Polyamide

Polyamide takes up dye readily; the difficulty is not uptake but evenness. Amino end groups pull dye in quickly, and the smallest difference between yarns shows on the fabric as a streak.

Why is levelness the real problem on nylon?

Polyamide is dyed with acid dyes. The dye binds to positive charges on the fibre end groups, and that bond forms fast. As in any fast-dyeing system, small differences in the bath are written large on the fabric.

The second difficulty comes from the fibre itself. Even within one lot, yarns can differ in draw history and in the number of amine end groups. The difference is invisible before dyeing and appears afterwards as barriness. The fabric is sound, the dyeing was even, and the result is streaky.

What do barrier and levelling agents do?

The two effects usually come from one product, in the dispersing and levelling agents group.

• The levelling effect slows dye transfer to the fibre, so the bath dyes more evenly over a longer time.
• The barrier effect temporarily blocks the fibre regions that take up more dye, reducing how far yarn differences carry into the fabric.

The right dosage is found by trial: too much not only slows dyeing, it also shifts the shade.

Why is fastness treated separately?

The bond between acid dye and fibre is ionic and loosens in washing. Harmless on pale shades, this becomes a wet fastness complaint on dark shades and in sportswear.

That is why nylon dye fixing agents exist as their own family. A fixing agent seals the dye on the fabric and makes it harder to wash out. It is applied after dyeing in a separate bath; because dosage and pH can shift the shade, it is trialled first on dark colours.

Can pre-washing be skipped?

No. Nylon yarn is oiled before knitting and weaving, and that oil is not evenly spread on the fabric. If it is not removed before dyeing, the areas where it remains take dye differently.

The groups that do the work: wetting agents and oil stain removers. Nylon is less alkali-resistant than cotton, so pre-washing is run near neutral and at a moderate temperature.

What changes with elastane in the blend?

Nylon-elastane is standard in hosiery, swimwear and sportswear. Elastane limits the dyeing conditions: high alkali and long exposure to high temperature damage its elasticity.

Both the temperature and the pH window therefore narrow, levelness gets harder still, and auxiliaries are chosen around the elastane's limits. See the elastane page.

Which product groups are used on polyamide?

Wetting agents, dispersing and levelling agents, washing agents, fixing agents, antifoaming agents and finishing agents.