Continuous direct esterification of Hengli Petrochemical (Huizhou) PTA with monoethylene glycol in a polyester staple fibre line is operated with an EG/PTA molar ratio held between 1.12:1 and 1.15:1 at the first esterifier feed. Antimony trioxide is metered into the paste at 250–300 mg Sb/kg of polymer, and the first esterifier is maintained at 260–265 °C with water removed overhead while the second esterifier raises the temperature to 272–275 °C to push esterification conversion above 97%. Polycondensation occurs in a finisher at 285–290 °C under an absolute pressure below 100 Pa, and the resulting melt is spun into partially oriented yarn with intrinsic viscosity controlled at 0.62–0.68 dL/g as determined by ASTM D4603-18 in 60:40 phenol/1,1,2,2-tetrachloroethane at 25 °C. For staple fibre, IV is held at 0.63–0.66 dL/g to balance spinning continuity and fibre strength. Hengli PTA carrying 4-carboxybenzaldehyde above 25 mg/kg functions as a monofunctional chain terminator; the resulting conjugated aldehyde species shift the b* colour coordinate of undyed fibre above 2.0, and each 10 mg/kg increase in 4-CBA above specification has been reported to reduce batch IV by 0.03–0.05 dL/g at constant finisher torque. PTA particle size measured by ISO 13320:2020 should remain in the median range of 100–150 μm; particles below 80 μm create paste line dust and poor wetting, while particles above 180 μm slow dissolution in the first esterifier and elevate filter pack pressure in downstream spinning. Moisture in the PTA silo is maintained below 0.2 wt% by ISO 15512 because free water hydrolyzes ester linkages during preheating and destabilizes melt viscosity. In a typical single-screw spinning extruder with L/D 30:1 and screw speed 90–120 min⁻¹, the melt is spun through spinneret holes of 0.20–0.28 mm and quenched with air at 0.4–0.6 m/s. Plant data from humid coastal locations show that inadequate dry-air blanketing of PTA silos causes paste density oscillation and first esterifier level swings of ±3%, which reduce day-to-day IV reproducibility.

What Limits Acetaldehyde Reversion in Bottle-Grade PET Made from PTA?

Container-grade PET from Hengli PTA and monoethylene glycol is copolymerised with 2.0–2.5 wt% isophthalic acid to disrupt crystallisation and improve preform clarity. The esterification and melt polycondensation sequence yields amorphous chips with an intrinsic viscosity of 0.63–0.66 dL/g, which are then solid-state polymerised at 210–220 °C under nitrogen with residual oxygen below 20 ppm for 12–18 h to reach 0.82–0.86 dL/g for carbonated soft drink bottles. Acetaldehyde reversion during preform injection is the dominant quality constraint: melt temperature must remain within 270–285 °C and barrel residence time below 5 min to keep preform acetaldehyde below 1.0 μg/g when measured by headspace GC according to ASTM F2013-10. The same standard is used for routine release of water-contact PET, because acetaldehyde concentrations above 2.0 μg/g impart sensory taint in still water within 24 h at 40 °C. PTA-derived PET with a high population of hydroxyl end groups, caused by premature chain termination from excess 4-CBA or p-toluic acid, generates additional acetaldehyde precursors during melt processing. Therefore the PTA feed for bottle resin is controlled for 4-CBA ≤ 25 mg/kg and p-toluic acid ≤ 150 mg/kg under GB/T 32685. Injection moulding tools with hot runner temperatures above 290 °C show acetaldehyde spikes exceeding 2 μg/g within 30 s of flow stagnation; this is a documented failure mode in high-cavitation preform tools with unbalanced runner geometries.

Compliance for bottle-grade PET relies on both polymer chemistry and processing history. PTA-based PET resin with antimony catalyst requires a phosphorus stabiliser addition of 10–40 mg P/kg to prevent catalyst re-activation and acetaldehyde formation during SSP and injection. Chips exiting the solid-state reactor are moisture-conditioned below 50 ppm before drying at 170–180 °C for 4–6 h; drying air with a dew point above -40 °C leaves residual moisture that reduces preform IV and increases haze. The combination of low residual acetaldehyde and regulated catalyst migration makes PTA-derived PET the default resin for still water, carbonated soft drink, and aseptic dairy packaging in Asia-Pacific markets.

PBT Condensation Kinetics and THF Suppression

For PBT condensation, esterification of Hengli PTA with 1,4-butanediol requires a molar excess of BDO between 1.30:1 and 1.50:1 to compensate for acid-catalysed dehydration of BDO to tetrahydrofuran. Tetrabutyl titanate is used at 50–150 mg Ti/kg of polymer; esterification proceeds at 220–240 °C in a stirred reactor with a packed column to return entrained BDO while venting THF and water. Polycondensation is completed at 250–260 °C under an absolute pressure below 60 Pa until the intrinsic viscosity reaches 0.90–1.20 dL/g as measured by ISO 1628-5. Because THF formation is promoted by carboxyl end groups, incomplete first-stage esterification increases BDO consumption and reduces molecular weight; plant runs with PTA of higher p-toluic acid content show elevated THF make and higher vacuum load on the finisher. For extrusion and injection moulding, PBT pellets must be dried to a moisture content below 0.02 wt% at 120–140 °C for 4 h with a desiccant dryer dew point of -40 °C; residual moisture above 0.04 wt% causes hydrolysis at 250–260 °C and reduces tensile strength by more than 10%. Glass-fibre-reinforced grades are compounded on a twin-screw extruder with L/D 40:1 at barrel temperatures of 240–270 °C; glass roving is side-fed after the melting zone. A 30 wt% glass-fibre PBT compound typically shows a tensile modulus above 9000 MPa by ISO 527-2, flexural strength above 180 MPa by ISO 178, and heat distortion temperature above 200 °C by ISO 75-2/B. Electrical connector grades require a comparative tracking index above 400 V when tested according to IEC 60112. Melt temperature must remain below 270 °C and residence time under 5 min to limit thermal regeneration of THF and carbonyl discolouration.

In batch DOTP esterification units, Hengli PTA is reacted with 2-ethylhexanol at a stoichiometric alcohol excess of 2.2:1 to 2.5:1 and catalysed by tetrabutyl titanate or tetraisooctyl titanate at 0.05–0.20 wt% of PTA. Reaction temperature is ramped from 180 °C to 230 °C while water is removed through a packed entrainment column; the final acid value is reduced below 0.1 mg KOH/g by vacuum stripping below 5 kPa at 230 °C. Low 4-carboxybenzaldehyde and p-toluic acid in the PTA minimise colour-forming side reactions, allowing the DOTP crude ester to meet an APHA colour below 20 after activated carbon treatment. DOTP is compounded into flexible PVC insulation at 60–80 phr as a non-phthalate plasticiser and is not subject to the REACH Annex XVII restriction entries covering DEHP, DBP, BBP, and DIBP. A DOTP-plasticised PVC compound typically retains a volume resistivity above 1×10¹² Ω·cm when measured by ASTM D257 after immersion in water at 70 °C for 48 h; tensile strength retention after aging at 100 °C for 168 h is above 80% when tested by IEC 60811-401. Residual titanate catalyst above 0.10 wt% in the finished ester can promote hydrolysis of the plasticiser during cable service; neutralisation with dilute aqueous alkali followed by water washing and filtration is therefore required. The process is run under nitrogen blanketing to prevent colour development, and the PTA feed moisture is controlled below 0.2 wt% by ISO 15512 to avoid ester yield loss.

When Terephthalate Backbones Raise Compressive Strength in PU Foam

Aromatic polyester polyols prepared from Hengli PTA and diethylene glycol are incorporated into rigid polyisocyanurate foam formulations to replace part of phthalic anhydride-derived polyol. Esterification is performed in a forced-circulation reactor at 200–230 °C until the acid value falls below 2 mg KOH/g and the hydroxyl number reaches 250–350 mg KOH/g; the final viscosity at 25 °C is typically 3000–6000 mPa·s. The aromatic terephthalate backbone raises foam compressive strength when the polyester polyol is used at 20–40 wt% of the total polyol blend, with compressive strength measured by ASTM D1621 increasing by 10–25% relative to ortho-phthalate polyols at the same density. However, the high melting point and slow dissolution of PTA require longer cook times than phthalic anhydride; local overheating above 230 °C produces dark colour and free diethylene glycol, which increases foam friability. Free DEG in the polyol must be held below 1.5 wt% and residual water below 0.1 wt% to prevent excessive CO₂ generation during isocyanate mixing. In a typical PIR panel formulation, the PTA-based polyol is reacted with polymeric MDI at an isocyanate index of 250–350, with cyclopentane as the physical blowing agent and potassium octoate as trimerisation catalyst. Closed-cell content above 90% is confirmed by ISO 4590, and initial thermal conductivity below 0.022 W/(m·K) is measured by ASTM C518. The higher blend viscosity requires heated polyol lines at 40–50 °C and static mixers with pressure drop above 2 MPa to maintain component ratio control.

Outdoor-Durable Polyester Resins Synthesized from PTA

Because PTA imparts a high glass transition temperature and excellent hydrolytic stability to carboxyl-terminated polyester resins, it is used in powder coatings for architectural aluminium profiles and outdoor furniture. The resin is synthesised from PTA and neopentyl glycol at 235–240 °C with xylene azeotropic distillation under nitrogen; the acid value is controlled at 30–35 mg KOH/g and the glass transition temperature at 55–65 °C for triglycidyl isocyanurate curing. Curing at 180 °C for 15 min produces a coating with pencil hardness above H by ASTM D3363 and flexibility of 3 mm by ASTM D522. PTA concentrations above 50 wt% of the diacid charge increase melt viscosity and require a reactor with an anchor agitator and heated vapour line above 100 °C to prevent sublimed PTA from blocking the condenser. Weathering performance under ASTM D3451 and Qualicoat Class 1 requires that the cured film retain at least 50% of its original 60° gloss after 1 year of Florida exposure; resins with excess residual carboxyl groups above 35 mg KOH/g show lower boiling-water resistance and pinholing on aluminium edges. In HAA curing systems, acid values are adjusted to 20–25 mg KOH/g to limit gas evolution from the β-hydroxyalkylamide reaction, but the higher acid value resin from PTA is typically reserved for TGIC systems because of its faster gel time at 180 °C.

Biaxially oriented polyester film production from Hengli PTA typically requires chip intrinsic viscosity of 0.60–0.65 dL/g measured in 60:40 phenol/1,1,2,2-tetrachloroethane at 25 °C by ASTM D4603-18. The dried PET resin is extruded at 280–290 °C through a slot die onto a chilled roll at 30 °C, and the cast amorphous sheet is sequentially stretched in the machine direction at 95–105 °C and in the transverse direction at 105–115 °C, with draw ratios of 3.0:1–3.5:1 in each axis. PTA-derived PET must contain diethylene glycol below 0.05 wt% to avoid depression of tensile modulus and breakage in the stenter; the biaxial stretching line tension typically exceeds 50 N/mm at widths above 4 m. Inorganic slip additives such as silica are masterbatched at 0.1–0.3 wt% to control surface roughness below 20 nm for capacitor-grade film, while coextruded layers permit a highly transparent core with low haze. Moisture in the PET chip must be dried below 50 ppm before extrusion because residual water beyond 0.01 wt% reduces IV during melting and creates pinholes. Packaging film is tested for tensile strength and elongation by ASTM D882, and capacitor film grade is supplied under IEC 60674-3 with a dissipation factor below 0.005 at 1 kHz. The PTA impurity profile affects film colour: 4-CBA above 25 mg/kg shifts the yellow index above 1.5 after extrusion, which is unacceptable for optical applications.

When high-tenacity polyester industrial yarn is produced from Hengli PTA, solid-state polymerised chips with an intrinsic viscosity of 0.95–1.05 dL/g by ASTM D4603-18 are used. The dried chips are melted in an extruder at 295–305 °C and spun through spinneret orifices of 0.25–0.35 mm at take-up speeds around 2500–3000 m/min. A two-stage hot drawing process at draw ratios of 5.8:1–6.3:1 with godet temperatures of 80–120 °C for the first draw and 200–230 °C for the second draw produces a tenacity of 8.0–8.5 cN/dtex and an elongation at break of 12–14%. For tyre cord applications, carboxyl end group concentration is maintained below 20 meq/kg to resist hydrolysis during vulcanisation at 150–170 °C; cords are tested for tensile strength and elongation by ASTM D885. After resorcinol-formaldehyde-latex dipping, the cord retains more than 90% of its original strength after 24 h at 150 °C when the polyester contains diethylene glycol below 1.2 wt%. DEG units create flexible chain segments that reduce thermal dimensional stability; tyre cord made from PTA-derived PET with elevated DEG content above 2.0 wt% shows increased thermal shrinkage and lower modulus under cyclic loading. The PTA feed for industrial yarn is therefore specified with low p-toluic acid and 4-CBA levels under GB/T 32685, and the polyester line includes a separate solid-state polymerisation stage to reach the required IV without excessive melt residence time.