Understanding Consequences and Tradeoffs of Melt Processing as a Pretreatment for Enzymatic Depolymerization of Poly(ethylene terephthalate)

Abstract

Melt extrusion pretreatment of poly(ethylene terephthalate) (PET) prior to enzymatic depolymerization with an unpurified leaf branch compost cutinase enzyme cocktail is explored to ascertain the efficiency gained by different processing methods on the enzymatic depolymerization of PET. Specific surface area (SSA) is investigated as a key factor in reducing depolymerization time. Higher SSA substrates (>5.6 mm2 mg−1) show higher depolymerization rates (≈0.88 g L−1 terephthalic acid [TPA] per day) and no induction phase, while lower SSA substrates (≈4.3, 4.4, and 5.6 mm2 mg−1) show, after an initial induction phase, similar depolymerization rates (≈0.46, 0.45, and 0.44 g L−1 TPA per day) despite increases in SSA of up to 30%. The mechanism of enzymatic depolymerization manifests in the appearance of anisotropic pitting. Longer incubation time used to overcome the induction phase in low SSA substrates allows for nearly full recovery of monomeric products, but manual pregrinding of extruded PET sharply increases SSA, depolymerization rate, and substrate crystallinity which may decrease the maximum recycled yield of the product materials. An estimate of the energy cost of increasing SSA is made and its effects on material properties are discussed. This work highlights key material structure and pretreatment aspects influencing the enzymatic recycling of PET.

Melt Processing Pretreatment Effects on Enzymatic Depolymerization of Poly(ethylene terephthalate)

“…28,30,32,33 Recent advances in our understanding of enzyme technologies have led to economical and scaled demonstrations of enzymatic depolymerization of PET into TPA and EG. 15,28,30,34 A variety of enzymes (e.g., PETase, MHETase, and LCC) have been explored to ascertain their effects on PET degradation. 11,28,30,32,35−37 As compared to the mechanical and chemical recycling methods, the current enzymatic recycling technology still faces challenges of accelerating the rate and improving the yield so that it can be implemented at an industrial scale.…”

“…Researchers have performed various pretreatments to adjust the material properties (i.e., particle size/surface area, surface hydrophobicity, glass transition temperature, crystallinity percentage, and molecular weight distributions) of PET to accelerate the enzymatic depolymerization. 11,30,34,35 These material properties directly influence the conversion efficiency of enzymes. Low crystallinity or high amorphous fraction enhances the enzyme penetration inside the crystalline lamellae, which enhances the depolymerization rate.…”

“…28,46 The higher specific surface area of the PET substrate provides more active binding sites for enzymes to attach to a plastic surface for further degradation. 30,34 Many prior studies use laboratory-grade PET powders of low crystallinity as the substrate for the enzymatic reaction in order to maximize the degradation efficiency. Despite the current wealth of knowledge, scale-up, and industrial acceptance of biochemical recycling, it still suffers from the high capital investments in feedstock pretreatment, and complex systems are required to achieve efficient conversion.…”

“…The cell lysis product (unpurified LCC) was used for the enzymatic degradation experiments in this study. The synthesis and extraction method of LCC was adopted from Chang et al 34 Twin-Screw Extrusion Setup. The ultra-high-speed TSE employed in this study consists of a corotating, completely intermeshing twin-screw with a screw diameter of 15 mm and a length to diameter (L/D) ratio of 60:1.…”

“…The column temperature was maintained at 40 °C. The analysis method follows the one described in Chang et al 34 The flow rate was held constant at 0.6 mL min −1 . For the target analyte, a DAD wavelength of 240 nm was utilized.…”