The popularity of polyester fabrics led to the development of a completely new form of printing: heat transfer printing, which prints the pattern on paper with carefully selected dyes. The paper is then applied to the fabric by passing the two together through a type of hot calender, and the pattern is transferred from one to the other. This method opens up new possibilities, such as the production of halftone effects.
In all textile printing, the nature and, particularly, the viscosity of the print paste are important, and the thickeners employed must be compatible with all the other components. For conventional methods the thickeners are such reagents as starch, gum tragacanth, alginates, methyl cellulose ethers, and sodium carboxymethyl cellulose. Many types of dye can be applied, including direct cotton, vat, mordant, and reactive dyes, as well as pigment colours. Most dyes are fixed by steaming or aging, by a batch or continuous method, and more rapid fixation is effected by flash aging—e.g., allowing a shorter steaming period by employing smaller machines. After steaming, the fabric must be thoroughly washed to remove loose dye and thickener, ensuring fastness to rubbing.
Most textile materials can be printed without special pretreatment, but wool cloths are generally chlorinated before printing. Tops (long, parallel wool fibres), printed in stripes, are used for mixed effects, and printed warps produce shadowy effects. Tufted carpets are printed by a process designed to ensure good penetration.