Colouration

The fibre can be dyed in the form of fibre, yarn, fabric or slivers. They are usually processed as blends-with cotton, rayon, wool or acrylics.

Polyester fibres are dyed by disperse dyes due to their firm inner structure, large proportions of crystalline regions, smaller numbers of attaching points for different dye types and pronounced hydrophobic nature. The dyeing is chemically called solid solution theory, as the disperse dyes do not dissolve but remain finely dispersed in water. The crystalline areas adsorb the dye at high temperature (130-140C).

Pale to medium dyeing can be obtained by using carriers at the lower boiling point of water (100C), under atmospheric pressures. However, carriers (based on chlorobenzenes di or tri) are not eco-friendly, and banned. Hence, loosening intermolecular links helps to dye darker shades, as the fibre structure allows dye molecules to diffuse easily and rapidly.

The thermosol process is the third method. In it, disperse dyes dissolve rapidly in the polyester fibre, producing dyeing with good fastness properties, when heated to approximately 200C. Dyeing polyester from organic solvents and using liquefied CO₂ (which is recovered and reused) are catching on.

Problems:

• Variations in the dyeing behaviour of the fibres due to variations in their production or thermal pretreatments
• Hard water can have significant effect on the colour value of disperse dyes
• Moir effect due to heat setting problems and uneven relaxation of fabric tensions
• Off shades is the biggest problem and it occurs due to combination of various factors like:
  • Choice of dyes
  • Choice of auxiliaries like dispersing agent, leveling agent, carrier etc.
  • Choice of machinery
  • Operation negligence during various processing steps like heat setting, washing and reduction clearing, etc
  • Dyeing method
  • Pretreatment will still leave behind some sizes, coning oil, etc.
  • Concentration of dyes and various auxiliaries
  • Rapid dye uptake by polyester above glass transition temperature may cause off shades The dye uptake is less than 10% below 75C and it increases rapidly above it causing higher strike rate
  • Inappropriate increase rate of temperature in the HTHP dyeing method
  • Dispersion stability of disperse dyes
  • Density of fibre structure
  • Dye bath pH
  • Dye bath circulation efficiency
  • Simmering temperature and rising temperatures, as well as substrate to liquor ratio

Other problems include: o

• Oligomers
• Pilling
• Migration