Average heat release rate

FIGURE 16.8 Average heat release rate (HRR, 60s average) versus external irradiance (heat flux) for four HIPS formulations. The FR in 2-HIPS was bromine/antimony oxide, 3-HIPS contained no FR, 9-HIPS contained bromine/antimony oxide, and 18-HIPS contained a nonhalogen FR. The UL 94 rating for each formulation is indicated on the plot. The uncertainty in the HRR is 5% (2o). 

A comparison of the combustion properties of EPDM composites containing wet ground and precipitated ATH (Table 17.4) shows significant differences. It is likely that the lower average heat release rate (HRR) and average mass loss rate for the wet... 

Average heat release rate in flaming combustion versus char yield of polymers. 

Fig. 18. Average heat release rate versus heat release capacity of polymers.

The average heat release rate, expressed in J/m, corresponds to the total heat release averaged over a specific period of time (usually 5 minutes) it can be understood as a measure of the heat contribution to a sustained fire. 

The effect of the additives on the oxygen index of PMMA, PS and nylon-6, 6 has also been measured. [227). The results are shown in Table 5.5. The trend in oxygen index response for PMMA and PS is similar to the trend in the peak and average heat release rate data from the cone calorimeter. Scung reports similar results fo r polypropylene with additives or fillers, in their comparison between cone calorimeter and traditional tests (oxygen index, glow wire test, etc.) [229]. The correlation is very poor, however, between the LOI and rate of heat release for nylon-6,6 with silica gel / KjCOj additives. 

The average heat release rate, during the overpressure is approximated by... 

Because the release rate of flammable fuel is reduced, the heat release rate is reduced as well. However, the char-clay barrier only slows the release of fuel—it does not fully prevent it—so a polymer nanocomposite will slowly bum until almost all the carbon mass has been pyrolyzed and combusted, which means that the total heat release for a polymer-clay nanocomposite is unchanged from that of the base polymer, but the peak heat release and average heat release rates are lowered. 

Figure 13 shows representative HRR histories for thermally thick and thin samples of polymers that gasify completely or form a char during burning. It is apparent that none of these heat release rate histories show a constant (steady-state) value of heat release rate over the burning interval as presumed in equation 45. Consequently, an average heat release rate for the entire test or over some time interval is typically used in place of a steady HRR. In fact, the... 

A typical heat release rate curve for a neat epoxy system and the respective layered silicate nanocomposite, is shown in Fig. 2.12. Both peak and average heat release rate, as well as mass loss rates, are all significantly improved through the incorporation of the nanopartieles. In addition, no increase in specific extinction area (soot), CO yields or heat of combustion is noticeable. However, the mechanism of improved flame retardation is still not clear and no general agreement exists as to whether the intercalated or exfoliated structure leads to a better outcome. The reduced mass loss rate occurs only after the sample surface is partially covered with char. The major benefits of the use of layered silicates as a flame retardation additive is that the filler is more environmentally-friendly compared to the commonly used flame retardants and often improves other properties of the material at the same time. However, whilst the layered silicate strategy is not sufficient to meet the strict requirements for most of its application in the electrical and transportation industry, the use of layered silicates for improved flammability performance may allow the removal of a significant portion of conventional flame retardants. 

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