Econtrol® the modern art of continuous dyeing

Citation

(2006), "Econtrol® the modern art of continuous dyeing", Pigment & Resin Technology, Vol. 35 No. 5. https://doi.org/10.1108/prt.2006.12935eab.019

Econtrol® the modern art of continuous dyeing

Econtrol^® the modern art of continuous dyeing

Keywords: Dyes, Colours technology, Textile technology

Franz Peter Tolksdorf, DyStar Textilfarben GmbH & Co. Deutschland KG, Frankfurt am Main, Germany.

In recent years, only minor technical application developments have been made to the continuous processes used to apply reactive dyes. There have not been any fundamental changes or real innovations, or where there have been, they have not become established in the industry.

That was true, at any rate, until ITMA 1995 in Milan when Monforts and Zeneca (now part of DyStar) unveiled a joint development – the Econtrol process. This process delivers customer Controlled Coloration^® benefits combined with the innovative machine design from Monforts. Since then 65 units have been installed worldwide, allowing more economical and more ecologically friendly dyeing by continuous application.

So what made this development necessary?

To understand the background to the Econtrol process and its success, it makes sense to start by looking at the situation in the textile finishing industry in recent years:

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  the number of shades to be dyed has risen steadily;

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  at the same time, textile finishers are expected to meet higher fastness specifications;

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  environmental regulations have become far more stringent in many parts of the world;

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  batch sizes have declined dramatically; and

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  price pressure has risen enormously and delivery times are far shorter than in the past.

One of the major challenges facing textile finishers is the reduction in batch sizes. After all, capital investments tend to be geared to a defined production capacity. Output levels thus have a lasting impact on the capacity utilisation of a unit. Figure 1 shows the change in average batch sizes in various regions between 1960 and 2000. This overview clearly shows that the most dramatic change has taken place in Europe and Asia.

Figure 1 Lot size development in recent decades

Such changes evidently have a direct impact on textile mills. In order to maintain output levels, far more batches have to be dyed within the same time (Figure 2). Naturally, that also increases the number of lab matchings required (Figure 3).

Figure 2 The daily number of bulk dyeings required to maintain full production

Figure 3 The daily laboratory dyeings required to support the bulk production

Let us look at capacity utilisation in more detail. Many mills aim for 75 percent capacity utilisation. Assuming an average batch size of 11,000m, that means about 60min are available to change shades. By contrast, if the average batch size were 1,000m, a 60- min changeover time would reduce capacity utilisation to under 20 percent, making economical operation impossible. To achieve 75 percent capacity utilisation in these conditions, the changeover time would have to be cut to 5min (Figure 4).

Figure 4 Downtime effect on efficiency

Dyeing processes and products therefore have to be adjusted to reflect these conditions. Production personnel also need to adapt.

That was the trigger for the development of the Econtrol process.

The basic principle of the Econtrol process

The Econtrol process comprises just three steps.

Application of the dye – drying – washing-off (Figure 5).

Figure 5 Econtrol process route

The dyeing system must be carefully balanced to ensure complete fixation of the Remazol^® or Levafix^® dyes within the drying time.

However, complete fixation of reactive dyes is not possible in normal drying conditions because of the wet bulb temperature, which is a characteristic of all convection-based drying systems. It means that during drying the temperature of the moist goods is far lower than the surrounding temperature.

In normal drying processes the temperature on the goods can drop to 50-55ºC. The exact temperature depends on the humidity of the air used to dry the goods. If humidity is 25 percent and the air temperature is 120ºC, the temperature on the surface of the goods is around 68ºC. If humidity is 30 percent, the temperature on the goods is around 71ºC.

The Econtrol process uses this physical fact to fix the reactive dye during drying. Three key variables – time, temperature and humidity – are used to ensure accurate control of the dyeing process in the Thermex^® hotflue. Figure 6 shows a typical drying curve and the resultant temperature of the goods throughout the Econtrol dyeing process.

Figure 6 Econtrol – fabric temperature and moisture content

The goods should spend about 2-3min in the hot flue. However, it should be noted that these are average values: the actual drying time depends principally on the material to be dried.

Dyes, auxiliaries and alkali systems for the Econtrol process

Recommended dyes

Evidently, the dyes used in the Econtrol process have to be suitable for the process conditions. Since dyeing is completed in approximately 2-3min, reactive dyes with high to medium reactivity are particularly suitable. All Levafix and most Remazol dyes can be used.

In practice, Levafix dyes are used for pale to medium shades, while Remazol dyes are used for medium to very deep shades.

Dyes with double-MCT anchors such as Procion^w H-EXL and Procion PX are not recommended for the Econtrol process.

Alkali and auxiliaries

Soda ash or soda ash/NaOH is normally used as the alkali system.

To ensure optimum reliability, it is essential to dose the alkali and dye solutions separately.

For example, see Table I.

Table I Amounts of alkali recommended for Levafix CA dyes

Common auxiliaries used in continuous dyeing processes can be used, e.g. wetting agents, antimigration agents and, if necessary, sequestering agents.

A mild oxidant can be added to prevent reduction of the dyes.

Urea is not essential for the Econtrol process but up to 50g/l may be advantageous in some circumstances.

Guide recipe

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  Levafix^® Amber CA 2.15g/l

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  Levafix^® FastRed CA 1.59g/l

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  Levafix^® Olive CA 100 6.03g/l

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  Sera^® Wet C-AS 2-4g/l (wetting agent)

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  Sera^® Gal M-IP 0-10g/l (antimigrant)

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  Sera^® Con M-LU 2g/l (mild oxidant)

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  Soda ash 20g/l

Features of the Econtrol process

The demands made on future-oriented dyeing systems include the following criteria:

economical, even when dyeing small batches; rapid colour changes; simple to use; suitable for brilliant, very pale and very heavy shades; high reliability due to excellent reproducibility; a simple lab dyeing method must be available; environment-friendly; process should not restrict the obtainable fastness properties; and short processing times

In other words, the aim is to achieve optimum results in the shortest possible time. The fewer the variables affecting the dyeing process, the higher the probability of achieving this goal.

The PDPS process in Figure 7 shows what is meant by process variables.

According to this definition, 25 variables mean 25 parameters that can adversely affect the outcome of the dyeing.

Figure 7 Process variables in continuous dyeing processes

By contrast, Figure 8 shows the process variables for the Econtrol process. This process contains 40 percent fewer variables, which means the possibility of making a mistake is reduced by 40 percent. That is only possible because the Econtrol process does not use a separate fixation step. By contrast, all other continuous dyeing processes require separate fixation of some sort.

Figure 8 Process variables in Econtrol

There are also a number of other variables that are not directly related to the dyeing process. As a rule, far too little attention is paid to these parameters, especially the quality of the dyes and chemicals used.

Product quality

It is amazing how many mills invest in very expensive machines yet use them to apply poor-quality dyes, auxiliaries and chemicals.

However, it is a well-known fact that a process is only as good as the weakest link in the chain. In practice, using inferior products prevents mills getting the best out of their machinery.

No one would dream of running a racing car on low-grade petrol and oil because they know they would have no chance of winning the race.

The same applies to textile dyeing with the Econtrol process. The advantages of this process, which include excellent reproducibility, can only be fully achieved by using high- quality dyes and chemicals.

Lab processes

To maximise the efficiency of a dyeing process, a good lab process is needed to ensure that reproducible dyeings can be produced quickly and easily. In this respect, attention is drawn to Figure 3, which demonstrates how declining batch size increases the number of samples that have to be dyed in the lab.

The more reliable a dyeing process can be transferred from lab to production conditions, the more effective use is made of the money invested in the machinery.

The more accurately the lab method simulates bulk dyeing, the better the results. In collaboration with Mathis, DyStar has therefore developed a method that allows accurate reproduction of the Econtrol dyeing conditions in the lab. This comprises a Type DH lab steamer especially adapted for the Econtrol process. This allows regulation of all process parameters that influence the drying and dyeing results on the Monforts Thermex Hotflue (Figure 9):

Figure 9 Time consumption for lab dips

temperature; steam content; air flow; and time.

As an optional extra, a radiation pyrometer can be used to monitor the temperature of the goods. This allows optimum adjustment of the dyeing parameters to meet the requirements of different types of goods.

Using the Econtrol process can greatly improve efficiency in the lab. To illustrate this, Figure 10 compares the time required for an Econtrol dyeing and a pad-dry-pad-steam dyeing.

Figure 10 Mathis laboratory steamer for the Econtrol process

In this comparison, the Econtrol process requires 35 percent less time.

The process also has a logistical benefit: since the goods are dry at the end of the Econtrol dyeing process, they do not have to be washed off immediately and can be stored without problem.

If the comparison is extended to the cold pad-batch process, an additional batching time of at least 8h per dyeing has to be taken into account.

This extremely simple yet accurate lab process therefore helps meet the need to speed up processing times and injects a new meaning into the over- used cliché “quick response”.

As an alternative to this laboratory steamer, all lab processes commonly used for cold pad-batch processes can be used. However, the performance is in no way comparable and these processes greatly increase the overall time required for the process.

Lab-to-bulk reproducibility

A quick lab process is no use if it cannot be transferred reliably to production conditions. Special attention was therefore paid to this in the development of the process. Figure 9 shows the lab unit as used by DyStar to work out production conditions for customers.

Figures 11 and 12 compare lab and bulk dyeings on various materials. All lab dyeings were produced on the Mathis unit. The illustrations show very clearly that there is a very high standard of lab-to-bulk reproducibility on a wide variety of articles. Values below DE 1.0 (measured in accordance with CMC) can be achieved without difficulty and can be optimised on production machinery.

Figure 11 Lab-to-bulk reproducibility with Levafix dyes on Merc. Co

Figure 12 Lab-to-bulk reproducibility with Levafix dyes on corduroy

What shades can be dyed using the Econtrol process?

Econtrol can be used to dye all shades that can be obtained with reactive dyes. That includes brilliant turquoise, deep bordeaux, deep navy and, of course, black.

What materials and articles can be dyed in the Econtrol process?

So far this process has been used for fabric weights from 70 to 500g/m². All cellulosic fibres can be dyed. DyStar has extensive experience of this process on cotton, Tencel^®, viscose and linen.

Energy requirements

The Econtrol process needs far less energy than the, e.g. PDPS process because it cuts out the extra fixation step (steaming).

Chemical requirements

Unlike the PDPS process, the Econtrol process does not require any salt. The amount of alkali is similar. The calculation below shows how high the salt requirements can be in the PDPS process.

A total of 61,000kg salt is needed in the PDPS process, to dye 1,000,000m of fabric assuming a fabric weight of 350g per running metre, a liquor pick- up of 70 percent and 250g/l salt (Figure 13).

Figure 13 Consumption of salt and alkali, Econtrol vs PDPS

Cost efficiency and environmental profile

The environmental profile of industrial production processes is becoming an increasingly important factor. It therefore has to be considered when selecting the optimum dyeing process. The key criteria are energy consumption, chemical requirements, effluent contamination and pollution of exhaust air. One factor that is often overlooked is that the most environmentally friendly process is often also the most economical. Econtrol is a case in point.

By reducing energy requirements, it cuts the amount of energy that has to be generated and thus paid for.

Similarly, dyehouses do not have to purchase chemicals they do not need. Moreover, they do not have to wash out so they cannot contaminate effluent or exhaust air.

Econtrol is therefore both economical and eco-friendly. A comparison of production cost with the pad-dry-pad-steam-process is shown in Figure 14.

Figure 14 Cost calculation for 1,000m lot size

Limits

Nevertheless, like all dyeing processes, Econtrol does have its limitations.

The first is that Indanthren dyes can be dried by using the Thermex^w hotflue but not fixed as an air free steamer is required for this.

Pale to medium shades can be dyed on PES/CO blends using a modified one-bath TTN process but a separate thermosol step is required after the Econtrol step. Consequently, at least two Thermex^® chambers are required.

More experience is needed to obtain reproducible dyeings on very lightweight qualities (<60g/m²) than on heavier fabrics.

The Econtrol process is not entirely suitable for heavyweight pile and loop pile goods such as terry towelling because there is a risk of frosting.

The appearance of some goods dyed under Econtrol conditions differs from their appearance in the PDPS process. This means the two processes are not always interchangeable.

Conclusion

The Econtrol process combining Controlled Coloration from DyStar with the Monforts Thermex machine technology incorporating humidity control is a very progressive continuous dyeing process and an interesting alternative to conventional processes.

The simple lab process and excellent reproducibility allow a quick response, rapid batch changes raise capacity utilisation, and lower energy costs and the reduced effluent loading supports sustainability.