Pretreatment of Wool - Part-1
After shearing, wool is mechanically treated to shake out dirt and open the fleece in order to improve the efficiency of subsequent scouring. The raw wool is then scoured by aqueous washing solvent scouring is less widely practiced.
The most common treatments before dyeing wool are:
- Raw wool scouring; aqueous and/ or solvent washing
- Scouring (desizing)
- Fulling /crabbing/thermo fixing
- Easy-care treatments
- Anti-felting anti-shrinking treatments
- Wool Bleaching
Raw Wool Scouring: (The washing of raw wool)
Aim of Scouring:
- To remove natural impurities such as wool grease, suit (dried perspiration) and dirt.
- To remove residues of insecticide, acaricides or insect growth regulators ( veterinary medicines to protect sheep from ectoparasites, such as lice, mites etc.)
- To make the wool suitable for further processing.
When considering the ability of solving the different contaminants present in raw wool, one may be aware of the specificity of wool washing treatments. Wool grease is insoluble in water ( but soluble in non-polar solvents such as dichloromethane or hexane) and needs addition of special surfactants to water. Suit arises from the secretion of the sweat gland in the skin and is soluble in polar solvent such as water and alcohol. Dirty materials present on raw wool can be mineral dirt, sand, clay, dust etc.
- Water wash with soda and a suitable surfactant at pH 10-11 at moderate temperatures, is the most common treatment of scouring.
- Organic solvent treatments are not practically in vogue. However perchloroethane, dichloromethane, Freon etc may be used as solvent scouring agents for removing the grease in the wool.
- Combined water/ Iso-propanol / hexane treatment is usually followed for removing grease.
- However it should be remembered that the total removal of natural grease from wool makes it brittle and unsuitable for further operations such as spinning, knitting, weaving etc. So some lubricants are added during the above processes and there are to be removed before taking the fabric for further wet processing operations.
Wool Scouring with Water:
This process is done by passing the wool through a series of wash bowls and subsequent squeeze presses. The machine is called Leviathan scouring machine, where clean water is added to the last bowl and passes via a counter flow system from bowl to bowl. The final discharge in the first bowl occurs in a controlled manner, as wool fat-Lanolin is recycled and effluent may be less contaminated of impurities.
Washing Bowl (1)
1. Washing - hot water (40-45C)
2. Soda ash or other alkali is added as detergent builder (from fatty acids) at pH 10-11
3. A suitable detergent (non-ionic) 1-2 g/e (1 gram / e = 367.879441 milligrams)
Washing Bowl (2)
1. Hot wash nearly 38C ( melting point of wool fat)
2. Some builders and detergents as above mentioned, usually at lower concentration - 5-6 g/e
3. Sodium bi-sulphate solution (optional)
Rinsing Cycle (3)
- Hot Wash ( 38C)
- Anti-electrostatic agents ( last rinsing step)
- pH adjustments ( last rinsing step) with 4-5 g/e of detergent.
Washing occurs near pH 4.5, the isoelectrostatic point of wool - the fibre damage is minimized, special acid resistant detergents are used for washing. However, this acid wool scouring treatment is not used in practice as the acid washed wool may have the less suitable properties when spun.
Before wool is subjected for further processing, the pH of the wool fibres must be readjusted to isoelectrical point in the last rinsing bath. Anti-electrostatic agent may also be added to facilitate easier subsequent spinning. The removing of residues like insecticides from raw wool will have important implications for the discharge of scouring effluent.