Humans use a range of technologies to produce color: solid surfaces are painted, paper and packaging is printed, color images are viewed on screens, and they color the textiles and leather used in their clothing and around homes. Screen color is transient, and few paper products are expected to last long. More durable color is found on painted surfaces and textiles, and the color is provided to them by colorants. That durability is expressed as “fastness.”
The colorant remains in place but is permanently changed or destroyed.
The colorant is unaltered, and maintains its color, but is removed, for example, by washing.
Such a change or loss of color is described as a lack of fastness. If the colorant is removed, for example, during laundering, it can stain other items with which it comes into contact. Even when the color of the original item is not appreciably altered, this is also considered a lack of fastness.
Color Fastness May Thus Be Defined as the Resistance to Change or Removal of the Color of an Item
Colorants are divided into dyes and pigments. The difference between them affects both their application and the likelihood of their permanence ( Colorant, Natural).
A pigment consists of molecular aggregates, usually in the micron range, with minimal solubility in any solvent with which it is likely to come into contact with ( Pigment, Inorganic). Its removal by dissolution is therefore unlikely. If a few of the pigment atoms or molecules on the surface of an aggregate particle are removed or changed, the overall color of the particle (and hence of the substrate it is coloring) is little affected. Low solubility and particulate nature thus mean that fastness in coloration by pigments is readily achieved.
In contrast, a dye is characterized by its solubility, most often in water, in its application ( Dye). In many cases its resistance to removal derives from the forces that drive its absorption by a substrate, and it remains soluble when the colored item is in use. The details are discussed in greater detail below, but the solubility may limit the resistance of the colorant to removal, and the monomolecular dispersion makes the effect of any removal or destruction more apparent.
Dyes are little used for non-textile items. Pigments have some use on textiles, but dyes and textiles are largely mutually inclusive. Solid surfaces typically do little flexing or moving, whereas textiles face a greater range of challenges than most other colored objects: as clothing they must have sufficient flexibility to move with the wearer and be soft enough not to irritate. They rub against each other. Curtains hang in sunny windows. Textiles are regularly cleaned and must withstand hot water and detergent or the solvents used in dry-cleaning. For these reasons, the subject of color fastness is most widely studied for textiles. As discussed below, truly permanent color is an unrealistic expectation, and the extent to which color change is tolerable varies widely and is affected by the cost of the item, the cost of replacing it (or its color), and the possible effect of the color loss on other items. Problems of fastness are therefore strongly associated with dyed textiles, and fastness in other colored materials is often tested using methods and principles drawn from textile tests.
As outlined in the definition, the color of an item may change when either the colorant remains in place but is permanently changed or destroyed, or when the colorant is unaltered and maintains its color, but is removed. These two cases are considered below.
Color Change: Colorant Destruction
The colorant on an item may undergo a chemical reaction that changes or destroys its color. Such a change is perceived as a lack of fastness. The reaction may take place slowly and only become apparent after a long period of time or may be quite rapid.
The former relates most often to the chemical reactions that occur as a result of exposure to light. Absorption of light raises a colorant molecule to an excited state in which condition it is susceptible to decomposition, or reaction with oxygen, water, or other available reactants. The reactions involved are complex. They have been the subject of extensive study but much remains to be understood: the factors that affect the reaction are many and varied (light intensity, spectral power distribution, temperature, humidity, etc.); it is also clear that color change is not simply a property of the chemical identity of the colorant alone, but is affected by colorant-substrate interactions, by colorant aggregation, and (in mixtures) by colorant-colorant interactions [1, 2]. The same dye can have different fastness on different fibers. Pigments are less involved in these external interactions, but their physical form (particle size, crystalline form, etc.) may affect their fastness.
Whether dye or pigment, for a constant exposure, the destruction of colorant occurs at a fairly constant rate. The effect on the color, however, is not constant. A darker color, resulting from a greater amount of colorant, will be affected less by a given exposure than a light one, where the same amount of colorant destroyed represents a greater proportion of the colorant present. In other words, a pale shade of a given colorant will have lower fastness to colorant destruction. This is in contrast to the lack of fastness represented by staining as a result of colorant removal discussed below.
The more rapid destruction of dye is usually the result of exposure to a chemical agent: most obviously this might be sodium hypochlorite or hydrogen peroxide used as a bleach, but other chemicals such as benzoyl peroxide used in skin medication will effectively decolorize many dyes. Again, pigments are less prone to such reactions, given their particulate nature and insolubility.
Color Change and Staining: Colorant Removal
Color may be removed from an item, particularly a textile, for a number of reasons. Unlike the case of colorant destruction, the color remains, and the removed colorant may color an item with which it comes into contact: such staining is a component of (a lack of) fastness. For reasons that will become clear, once again, this is best examined from the point of view of dyes.
Dyes are applied to textiles in a number of ways: batch dyeing, continuous dyeing, and textile printing ( Coloration, Textile). The principles of the interaction are common to all, but best illustrated in the case of batch dyeing [3, 4]. In a batch dyeing process, dissolved dye molecules are preferentially sorbed from the external dye solution at the fiber surface and penetrate the interior of the fiber as a result of the intermolecular forces that comprise the “substantivity” of the dye. The relative motion of the dyebath and the substrate replenishes the dye-depleted solution at the fiber surface, and the rate-determining step of the overall process is typically the rate of sorption into the fiber. If dyeing conditions are maintained long enough, an equilibrium is established between dye in fiber and dye in solution, and all fibers are fully and equally penetrated. In practice this situation is rarely achieved, and microscopic examination of a dyed textile will reveal fibers and yarns with only partial penetration of dye. Such microscopic unlevelness is tolerable.
An essential requirement of a colored textile is that the color be level at the macroscopic scale. Assuming a clean and absorbent substrate from earlier preparation processes, and homogeneously dyeable fibers, levelness results from a balance of two possible routes. Either the dye is absorbed in a level manner, or an initial unlevel sorption is made level by the migration of dye .
The substantivity of the dye is based on the molecular size and shape of the dye and functional groups present. Together, these allow the formation of a range of non-covalent bonding that makes up the attraction of dye for fiber and which enables dyeing to take place . If nothing else happens, these same forces will provide the fastness the dye has in use, in competition with the solubility of the dye in the particular medium (e.g., laundry liquor).
For fastness (as resistance to removal), a high substantivity (strong dye-fiber bonding) is required. But dyes with high substantivity do not migrate readily, and if initial application is unlevel, it is difficult to achieve levelness via migration. To use a high-substantivity dye, the dyer must control the initial uptake (“strike”) of the dye to be as level as possible. For a given system (machine, dye, fiber), dye uptake can be controlled by control of temperature (rate of rise and ultimate temperature), time, and dyebath conditions of ionic strength (salt) pH, and auxiliaries that moderate the dye-fiber interactions (often surfactant based). However, the demands of levelness may require that lower-substantivity (and thus less fast) dyes be used.
As discussed earlier with colorant destruction, the fastness of a particular dyeing will vary with the depth of shade. For the case of colorant removal, a dark shade will exhibit lower fastness than a pale one: the same percentage of colorant removed from a dark shade will have the propensity to stain more heavily than from a pale shade.
For some dye-fiber systems, the substantivity of the dye that drives dyeing is all that provides fastness in later use. Most notably, acid and metal-complex dyes on protein and polyamide fibers (wool, silk, and nylon) direct dyes on cellulosic fibers and disperse dyes on acetate. In essence, a subsequent aqueous challenge represents the equivalent of a dyebath and the (beginnings of) reestablishment of an equilibrium in which some proportion of the dye is lost from the substrate.
Several dye-fiber systems have fastness attributable to reasons beyond those provided simply by the dye-fiber interactions. Disperse dyes on polyester have fastness greater than the same dyes on acetate. The high glass transition temperature of polyester means that practical dyeing takes place only at temperatures considerably higher than that of boiling water: these are found in pressure dyeing machinery. After dyeing (and in use), the fiber is cooled below Tg, and the dye is essentially trapped in the fiber: the inaccessibility of the dye to outside agencies is reflected in the use of “reduction clearing” to remove any surface deposits of the low-solubility dye. Similar arguments apply to the fastness of basic dyes applied to acrylic fibers.
While a dye must be soluble in application, greater fastness can be achieved if the dye is later converted (or reverts) to an insoluble or less soluble form. This form of the dye may also comprise molecular aggregates and/or larger molecules for which physical entrapment may contribute to the fastness. Vat dyes for cellulosic fibers are produced and sold in insoluble form and must be chemically reduced to a soluble and substantive form for application. Once on the fiber, they are oxidized to their original insoluble form and form aggregates inside the fiber: essentially they are now pigments. Several dye types are formed inside the fiber as low-solubility moieties as part of their application. Sulfur dyes for cellulosic fibers are supplied as a reduced solution; once applied, they undergo a similar oxidation to vat dyes to form a low-solubility dye. Azoic colorants are generated by reaction within the fiber of a soluble coupling component and a soluble diazo component. The resulting azo dye is insoluble and, like a vat dye, aggregates into pigment form. Mordant (“chrome”) dyes on wool rely on a reaction of dye and mordant in situ within the fiber to form a low-solubility complex of good fastness, and its fastness is enhanced by the entrapment of a physically large molecule.
A further reason for additional fastness may arise from the presence in the dye of functional groups capable of reaction with the substrate to form covalent bonds. So-called reactive dyes achieve their fastness in this way and since their introduction in 1956 have become the dominant dye type for cellulosic fibers, with additional usefulness for wool dyeing where they can replace the mordant dyes and their associated use of heavy metals . It should be noted that the reaction efficiency is not 100 %, and at the end of the dyeing process, any unfixed dye that is not removed by washing will be held only by the forces of substantivity and may be readily removed in later use. In such a case the dyeing as a whole will be considered to have poor fastness.
Colorfastness, whether as resistance to destruction or resistance to removal, is largely achieved by choice of colorant. Dye manufacturers will supply customers with “pattern cards” of materials colored with individual dyes, together with the results of key fastness tests (see below) conducted on those materials. Colorists may thus select colorants that are likely to meet fastness requirements, although as mentioned earlier, the depth of shade will affect the fastness achieved. Resistance to removal may additionally depend on the ability of the process to eliminate any dye that is not fixed or that is loosely held on the fiber surface.
Pigments are used for textiles in one of two very different ways. A pigment can be added to the melt or solution used to form manufactured fibers in the same way that a pigment is added to a melt used to mold plastic items. Subsequent solidification traps the pigment within the polymeric matrix. Polymers colored this way are among the most colorfast of all. However, for textiles, the time between fiber manufacture and ultimate sale may be a year or more, and the holding of multiple color stocks of fiber and yarn means that this coloration method is of limited use in this context.
Pigments can also be bound to a textile material by use of a polymeric binder: this is akin to the application of paint to a surface. The fastness of such colored materials is related to both the colorant and the binder and the strength of the pigment-binder-fiber interactions . Once again, textiles tend to face greater challenges as they flex and move.
The permanence of color in an item is desirable, but the challenges it might face in use are many and varied. It is unrealistic and expensive to demand the best fastness in every case, so compromises are inevitably made. The questions then become, what are the likely challenges this item will face? How well should it be able to resist them? How well does it resist them? The answer to the first derives directly from the intended end use, while the answers to the second and third require some way of presenting the challenge and assessing the resistance. The most realistic way to do this is to conduct a trial with the item in its intended use to see when and how failure occurs. Such real-life trials are occasionally performed, but they are very expensive and take a long time to produce results. Standardized laboratory fastness testing provides an economical and useful alternative. Standard test methods are developed that are designed to approximate to a single real-life challenge and to predict how an item will respond. Such tests typically provide results rapidly, either because the challenge itself is a rapid one (e.g., color being rubbed off) or because the test accelerates the challenge (e.g., using a very intense light instead of real daylight).
A fastness test is developed to provide the challenge and a way of assessing the result. The result is then compared to a requirement that forms part of an overall product specification. The tests may be widely applicable or represent a challenge that is more specific. Tests are developed by standard setting organizations. Much of the original work to develop these tests for textiles was carried out by the Society of Dyers and Colourists (SDC)  in the UK and the American Association of Textile Chemists and Colorists (AATCC) in the USA . With the rise in global trade, fastness tests have become more international, and many countries now rely on tests published under the auspices of the International Organization for Standardization (ISO) .
A good test should be valid: the property it measures should have some real-life relevance, and the test should predict in-service suitability. Additionally, a good test should be simple in terms of how it is performed and how easy the instructions are to understand. It should also be reproducible giving the same results from operator to operator and lab to lab. In some cases the control of test conditions may be difficult, and standard fabrics of known susceptibility are used as control materials.
Issues of environmental, health, and sustainability have come to the fore in recent years. Several certification schemes are intended to reassure the consumer that an item has been produced in an environmentally friendly manner and that it does not contain (or will release) substances of concern. Colorants are among such substances, and the measure of colorant release is often based on standard color fastness testing. Thus, for example, a range of color fastness tests is included in the OEKO-TEX 100 certification  and in the Global Organic Textile Standard .
Testing for Fastness: Challenges
As discussed earlier, fastness tests are designed to reproduce the challenges that a colored item would face in real life. Again, the subject tends to emphasize textiles. Some tests relate to challenges faced in the sequence of manufacturing steps that a textile item undergoes before it gets to the consumer. Most relate to the challenges encountered after the item is sold: these can be divided into those met when the item is being used and those involved in its cleaning or refurbishment. Full details of the tests are sold by from the relevant standard setting organizations. The titles and scope are available in texts and on the organizations’ web sites [11, 12, 15].
Fastness to (Textile) Production Processes
Textiles woven from multicolored yarns may be scoured and may include a white portion that requires bleaching, and thus a test for fastness to bleaching may be needed. Mercerizing can alter color. Dry heat can cause color changes. Wool undergoes a variety of wet treatments: tests determine the color changes caused when wool is carbonized, boiled in water, bleached, set with steam, or milled. The fastness challenges of the last are reflected in the naming of certain acid dyes that will withstand this as “milling acid dyes.”
After makeup, garments may be pleated or hot-pressed: these can cause thermochromic or sublimation-based color changes. Silk fabrics may be degummed in hot alkaline soap solution, and a fastness test can predict any changes to colored materials that undergo this process.
Fastness in Use
In use, garments may be rubbed, and their color transferred to another item: fastness to rubbing (also known as “crocking”) is one of the most common tests.
Spots of liquid can cause a color change, either by moving dye within the textile or removing the dye and transferring it to an adjacent material. Thus there are tests for spotting by water, dry-cleaning solvent, seawater, perspiration, acids, and alkalis. Swimming pool water with chlorine represents a destructive challenge.
Atmospheric contaminants, notably ozone and oxides of nitrogen (“burnt gas fumes”) derived from combustion and sunlight, can destroy colorants.
Light represents a particular challenge. Real-life exposure is both slow to produce change and is highly variable (but such real-life exposure is included among standard tests). More usually, an accelerated test uses a xenon arc lamp and controls the temperature and humidity of exposure. The test may be extended by the inclusion of water sprays to cover “weathering.”
Fastness to Refurbishment
Textile items get dirty and require periodic cleaning. The processes to be used are given in a care label, and the suggested cleaning method should have been shown by testing to be appropriate to restore the item to an acceptable level while not causing any damage. However, such refurbishment represents another set of challenges to the color.
Laundering is conducted in aqueous surfactant solutions with agitation. The details can vary considerably: the detergent (type and amount used), the temperature, the amount of water and the extent of agitation (based on the type of machine used), the nature of the materials used to make up the bulk of the load (“ballast”), and the presence or absence of bleach. The tests to assess fastness reflect this variation in practice. Full-scale launderings, repeated three or five times, may be used, but since each item should be tested individually to avoid possible confusion in results, an accelerated test is used. A small sample, usually with a standard multifiber adjacent fabric, is agitated in standard detergent solution in a small cylinder for 45 min or so. The effect on color will approximate to that obtained in five real cycles of laundering. Similar considerations apply to dry-cleaning.
Ironing can affect color: tests examine the effects of dry, moist, and wet heat.
Assessing the Results of Fastness Tests
The tests produce color changes and thus assessment of the results is an assessment of color difference . This might be the difference between the original color and the challenged color or the difference between an unstained fabric and one stained by color removed from the test sample.
Color difference assessment has been widely studied for judging colors for acceptable closeness to a standard color (“a match”). That use concentrates on small color differences and is as much concerned with the quality of difference as the quantity of difference. However, the assessment of color differences from fastness testing covers a much wider (quantity) range of color differences and is rarely concerned with the quality of difference. These two aspects of color difference measurement have thus tended to go their separate ways.
The results from a fastness test can be assessed subjectively, by reference to physical color standards (“gray scales”) or objectively, based on spectrophotometric or digital camera measurements. Physical gray scales have been so extensively used that even when an instrument is used, its output is converted to gray scale ratings.
The correct use of each of these scales is described in the relevant ISO and AATCC documents. The angle of viewing, the quality and intensity of the illumination, and the masking of the test specimen/gray scale pair are specified to eliminate variables and increase interlaboratory agreement.
Instrumental Color Measurement of Fastness Test Data
A numerical color difference equation might reproducibly express the color change or staining resulting from a fastness test. The need for this to correspond to human visual data over a wide color range and to make it agree with the results generated visually with gray scales has made this somewhat challenging. Nonetheless, while some deficiencies are recognized, mathematical formulae for gray scale ratings derived from spectrophotometric measurements or digital camera data are published and used. Work to develop better formulae continues.
Color is an important property of many items, and its durability in items that are expected to last over a period of time is a matter of concern. Textile colorants have the greatest potential to produce unwanted changes in the textiles they color and to stain adjacent materials, and textiles are subject to the greatest range of potentially damaging agencies. Thus color fastness testing is most widely studied and performed on textiles. The tests reflect real-life challenges met in textile processing, in use, and in care, and many of them are accelerated. ISO and AATCC are the most active organizations in the development of such tests.
The tests require a measurement of the changes produced by the test challenge and the stains caused by removed color. For many years, measurement has been based on visual comparison with standard gray scales. Compared to other situations involving color assessment, the adoption of instrumental methods to assess fastness results has been slow. Standard methods for such instrumental assessments are in place, if yet to be widely adopted.
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