A Concept for the Dyeing Of Cellulosic Substrates at Ultra Low Liquor Ratios

Ever increasing demands on textile finishing processes require innovation in machinery as well as in process technology. The Luft-roto plus, developed by Thies in co-operation by DyStar, is the answer to recurring demand from dyers for shorter processes which save water and energy.

The starting point for the development of the Luft-roto plus was a consideration of how to apply dyestuffs in an exhaust dyeing process, by maintaining the hydrodynamic transport of the fabric whilst at the same time optimising the use of water, energy, chemicals (ecological impact) and time (economic impact).

In today's conventional jet machines the rope, together with the liquor that transports, is circulated from the jet to the dyeing kier. The liquor that is not retained by the substrate runs off the goods to the bottom of the dyeing kier and is then re-circulated by the pump.

The Concept:

In the Luft-roto plus a new design concept has been used in which the soft-flow jet is positioned on the lifting side of the machine, and a skimming unit before the winch permits an optimisation of the carry-over of the liquor on the goods, and hence an ultra low liquor ratio. ("Carry-over" denotes the liquor retention characteristics of the textile material). The wet rope is opened by means of an air transport system and plaited in the water-free dyeing kier by a synchronised plaiter (already well known from its use in the Thies roto-stream). The rotating fabric chamber which functions by weight and requires no external drive of the rope, produces a soft handle on the surface of goods (figure 1).

This modified liquor circulation process, which means there is no liquor in the dyeing kier, produces a free liquor volume of between 200 and 360 l depending on the loading capacity (100 kg to 180 kg) which leads to very rapid liquor exchange and very short heating times.

Other options such as integrated CCR (combined cooling and rinsing system), analogue dosing, 100% addition tank, automated salt solution preparation as well as fully automatic machine control permits perfect bleaching and dyeing results with considerable savings of water, energy and time, leading to increased productivity. Due to its gentle handling on the goods the Luft-roto plus is suitable not only for normal woven and knitted goods but also for strech articles which contain a high proportion of elastane. The dyeing process of water soluble dyes is characterised by the substantive exhaustion of the dye from the aqueous medium to the textile substate promoted by the addition of electrolyte such as sodium chloride or sodium sulphate.

Dyeing with reactive dyes

The exhaustion of the dye and the establishment of equilibrium between the concentration of the dye on the fibre and the dye in the bath is dependent on the liquor ratio, the quantity of electrolyte, and the substantivity of the dyes. Normally this equilibrium is reached after 30 minutes at the dyeing temperature. By then, depending on the factors mentioned, 30 - 80 % of the dye is exhausted onto the fibre. Shorter liquor ratios shift the equilibrium in favour of the dye on the fibre and the equilibrium is also reached more rapidly. The alkali required for fixation of the dye is then added portion-wise or by dosing typically over 60 minutes, and the dye is fixed on the fibre by running for a further 30 minutes.

As already mentioned our goal was to develop an ecologically and economically optimised jet dyeing machine as well as an optimised process for the dyeing of cellulosic fibres. The circulating liquor volume is so low that, in comparison to conventional jet machines, a much shorter dyeing time is possible.

New process

The dye is pre-dissolved in fresh water or water from the circulating liquor in the machine, half the desired amount of salt is added and then linearly dosed into the machine over a short period. A short holding time follows the dosing phase during which time solutions of alkali (preferable soda ash) and the remaining salt may be prepared in the addition tank. Depending on the press fresh water or liquor from the circulation in the achien can be used for this. These solutions are then dosed linearly over a short period into the bath and the dyeing continued for a period of 45-60 min. Finally, the dyeings are rinsed and soaped as usual. The preferred dye ranges for this process are the medium-high Levafix or selected Remazol dyes.

In addition successful trials have been carried out with Procion H-EXL dyes in pale shades. Further work with Procion H-EXL dyes are planned. Load to unload times of 4 hours can be achieved by the process described (Figure 2).

The new process was developed in the Thies Technikum in co-operation with DyStar. As a volume of circulating liquor is so small and, unlike conventional machines, is not resting in the fabric sump, rapid bath changes and short heating times are possible. By positioning the skimming unit before the winch, efficient removal of the unfixed hydrolysed dye is possible which normally requires longer liquor ratios.

Of course, a traditional process at short liquor ratio is also possible with addition of salt and dyes and linear or progressive dosing of alkali. Under these conditions, savings in water, energy and time are possible compared to other jet machines on the market which operates at liquor ratios between 1:6 and 1:10.

As with all dyeing processes the preparation of the goods plays a significant role. In order to fully exploit the efficiency of the Luft-roto plus, high temperature bleach with a subsequent CCR rinse process is recommended (Atmospheric bleaching at 95�C also possible). The liquor ratio can be extended to 1:8 for these processes without losing the benefits of the dyeing stage.

Final remarks

The introduction of the Luft-roto plus and the associated dyeing process represent a step change comparable to the changeover from winch to jet dyeing. New questions will arise from practical application of the new technology for which it will be necessary to work out locally applicable solutions.

Levafix, Remazol, Procion = registered trade mark