Retardant fillers aluminium hydroxide

Antimony trioxide and chlorinated paraffinic derivatives are common materials used as fire retardants, as are intumescent zinc (or calcium) borate, aluminium hydroxide and magnesium hydroxide. These inorganic materials, used as bulk fillers, act to reduce the fire hazard. Halogenated materials release chlorine, which then combines with the antimony trioxide to form the trichloride, which is a flame suppressant. 

The carbon black generated by a fire from a rubber source increases the smoke density other products are highly toxic and often corrosive. The halogens, phosphates, borates, and their acids evolved during a fire corrode metals and electrical and electronic equipment. Hence many of the fire retardants described below cannot be used in situations where the toxic gases evolved will create their own hazards. In these cases inorganic hydroxides are used, at filler-type addition levels. Aluminium hydroxide and magnesium hydroxide are used as non-toxic fire retardant systems. 

Aluminium hydroxide is essentially non-toxic, but does require high addition levels to be effective. As a result, the physical properties of the compound usually suffer. Its fire retardancy action results from the endothermic reaction which releases water under fire conditions and produces a protective char . The endothermic reaction draws heat from the rubber/filler mass and thus reduces the thermal decomposition rate. The water release dilutes the available fuel supply, cooling the rubber surface and mass. 

The performance of aluminium hydroxide/magnesium hydroxide-filled systems can be enhanced by incorporation of zinc hydroxystannate in halogen-free rubbers giving reduced smoke and toxic gas emission, coupled with higher flame retardancy. This action will be complimentary to the water release and endothermic effects of aluminium hydroxide/magnesium hydroxide filler systems. 

Aluminium hydroxide has a Moh hardness of about 3 and a specific gravity of 2.4. It decomposes endothermically with the release of water at about 200 °C and this makes it a very useful flame retardant filler, this being the principal reason for its use in polymers. The decomposition temperature is in fact too low for many thermoplastics applications, but it is widely used in low smoke P VC applications and finds some use in polyolefins. For these applications low aspect ratio particles with a size of about 1 micron and a specific surface area of 4-10 m g are preferred. The decomposition pathway can be diverted through the mono-hydrate by the application of pressure, and this may reduce the flame retardant effect [97]. This effect can be observed with the larger sized particles. Although it is chemically the hydroxide, it has for many years been known as alumina trihydrate and by the acronym ATH. 

Consideration is given to the influence of combinations of zinc hydroxystannate (ZHS) with hydrated fillers, on the fire properties of plasticised PVC and polychloroprene. It is shown that magnesium and aluminium hydroxides specially coated with ZHS, confer significantly increased combustion resistance and lower levels of smoke evolution to these polymers. This permits large reductions to additive loading relative to unmodified filler, without sacrificing flame retardant or smoke suppressant performance. 10 refs. 

Intrinsically fillers can be divided into two types, reactive and inert. Reactive fillers will react with their environment. A good example of this is gibbsite (aluminium hydroxide), which will react with both acidic and basic substances. Aluminium hydroxide also loses its water of crystallisation at around 200 °C and this enables it to provide fire retardancy in polymer formulations. The silicate minerals (kaolin, mica, talc, quartz, etc.), are, in classical chemical terms, virtually inert, only being attacked by very strong acids and alkalis. The carbonate minerals and the hydroxide minerals are very reactive to acids. 

This is by far the most widely used flame-retardant filler, being available at relatively modest cost and with a wide range of particle sizes, shapes and surface treatments to suit various applications. Although its chemical structure is that of the hydroxide, it is often referred to as alumina trihydrate (AI2O3.3H2O) or simply ATH. There is more than one crystal form of aluminium hydroxide, but that used as a flame retardant is gibbsite. For convenience the common acronym, ATH, will be used throughout this book. 

The first example examines the adsorption of maleanised polybutadiene (MPBD) onto aluminium hydroxide (ATH) and magnesium hydroxide. These are two very effective flame retardant fillers, ATH is the most commonly used, but magnesium hydroxide with... 

Silane coupling agents are widely used in thermoset systems, especially unsaturated polyesters, acrylics and epoxies. The silanes most commonly used are vinyl, methacryl, epoxy and amino. Among the fillers commonly treated are various silicas and silicates and aluminium hydroxide. The latter is particularly widely used for its flame retardancy. The in situ treatment method is frequently used with thermosets. 

These fillers are of great industrial importance, and are the main subject of this chapter. They owe their fire retardant effectiveness to their ability to decompose endothermically at polymer pyrolysis temperatures, with the release of inert gases such as water. Thus, unlike some other flame retardants, they are able to combine a high level of flame retardancy, with low smoke and low toxic and corrosive gas emissions, and are thus becoming of increasing importance. One of the simplest such materials is aluminium hydroxide (also known as alumina trihydrate, ATH). 

The commercial use of hydrated fillers was given a great boost in the mid-1970s, by legislation in the USA requiring carpet backing to be flame retarded, an application for which aluminium hydroxide was ideally suited. 

Stearic acid and metal stearates are widely used as dispersants, especially in cases where high filler loadings are required. Examples are polyolefins filled with aluminium hydroxide or magnesium hydroxide where 60 weight percent of filler or more may be needed to achieve sufficient flame retardancy [129, 130]. Of course the correct level of addition depends upon the amount of filler surface to be covered, and therefore upon the amount of filler, and its specific surface area. Excess additive is to be avoided as it can seriously destabilise some polymers and give yellowing problems [127]. 

Lastly, a major use of fillers is for flame-retardant applications, with aluminium hydroxide being extensively used, especially in unsaturated polyesters. This is dealt with in depth in Chapter 6. 

In order to consider effective strategies for the recovery and reuse of plastics recyclates, a quantitative method is described for the characterisation of polyester-based moulding compounds. Analytical procedures are described for the quantification of fibre, filler and fire retardant contents in polyester moulding compounds containing calcium carbonate filler, glass fibre reinforcement, and aluminium hydroxide flame retardant. 3 refs. 

One of the emerging technologies that is showing great promise is the use of hydrated mineral fillers such as aluminium and magnesium hydroxides, as such materials can provide high levels of flame retardancy without the formation of smoke or corrosive and potentially toxic fumes. The use of fillers as flame retardants has recently been reviewed by Rothon [23]. Essentially the key features are an endothermic decomposition to reduce the temperature, the release of an inert gas to dilute the combustion gases and the formation of an oxide layer to insulate the polymer and to trap and oxidise soot precursors. 

The fillers most commonly treated are silicas, clays and other silicates and flame retardants such as aluminium and magnesium hydroxides. While both in situ and pre-coating methods are utilised, pre-coating is most popular. This is in part at least due to the problems that can be encountered due to alcohol release in compounding machinery when the in situ process is used. 

The effectiveness of intumescent flame retardants is frequently reduced when fillers are added. Interactions can be either chemical or physical. Materials which are basic in character such as aluminium and magnesium hydroxides and calcium carbonate tend to interfere chemically with the phosphoric acid precursor in the intumescent system, presumably forming inorganic phosphates. Such antagonistic behaviour can be easily recognized by an almost complete lack of char formation. 

It is also, of course, possible to observe mineral filler decompositions, such as those of aluminium and magnesium hydroxide, which are frequently used in cable sheathing as fire retardants (see Figure 5.15). Both materials quantitatively decompose to liberate water and their oxides for example... 

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