A Study on Mechanical Properties and Photo-degradation Behavior of HDPE Subjected to Shearing

Abstract

In this study, we focused on high-density polyethylene (HDPE), which is commonly used for a lot of applications, and aimed to clarify changes in the mechanical properties of HDPE depending on shear deformation, as well as its resistance to photo-degradation. Also, the purpose of this study is to clarify the deterioration of resins that have been subjected to shearing during injection molding, etc., and the issues involved in remixing and reusing HDPE as recycled materials. The results revealed that HDPE subjected to shear deformation deteriorates in terms of mechanical properties, especially elongation at break. Furthermore, it was revealed that shearing reduces entanglement. In addition, although the mechanical properties of HDPE subjected to shear deformation, especially fracture strain, decrease markedly with the light irradiation time, there is no noticeable difference in the molecular weight of HDPE subjected to shear deformation and HDPE not subjected to shear deformation. Taking these results together, it is thought that the application of shear reduces entanglement between PE molecular chains, and as a result, the number of tie molecules connecting crystal lamellae during crystallization decreases. Moreover, it is deduced that the fracture strain was greatly reduced in a short period of time due to the cleavage of fewer tie molecules resulting from light irradiation (deterioration). This result also shows that in HDPE that has been subjected to shear deformation even once, the generation of tie molecules is suppressed due to a decrease in entanglement, and when light irradiation is performed, the few tie molecules are severed, resulting in an extreme decrease in mechanical properties. This suggests that there are concerns about the long-term durability (extension of service life) required by the “Law for Recycling Plastic Materials”in the future.