Dyeing fabrics? Want to know how chemical forces of attraction work?
For a start, how do we perceive colour? The colour of dyed textiles, as for paint pigments on paper or objects, is dependent upon the reflection or absorption of light waves by the dye molecules within the polymers of the fibre or fabric. The configuration, or shape, of the dye molecule determines its effectiveness in entering and becoming attached to the polymer system of the fibres. The most efficient and compatible shape appears to be flat, like a sheet of paper, which can then slide between molecules. Forces of attraction and repulsion combine with the shape of the dye molecule to assist its entry and attachment to the fibre.
Water is an important ingredient when dyeing fabrics, although the dye molecules might resist leaving the water unless they are encouraged to do so by the application of heat or a chemical – often in an alkaline solution. The choice of dyes varies according to the fibre - eg nylon, polyester and other man-made fibres require a different class of dyes to cellulosic fibres such as cotton or protein fibres such as silk or wool. Excess dye left on the surface of fibres must be removed by thorough rinsing and washing.
There are many classes of dyes, each chosen for its suitable properties for the particular fibre being dyed. Electrical and chemical bonds, such as salt links, ionic bonds, hydrogen bonds and van der Waals forces come into play to attract the dye to the polymers and keep it there. Various properties of light and wash fastness result, according to the type of dyestuff and fibre used. Dyes may be classed as – acid, azoic, basic, direct, mordant, disperse, pre-metallised, reactive, sulphur, vat and solvent – according to their chemical characteristics and method of application. When dyeing fabrics I now use fibre reactive dyes almost exclusively for silk, cotton, cashmere or bamboo; “Drimarine K” best suits the way I work.
Sometimes I add a wetting agent such as urea to the dye bath to open the molecules and allow the dye to enter, then after a period, an alkaline solution such as soda ash ( sodium bicarbonate for silk) to close them firmly! On other occasions I paint the fabric with dissolved dye or print it, adding a printing medium to the dye, later coating or painting it with an alkaline substance, such as sodium silicate or sodium bicarbonate. In both instances, when dyeing fabric, time is a factor to be considered, so that the molecules in the fibre can absorb as much of the dissolved dyestuff as possible. Thorough rinsing and washing, then neutralising with a few drops of vinegar completes the process.
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