Modacrylic

Theory of Synergistic FR & Char Formation

Not only are modacrylic fibers inherently flame retardant (FR), but those with certain additives have two additional mechanisms that create a synergistic flame retardant system.

First is the "free radical capture" process that takes place in the vapor phase. On combustion at a temperature of over 600° F, oxyhalides are formed. The flame retarding action takes place in the vapor stage above the burning material. The oxyhalides act as "free radical traps", and take up free radicals. They inhibit ignition and pyrolysis in the solid, liquid, and vapor phases. This chemical reaction is endothermic, drawing heat away from the substrate.

For modacrylic fibers, the halides are in the form of chloride molecules in their acrylonitrile vinylidene chloride co-polymer.

A second process occurs in a solid phase. This is the "char process". It promotes the formation of "char" (essentially carbon) on the substrate which reduces volatile gas formation. The barrier between the substrate and the vapor phase reduces the available oxygen to the underlying substrate. Modacrylic also acts as a dehydrating agent that increases charring. Charring is enhanced when modacrylic is blended with cellulosic fibers, such as cotton or rayon.