticized with tricresyl phosphate in following concentrations (given in mole of plasticizer per primary mole of PVC):
1.: 0.102; 2.: 0.086; 3.: 0.0339; 4.: 0.0676; 5.: 0.0486.
(The extent of dehydrochlorination, x, is defined as the amount of HCl
eliminated to that initially present in the undegraded polymer.)
i
0,s %HCl/h
Q3-
0
Fig. 2.
188
moles plasticizer
primarymoles of PVC
I
I
I
0.04
0,oa
0,12
Effect of plasticizer content on the rate of degradation of PVC foils a t
180OC.
: Dioctyl sebacate,
0 : Tricresyl phosphate,
A : Dibutyl phthaIate.
Dehydrochlorination of Poly (Vinyl Chloride)
The most remarkable result is the fact that there is a minimum of the rate of
degradation a t a definite ratio for each plasticizer. An explanation of this could
be that a t lower concentrations of plasticizer the interaction between polar
groups of polymer and plasticizer molecules is stronger than the interaction
between the polymer chains; a s a result of this the degree of order in the polymer chains is increased. Therefore the C-C1 bond should be more stable and,
consequently, more energy would be necessary to eliminate HC1. At a higher
concentration of plasticizer there must be a maximum of steric order in the
polymer, above which the chains become more flexible due to solvating power
of the plasticizer; this would correspond to the minima in the curves of Fig. 2.
After these minima, the .increasing amounts of plasticizer molecules should
introduce an effect of solubility, by which the polymer chains become more
flexible. Here the C-C1 bonds become less stable, because the interaction bctween the PVC chains is reduced by interjection of plasticizer molecules.
These considerations agree with the observation reported in the literature374
that only a small amount of plasticizer is held firmly by the polymer, and that
with increasing plasticizer content the forces holding the plasticizer become
weaker. There is a point in this process where a marked break in the curve of
some physical property plotted against plasticizer content occurs. This seems
to be the case also in the rate curve of degradation against plasticizer content.
Acknowledgement
One of the authors (I.M.) thanks the MAX PLANCK
GESELLSCHAFT
for the
financial aid granted.
N E R. DE BROUTELLES,
R. G.C. P. Plastiques 5 (1968), No. 5, 315.
L. D W C H ~and
J. STEPEK,
C. JIREAL,and J. HENNIKER,
Plastiques Modernes et ElastomBres,
Dez. 1968, 119.
3
D. BRAWN
and M. THALLMAIER,
Kunststoffe 56 (1966) 80.
W. S. PENN,PVC Technology, Maclaren & Sons, London 1962, p. 49.
189
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