Performance evaluation of textile chemicals

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.

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.