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.

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

Part 3 of 4: Monitoring of the Indigo Dye baths

As discussed in Part 2 of this series, most of the control of Indigo dyeing must be managed before the dye and chemicals reach the dye boxes in the machine. In order to measure the effectiveness of control measures of the dye and chemical mixes, it is necessary to measure concentrations of Indigo, reducing agent and alkali in the Indigo dye boxes. If the methods of measuring these concentrations in the dye boxes are accurate and reliable, then those measurements will be able to predict the Indigo color of the yarn and the colorfastness of the denim in the garment laundry as well as providing a benchmark of dyeing management capability. By analyzing the data for each dyeing method with simple statistical tools such as Standard Deviation, the level of control for each color, for each dye lot and each group of workers can be measured and improved. In this way, continual progress can be made towards perfect denim color which has been achieved in a few companies. This wins business with jeans producers that pay the highest prices.

In 1992, I published an article, “Quality Assurance Methods for the Continuous Dyeing of Cotton Yarn with Indigo” that was the most advanced treatment of the subject. After years of research into test methods for Indigo dyeing, several conclusions could be made:

• 1) the most commonly-used methods of measurement, pH and millivolts were unreliable and could not be correlated with the dyeing results. They gained popularity because of simplicity and because that the results were consistent- the color changed, but the pH and millivolts were consistent. Dye managers used these numbers to claim that they had good dyeing control when customers complained about color variation.
• 2) Statistical Analysis comparing test results with the actual fabric color and wash-down demonstrated that the glass plate, vat-ometer and a special alkali titration correlated perfectly with instrumental color measurements of denim(L*a*b* or L*c*h*) and were the only reliable test methods for correcting Indigo dyeing problems.

The primary cause of Indigo variation is changes in the reduction potential of the dyebath which consists of a combination of reduced Indigo and free sodium hydrosulfite. The total hydrosulfite in the Indigo bath is divided between the hydrosulfite that is consumed for reduction of the indigo dye molecule and free hydrosulfite that is not consumed in dye reduction. Where the dye circulation is adequate and the dye feeding concentrations are consistent, the grams per liter of reduced Indigo and the alkali levels are consistent. The free hydrosulfite is unstable and requires the most effort in control for Indigo dyeing. As the concentration of free hydrosulfite increases, the Indigo shade becomes greener and more wash-fast; as the free hydrosulfite decreases, the indigo appears less green or redder and loses color faster after washing. If the free hydrosulfite changes by 0.3 grams per liter in the indigo bath, a different indigo shade results. Many denim companies have 10-15 shades per dye lot where a true color difference of 0.2 Delta is used as a measure.

In the above-mentioned article that I published, I mentioned a number of possibilities for measuring reduced Indigo and free hydrosulfite in Indigo dye boxes. One was the permanganate method that uses a 2-endpoint, potentiometric reduction-oxidation titration.This method has come into common use in many companies, but is not reliable for measuring the Indigo dye. The method has been in use for over a hundred years and I found it useful in conducting research into Indigo dyeing in order to develop Indigo formulations in the laboratory. This method was occasionally useful at the Indigo machine when developing new Indigo colors, but only with manual titration. Automatic titrators usually proceed too quickly and produce erroneous endpoint identifications. Also, the sample sizes used are too small to be representative of the average Indigo and hydrosulfite concentrations for all of the dye boxes. In addition, the test is too slow for quick response in dyeing and laboratory personnel often are not aware of maintenance and calibration requirements.

The vat-ometer, on the other hand, offers simplicity, speed and reliability. This device was invented about 150 years ago and research into Indigo dyeing has demonstrated that the vat-ometer results are consistent and are predictive of actual color variations in Indigo dyeing. The vat-ometer consists of a rounded glass flask and a measuring tube. Water is added to the vat-omter, then dyebath is entered, the vat-ometer is sealed to prevent entry of additional air and then the mixture is shaken for about 1 minute. The principle involved is to titrate the hydrosulfite with oxygen from the air. This causes oxygen trapped in the flask to be consumed by the hydrosulfite,which creates a partial vacuum in the flask. Then water is added to the measuring tube, the stopcock on the measuring tube is opened and a quantity of water is pulled into the flask that equals the number of cc’s of oxygen consumed. The number of cc’s of oxygen consumed multiplied by 1.1 yields an accurate and reliable measure of total hydrosulfite in the dye. Repeating the test after adding some formaldehyde to the water will provide the grams per liter of reduced Indigo. By subtracting the reduced indigo number from the total hydrosulfite number, the grams of free hydrosulfite is provided- this number determines the color and wash-fastness of the Indigo.

As mentioned previously, the practical number is the average hydrosulfite for all of the dye boxes. As a rule the hydrosulfite concentrations in indigo dye boxes will vary from box-to-box, from top-to-bottom and from side-to-side. In order to know the machine average, at least 2 dye boxes should be sampled. For example, on a 6-box Indigo machine, normally an average of the 2^nd and 5^th box will equal the machine average.

The vat-ometer offers the advantages of larger sample sizes for better accuracy faster results. The vat-ometer can be constructed in the laboratory to use dye samples as large as 100 cc’s, however the portable vat-ometer which yuses a 10 cc sample has long been proven to be accurate enough for Indigo dyeing production eg the one from Tudorscientific here.

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.

This is a guest post by Harry Mercer on Indigo dyeing. It is second part of the article in series. The first part can be seen by clicking here

Preparation For Dyeing

In the previous article, the basic machine factors in Indigo dyeing were discussed. There are many other details required to achieve the highest quality Indigo dyeings , but ultimately the most important factors involve the preparation of Indigo and chemical feeds to the machine. Approximately 80% of Indigo dyeing control depends on the stability and consistency of the dye and chemicals being sent to the machine.

1) Raw materials: The basic ingredients for Indigo dyeing are the Indigo dye, sodium hydroxide and the hydrosulfite (sodium dithionite). Indigo is an insoluble vat dye which means that it cannot enter the cotton fiber until it is made soluble by the process of reduction .Reduction is basically a process where hydrogen is produced which opens up the Indigo dye molecule allowing it to attach to a water molecule which carries the vat dye into the fiber. The most commonly-used

2) The reducing chemical is known as “sodium hydrosulfite”, but this nomenclature is incorrect because the molecule does not contain hydrogen. The “hydrosulfite acts on the sodium hydroxide to split it into NaO and hydrogen, both of which attach to the dye molecule in the reduction process.

3) The reduction of Indigo with sodium hydroxide and sodium dithionite is known as vatting and has been used for thousands of years. Vatting refers to mixing the dye and chemicals into a tank or “vat” with some stirring and then waiting from 1-4 hours usually for the complete reduction of the dye to occur which is noted when the solution color is a clear, yellow-brown.. The solution then is referred to as “leuco” Indigo, a Greek word meaning “without color”. The concentrated Indigo mix is then ready to pump into the dye machine for dyeing.

4) Most of the variation in Indigo dyeing is a result of instability in this concentrated mix. Sodium dithionite can be extremely unstable, with the concentrations in this feeding mixture becoming smaller with the passage of time. For example, the initial recipe may specify 100 grams per liter of sodium dithionite, but by the time the last liter goes into the machine, the concentration often drops to 20 to 30 grams per liter and each 5 gram per liter loss in dithionite concentration produces a small Indigo color variation. This is evidenced in many denim operations that suffer 10-15 colors after fabric washing per dye lot.

5) There are several causes for the decomposition and strength losses of the reducing agent in the feeding mix: Oxidation at the surface of the tank, unnecessary stirring and high concentrations of ingredients. The stirring should be only enough to maintain consistent concentrations of dye and chemicals from the top of the feeding tank to the bottom. Stirring beyond that will result in more reducing agent being oxidized. Also, in many Indigo operations the stirring units are badly designed with small propellers that turn at high speeds. The Indigo feeding mix is of very high viscosity and in order to stir the entire mix out to the edge of the tank, large propellers that cover the tank diameter are needed. These stirrers should turn at only 10-15 RPM in order to avoid turbulence that would lower the strength of the mix. With regard to concentrations, if the viscosity of the dye mix is too high, the reduced Indigo will not disperse uniformly resulting in areas of varying concentration in the tank that will cause color change as the mix is fed to the machine. Concentrations above 23% solid have a tendency to settle, so that there are very high concentrations of reducer in the bottom of the tank, making a greener Indigo tone when pumped to the dye boxes, and lower concentrations of reducer at the top of the tank, making a redder Indigo tone later in the dyeing. No more than 80 grams per liter of indigo should be added to a feeding mix as this is the maximum amount that has long been proven that can be completely reduced. The amount of reducing agent should also be limited to 80 grams per liter since greater amounts will cause more rapid decomposition due to aerobic and anerobic decay.

6) The concentrations of indigo and reducing agent must be actively managed so that the same concentrations of dye and reducer are feeding to the machine every minute,otherwise the color will change. Management of the feeding mix requires an understanding of the chemistry of reduced dye solutions, measurement of concentrations and skill in correcting strength losses of ingredients in the feeding mix especially of reducing agent and alkali. There are 2 simple , but special test methods to measure the concentration of alkali and sodium dithionite in the feeding mix: a 2-endpoint titration for alkali and the glass plate test which have been in use by the best denim companies for over a century and will be covered in a future article.

Conclusion:

The problem of Indigo color variations is principally a result of inconsistent dye and chemical concentrations going to the machine. A glance at the design of flow of dye and chemicals into Indigo dye machines should make this obvious. The multiple dye box arrangement and circulation in the dyeing section of indigo machines allow the blending of indigo and reducing agents, so the problem of variation obviously starts at the mixing tank.

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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.

This is a guest post by Harry Mercer. It tells about the  best practices in Sulphur dyeing for fashion denim and was  a part of the presentation made at Dhaka  by him on a seminar on Sulphur Dyes by Fenazol

The Nature of Sulfur Dyes

• Sulfur dyes are a form of vat dyes – they are water-insoluble and in order to apply to fibers must be made water-soluble through the process of reduction. 
• Reduction is a chemical process in which hydrogen is liberated. The hydrogen reacts with the dye and permits a water molecule to attach to the dye. The dye is transported into cotton fiber by the water.
• This reduced dye must then be oxidized. Oxygen reacts with the hydrogen producing water. Removing the hydrogen makes the dye insoluble, which results in the dye becoming physically trapped inside the fiber.

The Uses of Sulfur Dyes

• Sulfur dyes are commonly used for black, blue, brown, khaki and green colors.
• Sulfur dyes are applied to cellulosic fibers and blends of cellulosics with polyester, nylon and acrylics.
• Sulfur dyes can be applied with little difficulty and with excellent results at a relatively low cost.
• With good application methods, sulfur dyes are extremely wash-fast, however, they are usually not fast to chlorine bleaching.

Dark and medium colors are the most common with sulfur dyeing. Light colors can be produced by special procedures involving blended reducing agents, blends of surfactants, controlled oxidation and preparation methods like Mercerizing and bleaching.

Dark, dull sulfur dyeing

•Sulfur blacks are commonly applied in the 1st box on Indigo machines.

•The sulfur black dye molecule exists as very large polymers which require high temperatures for penetration into fibers (90-95 degrees).

•The maximum amount of sulfur black dye that can be efficiently applied is 4%on weight of cotton of a pure sulfur powder or 20% of a standard liquid (20% dye strenght). Darker sulfur blacks can be achieved by adding a sulfur blue.

•Sulfur blacks are best oxidized with an air passage like Indigo.

Boxes :
1. 20% Caustic, 60 degrees
2.Wash 60°C
3.Wash 60°C
4. Pad sulfur dye with reducer
Drying cylinders cold
Steamer hot
Bypass boxes 5-10
11. Wash 60°C
12. Wash 60ËšC
13.Oxidize
14. Wash 50°C
15. Wash 50°C
16.Wash 50°C

Light Sulphur Colors

• Sulfur dyeing procedures have been used to create a full range of colors including browns, yellows, greens, violets, reds, orange and neutrals  (light/medium greys)
• Producing special colors with these processes involves the use of true sulfur dyes alone or in combination with sulfurized vats or special direct dyes that  can be applied with special sulfur dyeing procedures.
• For success in fashion markets with these colors, marketing is focused on color “themes” such as “earth tones”, “urban”, sport or university colors.

Control Of Light Sulphurs

• In order to match specific colors, dyes with greatly different affinity factors are mixed.
• Different affinity factors result in these dyes being picked up at different rates, so special methods are necessary to avoid color variation.
• Sulfur colors should be applied from 30 to 60 degrees to avoid color variation and must be chemically oxidized.

 

 
Boxes:

1. 20% Caustic, 60 degrees
2.Wash 60°C
3.Wash 60°C
4. Pad sulfur dye with reducer
Drying cylinders cold
Steamer hot
Bypass boxes 5-10
11. Wash 60°C
12. Wash 60ËšC
13.Oxidize
14. Wash 50°C
15. Wash 50°C
16.Wash 50°C

Sulphur Bottoming

• Bottoming of Indigo dyed yarns produces a darker, brighter effect with Indigo.
• The purpose of bottoming originally was to produce a darker color with less Indigo.
• While large amounts of sulfur dye can be applied in the 1st box, most of the sulfur is removed by hydrosulfite in the Indigo dye.
• Unlike other sulfur dye applications, a pH of 12 with sodium hydroxide is helpful to retard the dyeing for color consistency.
• A temperature of 60 degrees or less should be used to lower the affinity factor for consistent color.

Problems With Sulphur Bottoming

• Sulfur bottoming can provide lighter shades because most of the dye applied in the 1st box is removed by the hydrosulfite in the Indigo boxes.  
• The affinity factor (rate of dyeing) at high temperatures of sulfur dyes results in shade variation.
• If sulfur black is applied as a bottom, the Indigo boxes are contaminated with a yellow compound which causes discoloration of pure Indigo shades. Sulfur blacks are typically produced by polymerizing 2,4 –dinitrophenol(reddish-yellow) with sulfur. Hydrosulfite in the Indigo boxes-reduces the sulfur black to this yellowish compound.
• Sulfur bottoming should be conducted at temperatures of 30-60 degrees for consistency. Unlike other sulfur dyeing which should be conducted at pH 11, sulfur bottoming can be conducted better at pH 12 with sodium hydroxide which acts as a retarder to slow the rate of dyeing.

Boxes

1. Pre-wet 2% caustic 90°C
2.Wash 60°C
3.Wash 60°C
4. Wash cold
By pass drying cylinders
Bypass steamer
Boxes 5-10 Indigo
11. Wash 60°C
12. Pad sulfur topping
Steamer hot
13. Wash cold
14. Wash 50°C
15. Wash 50°C
16.Wash 50°C/Softener

Black-on-Black Dyeing

• Sulfur black dyes are low-intensity dyes that require large amounts for dark shades.
• The amount of sulfur black that will produce the darkest shade when applied properly is 4% of a 100% powder or 18% of a 20% liquid.
• It is difficult for cotton to easily absorb that quantity of dye in a way that produces a very dark shade. 
• One method that has been used to produce darker, more colorfast sulfur blacks on yarn is to apply 50% of the dye, then dry the cotton, air oxidize and without washing, proceed to a second dye-box where the dye is applied again, dried oxidized with air again and washed.

Boxes:

1. 4% caustic 90°C
2.Wash 60°C
3. Pad ½ sulfur black
Drying cylinders hot
4. Pad ½ sulfur dye
Steamer hot
Bypass boxes 5-11
12. Wash 60°C
13.Wash 50°C
14. Wash 50°C
15. Wash 50°C
16.Wash 50°C /softener

Reducing Agents For Sulphur Dyes

• Because sulfur dyes cannot be dissolved in water directly, reducing agents must be utilized to produce hydrogen which allows the dye to attach to water.
• Any standard reducing agent can be used to dye sulfurs including reducing sugars (dextrins), sodium hydrosulfite, sodium bisulfite, sodium sulfide, sodium polysulfide and sodium hydrosulfide.

Selection of reducing agents

• Reducing agents for sulfur dyes should be selected according to the type of dyeing (batch or continuous), temperature of dyeing (cold or hot) and depth of color (light or dark).
• The reduction potential measured in millivolts for continuous sulfur dyeing as on Indigo machines should be at least -600 for dark, dull shades and at least -700 for bright, colorfast shades.
• The best sulfur dyeing for bright, consistent and colorfast shades requires blends of reducing agents.

Advances in sulfur dyeing

• Cold dyeing of sulfurs is possible on Indigo machinery in a full range of colors.
• This procedure has long been utilized by craft dyers in Asia.
• Cold dyeing of sulfurs have the advantage of requiring only 50% as much sulfur dye,
• Energy savings, excellent colorfastness, no polluting dye waste and less cotton damage for a soft touch and higher weaving efficiency.

Special Notes

• Dyeing with 100% sulfur blacks on yarn results in lower weaving efficiency. Normal yarn break levels with black can be achieved with improved washing and the use of buffers in dyeing.
• Sulfur blacks can be dyed with special procedures that provide wet rub-fastness of 4 and good washfastness even after 20 launderings.

Caution And Care

• The recommendations of many sulfur dye suppliers result in dye waste of 50% or more, which increases dyeing costs by twice that is necessary. Also, most of the additional chemicals used in sulfur dyeing, especially for blacks, are unnecessary.
• The use of fixatives after sulfur bottoming are often recommended to increase dye shades, but these are destroyed by the hydrosulfite in the Indigo boxes.

For more details on sulphur dyeing , check the site of FENAZOL

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. 

 

 

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