High-Performance Hybrid Acrylic Epoxy System for Water-Resistant Coatings

The global coatings market represents approximately 80 billion pounds (36 MM metric tons) of finished product worth USD 120 billion (1). The coatings market is typically divided into three segments: Decorative, OEM (original equipment manufacturer) and specialty applications. The growth of environmentally friendly coatings continues to the point where approximately sixty percent of all coating volumes today are water-based, powder or solvent-based coating alternatives. (1)

Figure 1. Global Coatings - By Technology Type

In the US coatings market, a blended approach has been successfully used for years to enhance the performance of traditional thermoplastic acrylic dispersions (for example; vinyl acrylic, pure acrylic or VAE). This approach involves a two-component (2K), acrylic-epoxy coating based on high acid functional acrylic resin and a water-based epoxy resin. Because there is a reaction between the acid functionality and epoxy groups, these coatings are cross-linked and therefore suitable for higher performance or protective maintenance applications. Compared to water-based or solvent-based acrylic paints, the two-component acrylic-epoxy coating offers enhanced resistance to food stains, industrial and household cleaners, and disinfectant solutions (for example, bleach - sodium hypochlorite). Hybrid acrylic-epoxy formulations provide lower odor, faster drying and easy cleaning with water (if needed, soap and water). Because these coatings are typically based on acrylic resin at 70-80 percent by weight, they have longer pot life compared to WB epoxy/amine coatings, and many painters believe this is "nearly the same as household paint." Water-based acrylic epoxy coatings can be used in schools, mosques, general toilets, dental and medical offices, routine maintenance and extended maintenance facilities. When modified with special additives that enhance antibacterial and antimicrobial resistance, they may be suitable for examination rooms, operating rooms and food preparation areas in hospitals. Coatings based on water-based acrylic/epoxy can be developed for concrete, walls, drywall, slag block, fiber-reinforced cement, wood, plaster, mortar or metal. Because the high acrylic resin content results in better resistance to yellowing and whitening compared to standard epoxy-amine coatings, hybrid acrylic-epoxy coatings may be suitable for interior and exterior applications.

Epoxy Emulsion Technology

Two-component, water-based acrylic-epoxy coatings are based on carboxyl-functional acrylic resin cross-linked with water-based epoxy emulsions. These epoxy emulsions are white, opaque liquids containing resin particles dispersed in water. These resin particles are stabilized with a surfactant (Figure 2). The average size of dispersed resin particles can range from approximately 0.3 to 5 microns. Epoxy emulsion viscosity typically ranges from less than 1,000 cP to 15,000 cP at 25°C.

Figure 2. Epoxy Emulsion

Figure 3. Potential Stability with Water-Based Emulsions

Emulsions can be developed using resin, surfactant and "high-speed mixer blades," but there are various stability issues that could potentially arise from this approach (Figure 3). By combining polymer chemistry with surfactants and manufacturing techniques optimized through colloidal science, raw material suppliers have developed epoxy emulsions with improved product stability. For example, if you compare the particle size distribution of a commercial emulsion to a self-produced version, the produced emulsion offers better product stability and consequently improved coating performance due to smaller average particle size, narrower particle size distribution, narrower solids range or tighter particles. Hexion recently developed a new epoxy emulsion offering more cost-effective and superior product stability and performance. It is specifically designed for use in WB acrylic epoxy coatings. EPI-REZ™ Resin 7510-W-60 is based on a standard Bis-A liquid epoxy resin (for example, EPON™ Resin 828). This emulsion is supplied at 60 percent solids in water (without co-solvent). This unique emulsion has a minimum product shelf life of 18 months (compared to 6-12 months in industry standards). The product is easy to use and is supplied with higher solids content (50-55 percent by weight) than alternative commercial emulsions. Typical property ranges for this unique emulsion are summarized in Table 1. Table 1. Typical Property Ranges for EPI-REZ Resin 7510-W-60* * Sold as EPI-KOTE™ Resin 7510-W-60A in Asia Pacific. One known limitation of acrylic-epoxy coatings is their tendency to cure more slowly compared to epoxy/amine coatings. This slow film hardness development is associated with the relatively slow rate of the acid/epoxy reaction at room temperature. To address this known limitation, a glossy white enamel formulation has been developed. This new formulation provides: • Improved early film hardness (overnight), • Superior film hardness (after 7 and 30 days cure), • Excellent rub resistance, • Higher gloss (70-80 units at 60°), • Lower VOC (122 g/L) and reduced odor compared to solvent-based alternatives. Using standard drying conditions (25°C, 50% R.H.), water-based acrylic epoxy coating yields good performance results (Table 2).

Table 2. Typical Performance Properties *

*Performance on cold-rolled steel (CRS) cured for 21 days at 25°C and 50% R.H. As a comparison, the application and performance properties of water-based acrylic-epoxy coatings are between a standard water-based acrylic and a standard epoxy-polyamide (Table 3).

Table 3. Comparison of Coating Technologies (-- very poor - poor + strong ++ very strong)

A Case Study from South America

Hexion worked with a local South American paint company to develop a practical alternative to solvent-based epoxy coatings used by hospitals in Colombia as a case study for this water-based acrylic-epoxy coating.

Use and Benefits

Health care facilities such as hospitals and government institutions, operating rooms, work areas and nursery areas requiring high levels of disinfectant, the local paint company identified that these advantages would be attractive to their customers: • Washable, resistant to rubbing, easy to clean, stain-resistant, • Light colors (white, green, blue) in the gloss range (between satin and enamel), • Resistant to household and strong chemical disinfectants, • Durable and low odor (VOC <120 g/L), • Fast drying and ready for reuse overnight, • Bacterial resistant (Escherichia coli, Staphylococcus aureus). Following the successful development of the customer's proprietary coating formulations, Hexion, together with representatives of this paint company, made a joint visit to a university hospital in Colombia where the company's water-based acrylic-epoxy hybrid coating had been applied to the walls and ceilings of one of the clinical procedure rooms. The hospital administrator reported that after four months of continuous use and daily cleaning with strong detergents, "the water-based coating performed well and looked very good." The same coating was also used in food preparation areas and various corridors, with the reason stated as "having low odor, drying quickly and eliminating the need to close the area for several days until the solvent odor disappeared." During a subsequent follow-up visit, we spoke with one of the senior nurses at the university hospital. Her comments were as follows: "My name is Jesus Lopez, and I work as a specialist nurse at Hospital Universitario del Valle (HUV). HUV is located in the southwestern region of Colombia and is the largest active public hospital. The walls in the brachytherapy area are washed daily with detergent and every Saturday are cleaned with 10 percent sodium hypochlorite. The entire wall is saturated and after being left for 30 minutes begins to rinse. We have had no complaints about the special water-based paint. After application it retained its resistance, did not fade, and requires us to check it twice yearly for standard microbiology. When examining the area with a Petri dish, samples were taken from the corners as well as from any location the official representatives wanted to sample. Despite having been in service for more than 2 years, no test conducted in this area has yielded a positive result for any bacteria."

Conclusion

This article presents an example of an improved two-component, water-based acrylic-epoxy hybrid coating. These hybrid coatings offer significant performance improvements over acrylic latex paints and are therefore suitable for high-performance coatings and light industrial maintenance coatings. Among the advantages of this new system are durability (tile-like surface), abrasion resistance, stain and chemical resistance. Additionally, because these hybrid coatings are predominantly acrylic-based, these coatings may be suitable for exterior applications due to their low yellowing rates. A new, low formulation cost epoxy emulsion designated as EPI-REZ Resin 7510-W-60 has been developed. This product has approximately 1000 cP viscosity, contains no organic auxiliary solvents and has minimum 18-month shelf life stability. A new starter formulation providing early curing has been developed. A real-world case study demonstrates that these water-based acrylic-epoxy hybrid coatings can be developed for healthcare applications. This case study has shown successful use for over two years in a hospital environment where daily cleaning with strong detergents is required. Although this formulation development and case study was conducted in South America, the green building trend and the driving force to reduce LEEDS and VOC content continues to show that acrylic/epoxy coatings are gaining popularity in the US, Canada and globally. Characteristics such as low odor, non-flammability, improved durability, lower yellowing and resistance to harsh cleaning programs make these WB acrylic-epoxy systems an excellent choice for making acrylic paints a high-performance, protective coating when needed. Zafer Arslan / Technical Sales Manager / Coatings, Inks & Adhesives Industrial Business Unit / IMCD Türkiye References 1. Kusumgar, Nerlfi and Growney (KNG), "Global Paint and Coatings: 2013-2018", Feb. 2014, pp. 2 & 14. 2. Kurt G. Olson "Environmentally Friendly Coatings: Historical Perspective and Future Outlook" PCI- Paint & Coatings Industry, Sept. 2016, p. 64.