What is Pretreatment Process in Textile?
In textiles, pre-treatment is a processes applied on textile material (viz. fibres, yarn, woven and knit fabrics) to prepare the material for dyeing, printing or finishing. Pretreatment plays a crucial role in achieving consistent and high-quality results in textile processing. As the grey material will have lot of impurities, these impurities hampering the subsequent processes has to be removed and made absorbent to ease the further processes like bleaching, dyeing, finishing, etc. For all practical purposes pre-treatments are carried out in continuation of dyeing or printing and their equipment is part of the wet processing plant.
Purposes of Pretreatment:
The purpose of pretreatment is to ensure that the textile material is clean, even in color, and ready to accept the desired dye or finish. The main object of pre-treatment is to impart a uniform and high degree of absorptivity for aqueous liquors with the minimum possible damage to the material. The cotton fabric, for example, after the pre-treatment should become free of all natural impurities like pectin, wax, protein and husks and the sizing chemicals comprising of adhesives and softeners. Besides high and uniform absorptivity, the textile materials should have adequate degree of whiteness so as not to hamper color and brilliance of the applied colors. Normally achievement of whiteness of about 80% remission (As. C 100% reflectance from barium sulphate) and a D.P. of 1,600–2,000 are aimed at for the cotton goods.
Flow Chart of Pretreatment Process in Textile (Cotton):
Inspection and Marking
↓
Shearing or Cropping
↓
Singeing
↓
Desizing
↓
Scouring
↓
Mercerizing
↓
Bleaching
Shearing and singeing are always carried out in open-width but desizing, scouring and bleaching of woven fabric may be done either in rope or in open-width forms by batch, semi-continuous or continuous systems. In the rope form, woven and knit fabrics are scoured and bleached in kiers (autoclaves), winches and jets for batch and in J box or a similar storage system for continuous working which are not so common these days. In open-width processing, jiggers, pad-batch and pad- roll machines are used for batch system and J-box, U-box, normal steamer, pressure steamer, roller bed conveyor steamer or perforated belt steamer are employed for the continuous working. Choice of the equipment depends mainly on the volume of cloth to be processed but the present clay trend is to install the open-width continuous pre-treatment machines.
Factors of Pretreatment Process in Textile:
Satisfactory preparation of the textile materials before any value addition process such as bleaching, dyeing, printing and finishing makes major contributions to consistent attainment of the desired end-product quality. In order to get the best results, we should conduct right pretreatment right at first instance.
Successful preparation depends on four factors:
- The amounts of the various impurities present
- The purity of the water supply
- The chemicals used in the various preparation processes
- The machinery available for processing of the goods
It is necessary to remove the unwanted materials from the textile materials which hinder the value addition processes. They may be referred to as impurities from the point of view of value addition processes. Let us see the normal unwanted materials in a fabric that hinders value addition processes.
- Cotton impurities and preparation chemicals
- Pectins are polygalacturonic acids and their calcium, magnesium and iron salts
- The inorganic ash containing calcium, magnesium and potassium phosphates and carbonates
- The spin finish and knitting oil containing mineral oil and surfactants applied to decrease friction on machinery parts
- Sizing agents, which are film-forming polymers applied to warp yarns before fabric weaving in order to minimize yarn breakage
- Metallic ion contamination, particularly iron and copper, is of serious concern during oxidative bleaching processes
Important factors to select and use of surfactants in preparation are as follows:
- Wetting agents used in the desizing stage must be compatible with the enzyme preparation.
- Detergents selected for scouring must be stable at the temperature and concentrations of alkali and electrolyte required.
- Surfactants added to bleach liquors must be stable at strong oxidizing conditions.
- Residual surfactants which are retaining must not cause problems in subsequent printing or water-repellent finishing.
- The cloud point of any non-ionic surfactants used must be high enough to avoid impairing the wetting or detergency performance.
- Surfactants must be low-foaming to avoid risks of pump cavitation in circulating liquor systems and loss of traction in conveyor or rollerbed steamers.
- The viscosity of the surfactant solution should allow satisfactory performance in automatic dosing systems.
Conclusion:
Pretreatment in textile is a series of preparatory processes that are carried out on textile materials before they undergo dyeing, printing, or finishing. The specific pretreatment steps and chemicals used can vary depending on the type of textile material and the desired end product. Proper pretreatment is essential to ensure that dyes or finishes adhere evenly to the fabric, resulting in the desired color, texture, and performance characteristics in the final textile product. The various pre-treatments are singeing, desizing, scouring, bleaching, mercerizing for cotton fabrics. The degumming is a special pretreatment for natural silk fabrics.
References:
- Pretreatment of Textile Substrates by Mathews Kolanjikombil
- Handbook of Value Addition Processes for Fabrics By B. Purushothama
- Chemical Technology in the Pre-Treatment Processes of Textiles by S. R. Karmakar
You may also like: Textile Wet Processing: Stages, Flowchart and Importance
