Within the semicrystalline polyester family, the operational distinction between Polybutylene Terephthalate (PBT) and bottle-grade polyethylene terephthalate (PET) is governed by crystallisation kinetics and melt rheology rather than by the presence of the terephthalate linkage alone. PBT is polymerised from dimethyl terephthalate and 1,4-butanediol; the four-methylene flexible spacer permits rapid chain folding from the melt. Polybutylene Terephthalate vs PET: One Moulds in Seconds, the Other Becomes a Bottle—this distinction defines the processing route, tooling economics, moisture tolerances, and mechanical testing requirements for each polyester. Commercial unfilled PBT grades exhibit a melting temperature of 220–225 °C and a glass transition temperature of 45–60 °C when measured under ISO 11357-2:2020 and ISO 11357-3:2018. The rapid crystallisation permits ejection from tools at 80–110 °C, whereas bottle-grade PET requires higher mould temperatures and longer holding times unless nucleating agents are compounded into the resin.

Polybutylene Terephthalate vs PET: One Moulds in Seconds, the Other Becomes a Bottle

The short cycle time of PBT is not primarily a function of melt fluidity; it is the rate at which the resin attains crystallinity sufficient for part ejection. Isothermal crystallisation data under ISO 11357-7:2022 show that PBT reaches its maximum crystallisation rate at roughly 170–185 °C, and unfilled grades can be ejected from thin-wall moulds within 5–15 s. By contrast, bottle-grade PET without nucleators crystallises slowly, requiring mould temperatures above 130 °C and cycle times often exceeding 30 s for similar wall sections. PET is therefore converted as an amorphous preform and subsequently biaxially oriented into a bottle, while PBT is injected directly into a crystallising tool.

Mould temperature control units with closed-loop oil or pressurised water circuits are set between 60 °C and 110 °C depending on part thickness and filler content. For unfilled thin-wall components, a tool temperature of 60–80 °C is common; glass-reinforced PBT at 15–30 wt% glass requires 90–110 °C to control surface finish and crystalline morphology. Hot-runner manifolds are maintained at 250–260 °C, and valve-gate systems are used to move weld lines away from load-bearing sections. Injection pressures for unfilled grades normally fall between 60 MPa and 100 MPa, while glass-filled grades require higher packing pressure to compensate for fibre orientation and shrinkage anisotropy.

Why Does Crystallisation Rate Dictate Mould Cycle Time?

In short-cycle injection moulding, the limiting bottleneck is not injection velocity but the time required for the polymer to reach a crystalline solid fraction sufficient for ejection. PBT achieves this rapidly because its molecular chain contains four methylene groups between ester linkages, granting backbone mobility that accelerates chain folding. Differential scanning calorimetry data generated under ISO 11357-7:2022 isothermal crystallisation methods show that PBT crystallises at rates suitable for cycle times of 5–15 s in thin-wall connectors; bottle-grade PET without nucleators requires hot mould temperatures above 130 °C and cycle times frequently exceeding 30 s for comparable wall sections. The divergence explains why PBT is injection moulded, while amorphous or strain-hardened PET is stretch blow moulded into containers.

Mechanical Benchmarks Under ISO 527-2 and ISO 178

When comparing tensile data across PET and PBT, the key variables are not just molecular weight but also crystallinity, orientation, and moisture conditioning. Commercial unfilled PBT datasheets generally list tensile stress at yield of 50–60 MPa and flexural modulus of 2.3–2.7 GPa when tested according to ISO 527-2:2012 or ASTM D638-14 at 23 °C. Bottle-grade PET in the amorphous state has similar tensile strength, but the biaxially oriented bottle wall develops much higher tensile modulus in the hoop direction. Glass-fibre reinforcement at 30 wt% raises PBT tensile strength to 90–140 MPa and flexural modulus to 7–10 GPa, depending on fibre length retention and coupling chemistry. Notched Izod impact values under ISO 180:2019 are 4–6 kJ/m² for unmodified PBT, but toughened grades can exceed 10 kJ/m².

The semicrystalline structure of PBT is also measured by heat of fusion under ISO 11357-3:2018; unfilled injection-moulded parts typically show a heat of fusion of 45–55 J/g, corresponding to a crystalline fraction of 30–40% when referenced to the theoretical heat of fusion. In comparison, PET bottle walls are biaxially oriented and strain-hardened in the amorphous state; the bottle wall does not rely on thermal crystallisation for its hoop strength.

Moulded PBT parts can exhibit post-mould shrinkage of 0.3–0.7% in the flow direction and 1.0–1.5% transverse to flow for glass-reinforced grades depending on fibre orientation. Dimensional checks after 48 h conditioning under ISO 291:2008 are specified when parts must fit into sealed automotive housings. The rapid crystallisation of PBT tends to freeze in less molecular orientation than PET, but anisotropic glass orientation remains the dominant contributor to warpage.

Electrical connector bodies moulded from PBT are tested for dielectric strength under IEC 60243-1:2013; unfilled grades typically exhibit 20–25 kV/mm at 1 mm thickness. Because PBT retains low equilibrium moisture at 0.1–0.2%, insulation resistance remains stable under humid conditions relative to polyamide resins. Unmodified PBT is classified as UL 94 HB. Flame-retarded grades compounded with halogenated or halogen-free additives are used where UL 94 V-0 certification is required, and the comparative tracking index is measured under IEC 60112:2003 to verify reliability in creepage-sensitive connector designs.

In electrical motor brush holders and bobbins, PBT is selected because the moulded part must retain insulation after thermal ageing and vibration. The test programme includes heat ageing at 150 °C for 500 h followed by dielectric strength measurement under IEC 60243-1:2013. If the dielectric strength falls below 15 kV/mm, hydrolysis stabilisers or alternative resins are evaluated. This sequential test is more relevant than single-point data because it mirrors the combined thermal, oxidative, and mechanical stress seen in wound components.

If the Part Must Survive Underhood, Thermal Oxidative Resistance Changes the Stabiliser Suite

PBT is selected for automotive connectors, ignition housings, and sensor bodies because it withstands continuous use temperatures of 120–140 °C. Long-term heat ageing under ISO 2578 is used to establish relative thermal index values, but the result depends on glass loading, stabiliser package, airflow, and part thickness. Unfilled PBT exposed to air at 140 °C may retain tensile strength for several thousand hours, whereas contact with hot engine oil, calcium chloride road salts, or brake fluid changes the failure mode. In underhood electrical systems, a 30 wt% glass-reinforced PBT compound may achieve a comparative tracking index of 250 V to 600 V depending on flame-retardant additives; the exact value must be verified by the compounder under IEC 60112:2003.

Chemical resistance is usually assessed by immersion testing under ISO 175:2021. PBT resists aliphatic hydrocarbons, mineral oils, esters, and many alcohols at room temperature, but strong bases and hot concentrated acids attack the ester linkage. Fuel-system exposure to methanol or sour gasoline requires volume swell below 5% after 7 days at 60 °C; published data for this specific configuration is limited when the fuel blend contains peroxides or dissolved water.

Melt stability is not a single failure mode in hot-runner production.

Molecular weight degradation in PBT occurs by thermal decomposition and hydrolytic chain scission. The ester linkage reacts with water to form carboxylic acid and alcohol end groups, and the carboxyl end groups autocatalyse further hydrolysis. For this reason, PBT must be dried to residual moisture below 0.02% before melt processing. Desiccant dryers with a dew point of −40 °C to −50 °C and air temperature of 120–140 °C for 3–4 h are standard. PET bottle grades require higher drying temperatures of 140–160 °C and longer residence because moisture above 0.004% can hydrolyse the polyester in the barrel and reduce intrinsic viscosity before blow moulding.

Regrind ratios for unfilled PBT are normally limited to 20–30% because repeated extrusion lowers intrinsic viscosity and increases carboxyl end-group concentration. Higher regrind fractions reduce melt viscosity and can cause sink marks, short shots, or a loss of impact strength under ISO 180:2019. In glass-reinforced compounds, regrind addition also reduces average fibre length, and the resulting notched impact strength may drop by more than 20% compared with virgin compound. The melt volume-flow rate, measured under ISO 1133-1:2022 with a 2.16 kg load at 250 °C, is used to track lot-to-lot consistency; a shift from 15 cm³/10 min to 25 cm³/10 min indicates molecular weight loss.

Glass-fibre-reinforced PBT is typically compounded on a co-rotating twin-screw extruder with an L/D of 40:1 to 44:1 and segmented screws that allow downstream feeding of glass roving. The base resin is melted before the side-stuffer to prevent excessive screw wear and fibre attrition. Barrel zones are set between 240 °C and 270 °C, with vacuum venting at −0.06 MPa to −0.08 MPa to strip residual moisture and volatiles. Hydrolysis stabilisers and epoxy-based chain extenders are dosed at 0.1–0.5 wt% when the compound must survive warm, humid service conditions.

In automotive door-lock housings, PBT is moulded at a wall thickness of 1.5 mm with a cycle time of 12 s; the same tool would not produce a dimensionally stable bottle-grade PET part because the crystallisation rate is insufficient to build stiffness before ejection. The moulded housing is then conditioned for 48 h at 23 °C and 50% RH under ISO 291:2008 before dimensional inspection and creep testing at 80 °C. This application demonstrates the limits of the comparison: PET can be injection moulded, but only with nucleating agents, higher mould temperatures, and longer cycles that erase the economic rationale for thin-wall connector production.