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Mineral Filler Fire Retardants
Mineral fillers represent an important class of alternative, “halogen-free” fire retardants. While the Oxford English Dictionary defines “mineral” as “a solid, naturally occurring, usually inorganic substance with a definite chemical composition and characteristic physical structure and properties (such as crystalline form),” within the scope of this chapter, this definition has been extended to include synthetic inorganic substances with fire retardant applications, in line with usage within the industry. Mineral fillers are more difficult to incorporate into a polymer at the loadings necessary to achieve the required level of fire retardancy than halogenated flame retardants, so it is vital to understand their mode of action and the options available.
The materials of interest include: aluminum hydroxide, magnesium hydroxide, magnesium carbonate, and mixed magnesium/calcium carbonates and hydroxides, such as naturally occurring mixtures of huntite and hydromagnesite. Of course, several other inorganic materials with fire retardant applications fit the definition of “mineral fillers,” such as nanoclays, zinc borates, and zirconium phosphates, although they are typically used at lower loadings.
All mineral fillers act in the condensed phase, diluting the combustible polymer, forming barriers to heat and mass transfer, and resulting in a subsequent reduction in the rate of mass loss and heat release. However, the focus of this chapter is those mineral fillers that also contribute to fire retardancy through endothermic decomposition in the condensed phase, releasing inert gas and leaving a solid residue. In order to be effective, the decomposition must occur in a narrow window above the polymer processing temperature, but at or below its decomposition temperature. The different types of mineral filler fire retardants are described, and their modes of action and applications discussed. The common test methods used to assess the performance of fire retardant polymer composites are also described, and their environmental sustainability and recycling potential, a topic of increasing concern and relevance, is evaluated.
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