Water Hardness in Textile Dyeing: Why Shades Do Not Repeat and What a Sequestrant Does

What hardness is

Water hardness is the content of dissolved calcium and magnesium salts. It is reported as milligrams per liter of calcium carbonate (mg/L or ppm CaCO3), or in German degrees (1 °dH = 17.8 mg/L CaCO3) or French degrees (1 °fH = 10 mg/L CaCO3).

As a reference, in mg/L of CaCO3:

• Soft: under 60
• Moderately hard: 60 to 120
• Hard: 120 to 180
• Very hard: over 180

Hardness is only part of the problem. The same water usually carries iron, copper and manganese, in smaller amounts but with a much larger effect per milligram. They come from well water, from rusted pipes and from the raw cotton itself, which naturally contains calcium, magnesium and iron. That is why even a plant with soft water has metals in the first bath.

Lima's municipal water is hard. In Colombia it varies by city and by source; plants that pump their own well water usually have the hardest water, and the hardness changes between the dry and rainy seasons. That seasonal change is the reason a recipe that worked in March gives a different shade in August.

What hardness and metals do in each step

Scouring and bleaching. Calcium and magnesium react with anionic surfactants and with the fatty acids saponified from the cotton wax to form insoluble lime soaps. These deposit on the fabric and the machine, lower the detergency of the bath, and leave a harsh hand and a dull, grey white. Iron, copper and manganese catalyze the decomposition of hydrogen peroxide. Instead of bleaching the fiber, the peroxide breaks down in seconds around each metal particle, oxidizing the cellulose at that point. The result is pinholes and holes that only show up after dyeing or after the first wash. Iron also leaves yellow and brown stains.

Reactive and direct dyeing. The sulfonic groups that make these dyes soluble form calcium and magnesium salts of much lower solubility. In hard water part of the dye precipitates: lower yield, dye spots and a shade that moves with the water. Iron and copper ions complex with some chromophores and shift the hue, most visibly in turquoise, blue and red shades.

Soaping after reactive dyeing. Hardness makes the hydrolyzed dye redeposit on the fabric instead of staying in the bath, and the wash fastness drops.

Vat, indigo and sulfur dyeing. The reduced leuco forms of these dyes precipitate as calcium salts, which causes streaks and uneven shade on continuous ranges.

Machines and steam. Hardness scales heat exchangers and nozzles, raises the alkali consumption of every bath, and leaves calcium carbonate on the fabric that later interferes with finishing.

Industrial laundries. Hardness is the main consumer of detergent. Part of each dose goes to precipitating calcium before any washing happens, and the lime soap builds up on the linen as greying.

What a sequestrant does

A sequestrant, or chelating agent, binds metal ions in a soluble complex so they can no longer precipitate surfactants, decompose peroxide or react with dyes. Most textile sequestrants also disperse: they keep the precipitates that do form suspended in the bath until it drains.

Not every sequestrant works in every step. A sequestrant for bleaching has to be strong against iron and copper. A sequestrant for the dye bath must bind hardness without touching the dye: a very strong one pulls the metal out of metal-complex dyes such as turquoise, which changes the shade and lowers the fastness.

Where to dose, process by process

• Scouring and combined scour-bleach. 0.5 to 2 g/L in exhaust, 2 to 5 g/L on continuous ranges, together with the peroxide stabilizer. If the cotton or the water carries a lot of iron, run a demineralizing acid wash before bleaching; it removes the metals before the peroxide meets them.
• Neutralization after bleaching. Acid plus a sequestrant removes the alkali trapped inside the fiber and the hardness and metals left on the fabric, so the dye bath starts clean. A catalase can go in the same bath to remove residual peroxide.
• Reactive dyeing. 0.5 to 1 g/L of a sequestrant that does not demetallize dyes. Do not use strong sequestrants with turquoise or metal-complex dyes.
• Soaping. 0.5 to 1 g/L of a sequestrant-dispersant improves removal of hydrolyzed dye and the wash fastness.
• Vat, indigo and sulfur on continuous. In the chemical pad, to keep the leuco compound in solution.
• Industrial laundry. In the main wash, dosed to the measured hardness, and in the last rinse where greying is a problem.

How to measure and how much to use

1. Test the water. Hardness with a titration kit or test strips, and iron with a colorimetric kit. Repeat the test each season and after any change of source or well.
2. Ask for the sequestering value. Suppliers report it in milligrams of CaCO3 per gram of product. With the water analysis and that figure, the dose for the water is a simple calculation.
3. Add a margin for the fabric. Raw cotton adds metals the water test does not show. In pretreatment the dose is usually set by the fabric, not by the water.
4. Do not overdose in the dye bath. More sequestrant than needed does nothing for hardness and raises the risk of demetallizing the dye.
5. Check the result. Fewer pinholes, higher whiteness, repeatable shades between lots, and less lime soap on the fabric and the machine.

Common problems and what they usually mean

• Pinholes after bleaching: iron or copper catalyzing the peroxide. Increase the sequestrant, check the water, and consider an acid demineralizing wash.
• Dull or grey whites, harsh hand: lime soap deposits. Sequestrant in scouring, and check whether the surfactant is anionic.
• Shade differs between lots with the same recipe: hardness changing with the season or the source. Test the water and dose by analysis, not by a fixed figure.
• Turquoise or blue shade shifts after adding a sequestrant: the product is stripping the copper from the dye. Change to a non-demetallizing type.
• Poor wash fastness after soaping: hydrolyzed dye redeposited in hard water. Sequestrant-dispersant in the soaping bath.
• Yellowing of nylon-elastane after bleaching: sodium acetate from acetic acid neutralization, or residual alkali inside the fiber. Use a non-volatile acid neutralizer in the last rinse.

Casditol 54-P and Casditol Acid

For pretreatment and dyeing we offer two sequestrants:

CASDITOL 54-P is an anionic blend of organic chelating agents that sequesters and disperses iron, copper, calcium and magnesium. It is used in causticizing, scouring and bleaching, and in dyeing with vat, indigo, sulfur and reactive dyes and other dyes that do not contain metal in their molecule. It works in exhaust and continuous processes and is compatible with anionic and non-ionic products.

CASDITOL ACID is an acid neutralizer and sequestrant-dispersant for cotton, nylon-elastane and their blends. It is free of acetic acid and other volatile weak acids, so it does not form the sodium acetate that yellows the fiber. It sequesters hardness and metals sub-stoichiometrically, is stable in acid and strongly alkaline conditions and against oxidants and reductants, and neutralizes the alkalinity inside the fiber. After peroxide bleaching it allows neutralizing the caustic soda and removing the peroxide with TERMINOX ULTRA 50L in a single step, and applied in the last rinse it minimizes the later yellowing of nylon-elastane.

To have your water tested and get a dosing recommendation for your process, contact our technical team.