Change in the physicomechanical properties of polyethylene on photo-oxidation

Physicomechanical properties of P E

257

~27. R. M. MYASNIKOVA, Vysokomol. soyed. AI9" 564, 1977 (Translated in P o l y m e r Sei. U.S.S.R. 19. 3, 651, 1977) 28. I. V. NIKITIN, E. F. VAINSHTEIN, M. Ira. KUSHNEREV, M. L BANATSKAYA a n d N. K. KOCHETKOV, Dokl. Akad. l~auk SSSR 247: 1158, 1979 29. 8.-8. A. PAVLOVA, L. V. DUBROVINA, N. V. I(TJMANOVA and S. N. SALAZKIN, Vysokomol. soyed. B19: 175, 1977 (lqot translated in P o l y m e r Sol. U.S.S.R.) 30. N. V. I(L/MA_NOVA, D. V. DUBROVINA, S.-S. A. PAVLOVA, T. M. BABCHINITSER, Ira. V. GENIN a n d V. V. KORSHAK, Ibid. A19: 2309, 1977 (Translated in Polymer Scl. U.S.S.R. 19: 10, 2649, 1977) 31. H. CHANZY, M. DUBE a n d R. H. MARCHE8SAULT, Polymer Letters 17: 219, 1979 32. L N. ANDREYEVA, Z. S. KHANIN, O. I. ROMANKO, A. V. VOLOKHINA, M. M. IOVLEVA, A. T. KALASHlgIN, S. P. PAPKOV a n d G. I. KUDRYAVTSEV, Vysokomol. soyed. B23: 89, 1981 (Not t r a n s l a t e d in Polymer Sci. U.S.S.R.) 33. M. M. IOVLEVA, V. N. SMIRNOVA, Z. S. KHA_NIN, A. V. VOLOKHINA a n d S. P. PAPKOV, I b i d . A23: 1867, 1981 (Translated in Polymer Sci. U.S.S.R. 23: 8, 2038, 1981) 34. S. P. PAPKOV and V. G. KULICHIKB[IN, Zhidkokristallieheskoye sostoyaniye polimerov (Liquid-Crystalline State of Polymers). Khimiya, Moscow, 1977 35. M. PANAR a n d L. F. BESTE, Macromolecules 10: 1401, 1977 36. S. P. PAPKOV, Vysokomol. soyed. B24: 109, 1982 (Not translated in P o l y m e r Sei.

U.S.S.R.) 37. 8. I. BANDURYAN, M. M. IOVLEVA, N. A. IVANOVA, Z. S. KHANIN, A. V. VOLOI(HINA and S. P. PAPKOV, Ibid. A22: 2300, 1980 (Translated in Polymer Sei. U.S.S.R. 22: 10, 2522, 1980) 38. J. PETERMANN, M. MILES a n d H. GLEITER, J. Polymer Sci., Polymer Phys. E d . 17: 55, 1979 39. S. P. PAPKOV, Ravnovesiyo faz v sisteme polimer-rastvoritel' (Phase E q u i l i b r i u m in P o l y m e r - S o l v e n t System). K h i m i y a , Moscow, 1981

Polymer ScienceU.S.S.R. Vol. 24. 1~'o.2, pp. 257-263. 1 9 8 2 -Printedin Poland

0032-3950/82/020257-07507.50/0 © 1983 Pergamon Press LMI.

CHANGE IN THE PHYSICOMECHANICAL PROPERTIES OF POLYETHYLENE ON PHOT0-0XIDATION* ~rE. 1~. SLOBODETSKAYA (dec.), T . V. M~kGOMEDOVA, O. ~ . ~ k R P U K H I ~ and V. G. PROTASOV Institute of Chemical Physics, U.S.S.R. A c a d e m y of Sciences

(Received 19 June 1980) The paper deals with the changes in the physicomechanical properties of various t y p e s of P E on photo-oxidation and shows t!~at these are determined, in the main, b y the kinetics of the radical reactions occurring in the polymer. I t is also established t h a t the non.radical degradation of the ketones contained in it makes a definite cont r i b u t i o n to change in the properties of P E . * Vysokomol. soyed. A24: 1~o. 2, 249-253, 1982.

9"58

Y E . M. SLOBODE'I~KAYA e$ ~ .

01r~ of the most important aspects of the task of improving the quality of polymer materials and prolonging their life is the development of ideas on the kinetics and mechanism of ageing of polymers enabling one to predict the life of the products and also predetermining scientifically grounded methods of stabilizing and modifying polymers and secondary polymer raw materials. The solution o f the problem of predicting the life of PE may be based on elucidation of the mechanism of its ageing and establishing the link between the kinetics of the chemical processes occurring in the polymer and changes in t h e important physicomechanical properties of the polymer. It is known that the main factor leading to loss of the complex of the working properties of PE products in natural conditions is photochemical action of solar radiation initiating oxidative processes. However, the literature provides no information on change in the physicomechanical characteristics of P E on exposure to light of different intensities. The information on the ageing of secondary polyethylene (SPE) is incomplete although it is important for validating the desirability of secondary processing. I t may be that on ageing PE accumulates oxidation products which accelerate the photo-ageing of the polymer after secondary processing and in such a case the life of SPE products will be very short. This set the task of the present work-study of the change in the physicomechanical characteristics of PE and different types of SPE on photo-oxidation. Earlier it was established that change in the physicomechanical characteristics of P P on photo-oxidation on exposure to light of different intensities and spectral composition is determined by the kinetics of the radical reactions, namely, the total amount of radicals formed in the time of irradiation ~ [1]. Within the kinetic scheme describing the photo-oxidation of P P [2] the value Q is the integral o f the sum of the acts of photo-degradation of the branching agents of hydroperoxides (POOH) and the ketones in the period of oxidation = S (2~ [POOH] ~f~' [ketene] dr,

(1)

0

where ~, ~' are the probabilities of photodegradation of POOH and ketones with the absorption spectra 4(2) and 8'(A), on exposure to light with an intensity and spectral composition 1(2) a = ~ ~ e(2)I(2)d), .h

a ' = q" ~ g(2)I(2)d).

(2)

~t

(~, ~' are the quantum yields of photolysis of POOH and ketones; f is the fraction of radicals released into the volume on photolysis of the ketones). It has also been established that photo-oxidation of PE like P P is a chain reaction with bimolecular termination and degenerate photobranching on POOH [3]. We tried in similar fashion to treat the data on change in the physicomechanical properties of PE and SPE on photo-oxidation and showed that as with P1>

Physicomechanieal properties of P E

25~

i t is, i n t h e m a i n , d e t e r m i n e d b y t h e k i n e t i c s o f t h e r a d i c a l r e a c t i o n s o f o x i d a t i o n and "non-radical" breakdown of the ketone groups in the main chain of the polyiner.

I n the work we used a commercial film of low density P E (LDPE) of grade 15802-020 GOST 51308.72 a n d also SPE~, SPEz and SPE8 representing aged, secondarily processed P E s containing a certain concentration of oxidation products. SPE1 was subjected to ageing over one a n d SPEz two seasons in conditions of a m o d e r a t e l y cold climate. SPEa was exposed to ageing over one season in the humid subtropical conditions of Sukhumi. The films were freed o f impurities and superpressed b y the technique in [3]. The physicomechanical characteristics of L D P E and S P E s and their ketone contents are given in the Table. PHYSmO-~C~A~r~CAL C ~ C T m a I S ~ O S Properties of initial film Polymer O'r,

Properties of the film after superpressing O'r~

MN/m'

e, %

LDPE

18.0

280

SPEx SPE.

15. 6 16.0 11.4

470 170 17

SPE8

Or FILMS A~D ~ E n ~ XETO~E CONT]~WrS Properties of films superpressed from extrudate

[Ketone] × × l0 s mole/1.

O'rs

M:hT/m ~

8, °/o

MN-/m z

e, °/o

15-5 14- 4 17.0 8.4

490 414 330 60

--12.5 1{~0

--330

-1 10

105

30

The literature provides information indicating t h a t the superpressed (superfused) aged P E possesses the same physicomechanical properties as the initial [10]. However, our in. vestigations showed t h a t to restore the properties of m a r k e d l y aged samples of SPEs and SPEs superpressing is not enough. After processing in a disc extruder the indices of the physicomechanical properties of these polymers substantially improved. F o r t h e tests we used samples obtained b y pressing crushed extrudate. Processing was carried out in a a ED-2-2 disc extruder at a turning speed of the moving disc of 3-75 rps, the gap between discs 10 -8 m and t e m p e r a t u r e 160°C. The concentration of the carbonyl compounds after processing did n o t change. The films were irradiated in air at room t e m p e r a t u r e with the light of a DB-30 low pressure m e r c u r y lamp (21rraa~-254 rim); I = 10 l° quanta/m2.sec. The intensity of light was changed with calibrated meshes. The optical density of the samples at the wavelength of irradiation d i d n o t exceed 0. 3. The mechanical characteristics of the samples were measured with the Instron rupturo machine. The speed of movement of the lower clamp was 50 ram/rain. The measurements were m a d e in air a t room temperature. The concentration of ketones in the polymers was determined b y 3JR spectroscopy b y the technique proposed in reference [4]. As is known, in the I R region of the s p e c t r u m {1700-1800) × l0 S m -1 absorption of different carbonylcontaining products is o b s e r v e d - - k e t o n e s , acids, complex esters, y-laetones [5]. On treating t h e fl]ms w i t h a s a t u r a t e d solution of K O H in isopropanol the lactones, esters and a c i d s pass into carboxylates and in the region 1720 × l0 S m -~ there remains a b a n d due only to the absorption of the ketones. The concentrations of the ketones was estimated from the f o r m u l ~ [ketone]----- Dl'SS ~lse-- c1881: 0.92DlTze, 8lvss DxeeT Dxse~

260

Ym M° SLOBOD]~TSKA.YAe~ a~.

where Din7 is the optical density at the peak 1367 × 102 m -1 matching the fa~ vibrations o f the CH~ groups taken as internal standard; C188~is their concentration equal to 36 mole/l.~ ~1~6T-----~24 1./mole.era. Figure 1 presents the kinetic curves of change in the relative elongation at rupture e (%) and strength ar on irradiation of LDPE, SPE1 and SPEa with light of different intensities. It will be seen that for high intensities e and ar change less effectively on conversion to a quantum of incident light. This is connected with the fact t h a t on bimolecular termination of the chains at high rates of initiation the effective length of the oxidation chain of the polymer decreases. As shown earlier [2, 3], in our conditions in P E the main initiating agent is POOH. Then $

£21~ S 2a[POOH]dt= [POOH]oo[at--2 (3--e -~'/') (1 --e-~t/2)],

(3)

where [ P O O H ] = 2 ( k p / ~- /-~ is the boundary concentration of POOH in the polymer; kp and k0 are the rate constants of the reactions o f continuation and termination of the chain. For large at

~1 ~ 2 (kp/ 4ko)~t,

(4)

where ztt is time of irradiation of the polymer. From the relation (4) it follows that for different intensities of light change in and ar must be described by a single dependence in the coordinates e, ar-t. Figure 2 presents the experimental data obtained for L D P E and SPE x as a function of the value Q~. From the Figure it is clear t h a t the light stability of these polymers is practically identical, i.e. the initial concentration of the ketone [ketone]=0.01 mole/1, in the SPE1 sample weakly influences ageing. I t is clear t h a t a fall in the value ~ to one fifth corresponds to the value Oer=0"2 mole/1. (Ocr is the critical value of ~ at which further exploitation of the polymer is impossible). On photo-oxidation of SPE2 and SPEs it was found that the same fall in e as for L D P E and SPE 1 occurs at smaller values of Ocr (0"1 and 0"02 mole/1. respectively, Fig. 2). This effect of fall in Oer with rise in the initial concentration of the ketone m a y be connected with non-radical degradation of the ketones by a Norrish type I I reaction leading to rupture of the polymer molecule. At a high initial concentration of ketones in the polymer, i.e. on fulfilment of the condition ~'f [ketone]>>2a[POOH] the ketone becomes the main initiating ~gent. In our experiments this condition is fulfilled for an initial concentration of the ketone i>0.5 mole/1. (eeooH=12"50, Sketone=8"0 (l./mole'm)-l; [POOH]0 10 -s mole/1. [1, 2]). The total number of acts of photodegradation of the ketones to the radicals ~ 2 and the total number of acts of photodegradation of the ketones by a Norrish !

Physieomeetlanieal properties of PE

261

reaction of type II Qa are respectively equal to $

t

~f[ketone],

02 = Sf£[ketone]dt

Q a = S a"[ketone]dt ~ [ketone]0,

0

0

wlmrc a" is the rate constant of the non-radical photodegradation of the ketones a " = .f ~Nne'(2)I(~)d,~, 2

+pN~is the quantum y i e h / o f the photodegradation of the ketones equal to 2 x 10 -2 [6]. After a time of the +~rder 1/~', the ketoncs are expended and P O O H again becomes the main initiating agent. In this time in the polymer Y2a ruptures of the polymer chains have accumulated. The value Q2 m a y be ignored (as compared with t~:~) since the value f in P E is low ( f a ' ~ 3 × 10 -a [7]). Evidently, if the vMue Qa is comparable ~dth Qer tim main contribution to the degradation of P E will be ma(le b y photolysis of the ketom,s 1)y a Norrish type Y[ reaction. In other cases the degradation occurs, in the mMn, during oxidation. This is il]ustra.ted by Fig. 3 showing that the change in the value e for all types of PE is determined hy the value Q re,presenting the sum of t)t, t)e and t)a. Figure 2 sho,~:s that at different temperatures change in e is determined solely by the value Q. Establishing this, one m a y calculate the life of P E and SPE. The calculation was nmde with a computer b y a programme proposed for caleuo/oo

e/eo

[" l.o ~xo

O'Z

oI

a

z

l.O

+d

O'Z

zx

IY

IT

7Z

216

1.o ~_~k

J60

J6

l.o l ~

C

4Y\ ZI5 OJZ

b

7Z

108 "

d

.+ 7ZO~t

18

36 ~t

~'io. I. Kinetic curves of change in strengih (a, c) told relative elongation (b, d) on photo-oxidatim, of L D P E m~d SPE~ (a, b) and SPE~ (c, d). I n t e n s i t y of incident light I x I0 -~s qnant~/ /mmsec,

I--2.0; II--l-0; III--0.3; 1-3--LDPE; 4-6--SPE~; 7, 8--SPE~.

262

YE. M, SLOBODETSKAYA et al.

of

tnZr 0.51 x,x

-Z

xJ

+

l

0"!

0"2

8"5-

AS

19

}

0"~

04

•

*[4

\

"

0"2

Q,, molelL

0"5

•, molelt.

FIo. 2

Fro. 3

Fro. 2. I)ep~,ndence, of the, relative eh)ngation of L D P E (1, 1') SPE1 (2, 2'), SPE2 (3) and SPE3 (4) on tile vahw of ~1 at 27 (1-4) and 50 (1', 2') °C. F~c. 3. R¢~lative elongat, ion of L D P E (1), SPE~ (2), SPE2 (3) and SPE.~ (4) on Q. l a t i n g t h e a c c u m u l a t i o n o f t h e v a l u e ~ on a g e i n g o f P P in n a t u r a l c o n d i t i o n s [8]. F i g u r e 4 shows h o w t h e v a l u e ~ a c c u m u l a t e s on a g e i n g o f P E in c o n d i t i o n s o f t h e temperate climate of Kiev and the subtropical climate of Sukhumi (start of e x p o s u r e 1 April). I t will be seen t h a t p r i m a r y P E loses its p r o p e r t i e s in ~ 100 a n d SPE:~ in 40 d a y s . Q~ rnole/l.

~2 mole/l.

g.z~ e 0,3 -

a

O.,~

b

/

/

g.z

0.1 L

04

O

/4

[4

J

'~

/ g.z

/

g.z

04

i ~0

/4

0"I

90

30

go

Time, days FIG. 4. Calculated curves of accumulation of ~2 (1), ~2~ (2), ~22 (3) and ~2a (4) for L D P E in conditions ~ff the temperate climate of Kiev (a) and subtropical climate of Sukhumi (b) aud also for 8PE 3 in the same conditions (c, d).

Physicomechanieal properties of PE

263

T h e results of c a l c u l a t i o n agree w i t h the p u b l i s h e d d a t a oil the ageing of P E in t h e conditions of a m o d e r a t e l y cold c l i m a t e [9]. F r o m this s t e m s a certain criterion of the q u a l i t y of s e c o n d a r y P E , n a m e l y its k e t o n e content. I f the c o n t e n t of k e t o n e s in S P E is such t h a t on t h e i r p h o t o d e g r a d a t i o n the v a l u e ~3 w h i c h a c c u m u l a t e s is c o m p a r a b l e w i t h ~ t h e n this p o l y m e r will be d e g r a d e d r a p i d l y on e x p o s u r e to light a n d is not s u i t a b l e for e x p l o i t a t i o n in m l t u r a I conditions. According to t h e relation ( 5 ) t h e v a l u e [ketone]0 cr ~ 0"1 mole/l. A t c o n c e n t r a t i o n s of the k e t o n e s 5 x 10-" mole/1, t h e v a l u e is less t h a n 25~/o of ~2er a n d t h e contrib u t i o n of this process m a y be d i s r e g a r d e d . T h u s , t h e results of i~tvestigation show t h a t t h e change in the p h y s i c o m e c h a n ical c h a r a c t e r i s t i c s of the p r i m a r y a n d s e c o n d a r y P E s on p h o t o - o x i d a t i o n is due to the kinetics of the r a d i c a l processes occurring in it. I n the case of p r i m a r y P E t h e process of p h o t o - o x i d a t i o n is f u n d a m e n t a l a n d in t h e case of the s e c o n d a r y p o l y m e r a m o r e significant c o n t r i b u t i o n to ageing is m a d e b y rlon-ra(tical p h o t o d e g r a d a t i o n of t h e ketones. REFERENCES

1. 0. N. KARPUKHIN and Ye. M. SLOBODETSKAYA, Vysokomol. soyed. AI8: 2700, 1976 (Translated in Polymor Sci. U.S.S.R. 18: 12, 3084, 1976) 2. O. N. KARPUKHIN, Ye. M. SLOBODETSKAYA, V. V. AMEKIK, T. M. FES'KOVA,

and M. G. VOROB'EV, Ibid BI7: 749, 1975 (Not translated in Polymer Sei. U.S.S.R.) 3. O. N. KARPUKHIN, Ye. M. SLOBODESTSKAYA and T. V. MAGOMEDOVA, Ibid. B22: 595, 1980 (Not traaslated in Polymer Sci. U.S.S.R.)

4. J. H. ADAMS, J. Polymer Sci. 8: 1279, 1970 5. B. RANBI and Y. RABEK, Fotodestruktsiya, fotookisleniye, fotostabilizatsiya polimerov (Photodestruetion, Photo-Oxidation and Photostabilization of Polymers). p. 263, Mir, Moscow, 1978 6. M. U. AMIN, G. SCOTT arid L. M. K. TILLECERATHNE, Europ. Polymer J. 11: 65, 1975 7. J. E. GUILLET and G. H. HARTL.EY, Macromolecules 1: 165, 1968 8. O. N. K A R P ~ N , Ye. N. MOSTOVAYA, B. V. NOVOZHILOV and Ye. M. SLOBODETSKAYA, Plast. massy, No. 6, 17, 1980 9. V. A. BRYZGALOV ~nd G. S. OSIPOVA, Ibid., No. 12, 26, 1979 10. A. I. TOVSTANOVSKII, [bid., N~). 5, 66, 1977