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Analysis of weathering of a thermoplastic polyester elastomer II. Factors affecting weathering of a polyether–polyester elastomer
Polymer Degradation and Stability 65 (1999) 217±224
Analysis of weathering of a thermoplastic polyester elastomer II. Factors aecting weathering of a polyether±polyester elastomer Y. Nagai*, T. Ogawa, Y. Nishimoto, F. Ohishi Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka City, Kanagawa 259-1293, Japan Received 30 June 1998; accepted 7 October 1998
Abstract To compare the contribution of degradative factors to weathering of a thermoplastic polyester elastomer (TPEE) photo-, thermal conditions and water were selected as the degradative factors. Experiments on photodegradation, thermal degradation and hydrolysis of TPEE were carried out. Spectral irradiation using sharp-cut ®lters with a xenon lamp, photodegradation under several temperature conditions and immersion in water during photoirradiation were also performed. The results of these experiments were compared with those of outdoor and accelerated weathering tests. The following conclusions were obtained under the experimental conditions: (1) a major factor of the degradation are ultraviolet rays below 310 nm; (2) an increase in temperature accelerates the rate of degradation reaction; and (3) the presence of water increases the amount of gel formation caused by cross-linking reaction. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Thermoplastic polyester elastomer; Polyether±polyester elastomer; Weathering; Degradation; Wavelength sensitivity
1. Introduction Thermoplastic elastomers (TPEs) are growing in importance as new materials which cover the boundary between rubbers and plastics. Therefore, study of the weatherability of TPEs is important. Speci®cally, thermoplastic polyester elastomers (TPEEs) among TPEs have high performance with respect to thermal and mechanical properties. Thus, a study on weatherability of TPEEs has been required. In our previous paper [1], we reported on the weathering of the TPEEs±polybutylene terephthalate and polytetramethyleneglycol. In that paper, the following conclusions were obtained from outdoor exposure tests and an accelerated weathering test by using a sunshine weathermeter (SW test): (1) the ether parts of the soft segment in the polymer were attacked selectively and the ester bonds formed were caused by degradation; (2) in both the outdoor exposure tests and the SW test, main-chain scission and crosslinking reactions occurred simultaneously; and (3) a clear dierence of the degradation was observed in the outdoor exposure tests and the SW test. The amount of cross-linked products formed in the SW test was larger than that in the outdoor exposure tests. * Corresponding author.
We presumed that the wavelength range of light, temperature, the presence of water and atmospheric conditions may be factors aecting TPEE degradation. In order to specify the principal factors aecting the weathering of TPEEs, we tried photodegradation, thermal degradation and hydrolysis experiments. Studies on thermal aging, photodegradation or photoyellowing of TPEEs have already been reported [2±4]. However, in the case of photodegradation of TPEEs, studies of the dependence on the wavelength of UV light by using sharp-cut ®lters and of the in¯uence of water have not been reported. In this paper, we aimed to clarify the major factors which aect the weathering of a TPEE and discuss the eect of wavelength range of light, temperature and water on photodegradation. 2. Experiments 2.1. Test samples Co-polymer of polybutylene terephthalate and polytetramethyleneglycol was molded by injection into the form of 2-mm-thick plaques, 100 mm in length and 100 mm in width (TOYOBO Co. Ltd, Ohtsu City, Japan).
0141-3910/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0141 -3 910(99)00007 -5
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Y. Nagai et al. / Polymer Degradation and Stability 65 (1999) 217±224 Table 1 Optical properties of sharp-cut ®lters
Fig. 1. Chemical structure of the thermoplastic polyester elastomer (TPEE).
Fig. 1 shows the chemical structure. None of the samples contained stabilizer. Film samples (100 mm thick) were cast on a Petri dish from a chloroform (CHCl3) solution and dried under reduced pressure at ambient temperature for 12 h. Sheet samples were used for the thermal degradation experiments, hydrolysis experiments and the measurement of gelation ratio caused by photodegradation; and ®lm samples were used for the photodegradation experiments, excluding the measurement of gelation ratio [1]. 2.2. Photodegradation experiments Photodegradation tests (Xe test) were carried out using a Heraeus Industrietechnik Model SUNTEST CPS+. This apparatus is equipped with a 1500-W xenon lamp and a series of quartz glass ®lters and UV special glass ®lters which cut wavelengths shorter than 290 nm. To study the eect of wavelength range of UV rays on degradation, sharp-cut ®lters were used in the Xe test. The optical characteristics [5] of the ®lters are shown in Table 1. Table 2 shows the irradiation time and radiant exposure in the Xe test. The wavelength range of radiant exposure was measured from 300 to 400 nm by a Radialux UV sensor. In order to study the eect of temperature during irradiation on the degradation, the Xe test was carried out at 63, 73, 83 and 93 C. In order to examine the in¯uence of water, the irradiation test was carried out using the SUNTEST CPS+ combined with an immersion unit. A sample was placed on a sample table consisting of a ¯ooding tub and a perforated plate and was irradiated and immersed in distilled water. The temperature of water was 30 or 40 C. Samples were irradiated each for a total irradiation time of 10 h with dierent immersion times of 18, 36, 54 and 72 min, respectively, per 2-h. The total irradiation time was divided into ®ve 2-h time segments. At the beginning of each time segment, samples were immersed in distilled water for 18, 36, 54 and 72 min, respectively, and this process was repeated ®ve times to make the total irradiation time 10 h.
Symbol
Absorption Transition High transition wavelength wavelength wavelength (nm) (nm) (nm)
Transition interval (nm)
UV-30 UV-31 UV-32 UV-33 UV-34 UV-35 UV-37 L-39
296.0 298.0 313.5 312.0 327.0 344.0 358.0 372.0
19.0 31.0 20.0 30.0 23.5 19.0 28.0 39.0
305.5 313.5 323.5 327.0 338.8 353.5 372.0 391.5
315.0 329.0 333.5 342.0 350.0 363.0 386.0 411.0
Table 2 Radiant exposure through sharp-cut ®lter Irradiation time (h)
Quartz UV-30 UV-31 UV-32 UV-33 UV-34 UV-35 UV-37 L-39
Radiant exposure (MJ/m2) 0
5
10
20
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
1.0 1.0 0.9 0.8 0.7 0.6 0.3 0.1 0.0
2.0 2.0 1.8 1.7 1.5 1.2 0.6 0.2 0.0
4.1 4.0 3.6 3.4 2.9 2.5 1.3 0.4 0.1
2.3. Thermal degradation experiment The thermal degradation test was carried out using a Model G0 S Oven TG1-200, Suga test instruments Co., Ltd (Tokyo, Japan), at 100 C for 30 and 60 days, respectively, in air. 2.4. Hydrolysis experiment The sample was hydrolyzed in a methanol solution of 0.01 mol/l potassium hydroxide at 50 C for 24, 48, 72, 96 and 120 h, respectively. In addition, the sample was hydrolyzed in a 0.01 mol/l potassium hydroxide aqueous solution, 0.01 mol/l sulfuric acid, 0.02 mol/l nitric acid, and 0.02 mol/l hydrochloric acid at 50 C for 120 h, respectively. 2.5. Measurements of molecular weight and molecular weight distribution The molecular weight and its distribution were measured by a TOSOH GPC system (Degasser: Model SD-8000; pump: Model CCPS; column oven: Model CO-8020; detector: Model UV- 8020; data processor: Model Chromatocorder 21) with chloroform as solvent. Chloroform was used as the mobile phase at a ¯ow rate of 0.7 ml/min. The wavelength of the detector was
Y. Nagai et al. / Polymer Degradation and Stability 65 (1999) 217±224
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254 nm. In every case, the molecular weight of the sample is relative to polystyrene standards.
2.7. Analysis of chemical structure by nuclear magnetic resonance (NMR)
2.6. Measurement of IR spectra
The solution 1H-NMR spectra were recorded by a JEOL Model JNM-EX400. The chemical shifts were determined in CDCl3 solutions, in parts per million from internally added tetramethylsilane (TMS).
The infrared absorption spectra were obtained using a Model FT-IR 300 (JASCO Corporation) by means of the diuse re¯ectance method.
3. Results and discussion 3.1. Photodegradation test
Fig. 2. Chromatograms of original sample and a sample irradiated for 5 h through quartz.
Fig. 2 shows that the average molecular weight shifted to a lower value and the GPC chromatogram became broader on photo-irradiation. The IR absorption spectra of the original sample and the sample irradiated for 5 h through quartz are shown in Fig. 3. The IR spectrum of the irradiated sample showed the formation of aliphatic ester bonds like the outdoor exposed sample. The 1H-NMR spectra of the original sample and the sample irradiated for 5 h through quartz are shown in Fig. 4. The ratios of the integrated intensity at 4.4 ppm to that at 8.1 ppm changed a little in the photodegradation. Nevertheless, the ratios of the integrated intensity at 3.4 ppm to that of 8.1 ppm decreased with an increase in the radiant exposure (Fig. 5). The peaks at 4.4 and at 3.4 ppm are assigned, respectively, to the protons of methylene groups bonded to aromatic esters and of methylene groups adjacent to ether oxygens. This
Fig. 3. IR spectra of the original sample and sample irradiated for 5-hour through quartz.
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suggested that selective degradation of ether parts in soft segment occurs in photodegradation. The new peak (4.2 ppm), which was assigned to the protons of methylene groups bonded to aliphatic ester oxygens, appeared during photodegradation. In addition, a peak based on the protons of aldehyde groups, appeared at about 9.8 ppm. These peaks were assigned by comparison with the peak of n-butyric acid, n-butylester or n-butyl aldehyde. The results from IR and NMR measurements for the photodegraded samples led to the conclusion that the selective degradation of ether parts happened similarly to the results of the outdoor exposure tests. Moreover, hydroperoxides were detected by a potassium iodine paper test after photodegradation. These correspond to the hydroperoxides formed in the a-position of the ether oxygen. The hydroperoxide may correspond to the methine protons (4.7 ppm) which were detected by NMR for irradiated samples. It is generally known that low molecular weight ethers form ester and hydroperoxide by photo-oxidation. For block co-polyethers, formation of hemi-acetals, esters and formates by decomposition of hydroperoxide have been reported [3,6±8]. The formation of aliphatic ester, aldehyde and hydroperoxide, con®rmed by the photodegradation test in our study, is consistent with the test results reported
in these references. However, little gel was formed in irradiation, in sharp contrast to the results of outdoor exposure and SW tests. 3.2. Thermal degradation experiments Figs. 6 and 7 show changes in the weight average molecular weight with an increase in the thermal degradation period, and IR spectra of the original sample and a sample thermally degraded at 100 C for 60 days. The weight average molecular weight decreased a little in the thermal degradation test. The IR absorption peak at 1175 cmÿ1, which appeared in the outdoor exposure tests and the SW test, did not appear in the thermal
Fig. 5. Relationships between integrated intensity ratios (4.4/8.1 ppm and 3.4/8.1 ppm) and radiant exposure. ~, 4.4/8.1 ppm; *, 3.4/8.1 ppm.
Fig. 4. (a) NMR spectra of the original sample and (b) sample irradiated for 5 h through quartz.
Fig. 6. Relationship between the weight average molecular weight and thermal degradation period.
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Fig. 7. IR spectra of the original sample and sample thermally degraded at 100 C for 60 days.
such as the selective degradation of ether bonds and formation of the ester bonds was not caused by the thermal degradation test. Therefore, it was con®rmed experimentally that the main reaction in the weathering of the TPEE was not similar to that in the thermal degradation test. 3.3. Hydrolysis experiment
Fig. 8. Relationships between integrated intensity ratios (4.4/8.1 ppm and 3.4/8.1 ppm) and thermal degradation period. ~, 4.4/8.1 ppm; *, 3.4/8.1 ppm.
degradation test. Moreover, the peaks at 4.2 and at 9.8 ppm were not detected in 1H-NMR. From these results, it was clear that the aliphatic ester bonds and aldehydes were not produced by thermal degradation. Fig. 8 shows the changes in the ratios of the integrated intensity at 4.4 ppm to that of 8.1 ppm and the ratios of the integrated intensity at 3.4 ppm to that of 8.1ppm with an increase in the thermal degradation period. From Figs. 6±8 we suggest that the degradation reactions
In the case of the hydrolysis test using a methanol solution of 0.01 mol/l potassium hydroxide, the solution which contained the hydrolyzed products was dried under reduced pressure. In other hydrolysis tests, the samples were washed with ethanol after the test and dried under reduced pressure because the samples kept their original shape after hydrolysis. Then the dried precipitate was dissolved in CHCl3. The solution was ®ltered to 10 mm pore size. GPC and NMR measurements were carried out for the part dissolved in CHCl3. Table 3 shows the weight average molecular weight of the original sample and 120-h hydrolyzed samples. For the samples which were hydrolyzed in a 0.01 mol/l potassium hydroxide methanol solution and in nitric acid, weight average molecular weight decreased. In the measurement of 1H-NMR, the peaks at 4.2 and 9.8 ppm, which appeared in the outdoor exposure tests and the SW test, did not appear in the hydrolysis tests. Therefore, it was clear that the aliphatic ester bonds and aldehydes were not formed by hydrolysis. Table 4 shows the ratio of the integrated intensity at 3.4 ppm to that at
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Table 3 The weight average molecular weight of the original and the 120-h hydrolyzed samples Mw Original Potassium hydroxide methanol solution (0.01 mol/l) Potassium hydroxide aqueous solution (0.01mol/l) Sulfuric acid (0.01 mol/l) Nitric acid (0.02 mol/l) Hydrochloric acid (0.02 mol/l)
102 000 6000 102 000 98 000 48 000 102 000
Table 4 The ratio of the nuclear magnetic resonance (NMR) integrated intensity at 3.4 ppm to that at 4.4 ppm of the original and the 120-h hydrolyzed samples Integrated intensity ratio Original Potassium hydroxide methanol solution (0.01 mol/l) Potassium hydroxide aqueous solution (0.01 mol/l) Sulfuric acid (0.01 mol/l) Nitric acid (0.02 mol/l) Hydrochloric acid (0.02 mol/l)
7.01 17.1 6.74 6.3 6.14 6.57
4.4 ppm. For the sample which was hydrolyzed in a 0.01 mol/l potassium hydroxide methanol solution, the ratio of integrated intensity at 3.4 ppm to that at 4.4 ppm was increased when the fact that ester bonds of the mainchain were broken down by hydrolysis is taken into consideration. In the cases of the outdoor exposure tests, the SW test and photodegradation test, the ratio of integrated intensity at 3.4 ppm to that of 4.4 ppm decreased with an increase in radiant exposure, whereas in the case of the hydrolysis tests, that ratio was little changed or even increased. For the samples which were hydrolyzed in a 0.01 mol/l potassium hydroxide aqueous solution, 0.02 mol/l nitric acid and 0.01 mol/l sulfuric acid, changes in chemical structure were not observed by IR spectroscopy or NMR. The results of the hydrolysis tests showed that the tendency for degradation in the hydrolysis tests diered from that in the outdoor exposure tests and the SW test. Therefore, it was clear that the basic and acidic solutions were not principal factors aecting the weathering of the TPEE. 3.4. Eect of wavelength range of light on photodegradation Photodegradation, thermal degradation and hydrolysis tests proved that light is the principal factor that aects the weathering of our TPEE. In order to investigate the wavelength sensitivity, a photodegradation test was carried out through sharp-cut ®lters.
Fig. 9. Relationships between the number average molecular weight and the absorption wavelength of the each sharp-cut ®lter. , 5, 10-h irradiation; , 20-h irradiation. h irradiation;
Fig. 10. Relationships between the absorbance ratio (1175/1500 cmÿ1) and the absorption wavelength of the each sharp-cut ®lter. , 5h irradiation; , 10-h irradiation; , 20-h irradiation.
Fig. 9 shows the relationships between the number average molecular weight of the samples which were irradiated through various ®lters for 5, 10 and 20 h and the absorption wavelength of the each ®lter. The absorption wavelength of ®lters is the wavelength corresponding to the upper limit where the transmittance of the ®lters does not exceed 5%. Therefore, it is considered that the eect of the light below the absorption wavelength of the ®lters on the degradation is small. From Fig. 9, we can see that light below 310 nm caused main-chain scission at an early stage of irradiation. The wavelength which decreased the number average molecular weight shifted to longer values. This may be related to the fact that the UV absorption wavelengths of
Y. Nagai et al. / Polymer Degradation and Stability 65 (1999) 217±224
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Fig. 13. Photodegradation mechanisms of the thermoplastic polyester elastomer (TPEE). Fig. 11. Relationships between the integrated intensity ratio (3.4/8.1 ppm) and the absorption wavelength of the each sharp-cut ®lter. , 5-h irradiation; , 10-h irradiation; , 20-h irradiation.
Fig. 14. Relationships between the gelation ratio and immersion time during 10 h irradiation. , 30 C; , 40 C.
Fig. 12. Relationships between the integrated intensity ratio (4.2/8.1 ppm) and the absorption wavelength of the each sharp-cut ®lter. , 5-h irradiation; , 10-h irradiation; , 20-h irradiation.
degradation products caused by irradiation were extended to longer values than that of the original sample. The ratios of absorbance at 1175 cmÿ1 to that 1500 cmÿ1 (in-plane skeletal vibration of benzene ring) increased at 310 nm as shown in Fig. 10. It is suggested that the aliphatic ester bonds were formed by irradiation by radiation below 310 nm. Fig. 11 shows the ratios of the integrated intensity at 3.4 ppm to that of 8.1 ppm against the absorption wavelength of the each sharp-cut ®lter. In this case, the decrease in the integrated intensity ratio of irradiated samples started at about 310 nm of the absorption wavelength. Fig. 12 shows that the ratios of the integrated intensity at 4.2 ppm to that at 8.1 ppm for the irradiated samples
increased at about 310 nm. These results suggest that the ether parts of soft segments in the polymer were degraded by the irradiation with UV rays below 310 nm, and ester bonds and aldehyde were formed. When the results of weathering were compared with those of photodegradation, it was clear that the weathering of the TPEE was mainly induced by the radiation below 310 nm. The experimental results indicate the reaction mechanisms shown in Fig. 13 under photodegradation conditions. 3.5. Eect of water on photodegradation during irradiation Fig. 14 shows the relationships between the gelation ratio and the immersion time during a 10-h irradiation. It suggests that the presence of water during irradiation increases gel formation. This behavior is not fully understood, but it appears that de®ciency of oxygen induced by immersion in water during irradiation gives rise to a gel formation reaction.
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2. In the degradative reactions caused by the outdoor exposure tests, selective degradation of the ether parts of the soft segment occurs in the polymer; ester and aldehyde are formed, and main-chain scission is caused by the action of UV rays below 310 nm. 3. The degradative reactions are accelerated by temperature rise during light irradiation. 4. Gel formation caused by light irradiation is hardly observed. In contrast, the presence of water during irradiation increases gel formation, possibly by excluding oxygen. 5. The mechanism of the degradation caused by hydrolysis diered from that of the outdoor exposure tests and the accelerated weathering test. Fig. 15. Relationships between the NMR integrated intensity ratio (3.4/8.1 ppm) and radiant exposure under several temperature conditions. , 63 C; , 73 C; , 83 C; , 93 C.
3.6. Eect of temperature during photo-irradiation The relationships between the NMR integrated intensity ratio (3.4/8.1 ppm) and radiant exposure under several temperature conditions are shown in Fig. 15. The result indicates that the selective degradation of the ether parts in the soft segment was accelerated by temperature rise during the exposure.
Acknowledgements The authors are indebted to Mr Reiichi Someya of Kasho Co. Ltd., Japan, for measurements of the radiation exposure and to Mr Junichi Ikeda and Mr Tomio Yamaguchi of the Japan Weathering Test Center for the thermal degradation tests. We are grateful to TOYOBO Co. Ltd., Japan, for supplying the samples used in this work.
References 4. Conclusions This paper presents the results of analysis of the degradation by photodegradation, thermal degradation and hydrolysis for the TPEE. The following conclusions can be drawn from the experimental results: 1. The degradation mechanism caused by the photodegradation experiments is a selective degradation of the ether parts of the soft segment in the polymer and resulting formation of ester bonds, which is almost the same as that of the outdoor exposure tests.
[1] Nagai Y, Ogawa T, Liu Y, Nishimoto Y, Ohishi F. Polymer Degradation and Stability 1997:56;115. [2] Pan Jiang-Qing, Zhang Jie. Polymer Degradation and Stability 1992:36;65. [3] Tabankia MH, Philippart JL, Gardette JL. Polymer Degradation and Stability 1985:12;349. [4] Tabankia MH, Gardette JL. Polymer Degradation and Stability 1987:19;113. [5] Japanese Industrial Standards B 7113, Photographic Sharp-cut Glass Filters. Tokyo: Japanese Industrial Standards Committee, 1975. [6] Hughes OR, Coard LC. Journal of Polymer Science A 1969:7;1861. [7] Gauvin P, Lemaire J, Sallet D. Makromol Chem 1987:188;1815. [8] Gardette JL, Sabel HD, Lemaire J. Angew Makromol Chem 1991:188;113.
Analysis of weathering of a thermoplastic polyester elastomer II. Factors aecting weathering of a polyether±polyester elastomer Y. Nagai*, T. Ogawa, Y. Nishimoto, F. Ohishi Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka City, Kanagawa 259-1293, Japan Received 30 June 1998; accepted 7 October 1998
Abstract To compare the contribution of degradative factors to weathering of a thermoplastic polyester elastomer (TPEE) photo-, thermal conditions and water were selected as the degradative factors. Experiments on photodegradation, thermal degradation and hydrolysis of TPEE were carried out. Spectral irradiation using sharp-cut ®lters with a xenon lamp, photodegradation under several temperature conditions and immersion in water during photoirradiation were also performed. The results of these experiments were compared with those of outdoor and accelerated weathering tests. The following conclusions were obtained under the experimental conditions: (1) a major factor of the degradation are ultraviolet rays below 310 nm; (2) an increase in temperature accelerates the rate of degradation reaction; and (3) the presence of water increases the amount of gel formation caused by cross-linking reaction. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Thermoplastic polyester elastomer; Polyether±polyester elastomer; Weathering; Degradation; Wavelength sensitivity
1. Introduction Thermoplastic elastomers (TPEs) are growing in importance as new materials which cover the boundary between rubbers and plastics. Therefore, study of the weatherability of TPEs is important. Speci®cally, thermoplastic polyester elastomers (TPEEs) among TPEs have high performance with respect to thermal and mechanical properties. Thus, a study on weatherability of TPEEs has been required. In our previous paper [1], we reported on the weathering of the TPEEs±polybutylene terephthalate and polytetramethyleneglycol. In that paper, the following conclusions were obtained from outdoor exposure tests and an accelerated weathering test by using a sunshine weathermeter (SW test): (1) the ether parts of the soft segment in the polymer were attacked selectively and the ester bonds formed were caused by degradation; (2) in both the outdoor exposure tests and the SW test, main-chain scission and crosslinking reactions occurred simultaneously; and (3) a clear dierence of the degradation was observed in the outdoor exposure tests and the SW test. The amount of cross-linked products formed in the SW test was larger than that in the outdoor exposure tests. * Corresponding author.
We presumed that the wavelength range of light, temperature, the presence of water and atmospheric conditions may be factors aecting TPEE degradation. In order to specify the principal factors aecting the weathering of TPEEs, we tried photodegradation, thermal degradation and hydrolysis experiments. Studies on thermal aging, photodegradation or photoyellowing of TPEEs have already been reported [2±4]. However, in the case of photodegradation of TPEEs, studies of the dependence on the wavelength of UV light by using sharp-cut ®lters and of the in¯uence of water have not been reported. In this paper, we aimed to clarify the major factors which aect the weathering of a TPEE and discuss the eect of wavelength range of light, temperature and water on photodegradation. 2. Experiments 2.1. Test samples Co-polymer of polybutylene terephthalate and polytetramethyleneglycol was molded by injection into the form of 2-mm-thick plaques, 100 mm in length and 100 mm in width (TOYOBO Co. Ltd, Ohtsu City, Japan).
0141-3910/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0141 -3 910(99)00007 -5
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Y. Nagai et al. / Polymer Degradation and Stability 65 (1999) 217±224 Table 1 Optical properties of sharp-cut ®lters
Fig. 1. Chemical structure of the thermoplastic polyester elastomer (TPEE).
Fig. 1 shows the chemical structure. None of the samples contained stabilizer. Film samples (100 mm thick) were cast on a Petri dish from a chloroform (CHCl3) solution and dried under reduced pressure at ambient temperature for 12 h. Sheet samples were used for the thermal degradation experiments, hydrolysis experiments and the measurement of gelation ratio caused by photodegradation; and ®lm samples were used for the photodegradation experiments, excluding the measurement of gelation ratio [1]. 2.2. Photodegradation experiments Photodegradation tests (Xe test) were carried out using a Heraeus Industrietechnik Model SUNTEST CPS+. This apparatus is equipped with a 1500-W xenon lamp and a series of quartz glass ®lters and UV special glass ®lters which cut wavelengths shorter than 290 nm. To study the eect of wavelength range of UV rays on degradation, sharp-cut ®lters were used in the Xe test. The optical characteristics [5] of the ®lters are shown in Table 1. Table 2 shows the irradiation time and radiant exposure in the Xe test. The wavelength range of radiant exposure was measured from 300 to 400 nm by a Radialux UV sensor. In order to study the eect of temperature during irradiation on the degradation, the Xe test was carried out at 63, 73, 83 and 93 C. In order to examine the in¯uence of water, the irradiation test was carried out using the SUNTEST CPS+ combined with an immersion unit. A sample was placed on a sample table consisting of a ¯ooding tub and a perforated plate and was irradiated and immersed in distilled water. The temperature of water was 30 or 40 C. Samples were irradiated each for a total irradiation time of 10 h with dierent immersion times of 18, 36, 54 and 72 min, respectively, per 2-h. The total irradiation time was divided into ®ve 2-h time segments. At the beginning of each time segment, samples were immersed in distilled water for 18, 36, 54 and 72 min, respectively, and this process was repeated ®ve times to make the total irradiation time 10 h.
Symbol
Absorption Transition High transition wavelength wavelength wavelength (nm) (nm) (nm)
Transition interval (nm)
UV-30 UV-31 UV-32 UV-33 UV-34 UV-35 UV-37 L-39
296.0 298.0 313.5 312.0 327.0 344.0 358.0 372.0
19.0 31.0 20.0 30.0 23.5 19.0 28.0 39.0
305.5 313.5 323.5 327.0 338.8 353.5 372.0 391.5
315.0 329.0 333.5 342.0 350.0 363.0 386.0 411.0
Table 2 Radiant exposure through sharp-cut ®lter Irradiation time (h)
Quartz UV-30 UV-31 UV-32 UV-33 UV-34 UV-35 UV-37 L-39
Radiant exposure (MJ/m2) 0
5
10
20
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
1.0 1.0 0.9 0.8 0.7 0.6 0.3 0.1 0.0
2.0 2.0 1.8 1.7 1.5 1.2 0.6 0.2 0.0
4.1 4.0 3.6 3.4 2.9 2.5 1.3 0.4 0.1
2.3. Thermal degradation experiment The thermal degradation test was carried out using a Model G0 S Oven TG1-200, Suga test instruments Co., Ltd (Tokyo, Japan), at 100 C for 30 and 60 days, respectively, in air. 2.4. Hydrolysis experiment The sample was hydrolyzed in a methanol solution of 0.01 mol/l potassium hydroxide at 50 C for 24, 48, 72, 96 and 120 h, respectively. In addition, the sample was hydrolyzed in a 0.01 mol/l potassium hydroxide aqueous solution, 0.01 mol/l sulfuric acid, 0.02 mol/l nitric acid, and 0.02 mol/l hydrochloric acid at 50 C for 120 h, respectively. 2.5. Measurements of molecular weight and molecular weight distribution The molecular weight and its distribution were measured by a TOSOH GPC system (Degasser: Model SD-8000; pump: Model CCPS; column oven: Model CO-8020; detector: Model UV- 8020; data processor: Model Chromatocorder 21) with chloroform as solvent. Chloroform was used as the mobile phase at a ¯ow rate of 0.7 ml/min. The wavelength of the detector was
Y. Nagai et al. / Polymer Degradation and Stability 65 (1999) 217±224
219
254 nm. In every case, the molecular weight of the sample is relative to polystyrene standards.
2.7. Analysis of chemical structure by nuclear magnetic resonance (NMR)
2.6. Measurement of IR spectra
The solution 1H-NMR spectra were recorded by a JEOL Model JNM-EX400. The chemical shifts were determined in CDCl3 solutions, in parts per million from internally added tetramethylsilane (TMS).
The infrared absorption spectra were obtained using a Model FT-IR 300 (JASCO Corporation) by means of the diuse re¯ectance method.
3. Results and discussion 3.1. Photodegradation test
Fig. 2. Chromatograms of original sample and a sample irradiated for 5 h through quartz.
Fig. 2 shows that the average molecular weight shifted to a lower value and the GPC chromatogram became broader on photo-irradiation. The IR absorption spectra of the original sample and the sample irradiated for 5 h through quartz are shown in Fig. 3. The IR spectrum of the irradiated sample showed the formation of aliphatic ester bonds like the outdoor exposed sample. The 1H-NMR spectra of the original sample and the sample irradiated for 5 h through quartz are shown in Fig. 4. The ratios of the integrated intensity at 4.4 ppm to that at 8.1 ppm changed a little in the photodegradation. Nevertheless, the ratios of the integrated intensity at 3.4 ppm to that of 8.1 ppm decreased with an increase in the radiant exposure (Fig. 5). The peaks at 4.4 and at 3.4 ppm are assigned, respectively, to the protons of methylene groups bonded to aromatic esters and of methylene groups adjacent to ether oxygens. This
Fig. 3. IR spectra of the original sample and sample irradiated for 5-hour through quartz.
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suggested that selective degradation of ether parts in soft segment occurs in photodegradation. The new peak (4.2 ppm), which was assigned to the protons of methylene groups bonded to aliphatic ester oxygens, appeared during photodegradation. In addition, a peak based on the protons of aldehyde groups, appeared at about 9.8 ppm. These peaks were assigned by comparison with the peak of n-butyric acid, n-butylester or n-butyl aldehyde. The results from IR and NMR measurements for the photodegraded samples led to the conclusion that the selective degradation of ether parts happened similarly to the results of the outdoor exposure tests. Moreover, hydroperoxides were detected by a potassium iodine paper test after photodegradation. These correspond to the hydroperoxides formed in the a-position of the ether oxygen. The hydroperoxide may correspond to the methine protons (4.7 ppm) which were detected by NMR for irradiated samples. It is generally known that low molecular weight ethers form ester and hydroperoxide by photo-oxidation. For block co-polyethers, formation of hemi-acetals, esters and formates by decomposition of hydroperoxide have been reported [3,6±8]. The formation of aliphatic ester, aldehyde and hydroperoxide, con®rmed by the photodegradation test in our study, is consistent with the test results reported
in these references. However, little gel was formed in irradiation, in sharp contrast to the results of outdoor exposure and SW tests. 3.2. Thermal degradation experiments Figs. 6 and 7 show changes in the weight average molecular weight with an increase in the thermal degradation period, and IR spectra of the original sample and a sample thermally degraded at 100 C for 60 days. The weight average molecular weight decreased a little in the thermal degradation test. The IR absorption peak at 1175 cmÿ1, which appeared in the outdoor exposure tests and the SW test, did not appear in the thermal
Fig. 5. Relationships between integrated intensity ratios (4.4/8.1 ppm and 3.4/8.1 ppm) and radiant exposure. ~, 4.4/8.1 ppm; *, 3.4/8.1 ppm.
Fig. 4. (a) NMR spectra of the original sample and (b) sample irradiated for 5 h through quartz.
Fig. 6. Relationship between the weight average molecular weight and thermal degradation period.
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Fig. 7. IR spectra of the original sample and sample thermally degraded at 100 C for 60 days.
such as the selective degradation of ether bonds and formation of the ester bonds was not caused by the thermal degradation test. Therefore, it was con®rmed experimentally that the main reaction in the weathering of the TPEE was not similar to that in the thermal degradation test. 3.3. Hydrolysis experiment
Fig. 8. Relationships between integrated intensity ratios (4.4/8.1 ppm and 3.4/8.1 ppm) and thermal degradation period. ~, 4.4/8.1 ppm; *, 3.4/8.1 ppm.
degradation test. Moreover, the peaks at 4.2 and at 9.8 ppm were not detected in 1H-NMR. From these results, it was clear that the aliphatic ester bonds and aldehydes were not produced by thermal degradation. Fig. 8 shows the changes in the ratios of the integrated intensity at 4.4 ppm to that of 8.1 ppm and the ratios of the integrated intensity at 3.4 ppm to that of 8.1ppm with an increase in the thermal degradation period. From Figs. 6±8 we suggest that the degradation reactions
In the case of the hydrolysis test using a methanol solution of 0.01 mol/l potassium hydroxide, the solution which contained the hydrolyzed products was dried under reduced pressure. In other hydrolysis tests, the samples were washed with ethanol after the test and dried under reduced pressure because the samples kept their original shape after hydrolysis. Then the dried precipitate was dissolved in CHCl3. The solution was ®ltered to 10 mm pore size. GPC and NMR measurements were carried out for the part dissolved in CHCl3. Table 3 shows the weight average molecular weight of the original sample and 120-h hydrolyzed samples. For the samples which were hydrolyzed in a 0.01 mol/l potassium hydroxide methanol solution and in nitric acid, weight average molecular weight decreased. In the measurement of 1H-NMR, the peaks at 4.2 and 9.8 ppm, which appeared in the outdoor exposure tests and the SW test, did not appear in the hydrolysis tests. Therefore, it was clear that the aliphatic ester bonds and aldehydes were not formed by hydrolysis. Table 4 shows the ratio of the integrated intensity at 3.4 ppm to that at
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Table 3 The weight average molecular weight of the original and the 120-h hydrolyzed samples Mw Original Potassium hydroxide methanol solution (0.01 mol/l) Potassium hydroxide aqueous solution (0.01mol/l) Sulfuric acid (0.01 mol/l) Nitric acid (0.02 mol/l) Hydrochloric acid (0.02 mol/l)
102 000 6000 102 000 98 000 48 000 102 000
Table 4 The ratio of the nuclear magnetic resonance (NMR) integrated intensity at 3.4 ppm to that at 4.4 ppm of the original and the 120-h hydrolyzed samples Integrated intensity ratio Original Potassium hydroxide methanol solution (0.01 mol/l) Potassium hydroxide aqueous solution (0.01 mol/l) Sulfuric acid (0.01 mol/l) Nitric acid (0.02 mol/l) Hydrochloric acid (0.02 mol/l)
7.01 17.1 6.74 6.3 6.14 6.57
4.4 ppm. For the sample which was hydrolyzed in a 0.01 mol/l potassium hydroxide methanol solution, the ratio of integrated intensity at 3.4 ppm to that at 4.4 ppm was increased when the fact that ester bonds of the mainchain were broken down by hydrolysis is taken into consideration. In the cases of the outdoor exposure tests, the SW test and photodegradation test, the ratio of integrated intensity at 3.4 ppm to that of 4.4 ppm decreased with an increase in radiant exposure, whereas in the case of the hydrolysis tests, that ratio was little changed or even increased. For the samples which were hydrolyzed in a 0.01 mol/l potassium hydroxide aqueous solution, 0.02 mol/l nitric acid and 0.01 mol/l sulfuric acid, changes in chemical structure were not observed by IR spectroscopy or NMR. The results of the hydrolysis tests showed that the tendency for degradation in the hydrolysis tests diered from that in the outdoor exposure tests and the SW test. Therefore, it was clear that the basic and acidic solutions were not principal factors aecting the weathering of the TPEE. 3.4. Eect of wavelength range of light on photodegradation Photodegradation, thermal degradation and hydrolysis tests proved that light is the principal factor that aects the weathering of our TPEE. In order to investigate the wavelength sensitivity, a photodegradation test was carried out through sharp-cut ®lters.
Fig. 9. Relationships between the number average molecular weight and the absorption wavelength of the each sharp-cut ®lter. , 5, 10-h irradiation; , 20-h irradiation. h irradiation;
Fig. 10. Relationships between the absorbance ratio (1175/1500 cmÿ1) and the absorption wavelength of the each sharp-cut ®lter. , 5h irradiation; , 10-h irradiation; , 20-h irradiation.
Fig. 9 shows the relationships between the number average molecular weight of the samples which were irradiated through various ®lters for 5, 10 and 20 h and the absorption wavelength of the each ®lter. The absorption wavelength of ®lters is the wavelength corresponding to the upper limit where the transmittance of the ®lters does not exceed 5%. Therefore, it is considered that the eect of the light below the absorption wavelength of the ®lters on the degradation is small. From Fig. 9, we can see that light below 310 nm caused main-chain scission at an early stage of irradiation. The wavelength which decreased the number average molecular weight shifted to longer values. This may be related to the fact that the UV absorption wavelengths of
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Fig. 13. Photodegradation mechanisms of the thermoplastic polyester elastomer (TPEE). Fig. 11. Relationships between the integrated intensity ratio (3.4/8.1 ppm) and the absorption wavelength of the each sharp-cut ®lter. , 5-h irradiation; , 10-h irradiation; , 20-h irradiation.
Fig. 14. Relationships between the gelation ratio and immersion time during 10 h irradiation. , 30 C; , 40 C.
Fig. 12. Relationships between the integrated intensity ratio (4.2/8.1 ppm) and the absorption wavelength of the each sharp-cut ®lter. , 5-h irradiation; , 10-h irradiation; , 20-h irradiation.
degradation products caused by irradiation were extended to longer values than that of the original sample. The ratios of absorbance at 1175 cmÿ1 to that 1500 cmÿ1 (in-plane skeletal vibration of benzene ring) increased at 310 nm as shown in Fig. 10. It is suggested that the aliphatic ester bonds were formed by irradiation by radiation below 310 nm. Fig. 11 shows the ratios of the integrated intensity at 3.4 ppm to that of 8.1 ppm against the absorption wavelength of the each sharp-cut ®lter. In this case, the decrease in the integrated intensity ratio of irradiated samples started at about 310 nm of the absorption wavelength. Fig. 12 shows that the ratios of the integrated intensity at 4.2 ppm to that at 8.1 ppm for the irradiated samples
increased at about 310 nm. These results suggest that the ether parts of soft segments in the polymer were degraded by the irradiation with UV rays below 310 nm, and ester bonds and aldehyde were formed. When the results of weathering were compared with those of photodegradation, it was clear that the weathering of the TPEE was mainly induced by the radiation below 310 nm. The experimental results indicate the reaction mechanisms shown in Fig. 13 under photodegradation conditions. 3.5. Eect of water on photodegradation during irradiation Fig. 14 shows the relationships between the gelation ratio and the immersion time during a 10-h irradiation. It suggests that the presence of water during irradiation increases gel formation. This behavior is not fully understood, but it appears that de®ciency of oxygen induced by immersion in water during irradiation gives rise to a gel formation reaction.
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2. In the degradative reactions caused by the outdoor exposure tests, selective degradation of the ether parts of the soft segment occurs in the polymer; ester and aldehyde are formed, and main-chain scission is caused by the action of UV rays below 310 nm. 3. The degradative reactions are accelerated by temperature rise during light irradiation. 4. Gel formation caused by light irradiation is hardly observed. In contrast, the presence of water during irradiation increases gel formation, possibly by excluding oxygen. 5. The mechanism of the degradation caused by hydrolysis diered from that of the outdoor exposure tests and the accelerated weathering test. Fig. 15. Relationships between the NMR integrated intensity ratio (3.4/8.1 ppm) and radiant exposure under several temperature conditions. , 63 C; , 73 C; , 83 C; , 93 C.
3.6. Eect of temperature during photo-irradiation The relationships between the NMR integrated intensity ratio (3.4/8.1 ppm) and radiant exposure under several temperature conditions are shown in Fig. 15. The result indicates that the selective degradation of the ether parts in the soft segment was accelerated by temperature rise during the exposure.
Acknowledgements The authors are indebted to Mr Reiichi Someya of Kasho Co. Ltd., Japan, for measurements of the radiation exposure and to Mr Junichi Ikeda and Mr Tomio Yamaguchi of the Japan Weathering Test Center for the thermal degradation tests. We are grateful to TOYOBO Co. Ltd., Japan, for supplying the samples used in this work.
References 4. Conclusions This paper presents the results of analysis of the degradation by photodegradation, thermal degradation and hydrolysis for the TPEE. The following conclusions can be drawn from the experimental results: 1. The degradation mechanism caused by the photodegradation experiments is a selective degradation of the ether parts of the soft segment in the polymer and resulting formation of ester bonds, which is almost the same as that of the outdoor exposure tests.
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