Shrinkage And Combustion Behaviors Of Flexible Polyurethane Foam

Polyurethane(PU)is a class of polymers usually made by the polyaddition of polyisocyanates and macro-polyols.PU materials have an excellent performance.The fire risk analysis and fire prevention have been received extensive attention from scholars.Flexible polyurethane foam(FPUF)has the shrinkage response characteristics in fire conditions.There is a huge demand for FPUF in life.When FPUF was ignited,it will melt and formed fluid,then caused poo fire owning to its extreme flammability.FPUF has the flow characteristic so that So the controllability is very difficult.Based on the above reasons,FPUF was chosen to study in this paper and foused on its shrinkage behaviors and combustion behaviors in fire conditions.The shrinkage and combustion behaviors of flexible polyurethane foam(FPUF)under fire conditons were studied in this paper and the effects of density of FPUF and external heat radiation flux on the shrinkage and combustion of FPUF were considered.Through the above study,the mechanism of shrinkage was discussed and the model of shrinkage mechanism was established.In addition,the combustion behaviors of FPUF was researched by the cone calorimeter and the horizontal flame propagation process of FPUF was studied on the self built fire spread device.The cone calorimeter was used to research the shrinkage process of FPUF before ignition.The effects of density and the heat radiation flux on the shrinkage process were discussed respectively.The results shows that as the samples density increased,the shrinkage rate of PFUF before ignition decreased,and as the external radiaion heat flux increased the shrinkage rate of FPUF before ignition increased.The critical shrinkage temperature of FPUF is 236.3°C which is measured by hot microscope.The mechanism of shrink is the cell wall of FPUF pyrolysised to form the tar under the radiation heat flux and then the viscous tar flow to fill the porous,which caused the collapse of cell structure and represented shrink.A black tar layer was generated in the surface of FPFU when received the external radiation heat flux and as the density of FPUF increased the thickness of tar layer decreased.The combustion process of FPUF can be divided into three stages:the shrinking before ignition,the combustion with shrinking and the pool fire.All curves have only one peak and the time to the HRR peak increases with increasing the foam density.For FPUF30,FPUF40 and FPUF50 there is a platform before the peak.For FPUF15 the Figra curve has only one peak.For FPUF30,FPUF40 and FPUF50 there are two peaks,and the first peak decreases and the second increases with increasing the foam density.For FPUF30 the first peak is higher than the second while for FPUF40 and FPUF50 the second peak is higher.It indicates that as the foam density decreases the hazard of fire growth becomes important early.The critical heat flux of FPUF was 11.9kW/m~2 when external ignition source existenced,and the critical ignition heat flux is 18.8kW/m~2when there is no external ignition source.Besides,The limiting oxygen index of FPFU did not change with the density of FPUF.The above results showed that the hazard of fire growth increased with the density and the external radiation heat flux,which suggested that the storage of FPUF for different densities should be classified.The experimental results of horizontal combustion of FPUF showed that as the FPUF samples height and width increased the fire spread rate and the mass loss rate increased respectively.The pyrolysis of FPFU could be divided into two stages:the decomposition FPUF and generated the polyols and isocyanates and the redecomposition of polyols.The existence of oxygen promoted the occurrence of thermal decomposition reaction of FPUF and made the reaction ahead of time which was compared the two kinds of pyrolysis environments.