Acrylic and Modacrylic
BISFA defines acrylic fibres as "fibres composed of linear macromolecules having in the chain at least 85% (by mass) of acrylonitrile repeating units". Modacrylic fibres have, in the chain, at least 50% and less than 85% by mass of acrylonitrile. The first commercial fibres were introduced in the USA and Germany in 1948.
The starting materials for acrylonitrile are propylene and ammonia, which are reacted with oxygen in the presence of catalysts. The acrylonitrile is then polymerised to produce polyacrylonitrile (PAN). The PAN is then spun into fibres from a solution in a solvent. Two process routes are used, wet spinning in which the fibres are spun into an aqueous coagulation bath and dry spinning in which the fibres are spun into hot air.
The fibres are then stretched, washed and crimped. The modacrylic fibres contain halogen comonomers such as vinyl chloride or vinylidene chloride, and have flame-retardant properties.
Properties and End-Uses
Acrylic fibres are soft, flexible and have a high loft. For this reason they are widely used in knitted apparel end-uses such as sweaters and socks. In addition to knitted apparel, home furnishing and blankets are other important applications due to its excellent heat retention.