Printing

Modern procedures for printing textile goods may be traced back to the block printing of silks in ancient China. In this method a wooden block with a raised pattern on the surface was dipped into the printing colorant and then pressed face down on to the fabric. The desired pattern was obtained by repeating the process using different colours. Printing by brushing colorant through thin metal stencils and the transfer of illustrations to the printed page from engraved rollers in a printing press were also widespread by the fifteenth century.

Block printing remained a practical proposition until the roller printing machine was invented by James Bell in 1783. This enabled six colours to be printed at a rate equivalent to that of 40 hand-block printers. The success of the machine depended on the hard rollers, each of which bore an engraving (i.e. an intaglio engraving, in which the depth of the recess on the roller determines the intensity of the print produced) corresponding to a particular colour component of the design. The machines were capable of continuously printing six different colours in sequence, with the rollers pressed against the fabric.

PRINTING PASTE

In textile printing the printing paste on the fabric may be regarded as a miniature localised dyebath containing the dye and all the additives necessary for the coloration process. Thus print pastes for reactive dyes will contain alkali, those for vat dyes will contain reducing agent, and so on. Dye diffuses into the fibre from the paste, aided either by a rise in temperature and absorption of water within a steamer, or by the fibre becoming a more favourable environment as the paste becomes affected by the heat of a dry fixation process. To a certain extent the nature of the fixation process to be used influences the type of additive mixed with the print paste.

Textile printing pastes are usually water-based, but for paper printing a solvent base may be chosen since for this purpose rapid drying through evaporation is more important than substrate penetration. Compared with a dyeing process on a similar fabric, dyes of lower substantivity and high water solubility are often chosen, so as to ease the final washing-off process. The viscosity of the print paste needs careful attention to ensure that it flows smoothly from the source of application on to the fabric. Too high a viscosity impedes transfer and penetration, and if the paste does not spread evenly on the fabric the surface appearance will be unsatisfactory. Most thickening agents are polymeric in nature, and the overall chemical composition of the paste must be compatible with the dye fixation mechanism. For example, there is little point in using a polysaccharide thickening agent (such as starch) for reactive dyes, because of its chemical similarity