Polyester

Polyester does not like water and carries dye inside the fibre rather than on it. Dyeing therefore runs near 130 °C under pressure, and the auxiliary's job is to keep the dye well dispersed in the bath before it ever reaches the fibre.

Why do auxiliaries behave differently on polyester?

On cotton the dye bonds to hydroxyl groups inside the fibre. On polyester there is no such bond: disperse dye is almost insoluble in water, works its way into the fibre structure as it loosens with heat, and stays trapped there.

Two consequences follow. First, the dye is suspended rather than dissolved; the moment it stops being suspended it agglomerates and spots the fabric. Second, the dyeing temperature is high — an auxiliary that is not stable at 130 °C breaks down and leaves a mark.

Dispersing agent or levelling agent?

Both sit in the dispersing and levelling agents group, but they do different jobs:

• A dispersing agent keeps dye particles apart in the bath. Its target is settling and agglomeration.
• A levelling agent slows the rate at which dye transfers to the fibre. Its target is unlevelness and centre-to-selvedge variation.

Needing both in one HT bath is normal. Raising only the levelling agent when unlevelness appears is the usual reflex; done without reading the pattern it does not fix the cause. The unlevelness article sets out four patterns pointing at four different places.

What is the oligomer problem?

Short cyclic molecules left over from polyester manufacture are called oligomers. At high temperature they migrate out of the fibre into the bath, and as the bath cools they deposit as a white, dusty residue on machine surfaces and on the fabric. They show up most on dark shades and on blends.

There are two ways to limit it: drain hot, so the fabric is not in the machine during the cooling window when oligomer precipitates; and use an auxiliary that keeps oligomer suspended. For build-up on machine surfaces, machine cleaning agents are a separate group.

Why is reduction clearing not skipped?

After dyeing, dye remains stuck on the fibre surface without having entered it. That fraction comes off in washing and rubbing, so it lowers fastness directly.

Reduction clearing reduces that surface dye to a soluble form and takes it into the bath. Skipping it on dark shades makes a wet rub fastness complaint all but certain. On elastane blends the alkaline conditions are limited; there, special-purpose washing agents that work near neutral are preferred.

Foam and creasing in jet machines

In high-speed jet and overflow machines foam is a genuine production problem: it disturbs fabric flow, misleads sensors and causes overflow. Antifoam selection follows the bath temperature and the surfactants already in it; the wrong choice comes back as a silicone spot. See choosing an antifoam.

Crease marks also appear on microfibre and heavy polyester. Anti-creasing agents do the same job here.

Finishing

Handle on polyester is usually built with silicone emulsions. The difference between micro, macro and elastomeric types, and the yellowing risk, is covered in choosing a silicone softener.