Petroleum paraxylene is fed to a continuous liquid-phase oxidation reactor operated at 175–205 °C and 1.5–3.0 MPa air partial pressure using acetic acid as solvent, cobalt(II) acetate and manganese(II) acetate as primary redox catalysts, and a bromine source as free-radical promoter. The molar ratio of catalyst metals to paraxylene is maintained within narrow bands—commonly 100–400 ppm cobalt, 100–400 ppm manganese, and 200–1200 ppm bromine relative to reactor charge—because excess promoter accelerates solvent burn to carbon dioxide and carbon monoxide while depleting acetic acid. Oxidation proceeds through the intermediate p-toluic acid and 4-carboxybenzaldehyde, with crude terephthalic acid precipitated and recovered by pressure filtration. The crude acid is then purified by hydrogenation in aqueous solution over a fixed-bed palladium-on-carbon catalyst at 250–300 °C and 6.0–8.0 MPa, converting 4-carboxybenzaldehyde to p-toluic acid for subsequent crystallization. Product specifications for purified terephthalic acid entering polyester melt-phase reactors typically require 4-carboxybenzaldehyde below 25 mg/kg, total ash below 10 mg/kg, and Hunter b* color below 1.0. In continuous reactors, oxygen starvation in the final oxidation zone produces elevated 4-carboxybenzaldehyde, which acts as a chain terminator in polyester polycondensation and lowers intrinsic viscosity stability. Plant-scale rotary vacuum filters handling crude terephthalic acid slurries at 60–70 °C exhibit cake cracking when mother liquor solids exceed 40 wt%, causing wash efficiency loss and residual acetic acid carryover. Published data for newer low-solvent oxidation processes indicates that water content of the reaction medium must be controlled between 4% and 8% to avoid catalyst precipitation while maintaining para-xylene conversion above 99.0%. Off-gas from the oxidation reactor contains methyl bromide, carbon monoxide, and unreacted paraxylene; catalytic oxidation of the off-gas stream above 300 °C recovers energy and reduces volatile organic compound emissions under local permit thresholds.

What Constrains the Melt Filtration Index After Liquid-Phase Polycondensation of Bottle-Grade PET?

The conversion of purified terephthalic acid and ethylene glycol proceeds through a two-stage melt process: esterification at 260–270 °C with a glycol-to-acid molar ratio of 1.15–1.25, followed by polycondensation at 275–285 °C under vacuum below 150 Pa using antimony trioxide catalyst at 150–250 ppm Sb. The resulting amorphous melt-phase resin typically has intrinsic viscosity around 0.60–0.65 dL/g, measured according to ASTM D4603-18 in 60:40 phenol/1,1,2,2-tetrachloroethane at 30 °C. Bottle preform injection requires intrinsic viscosity of 0.80–0.84 dL/g, achieved by solid-state polymerization in a continuous crystallizer and reactor train at 210–230 °C under nitrogen flow for 12–18 hours. Acetaldehyde generation in the melt phase must be kept below 1 ppm in the final preform because acetaldehyde migrates into packaged water and carbonated soft drinks and alters sensory profile. Bottle-grade formulations add a polycondensation catalyst and a color-control package containing cobalt acetate and organic toner; the molar ratio of ethylene glycol to total diacid is adjusted to control diethylene glycol content between 1.0 and 1.5 mol%, since higher diethylene glycol depresses crystallinity and oxygen barrier performance. Preform injection molding on high-cavitation systems with hot-runner valve gates operates at melt temperatures near 275–285 °C and clamp force settings above 3,000 kN for 48-cavity tools; premature gate freeze and shear-induced acetaldehyde spikes occur when injection speed exceeds 150 mm/s. Material with residual 4-carboxybenzaldehyde above 8 mg/kg can be processed, but it produces a slight yellow shift that is unacceptable in water-white applications. A purge of 0.5–1.0 kg of resin per barrel residence time unit is required after color changes to prevent cross-contamination. The European food-contact framework EU Regulation (EU) No 10/2011 and US FDA 21 CFR 177.1630 define migration limits; compliance testing typically follows EN 1186-1:2002 and US FDA migration cell protocols.

When Polyester Staple Fiber Spinning Demands Low Degradation and Tight Molecular Weight Distribution

In staple fiber production, purified terephthalic acid is esterified with ethylene glycol and polymerized to an intrinsic viscosity of 0.62–0.68 dL/g. The polymer is pumped directly from the final polycondensation reactor to a manifold feeding multi-spinneret spin beams. Melt temperature at the spinneret is typically 285–295 °C. The spinning assembly uses 0.15–0.35 mm capillary diameters and filtration media rated at 20–40 µm. Hydrolytic degradation during melt transport is managed by maintaining moisture below 50 mg/kg in the dried polymer and using a short residence time between final reactor and spinning pack, typically less than 8 minutes. Spinning speed for partially oriented yarn ranges from 2,800 to 3,400 m/min; for fully drawn yarn the process couples high-speed spinning at 4,000–5,000 m/min with drawing at draw ratios of 1.4–1.8. The birefringence of the as-spun fiber increases with spinline stress, so molecular weight distribution influences elongation at break and dye uptake. Batch-to-batch variability in PTA particle size distribution above 250 µm leads to feeding interruptions in continuous esterification. In practice, a PTA lot with mean particle size 110–130 µm and fines below 5% through a 45 µm screen is favored. Dyeability with disperse dyes follows diffusion-controlled uptake; an excessive diethylene glycol content above 2.0 mol% increases amorphous orientation and can cause dye non-uniformity under high-temperature exhaust-dyeing cycles at 130 °C. Tensile testing on finished yarn uses ISO 2062:2009; tenacity values for high-tenacity polyester industrial yarn are commonly 7–8 cN/dtex, but values depend on molecular weight and spinning technology.

Biaxially oriented polyester film converts PTA-derived PET chip into film through extrusion on a chill roll at 20–40 °C, sequential stretching in machine direction at 80–95 °C and transverse direction at 90–110 °C, followed by heat-setting at 210–230 °C. The melt filtration index is measured by pressure rise across a 15–25 µm pore-size segmented screen pack; pressure increases above 0.5 MPa/h indicate oligomer agglomeration or gel particles. PET resin for film typically uses intrinsic viscosity of 0.62–0.65 dL/g, as higher intrinsic viscosity increases haze and requires higher extruder torque. Extruder barrel temperatures are ramped from 250 °C at the feed section to 275–285 °C at the metering section, with a melt pump maintaining 5.0–8.0 MPa die pressure. Anti-block particles such as silica are added at 500–2,000 mg/kg, and their concentration directly affects surface roughness and coefficient of friction. In the transverse stenter, film enters at 80 °C and is accelerated through a draw ratio of 3.0–4.0 in machine direction and 3.5–4.5 in transverse direction. Film thickness uniformity below ±2% at 12 µm thickness requires high-quality PTA with low metal fines; a single 20 µm iron particle can generate a visible void in metallized barrier film. For food-contact film, migration of residual ethylene glycol and terephthalic acid monomers is assessed under EU Regulation (EU) No 10/2011 with simulant D for fatty foods; optical and mechanical properties are tested under ASTM D882-18 and ASTM D1003-21.

Dimethyl Terephthalate Transesterification Route to PBT: Stoichiometry and Titanate Catalyst Deactivation

Paraxylene-derived dimethyl terephthalate is used in polybutylene terephthalate production rather than purified terephthalic acid because the methyl ester allows direct transesterification with 1,4-butanediol without producing water, avoiding hydrolysis side reactions that form tetrahydrofuran. Stoichiometry uses a 1,4-butanediol-to-dimethyl terephthalate molar ratio of 2.0–2.4 in the first stage under atmospheric pressure at 150–200 °C. Tetra-n-butyl titanate catalyst is charged at 50–150 ppm titanium relative to polymer. The transesterification step releases methanol and requires an efficient distillation column to maintain methanol overhead quality above 99.5%; residual methanol in the prepolymer above 0.1 wt% modifies the ester interchange equilibrium and suppresses molecular weight build. Polycondensation proceeds at 245–255 °C under vacuum below 100 Pa. The activated catalyst hydrolyzes in the presence of moisture, so the dimethyl terephthalate feed should be stored under dry conditions and the 1,4-butanediol moisture content controlled below 200 mg/kg. PBT resin for injection molding typically exhibits melt volume-flow rate 15–25 cm³/10 min at 250 °C/2.16 kg according to ISO 1133-1:2022; engineering grades with intrinsic viscosity 0.9–1.2 dL/g are used for electrical connectors and automotive under-hood parts where heat deflection temperature exceeds 160 °C after 30% glass-fiber reinforcement. Titanate residues can interact with carbon black or copper-based stabilizers; formulations should avoid amine-based heat stabilizers because these accelerate hydrolysis at processing temperatures above 250 °C.

The Terephthaloyl Chloride Route to para-Aramid Fiber Begins with Molten PTA Chlorination

For para-aramid polymer production, purified terephthalic acid is converted to terephthaloyl chloride using thionyl chloride or phosgene in the presence of a small amount of N,N-dimethylformamide catalyst at 80–120 °C. The resulting diacid chloride is distilled under vacuum; the specification for para-aramid condensation requires terephthaloyl chloride purity above 99.5% and hydrolyzable chlorine below 100 mg/kg. Polymerization with p-phenylenediamine occurs in N-methyl-2-pyrrolidone containing dissolved calcium chloride at temperatures between -5 °C and +5 °C in a low-temperature solution polycondensation. The stoichiometric imbalance between diacid chloride and diamine is maintained below 0.1 mol% to prevent end-group imbalance; an excess of either monomer rapidly limits molecular weight. The resulting poly(p-phenylene terephthalamide) solution exhibits anisotropic liquid-crystalline behavior above a critical concentration around 8–12 wt%. It is spun through an air gap into an aqueous coagulation bath at 0–10 °C using spinnerets with 50–100 µm orifices. Fiber tensile properties are measured according to ASTM D7269-19. The continuous process uses PTA as an indirect source; trace 4-carboxybenzaldehyde in PTA converts to mono-functional acid chloride and acts as a chain terminator, so the upstream oxidation step must deliver high-purity product. Temperature control in the polymerization vessel is critical: an adiabatic temperature rise above 15 °C during diacid chloride addition increases branching and gelling. Post-polymerization neutralization with calcium oxide removes hydrogen chloride and stabilizes the polymer solution for spinning.

In unsaturated polyester resins for corrosion-resistant fiberglass-reinforced plastic, terephthalic acid is used as the aromatic diacid component in a polyesterification with maleic anhydride and a glycol such as propylene glycol or neopentyl glycol. The reactor is a jacketed stainless-steel vessel equipped with a packed distillation column; the reaction proceeds at 180–220 °C until acid number falls below 25 mg KOH/g. The aromatic ring from paraxylene-derived terephthalic acid imparts higher glass-transition temperature and lower styrene solubility relative to orthophthalic resins, requiring styrene content to be adjusted downward by 5–10 wt% to achieve a workable viscosity of 300–600 mPa·s at 25 °C. Resin producers select terephthalic acid when the final composite must resist attack by dilute acids, fuels, or solvents; laminate testing under ISO 175:2010 can be specified. Curing uses methyl ethyl ketone peroxide at 1.0–2.0 phr with cobalt octoate accelerator at 0.2–0.5 phr. Exothermic temperature rise in thick laminates exceeding 120 °C can cause internal cracking, so filler loadings of 20–40 phr calcium carbonate or aluminium trihydrate are used to absorb heat and reduce shrinkage. The use of terephthalic acid in this segment is less sensitive to 4-carboxybenzaldehyde content than PET production, but residual catalyst metals above 20 mg/kg can affect gel time and cure stability.