Chemical and biological catalysis for plastics recycling and upcycling

Abstract

At the molecular level, polymers are long chains in which the emergent material properties are dictated by the movement, arrangement, and interactions of these chains. Key factors that contribute to how the polymer chains move and rearrange are the molecular identity and arrangement, crystallinity, and molecular weight. Generally, the monomer identity influences the final application of the polymer by dictating many properties, such as the glass transition temperature (Tg). 12 As the Tg represents a softening of the material, it is a prime factor in determining the final polymer application. Flexible molecules in the backbone, which can relax faster, may result in low Tg materials with applications such as PE bags or rubber (i.e. polybutadiene). 13 Meanwhile, rigid molecules or molecules that result in stronger interchain interactions (and relax on longer timescales) can result in high Tg materials, ideal for reinforced applications. In general, when materials are at temperatures below the Tg, the polymer chains are kinetically arrested, exhibiting higher strengths. Even though monomer identity is often the largest contribution to Tg, it is not the only factor, as molecular weight, 14 tacticity, 15 and crystallinity 16 also contribute. While nearly all polymers exhibit a Tg characteristic of their amorphous region, semi-crystalline polymers will also exhibit concomitant melting behaviour crystalline in their crystalline regions, making them semi-crystalline. Crystallinity has a direct impact on polymer properties, as increases in crystallinity augment the strength of the final product and reduce the permeability of liquids and gases. Co-monomers (e.g., isophthalic acid in poly(ethylene terephthalate) (PET)) are often used to lower or completely remove crystallinity to make polymers easier to process or more transparent. 17 Finally, molecular weight, and the distributions of molecular weights, have some effect on the thermomechanical polymer properties (e.g., increasing molecular weight leads to higher Tg, modulii, etc.). However, over a critical molecular weight, nearly all thermomechanical polymer properties are constant. The exception to this generalization is the viscosity of a polymer melt, which scales with the molecular weight to the 3-3.5 power (η ~ MW 3-3.5 ) and also encapsulates properties such as diffusivity. These factors together contribute to polymer recalcitrance by limiting polymer mobility and accessibility to chemical linkages, posing a challenge for catalytic plastics deconstruction.

Sustainability of Synthetic Plastics: Considerations in Materials Life-Cycle Management

“… 11 Chemical recycling can occur through pyrolytic, catalytic, enzymatic, or solvent-based routes. 23 − 26 The general case of conversion of polymeric materials to small molecules is considered deconstruction, 27 while the more specific case of conversion to monomers is termed depolymerization. 28 Each of the chemical recycling approaches has advantages and disadvantages in terms of substrates, tolerance to impurities/additives and functional groups, yields/product distributions, energy usage, scalability, and environmental impacts.…”

Processive Enzymes Kept on a Leash: How Cellulase Activity in Multienzyme Complexes Directs Nanoscale Deconstruction of Cellulose

“… 9 , 10 Even in xenobiotic materials such as polyethylene terephthalate, the naturally evolved enzyme system for biological deconstruction of the plastics exploits the synergy between two hydrolases. 13 , 14 , 16 , 17 …”

Pretreatment of Plastic Waste: Removal of Colorants from HDPE Using Biosolvents

“…Depolymerization is an interesting approach since the polymer is degraded into monomers that can be used to remake virgin-grade material, while preserving the initial material properties [ 6 ]. Using (bio)catalysis to convert plastic waste into circular material streams is distinctly different from converting plastics into fuels or using them for energy recovery [ 9 ]. Thus, this method has the advantage of ensuring that the material value is retained within the plastic economy indefinitely, contributing to closed-loop recycling [ 10 ].…”

“…Fourth, the recovered polymer was characterized to evaluate if its thermal properties were maintained after the pretreatment. Fifth, the crystallinity of the recovered HDPE was also evaluated, since it has a direct impact on the mechanical properties of the polymer, such as yield stress, modulus of elasticity and impact strength [ 9 , 20 ]. Finally, the recyclability of the most promising solvents system (solvent:antisolvent) was studied.…”