Washing - Off reactive dyes after dyeing: Shortening the process

1. Abstract

The basis for this article arises from a question by a prominent Turkish producer:

"The rinsing and washing processes in my dyehouse are toolong. Is there any way that we can utilise our existing machinery to improve the efficiency and reduce the rinsing time without compromising fastness properties? Our machine supplier is telling us that the latest Jet dyeing machines automaticallyreduce the washing off time. Is this true?"

In this article Mr. Steve Moorhouse, Technical Director of DyehouseSolutions International., responds:

2. Article

The rinsing process can be shortened as follows:

Various people (machinery suppliers or chemical suppliersfor example) will try to convince you that it is simple, and that only one approach holds the key. It is upto the concerned person to decide whether or not that opinionderives from a "vested interest". Experience wise, the objective should be notonly to shorten the rinsing process time, reduce the number of rinse baths, butto do so without any compromise to the safety of the process or to the level"Right First Time" production.

Thus, many chemical suppliers will seek to convince that thenumber of rinses after bleaching and before dyeing can be reduced by the use ofa peroxide killer. But this is only true if the modified rinsing process leaves the substrate "Fit for Dyeing". Peroxide is only one of the potentially harmfulresidual chemicals which must not be carried forward into the dyebath. If theuse of a peroxide killer leads to a reduced number of rinse baths, will thatreduced number still be sufficient to remove residual alkali ?

In our experience not necessarily so. See Case Study 1.

Thus also, the number of rinse baths after dyeing can easilybe reduced. But what is the effect on critical wash-fastness? See Case Study 2.

There is no doubt whatsoever that machinery suppliers havemade great strides in the last decade to shorten rinsing process timesconsiderably with such techniques as CCR (Combined Cooling & Rinsing) andSmart Rinsing.

Much reliance on machinery should not be placed withouttaking full consideration of a full understanding of all the critical successfactors involved.

So far as machinery developments are concerned, attempts aremade in this article to give an independent review without any vested interestof any kind.

2.2 Case Studies

Shortening rinse processes with Improvements in Quality and Safety

To emphasise the importance of Quality and safety we posetwo very simple, but "biting" questions.

a) Consider two processes A and B

Which process is the shorter?

b) Consider two processes A and B

Which Process is the shorter?

Let us now consider two case studies to illustrate thepoint.

2.6. Practical Bulk Experience

Figures 4 and 5 illustrate the same dark red shade washed off on the same machine using either conventional or Smart Rinsing techniques. The machine was a Thies Roto-stream which was fitted with a controllable hot water feed and a low level overflow weir.

The improved efficiency from Smart Rinsing in the initial dilution phase is clearly illustrated by the time taken to reduce the salt concentration below 2 g/l, approximately 10 minutes, compared with the Drain/fill system that required three discrete rinses to achieve the same end point. This could take over one hour when filling, heating (cold water) and draining times are all taken into account. The benefit of the hot water (70C) in the early rinse is also clearly demonstrated with respect to the rapid removal of unfixed dyestuff leaving little to be diffused out in the re-circulation 'soaping' bath.

The exploitation of such technologies is vital to those companies who wish to reduce their total costs of production and gain competitive advantage. However, the first step in any productivity improvement program should be a complete audit (and possible upgrading) of utilities to ensure that the benefits of the new equipment can be exploited to the full.

Further process optimisation may see these Smart systems, which have already seen the on-line use of conductivity meters to signal the attainment of pre-set salt concentrations and step on the process automatically, develop into truly 'intelligent' systems through the introduction of colorimeters to facilitate rinsing to the optimum end point.

There are several such 'smart rinsing' systems available now from the leading machinery suppliers.

2.7. Concluding Remarks

• Rinsing and soaping processes can be optimised to a limited extent to improve efficiency and maintain high wet fastness levels without capital investment. Careful audits of existing procedures and equipment will identify how.
• Some elements of the newer machinery developments can be retro- fitted to existing machinery, allowing further efficiency gains.
• Any dyehouse considering upgrading dyeing machinery or increasing capacity should seriously consider the benefits offered by machines fitted with the latest rinsing technology.
• It is important that the commissioning and optimising processes for such machinery are done alongside a deep and full understanding of Reactive Dye design, manufacture and application technology.

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Case Study 1

Customer Profile:

• Yarn Dyer in Peru.

• The only dyeing operation in the market exporting directly to Europe.

Customer History:

• Operating without level dyeing problems (with preparation before dyeing indicated by Profile 1).

• Visited by leading European Chemical Supplier whose literature proudly states (unequivocally) that they are the World's Number 1 experts in Cellulosic Preparation.

• World Number 1 preparation experts recommending cutting 4 rinse baths to 3 and incorporating a peroxide killer in final rinse bath. (Profile 2).

• Level Dyeing problems occur immediately. Dyehouse told by Preparation Experts that the problem was due "dye selection" despite the fact that the same dyes had not previously given level dyeing problems.

• Dyehouse Solutions International called in. We install a 2 bath preparation process (with significant reduction in water consumption and time). No level dyeing problems, and level dyeing (as quantified by knitted socks a significant improvement on original level dyeing performance.

Case Study 2

Customer Profile

• Yarn Dyer in Indonesia.

• Indonesian Management.

• 100% owned by Japanese Venture Capitalists.

• Exporting directly to Japan, to Japanese retail specification.

Customer Problems

• Unlevel Dyeing and fastness in critical shades (Heavy Red, Heavy Scarlet, Green / Turquoise). Critical Fastness test failed (60'C wash test in 5 g/l perborate).

• Lengthy Process and poor tensile strength of yarn (difficulties in knitting socks to quantify levelness).

Soaping Chemicals in 4^th and 5^th Baths

No Soaping Chemicals in any Bath.

Now let's consider what new capital investment could bring to a Dyehouse.

2.3. Background to Machinery Hardware Developments

This area of wet processing tends to get the least attention yet it has been estimated that 80% of the water consumption, 90% of the energy and 70% of the time of the total wet process is consumed in bleaching, washing and rinsing.

General improvements in production efficiency via the dyeing (colouration) stage have been realised through a more widespread adoption of Controlled Coloration principles that lead to higher Right-First-Time (RFT) performance and reductions in costly and wasteful reprocessing. When RFT performance has been optimised the focus should change to increasing productivity by utilising the latest generation of textile machinery and machine controllers, in order to reduce the fixed costs per unit of production. The emphasis is on that such drives should follow, and never precede, optimisation of the basics.

The introduction of newer elements of application technology has already led to significant productivity gains in the exhaust dyeing of cellulosic substrates in pre-treatment and dyeing.

The essential machinery developments in this respect have been:

• low liquor ratio machines
• multi-tasking controllers
• and heat recovery systems and hot water supplies.

The increasing costs of water and waste-water treatment, coupled with the need to drive down production cycle times and costs, forced the industry to re-evaluate the way in which this vital commodity was utilised.

ITMA 1995 thus saw the unveiling of a range of machines from the major manufacturers with 'smart' rinsing systems in which fresh hot water could be metered into the dyeing machine to dilute and remove fabric contaminants, dyes and processing chemicals. In addition to significant time savings and much more effective use of water, these systems offer a scientific approach to a stage of the process that is normally left to subjective judgement. Some of these systems can be 'retro-fitted' to existing machines.

2.4. Rinsing Objectives

2.4.1 During Pre-treatment

Efficient rinsing will ensure that all of the unwanted natural impurities and processing chemicals removed from the fibre by the pre-treatment chemicals and process conditions are prevented from re-depositing onto the substrate.

The overall objective of all pre-treatment processes is to make the substrate "Ready for Dyeing".

2.4.2 Washing Off (After Dyeing)

In the exhaust application of reactive dyes to Cellulosic substrates, this mainly concerns the removal of hydrolysed dyestuff.

Although, with some reactive dye systems, there may also be some reactive dye to remove. There is a finite quantity of unfixed dye on the fibre at the end of the fixation stage, whether it is hydrolysed or unreacted. The unfixed dye must be effectively removed in order to meet the required wash fastness specification. With some reactive dyes also, the washing off stage can play a vital role in maintaining shade reproducibility, and pH control is often necessary.

The effective removal of unfixed reactive dye takes place in 4 phases:

1. dilution of dye and chemicals in solution and on the surface of the cellulose
2. diffusion out of the deeply penetrated unfixed hydrolysed dye to the fibre surface
3. dilution and removal of the diffused-out dye
4. prevention of re-deposition of the dye removed.

All of this takes place in the fibre-liquor interchange zone in the jet/overflow nozzle where the contaminated liquor is removed from the fabric rope through repeated passes. Each of the rinsing systems available follows this general pattern.

2.5. Rinsing Systems

2.5.1 Drain/Fill Rinsing

Washing after pre-treatment and dye fixation is achieved in conventional rinsing by successive fill, run, drain operations. The efficiency of these operations depends on; liquor ratio, temperature of rinsing liquor, electrolyte concentration (a function of depth of shade), draining time and effect on the liquor retention of the substrate, rope cycle time/jet passes, and dye chemistry. The efficiency of rinsing is also influenced by; water pressure, steam capacity and the bore of feed pipes. These factors affect filling and draining, and heating and cooling rates.

The liquor ratio has a major influence on the dilution effect. For example when dyeing 100kg of fabric; if the exhausted dye liquor contains 60 g/l electrolyte, the retention capacity of the substrate after draining is 3 litres/kg (300%), and the liquor ratio is 15:1, the concentration of electrolyte, in the subsequent rinse bath after one drain/fill operation can be calculated as 12 g/l, and after two drain/fill operations this will be 2.4 g/l. If the liquor ratio is reduced to 7:1, the values become 26 g/l and 11 g/l respectively for one and two subsequent rinses.

Table 1: Effect of Liquor Ratio

Rinsing at lower liquor ratios therefore requires more rinsing steps (and probably more time) to achieve the same end point but uses less water. The substrate retention capacity or carry-over, also has a major influence. This can be reduced by increasing draining times after the more heavily contaminated exhausted dyebath and 'soaping' bath.

During rinsing, the non-substantive chemicals such as electrolyte, and dye trapped in the interstitial liquor quickly saturate the rinse liquor, especially in the early stages of rinsing when their concentrations are highest. The optimum rinsing time depends very much on local conditions and guidance on an individual basis should form an integral part of a Dyehouse Audit.

A new bath should be set only once the optimum rinse bath time has been determined (see Fig 1).

Figure 1: Effect of Rinsing Time

The drain/fill sequence should include a boiling 'soap' (phase 2) to diffuse the deeply penetrated unfixed hydrolysed dye to the fibre surface. This process is aided by sequestering/disaggregating agents that prevent dye agglomeration caused by permanent hardness in the 'soaping' bath. For some reactive dyes, it should also include a careful pH control before 'soaping' to prevent the stripping of fixed dyestuff from the fibre. Again, advice on which dyes require such control forms part of an "Audit Service".

The number of rinses required is determined by the depth of shade. There should be sufficient rinses during phase 1 to reduce the electrolyte concentration to less than 2 g/l before the boiling 'soap', phase 2. This will ensure that the substantivity of the unfixed dye is at a minimum. Higher concentrations of electrolyte in the 'soaping' bath hinder dye diffusion and result in inferior wet fastness properties. A typical rinsing sequence for MCT dyes is illustrated in Fig 2:

Fig. 2: Conventional Drain & Fill Rinsing Sequence with MCT dyes

The main disadvantage of Drain/fill rinsing is the deposition of scum on the fabric surface between rinses as the fabric rope stops and the liquor drains. This is particularly problematical in rinsing after pre-treatment when the removed fats, waxes and processing chemicals can be re-deposited non-uniformly onto the fabric surface, especially if the machine is re-filled with cold water. The main benefit of Drain/fill rinsing is that it is relatively efficient in terms of water consumption.

2.5.2 Overflow or Flood Rinsing

Overflow or 'flood' rinsing is sometimes used as a corrective action to remove surface scum resulting from poor quality water or chemicals, or from inefficient pre-treatment. Clean water is fed into the machine and drained through an overflow weir usually set near the normal running level. It is inefficient in terms of water consumption as the clean water is usually introduced into the machine with the feed valve fully open. In addition, the efficiency of overflow rinsing can be variable from lot to lot due to variations in water pressure (depending on demand), and also between machines of different size due to differences in the number and frequency of rope passes through the jet nozzle. When overflow rinsing is used it tends to be in combination with Drain/fill rinsing, overflowing first of all for up to 10 minutes to remove surface scum and partially dilute contaminants, then running through a series of drain/fill treatments. This still constitutes a lengthy process and lacks a degree of control.

2.5.3 Combined Cooling and Rinsing

Combined Cooling and Rinsing (CCR) can be utilised at any stage in the process that requires the simultaneous cooling and rinsing of the fabric. This is achieved by introducing clean water into the main liquor circulation and through the jet/overflow nozzle after it has been pre-heated by passing it through either the machine's main heat exchanger or through an external highly efficient plate heat exchanger. The contaminated waste liquor is then drained through an overflow weir. The quantity of clean water consumed depends on the programmed rate of cooling, the temperature of the cooling water, and the pre-set end temperature and is controlled automatically by the machine controller.

CCR gives very efficient cleaning of the fabric because of the high starting temperature and reduces processing time and total water consumption (combined processing and cooling water). A further major benefit is that the contaminated rinse liquor is decanted away from the machine whilst the rope is turning during rinsing so there is no deposition of scum on the fabric surface.

2.5.4 Smart Rinsing

One of the main disadvantages of traditional overflow rinsing is the high bath volume during the rinsing stage. This was required on older jets to aid fabric transport from the back to the front of the dyeing kier. However, advances in fabric transport mechanisms, plating devices and chamber configuration, such as are available on ultra low liquor ratio (ULLR) machines, have eliminated the need for large liquor volumes. The volume can now be reduced to the minimum required to completely wet-out the fabric and of course to avoid cavitation of the main liquor circulation pump.

ULLR machines were introduced to reduce the 'dead' draining, filling, heating and cooling times as well as the cost of chemicals used. ULLR processing is now a specific requirement for effective use of the Smart Rinsing systems now available as an option on virtually all new jet and overflow machines.

It is clear that the dilution effect of a given flow rate of incoming clean water will be greater the smaller is the volume of contaminated liquor circulating within the machine. If the overflow weir is therefore set low down in the dyeing kier and if a supply of clean water can be regulated in through the jet nozzle at a rate equivalent to that of the liquor draining through the low level overflow, this offers the basis for an efficient rinsing system. This is particularly the case when hot water is used for rinsing because of the increased rates of diffusion-out of the unfixed hydrolysed dye at higher temperatures.

Several machinery manufacturers have designed 'piggy-back' stock tanks with internal heat exchangers large enough to accommodate the nominal running capacity of the machine (typically 5:1LR) for the purpose of storing and pre-heating the rinsing liquor (or indeed any subsequent liquor required in the process). Other manufacturers rely on the efficiency of the machine's own heat exchanger or the temperature of the draining waste liquor (via very efficient plate heat exchangers) to heat the incoming water to the required temperature. Whatever the source of clean water, the principle of the systems is very similar.

In Smart Rinsing the bath volume, flow rate of rinsing water, and rinsing temperature all influence the efficiency of the rinse.

2.5.6 Influence of Rinsing Temperature

The dilution effects so far considered have been appropriate for non-substantive chemicals and for dyes already desorbed from the fibre surface into the liquor.

However, temperature can influence dye diffusion even during the phase 1 dilution step. It has been shown that the rate and amount of unfixed dye removal during phase 1 increase with temperature from 50C to 70C. There is a similar increase in the quantity of dye removed between 50C and 60C in phase 3 (after 'soaping') but there is no improvement above 60C. The increasing rate of dye diffusion is assisted by the swollen fibres at the higher temperature, particularly under alkaline pH conditions. These effects are illustrated in figure 3

Figure 3: Influence of Rinsing Temperature

Following the drain and initial rinsing/dilution phase, the diffusion-out of the unfixed hydrolysed reactive dye in phase 2 can be achieved either by:

1. - continuing the Smart Rinsing up to the required temperature (ideally greater than 90C),
   - running at this temperature for a defined period,
   - cooling back at a pre-determined rate by Combined Cooling and Rinsing, and
   - continuing with Smart Rinsing at 60 - 70C or

2. - closing off the low level overflow weir,
   - filling to the nominal running level and carrying out a conventional boiling 'soap',
   - cooling back at a pre-determined rate by Combined Cooling and Rinsing, and
   - continuing with Smart Rinsing at 60 - 70C

The choice will depend upon the particular machine set up and the ability to continue feeding in hot water at the required rate and temperature. The relative water consumption will also determine the design of the rinsing sequence. Option ii) will normally consume less water and has the added advantage that any required chemicals (e.g., sequestrants) can be dosed into the 'soaping' bath and thus used more effectively.

Phase 3, is a continuation of smart rinsing to dilute the diffused out dyestuff to an acceptable level. As with phase one this is accomplished most efficiently with hot water, 60 - 70C (the colour content of cold rinse liquors give no indication of the extent of unfixed dye left on the fibre because the dye becomes 'frozen' into the constricted fibres). The rinsing time will again depend on depth of shade and will normally take 10 - 30 minutes. The rinsing can be continued down to 40C where after-treatments may be exhausted from a new discrete bath, if required. Whichever of the above options is used the entire controlled rinsing sequence should take only 45 - 90 minutes depending on depth of shade and flow rates. This represents a saving of more than 2 hours compared with conventional rinsing of dark shades.