Jens Schaller of Thüringisches Institut für Textil und Kunststoff Forschung e.V. (TITK), Rudolstadt (Germany) has been dissolving cellulose acetyated to varying degrees in ionic liquids prior to spinning. Over a range of degrees of substitution from 0.07 to the commercially standard 2.5, absorbency peaks between 0.5 and 0.7DS and then falls to the normal acetate value at 2.34. At peak the WI is 300-400%, and hence much higher than obtainable with pure cellulose fibres or their alloys with water soluble polymers. Furthermore the absorbency level is unaffected by ionic concentration, and the fibre properties are like viscose. The fibres have a rough surface and mercury porosimetry shows a porous structure similar to lyocell. 2.5DS “standard” acetate can also be spun, wet rather than dry, from the ionic liquid (1-butyl-3-methyl-imidazolium chloride or BMIMCl). This process avoids the explosion risks of standard acetate production.
Recycling the BMIMCl involves vacuum distillation of the 10% spinning solution up to 80%, and then using a thin-film evaporator to get to 97%.
Asked why the WI increases with DS up to 0.7, Mr Schaller said a few acetate groups destroy the hydrogen bonding and increased accessibility to water but over 0.7 DS, increasing hydrophobicity takes over. Could lithium chloride be used as the solvent for spinning? Yes but it’s more expensive and harder to recycle (thin film evaporation not possible) Why bother with acetate when SAPs are available more cheaply? This approach gives fibres more easily and they are not salt-sensitive.