Fibre-Reinforced Polyamides and the Influence of Water Absorption on the Mechanical and Thermomechanical Behaviour

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Polyamides tend to absorb water from environment especially due to their polar structure. Aliphatic polyamides absorb water faster and to a higher extent than polyamides, aromatic parts within their backbone. The absorbed water causes a weakening of the hydrogen bonds between adjacent polyamide backbones. For both, aliphatic and semi-aromatic polyamides, a significant shift of the glass transition was found using dynamic mechanical analysis. In tensile tests the water absorption and increase of testing temperature show a decrease of the tensile modulus and tensile strength for the aliphatic polyamide 66 (PA66) and the semi-aromatic polyamide (PPA) types. Whereas PA66-GF30 and PA66-GF50 also show an increase of elongation at break with water absorption and higher testing temperature, no such effect can be found for PPA-GF35. An efficient creep test methodology using the Stepped Isothermal Method (SIM) and the principle of time–temperature superposition show differences in the long-term creep behaviour of the investigated polyamides in wet materials condition. It can be shown, that, if the testing or reference temperature of creep tests is chosen at the beginning of the glass transition range, the short-term mechanical properties remain at a high level, but the creep tendency is significantly increased. With testing temperatures distinctly above glass transition, PA66-GF30 and PA66-GF50 show a lower creep tendency compared to PPA-GF35 which was tested at a temperature close to the glass transition range. Physical or chemical ageing effects induced by water absorption of polyamides, which may occur after long-term exposure, are not considered within the present test program. Further specific materials characterisation methods after long-term immersion are recommended for the investigation of possible ageing effects.