Dyeing and Finishing
Dyeing and Finishing examines colour application and the functional treatment of fabrics. The exam tests dye-fibre substantivity, exhaustion and fixation, levelness, fastness and the trade-offs of chemical and mechanical finishes.
Core concepts
- Reactive dyes form covalent bonds with cellulose; unfixed dye must be washed off to achieve fastness.
- Disperse dyes colour polyester by dissolving into the fibre; heat setting, carrier and thermosol conditions affect uptake.
- Exhaustion is dye removed from the bath; fixation is dye bound or trapped in the fibre. High exhaustion does not guarantee high fixation.
- Levelness depends on migration, agitation, temperature control and electrolyte addition.
- Finishes include softening, repellency, flame retardance, anti-pilling, compressive shrinkage and curing.
Worked example
A 500 kg fabric lot is dyed at 2% owf, so dye charged = 10 kg. If spectrophotometry shows 90% exhaustion, 9 kg leaves the bath. If fixation is 80% of the exhausted dye, 7.2 kg is fixed and 1.8 kg must be washed off. Reporting 90% as fixation overstates colour yield and understates effluent load.
Traps
- Equating exhaustion with fixation.
- Ignoring shade strength versus dye concentration.
- Overlooking pH and temperature sensitivity of reactive dyes.
- Applying a finish without checking handle, strength and fastness trade-offs.