Cotton

Cotton takes the longest route through finishing. Every bath it passes, from desizing to the softener, calls for a different auxiliary, and work left undone at one step shows up as a fault at the next.

What is used at each stage of cotton?

Cotton arrives as woven or knitted goods, but the chemical sequence is largely the same:

1. Desizing — the starch or synthetic size applied to the warp is removed. Desizing enzymes do this by breaking down starch; the alkaline route is harsher on the fibre.
2. Scouring and bleaching — natural colouring matter and pectin are removed with peroxide. A sequestering agent and a peroxide stabiliser work together in the bath.
3. Peroxide removal — residual peroxide is taken to zero before dyeing. Catalase does this in a single rinse.
4. Bio-polishing — surface microfibres are cut with acid cellulase, leaving a cleaner surface that pills less.
5. Dyeing — reactive dye, salt and alkali. Creasing and levelness are decided here.
6. Washing off — hydrolysed dye is removed from the fabric by reactive washing-off agents.
7. Finishing — handle is built with softeners and silicones.

Why is water hardness discussed more with cotton?

Cotton is processed with alkali and peroxide. In hard water, calcium and magnesium accelerate peroxide decomposition: the peroxide is spent without bleaching and leaves localised fibre damage. The same ions form insoluble salts with dye in the dyebath and cause unlevelness.

This is why the sequestering agent and the stabiliser are chosen together in the bleach liquor. They are not the same job: the sequestrant binds metal, the stabiliser governs how fast peroxide decomposes. Too much of one can undo the other — see choosing a sequestering agent and peroxide stabiliser.

Where is strength lost in bio-polishing?

Cellulase can cut cotton itself. While it removes surface fuzz it also acts on the fibre body, so the enzyme has to be stopped once the job is done.

The most common shop-floor mistake is skipping deactivation. An enzyme that is not stopped keeps working in the next bath, at a temperature nobody is watching, and strength loss overruns the figure that was measured. The reasoning is in acid cellulase anti-pilling and enzyme deactivation.

Why does washing off after reactive dyeing take so long?

Part of the reactive dye never bonds to the fibre: it reacts with water and hydrolyses. That part sits inside the fabric and pulls wet rub fastness down until it is washed out.

Washing off exists to remove it, and hot rinsing alone is not enough: the dye leaves the fabric, stays in the bath and redeposits as the temperature falls. A washing-off agent holds the dissolved dye in the liquor and prevents that. See washing off after reactive dyeing.

Why are crease marks more common on knitted cotton?

Knitted goods run in rope form and, in a jet, the same fold passes through the same point again and again. On heavy and mercerised fabrics the mark becomes permanent. Anti-creasing agents raise fabric lubricity so the fold does not always form in the same place; machine speed and liquor ratio matter just as much.

Which product groups are used on cotton?

The groups most used for cotton in the catalogue: enzymes, wetting agents, sequestering agents and stabilisers, washing agents, anti-creasing agents, fixing agents and finishing agents.