Modacrylic is a generic name of a manufactured fibre in which the fibre-forming substance is any long-chain synthetic polymer composed of at least 85% by weight of acrylonitrile chain. A modacrylic has properties that are similar to an acrylic: lightweight, soft and warm with a wool-like feel. However, modacrylics are flame retardant and do not combust. The fibres are difficult to ignite and will self-extinguish. In addition to a modacrylic's flame retardant properties it has a high durability and a very low tenacity. Moreover, flame retarding treatment is permanent and effective, even after the garment has been repeatedly washed.