Polybutylene terephthalate (PBT) is a semi-crystalline aromatic polyester with a melting point typically between 220 °C and 230 °C and a glass transition temperature commonly reported at 45–60 °C. It is used in automotive connectors, sensor housings, ignition components and industrial enclosures where hot-spot temperatures approach 150 °C. Selecting a PBT grade for continuous service at 150 °C requires separation of short-term thermal properties from long-term thermal endurance data. The relevant evaluation chain includes the relative thermal index assigned under UL 746B, heat-ageing response measured in forced-air ovens, and the margin between the mechanical-with-impact RTI and the peak service temperature of the moulded part. Heat deflection temperature and differential scanning calorimetry results do not provide adequate evidence for long-term oxidative stability in this service window.

Does UL 746B RTI define a continuous use temperature?

The relative thermal index is not a direct continuous use rating for a finished part. It is a comparative temperature index derived from long-term thermal ageing of standard specimens. Under UL 746B, RTI values are assigned for three property classes: RTI Electrical, RTI Mechanical without Impact, and RTI Mechanical with Impact. Each value is thickness-dependent and colour-dependent. A 0.8 mm natural grade may exhibit a different RTI from a 1.5 mm black grade of the same base resin. The rating represents the temperature at which the material retains 50% of the original selected property after 100 000 h of thermal ageing, extrapolated by Arrhenius regression from elevated-temperature oven data.

For structural parts operating at 150 °C, the RTI Mechanical with Impact value is frequently the limiting rating. Unfilled and impact-modified PBT grades often show mechanical-with-impact RTI values 20–30 K below the electrical RTI. Glass-reinforced heat-stabilized grades narrow this gap but may still have an impact RTI below the hot-spot target. RTI Electrical alone is not sufficient for load-bearing connectors, snap-fits or brackets because insulation integrity can be retained after the polymer has embrittled. Grade selection must therefore be based on the lowest relevant RTI for the actual stress and impact condition, not on the most favourable electrical index.

Heat ageing, failure criteria and Arrhenius regression

Thermal endurance programmes for PBT use multi-temperature ageing in forced-air ovens following the principles of IEC 60216-1 and UL 746B. Tensile bars, impact specimens and dielectric plaques are aged at three or four temperatures. The lowest ageing temperature is normally selected so that failure occurs between 5 000 h and 10 000 h, allowing extrapolation to 100 000 h without excessive departure from the experimental range. At each pull point, tensile properties are measured according to ISO 527-2 or ASTM D638-14, impact properties according to ISO 179-1 or ASTM D256, and dielectric strength according to ASTM D149 or IEC 60243-1.

The failure criterion is commonly a 50% absolute loss of the selected property, but this endpoint is not universally adequate for design. In snap-fit connectors, a loss of 30% in notched impact strength may already cause brittle fracture during assembly or vibration. Elongation at break is more sensitive to molar mass loss than tensile strength and can decline rapidly at 150 °C before the tensile strength reaches the 50% endpoint. A grade with an acceptable tensile RTI can therefore fail impact-limited service far earlier than the RTI suggests.

The extrapolation assumes a linear relationship between the logarithm of failure time and reciprocal absolute temperature. For PBT, apparent activation energies for thermo-oxidative degradation have been reported in the range 80–120 kJ mol⁻¹. A change in slope in the Arrhenius plot indicates a transition in degradation mechanism, such as stabilizer depletion followed by uncontrolled oxidation. Extrapolation below the lowest ageing temperature is valid only if the regression remains linear and no transition is observed. For 150 °C service, the lowest ageing oven is often at 160 °C or 170 °C, so the margin between experimental data and service temperature is narrow and requires close attention to scatter in the regression.

Specimen thickness also changes heat-ageing behaviour. Thin sections are more oxidation-limited because oxygen can penetrate the complete cross-section. Thick sections degrade from the surface inward, creating a skin-core oxidative profile. A 3.0 mm specimen may therefore show longer property retention than a 0.8 mm specimen under identical oven conditions. This is one reason RTI values are assigned per thickness and cannot be transferred between wall sections without verification.

In a 30 wt% glass-fibre reinforced PBT connector moulded on a 1 500 kN clamp force injection machine, production data show that processing history directly influences 150 °C heat-ageing margin. At the compounding step, a 40:1 L/D twin-screw extruder with atmospheric and vacuum venting is used, and glass fibre is side-fed after the polymer is molten to limit fibre attrition. Melt temperature at the die is held at 255–265 °C. Pre-drying is performed at 120 °C for 4 h in a desiccant dryer with a dew point at or below -30 °C, targeting residual moisture below 0.02 wt%. Mould temperature is controlled at 100–120 °C to stabilise crystallinity and reduce post-moulding shrinkage during subsequent thermal exposure.

Residence time in the barrel is kept below 5 min. Holdup in hot-runner dead spots produces black specks and lowers intrinsic viscosity from approximately 0.85 dL/g to 0.70 dL/g or lower when measured per ASTM D4603. This molar mass loss reduces the starting elongation and impact reserve of the moulded part. In subsequent 150 °C air-oven ageing, such parts can reach brittle failure long before a properly processed specimen of the same compound. Weld lines are an additional field variable. In glass-reinforced PBT, weld-line impact strength can be 30–50% lower than the bulk value, and standard ageing specimens without weld lines may overestimate part life.

When the hot spot is 150 °C, impact margin controls grade selection

The margin is calculated as the difference between the RTI Mechanical with Impact value for the actual production thickness and colour and the peak continuous hot-spot temperature under mechanical load. A part at 150 °C moulded from a grade with an impact RTI of 140 °C operates 10 K above its rating. No positive margin exists. Industrial practice for automotive and electrical connectors frequently requires an additional 10–15 K margin to account for compound batch variation, regrind content, filler orientation, weld-line location, and oven calibration scatter during thermal endurance testing.

The following representative screening values are compiled from publicly accessible UL Yellow Card entries for 0.8 mm natural PBT compounds. They are not specification values and must be confirmed for the exact production colour, thickness and supplier.

Black or laser-marked grades may carry RTI values 10–20 K lower than their natural equivalents because pigments and marking additives alter the oxidative stability of the matrix. If the production part contains regrind, the RTI cannot be assumed to remain unchanged without comparative ageing data. In the above screening table, no entry provides a positive impact margin at 150 °C. A high-heat grade with an impact RTI of 145 °C still leaves a -5 K gap. This means a 150 °C continuous service snap-fit or spring-arm design cannot be qualified solely on published RTI values unless the mechanical stress is reduced, the peak hot spot is lowered by thermal isolation, the wall thickness is increased, or a higher-temperature polymer is selected.

For purely compressive or low-impact applications, the RTI Mechanical without Impact value may be used as the design basis. In that case, some 30 wt% glass-filled high-heat PBT grades approach the required 150 °C rating, but the margin remains small. The designer should verify whether the actual part hot spot is truly 150 °C and not higher. Thermocouple data or thermal imaging on production parts is required; air temperature is not equivalent to surface temperature.

The following minimum screening matrix is applied when evaluating a PBT grade for 150 °C continuous service where published RTI data leave a negative or near-zero margin.

At 150 °C, humidity must be considered separately from dry-air RTI. Hydrolysis of the ester linkage competes with thermo-oxidative degradation when moisture is present. Under-hood and humid industrial environments can produce earlier embrittlement than dry-air ageing suggests. Hydrolysis-resistant PBT grades and designs that limit moisture ingress at the hot surface are required for such service. If the application is an electrical connector exposed to thermal cycling, the design should also account for post-crystallisation and the resulting shrinkage or dimensional change during initial service hours.

Published multi-point ageing data for exact production colours at 150 °C and 0.8 mm thickness are limited. Supplier UL Yellow Cards for the specific grade, colour and wall thickness remain the authoritative source, supplemented by part-level ageing because weld lines, gate orientation and moulded-in stress change the oxidation response. Where no material offers a positive impact margin at 150 °C, the part cannot be qualified for a 100 000 h service life with that design. The remaining alternatives are thermal derating, reduction of impact stress, increased section thickness, shortened service life with verified Arrhenius regression, or substitution with a higher-temperature polymer such as PPS or PPA for the affected component.