Standardisation of Dyes

Colourants are widely used in various industries including textiles. The ever evolving trends and consumer expectations in the apparel and textile industry call for standardisation of dyes. Ashok Athalye discusses the need for standardisation and the methods and processes followed by manufacturers to deliver consistent quality and avoid rejections.

The size of the global colourantindustry is estimated to be about $ 35 billion which includes dyestuffs,pigments, and their intermediates. The average person consumes is estimated tobe about 250 gm. Though synthetic colourant production began in Europe a coupleof centuries ago, over the last few decades, Asia has emerged as a major manufacturing hub contributing almost 75 per cent of the world demand.

India is the second-largest supplier of colourants with close to 1,000 manufacturers producing almost the entire gamut of dye intermediate, dyestuffs and pigments for various end-use applications. The size of the Indian colourant industry is estimated to be about Rs 45,000 crore of which almost 75 per centis exported. Textiles are among the major user industries of colourants and India has achieved distinction of being the second-largest global supplier of apparel and home furnishings. With the Make in India and Made in India initiative and drive by the government, there will be renewed focus to achieve exponential growth in this sector.

Need for standardisation: With changing fashion trends, user industries including textile and apparel marketing brands are aiming to ensure that products they deliver meet consumer expectations related to quality, consistency, durability,sustainability and conformance to ecology norms. This is hoped to be achieved by meeting the standard quality parameters as devised by various stake holderslike consumers, brands, regulatory bodies, textile processors, and the colourant manufacturers by ensuring stringent control over productst and ardisation.

Dyestuffs form a major part oftextile coloration and different classes of dyes are used for dyeing differenttypes of substrates on different types of machines by different methods ofapplication. Many dye manufacturersprovide dyes of same Colour Index which does not ensure same performance (due toimpurities and isomers resulting from the raw materials and the method ofsynthesis) and in case of patent or IPR protection from some manufacturer,prototype products with some modification are developed by other manufacturers,which tends to behave differently under different application conditions. Suchcomplexity has increased the onus on dyestuff manufacturers to devise methodsfor standardisation of dyes to suit textile processors' application needs.

The textile industry in India isbifurcated into the organised and unorganised sectors. The latter is highlyfragmented and geographically, widely spread through the country. Further,there are clusters (hand processors), SME's (process house-job workers) andcorporate (composite mills) clients, which may or may not have sophisticatedquality control labs for inspecting incoming dyes and chemicals. Therefore, itis expected that the dyestuff manufacturer ensures desired quality and providesdyes of adequate consistency to meet the processors needs.

Further, proper standardisation of dyes is essential for the textile processor in terms of receiving consistent quality supply to get uniform and reproducible results, increase in productivity by avoiding shade rejections and achieving reduction in water and energy as well as processing time. Similarly, consumer expectations in terms of desired level of fastness and safety are important criteria. Also, ecological norms in terms of generation and containment of solid waste, liquid effluent, and gaseous emission also need to be considered.

This article attempts to provide some of the commonly employed test methods and procedures followed by reputed manufacturers like Atul Ltd Colors Division, during standardisation of different class of dyes to ensure a high level of product quality and consistency.

Various check points and test procedures are used during the dye synthesis for quality assurance before the standardisation of the final finished product. These tests involve evaluation and analysis of incoming basic chemicals, raw materials and dye intermediates for conformance to specifications. In-process testing is also required to ensure stoichiometry during dye synthesis, to achieve optimum colour yield and avoid wastage of chemicals.

Standardisation methods include several testing parameters like physical, chemical, fastness, toxicological, effluent related etc and comparative analysis is made against an internal or external standard.

Physical Characteristics: Dyes are manufactured and supplied in various physical forms and the steps of standardisation involve ensuring consistency in maintaining this for effective handling and usage.

i. Appearance: Dyes are supplied in different forms like solid powder, flakes, granules, pearl/beads, viscous paste, liquid etc and consistency from batch to batch in maintaining the specific form is desired.

ii. Aspect: This involves measuring and maintaining desired form, like fluffiness of the dye powder, size and shape of flakes or granules, viscosity of the paste and liquids.

iii. Colour: Maintaining consistency in the physical colour of the dyestuff in terms of lighter or darker, shiny/brighter or duller is also important as it affects the first impression of the user.

iv. Odour: The unintended or un-reacted components during synthesis of the dye or subsequently added ingredients like diluents, dispersants, reducing agents, etc or the bacterial/ fungal growth in case of paste and liquids when stored for long duration of time may result in obnoxious smell.

v. Moisture content: Generally, dyes are sold by weight. In case of solid forms, the moisture regain depending on the storage conditions and prevailing atmospheric humidity would affect weighing accuracy during usage.

Chemical Analysis: For improved manufacturing efficiencies and productivity, help of various chemical and analytical techniques is taken during characterisation and standardisation. For this, many specialised processes and equipment are used.

i. Chromatographic techniques: Different methods are employed for identification, separation and differentiation of the dye and its components. Depending on the type of dye and the method of analysis, evaluation involves use of Gas Chromatography (GC), Thin Layer Chromatography (TLC), High Performance Liquid Chromatography (HPLC) etc.

ii. Spectroscopic techniques: Various sophisticated instruments are used for product analysis based on UV Visible Spectroscopy, IR Spectroscopy, Mass Spectroscopy and Transmission Spectroscopy to evaluate dyes under standardisation.

iii. Analytical techniques: For investigation and assessment of heavy metal content, analytical instruments like Atomic Absorption Spectroscopy (AAS) is used to determine and estimate dye particle size with Particle Size Analyser. A Dust Analyser is used to evaluate dye dusting characteristics.

Gravimetric Analysis: This involves quantitative determination of either the main colouring component or other ingredients of the dye. These methods are employed for ash content analysis, determination of salt concentration, water insoluble content etc.

Tinctorial Specification: Being a colouring component, the shade and strength analysis of the dye is the most important criterion during standardisation. Commonly used techniques for determining the tinctorial strength of dyes are:

i. Optical density: The aqueous dye solution is spectrophotometrically assessed and compared against the standard for colour value estimation.

ii. Visual assessment: This is one of the most important control parameters where the shade and strength of the dye under study are visually compared under different light sources.

iii. Instrumental measurement: Computer colour matching systems are used to measure reflectance data and monitor colour electronically. Here, help of sophisticated spectrophotomers is taken for assessment and standardisation within pre-determined limits for approval or rejection of the manufactured dye. Generally, the acceptable tolerance norms are colour strength variance within /- 2.5 per cent and shade deviation of less than DE 1.0 by CIE Lab measurement system.

Performance Evaluation: For the textile processor, being the actual user of the dyestuff, the application performance of the dyestuff is of paramount importance.

Dyes are grouped based on chemical composition, nature of dye fibre bond formation, ionic character, etc and further sub classified based on the method and conditions of application. Therefore, depending on the type of dye (Acid, Basic, Direct, Disperse, Reactive, Sulphur, Vat, etc), form of textile substrate to be dyed (loose fibre, top, yarn, woven, knit, towel, garment, etc), method of application (exhaust, semi-continuous, continuous) and the machinery on which it is intended to be dyed (based on conditions of liquor ratio, turbulence, temperature etc) test methods of standardisation and points of consideration would vary. Further, in case of newly developed or formulated dye molecule some additional tests to check suitability for different application needs are done before its commercial launch.

Apart from basic characterisation tests like ionic nature and pH of the dye solution, various other performance application tests are carried out. Given below are select test methods:

i. Solubility: Water soluble dyes have specific solubility parameters which varies based on dye characteristics. From the textile processing point of view depending on the intended method of application and machine in use, this test is important. Solubility is measured by preparing dye solution of specified concentration and then spotting it on filter paper to check whether the dye spreads uniformly or precipitates at the centre and the results are compared against the standard. Further, in case of reactive dyes, solubility under intended method of application is measured to check stability and suitability for specific use.

    Temperature: Extent of solubility of dye in water under varying temperature conditions from ambient to the final temperature of dyeing is evaluated.

      Electrolyte: Mainly in case of exhaust applications, the dye solubility in presence of varying concentrations of salt is measured.

      Alkali: Mainly in case of dyes intended for continuous and semi-continuous application of reactive dyes, the stability to different type of alkali are evaluated and compared. Commonly used alkali systems are soda ash, caustic soda, sodium silicate which are used separately or in different combination.

ii. Dispersion stability: In case of water dispersible dyes like Disperse and Vat which are applied by pigmentation process, this is a critical test parameter which reflects in uniformity and levelness of dyeing. Various test methods are employed depending on the type of dyes and intended application. Given below are a few test methods:

        Flow test: Aqueous dye dispersion is spotted on Whatman No 3 filter paper sandwiched between two glass plates. Uniform dye spread indicates good dispersion, while dye concentrate at the centre indicates poor dispersion.

        Separating funnel test: Dye dispersion is allowed to stand for 24 hours in a separating funnel and then the filter paper dispersion is checked for top and bottom layers.

        Suction filter test: Dye dispersion is heated at 710C for 10 minutes and poured under vacuum through Buckner funnel containing Whatman filter paper 2 on top and filter paper 4 at the bottom held by stainless steel ring. The time taken for filtration and the residual dye deposition on filter paper is assessed and rated

    Speck test: Mainly in case of Vat dyes intended for continuous application, dye dispersion is vigorously stirred at 2200 rpm (rotations per minute) and filtered through woven cotton filter cloth. Subsequently, the cotton filter is treated with caustic soda and hydros (reduction) followed by chemical oxidation using hydrogen peroxide. Then, the fabric is visually assessed for the number and size of visible specks and rated between 5 to 1, 5 being the best.

       Sandwich test: Dye dispersion is filtered through PET/COT (50:50) fabric, dried in the oven at 60oC, rolled in the form of a funnel, reduced with caustic and hydros and air dried. Then the both the fabrics are assessed for appearance of dye specks.

iii. Strength and shade assessment: The colour value and tone of the production batch is compared against the standard by practical dye application on the textile substrate in the laboratory simulating bulk dyeing conditions. Generally, comparison is done at LMD (Low, Medium and Dark) depths. Lower depth is assessed for tone or hue, middle depth for strength and higher depth for build-up comparison.

        Exhaust dyeing: The textile substrate is dyed as per intended bulk dyeing method, maintaining dye bath variables like MLR (material to Liquor ratio), time of dyeing, chemical dosing (basic chemicals like salt, alkali, reducing agent, etc and auxiliary chemicals like dispersing, levelling, sequestering, etc) and temperature conditions. The dyeing is followed by post dyeing process of washing off, oxidation, reduction clearing, etc depending on the dyestuffs. The substrate is dried, conditioned and assessed for comparative shade and strength.

     Continuous and semi-continuous dyeing: Different dyeing methods like CPB (cold pad batch), PS (pad steam), PDPS (Pad dry pad steam) and PDTPS (pad dry thermosol pad steam) are employed depending on the type of dye and intended application.

The shade and strength assessment is done both visually and spectroscopically. The shades are also viewed under different light source to assess extent of flaring or colour constancy. Further, in case of modification in dye formulation or a new molecule development, few other performance tests are carried out like, standard depth determination, substantivity, exhaustion, fixation behaviour, migration index, robustness to dye bath variables, wash off profile and dischargeability in case of reactive dyes and rate of reduction, half dyeing time, migration behaviour in case of vat dyes.

Fastness Assessment: From the textile end-user i.e the consumer's point of view, durability and colour fastness are major concerns. The cross staining of colour from one dyed area on to adjacent differently dyed or un-dyed part spoils the aesthetic appeal of the garment. Therefore, various comparative fastness tests like wash, light, rub, perspiration, saliva, etc need to be checked. Further, there are different test methods and standards like ISO, AATCC, M&S etc which need to be adhered to as per the specifications of brands or retailers. Depending on the requirement, some of these tests can be carried out at external specified accredited lab.

Storage Stability: There could be considerable time lag between the time of dye manufacturing and its usage in textile processing. This could be due to storage and stocking while distribution channelling as well as transportation during overseas supplies. Further, there could be varying temperature conditions depending on seasonality and location of storage. Considering these factors it is important to determine and evaluate storage stability and shelf life of the product.

        Accelerated storage stability (ASS test): To simulate varying weather conditions and duration of storage, the dye sample is kept in an oven at 60oC for seven days and then the performance properties are re-checked.

     Freeze Thaw (FT test): Dye sample is kept in refrigerator for eight hours and then in oven at 60oC for eight hours. This process of cooling and heating is repeated three times and the performance is evaluated at the end of each FT cycle. The comparative performance is considered to represent storage stability of one year.

Effluent-Related: The amount of coloured effluent depends on the residual or left over dyes which are unfixed or un-reacted at the end of textile application. The excess and superficially deposited dyes are removed by suitable washing off process to ensure adequate fastness properties. The effluent treatment depends not only on the decolourisation of water but also on the degradation of chemical components of the dye. The tests involve checking of COD (chemical oxygen demand), BOD (biochemical oxygen demand), TSS (total suspended solids), TOC (total organic content), AOX (absorbable organo halogen), extent of sludge formation, etc.

Sustainability Parameters: Various ecological and toxicological parameters and related tests of the chemical ingredients present in dyestuff and their impact on the health and safety of human beings, other animals and aquatic life is of major concern. The amount of heavy metal ions present in dyes and the acceptable tolerance limits, presence of harmful banned amines, phenolic compounds, non biodegradable dye components, etc need to be checked and conformance confirmed. With growing awareness about sustainability characteristics the impact of dyes during use in terms of consumption and conservation of natural resources like water and energy as well as their footprint in terms of emission of green house gases is under scrutiny by many global authorities. From the consumer or user point of view the dermatological as well as toxicological harmful effects in terms of carcinogenicity, skin sensitisation, rashes and other effects on biological immune systems are of prime concern. Therefore, various norms, test methods and tolerance limits are prescribed by various governing organisations. The parameters, test methods and specification limits vary depending on the end-user requirements related to health, safety and hygiene.

Given below are some of such organisations and their requirements

Considering the global dyes market for textile application, the challenges involved in sustaining a competitive edge depends on consistent quality supply to ensure suitability to meet intended or still to be developed demands. The standardisation and quality control puts emphasis on testing to avoid defects and make the decision to allow or deny product release, at the same time develop test protocols to improve and stabilise production to avoid or minimise issues which may lead to the defect in the first place.

The manufacture of a dye is normally a multi-stage synthesis from several intermediates and it is not always possible to ensure consistent reaction yields at every stage. Therefore, the quality control testing of each individual batch against a standard batch is an integral part of the dye standardisation process. The testing can also be carried out by an independent, accredited testing laboratory for few specific parameters,

The analysis, testing, evaluation and standardisation of a dye ensures achieving RFT (right first time) performance during textile coloration.

References

1. Scribd.com

2. J Park; Textiles Today; Feb 13, Vol 6 Issue 2