Molecular defoamers Resolving stability and compatibility problems. Wim Stout, Christine Louis. Though surfactants allow waterborne coatings to wet low surface energy substrates even in high-speed applications, they often stabilise foam. Traditional defoamers can cause surface defects or storage stability problems. New silicone-free defoamers eliminate foam and microfoam by attacking foam stabilisation on a molecular level, minimising side-effects and reducing the total additive package needed. Conventional defoamers, which operate via an incompatibility with the water phase, provide good defoaming properties in waterborne coatings but often generate unwanted side effects such as surface defects, poor recoatability and a loss of efficiency after relatively short storage periods. Using further additives to overcome these problems may adversely affect foam knock down. Molecular defoamers are a new type of multifunctional additive, which have the potential to simplify the challenges of water-based formulations. Technical studies have been carried out in various types of systems to illustrate the multifunctional behaviour of molecular defoamers. The results highlight enhanced wetting and defoaming characteristics, while reductions in minimum film formation temperature (MFFT), enhancement of gloss and improved flow and levelling were also observed. Changing the nature of foam-breaking Molecular defoamers are a new class of defoamers which combine foam control and dynamic wetting. The name derives from the fact that the defoamers are surface-active agents, which break foam at a molecular level rather than through incompatibility. As described in foam stabilisation theory [1], foam-stabilising components such as wetting agents, dispersants, emulsifiers and soluble resins stabilise foam due to one or a combination of effects including ionic forces, hydrogen bonding and Van der Waals forces. Molecular defoamers act to destabilise foam by disrupting these forces, and in consequence cause the foam to collapse as shown in Figure 1. At the foam interface, molecular defoamers reduce the film elasticity of bubbles to prevent their stabilisation. They also reduce the surface viscosity of the foam lamella and increase the rate at which liquid drains out of the bubble films by interrupting the even packing of the foam stabilising components. These combined effects enhance their defoaming efficacy. In addition, molecular defoamers are easily incorporated into aqueous systems, eliminating the need for high shear forces, which can entrap air during their incorporation. Conventional defoamers have an optimum particle size which ensures best performance. However, over time their particle size may increase due to incompatibility (the worst case will be complete separation) or may decrease as they become more compatible. In either case, they will lose their effectiveness in controlling foam over time. By contrast, since molecular defoamers work at a molecular level and do not rely on particle size, they remain effective in the system indefinitely. Benefits from surface tension reduction By the nature of their structure and chemistry, molecular defoamers are free to migrate in the system and strongly reduce many foam stabilisation mechanisms [2]. Molecular defoamers are also capable of lowering the surface tension, which is required to enter and penetrate the foam lamella for foam destabilisation. They are surface active, and are also effective in reducing both the equilibrium surface tension (EST) and dynamic surface tension (DST), providing good wetting properties, especially under dynamic conditions. Thus molecular defoamers can be regarded as foam control agents with additional wetting capabilities. A series of different molecular defoamers has already been developed and tests on these products, which are referred to as AD01, MD20, DF110C, DF110D, AE01, AE02 and AE03, are reported here. It is important to note that molecular defoamers not only prevent foam formation but also provide very good surface tension reduction of water both under equilibrium and more especially under dynamic conditions. Table 1 summarises the surface tension and Ross-Miles foam test data of aqueous solutions containing several different molecular defoamers. Surface tension was measured by the maximum bubble pressure method and the foam data was generated according to the standard Ross-Miles method ASTM D 1173. The combination of antifoaming, defoaming and wetting performance is a strong advantage with respect to limiting surface defect formation, allowing coatings formulators to simplify the additive package. The following discussion illustrates the multifunctional performance of the molecular defoamers in several applications. Waterbased UV wood lacquer: improved appearance The molecular defoamer AD01 was tested against a silicone defoamer in a water-based UV-curable wood lacquer. As shown in Table 1, the chosen molecular defoamer provides no foam stabilisation and the reduction in DST also suggests that this product may improve the aesthetic appearance of the coating once applied on white oak. In Figure 2 it can be seen that this molecular defoamer demonstrated defoaming control similar to that of the conventional silicone defoamer without creating any surface defects. In addition, it enhanced the aesthetic appearance of the wood coating due to the reduced dynamic surface tension. It should be noted that the new product was used at only half the concentration of the silicone defoamer in this particular formulation, thus helping to reduce costs. Furniture coatings: Reducing coalescents and the MFFT The emulsifiers present in the polymer dispersions used for water-based wood coatings can stabilise foam. These formulations are often applied by brush, roller or spray, which may generate significant foam during application and thus require the use of defoamers. Unfortunately, defoamers are often difficult to dose, affect gloss, flow and levelling and can even generate surface defects such as craters or fish-eyes due to their incompatibility. Different types of surfactants were tested in a clear furniture lacquer based on an acrylic dispersion in order to compare the defoaming capabilities of molecular defoamers with different surfactant chemistries. Figure 3 shows that the formulation containing the molecular defoamer AE03 has the most efficient foam control compared to the model formulation and formulations containing other chemistries such as acetylenic diol (TMDD), silicone, fluoro or branched-alcohol ethoxylate (BAE). The wetting performance was also studied. It was observed that the molecular defoamer AE03 as well as AE01 and AE02 aid film coalescence. These products have a strong effect on the minimum film formation temperature (MFFT) of a variety of polymer dispersions including a urethane-acrylic hybrid (A) and three acrylic dispersions (B, C, and D). Figure
4 shows the effect of these molecular defoamers when used at 2% by weight on the liquid polymer dispersions, without any additional coalescent. Thus in addition to the foam control and dynamic surface tension reduction (as already shown in Table 1), the AE-type molecular defoamers improve film coalescence, allowing for partial or complete coalescing agent removal and thus reducing the total VOC level of the formulation. These products are suitable for a wide variety of water-based and other systems but stability at ph extremes may be formulation-specific. PSA's: Defoaming without loss of adhesive properties Molecular defoamer MD20 was tested in an acrylic-based pressure sensitive adhesive (PSA) formulation for label application in order to eliminate foam stabilised by an anionic wetting agent present in the formulation. The foam arising from the anionic surfactant can be controlled by the use of traditional defoamers such as silicone. However, the incompatibility of such defoamers often generates craters in the adhesive film and the foam control deteriorates over time. Figure 5 shows a picture taken after agitating three PSA formulations containing the anionic surfactant at 0.75% by weight on total formulation and compares the effect on foam control of the molecular defoamer MD20 with a silicone based defoamer. The silicone defoamer is more efficient than the molecular defoamer in eliminating foam. However, as mentioned above, silicone defoamers generate craters in the film because of their incompatibility and reduce adhesion to the substrate. They are therefore rarely used in PSAs. It is also important that surfactants and defoamers do not adversely affect the adhesive properties of the formulations such as peel, tack and shear strength. Figure 6 shows that the molecular defoamer had little effect on these three key properties. It provides similar peel and tack strength and slightly superior shear strength (20% improvement) when compared to the same formulation containing only dioctyl sulfosuccinate (DOSS) as the surfactant. Automotive refinish: Additive packages can be simplified The shift from solvent-based to water-based automotive refinish basecoats has forced a different approach in formulating technology. Newly developed single or 2 component resin systems may contain foam stabilising ingredients which increase the need for foam control. The use of traditional defoamers is known to generate surface defects especially in low film thickness applications. Flow and levelling agents, which are used to decrease the adverse effects of traditional defoamers, often seriously influence the recoatability of the base coat by creating surface defects when a solvent-based automotive repair clearcoat is applied over it (see Figure 7). Molecular defoamers such as AE01 can replace both traditional defoamers and flow and levelling agents, and can provide defect-free foam control without influencing the recoatability of the basecoat, resulting in improved appearance. Such multifunctional problem solvers allow formulators both to enhance products and to simplify the additive package. Because these novel molecules defoam at the molecular level, they eliminate the longevity issues and accompanying defects commonly associated with traditional defoamers. The wetting properties, not present in traditional defoamers, enhance film appearance and good wetting performance is maintained under dynamic conditions. These unique chemistries allow for reduced use levels and simplification of the total additive package, helping formulators to create water based alternatives for solvent based coatings, inks and adhesives. Due to their high compatibility within the system, molecular defoamers can also be combined with traditional defoamers to optimise both overall use level and performance. Acknowledgements The authors wish to thank Roger Reinartz, Samir El Ajaji, Wilco Chaigneau and Yvonne Lavrijsen for their contributions to the results reported in this paper References [1] S. Y. Chan, C. Louis, New additives for water based coatings: a new class of defoamers is born, Eurocoat 2003, Lyon, France, September 23-25, 2003. [2] R. Reinartz, J. Reader, S. Sundaram, K. Lassila, New gemini surfactants as paint additives, 7th Nüremberg Congress, Nüremberg, Germany, April 7, 2003. Results at a glance - A range of new silicone-free molecular defoamers combines dynamic wetting with strong defoaming properties. - The mechanism by which they attack foam is fundamentally different from that of traditional defoamers: in particular, effectiveness is not dependent on particle size. - These products avoid the problems of surface defects or limited storage stability often associated with traditional defoamers. - Molecular defoamers attack the foam stabilisation mechanisms provided by surfactants and other formulation components, are easy to incorporate, retain their efficiency for long periods and improve surface appearance. - The new products can be combined with traditional surfactants or defoamers in order to enhance their performance, while reducing the total additive package needed. The authors: -> Christine Louis obtained a masters degree in Materials Sciences at the University of Nantes (France) in 1998. She joined Air Products and Chemicals, Inc. in March 1999 as a Laboratory Technician, since January 2001 she works as an Application Development and Technical Service Chemist. -> Wim Stout has over 14 years of experience in developing coatings and tinting systems as a paint chemist for Motip BV and Air Products and Chemicals, Inc. He works as a Senior Application Development and Technical Service Chemist at Air Products. Molecular defoamers are truly multifunctional Molecular defoamers provide multiple benefits in many formulations and in many different applications such as UV curing wood coatings, furniture coatings, adhesives and automotive coatings. These products control foam generation during production and application, irrespective of the application method used, because their surface-active character provides additional wetting effects.
Figure 1: Defoaming mechanisms compared. Conventional defoamer (left) utilises incompatibility to break the surfactant-stabilised film that forms the surface of bubbles; molecular defoamer (right) disrupts film through surface activity.
Figure 2: Performance of molecular defoamer AD01 (right) compared with silicone defoamer (left) in a UV-curable wood lacquer. Coatings defects appear as white marks in the pictures.
Figure 3: Comparison of the density after agitation of a wood lacquer formulation, with various additives present at 0.75% by weight. Key: AE03 = molecular defoamer; TMDD = acetylenic diol; BAE = branched alcohol ethoxylate.
Figure 4: MFFT (minimum film forming temperature) measurements on molecular defoamers in various polymer dispersions.
Figure 5: Foam control agitated PSA formulations containing different additives (DOSS = dioctyl sulfocuccinate).
Figure 6: Comparison of the effect of additives on a PSA: molecular defoamer MD20 gives adhesive properties similar to those with only a conventional surfactant (dioctyl sulfosuccinate, DOSS) in the formulation.
Figure 7: Spray applied water based automotive repair basecoat, showing difference in appearance between use of silicone-based defoamer (left) and molecular defoamer (right).
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