13.2.1 Reactive dye and dyeing technology

Reactive dye is the dye that can react with a fibre to form a covalent link, that is forming a permanent attachment in the fibre and could not be removed by repeated treatment with boiling water under neutral conditions. Consequently, the dyes become parts of the fibre, leading to outstanding colour fastness to wash.

Due to the advantages of full colour ranges, brightness, high fastness, low cost, easy application, etc., reactive dye became the predominant dye for cotton dyeing and printing in textile industry since it was invented. Compared with direct dye, reactive dye is applied as easy as direct dye but has very high levels of fastness, especially for wet fastness.

Properties of reactive dye

The characteristic structure of a common reactive dye includes the following components:

As the name implies, reactive dye has reactive group(s) to form covalent bonds chemically with cotton and become part of it, rather than as an independent coloured substance within the fibre. Under mild alkaline conditions, the reactive group(s) on the dye molecule can react with the oxygen atom in the cellulose hydroxyl group, either by nucleophilic substitution, or by addition, and sometimes by both mechanisms for a dye with two or more reactive groups (Broadbent, 2001). The representative substitution and addition reactions between dye and cellulose are shown in Fig. 13.1 and 13.2, respectively (Gordon and Hsieh, 2007).

Both the chromophore and reactive group in a reactive dye govern its application characteristics, such as reactivity, substantivity/affinity, diffusion coefficient and solubility. Substantivity, that is the tendency of dye to transfer from dyebath to fibre substrate, is determined by both but the former has a more direct influence. Reactivity and the stability of the dye–fibre bond are determined by reactive group(s). Recently, reactive dyes with two, even three, reactive groups have been widely used for achieving a greater fastness and deeper shade. Based on the varied reactive groups, reactive dyes can typically be classified as dichlorotriazine (DCT), difluorochloropy-rimidine (DFCP), vinylsulphone (VS), monochlorotriazine (MCT), dichlo-roquinoxaline (DCQ), and the mixtures of MCT + VS, and MCT + MCT, etc. Typical structures of reactive dyes with one reactive group are shown in Fig. 13.3, and the major properties of the different types of reactive dyes are listed in Table 13.4.

A conventional dyeing process of reactive dyes contains three stages: (1) adsorption and diffusion, (2) fixation, and (3) wash-off. Reactive dyes can be used in both batchwise and continuous dyeing methods. Generally, dyes with relatively high substantivity and slow diffusion are more appropriate for exhaust dyeing in the batchwise method, such as yarn dyeing with a package dyeing machine or hank yarn dyeing machine, as well as fabric dyeing with an overflow dyeing machine or jig dyeing machine. Unlike exhaust dyeing, dyes with low substantivity and quick diffusion are more suitable for the continuous pad dyeing method. Generally, there are two types of the continuous pad dyeing process, the simpler single-pad in which dyes and alkali are padded in one dyebath, and the versatile double-pad in which dyes and alkali are padded separately and sequentially.

Although exhaust dyeing and continuous dyeing are quite different in terms of process control, as listed in Table 13.2, there are common fundamental technology parameters, apart from the machinery variables, which influence the dyeing procedure. These parameters include temperature, time, pH, liquor ratio and auxiliaries.

Temperature influence

In a reactive dyebath, temperature has profound effects on both dye and cotton in aqueous solution. The increased temperature may bring about better dye penetration, more rapid diffusion, better evenness, but may reduce dye substantivity and increase the risk of dye hydrolysis. Also, raised temperature leads to the opening-up of the cellulose structure, which activates the dye–fibre interaction (Ibrahim and Sayed, 1993). Therefore, the dyeing temperature is determined by both the substantivity and reactivity of the dye and the structure of fibre. For high twisted yarn or tight fabric, or for those dyes being not easily level dyed, temperature could be raised to 98 °C to promote migration and penetration in the adsorption phase with low reactivity dyes, and cooled to 80 °C for fixation by adding alkali. For dyeing loose fabric, such as knitgoods, generally a warm dyebath with temperature between 50 °C and 65 °C is preferred. Good results can be obtained by carefully controlling the temperature increase rate during heating-up.

The pH influence

The pH primarily influences the concentration of the cellusate anion (cellulose-O^–) on the fibre, as well as the hydroxyl ion (-OH) concentration in the dyebath and on the fibre. Reactive dyes consume some alkali for both dye fixation and dye hydrolysis. The internal alkalinity of fibre also absorbs alkali. These lead to the dyebath pH at the dyeing end being always lower than its initial value. Generally increasing dyeing pH in the fixation stage can accelerate the reaction rate between dye and fibre. For batchwise dyeing method, the dyebath pH is recommended to be controlled ranging from 10.5 to 11. Even with the low reactivity dyes, a pH exceeding 11 is still not appropriate since an unduly high pH will enhance dye hydrolysis and reduce dyeing efficiency in terms of depth and fixation. NaOH, Na₂CO₃, NaHCO₃ or a combination of these are the conventionally used alkalis, among which Na₂CO₃ is the most commonly applied. The selection of alkali is usually related to the dyes applied and the dyeing method adopted. The fixation rate of dyes and the dyeing evenness can be controlled by the dosing rate of alkali. Dyes with high reactivity are sensitive to alkali concentration in dyebath, and have an optimal temperature range of between 40 °C and 60 °C. Typical examples are DCT, DFCP, DCQ and VS.

Electrolyte effect

When cotton fibre is in an aqueous solution, the fibre surface presents negative charge, mainly due to the dissociation of accessible cellulose hydroxyl (Cell-OH) groups and the rearrangement of the charge groups at the interface between the fibre and the water. Reactive dyes, as well as other soluble dyes for cotton, carry negative charges because of sulphate group(s) on their molecules for solubility. Therefore, the electrostatic repulsive force between fibre surface and dyes has to be overcome in order to diffuse the dyes through the fibre-water interface. The most common method to overcome the electronic repulsion in exhaust dyeing is to add large quantity of electrolyte (so-called salt), sodium chloride (NaCl) or sodium sulphate (Na₂SO₄), in the dyebath. The presence of electrolyte in the dyebath reduces the extent of the surface charge, which leads to a reduction in repulsion between ionised dyes and fibre, thereby increasing the substantivity of dyes. Trichloropyrimidine and aminochlo-rotriazine dyes are typical dyes exerting high exhaustion without alkali, thus it is necessary to control the salt-dosing rate carefully with those dyes to ensure levelling. There are four methods of adding salt in general: (1) portion adding/dosing salt; (2) adding salt at the start of dyeing (especially for dyeing dark shade or nonsensitive colour); (3) adding salt and soda at the start of dyeing; and (4) all-in process. The first two methods are used widely in production.

Liquor ratio

Water is an essential medium for a dyeing process in most dyeing methods. The liquor ratio in an exhaust dyebath is the ratio of the weight of the dry material being dyed to the water weight of the dyebath. Example, a liquor ratio of 1:10 implies 1 kg fibre is dyed in 10 L water. A large liquor ratio benefits dyeing levelness, but has negative impacts on production costs and environment, and results in more carbon emission. Therefore, a low liquor-ratio dyeing machine, such as 1:5 for cotton, is being applied more broadly (Shang, 2002; Shang and Zhuo, 2003).

Auxiliaries

Suitable auxiliaries in the dyebath could enhance dyeing qualities. Both the anionic surfactant and the non-ionic surfactant can increase dyeing evenness. The former can improve dye uptake, and the latter may decrease the exhaustion, but both may slow down the hydrolysis rate of reactive dye. The presence of triethanolamine can improve the swellability of the cellulose structure, and thereby enhance colour strength. Urea could increase the solubility of reactive dye in water and enhance the swellability of cellulosic fibre, which is critical in continuous dyeing (Shamey and Hussein, 2005).

Reactive dye selection and dye compatibility

Dye selection is the first important issue in dyeing, considering both the dyeing behaviour and production cost. Table 13.5 lists the majority of considerations in dye selection.

Nowadays, colour matching with three dyes, such as with the dyes of three primary colours in red, yellow, and blue, is very common in practice. Therefore the compatibility of the selected three dyes should be good, that is the dyes should have the similar dyeing behaviour, including tone-on-tone build-up properties, equal robustness to different dyeing conditions, and no blocking effect between them. Figure 13.4 shows a set of the typical commodity dyes of three primary colours in red, yellow, and blue applied for matching medium and deep shade. There are also sets of commodity dyes of three primary colours with higher colour fastness applied for matching light shade.

Reactive dyeing process control in the exhaust dyeing process

The exhaust dyeing method is mainly applied in the batchwise dyeing machines, such as jig or overflow dyeing machine for fabrics (Shang and Chong, 2002; Shang and Zhuo, 2003), and the package or hank dyeing machines for yarns.

In order to identify the tone-on-tone build-up properties of reactive dye in exhaust dyeing, the concept of SERF value is developed. As shown in Fig. 13.5, S is the exhaustion value before the addition of alkali, indicating the substantivity; E is the final exhaustion value, indicating the exhaustion ability; R is the fixation value when fixation lasts 10 min (another explanation is that R is the fixation rate when fixation lasts half of the total fixation time when fixation reaches the maximum value), indicating the fixation rate; and F is the final fixation value, indicating the reactivity.

Depending on the difference between E and S, the dyeing curve for exhaust dyeing can be classified into several categories.

For dyes with a large (E–S) value, it means the substantivity of these dyes is low. Salt can be added at the beginning of dyeing to increase substantivity, which accelerates the dye adsorption rate. Fixation can be controlled by either the rate of alkali dosing or temperature increase.

The dosing unit is widely equipped with dyeing equipment, and the dosing rate in some advanced dyeing machines can be controlled progressively or retrogressively (Fig. 13.6). The term ‘dosing’ here not only means simply the addition of chemicals by the dosing system, but also indicates that much more attention has to be paid to the ‘dosing’ stage, and a versatile dosing programme is necessary in order to obtain the best results.

Figure 13.7 shows the alkali-controllable process, in which alkali is added more carefully than with regular dosing, in a constant-temperature dyebath. Figure 13.8 shows the controlled temperature process in which alkali is regularly dosed at room temperature (assuming 30 °C) and the temperature rising is controlled more carefully. The other dyeing profiles are shown in Table 13.6. The symbol of the right-angled triangle ‘ ///inlineimage/// in all dyeing curves in this chapter denotes the linear dosing of chemicals and dyes. For alkali dosing, progressive dosing may be required for some dyeing processes in practice for achieving better dyeing results.

For dyes with low (E–S) value, indicating high substantivity, ‘salt-at-start’ is not recommended, because these dyes have the poor migration ability and unevenness is inevitable. As a result, controlled salt-dosing is necessary to control the substantivity. Salt-dosing combined with temperature increase is adopted to control the exhaustion rate (Fig. 13.9). Controlled dye-dosing can be adopted for dyes with moderate (E–S) value (Fig. 13.10). The other dyeing profiles are shown in Table 13.7.

When the dyeing process is ended, a wash-off process is necessary to achieve optimal colour fastness. The unfixed dye on fibre will be effectively removed under hot soaping treatment.

Figure 13.11 shows a general batch wise wash-off process for products dyed with medium shade. More washing-off may be needed for deep shade, until the required colour fastness is achieved.

Reactive dyeing process control in continuous process

The continuous dyeing method for woven fabric, especially for cotton or cotton blend, is an important processing technology for reactive dyeing, especially when a large amount of fabric needs to be dyed. Unlike exhaust dyeing, in which dyes of different substantivity are properly all available, dyes with only low to moderate substantivity are preferable in continuous dyeing. The continuous dyeing process can generally be classified into four types: (1) pad-dry-pad-steam method (Table 13.8), (2) pad-steam method (Table 13.9), (3) Econtrol process (Table 13.10), and (4) pad-dry-cure method (Table 13.11).

Note:

The process parameters of pad-steam method are similar to those of the pad-dry-pad-steam process. But more attention has to be paid to the stability of the mixed dyebath. Generally Na₂CO₃ is more appropriate for dyes with relatively lower reactivity, such as MCT dyes; NaHCO₃ is properly used with dyes of relatively high reactivity, for avoiding dye hydrolysis. It is recommended to use the continuous dyeing machine with the special chemical mixing equipment, which can mix dye and alkali at the point when they are just introduced into the pad-bath trough.

In the Econtrol process, recently developed to eliminate the drying process for energy saving and low carbon production, the padded fabric is directly steamed at 120 °C with hot air at a low relative humidity (25%). This process combines the advantages of short processing time and high fixation rate.

The pad-dry-cure process has the advantages of higher fixation rate, easy operation and good reproduction. The curing temperature is around 180 °C. Because little water is present on the fabric when curing, large amounts of urea must be added to the dyebath to help fibre swelling and dye fixation. One disadvantage of this process is the risk of urea decomposition during curing, especially when the temperature is extremely high. Recently, thermal-stable chemicals performing the same functions, such as dicyanoguanidine, are more appropriate (even though more costly) to replace urea to avoid harmful by-products generated during curing.

In the continuous dyeing range, the wash-off unit is usually installed at the end of the production line. The washing unit may include a set of washing boxes. The continuous washing process may involve three stages: (1) initial rinsing, (2) soaping, and (3) final rinsing. Initial rinsing employs either cold or warm water to remove superficial loose colour (sometimes HAc is required for neutralisation); soaping uses hot water (95 °C) containing detergent (1–3 g/L) to remove the unfixed dye inside the fabric; and final rinsing is to ensure that loose colour is cleaned thoroughly.

13.2.2 Other dyes available for cotton dyeing

In addition to reactive dyes, other available dyes for cotton include direct dyes, sulphur dyes, vat dyes and azoic dyes. The invention of direct dye, with substantivity for cellulosic fibres, greatly preceded the advent of reactive dyes. The weakness of direct dyes is poor colour fastness, especially the wet fastness. Although the wet fastness of direct dyes is generally not as good as vat dyes, due to easy application and a broader shade range direct dyes were the most important dye for cotton dyeing before the 1950s. Since then, the position of direct dyes in cotton dyeing has been replaced by reactive dyes. Direct dyes nowadays are only used in the applications where colour fastness is not important. An example of the direct dyeing process is shown in Fig. 13.12.

Sulphur, vat, and azoic dyes can be categorised together because they share the same dyeing mechanism, by which soluble forms of the dyes are first absorbed by cotton and then transferred into insoluble form in the fibre substrate. Vat dyes and sulphur dyes are still important for dyeing and printing (discharge and resistant style) of cotton. The dyed goods have very considerable fastness. Indigo dyes are the oldest vat dyes derived from natural sources. Indigo and sulphur dyes are the most important dyes applied in denim production. The dyeing application of vat dyes and sulphur dyes is similar, but the latter are particularly important for deep shades dyeing. The azoic dyes are of less importance in dyeing and printing because of their relatively low colour fastness.

13.3.1 Polyester and its dyeing properties

Polyester (PET) is a synthetic fibre and has the properties of hydropho-bicity, crystallisation and thermoplasticity. It becomes softer when being heated over glass temperature T_g (the transfer temperature of polymer from glassy state to a state of higher elasticity) and melts at temperatures above melting point T_m. When polyester fibre was discovered in 1947, none of the water-soluble dyes used at that time, such as direct, acid and basic dyes, could be applied for polyester dyeing, since there is no dyeing site on this fibre. Later, disperse dye, which was originally developed for the dyeing of cellulose acetate fibre, was found to be practical for dyeing polyester.

The diffusion of disperse dyes into polyester, including all the thermoplastic fibres, is described by free volume theory (Vrentas and Duda, 1977). According to this theory, disperse dyes are adsorbed on the fibre surface and diffuse into the fibre through transient passages, resulting from the segmental motion of polymer chains in material substrate when sufficient thermal energy is provided. When polyester reaches T_g, the lowest temperature at which the polymer chains begin to vibrate and slide past each other when a force is applied, disperse dye begins to diffuse into fibre by ‘jumping’ from one site to another. With continuous thermal energy provided dyes are gradually embedded into the fibre substrate, until the distribution in fibre and dyebath reaches equilibrium. When the system temperature is decreased after equilibrium, the vibration and sliding of polymer chains is weakened, and finally the dyes in fibre are blocked inside the fibre. Based on the free volume theory, dyeing temperature must be beyond T_g to generate enough free volume in fibre to accommodate the dye.

13.3.2 Disperse dye and polyester dyeing

Disperse dye, as indicated in Table 13.1, is the only dyestuff being available for polyester dyeing.

Properties of disperse dye

Unlike the anionic dyes for cotton, disperse dyes are relatively insoluble in cold water and have only limited solubility in boiling water. As their name implies, disperse dyes are present in the dyebath in a superfine aqueous suspension with dispersing agents. However, they possess substantivity for hydrophobic fibre, such as polyester and nylon, in which they are more ‘soluble’ than in aqueous. The slight water solubility of disperse dyes results from the presence of polar substituents in their molecular structures. It gives rise to the formation of dyes in their monomolecular forms in the true aqueous solution, which helps the disperse dye to penetrate into hydrophobic fibre during dyeing. Apart from a few bright pink and blue shades, anthraquinone disperse dyes (Fig. 13.13) are gradually being replaced by azo disperse dyes because of production costs and environmental threats in the synthesis process.

Disperse dyes are primarily marketed in the forms of powder or grain, and less frequently as paste or aqueous dispersion. All of these forms contain micro-fine dye particles and dispersants varying in dose and type. Dispersant is added, usually anionic surfactant, to clad each dye particle with a monomolecular layer of adsorbed dispersant. When the disperse dyes ‘resolve’ in the dyebath, hydrophobic chains in the dispersant structure embed into the hydrophobic dye particle, leaving the anionic groups exposed to the surrounding water. The overall negative charge on the surface of each particle prevents coalescence and aggregation. Consequently, the suspension of dye particles in the dyebath maintains stability and dyeing readiness (Broadbent, 2001).

Disperse dyes have distinctive dyeing properties on polyester. They are often classified in accordance with their dyeing rate and sublimation fastness, which are closely related to the polar group number and molecular weight of dye molecule. Table 13.12 shows the most common classification of disperse dye in the exhaust dyeing of polyester, as well as a set of the typical commodity dyes of three primary colours, red, yellow, and blue, applied for relevant shade. Disperse dyes with low molecular weight, classified as low energy disperse dyes, present more levelling and faster dyeing rate in the dyeing progress but poor sublimation fastness for its final products. On the other hand, those dyes with higher molecular weight, which exhibit weak migration and low dyeing rate but good sublimation fastness, constitute the high-energy disperse dyes. Low energy dyes are more suitable in the exhaustion dyeing, while high-energy types are preferred in thermofix dyeing.

Factors influencing disperse dyeing

The major influencing factors on disperse dyeing process are temperature, time, pH and auxiliaries. Polyester, in common with other synthetic fibres, is quite crystalline and very hydrophobic. Even at 100 °C, which is greater than the T_g of polyester by about 20 °C, dye diffusion is so weak that no satisfactory exhaustion can be obtained for most of the disperse dyes. As dyeing temperature increases from 100 °C to 130 °C under pressure, dyeing rate accelerates considerably, giving a better coverage of filament irregularities due to improved dye migration. For each particular dyeing, there always exists an optimum temperature/time profile, relating to the type of goods, dyes and the dyeing equipment involved. Dyeing time can be minimised by controlling the temperature to offer a reasonable exhaustion rate giving absorption uniformity. Conventionally, the dyeing time ranges from 30 to 60 min at 130 °C to obtain sufficient equilibrium. The pH of dyebath is usually adjusted within 4.5–5.5 using either acetic acid alone or in combination with other acids. Acidic conditions could minimise reduction and hydrolysis of some disperse dyes with chemical groups that are sensitive to alkali at high temperature.

A carrier, an organic compound dissolved or emulsified in the dyebath, can be used to assist the dyeing of polyester around 100 °C. The typical method is also called carrier dyeing. These substances enable the deep shades can be obtained at the boil within a reasonable dyeing time. Nowadays, carrier dyeing is declining for environmental and other considerations (Broadbent, 2001).

Disperse dyeing process control

Dyeing of polyester with disperse dye can technically be launched in either batchwise or continuous processing.

Exhaust dyeing

Batchwise dyeing for polyester is for dyeing polyester with batchwise dyeing machines, such as package dyeing machines for yarn, and jet or overflow dyeing machines for fabric, at high temperature (usually around 130 °C).

Figure 13.14 shows the high temperature dyeing profile for polyester. Dyes selected must be subject to the same energy class. The build-up of colour on shade requires that the compatibility of dyes should be good, that is having about the same dyeing rate, similar to reactive dyeing. When the dyebath temperature reaches 90 °C, at which dyeing initially begins, the rate of temperature increase should be slowed down to eliminate dye agglomeration. When dyeing equilibrium is obtained, slow cooling is much preferred to minimise the generation of oligomer, which would lead to deterioration in appearance of the dyed goods. Reduction cleaning is important to get rid of the loose colour in fabric, and colour fastness can be enhanced. Thorough cleaning is necessary to ensure the dyed goods possess satisfactory fastness. In practice, reduction cleaning at 80 °C for 15 min, followed by general cool rinsing, is usually enough to wash down the unfixed dye. To neutralise the dyed fabrics, acetic acid (60%) of 1–2 mL/L is required in the final rinse bath. The other related dyeing profiles are shown in Table 13.13.

Continuous dyeing

Continuous dyeing for polyester is usually carried out in the continuous Thermosol (thermofix) dyeing machine, which was invented by Du Pont in 1949. It involves first padding the disperse dyes into fabric, drying, and then fixing the dyes by dry heating at a temperature of about 190–205 °C. At this elevated temperature, the fibre molecular chains open up and the dispersed dyes vaporise and diffuse into the polymer. On cooling, the dyes are trapped within the fibre, yielding coloured fibres with good fastness. This process is rarely adopted for pure polyester dyeing, but is employed frequently in the dyeing of polyester/cellulose woven blends. The process control for the Thermosol process will be discussed later in Section 13.4.2.

13.3.3 Dyeing of nylon

As the second important synthetic fibre, nylon is becoming more and more significant throughout the textile industry. Nylon, with a high strength but low density, is a polyamide fibre with two kinds of chemical structure, namely nylon 6 and nylon 6,6. Their structures are illustrated in Fig. 13.15.

Acid dyeing

Nylon fibre is a hydrophobic polymer with an α-terminal amino group, so it can be dyed with disperse dyes as well as other water-soluble dyes such as acid dye and reactive dye. The exhaust dyeing process with batchwise dyeing machine is the most common method for pure nylon dyeing.

Acid dyes for nylon are divided into three types depending on their dyeing properties. These dyeing properties are shown in Table 13.14.

One of the most common defects of acid dyeing for nylon is the phenomenon of the barré result. Barré derives from chemical variation in manufacturing, oxidation of terminal amino in melt spinning and/or physical changes in spinning, drawing or heat-setting. It gives rise to the stripes on dyed goods. It is usual to add dyeing auxiliaries to control exhaustion to reduce the tendency of stripe formation and improve levelling. An anionic agent competes with an anionic acid dye for dyeing sites, while the cationic agent captures anionic acid dye first and then liberates it in proper condition, both of which decrease the dyeing rate to obtain levelness (Lewis and Marfell, 2004). A typical dyeing recipe is shown in Table 13.15, and the matched dyeing curves for different dye types are shown in Fig. 13.16 and 13.17.

It is difficult to obtain satisfactory wet fastness for dyed nylon with acid dyes for medium and dark shades; dye fixation is therefore a necessity. For example, tannin and tartar emetic are applied for acid dye fixation, and the process details are shown in Fig. 13.18. It is important to note that excessive tannin and tartar emetic will cause deterioration in handle.

Disperse dyeing

Disperse dye is another dye class that can be applied in nylon dyeing because of the hydrophobic property of nylon. The glass transition temperature (T_g) of nylon is 50–60 °C and it is easier to swell than polyester, thus dyeing at boil under atmospheric pressure can yield satisfactory exhaustion. Disperse dye has better coverage of barre than acid dye, but it is difficult to obtain dark shades, and some disperse dyes exert poor washing fastness, especially for relatively deep colour. Nowadays, with the increasing requirement of higher colour fastness, dyeing nylon with disperse dye is declining. The typical dyeing curve is shown in Fig. 13.19 and the dyeing recipe is shown in Table 13.16.

In some cases, to obtain better levelling and dark shade, disperse dye and acid dye are combined in nylon dyeing. A cationic agent is not recommended in disperse dyeing, since it would be absorbed by disperse dye particles, decreasing the stability of dyebath. The non-ionic type is greatly preferred.

13.4.1 Exhaust dyeing

Exhaustion dyeing processes for blends with disperse/reactive dyes are of two kinds. These are (1) the two-bath method, in which the two fibre in blends are dyed in two independent dyebaths separately with different dye-stuffs, and (2) one-bath-two-stage, in which the dyestuffs are mixed together in a single bath, but the dyeing process is divided into separate stages for each fibre.

Two-bath method

In the two-bath method, usually the polyester is dyed first, followed by the cellulose part, which can avoid negative influences under the polyester dyeing conditions, such as high temperature, acidity and reduction cleaning on cellulose reactive dyeing. This process is similar to that of pure polyester and pure cotton dyeing. The two-bath process ensures the final goods have maximum brightness and colour fastness. Moreover, because of the independent dyebath for the different classes of dyes involved, there is considerable freedom in dye selection. There is no risk of interaction between reactive and disperse dyeing system. Nevertheless, the biggest shortcoming of such process is that the total dyeing time is greatly extended, which rapidly boosts production costs.

One-bath-two-stage method

The one-bath-two-stage method is an alternative method for blends dyeing. The term ‘two-stage’ means reactive dyeing for cotton is implemented in two steps. Exhaustion of reactive dye is carried out first and then fixation is activated by pH change (or pH-sliding) from acidic to alkaline. The dyeing curve for this process has some variation depending on the exhaustion sequence of each dyestuff. Fig. 13.20 shows two dyeing curves; in process (I) disperse dye and reactive dye are first exhausted by blends simultaneously, then the reactive dye is fixed by alkali, while in process (II) the pH adjustment is achieved by a pH-sliding agent. The pH-sliding agent is a specific auxiliary to gradually change the dyebath pH, either from acidic to alkaline, or from alkaline to acidic. In process (II), the pH-sliding agent presents alkaline character initially in the dyebath, and then it decomposes and liberates an acidic substance as the temperature increases. Reactive dye exhaustion is completed before the addition of the pH-sliding agent, and the dye fixation is carried out after the addition of the pH-sliding agent. When reactive fixation is accomplished eventually, the temperature is further raised and the pH-sliding agent releases acids to shift the dyebath from alkaline to acidic, meeting the requirements of disperse dyeing. In this process, the total processing time can be shortened to four hours. But there are further requirements for dye selection in this one-bath method.

In the above two methods, reactive dye and disperse dye are simultaneously present in one dyebath at the same time. As a result, each must be stable enough to withstand the dyeing condition for the other dyestuffs, that is reactive dye must withstand the acidic bath at a high temperature, and disperse dye must display sufficient stability to alkali as well as high electrolyte concentration. Furthermore, it is essential to choose reactive dyes and disperse dyes that do not react with each other. After dyeing, the reduction cleaning for unfixed disperse dye must be avoided otherwise there will be decomposition of reactive dye from the dyed cotton composite.

A more versatile dyeing process for blends is of dyeing in a neutral bath in a single stage with special reactive dye and disperse dye. The Kayacelon React CN® reactive dye, which can be fixed in a neutral condition as the temperature ranges from 110 °C to 130 °C, has admirable compatibility with disperse dye. In this dyeing process,

pH adjustment is no longer a necessity and the total dyeing time is only about four hours.

13.4.2 Continuous dyeing

Continuous dyeing for woven polyester/cellulose blends is another significant process throughout the dyeing industry because of having large production capacity and high efficiency. There are three process flows as follows:

The first process avoids mutual interference between the dyeing of two fibres and is free of dye selection, but is more time-consuming. The condition for each stage is similar to that for pure polyester and pure cotton. One of the distinct characteristics of the continuous process is that the staining of disperse dyeing on cellulose is more severe than that in exhaust dyeing. To overcome this, an effective dispersing agent is necessary to transfer disperse dye from cellulose to polyester. High-energy dye is always preferred for the thermofix process although a complicated control is needed to obtain optimum dyeing results. Before the second padding of reactive dye solution, reduction cleaning of unfixed disperse dye is essential if high fastness is required. If the former disperse dye used is alkali-sensitive, such as those with vulnerable heterocyclic rings or hydrolysable ester groups, reduction cleaning may not be necessary, because these loose colours would be destroyed after alkali-padding and cleaned in the final rinsing, which is more economical. Reactive dyeing for a cellulose component can use either high reactivity dyes or low reactivity dyes following their characteristic processes.

The second process is relatively shorter than the first one, but there is a problem in dye selection, as in batchwise dyeing. The dyeing condition for this process is listed in Table 13.17.

The one-bath-one-stage employs disperse dye, reactive dye and alkali in a single pad-bath. The padded two dyes are fixed on fabric simultaneously in the thermofix unit in a continuous production line. The dyeing condition is shown in Table 13.18. Suitable disperse dyes for this process are those with low alkali-sensitivity and less cotton-staining tendency. The reactive dye with low reactivity and good thermal stability, such as MCT type, is thus suitable.

13.5.1 Package dyeing machines for yarn

In package dyeing, the most important dyeing method for yarn, yarns are wound on perforated cones. Before dyeing, yarn packages are first inserted onto vertical, perforated spindles in the dyeing machine. When dyeing begins, dye liquor can be pumped through the perforations in spindles and then forced to run into the package crosswise. Flow direction can be reversed from inside-to-outside to outside-to-inside automatically. A schematic of a package dyeing machine is shown in Fig. 13.21.

Evenness is always the predominant consideration in dyeing. For package dyeing, the evenness depends on good preparation of the yarn (proper package density, even winding), correct loading, right liquor ratio and appropriate liquor flow rate (good liquor circulation), and so on.

Package density

The porosity of the package must be uniform to prevent the uneven dyeing caused by the liquor short-cut. The evenness of winding has great influence on dyeing quality. Good winding requires the packages are constant in density, including in any single package from inner to outer, as well as that from package to package in the same batch. It is very important to find the proper package winding density. Too low a density would cause the collapse of yarn from the perforated tubes, while too high a density could result in difficulty for dye liquor penetration. The correct density of the yarn package is associated with the nature of material, yarn texture, yarn count, and the liquor circulation of package machine. The examples of package densities are listed in Table 13.19.

Mounting of package

After packages are prepared, they have to be mounted on a series of hollow, perforated, vertical spindles in the dyeing machine. It is critical to fix the packages firmly and compactly on spindles, without any slip space. Gaps between packages, top spacer and top package, as well as bottom spacer and bottom package, should be eliminated (Fig. 13.22). Otherwise a leakage may occur between these spaces when dye liquor flows pumped from either inside-to-outside or in reversed flow direction during the dyeing operation, which can lead to unevenness in dyeing.

Liquor circulation

Liquor circulation, as the most fundamental and critical requirement, must be sufficient and uniform (Shore, 1995). A better dyeing levelness can result by changing liquid flow direction from time to time. Table 13.20 gives the recommended flow parameters for package dyeing.

Dyeing quality of a dyed yarn depends on both having dyeing evenness and maintaining the yarn in good physical quality. Generally, the higher the liquor flow rate, the better the dyeing evenness that can be achieved (Shang, 2002). However, undue turbulence in the dyebath can cause pilling which declining in physical quality of the yarn (Shore, 1995). Therefore, the flow rate of dye liquor should be optimally controlled so as to balance the dyeing evenness and the physical quality of the dyed yarn (Fig. 13.23).

13.5.2 Overflow dyeing machines and jet dyeing machines for fabric

The principles of the overflow dyeing machine and the jet dyeing machine are same, that is fabric being dyed runs cyclically, and dye solution at the same time also flows cyclically. Fabric loaded in the dyeing machine is in rope form, and is moved by both the driving force from the lifting reel and the pushing force generated by nozzle liquor, which determines the fabric running rate. Dye solution is sprayed or jet-fed on to the fabric as the fabric passes through the nozzle, which is the main place that dye solution exchanges with fabric in terms of dye exhaustion. During repeated fabric circulation inside the machine, a dyeing process is carried out. Moreover, some machines are designed to operate above atmospheric pressure, which is particularly suitable for dyeing polyester at temperatures approaching 135 °C (Broadbent, 2001).

Machine classification

The overflow and the jet dyeing machine are distinguished on the basis of the fabric running rate inside the machine. Generally, a fabric running rate below 350 m/min is classified as an overflow dyeing machine, and that over 350 m/ min is called a jet dyeing machine. There are four main shapes of overflow or jet dyeing machines (Shang and Zhuo, 2003), that is O-shape (Fig. 13.24), U-shape (Fig. 13.25), up-L-shape (Fig. 13.26) and down-L-shape (Fig. 13.27). The comparison of these machines is listed in Table 13.21.

For the overflow dyeing machine, friction between fabric and machine, as well as that between fabric and liquor, is smaller than that of the jet dyeing machine, since the liquor pushing force generated by nozzle is gentle. The overflow dyeing machine is particularly suitable for those fabrics which cannot withstand high tension and high friction, such as cotton knits and their blends, as well as the elastic fabrics. Knitted fabrics should be dyed with overflow dyeing machines to reduce pilling and to retain its dimensional stability. This is because the nozzle liquor pushing force for fabric is moderate, and the slippage between fabric and lift reel is small. The jet dyeing machine is more suitable for pure synthetic fabrics, especially for thin fabrics and the easy crease-marked fabric.

Factors of dyeing process control

Overflow and jet dyeing machines are the most significant exhaust dyeing machines for fabric at present. Keeping the machines running under the optimal conditions has profound influences on the dyed goods. The important factors for dyeing process control include: (1) fabric loading weight, (2) liquor ratio, (3) fabric running rate in terms of the total actions of lifting reel speed and nozzle liquor pressure, (4) fabric cycle time, (5) chemical dosing rate, and (6) temperature/time profile.

Loading capacity

Generally, the maximum fabric loading capacity depends on the design capacity of the dyeing machine. However, the actual loading weight should be based on the fabric loading length. For a designed weight, the fabric length of thin fabric is longer, while the fabric length is shorter for thick fabrics. Since the requirement of shortening fabric cycle time for ensuring even dyeing, thick fabric could be loaded up to the designed capacity, while the loading of thin fabric should be discounted. The loading for easy crease marked fabric might also need to discount to avoid the un-removed crease marks happened. The actual loading capacity for a single chamber machine can be calculated as follows:

Fabric length = machine running rate × cycle time

The elongation of fabric and the slippage between fabric and lifting reel should be considered.

Actual fabric loading weight = fabric length × fabric weight per metre

For a machine with multi-chambers, its actual total loading capacity can be calculated as:

Total actual loading capacity = the actual loading weight of a single chamber x chamber number

The development trend of the overflow and jet dyeing machines is towards lower liquor ratio and shorter processing time for environmental and energy considerations, so-called low carbon dyeing. Right-first-time and reproducibility in dyeing for all batchwise dyeing machines are other important issues needed to be improved. Various modified overflow and jet dyeing machines approaching these targets are now appearing in the market.

Broadbent, A.D. Basic principles of textile coloration. Society of Dyers and Colourists: Bradford; 2001.

Gordon, S., Hsieh, Y. Cotton: science and technology. Cambridge: Woodhead; 2007.

Ibrahim, N.A., Sayed, W.A.E. Low-temperature dyeing of cotton fabrics with monochlorotriazine dyes. American Dyestuff Reporter. 1993; 82:44–49.

Ingamells, W. Colour for textiles: a user’s handbook. Society of Dyers and Colourists: Bradford; 1993.

Lewis, D.M., Marfell, D.J. Nylon dyeing. In: Hawkyard C., ed. Synthetic fibre dyeing. Cambridge: Society of Dyers and Colourists; 2004:82–121.

Shamey, R., Hussein, T. Critical solutions in the dyeing of cotton textile materials. Manchester: Woodhead Publishing Limited; 2005.

Shang, S.M. Package dyeing: Low Liquor Ratio and Short Cycle. International Dyer. 2002; 187:23–25.

Shang, S.M., Chong, C.L. Bleaching and dyeing cotton with a shorter process. AATCC Review. 2002; 2:31–33.

Shang, S.M., Zhuo, H.J. Breakthrough in Overflow Dyeing Machine Promotes the Development of Eco-friendly Dyeing Technology. China Textile Leader. 2003; 4:28–33.

Shore, J. Cellulosics dyeing. Society of Dyers and Colourists: Bradford; 1995.

Vrentas, J.S., Duda, J.L. Diffusion in polymer-solvent systems. I. Reexamination of the free-volume theory. Journal of Polymer Science: Polymer Physics Edition. 1977; 15:403–416.

Xi, D., Chen, J., Ma, C., The importance of water resource on development of dyeing and finishing industry. In: China Association of Dyeing and Finishing Industry, Annual symposium on environmental protection in dyeing and finishing industry, Qingdao, 2006, Beijing: China Textile & Apparel Press. 2006.

Yeung, K.W., Shang, S.M. The influence of metal ions on the aggregation and hydrophobicity of dyes in solutions. Journal of the Society of Dyers and Colourists. 1999; 115:228–232.