Polybutylene terephthalate (PBT) occupies a narrow but demanding envelope in automotive under-bonnet electrical systems, where ignition-coil components are subjected to continuous local metal temperatures near 150 °C, rapid thermal cycling between -40 °C and 160 °C, and simultaneous exposure to engine oil mist, brake fluid splash, and calcium chloride road de-icer residues. The 150 °C question for PBT is not whether the polymer passes a short-term deflection test but whether a 30% glass-fibre reinforced, heat-stabilised grade can retain dielectric strength, impact resistance, and dimensional stability over 3000–5000 h of accumulated engine-bay exposure. In ignition coils, PBT bobbins, terminal carriers, and housing shells function as electrical insulation systems rather than structural brackets; failure is therefore defined by primary-to-secondary breakdown, surface leakage, or flange cracking that releases winding tension.

Two architectures dominate: waste-spark coils with a single secondary winding firing two cylinders, and coil-on-plug pencils placed directly over the spark plug. In both, PBT is used for the secondary bobbin, primary bobbin, connector shell, or housing; each zone has a different failure criterion. The secondary bobbin sees peak secondary voltages of 30–40 kV and must maintain layer-to-layer insulation after the enamel film ages. The primary bobbin sees lower voltage but high di/dt, and the housing sees clamp load, vibration, and chemical exposure. The 150 °C question is most acute at the secondary tower adjacent to the engine head, where conduction from the spark-plug well and exhaust manifold radiation combine with ohmic heating from the primary winding.

Why Does the 150 °C Requirement Separate Ignition-Coil PBT from Standard Connector Grades?

The distinction arises from the difference between short-term heat resistance and oxidative ageing. A 30% glass-fibre PBT typically exhibits a heat deflection temperature (HDT-A) of 200–210 °C under 1.82 MPa in accordance with ISO 75-2 or ASTM D648. That value indicates ample short-term margin above 150 °C, but HDT-A is a creep-limited deflection test performed over minutes, not a long-term thermal endurance test. Under continuous air ageing at 150 °C, the polybutylene terephthalate matrix undergoes oxidative chain scission at the ester linkage and along the butylene segment. Stabiliser packages based on hindered phenols, phosphites, or thioesters delay carbonyl formation but do not eliminate it. The engineering acceptance criterion is the relative thermal index (RTI) assigned under UL 746B from comparative ageing of tensile, impact, and dielectric specimens. Heat-stabilised PBT grades often carry an RTI Electrical of 130–140 °C and an RTI Mechanical with Impact of 120–130 °C. Ignition-coil programmes requiring 150 °C continuous service therefore require grade-specific testing under UL 746B at 150 °C for 500–2000 h to establish retention of dielectric strength and elongation at break.

Thermal degradation of PBT proceeds through β-hydrogen transfer in the butylene segment, releasing 1,3-butadiene and producing carboxyl end-groups. The carboxyl end-groups promote further hydrolysis and autocatalytic chain scission. Heat-stabilised grades reduce this autocatalytic effect by acid scavenging, but over-stabilisation may increase mould deposit formation on tool surfaces. Under-bonnet temperature profiles are not uniform. Proximity to the exhaust manifold, cylinder head, and catalytic converter creates local hot spots; ISO 16750-4 and OEM-specific heat-soak profiles often include short excursions to 165–180 °C during trailer towing or idle after hot shut-down. PBT must therefore survive not only 150 °C continuous air exposure but also repeated short-term overshoots that approach the lower edge of its crystalline melting region.

Material Selection Logic and Comparative Experimental Data

Material selection for a waste-spark or coil-on-plug bobbin is driven by the interaction of electrical stress, thermal expansion, and moulded-in stress. In coil bobbins wound with 0.05–0.15 mm enamelled copper wire, flange warpage of only 0.10–0.20 mm across a 60 mm winding length can disturb layer-winding tension and increase the risk of wire crossover or secondary shorting. The property ranges below, drawn from published supplier technical data, illustrate why PBT is preferred where moisture-cycle dimensional stability and dielectric retention dominate over dry tensile strength. The values should be treated as typical injection-moulded ranges, not specification limits.

The moisture-absorption differential is the principal reason PBT retains insulation resistance across humidity cycles while PA66 undergoes plasticisation and dimensional drift. Moulded-in stress is another factor: the bobbin is often moulded over a steel or ferrite core, so differential shrinkage between the 0.8–1.2% transverse PBT shrinkage and the core creates residual tensile stress at the core surface. If core preheating is not controlled, the PBT may quench to an amorphous state at the interface, reducing adhesion and increasing the risk of axial cracks after thermal cycling.

Hydrolysis Limits 150 °C Service Life in PBT Coil Housings

PBT is a condensation polyester, and the ester carbonyl group is susceptible to hydrolysis at elevated temperature. At a moulded moisture content above 0.02%, hydrolysis during melt processing reduces molecular weight and produces splay marks on part surfaces, particularly near gates and end-of-flow zones. Ignition-coil bobbin production therefore uses desiccant dryers with a dew point of -40 °C to -20 °C and a material temperature of 120–130 °C for 3–4 h. Drying is verified by ISO 15512 water-content measurement, and hopper residence time is limited to prevent moisture regain. In service, under-bonnet humidity combined with 150 °C surface temperatures accelerates hydrolytic embrittlement; the rate depends on local relative humidity, wall thickness, and whether the part is encapsulated in epoxy or exposed to engine-bay air. PBT grades with hydrolysis-resistant additives and low carboxylic acid end-group concentration are specified for long-life coil housings. Separate damp-heat ageing at 85 °C/85% RH according to IEC 60068-2-78 is specified because UL 746B thermal endurance in air does not fully capture hydrolytic ageing.

Chemical exposure cannot be decoupled from thermal stress. Engine oil, urea-based corrosion inhibitors, and road salt create a conductive film that can reduce comparative tracking index. PBT shows useful resistance to aliphatic hydrocarbons and engine oils, but it is attacked by strong bases and by prolonged contact with hot ethylene glycol/water coolant. In coil housings, sealing gaskets and encapsulant compatibility are tested by immersion in ASTM D543 reference fluids at 60–100 °C before final part approval.

On a production-scale electric injection moulding machine with a screw L/D ratio of 20:1 to 25:1 and a check ring suitable for glass-filled material, barrel temperatures are typically set from 240 °C at the feed zone to 260 °C at the metering zone, with nozzle temperature at 260–265 °C. Mould temperatures of 80–100 °C are required to crystallise the PBT matrix sufficiently; mould temperatures below 70 °C produce under-crystallised skins and increase post-mould shrinkage in the winding area. Production experience shows that gas entrapment at blind pin locations creates burn marks, and vacuum-assisted moulding or porous vent inserts are used to mitigate this. For a cylindrical bobbin with a central iron core insert, balanced film gates or multiple pin gates around the secondary tower prevent weld lines from forming at the base of the high-voltage terminal. Holding pressure of 600–1000 bar and a hold time of 5–10 s are typical, although gate freeze-off must be confirmed by short-shot studies. Melt residence time is held below 8 min and melt temperature is measured by air-shot pyrometer to avoid exceeding 275 °C, above which PBT depolymerisation accelerates. Thin wall sections below 0.8 mm in terminal towers are generally avoided because glass-fibre orientation effects increase the risk of short-shot and weld-line weakness.

PBT crystallinity is controlled by mould temperature and cooling rate. A mould temperature in the 80–100 °C range is reported to yield crystallinity of 30–35% in filled PBT; lower mould temperatures create amorphous skins that post-crystallise during the first 150 °C service cycles, producing dimensional drift and possible cracking around the iron core. Injection is often profiled with a moderate initial velocity to avoid jetting, followed by a controlled pack phase. High injection speeds above 150 mm/s can cause glass-fibre breakage and reduce tensile strength, while low speeds below 40 mm/s may cause premature freeze-off in thin sections.

When PBT Replaces PA66 in Secondary-Side Coil Bobbins

The substitution is typically driven by moisture absorption. PA66 GF30 absorbs up to 2.0% moisture at 23 °C/50% RH and more at saturation, causing a dimensional change of 0.4–0.6% and a reduction in flexural modulus of 20–30%. PBT GF30 absorbs less than 0.25% at the same condition, so the wound bobbin retains primary-to-secondary insulation resistance across humidity cycles. PA66 also exhibits a dielectric constant shift from 3.3–3.8 dry to 4.0–4.5 at 50% RH, which may affect coil resonance; PBT remains around 3.2–3.5 over under-hood humidity ranges. However, PBT GF30 has lower dry tensile strength and lower notched impact toughness than PA66 GF30, so thin-wall terminal towers and snap-fit retention features require finite-element review to avoid brittle fracture during thermal shock from -40 °C to 150 °C. Charpy notched impact strength for PBT GF30 at 23 °C is typically 7–10 kJ/m² under ISO 179-1/1eA, falling to 4–5 kJ/m² at -30 °C. Sharp corners in terminal towers and snap-fit hooks are therefore radiused to a minimum of 0.5 mm to avoid brittle failure during assembly and thermal shock.

Comparative tracking index values around 500–600 V for PBT GF30 allow use in circuits where leakage-current paths develop across carbonised dust or condensed salt film; the specific grade must be validated according to IEC 60112. Volume resistivity of PBT GF30 at 23 °C is typically 1 × 10¹⁵ Ω·cm according to IEC 62631-3-1, and surface resistivity is typically 1 × 10¹⁴ Ω. After conditioning at 150 °C and 85% RH, resistivity may fall by one to two decades; the coil bobbin must still maintain sufficient insulation to prevent primary-to-secondary leakage. The final coil assembly is evaluated under IEC 60664-1 creepage and clearance criteria. Where flame-retardant V-0 is specified, the flame-retardant package may reduce CTI and tensile strength, so the balance is assessed by UL 94, IEC 60112, and ISO 527-1/-2 on the actual moulded part.

Validation on production tooling is usually structured as a matrix of thermal, electrical, and environmental tests. The table below lists representative tests for PBT ignition-coil insulation components at 150 °C continuous service.

Field failures from PBT ignition-coil housings are rarely simple melting. The dominant modes documented in under-bonnet returned parts are surface microcracking of the secondary tower after thermal-oxidative ageing, erosion of the moulded surface adjacent to high-voltage terminal bushings, and cracking of flange corners after thermal shock and vibration. The use of 30% glass fibre increases stiffness and HDT but reduces flow length and creates anisotropic shrinkage that can cause bore distortion if gate placement is not symmetrical. For continuous operation at 150 °C with transient excursions to 165 °C, the material specification must address not only the polymer grade but also the stabiliser package, the maximum moisture content before moulding, the mould temperature, and the copper-to-polymer thermal expansion mismatch. Published data for specific coil configurations is limited, and any change in winding tension, epoxy encapsulant, terminal geometry, or ignition voltage profile requires re-running thermal shock and dielectric withstand validation on production tooling.