DENIM DYEING

DENIM DYEING:

The classical jeans were 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 especiallyfor 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. The Indigo plant is used for preparing Indigo.
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. There are two processes in the practice for continuous dyeing:

• • Rope Dyeing
• • Slasher or Sheet Dyeing

ROPE DYEING

Indigo Rope Dyeing When dyeing according to the rope dyeing or cable dyeing method.350 - 400 warp threads are bound on the ball warper to very thick cables of 10000 - 15000 m length. On the continuous dyeing installation, 12 to 36 cables are led side by side, wetted, dyed and dried after the dyeing process on cylinders and put into cans. Then the cables are dissolved to warps on the long chain beamer. The warps are added to the sizing machine, sized and then led together to warp depending on the total numbers of threads. In practice, this method has proven to be very good through obtaining an optimum indigo dyeing. However it is important that the cables have a constant tension in order to avoid warp stripes. The disadvantage compared to other methods is that yarn breakages do occur more often. Size of the dyeing unit is between 60 - 80 m. Normally, 6 dyeing vats are in use. There are nevertheless variations with 3 to 8 dyeing vats.

SLASHER DYEING

Indigo Sheet or Double Sheet Dyeing (Slasher Dyeing).When dyeing according to the sheet dyeing method, instead of cables the warp threads are fed to the machine parallel next to each other. These are much smaller compared to the rope dyeing machines. Another advantage is that the cables don’t need to be open after dyeing. Moreover, each yarn wets much faster and in this way reduces the dipping and wetting times during dyeing. All in all, each thread has a larger surface compared to a dyeing cable and this requires somewhat more hydrosulphite to prevent a premature oxidation of the indigo. Whereas during rope and sheet dyeing the yarns are always led in succession to the dyeing baths, the fabric of the loop dye method is led several times through the same dyeing bath. The machine is then much shorter and the hydrosulphite consumption is lower. Depending on the number of dyed passages and the concentration of indigo in the dyeing vats, different dyeings of the yarn.

WORKING PRINCIPLE OF SLASHER

Warp beams are brought to the dyeing and sizing section in sheet form as per requirement. The warp sheet is taken to wetting tank (like mercerize, semi mercerize and causticize tanks depends upon the dyeing methods) which swells the fiber and increases its dye uptake. It is then taken to a hot washing tank whose temperature is maintained at 850C. It is then taken to cold washing tank whose temperature is maintained at 600C.
Warp sheet is then taken to dyeing tanks where dye is reduced to soluble leuco form by using sodium hydrosulfite, yarn is then contacted with the reduced dye, and the dye is adsorbed by the fibers. The fixed leuco indigo in the yarns of the warp sheet was oxidized to form pigment during transport of the warp sheet in the air space following the dwelling chamber, taking approximately 60 seconds. It is taken to two hot washing tanks for fixation. Following rinsing, warp sheet is dried on cylinders dryers prior to the application of sizing agent.

OXIDATION REACTION

Na2S2O4 + O2 + 2NaOH → Na2SO4 + Na2SO3 + H2O + C16H10O2N2Na2 + . O2 + H2O + Sodium Hydrosulfide C16H10O2N2 + 2NaOH

OXIDIZED INDIGO

During the slashing process, the sheet of yarns is passed through the size box which contains the hot water solution or mixtures of sizing agents. The yarns pick up the required quantity of size solution in the size box, excess size is squeezed off as the yarns pass through squeeze rolls. After the size box, the yarns go through the dryer section. The wet yarns are dried by using drying cylinders. Cylinder drying is done using steam heated hot rolls. The Teflon coating is done on all the cylinders to prevent sticking of yarns. Accumulator stores 150 meters of warp sheet. Due to the nature of sizing, the yarns in the sheet may be stuck together at the exit of the dryer section. They are separated into individual sheets by leasing rods and individual ends of yarns are separated by passing through comb then the yarns are wound on to a loom beam for weaving.