General theory of printing

The printing of textile materials is the application of colour according to a predetermined design.

The printing paste which is applied to textile material consists of dye, water, thickener and hydrocarbon solvent or oil. After the printing paste is applied, the textile material is usually steamed. 

The general theory of printing explains the interaction, on steaming, between the dye, fibre, water, thickener and hydrocarbon solvent. More specifically, it explains how within the printing paste:

1. forces of repulsion are developed between the dye molecules and the constituents of the printing paste; and

2. forces of attraction are developed between the dye molecules and the fibres of the textile material to be printed.

The dye molecule

This has already been covered under ‘The general theory of dyeing’.

The role of the water

A relatively small amount of water is used; enough to dissolve the dye into a paste. Water is used as it is a convenient and readily available medium to mix, or disperse, the dye molecules in the thickener.

The role of the thickener

The purpose of the thickener is to produce a medium for the dye paste and the resultant product is called the printing paste. The printing pastes are an emulsion of thickener and hydrocarbon. The success of printing textile materials depends very much on the type and quality of the thickener.

Thickeners can be any of the following:

a. natural gums, such as gum arabic, acacia gums or gums prepared from starches and other polysaccharides;

b.  the man-made, natural polymer-based gums, for example, water soluble cellulose ethers, such as carboxymethyl cellulose, methyl and ethyl cellulose, and sodium alginate; or

c.   occasionally, made-made, synthetic compounds such as polyvinyl alcohol.

The role of steam

After printing, it is usual to steam the textile material. This is done to achieve colour fastness.

Steaming ensures the adequate penetration of the fibre by dye molecules. This is possible because steaming:

a. generates sufficient energy in the dye molecules for them to enter the fibre polymer system; and

b. swells the fibre so that the dye molecules can enter the fibre polymer system.

Washing off

This has to be done to remove the thickener and other printing paste constituents which have not entered the polymer system of the fibre.