Traditionally, the glass transition temperature (Tg), as determined by thermal analysis, has been used to calculate the material use threshold. The main drawback to the use of Tg as a measure of upper use temperatures is that it results in use temperatures that are significantly higher than the use temperatures developed through mechanical testing. This study compared operating limits obtained from mechanical tests to glass transition temperatures obtained by several methods of thermal analysis.
A toughened epoxy composite, IM7/977-3 from ICI Fiberite, was tested in this study, along with the corresponding neat resin. The mechanical tests performed were interlaminar shear, resin compression, and material operating limit (MOL) testing (a modified open-hole compression test). Specimens were tested over a range of temperatures to determine the upper use temperature.
Thermal analysis testing included differential scanning calorimetry (DSC) and thermal mechanical analysis (TMA). Two types of dynamic mechanical spectroscopy (DMS) tests were performed: dynamic mechanical analysis (DMA) and Rheometrics dynamic analysis (RDA). The effects of moisture, heating rates, and test frequency on the measured Tg were examined.
The DMS tests were found to give more reproducible results than either the DSC or TMA. Higher heating rates and test frequencies resulted in higher measured Tg's. Shift factors were developed to allow comparison between data obtained at various heating rates and tests frequencies. Results from the mechanical testing corresponded more closely to the storage modulus measurements of the DMS tests than the values obtained from the tan delta curves.