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
  • Factors related to dyeing conditions

    • Dye concentration
    • Characteristics and amount of dyeing auxiliaries (dispersant, leveling agent, carrier etc.)
    • Dye bath pH
    • Quality of water used in dyeing
    • Dyeing temperature (temperature raising conditions, simmering temperature etc.)
    • Dyeing time
    • Volume of substrate in the dye bath
    • Dye bath circulation efficiency

 

Dye bath pH


90 per cent of disperse dyes are azo dyes, which decrease in the presence of alkaline pH; thus reducing the shade depth. Hence, acidic pH is very important to maintain in dye bath.


Time


Prolonged boiling or high temperature increases the particle size of the disperse dye through agglomeration of dye crystals. High agglomeration causes irreversible precipitation of the dye. Hence, to minimise the aggregation tendency, proper selection and control of dispersing agent and additives is essential.


Machine


Package or beam dyeing machines are strictly discouraged for polyester dyeing. A variety of jet dyeing machines-including the soft flow type and low liquor ratio type over flow units-are more or less suitable for dyeing polyester.


The chances of uneven dyeing can be removed by using high rate liquor circulation. Polyester fabrics are structured with a large number of small interstices which make it extremely difficult to wash out any unfixed dye and chemicals completely. Therefore the dyeing machine used should also have an efficient washing system.


The latest machines handle a wide range of fabrics including PET micro fibre fabrics. This machine helps maintain the quality associated with level dyeing of PET. In this machine, a new air jet attachment has been introduced to provide a solution to reduce the variation in dyeing. This newer jet dyeing machine with air jet attachment is especially good for dyeing the pure synthetic polyester fabrics at a low liquor ratio of 1:3


Skitteriness


Skitteriness occurs due to the differential dye uptake by the fibres in the fabric. Often one cannot dye two components in a fibre blended fabric in exactly the same shade if one of the fibres is highly lustrous while the other is devoid of luster.


Remedies include:


  • In piece dyeing, ensure that polyester of different deniers is not mixed together at any stage. The skittery could be due to mixing of polyester fibres of different deniers between 1.2-1.5 deniers.


 

  • Use polyester fibre with low oligomer content. Oligomer deposition on the surface of the fabric leaves behind whitish impurities, which remain on the fabric even after severe reduction clearing and dyeing with vat dyes on jigger. This happens mainly while beam dyeing of polyester. Another solution is to drop the bath at high temperature into a pond of cool water. Oligomers are soluble at such high temperature. Jet dyeing should be preferred to beam dyeing machine.


Streakiness


Usually in the warp direction, instances of weft streakiness can also be found. Fabric related warp streaks show a sharp boundary running along the warp, wherein single or multiple warp threads are present. Warp threads processing run parallel to the warp, but involve different warp threads along the fabric length. They generally do not involve a single warp thread. It is not always easy to identify the root cause of streakiness and all the causes mentioned above should be taken care of to avoid streakiness


Antipilling


The textile finishers role has become increasingly demanding. It requires careful balance between the compatibility of different finishing products and the treatment. Certain rubber-based latex compounds, non-ionic polyurethane resins, and self-cross linking binders can all be used as effective antipilling agents.


The enzymatic process biopolishing improves fabric quality. Cellulose enzymes processing cellulose-based fabrics gives superior handle, luster and novel finishes on cellulose. Hence, they can be effectively used as antipilling agents.


References


  • ND Iyer, Polyester the most versatile synthetic fibre, Colourage, September 01 to February 04 (series of 26 articles)
  • AK Samnta, Road map for wet processing of micro denier polyester fabric and relevant studies on its dyeability, Man Made Textiles, October 03
  • Dr Bharat Desai, Oligomer control in polyester fibre processing, Colourage June 05
  • Prof VA Shenai, Technology of dyeing
  • Mr CN Sivaramkrishnan, Dispersing Agent, Colourage, September 05
  • AD Sule, Practical Problems in dyeing of polyester off shades, Man Made Textiles, December 03
  • Search engine google.com



The author is associated with Resil Chemicals, Ahmedabad