Purified terephthalic acid supplied as Pengwei Petrochemical PTA enters continuous polyester fiber lines as the acid monomer in a direct esterification route rather than a DMT transesterification route. The paste mixer combines PTA with ethylene glycol at a molar ratio of 1.15–1.25 mol MEG per mol PTA, and the slurry is pumped to a first esterifier operating at 240–260 °C and 0.15–0.30 MPa gauge, where water of reaction is removed and EG is refluxed. The polymerization-grade acid specifications that control fiber quality most directly are 4-carboxybenzaldehyde content and p-toluic acid content; in high-speed winding lines, 4-CBA values above approximately 25 mg/kg and p-toluic acid values above 150 mg/kg are associated with elevated melt yellowness b* and reduced degree of polymerization at fixed residence time. A typical antimony trioxide catalyst addition of 150–300 mg Sb/kg polymer is metered into the oligomer stream, while a phosphate ester stabilizer at 10–40 mg P/kg protects the catalyst and reduces gray discoloration. The final polycondensation is carried out in a high-vacuum finisher with cage-type rotating elements, and the intrinsic viscosity of the melt-phase polyester for textile staple or filament is controlled to 0.60–0.68 dL/g using a 0.5 g/100 mL solution in 60/40 phenol/1,1,2,2-tetrachloroethane at 25 °C per ASTM D4603-18. Anatase TiO₂ delustrant at 0.3–0.5 wt% is dispersed into the esterification or oligomer line for semi-dull products; the resulting fiber tensile properties are evaluated by ASTM D2256-22 and linear density by ISO 1973:2021. Production-scale five-vessel trains—two esterifiers and three polycondensation reactors—commonly operate with the final reactor agitator torque determining line speed, not esterifier conversion. If the finisher temperature exceeds 285 °C, thermal degradation generates acetaldehyde, vinyl ester end groups, and color bodies, and if the finisher vacuum is not held below 1.5 mbar absolute the target intrinsic viscosity cannot be achieved. Moisture content of the solid PTA above 0.3 wt% can create bridging in storage silos and paste-mixing deviations, so the pneumatic conveying system is operated with dry air or nitrogen at a dew point below -20 °C in installations where ambient relative humidity exceeds 60%.

What Limits Acetaldehyde in Bottle-Grade PET at the Preform Stage?

In continuous bottle-grade production, the melt-phase resin is first pelletized at an intrinsic viscosity of 0.60–0.65 dL/g and then transferred to a solid-state polycondensation train. The amorphous pellets are crystallized at 140–170 °C until the crystalline density reaches approximately 1.38 g/cm³ measured by ASTM D1505-18, after which the bed is heated to 200–220 °C in a tower or continuous crystallizer-reactor supplied with counter-current nitrogen. The nitrogen gas is conditioned to a dew point below -40 °C and an oxygen content below 10 ppmv, which limits both hydrolysis and thermo-oxidative color formation. Under these conditions, the solid-state reaction raises the intrinsic viscosity to 0.80–0.84 dL/g for carbonated soft drink preforms or to 0.84–0.86 dL/g for high-pressure applications, measured by ASTM D4603-18. Acetaldehyde is the critical volatile for mineral water and CSD packaging; the preform specification is commonly below 1.0 mg/kg for water-grade pellets and below 3.0 mg/kg for CSD preforms, tested by headspace gas chromatography following melt extrusion or preform injection according to ASTM F2013-10. The solid-state rate slows below 200 °C because the condensation byproduct ethylene glycol cannot diffuse fast enough from the pellet core, while temperature excursions above 225 °C risk pellet fusion and a rapid increase in acetaldehyde. Antimony trioxide polyester catalysts contribute to grayness if overdosed, whereas excess phosphate stabilizer can compete with antimony and reduce solid-state kinetics; therefore the Sb/P molar balance is trimmed on the finisher and melt pump data rather than only on upstream mass flow. Predrying of the amorphous pellets at 160–170 °C for 4–6 h is mandatory when the silo outlet moisture exceeds 0.4 wt%, and the extruder screw is typically a 30:1–36:1 L/D single-screw or twin-screw unit with a mixing section to minimize melt-temperature variance at the die plate.

When biaxially oriented polyester film is cast from a Pengwei PTA-based PET melt, the amorphous sheet is produced by extruding resin with an intrinsic viscosity of 0.62–0.66 dL/g through a flat die at 270–285 °C and quenching it on a chilled drum at 15–30 °C. Sequential stretching uses a machine-direction draw ratio of 3.0–3.8× at 90–110 °C and a transverse-direction draw ratio of 3.5–4.5× at 110–140 °C, followed by heat setting at 210–230 °C for 5–15 s. The controlling PTA-derived quality parameter is the tendency to form cyclic ethylene terephthalate trimer, which volatilizes and deposits as a white powder on heat-setting rolls and metallization chambers; trimer deposition becomes operationally significant when the oligomer content of the film exceeds 1.0–1.5 wt% by boiling xylene extraction. Diethylene glycol content must be held below 1.2 wt% in the polymer because higher DEG acts as a crystallization retardant and shifts the stretching behavior, while 4-CBA residues contribute to surface defects and reduced electrical breakdown. The melt filter behind the extruder is typically a continuous screen changer with 20–40 µm filtration elements, and the die-lip heater zones are set within ±1 °C to prevent transverse thickness bands. Film-grade resin with a melt flow rate of 30–40 g/10 min at 295 °C and 2.16 kg as measured by ASTM D1238-23 is preferred because lower melt viscosity leads to uneven die distribution and higher melt viscosity generates excessive shear heat and gel formation. Tensile strength after biaxial orientation is evaluated by ASTM D882-18, haze by ASTM D1003-21, and dielectric strength by ASTM D149-20. A line observation commonly reported in metallized film plants is that melt-phase PET made from acid with high b* cannot be compensated by increasing the percentage of recycled film flake without lowering the oriented film’s break elongation below 100%, so virgin PTA quality sets the recycle-use ceiling.

Tetrahydrofuran Generation in PBT Polycondensation

In the production of polybutylene terephthalate, Pengwei PTA is esterified with 1,4-butanediol at a molar excess of 1.3–1.5 mol BDO per mol PTA in a continuous kneader-reactor or batch esterifier at 240–250 °C. Water is stripped out while BDO is refluxed; the acid number of the oligomer is driven below 15 mg KOH/g before the vacuum stage. The side reaction of BDO dehydration to tetrahydrofuran is accelerated by acidity and by local overheating; in an industrial reactor, the temperature at the reactor wall must be no more than 10 °C above the bulk temperature to avoid a THF excursion above 8–12 wt% of the BDO feed. Tetrabutyl titanate is metered at 50–120 mg Ti/kg of polymer as the polycondensation catalyst, and triphenyl phosphite may be introduced at 0.05–0.15 wt% to control color and catalyst residue. Final polycondensation in a disc-ring reactor or twin-screw polycondensation unit is carried out at 245–255 °C and below 1.0 mbar absolute until the intrinsic viscosity reaches 0.80–1.00 dL/g, measured in 60/40 phenol/tetrachloroethane by ASTM D4603-18. Pellets are dried at 120–130 °C for 4 h to reduce moisture below 0.02 wt% before injection molding; if moisture exceeds this limit, hydrolytic chain scission produces a melt-flow-rate increase of more than 10% relative to dry resin measured by ASTM D1238-23. Injection molds for connectors and housings typically use clamp forces above 800 kN and barrel temperatures of 240–260 °C, with a rectangular plaque mold run under 23 °C and 50% relative humidity for post-molding dimensional checks. Mechanical properties are verified by ISO 527-1:2019 tensile testing, ISO 179-1:2010 notched Charpy impact, and ASTM D648-18 heat deflection temperature. The operational limit for PTA quality in PBT is stricter for color than for fibers because yellowing cannot be hidden by delustrant; b* values above 3.0 in the melt create downstream batch rejection in white and natural connector stocks.

Dioctyl terephthalate synthesis from PTA and 2-ethylhexanol is operated as a batch or continuous esterification with a molar alcohol excess of 2.2–2.8 mol 2-EH per mol acid. Tetrabutyl titanate or tetrabutyl zirconate catalyst is charged at 0.1–0.5 wt% of the PTA mass, and the reaction temperature is held between 180–220 °C while water is removed by nitrogen stripping or vacuum-assisted distillation. The conversion endpoint is monitored by acid value rather than time alone; the crude ester is transferred to vacuum stripping when the acid value drops below 0.5 mg KOH/g, and after neutralization with sodium carbonate solution, steam stripping at 150–180 °C and 5–20 mbar absolute, and filtration through a 5–10 µm bag or plate filter, the finished DOTP typically exhibits an acid value below 0.1 mg KOH/g, an ester content above 99.5 wt% by gas chromatography, a density of 0.981–0.985 g/cm³ at 25 °C by ASTM D4052-22, and a viscosity of 60–80 mPa·s at 25 °C by ASTM D1045-14. Volume resistivity above 1×10¹² Ω·cm is measured by ASTM D257-14 for wire and cable plasticizer applications. DOTP is selected as an ortho-phthalate-free plasticizer where REACH Annex XVII restrictions on DEHP, DBP, BBP, and DIBP and RoHS 2011/65/EU create a compliance matrix; however, the plasticizing efficiency is lower than that of DOP in highly plasticized PVC formulations, so the compounder compensates by increasing plasticizer loading by 5–15 wt% for equivalent Shore A hardness. In a production-scale wiped-film evaporator, PTA-derived DOTP with residual titanium above 10 mg/kg can develop haze during storage, so post-stripping hydrolysis and filtration are not optional. Published data for the specific interaction of Pengwei PTA lot-to-lot particle size distribution and DOTP esterification rate are limited; therefore ester loadings are normally verified on the basis of the acid’s particle median diameter rather than assuming a single fixed reaction time.

When PTA Is Used in Aromatic Polyester Polyols for Rigid Foam

Because PTA-derived aromatic polyester polyols are used mainly as modifiers in rigid polyurethane and polyisocyanurate foam systems, the synthesis is formulated around a high aromatic ring content. A typical bench-to-plant recipe esterifies PTA with diethylene glycol at a molar ratio of 1.6–2.2 mol glycol per mol acid under a nitrogen sweep at 220–240 °C, using dibutyltin oxide at 0.05–0.2 wt% on total solids. The reaction is continued until the raw polyol achieves a hydroxyl number of 250–350 mg KOH/g by ASTM D4274-21, an acid value below 3.0 mg KOH/g by ASTM D4662-20, and a Brookfield viscosity of 2,000–5,000 mPa·s at 25 °C by ISO 3219:1994. The high aromatic content improves compressive strength and dimensional stability in cyclopentane-blown insulation panels, but it also raises the polyol’s viscosity above that of sucrose/glycerol polyether polyols; therefore formulators use preheat to 35–45 °C and low-shear mixing before metering into the high-pressure PU machine. In plant trials, the replacement of sucrose polyether polyol by 15–30 wt% PTA polyester polyol can increase foam compressive strength measured by ASTM D1621-16, but the isocyanate index must be recalculated because the aromatic polyol’s average functionality is lower than that of sucrose polyether and its hydroxyl number tolerance is ±10 mg KOH/g. Residual free diethylene glycol above 2.0 wt% in the polyol creates a viscosity rise and can increase friability; vacuum stripping at 150–180 °C and 10–30 mbar absolute is applied until free glycol is below the specification. Acid-terminated oligomers promote premature gelation with tertiary amine catalysts, so the esterification endpoint is not based on acid value alone but on combined acid value, hydroxyl number, and water content below 0.1 wt% by ASTM D4672-18. Storage tanks require low-speed agitation and nitrogen blanketing because moisture pickup above 0.15 wt% can react with isocyanates and create foam pinholes.

Unsaturated Polyester Resin Syntheses Accept Only Partial PTA Substitution

Partial replacement of phthalic anhydride by PTA in orthophthalic unsaturated polyester resin syntheses is technically feasible but shifts the kettle behavior from a relatively low-water azeotrope to a water-condensation-dominated schedule. A batch reactor charged with PTA, phthalic anhydride, propylene glycol, and maleic anhydride is heated stepwise: the first plateau at 180–200 °C drives off esterification water while the second plateau at 210–225 °C removes residual acidity and builds molecular weight until the acid number is below 30 mg KOH/g by ASTM D1045-14. The maximum practical PTA substitution in a typical orthophthalic resin is 20–40 mol% of the aromatic dicarboxylic acid; above that level the resin tends to crystallize on dilution with styrene, and the diluted resin’s shelf stability falls below 3 months at 25 °C. Xylene is not required as an azeotrope entrainer because PTA liberates water directly, but the reactor overhead must be designed for a higher water load than a phthalic anhydride-only cook. Propylene glycol excess of 1.02–1.10 mol per mol total acid is maintained to limit side reactions; the final resin is thinned with styrene to a solids content of 60–70 wt% and inhibited with hydroquinone at 50–150 mg/kg of resin. Gel time in a 1% methyl ethyl ketone peroxide system is measured by ASTM D7029-17, and the ratio of PTA-derived units increases hardness while reducing resilience; the performance boundary is that PTA-rich unsaturated polyester resins usually require higher styrene content to reach a casting viscosity of 0.4–0.6 Pa·s, which can conflict with volatile organic compound limits in EU regulations covering styrene emissions. Published data on the exact lamella distribution in cured laminates made from Pengwei PTA in unsaturated polyester are limited, so the replacement ratio is best confirmed by running an acid-number curve and curing a 4 mm cast sheet for flexural tests by ISO 178:2019 rather than relying on neat resin viscosity alone.