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In the Denim Industry today, achieving sustainability during manufacturing processes has become very important and the most talked about subject.
Indigo dyeing is the most difficult and complicated process among the various types of dyes applied on Cotton. It involves complex chemical reactions across multiple dye-baths. Controlling the shade consistency requires sound understanding of the basic concepts of chemistry that are used during the dyeing process.
Using right production parameters as well as effectively controlling them, gives substantial savings in the consumption of Indigo dyes and the auxiliary chemicals. Here, same depth of indigo shade with excellent ring dyeing effect on the yarn can be achieved with much reduced chemistry.
A better sustainable process not only reduces the load on the effluents, but also helps in saving the cost of chemicals, energy and water coupled with reduced cycle time during the subsequent garment washing / laundry process.
There are lot of technical papers and write ups available which explain that the chemistry of indigo dyeing process in great detail by showing the figures of molecular structure and chemical equations. To a non-chemistry background person, it is difficult to grasp as to what exactly happens during the dyeing process. Hence, here I will focus on explaining the mechanics of indigo dyeing process instead chemical reactions and equations. I am also from a non-chemistry background. My understanding of these concepts came from the several interactions I had with my shop floor colleagues and the experts in the dyestuff industry during my career.
Basic Concepts:
The yield of indigo color depends on the degree of its penetration in the yarn. Given the same amount of indigo; lesser the penetration of the dye, better is the ring dye effect resulting in darker shade depth.
The degree of indigo dye penetration depends on its solubility level. A lower solubility produces darker color. Solubility is determined by the pH of the dye-bath.
Lower PH levels of 10.5 to 11.5 in the dye-bath, changes the leuco indigo dye to the mono-anionic form, which is a state of dispersed clusters. In this form, the strike rate of dye is very high but it has poor penetration. This results in the dye getting concentrated more on the yarn cover giving a darker color hue and a better ring-dyeing effect.
On the other hand, as the pH level is increased beyond 11.6 (max. <13) the indigo dye starts to convert into di-anionic form which is a state of soluble clusters. In this form, the penetration is higher. This results in diffusion of the dye to the inside of the yarn core, resulting in poor ring-dyeing effect and lighter color hue given the same amount of dye.
Similar phenomenon of change in indigo shade depth is observed as the redox potential of the dye-bath changes. Lowering the redox potential of the dye-bath results in darker shade depths. Hence at low redox potential, less amount of dye and hydrosulphite is required to achieve the same shade depth. It is needless to say, that when you use less amount of chemistry, less water is needed to wash the substrate resulting in good savings of water.
Right parameters to dye dark indigo shades:
I have observed, that in the industry, in order to dye very dark shades of 3.5% to 4% Indigo depth, conventionally the technicians use parameters of about >12 pH and >800 redox potential. Invariably, the shade hue & tone obtained with such high-range process parameters is ‘deep indigo blue with a red cast’. Such Denim fabrics give a dull & flat blue color esthetic appearance; when garments made out it, undergo heavy wash-downs at the laundry stage.
I would strongly recommend from my personal experience, that for a good dark indigo shade (free from red cast) with proper ring dye effect on the yarn, one should use low PH and Low redox potential. This will give brighter hues of indigo blue with excellent ‘salt-pepper’ effect when garments undergo heavy wash-downs at the laundry stage.
From my practical experience I have found that, very dark indigo shade depths of 3.5% to 4% can be dyed at pH levels as low as 11.0 +- 0.1 and redox potential as low as 720 +- 20. The actual consumption of indigo and hydrosulphite at these dye-bath parameters are substantially lower as compared to dyeing the similar shade depth at pH >12 and redox potential >800.
Right methods to control parameters in the dye-bath:
Sodium hydrosulphite is sensitive to atmospheric oxygen. The hydrosulphite used in indigo dyeing decomposes under the presence of atmospheric oxygen especially in an alkaline medium.
The Leuco form of Indigo turns yellow when its reduction is complete. It turns to green very quickly within 12-15 seconds the moment it gets oxidized. To avoid undue oxidation of the dye-bath, some extra quantity of hydrosulphite is always added to maintain a buffer in the dye-bath.
Dyeing at low pH & low ‘redox potential’ means having a reduced amount of extra-buffer of the hydrosulphite in the dye-bath. This poses an increased risk of potential depletion of the ‘free hydrosulphite’ if monitoring is not done properly. For this reason, most of the technicians are tempted to use higher pH with higher redox potential in the dye-bath to avoid any catastrophes.
More often I have come across the dyeing technicians using the conventional old test method of measuring only the pH and the total redox potential at frequent intervals during the dyeing process. The readings of this method remain very steady during the dyeing of the whole set-length, giving them the false mental comfort of dyeing parameters being under control.
However, they do not realize that the above method does not give them any idea of the level of concentrations of ‘reduced indigo’ and ‘free hydrosulphite’ present in the bath. These are the two most relevant parameters besides the pH which are important for controlling shade consistency during indigo dyeing.
The above process control can be easily acheived by installing a ‘Potentiometric auto titration analyser’. This will help quickly and accurately measure the right concentration levels of the 'reduced indigo' and 'free hydrosulphite' in the dye-bath.
I believe that the above discussed approach will certainly result in making the indigo dyeing process more consistent and sustainable.
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