Utility of Chemical Upcycling in Transforming Postconsumer PET to PBT-Based Thermoplastic Copolyesters Containing a Renewable Fatty-Acid-Derived Soft Block

Plastic production is one of the most greenhouse gas intensive industries and a main contributor to climate change and worsening environmental destruction. The accumulation of plastics in our oceans, on our beaches, and in landfills has become a global crisis. To this end, developing effective strategies to avert the unabated global plastic pollution crisis is urgent. Whereas recycling has proved effective over the years, there is still an urgent need to promote and improve the recycling of valuable plastic materials like polyethylene terephthalate (PET) that are often discarded after short useful lifetime. Chemical transformation of plastic waste streams, otherwise known as upcycling process, hold potential for developing effective routes toward high-value materials. A good example is the utilization of PET recyclate (rPET) through chemical diversification.

Recently, generating thermoplastic copolyesters (TPCs) with remarkable mechanical properties has drawn significant research attention. TPCs are multiblock copolymers consisting of alternating flexible and rigid segments, which are semicrystalline polyesters and long chain polyol with low glass transition temperature, respectively. Their final properties depend on a range of factors, including polyol molar ratio, composition and the properties of the soft block. TPCs are critical structural components in engineering applications that require remarkable mechanical properties and outstanding thermal stability. Unfortunately, these segment copolymers are mainly prepared from pristine feedstocks obtained from fossil fuel sources.

The preparation of TPCs from more sustainable feedstock has been explored. Importantly, achieving accessible and comparable properties require responsibly source feedstock and a robust and innovative synthetic strategy. To this note, Apostolos Karanastasis, Victoria Safin, Subin Damodaran and Professor Louis Pitet from Hasselt University developed a direct one-pot synthetic approach for preparing TPCs with excellent mechanical performance. This reaction pathway started from a high molar mass rPET combined with a hydrophobic fatty acid dimer diol (FADD) flexible segment. Their work is currently published in the research journal, ACS Polymers Au.

Briefly, this strategy built on the previous works, and it utilized complementary routes to show the versatility of the final chemical structure by incorporating reactive comonomers. A multiblock architecture was created through transesterification. A detailed size-exclusion chromatography as well as carbon and proton nuclear magnetic resonance spectroscopy were utilized to characterize the high molar mass and segment distribution to uncover contrasting mechanical and thermal properties.

The research team demonstrated the possibility of chemically converting rPET to PBT via molecular exchange, resulting in the preparation of a series of copolymers with different compositions. Compared with their PET-based counterparts, PBT-based TPC copolymers crystallized faster and exhibited higher modulus for a wide range of copolymer compositions, making them ideal for practical applications requiring injection molding.

The redistribution of the building blocks was facilitated by the improved miscibility and molecular mobility due to the addition of the small molecule diols. These small molecule additives played a vital role in determining the final makeup of the polymer. The polymer makeup of multiblock copolymers prepared by adding ethylene glycol or 1,4-butanediol to rigid EG and PTB segments, respectively, were shown to have impressive implications on thermal and mechanical performance.

In summary, a straightforward and sustainable upcycling approach for expanding the utility of postconsumer rPET waste resources via transesterification-based chemical transformation was reported. The versatility of this approach in transforming hard polyester block from PET to poly(butylene terephthalate) (PBT) via innovative in situ chemical exchange was illustrated. In a statement to Advances in Engineering, Professor Louis Pitet, the lead author explained their new strategy could be extended to additional waste streams and would be pivotal in addressing the growing global plastic pollution crisis.