Many additives used in waterborne coatings and inks are complex formulations. For example, most defoamers are comprised of insoluble oils, compatibilizers, and, oftentimes, hydrophobic solids and surface-active species. Since their mechanism of action involves delivering incompatible droplets capable of spreading across and rupturing foam lamellae, it is important to balance this incompatibility with the equally critical requirement that the defoamer not cause defects or recoatability issues in the cured film. Additionally, it is important that the formulated additive resist phase separation under typical transportation and storage conditions. The objective of this article is to share specific details of how a project team developed unique automated test methods and used them, in combination with design of experiments, to accelerate the development of new defoamers and deaerators. Case studies in which the team optimized silicone polyether structures, as well as formulated defoamers, for both performance and stability will be discussed.

Introduction

Approximately 15 years ago, a “voice of the customer” survey was conducted to better understand unmet market needs in coatings and inks. One of the primary challenges formulators identified was that of finding the ideal additive to mitigate foam-related complications in a brand-new formulation, particularly after most other system components had already been established. Therefore, rather than try to develop new defoamers targeted to solve specific foam control problems in select formulations, the team chose to design a series of defoamers that spanned the entire performance space—ranging from strong, incompatible grind defoamers to very compatible defoamers required for low viscosity coatings intolerable of defects. Ideally, the set of defoamers would be designed in a structured manner so that each defoamer’s performance would be predictable relative to the others, thus, enabling formulators to more quickly home in on a suitable solution. However, to accomplish this goal it would be necessary to build a deep understanding of the structure-performance relationships of several key components used to create defoamers, specifically, the silicone polyethers, surfactants, and hydrophobized particles.

An automated liquid handling formulator capable of preparing multiple-component mixtures in batches of 48 samples was used to create formulations that could then be used in subsequent evaluations. To further build upon the increased number of prepared samples, an automated testing laboratory focused on surface science was built to accelerate these evaluations. The core of this automated laboratory consisted of testing platforms to automatically measure dynamic surface tension and turbidity, as well as to assess foam behavior following a shake test. Although originally intended to prepare and evaluate formulated surfactants in applications such as electronics and cleaning, the operator in charge of the system recognized the potential for its broader use. Other automated techniques were then added to the surface science and automation laboratory to develop defoamers and deaerators more efficiently.

Experimental

Automation Equipment and Test Procedures

A JANUS G3 automated liquid handling workstation (PerkinElmer/Revvity) that can dispense a combination of up to eight different materials was used to prepare 2-5 mL formulated aqueous samples for future characterization (Figure 1). Typically, the individual test tubes were arranged in a rack on the deck of the formulator and, after filling and capping each tube, the rack of 48 samples was mixed on a vortex mixer.

FIGURE 1 Automated liquid formulator.

To further utilize the capabilities of the formulator, an air sparge test was developed to take advantage of the controlled dispense feature of this equipment. Using this protocol, a set volume of air was delivered at a set flow rate into each sample. A Bassler high-speed camera was mounted to capture images before and after the air sparge (Figure 2). Images were captured over a set period, and then analyzed to quantify the amount of foam bubbles generated and how long the foam persisted. Each image was sliced into samples and analyzed using a proprietary technique developed for this purpose. The data could then be graphed to represent foam generation and decay over time. Data could also be compared along with reference to each sample’s initial turbidity and the density of the foam layer over time; this provided a more complete picture of each additive’s ability to prevent and control foam.

FIGURE 2 High-speed camera images of four 2-mL aqueous samples during the air sparge testing using the automated formulator (left) and an example of foam density analysis for one of the samples after air sparging was completed (right).

Another technique developed around the capability of the Janus formulator involved a multi-dispense of liquid onto a Leneta chart. For example, an aqueous resin could be diluted with water to achieve a low enough viscosity for testing; addition of an oil soluble blue dye enabled greater visibility of foam and incompatibility within the formulation. The Janus formulator was used to add set amounts of different defoamers to vials containing 2 mL of dyed resin and the set of samples was mixed using a vortex mixer. The resulting formulated resin systems were then automatically dispensed using individual syringes, four at a time, by aspirating approximately 0.7 mL of liquid and dispensing it at the top of a Leneta Form 5C chart that was mounted at a 45° angle. The dispensed liquids were allowed to flow down the chart, after which the dried strips of coating were examined. This “dropdown” test method demonstrated good reproducibility and directly correlated with results from a manual draw down to assess compatibility. An early sample dropdown test chart is shown in Figure 3. Results were assessed visually and each defoamer’s compatibility in each resin system would be rated on a scale of 0-10, with zero representing complete dewetting and 10 representing excellent compatibility with no visible craters or dewetting.

The paint and coatings industry gathered May 16–18 at Harrah’s Resort Atlantic City in New Jersey for the very successful 2023 edition of the Eastern Coatings Show. The technical conference and exhibit were hosted by three East Coast paint societies: the Philadelphia Society for Coatings Technology, the Metropolitan New York Coatings Association, and the New England Society for Coatings Technology. With more than 1,300 attendees, the conference had well-attended presentations and robust conversations on the exhibit floor.

The technical conference began on the first day with a coatings short course titled “Fundamentals of Coatings and Sustainable Materials in the Marketplace,” which was presented by James Rawlins and Robson Storey, both professors at the University of Southern Mississippi’s School of Polymer Science and Engineering. The second day started with a packed room for the keynote presentation on “Leading a Business through Challenging Times” by Dan Calkins, CEO and chairman of Benjamin Moore.

The afternoon of day two included an interesting panel discussion. Titled “Where Do We Go from Here? The Future of the Coatings Industry,” the panel included George Pilcher of the ChemQuest Group, Amanda Andrews of Michelman, Professor Dean Webster of North Dakota State University, and Professor Ray Fernando of the California Polytechnic State University. Each provided their unique view on what to watch for in the future and touched on topics such as sustainability, technical staffing, removing substances of concern from raw materials and formulations, raw material sourcing, recycling, and the role of artificial intelligence.

The technical program consisted of 42 technical presentations by industry scientists on a variety of topics, including advances in resins, pigments, additives, testing methods, and coatings formulation. This article examines some highlights and summaries of just a few of the presentations.

Sustainability

Sustainability is an important concern that is getting increased attention in the coatings industry. This includes the decades-long search for products with lower volatile organic content (VOC) and the more recent emphasis on biosourced raw materials, the industry has been interested in sustainable technologies for a long time. More than a buzzword, sustainability is becoming a way of life for the industry.

A full one-third of the presentations referred to sustainability in their abstracts and titles, and one of the concurrent tracks was titled “Driving Sustainable Coatings with Chemistry.” Even the short course presented on the first day of the technical program mentioned sustainable materials in its title.

In a presentation titled “Alkyd Emulsions and Their Contribution to More Sustainable Paint and Coatings Formulations,” Caroline Matthieson of Worlée-Chemie spoke about the use of biosourced raw materials in the production of alkyd resins. By introducing the 17 sustainable development goals set in the United Nations 2030 Agenda for Sustainable Development,^1,2 Matthieson first explained how Worlée is focusing on several of the goals in their own work, such as responsible consumption and production (#12), climate action (#13), and partnerships for goals (#17).

Figure 1. Typical structure of an alkyd resin, which is a polyester resin based on a polyacid (such as isophthalic acid, shown here) and a polyol (such as glycerol, shown here) and modified with a fatty acid.

The presentation then described the use of biosourced fatty acids in the production of alkyd resins. Alkyds are polyester resins, formed by the condensation reaction of polyacids and polyols and modified with fatty acids (Figure 1). The fatty acids used to make alkyds have biobased origins. For example, linseed oil is a common source of fatty acids used in alkyds and is produced from flax seeds. Matthieson explained that, although linseed oil is a great source of renewable and biobased raw materials, its use in coating resins competes with its use in the food supply, where it is employed as a source of alpha-linolenic acid (an omega-3 fatty acid).

Camelina oil was presented as a beneficial alternative to linseed. Table 1 shows a comparison of linseed and camelina oil compositions in terms of the fatty acids available in each. Matthieson explained how camelina oil is a more sustainable choice for Worlée because the camelina plant (Camelina sativa) is grown locally near their production sites in Germany, while flax is not. In addition to regional cultivation, camelina can be grown as a mixed crop with peas or as a secondary crop in temporarily fallow land, and thus, it does not compete with food production. Other advantages include that it provides a food source for pollinators, requires less fertilizer, has good resistance to pests such as aphids, and that the crop’s yield has a lower dependence on the weather.

Although there are limited choices of biobased polyacids for use in alkyd resins, some examples include furandicarboxylic acid and succinic acid. Many choices of biobased polyols exist, so depending on choice of raw materials, Matthieson explained that alkyds can be produced with 85% to almost 100% biobased raw materials. As an example, a waterborne alkyd emulsion with 85% renewable content and based on camelina oil was described as having very similar physical properties and performance compared to one prepared with linseed oil, and with the sustainability advantages of less competition with the food supply and use of regionally produced materials.

Table 1. Fatty Acid Distribution in the Composition of Oils

Another presentation on the topic of sustainable coatings was presented by George Daisey of Dow and was titled “Sustainable Coatings Technology That Works.” Daisey began with an introduction to Dow’s sustainability goals, which include combatting climate change, driving circular economy by designing for circularity, and innovating new materials that offer a more favorable health and environmental profile over their lifecycle. Dow has an ambitious goal of reducing carbon emissions and being carbon-neutral in its operations by 2050. In particular, Daisey described the sustainability benefits of roof coatings, which are defined as thick, white, monolithic, and solar-reflective elastomeric films.

The most important value proposition for maintenance with roof coatings is roof life extension, which lowers the lifecycle cost of the roof and decreases the amount of material associated with replacing a roof that is being sent to landfills. In addition, the use of solar-reflective roof coatings can help lower energy usage associated with air conditioning and reduce the “urban heat island” effect.² Daisey described how an effective roof coating must have both high reflectivity to prevent absorption of solar energy by the building, as well as high emissivity of the energy that is absorbed. According to Daisey, an uncoated roof can reach a surface temperature of approximately 180 °F on a hot summer day, with an effective roof coating dropping the temperature by 60 degrees to 120 °F. He emphasized that the drive for more sustainable roof coatings cannot ignore the other challenges that a roof coating must face, which include the need for resistance to biological growth and dirt pickup and adhesion issues on the varied roofing substrates.

According to Daisey, Dow is thinking about advancing roof coatings via multiple technologies, including delivering biological resistance without biocides, creating faster-setting acrylics to reduce labor and equipment time, enhancing durability, using biosourced raw materials, and developing hybrid technologies. He expanded on the hybrid technologies approach by describing new acrylic-urethane products for two global regions. One is a waterborne acrylic-urethane hybrid polymer designed for liquid-applied waterproofing membranes for flat roofs in the EMEAI markets (Europe, Middle East, Africa, and India). The hybrid polymer is designed to have a superior balance of cold temperature flexibility and traffic-ability at temperatures up to 90 °C. Roof coatings based on the acrylic-urethane hybrid can be formulated as a liquid-applied roofing membrane that meets European CE (Conformité Européene) marking requirements and passes the strict durability tests for an expected working life of 25 years, as set forth by the European Organisation for Technical Approvals (EOTA) in ETAG 005.³

Today, products meeting the ETAG 005 are mainly solventborne, with one-component (1K) polyurethanes being the most common. The hybrid offers a waterborne alternative in a market dominated by solventborne technology. Daisey presented data showing that the waterborne acrylic-urethane hybrid yields a coating with similar mechanical properties compared with a 1K polyurethane, while contributing to better durability. Tensile strength of the hybrid was close to that of a 1K polyurethane and higher than a standard waterborne acrylic coating. For elongation, after 14 days of thermal aging at 80 °C, the 1K polyurethane coating dropped to 137% from an initial value of 220%, while the hybrid started at 272% and only dropped to 245%, more in line with the performance of the waterborne elastomeric acrylic roof coating. The amount of water swelling exhibited by the hybrid coating (8.9%) was intermediate between the 1K polyurethane (1%) and the waterborne acrylic (16.4%). Accelerated and natural weathering data demonstrated that the hybrid maintains good mechanical properties on extended UV exposure and has improved durability versus the acrylic.

In an interesting presentation titled “Analytical methodologies and challenges for understanding paint emissions,” Michelle Gallagher, Ph.D., of Dow described some of the challenges facing the analytical chemist when attempting to quantify and identify volatile emissions. Emissions testing is becoming more important because health-conscious customers care about indoor air quality and emissions that originate from consumer products such as paint. In addition, Gallagher explained how the growth of green building certifications and their requirements for low VOC and low emission products has increased the industry’s need for such testing. Dow has been growing its emission testing capabilities to simulate the emission rate of paint VOCs after application, as well as to understand how its products affect emissions.

While bulk VOCs are measured on wet paints using ASTM D6886 and reported in units of g/L, emitted VOCs are measured on a paint after application using chamber methods and reported in units of μg/m³. There are several different certifications and standards dealing with emissions, such as the chamber method described in California Department of Public Health (CDPH) Standard Method v1.2, a widely used standard in North America for measuring emissions from building products such as paints.⁴ A typical chamber is made of stainless steel and has the ability to control humidity and airflow. The paint is applied to a plate and placed under constant airflow (e.g., 0.5 to 1 air exchange per hour). At various times, the emissions are sampled using an absorbent trap, which is then analyzed using GC-MS or HPLC methods.

Gallagher explained that one of the challenges in using these chamber methods is obtaining a clean background prior to testing for emissions. For example, each volatile should be under 2 μg/m³, and the total VOC should be under 25 μg/m³ in the CDPH Standard Method v1.2.⁴ Gallagher described how a robust cleaning procedure is required and how gloves should always be worn when working with the chamber and plates and holders. Even a single fingerprint can lead to the detection of volatiles (e.g., hexadecenoic acid) at a level above the threshold allowed for background emissions. Chambers must be cleaned and purged between each study, and the background emissions levels checked before taking a new series of measurements. In addition, Gallagher stressed that absorbent tubes should always be cleaned and background checked before collecting samples.

Other challenges include the choice of substrate specified by the method. While clean stainless steel and glass have very low background emissions, drywall or drywall with the edges taped is sometimes specified. Both the drywall and the edge tapes (such as foil or metallized polymer tape) can lead to contaminant emissions being measured that do not emanate from the coating. Calibration is also critical, and toluene is typically used for that purpose.

Identifying the source of the volatile emissions can also be challenging. Every material used in a paint formulation can have its own unique volatiles, so analyzing raw materials individually with GC-MS can help determine from where emissions originate. Understanding the source of emissions is necessary to better control them through both raw material and coating formulation design. ��Finally, it was stressed that it is difficult to predict emissions based on total VOC measurements, because ASTM D6886 measures the total VOC in the wet bulk paint, while emissions testing measures VOCs at various times as the coating dries.

Functional Coating

End-users are continuing to ask for more of their paints and coatings. In addition to their decorative and protective properties, there are numerous coatings that are also designed to provide other functions such as soft-feel haptics, sound damping, thermal insulation, or antimicrobial properties. For example, in a presentation titled “New Thermal Management Raw Materials Platform Gives Flexibility to Develop Next Generation Thermal Insulation Coatings (TIC) with Improved Performance,” Hrishikesh Bhide, Ph.D., of Evonik described new resin and filler materials for use in thermal insulation coatings. Thermal insulation coatings are a type of functional coating designed to provide personnel protection by reducing the surface temperature of hot surfaces and prevent skin burns, as well as to improve energy management. The coatings also provide direct protection of the substrate and lead to a reduced risk of corrosion under insulation (CUI).

Bhide described two new silica granules with low thermal conductivity that can be used in thermal insulation coatings. The first was described as a super-insulating granule (SIG) with a larger particle size (~300 μm) and high hydrophobicity. The SIG particles derive their insulation properties from a passivated amorphous silica composite core and have a thermal conductivity of 24 mW/m·K. The second granule material was described as a SIG synergist, having a particle size of ~30 m and thermal conductivity of 30 mW/m·K. The small particle size synergistic filler reduces the cracking tendency of highly filled insulation coatings and facilitates smoother coatings. When formulated with a waterborne acrylic binder, the combination of the SIG and SIG synergist granules provides a coating with lower thermal conductivity (about 5mW/m·K lower) than when either is used by itself and with little change in thermal conductivity after heat aging.

In addition to the insulating granules, Bhide also introduced a new waterborne silicone resin, which, along with the granules, can be utilized as the sole binder in thermal insulation coatings with higher heat resistance than standard binders such as acrylics. The combination of the silicone binder and the two insulating granules leads to coatings with good fire retardance and low thermal conductivity (57 mW/m·K at 25 °C). The silicone resin can also be blended with other waterborne binders such as acrylics to extend the heat resistance of insulation coatings based on traditional binders such as acrylics. Bhide also described the use of these materials in some field studies within chemical production facilities for condensation control, thermal management, and safe-touch properties.

In another paper titled “High-Touch Coatings with Bactericidal and Virucidal Properties,” Mark Langille, Ph.D., of Corning described a new copper-glass additive for use in antimicrobial coatings with high efficacy towards both bacteria and viruses. Langille began with a discussion of the expected performance of antimicrobial coatings that could be used to improve public health by killing microbes that come in contact with the coating surface. In contrast to liquid disinfecting agents, a dried coating would be expected to provide a surface that is continuously active, provides antimicrobial efficacy between regular cleaning cycles, and addresses a spectrum of real-world germs, including easier-to-kill bacteria and viruses (e.g., SARS-CoV-2) and harder-to-kill viruses such as non-enveloped viruses (e.g., norovirus).

In addition, Langille described how the U.S. Environmental Protection Agency (EPA) formalized guidance in 2022 for products claiming residual efficacy, including test methods for demonstrating both bactericidal and virucidal activity.^5,6 To better simulate real world contamination events, dry test conditions are utilized in which the surface is contaminated with bacteria or virus, allowed to dry, and then analyzed for how effectively it killed the microbes after only 2 hours. It is expected that at least a 3-log reduction occurs, or 99.9% of microbes are killed. In addition, durability of the antimicrobial properties is evaluated by putting the surface through simulated wear/cleaning cycles and then testing for efficacy.

Langille discussed how copper is a powerful, natural antimicrobial and is effective at killing both bacteria and viruses. Copper in the +1 oxidation state (Cu+1) is a particularly potent but less stable form of copper, and the innovation in the new additive is that Corning found a method of stabilizing Cu+1 in glass. Aluminoborosilicate glass and a copper source are melted together to form a glass, which is then milled to give the copper-glass additive, with an average particle size of 4 μm. The additive is a brown color but can be incorporated into a large variety of paint colors. A typical loading in a paint formulation is approximately 1% by weight of the copper-glass additive.

Data were presented showing results of antimicrobial activity (log kill) after dosing six commercial coating formulations with the copper-glass additive at various levels (0 to 40 g per gallon). Effectiveness at a particular dose varied amongst the coatings because of differences in formulation ingredients, but each coating demonstrated log-3 kill (99.9% kill) of Staphylococcus aureus at levels of approximately 1% additive, and all had log-5 kill (99.999% reduction) at a level of 40 g/gallon or below.

Langille also discussed how challenges in initial or long-term efficacy can occur for coatings that are high in polymer content due to their low porosity, which may prevent access of the Cu+1 in the coating to the microbe at the coating surface. An example presented was a waterborne direct-to-metal coating with 2% copper-glass additive, where efficacy dropped after 3 months. A study of compatibilizers to control the stability and availability of the copper-glass additive identified solutions that enabled long-term efficacy in the system. It was noted that compatibilizers can eliminate the need for significant formulation changes and thus make it easier for current commercial formulations to add antimicrobial functionality via the copper-glass additive.

Additive Technologies

Figure 2. Generic formula of alkyl aryl sulfonic acid salts used as corrosion inhibitors.

Coatings additives are always an important topic of discussion when the coatings industry gets together for technical conferences, and this year’s Eastern Coatings Show was no different, as multiple presentations covered new additives to enhance the performance of paints and coatings. In a presentation titled “Improving Corrosion Resistance of DTM Coatings Using Hydrophobic Alkyl Aryl Sulfonate Additives,” Matthew Gadman of King Industries talked about a class of easy-to-use liquid corrosion inhibitors that can be effective at low dosages of approximately 1 to 3%. Corrosion takes a large economic toll on the economy, to the tune of $2.5 trillion globally (about 3.5% of global gross domestic product), as Gadman described. The corrosion inhibitors he discussed have the generic structure shown in Figure 2 containing a hydrophobic naphthalene ring with alkyl substituents and a polar sulfonate salt group. The counterion can vary with both metal and amine cations being used.

The corrosion inhibitors can be used in both clear and pigmented coatings, including high gloss systems. They are compatible with a range of resins and can also act as catalysts for aminoplast coatings (for X = Zn and Ca). They can also aid in wet adhesion and pigment dispersion. In a coating on metal, the polar portion of the molecule orients toward the metal surface and passivates the surface, preventing the formation of anodic corrosion sites where oxidation reactions occur. The non-polar alkyl-substituted aryl group orients away from the metal surface and prevents water from reaching the surface.

Gadman shared the results of several experiments where the alkyl aryl sulfonate inhibitors were added to coating formulations and evaluated for corrosion resistance. For example, a barium sulfonate salt was incorporated at 1% on total formulation weight into a 1K solventborne acrylic/melamine clear baking finish and was found to aid in preventing blisters and rusting compared with the blank control, which had no inhibitor. In another set of experiments, the sulfonic acid salts were evaluated at 1% on total formulation weight in several pigmented formulations in combination with a variety of anticorrosive pigments. Thermoset and air-dry systems were evaluated, and all showed a synergistic effect on corrosion resistance when using the sulfonate salts with an inorganic anticorrosive pigment. Finally, in a third set of experiments, Gadman demonstrated that the addition of a barium sulfonate salt to aerosol paints improved the corrosion resistance and had no negative effect on appearance.

Helena Wassenius, Ph.D., of Nouryon presented an interesting paper covering a new type of cellulosic rheology modifier in a presentation titled “New Ultra-Low Viscous, Highly Associative Cellulose Ethers for Acrylic-Based Architectural Paints.” Wassenius explained how synthetic HEUR (hydrophobically modified ethoxylated urethane) rheology modifiers have low molecular weight and are highly associative, while HM-CE (hydrophobically modified cellulose ether) thickeners are cellulosic-based materials of high molecular weight and are typically moderately associative. HEUR thickeners provide excellent levelling, while HM-CE thickeners provide good sag resistance. The new cellulosic thickener falls between the two extremes and provides a high level of association.

Results of testing in 30% PVC semigloss architectural paints were detailed. Leveling performance of the formulation containing the new cellulosic ether was similar to one with HEUR thickener and much improved relative to those containing HM-CE thickener. Meanwhile, sag resistance was better than the HEUR and closer to that of an HM-CE thickener. In addition, viscosity loss on tinting, syneresis resistance, and color acceptance were much improved versus a HEUR thickened formulation. The new cellulose ether also shows excellent spatter resistance, and the hiding power, as measured by contrast ratio after roller application, was substantially better than HM-CE and close to that of a pure HEUR system. Finally, Wassenius alluded to significant sustainability benefits, as found in a lifecycle analysis that estimated the contribution of the thickener system to the carbon footprint of one ton of formulated paint is 40% lower for the new cellulose ether thickener compared to a synthetic HEUR thickener.

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) offer many performance benefits to paints and coatings, but the industry is actively trying to replace these materials due to their link to serious health effects and persistence in the environment. In a presentation titled “Free of PTFE! New Micronized Wax Additives for Scratch & Scuff Resistance,” Smriti Arora of BYK discussed new wax additives that were developed as replacements for polytetrafluoroethylene (PTFE). PTFE, also known by the tradename Teflon, is a fluoropolymer and a well-known PFAS example. Wax additives based on PTFE are often used in coatings because they impart scuff, scratch, and abrasion resistance.

Figure 3. Orientation of wax additives in a dried coating film. Most wax additives orient to the surface of the film (a), while PTFE-based waxes orient more homogeneously throughout the film (b) due to their higher densities.

Most waxes orient to the paint surface but can be removed from the surface during abrasion. Arora explained that due to their higher density, PTFE-containing waxes orient more evenly throughout the coating film and have a more durable abrasion resistance (Figure 3). Three new PTFE-free additives, based on either polyethylene (PE) or modified PE alloy waxes, were introduced, and the presenter explained that they also provide a homogenous distribution in the coating film. The additives improve both abrasion resistance and scratch resistance and produce a medium-to-strong reduction in coefficient of friction (COF). Due to their small particle size, the additives have minimal impact on gloss. According to Arora, the new additives are also food-contact compliant.

Examples of their effectiveness in Taber and Wazau abrasion resistance was demonstrated for a 1K waterborne acrylic industrial coating, where the new PTFE-free additives showed similar performance to the PTFE-containing controls at 2% loading on total formulation weight. Evaluation in a BPA-free clear can coating showed abrasion results and COF reduction comparable to PE/PTFE controls at 1% loading. For systems with low COF requirements, a combination with a softer wax (e.g., carnuba) or polysiloxane additive was also recommended, and examples of the strategy were shown for an epoxy/phenolic gold lacquer. Results of testing at 4% loading in an architectural interior flat deep base were also shared, and the PTFE-free wax additives had a positive effect on both scuff resistance and scrub resistance. Arora stressed the wide compatibility of the PTFE-free wax additives across waterborne, solventborne, and UV coating systems and their comparable technical performance to PE/PTFE-based wax additives.

Resin Technologies

Several conference presentations covered new resin technologies for both architectural and industrial coatings. One such presentation was titled “New Polyester Dispersion for VOC-Compliant 2-Component Waterborne Coatings,” in which Ashish Zore, Ph.D., of Coim USA described the use of a new waterborne polyester polyol dispersion for use in two-component (2K) polyurethane floor coatings with good light stability and near-zero VOC content. Zore described some of the challenges facing the floor coating industry, including restrictive VOC limits that are under 50 g/L in some regions, the phasing out of exempt solvents, and the continuing demand for higher performance. Floor coating end-users are asking for better light stability, chemical resistance, wear resistance, and weatherability.

The polyester polyol dispersion is supplied at greater than 60% solids in water and contains no co-solvents or surfactants. It can be formulated into 2K waterborne polyurethane coatings utilizing standard hydrophobic polyisocyanates, rather than the hydrophilically-modified polyisocyanates that are often required for waterborne systems. According to Zore, it is suitable for use in various industrial coating applications, including general industrial finishing, protective coatings, and floor coatings.

Zore described testing results comparing a 2K polyurethane coating based on the waterborne polyester polyol dispersion with polyurethanes based on two waterborne acrylic polyols, as well as two high-solids polyaspartic coatings. The coating based on the polyester polyol dispersion had an ultra-low VOC level of under 25 g/L. The gloss of the polyester-based polyurethane was very high, with a 20°/60° gloss of 82/92, comparable to the polyaspartic coatings (88/94) and much higher than the two acrylic-based polyurethanes (8/40 and 39/71). Flexibility of the polyester-based coating was excellent, but both pencil and pendulum hardness were lower than the other systems. Zore noted that hardness could be improved by manipulating the polyol/isocyanate ratio toward more isocyanate and commented that stoichiometries with higher isocyanate also yield better chemical resistance. Finally, the estimated applied cost ($/sq ft) of the waterborne polyester-based formulation was only 75% of the cost of the waterborne acrylic-based polyurethane and approximately one-third of the cost of the high solids polyaspartic system.

In a presentation titled “Enhance Performance of Waterborne Coatings Using Functionalized Binders with Novel Monomers,” Tiffany Chen of Solvay described the use of a functional specialty monomer for acrylic and styrene-acrylic latex polymers. Added during the emulsion polymerization process, with recommended levels of 0.5 to 2% based on total weight of monomer, the monomer offers performance benefits in both architectural and industrial coatings. Chen first described its use in an architectural latex with a BA/MMA backbone. A control latex was made with 2% methacrylic acid (MAA) and compared with a latex containing 1% MAA and 2% specialty monomer. In a semigloss formulation (22% PVC), the functional monomer resulted in improvements in opacity and tint strength, as well as in metal adhesion and the removal of household stains.

In another study, a flat architectural coating (49% PVC) was prepared with a latex containing 1% specialty monomer. The functional latex demonstrated better color acceptance for both initial and heat aged paints compared to a control. Scrub resistance was also better, as was dry and particularly wet adhesion to various substrates (e.g., glass, steel, and aluminum).

The specialty monomer was also evaluated in a styrene-acrylic composition designed for light duty direct-to-metal (DTM) coatings. The control used 2% acrylic acid (AA), while the functional monomer replaced half of the acrylic acid in the experimental latex (1% AA and 1% specialty monomer). Formulated into a gloss DTM coating (18% PVC), the functional monomer led to improvements in corrosion resistance as measured by salt fog exposure (ASTM B117) and blister resistance upon immersion in water. Adhesion over cold-rolled steel was greatly improved versus the control, and in addition the initial gloss was much higher, with a 60° gloss of 63 units compared to the control with a gloss of only 17 units. Overall, the specialty monomer facilitates the improvement in several properties for both architectural and industrial latex coatings.

References

1. United Nations. 2030 Agenda for Sustainable Development. (accessed July 11, 2023).
2. United Nations. The Sustainable Development Goals Report 2022. (accessed July 11, 2023).
3. European Organisation for Technical Approvals. ETAG 005, Guideline for European Technical Approval of Liquid Applied Roof Waterproofing Kits, 2004.
4. California Department of Public Health. Standard Method for the Testing and Evaluation of Volatile Organic Chemical Emissions from Indoor Sources Using Environmental Chambers, Version 1.2, 2017.
5. US Environmental Protection Agency. Guidance for Products Adding Residual Efficacy Claims. (accessed July 11, 2023).
6. U.S. Environmental Protection Agency. Test Method for Evaluating the Efficacy of Antimicrobial Surface Coatings, SOP Number MB-40-00, revised September 2022.

About the Author

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

Introduction

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

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

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

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

Experimental

Raw materials and testing panel preparation

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

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

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

Testing Procedures

Dry Time

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

Continue reading in the ���Ǵ���CoatingsTech.

While researching the world of paint and coatings additives for what was new and novel, an article written in June 2020 caught my attention. It was an in-depth analysis of the myriad of additives that are ubiquitous in paints, coatings, and adhesives.

Composing only 4% of a coating, chemical additives are instrumental to coatings manufacturing, shelf-life stability, successful application onto substrates, film formation, and ultimately, appearance, performance, and durability. Ironically, this seemingly endless array of additives is the smallest of the global raw material categories, as defined by World Coatings Council data, representing only 0.5% of total usage volume under the anonymous category of “Other.”

More importantly, consolidation of raw materials and coatings companies, coupled with environmental constraints, has led to decreased availability of “preferred tried-and-true” raw materials. This has forced formulators to embark on the uncomfortable task of extracting multifunctional properties from the remaining additives.

That article ended with the observation that there was “plenty of room for innovative thinking, by both the additive producers and the coatings formulators, with regard to the subject of polyfunctional additives for paints and coatings. The challenge is here . . . and the time to accept it is now.”

That last sentence challenging the status quo shouted that another treatise governing the myriad of chemical paint additives would be redundant at best. The academic literature and trade publications contain everything that is currently known about traditional additives for paints and coatings. Rather, a different analytical approach to examining the world of additives would be more interesting and perhaps a small step forward to “accepting the challenge.”

This musing led to the obvious question: “If not wetting/dispersing agents, rheological modifiers, foam control, surface modifiers, flow and leveling agents, catalysts, driers, adhesion promoters, biocides, stabilizers, plasticizers, et al., then what?” After all, Robert Ruckle, global marketing and sales director of Siltech Corporation, likened additives to the “flavors and spices” of a coating by comparing the plethora of choices to a “secret chili recipe.” Chili, like paint, derives signature characteristics from the special blend of secret additives. Clearly, another path of examination would be required.

ADDITIVE CHALLENGE ACCEPTED: MICROSPHERES

Another category of additives called microspheres has been gaining popularity in coatings and adhesive applications in recent years. Microspheres are functional additives that physically modify coatings and films, generating special final properties without chemically based “steroids.”

Microspheres have long been used in the medical community for cancer research, invitro drug release, biological protocols, flow visualization, and imaging. Industrial uses include LCD screen spacers, particle sieve standards, electrical potting compounds, and polymeric material lightweighting and/or strengthening.

The first official definition of a microsphere was developed by the International Union of Pure and Applied Chemistry (IUPAC) for the medical community:

Microsphere: A microparticle of spherical shape without membrane or any distinct outer layer. (See microcapsule.)

Note: The absence of outer layer forming a distinct phase is important to distinguish microspheres from microcapsules because it leads to first-order diffusion phenomena, whereas diffusion is zero order in the case of microcapsules.

Recently, as microspheres become more prevalent in a wide variety of industries, the definition has been clarified and expanded. Microspheres Online—the definitive data base supporting worldwide microsphere research—provides the following definition:

Microsphere is a term used for small solid spherical particles with diameters in the micrometer range (typically 1 micron to 1000 micron).

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Titanium dioxide is the most important white pigment in waterborne architectural paints. Even though it is one of the most expensive raw materials in the formulation, subject to numerous price fluctuations and turbulence related to changes in the hazard classification, titanium dioxide is the most effective pigment for providing opacity to white and light shades.

In addition to opacity, the structure, surface treatment, and grade of titanium dioxide pigments are valued in architectural coatings, as described in ASTM D476.

Titanium dioxide in waterborne paints is used in architectural paints, on wood, and on metal and other substrates. Thanks to its very high opacity, it has replaced zinc oxide and barium sulphate in many applications.

However, to maximize the effectiveness of titanium dioxide pigment in a formulation, it must be incorporated into the paint in a specific manner. This involves the deagglomeration of the titanium dioxide particles; in its commercial form, titanium dioxide is a powder with aggregated and agglomerated particles.

In architectural paints, titanium dioxide is usually incorporated into wall and ceiling paints, along with other fillers, in the form of slurries and is combined with other raw materials. The slurries for waterborne paints for walls and ceilings are prepared by dispersing the primary particles at a high speed with a Cowles dissolver equipped with a serrated disc.

The appropriate selection of surfactants and grinding aids helps separate the primary particles so the degree of dispersion is sufficient to obtain paints with a semigloss or gloss above 60% at 60°. This is usually because latex paint formulations for walls are prepared by dispersing the primary particles with other fillers, such as calcium carbonate, nepheline syenite, and quartz, which act as an additional grinding medium for the titanium dioxide particles. This result occurs even when the paint production process consists of adding separate slurries from each type of filler in the let-down process and mixing them with a binder and thickeners in an increased viscosity of suspension.

Many types of titanium dioxide cannot be used in high-gloss waterborne paints without grinding the particles in a bead mill. In some cases, paint producers do not have bead mills in their machine park, so they produce high-gloss paints by using a Cowles dissolver. It is very important that the formulator of the recipe select the appropriate type of titanium dioxide for grinding in a Cowles dissolver, ensuring proper dispersibility and obtaining the highest possible gloss level. The selection should be based on data from research and case studies prepared in various devices to ensure grinding to a level that maximizes gloss in paints with low PVC. It is also very important to note the type of titanium dioxide used to produce the level of gloss that was achieved with the resulting coating, along with the related characteristics of hiding power, gloss retention, durability of the coating against solar radiation and weather conditions.

TiO2 classification

Typically, paint formulators use the recommendations of raw material manufacturers in selecting ingredients partly based on how they can be used in paint. It is similar in the case of titanium dioxide. Raw material manufacturers’ recommendations are based on the titanium dioxide grades described in ASTM D476 and ISO 591-1.

Data from these standards can often be found in the technical materials of manufacturers of titanium dioxide pigments. These classifications include a list of different groups of anatase and rutile pigments, which are divided according to the TiO2 content and other pigment parameters; in the case of ASTM D476, they are also divided in terms of use in paints with a diverse range of PVC and paint types in terms of gloss, durability of the coating, etc.

Many additives used in the coatings industry are based on synthetic petroleum-derived raw materials. As concerns grow regarding the sustainability, naturality, and biodegradability of coatings, formulators are beginning to consider replacing non-renewable formula ingredients with alternate additives based on natural and sustainable chemistries.

This article reviews several new developments in natural additive powders and emulsions that offer a more favorable environmental profile while also demonstrating excellent performance properties.

WAX ADDITIVE TECHNOLOGY

Wax additives are an essential part of any ink and coating formulator’s toolkit. Micronized wax powders, dispersions and emulsions can improve the durability of all types of surface coatings, imparting slip, abrasion and scratch resistance, anti-blocking, and rub resistance. Most commercial wax additives are based on synthetic materials including polyethylene and polypropylene. In the past few years, there has been a growing trend to develop formulated systems that contain higher percentages of materials that are biobased, renewable, and/or biodegradable.

CLASSIC BIOBASED WAXES

Historically, the only commonly used wax additive that qualifies as biobased and renewable would be carnauba wax. This natural wax, derived from the Brazilian palm, is freshwater biodegradable based on OECD 302 testing. Dry powders based on carnauba wax can provide slip and lubricity with good film clarity. Emulsions of carnauba wax can improve water beading in high gloss coatings with slip, anti-blocking, and mar resistance.

Another class of wax additive that offers significant biocontent is ethylene bis-stearamide, or EBS wax. This material typically contains approximately 90% stearic acid, which can be derived from both animal and plant sources.

Most formulators prefer an EBS that is based on plant-derived stearic acid, especially for food packaging applications. EBS can be used as the sole wax powder but is often more effective when contained in a composite wax powder, where multiple components are melted together and homogenized prior to milling into an ultrafine powder.

Beyond carnauba wax and EBS, there have been limited options available to formulators looking to increase the natural content of an ink or coating.

NEW BIOBASED WAX TECHNOLOGIES: NANOCOMPOSITES

Micronized carnauba wax powder is a useful wax additive, but the performance attributes can be limited because of the softness and lower melting point of the wax. However, this flaw can be overcome by modifying the natural wax with other more durable materials.

Floor coatings are one of the most demanding end uses that require maximum resistance to scratching and surface damage caused by foot traffic, furniture movement, and even pets. Many high-performance floor coating formulations utilize a protective wear layer that is fortified with hard inorganic particles.

Aluminum oxide has been used to impart a highly durable surface that resists wear, abrasion, and scratching. The Mohs scale is a measure of mineral hardness, ranging from 1 (talc) to 10 (diamond). Aluminum oxide (also known as alumina or corundum) measures a 9 on the Mohs scale, making it an extremely hard and durable substance. With alumina at the surface of a floor coating, dramatic improvements in wear resistance can be achieved.

The structure of fumed aluminum oxide is a complex morphology of tightly fused aggregates of nanosized alumina particles, which subsequently attach to each other into agglomerates that are held together by weak interactions. These agglomerates can be broken down with sufficient shear energy into individual particles that can approach 300 nanometers.

Aluminum oxide is also a very heavy material, with a density on the order of 3.8–3.9 grams g/cc. Since the particles are heavy, they could settle in low-viscosity coating systems, leading to potential inconsistencies in performance when applied. These particles have a very high specific surface area (SSA) and can be extremely difficult to efficiently disperse into coatings. They are also dusty, difficult to handle, and could present health effects if lab or production workers are exposed to airborne dust particles.

Because aluminum oxide particles are so heavy, they require extra energy to get them to a coating surface during the drying process. Following a common wax design concept where HDPE/PTFE composite wax particles are used to get the heavy PTFE to the surface of a coating more efficiently, the heavy alumina can be combined with molten carnauba wax in a high-energy extrusion process, and then micronized to a precise particle size. The result is an alumina/carnauba wax nanocomposite powder, as shown in Table 1.

To compare the improvement in scratch resistance, the carnauba/alumina nanocomposite product was tested against a conventional carnauba wax powder. The wax was dosed at 1% on total formula weight in soft water-based PUD coating (Formula 1) and applied to aluminum panels. The dried panels were tested for pencil scratch hardness using a Taber linear abraser per ASTM D3363 (Figure 1).

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Formulating paints and coatings to provide the right balance of properties is a complex undertaking. Getting the rheology profile correct is just one important task on the formulator’s to-do list, but it is crucial to the ultimate success of the coating.

The rheology profile influences properties of the coating throughout its lifetime, from manufacture, storage, mixing, and application, to the resulting film properties. To control and optimize the rheology of liquid coatings, the formulator typically relies on additives, often referred to as rheology modifiers, thickeners, and thixotropes.

This article reviews some of the basics of rheology and rheological additives and includes comments from a roundtable discussion with industry experts on existing challenges and new developments.

BASICS OF RHEOLOGY

Rheology is defined as the science examining the flow and deformation of materials. When considering fluids such as liquid paints and coatings, the aspect of rheology discussed most often is viscosity. The viscosity of a fluid is a measure of its resistance to deformation under a given stress and is a material property that we associate with the coating as being either “thick” or “thin.” To understand viscosity and its importance for coatings, a quick review of the principles is helpful.

As Isaac Newton did many years ago, consider a fluid between two plates as in Figure 1, where the bottom plate is static, and the top plate is moving at a certain velocity (𝑣) due to an applied force (F). When a force is applied to the liquid, flow of the liquid will occur to relieve the strain from the force.

The shear stress (τ) is defined as the shear force (F) per unit area (A) that results in flow of the liquid and has units of Pascals (1 Pa = 1 N/m²) or dyne/cm². The shear strain (𝑦) describes the deformation of the fluid and is defined as the ratio of the horizontal displacement (ΔL) to the height (h). The velocity of a fluid layer near the top plate will be higher than the velocity of a fluid layer near the bottom static plate.

Shear rate (ẏ) describes the velocity gradient, or the change in liquid velocity per unit height between the shear plates, and is defined as 𝑣/h, where 𝑣= ΔL /t, and t is the time it takes the top plate to move the distance ΔL. Shear rate has units of reciprocal seconds (s^-1). Viscosity, which we already noted is a measure of the resistance of the fluid to deformation, is the ratio of shear stress (τ) to shear rate (ẏ), and is reported in units of Pa•s or dyne•s/cm² (where 1 dyne•s/cm² = 1 Poise (P), and 1 Poise equals 0.1 Pa•s.

When the viscosity of a fluid is constant at varying shear rates, it is said to exhibit ideal or Newtonian viscosity (Figure 2a). Newtonian fluids include simple fluids such as water, solvents, and oils. However, most liquid materials, including paints and coatings, are non-Newtonian in nature, and have a viscosity that changes with shear rate. The viscosity of paints and coatings generally decreases with increasing shear rate, also known as shear-thinning or pseudoplastic behavior (Figure 2b). Shearing the coating will break down structure within the liquid, and lead to a lower viscosity versus when the coating is at rest and unperturbed.

Some fluids, such as high-solids dispersions, can show an increase in viscosity with increasing shear rate, also known as shear-thickening or dilatant behavior (Figure 2c). Generally, dilatant flow is not desired, as it can cause problems during manufacture and in processes such as mixing and pumping.
 

As global environmental regulations continue to tighten restrictions on coatings containing volatile organic compounds (VOCs), the need for hydrolytically stable additives in waterborne coatings has never been greater.

Organofunctional alkoxysilanes are a class of widely used additives in the coatings industry. They act as a bridge between an organic coating and an inorganic substrate, providing adhesion promotion and other important performance improvements.

Given the high moisture sensitivity of organofunctional alkoxysilanes, most silane additives rapidly undergo condensation in waterborne coatings, leading to unworkable viscosities and gelling of the waterborne coatings within the first few weeks or months on the shelf. This has posed a significant barrier to using silane additives in waterborne coatings for all types of applications.

Two organofunctional silane additives that demonstrated positive hydrolytic stability over an extended period of time in waterborne acrylic roof coatings are investigated here.

These organofunctional silane additives include an epoxy-functional silane oligomer, VPS 4721, and an amine-functional silane monomer, Dynasylan^® 1505 (Figure 1).

The oligomeric structure of the epoxy-functional silane oligomer allows for slower hydrolysis and condensation rates in a waterborne system compared to a monomeric epoxy-functional silane (such as glycidoxypropyltrimethoxysilane).

Furthermore, the epoxy groups on this silane oligomer may break open into diols in the presence of water, allowing for further stability in water over time. The presence of only two ethoxy groups and one methyl group on the amine-functional silane monomer allows for a slower, two-dimensional crosslinking mechanism when hydrolysis and condensation occur in a waterborne system. The primary amine group on this amine-functional silane monomer also shows positive stability in waterborne systems.

As the liquid-applied, cool-roof coating market continues to grow (specifically liquid-applied waterborne acrylic cool-roof coatings), this article will focus on the liquid-applied waterborne acrylic roof coating market. It will demonstrate that with the use of stable organofunctional silane additives, several crucial performance characteristics of waterborne acrylic roof coatings can be improved.

The mechanism behind an organofunctional silane adhering to a roofing membrane surface is an important process to understand before investigating the performance of the organofunctional silane additives into the waterborne acrylic roof coatings.

Organofunctional silanes contain a hydrolysable alkoxysilane (Si-OR) functional group that can bond with inorganic surfaces. In this work, the organofunctional silanes to be investigated have silicon functional groups that consist of alkoxy groups, typically ethoxy or methoxy groups.

Organofunctional silanes also consist of an organofunctional group that can react with organic systems, such as an acrylic resin. 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 additive to provide adhesion promotion of a waterborne acrylic roof coating to a roofing membrane, hydrolysis must first take place at the alkoxy sites to form silanol groups. This process occurs within the first few hours of the organofunctional silane being added into a waterborne acrylic roof coating, given the excess water in this type of coating.

When the hydrolyzed organofunctional silane contacts 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 roofing membrane. These siloxane bonds provide the strong adhesion characteristics that organofunctional silanes are known to have.¹

Furthermore, siloxane bonds can form between silanol groups within the waterborne acrylic roof coating upon application, providing an increased crosslinking density for the coating that can lead to improved mechanical properties and other critical roof coating characteristics.

Several key coating properties will be investigated in accordance with ASTM D6083, the standard for liquid applied waterborne acrylic roof coatings.² This includes shelf stability, dry and wet adhesion properties to various roofing membranes, ponding water resistance, dirt pickup resistance, tensile strength, flexibility, and UV weathering resistance.

REFERENCES

CoatingsTech | Vol. 18, No. 9 | September 2021

Pinholes are a problem for formulators developing fast-drying, water-based coatings, especially in coatings cured at high temperature and applied at high wet film thickness. Pinholes are often caused by air or solvent vapor release from the film when the coating is too viscous to flow back and repair the holes. Pinholes also occur in baked coatings, as the trapped vapor regains mobility when the coating softens under heating, before crosslinking hardens the film, preventing flow back into the voids.

Hydrocarbon-based defoamers help eliminate the foam and pinholes in these formulations, but their limited compatibility results in lower gloss, poor leveling and surface appearance. This paper describes a new additive that combines both deaeration of microfoam and modified surface drying for pinhole elimination. This new additive shows comparable pinhole elimination compared with hydrocarbon-based defoamers, but without compromising formulation compatibility or final coating appearance.

INTRODUCTION

Pinholing has been described as “the formation of minute holes in the wet film of a coating material that form during application and drying, due to air or gas bubbles in the wet film that burst, giving rise to small craters that fail to coalesce before the film has set.”¹ These tiny defects disrupt the surface appearance, especially in high gloss formulations, and reduce the protective properties of the finished coating. Pinholes are a problem for both water-based and solvent-based coatings although the cause of the defect may be different. They are also frequently seen in oven-cured coatings when the defect may not appear until the coating is baked. The effect can be seen in Figure 1, which shows the cured surface of a black, solvent-based, automotive OEM basecoat where the coating film thickness increases from left to right.

The most common cause of pinholes is the release of volatile materials or trapped air from the drying film after application. These gases form bubbles in the film that move to the coating-air interface, driven by buoyancy forces or surface-tension-driven flow.² The movement of the bubble will be slowed by the increasing viscosity of the paint film, and a pinhole is formed when the coating cannot reflow to fill the void left behind by the escaping bubble (Figure 2). Bubbles may also remain trapped in the dry film but regain mobility when the coating is heated and softens before crosslinking. As the coating cures, viscosity rebuilds that prevents flow back into the voids. The gas pressure may also be enough to blow through a dry-coating film, either the original primer or a topcoat, in a multilayer application. This can also create pops, craters, and other effects. The defect can also be caused by air or gases released from the substrate (e.g., wood) or even absorbed by the substrate and released on heating.³

There are several ways to reduce or prevent pinholes (e.g. reducing the coating’s film thickness, using slow (co)-solvents), although the mechanisms for bubble release from a film are not fully understood and remain subject for considerable research.^2-7 The reduced film thickness reduces the amount of gas and volatiles present, as well as the distance and time needed for the bubbles to escape. Slow (co)-solvents delay the drying of the film and may also slow the buildup of viscosity, again allowing more time for the bubbles to escape. Reducing film thickness may result in insufficient hiding and protection or require multiple applications to achieve the required total film thickness, which may also lead to pinholes. Changing the (co)-solvent package can affect drying properties and conditions but may also not be possible within regulated VOC (Volatile Organic Component) limits or local environmental regulations.

Defoamers and antifoams are also used to control pinholes, although these additives can cause other problems, such as craters, fisheyes or poorer substrate adhesion, and/or intercoat adhesion.^8-9 Defoamers can also influence gloss, orange peel, and depth of image—critical properties in automotive coatings.¹⁰ Therefore, careful defoamer selection is required when choosing additives for pinhole control in such coatings.

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A new silica matting agent has been developed, featuring a mercapto silane-modified surface. Unlike traditional organically modified silicas, which utilize adsorbed waxes on the surface, silane modification offers a new opportunity to introduce reactive functionality into silica additives for coatings. With judicious pairing of surface functionality and resin chemistry, this functionalized surface opens the potential to react with the resin matrix. This paper will highlight this new class of functionalized silica for coatings and outline areas of potential application, with specific emphasis in cobalt (Co)-free alkyd formulations.

INTRODUCTION

Micronized, synthetic, amorphous silicas (SAS) are ubiquitous additives in the coatings marketplace for gloss reduction, rheology modification, and corrosion protection.¹ SAS have historically been a choice coating additive due to their high tailorability, ease of dispersion, and compatibility across a wide range of formulations. As a class, SAS are made of three major types of silica: precipitated, gel, and fumed; each provides their own distinct advantages and disadvantages in formulation.² Of these, precipitated silicas are particularly versatile as they are produced through a neutralization reaction of sodium silicate (water glass) and strong acids (e.g. sulfuric or hydrochloric). Through the modification of precipitation parameters, one can target different surface areas, pore structures, and grind resistance in the resultant silicas. Both surface area and pore structure have implications on binder demand in coatings formulations and will impact overall viscosity. Grind resistance refers to the structural integrity of the particles when energy is applied (i.e. milling).

The surface chemistry of SAS also plays an important role in formulation. While SAS surface chemistry is difficult to modify during synthesis, post-synthetic modification of the surface is traditionally done with the adsorption of wax materials. Importantly, these wax treatments aid in the settling and re-dispersion properties of SAS additives, key factors in formulation stability. Waxes can also impart other properties into the coating ranging from hapticity, mar/burnish resistance, and hydrophobicity.³ While a useful technique for surface modification, it only allows for physisorption of materials, risking disassociation of the surface treatment from SAS after formulation or when the system is subjected to shear.

While untreated and wax-treated SAS dominate the portfolios for coatings-grade additives, PPG has developed a versatile surface treatment methodology. Due to the synthetic conditions used during the precipitation, there is an opportunity to introduce reactive moieties into the batch to covalently modify the silica surface. Using this approach, we are able to introduce reactive triethoxy silanes into the precipitation to perform an in situ covalent surface treatment. In a typical process (Figure 1), a triethoxy silane is introduced to the aqueous mixture of partially neutralized sodium silicate. As the precipitation matrix is typically alkaline in nature, hydrolysis of the silanes is catalyzed to convert the ethoxy moieties to silanols (Figure 1a). In a simultaneous process, the newly de-blocked silanols can undergo condensation with silanols at the silica surface (Figure 1b). With the condensation complete, the remaining processing steps of filtration, drying, and milling can occur to achieve a unique functionalized silica additive. Depending on the solubility of the silane, a secondary “compatibilizing” agent can also be added to aid in dissolution.

For this current development opportunity, we chose to utilize a mercaptosilane as our functionalizing material. Unlike traditional wax treatment, wherein the surface is treated with a non-functional wax, this material is functionalized with reactive thiols. As it pertains to the functional group, thiols offer myriad potential synergies in coatings. Specific areas of interest are formulations that contain unsaturation (i.e., alkyd, UV-curable) and/or isocyanate functionality (i.e., polyurethane, polyurea). This article will focus on applications with alkyds, emphasizing formulations utilizing cobalt-free driers.

Alkyd resins are generally comprised of polyols functionalized with poly-unsaturated fatty acids. These poly-unsaturated units are key to the auto-oxidative cure mechanism that alkyds undergo for film formation.⁴ Alkyds themselves will undergo this auto-oxidative process under ambient conditions; however, this will occur over a long time frame (>24 h).

To accelerate cure, metal soaps/carboxylates are used to catalyze the auto-oxidation reaction, reducing drying times to less than 8 h. Until recently, the standard drying agents utilized in alkyd formulations were Co-based complexes due to their superior activity. As regulations continue to evolve, many of the Co driers have been identified as potential carcinogens and reprotoxins.⁵ To that end, formulators have been challenged to find non- or less toxic replacements for long-held industry standard driers. Not surprisingly, many of these driers alone are less active as compared to Co. Blends of various primary and auxiliary driers are necessary to achieve the required drying performance.

As drier chemistry continues to evolve, formulators are challenged in matted systems as driers can adsorb to the matting agent surface and become deactivated, leading to increased dry times. SAS are excellent matting agents, but they also introduce large amounts of surface area into the formulation that can increase the probability of deactivation. Surface-treated SAS are obvious candidates for matting agents in alkyd systems due to their blocked surfaces. Unlike a traditional wax-treated SAS, the silane-treated materials have no chance to lose surface treatment via desorption due to the synthetic pathway and covalent nature of the modification. With the enhanced surface modification, these SAS additives tend to have reduced binder demand and are less prone to indiscriminate adsorption of formulation additives (i.e., drier chemistry). With the introduction of thiol moieties, this also opens the possibility of thiol-ene type reactions to occur during the cure process in alkyd systems. To evaluate our new matting agent, a series of tests focused on dry time, viscosity, gloss reduction, and weathering were conducted to compare silane-treated SAS to the industry standard SAS matting agents.

GENERAL EXPERIMENTAL DETAILS

In cases where coated substrates are used, pine panels are coated to target a dry film thickness of 45-55 mm. Drying time evaluation on wood panels is done directly with a finger test for tackiness. Viscosity is measured using a Haake rotational viscometer equipped with a SV1ST spindle. Gloss is measured directly on wood substrates using a Byk Mirco Tri Glossmeter. The accelerated aging test involved exposure of doubly coated panels to continuous ultraviolet (UV) light radiation (UVA-340 lamp) and condensation in a weathering chamber. Coated panels were exposed to 8 h of illumination dry at 60 °C, followed by 4 h at 50 °C with 100% humidity.

RESULTS AND DISCUSSION

1. Drying Time

A screening study of various untreated and treated SAS was designed to compare drying times with alternative drier blends. Table 1 lists the generic formulas for the alkyd systems. Aside from our silane-treated SAS, a gel, thermal, wax-treated, and fumed SAS will also be screened in this formulation.

Drying time was measured for 2 applications on pine panels, with a target dry film thickness of 45-55 μm for each application. Overall, the silane-treated SAS demonstrated consistently low drying times (<6 h) in both drier formulations (Figure 2). Thermal-treated SAS demonstrated commensurate performance to the silane-treated SAS. As this material relies on heat to modify the surface silanol concentration, one can imagine less deactivation may occur with these additives. However, for fumed SAS, with the lowest silanol density, the Fe formulation does not cure, whereas the Mn formulation hasan excellent dry time. Most surprising is the performance with wax-treated SAS, as they are most akin to the silane-treated SAS. In both Fe and Mn, films did not cure within 8 h, whereas silane-treated SAS formulations cured in less than 6 h. This ability to work universally across various drier compositions is a unique property of the silane-treated material.

2. Viscosity

While drying time is important, formulation viscosity is another key parameter for proper film application. Of the drying times, silane-treated and thermal silica were the best-performing additives. The two materials diverge in performance when viscosity is evaluated (Figure 3). Formulation viscosity in both Fe and Mn systems with silane-treated SAS are roughly 50% less that of thermal SAS, similar to fumed SAS. While gel and wax-treated silicas have similar viscosities to the silane-treated SAS, the drying times are unacceptable. The unique behavior of silane-treated SAS in these alkyd formulations is a powerful tool for formulators when building a drier system. Not only can one achieve low drying times with low viscosity—these can be achieved in multiple drier systems interchangeably.

3. Gloss Reduction

As formulation parameters go, the silane-treated materials have been shown to enhance the cure of Co-free alkyd systems relative to the industry standard SAS matting agents. To compare the post-application performance, matting efficiency was measured for the various silicas in both Fe and Mn drier formulations. The gloss at a specular angle of 60° (60° gloss) was measured after the initial drying of two applications on pine panels, and again 14 days post-application (Figure 4). Between initial and 14 days, all formulations showed slight decreases in gloss value. The silane-treated SAS demonstrated excellent matting efficiency with 60° gloss <57 in both formulations. Compositionally, the closest comparison, wax-treated SAS were the poorest-performing matting agents with gloss values nearly double that of the silane-treated SAS material. Thermal SAS was comparable to silane-treated SAS, with only slightly higher gloss in the Mn formulations. Both gel and fumed SAS struggled with matting in Fe formulations but had acceptable performance in the Mn system. The matting data demonstrates the superior nature of the silane-treated SAS in Co-free alkyd systems when compared to other industry standard SAS matting agents, not only delivering excellent matting efficiency, but also formulation flexibility for drier selection

4. Initial Weathering Test

As a final evaluation of performance, accelerated aging tests were completed using a QUV-A accelerated weathering test to evaluate the durability (i.e., gloss retention) as a function of UV-A exposure.⁶ In the case of Fe-based compositions (Figure 5), the worst-performing materials for gloss retention were wax-treated and fumed SAS. Over the course of 360 h of exposure the two systems retained less than 20% of their initial gloss values, with fumed only retaining 6%. The gel and thermal SAS materials stand out as outliers in this study with considerable gloss reductions from 0 to 120 h, followed by an unusual gloss increase from 120 to 360 h. Using the 120 h data points for limited insight, gel SAS retained 73% while thermal only retained 45%. The silane-treated SAS, which has a gloss retention of 76% at 120 h and 64% over 360 h, demonstrated robust durability to accelerated aging.

Many of the same trends were observed in the Mn-based formulations (Figure 6). Again, both wax-treated and fumed SAS performed poorly with regard to gloss retention, with less than 30% retained in both cases. Again, the gel and thermal SAS had unusual behavior upon extended exposures. Using the 120 h mark as an initial indicator of performance, these show the reverse performance as compared to the Fe-system. Gel SAS has a gloss retention of 50% while thermal maintains 72% of the initial gloss at 120 h QUV-A exposure. Comparable to the Fe-based compositions, the silane-treated SAS in Mn-based compositions retained 60% initial gloss over the full 360 h exposure cycle. Taking the Fe- and Mn-based compositions together and keeping in mind the unusual behavior of the gel and thermal SAS, the silane-treated SAS had better gloss reduction and consistent gloss retention across both formulations. Future studies are planned to further explore the gel and thermal silicas to better understand the cause of the gloss increase with extended exposure.

CONCLUSION

In summary, we have shown the application of a thiol-containing silane-treated silica as a functional matting agent in alkyd-based coatings. This silane-treated silica has significantly enhanced the properties of a non-Co drier alkyd system. Co-free formulations utilizing this new silane-treated SAS have been enhanced with robust and fast drying times, low viscosity, high matting efficiency, and predictable durability as compared to other industry-standard SAS matting agents. The studies presented here have enabled formulators to extend the use of silane-treated silica matting agents to a variety of other Co-free drier alkyd systems. Work is ongoing to further understand the interactive mechanisms of this differentiated coatings additive in resin matrices. Outside alkyd formulations, additional studies are underway to explore advantages in other unsaturated systems (e.g., UV-cure formulations) and will be disclosed in a future report.

Acknowledgements

The alkyd resin (QUL 5120), starting point formulations, and testing data were all generously provided by Stefan Bomballa of Synthopol. His work and insight were greatly appreciated in the development of this material.

References

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2. Iler, R. K.��Chemistry of Silica—Solubility, Polymerization, Colloid and Surface Properties and Biochemistry.John Wiley & Sons: New York, 1979.
3. Bower, J.D. Waxes. In Coatings Technology Handbook; Tracton, A. A., Ed.; Taylor & Francis Group: Boca Raton, FL, 2006; pp 66-1–66-6.
4. Van Gorkum, R., and E. Bouwman. Coord. Chem. 2005, 249, 1709–1728.
5. De Boer, J. W.; Wesenhagen, P. V.; Wenker, E. C. M.; Maaijen, K.; Gol, F.; Gibbs, H.; and R. Hage. Eur. J. Inorg. Chem. 2013, 3581–3591.
6. The accelerated aging test involved exposure of doubly coated panels to continuous UV light radiation (UVA-340 lamp) and condensation in a weathering chamber. Coated panels were exposed to 8 hours of illumination dry at
   60° C, followed by 4 hours at 50° C with 100% humidity.

FEBRUARY 2021 | VOL. 18, NO. 2