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Cone Calorimetry

The cone calorimeter (ISO 5660) is a widely used technique for physically modeling flaming combustion penetrating into the bulk of a polymeric material. The sample (100 mm x 100 mm) is subject to uniform irradiation from the conical heater, typically with a flux of 25, 35, or 50 kWm~2, in the presence of a spark igniter. The effluent is drawn into a hood, and the oxygen (O2) consumption is precisely determined. For most materials containing carbon, hydrogen, oxygen, and nitrogen, the heat release may be calculated as 13.1 kJ per g O2 consumed.

Results from the cone calorimeter increasingly dominate reports of the efficacy of fire retardant systems, although it features in few regulatory requirements. A detailed description of its use for assessment of fire retardant behavior has been given elsewhere (Schartel and Hull 2007). Unlike the LOI, the effects of dripping are negligible, since the solid/liquid sample is contained within a foil tray. The key flammability parameters obtained from cone calorimetry are the time to ignition (TTI) and the heat release rate per unit area (HRR). As flame spread can be viewed as a series of repeated ignitions, surface spread of flame is likely to be controlled by the time to ignition. The overall heat release rate (Q) is the most important parameter controlling the fire growth rate. In the cone calorimeter and other standard protocols, only the heat release rate per unit area (HRR) is measured, since surface spread of flame is prevented by the small sample size (100 x 100 mm). However, uncontrolled fires spread across surfaces as well as penetrating into them, and the overall heat release rate (Q) is the product of the heat release rate per unit area (HRR) and the burning area (A). If the speed of flame spread is assumed constant (vf and the fire spreads as a growing circle (of radius r), the heat release rate will increase in proportion to the square of time (Celzard et al. 2011).

Fire safety engineers frequently refer to this as a “t2 fire.” Thus a major limitation of cone calorimeter data is that the only indication it gives of flame spread rate is through the time to ignition parameter.

The time to ignition will be a function of the time taken for the surface temperature to reach the critical value for ignition. This depends on the thermal inertia of the material (the product of heat capacity, thermal conductivity, and density); on the absorption of radiation, dependent on the absorptivity and emissivity of the sample; all of which will change on incorporation of a filler, though only the heat capacity and the decomposition endotherm of the filler, contributing to the heat capacity, have been included in this simplified analysis.

After ignition, the heat release rate per unit area increases to a peak value (pHRR), an important parameter controlling fire growth, provided the fuel has significant thickness, and the rate of burning increases as it penetrates into the bulk of the material. At the pHRR, adjacent flammable articles are most likely to have their own critical heat fluxes for ignition exceeded, and thus ignite, contributing to the conflagration. However, this mode of burning is essentially penetrative, with the flame front moving towards the fuel above the gas-polymer interface. In this case, the physical presence of the filler residue will exert a greater influence on the burning behavior (seen as a dramatic reduction of pHRR) through its heat capacity, its absorption and emission of radiation, and its physical blocking of the route from fuel to flame. Thus a polymer containing a mineral which formed a coherent residue would be expected to show a HRR curve like that of a char forming material (such as wood) rather than that of a normal thermally thick burning (Schartel and Hull 2007). This is shown in Fig. 5. In this case the endothermic release of diluent gas may be counterproductive, encouraging greater flow of the protected fuel through the filler residue towards the surface, as the filler decomposes and gas release from below disrupts the protective layer.

In a study in the effect of nanoparticulate fillers, Schartel and Weip (2010) concluded that their major effect as fire retardants was to absorb and reemit radiation, and this effect increased with increasing surface temperature (following Stefan’s Law) / T4. While the conclusions of Schartel’s work are controversial, the capacity of fine or ultrafine particles to reach high temperatures, where they become highly

Idealized heat release curves for a range of sample types and behaviors

Fig. 5 Idealized heat release curves for a range of sample types and behaviors

effective radiation shields (as absorbers and reemitters), has an obvious bearing on the present work. However, other sources cite different reasons for the particle size effect (Hughes et al. 1993).

It is worth noting that while the burning behavior of polymers containing mineral fillers will be modified, not all these effects will be evident in the cone data. It has been observed that the particle size effect, evident in the LOI and UL 94, is less evident in the cone calorimeter (Herbert 1994). The time to ignition will be delayed, and the peak heat release rate may be delayed and smaller then if the same mass of polymer was burned in the absence of the mineral filler, but the total heat released and hence the effective heat of combustion will not be reduced by the heat absorbing effects of the filler (typically twice the decomposition endotherm). This arises through the use of oxygen depletion calorimetry, which relates the heat release to the oxygen consumption as 13.1 kJ of heat released per gram of oxygen consumed. Any endothermic effects, which would be seen by a thermometric device, or in a real fire, will not be observed using oxygen depletion calorimetry. For example:

For a PE-ATH mixture (33.3:66.6%), the effective heat of combustion (EHC) for PE is 43 kJ g-1; in this mixture, the cone calorimeter would record an EHC value of

  • 14.3 kJ g-1 (43 x 0.33). However, the actual EHC would be that recorded by the cone minus the endothermic effects of the mineral filler, in the case of ATH,
  • 2.3 kJ g-1 (14.3-2.3 = 12.0 kJ g-1) (from data in Fig. 2). This results in an error of approximately 16%.

This is an artifact of the technique and corrections need to be incorporated into measurement of total or effective heat of combustion, wherever oxygen depletion calorimetry is used to assess fire performance of materials containing endothermically decomposing mineral fillers. However, the delay in the time to ignition and the flattening of the heat release rate curve will still be apparent in cone calorimeter tests on polymers fire retarded with mineral fillers.

 
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