When an under-hood connector, a thermal-management component or a bearing housing has to survive at elevated temperature, the shortlist almost always starts with two candidates: PA6.6 and PPA. Both are polyamides, both offer glass-fibre reinforcement, and both have long track records in automotive and electrical applications. They are not interchangeable, and specifying the wrong one has real consequences. This article sets out the key differences, the temperature range where PPA becomes necessary, and the application factors that should drive the decision.

The thermal ceiling of PA6.6

PA6.6 is an excellent engineering polymer. Reinforced with 30–35% glass fibre, it routinely reaches a Heat Deflection Temperature (HDT-A, 1.8 MPa) of 245–255 °C, its long-term heat-ageing behaviour is well documented, and decades of processing data make it one of the most reliably manufacturable engineering thermoplastics in use.

It has one structural limitation that matters above roughly 130 °C in continuous service: moisture. PA6.6 takes up around 2.5% moisture at standard 50% RH equilibrium, rising further toward saturation in wet conditions, and that absorbed moisture acts as a plasticiser, lowering stiffness and strength at temperature. For parts that see heat and humidity together (common under the bonnet), real-world performance can fall well short of the dry datasheet values.

PA6.6 is also subject to thermal oxidation in long-duration service above about 150 °C. Heat-stabiliser packages extend that ceiling: Politem’s heat-stabilised PA6.6 grades are formulated for prolonged service at elevated temperature. Beyond a practical limit, however, PA6.6 is no longer the right specification.

Where PPA changes the equation

Polyphthalamide (PPA) is a semi-aromatic polyamide. The aromatic rings in the backbone give it fundamentally different thermal and moisture behaviour from aliphatic polyamides such as PA6.6.

• Higher melting point: PPA melts at about 305–310 °C versus 262 °C for PA6.6, giving more headroom in heat-critical parts.
• Lower moisture uptake: roughly 1.0–1.8% versus 2.5–3.5% for PA6.6, so mechanical properties stay more stable in humid service.
• Higher HDT: GF-reinforced PPA reaches HDT-A of 280–295 °C versus 245–255 °C for equivalent PA6.6.
• Better chemical resistance: PPA resists automotive fluids, lubricants and coolants more effectively, particularly hot.
• Better dimensional stability: lower moisture uptake means less dimensional change with humidity cycling.

A decision framework

Processing considerations

PPA runs hotter than PA6.6: typically 310–330 °C melt versus 270–290 °C, with mould temperatures of 120–150 °C versus 60–90 °C to ensure crystallisation and surface quality. That has implications for tooling and energy use, and belongs in the total-cost assessment. Pre-drying is also more stringent: moisture below about 0.1% before moulding to avoid hydrolytic degradation. Politem’s RugoPPA families ship with application-specific processing data to guide set-up.

The intermediate space

Where the thermal demand sits at the borderline, roughly 130–150 °C continuous, two routes bridge the gap without full PPA processing complexity: a heat-stabilised PA6.6, or a lower-aromatic partially aromatic (PPA-family, e.g. PA6.6/6T-type) grade. Long-chain polyamides such as PA6.12 (RUGOPA Z13) are a different tool: they are chosen for low moisture uptake, ductility and dimensional stability at moderate temperature, not for a higher temperature ceiling. Politem develops custom formulations in this intermediate space for cost-performance targets.

Conclusion

PA6.6 and PPA are both outstanding polyamides serving different spaces. PA6.6 remains the right choice for most structural applications below 130 °C. Above that, and especially in combined heat-and-humidity service, PPA becomes technically necessary rather than merely preferable. The right call needs an honest read of the service environment, not just the datasheet HDT. Politem’s technical team can provide a comparison based on your actual load case and service conditions.