Effect of morphology on the photooxidation of polypropylene films

Polymer Photochemistry 6 (1985) 425-435

Effect o| Morphology on the Photooxidation of Polypropylene ]Films

F. P. L a M a n t i a , G . S p a d a r o a n d D . A c i e r n o Istituto di Ingegneria Chimica, University of Palermo, 90128 Palermo, Italy (Received: 23 May 1984)

ABSTRACT Photooxidative degradation of polypropylene films with different morphology (degree of crystallinity and orientation) has been followed by means of mechanical tests. The kinetics o[ the photooxidation are signiOcantly depressed on increasing crystaUinity and especially orientation. This behaviour has been interpreted in terms of the reinforcing action due to the oriented crystalline fibrils. A correlation has been attempted between the photooxidative kinetics and a morphological parameter.

INTRODUCTION T h e literature concerning the role played by crystallinity and orientation in the photooxidative process of the polyolefines is very conflicting. A n d indeed, while i m p r o v e d resistance to the photooxidative attack and higher crystallinity are well correlated, 1-3 the effect of orientation still seems uncertain. B o t h acceleration 4~ and slowing 6-8 of the photooxidative kinetics have b e e n found, and s o m e authors 9 did not o b s e r v e any effect. In particular, A k a y 6 s h o w e d that while H D P E and L D P E samples improve their resistance to U V oxidation 425

Polymer Photochemistry 0144-2880/85/$3.30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Northern Ireland

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F. P. La Mantia, G. Spadaro, D. Aciemo

with higher orientation, drawing increases photooxidative degradation in PVC. McTigue and Blumberg 1° in natural weathering tests of uniaxially oriented polypropylene found an improved U V stability up to a given orientation. Further drawing first does not influence significantly, and then decreases, the weathering resistance. An assessment of this subject seems necessary because uniaxial and biaxial orientation is often induced in the finished products in order to improve many characteristics, in particular their mechanical properties. In this work the influence of uniaxial and biaxial orientation and of different induced crystallinity on the photooxidation of polypropylene films has been studied. Photooxidative degradation has been followed mainly by means of mechanical tests.

EXPERIMENTAL Commercial films of unoriented polypropylene, Moplefan BT (Moplefan, Italy), have been used in order to produce uniaxially oriented specimens. The raw material has the following characteristics: Mw = 450 000, M,,,/M. = 9, M F I = 1.75 and 0 = 0.905 g/cm 3. The films were used as received and then with their U V antioxidant stabilizers. Unoriented samples, 6 cm long, were drawn at 8 0 ° C and at a drawing velocity of i cm/min, by means of an Instron Universal Testing Machine equipped with a temperature cabinet. After reaching the desired draw ratio, A, air was blown against the sample to cool it quickly to the ambient temperature to freeze the orientation. Draw ratios, A, of 2 and 4 have been applied to the samples of original thickness of 50 and 100 ~ m respectively. This was to obtain after drawing specimens with a thickness of about 25 + 2 ~m, like that of the unoriented films, thus avoiding the influence 9 of thickness on the photooxidation kinetics. A commercial biaxially oriented film, Moplefan OT, 25 ~ m thick, was also used as received. The photooxidation was carried out, for various time periods, by exposing the specimens to a xenon lamp (Osram X B O 150 W / l ) without any filter, at a distance of about 30 cm. The dose rate was 120 W / m 2 as measured by means of a thermopile Eppley model G3. The test temperature was 25 °C.

Effect of morphology on the photooxidation of polypropylene films

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T h e specimens for the tensile tests w e r e cut in the stretching direction (SD) and in the cross direction (CD) for the uniaxially oriented polymer. B e c a u s e the mechanical properties of the biaxially oriented films are different in the two directions, in this case also specimens were cut o u t in b o t h directions: in the following results samples with higher mechanical properties are called SD, those cut in the cross direction are called C D . Mechanical tests were p e r f o r m e d at r o o m t e m p e r a t u r e in an Instron Testing Machine at an elongation rate of 0-33 min -1. All the results w e r e d e t e r m i n e d as the average of several m e a s u r e m e n t s , usually five. Calorimetric tests were p e r f o r m e d with the aid of a Perkin E l m e r D S C l B . The heating rate was 8 °C/min. The gel content was d e t e r m i n e d by means of a soxhlet extractor. The solvent was xylol and the extraction time was 72 h.

RESULTS AND DISCUSSION The effect of U V irradiation on the stress-strain curves (tr-e) is shown in Fig. 1 for samples with A = 4 cut in the stretching and in the cross direction. The curves are relative to the u n e x p o s e d samples and to the samples irradiated for 100 h (SD) and for 50 h (CD). In this case no longer exposure time was possible b e c a u s e the r e m a r k a b l e brittleness of the samples did not allow the handling of the film. It can b e seen that the ultimate properties, and in particular the elongation at break, are dramatically d e p e n d e n t on the p h o t o o x i d a tive degradation. H o w e v e r , on increasing the orientation, the deterioration of the ultimate properties is much lower. Also the elastic m o d u l u s d e p e n d s on the U V irradiation: a rise is well evident for b o t h the samples. In Fig. 2 the dimensionless m o d u l u s /~ (modulus of the irradiated sample divided by that of the corresponding unirradiated one) is plotted versus the exposure time for all the specimens. As o b s e r v e d previously, the modulus always increases; however, the increase d e p e n d s u p o n the orientation. Indeed, considering the pair of samples cut o u t from the same films in different directions, b u t with the same crystallinity, the rise is larger the lower the orientation. B e c a u s e no crosslinking is induced in the p o l y p r o p y l e n e by U V radiation, as revealed by the extraction tests, the increase of the

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elastic modulus can be explained by the greater crystallinity developed during photooxidation. 9 Secondary chemocrystallization resuits assuming that the chain segments derived from photooxidative scission of the original chains in a m o r p h o u s zones can further crystallize because of their increased mobility. D e t e r m i n a t i o n of the crystallinity from calorimetric m e a s u r e m e n t s on the same samples confirms this hypothesis. Increases in the degree of crystallinity of the order of 15% have been found, and differences a m o n g the different samples are not very significant. The effect of the orientation can be explained by assuming that the macromolecular segments crystallize isotropically during photooxidation and then the orientation in the crystalline phase decreases in the oriented samples, while no effect occurs in u n o r i e n t e d specimens. T h e r e f o r e the absolute value of the modulus rises because of the increased crystallinity; however, the increase is lower for the m o r e

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oriented samples for which a decreased orientation of the crystalline phase results. An indirect confirmation of this hypothesis is given by the thermograms. Figure 3 represents, as an example, the results relative to the unoriented and the uniaxially oriented sample. It is well evident that the endothermic peak is broader and the melting temperature is lower for the oriented samples after U V irradiation. Both these effects can be due to the presence of less oriented and less perfect crystals grown during the photooxidation. Tensile strength, #r, and elongation at break, ~r, are reported in Figs 4 and 5 as a function of the exposure time for all the samples, also in this case in dimensionless form. Both the ultimate properties show a rapid decrease with the irradiation time; the degradation

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kinetics are clearly slowed by the or i e nt at i on. Also the fract ure surfaces are v e r y different f or u n o r i e n t e d and o r i e n t e d samples. P h o t o g r a p h s of two f r a c t u r e d specimens are show n in Fig. 6. It is well e v id en t that at a high d e g r e e of o r i e n t a t i o n the cracks are of the interfibrillar type, being m o r e or less parallel to the draw direction. Similar b e h a v i o u r has b e e n f o u n d 7"11 in d r a w n stabilized H D P E and p o l y p r o p y l e n e films. This p h e n o m e n o n m a y be r e l a t e d to the fact that w e a k sites are g e n e r a t e d during the p h o t o o x i d a t i o n resulting f r o m chemical and physical het er oge ne it i es. T h e s e sites and their gradient act as stress c o n c e n t r a t o r s 8"12"13and of course are localized in the a m o r p h o u s zones b e t w e e n the fibrils of the draw n p o l y p r o p y l e n e .

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The w e a k sites are elongated during the mechanical test causing the interfibriUar cracks, b u t the fibrils act as a reinforcing agent giving rise to an i m p r o v e d resistance to photooxidation with respect to an oriented sample. It is worth noting, however, that in our view the bulk properties of the film also, and not only the surface defects, play an important role in the fracture b e h a v i o u r of photooxidized polymeric material. As for the fracture surface of the biaxially oriented films, no peculiar b e h a v i o u r is revealed and a regular surface is noticed after breaking. This is p r o b a b l y d u e to the fact that the fibrils in this case are not oriented in o n e direction only. T h e photooxidation kinetics can be followed by the elongation at break, ~r, versus time, t, curves through the first-order equation: In ~ = -Kst

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where Ks is the kinetic constant depending on the morphology and reported for the different samples in Table 1. For all the curves the coefficient r2>0.995. In order to give a quantitative assessment of the effect of the morphology on the photooxidation of the polymers, we have used a

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Fracture surfaces of photooxidized oriented samples, a, Uniaxially oriented, )t = 4, irradiation time 100 h; b, biaxially oriented, irradiation time 100 h.

Effect of morphology on the photooxidation of polypropylene films

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TABLE 1 Morphological Parameter, p, and Kinetic Constant, Ks, for the Samples

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The kinetic constant Ks depends upon the morphology, as stated above, and so can be correlated with the morphological parameter, in particular through the relation Ks = a - b In (p + c)

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where for polypropylene the values of a and b are 0.018 and 0.013 h -1 respectively and c is 0-015. Figure 7 reports the exposure time, t½, necessary to reduce to one half of the initial value both tensile strength and elongation at break versus the morphological parameter p. It is well evident that this time, and so the timelife of the polymeric films, increases remarkably with p. This means that an increase of the crystallinity and especially a better orientation (compare SD and CD samples of the same film) significantly depresses the photooxidation kinetics.

CONCLUDING REMARKS The results presented in this work show that by increasing the degree of crystallinity and the orientation, the photooxidative degradation kinetics of semicrystalline polymers, as seen by mechanical tests, are lowered. However, we think that this result cannot be generalized to all polymeric systems, because the deterioration of the mechanical properties of the photooxidized polymers is strongly dependent on the response of the polymer structure to the irradiation. Indeed, several cases can be imagined; in particular: (i)

(ii)

when the polymer is crystalline and its molecules break and crystallize during irradiation, the modulus increases and the photooxidative degradation of the ultimate properties is depressed with increasing orientation and crystallinity; when the polymer is amorphous, the reinforcing action of the crystalline fibrils is not present and a quicker degradation of the mechanical properties can be expected while the effect of the orientation should be very modest. REFERENCES

1. Scott, G., Atmospheric oxidation and antioxidants, Elsevier, Amsterdam, 1966.

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2. Ramby, B. and Rabek, J. F., Photodegradation, photooxidation and photostabilization of polymers, Wiley Interscience, New York, 1979. 3. McKeUar, J. F. and Allen, N. S., Photochemistry of man-made polymers, Applied Science Publishers, London, 1979. 4. Benachour, D. and Rogers, C. E., ACS Syrup. Ser., 151 (1981) 263. 5. Benachour, D. and Rogers, C. E., Polymer Preprints, 23 (1982) 209. 6. Akay, G., Tincer, T. and Aydin, E., Europ. Polym. J., 16 (1980) 597. 7. Akay, G. and Tincer, T., Polym. Eng. Sci., 21 (1981) 8. 8. Raab, M., Kotulhk, L., Kolarik, J. and Pospisil, J., J. Appl. Polym. Sci., 22 (1982) 2457. 9. Garton, A., Carlsson, J. and Wiles, D. M., J. Polym. Sci., Polym. Chem. Ed., 16 (1978) 33. 10. McTigue, F. H. and Blumberg, M., Appl. Polym. Syrup., 4 (1967) 185. 11. Blais, P., Carlsson, J. and Wiles, D. M., J. Polym. Sci., Polym. Chem. Ed., 10 (1972) 1077. 12. Pabiot, J. and Verdu, J., Polym. Eng. Sci., 21 (1981) 32. 13. La Mantia, F. P., Europ. Polym. J., 20 (1984) 993.