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The classical jeans was produced out of indigo-dyed Denim fabric. The special character of this fabric – only the warp thread is dyed – makes it necessary to carry out dyeing in yarn form. The yarns applied for Denim were exclusively produced on ring spinning machines in former times. The development of OE(open end) yarns – by applying smaller rotors with a spinning speed of up to 200 m/min – has led to the application of OE rotor yarns both for warp and weft. The yarns applied for weaving must be of high quality: a high fiber for strength, regularity as well as a small part of short-stapled cotton fibers belong to the basic features of the denim yarn. For regular jeans qualities the warp yarns are spun in a fineness of 50 to 90 tex, for the weft yarn the fineness ranges are mainly 75 to 120 tex. If Denim is made out of Tencel or Modal especially for jeans shirts the finenesses are up to 25 tex. Indigo, sulphur and indanthrene are mainly used in the dyeing process. Two methods are applicable for continuous dyeing with indanthrene dyes: rapid dyeing and vat dyeing. While processing the basic colored denim, reactive dyes are used and fixed with hot caustic soda solution. The dyeing process is mainly influenced by the dyestuff characteristics, dyeing temperature and necessary chemicals used in the process. Indigo dye is the most popular choice as it has good depth of shade and suitable rubbing and washing fastness. When cotton yarn is dyed with indigo, it leaves a ring-dyeing effect, because of which the outer layer of warp yarn is coated with indigo, and the core of the yarn remains undyed. This gives the denim garment a unique ‘faded look’ and a rich blue shade after repeated use and wash.

Originally, the warp yarns or ends were put through the dye bath side by side to form a sheet of yarn, which passed continuously through several dye baths, squeeze rollers or airing sequences.

However, if there were breaks in the yarn (and there would be, as each yarn had to take the tension of being pulled through these processes virtually on its own), the dyeing process had to be stopped. The yarns would be then mended, or else it would lead to very bad tangling. These stoppages would in turn cause large shade variations, and the yarn breaks would show up as bad faults in the fabric. Now, an infinitely more efficient system has been introduced.

Special attention shall be paid here to Indigo, the „king of dyestuffs“since it plays an important role in obtaining the jeans effect. Indigo belongs to the category of water-insoluble dyestuffs. It was first mentioned in a book 13 BC; at that time the name Indian blue indicated the country the color came from. It is said to have been used for dyeing in India and China 2000 years BC already.

PRESTON UNIVERSITY

The Indigo plant is used for preparing Indigo

(Indigo tinctoria L.)

This plant came to Europe in the 16th century via India and gradually replaced the woad which was one of the most important dyeing plants up to this time. Only the leaves were used for good qualities whereas the leaves together with the stalks were applied for normal qualities. In a vat filled with water and partially with human fermented urine as alkali donor stems and leaves were exposed to a putrefactive process. During this putrefactive process hydrogen was created by means of micro-organisms which, as a reduction agent, transformed the dyestuff contained in the Indigo plant into a water-soluble form.

When this process was over the whole mass was filled into a liquid where the fermented mass was stirred with poles. The reason for doing so was to transform the Indigo into its water insoluble form again by air oxidation. In a last step the water-insoluble dyestuff particles could then deposit on the bottom of a stationary vat.

Then the liquid standing above was drained and what was left was a thin mash which was dried in the open air and was put on the market in pressed or in powder form

PRESTON UNIVERSITY

As shown in figure trading form of Indigo at that time, approx. 9 cm length of edges and 163 g in weight. When looking at these methods one can easily imagine that the reduction of Indigo was considered to be an evil-smelling trade. In 1880 Adolf von Baeyer succeeded in carrying out the first synthetic production of Indigo. In the year 1897 the

“Badische Anilin- und Sodafabrik“in Ludwigshafen -hich is nowadays called BASF - was able to carry out an industrial-scale production of the Indigo dyestuff for the first time. A few years later this synthetic dyestuff replaced the indigo coming from British-India almost completely. As was already mentioned Indigo is a dyestuff insoluble in water. In order to be able to apply it on cotton it must be transformed into a water-soluble form. Similar to the former production of Indigo this is done by reducing the dyestuff (ill. 4). In practice this is nowadays carried out with sodium dithionite or hydroxiacetone in the alkaline range.

INDIGO REDUCTION

C¹⁶H¹⁰O²N² + Na²S²O⁴ + 4NaOH

.

OXIDATION REACTION

Na2S2O4 + O2 + 2NaOH

.

Sodium Hydrosulfide C16H10O2N2 + 2NaOH

PRESTON UNIVERSITY

Fiber cross-section of a yarn dyed with Indigo

This is Fiber cross-section of a yarn dyed with Indigo simplified description of the reduction/oxidation of Indigo In former times dyeing with Indigo was carried out in wood or metal vats, normally in rope form.

Indigo sample dyer as very clearly visible on the above picture, at the bottom side of the rope the water-soluble Leuco form of the indigo is yellowish and on the side of the rope oxidized with air the indigo blue can be seen again. Nowadays, yarn dyeing with indigo is done continuously. Here the various dyeing processes with different concentrations of chemicals as well as the subsequent yarn sizing exert an influence on the quality and the appearance of the ready fabric.

PRESTON UNIVERSITY

HOW TO MAKE INDIGO SOLUTION In a tank of 1000 liters:

a. take 400 litres of water (soft)

b. add setamol ws--> 4 g/l (stirring) ( dispersing solution)

c. Add 100 kg of Indigo ( at 1.8 % shade -see the indigo calculations- stirring) d. add caustic soda --> stirring ( for solubilising and pH)

e. allow to cool it for 2/3 hours

f. Add hydrosulphide ( As reducing agent) g. Make the solution to 1000 l by adding water.

If pH is fluctuating, if it is > 11.7 then hydro is added (2-3 kg), if (<11.2) then caustic is added. at 24 m/min, a lot of 14000 m will be completed in 14000/24 = 583.3 min at 1.8% shade

1 litre would be completed in = 583.3/900= 38.8 seconds so flow rate will be 38.8 seconds / litre

Similarly flow rate of caustic and hydro can be determined Hydro is taken around 100 gpl

caustic is taken around 90 to 100 gpl

PRESTON UNIVERSITY

There are three processes in the practice for continuous dyeing:

1) Rope Dyeing

2) Slasher Or Sheet Dyeing 3) Loop Dyeing