The conversion of raw wool into a textile fabric or garment involves a long series
of separate processes. There are two main processing systems, worsted and
woolen. A significant volume of wool is also processed on the semiworsted system,
for carpet use. Some wool is processed on the short-staple (cotton) system. Woolen
spinning is the mechanized form of the traditional system.
Scouring
The first stage in wool processing is to remove the fieece
impurities by scouring. The impurities consist principally of wool wax, dirt, pro-
teinaceous contaminants, and suint.
Scouring is carried out by washing the raw wool in an aqueous medium, a
solvent medium, or a combination of the two. Aqueous scouring is used to process
99% of the world’s wool. This involves washing the raw wool in aqueous solutions
(neutral or slightly alkaline) of nonionic detergent or, less frequently, soap and
alkali. This is followed by rinsing in water.
In most situations, wastewater discharged from a scouring machine contains large quantities of wool wax and both water-soluble and water-insoluble
material (organic and inorganic). The pollution load of a single modern scouring
machine is equivalent to a population of 50,000 people. Disposal or treatment of
the wastes to comply with environmental requirements is expensive.
Carbonizing
Carbonizing is a process used to remove excessive
amounts of cellulosic impurities, eg, burrs and vegetable matter, from wool. It
is carried out on loose wool, rags and fabric. With loose wool and fabric,
the wool is treated with aqueous sulfuric acid and then baked. After acid treatment, the carbonized vegetable matter
is crushed to facilitate its removal. The wool is then normally neutralized in
alkali, although some mills omit this stage to facilitate subsequent dyeing
under acid conditions.
Processing on the Worsted and Woolen Systems
The wool is
dried after scouring and blended before carding. Carding individualizes the
entangled fibers and reassembles them into a sliver weighing about 25 g/m.
At the same time, up to 90% of the vegetable matter (VM) is removed. Lubrication is used to minimize fiber breakage. After carding, there are three stages
of gilling, to align fibers and remove hooked fiber ends before combing.
Long-staple ring spinning is similar to short-staple (cotton) ring spinning,
except that the components are increased in approximate proportion to the
length of the fibers. On the spinning frame, the rovings are drafted by a factor
of about 20, twist is inserted, and at the same time the yarn is wound onto bobbins. The yarn on the bobbins is usually steam-set in an autoclave to reduce its
twist liveliness, then wound onto packages or cones. Detection and removal of
thick, thin and colored faults can occur during winding. Following twisting
into a two-fold yarn and further setting, the yarns are ready for knitting or weaving. Open-end (rotor) spinning is rarely used for wool and spinning and twisting
are very costly operations, because of their low productivity: typically 40 spindles
produce 1 kg/h of yarn.
Setting
Setting operations form an important part of the processing
of wool yarn, fabric, and garments. Yarn is set in steam to stabilize twist and prevent snarling during winding and warping, while fabrics may be set after weaving to prevent the formation of distortions during wet processes, such as scouring
and dyeing. Normally, fabrics are fiat-set near the end of the finishing routine to
impart dimensional stability and to confer the required handle. Finally, garments are set by steam pressing to form their desired shape, for example, to
insert pleats and creases.