Washing-off after reactive dyeing: getting the hydrolysed dye out

What decides wet fastness on reactive dyeings?

One thing decides it: how much hydrolysed dye is left on the cloth.

A reactive dye forms a covalent bond with the hydroxyl groups of cellulose. But water in the bath is present in vast excess over cellulose, and part of the dye's reactive group reacts with water instead of fibre. The molecule that results has the same colour and looks identical, yet it cannot bond. It sits on the fibre held only by substantivity.

That is the dye that bleeds in a wet fastness test. Fixed dye does not come off in washing; what comes off is always the hydrolysed fraction. Which makes a fastness problem on reactives a washing problem, not a dye problem.

How much hydrolyses depends on the dye class, the depth of shade, liquor ratio, alkali and temperature. The relationship worth carrying onto the floor is simple: the deeper the shade, the more unfixed dye stays on the cloth, and the heavier the washing-off has to be. A deep navy and a pale blush cannot share a washing programme.

The dye class shifts the load as well. The hydrolysed form of a vinyl sulfone dye holds onto the fibre weakly and comes off comparatively easily. With monochlorotriazine types the hydrolysate is more substantive, so the same depth needs more soaping. Bifunctional dyes give higher fixation and therefore leave less unfixed dye on the cloth. When you inherit a recipe, the first question is which dye class it was built on.

Why is rinsing alone not enough?

Hydrolysed dye is anionic and has a natural affinity for cotton. It does not stay in the liquor once it is released; part of it re-adsorbs onto the fibre. Rinsing dilutes, but it does not change that equilibrium.

Two conditions strengthen the affinity: salt in the bath and high temperature. At the end of a dyeing, both are present. The moment the dyeing finishes is the moment hydrolysed dye clings hardest.

A washing-off agent is what shifts that equilibrium. A good one does three things. It holds the released dye dispersed in the liquor, it suppresses re-deposition onto the fibre, and it binds hardness ions so that dye-calcium precipitates do not form. In effect it keeps the dye in solution and puts itself between the dye and the cloth.

Foaming behaviour also comes into the choice. In a high-speed jet, foam causes overflow and disturbs circulation, so foaming types belong on machines where that is not an issue and in particular washing steps.

At Derin Kimya, DK WASH RWS 1038 and DK WASH RWS 40 are the reactive washing-off agents. DK WASH CT covers applications that need a foaming wash, and DK WASH 2K sits in general washing steps. Temperature, time and dosage follow the depth of shade: pending from the client.

Why does the first rinse temperature matter so much?

The most common washing-off mistake is heating the bath and starting to soap while salt and alkali are still in it.

The logic looks reasonable: heat washes. What actually happens is different. Salt is still present, pH is still alkaline, and under those two conditions hydrolysed dye has its highest affinity for the fibre. Heating drives the dissolved dye deeper into the cloth. The result is re-deposition that no later step fully reverses.

The correct order is the other way round. Clean up the bath first, then apply temperature. Drop the dyebath hot; cooling it in the machine only extends the contact time between the cloth and the dye dissolved in it. Take out most of the salt and alkali with warm and cold rinses. Neutralise. Only then soap, in clean water, near neutral pH, with a washing-off agent present.

The literature puts the typical soaping temperature in the 90 to 98 °C band. That is a class range, not a product specification; the fibre blend and the limits of the machine decide where in that band you stop.

One caution the other way. Cooling too fast is not free either. Viscose, heavy knits and elastane qualities crease when the temperature drops sharply. Set the cooling rate by the fabric.

How does hard water break the washing-off?

Calcium and magnesium form insoluble salts with the anionic dye released into the liquor. The precipitate settles on the fabric surface and does two things: it dulls the shade and it lowers rub fastness. On the floor this usually gets logged as "the shade came out flat", and nobody looks further.

Hard water also works against the washing-off agent. When part of the anionic system reacts with hardness ions, its dispersing capacity falls, so the same dosage does less work.

One detail gets missed. The soaping bath draws the same water as the dyebath, but at a different temperature and pH. Calcium precipitation increases with temperature, so the same hardness causes more trouble in soaping than it did in dyeing.

The fix is a sequestering agent in the soaping bath. The logic already used in bleaching applies here, and the dominant ion in your water decides which class to reach for.

In what order is the washing-off run?

1. Drop the dyebath hot. Cooling before draining extends the contact time.
2. Rinse warm, then cold. The purpose here is removing salt and alkali, not removing dye.
3. Neutralise with acetic acid and confirm by measuring pH. Never start soaping with alkali still in the cloth.
4. Soap in clean water with a washing-off agent, adding a sequestering agent where the water calls for it. On dark shades one soaping is not enough; the second bath has to come out cleaner than the first.
5. Rinse hot after soaping, then rinse cold.
6. Apply a fixing agent if the specification calls for one. This step is optional and it does not replace washing-off.
7. Look at the colour of the last bath. If it is still coloured, the washing-off is not finished. The bath decides that, not the programme.

Every step in that sequence spends water and steam. When pressure comes to shorten the cycle, the intermediate rinses are usually the first to go, and those rinses are exactly what makes the soaping efficient. If something has to be cut, argue about bath volume and liquor ratio, not about the number of soapings.

When is a fixing agent appropriate, and what does it cost?

A fixing agent is a cationic polymer. It forms salt links with the anionic dye on the fibre, lowers the dye's water solubility and raises washing, water and perspiration fastness.

Application goes two ways. On exhaust it is dosed into the final bath and the cloth takes up the liquor itself. On the padder it is impregnated, squeezed and dried on the stenter, where pick-up governs everything, so a change in nip pressure changes the effective dosage. Dosages from one route do not convert to the other.

It comes with costs, and each one gets weighed per order.

The shade shifts. Most fixing agents dull or deepen the colour to some extent. Do not go to production before a sample with and a sample without the fixing agent have been approved side by side in the lab.

Light fastness can fall. The effect is most noticeable with older cationic condensate types. On articles going into outdoor use, test that separately.

The handle can stiffen, and compatibility with the next step is not guaranteed. A cationic fixing agent precipitates when it meets an anionic softener or anionic finish in the same bath.

And the one that matters most: a fixing agent does not rescue badly washed cloth. It locks the hydrolysed dye in place where it sits. Washing fastness reads a little better, rub fastness gets worse, and the complaint comes back from the field.

Why do buyers now ask for formaldehyde-free fixing agents?

Some classic fixing agents are condensation products containing formaldehyde, and they leave free formaldehyde on the fabric. Brand and retailer restricted substance lists cap that figure, and the caps are tighter on babywear and childrenswear. It is also one of the items measured in OEKO-TEX and comparable certification work.

That is why buyers now raise it at the quotation stage. Saying a product is formaldehyde free is no longer enough; it has to be supported by documentation. In the Derin Kimya range, DK FX DR 45 and DK FX DRS 45 are formaldehyde-free fixing agents, DK FX R is a reactive dye fixing agent and DK FX DR CONC is the concentrated type. Application conditions are product specific: pending from the client.

From the fastness result back to the cause

Every row in that table assumes one thing: that washing-off is a measurable step. Looking at the colour of the last bath, recording how many soapings were run and filing the adjacent fabric result with the batch takes the fastness discussion out of the realm of opinion.