Retardant fillers particle size effect

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. 

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. 

Antimony pentoxide is an alternative to antimony trioxide. It finds applications in semi-transparent materials and dark colors because of its low tinting strength. As with antimony trioxide, antimony pentoxide must be used together with halogen-containing compounds to function as a flame retardant (sec discussion under antimony trioxide). The other advantages of antimony pentoxide include its refractive index which is closer to most materials, its very small particle size, its high specific surface area, and its substantially lower density. Because of its small particle size, its is frequently used in the textile industry since its addition has only a small effect on color or on mechanical properties. Production of fine-denier fibers requires a stable dispersion and a small particle size filler. The flame retardancy of laminates is also improved with antimony pentoxide because small particles are easier to incorporate in the interfiber spaces. 

The effect of fillers on smoke suppression depends on the particle size and crystalline structure of the filler. A new fire retardant, based on a hydrated potassium-magnesium aluminosilicate, in two grades - coarse and fine is available. The fine grade is twice as effective as the coarse grade. ... 

For carbon-black fillers, structure, particle size, particle porosity, and overall physico-chemical nature of particle surface are important factors in deciding cure rate and degree of reinforcement attainable. The pH of the carbon black has a profound influence. Acidic blacks (channel blacks) tend to retard the curing process while alkaline blacks (furnace blacks) produce a rate-enhancing effect in relation to curing, and may even give rise to scorching. 

Fillers. Fillers are used in compounds to increase viscosity, increase solids, and to lower cost. Most fillers used in latex do not exhibit the reinforcing effect that is characteristic of their use in dry-type polychloroprene. Water-washed whiting (calcium carbonate) can be added directly to the latex. Most clays are acids and must be neutralized and slurred before adding to the latex. When used at levels ranging from 10- to 20-phr, fine clays such as DIXIE Clay (R. T. Vanderbilt Co.) can add some degree of reinforcement. Hard clays have much smaller particle size than soft clays. Feldspar can be added directly to the latex but will tend to settle quickly. Hydrated alumina is used primarily to improve flame retardancy and improve water resistance. Large-particle-size hydrated alumina can be added directly to the latex. 

The interpretation problems outlined previously and the so far unexplained effects of particle size, make comparisons of the performance of various potential flame retardant fillers reported in the literature very questionable. This is graphically illustrated by the case of MCS referred to earlier (Section 6.4.2). 

Besides the use of melamine as a component of intumescent systems it has also been advocated as a flame retardant in its own right. It has the advantages of being inexpensive, readily dispersible in most thermoplastics and is commercially available in grades of varying particle size. The flame retardant effect is largely due to a combination of heat sink effects and vapour phase dilution effects so in these respects it has some similarities with the hydrated filler flame retardants. 

The use of layered silicate nano-particles (i.e., clay), which have a much larger surface area (-750 mVg) and a much higher aspect ratio (>200) than conventional, macro-sized fillers, to reinforce polymers, has drawn a great deal of attention in recent years [14-18]. Adding a small amount of nano-clay can dramatically improve a number of properties such as stiffness and strength, thermal and dimensional stability, flame retardance, and barrier properties [17,18]. Thus, the introduction of nanoclay into WPCs could prove interesting from the perspective of improving their mechanical properties and flame retardance, which are desirable effects, particularly... 

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. 

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