ľţ˛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.

Anyone who has had the pleasure of scraping ice off a car windshield, looking nervously at beautiful but deadly ice spears hanging from winter rooftops, or waiting on a slow and interminably long aircraft deicing line at an airport has dreamed of having some sort of coating that would eliminate inconvenient ice and snow accumulation. The desire to triumph over ice and snow accumulation has, in fact, been the focus of significant technical work that has been going on since the 1940s in an attempt to make all kinds of surfaces ice and snowphobic.

Icephobicity is defined as the ability of a solid surface to repel ice or prevent ice-film formation due to topographical structures at the ice-surface interface. Ice-repelling characteristics are attributed to the surface structure (sometimes called surface roughness) and/or low surface energy leading to poor ice adhesion allowing for easy removal.

Icephobic coatings are formulated to ensure that ice doesn’t accumulate on or strongly attach to a surface. In fact, fully functional icephobic coatings operate on several different levels. They can have the ability to repel water droplets, delay ice nucleation, and/or reduce ice adhesion.

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 SelektopeÂŽ – 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.

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

Two-component (2K) epoxy coatings have long been used for industrial protective applications due to their excellent adhesion to a wide range of substrates, superior chemical resistance, and good mechanical properties.¹ Today, those in the industry who are looking for innovations that can boost coating performance heavily prioritize high performance and productivity in particular. Their main goal is to extend coating life service time and use the fewest coating layers possible. Additives, or performance boosters, are commonly used for this purpose in a formulation. One example of an additive is tougheners, which are used in many high-performance industrial protective coatings to improve their toughness, flexibility, adhesion, and long-term performance.

Phenoxy resins are unique polymer-based additives that have become available to formulators. Phenoxy resins are the product of bisphenol A and epichlorohydrin, with the epoxide ring opened. They are a tough and ductile thermoplastic material with high cohesive strength and good impact resistance. The backbone ether linkages and pendant hydroxyl groups promote wetting and bonding to polar substrates and fillers. The typical phenoxy resin structure is shown in Figure 1.

The structure of the phenoxy resin is a polyhydroxyether with terminal alpha-glycol groups. Weight-average molecular weights range from approximately 25,000 to above 60,000, with n ranging from 30 to above 60. This long chain linear polyhydroxylether structure allows for excellent adhesion, impact and abrasion resistance, flexibility, and chemical resistance in coating applications.

Phenoxy resins are soluble in a variety of materials, including ketones, glycol ethers, and glycol ether esters. This article discusses the use of a phenoxy solvent solution, in a solventborne zinc-rich primer formulation. A performance comparison with a commercially available zinc-rich primer will also be discussed, specifically comparing adhesion, dry time, hardness, impact resistance, and salt spray corrosion resistance.

Experimental

Raw materials and testing panel preparation

A commercial, two-component (made up of part A and part B), solventborne zinc-rich primer was chosen in this study as the commercial control, referred to as Control. A solventborne phenoxy solution, referred to as PKA (the phenoxy resin additive), was added to part B (the amine curing agent side) as an additive. The physical properties of the Control and PKA are described in Tables 1 and 2, respectively.

The two-component coatings were prepared by mixing part A and part B in an overhead mixer on the 100- to 300-gram scale. By adding different amounts of PKA, which was based upon the total formulation weight, to part B, several phenoxy-modified coating formulations were obtained, and those three are referred to as 1%, 2%, and 4%.

Testing panels were prepared by drawing down the two-component coatings over xylene degreased steel panels, generally using a 10-mil gap 3-inch drawdown bar. The panels were typically allowed to cure for 7 days at 23 ˚C and 50% relative humidity before testing, unless otherwise noted. The dry film thickness of each testing panel was about 3–5 mil.

Testing Procedures

Dry Time

Coatings were drawn down onto glass substrates with a wet film thickness of 75 Âľm and set on a B.K. Linear Drying Time Recorder. The dry-to-touch time was visually assessed after dragging a needle through the coating over the course of 24 hours, according to ASTM D5895.

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INTRODUCTION

Concrete is one of the most used construction materials due to its strength, durability, resilience, safety, and low cost. In the flooring applications, demand for concrete for interior and exterior use in residential and commercial settings is growing significantly thanks to the strong construction market around the globe.

Coatings for concrete floor not only protect concrete from wear, deterioration, and contamination, but also enhance the physical performance, provide chemical resistance, and improve aesthetics. Today, architects, specifiers, and end-users can select from a broad range of technologies and finishes to protect and enhance the aesthetics of the concrete floor. Each technology or finish brings specific benefits and introduces trade-offs in other areas. Polymer technology is among the most popular choices for concrete flooring and include thermoset chemistries such as amine-cured epoxy, urethane, urea, methacrylate, and acrylic. The preferred option depends on the type of application and the required performance properties.

High-performance floor installation using polymer technology requires at least two stages (primer and topcoat), and preferably three (including a midcoat) as shown in Figure 1. With each stage needing time to cure, fast-cure speed to reduce the total installation time translates into minimum downtime and fast return to service. This has been a key market driver across the coating industry. Yet, fast-cure speed often comes as a trade-off for short working time. Balancing fast cure and good working time remains a challenge for coating industry. This article describes fast-cure amine technologies using a system approach that enables the installation of a multistage floor system within one day while maintaining good working time at each stage and high-performance of the entire system.

FIGURE 1—Representative examples of a high-performance flooring system.

As shown in Figure 1, the concrete primer serves to penetrate and seal the concrete pores because concrete is a porous and permeable material. Primer establishes good adhesion and bonding between the concrete substrate and the polymer overlayment. Cured concrete traps various amounts of moisture, about 1% to 2% in ambient dry concrete, and 4% to 5% in damp and wet concrete; however, the corresponding relative humidity inside concrete is as high as 75% to 95%. When a primer is applied at low temperature, applicators also need to consider the dew point to avoid moisture condensation during or shortly after the application of the primer. Two-part amine-cured epoxy primers have proven to tolerate the challenges introduced by concrete as a substrate and provide good adhesion to dry and damp concrete even at low temperature and high humidity. In addition, advancement in waterborne technology equips the formulators with waterborne epoxy system as a new tool to meet performance, low volatile organic components (VOC) and low emission requirements.^1–3 It has gained wide acceptance as an environmentally friendly alternative to the solventborne system due to the improved performance made during the past two decades.

Coated on top of the primer is the polymer midcoat and then topcoat. Until recently, topcoats have been dominated by two key technologies: amine-cured epoxy and polyurethane based on polyol-cured isocyanate. Lately, another two-component (2K) aliphatic polyurea, referred to as polycarbamide or polyaspartic technology, has been developed to practical industry use. It is derived from polyamine cured isocyanate chemistry.^4–7

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Industrial wood coatings encompass several market areas, including furniture, kitchen cabinets, building products, and decorative coatings. Requirements for these markets largely depend on their field of application.

Exterior performance is focused on high durability and protection against humidity, while interior coatings require properties such as scratch, chemical and abrasion resistance. One of the largest sectors of the interior market is the furniture industry.

Several resin technologies are being used by coatings formulators in this market, including solventborne (SB), waterborne (WB) UV polyurethane dispersions (PUDs), and self-crosslinking acrylics. Several criteria are of importance in considering which technology to use.

Each technology has advantages and disadvantages, and a comparison has been summarized based on the criteria in Figure 1. The dominant technology used in North America is solventborne, including nitrocellulose (NC) and acid cure conversion varnish. These coatings have many benefits, including fast dry time and very high gloss, and they enhance the wood appearance. They are also very economical and can be easily applied by spraying, rolling, curtain coat, and dipping.

However, a significant disadvantage of using these materials is the high level of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) and the limited varnish pot life. Due to increasing regulations for lower VOC and formaldehyde emissions, more environmentally friendly coatings are now in demand.¹ This shift has opened the door to waterborne technologies, including UV PUDs and self-crosslinking acrylics.

UV PUDs are increasingly gaining acceptance in the market as a replacement for solventborne because they have very low emissions. They offer high-end performance with minimal process issues. Since UV PUDs are high-molecular-weight polymers, the crosslink density of the cured networks compared to 100% solids is lower, limiting shrinkage after cure and resulting in excellent adhesion to most substrates. They inherently yield good mechanical performance because they have hard urethane and urea domains that can have hydrogen bonding, coupled with softer domains that come from the choice of raw material building blocks such as the polyols.

Some of the challenges with using WB UV are related to processing. It is essential that water is completely released prior to cure, and factors such as humidity must be considered to minimize production of defective parts due to incomplete drying. Additionally, the cost of this technology is higher compared to SB.

Self-crosslinking (SC) acrylic dispersions are also included amongst WB resin technologies. Overall, they have good durability and can be formulated into high-performance coatings with a low coalescent demand due to phase-separated morphologies in the polymer particles. Several types of morphologies can be achieved depending on the polymerization strategy that is applied, which also influences film properties such as block resistance. These materials can also be blended with WB UV resins to offer a more economical formulation while maintaining excellent performance.

Areas of concern include the presence of surfactants that are required for the colloidal stabilization of the polymer particles. Such components can migrate to film surfaces imparting water sensitivity into the film or may lead to foaming issues during formulation. Regarding aesthetics, acrylics also are not especially noted for enhancing the appearance of the wood substrate and most often they lack wet clarity. While WB acrylics are higher solids compared to SB/NC lacquer, they generally do not produce a smooth haptic touch.

In this article, water-based technologies, including UV-curable PUDs and SC acrylics, have been evaluated for industrial wood coatings. These resins have been designed to fulfill the range of requirements needed for adequate protection of the substrates, minimizing formulation issues and ease of processing. Table 1 provides basic properties of the resins used to formulate these coatings. This investigation will further detail the comparison of these resins to traditional resin types used in these markets.

In the coatings industry, the concept of sustainably is becoming increasingly important, whether to meet regulatory requirements or growing customer and consumer expectations.^1,2 Sustainability is also a significant driver for coatings innovations.^3,4,5

The United Nations has adopted a list of 17 “Sustainable Development Goals”⁶ that seek to define a blueprint for sustainability on a global scale. Many of these relate to sustainability in coatings, such as reducing volatile organic compounds (VOCs) and greenhouse gases and replacing fossil fuel-based products with biosourced materials.^7,5

With these UN Sustainable Development Goals in mind, a new coalescent technology for waterborne paints that meets the zero-VOC requirement as measured by ASTM D6886 and has a high biobased carbon content of 96% as measured by ASTM D6866 has been developed.

This goal of the study described in this article is to evaluate the performance of this product in several representative architectural and industrial waterborne paint formulations and benchmark performance against other VOC and zero-VOC coalescents available on the market.

Performance of this new coalescent was evaluated in a range of waterborne binder chemistries, including all acrylic, vinyl acrylic, styrene acrylic hybrid, and self-crosslinking acrylic systems in several architectural and industrial maintenance paint formulations.

This new coalescent technology was found to compare favorably with commercial zero-VOC coalescents in architectural coatings formulations. In industrial coatings formulations, substitution of VOC solvents, in whole or in part, with this new technology allows for significant lowering of overall formulation VOC while maintaining good coating performance. Improved metal adhesion and impact properties were observed with retention of high gloss and good corrosion resistance.

Experimental Details

Test Methods

Viscosity and Heat Age Stability: The initial Stormer viscosity—measured with a Krebs Stormer viscometer according to ASTM D562 and reported as Krebs Unit (KU)—was measured at room temperature prior to placing the paint can into a 120 °F oven. The can was removed after 2 weeks and allowed to return to room temperature. The Stormer viscosity in KU was measured again.

Gloss measurements and yellowness index: Gloss and yellowness of paint films were measured using micro-TRI-gloss and color-guide 6805 (both meters from BYK-Gardner), respectively.

Minimum Temperature Film Forming (MFFT) testing: MFFT testing was carried out on a Rhopoint instrument equipped with a variable temperature MFFT bar. A temperature range of -4.5 °C to 13 °C was used. Latex films were drawn down at 75 microns thickness and allowed to dry under flowing N₂ gas for 30 minutes. MFFT temperature readings were taken at the point where the latex film transitioned from clear cohesive film to white powder.

Tint strength: Five grams of Colortrend Phthalo Blue was weighed into a half-pint can containing 250 grams of test paint. After the colorant was added, the paint can was shaken on a Red Devil shaker for 3 to 5 minutes. Paint drawdowns using the tinted paint compositions were then prepared on Leneta B charts using a 3 mil bird bar. These were allowed to dry for 1 day in a controlled temperature and humidity chamber at 25 °C and 50% relative humidity. The Y% brightness value was measured on a colorimeter and the percent tint strength was calculated by the Kubelka-Munk (KM) formula.

Washability and Stain Removal: Paints were applied at a wet film thickness of 7 mils to black Leneta Scrub Charts and allowed to dry for a minimum of 3 days. CIELAB color values were measured prior to application of stains. Stains were allowed to set for 2 hours and excess stain was removed by rinsing under cold water. Samples were placed into a Garner Straight Line Washability and Wear Abrasion Machine and scrubbed for 50 cycles with the addition of 10 mL of Formula 409 solution to the premoistened sponges. Samples were rinsed with water and dried. CIELAB color values were measured on areas where the samples were scrubbed and delta E values are reported.

Mudcracking: Paint drawdowns were made using a Sag bar on 1B Leneta charts and allowed to dry for 24 hours. The greatest thickness that did not show cracking is reported.

Dry Adhesion: The test paints were applied to untreated aluminum, cold rolled steel and galvanized steel using a 4 mil Bird applicator. The paints were allowed to dry for 7 days before the lattice pattern described in ASTM D3359B was cut and the pressure sensitive tape applied. The removal of the coating from the substrate was rated using the classification charts described in the ASTM Standard.

Flash Rust: The cold rolled steel panel from the adhesion test was examined after drying overnight. The panels were rated on a scale where 10 = no rust and 0 = heavy rust.

Other standard test methods used: Scrub Resistance ASTM D2486-06, Test Method B; Wet Adhesion to alkyd paints ASTM D6900; Block Resistance ASTM D4946; Chemical Resistance, ASTM D1308, 1 Hour Contact; Impact Resistance, ASTM D2794, 1 Week, R612 CRS Panels; Humidity Resistance, ASTM D2247, R46 Panels; ASTM B117-09 Standard Practice for Operating Salt Spray (Fog) Apparatus – R46 Panels; Prohesion, ASTM G85 Annex, R46 Panels.

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References

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2. Challener, C. “An Update on Sustainability in the Coatings Industry.” CoatingsTech. April 2018. /coatingstech-magazine/articles/an-update-on-sustainability-in-the-coatings-industry/ (accessed Sept 9, 2021).

3. Challener, Cynthia “Industry Update: The State of Coatings R&D.” CoatingsTech. August 2017. /coatingstech-magazine/articles/industry-update-state-coatings-r-d/ (accessed Sept 9, 2021).

4.Ěý Challener, C. “Resin Technologies for Industrial Maintenance Coatings.” CoatingsTech. January 2018. /coatingstech-magazine/articles/resin-technologies-industrial-maintenance-coatings/ (accessed Sept 9, 2021).

5. Challener, C. “Innovation in Architectural Coatings: Meeting High-Performance and Sustainability Expectations.”CoatingsTech. April 2021.

6. United Nations Department of Social Affairs. https://sdgs.un.org/goals (accessed Sept 9, 2021).

7.Ěý Pilcher, George “Sustainability in the Paints and Coatings Industry: Far More Than Just a ‘Good Idea.’” CoatingsTech. May 2021.

8. Arkema. https://www.arkemaepoxides.com/en/ (accessed Sept 9, 2021).

9. Flack, K. et al. “Driving Performance via Permanent Coalescent Choice in Low-VOC Architectural Paints” CoatingsTech. April 2017.

10. Hansen, C. “Solubility Parameters.” Paint and Coating Testing Manual. Ed. Koleske, J. V.; ASTM 1995.

11. “The famous factor of 4—Dr. Hansen’s view” https://www.hansen-solubility.com/HSP-science/4factor.php (accessed Sept 9, 2021).

12. Auld, K., Padaon, M., Procopio, L. “Direct-to-Metal Coatings Under 25 g/L VOC.” CoatingsTech. Sept 2020.

13. “Roughness and Surface Coefficients” Engineering Toolbox. https://www.engineeringtoolbox.com/surface-roughness-ventilation-ducts-d_209.html (accessed Sept 9, 2021).

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15. Pujala, S. B., Chakraborti, A., “Zinc(II) Perchlorate Hexahydrate Catalyzed Opening of Epoxide Ring by Amines” J. Org. Chem, 2007, 72, 3713-3722.

16. Monaghan, G., Swaim, N., Estill, B. “Metal Adhesion and Corrosion Resistance of Coatings.” PCI. May 2018.

17. Goldschmidt, A., Streitberger, H.J. BASF Handbook: Basics of Coating Technology, 3^rd ed. BASF Coatings, 2018.

A new application of Michael Addition (MA) chemistry was recently explored and developed for use in industrial flooring. Historically, MA was not applied in protective coatings due to the extremely fast reactivity which resulted in poor appearance and unacceptably short pot life.

Now, the development of a blocked catalyst and incorporation of kinetic additives have allowed formulators to tame and control the speed of the MA reaction. This novel technology exhibits excellent chemical properties compared to traditional systems, providing the best balance between working time, appearance, and fast return to service.

This MA technology is comprised of a wide variety of donor and acceptor resins, giving the formulator the ability to tailor coatings to meet specific performance goals. Polyaspartic (PA) technology is used when fast cure and hardness development is required in flooring systems. However, this contractor-applied system requires highly skilled labor due to its very short pot life and sensitivity to adverse application conditions such as high environmental moisture and temperature.

In this study, different combinations of resins and kinetic additives were explored to match properties of a commercial polyaspartic for industrial flooring. The MA-based system produced coatings that displayed outstanding chemical and physical properties, exterior durability, longer working times, excellent appearance, low volatile organic compounds (VOCs), and fast return to service.

Another added value of this technology over traditional systems is the offering of a non-isocyanate (NISO) solution to customers who are looking to comply with increasing regulatory restrictions. This study also presents the rheology study of various formulations and demonstrates that lapping time and flow-leveling of MA coatings can be modified to meet industrial market needs.

INTRODUCTION

Floor coatings are of great importance in the industrial, commercial, and architectural markets because they provide protection to the substrate and aesthetics to the environment. Concrete floors are the preferred substrate due to hardness, durability, relative low cost, and ease of installation. However, concrete floors lack chemical resistance and flexibility to achieve different degrees of finish (high gloss, colors, special effects, etc.).

A wide range of polymer technologies are used as protective coatings ranging from acrylic emulsions or thermoplastic acrylics to high-performance two-component (2K) systems such as epoxies, polyaspartics, and polyurethanes (PUs).

High-performance 2K protective coatings for flooring exhibit unique properties such as excellent appearance, high-mechanical properties (such as toughness and high hardness), and chemical resistance. Epoxies that are 100% solid are widely used due to their low cost, near-zero VOC capability, and exceptional adhesion to several substrates.

On the downside, epoxies can be brittle, exhibit poor UV resistance, and long curing times.ĚýIn commercial and industrial flooring maintenance, fast return to service is essential because floors require high-efficiency installation to avoid expensive long downtimes in production or temporary closure of facilities.

Most applications in construction or maintenance require special equipment and a large team of highly skilled personnel. This translates into a high cost per square footage. Economical losses are also produced when asset owners must wait to resume activities to allow the coatings to develop the necessary chemical and mechanical properties.

One of the fastest cure technologies for flooring is based in the radical polymerization of methyl methacrylate (MMA). This type of reaction offers a fast return to service, though it lacks ease of application. Workers must be highly trained to avoid errors regarding in situ stoichiometry calculations, evaluation of substrate conditions (moisture in concrete), and preventing accidents due to the hazardous materials handled. Another big complaint from end-users is the strong smell lingering during and after application.

Until now, PA technology could be considered the best compromise between ease of application, high-performance properties, and fast return to service. The latter characteristic has positioned this technology as the preferred product to use among contractors in North America.ĚýPAs are based on the reaction of an aliphatic polyisocyanate and a polyaspartic ester, which is an aliphatic diamine.

PAs used in fast return-to-service flooring applications have 15- to 20-minute pot life, requiring a large crew of contractors to handle the application to achieve good appearance, eliminate lap marks, and avoid wasting expensive material. Sometimes, applications must be rescheduled due to high environmental temperatures and high humidity. These conditions shorten working time even further and could produce undesirable effects such as microfoaming, haziness, and blistering on the coating.

A novel technology launched in 2015 has proven the possibility of using MA chemistry in the industrial metal protective market.ĚýMA decouples the relationship between pot life and speed of cure, producing polymers with excellent appearance and outstanding chemical properties. MA relies on the formation of carbon-carbon bonds when an acidic proton is subtracted from a malonate group through a strong base. The resulting carbanion reacts with the partially positive carbon in the pi bond of an acrylate.

Figure 1Ěýshows an example of how MA chemistry is translated to crosslinking in polymers. A resin bearing a malonate functional group (donor resin) is deprotonated by a strong base (catalyst) and reacts with the acrylate group contained in a monomer or polymeric resin (acceptor resin). In this 2K MA technology, both donor and acceptor resins are blended (Part A), and then the mechanism of polymerization is triggered by the addition of a blocked catalyst (Part B).

References

To achieve that goal, the company had to overcome two key challenges presented by applying coatings in the field. Factory coatings are customized to the surface being painted, and they are applied under highly controlled conditions that are optimal for painting. Therefore, customization of coatings to the surface being painted and adjustment of the coating formulation to match the environment would be needed to create a similar applied appearance in the field.

How can that be accomplished? Only with a deep understanding of coatings chemistry and the ability to apply advanced software technology to solving the problem. “Our motto is to use chemistry to create smart renovation solutions that make our customers happy by making their lives easier,” says Marsala. After thousands of hours in the lab, the company developed a solution that enables the adjustment of a paint formulation based on the expected weather conditions, the nature of the surface to be painted and the customer’s color selection. “Some surfaces need flexibility and breathability, while others need heat resistance and hardness. We customize each coating to perform based on a substrate’s unique properties for the best results,” Marsala explains.

Their solution is patented, thus making Spray-Net the only exterior painting company that can provide a real-time, weather-adjusted paint job, according to Marsala. The result, he adds, is a long-lasting finish that won’t peel, does not require maintenance every 2 to 5 years and looks like-new, not repainted. Spray-Net delivers a factory finish on aluminum and vinyl siding, stucco, brick, fiber cement, roofs, and even surfaces that are not traditionally painted on-site, like front and garage doors, windows, and kitchen cabinets.

When homeowners Spray-Net their homes, they get more than a regular paint job, Marsala stresses. “Leveraging chemistry and technology makes us a different kind of painter, and it’s what allows us to innovate new renovation solutions that provide homeowners with value,” he asserts. “Building owners get a real and cost-effective alternative to replacement with Spray-Net, because our coating solutions instantly transform the look of a home for a fraction of the cost of brand-new siding, doors, and windows,” Marsala observes. “We offer a whole new way to renovate.” Spray-Net’s goal is to provide every homeowner with a renovation that increases property value, is practical, cost-effective, and makes them fall in love with their home all over again, he concludes.

Spray-Net, because it is selling home transformations and not paint itself, invest in the latest coating technologies and top-of-the-line ingredients to equip its painters with the premium paint they need to deliver factory-quality results on siding, doors and windows, according to Marsala. Its paint line includes acrylic dispersions for aluminum, fiber cement and engineered wood siding, two-component polyurethanes where protection from UV rays, abrasions and regular washing are required, breathable elastomeric copolymer emulsions for stucco and a silicate stain that forms a chemical and mechanical bond with brick. Proprietary software enables customization to real-time weather conditions at the time of application (temperature, humidity and wind levels) to ensure optimal film formation, end properties and lasting results.

Coating formulation at Spray-Net’s in-house paint lab takes approximately 2 weeks, including building the customer’s color selection into the paint by balancing the resin-to-pigment ratio for each color, making sure there is sufficient pigment for optimal coverage, making sure there are enough resins to encapsulate pigments for maximum fade protection, according to Marsala. In addition, the company also uses high-grade, inorganic, solar-reflective pigments. The coatings are also optimized for exterior spray application and atomized at an optimal viscosity and custom thickness to achieve streak-free, sag-free, high-build finishes in one coat, according to Marsala. When the work on site begins, typically the building is power-washed and window and door surfaces prepped on day one, with painting completed on day two. For smaller homes, however, projects can be completed in as little one day. Paints are dry to the touch within 15 to 30 minutes and then finish curing with exposure to ambient temperature. When needed in unideal weather conditions, the company can produce a forced bake for front doors in its mobile spray booth given that this high-traffic surface requires a quick, full cure.

Marsala is not done innovating yet, either. “We might have succeeded in bringing a factory finish on-site, but there’s so much more we can’t wait to do,” Marsala insists. “Imagine if we could customize the coating on your front door according to how much direct sun exposure it gets? The possibilities are endless (and exciting)!”

Spray-Net has locations across Canada, and several new franchises in the United States, including in Columbus, OH; Chattanooga, TN; and St. Louis, MO. ĚýThe goal is to expand nation-wide in the United States, and the franchise is actively seeking new partners.