Flammability of Wood

The FSI for ordinary wood species is typically between 100 and 200, for some special cases it is as low as 60-70 (Table 14.1) To burn, wood should be exposed to heat 

TABLE 14.1 ASTM E 84 flame spread indexes for 19-mm-thick solid lumber [1] 

Species Flame spread index Smoke developed index 

Generally, the same stages take place in burning of WPCs. However, composites provide much more variations in their chemical composition, density, the nature of plastics, plastic content, amount of fillers, and so on. 

Composite materials, as well as plastics, do not ignite, per se. Ignition happens when the flammable material reaches a certain temperature in atmosphere that contains sufficient amount of oxygen. Ignition can be piloted, that is, in the presence of a flame (or another ignition source), or unpiloted, such as in a furnace with temperature at or above the ignition point. 

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Alum used to reduce the flammability of wood by Egyptians. About 450 B.C. 

Li etal. (2000a, 2001b) smdied the flammability of wood modified with acetic or propionic anhydrides, finding that this parameter, as determined by limiting oxygen index (LOI), was the same as that found for unmodified wood. Samples were also prepared which were impregnated with sodium silicate prior to anhydride modification. The latter treatment resulted in a decrease in flammability, as shown by an increase in the LOI of the wood. 

This sodium borate, known as Polybor , is an amorphous material and thus can be dissolved into water rapidly (solubility 9.7 wt.% at room temperature and 21.9% at 30°C). It is particularly effective in reducing the flammability of wood/cellulose/paper products. 

Boron compounds have been used in several ways to achieve reduced flammability of wood products. Borax and boric acid can be incorporated into particle board chips before addition of a dicyan-diamide, phosphoric acid, amino-resin system 85). They can also be used to produce a fire-retardant hardboard. Riem and Dwars 86) added water-soluble ammonium borate to wood fibers before the board was formed. A 6-7% boron content produced a hardboard that had a flame spread of 25 or less. 

The addition of fire retardants can reduce the flammability of wood however, this may occur at the expense of related wood prop-... 

Wood or wood-based products can be treated with fire-retardant chemicals. When so treated, the flammability of wood and wood-based products is reduced. The flammability of these products can also be reduced when they are used in combination with other materials, such as insulation. 

Extractives have various effects on other physical properties. Extractives with special characteristics, such as quinones, seriously affect the adhesive and finishing qualities of the wood (9.4.2). The nonpolar extractives with a lower oxygen content such as terpenoids, oils, fats, and waxes, affect the hydroscopicity, and permeability. This causes trouble in adhesion and finishing by the inhibition of the wetting of the wood. On the other hand, since these nonpolar extractives have high caloric value, they increase the flammability of wood and make these woods valuable as fuel. 

The control of polymer flammability, which has enjoyed considerable success in the last forty years, had its beginnings in antiquity with the first early attempts to reduce the flammability of natural cellulosic materials such as cotton and wood (1-3). Some of these early developments are summarized in Table I. Perhaps the... 

Inorganic boron compounds are generally good fire retardants (59). Boric acid, alone or in mixtures with sodium borates, is particularly effective in reducing the flammability of cellulosic materials. Applications include treatment of wood products, cellulose insulation, and cotton batting used in mattresses (see Flame retardants). 

The most widely accepted theory of the mechanism of fire-retardant chemicals in reducing flaming combustion of wood is that the chemicals alter the pyrolysis reactions with formation of less flammable gases and tars and more char and water (4,5,8,21,24-29). Some fire retardants start and end the chemical decomposition at lower temperatures. Heat of combustion of the volatiles is reduced. Shafizadeh (21) suggests that a primary function of fire retardants is to promote dehydration and charring of cellulose. 

Other Methods. Other methods of recovery of traces of flammable liquids used infrequently are vacuum distillation and soaking in water. Most items of arson evidence are physically more susceptible to analysis by other methods than by vacuum distillation. Soaking in water will sometimes allow residues from small pieces of wood to surface they can then be collected and analyzed. These methods are usually highly inefficient and little success is experienced with them. 

Borax pentahydrate is an effective flame retardant for wood/cellulosic materials in terms of surface flammability. However, due to the Na20 moiety, it can promote smoldering combustion in cellulose. Thus, in cellulosic material and wood products, it is commonly used in combination with boric acid, which is an effective smoldering inhibitor. For example, the treatment of wood fibers with a partially dissolved boric acid and borax pentahydrate slurry (-1.75% by wt. of boron) results in Medium Density Fiberboard (MDF) that is claimed to pass the ASTM E-84 Class 1 surface flammability standard.12 The additional examples of using borax pentahydrate and boric acid combination are presented in Section 9.2.2.1. 

It is well known that wood samples containing phosphorus compounds can release phosphoric acid that accelerates the dehydration and carbonization of wood (i.e., with decreased threshold temperature and activation energy). As a result, phosphate renders the main decomposition of wood at lower temperatures (<300°C) and results in the formation of less flammable products and correspondingly more char. On the other hand, boric acid can increase the thermal stability of wood via a different pathway (i.e., increase in threshold temperature and activation energy), and thus suppresses the mass loss and stabilizes the char. 

Experiments were conducted on the pyrolysis products of wood samples to affirm that the increased amounts of char involved a decrease in the amount of combustible tars (52). The chemicals increased the yield of char, water, and noncondensable gases at the expense of the flammable tar fraction. These results confirmed that the increased amount of residual char in TG results was associated with the reduction of the combustible volatiles. 

Flammability of materials is characterized by many different ways, one of them is the flame spread index (FSI). As reference values, FSI for inorganic reinforced cement board surface is arbitrarily set as 0, and for select grade oak surface as 100 under the specified conditions. FSI for ordinary wood species is typically between 100 and 200, and for some special cases it is as low as 60-70. An average FSI for about 30 different wood species is 125 45. 

Cellulose hber is flammable. Flammability of various types of wood is described in Chapter 14. 

Most of the ASTM recommendations applicable for WPCs have been developed either for plastics (ASTM Committee D20) or for wood (ASTM Committee D7). ASTM procedures developed specifically for WPCs have started to appear only lately. This situation is applicable for majority of WPC properties—slip, flammability, microbial degradation, oxidative degradation, among others, but test methods for some of them are essentially the same, regardless of wood or plastic for some they are quite different. Recommended test methods for water absorption are quite different for wood or plastics. The principal ASTM procedures for both of them are given below. 

FLAMMABILITY AND FIRE RATING OF WOOD-PLASTIC COMPOSITES... 

This chapter explains the meaning of the above statements. It describes flammability and smoke/toxic gases evolntion at burning of wood compared to wood-plastic composite (WPC) materials and products of different compositions and profiles. It also explains flammability and fire ratings and indexes as quantitative measures for fire hazard and fire safety, and fire performance characteristics in general of wood and composites. 

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