- Email: [email protected]
Effect of stereoregularity on the photo-oxidative degradation and weathering of polypropylenes
Polymer Degradation and Stability 67 (2000) 1±5
Eect of stereoregularity on the photo-oxidative degradation and weathering of polypropylenes M. Kato, A. Tsuruta, S. Kuroda, Z. Osawa* Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan Received 16 April 1999; accepted 14 May 1999
Abstract Eect of stereoregularity on the photo-oxidative degradation and weathering of polypropylenes (PPs) (isotactic: IPP and syndiotactic: SPP) was examined compared with that of high-density polyethylene (HDPE) by means of spectrophotometries and molecular weight measurement. It was found that SPP is more stable than IPP against photo-oxidative degradation and to weathering, and HDPE is the most stable among the examined polymers. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction We have been concerned with the eect of stereoregularity on the degradation of polymers, especially polypropylenes [1±3] and reported that the stereoregularity of the polypropylenes de®nitely aects their thermal oxidative degradation and syndiotactic polypropylene (SPP) is much more stable than isotactic polypropylene (IPP). Our concern, naturally, has been focussed on the eect of the stereoregularity on the photo-oxidative degradation and weathering of the polypropylenes. In this work, we have examined photo-oxidative stability and weatherability of IPP and SPP compared with that of high-density polyethylene (HDPE) by means of infrared and ultraviolet spectrophotometries and molecular weight measurement. The results indicated that SPP is more stable than IPP to photo-oxidative degradation and weathering, and HDPE is the most stable among the examined polymers. 2. Experimental 2.1. Materials Isotactic polypropylene (IPP) and syndiotactic polypropylene (SPP) and high density polyethylene (HDPE) * Corresponding author.
used in this work were the same as that used in previous papers [2,3]. 2.2. Photo-irradiation and weathering 2.2.1. Photo-irradiation Each sample ®lm ca. 100 mm in thickness was ®xed on a black frame, and placed at right angles and 20 cm from the light source, a 300 W high pressure mercury lamp of Orc-OHD-320 M, main wavelength 254, 290 and 365 nm. Photo-irradiation was carried out in ambient atmosphere with or without a cut-o ®lter of Pyrex glass plate for various times. 2.2.2. Weathering Each ®lm was outdoor exposed on the roof of our research building from 11 September, 1997 to 21 January, 1998. Weathering conditions were based on JIS K7219 [4]. Namely, sample ®lms were placed 30 cm height from a stand ®xed at ca. 50 cm height on the roof to the south direction at an angle of elevation of 30 . 2.3. Measurement 2.3.1. Spectrophotometry Infrared spectra of the sample ®lms were recorded on a JASCO Fourier Transform Infrared (FT-IR) 8000 spectrophotometer under resolution power of 2 cmÿ1 and integrating number of 64. Ultraviolet spectra were recorded on a double beam grating spectrophotometer, Hitachi 228 type.
0141-3910/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(99)00098-1
2
M. Kato et al. / Polymer Degradation and Stability 67 (2000) 1±5
2.3.2. Molecular weight The molecular weights of the samples were determined by gel permeation chromatography (GPC) using a Senshu Kagaku SSC-7100 type instrument. The instrument conditions were the same as described in a previous paper [3]. 3. Results and discussion 3.1. Photo-irradiation with a glass ®lter Infrared spectra in carbonyl regions of the photoirradiated polypropylenes are shown in Fig. 1. Although precise comparison of each carbonyl group between IPP and SPP is dicult, it is quite obvious that the formation of the carbonyl groups in IPP is remarkably faster than that of SPP during the photo-irradiation with light longer than 300 nm in ambient atmosphere. However, in the case of HDPE no appreciable change in IR spectra was observed under the same photo-irradiation conditions. Plots of absorbance at 1715 cmÿ1 against photo-irradiation time for each sample apparently demonstrate the
Fig. 1. Infrared spectra of isotactic polypropene (a) and syndiotactic polypropene (b) during photo-irradiation in air. (Irradiated with a mercury lamp through a Pyrex glass ®lter.)
facts mentioned above (see Fig. 2). For example, after ca. 100 h photo-irradiation the absorbance of SPP is extremely small, while that of IPP is ca. 0.1, and more than 200 h is required to attain the equal absorbance level (that is ca. 0.1) for SPP. Similar phenomena were also observed in hydroxyl group regions in IR spectra. Detection of dierence in formation of hydroperoxide groups between IPP and SPP was unattainable although dierence IR spectra revealed very little formation of hydroperoxide groups in IPP. In the case of HDPE little change in IR spectra was observed within irradiation time. As shown in Fig. 3, individual plots of the absorbance at 3420 cmÿ1 due to hydroxyl groups against photo-irradiation time clearly demonstrate dierence in photostability among the samples examined. Namely, HDPE is the most stable and SPP is much more stable than IPP to the photo-oxidative degradation. Successively, changes in ultraviolet spectra were examined and representative UV spectra of IPP and SPP during the photo-irradiation are shown in Fig. 4, respectively. In the case of IPP, absorbance between 200 and 250 nm increases steadily with time. On the other hand, the UV spectrum of unirradiated SPP shows a broad absorption around 230 nm which decreases up to 100 h and then increases again with the photo-irradiation time. In the case of HDPE, little change in UV spectra was observed within 250 h. Plots of absorbance at 230 nm against time shown in Fig. 5 clearly demonstrate the results described above. Identi®cation and roles of the substances corresponding
Fig. 2. Changes in absorbance in carbonyl region (at 1715 cmÿ1) of isotactic and syndiotactic poly(propylene)s and (at 1717 cmÿ1) of high density polyethylene during photo-irradiation in air. (Irradiated with a mercury lamp through a Pyrex ®lter.)
M. Kato et al. / Polymer Degradation and Stability 67 (2000) 1±5
Fig. 3. Changes in absorbance in >O-H region (at 3420 cmÿ1) of isotactic polypropylene, syndiotactic polypropylene and high density polyethylene during photo-irradiation in air. (Irradiated with a mercury lamp through a Pyrex ®lter.)
3
Fig. 5. Changes in absorbance at 230 nm of isotactic polypropene, syndiotactic polyupropene and high density polypropene during photo-irridation in air. (Irridated with a mercury lamp through a pyrex glass ®lter.)
involved with the photostability of SPP in addition to the eect of the stereoregularity. In the previous papers we reported that SPP is much more stable to thermo-oxidative degradation than IPP and it is probably ascribable to the favorable con®guration of IPP to the autoxidation through a back biting mechanism in which a formed peroxy radical (RO ) easily abstracts an adjacent tert-hydrogen in the same polymer chain [2,3,5]. Therefore, in the photo-oxidative degradation of polypropylenes, the photostability of SPP can be similarly explained by the same mechanism. 3.2. Photo-irradiation without a glass ®lter
Fig. 4. UV spectra of isotactic polypropylene (a) and syndiotactic polypropylene (b) during photo-irradiation in air. (Irridated with a mercury lamp through a pyrex ®lter.) ÐÐ: 0 h, .........: 48 h, - - - -: 144 h, ± . ±: 216 h, Ð Ð: 240 h.
to a broad absorption at ca. 230 nm are not clear at present. However, since the induction period, ca. 100 h observed in UV spectra, some impurities may be
When photo-irradiation was carried out with total light emitted from a mercury lamp without using a Pyrex glass ®lter, the phtoto-oxidative degradation of the samples became, naturally, much faster than that in the irradiation with a glass ®lter mentioned in the previous section. In the case of polypropylenes, oxygenated groups such as carbonyl and hydroxyl groups were easily formed and accumulated during the photo-irradiation. In addition, as shown in Fig. 6, there are no appreciable dierences in the rate of the formation of carbonyl groups between IPP and SPP. However, the absorbance of SPP is slightly lower than that of IPP within ca. 6 h. Similar phenomena were also observed in the formation of hydroxyl groups and changes in absorbance at 230 nm in UV spectra. Furthermore, photo-oxidative degradation of HDPE commences easily and carbonyl groups are formed in the early stage
4
M. Kato et al. / Polymer Degradation and Stability 67 (2000) 1±5
and the absorbance at 1715 cmÿ1 increases linearly with photo-irradiation time. The results mentioned above apparently indicate that total irradiation energy is too large to examine photostability precisely between IPP and SPP. In this context, the photo-irradiation with monochromatic light, 260, 280, 300, 320, 340 and 360 nm was kindly carried with a computer-operated large-scale spectrograph in the
National Institute for Basic Biology at Okazaki, Japan [6,7]. It was found that lower wavelength light of 260 and 280 nm was most signi®cant for the photo-oxidative degradation of the polypropylenes. Furthermore, in the case of g-ray irradiation of polypropylenes there was also little dierence in the oxidizability between IPP and SPP [8]. 3.3. Outdoor exposure Since it has been found that the photo-stability of SPP is superior to that of IPP during the irradiation with light longer than ca. 300 nm, outdoor exposure was performed for four months to compare the weatherability of the three samples. IR spectra of the outdoor exposed samples also showed the formation of oxygenated groups such as carbonyl and hydroxyl groups, and their increase during outdoor exposure. Individual plots of absorbance of each sample in the carbonyl
Fig. 6. Changes in absorbance in carbonyl region (at 1715 cmÿ1) of isotactic and syndiotactic poly(propylene)s and (at 1717 cmÿ1) of high density polyethylene during photo-irridation in air. (Irradiated without a ®lter.)
Fig. 7. Changes in absorbance in carbonyl region (at 1715 cmÿ1) of isotactic and syndiotactic poly(propylene)s and (at 1717 cmÿ1) of high density polyethylene during outdoor exposure under ambient conditions. (From 11 September 1997 to 21 January 1998.)
Fig. 8. Changes in Mn (a), Mw (b) and Mw/Mn (c) of isotactic and syndiotactic poly(propylene)s and high density polyethylene during outdoor exposure under ambient conditions. (From 11 September 1997 to 21 January 1998.)
M. Kato et al. / Polymer Degradation and Stability 67 (2000) 1±5 Table 1 Molecular weight and its distribution during outdoor exposurea Mn, Mw, Mw/Mn of isotactic polypropylene, syndiotactic polypropylene and high density polyethylene during outdoor exposure under ambient conditions Sample
Exposed time (month)
Mn
Mw
IPP
0 1 2 3 4
50 20 17 16 16
000 000 000 800 500
250 144 96 68 66
000 000 000 000 000
5.0 7.2 5.6 4.0 4.0
SPP
0 1 2 3 4
45 36 36 33 32
000 800 800 600 800
76 76 76 75 76
500 000 000 000 000
1.7 2.1 2.1 2.2 2.3
0 1 2 3 4
1700 12 960 12 690 12 600 12 150
81 75 75 72 72
000 600 600 900 900
4.8 5.8 6.0 5.8 6.0
HDPE
a
- M/Mn
5
To con®rm the dierence in weatherability of the samples estimated by the spectrophotometry, changes in molecular weight and its distribution were examined. The results obtained are summarized in Table 1 and are graphically shown in Fig. 8, respectively. As can be seen a drastic decrease in the molecular weight occurs in IPP during the outdoor exposure while its decrease is extremely less in SPP and HDPE. 4. Conclusions Eect of stereoregularity of polypropylenes (IPP and SPP) on the photostability and weatherability was examined, comparing with high-density polyethylene (HDPE). It was found that SPP is more stable than IPP to photo-oxidative degradation with glass ®lter irradiation, light longer than ca. 300 nm and also to the weathering when exposed outdoors. HDPE is the most stable among the three samples examined.
From 11 September 1997 to 20 January 1998 in Kiryu, Japan.
Acknowledgements region are shown in Fig. 7. It is quite apparent that the weatherability of SPP is much better than that of IPP and also HDPE is the most stable among the examined samples. Similar results were also observed for the formation of hydroxyl groups in IR spectra. Furthermore, changes in absorbance at 230 nm in UV spectra showed similar phenomena observed in the photo-irradiation with a glass ®ltered light described in Section 3.1. Namely, in the case of IPP, absorbance of the UV spectra increased with exposure time while in the case of SPP the absorbance decreased until ca. 2 months then gradually increased. In addition to the changes in the chemical structure, a remarkable dierence in mechanical properties was noticed between IPP and SPP. For example, IPP ®lm became very brittle after 3.5 months outdoor exposure and it was too brittle to measure IR spectrum in a ®lm state. In contrast, SPP kept its original ®lm form and was ¯exible even after 4 months.
The authors gratefully acknowledge Professor M. Terano and his colleagues, School of Materials Sciences, Japan Advanced Institute of Science and Technology for the donation of samples and measurement of molecular weight. References [1] Osawa Z, Saito T, Kimura Y. J Appl Polym Sci 1968;22:563. [2] Osawa Z, Kato M, Terano M. Macromol Rapid Commun 1997;18:157. [3] Kato M, Osawa Z. Polym Degrad Stab 1999;65:457. [4] JIS K7219. [5] Hatanaka T, Mori H, Terano M. Macromol Rapid Commun 1997;18:667. [6] Watanabe M, Furuya M, Miyoshi Y, Inoue Y, Iwahashi I, Matsumoto K. J Appl Polym Sci 1970;14:765 and Photochem Photobiol 1982;36:491. [7] Fukuda Y, Osawa Z. Polym Degrad Stab 1991;34:75. [8] Osawa Z. Unpublished results.
Eect of stereoregularity on the photo-oxidative degradation and weathering of polypropylenes M. Kato, A. Tsuruta, S. Kuroda, Z. Osawa* Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan Received 16 April 1999; accepted 14 May 1999
Abstract Eect of stereoregularity on the photo-oxidative degradation and weathering of polypropylenes (PPs) (isotactic: IPP and syndiotactic: SPP) was examined compared with that of high-density polyethylene (HDPE) by means of spectrophotometries and molecular weight measurement. It was found that SPP is more stable than IPP against photo-oxidative degradation and to weathering, and HDPE is the most stable among the examined polymers. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction We have been concerned with the eect of stereoregularity on the degradation of polymers, especially polypropylenes [1±3] and reported that the stereoregularity of the polypropylenes de®nitely aects their thermal oxidative degradation and syndiotactic polypropylene (SPP) is much more stable than isotactic polypropylene (IPP). Our concern, naturally, has been focussed on the eect of the stereoregularity on the photo-oxidative degradation and weathering of the polypropylenes. In this work, we have examined photo-oxidative stability and weatherability of IPP and SPP compared with that of high-density polyethylene (HDPE) by means of infrared and ultraviolet spectrophotometries and molecular weight measurement. The results indicated that SPP is more stable than IPP to photo-oxidative degradation and weathering, and HDPE is the most stable among the examined polymers. 2. Experimental 2.1. Materials Isotactic polypropylene (IPP) and syndiotactic polypropylene (SPP) and high density polyethylene (HDPE) * Corresponding author.
used in this work were the same as that used in previous papers [2,3]. 2.2. Photo-irradiation and weathering 2.2.1. Photo-irradiation Each sample ®lm ca. 100 mm in thickness was ®xed on a black frame, and placed at right angles and 20 cm from the light source, a 300 W high pressure mercury lamp of Orc-OHD-320 M, main wavelength 254, 290 and 365 nm. Photo-irradiation was carried out in ambient atmosphere with or without a cut-o ®lter of Pyrex glass plate for various times. 2.2.2. Weathering Each ®lm was outdoor exposed on the roof of our research building from 11 September, 1997 to 21 January, 1998. Weathering conditions were based on JIS K7219 [4]. Namely, sample ®lms were placed 30 cm height from a stand ®xed at ca. 50 cm height on the roof to the south direction at an angle of elevation of 30 . 2.3. Measurement 2.3.1. Spectrophotometry Infrared spectra of the sample ®lms were recorded on a JASCO Fourier Transform Infrared (FT-IR) 8000 spectrophotometer under resolution power of 2 cmÿ1 and integrating number of 64. Ultraviolet spectra were recorded on a double beam grating spectrophotometer, Hitachi 228 type.
0141-3910/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(99)00098-1
2
M. Kato et al. / Polymer Degradation and Stability 67 (2000) 1±5
2.3.2. Molecular weight The molecular weights of the samples were determined by gel permeation chromatography (GPC) using a Senshu Kagaku SSC-7100 type instrument. The instrument conditions were the same as described in a previous paper [3]. 3. Results and discussion 3.1. Photo-irradiation with a glass ®lter Infrared spectra in carbonyl regions of the photoirradiated polypropylenes are shown in Fig. 1. Although precise comparison of each carbonyl group between IPP and SPP is dicult, it is quite obvious that the formation of the carbonyl groups in IPP is remarkably faster than that of SPP during the photo-irradiation with light longer than 300 nm in ambient atmosphere. However, in the case of HDPE no appreciable change in IR spectra was observed under the same photo-irradiation conditions. Plots of absorbance at 1715 cmÿ1 against photo-irradiation time for each sample apparently demonstrate the
Fig. 1. Infrared spectra of isotactic polypropene (a) and syndiotactic polypropene (b) during photo-irradiation in air. (Irradiated with a mercury lamp through a Pyrex glass ®lter.)
facts mentioned above (see Fig. 2). For example, after ca. 100 h photo-irradiation the absorbance of SPP is extremely small, while that of IPP is ca. 0.1, and more than 200 h is required to attain the equal absorbance level (that is ca. 0.1) for SPP. Similar phenomena were also observed in hydroxyl group regions in IR spectra. Detection of dierence in formation of hydroperoxide groups between IPP and SPP was unattainable although dierence IR spectra revealed very little formation of hydroperoxide groups in IPP. In the case of HDPE little change in IR spectra was observed within irradiation time. As shown in Fig. 3, individual plots of the absorbance at 3420 cmÿ1 due to hydroxyl groups against photo-irradiation time clearly demonstrate dierence in photostability among the samples examined. Namely, HDPE is the most stable and SPP is much more stable than IPP to the photo-oxidative degradation. Successively, changes in ultraviolet spectra were examined and representative UV spectra of IPP and SPP during the photo-irradiation are shown in Fig. 4, respectively. In the case of IPP, absorbance between 200 and 250 nm increases steadily with time. On the other hand, the UV spectrum of unirradiated SPP shows a broad absorption around 230 nm which decreases up to 100 h and then increases again with the photo-irradiation time. In the case of HDPE, little change in UV spectra was observed within 250 h. Plots of absorbance at 230 nm against time shown in Fig. 5 clearly demonstrate the results described above. Identi®cation and roles of the substances corresponding
Fig. 2. Changes in absorbance in carbonyl region (at 1715 cmÿ1) of isotactic and syndiotactic poly(propylene)s and (at 1717 cmÿ1) of high density polyethylene during photo-irradiation in air. (Irradiated with a mercury lamp through a Pyrex ®lter.)
M. Kato et al. / Polymer Degradation and Stability 67 (2000) 1±5
Fig. 3. Changes in absorbance in >O-H region (at 3420 cmÿ1) of isotactic polypropylene, syndiotactic polypropylene and high density polyethylene during photo-irradiation in air. (Irradiated with a mercury lamp through a Pyrex ®lter.)
3
Fig. 5. Changes in absorbance at 230 nm of isotactic polypropene, syndiotactic polyupropene and high density polypropene during photo-irridation in air. (Irridated with a mercury lamp through a pyrex glass ®lter.)
involved with the photostability of SPP in addition to the eect of the stereoregularity. In the previous papers we reported that SPP is much more stable to thermo-oxidative degradation than IPP and it is probably ascribable to the favorable con®guration of IPP to the autoxidation through a back biting mechanism in which a formed peroxy radical (RO ) easily abstracts an adjacent tert-hydrogen in the same polymer chain [2,3,5]. Therefore, in the photo-oxidative degradation of polypropylenes, the photostability of SPP can be similarly explained by the same mechanism. 3.2. Photo-irradiation without a glass ®lter
Fig. 4. UV spectra of isotactic polypropylene (a) and syndiotactic polypropylene (b) during photo-irradiation in air. (Irridated with a mercury lamp through a pyrex ®lter.) ÐÐ: 0 h, .........: 48 h, - - - -: 144 h, ± . ±: 216 h, Ð Ð: 240 h.
to a broad absorption at ca. 230 nm are not clear at present. However, since the induction period, ca. 100 h observed in UV spectra, some impurities may be
When photo-irradiation was carried out with total light emitted from a mercury lamp without using a Pyrex glass ®lter, the phtoto-oxidative degradation of the samples became, naturally, much faster than that in the irradiation with a glass ®lter mentioned in the previous section. In the case of polypropylenes, oxygenated groups such as carbonyl and hydroxyl groups were easily formed and accumulated during the photo-irradiation. In addition, as shown in Fig. 6, there are no appreciable dierences in the rate of the formation of carbonyl groups between IPP and SPP. However, the absorbance of SPP is slightly lower than that of IPP within ca. 6 h. Similar phenomena were also observed in the formation of hydroxyl groups and changes in absorbance at 230 nm in UV spectra. Furthermore, photo-oxidative degradation of HDPE commences easily and carbonyl groups are formed in the early stage
4
M. Kato et al. / Polymer Degradation and Stability 67 (2000) 1±5
and the absorbance at 1715 cmÿ1 increases linearly with photo-irradiation time. The results mentioned above apparently indicate that total irradiation energy is too large to examine photostability precisely between IPP and SPP. In this context, the photo-irradiation with monochromatic light, 260, 280, 300, 320, 340 and 360 nm was kindly carried with a computer-operated large-scale spectrograph in the
National Institute for Basic Biology at Okazaki, Japan [6,7]. It was found that lower wavelength light of 260 and 280 nm was most signi®cant for the photo-oxidative degradation of the polypropylenes. Furthermore, in the case of g-ray irradiation of polypropylenes there was also little dierence in the oxidizability between IPP and SPP [8]. 3.3. Outdoor exposure Since it has been found that the photo-stability of SPP is superior to that of IPP during the irradiation with light longer than ca. 300 nm, outdoor exposure was performed for four months to compare the weatherability of the three samples. IR spectra of the outdoor exposed samples also showed the formation of oxygenated groups such as carbonyl and hydroxyl groups, and their increase during outdoor exposure. Individual plots of absorbance of each sample in the carbonyl
Fig. 6. Changes in absorbance in carbonyl region (at 1715 cmÿ1) of isotactic and syndiotactic poly(propylene)s and (at 1717 cmÿ1) of high density polyethylene during photo-irridation in air. (Irradiated without a ®lter.)
Fig. 7. Changes in absorbance in carbonyl region (at 1715 cmÿ1) of isotactic and syndiotactic poly(propylene)s and (at 1717 cmÿ1) of high density polyethylene during outdoor exposure under ambient conditions. (From 11 September 1997 to 21 January 1998.)
Fig. 8. Changes in Mn (a), Mw (b) and Mw/Mn (c) of isotactic and syndiotactic poly(propylene)s and high density polyethylene during outdoor exposure under ambient conditions. (From 11 September 1997 to 21 January 1998.)
M. Kato et al. / Polymer Degradation and Stability 67 (2000) 1±5 Table 1 Molecular weight and its distribution during outdoor exposurea Mn, Mw, Mw/Mn of isotactic polypropylene, syndiotactic polypropylene and high density polyethylene during outdoor exposure under ambient conditions Sample
Exposed time (month)
Mn
Mw
IPP
0 1 2 3 4
50 20 17 16 16
000 000 000 800 500
250 144 96 68 66
000 000 000 000 000
5.0 7.2 5.6 4.0 4.0
SPP
0 1 2 3 4
45 36 36 33 32
000 800 800 600 800
76 76 76 75 76
500 000 000 000 000
1.7 2.1 2.1 2.2 2.3
0 1 2 3 4
1700 12 960 12 690 12 600 12 150
81 75 75 72 72
000 600 600 900 900
4.8 5.8 6.0 5.8 6.0
HDPE
a
- M/Mn
5
To con®rm the dierence in weatherability of the samples estimated by the spectrophotometry, changes in molecular weight and its distribution were examined. The results obtained are summarized in Table 1 and are graphically shown in Fig. 8, respectively. As can be seen a drastic decrease in the molecular weight occurs in IPP during the outdoor exposure while its decrease is extremely less in SPP and HDPE. 4. Conclusions Eect of stereoregularity of polypropylenes (IPP and SPP) on the photostability and weatherability was examined, comparing with high-density polyethylene (HDPE). It was found that SPP is more stable than IPP to photo-oxidative degradation with glass ®lter irradiation, light longer than ca. 300 nm and also to the weathering when exposed outdoors. HDPE is the most stable among the three samples examined.
From 11 September 1997 to 20 January 1998 in Kiryu, Japan.
Acknowledgements region are shown in Fig. 7. It is quite apparent that the weatherability of SPP is much better than that of IPP and also HDPE is the most stable among the examined samples. Similar results were also observed for the formation of hydroxyl groups in IR spectra. Furthermore, changes in absorbance at 230 nm in UV spectra showed similar phenomena observed in the photo-irradiation with a glass ®ltered light described in Section 3.1. Namely, in the case of IPP, absorbance of the UV spectra increased with exposure time while in the case of SPP the absorbance decreased until ca. 2 months then gradually increased. In addition to the changes in the chemical structure, a remarkable dierence in mechanical properties was noticed between IPP and SPP. For example, IPP ®lm became very brittle after 3.5 months outdoor exposure and it was too brittle to measure IR spectrum in a ®lm state. In contrast, SPP kept its original ®lm form and was ¯exible even after 4 months.
The authors gratefully acknowledge Professor M. Terano and his colleagues, School of Materials Sciences, Japan Advanced Institute of Science and Technology for the donation of samples and measurement of molecular weight. References [1] Osawa Z, Saito T, Kimura Y. J Appl Polym Sci 1968;22:563. [2] Osawa Z, Kato M, Terano M. Macromol Rapid Commun 1997;18:157. [3] Kato M, Osawa Z. Polym Degrad Stab 1999;65:457. [4] JIS K7219. [5] Hatanaka T, Mori H, Terano M. Macromol Rapid Commun 1997;18:667. [6] Watanabe M, Furuya M, Miyoshi Y, Inoue Y, Iwahashi I, Matsumoto K. J Appl Polym Sci 1970;14:765 and Photochem Photobiol 1982;36:491. [7] Fukuda Y, Osawa Z. Polym Degrad Stab 1991;34:75. [8] Osawa Z. Unpublished results.