Improving the flame retardancy of polypropylene foam with piperazine pyrophosphate via multilayering coextrusion of film/foam composites

A series of 16-layer polypropylene/flame retardant (PP/FR) film/foam composite structures were produced by microlayer coextrusion. A highly branched PP was used in the foam layers to increase strain hardening and cell stability, while the PP used in the film layers was a high shear viscosity grade to confine bubble growth. In addition to improved tensile properties, the PP/FR composite film/foams exhibited five times the compression modulus of PP/FR composite foams at each FR loading level. The thermal stabilities of the composites were investigated, exhibiting three step decompositions. The FR particles were effective in decreasing flammability by forming intumescent char. The PP/FR-film/foam-20 showed self-extinguishing behavior in a modified vertical burn test, while the PP/FR-foam-20 sample continued to burn. Cone calorimetry demonstrated that PP/FR film/foams had lower heat release than PP/FR foams due to the unique alternating film/foam structure of PP/FR film/foams. Scanning electron microscopy imaging of the residual chars from fire testing that the PP/FR composite film/foams showed a more continuous protective char surface when compared with PP/FR composite foams at each FR concentration. The combined data indicate that the formation of a surface film on top of a foam ensures a robust intumescent fire protective barrier for partly foamed materials and shows a new way toward lightweight materials with improved fire safety performance. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48552.