Pretreatment and dyeing of cotton knit fabric in exhaust dyeing machines with a very short process sequence

Technical description

In literature, different levels of water requirement have been reported to dye one kilogram of textile material, depending upon the fibre type, depth of colour needed, dyestuff chemistry, and machinery used for dyeing.

Exhaust dyeing, mostly used for knit fabric dyeing, are batch process and are most water-consuming technology to impart colour to textile material. In exhaust dyeing, a certain amount of textile material is loaded into a dyeing machine and brought to equilibrium with a solution containing the dye and the auxiliaries over a period of time. The dye liquor, with initial high dye contents, becomes gradually exhausted while the dye shifts from the dyeing liquor to the fiber. At the end of the process, the material is washed or rinsed to remove the unfixed dye to comply with given fastness standards.

Exhaust dyeing techniques, also called batch or discontinuous dyeing, use more than 50% to 200% water compared to the continuous dyeing process to dye a kilogram of fabric. A German machine company, Thies, examined the water use and water consumption of different reactive dyeing processes for cotton in their exhaust dyeing machines used in Pakistan by several dye houses. With a water metre that is part of all the machines, it has been found that water use varies from 15 L to almost 70 L water per batch per kg of cotton fabric due to various reasons.

For knit fabric processing some common steps are used as given in Figure 1. Each of these process steps is usually followed by intermediate washing steps. In the past, the number of baths for finishing cotton knitwear in exhaust dyeing machines was considerable such as 3-4 for pre-treatment and up to 8 or 9 baths for the complete dyeing process.

Figure 1: Conventional process sequence for knit fabric

More recently, the number of baths could be drastically reduced down to 4 – 6 by

  • Combining scouring and bleaching
  • Reducing biopolishing
  • Semi-scouring and semi-bleaching
  • Enzymatic processing at dyebath pH
  • Removing bleaching for dark shade
  • No discharge of the pre-treatment bath but directly dosing of dyestuffs (with prior cationic treatment) or discharge of one bath before dyeing
  • After pre-treatment and dyeing – first drain
  • Soaping at 60°C (commonly at boiling temperature) – second drain
  • 2 rinsing baths (for few specific dyestuffs 3 rinsing baths)

Example of conventional dyeing process (Figure 2)

Figure 2: Example for conventional dyeing process of cotton knit fabric

An example of conventional cotton knit fabric dyeing is shown in Figure 2. It can be seen that during pretreatment, the water consumption is 51 L/kg and aftertreatment would take around 71 L/kg, which mostly consists of rinsing, soaping, hot wash etc. The dyeing process itself takes only 6 L/kg, since the liquor ratio is 1:6. Nevertheless, the total water consumption is 128 L/kg.

Shortening of Process - Example of full dyeing procedure

Figure 3: Example for an optimised, i.e. shortened dyeing process of cotton knit fabric

When the processes are shortened and modified, the water consumption goes down as follows:

  • In pretreatment, it is 28 L/kg
  • Dyeing water remains the same at 6 L/kg
  • After treated water consumption reduced to 30 L/kg

In such a shortened process, several processes are combined such as scouring and bleaching, the number of washing steps is reduced or the liquor ratio is reduced. The total water consumption comes down to 64 L/kg (Figure 3).

Shortening of pretreatment – Example 1

Figure 4: Example for shortening the pretreatment of cotton knit fabric

Figure 4 is an example of shortening process sequences by avoiding scouring and bleaching by bio-scouring.

The bio-scouring process results in textiles being softer than those scoured in the conventional sodium hydroxide process. In this process, pretreatment steps are carried out with bio-scouring only with no bleaching, followed by two rinses – hot and cold. The water consumption is approximately 18 L/kg. As it is seen earlier, in the conventional dyeing process it may require up to 51 L/kg for pretreatment only.

This type of single-stage bio-scouring is possible for medium to dark colour, such as black.

In this second example, bio-scouring and bio-polishing are carried out in the same bath with only one hot rinse and drain to continue dyeing. The water consumption is further reduced to 14 L/Kg (Figure 5).

Shortening of Pretreatment – Example 2

Figure 5: Shortening of pretreatment of cotton knit fabric

Achieved environmental benefits:

  • Water conservation
  • Reduction in greenhouse gases
  • Reduction in hydraulic load of wastewater treatment plant

Operational data:

The very short sequence of processes (minimized number of baths) requires effective chemicals to disperse all the impurities. So far, specific information on these new chemical products is not available yet.

Cross-media effects:

Probably more chemicals have to be applied to minimize the number of baths but information on that is still limited. It is also not known whether the additional chemicals are easily or heavily or even non-biodegradable.

Technical considerations relevant to applicability:

This measure is applicable to all the pre-treatment and dyeing of cotton and cotton/polyester knit fabric in exhaust dyeing machines.

The specific operating conditions are not known yet. The same is true for dyestuffs with low fixation rates (especially phthalocyanine reactive dyes) or for dyestuffs that are more difficult to dye such as many reactive red dyes.

Economics:

So far, there is no detailed information available on the economic aspects. Certainly, there will be a big advantage due to saving time, water and energy.

Driving force for implementation:

  • Financial benefits
  • Resource conservation
  • Sustainability of textile business

Reference industry:

Various companies in Bangladesh, Pakistan, China, India and other parts of the world.

Reference:

Not available so far


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