Due to their high strength, good dimensional stability under heat and high wear resistance, polyamide materials are among the most widely used materials in the group of engineering plastics. However, when using polyamides, the strongly pronounced water absorption must be considered, as this has a major influence on the mechanical properties of the material. The aim of this work is to perform a comprehensive analysis of the time- and temperature-dependent behavior for an unreinforced and a 30% glass-fiber-reinforced polyamide 6 in different material conditions (dry, humid, wet). Conditioning for the wet material state was carried out according to ISO 1110 at 62 % RH and 70 °C. Conditioning for the wet state was carried out in a water bath at 70 °C.
DMA tests at different frequencies were performed to characterize the temperature and frequency dependent properties. The shift of the glass transition temperature as a function of the test frequency allows the determination of the activation energy for the main softening range. For the characterization of the long-term deformation behavior under constant load, creep tests were carried out according to the Stepped-Isothermal Method (SIM). For this purpose, creep tests were performed at different test temperatures in the range of 30 °C to 80 °C, with stepwise temperature increase, and the creep-deformation curve was measured. By means of time-temperature shifting of the isothermal curve segments, the corresponding master curve for the creep behavior was generated over the application-relevant long-term range. The horizontal displacement was calculated using the Arrhenius approach according to the activation energies determined in the DMA for the glass transition. Due to the specific evaluation methodology of the SIM as well as material- and stress-related influences, an additional vertical displacement component was required to generate the master curves for the humid material state. In general, the creep tests were performed within or close to the linear viscoelastic range at test stresses of 2 and 4 MPa for the unreinforced and the glass fiber reinforced material. Linearity limits were determined for all material conditions (dry, humid, and wet) and test temperatures in series of short-term creep tests. Clearly nonlinear viscoelastic behavior was found from low loads, especially for the humid and wet states.
The absolute values of the resulting SIM master curves sometimes showed significant deviations from the corresponding data sheet specifications, especially for stiff specimen states (dry state and fiber-reinforced state), mainly due to equipment limitations. Nevertheless, the creep behavior of the investigated material states could be well characterized with the SIM method used. For example, modulus drops of 75 % (PA6, unreinforced) and 45 % (PA-GF30) were determined for the dry state of the materials at a reference temperature of 30 °C over a period of 50 years. The corresponding SIM master curves for the wet state gave modulus drops of 43 % (PA6, unreinforced) and 32 %(PA-GF30), respectively, over this time range. Comparison of the SIM master curves with conventional long-term creep tests also performed showed very good agreement between the creep modulus curves, especially for the humid condition. For the wet material state, it was not possible to perform SIM tests due to the high heat capacity of the water and the corresponding long heating times. Nevertheless, by performing conventional creep tests in a water bath at different temperatures, meaningful master curves could be obtained by means of time-temperature shifting.
In general, the SIM method proved to be well suited for characterizing long-term creep behavior, considering equipment limitations, especially with regard to specimen stiffness. The significant influence of the moisture content on the creep behavior of the polyamide materials investigated was demonstrated.