Direct print coloration

Printing can be considered as a controlled form of localised dyeing and, in principle, any dyes used to produce plain-coloured fabric could be used to print that fabric. The same mechanisms of dye fixation apply in both dyeing and printing. Careful selection of appropriate dyes for a particular dyeing process is desirable, however, and selection for printing is essential. The primary reason is that in printing dye solubility is more critical, even more so than for continuous dyeing processes, which admittedly compare closely with printing. Not only is the amount of water in the print paste severely limited but, at the fixation stage, the dye must be redissolved in a small volume of condensed steam. (Fixation and other aftertreatment processes are fully discussed in Chapter 8.) There are further reasons for careful selection. The dye must diffuse through the film of thickener before adsorption on, and diffusion into, the fibre. Unless the dye diffuses rapidly, therefore, the fixation time must be extended. Consequently the lowerr.m.m. dyes will often be preferred, provided that their fastness levels are adequate. It is also necessary to choose dyes that can be washed off satisfactorily, without staining the ground or other printed areas in the fabric. Not only must the dye manufacturer select the dyes that show satisfactory printing properties, but when one dye is recommended for both dyeing and printing the standardisation will be different. For example, lower electrolyte content is desirable for printing, especially where electrolyte-sensitive thickening agents are used. In this chapter, therefore, the factors that determine the selection of colorants for particular fabrics and fixation methods are considered. The term ‘colorant’ is used here because it covers both dyes and pigments. Pigments, of course, are used for the production of plain-coloured fabrics, but are of much greater importance in printing. The principal reason is that the stiffening effect of the pigment binder is less noticeable where significant areas of fabric are left uncoloured and flexible. For this reason, a far higher proportion of low-cover designs are printed with pigments than of high-cover prints. Secondly, and perhaps equally importantly, the elimination of the washing-off stage has extra value to the printer because this is a more critical operation for prints than for plain-dyed fabric. Thirdly, rub marking, to which pigment prints are susceptible, is more objectionable on dyed fabrics than on prints, where the pattern conveniently distracts the eye. Because pigments can be applied to all textile fibres, including glass fibre for which there is no alternative, and because pigments are used more than any other colorant type, pigment printing is here considered first