2.6.2. Mediator concentrates technique:

The contents of the dye bath are not circulated through the electrochemical cell in this technique, nor does a dye solution flow through it. The technique is analogous to the metering of dithionite solution in indigo, vat and sulphur dying. As far as the dye bath potential is concerned, metering a similar to that of closed circuit technique. However, the quantity of mediator required in dye bath is correlated with the composition of dye bath liquor, as is the case with conventional reducing agent. Here, an upper limit to the volume of reducing agent that can be metered is imposed. If this limit is exceeded, the dye bath will overflow unless additional technical measures are adopted to prevent it. The great advantage of this technique is that the reduced mediator is stored in the tank. The mediator from this tank can be supplied to several installations with different colouristic settings.

2.7. Future outlook: electro chemical dyeing

Reducing agents should be dispensed with completely on ecological grounds. This is the aim of the most recent development, which is still in the laboratory stage in fact, but could become the dyeing process of the future in BASFs view.

In electrochemical dyeing the chemical reducing agent is replaced by electrons from the electric current introduced into the dye bath via. a special cathode. A distinction drawn here between direct and indirect electrolysis.

In the case of sulphur dyes direct electrolysis is successful i.e. the electrons are transferred directly to the dye, reducing it to the active dyeing species. With vat and indigo, which are present as pigment, and therefore have inadequate interaction with the electrode surface, indirect electrolysis is employed, in which a mediator, which is easily soluble and can be regenerated, transfers the electrons from the cathode to the dye molecules.

3. Plasma technology in textile processing^{1, 2, 3}:

Plasma has been known from the dawn of mankind from its natural appearance in lightning displays, the solar corona and the northern lights. Plasma is the fourth state of matter, after solids, liquids and gases, and this fourth state was first proposed by Sir William Crooke in 1879 as a result of his experiments in the passage of electricity through gases. The word plasma comes originally from a Greek term meaning something formed, fabricated and molded and was first used by Irving Langmuir in 1929.

3.1. What is plasma? ⁴

The physical definition of plasma is an ionized gas with an essentially equal density of positive and negative charges. And today the term is recognized as being generated by electrical discharges through a gas and it consists of a mixture of positive and negative ions, electrons, free radicals, ultraviolet radiation and many different electronically excited molecules. Thus, gas plasma treatment differs in nature according to the specific gas or gases, e.g. Air, ammonia, argon, etc. Any gas plasma contains a complex mixture of species that can interact with textile fibers placed in the vicinity of the plasma, and this can lead to a variety of fiber-surface treatments. The nature of the gas composition, the type of textile fiber, and machine parameters such as the pressure within the plasma chamber, the treatment temperature and time, and the frequency and power of the electrical supply, can be used to vary the type and degree of fiber modification.