Innovative, Gemini-type molecular defoamer technology for improved coating aesthetics.

The use of conventional surfactants and defoamers in waterborne coatings can often create an undesirable cycle, whereby each imparts a negative effect that requires the other to solve. Molecular defoamers represent a novel class of defoamers that break foam on a molecular level, which is unlike conventional defoamers that utilize macroscopic physical incompatibility within a coating formulation. This is accomplished by the ability of the molecular defoamer to adsorb at the air-water interface of a foam bubble and, thereby, displace some of the surfactants that stabilize the foam. Based on a fundamental understanding of the effects of basic adsorption parameters, innovative nonsilicone molecular defoamers were developed that provide long-term foam control, effectively reduce both surface tension and foam in a variety of emulsion polymers, and decrease significantly the level of defects in waterborne coating applications. A synergistic performance benefit was obtained when these molecular defoamers were used in combination in an acrylic, industrial maintenance formulation.

Keywords: Defoamers, wetting agents, surface tension, waterborne, foam control, gemini surfactant, molecular defoamer, acetylenic diol, surface adsorption

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Waterborne coating formulations are a complex mixture of resin dispersions, pigments, solvents, and additives such as surfactants and defoamers, which are often required to solve flow, leveling, and foam problems that are exacerbated in waterborne technology because of the relatively high surface tension of the continuous water phase. In addition, surfactants are also necessary to stabilize resin and pigment dispersions. Commonly, these stabilizing surfactants are high HLB (hydrophile-lipophile balance) ethoxylate or ionic types that are prone to stabilize foam as well. Examples include ethoxylates of alkylphenols and various alcohols and alkyl sulfonates. Thus, the use of these surfactants necessitates the need to add defoamers to curtail the development and subsequent build-up of unwanted foam. Defoamers, however, often produce negative consequences, such as surface defects (e.g., craters) and poor flow and leveling (e.g., orange peel). As a result, more surfactants are added to the system to eliminate the problems caused by the defoamers. In turn, these surfactants can reaggravate the foam problem. Thus, traditional defoamers and surfactants can perpetuate a negative feedback loop, which might ultimately compel the waterborne coatings formulator to make undesirable compromises in the final coating formulation.

Because of this negative feedback cycle, it is beneficial for additives to perform multiple functions such as both wetting and defoaming to minimize any potential detrimental effects. Preferably, the additives should also have higher efficiency. Nonionic surfactants that are based on acetylenic diol technology have been reported to provide higher efficiency and multifunctionality and, therefore, have been widely used in coatings. (1-9) The unique properties of the acetylenic diol surfactants arise in part due to their Gemini ("twin" or dimeric) surfactant structure which, unlike conventional monomeric surfactants that have a single, hydrophobic group (e.g., a hydrocarbon tail) connected to a hydrophilic head (e.g., a hydroxyl group or a polyethylene oxide tail), is comprised of two hydrophilic heads that are connected by a molecular segment or "spacer" and two (most commonly) hydrophobic tails. The generic structures of monomeric versus Gemini surfactants are shown in Figure 1. The unique structure of Gemini surfactants produces highly efficient, multipurpose additives.

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In coatings, the most widely used acetylenic diol surfactant is 2,4,7,9-tetramethyl-5-decyne-4,7-diol (TMDD) (Figure 2) and its derivatives (e.g., alkoxylates). This surfactant has been shown to provide multipurpose benefits in a variety of waterborne coatings. (1,2,4,5)

Foam Stabilization and Acetylenic Diols

To understand why acetylenic diol surfactants show low foam tendencies, it is beneficial to discuss their properties with respect to the stabilization mechanisms of foam. In a simple air-liquid system, foam does not spontaneously occur, but its generation requires the input of mechanical energy, such as mixing, to disperse the air into the liquid. (Of course, gas bubbles can also be introduced into a liquid through other means such as chemical reactions, displacement from a porous surface, or through direct injection.) In a low viscosity pure liquid, when the mechanical energy is released, bubbles of the dispersed gas will quickly return (due to buoyancy) to the liquid-gas (L/G) interface and be released rapidly back into the air. This occurs because the foam is thermodynamically unstable, since the free energy of the foam-containing system is higher due to the increased L/G interfacial surface area of the bubbles. In addition, once a given bubble reaches the L/G interface, there is a low energy barrier for drainage (affected by gravity and surface tension) of the liquid surface layer (i.e., lamella) of the bubble; consequently, the lamella will promptly thin to a critical thickness at which the bubble bursts because the internal pressure of the bubble exceeds the strength of the lamella. Thus, one of the critical steps with regard to foam stability is the rate of liquid drainage from the bubble lamella. (10) This is also important for bubble coalescence in the bulk liquid because bubble coalescence leads to a stronger buoyant force, so that the bubbles can rise to the L/G surface more rapidly. Herein is where many conventional surfactants can exert their greatest influence by providing mechanisms to significantly raise the energy barrier and, thereby, retard liquid drainage. A brief discussion of the main stabilizing mechanisms imparted by conventional surfactants in water is provided below and compared to those of acetylenic diol surfactants (specifically TMDD).

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** Electrostatic Repulsion: Ionic surfactants adsorbed on the bubble lamella walls can lead to a strong stabilization of lamella thickness (and thus foam) through mutual charge repulsion. Acetylenic diols are nonionic, so this mechanism does not apply.

** Steric Hindrance: Surfactants with relatively long hydrophilic chains (e.g., ethoxylates) retard the thinning of lamella due to the steric repulsion of those chains. Compared to conventional ethoxylates, which may have five or more ethylene oxide groups for example, the hydrophilic groups on the TMDD molecule are merely hydroxyls, which impart virtually no steric hindrance. However, foam stabilization increases as the degree of ethoxylation of TMDD increases, and highly ethoxylated versions in water are quite foamy.

** Gibbs-Marangoni Effect/Surface Transport (11): A bubble lamella possesses an "elasticity" that arises in theory from local increases in surface tension as a region of a lamella thins. The consequent surface tension gradient (d[gamma]/dA; [gamma] = surface tension, A = surface area) causes liquid to flow from the thicker region into the thinner region. The surface tension gradient is induced by the increase in surface area of the thinned region, which initially reduces the surfactant concentration on the lamella walls and, subsequently, in the lamella liquid adjacent to the thinned region. In addition, transport of surfactant at the surface will drag a significant amount of liquid to the thinned region. The net effect is thickening and, thus, reinforcing of the thinned region. Because of its rapid diffusion, TMDD minimizes the surface tension gradients that arise due to a thinning lamella by diffusion from the bulk liquid rather than by surface transport. Thus, the driving force for healing of the lamella is minimized, and liquid drainage can occur.

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** Surface Viscosity/Cohesive Strength: At sufficient concentrations, surfactant molecules adsorbed at the L/G interface orient to allow maximum packing. The resulting structure leads to an increase in the surface viscosity, which slows drainage, and a higher film cohesive strength due to intermolecular attractive forces. Because of the branched hydrophobes on the TMDD molecule, L/G surface packing is less efficient compared to conventional surfactants with linear hydrophobes. Therefore, the overall surface concentration would be lower for TMDD, and that results in a reduced surface viscosity and film cohesive strength.

As the preceding discussion suggests, TMDD should oppose the effects of surfactants that stabilize foam and, by itself, should not impart foam stability. Indeed, foam studies have shown that TMDD provides relatively low foam wetting to a variety of emulsion polymer systems. (8,9,12)

Defoaming

As stated previously, conventional defoamers are physically incompatible within the coating formulation. Typically, they are comprised of a primary liquid, emulsifiers/wetting agents, and other ingredients which help to improve efficacy, stability, and compatibility. The primary liquids are generally hydrophobic liquids like mineral and silicone oils that serve as the defoaming agents and may contain hydrophobic particles such as modified silicas. An important function of the emulsifiers/wetting agents is to aid the efficiency of the defoaming agent by affecting the entering into the lamella and the spreading at the L/G interface. The properties of entering and spreading are dependent on the surface tensions of the liquid film ([[gamma].sub.L]) and the defoamer ([[gamma].sub.D]), and the interfacial tension ([[gamma].sub.LD]) between the two. Entering (E) and spreading (S) coefficients can be defined such that, when maximized, optimum defoaming is achieved. (13,14)

E = [[gamma].sub.L] + [[gamma].sub.LD] - [[gamma].sub.D] (1)

S = [[gamma].sub.L] - [[gamma].sub.D] - [[gamma].sub.LD] (2)

For best entering and spreading, the defoamer should have a low surface tension ([[gamma].sub.D]) relative to that of the liquid ([[gamma].sub.L]). In contrast, the interfacial tension ([[gamma].sub.LD]) has a positive effect on entering but a negative influence on spreading. Thus, the general effectiveness of silicone defoamers can be explained by their low [[gamma].sub.D]. On the other hand, the low [[gamma].sub.D] of silicone defoamers is also responsible for their tendency to induce surface defects in the coating film due to their relative incompatibility (phase separation) and the resultant Marangoni flow.

Molecular Defoamers

Although TMDD and some of its modifications have been proven to provide good surfactant properties (both dynamic and equilibrium surface tension reduction) with low foam tendency, there is still a need for more efficient defoamers that can provide surfactant-like wetting performance to reduce or eliminate the tendency to cause coating defects that are often created by silicone defoamers. One class of defoamers that offers the potential for improved performance is the so-called molecular defoamer (MD). (12,15) Unlike conventional defoamers, which utilize macroscopic physical incompatibility within the coating formulation, MDs break foam on a molecular level by adsorbing at the L/G interface of foam bubbles, thereby displacing some of the foam-stabilizing surfactants with surfactants (molecular defoamers) that behave antagonistically. The adsorbed MD molecules disrupt the foam stabilizing mechanisms that are imparted by the conventional surfactants adsorbed at the bubble L/G surface, and thus allow liquid drainage to proceed until the bubble bursts. The differences between conventional and molecular defoamers are schematically illustrated in Figure 3.

In this article, the basic properties of a series of novel MDs of different molecular structures are reported. Further, this article describes how those basic properties are related to their utility as MDs and ultimately to their performance in waterborne emulsion polymer systems.

EXPERIMENTAL

The generalized molecular structures for the molecular defoamers evaluated in this study are shown in Figures 4 and 5. MD1 is an alkane diol, while MD2 is a hydrophilic-hydrophobic modified acetylenic diol. MD3 is 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol.

The properties of the molecular defoamers evaluated in this study are provided in Table 1. MD1 and MD2 are 100% active, low viscosity liquids with low HLBs ([less than or equal to] 4) and no solvents. MD3 and TMDD are solids at room temperature and were evaluated as solvent solutions, as indicated in the footnotes in Table 1. These products were used as commercially supplied. Refer to the Appendix for material identification and suppliers.

All coating formulations were prepared and applied using standard techniques. Films were applied on various substrates (usually sealed Leneta charts) by drawdown to a wet film thickness of about 152 [micro]m (6 mils). Equilibrium surface tension (EST) measurements were performed using the Wilhelmy plate method or the pendant drop method. (16,17) Dynamic surface tension (DST) data were obtained by the maximum bubble pressure method using a Bubble Pressure Tensiometer BP2 (Kruss USA). Evaluations for foaming properties were done by vigorously agitating the surfactant-containing sample using a Waring blender on high speed for one minute and then immediately (or after a prescribed time as noted) measuring the density (referred to as "agitated density") of the agitated liquid, or by pumping air through the solutions at a fixed rate over specific time intervals and measuring the initial amount of foam and the rate of foam decay.

RESULTS AND DISCUSSION

Molecular Defoamer Properties

As the structure for MD3 shows, it is an acetylenic diol similar to TMDD, but the hydrophobic, alkyl chains on MD3 are one carbon longer than those on TMDD. The longer hydrocarbon chains on MD3 impart greater hydrophobicity to the molecule and, therefore, better defoaming capabilities compared to TMDD, which represents an earlier generation MD. While MD3 is an extension of acetylenic diol technology, MD1 and MD2 represent novel structures based on nonacetylenic and highly modified acetylenic technologies, respectively. Both MD1 and MD2 are pourable liquids at room temperature and, thus, do not require solvents, which are necessary to dissolve the solid products MD3 and TMDD. Since all of the molecular defoamers have very low HLBs and low water solubilities, they would not be expected to impart water sensitivity to coatings. In addition, all of these products are surface active molecules that provide low equilibrium surface tensions, which are desired for the entering and spreading of defoamers in bubble lamella, and they have low tendency to cause surface defects in applied coatings. In the following section, the relationship of the surface activity to the defoaming capabilities of MD1, MD3, and TMDD are discussed. The fundamental understanding of this relationship formed the basis for the development of the latest generation of molecular defoamer products--MD1 and MD2.

Surface Adsorption Parameters

In order for MDs to be effective, they must exhibit system compatibility and a strong tendency for adsorption at the L/G interface. Surfactant adsorption parameters such as surface packing efficiency, interaction energies, and adsorption constants can be obtained by measuring the surface tensions of surfactant solutions at various bulk concentrations in water. The resulting adsorption isotherms can be fit to the appropriate equations to obtain the adsorption parameters of interest. Two useful expressions for this purpose are given below. (12,17-20)

[d[GAMMA]]/[dt] = 0 = [beta][[GAMMA].sub.[infinity]]C(1 - [GAMMA]/[[GAMMA].sub.[infinity]]) - [alpha][GAMMA][e.sup.K[GAMMA]/[[GAMMA].sub.[infinity]]] (3)

[gamma] = [[gamma].sub.o] + RT[[GAMMA].sub.[infinity]][ln(1 - [GAMMA]/[[GAMMA].sub.[infinity]]) - [K/2]([GAMMA]/[[GAMMA].sub.[infinity]])[.sup.2]] (4)

In the above equations, [beta] and [alpha] are the adsorption and desorption rate constants, respectively. In addition, C is the bulk surfactant concentration, [GAMMA] is the surfactant surface-concentration, and [[GAMMA].sub.[infinity]] is the limiting surfactant surface-concentration at maximum packing (i.e., a monolayer). The term [gamma] is equilibrium surface tension of the aqueous surfactant solution at a given concentration, and [[gamma].sub.o] is the surface tension of pure water. R is the gas constant, and T is the temperature. K is termed the Frumkin parameter, which equals to -E/RT, where E is the interaction energy and RT is the thermal energy. For the surfactant adsorbed on the surface, K relates to the attractive forces (van der Waals) between the hydrophobic tails and to the repulsive forces due to hydration of the hydrophilic head groups. E has a positive value when the attractive energy of hydrophobic tails dominates; therefore, K has a negative value under that circumstance.

The surface adsorption parameters for the molecular defoamers (MD1 and MD3) and a series of monomeric, ethoxylated-alcohol (1[degrees] alcohol groups) surfactants were obtained using the above analysis. (12,17,20) The results are given in Table 2.

The term [[GAMMA].sub.[infinity].sup.-1] represents the surface area that a given surfactant molecule occupies at surface saturation. For the monomeric ethoxylated-alcohol surfactants, the area occupied by each molecule is relatively low compared to the Gemini-type surfactant/molecular defoamers. This can be explained by the highly branched structure of the Gemini surfactant/molecular defoamers. Also, as the number of ethoxylate groups increases on the ethoxylated-alcohols, the occupied area increases due to steric repulsion of the longer hydrophilic groups. However, even for an 8-mole ethoxylate, the area occupied is still significantly less than for the Gemini products. Thus, as discussed previously, due to their lower surface concentration, the Gemini products should provide lower surface viscosity and cohesive strength, which, in turn, should reduce the stability of foam bubbles.

With regard to surface adsorption and desorption, a higher value of [beta]/[alpha] represents a greater tendency for surface adsorption. The monomeric surfactants show greater adsorption as the number of ethoxylate groups increases. For the Gemini materials, TMDD and MD1 have similar values of [beta]/[alpha], while the MD3 material has a [beta]/[alpha] value about an order of magnitude greater. For molecular defoamers to be effective, they should have adsorption constants that are competitive with those of conventional surfactants that stabilize foam. The molecular defoamers have significantly higher constants compared to the [C.sub.10][E.sub.0] alcohol and, therefore, would be expected to adsorb more efficiently onto the surface. Thus, based on adsorption, the molecular defoamers would be expected to be more efficient surfactants than [C.sub.10][E.sub.0] and to compete to some extent with the ethoxylated versions (e.g., [C.sub.10][E.sub.4]).

As mentioned above, the Frumkin parameter, K, is negative for the case where the attractive energy between the hydrophobic groups is greater than the repulsive energy of the hydrophilic groups. The negative value of K for [C.sub.10][E.sub.0] is the result of the relatively small repulsive energy from the single hydroxyl group on each molecule compared to the attractive energy of the [C.sub.10] chain. However, by adding ethoxylate groups, the repulsive energy sharply increases, and a positive K value is seen for even the 4-mole ethoxylate. This has implications for foam stability. As a foam lamella thins due to drainage in a given area, the lamella curves, and the hydrophilic groups of the adsorbed surfactant are compressed. For surfactants with K > 0, the repulsive energy of the hydrophilic groups will resist compression due to the consequent increase in energy. Thus, surfactants with positive K values should stabilize foam, whereas those with values of K < 0 should destabilize foam from an overall surface energy perspective.

Of course, for a given surfactant, the stabilization of foam is dependent, in large part, on the net influence that the three parameters discussed previously have on the foam stabilization mechanisms. For example, the effects of the surface area per molecule and the Frumkin parameter favor the Gemini surfactants (K also favors 1-decanol) as defoamers versus the ethoxylated alcohols. On the other hand, the ethoxylated alcohols, in general, have a higher surface adsorption. the relative effects of these adsorption parameters were studied by generating foam in aqueous solutions of these surfactants by pumping air through the solutions at a fixed rate (Q in ml/min) over specific time intervals ([t.sub.f] in min). The initial amount ([f.sub.m] in ml; foam amount immediately after air injection was stopped) of foam and the rate (df/dt) of foam decay were measured. The results are provided in Table 3.

The data in Table 3 show that the solutions containing the ethoxylated alcohol surfactants have significantly more initial foam than the other surfactants. In addition, the foam stabilized by those surfactants has about an order-of-magnitude slower rate of decay when compared to that of TMDD. On the other hand, MD3 and 1-decanol did not show any initial foam, even at a very high rate of air injection. TMDD did show some initial foam, but this was at a significantly higher concentration than used for any of the other surfactants. Thus, the foam stabilizing tendency of the ethoxylated surfactants can be attributed to their positive K values, while the low- or nonfoam characteristics of 1-decanol, MD3, and TMDD can be explained by their K values being [less than or equal to] 0.

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Similarly, the effect of surfactant adsorption, [beta]/[alpha], was examined by measuring the foam properties of aqueous solutions containing a "foamy" nonionic ([C.sub.10][E.sub.4]) or anionic (sodium octyl sulfonate = [C.sub.8]S[O.sub.3]) surfactant, to which were added either TMDD or the MD3 molecular defoamer. The results are given in Table 4. Even though both MD3 and TMDD have the same K value (= 0), the data show that MD3 is a stronger defoamer than TMDD. The initial foam heights for MD3 are lower than those of TMDD, and that holds true even for relatively lower concentrations of MD3. Furthermore, the foam decay rate for MD3 is generally faster than that of TMDD. These findings can be rationalized by the much higher adsorption constant for MD3 versus TMDD. Additionally, the adsorption constant of the foamy surfactant has an effect on foam. In these binary mixtures, the initial foam heights for the anionic surfactant ([C.sub.8]S[O.sub.3]) are somewhat lower than for the nonionic [C.sub.10][E.sub.4], even though anionic surfactants are well known to be very foamy surfactants due to strong electrostatic foam stabilization. However, because [C.sub.8]S[O.sub.3] has a relatively low adsorption constant ([beta]/[alpha] = 3.2 x [10.sup.6] [cm.sup.3]/mol; [[GAMMA].sub.[infinity].sup.-1] = 37.0 [[Angstrom].sup.2]/molecule, which is close to that of [C.sub.10][E.sub.4]), the nonfoamy surfactants can compete effectively for the surfaces of the foam lamellar walls, and thereby allow drainage to destabilize the foam.

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Surface Tension

Because of their nature, molecular defoamers have a number of potential advantages over traditional defoamers. In order to preferentially adsorb on the bubble lamella surfaces, they need to impart a comparatively low surface tension. The data in Table 1 indicate that the MDs have ESTs in water ranging from about 28 to 35 mN/m, which represents a significant reduction compared to the EST ([gamma] = 72 mN/m) of pure water. Figure 6 shows that the MDs have relatively low DST values compared to the conventional defoamers, which basically do not reduce DSTs over that of pure water at or above even moderate bubble frequencies (~ 4 bubbles/sec). The high DSTs for the conventional defoamers most likely result from their incompatible nature and their relatively low diffusion rates (low mobility) in water. Thus, it appears that another important characteristic of molecular defoamers is relative compatibility on a molecular level in the coating system. The combination of low surface tension and good compatibility should help provide good wetting, flow, and leveling, and thereby reduce or eliminate the surface defects which are often associated with standard defoamer technology. In addition, since their bubble-breaking mechanism does not depend on incompatibility, molecular defoamers can provide defoaming longevity relative to traditional defoamers. Also, molecular defoamers can be particularly effective in microfoam control.

Defoaming Emulsion Polymers

The performance of the MD2 material has been tested relative to organic- and silicone-based defoamers in various emulsion polymers. As shown in Figure 7, the MD2 product displayed more efficient defoaming than either the organic or silicone defoamers in a urethane-acrylic hybrid polymer emulsion (polymer emulsion A). Because this emulsion polymer contains no external surfactants or added solvents, it is particularly sensitive to defects caused by additives such as defoamers. The MD2 material was the only material that provided defect-free films when the defoamer-containing emulsion samples were coated and dried on Leneta charts. These observations illustrate the excellent compatibility and surface tension reducing characteristics of the MD2 product. Another example of the defoaming capability of MD2 is illustrated in Figure 8, which shows MD2 compared to several commercial defoamers in a series of acrylic polymer emulsions. The commercial defoamers were chosen because they were included in the starting formulations from the emulsion polymer supplier. As can be seen, MD2 provided improved defoaming (higher agitated density = less foam) in each of these systems versus other defoamer technologies.

In separate experiments, the MD products were tested for defoaming, surface tension, and wetting properties in a number of polymer emulsions used in wood coatings. Figures 9 and 10 provide the surface tension and defoaming results, respectively. Emulsions E, F, and G were urethane-acrylic hybrids, while emulsions H and I were acrylics. In all of the polymer emulsions tested, the MDs lowered the EST; MD2 provided the lowest values. Likewise, for defoaming, all of the MDs were found to reduce the level of foam compared to the neat polymer emulsions. With the exception of emulsion F, MD2 was found to be the best defoamer. MD2 was significantly more effective in the acrylic systems than either MD1 or MD3. All three MDs were relatively strong defoamers for the urethane-acrylic hybrid emulsion E. Table 5 provides data for the appearance of coatings (drawdowns on sealed Leneta charts) prepared from the emulsion polymers containing 0.5 to 1% by weight of the MDs. Despite the relatively high addition levels tested, the appearance of many of the coatings containing the MDs compared favorably to those of the neat emulsions. Only minor defects (a few small craters or minor orange peel) were observed in any of the coatings containing the MDs. These observations illustrate the capability of the MDs to act as both defoamers and wetting agents.

Defoaming Longevity of MD2

A study was conducted to evaluate the defoaming longevity of MD2 versus a conventional mineral oil defoamer in a standard pressure sensitive adhesive (PSA) formulation (acrylic copolymer without tackifying resins, 56% solids, viscosity of 1000 cP at 20[degrees]C). In this study, the PSA contained a strongly foaming surfactant (DOSS--dioctyl sodium sulfosuccinate), which is a standard wetting agent used in PSA formulations. The defoamers were added to the samples, which were then aged in an oven at 50[degrees]C for up to two weeks. The level of defoaming was measured periodically (Waring blender test). The results are shown in Figure 11. MD2 retained or even slightly improved in defoaming effectiveness, while the mineral oil defoamer lost efficacy over the two-week span of the study. The reduced performance of the mineral oil defoamer is probably the result of increased compatibility in the system, while MD2 remained effective

because of its inherent compatibility in the system.

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Performance of MD1 in Coating Formulations

In applications studies, MD1 has shown effectiveness as a wetting agent. An example is provided in Figure 12, which compares MD1 to a silicone defoamer in a water-borne UV-curable wood lacquer formulation on white oak panels. The example clearly shows that MD1 provides improved coatings aesthetics at a lower use level than the silicone defoamer.

In another study, the performance of MD1 was compared in an automotive primer formulation based on a saturated polyester resin (SPR) and a fatty acid-modified polyurethane resin (FAPUR). The formulations were ranked for a variety of aesthetic properties, and the results are shown in Figure 13. Compared to the control and the standard additive blend, MD1 showed the best results at an equal use level. These results illustrate the utility of the MD1 material to produce coatings with relatively low defect levels.

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Synergy of MD1 and MD2 in an Acrylic Industrial Maintenance Formulation

A study in an acrylic industrial maintenance primer formulation has shown that the combination of MD1 and MD2 provides a synergistic improvement in coating aesthetics versus TMDD and/or silicone defoamer combinations. Photographs that illustrate this synergy are shown in Figure 14. Coating films prepared by airless spray had craters and microfoam with the silicone defoamers and TMDD. The combination of TMDD plus MD2 alone showed an improvement, but there were still defects. When the MD2 was used alone, significantly fewer defects were observed. Finally, when MD2 was combined with MD1, the defects were virtually eliminated. This synergy can be explained by the complementary nature of MD1 and MD2 in surface tension and defoaming behaviors. With regard to surface tension, MD1 has a low DST, and MD2 has a very low EST. So, blends of MD1 and MD2 have desired low EST and DST values, which are important for coating defect (e.g., craters) elimination. In terms of defoaming, our experience is that MD1 reduces macrofoam well, while MD2 provides excellent microfoam reduction. Thus, the combination is useful for reducing different types of foam that can lead to gloss reduction or pinholes.

SUMMARY AND CONCLUSIONS

Molecular defoamers represent a novel class of defoamers that break foam on a molecular level rather than by macroscopic physical incompatibility like conventional defoamers. This is accomplished by the ability of the molecular defoamer to adsorb at the air-water interface of a foam bubble and, thereby, displace some of the surfactants that stabilize the foam. Fundamental studies of surface adsorption have shown the importance of the adsorption constant ratio ([beta]/[alpha]), [[GAMMA].sub.x.sup.-1] (the surface area covered by each surfactant molecule at saturation), and the Frumkin Parameter (K) on the foaming/defoaming behavior of surfactants and how those parameters depended on molecular structure. Surfactants with high values of [beta]/[alpha] and [[GAMMA].sub.[infinity].sup.-1] and K [less than or equal to] 0 were found to behave well as defoamers. Based on this fundamental understanding, innovative, nonsilicone molecular defoamers were developed that provided foam control longevity, a combination of low surface tension and low foam in a variety of emulsion polymers, and reduced levels of defects in waterborne coating applications. The surface activity (low equilibrium and dynamic surface tension) of these molecules allows them to serve a dual function as wetting agents and defoamers in the systems studied. A combination of these molecular defoamers was found to provide a synergistic performance benefit (low level of surface defects versus the control) in an acrylic, industrial maintenance formulation: This benefit is probably the result of an advantageous combination of complementary surface tension profiles (low EST and DST are important for coating defect elimination) and defoaming capabilities (macrofoam and microfoam reduction).

ACKNOWLEDGMENTS

Many people have contributed to this article, and the authors extend their gratitude to all of them. Special mention and thanks to the following people: Kevin R. Lassila, Jose M. Lorenzo, Christine Louis, Phuong Nguyen, Robert K. Pinschmidt Jr., Jim Reader, Roger Reinartz, Koty Schreffler, Jeanine M. Snyder, Robert E. Stevens, Wim P. Stout, and Sekhar Sundaram. Also, many thanks to Rohm and Haas Company and NeoResins for supplying samples of the emulsions used to test the MD products.

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(14) Hill, R.M. and Fey, K.C., "Silicone Polymers for Foam Control and Demulsification," Silicone Surfactants, Surfactant Science Series, Vol. 86, Hill, R.M. (Ed.), Marcel Dekker, Inc., New York, 166, 1999.

(15) Breindel, K., "Sugar Coating," Mod. Paint Coat, 18-22, May (2000).

(16) Rotenberg, Y., Boruvka, L., and Neumann, A.W., "Determination of Surface Tension and Contact Angle from the Shapes of Axisymmetric Fluid Interfaces," J. Colloid Interface Sci., 93, 169-183 (1983).

(17) Lorenzo, J.M., Pinschmidt, R.K., and Sundaram, S., "Inhibition of Aqueous Foams by Acetylenic Diol Based Gemini Surfactants," manuscript to be submitted for publication in Colloids Surf., A: Physicochem. Eng. Aspects.

(18) Pan, R., Green, J., and Maldarelli, C., "Conditions for Promoting Kinetic Influence During Surfactant Exchange at an Interface and Their Use for Measuring Kinetics Parameters by the Pendant Bubble Method," J. Colloid Interface Sci., 205, 213-230 (1998).

(19) Rosen, M.J., Surfactants and Interfacial Phenomenon, 2nd Ed., John Wiley and Sons, New York, 83-84, 1989.

(20) Lorenzo, J. and Sundaram, S., "Inhibition of Aqueous Foams by Acetylenic Diol Based Gemini Surfactants," AIChE 2002 Annual Meeting, Indianapolis, IN, November 3-8, 2002.</p> <pre> APPENDIX -- List of Raw Materials and Suppliers Reference Code Raw Material

Supplier MD1 EnviroGem[R] AD01 Air Products and Chemicals, Inc. MD2 SURFYNOL[R] MD-20 Air Products and Chemicals,

Inc. MD3

SURFYNOL[R] 124 Air Products and Chemicals,

Inc. TMDD SURFYNOL[R] 104 Air Products and Chemicals,

Inc. Polymer Emulsion A HYBRIDUR[R] 870 Air Products and Chemicals,

Inc. Polymer Emulsion B Maincote[R] HG-54 Rohm and Haas Polymer Emulsion C Maincote[R] PR-71 Rohm and Haas Polymer Emulsion D Maincote[R] HG-86 Rohm and Haas Polymer Emulsion E HYBRIDUR[R] 580 Air Products and Chemicals,

Inc. Polymer Emulsion F NeoPac[R] E106 NeoResins Polymer Emulsion G NeoPac[R] E125 NeoResins Polymer Emulsion H RoShield[R] 3188 Rohm and Haas Polymer Emulsion I RoShield[R] 3275 Rohm and Haas Conventional Defoamers DREWPLUS[R] L-405 (Figure 14)

DREWPLUS[R] Y-250 Ashland/Drew Industrial FAPUR

Bahydrol[R] FT 145 Bayer MOD DREWPLUS[R] Y-250 Ashland/Drew Industrial SD Harcros 585

Harcros SD2 DREWPLUS[R] L-405 Ashland/Drew Industrial SD3 Sag[R] 5440 GE Silicones SD4 TEGO[R] Foamex 1488 Tego Chemie Service GmbH Silicone (Figure 12) Dehydran[R] 1293 Cognis SPR

Bayhydrol[R] D 270 Bayer UV-Curable Resin NeoRad[R] R-440 NeoResins </pre> <p>Ernest C. Galgoci, ([dagger]) Steven Y. Chan, and Khalil Yacoub -- Air Products and Chemicals, Inc.*

Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27-29, 2004, in Chicago, IL.

* 7201 Hamilton Blvd., Allentown, PA 18195.

([dagger]) Author to whom correspondence should be addressed. email: [e-mail].

Table 1 -- Properties of the MD1, MD2, and MD3 Molecular Defoamers and
TMDD

Property                              MD1      MD2

Activity, wt%                          100     100
Viscosity, cP, 25[degrees]C           2000     200 (21[degrees]C)
HLB (a)                                  4      <4
Water Solubility, %                      0.06    0.003
EST (b), mN/m, 25[degrees]C, 0.1 wt%    35.2    28.5
VOC solvents (neat material)          None     None

Property                              MD3        TMDD

Activity, wt%                         100 (c)    100 (c)
Viscosity, cP, 25[degrees]C           solid (c)  solid (c)
HLB (a)                                 3          4
Water Solubility, %                     0.03       0.1
EST (b), mN/m, 25[degrees]C, 0.1 wt%   34.1       33.1
VOC solvents (neat material)          None (c)   None (c)

(a) HLB = hydrophile-lipophile balance determined using the Water-
Solubility Method, "The HLB System," ICI Americas, Inc., 1992.
(b) Equilibrium surface tension (EST) at the surfactant concentration
listed in water was measured using the Wilhelmy plate method.
(c) MD3 was evaluated as a 32% solution in dipropylene glycol (DPG).
TMDD was used as a 50% solution in dipropylene glycol monomethyl ether
(DPM).

Table 2 -- Surface Adsorption Parameters for the MD1 and MD3 Molecular
Defoamers, TMDD, and a Series of Ethoxylated-Alcohol Surfactants at
25[degrees]C

                     [[GAMMA].sub.[infinity].sup.-1]  [beta]/[alpha]
Surfactant           ([[Angstrom].sup.2]/molecule)    ([cm.sup.3]/mol)

[C.sub.10][E.sub.0]  25.3                             7.7 x [10.sup.6]
  (22[degrees]C)
[C.sub.10][E.sub.4]  41.7                             4.3 x [10.sup.8]
[C.sub.10][E.sub.8]  54.1                             7.7 x [10.sup.9]
TMDD                 71.0                             1.8 x [10.sup.8]
MD1                  72.2                             1.3 x [10.sup.8]
MD3                  63.5                             1.4 x [10.sup.9]

Surfactant           K

[C.sub.10][E.sub.0]  -3.9
  (22[degrees]C)
[C.sub.10][E.sub.4]   2.5
[C.sub.10][E.sub.8]   9.6
TMDD                  0.0
MD1                   0.0
MD3                   0.0

(a) The ethoxylated-alcohol surfactants are denoted by
[C.sub.x][E.sub.y], where x is the number of carbons in an aliphatic
group and y is the number of repeat ethoxylate end groups
(-C[H.sub.2]C[H.sub.2]O-). For example, [C.sub.10][E.sub.0] is 1-
decanol.

Table 3 -- Initial Foam and Foam Decay Results for Aqueous Solutions of
Surfactants

                     Concentration     Q      [t.sub.f]
Surfactant           (mM)           (ml/min)  (min)

[C.sub.10][E.sub.0]  0.14           450       1
[C.sub.10][E.sub.4]  0.16           100       1
[C.sub.10][E.sub.8]  0.17           100       1
TMDD                 2.00            75       2
TMDD                 2.00           125       2
MD3                  0.12           100       2
MD3                  0.12           450       2

                     [f.sub.m]  df/dt
Surfactant           (ml)       (ml/sec)

[C.sub.10][E.sub.0]    0        NA
[C.sub.10][E.sub.4]  117        -0.17
[C.sub.10][E.sub.8]  115        -0.2 (a)
TMDD                   0        NA
TMDD                  49        -1.5
MD3                    0        NA
MD3                    0        NA

(a) Foam decay was nonlinear.

Table 4 -- Foam Results for Binary Mixtures of Foamy and Nonfoamy
Surfactants in Water

Foamy Surfactant/          Defoaming Surfactant/  [f.sub.m]  df/dt
Concentration (mM)         Concentration (mM)     (ml)       (ml/sec)

[C.sub.10][E.sub.4]        TMDD
                     0.16        0.03             118        -0.4
                     0.16        2.00             104        -1.5 (a)
[C.sub.10][E.sub.4]        MD3
                     0.14        3.6 x            120        (>180
                                 [10.sup.-3]                 sec) (a,b)
                     0.14        0.10              70        -1.3
[C.sub.8]S[O.sub.3]        TMDD
                     1.89        3.83              67        -1.2
                     4.1         3.82              72        -1.0
[C.sub.8]S[O.sub.3]        MD3
                     3.7         3.6 x             45        -1.2
                                 [10.sup.-3]
                     3.7         0.10              45        -1.7

(a) Foam decay is nonlinear.
(b) Foam persisted for >180 sec.
(c) Air was injected into the samples at a rate of 100 ml/min for 1 min.

Table 5 -- Appearance Ratings for the Drawdowns of the Polymer Emulsions
Containing the MDs (a)

Emulsion  MD1  MD2  MD3  Neat

E          9    9   10   10
F          7   10    8    7
G         10    8   10   10
H         10   10   10   10
I          8    8    8   10

(a) The ratings are from 0 to 10; a 10 indicates no defects. MD1 and MD2
were tested at 1% by weight, while MD3 was tested at 0.5% by weight due
to its low solubility. Neat indicates no MD added.