Wood materials smoldering

Some materials may bum quite slowly but may propagate a flame rapidly over their surfaces. Thin wood paneling will burn readily, yet a heavy timber post will sustain a fire on its surface until it is charred, then smolder at a remarkably slow rate of burning. Bituminous materials may spread a fire by softening and running down a wall. Steel of course does not burn, but is catastrophically weakened by the elevated temperatures of a fire. PVC does not bum, but it softens at relatively low temperatures. Other plastics may not burn readily but still emit copious amounts of smoke. And some flammable plastics, such as... 

Lignocellulosics reach temperatures around 80 C in many ways besides. For example, planer shavings and peat insulation around hot pipes or in walls of dry kilns easily attain 100 C, and forest product industries stack still hot wood-base panels at temperatures around 80 C. In many cases the temperatures of the hot materials later on rose above the initial 80 or 100"C, first to levels of smoldering combustion, and finally to those of open flames. In air-exposed and ventilated materials oxidation could cause the heating above 80 or 100 C, but inside tight packs of panels pyrolysis must have been the heat source. 

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. 

Unless this concentration of gas is maintained for an extended period, CO2 does not normally extinguish fires in materials that smolder or produce glowing embers, such as paper and wood. Its greatest effectiveness is on flammable liquid fires that do not involve material that might reignite after the CO2 has dissipated. It is especially suitable for use in laboratories and in the protection of electrical and electronic equipment. 

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