Abstract
Polyethylene terephthalate (PET) is the most important mass‐produced thermoplastic polyester used as a packaging material. Recently, thermophilic polyester hydrolases such as TfCut2 from Thermobifida fusca have emerged as promising biocatalysts for an eco‐friendly PET recycling process. In this study, postconsumer PET food packaging containers are treated with TfCut2 and show weight losses of more than 50% after 96 h of incubation at 70 °C. Differential scanning calorimetry analysis indicates that the high linear degradation rates observed in the first 72 h of incubation is due to the high hydrolysis susceptibility of the mobile amorphous fraction (MAF) of PET. The physical aging process of PET occurring at 70 °C is shown to gradually convert MAF to polymer microstructures with limited accessibility to enzymatic hydrolysis. Analysis of the chain‐length distribution of degraded PET by nuclear magnetic resonance spectroscopy reveals that MAF is rapidly hydrolyzed via a combinatorial exo‐ and endo‐type degradation mechanism whereas the remaining PET microstructures are slowly degraded only by endo‐type chain scission causing no detectable weight loss. Hence, efficient thermostable biocatalysts are required to overcome the competitive physical aging process for the complete degradation of postconsumer PET materials close to the glass transition temperature of PET.
Highlights
Compared to the previously reported expression of a highly similar enzyme TfH (99% sequence identity to TfCut2) in Bacillus megaterium,[14] TfCut2 could be obtained in higher purity in the growth medium of B. subtilis thereby enabling a straightforward purification of the recombinant enzyme by a single size exclusion chromatography step to yield a highly pure enzyme preparation (Figure S1c, Supporting Information)
differential scanning calorimetry (DSC) analysis confirmed the presence of a mobile amorphous fraction (MAF) fraction of the Polyethylene terephthalate (PET) which was more accessible to enzymatic hydrolysis and a RAF fraction which was more difficult to be hydrolyzed by the enzyme
During hydrolysis at 70 °C, a transition of MAF to RAF as a result of a physical aging process was observed which resulted in drastically decreased degradation rates during further incubation
Summary
In the first 24 h of degradation, weight losses of CP-PET (15.3 ± 1.5%) and GF-PET (22.3 ± 0.9%) chips sampled from different parts of the packages and films were detected corresponding to absolute weight losses of 4.8 ± 0.5 mg and 9.9 ± 0.1 mg, respectively. AP- and CP-PET chips from different parts of the postconsumer packages showed a strong discrepancy in their degradability after 24 h incubation, as indicated by wide ranges covered by the degradation curves (Figure S2b,c, Supporting Information). Minimum weight losses of 8.2 ± 1.2% for AP-PET and 23.9 ± 2.4% for CP-PET were determined (Figure S2b,c, Supporting Information) These results suggest a heterogeneous distribution of amorphous and crystalline microstructures correlating with different enzymatic degradability within the PET samples. While crystals smaller than the wavelength of the visible light will not reduce the transparency of the PET material, they will still restrict the mobility of the amorphous polymer chains in their neighborhood thereby affecting its overall biodegradability.[20]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
