Acrylic

On acrylic, the rate of dyeing changes abruptly with temperature. Above a certain point the fibre softens and takes dye up quickly, and the few minutes spent around that threshold decide whether the lot comes out level.

Why is temperature so critical on acrylic?

Below its glass transition temperature, acrylic is all but closed to dye. Above that threshold the fibre structure loosens and cationic dye enters rapidly. The transition falls roughly between 78 and 82 °C and shifts with fibre type.

The consequence: the heating rate matters, especially between 75 and 90 °C. A machine that races through that band dyes the part of the fabric that enters the bath first darker than the part that follows. For the same reason cooling is slow; rapid cooling both builds internal stress and sets crease marks permanently.

What does a retarder do, and when is it too much?

Cationic dye is positively charged and binds to anionic sites in the acrylic fibre. A retarder competes for the same sites and slows transfer, buying time for the dye to distribute evenly.

Too much comes back in two ways: colour yield falls (dye stays in the bath rather than on the fibre) and the shade shifts. The right dosage follows the dye class and target depth, and is confirmed in a trial bath.

In the catalogue these products sit under dispersing and levelling agents, in the levelling agents family.

Are crease marks on acrylic permanent?

Usually yes. Acrylic is thermoplastic: the shape it takes while hot is held as it cools. Fabric folded at dyeing temperature leaves the machine with that fold set into it, and ironing will not take it out.

Prevention is on the production side: do not cut the liquor ratio, match machine speed to the fabric, and cool slowly. Anti-creasing agents raise lubricity so the fold does not always form at the same point; the mechanism is in the crease marks article.

Pre-washing and finishing

Acrylic yarn is spin-finished with oil and antistatic. Left on the fabric, it blocks dye transfer in patches. Wetting agents and washing agents do this step, near neutral and at a moderate temperature.

In finishing, the handle expected from acrylic is soft and bulky. Blankets, knitted outerwear and plush use softeners and silicone emulsions. Stenter temperature is kept below the fibre's softening point; above it the handle turns harsh and the lustre is lost.

Which product groups are used on acrylic?

Wetting agents, dispersing and levelling agents, anti-creasing agents, antifoaming agents, washing agents and finishing agents.