Combustion behaviour and dominant shrinkage mechanism of flexible polyurethane foam in the cone calorimeter test

The shrinkage is an important thermal response of flexible polyurethane foam (FPUF) in fire, which couples its first combustion stage and influences the initial fire spread. In this paper the combustion and shrinkage behaviours are quantitatively investigated and the shrinkage mechanisms are discussed. The critical heat flux for the shrinkage is about 13 kW/m², between the critical heat flux for piloted ignition and that for non-piloted ignition. Above the critical value the shrinkage rate increases linearly with increasing the heat flux. As the foam density decreases both the shrinkage rate and the first peak of Figra curve which reflects the initial fire spread rate increase. The perceptible shrinkage originates from the decomposition via which the struts convert to the melts. Both the shrinking of struts and the gasification play a minor role. The collapse of porous structure, namely the melts filling into the pores to form the tar layer, dominates the shrinkage. The tar is composed of polyols produced after the first decomposition stage. Beneath the tar layer the porous structure is unchanged. To reduce the fire hazards of FPUF by its heat insulation merit it needs to maintain the porous structure at the first decomposition stage.