Flammability and vertical flame spread for metal-polymer layered composites

The flammability and vertical flame spread behaviours of metal-polymer composite panels were investigated experimentally. Experiments were performed on specially created composite products comprising alternate layers of polymer (polymethyl methacrylate) and metal. Where polymer layers were thick they behaved akin to homogeneous plastic. However, where the polymer layers were thinner, the influence of the metal began to affect both the ignition and burning behaviour – delaying onset of ignition, and increasing the heat release rate per unit area of plastic. As the polymer layers became thinner, it was found that the composite product began to act as one ‘bulk’ composite, which was highly influenced by its thermal boundary conditions. A classification framework was set out to conceptualise and classify these behaviours, based on the idea of ‘local’, ‘product’, and ‘system’ behaviours. These classifications were also found to be applicable for vertical flame spread where composites transitioned between different behaviours based on the applied heat flux and the heating time. It was found that the transition between behaviours could also be explained using existing flame spread theory. Overall the study found that the flammability and flame spread of metal-polymer composites was largely controlled by either local behaviours or system boundary conditions.