Remedies of colouration
Thermosetting
Improper dyeing can be traced to improper heat setting. Good
heat setting is the chief quality criterion in superior quality fabrics. The
heat setting temperature should be accurately measured using an infrared sensing
thermometer. The ideal range is 180-210C, 40-50C above
the dyeing temperature. It has an effect on the equilibrium dye uptake, more in
case of HT dyeing as compared to carrier dyeing.
The research shows that residual shrinkage is 0 per cent at
180C and thus thermosetting should be carried out at this
temperature only. It also shows minimum dye uptake at this temperature as
number of crystals has increased and the free volume has decreased. A reverse
trend is observed above this temperature-the number of small crystals
agglomerates to form larger crystals; thus increasing dye uptake.
Preventive measures include:
- Heat setting the entire lot in one go, rather than in
portions
- Distributions of hot air within the stenter should be
checked with the help of thermo papers periodically
- Dyes sensitive to variations in heat setting should be
substituted by less sensitive dyes
Dye dispersion
Since a small part of dye dissolves in water, the dispersing
agent is added for uniform dispersion. Increasing the dispersing agent amount,
results in reduced depth of shade. The dispersion stability also depends on the
particle size. Hence, optimum use of dispersing agent is important. In order to
ensure good dispersion stability, it is necessary to take certain precautions
like:
- No auxiliary used should have cloud point below 1300C
- Carrier, if added, should not be used in the beginning
of dyeing
- All impurities such as sizes, etc. should be removed
from the fabric
- For higher rate of heating as well as for higher rate
of liquor circulation, special dyes should be used
Besides oligomers, there are other factors that affect the
dispersability of dyestuff and affect the disperse dyeability of polyester
under high temperature. These complex factors are dependent on dyestuff,
substrate and dyeing conditions. They include:
- Factors related to dyestuff
- Chemical structure
and related physical characteristics
(the melting point, solubility in water
etc.)
- Physical characteristics
of dispersed dyestuff particles
- Type and
characteristics of dispersants in dyestuff
- Factors related to substrate
- Dyeability of
the fibre
- Density of fibre
structure
- Amount of
oligomer contained in fibre
- Presence of
oil/thickener adhering to the fibre surface
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