This is a technical article by Harry Mercer on Indigo dyeing. It is the fourth and the last part of the article in series. The first part can be seen by clicking here and the second one here and the third one here

Part 4 of 4: Denim Dyeing Done Right

Indigo dye is unique among textile dyes with the unusual color and appearance that it imparts to any fabric, and is along with the extreme comfort of 100% cotton denim is the source of denim’s enduring appeal. Attempts to replicate the Indigo color with other dyes ,like sulfurs, only results in a poor imitation.The process of dyeing Indigo is also unique because of the necessity for repeated applications in multiple dye baths. Indigo is actually a better wool and silk dye and has been used to dye those fibers for centuries.

All other cotton dye types- reactives, directs, sulfurs, naphthols and vats- have simple application methods usually only requiring only a single bath application and simple machinery arrangements.

Indigo dyeing on the other hand presents complexities with regard to machinery design that includes multiple dye baths and a circulation system that is needed to keep the Indigo dye in motion with pipes through which the dye flows into and out of each box and in a circuit around the dyeing section which can have a total dye volume of from 1000 to 30000 liters while ordinary dyes need only a few hundred liters for application. This is necessary because Indigo dye, exists not in a true solution like acid or basic dyes, but are in colloidial state- clusters of dye molecules in a reduced condition that will sink to the bottom of the dye bath due to gravity if not stirred constantly. The degree of circulation pressure must be balanced so that there is a uniform concentration of Indigo from top-to-bottom and from side-to-side for consistent dyeing. If the circulation pressure is too high there will be excessive turbulence in the dye baths which will result in the reduced Indigo and the reducing agent breaking down. In engineering terms (Reynold’s Number), the flow should be greater than laminar, but in the low transitional range and never approaching turbulent conditions. Unfortunately, a number of Indigo dyeing machines offered in the last 20 years, are badly designed for maintaining uniform bath circulation, especially sheet ranges which usually have entry- and exit pipes that are too small to achieve even dye distribution in the boxes, which results in serious problems like Cross-Shade Variation. These problems have been complicated in many denim companies by the feeding of sodium hydrosulfite as a dry powder which is highly unstable when mixed into the dye, most of it being wasted through unnecessary decomposition. Hydrosulfite should never be added to the dye unless in a stable liquid mixed with alkali, which was the method used by all U.S. denim companies.

More importantly for achieving the desired tone(red versus green), consistency of color and color-fastness (to rubbing , washing and staining) is understanding the complicated chemistry of Indigo. Other dyes are mixed and applied in basically the same chemical state. When Indigo is mixed with sodium hydroxide and sodium dithionite (hydrosulfite) a series of reactions occurs in which the Indigo is combined with hydrogen and sodium to form the reduced indigo form. Additionally, approximately half of the sodium hydroxide is converted into sodium carbonate and other alkalies while the sodium dithionite reducer decomposes into an acid and simpler reducing agents like sodium sulfite. Three ingredients are mixed and between 9 and 12 new chemicals are created. Because of this phenomenon and the special design of the circulation system, Indigo dyeing can only be correctly understood with a basic knowledge of chemical engineering. I have found in training programs in many denim companies, that only trained chemical or mechanical engineers are readily capable of understanding the Indigo dyeing problem well enough to have perfect results

In order to measure the chemical concentrations for Indigo dyeing there are only certain methods that have proven to correlate with the yarn color, which is after all, the measure of effectiveness for any test method. If test results are plotted comparing the measures of dye and chemical concentrations vesus the yarn color and colorfast-ness and changes in concentration measurements do not reflect similar changes in the color, then the wrong test method is being used.

Extensive research has demonstrated that for testing the strength of the Indigo mix, the glass plate method and the 2-endpoint titration for caustic/carbonate have always proven reliable. For testing the concentrations of reduced Indigo and hydrosulfite in the dye bath, the vat-ometer , mentioned in a previous article will correlate well with the color produced and can also be used to control color-fastness.

Unfortunately, the most commonly used methods, will not correlate with color and therefore are not useful for solving Indigo dyeing problems. These include:

–pH and millivolt meters which are incapable of quantitatively measuring concentrations of sodium hydroxide and sodium hydrosulfite. They provide only a general qualitative measure of the number of particles in a liquid indirectly by measuring electrical currents. The only difference between a pH meter and a millivolt meter, other than the electrode used is that one converts electrical measurements into a logarithmic scale(pH) and the other to a linear scale(ORP) which reflects a buffered condition and not concentrations-only concentrations matter in chemistry.

Automatic titrators for use with the 2-endpoint ORP titration are not useful in monitoring Indigo dyeing for production. The sample size is too small for accurate results and there is a persistent problem with the titrator trying to identify the correct endpoints. 20 years ago, I introduced the potassium ferricyanide method to the public in an article. It had been in use by a small group of denim experts in the U.S. for many years, but only worked with manual titration with a large sample size. As a denim consultant for the last 18 years, I have never observed results from automatic titrators that correlated with the Indigo color.

Dye strength measurement by colorimetric transmission measurement is false sophistication. In order to measure the strength of a dye by transmission and converting it to absorbance requires application of the Beer-Lambert Law. The basic principle is that light absorbance is directly proportional to the concentration of a dye is correct, however there are limitations which include the need for a true solution at very specific concentrations (<0.01 M) that avoid light scattering. Indigo dye, as mentioned earlier, exists as a colloidial dispersion which produces turbidity, which, in turn, causes light-scattering and defeating attempts to measure absorbance.

Indigo dyeing can, in fact, be conducted at nearly perfect levels with a properly designed training program that illustrates the chemistry, mechanics and management methods required to get the dyeing right which I have provided for many years.

This is a guest post by Harry Mercer. Mr. Mercer has 30 years experience in the denim business including 3 prominent U.S. denim companies.He is an expert colorist for measurement and color matching as well as textile testing and is considered to be a leading authority for denim dyeing, finishing and fashion denim development.