High-Performance Waterborne Coatings for Roofing
By Eric Bennung, Acrymax Technologies Inc.
Roof coatings have evolved dramatically over the past 100-plus years.
The following formulation was listed in The Tinsmith’s Helper and Pattern book by H.K. Vosburgh from the late 1800s as a good remedy “to stop a leaky roof”: “25 lbs. yellow ochre, 1 lb. litharge, 6 lbs. black lead, 1 lb. fine salt; boil well in oil. Soak strips of cloth in the above and paste over the seams. Good where solder is not practicable.”
Note the litharge (red lead), the black lead and the fact that it is recommended to be boiled well in oil. The mixture undoubtedly would solve the leak, but today we would cringe at the thought of the environmental and safety concerns.
This formulation would not be what we would think of as a roof coating in the modern sense, but it and other similar concoctions were the forerunners to the modern liquid-applied products that we now utilize to repair and revitalize roofs.
In the late 1970s, the game-changing technology of waterborne 100-percent acrylic elastomeric polymers was introduced to the roofing market by the Rohm and Haas Company. To understand the evolution of this development, we need to step back as, until this point, the fluid-applied roofing market was dominated by bituminous and solvent-based rubber coatings.
In the early 1950s, the roof coating market took a major leap forward with the introduction of polymers as the foundation for formulations. The shortage of natural rubber associated with World War II led to an explosion in the development of synthetic polymers. Some of these polymers—initially neoprene and then chlorosulphonated polyethylene (CSPE)—found their way into new coatings for roofing, revolutionizing the fluid-applied roofing market. They were flexible, fully conformable coatings that allowed for previously unattainable roofing solutions, but they did have their drawbacks: low solids and, most importantly, high solvent content.
Application of a solventborne neoprene coating on a plywood roof in the 1950s. Black and gray were the only colors available.
In 1970, the creation of the Environmental Protection Agency and passage of the Clean Air Act created federal government programs aimed at meeting public demand for improved air quality. One of the main contributors to air quality issues are volatile organic compounds. They react with nitrogen oxides to create smog and ground-level ozone. In 1966, when Rule 66, the Californian predecessor to the Clean Air Act of 1970, was issued by the Los Angeles County Air Pollution Control District (APCD), it was estimated that more than half of the area’s solvent emissions were attributable to the application of architectural coatings.
This era of environmental consciousness ushered in a transition away from solvent-based coatings. However, the early waterborne roof coatings severely lagged in performance when compared to their solvent-based counterparts. They were essentially thinned-down, heavily-plasticized acrylic caulking formulations that had a range of problems from embrittlement to severe dirt pickup as they aged. The new regulatory requirements forced innovation within the waterborne realm and eventually led to new water-based coatings that equaled or outperformed their solvent-based predecessors. VOC reductions from 500-plus grams per liter (g/L) to under 100 g/L were achieved. While the main aim was compliance with the law through VOC reduction, additional benefits were also attained: higher solids and easier application.
Completed while the VOC laws were being phased in during the 1980s, a reflective roof coating application on the Chicago Bulk Mail Center highlights the improvements waterborne coatings were offering. This project involved roughly 500,000 square feet of roofing surface and was completed with a solvent-based CSPE rubber coating. To complete this project, 7,500 gallons of the rubber coating were required—of which 5,625 gallons were highly volatile solvent. Conversely, if the project had been completed with the relatively new acrylic waterborne technology, then it would have required only 3,750 gallons of acrylic coating to achieve the same dry film thickness, and less than 200 gallons of that would be VOC.
The reductions in VOC emissions earned the coatings industry praise not only for its compliance (that was mandatory), but also its willingness to take to heart the spirit of the regulatory laws. In a 1968 review of their Rule 66, the LA County APCD commended the National Paint, Varnish and Lacquer Association for its active participation in the program’s creation.
Coatings manufacturers continue to do their part to improve their products for both better performance and lower environmental impact. The national EPA limit for roof coatings is 250 g/L, while California—the original driver of the clean air laws from late 20th century—has pushed VOC limits to less than 50 g/L. One important note: Compliance with the 250 g/L limit, or even the lower 50 g/L limit, does not necessarily mean that the coating is waterborne, nor does it mean it is low-odor. There are many solventborne coatings in use today that achieve compliance but still have noxious odors and safety concerns. These products fail to offer a user-friendly experience.
Spray application of a waterborne 100-percent-acrylic elastomeric roof coating.
The push toward waterborne coatings had other major benefits, but none more evident than ease of application. They offer excellent functionality and are non-hazardous, making them safe and easy to apply. There are no harsh odors or solvent exposure for workers to contend with, leading to more pleasant and safer working conditions. With roofing, the devil is in the details, and while any fluid-applied roofing system lends itself to addressing details well, applicators working with waterborne coatings are more willing to work closely with the products, especially at penetrations and other critical roofing junctures.
Over the past three-and-a-half decades, waterborne roof coatings have become an industry norm, and within the waterborne coatings category, there is certainly a subset that attain a high-performance level. But what sets them apart?
The answer is: It depends on the purpose of the roof coating. Goals for a roof coating project can range from waterproofing and leak prevention to more aesthetic concerns such as color and gloss retention. There generally is not a silver-bullet coating that will solve all aspects of your roofing project. Most successful coatings projects center around the idea of combining different sets of materials, each with its own purpose, to create an overall system that has functional layers to achieve the overall goals of the project. Roof coating manufacturers design specific primers, reinforcement, flashing compounds, intermediate coats and topcoats for this very purpose.
Flexibility and Long-Term Performance
One of the most crucial characteristics needed for high performance is flexibility. The world is a dynamic place—temperatures on roofs can fluctuate from well below freezing to in excess of 180 degrees Fahrenheit, not to mention thermal shock when a summer thunderstorm rolls across an area, dropping 50- or 60-degree water on a roof surface that has been baking under the late afternoon sun. The tremendous stress that the dramatic and rapid temperature change causes will test the durability of even the best systems.
Low-temperature flexibility is tied to a property of the polymer within the coating called the glass transition temperature, or Tg. This is the temperature at which a polymer ceases to be flexible but instead would shatter brittlely like glass. With most high-performance roof coatings, Tg is desired to be very low (-15 F or lower). This ensures that the roof coating will not crack or fail when subjected to stresses at low temperatures.
Long-term flexibility is also tied to the polymer within the coating. Most high-performance waterborne roof coatings are based on polymers that are internally plasticized, meaning that the flexibility is inherently built into the backbone of the polymer. As such, they retain their flexibility as the coating ages.
One of the exceptional benefits of well-designed roof coating systems with high-performance coatings is their end-of-life consideration. When the time comes, a simple recoat can extend the system, but only if the coating retains its basic physical properties, including flexibility.
Historic metal roofs at Fort McHenry in Baltimore were recovered using an elastomeric acrylic coating system formulated with recycled content.
The Bakes Athletic Center at Ursinus College in Collegeville, Pennsylvania, represents an excellent case study on the recoatability and life-cycle extension of a high-performance waterborne coating system. Originally installed in the early 1980s, the fully reinforced waterborne acrylic coating system is still going strong today. The 100-percent acrylic coatings used there have retained their flexibility and have allowed for periodic maintenance coatings to extend the life of the roof. Thirty years after the original application, this coating system is providing effective, waterproof protection and has proven to be very cost effective.
SPECIALIZATION OF WATERBORNE ROOF COATINGS
One-hundred-percent acrylic elastomerics are known for their weatherability, dirt pickup resistance and high solar reflectivity. The 100-percent acrylic is an important distinction when specifying a high-performance coating. Acrylic blends with other polymers, such as vinyl, styrene and other less expensive polymers, are often used to lower costs. While they can bring certain benefits to the coating—styrene can assist in providing improved alkali resistance for coating concrete, for instance, making styrene/acrylic copolymer paints popular for masonry wall applications—the sacrifices made in flexibility, long-term durability and other key roofing properties are dramatic.
Although currently withdrawn at the time this article is was written, ASTM D-6083 Standard Specification for Liquid Applied Acrylic Coating Used in Roofing offers a minimum set of properties desired for an acrylic roof coating to meet. And while benchmarks are good at helping separate the commodity-type products from those that attain agreed-upon performance standards, they don’t do a good job at providing a grading of the higher-performing roof coatings available on the market. For example, D-6083 calls out a minimum of 100 percent elongation and 200 psi, but there are top-of-the-line acrylic roof coatings with elongation of greater than 400 percent and tensile strength in excess of 600 psi. These ultra-high-performance waterborne acrylics should not be but are often lumped into the same classification.
Waterborne roof coatings offer reflective benefits capable of saving building and home owners in cooling costs.
The roofing market presents a wide range of substrates for application—from asphalt to thermoplastic polyolefin (TPO) to metal, each with its own set of challenges. On asphalt, one of the concerns was bleed-through of the underlying asphaltic oils though the applied coating. On TPO, the concern was the low surface energy of the TPO substrate leading to lack of adhesion by the coating. These were both solved by modifications to the polymers utilized in the coating.
Waterborne coatings for metal had to overcome not only a technology hurdle, but also a long-standing perception regarding corrosion. In the scientific community, we are taught from a young age that water on metal will create corrosion; remove the water, you’ll remove the electrolytic pathway for electron transfer, and the corrosion will cease. So how on earth would you apply a waterborne coating to metal? Well, provided they are manufactured with flash-rust inhibitors, waterborne coatings for metal roofing are an ideal choice. These flash-rust inhibitors will temporarily passivate the metal while the water evaporates, and in some situations, contribute further to the corrosion resistance of the coating as well.
Ponding water, while undesirable for many reasons, is nearly impossible to completely eliminate from every roof surface. Under ponding water conditions, waterborne coatings absorb water, swell and eventually delaminate from the roof surface. For years, this was one of the major drawbacks of waterborne coatings. Recent advancements in acrylic polymer technology allow for a reduction in water swelling experienced under ponding conditions and have helped move the needle toward a ponding-water-resistant waterborne coating.
Weather conditions are another major challenge in roofing. While solventborne coatings may have some forgiveness when compared to waterborne, rain, heavy dew and low temperatures can cause major issues ranging from wash-off to poor curing no matter the type of coating applied. While not a universal solution, quick-setting technology has allowed waterborne coating to develop rain resistance in as little as 20 minutes after application. This has allowed for the expansion of waterborne coatings in markets such as the tropics, where rainfall can be unpredictable.
HYBRIDS FOR PERFORMANCE
Hybridization can be done to value-engineer a product, reducing cost with the consequence of sacrificing performance. However, performance-driven hybrids—with waterborne field-applied acrylic-modified fluoropolymers at the forefront of this push—are currently at the leading edge of the high-performance roof coating market. Polyvinylidene fluoropolymers (PVDFs) have been used in roofing for 50-plus years. They are known for their exceptional durability and robust color retention due to the carbon-fluorine bond that makes up the backbone of their molecular structure. The C-F bond is the third strongest bond in all of organic chemistry.
Originally coated in the 1980s, the Bakes Athletic Center roof is still going strong three decades later.
PVDF coatings are traditionally baked-on finishes that need temperatures in the 450-degree range to properly cure. However, by hybridizing the PVDFs with acrylic resins, these new waterborne versions do not require these high temperatures. They can be applied at ambient conditions, allowing for field applications. Depending on the level and method of hybridization, various results can be achieved. If a more flexible acrylic resin is used, then the resulting coating could be used as topcoat over an elastomeric acrylic system for increased durability, dirt pick-up resistance and long-term solar reflectivity. If a harder acrylic resin is used, then the resulting coating would be more apt for the restoration of an aged metal roof that had a PVDF coating on it from 30 years ago that is showing its age.
Additional benefits beyond field-applied came with this new waterborne fluoropolymer advancement. Waterborne PVDF coatings manufactured with bright organic color, particularly in the red family, have been seen to show better color retention than their factory-applied baked-on counterparts. It is believed that the high bake temperature of the factory process negatively impacts some organic pigments’ chemical structure and, as a result, reduces their colorfastness. Additionally, the lower-temperature cure has opened the ability to apply PVDF coatings to substrates other than metal. Today, waterborne PVDF coatings are being used as topcoats to roof coating systems for a wide range of surfaces.
In the coming years, we can only expect waterborne roof coatings to develop further. Hybridization of polymers such as acrylic, urethanes and silicones are on the horizon, and specialty additives will drive performance higher in specific areas such as ponded water resistance, color retention and adhesion. A new wave of environmental consciousness is requesting greener technologies, and coatings manufacturers are meeting that demand with the use of recycled content and bio-based ingredients within their coatings.
As an engineer, I am truly excited to be an active member of the waterborne coatings field as we look forward to the next surge in technologic advancement of waterborne coatings.
ABOUT THE AUTHOR
Eric Bennung is a product development engineer and vice president with Acrymax Technologies Inc., a third-generation protective coatings firm with over 65 years of experience in formulating and manufacturing flexible protective coatings. An honors graduate with distinction of University of Delaware’s rigorous chemical engineering program in 2009, Bennung has been engineering both coatings and coating systems with Acrymax for close to 10 years. This includes work not only for Acrymax on its commercial and industrial pursuits but also historic preservation projects though Acrymax’s sister company Preservation Products Inc. Bennung was honored by the Journal of Protective Coatings and Linings as part of its spotlight on the next generation of coating professionals in 2015. He is a member of the Philadelphia Society for Coatings Technology (PSCT), the Delaware Valley Green Building Council (DBGBC) and SSPC: The Society for Protective Coatings. He currently is serving as president of the PSCT.