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6 Tensile testing The tensile testing was carried out on H25K-S universal testing machine (Hounsfield, UK). The test temperature was 23℃ and the crosshead speed is 50mm/min. Tensile strength retention was calculated by equations (3): （3）.
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Results and discussion.
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1 Sample Characterization In order to study the photodegradation behavior of α-PP and β-PP, it is necessary to characterize whether α-PP and β-PP used as sample in this work crystallized mainly in α- and β-modification, respectively. Fig. 1 shows the DSC melting thermograms and WAXD patterns of pure PP, α-PP and β-PP after non-isothermal crystallization from melt. It is well recognized that the characteristic melting peak of α- and β-crystal of PP locate at 165℃ and 155 ℃, respectively, the diffraction angle (2θ) of α-crystal of PP locates at 14. 1˚, 16. 9˚ and 18. 5˚ while β-crystal locates at 16. 2˚[9]. Therefore, it can be deduced from Fig. 1 that PP and α-PP mainly composed of α-modification while β-PP mainly composed of β-modification. (a) (b) Fig. 1 DSC melting thermograms (a) and WAXD patterns (b) of PP, α-PP and β-PP.
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2 FTIR analysis The result of photodegradation of PP is formation of hydroperoxides and carbonyl species such as ketones, esters, and acids. Carbonyl index were calculated to quantitatively investigate the photodegradation extent of PP, α-PP and β-PP. The relationship of carbonyl index with exposure time is shown in Fig. 2. It can be seen that the increase trends of carbonyl index for PP and α-PP are similar, which may be because PP and α-PP has the same crystalline modification. The carbonyl index for PP and α-PP increase sharply firstly, and then increase slowly as the irradiation time increasing, on the contrary, carbonyl index for β-PP increases slowly firstly, and then increase sharply as the irradiation time increasing, which revealed different photodegradation behavior of α-PP and β-PP. At the same irradiation time, the carbonyl index arranged as α-PP>PP>β-PP, which means α-PP is relative most oxidative, and β-PP is relative least oxidative among the three samples. The phenomena can be understood from the different crystal structure and size. The spherulite size of α-crystal is smaller than β-crystal , thus α-PP is more transparent than β-PP, so more UV light can permeate through α-PP, which is benefit for the photodegradation of α-PP, as a result, the carbonyl index of α-PP is the largest. Besides, there are different crystal structures for α-modification and β-modification, photodegradation behavior may be dependent of crystal structure, but it needs further investigation. Fig. 2 Plots of carbonyl index versus irradiation time.
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3 Tensile property At the early UV irradiation period, the tensile strength of PP, α-PP and β-PP increased as shown in Fig. 3, which ascribed to fact that the annealing effect caused by the exposure environment temperature which can improve the tensile strength. Then the tensile strength began to decrease as the exposure time increasing, the tensile strength retention decreased linearly with exposure time for PP and α-PP which are both mainly in α-modification. The tensile strength retention of PP is higher than α-PP and β-PP, which revealed PP is less photodegradable than α-PP and β-PP. The irradiation time at which the tensile strength began to decrease are also different for PP, α-PP and β-PP, the irradiation time is 480h for PP while 312h for α-PP and β-PP, which are the same as the irradiation time at which the crystallization and melting thermograms began to obviously change, in another word, the mechanical property change is in consistent with structure change for PP, α-PP and β-PP. The retention of the elongation at break of PP, α-PP and β-PP are shown in Fig. 4. It can be seen that the effect of UV irradiation on retention of the elongation at break of PP, α-PP and β-PP is similar as the effect on the retention of tensile strength. The retention of the elongation at break arranged as PP > β-PP > α-PP. Fig. 3 Plots of tensile strength retention versus exposure time Fig. 4 Plots of retention of the elongation at break versus exposure time.
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5 Relationship of tensile strength with carbonyl index Plots of tensile strength retention versus carbonyl index for PP, α-PP and β-PP are shown in Fig. 5. From Fig. 5, it can be seen that the tensile strength retention of PP and α-PP decayed exponentially with carbonyl index increasing, while tensile strength retention of β-PP decayed in sigmoid form. The different decay behavior may reflect the different photodegradation mechanism of α-PP and β-PP. (a) (b) (c) Fig. 5 Plots of tensile strength retention versus carbonyl index, (a)PP; (b) α-PP; (c) β-PP.
DOI: 10.17816/brmma62811-38009
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6 Stability of β-crystal of PP during photodegradation The relative content of β-modification (Kβ) during photodegradation is shown in Fig. 6. It can be seen that the Kβ keeps nearly constant during the exposure, which revealed that the relative content of β-modification was nearly not affected by the photodegradation. Fig. 6 Effect of exposure time on Kβ value for β-PP.
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Summary Xenon lamp UV irradiation has obvious effect on the molecular structure and mechanical property of PP, α-PP and β-PP, but the UV irradiation has little effect on the relative content of β-crystal in β-PP. The photodegradation behavior of PP, α-PP and β-PP are different, which may be due to their different crystalline structure, α-PP has the highest carbonyl index while β-PP has the lowest. However, pure PP is less photodegradable than α-PP and β-PP deduced from their mechanical property. Acknowledgements The authors gratefully acknowledge National Basic Research Program of China （973 Program， Projects No. 2012CB724605）, the National Natural Science Foundation of China (Grant No. 51133005) and Guangdong Provincial Natural Science Foundation (Project No. 9151027501000072) for financial supports. References.
DOI: 10.1016/j.ifacol.2016.07.227
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