**Dyeing with Disperse Dyes**

Joonseok Koh *Konkuk University South Korea* 

#### **1. Introduction**

194 Textile Dyeing

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Before the First World War, almost all dyes were applied from solution in an aqueous dyebath to substrates such as cotton, wool, silk and other natural fibres. However, the introduction of a man-made fibre, cellulose acetate, with its inherent hydrophobic nature, created a situation where very few of the available dyes had affinity for the new fibre. Water-soluble anionic dyes had little substantivity for the fibre and the alkaline conditions required for the application of vat dyes brought about a loss in tensile strength and deterioration in fibre appearance due to the rapid hydrolysis of acetyl groups.

The development of disperse dyes for dyeing secondary cellulose acetate fibres in the early 1920s was a major technological breakthrough although their major use today is for the coloration of polyesters, the most important group of synthetic fibres (Broadbent, 2001). The first systematic study of dyes that was suitable for application to cellulose acetate by a direct dyeing process was carried out by Green. The presence of hydroxyl and amino groups, a low relative molecular mass and an almost neutral or basic character were found to be advantageous. As a result of these investigations, in 1922, Green and Saunders developed the Ionamine Dyes (British Dystuffs Corporation) for application to acetate fibres (Green & Saunders, 1923; Green, 1924) (Fig. 1). These water-soluble dyes were hydrolyzed in the aqueous dyebath to produce the sparingly soluble free base in a very fine suspension that was then absorbed by the fibre. This discovery, that aqueous dispersions of almost waterinsoluble dyes were highly suitable for the dyeing of secondary acetate, lead to the rapid development of other such dyes for dyeing cellulose acetate.

Fig. 1. An Ionamine dye.

In 1923, aqueous dispersions of dyes were examined independently by the British Celanese Corporation and the British Dyestuffs Corporation and Ionamine dyes were superseded by ranges of disperse dyes, such as SRA (British Celanese Corporation) and Duranol (ICI), that were devoid of ionic solubilising groups (Fig. 2). These sparingly water-soluble acetate dyes were applied to cellulose acetate in the form a fine aqueous dispersion (Burkinshaw, 1995). The advent of other man-made fibres, such as nylon in 1938 and acrylic in the early 1940s, both of which possess a significant hydrophobic nature, further increased the use of disperse dyes. However, it was the discovery in 1941 and subsequent commercial introduction in

Dyeing with Disperse Dyes 197

desirable dyeing properties, whereas Class D dyes had high relative molecular masses and therefore good sublimation fastness, but somewhat poorer dyeing properties. Classes B and C were between these two extremes. Other dye manufacturing companies such as BASF

The Disperse Dye Committee of the Society of Dyers and Colourists has now classified the dyeing characteristics of disperse dyes according to the results of several tests which can be performed on the dyes. These tests assess characteristics such as the build up, levelling properties, rate of dyeing and temperature range properties of the dyes on a range of hydrophobic fibres. Small dye molecules with low polarity are levelling, rapid dyeing dyes with poor heat resistance. These are called low energy disperse dyes. More polar, higher molecular weight dyes have low dyeing rates, poor migration during dyeing but good heat and sublimation fastness. These constitute the high energy disperse dyes

A chemical classification by chromogen is very useful to dye chemists, particularly those who intend to synthesize dyestuffs, while the latter classification is more valuable for dyers. The Colour Index listed around 1,150 disperse dyes in 1992 by chemical class. i.e. azo, anthraquinone, meethine, nitrodiphenylamine, xanthene, aminoketone, quinoline and

Azo dyes are defined as compounds containing at least one azo group attached to sp2 hybridized carbon atoms, such as benzene, naphthalene, thiazole and thiophene. Under normal conditions, azo dyes exist in the more stable planar tras-form so that the carbon-

**N N**

Azo dyes are by far the most important class and account for more than 50% of the total commercialized disperse dyes in the world. There are several reasons why azo dyes have dominated the markets and have replaced many of the conventional anthraquinone dyes. In principal, the azo group is an inherently intense chromophore in terms of the tinctorial strength, when compared with the comparatively weak anthraquinone chromogen, normally 2 or 3 times stronger in tinctorial strength. By virtue of their simplicity and the ease of manufacture, unlike the other types of dyestuffs such as anthraquinone and styryl, the cost of manufacturing azo dyes is comparatively lower than the expensive

Azo dyes cover a whole gamut of colours, from yellow to blue-green hues, by varying the intermediates especially when heterocyclic diazo components are coupled to aminobenzene couplers substituted with powerful electron donating groups, giving bright blue-green colours. Although some deficiencies of azo dyes are not easily overcome, such as duller shades, lower fastness to light and breakdown into carcinogenic amines derived from the cleavage of the azo linkage, and have somewhat restricted their extensive usage against anthraquinone dyes, the cost effectiveness undoubtedly compensates for the drawbacks

**<sup>R</sup> .. ..**

nitrogen bond angle is approximately 120 (Fig. 3) (Gordon & Gregory, 1983).

**R**

soon introduced their own similar systems of disperse dye classification.

(Broadbent, 2001).

miscellaneous.

**2.1.1 Azo disperse dyes** 

Fig. 3. The trans-form of an azo dye.

mentioned above (Gordon & Gregory, 1983).

anthraquinone dyes.

1948, of polyester fibres that triggered an intensive research effort into disperse dyestuffs. Due to the highly crystalline and hydrophobic nature of polyester, the fibre is only slightly swollen by water up to the atmospheric boil which, in the 1950s was the maximum practical temperature available for dyeing. Consequently, new methods of application had to be developed. These new methods involved opening up the fibre structure temporarily so as to facilitate dye penetration (Fourness, 1979).

SRA Golden Yellow IX

Fig. 2. SRA range dyes by British Celanese Corporation.

When cellulose acetate is dyed at 85-95℃, small molecular size dyes are required which have an adequate solubility to dissolve and diffuse into the fibre. Such dyes possess high vapour pressure that did not normally cause a problem as the resulting dyeings were not usually subjected to high temperatures. However, with the advent of polyester fibres, problems with fastness arose due to dye sublimation during the subsequent heat setting and pleating processes. The vapour pressure of disperse dyes is related to the relative molecular mass of the dye and can therefore be decreased by increasing the relative molecular mass of the dye and by incorporating polar substituent groups into the dye molecule, such as benzoyl or amide groups. Incorporation of polar groups into the dye molecule causes a slight increase in water solubility in addition to increasing the relative molecular mass, thus giving rise to slower rates of diffusion. The decrease in rate of diffusion means that dyeing times have to be increased. However, the use of higher temperatures or carriers leads to increases in the rate of diffusion of dye molecules into the fibres.

#### **2. Disperse dyes**

Disperse dyes are traditionally non-ionic chemicals with sparing solubility in water which, consequently, are able to retain comparatively better substantivity for hydrophobic fibres, such as polyester, nylon and acetate. For the sake of efficient diffusion into textiles, the particles of disperse dye should be as fine as possible comprising low molecular weight molecules in the range of 400 – 600. It is essential for disperse dyes to be able to withstand various dyeing conditions, pH and temperature, resulting in negligible changes in shade and fastness (Aspland, 1992, 1993).

Disperse dyes are often substituted azo, anthraquinone or diphenylamine compounds which are non-ionic and contain no water solubilising groups. The dye particles are thus held in dispersion by the surface-active agent and the dyes themselves are called disperse dyes. They are marketed in the form of either an easily dispersible powder or a concentrated aqueous dispersion and are now the main class of dye for certain synthetic fibres (Ingamells, 1993).

#### **2.1 Classification**

In the 1970s, ICI developed a method for the classification of disperse dyes according to their sublimation fastness and dyeing properties, placing them into categories named A to D. Class A dyes had low relative molecular masses and hence poor sublimation fastness, but

1948, of polyester fibres that triggered an intensive research effort into disperse dyestuffs. Due to the highly crystalline and hydrophobic nature of polyester, the fibre is only slightly swollen by water up to the atmospheric boil which, in the 1950s was the maximum practical temperature available for dyeing. Consequently, new methods of application had to be developed. These new methods involved opening up the fibre structure temporarily so as to

N

NH2

NO2

SRA Golden Yellow IX

When cellulose acetate is dyed at 85-95℃, small molecular size dyes are required which have an adequate solubility to dissolve and diffuse into the fibre. Such dyes possess high vapour pressure that did not normally cause a problem as the resulting dyeings were not usually subjected to high temperatures. However, with the advent of polyester fibres, problems with fastness arose due to dye sublimation during the subsequent heat setting and pleating processes. The vapour pressure of disperse dyes is related to the relative molecular mass of the dye and can therefore be decreased by increasing the relative molecular mass of the dye and by incorporating polar substituent groups into the dye molecule, such as benzoyl or amide groups. Incorporation of polar groups into the dye molecule causes a slight increase in water solubility in addition to increasing the relative molecular mass, thus giving rise to slower rates of diffusion. The decrease in rate of diffusion means that dyeing times have to be increased. However, the use of higher temperatures or carriers leads to

Disperse dyes are traditionally non-ionic chemicals with sparing solubility in water which, consequently, are able to retain comparatively better substantivity for hydrophobic fibres, such as polyester, nylon and acetate. For the sake of efficient diffusion into textiles, the particles of disperse dye should be as fine as possible comprising low molecular weight molecules in the range of 400 – 600. It is essential for disperse dyes to be able to withstand various dyeing conditions, pH and temperature, resulting in negligible changes in shade

Disperse dyes are often substituted azo, anthraquinone or diphenylamine compounds which are non-ionic and contain no water solubilising groups. The dye particles are thus held in dispersion by the surface-active agent and the dyes themselves are called disperse dyes. They are marketed in the form of either an easily dispersible powder or a concentrated aqueous dispersion and are now the main class of dye for certain synthetic fibres (Ingamells,

In the 1970s, ICI developed a method for the classification of disperse dyes according to their sublimation fastness and dyeing properties, placing them into categories named A to D. Class A dyes had low relative molecular masses and hence poor sublimation fastness, but

O2N <sup>H</sup>

facilitate dye penetration (Fourness, 1979).

Fig. 2. SRA range dyes by British Celanese Corporation.

increases in the rate of diffusion of dye molecules into the fibres.

**2. Disperse dyes** 

1993).

**2.1 Classification** 

and fastness (Aspland, 1992, 1993).

desirable dyeing properties, whereas Class D dyes had high relative molecular masses and therefore good sublimation fastness, but somewhat poorer dyeing properties. Classes B and C were between these two extremes. Other dye manufacturing companies such as BASF soon introduced their own similar systems of disperse dye classification.

The Disperse Dye Committee of the Society of Dyers and Colourists has now classified the dyeing characteristics of disperse dyes according to the results of several tests which can be performed on the dyes. These tests assess characteristics such as the build up, levelling properties, rate of dyeing and temperature range properties of the dyes on a range of hydrophobic fibres. Small dye molecules with low polarity are levelling, rapid dyeing dyes with poor heat resistance. These are called low energy disperse dyes. More polar, higher molecular weight dyes have low dyeing rates, poor migration during dyeing but good heat and sublimation fastness. These constitute the high energy disperse dyes (Broadbent, 2001).

A chemical classification by chromogen is very useful to dye chemists, particularly those who intend to synthesize dyestuffs, while the latter classification is more valuable for dyers. The Colour Index listed around 1,150 disperse dyes in 1992 by chemical class. i.e. azo, anthraquinone, meethine, nitrodiphenylamine, xanthene, aminoketone, quinoline and miscellaneous.

#### **2.1.1 Azo disperse dyes**

Azo dyes are defined as compounds containing at least one azo group attached to sp2 hybridized carbon atoms, such as benzene, naphthalene, thiazole and thiophene. Under normal conditions, azo dyes exist in the more stable planar tras-form so that the carbonnitrogen bond angle is approximately 120 (Fig. 3) (Gordon & Gregory, 1983).

> **N N R <sup>R</sup> .. ..**

Fig. 3. The trans-form of an azo dye.

Azo dyes are by far the most important class and account for more than 50% of the total commercialized disperse dyes in the world. There are several reasons why azo dyes have dominated the markets and have replaced many of the conventional anthraquinone dyes. In principal, the azo group is an inherently intense chromophore in terms of the tinctorial strength, when compared with the comparatively weak anthraquinone chromogen, normally 2 or 3 times stronger in tinctorial strength. By virtue of their simplicity and the ease of manufacture, unlike the other types of dyestuffs such as anthraquinone and styryl, the cost of manufacturing azo dyes is comparatively lower than the expensive anthraquinone dyes.

Azo dyes cover a whole gamut of colours, from yellow to blue-green hues, by varying the intermediates especially when heterocyclic diazo components are coupled to aminobenzene couplers substituted with powerful electron donating groups, giving bright blue-green colours. Although some deficiencies of azo dyes are not easily overcome, such as duller shades, lower fastness to light and breakdown into carcinogenic amines derived from the cleavage of the azo linkage, and have somewhat restricted their extensive usage against anthraquinone dyes, the cost effectiveness undoubtedly compensates for the drawbacks mentioned above (Gordon & Gregory, 1983).

Dyeing with Disperse Dyes 199

A whole range of colour shades from yellow to even turquoise blue can be provided by variation of the substituents in different positions. Thus, 1,5-dihydroxyanthraquinone has a yellow shade and an absorption maximum at 425 nm, whereas 1,4,5,8 tetraaminoanthraquinone absorbs maximally at 609 nm, giving rise to a greenish-blue color. The inherent brightness of anthraquinone dyes, which exceeds that of typical azo dyes, may be accounted for by their unique fluorescence emitted by the transition from the first singlet

Simple anthraquinone disperse dyes containing alkylamino or hydroxy groups (e.g. C.I. Disperse Violet 4, C.I. Disperse Red 15) can be used for the coloration of cellulose acetate, although of limited color range and of moderate fastness. When more hydrophobic substituents are introduced to enhance the affinity for polyester fibre, various shades can be produced. Among these types of anthraquinone dyes, a bright red dye (C.I. Disperse Red 60) and a violet dye (C.I. Disperse Violet 26) have been widely used in the coloration industry, particularly C.I. Disperse Red 60 which acts as a basic colour for trichromic combinations in pale shades and is one of the best dyes available for heat transfer printing. These dyes show exceptional brilliance and high light fastness against similar colours of azo

Recent literature has revealed that more bathochromic shifts can be obtained by condensing various arylamines with 4-arylamino-5-nitro-1,8-dihydroxy anthraquinone or with 4,8 dichloroquinizarin or 4,5-dinitrochrysazin to give deep blush-green shades on polyester (Peters & Chao; Yu & Chao]. Although anthraquinone disperse dyes have many advantages such as bright shades, high light fastness, good stability in dyeing and excellent levelling, serious economic drawbacks and inevitable pollution problems ensure the continuing displacement of these dye by other types, such as benzodifuranone dyes and azo dyes having heterocyclic diazo components (Griffiths, 1984; Weaver & Shuttleworth, 1982).

Although anthraquinone, monoazo and disazo disperse dyes are the most important classes of disperse dyes in terms of market share, there are a number of other important classes as

excited state to the ground state (Gordon & Gregory, 1983).

Fig. 7. Commercial anthraquinone disperse dyes.

**2.1.3 Other disperse dye classes** 


follows ;

dyes (Fig. 7).

Aminoazobenzene dyes, which can be represented by the general structure (Fig. 4), have been by far most important disperse dyes particularly during the early period of progress for the coloration of polyester fibres.

Fig. 4. General structure of aminoazobenzene dyes.

As a typical donor-acceptor chromogen, the electron-accepting substituents, X, Y and Z and the electron donating substituents R1 and R2 are favourably sited to create visible colors from yellows to reds, and more recently, blues (Fig. 5). In general, aminobenzenes are easily diazotized by normal diazotization reagents and the resultant diazonium salts are comparatively stable so that high yields of dyes of good purity are obtained.

Fig. 5. Some typical azo disperse dye structures.

#### **2.1.2 Anthraquinone disperse dyes**

Anthraquinone disperse dyes were among the early 'acetate' dyes and have made an important contribution to the violet and blue shade range. They produce bright dyeings of excellent light fastness and cause no dye stability problems during dyeing.

From a historical point of view, anthraquinone dyes are the oldest to mimic a natural chromogen. As seen from the Colour Index Classification, anthraquinone systems of basic formula (Fig. 6) are second only to the azo chromogen for the manufacture of disperse dyes.

Fig. 6. Anthraquinone systems of basic formula ; 9,10-anthraquinone.

Aminoazobenzene dyes, which can be represented by the general structure (Fig. 4), have been by far most important disperse dyes particularly during the early period of progress

As a typical donor-acceptor chromogen, the electron-accepting substituents, X, Y and Z and the electron donating substituents R1 and R2 are favourably sited to create visible colors from yellows to reds, and more recently, blues (Fig. 5). In general, aminobenzenes are easily diazotized by normal diazotization reagents and the resultant diazonium salts are

Anthraquinone disperse dyes were among the early 'acetate' dyes and have made an important contribution to the violet and blue shade range. They produce bright dyeings of

From a historical point of view, anthraquinone dyes are the oldest to mimic a natural chromogen. As seen from the Colour Index Classification, anthraquinone systems of basic formula (Fig. 6) are second only to the azo chromogen for the manufacture of disperse dyes.

excellent light fastness and cause no dye stability problems during dyeing.

Fig. 6. Anthraquinone systems of basic formula ; 9,10-anthraquinone.

comparatively stable so that high yields of dyes of good purity are obtained.

for the coloration of polyester fibres.

Fig. 4. General structure of aminoazobenzene dyes.

Fig. 5. Some typical azo disperse dye structures.

**2.1.2 Anthraquinone disperse dyes** 

A whole range of colour shades from yellow to even turquoise blue can be provided by variation of the substituents in different positions. Thus, 1,5-dihydroxyanthraquinone has a yellow shade and an absorption maximum at 425 nm, whereas 1,4,5,8 tetraaminoanthraquinone absorbs maximally at 609 nm, giving rise to a greenish-blue color. The inherent brightness of anthraquinone dyes, which exceeds that of typical azo dyes, may be accounted for by their unique fluorescence emitted by the transition from the first singlet excited state to the ground state (Gordon & Gregory, 1983).

Simple anthraquinone disperse dyes containing alkylamino or hydroxy groups (e.g. C.I. Disperse Violet 4, C.I. Disperse Red 15) can be used for the coloration of cellulose acetate, although of limited color range and of moderate fastness. When more hydrophobic substituents are introduced to enhance the affinity for polyester fibre, various shades can be produced. Among these types of anthraquinone dyes, a bright red dye (C.I. Disperse Red 60) and a violet dye (C.I. Disperse Violet 26) have been widely used in the coloration industry, particularly C.I. Disperse Red 60 which acts as a basic colour for trichromic combinations in pale shades and is one of the best dyes available for heat transfer printing. These dyes show exceptional brilliance and high light fastness against similar colours of azo dyes (Fig. 7).

Fig. 7. Commercial anthraquinone disperse dyes.

Recent literature has revealed that more bathochromic shifts can be obtained by condensing various arylamines with 4-arylamino-5-nitro-1,8-dihydroxy anthraquinone or with 4,8 dichloroquinizarin or 4,5-dinitrochrysazin to give deep blush-green shades on polyester (Peters & Chao; Yu & Chao]. Although anthraquinone disperse dyes have many advantages such as bright shades, high light fastness, good stability in dyeing and excellent levelling, serious economic drawbacks and inevitable pollution problems ensure the continuing displacement of these dye by other types, such as benzodifuranone dyes and azo dyes having heterocyclic diazo components (Griffiths, 1984; Weaver & Shuttleworth, 1982).
