The mechanism of the influence of radiation heat flux on the combustion behavior of raw rubbers.

• BR and SBR have two HRR peaks at heat fluxes lower than 20 kWm−2. • At the ignition time, the crosslink degree of surface layer reaches 84.23–100 %. • The model considering the crosslink reaction can predict the two-peak HRR curve. • At low heat flux the crosslink reaction has a long residence time before ignition. • The first HRR peak results from the crosslink reaction. Understanding the combustion behavior of rubbers is crucial for predicting their fire hazards. The cone calorimeter experimental results in this study confirm that the combustion behavior of raw rubbers depends on the rubber type and the heat flux. Natural rubber and ethylene-propylene-diene monomer rubber exhibit heat release rate (HRR) curves with only one peak at the end of the burning stage under all tested heat fluxes. At higher heat fluxes, the HRR curves of polybutadiene rubber (BR) and styrene butadiene rubber (SBR) exhibit a single peak; at heat fluxes lower than 20 kWm−2, two HRR peaks were observed. The proposed mechanism model with the crosslink reaction as the key factor can explain the emergence of two-peak HRR curve and has been proved through experiments and simulations. For BR and SBR at 15 kWm−2, the specimens exhibit two upper layers at ignition time. The surface pyrolysis layer seems to be covered by black film or char, with the crosslink degree as high as 84.23–100 %. The second crosslink reaction layer exhibits a higher decomposition temperature T d, 5 % than the original rubber. Based on reaction kinetics and thermodynamics data derived from TG/DSC experiments, the novel pyrolysis model taking crosslink reaction and thermal decomposition of crosslinked intermediate into account well predicts the HRR curves obtained at varied heat fluxes. The conversion of raw rubber to the crosslinked intermediate plays an important role in the reduction of HRR immediately after ignition, resulting in the first HRR peak. [ABSTRACT FROM AUTHOR]