Fire retardant fillers

FIRE RETARDANT FILLERS. The next major fire retardant development resulted from the need for an acceptable fire retardant system for such new thermoplastics as polyethylene, polypropylene and nylon. The plasticizer approach of CP or the use of a reactive monomer were not applicable to these polymers because the crystallinity upon which their desirable properties were dependent were reduced or destroyed in the process of adding the fire retardant. Additionally, most halogen additives, such as CP, were thermally unstable at the high molding temperatures required. The introduction of inert fire retardant fillers in 1965 defined two novel approaches to fire retardant polymers. 

In addition to the fire retardant fillers which are effective in their own right, a number of mineral fillers are used as components of fire retardant systems for thermoplastics. The principal one is antimony oxide. 

The viscoelastic properties of polypropylene melts containing magnesium hydroxide fire retardant fillers have been studied using parallel plate dynamic rheology [36]. In this work the filler variants differed in particle size, surface area and morphology, ranging from approximately spherical particles formed... 

A major drawback to the industrial use of fire-retardant fillers is the high addition levels needed in most polymers to confer adequate fire retardancy. This can detrimentally influence processability and melt rheology, and, when used in load-bearing situations, the presence of the filler generally... 

In this chapter, an overview is presented of the principal fire-retardant filler types, including details of their origin, characteristics, and application. Consideration will then be given to their mechanism of action both as flame retardants and as smoke suppressants, and to means for potentially increasing their efficiency using synergists and nanoscale variants. 

The following properties are ideally required for the successful commercial use of a fire-retardant filler 1... 

TABLE 7.1 Current and Potential Fire-Retardant Fillers Candidate Material Approximate Onset Approximate Enthalpy ... 

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. 

Boehmite is, in effect, partly decomposed aluminum hydroxide, where two-thirds of the water has been removed. Although it has been promoted as a fire retardant in its own right, but because of the relatively low water content, does not seem to be very effective for this purpose. However, it does seem to have some potential in mixtures with other fire-retardant fillers and this is where it is now being targeted.6... 

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. 

Fire retardant fillers affect smoke formation. " Table 12.5 gives some data on the specific extinction area. The data show that, with the exceptions of A1(OH)3 and Mg(OH)2, fillers have a small effect on smoke suppression. 

Major results General fillers do not affect smoke formation by any means other than simple dilution. Fire retardant fillers such as Mg(0H)2 decrease smoke formation only at high concentrations." Materials which are known catalysts of degradation (e.g., copper) increase smoke formation. ... 

Methods of filler pretreatment coating with thermoplastic polymer zinc hydroxystannate coating of fire retardant fillers increases their performance... 

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