The flammability of hybrid viscose blends

Visil (manufactured by Kemira Fibres Oy, Finland) is a hybrid of organic-inorganic components to give a cellulosic fibre containing polysilicic acid produced by a modified viscose process. Viscose on its own will burn, but non-flammable polysilicic acid which is present in Visil, when heated causes the cellulose to char and the silica formed to provide a flame and heat barrier. Thus the fibre is only partly combustible and reduced volatiles from the cellulose emit only minor amounts of smoke along with some carbon monoxide and carbon dioxide. When blended with other fibres which have low flammabilities themselves, non-additive levels of flame retardance may be observed compared with those expected from averaging of individual component properties. Initial results of a larger research programme show that blends of Visil and modacrylic in yarn form show either higher or lower-than-expected Limiting Oxygen Index (LOI) values depending upon the yarn structure. These effects are corroborated by char length studies following vertical strip testing which show that either synergistic or more additive effects are possible depending on yarn physical structure.