Clearing residual peroxide after bleaching with catalase

What residual peroxide does to a reactive dyeing

When the bleach bath is dropped, hydrogen peroxide stays inside the cloth. You cannot see it, and nothing on the machine panel reports it. It announces itself once dyeing starts.

Reactive dye chromophores are open to oxidation. At dyeing temperature and in an alkaline bath, peroxide degrades the dye: colour yield drops and the shade shifts. The shift is most obvious on turquoise and blue, because those dyes are more sensitive to oxidation than the rest of the palette.

The second consequence costs more. Residual peroxide is never evenly distributed. It sits at different concentrations at the selvedges, in creases and wherever the squeeze was uneven, so the dye degrades by different amounts across the piece. The result comes out as unlevelness, often reading as a streak or a cloud after drying. At that point the dyehouse argues about levelling agent dosage and the dyeing curve, while the fault was made in the previous bath.

How badly it bites depends on the dye class, so the same amount of residual peroxide does not give the same result on every shade. That makes the diagnosis harder. A residue that passes unnoticed on pale shades produces rejects on a deep turquoise the same week, and nothing except the recipe appears to have changed between the two batches.

There is a quieter third effect. Degraded dye does not fix to the fibre; it stays in the liquor and adds to what the washing-off agent has to remove. If fastness results wander from batch to batch on an unchanged recipe, this is one of the places to look.

Hot rinsing or catalase?

Both routes get rid of the peroxide. The difference is in what they spend.

Catalase breaks hydrogen peroxide down into water and oxygen. Neither product interferes with dyeing, and the oxygen leaves the bath. As a class, catalase works around pH 6-8 at moderate temperatures. That is a class range, not a product specification. Derin Kimya's own figures sit on each product page: pending from the client.

Hot rinsing has one genuine advantage worth stating plainly: along with the peroxide, it carries away residual alkali, stabiliser and surfactant left from bleaching. Catalase only targets the peroxide. A mill moving to a single bath has to account separately for what those residues do to the dyeing.

Why a reducing agent is the wrong tool here

Hydrosulphite, bisulphite and thiourea dioxide really do destroy peroxide. The problem is that nobody knows how much is needed.

How much peroxide the cloth holds depends on the batch, the squeeze and the bleaching recipe. The reducing agent gets dosed over that uncertainty with a "safe" excess, and the excess survives into the dyebath. There it reduces the azo bond of the reactive dye. The shade goes weak, and in three-dye combinations one component is usually hit harder than the others, so the recipe stops reproducing.

Catalase limits itself. When the substrate is gone the reaction stops, and leftover enzyme does not harm the dyeing. A reducing agent has no such brake.

The second argument is on the effluent side. Sulphur-based reducing agents add load to treatment, and sulphur compounds bring odour and corrosion problems in the drain line. The third is the fibre and the dye: a strongly reducing bath also acts on coloured material already on the cloth, which makes it a risk on yarn-dyed constructions and on anything previously dyed.

Reducing agents have a place. That place is stripping a failed dyeing, not clearing peroxide before a good one.

The usual defence is "dose a little and there will be no excess". That manages an unmeasured process with an unmeasured judgement. If you do not know how much peroxide is left, you cannot know how much reducing agent it needs: dose light and peroxide survives, dose heavy and the dye suffers, and the window between the two moves from batch to batch. With catalase there is no window to find, because the reaction finds its own end.

How the step is set up on each machine

In exhaust the catalase gets its own bath and every condition can be set properly. Temperature is dialled in, pH is measured, time is held, and the test comes at the end. The one difficulty is cooling: dropping from bleaching temperature to the enzyme band ties the machine up and gives back part of the saving. Mills that track their heating and cooling curves put that time into the plan instead of waving it through as short.

A continuous washing range behaves differently. The enzyme is dosed into a wash box and contact time is set by line speed, so it is measured in seconds rather than minutes. The shortened time is made up with concentration, which makes the whole thing depend on the dosing pump running properly. On a shift where the pump stalls or blocks, the fact that the cloth left the range carrying peroxide only surfaces in dyeing.

On a jigger the fabric runs wound on a roll at a low liquor ratio, and the enzyme does not reach every layer at the same moment. Sample both ends of the piece if you want to see the head-to-tail difference.

One thing holds on all three machines: how much peroxide the cloth carries is set by the squeeze after bleaching. If the squeeze setting has changed, the catalase recipe has to be re-proven.

How do you prove the peroxide is gone?

Verification is not an optional step. Dosing the enzyme does not prove the peroxide left: the enzyme may have gone into the wrong temperature, the wrong pH or a peroxide concentration high enough to kill it, and none of that shows on the machine panel.

Two methods are used on the floor. A peroxide test strip gives a semi-quantitative reading against a colour scale. Starch-iodide paper is cheaper: acidify the sample with acetic acid, dip the paper, and an oxidiser turns it blue-violet.

Sequence:

1. Take the sample from the circulating liquor, not off the surface of the machine. Surface water is the most misleading point you can sample.
2. Take a fabric sample at the same time. Squeeze a piece of cloth and test the expressed liquor separately.
3. If you use strips, respect the reading time. A strip read early reads low.
4. With starch-iodide paper, acidify first. A neutral sample gives a weak colour.
5. Record the result against the batch. An unrecorded test counts as a test that was not done.

Testing the liquor squeezed out of the cloth is the step most often skipped. The reservoir of peroxide is the fabric, not the bath. The bath can read clean while the cloth is still carrying peroxide, and this happens most on heavy weights and tight constructions.

What has to be true before you dye in the same bath?

A single bath is the main argument for catalase. But carrying the bath forward means carrying everything in it forward.

Four things have to be right before dyeing starts. The peroxide test has to read negative. Residual alkali from bleaching has to be neutralised and the pH brought to the dyeing start value. The bath has to be cooled to the dyeing start temperature. And the carry-over of stabiliser, sequestering agent and wetting agent from the bleach has to be known to be compatible with the dyeing recipe; that last one is normally proven once in the lab and then held valid for that quality.

On dark and critical shades, some mills put a rinse back in despite running a single-bath scheme. The decision comes from the gap between the cost of water and the cost of a reject, not from the enzyme.

Why catalase is killed by the peroxide it destroys

Catalase is inactivated at high hydrogen peroxide concentrations. The very substance it is dosed to destroy will finish the enzyme if the concentration sits above a certain level. This is the source of the most common mistake in catalase use: dosing the enzyme straight into a bleach bath that has not been dropped.

The second limit is temperature. Bleaching finishes somewhere around 95-100 °C and the enzyme denatures instantly at that temperature. An enzyme dosed before the bath is cooled does nothing, even where the peroxide concentration would have been acceptable.

The correct sequence falls out of those two limits:

1. Drop the bleach bath. Most of the peroxide leaves at this step.
2. Run one hot rinse. The aim is not to reach zero peroxide; it is to bring the concentration down to a level the enzyme survives and to lower the alkali.
3. Fill the bath, bring it to the enzyme's working temperature and set the pH.
4. Dose the enzyme diluted, with circulation running.
5. At the end of the run, test for peroxide in both the bath and the cloth.
6. If the test is clean, go to dyeing. If not, check temperature and pH before extending the enzyme time.

Most "the enzyme did not work" cases are the second or third step being skipped. Raising the dosage where the enzyme is dying in too concentrated a peroxide bath only kills more enzyme.