In a continuous polyester fibre polymerisation line supplied with Jiaxing Petrochemical (Tongkun) PTA having a mean particle size of 120–140 µm, the slurry preparation station is set to maintain an ethylene glycol-to-PTA molar ratio of 1.10–1.20:1. The slurry density is held between 1,350 kg/m³ and 1,420 kg/m³ before injection into the first esterification reactor. The esterification train operates at 260–270 °C and 0.2–0.4 MPa, with water removal driving conversion above 96%. Antimony trioxide is charged at 150–300 ppm Sb on polymer, and triphenyl phosphite is added at 10–50 ppm P to limit thermal degradation in the finisher. The finisher pressure is maintained at 1.5–3.0 mbar until the intrinsic viscosity reaches 0.63–0.68 dL/g for filament grades or 0.60–0.65 dL/g for staple grades. On production units above 600 t/day, lot-to-lot variation in PTA mean particle size beyond ±5 µm delays esterifier thermal stabilisation by several hours, which is observed as a widening of the diethylene glycol distribution in the pre-polymer. Compliance for textile materials is typically demonstrated against OEKO-TEX Standard 100 Annex 4, which limits total extractable antimony to 30 mg/kg in final textile articles, and against ZDHC MRSL for spinning finish components; melt flow characterisation follows ISO 1133-1:2022. The formulated melt is converted by spinning beams into partially oriented yarn at wind speeds in the 3,000–3,500 m/min range, or into fully drawn yarn and drawn textured yarn through heated godets and texturing discs. Terminal finished product types include polyester staple fibre for ring and open-end yarn, high-tenacity filament for tyre cord, POY, DTY, FDY, and needle-punched nonwoven web.

What Limits Solid-State Polycondensation Throughput in Bottle-Grade PET?

Bottle-grade PET synthesis begins with PTA and ethylene glycol at a molar ratio of 1.05–1.15 mol EG per mol PTA. Purified isophthalic acid is co-charged at 1.0–3.0 wt% of the total acid fraction to reduce crystallisation rate and widen the injection stretch-blow moulding window. The melt-phase line runs at 265–285 °C with a final polycondensation pressure of 0.5–1.5 mbar, producing amorphous pellets with intrinsic viscosity 0.58–0.65 dL/g. Acetaldehyde, the principal regulatory and sensory problem in bottle resin, is formed by thermal degradation of polyester chains and remains above 1 ppm if pellet transfer from the granulator to the crystalliser is too slow or if melt temperature exceeds 285 °C at the die. The pellet is therefore crystallised at 170–180 °C and passed to a solid-state polycondensation reactor under nitrogen circulation at 205–215 °C for 12–18 h. Inlet crystallinity must exceed 34–38%; otherwise the pellets fuse and foul the SSP vessel walls. Outlet intrinsic viscosity is raised to 0.78–0.84 dL/g. Compliance for food-contact use is anchored to FDA 21 CFR 177.1630, EU Regulation No 10/2011 Annex I with an overall migration limit of 10 mg/dm², and GB 4806.7-2016 for food-contact plastic resin in China. Catalyst selection for low-haze bottle grades uses either antimony trioxide at 150–250 ppm Sb or titanium-based catalysts at 5–15 ppm Ti, with the latter preferred when preform clarity is critical and where antimony limits are under review. Downstream conversion occurs on injection stretch-blow moulding machines with preform barrel temperatures of 270–285 °C, preform cooling to 10–15 °C, and blow mould temperatures of 145–155 °C. Terminal finished product types include carbonated soft-drink bottles, still water containers up to 5 L, hot-fill beverage bottles, edible oil containers, and pharmaceutical dispensing bottles.

Biaxial Orientation Film Casting and Oligomer Control

Film-grade PTA polymerisation uses a lower diethylene glycol content than textile fibre to reduce melt degradation and oligomer deposition. The charge is ethylene glycol to PTA at 1.05–1.10:1, with DEG controlled to 1.0–1.5 wt%. Silica slip agent is incorporated at 500–3,000 ppm, and static melt filters of 20–40 µm are installed after the final polycondensation reactor. The intrinsic viscosity target is 0.62–0.68 dL/g, measured by capillary viscometry. Film casting lines above 4 m width are reported to develop die-lip oligomer deposits when DEG exceeds 1.5 wt% or when melt temperature at the die exceeds 285 °C, leading to die lines and transverse thickness variation. The cast sheet is quenched on a chill roll to below 45 °C, then stretched in the machine direction at 75–85 °C and in the transverse direction at 95–110 °C. Stretch ratios of 3.0–4.0:1 are applied in both directions. Heat setting at 220–240 °C reduces free shrinkage at 150 °C to 1.5–4.0% depending on grade. Compliance requirements follow FDA 21 CFR 177.1630 and EU Regulation No 10/2011 for food-contact films, while electrical insulation film is tested according to IEC 60674-3-4 for dielectric breakdown voltage and thickness; photovoltaic backsheet film is assessed under IEC 61215 for damp-heat and thermal-cycling stability. The film is subsequently metallised, coated, or laminated. Terminal finished product types include flexible food packaging film, metallised barrier film, capacitor dielectric film, thermal lamination film, and photovoltaic backsheet film.

Because direct esterification of PTA with 1,4-butanediol proceeds through a homogeneous acid-catalysed step in which water removal shifts the equilibrium, PBT synthesis is highly sensitive to titanium catalyst activity and to the suppression of tetrahydrofuran side reaction. A continuous or batch PBT plant charges 1.35–1.50 mol 1,4-butanediol per 1.00 mol PTA, with tetrabutyl titanate or tetraisopropyl titanate at 20–80 ppm Ti based on polymer. The esterification phase is run at 235–250 °C and 0.1–0.3 MPa, while the polycondensation phase at 245–255 °C and 0.5–2.0 mbar raises intrinsic viscosity to 0.80–1.20 dL/g for injection moulding grades or 1.05–1.30 dL/g for extrusion grades. The THF side reaction is monitored by headspace gas chromatography; when THF generation exceeds 2.5–4.0 mol% of the butanediol feed, the plant typically reduces the esterification temperature or shortens residence time in the first esterifier. PBT resin exiting the final polymerisation vessel must be pelletised under nitrogen and dried to below 0.05 wt% moisture before compounding. Glass-fibre reinforced compounds are produced on twin-screw extruders with L/D ratios of 40:1 to 48:1 at barrel temperatures of 240–260 °C, using 15–30 wt% chopped glass fibre. Compliance for electrical and automotive parts includes UL 94 V-0 at 0.75 mm for flame-retarded compounds, IEC 60695-2-11 for glow-wire ignition temperature, and ISO 527-2 for tensile modulus. Terminal finished product types include automotive wiring harness connectors, electronic relay sockets, sensor housings, coil bobbins, fibre optic buffer tubes, and high-temperature appliance components.

Polyester Polyols for Isocyanate Crosslinking: A Low-Acid Route

For aromatic polyester polyols used in two-component polyurethane systems, PTA is condensed with diethylene glycol, monoethylene glycol, dipropylene glycol, or trimethylolpropane. The charge ratio is governed by the target hydroxyl number and functionality. A typical flexible-grade formulation places PTA at 40–55 wt% of the total charge, with trimethylolpropane at 10–20 wt% for branched functionality and diethylene glycol at 30–50 wt% for flexible chain extension. The esterification reactor reaches 220–240 °C, with final vacuum of 20–50 mbar and a nitrogen sparge to strip water. Acid value is reduced to 2–5 mg KOH/g, and hydroxyl number is held in the 150–250 mg KOH/g range for coatings and adhesives. Production batches of 10–20 t are cooled below 60 °C before packaging because residual water above 0.1 wt% reacts with isocyanate during curing and generates carbon dioxide pinholes in cast elastomers. Compliance for polyurethane adhesives in food-contact packaging relies on FDA 21 CFR 175.105 for indirect food additives, while general industrial systems are registered under REACH; isocyanate content determination follows ISO 14896:2009. Downstream processing of the polyester polyol includes mixing with pure 4,4′-diphenylmethane diisocyanate or toluene diisocyanate, degassing at 30–50 °C, and casting or metering. Terminal finished product types include two-component polyurethane adhesives, coil coatings, cast elastomer wheels, flexible slabstock foam, and moisture-cure sealants.