Viscose yarn/fabric Dyeing: Viscose is a regenrated cellulose fiber and can be dyed with the following dyestuffs., viz., Direct, Reactive, Vat, Suplur, Pigment and Solubilized vat dyes.
Physical Properties of Viscose: Before we go in to the details of wet processing of viscose, its better to know the basic physical and chemical properties of this fiber, so that one can understand how careful he should handle this material.
- Viscose has lower tenacity in both wet and conditioned state than cotton – more care is necessary to prevent fabric breakages and tears in wet processing
- Viscose has greater elongation in both wet and conditioned state than cotton – it will be stretched or distorted more under tension.
- Viscose and Modal fibres are supplied in a pure state and with a higher degree of whiteness than cotton. Bleaching is only required for a full white or pastel shades. Viscose/cotton blends require bleaching baths with a reduced chemical content.
- The water retention value (swelling index)
of viscose is very much higher than that of
In aqueous liquors, viscose fibres tend to swell more strongly than Modal fibres or cotton. This swelling process happens very quickly and is almost complete after ten seconds at the lower temperature range.Fabrics become much more stiff when wet because the fibres are so swollen.
In their swollen state, viscose fibres can become set to a certain extent. This is called hydro
- Viscose has higher dye affinity than cotton.
- 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 < 30 seconds.
- 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.
- Dyes have higher substantivity and faster
fixation. Use ‘Migration’ dyeing techniques (at up to
110° C). Add salt after dye.
Refer viscose pretreatment for details.