Direct & Reactive Dye Fixing Agents

A fixing agent binds the anionic dye the fibre never held into a cationic complex, making it harder for that dye to move into water. The gain shows in wet fastness; the cost is paid in light fastness, shade and handle. The Derin Kimya range covers eight products.

How does a fixing agent raise fastness?

Direct and reactive dyes are water-soluble, and the sulphonate groups that make them soluble stay on the cloth after dyeing. In washing, in perspiration or in damp storage those groups pull water in, and dye that is not held by the fibre leaves the cloth. The result is bleeding, staining of adjacent fabric and poor wet fastness.

A fixing agent is cationic. It pairs with the anionic groups on the dye and forms a complex that does not dissolve in water. The dye can no longer move into water as readily, and wet fastness figures improve.

That also marks the limit of the job. A fixing agent locks down dye the fibre never held; it does not repair a badly washed-off dyeing. On reactive shades, washing the hydrolysed dye out is still the first step.

Direct dyes or reactive dyes?

A direct dye is held on the fibre by physical forces alone, with no chemical bond. It is therefore the dye class that gains most from fixing, and DK FX, DK FX 2 and DK FX D are the products set aside for it.

A reactive dye forms a covalent bond with cellulose, so its base fastness is already high. The trouble comes from unfixed and hydrolysed dye. DK FX R serves the reactive side, while DK FX DR CONC and DK FX NEW are concentrates that work on both classes.

Formaldehyde-based and formaldehyde-free agents

Part of the classical fixing-agent range is built on formaldehyde condensation products. These give high fastness but can leave free formaldehyde on the cloth, and many brand specifications now cap it.

Formaldehyde-free grades are built on polymeric cationic structures instead. DK FX DR 45 and DK FX DRS 45 belong to that group. The formaldehyde status and documentation of each product are given on its product page: pending from the client.

What does fixing cost?

Fixing is not a free improvement. It is paid for in three places.

The first is light fastness: on some dyes the cationic complex brings it down. The second is shade; a fixed dyeing usually reads slightly deeper or shifts in tone, which is why shade approval is taken on fixed cloth. The third is handle, since a cationic polymer can stiffen the fabric.

Where a cationic softener follows the fixing agent, the two should not share a bath. A separate bath, or an intermediate rinse, prevents precipitation and spotting.

Does every lot need a fixing agent?

No. On pale and medium shades a well washed-off reactive dyeing can meet the specification without one. Fixing is used on deep shades, at high dye concentrations, and where the wet fastness limit is tight.

Two things settle the decision: the wet fastness level the customer asks for, and how the dyeing behaves on its own. With both known, the fixing dosage is set from measurement rather than held as a safety margin.

Application

In exhaust, the fixing agent goes in after the final rinse, into a warm bath taken to the acid side. On the padder it is applied in a single pass and the effect follows the pick-up. Either way it is dosed diluted rather than as a concentrate. Application figures for Derin Kimya products are given on the product pages: pending from the client.

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