Thermo-Oxidative Degradation of High Temperature Polyimide Composites : Characterization and Modeling of Composites Affected by an Extreme Environment
Abstract: Carbon fiber (CF) 8-harness satin weave, T650/Neximid system of [(+45/-45)/(0/90)]2S and [(0/90)]4S layup was manufactured using resin transfer molding (RTM). The material was cut into 3-point bending specimens and treated for 24 hours in a burn oven at T=(320,350,375,400,450 & 500)°C. The material was tested according to ASTM E1640-13 using dynamic mechanical thermal analysis (DMTA). Un-treated material showed Tg levels of 384°C and 392°C for the respective layups. It was found that pre-Tg treatment between 320-375°C affected this material parameter up to similar levels as previous studies of post Tg exposure for 2h to ~420°C . Subjecting the material for post-Tg exposure at 400°C showed a rapid change up to ~480°C for [(0/90)]4S laminate. Indications that this resin system could reach levels above 500°C was found for [(+45/-45)/(0/90)]2S layup. However, one of these specimens were unfit for testing. DMTA tested material for 400°C showed indications of degradation, found by a broadening of the tan delta peak. This can be put in relation to epoxy where a similar behavior appear after 24h exposure at 150°C. Furthermore, it was showed that poor quality laminate, elevated mass loss at this temperature. When the material was subjected to as high temperatures as 450°C only remaining fibers were found. At 500°C these were almost fully oxidized. 400°C data was predicted by the use of activation energy along with TG extrapolation. It was possible to show that ~1/8 out of this 8-layered structure, (½ of each surface layer), was degraded after 400°C exposure for 24h, resulting in diffusion limited oxidation (DLO). Last but not least, DLO assumptions was used to predict the storage modulus change for thermo-oxidative degradation of 400°C samples with Classic Laminate Theory (CLT). A ~4% stiffness decrease was predicted by this method. The drop is regarded as a loss in tensile stiffness of the outer damaged layer. This was compared by 3-point bending DMTA data showing a ~7% decrease. This model could thus be regarded as a contributing factor for the stiffness decrease of this complex degradation process.
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