The production of bottle-grade polyethylene terephthalate from INEOS Aromatics Geel PTA begins with a slurry of purified terephthalic acid in ethylene glycol at a PTA-to-MEG molar ratio of 1:1.15 and a solids content of 65–70 wt%. The slurry is prepared in an agitated paste mixer and transferred to a continuous esterification train operating at 240–260 °C and 0.2–0.4 MPa with a mean residence time of 2–4 h. Conversion to bishydroxyethyl terephthalate is driven by water removal, and the degree of esterification is controlled between 96.5% and 98.5%. Residual 4-carboxybenzaldehyde, specified at ≤25 mg/kg for high-purity PTA, functions as a monofunctional chain terminator; levels above this threshold suppress intrinsic viscosity build, widen molecular weight distribution, and increase CIE b* colour in amorphous pellets. Melt-phase polycondensation is operated at 270–290 °C under a vacuum below 100 Pa with antimony trioxide catalyst dosed at 150–300 ppm Sb and a phosphorus-based thermal stabilizer at 50–100 ppm P. Discharge from the final finisher is filtered through a 30–40 μm screen before underwater pelletisation. The amorphous melt-phase product is produced at an intrinsic viscosity of 0.62–0.65 dL/g measured by ASTM D4603-18 and then advanced by solid-state polycondensation at 200–220 °C under nitrogen or vacuum for 12–24 h to a bottle-grade intrinsic viscosity of 0.80–0.84 dL/g. Acetaldehyde in preforms is controlled below 8 μg/g when measured by headspace gas chromatography according to ASTM F2013-17, and residual acetaldehyde after blow moulding is maintained below 2 μg/g. Compliance for food contact is evaluated under FDA 21 CFR 177.1630, EU Regulation (EU) No 10/2011 Annex I, and European Food Safety Authority migration limits for terephthalic acid and ethylene glycol. The bottle resin process boundary is narrow because melt temperature above 295 °C accelerates degradation to acetaldehyde and crotonaldehyde, while solid-state polycondensation above 225 °C induces sintering and agglomeration in the reactor.

On a continuous 200 t/day melt-phase line, the most frequent process deviation is drift in the esterification degree caused by changes in PTA particle size distribution. If the median particle size exceeds 180 μm, slurry viscosity at 65–70 wt% solids can rise from 0.8 Pa·s to above 1.5 Pa·s at 40 °C, increasing loader load on the paste mixer and reducing conversion in the first esterifier. The second operational boundary is the antimony-to-phosphorus balance; a phosphorus dose above 100 ppm P precipitates antimony and lowers final intrinsic viscosity, while a phosphorus dose below 50 ppm P raises melt thermal degradation and yellowing. Solid-state polycondensation reactors use nitrogen with a dew point below -30 °C; moisture above 20 ppmv in the carrier gas slows the SSP rate and can cause pellet surface hydrolysis. Pre-drying is required at ambient relative humidity above 60%, and pellets must be conveyed with dried air to prevent moisture pickup above 50 ppm before injection moulding or blow moulding.

What Limits the Hydrolytic Stability of PBT Polymerised from PTA and 1,4-Butanediol?

Polybutylene terephthalate synthesis from PTA and 1,4-butanediol is operated at a diol-to-diacid molar ratio of 1.10:1 to 1.25:1 in a two-reactor melt process. The first stage is esterification at 220–250 °C under atmospheric or slight vacuum, followed by polycondensation at 245–260 °C and 0.05–0.10 kPa absolute pressure with tetrabutyl titanate catalyst at 50–150 ppm Ti or tetrabutyl zirconate at equivalent metal loadings. A critical side reaction in the esterifier is the acid-catalysed dehydration of 1,4-butanediol to tetrahydrofuran. THF generation increases with residual PTA acidity, water concentration above 0.10 wt% in the diol feed, and hot spots above 250 °C. Excess THF reduces 1,4-butanediol availability, lowers the esterification degree, and shifts carboxyl end-group concentration upward; this reduces melt-phase intrinsic viscosity and later hydrolytic stability. The melt discharge is filtered through a 20–35 μm steel mesh and pelletised under dry nitrogen. Commercial PBT grades for injection moulding target an intrinsic viscosity of 0.85–1.20 dL/g by ASTM D4603-18, with a carboxyl end-group concentration below 35 mmol/kg. Before injection moulding on an 800–1,500 kN clamp force machine with barrel temperatures of 240–260 °C and mould temperatures of 30–80 °C, pellets are dried to a moisture content below 0.02 wt% using a desiccant dryer with a dew point of -40 °C. Hydrolytic stability is assessed by tensile strength retention according to ISO 527-1:2019 and ISO 527-2:2012 after exposure at 85 °C and 85% RH for 500 h; unreinforced PBT typically shows a tensile strength loss of 10–20% under these conditions. Electrical and automotive housing compounds use glass-fibre reinforcement at 15–30 wt% and are evaluated for heat distortion temperature under ISO 75-1:2020 and ISO 75-2:2013 at 1.80 MPa. The process boundary is tight because titanium catalyst is sensitive to hydrolysis; exposure of the melt line to ambient relative humidity above 60% RH before pelletisation increases haze and reduces filtration performance.

For semi-dull polyester staple fibre, the PTA-based melt is prepared with anatase titanium dioxide at 0.30–0.50 wt% of polymer, introduced as a 20–40 wt% masterbatch or metered slurry after esterification. The continuous spinning line operates at 285–295 °C with melt filtration through 20–40 μm sintered metal or woven wire filters. Titanium dioxide agglomerates above 10 μm increase spinneret pack pressure and cause filament breaks; the melt is therefore filtered twice before the spinneret. The base polymer for cotton-type staple is produced with an intrinsic viscosity of 0.62–0.68 dL/g under ASTM D4603-18 and a titanium dioxide content measured by ashing according to ISO 3451-1:2019. The filaments are quenched with conditioned air at 18–24 °C and 65–75% RH, combined into a tow, drawn at a draw ratio of 3.0–4.0, heat-set at 140–180 °C, crimped, and cut to 32–38 mm staple length. A spin finish is applied at 0.3–0.7% active dry-on-fibre; the finish chemistry is selected to be emulsifiable and thermostable at 200 °C without generating smoke or yellowing during downstream carding. Tensile properties are measured on single fibres according to ISO 5079:2021, and linear density is checked by ISO 1973:2021. Batch-to-batch drift in PTA particle size changes slurry viscosity and the degree of esterification in the first reactor; if the median particle size exceeds 180 μm, esterification residence time must be increased by 10–15% to maintain equivalent conversion. The fibre process boundary is set by the balance between molecular weight and spinline stress; an intrinsic viscosity below 0.58 dL/g reduces tenacity below typical textile values, while an intrinsic viscosity above 0.72 dL/g increases spinning pressure and may produce undrawn filaments with high elongation at break.

Oligomer Deposition on Cast Roll and Die Lip in Biaxially Oriented Film Production

Biaxially oriented polyester film made from PTA and ethylene glycol is extruded at 270–285 °C through a flat die with a lip gap of 1.5–3.0 mm onto a chilled cast roll held at 20–30 °C. Electrostatic pinning is applied with a wire voltage of 5–10 kV to prevent air entrapment and maintain contact between the melt web and the cast roll. The cast amorphous sheet has a thickness of 0.4–3.0 mm and is then stretched in the machine direction at 2.8–3.6× on heated rolls at 70–100 °C, followed by transverse stretching at 3.0–4.0× in a tenter oven at 100–140 °C. Heat-setting is conducted at 220–240 °C for 2–8 s to crystallise the oriented film and reduce shrinkage. Film-grade polyester typically uses a melt-phase intrinsic viscosity of 0.64–0.70 dL/g by ASTM D4603-18 and a diethylene glycol content below 1.5 wt% to limit thermal degradation during tenter processing. Cyclic ethylene terephthalate trimer migrates to the surface during stretching and heat-setting, forming a white powder that accumulates on the transverse stretching clips, the tenter chain, and the downstream cooling rolls. If oligomer deposition is not controlled by surface extraction or low-oligomer copolymerisation, film haze increases from below 1.0% to above 3.0% when measured by ASTM D1003-21, and roll wrap frequency rises. The final film is evaluated for tensile strength and elongation according to ASTM D882-18, for shrinkage at 150 °C for 30 min according to ASTM D1204-14, and for food-contact compliance under FDA 21 CFR 177.1630 and EU Regulation (EU) No 10/2011. The processing window is narrow because a cast roll temperature above 40 °C increases crystallinity before orientation and produces uneven stretching, while a heat-set temperature below 210 °C results in a film with shrinkage above 5% in the transverse direction.

Thermal Degradation During Polyester Polyol Esterification Dictates Hydroxyl Number Drift.

Aromatic polyester polyols are produced from PTA with diethylene glycol, neopentyl glycol, or their mixtures in a stirred batch reactor with a partial condenser, a distillation overhead, and a vacuum system capable of 0.005 MPa. The charge ratio is designed to produce a hydroxyl number between 200 and 400 mg KOH/g; a typical formulation for rigid polyurethane foam uses a molar ratio of total glycol to PTA of 1.8:1 to 2.2:1 with a small amount of trimethylolpropane as branching agent. Esterification is started at 180–200 °C and advanced stepwise to 220–240 °C under nitrogen, with water removed through a packed column and a reflux control loop. The reaction is monitored by acid value according to ASTM D4662-20; when the acid value falls to 5–15 mg KOH/g, vacuum is applied at 20–50 mbar to strip residual water and drive final conversion. The product is discharged when the acid value is below 2.0 mg KOH/g and the hydroxyl number measured by ASTM D4274-21 is within the specified range. Thermal degradation during the final vacuum stage produces carbonyl by-products and acidic species from diethylene glycol and causes the hydroxyl number to drift below target while the acid value increases. The reactor is therefore limited to a wall temperature below 250 °C and a total batch time below 24 h; the heat-transfer oil inlet temperature is maintained 10–15 °C above the reactor set point. The resulting polyester polyol is used in rigid polyisocyanurate foam systems with a viscosity at 25 °C of 1,000–8,000 mPa·s; reactivity is assessed in hand-mix foam tests with an isocyanate index of 250–350 for PIR systems. The process boundary is set by the diethylene glycol content; above 30 mol% of total glycol, the polyester polyol is more viscous and rigid foam friability increases, while below 10 mol% storage stability at 10 °C can be compromised by crystallisation.

When PTA Replaces Dimethyl Terephthalate in Saturated Polyester Powder Coating Resins

In saturated carboxyl-functional polyester resins for powder coatings, PTA is combined with isophthalic acid, neopentyl glycol, and a branching agent such as trimellitic anhydride. The substitution of dimethyl terephthalate with PTA removes methanol from the condensation reaction and generates water as the by-product; this requires a slower initial esterification profile and a larger overhead condenser relative to the monomer feed. The formulation is built to an acid value of 30–50 mg KOH/g for triglycidyl isocyanurate cure and 20–30 mg KOH/g for β-hydroxyalkylamide cure, according to ASTM D4662-20. A typical aromatic acid content is 45–60 mol% of total acid monomers, with PTA as the main terephthalic building block; the neopentyl glycol content is 40–55 mol% of total polyol, and the branching agent is limited to 0.5–4 mol% to prevent gelation. Melt polymerization is carried out at 230–250 °C under vacuum below 100 mbar in a reactor with an anchor stirrer and a scraped-wall condenser. The resin is discharged and flaked when the glass transition temperature is between 55 and 70 °C by differential scanning calorimetry according to ISO 11357-2:2020. The flake is then extruded with the crosslinker on a twin-screw extruder with an L/D ratio of 32:1 at a barrel temperature of 90–110 °C, ground, and classified to a particle size distribution of 10–100 μm. The curing reaction with triglycidyl isocyanurate is run at 180–200 °C for 10–15 min; gel time is measured at 200 °C according to ISO 8130-6:2021 and is typically between 60 and 180 s. The final powder coating is tested for flexibility by ASTM D522-17, for impact resistance by ISO 6272-1:2011, and for salt-spray resistance by ISO 9227:2022. The processing window is limited by the high melting point of PTA; if the initial esterification temperature is raised too rapidly, unreacted PTA sublimes into the overhead line and blocks the condenser. The esterification profile is therefore stepped at 180 °C for 2–4 h before the final 230–250 °C cook.