Designed from Recycled: Microwave-Assisted Upcycling of Plastic Waste

Abstract: Mismanagement of plastic waste is of great concern. To reach a more sustainable society, methodologies to handle materials in a more circular way needs to be developed. The work presented in this thesis focused on the development of chemical recycling routes for common plastic materials and demonstration of further utilization of the obtained chemicals in material applications. Methods to chemically recycle model waste consisting of low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyethylene terephthalate (PET) and polyamide (PA) /polypropylene (PP) carpet by microwave-assisted reactions were developed. Well defined carboxylic acids were produced by microwave-assisted oxidative recycling of LDPE and HDPE waste using aqueous nitric acid as reaction media. The mixed dicarboxylic acids, obtained from HDPE recycling, were further reacted to produce plasticizers with crotonate end-groups. The synthesized plasticizers were evaluated for their ability to plasticize polylactide (PLA) by blending or grafting onto the PLA backbone. PLA was also plasticized by blending with terephthalamides obtained from microwave-assisted aminolysis of PET. Both methods of plasticization improved the flexibility of PLA. Terephthalamides with terminal unsaturated groups were also produced by aminolysis of PET and utilized as reactants to manufacture plastic films by a radical thiol-ene reaction. A microwave-assisted hydrothermal process in aqueous hydrochloric acid (HCl) was utilized to selectively hydrolyse the PA part of a PA/PP carpet to monomeric products leaving the PP part relatively intact. The retained PP could be directly mixed with virgin PP by extrusion and hot-pressed to films. Films with 25 wt% recycled content showed no deterioration of thermal and mechanical properties compared to 100% virgin PP. The demonstrated approaches for microwave-assisted chemical recycling of plastic waste and further usage of the products show great promise for development of more circular plastic waste streams.