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.

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 theĚýĚý´Ç´ÚĚýCoatingsTech.

Corrosion is a process where a refined metal, such as aluminum or iron or alloys such as steel, undergoes degradation. In scientific terms, corrosion is an electrochemical process leading to oxidation of the metal and returning it to its natural state, such as iron being converted to iron oxides. Electrochemical reactions eat away at the underlying metal substrate, sometimes degrading it to such an extent that it creates aesthetic, mechanical, or structural failures.

The economic impact of corrosion is enormous. In a recent report, corrosion was estimated to cost the global economy more than $2.5 trillion annually, or approximately 3.4% of global gross domestic product (GDP).¹ Preventing corrosion is clearly an important goal, and paints and coatings play an important role in this never-ending battle.

Coatings applied to metal substrates can have a variety of purposes, including the enhancement of aesthetics such as gloss and color, or various functional purposes such as thermal insulation, sound dampening, or antimicrobial properties.

However, in many cases the most important role of a coating on a metal substrate is protection of the metal surface from the environmental effects that lead to corrosion. Waterborne direct-to-metal (DTM) coatings are just one category of coatings designed to protect metal substrates.

We begin by considering the science behind the corrosion process and the methods by which a coating can prevent corrosion. Then we will examine what is meant by direct-to-metal and how the term “DTM coating” is typically discussed in the industry, followed by a description of one-component (1K) DTM coatings based on acrylic latex polymers, which are the most common type of waterborne DTM coatings. We will also discuss principles of formulating waterborne acrylic DTMs for maximum performance over metal substrates. Finally, we will present a two-part, roundtable Q&A, where industry experts discuss the current trends and technology advances.

THE SCIENCE OF CORROSION

For corrosion to occur, four basic elements of an electrochemical cell must be present, as shown in Figure 1. The first element is a thermodynamically unstable metal (the anode) that will undergo oxidation (i.e., the base metal gives up electrons and is converted to the corresponding metal cations). In Figure 1, the anode is zinc, which oxidizes to form Zn⁺² ions. The electrons travel via the second element, a metallic conductive pathway, to an electron acceptor. The cathode is the third necessary element, where the electrons take part in a reduction reaction.

In Figure 1, electrons travel to the iron cathode, where oxygen, water and electrons combine to generate hydroxyl ions. The metal cations formed at the anode, and hydroxyl anions formed at the cathode, need a medium that allows ion movement. That medium, an electrolyte, is the fourth element to complete an electrochemical cell. In its simplest form, an electrolyte is a polar solvent such as water containing some soluble salts. In the presence of an electrolyte, the cations can move towards the cathode and the anions can move towards the anode. Corrosion products, such as zinc hydroxide shown in Figure 1, will form as the two ions form a salt and precipitate from solution.

In the case of dissimilar metals, such as zinc and iron in Figure 1, the galvanic series (shown in Figure 2) dictates which metal acts as the anode and corrodes, and which metal acts as the cathode and is protected from corrosion. Zinc is more active than iron, and when in electrical contact, zinc will act as the anode and will preferentially corrode.

One might ask, “Why does a piece of steel (an iron alloy) corrode if it is by itself and not in contact with another metal?” It will undergo corrosion because it is heterogeneous in nature; that is, there are domains within the steel structure that have slight differences in electrochemical potential. This slight difference is enough to allow one domain within the steel to act as the anode and another to act as the cathode as seen in Figure 3.

Mild or low-carbon steel is a very common material used in manufacturing and construction and corrodes quite readily if not protected. The corrosion products are iron oxides and hydroxides, commonly referred to as rust. Three types of coatings can be used to protect steel and other metals from corrosion—sacrificial, inhibitive, and barrier.²

A sacrificial coating works by making itself the anode and sacrificing itself for the metal substrate. A sacrificial coating is formulated with a high level of metal particles, such as zinc dust, which in the dry film are in electrical contact with each other and the metal substrate. If the metal particles are a more active metal than the substrate, they will act as the anode in a corrosion cell and sacrificially corrode and protect the substrate (i.e., the cathode). When a piece of steel is coated with a zinc-rich primer, the zinc particles sacrificially protect the steel substrate. These types of primers are often used in heavy duty service environments, such as in industrial maintenance applications, and are very effective at protecting steel from corrosion.

An inhibitive coating interferes with the corrosion process by introducing chemical species that interact with the metal surface. Inhibitive primers typically contain pigments that are slightly soluble, and once solubilized in the presence of water, can create a passivating layer that disrupts the electrochemical reactions at the anode or cathode. This type of inorganic pigment is often called a reactive, inhibitive, or anticorrosive pigment, and an example is basic zinc phosphate. Various organic corrosion inhibitors may also be used in formulating metal primers. Primers containing inorganic and organic corrosion inhibitors can be very effective at protecting metals from corrosion, and waterborne, solventborne and high-solids versions based on various resin technologies (e.g., epoxies, alkyds, etc.) are available.

Both sacrificial and inhibitive coatings must be in direct contact with the metal surface to provide effective protection. A barrier coating, on the other hand, can be the first layer in contact with the metal, or can be a subsequent layer on top of a primer. Barrier coatings disrupt the corrosion process by preventing either the fuel for the cathodic reaction (water and oxygen, or other chemicals) or the electrolyte from coming into contact with the substrate. Most coatings provide some barrier properties to a coating system, but their effectiveness will depend on factors that influence permeability to water and electrolyte, such as polymer hydrophobicity and coating porosity.

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Siloxane-based additives are critical tools in coating applications because their structures can be varied to provide a broad range of performance benefits in many types of formulations and chemistries. Surfactants and defoamers are some of the more commonly recognized additive classes, but many other functionalities can be derived from siloxane chemistries, particularly attributes related to surface control such as flow and leveling, slip, scratch resistance, and haptic properties. This extensive range of performance attributes is achievable due to the broad flexibility inherent in siloxane chemistry, allowing a fine-tuned balance of compatibility, incompatibility, and surface activity.

As with many additive types, a broad range of functionalities creates many options for improvement and innovation but also presents challenges in finding the right additive and optimizing to achieve the desired performance. This article will attempt to clarify the general structure-
property relationships that drive the performance attributes of siloxane additives and detail the continuum that exists between wetting, leveling, defoaming, and slip within this chemistry class. Surface control properties and testing will be reviewed and related to recent evaluation work conducted in developing novel siloxane surface control additives.

Introduction

Siloxane Chemistry

Siloxanes are molecules that contain Si–O–Si linkages, and their structures can be tailored to create materials with a broad spectrum of properties. While siloxanes can be derivatized with many different organo-functional groups to create a variety of small molecules and polymers, the siloxane backbone itself contributes some of the most unique attributes. The structure of underivatized polydimethylsiloxane (Figure 1) illustrates how this backbone, comprised of repeating –Sľ±(°ä±á₃)₂–O– units, results in a molecule that is both highly methylated and extremely flexible. It is this feature of siloxanes that enables their high level of surface activity because the surface energy of a “methyl-
saturated” surface is ~20 mN/m.^1-3

Siloxane-based wetting agents are generally smaller molecules with relatively short siloxane backbones. As shown in Figure 2, they are typically either trisiloxanes (n = 0, m = 1) or have comb structures with very few –Sľ±(°ä±á₃)₂–O– units (n) and only a very few organo-modifications (m); generally, n + m is less than 5. Hydrophilicity can be tuned by varying the nature and length of the pendant organic groups and by modifying the ratio of hydrophilic and hydrophobic moieties present in the molecule. In aqueous systems, these amphiphilic siloxanes are driven to the air–water interface and can lower surface tension within a time frame consistent with their molecular mobility. Molecules with longer siloxane backbones can also be rendered oleophobic and may also be driven to the air–liquid interface in nonaqueous, organic-based systems. Siloxane-based wetting agents are most often used to lower the surface tension of aqueous and solventborne, as well as high-solids and 100% solids, formulations to improve substrate wetting. Occasionally, these wetting agents are also used to compatibilize insoluble oils—particularly silicone contaminants—to prevent problems such as dewetting and defects.

Siloxane defoamers can also be created using comb siloxane structures like that shown in Figure 2; however, siloxane defoamer molecules are higher in molecular weight than siloxane wetting agents, and they usually have longer
–Sľ±(°ä±á₃)₂–O– segments (larger n) as well as more pendant groups (m) that are more hydrophobic in nature (i.e., c = 0 or d > c). A wide variety of molecular structures can be synthesized via formation of Si–O–C or Si–C linkages. Linear siloxanes like that shown in Figure 3 (also known as a, w- or bolaform structures) can be used to create defoamers and deaerators that range in performance attributes as the length of the siloxane backbone and lengths and chemical natures of organo-modifications are varied. In general, defoamers are designed to be incompatible in the surrounding media because this enables them to be driven to the lamellae of foam bubbles where they can spread and destabilize them, causing bubble rupture.

As one increases the length of the siloxane block within a molecule, the interfacial activity of the molecule also increases. Therefore, polyether-modified siloxanes that can improve flow and leveling, increase surface slip, and impact other surface properties are also possible. These surface control additives are generally of moderate molecular weight (between 1,000–20,000 g/mol), and compatibility within the intended liquid matrix is achieved through a diversity of organo-modifications. Thus, surface control additives that function well in waterborne, solvent-based, and 100% active (solvent-free) systems now exist. Three examples of organo-modified siloxane-based surface control additive structures are shown in Figure 4. Additionally, it is possible to crosslink and emulsify or disperse siloxanes to create emulsions or dispersions of even higher molecular weight and more hydrophobic moieties. Unlike the 100% active liquid siloxanes that exist as nanometer-scale polymers, these siloxane emulsions or dispersions exist as micrometer-sized droplets or particles, as shown in Figure 5.

Siloxane-based Surface Control Additives in Coating Applications

Due to their unique interfacial activity, siloxane-based surface control additives have become invaluable tools in the coating formulator’s tool box. Many surface control additives are capable of mitigating surface energy gradients that can exist within a liquid film immediately after application; thus, they can improve flow, prevent retraction, minimize cratering, enable better surface leveling, and ensure a flawless surface appearance. As their surface activity increases, surface control additives can also render special properties to the surface of the coating. For example, depending on their chemical structures, certain surface control additives can impact surface slip, affect haptic properties (surface “feel”), impart scratch resistance, act as antiblocking agents, and even create a release effect on the surface of the cured coating.

The property of “slip” is characteristic of a smooth sliding motion across the coating that results from a reduced coefficient of friction. It is quantified as the force needed to slide a mass across the coating surface. The material properties of both the substrate and the object to be moved are reflected in the static and dynamic coefficients of friction, and the chemical composition of the coating and the interactions arising from it, as well as surface roughness, all contribute. Surface control additives with large polydimethylsiloxane segments and a high degree of surface activity can ensure particularly slippery surfaces. Additionally, coatings with a high slip often feel smooth and silky.⁴

Coatings may resist scratching when the scraping object slips off the surface rather than penetrating the coating film; however, force-dependent scratch resistance of a coating can only be significantly improved if the surface control additive used contains functional groups that do not interact strongly with one another.^4,5 Organo-modified siloxanes, with a high percentage of polydimethylsiloxane domains, exhibit particularly weak interactions, both with each other and with other materials. This can make them ideal for this purpose. Moreover, during the drying process, organo-modified siloxanes continually migrate to the air–liquid interface, producing a lubricating film that significantly reduces the coefficient of friction of the coating. When a cured coating surface has a strong polydimethylsiloxane character due to the use of a surface control additive it is also more likely to resist blocking—adhesion of the dried coating to another freshly coated surface or other substrates. Liles has described the vast differences in, as well as the unique features of, silicone chemistry in two review articles; the reader is encouraged to consult these for additional background information on this fascinating topic. ^6,7

The Need for Recoatability: The Balancing Act

While it is possible to achieve a variety of desired benefits by using siloxane-
based surface control additives that have large polydimethylsiloxane segments, the need for most coatings to be able to be eventually recoated remains. The downside to employing these incredibly hydrophobic and oleophobic surface control additives is the tendency for the resultant coating to have a very low surface energy and to be very difficult—if not impossible–to recoat. Even if a second coating layer can be applied to such a surface, lack of adhesion and the tendency for the second coating layer to show craters are true problems.^6,7

To address these seemingly conflicting requirements, a series of new organo-modified siloxane-based surface control additives has been developed. This article will describe both the chemical attributes and the achievable benefits of these novel surface control additives.

Results and Discussion

Experimental

Three different waterborne self-crosslinking acrylic wood coating formulations were prepared using the formulations presented in Tables 1 through 4. Siloxane-based surface control additives were commercial products sold by Evonik Corporation under the TEGO® Glide brand, and they were used as received. The 35% active wax emulsion (AQUACER® 539) used as a control surface control additive in the formulation in Table 1 was a commercial product supplied by BYK USA. The water-based oil-modified polyurethane clear semi-gloss (MINWAX, Product code: 63020/71032) was purchased from a local retail store.

The coating formulations in Tables 1 and 2 were applied to plain black, sealed Leneta charts that measured 8 5/8 x 11 1/4 in.
(219 x 286 mm) using a #28 wire wound rod. After seven days of drying at ambient temperature and humidity, the coatings were inspected and evaluated for visual defects and ranked on a scale of 1 to 10 with 10 being perfect and 1 having many defects (Figure 6). Defects observed included pinholes, craters, de-wetting, orange peel, and smearing. Gloss measurements at 20°, 60°, and 85° were taken on the coated black Leneta charts using a Micro-TRI-gloss (Cat. No. 4446 Ser. No. 1083656) from BYK Additives & Instruments.

Relative Qualitative Slip was assessed by taping a Leneta chart coated (once) with the formulation containing no surface control additive and a second Leneta chart coated (once) with the comparative coating onto a single glass plate. A clean coin was placed on each chart and the glass plate was lifted relatively slowly until one of the coins slid off. This test was repeated several times to help assess whether the coated surface was truly more slippery than the blank. The coins were switched, and testing was repeated. The blank (no additive) coating was assigned a “0” and scores of “+” and “++” were given for increasing degrees of slipperiness while a “–” rating was assigned to coatings that were less slippery than the blank. Similar ratings were used for Relative Qualitative Smooth Feel. In these evaluations, the technician compared the feel of each coating and rendered a judgment based on her subjective opinion.

Red oak panels were lightly sanded using 220-grit fine sandpaper and they were brushed off with a clean high quality synthetic bristle brush before each application. Three panels were used for the blank formulation: two in the beginning and one at the end. The remaining panels were coated with the formulations containing 0.5 wt% of the surface control additives. The order of the coating process was randomized to reduce the effect of the operator. The brush was dipped into the coating sample and the entire panel was coated for the first coat. After two hours, the panel was lightly sanded, and 1 mL of the coating was pipetted to the panel and evenly distributed using the designated brush in a back and forth motion. This process was repeated after another two hours but 2 mL were added in the final coat. The brush was placed in aluminum foil and tightly sealed between each coat to avoid drying. The panels were left to dry for about five days before scratch and mar testing.

Scratch resistance was evaluated using an internally developed method (Figure 7). A plastic cuvette stirrer (5-in. long with a flat rounded edge) was used to quickly and with moderate pressure scrape the coated wood panel perpendicular to the grain pattern all the way across the short length of the panel and back. This motion was repeated rapidly 10 times and then the panel was inspected for any scratching or marring. Ratings were assigned using the following scale: 0 = no scratching, 1 = mild scratching, 2 = moderate scratching (like blank), and 3 = severe scratching (worse than blank). This method was found to be more reproducible than a fingernail scratch test.

For blocking studies, coatings prepared according to the formulation in Table 4 and containing 0.3 wt% surface control additives were applied as 200 mm wet drawdowns on primed beech panels. The coatings were dried for 5 h at room temperature. Blocking conditions used were: 1.6 kg/cm² with Osimeter, 20 h at room temperature, one hour recovering; two coated wood panels were positioned coating-to-coating.

Design of the New Surface Control Additives

With the ambitious goal of designing organo-modified polydimethylsiloxane surface control additives that could provide the required surface effects while being recoatable, synthetic efforts turned toward the variety of molecular structures achievable with this chemistry. Not only are linear, comb, and branched structures possible, but variations in the length of the siloxane backbone, the position of polyether groups (organo-modification), and the length and chemical nature of these polyether groups are also possible.

To meet all objectives of this study, several key molecular attributes would need to be realized. First, the polydimethylsiloxane segments would need to be large enough to drive the molecule to the coating–air and coating–substrate interfaces in order to provide substrate wetting and adhesion as well as the benefits for which surface control additives are typically used (slip, scratch resistance, antiblocking, etc.). Second, there would need to be a means of compatibilizing the siloxane surface control additive in coatings formulations. In the case of waterborne coatings, one can achieve this by designing a water-based emulsion containing the active surface control additive. However, a 100% active surface control additive that is compatible in both aqueous and nonaqueous systems requires the introduction of a high degree of organo-modification in order to impart this compatibility.

After many iterations within this synthetic space, four new surface control additives were developed; these include three siloxane emulsions (SCA#1, SCA#2, and SCA#3) as well as one 100% active siloxane (SCA#4). The characteristics of these four new materials are shown in comparison to two benchmark surface control additives (SCA#5 and SCA#6) in Table 5.

Evaluations of New Surface Control Additives in Waterborne Wood Coatings

To better understand the performance of these new surface control additives, they were evaluated in several different waterborne wood coating formulations. These included a commercial water-based oil-modified polyurethane clear semi-gloss as well as three internally prepared waterborne wood coating formulations—two clears and one pigmented—based on different self-crosslinking acrylic resins (see Experimental). It should be noted that the original objectives for each study differed; some were simple screens of the new additives with minimal comparison to other surface control additives, while others were more thorough comparative studies. As a result, SCA#5 and SCA#6 were not evaluated in all systems and some of the surface control additives were evaluated at fewer use levels than others.

The first study undertaken was a screen of the surface control additives at 0.25 wt%, 0.5 wt%, and 0.75 wt% as post-additions to a commercial do-it-yourself (DIY) wood coating.Ěý Results for coatings applied to Leneta charts are shown in Table 6 and those for coatings applied on red oak are shown in Table 7. Clearly, all the surface control additives at either 0.5 or 0.75 wt% significantly improve the appearance of the initial coating; however, SCA#1 and SCA#2 impart considerable hydrophobicity to the coating surface and the appearance of the second coating layer is very poor, resulting in moderate dewetting. All the surface control additives increase the gloss as well as the relative qualitative slip and smooth, slippery feel. They all improve scratch resistance of this coating when post-added at 0.5 wt%, and SCA#1 and SCA#2—the most incompatible additives—provide the greatest improvement in scratch resistance. Overall, at 0.5 wt%, SCA#6 provides more benefits than the other surface control additives in this formulation; it increases gloss, slip, and scratch resistance of the coating while rendering it recoatable. SCA#3 and SCA#4 at higher use levels also improve the coating’s recoatability.

The second formulation studied was a water-based self-crosslinking modified acrylic clear wood coating based on NeoCryl XK-12 (Table 1). Results of post-adding 0.5 wt% and 0.75 wt% of the surface control additives to this coating are shown in Table 8. At 0.75 wt%, SCA#2 outperforms the other surface control additives with regard to improving blocking and scratch resistance; however, it seems to introduce more foam to the formulation. SCA#1 provides the best appearance when the coating is applied on red oak, and it provides a moderate improvement in scratch resistance at both 0.5 wt% and 0.75 wt%. The 100% active organo-modified siloxane SCA#4 appears to be too compatible in this coating formulation; therefore, it is not able to significantly improve scratch resistance of the coating when used at use levels up to 0.75 wt%.

The third coating formulation studied was a water-based self-crosslinking modified acrylic clear wood coating based on Alberdingk AC 3630 (Table 2). Results of post-adding 0.3 wt% and 0.5 wt% of the surface control additives to this coating are shown in Table 9. Again, 0.5 wt% SCA#1 and SCA#2 provide the best scratch resistance, while SCA#3 hurt scratch resistance at 0.3 wt% and did not improve it at a 0.5 wt% use level. While SCA#4 was only tested at 0.3 wt% (the same actives level as was delivered by using 0.5 wt% of the three emulsions), it did not improve scratch resistance at this use level.

The final coating studied was a water-based pigmented acrylic wood coating based on Acronal LR 9014 (Table 4). Figure 8 shows the results of blocking tests performed using primed beech panels that had been coated (200 mm wet) using coatings containing 0.3 wt% active surface control additives. The Competitive SCA is an ultra-high molecular weight silicone dispersion in water that is promoted as a slip and antiblocking agent for waterborne wood coatings; it was used at 0.5 wt% in order to deliver 0.3 wt% actives on total formulation.

The effects of the new 100% active organo-modified siloxane surface control additive SCA#4 on recoatability in this pigmented water-based acrylic wood coating were also studied.Ěý A comparison of the cross-cut adhesion test panels is shown in Figure 9. At 0.5 wt%, SCA#4 shows excellent recoatability and no delamination of the second coating layer. However, the panels prepared using the coatings containing 0.5 wt% SCA#5 and 0.5 wt% Competitive SCA both show some delamination in regions near the cross cuts.

Discussion

This article has uncovered a few findings that should be mentioned. First, SCA#5 is extremely hydrophobic and proved to be quite incompatible in the two waterborne formulations in which it was evaluated. While SCA#5 can be used to provide a high-surface slip and silky feel, it is difficult to recoat with a waterborne coating; therefore, it might be more easily used in waterborne formulations that are intended as topcoats or overprint varnishes, and it is probably very advantageous in solvent-based and high solids coatings where its oleo-phobicity should render it very surface active. If a coating that contains SCA#5 requires recoating with a second waterborne formulation, the second coating should include an organo-modified siloxane designed to provide anticratering benefits. Also, intercoat sanding to improve adhesion and surface appearance should be considered if possible.

The more compatible 100% active organo-modified surface control additives, SCA#4 and SCA#6, can perform well—being sufficiently compatible with and providing significant surface benefits—in certain waterborne formulations. SCA#4 appears to be slightly more compatible than SCA#6 in the commercial oil-modified polyurethane wood coating, and SCA#4 showed better slip and feel at higher concentrations while SCA#6 provided better scratch resistance. With that said, SCA#6 was probably too compatible in the pigmented water-based acrylic wood coating because it was unable to significantly improve blocking. SCA#4, on the other hand, was able to provide antiblocking when used in the same formulation.

A comparison of the three new siloxane emulsion surface control additives has shown that, in general, SCA#1 appears to be most incompatible, SCA#2 a bit more compatible, and SCA#3 the most compatible in the waterborne coatings studied here. The additive that performs “best” in a formulation will really depend on what specific benefits are needed. For example, in a coating that employs a binder that has excellent blocking properties but is susceptible to scratching, a surface control additive that improves scratch resistance but has less of an impact on blocking might be the ideal choice. Other properties such as foam stabilization and anticratering properties may become important in coatings that require perfect mirror-like surfaces. Therefore, the formulator should consider several different surface control additives in order to identify the optimum one for the formulation under development.

Conclusions

Clearly there is no “universal” surface control additive. Each formulation and the application in which it will be used have different demands and the compatibility and properties of a surface control additive need to be matched to the specific system and its performance needs. The ideal surface control additive should provide the optimal balance between system compatibility and interfacial activity that minimizes issues and maximizes the benefits that these powerful tools can provide. Improvements in surface properties, including scratch and abrasion resistance, blocking behavior, and slip and haptic qualities, can be significantly enhanced with siloxane surface control additives. While compatibility and recoatability are significant concerns with this additive chemistry, these concerns can be readily overcome with a properly designed product.

The work described in this article highlights the fact that surface control additives can be engineered to provide a range of benefits in waterborne coating formulations. Of the four new surface control additives introduced in this work, two of the emulsions of crosslinked, high molecular weight polydimethylsiloxanes (SCA#1 and SCA#2) were found to provide very good scratch resistance while being less compatible in the water-based wood coatings. Interestingly, the third siloxane surface control additive emulsion (SCA#3) proved to be more compatible, resulting in very good surface appearance and haptic feel but little scratch resistance in the four wood coatings used in this study. While probably not the ideal characteristics for a wood coating, these properties are sought after in other coatings applications like leather coatings and graphic arts. The new 100% active organo-modified siloxane has good compatibility in the waterborne wood coatings studied as well as in other coating formulations. As a result, coatings containing SCA#4 may be recoatable and properties like antiblocking may be achieved in certain systems.

Clearly, siloxane-based surface control additives offer the formulator useful tools that can be employed to create high-performance coatings. This work has merely scratched the surface of what is undoubtedly an area that deserves further exploration, and additional fundamental studies to better elucidate chemical structure-property relationships are warranted.

References

1. Owen, M.J., “Interfacial Activity of Polydimethylsiloxane,” in Surfactants in Solution, 6, Mittal, K.L. and Botherel, P. (Eds.), New York: Plenum Press, 1557–1569, 1986.
2. Hill, R.M., “Siloxane Surfactants,” in Specialist Surfactants, Robb, I.D. (Ed.), London: Chapman & Hall, 143–168, 1997.
3. Hill, R.M., Silicone Surfactants, New York: Marcel Dekker, Inc., 1999.
4. The Big TEGO: Products—Services—Data Sheets, 4th ed., Evonik Industries AG, Essen, Germany, 68–75, 2014.
5. Chen, H.H., “Scratch Resistant Low Friction/Low Surface Energy Coating for Silicon Substrate,” J. Appl. Polym. Sci., 37, 349–364 (1989).
6. Liles, D.T., “The Fascinating World of Silicones and Their Impact on Coatings: Part 1,” CoatingsTech, 9, No. 4, 58–66 (2012).
7. Liles, D.T., “The Fascinating World of Silicones and Their Impact on Coatings: Part 2,” CoatingsTech, 9, No. 5, 34–46 (2012).

CoatingsTech | Vol. 17, No. 3 | March 2020

As discussed in the CoatingsTech June 2019 article “Durability Improvement Underlies Much Coating Innovation,” most coatings must serve two objectives—to protect and decorate. Although a balance must be achieved between durability and decoration, improving durability is always an important goal of new product development efforts. As a result, several new products and technologies are being introduced to enhance durability and protect against the harmful impact of weathering.

In the testing area, Atlas Material Testing has introduced the Ci4400 xenon arc Weatherometer^®, which offers a high level of test precision, repeatability, and reproducibility for better laboratory testing when differentiating coatings performance, according to Allen Zielnik, global weathering applications manager with Atlas Material Testing. For outdoor accelerated testing, the company has introduced the UA-EMMAQUA and hybrid-EMMAQUA concentrated solar tracking technologies, which provide higher degrees of test acceleration than was previously possible with most outdoor techniques.

For the automotive market, Covestro continues to focus on simultaneously achieving a luxurious look and feel and chemical resistance in automotive interior coatings, according to Scott Grace, market segment technical support for Automotive with Covestro LLC.

BASF has developed Joncryl OH 8314, an adaptive direct-to-metal (DTM) hydroxyl functional epoxy-modified acrylic hybrid dispersion for two-component polyurethane coatings designed with an indication of end of useful life (pot life). The unique synthesis platform allows for performance advantages and provides characteristics comparable to solventborne systems in an ultra-low VOC package, according to Shiona Stewart, industry marketing manager, Transportation, Industrial, Furniture and Floor Coatings at BASF. Coatings formulated with Joncryl OH 8314 exhibit good durability and feature a low viscosity, low isocyanate demand with excellent flow and leveling, as well as low foaming and reduced water sensitivity.

In the protective and marine space, PPG is focused on self-healing coatings, self-cleaning coatings, ultra-durable weathering resistance, and novel corrosion mitigation, according to Jim McCarthy, technical director with PPG’s protective and marine coatings business. One example is PPG Pitt-Char NX, an epoxy intumescent coating system that offers reduced coating thickness, lower weight, and faster application while providing excellent strength and durability due to its unique flexible technology, McCarthy says. The patent-pending technology is designed to protect against severe hydrocarbon hazards, including pool fires, jet fires, and explosions, for both onshore and offshore environments in the oil, gas, and petrochemical industries.ĚýPPG NOVAGUARD 810 ER is a new two-component, high-solids, amine rapid-cured novolac epoxy coating. The one-coat, DTM formulation delivers very fast application times and faster return to service after coating (with touch-ups possible using a thin-film epoxy), and is ideal for protecting fuel tanks, ballast tanks, and the internal and external superstructures of tugs and barges, according McCarthy. The PPG ONE Series is a collection of two individual coating products (AMERCOAT^® ONE 1K epoxy and PSX^® ONE 1K isocyanate-free acrylic-siloxane coating) developed to provide industrial-level protection in an easy-to-use, one-component package.

PPG has also continued to advance polyester coating chemistries for the exterior building markets, according to Sarah Mueller, coil technical manager for Industrial Coatings at PPG’s Springdale plant. “The key is identification of the ideal combination of polymer architecture, crosslinker selection, and UV stabilizer enhancements. Certain combinations of pigment chemistries have also shown advances in color fastness not seen in the past,” she says.

The newest offerings from allnex include the ACURE^® platform, an exterior, durable offering that is isocyanate-free yet improves application drawbacks of existing technologies by extending the pot-life. It also improves the return to service with extremely fast cure times, according to Tim Kittler, global segment lead for Marine and Protective from allnex. The low-VOC, user-friendly system has been targeted across key markets such as flooring, industrial metal and wood, ACE, and marine and protective.

Sun Chemical recently launched the DEFENSA OP series of UV-reactive low refractive index resins with high transparency for products that make use of optical materials, such as polymer clad fiber and clad sheathed materials. “This technology is a major advancement in easy-clean, stain-resistant hardcoat coatings and is suitable for a wide range of substrates, providing great durability for stainless steel, flooring, lenses, and glass decoration,” says Michael T. Venturini, marketing director for Coatings at Sun Chemical.

At Axalta Coating Systems, advances in exterior products have focused on improving durability and color space in exterior products across a wide range of substrates and markets, many of which are highly customer-specific solutions, according to James Hazen, director of Industrial Technology Planning at Axalta Coating Systems. In the interior products space, he notes that advances include specialized improvements in casegood paints and building components, not only for performance but also appearance and added benefits relating to increased customer efficiencies, often as more environmentally friendly solutions. ĚýNew introductions for flooring surfaces offer improved scratch, mar, and moisture resistance.

Coil coating advances at AkzoNobel include CERAM-A-STAR^® Expressions and POLYDURE^® Expressions products, which combine wrinkle technology with print to provide a unique appearance with a textured feel, according to Chris Bradford, AkzoNobel’s segment marketing director for Coil and Packaging Coatings. CERAM-A-STAR Matte and TRINAR^® Matte coatings provide low gloss/sheen characteristics that reduce glare but also provide a rich soft appearance. They are also beneficial for reducing the appearance of “oil canning” in metal roofing.

For wood coatings, AkzoNobel introduced the Rubbol WF 3310 system in 2018 for exterior use. Based on novel core-shell acrylics technology that enables a low particle size and low particle size distribution and an optimized formulation ensures long durability, according to Anthony Woods, AkzoNobel’s segment marketing director for Wood Coatings. NaturaMatte™ is a new low-gloss UV coating system for flooring that provides a long-lasting natural matte finish with excellent scratch- and mar-resistance and angular gloss, which provides a uniform appearance when viewed anywhere in a room.

Dow Coating Materials (DCM) has a product line and toolkit to draw on to continually advance the performance of interior and exterior architectural coatings. Recently, DCM has been looking to leverage technologies from its Industrial group for the development of versatile products designed for formulation into architectural coatings, according to Stan Cook, North American architectural marketing director at Dow Coating Materials.Ěý He further states that understanding the direction of the market is one thing, but working closely with customers to deliver specialized raw materials for their specific needs really enables formulators to be able to put their individual stamps and product claims on the can as well. This is generally a win-win for everyone, he notes.

A new dispersing agent from Evonik for exterior wall paints functions via electrostatic and steric stabilization mechanisms, particularly for red and yellow iron oxide pigments, according to BrĂĽnink. As a result, a much higher color strength can be achieved along with increased hydrophobicity for greater wet scrub resistance.

In addition to its resins for industrial and protective applications, BASF has developed a high performance, all-acrylic latex designed for exterior flat to semi-gloss paints. Acronal EDGE 4247 is an ultra-low VOC latex with excellent primer and topcoat attributes, including excellent film formation and adhesion capabilities and superior dirt pickup resistance, surfactant leaching resistance and grain cracking resistance for good washability, scrub resistance, and stain blocking performance, according to Charles Johnson, market segment manager for Architectural Coatings at BASF.Ěý Acronal EDGE 4750, meanwhile, can be formulated into interior coatings with excellent stain resistance, dirt pickup resistance, and scrub resistance.

PPG’s new products for the architectural market include OLYMPIC^® SmartGuard clear multi-surface sealer, a super-concentrated formula in a lightweight, easy-to-handle pouch package that covers as much as two-gallon cans, according to Brian Osterried, product marketing manager for the PPG paint brand. It seals wood, concrete, masonry, and weathered composite, penetrating the substrate to provide advanced durability and protection. It can be applied after rain or cleaning and dries in one hour (compared to 24–48 hours for traditional products). It is formulated to be extremely fast and easy to apply, especially from a pump sprayer, but also applies easily with brush or roller. The newly reformulated PPG PAINTS™ PERMA-CRETE® product assortment for interior and exterior concrete and masonry help protect and improve appearance while extending the painting season and reducing costs. Osterried notes that several of these products now come in low-VOC formulas while retaining their original performance features, including PERMA-CRETE Plex-Seal™ Clear Sealer and PERMA-CRETE Color-Seal™ Acrylic Concrete Stain, which now have a VOC content of < 100 g/L and PERMA-CRETE High Build and PERMA-CRETE Alkali Resistant Primer, which are now available with a VOC content of < 50 g/L.

AkzoNobel has also introduced new powder coatings for the architectural market. Interpon D2015 Précis is a weather-resistant, super-durable, ultra-matte powder coating with a gloss level of 5–15 (a typical matte is 25) that can mimic the anodizing finishes currently popular on commercial buildings, but without the common problems associated with anodizing, according to Jean-Paul Moonen, AkzoNobel’s global segment manager for Architectural Coatings in the Powder Coatings business. Interpon D3020 Brilliance is a single-coat, hyper-durable powder, bright, sparkling metallic architectural powder coating designed to meet the 10-year Florida requirements of AAMA 2605 and Qualicoat Class 3.

An important new pigment from Sun Chemical is Benda Lutz COMPAL PC, a highly concentrated aluminum preparation for powder coatings that comes in a pelletized form and virtually eliminates dusting to minimize housekeeping, simplify equipment clean-up, and reduce worker exposure, according to Venturini. It is easy to handle and store and can be added just like aluminum powder during the paint manufacturing process. Notably, the pellet is not classified for shipping and storage in the European Union and is shipped in compact bag-in-box packaging that is eco-friendly and recyclable.

Clariant has added Hostatint™ A-100 ST, a range of dispersions made from highly transparent pigments that do not contain halogens in their molecular structures, which is a major requirement for the recovery of precious rare earth metals in the recycling process, according to Romesh Kumar, senior technical sales manager for Clariant Plastics & Coatings USA LLC. The line includes nine highly chromatic and bright “candy color” shades for metallic and non-metallic (e.g., wood stains) formulations. The company also now offers a Hostatint UV line comprising more than 20 unique products that are compatible in many resin systems, have very high shelf life and/or storage stability, and crosslink via a UV curing processes. Kumar notes that in UV-cured wood coatings, the specified film thickness can often be reached in just one pass with products from this line.

As global environmental concerns continue to overshadow the use of well-established metal surface pretreatment processes such as chromate treatment and phosphatization, the need for environmentally friendly corrosion protection systems has never been greater. A promising solution to this worldwide regulatory issue is waterborne silane technology, which can offer a heavy metal-free, volatile organic compound (VOC)-free alternative to protecting metals from corrosion. The mechanism behind this corrosion protection can best be explained by the passivation of a metal surface with a waterborne silane film, which acts as a barrier to water, salts, and other corroding materials in the surrounding environment. It is important to note that the waterborne silaneĚýtechnology investigated in this workĚýcan be viewed as a type of conversion coating or pretreatment to the metal surface, rather than a conventionalĚý waterborne coating or primer. Certain waterborne silane technology requires high-temperature curing procedures for optimal results, which can be difficult to achieve in certain applications or industries. With the use of bipodal silanes, the additional crosslinking introduced into the system can alleviate the need for this high-temperature curing procedure. In this novel work, we demonstrate that the incorporation of a bipodal silane into waterborne silane systems improves the surface passivation of the metal surface, enhances the hydrophobicity of the system, and increases the crosslinking density of the system, leading to significant improvements in the corrosion resistance of waterborne silane technology.

INTRODUCTION

Whether it be for a bridge, a tunnel, an automobile, an electronic component, or a building, corrosion protection technology plays one of the most important roles in maintaining the integrity and longevity of the world around us. There are many well-established methods for protecting metals from corroding over time, including chromate treatment and phosphatization, which have been widely used for corrosion protection across the globe for decades.^1,2 While these processes are inexpensive and well-known, governmental regulations and overall awareness of the hazards associated with these methods are growing. In particular, hexavalent chromium, a key material used in chromate treatment for the past 90 years, has recently been subject to new far-reaching restrictions. After the European Union classified hexavalent chromium as a carcinogen and mutagen in 2013, Europe’s Registration, Evaluation, Authorisation & Restriction of Chemicals (REACH) regulations have forced hexavalent chromium to be phased out of most industry applications across Europe. While most industries had to stop using hexavalent chromium by January 2019, some industries, such as the aerospace industry, have been allowed to continue using hexavalent chromium through 2026. However, corrosion protection technology for aerospace applications takes several years of research, development, and qualification, which is why the time for investigating chromium-free corrosion protection technology is now.

One viable alternative to these hazardous corrosion protection systems is silane technology. While organofunctional silanes have been widely used as adhesion promoters for several decades now, the use of these materials in corrosion resistant coatings is a more recent development. When properly prepared and applied, organofunctional silane coatings have the ability to form protective barriers on metal substrates, which subsequently protect the metals from corroding over time.³ Previous studies have shown that a only a small amount of active silane content, ranging from 0.2–2.0 wt% solids, is necessary for improving the adhesion of a coating system.⁴ For this reason, the incorporation of an organofunctional silane into a coating system can provide adhesion promotion or corrosion resistance without significantly increasing the volatile organic content (VOC) of the system. This is one of the many reasons why waterborne silane technology offers excellent corrosion resistance performance without the need for hazardous pretreatments, volatile solvents, or heavy metals.

The mechanism behind an organofunctional silane adhering to a metal surface is an important process to understand before investigating the corrosion resistance performance of waterborne silane technology. Over the past several decades, organofunctional silanes have been used as coupling agents for organic and inorganic materials across many different industries. Organofunctional silanes contain a hydrolyzable alkoxysilane (Si–OR) functional group that can bond with inorganic surfaces. In this work, the organofunctional silanes to be investigated have silicon functional groups comprising of alkoxy groups, specifically methoxy and ethoxy groups. Organofunctional silanes also consist of an organofunctional group that can react with organic systems. The simultaneous reaction of the silicon functional groups and organofunctional groups allow organofunctional silanes to act as an adhesion promoter between inorganic and organic materials.

For an organofunctional silane-based system to adhere to an inorganic substrate, hydrolysis must first take place at the alkoxy sites to form silanol groups. When the hydrolyzed organofunctional silane comes into contact with an inorganic surface, the silanol groups can initially form hydrogen bonds with the hydroxyl groups on the inorganic surface. Upon removal of moisture from the system, these hydrogen bonds can form siloxane bonds between the organofunctional silane and inorganic surface. These siloxane bonds provide the strong adhesion characteristics for which organofunctional silanes are well known.⁵ With proper surface preparation and material selection, organofunctional silane-based coatings can form siloxane bonds to many sites on the inorganic surface, forming an organofunctional siloxane network in the process (Figure 1). This organofunctional silane film can passivate the surface of a metal substrate, providing a barrier to keep water and salts from coming in contact with the metal surface. Furthermore, the organofunctional groups can provide additional hydrophobicity and adhesion promotion of any subsequent organic topcoats that may be applied for further corrosion protection.

As previously mentioned, an elevated temperature curing procedure is typically required to drive off all the moisture in an organofunctional silane coating. This thermal curing procedure is not always feasible depending on the specific application or industry, and this has led to the exploration of alternative curing methods for organofunctional silane coatings. With the use of organofunctional bipodal silanes, the additional crosslinking density introduced into the system may alleviate the need for this elevated temperature curing procedure. This additional crosslinking density stems from the influx of alkoxy groups from the organofunctional bipodal silane. While it is possible for these additional alkoxy groups to undergo crosslinking at room temperature, the condensation of silanol groups is significantly accelerated at elevated temperatures.⁶ Additionally, the rate of crosslinking depends on several other factors, including pH, presence of solvents, and the concentration of silanes in the system.⁷ Although organofunctional trialkoxy silanes are commonly used in a wide variety of coating applications, organofunctional bipodal silanes, such as 1,2-bis(triethoxysilyl)ethane (Figure 2), can have six or more alkoxy groups. As these alkoxy groups undergo hydrolysis and condensation in the system, the additional siloxane bonds formed can accelerate the curing process of the system.⁸

It is important to note that a two-carbon spacer links the six alkoxy groups on each side of 1,2-bis(triethoxysilyl)ethane. These alkyl chains are responsible for the hydrophobic nature of this organofunctional bipodal silane. For this reason, 1,2-bis(triethoxysilyl)ethane is commonly used in solvent-based systems, where the hydrophobic nature of this organofunctional bipodal silane does not interfere with its solubility in alcohol-based systems.⁹ Although it is rather difficult for hydrophobic organofunctional silanes to exhibit good stability in waterborne systems, optimizing the pH of the system to a slightly acidic value (pH 4–5) can maximize the hydrolysis rate and minimize the condensation rate of organofunctional bipodal silanes.^10,11 This allows for improved solubility and hydrolytic stability of organofunctional bipodal silanes in waterborne systems.

The two waterborne systems that are presented in this work include a waterborne organofunctional silanol system with functionalized colloidal silica and a waterborne organofunctional silanol system without functionalized colloidal silica. Both these waterborne systems do not contain any volatile organic compounds, which is why they are commonly used as environmentally friendly alternatives to harmful corrosion resistance technology. The waterborne organofunctional silanol system with functionalized colloidal silica can also be used as a transparent sol-gel topcoat, while the waterborne organofunctional silanol system without functionalized colloidal silica can be used as a surface modifier for organic materials or as an adhesion-promoting additive into waterborne polymer systems. While these waterborne systems provide excellent corrosion resistance on their own, organofunctional bipodal silanes will be explored as performance-enhancing additives to these waterborne systems, in hopes of better understanding how to improve this new technology.

EXPERIMENTAL METHODS

Materials

1,2-Bis(triethoxy)silylethane (Dynasylan® * BTSE), waterborne organofunctional silanol system with functionalized colloidal silica (Dynasylan® SIVO 110), and waterborne organofunctional silanol system without functionalized colloidal silica (Dynasylan® HYDROSIL 2926) are available from Evonik Industries AG. Sodium hydroxide (99.99% pure) and ethyl alcohol (99.5% pure) were purchased from Sigma Aldrich. Bulk Kleen® * 737G (a proprietary alkaline powder cleaner) was purchased from Bulk Chemicals. Deionized (DI) water was obtained with a water purification system (WaterPro®  † Plus) originally purchased from LabConco Corporation. Aluminum 6061T6® ‡ substrates were purchased from ACT Test Panels, LLC.

Formulation Preparation

The waterborne coatings evaluated in this article were formulated in 150 mL glass beakers (see Table 1 for ingredient breakdown) and allowed to mix for 96 h before use. This extensive mixing time is preferred to allow adequate time for the silane molecules in the formulation to hydrolyze and condense in the presence of water. After enough time, the condensation of silanol groups in the formulations will start to have a considerable impact on the viscosity of the coating, eventually leading to decreased film formation properties. This condensation rate is particularly low at pH 4–5 for the waterborne coatings evaluated in this article, allowing for approximately three weeks of sufficient stability before the onset of minor visual changes to the solutions. These visual changes could include precipitation, haziness, and increases in the viscosity of the system while mixing.

1,2-Bis(triethoxysilyl)ethane is 100% active solids, while the waterborneĚýorganofunctional silanol system with functionalized colloidal silica is 36% active solids, and the waterborne organofunctional silanol system without functionalized colloidal silica is 30% active solids. The final wt% solids of the waterborne coating formulations in this article were chosen to obtain transparent films that do not cause any negative optical characteristics to the metal surfaces. It is also important to note that the optimal silane concentration in a waterborne protective coating directly depends on the surface roughness of the metal substrate.¹²

Cleaning and Application Procedures

Metal Surface Cleaning Procedure

Before applying the waterborne formulations described above, it is crucial that the metal substrates are properly cleaned for optimal surface wetting properties. The metal substrates were first wiped with an ethyl alcohol-soaked paper towel. Following this solvent wipe, the metal substrates were dried with an air gun and placed in an alkaline washing solution (prepared by adding 15 g of Bulk Kleen® 737G to one liter of DI water and stirring for several hours before use) for 3 min at 140–150°F. The aluminum substrates were rinsed with DI water and dried with an air gun following the alkaline washing procedure.

Coating Application Procedure

After the metal substrates were properly cleaned and the waterborne coating formulations were allowed to fully hydrolyze, the coatings were applied via a dip coating procedure. The metal substrates were fully immersed in the waterborne silane formulations for 60 sec at room temperature, removed from the solution, and hung vertically in a fume hood for 10 min to allow the excess liquid to drip off the metal surface. Although some of the waterborne coating formulations were milky white, all the coatings evaluated in this article formed transparent films upon application.

Curing Procedure

After being allowed to dry at room temperature for 10 min following the dip coating procedure, the coated metal substrates were either left in the fume hood at room temperature to dry for an additional 48 h, or placed in an oven for 30 min at either 80°C (formulation WB1) or 180°C (formulations WB2, WB3, WB4, and WB5). It is important to note that after curing, the dry-film thicknesses of the waterborne organofunctional silanol system with functionalized colloidal silica and the waterborne organofunctional silanol system without functionalized colloidal silica were less than 1 mm.

Testing Procedures

Contact Angle Measurement Procedure

Once the coatings were fully cured, a goniometer (Ramé-Hart, Inc.) was used to measure the contact angle of DI water on the coated substrates. Each measurement reported in this article is the average of 10 contact angle measurements to ensure the accuracy of this method. The standard deviation of each set of 10 contact angle measurements represents the statistical error reported in this data. It is important to note that although the metal substrates were slightly bent during the production process, all measurements were done on aluminum substrates of the same production batch and at the same locations on each substrate.

Neutral Salt Spray Testing Procedure

Before evaluating the coated metal substrates in a neutral salt spray test, wax (IGI 1334 paraffin wax supplied by Lone Star Candle Making Co.) was used to coat the edges of the metal substrates. Corrosion resistance was evaluated in a Q-Fog® ^§ Cyclic Corrosion Tester (The Q-Panel Company) according to ASTM B117.

Alkaline Resistance Testing Procedure

A solution containing 10% sodium hydroxide (NaOH) and 90% DI water was prepared by stirring at room temperature until all the NaOH pellets were fully dissolved. After properly applying and curing the waterborne coating formulations on the aluminum substrates, the panels were immersed in the alkaline solution for 10 min at room temperature. The panels were then removed, rinsed with DI water, observed visually, and then placed in a neutral salt spray test as described above.

Electrochemical Impedance Spectroscopy (EIS) Testing Procedure

EIS testing was performed by Matergenics, Inc. Gamry PCI4/750™*Ěýpotentiostats were used to record the impedance spectra at frequencies of 0.1–100,000 cycles/sec. The coated metal panels were immersed in an aqueous conductive 3.5% NaCl solution during testing. To achieve a relatively stable open circuit potential for EIS measurements, the coated metal panels were immersed in the conductive 3.5% NaCl solution for 20 min before collecting impedance data. All measurements were performed in a grounded Faraday cage at room temperature.

RESULTS AND DISCUSSION

Surface Contact Angle Analysis

As mentioned previously, incorporation of an organofunctional bipodal silane into a waterborne system is expected to increase the hydrophobicity, surface passivation, and crosslinking densityĚýof the system, leading to improvedĚýcorrosion resistance performance.ĚýIn particular, 1,2-bis(triethoxysilyl)ethane was chosen for investigation in waterborne systems because of the two-carbon spacer group between the silicon functional groups on either side of the organofunctional bipodal silane. This two-carbon alkyl chain not only provides significant hydrophobicity, but is also a short enough chain to allow for sufficient solubility in waterborne systems.

While there are many ways of characterizing the hydrophobicity of a metal coating, valuable insight can be gained by visually observing the behavior of water on the coated surface. Water tends to spread out when placed on an uncoated aluminum surface, indicating a hydrophilic surface. On an aluminum surface that has been coated with a waterborne organofunctional bipodal silane coating, the water tends to bead up (Figure 3).

While visually observing the behavior of water on a metal surface is typically a good indication of whether the metal surface is hydrophilic or hydrophobic, contact angle measurements of water droplets on a metal surface can better quantify this behavior. The average contact angle of a DI water droplet on uncoated aluminum is 44° ± 1.5°, while the average contact angle of a DI water droplet on WB1-coated aluminum was 72° ± 1.6° (Figure 4). Thus, coating the aluminum surface with a waterborne organofunctional bipodal silane coating increased the contact angle of DI water by ~64%. Because a surface is typically considered hydrophobic when the contact angle of water on the surface is > 90°, it can be said that the this waterborne organofunctional bipodal silane system made the surface less hydrophilic.

While the average contact angle of a DI water droplet on WB2-coated aluminum was 41°± 1.6°, adding an organofunctional bipodal silane into the system increased the average contact angle of a DI water droplet to 50° ± 2.0° (Figure 5). This ~22% increase in the average contact angle of DI water on WB3-coated aluminum indicates a significantly less hydrophilic surface than the WB2-coated aluminum.

A ~40% increase in the average contact angle of DI water was observed when an organofunctional bipodal silane was incorporated into the waterborne organofunctional silanol system without functionalized colloidal silica. This indicates a significant decrease in the hydrophilic characteristics of the aluminum surface. The average contact angle of a DI water droplet on WB4-coated aluminum was 40° ± 1.7°, while the average contact angle of a DI water droplet on WB5-coated aluminum was 56°± 1.4° (Figure 6).

As confirmed by contact angle analysis, the incorporation of an organofunctional bipodal silane into the waterborne coatings described above decreased the hydrophilicity of the systems. This reduction in hydrophilicity should help prevent water droplets in the surrounding environment from properly wetting and coming into contact with the metal surface. While it can be speculated that less contact between water and the metal surface leads to lower corrosion rates, further corrosion testing was necessary to confirm this hypothesis.

Neutral Salt Spray Testing

While it has been debated that cyclic prohesion testing provides a more accurate prediction of corrosion resistance in real life conditions, neutral salt spray testing has been an industry standard for evaluating corrosion resistance for decades.¹³Ěý However, it is important to note that the corrosion mechanism occurring in a salt spray chamber is fundamentally different than corrosion mechanisms in the real world. In a controlled salt spray chamber, the humidity, temperature, and sprayed salt solution is precisely controlled and monitored in a closed environment. In the field, the humidity, temperature, and exposure to varying weather patterns introduce many more variables when evaluating the performance of a corrosion protection coating. Although a coatings’ ability to prevent corrosion in a controlled humid and salty environment is often a good indicator of corrosion resistance performance out in the field, this will have to be confirmed with outdoor weatherability studies during future testing.

After coating aluminum substrates with the waterborne sol-gel silane coatings, three different curing procedures were employed. One set of panels was left to dry at room temperature for 72 h, while the other two sets of panels were baked in the oven for 30 min at either 80°C or 180°C. Without any heat to drive off the moisture in the waterborne sol-gel silane coatings cured at room temperature, poor adhesion and corrosion resistance was expected due to the low condensation rate between silanol groups in the coating, which inhibits the surface passivation of the metal surface. Additionally, such low condensation rate typically results in insufficient adhesion between the coating and the metal surface.

By incorporating an organofunctional bipodal silane into the waterborne organofunctional silanol system with functionalized colloidal silica, the influx of silanol groups in the system should increase the rate of condensation between silanol groups in the coating, leading to better surface passivation of the metal surface. Furthermore, these additional silanol groups should also increase the rate of condensation between the silanol groups in the coating and the hydroxyl groups on the metal surface, resulting in better adhesion to the metal, and better corrosion resistance.

These hypotheses can be supported by the superior corrosion resistance demonstrated by the waterborne organofunctional silanol system with functionalized colloidal silica and organofunctional bipodal silanes in the system (Figures 7 and 8).

This holds true for the coatings cured at room temperature for 72 h and elevated temperatures as well. In particular, no corrosion or defects were found on the WB3-coated aluminum substrate cured at 180°C after 250 h in a neutral salt spray test. It is important to note that no additional organic topcoats were applied on the waterborne coatings for this neutral salt spray test data.

Just as the presence of an organofunctional bipodal silane in the waterborne organofunctional silanol system with functionalized colloidal silica improved the corrosion resistance of the coating, similar performance improvements were observed in the waterborne organofunctional silanol coatings without functionalized colloidal silica as well. Although these systems are not identical, the addition of an organofunctional bipodal silane into the waterborne organofunctional silanol system without functionalized colloidal silica should also increase the crosslinking density, hydrophobicity, and surface passivation of the system. These hypotheses are supported by the superior performance of the waterborne organofunctional silanol coatings without functionalized colloidal silica and organofunctional bipodal silane additions in neutral salt spray testing (Figures 9 and 10).

It is important to note that although only a few hundred hours of neutral salt spray performance are shown here, the trends regarding better corrosion resistance of systems with organofunctional bipodal silane additions were consistent for over 1000 h of neutral salt spray testing. Additional layers of organic topcoats would also significantly improve the corrosion resistance of the aluminum substrates over time. While this is a promising glimpse of performance improvements achievable with organofunctional bipodal silane additives, other corrosion-related testing procedures are crucial in determining the full scope of the corrosion resistance performance for these systems.

Alkaline Resistance Testing

When it comes to the automotive industry, the alkaline resistance of a coating is often just as important as the corrosion resistance performance. This is due to the wide variety of automotive coatings that are subject to alkaline detergents, which can be found in most car wash fluids on the market nowadays. For exterior automotive coating applications, sealants are often applied on aluminum surfaces to provide corrosion resistance and alkaline resistance.¹⁴ As previously hypothesized, the addition of an organofunctional bipodal silane into a waterborne organofunctional silanol coating with functionalized colloidal silica increases the crosslinking density, hydrophobicity, and surface passivation of the system. While it has already been demonstrated that these additional characteristics from the organofunctional bipodal silane boost the corrosion resistance of the system, the alkaline resistance of the system can also be similarly improved.

It is important to note that the alkaline solution (pH 14) used in this test produces a much harsher environment than the neutral salt spray test environment. Within minutes of immersion, all the waterborne organofunctional silanol systems with functionalized colloidal silica and no organofunctional bipodal silane additives began to bubble violently and blackened the alkaline solution (Figure 11).

These bubbles are likely to be hydrogen gas, which is generated when bare reactive aluminum comes into direct contact with water (Scheme 1). This reaction occurs when the waterborne coating and protective aluminum oxide surface on the bare aluminum metal are removed by the high concentration ofĚý–OH groups in the alkaline solution.¹⁵

^ĚýThe addition of an organofunctional bipodal silane into the waterborne organofunctional silanol system with functionalized colloidal silica significantly reduced the amount of observable bubbling on the surface of the aluminum substrates. This can be explained by the lack of bare reactive aluminum metal exposed to the alkaline media due to the additional crosslinking and surface passivation from the organofunctional bipodal silanes in the coating. Upon removing the aluminum substrates from the alkaline media after 10 min of immersion, the waterborne organofunctional silanol coating with functionalized colloidal silica and no organofunctional bipodal silane additives was completely dissolved (Figure 12), and the aluminum surface was observably deformed. On the other hand, the waterborne organofunctional silanol system with functionalized colloidal silica and organofunctional bipodal silane additives only showed minor surface defects after the alkaline resistance test (Figure 13).

After prolonged exposure to alkaline media, the ability for a coating to continue exhibiting sufficient corrosion resistance is crucial in the automotive industry. For this reason, the coated aluminum substrates were rinsed with DI water following the alkaline resistance tests, then immediately placed in a neutral salt spray for 100 h, and evaluated for corrosion resistance. The aluminum substrate coated with the waterborne organofunctional silanol system with functionalized colloidal silica and no organofunctional bipodal silane additives showed significant change in appearance after the neutral salt spray testing. These changes included a significant discoloration of the entire aluminum surface, indicating rust build-up due to the lacking presence of the corrosion protection coating that was eradicated by the alkaline media. On the other hand, the aluminum substrate coated with the waterborne organofunctional silanol system with functionalized colloidal silica and organofunctional bipodal silane additives in the system exhibited much better corrosion resistance (Figure 14). Although ~50% of the aluminum surface showed signs of minor corrosion, no significant discoloration due to rust build-up was observed.

While this corrosion resistance ofĚýthe organofunctional bipodal silane-containing system was not perfect, higher loading levels of the organofunctional bipodal silane could further enhance this post-alkaline test corrosion resistance performance.

Electrochemical Impedance Spectroscopy

While contact angle measurements have shown the impact of organofunctional bipodal silanes on the hydrophobicity of waterborne coatings, and salt spray tests have demonstrated the corrosion resistance performance of the coatings, EIS can also offer valuable insight into the barrier properties of these organic coatings on metal substrates. EIS is a useful characterization technique for analyzing coated metal substrates because the deterioration of a metal coating in the presence of an electrolyte solution can be monitored in real time.¹⁶ By simultaneously measuring the resistance and capacitance of an organic coating on a metal substrate, the impedance of the system can be calculated. As organic coatings are nonconductive in nature, they typically exhibit very high impedances in the presence of an electrolyte. The better the barrier properties of an organic coating, the lesser is the amount of electrolyte solution that is directly exposed to the metal substrate, which subsequently results in high impedance measurements for the system. While an organic coating may exhibit high impedances immediately after the coated metal substrate is immersed in the electrolyte solution, the impedance of the system will drop as the electrolyte continuously penetrates the organic coating and reaches the underlying metal substrate.¹⁷ Even though this initial corrosion of the metal substrate may only be occurring over microscopic surface areas, EIS can detect these small changes in impedance when no visible corrosion has appeared on the surface of the coated metal substrates.¹⁸ Impedance measurements are typically carried out across a broad range of frequencies, and are represented in a Bode plot (Figure 15).

One of the most evident observations of EIS measurements is the difference in the magnitude of the impedance for each corrosion protection system. Comparing the impedance magnitude (in ohms, Ω) of these systems at 0.1 Hz is a quick, well-established prediction of how well a corrosion-protection coating will perform as a barrier over time.¹⁹ At 0.1 Hz, the impedance of the thermally cured waterborne organofunctional silanol coating with functionalized colloidal silica and an organofunctional bipodal silane was ~82 kΩ (82,000 ohms). Without the organofunctional bipodal silane in this thermally cured waterborne organofunctional silanol coating, the impedance at 0.1 Hz was ~54 kΩ, a ~34% reduction in impedance. Addition of an organofunctional bipodal silane into the room temperature cured waterborne organofunctional silanol coatings with colloidal silica improved the impedance of the system from ~33 kΩ to 40 kΩ at 0.1 Hz, a ~17% improvement in impedance. It is important to note that the dry-film thicknesses for all these pretreatment systems were approximately the same (less than 1 µm), thus not contributing to the difference in impedance observed in these EIS measurements.

The lowest impedance across all the frequency measurements was observed with the uncoated aluminum substrate. Without a coating to act as a barrier to the water and salts in the surrounding environment, corrosion can form rapidly and degrades the surface over time.
With the application of a waterborne organofunctional silanol coating with functionalized colloidal silica on the aluminum surface, the barrier effect is increased, subsequently increasing the impedance of the system over a wide range of frequencies.

Addition of an organofunctional bipodal silane into the waterborne organofunctional silanol coating with functionalized colloidal silica significantly increased the impedance of the system over the range of 0.1 Hz to 20,000 Hz. Additionally, coatings cured at 180°C exhibited significantly higher impedance measurements than the coatings cured at room temperature. This can best be explained by the additional crosslinking introduced into the waterborne coating by both the organofunctional bipodal silane and the heat from the thermal curing procedure. The organofunctional bipodal silane increases the crosslinking density and surface passivation with additional alkoxy groups being introduced into the system, while the thermal curing procedure accelerates the formation of siloxane bonds between the aluminum substrate and the coating through driving condensation. Both the organofunctional bipodal silane additives and elevated temperature cure increase the impedance of the system by making the coating a more efficient barrier to water and salts in the surrounding environment. While contact angle measurements gave insight into the hydrophobicity of these waterborne systems, and salt spray testing gave observable corrosion resistance evidence, these EIS measurements further confirmed the hypothesis that organofunctional bipodal silanes can improve the corrosion resistance of waterborne systems by increasing the surface passivation of the coatings.

CONCLUSION

As waterborne silane technologyĚýcontinues to gain interest as an environmentally friendly alternative to well-established corrosion-protection technology, investigations intoĚýperformance-enhancing additives are crucial for supporting this market growth. It can be concluded that organofunctional bipodal silanes are viable performance-enhancing additives in waterborne corrosion-protection systems due to the increase in hydrophobicity, crosslinking density, and surface passivation that these materials can provide. In particular, the waterborne organofunctional silanol system with colloidal silica and organofunctional bipodal silane additives exhibited the best corrosion resistance in neutral salt spray testing, the best alkaline resistance in alkaline testing, and the highest impedance during EIS testing. However, it is important to note that adding in an organofunctional bipodal silane into a room temperature cured waterborne organofunctional silanol coating did not outperform a thermally cured waterborne organofunctional silanol coating without organofunctional bipodal silane additives. While contact angle measurements, salt spray testing, alkaline resistance testing, and EIS data support this claim of increased corrosion resistance with the use of organofunctional bipodal silane additives, further research is necessary to understand the complete scope of waterborne silane technology and its interactions with organofunctional bipodal silanes. Additional experimentation, including outdoor weatherability testing in real life conditions, is underway to better understand this technology in hopes of further improving the performance, affordability, and reliability of waterborne silane coatings for corrosion resistance applications.

Presented at the 46^th International Waterborne, High-Solids, and Powder Coatings Symposium, February 24–March 1, 2019, in New Orleans, LA.

*Dynasylan® is a registered trademark of Evonik Degussa GmbH.

*Bulk Kleen® is a registered trademark of Bulk Chemicals Inc.

† WaterPro® Plus is a registered trademark of Bulk Chemicals Inc.

‡ Aluminum 6061T6® is a registered trademark of ACT Test Panels, LLC.

^ §Q-Fog® is a registered trademark of Q-Lab Corporation.

*Gamry PCI4/750™ is a trademark of Gamry Instruments.

References

1.ĚýĚý Kendig, M., Jeanjaquet, S., Addison, R., and Waldrop, J., Surf. Coat. Technol., 140 (1), 58–66 (2001).

2.Ěý Fedrizzi, L., Deflorian, F., Rossi, S., Fambri, L., and Bonora, P., Prog. Org. Coat., 42 (1-2), 65–74 (2001).

3.Ěý Zhu, D. and van Ooij, W.J., Prog. Org. Coat., 49 (1), 42–53 (2004).

4.Ěý Cave, N. and Kinloch, A., Polym., 33 (6), 1162–1170 (1992).

5.Ěý Plueddemann, E., J. Adhesion, 2 (3), 184–201 (1970).

6.Ěý Monticelli, F., Toledano, M., Osorio, R., Ferrari, M., Dental Mater., 22 (2006).

7.ĚýĚý Liu, Q., Ding, J., Chambers, D., Debnath, S., Wunder, S., and Baran, G., J. Biomed. Mater. Res., 57 (3), 384–393 (2002).

8.Ěý Song, J. and van Ooij, W.J., Adhes. Sci. Technol., 17, 2191–2221 (2003).

9.Ěý Zhendong, S., Yanning, Y., Qingpeng, L., Jianguo, L., and Chuanwei, Y., Adv. Mater. Res., 971–973, 135–138 (2014).

10. Salon, M.-C., Bayle, P.-A., Abdelmouleh, M., Boufi, S., and Belgacem, M., Coll. Surf. A: Physicochem. Engin. Aspects, 312 (2-3), 83–91 (2008).

11. Brinker, C.J., J. Non-Cryst. Solids, 100 (1-3), 31–50 (1988).

12. Zhao, H., Yu, M., Liu, J., and Li, S., J. Electrochem, Soc,, 162 (14), C718–C724 (2015).

13. Skerry, B., Alavi, A., and Lindgren, K., Coat. Technol., 60, 97 (1988).

14. Manavbasi, A., Bodily, K., Clarke, T., Johnson, K., and Estes, B., Met. Finish., 111 (6), 12–15 (2013).

15. Zhang, J., Klasky, M., and Letellier, B., J. Nucl. Mat., 384, 175–189 (2009).

16. Bierwagen, G., Tallman, D., Li, J., and He, L., Prog. Org. Coat., 46, 148 (2003).

17. Sharer, Z. and Sykes, J., Prog. Org. Coat., 74 (2), 405–409 (2012).

18. Chico, B., Galvan, J.C., de la Fuente, D., and Morcillo, M., Prog. Org. Coat., 60, 45–53 (2007).

19. Gray, L. and Appleman, B., J. Protect. Coat. & Linings, 66 (2003).

CoatingsTech | Vol. 16, No. 4 | April 2019

In the challenging world ofĚýĚýindustrial, protective,Ěýand marine coatings, two-component (2K) epoxy systems are established as the benchmark technologies due to the combined offerings of excellent corrosion protection and compliance with regionalĚývolatile organic compound standards. Today, productivity has emerged as a major driver, and innovation focuses on developing epoxy coatings that have greater application versatility while providing enhanced performance properties, such as dry speed, rapid recoat, and through-cure. For marine and protective coatings, the need is for faster cure and blush-resistant coatings when applied under adverse, low temperatures conditions. In the OEM sector, the wet-on-wet application process means there is a requirement for epoxy systems to provide rapid overcoatability with polyurethane and/or polycarbamide topcoats within minutes after initial application of the primer. In this sector, the drivers are to increase overall productivity with faster application of multiple layers coupled with lower bake temperatures which can provide energy savings. This article will focus on the performance attributes of a novel polycyclic-aliphatic amine and its use in the development of new epoxy curing agents designed to provide benefit in the above markets. The supporting data includes the functional properties thermal analysis, glass transition (T_˛µâ€…), and infrared (IR)-cure profile, confirming the rapid through-cure and crosslinking capabilities in epoxy systems. In addition, a review of model coating formulations and their key performance attributes, including the rapid recoat times, improved intercoat adhesion, and excellent corrosion protection properties will be discussed.

Introduction

Global megatrends are reshaping the world we live in, driving common requirements of improved productivity and reducing costs, while addressing emerging environmental concerns. Epoxy coatings used in marine and protective coatings are based on either solid or liquid epoxy resins, derived from bisphenol A digylcidylether and cured in combination with polyamides or modified aliphatic or cycloaliphatic amine hardeners. Typical modifications include amine adducts, Mannich bases, phenalkamines, and specialty ketimine curatives,¹ designed to ensure an optimum balance of handling and end performance properties. With the introduction of maximum volatile organic compound (VOC) limits in coating applications, development work in epoxy coatings has moved away from using traditional solid epoxy resins (SER) to systems based on the lower viscosity liquid epoxy resin (LER). A standard solvent-free LER has a viscosity of 10,000 mPa.s and is characterized by an epoxy equivalent weight (EEW) ±190 (functionality ±2). The use of LER enables the formulator to achieve higher coating solids (≥80%) vs the traditional solventborne SER systems where solids are in the 40%–60% range. This approach influences the handling and performance characteristics of the formulated coating. Reaction kinetic studies demonstrate that there is a negative impact on the workable pot life coupled with an extension of the dry time. The latter effect is due to the polymeric network having to react and build up sufficient molecular weight to reach the gel point or dry-to-touch state, whereas with the SER systems, these are already high molecular weight polymers and dry-to-touch or lacquer dry is observed as soon as the solvents evaporate from the coating film.²

To overcome the slower dry speed, formulators typically incorporate tertiary (3°) amine accelerators (e.g., tris -2,4,6-dimethylaminomethyl phenol) at ±5% by weight based on active curing agent into the formulation. The acceleration mechanism of the 3° amine is that it polarizes the C–O bond in the epoxy group and makes it more susceptible to nucleophilic attack with the primary (1°) and secondary (2°) amines present in the additional curing agent.³

Formulators can only use small quantities of this type of accelerator because a high concentration can result in driving the homopolymerization of the epoxy resin in favor of the preferred crosslinking reaction between the epoxy resin and amine curing agent. Excessive homopolymerization often results in brittle coatings and free unreacted amines, the presence of which may result in a decrease in the corrosion resistance properties of the cured film.

Our approach to amine design has resulted in the development of a new polycyclic amine, Poly [HCA], that has a balance of 3°, 2°, and 1° amines built into the polymeric backbone.⁴ Poly [HCA] delivers a unique balance of properties by acting as a reactive accelerator and co-curative all in one. The amine is capable of enhancing the molecular weight build up in applied epoxy so coatings can reach their dry-to-touch state faster, while driving the crosslinking reaction so that coatings achieve rapid through-cure at both ambient and low-temperature cure conditions. This is evident when monitoring the degree of reaction via infrared (IR) spectroscopy. The Poly [HCA] shows a higher level of hydroxyl formation compared to the standard tertiary amine accelerator, which in the presence of liquid epoxy resin is shown to form ether linkages (Figure 1). High ether linkage formation is a clear indicator that the epoxy system undergoes homopolymerization rather than amine-epoxy crosslinking during the curing process.

The novel polycyclic amine technology enables rapid property development when formulated with conventional polyamides, modified polyamides, aliphatic, and other formulated epoxy curing agents. Examples of curing agents developed based on Poly [HCA] amine include two new polycyclic amine polyamides, RDPA-1 and RDPA-2. The new polyamides undergo rapid cure, which enables application of topcoats based on similar epoxy or polyisocyanate technology within 15–30 min while maintaining excellent intercoat adhesion and corrosion protection. It provides excellent film appearance without loss of gloss, distinction of image and surface wrinkling, and dive-back of topcoats into the primer. This performance attribute allows applicators to spray-apply multiple layers within quick succession, to increase the overall productivity of the coating application at the job site.

Key Features and Benefits of the Poly [HCA] Amine Building Block

Common amines used in the design of amine curing agents are classified as aliphatic or cycloaliphatic amines, examples of which include diethylenetriamine (DETA), triethylenetetramine (TETA), diaminocyclohexane (DACH), and isophoronediamine (IPD). Although aliphatic amines such as DETA and TETA have high functionality and reactivity, they have a strong tendency to blush and form amine carbamate due to poor compatibility of the amine with the epoxy resin. On the other hand, cycloaliphatic amines have excellent compatibility with epoxy resin because of the cyclocaliphatic backbones but have slower reactivity than aliphatics, especially at low temperature. Good compatibility between curing agent and epoxy resin is essential to provide coatings with good surface appearance, excellent overcoatability, and corrosion resistance. It has been a challenge in epoxy coatings to design a new amine building block that possesses the benefits of both the aliphatic and cycloaliphatic amines: the reactivity of an aliphatic amine and the resin compatibility of a cycloaliphatic amine. As highlighted, Poly [HCA] is a novel polyheterocyclic amine that crosslinks with an epoxy resin delivering fast through-cure while maintaining good resin compatibility over a range of application temperatures. Poly [HCA] is a polymeric amine with moderate viscosity, has a low color (water white), and can be used as a sole curing agent or as a co-curing agent with other amines. The general handling properties are outlined in Table 1, and the following sections detail the amine’s unique performance properties.

ĚýFast Property Development of Poly [HCA] Amine

Poly [HCA] amine was evaluated against aliphatic amines, cycloaliphatic amines, and Mannich base curing agents to exemplify its fast development of coating properties. For this, Poly [HCA] amine was plasticized with ±30 wt% of benzyl alcohol, comparable to the concentration contained in many commercial curing agents, and cured with standard bisphenol A liquid epoxy resin (EEW=190) at 1:1 stoichiometry. Clear coating formulations for thin film set time (TFST), Persoz hardness, and gloss were deposited on glass substrates at 150 mm wet film thickness using a bird applicator. The TFST was determined using a Beck-Koller recorder, in accordance with ASTM D5895. Persoz hardness was conducted in accordance with ASTM D4366 after coatings were cured at 23°C, 10°C, and 5°C, and 50% RH for designated cure duration of one day, two days, and seven days. Gloss was determined at an angle of 20° and 60° using a Gardner gloss meter according to ASTM D523. Measurements were made with the glass panel placed on a black cardboard background to minimize reflection.

The test summary in Table 2 clearly shows that clear coatings based on Poly [HCA] delivered fast dry speed and Persoz hardness development at ambient and low temperatures. The Poly [HCA] amine demonstrated faster dry speed than a Mannich base curing agent, which is typically used as fast curative especially for low-temperature cure. Poly [HCA] coatings also exhibit good coating appearance similar to cycloaliphatic amine, and better than the Mannich base curing agent. High gloss coating is an indication of good compatibility between resin and curing agents. By comparison, the aliphatic amine exhibited poor compatibility with the epoxy resin and, as such, the resultant coatings were greasy and the Persoz hardness could not be obtained.

In addition, Figure 2 shows the cure and property development of clear coatings across a range of application temperatures, from ambient temperature to low temperature down to 5°C. The fast cure property is demonstrated by the early hardness development and ultimate hardness both at ambient and low application temperatures, thus making the Poly [HCA] amine an ideal amine building block for a variety of coating systems where low temperature cure is critical. Poly [HCA] amine not only delivers fast cure speed, but also shows good compatibility with epoxy resin.

ĚýFundamental Study of Fast Cure Mechanism of Poly [HCA] Amine

To understand the cure mechanism of Poly [HCA] amine, dynamic mechanical analysis (DMA) was utilized to monitor the cure process. DMA provides the mechanical property information and the crosslinking density of the cured samples. The crosslinking density is expressed as the average molecular weight between crosslinking points, Mc, and is calculated from the analysis data, while mechanical property such as storage modulus was measured during the analysis.⁵ In this experiment, Poly [HCA] amine was compared with a cycloaliphatic amine and an aliphatic amine. The amines were cured with standard bisphenol A liquid epoxy resin at 1:1 stoichiometry, and the samples were prepared by making the plaques in silicone rubber molds and allowed to cure at ambient temperature and humidity for a week.

Figure 3 compares the average molecular weight between crosslinking points, Mc, of the samples of the Poly [HCA] amine, an aliphatic amine, and a cycloaliphatic amine after one-day, three-day, and seven-day cure. Mc of Poly [HCA] amine is low and remained unchanged after one-day cure, while Mc of aliphatic amine and cycloaliphatic amine are higher after day 1 and then decrease over time, indicating lower initial degree of crosslinking. However, further crosslinking develops over time, thus a longer reaction time is necessary to achieve full through-cure. Mc of cycloaliphatic amine was the highest, suggesting the slowest reaction rate and lowest crosslinking density.

Furthermore, Figure 4 shows the comparison of storage modulus, G’, after one-day cure. G’ of Poly [HCA] remained relatively flat and was the highest among the samples, indicative of highest degree of cure, and no post cure in the rheometer. However, G’ of aliphatic amine and cycloaliphatic amine shows a greater increase, indicating significant post cure in the rheometer. Cycloaliphatic sample exhibited the greatest increase in G’ in the rubbery region, suggesting that it had the lowest degree of cure at ambient temperature. The DMA data of Mc and G’ demonstrate that Poly [HCA] is a much faster curing agent than aliphatic and cycloaliphatic amines, and can reach a high degree of through-cure and crosslinking within a shorter time period.

New Polycyclic Polyamide Curing Agents

The Poly [HCA] amine is further derivatized to prepare a range of amine curing agents. Of specific interest is the use of the amine is the synthesis of polycyclic-polyamide curing agents that deliver enhanced rapid dry characteristics. Two examples, RDPA-1 and RDPA-2, have been developed and will be reviewed in this article. Both new curing agents provide rapid through-cure at both ambient and low application temperatures, and are capable of being rapidly overcoated either self-on-self or with polyisocyanate-based technologies within a 15 min window after initial application. The technology also delivers the high levels of corrosion resistance properties demanded by the marine, protective, and industrial maintenance coating markets. The typical handling and performance properties of the new curing agents are summarized in Table 3 and subsequent sections below.

Near FTIR spectroscopy (NIR) is a powerful and versatile technique for monitoring transient chemical change during the cure process.⁶ It offers a unique possibility to obtain detailed information about molecular orientation and relaxation behavior and is an effective tool to monitor the extent of the epoxy-amine reaction. Using an NIR spectrometer, Model 6500, the conversion of oxirane (epoxy) and primary amine during the cure was monitored by the C–O stretch of oxiran ring at 1646 cm^-1, and the N–H stretch of the primary amine at 2026 cm^-1.

Figures 5 and 6 show the conversion of primary amine and epoxy during the cure process, comparing the RDPA-1 with a standard high solid polyamide (HSPA-1) and the same polyamide blended with a tertiary amine (HSPA-1a). In Figure 5, the primary amine conversions were similar among the three samples due to the fast reactivity of available primary amines with the epoxy. However, in Figure 6, RDPA-1 exhibits the fastest epoxy conversion—faster vs the HSPA-1a and significantly faster vs the unmodified polyamide HSPA-1. Although HSPA-1a showed a fast epoxy consumption vs HSPA-1, there was a lower level of hydroxyl formation in the matrix. The data indicates that a high percentage of the epoxy conversion is a result of the epoxy homopolymerization reaction instead of the amine crosslinking with the epoxy resin.

The conclusion is further collaborated by dynamic mechanical analysis. Table 4 shows the T_g and Mc of the three polyamide systems. The coating system HSPA-1a containing the tertiary amine accelerator exhibited the highest T_g and lowest Mc, followed by RDPA-1, whereas the standard polyamide HSPA-1 exhibited the lowest T_g and highest Mc values. Both modified polyamides had lower Mc vs HSPA-1, indicating a higher degree of crosslinking. However, in the case of HSPA-1a, the presence of the tertiary amine also drives the competing homopolymerization reaction, which in turn, leads to lower Mc, a higher T_g, and potentially an increase in coating brittleness.

Further evidence supporting the excellent low-temperature cure characteristics of RDPA-1 is shown by the faster dry speed development obtained in clearcoat formulations (Figure 7). When used with liquid epoxy resin, the thin film set times as measured using a Beck Koller (BK) instrument offer a significant improvement over both HSPA-1 and a special modified high solid polyamide adduct (HSPA-2), commonly promoted for low-temperature cure applications. At room temperature, the phase III, thin film set time is 4 h compared with 10 h and 7 h, respectively for HSPA-1 and HSPA-2. At lower application temperatures, the performance benefits of RDPA-1 are clearly demonstrated where a phase III dry time of 14 h is achieved, compared with 48 h for HSPA-1 and 30 h for HSPA-2.

Additional analysis via DSC highlights the differences in the degree of cure development of the polyamides at low application temperatures. Analysis was conducted via measurement of the residual exotherm, during the curing process. Samples were prepared at ambient temperature and then the sealed DSC cells were immediately stored at 5°C in a climate chamber for one to seven days. After the allotted cure time, samples were removed and scanned by DSC (TA Instruments–model Q200) at a ramp rate of 10°C/min. The percentage cure for each sample was calculated by using the following equation.⁷

When cured with the standard liquid epoxy resin, DSC analysis shows that the new curing agent RDPA-1 undergoes excellent cure development at 5°C (Figure 8). When compared with HSPA-1 and HSPA-2, RDPA-1 achieves a degree of cure after one day of 64%, which is two times faster than HSPA-1 which only achieved 30% conversion. HSPA-2 achieved 40% degree of cure, however, still significantly slower vs the new curing agent technology. After seven days’ cure at 5°C, the extent of cure for RDPA-1 was >95%, compared with 55% and 83% for HSPA-1 and HSPA-2 respectively.

The RDPA-1 curing agent offers additional performance benefits vs existing high solid polyamides used for protective coating applications. The benefits are summarized in Table 5 and include a lower initial curing agent viscosity, thus allowing for high solid coatings where VOC <250 g/L can be achieved. More importantly, the surface appearance and early film integrity, as determined by the ability of the coating to withstand attack from solvents at lower application temperature, is significantly improved.

With RDPA-1, the cured coating is smooth and glossy when systems are applied with a minimum induction time of 5 min. With HSPA-1, film appearance is hazy, and an inherent tackiness is present. MEK resistance using the double rub methodology was employed as a way of assessing early through-cure and solvent resistance. In the case of RDPA-1, clearcoats provided >200 MEK rubs with no down glossing after three days at 5^°C, whereas the polyamide systems HSPA-1 and HSPA-2 exhibited no resistance to MEK after one day and after three days the coatings were destroyed, exhibiting < 100 double rubs.

ĚýNovel Polyamide Curing Agents for Wet-on-Wet Applications Ěý

For OEM applications, there is a growing demand for 2K systems to be applied wet-on-wet to enhance throughput and reduce overall application time and costs. The incumbent technology utilized in sectors where rapid development of cure properties is essential is typically based on Mannich base (phenalkamine type) curing agents that are derived from low molecular weight ethylene diamines. This technology while providing fast dry, often results in coatings with amine blush. This subsequently interacts with the polyisocyanate topcoats, causing surface defects such as down glossing and excessive wrinkling, as shown in Figure 9A. Utilizing the polycyclic amine Poly [HCA], the new rapid cure polyamide, RDPA-2 curing agent is shown to eliminate this phenomenon. When formulated with LER, the surface appearance of the epoxy primer based on RDPA-2 is blush free. This was confirmed via both a visual inspection as well as surface analysis adding a drop of 1% aqueous solution of phenolphthalein indicator to the coating and conducting a swab test.

Upon application of a polyurethane topcoat following an initial cure time of 30 min for the base epoxy primer, the topcoat based on a solvent containing polycarbamide dries wrinkle free and with a high surface gloss, as shown in Figure 9B.

For coatings to be used in wet-on-wet applications, systems must build up sufficient coating integrity, which requires coatings to be touch dry or be resistant to dust pickup as quickly as possible after spray application of the primer coating. To demonstrate the performance value of the new polyamide technology, anticorrosive primers were applied on Bondrite B-952 panels at a wet film thickness of 180 µm. Cure properties were evaluated using the thumb twist method, which is an objective test method. In this method, the operator places a thumb on the coating and twists the thumb 90^o. If no finger print was observed, the coating was deemed as the minimum dry time required for over coating with the next layer. For this study, the properties of RDPA-1 and RDPA-2 were compared to three alternative technologies. The LOPA-1 polyamide represents a polyamide with an established track record in the coatings industry where it provides an extended recoat window greater than six months.⁸ HSPA-1 was used as the generic high solids polyamide, and a commercial system (CS-1) was used within the OEM market, based on an accelerated polyamide. The results of the dry film tests are summarized in Table 6.

These results clearly demonstrate the faster surface dry characteristics inherent in RDPA-1 and RDPA-2. Both technologies reach the rapid surface dry state significantly faster compared to a standard polyamide, but also faster vs the commercial system CS-1. AlthoughLOPA-1 was faster vs CS-1, this product contains solvent and as such makes it difficult for formulators to meet the need for lower VOC systems. When comparing the reaction kinetics using infrared spectroscopy, RDPA-2 shows a faster conversion of the available primary amine content and higher molecular weight buildup vs comparative samples used in this study.

The dry and wet adhesion properties of the epoxy coatings were also evaluated. Tests were conducted on Bondrite B-952 steel panels by applying primers at a wet film thickness of 75 µm followed by a second epoxy white topcoat at 15 min and 60 min intervals. Examples of the primer (F1) and topcoat (F2) are provided in Appendix A. While examples are based on RDPA-1, the same Component A was used for all the curing agents tested, with Component B adjusted for loading based on the AHEW of the corresponding curing agent.

Following seven days cure at ambient temperature, intercoat adhesion was measured as follows: For the wet adhesion test, the panels were immersed in water for additional 24 h. After the immersion time, panels were wiped dry with a paper towel and tested in accordance with ASTM D3359 method A. The results are listed in Table 7.

Rapid Recoat Study Using Polycarbamides

The above study highlighted the excellent rapid dry and overcoatability of RDPA-1 and RDPA-2 with a standard epoxy topcoat. As discussed in the next section, epoxy primers were evaluated for their ability to be rapidly overcoated with a fast cure isocyanate-based system. For this example, a polycarbamide⁹ (polyaspartic) topcoat based on HDI trimer and a cycloaliphatic diethyl maleate ester-curing agent was used. The rapid recoat properties of RDPA-1 polyamide are compared with that of conventional polyamide HSPA-1 by applying 75 mm of formulated epoxy primer as shown in Appendix A (F1) on Bondrite B-952 panels and cured for 15 min and after 60 min on separate panels. Formulated polycarbamide topcoat Appendix A (F3) was applied (150 mm) after 15 min and 60 min on primed Bondrite panels. The panels were cured for 24 h, and the cross-hatch adhesion test as per ASTM D3359 was conducted. The results are given in Table 8, and the appearance following the dry adhesion tests are depicted in Figures 10 and 11.

With the modified polyamide RDPA-1, the epoxy primer is dry to touch (finger press) after 15 min cure at 23°C. After application of the polycarbamide topcoat, the surface appearance is smooth and wrinkle free, and high gloss is obtained. The intercoat adhesion results are excellent with a rating of 5A being achieved. With the HSPA-1 curing agent, the epoxy primer is still wet to touch after 15 min and the applied topcoat showed both base primer bleed through and a down glossing after 24 h cure. With this system, the polycarbamide topcoat exhibited very poor intercoat adhesion with a significant surface area delaminating following application and removal of the adhesive tape during the cross-hatch test. When base primers were subjected to a 60-min cure at 23°C followed by application of the polycarbamide topcoat, again excellent surface appearance and adhesion was observed for the RDPA-1 curing agent. There was some improvement with the HSPA-1, with no primer bleed through and an improvement in surface gloss; however, the dry/wet adhesion of the polycarbamide coating was poor with a rating of 2A only. With HSPA-1, the epoxy primer requires approximately 2 h cure¹⁰ prior to application of the topcoat to provide the intercoat adhesion performance exhibited by RDPA-1.

Anti-corrosive Primer Formulation

Appendix A (F4) contains a starting point formulation based on RDPA-1 for an anti-corrosive primer. The formulation is a red iron oxide primer based on a modified zinc calcium polyphosphate and offers 86% volume solids at VOC of 198 g/L. Further, the primer formulation has a low mix viscosity of approximately 550 mPa.s, with a pot life of 1.5 h. The primer can be spray applied with conventional spray equipment following 5–10 min of mixing or brush applied to a steel substrate without the addition of extra solvents. The BK phase III dry time is achieved after 2 h, while a dry-to-handle time (thumb twist) is obtained within 8 h.

Accelerated Corrosion Resistance

The anti-corrosive primer formulation based on RDPA-1 was applied to grit blasted (SA 2.5), hot-rolled steel substrate panels. Using conventional spray equipment in double coats, a 180–220 mm dry film thickness resulted. Panels were left to cure at ambient temperature for 10 days prior to testing in salt spray. Panels were scribed and evaluated for field blisters using ASTM B117. Evaluation of scribe creep was rated in accordance with ASTM D1654. At 1000 h intervals, one set of duplicate panels was removed from the test cabinet and evaluated for blistering and rusting. After the visual evaluation was completed, the scribe areas were scraped to expose the underlying metal substrate, allowing for accurate scribe creep measurements. Results for the 2000 h exposure are shown in Figure 12 and reported in Table 9.

All ratings and scribe creep values reported are the average of the two test panels in each set. The results show that throughout the 2000 h exposure period, the RDPA-1 epoxy primer had similar resistance to blistering and rusting as the standard epoxy-polyamide control coating based on HSPA-1. During the test, no blistering or field rusting was observed on the test panels, thus demonstrating RDPA-1 delivers excellent corrosion resistance properties.

Cathodic Disbondment

The experimental setup is shown in Figure 13. Each test panel is fitted with one intentional holiday of 3 mm diameter in the paint layer, and then covered with a glass cylinder (inside diameter = 99 mm; high 155 mm) on the painted side. The glass cylinders are placed so that from every panel the intentional holiday is situated in the middle of the test area. The glass cylinder is filled with about 1000 mL of electrolyte (artificial seawater). To establish the electrical circuit, a connection to the steel panel is made with a platinum or graphite electrode that is placed in the center of the tank containing the electrolyte. This acts as the anode and is connected to the positive lead from the power supply (Top Hex Cathodic Disbondment Tester). The bare steel of the panel (cathode) is connected with copper wire to the negative lead of the power supply. A reference electrode (saturated calomel) is placed in the test tank for measuring a continuous potential of 1.5 V. After 28 days at room temperature (23°C), the test was stopped. The exposed coatings were checked for loss of adhesion, blistering (ASTM D714) and other defects (discoloration, cracking, etc.). Loss of adhesion was determined by cutting eight radial pies, extending 3 cm from the center of the intentional holiday, by using a sharp-bladed knife. Starting at the intentional holiday and working outward, the degree of disbondment was measured. The disbondment cell and panels after 28 days’ exposure are shown in Figure 13.

Coatings formulated with RDPA-1 and the benchmark HSPA-2 resulted in an average radial creep of 1 mm and 3 mm respectively. This is well within the requirements of the test and, as such, both coating systems are fit for purpose and meet the ASTM G8-96 standards for pipe coatings requiring excellent cathodic disbondment resistance.

Conclusions

In this article, the utility of a new polycyclic amine, Poly [HCA] that acts as both accelerator and crosslinker for epoxy coatings has been explored. It was demonstrated that the amine allows for a rapid conversion of both epoxy and primary amine functionalities within a 2K epoxy coating system. Thus, coatings achieve a high degree of conversion compared with other aliphatic and cycloaliphatic amines. The buildup of crosslink density is driven by the amine-epoxy reaction compared to the epoxy homopolymerization reaction that is the dominant mechanism observed when conventional tertiary amine accelerators are employed. The new amine technology has further enabled the development of advanced polyamide epoxy curing agents that now exhibit an excellent balance of low-temperature cure, while maintaining the excellent adhesion and corrosion resistance properties required for long-term corrosion protection. In addition, the ambient cure properties of the new curing agents are such that they allow for the development of epoxy primers that exhibit ultra-fast, tack-free set times. The fast set-to-touch combined with a blush-free appearance and excellent early solvent resistance enables the epoxy basecoats to be topcoated with a range of different coating technologies within a very short application window. This property makes the technology ideal for use in wet-on-wet applications for automotive and industrial maintenance applications, where faster technology can provide faster throughput, increasing productivity in the field projects and reducing system and application costs while providing long-term asset protection.

Acknowledgment

The authors would like to thank Evonik colleagues Mike Oberlander, Tom Corby, Aziz Gaffar, and Marcel Peters for carrying out the experimental work, as well as Daniel Totev, Wei Cao, Rob Rasing, and Marcelo Rufo for their technical input.

References

1.Ěý Lee, H. and Neville, K., Handbook of Epoxy Resins, McGraw-Hill: New York, NY, 1967.

2. Walker, F.W., Cook, M., Vedage, G., and Rasing, R.,
J. Coat. Technol. Res., 6 (3), 283-313, (2009).

3. Rozenberg, B.A., Adv. Polym. Sci., 75, 113 (1985).

4.Ěý Zheng, S., et al., European Patent publication, 3170849A1 (2017).

5. Vratsanos, M., “Rheological and Thermal Characterization of Polymer Coatings,” CoatingsTech, 7, 28-38 (2017).

6. (a) Xu, L., Fu, J.H., and Schlup, J.R.J., Am. Chem. Soc.,116, 2821-2826 (1994); (b) Fu, J. H. and Schlup, J.R.J., Appl. Polym. Sci., 49, 219-227 (1993).

7.Ěý Weinmann, D.J., Dangayach, K., and Smith, C.,”Amine-Functional Curatives for Low Temperature Cure Epoxy Coatings,” J. Coat. Technol., 68 (863), 29-37 (1996).

8. Cook, M. and Rasing, R., 19th SLF Congress, 2009.

9. Amicure® IC-133–Evonik Materials.

10. Evonik Materials; unpublished results.

Appendix A—Start Point Formulations

CoatingsTech | Vol. 16, No. 3 | March 2019

Corrosion prevention is a major priority for the agency. As a result, they are also focusing on structures on Earth that are generally located in hot, humid climates and in close proximity to the ocean, with constant exposure to salt spray, as well as structures within engines exposed to extreme temperatures and environments. Coating technologies have been developed to address both issues. Because the latter are typically ceramic or metallic systems, this article will focus on the former.

The first example is from NASA’s Kennedy Space Center (KSC), which is seeking licensees for its Anti-Corrosive Powder Particles. The Powder Particle technology combines metallic materials into a uniform particle that can be applied to prevent corrosion of rebar embedded in concrete. The powder is sprayed simultaneously with a liquid epoxy binder onto the surfaces of concrete structures to achieve a uniform distribution of the metallic pigments and provide cathodic protection of the underlying steel in the concrete. After the coating is applied to the outer surface of reinforced concrete, an electrical current is established between the metallic particles and the surfaces of the embedded steel rebar. This approach is advantageous because the coating is applied (and reapplied as needed) via brush or spray to the exterior of the concrete (not to the rebar) after construction is complete. According to NASA, it has applications for parking decks, ramps, and garages; highway and bridge infrastructure; concrete piers, offshore platforms, and other marine structures; cooling towers; pipelines; and buildings, foundations, and other engineered structures.

NASA’s KSC has also developed and is seeking licensees for its newest liquid-applied coating for corrosion prevention of rebar embedded in concrete. According to the agency, the coating is made of inexpensive, commercially available ingredients and is easily applied to the outer surfaces of reinforced concrete by brush or spray. It is an inorganic, galvanic coating containing various metallic particles including magnesium, zinc, and/or indium along with moisture-attracting compounds that facilitate the protection process. After the coating is applied to the outer surface of reinforced concrete, an electrical current is established between the metallic particles and the surfaces of the embedded steel rebar. Early tests of the coating have shown that it meets NACE International RP0290-90 100-millivolt (mV) polarization development/decay depolarization criteria for complete protection of steel rebar embedded in concrete. NASA has also demonstrated that the rebar becomes negatively polarized, indicating the presence of a positive current flow with a shift in potential of over 400 mV. Like the Powder Particle Technology, the liquid coating is applied after construction is complete. It typically lasts for 10 years and can be reapplied to provide extended protection. This coating has similar potential applications, from concrete balconies and ceilings to bridges, piers, parking garages, cooling towers, and pipelines.

Mechanical damage to the coating triggers the release of film-forming compounds to repair the damage . . . The technology has the potential for use in corrosion prevention coatings for bridges and other infrastructure, automobiles, ships, aircraft, pipelines, and machinery.

Another approach to corrosion control taken by the KSC scientists at NASA involves development of High-Performance Polyimide Powder Coatings. Polyimides with melting points much lower than those of traditional polyimides used for insulation have been formulated into powder coatings with excellent thermal stability, chemical resistance, and electrical properties. The coatings, which are based on polyamic acid resins, are currently undergoing testing, with encouraging early results regarding salt spray corrosion resistance. NASA believes they have potential applications for the protection of pipelines and other infrastructure, machinery, exposed metal parts, and automotive components. The agency is looking for partners to commercially develop this coating technology.

The researchers at NASA’s KSC have also developed a smart, environmentally friendly coating system for early detection and inhibition of corrosion and self-healing of mechanical damage without external intervention. The coating is designed to inherently detect the onset of corrosion in the coated substrate and respond autonomously to control it through the release of corrosion inhibitors and indicators from specially formulated microcapsules and particles. The onset of corrosion triggers the release of compounds that indicate and inhibit corrosion. Mechanical damage to the coating triggers the release of film-forming compounds to repair the damage. NASA believes the multifunctional coating system will reduce maintenance costs and improve safety by preventing catastrophic corrosion failures. The technology has the potential for use in corrosion prevention coatings for bridges and other infrastructure, automobiles, ships, aircraft, pipelines, and machinery.

In addition to corrosion prevention, the prevention of insect adhesion is another ongoing challenge for NASA. Accumulation of insect strikes on the leading edge of airplane wings is a more serious problem than one might realize. Depending on the magnitude, such accumulation changes the aerodynamic characteristics of the wing, causing a change from laminar to turbulent flow and resulting in decreased lift, increased drag, and reduced fuel efficiency. Compared with other possible solutions, coatings offer several benefits, including ease of application, potentially negligible weight penalty, reduced environmental concerns, better economics, and continual function throughout the flight profile, according to the agency.

NASA’s Langley Research Center (LRC) has developed a fluorinated alkyl ether containing epoxy system designed as a robust anti-insect coating to improve aircraft efficiency. It believes the coating will also be useful in other applications where reduction of insect residue adherence is desirable, such as in the automotive and wind energy industries.Ěý The easily applied epoxy coating has improved hydrophobicity even when the concentration of the fluorinated aliphatic component is as low as 1 wt% due to preferential migration of the functional groups to the surface of the polymer and incorporation of nano- to microscale particle fillers. Several formulations of this coating technology were flight tested on Boeing’s ecoDemonstrator in April and May of 2015.

NASA has also developed contaminant-resistant polyurethane coatings containing fluorine groups for use in extreme environments.Ěý These systems can be used not only for prevention of the adhesion of insect residue on aircraft, automobiles, and wind turbine systems, but as general anti-soiling coatings and coatings that provide improved stain and corrosion resistance and greater weatherability, according to the agency. They have been tested for adhesion mitigation of insect residues in a controlled insect impact facility and on the ecoDemonstrator Boeing 757 aircraft. Insect reside was lower than that observed on a non-coated surface, and the coatings were shown to be hydrophobic with durability comparable to current state-of-the-art polyurethane formulations. The coating also meets current aircraft manufacturing requirements.

Prevention of ice buildup on aircraft is another area where NASA has found that novel coating technology can help overcome real challenges. Scientists at LRC have developed novel monomers and polymers for the formation of coatings used to de-ice commercial aircraft and wind turbines. The coating prevents ice formation rather than removing it, minimizing the need to apply deicing agents.Ěý It works by mimicking the behavior of anti-freeze proteins (AFPs) found in certain fish and amphibians, imparting properties including reduction of the freezing point, ice recrystallization inhibition, and ice restructuring (changing ice crystal morphology), according to NASA. The result is ice growth inhibition and ice formation prevention via an adsorption mechanism.

Other interesting coating technologies developed by NASA include a low-cost, temperature-sensitive coating (up to 600°C) based on hematite (iron(III) oxide) in a binder that can serve as an alternative to photoluminescence techniques; conductive inks containing carbon nanotubes and metallic particles for inkjet printing that have resistances in the kilo-ohm range; durable yet flexible polyamide/polyimide aerogels that are 500 times stronger than conventional silica aerogels and useful for thermal insulation and lightweight structures; and thin-films with integrated structural and functional elementsĚý with increased damage tolerance to tearing and ripping produced using commercially available additive print manufacturing equipment.

CoatingsTechĚý |Ěý Vol. 16, No. 1 |Ěý January 2019

The process industries are well aware of the challenges posed by CUI, but as yet no foolproof solution has been developed. Conditions in processing plants by their nature will lead to corrosion. Once water penetrates insulation, CUI can progress via a number of different mechanisms depending on which chemicals are present. Electrolytes or salts, acidic or basic compounds, and leachable chlorides contribute to galvanic, acidic/alkaline, and chloride mediated CUI, respectively. Water penetration can result from many causes including monsoons, rain, flooding, wash downs, and sprinkler systems, as well as exposure to steam, humidity, or frequent condensation and evaporation of atmospheric moisture. CUI can appear as general corrosion, pitting, or stress corrosion cracking depending on the type of metal and the environmental conditions. Carbon and low-alloy steels maintained at higher temperatures in the presence of any moisture are at risk for corrosion. When moisture penetrates and is trapped beneath insulation, the corrosion process can be accelerated, resulting in aggressive CUI, particularly for metals heated at or above 100°C, where intermittent boiling and flashing of water occur.

Chemicals, oxygen, and moisture cannot penetrate this layer, even if it is scratched or gouged

To prevent, or at least delay, corrosion under insulation, it is necessary to keep water and reactive chemicals from coming in contact with the metal surfaces of processing equipment. Protective barrier coatings are most commonly used. Insulation can also be designed to direct water away from rather than penetrate the surface, comprised of materials such as fiberglass that do not retain water, and be devoid of leachable chemicals that participate in corrosion processes. Ongoing maintenance coupled with an effective inspection strategy is also essential. Inspection methods range from removal of the insulation to view the surface of the equipment to evaluation of the surface through the insulation using x-ray analysis, neutron backscatter and infrared thermography, ultrasonic thickness measurement, pulse eddy current analysis, and other nondestructive techniques. The latter approaches can reduce the cost of inspections, but their limitations must be kept in mind.

Polymer-based epoxy, polyurethane, and siloxane-based coatings are widely used for corrosion prevention. These types of coatings, however, only provide protection as long as they remain unblemished. Scratches, chips, and even tiny pinhole defects allow sufficient ingress of water and chemicals that can lead to corrosion. Once the coating is breached, it—and the insulation on top of it—can trap the water and any chemicals that promote corrosion, leading to CUI.

Recognizing the limitations of organic, polymer-based protective coatings, EonCoat, based in Raleigh, NC, developed an inorganic alternative that addresses these shortcomings for steel processing equipment. Spray-applied EonCoat is a chemically bonded phosphate ceramic (CBPC) that bonds through a chemical reaction with the substrate. The presence of a low level of surface oxidation provides a source of iron that promotes the reaction, which results in the passivation of the steel surface due to the formation of a magnesium iron phosphate alloy layer, according to Merrick Alpert, president of EonCoat. Chemicals, oxygen, and moisture cannot penetrate this layer, even if it is scratched or gouged. The chemical transformation of the steel surface into an alloy of stable oxides prevents corrosion from spreading beyond the damaged area. The alloy barrier layer is covered with a ceramic layer that further resists corrosion, water, fire, abrasion, impact, chemicals, and temperatures up to 400°F, Merrrick notes. “CBPC coatings can control corrosion for decades, reducing the need for maintenance, downtime, and the potential for incidents and therefore leading to cost savings and increased efficiency and safety of processing operations,” he asserts.

The EonCoat formulation is 100% solids, water-based, non-toxic, non-flammable, odorless, and has zero volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). It can be applied at temperatures ranging from 40–120°F and humidities of 30–95% at a minimum thickness of 20 mils, with no upper limit. Acrylic, epoxy, and polysiloxane coatings have all been used as topcoats for the ceramic CBPC coating. EonCoat conducts training and certification for all applicators, which is typically completed in one day.

One company looking to leverage these benefits is China Petroleum & Chemical Corporation (Sinopec Corp.), one of the largest integrated energy and chemical companies in the world, with upstream, midstream, and downstream operations.Ěý Many of the company’s facilities for oil and gas extraction, transport, and storage are located in China’s Jianghan Plain in Hubei province, which has a sub-tropical monsoon climate. CUI and corrosion are serious issues that shorten equipment life and require excessive maintenance.Ěý In this environment, traditional coatings have been ineffective, and stopping corrosion that is already underway is often the last resort.

Sinopec thus elected to explore the potential of the EonCoat CBPC coating. Its first project involved coating of a 500 m³ petroleum storage tank in an oil-extraction facility in the Jianghan Oilfield. The storage tank’s original coatings were wrapped beneath a mineral wool insulating layer, but due to rain, condensation, and moisture invasion through the damaged insulating layer, these coatings had failed, allowing CUI to occur in a number of areas. The insulation was removed, the metal surface sandblasted, and then the CBPC coating applied. The application has effectively stopped the CUI issue and is expected to extend the storage tank’s functional life for years to come, according to Merrick.

The positive results of this project have led Sinopec to use EonCoat on a container-type water injection pumping station in another Jianghan Oilfield facility.Ěý The soil surrounding the pumping station has a high saline alkali content and the overall environment is very humid, and shutdown of the station for corrosion maintenance was typically required every three years. The pumping station generally had to be shut down for at least seven days to allow for surface preparation and application and drying of the conventional three-coat barrier coating system.

For the CBPC coating, preparation will involve a NACE 3 / SSPC-SP 6 commercial blast (compared to a NACE 2 / SSPC-SP 10 white metal blast cleaning for conventional anti-corrosion coatings). In addition, because surface oxidation, or “flash rust” is desirable with EonCoat, there is no need to prepare small sections of the surface at a time. The surface preparation time is therefore reduced, according to Merrick. Furthermore, only a single coat is needed, and a coating applied at a thickness of 20 mils dries to the touch in 2–50 minutes, can be returned to service in approximately one hour and is fully cured within about 24 hours depending on the temperature and humidity. “Because return to service can be achieved in as little as one hour, users can potentially save hundreds of thousands of dollars per day in reduced processing facility downtime,” Merrick observes. Total coating application including surface preparation of the Sinopec pumping station will take two days, and the company will be able to put the asset back into service immediately.

CoatingsTechĚý |Ěý Vol. 15, No. 11 |Ěý November-December 2018

The importance of paints and coatings to the Navy was highlighted in early September 2018 when Under Secretary of the Navy, Thomas Modly, visited the Sherwin-Williams research and development lab in Warrensville, OH as part of Cleveland Navy Week—a week-long celebration that brought the Navy closer to the people it protects. Members of the Sherwin-Williams Protective & Marine Coatings division and the company’s research and development (R&D) leadership teams met with Naval officials to discuss the role protective coatings play in helping to ensure the readiness of the Navy’s fleet around the globe.

Sherwin-Williams marine group offers fast-drying, general maintenance coatings for quick return-to-service to high-solids coatings designed for long-term asset protection. Of particular note is the close collaboration between the company and the Navy to develop novel coating solutions including Fast Clad^® ER, an ultra-high-solids, rapid cure, single-coat epoxy that replaced the Navy’s traditional three-step coating practice to enable faster maintenance and a 24-hour return-to-service for ballast and fuel storage tanks and other vessel assets.

In addition, Sherwin-Williams is the primary paint supplier for the new USS Gerald R. Ford (CVN-78) aircraft carrier and for restorations being performed on the USS George Washington (CVN-73) aircraft carrier. The Navy is currently using the company’s SeaVoyage® Copper Free antifoulant coating on the USS Nimitz (CVN-68) to deter fouling of its underwater hull. In addition, Sherwin-Williams has a five-year just-in-time (JIT) coatings contract with four public naval shipyards in Norfolk, Portsmouth, Puget Sound, and Pearl Harbor. The JIT contract enables Sherwin-Williams to manage the shipyards’ coatings inventory and deliver supplies as needed to not only ensure a steady delivery of coatings from local inventory but also reduce onsite storage challenges for the shipyards.

Meanwhile, ONR is sponsoring efforts by Dr. Anish Tuteja, an associate professor of materials science and engineering at the University of Michigan, to develop an omniphobic coating. The clear coating, which can be applied to many different surfaces, has been shown to be durable and repel most types of liquids, from oil to water to peanut butter. The coating has significant potential to reduce friction drag on ships, submarines, and unmanned underwater vessels. Because up to 80% at lower speeds and 40–50% at higher speeds of a ship’s fuel consumption goes toward maintaining its speed and overcoming friction drag, significantly reducing friction drag would result in reduced fuel consumption or required batter power, saving money and also extending the potential range of operations, according to Dr. Ki-Han Kim, a program officer in ONR’s Sea Warfare and Weapons Department.

The challenge has been to develop an omniphobic coating that is durable enough for use on ship hulls and capable of repelling all of the types of liquid a ship might come in contact with. Simply mixing polymers and fillers with the right properties does not provide the best coating. Tuteja’s team searched through large databases of known chemical substances and evaluated their performance in mixtures using computer models that considered a wide rage of molecular properties. The right combination was identified after investigating hundreds of combinations.

The rubber-like formulation is optically clear and binds tightly to many different types of surfaces. It can be applied by spraying, brushing, dipping or spin-coating and has excellent resistance to scratching and denting. In addition to its potential for reducing friction drag, Tuteja believes the coating could also be used to protect high-value equipment like sensors, radars, and antennas from damage due to harsh weather. Tuteja expects to have the coating available for small-scale military and civilian use within the next couple of years.

ONR is also sponsoring research into other types of protective coatings, from anti-corrosion systems to coatings that prevent biofouling (the buildup of marine organisms) and ice buildup on ship hulls.