Foam Technology in Textile Processing

Foam Technology in Textile Processing M Capponi, A Flister, R Hasler, C Oschatz, G Robert, T Robinson, H P Stakelbeck, P Tschudin and J P Vierlina Sandoz Ltd Dyes Division CH-4002 Basle Switzerland

INTRODUCTION Foam can be defined as a gas dispersed in a liquid phase of another substance and may be in a state of equilibrium or disequilibrium [l]. Many of the 'classic' and some of the more recent wet processes for textiles demonstrate the adverse effects of foam formation, as is clear from the following examples.

In partially flooded jet dyeing machines foam may give rise to bubbles in the pump assembly that causeafalling- off in the pump delivery rate and may even bring the liquor flow and the motion of the goods to a standstill. If this happens the liquor has to be cooled, and the rope of fabric drawn out of the machine by hand and disentangled, a procedure which often results in unlevel dyeing. Other factors that encourage foaming in jet machines are the high fabric speed and the rapid liquor flow.

In continuous pad dyeing processes, foaming of the liquor is highly detrimental to the quality of the dyeing. It stems from the action of the liquor recirculation pumps, or on multi-bowl machines from air entrapped in the interstices of the fabric that is set free as the fabric is squeezed in the nip.

In textile printing, foam formation is a well-known phenomenon, occurring, for example, in pastes during stirring at high speed, and is reflected in unlevel prints in roller and screen printing. In the rotary-screen process, foam formed during printing may build up in the screen, so it is usually advisable to include a foam inhibitor in these printing pastes.

At the same time, it will be recognized that foams have properties that can be usefully employed in wet processing.

The pioneering work was carried out by Lister', who in 1972 [2] replaced some of the water used in batchwise dyeing by air in the form of foam, thereby laying the foundation for low liquor ratio processes subsequently developed. From his initial experiments with pad dyeings and techniques employing organic solvents, he saw that the actual dye transfer process takes place in the small spatial region where the dye-liquor comes into direct contact with the surface of the substrate, and that the function of water is merely that of a heating and cooling medium and a transporting medium for the goods, plus a means of promoting migration to assist level dyeing. He therefore put forward a technique in which a small volume of water emulsified in an organic solvent would perform the actual work of dyeing. The next step was to replace the water-solvent emulsion by an air-water mixture, that is to say, by a microfoamt that would deposit uniformly on the substrate the minimum amount of aqueous dye-liquor necessary for dyeing. These ex-

' G H Lister, former R & D Dept, Sandoz Ltd, Horsforth, Leeds t A foam with an average bubble diameter of 0.05-0.1 mm is termed a microfoam. Normal foams have a bubble diameter of 0.05-0.50 mm.

periments were carried out in a drum dyeing machine with a surface active agent as foam former, and gave results which confirmed the correctness of the reasoning behind the technique. As the Sancowad process for dyeing with a minimum of water and controlled microfoam, it was later patented in numerous countries [3].

At the outset, the Sancowad short-liquor foam process was employed in the main for dyeing fully fash- ioned goods in drum machines [4]. It was later adapted to the conventional winch and used for the preparative wet processing of piece goods and for the application of fluorescent brighteners and finishing agents, as well as for dyeing [5]. Thus the Sancowad short-liquor technique with controlled microfoam formation was an important contribution to saving water and energy, as well as dyes and chemicals [6].

As already mentioned, in certain types of jet machine, foam obstructs the free movement of the goods. Yet experience with the Sancowad technique on the winch has proved conclusively that the presence of controlled microfoam facilitates the piling down of fabric dyed in rope form and has a beneficial effect on levelness of shade and the handle of the material. So, in due course, jet machines of new design were launched in which the run of the fabric was not impeded by foam; one example is the Molin6 model [7]. Other machines, such as the Gaston County Aqualuft II, are built with an integral foam generator [8 ] . It appears that the development of machines for piece dyeing using foams at short liquor ratios has not yet reached the definitive stage.

The purpose of this review of wet processing with the aid of foam is to show that the technique has a promising future, quite apart from its initial employment for piece dyeing.

FOAM AND ITS GENERATION

Definition Foam is an agglomeration of cells separated by thin liquid lamellae, is formed by gas dispersed in the liquid and congregates to form bubbles [9]. The foams used for textile dyeing and finishing are invariably generated from a dispersion of air in the aqueous dyeing or finishing liquor.

Foam Generation By definition, air has to be dispersed in a liquid, normally water, in order to generate foam. This is usually done by vigorous mechanical agitation, the air being either cap- tured from the atmosphere by the turbulence of the liquid or injected under pressure.

For the discontinuous preparation of small batches, mixers fitted with a beating mechanism are suitable, while for a continuous supply foam generators are employed. The principle of foam generation is practically the same in all the mixers that have been marketed to date. A delivery pump, generally one of the axial screw type, drives the liquid into a mixing head, in front of which compressed air is blown into the stream. The air delivery rate is adjusted so that a foam of a given weight per litre is generated. A rotor in the mixing head, set at

48 REV. PROG. COLORATION VOL. 12 1982

the requisite rotational speed, blends the air and the liquid under steadily increasing system pressure, which is built up by a counterpressure regulator or by the resistance of the foam in the outlet pipe. In the latter case the pressure is controlled by the length and cross- sectional area of the pipe. It follows that the stability of the foam depends only partially on the foam generating agent, and to a greater extent on the system pressure, the rotor speed, the volume of air supplied to the liquid (the weight per litre) and the pump capacity.

Continuous mixers deliver foam at a uniform rate, the output being set by the proportioning pump or machine speed. The fineness and the weight per litre are determined by the air-injection rate and the rotor speed. At constant rotor speed and constant blow ratio, fluctuations in the concentration of the foaming agent lead to only minor variations in the foam half-life or collapse rate. But if the rotor speed in the mixing head is changed, the system pressure increases automatically and more homogeneous mixing takes place giving a foam of higher stability. Egon has analysed in depth the complex of problems posed by this foam generating method [ lo] .

Pure liquids do not foam. When such a liquid is agitated the entrapped air bubbles rise to the surface and burst. If the liquid contains a surface active agent this is adsorbed on the surface of the bubbles to form a film. As the bubbles break through the liquid/air interface, a double film is formed - a foam lamella consisting of two monomolecular films of surfactant and the interlamellar liquid [l 1 1. The accumulation and reciprocal contact of the bubbles in the liquid also plays a part in this film- formation process.

Foam Stability Thermodynamically, foams are unstable systems as they have a higher surface-energy potential than the liquid and gas from which they are formed.

The main phenomena leading to the collapse of foam are drainage of the interlamellar liquid, breakage of the liquid lamellae, and gas diffusion through the lamellae [12]. Thus two types of foam can be differentiated.

Unstable Foam As the interlamellar liquid drains off, the lamellae grow thinner and burst causing progressive collapse of the foam. The collapse can be retarded by selecting the type and amount of surfactant that will give a more persistent adsorption film, and also by the viscosity of the liquid.

Metastable Foam After a certain time the liquid ceases to drain and a metastable structure comes into being. This state is achieved by retarding the thinning of the lamellae so that bursting of the bubbles is delayed.

The major factors in foam stabilization are:

Surface Viscosity As Plateau [13] recognized, it is generally assumed that increase in the viscosity of the liquid and the surface viscosity retard drainage of the liquid and improve the stability of the lamellae.

Marangoni-Gibbs Effects Marangoni [14] was the first to assume that a mechanically disturbed foam lamella forms a new surface with higher surface tension, which pulls the edges of the disrupted area together. Gibbs [15] advanced the con- cept of the elastic film in support of this theory. Ewers and Sutherland [16] explained this restorative phenom-

enon as being due to transfer of surfactant from the undisturbed surface to the thin areas with higher surface tension. This movement draws with it a certain amount of the liquid beneath, thus balancing out the thickness of the lamellae. The stability of foam is affected by the electric charge on the interfaces, evaporation of the liquid, crystallization of chemicals on the interfaces, and by external action such as vibration, pressure and mechanical shock.

The simplest method of destroying foam is to displace the foam former from the lamella surface by a substance incapable of forming a stable film. According to Ross, the defoaming agent must be able to form gaseous films and must spread a solution of the foaming agent on the surface [17].

Ewers observed that, as the defoaming agent is adsorbed on the lamella, there is a local increase in surface tension. The defoamer spreads out, carrying with it a thin film of subjacent liquid and thereby thinning the lamella until finally it bursts [16].

Another method of foam destruction proposed by Ross and Manegold [18] is to accelerate the drainage rate of the interlamellar liquid.

Generally, a defoaming agent should (a) be sparingly soluble in water, so that it is already present at low concentration on the lamella surface, (b) have a positive spreading coefficient relative to the surfactant solution and (c) should form quasi-gaseous interfacial films [19].

- Fats, oils and waxes - Aliphatic acids and esters

- Free amines and amides - Phosphoric acid alkyl esters, especially tributyl phos-

phate - Alkyl polysiloxanes.

A large number of the commercially available products are emulsions of an insoluble active substance. Many products contain an organic solvent to bring about rapid spreading of the defoaming agent itself.

Mechanical foam destruction plays an important part in continuous dyeing, where it is accomplished mainly by the application of a vacuum [20, 211.

Effective antifoaming agents are:

- Alcohols

Structure and Properties of Foam According to Manegold [20], two types of foam can be differentiated: spherical foam and polyhedron foam.

Spherical foam is a concentrated assembly or accumulation of discrete bubbles in a liquid. It may consist of 'single bubbles' in film or lamellar form and may assume, through external action, a non-spherical or polyhedron- like shape, which is termed 'honeycomb foam'.

Polyhedron foam, by contrast, is an aggregation of polyhedron-shaped bubbles which have lost autono- mous existence. Polyhedron foam may be formed through drainage of the interlamellar liquid from a spher- oid foam.

The principal properties of foam are as follows.

Blow Ratio This is defined as the ratio of the weight of a given volume of liquid prior to foaming to the weight of the same volume of foam. Frequently the density of the foam is reported along with the blow ratio.

Half-life Period of Decay The most sidely used and technically simplest method of determining the stability is measurement of the half-life

REV. PROG. COLORATION VOL. 12 1982 49

of the foam. The time recorded is that required for one half of the liquid in the initial foam to separate from the foam by self -drai nage.

Wetting Action Foam does not wet out textiles at the same speed as the starting liquid. It has to spread out on the substrate first, after which it breaks, releasing the liquid to penetrate the fibre. A very stable foam may remain on the substrate for some length of time. The wetting action can be evaluated by measuring the spreading and drainage times. Drainage of the foam on the textile can be characterized by measuring the growth in the wetted surface area [22].

viscosity The viscosity of foam can be determined with a rotary viscometer. The behaviour is similar to that of non- Newtonian liquids. The effect on the viscosity of various additives has been fully described in the literature [23].

Bubble Size The bubble size and size distribution are determined by the method of foam generation, the composition of the liquid, and the drainage behaviour. Fine foams are more stable than coarse.

APPLICATIONS OF FOAM IN TEXTILE PROCESSING

Applications in Dyeing Foam is formed wherever surface-active agents are used for the wet processing of textiles. It may be highly desirable or just useful, or it may cause serious interfer- ence. Owing to the operational diversity of wet-processing machines, on some occasions it is necessary to use foam-inhibiting synergistic mixtures of surfactants (on enclosed dyeing machines with high liquor circulation rates), while on other occasions controllable foam- forming surfactants are required (on open equipment (winches) with little or no liquor circulation).

Foam performs a number of important functions, both before and during the actual dyeing process: - Rapid wetting-out of the substrate - Protection of the fibre structure against mechanical

- Uniform distribution of dye-liquor in the substrate. The selected surfactant should not have any detrimen-

tal effect upon the dye-liquor or enter into any reaction with it. Its prime function is to minimize potentially damaging action and to improve the handle and appearance of the goods.

Batch Dyeing To ensure good handle and appearance, batchwise exhaust dyeing was for many years confined to jigs and winches. The great changes that have been made in fibre types and the demand for higher productivity, coupled with economic and environmental factors, have led to the adoption of dyeing methods at shorter liquor ratios in high-temperature beam, jet and soft-stream machines. The availability of the Sancowad technique with controlled foam made it possible for dyeing at short liquor ratio to be extended to conventional winches [24].

Since an evaluation of the effectiveness of any treatment has to take into account the handle and appearance of the fabric as well as the quality of the dyeing, the mode of transport of the goods on the winch had to be given close attention.

Foam, traditionally seen as an unwelcome phenomenon, enabled piece goods to be dyed using short liquor

action

ratios while maintaining good handle and appearance. Dyeing itself takes place in the foam or in the liquor with a controlled amount of foam. During dyeing, foam is constantly formed and drained as the bubbles break and donate dye-liquor to the substrate. It is this continuous foaming and draining that makes possible the actual dyeing and dye fixation processes.

Continuous Dyeing Continuous and semi-continuous pad-fixation processes remain unrivalled for productivity and cost effectiveness. Given the correct liquor-feed rate, they run reliably and give reproducible colours at low water- and energy-consumpiton levels.

An analysis of the dyeing costs for continuous processes, taking the pad-batch process at 30m/min as having a base value 1, shows that the energy costs are greater by these factors:

3.7 for the pad-dry process 4.1 for the pad-dry-steam process 4.1 5 for the pad-chemical pad-steam process 2.4 for the pad-steam process. Clearly pad-batch dyeing is unequalled for energy

economy. If the energy input for the other processes is to be cut down, the only way of doing so is to pad with a foamed liquor. This enables the pick-up to be reduced from the otherwise excessively high levels to the base value or below [25].

When using normal pad liquors, fluctuations in pick- up remain a possibility even when the liquor feed is accurately metered. As padding is a technique involving application of excess liquid, any variations in the fabric weight, liquor affinity and, in particular, in dye migration may easily lead to unlevel dyeing. Excess pick-up of liquor is most likely to occur at low percentage expres- sion (see Figure 1). The large amount of liquor on the fabric may lead to unduly high expenditure on energy for drying, or may induce unwanted migration during drying and fixation.

By padding at minimum pick-up (see Figure 1). un- controllable liquor movement on the fabric is ruled out, which reduces the risk of undesirable effects in drying or fixation. This is practicable with foamed liquor, without the aid of electronic measuring instruments. Very small amounts of dye can be applied at pick-up values down to well below 50% (deficit application, see Figure 1 ). There is, however, a lower limit value which is given by the amount of dye necessary for penetration of the fabric [22]. Level dye distribution is more difficult with foamed liquor than with conventional padding liquors because virtually no pressure is applied to the fabric. In industry

Excess pick-up More than 50%

Figure 1 - Wetting-out of fabric at different liquor pick- ups


several different approaches have been taken to over- come this difficulty [26].

As in all continuous processes, it is essential that the liquor - in this case the foamed liquor - should be fully homogeneous and evenly distributed on the goods. This prerequisite can only be met by employing a foam- generating unit in conjunction with a foam-forming surfactant, so that the foam supplied throughout the production run is constant in bubble size, homogeneity and fineness, and in the weight per litre and half-life period. To generate foams with reproducible values, a continuous automatic mixer is indispensable. Connected to a flow meter, such a mixer delivers homogeneous foams of constant weight per litre.

Given this facility, the job of the application machine is to distribute the foamed liquor evenly across and along the material. It may be applied to one side or both sides of the fabric. The choice between the two methods depends primarily on the drainage rate of the foam and/or on the wettability of the fabric. Other variables that call for attention are the fibre content of the fabric, which may be a blend or of one fibre only, the possible presence of fibres in the face and back that are dyeable with different dye classes, and, with heavy qualities, the fabric weight per unit area. The foamed liquor can be applied to one side of the goods with a doctor blade or a doctor-roll coater, or by pressure or suction; for two- sided application a horizontal pad or a pair of transfer rolls is suitable (see Figure 2).

For example, one-sided application is mostly suitable for bulky structures such as carpeting and imitation furs. These materials require relatively large amounts (80-130%) of foam to be applied, which can only be satisfactorily carried out with a doctor blade or doctor- roll coater.

Whether application is to be to one or to both sides of pile fabrics, flat fabrics and knits will depend on the drainage of the foam. If the foam is applied without mechanically assisted drainage on articles weighing over 100 g/mZ, two-sided application is preferable. If however, drainage is mechanically assisted, then even articles of over 100 g/m2 can receive the foam application on one side only.

One-sided

&&; :$:? Two-sided L +$;?q(

(fyy , L .< l,

Doctor coater

. +,& Roller coater

Twin foam transfer rolls Flow coater

1 Two-bowl pad

Figure 2 - Methods of application for foamed media

The foam has to break and the liquid drain before the dye-liquor can diffuse into the substrate. If the foam did not collapse speedily to permit fixation the process would not be practicable. The methods of foam drainage are as diverse as the application methods themselves. In choosing a method, the guiding factors are the foam application method and the substrate. The first require- ment for all drainage methods, including self drainage, is that the fabric should be supplied with dye and dyeing chemicals evenly over its whole area (see Figure 3). In addition to self drainage, pressure, vacuum and squeeze rolls are employed. It is of advantage to destroy all the foam on the surface to prevent any accumulation of residual foam in later processing. After the foam has drained the fabric should be fully penetrated with liquor, but without completely preventing foam formation during fixation.

On bulky goods, such as plush and carpets, complete drainage on the surface only should be the aim. Because of the condensation which forms in the steamer, a residual foam cushion is important to promote root-to- tip levelness of colour in the pile.

After the foam has drained, a predominantly liquid dyeing medium remains, which differs from normal by being of smaller volume, so fixation proceeds in much the same way as in conventional continuous processes. As there is less liquid, drying and fixation times can be shorter, which means higher output rates. The general pattern of the process, apart from the foamed-liquor application, is not radically different from normal.

In order to apply foam technology to industrial production, knowledge of the intrinsic properties of foaming agents and foaming agent combinations, and of their behaviour in the selected foam generator, is essential. In the early days, formulations were worked out in the laboratory using kitchen mixers, which led to incorrect deductions and recommendations that were not trans-

One-sided

1 Self-drainage

Pressure drainage

m -

Pressure drainage

Vacuum drainage

Figure 3 - Methods of foam drainage


ferable to full-scale production. Today foams of opti- mum fineness are demanded for the application of dye- liquors, though no very high stability is specified. Stabil- izing additions such as protein decomposition products or thickening agents should be avoided, as they add to the pollution level in the effluent. The more stable the foam, the more slowly it drains and the longer it takes to wet out the substrate.

Since every dyeing process, foamed-liquor application included, requires chemical auxiliaries, the levelling, dispersing and fixing agents, alkali, acid, salt, urea, etc. in the liquor must be regarded as additives. If there are only slight differences in the ionic character of these additions, these have only minimal effects on the foam consistency (see Figure 4). Strongly cationic and anionic products render the foam unstable, as do the majority of fixation accelerants. Another point to be noted is that salts increase foam stability owing to their high solid content. Variations in pH bstween 3 and 10 have no adverse effect on foam stability provided suitable foaming agents are used.

Given the very wide range of dyes applicable by the technique, variations in the time of foam collapse or in the drainage rate are likely, but by regulating the rotor speed, as mentioned above, thedesired stability can be achieved. Representative disperse, reactive and substantive dyes give a marginal increase in foam stability on account of their particle size and the standardizing materials they contain, whereas acid, metal-complex and basic dyes have no significant effect on stability (see Figure 5). Special account has to be taken of the solubility of the chosen dyes, because in foamed liquors the dye concentration is at least twice that in conventional liquors.

Summing up, it can be said that when foaming agents that work efficiently in conjunction with the particular class of dye are used, and the operation of the foam- generating unit is properly controlled, any fluctuations in the applied amounts of dye and dyeing assistants will be levelled out during the process. This means that proven continuous dyeing recipes can be employed as a basis for a foamed liquor process, provided the correct foaming agent and the correct method of foam application are chosen.

Despite the relative ease with which continuous dyeing recipes can be adapted to foam technology, serious consideration should be given to both the advantages and problems of this technique before adopting it.

Advantages of foam application include: - Further reductions in energy consumption - High running speed - One-sided application possible

- Less pile deformation - Savings in chemicals - Lower dye migration - Less streaking. Problems occurring with foam application include: - Inadequate foam stability and reproducibility - Incomplete wetting when too little foam is applied to

the fibres - Fluctuations in wetting due to differences in moisture

content of the fabric - Fluctuations in wetting due to uneven foam drainage - Lower liquor stability due to higher concentrations of

dye in the liquor. High dye-liquor concentrations become especially ev-

ident when processing carpets (Table 1 ). However, such reductions in liquor pick-up cannot be achieved in other continuous processes, even though the concentration of dye in the liquor may be doubled.

Fabric Appearance Whether a conventional dyeing process or foamed- liquor application should be employed is largely a ques- tion of cost effectiveness and quality standards, plus the relative importance of the energy- and water-consumption levels. The suitability of a fabric for foam-assisted dyeing depends partly on whether equal or better levelness compared with normal processes will be produced, and partly on whether the final quality will be better. Since the liquor volume for padding processes is already very small in relation to that required for batch dyeing, it would seem that the possible improvement in quality is the most important consideration, even though the savings are significant at a time of energy shortages.

If, for example, the structure of a pile fabric is damaged in conventional padding processes, the obvious course is to consider foamed-liquor application to see whether it affords better protection. Similarly, fabrics that are prone to felting, or are made of fibres whose glass- transition characteristics could introduce the risk of pile deformation, should be included in any survey of the potential uses of foam technology.

With certain fabrics of low volume per unit weight a sufficiently high pick-up cannot be achieved because of their low absorptive capacity. They are not easy to dye successfully by conventional padding processes, but the foamed-liquor technique might well provide an answer to the problem.

Not only the energy savings but also, and more importantly, the enhancement of quality has been re- sponsible for the notable success of foamed-liquor dyeing with three very different classes of textiles:

10

2 6 10 Conranllallo" II ,

Figure 4 - Foam stability, (a) effect of foaming agent concentration (blow ratio = 1 : 1 1 , rotor speed = 140 rev./min, system pressure = 2.7 bar) (6) effect of rotor speed in the mixer (blow ratio = 1: l l )

11

t

5

: B

r !

Figure 5 - Effect of dyes on foam stability, (a) anionic and disperse dyes (blow ratio = 1: l l ) (b) cationic dyes (blow ratio = 1: l l )


TABLE 1

Component Concentration in Pad Liquor (g/l)

Conventional Process Foam Application (500% Liquor Pick-up)

Dye (e.g. for a 5 25 dark brown) Assistants 4 8 Thickener 3

(100% Liquor Pick-up)

- Nylon cut-pile carpeting - Acrylic fibre/cotton imitation furs - Acrylic fibre/cotton upholstery fabrics.

In the production of solid colours on nylon cut-pile carpets, unlevelness due to variations in liquor distribution is less pronounced due to the smaller volume of liquid applied. This effect has been noted with acid levelling and metal-complex dyes. The fact that less liquid is supplied and migration thereby restricted means of course that the coloristic effects obtainable on carpets containing differential-dyeing yarn is more limited than otherwise would be the case.

Imitation furs of acrylic fibre/cotton are more suscepti- ble to pile deformation during padding, which can be such as to make them unsaleable. When dyed using the minimum pick-up of foamed liquor they escape deformation as they retain very little liquid to be heated during fixation.

On acrylic fibre/cotton upholstery fabrics, improve- ments in the fastness properties, particularly rubbing fastness, are attainable when the cationic and anionic dyes are foamed and applied separately to the acrylic fibre pile and the cotton back respectively. This procedure results in much better fixation, which is not impeded by complexing of the two dye types (Table 2).

costs The low costs of continuous and semi-continuous processes in relation to batch dyeing have been illustrated in the literature [27]. It will be readily seen that any further

TABLE 2

Improvement in Rubbing Fastness on Acrylic Fi- bre/Cotton Upholstery Fabric Dyed from Foamed Liquor

Process Rubbing Fastness

Dry Wet

Conventional continuous 4-5 3 process Foamed liquors applied 5 4-5 separately to the pile and the back

reduction in the liquor ratio would add very little to the savings in energy consumption. The only exceptions are processing systems where a high pick-up is used, as are found in carpet dyeing. Even so, the possible further reduction in liquor volume should not be discounted, for in certain processes it allows the salt and chemical additions to be reduced.

The rule of thumb is that the amount of foamed liquor applied may be 50% less than that used in conventional continuous processes. Nylon carpeting is an exception, where a 100% reduction (to 200% pick-up) is feasible. This brings down the overall production costs by 20% (Table 3).

TABLE 3

Possible Cost Savings Using Foamed Liquors Relative to Continuous Processes

Substrate Energy Effluent Dyeing Dyeing Dyes Treatment Assistants Chemicals

iqylon carpeting *** ** * * Nylon woven and ** knitted fabrics Acrylic woven and ** * * knitted fabrics Polyester woven and * * knitted fabrics Acrylic/cotton ** *** *** upholstery fabrics Polyester/cotton * ** ** ** (reactive dyes) Cotton * ** *** (reactive dyes)

L ~~

Key: ZZL = Substantial cost saving

+* = Moderate cost saving *=Minor cost saving


Outlook for Foamed-liquor Dyeing A major consideration influencing the choice of a continuous dyeing process is the specifications laid down by the textile buyers for levelness and quality of finish. The same holds good for the foam technique, where full coverage and penetration of the substrate by the dye- liquor is an essential condition for achieving the required levelness. The technique is still fairly new and it demands considerable skill and forethought on the part of the textile dyers and finishers and the manufacturers of the machinery, dyes and dyeing chemicals. The first successful trials were carried out on carpets and upholstery fabrics, and gave good grounds for believing that the technique could be employed for other textiles. Al- though the cost savings are fairly modest, it is certain that application of the technique to other classes of goods will be investigated with a view to achieving an improvement in quality, whether in levelness, fabric handle or general appearance. The use of foam should play an important part in this development.

Applications in Textile Print ing

Patents and Technical Literature The use of foam has always aroused mixed feelings in printers; poor colour coverage, unlevel prints and length- wise streaks are among the adverse effects attributed to it. So it is understandable that a technical innovation involving the deliberate use of foam in the printing system should be met with reserve and scepticism. Nevertheless, the idea of replacing part of the printing paste by air has a certain fascination for printers.

Current development projects are all aimed at increasing the cost effectiveness of printing processes. The technical consequences of working with foamed pastes are interpreted differently and with varying emphasis by different writers on the subject.

As early as 1957, Kumins [28] described a foam- assisted printing process for use with several different dye classes. Higher yields and greater economy were achieved with most of the dyes than when conventional systems were employed, for example, oil-in-water emulsions. A specially designed, enclosed foam applicator was described that prevents changes in the foam density during the printing process.

In 1973 Fabbri and Frauenknecht [29] developed a dyeing and printing process using liquors or pastes set with a foaming agent. The foam is generated on the substrate by applying pressure, for example by the massage action of surface-profiled rollers during or after application of the foamed medium. The merits of the process, which was specially developed for carpets and bulky fabrics, are the low liquid content of the medium, superior penetration of prints owing to the low viscosity of the pastes, and the absence of frosting. The smaller amount of thickener in the printing pastes makes washing-off, ordinarily a cost-intensive treatment, very much easier.

In a patent application of Textilana, Liberec [30], foamed printing pastes are described. They can be foamed by various means, e.g. gas injection with stirring or the expansion of gas occluded in the paste or formed in it by chemical reaction of two substances. Subsequent patent specifications in the names of Toyo Spinning KK [31], Bayer AG [32], Tennants Textile Colours [33] and the lndustrie des Peintures Associbes [34] disclosed further methods of preparation and application for foamed printing pastes.

Printing methods using foamed pastes have also been reported in the technical journals. A paper by Kiselev et

al. [35] describes successful rotary-screen printing trials on acetate fabric using foamed disperse dye pastes. Other authors [36] describe stable foamed systems that have been developed, principally for pigment printing. Guth [ lo ] holds that the selective use of foamed printing pastes in direct printing offers promise of higher-quality prints on all known substrates. A serious obstacle to the setting-up of production trials in the printworks is the fact that a separate foam generator is necessary for each colour.

It is evident from these patents and papers that work is continuing in various quarters to utilize foam profitably in textile printing. Further pioneering work will undoubt- edly be necessary to bring about wider acceptance of foamed systems in printing.

Applications of Foam Technology in Textile Printing Four possible applications will be considered, with due account being taken of the prerequisites for success with foamed printing pastes.

PIGMENT PRINT1 NG In pigment printing pastes, foam can take over the function of the thickener or the benzine emulsion. The binder takes care of film formation and prevents dusting or marking-off of the colours after the print has been dried. A certain amount of a natural or synthetic thickening agent is necessary for satisfactory foam stability.

PRINTING WITH SOLUBLE DYES Fairly large amounts of water are required to dissolve the dyes, so there is very little scope for foamed application. Moreover, the pastes have to be set with a conventional thickener or other film former to ensure good colour adhesion of the dye to the fabric after drying.

PRINTING WITH DISPERSE DYES With disperse dyes, conventional thickeners or film for- mers are indispensable. Adverse effects on foam formation by the dispersing agent in these dyes cannot be ruled out. Since the members of the disperse class differ in chemical and physical constitution and are printed at different concentrations, these variables too have to be taken into account in foamed recipe formulation.

CARPET PRINTING WITH SOLUBLE DYES The volume of water necessary for carpet printing is substantially less than for fabric prints, because the dyes are applied at relatively low concentrations. Film formation is unnecessary as there is no intermediate drying step before fixation.

These considerations suggest that in addition to pigment printing, for which promising foamed recipes have already been formulated, carpet printing offers favoura- ble preconditions for the use of foamed systems. In addition, together with big savings in thickening agent and energy, there are important technical merits: more rapid heating-up and easier washing-off giving higher output rates, higher fixation values and fastness properties. Special applicators have been designed and launched on the market that provide for thorough penetration of the carpet structure by the foamed paste WI.

For some time to come the majority of printers will probably continue to look upon foam as an unwelcome factor in printing systems that must be suppressed. But once the advantages and the limitations of foamed printing colours are recognized there is good reason to believe that further progress in the development of the technique will lead to successful applications, particularly for the bulkier qualities of fabric.


Applications of Foam Technology in Finishing

Discontinuous Processes The Sancowad short-liquor technique with controlled foam formation, outlined above, is highly suitable for the application of fluorescent brighteners, softeners and other finishing agents.

Continuous Processes These processes differ from the Sancowad technique in that the foam is generated outside the application machine. Continuous finishing using foamed liquors has hitherto been practiced mainly for applying resins. Its use in dyeing and printing, as previously mentioned, is still largely at the trial stage. To date, the following application systems for foamed finishing liquors have been adopted by industry or are being tested under industrial conditions. - United Merchants & Manufacturers; Valchem: foamed

liquor applied with a doctor coater, foam destruction effected with squeeze rolls or by suctioning with a vacuum slit nozzle.

- Gaston County; Union Carbide: foamed liquor applied through pressure nozzles.

- Kusters: machine for applying a minimum amount of foamed liquor from transfer tolls to both sides of the fabric.

- Stork: foamed liquor application on a rotary-screen coater. A comparative evaluation of the performance of these

systems to ascertain their versatility and the quality of the finishes obtained has not yet been possible, as the trial period has been different for each one. It can be expected that further machines will be launched. The factor that will determine their acceptance will be their ability to meet the higher specifications for levelness of colour and penetration in the dyeing of piece goods.

The applications suitable for foamed finishing liquors are as follows: - Resin finishing - Finishes for dimensional stability and crease resistance - Flame-retardant finishes - Softening - Antistatic finishing - Finishes for improvement of handle - Fluorescent brightening - Soil-release and water-absorbent finishes.

Difficulty is likely to be experienced in water-repellent finishing, as the currently known foams reduce repel- lency. Surface-active agents that break down progres- sively after the foam has been applied will have to be used, so as not to impair this property. The advantages can be summarized in the following terms: - Lower energy costs - Savings in finishing agents (around 10-20% for res-

- Higher drying speeds - Less migration of resins - Suitability for wet-on-wet application - Absence of deformation in structured fabrics such as

The problems are: - Poor stability and reproducibility of the foam - Inadequate penetration at low pick-up values (below

25%) - Satisfactory control of foam application to ensure

levelness of treatment. In formulating foamed liquors it has to be borne in

mind that the concentration of each ingredient must be higher than in normal liquors to compensate for the

ins)

corduroys and velvets.

smaller volume of available liquid. This may give rise to stability problems, with foamed liquors based on estab- lished recipes showing widely different behaviour from that displayed in conventional application. The following example will aid understanding of this situation.

Recipe for 80% pick-up

Resin finishing (gA) Synthetic resin (50% active substance) 1 00 Softener 10 Agent for improvement of handle 20 Catalyst (e.g. magnesium chloride, 15% of resin) 15 Wetting agent 1 Water 854

1 000 -

In foamed liquor application at 20% pick-up the above recipe has to be strengthened by a factor of four: Synthetic resin (50% active substance) 400 Softener 40 Agent for improvement of handle 80 Catalyst 60 Wetting agent 4 Foaming agent, depending on requirements 16 Water 400

1000 The problem is to dissolve 600 g of material in 400 g of

water to form a stable solution that can be converted into foam of the desired stability. The more highly concentrated the starting solution, the more problematic is the stability. Dye solutions can more easily be converted to foam than finishing liquors with a high content of active substances. It will be seen from the preceding example that the products applied from a foamed solution must be highly soluble and must remain in solution, even if present at high concentrations. Under these conditions many products settle out or fail to dissolve completely.

Trials have demonstrated that with colourless foams, e.g. liquors for resin finishes, softening or improvement of fabric handle, acceptable levelness is obtained even if the amount of liquor deposited on the fabricvaries by asmuch as 20-30% (measured from the differences in moisture absorption). Coloured foams are much more sensitive to such fluctuations; a difference of 5% can be enough to spoil the appearance of the goods. In liquors containing fluorescent brighteners the sensitivity is less than with dye solutions; variations of up to 10% or so are tolerable.

The effect of 20-30% variations in the pick-up of a foamed-resin finishing liquor was examined by testing the following properties: - Dry- and wet-creasing angles (warp and weft) at

several points on the right- and left-hand selvedges - Abrasion resistance of the face and back of the fabric - Tensile strength at various points - Levelness of the finish (colour reaction with Neocar-

mine MS and Tollens reagent) - Penetration of resin (determination of the abrasion

resistance of the face and back of the fabric). The results showed that the effect of varying the pick-

up was not significant; the results of tests on conven- tionally finished fabric were virtually the same. This invalidates some of the possible objections to the use of foamed liquors in finishing.

SUMMARY AND OUTLOOK The examples that have been presented here are evi- dence that foam-based processing has highly interesting


features and advantages compared with conventional wet-processing systems. On the other hand, the relatively short period of experience with this technology has already brought to light certain criteria that set limits to its applicability. What is urgently needed is a compre- hensive evaluation of foamed media v i s - h i s conventional and other new wet-processing techniques.

On considering batch dyeing from foamed liquor, it will be noted that the main rewards are the savings in chemicals, water and energy. However, short-liquor dyeing machines generally offer comparable advantages. The feature that has so far been absent in most of these machines is foam, which has been seen as a hindrance to smooth running. But now there is proof that the use of specific foaming agents can result in higher quality standards, i.e. improved fabric handle and levelness of shade. The short-liquor machines of the next generation will in our view represent a real breakthrough in textile- machinery design [38].

At this point a few words on dyeing from solvent media are relevant. This technique, which was explored in the 1970s by several dye manufacturers, offers no advantages over the short-liquor or foamed-liquor dyeing processes, except with regard to the cost of drying, as the commonly used solvents (chlorinated hydrocar- bons) require far less heat for evaporation. Also, polyester substrates showed greatly improved dyeability owing to swelling of the fibre by the chlorinated hydrocarbon solvent. But serious drawbacks were apparent too, i.e. the substantial capital investment for a closed-circuit dyeing machine with attached solvent-recovery unit, and the fairly high cost of eliminating the solvent. In our estimation, this technique is not likely to return to favour for batch dyeing. It has, however, found a special use for scouring synthetic-fibre knitted goods in tumblers and continuous machines to remove the winding oil, which is present in amounts of up to 10%. It has also been taken up for washing-off polyester prints produced using synthetic thickeners. Exceptionally, the solvent technique is used to apply speciality finishing agents, which are sparingly soluble or difficult to disperse in water, but are readily soluble in solvents.

Finishing processes normally include a drying stage, a high-cost element, largely because of the amount of liquid that has to be removed from the goods. This is why there is such keen interest in low-pick-up techniques. Several techniques have been developed; examples are the Triatex MA process, electrostatic spraying, the Monforts Matex VAC, rotary screens, the Es- cher-Wyss Nipco roller and the Kusters floating roller. In this field the foamed-liquor technique is making an important contribution to progress; in the United States it has already met with wide acceptance [39]. As a mini- pick-up technique, foamed-liquor application can be expected to make further headway. On the product side, a wide range of foaming and finishing agents has already appeared on the market.

In the continuous dyeing sector, foam application now plays an important part. In carpet dyeing, where the continuous padding of dye-liquor has been almost completely superseded by pressureless flow coating, the advantages of the foamed-liquor technique, conferred by the low quantities of liquid used, are lower energy consumption in heating up the carpet in the steamer and better fixation, which in turn brings down the pollutant level in the effluent. With the more recent machines for the one-sided application of foams, the technique will come into use for dyeing acrylic fibre/cellulosic fibre pile fabrics as well, with basic dyes applied to the acrylic pile and direct dyes to the cellulosic fibre back. In this way

higher yields will be reached with both classes, and there will be no stability problems such as occur with liquors containing both basic and direct dyes. Foamed liquors also appear to have interested technical potential for corduroy fabrics.

In printing, the great merit of foam technology in our view is the economy in energy consumption for drying. It opens up attractive prospects in the printing of carpets and bulky fabrics, which can be printed with pastes containing greater amounts of dye in the same amount of liquid used in conventional processes.

While the new dyeing and printing methods using foamed liquors and pastes have scored some promising initial successes, a number of problems have yet to be solved. In our view, the answers are most likely to come in the shape of new machines, for there are many possible design variations for foam-generating and application systems. This range of variation is, we think, due to the fact that foam systems cannot be so easily defined as liquids, and to the diverse parameters such as blow ratio, average bubble diameter, foam stability (the half-life period), and the effects of the process temperature, chemicals and dyes, all of which have to be given due consideration. Mention of these also points to the lines of research and development that will have to be pursued to give foam systems tailored to the intended application, whether it be dyeing, printing or finishing. As is always the case with new technologies, we think that the foam technique has brought about progress not only in the fields reviewed here, but also in marginal areas; many conventional technologies may benefit from its development. Synergistic effects have to be taken into account here as elsewhere. Meanwhile we can be confi- dent that the use of foams will in time gain an assured place in many sectors of textile processing.

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Foam Technology in Textile Processing