Measurement and investigation of intumescent char strength: Application to polyurethanes

This work investigates the parameters influencing the mechanical resistance of intumescent chars. A polyurethane matrix containing 30 wt% of ammonium polyphosphate was used in this study as intumescent model system. Three heating devices (rheometer furnace, mass loss calorimeter, and tubular oven) were used as heat treatment scenarios to obtain a charred structure. This char is then analyzed by measuring its compression resistance using a protocol involving a plate–plate rheometer (estimation of the char strength). The study of the char strength reveals that the presence of a flame, the heating type (convective or radiative heating), and the quenching temperature do not modify the mechanical properties of the resulting intumescent char. On the contrary, it was found that the critical parameters involved in the formation of a resistant char are the heating temperature and the heating rate. The strongest charred structure is reached after a heat treatment at 10°C/min until 500°C. Additionally, the modification of the temperature at which the mechanical test is performed (linked to the experimental protocol) leads also to different mechanical resistance of the tested materials.