Indigo dyeing is unique and because of the complex chemical reactions should be correctly viewed a a form of chemical engineering. Only Indigo dyeing requires multiple dye applications for a dark shade. Color consistency of Indigo in recent decades has been unsatisfactory as a result of machine designs that do not apply basic principles of fluid mechanics properly and unstable dye mixes. Commonly, a single dye lot will have between 8 and 15 visually different shades from beginning to end and also have shade differences from one side to the other.
Dark Indigo Color
This was an example of a typical method used in the U.S. for a very dark shade. In order to produce the same depth of color as 1.2% in the U.S., in Latin America 2.0% is used and in Asia from 2.4 to 2.8%. The U.S. method results in more surface (ring dyeing), which loses color faster.
Darkest Indigo Shades
Very dark shades of Indigo are in demand currently around the world. Many companies use 4% or more Indigo on weight of yarn, which is expensive. 2% Indigo will produce the same depth if low levels of caustic are used(0-0.4%). For dark Indigo that does not lose color 2% applied normally, with an Indigo bottom.
Light Indigo Shades
Dyeing Indigo in light shades results in a sky-blue impossible with any other dye. This is useful for shirting fabrics that are not strong enough for stonewashing, bleaching or cellulase treaments. Special procedures are necessary in order to avoid colorfastness problems.
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.
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.
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.
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.
In topping the sulfur dye is applied after the Indigo dyeing. Sulfur topping permits much darker color than a sulfur bottom, but is duller. Sulfur topping colors include black, blue-black, yellow brown and green. Sulfur toppings are used to produce slub appearances in normal yarn.