Dyeing

Cotton and other cellulosics take reactive (or direct for low-cost). Wool, silk, and nylon take acid. Polyester and other synthetics take disperse. Acrylic takes cationic. Deep indigos and sulfur blacks are vat dyes. The dye is chosen to match the fiber's functional groups and the wash-fastness target.

Direct dyes adsorb onto cellulose with weak intermolecular forces — they are cheap but wash out over time. Reactive dyes form a covalent bond with the cellulose hydroxyl group — the bond is permanent under home laundering, which is why reactive is the standard for cotton apparel.

Bleeding is dye that was not fully fixed or not thoroughly washed off after fixation. Reactive dyes need a hot post-soaping rinse to remove hydrolysed dye; skipping that step leaves dye that will bleed into the next wash.

Crocking is color transferring by rubbing. The standard test is AATCC 8 (or ISO 105-X12), which uses a crock cloth rubbed against the fabric under controlled pressure. The stain on the cloth is rated 1-5; commercial-grade fabric should rate 4 or higher dry and wet.

Polyester is hydrophobic and glassy at room temperature. Above its glass transition (~70 °C in water, lower with carriers), the amorphous regions swell enough for disperse dye molecules to diffuse in. 130 °C gives the kinetic energy and swelling time needed for full saturation in 30-60 minutes.

Three main sources: (1) liquor ratio drift between machines, (2) temperature probe calibration differences, and (3) salt-to-liquor ratio variation that changes the exhaustion curve. The fix is locking the recipe to a target LR ±5%, reprobing thermometers monthly, and pre-dissolving the salt in a side tank before adding to the main bath.