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Bangladesh Textile Today - A Comprehensive Publication for the Textile & Apparel Industry
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Palette generation and engineered textile colour standards (ETCS)

Introduction The generation of colour palettes and the production of ‘engineered textile colour standards (ETCS)’ are now mature technologies and are able to shorten significantly the colour selection and manufacturing process, giving improvements in colour quality control and cost reductions. These advances would not have been possible without the development of optimised colour-difference equations, improvements in laboratory dyeing techniques and computer colour matching together with the availability of digital communication. The importance of colour in textile products There are relatively limited ranges of fibre types, yarn constructions, spinning methods and fabric-forming techniques from which to select the basis for designing a textile substrate. By contrast, the range of colours that can be applied to such substrates is extremely wide. It has been suggested that there are at least a million colours which are perceptibly different, although little evidence exists that this many colours have been physically identified, far less produced, on textile materials. A visit to any retail clothing store will confirm that actual garments are manufactured from a relatively few, almost standard, fabric types. The coloration of textiles and the selection of the individual colours in a shade range are major methods whereby manufacturers and retailers can differentiate their products from those of their competitors. Those retailers that are seen as being successful in terms of volume of sales and profitability attribute this success to having well-designed and well-manufactured products in a range of colours that appeal to the purchaser. The selection of fabrics for garment design is an important aspect of the product development process carried out by the design team. Perhaps even more critical is the selection of a colour range to create a colour palette for a range of merchandise. From this range, master ‘engineered’ standards are produced for the colours selected for bulk production so that suppliers can match the correct colour, to give reproducibility with a given product and on products made from different fibres and blends, especially co-ordinates. A changed industry The textile industry has changed dramatically over recent years from mainly vertical organisation to retail-specified manufacturing. In traditional vertical manufacturing, garments were designed by manufacturers who had the responsibility of selecting yarns, fabric constructions, colour, finish and the final garment design. In theory at least, a garment was never designed that could not readily be made and costs were kept within a pre-determined budget to arrive at a profitable selling price. Many of these factors are now specified by the retailer but nevertheless an informed input is required from other members of the manufacturing chain, including dyers and finishers. Furthermore, globalisation of the textile processing industry, particularly in favour of developing countries in Asia, has resulted in lengthy supply chains. Retailers who have adopted the concept of using ‘engineered’ master standards have seen major improvements in colour reproducibility and consistency of products between suppliers, as a direct result of all colour matching being carried out using one master standard. These retailers perceive the major advantages as reducing the lead times for product development, improving in-store colour quality and reducing associated costs (including travelling and courier costs) of the product development cycle. Distance quality control makes a major contribution to cost reduction. Typical savings being achieved are a 30 to 50% reduction in the number of laboratory submissions from suppliers, with a similar reduction in the time required. It has been claimed by a major retailer that the savings in courier charges alone in one year paid for the capital investment in measuring and computer equipment. Instrumental colour measurement Visual methods of colour matching can be haphazard and are notoriously unreliable for a number of reasons [1]. These include the absence of standard illuminants and viewing conditions, the subjective and psychological nature of the human observer. There is often difficulty in agreeing standards with the added difficulty of establishing standard tolerances, whilst physical standards quickly become soiled. There can thus be a significant quantity of wrong decisions. Even when visual assessments are carried out in standard matching cabinets with standard illuminants under virtually darkroom conditions to eliminate extraneous illumination, the other factors listed can result in a significant proportion of ‘wrong decisions’ in which acceptable matches are rejected and vice versa. Wrong decisions can exceed 20% of assessments with trained colourists reversing their decisions on a similar proportion if given the opportunity to re-assess standard and batch pairs. An improvement in the quality of colour matching, in terms of accuracy, consistency and reproducibility can only be achieved by the use of instrumental methods based on spectrophotometry and a colour difference equation which gives results that correlate with visual assessment by a panel of experienced colour matchers. A major advance was the development of the so-called optimised colour difference equations which effectively compensated for the visual non-uniformity of the colour solid. The most important of these was the JPC 79 equation, and this was suggested as a viable alternative to visual colour matching [1]. This equation was soon adopted for routine quality control of bulk production using single number shade passing (SNSP) and with minor modifications this equation became the CMC (2:1) equation which gained the status of a British Standard [2] and eventually became an international standard. With the availability of optimised colour-difference equations, the difficulties associated with visual assessment can be eliminated and this was a major factor in the development of engineered standards, leading to major improvements in colour quality. SNSP was established which allows a single numerical tolerance to be applied to any component of colour difference (such as total colour difference, ΔE) and this tolerance is then satisfactory for all areas in colour space. The numerical tolerance is defined only by the end-use criteria as regards the tightness of matching. Traditional methods of colour selection Traditionally, a shade range was created (for example, by designers or retailers) by collecting coloured samples from various sources in readily available materials. These initial colour ideas could be composed of a number of materials, including plastic, paper, leather or metallic in addition to textiles. These highly-variable, physical samples were often sent to the matching laboratory of each potential supplier for conversion of these colour ideas on to a textile substrate. Each laboratory produced its own interpretation of a match and this was an obstacle to achieving continuity of colour between suppliers and over a range of products. The problems associated with visual assessment, together with the interpretative nature of this process and possible conflict with the designer’s personal preferences, often made this a lengthy and involved procedure. This situation was rationalised by the product-specifier sending target colours to only one independent laboratory for matching and the eventual production of master or ‘engineered’ standards for the approved colours. The traditional approach to palette generation The methodology for palette creation and master standard production by the traditional approach consists of two phases as outlined in Table 1. The master standards from phase 2 are issued to the suppliers by the retailer. These suppliers must match the master standard on the substrates required for the product, using dyes to meet the dye specifications listed in Table 1. Table 1 – Outline of traditional process  Matching specification for phase 1 DE (CMC) below 0.5 in primary illuminant only Colour constancy in other illuminants of interest (for example TL84, illuminant A) Substrate and dye selection Dyeings carried out on cotton fabric, since colours on this substrate can be reproduced on other substrates, using a rationalised range of dyes to give robust, stable, consistently reproducible and level dyeings with colour constancy in different illuminants, producing a complete gamut of colours with the required fastness properties Matching specification for master standard for phase 2 DE (CMC) of 0.8 or less in all illuminants of interest using the dyes from phase 1 As discussed in a recent paper [3], the use of colour atlases or colour-specification products illustrating a large population of coloured patterns, preferably on a textile substrate, greatly assisted in the initial colour selection decisions and in the matching process. Designers and colourspecifiers often did not favour the use of such colour-specification products, considering that they detracted from the ‘artistic talents’ of the designer. Many such products have been produced ranging from within-house atlases, often compiled by textile printers, to commercial products with a large sales distribution. Many of these products are often less than perfect, since they illustrate the colours on a non-textile substrate or on a less than ideal textile material. Atlases based on colour order systems are preferred and such systems were described [3]. Even although on a percentage basis its worldwide usage has declined, cotton is still the preferred substrate for textile colour atlases. Apart from the pleasing appearance and drape of the fabric, colours dyed on cotton can be matched on other fabric types using appropriate dye classes, almost without exception. The reverse is not always the case. The successful utilisation of such colourspecifiers is proportional to the number of units that are sold worldwide. A uniformly-spaced colour atlas based on the CMC colour solid was developed (the PCC) [3]. The cotton specimens were dyed with commercially available reactive dyes, to ensure adequate fastness of the product in use so that these colours could be reproduced accurately in bulk-scale procedures and of the necessary fastness. Further equally-spaced colours can be readily inserted between existing colours. A major high-street retailer found that, by using this colourspecifier, the time taken to generate master standards, to which suppliers have to match products, was reduced from eight to only two weeks. Digital colour matching Computer colour matching (CCM) technology has been well-established for several decades for recipe prediction and correction, together with quality control of coloured materials against a standard. In addition to the contribution that CCM makes to quick response and right-first-time (RFT) processing, considerable financial savings are possible and these were demonstrated [4]. Physical standards become quickly soiled and the concept of using a non-physical standard (NPS) based on reflectance measurements is not new [5] and the development of the engineered textile colour standard (ETCS) was illustrated [3]. Developments in spectrophotometry, together with improved accuracy in CCM and lab dyeing, have allowed non-physical standards in the form of reflectance data to be communicated by fax or e-mail as part of the matching process [6]. When spectrophotometers from different manufacturers are involved, the variability between instruments is an average of only DE (CMC) = 0.3, when assessed by means of ceramic tiles [7]. The repeatability and reproducibility achieved using modern spectrophotometers with better inter-instrument agreement assures that the same spectral data would be obtained when measuring a sample at different times and with different instruments. The use of reflectance data is thus a feasible method of specifying colour provided that measurement techniques and the condition of the instruments are standardised and controlled. Self-approval and accreditation becomes possible leading to a quick response in colour specification. High levels of colour acceptance are achieved provided that a match of DE (CMC) below 0.5 is obtained in the primary illuminant and that colour constancy is achieved in secondary illuminants. In a study involving 2500 colours, matching difficulties were experienced with only three colours: one was a paper standard, a second contained a fluorescent brightening agent and a third was a multi-fibre mixture of dye classes. This first approach to ‘colouring by numbers’ allowed a significant shortening of lead times and a quicker turn-round in matching [6].   Digital colour communication So-called digital colour communication has become established as another means of achieving quick response in product development, including colour selection, together with the ability to monitor development and production without the need to ship physical samples. Major time-savings and cost benefits are achieved in colour communication for matching, standard generation, communication with suppliers and quality control in the end-product from either the laboratory or bulk production – distance quality control. An outline of the procedure is given in Figure 1. The standards can be loaded on to a web site and password-protected, giving availability to only authorised users. A further advantage of this procedure is that the ‘virtual’ match (illustrated on the calibrated screen) will indicate the limit of shades that can be matched using the commercially available dyes contained in the prediction programmes of the CCM system. This avoids attempts to obtain colours which are unattainable using these dyes.  Figure 1 - Specifying, Communicating and Matching Standards Electronically   Precise colour is generated on calibrated screen by retailer Matching lab is requested to produce virtual matching and an appropriate dye formulation is predicted Virtual matching is returned within minutes by e-mail to retailer Retailer assesses virtual match which is Accepted or modified Laboratory dyeing within agreed tolerances is produced of approved virtual match Approved laboratory dyeing is identified as ‘master standard’ and remains so until this colour is discontinued A sufficient quantity of ‘engineered standards’ are produced and matched against this ‘master standard ‘Engineered standards’ are distributed by retailer to suppliers Production dyeings and initial lab dyeings on other substrates or carried out by different coloration processes or using different dye formulations are assessed against the ‘engineered standard’. Distance quality control procedures can be used. The necessary hardware and software are available but ultimate success depends on adequate training of the personnel involved and the use of standard operating procedures (SOP) by the operators and the systems manager. Calibration of spectrophotometers, monitor screens, viewing booths and colour printers enables colour to be viewed and evaluated in various formats. A major development in digital colour communication and the use of non-physical standards is the ability to communicate, visualize, evaluate and manipulate colour on screen. Following the change to retail-specified manufacturing, there is an increasing demand for digital communication techniques for both colour and design. This is not to say that the dyed standard or sample will disappear, since it is impossible to touch non-physical samples. Even with high-quality matches, as judged visually or instrumentally, designers will often modify 10 to 15% of colours to fit the overall visual appearance of the palette. Although specifiers, buyers and technologists have generally welcomed digital communication, designers (as with physical colour specifiers) tend to see digital colour communication as a denigration of their artistic flair and talents, some even refusing to examine colour on screen. Non-physical standards (NPS) are not more accurate than physical data nor are they a replacement for trained colourists. Physical samples are required for handle and colour visualisation. For the successful use of NPS, there is a need for standardisation, maintenance and calibration of equipment together with SOP since otherwise differences can occur for a variety of reasons. These include the spectrophotometer, the operator, method of preparing and presenting samples, sample conditioning, the substrate type and history. Many of the companies involved in the production of engineered standards have produced some form of colour atlas or standard shade range. Many designers are now being forced to select colours from these products rather than have ‘custom colours’ dyed, if the tight delivery times, which are often involved, are to be met. Digital colour communication has to be accepted for the same reason. Standard operating procedures (SOP) An SOP must be available for carrying out colour measurement to include the preparation, handling and presentation of samples for the creation of NPS, engineered master standards and for quality control procedures. The parameters that must be considered include those listed in Table 2. Table 2 - SOP for colour measurement   Significant variability can occur if SOP are not in place. To illustrate the importance of SOP, colour differences due only to variations in moisture content, humidity and degree of conditioning are illustrated in Table 3. Table 3 – Colour differences in D65 due to fabric condition   This shows the importance of a conditioning cabinet, installed in an air-conditioned laboratory, since otherwise many of the colour differences will exceed the agreed tolerances. To obtain reproducible results, both temperature and humidity must be controlled. Photochromism can be controlled by a standard illuminant in the conditioning cabinet.   Adverse long-term storage conditions can also lead to significant colour changes. A number of colours stored in plastic covers produced from BHT-containing material exhibited a colour change of 0.5 units in DE (CMC) after only seven days, compared with uncovered samples. A range of colours stored in an uncontrolled and unconditioned atmosphere exhibited colour differences of 1.5 to 2.0 units in DE (CMC) after six months, compared with standards held either as reflectance data on computer or as physical standards stored in a controlled atmosphere (see Table 2). Practice and problems in standards generation The generation of textile colour palettes and the production of the subsequent engineered standards have become high-tech operations, demanding that dyeing must be carried out to high levels of accuracy and reproducibility. The important factors that must be controlled include the following. Dye selection The major factors which must be considered in dye selection have been briefly mentioned in Table 1 and include the use of robust, compatible, consistent and stable dyes to give reproducible and accurate dyeings. Colour constancy is required over a full gamut of colours and no problems should be encountered in scaling-up into bulk production [8]. Level dyeings of the necessary fastness must be achieved. From the dyes selected, combinations are chosen to give RFT processing, to meet performance standards and minimise flare between various illuminants, depending on the priorities of the individual retailer.The insistence by designers to match certain unusual shades may require dye combinations that are not necessarily optimum for bulk production, sometimes exceeding the limiting depth expected to give satisfactory fastness. Production dyeings on other substrates or carried out at different stages of processing or by different application techniques will often necessitate using alterative combinations of dyes. Thus the provision of the recipes used to produce the master standard may be of little value and, in any case, would have to be checked under local conditions of application. Local circumstances may also result in certain dyes being unavailable. Dye deliveries are routinely tested for strength and hue against the standard.   Substrate variability Substrates for palette generation and standards production should be prepared thoroughly under bulk-scale conditions. The consistent preparation of cotton fabrics has become more difficult in recent years. The increasing use of pesticides, fertilisers and harvesting aids to enhance crop yields has resulted in significant contamination of raw cotton. These impurities are not always successfully removed in the preparation process. As an example, each roll of prepared cotton fabric is routinely tested for pH of aqueous extract, size and hardness content together with dyeability in terms of total colour difference (DE) and percentage strength against the standard. Even with satisfactory preparation as assessed by other factors, short-term variations in dyeability across the width and along the length in DE(CMC) of at least 1.2 units and colour strength variations of + 15% have been found [9]. Dye application methods Standard operating procedures (SOP) are required for the various parameters of dye application, including weighing and measuring, application variables and machine operation. Where appropriate, accurate and cost-effective automation and robotics can usefully be employed to improve accuracy and reproducibility. Master standards The approved laboratory dyeing is the master standard against which all future production of engineered standards is assessed. The master standard is measured by the SOP (as outlined in Table 2) and retained on computer. Physical standards should be stored in conditioned and darkroom facilities. As mentioned earlier, colour can also be affected by storage in polythene bags and BHT-free bags must be used. The card used for mounting standards must also be selected with care..   Conclusion   The generation of colour palettes and the production of ‘engineered’ master standards has become a mature activity carried out by independent matching laboratories. This is an exacting service in terms of quality and speedy delivery, supported by advanced technology, including digital colour communication techniques, with a high level of expertise. This is a cost-effective service for the provision of standards to which suppliers must produce colours for the continued success of both the supplier and the retailing organisation.   References [1] J. Park, Textile Today, 4 (2011) May-June [2] BS 6923:1988 (1994) [3] J. Park and K. Park, Textile Today (awaiting publication) [4] J. Park, Textile Today, 4 (2011) July-August [5] J. Park and T. M. Thompson, J. Soc. Dyers and Col., 97 (1981) 523 [6] R. I. Fenn and J. Park, J. Soc. Dyers and Col., 113 (1997) 56 [7] Ceramic Colour Standards, Seris II, (BCRA) [8] J. Park and K. Park, Textile Today, 7 (2014) May [9] J. Park, K. Park and J, Glover, Textile Asia, 35 (April 2004) 53; International       Dyer, 189 (July 2004) 32

 

November 2014
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