Choosing a sequestering agent and peroxide stabiliser for the bleach bath

Why does peroxide decomposition run away?

In an alkaline bath hydrogen peroxide dissociates to the perhydroxyl anion, and that anion does the bleaching. The problem is that there is a second route. Iron, copper and manganese in the bath break peroxide down catalytically, and that route releases the hydroxyl radical instead. The radical does not pick its target; it oxidises cellulose as well as the impurities.

What the damage looks like on cloth depends on where the metal sits. A rust particle resting on the fabric concentrates the decomposition at that point, the fibre dissolves there, and after drying you have pinholes. Metal dispersed through the bath spreads the loss instead: tensile and tear strength drop while the cloth still looks normal to the eye, and the lab result is the first hard evidence. The third form is local over-bleach. Nobody notices the difference in whiteness at the time; it comes back as unlevelness after dyeing.

The working band quoted in the literature for exhaust peroxide bleaching is pH 10.5 to 11.5 at 95 to 100 °C. That is a class range, not a product specification, and where you stop inside it depends on the cloth, the machine and the time. The point that matters: peroxide already decomposes quickly under those conditions, and metal catalysis is a second rate stacked on top of the first.

Metal reaches the bath from several places. Raw cotton carries it in from soil and harvesting machinery. Process water brings iron and manganese. Pipework and machine parts corrode. Impurities in caustic, soda ash and salt add their share, and residue from the previous batch adds more. No recipe closes those sources. What the recipe does is take the ions that arrive out of play.

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

They are not the same, and they do different jobs.

A sequestering agent binds metal ions in the bath and holds them inside a water-soluble complex, so the ion stops acting as a catalyst. Because the same molecule also binds hardness salts, it reduces lime deposits, machine scale and harsh handle along the way.

A peroxide stabiliser controls the rate of decomposition. Decomposition is what bleaches, so the aim is not to stop it but to spread it evenly across the cycle instead of spending it in the first ten minutes. The classic stabiliser is sodium silicate. It is cheap and it works, but it leaves silicate deposits on the machine and on the cloth, stiffens the handle and causes spotting in later steps. That is why organic stabilisers took over.

Most commercial products carry both functions. The reasoning is practical: the bleach bath needs both, dosing one product is easier than dosing two, and the components can be balanced against each other in the formulation. When a product description says "sequestering agent and stabiliser", this is what it means; DK SEQUA ACS is an example of the type.

Acrylate, phosphonate, heptonate: which ion does each handle?

Derin Kimya's families follow the same split. DK SEQUA AC and DK SEQUA AC MOD are sodium acrylate based. DK SEQUA ACP and DK SEQUA ACP MOD are acrylate and phosphonate blends. DK SEQUA 1624 is heptonate based, DK PML K is a polymeric concentrate and DK SEQUA PS PLUS is a concentrated blend. How much of which one depends on the water and the process: pending from the client.

One question narrows the choice: what dominates your water? High hardness with low iron is acrylate territory. Iron or manganese present means you need phosphonate in the product. Where caustic concentration is high, heptonate types survive while many other complexes fall apart.

Why the water analysis comes before the recipe

Hardness is quoted in French degrees (°fH) in Turkey and in German degrees (°dH) across parts of Europe. The conversion is fixed: 1 °dH is about 1.79 °fH. That is a unit conversion, not a product specification.

Total hardness alone does not decide anything. Temporary hardness precipitates on heating and lands on the cloth; permanent hardness stays in solution. And the two metals that matter most for peroxide, iron and manganese, do not appear in the hardness figure at all. They are measured in ppm and they do damage at concentrations far below where calcium becomes a problem.

Sequestrant demand rises with hardness. Nobody argues with that. What gets skipped is the consequence: a recipe proven on well water does not behave the same way on mains supply, and in the dyehouse that difference usually gets blamed on the dye.

The list to hand the lab is short. Total hardness, calcium and magnesium separately, iron, manganese, copper, bicarbonate alkalinity, conductivity, pH. Take the sample at the machine fill line, not at the tank inlet. The pipework between the two is exactly where the iron your analysis missed comes from.

One analysis a year filed in a folder is not a water programme. Well water shifts with the season, mains sources change, and filters and softener resin wear out.

What does over-sequestering break?

This is the part the technical brochures leave out. A sequestering agent is not selective. It binds whatever metal it can reach, and some processes need the metal it takes.

Alpha-amylase desizing enzymes depend on calcium for structural stability. Pull free calcium down towards zero with a strong complexing agent and activity drops, so the size is not fully removed. You cannot see it at that step. It surfaces as poor absorbency after bleaching, one stage later.

With metal complex dyes the effect is more direct. A strong complexing agent can strip the metal out of the dye molecule, which shifts the shade and costs fastness. That is why sequestrant choice and dosage get separate thought when 1:2 metal complex dyes are being run on wool or polyamide.

The third effect sits on the stabiliser side. Overdose the stabiliser and peroxide never decomposes far enough: whiteness falls short, seed husk stays in the cloth, and the ground reads yellow under the next dyeing. Because the floor reflex is to add more stabiliser, this mistake repeats.

The fourth is simple carry-over. Anionic polymer left on the cloth is incompatible with the cationic softener or fixing agent in the next step. Silicone spots and fixing agent precipitate often start here, in a rinse that was cut short.

The rule that falls out of this is short: set the dosage against the water, and do not double it as a safety margin. The excess costs money and it costs the next step.

Symptom to cause: a decision table

In what order is the recipe built?

1. Have the water analysed from a sample taken at the machine fill line. Ask for iron, manganese and copper alongside hardness.
2. Choose the product class by the dominant ion: acrylate where hardness leads, phosphonate-containing where heavy metals are present, heptonate where caustic concentration is high.
3. Dose the sequestering agent into the bath before the peroxide and before the alkali. It has to bind the ions before they meet peroxide.
4. Never pour concentrate onto the cloth. Dilute it and dose with circulation running.
5. Measure residual peroxide and pH at the end of the cycle and write both into the batch record. You cannot argue about a figure you never took.
6. Do not skip peroxide removal. Going into a reactive dyeing with residual peroxide produces a colour loss that never gets charged to the bleaching step.
7. Record the day's water hardness in the batch file. Without tying the recipe to hardness there is no repeatability to defend.

Dosage figures depend on the product and on the water: pending from the client.