Melt Viscosity Control in PBT Moulding Resins with PTA Butanediol Esterification

In continuous PBT production based on purified terephthalic acid and 1,4-butanediol, direct esterification proceeds as a heterogeneous reaction system in which solid PTA dissolves into molten oligomeric bis(4-hydroxybutyl) terephthalate while water and tetrahydrofuran are stripped through a rectification column. The BDO/PTA molar ratio is generally maintained between 1.15 and 1.40 because the acid-catalyzed dehydration of free 1,4-butanediol to THF consumes 1–4 mol% of the diol before the polycondensation stage; this side reaction shifts the effective stoichiometry and increases the carboxyl end-group burden if the diol excess is too low. Melt viscosity control in the downstream polycondensation reactor is therefore not a simple function of residence time and vacuum level but is coupled to the esterification stoichiometry, residual catalyst activity, dissolved water, and the thermal history of the melt. Polycondensation is normally conducted at 245–260 °C under absolute pressures below 1.5 mbar with titanium alkoxide catalyst added at 50–150 mg/kg as Ti; under these conditions, the prepolymer melt viscosity rises from 5–10 Pa·s at low conversion to 80–180 Pa·s at moulding-grade molecular weight when measured at 260 °C and 1000 s⁻¹ by capillary rheometry according to ISO 11443. The corresponding intrinsic viscosity measured in phenol/1,2-dichlorobenzene at 25 °C according to ISO 1628-5 is typically 0.80–1.20 dL/g for injection-moulding grades.

Esterification in the first paste reactor is intentionally incomplete, with target conversion of carboxyl groups commonly in the range 90–98%, because quantitative conversion at high temperature would increase THF formation and thermal degradation to unacceptable levels. Water generated during esterification is removed at column top pressures between 101 kPa and 130 kPa; the reflux ratio is adjusted to return 1,4-butanediol to the reactor while rejecting low-boiling THF and water overhead. A typical continuous esterification train includes a vertical paste preparation vessel, two to three horizontal agitated reactors with vapour disengagement, and a catalyst make-up point located after the first reactor. Reported operating temperatures decline from 240–250 °C in the first esterification reactor to 230–240 °C in the final oligomerization vessel; this decrease limits the acid-catalyzed cyclodehydration of butanediol. The oligomeric prepolymer entering the polycondensation reactor usually has a degree of polymerization below 10 repeat units and a carboxyl end-group concentration in the range 150–400 meq/kg; the final melt viscosity is subsequently developed by removing the excess diol and the condensation water under high vacuum.

Thermal Degradation Pathways in Butanediol-Rich Esterification Systems

Thermal degradation in PBT synthesis and compounding above 250 °C proceeds mainly through ester pyrolysis and β-hydrogen transfer, producing carboxyl end groups and unsaturated chain ends; these reactions reduce molecular weight and therefore lower melt viscosity. The acid-catalyzed dehydration of free 1,4-butanediol to THF continues during the early stages of polycondensation and generates water, which must be removed below 1.5 mbar to prevent hydrolytic chain scission. The kinetic competition between chain growth and degradation becomes critical when the melt temperature exceeds 260 °C; published thermal degradation kinetic data for poly(butylene terephthalate) report activation energies in the range 150–220 kJ/mol, with the rate constant depending on residual titanium catalyst and carboxyl end-group concentration. Because the melt viscosity at constant shear rate scales with weight-average molecular weight raised to approximately 3.4 power, a small number of chain scission events can produce a large reduction in capillary pressure drop and an increase in melt volume-flow rate. For moulding resins, the melt viscosity measured after a compounding or injection-moulding cycle is therefore a more sensitive indicator of thermal history than a single pellet intrinsic viscosity determination.

Carboxyl end-group concentration in dry moulding-grade PBT is commonly specified below 30 meq/kg when determined by potentiometric titration in o-cresol, adapted from ASTM D7409-15; higher values correlate with reduced hydrolysis resistance and with melt-volume-flow-rate drift during multiple processing passes. Phosphite and phosphonite antioxidants at loadings between 0.05 wt% and 0.30 wt% are added in the finishing section or during compounding to complex residual titanium and to scavenge hydroperoxides; triphenyl phosphate and tris(nonylphenyl) phosphite are frequently reported in public product formulations. The sequence of stabilizer addition matters because phosphorus compounds can deactivate the titanium catalyst if added too early in polycondensation; industrial practice places the antioxidant addition after the main polycondensation reactor or in the pelletizing feed zone. A poorly stabilized charge can display a melt viscosity reduction of more than 10% between the first and third processing heat history, whereas a stabilized resin typically shows ≤5% drift when measured by ISO 1133-1:2022 at 250 °C and 2.16 kg.

How Do Capillary Rheometry and ISO 1133-1:2022 Interrogate Moulding-Grade PBT?

Capillary rheometry according to ISO 11443 provides the apparent shear viscosity curve needed to distinguish between low-shear molecular-weight effects and high-shear processability; the standard measurement is often conducted at 260 °C with a capillary die having a length-to-diameter ratio of 16:1 and an entrance angle of 120° or 180°. PBT melts exhibit pronounced shear thinning, with the apparent viscosity falling from 150–300 Pa·s at 100 s⁻¹ to 50–150 Pa·s at 1000 s⁻¹ for medium-viscosity moulding grades; the power-law index is typically 0.60–0.80 over this shear-rate window. Melt volume-flow rate measured with ISO 1133-1:2022 at 250 °C and 2.16 kg is a single-point quality-control tool rather than a full viscosity measurement; it is nevertheless sensitive to molecular weight, moisture, residual catalyst, and thermal degradation. An increase in melt volume-flow rate from 15 cm³/10 min to 30 cm³/10 min indicates significant molecular weight loss or hydrolysis, and the corresponding reduction in melt viscosity can be confirmed by a capillary pressure drop that is 20–30% lower at constant shear rate. The test method requires drying of the specimen below 0.02 wt% moisture before measurement; otherwise the result is confounded by water-induced chain scission during the preheat.

Capillary rheometry data are also used to compute Bagley and Rabinowitsch corrections; because PBT melts are viscoelastic, the true shear viscosity at 1000 s⁻¹ is typically 5–15% lower than the apparent value after correction. Entry pressure loss from capillary rheometry provides an indication of extensional viscosity, which controls gate filling and weld-line formation in multi-cavity moulds. In-line rheometers and machine pressure transducers detect shot-to-shot viscosity changes that are not observable from pellet melt-flow tests alone; a progressive reduction in injection pressure at constant fill time indicates molecular weight loss, while an increase in pressure requirement may signal glass-fibre dispersion instability or moisture-induced volatiles.

Pelletized PBT moulding compound must be dried in a desiccant dryer to a water content below 0.02 wt% before compounding or injection moulding; typical drying conditions are 120–130 °C for 4–6 h with a dew point below -40 °C. In a twin-screw compounding line with an L/D ratio of 40:1, the barrel temperature profile is set between 240 °C and 270 °C, with screw speeds of 200–400 rpm and a residence time below 90 s to limit thermal degradation. Glass-fibre reinforcement at 30 wt% according to ISO 3451-1 raises the apparent melt viscosity at 1000 s⁻¹ by 30–70% relative to the unfilled base resin due to hydrodynamic and fibre-length effects; the exact increase depends on fibre diameter, sizing chemistry, and the degree of fibre breakage. Injection moulding of PBT is commonly performed at melt temperatures of 250–270 °C, mould temperatures of 80–120 °C, and injection pressures of 70–140 MPa; these conditions are used for parts with wall thicknesses from 0.5 mm to 4.0 mm. Mould temperature below 80 °C reduces crystallinity and can lower heat distortion temperature measured by ISO 75-2 at 0.45 MPa; mould temperature above 120 °C can extend cycle time without significant viscosity benefit.

Hot runners with large pressure drops can generate melt temperatures exceeding the barrel set point by 10–20 °C, creating local viscosity gradients that are not visible in standard melt-volume-flow-rate testing. Residual moisture above 0.04 wt% causes rapid hydrolysis during plastication, with a measurable drop in intrinsic viscosity after one injection-moulding cycle; the resulting surface defects and reduced mechanical strength are reflected in tensile tests according to ISO 527-2. On production machines, the melt temperature measured at the nozzle frequently exceeds the set barrel temperature by 5–15 °C due to viscous dissipation; this reduces melt viscosity and can shift the effective processing window by several degrees, particularly in thin-wall moulds where shear rates exceed 5000 s⁻¹.

When Residual Tetrabutyl Titanate Exceeds 150 mg/kg in the Melt

If the titanium alkoxide charge is allowed to exceed 150 mg/kg as Ti, the residual catalyst remains active after pelletization and promotes transesterification and hydrolysis during every subsequent melt-processing step. The melt viscosity then becomes unstable even when the pellet intrinsic viscosity is within specification: capillary pressure at constant shear rate can decline by 5–15% during a single compounding pass, and the carboxyl end-group concentration can rise because titanium-catalyzed transesterification consumes ester linkages without producing chain growth. Phosphorus-based stabilizers are used to complex residual titanium, but the stoichiometry must be adjusted carefully; an excess of phosphate or phosphite can reduce melt stability and colour, while an insufficient amount leaves active titanium that accelerates melt-volume-flow-rate drift. Published data for this specific configuration is limited, but industrial experience indicates that the residual titanium target for moulding resins should remain below 150 mg/kg as Ti and preferably below 100 mg/kg when high thermal cycling resistance is required. Operational boundaries include avoidance of amine-based additives unless specifically formulated for PBT, because basic species can catalyze transesterification and produce uncontrolled melt viscosity reduction. Pre-drying to 0.02 wt% moisture and limiting melt residence time above 260 °C to ≤10 min are necessary control points; combination with certain brominated flame retardants and antimony trioxide can further increase shear sensitivity and require a reduction in barrel temperature of 5–10 °C.