Accelerated thermal aging of thermoplastic materials for the motor compartment: Characterization, degradation model and lifetime prediction

Changes in structure and properties of polyamide 6.6 with glass fibers-as an example for a semicrystalline thermoplastic polymer material-were investigated after defined periods of thermal aging by optical and scanning acoustic microscopy, gel permeation chromatography, time-of-flight matrix assisted laser desorption/ionization-time of flight mass spectrometry, nuclear magnetic resonance and infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, dynamic-mechanical analysis, tensile, and cyclic fatigue testing. Combining the different characterization methods allows differentiating between aging mechanisms in semicrystalline thermoplastic polymers such as chain scission, crosslinking or changes in the crystalline structure. The results were analyzed in terms of Arrhenius-type thermal activation. The underlying aging mechanisms and activation energies were found to be different for the different properties. Equivalent aging temperatures and acceleration/prolongation factors for accelerated testing are calculated for the different properties based on real-drive microclimate data from the motor compartment using property specific activation energies. The limitations of this approach are discussed.