Performance modifying techniques for recycled thermoplastics

Petro-derived commodity thermoplastics are relatively inexpensive, lightweight, and non-biodegradable materials, which can be readily molded at high temperatures into a range of products. The manufacturing of such thermoplastic resins and products has increased dramatically over the last 70 years. The plastics based on polyolefins, polystyrenes, and polyesters occupy the largest share (80%) of the world's plastic markets. However, the disposal of waste plastics has created considerable environmental concerns. Therefore, environment protection agencies and plastic manufacturers are constantly seeking appropriate techniques for recycling or upcycling waste plastics into new products. In recent years, the recycling rates are approximately 9 to 15% out of total plastics produced annually in the United States and Europe, which are predominantly limited to individual plastic fractions such as HDPE and PET. The recycling process is associated with various expensive and time-consuming sorting techniques, which are not economically attractive to the recycling industries. A significant issue is when multiple plastics are blended, they often are not chemically compatible, resulting in phase separation and inferior-quality materials. Therefore, it is important to apply certain performance modifiers during the re-extrusion of waste thermoplastics, which include compatibilizers, coupling agents, impact modifiers, and many others. There are different additives and techniques available, suitable to improve the mechanical and other associated properties of polymer blends and composites, and these can also be used for improving the performance of recycled thermoplastics. This review article summarizes various chemical additives and approaches, which can be used in thermomechanical upcycling of waste thermoplastics to new materials with superior mechanical performance via improving interfacial adhesion or phase homogeneity of polymer blends.