Aqueous thiocyanate–urea solution as a powerful non-alkaline swelling agent for cellulose fibres

• Concentrated aqueous NaSCN/urea solutions lead to strong swelling of cellulose fibres.
• Sodium thiocyanate is more effective than potassium or ammonium thiocyanate.
• Replacement of urea by N-substituted urea does not lead to increased swelling.
• The swelling treatment increases water retention value of treated cellulose fibre.
• No indication of derivatisation or modification of cellulose is found in FTIR spectra.

For many applications cellulose fibres are treated with concentrated solutions of swelling agents to increase reactivity and to achieve reorganisation of fibre structure. Representative examples are caustic soda, potassium hydroxide solution or liquid ammonia. These highly concentrated media bear considerable safety hazards during the technical handling thus alternative swelling agents are of interest. The thiocyanate–urea system investigated in this work offers high swelling potential for regenerated cellulose fibres. Experiments with different cations of M+ in M+ SCN− demonstrate the significant influence of the cation on the degree of fibre swelling. In concentrated NaSCN/urea solutions, at 80 °C, lyocell fibres expand the diameter from 12–14 to 100 μm. The treatment in the swelling agent also led to a significant increase in the water retention value which was accompanied by a strength loss of 20–40% of the initial value. FTIR analysis of treated fibres did not indicate substantial changes in structure of the cellulose polymer. Limited weight loss of up to 20% was observed despite the high expansion of the fibre.