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Fire Retardant Effects of Mineral Fillers
It is necessary to ensure that the decomposition of the mineral fillers intended for use as fire retardants occurs within a temperature range similar to that of its host polymer. Table 3 shows typical melting and decomposition temperatures of some common polymers - this is important information for selecting an effective fire retardant for the particular polymer matrix. The “processing window” lies between the melting
Table 3 Decomposition temperatures of some common polymers
point and decomposition temperature; the filler must not decompose within the processing window.
The polymers listed in Table 3 demonstrate a considerable range of melting and decomposition temperatures. Comparing these with the decomposition temperatures of the mineral fillers in Table 2, it could be said that ATH, MDH, hydromagnesite, and HMH mixtures tend to be suitable for most of these materials. Due to its low cost, ATH is a popular choice, but for those polymers that are processed above its decomposition temperature, MDH and HMH mixtures would be more suitable. For polymers that decompose at higher temperatures, for example, PEEK and PTFE, fillers such as ATH and MDH would undergo decomposition too early and be unable to interfere with the decomposition of the polymer matrix. These polymers would be better suited to mineral fillers with higher decomposition temperatures, although few are in common use.
The fire retardant effects of mineral fillers can be illustrated using ATH as an example. ATH decomposes to form alumina (Al2O3) with the release of water. It breaks down endothermically forming water vapor, diluting the radicals in the flame, while the residue of alumina builds up to form a protective layer.
Organic polymers have heat capacities (Stoliarov et al. 2009) ranging from 0.9 to 2.1 J K-1g-1, thus the decomposition enthalpy of a fire retardant mineral filler is a factor of 1000 larger - the decomposition enthalpy of 1 g Al(OH)3 is equal to the heat (q) required to raise the temperature of a mass (m) of 1.5 g of low density polyethylene (LDPE) from ambient temperature to decomposition (400 °C) (Д0), assuming constant heat capacity (c) during heating (q = m c Д0, soq = 1.5 x 2.3 x 375 = 1.29 kJ). ATH is also an effective heat conductor and acts to reduce local hotspots, which are responsible for starting fires (Table 4).
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