**2.2.2 Fastness to dry heat**

Sublimation, or dry heat, fastness can be an important property of disperse-dyed polyester because of the use of heat treatments in the finishing of the fabric; disperse dyes must be small, non-ionic molecules of low molecular weight. As such, they often exhibit a significant vapour pressure. Therefore, if heat treatments during fixation, or subsequently, e.g. ironing, are involved, the dye may sublime and cause contamination of equipment or adjacent undyed material (Gordon & Gregory).

The heat fastness of dyeings of equal dye concentration on the same substrate will, at a specific temperature and time, be dependent on the size and polarity of the molecules of the

Dyeing with Disperse Dyes 203

Many dyes are known to fade when left exposed to sunlight for prolonged periods. Such photofading is a well known technological phenomenon within the textile industry (Zollinger, 1991). In particular, certain applications of textiles dyes require higher light fastness quality control than others and this has led to an accepted light fastness scale that facilitates selection of a dye for a give application (Oakes, 2001). Although a close relationship does exist between the chemical structure of a dye and its light fastness rating, it is important to remember that other factors are of relevance, the most critical of which are

a. The inherent photostability of the dye molecule; in general, the chromophoric nucleus is the most important element in determining the light fastness of a dye, but nuclear

b. The concentration of the dye within the fibre; usually the fastness of a dyed fibre

c. The nature of the fibre in which the dye is dispersed; different fibres contain different chemical groups and these substituents can have a significant effect on the light fastness

d. The wavelength distribution of the incident radiation; not all the absorption are equally

e. The composition of the atmosphere; the moisture content of the atmosphere can have a

A survey of commercial disperse dyes applied to various substrates has revealed that the average light fastness is highest on polyester, followed by di-, tri-acetate and then nylon, although this could simply be an artefact of commercial usage and research concentrating on dyes for polyester and acetate as these are the more important substrates for this dye

However, anomalously low fastness arises with *o*-nitro substitution. The deleterious effect has been ascribed to the proximity of the negative charge on the nearest oxygen of the nitro function to the azo group preventing electron delocalization, ultimately lowering the bond order of the -nitrogen-carbon link and therefore increasing susceptibility to photolytic

In the 4-aminoazobenzene series, variation of the terminal amino groups provides an important means of increasing light fastness on polyester. The use of -cyanoethyl groups appear advantageous, while that of -hydroxyethyl groups, useful in dyes for diacetate, is

Fig. 9. Modified dyes for improved sublimation fastness.

substituents may alter the fastness significantly.

marked effect on the fading rates of certain dyes.

increases with increasing dye concentrations.

rating of a dye on a given fibre.

cleavage (Mehta & Peters, 1981).

effective in starting a fading process.

**2.2.3 Fastness to light** 

(Giles & McKay, 1963):

class.

dyes involved which in turn determine the rate of diffusion of the dye within the substrate and the volatility of the dye. Increases in size and/or polarity will tend to reduce the rate of diffusion out of the fibre and the vapour pressure of the dye and thus increase sublimation fastness. The same changes will tend to make application by exhaustion methods more problematical as the lowered diffusion coefficient will mean slower fibre-penetration, reduced levelling power and lower colour yields. Therefore, a compromise between the decrease in hydrophobicity (and consequent loss in fibre affinity) and polarity has to be made. Because of this property, disperse dyes have been classified into types of excellent dyeing properties but poor heat fastness (Class A) through to those with excellent heat fastness but poor dyeing properties (Class D) (Nunn, 1978).

Fig. 8. Recently developed disperse dyes with high wash fastness

The structural changes useful in raising heat fastness include modification of the side chains of the terminal amino nitrogen (Hallas, 1979). However, other balances must be kept in mind; the hydroxyl group is a particularly effective group, but can have an adverse effect on light fastness on polyester. Increasing mass may also play a small part in the effectiveness of these groups. A 2-acetylamino group in the donor ring can have a beneficial effect on heat fastness (Gordon & Gregory, 1983), the tendency to sublime decreasing as the polarity and the number of potential hydrogen-bonding groups increases .

Generally the effects of substitution in the diazo residue are less marked. Replacement of either carbocyclic ring for a heterocycle can raise fastness, as well as substitution on the heterocycle itself, although in some cases, little difference has been found, for example between some thiazole and phenyl analogues (Peters & Cheung, 1985).

Improved sublimation fastness can be achieved by increasing the molecular size and by incorporating additional hydrophobic groups into anthraquinone ring. Thus, the modified dyes such as C.I. Disperse Red 92 and C.I. Disperse Red 229 have been designed to improve the moderate sublimation fastness of C.I. Disperse Red 60 by introducing additional substituents that leave the brilliant hue intact (Fig. 9).

dyes involved which in turn determine the rate of diffusion of the dye within the substrate and the volatility of the dye. Increases in size and/or polarity will tend to reduce the rate of diffusion out of the fibre and the vapour pressure of the dye and thus increase sublimation fastness. The same changes will tend to make application by exhaustion methods more problematical as the lowered diffusion coefficient will mean slower fibre-penetration, reduced levelling power and lower colour yields. Therefore, a compromise between the decrease in hydrophobicity (and consequent loss in fibre affinity) and polarity has to be made. Because of this property, disperse dyes have been classified into types of excellent dyeing properties but poor heat fastness (Class A) through to those with excellent heat

fastness but poor dyeing properties (Class D) (Nunn, 1978).

Fig. 8. Recently developed disperse dyes with high wash fastness

the number of potential hydrogen-bonding groups increases .

substituents that leave the brilliant hue intact (Fig. 9).

between some thiazole and phenyl analogues (Peters & Cheung, 1985).

The structural changes useful in raising heat fastness include modification of the side chains of the terminal amino nitrogen (Hallas, 1979). However, other balances must be kept in mind; the hydroxyl group is a particularly effective group, but can have an adverse effect on light fastness on polyester. Increasing mass may also play a small part in the effectiveness of these groups. A 2-acetylamino group in the donor ring can have a beneficial effect on heat fastness (Gordon & Gregory, 1983), the tendency to sublime decreasing as the polarity and

Generally the effects of substitution in the diazo residue are less marked. Replacement of either carbocyclic ring for a heterocycle can raise fastness, as well as substitution on the heterocycle itself, although in some cases, little difference has been found, for example

Improved sublimation fastness can be achieved by increasing the molecular size and by incorporating additional hydrophobic groups into anthraquinone ring. Thus, the modified dyes such as C.I. Disperse Red 92 and C.I. Disperse Red 229 have been designed to improve the moderate sublimation fastness of C.I. Disperse Red 60 by introducing additional

Fig. 9. Modified dyes for improved sublimation fastness.

## **2.2.3 Fastness to light**

Many dyes are known to fade when left exposed to sunlight for prolonged periods. Such photofading is a well known technological phenomenon within the textile industry (Zollinger, 1991). In particular, certain applications of textiles dyes require higher light fastness quality control than others and this has led to an accepted light fastness scale that facilitates selection of a dye for a give application (Oakes, 2001). Although a close relationship does exist between the chemical structure of a dye and its light fastness rating, it is important to remember that other factors are of relevance, the most critical of which are (Giles & McKay, 1963):


A survey of commercial disperse dyes applied to various substrates has revealed that the average light fastness is highest on polyester, followed by di-, tri-acetate and then nylon, although this could simply be an artefact of commercial usage and research concentrating on dyes for polyester and acetate as these are the more important substrates for this dye class.

However, anomalously low fastness arises with *o*-nitro substitution. The deleterious effect has been ascribed to the proximity of the negative charge on the nearest oxygen of the nitro function to the azo group preventing electron delocalization, ultimately lowering the bond order of the -nitrogen-carbon link and therefore increasing susceptibility to photolytic cleavage (Mehta & Peters, 1981).

In the 4-aminoazobenzene series, variation of the terminal amino groups provides an important means of increasing light fastness on polyester. The use of -cyanoethyl groups appear advantageous, while that of -hydroxyethyl groups, useful in dyes for diacetate, is

Dyeing with Disperse Dyes 205

A classic anthraquinone dye, C.I. Disperse Blue 56 was first synthesized to dye cellulose acetate during the late 1950s, featuring a bright neutral blue shade and excellent level dyeing. However, the manufacture of this dye gives rise to environmental problems and is economically disadvantageous. Nevertheless, this dye remains of limited application in

Disperse dyes can be applied to a whole range of chemically diverse, hydrophobic manmade fibres, which include acetate, acrylic, modacrylic, nylon, polyester and polyurethane fibres. However, of these, the most important fibres for disperse dyeing are polyester and

Polyester fibre is a manufactured fibre composed of synthetic linear macromolecules having in the chain at least 85% (by mass) of an ester of a diol and benzene-1,4-dicarboxylic acid(terephthalic acid) (Denton & Daniels, 2002). Fibres of the most common polyester, Poly(ethylene terephthalate) (PET or PES) is generally made from either terephthalic acid or dimethyl terephthalate together with ethylene glycol (Fig. 12) (Lewin & Pearce, 1985).

The first commercial fibre-forming polyester was developed by Dickson and Whinfield working at the Calico Printers' Association in England in 1941. It was produced by condensation of ethylene glycol and terephthalic acid. Rights to manufacture this product were bought by ICI and DuPont. The ICI product was named Terylene and the DuPont

Polyester fibres are produced as medium- and high-tenacity filament yarns and as staple fibres of various lengths and fibre colour to suit the kinds of spinning machinery found in the textile trade. Polyester fibre not only offers the typical features of a synthetic fibre, such as high chemical resistance, high moth-proofness, excellent wash and wear and permanent press characteristics, but also, when blended with cotton and wool, gives rise to fabrics of high quality. These unique properties make it the largest commodity fibre in the synthetic fibre world, in 1990, polyester production exceeded the total production of both polyamide

Recent developments include new polymer compositions, physical characteristics improvement, enhanced aesthetic quality and improved dyeability (Fig. 13). Developments in physical characteristics include decreasing the fibre's intensity so as to facilitate the rupture and removal of objectable 'fuzz balls' (pilling-resistant fibre) and anti-static finishing through the application of the hydrophilic finish which creates capillarity in the interfibre spaces (Brunnschweiler & Hearle, 1993). As for improvements in dyeing properties, modified polyester has been introduced that contains ionic sites to facilitate ionic

areas that require good light fastness in pale depths.

secondary cellulose acetate (J. R. Aspland, 1997).

Fig. 12. Polymerization of poly(ethylene terephthalate).

and polyacrylic fibres (Brunnschweiler & Hearle, 1993).

**3. Hydrophobic fibres** 

**3.1 Polyester fibres** 

**3.1.1 Historical background** 

product Dacron (A. D. Broadbent).

here inexpedient, although *o*-acylation brings about a striking increase in fastness (Kassim & Peters, 1973)

Oxidation and reduction reactions are known as the two most important pathways for the fading of dyes. Although photooxidative processes are generally accepted as being responsible for fading on non-protein substrates, such as polyester and acetate, there is little direct evidence (Fig. 10).

Fig. 10. Oxidative fading mechanism of azo dye : the conversion of the azo group to an azoxy group.

Deductions from Hammett plots provide indirect affirmation (Gordon & Gregory, 1983) as well as increased fading in the presence of oxygen. Direct evidence has however been obtained of an oxidative pathway, the dealkylation of 4-*N*,*N*-dialkylaminoazobenzenes on polyester and nylon in which dye-sensitized singlet oxygen attacks the terminal nitrogen lone pair electrons of the dye in the ground state. While the dealkylated products will not differ radically in colour, the creation of carbonyl species and peroxides may cause destruction of the azo group, leading to more significant fading.

Intensive investigations have concentrated on improving the molecular stability to light, and elucidating some empirical correlations between the electronic behaviour of substituents and their resistance to light. Anthraquinone dyes containing electron-donating substituents are more susceptible to fading than those containing groups, such as –NHCOPh, which contribute towards better light fastness (Dawson, 1983).

Further incorporation of less basic substituents, notably heterocyclic moieties, into positions of 1,4-diaminoanthraquinone leads to bright turquoise blue dyes, for example C.I. Disperse Blue 60 and Blue 87 that were originally developed by DuPont and BASF in 1955 and 1963, respectively (Fig. 11). The superior light fastness of these dyes is attributed to the electron-withdrawing effect exerted by the imide groups (Dawson, 1983). These two bright dyes are still preferred for the coloration of polyester in intrinsic turquoise shades and in a bright green by the addition of an appropriate yellow dye.

Fig. 11. Modified dyes for improved light fastness.

here inexpedient, although *o*-acylation brings about a striking increase in fastness (Kassim &

Oxidation and reduction reactions are known as the two most important pathways for the fading of dyes. Although photooxidative processes are generally accepted as being responsible for fading on non-protein substrates, such as polyester and acetate, there is little

Fig. 10. Oxidative fading mechanism of azo dye : the conversion of the azo group to an

destruction of the azo group, leading to more significant fading.

contribute towards better light fastness (Dawson, 1983).

bright green by the addition of an appropriate yellow dye.

Fig. 11. Modified dyes for improved light fastness.

Deductions from Hammett plots provide indirect affirmation (Gordon & Gregory, 1983) as well as increased fading in the presence of oxygen. Direct evidence has however been obtained of an oxidative pathway, the dealkylation of 4-*N*,*N*-dialkylaminoazobenzenes on polyester and nylon in which dye-sensitized singlet oxygen attacks the terminal nitrogen lone pair electrons of the dye in the ground state. While the dealkylated products will not differ radically in colour, the creation of carbonyl species and peroxides may cause

Intensive investigations have concentrated on improving the molecular stability to light, and elucidating some empirical correlations between the electronic behaviour of substituents and their resistance to light. Anthraquinone dyes containing electron-donating substituents are more susceptible to fading than those containing groups, such as –NHCOPh, which

Further incorporation of less basic substituents, notably heterocyclic moieties, into positions of 1,4-diaminoanthraquinone leads to bright turquoise blue dyes, for example C.I. Disperse Blue 60 and Blue 87 that were originally developed by DuPont and BASF in 1955 and 1963, respectively (Fig. 11). The superior light fastness of these dyes is attributed to the electron-withdrawing effect exerted by the imide groups (Dawson, 1983). These two bright dyes are still preferred for the coloration of polyester in intrinsic turquoise shades and in a

Peters, 1973)

azoxy group.

direct evidence (Fig. 10).

A classic anthraquinone dye, C.I. Disperse Blue 56 was first synthesized to dye cellulose acetate during the late 1950s, featuring a bright neutral blue shade and excellent level dyeing. However, the manufacture of this dye gives rise to environmental problems and is economically disadvantageous. Nevertheless, this dye remains of limited application in
