Dyeing Problems | Aggregation of Dyes | Measurement of Dye Aggregation | Reasons of Dye Aggregation in Dyebath | Prevention of Aggregation

Many problems take place during dyeing operation . Some problems are given below:
Dyeing Problems

1. Aggregation of dyes

2. Unlevelness (due to material)
  • Barriness
3. Unlevelness (due to other reasons)
  • Skitterness
  • Listing
  • Pale areas after dyeing
4. Reproducibility
  • Deviation of shade
  • Dyeing too pale
  • Precipitates in the dyebath
  • Change of shade
5. Fastness properties
  • Unexpectedly poor light fastness
  • Unexpectedly poor wet and rub fastness
6. Spots, marks
  • Precipitates in the dyebath
  • Singeing droplets
  • Change of shade (e.g. blue spots in brilliant red shades)
  • Dark spots
  • Specks
  • Dirt spots
7. Appearance of the goods
  • Dimensional stability (shrinkage)
  • Creasing
  • Chafe marks
  • Stitch distortion (knits)
  • Moire effects (on beam)
  • Lustre
  • Handle
  • Pilling (staple fibres)
8. Other problems
  • Difficulty with heavy shades on fine denier yarn
  • Soiling of machine
  • Frosting on goods
  • Poor running of material in subsequent processing
9. Thermosol dyeing problems
  • Listing
  • Two-sidedness
  • Dark or pale selvedges
  • Barry dyeings
  • Spots, specks
  • Ending
  • Pale points of intersection
  • Foam formation
  • Frosting 
10. Unlevelness(due to dyeing conditions)
  • Ending problems
  • Cloudy dyeing
  • Pale areas 
Aggregation of Dyes
Main problems during dyeing is Aggregation of dyes. Dye-dye self association in solution is called dye aggregation, which is important phenomenon where dye molecules or ion takes part. In general, the term aggregation is used for dye-dye interaction and dye association for interaction of dyes with other compounds e.g. polymers.

Generally dye molecules form aggregation in aqueous solution at room temperature and to an extent which depend on

  • Size of dye molecules
  • No of solubilizing groups in the dye molecules
In dye aggregation multiple equilibria need to be considered i.e. diametric, trimetric etc, aggregates are formed.

Measurement of dye aggregation

  1. Conductometry
  2. Calorimetry
  3. Polarography
  4. Solubility
  5. Sedimentation
  6. Fluorescence
  7. X-ray diffraction
  8. Measurements of diffusion coefficients
  9. Activity of counter (Sodium)
  10. Light scattering
  11. Evaluation of colligative properties
  12. Visible light adsorption
  13. H and 19F NMR
Reasons of dye aggregation in dyebath

1. Dyes are consists of

  • Hydrophobic aromatic portion
  • Polar groups (OH, amino etc.) for water solubility and charged groups (sulfonic or positive charged groups) for rendering molecule water soluble
When dye molecules dissolved in water a new interface is created between the hydrophobic portion and water. Dye can reduce the size of the interfacial water by overlapping of the hydrophobic areas and there will be a tendency to aggregate.

2. Usually linear and planar dye molecules should tend to stack one molecule upon another with the ionized groups arranged so as to give minimum free energy condition causes aggregation.

3. Dyes with long aliphatic chains form micelles of a spherical form in which the flexible chains associate in the interior with the sulfonic acid groups exposed on the surface of sphere.

4. Aggregation of dimer is more obvious as aromatic ring system have maximum overlap (van der waals forces) because the distance between the anionic charges is larger (minimum electrostatic repulsion).
As dye concentration increases there will be an increased tendency for trimers, tetramers etc. to be formed.

5. Aggregation is also expected from the unusual structure of water. When the interface is formed on dissolution of the dye molecule, the water molecules adjacent to the hydrophobic portion form an ‘iceberg’ type structure accompanied by a reduction in entropy. When the dye molecules aggregate not only will energy be gained from the reduction on the interfacial energy but also an increase in energy will rise from the melting of the iceberg structure.

6. Calculation shows that below concentration of 10-5 mole/L various higher aggregates appear, giving a polyassociated system.

7. Higher ionic strength, ionic dye aggregation becomes more dominant.

Prevention of aggregation

1.By raising the temperature of dyebath
2.Liberation and existence of monomers by circulations or stirring and keep concentration below 10-5 mole/L of dye.

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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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