Recycled Polyester (rPET) in Knits
rPET in knits: mechanical vs chemical recycling, the GRS/RCS chain-of-custody concept, and performance parity with virgin PET.
Recycled polyester (rPET) is the leading answer to sustainability demand in performance knits. Usually made from recycled PET bottles or textile waste, the yarn shares the same chemical structure as virgin polyester. When produced correctly, it therefore yields a fabric that is largely equivalent to virgin PET in performance terms.
Mechanical vs chemical recycling
- Mechanical recycling: PET bottles are washed, flaked, melted and re-extruded into chip/yarn. It is common, lower-energy and economical; however, the polymer chain can shorten slightly each cycle, which can introduce small variations in colour and strength.
- Chemical recycling: PET is broken down to its monomers (depolymerisation) and re-polymerised. More costly and energy-intensive, it delivers purity and consistency very close to virgin and can process mixed or dyed waste streams.
GRS and RCS: the chain-of-custody concept
For an rPET claim to be credible, the recycled content must be traceable from source to finished fabric. GRS (Global Recycled Standard) and RCS (Recycled Claim Standard) are certifications that document this chain of custody. GRS additionally covers social, environmental and chemical criteria, whereas RCS focuses solely on tracking recycled content.
- Every link in the supply chain (yarn, knitting, dye/finish) must be certified and tied to the next link via a transaction certificate
- The recycled-content percentage is tracked and verified
- Certification makes the 'recycled' claim to the end user auditable
Performance parity with virgin PET
Because the polymer chemistry is identical, quality rPET yarn can produce fabrics practically equivalent to virgin PET in strength, moisture management, dimensional stability and colourfastness. All standard processes — disperse dyeing, microfibre fineness, DTY texturing — apply to rPET as well. The point to watch, especially with mechanical rPET, is feedstock consistency: source cleanliness and dye control matter for within-lot and lot-to-lot colour matching.
Summary for buyers
rPET is a practical way to cut carbon and waste footprint without sacrificing function. When buying a claim, asking about the recycling type (mechanical/chemical), the certification (GRS or RCS) and the verified recycled-content ratio safeguards both performance and the integrity of the sustainability claim.
In depth: the mechanical recycling process and its limits
Mechanical recycling does not break PET down chemically; it melts the flake and re-forms it into fiber or pellet. The whole story therefore hinges on one quantity: the length of the polymer chain. The practical measure of that length is intrinsic viscosity (IV), determined by glass-capillary viscometry in a 60/40 phenol/1,1,2,2-tetrachloroethane solution (ASTM D4603, ISO 1628-5) and reported in dL/g. Every thermal and mechanical pass lowers this number a little; all of rPET engineering is about managing that drop and recovering it when needed.
The flow runs: collection, sorting, grinding (flake), hot caustic washing (removing PVA glue, labels and food residue), rinse-dry, melt extrusion, melt filtration (capturing gels and solid contaminants), and finally pelletizing or direct melt-to-fiber spinning. The critical point: PET is hygroscopic, and at melt temperature (around 280 °C) moisture in the chain hydrolytically cleaves the ester bond. Pre-extrusion moisture is therefore typically dried below about 50 ppm; otherwise every added increment of moisture accelerates chain scission.
Two degradation mechanisms run in parallel. Hydrolytic chain scission is driven by water in the melt; thermo-oxidative degradation by oxygen and heat; both cleave the ester bond, lower IV and raise the carboxyl end-group (COOH) count. A by-product is acetaldehyde, a key constraint in food-contact use; typical rPET is reported on the order of roughly 10–80 ppm, sensitive to extruder residence time and temperature. On color, repeated thermal processing and residual contaminants cause yellowing, quantified by the b* value in the CIE L*a*b* system (positive b* is yellow, while L* tracks lightness).
| Material / state | Typical IV (dL/g) | Note |
|---|---|---|
| Virgin bottle-grade resin | approx. 0.72–0.84 | High molecular weight for preform/injection |
| Post-consumer flake (washed) | below approx. 0.70–0.78 | Wash hydrolysis and use fatigue lower IV somewhat |
| After melt extrusion (no SSP) | further drop each pass | Thermal + mechanical scission is cumulative |
| Fiber-grade target | approx. 0.55–0.70 | Staple/POY spinning tolerates lower IV |
| Rebuilt by SSP | near-virgin | Sub-Tm solid-phase chain extension recovers IV |
Recovering IV: solid-state polycondensation (SSP) and chain extenders
There are two main routes to rebuild the lost molecular weight. The first is solid-state polycondensation (SSP): pellet or flake is held below the melting point (Tm around 250–260 °C), typically at roughly 200–220 °C, under a nitrogen sweep or vacuum for an extended residence. Under these conditions chain ends re-react and join, the reaction by-products (water, ethylene glycol, acetaldehyde) are swept away by the inert flow or vacuum, and IV climbs back toward virgin levels. Vacuum accelerates molecular-weight build-up while a nitrogen sweep is slower but an industrially common route. A secondary benefit of SSP is decontamination: high temperature and low pressure drive off acetaldehyde and volatile migrants, delivering the cleanliness required for food-contact approval.
The second route is chain extenders in reactive extrusion: multifunctional epoxy-functional oligomers (for example, Joncryl-type styrene-acrylic epoxy resins used in the sector) or dianhydrides (for example, pyromellitic dianhydride, PMDA) bridge the COOH and OH ends of broken chains and raise molecular weight within seconds. Whereas SSP is an hours-long equilibrium process, a chain extender acts in a single pass; in practice the best result comes from combining the two, since SSP alone can fall short of some target IVs.
The intrinsic limits of mechanical recycling and the bridge to chemical
The mechanical loop has honest limits. Each pass pulls IV down and pushes b* (yellowing) up; mixed-color streams yield gray/dull resin, so dyed textile waste often goes to lower-value uses (downcycling) rather than high-value clear rPET. Elastane, dyes, coatings and especially cotton blends in textiles clog the mechanical line; separating PET from cotton is not practical mechanically. This is where chemical recycling steps in: glycolysis, methanolysis, hydrolysis or enzymatic depolymerization break the polymer back to its monomers (BHET, DMT, or TPA + EG) and reset the color/IV history, so output can be equivalent to virgin. The cost is higher energy and expense; the payoff is returning the dirty/mixed/dyed streams that mechanical recycling rejects to the loop. The two are complementary rather than rival: clean streams cycle cheaply in mechanical, dirty streams are rescued in chemical.
Chain of custody: GRS / RCS and content verification
Recycled content is as much a supply-chain claim as a quality parameter, and it must be verifiable. Two Textile Exchange standards govern this: the Recycled Claim Standard (RCS) and the Global Recycled Standard (GRS). RCS tracks the content claim from a minimum of 5% recycled input; GRS sets a higher threshold (minimum 50%) and additionally covers social, environmental and chemical requirements. At the core of both is chain of custody: the identity of the recycled material is protected from raw input to finished product under third-party audit and is passed at each stage via transaction certificates.
The practical engineering takeaway: when evaluating rPET, ask for two documents together. First, the technical identity of the material (IV, b*, acetaldehyde, moisture, contaminant level); second, proof of content (a GRS/RCS certificate and transaction certificates). The first tells you how the fabric will behave, the second tells you the recycling claim is real; without both, neither a quality nor a sustainability claim is defensible.