A flame-retarding agent is a substance or mixture that does not burn or support combustion and that inhibits the combustion of materials in which it is incorporated or to which it is applied. Fabrics that are treated with flame retardants, such as Pyrovatex and Proban, resist ignition and will not support combustion. Consequently, if they are present at the scene of a fire, they do not contribute to the spreading of flames.
In installations where hot or inflammable materials are handled as a matter of routine, such as in some types of factories, completely nonflammable materials such as steel and concrete are the predominant construction materials; potentially inflammable articles, such as carpets and wall coverings, are avoided. In locations where the risk of fire is slight but its consequences could be disastrous—domestic, leisure, and office environments, for example—all decorative finishes and fabrics ideally should be modified or treated to make them resistant to the spread of fire.
Noncombustible fabrics
Few completely noncombustible materials can be formed as fibers. Glass and some other ceramics form noncombustible fibers, but textiles produced from these fibers tend to splinter and are suitable for only a limited range of applications.
Some synthetic fibers have extremely low inflammability. Examples include polyaramids (aromatic polyamides, such as Kevlar), polychloroethene (polyvinyl chloride, PVC) and copolymers of propenonitrile (acrylonitrile, CH2CHCN) with chloroethene (CH2CHCl) or 1,1-dichloroethene (vinylidene chloride, CH2CCl2). Fabrics produced from these synthetic fibers are similar in textile characteristics to polyamide (nylon), acrylic, and polyester fabrics, but they have several disadvantages. Such fabrics are difficult to color, since they require unconventional dyes, expensive dyeing machinery, and higher dyeing temperatures than other fabrics. They are also awkward to launder, since their thermoplastic nature makes them sensitive to high temperatures; thus, they cannot be boiled to remove stains, and drying at too high a temperature can cause permanent wrinkles.
The most significant disadvantage of such materials stems from their chemical compositions. Although flame-retardant synthetics do not burn as such, the high-temperature decomposition of nitrile-based fibers can produce deadly hydrogen cyanide gas, while chlorine-containing polymers can form highly toxic dioxins.
Flame-retarding additives
The susceptability to combustion of many conventional materials can be greatly reduced by the addition of small percentages of flame-retarding chemicals, which act by modifying the reaction of the base material to heat so that it burns less well or by interfering in the combustion process in some other manner. The treated material usually suffers some damage in the event of fire, but its contribution to the spread of fire is reduced.
Dehydrating agents
Cellulosic materials, such as cotton, linen, and viscose rayon, can be modified by chemicals that encourage the surface layer of the material to char when heated. Untreated cellulose burns well because its thermal decomposition, or pyrolysis, causes the evolution of inflammable gases at temperatures above 570°F (300°C). Above 660°F (350°C), these gases ignite, producing heat and perpetuating the burning process. The overall formula for this reaction is
Some flame-retarding agents function by releasing powerful dehydrating agents, such as boric acid or phosphoric acid, in the heat of a fire. The reaction then proceeds as follows:
This reaction, which occurs at 480 to 550°F (250–290°C), releases no inflammable gases; the carbon that results forms a layer of char that burns only at temperatures significantly higher than the burning temperature of untreated cellulose.
Flame-retarding agents that act by dehydration include monoammonium phosphate and dicyandiamide phosphate, which both release phosphoric acid when heated, and mixtures of borax and boric acid, whose active agent is boric acid. Finishes of this type do not affect the normal textile characteristics of treated fabric.
Cellulosic fabrics can also be made fire resistant by reacting them with organophosphorus compounds. These compounds, which release dehydrating phosphoric acid when heated, form chemical bonds with the fiber that make the flame-retarding agent highly resistant to washing and dry cleaning. Yarns and woven or knitted textiles can be treated by immersion in a bath of reactive phosphorus compounds; in the case of viscose rayon, the flame-retarding agent can be incorporated during the manufacture of the fiber.
