By Eman Abdallah Kamel
Eman is a writer and engineer. She holds a bachelor’s degree in textile sciences from the Faculty of Applied Arts in Egypt.
This article discusses how wool is classified and the various steps involved in its production, including growing, shearing, scouring, carding, drawing, filling, spinning, weaving, and finishing.
Wool Fiber
Wool is a natural fiber obtained from the soft, hairy undercoat of sheep, goats, alpacas, camels, and llamas.
In newly sheared wool, the tallow and lanolin hold the basic threads of wool together.
The microstructure of wool fibers consists of three main parts: cuticle, cortex, and medulla. The fibers are made up of protein molecules. Keratin protein is a crystalline polymer; the repeating units are amino acids. Wool has two structures. The first is alpha-keratin, and the second is beta-keratin.
Wool fibers have remarkable elasticity, increasing in length by about 30% under low tensile stress and returning to their original state when the stress is removed.
Wool has many applications, including upholstery, insulation, blankets, and clothing.
Wool Grades
The wool classifier classifies wool into five main categories:
- Fleece,
- Necks,
- Cuts,
- Bellies,
- locks.
The fleece is also sorted into groups according to micron. The sorted wool is placed in nylon bags and pressed in a wool press to make bales. All bales are sealed and marked with the wool farms, categories, types, and wool quality.
Production Stages
The stages of wool manufacturing include the following basic steps:
- Growing,
- Shearing,
- Scouring,
- Carding,
- Drawing,
- Gilling,
- Spinning,
- Weaving,
- Finishing.
1. Growing
Wool-producing sheep are raised in barns or extensive pasture systems with supplemental feeding during dry conditions. Sheep are graded according to the fineness of their wool, and wool farmers aim to raise sheep with a specific type and quality of wool.
Shorter wool breeds will have denser fibers, while longer wool breeds will have less dense, longer wool. For example, Lincoln sheep will have locks that are up to 8 inches long, while breeds such as Finn will be around 5 inches long. Sheep with shorter locks, like Southbound, will have very short locks.
Most Merinos have woolly wrinkles in their skin, particularly around the tail and rump area, which become damp with urine and soiled with feces. Flies are attracted to this damp area where they lay their eggs. The eggs hatch into larvae within 12-24 hours and feed on the lamb’s flesh for up to 3 days – this is called fly strike, which, if left untreated, can be fatal. To treat this problem, the skin is sheared, which involves cutting a crescent-shaped flap of skin around the lamb’s rump and tail using specially designed sharp scissors. Merinos with a flat body have little or no wrinkles and do not require this process.
2. Shearing
The timing and amount of shearing have a significant impact on the quality and quantity of wool produced, as well as on the health of the sheep. In one study, the most common reason for timing shearing was when shearers were available. The second most common reason was concern about the increased risk of fly infestation in unsheared sheep during the summer. Shearing timing has been shown to affect staple strength, wool weight, yield, and fiber diameter. It can also have important consequences for the nutritional requirements of the flock and the risk of disease.
When plant material accumulates in wool, it reduces its quality and can threaten the animal’s health.
The length of the wool affects its wet and dry characteristics. After about six months, the wool is still wet but dries slowly. If shearing time results in such a length of wool during a wet but warm time of year, the wool is susceptible to fleece rot. Although wool rot usually causes a slight reduction in wool quality, it is the main risk factor for flystrike. Although flystrike can be prevented or controlled chemically, the shearing time that requires additional chemical treatments does not help achieve the industry goal of reducing pesticide residues in wool.
Sheep that have been sheared can suffer significant losses when exposed to cold weather. Sheep can suffer from severe hypothermia in the two weeks following shearing, especially in the first three days. Sheep are not adapted to cold during the warmer months, so a cold snap during these times greatly increases the risk of death.
Shearing frequency affects the length of the staple wool. In some countries, such as Australia, shearing is generally done once a year. Wool manufacturers prefer a staple length of between 60 and 90 millimeters. Some producers have changed the shearing frequency to three times every two years to meet market demands.
Two methods of shearing exist:
- Hand shearing: This method involves using various kinds of scissors, which takes a lot of time and labor and can result in uneven wool shearing and animal injuries.
- Shearing by electric machines: This technique is common in many countries because it saves time and effort, yields high-quality shearing, and, when performed by trained shearers, does not harm sheep.
Shearers remove the wool with many long strokes. After the belly, leg, and face wool is removed, the body of the fleece is separated as one piece.
Once the wool is sheared, the wool processor gathers the wool and throws it onto a large table. The wool processors skirt the wool. Skirting removes the edges of the poor-quality wool, which are kept separate from the main wool. This can include necks, cuts, and dirty wool. The remaining wool is given to the wool grader after being rolled up.
3. Scouring
The main goal of cleaning is to remove pollutants such as dust, dirt, sweat, and natural oils present in wool. Pollutants make up 30 to 70 percent of the total weight of wool.
A sequence of alkaline baths with water, soap, and alkali is used to clean the wool. The cleaning machines’ cylinders remove extra water from the wool while preventing it from drying fully. The wool is then treated with oil.
The scouring stage is followed by other operations to completely clean the wool.
- Carbonization is the formation of a solid residue with an increased carbon content from an organic material, usually by thermal decomposition in an inert atmosphere. Carbonization is performed on wool to remove traces of unwanted plant matter, which remains entangled with the raw wool. These must be removed after the cleaning process; they may damage the spinning machinery. Sulfuric acid is the chemical used to destroy the molecules of these plants. The carbonization process consists of soaking the material in dilute sulfuric acid, followed by neutralization with sodium carbonate. The material is then dried, and the brittle cellulose material is mechanically removed. The acid carbonization process has disadvantages such as fiber damage and environmental pollution.
- Bleaching: wool needs to be bleached to get rid of yellowing or dyed in vibrant pastel colors. Wool cannot be bleached with sodium hypochlorite solutions, as it deteriorates to the point that it dissolves in the solution. Sulfur dioxide gas was once used to bleach wool wet, but this technique is no longer effective. Today, hydrogen peroxide is considered the most effective bleaching agent. Bleaching is done in a weak alkaline solution containing a stabilizer and a buffer, which gives the best quality white fibers. After carefully bleaching wool with an alkaline agent, the wool should not be boiled, as bleached wool is sensitive to yellowing. Wool is bleached using oxidation, reduction, or a combination of oxidation and reduction.
- Oxidative Bleaching: Chlorine-based oxidizing bleaches are not suitable for wool. The alkali used for activation must be mild. Trisodium phosphate, tetrasodium pyrophosphate, and phosphates are used as stabilizers. Wool can be damaged under alkaline conditions; the alternative is to bleach under acidic conditions using a peracid activator. The wool is bleached using a fixed solution of hydrogen peroxide (0.75% w/w) at a pH of 8–9 for one hour at 60°C.
- Reductive Bleaching: Stabilized sodium dithionite and thiourea dioxide are the two most often utilized chemicals in the reductive bleaching of wool. Bleaching is done for one hour at 45–65°C and pH 5.5–6 using 2–5 g/L sodium dithionite. Thiourea dioxide (1-3 g/L) can also be used for one hour at 80 °C and pH 7.
- Bleaching with Oxidative-Reductive Combination: The wool is first bleached with an alkaline water solution at 60°C for 1 hour. Thiourea is added to the bath, and the pH is adjusted to 4.5-5.5. Thiourea is then converted to thiourea dioxide. The bleaching process is carried out at 60°C for another 25 minutes. The fleece is then washed and rinsed.
4. carding
The fibers are passed through a series of metal teeth that straighten and fuse into slivers. The combing process removes any remaining dirt and other materials from the fibers. The combed wool intended for spinning is passed through gilling and carding, processes that remove the short fibers and place the longer fibers parallel. The softer slivers are compressed and loosened through the drawing process. The combed wool is then sent to spinning.
5. Drawing
The drawing process is the pulling of a fiber group, such as a sliver, top, or roving, to form a thinner yarn. The result is a longer continuous strand of fibers with a lower linear density than before. Roll drawing is performed by passing a sliver between two pairs of driven rollers. Delivery rollers have a higher surface speed than feed rollers.
What is Doubling?
Doubling is introducing two or more roving threads side by side into the drawing area so that they are combined and delivered as a single thread. The goal of doubling is to promote uniformity, fiber mixing, and fiber alignment and maximize machine productivity.
6. Gilling
In the wool industry, the card sliver undergoes several preparatory operations of gilling or pin drawing before carding, to straighten and improve the alignment of the fibers, provide more mixing, and reduce fluctuations in the linear density of the sliver. A final gilling operation is also performed after carding, to give a very uniform sliver called the top. The main purpose of the final gilling operation is to redistribute the leading fiber ends randomly by drawing, provide more mixing, straighten and align the fibers, and add moisture and oil, to produce a top of the desired linear density and evenness.
The fallers consist of metal rods with about 100 sharp steel spikes protruding from their working surfaces, evenly spaced along the length of the rods. The spikes may be round or flat; round spikes are stronger, but flat spikes provide better control of the fibers. This is because they have more free space to fasten more fibers of the same number per unit length.
The gill boxes can be equipped with mechanical or electronic aeration devices. Adding moisture during the high-speed gilling process is important to achieve the required recovery for subsequent processing. A lubricant (0.1-0.3%) can also be sprayed on the sliver during the first or second gilling operations to maintain recovery, reduce static effects, and adjust the fiber cohesion.
Mechanism: Some doubling slivers pass between the back rollers, merge into one sliver, and are gradually penetrated and held between the pins fixed to the fallers. The upper and lower sets of crossed fallers move forward toward the front rollers at a linear speed slightly higher than the surface speed of the back rollers. This difference in speed applies tension to the sliver. At the end of its journey, each faller is withdrawn and transported back to its starting point, where it gradually penetrates the fibers as it moves forward again.
What is the Meaning of Ratch?
Ratch is the drafting zone’s length. It is the distance between the nip lines of the front and back rollers.
If the draft in the back ratch area is too low, the fibers may not be stretched enough to remove their natural crimp. The drafting force in the front ratch area then reaches a level sufficient to break the fibers due to the greater frictional resistance created by the crimp held by the fibers. Pins are used to control the fibers as they help maintain friction between the fibers. The back ends of the fibers being withdrawn are combed by the pins, which greatly speeds up the straightening and alignment process.
7. Spinning
A mule spinning machine is usually used to spin woolen yarns, but any number of spinning machines can be used to spin worsted yarns. The spun yarn is then wound around commercial drums, bobbins, or cones.
Why are mule spinning machines used for woolen yarns?
The variable lengths of the individual wool fibers are unsuitable for attenuation by roller drafting. So, woolen fibers are carded using condenser cards, which rub the carded fibers together rather than drafting them. They are then spun on mule-type machines, which have no roller drafting but create the draft by the spindles receding from the delivery rollers. To guarantee the best possible treatment of the yarn, these mules include incorporating various spindle speeds, receding motions, etc.
8. Weaving
There are two basic types of wool weaves: plain and twill. Plain weaves turn wool yarns into fabric, which results in a more flexible fabric and a soft surface due to the napping.
Did You Know?
Worsted yarns can produce soft fabrics using a twill weave. They are more durable than woolen and therefore more expensive.
9. Finishing
After weaving, woolen fabrics and worsted undergo a series of finishing operations, such as:
- Fulling: The fabric is submerged in water to cause the fibers to interlock.
- Crabbing: Permanently setting the interlock.
- Decatising: It is a process of continuously removing moisture from textile materials in rolls by wet cleaning and pressing damp materials to stabilize the woven or knitted fabric. The wool or wool blend fabric is washed and drained. This is followed by a reduction of the residual moisture to between 20 and 40%. The process allows the wool or wool blend fabric to be polished while maintaining its shape.
- Dyeing: Wool fibers can be dyed either before the carding process or following the weaving of the wool into the fabric.
Sources
- Raising Sheep for Fiber & Naturally Dyeing Wool
- Time, frequency, and preparation for shearing
- Shearing
- textiles committee. nic.in/Wool Cleaning and Scouring Supervisor.pdf
©Eman Abdallah Kamel, 2024
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