"Well prepared is half finished", the age old and universal adage probably has roots in pretreatment of textile material. Ashok Athalye attempts to cover aspects related to the evaluation of selection, application and performance (SAP) properties of chemicals used in textile wet processing.

The aesthetic appeal and appearance, feel and comfort, fitment and shape retention of the finished fabric are dependent on the effective preparation and colouration of the textile substrate.

The final textile product, be it a ready-to-wear apparel, home textile item or a technical performance material, undergoes many stages of processing. The steps involved, depend on various factors, such as source of fibre - plant, animal or synthetic; form of substrate - yarn, woven, knit, towel, garment or carpet; method of processing - exhaust, semi-continuous or continuous; and use of machinery under low/high turbulence or low/high temperature.

Fig 1. Processing of Cotton

The processing steps are aimed at optimising extraction of impurities, controlling residual moisture content, achieving the desired extent of absorbency, maintaining uniform standard of white, minimising the damage and loss of strength of the fibre, and ensuring consistent pH from core to fibre surface.

Irrespective of the textile material used, the pre-treatment process is highly chemical-intensive and the desired result is possible only when adequate chemicals are used. Apart from basic chemicals like caustic soda, soda ash, acetic acid and hydrogen peroxide, many other specialised auxiliary chemicals are used during textile wet processing and broadly categorised based on the intended purpose or processing step for which they are used. A broad list of chemicals generally used during cotton processing is given below

  •         Desizers and dispersants

  •          Emulsifiers and detergents

  •          Demineraliser and sequesterants

  •          Bleach stabiliser and quenchers

  •          Deaerators and defoamers

  •          Acidic and alkaline pH controllers

  •          Levelling and dispersing agents

  •          Dye bath controllers and migration inhibitor

  •          Dye accelerator and penetrants

  •          Anti-crease lubricant and friction controllers

  •          Dyefixers and wash-off improvers

  •          Reducing and oxidising chemicals

  •                Colour fastness improvers

Abundant literature is available on each of these processing steps and the chemicals used, depending on the machinery in use. Brief information is offered below on only the critical parameters which govern selection, application performance evaluation of these auxiliary chemicals.

The criteria for a specific chemical selection depends on its suitability for use under intended thermo-mechanical conditions and the following end-use performance evaluation indicators:

1. Physical characteristics

  •          Aspect: solid, paste, liquid

  •          Appearance: flakes, viscous

  •          Colou'r: light or dark, shiny | duller

  •          Odour: ammoniacle, fishy, acrylic

2. Gravimetric analysis

  •          Non volatile matter, solid | active content

  •          Density and specific gravity

  •          Extent of impurity, insoluble matter

3. Chemical analysis

  •          pH of self and diluted solution

  •          Ionic nature: cationic, anionic, non-ionic, amphoteric

  •          UV-VIS spectroscopy: fluorescence, tint

  •          IR, FTIR nomenclature

  •          NMR molecular identification

  •          Atomic absorption for heavy metal ion identification and determination

4. Ecological aspects

  •          Biodegradability and bio elimination

  •          COD, BOD and AOX level

  •          Toxicity to aquatic animals

  •          Presence or extent of components from restricted substance list

  •          Boiling, melting and flash point

  •          Storage conditions

  •          Disposal considerations

Apart from the basic and general qualitative chemical analysis, the efficacy of these chemicals is determined and assessed by the functional performance evaluation. Given below in brief are some of the most commonly used process chemicals and their selection criteria and performance evaluation methods.

1. Desizers: Starch and its derivatives are the most widely used ingredient for enhancing strength of cotton yarn during sizing process. After fabric weaving, such temporarily applied sizing chemical needs to be removed to facilitate further processing. Earlier, some simple but inaccurate methods, such as rot steeping or acid steeping, were performed. However, with the advent of enzyme-based technology, a simple, easy-to-use, accurate and eco-friendly process for starch based size removal has gained popularity.

This process consists of use of a bacterial enzyme that has specific selectivity towards starch and which effectively destroys starch without harming the cellulosic backbone of cotton fibre. The selection, application and performance evaluation of such enzymatic desizing chemical depends on

  •          Type and nature of size

  •          Quantity of size

  •          Fabric construction

  •          Process and equipment used

  •          Water conditions, water hardness

  •          Compatibility with other chemical components

  •          Enzyme activity | Assay

  •          Weight loss estimation

  •          Absorbency and wetting

  •          TEGEWA scale rating of ? 7

Fig 2. TEGEWA Scale

2. Emulsifiers and detergents: During the scouring and bleaching processes, uniform extraction of impurities is desired. The basic criteria for selection of such chemicals depends on how quickly it gets adsorbed and wets out the fibre surface. Other important parameters are:

  •          It should not generate undesired foam.

  •          It should retain rewetting property.

  •          It should effectively clean and uniformly remove dirt and soil.

  •          It should be stable to varying temperature, pressure, turbulence and pH conditions.

The important selection and performance evaluation criteria include:

  •          Ionic nature

  •          Shear stability

  •          Surfactancy: surface tension, cloud point

  •          Wetting efficiency: absorbency by Drave's test AATCC-17, wicking, detergency

  •          Foaming: turbulence, foam height, suppression time

  •          Density: auto dispensing, alkali stability

  •                 Eco conformance: APEO free, bio degradability

Fig 3. Assessment of an Emulsifier

3. Chelating agent: Generally used for forming a complex with alkali earth metals like Ca and Mg and heavy metal ions like Fe, Cu and Cr, this helps overcome metal salt scale formation and deposition, thereby avoiding undesired contamination of machine parts. It enhances dispersion stability, avoids tendering or tensile strength loss of cotton yarn and minimizes risk of pin hole formation during continuous bleaching process.

The commonly used terms are demineraliser (removal of metal ions from textile fibre) and sequesterant (removal of metal ions from process water). The performance evaluation criteria are chelation value by titration, stability under varying pH conditions, hardness testing by Merck strips and dispersing effect by filteration or spot test.

4. Bleaching assistants: Generally, two types of auxiliary chemicals are used: one at the beginning, for enhancing the bleaching efficiency and another at the end for suppressing the residual bleaching agent. The chemical used during bleaching acts as a stabiliser and helps control uniform decomposition of the main bleaching agent, while the chemical used after the bleaching process helps neutralize the bleaching component and ensure no residual component remains in the bath as well as on the cotton substrate, which could affect subsequent dyeing process. The performance assessment is usually carried out by using indicator paper strips, spotting with titanyl chloride or volumetric titration.

5. Deaerator and Defoamer: Foam is a stable dispersion of gas bubbles in a liquid. The atmospheric air can possibly enter water during the preparatory process due to high speed agitation (package or soft flow machines), chemical mixing, machine loading and bath filling, while some extent of air remains entrapped within the interstices of the fibre which eventually entre into the process water. Generally, the term deaeration is used for removing or expelling entrapped air from the fibre while the term defoaming is used for removing foam in the form of air bubbles from the liquid bath.

The chemicals used for deaeration work on the mechanism of rapid surface wetting and diffusion inside the fibre, thereby releasing the entrapped air. While the defoamers effectively block or hinder the air-water interface and help collapse air bubbles formed inside the water. Both these types of chemicals help improve uniformity and levelness of the intended process, whether bleaching or dyeing, avoid usually encountered problems of dye specks or streak marks, and overcome premature oxidation of vat and sulphur dyes.


The commonly used performance evaluation test methods involve

  •          Cylinder shake - extent of foam generation and time for foam suppression

  •          Foam rig agitation - measurement of foaming tendency

  •          Package immersion - dip yarn package in water and observe release of air bubbles

Fig 4 Defoaming test

5. pH controllers: During various stages of pretreatment and coloration, use of varying concentrations of acid and alkali is required. As certain specific pH conditions are desired during various processing steps, it is necessary to adjust, control and uniformly maintain the pH for effective processing. Various formulated auxiliary chemicals are used for such purpose. Owing to the controlled buffering capacity, these chemicals maintain specified pH under the treatment conditions. Some of the actions expected are

  •          Remove surface and core alkali from the fibre

  •          Ensure slightly acidic pH at the start of dyeing

  •          Avoid premature dye hydrolysis of alkali sensitive reactive dyes

  •          Release alkali during dye fixation stage

  •          Impart adequate alkalinity required for fixation during CPB dyeing

  •          Ensure adequate acidic pH to avoid disperse dye reduction

  •          Avoid formation of disturbing neutralization salts

  •          Minimise fibre damage

  •          Avoid corrosion of machines parts

The performance assessment is done based on acid value, pH after drop-wise alkali addition, estimated thermal stability, volatility, spot on filter paper and iron to check degradation, observed precipitation of salts, foam formation and time of suppression, chelation value using titration method, stability under varying temperature conditions and impact on depth and tone of alkali-sensitive dyestuff.

Dye bath chemicals: The colouration of textile material involves dyeing as well as printing using various application methods. The performance in terms of uniformity and levelness, depth and shade achievability, coverage and sharpness of motifs and attainment of desired fastness levels depends on the optimum performance of auxiliary chemicals apart from the dye in use.

Different textile substrates need different types of colorants and accordingly, the chemicals involved also vary. The major application areas involve pH control by buffering, levelling aids, dye dispersion enhancers, washing off agents, dye fixators, migration inhibitors, metal ion sequesters, rheology modifiers, diffusion accelerators and crease preventers.

Further, the use of chemicals depends on the type of textile material being used, whether in self or in blends, machinery in use, process sequence followed and the end use fastness expectations.

6. Levelling, dispersing and anti-migrating agents: For ensuring and maintaining adequate dispersion of water insoluble dyes and achieving uniform level dyeing, certain chemicals are added during dyeing. These chemicals help achieve optimum dye diffusion inside the fibre, avoid dye agglomeration or precipitation and attain the desired shade brilliance. Further, the success of the dye being evenly exhausted and uniformly applied depends on

  •          Rate of liquor flow through the package

  •          Consistency of flow rate through all packages

  •          Rate of temperature rise

  •          Time and temperature of migration phase

  •          Exhaustion rate and migration properties of the dye

  •          The dispersion stability of the dye

  •          Preventing dye migration during continuous dyeing

  •          No gelling or agglomeration in hard water

  •          No sticky film on rolls

Some methods of performance assessment include filter paper spot test, flow test, suction filter test, separating funnel test, speck test and sandwich filter test.

7. Anti-crease lubricants: These help minimise fibre-to-fibre and fibre-to-metal or -machine part friction during the high turbulent movement of the textile material over machine parts, especially fabric processing on Jet or soft-flow machines, thereby avoiding risk of crease mark or chafe mark during dyeing. These also help avoid the Moire effect in beam and pad-batch dyeing.

The performance efficiency of such chemicals is assessed by visual inspection of the dyed fabric or instrumental analysis of coefficient of friction.

8. Dye clearing and fixing agents: The desired shade and depth is achieved only when the unfixed superficial dye is thoroughly washed off and removed from the fibre surface. Similarly, the desired wet fastness properties are achieved when there is no bleeding or extraction of dye in aqueous medium under thermo-mechanical conditions and colour staining on adjacent surface. This is achieved by use of a suitable chemical for efficient washing off of superficial dye and subsequent treatment with another chemical for not allowing any dye molecule to break away from the fibre and come out during end use application.

The performance evaluation of these chemicals is conducted by testing various wash fastness properties, dispersing and chelating power, foaming behavior, effect of colour depth and hue, effect on absorbency and handle of the textile material and compatibility and stability under varying pH conditions.

Further, such auxiliary chemicals are expected to be stored in closed containers under cool and dry conditions (a temperature of 20 /-20C) and at relative humidity of 65 per cent so as to maintain shelf life of about a year. However, the shelf life may vary depending on the chemical constituents and their concentration. Therefore, it is expected that the chemical supplier specifies shelf life for their product. Some of the assessment methods are

  •          Freeze-Thaw test: Should pass at least five cycles of storage at 500C for 8 hours, followed by refrigerator for 8 hours

  •          Centrifuge test: No layer separation after centrifuging at 20,000 RPM for 30 minutes

  •          Foam-rig test: No layer separation after 30 minutes circulation

  •                 Accelerated storage stability or ASS test: Keep at 600C for a week and reevaluate its performance

At the end of textile processing, these chemicals need to be removed and drained into the effluent stream. This necessitates understanding of their disposal as well as effect on solid, liquid and gaseous effluent.

The selection of adequate chemicals is key to the success of optimum textile preparation and colouration. The criteria for performance evaluation, storage conditions and disposal properties of these chemicals vary based on their chemical composition as well as concentration in use.

About the author: Dr Ashok Athalye is from the technical service, Atul Ltd, Colors Business, Valsad, Gujarat.