Wood, by virtue of being a renewable material, and when sourced from responsibly managed forests, constitutes a sustainable and responsible building material. If continuously exposed to water, wind, and sunlight, though, wood substrates are subject to damage, manifested as discoloration, cracking, and decay. Protective coatings can delay or prevent damage, improving the durability and prolonging the life of this vital natural resource. There is a demand for wood coatings with low-volatile organic compound (VOC) content, a requirement often achieved with “VOC exempt” solvents.

Recognizing that the use of certain, widely used VOC exempt solvents may be permanently restricted in many applications because of proposed state-level environmental regulations—for example, by the South Coast Air Quality Management District in California as early as 2026— creative solutions to match VOC and performance targets are critical. A proprietary polymer has been developed that enables the replacement of these VOC exempt solvents with water. Using this unique technology, a low-VOC waterborne coating for exterior wood substrates has been formulated to provide exceptional protection, which meets or exceeds that of conventional wood coatings.

Introduction

As an exterior substrate, wood has both practical and aesthetic advantages. Not only does wood offer an organic, warm look, it readily accepts color, ranging from semi-transparent stains to fully opaque coatings. Notably, wood can act as a natural insulator. Wood substrates, therefore, are an attractive and environmentally sound alternative to brick, concrete and vinyl.

Wood substrates, however, can be an expensive option, because regular maintenance is required to prevent decay, damage from insects, and deterioration caused by weather. Wood is particularly susceptible to fluctuations in environmental moisture. Continuous high humidity and periodic severe and wet weather can result in warping, cracking, and rot, which, if not addressed by timely repairs, can lead to irreversible structural damage.^1,2

Protective coatings are therefore critical to maintaining exterior wood substrates. One such coating, spar varnish, is particularly effective. Spar varnish, also known as “marine varnish” or “yacht varnish,” was originally developed for use on the wooden poles that support the sails (“spars”) of sailing ships.³ Generally, a spar varnish is composed of an oil, like tung oil or linseed oil, which penetrates the substrate, and a resin, such as an alkyd or a polyurethane, which provides hardness. These components are solubilized in a compatible solvent, which contributes to ease of application. Spar varnishes are formulated to be flexible, allowing the coating to expand and contract in concert with changes occurring within the wood substrate, and water resistant, allowing the coating to provide a barrier to environmental moisture. Spar varnishes are also typically formulated with UV-absorbing compounds, which both extend the lifetime of the coating and prevent the substrate underneath from degrading.

As of 2023, the global market size for marine spar varnish was valued at approximately USD 1.2 billion. This market is projected to reach around USD 2.5 billion by 2032, growing at a compound annual growth rate (CAGR) of 7.5%.⁴ This growth is the result of increasing demand for protective coatings in marine applications and outdoor wood furniture, as well as increased awareness about the importance of safeguarding wooden structures against harsh environmental conditions.

Solvents typically used to formulate a spar varnish include mineral spirits, aliphatic hydrocarbon-based solvents, such as naphtha, and aromatic solvents, such as xylene. While the volatile organic compound (VOC) content can vary, VOC content for a standard spar varnish using traditional solvents usually ranges close to 475 g/L. The VOC limit for spar varnishes under the South Coast Air Quality Management District (SCAQMD) in California and the Ozone Transport Commission (OTC; Mid-Atlantic and Northeast) Model Rule 2010, however, is 275 g/L.^5,6 For a spar varnish to meet lower VOC requirements, “VOC exempt” solvents, such as parachlorobenzotrifluoride (PCBTF) or tert-butyl acetate (t-BAc), must be incorporated.

The U.S. Environmental Protection Agency (EPA) granted PCBTF and t-BAc exempt status because these solvents have negligible photochemical reactivity and do not significantly contribute to ground-level ozone formation.^7,8 PCBTF is the most widely used VOC exempt solvent in the coatings and adhesives industry. Another VOC exempt solvent, tert-butyl acetate, has been promoted as a potential replacement for the halogenated PCBTF. In 2022, however, the SCAQMD included a provision in Rule 1168 (Industrial Adhesives and Sealants) that prohibited the use of PCBTF and t-BAc.^9,10 The District intends to follow through with identical rulings for other coating rules, including Rule 1136 (Wood Products Coatings) as early as 2026.¹¹ Recognizing that the use of VOC exempt solvents may be permanently restricted in many applications, creative solutions to match VOC and performance targets are needed.

Correlating paint film integrity in the lab with wood grain crack resistance in real-world exposure is a challenging task. In 2020, we reported on early correlations between adhesion and film mechanics that when combined could fit a grain cracking predictive model to outdoor weathering.¹ In this study, we sought to further challenge our own model by greatly expanding our study to include 79 more paints on various substrates. Our aim was to elucidate potential correlations between paint film mechanical properties and realworld exposure performance and to test the validity of our early model.

Through our analysis, we identified several significant factors and developed a statistical model. Although it is still in its early stages, this model shows promise in providing insights into the performance of paints under real-world exterior conditions. Further refinement and validation of this model could have significant implications for the paint industry. Such a model could enhance the durability and quality of wood coatings and accelerate the development timeline.

Introduction

The correlation between laboratory tests and real-world applications of paint is a critical area of research within the paint industry. While laboratory tests provide valuable insights into paint properties and performance in controlled environments, they often fail to accurately replicate the complex and dynamic conditions found in real-world environments.² Consequently, investigating the correlation between laboratory results and real-world scenarios, particularly concerning grain cracking in paint, becomes essential.

Grain cracking is a prevalent issue that occurs when paint applied to dimensionally unstable wood substrates undergoes cyclic stress from temperature fluctuations, moisture, and other environmental factors. However, reproducing these conditions accurately within a laboratory setting proves challenging, potentially leading to results that do not truly reflect the behavior of paint in the field. Understanding the correlation between laboratory tests and real-world applications of paint becomes critical in developing more accurate and reliable testing methods, as well as improving paint design and formulation. By studying this correlation, researchers and professionals in the paint industry can develop more effective strategies for preventing grain cracking and other forms of paint degradation. This, in turn, leads to enhanced durability, aesthetics, and overall quality of paint products. Therefore, investigating the relationship between laboratory tests and real-world applications is a vital endeavor that holds significant implications for the paint industry and its consumers.

In our previous study, we focused on evaluating the mechanical performance of 18 different paints.¹ The aim was to establish correlations between the performance of these paints and their real-world exposure results. To achieve this, we employed multiple linear regression (MLR) models. During the course of our research, we discovered that the results obtained from various laboratory and accelerated test methods were significantly confounded. These results did not yield easily decipherable trends.

To overcome these challenges, we expanded our study to include additional parameters. Specifically, we measured the tensile elongation at multiple conditions, adhesion to water-conditioned surfaces, and film hardness. By incorporating these factors, we were able to develop a predictive model that exhibited a high degree of accuracy. It is important to note, however, that this model is currently limited to the scope of a single commercial paint study.

To progress the development of highperformance exterior wood coatings, it is crucial to validate and expand on our findings. This entails conducting a similar study that encompasses a broader range of paints, covering a wider spectrum of mechanical performance. By doing so, we can further investigate and identify correlations between mechanical properties and real-world exposure results. This expanded research will ultimately serve to enhance the predictive model and transform it into a valuable tool for the development of high-performance exterior wood coatings.

In this study, we examined 79 paints with different formulations, colors, sheens, and significant variations in mechanical performance. These paints were applied to various substrates and evaluated every six months to determine if we could establish a correlation between accelerated testing and long-term real-world exposure results. This comprehensive analysis aims to provide a more robust understanding of the correlation between laboratory tests and real-world applications of paint, ultimately contributing to the advancement of paint technology and the development of superior paint products.

Experimental Setup
Paints
In this study, the 79 paints used included 16 experimental waterborne acrylic architecture paints and 63 commercial waterborne architecture paints purchased from hardware or paint stores. The paints were selected to cover a wide range of formulations, pigment volume concentrations (PVC), mechanical properties, and historical exposure results based on our own in-field benchmarking results. Table 1 provides information on the number of paints used in the study based on different sheens and bases. The sheens include semi-gloss, satin, and flat, while the bases include a white and deep (tinted with 12 oz per gallon black colorant) from each sheen.

Fast-curing acid-catalyzed alkyd-aminoplast coatings are often utilized as protective coatings for wood cabinet coatings. Quantitative assessment of impact of resin, crosslinker, and catalyst concentration on the curing process can be challenging for these and other coatings as well.

In the development of these coatings, curing is often assessed after the coating film has been exposed to a given cure profile by evaluation of physical properties such as hardness development as a function of time after exiting an oven. Curing under ambient conditions is often assessed by evaluating coating hardness development as a function of ambient drying-time conditions.

Evaluation of the influence of solvent on cure and crosslinking is often assessed by means of the finger-touch-down method as the sample is drying. Although these methods are very useful, they do not give quantitative insight into the drying and curing of the coating during the early drying process.

The current work investigates the effect of resin and crosslinker composition, and catalyst concentration, on the kinetics of curing of solvent-based acid catalyzed alkyd-aminoplast wood coatings. Further, the research investigates dynamic mechanical thermal properties of the resulting coatings after exposure to a given cure profile.

Introduction

Acid-catalyzed alkyd-aminoplast coatings for wood coatings applications are commonly based on alkyd resins crosslinked with urea- and/or melamine-formaldehyde crosslinkers and resins. Economical, “water white,” color stable varnishes typically consist of coconut fatty-acid-modified alkyd resin in combination with urea- and/or melamineformaldehyde crosslinkers. Curing at low temperatures is possible by the addition of a strong acid catalyst such as para-toluene sulfonic acid (PTSA). Rate of cure, development of physical properties, and long-term durability of these varnishes is dependent upon a variety of factors, such as alkyd resin chemistry, crosslinker composition, solvent selection, and catalyst chemistry and concentration.

Alkyd resins may vary in oil type and length, hydroxyl number and its reactivity (i.e, primary, secondary, etc.), molecular weight, molecular weight distribution, and degree of branching, as well as other parameters.

Recent work has identified interesting synergies of a matting technology that pairs the efficiency and clarity of silica matting agents with the physical durability and protective ability of organic particles.

Matting of clear wood coatings is a critical aspect in capturing the natural beauty of the substrate and appealing to consumer demands. Maintaining this visual appearance is vital to coating during product lifespan, and with deep-matte finishes, resistance to burnish and polishing is of particular concern. Recent work has identified interesting synergies of a matting technology that pairs the efficiency and clarity of silica matting agents with the physical durability and protective ability of organic particles. The plastic deformation and energy dissipation of these particles resist polishing forces and protect the matting structure of the silica. This combination has shown excellent synergy and allows formulation of durable deep-matte finishes with excellent burnish resistance. There are also significant benefits in reduced viscosity build at high matting-agent levels. This article will review the technical details of the synergy and characteristics in different wood coating systems.

Introduction

The use of particles to reduce the gloss of coatings is a very well-known practice. A wide variety of products and technologies are available to develop wood coating formulations. The use and choice of a specific product is often driven by a variety of factors, but fundamentally particle technologies all function through a similar mechanism. The particles, dispersed in the liquid coating, provide a surface roughness after application and drying/curing that serves to scatter light providing a visual differential and reduction in gloss (Figure 1). Their effectiveness is heavily contingent on film shrinkage around the particles during the drying and curing of the coating, and their utility is often judged based on secondary effects to the coating film such as rheology, clarity, haptic properties, chemical resistance, and resistance to burnish.¹

FIGURE 1 Light reflection on smooth surface develops a glossy coating while a matted coating with a rough surface scatter lights in different angles.

Deep-matte coatings, where the gloss has been significantly reduced, typically require higher loads of particles to achieve the necessary surface roughness. These higher loadings often result in greater impact on the secondary effects to the coating film, and require far more attention to product selection and formulation to balance the performance needs of the coating. Of particular interest is the aspect of burnish resistance—the increase in gloss caused by the polishing of the finished coating by handling, touching, brushing, or other physical contacts. Burnishing and reduction of the matting effect can be an issue with all matted coatings, but is particularly problematic with deep-matte coatings, as the visual differentiation can be significant. Burnish resistance of a deep-matte coating is also very dependent on the nature and chemistry of the particles themselves due to their higher concentration on the coating surface that is needed to achieve the necessary surface roughness. The morphology of the particles, their chemical and physical properties, and interaction with the coating film can play a pronounced role in resistance to burnish and is an area of significant interest.

The work described in this article is part of an ongoing effort to better understand the role of matting agents in achieving a deep matte-finish with higher resistance to burnishing. While only a snapshot of the broader investigation is reviewed here, the best-in-class products within a variety of technologies were evaluated in different systems and the results are compared and assessed. Matting-agent combinations are also explored, and results suggest synergistic benefits are possible when pairing silica matting agents with organic particles. Deep-matte water-based UV-curable coatings, water-based air-dry systems, and deep-matte 100%-solid UV-curable coatings will be discussed.

Results and Discussion

The matting capability of three different matting agent types of similar particle size were tested in the water-based UV-curable formulation shown in Table 1. In this formulation, UCECOAT® 7788 (Allnex) is a UV-curable aliphatic polyurethane dispersion and Omnirad 500 (IGM Resins) is a water-soluble photoinitiator. TEGO® Foamex 822 (Evonik Corporation) is a compatible defoamer emulsion based on polyether siloxane technology.

Figure 2 shows the initial matting and burnishing results for coatings containing particles of (A) surface-treated thermal silica, (B) polyamide, and (C) micronized polypropylene matting agents in this system. A deep-matte sheen and excellent burnish resistance can be obtained by using 10 wt % of the surface-treated thermal silica in the formulation, as visible in Figure 2A.

FIGURE 2 Gloss and burnish resistance of a water-based UV-curable coating with dried film thickness 29 microns containing 10 wt % of matting agent based on (A) surface-treated thermal silica, (B) polyamide, and (C) micronized polypropylene. The burnish test was done using a Crockmeter.

Program Highlights

The Wood C&S Conference gathers professionals in the wood coatings industry to engage with cutting-edge research and developments in wood substrates and coatings science. Key topics include advancements in raw materials and technology for wood coatings.

Keynote Speakers

This year’s conference features two keynote addresses. Presenting Thursday’s address is Dr. Mojgan Nejad, an associate professor with a joint appointment in the Department of Forestry and Chemical Engineering and Materials Science at Michigan State University. Dr. Nejad’s keynote address, titled “Sustainable Coatings: Utilizing Lignin for the Development of Biobased Resins,” will discuss her groundbreaking research on lignin-based coatings, adhesives, foams, and composites. She is recognized for her collaborative work with industry partners and has been honored with the Adhesive and Sealant Council Innovation Award for her innovative phenolic resin formulations.

Dr. Véronic Landry, a professor in the Department of Wood and Forest Sciences and Director of the Research Center on Renewable Materials at Laval University, will deliver the Friday keynote. Dr. Landry’s address, “The Power of Confocal Raman Microscopy in Coating Analysis,” will highlight her work in developing environmentally friendly coatings and wood protection through finishing and chemical modification. Her research includes the creation of water-based, photopolymerizable, and biobased coatings, as well as functional coatings with self-healing and self-stratifying properties.

Event Details

Registration: The conference is free to attend, and on-site registration is available.
Lodging: A special rate of $125 is available at the Marriott Courtyard Greensboro Airport for reservations made by September 2.
Parking: Parking is available for $10.00 per day in the Oakland Avenue and Walker Avenue Parking Decks on the UNCG campus.

Networking Opportunities

Wood C&S Fellowship Dinner: Open to all attendees on Thursday evening. Cost and venue details will be announced soon.
Friday Lunch: Lunch will be provided on Friday, offering a valuable networking opportunity and a chance to visit sponsor booths.

Venue Information

Location: Elliott University Center, Cone Ballroom, UNCG, 540 Stirling Street, Greensboro, NC 27412

Who Should Attend?

The conference is designed for a wide range of professionals, including chemists, formulators, raw material and equipment suppliers, wood manufacturers, scientists, technologists, students, educators, and end users interested in wood coatings.

Additional Information

Don’t miss this opportunity to connect with industry experts and explore the latest advancements in wood coatings and substrates. For more details on the program, abstracts, registration, directions, and corporate sponsorship, please contact Ronald Obie at r.obie@woodcoatingsresearchgroup.com. More information can also be found on the .

Introduction

Knots—the circular imperfections in the wood grain—often cause issues in coating appearance due to their high concentration of potent extractives that can degrade a coating system over time. Unfortunately, knots are unavoidable in softwood species, such as pine, which is often used for furniture and cabinetry applications.

Knots occur naturally when branches die and become encapsulated as the tree grows. During the encapsulation process, extractives—the tree’s natural resins—seal off the former branch’s location and provide a biological boundary between this area and the rest of the tree. Wood typically contains up to 4 wt % extractives of total weight of the wood. However, in knots, the extractives are concentrated and can reach as high as 40 wt %. When the wood is coated, the knot area is prone to exudate the extractives and bleed through the coating. Combined with the effect of UV radiation oxidizing some of these components, the typical result is brownish discoloration bleeding through the coated surface at the knot area.¹ Figure 1 shows the knots of pinewood, wood extractives, and the resulting knot bleeding effect through the coating.^2,3,4

Knot bleeding effects are a problem in the wood coating industry because they can significantly impact the appearance of the coatings, as shown in Figure 1. In addition, bleeding can compromise coating performance, causing a coating to become brittle, crack, or peel.⁵ The discoloration over the knots is typically not seen immediately but can take months. The time it takes for the discoloration to appear depends on several factors, including the wood species, environment humidity, temperature, and exposure to UV radiation.^6,7,8

Knot bleeding should be differentiated from tannin bleeding, which is another very common defect from extractives causing discoloration. Tannin bleeding is typically a problem for hardwoods such as oak, cedar, or merbau, where knots are not typically an issue.⁹ Figure 2 is an example of coated panels that show tannin bleeding.

Figure 3 shows an overview of different structures of wood extractives. Extractives can be grouped as either hydrophobic extractives, such as terpenes and resin acid, and hydrophilic components, such as tannins. Depending on the wood types and growth conditions, the exact composition of the extractives in the wood can vary.¹⁰

There are a few approaches historically used to prevent knot bleeding through a coating. One solution is to use a solvent-based two-part (2K) coating. These coatings typically have high crosslinking densities, which provide good barrier properties to hinder the extractives from migrating to the surface. However, their solvent emissions and high VOC contents are disadvantages. A second option is to use water-based cationic dispersions; however, this technology is not commonly used for industrial wood coatings, because they can require special equipment or additives.¹¹ A third solution is to use UV primers and topcoats. Like 2K coatings, UV coatings have high crosslinking densities, providing good barrier properties, but UV coatings require special curing equipment, which is often cost-prohibitive.

In this article, a new approach—an anionic acrylic dispersion—is introduced.

Continue reading in the of CoatingsTech.

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

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

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

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

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

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

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

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

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

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

Demand for high-performing, water-based coatings continues to be the trend throughout the many coatings market segments. Wood flooring, cabinetry, and furniture coatings segments are a major component of this movement. To support this transition, development of innovative solutions is required to provide improved coatings performance. End users are seeking increased durability and lower environmental impact, which comes with water-based technology. Improved scratch resistance will improve the long-lasting aesthetics and protection that wood coatings provide. A novel, easy-to-use approach to improve scratch resistance in water-based wood coatings has been demonstrated in multiple resin technologies. Performance testing of improved scratch resistance while maintaining good film properties will be presented in a water-based, ultraviolet (UV)-curable system that exceeds performance of current market offerings.

INTRODUCTION

Durability is sought out in all coating applications, but the meaning of that term can vary greatly depending on the end use of the coating and its desired protective property. Exterior architectural coatings require exceptional durability in terms of UV and moisture resistance, while a high-performance industrial coating interprets durability to be long-term corrosion protection. Durability is also valued for industrial wood coatings, which require an aesthetically pleasing appearance that is resistant to scratch deformation. Beyond aesthetics, a scratch in a film can lead to early failure of a coating’s protective properties.¹ Superior scratch resistance has been identified as an unsatisfied need in the OEM industrial wood coatings market.

Many of the scratch additives used today rely on variations of waxes to be active at the coating’s surface. The density of polyethylene waxes can be altered to give slip properties and allow them to migrate to the surface of the coating. Due to their hydrophobic nature, formulating with waxes can bring complexities such as difficult dispersion and surface tension changes, leading to surface defects. The resin system, melt point, particle size, and density of the wax must be understood to provide the benefits touted by wax manufacturers.² PTFE, polytetrafluoroethylene, is a tough, wax-like synthetic resin that is used in many applications, including scratch resistance, because of its slick surface and hydrophobic properties.³

Other scratch additives rely on minerals with high hardness to provide scratch improvement. Aluminum oxide, zirconium, and silicates are common materials known to have high Mohs hardness values. These types of materials also come with a high density, making it difficult to suspend them at the coating’s surface, where scratch resistance is most impacted. They also traditionally have larger particle sizes, imparting haze and lowering gloss.

Recent advancements in nanoparticle technology has opened the door to improvements in high-gloss clear formulations.⁴ Scratch resistance is most critical in high-gloss coatings because defects are so easily visible. Nanometer-sized materials will impart less gloss reduction and maintain clarity, while providing the same hardness values. But the high-surface area of nanoparticles often makes them difficult to disperse and can create respiratory health hazards when used in the dry form.

ICL has created a scratch additive that combines the benefits of a hard silicate material along with nanoparticles that impart scratch resistance properties while also suspending the dense silicate material at the coating’s surface. This product is in an easy-to-use liquid form intended for water-based coatings systems. An examination of the current market offerings versus this newly developed product has been completed.

EXPERIMENT

Six commercially available anti-scratch additives were selected to represent popular chemistries utilized by coatings manufacturers to improve scratch resistance of industrial wood coatings. The products are identified in Table 1. To begin this study, a minimum three-point ladder study was conducted on each additive to determine the optimal usage level in the test coating, focusing only on scratch resistance improvement. The coating containing the optimized level for each product was put through the full range of tests listed below. A coating formulation without a scratch additive, the blank, was used as the control. For ease of identifying the products, sample IDs will be utilized throughout this article.

These additives were tested in a water-based, UV-curable urethane acrylate resin formulation. This type of formulation provides protection and beautification to wood cabinetry and furniture. The coating was subjected to critical property tests identified by the Kitchen Cabinet Manufacturers Association (KCMA) and office furniture standards to demonstrate durability, cold crack resistance, chemical resistance, pencil hardness, water immersion, Taber abrasion, adhesion, gloss, haze, and the property of focus, scratch resistance.⁵

The components of the UV-curable wood formulation used for the evaluation can be found in Table 2. This is a 25% solids formulation that utilizes a resin with T_g of 51 °C. Each coat of the system was cured using a three-step method; air dry for 15 min at ambient temperature, oven dry at 66 °C for 15 min, and three passes through an American Ultraviolet curing system utilizing a standard mercury halogen lamp projecting approximately 300 mj/cm² irradiance per pass.

Hardness, like durability, is another term that brings much ambiguity. Many test methods are employed throughout the coatings industry, and convergence on a single method is not likely. Hardness can be interpreted many ways, be it resistance to wear, to penetration by an object, or to scratch. For this reason, multiple test methods are often used to characterize coating properties.⁶ In this study, scratch resistance was measured by comparing the change in 20 ° gloss values after 10, 25, and 50 double rubs of #1 steel wool under a two-pound weight. Taber abrasion resistance was run for 1000 cycles using CS-17 abrasive wheels under a 1000-gram weight. Birch veneer panels were used to test Taber with three coats of the coating applied before the test. Pencil hardness measurements were taken on a single coat over glass.

To evaluate gloss and haze, a 3-mil wet film was applied to a Leneta card and cured under the conditions previously discussed. Values were obtained using a tri-gloss meter. Haziness of the film was also visually evaluated by drawing down the coatings over a glass plate.

Adhesion was measured according to ASTM D3359 over birch wood and glass. Wood panels received four coats of the clear coating using an HPLV sprayer, applying 1–1.25 mils per coat.

The method to test cold crack resistance included cycling coated birch panels through cold and then hot conditions. The coated boards were placed in the freezer for one hour at –20 °C. Then the panels were immediately transferred to an 80 °C oven for one hour. Panels were observed for discoloration or cracking after each of these cycles.

Common household food and chemicals were used to evaluate the coatings resistance to deformation by applying a spot under a watch glass for 24 h. Substances tested were water, 50% 409 solution, red wine, vinegar, lemon juice, orange juice, grape juice, mustard, ketchup, coffee, olive oil, and 100% ethanol. Approximately 24 h after the spots were removed, the coatings were rated for recovery.

In addition to the water spot test, coatings were tested by applying a single coat over a glass plate, curing the panels, and then submerging them in water for observations at four and 24 h, and after a 24-h recovery. Coatings were observed for blushing and wrinkling of the film.

RESULTS

The resin system is the primary component providing the level of scratch resistance that is needed for a given coating application. UV curable coatings are able to achieve high hardness levels as soon as the UV curing stage is complete. Traditional water-based coatings will require longer dry times at ambient conditions, or forced air drying to achieve comparable results. Formulations can be further enhanced by adding a scratch resistance additive like those identified in this study. Figure 1 displays the scratch resistance properties of the UV-curable coatings system with each of the additives at their optimal loading level. The key measurement graphed is the percentage of 20° gloss loss that was obtained after the set number of steel wool double rubs. The nano-stabilized silicate dispersion (NSSD) shows a marked improvement over all other products in the set. The hardness of the key components of the NSSD combined with the rheological properties that suspend it at the coating’s surface allow it to demonstrate superior scratch resistance properties in this high-gloss clear formulation. The next best performing additive, which shows three times more gloss loss than the NSSD, is the PTFE-containing wax dispersion. The NCS displayed similar scratch resistance to the PTFE. All other additives provide marginal improvement over the blank control.

ASTM D3363, commonly known as pencil hardness, describes the controlled method of using leads of known hardness to measure the mar or gouge resistance of a coating. Although results can vary between operators and leads themselves, the test is a valuable tool when care is taken to control the testing technique within a data set.⁷ In this data set, the hardness displayed in the scratch test is also echoed in the pencil hardness results. The NSSD displays a hardness twice as high as the nearest competitor. Figure 2 demonstrates the effect of each additive on the coating’s hardness level.

The third method of measuring coating hardness is Taber abrasion resistance, ASTM D4060. Mass loss of the coatings was measured after 500 and 1000 cycles as shown in Table 3. This test measures the coating’s ability to resist gradual wear versus the deformation caused by a scratch. It is a valuable trait to determine a balance of scratch and abrasion properties. Results indicate that all additives provide a neutral or positive benefit to the coating system. Improvement in abrasion can be achieved with chemistries HDPE, NAOD, and NCS, while the NSSD had a neutral effect. Abrasion resistance is predominantly controlled by the hardness of the resin system, but results shown here demonstrate an additive can also have a positive effect.

The remaining properties tested in this series are important to create a high-performance, balanced wood coating, but are not intended to be improved by the additives in this study. When formulating a coating, improvement in one property can often diminish another. The balance of formula properties can be valued differently between each formulator, resulting in strengths and weaknesses within any given coating. All key properties must be tested to get a true measure of the coating’s durability.

One key property that can be impacted by a scratch additive is gloss development (Figure 3). Initial gloss readings show that many of the additives produce minimal loss in gloss properties at their optimal loading level, including HDPE, NAOD, NSD, and NCS. Three of these four products are based on nanoparticle technology, which may explain their minimal impact on gloss. The NSSD showed a slight reduction in gloss but remains in the high-gloss category. Particle size can be the key property that determines the degree of gloss reduction. The PEW has the largest particle size of the additives tested, and consequently demonstrates a significant reduction in gloss. Similar takeaways can be said for the degree of haze, or opacity of a clear coat. It is important that a clear coat imparts as little distortion of the substrate as possible. Clarity is controlled by the refractive index of materials used and their primary particle size. Haze can be interpreted by the change in 20° gloss values vs the control. A visual representation of haze was collected by applying a film over glass and looking for distortion when placed over an image, as seen in Figure 4. PEW is the only additive type to show a visual distortion.

No matter the coating type, adhesion is a critical property that must be maintained when improving other performance attributes. Testing according to ASTM D3359, the cross-hatch tape-pull test, provides a measurement of large differences in adhesion between samples, indicated by the zero to five rating scale.⁸ Generally, adhesion to a wood surface is easy to achieve because of its porosity. All additives were able to achieve a 5B rating for wood adhesion. PTFE and HDPE demonstrated inter-coat adhesion failures over wood, which indicates that the additive has significant effect on the coatings surface energy. This is a critical failure in applications where more than one coat of the formulation is applied. To find differentiation between the samples, adhesion over glass was also measured. Table 4 lists the results for glass, wood, and inter-coat adhesion.

The chemical resistance spot test was rated after 24 h of exposure to the substances under a watch glass, a severe scenario. Results are tabulated in Table 5. The following scale was used to classify results:

5    No effect

4    Ring left

3    Color and/or gloss change

2    Soft film

1    Blistering

0    Film lifting

The critical difference between the blank control and the additive formulations is the slight decrease in resistance to water. Only the PTFE sample displayed the same rating as the blank control. When looking at the scores cumulatively, only one product, NSD, achieved a score higher than the control. The PEW caused slightly less chemical resistance and all others have no significant impact.

Recovery ratings were also considered (Table 6). After 24 h, NSSD provides the same cumulative score as the control. In fact, all products see an improvement in their ratings after the recovery period. Three of them, PTFE, PEW, and NSD are better than the control.

Water resistance was measured in the spot test and was further examined in the water immersion test over glass. Adhesion to glass has already been demonstrated as a difficult substrate for most of the sample coatings. A four-hour and 24-h immersion in ambient water further demonstrates differences between the coating’s hydrophobic nature. Results were difficult to photograph but characteristics of the immersed coatings can be found in Table 7. This test is further indication that all products have varying degrees of impact on the water resistance properties of this water-based, UV-curable coating.

The additives had no effect on cold crack resistance. All samples passed eight cycles of temperature changes between extreme cold and hot. This property is important to ensure coated materials can withstand the temperature fluctuations when shipped through multiple climates.

CONCLUSION

Scratch resistance is a critical property that can lead to longer service times for wood cabinetry and furniture. Additives are important formulating tools that can lead to higher performance. Examining the test results in sum demonstrates the overall impact an additive can have on a high gloss clear coat’s performance. The loss of adhesion found with the PTFE and HDPE is a critical failure that cannot be compensated for and demonstrates the challenges that can be caused by using a wax-based product that significantly effects the surface energy of the coating. Minor detriments in chemical and water resistance can be seen with most of the scratch additives tested. The intended effect of these additives is to improve the durability of the coating by reducing the likelihood of scratch. Only the NSSD achieves a significant improvement in scratch resistance while showing minimal diminished performance in other properties. These results exceed the performance of chemistries currently employed by the coatings market.

References

1. Smeets, S. “Evaluation of Scratch Resistance Test Methods for Organic Coatings.” PCI Magazine, March 2004. (accessed July 22, 2020).
2. Czarnecki, R. Nanoalumina Wax Composites for Improved Surface Durability. Paper presented at Coatings Trends and Technologies, Sept. 2019.
3. Augustyn, A. “Polytetraflouroethylene.” Encyclopedia Britannica, Encyclopedia Britannica, Inc., April 2019. (accessed July 22, 2020).
4. Shanbhag, D., and Dhamdhere, P. (June 2012). “Recent Developments to Improve Scratch and Mar Resistance in Automotive Coatings.” PCI Magazine, June 2012.
5. Morris, L. Waterborne UV Curable Resins for Industrial Wood Applications. Paper presented at Coatings Trends and Technologies, Sept. 2019.
6. Guevin, P. R. “Hardness.” Paint and Coating Testing Manual: fourteenth edition of the Gardner-Sward handbook, 1995. Philadelphia, PA: American Society for Testing and Materials.
7. ASTM D3363-05(2011)e2, “Standard Test Method for Film Hardness by Pencil Test,” ASTM International, West Conshohocken, PA, 2011, (accessed July 22, 2020).
8. ASTM D3359-17, Standard Test Methods for Rating Adhesion by Tape Test, ASTM International, West Conshohocken, PA, 2017, (accessed July 22, 2020).

CoatingsTech | Vol. 17, No. 8 | August 2020

The fastest growth is occurring in the Asia-Pacific region (including India, Korea, and Japan), which will account for nearly 60% of the expansion during that timeframe. This trend is not surprising given that this region accounted for approximately 55% (Europe: 26%, United States: 14%) of the global furniture market in 2015, according to market research firm CSIL, Milan. While there was a slight slowdown in the market at the end of 2018 and early 2019, Anthony Woods, AkzoNobel segment marketing director for Wood Coatings in Akzo- Nobel’s Industrial Coatings business, expects improvement over the rest of this year. “There is still some volatility due to impact of the recent U.S.-China trade impasse, with U.S. importers looking for alternative manufacturing locations other than in China. Vietnam has benefited, and the market there is still growing strong. There is also strong expansion in other Southeast Asia countries,” he observes. In addition, the market for products manufactured in Asia for non-U.S. destinations is stable, achieving expected levels of growth.

That growth is being driven by many trends in the furniture industry. Jacobs points to three key contributors: growing interest in more eco-friendly furniture and a shift to more sustainable solutions, including a move to coatings with lower VOC emissions and renewable raw materials; the move away from solid wood due to global wood logging bans; and increases in online ordering and expectations for shorter delivery times, which are driving just-in-time production and overall demand for more time- and cost-efficient processing. Bakker agrees with Jacobs that one of the main trends is the development of new water-based resins with bio-based content and low VOCs. “Manufacturers are looking for new sustainable processes and products that minimally affect the environment,” he says. He notes, too, that the rise of natural coating solutions is also increasing the prospects of the furniture wood coatings industry.

The overall growth of the market for wood furniture coatings can be attributed to the rising global population and recovery of the housing market, which are both leading to increasing demand for chairs, tables, beds, shelves, etc.

More flexible manufacturing processes and just-in-time business models, adds Woods, are also enabling much higher levels of customization, so manufacturers can offer more products without needing to increase their inventories. This reduces the pressure on working capital and lowers the total cost of ownership. Pressure on costs is also leading customers to move away from liquid coatings as they seek additional efficiencies, according to Woods. “In particular,” he observes, “this trend is leading to a growth in powder coatings in lower tier applications. In addition, innovations around aesthetic and soft touch in foil mean that higher-end furniture can now be coated with warm wood effect foils, which again helps to reduce costs.” Woods notes that the development of advanced curing technologies is another key driver that is helping to strengthen process effectiveness and deliver efficiencies, while at the same time improving gloss control and reducing energy requirements. He also points to ongoing growth of automation, which is continuing to reduce operating costs and strengthen efficiencies throughout the value chain, and an increasing focus on design, particularly in terms of services like enhanced color styling. “It is vital to stay on top of the latest design trends, to deliver leading edge solutions for customers that reflect the demands of modern lifestyles,” Woods asserts.

The dominant resin chemistries found in coatings used on wood furniture include polyurethanes, nitrocellulose, acrylics, and polyesters formulating as solventborne, water-based, radiation-cured, and powder coatings. Solventborne coatings predominate, but waterborne solutions are available that offer excellent performance without loss of properties or aesthetic features, according to Bakker. Globally, according to Jacobs, two-thirds of furniture wood coatings today are still based on solventborne resins. The remainder is split between waterborne technologies (1K/2K/WB UV) and 100%-UV technologies. She notes that, currently, powder resins for wood applications only have a small share in the total wood furniture coating market. “Because of ecological footprints, environmental legislation, and increased quality performance, there is a noticeable move from solventborne coatings to solvent-free or solvent-reduced systems.

The choice of coatings technology depends on the construction of the finished article and the capability and resources of the manufacturer, according to Woods. For assembly-type residential applications, traditional solvent-based lacquers still have their place in the market. In Asia-Pacific, there is a continued push towards waterborne chemistries, as has been observed with China’s move to waterborne technologies. In the automated manufacturing environment, which involves more flat-line operations, UV and waterborne UV technologies are becoming the leading technologies due to their improved durability and application productivity, Woods comments.

Recent advances in wood furniture coating technology have focused on the development of resins with high performance and bio-based content that will allow the formulation of sustainable and low VOC coatings with a top quality and high level of properties, according to Bakker. Renewable resources are starting to replace petrochemical products in wood coatings, improving their sustainability value, agrees Jacobs. She adds that “this process is slowly evolving and will take some time to become mainstream, but the technology offers a new horizon for the development of eco-
friendly coatings around the world.”

Important properties include:

• Excellent stain and chemical resistance
• High scratch resistance
• High impact resistance
• High gloss and transparency
• Very good blocking resistance at high coating thicknesses
• Low dirt pickup
• Low water absorption
• High water vapor permeability
• Easy and quick sandability
• Very good exterior durability with high UV and humidity resistance for outdoor furniture
• Non-yellowing after UV exposure

For Woods, the application of UV LED coating technology, which has been talked about for many years, is now occurring and having a measurable impact. “The use of UV LED coating technology is being adopted now because the total investment costs for UV LED equipment have come down, making the technology more affordable, and furniture manufacturers are more aware of the value of energy savings and the reduction in total cost of ownership,” he says. Another important development, according to Woods, is the trend for coatings suppliers to take a more holistic approach to product development. “Rather than developing solutions in isolation without considering how other stages in the process, like application or curing methods, may change, coatings suppliers are partnering with other players in the value chain,” he explains. “By collaborating and sharing knowledge with application equipment manufacturers, machinery suppliers, and software developers, for example, they are able to develop total process and product solutions that deliver greater cost efficiencies and productivity improvements for furniture manufacturers,” Woods states.

In the coming year, Jacobs expects the wood furniture coating market to continue showing strong growth in line with the increasing consumer demand for wood furniture and the world’s expanding middle class. Beyond that, she believes there will continue to be long-term growth, especially within more sustainable and eco-friendly solutions. “There is a growing awareness among consumers for the need to adopt sustainable living practices. I can only see this increasing in the coming years,” she states. Bakker agrees: “There is a huge potential market for water-based coatings thanks to the increasing awareness of eco-friendly products. This trend creates an opportunity for the market today and into the future.”

With respect to specific furniture segments, Woods expects in the residential market that overseas imports will increase their share in mature markets over the mid- to long-term due to their strong cost-competitiveness. In the office market, the move towards more open office concepts with fewer items of furniture could change the market landscape, according to Woods. Digital technology and agile working strategies will also continue to reduce the need for storage in the workplace, while customization and just-in-time delivery will become more established features of the market in the near to mid-term. In some hospitality markets, such as international hotel chains, AkzoNobel anticipates that Asia will become an ever-bigger player due to the increased cost-competitiveness it can offer in situations where the global footprint of the customer can be leveraged to provide economies of scale.

Product Highlights

AkzoNobel: AkzoNobel has worked with the North Carolina State University Materials Science Department to develop a unique spray stain system called PurTone™. According to Woods, PurTone is a multi-step process that delivers the aesthetics of hand-wiped stains through a spray application. As manufacturers continue to search for efficiencies, the PurTone stain system is another way to lower costs by delivering a consistent, high-quality finish, while still reducing waste and labor costs, observes Woods.

DSM Coating Resins: DSM has introduced a range of wood furniture coatings based on its renewable Decovery® plant-based resins for the formulation of coatings targeting the growing segment of sustainable furniture consumers that offer high-performance with reduced environmental impact. The resins, which are appropriate for both for indoor and outdoor applications, contain nearly 50% plant-based content and are manufactured with the use of green energy, enabling a carbon footprint reduction of up to 34% compared to alternative coating systems, according to Jacobs. Notably, Decovery resins are low in VOCs (less than 5%) and odor and compliant with Ecolabel, Nordic Swan, The Blue Angel (Der Blaue Engel), and AgBB labelling, keeping coating formulations in line with—and in some cases ahead of—the demands of regulatory authorities, notes Jacobs.

Stahl Holdings: Stahl recently launched Relca HY-288, a new water-based, APEO-free and self-crosslinking acrylic/polyurethane hybrid with excellent stain resistance (coffee, red wine, mustard) in one-component white pigmented coatings for wood, according to Bakker. The resin allows the formulation of high-performing coatings without the use of crosslinkers, thereby enabling simpler, more efficient, and less costly solutions, he notes. It is ideal in furniture and interior joinery coatings with high chemical and scratch resistance, good early block resistance for faster line speeds and overall good appearance.

Relca Bio PD-814 is a water-based matting polyurethane resin containing biobased material and is considered a next-generation resin for the formulation of coatings that provide a matt or satin effect without the use of standard matting agents like silica or waxes, according to Bakker. It can be used on plastic, wood, or metal, and provides a long-lasting matt surface with gloss levels down to one at 20º or 45 at 85º. Bakker notes that it is also possible to achieve a warm soft feel and a pleasant touch effect with high flexibility using this binder.

CoatingsTech | Vol. 16, No. 8 | August 2019

The main objective of most coatings is to both protect and decorate. The balance that must be achieved between decoration and durability depends on application. Even so, the durability of paints and coatings continues to be a key performance characteristic. Therefore, improving durability is always an important goal of new product development efforts. The challenge is to provide performance over the longer term without increasing cost. On the other hand, significant cost savings can be obtained over the life of an asset by increasing coating durability, particularly for applications where the surfaces to be painted are difficult to access and coating application is more complex (monumental building exteriors, bridges, and offshore structures), for assets that must remain in service (trains and airport terminals), and where extensive surface preparation is required prior to coating. In most cases, the cost of the coating itself is relatively small compared to the costs of application, failure of the coating, and down time.

Specialty coatings specifically designed to address the environmental conditions within which an asset exists will provide protection much more effectively than commodity-type coating solutions. Whether on a car, house, ship, bridge, tractor, or home appliance, reducing the rate of applied coating failure and the interval between repainting benefits all users. By extending asset lifetimes, coatings technology can be a significant contributor to facilitating resource, energy, and ultimately global sustainability. The key for coating manufacturers is to understand customer expectations before selecting their final formulations.

Many Drivers for Greater Durability

Coating end users look to improve durability and weatherability for different reasons depending on their specific applications. Automotive OEMs are looking for interior coatings that display improved resistance to chemicals and other staining agents as the future of mobility shifts toward ridesharing, particularly autonomous ridesharing. As these models are expected to grow and experience greater utilization, the need for increased resistance of coated surfaces to food and beverage staining agents and other cleaning or disinfection products is also rising, according to Scott Grace, market segment technical support for Automotive with Covestro LLC.

In the industrial sector, customers are looking for products that last longer, perform well over an extended period of time, and maintain their appearance without requiring regular maintenance. These expectations are driving the development of more robust and specialized coating solutions, observes James Hazen, director of Industrial Technology Planning at Axalta Coating Systems. Agricultural equipment manufacturers want their coatings to “look like new” for a longer period of time because much of this equipment is re-sold after a few years. A great paint job with a fresh appearance helps increase the resale value of that equipment, notes Phil Jones, head of Industrial Coatings Market and Growth Project Management for Covestro LLC. He adds that in the window market, manufacturers are increasingly using more color to differentiate their products, which has resulted in increased demand for coatings that do not fade or chalk easily.

Sustainability in the form of longer-lasting or recyclable (no heavy metals) coatings is also increasingly important for coil coatings for transport and construction applications, according to Chris Bradford, AkzoNobel’s segment marketing director for Coil and Packaging Coatings. Marine and protective coating formulators also face the challenge of maintaining high durability with more sustainable solutions as environmental regulations become increasingly stringent, notes Shiona Stewart, industry marketing manager, Transportation, Industrial, Furniture and Floor Coatings at BASF. There is also growing demand for greater functionality (e.g., anti-microbial or dirt-shedding technology) to reduce cleaning and maintenance, particularly for metal roofing, according to Bradford. “We have to be able to provide different solutions for different applications, which requires finding a balance between functional and aesthetic innovation,” he states. PPG is also experiencing demand for stronger warranties in terms of both year and stated value for attributes such as film integrity, color fade, and chalk resistance.

While architectural coatings do not typically have the same protective demands as industrial coatings, end users do want architectural paints to maintain a freshly painted look, according to Stan Cook, North American architectural marketing director at Dow Coating Materials (DCM). Current socio-economic, demographic, and regulatory factors are also driving the need for longer product lifetimes, asserts Camilo Quiñones-Rozo, market segment manager for Architectural Coatings with BASF. On the professional end of the architectural market, he notes that the increasing reference to high-performance standards in specifications (e.g., MPI’s) has played an important role, especially for commercial and institutional applications. Homeowners, meanwhile, are demanding products that can weather the elements for a longer period of time and reduce repaint frequency. On the DIY front, Quiñones-Rozo says busy customers want a forgiving painting experience to accommodate their limited skills and provide a longer “fresh paint” look and a worry-free post-application experience. For the interior architectural market, end users are looking for an attractive appearance, stain resistance, easy stain removal, and high consumer reports scores, according to Charles Johnson, market segment manager for Architectural Coatings at BASF. Professional contractors expect high performance in product application, color matching accuracy, durability, and a beautifully finished project, adds Brian Osterried, product marketing manager for the PPG paint brand.

For architectural powder coatings, desire for greater sustainability is driving demand for more durable products, according to Jean-Paul Moonen, AkzoNobel’s global segment manager for Architectural Coatings in the Powder Coatings business. “When coatings extend the lifecycles of coated materials, that translates into ‘green credentials’ (like LEED points) for architects, opportunities for window and door manufacturers to upgrade their products, and lower maintenance cost for building owners,” he explains. AkzoNobel is also seeing strong demand for longer warranties with its powder coatings used in the construction industry on window frames, doors, and facades with expected lifetimes of 30 years.

Four factors are driving demand for more durable wood coatings, observes Anthony Woods, AkzoNobel’s segment marketing director for Wood Coatings. In addition to improved protection in any weather conditions, users of exterior wood coatings are also looking for reduced repair and maintenance, which also results in increased sustainability through reduced repair and maintenance cycles. Manufacturers of interior wood products are looking to use wood coatings to create value through differentiation in terms of greater durability, ease-of-care, and longer lifespans. “The positioning of these products goes directly to helping the consumer/end user and positively contributing to their lifestyles and busy schedules,” Woods comments.

 End Use Drives Performance Expectations

The properties that determine durability depend heavily on the environment in which a coating must perform. As a result, the specific application will dictate the most important attributes required to ensure long-term product performance, according to Tim Kittler, global segment lead for Marine and Protective from allnex. “The needs on an oil rig will differ dramatically from those of a residential application,” he says. A high degree of corrosion protection against salt is required in coastal marine climates and also where snow removal salts are used. A higher degree of color and gloss appearance retention may be placed on coatings used in locations such as Florida, where tourism is the number one industry and companies such as Walt Disney set a high bar, notes Allen Zielnik, global weathering applications manager with Atlas Material Testing. School hallway interiors, on the other hand, may place a high value on burnish and mar resistance. Moisture-sensitive substrates such as wood may require a high degree of barrier protection, while other substrates such as concrete masonry units may need breathability. Monumental building exteriors may require extremely durable coatings such as those based on fluoropolymers or other UV- and weather-resistant durable chemistries. “The choice of a coating often depends on a matrix of many factors including material cost, installation cost, substrate, desired property retention, and service environment,” he concludes.

One approach is to segment the market according to the type of performance that is expected. At the top level are exterior and interior paints, which primarily are intended to serve a decorative function. Next are protective coatings, which primarily serve a protective function, such as an anti-corrosion industrial coating or a wooden deck water sealer. The next level includes functional coatings, such as antibacterial or flame-retardant intumescent products. These categories are not mutually exclusive in that many protective and functional coatings must also serve a decorative function, even though it may be secondary in importance, according to Zielnik.

For coatings that will be employed in exterior applications, weathering and corrosion are the two main performance factors, according to Andy Francis, weathering and corrosion technical manager at Q-Lab Corporation. Weathering depends on sunlight, heat, and water, while corrosion involves the reactions of water, oxygen, and ionic species. “Ultraviolet light is ultimately responsible for nearly all outdoor degradation of durable materials such as coatings, although the heat of an environment can accelerate that degradation, and the presence of water can exacerbate certain effects,” Francis remarks. In fact, he points out that there has been an increased focus on water as a failure mode for paints and coatings. “Advances following years of testing have resulted in coatings that are very resistant to degradation from UV light. These systems still, however, can experience water-based failures such as blistering, swelling, cracking, and plasticizing,” he explains. In recent years, test protocols have been implemented to evaluate coatings for these types of failures.

For protective coatings in infrastructure applications, atmospheric corrosion resistance is the baseline function of the coating system, so high-end durability is primarily determined by color and gloss retention, according to Aaron Lockhart, technical market development director for Protective & Marine/Construction at Covestro LLC. He adds that adhesion is also essential, but adhesion loss is a very common cause of field failures, especially in regions where the coated structure experiences low temperatures. Aesthetics have become increasingly important as well, adds Jim McCarthy, technical director with PPG’s protective and marine coatings business. “As a result, demands for high color/gloss retention have driven development of new classes of coatings, such as polysiloxane and other technologies,” he says. In addition to UV resistance, coatings used in the aerospace market typically also require very good chemical resistance properties to combat the chemicals used in this industry, notes Jones.

For industrial applications, Hazen says the key attributes for weatherability of exterior coatings that are most requested include, but are not limited to, color and gloss retention, larger color pallet selection,��self-cleaning,��microbial, moisture and UV exposure resistance, as well as paint film robustness for adhesion, scratch, and mar. Dominant attributes of durable interior industrial coatings include cleanability and sheen retention, scratch and mar resistance, stain and chemical resistance, and color fastness. Bradford adds that in “high-touch” environments or where fabrication is required, such as in the steel door market, users are looking for flexibility and scratch/mar resistance, while manufacturers of products with extended warranties, such as metal roofing, look for long-term gloss and color stability.

One of the biggest challenges with automotive interior coatings is simultaneously achieving a luxurious feel and appearance while increasing stain and chemical resistance (suntan lotion, insect repellant, etc.) requirements, according to Grace. The industry is working to drive improvement in soft feel coatings, especially when tested in primerless application scenarios.

Expectations also vary widely for different architectural coatings. Scratch and mar resistance are important for a door knob, while decorative coatings for a pool require resistance to specific chemicals like chlorine. On the exterior side of the equation, according to Cook, properties like tint and gloss retention, balancing dirt pickup resistance and cracking, as well as robust adhesion to a variety of (and changing) substrates, contribute to durability.  “As coatings have transitioned to lower and lower VOCs over time,��much of our research revolves around improving and balancing these exterior attributes. It’s a combination of maintaining performance properties and looking to surpass the higher VOC paints from decades ago as well as the raw material selection and molecular technology that goes into them, which is a critical component,” he notes. For Jack Johnson, a technical specialist in Architectural Coatings at BASF, the most meaningful measure of durability is real-world damage resistance. For interior coatings, a film must be able to withstand rigorous washing, impacts, and have resistance to burnishing.  Exterior coatings must be able to withstand light-based damage (color retention, gloss retention) and resist raining and freezing effects on the coating and substrate (erosion, grain crack resistance). Because time is money in the professional market, contractors look for high-quality, long-lasting paints that minimize the need for repainting and callbacks, according to Charles Johnson.

In most applications, Woods stresses that it is sometimes necessary to compromise between different performance attributes to find the right balance for customers. For wood coatings, he notes that while it is important to protect the surface and ensure top performance, it is also important to maintain the natural beauty of the wood substrate. In all types of coatings, it is the resin chemistry that plays the largest role in determining durability and performance. Where color retention is also important, and particularly for coatings used in exterior applications, precise and innovative pigment chemistries are also required, according to Romesh Kumar, senior technical sales manager for Clariant Plastics & Coatings USA LLC. He also notes that new UV additives at relatively lower prices are providing new levels of performance.

Testing Challenges

“Overall,” says Hazen, “regardless of the end use of a coating, most durability is measured via controlled standardized test methods that have been developed and vetted over the years. There are global and national standards as well as customer or end-user specific methods that require meeting minimum performance targets.”

Although companies are increasingly pushing for shorter test times and faster results, natural outdoor testing remains the best way to test for performance in outdoor conditions, according to Francis. Michael T. Venturini, marketing director for Coatings at Sun Chemical, agrees: “There is no substitute for real exposure. All other testing is an approximation because there are a wide range of environmental factors that play a role in the breakdown process. One single test method doesn’t work well, so we need to consider a variety of tests together to determine the best solution.” Static exposure test rack exposures, including North facing vertical indirect exposures for mold growth, are frequently performed in subtropical environments such as South Florida. Depending upon the product, other environments such as temperate freeze/thaw, coastal corrosion, and high altitude may be used as appropriate for the product and the target service environments.

DCM, according to Cook, is a big believer in real world tests for developing its products and leveraging a global network of exposure stations to demonstrate performance. “We have a robust program that not only tests current global paint brands on the market, but is also instrumental in developing the next generation of binders and additives. The ability to compare testing across substrates and geographies is a critical step for us,” he explains. DCM also continues to develop its eXposure Vision™ Viewer tools, which provide customers with the ability to monitor panel images in high resolution.

Even if companies are using an accelerated testing program, Francis recommends exposing samples outdoors long-term to build a library of data for comparative purposes. None of the variety of accelerated tests available today that are designed to provide predictive results for coatings performance such as corrosion resistance and UV durability are able to perfectly correlate to actual results, adds Kittler. There are several ASTM and DIN test methods for demonstrating improvement in properties that are generally accepted by paint manufacturers and their customers, but Kumar believes that to ensure the highest possible performance, color panels should be exposed in real time to extreme weather conditions in Florida for up to five years or more to ensure strong color retention. “This testing is critical because paint companies rely heavily on their reputation with end customers, and end customers know that repainting due to poor performance (e.g., poor adhesion, color fading, etc.) is much more expensive than the paint itself,” he says.

In all types of coatings, it is the resin chemistry that plays the largest role in determining durability and performance.

However, because testing in the service environment takes an inordinate amount of time, companies often rely on battery of accelerated durability tests. Zielnik comments that even if they cannot guarantee predictive ability in all circumstances, they at least can serve as a relative comparison basis that hopefully will carry over to field performance. Tests for interior coating performance include hiding, scrubability, mar and burnish resistance, low odor, and VOC content, etc. The most common tests for exterior products evaluate UV and weather resistance, dirt pickup and retention, color and gloss retention, and adhesion. Weather resistance is often determined using Xenon arc laboratory artificial accelerated weathering to provide earlier data on both color and appearance as well as mechanical properties of both interior and exterior coatings. These tests are often augmented with mechanical tests such as Taber abrasion, scrubability, crosshatch, and X-scribe adhesion, particularly in combination with various salt fog corrosion tests. “One of my favorite non-standard tests for gauging moisture hydrolysis and de-adhesion on metal test panels is to place them in a dishwasher under the pot scrubber cycle for 24 hours,” remarks Zielnik.

Depending on the application, there may also be specialized tests for evaluating coating durability. For instance, automotive OEMs have developed standard chemical and stain resistance testing requirements for interior coatings used in traditional vehicles, according to Grace. He adds that as ridesharing continues to grow in popularity, it is important that additional testing requirements be developed to meet higher durability targets. AkzoNobel, meanwhile, is refining its “global weathering approach” for exterior wood coatings. The method includes accelerated weathering testing and external weathering all over the world in some of the harshest climates, according to Woods. “This approach ensures we have the appropriate data to have the highest confidence in our coatings’ exterior durability performance,” he states.

With respect to recent advances in testing, Francis points to ASTM D7869, first released in 2013, as the best recent example from the field of weathering. “This test standard was the first to actually quantify the forces of outdoor weathering, apply them in a realistic yet accelerated test cycle, and verify the results with a broad range of automotive and aerospace coatings,” he says. A major breakthrough from this work, he adds, was the realization that delivery of water in most accelerated cycles was completely inadequate to reproduce the long time of wetness experienced by materials outdoors, primarily through overnight dew formation. In corrosion testing, breakthroughs have been driven by the automotive industry, according to Francis. “Most OEMs have determined that control of relative humidity is critical to reproducing the array of corrosion behaviors and products observed outdoors and have developed test cycles around that concept,” he notes.

There have been other advances as well, but many have not yet been widely applied due to their cost and complexity, according to Zielnik. For instance, combinatorial testing allows evaluation of multiple parameter values rather than the commonplace “pairwise testing” to evaluate the impact of interactions between many different conditions on system failures. The use of robotic technology to measure large numbers of test panels for several parameters such as color and gloss is still slow to gain traction due to the lack of commercial systems. A second methodology uses sensitive analytical techniques to detect material degradation well before it becomes manifest, such as Fourier transform infrared (FTIR) spectroscopy, which can detect changes to the coating resin film such as photooxidation and hydrolysis well before they can be visually observed on a test fence. In fact, Zielnik notes that this technique and others were used in the development of the ASTM D7869 laboratory Xenon-arc weathering test for transportation coatings.

Another important standard, according to Stewart, is ASTM D5894, a UVA/salt spray cyclic test developed to more closely match real-world conditions for coated metal.  McCarthy points to the development of electrochemical techniques, such as electrochemical impedance spectroscopy (EIS). “At PPG, we focus on deeply understanding the fundamental science behind the corrosion susceptibility of the substrates, and several levels of interactions—between the substrate and the coating, between coating layers, as well as the coating and the environments. By fundamentally understanding both the corrosion mechanism and the protection mechanism, we can look beyond the standard tests. These understandings also provide significant insights in the development of next generation coating solutions,” he adds. DCM is creating test substrates that will accurately represent tilt-up construction surfaces for use in the testing of coating surfaces because the use of this substrate, which often has a less-than-ideal surface, is increasing in North America. In the wood coatings area, the European Union is funding inter-company research into Big Data and accelerated testing methods to simulate natural weathering, according to Woods.

 Evolving Coating Technology

The biggest impact on the durability of coatings occurs when advances are made in all components of a formulation, including the resins, additives, and pigments, along with the evolution of formulation strategies and application techniques, according to Hazen.

Sarah Mueller, coil technical manager for Industrial Coatings at PPG’s Springdale plant, points to developments over the past 15 years that have resulted in improved UV resistance for polyester coil coatings. She also notes that layered systems that include protective clearcoats over pigmented films allow for wider color spaces with longer-term color stability. To improve mar resistance, she highlights a new technique that involves manipulating the coating surface through the use of surface modifiers and texturing and dewetting agents, leading to a protective barrier or harder films. In wood coatings, Woods has observed growing use of core-shell technology for resin production that provides better wet adhesion promotion while maintaining the necessary process efficiency for industrial coated wood.

In the protective and marine coating segment, polysiloxane coatings technology has provided significant improvements in weathering resistance (color/gloss retention) of topcoats.

In the protective and marine coating segment, polysiloxane coatings technology has provided significant improvements in weathering resistance (color/gloss retention) of topcoats, according to McCarthy. “The inherent corrosion resistance of this technology has facilitated the implementation of compact processes, achieving performance properties of three-coat systems with two coats, saving asset owners the application cost of the third coat,” he adds. Stewart points to developments in anti-corrosive pigments, functionalized resin technologies, and UVA absorbers that do not migrate to the substrate as important advances.

For architectural coatings, synergistic solutions have been important to advancing durability, according to Quiñones-Rozo. One example he notes is the impact on dirt pickup resistance caused by the use of softer binders
and/or permanent-coalescing agents needed to attain good film formation in low-VOC formulations. BASF has developed a solution that combines new resin and flow/leveling additive technologies to overcome this issue in formulations with < 25 g/L VOC. Cook stresses that while getting the right polymer technology for a given application is paramount, it is also important to minimize the other ingredients in the paint film. He points to the use of resin technology that can deliver the same hiding with less TiO₂ content as an important example. “We have applied our understanding of formulation nuances and overall impacts to the development of resin and additive technologies that influence the choice of all ingredients in a paint can to best maximize ultimate durability,” he says.

For Brünink, one important advance has been the development of new silicone resins as hydrophobing agents, which has achieved a significant effect on water uptake for exterior facade paints. New polymeric dispersing agents that help to balance hydrophobicity and hydrophilicity in wood coating formulations are also important to Woods. Mineral oil-free defoamers, meanwhile, have enabled the formulation of interior paints that do not suffer from fogging, even when heaters are close to the wall, according to Brünink. New film-forming enhancers are also helping to improve film formation and increase the durability of low-VOC paints. Novel alkyd-hybrid technology, such as high-solid alkyd-acrylics and polyurethane-modified alkyds, have improved the durability and process efficiency of coatings for claddings and decking, according to Woods. “Bringing the new resin developments and new dispersing additives together ensures that we can now formulate waterborne, one-component coatings with long durability, low water-uptakes, and with good wet adhesion on multiple wood substrates,” says Woods.

Bringing the new resin developments and new dispersing additives together ensures that we can now formulate waterborne, one-component coatings with long durability, low water-uptakes, and with good wet adhesion on multiple wood substrates.

With respect to pigment technologies, Kumar believes that opacity enhancement, ease of pigment dispersion, and color permanency while maintaining the cost of use have been the primary improvement targets. “Higher opacity pigments offer single-coat hiding, ease of dispersion offers lower manufacturing cost, and color-permanent pigments offer better color retention. Opaque pigments also have lower binder demand and are usually dispersed at a higher loading with much better shelf stability and relatively lower VOCs,” he explains. Better control of the crystal morphology and size of organic pigments has contributed to the improved durability of many coatings, according to Venturini. He also observes that new encapsulation techniques for effect pigments are also important and have enabled the use of effect pigments in durable coatings for exterior applications.

More Work to Do

Increasingly strict environmental regulations and growing customer and consumer expectations for more sustainable products continue to create challenges for coating ingredient manufacturers and formulators that also need to provide more durable coating solutions. As the industry continues to move toward more environmentally friendly and sustainable coating systems, gaps in some performance attributes require action, says Hazen. “This situation particularly arises when technology jumps are made, such as moving from solventborne to water-based coatings. As customers continue to become more demanding in their needs, gaps occur that must be addressed as quickly as possible. The entire industry wins from the knowledge and innovation that occurs when this happens.” In general, Cook sees the balance between low-VOC content and exterior durability as a fine line and does not expect low-VOC coatings in which exterior durability demands are completely satisfied to be achieved for some time.

The biggest gap, according to McCarthy, is the disparity in performance between water-based technologies and solvent-based technologies with respect to corrosion and water resistance. “Restrictive regulations on VOC content are driving development of water-based technologies, and performance has improved greatly in recent years. But for immersion and high-corrosion environments, solvent-based technologies are still the performance standard,” he observes. On the other hand, Jones points out that the simultaneous need to develop environmentally friendlier coatings with equivalent or better durability has led to advances in highly durable waterborne products, such as waterborne polyurethanes. In addition, Woods remarks that “by focusing on new novel monomers for acrylic resins, optimizing wet adhesion promotion, and looking into how to
balance hydrophobicity and hydrophilicity in the polymer, the gap between one-component waterborne coatings and two-component solventborne or waterborne coatings is being closed.”

Gaps also remain in the technology for lower-VOC solventborne coatings formulated without the use of exempt solvents, notes Stewart. “As the regulatory environment changes in Southern California and the Northeast, technologies that do not utilize exempt solvents will need to be developed,” she says. There is also a need for coatings that do more with less, meaning thinner film builds that provide equal or better corrosion and weathering resistance. In the powder coating sector, Moonen notes that there is a significant opportunity for development of lower-temperature cure technologies. For wood coatings, two ongoing issues relate to bleeding from substrates and inadequate color and gloss retention, according to Woods. “While we are close to getting full protection regarding bleeding substrates, a deeper understanding of the mechanism is still necessary. For now, new crosslinking chemistry has been developed to ensure that we can stop the bleeding from various substrates, including in harsh environments with high humidity and high temperatures,” he comments.

A specific color issue is the inability for high performance in opaque and bright orange shades. There is a demand for these coatings in the automotive industry because the brand shades of many utility vehicles include bright orange.

With respect to pigment technology, increased expectations for aesthetics and color stability, particularly for organic pigments in exterior coatings, still remain an issue, according to Brünink. “Customers require much brighter colors and

would like to select from a broader range of different colors. Offering these choices in architectural coatings can only be achieved by using more difficult-to-stabilize organic pigments instead of inorganic pigments. Venturini agrees that there are always gaps in color space as formulators balance durability and costs. In some cases, he notes that resin chemistries or additives can improve the performance of less stable offerings. Accord-ing to Kumar, a specific color issue is the inability for high performance in opaque and bright orange shades. There is a demand for these coatings in the automotive industry because the brand shades of many utility vehicles include bright orange.

Testing requirements also pose hurdles that have yet to be addressed. A challenge within the building products industry is understanding durability of a coating system in a t

imely fashion, according to Mueller. “Many industry specifications require outdoor exposure data, which can take decades to obtain. Accelerated testing equipment can supplement this process, but the time needed to collect comparable information can still take months to collect, and still may not reflect outdoor exposure conditions,” she notes. There is also a need for reliable accelerated tests for coil coatings, particularly for UV and corrosion, that correlate to real world results, Bradford asserts.

Finally, Zielnik observes that protective coatings that can be applied to surfaces that do not require extensive substrate preparation, such as abrasive blasting and other similar methods, yet still exhibit long-term durability, would be a major advance. Self-healing coating technology that allows the self-repair of minor damage within an applied coating is also desirable in many applications, according to Zielnik. Initial products for the automotive sector have been introduced that are able to self-heal minor scratches.

To learn more about the products and technologies that are being introduced to enhance durability and protect against the harmful impact of weathering, visit paint.org/coatings-tech/june-2019/.

CoatingsTech | Vol. 16, No. 6 | June 2019