The construction industry is experiencing a paradigm shift from focusing solely on sustainability to embracing comprehensive resilient design, driven by increasingly severe weather events and rising financial risk. While sustainable design emphasizes minimizing environmental impact and resource conservation, resilience—the capacity to adapt and maintain functionality after a disturbance—demands a systems-based approach that addresses future-looking hazards like high winds, hail, fire, and flooding. This article argues that true durability requires building materials, including advanced coatings, to work collaboratively as integrated systems to resist extreme loads that exceed minimum building code requirements. It explores current design resources like LEED v5 and FM Global standards, and provides specific examples of how materials are engineered to resist hazards. These examples include multilayer roofing systems designed for very severe hail, and innovative coatings and membranes used in water-retention assemblies to manage urban storm runoff. Ultimately, resiliency is redefining what it means to create durable, lasting buildings, positioning systems-level thinking—rather than isolated product properties—as the foundation for a future-proof built environment.

Introduction

Resilient design has become a catchphrase within the construction and architecture communities. Over the last two decades, forward-thinking designers and building owners have focused not just on the now, but on the future, when determining their designs. This challenge started with a focus on sustainability. The U.S. Green Building Council (USGBC) defines sustainable design as “creating places that are environmentally responsible, healthful, just, equitable, and profitable.”¹ĚýSustainable solutions often refer to minimizing the burden on the natural environment, recycling, and conserving energy and other natural resources. These goals have created a multitude of industry buzzwords, including durability, recycled content, energy efficiency, and carbon neutral. However, sustainability is not the same as resiliency.

Resilience is defined by the Resilient Design Institute as “the capacity to adapt to changing conditions and to maintain or regain functionality in the face of stress or disturbance. Resilient design solutions often consider durability as well as the ability to keep a building functional after a weather event.”²ĚýSolutions such as having a generator to maintain power are very resilient, though not necessarily sustainable (Figure 1). To be truly resilient, designers and building product manufacturers must look at more than product properties such as Volatile Organic Compounds (VOC) and embodied carbon, often the go-to for sustainable design, and more at materials working together as systems. There is no one property that ensures resilience. Designers and manufacturers need to collaborate to create complete systems of materials that work together to achieve a successful outcome. The ultimate goal is for designed solutions to meet both sustainability and resiliency targets, such as slowing the release of storm water to prevent overloaded sewers while also using some of the captured rainwater for irrigation.

One example of resilient design is the Sand Palace, which was one of the only structures left standing in its area after Hurricane Michael in 2018. Built specifically to withstand severe storms, the house utilized advanced materials like insulated concrete forms (ICFs) and was designed to resist winds of up to 250 mph, significantly exceeding state building codes at the time. The homeowner explained that they deliberately went “above and beyond code” when making material and design decisions by consistently asking, “What would survive the big one?” It is estimated that the house cost 15-20% more as a result of these decisions. Although they did have to replace utilities and experienced the loss of the first floor along with one of the air handlers, the overall damage was minimal compared to the surrounding properties.

Resiliency has become an important design strategy for many reasons, but the primary driver is money. It is expensive to rebuild after severe weather events, and insurance companies are noticing. In some parts of the United States, it is becoming more expensive and more difficult to get insurance, particularly in coastal regions and areas prone to wildfire. For example, a 2024 report from the Senate Budget Committee shows that the nonrenewal rate in Florida increased 280% between 2018 and 2023.³ĚýAdditionally, FM Global, one of the largest insurers of commercial properties, continues to expand the areas where their buildings must meet very severe hail requirements.

On the residential side, prospective homebuyers are paying attention to the potential weather impacts on properties. In 2024, Zillow started posting hazard ratings for climate-related impacts such as flood, wildfire, wind, heat, and air quality on property listings.⁴ĚýAs with other trends within the construction industry, significant attention is paid when there are clear drivers to profits and losses. This article introduces published resources and references being used by designers to design for resilience. It then looks closely at specific examples in which coatings and other building materials work together as systems to withstand increased building loads.

µţ˛âĚýC. Jim Reader, Evonik Corporation

Many surface defects, such as fisheyes, edge pull, and retraction are caused when the liquid film dewets after application. Application of the coating by brush, roller, or spray may effectively force wetting and spread the film across the substrate, but defects may form soon after application. There is competition between the hydrodynamic inertia of the applied film and the interfacial tension forces that can cause the coating to dewet or retract.

Additives can be used to prevent these defects by reducing the interfacial forces that drive retraction. However, with many different additives to choose from, that may also cause unwanted side effects, the formulator can find additive selection difficult. This paper will compare the advantages and disadvantages of different additive chemistries used to prevent defects caused by dewetting.

Introduction

The role of surface tension in the formulation and application of surface coatings has been acknowledged for many years. The surface tension of the coating is important to its ability to wet a surface, and many surface defects can be attributed to poor wetting or surface tension gradient driven flow.^1,2 These include crawling or edge retraction, craters, fisheyes, orange peel and leveling patterns, picture framing, and poor recoatability.

Wetting is the displacement of one fluid (or gas) by another at a solid interface. It reflects the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. When a drop of liquid is placed on a solid surface, it forms a sessile drop in the shape of a sphere sectioned by the surface³ with a discrete and measurable contact angle between the sphere and the surface at the three-phase contact line (Figure 1). Thomas Young observed that there is an “angle of contact” for every solid/liquid pair and observed that this concerned a balance of forces that was later expressed in equation form where Îłsv, ÎłLv and Îłsv are the surface free energies (not forces) for the solid-vapor, liquid-vapor, and liquid-solid, respectively. Wetting deals with all three phases of matter: gas, liquid, and solid.

FIGURE 1 Contact angle at equilibrium.

Young’s equation describes the situation at equilibrium, where the forces are equally balanced. Gutoff and Cohen describe that for spreading (wetting) to occur, “the forces to the left must be stronger than those to the right.”⁴ This leads to the well-known, and still applicable, rule of thumb that the surface tension of the liquid and the interfacial tension between the liquid and solid should be less than the surface energy of the coating. That said, Gao and McCarthy’s “Wetting 101” article highlights the limitations of this simplistic treatment and the need for better terminology regarding this complex subject.³ Unfortunately, only two of the four parameters in this equation can be measured experimentally and these measurements have limitations and require skilled interpretation.⁵ More critically, in real-world situations of non-ideal surfaces, the substrate surface roughness and inhomogeneous surface energy play a vital role in wetting, film stability, and dewetting.

When a liquid first contacts a surface, the initial contact area with the substrate is created by the forces applied. In the simple case of a droplet hitting a surface, the initial contact area is influenced mostly by the hydrodynamic forces applied during impact on the substrate—particularly the size, velocity, density, and kinematic viscosity of the droplet^6-8—surface tension acts as a restraining mechanism. This can be visualized with high-speed photography, where a droplet of water initially spreads rapidly on a hydrophobic surface before retracting. When the surface tension of the droplet is changed, using different surfactants, the behavior of the droplet on impact and after also changes.

When coatings are applied, the initial wetting of a substrate occurs through the force used to apply the coating to the surface. Figure 2 shows the application of an overprint varnish over an oil-based lithographic ink. The drawdown application ensures that the varnish fully covers the hydrophobic surface; however, less than a second after the drawdown is completed, dewetting begins rapidly as the cohesive forces within the liquid cause the liquid to contract. This highlights how liquids may retract or dewet after application, and this is usually what happens when a coating is applied. The force of the application method may ensure that a surface is initially covered by a liquid but does not ensure that the liquid will remain in place.

FIGURE 2 Dewetting of an overprint varnish over lithographic ink after application.

Biocides, especially isothiazolinones, can often trigger allergic reactions, which, in recent years, has driven the demand for water-based biocide-free paints for interior and exterior areas, as well as regulation. Silicate paints are an alternative, as they use liquid glass (i.e., liquid potassium silicate), rather than biocides, as a mineral/inorganic binder to adjust the paint pH so that bacteria growth is inhibited. To support paints and coatings formulators in their binder decision-making, we investigate the influence of binder choice on the wet scrub resistance of organosilicate paint (a silicate paint with a binder modification of more than 5%, which shows good adhesion to a variety of substrates) under various storage conditions, comparing four vinyl acetate ethylene (VAE) binders against each other and against the performance of several styrene acrylate binders.

Silicate paints for architecture

There are three categories of silicate paints. First, those that do not have an organic component and that typically contain 10–30% potassium silicate; these are historically used in buildings for beauty and protection. Second, standard silicate paints that have less than 5% organic content and a similar level of potassium silicate. These can be used in interior and exterior applications and adhere to a wider range of substrates.Ěý Finally, there are organosilicate paints with a binder modification of more than 5% and a lower level of potassium silicate. To enable the handling and suitability of these mineral paints for a wide variety of applications, organic binders are added to the paints as a complement to the liquid glass. These can also be used indoors and outdoors and show good adhesion to a variety of substrates.

Binders for high-pH systems

A recognized disadvantage of biocide-free paints containing liquid glass is that, due to their rheological behavior, they can be difficult to formulate and apply using a paint roller or a brush application. Here, the choice of the binder is crucial. It should have a compatibility with liquid glass/resistance to high-pH conditions, otherwise stability issues and loss of performance such as a lower wet scrub resistance will most likely be observed.

While styrene acrylate binders are often used in organosilicate paints, the performance of other types of binders, such as vinyl acetate ethylene (VAE) binders, is often discussed in the industry since these tend to saponify due to the high-pH conditions.

To better understand this effect, in the following study we investigate the performance of four VAE binders with different chemistries and physical properties. As well as comparing the performance between the four VAE binders, within this study the performance of styrene acrylate binders in a similar organosilicate paint system is presented as a comparison to VAE binders.

We chose wet scrub resistance as a parameter for the study because a significant increase in coating layer thickness loss after wet scrubbing is a clear indicator demonstrating that a binder has saponified. Wet scrub resistance is also a highly valued paint feature serving as a major selection criterion for interior paints for the DIY customer.

Results at a glance

The use of VAE and styrene acrylate binders for silicate-based paint systems has been proven feasible. Both binder types show major differences in quality with regard to wet scrub resistance. Good performance can be achieved with selected raw materials in biocide-free organosilicate paint systems.

The following recipes were used to prepare the organosilicate paints containing the VAE binders (Table 1) and the styrene acrylate binders (Table 2). The paints were prepared using a Dispermat CN 10 dissolver from VMA Getzmann company.

All the binders used in this study are commercial binders and their physical and chemical properties are listed in the following tables. To make Table 3 easier to review, the VAE binders (Binders 1-4) are highlighted in yellow and the styrene acrylate binders (Binders 5-10) are highlighted in pink.

Testing at different conditions

All the formulations and tests described below in the frame of this study were carried out by the same person on the same equipment, raw materials and consumables, to limit the influence of external factors on the results. In order to study the effect of the storage condition on the performance of the binder, all the paints were split and stored at various temperatures: room temperature 25° C, 40° C, and 50° C. Samples were then taken out of each of the stored paints at different storage times in order to perform wet scrub resistance tests, according to ISO DIN EN 13300. Wet scrub resistance tests were performed with PVC cards (following DIN 53778) and an abrasion and scrub resistance tester (Erichsen washability and scrub tester model 494) while using 2,5g/L Hostapur SAS 60 (Clariant) as a washing solution.

By Cynthia A. Gosselin, Ph.D., The ChemQuest Group

In 2022, marine coatings were an $8.7 billion market, with a growth projection of 5–7% through 2028.¹ However, there is a lot more to marine coatings than great aesthetics. At approximately a 64% share, anti-fouling/fouling release coatings were the largest portion of the marine coating market, followed by anti-corrosion and self-cleaning/self-polishing coatings. In addition, marine coatings have special and specific functionalities to protect watercraft above and below the waterline. Finally, marine coatings are specially formulated to be easily cleaned.

The two most significant drivers for the growth of the marine coatings market are transportation of goods by sea and recreational sailing. Unlike relatively new, speedy, expensive air shipping, transportation of goods by sea is a centuries-old tradition. To this day, it remains the preferred method for heavy or bulk products. Interestingly, it is also the most cost effective and it tends to have a lower carbon footprint and emission standards. Additionally, safe harbors are widely available almost everywhere there is water, making it a practical and accessible choice.

Recreational or leisure boating is the second largest driver of the marine coatings market. Whether for routine maintenance, hull cleaning, racing efficiency, fuel economy, or craft longevity, marine coatings are top-of-mind for large- and small-craft owners. In both the shipping and recreational categories, shipbuilding and repair are enjoying a resurgence in the post-COVID market.

With all these factors contributing to the growing demand for marine coatings, it’s essential to understand what they are and why they’re so vital. Simply put, marine coatings are broadly defined as waterproof protective layers that are applied to surfaces exposed to or immersed in fresh, brackish, or saltwater. Boats, ships, ferries, small watercraft, and marine structures such as offshore oil rigs and bridge structures employ some kind of marine coating both above and below the waterline.

A wide variety of coating chemistries can be used depending upon the specific substrate and service application, with the exception of unsaturated polyester resin (the type often used in fiberglass). Most marine coatings contain varying degrees of volatile organic compounds and can be applied by brush, spray, roller, or any other convenient method.²

Topside boat paints are usually 1K or 2K polyurethanes, buffable 2K polyurethanes, or alkyd marine enamels that protect the boat from UV damage. Bottom boat paints are antifouling coatings designed to reduce the attachment of aquatic organisms to the hull. Bottom paints include ablative and hard coat paints as well as primers. These types of coatings are best removed with paint removers especially designed for these chemistries.

The inability to escape the effects of biofouling beneath the waterline is one of the main reasons for the strength of the antifouling paint sector. Many boats and ships sat idle during the COVID-19 pandemic and exacerbated the problem, especially in relatively still, warm waters. Sitting idle in the harbor allowed the bottom of watercraft to accumulate barnacles, tube worms, algae, sea squirts, and slime.³ While this is a natural aquatic process, it is one of the most significant hurdles for boat and ship owners alike—particularly when speed and fuel economy are compromised. The commercial reason for using antifouling paints is for improving the flow of water passing the hull, and thereby maximizing fuel economy.

When a hull is covered by only 10% barnacle fouling, 36% more power from the engine is required to maintain the same speed through the water and could be responsible for 110 million tons/year of excess carbon emissions and $6 billion of addition fuel cost in the international shipping industry.⁴

Even recreational sailing is affected. Tapio Lehtinen was racing the 2019 Golden Globe non-stop, around-the-world race when he was almost stopped dead in the water. Racing furiously with his rival, he surprisingly noticed that he was being left behind. Thinking something had happened to his propellor, he dived in to check. To his dismay, barnacles were growing all over the hull. He did finish the race, but 110 days behind the winner. Barnacles had encrusted the entire bottom of the hull, stealing his speed. More had colonized on his self-steering blade, causing it to shear off when the load became too massive.⁵ Whether recreational or commercial enterprises, billions of dollars are spent every year to increase the usefulness, fuel economy, and longevity of watercraft by reducing biofouling.

Besides the economic and convenience factors, biofouling presents another important ecological concern. Marine life hitchhiking along the bottom of a boat can be the mechanism for translocating invasive species to the wrong ecosystem, far from natural predators.

Striped zebra mussels are a species native to the Caspian and Black Seas of Russia and Ukraine that have been distributed by ships as invasive species in Ireland, Italy, Sweden, Spain, the United Kingdom, and the United States. Root-like threads of protein called “byssal threads,” enable the zebra mussels to adhere very tightly to hard surfaces (like boat hulls, native mussels, and rocks). They colonize rapidly in the absence of natural predators, filter out algae needed by the “locals,” and are the succubus that attaches to and incapacitates native mussels. The zebra mussels form dense clusters that cut off water flow, clog pipes, and damage equipment. Their sharp edges can injure swimmers, and the infestation may lead to the devaluation of the boat. Since the 1980s, the Great Lakes in the United States have been struggling to eliminate this nuisance that just came along for the ride.⁶

Because of all the ship and yacht traffic, the Mediterranean Sea has more than 800 identified invasive species. In one audit of leisure vessels sailing through that area, 71% of leisure vessels were harboring at least one non-native species.⁷

Despite the problems plaguing biofouling of hulls, the landscape for marine coatings is moving toward sustainability measures and stricter environmental regulations. Copper-based, biocide-boosted antifouling paints have been the dominant performers in reducing biofouling. However, the effect of these paints does not segregate itself to affecting only those aquatic hitchhikers that attach themselves to the hull. Rather, cuprous oxides carrying biocides leach into the water, where they ultimately settle at the bottom of the sea or lake, also poisoning oysters, welk, clams, and other bottom-dwelling organisms.

Marine coatings manufacturers are now emphasizing improved sustainability along with operational performance factors of reduced power demand, lower fuel consumption, and carbon emissions. Manufacturers have had to become more cognizant of the environmental effects of biofouling agents because of the push by the regulatory agencies resulting from harbor contamination studies conducted during pandemic idle time.

Earlier, paints that contained the organitin biocide tributyltin (TBT) were banned on January 1, 2008, by the International Maritime Organization. For more than 40 years, TBT had been used successfully to boost the performance of cuprous oxide through controlled release. The problem with this effective method was that TBT would leach out of the paint, damaging the aquatic hitchhikers and contaminating the surrounding water. Once there, it would accumulate at the bottom, affecting the endocrine systems of shellfish. This led to abnormal developments, such as female snails taking on male sex characteristics, and severe deformities in oyster shells, making some beds almost extinct.

Since the TBT biocide ban, the number of effective biocides that meet regulatory requirements has decreased substantially. As a result, self-polishing copolymers (SPC), controlled depletion polymers (CPD), and foul release (FR) coatings are gaining popularity. Right now, the biocides in SPC and CPD are evenly dispersed in the matrix and not bound to anything. Release occurs when the matrix erodes (polishes) or by dissolution when water penetrates the paint film. These reactions are relatively uncontrolled and could lead to premature dissolution or over-leaching. Research is underway to find a way to attach the biocide molecules to a polymer carrier. Using hydrolysable covalent bonds could control the release rate of the antifouling constituents— mimicking the long-lived controlled performance of TBT.³

It is clear that a totally different strategy will be needed in the future to satisfy ultimate environmental concerns and regulations in harmony with antifouling performance characteristics. On the one hand, something is needed that will keep marine organisms from damaging and compromising marine craft below the waterline. On the other hand, even the most hardened industrialist agrees that the solution must keep from damaging everything else in its wake.

Nanotechnology that mimics the surface texture of algae that inhibits attachment of marine organisms is one outside-the-box thought. But this is still in the research stage and not yet available for mass production.⁸

There is one other approach that uses a totally different point of view and has proven commercially and environmentally successful. Ninety studies have been completed that pass the EU regulation Biocides Product Directive 98/8/EC (BPD) governing “biocides.” Regulating bodies in Japan (CSCL and JPMA), Korea (NIER), and China (MEP, Order 7) have also approved this material, and notifications have been provided to other relevant shipping regions.

Rather than using the typical paradigm of killing off the barnacle hitchhiker (and ultimately everything in the vicinity), an attempt was made to modify the attachment mechanism of the larvae.

Instead of using metal oxide biocides, medetomidine, a mammal anesthetic, was added to bottom paint. When exposed to medetomidine leaching out from a wet coating, the cyprid larvae of the barnacle species Balanus improvises were repelled from the surface. How? Medetomidine in tiny concentrations stimulates a receptor in the larvae causing hyperactive swimming behavior. Instead of settling down on the surface, the legs move at 100 kicks/minute, forcing it to swim away from the bottom of the boat. The effect is reversible. As the barnacle larvae move away, the kicking stops. This makes it impossible for the organism to attach to the surface.⁹

Since 2016, a thousand governing-body-approved commercial ship applications of this “biocide” were used successfully in several different biofouling paint formulations. This may be the first marriage of true environmental sustainability through benign influence on organisms and the desired performance from an antifouling marine paint.

About the Author

Cynthia A. Gosselin, Ph.D., is director at The ChemQuest Group, ChemQuest Technology Institute, ChemQuest Powder Coating Research. Email: cgosselin@chemquest.com.

References

1. IMARC Impactful Insights. “Antifouling Paints and Coatings Market: Global Industry Trends, Share, Size, Growth, Opportunity and Forecast 2023-2028.”
2. “Marine Coatings Selection Guide: Types, Features, Applications | GlobalSpec.” https://www.globalspec. com/learnmore/materials_chemicals_ adhesives/industrial_coatings_ sealants/marine_coatings#.
3. Koch, S. “Invasive Zebra Mussels.” National Park Service, April 2, 2021. https://www.nps.gov/articles/ zebra-mussels.htm (accessed July 27, 2023).
4. “Sustainable Antifouling by Controlled Release from Polymer-Bound Selektope.” ITECH-Technical-Paper_ November-2022-1.pdf, November 2022, selektope.com (accessed July 27, 2023).
5. Strickland, K. “Tapio Lehtinen’s Barnacle Blight.” Yachting Monthly, May 22, 2019.
6. “The 5 Most Common Marine Fouling Organisms and the Effect They Can Have on Your Boat.” Electronic Fouling Control, Antifouling Tips, June 21, 2023.
7. Rotter, A.; et al. “Non-indigenous Species in the Mediterranean Sea: Turning from Pest to Source by Developing the 8Rs Model, a New Paradigm in Pollution Mitigation.” Front. Mar. Sci., 2020, 7, Marine Pollution Section, March 24, 2020.
8. Kumar, S.; et al. “Nanocoating is a New Way for Biofouling Prevention.” Front. Nanotechnol., 2021, Environmental Nanotechnology Section, Nov. 22, 2021.
9. “About Selektope® – A Sustainable Biocide Used in Antifouling Coatings.” https://selektope.com/ about-selektope/ (accessed July 27, 2023). When exposed to medetomidine leaching out from a wet coating, the cyprid larvae of the barnacle species Balanus improvises were repelled from the surface.

Maximizing a coating’s durability is a complex equation that must account for multiple factors.¹ Making matters even more challenging is the fact that formulators have been missing a critical piece of information about how titanium dioxide (TiO₂) can influence durability—until now.

Today’s paint producers and formulators are under heightened pressure to create highly durable coatings for exterior architectural and industrial applications capable of standing up against harsh UV rays, rising temperatures, and severe weather events. At the same time, many applications also demand greater efficiency via longer-lasting coatings that increase the length of service before recoating and reduce material and labor costs.

TiO₂, a critical component of high-quality, highly protective coatings, is known to have several effects on paint durability that formulators must consider.^2-10 As a strong UV light absorber, TiO₂ protects underlying resin from direct interactions with the UV component of sunlight. However, TiO₂ can convert some of the UV light energy into chemical energy in the form of radicals, which can then attack the binder or react with other molecules at the surface that can lead to further paint degradation. In addition to the inherent photoactivity of the TiO₂ pigment, the degree of TiO₂ dispersion also plays an important role in determining a paint’s durability.

It is logical for formulators to assume that by decreasing the photoactivity they can create a more durable coating. However, a more nuanced approach shows that TiO₂ grade can affect paint durability beyond simple TiO₂ photoactivity.

Closing a Crucial Information Gap

It has been previously established that initial paint gloss—a paint characteristic that is highly representative of pigment dispersion—has an effect on gloss retention independent of TiO₂ photocatalytic activity.¹¹ However, the effect of degree of dispersion on color stability (fade) of paints has not been studied in the same way. This is because the durability of paints with poor TiO₂ dispersion is seldom measured. Durability testing is expensive and time-consuming, whereas TiO₂ opacity testing—which is also related to degree of dispersion—can be used to identify and reject poorly dispersed paints quickly and for little cost.
As such, there is limited information available that separates the effect of TiO₂ dispersion on paint durability from the effect of TiO₂ photocatalytic activity. This information gap has made creating coatings that maximize paint durability challenging for formulators.

To create high-quality paints that provide a high level of protection from the elements, formulators must fully understand the relative importance of photoactivity and TiO₂ dispersion on paint durability. At Chemours, we wanted to measure and quantify this relationship to help formulators make the highest-quality products possible. To do so, we conducted an experiment to shine a light on this process and uncover exactly how the degree of TiO₂ dispersion can affect color stability, a critical aspect of a coating’s durability.

Continue reading in the ofĚýCoatingsTech.

By Leo Procopio,

Bridges are critical to society because they facilitate connections between people and businesses. People need to see their friends and family, workers need to get to their jobs, and goods need to get to their market. Allowing the transport of people and goods across physical barriers such as waterways, ravines, and highways is necessary for our social and economic health.

Due to their importance, it is in the best interest of society to build new bridges when needed, keep existing bridges in safe and working condition, and protect them against excessive degradation from overuse, weather, and corrosion. However, therein lies the problem, as bridges are also very expensive to build and maintain.

Coatings certainly have an important role to play in the protection of bridges from the elements of the weather and corrosion, and effective coating systems can extend their service life and increase the time between maintenance cycles. This article will explore some of the challenges facing our bridges, the role of coatings in bridge construction and maintenance, and the types of coating systems that are currently used on bridges. CoatingsTech also reached out to several experts in the field of bridge coatings for their thoughts on topics such as challenges for bridge maintenance, trends in the bridge coatings market, and where the market is headed in the future. Their comments will be presented in the Q&A roundtable section of this article.

Figure 1. Number of bridges in the current U.S. bridge inventory and their condition, according to their age (year built). Based on data from the National Bridge Inventory as of June 2022.¹

The State of Bridges in the U.S.

According to the National Bridge Inventory (NBI), a database administered by the Federal Highway Administration, there were more than 620,000 road and highway bridges in the United States as of June 2022.¹ The inventory of bridges has been steadily growing over the decades. For example, 30 years ago, there were slightly more than 570,000 bridges. The inventory of road and highway bridges includes major bridges across mighty rivers, small bridges of at least 20 feet, as well as highway bridges and overpasses spanning other roads or other obstructions such as railroad tracks or gullies. The NBI database doesn’t cover bridges dedicated for rail traffic, and although there is not an exact accounting of railroad bridges in the United States, there are at least 61,000 bridges used for Class 1 rail traffic,² i.e., the largest rail carriers that account for about 94% of freight rail revenue and 67% of freight rail mileage.

The condition of infrastructure in the United States gets a lot of attention, especially when it’s not in good repair, and road and highway bridges are no different. The American Society of Civil Engineers provides an annual Infrastructure Report Card on various infrastructure segments, and in 2021, it gave the bridge segment only a C grade.³ We tend to take the condition of bridges for granted, but high profile bridge failures, such as the 2007 collapse of the I-35W Mississippi River bridge in Minneapolis or the more recent 2022 collapse of the Fern Hollow bridge in Pittsburgh, focus public attention on the state of our bridges.

Figure 2. Bridge condition for all US bridges and the subset of National Highway System (NHS) bridges, by bridge count (chart on left) and by bridge deck area (chart on right). Based on data from the National Bridge Inventory as of June 2022.¹

As might be expected, older bridges tend to have more wear and tear, and a higher percentage of older bridges are rated at a lower condition. Figure 1 shows data for the entire inventory of 620,669 bridges in the NBI, broken down by age (year built) and the overall condition of the bridge. The condition ratings of good/fair/poor are based on the National Bridge Inspection Standards,⁴Ěýand they consider the combined condition of the deck (i.e., road surface), sub-structure, and super-structure of the bridge. A rating of poor does not necessarily mean that a structure is unsafe, but rather that it needs attention. Approximately 40% of all bridges were constructed before 1970 and are more than 50 years old. It is clear from Figure 1 that a higher percentage of those older bridges are in fair or poor condition compared with those built more recently.

Figure 2 shows the conditions of the entire inventory of U.S. bridges and a comparison with the subset of bridges that are part of the National Highway System (NHS). The National Highway System consists of roadways and major arteries deemed important to the nation’s economy, defense, and mobility. Bridges carrying NHS roads make up about 24% of all bridges by number. However, because these bridges tend to carry larger roads, they make up a larger percentage of the inventory, about 58%, when considering the deck area.

Most are owned by state, county, and local governments (Figure 3). States own about 48% of bridges in number, and local governments own about 50%. However, states generally own the larger and more heavily travelled bridges, and so own approximately 76% of bridges by deck area. The federal government owns a little under 2% of all bridges, such as those on federal lands.

Figure 3. Distribution of bridges by type of owner, according to bridge count (left) and bridge deck area (right). Based on data from the National Bridge Inventory as of June 2022.¹

Although the number of bridges characterized as in poor condition has been trending downwards over many years, there are still about 43,000 bridges listed in the recent NBI data as being in poor condition. Funding for bridge construction and repairs has been a perennial problem, and while state and local governments own most bridges, funding is largely from federal sources. In 2021, approximately $8.6 billion was obligated towards bridge projects from federal highway program sources. There had been no standalone federal funding sources dedicated to bridges since 2013, but the recent Infrastructure Investment and Jobs Act (IIJA), enacted at the end of 2021, now provides two funding programs dedicated to bridge projects. It is estimated that these new programs could roughly double the annual spending by states for bridge projects relative to 2021 levels (not adjusted for inflation).⁵

Painting Bridges

Coatings are utilized on bridges to improve their aesthetics, but protection is their most important function. Bridges in the United States are most commonly constructed from reinforced concrete (about 75%) and steel (about 25%). There are reasons for coatings both substrates.

Problems with concrete occur when water and soluble salts penetrate the concrete and cause the steel reinforcing bar to corrode. As the rebar corrodes, the expanding corrosion products can cause cracks in the concrete. The use of powder coated rebar can help mitigate the issue, and sometimes silane-based sealers and water repellants are applied to the concrete to prevent the water/salt penetration. Pigmented coatings such as epoxies, acrylics, and polyurethanes are also applied to concrete bridges for both aesthetics and barrier protection. However, concrete bridges are largely left uncoated.

Steel bridges, on the other hand, are almost always painted to protect the steel from corrosion. Some bridges constructed from a specific steel alloy, referred to as weathering steel, can be left unpainted, as the steel alloy develops a patina or protective layer during weathering. But in most cases, steel will corrode rapidly if left unpainted. Corrosion can be accelerated when soluble salts are present, such as from road de-icing salts.

Certain areas of bridges are more susceptible to corrosion due to their micro-climate. Steel located under leaking deck joints is in an aggressively corrosive environment. Splash zones, which are more prone to having water and salt splashed onto the steel surface, exist both above the bridge deck surface and sometimes underneath a bridge where another road passes. Any areas that can trap water/salt and remain wet for longer periods of time, such as bottom flanges on I-beams, also have an aggressive micro-climate.

Coatings for Steel Bridges

Table 1. Common Coating Systems for Painting Steel Bridges Note: IOZ = inorganic zinc rich, OZĚý= organic rich zinc, MCU = moisture cure urethane, CSA = calcium sulfonate alkyd

Historically and up to the mid- to late 1970s, many steel bridges were painted with multiple thin coats of alkyd coatings containing toxic corrosion-inhibiting pigments such as chromates and red lead and which were applied directly over the mill scale. Mill scale is a thin layer of iron oxides that forms on hot rolled steel during the milling process. It adheres to the steel surface, and as long as there is no break in the layer, protects the underlying steel. However, because it is electrochemically cathodic to the steel, any breaks in the mill scale can lead to corrosion of the underlying steel. Red lead alkyd coatings were effective, but eventually replaced due to regulation of the toxic lead and chromate pigments and recognition of the benefits of surface preparation (i.e., removal of mill scale via abrasive blasting).

Coating systems comprising a zinc-rich primer and solventborne vinyl finish coat replaced the red lead alkyds in the 1980s. However, the vinyl finish coats were eventually replaced with other topcoats due to their extremely high VOC levels. Today, the bridge industry uses many types of coatings systems, but the most common is a three-coat system comprised of a zinc-rich primer, an epoxy intermediate coat, and an aliphatic polyurethane topcoat.

There are various scenarios in which a steel bridge might be painted, and the choice of process has a large effect on the overall cost of the painting process. The cost of paint materials is typically only a relatively small percentage of the total project cost, with the main contributions coming from labor and the type of surface preparation. If removal of old lead-based coatings is required, the health and safety requirements for the lead abatement process will drive costs up further.

Figure 4. New bridge spanning the Mississippi River and carrying I-74 between Bettendorf, IA, and Moline, IL. Opened to traffic in 2021, all exterior steel surfaces were coated with a three-coat system using a zinc-rich primer, epoxy midcoat, and fluorourethane topcoat. Photo courtesy of AGC Chemicals Americas.

For new construction, it is common to paint the structural steel in a shop setting, where surface preparation and paint application are most easily controlled. After abrasive blasting in the shop, typically at least a zinc-rich primer is applied. Sometimes the full coating system including intermediate and finish coats is also applied in the shop, or alternatively the remaining coats can be applied in the field after the steel is erected.

For maintenance painting, several possible scenarios exist.⁶ Spot repair and touch-up is used when there are smaller areas of corrosion or paint failures, and those areas can be treated by various surface preparation methods and painted. Zone painting is used to remove and replace coatings in specific areas or zones, such as steel within a splash-zone or within a certain distance from expansion joints. Spot repair and overcoating is used when the original paint system is still in relatively good shape and well adhered. After partial removal of failed coating and rust, spot priming is done, and a full coat of topcoat is applied. Finally, full removal of the old coatings and replacement with a new multi-coat system can be done when the original coating is in poor shape.

Although less expensive on a square foot basis compared with spot repair and zone painting, full removal and replacement is generally the most expensive maintenance painting scenario overall, due to the large surface area being prepared and painted. Overcoating is a less expensive option than full removal/replacement because of the lower amount of surface preparation. Spot repair and zone painting have the highest cost per square foot, but are the lowest overall cost due to the smaller areas being repaired.⁷

Figure 5. The Tokiwa Bridge, located in a mountainous region near Hiroshima, Japan, was painted with a three-coat system in 1986. The coating system uses a zinc-rich primer, epoxy midcoat, and fluorourethane topcoat, and has performed well for over 30 years. Photo courtesy of AGC Chemicals Americas.

Table 1 lists some of the common coating systems used for steel bridges. The ultimate choice of coating system depends on factors such as the painting scenario (e.g., new construction, full removal or replacement, or spot removal or overcoating), expected durability, as well as immediate and/or life-cycle costs.

For new construction, an inorganic or organic (e.g., epoxy or moisture-cure urethane) zinc-rich primer is typically applied in a shop setting. For field-applied systems, organic zinc rich primers would be utilized. Sacrificial zinc-rich primers are used because they are the most effective primers at preventing corrosion of properly prepared (abrasive blasted), clean steel. There are several types of two- and three-coat systems used for new construction and full removal/replacement of old coatings, as shown in Table 1. As mentioned previously, the most common is a three-coat system using a two-component epoxy intermediate coat and a two-component aliphatic polyurethane topcoat over the zinc-rich primer. The zinc-rich primer provides the corrosion resistance, the epoxy midcoat protects the primer and adds to corrosion resistance via its excellent barrier properties, and the polyurethane topcoat provides a highly durable finish with excellent gloss and color retention.

There is a trend towards using ultra-durable topcoats with excellent gloss and color retention, and one approach is to use a polyurethane based on a fluoropolymer polyol, such as FEVE (fluoroethylene vinylether). An example of a recent bridge coated with such a system is shown in Figure 4. The bridge spans the Mississippi River, carrying I-74 between Iowa and Illinois, and was opened to traffic in December 2021. Protecting the steel on this $1 billion project is a three-coat system consisting of a zinc-rich primer, epoxy midcoat, and fluorourethane topcoat.

Similar coating systems based on fluoropolymers have been used elsewhere in the world, specifically in Japan, for many years. Figure 5 shows an example of such a bridge near Hiroshima, Japan, which was coated with a three-coat system using an FEVE-based fluorourethane topcoat in 1986.⁸

Table 2. Gloss and Gloss-Retention Data for the Fluorourethane Topcoat on the Tokiwa Bridge Near Hiroshima, Japan, Over a 30-Year Period

Table 2 shows some gloss and gloss retention readings made on the Tokiwa Bridge over the course of three decades, illustrating the excellent gloss retention with these systems. After wiping to remove dirt from the painted surface, it was found that the 60° gloss retention was 97% after 30 years. The photographs in Figure 6 illustrate the excellent gloss observed on an external-facing fascia beam, which is expected to receive the most exposure to sunlight and weather, over the 30-year exposure.

Another trend has been to use two-coat systems, in effect replacing the epoxy midcoat and polyurethane topcoat with a single finish coat.⁹ Utilizing a bridge coating system with fewer coats is advantageous for its lower cost of labor and faster return-to-service (e.g., less downtime for traffic lanes). Polysiloxanes are used directly over zinc-rich primers in a two-coat system and also provide superior durability.¹⁰

Figure 6. Photos of a steel fascia beam on the Tokiwa Bridge taken at various exposure times, and showing excellent gloss retention of the fluorourethane topcoat. The photo taken in 2016 represents 30 years of exposure. Photos courtesy of AGC Chemicals Americas.

Fluorourethanes and polysiloxanes are among the most UV-resistant and durable finish coats available today, and both have excellent gloss and color retention. Polysiloxanes are also used in other market segments where durability is important, such as marine coatings.

Polyaspartic finish coats,¹¹Ěýbased on amine-functional aspartate ester resins (e.g., Figure 7) crosslinked with aliphatic polyisocyanates, are also utilized in two-coat bridge coating systems.¹² Relative to polyurethanes, polyaspartic coatings can offer faster dry times with reasonable pot lives, can be applied at higher film thicknesses, and offer equivalent gloss and color retention. The thicker films facilitate moving to a two-coat system without sacrificing corrosion resistance.

Work on two-coat bridge coating systems continues. Figure 8 shows a highway bridge in Missouri that was recently painted with a proprietary 2-coat system based on new technology from Carboline.

Figure 7. Molecular structure of an amine-functional polyaspartic resin, which can be crosslinked with polyisocyanates (X = aliphatic or cycloaliphatic bridging group, and R = alkyl).

Waterborne acrylic topcoats also get some use in both two- and three-coat systems over zinc-rich primers for full removal/replacement scenarios, as well as find use in multi-coat overcoating scenarios. Acrylics have good exterior durability, the benefit of low VOC, and testing has shown that they can perform well in aggressive environments typical for bridges.^13,14

As regulations have pushed end-users towards lower VOC coatings, they have several options to move away from high VOC solventborne coatings, including the replacement of traditional solvents with VOC-exempt solvents and the use of either high solids solventborne coatings or waterborne coatings. A number of states, such as California and North Carolina, have specifications that allow the use of waterborne acrylics on steel bridges. California has recently pioneered the use of several waterborne coatings based on acrylic/FEVE blends as higher durability waterborne options.^15,16

State departments of transportation (DOTs) maintain qualified product lists (QPL) or approved product lists (APL) of coatings and coating systems that are approved for use on bridges in their state. A coating typically undergoes a long process of testing prior to being placed on a QPL. State DOTs can do their own testing, and there is also a national program administered by the American Association of State Highway Transportation Officials (AASHTO) that generates data on coating systems that states can use in determining whether a system can be approved for use in their bridge projects.

Figure 8. Photo of bridge carrying I-270 over Dorsett Road in Maryland Heights, just outside of St. Louis, MO. The steel structure was painted with a new proprietary two-coat system. Photo courtesy of Carboline.

The AASHTO National Transportation Product Evaluation Program (NTPEP) evaluates materials and products across a variety of applications and includes a program for steel bridge coatings. The program helps prevent duplication of testing efforts across state DOTs and provides paint manufacturers with a single testing protocol rather than having to support testing done by each state separately. The NTPEP Structural Steel Coatings program evaluates products via an independent third party laboratory according to a consensus-based project work plan that describes the laboratory and field test protocols.¹⁷ Testing includes demanding accelerated weathering tests such as 5000 hours in ASTM B117 salt spray and 15 cycles (5040 hours) in ASTM D5894 cyclic salt fog/UV exposure, as well as slip coefficient testing for coatings applied to faying surfaces (i.e., surfaces being bolted together), among others. Results are shared with member states via an online database¹⁸ and used to make decisions about qualifying products for inclusion on a state’s QPL/APL.

Conclusions

The vast importance of bridges to society lies in the numerous social and economic connections that they facilitate. The United States currently has over 620,000 road and highway bridges that allow people and products to get over obstacles such as waterways, valleys, and roads which would otherwise be difficult to traverse. Unfortunately, many of them are rated as being in poor condition. Building new bridges and replacing unsafe or obsolete bridges is very expensive, so protecting and maintaining existing bridges against deterioration is incredibly important.

Coatings have a key role to play in protecting critical infrastructure such as bridges. Bridge coatings have evolved over many decades to become safer and more effective, and today, a three-coat system based on a zinc-rich primer, epoxy intermediate coat, and polyurethane topcoat is most common. However, many other systems are used, including ones utilizing more durable finish coats (e.g., fluorourethanes and polysiloxanes), environmentally friendly waterborne coatings, and systems with fewer coating layers. The challenges facing bridges and other infrastructure are many and daunting, but there is no doubt that future innovations in coatings will be part of the solution.

References

1. Information on the National Bridge Inventory database, maintained by the Federal Highway Administration, can be found at .
2. “How Freight Railroads Keep More Than 61,000 Bridges Safe,” Association of American Railroads
3. “2021 Report Card for America’s Infrastructure,” American Society of Civil Engineers, accessed at .
4. Info on the National Bridge Inspection Standards can be accessed at .
5. “Infrastructure Investment and Jobs Act: Highway Bridges,” Congressional Research Service, May 2022.
6. Ault, J.P.; Kimmer, C.; Shoyer, E., “Maintaining Modern Bridge Coatings Systems,” J. Protective Coatings & Linings, 40(1), pp. 18-27, January 2023.
7. Richards, G.; Grisso, B., “Maintenance Painting- Protective Coatings and Coating Systems for Bridges,” presentation at the 2013 Southeast Bridge Preservation Partnership annual meeting, 2013.
8. Darden, W., “Long life coatings for steel bridges,” Proceedings of AREMA Annual Conference, 2019.
9. O’Donoghue, M.; Datta, V.; Walker, S.; Wiseman, T.; Roberts, P.; Repman, N., “Innovative Coating Systems for Steel Bridges: A Review of Developments,” J. Protective Coatings & Linings, 30(1), pp. 34-52, January, 2013.
10. Calzone, T., “Ultra-Durable Finished for Zinc-Primed Steel Bridges,” Proceedings of the World Steel Bridge Symposium, 2005.
11. Squiller, E.P.; Reinstadtler, S., “Polyaspartic Coatings,” Chapter 14 in ASM Handbook Volume 5B: Protective Organic Coatings, K.B. Tator (Ed.), ASM International, 2015.
12. Olsen, A.; Williams, C.T.; Hudson, M.; Fleming, C.W., “Two-coat Polyaspartic Urethane Coatings Protect Virginia Steel Bridges for Over a Decade,” J. Protective Coatings & Linings, 33(1), pp. 56-63, January 2016.
13. Medford, W., “Testing Low VOC Coatings in Aggressive Environments: North Carolina’s Experience,” J. Protective Coatings & Linings, pp. 23-29, May 1995.
14. Peart, J.; Kogler, R., “Environmental Exposure Testing of Low VOC Coatings for Steel Bridges,” J. Protective Coatings & Linings, pp. 60-69, January 1994.
15. Marcks, B., “Improvements of Waterborne Acrylic Latex Finish Paint Properties by Incorporating Fluoroethylene Vinyl Ether (FEVE) Emulsion Technology,” Proceeding of the SSPC Coatings+ Conference, 2020.
16. For example, see State of California Department of Transportation Specification PWB-182B, “Dark Green Finish Paint Waterborne Acrylic Latex/ FEVE Blend Vehicle,” February 2023.
17. “NTPEP Committee Work Plan for Evaluation of Structural Steel Coatings (SSC-18-1),” AASHTO, 2019, accessed at https://ntpep.transportation.org/technical-committees/protective-coatings-ssc-ccs.
18. NTPEP DataMine for Structural Steel Coatings (SSC) can be accessed at .

Roundtable Q&A: Industry Experts Discuss Bridge Coatings

CoatingsTech asked several industry experts for their thoughts on the present and future of the bridge coatings market. Topics ranged from challenges facing the bridge construction and maintenance industry to trends affecting the market for bridge coatings. They also discuss the role of sustainability in this industry, as well as their thoughts on what the future holds for bridge coatings.

Participants in the Q&A roundtable discussion include experts in the bridge coatings industry from raw material suppliers, coatings manufacturers, engineering firms, and facility owners. The industry experts providing comments include:

• Peter Ault, president at KTA-Tator
• Andrew Birnie, North American market development manager for industrial coatings at Covestro LLC
• Winn Darden, business manager for AGC Chemicals Americas, Inc.
• Vijay Datta, technical leadership and business development manager at International Paints at AkzoNobel
• Justin Manuel, global product line director for Carboline
• Barry Marcks, associate chemical testing engineer with the California Department of Transportation (Caltrans)
• Kevin Morris, director of strategic segments and business development for protective and marine coatings at PPG

Q&A

Q1: From your perspective, what are some of the key challenges facing the bridge construction and maintenance industry?

Ault (KTA-Tator): At this time, the availability of skilled labor and supply chain issues are the prominent problems. As we digest the funding made available through the Infrastructure Investment and Jobs Act (IIJA), we will be back to the enduring challenge of balancing present-day cost with life-cycle costs. Often the life-cycle costs are too obscure to justify higher present-day expenditures.

Darden (AGC): Keeping older assets viable for longer periods of time is a key challenge. Also, new bridge construction has become extremely expensive, as has repainting.

Datta (AkzoNobel): The key challenge is the expense, length of time, and exclusion of emerging technology in the current approval protocol. This is applicable to both new construction and maintenance projects.

Manuel (Carboline): While there are many industries that face significant funding challenges, I believe the bridge construction and maintenance industry is actually in a prime position to thrive in this regard—particularly given the growing emphasis on tackling our nation’s infrastructure corrosion challenges. If more funding continues to be allocated to this industry, I expect that we will see an abundance of new bridge projects added to the DOT dockets in coming years. As a result, this would necessitate the hiring of more skilled labor in order to support these contracts.

Marcks (Caltrans): Some challenges in the construction and maintenance markets include the promotion of unqualified contractors through the low bid process and a lack of experienced QC/QA inspectors. A grey tsunami, involving the retirement of institutional knowledge, and a small pool of qualified new candidates and hires are other issues.

Morris (PPG): The two greatest challenges that I see right now are lack of funding (temporary correction with the IIJA bill) and the need to maximize life cycles so the budget money can address more needs.

Q2: What do you see as some of the important market trends and drivers affecting paints and coatings for bridges?

Birnie (Covestro): The top market driver on everyone’s mind is the potential elimination of exempt solvents, such as PCBTF, which is the most widely used exempt solvent in the industry. The challenge is keeping pace with the productivity demands of the industry and remaining compliant with current and potential future regulations. There are a few approaches coatings companies could take in anticipation of this change—finding a new exempt solvent or developing new coatings systems with waterborne or high solids resins which don’t require any at all. Coatings manufacturers are hesitant about finding a replacement solvent for fear that it could also end up being eliminated someday. Waterborne is an option in this case but could fall short of the productivity needed to be a true replacement. Finally, a high solids option, like polyaspartate chemistry can be formulated to most VOC levels and offers several other benefits, such as fast dry times, early property development, and a faster return to service. As lawmakers work to finalize their regulations, working with an existing chemistry with a proven record of performance is likely one of the most attractive choices for formulators.

Manuel (Carboline): One of the biggest drivers of trends within the coatings industry is a major push toward the development of more sustainable solutions that offer lower VOCs and a reduced environmental footprint. This has been a bit of a challenge for the bridge industry, which has predominantly used solvent-based, higher-VOC coatings in both field and shop environments for the past 50+ years. While this push for sustainability is still fairly new as it relates to the bridge industry, it will certainly be important for coating manufacturers and DOTs to anticipate this “green wave” based on other market trends we’ve seen in recent years.

Ault (KTA-Tator): Since the 1990s, coatings selection for bridges has largely been driven by issues surrounding lead-based paint. Even for new structures, the three-coat zinc-rich/epoxy/urethane (Z/E/U) became the standard replacement for coated steel bridges (uncoated steel also became a popular option). In the past 5 to 10 years, owners have begun optimizing their approaches to coated steel. As the initial Z/E/U bridges are approaching their first maintenance interval, the maintenance community is looking at new maintenance painting procedures—zone painting and new overcoating strategies are becoming common. For both new and maintenance painting, there is increasing interest in and use of color and gloss retentive topcoats. For new structures, the owners are beginning to refine their corrosion control strategies to use both less expensive systems (e.g., single coat inorganic zinc), and pricier systems offering more durability (e.g., duplex coatings consisting of a metallic coating in combination with organic coating(s)). Finally, owners have begun to combine coatings systems to optimize cost and performance. For example, on simple overpasses, several states will apply a full coating system on outside members and omit the finish coat on interior beams.

Marcks (Caltrans): Market trends affecting bridges coatings include the loss of raw materials and products used in making coatings. Supply chain disruptions have caused a lack of availability of some raw materials. Corporate takeovers and acquisitions of specialty materials suppliers is another trend. Inflation and rising costs affect the ability to plan projects. On the EH&S front, Go Green policies are advocating the removal and discontinuation of some products from use.

Darden (AGC): It appears that many DOTs and bridge authorities are more open to concepts like life cycle cost and trying to quantify those costs. Cost per gallon of paint isn’t the relevant metric for coatings, it’s applied cost per square foot per year of coating life. This means that more expensive products like fluoropolymer coating systems are more common today.

Morris (PPG): I think cost and budgets continue to be major impacts for bridges, and during this recent inflationary period several have speculated that the funds that will pour in from the IIJA Bill will only cover the inflationary costs that have been witnessed in the workplace.

Datta (AkzoNobel): The new upcoming trend could be surface prep and application by robotics. The inspection and maintenance can be done by drones. We also believe the current testing protocol should be replaced by ISO 12944 testing. I think ISO has classified environments based on severity, required durability, and expected service life. Their testing protocol is based on these requirements. This new testing/approval protocol should answer all the above questions.

Q3: What do you consider the top challenges for the bridge coatings industry today, and how are they being addressed?

Morris (PPG): What is unique in our world are the silos built around end use segments and the lack of data transfer from one segment to another. The bridge and highway market segment continues to look for ways to meet longer life cycle performance and to achieve the elusive “100-year design life” for bridges. Other branches of government, such as the Navy, looked for life-cycle improvements decades ago, and through technology, they realized improvements of 3x to 4x. Perhaps this rests as much on manufacturers to do a better job telling the story as it does on the bridge industry to become early adopters of new technology so that they realize improvements in performance. Two examples that I would give are edge retentive, ultra-high solids coatings, which could improve protection on areas where corrosion commonly starts on a bridge, and ultra-weatherable topcoats that are more resistant to degradation from UV light.

Datta (AkzoNobel): A key challenge is the cost of being a qualified contractor (QP 1 and QP 2 certifications from AMPP), including capital investments in equipment and containment and the ability to find the proper labor resources. In addition, recent supply chain constraints have either canceled or postponed several bridge projects.

Darden (AGC): One key challenge is trying to get new products qualified in the bridge coating market.

Manuel (Carboline): Although we have seen marked improvement in recent months, disruptions to the supply chain continue to be the biggest challenge that is faced by the bridge coatings industry. Because DOT specifications often follow rigid testing requirements, the approved list of coating solutions from which a contractor can select is very narrow and specific. As a result of ongoing raw material shortages, it has become increasingly difficult to manufacture these products. Ultimately, this can impact the completion timeline for bridge coating projects.

Ault (KTA-Tator): In addition to those mentioned above, the biggest challenge is deciding when new approaches make sense in the absence of long-term performance data to validate the decisions. Public perception often incentivizes near-term performance at minimal cost over solutions that are more cost-effective over the longer-term.

Marcks (Caltrans): Inflation with its rising costs has made everything more expensive. And a small pool of qualified labor has made it more difficult to get the people needed to complete projects. Supply chain issues have caused shortages, even allocation of products and materials. This delayed some projects. The problems seem to have been resolved for now, but will it happen again?

Q4: Are there any new and innovative products or technologies for the bridge coatings industry that you would like to highlight?

Darden (AGC): Newer concepts like duplex coatings can enable significant extension of coating system life. Using long-life fluoropolymer topcoats over galvanized or metallized surfaces can give coating system life beyond that offered by each technology alone.

Marcks (Caltrans): Caltrans has developed UV-resistant, one-component waterborne finish coatings using an acrylic/FEVE blend for use on our structural steel bridges.

Ault (KTA-Tator): As mentioned above, some key innovations include color and gloss retentive topcoats (e.g., polysiloxanes and fluoropolymers), less expensive systems (single coat inorganic zinc-rich, rapid-cure coatings), and pricier systems offering more durability (duplex coatings consisting of a metallic coating in combination with organic coatings).

Morris (PPG): This market segment is one in which it is difficult to innovate. The industry utilizes qualified and approved product lists (QPL/APL) that have to be met, and they either call for specific products/chemistries with minimum performance requirements, or they are formulary in nature.

Datta (AkzoNobel): To our knowledge, coatings innovation for the bridge market is somewhat stagnant due to the long and expensive testing procedures necessary for qualification, coupled with a low margin business. Adapting the new ISO 12944 standard for qualification of bridge coatings may inspire new technology development.

Manuel (Carboline): Carboline will soon be launching an innovative, new two-coat system that will revolutionize the construction and rehabilitation of steel bridge structures. Developed to provide state DOTs with the ultimate corrosion protection, this system will significantly extend the asset life cycle over more traditional, three-coat systems. It will also eliminate the need for block-outs (masking areas where bolted connections are made, so only zinc-rich primers are on faying surfaces) as the entire coating system will adhere to Class B slip coefficient standards.

Q5: How is the bridge coatings industry dealing with the concept of sustainability? Are there any advances in surface preparation, coatings, application methods, etc., that show promise for improving the sustainability of the bridge coatings process.

Manuel (Carboline): While it has been fairly slow to evolve, the bridge industry is seeing a shift from solvent-based, higher-VOC coatings to more sustainable technology. Because new coating systems within the bridge industry are subject to rigid testing standards, some manufacturers may be hesitant to innovate new systems that compromise their current specification positions. That said, as the global coatings industry—and the world—increasingly push for more sustainable coating solutions, I expect that will accelerate the adoption of this trend within the bridge industry.

Ault (KTA-Tator): From my perspective, addressing sustainability is in its infancy. While coatings do have an environmental impact that is magnified with multiple maintenance cycles, the industry doesn’t have a firm grasp on the frequency of such intervals. Perhaps the more significant environmental cost is the so-called user impact. Slowed traffic, underutilized human resources, delayed deliveries are examples of costs associated with bridge maintenance that have a significant sustainability impact.

Morris (PPG): Sustainability is primarily addressed through VOC compliance such as but not limited to Ozone Transport Commission (OTC) Phase II regulations.

Birnie (Covestro): Over the last several years, sustainability has evolved well beyond a buzzword with theoretical implications for the distant future. The bridge coatings industry has begun to define sustainability and establish standards, goals and best practices that are all intended to push the industry toward a more sustainable existence.

Carbon neutrality is perhaps the most tangible example of sustainability drivers the industry is looking to improve. Paint companies are now shifting focus on carbon reduction from embodied carbon, making the choice of building materials, such as bridge coatings, even more critical than it has been in the past. Coatings derived from bio-renewable or recycled raw materials will not only reduce the amount of embodied carbon of a bridge project, but also help companies across the value chain achieve their carbon neutrality goals.

This progress on reducing environmental impact does not mean a sacrifice on performance specifications. Coatings made from bio-renewable raw materials will perform just like their fossil-based versions. When these coatings begin to prove themselves in the field, accelerated adoption is likely to follow.

Marcks (Caltrans): Sustainability is today’s buzzword, and many times it is a vaguely defined term.

Caltrans uses a waterborne FEVE/acrylic latex blend coating that lasts longer and doesn’t have to be repainted as often. This saves tax dollars and enhances sustainability through increased longevity of our coatings and steel structures.

Caltrans has looked into using laser blasting as a surface preparation method to remove existing coating and corrosion to achieve SSPC-SP 10 standards (Near White Metal Blasting). Caltrans is also utilizing UAS or Unmanned Aircraft Systems (drones) for bridge inspection and coating assessment.

Darden (AGC): The use of longer life coatings is one way to meet sustainability standards. Minimizing the need for repainting over multiple cycles reduces the amount of energy and carbon dioxide emissions from manufacturing of the coating, application of the coating, disruptions to traffic, etc.

Q6: Putting your futurist hat on, what do you see as the future of bridge maintenance and the role of coatings? What would you like to see the industry accomplish in the next 10 or 20 years?

Datta (AkzoNobel): We think the biggest barrier for the next 20-plus years is low return on investment due to R&D, testing costs, and final independent lab testing and approval.

Marcks (Caltrans): Coatings provide the primary corrosion protection system for steel bridges. Bridge painting is a cost-effective means of extending the service life of our infrastructure. I would hope to see more funding available for bridge maintenance programs and see the need for higher wages and better incentives to attract and hire painters.

Darden (AGC): The industry has started moving in the direction of more durable materials in coating systems. The ultimate goal should be to use coating systems that protect steel bridges for 50-plus years, moving as close as possible to 100-year coating systems.

Manuel (Carboline): Especially in more recent years, the bridge industry has increasingly shifted away from carbon steel bridges painted with the traditional three-coat system in favor of alternatives like concrete, weathering steel and metallizing (also known as thermal spray). I expect this to prompt a push for new and innovative advancements in bridge coatings technology and data to support these trends and bolster coating systems’ position as the ultimate corrosion control mechanism on steel bridges into the next generation.

Morris (PPG): The future of bridge maintenance and role of coatings does not show much in the way of expected change going forward. The one exception would be the potential addition of ultra-durable topcoats. I would like to see an acceleration of trials/demos for new technologies from coatings manufacturers to prove increased life cycles.

Ault (KTA-Tator): In 10 to 20 years, major bridge rehabilitation will be limited to functional issues (capacity constraints, road realignments, etc.). A properly designed bridge will be able to endure its design life with minimal coating maintenance.

About the Author

Leo J. Procopio, Ph.D., is president and owner of Paintology Coatings Research LLC. For more information, visit or email leo.procopio@scienceofpaint.com.

Introduction

Alkyd-based coatings cure via both physical and chemical drying processes. The natural drying time of an alkyd can be weeks to months, which is not desirable from a practical point of view. In practice, the chemical crosslinking process is accelerated using catalysts, commonly referred to as driers. Generally, these are transition metal complexes with organic ligands. The most widely known and commonly used driers are based on cobalt carboxylates. While cobalt driers lead to highly crosslinked hard films, cobalt-based siccatives have recently faced reclassification as class 1b carcinogens by the Cobalt REACH consortium,¹ a nonprofit group tasked with preparing the registration dossiers for cobalt and cobalt compounds. As a carcinogen, cobalt that is used to cure coatings and inks can be a risk to human health as humans can be in frequent contact with these substances (especially when applying paints or scraping off old paint layers). Many regions recognize that the use of cobalt in this industry must be reduced as part of a movement toward a sustainable future.

The two leading technologies to replace cobalt driers are based on either manganese or iron. Manganese carboxylates (Mn³⁺) have long been known to exhibit good drying activity, although generally to a somewhat lesser degree than that observed with cobalt. Furthermore, the formation of Mn³⁺ species often leads to a brown coloration of the formulation, which precludes its use at high levels or in light-colored coatings.

Most iron driers show poor activity, especially in relation to cobalt or manganese analogs at ambient temperatures. Iron carboxylates work well at high temperatures and are often used in stoving enamels where curing occurs at 80–250 °C. Iron has a distinct yellow-brown color that can lead to significant discoloration in light-colored formulations as well.

An exception to this is the recent invention of an iron-bispidon complex (Figure 1), which shows vastly improved catalytic activity at very low iron weight percentages (wt %).² For example, in a medium-oil alkyd resin, the level of iron needed to obtain a good level of curing activity can be reduced from 0.08 wt % on resin solids, a typical drier loading for many formulations, to 0.0007 iron wt % on resin solids.³ As a result of this low dosage, color intensity in a paint formulation is greatly reduced, often to a level below that of cobalt-based coatings.

Fe-bispidon complexes have emerged as a preeminent cobalt alternative for the drying of waterborne and solventborne alkyd paints. Due to very high efficiency at low metal wt %, these complexes are often referred to as High-Performance Catalysts (HPC). When tested in model systems, HPCs often provide superior performance in drying speed as well as many physical coating properties when compared with cobalt-based driers.4-6 While model systems provide valuable insight into the underlying chemistry, the goal in this study was to understand the effect of HPCs on the chemistry and coating properties of real-world alkyd paint formulations made with industry-leading resins. A series of experiments was conducted to determine the optimum dosage of HPCs for four commercially available alkyd emulsions using representative formulations of the types used in architectural trim paints. These formulations were then evaluated in an array of standard industry tests versus equivalent cobalt-catalyzed formulations. Differences in the types and levels of chemical crosslinks were studied through Fourier Transform Infrared (FTIR) spectroscopy experiments on cured films. Possible variations in the β-scission reaction pathway were evaluated through gas chromatography/mass spectroscopy (GC/MS).

Finally, to demonstrate the relevance of these formulations for architectural trim applications, a section of popular commercial alkyd and acrylic paints was benchmarked and compared.

Continue reading in theĚýĚýofĚýCoatingsTech.

Durability remains a prime driver of exterior coating purchases in all market segments where coatings are exposed to severe conditions, including wood decks, all types of siding, industrial metals that are more susceptible to damage in harsh environments, and many others.

These coatings must also be cost-effective, easy-to-apply, and sustainable. Customers demand coatings that are durable and maintain their original brilliance over time, regardless of weather conditions.

Meeting these expectations requires that the coatings are formulated not to peel, blister, erode, or discolor due to chalking, yellowing, or excessive fading, according to Craig Lyerly, sales director of industrial wood coatings at Axalta.

“Facility managers, professional painters and homeowners alike want to invest in exterior paint technologies that are durable, resistant and require minimal upkeep, helping them save time, resources and money,” adds Catie Landis, PPG senior marketing manager for architectural coatings.

MANY CHALLENGING APPLICATIONS

Each segment of the coatings industry has its own specific definition for long-term durability and its own requirements.

Hitting the value-durability balance for high-volume applications can be as technically challenging as those for 20-year warranty products, says Michael T. Venturini, marketing director for coatings with Sun Chemical.

There are, however, several common requirements for a durable coating that many people will agree on, including lightfastness, stain/chemical resistance, and substrate protection, he says. “Generally, those need to be maintained for the life of the item, whether it is an exterior paint for the home, a monumental building, or a fleet vehicle,” Venturini says.

Wood substrates seem to pose some of the greatest challenges, particularly in horizontal applications such as decks, but also when used as exterior siding. Some of the more challenging situations are wood (or concrete) substrates that are saturated with moisture, according to John Gilbert, chief research and development officer at Behr Paint Company.

“In some geographic locations, it is difficult for the substrate to dry out.ĚýMoisture trapped underneath the coating can cause blistering defects after the coating is dry,” he explains. If the substrate is cold, the latex particles could be prevented from coalescing properly, resulting in a poorly formed film.”

The challenges are compounded for traditional wood deck coatings, notes Tim Kittler, global marketing manager at allnex. “Wood is a dynamic substrate that continually moves with expansion and contraction, has varying rates of absorption, and will decay if not properly protected,” he explains.Ěý“These large horizonal surfaces are subject to direct UV exposure, temperature swings from extreme heat to freezing, wet and dry periods, foot traffic, furniture being moved, and cleaning chemicals.”

Achieving durability with transparent deck stains can be particularly difficult, adds Jack R. Johnson III, market segment manager for exterior architectural coatings with BASF.
“The challenges posed by decks not readily shedding water or dirt, being subjected to more mechanical abuse than most coated surfaces around the home, and the consistent exposure to UV radiation throughout the day are amplified for transparent and semitransparent coatings because there is little (if any) colorant or pigment to refract or absorb light that enters the film,” he explains. “As a result, the lignin fibers on wood surfaces are degraded, leading to not only overall degradation of the surface you are trying to protect, but also loss of coating adhesion.”

There are challenges in maintaining long-term appearance for exterior wood coatings as well, particularly when the siding is aged wood, according to Chantal Roidot, global marketing manager for architectural coatings at Arkema. Some of the issues are the same as those for wood decks—variation in humidity, extraction of substances, and the flexibility needed to follow the wood movements.

Roidot also says adhesion is an issue on aged wood on which old paints remain. “Exterior wall paints based on acrylic emulsions have proven their durability due to their excellent UV resistance, but it is still a challenge to combine flexibility and dirt pickup resistance with a large choice of colors and no surfactant leaching,” she notes.

In addition, Lyerly notes, siding manufacturers are somewhat unique when it comes to their builder customers. “Builders want products that save time without sacrificing product quality, durability, or design,” he says. “One solution is pre-finish coatings applied by siding manufacturers to meet builders’ needs.”

FINDING THE RIGHT BALANCE

One of the biggest hurdles for achieving performance is finding an acceptable balance of all critical application needs, particularly in a dynamic regulatory and market environment, says Mike Peck, senior manager of applied research and technology for wood and floor coatings, Americas with Evonik Corporation.

“Formulators increasingly need to build systems that meet the performance demands but also employ resin and additive technologies that will not only meet current regulatory and market requirements but continue to do so for the lifespan of the formulations,” he says.

Roidot agrees that the evolution of regulations—such as VOC reduction and substances under HSE review (benzophenone, PFAS, etc.)—force the researcher to be more innovative to offer new sustainable solutions.

End-users, too, are prioritizing sustainable paint products that contribute to not only their own sustainability, but also customers’ environmental, social, and governance (ESG) goals in addition to durable performance, according to Landis.

“Today’s customers, employees and investors continue to prioritize increased transparency and value impact from sustainable practices—not just within the environmental space, but increasingly in the areas of governance and social impact, aligned with the Global Reporting Initiative (GRI), the Task Force on Climate-related Financial Disclosures (TCFD) framework, and Sustainability Accounting Standards Board (SASB) frameworks,” she says.

Kittler adds that sustainable solutions are becoming more important in the development of new technologies and are being requested as formulators initiate evaluations for next generation coatings.

“As we develop products with extended durability, we are now also seeking to introduce sustainable solutions, and these combined requirements can make the formulator’s job even more difficult,” he observes.

Axalta also designs coatings to limit waste, save time, and increase operational productivity, says Lyerly, citing as an example Axalta’s high-solids, single-coat, pre-finish industrial wood coatings solution that allows users to get more square feet per gallon at an equal dry film level.

UNDERSTANDING THE CHEMISTRY

For exterior applications, latex resins are needed that can coalesce at low temperatures so that paint can be applied at low temperatures (i.e., 35° F), Gilbert notes. He adds that paints that are designed to coalesce quickly and completely allow for early resistance to rain, an important property for exterior paints. It is also important to avoid unsightly spotting by minimizing the content of low-
molecular-weight, leachable species in formulations.

Johnson says that prevention of coating delamination due to UV exposure, which he identifies as the largest contributor to the failure of exterior coatings—particularly transparent systems—is difficult to control with binder chemistry alone.

In addition, on wood decks, he remarks that even if this damage can be mitigated, the binder itself still faces many challenges from a performance standpoint, including the need to be sufficiently hard to withstand foot traffic yet flexible enough to withstand wood movement and stable to free radical attack from UV-initiated free radicals.

The best protection is afforded by combinations of UV absorber (UVA) and hindered amine light stabilizer (HALS) additives, which absorb the UV light and prevent free radicals from propagating through the film or wood surface, respectively. Regardless of the application, where harsh conditions exist, Behr recommends applying two coats of topcoat on exterior surfaces even when hiding can be achieved with one coat to ensure long-term performance, according to Gilbert.

One of the best tools for a formulator, according to Venturini, is accelerated predictive testing to simulate the conditions of use for exterior coatings. “Although there is no substitute for real-time testing, accelerated tests hold significant value in bringing new products to market. Understanding the chemistry of the materials and the break-down mechanisms can be critical to mitigating weaknesses and extending the durability of products,” Venturini explains.

He believes that basic research like that performed at Sun Chemicals has helped expand the understanding of resin and pigment chemistries, which in turn has been instrumental in improving durability. As an example, Venturini points to expanded knowledge about aluminum and pearlescent pigments and the post-treatment technologies required to enable their use in some cases in single-layer coatings (i.e., no need for a protective clear topcoat), affording significant cost and environmental savings.

Axalta, meanwhile, subjects its coatings to extensive exposure testing to ensure consistent performance of durable products. Roidot agrees that no accelerated tests can fully replace real-life tests on the coatings applied on wood or walls in different weather conditions. “All projects to extend the exterior durability of coatings are long,” she notes.

HYBRID AND SYNERGISTIC ADVANCES

Despite the effort involved to improve durability, resin and additive suppliers and coating formulators have focused on achieving greater long-term performance given the high demand for such improvements by end users. Using hybrid and synergistic approaches seem to predominate.

Recent improvements in resin technologies, according to Kittler, have been achieved by creating hybrid chemistries that take advantage of the unique benefits of each while managing the costs.

“We are seeing blends of acrylics, alkyds, and urethanes that enable formulators to develop systems that have extended durability, while also demonstrating the preferable failure mode of erosion versus flaking of the film once it begins to fail,” Kittler says.

The combination of technologies such as waterborne alkyds that penetrate wood with acrylic emulsions improves the adhesion on wood, Roidot notes. She explains that acrylics with specific functionalities also enhance adhesion on aged wood and aged alkyds, while the newest acrylic emulsion technologies offer improved water resistance.

Component synergies, meanwhile, represent some of the most valuable and technically interesting developments, according to Peck. “These new solutions enable multifunctionality and contribute to the increasingly complex roles that additives play not just in the application of coatings, but their performance and durability over time,” he says.

Peck says that increasing demands for chemistry and performance have also brought greater scrutiny on the positive and often negative contributions of even minor components. “Finding additives that can play multiple roles, allow reduction in use levels, and increase coating film performance is a growing area of focus and research that is providing an important tool for optimizing durability in exterior systems,” he says.

AREAS FOR IMPROVEMENT

Definitions of durability vary considerably, but perhaps one of the most common is the length of time before a coating needs to be replaced because it can no longer sufficiently protect the substrate, or its appearance has degraded. End-users always want coatings that last longer and require less frequent replacement, and there is consequently opportunity for continued development.

For instance, new flexible acrylic emulsions with good dirt pickup resistance are needed. Even so, Roidot believes their development has been impeded somewhat by health, safety, and environmental issues related to some of the substances that would be used in these systems. Another gap she points to is flexible coatings for wood that also have good blocking resistance.

Johnson agrees, adding, “Even when binders are appropriately protected with proper UVA/HALS additive packages, the challenge still remains to develop resin systems that allow for weathering, flexibility, and durability.”

Alkaline resistance of metallic pigments in exterior architectural coatings has been another key target, Venturini adds.

Sun Chemical has approached this problem by developing encapsulated pastes and powders that provide resistance to alkali and acid staining. “One issue,” Kittler says, “is that the ability to offer better-performing products exists, but current solutions drive costs outside what is considered viable for consumer pricing.”

CONTINUING INNOVATION

Despite the various challenges to developing more durable coatings, ingredient suppliers such as BASF and their coating customers continue to focus on improving their offerings in the exterior market, with an emphasis on creating long-term durability for all exterior coatings, says Johnson.

Companies are also looking to find solutions that are economically feasible, through careful raw material selection, strategic sourcing, and manufacturing optimization, adds Kittler.

“The goal,” Landis explains, “is to develop coatings, materials and technologies that extend the useful life of products and help customers reduce energy usage and emissions, protect their employees, and minimize waste and water consumption.”

One example from PPG is a new matt exterior masonry paint with reduced weight compared to standard products, which translates, according to Landis, to improved ergonomics for professional painters and reduced carbon dioxide emissions over the full life cycle of the paint.

Ultimately, it comes down to commitment, according to Venturini. “Commitment to the programs, commitment to the market, and commitment to the customer are certainly important factors in successfully achieving durable exterior coatings. Commitment to the future leads Sun Chemical and the entire coatings industry to success,” he concludes.

CoatingsTech August 2021 | vol. 18, no. 8

Several high-profile buildings in New York City benefit from advances in powder coating technologies offered by PPG. These include Pier 17 at South Street and several structures in the Hudson Yards development, among others.

Located in Lower Manhattan’s South Street Seaport, Pier 17 anchors an extensive redevelopment project that is focused on transforming the historic neighborhood into a thriving cultural, retail, and entertainment center. Due to Pier 17’s location on the East River, any coating selected had to include a long-term warranty covering film integrity, chalk, and color fade.

The project also required metal coatings free of solvents, which could contribute points towards LEED^® certification for the exterior curtainwall components. In the end, the Pier 17 project represented the first use of PPG DURANAR^® powder coatings in a challenging marine environment.

To achieve the warranty required by the architect, the powder coating was applied over a proven liquid primer to optimize corrosion protection. To make that possible, Spectrum Metal Finishing, which is a PPG/CAP certified applicator and ISO-certified, was chosen because it had the capability to operate both liquid and powder coating lines in the same facility.

Most recently, 55 Hudson Yards in Manhattan became one of the first skyscrapers in the United States to be finished with textured powder coatings. The 51-story building is a landmark in the country’s largest-ever private real estate development.

For the Hudson Yards project, both smooth and textured PPG CORAFLON® powder coatings were used on the metal curtainwall and window frames in the custom color Black Flower, which was developed to mimic the classic cast-iron facades of neighboring SoHo and commemorate the industrial heritage of the building’s Meatpacking District setting. Nearly 20,000 pounds (9.1 metric tons) of the coatings, which are based on a proprietary fluoroethylene vinyl ether (FEVE) resin to provide satin and glosses with outstanding color retention and superior chalk resistance, were applied by Spectrum Metal Coatings, based in Youngstown, Ohio.

The New York chapter of the American Institute of Architects (AIA) named 55 Hudson Yards a 2020 Design Award recipient. It is one of three skyscrapers in the Hudson Yards development finished with PPG coatings.

Exterior surfaces experience degradative environmental conditions, such as intense UV exposure, rain, and temperature swings, leading to deterioration. Coatings are low-cost solutions offering decades of protection and preventing significant repair costs for buildings. The coating must withstand UV, mitigate water damage, and express the flexibility required to maintain adhesion to dimensionally unstable substrates (i.e., wood) as they undergo thermal expansion and contraction through the days and seasons.

BASF has investigated paint film mechanics through accelerated thermal cycling grain crack and tensile testing with intent to correlate the film properties to exterior exposure data. In this article, we demonstrate that adhesion after accelerated weathering, combined with tensile elongation testing, can be used to model outdoor weathering.

Introduction

The construction of new residential and multi-family housing units is on the rise in the United States. For example, construction of new privately owned housing units rose 7.3% from November 2018 to November 2019.¹ Common to all these buildings is some type of exterior cladding; vinyl siding, brick, and veneers are fairly low-maintenance and do not typically require subsequent protection. In 2018, 50% of single-family houses completed used exterior cladding (stucco, fiber cement, or wood siding/trims) that require an architectural coating for both beautification and protection.² In addition to new construction, many existing residential structures require recoats of paint to ensure continued protection and cosmetic appeal when damage to the substrate occurs. In 2019, this demand approximately amounted to 130 million gal (as calculated by BASF using American Coatings Association data) of architectural paint sold in the United States.³

Wood is a very popular building material due to its abundant supply, relatively low cost, wide choice of dimensional pieces, and ease of use in construction. Approximately 22% of single-family houses completed in the past 40 years feature wood siding.⁴ While there are many species of wood with varying properties, the most commonly used are spruce, pine, and cedar. Wood cladding will be subject to stress from thermal contraction and expansion throughout the seasons depending upon its specific thermal expansion coefficient (α). The thermal coefficients for either radial or tangential grain direction can be 5–10 times greater than thermal coefficient of the parallel grain direction.⁵ Additionally, the wood is subjected to UV, which can act to degrade the wood fibers through high-energy radical generation, while water can cause swelling and further expansional strain. Without protection, the combined effects of UV degradation at the molecular level, increased stress from water hydrostatic pressure, and thermally induced movement will generate substantial degradation of wood cladding over time.

An architectural paint not only provides beautification to the building envelope, it also can act to mitigate water absorption and UV exposure for the cladding, reducing the overall stress the cladding is subjected to, and thus extending the life of the cladding, which in turn helps extend the life of the building. When it comes to wood cladding, a protective coating is required to prevent accelerated deterioration. Traditionally, solventborne paints have filled this role, but with the shift over the last few decades to more environmentally friendly paints, waterborne acrylic-based paints have come to dominate residential architectural coatings. Acrylic polymers offer a broad set of monomers, morphologies, polymer glass transition temperature (T_g), and functionality tools for chemists to achieve excellent cost-performance balance.⁶

When developing and marketing waterborne acrylic paints, paint manufacturers often use guarantees claiming up to 25 years of performance to consumers and painters. To back such claims, the industry must continually improve the use of accelerated weathering and laboratory tests to more reliably predict the performance of the coating in the field; waiting 25 years for a coating to fail is not practical. Correlating accelerated testing with natural exposure is no trivial task and requires many testing variables to be taken into account. Given the variation in the North American climate and the high degree of variation within wood species and geometries, there is very unlikely to be one, single predictive test for long-term durability. Rather, the selection of a host of variables and coating performance properties will need to be combined into one predictive model.

Using data from samples of acrylic binders on pine exposed for two years at a 45° south configuration at its Limburgerhof, Germany exposure facility, BASF sought a correlation between lab test data on binders and formulated clear stains and their real-life performance.⁷ While this location is not considered to have extreme weather conditions, it represents a true four-season climate with snow, freeze-thaw cycles, heat, moderate UV exposure, and moderate rainfall. Samples were evaluated after two years on cracking, flaking, and mold growth, among other important attributes, and the candidates were grouped into three performance groups—”no damage”, “starting damage”, and “bad damage”. Lab testing included hardness, water resistance, initial wet adhesion, and film elasticity at 0° C and 23° C. It was found that there was no single correlation between lab testing and the exterior rankings. The better performing samples with “no damage” tended to have higher elongation at break at 0° C, better wet adhesion, and lower water adsorption. However, this trend did not hold true in all cases, and looking across the best (“no damage”) data set, individual coatings exhibit different confounded factors. The findings overall give an indication of important factors to consider in the design of high-performance exterior coatings—most of which align well with common intuitions and expectations—but are unable to assemble a robust predictive model.

Recently, we sought to expand upon these findings by comparing the exterior exposure results of commercial paints after four years in Charlotte, NC at 45° south. Similar to the Germany location, Charlotte is also an interesting exposure site that offers a true four seasons and all the associated weather challenges. The exposure project offered a series of semi-gloss and flat paint weathered samples on wood substrates displaying a balanced and distributed range of cracking behavior, thus making it an interesting candidate set for studying predictive failure. Cracking was exclusively the focus of the exposure series for this study, as this type of failure particulary Ěýleads to loss of protection for the building envelope and results in the costliest claims for paint manufacturers. For the lab studies, we measured adhesion properties to wood, thermal cycling testing by ASTM D6944, and film mechanics at varying conditions. The goal was to determine the accelerated and mechanical lab tests that best correlated to the real-world coating performance. However, we found no single test correlated at all with the four-year exposure results. Rather, the combination of several tests could be used to construct a model that predicted the cracking behavior within the scope of this single board series. This work lays the foundation for our future aims to build, test, and refine a broader and more comprehensive predictive model of exterior wood coating failure.

Experimental Setup

Adhesion Testing

The adhesion of paints was measured using the method described in ASTM D 3359-09e2 entitled “Standard Test Methods for Measuring Adhesion by Tape Test.” Test method B was used with a 255 mm clearance rectangular applicator applied to 0.75-in. southern yellow pine wood. Paints were air-dried for seven days at 72° F and 50% humidity and then placed in fog box for seven days with mist spraying at a flow rate of about 2.3-in./hr. Samples were allowed to be air-dried at 72° F and 50% humidity for four hours before the adhesion testing. A visual adhesion rating was noted for each coating (0B, little or no adhesion; 1B, 20% adhesion; 2B, 40% adhesion; 3B, 60% adhesion; 4B, 80% adhesion; 5B, 100% adhesion).

Accelerated Thermal Cycling Grain Crack Test

All paints were applied to 0.75-in. southern yellow pine as two coats at 72° F and 50% humidity. The second coat of paint was applied 24 h after the first coat. After conditioning for a total of seven days at 72° F and 50% humidity, the specimens were placed into the thermal cycling apparatus, and the cycling procedure described in the Freeze / Thaw / Immersion Cycle Accelerated Testing section was begun. The cycle was repeated for 30 days and rated by visual evaluation following the guidance described in ASTM D661 entitled “Standard Test Method for Evaluating Degree of Checking of Exterior Paints.”

Exterior Exposure

For the purpose of this study, all paints were applied to 0.75-in. southern yellow pine as one coat at 72° F and 50% humidity. After conditioning for a total of seven days at 72° F and 50% humidity, initial surface properties such as gloss, yellowness, and whiteness were measured. All boards were then placed outside at 45° south in Charlotte, NC starting in October 2015. Subsequently, every six months for the following four years, the boards were evaluated for gloss, whiteness, yellowness, cracking, checking, chalking, mildew growth, and dirt accumulation.

Tensile Elongation Testing

Tensile specimens were made by applying paints to Teflon^™-coated panels using a 20-mil clearance square applicator, then allowed to cure at 25° C and 50% humidity. After one day, the films were flipped and allowed to dry for seven days. Dog bone-shaped specimens were then cut from the film, having a width of 0.15 ± 0.01 in. and a gauge length of 1 ± 0.01 in. The thickness of each sample was measured using a micrometer; film thickness ranged from approximately 0.05–0.200 mm. Tensile was measured in triplicate using Instron^® model number 3382. Deformation was applied at 1 in./min until sample film ruptured.

Tensile Elongation Testing Variables

The above film curing, film preparation, and machine protocols were used in all tensile testing. Depending upon the requirement, films were exposed to different environments and include the following: 1) exposure to QUV-A (Q-Lab Corporation, Model Number: QUV/SPRAY; 0.89 W/m²/nm @ 340 nm) for seven days, 2) exposure to a fog box at flow rate about 2.3 in./h for seven days. Once the films were exposed to a desired condition, they were pulled to break at either 25° C, 0° C and/or -20° C. The percent elongation and tensile stress were recorded at the point of film breakage.

Statistical Data Analysis

The data were analyzed by statistical analysis software Modde (Version 12, MKS Umetrics AB Company) using its multiple linear regression model. The grain cracking rating was treated as a continuous response. Factors with a P-value less than 0.05 were considered as significant. Factors with P-value larger than 0.05 were removed from the model. The factors were selected manually to maximize predicted R-squared (Q²).

Results and Discussions

The relationship between physical properties of a coating and film cracking under natural weather conditions has been extensively studied. Once the film cracks, it exposes the wood to more intense degradative effects of the environment. Coating elasticity is, therefore, absolutely required to endure the dimensional changes that both the coating and the wood substrate undergo. But how much elasticity is required is unknown and highly dependent upon the conditions and physical properties of the wood substrate.

To develop a basic sense of how much dimensional instability a wood coating could be subjected to, we measured the expansion of one piece of ~3-in. wide pine wood cut ~70° perpendicular to the annular rings after soaking it in water for three days. By soaking in water, we exceeded the MC_fs (moisture content of fiber saturation), allowing us to measure the maximal expansion at ambient temperature; beyond the MC_fs the wood is known to be dimensionally stable.⁸ We measured a 4.5% width increase for the entire wood section; a typical, if not slightly low value for pine. However, as shown in Figure 1, we also measured the expansion of 40 individual annular rings in (predominantly) the radial direction via microscope. The individual radial expansion and contraction values are graphed in Figure 2. Approximately 70% of the early rings were measured to contract, while 65% of the neighboring, higher-density, late rings were observed to expand. While the overall wood section expanded by 4.5%, the local annular rings experienced a dimensional change from -36% to 43%. This means that discrete sections of a coating can experience these extreme degrees of dimensional strain.

With these results in mind, we considered a range of lab tests that we could use to attempt to make correlations to our four-year commercial paint study. Because wood coating mechanical integrity is clearly a requirement, we put much emphasis on studying the mechanics of the coating via tensile elongation and accelerated thermal cycling tests to understand the impact of water and temperature-induced dimensional stress on a coated piece of wood. Additionally, we incorporated adhesion to water-conditioned boards because the dimensional stress between the coating and the wood is highest under water-induced expansion and contraction.

Exposure Series

Twelve flat and five semi-gloss, untinted, white waterborne commercial paints from several paint manufacturers were used in this study dating back to 2015. The paints were applied with one coat by brush on 0.75-in. southern yellow pine, with at least one duplicate, placed on a test fence facing south at 45°, and tracked for four years in BASF’s Charlotte, NC exposure facility. The grain cracking of paints was rated by visual evaluation on a 0–10 scale (10 being the best) according to ASTM D661. Images referenced in ASTM D611 are from the Pictorial Standards of Coatings Defects⁹ as reproduced in Figure 2. Table 1 shows the paints and the average crack rating after four years of exposure.

Freeze / Thaw / Immersion Cycle Accelerated Testing

A variety of accelerated weathering methods have been developed to predict the coating performance in the real world. However, correlations between these accelerated methods and real-world exposure are not well established. For example, ASTM D6944 describes an accelerated thermal cycling method that includes freezing, thawing and immersion steps to determine the coatings resistance to such conditions. Properties such as checking, cracking, blistering, or adhesion loss can be gauged with an accelerated stress test like ASTM D6944. However, the method itself does not purport that it can provide a quantitative prediction of service life of a coating. We thus first wanted to test if ASTM D6944 bears any correlation to real-world exterior performance.

In this study, we slightly modified the example cycling parameters of ASTM D6944 Method A to fit our lab schedule. The 17 selected paints were applied by brush at two coats at natural spread rate on southern yellow pine and air-dried in a controlled temperature and humidity room (23^o C, 50%). After curing for seven days, the specimens were placed into the thermal cycling apparatus and the cycling procedure shown in Table 2 began.

The cycle was repeated for 30 times and rated by visual evaluation. Only three paints (Flat-5, Flat-8, and SG-1) showed noticeable cracking with respective ratings of 6, 8, and 8, while the other paints had no cracking after accelerated testing (rating=10). The relative crack rating obtained from this testing does not correlate with the four years of exposure, showing that, with its current parameters, it is not a sufficient method to predict grain cracking.

It is also worth noting that the cracks we observed in the thermal cycling test were always preceded by blister formation immediately after several cycles. The blisters could be observed immediately after removal from the water bath and then would recover over time with exception of FL-5, which suffered irreversible, persistent blister deformation. The observation indicated that the failure mode in this testing could not be deconvoluted from either thermal expansion of the substrate or blistering. It is entirely possible that the cracking we observed was the result of stress caused by hydrostatic deformation from blister formation and not thermal expansion of the wood substrate. Meanwhile, in our real-world exposure testing, we were not able to capture blister formation, but did observe a range of cracking behavior. Because the boards are rated on a biannual basis, it is entirely possible that blisters formed and recovered. Again, in the real-world exposure, we could not deconvolute from either thermal expansion of the substrate or blistering. These results suggest ASTM D6944 is not sufficient as a predictor of real-world cracking behavior. It is possible that other lab or accelerated tests may help serve as better predictors of cracking. In the following sections, a range of other test methods are explored as means to find better correlation of the lab results with real-world performance.

Tensile Strength and Elongation Testing at Room Temperature

Despite the general agreement that the mechanical performance of a coating, such as strain at break, has a strong influence on the grain cracking resistance, a correlation and prediction model has not been well established.¹⁰ Tensile strength and elongation testing is typically used to characterize the brittleness and ductility of a paint film. This test can provide very useful information of the mechanical properties of specimens, including elastic modulus, strain, and stress at break, total energy to break, etc. In the first testing series, the stress and strain response at break were measured for all 17 paints at room temperature as described in the experimental setup. For all our tensile elongation testing, dog bone-shaped films were chosen over rectangular strips, as previous comparative studies of film geometry have shown that this shape yields a lower standard deviation.¹¹ To further help reduce errors, we also followed the recommendations from this study to maintain consistent tensile test speed rates and a minimum dry thickness of 80–100 mm.

It is often assumed that higher strain at break at room temperature implies that the coating is more ductile and that, in turn, this property should translate to suppressed cracking over the lifetime of the exterior coating. Figure 4 is the plot for grain cracking rating after four years of exposure vs strain at break at room temperature for the 17 paints. Based on this data, it is clear there is no obvious correlation. We can see that higher strain at break does not necessarily lead to better cracking resistance. For some paints, a good crack rating can be achieved even though the strain at break was as low as ~25%.

The results suggest that the tensile elongation test for a fresh film at room temperature is not sufficient to predict the cracking resistance. During outdoor exposure, coatings experienced varieties of weathering condition such as sunlight, raining, etc. The greatest substrate deformation can happen at any time of the year depending upon the conditions. In Charlotte, NC, the boards in this study would have experienced typical temperature ranges from -10 ^oC to 30 ^oC, average annual rainfall of 42 in. and an average of 40 freeze-thaw cycles per year. Therefore, it was prudent to explore the tensile elongation performance for specimens after different accelerated artificial weathering conditions and test them at different temperatures. The combination of results of tensile elongation under different accelerated weathering and temperature conditions may provide more insight of grain cracking in real-world.

Tensile Strength and Elongation Testing at Low Temperature

Typical polymer glass transition temperatures (T_g) of an exterior architectural paint are between -10^o °ä–30^o C. At temperatures close to and below their glass transition temperature, films are more brittle. Therefore, it is important to test tensile elongation at low temperatures. In this section, all specimens were prepared as described in the experimental setup. The tensile elongation tests were performed at two low temperatures: 0^o C and -20^o C. The strain at break vs grain cracking ratings at both temperatures are shown in Figure 5 and Figure 6. Once again, we can see that neither of these testing conditions alone correlated well with the grain cracking ratings.

Tensile Strength and Elongation Testing After Accelerated Weathering

In the real world, UV light from the sun and water can combine to change the mechanical properties of paint film over time. Typically, the flux of substrate, the coating dimensional instability (driven by hydrostatics and thermal expansion) and the increased frequency of free radical generation from UV lights act to accelerate damage to the coating. Therefore, it is common to use several accelerated weathering conditions to simulate natural weathering. This section describes QUV, or fog box, methods applied to specimens to study the UV and water damage, respectively.

Samples After QUV Exposure

The tensile test specimens were prepared as described in the experimental setup and specimens were then placed in a QUV chamber. To isolate the photodegradation effect from water damage, no condensation cycle was used in the procedure. After seven days UV exposure, specimens were taken out of the QUV chamber and allowed to equilibrate in a controlled temperature and humidity room (23^o C, 50%) and then tested by tensile elongation. Figure 7 shows the grain cracking rating after four years exposure vs room temperature strain at break after seven days QUV. We can see that there is still no strong correlation demonstrated in the plot.

Samples After Fog Box Exposure

In this study, an in-house-developed fog box method was used to simulate water damage of the coatings. In the fog box, water mist was sprayed continuously from the top, and all specimens were laying horizontally on a piece of polyolefin substrate. After seven days in the fog box, specimens were removed from the fog box, air-dried for one day in a controlled temperature and humidity room (23^o C, 50%) and then tested by tensile elongation. Figure 8 showed the grain cracking rating after four years exposure vs room-temperature strain at break after seven days in the fog box. Again, there was no strong correlation demonstrated in the plot.

Adhesion After Water Conditioning

R. Sam Williams et al. discussed the importance of adhesion to substrates.¹² They demonstrated that weaker adhesion to substrates leads to earlier cracking. Thus, we elected to test paint adhesion to pine wood boards after extensive water conditioning of the painted board in the fog box to simulate natural conditions. After seven days in the fog box, specimens were air-dried for four hours in a controlled temperature and humidity room (23^o C, 50%). After four hours, the adhesion was evaluated following ASTM D3359 test method B. Figure 9 showed the grain cracking rating after four years exposure vs adhesion after seven days
in the fog box; no simple correlation can be observed.

Developing New Methods to Predict Grain Cracking

As shown above, no single tensile elongation or accelerated weathering test has a clear correlation with grain cracking. In the real-world, the failure of a coating is more likely a combination effect of different failure modes. All lab tests in the Experimental Setup section explore the failure under one single condition; therefore, it is not surprising that the correlation between any single condition testing result and the real world is weak. In this section, we combined results from the Tensile Strength and Elongation Testing Variables, and Adhesion After Water Conditioning sections, and used MLR to fit the grain cracking rating. We found good correlation when we considered all factors together in the regression fitting.

Stress and strain at break under different accelerated weathering and testing conditions (as described in the Exterior Exposure, Tensile Elongation Testing, and Tensile Elongation Testing Variables sections), adhesion rating (as described in the Statistical Data Analysis section), and pendulum hardness were used as the factors to fit real-world grain cracking ratings for the 17 commercial paints. Again, the data were analyzed by statistical analysis software Modde using its MLR model. Table 3 lists the significant factors for grain cracking and Figure 10 shows their coefficients in MLR fitting. Compared to single-factor effect discussed in the Tensile Elongation Testing Variables and Statistical Data Analysis sections, the multifactor regression model improved fitting with predicted R-squared (Q²) of 0.879. Figure 11 showed good agreement between the observation and model prediction.

It was encouraging that combining several test results provides a better fit for the grain cracking rating, which was not achievable from a single lab testing. The results may guide us to a potential method to predict grain cracking in natural weathering conditions. Unfortunately, using the coefficients to investigate the factors’ influence is limited since some factors were correlated (multicollinearity). For example, it was expected that strain at break for a fresh specimen correlated with strain at break for a specimen after sevenĚý days QUV exposure, as shown in Figure 12. When we included correlated factors in the model fitting, the contribution from each individual factor cannot be investigated through the coefficients. However, if the model is valid, it can still be used as a predictor for grain cracking.

The square term of adhesion (adh^2) was considered significant and included in the model and exhibited a stronger correlation than the adhesion term alone. Since adhesion to a water-conditioned, painted sample has no strong correlation with other tensile elongation tests, we can investigate the contribution from adhesion. If the model is valid, the square term in this model indicated that the grain cracking was not a monotonic function of adhesion. This suggests the case that not too weak, but not too strong, adhesion may lead to better grain cracking resistance.

In the development of the model, we were concerned about overfitting since the R-squared (R²) seems to be high for this study, while tensile elongation testing and a natural weathering test usually has large standard deviations. In this model, the difference between R-squared (R²) and predicted R-squared (Q²) is only 0.106, which in general indicates overfitting was not observed. Nonetheless, further model validation with new data sets is critical if we are to develop a robust predictive tool.

The MLR model fitting was based on the performance of 17 paints in a single study design at, and only at, the four-year point in Charlotte, NC. The results from the model may not be applicable to other locations, climate, or durations. We expected that coefficients to be a function of weathering history. Under different weathering history, the contribution of each factor may also be very different. It is our intent to further investigate these variables and use them to refine future predictive models.

Summary and Conclusions

Effective exterior wood coatings can significantly extend the life span of the wood substrates if they can resist cracking and substrate adhesion loss. Within small loci, wood coatings can experience tremendous dimensional stress at the interface of the coating and the wood with varying temperatures and moisture uptake. Film flexibility is a must for an exterior wood coating, but direct correlations of this flexibility measured at varying conditions to real-world exposures is very weak.

Separate studies by two of our coatings development research groups show that the results from multiple lab and accelerated test methods are significantly confounded and do not generate easily decipherable trends. By measuring tensile elongation at multiple conditions along with adhesion to water-conditioned surfaces and film hardness, we were able to build a predictive model with a very good fit. The resulting model, however, is at this time limited to the scope of a single commercial paint study. Further, significant validation and expansion to other test methods, paints, and exposure parameters is required to progress the predictive model to the point where it can be a useful tool in the development of high-performance exterior wood coatings.

Acknowledgements

Deep gratitude to our colleague Dr. Roland Baumstark, who has contributed many years of coatings experience and fundamental work to achieve key insights in our understanding of exterior coating performance. Special thanks to Dr. Bas Lohmeijer and Robert Wrazidlo for stimulating discussions and wood swell measurements. Great support was received from Dr. Keith Task for statistical data analysis and model validity interpretation.

References

1. United States Census Bureau Public Information Office; Monthly New Residential Construction, November
2019, December 17, 2019. United States Census Bureau. www.census.gov/construction/nrc/pdf/
newresconst_201911.pdf

2. United States Census Bureau, Survey of Construction—Annual Characterisitcs of New Housing, 2018. United States Census Bureau. www.census.gov/construction/chars/

3. American Coatings Association. (2019). Architectural Industry Pulse.

4. United States Census Bureau, Survey of Construction—Annual Characterisitcs of New Housing, 2018. United States Census Bureau. www.census.gov/construction/chars/

5. U.S. Deptartment of Agriculture. (2010). Wood Handbook. Madison: The Forest Products Labratory.

6. Schwartz, M., & Baumstark, R. (2001). Waterbased Acrylates for Decorative Coatings. Hannover: Vincentz Press.

7. Baumstark, R. (2014). How to Predict the Durability of Water-based Exterior Wood Coatings? PRA 9th Wood Coatings Congress, (p. 12). Amsterdam.

8. U.S. Dept. of Agriculture. (2010). Wood Handbook. Madison: The Forest Products Labratory.

9. Federation of Societies for Coatings Technology. (1979). Pictorial standards of coatings defects. Philadelphia: Blue Bell.

10. Floyd, F. L. (2001). Modeling and Laboratory Simulation of Grain Cracking of Latex Paints Applied to Exterior Wood Substrates. ACS Symposium Series.

11. Schirp, C. (2016). News about testing of film mechanics of exterior wood coatings–Parameters, challenges and possibilities. PRA’s 10th International Woodcoatings Congres. Amsterdam.

12. Williams, R. S. (2004). Comparison of Traditional Methods for Testing Paint Service Life with New Methods for Service Life Prediction. 3rd International Symposium on Surfacing and Finishing of Wood. Kyoto.

CoatingsTech | Vol. 17, No. 10 | October 2020