Polybutylene Terephthalate (PBT): Substituting PBT for PPS in a Hot Electronic Part: What Breaks First

Polybutylene terephthalate (PBT) is a semicrystalline thermoplastic polyester produced from 1,4-butanediol and terephthalic acid or dimethyl terephthalate. In hot electronic parts, 30% glass-fibre-reinforced PBT grades are specified for connectors, relay bases, ignition housings, sensor bodies, and power module carriers where surface temperatures remain below the material’s published UL 746B relative thermal index. The phrase “Substituting PBT for PPS in a Hot Electronic Part: What Breaks First” defines an engineering comparison in which the replacement material reduces melt temperature and energy consumption but must survive continuous electrical load, thermal aging, moisture, and mechanical stress. A 30% glass-fibre-reinforced PBT grade processes at 250–270 °C melt temperature and 80–120 °C mold temperature; a 40% glass-fibre-reinforced polyphenylene sulfide (PPS) grade typically processes at 320–340 °C melt temperature and 140–160 °C mold temperature. The lower processing envelope reduces energy input, but the substitution moves multiple degradation mechanisms closer to threshold. The first failure is usually mechanical embrittlement and crack growth at metal-insert interfaces, not bulk dielectric breakdown.

When the Maximum Service Temperature Exceeds the UL 746B Relative Thermal Index

Thermal oxidative embrittlement is the primary failure mechanism when a part molded from 30% glass-fibre-reinforced PBT operates continuously above its mechanical impact relative thermal index. UL 746B assigns a mechanical impact RTI of 130 °C to many 30% GF PBT grades and a mechanical without-impact RTI of 140 °C; by comparison, 40% GF PPS grades are rated at 200–220 °C mechanical with impact and 220 °C electrical. At a hot-spot temperature of 155 °C, the PBT part is therefore 25 °C above its mechanical impact RTI, while the PPS part remains at least 45 °C below its mechanical impact RTI. The UL 746B protocol uses a 50% property retention criterion. Accelerated aging at 160 °C in a forced-air oven is not equivalent to an RTI rating, but it indicates that tensile elongation at break measured according to ISO 527-2 falls below 2% before bulk tensile strength drops below 50% of its initial value. Cracking initiates at weld lines, snap-fit roots, and metal-insert edges because these features contain residual stress from injection molding. On a production part molded on a 100 t clamp force machine with a 22 mm diameter barrier screw, the first visible failure is often a hairline crack at the gate or at the contact retention lance, followed by loss of spring retention force and fretting motion at the terminal interface.

Thermal endurance under IEC 60216-1 confirms that the activation energy for oxidative degradation of PBT is grade-dependent, typically 90–120 kJ/mol. An increase of 20 °C above the RTI can reduce lifetime to 25–50% of the expected value at the RTI temperature, depending on the antioxidant package. In a hot electronic part such as a power module carrier, the local temperature at the insert-molded heat sink tab can exceed the average housing temperature by 10–20 °C. The practical result is that a PBT housing may pass a component-level thermal cycling test at 140 °C and then fail in continuous operation at 160 °C because the degradation mechanism shifts from surface oxidation to internal chain scission. Published data for this specific configuration is limited; however, the relative thermal index gap between PBT and PPS is sufficient to predict that thermal mechanical embrittlement occurs before dielectric breakdown under dry heat.

In a sealed under-hood connector exposed to 85 °C and 85% RH, the failure ranking shifts from thermal oxidation to hydrolysis. PBT contains ester linkages that undergo hydrolytic chain scission at elevated temperature; PPS does not contain ester groups and is not subject to the same hydrolysis mechanism at the same rate. Moisture absorption measured according to ISO 62 at 23 °C saturation is typically 0.2–0.6% for 30% GF PBT and 0.02–0.05% for 40% GF PPS. Under humid aging, water diffuses along the glass-fibre interface and attacks the ester bond, generating carboxylic acid end groups that autocatalyse further scission. The number-average molecular weight of PBT falls, and notched Charpy impact strength measured according to ISO 179-1/1eA drops from 8–10 kJ/m² to below 4 kJ/m². For a connector retention lance, the first visible failure is a brittle crack at the thinnest wall because the strain-to-fracture has dropped below the local molding strain. Residual moisture above 0.02% before melt processing is sufficient to cause additional chain scission during molding, so the resin must be dried at 120 °C for 4 h in a desiccant dryer with a dew point below −40 °C. The operational boundary is that combined heat and humidity causes mechanical failure before the CTI or dielectric strength of the PBT part becomes the limiting electrical property.

The Limit of Comparative Tracking Index in Cracked, Contaminated Surfaces

The CTI of a 30% glass-fibre-reinforced PBT grade, measured according to IEC 60112, lies between 250 V and 600 V depending on flame retardant and impact modifier; many PPS grades fall between 125 V and 225 V. On a CTI-only comparison, PBT appears to have an electrical advantage. The CTI test, however, evaluates a clean surface under controlled electrolyte droplets, not a hot electronic part that has developed microcracks, carbonized dust, flux residues, and condensed moisture. In a relay base or power connector running at 150–170 °C, the first electrical failure is usually intermittent leakage across a mechanically cracked surface and carbon track formation at the crack tip. Dielectric strength measured according to IEC 60243-1 on 3 mm specimens is 16–22 kV/mm for PBT and 17–24 kV/mm for PPS; the initial difference is small. After 500 h at 160 °C, the surface of the PBT part becomes more polar as oxidative products accumulate; surface resistivity measured according to ASTM D257 can decline by two to three decades. The PPS part retains its surface resistivity longer because the aromatic sulfide backbone has a higher thermal-oxidative threshold. The failure that breaks first in contaminated service is therefore not CTI failure; it is arc tracking promoted by the cracked, oxidized PBT surface.

Under sustained load at 150–170 °C, contact normal force decays because the CLTE of 30% GF PBT is roughly twice that of 40% GF PPS in the transverse direction. The CLTE flow direction measured with ISO 11359-2 is 25–35 ppm/K for PBT and 15–20 ppm/K for PPS; transverse CLTE is 70–90 ppm/K for PBT and 40–50 ppm/K for PPS. For a 10 mm long retention wall heated from 23 °C to 160 °C, the flow-direction expansion is about 0.34 mm; the same wall in PPS expands about 0.21 mm. At the metal terminal pocket, this differential expansion reduces the interference fit and lowers the normal force. In a production relay terminal cavity, a 30% GF PBT part can lose more than 30% of insertion retention force after thermal cycling from −40 °C to 150 °C; PPS typically retains a higher fraction of original retention force because of lower CLTE and higher creep resistance. The failure mode is intermittent contact open during vibration, not a visible crack. Post-mold shrinkage after 1000 h at 150 °C is also greater for PBT, typically 0.2–0.5%, while 40% GF PPS shows less than 0.1% under similar conditions. This dimensional drift changes contact pitch and accelerates fretting corrosion at the contact interface. The first broken function is therefore often loss of low-voltage signal continuity, even before the housing shows a structural crack.

Processing Boundaries for Thin-Walled Connector Housings

Mold temperature and crystallinity determine whether a thin-walled PBT connector housing starts with a stable morphology. PBT crystallizes rapidly, but mold temperatures below 80 °C produce an amorphous skin that can post-crystallize in service, producing warpage and tolerance shift. A mold temperature of 90–120 °C is recommended for 30% GF PBT, compared with 140–160 °C for 40% GF PPS. In an eight-cavity hot-runner mold with valve-gated filling, filling pressure for 30% GF PBT at 250–270 °C melt temperature is typically 80–120 MPa; a PPS part of the same flow length requires 120–180 MPa at 320–340 °C. The lower melt temperature reduces energy input, but it also places the PBT service temperature closer to the melting range. Local hot spots above 200 °C caused by a mounted MOSFET or power resistor can produce creep, compression set, and local deformation in PBT; PPS retains higher modulus at short-term excursions to 260 °C. In a part with a metal insert, the insert temperature should be controlled to prevent excessive melt freezing and sink marks. For PBT, insert preheating at 100–120 °C is used on production equipment; for PPS, insert preheating is typically 150–170 °C. The first failure from a processing condition is usually a weak weld line behind the insert because the melt front from two gates meets at a lower temperature in PBT than in PPS. The resulting weld-line strength measured by ISO 527-2 can be 50–70% of the nominal tensile strength, and this weakened weld line becomes the preferred initiation site for thermal cracks.

In a relay coil bobbin application molded from 30% glass-fibre-reinforced PBT and operated at 155 °C with a 12 V coil load, the observed failure sequence begins with discoloration and surface microcracking at the terminal insert between 500 h and 1000 h. The first broken function is erosion of the contact surface caused by micro-motion after retention force decay, followed by intermittent arcing and carbon tracking along the insert-to-housing interface. By the time the part fails a continuity test, the bulk tensile strength retention measured according to ISO 527-2 may still be above 50%; the local crack is the limiting factor. A subsequent failure in the same relay bobbin design occurs when the contact normal force drops below the terminal retention specification; intermittent electrical open during vibration is detected before any visible crack runs across the housing. The failure is attributed to the lower CTE match of the PBT housing and the metal terminal, confirming that the first broken function in this configuration is mechanical retention loss rather than dielectric breakdown.