Poly(ethylene terephthalate) (PET) is the most abundant polyester manufactured in the world and it is mechanically recycled at a rate of ~25%. The most widely recycled form of PET is rigid packaging, such as used for water bottles, food trays and containers. However, this represents only about 55% of the PET end use in the U.S., with the remainder mostly film and sheet, and fibers for textiles and carpet. Coloring agents, processing aids, and laminates with other polymers, paper and metal are prevalent. Thus, the PET waste stream is complex and heterogeneous, presenting major issues for the purity and quality of recyclates obtained through mechanical processing. While chemical recycling of PET has been known for decades, the cost and energy intensity of the process is too high for practical application. Recent breakthroughs in bioprocess engineering have demonstrated industrially relevant rates of degradation of PET using thermophilic hydrolysis enzymes with more favorable economics. This new paradigm in polymer deconstruction still suffers from low efficiency, slow kinetics, and unknown efficacy on real PET waste materials. In this research we are investigating a mechano-chemical pretreatment process to alter the molecular weight, crystal structure, and particle size of PET secondary feedstocks prior to enzymatic deconstruction. Three forms of PET, both from waste and prime sources, were melt processed with and without hydrolytic additives, and the molecular weight, thermal properties and crystallinity were evaluated. It was found that incorporation of cyclohexanedimethanol decreased the molecular weight by 50% and the glass transition temperature by 10 oC. The effects of the pretreatment and PET particle size on enzymatic depolymerization were studied using modified leaf compost cutinase enzymes expressed in E. coli and B. subtilis. The yield of terephthalic acid was increased for the pretreated PET. Results from this work will inform process intensification through coupling of the mechanical and biological recycling strategies.

Polyester recycling to recover high value chemicals: Effects of pretreatment on conversion efficiency

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.