Effect of calcium and zinc carboxylates on the thermal stabilisation of PVC

Fur. Polym. J. Vol. 20, No. 1, pp. 95-98, 1984 Printed in Great Britain. All rights reserved

0014-3057/84$3.00 + 0.00 Copyright © 1984 Pergamon Press Ltd

EFFECT OF CALCIUM AND ZINC CARBOXYLATES ON THE THERMAL STABILISATION OF PVC* M. K. NAQVIt, P. A. UNNIKRISHNAN~, Y. N. SHARMAt a n d I. S. BHARDWAJ~ tResearch Centre, Indian Petrochemical Corporation Ltd, P.O. Jawahar Nagar, Vadodara-391 346, Gujarat and :~Department of Applied Chemistry, University of Cochin, Cochin-22, India

(Received 24 January 1983; in revised form 29 March 1983) Abstract--Metal carboxylate stabilisers are believed to replace labile chlorines in PVC with more stable ester linkages resulting in an increase in the stability of the polymer. In the present work, effects of combinations of stearates, palmitates and laurates respectively of zinc and calcium, in various proportions, on the thermal stability of PVC were studied. Combinations of palmitates and stearates having more than 75 molto of calcium salt were found to increase the stability of the polymer. The combinations of the three carboxylates showed the following order of stabilising: palmitate > stearate > laurate. This effect is explained in terms of a critical chain length of the n-alkyl group of the carboxylate anion which is most effective in the stabilising process. Highly crystalline, low molecular weight polyethylenes are used as plasticisers for PVC. They were found to have a stabilising effect explained in terms of a dilution effect by the non-polar polyethylenes on the polar interactions in PVC; compatibility of polyethylenes with PVC is the limiting factor in this stabilisation.

INTRODUCTION

were taken as examples o f long n-alkyl chains a n d their effect on the thermal stability of PVC was studied.

Studies on thermal d e c o m p o s i t i o n of the P V C models 1,3,5-trichlorohexane a n d 2,4-dichloropentane show t h a t the idealised head-to-tail structure for P V C is intrinsically stable below 300 ° [1,2]. Commercial PVC starts degrading at a b o u t 120 °. This low t h e r m a l stability o f PVC is a t t r i b u t e d to defects in the polymer structure [3-5]. M o d e l c o m p o u n d studies have s h o w n that allylic chlorines arising from internal u n s a t u r a t i o n a n d tertiary chlorines situated at b r a n c h - p o i n t s are the m o s t labile; they are accepted as the most i m p o r t a n t centres for the initiation o f d e h y d r o c h l o r i n a t i o n d u r i n g t h e r m a l d e g r a d a t i o n of the polymer. Frye a n d H o r s t [7-10] showed t h a t the most effective stabilisers for P V C are those which function by replacing the weakly b o u n d labile chlorine a t o m s with more stable groups, Ocskay et al. [11] studied the effect o f varying c o m b i n a t i o n s o f b a r i u m and c a d m i u m laurate on the stability o f PVC. Using degree o f discoloration as a measure o f stability, they suggested ratios 2:3 a n d 4:1 for good initial stability a n d longer life-time respectively. C z a k o et al. [12] studied the effect o f b a r i u m a n d c a d m i u m butyrates, 2-ethylhexanoates, laurates a n d stearates on the stability of P V C a n d d e m o n s t r a t e d synergistic efl'ects for their c o m b i n a t i o n s . In this work the c o m b i n a t i o n o f zinc a n d calcium carboxylates is e x a m i n e d with the view of studying the influence o f the length of the n-alkyl g r o u p o f the a n i o n on the stabilisation of the polymer. C o m b i n a tions of stearates, palmitates a n d laurates o f Z n a n d Ca were mixed with PVC (total stabiliser c o n t e n t 3 phr) a n d d e g r a d a t i o n characteristics of the polymer were studied. Low molecular weight highly crystalline polyethylenes are used as plasticisers for PVC. They

EXPERIMENTAL

Preparation of stabilisers The carboxylates of Zn and Ca were prepared by the precipitation method [13]. A soluble sodium soap was first prepared by reaction of the fatty acid with NaOH in the presence of excess water. A solution of the chloride of the desired metal was then added. The metal carboxylate precipitated out. It was washed with hot water and dried. C and H analyses of the salts showed them to be pure. Polymer-additive compounding Compounding of PVC with the additives was done in the planitary mixer of the Brabender-Plasticorder (PLV-151). Three-hundred grammes of PVC resin were charged into the mixer at 100° and stirred for 1 min at 75 rpm. The desired amount of stabiliser was then added and allowed to mix at the same speed for 10rain. Polymer degradation Degradation of the polymer was studied by monitoring the amount of HC1 evolved using a potentiometric technique. The polymer (0.5 g) was heated in a glass test-tube at 190 + 0.5 °. A continuous flow (60 cm 3 rain-1) of dry preheated (190°) N 2 was maintained through the degradation tube and then the HCI from the degrading polymer was bubbled through sintered glass into 2.4 × 10-3NNaOH solution (250 cm3). The solution was constantly stirred at 50°, and the change in pH was monitored using a digital pH meter. The amount of HCI evolved was determined from a calibration curve of change in pH (ApH) vs the amount of HCI added. The period between start of heating and the first detection of HCI evolution (the induction period) and the rate of HCI evolution at 3% conversion were taken as measures of the effectiveness of the stabilisers. Rate of HCI evolution was calculated from the slope of the tangent to the degradation curve at the point where the polymer had lost 3% of the total chlorine present

*IPCL communication No. 55. 95

96

M. K. NAQVIet al. Table

1. Inductionperiodsand degradationrates of PVC for various combinationsof C~Zn stearates,palmitates and laurates Stearate

% Ca 25 50 75 91.6

Palmitate

Laurate

IP (min)

Rxl0 6 (tool HCl/min)

IP (rain)

R×I0 6 (tool HCl/min)

5 10 15 15

12.3 9.0 3.4 0.8

10 15 25 40

7.57 3.42 0.75 0.26

.

IP (min)

R×10 6 (mol HCl/min)

10 20 35

48.0 7.8 2.5

.

.

.

.

.

C a - Z n mixtures of the three salts containing 75 and 91.6 mol% of Ca were also studied. In the former When PVC is subjected to thermal degradation, case only the palmitate mixture showed definite stainitially only HCI is evolved but in the latter stages bilisation with the dehydrochlorination rate half that various other degradation products (mainly polyenes and aromatics) are also formed. Hence the rate of of unstabilised PVC. Mixtures of laurates and palthermal dehydrochlorination, taken as the measure of mitates were found to reduce the stability. In the case thermal stability, is measured during the early stages of 91.6 mol% of calcium, mixtures of stearates and of degradation. In the present study the rates of palmitates were found to increase the stability of PVC thermal dehydrochlorination were measured from the considerably, as shown by the dehydrochlorination degradation curves at points representing the loss of rates in Table 1. The laurate mixture was found to 3 ~ of the total chlorine initially present in the have a destabilising effect even at this ratio. In all the above studies, the PVC compounds polymer. For unstabilised PVC which had been subjected to containing the Ca-Zn mixtures of the three salts the compounding treatment in the Brabender, the exhibited the following order of stability: rate of dehydrochlorination at 3% degradation was palmitate > stearate >laurate. The mechanism of 1.39 x 10 -6toOl/rain and the induction period was stabilisation of PVC by carboxylates is complex. It is generally believed that only Zn carboxylate is able to 30rain. The rates of dehydrochlorination (at 3% undergo the exchange reaction with the labile degradation) and the induction periods for various PVC-stabiliser combinations are given in Table 1. chlorines in PVC. The resulting ZnCl 2 which is a degradation catalyst is neutralised by exchange reacFigure 1 shows typical degradation curves obtained tion with the Ca carboxylate resulting in the less for PVC stabilised with C a - Z n carboxylates containharmful CaC12 and regenerating the Zn carboxylate. ing 75 tool% of the Ca salt. The calcium carboxylate reacts preferentially with PVC-stabiliser combinations containing 25 mol% HCI and reduces its autocatalytic effect on the dehyof the Ca salt were found to be less stable than drochlorination, the reaction products being CaCI2 unstabilised PVC. The composition containing paland the carboxylic acid. The work of Anderson and mitates is comparatively more stable than that conMcKenzie [14], Briggs and Wood [15] and Onozuka taining stearates. The composition containing lau[16] has shown that the chlorides of Ba and Ca are rates was the least stable and degraded rapidly. The weaker degradation catalysts than those of Cd and induction period and dehydrochlorination rate were Zn. not determined for this composition (Table 1). Grassie and coworkers [17] studied the thermal PVC-stabiliser combinations containing 50 mODo stabilities of vinyl chloride-vinyl acetate copolymers of the Ca salt were also less stable than unstabilised and showed that the introduction of the acetate PVC. In this case also PVC containing palmitates was group has a destabilising effect on PVC. This effect found to be the most stable. was explained on the basis of an intramolecular neighbouring group effect. RESULTS AND DISCUSSION

~CH2--CH--CH

/

\

0

H

CH,,~, ~ ~,,CHf---CH=CH

CH~,~

CI

C1

CH3COOH

(1)

~c--6"" / CH3

H ~,~CH2--CH

C1

CH

CH,~--* ~ , , C H 2 - - C H - - C H ~ C H , ~ z

\o

o"

Io \ c = o + HCl

\c// CH 3

CH3

(2)

97

Effect of Ca and Zn carboxylates on PVC

%

o 28 I

E o w

~0 i

] 30 Degrcldall0q

j

i

60

90

time

( mln )

Fig. 1. Degradation of PVC stabilised wilh a mixture of Ca-Zn carboxylates containing 75 moP';;of the calcium salt. (Z]-- ~, (a-Zn lauratc; O - - - - O , Ca Zn stearate; A -A, Ca Zn palmitate.

36~ i

32~

/

/

28~

z

//

0

4.0

80

i 120 160 200 Degradation

I 240 time

I 280

I I I 320 360 400

I 440

( rain )

Fig. 2. Degradation of PVC mixed with AC Polyethylenes. OO, ACPE-gA (1 phr): 0 - - - 0 , ACPE-9A (2 phr); A--A, ACPE-540A (1 p h r ) ; • - - - A , ACPE-540A

(2 phr): x ~ - - x, unstabilised PVC.

Grassie and McGuchan [18] studied vinyl chloride acrylonitrile copolymers and showed that acrylonitrile exerted a neighbouring group destabilising effect similar to acetate on the stability of PVC. McNeill et aL [19] observed a substantial destabilisation of PVC in the temperature range where polyacrylonitrile was stable in a PVC-PAN blend. It has been recently shown that substitution of all labile chlorines by acetate groups in PVC resulted in a polymer that was less stable than the original polymer [20]. In view of the findings discussed above, introduction of highly polar groups like C ~ O and C ~ N greatly destabilises PVC. The ability of oxygen and

nitrogen to undergo hydrogen bonding may also contribute towards destabilising the polymer. Replacement of labile chlorines in PVC by carboxylates should exert a destabilising effect similar to that of acetate because of the carbonyl group. In the stabilisation of PVC by metal carboxylates, the long linear alkyl chain can be considered to play a significant role in the stabilisation. In the present work the effect of the length of alkyl chain on the stabilisation process has been demonstrated. The three salts chosen differed only in the length of the alkyl chain. The stabilisation may be due to the dilution of polar-polar interactions in PVC by the non-polar alkyl chain of the carboxylate just as highly polar groups tend to destabilise the polymer, probably by intensifying the same interactions. The length of the alkyl chain would also be expected to influence the ease of substitution with labile chlorines and compatibility with the polymer. The results show that palmitate with CI5H3~ alkyl chain is most suitable for the overall stabilisation of the polymer. The longer stearate also stabilises but not as effectively, whereas the shorter laurate results only in destabilising the polymer. A maximum in the stabilisation at an intermediate composition of the salt mixtures is not observed in this work although there is some evidence for synergism. In each column of Table 1 the rate of dehydrochlorination increases by a lector which is greater than the increase in the zinc content and this can be attributed to the decreased amount of stabilising synergistic calcium. Two low molecular weight crystalline polyethylene samples (Allied Chemicals Corporation, AC Polyethylene 9A and AC Polyethylene 540A) were taken as examples of non-polar alkyl chains; their effect on the thermal stability of PVC was studied. Figure 2 gives the degradation curves along with that of unstabilised PVC. The rates of dehydrochlorination are given in Table 2. Both the polyethylenes were found to improve the thermal stability of the polymer. AC Polyethylene 9A was found to stabilise PVC considerably. For AC Polyethylene 540A the rate of HC1 evolution was higher than that of unstabilised PVC in the early stages of degradation (Fig. 2). At 3% degradation however the rates of dehydrochlorination for the polyethylene-stabilised PVC were lower than that of unstabilised PVC (Table 2). Hence despite some initial destabilisation, the overall effect of AC Polyethylene 540A can also be considered to be stabilising. The manufacturers of AC Polyethylenes also claim that these polymers exert a stabilising influence on PVC. This effect may also be explained in terms of the diluting effect of" the nonpolar polyethylenes upon the polar effects within PVC. The compatibility of the polyethylenes with PVC would be a limiting factor in the stabilisation.

Table 2. Inductionperiods and degradationrates of PVC stabilisedwith AC Polyethylenes ACPE-9A

ACPE-540A

IP (rain)

(tool HC1/rnin)

IP (rain)

R x 106 (tool H C I / m i n )

30 30

0.76 1.11

25 25

1.21 1.16

R x l0 n

Concentration I phr 2 phr

EPJ 20/1 G

M. K. NAQVI et al.

98 REFERENCES

1. H. V. Smith, Rubber J. inst. Plast. 138, 969 (1960). 2. M. Asahina and M. Onazuka, J. Polym. Sci. A2, 3505 (1964). 3. E. J. Admen, J. Polym. Sci. 12, 547 (1954). 4. A. Crosato-Arnaldi, B. Obereigner and D. Lim, Eur. Polym. J. 7, 111 (1971). 5. G. S. Park and C. L. Skene, J. Polym. Sci. C33, 269 (1971). 6, D. Braun, J. Polym. Sci. C16, 2351 (1967). 7. A. H. Frye and R. W. Horst, J. Polym. Sci. 40, 419 (1959). 8. A. H. Frye and R. W. Horst, J. Polym. Sci. 45, 1 (1960). 9. A. H. Frye, R. W. Horst and M. A. Paliobagis, J. Polym. Sci. A2, 1765 (1964). 10. A. H. Frye, R. W. Horst and M. A. Paliobagis, J. Polym. Sci. A2, 1801 (1964). 11. Gy. Ocskay, Zs. Nyitrai, F. Varfalvi and T. Wein, Eur. Polym. J. 7, 1135 (1971).

12. E. Czako, Z. Vymazal, Z. Vymazalova, J. Skalsky and J. Stepek, Eur. Polym. J. 14, 1011 (1978). 13. J. Lery, Utilisation of fatty acids in metallic soaps and greases. Fatty Acids and Their Applications, Chap. 8. Marcel Dekker, New York (1968). 14. D. F. Anderson and D. A. McKenzie, J. Polym. Sci. Al 8, 2905 (1970). 15. G. Briggs and N. E. Wood, J. appl. Polym. Sci. 15, 25 (1971). 16. M. Onozuka, J. Polym. Sci. A1, 5, 2229 (1967). 17. N. Grassie, I. F. McLaren and I. C. McNeill, Eur. Polym. J. 6, 679, 865 (1970). 18. N. Grassie and R. McGuchan, Eur. Polym. J. 9, 507 (1973). 19. I. C. McNeill, M. Grassie, J. N. R. Samson and T. Straiton, J. Macromol. Sci.-Chem. A12, 503 (1978). 20. J. Lewis, M. K. Naqvi and G. S. Park, ACS Polym. Prepr. 23, 140 (1982).