Colour Fastness

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COLOUR FASTNESS1.1. COLOUR FASTNESS It is a fundamental requirement that the colored textiles should withstand the conditions encountered during processing following coloration and during their subsequent useful life. Consideration of the subject of color fastness here is deliberately directed to an examination of the factors which determine the behavior of textile materials, when subjected to the conditions encountered during processing and use, and the principles upon which color-fastness testing must, in consequence, be used When a colored textile is subjected to particular conditions, e.g. light, washing, milling, and bleaching, one or more of several things may happen. As far as the color of the material itself is concerned there may be an alteration in depth, or in hue, or in brightness. In certain cases there may be alteration in all three. Thus a red material may become paler, yellower and duller. Further, under certain conditions, e.g. during washing, adjacent white material may become colored and colored material may acquire new color due to the transfer of dye from the original dyed material. This is generally described as 'staining or 'marking-off . The color fastness of a textile is therefore defined as its resistance to these changes when subjected to a particular set of conditions. It follows that color fastness must be specified in terms of these changes and expressed in terms of magnitude.1 1.2. HISTORICAL DEVELOPMENT Systematic color-fastness testing began with the efforts of individuals within particular firms. Thus when in 1902 James (later sir James) Morton discovered how fugitive were the dyeing of many of the synthetic dyes, he set out to produce a range of tapestries and furnishing fabrics which would stand with repeated washing and prolonged exposure to light. The first serious attempt to establish standard methods of color-fastness testing was made by the German Society of Chemist, who in 1911, set up a Fastness Committee. The Society of dyers and Colorists, who in 1927, set up a fastness committee, which presented its first report in 1934, under took the pioneer work. In 1947 the International Organization for Standardization (ISO) was set up. 1.3. FACTORS AFFECTING CHANGE IN COLOUR AND STAINING

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When a coloured material is subjected to particular conditions e.g. light, washing, milling and bleaching, the Colour may change in depth, hue or brightness. In some cases there may be alteration in all three. Thus a blue dyed fabric may become paler, yellower and duller. The resistance of a coloured material to any such change in colour is termed as its colourfastness. Also, during wet treatments such as washing and dry-cleaning, adjacent undyed material may take up colour due to transfer of dye from the original dyed material. This is known as staining. The substrate to which dye has been applied also affects the fastness properties. It is thus essential that while reporting colorfastness grades, the substrate, as well as the change in hue, value and chroma must be specified.2 The color changes which occur when dyed or printed textiles are subjected to a particular agency during processing are due to one, or both, of the following main causes. The first is breakdown of the colorant itself inside the fiber whereby it is converted in to colorless or differently colored compounds --- a very complex matter indeed. The second is detachment of the colorant, as such, from the fiber. Staining of surrounding areas in the same material occurs if the detached colorant is substantive towards either the original substrate or any other fibrous material with which it comes in contact during exposure to the agency. Staining is particularly likely to occur during exposure to conditions similar to those encountered when the material was being dyed or printed, i.e. during contact with water, wash-liquor etc. The possible effect on color fastness of a change in the color of the fibrous substrate itself is a result of exposure to the particular conditions must not be overlooked. 1. The chemical structure of the colorant: The resistance of a dye or pigment to chemical or photochemical attack is directly related to its chemical structure. Thus the relatively high fastness to light of dyeing of anthraquinone acid dyes on wool and the poor light fastness of triarylmethane acid dyes on the substrate are directly attributable to the stability of the one and the instability of the other to photochemical attack. Similarly, the good fastness to oxidizing/ bleaching agents of anthraquinonoid vat dyes on cellulosic fibre is related to the high stability of such compounds to oxidation. Where two or more dyes are present in the fibre, one may catalyze the breakdown of another. A well known example of this, but certainly not the only one, is the catalytic fading of dyeing of mixtures of vat dyes on exposure to light, particularly greens produced using mixtures containing certain yellow dyes. The progressive yellowing which occurs on exposure to light arises from acceleration in the rate of breakdown of the blue component due to the presence of the active yellow dye. Incidentally, when such active dyes are present alone or in presence of dyes, which are not so affected, photochemical attack on the fibrous substrate, it self may be promoted with consequent serious tendering of the fibre and the weakening of the material.2

The resistance of a dye or pigment to chemical or photochemical attack is an inherent property of the dye chromophore but the auxochromes may also substantially alter the fastness either way. Thus the good lightfastness of anthraquinone base natural dyes on wool and silk and the very poor fastness of curcumin and annatto dye on the same substrate are directly attributable to the stability of the one and the instability of the other chromophore to photochemical attack. The light fastness of hydroxyl anthraquinones and concluded that the fastness decreases as the number of hydroxyl groups increases. The decrease of light fastness depending on position of substituent. In anthraquinone nucleus, SH, -NH, -NHR or aquinoline nucleus decreases the light fatness, while NO2 has a favorable effect.2 When two or more natural dyes are present on the fibre, there might be a complete change in tone of colour with time because of dissimilarities in the fading behaviour. This effect is readily observed in old tapestries. In these historic textiles, green colours produced by over dyeing indigo with a natural yellow dye, inevitably to a bluer hue of the higher light fastness of the indigo (blue) component.3

2. The state of the colorant in the fibre: The state of the colorant in the fibre is obviously important. The superiority of the reactive dyes over the direct dyes in respect of fastness to wet treatments on cellulosic fibres is the direct result of the covalent attachment of the reactive dye to the fibre compared with the attachment of the direct dye through reversible forces, such as hydrogen bonds and other secondary attractive forces. In a dyeing or print of a reactive dye the colorant molecule and the fibre molecule become one-entity. In the case of dyeing of direct dyes on cellulosic fibres reversal of the dyeing process is fairly easily initiated since dye absorption and retention is due to weak forces of attraction which are easily over come. The very high fastness to wet treatments of dyeing and prints of, e.g., vat dyes, azoic combinations, and the small collection of dyes classified as Ingrain dyes in the Color Index, is due primarily to the fact that the dye inside the fibre is in the form of relatively large particles of insoluble colorant which, resistant to removal during wet treatments e.g. washing. At the same time the presence of the colorant in the fibre in this form also results in an improvement in its stability to chemical attack owing, presumably, to the in accessibility of the bulk of it to the attacking chemical. The overall high fastness of the anthraquinonoid vat dyes on cellulosic materials, which has led to their setting the standard of performance, is the net results of their high chemical stability and their presence in the fibre as particles which are insoluble in water and in aqueous solutions of the majority of chemicals used in textile processing, with the notable exception of alkaline solution of reducing agents. 3. The amount of colorant present on the fibre3

The colour fastness of a deep dyeing or print of a particular dye often differs markedly from that of a pale dyeing or print of the same dye on the same material. Where the principal effect of exposure to the particular conditions is to produce a change in the colour of the material, e.g. as is the case with exposure to light, it is generally found that the deeper the dyeing or print (i.e. the greater the amount of dye present on the fibre) the higher is its fastness in respect of change in colour on exposure. In certain cases the fastness to light of a deep dyeing may be two or more points higher (on the 1-8 scale) than that of a pale dyeing of the same dye. This is explained in part by the fact that the deeper the dyeing the greater the amount of dye, which must be destroyed before a visible change in the colour of the material, becomes apparent. In certain cases the situation is complicated by the fact that the colorant is present in the fibre in the form of large particles of pigment. In this case even a pale dyeing or print contains a relatively large amount of colorant in a state in which it is least susceptible to photochemical attack and thus the effect of depth of colour on light fastness is much less pronounced, or even absent. The high fastness to light of pale dyeing and prints of pigments resin-bonded to the fibre is a very good example of this. In the case of conditions such as washing, water, dry heat (disperse dyes) etc., the fastness of a dyeing or print in respect of sta