Disperse dye; Introductive article

INTRODUCTION TO DISPERSE DYES

Cotton can be dyed with anionic direct, sulphur, vat, reactive and azoic dyes. These types of dyes, however, are of little use for the dyeing of synthetic fibres. Disperse dyes, on the other hand, are non-ionic. They dye all the synthetic and cellulose acetate fibres using a direct dyeing technique. Only the dyeing temperature varies from one fibre to another. They are thus one of the major classes of dyestuff. The development of disperse dyes for dyeing secondary cellulose acetate fibres in the early 1920s was a major technological breakthrough. Their major use today is for the coloration of polyesters, the most important group of synthetic fibres.

What is a disperse dye? These non-ionic dyes are relatively insoluble in water at room temperature and have only limited solubility at higher temperatures. They do, however, possess substantivity for hydrophobic fibres such as nylon and polyester, in which they are quite soluble. As their name implies, these dyes are present in the dyebath as a fine aqueous suspension in the presence of a dispersing

agent. The water dissolves a small amount of the dye in monomolecular form. The hydrophobic fibres then absorb the dye from the solution. Because these dyes are non-ionic organic compounds of relatively low molecular weight, many sublime on heating and dyeing by absorption of the dye vapour is also possible.

When cellulose diacetate fibres first appeared in 1921, few of the available ionic dyes were able to successfully colour them. Secondary acetate fibres absorb little water, do not swell and have only small pores. In addition, their surface potential is much more negative than that of cotton and therefore they repel anionic dyes. Although cellulose diacetate fibres will absorb some cationic dyes, and a few acid

dyes, there is little or no penetration of the dye unless the fibre is pre-swollen. The dyeings produced using ionic dyes also have poor fastness properties. The first dyes for cellulose acetate fibres were water soluble.

The dye molecules contained a methylamino sulphonate group (–NHCH2SO3Na) introduced by reaction of a primary amino group with formaldehyde and sodium bisulphate (Ionamine dyes, 1922). During dyeing, this group hydrolysed to the less soluble parent amine. It was soon recognised that it was this compound that the cellulose acetate absorbed. The first true disperse dyes were simple, relatively insoluble azo and anthraquinone compounds dispersed in water using the sodium salt of sulphated ricinoleic acid. Many of these dyes are obsolete but their development provided the technology for preparing fine aqueous dispersions by grinding the dye with dispersing agents. A fine dispersion is essential for rapid dyeing and avoids deposition of larger dye particles on the material. Disperse dyes have slight water solubility because of the presence of polar substituents in their molecular structures. During dyeing, a small quantity of dye is present in true aqueous solution in a monomolecular form. These dye molecules are able to penetrate into hydrophobic artificially-made fibres such as those of cellulose acetate, nylon or polyester. The dye is much more soluble in the fibre than in water so deep dyeings are possible. The dye particles in dispersion are very small. Their large specific surface area ensures rapid solution to maintain saturation of the aqueous solution as the soluble dye transfers to the fibre. Dyeing consists of a solubility equilibrium coupled to solid solvent extraction equilibrium 

Dyeing isotherms, graphs of Cf (concentration of dye in the fibre) as a function of Cs (concentration of dye in solution) at a given constant temperature, are of the linear Nernst type. This is true whether the equilibrium is established starting with undyed fibres and an aqueous dispersion of the dye, or with the dyed fibre and water. The final point on the linear portion of the isotherm gives the solubilities of the dye in the fibre and in water. Beyond this point, the dye saturated fibre is in equilibrium with the saturated aqueous solution, which is in equilibrium with solid dye particles. The slope of the isotherm gives the value of the partition coefficient (Cf/Cs). This dyeing equilibrium constant decreases with increasing temperature since dyeing is exothermic.