Dyes used for denim and why does Indigo is so popular for Denim?

Dyes used for dyeing denim are:

• Indigo
• Vat
• Direct
• Sulphur
• Reactive
• Pigment Colors

Among all these dyes indigo dye is the most popular and widely used consisting of almost 67% of all the dyes used for denim production.

Indigo for Denim

The indigo color is the principal source of the almost magical appeal of denim. The dye imparts a brilliant blue hue to fabric. In fact, there is no other dye that creates such an intense blue color with so few carbon atoms in its molecule Indigo is unique in its ability to impart surface color due to partial penetration in cotton fibers. When cotton yarn dyed with indigo is untwisted, the inner layers remain uncolored. The dye also fades and abrades continually. This characteristic of indigo lets denim fabric have its final worn look with different types of washing and finishing applications. It enables denim fabric to respond to finishing applications that give a real life to the fabric.

Despite many other blue dyestuffs, indigo has kept its popularity for denim dyeing. This no doubt is achieved by the fact that indigo has several properties that have not yet been achieved by another single dyestuff. The unique feature of indigo dyed denim is the possibility of achieving wash down effects on repeated washing without losing the freshness of the color. Another important feature of indigo is that unlike many other dyes, indigo dyed denim does not pose health hazards. In fact, indigo is so safe for living things that it has long been used to color polyester medical sutures. Indigo is also used as food color and as a medical indicator applied intravenously.

Technical features of indigo include possibility of dyeing at room temperature, favored shades from black to navy to sky blue that are not too brilliant or not too dull, reduced indigo solution is not sensitive to water hardness and this allows the dyeing of greige or minimum pretreated cotton, does not strongly bond to the fiber, repeated washings slowly remove the dye without losing freshness and most importantly the competitive price. In fact, many synthetic blue dyes have superior qualities compared to indigo, and this would have ruined synthetic indigo as well if denim had not begun its invasion to Western culture after the Second World War.

Other Dyes Used for Denim

Since denim is made of 100% cotton, therefore in addition to indigo it can be dyed with dye classes such as direct, vat (other than indigo), reactive, sulphur dyes and pigment colors. In the last decade, denim was mainly dyed with indigo (67%), sulphur black (26%) and other sulphur colors (6%). The changing fashion trends have also led to vat, reactive and direct dyestuffs, as well as pigments being used to color denim.

Direct dyes 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 formulation. Reactive dyes are often used in garment dyeing. There have been attempts to apply reactive dyes 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 react with cotton. Therefore, reactive dyes should be applied in a small box, then dried, chemical padded and steamed. The only practical alternative for most denim companies is to apply reactive dyes during sizing with special procedures. There are 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 with an additional set of drying cylinders, steamer and small pad boxes for dye pad, dry, chemical pad, steam methods. However, vat dyes produce shades with high fastness properties and therefore it is difficult to get the classic denim look.

Sulphur dyes are of low cost and can be applied on indigo machinery, fabric dyeing machinery, jiggers, pad-batch and garment dyeing machinery with properly designed methods. Sulphur dyes have an appearance that is more natural than reactives or directs, having a softer appearance and allowing versatile wash down effects in laundering. Therefore, among the different dye classes other than indigo, sulphur dyes are more popular and are used for bottoming or topping or over dyeing of finished indigo dyed garments to produce a variety of shades with fancy looks. Pigments are relatively easy to apply, and since they are a surface coloration, can produce a distressed look, but they present color fastness problems in darker shades and have a harsher feel than dyed garments.