Colour and protection: dyes, pigments, paints and coatings: Dye classes, fibres and the dyehouse

3.1 The fibre decides the dye class

Kent groups fibres by origin. Cotton, flax and viscose are cellulose: long chains of glucose units covered in hydroxyl (-OH) groups that soak up water. Wool and silk are proteins, with amine and acid groups that can carry a charge; nylon, a polyamide, has similar amine ends. Polyester, polypropylene and acrylic are largely water-repellent. Kent's moisture contents at 65% relative humidity show the range: viscose 13%, wool 13 to 15%, cotton 7%, nylon 3 to 5%, acrylic 1 to 1.5%, polyester 0.3 to 0.5%, polypropylene almost nothing. Since most dyeing is done from water, a fibre's water uptake governs how a dye gets in.

Kent then lists how dyes are held. Acid dyes form ionic bonds with charged sites on wool or nylon. Reactive dyes form covalent bonds with cotton. Vat and sulphur dyes are trapped as insoluble particles. Direct dyes cling to cotton by weaker forces such as hydrogen bonds. Disperse dyes dissolve in polyester as a solid solution, like sugar in a boiled sweet. Basic (cationic) dyes bond ionically to negative sites built into acrylic.

The bonding explains the trade-offs in the table. A covalent bond survives washing, so reactive dyes are very washfast. Hydrogen bonds are weak, so direct dyes wash out unless after-treated. Polypropylene has almost no dye affinity, so pigment is mixed into the molten polymer before spinning.

Volume follows the fibre market. A 2002 DyStar survey quoted by Kent puts world textile dye use at 638,871 tonnes. Reactive dyes (for cotton) and disperse dyes (for polyester) were 28% each, sulphur dyes 15%, mostly cheap black on cotton, and indigo 6%. Every other class was 4% or less.

• Kent (2012), Ch 13 Dye Application, Manufacture of Dye Intermediates and Dyes · pdf pp. 487-490, 494-497, 556

3.2 Reactive and disperse dyes: the two big classes

Reactive dyes, commercialised by ICI in 1956, are the main colorants for cotton. Kent describes the molecule in five parts: a solubilising group (sulphonate), a chromogen (often azo or anthraquinone), a bridging group such as -NH-, a reactive group (chlorotriazine and vinyl sulphone are the commonest), and a leaving group (-Cl, -F or sulphate) that departs when the dye bonds. In alkali, cellulose's hydroxyl groups attack the reactive group and form a covalent bond, giving excellent washfastness and bright shades.

Two costs are built into the chemistry. First, reactive dyes are small and have little natural pull towards cotton, so the bath needs 200 to 300 grams of salt per litre to drive them on. That is 200 to 300 kg of salt per 1,000 litres of bath, and it leaves with the wastewater. Second, the alkali that makes cotton react also lets water react: some reactive groups hydrolyse before reaching the fibre. Hydrolysed dye can never fix, so it is wasted and colours the effluent, which must be treated before discharge. Chemists responded with bireactive dyes carrying two reactive groups, so fixation can still happen if one is lost.

Disperse dyes were invented in the 1920s for cellulose acetate, the first water-repellent fibre, and are now the polyester dyes. They are almost insoluble, so they are milled to 1 to 3 micrometres and kept suspended with a dispersing agent (a surfactant); single molecules then migrate into the fibre and dissolve in it. Kent classifies them by molecular size, as in the table: small dyes go in easily but sublime (turn to vapour) at low temperatures, while large dyes need pressure dyeing at 130 C but withstand heat. Most are azo (about 59%) or anthraquinone (about 32%).

• Kent (2012), Ch 13 Dye Application, Manufacture of Dye Intermediates and Dyes · pdf pp. 485, 498-502, 555
• Shreve (1984), Ch 39 Cyclic Intermediates and Dyes · pdf p. 807

3.3 Vat and sulphur dyes: insoluble colours made soluble for a while

Some of the most durable colours on cotton are insoluble in water, which seems to rule out dyeing from water. The trick is chemical reduction. Vat dyes, mostly anthraquinones (82%) or indigoids (9%), are reduced with caustic soda and sodium hydrosulphite (Na₂S₂O₄, 'hydro' in dyehouses) to a soluble leuco form, which soaks into cotton like a direct dye. Air or hydrogen peroxide then oxidises it back to the insoluble form, now locked inside the fibre. Kent says vat dyes have the best light and bleach fastness of the dyes for cotton, which is why Shreve recommends them, despite their cost, for fabrics washed often, such as shirts.

Sulphur dyes use the same idea with cheaper chemistry. Their sulphide (S-S) links are split by sodium sulphide at pH above 10, giving a soluble leuco form that exhausts onto cotton with salt; hydrogen peroxide reoxidises it. Kent says they are used for economy and washfastness, give dull shades, and matter most for washfast black: C.I. Sulphur Black 1 is the main commercial dye. They are made by heating aromatic amines or nitro compounds with sulphur or sodium polysulphide, and their exact structures are still largely unknown.

Indigo, the denim blue, is the biggest vat dye: 35,159 tonnes in 2002 in Kent's table. It has low affinity, so deep shades are built by repeated dipping and airing. Shreve remarks that its modest fastness on cotton, the fading of jeans, has become prized. The synthetic route in the figure replaced the plant crop once grown in India and the United States.

The cost of these classes lies in the chemistry around the dye. Reducing agents, alkali, sulphide and oxidants all pass into the effluent, and every lot goes through reduction, rinsing, oxidation and soaping.

• Kent (2012), Ch 13 Dye Application, Manufacture of Dye Intermediates and Dyes · pdf pp. 502-507, 549-551, 556
• Shreve (1984), Ch 39 Cyclic Intermediates and Dyes · pdf pp. 808, 810

3.4 How dyeing is done, and what leaves in the water

Kent describes four steps in any dyebath. In exhaustion, dye moves from the bath to the fibre surface. In diffusion, it moves into the fibre's disordered (amorphous) regions. In migration, it spreads out until even. In fixation, it bonds or locks in place. Dyers add helpers to control the pace: salt to push anionic dyes onto cotton, retarders that compete for dye sites to slow a dye that strikes too fast, and carriers such as biphenyl that swell polyester. Matching a shade may take two to a dozen dyes.

Batch, or exhaust, dyeing covers most goods: yarn on perforated tubes in package machines, fabric in becks, jigs and beam machines, and delicate fabric in jet machines that drive a loop of cloth through a jet of liquor. Kent notes newer machines use lower liquor ratios (less water per kilogram of goods), saving energy. Continuous dyeing suits long runs; the classic case is polyester-cotton, shown in the figure, where one pass handles both fibres. Printing is localised dyeing, mostly by rotary screens with one screen per colour, with ink-jet printing for short runs.

Effluent is the dyehouse's chronic cost. Unfixed reactive dye, salt, alkali, sulphide, spent reducing agents and carriers all end up in the wastewater. Shreve notes that dyeing from solvents such as perchloroethylene would greatly ease the removal of unabsorbed dye from outfall waters, but US dyers would not buy the new machines needed. On the supply side, Kent records that international dye makers put major plants where environmental laws were less stringent, and Dryden says high costs and stricter laws pushed production towards countries such as India.

• Kent (2012), Ch 13 Dye Application, Manufacture of Dye Intermediates and Dyes · pdf pp. 486, 489-490, 507-510
• Shreve (1984), Ch 39 Cyclic Intermediates and Dyes · pdf p. 808
• Dryden (1997), IVF Chemicals from Aromatics (including Dyes) · pdf pp. 550, 552-553
• Dryden (1997), IB Indian chemical industry · pdf p. 15

See it in the table

Check your understanding

Why does reactive dyeing of cotton use 200 to 300 g of salt per litre?

Which dye class is used for polyester, and how is it held?

What makes an insoluble vat dye usable in an aqueous dyebath?