Viscose Rayon Pretreatment -woven fabric

Woven fabrics
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.

Desizing:

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.

Scouring & Bleaching:

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
3 g/l Wetting and scouring agent
2 g/l Soda Ash
1 g/l Sequestering/chelating agent
at 95°C

Box 2
1 g/l Sequestering/chelating agent at 95°C

Box 3
2 g/l Sequestering/chelating agent at 95°C

Box 4
Water only at 70°C

Box 5
Water only – cold

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 Core neutralizing agents, is also advisable.