The Key to Enzymatic Degradation of Polybutylene Terephthalate (PBT): Influence of Semicrystalline Properties on Degradation
Annalena Pongratz, Andreas Gagsteiger, Birte Höcker, Holger Ruckdäschel
Abstract
The rapidly growing plastic production and its environmental impact necessitate closed-loop recycling strategies for all polymers. For polyesters, which are susceptible to hydrolysis, enzymatic degradation offers a promising solution. While amorphous polyethylene terephthalate (PET) is already fully degradable, enzymatic breakdown of polybutylene terephthalate (PBT) remains challenging and underexplored. This study investigates the enzymatic degradability of PBT by systematically modifying its semicrystalline structure and optimizing enzymatic degradation conditions. Several PET-hydrolases were tested for their capability to degrade PBT, with LCC variants proving most effective. Increasing incubation temperatures drastically improve the release of soluble degradation products of PBT, with 80 °C as optimum. To assess the impact of semicrystalline properties, PBT substrates with varying contents of mobile amorphous fraction (MAF), rigid amorphous fraction (RAF), and crystalline fractions, as well as different lamellar morphologies, are prepared. Degradation trials reveal that lowering crystallinity significantly improve enzymatic PBT hydrolysis, yielding up to 1.7 mM of soluble products within 24 h. Both MAF and RAF are hydrolyzed at comparable rates, whereby incubation temperatures near the glass transition of RAF are required for significant hydrolysis. These findings demonstrate the interplay between material and enzyme properties, showcasing that fine-tuning both is key to advancing efficient enzymatic PBT recycling.