Pretreatment of Viscose fabrics - woven & knit

Properties:

  • 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 cotton.
  • 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 plasticity.

Pretreatment:

  • 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 or Hisaka). The more “soft flow�? the dyeing machine – the better
  • Better base white than cotton, therefore bleaching mainly required only for full white or pastel shades.

Woven fabrics

  • Desizing: Warps of viscose rayon can be sized with a variety of agents : starch based systems, polyvinyl alcohol systems or a combination of both, modified starch, starch ethers or CMC or combinations of these.

Identification of the size present is important prior to deciding on a suitable desizing procedure, however, many factories will opt to use their standard cotton desize treatment:
e.g. applying in the quench box of a singeing machine :
pad      2 - 4 g/l amylase enzyme
1 - 2  g/l wetting agent
Impregnate at 60°C ; pick-up 80 % ; batch minimum 4 hours - normally overnight ; wash-off.

  • Relax / scouring: Viscose rayon fibres, unlike natural cellulosic fibres, are free from natural fats and waxes, motes and seeds, and the scouring process, therefore, need not be as severe as for cotton, and can be based on soda ash or tetra sodium pyrophosphate recipes rather than caustic soda. A typical scouring recipe on a 5 box continuous open width washing range would be :

Box 1                                                           Box 3
3 g/l Detergent                                             2 g/Sequestering agent at 95°C
2 g/l Soda Ash
1 g/l Sequestering agent                             Box 4
at 95°C                                                         Water only at 70°C
Box 2                                                           Box 5
1 g/l Sequestering agent at 95°C                 Water only – cold

Knitted fabric – on jet or overflow machine
2 – 4 ml/l Lubricant
0.5 – 1 g/l very low foaming detergent
0.5 – 2 g/l Dispersing cum sequesterant
1 – 3    g/l Soda ash (about pH 9)        
Run 20 to 30 minutes at 60-90° C    
Hot rinse    

Peroxide bleach on jet or overflow machine
For full white
1.0 – 3 ml/l Lubricant
2 – 4     g/l All in one wetting/scouring/sequestering/stabilizing agent
2 – 6   ml/l Hydrogen peroxide 35%
0.3 – 0.7 % Optical whitener
Run 40 minutes at 85° C
Hot rinse at 60 – 80° C
Neutralize with sequestering agent for 10 – 15 minutes at 50-60° C

Causticising:
Causticisation modifies the viscose fibre surface or skin to enable more rapid diffusion of dye into the fibre. Benefits are therefore more obvious in printing than in dyeing, where long diffusion times are employed, and
with selected reactive dyes, yield gains of up to 50% are possible. Typical conditions for pure viscose are treatment with 6° - 8° Bé caustic soda at 25° - 30° C for at least 2 minutes followed by low tension washing
with boiling water to assist the rapid removal of alkali.

Pad-Batch processing is popular although dedicated continuous plant, employing a scray or conveyor for tensionless swelling and reaction, is preferable. Liquor pick-up of about 120% should be achieved. The best
after-washing device is probably a sieve drum continuous open-width range. Low uniform processing tensions are essential for consistent results and good quality. Rinsing should be done as hot as possible to minimise swelling and ensure rapid and complete removal of caustic soda. An addition of 2 – 4 g/l soda ash in the initial wash boxes will promote removal of alkali, and help maintain a good fabric handle. Neutralisation
with an acidic sequesterant.

Scouring of blends containing viscose:
Standard viscose and polynosic rayons are blended with cotton for improved physico-chemical properties of the blended fabric. The increase in tensile strength brought about by blending wool with viscose is well-known. Fabrics containing around 65% triacetate with viscose may be given durable pleats. Bulked or textured nylon is sometimes used with viscose rayon for fabrics.
Scouring Conditions for Blends Containing Viscose
Blend Treatment conditions


Viscose/Cotton
Viscose/Wool
Viscose/Acetate
Viscose/Acrylic
Viscose/Nylon
Viscose/Regenerated
protein
0.1% soap ; 0.1% NaOH ; 1 h at boil.
3% non-ionic detergent; 1 ml/1NH 3 (0.88 sp. gr.);
15-20 min at 40-45~
0.2% soap, 0.2% ammonium hydroxide (sp. gr.
0.88) ; 30 min at 90~ Temperature should not
exceed for bright acetate.
0.25% synthetic detergent; 0.5% sodium acetate
or acetic acid, adjusted to pH 5.5-6 ; 30 min at
90~
2-5% (o.w.f.) soap or synthetic detergent; 2-5%
trisodium phosphate ; 30 min at 40-70~
Solution is same as for viscose/acrylic blend ; 30
min at 50~