Nylon Dye Fixing Agents

Nylon dye fixing agents cap the free amino groups on the surface of polyamide, making it harder for acid dye to leave the fibre in perspiration and washing. Unlike the classical cotton agents they are anionic. The Derin Kimya range covers three products.

Why does fastness fall away on nylon?

Polyamide holds acid dyes on the amino end groups of the fibre. Those groups are limited in number; on deep shades more dye is applied than the fibre can hold, and the excess sits on the surface. In perspiration, in hot water or in damp storage that dye leaves the fibre.

The visible form of the problem usually appears on a second fabric. Dye that crosses onto a white ground, or onto the elastane in swimwear and hosiery, pulls the wet fastness report straight down. On polyamide blends, elastane picking up dye is a complaint in its own right.

How does a nylon fixing agent work?

Unlike the cationic agents used on cotton, nylon fixing agents are anionic structures, known in the trade as syntans. They bond to the free amino groups on the fibre surface and cap them.

That has two consequences. Dye left at the surface is held in a stronger bond with the fibre, and because the sites an anionic dye would reach are now blocked, cross-staining is reduced. The same mechanism, applied before dyeing, is used for stripe control in hosiery and swimwear production.

What changes on nylon-elastane?

Elastane picks up acid dye but does not hold it firmly; that dye comes back out in washing and in perspiration. A nylon fixing agent will not close this on its own. The recipe usually runs in two steps: an after-wash that clears dye from the elastane, then fixing.

Reverse the order and the result suffers. Once the cloth is fixed, stripping dye out of the elastane becomes harder, because the fixing agent holds that dye in place too.

How is the result measured?

Two tests dominate on the nylon side: perspiration fastness and wash fastness. In both, what actually counts is the dye that crosses onto the adjacent white cloth rather than the change in the shade itself. For swimwear and underwear, chlorine and sea-water fastness enter the specification as well.

To see what the fixing agent contributes, two swatches from the same dyeing are compared, one fixed and one not. The recipe is discussed against the difference between those two swatches.

What sets the dosage?

Depth of shade is the deciding factor. A deep shade leaves more dye at the surface, so it needs more fixing agent. Running the same dosage on a pale shade means cost and handle given away for nothing, so the dosage is stepped against the shade card and confirmed in the lab.

What to watch during application

Nylon fixing agents work on the acid side, and pH is the main variable behind the result. Alkali left in the bath, or a heavy electrolyte load, both lowers uptake and raises the risk of spotting.

They should not share a bath with cationic softeners; the two opposite charges precipitate and mark the cloth. Application figures for Derin Kimya products are given on the product pages: pending from the client.

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