Water-Based Paint Technologies

Water-Based Paint Technologies According to the EPA definition, volatile organic compounds or VOCs are organic chemical compounds whose compositions can evaporate under normal indoor temperature and pressure conditions. Because of their low boiling points, they easily evaporate and pose hazards in nature and industry (Web 1). Today, water-based paints contain significantly fewer chemical gases that could harm the environment and human health compared to other paint formulations, which is why they are prioritized in industrial applications due to current legal requirements or pending regulations. Water-based resins are receiving increasing attention because they are non-flammable and non-polluting during production and transportation processes (Jiao et al., 2021). When properly engineered and applied, water-based paints can provide the same end-use properties as solvent-based counterparts (UV resistance, abrasion resistance, and moisture resistance). Resins, which are the primary input of paint, are fundamentally structured as emulsions/dispersions in water-based paints. Emulsion polymerization, which is a heterogeneous free radical polymerization, consists of emulsifying a relatively hydrophobic monomer with low water solubility using surfactants (o/w) and then initiating the reaction either with a water-soluble initiator (e.g., sodium persulfate (NaPS)) or an oil-soluble initiator (e.g., 2,2′-azobisisobutyronitrile (AIBN)). Ionic, non-ionic surfactants, or colloids (for example, hydroxyethyl cellulose, polyvinyl alcohol, polyvinyl pyrrolidone, and dextrin) are generally necessary to stabilize this system; otherwise, latex particles nucleated at the beginning of polymerization may coagulate to reduce interfacial free energy. Stabilization can be electrostatic, steric, or both. In emulsion polymerization used for producing polymeric materials, the level of surfactant is generally kept to a minimum for water and adhesion resistance. However, if insufficiently stabilized, they lose colloidal stability during aging, coagulate, and form large structures. Emulsion polymerization performed without surfactants yields monodisperse latex products with large particle sizes and excellent water resistance and adhesion properties (Chern, 2008). The surface activities of surfactants vary according to their hydrophilic/lipophilic balance (HLB) values, which depend on the molecular structure of the surfactants. The HLB value is generally a measure of water and oil solubility in non-ionic surfactants. These values range from 1 to 20, with low HLB values generally being good for water-in-oil emulsions, while hydrophobic surfactants with high HLB values work effectively in oil-in-water emulsions. The ranges of HLB values of surfactants for various applications are shown in Table 1 (Heilen, 2014, Myers, 2020). Film formation is a process in which the paint/coating transforms the wet film into a solid polymeric structure through water evaporation after application (Figure 1). During this process, particle mobility decreases as the structure transitions from heterogeneous to homogeneous (Brownian Motion) (Dihang et al., 2008). Coalescence occurs when forces supporting film formation such as van der Waals and capillary forces are greater than repulsive forces such as electrostatic repulsion. Proper film formation is necessary for the coating to fully develop its mechanical properties. When the application temperature falls below the minimum film formation temperature (MFFT), emulsion polymers cannot form a proper film, and consequently final performance properties cannot be achieved. Furthermore, it can create adverse effects such as cracking, low gloss, poor adhesion, and decreased water resistance. While some softening of the polymer may overcome these problems, it can in turn create adverse effects on mechanical properties. Another approach is to add a small amount of high boiling-point organic solvent to the formulation. These solvents are located between the water phase and polymer particle phase. By remaining in the latex structure and lowering the minimum film formation temperature, they improve the film formation process through their plasticizing effect. The plasticizing effect of solvents depends on the solubility parameters of the solvent and polymer. The smaller the difference between the solubility parameters of the solvent and the emulsion polymer, the higher the rate of solvent migration to the polymer phase. The portion remaining in the water phase improves the paint's spreading properties by reducing the evaporation rate of water. However, despite the properties it provides, it should be noted that solvents added to paint reduce the drying speed of the paint and increase its VOC value. While solvent-based systems can use numerous solvents with different evaporation heats and boiling points, water-based coatings can use only a limited number of solvents (Heilen, 2014). Common solvents used in water-based paints include 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (texanol), ethylene glycol monobutyl ether (butyl glycol), and methoxypropanol (propylene glycol monomethyl ether). Solvents with different solubility and boiling points can be used together for an optimal formulation (Chern, 2008, Ludwig et al., 2007). One of the main concerns in water-based paints is foam formation, which can occur during production, during transportation/shipping, or during application, and this reduces paint quality. This is due to the high surface tension at the water-air interface and the stabilization of air bubbles formed by surfactants in the formulation. In this case, adding a small amount of foam control agents to the system will also be beneficial in terms of the surface quality to be obtained from the final product. Defoamers are relatively hydrophobic surfactants. Defoamers work by replacing the surfactants that stabilize foam, causing the air bubble to break apart, or by replacing the surfactants that stabilize foam by spreading in a lens-like manner at the air-water interface. Typical mineral oil-based or silicone-based defoamers have low HLB values. Silicone-based defoamers are the most active; however, since they are incompatible with the system, they must be added using high shear forces, and when used in high amounts, they can cause surface defects such as fish-eyes and craters (Chern, 2008, Heilen, 2014). Another important criterion for water-based paints is surface wetting because successful surface wetting is the first step in proper coating. Surfaces with low surface energy or those contaminated with oil and dirt are the hardest surfaces to wet with water-based paints. Improperly wetted surfaces suffer from problems such as crawling, cracking, and poor adhesion. The smaller the angle formed when a drop of water-based paint is applied to a surface, the more successful the wetting (Figure 2). General rules for surface wetting are as follows: 1. Wetting of high surface energy solid surfaces is easy, 2. Water-based paints with low surface tension wet surfaces more effectively, 3. The best wetting occurs when the surface energy of the paint/coating material is lower than that of the surface. The reason wetting is difficult in water-based paints is due to the fairly high surface tension of water (72.8 dyne/cm). Water has a dipole moment and is a relatively small molecule, and there is high attraction between its molecules. This causes high surface tension. There are two things that can be done to ensure proper wetting of the surface. One is to increase the surface energy of the surface by cleaning oil and dirt and/or performing surface treatments (acid washing, caustic washing, etc.). The other is to reduce surface tension of water-based paints by using surface wetting agents. Similar to conventional surfactants, wetting agents contain both hydrophilic and hydrophobic components. Hydrophilic components are mostly ionic or non-ionic polyethylene glycols; hydrophobic components are hydrocarbon chains. Wetting agents containing fluorine groups or polysiloxane chains are quite effective in reducing surface tension of water-based paints at low concentrations (Chern, 2008, Davison et al., 2003). When a latex product freezes, ice crystals tend to phase separate from the colloidal system, and as a result, the concentration of polymer particles in the liquid phase continues to increase as the freezing process progresses. Subsequently, phase change occurs and the likelihood of polymer particles coagulating greatly increases. This is more applicable to polymers whose glass transition temperature (Tg) is below the freezing temperature. In water-based paints, slow freezing creates larger ice crystals and thus produces more adverse results compared to rapid freezing. Acrylic acid/methacrylic acid copolymers (<3%) and non-ionic surfactants added in small amounts to the latex improve freeze-thaw capacity. Hydrophilic co-solvents such as ethylene glycol, propylene glycol, and glycerol also improve freeze-thaw stability but affect drying time and mechanical properties. Freeze-thaw stability is generally measured by, for example, freezing at approximately -12°C for 16 hours and then keeping at approximately 24°C for 8 hours. Commercially available water-based paints are expected to undergo the freeze-thaw cycle mentioned above 3 to 5 times (Chern, 2008). Paint formulation development is a balancing act. Surfaces coated with paint are generally expected to have satisfactory physicochemical properties. The desired properties from the paint to be developed (surface appearance such as matte/gloss, scratch/abrasion resistance, water and chemical resistance, etc.), application method, and environmental conditions should be considered, the parameters mentioned above should be taken into account, and good balance should be established among them to create an optimal formulation. From the perspective of a paint developer, a simple formula is ideal. However, since the desired properties cannot be developed without additives, an increase in the number of inputs in the formulation is likely. At this point, the formulator must examine the inputs selected in multiple ways. Kanat Paints & Coatings has established a specialized team for its water-based division and is specializing in water-based industrial paints and meeting industry demands.

References

• Chern, C. S. (2008). Principles and applications of emulsion polymerization. John Wiley & Sons. • Davison, G., Lane, B. C., & Lane, B. (Eds.). (2003). Additives in water-borne coatings (No. 290). Royal Society of Chemistry. • Dihang, H., & Brunel, L. (2008). Film formation analysis by optical methods. Technical Update, Surface Coatings International, Issue, 1-4. • Heilen, W. (2014). Additives for waterborne coatings. In Additives for Waterborne Coatings. Vincentz Network. • Jiao, C., Sun, L., Shao, Q., Song, J., Hu, Q., Naik, N., & Guo, Z. (2021). Advances in waterborne acrylic resins: Synthesis principle, modification strategies, and their applications. ACS omega, 6(4), 2443-2449. • Ludwig, I., Schabel, W., Kind, M., Castaing, J. C., & Ferlin, P. (2007). Drying and film formation of industrial waterborne latices. AIChE Journal, 53(3), 549-560. • Myers, D. (2020). Surfactant science and technology. John Wiley & Sons. • Web1 https://www.epa.gov/indoor-air-quality-iaq/technical-overview-volatile-organic-compounds   Dr. Suna Koçyiğit Senior R&D Engineer Water-Based Paints Kanat Paints & Coatings