Nuchem Newsletter

Viscose Rayon Properties

Different ratio of crystalline and amorphous regions compared to cotton	Viscose has higher dye affinity than cotton.
Viscose has irregular serrated skin and cross section.	Inferior diffusion and penetration. More kinetic energy needed. Hot reactive dyes.
Viscose loses tenacity when wet.	More care needed to avoid damage.
Wet swelling increases with temperature	Very important in package dyeing. Liquor circulation should mainly be IN to OUT. OUT to IN should be <>
Swelling of fibres makes wet fabrics stiff. Swelling and heat can set creases.	Use longer L.R. than for cotton. Keep liquors above 50° C, Cool at maximum 1° C per min. Use suitable anti-crease lubricants.
Viscose may contain residues of sulphur.	Mild peroxide bleach may be necessary to remove sulphur.
Viscose is creamy white in colour, naturally clean. Fabrics free from natural fats and waxes, motes and seeds.	Little preparation required. Bleaching chemicals can be reduced. Always give SOME preparation to avoid carry over of sulphur.
Dyes have higher substantivity and faster fixation	Use ‘Migration’ dyeing techniques (at up to 110° C). Add salt after dye.

Pretreatment of Viscose

• Scouring and bleaching need to be mild in nature. The fabrics should always be scoured, and never put straight into dye because it is important to remove any residual sulphur to prevent dye reduction. Spinning lubricants used on viscose tend to yellow with heat, and so should be removed for best whites and bright pastels.
• The liquor ratio may need to be a little higher than for cotton, because of the higher water retention but also because of the high swelling. (Reduce jet capacity).
• Break outs are always an issue on viscose, particularly where there is lengthways tension on the fabric. Best avoided by sewing the fabric at an angle, not straight across. Use multiple "zig-zag" stitching for additional security.
• Fabrics will tend to stiffen in tight constructions so lubrication is important. There should be as little tension on the fabric as possible, because of the low wet modulus it will stretch easily and dimensional stability may never be achieved. Usually means slower turn around times and dyeing machines with winches to help the fabric into the jet, or the cigar-type machines with the fabric movement into the jet being downwards (like the Gaston County Futura). The more “soft flow”the dyeing machine – the better.