**4. Disperse dyeing**

For dyeing polyester fibres, in practical terms only disperse dyes are suitable. Through their hydrophobic properties, these dyes are capable of penetrating into the similarly hydrophobic polyester fibre. This class of dyes has only extremely poor solubility in water (Rouette, 2000); for this reason, dispersing agent is added to the dyebath to maintain dispersion stability, especially in the case of high temperature dyeings (Burkinshaw, 1995).

#### **4.1 Basic principle**

The inclusion of dispersing agent in the dyebath is a crucial factor in the application of disperse dyes. Once such a compound is added to water, its dual character results in the formation of micells above critical, but low, concentration. The hydrophobic tails of the

greater electronegative surface charge in water, there is no response to direct dyes. The absence of basic groups affords no sites of attachment for acid dyes, but the yarn does show some substantivity for basic dyes. In the early days, limited ranges of water-soluble dyes, selected from a variety of sources by trial and error, were placed on the market. In many

Cellulose di- or triacetates have no ionic groups. They are quite hydrophobic fibres. When introduced in the 1920s, cellulose diacetate was initially difficult to dye satisfactorily with existing ionic dyes. Fine dispersion of simple, non-ionic azo and anthraquinone compounds, of limited water solubility, however, efficiently dyed this fibre. These so-called disperse dyes are slightly soluble in water and are extracted from the aqueous solution by the solid

These fibres in no way resemble fibres of cellulose. Both cellulose diacetate and triacetate are relatively hydrophobic and thermoplastic. The standard regains of cellulose di – and triacetate are 5.0 and 2.5%, respectively. They hardly swell in water(Broadbent, 2001). Cellulose diacetate is thermoplastic in the hot dyebath and care should be taken to ensure the goods are not subjected to stretching. They are approximately 40% weaker wet than dry, and their extensibility is increased by about 40% when wet. They are permanently glazed at temperatures above approximately 175C, soften at 205C and melt at approximately 230C. The low temperature of softening may be of advantage for embossed finishing (Trotman,

The mechanical properties of triacetate fibres are close to those of the secondary acetate but they do not lose a great deal of strength when wet. It gives fabrics with a firm crisp handle,

Both cellulose acetates are resistant to dilute solutions of acids but are sensitive to alkaline solution, which cause hydrolysis of the acetate ester to hydroxyl groups, especially at higher temperatures. The triacetate is more hydrophobic and more compact and crystalline than the diacetate and has better resistance to alkaline solutions and to solvents (Broadbent,

For dyeing polyester fibres, in practical terms only disperse dyes are suitable. Through their hydrophobic properties, these dyes are capable of penetrating into the similarly hydrophobic polyester fibre. This class of dyes has only extremely poor solubility in water (Rouette, 2000); for this reason, dispersing agent is added to the dyebath to maintain dispersion stability, especially in the case of high temperature dyeings (Burkinshaw, 1995).

The inclusion of dispersing agent in the dyebath is a crucial factor in the application of disperse dyes. Once such a compound is added to water, its dual character results in the formation of micells above critical, but low, concentration. The hydrophobic tails of the

cases both their fatness and exhaustion left much to be desired (Trotman, 1984)

fibre in which the dyes are quite soluble (Broadbent, 2001).

less soft than that of the diacetate (Broadbent, 2001)..

**3.2.2 Properties of acetate fibres** 

**Physical properties** 

**Chemical properties** 

**4. Disperse dyeing** 

**4.1 Basic principle** 

1984).

2001).

dispersing agent molecules are inside the micelle which, as a consequence, is able to solubilise the disperse dye molecules, so conferring a higher apparent solubility on the dye. The dye transfer to the fibre from the micelles. As micelles empty their dye, they re-from and dissolve more dye from the solid particles (Ingamells, 1993)

Much of the evidence that is available on the subject suggests that in dyed polyester fibres the disperse dyes are present chiefly in the monomolecular state [Schroeder & Boyd, 1957; Hoffman et al, 1968]. At the end of the dyeing process, the dye that has been absorbed by the fibre is in a state of dynamic equilibrium with the dye that remains in the bath, and the fraction of the latter that is in aqueous solution must be present in the same state of aggregation as the dye in the fibre. It is reasonable to infer that the transfer of the dye to the fibre takes place from a monomolecular aqueous solution, the concentration of which is maintained during the first phase of the dyeing process by the progressive dissolution of solid dye from the particles in dispersion in the bath. In the presence of dispersing agents the following equilibrium situation is set up (Fig. 15) (Johnson, 1989).

Fig. 15. Disperse dyeing mechanism

The four stages of the process mechanism are as follows (Murray & Mortimer, 1971):


The process of transfer from the aqueous solution to the fibre is comparable with the extraction of a solute from one solvent by a second, immiscible solvent and similar laws of partition are applicable. Distribution coefficients that are related to the solubilities of the dyes in the aqueous and fibre phases can be determined for different processing temperatures, although they may be affected by the simultaneous equilibrium between the aqueous and solid phases of the dye. The rates of the first and second stages of the process mechanism are governed by these solubilities.

It was established that the disperse dyeing system was truly reversible and that the results conformed to a rectilinear isotherm. Typical results show linear relationship in distribution of dye between polyester and water. It is well established that dyeing with disperse dyes is

Dyeing with Disperse Dyes 211

The possible different crystal forms (e.g. , , , and ) of disperse dyes have been suggested to influence the saturation values achieved on polyester, due to differences in the vapour pressure and solubility of the different forms of dye in both water and fibre

Disperse dyes are sparingly soluble in water and often crystalline with varying particle size. These characteristics are inadequate for dispersing the dyes in water and cause unlevel dyeing. In order to achieve the required particle size and distribution (Heimanns, 1981), the disperse dye is milled, usually in the presence of a dispersing agent (Derbyshire et al, 1972). Generally, the dispersing agents are anionic, ligninsulphonates or polycondensates of arylsulphonic acids with formaldehyde which facilitate milling by preventing

The aqueous solubility of disperse dye particles in a dispersion increases with decreasing particle size (Kenneth & Skelly, 1973). Thus an increase in the severity of milling that accompanies a reduction in the particle size of the dye enhances the solubility and

Generally, in the commercial dyeing of polyester fibres with disperse dyes, dyeing is carried out within the pH range 5.5 to 6.5. Strongly alkaline or acidic conditions, such as higher than pH 9 and lower than pH 4, induce hydrolysis of the fibre as well as decomposition of azo disperse dyes (Nunn, 1979). In the case of high temperature dyeing, this degradation of

The substantivity of disperse dyes towards polyester fibres is one of the most critical factors in determining dyeing behaviour and there have been many studies carried out to evaluate the substantivity of disperse dyes towards hydrophobic fibres, including polyester, in order to select suitable dyes. These attempts include the 'Solubility Parameter Concept'

The adsorption and diffusion of disperse dyes on polyester are greatly influenced by temperature as an increase in temperature increases the mobility of the polymer chains in the amorphous regions of the fibre. Polyester fibres dye very slowly at temperatures much below 100C. At 85C, the temperature normally used for the dyeing of secondary cellulose acetate, it was found that polyester fibre dyed at rates between 700 and 1000 times slower than those measured for the same dyes on secondary cellulose acetate and nylon (Nunn, 1979). When, however, sufficient time was allowed for the polyester fibre to come nearly to equilibrium with the dyebath, it was found that it had taken up approximately the same amount of dye as had secondary acetate and in most cases about twice as much as nylon (Waters, 1950). For the dye-hydrophobic fibre system, the affinity of the disperse dye decreases with increase in dyeing temperature while the saturation value of the dye in the

(Ingamells, 1990) and 'IOR (Inorganic/Organic) values' (Kim et al, 2003).

fibre increases with increasing temperature (Bird et al, 1959; White, 1960).

**Crystal form of the disperse dyes** 

agglomeration of the dye particles. **Particle size in dispersion of the dyes** 

polyester fibre and dye is accelerated.

**Substantivity of disperse dyes** 

adsorption of disperse dye.

**pH of dyebath** 

**Temperature** 

(Biedermann, 1971, 1972).

**Dispersing agent** 

the transfer of dye molecules from a molecular dispersion into the fibre and because of the linearity of the isotherms obtained, the amount of dye adsorbed [D]ad relative to the concentration in the bath [D]s can be expressed by a partition coefficient K (A. Johnson, 1989), i.e.

$$\frac{[D]\_{al}}{[D]\_{\text{.}}} = \text{K} \tag{1}$$

As more dye is introduced into the system a point will be reached at which the amount of dye in the dyebath at equilibrium exceeds the solubility of dye. In the ideal case further additions of dye will produce no further change in the concentration of dye in solution, and hence no change in the concentration of dye on the fibre. At this point, therefore, if the abscissa denotes total dye in the bath rather than dye in solution, then the isotherm will become horizontal (A. Johnson, 1989).

Dye molecules that have been adsorbed on the fibre surface diffuse into the interior of the fibre by a relatively simple mechanism, which appears to obey Fick's equation (Patterson & Sheldon, 1959). That is to say, the rate of diffusion of dye through unit area (transverse to the direction of diffusion) at any point in the fibre is directly proportional to the concentration gradient of the dye at that point. As would be expected, the amount of dye taken up by polyester fibres from a bath of constant concentration is found to be proportional to the square root of the dyeing time, until a saturation value is approached. Very similar results are observed during the earlier stages of the process in dyebaths of normal composition and concentrations, such as are employed in commercial 'exhaustdyeing' processes. It is found that the rate of dyeing is quite independent of the concentration of the dyebath, practically up to the point at which equilibrium is established (Waters, 1950). For dyeings carried out at a constant temperature, a plot of the instantaneous fractional 'dye uptake' (Ct/C) against time of dyeing gives a steeply-rising asymptotic curve, which appears to fit a law based on the hyperbola or, possibly, on the hyperbolic tangent (Cigarra & Puente, 1967).

An important difference between the dyeing behaviour of polyester fibres and that of other fibres such as nylon and secondary acetate, which also accept disperse dyes, is in their rates of dyeing. Polyester fibres dye very slowly at temperatures much below 100C (Waters, 1950). Disperse dyes can be applied to cellulose secondary acetate readily over approximately 1 hour at 80C. Higher temperatures are avoided as otherwise acetate groups on the cellulosic fibre can be hydrolysed to hydroxyl groups, which can spoil the surface of the fibres and reduce their substantivity towards the disperse dyes. Cellulose triacetate is more difficult to penetrate with disperse dyes because of its more compact molecular structure, but it can be dyed at the boil. Nylon fibres can be dyed under conditions similar to those used for cellulose acetate fibres. In case of acrylic fibres, the presence of anionic groups such as –SO3H and –COOH permit only pale shades to be obtained under normal conditions with disperse dyes (Ingamells, 1993).

#### **4.2 The effects of variations in disperse dyeing**

Several factors affect the dyeing of polyester fibre with disperse dye such as crystal form of the dye, dispersing agents, particle size of the dye, pH of the dyebath, temperature of dyeing and heat setting, and fibre fineness.

the transfer of dye molecules from a molecular dispersion into the fibre and because of the linearity of the isotherms obtained, the amount of dye adsorbed [D]ad relative to the concentration in the bath [D]s can be expressed by a partition coefficient K (A. Johnson,

As more dye is introduced into the system a point will be reached at which the amount of dye in the dyebath at equilibrium exceeds the solubility of dye. In the ideal case further additions of dye will produce no further change in the concentration of dye in solution, and hence no change in the concentration of dye on the fibre. At this point, therefore, if the abscissa denotes total dye in the bath rather than dye in solution, then the isotherm will

Dye molecules that have been adsorbed on the fibre surface diffuse into the interior of the fibre by a relatively simple mechanism, which appears to obey Fick's equation (Patterson & Sheldon, 1959). That is to say, the rate of diffusion of dye through unit area (transverse to the direction of diffusion) at any point in the fibre is directly proportional to the concentration gradient of the dye at that point. As would be expected, the amount of dye taken up by polyester fibres from a bath of constant concentration is found to be proportional to the square root of the dyeing time, until a saturation value is approached. Very similar results are observed during the earlier stages of the process in dyebaths of normal composition and concentrations, such as are employed in commercial 'exhaustdyeing' processes. It is found that the rate of dyeing is quite independent of the concentration of the dyebath, practically up to the point at which equilibrium is established (Waters, 1950). For dyeings carried out at a constant temperature, a plot of the instantaneous fractional 'dye uptake' (Ct/C) against time of dyeing gives a steeply-rising asymptotic curve, which appears to fit a law based on the hyperbola or, possibly, on the hyperbolic

An important difference between the dyeing behaviour of polyester fibres and that of other fibres such as nylon and secondary acetate, which also accept disperse dyes, is in their rates of dyeing. Polyester fibres dye very slowly at temperatures much below 100C (Waters, 1950). Disperse dyes can be applied to cellulose secondary acetate readily over approximately 1 hour at 80C. Higher temperatures are avoided as otherwise acetate groups on the cellulosic fibre can be hydrolysed to hydroxyl groups, which can spoil the surface of the fibres and reduce their substantivity towards the disperse dyes. Cellulose triacetate is more difficult to penetrate with disperse dyes because of its more compact molecular structure, but it can be dyed at the boil. Nylon fibres can be dyed under conditions similar to those used for cellulose acetate fibres. In case of acrylic fibres, the presence of anionic groups such as –SO3H and –COOH permit only pale shades to be obtained under normal conditions

Several factors affect the dyeing of polyester fibre with disperse dye such as crystal form of the dye, dispersing agents, particle size of the dye, pH of the dyebath, temperature of

*<sup>D</sup>* (1)

[ ] [ ] *ad s <sup>D</sup> <sup>K</sup>*

1989), i.e.

become horizontal (A. Johnson, 1989).

tangent (Cigarra & Puente, 1967).

with disperse dyes (Ingamells, 1993).

**4.2 The effects of variations in disperse dyeing** 

dyeing and heat setting, and fibre fineness.
