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Pretreatment of Wool:
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
- Carbonizing
- Scouring (desizing)
- Fulling
/crabbing/thermo fixing
- Easy-care treatments
- Anti-felting
anti-shrinking treatments
- Wool Bleaching
1. 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.
Scouring treatments:
- 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.
Treatment steps:
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.
External Link: Wool
processing
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