C H L O R I D E F O R M A T I O N A N D P O L Y M E R C O L O R 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 O F PVC S T A B I L I S E D

W I T H A Z N - C A R E C I P E

TRAN VAN HOANG. ALAIN MICHEL & ALAIN GUYOT

CNRS--Laborato ire des MatOriaux Organiques, 79. Boulevard du I 1 Nocembre 1918, 69626 Villeurbanne Cedex, France

(Received: 23 October. 1978)

A B S T R A C T

The part played by metallic chlorides, and especially zinc chloride generated during the stabilisation process of PVC with the zinc-calcium recipe, is analysed through a brieJ" ret,iew of the mechanism established preciously, a few additional data being presented. Correlations between polymer discoloration and chloride formation, estimated through a coulometric method based upon Ag ~ ion precipitation in the presence oJ chloride ions, are analysed Jor diJ]erent short-term and long-term secondary stabilisers. The)' allow the exact part played by each component to be explained.

INTRODUCTION

It was early recognised that the thermal degradation of PVC, observed during processing, is caused by a dehydrochlorination reaction. However discoloration may be prevented by using a suitable stabilising recipe.

A few stabiliser systems, more or less complex, were developed empirically. Among them, the zinc-calcium recipe, which incorporates a mixture of zinc and calcium carboxylates as the primary stabiliser, together with a combination of organic secondary stabilisers, may be used in food packaging. Most of its components react with hydrochloric acid. Obviously, zinc and calcium chloride are among the final products. This paper aims at contributing to the understanding of the part played by these chlorides in degradation and stabilisation reactions. First, a review of our knowledge of stabilisation mechanisms is given, together with some new results on some particular points. Then, it is shown that a close parallelism may be observed

237 Polymer Degradation and Stability 0141-3910/79/0001-0237 $02.25 '~ Applied Science Publishers Ltd, England, 1979 Printed in Great Britain

238 TRAN VAN HOANG, ALAIN MICHEL, ALAIN GUYOT

between colour development during processing or thermal ageing and chloride ion formation in the bulk of the polymer.

THE MECHANISM OF PVC STABILISATION

Only a few studies have been carried out in order to understand the mechanism of thermal stabilisation of PVC with the zinc-calcium recipe, and using a direct analysis of the products formed during processing or thermal ageing of the formulated polymer. For that purpose, Frye and Horst t used labelled zinc carboxylates as well as cadmium carboxylates. They showed that the carboxylate moiety, labelled with ~'~C, remains attached to the polymer backbone after processing. The amount of permanent radio-activity varies with heating time: it goes through a sharp maximum with cadmium carboxylate and tends to decrease from an initially high value in the case ot" zinc carboxylate. The authors concluded that substitution of labile chlorine atoms by ester groups of the metal carboxylate occurred: the substituted ester group is more stable than the initial chlorine atoms but may be liberated by reaction with HCI. These reactions are more rapid ~vith zinc than with cadmium.

These findings were substantiated by a number of studies using model compounds with allylic chlorine atoms such as 4-chloro-2-hexene. Thus Onozuka and Asahina 2 give a first explanation of the well-known synergistic effect between metal soaps based upon ionisation potential values of carboxylates. As a synergism mechanism they suggest that a metal soap having a low ionisation potential value, such as calcium or barium soaps, acts as an ester-exchanger of metal chlorides, such as zinc or calcium chlorides. This mechanism was also suggested by Anderson and McKenzie, 3 by Tran Van Hoang e t a l . "~ using model compounds, and also by Briggs and Wood 5 in experiments on degrading poly(vinylchloride) in the presence of barium and cadmium laurate and their synergistic mixtures or in the presence of calcium and zinc laurate and their synergistic mixtures. Evidence was also obtained by Tran Van Hoang e t a l . 6 during processing of PVC in mixers (Brabender plasticorder and rolling-mill) using a coulometric method to follow the formation of chloride ions. This exchange reaction between zinc chloride and calcium carboxylate restores the active zinc carboxylate and gives CaCI_, which does not have the same deleterious effect on PVC thermal stability as ZnCI,.7 So calcium carboxylates serve as a reserve to increase the lifetime of zinc carboxylates and to delay the polymer blackening which appears less suddenly as the Ca/Zn ratio increases. For long-term stabilisation, it is necessary to use a recipe with a Ca~Zn ratio larger than one, and Tran Van Hoang e t a l . "~ showed that with 4-chloro-2-hexene the substitution reaction of allylic chlorine atoms is favoured compared with the elimination of HC1 as this ratio is increased.

However, during processing of polyvinylchloride, a yellow colour appears when

PVC STABILISED WITH A Zn-Ca RECIPE 239

this ratio is larger than one, and zinc carboxylate reactivity decreases with respect to the substitution reaction (see Authors" Note at the end of the paper). An explanation of thisphenomenonwas first given by Onozuka andAsahinaZ who demonstrated com- plex formation between zinc and calcium carboxylate (2Zn, lCa). It was suggested that this complex prevents a rapid substitution of allylic chlorine atoms in PVC, but its exact role in the stabilisation mechanism has not been clearly demonstrated. In fact, Tran Van Hoang e t a l . ~ proved the existence of another complex between zinc and calcium chlorides with the same stoichi0metry (2Zn, lCa): this complex may contribute to the inhibition of the catalytic activity of ZnCI., with respect to the substitution reaction and thus favours the HCI elimination reaction which results in

2

I

Vohs

0 0.5 1.0 1.5 2.0

Fig. Polarogram of metal chlorides in dimethylsulphoxide (DMSO). 1. pure D M S O 2, DMSO + ZnCI_,: 3, DMSO + ZnC1, ~- CaCl_, (2Zn. ICa).

polyene sequences and polymer discoloration. Further evidence of the existence of this complex between the chlorides is given in Fig. 1 where the polarographic curves of ZnCI 2 and of a ZnCI2-CaCI _, mixture (2Zn, 1Ca) in solution in dimethylsul- phoxide are given. The wave at 0.5V corresponding to ZnCI_, ionisation is eliminated when CaCI 2 is added in the proportion of the complex. Another complex between IZnC12 molecule and 2 calcium carboxylate molecules was involved where ZnCI 2 catalytic activity is strongly reduced. "~

240 TRAN VAN HOANG, ALAIN MICHEL. ALAIN GUYOT

The simultaneous catalytic power of zinc chloride on the competitive substitution and elimination reaction from allylic carbocation structures is thought to be associated with the synergistic effect bet~een metal soaps of different ionisation potential value. The ester exchange reaction between metal chlorides which are strong Lewis acid, such as ZnCI_, or CdCI,, and calcium or barium soaps give a synergistic effect with respect to the time of polymer blackening. On the contrary. possible complexes between zinc chloride and calcium chloride or calcium soaps inhibit ZnCI 2 catalytic activity with respect to the substitution reaction and thus encourage the initial polymer discoloration.

In our laboratory, we have studied more thoroughly the catalytic action of zinc chloride in the various reactions involving both main and secondary stabilisers, such as phosphites, s epoxy compounds, 9 ~-phenylindole t° and fl-aminocrotonate esters tt using the calcium-zinc recipe. It was shown tz first that ZnCI z is a strong catalyst both for the elimination reaction giving HCI and hexadiene from the model compound for allylic chlorinated structures, 4-chloro-2-hexene, and in conjunction with HCI for diene polymerisation. These two reactions can be considered as models for the dehydrochlorination and crosslinking reactions through polyene sequences during thermal degradation of PVC.

The elimination mechanism involves an ion pair, ZnCI3 e and the allylic carbocation precursor. Cationic polymerisation of diene is initiated by the ZnCI3eH ~ ion pair from ZnCI z and HCI. Simultaneous coloration and elimination of HCI can be explained by a charge transfer complex between HCI (or ZnCI 2) and the diene in the case of the model compound, or the polyene sequence in the case of the polymer. Such a complex of polyene with metal chloride was previously suggested by Iida and Goto z3 to explain polymer coloration in the presence of stabilisers such as metal soaps.

In the presence of secondary stabilisers ZnCI 2 is a strong catalyst for the following reactions:

(a) esterification of allylic chlorine atoms by zinc carboxylates ~

2CH3---CH==CH---CH---C2H 5 + Zn(OCOR) 2

I CI

2CH3---CH==CH--C--C2H5 + ZnCI_, I

O t

O==C--R

(b) etherification of allylic chlorine atoms by epoxy compounds 9 already established

PVC STABILISED W I T H A Zn-Ca RECIPE 241

by Anderson and McKenzie 3

CH3---CH~---CH--CH--C2H 5 + C H 2 - - C H - - R ' ~ C H 3 - - C H = = C H - - C H - - C 2 H 5 I \ / r C1 O O

I CH_, I CHC1 I R'

(c) phosphonatation of allylic chlorine atoms by alkylphosphites s

C H 3 - - C H = C H - - C H - - C a H 5 + P(OR)3 --+ CH3--CH~---CH--CH--£72H 5 + RCI I l C1 P(OR),

,! O

(d) C-alkylation of atlylic chlorine atom by ~-phenylindole '°

H

3CH 3--CH-~-CH---CH--C2 H s + -+

C1 H

R R

+ 3HC1

R = (CH3--CH~---CH--CH--C:Hs)

(e) N- and further C-alkylation of/~-aminocrotonate esteI:s in the presence of allylic chlorinated structures t

[CH3--C:CH--C--O(CH2):] 2 + 2CH3--CH~CH--CH~:H 5 -+

I H 1 NH 2 O CI

[CH3--C~-----CH--C--O(CH_,),]2 + 2HCI

NH O

C H 3 - - C H = C H - - C H - - C 2 H 5

242 TRAN VAN HOANG, ALAIN MICHEL, ALAIN GUYOT

[CHa--C==CH--C--O(CH2)2]2 + 2CHz--CH~-----------~H---CH---C2H 5 I iL L

NH O CI I

C H 3 - - C H ~ H - - C H - - C 2 H 5

CH ~--CH==CH---CH--C ~ H 5 I

[CHs--C~-----C---C--O(CH2)2] 2 +" 2HCI

NH O l

CH 3--CH~---CH--CH---C2 H 5

For most of these reactions, kinetic studies show that the rate determining step is the carbocation formation above mentioned, followed by nucleophilic attack of the reactant (epoxy compounds, phosphites, carboxylates. . . ) . The first three reactions did not lead to HCI evolution but it is formed in the last two. Consequently. esterification, etherification and phosphonatation, i.e. O-alkylation reactions, are sensitive to an excess of HC1, which causes scission of C -O bonds and finally formation of a new allylic chlorine atom which participates in the competitive elimination reaction. Thus, these stabilisation reactions are not definitive and this characteristic accounts for the sensitivity of the zinc-calcium recipe to sudden blackening of the polymer when the stabiliser has been consumed.

In the esterification reaction, a large difference was observed between zinc and calcium carboxy[ates. ~ When both are studied in the presence of ZnCI z as catalyst. the esterification rate for zinc stearate is 16 times larger than for calcium stearate. Such a difference may be surprising since the rate determining step is not the substitution itself, but the carbocation formation, which is not dependent on stearate concentration. A first obvious explanation could be the exchange reaction. which should decrease the actual catalyst concentration, producing CaCI_,, which has no activity.

However. the study of the exchange reaction itself did show that it was incomplete and reversible. Even if the equilibrium is immediately established, it might account for reduction of the ZnCI, concentration by a factor of 5 only. So we prefer to conclude that the formation of a complex of ZnCI 2 and calcium stearate strongly reduced the catalytic activity of ZnC12.

The substitution reaction involving ~-phenylindole or fl-aminocrotonate esters should be noticed. In the latter case, the direct reaction between stabiliser and HCI leads to NH~CI which strongly inhibits the catalytic action of ZnC1, by complexing with it. In the case of epoxy compounds, a polymerisation reaction of epoxy through a ring opening cationic mechanism was demonstrated. 9 It is initiated by ZnCI 2 and a zinc-oxygen bond is formed together with a chlorinated chain end. Thus a part of

Pvc STABILISED WITH A Zn-Ca RECIPE 243

the ZnCI, is consumed. Similarly the formation of zinc phosphonate was suggested to account for the phosphite synergistic effect. 5 However, this reaction probably takes place only at high temperatures, and cannot be observed in the experiments with model compounds, which are limited to rather low temperatures (60-80°C). due to the low thermal stability of allylic chlorine models. These limitations point out the need for direct studies on the polymer under processing conditions. A next step in our laboratory was to develop new methods for studying stabiliser consumption. Semi-quantitative results were obtained on organic secondary stabilisers by means of liquid chromatography, t'~

Using this technique, with a double detector (a refractometer which is sensitive to both polymer and ~-phenylindole and a uv detector which is sensitive to z~- phenylindole only), it was possible to show that a part of this compound was grafted on to the polymer during processing at 180 °C on a rolling-mill (Fig. 2). The uv trace shows that :~-phenylindole is grafted from the beginning of heating (chromatogram a) and that a large proportion is grafted after 33 minutes. This result may be correlated with the main conclusion of the studies with the model compound, which showed ~-phenylindole can react with allylic chlorine atoms.

More interesting data were obtained using acidimetric titration of the carboxylic acids, and coulometric titration of metal chlorides formed from interaction of HCI

a-phenylindole ~I~ " Ii

._ j" ',, / ",..

w

--PVC

cc-phenylindole i

/

,/ ~'~

--PVC

Fig. 2. GPC chromatogram after PVC processing on a rolling-mill at 180°C (PVC RSS000, 100g; calcium stearate, 1-60 phr; zinc stearate, 1-66 phr: x-phenylindole, 1.02 phr). (a) Heating time at 180 °C,

3 minutes, (b) heating time at 180°C, 33 minutes. ( . . . . . uv traces, - - refractometer traces.)

2 4 4 TRAN VAN HOANG, ALAIN MICHEL, ALAIN GUYOT

and metal carboxylate. Coulometric titration was also used by Stepek et al. tS to study the barium-cadmium recipe. Our methods, initially developed to study stabiliser consumption during the various steps of industrial processing ~ such as extrusion to produce bottles or calendering to get sheets of rigid PVC, were applied to complete our study of the stabilisation mechanism. In the absence of organic stabilisers, the amount of carboxylic acid produced is larger than the amount of chloride. 6 A tentative explanation of this unexpected result involves the formation of intermediate monochlorides such as C1--Zn--OC OR and CI---Ca--OCOR, but a further study is necessary to confirm the existence of such chloro organo compounds.

In the presence of organic stabilisers the situation is different and the results are strongly dependent on the nature of the organic stabiliser. Typical results are shown in Fig. 3, in which the stearic acid and metal chloride liberated (expressed as a percentage of the carboxylate in the recipe) are related to the time of heating at 180 °C on a rolling mill.

The difference between acid and chloride titration is very large initially and remains so with 7-phenylindole which does not fix HCI formed in the substitution reaction as 3-aminocrotonate esters and epoxy compounds do. With epoxidised soya bean oil the difference between acid and chloride titration is also large initially, but decreases with heating time, because carboxylic acid reacts with epoxy compounds.

The accumulation of chloride is delayed in the presence of epoxidised soya bean oil compared with ~-phenylindole. This may be explained by a direct reaction

Metal chlorides formation

5tearic acid formation

"i,, $ /

I / / . /

50, / / : / / / " /

l I l t _ _

20 30 40 60 80

Fig. 3. Metal chloride ( - - , 1, 2, 3) and stearic acid (- - -, 4, 5, 6) formation on a rolling-mill during PVC processing at 180 °C. t and 4: basic recipe: PVC RS8000, 100 g: calcium stearate. 1-60 phr: zinc stearate, 1-66 phr. 2 and 5: basic recipe and x-phenylindole, 1.02 phr. 3 and 6: basic recipe and epoxidised soya

bean oil. 6.6 phr.

PVC STABILISED WITH A Z n = C a RECIPE 2 4 5

Metal chlorides

Conversion %

-1oo

A / A /

o ~-~'~o • o ~-50 /~_o o ' / •

1/~/ jA.A...-~ A / / ' / ' ~ Heat,ng time (rain)

. . l 1 J t I

0 20 40 60 80 100

Fig. 4. Zinc or calcium chlorides reaction with pure l-2-epoxy butane at 60°C. @. (CaCI:), 18mmoIesl-I: O, (CaClz). 36mmoles I-1; ~, (ZnClz), IVmmolesl-t: A, (ZnClz), 34mmoles1-1.

Chlorides (%)

75'

Heetin~ time (rain) [

10 20 30 40 5o

Fig. 5. Chlorides formation during PVC processing at 180°C on a rolling-mill, x, Basic r~ipe (PVC Rhone-Poulenc Polymeres RS8000, 100g; calcium stearate, 1.60 phr; zinc stearate, 1-66 piar). ~ , Basic recipe and :~-phenylindole, [.02phr, /M Basic recipe and epoxidised soya bean oil, 6-6phr). ©, Basic

recipe and sorbitol, 0-96 phr, O, Basic recipe and benzoylacetone. 0-86 phr.

246 TRAN VAN HOANG. ALAIN MICHEL. ALAIN GUYOT

between chloride and epoxy compounds. It was shown that the zinc chloride or calcium chloride concentration in solution in epoxidised soya bean oil quickly decreases in a few minutes on heating at 180°C and reacts completely. 6 Some additional results are presented in Fig. 4, which shows the conversion of zinc and calcium chlorides, determined from their coulometric titration, in 1.2-epoxy butane at 60 °C. The reaction with zinc chloride, initially slower, tends to be more complete

t h a n with CaC12, which rapidly levels off at moderate temperature. As already described in the case of zinc chloride, 9 a white precipitate is formed with CaCI, and its infra-red spectrum suggests the presence of Ca--Cl and Ca - -O bonds which may come from the opening of the oxirane ring. In the case of ZnCI 2 a viscous liquid is obtained and is due to cationic polymerisation of epoxy-butane. These secondary reactions of chlorides offer a possible explanation for synergistic effects of epoxy compounds used as secondary stabilisers because they delay the accumulation of zinc chloride, a catalyst of dehydrochlorination and responsible for sudden blackening. It is to be noted that metal chloride fixation to epoxy compounds is not decisive and may be liberated by HCI (Fig. 4).

CORRELATION BETWEEN METAL CHLORIDES FORMATION AND POLY(VINYLCHLORIDE)

DISCOLORATION

In this study, a basic recipe was prepared, using 1-60 g (2.64.10- 3 mole) of calcium stearate and 1-66 g (2.54.10 - 3 mole) of zinc stearate for 100 g of PVC; 5-18.10 - 3 mole of each organic secondary stabiliser were added (either epoxidised soya bean oil, :t-phenylindole, sorbitol or benzoylacetone): mixing was carried out on a rolling mill at 180°C and samples were taken at three-minute intervals. For each sample chloride coulometric titration was carried out: the results are expressed (Fig. 5) as the percentage of chloride liberated with respect to complete ,transformation of stearates into chlorides versus dine. Without secondary stabilisers, ~6 the yellow colour (see Authors" Note at the end of the paper) appears at about 10 °, D chloride yield and complete blackening is observed at about 50 % yield. :~-phenylindole retards initial discoloration and a pink colour appears at about 25 % yield, the blackening being observed later at about the same chloride yields as for the basic recipe (50 °o).

The pink colour is the same as was previously observed t° during interaction of :t- phenylindole with 4-chloro-2-hexene or HC1 in dichloroethane and due to a hydrogen chloride-:t-phenylindole complex. Because of the substitution reaction catalysed by zinc chloride, ~-phenylindole may prevent formation of polyene sequences, and their coloured complexes with metal chlorides,t3 at the beginning of heating. However, since the substitution reaction leads to HCI formation, retardation of chloride formation is not very efficient. :~-phenylindole is typically a short-term stabiliser especially useful to prevent initial discoloration during processing.

PVC STABILISED *~VITH A Zn-Ca RECIPE 247

The same conclusion is true for benzoylacetone, which is a model compound for a new secondary stabiliser. 16 It accelerates chloride formation, and shortens the blackening time, although its effect in preventing initial discoloration is good. Epoxy stabilisers give different results. Chloride formation is much slower (and even with a large amount of epoxy compound, an induction period may be observed 6) but discoloration (yellow, then orange) may be observed at tow chloride levels. This is due to the consumption of metal chlorides, and especially zinc chloride, by epoxy compounds and has previously been noted. 6"9 However, inhibition by metal chloride is not decisive because of interaction of HC1 with Zn-O or Ca-O bonds formed by epoxy ring opening, and consequently blackening is observed after 75 minutes when about 75 "o metal chloride is liberated. Epoxy compounds are long- term stabilisers.

Sorbitol delays formation of metal chlorides and consequently delays their prodegradant effect and especially the zinc chloride effect. Sorbitol, too, is a long- term stabiliser although it is less efficient than epoxy compounds. So, polyols play a direct part in the stabilisation mechanism and probably do more than act simply as metal chloride complexing agents as previously proposed by Briggs and Wood. 5 Attempts to react sorbitol with the model compound 4-chloro-2-hexene show no reaction at moderate temperatures even in the presence of zinc chloride. Further experiments are necessary for a better understanding of the rote played in PVC stabitisation by polyols and especially in the synergism mechanism in the presence of metal soaps.

In conclusion, the above results allow an explanation of the exact part played by the various components of a stabilising recipe. To avoid any discoloration, at least during processing and the beginning of the thermal ageing, it is necessary to catalyse the substitution reaction strongly, To do that, ZnCI, must be quickly produced, for instance by using a short-term organic stabiliser, ~vhose reaction with PVC leads to HCI formation: this might be complemented by complexing with ZnCl, ~vhich does not reduce its catalytic activity too much, but avoids the formation of strong chromophores with polyenes. However, the chloride concentration must be limited in order to avoid a sudden blackening and for that reason calcium carboxylate as well as organic long-term stabilisers are introduced.

This study also shows clearly that the secondary stabilisers must be carefully selected to obtain the best compromise between final coloration of the polymer after processing and sudden blackening during processing.

ACKNOWLEDGEMENTS

The authors wish to thank Dr M. Gay, from the Rhone Poulenc Company, Research and Development Department (Centre de Recherches des Carri~res, Saint- Fons, France) who made the sheets on a rolling-mill for colour card and chloride titration. We are also indebted to Dr A. Revillon for GPC experiments.

248 TRAN VAN HOANG, ALAIN MICHEL, ALAIN GUYOT

AUTHORS' NOTE

On direct application to Professor Guyot. colour cards are available which illustrate the colour changes described in the text.

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