What Limits Carboxy End-Group Formation in Continuous PET Fibre Polymerisation?

In continuous polyester fibre melt polymerisation based on purified terephthalic acid (CAS 100-21-0), the glycol-to-acid feed ratio is maintained at 1.15:1 to 1.25:1 ethylene glycol per mole PTA, corresponding to a PTA charge of 85.0–87.0 parts by mass per 100 parts final poly(ethylene terephthalate). The slurry contains antimony trioxide at 190–240 mg/kg Sb and phosphoric acid at 15–40 mg/kg P. Esterification in the primary esterifier runs at 260–270 °C and 100–200 kPa absolute, with 2.5–4.0 h residence time; the oligomer reaches a degree of polymerisation of 4–8 and a carboxyl end-group concentration of 15–30 meq/kg. Polycondensation proceeds at 270–285 °C under <100 Pa, producing fibre-grade chips with intrinsic viscosity 0.62–0.68 dL/g measured by ASTM D4603. Plant-scale observation shows that PTA with median particle size above 180 µm increases slurry settling in the esterification feed legs and causes lobe-pump suction pressure oscillation; 20–40 µm spinneret filtration and melt-pump pressure of 8–14 MPa are required to avoid particle-related filament breaks. The melt is extruded through spinnerets, quenched at 0.4–0.7 m/s air velocity, drawn, crimped, and cut. Terminal products include semi-dull staple fibre, fully drawn filament yarn, low-pill fibre, and needle-punched nonwoven. Compliance is anchored to REACH (EC 1907/2006) Article 14 exposure scenarios, OEKO-TEX Standard 100 Annex 4 Class II, and yarn tensile testing per ISO 2062:2009.

A 600 t/d bottle-resin line processing amorphous copolymer precursor uses a diacid feed containing 1.5–2.5 wt% isophthalic acid and a molar glycol-to-PTA ratio of 1.08:1 to 1.12:1, which places PTA at 84.5–86.5 parts per 100 parts pellet mass after melt-phase losses. The melt-phase finisher discharges resin at intrinsic viscosity 0.60–0.64 dL/g; solid-state polycondensation in a continuous shaft dryer with nitrogen dew point −60 °C, temperature 205–215 °C, and residence time 12–18 h raises intrinsic viscosity to 0.82–0.86 dL/g and lowers pellet acetaldehyde to ≤1 mg/kg. Process constraints include nitrogen flow 4–6 m/s superficial velocity and pellet bed height 1.5–2.2 m to avoid dusting and local overheating. Terminal packaging grades include carbonated soft-drink bottles, hot-fill containers, aseptic dairy packaging, and food trays. The compliance matrix below defines the boundary for food-contact export documentation.

PBT Direct Esterification: Tetrahydrofuran Loss and Molar Balance Control

Polybutylene terephthalate is produced from PTA and 1,4-butanediol using a diol-to-PTA molar feed ratio of 1.30:1 to 1.45:1, with PTA representing 56–60 parts per 100 parts raw diol/diacid charge before recovery. The excess diol compensates for dehydration to tetrahydrofuran, which is collected in the overhead decanter and returned after distillation; tetrahydrofuran yield is typically 0.5–1.5 mol% on BDO. Esterification at 225–235 °C is followed by polycondensation at 245–255 °C and <100 Pa with tetrabutyl titanate catalyst at 50–120 mg/kg Ti. Injection-moulding grades exit with intrinsic viscosity 0.95–1.10 dL/g in 60:40 w/w phenol/tetrachloroethane, and melt volume-flow rate 12–18 cm³/10 min at 250 °C/2.16 kg per ISO 1133-1:2022. Reinforced compounds with 30 wt% glass fibre are produced on a L/D 40:1 twin-screw extruder with barrel profile 230–260 °C; tensile stress at break is 130–140 MPa per ISO 527-2. Pre-drying at 120 °C for 4 h to below 0.02 wt% moisture is mandatory; hydrolysis reduces molecular weight by 10–20% after 8 h at 250 °C if moisture exceeds 0.05 wt%. Terminal products include automotive ignition coil bobbins, electrical connectors, sensor housings, and pump rotors. Compliance is based on UL 94 V-0 at 0.75 mm, IEC 60112:2020 comparative tracking index, and ISO 527-2:2012 mechanical data.

At a 20 m³ jacketed esterification reactor operating with 2-ethylhexanol as the excess alcohol, PTA is metered at 39–43 parts by mass per 100 parts recovered dioctyl terephthalate, with a molar alcohol-to-PTA ratio of 2.20:1 to 2.50:1. The catalyst is tetrabutyl titanate at 0.05–0.15 wt% of PTA; esterification is maintained at 190–220 °C while water is removed as a 2-ethylhexanol azeotrope, returned through an overhead decanter, and the recovered alcohol is recycled. Esterification continues until acid value is below 0.10 mg KOH/g; neutralisation with 0.1–0.3 wt% aqueous sodium carbonate, filtration through 1 µm media, and vacuum stripping at 15–25 kPa reduce moisture to <0.05 wt% and colour to <30 Hazen. In flexible PVC compounding, DOTP is added at 35–70 phr to produce Shore A hardness from 60 to 85; twin-screw compounding at 160–175 °C with a L/D 40:1 configuration provides sufficient dispersion. Terminal products include 80 °C building wire insulation, appliance wire jackets, calender-made vinyl flooring, and artificial leather. DOTP is not listed under REACH Annex XVII Entry 51 restrictions for DEHP, DBP, BBP, and DIBP, and is not restricted under EU RoHS Directive 2011/65/EU Annex II; batch documentation must nevertheless include REACH registration and Prop 65 certification for North American wire exports.

When PTA Replaces Phthalic Anhydride in Aromatic Polyester Polyol Kettle Processing

When PTA replaces part of the phthalic anhydride in a batch aromatic polyester polyol cook, the charge is set at 15–45 wt% of the total diacid/diol mass, with diethylene glycol as the main glycol and 5–15 wt% adipic acid or phthalic anhydride as modifier. A 10–20 m³ kettle with a helical ribbon impeller operating at 0.5–1.0 m/s tip speed keeps PTA suspended until dissolution above 180 °C. Reaction proceeds at 220–240 °C under nitrogen with organotin catalyst at 50–100 mg/kg; vacuum is ramped to <500 Pa until acid number reaches 1.5–2.5 mg KOH/g and hydroxyl number is 180–320 mg KOH/g. The finished polyol has viscosity 8,000–15,000 mPa·s at 25 °C and water content <0.05 wt%. In rigid polyurethane foam, a formulation uses 100 parts polyester polyol, 2–4 parts water, 1–3 parts amine catalyst, 1–2 parts silicone surfactant, and polymeric MDI at an isocyanate index of 105–180. Compressive strength by EN 826:2013 is 250–450 kPa for 40–80 kg/m³ core density. Terminal products include laminated boardstock, pour-in-place appliance insulation, spray-applied insulation, and district heating pipe segments. Fire performance is evaluated under ASTM E84-20 Class A/B and EN 13501-1:2018 with flame retardant additions of 5–15 parts depending target Euroclass. PTA-based polyols crystallise below 25 °C; storage at 40–60 °C with recirculation is required to prevent solidification.

Biaxially oriented polyester film casting begins with a pellet feedstock whose intrinsic viscosity is deliberately raised to 0.65–0.68 dL/g by melt-phase polycondensation from PTA and ethylene glycol at a glycol-to-PTA molar ratio of 1.13:1, equivalent to 85–86 parts PTA per 100 parts final polymer. Film formulations include silica anti-block at 0.05–0.40 wt%, magnesium acetate at 0.03–0.05 wt%, and phosphoric acid at 0.03–0.06 wt%. The melt is filtered through 15 µm media and extruded at 275–285 °C through a slot die onto a casting drum at 20–30 °C. Sequential stretching uses machine-direction draw ratio 3.2–3.8:1 at 90–110 °C and transverse-direction draw ratio 3.5–4.2:1 at 110–130 °C, followed by heat setting at 220–230 °C and 1–3% relaxation. Terminal products include 1.5–6 µm capacitor film with surface roughness 0.030–0.050 µm, 50–250 µm solar backsheet film, 12–50 µm flexible packaging, and 25–125 µm release liner. Mechanical inspection uses ASTM D882-18 tensile stress 200–250 MPa in both directions, ASTM D1003-21 haze below 3%, and IEC 60674-3 film specifications. Food-contact status is governed by 21 CFR §177.1630 and Regulation (EU) No 10/2011; hydrolysis-resistant grades require 0.5–1.0 wt% carbodiimide-based stabiliser for extended thermal cycling in photovoltaic modules. Published data for specific capacitor-grade monolayer PTA-based PET from individual lines is limited; the cited ranges reflect equipment manufacturer technical bulletins and film-converter specifications.