Polybutylene terephthalate (PBT) is a semi-crystalline thermoplastic polyester produced by polycondensation of 1,4-butanediol with purified terephthalic acid or dimethyl terephthalate. The melting point of standard injection-molding grades is 223 °C to 225 °C, and the density is 1.30 g/cm³ to 1.32 g/cm³ when tested under ISO 1183-1:2019. Typical melt processing temperatures are 240 °C to 270 °C, with mold temperatures between 60 °C and 90 °C. Unfilled PBT absorbs 0.08% to 0.10% moisture after 24 h at 23 °C under ISO 62:2008, and 30% glass-fiber-reinforced PBT absorbs 0.05% to 0.07%. The material is specified for electrical connectors, relay bases, and sensor housings because it combines rapid crystallization, a comparative tracking index near 600 V in many unfilled non-flame-retarded formulations under IEC 60112:2020, and resistance to aliphatic hydrocarbons in immersion screening conducted according to ASTM D543-21.

Your Polybutylene Terephthalate Part Failed After Eighteen Months: Tracing a Claim

An 18-month field return requires separation of external chemical attack from intrinsic degradation before assigning responsibility. The claim file opens with a retained control from the same production lot stored in a sealed aluminum laminate at 23 °C and 50% RH. Tensile properties are measured under ISO 527-1:2019 and ISO 527-2:2012 at 5 mm/min for rigid injection-molded specimens. Melt volume rate is determined at 250 °C with a 2.16 kg load under ISO 1133-1:2022. Intrinsic viscosity is measured in a 60/40 phenol/1,2-dichlorobenzene solvent system according to ISO 1628-5. The first diagnostic question is whether the part has lost molar mass, oxidized, or cracked from an oversized insert or excessive assembly torque. Morphology is examined on a scanning electron microscope after gold-palladium sputter coating; fracture surfaces from embrittled PBT typically show relatively smooth crack-origin zones with limited shear lips.

Table 1 — Comparative property windows for PBT injection-molding grades.

Is hydrolytic chain scission the primary degradation mechanism in an 18-month underhood connector?

When the failure site is a connector shell exposed to road splash, hydrolysis should be investigated before impact overstress. PBT contains aliphatic ester linkages that undergo hydrolytic scission, and the reaction is accelerated by temperature, retained moisture, and acidic species. A molded PBT part pre-dried at 120 °C for 4 h to below 0.02% moisture retains its molar mass during molding, but an in-service seal leak can introduce liquid water at a local temperature of 80 °C to 110 °C. Under these conditions, chain scission reduces the number-average molecular weight within 18 months. The change is tracked by melt volume rate under ISO 1133-1:2022; an increase greater than 25% relative to the retained control indicates meaningful chain shortening. Intrinsic viscosity under ISO 1628-5 commonly decreases by 10% to 30% in severely hydrolyzed parts. Tensile strength at break falls below 60% of the as-molded value, and elongation at break drops below 2% in unfilled material that originally yielded 5% to 10%.

For a 40 mm twin-screw compounding extruder with an L/D ratio of 32:1 to 44:1, barrel zones are typically set to 240 °C to 260 °C, screw speed 300 rpm to 600 rpm, and melt temperature measured at the die by an immersion thermocouple. Hydrolysis during compounding is controlled by predrying PBT pellets at 120 °C for 4 h to a moisture content below 0.02%; a twin-screw vent with vacuum at -0.08 MPa to -0.09 MPa removes residual moisture and volatile degradation products. If the vent is blocked, melt volume rate increases and silver streaking appears on injection-molded parts. PBT should not be combined with basic amines or ammonia-containing additives in a moist environment because aminolysis opens ester linkages through a nucleophilic route that is faster than neutral hydrolysis at equivalent temperature. Published data for this specific configuration is limited, but the incompatibility is a recognized polyester degradation route.

In a 30% glass-fiber-reinforced PBT housing removed from an underhood enclosure, scanning electron microscopy often reveals fiber-matrix debonding at the fracture origin and a relatively smooth matrix fracture path between fibers. Differential scanning calorimetry under ISO 11357-3:2018 may show a melting peak at 223 °C and no change in melting temperature, but the enthalpy of cold crystallization can increase because degraded chains crystallize more easily during heating. This is not a standalone proof of oxidation or hydrolysis; it must be paired with molar mass data. Heat aging in an air-circulating oven at 120 °C for 500 h under ASTM D3045-18 is used to separate thermo-oxidative embrittlement from hydrolysis. A part that survives 500 h hot-air aging but fails after water immersion at 85 °C is consistent with hydrolytic degradation rather than simple oxidation.

When melt residence time exceeds the supplier limit at 270 °C, does the failure signature differ from hydrolysis?

Processing-induced degradation is visible before service, but its mechanical consequences can appear only after 18 months under load. Barrel zone temperatures, hot-runner manifold setpoint, screw recovery time, and gate geometry are extracted from the injection molding setup record. A hot-runner PBT melt held at 270 °C for more than 5 min can undergo thermal degradation that produces volatile by-products including tetrahydrofuran and aldehydes; adequate venting is required. The as-molded part may show yellowing, a drop in molecular weight, and mottled surfaces at the gate. Melt temperature is confirmed by a needle thermocouple in the purge puddle; if the reading exceeds 285 °C, barrel control or frictional shear is suspect. Thermo-oxidative embrittlement is distinguished from hydrolysis by oxidation induction time at 230 °C under ISO 11357-6:2018. An OIT decrease greater than 50% relative to the retained control indicates antioxidant depletion, while hydrolytic chain scission can occur without a large OIT shift.

Because a 0.8 mm wall section in a PBT relay base cools at rates above 100 K/min in a cold-runner mold, the frozen skin locks in orientation before the core crystallizes completely. Post-mold shrinkage measured under ISO 294-4:2018 at 23 °C and 50% RH reaches 0.1% to 0.2% for unfilled PBT and 0.03% to 0.08% in flow direction for 30% glass-fiber-reinforced PBT. Across flow, glass-reinforced PBT can exhibit 0.2% to 0.4% post-mold shrinkage, and this anisotropy remains measurable after 18 months. A metal insert installed after molding without an expansion slot can convert that differential shrinkage into hoop stress exceeding the material’s craze resistance, producing cracks that resemble environmental stress cracking but are primarily assembly and design driven.

Flame-retarded PBT terminals, contact corrosion, and comparative tracking index after 18 months

Flame-retarded PBT grades classified as UL 94 V-0 at 0.75 mm or 0.4 mm may contain brominated flame retardants and antimony trioxide synergists. In an electrical terminal application, an 18-month field return may show no thermal deformation but a reduction in surface insulation resistance. Comparative tracking index is tested under IEC 60112:2020; unfilled non-flame-retarded PBT often shows 600 V, while glass-filled flame-retarded formulations can be lower. A shift from 600 V to 250 V after aging indicates surface contamination or bulk degradation. Contact corrosion is evaluated by flowing mixed gas exposure under IEC 60068-2-60:2015; published data for PBT contact-corrosion response under all four gas species is limited, so qualification must be conducted against a retained control. If a brominated flame-retarded PBT terminal is claimed to meet halogen-free customer specifications, compliance with IEC 61249-2-21 halogen limits may also be part of the claim.

For food-contact and automotive specifications, the claim file must record the regulatory boundary conditions. FDA 21 CFR 177.1660 covers linear polyesters and imposes compositional restrictions for food-contact use. Under EU Regulation (EU) No 10/2011, migration limits apply to 1,4-butanediol and terephthalic acid. Under the RoHS Recast 2011/65/EU, restricted polybrominated biphenyls and polybrominated diphenyl ethers must not exceed 1000 ppm in homogeneous materials. REACH candidate-list screening must be performed on stabilizer packages and flame-retardant additives. These requirements do not explain mechanical failure, but they determine whether an 18-month return has a separate compliance failure.

Melt flow ratio before and after field aging separates chain scission from additive migration

The melt volume rate alone can be misleading because a small part with a highly degraded surface layer and an intact core may show only a modest global increase. A failed-part MVR to retained-control MVR ratio above 1.25 under ISO 1133-1:2022 indicates substantial molar mass loss. However, additive migration can also shift flow by plasticization or surface slip. The claim must therefore include extraction and gravimetric analysis of acetone-soluble surface residues. Thermogravimetric analysis under ISO 11358-1 measures filler content and decomposition onset; a filler content below the specification by more than 2 wt% indicates a compounded batch error rather than an environmental failure. Fourier transform infrared spectroscopy of the fracture surface can identify carboxylate bands from hydrolytic degradation. Capillary rheometry under ISO 11443:2021 provides shear viscosity data for flow simulation; fiber-filled PBT exhibits higher viscosity than unfilled PBT with pronounced shear thinning.

Table 2 — Minimum analytical matrix for an 18-month PBT failure claim.