Purified terephthalic acid sourced from Yisheng Petrochemical (Dalian) is charged as the aromatic dicarboxylic acid feedstock across melt-phase polycondensation, solution esterification, and thermoset resin kettle operations. The application boundaries are set by 4-carboxybenzaldehyde (4-CBA), para-toluic acid (p-TA), particle size distribution, colour, and residual catalyst metals. These parameters interact with downstream reactor design and final regulatory compliance in ways that vary substantially by segment.

What Purity Thresholds Govern Bottle-Grade PET Solid-State Polymerization?

Bottle-grade polyethylene terephthalate production assigns the highest sensitivity to 4-carboxybenzaldehyde because the monofunctional aldehyde acts as a chain terminator during melt polycondensation and forms colour bodies under thermal stress. In commercial PTA specifications derived from hydrogenated crude terephthalic acid, 4-CBA is typically controlled below 25 mg/kg and para-toluic acid below 150 mg/kg. The esterification train is fed as a paste with ethylene glycol at a molar ratio between 1:1.05 and 1:1.15, with excess glycol removed during pre-polycondensation. Antimony catalyst is carried at a level that keeps residual antimony in amorphous pellet below 300 mg/kg, because higher metal residues accelerate thermal degradation and acetaldehyde regeneration during injection moulding. Melt-phase polymer is then solid-state polymerised in vertically staged cylindrical vessels with nitrogen purge, with bed temperature ramped between 200 °C and 220 °C. The solid-state train raises intrinsic viscosity from the melt-phase value of roughly 0.60–0.65 dL/g to bottle-grade values of 0.80–0.85 dL/g, measured according to ASTM D4603 in 60:40 phenol/1,1,2,2-tetrachloroethane at 30 °C. Since bottle preforms are food-contact articles, compliance is assessed under FDA 21 CFR 177.1630 and the European positive list under Regulation 10/2011; the resin supplier and converter jointly establish migration limits for terephthalic acid and ethylene glycol under the intended food simulant. Residual acetaldehyde in water-contact preforms is typically required not to exceed 8 ppm.

Pelletised amorphous resin transferred to the solid-state reactor must be dried under closed-loop nitrogen because residual water above 0.005 wt% hydrolyses ester bonds during the intrinsic viscosity build. The SSP cycle is validated by measuring carboxyl end-group concentration, diethylene glycol content, and bulk density; deviations in PTA particle size shift the amorphous pellet density and therefore gas distribution through the bed. If bed pressure drop rises above the design envelope, the solid-state reactor enters an abnormal cooling mode and the IV distribution across the vessel widens. Injection moulding of preforms on accumulator-type machines with clamping force between 1500 kN and 6000 kN requires barrel melt residence time to be minimised; prolonged heating regenerates acetaldehyde from vinyl ester terminations. These process limits are not resolved by changing the PTA specification alone, because preform quality depends on the combined response of PTA purity, glycol ratio, antimony residual, and SSP history.

During start-up of a polyester staple fibre melt line, the PTA particle size distribution and bulk density determine how uniformly the slurry feed system maintains stoichiometric precision in the esterification train. If the fines fraction below 45 µm shifts upward, slurry pump cavitation and feed line settling become more probable, producing batch-to-batch variation in diethylene glycol content and carboxyl end-group concentration. Textile-grade PET is normally polymerised to an intrinsic viscosity of 0.60–0.65 dL/g, with DEG controlled below 1.2 wt% to preserve tensile strength and crystallisation rate. Titanium dioxide delustrant at 0.3–0.5 wt% is metered as a predispersed ethylene glycol slurry just before the spin pack; spin pack filtration uses multilayer screens of 20–40 µm absolute rating to capture agglomerates. Melt spinning from the finisher to the spinneret is maintained above 280 °C, and fibre-line operators monitor spin finish uptake and interlace pressure against final tenacity measured under ASTM D2256. A shift in PTA colour b* above 1.5 is particularly visible in undyed fibre because it transfers directly into yellow reflectance and forces downstream optical brightener rebalancing. On multi-line staple fibre plants, a change in PTA median particle diameter from 120 µm to 140 µm can be absorbed if the slurry density control loop is retuned, but a shift below 100 µm tends to increase dusting losses in the vacuum system and changes reactor wall wetting. The better corrective action is upstream feed screening or blend management of PTA lots rather than raising ethylene glycol ratio, because excess glycol increases DEG formation. Melt viscosity is measured by melt flow index under ISO 1133-1 at 280 °C with 2.16 kg load; the value is used only as a trending tool because molecular weight distribution and shear history dominate spinning behaviour.

Low-DEG Molecular Architecture Limits BOPET Optical Haze

Optical-grade biaxially oriented polyester film production begins with PTA selected for low etherification tendency in the melt, because diethylene glycol units disrupt chain symmetry and operate as covalent defects that depress crystallinity. A DEG concentration above 1.0 wt% in the cast sheet is usually inconsistent with low-haze film for display and photovoltaic backsheet applications. The melt is extruded through a flat die at 270–280 °C, cast onto a chilled roll held at 20–40 °C, and pinned by electrostatic wire electrodes. The cast sheet is then stretched at draw ratios of 2.5–4.0 in the machine direction and 3.0–4.5 in the transverse direction. Haze is tested using ASTM D1003, and tensile properties are determined under ASTM D882. For film lines running above 300 m/min, the presence of PTA fines can generate periodic gel defects that are visible only after transverse stretching, which forces an increase in melt filtration grade from 20 µm to 10 µm absolute. Polyester film also requires a carefully controlled additive package: slip agents such as silica at 500–1500 ppm are used to reduce roll blocking without raising haze beyond specification. BOPET film formulations for capacitor or photovoltaic backsheet use increasingly exploit co-monomer additions and PTA purity to manage dielectric loss and hydrolysis resistance. If PTA p-TA exceeds the production limit, the resulting film shows higher extractables and lower breakdown voltage after biaxial orientation. A thicker cast sheet and slower line speed allow more uniform crystallinity development but reduce overall output; therefore film producers select PTA with low DEG generation rather than relying only on process compensation.

In a PBT polycondensation train, PTA is reacted with 1,4-butanediol in an excess diol environment because the acid-catalysed dehydration of 1,4-butanediol to tetrahydrofuran is kinetically significant above 180 °C. The esterification temperature is normally held between 220 °C and 250 °C, and the molar ratio of diol to acid is managed between 1.3:1 and 1.7:1 to compensate for THF loss. Tetrabutyl titanate is added at 50–150 ppm titanium; the catalyst is hydrolytically sensitive, and PTA with elevated free water above 0.1 wt% increases catalyst deactivation and shifts carboxyl end-group concentration upward. Polycondensation is completed in a horizontal finisher under vacuum below 1 mbar. The resulting PBT resin is pelletised and typically dried at 120 °C for 4–6 h before injection moulding. Mould temperatures of 40–80 °C are required because PBT crystallises too rapidly for cold moulds, generating weld-line weakness in electrical connectors. Mechanical characterisation follows ISO 527-1 and ASTM D638; flammability is rated under UL 94. For applications requiring hydrolysis resistance, carboxyl end-group content in the compounded resin is monitored below 30 mmol/kg. Published line-speed data for Dalian PTA specifically in PBT compounding is limited, but the purity constraints are widely transferable.

DOTP Esterification and Acid Value Termination

When PTA is esterified with 2-ethylhexanol to produce dioctyl terephthalate, the reactor charge is typically set at a slight alcohol excess, with the mole ratio between 1:2.2 and 1:2.5 to drive conversion while leaving sufficient alcohol for recycle. Titanium tetraisopropoxide is introduced at 0.05–0.15 wt% based on PTA; the reaction is staged from 180 °C to 230 °C with continuous water removal. The endpoint is defined by acid value rather than reaction time: the crude ester is neutralised with dilute alkali and steam-stripped until acid value falls below 0.07 mg KOH/g, with ester content typically above 99.5%. Residual titanium is removed by filtration after hydrolysis, otherwise haze and secondary catalytic activity can persist during PVC compounding. DOTP produced from PTA is used as a non-phthalate plasticizer in flexible PVC wire and cable jackets, where low volatility and electrical resistivity are critical. Plasticiser characterisation follows ASTM D1045 for acid value and specific gravity; compounded PVC hardness is measured under ASTM D2240, and tensile properties under ASTM D638. Long-term heat ageing is evaluated according to ISO 188 at 100 °C for 168 h, with mass loss and retained elongation reported against formulated references. The esterification of PTA with 2-ethylhexanol is more sterically hindered than ortho-phthalic anhydride esterification, so the first acid group reacts faster than the second; monooctyl terephthalate persists in the reactor until the acid value falls below the target. If the reactor is discharged too early, residual acid groups accelerate PVC dehydrochlorination during cable extrusion. The overhead water stream contains 2-ethylhexanol and must be separated in a decanter; entrained PTA fines can stabilise emulsions and increase chemical oxygen demand in the same manner observed in polyester filament slurry systems. If the PTA contains excessive coarse particles above 250 µm, esterification conversion drops because particle surface area is lower and the sludge contains unconverted PTA cores.

Polyester polyol kettles charged with PTA, diethylene glycol, adipic acid, and a branching triol such as trimethylolpropane produce hydroxyl-functional oligomers for rigid polyurethane foam and CASE polyurethane systems. The aromatic terephthalate unit raises glass transition temperature and compressive strength relative to adipate-only polyols, but increases blend viscosity and reduces compatibility with some hydrocarbon blowing agents unless the polyol is formulated with enough aliphatic co-monomer. Hydroxyl number is typically controlled between 180 mg KOH/g and 350 mg KOH/g for rigid foam systems, and acid number is held below 2 mg KOH/g; water content is dried below 0.05 wt% before reaction with isocyanate. ASTM D4274 and ASTM D4662 are the standard methods for hydroxyl number and acid number. In continuous rigid foam lamination lines, the aromatic content from PTA influences cream time and rise profile; metering ratio between polyol and isocyanate must compensate for the higher aromatic ring concentration, typically requiring a lower isocyanate index than aliphatic polyols with equivalent hydroxyl number. Because PTA is a solid diacid with high melting point, polyester polyol synthesis requires a long cook-out phase under partial vacuum to remove water of esterification from the viscous medium. If the reactor lacks a thin-film evaporator, final water removal can require 6–12 h at 220–250 °C. The branch triol concentration is limited to avoid gelation; trimethylolpropane levels above 5 wt% of total charge can produce a molecular weight distribution with insoluble microgel fragments that clog transfer pumps. The resulting polyol is often blended with sorbitol- or sucrose-based polyols in rigid foam systems to meet flame spread requirements such as ASTM E84.

When Carboxyl-Functional Polyester Resins Crosslink with HAA or TGIC

Powder coating polyester resins synthesised from PTA and neopentyl glycol are terminated to defined acid values so that they can be crosslinked with hydroxyalkylamide or triglycidyl isocyanurate. The typical acid value for TGIC-cure systems is 33–45 mg KOH/g, while HAA-cure systems are formulated higher, between 45 mg KOH/g and 75 mg KOH/g, because the β-hydroxyalkylamide chemistry consumes carboxyl groups through a different stoichiometry. The resin glass transition temperature is maintained between 55 °C and 70 °C to prevent powder sintering during storage and to ensure adequate flow during cure. Extrusion melt blending on co-rotating twin-screw equipment is conducted at 80–120 °C, below the curing agent threshold, and gel time is measured under ISO 8130-6. Cured film testing includes ASTM D2794 for impact resistance and ASTM D3359 for adhesion. PTA aromaticity improves exterior durability relative to isophthalic-acid-deficient resins but can reduce chain flexibility if the terephthalic acid fraction exceeds the formulation window; coil coating type polyester resins therefore often blend terephthalic and isophthalic acid to balance hardness and formability.