**2.1.3 Other disperse dye classes**

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 follows ;


Dyeing with Disperse Dyes 201

treatments made fastness more of a problem in the case of some dyes; the high temperatures lead to thermomigration where dye molecules tend to move from the core of the fibre towards the surface, and thus counteract the reduction clearing and lower the wash fastness. In the early 1980s, an official range of European wash test (the ISO 105 C06 series) was published that reflected the widespread adoption of detergents for domestic laundering, which with dyed polyester often gave lower ratings than with soap-based tests: any dye stripped from the fibre by the soap tends to remain solubilised through the powerful surfactant properties of the soap present in a relatively large concentration. Detergent, containing much less surfactant, is able to remove surface dye, but is not as effective at solubilising it, promoting staining although the inclusion of perborate bleaching agents can counteract this. This together with the now routine use of multifibre adjacent fabrics, has rendered many previously satisfactory dyes unsuitable and made dye selection far more important in this respect. An extreme requirement is that for polyester microfibres: the high surface area to mass ratio of these materials means that higher than normal depths of dyeing are needed for satisfactory shades. An investigation has suggested that planar dyes of low relative molecular mass (for satisfactory build up) and of high tinctorial strength may meet the challenge (Chao & Chen, 1994). During the 1990s, the development of novel wash test procedures has continued in order to counter perceived changes in the market such as innovations in detergent formulation. The appearance of a new generation of protocols like BS 1006 UK-TO and ISO 105:C08 is a sign that fastness criteria will keep evolving and so present fresh challenges to which disperse dye technology must respond

A range of test has been devised to simulate the action of various other wet agencies including perspiration. Much effort has been expended on trying to find ways to mimic human sweat, which is a complex mixture containing salt and amino acids (Heir et al, 1946). Fresh perspiration is slightly acidic, but goes alkaline with bacterial action (Perspiration Fastness Subcommittee, 1952), hence the development of two test, one at pH 5.5, the other at pH 8. Disperse-dyed polyester generally performs well because its structure is still close-packed at body temperature (the level at which the test samples are

Recently, improved wet fastness was achieved by incorporating easily washable groups into dye structures. Thus, the modified dyes containing diesters, thiophenes, benzodifuranones, phthalimides and fluorosulfonyl group have been designed to improve the wash fastness (Fig. 8) (Koh & Kim, 1998a, 1998b; Koh & Greaves, 2001). The most often patented dyes with enhanced wash fastness and build-up properties were based on heterocyclic amines such as 2-aminothiophene, and 2-aminopyrazole, and were used separately or in combination

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

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

(Choi et al, 2000).

maintained).

(Freeman and Sokolowska, 1999).

undyed material (Gordon & Gregory).

**2.2.2 Fastness to dry heat** 


#### **2.2 Constitution and fastness properties**

Many attempts have been made to trace relationships between the chemical structure of a dye and its fastness properties. However, such inter-relationships can be very complex. Some general trends can be distinguished but, within these, more subtle interactions involving electronic and steric effects make it very difficult to set down hard and fast rules. In recent years, more precise information has been obtained by studying groups of very closely related dyes in which minor structural changes have been shown to influence fastness properties.

Satisfactory fastness to light, washing, rubbing, sublimation and (on acetate) burnt gas fumes, is particularly significant for disperse dyes (Dawson, 1984).

### **2.2.1 Fastness to wet treatment**

The demand for environmentally friendly dyes of high wet fastness on polyester is increasing. As a result of current trends towards the increased use of finishing treatments, microfibers and stringent wash fastness test criteria, previously satisfactory colorants are no longer suitable and the performance of agents of relatively high fastness can be rendered ordinary (Choi et al).

Fastness to washing is similarly dependent on diffusion as it is unlikely during the limited time of a washing cycle or test that an equilibrium between the dye inside and outside the fibre will be reached. Consequently, the fastness will be subject to kinetic control: other things being equal, increases in size and polarity of the dye molecule will reduce its mobility within the fibre and be reflected in a lower diffusion coefficient and an attendant increase in wash fastness. The nature of the disperse-dyed substrate is also very important. Polyester tends to have better fastness to wet treatments such as washing and perspiration fastness than nylon and acetate with a given dye since its structure is more crystalline and hydrophobic. Both properties restrict diffusion, the former directly, by presenting a closepacked structure as an obstacle to movement, and the later directly by reduced fibreswelling which serves to keep the structure tightly packed. Heat fastness/dyeing trade-off, changes in diffusion characteristics to improve wash fastness will have an opposite effect on dyeability.

In terms of providing satisfactory wash fastness on polyester, dye selection has become far more critical than it ever has been, because of the more demanding wash fastness tests employed currently and the now widespread use of aftertreatments. Before the 1970s, the universal wash-fastness trial for dyed polyester was the ISO 105 C03 wash test, which simulated domestic washing with soap and examined staining on polyester and cotton or wool. Nearly all disperse dyes gave very good to excellent results. However, from the 1970s onwards, achieving satisfactory fastness to washing became more problematic as the result of a customer-driven rise in standards being implemented in the guise of more severe testing protocols (Baldwinson, 1989). Stringent conditions were stipulated in the late 1960s by Marks and Spencer, who required their suppliers to use only those dyes that met criteria using adjacent nylon, which showed a greater tendency to stain than wool or cotton. Heat

Many attempts have been made to trace relationships between the chemical structure of a dye and its fastness properties. However, such inter-relationships can be very complex. Some general trends can be distinguished but, within these, more subtle interactions involving electronic and steric effects make it very difficult to set down hard and fast rules. In recent years, more precise information has been obtained by studying groups of very closely related dyes in which minor structural changes have been shown to influence

Satisfactory fastness to light, washing, rubbing, sublimation and (on acetate) burnt gas

The demand for environmentally friendly dyes of high wet fastness on polyester is increasing. As a result of current trends towards the increased use of finishing treatments, microfibers and stringent wash fastness test criteria, previously satisfactory colorants are no longer suitable and the performance of agents of relatively high fastness can be rendered

Fastness to washing is similarly dependent on diffusion as it is unlikely during the limited time of a washing cycle or test that an equilibrium between the dye inside and outside the fibre will be reached. Consequently, the fastness will be subject to kinetic control: other things being equal, increases in size and polarity of the dye molecule will reduce its mobility within the fibre and be reflected in a lower diffusion coefficient and an attendant increase in wash fastness. The nature of the disperse-dyed substrate is also very important. Polyester tends to have better fastness to wet treatments such as washing and perspiration fastness than nylon and acetate with a given dye since its structure is more crystalline and hydrophobic. Both properties restrict diffusion, the former directly, by presenting a closepacked structure as an obstacle to movement, and the later directly by reduced fibreswelling which serves to keep the structure tightly packed. Heat fastness/dyeing trade-off, changes in diffusion characteristics to improve wash fastness will have an opposite effect on

In terms of providing satisfactory wash fastness on polyester, dye selection has become far more critical than it ever has been, because of the more demanding wash fastness tests employed currently and the now widespread use of aftertreatments. Before the 1970s, the universal wash-fastness trial for dyed polyester was the ISO 105 C03 wash test, which simulated domestic washing with soap and examined staining on polyester and cotton or wool. Nearly all disperse dyes gave very good to excellent results. However, from the 1970s onwards, achieving satisfactory fastness to washing became more problematic as the result of a customer-driven rise in standards being implemented in the guise of more severe testing protocols (Baldwinson, 1989). Stringent conditions were stipulated in the late 1960s by Marks and Spencer, who required their suppliers to use only those dyes that met criteria using adjacent nylon, which showed a greater tendency to stain than wool or cotton. Heat

fumes, is particularly significant for disperse dyes (Dawson, 1984).


fastness properties.

ordinary (Choi et al).

dyeability.

**2.2.1 Fastness to wet treatment** 

**2.2 Constitution and fastness properties** 

treatments made fastness more of a problem in the case of some dyes; the high temperatures lead to thermomigration where dye molecules tend to move from the core of the fibre towards the surface, and thus counteract the reduction clearing and lower the wash fastness. In the early 1980s, an official range of European wash test (the ISO 105 C06 series) was published that reflected the widespread adoption of detergents for domestic laundering, which with dyed polyester often gave lower ratings than with soap-based tests: any dye stripped from the fibre by the soap tends to remain solubilised through the powerful surfactant properties of the soap present in a relatively large concentration. Detergent, containing much less surfactant, is able to remove surface dye, but is not as effective at solubilising it, promoting staining although the inclusion of perborate bleaching agents can counteract this. This together with the now routine use of multifibre adjacent fabrics, has rendered many previously satisfactory dyes unsuitable and made dye selection far more important in this respect. An extreme requirement is that for polyester microfibres: the high surface area to mass ratio of these materials means that higher than normal depths of dyeing are needed for satisfactory shades. An investigation has suggested that planar dyes of low relative molecular mass (for satisfactory build up) and of high tinctorial strength may meet the challenge (Chao & Chen, 1994). During the 1990s, the development of novel wash test procedures has continued in order to counter perceived changes in the market such as innovations in detergent formulation. The appearance of a new generation of protocols like BS 1006 UK-TO and ISO 105:C08 is a sign that fastness criteria will keep evolving and so present fresh challenges to which disperse dye technology must respond (Choi et al, 2000).

A range of test has been devised to simulate the action of various other wet agencies including perspiration. Much effort has been expended on trying to find ways to mimic human sweat, which is a complex mixture containing salt and amino acids (Heir et al, 1946). Fresh perspiration is slightly acidic, but goes alkaline with bacterial action (Perspiration Fastness Subcommittee, 1952), hence the development of two test, one at pH 5.5, the other at pH 8. Disperse-dyed polyester generally performs well because its structure is still close-packed at body temperature (the level at which the test samples are maintained).

Recently, improved wet fastness was achieved by incorporating easily washable groups into dye structures. Thus, the modified dyes containing diesters, thiophenes, benzodifuranones, phthalimides and fluorosulfonyl group have been designed to improve the wash fastness (Fig. 8) (Koh & Kim, 1998a, 1998b; Koh & Greaves, 2001). The most often patented dyes with enhanced wash fastness and build-up properties were based on heterocyclic amines such as 2-aminothiophene, and 2-aminopyrazole, and were used separately or in combination (Freeman and Sokolowska, 1999).
