wavelength sensitivity of photooxidation of styrene-butadiene-styrene copolymer

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ELSEVIER Polymer Degradation and Stability 48 (1995) 99-102 0 1995 Elsevier Science Limited 0141-3910(95)00011-9 Printed in Northern Ireland. All rights reserved 0141-3910/95/$09.50 Wavelength sensitivity of photooxidation of styrene-butadiene-styrene copolymer Hu Xingzhou Institute of Chemistry, Academia Sinica, Beijing 100080, China Luo Zubo Research Institute of Synthetic Materials, Department of Chemical Industry of China, Guangzhou 510630, China (Received 17 October 1994; accepted 24 December 1994) The wavelength sensitivity of SBS exposed to filtered xenon arc radiation and sunlight was determined by the sharp cut filter technique based on the changes in IR absorption and mechanical properties. The spectral response curves of SBS exposed to both filtered xenon lamp and sunlight were similar. They showed that wavelength region, responsible for photooxidation of SBS, is in the range of approximately 290-380nm. The wavelength sensitivity of polystyrene exposed to filtered xenon arc radiation was also determined and was different from that of SBS. It is suggested that the wavelength sensitivity of SBS is determined mainly by the polybutadiene component. INTRODUCTION Styrene-butadiene-styrene (SBS) copolymer is a thermoplastic elastomer, widely used in the plastics, coatings and polymer composite in- dustries. Like other polymers containing poly- butadiene segments, SBS is susceptible to photo- oxidation. Our previous study’ on photooxidation of SBS and a PP-SBS blend showed that the photooxidation of SBS starts from the polybuta- diene component and that its oxidation will lead to photodegradation of both the polystyrene component in SBS and the PP component in a PP-SBS blend. Therefore, an understanding of the wavelength sensitivity of SBS is important. to the stabilization of both SBS itself and the SBS-containing composites. In the present study the wavelength sensitivity of photooxidation of SBS was determined in terms of the changes in IR absorption and mechanical properties when exposed to borosili- cate glass-filtered xenon arc radiation and direct sunlight. The relative effect of individual spectral regions was determined by the sharp cut filter technique. EXPERIMENTAL Sample preparation SBS resin (YH 802) was supplied by Rubber Plant, Yueyang Petrochemical Company, China. It was a star block copolymer with a styrene- butadiene ratio of 40/60. SBS films used for determination of IR absorption change during photooxidation were prepared by evaporating the SBS chloroform solution on a PTFE plate. The thickness of the film was 35 pm. Two types of SBS sheet were prepared for determination of mechanical property change in exposure tests with xenon lamp and sunlight, respectively. SBS sheets O-5 mm thick were prepared by pressing the resin granules at 190°C between two PTFE sheets. SBS sheets were cut into dumbbell shaped specimens for exposure to the xenon 99

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Page 1: Wavelength sensitivity of photooxidation of styrene-butadiene-styrene copolymer

ELSEVIER

Polymer Degradation and Stability 48 (1995) 99-102 0 1995 Elsevier Science Limited

0141-3910(95)00011-9

Printed in Northern Ireland. All rights reserved 0141-3910/95/$09.50

Wavelength sensitivity of photooxidation of styrene-butadiene-styrene copolymer

Hu Xingzhou Institute of Chemistry, Academia Sinica, Beijing 100080, China

Luo Zubo Research Institute of Synthetic Materials, Department of Chemical Industry of China, Guangzhou 510630, China

(Received 17 October 1994; accepted 24 December 1994)

The wavelength sensitivity of SBS exposed to filtered xenon arc radiation and sunlight was determined by the sharp cut filter technique based on the changes in IR absorption and mechanical properties. The spectral response curves of SBS exposed to both filtered xenon lamp and sunlight were similar. They showed that wavelength region, responsible for photooxidation of SBS, is in the range of approximately 290-380nm. The wavelength sensitivity of polystyrene exposed to filtered xenon arc radiation was also determined and was different from that of SBS. It is suggested that the wavelength sensitivity of SBS is determined mainly by the polybutadiene component.

INTRODUCTION

Styrene-butadiene-styrene (SBS) copolymer is a thermoplastic elastomer, widely used in the plastics, coatings and polymer composite in- dustries. Like other polymers containing poly- butadiene segments, SBS is susceptible to photo- oxidation. Our previous study’ on photooxidation of SBS and a PP-SBS blend showed that the photooxidation of SBS starts from the polybuta- diene component and that its oxidation will lead to photodegradation of both the polystyrene component in SBS and the PP component in a PP-SBS blend. Therefore, an understanding of the wavelength sensitivity of SBS is important. to the stabilization of both SBS itself and the SBS-containing composites.

In the present study the wavelength sensitivity of photooxidation of SBS was determined in terms of the changes in IR absorption and mechanical properties when exposed to borosili- cate glass-filtered xenon arc radiation and direct sunlight. The relative effect of individual spectral

regions was determined by the sharp cut filter technique.

EXPERIMENTAL

Sample preparation

SBS resin (YH 802) was supplied by Rubber Plant, Yueyang Petrochemical Company, China. It was a star block copolymer with a styrene- butadiene ratio of 40/60. SBS films used for determination of IR absorption change during photooxidation were prepared by evaporating the SBS chloroform solution on a PTFE plate. The thickness of the film was 35 pm. Two types of SBS sheet were prepared for determination of mechanical property change in exposure tests with xenon lamp and sunlight, respectively.

SBS sheets O-5 mm thick were prepared by pressing the resin granules at 190°C between two PTFE sheets. SBS sheets were cut into dumbbell shaped specimens for exposure to the xenon

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Page 2: Wavelength sensitivity of photooxidation of styrene-butadiene-styrene copolymer

100 Hu Xingzhou. Luo Zubo

lamp. For sunlight exposure SBS sheets 5 mm thick were prepared in the following manner: SBS resin was rolled at 115°C for 5 min. The plates thus obtained were then pressed at 145°C

was measured by an Integrated Illuminometer, model PH-llM-2AT (Suga Test Machine Co.).

Measurement of photooxidation between two polyester sheets into sheets that were cut into specimens. The changes in absorption of SBS and PS films

during photooxidation were followed by a Perkin-Elmer IR spectrophotometer, model PE 683 for xenon arc irradiated films and model PE 1700X for outdoor exposed films, respectively. The measurements of mechanical properties of SBS specimens were carried out by a Shinkch Tensile Test Machine. UV absorption curves of filters were measured by a Hitachi 340 UV- spectrophotometer.

Polystyrene (PS) resin was supplied by Yanshan Petrochemical Company, China. PS films 35 pm thick were prepared by casting from chloroform solution.

Exposure

The exposure sources were a xenon arc lamp and solar radiation. Xenon arc exposure was carried out in a Xenontest 450 Light and Weather Fastness Tester with a borosilicate glass-filtered 4500 W xenon lamp as light source. The testing temperature was 45°C and relative humidity was 40%. To determine the wavelength sensitivity, the sharp cut filter technique was used with a set of 11 filters, 4 X 4 cm* each. The spectral transmittance curves of these filters are shown in Fig. 1. The specimens were mounted about 2 mm behind the different filters and backed with black paper to prevent unfiltered scattered light from irradiating the back side of the sample.

Outdoor exposure was carried out in Tian- xinggang exposure site, Guangzhou, during the summer of 1993. The specimens were mounted behind the different filters and backed with filter paper and aluminum foil. The filtered specimens were exposed facing south at an angle of 23” (latitude angle). Total energy of UV exposure

100

90

80

300 400 ml

Wavelength (nm)

Fig. 1. Transmittance curves of sharp cut filters.

RESULTS AND DISCUSSION

During photooxidation of SBS film, the following changes in IR absorption were observed: increase in carbonyl (maximum at 1720 cm-‘) and hydroxyl (maximum at 3420 cm-‘) regions; decrease at 910 cm-’ (end vinyl) and 967 cm-’ (truns-vinylene); and increase in region of 1000-1300 cm-’ (cross link structure). These changes were consistent with the photooxidation of the polybutadiene component.“’

The spectral response curves of SBS exposed to xenon arc radiation for different times, based on IR absorption of carbonyl is shown in Fig. 2. Each point on the curve represents the increment of IR absorption at 1720cm. ’ of SBS exposed behind the cut-on filter, plotted in terms of its 10% transmittance wavelength. It can be seen that the main wavelength region responsible for photooxidation of SBS is in the range of about 290-380 nm. The different shapes of curves 1 and 2, obtained at different irradiation times, are due

_

250 300 350 400 450 SO0 550

IO%- transmittance wavelength of sharp cut filter (nm)

Fig. 2. Spectral response curves of SBS exposed to a filtered 4500 W xenon lamp. Irradiation time: (1) 20 h. (2) 27 h.

Page 3: Wavelength sensitivity of photooxidation of styrene-butadiene-styrene copolymer

Wavelength sensitivity of SBS copolymer photooxidation 101

mainly to the presence of some induction period for photooxidation of SBS. During this induction period, the amount of oxidation products is too small to be measured by IR absorption. Only after the end of the induction period can the oxidation products be accurately recorded. Therefore, curve 2 (obtained when the oxidation of all samples capable of photooxidation has passed the induction period) should reflect more correctly the wavelength dependence of photo- oxidation of SBS. The difference between curves 1 and 2 does not, in our opinion, mean that effective wavelengths are shifting towards a longer wavelength range, although some exten- sion of effective wavelength range to longer wavelength side may occur due to the yellowing of SBS films (gradual yellowing of SBS samples with increasing exposure time was observed in our experiments), as indicated by Searle et al. in the case of ABS photooxidation.

The spectral response curves of SBS exposed to xenon arc radiation in terms of IR absorption changes of three peaks (1720, 3420, and 967 cm-‘) are shown in Fig. 3. As seen from this figure, the three curves have the same tendency.

The wavelength dependence of photooxidation of SBS specimens exposed to xenon arc radiation, in terms of changes in elongation at break, is shown in Fig. 4. The main wavelength region, responsible for loss of elongation is found in the range 290-400nm. Figure 5 compares the spectral response curves of SBS exposed to xenon arc radiation, expressed in terms of changes in carbonyl IR absorption and tensile strength, respectively. As seen from the figure, two curves show quite similar wavelength effect, meaning that both changes of SBS samples are caused by the same mechanism.

5 ‘J

& 500

2 W

t 01 I I I I I

250 300 350 400 450 500 550

10% transmittance wavelength of sharp cut filter (nm)

Fig. 4. Spectral response curves of SBS exposed to a filtered xenon lamp. Irradiation time: (1) 12 h, (2) 18 h.

E

- 20 Ef e! yi 0

5 0.5 - - IO 5

c-

250 300 350 400 450 500 550

Fig. 5. Spectral response curves of SBS exposed to a filtered 4500 W xenon lamp. Irradiation time: (1) 27 h, (2) 12 h.

The spectral response curves of SBS exposed to sunlight in Guangzhou, expressed in terms of carbonyl index, elongation at break and tensile strength, are shown in Figs 6-8, respectively. Comparing Fig. 6 with Fig. 2, we find that the spectral response curves in both exposure conditions are quite similar. The same similarity can be observed when Fig. 7 is compared with Fig. 4, and curve 1 in Fig. 8 with curve 2 in Fig. 5.

3.01 2.5

1.5 , I

; 2.0

u

E I.5

8 a 1.0

250 300 350 400 450 500 550

10% transmittance wavelength of sharp cut filter (nm)

Fig. 3. Spectral response curves of SBS exposed to a filtered xenon lamp for 27 h.

10% transmittance wavelength of sharp cut filter (nm)

0.5

0 -I

2.50 300 350 400 4.50

10% transmittance wavelength of sharp cut filter (nm)

Fig. 6. Spectral response curves of SBS exposed to Guangzhou sunlight. Total energy of UV exposure: (1)

3-69 MJ/m’, (2) 8.34 MJ/m*.

Page 4: Wavelength sensitivity of photooxidation of styrene-butadiene-styrene copolymer

102 Hu Xingzhou, Luo Zubo

0 L I I I _I 250 300 350 400 450

10% transmittance wavelength of sharp cut filter (nm)

Fig. 7. Spectral response curves of SBS exposed to Guangzhou sunlight. Total energy of UV exposure:

(1) 3.69 MJ/m*, (2) 8.34 MJ/m2.

::

0 1 I I I

250 300 350 400 450

10% transmittance wavelength of sharp cut filter (nm)

Fig. 8. Spectral response curves of SBS exposed to Guangzhou sunlight. Total energy of UV exposure:

(1) 3.69 MJ/m*, (2) 8.34 MJ/m*.

For comparison, we also investigated the wavelength dependence of photooxidation for polystyrene. During photooxidation of PS films, the IR absorption increased in the carbonyl region (two marked peaks located at 1690 and 1725 cm-‘) and the hydroxyl region (maximum at 3540cm-I), and decreased at the phenyl group (695 and 755 cm-‘). The spectral response curves of PS films exposed to xenon arc radiation in terms of the increase in IR absorption are shown in Fig. 9. In the range 290-350nm, a linear wavelength dependence is observed.

Both the IR absorption change and its wavelength dependence for PS photooxidation were different from that of SBS. In addition, the difference in photooxidation rate between SBS and PS was very large. SBS film placed behind a filter with 10% transmittance wavelength 266 nm

250 300 350 400 450 500 5x3

10% tran.\mittance wavelength of sharp cut filter (nm)

Fig. 9. Spectral response curves of PS exposed to a filtered 4500 W xenon lamp for 389 h.

breaks after irradiation with xenon lamp for 27 h. But for PS film exposed behind the same filter, cracks appeared only after 299 h irradiation. Obviously, the photooxidation of SBS was much more rapid than that of PS. From these differences it can be reasonably concluded that the wavelength dependence of photooxidation of SBS is determined mainly by the polybutadiene component. During exposure of SBS the photooxidation of the PB component occurred rapidly. This led to the formation of different oxidation products, degradation and cross linkage of polymer chains, and changes in mechanical properties. Since the changes in oxidation products and mechanical properties were caused by the same source (photooxidation of the PB component), similar wavelength dependence of SBS photooxidation based on these changes was observed.

ACKNOWLEDGEMENT

The authors are grateful to the National Natural Science Foundation of China for supporting this research.

REFERENCES

1.

2.

3.

Hu Xingzhou and Qi Jiying, Photogr. Sci. Photochem. (Ch.), 4 (1990) 307. Adam, C., Lacoste, J. & Lamaire, J., Polym. Deg. Stab., 24 (1989) 185. Searle, N. D., Maecker, N. L. & Crewdson, L. F. E., J. Polym. Sci., Part A: Polym. Chem., 27 (1989) 1341.