Sunday, August 1, 2010

Viscose Dyeing - Method & Tips for yarn and fabric

Dyeing Yarn:
Without salt, swelling of viscose increases with temperature. This must be remembered in package dyeing where high swelling and high pump pressure can lead to flattening of the yarn cross section at cross over points. Flow should be predominantly IN to OUT so that the yarn layers float apart and the liquor can circulate more easily. With OUT to IN flow the yarn layers are compressed and flow is reduced. OUT to IN flow cycles should therefore only be 30 seconds or so.

Conical cones:
The cones must be homogeneously wound to ensure an even dye result and the bobbin edges should be carefully rounded off (bumped). The winding hardness should be approx. 25 Shore. Taking the relatively highswelling of the material into account this equals a volume of approx. 360 – 380 g/l. Due to the higher swelling of the material we recommend that the windingdiameter is limited to 160mm.  Dyeing problems have been experienced where the yarn tube diameter has reached 170mm (approximately 850g / cone).

Cylindrical Cones:
Lighter dyeing results on the edges can be avoided by using winding hardness's of approx. 25 Shore and a pressure rate of 20 %.

Dyeing Fabric:
As viscose is a high affinity fibre, special consideration must be given to the choice of dyestuff and exhaust application technique. Hot dyeing reactive dyes are preferred for exhaust application in order to secure the highest migration and diffusion through the high

affinity fibre when dyeing in rope form. Dye suppliers will thus generally recommend a migration technique to give levelness and reproducibility. This technique offers high dye mobility at temperatures as high as 110° C before cooling to 80° C for optimum fixation of the reactive dye to the cellulose chain.

For higher substantivity dyes, the salt should be added gradually (over 30 mins) at 95° C, or at the highest temperature at which additions can be made.

The bath is set at 50° C with the required amount of electrolyte, auxiliaries and the pH adjusted with acetic acid to around pH 6.0. Predissolved dyes are then added in a linear manner over 15 – 20 minutes. The temperature is raised to 95° C (or even up to 110° C) at 1.5 to 2° C per min. and held for 20 minutes at 95° C before cooling back to 80° C at 1° C per min. Hold at 80° C for 10 minutes before adding the alkali in a linear manner over 15-20 minutes. Continue for 45-60 mins.

Further precautions
Under alkaline conditions, in enclosed machines, viscose is especially prone to cause problems of dye reduction. It is essential that viscose is thoroughly desulphurised, otherwise appreciable colour value will be lost. Mild peroxide bleaching and addition of reduction inhibitor Resistsalt  from the start of dyeing is recommended.

When dyeing in rope form, there must be sufficient displacement and this is best achieved for woven fabrics in jets with aerodynamic systems. The liquor temperature should not fall below 50° C and a suitable running  crease inhibitor should be used in all hot baths.In order to guard against the danger of formation of running creases, the size of the load should not exceed approx. 80% of the maximum load.

Fabric circulation speed should be set to between 80 – 120 seconds. To prevent abrasion marks, the slippage (difference between winch speed and fabric speed) should be as low as possible.
Levelling & migrating agent (yarn and fabric) can be used to increase migration – especially with difficult large molecule dyes.

If dyed viscose fabrics are allowed to begin to dry in patches – top of trolley – edges of folds – these areas may be visible as a physical/optical patch after finishing. Keep fabric thoroughly  wetted until drying machine/ stenter is available

Preventing predrying of the fabric is important, but it is far better to ensure that production flows quickly and fabric does not stand wet for a long time.

Dyeing - Summary
• Remove oils and waxes in preparation
• No twists in loading – load machine below capacity
• Sewing – sew more than once – at an angle
• Abrasion – avoid slippage – always run at 50° C or higher
• Cooling creases – cool at 1° C per minute
• Danger of sulphur residues and reduction
• Tension in dyeing machine – problems with shrinkage
• Migration of unfixed dye while waiting for drying
• Fabric drying out in patches – watermarks
(physical/optical effect). Keep fabric wet right up to
the dryer

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