Novel intumescent composition

DETAILED DESCRIPTION It is difficult to impart durable flame retardance to a wood substrate.
Wood substrates present a flameproofing problem which is materially different from that presented by fibrous hydrophilic organic substrates.
According to U.
S.
Pat.
No.
2,927,050, in the latter substrate " .
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substantially independent fibers are tangled together, leaving interstices capable of being filled by an aqueous medium by capillary action between all of their surfaces.
The individual fibers contain a relatively small amount of cellulose, and the materials composed of them have a relatively low ignition temperature.
" In wood substrates, however, " .
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the cellulosic fibers are bonded together to form a relatively impenetrable block susceptible to little capillary action.
A piece of wood has small surface area in relation to the amount of surface area it contains and has a relatively high ignition temperature.
" Because of these factors, " .
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a flame-proofing agent which flameproofs fibrous hydrophilic organic materials is not likely to flameproof wood because its capacity to inhibit burning is likely to be destroyed by the time it is heated to the ignition temperature of the wood and/or because of the difficulty of causing a non-volatile substance to penetrate into the volume of a block of wood.
" Impregnation of a wood substrate with a fire-retardant is known in the art.
U.
S.
Pat.
No.
3,398,019 teaches that this method must be used to impart a satisfactory degree of flame retardancy to wood fiber insulation, stating that "as far as is known only by the use of a chemical retardant which impregnates the board can a commercially acceptable building material be produced which is capable of securing a nonflammable rating.
" U.
S.
Pat.
No.
4,049,849 teaches that this method, although well known, presents several substantial problems.
According to this patent, the use of a wood substrate impregnated with a fire-retardant salt is restricted to low humidity applications " .
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due to the water solubility and hygroscopicity of most known fire retardant salts