Adding PBT to PC/ABS: You Gain Chemical Resistance, You Risk Paint Adhesion

What PBT fixes

The weak point of PC/ABS is the sensitivity of the PC phase to oils, organic solvents and polar chemicals, leading to environmental stress cracking under load. Introducing PBT markedly improves oil, solvent and hydrolysis resistance while retaining useful low-temperature impact.

For parts contacting cleaning agents, petrol or skin oils — power tool handles, appliance panels, automotive exterior trim — PC/ABS+PBT is frequently a better fit than neat PC/ABS.

Why adhesion drops

PBT is semi-crystalline. Crystalline regions have lower surface free energy than amorphous ones, so wetting and anchoring of the coating weaken; cross-hatch results (ASTM D3359) commonly fall from 5B to 3B or below.

PBT crystallisation also needs higher mould temperature to complete. With insufficient mould temperature the skin stays poorly crystallised and continues to crystallise and shrink during the paint bake, cracking the film or further reducing adhesion.

How to compensate

Raise mould temperature to 80-100 °C so the skin crystallises first, then combine pre-treatment with a primer designed for polyester systems — PP/PA primers are not necessarily suitable. Plasma or flame treatment to raise surface energy is usually effective.

On the formulation side, PBT content can be cut to the 15-25% range to balance chemical resistance against paintability. If painting is a hard requirement, validate on test plaques before cutting the tool.