Enzymatic textile recycling

Polyester — poly(ethylene terephthalate), a condensation polymer of terephthalic acid and ethylene glycol — is hydrolysable in principle: the ester bond splits with water and returns the starting monomers. Chemical routes achieve this with hot alkali, glycol or methanol at elevated temperature and pressure. The enzymatic route asks a different question: can the same reaction run in water near 70 °C and yield monomers clean enough to repolymerise without deep purification?

What cuts polyester#

The active enzymes descend from cutinases, serine hydrolases that degrade cutin, the polyester of hydroxy fatty acids in the plant cuticle. Their serine–histidine–aspartate triad attacks the carbonyl carbon of an ester bond, and the same active-site geometry accepts the aromatic ester of PET. The key families are a cutinase from leaf-branch compost and PETase from Ideonella sakaiensis, a bacterium isolated from debris at a PET recycling site. Wild-type variants are slow and thermally fragile; engineering pursues two goals at once — widening the binding cleft around the aromatic ring, and raising the protein’s melting point far enough that it can work near the polymer’s glass transition.

Why crystallinity decides everything#

Hydrolysis happens only at the surface of a solid particle, and only where the chains are mobile. PET’s glass transition sits near 76 °C: below it the amorphous regions are glassy and the enzyme makes little headway; approaching it the chains gain mobility and rates rise by orders of magnitude. Crystalline lamellae stay inaccessible at any temperature at which the protein is still folded.

That sets the process logic. Textile polyester is drawn during spinning, and drawing orients and crystallises the chains — precisely what hydrolysis does not want. Feedstock is therefore milled, amorphised by rapid melting and quenching, and presented with high specific surface area. This preparation consumes energy, and it, rather than the fermentation step, usually dominates the process balance. The products are terephthalic acid and ethylene glycol; the acid is precipitated by acidification and filtered off, which also separates it from dyes.

Polycotton#

Cotton–polyester blends are the most common and the most awkward textile. Here enzymes are applied from the other side: a cellulase cocktail of endoglucanase, cellobiohydrolase and β-glucosidase hydrolyses the cotton fraction to glucose, leaving polyester filaments intact and recoverable by filtration. The glucose goes to fermentation, the polyester to remelting or to hydrolysis. The constraint is the one familiar from all cellulosic bioprocessing: cellulose I crystallinity, surface accessibility, and irreversible enzyme adsorption onto residual dyes and finishes, which prevents the protein from being recycled through the process.

Where the boundary runs#

The bottlenecks are economic but have physical causes. Enzyme dose per tonne is set by the fact that protein cannot be fully washed off the solid phase; the product competes with virgin terephthalic acid, whose price tracks oil; and dyes, flame retardants and above all elastane contaminate the stream. The EU’s circular textiles agenda creates demand for textile-to-textile output but suspends none of these conditions. The enzymatic route wins where mild conditions preserve monomer quality — not where raw throughput is the objective.