Modacrylic is polyacrylonitrile fiber (LOI = 33), contains 35% - 85% 2-propenenitrile which enables this fiber to possess very good dyeability and FR property.  Many synthetic fibers melt and turn into liquid form when in contact with heat, which can create serious burn on skin.  Modacrylic fiber possesses permanent FR property, it does not melt and can be easily blended with other FR fibers.