NEW SUPERWETTING SURFACTANTS FOR COATINGS APPLICATI ONS

C. Jim Reader and Jeanine M. Snyder - Air Products and Chemicals, Inc., USA Roger Reinartz - Air Products Nederland BV, The Netherlands

Ye Dai - Air Products and Chemicals (China) Investment Co. Ltd., China ABSTRACT The increasing use of new resin technology and non ferrous substrates to meet today’s product standards presents a significant challenge for formulators. Substrate wetting and recoatability are day to day issues while battling to improve surface appearance. Many flow and leveling agents as well as wetting agents do a great job but introduce a foam problem. Effective defoamers in such formulations provide craters and a vicious circle is hard to break. New, low-foaming optimized siloxane superwetting surfactants have been developed to provide premium equilibrium and dynamic surface tension reduction as well as improved flow and leveling for difficult to wet surfaces like wood, plastics and dirty or non ferrous metal. This paper discusses the importance of wetting, flow and leveling, and focuses on the superior performance of superwetting surfactants breaking the vicious circle.

INTRODUCTION Surface chemistry is responsible for wetting, flow and leveling of a coating, as well as other associated properties. When a liquid coating is applied to a substrate, it should wet it easily and evenly to ensure a good appearance and adhesion. The wetting of the substrate by the coating is most strongly influenced by the surface tension of the material and the substrate. The coating will exhibit both an adhesive (degree of association between the coating and the substrate) and a cohesive force to itself. For spreading, or good substrate wetting to occur, the coating must have a greater association with the substrate than itself. This is shown mathematically in Figure 1.

Figure 1. Adhesive and Cohesive Forces in a Coating When the coating-substrate system exhibits favorable surface chemistry, a desirable and well- functioning coating can be formed. If not, a myriad of defects and failure modes can occur, such as craters, crawling, picture framing, orange peel, Bénard Cells, cissing, pinholing, foaming, pigment flooding and floating [1-2], all of which can lead to other catastrophic deficiencies. Many coatings formulators have started to embrace waterbased technology as a means to meet various environmental and legislative demands, although formulating waterbased systems can pose many problems. Water has a high cohesive strength and high surface tension (72 mN/m) compared with most solvents; therefore additives are needed to reduce the surface tension of water to enable good substrate wetting. SURFACE ACTIVE AGENTS Surface active agents, or surfactants, are materials that can adsorb onto surfaces or at interfaces and lower the surface or interfacial free energies of aqueous formulated systems. Traditionally, surfactants have a characteristic molecular structure which includes a water-loving, hydrophilic head and an oil-loving, lipophilic tail as shown in Figure 2.

Figure 2. Building Blocks of a Surfactant

Because of their amphiphilic nature, surfactants will migrate to and accumulate at interfaces and reduce surface and interfacial tensions even when used at very low concentrations. Conventional surfactants have a polar or ionic, hydrophilic head group connected to a hydrophobic, hydrocarbon tail group. In contrast, a particular type of specialty surfactant structure, termed Gemini, has two hydrophobic tails connected to two hydrophilic head groups on the same molecule. As shown in Figure 3, each half of the “twin” surfactant is joined together by a spacer group, forming the Gemini structure. Because of their unique molecular architectures, these Gemini surfactants are much more surface active than their “monomeric” components [3]. Gemini surfactants are often used in waterborne coating, ink and adhesive formulations for both dynamic surface tension reduction and foam control.

Figure 3. Schematic Structure of Gemini Surfactant s There are many different interfaces in a coating formulation where surfactants can concentrate (Figure 4). Surfactants are still widely used to stabilize the resin emulsions used in waterborne systems and other surface active materials are used to disperse and stabilize pigments. However, for effective substrate wetting the surfactants must be active at the moving boundary between the liquid and substrate.

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Figure 4. Various Surfactant Interactions in a For mulation

Many surfactants can act to facilitate substrate and particle wetting by reducing the surface tension of the formulation to a level that is equal to, or lower than the substrate being coated. As the surface tension of water is 72 mN/m and typical coating substrates are in the realm of 35-45 mN/m, surfactants must be used to decrease the surface tension of the water-based paint to allow it to flow smoothly onto the substrate. Additionally, in order for wetting to occur, the contact angle, θ, must be less than ninety degrees; contact angles greater than ninety degrees result in beading of the coating onto the substrate. Figure 5 illustrates how choosing a surfactant that can lower the surface tension of the coating to below the surface energy of the substrate results in a final film with excellent wetting. In some cases, surface energies of substrates may be less than 35 mN/m and present a significant problem to the applicator. When this occurs, two pathways need to be followed. First, the substrate may be treated to raise the surface energy, making it easier for the coating to wet out the surface. When this is not a viable option, the formulator is then forced to look at a stronger set of surfactants, including superwetters, to achieve the low surface tensions needed to wet out the substrate.

Figure 5. Wetting Agents Facilitate Substrate and Particle Wetting γSA, γSL and γLA are Solid-Air, Solid-Liquid and Liquid-Air Inter facial Tensions

TRADITIONAL SURFACTANTS Surfactants can be classified in several ways. They may be nonionic, anionic or cationic in nature and the choice of which type to use is often determined by the application and other

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components in the formulation. The largest and most commonly used class of surfactants, the anionic surfactants, include chemistries like alkylbenzene sulfonates, fatty acid soaps and dialkyl sulfosuccinates [4]. While these surfactants offer excellent wetting at a relatively low cost in use, their main drawback is their tendency to foam in coatings thus requiring the formulator to include strong defoaming agents in his system which can lead to film defects and aesthetic issues. Nonionic surfactants, many of which are based on polyethoxylated materials, can be excellent wetting agents and emulsifiers [4]. This group contains the before mentioned Gemini surfactants and their derivatives as well as siloxane-based surfactants and fluorosurfactants. The final and smallest group, the cationic surfactants, is used less often in coatings applications as they will react with any anionic species in the system; however, they are sometimes used as solid particle dispersants or emulsifiers [4]. While all three types of surfactants can offer excellent wetting, they perform differently when subjected to dynamic conditions and exhibit different foam profiles. SUPERWETTING SURFACTANTS What happens when the surface tension of the substrate falls well below the typical 35 mN/m range where traditional surfactants are no longer effective? Such low-energy surfaces might include plastic, wood or even poorly prepared or oil-contaminated metal surfaces. This becomes even more challenging with lower VOC coatings, which are now formulated to well below 100 g/L and even down to 0 g/L, with less solvent to help flow, leveling, and appearance of the coatings. In such cases, the formulator may employ the use of superwetting surfactants to reach his desired level of wetting. Superwetting surfactants can be siloxane-based or organic in nature and they differ from traditional surfactants in several ways. Superwetters are surfactants with structures that allow them to efficiently adsorb and pack at interfaces, resulting in low dynamic surface tensions, fast wetting times and low contact angles on low surface energy substrates, as can be seen in Figure 6. It has been recognized that it is actually the compact structure of the surfactant hydrophobe that plays a major role in a molecule performing as a superwetter [5-9].

Figure 6. Wetting of a Traditional Siloxane Surfac tant vs. Organic Superwetter 0.1 wt% Aqueous Solution, Oily Metal Surface, 10 Se conds Wetting Time, 23°C

Traditional Siloxane Surfactant

Organic Superwetter

ORGANIC SUPERWETTING SURFACTANTS Many coating related processes, such as paint manufacture (e.g., pigment dispersion), mixing, and application, are high speed agitation processes, which disrupt the surfactant concentration and alignment at the surface, thereby causing higher surface tension during and after agitation, which is also known as dynamic surface tension. Surfactants that quickly diffuse back to the interface, align and lower surface tension have low dynamic surface tension, i.e., they provide low surface tension during and shortly after these dynamic agitating processes. They are effective and efficient at providing good wetting and leveling, and avoiding problems such as craters, fisheyes, and other defects. Organic superwetting surfactants are able to achieve reasonably low equilibrium and extremely low dynamic surface tensions; therefore, they are often excellent alternatives to traditional siloxane and fluorosurfactants. While siloxane and fluorosurfactants can achieve very low equilibrium surface tensions, their dynamic surface tension performance is much poorer because they are unable to quickly migrate to the newly created interfaces under dynamic conditions and maintain that low surface tension state. When used in a waterborne formulation, traditional siloxane-based and fluorosurfactants will migrate to the air-coating interface, as they tend to be organo-phobic. This may lead to re-coatability issues should a second layer of coating be applied. In contrast, organic superwetting surfactants maintain their low dynamic performance even during high speed coating processes. One other benefit to using organic superwetting surfactants is that they are generally more compatible with most coating systems, have no or minimal foam stabilization and present no issues regarding re-coatability that may be observed with the silxoane-based and fluorosurfactants. Figure 7 highlights the equilibrium and dynamic surface tension performance of some organic superwetters compared to siloxane-based and fluorosurfactants.

Figure 7. Dynamic Surface Tension (DST) Comparison of Aqueous Solutions Containing 0.1 wt.% Surfactant (Measured using a Kr üss BP-II Bubble Tensiometer)

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NEW OPTIMIZED SILOXANE BASED SUPERWETTING SURFACTAN TS Many waterborne resin systems suffer from application limitations that can only be effectively eliminated by using high quality surfactants and defoamers. In particular, poor substrate wetting, edge retraction, pinholing and other defects result in high product failure rates that are costly to the manufacturer. Choosing the right surfactant that provides effective wetting and leveling while minimizing foam improves the overall performance of the coating formulation. It is well known that siloxane-based surfactants have the ability to spread on surfaces and promote excellent flow and leveling, often exceeding the flow and leveling achievable with organic superwetting surfactants. However, it has also been shown that organic superwetting surfactants are capable of rapidly migrating to newly created interfaces to maintain low surface tension levels during application – a benefit not achievable with a traditional siloxane surfactant [10]. Recently, new optimized siloxane superwetters have been developed; these new products exhibit the outstanding spreading, flow and leveling usually associated with siloxanes as well as the low-foam, dynamic wetting benefits attributed to organic superwetting surfactants. Figure 7 compares the dynamic surface tension performance of the organic superwetters, a siloxane-based surfactant, a fluorosurfactant, and the new optimized siloxane superwetters. Because they possess both the siloxane and the organic functionalities, these superwetting surfactants provide superior flow and leveling and low-foam wetting. Additionally, these surfactants offer improved formulation compatibility and do not suffer the re-coatability issues that may be seen with traditional siloxane surfactants. When trying to coat a very hydrophobic surface such as plastic or wood, particularly when spray applying a coating, it is critical that the wetting package be able to perform while under dynamic shear and then again when the coating flows and levels across the substrate. The unique structure of these new optimized siloxane superwetters allows the formulation to effectively wet the surface of the substrate and flow evenly across the surface. Figure 8 illustrates how these new optimized siloxane superwetters hit the bulls-eye with respect to both equilibrium and dynamic surface tension reduction.

Figure 8. Equilibrium vs. Dynamic Surface Tensions for Select Surfactants

APPLICATIONS DATA PLASTIC COATINGS When coating hydrophobic substrates with very low surface energies, like plastic, perfect wetting is difficult to achieve particularly when spray applying the coating. Efficient dynamic and equilibrium surface tension reduction are key factors in affecting final wetting performance. The new optimized siloxane superwetters offer these benefits together with low foam and excellent compatibility, which can help achieve a perfect appearance as seen in Figure 9 and 10. A two component polyurethane coating crosslinked with isocyanate containing 0.2wt% surfactant was prepared and spray applied to respectively polypropylene panels and a silicone release liner at a 75 µm wet film thickness. Figure 9 and 10 show that with no surfactant the system completely de-wets. Using traditional siloxane-based surfactants also results in poor wetting and defects in the film as well. By incorporating the new optimized siloxane superwetters, excellent dynamic and equilibrium surface tension reduction is achieved resulting in excellent wetting with no defects.

Figure 9. 2K Polyurethane Waterborne Clearcoat App lied to Polyprolyne Panels

Figure 10. 2K Polyurethane Waterborne Clearcoat Ap plied to Silicone Release Liner

WOOD COATINGS Wood substrates are inherently difficult to wet out due to the porosity of the substrate, surface contaminates such as glue or sap, and irregularity in the surface roughness. As the coating flows over the surface of the wood, penetration of the coating into the substrate occurs; this can lead to surface imperfections from both inadequate wetting and foam generation. When the coating penetrates into the wood and wets out the grain, air is displaced which rises to the surface and can become trapped in the dried film. Selecting a wetting agent that offers excellent flow and leveling as well as foam control are critical for a high quality finish. Figure 11 illustrates a waterborne wood coating based on a polyurethane-acrylic hybrid polymer system that has been brush applied on red oak. When no surfactant is added to the system, severe orange peel is seen in the coating. In comparison, when incorporating Organic SW1 or a siloxane-based surfactant, the surface appearance is greatly improved. However, only Organic SW1 can migrate quickly to the newly formed interfaces during application and not only lower the surface tension but also eliminate any foam generated by the wetting of the substrate.The siloxane surfactant offers excellent wetting but stabilizes foam which is still apparent in the dried coating.

Figure 11. Polyurethane Acrylic Hybrid Coating App lied to Red Oak

To demonstrate the additional benefits seen with the new optimized siloxane superwetters, a second formulation was prepared and tested. This self-crosslinking acrylic emulsion based wood coating was prepared and spray applied onto wood at a wet film thickness of 100µm. As shown in Figure 12, compared to the siloxane-based surfactant and Organic SW1 surfactant, the new optimized siloxane superwetters 1 and 2, show improved crack filling and foam control when spray applied onto the wood surface. The ability of the new superwetters to migrate faster than conventional surfactants to problem areas to lower the surface tension and the ability of the new superwetters to then aid in the flow and leveling of the final coating is clearly apparent in Figure 12.

Figure 12. Acrylic Wood Coating Containing 0.2 wt% Surfactant Spray Applied on Wood

SELF LEVELING EPOXY FLOORING The multifunctional benefits and broad formulation latitude of these new optimized siloxane superwetters can be nicely demonstrated in a highly filled solvent-free epoxy/cyclo aliphatic amine self leveling flooring system. The original starting formulation had poor stability, appearance and suffered from pigment floating and pinholes, as can be seen in Figure 13. Various surfactants and superwetters were incorporated into the blank formulation (5min. @ 1000rpm) at both 0.2 and 0.8% dosage level. The self leveling flooring systems (10 x 10 cm) were applied on a plastic foil and after 15 min. air was introduced using a notched trowel (upper half of the samples in Figure 13).

Figure 13. Self Leveling Solvent Free Epoxy Floori ng Containing 0.2% Surfactant As can be clearly seen, siloxane-1 has poor leveling, suffers from pigment floatation and has a significant amount of pinholes. Siloxane-2 gave a viscosity increase and resulting poor leveling performance. Amongst all the evaluated additives only the new optimized siloxane superwetters gave a robust performance, independent of the dosage level and evaluated defoamer combinations. CONCLUSION The application of coatings, inks and adhesives on difficult-to-wet substrates such as porous wood, plastics, films and oily metal presents significant challenges for coatings formulators. A proper surfactant is critical to maximize wetting and minimize defects such as pinholes, edge retraction, and orange peel. New optimized siloxane superwetters have been developed to offer the formulator the ability to achieve low dynamic surface tensions with minimum foam generation while promoting excellent flow and leveling for a wide variety of coating systems. REFERENCES [1] Pierce, P.E., Schoff, C., Coating Film Defects, Federation of Series on Coatings Technology, Federation of Societies for Coatigns Technology, Philadelphia 1991. [2] Bierwagen, G.P., Prog. Org. Coat., 1991, 19, 59. [3] Reinartz, R., Reader, C.J., Sundaram, S. Lassila, K.R., New Gemini Surfactants as Paint Additives, 7th Nürnberg Congress, Volume 1, Paper III. 1, 217-229. [4] Salager, Jean-Louis, Surfactants: Types and Uses, FIRP Booklet #E300-A, UNIVERSIDAD De Los ANDES, 2002 [5] Hill, R.M., He, M., Davis, H.T., Scriven, L.E., Langmuir 10, 1994, 1724. [6] Goddard, E.D., Ananthapadmanabhan, K.P., Chandar, P., Langmuir 11, 1995, 1415. [7] Hill, R.M., He, M., Davis, H.T., Scriven, L.E., Langmuir 11, 1995, 1416. [8] Stoebe, T., Lin, Z., Hill, R.M., Ward, M.D., Davis, H.T., Langmuir 12, 1996, 337. [9] Rosen, M.J., Song, L. D., Langmuir 12, 1996, 4945. [10] Snyder, J.M., Marcella, P.C., “A New, Environmentally Friendly Wetting Agent for Architectural Coatings,” PCI, April 2011.