Polyethylene terephthalate (PET) chemical recycling is defined by the molecular outputs generated through solvolytic chain scission rather than by a single process route. Post-consumer bottle flake with an intrinsic viscosity of 0.70–0.84 dL/g measured by ASTM D4603-18 or ISO 1628-5:1998 in 60/40 wt/wt phenol/1,1,2,2-tetrachloroethane at 30 °C can be converted to dimethyl terephthalate (DMT) and monoethylene glycol (MEG) via methanolysis, to recycled purified terephthalic acid (rPTA) via hydrolysis, or to bis(2-hydroxyethyl) terephthalate (BHET) and higher oligomeric fractions via glycolysis. The production constraint is not depolymerization yield alone; it is whether the output monomer stream meets insertion specifications in continuous melt-phase polymerization and solid-state polymerization equipment.

Mechanical recycled PET retains molecular weight distribution and non-PET contamination. Chemical recycling outputs are re-fed as raw materials to esterification or polycondensation reactors. Consequently, the analytical panel for rPTA and DMT includes ester or acid end-group assays, metal and halide residues, color coordinates, volatile organic content, and particle size distribution. Each specification is tied to a standard method or to a specific reactor design requirement.

What constitutes chemically recycled PET monomer purity under typical industrial methanolysis?

Methanolysis reverses PET ester linkages with excess methanol. The stoichiometric equation consumes 2 mol methanol per PET repeat unit and produces 1 mol DMT and 1 mol ethylene glycol. Published continuous methanolysis data report reactor temperatures of 220–270 °C, methanol-to-PET repeat unit molar ratios of 3:1–6:1, and absolute pressures of 2–5 MPa to maintain methanol in liquid or dense-phase contact. Zinc acetate or magnesium acetate catalysts at 0.1–0.5 wt% relative to PET are common, and residence times range from 30 min to 120 min depending on reactor configuration.

Crude DMT is purified by vacuum distillation and melt crystallization. Polymer-grade DMT specifications typically require ester content of ≥99.9 wt%, with monomethyl terephthalate and methyl 4-formylbenzoate kept below 50–100 ppm because carboxylic acid or aldehyde end groups act as chain stoppers. MEG purification requires fractional distillation under vacuum; diethylene glycol content must remain below 0.1 wt% where subsequent PET esterification is designed for PTA or DMT routes, because diethylene glycol enters the polymer backbone and lowers melting point.

Industrial methanolysis trains typically use a high-pressure stirred reactor cascade, a flash drum for methanol recovery, a wiped-film evaporator for oligomeric residue removal, and structured packing columns for DMT and MEG separation. Batch-to-batch variance in flake color and polyvinyl chloride content shifts MEG distillation column fouling rates. Chloride released from polyvinyl chloride can be neutralized with sodium carbonate before distillation to protect stainless steel column internals. Methanolysis does not produce rPTA directly. rPTA is formed by hydrolysis of PET or by subsequent hydrolysis of DMT. Plants with dimethyl terephthalate esterification infrastructure can use DMT; plants with terephthalic acid paste feed systems require rPTA.

Hydrolysis-derived rPTA and its specification boundaries for re-polymerization

Neutral hydrolysis of PET in liquid water at 200–300 °C and autogenous or imposed pressures of 1.5–4 MPa cleaves the ester linkages to yield terephthalic acid and ethylene glycol. A water-to-PET mass ratio of 5:1–12:1 is typical in published batch and semi-continuous studies. Alkaline hydrolysis operates at lower temperatures, often 80–180 °C with sodium hydroxide, forming disodium terephthalate, which is subsequently precipitated by acidification to crude terephthalic acid. Acid hydrolysis is less preferred because sulfuric acid recovery adds unit operations.

Crude terephthalic acid is converted to rPTA by hot water washing, hydrogenation where colored impurities must be reduced, crystallization, filtration, and drying. Merchant PTA specifications frequently set 4-carboxybenzaldehyde below 25 mg/kg, p-toluic acid below 150 mg/kg, ash below 15 mg/kg, and CIE b* color below 1.0 measured by ASTM D6290-19. rPTA used for bottle-grade PET must meet equivalent thresholds because 4-carboxybenzaldehyde and p-toluic acid act as monofunctional chain terminators and reduce attainable intrinsic viscosity.

Purification lines use a rotary vacuum drum filter followed by reslurry washing at 80–95 °C; multiple reslurry stages reduce sodium, sulfate, and catalyst residues. Drying to moisture below 0.2 wt% is necessary before pneumatic conveying to esterification silos. Particle size distribution is controlled because fines increase dust and feeding variability, while coarse particles above 500 µm can reduce esterification mass transfer. PET bottle resins containing isophthalic acid as a comonomer release isophthalic acid during hydrolysis. Repeated recycling loops without isophthalic acid separation can raise copolymer comonomer content, and at 2–3 wt% isophthalic acid the melting point depression becomes measurable by ASTM D3418-15 differential scanning calorimetry.

Food-contact compliance of recycled PET produced from chemical recycling outputs is assessed under 21 CFR 177.1630 in the United States and Regulation (EC) No 282/2008 in the European Union. Recovered monomers placed on the market are handled within the registration obligations of EC 1907/2006.

Glycolysis of post-consumer PET in excess ethylene glycol at 180–240 °C with zinc acetate, tin chloride, or titanium phosphate catalysts produces BHET. The reaction is typically run at an ethylene glycol-to-PET repeat unit molar ratio of 2:1–4:1, and oligomeric residues are separated by hot filtration. Crude BHET is crystallized and washed with deionized water until the melting point by differential scanning calorimetry reaches 110–115 °C; residual ethylene glycol and diethylene glycol must be below limits because they modify glycol-to-acid ratio during repolymerization. BHET can be used directly in a prepolymerization reactor or hydrolyzed to rPTA when the downstream plant is designed for PTA paste feed. Enzymatic hydrolysis with cutinase-derived PET hydrolases at 65–75 °C yields terephthalic acid and ethylene glycol with high selectivity, but reaction rates remain limited by crystallinity and particle surface area; published data for commercial-scale enzymatic rPTA output is limited.

When recycled monomers are re-incorporated: extrusion and solid-state polymerization constraints

Repolymerization of rPTA and MEG proceeds through the same esterification and polycondensation sequence as virgin PTA. In melt-phase operations, rPTA is slurried with MEG at a molar ratio of 1.10–1.40:1 MEG to terephthalic acid and esterified at 240–265 °C under atmospheric or slight positive pressure. Antimony trioxide is added at 150–250 mg/kg as antimony; titanium-based catalysts may be used at lower loadings to avoid antimony accumulation in repeated recycling loops. Polycondensation is completed in a high-vacuum finisher at 270–290 °C and pressures below 1 mbar. The design of the finisher—whether cage, disc-ring, or horizontal high-surface-area—determines residence time distribution and residual acetaldehyde.

Continuous lines producing bottle-grade PET from rPTA must manage carboxyl end group concentration; values above 30 mmol/kg by ASTM D7409-15 indicate insufficient esterification or chain termination from impurities. The amorphous pellets are then crystallized at 150–180 °C and solid-state polymerized under vacuum or inert gas at 200–220 °C for 12–18 h to raise intrinsic viscosity from approximately 0.60 dL/g to 0.80–0.84 dL/g. Residual moisture in pellets must be reduced below 50 ppm before injection molding or blow molding because hydrolytic degradation during melt processing can consume intrinsic viscosity.

When DMT from methanolysis is used, the ester interchange is conducted with MEG at 150–220 °C using manganese, zinc, or titanium organometallic catalysts; methanol is removed as a vapor. DMT-based plants are more tolerant of 4-carboxybenzaldehyde because the ester interchange route does not generate water during the initial reaction, but DMT purity below 99.9 wt% shifts glycol-to-ester balance and limits molecular weight growth.

Methanolysis economics are governed by methanol recovery and by the separation of DMT from methyl 4-formylbenzoate and monomethyl terephthalate. The equilibrium between PET, methanol, DMT, and MEG is pressure-sensitive; insufficient methanol excess drives recrystallization of oligomeric intermediates inside transfer lines. A methanol recovery column operating with a reflux ratio above 2:1 can consume more low-pressure steam than the depolymerization reactor itself. In operations where post-consumer flake contains polyolefin caps and labels, pyrolysis-derived waxes accumulate in the wiped-film evaporator and require thermal cleaning every 300–500 h. Published data for this specific maintenance interval varies by feedstock quality.