2022 Annual Meeting
Improving the Recyclability of Polyethylene Terephthalate
Polyethylene terephthalate (PET) is a semi-crystalline polymer composed of aromatic rings connected by small glycol aliphatic linkages. PETâs chemical structure attributes certain physical properties, namely resilience to degradation and pliability at moderate temperatures, which results in an easily processable, durable material. Because of this, PET is used to manufacture many common and useful materials, such as plastic bottles and food packaging. However, contrary to popular belief, the majority of PET is not recycled due to the inherent difficulties posed by conventional recycling processes. Mechanical recycling employs high temperature in a moisture and oxygen-rich environment. These conditions cause hydrolysis and concomitant oxidation, which results in the degradation of the properties that make PET so useful. Chemical recycling strategies such as solvolysis provide an opportunity to recycle PET efficiently without property degradation but are far too energy intensive to be economically feasible. Chemical recycling of PET can be made less energy intensive by altering its chemical makeup slightly. The energy input to chemically recycle PET is exorbitant because of its lack of mobility at temperatures lower than its glass transition, allowing only end-chain hydrolysis. By inserting long-chain unsaturated aliphatic molecules, called âtrojan horseâ units, into PETâs backbone, the altered polymer now has sites that will hydrolyze and oxidize with ease under basic conditions at lower temperatures. Trojan horse units are hypothesized to aggregate in and enlarge the amorphous regions of PET; these amorphous regions are randomly distributed throughout PETâs backbone, causing random chain cleavage upon oxidative hydrolysis. The altered PET polymer is broken into much smaller chains, allowing for bulk depolymerization to occur with a fraction of the energy input required for traditional PET. The trojan horse molecules are cleaved into smaller diacids during depolymerization and can be used to create polyamides for nylon production, creating a process with virtually zero discharge.