Natural fibers contain oils, fats, waxes, minerals, leafy matter and motes as impurities that interfere with dyeing and finishing. Synthetic fibers contain producer spin finishes, coning oils and/or knitting oils, Mill grease used to lubricate processing equipment, mill dirt, temporary fabric markings and the like may contaminate fabrics as they are being produced.
The process of removing these impurities is called Scouring. Even though these impurities are not soluble in water, they can be removed by Extraction, dissolving the impurities in organic solvents, Emulsification, forming stable suspensions of the impurities in water and Saponification, Converting the contaminates into water soluble components.
Scouring is usually carried out by means of continuous or discontinuous systems, with the same machines used for downstream treatments; temperature, processing time, pH, concentration of reagents, depend on the fibre,whiteness required and on the machine used.Incomplete scouring processes usually originate dyeing and printing defects due to different degrees of wettability and to inconsistent affinity for dyes of the material.
Types of Scouring Treatments
1.Scouring By Chemicals Means
2.Bio scouring or scouring with Enzymes
Scouring of Cotton
In the scouring process the cotton cellulose material is treated with a solution containing alkali ( soda ash and caustic soda), an anionic and/or nonionic detergent, a wetting agent, a complexing and sequestering for the removal of metal ions and polyacrylates or polyphosphonates as special surfactant free dispersing agents, at high temperature.
The scouring operation was conventionally done in kier machines and hence the process was called kier boiling , but now a days the scouring is done mostly in the dyeing machines itself as a part of combined or single operation such as scouring and bleaching.
I. Saponification of fats into water soluble soap and water miscible glycerin under alkaline conditions.
II. Hydrolysis of proteins into water soluble degradation products.
III. Dissolution of amino compounds.
IV. Solubilising of pectose and pectins by converting into soluble salts .
V. dissolution and extraction of mineral matter.
VI. Emulsification and solubilisation of natural oils and waxes.
VII. removal and dispersion of dirt particles and kitty by the action of alkali and detergent.
The material after scouring is more absorbent , free from natural impurities and coloring matter.This treatment can be carried out on filaments, yarns and fabrics.
Scouring of Silk
The scouring of pure silk is a degumming process used to remove sericin (silk gum) from fibroin floss. Sericin is the gummy element which keeps together the fibroin floss and gives the silk a hard hand and dull appearance. It is carried out on yarn, on dyed yarn, piece-dyed fabric or on products ready for printing. The treatment, which causes a loss of weight ranging between 24 and 28%, gives the degummed silk a lustrous appearance and a soft hand; the treatment is carried out with soapy solutions or with buffer dissolving agents. It is also possible to use enzymes (protease), which hydrolyses sericin.
Scouring of Wool
On wool, the scouring process removes oils and contaminants accumulated during upstream processing steps and can be carried out on slivers, yarns and fabrics with solutions containing sodium carbonate with soap or ammonia, or anionic and non-ionic surfactants, which carry out a softer washing to avoid any damage to the fibres.
Scouring of Synthetics
The scouring process applied to synthetic fibres removes oils, lubricants and anti-static substances, dust, contaminants and can be carried out on yarns and fabrics (when warp yarns have been bonded, the treatment is called debonding). It is carried out by means of surfactants, detergents and emulsifying agents.