Enzymatic deconstruction of polyester textiles

Novel means of recycling end-of-life polyesters are required if we are to convert the current, linear textile economy into a circular one with a reduced reliance on fossil-based feedstocks.

Polyesters are a type of polymer, chains of building blocks linked together by chemical bonds; in this case, they are ester bonds. Natural polymers are commonplace in all kingdoms of life, and for every natural polymer, there exists a natural enzyme that can deconstruct it back into its constituent building blocks. These deconstructing enzymes give circularity to life; the building blocks are reused with nothing going to waste.

This project will develop enzyme-based technology for the breakdown of waste polyester textiles into their chemical building blocks for subsequent reuse in a circular bioeconomy. We will develop robust enzymes that can deconstruct the most commonly-used polyester polyethylene terephthalate (PET) in waste textiles, tolerating the challenges that this feedstock poses, namely its toughness and the presence of dyes and additives. Our research will establish the feasibility of using enzymes to deconstruct the PET in waste textiles into a soup of simple building blocks for conversion back into new polyesters, thus circularising the existing linear economy, and reducing the need to produce virgin PET from fossil-fuel based chemicals.

Current schemes for enzymatic recycling of polyesters - predominantly beverage bottles and food packaging - rely upon energy-intensive pre-treatment regimes to reduce the toughness of the polymer in order for it to be deconstructed by the current best-performing enzymes. Polyester textiles present further potential challenges for enzymatic deconstruction in that they contain dyes and other additives that may impede the activity of enzymes. Ultimately, industry's adoption of enzyme-based technology for deconstruction of polyester textiles will require thorough assessments of the technology, including proof-of-concept studies in laboratory-scale bioreactors that mimic the proposed industrial application.

Our project will address these challenges in three related work packages, at each point guided by input from an impact advisory group. We will use a combined approach of optimising both enzyme performance and green, physicochemical textile pre-treatment to overcome the resistance of tough polyesters to enzymatic deconstruction. We will test the compatibility of our engineered enzymes with additives, dyes and solvents to select those enzymes that are best suited to polyester textile deconstruction. We will apply our most robust enzymes to appropriately pretreated waste polyester textiles in laboratory-scale bioreactors to evaluate the potential and limitations of scaling up the enzymatic deconstruction technology. Throughout, we will engage with the global research community through the BOTTLE consortium and through Biomimicry Institutes' Design for Decomposition project. We will also work to build synergy with the requirements of UK businesses in the textiles value chain through our partnership with Endura, a UK cycling clothing brand.

At the end of the project, we will hold workshops with our project partners and other industrial stakeholders, to showcase the outcomes from the above study and to inform the development of a technology development roadmap for future application of enzymatic polyester textile deconstruction.

Enzymes hold great promise for the low-energy deconstruction and reuse of waste polymers such as the polyester polyethylene terephthalate (PET) which is commonplace in synthetic fabrics. However, applying enzymes to the deconstruction of waste PET in textiles may be hampered by the material's challenging characteristics: its crystallinity (the degree of structural order in the polymer), and the presence of dyes and additives both of which could inhibit the enzyme activity. The current approach to overcome the PET crystallinity problem is to amorphise it by melting and extrusion quenching, an energy intensive pretreatment method with a high carbon footprint. Alternative approaches are needed if the enzymatic deconstruction of polyester textiles is to achieve its low-carbon potential.

At the Centre for Enzyme Innovation, our researchers will overcome the crystallinity problem using a dual approach of: (a) using structure-guided engineering to generate more robust variants of a newly discovered cutinase enzyme (SfCut) that has a natural tolerance to higher crystallinity PET; and (b) developing a green, chemical pretreatment process, using dissolution-reprecipitation, to reduce the PET crystallinity to a level more compatible with enzymatic deconstruction. We will establish which commonly-used polyester dyes and additives are compatible with our SfCut variants by measuring the enzyme's thermostability (DSC), their intrinsic catalytic activity (ITC), and their ability to produce PET deconstruction products (HPLC), in the presence of these compounds. Finally, we will test the thermostable, dye/additive-resistant SfCut variants for their ability to deconstruct polyester textiles at the laboratory scale (0.2-1L) in pH-controlled bioreactors. Together with comprehensive technoeconomic and life cycle assessments, these proof-of-concept studies will inform the feasibility of applying enzymatic deconstruction of polyester textiles in industry.