Makromol. Chem., Rapid Commun. 1,389 - 396 (1980) 3 89

Light-induced Degradation of Chlorinated Poly(viny1 chloride) in Solution

Christian Decker, Michel Balandier

Laboratoire de Photochimie Gknkrale, Equipe de Recherche Associte au CNRS, Ecole Nationale Supkrieure de Chimie, 68 093 Mulhouse Cedex, France

(Date of receipt: April 28, 1980)

Introduction

Poly(viny1 chloride) (PVC) is known to be a thermally unstable material, undergoing easily at elevated temperatures a “zip” dehydrochlorination process which is responsible for the conjugated double bond structures and for the discolouration of the degraded resin. One of the possible ways to palliate this major deficiency is to introduce additional chlorine atoms into the polymer backbone. This treatment imparts superior high temperature properties and also improves the toughness and impact resistance of the chlorinated material.

While the thermal degradation of chlorinated PVC (CPVC) has been thoroughly investigated - 3) , very little information is available on the influence of light on this polymer. The objectives of this work were thus to evaluate the light-sensitivity of CPVC and to gain a better understanding of the different processes involved in its photo-oxidation. Alike PVC, chlorinated PVC undergoes upon UV irradiation a fast dehydrochlorination leading to polyene sequences. Besides, other competitive degradation reactions occur, mainly chain scissions and crosslinkings, their relative importance depending on the concentration of oxygen in the material.

We report here the first determination of the quantum yields of the different processes which develop in chlorinated PVC photolysed in tetrahydrofuran (THF) or in 1,2-dichloroethane (DCE) and compare their values to those obtained previously4) for PVC irradiated under similar conditions. A succeeding paper will extend this study to the photodegradation of films of chlorinated PVC.

Experimental Part

Materials: The polymer used was a commercial material (CPVC RB 8065 from RhBne- Poulenc) prepared by photochlorination of bulk PVC, with number and weight average molecular weights of 34 OOO and 67 400, respectively. The chlorine content of the polymer was measured to be 64,8% by using the Stoeckhert’s methods) modified recently by Quenum et a1.6). At this degree of chlorination, the polymer consists mostly of CHC1-CH, and CHC1-CHCI units, roughly in the one to one proportion and of only a few CC1,-CH2 structures, as indicated by 13C NMR analysis.

THF (Fluka puriss) was purified, just before use, by refluxing 3 h over KOH under nitrogen and two subsequent distillations. DCE (Fluka puriss) was used without further purification.

390 Ch. Decker, M. Balandier

Irradiation: THF or DCE solutions of CPVC (1 5 g/l) were exposed in a quartz cell to the UV light of a high pressure mercury lamp (Philips HPK, 125 W) under a pressure of 1,2. lo5 Pa of pure nitrogen or oxygen. The light was filtered by a solution (35 weight-%) of CoS04/NiS04 (2: 1) in water in order to isolate the 250 - 350 nm wavelength region. The potassium ferrioxalate actinometer') was used to determine the incident light intensity ( = 2 . 10- * Einsteids . cm2)*). To evaluate the number of photons absorbed by the polymer, the actinometer was placed just behind the cell and the percentage of light absorbed was calculated by taking the difference of the light intensities determined with the cell containing the solvent and the solution, respectively. The fraction of incident light absorbed by the polymer increased continuously with irradiation time (Fig. 1) as a result of an efficient dehydrochlorination process, leading to the formation of polyene sequences; this increase is slightly more pronounced in the presence of pure oxygen than in a nitrogen atmosphere. The amounts of hydrogen chloride, evolved by this reaction, were measured by gravimetric titration, as described previously4).

Change in molecular weight

Crosslinking: In the absence of oxygen, the viscosity of the CPVC solution increases with irradiation time leading to the formation of an insoluble gel. Determination of the sol fraction S for various absorbed doses allows the quantum yield of crosslinking to be evaluated from the slope of the straight line obtained by plotting S + S ' / 2 against reciprocal dose8).

Chain scission: In the presence of oxygen, the viscosity of the CPVC solution drops rapidly with irradiation time, indicating the occurrence of a predominating chain scission process. The average number of chain scission N per initial macromolecule was calculated from the relation: N = ( [q0]/[q]) ' /" - 1 where [qO] and [q] are the intrinsic viscosities before and after degrada- tion, respectively, and a the exponent of the Mark-Houwink relation. Values of [qO] and [q] were determined by extrapolating qSp /c to zero concentration. Since the Mark-Houwink relation was unknown for CPVC in THF or DCE solutions, at a first approximation, we used for the exponent a the value relative to PVC: a was taken as 0,77 for PVC in THF9) and was measured to be 0,66 for PVC in DCE**).

Gel permeation chromatography analyses of the photo-oxidized solutions show a slight broadening of the molecular weight distribution towards the higher molecular weights, thus indicating that, besides the chain scission process which predominates, some intermolecular crosslinks are formed. The values of N calculated from the above equation will thus correspond to net chain scissions and constitute the lower limits of the true amount of main chain scissions.

Photolysis of CPVC in the Absence of Oxygen

Dehydrochlorination

Large amounts of hydrogen chloride are evolved when DCE or THF solutions of chlorinated PVC are exposed to UV light in a nitrogen atmosphere. The conjugated

*) 1 Einstein = 1 mol of photons; energy = L . h * v = 6,02. 6,63 . v (in J); here

**) A value of 0,88 for a has been obtained previously4) by using PVC solutions, filtered on 0,2 wm pore size membrane filters, which apparently retained the high molecular weight fractions. The correct Mark-Houwink relation for PVC in DCE is [q ] = 5,56. 10- 2. M$66 m1.g-l.

v = i , i 8 - loi5 s - I .

Light-induced Degradation of Chlorinated Poly(viny1 chloride) in Solution 391

unsaturations formed in this dehydrochlorination process are responsible for the increasingly growing number of photons absorbed by the irradiated solution (Fig. 1).

Fig. 1 . Fraction of incident light absorbed by CPVC as a function of irradiation time

Fig. 2 shows how the UV absorption moves towards longer wavelengths as polyene sequences of increasing lengths are building up. The quantum yield of dehydro- chlorination, QHCI, was calculated from the slope of the straight line obtained by plotting the amount of HCl evolved as a function of the number of photons absorbed by the polymer (Fig. 3). In THF solution, QHCI remains constant at 0,4 throughout the photolysis, whereas in DCE solution the value of QHCI, initially constant at =0,6, drops to 0,05 after 3 h of irradiation. Dehydrochlonnation of CPVC appears thus to proceed more efficiently than in PVC, for which QHcI was 0,14 in THF solution and 0,OS in DCE4).

Crosslinking and chain scission

The intrinsic viscosity of THF solutions of CPVC, photolysed in the absence of oxygen, increases steadily with irradiation time up to 5 h, when an insoluble gel appears (Fig. 4). By measuring the soluble fraction as a function of the energy absorbed, the quantum yield of crosslinking, QCL, can be calculated from Eq. ( l ) * ) .

(1)

where S is the soluble fraction after absorption of a dose r in Einstein per gram of polymer, G$ is the initial average weight average molecular weight, po is the number of main chain fractures per monomer unit radiation dose and qo is the proportion of monomeric units crosslinked per unit radiation dose. As expected, a plot of S + 1/s against the reciprocal dose yields a straight line (Fig. 5 ) ; QcL appears to be substantially higher in THF (5,3 The non-zero

1 s + p ' 3 - + - Po 40 @ c L G $ r

than in DCE solution (2 .

392

0.8

0.6

CI U C 0 -4

v) -4

0.4

T

0.2

0

Ch. Decker, M. Balandier

h l n m

Fig. 2. UV absorption spectra of CPVC in THF. (. . . . .): Unirradiated pure THF; (0) (-): unirradiated solution; 1, 2, 4, 6, 15: solution irradiated during 1, 2, 4, 6, and 15 h in the presence of nitrogen (-) or of oxygen (- - - )

extrapolation at infinite dose indicates that crosslinking is accompanied by a competitive chain scission process. The quantum yield of chain scission, Gcs, can be deduced from the relation (Eq. (2)):

@cs 40 2@CL Po -

and was estimated to be 5,s * 1 0 - 3 in DCE and 1,8. in THF solution. It thus appears that, even in the absence of oxygen, degradation of chlorinated PVC by scission of the main chain proceeds effectively on UV irradiation. This process develops more extensively than in PVC solutions irradiated in a nitrogen atmosphere where Gcs = 2 . 4). Similarly crosslinking takes place more efficiently in CPVC than in PVC where no insoluble gel was observed even for prolonged irradiation of nitrogen saturated solution of PVC4).

An interesting feature was noticed for DCE solutions of PVC photolysed in a nitrogen atmosphere. The intrinsic viscosity of the solution dropped slightly during

Light-induced Degradation of Chlorinated Poly(viny1 chloride) in Solution 393

Einstein per g of CPVC

Fig. 3. Amount of HCI evolved from the polymer as a function of the number of photons absorbed by the polymer in photolysis of THF (-) or DCE ( - - - ) solutions of CPVC in nitrogen (0) and in oxygen ( 0 )

Fig. 4. Intrinsic viscosity of THF (-) or DCE ( - - - ) solutions of CPVC as a function of time of photolysis in nitrogen. (0): 7,s g/l; ( 0 ) : 15 g/l

T i m e in h

the first 2 h of irradiation and increased sharply by further exposure to light (Fig. 4). This behaviour can be attributed to an intramolecular crosslinking process which, by reducing the hydrodynamic volume of the polymer coil, would lead to a decrease of

394

s + 6 - 2 -

1.5-

1

Ch. Decker, M. Balandier

'

-0-4

o/.-o OOO*A

00° 04

00'

0 / A ,/ Fig. 5 . Plot of

,/ s + S ' l 2 against reciprocal dose in the photolysis of THF (-) or DCE ( - - - ) solutions of CPVC in nitrogen

[q ] . This effect is maximum for the first bridges formed, the shrinking of the coil being less pronounced for highly crosslinked macromolecules. Consequently, intramolecular crosslinking is expected to compete successfully at the beginning of the photolysis leading to the observed initial drop in viscosity, whereas intermolecular crosslinking will prevail at the later stages of the irradiation. By changing the polymer concentration it is possible to influence the competition between these two crosslink- ing processes. As shown in Fig. 4, a decrease of CPVC concentration from 15 to 7,5 g/l which favors intramolecular reactions leads to a more pronounced initial drop of [q], in agreement with our explanation. As expected, this intramolecular crosslinking is enhanced in a poor solvent like DCE, while in a good solvent like THF the inter- molecular crosslinking prevails already from the very beginning of the photolysis (Fig. 4).

Photooxidation of CPVC

Dehydrochlorination

Our kinetic results indicate that, as for PVC, oxygen enhances the dehydro- chlorination of chlorinated PVC photolysed in THF or DCE (Fig. 3). The shift of the UV absorption towards longer wavelengths, which results from the production of polyenes, is slightly more pronounced in the photolysis of CPVC in the presence of oxygen than of nitrogen (Fig. l), while larger amounts of HCl are evolved during the photo-oxidation. The quantum yield of HCl formation remains constant at 0,45 in THF, compared to 0,4 in nitrogen, whereas in DCE QHc, drops from 0,7 initially to 0,l l after 3 h of photolysis. These values of QHc, are not depending on the light- intensity in the range investigated (lo-* to lo-' Einstein. s- ' * cm-2), thus indicating that HCl is formed by a reaction that is first-order in rate of initiation.

Light-induced Degradation of Chlorinated Poly(viny1 chloride) in Solution 395

The fact that dehydrochlorination proceeds with a higher quantum efficiency when CPVC is photolysed in the presence of oxygen was unexpected, since 0, is known to be very reactive towards polyenyl radicals and polyenes; this bleaching reaction should thus reduce the extent of the chain dehydrochlorination process. We then assume that the net increase of cPH,-, observed in the presence of oxygen is the result of two opposite effects: the scavenging by 0, of the growing polyenes is apparently more than offset by the additional formation of HCI by a degradation process, still to be defined, which would develop only in the photo-oxidation of CPVC.

Chain scission

The sharp drop in viscosity of CPVC solutions irradiated in the presence of oxygen reveals that an efficient chain scission process is taking place. In contrast with the photolysis in an inert atmosphere, no insoluble gel was detected, even with prolonged exposure time, thus indicating that chain scissions always predominate over crosslinking. From plots of Nagainst the absorbed dose (Fig. 6), the quantum yield of

2,

Fig. 6. function of the number of photons absorbed by the polymer in the photolysis of THF (-) or DCE ( - - - ) solutions of CPVC in oxygen

Chain scission formation as a

10' Emstem per macromolecule I

net chain scission was calculated to be 8 * in DCE, values which are comparable to those measured in the photo-oxidation of PVC4). It should be noticed that, if some intermolecular peroxide bridges are formed, like in oxidized PVC, the true value of @scission will be even higher. Furthermore, an additional factor of inaccuracy lies in the fact that we assumed the Mark-Houwink exponent a to be the same as in PVC. The influence of a on the value of !Dscission can yet be estimated: by taking a in the range 0,6 - 0,7 for DCE solutions, instead of 0,66 like in PVC, @scission will fluctuate between 2,7 and 2,2 . respectively; similarly in THF if 0,7 < a < 0,85 instead of 0,77, we obtain 1 .

All the results obtained in this study clearly indicate that, if chlorination permits to improve the thermal stability of PVC, it has not, unfortunately, a similar positive effect on the photostability of this polymer. Chlorinated PVC appears indeed to be

in THF solutions and 2,5 .

> @scission > 6 .

396 Ch. Decker, M. Balandier

more sensitive to light as PVC, a threefold increase being observed in the quantum efficiency of the dehydrochlorination process. This work is now being extended to the photodegradation and photooxidation of CPVC in films.

The authors wish to thank RhSne-Poulenc Industries for supporting this work.

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