This is the second part of guest article by Harry Mercer. The first one can be seen here..

As mentioned in the first part, producing colors on denim machinery, especially Indigo dyeing equipment has usually resulted in failure to achieve denim colors that are without variation. Sulfur dyeing on Indigo machinery typically results in 10 or more different shades per dye lot, which is true of sulfur bottoming, sulfur topping and sulfur blacks. Other colors are even more sensitive to chemical and machine conditions.

The options for overcoming these problems and experiencing success in the market are:
1) To invest in yarn dyeing machinery with an additional set of drying cylinders and a steamer which permits precise dyeing control for all cotton dyes or
2) Establish a research effort internally in which the actual conditions of the Indigo machine are duplicated in the laboratory so that dyeing formulations and methods can accurately predict results on the yarn during production.

The options for producing denim colors as far as dyes are concerned are:

Sulfur dyes: These are low–cost and can be applied on Indigo machinery, fabric dyeing machinery, jiggers, pad-batch and garment dyeing machinery with properly designed methods. Sulfurs are highly versatile in application methods including space dyeing, printing, spray dyeing etc. Sulfurs have an appearance that is more natural than reactives or directs, having a softer appearance and allowing versatile wash down effects in laundering.

Vat dyes: These are are more costly, but they result in bright, highly colorfast shades. There are a few vat dyes that can be mixed with Indigo for casting, however pure vat colors are best applied on denim with specially-designed rope ranges, like the Spectrum, with an additional set of drying cylinders, steamer and small pad boxes for dye-pad, dry, chem-pad, steam methods.

Direct dyes: These are currently being applied in garment dyeing of jeans in light colors for the higher fashion market. Direct dyes can also be used for tinting denim fabrics by adding to finishing formulas.

Reactives: These are very expensive to use. They are often used in garment dyeing, because they do not require an understanding of dye chemistry. There have been attempts to apply on indigo machinery, but control of dyeing is nearly impossible because of the large dye boxes on most indigo equipment, much of the dye being hydrolyzed before it can penetrate the cotton. Again, as with vat dyes, reactives should be applied in a small box, then dried, chem-padded and steamed. The only practical alternative for most denim companies is to apply reactives in sizing with special procedures.

Affinity Factors: Key to controlling sulfur dyeing

Sulfurs are a type of vat dye like Indigo, both of which present control problems on Indigo machinery. Vat dyes like sulfurs are known as “substantive dyes” which means that they are absorbed by cotton at a faster rate than water. How much faster a substantive dye is absorbed by water is known as the “Affinity Factor”.

The affinity factor changes depending on the chemical nature of the sulfur dye, the concentration of each dye and the temperature of dyeing. For example, a sulfur black applied in continuous dyeing at a temperature of 90C and a concentration of 180 g/L will have an affinity factor of 2, which is very low. This means that the dye is absorbed twice as fast as water, so in order to maintain a concentration of 180 g/L in the box for a gravity feed, the concentration in the feeding tank should be about 360 g/L. If you were to use 30 g/L for a sulfur bottom at 90 C, the affinity factor would be perhaps 20. This means that at 90C, the dye is absorbed about 20 times faster than water, which makes dyeing control impossible and is why sulfur bottoms are usually inconsistent and light. Sulfur bottoms are a form of sulfur color and in order to establish dyeing control of any color, the affinity factor must be reduced by using lower temperatures. It is well-known that a 10C increase in temperature results in a doubling of reaction rates, so the solution is to dye at the lowest temperature possible. Commonly-used reducing agents like sodium sulphide and dextrose require temperatures around 90 C, therefore these should be replaced by reducers that allow dyeing at no more than 60C for any sulfur color other than black.

There is also a widespread problem with the quality of sulfur dyes in the market. Liquid sulfur dyes often have a pH of 14, which results in variation even with blacks as well as poor washfastness, poor rubbing- fastness and large amounts of dye waste during wash cycles. Sulfur dyeing is best at pH 11. Also for continuous dyeing, the reduction potential is critical for consistency and color-fastness. The higher the reduction potential of the dye and of the dye application, the better the results. Sulfur liquids with the highest reduction potential, around -750 mV offer the best continuous dyeing control, while dyeing at very low millivolts (-585) as with sodium sulphide result in very poor quality dyeings.

A note by Harry Mercer:
I have been involved in producing successful denim colors for the fashion markets for over 20 years. Until recently, activity in this area has been limited because of technical difficulties in applying colors on denim equipment, however interest in denim colors has become explosive recently. I have collected many of the denim colors available internationally and can refer garment producers to the leading fabric suppliers. I also have evaluated the dye and chemical suppliers with the best products and technical service for yarn, fabric and garment dyeing that I will be happy to refer interested parties to.

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.

Sponsored Link:
Color Denim Collection Spring/ Summer 2013

This is a guest post by Harry Mercer.

For denim executives that are trying to anticipate the next big fashion trend, there are clear indications of what that trend will be. The indications are that denim in a wide range of colors is emerging as the most promising path to significantly improved denim business by 2013. These are other than the pure Indigo blue, sulfur bottoms, sulfur tops and black shades that dominate the denim market. The availability of denim colors in the world market is presently very small because only a very few denim companies have overcome the technical challenges of dyeing colors on standard denim equipment. These problems along with the route to success in producing colors will be explained in following article.

The technical aspects of controlling colors on Indigo machinery consist of:

1) Understanding affinity factors( the velocity of dye particles) of indigo and sulfur dyeing

2) Control of reduction potential(millivolts) and alkalinity(pH) of indigo and sulfur dye

3)Dye box volumes( larger boxes are better for Indigo, smaller are better for sulfurs)

4) Control of wet pick-up where dyes are applied to dry cotton and the concept of “liquor exchange” where dye is applied to wet cotton

5) Effective washing for maintaining color consistency

6) Control of oxidation, especially with sulfur dyes to avoid strength loss with blacks and the extreme color variation with colors other than black. These will be addressed in detail in the following article.

These color denims are appearing in increasing volumes in higher-fashion retail stores such as Dillards in the U.S. as well as in Europe and Japan. These include yarn-dyed denims with the white weft preserved and garment dyes which can be produced with sulfur dyes, reactives, directs or pigments. One constant in yarns dyed by Indigo machinery is that there will be an undyed, white center in the yarns regardless of which class of dye or dyeing method is used. The reserved, undyed cotton in the center of the yarns is responsible for the unique brightness of denim garments. Light is reflected from the dyed fibers on the outside of the yarns, but also from the white fibers in the center which results in a special brightness that has been described as “blooming”.

Sulfur dyed garments seem to have greater appeal because of the more natural looking “earth tones” that are generated. Reactives have a more intense appearance because of the higher chroma, but are more expensive and are perceived as being artificial. Direct dyes are the lowest cost to use and there are pink and light greens that have penetrated the market, but present colorfastness problems in dark shades. Pigments are relatively easy to apply and since they are a surface coloration, can produce a distressed look, but also present colorfastness problems in darker shades and have a harsher feel than dyed garments. Sulfur colors offer easy application, are very low cost and can produce a wide range of colors that imitate colors in nature which have a more durable appeal in the higher-fashion markets. The basic procedure for producing sulfur dyes is to take “any organic compound with color and react it with sulfur”, so any naturally occurring material can be used to produce sulfur dyes. When sulfur dyes were introduced in the 1870’s, the first colors were brown dyes produced from sawdust, animal blood and horse manure reacted with sulfurs. More sophisticated methods since then have resulted in a full palette of colors with sulfur dyes.

The expression “sulfur colors” refers to colors like browns, greens, yellow, violets, greys, pinks and oranges, colors that will stand out in stores and catch the attention of retail consumers that are ready for something fresh to wear, but are in the unique denim fabric constructions.

The presence of white weft-yarns behind the colored weft yarns makes for a brighter, cleaner appearance and because the weft yarns have not been affected by dyes or chemicals, the feel of natural cotton against the skin is one of the important reasons for the appeal of denim.

Denim colors first became a huge success in the fashion denim area about 20 years ago, when U.S. companies invested in research programs to solve the mysteries of the physical chemistry involved in continuous dyeing on Indigo machinery, which none of the dye suppliers was able to solve. A serious effort is required which involves a basic understanding of color science, the chemistry of dyeing, especially the selection of reducing agents and special buffers to maintain stability in the large dye boxes of Indigo machinery, but the profit margins to be realized will make the effort financially worthwhile, especially for smaller, more flexible and technically capable denim companies.

The business cycle in fashion has always demonstrated that the fabric supplier who enters the market first with an entirely new product gains the greatest share of the profits form product development. Those who try to catch up and wait to follow the leaders will continue to struggle.

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.