Dyeing can be described as the uniform application of colorant(s) to a coloring medium. The coloring of textiles may involve mass pigmenting (involving compounding), dyeing, and printing processes. The coloring medium in textile dyeing may take different physical forms (such as loose fiber, yarn, tow, top, woven, nonwoven and knitted substrates in open width or rope form), whereas in printing colorants are added to selected regions of the medium which is usually in a fabric form. Dyeing of homogenous fibers should result in a uniform solid color. Multicolored effects may be obtained by dyeing a blend of different fibers, bi- or multicomponent synthetic fibers, or via multiple colorant or illuminated discharge printing. In a technique known as space dyeing, colorant(s) can be applied from nozzles that inject different dyes to yarns and force steam in the vessel, thus generating a multicolored pattern.
There are difficulties encountered in controlling the physicochemical changes that occur during dyeing when attempting to maximize color yield, levelness of dyeing, color fastness, etc. Therefore, a vast sub-technology of specialty chemical auxiliaries is used in preparation for dyeing and in the dyeing process itself, such as levelling agents, dispersing agents, antifoams, etc., as well as auxiliaries specifically designed for aftertreatments. Moreover, from the practical standpoint, methods of applying colorants to the coloring medium can be broadly divided into batch, semicontinuous, and continuous processes. These are discussed in more detail in the following sections.
The science of coloration entails several domains including chemistry, physics, physical chemistry, mechanics, fluid mechanics, thermodynamics, and others. Devising the most efficient dyeing process typically involves several concerns including machinery design, preselection of dyes of compatible properties, use of pH versus time profiles, selection of liquor ratio, flow rate and flow direction reversal times, and design of temperature versus time profiles. It is thus clear that dyeing is a complicated process with many different phenomena occurring simultaneously that require a suitable level of control.
When a textile fiber comes in contact with a medium containing a suitable dye under suitable conditions, the fiber becomes colored, the color of the medium decreases and that of the fiber increases, thus resulting in the dyeing of the substrate. True dyeing occurs when the dye is absorbed with a decrease in the concentration of dye in the dyebath and when the resulting dyed material possesses some resistance to the removal of dye by washing , rubbing, light, and other agencies.
Coloration of textiles is not limited to the simple impregnation of the textile fiber with the dye that occurs during the initial phase of the dyeing process. A dye is taken up by a fiber as a result of the chemical or physical interactions between the colorant and the substrate. Many dyes for textiles are water soluble, and their molecules are dissociated into positively and negatively charged ions in aqueous environments. The uptake of the dye by the fiber will depend not only on the nature of the dye and its chemical constitution but also on the structure and morphology of the fiber.
A fiber is characterized by its high ratio of length to thickness and by its strength and flexibility. Fibers may be of natural origin or formed from natural or synthetic polymers. They are available in a variety of forms. Staple fibers are short, with length-to-thickness ratios around 103–104, whereas this ratio for continuous filaments is at least several millions . The form and properties of a natural fiber such as cotton are fixed, but for man-made fibers, a wide choice of properties is available by design. The many variations include staple fibers of any length, single continuous filaments (monofilaments), or yarns comprised of many filaments (multifilaments). The fibers or filaments may be lustrous, dull or semi-dull, coarse, fine or ultra-fine, circular or of any other cross section, straight or crimped, regular or chemically modified, or solid or hollow.
From a processing standpoint, natural fibers have a number of inherent disadvantages. They exhibit large variations in staple length, fineness, shape, crimp, and other physical properties, depending upon the location and conditions of growth. Animal and vegetable fibers also contain considerable and variable amounts of impurities which must be removed prior to commencing dyeing. Man-made fibers are much more uniform in their physical characteristics. Their only contaminants are small amounts of slightly soluble low Mw polymer and some surface lubricants and other chemicals added to facilitate processing. These are relatively easy to remove compared to the difficulty of purifying natural fibers.
Water absorption is one of the key properties of a textile fiber that influences their coloration. Protein or cellulosic fibers are hydrophilic and absorb large amounts of water, which causes radial swelling. Hydrophobic synthetic fibers, such as polyester, however, absorb almost no water and do not swell. The hydrophilic or hydrophobic character of a fiber influences the types of dyes that it will absorb. The ability to be dyed to a wide range of hues and depths is a key requirement for almost all textile materials.
Another important property of a textile fiber is its moisture regain, which is the mass of water absorbed per unit mass of completely dry fiber, when it is in equilibrium with the surrounding air, at a given temperature and relative humidity. The regain increases with increase in the relative humidity but diminishes with increase in the air temperature. Water absorption by a fiber liberates heat (exothermic) and will therefore be less favorable at higher temperatures. The heat released is often a consequence of the formation of hydrogen bonds between water molecules and appropriate groups in the fiber. When the final regain is approached by drying wet swollen fibers, rather than by water absorption by dry fibers, the regain is higher. For hydrophilic fibers such as wool, cotton, and viscose, the relatively high regain values significantly influence the gross mass of a given amount of fiber. This is significant in dyeing. Amounts of dyes used are usually expressed as a percentage of the mass of material to be colored. Thus, a 1.0 % dyeing corresponds to 1.0 g of dye for every 100 g of fiber, usually weighed under ambient conditions. For hydrophilic fibers, the variation of fiber mass with varying atmospheric conditions is therefore an important factor influencing color reproducibility in repeat dyeings.
Stages of Coloration Process
Most textile dyeing processes initially involve transfer of the colored compound, or its precursor, from the aqueous solution onto the fiber surface, a process called adsorption. From there, the dye may slowly diffuse into the fiber. This occurs through pores or between fiber’s polymeric chains, depending on the internal structure of the fiber. The overall process of adsorption and penetration of the dye into the fiber is called absorption. Absorption is a reversible process. The dye can therefore return to the aqueous medium from the dyed material during washing, a process called desorption. Besides direct absorption, coloration of a fiber may also involve precipitation of a dye inside the fiber, or its chemical reaction with the fiber. These two types of processes result in better fastness to washing, because they are essentially irreversible.
Dye Transport from the Bulk Solution to the Fiber Surface
When a fibrous assembly is immersed in a dye solution, the rate at which the dye is taken up is generally dependent upon the extent to which the liquor is agitated and tends to approach a maximum value when the stirring is vigorous.
The transfer of dye from the bulk solution to fiber surface is fast, and the rate generally increases with increasing the flow rate. The adsorption equilibrium is also rapid, so it is usually assumed that the overall rate of dyeing depends on the rate of diffusion of the dye into the fiber. Inadequate control of the rate of dye adsorption will result in unlevel dyeings unless the dye can subsequently migrate from deeply dyed to lightly dyed regions of the substrate . Therefore, the control of the first two stages of the process, namely, the initial mass transfer from the bulk solution to the fiber surface and adsorption of the dye on the surface, is important for a level dye distribution throughout the substrate to be achieved.
A fundamental issue in dyeing is to ensure that the dye liquor penetrates all parts of every fiber and is distributed within the substrate as evenly as possible. In practice, however, there may be differences in fibers, to a greater or lesser extent, because of natural or process-related parameters. These variations are of major importance, since, in wet processes, changes in regional density of the substrate result in variations in the degree of dye penetration and differences in flow rate which can lead to shade differences in dyeing.
The influence of agitation in increasing the rate of dye uptake is dependent in large measure on the hydrodynamic complexity of the system. Unfortunately, flow through the substrate cannot be described in any simple fashion with fibrous assemblies, due to the extreme complexity of defining the flow of liquor through a mat of fibers or through yarns or cloth.
In spite of the complications of real systems, some of the principles governing the process may be elucidated by consideration of simple ones, e.g., a plane sheet or film of material immersed in dye liquor whose direction of flow is parallel to the sheet. On making contact with the dye liquor, dye is adsorbed by the film so that the neighboring liquor becomes deficient in dye; dye is transported to the surface by dispersion from the bulk, but the quantity transferred is modified by the speed at which the liquor passes through the film.
Calculations of the rate at which the fibers can take up dye require knowledge of the flow pattern of the liquor before analysis of diffusion may be attempted. Experimental investigations of flow of dye liquor through masses of fiber lead to the conclusion that, at the common rates used in dyeing, the flow is streamline rather than turbulent, so attention may be confined to streamline conditions.
Intermolecular Forces Operating Between Colorants and Textile Material
The strongest dye-fiber attachment is that of a covalent bond. Another important interaction between dyes and fibers includes electrostatic attraction, which occurs when the dye ion and the fiber have opposite charges. Hydrogen bonds may also be formed between a specific range of colorants and textile fibers. In addition, in nearly all dyeing processes, van der Waals forces and hydrophobic interactions are involved. The combined strength of the molecular interactions is referred to as the affinity of the dye for the substrate. The substantivity of the dye is a less specific term and is often used to indicate the level of exhaustion (which is described in the following sections) . Thus, substantivity is the attraction between the substrate and a dye under precise conditions where the dye is selectively extracted from the medium by the substrate. Different types of textile fibers require different kinds of dyes, and in general, dyes which are suitable for one type of fiber may not dye other types effectively.
Levelness is the uniformity of dye distribution (and hence color) on textiles. Two fundamental mechanisms contribute to a level dyeing. One is the initial sorption of dye during the dyeing; the other is the migration of dye after initial sorption on the fiber. An initial level sorption will lead to a level dyeing. An unlevel sorption may be corrected if sufficient migration takes place. These mechanisms are affected by dyes and chemicals, by textile substrate, and by controlling the parameters of the dyeing process such as dyebath pH, liquor ratio, flow rate, and temperature. Some dyes are more likely to level out, especially if they are small and do not have a high degree of affinity towards the substrate; other dyes on the other hand tend to be much less likely to migrate. These are often dyes of large size or with strong affinity towards the substrate.
Dyeing of Various Textile Materials
Textile fibers can be dyed at various stages of production such as loose fiber, top, tow, yarn, fabric, or garment. The levelness requirements for dyeing loose fibers are less strict than for dyeing at the yarn stage or at later stages, since further processing of the loose fiber results in some mixing of the dyed fibers, e.g., during carding or gilling, which improves the levelness of the resultant color. For yarn dyeing levelness is more critical because while the yarn will be made into fabric, either by knitting or weaving, or used in the construction of carpets, any unevenness in the dyeing of the yarn will show in the finished goods. In the case of fabric and especially garment dyeing, the process will be less forgiving and variations will have to be remedied in different ways, either by overdyeing the substrate to a darker shade or by stripping the color and reapplying the colorant. Dyeing textiles at earlier stages of production such as loose fiber also reduces the degree of flexibility in response to market needs. Garment-dyed material can reach the market quickly and meet the rapidly changing demands of the market, whereas in the case of fibers or yarns, typically several additional processes have to be completed before the dyed material reaches the consumer. In the case of substrates with striped patterns yarn dyeing is quite common. When blending dyed yarn with undyed yarn to produce a woven pattern, it must be considered that subsequent bleaching and scouring may be needed. Thus, dyes of suitable fastness properties, to bleach and various agencies, have to be employed to ensure no staining of adjacent white regions occurs. Dyeing of fabrics and garments with variations in fabric density or stress can also result in systematic unlevelness. A common type of unlevelness in dyeing of nylon fabrics is known as Barré which appears as stripes across the substrate. A specific visual appearance test method to rate the degree of Barriness of dyed substrate has been introduced over the years.
Batch, Semicontinuous, and Continuous Coloring Processes
Semicontinuous dyeing is carried out in a continuous range where the substrate is fed into the dyeing range from one end and collected at the other. In semicontinuous processes typically fixation and washing steps are carried out discontinuously. Pad rolls transfer the colorant that is picked up from a trough into the substrate, and the process is based on impregnation of the substrate followed by fixation of the colorant. The pressure applied by rolls to impregnate the substrate can be adjusted to squeeze the excess liquor out of the substrate and obtain the required wet pickup percentage. Since the initial uniformity of dye deposition on the substrate is critical for a level dyeing, padding rolls responsible for the transfer of the colorant onto the substrate must have a uniform surface with no indentations. The pressure across the rolls should also be adjusted and controlled regularly to ensure the uniformity of color transfer onto the substrate. The dyeing range may include cans or alternative forms of dryers to prevent migration of dye across the substrate prior to fixation. At the end of the range, the substrate may be rolled onto a beam, covered with plastic sheets, and kept overnight, to enable fixation of the dye, or transported to other sections for subsequent fixation, washing, and aftertreatment operations. Pad-batch dyeing is a specific type of semicontinuous coloration process which is common in the application of reactive dyes to cellulosic substrates. In this process the batch is left overnight to enable the colorant to react with the substrate. Other forms of pad-fix processes may include pad-bake and pad-steam.
Continuous dyeing refers to operations at constant composition involving several application and wash boxes (troughs), where a long length of textile fabric is pulled through each stage of the dyeing process including fixation and aftertreatment. Continuous dyeing operations are common when dyeing large quantities of substrate. This is often carried out on cotton and its blends with synthetic fibers such as polyester. In continuous dyeing processes, fixation and subsequent wash and rinse operations are combined with the coloration process to enable rapid throughputs. Thermofixation (commercially known as Thermosol) which is commonly carried out on polyester and polyester blends is particularly suited to continuous dyeing processes, since it involves padding the dyestuff from dispersion onto the fabric, drying, and then heating the padded substrate to a temperature of 180–220 °C. A specific category of disperse dyes capable of sublimation is employed for this purpose. At such temperatures, diffusion rates of sublimated dyes are so high that a few seconds suffice for adequate penetration of dye molecules into the substrate. Several variables, however, affect the final shade of the dyed fabric during the thermofixation process. Some of these variables are Thermosol period, temperature, type of disperse dyestuff, and pad bath auxiliaries.
In continuous printing processes, colorants are applied to specific sections of the cloth using a number of techniques that may include roller, flatbed, and rotary screen printing systems to obtain a preset design. Dye fixation is carried out by steaming or baking the printed material followed by washing to remove surplus dye, thickeners and any other auxiliaries.
The details of the dyeing process can vary considerably between different types of textile materials employing different types of dyeing equipment. For example, the maximum permissible rate at which the temperature of the bath can be raised may be determined by the relationship between the rate of circulation of the dye liquor and the rate of transfer of the dye from bath to fiber such as in the dyeing of yarn in hank dyeing machines.
Quality requirements and control
The aim of a successful dyeing process is to achieve the desired shade, at the right price, with sufficient levelness, whether dyeing loose fiber, yarn, or piece goods, with sufficient color fastness to withstand both processing and consumer demands, but without adversely affecting the fiber quality. Of these, an acceptable level of uniform dye uptake at all parts of the substrate may be the most important criterion.
Establishment of equilibrium between associated molecular dye and single molecules of dye in solution
Diffusion of monomolecular dye to the diffusional boundary layer at the fiber surface
Diffusion of dye through the boundary layer at the fiber surface
Adsorption of dye at the fiber surface
Diffusion of the dye into the fiber interior
Desorption and readsorption of dyes (migration)
These steps form a reversible equilibrium system. Each of the six steps can influence the levelness of dyeings. A complete quantitative analysis of the effects of many factors which influence the levelness of dyeing would require the development of a mathematical model involving a significant number of parameters. This is, however, a very difficult task. For practical applications, one may initially try and identify a few variables, which are thought to have a larger impact on levelness and restrict the development of the model to the effects of these few most important variables.
The Donnan equation involves nine factors: the concentration of dye in fiber, the concentration of dye applied, the liquor ratio, the distribution of ions between solution and fiber, the ionic charge on the dyestuff molecule, the internal volume of the fiber, the affinity of the dye, the gas constant, and the dyeing temperature. All together, these terms describe the dyebath conditions.
Quality of water supply
Preparation of substrate
Dyeability of substrate
Weight of substrate
Moisture content of substrate at weighing
Selection of dyes
Standardization of dyes
Weighing of dyes and chemicals
Dispensing method for dyes and chemicals
Moisture content of dyes
Liquor to goods ratio
pH of dyebath
Machine flow and reversal sequence
Various workers have placed different degrees of emphasis on each of the above factors and also on the factors which are not listed above. There may also be disagreement on the mechanism by which any one of these factors operates. Furthermore, it should be noted that the parameters in this list are not quite independent, and in some cases the effect of two of these factors may be considered as one effect.
The slope of a dyeing exhaustion curve (Fig. 3) defines the rate of dyeing at any instant during the process. The rate of dyeing gradually decreases until, if dyeing is continued long enough, an equilibrium is reached where no more dye is taken up by the fibers. There is now a balance between the rates of absorption and desorption of the dye. The equilibrium exhaustion is the maximum possible exhaustion under the given conditions. The lack of any further increase in exhaustion does not necessarily mean that a true equilibrium exists. It is possible for the dye in solution to be in equilibrium with the dye located on the outer surfaces of the fibers. True equilibrium only exists when the dye in solution is in equilibrium with the dye that has fully penetrated into the center of the fibers. Dyeings rarely continue to this point since it may take a relatively long time to attain. In fact, many commercial dyeings barely reach the point of constant exhaustion.
There are two basic methods of achieving a level exhaustion dyeing in any dye/fiber system; the first is by dye migration, and the second is by controlled dye exhaustion. The first method involves exhausting all of the dye onto the fiber and then allowing it to migrate between the fibers in order to “level out” the dyeing. These are dyes which are able to migrate from the fiber back into the liquor and then transfer back to the fiber. This redistribution of dye improves the levelness of the dyeing and normally takes place when the dye liquor is at the boil. In this method the dye is not completely exhausted onto the substrate, and this can lead to poor reproducibility of color, and hence, additions of dye to correct the final shade are often necessary.
The second method is to ensure that the dye is exhausted in a level manner from the start of the dyeing. In this method, the dyeing rate is controlled by changing the parameters of the dye bath at a controlled rate so that the dye is deposited on the yarn in a uniform manner throughout the substrate. Careful control of these parameters, such as dyeing temperature, pH, or amount of electrolyte and flow rate, is often necessary to obtain level, well-penetrated dyeings. This is essential if the dye initially absorbed is unable to migrate from heavily dyed to poorly dyed areas during the process.
Variation of the concentration of dye in the dyebath during the dyeing is referred to as the exhaustion profile, and the shape of this profile has been believed by many researchers to be the most determining factor in levelness of dyeing.
Exhaustion control has been developed theoretically and in the laboratory by several workers. These workers used knowledge of the dyeing kinetics to devise a time/temperature profile to give a particular exhaustion profile; others have attempted a direct control of the exhaustion rate.
Studies of the theoretical basis of the relationship between levelness of dyeing and the rate of dye uptake by textile substrates were initiated in the 1950s and the 1960s. Since then there have been many investigations into the methods of controlling the exhaustion of dye bath in order to improve levelness. This has been an area of much disagreement among researchers.
Linear Exhaustion Profiles
Carbonell et al.  developed a mathematical representation of various exhaustion profiles and went on to calculate practical time/temperature profiles that would result in linear exhaustion profiles. Later work aimed at establishing detailed kinetic relationships in order to carry out “isoreactive” dyeings, in which the dyeings have a linear exhaustion profile.
Cegarra et al.  later modified this approach to apply it to dyeings that used continuous addition (or integration) of dye into the dyebath. These dyeings were carried out at constant temperature, using a predetermined dye addition profile to achieve linear exhaustion. This method was defined as Integration Dyeing, which can be used to control the dye absorption during the integration, so as to avoid the possibility of initially fast and anomalous absorption, which may cause unlevel dyeings. In practice, this method is often used to improve the levelness, when all the dyes are added at the beginning of the process.
Several authors have stated that linear exhaustion is most likely to give a level result and developed a control strategy for automation of a dyeing machine such that the percentage exhaustion per circulation never rises above the critical value for levelness.
Other Exhaustion Profiles
The use of linear exhaustion profiles for the control of dyeing process is by no means generally accepted. A number of researchers have stated that a rapid uptake at the start of the process, with a gradual slowing of the exhaustion thereafter, should give a better result than a linear profile. The argument has been that the critical part of the exhaustion phase is the final phase, where the amount removed from the bath is large compared to that remaining, leading to a greater risk of unlevelness. Two profiles of this type, i.e., exponential and one with the exhaustion proportional to square root time, have been suggested.
Experimental work suggests that both exponential and square root profiles give a clear improvement over both a linear profile and a standard constant temperature ramp dyeing. It has been recommended to devise a reliable concentration monitoring system to control the exhaustion process. Despite the suggested strategies this is still an area requiring further research and development.
Practical Difficulties Involved in the Dyeing Process
There are some practical difficulties in achieving a quality dyeing when considering dyeing process control. These are briefly described in the following sections.
Dyeing rates are of greater practical significance than the exhaustion at equilibrium because continuation of dyeing to equilibrium is not economical. Long dyeing times increase the risk of fiber damage and dye decomposition, particularly at higher dyeing temperatures. On the other hand, very rapid dyeings will usually result in the color being unlevel. This implies that dyeing processes should be neither too slow nor too fast. In order that dyes are used economically and as little as possible is wasted in the dyehouse effluent, the dyer prefers a high degree of exhaustion in a relatively short dyeing time. However, dyeing must be controlled so it is not so rapid that it is difficult to produce a level dyeing. If there is a strong affinity between dyes and fibers and the conditions are not controlled, a rapid strike of dyestuff will occur which will often result in unlevel dyeings. To control the rate of dye update, a number of dyeing parameters, including temperature, pH, electrolyte concentration, and agitation among other parameters may have to be controlled.
The slope of the exhaustion curve gives information on the rate of dyeing. The determination of these curves, however, requires much work, and they are dependent on the dyeing conditions and the nature of the goods. The dyeing rate is influenced by the temperature and by chemicals such as salts and acids, all of which also influence the final exhaustion. A clear distinction of the effects of process variables on the dyeing rate and on the final exhaustion at equilibrium is essential in the successful control of the dyeing process.
Initial Stage of Dyeing
The initial rate of dyeing (the initial slope of exhaustion versus time) is called the strike. Rapid strike by a dye often results in initial unlevelness and must be avoided for those dyes that cannot subsequently migrate from heavily to lightly dyed areas of the fabric. For dyes of rapid strike, the dyeing conditions must limit the initial rate of exhaustion and therefore improve the levelness of the dyeing. The strike depends on the dyeing temperature, the dyeing pH, and the presence of auxiliaries.
Even for dyes of moderate and low strikes, the objective of uniform dyeing of the fiber mass is rarely achieved during the initial stages of the operation. This is because of irregularities in the material’s construction, in the fiber packing, and in the distribution of residual impurities, as well as differences in temperature and flow rate of the solution in contact with the fibers.
Dye/Fiber Types and Dye Migration
Different types of textile fiber require different kinds of dyes, and in general dyes which are suitable for one type of fiber will not dye other types effectively.
The degree of exhaustion of a dye at equilibrium is higher the greater the substantivity of the dye for the fiber being dyed. Often, a very substantive dye will give a high initial rate of absorption, or strike. Substantivity is the “attraction” between dye and fiber whereby the dye is selectively absorbed by the fiber and the bath becomes less concentrated.
The ability of a dye to migrate and produce a level color, under the given dyeing conditions, is obviously an important characteristic. It can overcome any initial unlevelness resulting from a rapid strike. Migration of the dye demonstrates that the dye can be desorbed from more heavily dyed fibers and reabsorbed on more lightly dyed ones. This is especially important in package dyeing where uniform color of the yarn throughout the package is essential.
While migration is important for level dyeing, it has two major drawbacks. Firstly, dyes with greater ability to desorb from dyed fibers during migration will usually have lower fastness to washing. Dyes of very high washing fastness are essentially nonmigrating dyes for which level dyeing depends upon very careful control of the rate of dye uptake by the material. The second problem with migrating dyes is that good migration may result in lower exhaustion, again because of their ability to desorb from the fibers.
Factors Affecting Levelness of Dyeing
As has been indicated dyeing is a very complicated process with different phenomena occurring simultaneously. Unlevelness can arise in many forms such as the unlevelness between sides, at ends of a fabric, or on the layers of a yarn package. The causes are as numerous as the effects. A quantitative analysis of the effects of many factors which influence the levelness of dyeing is a very difficult task since it would require the development of a mathematical model involving a significant number of parameters. Also, to investigate the transfer of dye through the substrate will involve the solution of nonlinear partial differential equations. Little work has been done on this aspect, according to the literature.
During the dyeing process, the supply of dye through the solution to the surface of the fibers/yarns can occur in two ways, either by aqueous diffusion of dye through the liquor or by convective movements of the fluid which replace the depleted solution by fresh solution. Diffusion is a much slower process than the convective transport of dye, except at very low velocities of liquor flow.
However, an exact solution to the problem of convective diffusion to a solid surface requires first the solution of the hydrodynamic equations of motion of the fluid (the Navier-Stokes equations) for boundary conditions appropriate to the mainstream velocity of flow and the geometry of the system. This solution specifies the velocity of the fluid at any point and at any time in both tube and yarn assemblies. It is then necessary to substitute the appropriate values for the local fluid velocities in the convective diffusion equation, which must be solved for boundary conditions related to the shape of the package, the mainstream concentration of dye, and the adsorptions at the solid surface. This is a very difficult procedure even for steady flow through a yarn package of simple shape.
Measurement of Levelness
Although objective measurements have been proposed, the measurement of levelness and its causes is difficult. The levelling ability of dyes is routinely tested under a fixed set of circumstances, but the effect of changing circumstances is less often reported. Similarly, a distinction between levelness of strike and levelness from migration is not usually studied. A relatively simple means of determining levelness is via colorimetric measurement of dyed object in several locations representative of the entire substrate. A color difference metric may be used to determine the degree of variability across the substrate, and a tolerance volume may be established for this purpose. Visual assessments are also common, although these would be subjective and open to interpretation. A common procedure is to place side center side of a full width of dyed fabric to determine variations from center to sides due to variations in processing conditions. Such variability is called tailing. Another common type of variability in continuous operations is due to variability in the composition of the trough during the dyeing process that may generate shade variation between the beginning and the end of a roll, commonly denoted ending. Some online measurement systems have been proposed and used in some cases, in which digital imaging or colorimetric systems are installed in a continuous processing line which can alert the operator to “larger than expected” variations in color during the operation.
A dye is a substance capable of imparting its color to a given substrate, such as a textile fiber. A dye must be soluble in the application medium, usually water, at some point during the coloration process. It must also exhibit some substantivity for the material being dyed and be absorbed from the aqueous solution.
For diffusion into a fiber, dyes must be present in the water in the form of individual molecules. These are often colored anions, for example, sodium salts of sulfonic acids. They may also be cations such as mauveine or neutral molecules with slight solubility in water, such as disperse dyes. The dye must have some attraction for the fiber under the dyeing conditions so that the solution gradually becomes depleted. In dyeing terminology, the dye has substantivity for the fiber and the dyebath becomes exhausted.
Solubility in water at some point during the dyeing cycle
Substantivity for the fiber being dyed
Reasonable fastness properties of the dyeing produced
The structures of dye molecules are complex in comparison with those of most common organic compounds. Despite their complexity, dye structures have a number of common features. Most dye molecules contain a number of aromatic rings, such as those of benzene or naphthalene, linked in a fully conjugated system. This means that there is a long sequence of alternating single and double bonds between the carbon and other atoms throughout most of the structure. This type of arrangement is often called the chromophore or color-donating unit. The conjugated system allows extensive delocalization of the p electrons from the double bonds and results in smaller differences in energy between the occupied and unoccupied molecular orbitals for these electrons. At least five or six conjugated double bonds are required in the molecular structure for a compound to be colored.
Classification of dyes according to chemical constitution and usage
Classification of dyes according to chemical constitution
Classification of dyes according to textile usage
Mordant (metal complex)
The type of fibers to be dyed
The form of the textile material and the degree of levelness required – level dyeing is less critical for loose fibers, which are subsequently blended, than it is for fabric
The fastness properties required for any subsequent manufacturing processes and for the particular end use
The dyeing method to be used, the overall cost, and the machinery available
The actual color requested by the customer
The approximate relative annual consumption of the major types of fibers and dyes estimated in the year 2000 indicates that dyes used for cotton (the most widely used natural fiber) and for polyester (the most widely used synthetic fiber) dominate the market. In the case of cellulosic fibers including cotton, reactive dyes due to possessing excellent fastness properties upon fixation and demonstrating bright and brilliant shades occupy the lion’s share of the market for this fiber category. Disperse dyes also occupy a large sector of the market due to their use on polyester fibers. Other colorants occupy smaller sections of the market and their applications are specific and less common.
In order to determine the thermodynamics of dye sorption by various fibers three main models have been proposed. These are known as Nernst, Langmuir, and Freundlich sorption isotherms . These models determine the relationship between the concentration of dye in fiber and that in solution under isothermal dyeing conditions. In the simplest form the relationship is linear (Nernst). The Nernst isotherm indicates that the distribution of dye between fibers and bath is simply due to the partition of the dye between two different solvents until one becomes saturated. In a more complex scenario, dye “sites” are present in the fiber which result in an apparent saturation value. These sorption processes are defined by the Langmuir isotherm. In some cases an empirical power function, represented by the Freundlich model, can be used to determine the relationship between the amount of dye in solution and that in fiber. An examination of this model shows that the exhaustion of dye drops towards the end of the dyeing cycle. The drop in percent exhaustion with an increase in the depth of dyeing is well known and reflects the loss of activity of the dye in solution with increasing concentration. According to this model there appears to be no saturation value for the fiber, and as more and more dye is added to the bath, more and more is taken up. There is of course a practical limit as to how much dye may be placed on the fiber. An example of a system that can be described by the Nernst isotherm is the dyeing of polyester fibers with disperse dyes. Acid dyes on wool or basic dyes on acrylic are attracted to specific dye sites with opposite charge, and these would exhibit Langmuir-type relationships. The adsorption of direct dyes on cotton may be described by a Freundlich model however, where no specific dye sites are present in fiber but a gradual decrease in the overall rate of dye adsorption is witnessed over time. In general Freundlich models are indicative of nonionic or relatively weak bonding possibilities between dye and fiber.
Many aspects of dyes and dyeing have not been covered. This is due to the complexity of the process and the large number of variables and processes involved. Textile coloration is a large industry, and a number of excellent resources are available that cover the fundamentals of coloration and the dyeing of specific types of fibers. The reader should refer to additional resources for a detailed examination of the topics.
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