The structure of the aged layer in polyvinylchloride photodegraded in the presence of oxygen

Polymer Science U.S.S.R. Vol. 25, ~No. 1, pp. 91-97, 1983 Printed in Poland

0032-3950/83 $10.00÷.00 ~ 1983 P e r g a m o n Press L t d .

THE STRUCTURE OF ~ AGED LAYER IN POLYVINYLCHLORIDE PHOTODEGRADED IN THE PRESENCE OF OXYGEN* N. M. BITYURIN, V . N . GENKIN, V. I). LI~BEDEV, K. V. NIKITIN, V. V. SOKOLOV, L. D. STRELKOVA and G. T. FEDOSEYEYA Institute of Applied Physics, U.S.S.R. Academy of Scienes

(Received 24 July 1981) The structttre of the surfi~celayer of photodegraded PVC has been studied. It has been shown that next to the surface there lies a region of accumulated photo-oxidation products which is characterized by a small change in the absorption coefficient and by the absence of crosslinks. The size of this region increases with decreasing irradiation intensity (at constant dosage). Next to the oxidized region lies another of accumulated polyencs ~hich is characterized by aa increase in abserption and crosslinking of the chains. A model which describ(:s t h,, structure of the layer observed experimentally has been proposed. THE photodegradation of PVC, as with the majority of other polymers, is closely connected with radical reaction processes [1]. Under conditions of oxygen deffi: ciency, the radical reactions in PVC are accompanied by dehydrocMorination. This process has been studied well, in the ease of thermal degradation [2]. I t is not. so with photodegradation where the processes of radical generation and possibly growth of polyenes and polyene radicals, are an absorption study. Dehydrochlorination occurs ~dtll formation of absorbing polyone structures, leading in the end to formation of an aged boundary layer, whoso space-time evolution has a wave nature. Reference [3] contains a general analysis of these types of phenomena and the connection of the layer st.m~cture with chemical reaction mechanisms. The possibility of applying this analysis to PVC is discussed in [4, 5]. We shall add t h a t the existanee of a boundary aged layer in PVC was shown by experiments [5, 6], carried out in air and therefore with the examination of growth of polydiene structures, attention1 should be paid to the competing radical oxidation processes [711 I t might be expeeted (and this will be shown below) t h a t oxygen defficiency will arise only in the depth of the sample, at a rat.lxer high light intensity, when the consumption of oxygen in the photo-oxidati(m reactions exceeds its supply, on account of diffusion. The molar absorption of the photo-oxidation products (peroxides, ketones) is several orders less than for polyene structures and this causes the appearance in * Vysokomol. soyed. A25: No. 1, 80-85, 1983.

91

92

N . M . BITYURIN et al.

t h e s a m p l e surface of a w e a k l y a b s o r b i n g l a y e r which shifts t h e high a b s o r p t i o n region, whose m e c h a n i s m o f f o r m a t i o n was discussed earlier [3]. T h e aim of t h e p r e s e n t w o r k was to s t u d y t h e s t r u c t u r e of t h e aged l a y e r a n d its relation to P V C d e g r a d a t i o n m e c h a n i s m s . We studied films, 210/~m thick, t)repared by the Valtsev-Kalandrov method at 453°K from grade C-55 PVC, stabilized with barium and cadmium stearates (up to 2 wt. %) and with octylepoxy stearate (3 wt. %). Irradiation was carried out in air with UV light of wavelength 254 nm (a BUV-30P lamp) at 3 intensities. The intensities were varied by changing the separation between films and lamp and were measured by a ferrioxalate aetinometer. The irradiation time was chosen so that the dose given to each film was the same and exceeded that necessary to obtain a boundary aged layer [5, 6]. Because of this, sample 1 was irradiated at an intensity of 1=1.58)<10 ~5 quanta/sec.cm 2 over 100 hr; sample 2 with I = 0.46 × 10x5quanta/sec •cm *over 400 hr; sample 3 with I = 0.22 × 10 ~5quanta/sec •cm ~ over 800 hr. The optical density of the thus-aged fihns (with successive removal from the irradiated surface of polymer layers, of the order of several ] umdredths of a millimetre thick) were measured on an SF-26 spectrophotometer. Moreover, the optical densities of the aged samples were reckoned from those of the original films which permitted a widening of the dynamic measurement range. The effect of scattering by sample surfaces was reduced by using paraffin oil as an immerson liquid. The content of b~soluble fraction was studied by successive scraping of the surface layer. Samples were dissolved in cyclohexanone at 50°; the residues were dried to constant weight. F i g u r e 1 shows t h e change in optical d e n s i t y of aged films w i t h successive r e m o v a l of t h e surface layers. I t can b e seen t h a t in t h e d i s t r i b u t i o n of a b s o r p t i o n coefficients a(x) according to s a m p l e thickness, d e t e r m i n e d f r o m t h e d e r i v a t i v e of optical density, dD/dx, t h r e e regions c a n be distinguished. T h e e x t e r i o r p a r t is c h a r a c t e r i z e d b y a r a t h e r smaller t h a n a v e r a g e change in t h e a b s o r p t i o n coefficient a(x), since t h e size o f t h e o u t e r section a p p r e c i a b l y d e p e n d s on t h e i n t e n s i t y of t h e fixed i r r a d i a t i o n dose. I n t h e end, a t a d e p t h o f >~70/~m, t h e curves are close t o g e t h e r a n d t h e a b s o r p t i o n coefficients a r e here a p p r o x i m a t e l y equal to t h e a b s o r p t i o n coefficient of t h e original film. This gives a basis for e x a m i n i n g a p h o t o d e g r a d e d s a m p l e s a a m u l t i l a y e r s t r u c t u r e (Fig. 2). I t is n o t e d t h a t t h e small size of t h e w e a k l y a b s o r b i n g region w i t h m a x i m u m i n t e n s i t y , in this case, allows one to neglect it in t h e analysis of t h e a b s o r p t i o n profile, as was done in [4, 5]. T h e c o n t e n t of insoluble f r a c t i o n was d e t e r m i n e d for all 3 t y p e s in a g e d s a m p l e s a n d a m o u n t e d t o 27~/o tbr s a m p l e 1, 14~o for s a m p l e 2 a n d 9 % for s a m p l e 3. I t is a p p a r e n t t h a t t h e q u a n t i t y of insoluble fraction is d e t e r m i n e d nov o n l y b y t h e i r r a d i a t i o n dose b u t depends on its intensity. T h e results of m e a s u r i n g t h e insoluble f r a c t i o n c o n t e n t b y r e m o v a l of surface layers are g i v e n in Fig. 3. I t can be seen t h a t in s a m p l e s 2 a n d 3, t h e r e l a t i v e insoluble f r a c t i o n c o n t e n t s initially are increased. A q u a n t i t a t i v e analysis indicates t h e absence of crosslinks in t h e first region. T h e location o f t h e m a x i m u m on t h e curves corresponds t o t h e s t a r t o f t h e crosslinked l a y e r a n d coincides w i t h

Structure of aged layer in polyvinylchloride

93

the inflection on the optical density curves (Fig. 1). The small thickness of the oxidized layer in sample I does not permit disclosure of a small content (or absence) of crosslinks in it. Thus the outer layer is characterized not only by a small change in the absorption coefficient but by the absence of crosslinks, whereas in the regions where strongly absorbing polyenes are formed," spatially crosslinked material results. The thickness of the oxidised part grows with decreasing intensity which leads ¢o the observed decrease in amount, of insoluble fraction. // q

4 1

i _

I I I I lI

IOOx,,urn

l 2 50

0

l

FIG. 1

xo

~"

FIG. 2

FIG. 1. Dependence of optical density D of photodegTaded PVC films on thickness of a skimmed off surface layer x at different irradiation intensities I × 1015 quanta/ /sec.cm~; 1--1.58; 2--048; 3--0.22. FIe. 2. Structure of aged layer: 1 --region of accumulated photo-oxidation products, 2--localized polyene and crosslinked region, 3--practically non-aged polymer. We will examine a simplified reaction scheme for PVC photo-oxidation, assuming t h a t it is based on a photo-oxidative initiating system [8, 9] S hv)R"

initiation

R'+02 k~ R00" chain propagation

R00"-i-RH

k,) R00H~-R"

ROOH hv K + 2 R ' + H , O ~0

degenerative chain branching

ROO'WROO"-----~ inactive products--secondary chain breaking

(1)

N. ~[. BITYURIN et al.

94

(S is the initiating process, K is ketone). Secondary breakdown of R" radicals one after another or breaking by W-ROO" transfer were not studied hero. We shall discuss this below. The system of kinetic equations, describing this process may be represented in the form: dR1 dt --=tl~I-- t:~ R1C 2V k2 R= P -J- 2 ~ tlp H I

dR2 dt

__

kl R l c - - k 2 R 2 P - - k o R~

(2)

dH

dt

~c

Px, %

~2c

/

i 20

°

1 o

2

10 °

3

I

50

100

/

X , t.Z/; 7

0.5 ~o.

3

1.0 b 2

Fro. 4

Fro. Change of relative content of crosslinked fraction in the polymer Px with successive layer removed in samples 1 (1), 2 (2) and 3 (3). FIG. 4. Solution of equation (8) as a function of the parameter b.

The first equation describes the concentration R1 of R" radicals, which is formed by the absorption of light of intensity 1 (x); ~.(x) is the absorption coefficient, ~ is the quantum yield of the process forming R radicals. The primary radicals are broken up in radical photo-oxidation (klRlC), the rate of which is determined by oxygen concentration c and are regenerated in the chain propagation reactions and in hydroperoxide tt decomposition. The peroxide Rz radical concentrations which thereby arise, are decomposed in cleavage reactions (koR~) and in chain propagation (k2R2P), whilst hydrogen is split off from polymer P forming hydroperoxides and regenerating R" radicals. The concentration of hydroperoxide H i~ broken down by the action of light, with a quantum yield of t/o (ev is the molar

Structure of aged layer in polyvinylchloride

95

absorption coefficient). Finally, the last equation represents the oxygen balance, whoso concentration is determined by its consumption in radical oxidation ~2c reactions and its intake on account of diffusion D c ~ 2 (where D is the oxygen diffusion coefficient). The analysis of system (2) will be carried out, assuming quasistationary reagent concentrations and t h a t at/= eonst., i.e. the concentration of centros, responsible for radical regeneration and their properties, is little changed. From the second system (2) equation we find klRlC k~P+koR~ Substituting tiffs expression in the first equation, we get /

3kzP

1)

=0

Further examination leads to the suggestion t h a t the first term in the brackets is m u c h less t h a n 1 k0R~ >> 2k~P

(4)

I t m a y similarly be assumed t h a t the secondary cleavage of peroxy radi(-als is dominant over chain propagation and chain branching. Using expressions (4), (3) and (2) we find: R1----- at/__II klC

/

(5)

a~I

Substituting R2 from equation (5) into expression (4), we get another form of the, inequality (4) atlI

4k P

>> - k0

(6}

Thus t o satisfy conditions (4) or (6), a sufficiently high intensity is necessary. I t is permissible to neglect the R" radical breakdown processes if ]clR1 C :

at/f<<--k~c* k0

(7)

The inequalities (4) and (7) are compatible only for a fairly high oxygen concentration. Substituting expression (4) in the last equation of system (2) and p u t t i n g I = I o e - ~ we obtain an equation for c (x) (x is reckoned from the sample boundary)

96

N.M. BITYURr~ etal. Dc ~ x2 : f f a l o e-~z

Using the boundary conditions, cl~=o=c 0 and ~c/ax=O with c=O, we obtain a transcendental equation for determining the separation l, where the oxygen concentration is reduced to nil

b2=l--(al+l)e l z ,

(8)

and the parameter b

~

.

.

.

.

.

.

.

.

This equation was solved numerically and its curve is given in Fig. 4. I t is apparent that equation (8) has a solution with b ~<1. Thus there are two regions at high intensities, in one of which photo-oxidation occurs and in the other, photoageing in the absence of oxygen. Between these regions, naturally, there is a transfer layer, whereby virtue of a small oxygen concentration, the inequality (7) is not satisfied and needs a more complex analysis. However, because of large kl values, this transfer layer is relatively small. With b= 1, the diffusional length for oxygen ~/Dc co/tlalo is equal to the characteristic penetration depth of the light in the film a -1 and therefore, oxygen penetrates the entire volume of the aged layer. We will compare the results obtained with that of the experiments. The magnitude of Deco represents the gas permeability coefficient for PVC b y oxygen and equals 3 × 100 cm -1 sec -1 [10]. Then for a quantum yield o f ~ = ]0 -2 for primary radical formation, at 3 values of the intensity of sample illumination, we get the following depths for the oxidized layer: l ~ : 18/xm,/2~35/~m and/3~55/~m. The graphs in Fig. 1 give l~ ~ 16/xm, l~.~-35/zm, and 13~ 50/zm which agree well with the theoretical results. It is noted that with Jl z 10-2 and the experimental parameters, inequality (8) does not contradict the value of the ratio k~/k0, quoted in [8]. It is possible to estimate, from the condition b ~ 1 what intensity well cause oxidation of the entire sample volume. This estimate is I 0 ~ 4 . 5 × l0 n quanta/soc-cm 2. It follows from the results given above that the size of the spatially crosslinked region in polymers under photodegradatioa and the localization of this region in the material depends considerably on irradiation intensity for a similar dose and spectrum. These conclusions are important for comparing the results of sample ageing exhibited under different conditions (e.g. in different climatic regions) and for comparison of accelerated experiments with natural ones. For a correlation of the results of natural and accelerated ageing, increasing the rate of radical formation, it is necessary to increase correspondingly the rate of other cocurrent reactions, in particular oxidation, as examined hero.

Translated by C. W. CAPP

Struchlre of aged layer in potyvinylehloride

97

R£FER£NC~

I. B. RKNB¥ a n d Ya. RARE][[ Fotodestrukstia, fotookis].enie, fotostabilizastia polimerov (Photodegradation, Photo-oxidation and Photostabilization of Polymers). Mir, ] 19, 1978 2. K. S. MINSKER and "G. T. FEDOSEYEVA. Destruktsia i stabilizatsia polivinilkhlorida (Degradation and Stabilization of PVC). Khimiya, 38, 1979 3. N. M. BITURIN, V. N. GENKIN a n d V. V. SOKOLOV, Vysokomol. soyed. A24: 748, 1982 (Translated in Polymer Sei. U.S.S.R. 24: 4, 832, 1982) 4. N. M. BITURIN, V. N. GENKIN and V. V. SOKOLOV, Vysokomol. soyed. B23: 221, 198] (Not translated in P o l y m e r Sei. U.S.S.R.) 5. N. M. BITURIN, V. N. GENKIN, V. P. LEBEDEV, V. V. SOKOLOV, L. D. STRELKOVA and G. T. FEDOSEYEVA, Vysokomol. soyed. B24: 101, 1982 (Not translated in Polymer Sei. U.S.S.R.) 6. L. D. STRELKOVA, G. T. FEDOSEYEVA and ]K. S. MINSKER, Vysokomol. soyed. A18: 2064, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 9, 2360, 1976) 7. F. MORY, M. KOYAMA and Y. OKI, Angew. Makromolek. Chemie 64: 89, 1977 8. O. N. K A R P U K H I N and Ire. M. SLOBODETSKAYA, Uspeki Khimii 42: 391, 1973 9. V. Ya. SHLYAPINTOKH, Potokhimicheskie prevrashchenia i stabilizatsiya polimerov (Photochemical Transformations and Stabilization of Polymers). p. 66, K h i m i y a , ]979 10. S. A. REITLINGER Pronitsaemost' polimernikh materialov (Permeability of Polymeric Materials). p. 66, Khimiya, 1974