Hygrothermal durability and thermal aging behavior prediction of high-temperature polymer-matrix composites and their resins

Prediction of the cure-induced T-g increases and associated matrix and composite mechanical property deterioration of BMI systems in real service environments is attempted by network structure interrelations with mechanical and thermal properties as a function of composition (initial monomer ratio) and time-temperature cure cycles. Tensile and flexural properties of four BMI compositions at six different cure cycles (or degree of cure) have been determined at three temperatures, 23 degrees C, 177 degrees C, and 250 degrees C, and correlated to T-g and density of the systems. Systematic studies on hygrothermal durability of bismaleimide (BMI) and various polyimide (PI)-carbon fiber composites and neat resins were conducted. The combined effects of moisture and thermal exposures such as hygrothermal spikes up to 250 degrees C and hygrothermal aging under various time-temperature-moisture conditions including accelerated aging at saturated steam environment(160 degrees C and 110 psi) on microscopic damage, polymer-matrix physical structural state, and residual properties of those composites and neat resins are presented. The onset of blistering in moist K3B PI C fiber composites occurred at 229 degrees C during the thermal spiking. It is evident that the hygrothermal performance stability is one of the prime guidelines in future aerospace applications, especially for PI composites. The physical and chemical structural state of PI matrices such as K3B and AFR700B as a function of hygrothermal exposure are discussed in terms of hydrolytic chemical degradation, moisture vapor-induced physical damage, and molecularly locked-in water. Those structural stares are characterized by systematic weight monitoring H-2 NMR, and various thermal, mechanical property measurements using D2O water environment.