Polyester Recycling: Mechanical and Chemical Routes

Polyester (PET) recycling follows two fundamental routes: the mechanical route, which re-melts the existing macromolecule under heat and shear, and the chemical (depolymerization) route, which cleaves the ester bonds back to monomer. The technical distinction between them largely collapses into a single quantity — intrinsic viscosity (IV, dL/g) — because IV tracks chain length, and therefore spinnability and tenacity. Viscosity is measured per ASTM D4603 (glass capillary viscometer at 0.50% concentration in a phenol/tetrachloroethane solution; this strictly yields the inherent viscosity, which is closely related to IV) and ISO 1628-5.

Mechanical route: melt, extrude, but lose a little each cycle

Mechanical recycling mostly collects bottle-sourced PET, washes it, reduces it to flake, and re-extrudes. The problem is thermomechanical: at melt temperature (roughly 250–280 °C) residual moisture and oxygen trigger hydrolytic and thermal chain scission at the ester bonds. The measurable consequence is an IV drop — typically from a bottle-grade input of ~0.80 dL/g down to ~0.65 dL/g after processing, and to the 0.5–0.7 range in some reports. Falling IV means falling molecular weight and falling fibre strength.

This is why a pure mechanical route is usually a 'downcycle': the material steps down one grade per loop (e.g. fibre to fill/interlining). Contamination (PVC, adhesives, other polymers), residual dye, and limited cycle count set the practical ceiling. Once PET goes from bottle to fibre it cannot, in most current technology, return to food-contact bottles — it is a one-way flow.

Restoring IV: solid-state polycondensation (SSP)

The standard way to compensate for the mechanical IV loss is a solid-state polycondensation (SSP) step. Pellets are held below melt (roughly >200 °C) under inert gas or vacuum; transesterification/esterification re-lengthens the chain and strips volatile contaminants. SSP can lift IV back to ~0.7–0.9 dL/g and reduce colour deviation (ΔE). SSP is effectively mandatory for food-grade rPET and often optional for fibre-grade; alternatively, reactive chain extenders can partly offset IV loss.

Chemical route: go back to monomer, build from scratch

Chemical recycling breaks PET into its building-block monomers; because those monomers can be polymerized into resin indistinguishable from virgin, the IV loss and colour problem are effectively 'reset'. There are four main depolymerization chemistries: glycolysis, methanolysis, hydrolysis, and enzymatic depolymerization. Each differs in target product, conditions, and maturity.

Glycolysis is the most mature and industry-ready route: it generally runs at atmospheric pressure and moderate temperature and yields bis(2-hydroxyethyl) terephthalate (BHET). Methanolysis produces dimethyl terephthalate (DMT) — for example Eastman's molecular/methanolysis line lands on this monomer pair (DMT + MEG); the company claims, depending on its life-cycle assumptions, roughly 29% lower greenhouse-gas intensity than virgin for conventional methanolysis and over 70% for its next-generation process (including avoided emissions). These figures are company/LCA-based claims rather than independently verified absolute values. Hydrolysis is attractive for food-grade because it yields terephthalic acid (TPA) directly, but its energy and chemical demand strain scalability. The enzymatic route runs at low temperature with high selectivity; Carbios' Longlaville plant, for instance, uses enzymatic hydrolysis to reduce PET to TPA and ethylene glycol and targets ~50,000 tonnes/year of prepared waste at full capacity.

The real barriers to textile-to-textile recycling

The true bottleneck is not the bottle but the textile feedstock. Garments are rarely pure PET: polyester-elastane blends make up much of contemporary apparel, and even at low loading elastane can clog machinery, cause clumping, and hamper efficient recovery of the other polymers. Dyes are the second barrier — disperse dyes (azo/anthraquinone) are physically and thermally locked to the fibre; in the mechanical route they carry through and make colour uncontrollable, and in the chemical route they must be separated from the monomer stream. The third barrier is feedstock: a consistent, sorted, pre-processed supply of textile waste is still fragmented and insufficient.

This is why chemical/enzymatic routes are more promising for textile-to-textile: dye and blend impurities can be separated at the monomer level, and selective depolymerization can, in principle, recover polyester without destroying the elastane. The mechanical route stays most efficient on pure, single-component, light-coloured streams.

Chain of custody: GRS and RCS

A recycled-content claim is a matter of traceability, not chemistry. Textile Exchange's Recycled Claim Standard (RCS) requires at least 5% recycled input and only verifies content/traceability; the Global Recycled Standard (GRS) requires at least 20% recycled content and adds social, environmental, and chemical criteria (use of the GRS logo, however, generally requires ≥50% content). Both are built on the Content Claim Standard (CCS) chain of custody: the material's identity is documented from feedstock to finished product. Under a mass-balance approach, virgin and recycled feedstock may be mixed at the facility, but the product can only be labelled with the actual allocated recycled percentage.

The practical upshot: 'recycled polyester' is not one thing. Mechanical rPET (mostly bottle-sourced; commercial rPET examples like REPREVE sit in this category) is fast and low-cost but limited in IV and colour; the chemical/enzymatic route is more expensive and less mature but promises virgin-equivalent quality and a real textile-to-textile loop. For a buyer, the right question is not 'is it recycled?' but 'which route, what IV, and with which chain-of-custody certificate?'