Bottle-Grade Polyethylene Terephthalate Melt Polymerisation and Solid-State Polycondensation

Continuous bottle-grade PET production from Haldia PTA is governed by food-contact migration limits and by incoming monomer purity constraints that are measured before esterification. In the European Union, finished PET articles must conform to Commission Regulation (EU) No 10/2011 Annex I, which assigns a specific migration limit of 7.5 mg/kg for terephthalic acid and 30 mg/kg for ethylene glycol; antimony introduced via catalyst is limited to 0.04 mg/kg. In the United States, FDA 21 CFR §177.1630(f) covers PET homopolymers and copolymers intended for food contact. PTA feed for continuous plant operation is specified with 4-CBA below 25 mg/kg, because 4-carboxybenzaldehyde truncates chain growth and shifts product colour toward yellow. An incoming particle-size distribution with a D50 of 100–140 µm is preferred because finer material increases paste viscosity and dust load, while coarser material slows homogenisation and can settle in the slurry line before the first esterifier. Intrinsic viscosity is measured by ASTM D4603-18 in 60/40 phenol/1,1,2,2-tetrachloroethane at 30 °C; residual acetaldehyde in preforms is measured by ASTM F2013-10.

Addition ratios at the paste make-up stage are set at a PTA:monoethylene glycol molar ratio of 1:1.15 to 1:1.20, equivalent to a mass ratio of approximately 70:30 PTA:EG, because ethylene glycol is continually removed in the polycondensation vacuum system and must be present in excess to maintain esterification rate. Antimony trioxide is metered at 0.015–0.025 wt% Sb, phosphorous acid or triethyl phosphate is added as a stabiliser at 15–50 ppm P, and cobalt acetate toning is applied at 10–30 ppm Co when the melt-phase polymer requires blue correction. The downstream production process starts with dense-phase pneumatic conveying of PTA into a paste mixing vessel at 60–70 °C, followed by a two-stage esterification system operating at 260–270 °C and 1.2–1.5 bar(a). The oligomer is then transferred to a finisher at 280–290 °C under 0.5–1.5 mbar absolute, where melt-phase IV reaches 0.60–0.65 dL/g before pelletising. Amorphous pellets are crystallised at 130–150 °C and solid-state polymerised at 210–220 °C under nitrogen to raise IV to 0.80–0.84 dL/g and reduce residual acetaldehyde below 1.0 ppm. Preform injection moulding requires drying to less than 30 ppm moisture, barrel temperatures of 270–290 °C, hot-runner temperatures held below 290 °C to avoid acetaldehyde reformation, and mould wall cooling at 8–12 °C. Terminal finished product types include single-serve water bottles, carbonated soft-drink containers, hot-fill tea and juice bottles, and wide-mouth food jars.

Conversion of Haldia PTA into polyester staple fibre for ring-spun yarns imposes a different set of processing limits than bottle resin because the final article is a high-surface-area fibre mass rather than a barrier container. Compliance standards for textile polyester staple include REACH Regulation (EC) No 1907/2006 Annex XVII, the ZDHC Manufacturing Restricted Substances List for spin finishes, and Oeko-Tex Standard 100 Annex 4 Class I limits when fibre is sold into infant textile applications. The formulation addition ratio is dominated by a PTA:EG molar ratio of 1:1.15; titanium dioxide delustrant is injected as 0.3–0.5 wt% TiO₂ in the final polymer, and diethylene glycol is controlled at 0.8–1.2 wt% by the esterification temperature profile. Melt from the polycondensation finisher at IV 0.60–0.64 dL/g is pumped to multiple spinning beams with spinneret orifices of 0.15–0.25 mm, quenched by conditioned air at 18–24 °C and 60–70% relative humidity, and drawn at a total draw ratio of 3.0–3.5 to produce 1.2–1.5 dtex fibres. Crimping, heat-setting and cutting on a downline rotary cutter produce staple of 32–51 mm cut length. Terminal finished product types include ring-spun blended yarns, open-end yarns, needle-punched nonwoven felts, and polyester fiberfill for bedding and upholstery.

What Limits Hydrolytic Stability in PBT Moulding Grades Produced from Terephthalic Acid?

Hydrolytic stability in PBT moulding compounds begins with the esterification of Haldia PTA and 1,4-butanediol in a molar ratio of 1:1.35 to 1:1.45, because the acid-catalysed dehydration of 1,4-butanediol produces tetrahydrofuran as a side product and consumes part of the added diol. Tetrabutyl titanate is metered at 40–120 ppm Ti; the excess 1,4-butanediol is stripped at reduced pressure after esterification at 220–235 °C. Polycondensation continues at 245–255 °C and 0.3–1.5 mbar absolute until the melt volume-flow rate reaches 15–25 cm³/10 min when tested according to ISO 1133-1:2022 at 250 °C with a 2.16 kg load. The main production-scale failure mechanism in unfilled PBT injection moulding is hydrolytic chain scission during melting, which occurs when pellets contain more than 0.02 wt% moisture and manifests as splay marks, reduced notched impact and screw torque variations. Drying must therefore be carried out in a dehumidifying hopper dryer at 120 °C for 4 h or until moisture falls below 0.02 wt%. Barrel temperatures are set to 235–250 °C, mould temperature to 40–80 °C, and residence time below 5 min to limit thermal degradation. Where PBT compounds are specified for hot-moist service, 0.2–0.5 wt% polymeric carbodiimide or glycidyl methacrylate chain extender is compounded on a co-rotating twin-screw extruder with an L/D ratio of 40:1, barrel set points of 240–260 °C, and screw speed of 300–500 min⁻¹.

Industry compliance standards for PBT moulding grades include ISO 527-2 for tensile modulus, ISO 179-1/1eA for notched Charpy impact, and UL 94 V-0 classification at 0.8 mm wall thickness for flame-retarded compounds. Electrical and electronic components must satisfy RoHS Directive 2011/65/EU and REACH Annex XVII restrictions; connectors subjected to elevated service temperatures are additionally evaluated by IEC 60695-2-11 glow-wire testing at 850 °C. Terminal finished product types include automotive relay sockets, ignition coil covers, motor end caps, sensor housings, pump impellers, and fibre-optic connector bodies.

When Biaxially Oriented Polyester Film Requires Submicron Filtration of Oligomer Gel

For biaxially oriented polyester film, Haldia PTA is reacted with ethylene glycol at a molar ratio of 1:1.15, with silica antiblock particles dispersed in ethylene glycol at 0.02–0.2 wt% and diethylene glycol controlled at 0.8–1.1 wt% because higher diethylene glycol content disturbs crystallinity and lowers tensile modulus. Film-grade PET is polymerised to an intrinsic viscosity of 0.62–0.65 dL/g before extrusion at 275–285 °C through a flat die; the melt is forced through 15–30 µm sintered metal or depth filtration media to remove oligomer gel and silica agglomerates that would otherwise appear as surface protrusions and reduce winding yield. The cast sheet is quenched on a chilled drum at 20–30 °C, then stretched 3.0–3.6 times in machine direction at 95–110 °C and 3.2–3.8 times in transverse direction at 100–120 °C, followed by heat-setting at 200–225 °C to control shrinkage. Dimensional stability is assessed by ASTM D1204 at 150 °C for 30 minutes; tensile properties are assessed by ASTM D882; optical haze is measured by ASTM D1003; finished food-contact films comply with Commission Regulation (EU) No 10/2011. Terminal film products include retort pouches, capacitor dielectric film, holographic film base, thermal transfer ribbons, and photovoltaic backsheet laminates.

In flexible PVC compounds that must avoid ortho-phthalate classification, direct esterification of Haldia PTA with 2-ethylhexanol produces dioctyl terephthalate, a non-phthalate plasticiser route in which the feedstock molar ratio is set at 1:2.4 to 1:2.6 PTA:2-ethylhexanol to maintain sufficient alcohol excess for complete conversion. Tetrabutyl titanate or tetra-n-butyl zirconate is metered at 0.05–0.15 wt% of PTA. The esterification is conducted at 210–230 °C under nitrogen with continuous removal of water; after the acid value falls below 0.5 mg KOH/g, excess alcohol is stripped at 160–200 °C and reduced pressure. The crude ester is neutralised, washed, and filtered to achieve an acid value below 0.05 mg KOH/g and moisture below 0.1%, because residual acidity accelerates PVC dehydrochlorination and unreacted PTA crystals block downstream filter presses. Regulatory compliance is defined by REACH Regulation (EC) No 1907/2006 and RoHS Directive 2011/65/EU; plasticizer volatility and compatibility are evaluated by ASTM D1045, and permanence in contact with liquids is evaluated by EN ISO 175. Terminal products include flexible PVC cable sheathing, floor coverings, automotive interior skins, and children’s articles where DOTP replaces ortho-phthalate esters.

PTA-Based Carboxyl-Polyester Resin Acid Value Falls to 30–50 mg KOH/g Before Mill Classification

PTA serves as the aromatic diacid in saturated carboxyl-functional polyester resins for powder coatings, where it is combined with isophthalic acid, neopentyl glycol and trimethylolpropane to raise glass-transition temperature and storage stability. Published formulation practice places PTA at 30–45 wt% of total acid monomers, neopentyl glycol at 40–55 wt% of total polyol monomers, and trimethylolpropane at 0.5–3.0 wt%; excess glycol is removed until the resin acid value reaches 30–50 mg KOH/g and the glass-transition temperature reaches 55–65 °C. The synthesis reactor operates at 240–250 °C with nitrogen sparging and xylene azeotropic water removal; the hot resin is then co-extruded with TGIC or β-hydroxyalkylamide curative at 90–110 °C, cooled, flaked, milled, and classified to 35–100 µm. Compliance standards include EN ISO 8130-6 for gel time, ASTM D3451 for testing thermosetting coating powders, Qualicoat Specifications for architectural aluminium, and RoHS Directive 2011/65/EU for heavy-metal limits. Terminal products are aluminium architectural cladding, appliance housings, automotive alloy wheels, and corrosion-resistant furniture finishes.