Eliminating PFAS, while maintaining coating performance, requires unique binder chemistry that is both flexible and durable.
By , , and Arkema Inc.
Although exterior architectural coatings improve the appearance of a structure, they are ultimately intended to be protective, preserving the integrity of a substrate by isolating it from environmental exposure. As the regulatory landscape shifts, however, coating composition is becoming equal in importance to coating performance. Perfluoroalkyl substances (PFAS), defined in a 2022 Organization for Economic Co-operation and Development (OECD) report as “fluorinated substances that contain at least one fully-fluorinated methyl or methylene carbon atom (without any H/Cl/Br/I atom attached to it),” are an effective tool for improving flow, gloss, adhesion, and water- and stain-resistance of coatings, but their use has a negative impact on both human health and the environment. Eliminating PFAS, while maintaining coating performance, requires unique binder chemistry that is both flexible and durable. An all-acrylic waterborne latex for exterior architectural applications has been engineered without PFAS. This new, low-coalescent demand binder has improved surfactant leach resistance when compared to a similar PFAS-containing product, while maintaining other performance properties, such as block resistance and dirt pick-up resistance. Through creative polymer design, a more ecologically sound waterborne all-acrylic latex with improved exterior performance is possible.
Introduction
Although liquid suspensions of pigment were first used more than 30,000 years ago in a decorative capacity, the utility of paint as protection was not fully realized until the dawn of the Industrial Revolution, when the first paint and varnish factories began mass-production of readymade coatings.1 While the automotive industry introduced the need for anti-corrosive coatings, special-purpose coatings were eventually developed for other products and industries, including farm equipment, children’s toys, furniture, and food production.2 Historically, in addition to innovation, the coatings industry has recognized and responded to potential negative impacts of product chemistries on human health and the surrounding environment. For example, during the years leading up to World War II, as consumers began to fully comprehend the inherent health and environmental risks associated with a common paint ingredient, lead, paint manufacturers ultimately found a safer alternative, titanium dioxide. The use of lead pigments in consumer paints was limited in the United States by the 1950s and fully banned in 1978.3
As academic institutions, governmental agencies, corporations, and consumer groups began to acknowledge the extent of coatings’ impact on the environment in the early part of the 21st century, demand increased for eco-friendly solutions that both comply with environmental regulations and maintain product performance. Coating manufacturers first met these calls for change by either reducing or eliminating volatile organic compounds (VOCs) in solvent-based and water-based coating formulations, but recent regulatory developments and industry trends have led to more aggressive measures to reduce industrial environmental footprints.4,5 To continue to expand this $26.1 billion industry, development of new, high-performance coatings and a commitment to sustainable chemistry and manufacturing practices is now paramount.6
In February 2021, the U.S. Environmental Protection Agency (EPA) began developing the Fifth Unregulated Contaminant Monitoring Rule (UCMR 5) to provide new data on 29 perfluoroalkyl substances (PFAS) and to clarify their impact on community drinking water. Since then, the EPA Council of PFAS has been created to better understand, and ultimately reduce, the potential risks of PFAS. EPA has also released preliminary Toxic Release Inventory (TRI) data, enhanced TRI reporting requirements, and begun development of a national testing strategy for PFAS. Additionally, in June 2022, EPA released four drinking water health advisories for PFAS; as a direct result, $1 billion in Infrastructure Law grant funding has become available to address PFAS contaminants in drinking water. More recently, in January 2023, EPA proposed a rule that would prevent starting or resuming the manufacture, processing, or use of an estimated 300 PFAS that have not been made or used for many years, known as “inactive PFAS,” without a complete EPA review and risk determination.7
Earlier generations of acrylic latexes were created with the intention of meeting the consumer expectation that a fully formulated coating would contain no more than 50 g/L volatile organic content (VOC). Reduced VOC targets were readily met by latexes having lower glass transition (Tg) temperatures and minimum film-forming temperatures (MFFT), but their use resulted in softer films. Softer films were deficient in several performance areas, including surface feel, dirt pick-up resistance, block resistance, and stain removal. Surfactant leaching was also observed. Surfactant leaching is defined as the unsightly staining caused by coating components migrating through the film and streaking down the coated surface. This defect is unique to lower-VOC exterior coatings, particularly when a deep tint coating is applied in low-temperature and high-humidity environments.
Improved surfactant leach resistance was identified as a product improvement target. Built on an exterior acrylic platform recognized for its durability and color acceptance, a new polymer was designed specifically to resist surfactant leaching. While surfactant leach resistance was successfully achieved through polymer design, certain performance properties were compromised. Block resistance, in particular, was found to be deficient. Performance improvement was realized, in part, through the incorporation of a specific class of PFAS, fluoropolymers, into the polymer recipe. Fluoropolymers have non-wetting and non-sticking properties, and are a well-established way to introduce block resistance.8 The release of the PFAS Strategic Roadmap by EPA in October 2021, however, indicated a dramatic shift in the regulatory landscape.9 In response, an improved exterior all-acrylic latex produced was developed to exceed the performance of its PFAS-containing predecessor, without the incorporation of PFAS.
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