Fire-retardant fillers combustion

It is well known that freshly formed oxides have high surface areas and in addition, can be cata-lytically active,52 thereby promoting both carbon deposition and subsequent oxidation processes.53 The reduced combustion rate arising from the effects of the fire-retardant filler also contributes to lowering the rate of smoke evolution and, by improving oxygen to fuel ratios, further limits levels of smoke density.1... 

Using this concept, it has been shown by cone calorimetry that over a 3 min combustion period, 3 and 6 mm thick laminated structures, made with different fire-retardant skin and unfilled core combinations can give similar resistance to ignition and comparable HRR and smoke extinction area (SEA) results to fully fire-retardant compositions (Table 7.4). Mechanical properties, in particular impact strength, were also found to be greatly enhanced by this approach, since less fire-retardant filler is present in the material. Whereas this approach has been demonstrated to be effective with hydrated fillers, it is applicable to all fire-retardant types. 

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. 

Nanocomposites refer to the combination of nanosized fillers (10 m diameter) with polymers, rather than the combination of polymer matrix (filled with nanoparticles) and fiber reinforcement The most popular fillers used as fire retardants are layered silicates. Loading of 10% or less (by weight) of such fillers significantly reduces peak heat release rates and facilitates greater char production [7]. The char layer provides a shielding effect for the composites below and the creation of char also reduces the toxicity of the combustion products, as less carbon is available to form the CO and CO2. 

Polypropylene (PP), for example, burns very easily and dripping is observed during its combustion. The use of virgin PP is thus limited when flammability properties are required. Several approaches have been developed to increase its fire-retardant properties. Hornsby [1] reviewed the approach of using classical fillers in PP to increase its fire-retardancy behaviour. With classical fillers, the main problem is the loading (typically between 40% and 60% of total mass), which directly affects the mechanical properties of the polymer. Another problem is that the filler must be treated to increase its interfacial adhesion with the matrix. 

Smoke suppressants are also important additives in fire retardancy. Resulting from an incomplete combustion, opaque smoke may evolve that leads to panic and slow the rescuers progress. Additives, such as antimony oxides, metal borates, hydrates of magnesium or aluminium and magnesium oxychlorides, are used as fillers and flame retardants, but they are also good smoke reducers. 

Before discussing the special effects of fillers that are active fire retardants, it is useful to recognise that the addition of any particulate, non-combustible, filler to polymers can considerably affect their thermal stability, resistance to ignition and combustion, and the amount and nature of the combustion gases in terms of smoke, corrosion and toxicity. The main general effects are ... 

The polymer may be modified significantly by the use of fire-retardant additives, which may act as fillers, plasticisers, and so on [27]. The combustion behaviour of fillers is discussed in Section 7.6.3.6... 

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. 

Combinations of inorganic and organic flame retardants are discussed hcrc. " Figure 13.6 shows than the addition of regular fillers, such as talc and CaCOs, to ammonium polyphosphate increased the fire resistance of PA-6. The function of filler in these combinations is to increase char yield and increase insulation properties of char. On the other hand, ammonium polyphosphate protects char Ifom oxidation and hinders diffusion of combustible gases to the flame. 

Highly interesting properties exhibited by polymer-layered silieate nanoeomposites include their inereased thermal stability and also their ability to promote flame retardancy at very low loadings. The formation of a thermally insulating and low permeability ehar to volatile combustion products caused by a fire is responsible for these improved properties. The low filler contents in nanoeomposites that account for the dramatic improvement in thermal stability are highly attractive, since FR end-products can be made cheaper and be easier to process. 

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