Fire-retardant fillers decomposition

This is the second most widely used fire-retardant filler. It is more expensive than aluminum hydroxide, but has a higher decomposition temperature (about 300°C), making it more suitable for use in thermoplastic applications where elevated processing temperatures are encountered. 

Since the decomposition reaction occurs at a specific temperature, the performance of these fillers depends on the properties of the polymers in which they are used. For example, Mg(0H)2 performs better in polyethylene than AlfOI I) because it remains stable during compounding and decomposes at a temperature closer to the decomposition of PE (300-400 C). In unsaturated polyesters, Al(0H)3 starts to release water at 200°C. The major endothermic peak occurs at 300°C with a heat of decomposition of 300 kJ/mol. About 90% of the water is released between 200 and 400 C. A considerable amount of heat is absorbed before the polymer is affected. The water also dilutes combustible gases and hinders the access of oxygen to the polymer surface. Figure 12.8 shows the difference between talc and a fire retardant filler in PP." Talc causes an increase in the combustion rate as its concentration increases, whereas Mg(OH)2, used at a sufficient concentration (above 20%), decreases the rate of combustion. 

Fire Retardant Fillers that Rely on Endothermic Decomposition... 

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. 

Furthermore, the effect of hydrated fillers on polymer fire retardancy will depend not only on the nature of the filler, including its particle characteristics (size, shape, and purity) and decomposition behavior, but also on the degradation mechanism of the polymer, together with any filler/ polymer interactions that might occur, influencing thermal stability of the polymer and possible char formation. 

This represents the key aspect of polymer fire retardancy using hydrated fillers, and involves energy changes that occur on the decomposition of the filler, related heat capacity effects, which influence the degradation profile of the polymer and thermal barrier formation resulting from the residue remaining from degraded filler. 

There is evidence to show that the particle size of the filler also plays a significant role in flammability resistance. For example, below a certain particle size (about 1-2 pm), in many tests, including oxygen index, aluminum hydroxide shows enhanced fire-retarding performance,34 which may be associated with the rate of filler decomposition and/or with the formation of a more stable ash. However, it has been found that the particle size effect is absent, or less evident, in the cone calorimeter test.35 Similarly, particle size reduction has been shown to enhance fire retardancy in magnesium hydroxide-filled PP in this case, samples were characterized by the UL94 test.36 This raises the question as to whether further reductions in particle size to the nanoscale will lead to an additional increase in flammability performance, and perhaps enable filler overall levels to be significantly reduced. This aspect is considered in a later section. 

Hornsby, P.R., Wang, J., Rothon, R., Jackson, G., Wilkinson, G., and Cosstick, K., Thermal decomposition behaviour of polyamide fire retardant compositions containing magnesium hydroxide filler, Polym. Deg. Stab., 51, 235-249, 1996. 

Electrical and electronic devices are made utilizing several various types of plastic materials, thus when discarded their waste is difficult to recycle. The plastics employed in housing and other appliances are more or less homogeneous materials (among others PP, PVC, PS, HIPS, ABS, SAN, Nylon 6,6, the pyrolysis liquids of which have been discussed above). However, metals are embedded in printed circuit boards, switches, junctions and insulated wires, moreover these parts contain fire retardants in addition to support and filler materials. Pyrolysis is a suitable way to remove plastics smoothly from embedded metals in electrical and electronic waste (EEW), in addition the thermal decomposition products of the plastics may serve as feedstock or fuel. PVC, PBT, Nylon 6,6, polycarbonate (PC), polyphenylene ether (PPO), epoxy and phenolic resins occur in these metal-containing parts of EEW. 

Flame-retardants are used as additives in the preparation of fire retardant paints. They are decomposed by heat to produce nonflammable components, which are able to blanket the flames. Both inorganic and organic types of flame-retardants are available in the market. The most widely used inorganic flame-retardants are aluminum trihydroxide, magnesium hydroxide, boric acid, and their derivatives. These substances have a flame-retardant action mainly because of their endothermic decomposition reaction and their dilution effect. The disadvantage of these solids is that they are effective in very high filler loads (normally above 60 percent). 

Magnesium hydroxide is an emerging filler for fire retardant applieations. In this area, it eompetes with aluminum trihydroxide, antimony oxide, and other fillers based on zine. Magnesium hydroxide has a different deeomposition temperature from aluminum trihydroxide, it is more suitable for polymers with higher decomposition temperature. These aspects and current findings are discussed in detail in Chapter 10. 

In general, when compared with the conventional polymer composites, polymer nanocomposites exhibit significant improvements in different properties at relatively much lower concentration of filler. The efficiency of various additives in polymer composites can be increased manyfold when dispersed in the nanoscale. This becomes more noteworthy when the additive is used to address any specific property of the final composite such as mechanical properties, conductivity, fire retardancy, thermal stability, etc. In case of polyolefin/LDH nanocomposites, similar improvements are also observed in many occasions. For example, the thermal properties of PE/LDH showed that even a small amount of LDH improves the thermal stability and onset decomposition temperature in comparison with the unfilled PE [22] its mechanical properties revealed increasing LDH concentration brought about steady increase in modulus and also a sharp decrease in the elongation at break [25]. While in this section, fire-retardant properties and electric properties of polyolefin/LDH nanocomposite were described in detail. 

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... 

Alumina Trihydrates Alumina trihydrate is used to improve flame retardancy and reduce smoke emissions of specific resin systems. It is a fine, white filler material which, when added in the proper amount, can improve flame retardancy of halogenated or non-halogenated resin systems. When a properly filled RP is exposed to fire, it decomposes into water and anhydrous alumina. The water cools the RP thus slowing the rate of decomposition or burning. 

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