Vat dyeing

The vat dyeing (also known as Küperei ) usually uses water-insoluble pigments for dyeing cellulose- based textiles . Although this class of dyes also shows an affinity for protein and polypeptide fibers (wool, silk and polyamide ), it is still used almost exclusively on cotton and linen, rarely on regenerated cellulose fibers and their mixtures with polyethylene terephthalate fibers (polyester).

history

The vat dyeing is one of the oldest dyeing processes. Real dyeings with fermented woad , indigo and purple were already known in ancient times .

The dye liquor is called the vat . The term is originally derived from the name of the vessel called a "vat", a vat .

Principle of dyeing

The dyes are converted into water-soluble, differently colored leuco compounds using reducing agents . The prerequisite is that the dye is finely distributed in an aqueous medium. The grain size of the dyes is of decisive importance for the speed of the reduction reaction as well as for the uniform distribution on the textile material. The dyes dissolved in water, predominantly in reduced form, are known as vats . In other dyeing processes, the analog is usually called a dye liquor .

In the simplest case, which is to be dyed fabric easily into the vat dipped, wrung out and air- imposed . The oxygen in the air oxidizes the (soluble) leuco compound again to the original, water-insoluble pigment, which adheres firmly to the fiber. Instead of atmospheric oxygen, suitable oxidizing agents which hardly damage the fiber can be used to oxidize the leuco compound. The colors are very real, cannot be attacked or changed by air and adhere very well to the fiber.

Dyeing process

The choice of the right dyeing process is, in addition to the fundamentally important selection of the dye combination, of decisive importance in order to obtain a satisfactory dyeing result. In addition to the depth of color to be achieved, it is primarily based on the dyeing machine or apparatus on which the dyeing is to be carried out, depending on the form of the textile substrate. Only the pad steaming process (pad dry pad steam process) and the wet steam process (pad steam process) are of industrial importance today.

See also : dyeing with real purple  and dyeing process for indigo

Reducing agent

Various urea- based mixtures were used as reducing agents until the 19th century , particularly urine directly. Other possible reducing agents are: sodium hydrogen sulfite (NaHSO ₃ ), sodium dithionite (Na ₂ S ₂ O ₄ ), sodium hydroxymethanesulfinate or boron hydrides . However, only sodium dithionite has achieved industrial importance, for example in the conversion of water-insoluble indigo into water-soluble indigo white.

${\displaystyle \mathrm {Na_{2}S_{2}O_{4}+2\ NaOH+Indigo\longrightarrow 2\ Na_{2}SO_{3}+Indigoweiss+2\ H_{2}O} }$

Electrochemical processes work technically, but have not attained any economic importance until today.

Oxidizing agent

In addition to atmospheric oxygen, inorganic per-compounds ( hydrogen peroxide , H ₂ O ₂ ), potassium dichromate (K ₂ Cr ₂ O ₇ ), alkaline sodium hypochlorite solution (NaClO) or mixtures of sodium chlorite (NaClO ₂ ) and acetic acid are used for this purpose (especially in the case of the Leukoküpen dye esters) (CH ₃ COOH). Hydrogen peroxide is of particular industrial importance today. Oxygen in the air plays an important role in the dyeing of yarn as a package for some dyes. It is sucked through the coil by means of a vacuum.

Electrochemical reduction and oxidation

According to the results of a research group at the Research Institute for Textile Chemistry and Textile Physics at the University of Innsbruck , it is possible to reduce and oxidize certain vat dyes using electrochemical processes. This would lead to a reduction in chemicals and an improvement in environmental protection in the dye works.

Suitable dyes

→ Main article : vat dyes

The following dyestuffs and groups of dyestuffs are suitable for vat dyeing, which can be brought into a form suitable for dyeing by reduction and into an adhesive, pigment-like form by oxidation.

• Indigoid dyes
  • Indigo,
  • Indirubin ,
  • Isoindigo,
  • Purple (dibromo indigo)
• Indanthrene dyes
• anthraquinone dyes
• Phthalocyanine dyes with a special reaction group
• Naphthalene dyes

literature

• Heinrich Zollinger: Chemistry of the dyes . Zurich.

Web links

• Dyeing with a vat

Individual evidence

1. ^ Franz Maria Feldhaus : The technology of prehistoric times, the historical time and the primitive peoples. Engelmann, Leipzig and Berlin 1914, p. 843.
2. ↑ ^{a b} Entry on vat dyeing. In: Römpp Online . Georg Thieme Verlag, accessed on June 15, 2014.
3. ↑ Eureka! LFU research project is exhibited . March 20, 2007. Retrieved March 15, 2012.