Yes, We Have a Foaming Problem!
He has authored about 70 publications which include original research papers, review articles, book chapters book reviews and symposium proceedings. He also served as the reviewer of technical and scientific papers of eight international publications. In 1989, he edited a special issue of Colloids and Surfaces (Aspects of Interfaces) as a guest editor. His biography is featured in "Marquis Who's Who in Science and Engineering (1997). Currently, Dr. Kunjappu is working as a research chemist at Propper M.C. Inc., New York. He may be reached email@example.com and 212-942-4828.
Foam has been symbolized in literature for the sea and the transient nature of human beings. In real life, some times it is appreciated and at times it is annoying. Foam is a boon in many materials and processes: for example, it is considered to be beneficial in some food stuffs, beverages, detergents, cosmetic formulations, personal care products like shampoos and tooth pastes, fire fighting fluids, enrichment of minerals by froth-flotation, etc .
But, foam is detrimental in many industrial processes and products : in the oil industry, it hinders the oil drilling process; in the paper industry, it causes harmful effects to the finished product. Similarly, in the textile and pharmaceutical industries, the negative effects of foam are disdained. Finally, in inks, paints and coatings, foam is considered to be an intruder causing trouble at various stages of production, application and end use.
Foam control is a matter of prime concern in inks. It is a mind-boggling phenomenon for the perplexed novice and a matter of chronic concern to the established expert. Foam becomes instrumental in causing surface defects to the ink film which mar the aesthetic, communicative and protective functions manifested in the form of craters, pinholes, fish eyes, orange peel, etc.
Foam can be considered as a colloidal dispersion of a gas (disperse phase) dispersed in a liquid or solid (dispersion medium). The former case is referred to as foam or froth, and the latter case is referred to as solid foam. We generally come across with the first type that we call as foam. An example for the second type is the insulating packaging material (polystyrene, polyurethane, etc.)  used for hot takeaway food.
There are three important sources of foam in an ink dispersion. They are the adsorbed air displaced from pigments during the dispersion stage, the air introduced into the system during the mixing process, and the surfactants that function as foam stabilizing agents. The air displaced from organic pigments makes the sample excessively voluminous, rendering it difficult to process further. Surfactants are usually added into the system as dispersants, wetting agents, component of emulsions, etc. and to impart specific attributes such as color acceptance, compatibility, etc.
Thus, it may be seen that foam is inevitable due to the very nature of ink composite where one cannot avoid air and foam-stabilizing ingredients and conditions. But, this problem can be tackled by controlling the foam-causing factors through prevention/cure operations. This is achieved by incorporating antifoaming/defoaming agents at different stages of ink making.
Observation of foam generation and destruction during ink making is a source of entertainment to the ink chemist. If one shoots the ink making sequences as a movie, one can see the making and breaking of foam in different frames: some times, mere pH adjustment will destabilize the foam; yet another time, polymer-surfactant interactions will stabilize it . It is the purpose of this article to understand the foaming and defoaming processes so as to gain useful insights into combating the foam.
Having defined earlier the elements of foam from the point of view of a colloidal and interfacial chemist, let us develop a basic understanding of it . If we shake a surfactant solution in a cylinder and allow it to stand, a foam column will be visible above the liquid. The lower portion of the foam contains spherical bubbles with relatively low gas volume fraction. They are classically referred to as kugelschaum. The upper portion consists of foam with a relatively high gas volume fraction where the liquid drainage distorts the foam into a polyhedral form, classically referred to as polyederschaum. They are pictorially represented in Figure 1.
The gas cells in foams are made up of thin liquid film walls called lamellae that have approximately plane parallel sides. When three or more adjacent gas bubbles meet, these thin films intersect to form a region called plateau border as represented by (A) and (B) in Figure 2. Because of negative curvature, the pressure at plateau border will be less than that in the adjoining thin films.
This difference in pressure results in the drainage of the liquid from the lamellae into the plateau borders (plateau border suction). Thus the lamellae becomes thinner with time due to the drainage of the liquid from it and finally the film ruptures (foam collapse) when the film thickness is about 100 Angstroms. The pressure difference DP instrumental in drainage of liquid into the plateau border at point B from point A is given by the Laplace equation,
DP = g (1/RB + 1/RA)
where g is the surface tension, BA and RB are the radii of curvature of the lamellae at points A and B, respectively .
It is a common experience that the gas bubbles produced inside a pure liquid are transient, rise to the top and break spontaneously. The velocity of rise V is related to the radius of the bubble r, and viscosity of the medium h, by the Stokes' expression:
Formation of foam is accompanied by an increase in the interfacial area of the system and hence in its total free energy. Thus the foam is thermodynamically unstable since it would like to attain a state of low energy.
In fact, stabilizing conditions are essential for its longevity. One such stabilizing factor is provided by surfactants that adsorb at the liquid-gas interface of the foam.
There are several factors governing the stability of the foam films. For example, the surfactant adsorbed films stabilize under the influence of film elasticity. Film elasticity has its origin in the concentration dependent and time dependent changes in the surface tension of the surfactant, respectively known as Gibbs effect and Marangoni effect.
Since drainage of liquid from the lamellae is one of the causes for the thinning of the films, factors such as bulk viscosity and surface tension gradients alsp play a role in stabilizing the foam. Higher bulk viscosity stabilizes the form better. Thus, it is a common observation that it is difficult to break the form after the thickener is added into inks to get higher viscosities. Similarly, the diffusion of gas through the lamellae, surface film viscosity, electrical double layer repulsion of the adsorbed surfactant layers, and Ostwald Ripening (disproportionation, i.e. the loss of gas from a small bubble at high Laplace pressure to a larger bubble at lower pressure) are all important in specific instances in stabilizing the foam. Halling depicts the general routes leading to foam degradation as in Figure 3 .
Even though many chemical engineering departments are involved in exploring the intricacies of foams, there are many unanswered questions left behind. Lack of uniformity in defining the foam parameters has made it difficult to make ready comparison of experimental results.
The foam generating power of a liquid is expressed by the term foamability, which includes the factors that help to attain immediate stabilization of the foam . Techniques of generating foams are quite diverse. In several studies, foam is generated by supersaturating a liquid with a gas under pressure and then releasing the pressure, or by forcibly injecting a gas into a liquid .
Terms such as expansion ratio, dispersed-phase volume fraction, density, and foam quality are used to express the relation between the volume of liquid to be foamed and the final foam volume. Form Quality T, is related to foam volume Qf, volume of the gas to generate the foam Qgas, volume of the liquid contained within the foam Vliq, and time over which foam is generated t as 
Qf = Qgas + T dVliq/dt
Foams are some times distinguished as dynamic foam and static foam. In dynamic foam, the foam has reached a state of dynamic equilibrium between rates of formation and decay. Volume of foam at equilibrium will serve as an effective parameter to represent this. In static foam, the foam has attained a state in which the rate of foam formation has become zero. The rate of foam collapse is indicative of this.
It is possible to study the form collapse process by examining it under a powerful microscope. The dynamic events occurring in foams can be captured by video cameras as well as electron and X-ray microscopes. Even magnetic resonance imaging method has been applied to foams where dynamic events such as gravitational separation, disproportionation and coalescence are studied. Here, the spatial dependence of the NMR parameters responsible for signal intensity yields microstructural information .
There are many experimental tools to assess the foam stability. In the sparge tube technique , this information is obtained from the average life time of foam. In the pressure method , the rate of increase in pressure associated with collapse of the foam confined in a sealed vessel serves as an index of the foam stability. This is based on the Laplace equation described earlier and simplified as DP=2g/r, where DP is the pressure difference between the outside and inside of gas bubbles in the foam, r is the radius, and g is the surface tension. In the conductimetric method, conductivity change between electrodes containing the foam is measured which is useful in the continuous monitoring of the drainage rates ,
The deleterious effects of foam on a waterborne system such as inks can be eclipsed by incorporating into the ink system certain chemical additives. This can be realized either by destroying the foam already formed or by preventing the creation of foam. The former type of additives are called as defoaming agents and the latter type as antifoaming agents, although the literature is replete with terms such as foam inhibitors, bubble breakers, air releasing agents etc. It may be noted that foam destabilization may be achieved by mechanical and thermal means also; but the chemical method is more popular and practical due to the ease of operation.
An effective antifoaming agent should be capable of neutralizing the foam stabilizing factors. The antifoaming action operates in two levels: by intercepting the factors responsible for stabilizing the outer film of the foam and by weakening the factors involved in the internal structure stability of the foam.
Different classes of chemicals are used as effective antifoaming agents. The effect of mineral oils as antifoaming agent has been known for a long time .
Hydrophobic particles are also found to be effective in destroying foam. In principle, any insoluble liquid that has a lower surface tension than the foamed. surfactant-containing liquid can act as an antifoaming agent. Fatty acids. phosphate esters, capped polyethylene alcohols and the like come under this category. To be effective, these materials, insoluble in the foaming media. should form discrete droplets. Also. they should have low surface tension and spreading ability in order to enter the surfactant layer and to let hydrophobic particles interact with the surfactant layer.
Robinson and Woods , by investigating the surface tensions of the foaming medium and antifoam, as well as that of the interfacial tension between them, arrived at the concept of rupture or entering coefficient under the assumption that the antifoam acts by penetrating into the gas-liquid interface of the foam
E = gF + gDF - gD,
where E is the entering coefficient, gF is the surface tension of the foaming medium, gDF is the interfacial tension between the antifoam and foaming medium and gD the surface tension of the antifoam. These authors maintain the view that E should have a value of greater than zero (positive) to be effective as an antifoam.
Ross  pointed out the importance of Harkins  spreading coefficient, S, in relation to the above surface tension parameters and suggested that a positive value for S is the requirement for an efficient antifoam. This stems from the idea that the antifoam should spread over the foaming liquid for its efficacy. In this case the appropriate relation is given below: S = gF - gDF - gD.
In reality, all of these factors are considered to be important in deciding the effectiveness of an antifoam.
It was found that a combination of mineral oil and hydrophobic particles served more effectively as an antifoam. When the foam lamellae comes in contact with the hydrophobic particle surface, the film adopts a certain contact angle. If this contact angle is >900 for a spherical particle, the foam lamellae gets thinner due to the Laplace pressure at the interface, and finally the particle is dewetted and the foam lamellae ruptures.
The literature is rich with studies on the influence of hydrophobic particle geometry, size and shape on foam stability .
In another explanation, the oil-coated particles operate at the plateau borders of the draining foam instead of working in the film lamellae. According to Kulkarni et al. , the hydrophobic particles in an antifoam acts by adsorbing surfactant molecules from the bubble surface and carrying them into the aqueous phase. The antifoaming mechanism may be represented schematically as in Figure 4.
In some instances, the spreading ability of the mineral oil based antifoams seems to be insufficient. Liquids with lower surface tension are needed in these situations.
Polysiloxanes (Figure 5) are found to meet this criterion. They are found to be effective in many cases as they are very inert and act independent of system conditions.
The effectiveness of silicone antifoams could be improved by incorporating in them a small quantity of hydrophobic silica (-3-4 percent).
The resulting dispersion problem was overcome by using suitable emulsifiers to stabilize them. More recently, organically modified polysiloxanes containing polyethers are found to be exceptionally effective in many systems (Figure 6).
These materials help in controlling the compatibility with the system, a prime concern in circumventing coating defects.
Thus, antifoams contain ingredients like hydrocarbons, siloxanes, hydrophobic particles, etc. often in the dispersed state assisted by lowfoaming surfactants.
More specificaliy, they are hydrocarbon/fatty acid/ester blends, hydrophobic particle! hydrophobic oil mixtures, hydrophobic particle/silicones, etc.
Some problems associated with the heterogeneous nature of antifoams could be overcome by using single component additives. Polyoxyethylene-polyoxypropylene-derived antifoams introduced for this purpose have only a limited utility in coating industry, although this class has been serving as a powerful material in dishwashing and fermentation applications .
An immediate consequence of uncontrolled foam in the ink system is the development of serious surface defects in the dried film. If the bubbles burst well before the application, then the ink film will be intact. In case the bubbles persist and break at or near the point of film formation, either small circular defects known as 'pinholes' appear due to small bubbles or large irregular defects known as craters' appear due to collection of polyhedral bubbles.
If the bubbles persist in the dried film without breaking, it will affect the parameters related to reflectivity, such as gloss.
Antifoaming agents added to combat foam can cause unwanted side effects also. It is important to select an antifoam that does not affect any of the desired properties of the ink film required in the end application. Antifoams are known to produce wetting-out problems on the substrate. Sometimes, they cause loss of color due to flocculation of the pigment particles resulting from the redistribution of surface active agents from the solid surface. Often, gloss reduction is observed while using antifoams with a high concentration of hydrocarbon oil. Surface defects usually noted with silicone oil based antifoams are being eliminated using polysiolxanes modified with hydrophobic polyethers without sacrificing gloss. Presence of silicones may result in adhesion problems since they can act as release agents. Thus the judicious choice of antifoams is important to achieve the best results.
The incorporation of antifoams in the formula at the appropriate stage reduces many problems. While making pigment dispersions, it is advantageous to add a portion of the antifoam in the premix stage to regulate the adsorbed air displaced from the pigment. Fresh amounts of antifoam may be added during the grinding stage. Control of intensity and time of shear force will help disperse the oil droplets in the antifoam to get optimum performance .
Various procedures are used to assess the foamability of an antifoam. In its simplest form, foamability may be expressed as the height of foam column formed after mixing the sample-antifoam mixture for a definite time.
More accurate parameters of foamability can be derived from density measurements of the sample before and after mixing with the antifoam. This method is eminently suited for the screening of antifoams while developing dispersions . The best dosage of the antifoam is arrived at by trials and intelligent guesses based on the nature of the antifoam. Usually, a ladder study, choosing successively increasing amounts of the antifoam, is recommended.
The usual dosage of the antifoam is between 0.25-1.50 percent based on the chemistry. The lowest effective dosage is selected from considerations of cost and integrity of the film. At times, mixtures of antifoams are used for optimum results.
The screening of antifoams for a particular situation is an onerous task. Commercial antifoams are too many in number. Resorting to a trial and error approach without insightful experiments is both tiresome and frustrating. Here, an ounce of theory may salvage the need for tons of experiments. An ink chemist seeking solutions to the latest foaming problem arising out of improper processing or inefficient formulation needs to open mind, eyes, and ears to the latest understanding on the molecular level interaction occurring within the complex world of ink chemistry  and their consequences in relation to the function/performance of the foam controlling agents.
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(4.) Joy T. Kunjappu, American Ink Maker, March, 1999.
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(21.) Joy T. Kunjappu, Ink World, August 1998, p.32.
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|Author:||Kunjappu, Dr. Joy T.|
|Article Type:||Statistical Data Included|
|Date:||Aug 1, 1999|
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