Sequestering Agents and Peroxide Stabilisers

Calcium, magnesium, iron and manganese never appear on the recipe sheet, but they decide the result. A sequestering agent binds those ions and takes them out of the bath; a stabiliser does the same job in the bleach liquor, there to protect the peroxide. Three families, 8 products.

What does hard water actually break?

Mains and well water carry dissolved calcium and magnesium. Those ions form insoluble salts with anionic surfactants and soaps; the precipitate stays on the cloth, stiffens the handle and shows up as a stain in the next step. On the dyeing side the damage is quieter: some reactive dyes complex with calcium and lose solubility, which reads on the cloth as shade drift and unlevelness.

Iron and manganese are a separate heading. They contribute nothing to hardness but they catalytically decompose hydrogen peroxide in the bleach bath. Because the decomposition is local, so is the damage: pinholes and spot-wise strength loss.

How does a sequestering agent hold an ion?

The molecule wraps the metal ion at several points and forms a water-soluble complex. The bound ion can no longer precipitate with soap or attack peroxide. Three chemical bases are in common use:

Polyacrylates also behave as dispersants: rather than dissolving the precipitate they keep it finely distributed and off the fabric. That is why blended products work over a wider window than single-base ones.

Is a sequestering agent the same thing as a peroxide stabiliser?

Not the same, but they overlap. A peroxide stabiliser slows the uncontrolled decomposition of peroxide in the bleach bath, and the largest part of that job is binding the iron and manganese that trigger it. The classic answer is sodium silicate, which is cheap but leaves deposits on the machine and the cloth and hardens the handle. Organic stabilisers remove that deposit problem.

In practice one product covers both jobs: the stabiliser dosed into the bath holds the hardness and protects the peroxide at the same time. Keeping two separate items on the recipe makes sense when the water is very hard or when a heavy-metal load is known.

Which product goes in at which stage?

In pre-treatment a sequestering agent goes in at two points: into the desizing and scouring liquors to hold the hardness, and into the bleach liquor to protect the peroxide. In dyeing the reason is different. There the aim is to protect the solubility and distribution of the dye, which makes it levelling work.

In washing and rinsing, the sequestering agent keeps stripped dye and soap from settling back onto the cloth. In mills with hard water, a small dose into the rinse is a cheap intervention that shows up in the wet fastness figures.

What sets the dosage?

Chelating sequestering agents work stoichiometrically: they are consumed in proportion to the ions they bind. Polyacrylates behave differently and delay precipitation at a far lower dose. Either way the right dosage starts with a water analysis: total hardness, iron and manganese. Liquor ratio feeds in directly too; for the same fabric, a machine running 1:5 and one running 1:10 bring different total ion loads into the bath.

Overdosing has a price of its own. Too much sequestering agent can strip the metal out of metal-complex dyes and shift the shade, and it lowers colour yield on some reactive dyes. Per-product dosage ranges are given on each product page: pending from the client.