Mechanism of Dye Fixation Processes in Fabric

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Introduction:
If a typical textile print is washed soon after printing and drying, a substantial part of the colour is removed. An appropriate fixation step is therefore necessary. Complete fixation can rarely be achieved, however, and the removal of unfixed dye, thickening and auxiliary chemicals in a subsequent washing process is usually required.

The efficiency with which these processes of fixation and washing are carried out is vitally important, to both the quality and the cost of the prints. The proportion of faults in the final product that are introduced at this stage can be disastrously high. The objective of this chapter is to direct attention to the details of the processes and the understanding of their mechanisms. 
 
Steamer
Steamers:
Printed dyes are usually fixed by steaming processes, the steam providing the moisture and rapid heating that brings about the transfer of dye molecules from the thickener film to the fibre within a reasonable time.

Historically, the process of developing printed mordants was known as ‘ageing’ and took a long time, as the term implies. Printed fabric was draped over poles and left in a room with a warm and humid atmosphere for some days, allowing the processes of diffusion and chemical reaction to occur. The term has been retained in use for steaming treatments, especially for short processes and machines; it has given rise to the descriptive, and euphonious, terms ‘rapid ager’ and ‘flash ageing’. Some authors have attempted to distinguish between steaming and ageing (with steam). This can lead to confusion, so the two terms can be used interchangeably.

The time and conditions for fixation in steam vary with the properties of the dyes and fibres used, ranging from 10 s to 60 min in steam at 200 to 100 0C. Technical and economic factors have encouraged the use of higher temperatures and shorter times,and the change from batchwise to continuous processes. A constant feature in the design of all printing steamers, as distinct from steamers for other textile processes, is the need to prevent the marking-off of printed colour on to pale-coloured areas.

Batch Steamers:
For expensive fabrics and small quantities, there are obvious advantages in using lowcapacity steamers that can be quickly raised to their working temperature and that produce no creasing, stretching or other damage to the fabric. Batch steamers also show advantages when colour yields are improved by steaming at above-atmospheric pressure or for extended times, as in the case of deep colours on polyester fabrics. A successful design for such a textile steamer is shown as star or bell steamer. Up to 500 m of fabric is attached by hand, along one selvedge, to the hooks on a star-shaped carrier frame, to form a spirally wound load with a space of about 1 cm between the fabric ‘layers’. An interleaved back-grey is used to eliminate all risk of marking-off, as the weight of the fabric and the uniformity of winding are unlikely to be adequate to prevent adjacent layers from touching. The steam chamber is a cylindrical pressure vessel, mounted vertically and closed at the top (hence likened to a bell) with a door that can be swung into position at its base. It is elevated because steam is lighter than air, so that with this arrangement the bell can remain filled with steam and either the star frame can be raised into the steam, or the bell lowered on to the star frame. Less air is taken in with the fabric, and the air is more easily displaced by steam, than in any other possible arrangement. The steam supply should be air-free and, ideally, dry but saturated. If there is significant superheat in the steam, a humidifier is used to increase its relative humidity.Wet steam is undesirable because splashes and drops of water inevitably cause local bleeding of dye or auxiliary chemicals.The older-style cottage steamers were often larger vessels into which the printedfabric on a carrier was wheeled horizontally. Substantial flow of steam is required to displace air from such a steamer.

Continuous Steamers:
A logical development of the ageing of prints while looped on rods led to the continuous transport of printed fabric in festoon steamers . Long loops areformed on rods, touching only the unprinted face, which are moved slowly along atrack near the top of a large steam chamber, constructed in brick or steel. The rods maybe slowly rotated to avoid bar marks due to non-uniform accessibility to steam. Withloops (festoons) of up to 5 m in length, long steaming times or high through put velocities can be achieved without the tension and mark-off problems associated with top and bottom carrying rollers. Several ingenious mechanisms for the formation of festoons of equal length are available. At the end of the steaming period, the fabric is withdrawn at the same high speed as at the entry point. A fabric content of overall though put speed of 80 m min–1 with a 10 min steaming time. Capacity can be doubled by introducing two layers of fabric, with an intermediate back-grey, if the printed area is not large. The larger the steamer dimensions and the more densely it is packed with fabric, the more difficult it will be to maintain uniform steaming conditions.
Festoon steamers
It is now considered essential to have fan-assisted circulation of steam. In older designs a flow of steam through a water tank at the base, which reduces the superheat of injected steam, to exhaust ducts in the heated roof was used to help to maintain uniformity and provided a valuable cooling effect. In most early festoon steamers, the fabric entry and exit were through slots in the roof, provided with heated roller seals. The seals could never be perfect and escaping steam, absorbed by the print, increased the mark-off from the printed surface to the sealing roller. A doctor blade was therefore required to clean the roller surface, before it again contacted the fabric. In modern equipment the fabric entry and exit ports are usually positioned in or near the base of the steam chamber, which reduces the sealing problem significantly (as steam is lighter than air).

Popular steamers made by Stork or Babcock have steam circulation arrangements whereby steam is extracted by fans from the base of the steamer, passed through heatable radiators, sprayed with water as needed, and forced through ducts to the top of the steamer, whence it passes down the folds to the bottom. In this way festoon steamers are increasingly being produced as universal steamers, so that any temperature between 100 and 200 0C may be employed. Since many knitted fabrics can be satisfactorily handled in festoon steamers, provided the loops are not too long, the term ‘universal steamer’ can in fact be justified.

Where the steaming time required is short (up to 2 min), more compact machines with fabric-carrying rollers have been used. The term ‘Mather and Platt Roller Ager’ was often applied to such steamers, in which the fabric path resembled that in a roller curing oven. With the increasingly important screen prints, which usually have more surface colour than engraved-roller prints, marking-off via the rollers became more probable. The Krostewitz steamer overcame this difficulty by adopting a spiral movement of fabric, using rollers that contacted only the back face of the print . At the centre of the spiral it is, of course, necessary to withdraw the fabric by rotating it into a normal plane so that it can be taken through a slot in the side wall.

This is achieved by passage around a stationary sword bar (rod) at an angle of 45 to 55 the fabric path. A double rotation of plane and a second, outward-moving, spiral allows fabric exit through the entry slot as in Steamers of this type, with a fabric content of 60 m, are used to give 30 s steaming at speeds of 120 m min–1, but are suitable only for stable, woven fabrics. Some of the carrying rollers must be driven, to prevent the build-up of high fabric tensions.

Double-spiral steamer (Krostewitz):
For steaming times of about 30 s, again without touching the print face until fixation is complete, an alternative fabric transport system has been used, where a compact arrangement is not essential. This is the rainbow or arch steamer.

An important application of either type of such flash agers has been in pad–steam(sometimes called two-phase) processes. Vat prints on cotton, for example, are efficiently fixed by application of alkali and reducing agent solution to the printed and dried fabric immediately before steaming. A thickener that gels on contact with alkali is required, and the time of contact between print and solution must be limited to avoid bleeding. The application of a minimum add-on of solution (about 30% on mass of fibre) using one of the ‘MA techniques’ can give higher visual colour yields than obtained by conventional padding . In the case of prints obtained with reactive dyes, the advantage can be even greater . For short steaming times at low running speeds, the simplest possible arrangement is a chimney or tower steamer, mounted above the pad mangle.

Mechanisms of Fixation Processes:
Is a stimulating review of dye-fixation processes, has pointed out that when buying and running process machines we must not forget that the fundamental requirement is the efficiency of the molecular processes. This is a necessary reminder when considering the mechanism of steaming processes, where we find easily overlooked, but significant, molecular phenomena. It is desirable, in the first place, to understand the properties of steam itself.

Steam: Terms and Properties:
Water vapour at 1000C and standard atmospheric pressure is known as saturated steam, and as dry saturated steam if it contains no droplets of liquid water. Steam at 1000C is very rarely found in a printworks, however, and the differences are significant.Boilers are designed to provide steam at pressures substantially above atmospheric pressure. This allows the use of smaller-diameter pipes to convey the substantial weights of water vapour from the boiler to the steamer and other steam-using machines.Saturated steam at 35 kPa above atmospheric pressure (5 lbf in–2 gauge pressure) occupies three times the volume of the same mass of steam at 350 kPa. At pressures above atmospheric, water boils at a temperature above 100 0C, more heat is required to evaporate a given mass of water and the steam produced has a high temperature. The temperature of saturated steam at 350 kPa above atmospheric pressure is 1480C. Any cooling would produce condensation, and this is why steam at a certain pressure, and a temperature corresponding to the boiling point of water at that pressure, is known as saturated steam.

Saturated steam is often deliberately superheated in the boiler house, giving a gas which does not condense until it has given up its superheat. This ensures that steam pipes do not carry a significant volume of troublesome water. Superheat is also introduced when saturated steam is allowed to expand rapidly as it passes through a valve into a chamber at lower pressure. For example, steam at 700 kPa and 170 0C when allowed to expand at 70 kPa falls to 148 0C.

Dye Fixation in Steam:
Steam can be a convenient source of both water and heat as both are transferred rapidly and uniformly over the surface areas of printed fabric entering a steam chamber. As we have seen, however, steam may be wet or dry, saturated or superheated, and the conditions of use must be chosen and maintained.

The essential requirements in all print fixation processes using steam are:
  1. The pick-up of enough water to swell the thickener film, but not so much as to cause the print to spread.
  2. Dispersion and solution of the dye, and production of a liquid medium through which the dye can diffuse to the fibre surface.
  3. Absorption of water by fibres such as cotton, nylon and wool, which must be swollen to allow penetration of dye.
  4. Raising the temperature to a level that accelerates the processes of diffusion,especially into the fibre.
In some cases steam can satisfy all the requirements but, as in all coloration processes, auxiliary chemicals may be introduced to assist dye solution and diffusion, or to make the process less critically dependent on the maintenance of ideal conditions. In order to illustrate the phenomena that can occur during the steam fixation of prints, one of the most critical and best-studied processes is considered here in detail.

Vat Dye Prints (all-in process) on Cotton:
The insoluble vat dyes must be reduced to their soluble leuco forms, to allow diffusion into the fibre. A stabilised reducing agent, sodium formaldehyde sulphoxylate (CI Reducing Agent 2), is activated when the print temperature approaches 100 0C and reduction therefore occurs inside the steamer, the highly soluble potassium carbonate providing the required alkalinity. It has been shown that the vat dyes must be selected from those with aqueous leuco potentials smaller than –920 mV. A typical paste formulation is shown in Recipe; the print would be dried rapidly, cooled and then steamed for 8–20 min in air-free steam, before rinsing, oxidising, soaping and drying.

The steaming stage was known to be critical, especially where the cover of the design was high. Difficulty was experienced in keeping the temperature of the steam below 103 0C, and dye fixation was reduced when the temperature rose above this level. The incorporation of glycerol in the print paste, to act as a humectant, improved the fixation. Thorough and rapid drying of the print was, however, found to be essential because the stability of the reducing agent in air under damp conditions was not satisfactory . In practice, after thorough drying a cooling procedure was necessary.In this discussion the steam conditions can be assumed to be ideal, that is, no droplets of liquid water are present, and that it has a temperature of 100 0C, with no superheat. When the dry print enters the steam, three exothermic reactions occur.

Recipe :
  • Vat dye paste .........................7 g
  • Potassium carbonate............. 15 g
  • CI Reducing Agent .............. 2 8 g
  • Glycero..................................l 5 g
  • Thickener .............................24 g
  • Water .....................................41
  • Total .................................... 100 g
The steaming stage was known to be critical , especially where the cover of the design was high. Difficulty was experienced in keeping the temperature of the steam below 103 C, and dye fixation was reduced when the temperature rose above this level. The incorporation of glycerol in the print paste, to act as a humectant, improved the fixation. Thorough and rapid drying of the print was, however, found to be essential because the stability of the reducing agent in air under damp conditions was not satisfactory .

In practice, after thorough drying a cooling procedure was necessary. In this discussion the steam conditions can be assumed to be ideal, that is, no droplets of liquid water are present, and that it has a temperature of 100 0C, with no superheat. When the dry print enters the steam, three exothermic reactions occur. Firstly, steam will immediately condense on the cold fabric, giving up its latent heat

of 2260 kJ , and raising the fabric temperature very quickly to 100 0C. Cotton has a specific heat of 1.4 kJ kg–1 K–1 and, starting at 20 0C and 7% regain, 5.5 g of steam will condense as liquid water on 100 g of fibre.

As this water condenses more heat will be liberated, partly because of the heat of wetting of cellulose and partly because of the heat of solution of potassium carbonate. For mercerised cotton at 7.5% regain, the heat evolved (when fully wetted) is 25 kJ kg–1 (6 cal g–1). Starting at 2.5% regain, the heat is 50 kJ kg–1 (12 cal g–1), but at

13% regain it is only 8.5 kJ kg–1 (2 cal g–1). As the absorption of condensed water and the consequent generation of heat occur less rapidly than the condensation of steam, the fibre temperature rises above 100 0C by an amount that depends on the initial regain and on the cooling effect of the steam. Fell and Postle have recorded temperatures of 110 and 120 0C on cotton and wool respectively after absorption of steam at 100 0C.

Similarly, anhydrous potassium carbonate has a heat of solution of 27.6 kJ mol–1 (6600 cal mol–1), and the amount used in the print paste could provide up to 25 kJ kg–1 (6 cal g–1) fibre. The third reaction is the oxidation of active reducing agent (sulphoxylate ion) , which is strongly exothermic (+560 kJ mol–1).

HSO2– O→ HSO3

The air content of the steam should be kept low (less than 0.3%), but the required reduction of vat dye means the equivalent oxidation of sulphoxylate will occur and the production of heat is inevitable . If only 20% of the total sulphoxylate were oxidised in the early stages of the steaming process, about 50 kJ kg–1 (12 cal g–1) of dry fibre would be liberated.

It is clear, therefore, that a total heat input of about 85 kJ kg–1 (20 cal g–1), over and above the heat of condensation, is likely for a fabric of low moisture content and for 100% print cover. This could lead to a fabric temperature of 140 0C, but actual temperatures will not be so high because the exothermic reactions occur slowly and evaporation from the print and cooling by the surrounding steam also occur. Fabric temperatures of 115 0C have been recorded

The practical answer to the problem was the addition of glycerol and the use of the deliquescent potassium carbonate, rather than the cheaper sodium carbonate. Water absorption by the print paste is therefore substantially higher than by the cotton fibre, especially under adverse steam conditions. Some of the glycerol and carbonate will have entered the fibre, thereby increasing fibre swelling. Measurements of water content during steaming showed that a typical vat print paste film absorbed about 20% of water after 2 min and 30% after 10 min, under ideal steam conditions.

The steaming of any other class of printed fabric is less complex but may involve one or more of the interactions of physical and chemical factors discussed above. Forexample, nylon is very sensitive to superheat in the steam (which can arise from theheat of wetting of the fibre), lower colour yields being the consequence.

High-Temperature Steaming:
In some circumstances the use of superheated steam shows advantages of faster heating, shorter fixation time and less colour spread; this is the case if the print has not been dried and also in the pad–steam situation, where there is usually more than sufficient water in the fabric. The term high-temperature (HT) steaming, however, is normally restricted to the treatment of dry prints in superheated steam at temperatures substantially above 100 0C and at atmospheric pressure.

Lockett, the first to advocate this approach, showed that reactive dyes on cellulosic fibres wereefficiently fixed in 1 min in steam at 150 0C, provided that a suitable concentration of urea was included in the print paste . The same dyes might require 5 min in steam at 100 0C or in dry air at 150 0C. Reactive dyes can be of small molecular size and low substantivity, so that diffusion occurs more readily than in other dye–fibre interactions, but a liquid medium is required for transport and for chemical reaction. For fixation in dry air, it was known that urea was required to act as the liquid medium, providing good dye solubility both in the later stages of drying and at temperatures above its melting point (132 0C).

When a printed cotton fabric at 20 0C and 7% regain enters HT steam, the steam will rapidly give up its superheat and then condense on to the fibres. The amount of condensed steam will be similar to that for saturated steam (5.5% o.w.f.), the reduction due to the temperature rise before condensation being outweighed by the strong heat absorption (240 kJ kg–1, equivalent to 58 cal g–1) occurring as the urea goes into solution. If the print has provided 20% o.w.f. urea and 12.5% water is also present, the total liquid phase is substantial. A fraction will be retained by the thickener, but the major part will enter the cotton fibres, which can absorb 30% by mass of water at 20 0C.

The moisture content will then fall, as steam at 150 °C has only 20% RH and the equilibrium moisture content of pure cotton in this atmosphere is only 1%. Lockett pointed out that urea forms a eutectic mixture with water, however, and holds some water very tenaciously. The temperature of the dye–fibre system therefore rises rapidly to 100 0C, stays at that level as long as the loss of heat by evaporation is high, and then rises towards the temperature of the steam . Reaction between dye and fibre, therefore, proceeds efficiently because the fibre is swollen and molecules the diffusion of dye to ionised sites in the fibre can occur. Diffusion of the larger reactive dye into viscose rayon under these conditions is slow, however, and colour yields are often unsatisfactory. As reaction approaches completion the water content has dropped and the amount of dye–fibre bond hydrolysis may therefore be smaller than in saturated steam. The large amount of urea required adds to the cost of the process and some decomposition occurs, with the production of ammonia and biuret-type products.

Some reactive dyes give low colour yields under these conditions, perhaps as a result of reaction with ammonia and of loss of alkali. There is also a need to reduce the nitrogen level in effluents, and alternatives to urea have been sought . The controlled application of water, as a foam, before steaming may provide the ideal alternative .

The use of HT steaming for prints on polyester and polyester blend fabrics hasbecome extremely important because the only satisfactory alternative is the batchwise pressure-steaming method. Although continuous pressure steamers have been used (for continuous bleaching, for example), the difficulty of avoiding mark-off at the entry seal is so great that they have never successfully been employed for prints. Steaming at atmospheric pressure and 100 0C is possible if carriers are incorporated in the print paste, but colour yields are limited and only a few disperse dyes are suitable At a temperature of 180 0C it is possible to achieve satisfactory fixation of many disperse dyes in 1 min, as compared with 30 min pressure steaming at 120 0C or 1 min in dry air at 200 0C. With the increased availability of festoon steamers, longer times (5–20 min) at temperatures in the range 160–180 0C have been preferred. The presence of urea improves colour yields, but also increases the fixation of thickener and causes undesirable build-up of deposits in the steaming equipment. Urea can be substantially replaced by liquid ‘fixation accelerators’, typically nonionic surface-active agents of high boiling point and low water solubility, in which disperse dyes are soluble at high temperatures.

To understand the mechanism of fixation, it is important to recognise that, for polyester/cellulosic blends, there are three steps:
  • diffusion of dye through thickener film
  • diffusion across a vapour gap
  • diffusion into polyester fibre.
Comparison with the transfer print mechanism is clearly valid. The presence of steam probably has little effect on the passage across the gaps between fibres. It is in the diffusion through thickener films that the combined effect of condensed steam and liquid urea or other fixation accelerator will be important, both from within the thickener film to a surface where sublimation can occur and through the films surrounding polyester fibres. At a high temperature, such as 180 0C, this would be the slow step because the moisture content and thickener swelling would both be low. Lower temperatures and longer times allow the retention of more moisture and a better balance of diffusion rates in the three steps. Diffusion into the polyester fibre is faster in high-temperature steam than in dry air because of the increased molecular mobility.

Thermofixation:
The features of this method of dye fixation are mentioned below:

1. No steam is used.
2. Dye is fixed by subjecting the print to hot air at 2100C for 1 minute.
3. The fixation is carried out in a backing oven or in a stenter where heat setting can also be done simultaneously.
4. The process productivity is high.
5. The dye which have good sublimation fastness are subjected to this thermofixation process.
6. There is 10-15% loss of colour in thermofixation, so the shade becomes dull.
7. It is a continuous process of dye fixation which gives high production.

Reference:
  1. P Gregory, Chemistry and technology of printing and imaging systems
  2. T L Dawson, Color TECHNOLOGY
  3. J T MARSH
  4. R.S.PRAYAG
  5. TECHNOLOGY OF PRINTING BY V Ashenai 
Author of This Article:
Shubham Yadav
B.Sc. in Textile Engineering
Government Central Textile Institute,
Kanpur, Uttar Pradesh







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