Global shortages as well as sharply rising costs for resources used in the textile industry, such as water, wastewater, and fossil fuels, are leading to ever-increasing pressure in the textile industry.
There are many ways to counteract this pressure, for example by using highly efficient dyes and auxiliaries or by optimizing processes. However, there is also a need for machine optimizations and the successes achieved by machine manufacturers in this respect allowed to reduce water consumption through low and lower liquor ratios (FV).
Despite all the positive aspects of this trend towards lower liquor ratios, the practical effects on the dyeing result must not be ignored and the processes must be adapted accordingly to meet the requirements, for example regarding the fastness level.
General Information on Low and Ultra-low Liquor Technology
First, it is necessary to define terminology and show which developments have been implemented and where we push the borders of physical limitations.
In contrast to the liquor ratios of earlier times, when liquor ratios ranged normally between 15:1 to around 30:1, today’s usual liquor ratios have been reduced to at least around 8:1 to 12:1. Consequently, liquor ratios of over 12:1 are now considered high. Below this, the range between 6:1 and 8:1 is referred to as shorter liquor ratios, while liquor ratios of less than 6:1 to around 5:1 are regarded as low liquor ratios. The range below this can then be described as an ultra-low liquor. However, liquor ratios of less than 4:1 or even 3:1 is hardly achievable in reactive dyeing. An exception to this is dyeing on a jigger, where liquor ratios of 1:3 to 1:2.5 are perfectly possible, depending on the material.
The following specifically covers the challenges in reactive dyeing. In principle, the statements also apply to the dyeing of synthetic fibers of course. However, the liquor ratio has the greatest impact on this dye class, particularly due to the high water and time consumption of the rinsing process for reactive dyeing. In addition to saving water, fewer chemicals are also required for dyeing, which means that a reduced salt concentration in wastewater is also a significant advantage of short liquor technology. Shorter liquor ratios also allow the installation of smaller circulation pumps, which reduces power consumption and therefore electricity consumption. In addition, the design of the machines can be kept more compact, which means less material is used and therefore also helps to conserve resources.
It should also be mentioned that the amount of steam required to heat up dye baths is significantly reduced with smaller volumes.
Technology and Problems
A lower limit in the liquor ratio is given by the liquor retention capacity of cotton (“carry-over”). This depends on the fabric construction and is usually 200-250 % for cotton. It is therefore relatively high, especially compared to synthetic fibers. A liquor retention capacity of 250 % means that with a liquor ratio of 1:2.5, the entire liquor would be absorbed by the cotton and there would be no freely circulating liquor. Therefore, the liquor ratio for dyeing such an article must always be greater than 1:2.5. The liquor retention capacity for rinsing is at least as important as for dyeing. The lower the liquor ratio and the higher the liquor retention capacity, the less favorable the effect of a rinsing bath. This means that compared to higher liquor ratios, the number of rinsing baths must be increased to achieve the same fastness level. Although more rinsing baths are necessary, the amount of water required is reduced by the short liquor technology.