In continuous PET bottle-resin lines using direct esterification, Formosa Chemicals & Fibre (Longde) PTA is dispensed as a paste with ethylene glycol at a molar EG/PTA ratio of 1.10–1.15. The first esterification vessel operates at 260–270 °C and 1.5–2.5 bar, removing generated water through a reflux column while maintaining a low-pressure environment to drive oligomer formation to a degree of polymerisation between 5 and 7. Polycondensation is then conducted at 285–290 °C under a vacuum below 1.5 mbar in a finisher equipped with a twin-screw or disc-ring agitator; antimony trioxide is introduced at 190–250 mg Sb/kg of polymer and phosphoric acid at 15–30 mg P/kg to suppress ester-interchange discolouration. The amorphous melt reaches an intrinsic viscosity of 0.60–0.65 dL/g when tested according to ASTM D4603-18 in a 60/40 phenol/tetrachloroethane mixture at 30 °C. Bottle-grade conversion requires solid-state polycondensation at 210–215 °C under nitrogen for 10–14 h, raising intrinsic viscosity to 0.80–0.84 dL/g while reducing acetaldehyde to <1.0 ppm; conveyor-type SSP units with counter-current nitrogen flow of 0.8–1.2 Nm³/h per tonne of pellets are standard. 4-Carboxybenzaldehyde and p-toluic acid are monitored because their presence affects esterification kinetics and thermal stability; Longde PTA is controlled below 25 mg/kg 4-carboxybenzaldehyde and below 150 mg/kg p-toluic acid in the supplier certificate. When exposed to humid coastal air above 60% RH, PTA powder can adsorb moisture, and pre-drying in a hopper dryer at 110–130 °C for 4–6 h is required before entering the esterifier; otherwise local hydrolysis can shift the apparent melt pH and increase filter pressure in the prepolymer line. The polymer is also incompatible with strong amine-based additives if recycled into acid-sensitive formulations, because residual ester-interchange catalysts and free carboxyl groups can promote premature chain scission during drying.

How Does 4-Carboxybenzaldehyde Content Shift the Kinetic Window in Continuous Polyester Fibre Spinning?

Fibre-grade producers monitor 4-carboxybenzaldehyde not as a cosmetic specification but as a chain-terminating impurity that changes apparent melt viscosity and post-draw tension. In staple fibre lines using Longde PTA and ethylene glycol, esterification can be run at a lower molar ratio of 1.06–1.10 because the product has low residual ash; the prepolymer is then polymerised at 283–288 °C to an intrinsic viscosity of 0.63–0.68 dL/g. For partially oriented yarn, the melt is extruded through 0.15–0.28 mm spinneret orifices at 285–295 °C, quenched with air at 0.4–0.6 m/s, and wound at 2800–3300 m/min; a draw ratio between 1.5 and 1.7 produces fully drawn yarn with tenacity in the range 4.0–4.8 cN/dtex when measured by ISO 2062:2009. For high-tenacity staple, a two-stage hot-draw at 80–95 °C and 140–160 °C with a total draw ratio of 3.0–3.6 yields fibre tenacity of 6.5–8.0 cN/dtex and elongation at break of 12–18%, tested according to ASTM D2256/D2256M-21. Diethylene glycol concentration is deliberately held between 1.2 and 1.6 wt% because lower values reduce dye uptake in disperse dyeing and higher values depress the melting point below 250 °C; 4-carboxybenzaldehyde can react to form conjugated species that increase yellowness b* beyond 3.0 if the PTA feedstock is not fully oxidised. Spinning line operators routinely use differential pressure across the final filter as a real-time indicator of gel and foreign-particle accumulation; excursions above 0.25 MPa trigger screen-pack replacement. Pre-drying is required at 160–170 °C for 4–6 h if incoming moisture exceeds 0.1%; hydrolysis in an undried melt leads to intrinsic-viscosity drops of 0.02–0.05 dL/g and filament breaks at the godet.

Because film-grade polyester requires optical defect counts below 10 gel particles per 100 g and haze below 2.0% for packaging films and below 1.0% for optical-grade base film, PTA with low p-toluic acid and low b* is fed into twin-screw or single-screw extruders vented at 270–285 °C. The cast sheet is pinned to a chill roll held at 25–35 °C by electrostatic pinning and then stretched in the machine direction at ratios of 3.0–3.8 at 70–90 °C; transverse stretching follows at ratios of 3.5–4.0 at 100–115 °C, with heat-setting at 210–230 °C to stabilise shrinkage. Haze is measured by ASTM D1003-21, tensile modulus by ASTM D882-18, and thermal shrinkage at 150 °C by ASTM D1204-20. Edge trim and reclaim are re-fed at 30–50 wt% of total throughput, but regeneration of amorphous scrap requires crystallising at 140–160 °C before drying at 170–180 °C to below 50 ppm moisture, otherwise the high-shear extrusion zone hydrolyses ester bonds and increases carboxylic acid end groups, which in turn raises dielectric loss in capacitor film. Longde PTA supplied via dedicated food-contact and film-grade lots avoids the use of recycled packaging-side streams that may carry mixed-metal residues and can generate pinholes during transverse drawing. Published data for this specific configuration is limited; however, the film-forming differences among PTA grades are most visible in gel count and surface roughness after electrostatic pinning, not in intrinsic viscosity alone.

When THF Generation During PBT Polycondensation Narrows the Pressure Control Band

When the reactor vacuum band drops below 2.5 mbar, tetrahydrofuran generation in PBT polycondensation competes with chain extension, and the pressure control band must be tightened to prevent high-molecular-weight gelation. In PBT plants using Longde PTA and 1,4-butanediol, esterification is carried out at 220–240 °C with a slight overpressure of 0.2–0.5 bar to retain butanediol; titanium tetrabutoxide or tetraisopropyl titanate is metered at 50–150 mg Ti/kg of expected polymer to accelerate esterification without converting excessive 1,4-butanediol to tetrahydrofuran. The polycondensation finisher operates at 245–255 °C and below 1.0 mbar, with intrinsic viscosity controlled between 0.85 and 1.05 dL/g for injection-moulding grades and between 1.10 and 1.30 dL/g for extrusion grades; melt volume-flow rate is then tested at 250 °C with a 2.16 kg load using ISO 1133-1:2022. Injection moulding uses barrel temperatures of 240–260 °C, mould temperatures of 60–80 °C, and injection pressure in the range 60–120 MPa; pre-drying at 120 °C for 4 h to below 0.02% moisture is compulsory because hydrolytic chain scission at 250 °C can reduce molecular weight by 10–15% within 30 min of wet residence. The material meets UL 94 HB unreinforced and can be formulated with brominated flame retardants and antimony oxide to V-0 at 0.8–1.6 mm thickness; tensile strength is tested by ISO 527-2:2012, comparative tracking index by IEC 60112:2020 with expected values of 600 V or higher in halogen-modified grades, and heat deflection temperature by ISO 75-2:2013 at 0.45 MPa load. The main incompatibility is with extended hold-up in high-temperature runners: if residence time exceeds 8–10 min at 260 °C, tetrahydrofuran evolution regenerates odour-causing volatiles and the surface develops silver streaks. Published reactor-specific kinetic data for Longde PTA in tetrahydrofuran suppression is limited; the controlling variables are inferred from esterification-water removal rates and acid-value tracking common to PTA-based PBT processes.

Unsaturated Polyester Resin Formulation Limits with Longde PTA as an All-Aromatic Backbone Acid

Longde PTA is introduced into unsaturated polyester cook only as a partial replacement for phthalic anhydride or isophthalic acid because its high melting point and low solubility in glycol can freeze the top of a batch reactor if added too quickly. A standard two-stage process uses propylene glycol at 1.05–1.12 mol per total dicarboxylic acid, with PTA replacing 15–25 mol% of the saturated acid charge; the esterification temperature is raised from 160 °C to 210–220 °C over 4–6 h while water is removed by xylene azeotrope. Maleic anhydride is then added at 180–190 °C to control maleate-to-fumarate isomerisation, and the resultant unsaturated polyester is cut with styrene monomer at 35–45 wt% to a cone-plate viscosity of 300–600 cP at 25 °C; gel time is adjusted with cobalt naphthenate and methyl ethyl ketone peroxide to 10–20 min using ISO 2535:2001. Castings reach Barcol hardness of 40–50 when tested by ASTM D2583-13a, tensile strength of 40–70 MPa by ISO 527-2:2012, and heat deflection temperature of 70–90 °C at 1.8 MPa by ISO 75-2:2013. The PTA-modified backbone improves aromatic content and solvent resistance versus phthalic anhydride resins, but the styrene-compatibilised prepolymer remains incompatible with amine-based promoters when acid value exceeds 30 mg KOH/g; high acid value leads to exothermic decomposition during storage. Operators must also avoid charging more than 30 wt% of the total acid as PTA in a conventional non-pressurised stirred reactor, because unreacted PTA can accumulate on the agitator blades and cause batch-to-batch acid-value variance above 5 mg KOH/g.

Directly from glycolysis of post-industrial PTA-based PET, aromatic polyester polyols are produced by reacting clean bottle flake or fibre waste with diethylene glycol at 200–220 °C in the presence of zinc acetate at 0.5–1.0 wt% of feed; the resulting polyol has a hydroxyl number of 250–400 mg KOH/g and viscosity of 5000–15000 mPa·s at 25 °C. This intermediate is blended with aluminium hydroxide, TCPP, and surfactants to formulate polyisocyanurate rigid foam used in boardstock and pipe insulation; the isocyanate index is set between 250 and 350 to drive trimer formation, and the exothermic profile is measured by ASTM D3012-19. Foam density is held at 35–55 kg/m³ according to ASTM D1622-20, compressive strength at 150–300 kPa according to ASTM D1621-16, and closed-cell content above 90% by ASTM D6226-21. Fire performance is governed by EN 13501-1 for the European market and ASTM E84-23 for North America; formulations containing high aromatic PTA-derived polyols tend to char rather than melt-drip, but they also raise mix viscosity and require equipment capable of dispensing at pressure above 150 bar in high-pressure impingement mixheads. Moisture ingress into the polyol above 0.2% reduces storage stability and can liberate carbon dioxide during froth foaming; therefore nitrogen blanketing and closed-loop recirculation are used in tropical manufacturing sites. The inherent limitation of this route is that glycolysed PTA-based polyol cannot restore the exact carboxyl end-group distribution of a native diacid polyol, so hydroxyl titration by ASTM D4274-21 must be repeated per batch before blending to avoid index drift in continuous lamination.