An important, if not the most important, operation in the pretreatment of cotton is the scouring or alkaline boil-off process. The purpose of alkaline boil-off and the ensuing washing stage is to perform extensive fibre-cleaning by ensuring a high degree of extraction of pectins, lignins, waxes and grease, proteins, alkaline earth metals (Ca and Mg), heavy metals (iron, manganese and copper), low molecular weight cellulose fragments, dirt and dust; and softening of husks.
The result is an increased responsiveness of cotton to subsequent processing. The process removes water insoluble materials such as oils, fats, and waxes from the textile material. These impurities coat fibres and inhibit rapid wetting, absorbency and absorption of dyes and chemical solutions. Oils and fats are removed by saponification with hot sodium hydroxide solution. The process breaks the compounds down into water-soluble glycerols and soaps. Unsaponifiable material such as waxes and dirt are removed by emulsification. This requires the use of surfactants to disperse the water-insoluble material into fine droplets or particles in the aqueous medium.
Both of these processes (saponification and emulsification) take place in a typical scouring process. In addition, the scouring process softens and swells the motes to facilitate their destruction during bleaching. Depending on the amount of impurities and the reaction and wash conditions, the loss in weight of the raw cotton material due to boil-off can reach up to seven percent or even higher in case of high-impurity cotton.
The important parameters of the scouring process are as follows:
- Concentration of caustic soda.
- Type and concentration of auxiliaries.
- Treatment temperature.
- Reaction time.
The higher the caustic soda concentration, the shorter can be the dwell time. In other words, the shorter the dwell time, the higher the concentration required. The caustic soda concentration normally employed neither affects the ash content or the average degree of polymerisation of cotton. Too high a concentration may result in a reduction in DP as well as yellowing of the cotton fibre. The higher the concentration, the greater will be the fat removal. Due to the high degree of fat removal, the absorbency will also increase but there may be harshness in the handle of the material.
Two important auxiliaries used in scouring are chelating agents and surfactants. Other auxiliaries that may sometimes be employed include antifoaming and anticreasing agents. Chelating agents are used to eliminate water hardness and heavy metals, such as iron and copper which can affect the scouring process. These agents bind polyvalent cations such as calcium and magnesium in water and in fibres, thus preventing the precipitation of soaps. If polyvalent ions are present, insoluble soaps may form, settle on the fabric and produce resist spots.