Testing Polybutylene Terephthalate to IEC 60112: Conditioning and Judgement Rules

Polybutylene terephthalate (PBT) is a semi-crystalline aromatic polyester whose role in electrical insulation systems is frequently controlled by surface tracking resistance rather than by bulk dielectric strength alone. In connectors, terminal blocks, relay bobbins, motor brush holders and power-electronics housings, the material is specified under IEC 60112 because the method quantifies comparative tracking index (CTI) and proof tracking index (PTI) under defined electrolyte contamination. The test deposits ammonium chloride solution onto a horizontal specimen between charged platinum electrodes and records the formation of conductive carbonaceous tracks. Unfilled PBT grades can achieve CTI values at or above 600 V; glass-fibre-reinforced and brominated flame-retardant grades commonly fall between 250 V and 350 V. That spread is not a laboratory artefact but the result of polyester pyrolysis chemistry, additive mobility, fibre wetting and moulded-surface structure.

The IEC 60112 test cell uses two chisel-edge platinum or platinum-iridium electrodes with a rectangular cross-section of 5 mm × 2 mm. The electrode gap is set at 4.0 mm ± 0.1 mm, and the applied electrode force is 1.00 N ± 0.05 N. Test solution A is 0.1 % ammonium chloride in deionized water with a resistivity of 3.95 Ω·m ± 0.05 Ω·m at 23 °C ± 1 °C. Drop volume is 20 mm³ ± 2 mm³, and the interval between drops is 30 s ± 5 s. Voltage is selectable in 25 V increments up to 600 V. The current trip threshold is 0.5 A, with a 2 s duration criterion for electrical failure.

Does the IEC 60112 conditioning regime reach moisture equilibrium in glass-filled PBT?

IEC 60112 requires test specimens to be conditioned in a laboratory atmosphere of 23 °C ± 5 °C and 50 % ± 10 % relative humidity for at least 24 h. For neat PBT, the 24 h soak is usually sufficient because equilibrium moisture uptake at 23 °C and 50 % RH is typically below 0.2 % by mass. Glass-reinforced PBT can equilibrate at 0.4–0.6 % depending on glass content and coupling-agent chemistry. The moisture is not evenly distributed; it accumulates at the fibre-matrix interface and at exposed glass ends. The IEC 60112 test is surface-driven and therefore more sensitive to skin moisture than to average bulk moisture. A plaque conditioned at the upper humidity tolerance may carry a thin conductive surface layer, while a plaque conditioned at the lower tolerance can produce a higher CTI simply because the surface is drier. That is a conditioning artefact, not a material improvement.

Production-scale drying and moulding are separate from IEC 60112 conditioning. PBT pellets are routinely dried in desiccant dryers at 120–130 °C until moisture content falls to 0.02 % or less, typically after 4–6 h. Residual moisture above 0.04 % can hydrolyse the ester linkage during melt processing; the resulting molecular-weight reduction changes melt viscosity and can leave a higher concentration of oligomeric species at the plaque surface. These low-molecular-weight species influence initial surface leakage. Compounding of glass-filled PBT is usually performed on co-rotating twin-screw extruders with L/D ratios from 36:1 to 44:1. Glass is side-fed downstream to limit fibre fracture. Moulders adjust barrel temperatures between 245 °C and 265 °C; lower melt temperatures preserve molecular weight but can increase skin orientation, while higher melt temperatures reduce orientation but increase hydrolysis risk. The moulded plaque surface is therefore a process-dependent variable before any IEC 60112 test begins.

Electrode force, drop volume and carbon-track judgement: the only variables that matter

IEC 60112 failure is not simply carbon formation. The electrical result is a trip when leakage current exceeds 0.5 A for more than 2 s. The visual result may be a carbonaceous track, deep erosion, ignition or a combination. Erosion alone, even when it forms a trough, does not constitute tracking unless it creates a conductive bridge or activates the overcurrent relay. Visible flame is recorded separately. The comparative tracking index is the maximum voltage at which a specimen withstands 50 drops without failure; it is reported in 25 V steps. The proof tracking index is a pass/fail verification at one selected voltage, commonly 175 V, 250 V, 300 V, 375 V or 500 V.

Because the judgement rules operate on a discrete voltage staircase, a material with a real tracking threshold near 275 V may pass at 300 V in one laboratory and fail at 275 V in another. This step-size effect is not specific to PBT, but it becomes commercially important when a PBT grade is positioned just above an insulation-material group boundary.

Moulded-surface orientation is an uncontrolled variable in CTI testing

Surface structure dominates IEC 60112 repeatability. Injection-moulded PBT plaques develop a skin layer with crystallinity and orientation different from the core. Rapid cooling at the mould wall produces a fine-grained or amorphous surface skin, while the core crystallises more slowly. Glass fibres align in the flow direction and can create capillary channels for electrolyte transport. When the electrode gap is parallel to the dominant flow direction, the conductive path can travel along the fibre-rich skin. When the electrodes are placed transverse to flow, the path must cross fibre boundaries and the apparent CTI can increase by one or more 25 V steps. This orientation dependence is not eliminated by the 24 h conditioning soak. It is a moulding variable that should be stated on internal test certificates when PBT parts are audited against IEC 60112.

Connector specifications commonly link CTI to creepage distance through IEC 60664-1. A material with CTI of 600 V or higher is assigned to material group I; 400–599 V to group II; 175–399 V to group IIIa; and 100–174 V to group IIIb. A PBT compound that falls from 600 V to 300 V due to flame-retardant loading does not merely drop two material groups; it forces increased creepage and clearance spacing on the moulded connector. This is the regulatory consequence of the IEC 60112 judgement rules.

When a 30% glass-filled PBT grade is tested at 600 V drops

At 600 V, the interaction between PBT decomposition and glass reinforcement becomes severe. Each electrolyte drop can initiate a small surface arc; local temperatures may exceed the oxidative decomposition onset of PBT, which is above 300 °C in air and well above the crystalline melting point of 223–228 °C in a transient arc. The aromatic fragments carbonise rapidly. Glass fibres increase local thermal conductivity and can quench hot spots, but they also expose hydrophilic sizing residues that alter droplet wetting. The result is often a carbon track that follows fibre bundles rather than a uniform surface film. A 30 % glass-filled PBT grade therefore rarely retains a 600 V CTI unless the formulation contains mineral fillers, silicone-based anti-tracking additives or other char-management chemistry. System designers should not extrapolate unfilled PBT dielectric behaviour to glass-reinforced PBT housings.

Sealed power-electronics housings and EV charging connectors impose the most demanding field conditions for IEC 60112 data. A laboratory CTI derived at 23 °C / 50 % RH describes the material in a clean conditioned state, not after 85 °C / 85 % RH damp heat or thermal cycling. When PBT components are subjected to lot acceptance testing, conditioning history must be included on the certificate; otherwise the reported CTI may not transfer to a moulded part that has been exposed to humid environment, dust and salt residues. For laser-transparent PBT or PBT/polycarbonate impact-modified blends, published IEC 60112 data for production-scale moulded plaques are limited; validation test programmes are recommended before changing creepage distances.