Improved methods for the thermal analysis of cure in polymer composites.

Measurement of the degree of cure of composite materials is vital to both research and manufacture of these materials. The glass transition temperature (Tg) is a measurable material property that can be used as an indicator of the degree of cure. The three most common thermal analysis techniques used to measure Tg are DMA, TMA and DSC (i.e. dynamic mechanical analysis, thermomechanical analysis and differential scanning calorimetry). There is a current need to improve the experimental methods and analysis of data when using these techniques, where issues such as thermal lag can negatively impact data precision. In this work, a method using multiple tests at different heating rates has been applied to these three techniques to eliminate the effect of thermal lag as well as assess other variables that can influence test data; specimen moisture condition, specimen thickness and fibre type. It was shown that while thermal lag can be accounted for, there are remaining slight differences between DMA, TMA and DSC Tg data, which can be expected due to the different response modes involved (e.g. mechanical, thermal expansion, calorimetric). For DMA testing, a simple relationship has been proposed, relating heating rate and specimen thickness, which can account for the effect of thermal lag when comparing data obtained for specimens of different thicknesses or for the same thickness at different heating rates; the relationship is supported by relevant experimental evidence. It was shown for materials with different degrees of cure that the relationship between Tg and degree of cure followed the same trend regardless of differences in Tg measured by the three techniques. Preliminary experiments indicated that FTIR showed promise for measurement of the degree of cure of composite materials, in addition to measurements by DSC