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Quantifying the Fire Retardant Effects
The decomposition data of the mineral fillers in Table 2, together with the average values of heat capacities of the filler, its residue, and its gaseous decomposition products have been used to estimate the heat absorption by the filler (% Filler), the residue (% Residue), the evolved water vapor and carbon dioxide (% Gas), and the decomposition endotherm (% Decomposition Endotherm) shown in Table 5.
The figures generally show broad similarities - in all cases, the greatest contribution comes from the endothermic decomposition. The decomposition temperature clearly affects the relative contributions of the heat capacity of the filler and those of the residue and gas. It can be seen that in some cases, e.g., Al(OH)3, and MgCO3.3H2O, the heat capacities of the gas phase products exceed that of the condensed phase, where for others, e.g., Mg(OH)2 and Ca(OH)2, the solid phase contribution is greater. Comparing aluminum and magnesium hydroxide, it is
Table 4 Fire retardant effects of mineral fillers
Table 5 Relative contribution of heat absorbing effects for potential mineral filler fire retardants (Hull et al. 2011)
aUltracarb is the trade name for HMH mixtures containing hydromagnesite and huntite at a ratio of around 60:40
evident that the difference between their relative effects arises from the higher decomposition temperature of Mg(OH)2, giving a larger contribution to the heat capacity of the undecomposed filler, but a smaller contribution from the heat capacity of the residue and the heat capacity of the (greater volume of) water vapor released by the Al(OH)3 - even though the energy for such a release is similar for both fillers. Where two separate decomposition processes occur, such as in nesquehonite, which loses water at 100 °C and carbon dioxide at 450 °C, the heat capacity of the intermediate MgCO3 has been included in the residue contribution, together with MgO. The low decomposition temperature of nesquehonite and the high volume of volatiles make this an interesting candidate material for polymers with very low (~100 °C) decomposition temperatures. For the hydromagnesite, huntite, and Ultracarb mixture, which is often proposed as an alternative to Al (OH)3, it can be seen that the higher decomposition temperature of huntite gives a greater filler contribution, and overall Ultracarb is intermediate between aluminum and magnesium hydroxides.
As the data have all been calculated in energy units, the contribution to the individual fillers may also be compared in absolute terms. Figure 2 shows the energy absorption per gram of each of the processes undergone by the filler.
The higher decomposition temperature of magnesium hydroxide and particularly the greater contribution of the filler increase its energy absorbing capacity by about 250 J g-1, compared to aluminum hydroxide, while the lower endotherm and smaller volatile heat capacity of calcium hydroxide also indicates inferior potential as a fire retardant. The large endotherm and higher vapor heat capacity of nesquehonite suggest its potential as a superior fire retardant additive, except that it decomposes at such a low temperature (onset of decomposition occurs between 70 and 100 °C).
Fig. 2 Absolute estimation of heat absorbed by potential fire retardant mineral fillers
The behavior of hydromagnesite is very similar to that of aluminum hydroxide, where the smaller endotherm and vapor heat capacity of huntite reduce the apparent fire retardant potential of the Ultracarb mixture below aluminum hydroxide. In practice, the Ultracarb mixture is often found to outperform either hydromagnesite or aluminum hydroxide as a fire retardant. This has been ascribed to the platy morphology of huntite, reinforcing the barrier properties of the residual layer (Hollingbery and Hull 2010). The much lower energy absorbing capacity of boehm- ite, together with reports (Laachachi et al. 2009) of its successful application as an additive fire retardant, suggest that other mechanisms must also be operating.
It is evident from Fig. 2 and Table 5 that endothermic decomposition accounts just over half of the fire retardant effect for the mineral fillers considered. For the example of LDPE and Al(OH)3 considered earlier, the total heat absorbed by 1 g of filler could otherwise have heated almost 4 g of LDPE to its decomposition temperature.
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