Environmental concerns over the levels of volatile organic compounds (VOCs) released into the atmosphere have exerted pressure on the coatings industry to produce waterborne alternatives for almost all classes of solventborne coatings. As the use of waterborne coatings continues to grow, so do the demands for ever-faster production times and application speeds. These pressures intensify an inherent problem with aqueous formulations -- foam generation.
A number of different types of organic- and silicone-based foam-control polymers, compounds, emulsions and dispersions are employed to combat foam, and selecting the proper one, its use level and incorporation method for a specific coating or ink formulation can be quite a task. In addition to effective foam control, the compatibility of the antifoam in the ink or coating formulation must be balanced to avoid surface defects such as craters or fisheyes. Silicone-based antifoams and their performance in selected application will be reviewed in this article, with particular emphasis on polymeric silicone-polyether foam-control agents.
Causes of FoamFoam is a stable dispersion of a gas in a liquid medium that results when a surfactant layer forms around air bubbles and entrains them within it. Air can be incorporated into a coating by mixing during the polymer/pigment grinding and let-down steps, by pumping during package filling or by shear or spraying during application. Effective foam-control agents are beneficial in preventing or reducing many common coating problems such as:
Viscosity increase and loss of mechanical shearing power during milling (resulting in smaller batch sizes and poor pigment/polymer dispersion);
Volume increase during the letdown and mixing steps leading to overflowing;
Slower package-filling rates due to inefficient pumping;
Air incorporation during transport and handling;
Slower printing-press speeds or lower pressures during spraying;
Surface defects on coated substrates resulting in poor appearance, reduction in gloss or less substrate protection.
Foam-Destabilization MechanismsFoam-control agents can be classified as antifoams or defoamers. Although the two terms are often used interchangeably, strictly speaking, antifoams prevent the formation of stable foams, while defoamers act by destabilizing already existing foams. Foam-control agents function by a variety of mechanisms to prevent or rupture foam1. Individual antifoam efficiency is determined by three key factors:
Insolubility of the antifoam agent in the foaming medium;
Low surface tension, so that it can be uniformly dispersed throughout the formulation;
Ability to penetrate into the foam wall (or lamellae).
Foam-control agents or defoamers must be insoluble in the foaming medium. They function by being more surface active than the surfactant stabilizing the foam so that they are able to enter the surface layers of the potentially foaming liquid and displace it from the gas/liquid interface. The mixed surfactant layers now prevent close association of molecules and exhibit low elasticity. The presence of random, highly surface active, insoluble molecules in the surface film interrupts foam stabilization via the Marangoni effect, and thus foaming is prevented.
The four basic processes by which antifoams disrupt aqueous foam are: entering, bridging, dewetting and rupture as depicted in Figure 1. This mechanism1 proposes that droplets of the antifoam move to the foam lamellae where they provide a point source for rupture of first one air/water interface and then the other. An oil lens is then formed, which bridges the air-water-air foam film. Drainage in the oil lens and foam film takes place until eventually the film ruptures. The thermodynamic factors and surface properties influencing the foam-control mechanism and foam stability have been discussed in the literature.2,3 Higher bulk viscosity systems, such as formulations containing thickeners or high binder content, slow down the displacement of the liquid from the lamella and limit the mobility of the entrained air bubbles.
Surfactants orient at the air-liquid interface to create a higher surface viscosity in the lamella than the bulk in which film drainage can be impeded. The surfactant stabilizes foam by hydrogen bonding and electrostatic repulsion between surfactant molecules within the lamella. Increased surface elasticity, created by the thinning of the lamellae, also impedes liquid drainage and subsequent foam collapse. Movement of the surfactant layer can actually pump water back into the lamella causing further stabilization (Marangoni effect).
Why Use Silicones?The term silicone refers to a class of materials characterized by a Si-O-Si backbone. The simplest polymer, sometimes referred to as silicone oil, is polydimethylsiloxane (PDMS).
where n = 0 to >1000.
Two properties of silicones make them suitable as aqueous foam-control agents -- they are very hydrophobic and, therefore, incompatible with water; and they are also highly surface active, with liquid surface tension values of approximately 20 mN/m. Both these properties ensure that silicones will migrate to the air/liquid interface of bubbles within a coating.
Types of Silicone-Based DefoamersThe simplest silicone-based system is PDMS alone. It is known that PDMS will migrate to the surface of a film and remain there. If the surface consists of macro-bubbles, it may penetrate and spread over the foam lamellae, but its ability to destroy foam is limited. The main disadvantage is that PDMS is so insoluble that it is very difficult to disperse in waterborne systems and almost inevitably causes surface defects. Polydimethylsiloxanes are typically emulsified when added to aqueous coating systems using organic or silicone-based surfactants to assist in delivering the foam-control agent and to help with leveling/wetting of the applied coating. In addition, a hydrophobic particle may be incorporated into the fluid then used or emulsified to assist with antifoam entry and subsequent foam rupture. Blends of PDMS and silica are often referred to as silicone compounds. Experience has shown that they can cause problems; they are extremely incompatible with the foaming medium, which makes them difficult to disperse, but they are also incompatible with the resin binders leading to dewetting of the coating as it dries and leaving defects in the dried films often described as fisheyes, cissing etc.
So the problem of dispersion has been resolved, but the problems caused by the incompatibility of PDMS in the drying film still needs to be addressed. This has been done by incorporating modified PDMS in the form of silicone-polyether copolymers into the foam-control formulations. The copolymers are synthesized from reactive siloxanes and polyethylene/polypropylene glycol ethers. A range of structures is available, for example branched, block or pendant copolymers. Figure 2 illustrates both the rake structure where the polyether groups are pendant to the siloxane backbone and the ABA structure where the polyether groups endcap the siloxane polymer. By varying the hydrophilic/hydrophobic nature of the silicone polyether, these materials can be used in conjunction with PDMS fluids and compounds such as emulsifiers and wetting agent components of an antifoam compound or emulsion. Silicone polyethers have also been formulated with glycols to form easily incorporated dispersions for applications such as architectural paints, but they can also be designed to function as effective antifoams alone. Potential benefits for polymeric silicone polyether used as the sole antifoam in a coating or ink include:
- 100% active to allow greater formulation flexibility and lower use levels;
Self-emulsifying for easier incorporation into aqueous or polar coatings;
No hydrophobic particles to separate or cause surface defects;
Balances effective foam control and good surface appearance;
Stable polymer allows for incorporation under high shear, allowing for use during the pigment/polymer grind step and increases flexibility in addition point selection.
This paper focuses on two new organomodified silicones that have been developed in such a way that they are surface active and have selected incompatibility and solubility in the foaming liquid.
Performance in Selected FormulationsThe new silicone polyether foam-control agents, silicone polyethers #1 and #2, were evaluated in several ink and coating formulations. Assessments focused on two key properties: antifoam performance as measured by formulation density retention and surface appearance of the applied coating. Both properties were assessed after high-shear mixing using a dissolver blade for maximum air incorporation. Where available, results are presented for the silicone polyethers compared to varying types of silicone-based antifoams.
Waterborne Clear Overprint VarnishFigure 4 shows the performance of the different foam-control technologies in a varnish formulation after a high-shear stirring test. A high density indicates low air entrapment and good foam control. The control is without defoamer. The first product in the series is a simple emulsion based on PDMS and silica; the second product is more formulated and contains not only the PDMS and silica but silicone polyethers to help in controlling the stability and compatibility of the emulsion.
It can be seen that different foam control is obtained dependent on the type of technology. Silicone polyether #1 effectively retained density after shearing in the overprint varnish formulation relative to the control.
Waterborne Flexographic InkFigure 5 shows the results of silicone polyether #1 compared to a typical PDMS compound emulsion and a silicone polyether compound in a waterborne flexographic ink. The control without an antifoam shows good surface appearance in terms of no craters formed due to an incompatible antifoam presence, but did show several pinholes due to many entrapped air bubbles present in the dried ink film. The density of the ink after shearing 10 minutes at 3000 rpm also decreased significantly, retaining only 65% of the original weight for an equal volume of ink. Silicone polyether #1 proved to be effective in resisting foaming with a density ratio of 1.0 at 0.2% use level and good/acceptable appearance initially and after the ink was aged two days at 40 ?C. A 10% active PDMS compound emulsion added at 2% by weight of the formulation to achieve equal actives was effective at controlling foam initially and after aging (0.8 – 0.95 density), but resulted in a poor surface appearance. A silicone polyether compound containing a hydrophobic silica particle proved very effective at controlling foam (density = 1.0), but suffered from significant surface defects.
Waterborne Wood Parquet LacquerTable 1 shows results in a one-part glossy wood lacquer (G-2170 formulation). This time the best performance can be seen with silicone polyether #2
Automotive Waterborne Clear BasecoatSPE #2 performed very well, weighing 96 g/100 ml and 100 g/100 ml at 0.2 and 0.1% use levels, respectively, after shearing 1 minute at 2800 rpm using a dissolver blade. Little to no pinholing was observed when spray applied. The control without a foam-control agent weighed 56 gms/100ml, and spray-applied film showed some surface craters. (See Figure 6)
ConclusionsSilicone-based antifoams have progressed markedly since the first use of PDMS fluids in solventborne coatings and inks. Keeping pace with formulation changes and environmental drivers, silicone antifoams have evolved to comprise a variety of delivery systems and polymer types to meet the specific requirements of diverse formulations. For waterborne coatings and inks, the product offerings have been expanded to include novel silicone polyether-based antifoams that offer effective foam control balanced against ease of incorporation and good coated surface appearance in several coating and ink systems. As coating formulators are aware, no “universal” foam-control agent has been developed to date, therefore, the authors of this paper respectfully suggest any foam-control agent be evaluated in a particular formulation for suitability.
This paper was presented at the 7th Nurnberg Congress, European Coatings Show, Nurnberg, Germany.
For more information, contact Donna Perry, Dow Corning Ltd., Cardiff Road, Barry, South Glamorgan, CF63 2YL,United Kingdom; phone + 44 1446 723805; fax +44 1446 730495; or e-mail firstname.lastname@example.org.
AcknowledgementsJianren Zeng is a product-development specialist for Dow Corning, and Ginny O’Neil is a technical service specialist with Dow Corning. The authors also wish to thank Takahiro Miura, Keiichi Akinaga, Eiichi Kitaura, Atsushi Kasamatsu and Tsunehito Sugiura at Dow Corning Asia for their valuable contribution to this work.
References1 Hill, R.M.; Fey, K.C. Silicone Surfactants, Hill, R.M., Ed.; Marcel Dekker, Inc., New York, NY 1999, p. 165.
2 Garrett, P.R. Defoaming: Theory and Industrial Applications, Garrett, P.R., Ed.; Vol. 45, Surfactant Science Series Marcel Dekker, New York, 1993.
Porter, M.R. Handbook of Surfactants, Blackie & Son Ltd, Chapman & Hall, New York, NY 1991, p.38.