Anisotropy of expandable graphite to explain its behavior as a flame-retardant

Expandable graphite is used as a flame-retardant in polymers. Its expansion on heating leads to a network of graphite worms which acts as a thermal barrier. However, mechanisms of action of worms are not yet well known. An original experimental approach is performed to study the heat dissipation in the network of worms. The network is made by the burning of polypropylene with 10 wt% expandable graphite during cone calorimeter experiment. After the burning, a hot spot is applied on the char. Temperature is monitored at different locations of sample during the combustion and after the application of the hot spot. During cone calorimetry, the char develops homogeneously over the whole sample. The hot spot test evidences the anisotropy of the entangled network of graphite worms. This anisotropy of heat conductivity allows the dissipation of heat in-plane and poorly out of plane, which explains the thermal barrier effect made by entangled worms.