Foam Technology in Textile Processing

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<ul><li><p>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 </p><p>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. </p><p>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. </p><p>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. </p><p>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. </p><p>At the same time, it will be recognized that foams have properties that can be usefully employed in wet process- ing. </p><p>The pioneering work was carried out by Lister', who in 1972 [2] replaced some of the water used in batch- wise 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- </p><p>' G H Lister, former R &amp; 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. </p><p>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 micro- foam, it was later patented in numerous countries [3]. </p><p>At the outset, the Sancowad short-liquor foam pro- cess 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 tech- nique with controlled microfoam formation was an im- portant contribution to saving water and energy, as well as dyes and chemicals [6]. </p><p>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. </p><p>The purpose of this review of wet processing with the aid of foam is to show that the technique has a promis- ing future, quite apart from its initial employment for piece dyeing. </p><p>FOAM AND ITS GENERATION </p><p>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. </p><p>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. </p><p>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 </p><p>48 REV. PROG. COLORATION VOL. 12 1982 </p></li><li><p>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. </p><p>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 deter- mined by the air-injection rate and the rotor speed. At constant rotor speed and constant blow ratio, fluctua- tions 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] . </p><p>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. </p><p>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. </p><p>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. </p><p>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. </p><p>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. </p><p>The major factors in foam stabilization are: </p><p>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. </p><p>Marangoni-Gibbs Effects Marangoni [14] was the first to assume that a mechani- cally 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- </p><p>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 me- chanical shock. </p><p>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]. </p><p>Ewers observed that, as the defoaming agent is ad- sorbed 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]. </p><p>Another method of foam destruction proposed by Ross and Manegold [18] is to accelerate the drainage rate of the interlamellar liquid. </p><p>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]. </p><p>- Fats, oils and waxes - Aliphatic acids and esters </p><p>- Free amines and amides - Phosphoric acid alkyl esters, especially tributyl phos- </p><p>phate - Alkyl polysiloxanes. </p><p>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. </p><p>Mechanical foam destruction plays an important part in continuous dyeing, where it is accomplished mainly by the application of a vacuum [20, 211. </p><p>Effective antifoaming agents are: </p><p>- Alcohols </p><p>Structure and Properties of Foam According to Manegold [20], two types of foam can be differentiated: spherical foam and polyhedron foam. </p><p>Spherical foam is a concentrated assembly or accumula- tion 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'. </p><p>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. </p><p>The principal properties of foam are as follows. </p><p>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. </p><p>Half-life Period of Decay The most sidely used and technically simplest method of determining the stability is measurement of the half-life </p><p>REV. PROG. COLORATION VOL. 12 1982 49 </p></li><li><p>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. </p><p>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]. </p><p>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]. </p><p>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. </p><p>APPLICATIONS OF FOAM IN TEXTILE PROCESSING </p><p>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-process- ing 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). </p><p>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 </p><p>- Uniform distribution of dye-liquor in the substrate. The selected surfactant should not have any detrimen- </p><p>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 appear- ance of the goods. </p><p>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 con- trolled foam made it possible for dyeing at short liquor...</p></li></ul>