Dissolution of semicrystalline polyethylene: Contributions of decrystallization and disentanglement to kinetics revealed by integrated experiments and modelling

Abstract

Increased use of plastics and corresponding generation of plastic waste leads to growing pressure for recycling technologies that are both economically viable and environmentally sound. For plastic films, widely used in packaging and comprising mostly polyolefins, mechanical recycling is not practical, hence the interest in chemical recycling. Dissolution/precipitation recycling can recover polyolefins for re-use, with energy needs and emissions much lower than pyrolysis. The dissolution of polyolefins is key to this recycling process, however, the underlying phenomena which govern the dissolution of semicrystalline polymers are little studied. To address this gap in knowledge, the swelling and dissolution kinetics of high-density polyethylene (HDPE) films are investigated here. Experiments are designed to obtain the time evolution of HDPE dissolved mass and degree of crystallinity. A mathematical model is developed to describe the swelling and dissolution of semicrystalline HDPE based on the transport phenomena and thermodynamics governing the process. Experimental data are used to validate the model and to obtain values for the two key fitted parameters, decrystallization constant and disentanglement rate. The detailed information provided by the model, spatial and temporal composition and solvent diffusion, reveals the molecular mechanism of HDPE dissolution. A parametric analysis is performed using the validated model to simulate dissolution phenomena at varying conditions, including initial degree of crystallinity and film thickness. These insights on polyethylene dissolution facilitate the design of more energy efficient and environment-friendly dissolution-precipitation recycling processes. The model can be extended to probe the dissolution of other semicrystalline polymers.