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Natural Indigo Blue Dye Origin Dying Procedures Technology and Dye Recipes for Denim Fabrics


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Indigo dying procedures, technology and dye recipe.

Clariant recommends recipes –Indigo dyebath should be controlled by means of chemical parameters such as pH and reduction. Significant changes in these values could make way for variations in the reaction’s oxidation-reduction kinetics, which in turn might lead to differences in the diffusion of colour, shade and intensity. The passage of yarn through a denim range consists of impregnation in preparation, dyeing and washing vats.

The indigo is applied to the yarn by means of repeated impregnations and then passed through the sky to become gradually oxidized. After impregnation and squeezing in the squeezing mangle, the yarn should spend some time in the air duct, where the indigo dye becomes gradually oxidized. It becomes insoluble and fixed on the yarn by means of weak bonds (Eg Van der Waals bonds). This way the indigo is deposited in as many layers on the yarn, as impregnations take place.

In any case, the final effect of indigo on the yarn is superficial, due to the low diffusion of the dye. Regularity and consistency throughout the process are the main goals during dyeing. This implies controlling the parameters which considerably influence this regularity. Temperature: Diresul® RDT Liq dyes are used together with reducers such as Reducing agent D PW. It means that the minimum application temperature has to be 60ºC and the maximum temperature 90º-95ºC and it should be maintained constant.

Diresul® RDT Liq dyes have greater cotton affinity and exhaustion at elevated temperatures. That is why variations of ±10 ºC during the dyeing produce shade intensity differences. Dye concentration: due to the affinity of Diresul® RDT Liq dyes (pre-reduced liquid dyes) the dye concentration in the bath requires control, which is carried out visually in various ways:

  • By taking a sample of the thread (once the yarn is dyed) from any part of the denim range
  • By using an online spectrophotometer (colourimetric control of the yarn while it circulates in the denim range). This is visual colour control.
  • By taking a dyebath sample every now and then and applying it on yarn or fabric in the laboratory.

During denim dyeing, the initial concentration of the sulfur dye should be reinforced both in the case of dry on wet as well as wet on wet dyeing. To adjust the concentration of the feed it is possible to use the Clariant feed-up program. Speed: this parameter influences the time of contact of the fibre with the bath. The longer the contact, the greater the bath-material exchange and, consequently, a greater intensity of the dyeing and diffusion of the dye in the fibre. pH and the Redox potential in the dye bath: Diresul® RDT Liq dyes are in a soluble and pre-reduced state.

This implies that they already themselves present alkalinity and a sufficient and stable state of reduction in their liquid form. It is very important to keep this reduced state in alkaline conditions in the dye bath. That is why Clariant recommends recipes that assure optimal conditions of the dye bath during the entire process. Closed dye vat which can be used for both indigo as well as for sulfur dyes the pH and Redox potential values should fit between the following margins: pH: 11.5- 12.5 Redox Potential: -600 to -750 mV. Depending on the recipe and the conditions of application, the pH can be a little higher.

Foam Indigo Dyeing of Cotton Yarns: New Technology for an Ancient Dye

The research reported here developed a foam dyeing system that eliminates the oxygen until the dyeing process is completed and the yarns are ready to be oxidized. Results to date have demonstrated that speed of the dyeing process can be multiplied, dye uptake and dye fastness improved, water and energy use greatly reduced, floor space required for dyeing dramatically reduced, and all without the use of the sulphur compounds.

Dyeing in a nitrogen atmosphere

The nitrogen technology ensures less environmental impact and more efficiency during indigo dyeing. With its high concentration in the dye bath and under a nitrogen atmosphere, the dye diffuses and migrates more intensely into the fibre than in the case of conventional comparable procedures. The yarn can absorb three times more dye in a dye vat. In this way, it is possible to reduce the number and length of vats, with positive effects on the environment.

The chemical consumption can be reduced considerably, a minus of 50% is possible when using hydrosulphite and caustic soda, and there is also less yarn waste. Moreover, due to the good fixation of the dyestuff on the fibre, considerably less water is required during the washing process.

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