Terephthalate vs Terephthalic Acid: Where the PET Monomer Chain Ends

In polyethylene terephthalate, the aromatic dicarboxylic acid component is present in the polymer backbone as a terephthalate diester, not as free terephthalic acid. The distinction determines where the PET monomer chain ends: a terminal residue may remain as a free carboxyl group derived from terephthalic acid or as a hydroxyethyl ester group derived from ethylene glycol. This balance controls hydrolytic stability, solid-state polymerization kinetics, and acetaldehyde generation in downstream conversion. Production via purified terephthalic acid and ethylene glycol dominates modern capacity, while the dimethyl terephthalate route persists in specialty polymerizations where monomer purity or process flexibility justifies the added methanol recovery step. Both routes yield the same terephthalate repeat unit [—OCH₂CH₂OCOC₆H₄CO—]; the difference lies in the chain-end population retained after polycondensation.

The terminal terephthalic acid unit can be viewed as a para-substituted benzoic acid: one side participates in the ester linkage, the other remains a free carboxyl. The free carboxyl end is ionizable and hydrophilic, lowering the pH of polymer solutions in polar solvents and contributing to dye uptake in fiber applications. The hydroxyethyl end carries a primary alcohol that can participate in transesterification; molecular weight can be rebuilt in the solid state if acid and alcohol ends are sufficiently mobile. This dual reactivity means the end groups are not separate defect sites but paired reactants for post-polymerization.

Which terminal species persist after esterification and polycondensation?

During the purified terephthalic acid route, esterification of terephthalic acid with excess ethylene glycol produces bis(2-hydroxyethyl) terephthalate and linear oligomers at 240–260 °C and 1.5–3 bar. Polycondensation follows at 270–290 °C under vacuum below 2 mbar, typically with antimony trioxide at 0.02–0.03 % by weight as catalyst. Incomplete esterification leaves a fraction of the terminal terephthalic acid residues as free carboxyl groups, while excess glycol leaves hydroxyethyl termini. The dimethyl terephthalate route begins with transesterification of dimethyl terephthalate and ethylene glycol, generating methanol; incomplete transesterification can leave terminal methyl ester groups, and free carboxyl end concentration tends to be lower because free terephthalic acid is not charged directly. Melt-phase resin after polycondensation displays intrinsic viscosity between 0.45 and 0.65 dL/g and carboxyl end group concentration between 15 and 35 mmol/kg, measured respectively by ASTM D4603-18 and ASTM D7409-15.

Chain-end identity also affects melt filtration and extrusion. Compounding on a corotating twin-screw extruder with L/D ratio 32:1 and vacuum venting at −0.08 MPa can strip some acetaldehyde and water, but it does not achieve the carboxyl end reduction of solid-state polycondensation. Carboxyl end groups participate directly in ester interchange and hydrolysis; their concentration constrains melt residence time. At melt temperatures above 285 °C, carboxyl end content above 30 mmol/kg combined with residual moisture above 0.004 % accelerates intrinsic viscosity loss, with measurable decline after 5–10 min cumulative residence. Preform moulders therefore limit barrel residence to 5 min and pre-dry pellets to ≤0.004 % moisture using desiccant dryers with dew point below −40 °C.

Modern continuous PET lines use an esterification reactor followed by a prepolycondensation vessel and a finisher. In the finisher, a ring or cage reactor operating under high vacuum removes ethylene glycol and water while maintaining melt film thickness below 1 mm to maximize mass transfer. The resulting amorphous polymer is pelletized under water and dried. Immediately after pelletizing, the carboxyl end group concentration is at its highest in the melt-state resin; subsequent solid-state processing consumes a portion of these groups, which is why pelletized amorphous resin and bottle-grade solid-state polymerized resin are not interchangeable in demanding applications.

The measured carboxyl end group concentration is a lot-acceptance property in bottle-grade PET, not a research variable. ASTM D7409-15 specifies titration of polymer dissolved in hot solvent with alcoholic potassium hydroxide; results are expressed as millimoles of carboxyl ends per kilogram of polymer. Intrinsic viscosity is determined by ASTM D4603-18 in 60/40 phenol/1,1,2,2-tetrachloroethane at 30 °C using a Cannon–Ubbelohde glass capillary viscometer. Melt volume-flow rate is measured by ISO 1133-1:2022 at 285 °C with 2.16 kg load, but the value is rarely reported alone because PET is hydrolytically sensitive at that temperature. For bottle-grade resin at 0.82 dL/g, carboxyl end group concentration typically remains below 20 mmol/kg; for textile-grade resin at 0.63 dL/g, the specification may allow 25–35 mmol/kg. If pellets are stored at relative humidity above 60 % in unsealed silos, post-drying time must be extended because PET adsorbs moisture quickly.

Solid-state polycondensation converts chain ends into backbone ester linkages. Pellets are first crystallized at 140–170 °C to prevent agglomeration, then reacted at 210–220 °C for 10–20 h under flowing nitrogen with dew point below −50 °C. This operation consumes carboxyl and hydroxyethyl end groups in roughly equimolar quantities, releasing water and shifting intrinsic viscosity upward from 0.62 dL/g to 0.82 dL/g while reducing carboxyl end groups into the 10–20 mmol/kg range. The process is kinetically limited at the crystal–amorphous interface, which is why pellet crystallinity and particle size distribution affect the required residence time.

Where dimethyl terephthalate is used, the terminal chain-end landscape shifts. Transesterification with manganese or zinc acetates at 150–220 °C produces methanol, and subsequent polycondensation with antimony or titanium catalysts leaves fewer free carboxyl ends because no free terephthalic acid enters the monomer feed. Terminal methyl ester groups may persist at low concentration; these groups are less polar than carboxyl ends but alter adhesion and surface wetting in film and coating applications. Published data for this specific configuration is limited across modern high-capacity lines, because the purified terephthalic acid route dominates new investments.

When Hydroxyethyl Ends Govern Acetaldehyde Formation in Preform Molding

The hydroxyethyl ester chain end is the principal source of acetaldehyde during melt processing. At barrel temperatures between 270 and 285 °C, thermal decomposition of hydroxyethyl ends forms vinyl ester intermediates and free acetaldehyde; the rate rises sharply above 290 °C and with prolonged screw recovery time. Preform moulders using injection machines with clamping force in the 300–400 t range and hot runners set at 280–295 °C typically specify preform acetaldehyde at or below 8 ppm, measured by headspace gas chromatography per ASTM F2013-10. Resin pellets are pre-dried at 150–165 °C for 4–6 h to a moisture content of ≤0.004 %, because residual moisture hydrolyzes the terephthalate ester backbone and raises carboxyl end group content during molding.

The competition between chain-end acetaldehyde generation and diffusion-limited degassing creates a narrow processing window. Excessively low melt temperature extends recovery time and increases shear heating around the check ring; excessively high temperature drives acetaldehyde regain by 1.5–2.0 ppm above pellet value within 3 min of recovery time. Barrel residence time is maintained below 5 min, and hot-runner gate geometries with short land lengths and low-shear surface finishes reduce local temperature overshoot. These are operational boundaries: PET formulations with higher carboxyl end group concentration or higher isophthalate comonomer content shift the acetaldehyde generation rate upward, requiring a reduction in melt temperature or an increase in vacuum venting.

Hydrolytic degradation converts the terephthalate ester linkage back to free terephthalic acid and ethylene glycol, shifting the chain-end profile away from hydroxyethyl ends and toward carboxyl ends. In post-consumer recycled PET washed and extruded at 270–280 °C, carboxyl end group content commonly rises from a pellet value below 20 mmol/kg to 20–35 mmol/kg after a single melt cycle, depending on residual moisture and vacuum venting efficiency. Solid-state re-polycondensation of recycled pellet crystallized at 140–170 °C and reacted at 210–220 °C for 10–20 h restores intrinsic viscosity to 0.72–0.80 dL/g and reduces carboxyl ends to 15–20 mmol/kg. Mechanical recycling lines for bottle-to-bottle applications therefore include continuous solid-state reactors with nitrogen purge dew point below −50 °C.

Recycled PET intended for food-contact use is evaluated under FDA 21 CFR 177.1630 and the relevant European Union migration framework; intrinsic viscosity, carboxyl end group concentration, and residual acetaldehyde are used as lot acceptance data alongside migration testing.

Chain-End Windows in High-Speed Melt Spinning

Fiber-grade PET operates at a lower molecular weight window than bottle resin, typically 0.62–0.68 dL/g and carboxyl end group concentration 25–35 mmol/kg. Higher chain-end concentration per unit mass lowers melt viscosity and permits high-speed spinning on extruders with L/D ratio 25:1–30:1, spinning manifolds at 285–295 °C, and take-up speeds above 3,000 m/min. The same chain ends, however, reduce hydrolytic stability in hot-wet processing.

Dyeing at 130 °C in pressurized jet equipment with water pH 4–5 can reduce intrinsic viscosity by 0.02–0.05 dL/g when carboxyl end groups exceed 30 mmol/kg; the corresponding tenacity loss is measured by ASTM D2256-10. Fiber producers therefore set carboxyl end group limits on dry chips and monitor spin-line residence time.