When 1,4-butanediol (BDO) is esterified directly with purified terephthalic acid (PTA) rather than with dimethyl terephthalate (DMT), the reaction liberates water instead of methanol and the thermodynamic equilibrium for the two-stage polymerisation shifts accordingly. The stoichiometric conversion C₆H₄(COOH)₂ + 2 HO(CH₂)₄OH ⇌ C₆H₄(COO(CH₂)₄OH)₂ + 2 H₂O produces bis(4-hydroxybutyl) terephthalate (BHBT) and two moles of water; subsequent polycondensation of BHBT releases additional BDO and water. Because water is less volatile than methanol at the process temperatures of 230–245°C, the esterification reactor must be operated with a dedicated vapour-separation system that removes water while returning entrained BDO to the reaction mass. PTA has a melting/sublimation point above 300°C and negligible solubility in BDO below 180°C; DMT melts at 141°C and forms a homogeneous solution with BDO at 170–190°C. The PTA route therefore requires slurry handling, whereas the DMT route can feed a homogeneous liquid. The direct esterification route eliminates methanol recovery and the associated distillation equipment, but it increases the acid-catalysed formation of tetrahydrofuran (THF) from BDO, which must be recovered or controlled by feedstock ratio and temperature profile. Published comparisons of continuous polyester technologies indicate that the choice between PTA and DMT for PBT production depends on feedstock price differentials, site methanol infrastructure, and the acceptable colour and end-group stability window for downstream applications. The kinetic and thermodynamic data for this specific configuration are available in public industrial literature, but numerical rate constants vary with catalyst type, reactor design, and impurity profile.

Carboxyl End Group Concentration Increases Non-Linearly Beyond 97% Esterification Conversion

In the titanium-catalysed direct esterification of PTA with BDO, the effective rate expression is typically approximated as r = k_app [COOH] [OH], where k_app incorporates catalyst concentration, temperature, and mass-transfer limitations. Published kinetic studies for titanium alkoxide-catalysed PTA-BDO esterification report apparent activation energies in the range of 55–70 kJ mol⁻¹, with the lower values associated with homogeneous melt-phase conditions after initial slurry dissolution and the higher values with the early heterogeneous stage. Titanium concentrations of 50–150 mg/kg Ti relative to PTA are typical in continuous PBT lines; higher catalyst loadings accelerate esterification but accelerate catalyst hydrolysis and titanium dioxide formation. As conversion of carboxyl end groups exceeds 97%, the concentration of remaining carboxyl groups falls below roughly 100–150 mmol/kg, and the reaction becomes diffusion-controlled because the melt viscosity of the oligomer phase rises sharply with increasing degree of polymerisation. The carboxyl end group concentration therefore decreases non-linearly: reducing it from 97% to 98.5% conversion may require more residence time than the reduction from 90% to 97%. Concurrently, the acid-catalysed intramolecular dehydration of BDO to THF follows a rate that increases with protonated BDO concentration and temperature. Industrial control loops on continuous esterifiers typically hold the esterification temperature setpoint within ±3°C because a rise from 235°C to 245°C can increase THF formation by approximately 30–40% relative to the lower setpoint, while a drop below 230°C slows conversion and increases residual PTA solids. The result is a narrow processing window in which the esterification reactor operates between 230°C and 245°C and the pre-polycondensation stage begins only after solid PTA disappears from the reaction mass. Water content in the BDO feed above 500 mg/kg hydrolyses the titanium alkoxide catalyst to insoluble titanium dioxide, which increases filter pressure drop and reduces catalytic activity; BDO storage and feed lines are therefore blanketed with dry nitrogen and pre-dried to moisture below 50 mg/kg where high conversion is required.

Continuous melt-phase PBT trains using the direct esterification route discharge PTA from a silo through a rotary valve into a loss-in-weight feeder and combine it with preheated BDO at 150–170°C in a stirred slurry vessel fitted with an anchor agitator and an external recirculation loop. The slurry typically contains 20–30 wt% PTA and is pumped to the esterification reactor using a progressive cavity pump with heated jacketed lines to prevent BDO solidification. The first esterification reactor is a vertical disc ring reactor with a horizontal agitator, internal heating coils, and a vapour disengagement dome; the reactor pressure is maintained between 300 mbar and 700 mbar absolute to facilitate water removal while minimising BDO losses, and the vapour is condensed in a partial condenser that refluxes BDO to the reactor. Average residence time in the esterification stage is 2.5–4.0 h, and the discharged oligomer has a degree of polymerisation between 5 and 15. The oligomer is then transferred to a pre-polycondensation vessel operating at 245–255°C and 50–100 mbar absolute, followed by a horizontal finishing reactor at 250–260°C and 1–5 mbar absolute. The finishing reactor uses a rotating cage of perforated discs with a length-to-diameter ratio of 4:1 to 6:1 to generate surface renewal for efficient BDO removal; the polymer melt is discharged by a gear pump through a screen pack with filtration rating of 20–40 µm and pelletised under water. Batch-to-batch variation in PTA bulk density and particle size distribution alters slurry rheology; a median particle size d50 of 120–150 µm produces a stable slurry with viscosity of 500–1200 mPa·s at 160°C, whereas finer particles increase slurry viscosity and can blind the feed filter screens. Experienced production lines compensate by adjusting the BDO-to-PTA molar ratio between 1.05:1 and 1.30:1 to account for THF losses, and by monitoring the online near-infrared spectrum of the slurry to detect changes in PTA solid content before the esterification reactor.

Why Does 4-Carboxybenzaldehyde in PTA Govern Pre-Polymer Colour and Melt Stability?

The polymer-grade PTA specification places an upper limit of 25 mg/kg on 4-carboxybenzaldehyde (4-CBA) because this impurity introduces a mono-functional aldehyde group that cannot extend the polyester chain and, under melt-phase temperatures, participates in condensation side reactions that generate conjugated chromophores. The aldehyde group in 4-CBA can undergo aldol condensation with other carbonyl species, producing unsaturated structures that absorb visible light and increase the CIE b* yellowness value of the polyester. In parallel, p-toluic acid, which is present at up to 150 mg/kg in industrial PTA, acts as a monofunctional chain terminator that reduces number-average molecular weight at a given carboxyl end group concentration. The DMT transesterification route largely avoids 4-CBA because DMT is purified by distillation; its primary acidic impurity is monomethyl terephthalate, which has a different reactivity and colour impact. Quantitative published data correlating exact 4-CBA concentration with PBT intrinsic viscosity loss is limited, but industrial quality records indicate that PTA with 4-CBA below 10 mg/kg yields melt-phase polyester pellet b* values in the range of 0.5–1.5, whereas PTA with 4-CBA near 35 mg/kg can shift pellet b* above 2.5. The total metals content of PTA is also controlled below 5 mg/kg because residual iron, cobalt, or manganese catalyses oxidative degradation during crystallisation and SSP. Colour of the polymer is measured by ISO 11664-4; 4-CBA and p-toluic acid in PTA are determined by ASTM D7884-13; moisture in PTA is measured by ISO 15512:2019. The following table summarises typical polymer-grade feedstock specifications relevant to the PTA substitution.

For PBT grades requiring intrinsic viscosity above 1.10 dL/g, the amorphous melt-phase resin is pelletised, crystallised at 140–160°C in a continuous crystalliser, and then subjected to solid-state polymerisation (SSP) in a shaft dryer with countercurrent nitrogen at 200–215°C. Intrinsic viscosity is determined according to ISO 1628-5:2018 using a 0.5 g/dL solution in a 60:40 by weight mixture of phenol and 1,1,2,2-tetrachloroethane at 30°C; carboxyl end group concentration is measured by ASTM D7409-15, and melt flow rate is tested per ISO 1133-1:2022 at 250°C with a 2.16 kg load. Typical injection-moulding grades have an intrinsic viscosity of 0.85–1.00 dL/g, melt flow rates of 10–30 g/10 min, and tensile strength of 50–60 MPa when tested according to ISO 527-2; extrusion grades for pipe and film have intrinsic viscosities of 1.20–1.30 dL/g and lower melt flow rates. Before injection moulding, pellets are dried in a desiccant drier at 120°C for 4 h to a moisture content below 0.02 wt% because hydrolytic degradation at melt temperatures above 260°C causes rapid intrinsic-viscosity loss and regeneration of BDO and THF. Injection moulding of thin-wall connectors typically uses clamp forces of 800–1200 kN for four-cavity tools and melt temperatures of 240–260°C. The maximum recommended melt residence time in an injection-moulding barrel at 250°C is below 10 min; longer residence times produce black specks and carboxyl end group increases measurable by ASTM D7409-15. These limits are particularly important when glass-fibre-reinforced PBT compounds are processed on co-rotating twin-screw extruders with an L/D ratio of 40:1 to 44:1, because high shear and local temperature overshoot can accelerate thermal degradation.

When Titanium Alkoxide Is Replaced by Antimony Trioxide in High-Temperature Esterification

Direct esterification of PTA with BDO in a titanium-catalysed melt phase is typically conducted with tetrabutyl titanate at a concentration of 50–150 mg/kg Ti relative to PTA; substitution of antimony trioxide, which is common in polyethylene terephthalate (PET) polycondensation, alters the temperature profile and product colour balance. Antimony trioxide is less active for BDO esterification at the temperature window of 230–245°C and requires higher finishing temperatures, typically 270–280°C, to achieve the same polycondensation rate; at these temperatures BDO dehydration to THF increases sharply and the polyester yellows. Antimony-reduced grey discolouration is also observed when antimony trioxide is used at concentrations above 200 mg/kg Sb in the presence of residual aldehyde groups. Organotin compounds, such as monobutyltin oxide, catalyse esterification at lower temperatures but can increase THF formation and may be restricted for food-contact applications by migration limits under EU 10/2011. The titanium alkoxide system itself is moisture-sensitive: water from PTA and BDO hydrolyses the alkoxide to titanium dioxide, which remains as insoluble haze and shortens polymer filter life from approximately 8 weeks to 2 weeks when feed moisture doubles. Therefore BDO is pre-dried to less than 50 mg/kg moisture and PTA to less than 0.2 wt% before slurry preparation. The combination of titanium alkoxide residues with amine-based additives such as hindered amine light stabilisers in the same melt step is avoided because coordination of amine nitrogen to titanium can deactivate catalyst residues and reduce melt stability; such additives are introduced only during a separate compounding step after catalyst residues have been purged or capped. If DMT is partially reintroduced into a PTA-based melt stream, the esterification reactor must be configured for both methanol and water removal, and the vapour system cannot operate with a single condensation train; published data for this specific mixed-feedstock configuration is limited. The upper temperature boundary for the finishing reactor is therefore set at 260°C, and melt residence time above 255°C is restricted to less than 20 min in standard continuous lines.