Kinetics of low temperature oxidation of oriented polypropylene

1614 6. 7. 8. 9. 10. 11. 12. 13.

14. 15.

16. 17. 18. 19. 20.

N. YA. RAPOPO~T et al.

J. T. MAYNARD and W, E. MOCHEL, J. Polymer Sci. 13: 251, 1954 R. (3. FERGUSON, J. Polymer Sci. 2: 4735, 1965 E. O. AZttANDZItYAN, L. G. MELKONYAN, Arm. khimich, zh., No. 20, 105, 1967 Y. HOFMANN, Vulkanizatsiya i vulkaniziruyushchiye agenty (Vulcanization and Vulcanising Agents). Izd. " K h i m i y a " , 1968 A. S. STEVENSON (book) b y G. ALLIGER and I. S'YETUN, Vulkanizatsiya elastomerov (Vulcanization of Elastomers). Izd. " K h i m i y a " , 1967 R. M. MURRAY, D. S. THOMPSON, Die Neoprene, Wilmington (Dee), Du P o n t do Nemours, 1965 P. KOVACIC, Industr. and Engng. Chem. 47: 1090, 1955 N. A. ZHOVNER, N. D. ZAKHAROV, S. V. OREKHOV, A. G. ROMANOVA a n 4 R. G. KOSTRYKINA, Vysokomol. soyed. A16: 821, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 4, 9~:7, 1974) N. A. ZHOVNER, N. D. ZAKHAROV, S. V. OREKHOV, Vysokomol. soyed. A12: 2457, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 11, 2785, 1970) Ye. V. SAKHAROVA, Ye. E. POTAPOV, T. FARAGO, K. I. PETROV and I. A. TUTORSKII, T r u d y M I T K H T im M. V. Lomonosova, K h i m i y a i khimieh, tekhnologiya 6, 82, 1976 Primeneniye spektroskopii v khimii, Sb. statei pod. red. W. Westa, Izd. inostr, lit., 1959 (Use of Spectroscopy in Chemistry) G. F. UOKER, Formal'degid (Formaldehyde). Goskhimizdat, 1957 M. HORAK and P. TVARUZEK, Collect. Czechosl. Chem. Com. 40: 2741, 1975 P. KARRER, Kurs organicheskoi khimii (Course on Organie Chemistry). Goskhimizdat, 1962 S. J. ANGUAL and R. C. RASSACK, J. Chem. Soc. 10: 2700, 1949

Polymer ScienceU.S.S.I~.¥ol. 20, pp. 1614-1620. (~) PergamonPress Ltd. 1979.Printed in Poland.

0032-3950/78/0601-1614507.50]@

KINETICS OF LOW TEMPERATURE OXIDATION OF ORIENTEI) POLYPROPYLENE* N. YA. I~APOPORT, A. SH. GONIASHVrLLI, M. S. AKUTIN and V. B. M~T,ER I n s t i t u t e of Chemical Physics, U.S.S.R. Academy of Sciences

(Received 20 December 1977) I n order to explain the variation of kinetic parameters of elementary stages o f chain oxidation of P P during orientation, a study was made of kinetics of low temperature oxidation of pre-oxidized isotropie a n d orientated P P samples. I t was shown t h a t the parameter l c , / 4 k 8 determining oxidation properties of the polymer, varies slightly during orientation; the rate constant of peroxide radical loss k6 in oriented P P (2=800%) is about 5-7 times lower t h a n in isotropie PP. The rate c o n s t a n t of kinetic chain extension ks decreases somewhat more suddenly during orientatioi) * Vysokomol. soyed. A20: No. 6, 1432-1437, 1978.

Low temperature oxidation of oriented PP

1615

than x/f%. The activation energy of oxidation, kinetic chain extension and rupture during orientation remain unchanged in practice. Results confirm tim earlier conclusion that a reduction in the initial rate of oxidation of PP during orientation elongation is due to a reduction in tile rate of kinetic chain branching as a result of a reduction in the rate of hydroperoxide formation in the oriented polymer. IT has been shown previously [1-3] t h a t orientation elongation of isotaet,ie I,i, markedly increases the induction period and reduces the initial rate of auto-oxidation. I t was found t h a t in oriented I"i" the yield of hydroperoxide per mole of oxygen absorbed decreases and the rate of formation shows a corresponding reduction [3]; since during oxidation of I,i, hydroperoxide funtions as a branching agent, a reduction in the rate of formation lowers the initial rate of oxidation of I'P, i.e. orientation elongation of the polymer changes the rate of the stage of kinetic chain branching. I t was interesting to explain wheter kinetic parameters of the remaining elementary stages of chain oxidation of I'i" vary during orientation of I'i,; whether rates of oxidation of isotropie and oriented I'i" vary if hydroperoxide already exists in both samples? To answer these questions samples were needed containing hydroperoxide and differing only in the wesenee or absence of orientation. I,reviously oriented and then oxidized films are unsuitable for this study for two reasons: 1) noticeable (of the order of 4-5 × 10 -2) concentrations of hydroperoxide are only found after intensive breakdown of oriented films [3, 4]; 2) isotropie and oriented oxidized films differ in composition of solid products containing oxygen [5, 6]. T b s e complications m a y be avoided by changing the order of preparing oriented oxidized samples; isotropie films were previously oxidized at 100 ° until hydroperoxide accumulated with a concentration of the order of (4-8)× 10 --"~ mole/kg and then subjected to orientation elongation by "point t y p e " heating under a load at 130 °. Special experiments show t h a t with this method of orientation elongation the concentration of hydroperoxide groups in previously oxidized films remains unchanged in practice. These methods, unfortunately, do not allow for high concentrations of hydroperoxide since highly oxidized isotropie films become brittle and cease to be oriented. Oriented samples thus obtained correspond to the r~quirements formulated: t h e y contain a hydroperoxide concentration equal with t h a t of iostropie fihns and only differ from t h e m in the existence of orientation. I'i, hydroperoxide previously accumulated was used as initiator of low temperature oxidation, thus simulating the real process of auto-oxidation of the polymer, as proposed previously [7]. "Moplen" isotactic PP with [~]= 1.83 in deealin was used at 135° and a powder density of 0.92. Films 40 and 100-110 pm thick (190°, 3 rain, followed by sudden cooling) were compressed from the powder. These films were oxidized either at 100° without an initiator, or at 85° in the presence of AID as initiator (AID was added to the film by wetting it in~ a benzene solution of AID for 24 hr after oxidation the initiator and breakdown products.

]616

N. ~rA. RAPOPORT e$ at.

were carefully washed with benzene, which was then pumped out at room temperature; the extent to which benzene was removed from the films was controlled spectrophoto~aetrically). Films 100-110/lm thick were subjected to orientation elongation. The degree of elonga. tion of oriented samples was 450-500% and thickness was 30-40/an. We note t h a t no s t r u t . t u r a l parallel can be drawn between oriented samples with 1 = 4 5 0 % obtained from uno. xidized [1-3] a n d oxidized isotropic films. I n the latter case we have a much more rigid initial isotropie matrix and the same degree Of orientation elongation corresponds to another structttre more highly stressed from a conformation point of view. The rate of oxidation* of P P was studied in O3 atmosphere with a sensitive capillary pressure gauge in the interval of 60-90 ° b y methods previously described [7]. The rate of initiation was determined from initial linear sections of curves showing the consumption of the stable nitroxyl radical (2,2,6,6-tetramethylpyperidine oxyl R2NO) in inert atmosphere b y E P R . Special experiments show t h a t R2N() disappearance from both isotropie a n d oriented P P films is of zero order and the measured rate of initiation is independent of R~N() concentration on changing it from 4 × 10 -5 to 1.6 × 10 -3 mole/kg. At 70 ° the linear section of curves showing loss is retained up to a 50% radical conversion. Rate constants of peroxide radical loss k6 in oriented and isotropie films were determined b y a method of non-stationary kinetics proposed to determine k6 [9]. Experimental methods r e q u i r e the use of transparent samples, since R 6 , radicals are formed in films b y the action of UV radiation. Samples obtained b y orientation elongation of oxidized isotropie films were unsuitable for this study as they scattered light to a marked extent. To study kinetics of peroxide radical loss, transparent films (isotropie a n d oriented with 1 = 800%) were used, which had been previously oxidized at room temperature (7-radiolysis, I = 1-16 X 10-5 Mrad/ /hr, 500 hr). The concentration of hydroperoxide in the isotropie film was ~ 1 × 10 -1 mole/kg; i n oriented films it was lower b y a b o u t one order of magnitude. I n spite of the low content o f hydroperoxide, peroxide radical teoncentration under UV exposure in a n oriented film was sufficient to study loss kinetics. P P films previously oxidized were exposed to the full light of a DRSh-1000 mercury arc lamp in air at 26-47 ° in the resonator of a PA-100 type E P R spectrometer. During irradiation of previously oxidized P P in oriented a n d isotropic samples R()m radicals of the same form of spectrum were produced; after disconnecting the light a s t u d y was made of kinetics of R()3 loss in the dark. The rate of oxygen absorption Wo~ was measured under conditions, when hydroperoxide concentration remained practically unchanged during the experiment and the rate of initiation Wl could be regarded as constant. Under these con. ditions

We,

k3[RH]

Kinetics of oxygen absorption with oriented P P . The rate of oxygen absorption b y isotropic films at 60-90 ° does not differ in practice for films 40 (sample 1) and 110/2m thick (sample 2). The temperature dependence of the rate of oxygen absorption by these samples is shown in Fig. 1 by l~nes 1 and 2. It corresponds * Bearing in m i n d t h a t oxidation only takes place in the amorphous phase of P P [8], all rates were converted to 1 kg of the amorphous phase using crystallinities previously given [ 1]. * This is first of all due to a lower rate of loss a n d secondly, a possible increase in the q u a n t u m yield of R()2 radicals [10] according to the Norrish-1 reaction during the orientation of PP.

Low temperature oxidation of oriented PP

1617

to an activation energy Ea~22,4004-1500 kcal/mole. As a result of orientation elongation of sample 2 the rate of oxygen absorption decreases approximately 1.5-fold and the temperature dependence of the rate is practically unchanged and corresponds to Ea=21,900=t=1500 kcal/mole (Fig. 1, line 3). T h i s activation energy of oxygen absorption is also observed for isotropic (line 4) and oriented

iogw0[6

f

2.8

2.g

3.[1 lOfT, °g -1

Fro. 1. Temperature dependence of the rate of oxidation for isotropie PP pro. viously oxidized at 100° in O~ (with a film thickness of 40 (1) and 110 /~m (2)), oriented PP (~=450~o), obtained from sample 2 (3); isotropic PP, previously oxidized at 22° under conditions of y-radiolysis, then heated to 100° for 1 hr in air (4) and oriented PP (4= 1200~o), previously oxidized under the same conditions as sample 4 (5). (to 2 ~ 1 2 0 0 % ) films subjected to intensive decomposition (line 5), previously oxidized under conditions of y radiolysis ( I ~ 28 rad/sec, 100 hr) at room temperature; before determining We, these were further heated in air at 100 ° for 1 hr to anneal stabilized radicals. As can be seen from a comparison of straight lines 4 and 5, the rate of oxidation of an oriented sample with 2 = 1200% was also lower than that of a sample previously oxidized under similar conditions. In contrast to samples 1-3 these samples differed in both hydroperoxide concentrations and the composition of solid products of oxidation containing oxygen [5]. Differences in the rate of oxygen absorption b y oriented and isotropie films previously oxidized m a y be due to the different rate of initiation and diff2renees in parameters ]c2/~/} 6 characterizing the ability of the polymer to underg9 oxidation. The rate of initiation was determined using the same samples as these used for measuring We,. Corresponding results are tabulated. A comparatively high

iN. YA. RAPOPORTet al.

1618

error in measuring WI in oriented samples* hinders the calculation of parameter k~/~/ke with an accuracy exceeding 20%. I t can only be safely stated that a variation in k~/~/k8 as a result of orientation elongation is slight (parameter k , . / ~ 6 decreases not more than 1.5-fold, compared with the isotropic polymer). We observed the same slight reduction in ]c2/~/~6 [5] for highly oriented P P films during oxidation at room temperature. In irradiated oxidized samples 4 and 5 (Fig. 1) subjected to intensive decomposition parameter lc=/~/]c6 was ~somewhat higher than in films subjected to auto-oxidation under mild conditions (samplesl and 3). The length of kinetic chains of oxidation at 70 ° in an isotropic polymer subjected to auto-oxidation was 44 and that in an oriented polymer, 27. Orientation elongation of P P therefore does not markedly change parameter /c2/~/~6, characterizing oxidation properties of the polymer.

10-~/R kg/mole I0"0

7",

7"5

5"O

2"5' 0

I 300

I BOO

I 900

-/-[me, seo

FIG. 2. Kinetics of R()= loss in oriented PP (~=800~o) in coordinates of a second order reaction at 34.5 (1) and 42.1° (2). In order to establish whether constants in the parameter undergo any variation, one value had to be measured b y an independent method. We measured the rate constant of peroxide radical loss in isotropic and oriented samples, for which it was found that parameter k~/~/~6 shows a 1.5-fold variation at room temperature. Kinetics of RO~ loss in oriented PP. Kinetics of loss are satisfactorily described at all the temperatures studied b y a second order equation up to high degrees of conversion both in oriented and isotropic films (Fig. 2). At the same temperature the rate of radical loss in oriented samples is several times lower than in isotropic samples. The temperature dependence ke is shown in Fig. 3 for isotropic (1) and oriented (2) films. It can be seen that the activation energy of radical l o s s does not differ in practice in isotropic and oriented samples, while the diffe* In oriented samples with an average radical concentration, identical to that of isotropic samples local concentrations are somewhat higher since there are regions in which diffusion of the radical involves difficulty.

Low temperature oxidation of oriented PP

1619

rence in numerical rate constants of radical loss is due to a difference in pre-exponential factors 26,100-{-2000 log k6=20.0 (for 4 = 0 ) 4-57 T and 26,300±2000 log k6=19"3 (for ~----800 ~o) 4.57T Orientation elongation of P I ~ therefore reduces rate constants of peroxido radical loss several times, compared with isotropic PP. This is, probably, due to log,k~

o

A

1

2

3./5

3.25

3-35

IO~T,°K "I

Fro. 3. Temperature dependence of the rate constant of I~Os loss for isotropic (1) and oriented PP (2) (~=800~o). ~tricter conditions of the system. In spite of this parameter k~/,,/T¢ 8 in the samplo with )~-----800% is 1.5 times lower than in an isotropie film. This means that the rate constant of kinetic chain extension ]c2 in oriented P P is lower than in isotropic PP; the value of k,, apparently, decreases during orientation more markedly than -x/k6. Since the overall activation energy of oxidation in orientation elongation is unchanged in practice it m a y be assumed that a reduction in ks as also in k6, is due to a reduction of the pre-exponential factor. As a result of orientation elongation of P P rate constants of kinetic chain extension and rupture decrease so that parameter k2/~/k6, which determines tho ability of the polymer to undergo oxidation, varies slightly. This once moro confirms the earlier conclusion that a reduction in the initial rate of oxidation of P P during orientation elongation is mainly due to a reduction in the rate of formation of hydroperoxide groups [3, 6] as a result of a reduction in hydroperoxide yield per mole of oxygen absorbed. Another important practical conclusion follows from the study: if hydro-

N. YA. RAPOPORT ¢~ aZ.

1620

peroxide already existed in the oriented polymer (for example as a result of oxidation of the melt during extrusion), the instantaneous raCe of oxidation will not differ considerably from the rate of oxidation of the isotropic, polymer, KINETIC

P A R A M E T E R S OF O X I D A T I O N OF P R E V I O U S L Y O X I D I Z E D FILI~IS OF POLYPROPYI~ENE~

Wo, × 10 6, mole/kg/sec Method of obtaining of sample

Auto-oxidation a t 10O°

4.5 0

12

Orientation elongation of sample, 2----0 ~-Radiolysis, 22 °, I=0.1 Mrad/hr, 70hr, P o , ----600 torr, then additional oxidation a t 100 °, P o , ~ - 150 torr, 1 hi

Wl × 108, mole/kg/sec

k,/~/k, X 10 4, mole/kg/see

A

T, °C

7O

7O

80

1.9 4.9 1.1 1.8 18.0

9.1

12.3

11.0 41"8

3.9 5"0

11.4

6.1 8.0 7.1 4.0 5.5 17

12

--

45,0

5

4.0 5"5 50.0

2[o

2.4 3.3 11

26.0

27.0

77.2

7.4

3"2

with the same concentration of hydroperoxide (some differences will only exist in the auto-acceleration of the process). Structural stabilization of the product will therefore only be effective if the polymer is protected from oxidation during processing to oriented fibres and films. Translated by E. S~.M'ER~. REFERENCES

1. N. Ya. RAPOPORT, S. I. BERULAVA, A. L. KOVARSKII, L N. MUSAYELYAN, Yu. A. YERSHOV andV. B. MILLER, Vysokomol. soyed. A17: 2521, 1975 (Translated in P o l y m e r Sei. U.S.S.R. 17: 11, 2901, 1975) 2. N. Ya. RAPOPORT, N. M. LIVANOVA a n d V. B. MILLER, Vysokomol. soyed. A18: 2045, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 9, 2336, 1976) 3. N. Ya. RAPOPORT a n d V. B. MILLER, Vysokomol. soyed. A18: 2343, 1976 (Translated. in P o l y m e r Sci. U.S.S.R. 18: 10, 2678, 1976) 4. N. Ya. RAPOPORT a n d V. B. MILLER, Dokl. A N SSSR 227: 911, 1976 5. N. Ya. RAPOPORT, A. Sh. GONIASHVILI, M. S. AKUTIN a n d V. B. MILLER, Vysokoreel. soyed. A19: 2211, 1977 (Translated in P o l y m e r Sci. U.S.S.R. 19: 10, 2533, 1977), 6. N. Ya. RAPOPORT and V. B. MILLER, Vysokomol. soyed. A19: 1534, 1977 7. Ye. L. SHANINA, V. A. ROGINSKII and V. B. MILLER, Vysokomol. soyed. A18: 1160, 1976 (Translated in P o l y m e r Sci. U.S.S.R. 18: 5, 1334, 1976) 8. N. Ye. KORDUNER, T. A. BOGAYEVSKAYA, B. A. GROMOV, V. B. MILLER a n d Yu. A. SHLYAPNIKOV, Vysokomol. soyed. B12: 693, 1970 (Not t r a n s l a t e d in P o l y m e r Sci. U.S.S.R.) 9. V. A. ROGINSKII a n d V. B. MILLER, Dokl. A N SSSR 215: 1164, 1974 10. Ye. M. SLOBODETSKAYA and O. I. KARPUKH1N, ]:)old. A N SSSR 236: 677, 1977