Impurities in Wool and These Removing Process

Last Updated on 09/05/2021

Impurities in Wool and These Removing Process

Anju Singh
M.Sc. in Fabric and Apparel Science
Delhi University, India


Wool is an animal fiber obtained by shearing the fibrous covering of sheep. Raw wool is often dirty and contaminated with natural fats, grease and perspiration residues. All these impurities are removed during wool scouring and wool carbonising to get cleaned wool. Raw wool contains 40% or more by weight of impurities in the form of waxes, suint, cellulosic material such as straw and dried grass, dirt, and proteinaceous material. Besides, during spinning and weaving other impurities are added.

Impurities in Raw Wool:
Raw wool is dirty and about 50% of its weight consists of natural and other impurities. In general, the finer wools such as merino contains a higher proportion of natural impurities than the coarse wool. The contaminants also vary according to breed, neutrition, environment and position of wool on the sheep. Wax, suint and dried perspiration are natural and dirt (adventitious), grass, seeds, straw, burrs, brambles, sticks etc. (vegetable matter) are acquired impurities. The presence of proteinaceous contaminant layer (PCL) is also found.

impurities in wool
Fig: Impurities in wool

Impurities in Raw Wool (in %)

TypeFat and suintSand and dirtVegetable MatterWool fiber

Types of impurities of wool and its removal stages:

  1. Wool waxes are recovered from the grease during scouring. These waxes are comprised of a variety of monocarboxylic, dicarboxylic and hydrocarboxylic acids as well as steroidal alcohols. It has been determined that unscoured wool contains an unoxidized fraction of wool grease and other contaminants that is easily removed and readily recoverable and an oxidized fraction at the tip of the hair that is difficult to remove and separate from other oxidized contaminants.
  2. Suint is usually considered to be a variable composition of water-soluble materials that is readily removed by scouring.
  3. The dirt that is removed from the scoured wool consist of both inorganic and organic materials
  4. The proteinaceous material has recently been discovered to consist of skin flakes from the sheep and soluble peptides.

Impurities Removing Process in Wool
The process that can remove the impurities has various steps:

  1. Wetting
  2. Crabbing
  3. Scouring
  4. Carbonizing
  5. Milling
  6. Bleaching

(i) Wetting:
The first treatment given to wool is wetting. This treatment releases latent strains and gives permanent set provide, wet treatment is not done at temperatures higher than that used in crabbing.

(ii) Crabbing:
This treatment is given to woolens to eliminate the tendency to cockle or distort. The wool is wound tightly on a roll which is made up of iron. The roll is a perforated cylinder covered with cotton cloth in order to prevent staining. It is rotated during treatment. Steam is passed in the cylinder at 40-150 lb/inch square pressure (as and when required). Now the wool is unwound and rewound, so that the outer roll of wool after crabbing becomes the inner roll, and steam is passed again, steaming enhances affinity of wool for dyes. The pH value of crabbing water determines the setting of wool. A low pH produces little setting and maximum degree of setting is attained at pH 10.2.

(iii) Scouring:
Scouring of wool differs from cotton. Firstly, wool contains a high percentage (30-60%) of wool grease compared with 0.5% of oil and wax in cotton. Secondly, wool is degraded rapidly with alkali, hence it saponification of oils, and fats is to be done with alkali; it should be done very carefully and below the boiling temperature. Sodium hydroxide is replaced by sodium carbonate, ammonia or ammonium carbonate. Raw wool is scoured by the counter current method, using a machine with four or five bowls arranged in a sequence, so that the wool passes directly out of the first into second and so on. Each bowl has a wringer at the exit, a false bottom and rakes. Below the false bottom is a spirally fluted shaft which rotates and carries the deposited solid dirt to the central outlet for discharge. The rakes make the wool travel forward, beneath the surface of liquor and also agitate to keep the dirt and emulsified grease in suspension. The scouring liquor falls back in the bowl after the wool passes through the wringer. The process is repeated at each bowl. Finally, it is washed in water.

Soap in solutionSodium carbonateTemperature
First bowl2-3%3-4%49-52°C
Second bowl1-3%2-3%46-49°C
Third bowl1-1.5%1-2%43-46°C
Fourth bowlwater only40.5-43°C

The pH should never be above 10. Soap reacts with hard water and precipitates calcium and magnesium salts; hence they have been replaced by synthetic detergent, and moreover syndents are more Gardinol and Teepol. There are stable to hard water and acids and are not used up at all during scouring and can be reused. Moreover, some pressure on wet wool in presence of soap can felt it which is not desirable.

Synthetic detergentSodium carbonateCommon saltpHtemp
First bowl0.25%0.25%9.054°C
Second bowl0.2%0.2%0.4%10-10.552°C
Third bowl0.12%0.02%0.5%1049°C
Fourth bowl0-0.1%0046°C

After every 1000 lb of 454 kg of wool have passed over, the bowl should be fortified. Woven piece goods and knitted wool fabrics are sewn together to form an endless rope. These contain much less fats etc. and scoured with 0.5% soap solution or surfactant at 40°C. If alkali is required, ammonia is preferred.

(iv) Carbonizing (removal of burrs):
Burr is mainly cellulosic material. Some cellulosic materials which form burr on the animal have to be removed. The method consists of hydro extracting with 6-8% H2SO4 then drying at 60-70°C, heating at 105-110°C and finally raising the temperature to 150°C.

After carbonizing fabric is passed through milling machine when hydrocellulose and hemicelluloses falls down, thus removing all vegetable impurities from wool.

(v) Milling:
It is done after or before dying process of milling is based on proper of wool, that when it is wet and subjected to pressure it felts permanently specially in presence of soap, alkali and acid. Felting gives denser or more durable fabrics of more pleasing appeal. Milling can be of three different types: soap, grease, and acid.

(vi) Bleaching:
Yellowish color on the fabric may be removed only if the goods are to be sold as white or light colors whereas for dark colors are washed by dye. Bleaching can be done by-

  • SO2 It is a cheep process known as staving. In this case sulphur is burnt in chambers where the wool is hanging in loop form on wooden poles. Sulphur forms sulphur dioxide which acts on the yellow coloring matter.
  • Hydrogen peroxide- It gives a better white light out. Fabrics are treated in winch machine i.e. without tension. Hydrogen peroxide contains acid for preservation hence sodium silicate is added to neutralize. (i.e. it acts as buffer to stabilize). Hydrogen peroxide is heated to 40-50°C. The materials are left overnight in the bleach liquor for complete bleaching wash the fabric with water and then with dilute acetic acid, again wash with water. This is preferred to staving. Heavy weight woolens are given this treatment on a jigger machine. Cloth is rolled on rollers A and B; roller C can be shifted from roller A and B squeezing. Cloth moves from A to B a number of times.

Jigger washing is done (by changing the bath) then wool is dried. When bleaching is done on jigger machine the strength of H2O2 should be 4-5 times higher than for light out fabrics as on a winch machine.

You may also like:

  1. Physical and Chemical Properties of Wool Fiber
  2. Scouring Process of Cotton in Textile Industry
  3. Estimation of Scouring Effect by Weight Loss and Absorbency Test

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