FIRE RESISTANT

FIRE-RETARDANT TREATMENT OF WOOD
• The Romans first treated wood for fire retardancy in the first century A.D.
• They used solutions of alum and vinegar to protect their boats against fire.
• In 1820, Gay-Lussac advocated the use of ammonium phosphates and borax for treating cellulosic material.
• Many of the promising inorganic chemicals used today were identified between 1800 and 1870. Since then, the development of fire retardants for wood has accelerated.
• Commercially treated wood became available after the U.S. Navy (1895) specified its use in ship construction, and New York City (1899) required its use in buildings over 12 stories tall.
• Production reached over 65 million board feet in 1943, but by 1964 only 32 million board feet was treated annually. However, starting in 1979 changes were made in some building codes that allowed structural changes if fire-retardant (FR) treated wood was used in place of untreated plywood for roof sheathing. The structural changes resulted in sufficient savings such that FR-treated wood was utilized more extensively.
• Also, the use of FR-treated plywood was mandated at other specific sites such as prisons.
• Fire-retardant treatments for wood can be grouped into two general classes:
(1) those impregnated into the wood or incorporated into wood composite products &
(2) those applied as paint or surface coatings.
• Chemical impregnation has the greater use, primarily for new materials, whereas coatings have been limited primarily to materials in existing constructions.
• There are advantages and disadvantages to each class. Coatings are applied easily, and they are economical.
• Chemical impregnation usually involves full-cell pressure treatment and can be costly.
• A coating is subject to abrasion or wear that can destroy the effectiveness of the fire retardant.
• Chemical impregnations deposit the fire retardant within the wood, so that if the surface is abraded, chemicals are still present.
• On-site application of surface coatings requires strict control of the amount applied to ensure correct loading levels for a particular flame-spread rating.
• Both coating and impregnation systems are based on the same chemical compounds although the formulations for each vary.
• Among the most commonly used chemicals for impregnation treatments are:
1. Di-ammonium phosphate
2. ammonium sulphate
3. borax
4. boric acid
5. zinc chloride
6. and most recently the leach-resistant amino-resin systems.
• These compounds have different characteristics with respect to fire resistance.
 Ammonium phosphate, for example, is effective in checking both flaming and glowing;
 borax is good in checking flaming but is not a satisfactory glow retardant.
 Boric acid is excellent in stopping glow but not so effective in retarding flaming.
• Because of these different characteristics, mixtures of chemicals usually are employed in treating formulations