Burning rate char effect

Wood Wood and wood products, like pulp, paper, and cardboard, will burn. There is considerable variation in the heat value of different wood species and products. During combustion, wood will char. In charring, wood forms a layer of partially burned material. This layer insulates material below it from the heat of combustion and slows the burning rate. In fact, wood can retain many of its stmctural properties in a hre for some time. The stmctural properties remain because of the insulating effect of the char formation. 

With greatly reduced burning rate, heat release, and char formation, it is possible to combine nanofiller with traditional flame retardants to achieve satisfactory regulatory approvals such as various UL ratings. In fact, several studies have revealed synergistic effects of combinations of nanofillers with traditional microsized flame retardants. 

Myers et al. reported that partially dehydrated APB is an effective intumescent flame retardant in thermoplastic polyurethane.77 APB at 5-10 phr loading in TPU can provide 7- to 10-fold improvement in burn-through test. It is believed that in the temperature range of 230°C-450°C, the dehydrated APB and its released boric oxide/boric acid may react with the diol and/or isocyanate, the decomposed fragments from TPU, to produce a highly cross-linked borate ester and possibly boron-nitrogen polymer that can reduce the rate of formation of flammable volatiles and result in intumescent char. 

Wood is a combustible material. However, depending on moisture content, it may not collapse rapidly due to the insulating effect that char provides. Treatment of wood and some other combustible materials with chemicals or finishes can slow the rate of burning. 

The RHR plots for PP-MAPP-Cloisite 20A nanocomposite and PP at 35 kW/m heat flux shown in Figure indicate a 60% - decrease of peak of RHR (Fig. 11). Comparison of the Cone calorimeter data PP and PP-MAPP- 7% Cloisite 20A reveals that the specific heat of combustion (He), specific extinction area (SEA), a measure of smoke yield, and carbon monoxide yields are practically unchanged this suggests that the source of the improved flammability properties of these materials is due to differences in condensed-phase decomposition processes and not to a gas-phase effect. The primary parameter responsible for the lower RHR of the nanocomposites is the mass loss rate (MLR) during combustion, which is significantly reduced from the value observed for the pure PP (Fig. 12). It is supposed, that this effect is caused by ability to initiate the formation of char barrier on a surface of burning polymeric nanocomposites that drastically limits the heat and mass transfer in a burning zone. 

The primary effect of the nano-clays seems to be related to char formation. The workers at NIST have found that a reduction in mass loss and heat release rate only starts once the surface of the polymer is at least partly covered by char. Beyer reported that, while no char was produced by burning unfilled EVA, the filled composite formed a strong char early in the process [54]. Once the amount of clay is taken into account, final char levels are often similar to unfilled polymer, indicating that while a stronger, more insulating, char may form and retard combustion, it is eventually consumed in this test. 

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