Degradation mechanisms of CF/PPS, CF/PEI, and CF/PEEK under combined UV radiation and condensation

From 1990 to 2020, the total weight of composites integrated in airplane structures increased from 5 to 50% contributing to reduce fuel consumption and CO₂ emission. However, environmental weathering conditions alter the stability and durability of load-bearing composite components. Three carbon fibre-reinforced high-performance thermoplastic composites — polyphenylene sulphide (PPS), polyether imide (PEI), and polyether ether ketone (PEEK) — used in aviation are exposed to combined ultraviolet radiation (UV), humidity and temperature, following ASTM D4329-21 for up to 28 days. To support characterisation, the corresponding neat polymer films of PPS, PEI, and PEEK were aged under the same conditions. The polymer matrix of the composites degraded following two distinct mechanisms. CF/PPS predominantly cross-linked, which resulted in increased brittleness, reduced crystallinity and increased in tensile strength after 28 days. In contrast, CF/PEI and CF/PEEK underwent primarily chain scission, leading to decreased T_g (PEI), increased crystallinity (PEEK), and reduced tensile strength (CF/PEEK and CF/PEI) after exposure.