Mechanism of Microstructural Change of High-Density Polyethylene Under Different Outdoor Climates in China

This work aims to understand the microstructural change mechanism of high-density polyethylene (HDPE) exposed at five national standard natural exposure stations (at Qionghai, Ruoqiang, Lhasa, Qingdao, and Hailar) for four years, which represented the five typical climates over China. It was found that the natural weathering of HDPE was the synergistic result of multi-factors such as temperature, irradiation, oxygen, etc. Based on the carbonyl index, the degradation degree in decreasing order was Ruoqiang, Qionghai, Lhasa, Qingdao and Hailar, but the microstructural change mechanism of HDPE was similar. The molecular structure was modified and mass molecular defects formed such as carbonyl and hydrogen groups during the degradation. The new freed molecular chains released from the amorphous region self-nucleated, and then formed new imperfect crystals because of the suppression of molecular defects. With the deposition of molecular defects, the chemi-crystallization ceased. Positron annihilation lifetime spectroscopy indicated the free volume hole shrank continually with exposure time mainly due to the interaction between molecular defects, and a part of amorphous region transformed into crystalline region by chemi-crystallization. In addition, the crystallization and re-melting behavior of degraded HDPE samples had been investigated in order to promote the recycling of waste degraded polymer materials. The results indicated that the crystalline temperature and the second melting temperature decreased with exposure time.