Interview: “Some challenges remain
What are the remaining challenges in thetransition from solvent- to waterborne coatings?
Daniel J. Weinmann: Over the past few decades, waterborne epoxycoatings have significantly improved their capability to deliver epoxyperformance in real world applications. For commercial coating formulations,the solvent content in waterborne epoxy coatings has dropped from 150 g/L VOCto less than 50 g/L VOC: in some cases, even approaching zero VOC levels. Evenat ultra-low solvent content, these waterborne epoxy formulations providesuperior adhesion, higher chemical resistance, and excellent corrosionresistance. Although waterborne epoxy performance has advanced significantly overthe past 40 years, there are some challenges that remain when transitioningfrom solvent- to waterborne coatings. These challenges fall into two primary categories: formulation development and applicationproperties.
Informulation development, the main challenges are robustness, cost and componentstability. Waterborne epoxy coatingstypically require multiple additives to overcome specific hurdles related tothe presence of water as the carrier; specifically, high interfacial tension(requires wetting and dispersing agents, flow and levelling agents) and thepotential for accelerated corrosion (flash rust inhibitors). The choice of additives and their use levelis critical to develop a coating formulation that has acceptable componentstability, good application properties and superior film performance.Optimizing the additives in these formulations may increase the time (andcosts) needed for coatings development.
Regardingapplication properties, waterborne epoxy coatings are challenged when theend-use requires application at lower temperatures and higher relativehumidity. Under these conditions, the reactivity and drying of the two-partepoxy/amine system slow down significantly, and the high relative humiditymakes it more difficult for the water to evaporate from the coatings film. Some ways to address this issue are to useheaters to warm the substrate or to use warm air flow to drive the release ofwater from the coating film.
Are there any developments that could expandthe areas of application of water-based coatings?
Weinmann: To protect the environment, improveworker safety and provide longer service life, coatings manufacturers aredeveloping waterborne epoxy systems that deliver higher performance, fasterreturn to service and superior film formation without any added co-solvents.With ultra-low VOC epoxy systems, formulators can take their waterborne epoxycoatings all the way towards zero solvent emissions. Another development areais to explore ways to reduce the cost of waterborne epoxy coatings. Theseoptions include formulating acrylic/epoxy hybrid coatings for institutionalapplications; as well as, selecting waterborne amine hardeners that offer loweruse levels. For example, “Epikure Curing Agent 8530-W-75” has a use level of only24 PHR (parts per hundred weight resin).
Anotheroption to reduce cost is to select a more economical epoxy dispersion, or aliquid epoxy resin emulsion. When choosing to reduce cost, the resultantperformance may be somewhat reduced. However, for light duty or medium dutyapplications, these lower cost options can still meet the minimum performancerequirements depending on the service environment.
What is R&D in water-based coatingscurrently focusing on?
Weinmann: Our goal is to help the coatings industry makea successful transition to solvent-free, waterborne coatings; and by workingtogether, we can achieve this vision with our customers. The developmentsmentioned above are fully commercial so now, our R&D groups are focused ondeveloping waterborne epoxy coatings for more challenging applications. Theglobal R&D labs are working to improve lower temperature cure, to increaseproduct stability and to develop new waterborne epoxy dispersions and aminehardeners that are easier to formulate, as well as, to improve adhesion toultra-smooth steel, wood, and other difficult substrates. For improvedperformance, there are initiatives to develop very high corrosion resistanceand higher chemical resistance, waterborne epoxy systems.More information about expoxy resins are included in our refenrce book “Epoxy resins” in the European Coatings Webshop