Chemistry: Fireproofing from the Dead Sea

High on the list of Israel’s slim supply of natural resources are the brains of its scientists and the chemicals such as bromine that can be extracted from the Dead Sea. Making use of both resources, Chemist Menahem Lewin has developed a wood-fireproofing process that may create a new world market for Israeli bromine.

Bubbles & Salts. There are two conventional ways of fireproofing wood and wood products, including paper and fiberboard. One is to coat them thickly with paint that releases carbon dioxide when heated and forms a layer of protective bubbles. This process serves satisfactorily for mild fires, but the bubble layer cannot resist intense or prolonged heat. The other system is to impregnate wood with various salts, but this weakens the wood and adds as much as 25% to its weight.

Bromine has long been known for its fireproofing qualities, but if it combines with wood’s cellulose fibers, it weakens them seriously. Dr. Lewin’s process gets around this disadvantage by forcing the bromine to attach itself to the wood’s lignin, the cement that causes the fibers to stick to each other. The best grades of paper have no lignin, but the types of wood pulp used to make paperboard and wallboard retain enough of it to make Dr. Lewin’s process useful.

Pulp & Chlorine. To brominate wood pulp, Dr. Lewin simply adds sodium bromide, which is as stable as table salt, to the solution in a standard bleaching apparatus, then bubbles chlorine through it. The combination of chemicals releases active bromine in a form that attaches itself to the lignin in the pulp. Treating solid wood is a more complicated process, but the results are spectacular. When a piece of brominated wood is put in a hot fire, it does not burn. After a while, a layer of carbon forms on its surface, but carbonization stops as soon as the wood is taken out of the fire. Any structure of brominated wood or wood products is safe from fire unless it is stuffed with highly combustible contents. “We could fireproof Japan,” says Dr. Lewin, who sees no reason for false modesty about his achievement.

For all his studies, Dr. Lewin does not yet know in detail how bromine fireproofing works, but in general the action is connected with the way that wood burns. When heat is applied to natural lignin and cellulose, they give off combustible gases that form flames and spread the fire by heating more wood. Somehow, bromine seems to make those gases nonflammable. And with no flames to spread it, combustion stops as soon as the external heat source, such as a lighted match, is removed.