TransFIRe researchers at Northumbria University have developed a method to convert waste polyethylene terephthalate (wPET) into an unsaturated polyester for use in protective coatings applications, thereby also lowering the demand of virgin petrochemical feedstocks typically required to produce these polyesters.
State of the art mechanical separation of mixed plastics which are predominantly PET at recycling centres works by converting large objects into a “flake” which can be separated efficiently into very high purity recycled wPET content. However, a proportion of the throughput can only be purified to 90-99% PET purity due to the presence of small and often highly coloured flakes of other plastics, and hence has no commercial value. This fraction of the wPET process is incinerated or sent to landfill rather than used in closed loop recycling.
These so called “flake losses” account for around 160 ktpa of PET lost from the closed loop recycling process in Europe alone, and are independent of the successful recycling modality for wPET drinks bottle content.
A typical recycling plant in the U.K. generates over 200 tonnes per annum of PET flake contaminated with a percentage of polyolefin plastics.
This case study sought to valorise this specific waste stream by conversion of the wPET into an unsaturated polyester which is a drop-in alternative for existing alkyd resin used in protective and decorative coatings applications. As well as the specific output of this chemistry, the team saw this as an exemplar process for creating an alternative polymer for an existing use case in one partner’s portfolio using a waste material in another partner’s workflow.
To solve this problem TransFIRe researchers at Northumbria University developed a one-pot partial depolymerisation-repolymerisation reaction which converted this contaminated wPET into an unsaturated polyester with greater than 95% efficiency. The end product was then optimised with the addition of waste-derived or bio-derived monomers so that material performance matched commercially input benchmarks of product properties such as drying time, microhardness, tensile stress and thermal resistance. These assessments were performed in parallel with a control polyester prepared using typical petrochemical feedstocks.
To completely demonstrate the potential for these alkyd resins to be the basis of a commercial product, they were subsequently formulated into pigmented protective coatings, utilising inorganic foundation industry wastes (identified as surplus materials by TransFIRe collaborators of BGS) as fillers. A gloss white paint has been created with research ongoing to create a range of different colours and finishes. In addition, there was also investigation of alternative cure chemistries to reduce the need for heavy metal drying catalysts.
Characterisation of these coatings (which are made from 100% waste and bio-derived resins) show that their material performance is comparable to a similar coating derived from petrochemical sources.
Future work will look into the development of alternative cure chemistries and alternative fully formulated protective coatings suitable for a commercial market.
Conversion of Contaminated Post-Consumer Polyethylene Terephthalate into a Thermoset Alkyd Coating Using Biosourced Monomers
Bradley Thomas, Nicole D. A. Lopez, James Railton, Jamal Bousbaa, Justin J. B. Perry, and Matthew G. Unthank
ACS Sustainable Chemistry & Engineering 2024 12 (17), 6485-6493
DOI: 10.1021/acssuschemeng.3c07560