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.
• 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.
• Suint is usually considered to be a variable composition of water-soluble materials that is readily removed by scouring.
• The dirt that is removed from the scoured wool consists of both inorganic and organic materials.
• The proteinaceous material has recently been discovered to consist of skin flakes from the sheep and soluble peptides.
The process that can remove the impurities has various steps:
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.
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.