Solid base-assisted photocatalytic degradation of polyethylene via the Norrish mechanism through the generation of alternating polyketones

Abstract

Low-density polyethylene (LDPE) is widely used in plastic products. Due to its nonpolar molecular structure and inert hydrocarbon chains, it is resistant to degradation, leading to serious environmental problems. As a photocatalyst, TiO2 generates electron-hole pairs upon exposure to ultraviolet (UV) light. The holes interact with hydroxide ions OH− dissociated from a solid base to produce hydroxyl radicals (OH∙). These radicals then attack the PE chain, generating carbonyl groups (C=O), which facilitate Norrish reactions and promote PE degradation. In this study, a LDPE-1%TiO2 (LDPE-1T) composite film was first soaked in a NaOH solution to determine its photocatalytic degradation efficiency under alkaline conditions. After 800 h of UV exposure, the film exhibited a significant mass loss of ~87 wt%. In contrast, the LDPE-1T composite film without NaOH pretreatment presented a lower mass loss of ~55 wt%. To increase the applicability of this study, a solid base was incorporated to fabricate LDPE-5%TiO2-3%K2CO3 (LDPE-5T-K) and HDPE-1%TiO2-3%K2CO3 (HDPE-1T-K) composite films through hot pressing. This modification was also shown to increase photocatalytic degradation efficiency by increasing the concentration of OH∙ radicals in the system, promoting the formation of C=O groups and facilitating the Norrish type I reaction.