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.
There are four major types of sequestering agents to choose from: inorganic polyphosphates, amino carboxylic acids, organophosphonic acids, and hydroxycarboxylic acids. The inorganic polyphosphates such as sodium tripolyphosphate and sodium hexametaphosphate are probably the best overall in that in addition to sequestering most metals they also aid in cleansing the fibres. They may, however, hydrolyze at high temperature and lose their effectiveness.
The amino carboxylic acid types such as ethylenediaminetetraacetic acid (EDTA) are very good in that they sequester most metal ions and are very stable under alkaline conditions. They are the most used types. The organophosphonic acid types such as ethylenediaminetetra (methylene phosphoric acid) are also very effective but comparatively expensive. Oxalates and hydroxycarboxylic acids are excellent for sequestering iron but not effective for calcium and magnesium.
In order to quickly and effectively bring the chemicals to the textile material, i.e. to improve their wet ability and to ensure that the fibrous impurities will be removed as far as possible, it is necessary to add surfactants with good wetting and washing/emulsifying properties. A surfactant of optimal versatility to be used for preparation, and in particular for the scouring and bleaching processes, ought to meet the following requirements:
- It should have an excellent wetting ability within a wide temperature range.
- It should permit a good washing effect and have a high emulsifying power for natural fats, waxes and oils.
- It should be resistant to oxidants and reducing agents.
- It should be resistant to water hardening substances.
- It should be highly stable to alkalinity.
- It should be biodegradable and non-toxic.
Care should be taken in selecting the surfactants because of the inverse effect of temperature on the solubility of non-ionic surfactants. If the process temperature is above the cloud point of the surfactant, the surfactant may be ineffective and may actually be deposited on the substrate. The surfactant used should have a cloud point temperature just above the operating temperature, to be most effective. The cloud point of non-ionic surfactants decreases in the presence of alkalis and electrolytes and the degree to which it is lowered increases with concentration. The cloud point should therefore be checked under application conditions to ensure that the surfactant is effective under those conditions. The adverse effect of temperature on non-ionic surfactants can be reduced by the addition of an anionic surfactant. Crypto-non-ionic surfactants do not exhibit a cloud point. These are non-ionic surfactants that are capped with an ionic group and they exhibit the excellent emulsifying properties of non-ionic along with the good solubility properties of anionic.
Higher scouring temperatures will reduce treatment times and vice versa. At high temperature, however, there will be complete removal of fats and waxes, which will promote harsh handle of the material. Moreover, the cloud point of the surfactant also has to be taken into account while applying high temperature.
In the case of pad-steam scouring, a typical process consists of the following steps: Saturating the fabric with a solution of sodium hydroxide, surfactant and sequestering agent; steaming; and thorough washing. After scouring, the material is checked for thoroughness and uniformity of scouring as well as other scouring faults.
This article was originally published in the July issue of the magazine, New Cloth Market the complete textile magazines from textile technologists."