Flame retardants Aluminium hydroxide

Flame retardants, aluminium hydroxide, magnesium hydroxide, nucleating/anti blocking agents, synthetic wollastonite Color and pigment concentrates... 

Metal hydroxides provide an important alternative to halogenated flame retardants. Aluminium trihydroxide, sometimes known as alumina trihydrate, is the most widely used of all FRs in plastics. Magnesium hydroxide is also finding increasing acceptance, and calcium hydroxide is being marketed as an additive for different reasons. 

Among the metallic hydroxide flame retardants, aluminium trihydrate, A1(0H)3 or magnesium hydroxide Mg(OH)2 are popular flame retardants and smoke suppressants (8). 

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 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. 

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. 

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. 

The production of flame retardant quahty aluminium hydroxide has recently been reviewed [98]. Various crystal forms of aluminium hydroxide exist, but that used for polymer appHcations is Gibbsite. This occurs widely in nature, usually in the rock bauxite, but the natural form is usually not suitable for direct use and synthetic products are nearly always employed. Most aluminium hydroxide is manufactured through the Bayer process used to make alumina for refractory applications. 

M. Wladyka-Przybylak and H. Rydarowski, Flammability and mechanical properties of PP modified by nanoclay compounds and aluminium hydroxide flame retardant, in Proceedings of the 19th BCC Conference on Flame Retardancy, M. Lewin (Ed.), Business Communications Co Editions, Norwalk, CT, 2008. 

L. Haurie, A.I. Fernandez, J.I. Velasco, J.M. Chimenos, J.M. Lopez-Cuesta, and F. Espiell, Thermal stability and flame retardancy of LDPE/EVA blends filled with synthetic hydromagnesite/aluminium hydroxide/aluminium hydroxide/ montmorillonite mixtures, Polym. Degrad. Stabil., 2007, 92 1082-1087. 

In 2004, Sony and Mitsubishi Plastics teamed up to develop a flame retardant PLA biodegradable resin claimed to be as strong as ABS. The new material will be used in the front panel of Sony standalone DVD players. The resin employs an aluminium hydroxide flame retardant, is rated UL94 V-2 and complies with the EU s Restrictions on Hazardous Substances (RoHS) directive. Sony says the use of additives and modifications to moulding parameters allows it to process PLA compound on conventional injection presses in commercially viable cycle times. 

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. 

Metal hydrates such as aluminium trihydrate or magnesium hydroxide remove heat by using it to evaporate water in their structures, thus protecting polymers. Bromine or chlorine-containing fire retardants interfere with the reactions in flames and quench them. Mixtures of flame retardants antimony trioxide and organic bromine compounds are more effective at slowing the rate of burning than the individual flame retardants alone. 

During the last decade, environmentalists have fought strongly to ban the use of brominated fire retardants, and already a number of plastics processors have voluntarily switched to non-halogenated ones (such as phosphate esters, aluminium trihydride and magnesium hydroxide fire retardants). Still, there are two contradictory forces in the fire retardant industry, one is the constant push for stronger fire-safety standards, and the other is the move to eliminate flame-retardants seen as persistent, bioaccumulative or toxic [19]. 

Alumina trihydrate (or hydrated alumina, ATH or aluminium hydroxide), AI2O3X3H2O or A1(0H)3, is a bulk flame-retardant for plastics that can be filled at a high concentration and processed below the decomposition temperature of alumina trihydrate (cf. Table 5.1). 

Specially coated magnesium and aluminium hydroxides confer significantly increased combustion resistance and lower levels of smoke evolution, permitting large reductions in additive loading, without sacrificing flame retardant or smoke suppressant performance. 

Ethylene-vinyl acetate copolymers, usually known as EVA, are used in many applications, but especially for low voltage cables. These polymers are easily flammable and flame retardants are added to reduce their flammability. The classic solution is to incorporate aluminium hydroxide or magnesium hydroxide that develop endothermic reactions when heated. Nevertheless, large amounts have to be incorporated, often around 60% and this can lead to a loss of mechanical properties in the compound. Intumescent technology that works well with polypropylene has also been tried for EVA polymer systems. 

Reduction of flammability and smoke production when submitted to fire - flame retardant additives are used (e.g., antimony trioxide, phosphorus compounds, aluminium trihydroxide, magnesium hydroxide, zinc borates)... 

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. 

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. 

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. 

Despite these reservations, the presently available data suggests that, with the possible exception of nesquehonite, none of the candidate materials is a significantly more effective flame retardant than aluminium hydroxide (also known as alumina trihydrate or ATH), although as will be shown later, magnesium hydroxide may have some advantages in terms of smoke production. 

Table 6.2 A comparison of the flame retardant performance of aluminium hydroxide (ATH) and MCS [125 phr in crosslinked EVA] ...

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. 

Elsewhere in Japan, Mitsubishi and Sony have teamed up to develop a flame retardant PLA resin which uses a UL 94 [1] V-2 rated aluminium hydroxide RoHS compliant flame retardant. The new material is claimed to be as strong as ABS and, by using additives and by adjusting moulding parameters, Sony is able to process it to make front panels for its stand-alone DVD players using conventional... 

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. 

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