Mercerizing Wetting Agent

In mercerising the caustic has to enter the fibre quickly and evenly, and caustic solution on its own is a poor wetting medium. A mercerising wetting agent closes that gap and is formulated to stay soluble under a concentrated electrolyte load. One product in the family: DK WET MRS.

What is the wetting agent doing in mercerising?

Mercerising runs cotton under tension through a concentrated caustic solution. The fibre cross-section swells and rounds out; lustre rises, dye uptake improves and dimensional stability gets better. All of that depends on the caustic entering the fibre evenly and quickly.

Caustic solution on its own is a poor wetting medium. Its surface tension is high and it is slow to get past the waxy surface of raw cotton. Where wetting is slow, the caustic does not reach every part of the cloth at the same moment and the mercerising comes out uneven. It shows in the dyeing: different areas take up different depths.

Why a separate product family?

The caustic concentration used in mercerising is above that of every other pre-treatment bath, and the trade normally states it in degrees Baumé. That is a class condition, not a product specification: pending from the client.

At that concentration an ordinary surfactant does not stay in solution. Nonionic structures separate under the electrolyte load and collect on top of the liquor; some anionic structures salt out. Mercerising wetting agents are formulated to stay soluble under those conditions; the first requirement is electrolyte tolerance, not wetting speed.

What does foam do on a mercerising range?

Mercerising is a continuous line with the solution circulating throughout. Foam means caustic going over the edge: chemical lost and a safety problem. A foam layer also breaks the contact between the cloth and the solution and creates unevenness.

A mercerising wetting agent therefore has to be low-foaming. Holding high wetting power and low foam in the same product is most of the formulation difficulty in this family.

Where does uneven wetting show up?

A mercerising fault is not always visible to the naked eye; it appears when the cloth is dyed. Unevenly mercerised areas take up dye differently and read as bands across the width or as clouding.

Mercerising is therefore checked without waiting for the dyeing. The barium activity number is the standard laboratory way of measuring the degree of mercerising, and regular measurement on samples taken at the range exit catches the problem before dyeing.

When does the wetting agent act?

The wetting agent is dosed into the caustic solution and starts working the moment the cloth meets it. Contact time in the impregnation zone is short, and the product is expected to finish the wetting within it.

In the wash boxes the priority changes. There the aim is to get the caustic off the cloth and neutralise it; caustic left behind stiffens the fibre in drying and shifts the shade in the following dyeing. The contribution of the mercerising wetting agent in that zone is to help the wash water penetrate the fibre.

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