How dyes hold on

A dye has two jobs: to colour a fibre, and to stay on it, wash after wash.

Dyes are coloured for the same reason carrots are: each dye molecule has a long run of double bonds sharing their electrons, which absorbs some colours of visible light and sends the rest back to your eyes. Indigo, the blue of jeans, absorbs orange light, around 600 nm, so it looks blue.

Indigo hardly dissolves in water, so dyers first add hydrogen to it in an alkaline bath, a reduction, which turns it into a yellow-green form that dissolves. Cotton yarn soaks it up and is lifted out into the air. Oxygen in the air takes the hydrogen off again, an oxidation, and blue indigo forms inside the cotton, where it is trapped. The yarn turns from yellow-green to blue in a minute or two. The indigo stays mostly near the outside of the yarn, so the white core shows through as jeans wear, and they fade.

Holding on

A dye stays put when it can grip the fibre's chains, and each fibre offers its own groups to grip. Dyers choose a kind of dye to match.

Cotton's chains are covered in -⁠OH groups. A reactive dye carries a group that reacts with one of them, so the dye becomes joined to the fibre by a covalent bond. Reactive dyes for home use are fixed with washing soda, a base, which helps the reaction along.

Wool, silk and nylon carry amine groups, -⁠NH₂, on some side groups and at the ends of their chains. In an acidic dye bath, each picks up a hydrogen ion, H⁺, and becomes -⁠NH₃⁺. An acid dye carries negatively charged -⁠SO₃⁻ groups, and opposite charges attract, so the dye clings to the fibre.

Polyester's chains offer a dye almost nothing to grip. It is dyed with small, oily disperse dyes in water at about 130 °C, under pressure. The heat loosens the chains enough for the dye molecules to slip in between them, and as the fibre cools, the chains close up and trap the dye.