Polymers 2021 , 13 , 709
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The temperature at the upper surface of the sample rose sharply to about 700 ◦ Cafter ignition (Figure 8). The temperature at the bottom surface of the sample rose slowly and had a comparable course in all heat fluxes (Figure 8). The temperature curves have a smooth linear course and show dependence of the temperature at the upper surface on the time of exposure to radiant heat with heat fluxes of 44kW.m − 2 and46kW.m − 2 (Figure 8). After ignition, there was a sharp rise in temperature at the upper surface of the sample (Figure 8). The sharp rise in temperature was likely caused by two effects. The first effect was a higher heat flux from the cone calorimeter to the surface of the tested sample and the second effect was a higher reverse heat radiation from flares [35]. Rantuch et al. [21] divided the OSB combustion process into five phases based on extensive research on OSB (14 mm thickness) using a cone calorimeter with heat fluxes of 20, 30, 40, 50 and 60 kW.m − 2 . The first phase entails a period prior to the sample ignition. The heat received by the samples is consumed while heating them, but the concentration of gaseous combustible products during thermal degradation is not sufficient to initiate the flame combustion. The second phase occurs after ignition of the sample. The amount of released flammable gases is sufficient to maintain a steady-flame combustion and at the same time a carbonized layer has not yet formed on the surface of the OSB. The mass burning rate as well as the rate of heat release have, therefore, high values. After the formation of a carbonized layer (Figure 4e) on the exposed surface of the samples, a steady burning phase occurs. The rate of heat release is almost constant, and the weight loss is uniform. This is followed by the burning phase of the superheated sample, characterized by significant pyrolysis in its entire volume (Figure 4d). The increase in the concentration of pyrolysis products is manifested by a more pronounced combustion and, thus, also by an increase in the rate of heat release. After complete burnout of the gaseous pyrolysis products, flameless combustion of the sample occurs with low weight loss [21]. OSB is often a part of the structural elements in the exterior, and therefore it is also crucial to carry out large-scale fire tests [36–38]. Sultan [36] showed that the effect of insulation types on the fire resistance of exterior wall assemblies with OSB sheathing can be considered significant for specific conditions. 4. Conclusions The ignition process cannot be defined by a single characteristic. Our research mon- itored important parameters, specifically type of the sample (OSB), thickness, heat flux density, weight loss, ignition temperature and critical ignition temperature, and modified conditions specified in the test procedures. OSBs with a thickness of 12, 15 and 18 mm were analysed with a focus on the critical heat flux and the ignition temperature. The ex- perimentally determined value of the heat flux density was 43 kW.m − 2 , which represented the critical heat flux. In all cases, dependences of the ignition time on the external heat flux were confirmed and there was a correlation between the ignition time, weight loss and intensity of radiant heat. The following results were obtained from the conducted experiments: • As the heat flux density increased, the ignition time decreased in all thicknesses of analysed OSB. • The ignition time increased with increasing thickness of the OSB, and the weight loss decreased with increasing thickness of the OSB at a constant heat flux. • With increasing board thickness, the weight loss decreased. The largest average weight loss of 24.31% was recorded in a 12 mm OSB that was exposed to a radiant heat flux of 50kW.m − 2 ; the lowest average weight loss of 12.44% was recorded in a sample of 18 mm OSB that was exposed to a radiant heat flux of 43 kW.m − 2 . • The ignition time is significantly dependent on the thickness of the OSB sample and on the value of the heat flux. As the heat flux increases, the ignition time shortens; as the thickness of the OSB increases, the ignition time extends. The largest weight
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