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Home » Textile Processes  » Wool Processing

Wool Processing

Wool ProcessingWool is a natural fiber obtained from animal hair, mainly from sheep's hair. Wool processing involves many operations, both dry and wet processes. The processes carried out in a woolen mill can be grouped under the headings- Fiber Shearing and Grading; Fiber Preparation; Weaving Operations; and Finishing Operations.

Fiber Shearing and Grading
In this first stage of wool processing, fiber is collected from the body of the animals through Shearing and then their suitability for processing is determined through Grading.

Fiber Shearing and GradingShearing: Sheep are sheared by skillful shearers who do the job by using electric hand clippers similar to enlarged barber's shears. Smooth strokes close to the skin of the animals are used for obtaining lengthy fibers so that the fleece remains valuable.

Grading: Grading is done for establishing the quality of raw fiber in respect of fiber length, diameter, amounts of dirt, and other impurities. These factors are determined either by the experienced Graders or by the prospective buyers themselves who take the samples for the purpose. Graders can do the job simply through visual inspection. Fine and medium-fine wools having longer staple lengths of more than three inches are considered to be of good quality and are used for making light-weight worsted suit and dress fabrics. Coarser and short staples of less than three inches long are used for making bulky sweater and carpet yarns.

Fiber Preparation
Fiber Preparation60% of the raw wool fiber is composed of impurities and only 40% of it is usable wool fiber. The impurities present in wool are of three types- natural; acquired; and applied impurities. Natural impurities are the glandular secretions that adhere to the fleece. Suint and wool grease, together known as yolk, are such two major components. Suint is the dried perspiration of sheep that is water soluble. Wool grease is water insoluble and requires special scouring for its removal. The acquired impurities include soil, dust, dirt, straw, vegetable and fecal matter. Applied impurities are due to the treatments given to the animal against insects, pests etc. and due to the markings made on them with tar or paints for their identification.

Washing and Scouring: The water soluble suint and other heavy dirt particles are removed by washing the raw wool with water at a temperature of 32°C to 42°C. The water insoluble wool grease is removed by treating the desuinted wool with a mixture of detergent and sodium sulphate (or chloride) at the temperature of 65°C. As the detergent scouring yields waste of very high strength, some mills prefer to remove the impurities by solvent scouring wherein the desuinted wool is scoured with organic solvents such as benzene, carbon tetra chloride, ethyl alcohol, methyl alcohol or isopropyl alcohol. Although solvent scoring removes grease effectively, dirt is not removed. Thus, solvent scouring is generally followed by a detergent wash.

After scouring, the woolen fibers are rinsed with water at a temperature of 46°C to 50°C to remove the residual chemicals and remaining dirt.

BlendingBlending: Clean wool fibers from several different batches are often blended or mixed mechanically. Blending helps in unifying the slightly-different basic colors of raw wool, and also standardizes staple length and diameter for uniform quality.

Stock Dyeing: Wool fibers absorb dyes very efficiently at any processing stage. Wool dyed immediately after scouring is called to be a stock-dyed fiber. It is dyed either in open or pressure type machines, after which the dyed fiber is rinsed thoroughly with water. The various types of dyes used for woolen dyeing are direct dyes, acid chrome, metachrome, pure mordant dyes and premetallized acid dyes. Vat dyes are also used under special circumstances.

Oiling: The cohesion of fibers are increased by oiling, which also helps in spinning. Olive oil or a mixture of lard and mineral oil is used for the purpose. The quantity of oil used varies from 1 to 11% of the wool's weight. The applied oil is washed out during finishing operations.

Weaving Operations
After the dry operations, the wool fiber is woven into woolen fabric .

Weaving OperationsCarding: The clean and dry wool is passed through wire rollers to straighten the fibers and remove any remaining impurities. Carding results in a thin web of aligned fibers. Smooth steel fingers are then used to divide the web and roll the strands over onto one another which creates narrow continuous ropes of fibers known as "slivers". If the treated batch of wool is of coarser fiber and shorter staple length the machine gently twists the slivers into ropelike strands called "roving", and winds the roving into balls ready for spinning into woolen yarns that are used for making bulky sweaters and carpets. If the batch consists of finer fiber and longer staples then the slivers usually go through the combing and drawing steps which prepare them for spinning into worsted yarns that are used for making suits, dresses, and gabardines.

Spinning: Roving prepared for both woolen and worsted yarns goes through the spinning process for yarn formation. After spools of roving are mounted on the spinning frame, the ends of the roving are drawn through small rollers to extend the wool fibers still further. Then the spinning machines give repeated twists to the roving thus converting them into yarns having different properties of varying strength, firmness, size and ply.

Weaving and KnittingWeaving and Knitting: In weaving, two distinct sets of yarns called the warp and the weft are interlaced at right angles with each other to form a woven fabric. Knitted fabrics are produced by interlocking rows of yarns and loops. As new loops are formed, they are drawn through those previously formed. A circular knitting machine primarily manufactures jersey and a variety of double knits. Flat knitting machines make yard goods like tricot and raschel knits.

Finishing Operations
Finishing OperationsA thorough examination of the produced cloth brings out the imperfections such as broken threads, variations in color and other undesired effects. These are removed and the area is rewoven by hand if necessary. The fabric then undergoes certain wet processes to acquire the required finish and for the removal of any traces of impurities.

Fulling: This wet process is adopted for giving a controlled shrinkage to the woven cloth. It converts even a loosely woven cloth into a tight, closely woven woolen fabric. It improves the texture of the fabric which feels like a felted cloth. Both the types, acid fulling and alkali fulling are adopted by the wool mills. In acid fulling, dilute solution of sulfuric acid and hydrogen peroxide are used along with small quantities of metallic catalysts. It is usually carried out for heavy fabrics such as blankets, military uniforms, felt cloth etc. In alkali fulling, soap or detergent, sodium carbonate and a sequestering agent are used. It is meant for delicate woolen garments. After fulling, the fabric is washed extensively twice or thrice in order to remove the excessive chemicals.

Wool CarbonizingWool Carbonizing: It is the final process applied for removing burrs and other impurities remaining in the wool. The woolen fabric, impregnated with sulfuric acid solution, is oven dried at 100°C to 104°C followed by mechanical agitation. The acid degrades the cellulosic impurities without harming the wool. During drying, the sulfuric acid becomes more concentrated due to water evaporation resulting in burning of remaining impurities. The fabric is then passed through pressure rollers to crush the solid carbon residue. The loosened carbon residue is removed from the wool by passing it through the mechanical agitators called dusters. The fabric is then rinsed with water, passed through baths containing sodium carbonate solution to neutralize the residual acid, washed again and then dried.

Piece Dyeing: If the wool had not been died at fiber stage then it is dyed following the piece dyed method after carbonizing.

Bleaching or Brightening: At times, mild bleaching is also done with sulfur dioxide or hydrogen peroxide to whiten the natural yellow tint of the wool.