Reading a Polybutylene Terephthalate Datasheet: Six Numbers That Decide Survival

Compounding lines that switch from unreinforced to glass-fiber-reinforced PBT often expose a datasheet-reading problem: six numerical fields are extracted without tying them to survival in the mould, in the application environment, and under electrical load. The six fields are melt volume-flow rate, tensile stress at break, flexural modulus, notched Izod impact strength, heat deflection temperature, and comparative tracking index. A purchasing specification may list only 30% glass fiber and a UL 94 V-0 rating, but the part fails when the melt volume-flow rate cannot fill a 0.8 mm wall or when the comparative tracking index drops below 400 V in a high-density connector. The following technical discussion reads these six numbers against the processing and end-use conditions that determine whether a PBT component survives.

What Does Melt Volume-Flow Rate Predict About Thin-Wall Filling and Hydrolytic Degradation?

Melt volume-flow rate is not a viscosity surrogate; it is a single-point flow measurement under low shear that flags molecular weight and processability. Under ISO 1133-1:2022, PBT is tested at 250°C with a 2.16 kg load. Commercial unfilled grades typically report 12–25 cm³/10 min, while 30% glass-fiber-reinforced grades fall to 7–15 cm³/10 min. For wall sections below 0.8 mm, an MVR below 10 cm³/10 min produces short shots on 60–80 tonne injection moulding machines with 20–25 mm plasticising screws. But an MVR above 30 cm³/10 min in unfilled PBT usually indicates severe molecular weight reduction from hydrolytic chain scission. PBT hydrolyzes during processing if pellet moisture exceeds 0.02%. Pre-drying in desiccant dryers at 120°C for 4 h is required at ambient relative humidity above 60%. MVR tests on undried pellets show artificially high values because water attacks the ester linkage at barrel temperatures of 245–270°C. Production-scale twin-screw extruders with L/D ratios of 32:1 to 44:1 compound glass-fiber-reinforced PBT with residence times below 10 min; longer residence at the upper temperature limit shifts molecular weight distribution toward lower viscosity and lowers tensile and impact properties. The melt volume-flow rate therefore acts as both a filling capability indicator and a hydrolysis alarm.

Interpreting tensile stress at break without simultaneous elongation data creates a false picture of PBT suitability for snap-fits and living hinges. Under ISO 527-2:2012, unmodified PBT at 23°C yields 50–60 MPa tensile stress at break, but the same specimens may show 50–120% nominal strain. 30% glass-fiber grades invert this profile: 90–130 MPa tensile stress with 2–4% strain. A designer selecting a flame-retarded grade with only 45 MPa tensile stress and 3% strain may still accept it if the part is rigid, but the same number fails in a snap-fit because the outer-fiber strain exceeds the tested breaking strain. Weld lines reduce tensile stress by 30–50% in unreinforced PBT, yet published data for weld-line tensile strength in specific connector geometries is limited. Tensile test frames with non-contact extensometers measure elongation to break; the extensometer gauge length must follow the ISO 527-2 type 1A specimen geometry. A datasheet that reports tensile stress at break without elongation or weld-line data cannot be used to qualify a PBT grade for a living hinge or snap-fit application. When a part is conditioned at 50% relative humidity per ISO 291, the polar ester group absorbs moisture and can reduce tensile strength by 5–10% compared with dry-as-moulded values.

Flexural Modulus, Creep, and Moisture-Conditioned Stiffness

Flexural modulus values on PBT datasheets are obtained by three-point bending under ISO 178:2019 at 2 mm/min. Unfilled PBT reports 2.3–2.7 GPa; 30% glass-reinforced grades report 7.5–9.5 GPa. Stiffness-based design for a cantilever snap-fit uses the modulus to compute deflection, but the short-term flexural modulus is not a linear design modulus under continuous load. Creep data under ISO 899-2 are required when the part experiences sustained flexural stress. In service above 60°C, unreinforced PBT approaches its glass transition, and stiffness drops more rapidly than the room-temperature datasheet value suggests. Moisture-conditioned samples absorb less than 0.2% water, but cyclic moisture exposure combined with flexural load can produce stress cracking at gate vestiges or sharp corners. Moulding trials on 100 mm × 100 mm × 2 mm plaques with 0.5 mm edge gates show that glass-fiber orientation parallel to flow can raise flexural modulus in the flow direction but reduce transverse stiffness by 20–30%. Published data for anisotropic flexural modulus in specific PBT grades is limited, so datasheet values should be treated as flow-direction baselines unless transverse testing is performed.

A notched Izod value alone misleads when the failure mode changes from ductile to brittle across mould temperature and moisture content. Under ISO 180:2019, notched Izod impact strength for unfilled PBT typically falls between 4 kJ/m² and 6 kJ/m² at 23°C using a Type A notch with 2 mm depth and 0.25 mm tip radius. Glass-reinforced PBT can reach 7–11 kJ/m², but the fracture propagates differently through the fiber-matrix interface. Low mould temperature produces underdeveloped spherulites and brittle skins; PBT injection moulding generally requires mould temperatures of 60–100°C to stabilize crystalline morphology. A datasheet notched Izod value generated on dry-as-moulded specimens may be higher than the value obtained after conditioning at 50% relative humidity because moisture plasticizes the ester group but also reduces glassy state brittleness. The transition is not linear, and a grade that passes at 23°C may fail at -30°C with notched Izod values below 2 kJ/m². Impact-critical PBT parts therefore require low-temperature notched Izod data, not only room-temperature values.

Heat Deflection Temperature Is Misread When Load and Specimen Orientation Are Ignored

Published heat deflection temperature values apply only at a specified outer-fibre stress of 1.8 MPa and a heating rate of 120 K/h under ISO 75-2:2013 method A. Unfilled PBT typically shows HDT/A of 55–65°C, while 30% glass-fiber-reinforced grades show 195–210°C. Comparing HDT values between grades is valid only when the load, specimen thickness, and flatwise or edgewise orientation are identical. A datasheet citing only HDT/B at 0.45 MPa misleads for under-hood brackets where the part experiences continuous flexural stress. Dynamic mechanical analysis under ISO 6721-7 gives a tan δ peak in the 45–60°C range for unfilled PBT, indicating that short-term thermal resistance is already limited before the HDT/A value. The glass-reinforced PBT HDT/A value above 200°C creates a false impression of continuous use temperature; PBT above 120°C in oxidative environments undergoes thermal oxidative embrittlement unless stabilizer packages are specifically formulated. A datasheet HDT value is therefore a comparative single-point ranking, not a safe continuous-use temperature.

When Comparative Tracking Index Drops Below 400 V in High-Density Connectors

When comparative tracking index falls below 400 V, creepage distances in high-density connectors must be re-evaluated against IEC 60664-1. PBT tracking resistance is measured under IEC 60112:2020; unreinforced, non-flame-retarded PBT can achieve 600 V, while glass-filled and halogen-free flame-retarded grades may report 250–400 V. A value below 250 V places the material in material group IIIb and increases required creepage by 25–40% depending on pollution degree. Flame performance under UL 94 V-0 at 0.8 mm is often read alongside CTI, but the two values are not independent: halogenated flame retardants can raise flame resistance while depressing tracking resistance. Electrical failures in sealed relay housings occur when the CTI is taken from an unreinforced grade and applied to a flame-retarded 30% glass-filled compound without retesting. The test uses two platinum electrodes with a 4 mm spacing and 50 drops of 0.1% ammonium chloride solution; failure at 50 drops defines the CTI voltage. A datasheet that omits CTI for a flame-retarded PBT connector compound leaves creepage and clearance design without a usable electrical safety boundary.