The photodegradation of poly(vinyl chloride) films—II

European Polymer Journal, 1972, Vol. 8, pp. 1223-1230. Pergamon Press. Printed in England.

THE PHOTODEGRADATION OF POLY(VINYL CHLORIDE) FILMS--II W. H. GIBB and J. R. MACCALLUM Department of Chemistry, University of St. Andrews, St. Andrews, Fife, KY16 9ST, Scotland

(Received20 April 1972) Abstract--Cast poly(vinyl chloride) films have been irradiated by ultra-violet light at various intensities and temperatures under a nitrogen atmosphere. The dehydrochlorination reaction is shown to occur in two parts. During the first hour the reaction is dependent on intensity and temperature but later it is independent of these two parameters.

INTRODUCTION FURTHER investigations i n t o the p h o t o d e g r a d a t i o n o f t h i n films o f poly(vinyl chloride), PVC, i n a n a t m o s p h e r e o f n i t r o g e n have b e e n carried out a n d are reported here.

EXPERIMENTAL The preparation of films described previously (t) was slightly modified. Films were placed in a vacuum oven and dried for at least 60 days at 313 K. Treatment with methanol was found to have no beneficial effect and was therefore discontinued. The extent of degradation was followed using a Metrohm Combititrator 3D. The pH of the trapping solution was held constant at 7.5, and thus the errors involved in following pH are minimized and a degradation curve is directly obtained. The effect of varying the flow rate of nitrogen was also checked. It was found that above 50 cm 3 rain- t and up to at least 130 crn3 rain- 1 the flow rate made no difference to the measured results. All runs were conducted with a flow rate of 70-80 cm 3 rain- 1. Intensity measurements were made using potassium ferrioxalate actinometry. ¢2) The extended runs were conducted using a slightly lower intensity arc.

RESULTS (a) Temperature variation Samples, cut f r o m the same film, were irradiated in n i t r o g e n for 7 h r at temperatures r a n g i n g f r o m 273 to 333 K. The light intensity was c o n s t a n t at 3 . 6 x 1016 q u a n t a c m - 2 sec-1 d u r i n g these experiments. The results o f two sets o f experiments, carried out using two castings o f the same p o l y m e r sample (including the rate, AW/At, o f d e h y d r o c h / o r i n a t i o n ) are s h o w n in Tables 1 a n d 2; A W denotes the weight o f HCI evolved f r o m the sample. It c a n be seen that the data f r o m the two series are in fair agreement. 1223

0"171 0-297 0.407 0.475 0"521 0-557 0.592 0.617

0"5 1"0 2"0 3"0 4"0 5"0 6"0 7-0

AW

(mg)

0-132 0.244 0'359 0"414 0.467 0'501 0"524 0.544

t

(hr)

0"5 1-0 2-0 3"0 4-0 5"0 6"0 7"0

-0"244 0.115 0"055 0.053 0.034 0.023 0.020

(mg h r - 1)

-0.297 0.110 0.068 0.046 0'036 0.035 0.025

-0"313 0"116 0.063 0"043 0"033 0"028 0.022

(mg hr-0

(mg) 0"191 0.313 0.429 0"492 0"535 0.568 0.596 0"618

AW/At

T=299K

( m g h r - t)

(rag)

-0.330 0"116 0.062 0-043 0.036 0.027 0.022

( m g h r -1)

(mg) 0-214 0"330 0"456 0"519 0.562 0.596 0'625 0.647

AW]At

AW

-0.360 0.128 0.065 0.047 0-034 0-027 0.024

(mg hr-0

(mg) 0-225 0.360 0"488 0"553 0.600 0"634 0"661 0.685

AW/At

0'238 0"373 0"495 0.566 0-613 0'650 0.681 0.706

(mg)

-0"373 0.122 0.071 0"057 0.037 0"031 0"025

( m g h r - t)

T = 328 K AW AW[At

-0"359 0.130 0.071 0"052 0"037 0-032 0"027

( m g h r - 1)

AW]At

T = 333 K

0"217 0"359 0.489 0.560 0"612 0"649 0.681 0.708

(rag)

AW

AW

T=318K

-0.327 0"125 0-071 0.052 0-037 0"032 0-027

AW/At

T = 323 K

AW

0.196 0.327 0"452 0"523 0"575 0.612 0.644 0.671

T = 308 K

TABLE 2

-0"295 0"128 0.072 0.051 0.043 0'034 0'028

(mg h r - t )

(mg) 0.162 0"295 0"423 0.495 0.546 0.589 0"623 0.651

AW/At

T = 313K

AW

AW

(rag h r - 1)

AW/At

T = 273 K

(mg)

(hr)

AW/At

T = 303 K

AW

t

TABLE I

t~

.m

The Photodegradation of Poly(Vinyl chloride) Films--II

1225

The weight losses for fixed periods of the photodegradation are shown in Tables 3 and 4. F o r example, AW~_7 signifies the weight loss between 1 and 7 hr irradiation. TABLE 3

T (K)

A WI-7 (rag)

A W~_7 (nag)

A W3_~ (nag)

303 313 323 333

0-320 0"356 0.344 0"349

0"210 0.228 0.219 0.219

0.142 0'156 0-148 0.148

TABLE 4

T OQ

A W1_7 (rag)

A W,_7 (nag)

A Wa-7 (rag)

273 299 308 318 323

0"300 0.305 0"317 0.325 0"333

0"185 0-189 0.191 0.197 0"211

0"130 0-126 0.128 0.132 0.140

The activation energy o f the reaction can be calculated by the following method. The rate o f evolution o f hydrogen chloride (HC1) can be written, d(HC1)o/dt = --kf(HC1)o where (HC1)o is the bound HCI available for photochemical reaction, t is time and k is the rate constant for the degradation reaction. It follows that,

f [1/f(HCl)o] d(HC1)a

=

f --k dt

f ' ( A r l 0 ----- - - k t + c.

(1)

S i n c e f ' (AW) = 0 when t = 0, c = 0. The temperature dependence o f k is given by the Arrhenius equation, k = Ae -r'/Rr. (2) Substitution o f Eqn. (2) in Eqn. (1) yields: f ' ( A W ) = tAe -~/Rr.

(3)

Taking a constant value for AW and hence f o r f ' ( A W ) , Eqn. (3) can be written,

K = t'Ae -r'/Rr,

(4)

where K is the constant value o f f ' ( A W) and t' is the time taken to reach that value. E.Pd. 8111~B

1226

W . H . GIBB and J. R.. M A c C A L L U M

Thus, log K = --log t' -- log A -k E/RT

(5)

log t' = E/RT -- log AK.

Hence for a series of runs and by taking a constant weight loss, a plot o f log t' vs 1/T should yield a straight line with a slope E/R. Two plots, made at AW = 0.45 nag, using the data from Tables 1 and 2 are shown in Fig. 1. Several values of AW were taken and aU yielded about the same activation energy. The results from Table 1 give an activation energy o f 12.6 kJ m o l - 1, and those 2"25

0

2"20

2"15

o 2.1G

2.05

2.00 3.0

t

3.1 :3-2 I/femperoture xlO3K

FIG. 1. A plot of log t'

vs

!

:3.3

lIT to determine activation energy.

from Table 2 a value of 15.4 kJ mol-1. Because temperature variation has such a small effect on this reaction, it is difficult to obtain an accurate activation energy. However, it can be deduced from these results that the activation energy is of the order 14 kJ mol-1. (b) Intensity variation Samples, again taken from the same film, were irradiated for 7 hr at different light intensities but at a constant temperature, 293 K. The lamp intensity, I o, was varied by altering the arc to sample distance. The results of a set o f experiments, carried out using one casting of the polymer sample (including the rate, --AW/At, of dehydrochlorination) are shown in Table 5. Table 6 shows the AWI_~, AW2_~ and AW3_~ values.

0"177 0-283 0"385 0"435 0"473 0-499 0"519 0"538

0-5 1"0 2"0 3"0 4-0 5"0 6"0 7"0

-0'283 0"102 0'050 0"030 0'026 0"020 0'019

A W/At (rag h r - l)

2 . 8 3 x 10 t6

AW (rng)

Time (hr)

lo(quantacm -2 see-l):

-0-195 0"104 0"050 0-036 0-029 0'020 0"020

A W/At (rag h r - 1)

1 . 2 4 x I 0 t6

0.084 0'199 0"299 0-349 0"385 0"414 0'434 0"454

AW (rag)

TABLE 5

0'028 0"110 0'220 0"272 0'306 0-332 0'351 0"370

-0"110 0'110 0"052 0"034 0'026 0"019 0'019

A W/At (rag h r - 1)

0 - 7 1 X 1016

A I,V (rag)

0.008 0"048 0-149 0"203 0"238 0-266 0'287 0"305

-0-048 0"101 0"052 0"035 0"028 0"021 0"018

0'43 × 10 ~6 AW A W/At (rag) (rag h r - i)

F-,

,...

< ~' o ::r ~.

0

o

~"

o 0e.,

1228

W . H . G I B B a n d J. R. M A c C A L L U M

TABLE 6 Io ( q u a n t a c m - 2 sec - t ) 2"83 1-24 0-71 0"43

× x × ×

101~ 1016 1016 1016

AWl_7 (rag)

AWz_7 (rag)

AW3_, (rag)

0"255 0"259 0"260 0"257

0"153 0"155 0"150 0"156

0"103 0"105 0"098 0"102

°° F ~ 0.4

D

g

•

.~ 0.3

~ 0.2

0-1

]

0

I0 Time,

I

|

20

50

hr

FIG. 2. T h e evolution o f H C l for films irradiated for 30 h r at varying intensities. A. Intensity = 2.83 × 10 le q u a n t a cra - 2 sec -1. B. Intensity = 1.24. C. Intensity = 0-71. D. Intensity ---- 0.43. o

4-

A

B

o~

225

I 250

I 275 Wavelength,

I 300

I 325

550

nm

FXG. 3. Ultra-violet absorption spectra o f films irradiated f o r 1 hr at varying intensities. A. Intensity = 2.83 x 10 le quanta cm -2 sec -1. B. Intensity = 1-24. C. Intensity ---- 0.71. D. Intensity = 0.43. E. U n i r r a d i a t e d film.

The Photodegradation of Poly(Vinyl chloride) Films--II

1229

Another set o f experiments was conducted under the same conditions but extending the irradiation time up to 30 hr. The dehydrochlorination curves obtained are shown in Fig. 2. Figure 3 shows absorption spectra o f films irradiated for 1 hr at different intensities. DISCUSSION It can be concluded that there is an increase in the rate of dehydrochlorination with increasing temperature and that the reaction has an activity energy o f 14 kJ tool-1. This value agrees well with Reinisch and Gloria's value of 18 kJ tool- 1, and both agree with their theoretical value, t3) The increase is most marked during the first hour and thereafter the rate becomes almost independent of temperature. The run at 313 K shown in Table I is anomalous. Individual A W/At values appear to indicate that the rate is constant after 1 hr, however, a close inspection o f the figures A W~_7, A W2_7 and A Wa-7 shows that temperature is having an effect, albeit very slight. As expected, intensity has a large effect on the rate of dehydrochlorination but this effect only lasts about 1 hr. Thereafter the rate becomes independent o f intensity. The figures AWl_7, AW2_ 7 and AW3_ 7 are constant, for the defined experimental conditions. Two new factors emerge and require explanation when formulating a mechanism for the photodegradation o f PVC. It has been shown that the rate of degradation after 1 hr is independent o f intensity and almost independent o f temperature. The reaction must now be considered as taking part in two distinct stages viz. a relatively fast first stage dependent on intensity and temperature and a slower second stage independent of both these parameters. The reaction occurring after the first hour is very unusual. It has been shown previously that dehydrochlorination is confined to a thin surface layer. ¢1) The weight evolved after infinite time, A W~o, was approximately estimated by extrapolation of the runs shown in Fig. 2. The values obtained are shown in Table 7. These limiting values suggest that the depth o f the surface layer is dependent on the intensity o f irradiation. Furthermore, it is possible to calculate the percentage decomposition after 1 hr, AW~/AWo~ × I00, and the results, taking the A M values from Table 5 are also shown in Table 7. TAnLe7 /o (quanta cm =2 sec -1) ×10 -16

AW,o (nag)

AM (nag)

2'83 1.24 0"71 0-43

0"8 0-74 0'68 0"62

0"283 0.195 0.110 0'048

(AW1/AW® x 100)

35 26 16 8

It can be seen that, although there is the compensating factor of a slightly smaller AW® in the less intense case, the actual percentage decomposition is far smaller.

1230

W . H . GIBB and J. R. MAcCALLUM

T h e a b s o r p t i o n s p e c t r a s h o w n in Fig. 3 indicate t h a t the a m o u n t o f fight a b s o r b e d after 1 h r is m u c h greater when the light intensity is greater. T h e s p e c t r a also s h o w that, however great the t o t a l a b s o r p t i o n , the d i s t r i b u t i o n o f polyenes, the a b s o r b i n g species, r e m a i n s constant. K e n y o n ~4) has s h o w n t h a t only light o f a w a v e l e n g t h less t h a n 340 n m can d e g r a d e PVC, so only this p a r t o f the s p e c t r u m is shown. I t is likely t h e n t h a t this second stage is c o n t r o l l e d n o t b y a chemical process b u t b y the " a v a i l a b i l i t y " o f the HCI. T a k i n g the two extreme cases, in the m o r e intense i l l u m i n a t i o n a l t h o u g h a lot m o r e p o t e n t i a l l y d a m a g i n g light is a b s o r b e d a p p r o x i m a t e l y 35 p e r cent o f the available H C I h a s a l r e a d y evolved. H o w e v e r , for the less intense case, less light is a b s o r b e d b u t only 8 p e r cent o f the available H C i h a s evolved. Similarly the o t h e r two cited examples lie in between these two. I f these two factors cancel o u t t h e n the rate o f r e a c t i o n will be i n d e p e n d e n t o f intensity. Because this " a v a i l a b i l i t y " will be unaffected by t e m p e r a t u r e , the d e h y d r o c h l o r i n a t i o n will be independ e n t o f t e m p e r a t u r e as has been f o u n d to be the case. Acknowledgements--The authors acknowledge helpful advice from Mr. G. C. Marks, a studentship from the S.R.C., and financial assistance from B.P. (International) Ltd.

REFERENCES (1) w. H. Gibb and J. R. MacCallum, Europ. Polym. J. 7, 1231 (1971). (2) C. G. Hatchard and C. A. Parker, Proc. R. Soc. A235, (1956). (3) R. F. Reinisch, H. R. Gloria and G. M. Androes, Photochemistry ofMacromolecules, p. 185, Plenum, New York and London (1970). (4) A. S. Kenyon, Natn. But. Stand. Circ. 525, 81 (1953).

R~sum~---Des films de poly(chlorure de vinyle) pr6par6s par 6vaporation d'une solution de polym~re ont ~t~ irradi~s en lumi6re ultra-violette h des intensit~s et des temperatures diverses et sous atmosphere d'azote. On a montr~ que la r~action de d6chlorhydratation se fait en deux ~tapes. Pendant la premi6re heure, la r~action depend de l'intensit6 et de la temp6rature puis elle devient ind~pendante de ces deux param6tres. Sommario--Si sono sottoposte a raggi ultravioletti di varia intensith delle pellieole fuse di poli(vinil cloruro), a varie temperature e in atmosfera di azoto. Si mostra chela deidroclururazione avviene in due modi: durante la prima ora della reazione essa dipende dall'intensith delle radiazioni e dalla temperatura, mentre pica tardi non dipende pi~hda questi due parametri.

Zusammenfassung--Aus Polyvinylchlorid gegossene Filme wurden mit ultraviolettem Licht bei verschiedener Intensit/it und Temperatur in einer Stickstoffatmosph/ire bestrahlt. Es wird gezeigt, dab die Chlorwasserstoffabspaltung in zwei Teilen erfolgt. W~ihrend der ersten Stunde ist die Reaktion abh~mgig yon der Intensitat und Temperatur, im weiteren Verlauf jedoch ist sic von diesen beiden Parametern unabh/i.ngig.