A Non-Hygroscopic Fire Retardant Treatment for Wood

Treatment of wood with aqueous solutions of dicyandiamide and phosphoric acid (D:P) imparts a degree of fire retardance equal to that provided by the best of the current formulations. Unlike previously known treatments, however, D:P is non-hygroscopic and it reduces the swelling of wood by up to 40 percent. Solution strength may be varied according to the degree of fire retardance desired. The solution has a limited solubility and should be used at about 50?C to prevent crystallization. After impregnation, by soaking for thin veneers or by full cell pressure processes for larger pieces, the wood is dried in a conventional manner to fairly low moisture contents at temperatures of 40? to 70?C. The wood is then cured for several hours between 70? and 100?C. The fire retardant. properties of D:P treated wood are similar to those reported for comparable retentions of the ammonium phosphates. With pine, fire-tube weight losses of 25 percent or less may be obtained by treatment with D:P solutions of 10 percent concentration or stronger to retention levels of 10 percent. Retention values generally are a better criterion than solution strength. D:P treatments showed less swelling than the other retardant salts (monoammonium phosphate, ammonium phosphate, zinc chloride) and the untreated wood control tested. The D:P treatments provide lower moisture content than any of the above salts as well as untreated wood throughout the entire relative humidity range. The D:P treatment caused an increase in shear strength over untreated wood, and a small loss in impact strength.

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