What Limits Acetaldehyde Control When PTA-Based PET Is Polymerized Under High-Vacuum Finishing?

Continuous bottle-grade PET polymerisation from Mitsubishi Chemical PTA is carried out by direct esterification. The slurry feed is prepared with an ethylene glycol/PTA molar ratio of 1.10:1–1.20:1, a range that drives esterification to high conversion while limiting the diethylene glycol side reaction that would otherwise reduce the crystallisation rate during preform injection. PTA feed quality is held to 4-carboxybenzaldehyde ≤ 25 mg/kg and p-toluic acid ≤ 150 mg/kg because both impurities act as monofunctional chain terminators; a shift of only 10 mg/kg in 4-carboxybenzaldehyde is known to change melt viscosity and stretch-blow moulding behaviour in downstream preform production. The esterification stage is distributed between two stirred reactors at 260–270 °C and an absolute pressure of 150–250 kPa; water of reaction is removed through a rectification column that recovers entrained ethylene glycol. The oligomer then passes to a pre-polycondensation vessel at 270–280 °C and 2–10 kPa, followed by a horizontal high-vacuum finisher at 280–290 °C and 0.1–0.5 kPa, where intrinsic viscosity reaches 0.60–0.65 dL/g when measured according to ASTM D4603-18. Antimony trioxide is added at 200–300 mg/kg Sb, with a phosphorus stabiliser at a P/Sb molar ratio of 0.8:1–1.2:1 to protect colour; cobalt acetate is introduced at 10–30 mg/kg Co to offset yellow-blue tint. Melt-phase polymer is pelletised, pre-crystallised at 160–180 °C, and advanced in a solid-state polycondensation tower at 200–220 °C under nitrogen with a dew point below −40 °C. Residence time is adjusted to raise intrinsic viscosity to 0.78–0.85 dL/g for carbonated soft drink grades and to reduce acetaldehyde below 1 mg/kg. On production-scale SSP lines, uneven pellet size distribution from strand pelletisers causes gas channelling and local intrinsic viscosity scatter of roughly ±0.02 dL/g within a single silo; pellet sticking is observed when the bed temperature exceeds 220 °C. Amine-based scavengers are not introduced before solid-state polycondensation because basic residues inhibit the antimony catalyst and regenerate acetaldehyde during subsequent injection moulding at 270–290 °C. Food-contact compliance is documented under FDA 21 CFR 177.1630, EU Regulation 10/2011, and GB 4806.7-2023. Terminal products include still water bottles, carbonated soft drink bottles, edible-oil containers, and hot-fill containers processed at 85 °C.

In textile-grade polyester melt lines, the base PTA/ethylene glycol molar feed ratio is maintained at 1.15:1–1.25:1 to compensate for ethylene glycol losses during esterification and to limit diethylene glycol formation during fibre-grade polymerisation. Titanium dioxide delustrant is added at 0.25–0.40 wt% for semi-dull yarn and 1.5–2.0 wt% for fully dull staple, while the melt-phase intrinsic viscosity is controlled to 0.62–0.68 dL/g as measured by ASTM D4603-18. When chip feed is used, the pellets are pre-dried in vacuum rotary driers at 160–170 °C for 6–8 h to below 30 mg/kg moisture; at plant relative humidity above 60%, hopper residence times must be shortened to prevent moisture regain. Extrusion takes place through a single-screw extruder with an L/D ratio of 24:1–30:1, a melt temperature of 285–295 °C, and sintered metal filtration media rated at 20–40 µm. Partially oriented yarn is taken up at 3,000–6,000 m/min and draw-textured at a draw ratio of 1.6:1–1.8:1; staple fibre lines apply cross-flow quench air of 0.3–0.6 m/s, a draw ratio of 3.0:1–4.0:1, heat-setting at 180–220 °C, crimping, and cutting to 28–38 mm. The main line constraint is spin-pack life: increasing TiO2 loading is observed to accelerate pack pressure rise and require more frequent spinneret cleaning than bright fibre, altering day-to-day production scheduling. Compliance for fibre and filament is evaluated under REACH Regulation (EC) No 1907/2006; finished textile materials destined for consumer apparel are additionally certified under Oeko-Tex Standard 100, but certification is a finished-article status and is not conferred by PTA supply alone. Terminal products include spun yarn for sportswear, staple fibre for nonwoven hygiene fabrics, and automotive carpet backings.

Biaxially Oriented PET Film: Electrostatic Pinning and Tenter-Frame Stretching under Partial Crystallization

Cast film extrusion from PTA-derived PET chips requires film-grade intrinsic viscosity of 0.58–0.68 dL/g and controlled diethylene glycol content of 1.0–1.5 mol% because ether linkages widen the transverse draw window by retarding stress-induced crystallisation. The resin is pre-dried at 170–180 °C for 4–6 h to below 30 mg/kg moisture and extruded at 275–285 °C through a slot die with automatic gauge control. Electrostatic pinning transfers the extrudate to a chill roll held at 30–40 °C; if the pinning current is set too low for the sheet basis weight and line speed, air entrapment produces slip-stick banding, while excessive pinning can create surface mottle on the quenched cast sheet. Machine-direction orientation is performed at 80–100 °C with a draw ratio of 3.0:1–4.5:1, followed by transverse stretching at 90–120 °C and 3.5:1–5.0:1, then heat-setting at 200–230 °C with 2–5% relaxation to control thermal shrinkage. Silica antiblocking particles are added at 0.05–0.20 wt% with a median particle size of 0.8–3.0 µm to prevent blocking during winding; optical haze is measured per ASTM D1003-13, and tensile properties per ASTM D882-18. Process excursions above 230 °C in the heat-set zone are observed to raise film crystallinity and reduce tear resistance, particularly in thin films below 12 µm. Food-contact film compliance is anchored to FDA 21 CFR 177.1630 and EU Regulation 10/2011; electrical insulation film is qualified under IEC 60674-3-2 where specified. Terminal products include flexible packaging laminates, lidding films, photovoltaic backsheet carrier film, and capacitor-grade insulation film.

When PTA is esterified directly with 1,4-butanediol instead of dimethyl terephthalate, the PBT line eliminates methanol recovery but introduces a competing dehydration pathway in which 1,4-butanediol cyclises to tetrahydrofuran under acidic conditions. The molar feed ratio is held at PTA/BDO 1:1.25–1.50; higher 1,4-butanediol excess improves esterification completion but increases tetrahydrofuran by-product and yellowness. Tetrabutyl titanate catalyst is metered at 50–200 mg/kg Ti, and triphenyl phosphite stabiliser is added at 0.05–0.15 wt% to suppress thermal discolouration without completely quenching catalytic activity. Esterification proceeds at 220–240 °C at atmospheric pressure with water removal; polycondensation follows at 250–260 °C and a pressure below 0.5 kPa, producing PBT with an intrinsic viscosity of 0.80–1.00 dL/g per ISO 1628-5 and a melt mass-flow rate of 15–30 g/10 min at 250 °C/2.16 kg per ISO 1133-1:2022. Injection molding compounds are pre-dried at 80–120 °C for 3–4 h to a moisture content below 0.02%, because residual moisture hydrolyses the ester backbone and reduces tensile elongation during moulding; melt temperature is set at 250–270 °C and mould temperature at 60–100 °C. On production-scale moulding campaigns, glass-fibre reinforcement above 30 wt% is observed to increase hot-runner pressure drop and narrow the fill window, requiring injection velocity profiling and higher clamp force than unfilled PBT. Regulatory statements are aligned with RoHS Directive 2011/65/EU, REACH Regulation (EC) No 1907/2006, and UL 94 flammability classifications; unfilled PBT is commonly classified HB, while flame-retardant compounds can be classified V-0 depending on the additive package. Terminal products include automotive electrical connectors, electronic relay and motor components, fibre-optic loose tubes, and appliance housings.

DOTP Esterification with 2-Ethylhexanol Requires Aggressive Water Removal Below 20 kPa

The esterification of PTA with 2-ethylhexanol to dioctyl terephthalate is conducted at an alcohol excess of 2.2:1–2.8:1 relative to PTA and a tetrabutyl titanate catalyst loading of 0.05–0.30 wt% based on PTA. Reaction temperature is staged from 180 °C to 230 °C, while the final reactor pressure is drawn below 20 kPa to shift equilibria; the overhead partial condenser is held at 90–110 °C to return entrained 2-ethylhexanol and remove water of esterification. Excess alcohol is recovered by stripping under vacuum and recycled into the next batch. Crude DOTP is neutralised, washed, decolorised with activated clay or activated carbon, and filtered through 5–10 µm polishing media; titanate hydrolysis generates insoluble fines that progressively increase filter pressure differential across the campaign. The finished ester is controlled to ester content ≥99.5%, acid value ≤0.05 mg KOH/g, and moisture ≤0.05%. Compliance for the plasticiser itself is evaluated under REACH Regulation (EC) No 1907/2006 and, in electrical and electronic applications, Directive 2011/65/EU; food-contact and medical-grade acceptance is region-specific and must be verified against national positive lists because harmonized migration limits for DOTP are not uniformly established. Terminal products include flexible PVC wire and cable sheathing, artificial leather, flooring, and phthalate-free toy and medical tubing compounds.

Aromatic polyester polyols derived from PTA and diethylene glycol are produced in batch reactors where the anhydride-free route is selected to raise terephthalate content and reduce free phthalic anhydride carryover. A typical charge uses a PTA/diethylene glycol molar ratio of 1:1.5–1.7 with a tin- or titanium-based esterification catalyst; the reactor is heated stepwise to 220–240 °C under nitrogen, then vacuum is applied to reach an acid value below 2.0 mg KOH/g. The product is adjusted to a hydroxyl number of 220–320 mg KOH/g and a Brookfield viscosity of 2,000–10,000 mPa·s at 25 °C. In rigid polyisocyanurate foam production, the polyol is mixed with isocyanate, blowing agent, and surfactant under high-shear mixing, then poured onto facers and cured in a double-belt laminator; aromaticity from PTA contributes to char formation in fire-rated panels evaluated to ASTM E84, but published production-scale data for some regional PTA-based polyol configurations is limited. Finished foam is specified under EN 13165 for factory-made rigid polyurethane insulation boards and ASTM C591 for cellular polyurethane thermal insulation. Terminal products include boardstock insulation for commercial roofing, pipe insulation segments, and fire-rated PIR panels.