Textile Chemistry: Dyeing, Printing and Finishing
What the exam tests
Questions link fibre chemistry to the correct dye class, distinguish dyeing from printing, and ask what a finish does to the fabric at the fibre level.
Dye selection by fibre
- Cellulosic: reactive (covalent bond with cellulose in alkali, excellent wash fastness), vat (insoluble pigment reduced to a soluble leuco form with sodium hydrosulphite and alkali, then oxidised), direct and sulphur.
- Protein and nylon: acid dyes, with reactive dyes also used on wool.
- Acrylic: basic (cationic) dyes.
- Polyester: disperse dyes, applied at about 130 °C under pressure or with a carrier because the fibre is hydrophobic and compact.
- Pigments: insoluble, fixed on any fibre with a binder.
Printing
Printing is localised dye application using a thickener. Methods include block, flat screen, rotary screen, roller, transfer (sublimation, mainly polyester) and digital inkjet. After printing, goods are steamed to fix the dye and washed to remove thickener. Discharge printing removes ground colour; resist printing prevents dye uptake.
Finishing
Mechanical finishes include calendering, raising and sanforising (compressive shrinkage). Chemical finishes include mercerising with about 18–20% sodium hydroxide, which swells the cellulose, raising lustre, strength and absorbency; easy-care resin finishing; water repellency; flame retardancy; and antimicrobial treatment.
Worked example
A polyester t-shirt must match a cotton one in shade. Cotton takes reactive dyes, but polyester has few reactive sites and is hydrophobic, so a reactive dye would give a weak, poorly fixed shade. Disperse dye is chosen: fine particles dissolve into the fibre above its glass transition temperature, giving level, fast shades at 130 °C under pressure.
Traps that cost marks
Mercerising is a finish, not a dyeing stage. Printing is not simply dyeing the whole fabric. Vat dyes are insoluble as supplied. Transfer printing is largely limited to polyester. Do not name reactive dyes as the standard choice for polyester.