Indigo plant, named as "Indigofera tinctoria", was discovered around 1600s (B.C.). Indigo dyestuff, extracted from its leaves, had been used in various primitive dyeing processes for years. In 1880's, the first synthetic (unnatural) indigo dye was developed by German chemist Adolf von Baeyer. He identified the chemical structure of indigo. After 1900s, synthetic indigo was marketed. This has triggered off the indigo dyeing developments as a result of improvement in denim wear. In ancient years, indigo dyestuff was fermantated in wooden vats. This process which is called as "vatting", is supposed to be the origin of vat dyes. Fastness is the resistance of fabric to the action of external influences, such as light, washing, crocking, perspiration, acids, alkalies, etc. Indigo is supposed to have poor washing fastness, fair light fastness, good perspiration fastness and poor crocking fastness.
Properties of Indigo Dyestuff
Indigo dyestuff which is classified as vat dye is insoluble in water and has no affinity to the fibre. They have poor washing fastness which lets the color of denim fabric to change naturally. Indigo creates living colours on fabrics. Indigo dyestuff can never fully penetrate into the fibre, since its molecule is so big and it only adheres to the surface and remains at outer surface of the fibre. The inside stays white. It abrades or fades continually. This character of indigo lets denim fabric to have its final look with different types of washing and finishing applications. It enables denim fabric to response to finishing applications that gives a real life to the fabric. Indigo dye should be classified into two different chemical forms:
1. Natural form, insoluble in water (cannot dye the fibre)
2. Leuco form, soluble in water (can dye the fibre)
In natural form, indigo dyestuff has a color of blue but after reduced to leuco form, the color of the solution turns to yellow.
What is reduction-oxidation?
In order that indigo is able to dye the fibre, it needs to be activated (leuco-form). In other words, indigo should be converted into soluble form and the affinity to fibre should be increased. Some chemical reactions are necessary for converting indigo to leuco form. These reactions are called "reduction". Reduction takes place in certain conditions with the presence of hydrosulfite ini alkaline medium. To keep the solution alkaline (basic), caustic (NaOH ) is used.
After reducing and dyeing, dyed ropes have to be aerated so that the dye and fibre can be fixed together. This process is called "oxidation".