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Mineral filler fire retardants represent one of the most important classes of fire retardant additive, exceeding all others by weight, if not by value. This is partly because they are available at relatively low cost, and need to be used at high loadings, but also because they provide the most effective, sustainable solution. As “fillers” they can enhance a material, providing structural integrity to the polymer, as described elsewhere in this volume, and if they are cheaper than the host polymer, may also reduce the unit cost.

Unlike halogenated flame retardants, whose selection only requires compatibility with the host polymer and a decomposition temperature around that of the polymer, mineral filler fire retardants also place other demands on formulators, arising from unwanted changes in physical, mechanical, and electrical properties, and particularly compatibility at high loadings.

A simple model has been described to quantify the relative contributions to the fire retardant effects of mineral fillers. The model is clearly not the end of the story but allows simple physical phenomena to be quantified in order to isolate more complex chemical and rheological phenomena. For example, the predicted similarity in performance between HMH and ATH in EVA is not observed in practice in the cone calorimeter, where the platy nature of the huntite reinforces the residue, reducing the main peak of heat release rate (Hollingbery and Hull 2012).

All fire retardant formulations require careful screening and evaluation. In many cases, the results of small- or bench-scale tests cannot be directly correlated to the behavior of real unwanted fires. It is essential to fully understand the characteristics of the test methods being employed when formulations are being optimized and to understand their relationship to the large-scale test. In particular, the use of oxygen depletion calorimetry (in the cone calorimeter (ISO 5660), microscale combustion calorimeter (ASTM D 7309), the single burning item test (EN 13823), and the bunched vertical wire cable test (IEC 60332-3) means that the beneficial effect of endothermic dehydration is not accounted for in the total heat release calculation.

The future drivers in fire retardant development are likely to focus on sustainability - although this has yet to be clearly defined, and many developers claim advances in sustainability while only focusing on a single aspect. With time, product specifiers will become more precise about their sustainability requirements, and it will be more difficult to hide behind overhyped claims, or “greenwash.”

In conclusion, mineral filler fire retardants are an essential tool for reducing the flammability of synthetic polymers and in many cases are capable of doing so without adverse effects on the environment or toxicity of the fire effluent. The greater expense of developing suitable formulations is largely offset by the difficulties competitors face in trying to reverse engineer them in order to copy them.

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