Polyurethanes for potential use in transparent armour investigated using DSC and DMA

A material combination that may be applied as transparent armour is glass-clad polyurethane. These are comprised of a relatively thin glass strike face and a relatively thick (transparent) polyurethane backing layer. Three transparent polyurethane samples were investigated using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The DSC results identified the glass transitions and in some cases the melting and crystallisation processes. The DMA experiments were only performed on heating around the glass transition region to further investigate this transition. The experiments were performed at three different frequencies (1, 10 and 100 Hz); the shift of the glass transition with the frequency was clearly observed. The method of time-temperature superposition was used to extrapolate the results to higher frequencies as the magnitude of the strain-rate occurring within ballistic applications is in the order of 1000 s(-1) or higher. Polyurethane with a rubbery behaviour at normal (low) strain rates can be stiff and brittle when used as an armour component (temperature below its dynamic T (g) value).