Images courtesy of BioPowder

The company has been creating particles and powders from , a byproduct of the olive oil industry, to provide an alternative to traditional mineral-based coating additives.

Kathrin Schilling, BioPowder’s founder and director, says the shift toward biobased materials is gaining momentum, especially in the decorative coatings market. While some technical coatings still face challenges in this transition, decorative applications have found new opportunities in plant-based products. Schilling notes that the company’s work with olive stones offers coatings manufacturers a chance to create products that are not only more sustainable but also competitive in terms of performance and cost.

The company says the use of upcycled olive stones, BioPowder’s flagship product, offers benefits in both functionality and environmental impact. Olive stones come from a carbon-neutral ecosystem, where the carbon emissions from food production and byproducts are offset by the olive trees’ carbon absorption. The particles are lightweight, durable, and compatible with a variety of , including biobased epoxy, polyurethane, and acrylic formulations. These properties make them suitable for improving abrasion resistance, durability, and curing times in coatings.

BioPowder has also expanded its offerings to include biobased color particles, allowing coatings manufacturers to customize the appearance of their products. The company says it works closely with manufacturers to adjust particle properties, such as viscosity and texture, for specific applications.

In a recent collaboration, BioPowder partnered with Austrian company F/LIST, known for its luxury coatings used in private jets and residential projects. F/LIST’s R&D division, F/LAB, developed a biobased laminate called F/LAB Aenigma Eco, which incorporates upcycled olive stones alongside other sustainable materials such as plant-based pigments and mother-of-pearl. The laminate is designed for high-end decorative applications, including wall coverings and furniture.

Schilling says the project demonstrates the potential for biobased innovation in the coatings industry, combining traditional craftsmanship with cutting-edge material science. “Our mission is to help companies transition to biobased product lines,” Schilling says. “We aim to solve the challenges that come with reducing environmental impact, and our helps accelerate the design and development of new eco-friendly coatings.”

To learn more about BioPowder and its work with biobased ingredients, visit .

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

Alkyd resins are oil-based polyesters created by reacting alcohol and acid, which can further be modified through the addition of unsaturated oils. These oils can vary in length and branching, which will create different coating properties, allowing for high diversity and tailoring for specific applications. Alkyds form a film by undergoing a crosslinking reaction in the presence of oxygen in the surrounding air at sites of unsaturation in the fatty acids. If unaided, this autooxidative process can be extremely slow.

The application of alkyds spans a multitude of different market segments, from architectural to industrial, and can be used on various substrates, including wood and metal. The versatility of these resins keeps them relevant in the coatings market as new chemistries are continually being developed. Since the original synthesis in the 1900s, alkyds have faced increasing regulations, driving developments of more sustainable alkyd chemistries.

It is well documented that the emission of potentially harmful VOCs has resulted in an increase in the number of health problems such as asthma, allergies, and other breathing problems as well as environmental concerns associated with global warming.

The health and environmental hazards related to these potentially harmful VOCs have prompted government agencies to enact stringent regulations. The increase in public awareness regarding environmental effects and health issues drives the demand for low-VOC paints and coatings, which has led to the development of low-solvent, solvent-free, and waterborne coatings. Low-VOC paints and coatings typically use water as a carrier instead of petroleum solvents. In some cases, specialized solvents, such as de-aromatized (aromatic content less than 1%) solvents, are used to reduce VOC content.

To comply with these increasing regulations, there are three paths that formulators can choose when working with alkyd coatings.

The first path is to increase the solids content, reducing VOCs in solventborne alkyd formulations. This is the most straightforward choice, as it typically requires a minimal amount of reformulation. Resin manufacturers can increase solids in two ways. Manufacturers can keep the same molecular weight and reduce solvent content. However, viscosity will increase, leading to potential differences in application performance and increased film thickness, resulting in wrinkling. To keep viscosity similar, manufacturers can decrease the molecular weight of the resin and solvent content, but this could negatively affect drying and performance properties.

The second path is to move from a pure solventborne alkyd to a water-reducible alkyd. Formulators often choose this path because of the similarity in paint application to high-VOC solventborne alkyds. Manufacturers can also continue to use the same industrial equipment, and the resin is easy to produce. Water-reducible alkyds are created by incorporating carboxylic-acid groups into the structure. These acid groups are then neutralized with amines to enable solubility in water. Although these types of alkyds are high in solids, 70–75% by weight, they still have relatively high levels of VOCs, primarily hydrophilic glycol ether solvents.

The third path is to use waterborne alkyd emulsions. This technology was engineered to have extremely low VOC levels that are significantly lower than water-reducible or high-solids solventborne alkyd coatings. Initially, waterborne alkyds did not have the same chemical resistance, ease of application, or shelf stability as solventborne alkyds. However, through years of development and adjustments, the properties of waterborne alkyds have become like that of solventborne alkyds. This is due in part to utilizing the correct additives in the full coating formulation to increase corrosion resistance, shelf stability, and other performance properties. This can be a difficult transition for coating manufacturers because nearly all materials and additives will need to change from what is traditionally used in high-VOC solventborne coating formulations.

All three options have pros and cons (Figure 1), as well as potential adverse consequences when the VOC level of a formulation is reduced. Longer dry times, reduced shelf life, wrinkling, and increased formulation complexity are common challenges formulators need to overcome with these technologies. By utilizing HPCs, formulators are achieving low-VOC targets while maintaining paint quality and meeting customer demands.

HPCs are a unique technology that can outperform typical metal carboxylates in oxidatively cured alkyd systems. They are patented organometallic ligand technologies (WO2012093250A1) that work differently from traditional metal driers. There are many environmental benefits such as being cobalt-free, APEO-free, and CMR-free materials. HPCs are also compliant with the European Union’s Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH). These catalysts are globally registered and can be used in both solventborne and waterborne applications, making them highly versatile. This article will discuss one case study from each of the three low-VOC alkyd formulation paths that showcase the use of HPCs to overcome performance challenges.

Biobased materials are increasingly important in the coatings industry as more companies align their sustainability goals to reduce environmental footprints and develop more eco-friendly products to meet customer needs. However, widespread adoption of biobased materials in coating applications can be challenging due to the lack of available biobased materials that are both cost-effective and performance competitive.

To solve these challenges, we developed a series of cost-competitive biobased epoxy resins using distilled tall oil (DTO) as the feedstock. DTO, a bio-refinery product derived from crude tall oil, is a byproduct of the pinewood pulping process and is 100% biobased. DTO is a mixture of tall oil fatty acids (TOFA)—mainly oleic acid and linoleic acid—and tall oil rosin acids (rosin)—mainly abietic acid and its isomers. Figure 1 shows the molecular structures of the main components in DTO.

The properties of these novel DTO-based epoxy resins can be tuned by changing the ratio of TOFA to rosin in DTO. The three DTO-based epoxy resins listed in Table 1 are amber color liquids at room temperature, have a biocontent range of 40% to 50% and an epoxy equivalent weight (EEW) ranging from 500 to 700 g/eq. They are designed for use in applications such as coatings, composites and adhesives, and performance enhancement(s) can be achieved with proper formulation. This article will demonstrate how these novel DTO-based epoxy resins can help improve miscibility and compatibility, water resistance, adhesion, flexibility, impact resistance, and chemical resistance in 2K epoxy coatings.

EXPERIMENTAL SECTION

Coating model formulations
The 2K epoxy coating formulations for this study are listed in Table 2. The name of each formulation represents the combination of the epoxy resins used. For example, the first formulation that only contains diglycidyl ether of bisphenol A (DGEBA) (EEW = 187 g/eq) in its epoxy part is named as DGEBA and used as the control. The other three formulations, EP100/DGEBA, EP125/DGEBA and EP150/DGEBA, all have a mixture of 50% by weight of DGEBA and 50% by weight of one of the DTO-based epoxy resins listed in Table 1. The amine hardener used in this study is Versamid 140, a polyamidoamine hardener from Gabriel. In each formulation, the ratio of epoxy to amine hardener was kept at 1:1 equivalent ratio.

Characterization
The mixing study was conducted at room temperature (23 ± 2 °C) on a 150g formulation scale with a mechanical stirrer rotating at 300 rpm. For the curing behavior study, 150g of Part A (epoxy) and Part B (hardener) mixture in a plastic cup was placed in a 25 °C water bath and the viscosity and temperature of the mixture were closely monitored to obtain the gel time and exothermic peak temperature. The viscosity of the mixture was monitored by a Brookfield CAP 2000+ viscometer and the point at which the viscosity reached the maximum limit with a #03 cone spindle at 50 rpm and 50 °C was defined as the gel time.

For tensile, dynamic mechanical analysis (DMA), Shore D hardness and water absorption test samples, the formulation mixtures were first cured in silicon molding at room temperature overnight and then removed from the silicon mold and post-cured at 100 °C for two hours. The tensile test was performed on an Instron 3365 universal testing machine at a crosshead speed of 10 mm/minute according to ASTM D638.

The DMA test was conducted using a 3-point bending geometry on a TA Instruments DHR-2 rheometer at a heating rate of 2 °C/minute and the tan delta curve maximum was used as the glass transition temperature (Tg) of each cured sample. The Shore D hardness was measured with a Shore D durometer.

The water absorption test was carried out by immersing a cured sample (44mm x 13mm x 3mm in dimension) in water at room temperature. Every three or seven days, the sample was taken out and water on the sample surface was wiped off before measuring the weight gain. The water absorption percentage after t days of water immersion was calculated as:

where M₀ and M_t are the initial sample weight and the sample weight after t days of water immersion, respectively.

The 2K epoxy formulations listed in Table 2 were coated onto aluminum (type A, 3003 H14 alloy, smooth mill) and steel (type R, cold-rolled steel, dull matte) Q-panels using a 6-mil drawdown bar for coating-performance evaluation. The coating dry time was recorded with a GARDCO DT-5040 quadracycle electronic dry time recorder (ASTM D5895) on a 3-mil wet film drawn down on a Lenata 2A opacity chart. All Q-panels were cleaned with acetone and dried for 15 minutes before applying coatings.

The coated panels were kept at room temperature to cure for seven days with some coated panels going through an additional two hours of post-cure at 100 °C if needed before characterization. The dry film thickness was roughly 3 mils. The coating flexibility was measured with a TQC mandrel bend tester (ASTM D522). Impact-resistance measurements were performed with a BYK Gardner 1102 impact tester (ASTM D2794).

The tape test method (ASTM D3359) was used for coating adhesion evaluation. The chemical resistance of each coating sample was evaluated with a spot test method. Nearly one milliliter of each of the testing chemicals was placed on the surface of a coated panel and covered with a watch glass. Each chemical droplet was kept in contact with the panel for 24 hours and then the contact area was examined for damage after the residual chemical was wiped off.

In the North American coatings market, traditional solventborne resins are still used to a large extent, enabled by regulations that allow for the use of certain solvents exempt from restrictions placed on most volatile organic compounds (VOCs) in the United States. The future of the exempt status of parachlorobenzotrifluoride (PCBTF) and tertiary butyl acetate (TBAc) is a concern in certain districts of California, and other regions may follow suit.

Coating companies are ramping up their research efforts in formulating paint without exempt solvents. Researchers are opting to formulate with high solids resins.

High-performance acrylic polyols were studied to understand the effect of various resin parameters on coatings performance. A set of resins with 80-90% solids was tested in pigmented topcoat formulations.

These topcoats contained VOCs ranging from 200 to 250 g/L, without the use of exempt solvents. Various conventional and advanced film properties were evaluated, including weathering performance by xenon arc exposure, cure kinetics using infrared (IR) spectroscopy, and crosslink density by dynamic mechanical thermal analysis (DMTA).

Correlation was obtained between coatings performance properties and resin characteristics such as equivalent weight, glass transition temperature, etc. The understanding of structure property correlation offers tremendous value not only for North American coatings researchers but also to the global community.

INTRODUCTION

The U.S. Environmental Protection Agency (EPA) has regulated the VOC limit to 450 g/L in the United States, and Canada has a restriction of 340 g/L for industrial metal (IM) topcoats. The California Air Resource Board (CARB) has a limit of 250 g/L and many other states, such as Utah, are following suit.

In North America, formulators can use exempt solvents (e.g. PCBTF, TBAc, acetone, etc.) to achieve a lower VOC of 250��g/L. Among these, PCBTF has more favorable characteristics than other exempt solvents, as it evaporates slowly, has a higher flash point, and is therefore less flammable than many other exempt solvents such as acetone.

However, effective June 28, 2019, the Office of Environmental Health Hazard Assessment (OEHHA) added PCBTF to the list of chemicals known to the state of California to cause cancer for purposes of Proposition 65. In addition, the South Coast Air Quality Management District (SCAQMD) is considering delisting PCBTF from the list of exempt solvents.

As the future of PCBTF as an exempt solvent is a concern, it is beneficial if new coatings are formulated without the use of it. If coatings are formulated without any exempt solvent, paint formulators don’t have to reformulate when the exempt status of any solvent is changed. Formulating without the use of exempt solvent has one more advantage, especially for global companies, which is that one formula can be used throughout the global market.

It is a challenge to achieve performance of low-solids solventborne coatings with high-solids coatings. Among all the coatings components, resin plays most important role in lowering the VOC. This requires higher-performing, lower-molecular-weight resins.

Two component (2K) polyurethane chemistry provides excellent durability, chemical resistance, appearance, and speed of cure, which makes the urethane technology a valuable tool for the coatings industry^1-2 (Figure 1). In both protective and industrial maintenance (IM) coatings, solventborne 2K systems are predominantly used. For the same reason, this study focuses on two component (2K) polyurethane chemistry^3-4.

There are various synthesis techniques, such as group transfer polymerization (GTP), atom transfer radical polymerization (ATRP), and reverse addition fragmentation chain transfer (RAFT), that give excellent control over acrylic polymer architecture to produce high-performing resins^5-7.

These techniques pose some challenge in manufacturing and are not competitive from a commercial aspect. A commercially competitive polymerization process, controlled molecular structure polymerization (CMSP), has been developed that gives excellent control over molecular weight and polydispersity⁸.

If oligomeric polyols are synthesized by conventional route, a few oligomers will not contain hydroxyl functionality, and those oligomers will act as a plasticizer, degrading the film performance. The CMSP process gives guaranteed functionality to oligomeric polyols maintaining high performance of the resin.

To achieve good film performance, a balance of solids (which is indirectly related to molecular weight), equivalent weight, and glass transition temperature (T_g) is necessary. Exterior durability is dependent on the crosslink density (XLD) of the film as well as the monomer composition of the resin.

This paper is a presentation of results from coatings evaluation using resins with solids ranging from 80-90% by weight, hydroxyl equivalent weights (HEW) from 230 to 400, and glass transition temperatures (T_g) from 2 to 15��ºC.

REFERENCES

1. American Coatings Association. /wp-content/uploads/dlm_uploads/2019/12/aim-voc-map-may-2019.pdf (accessed April 26, 2021).
2. California Office of Environmental Health Hazard Assessment. https://oehha.ca.gov/proposition-65/crnr/chemical-listed-effective-june-28-2019-known-state-california-cause-cancer (accessed April 26, 2021).
3. Wicks Z.W.; Jones F.N.; Pappas S.O.; Wicks D.A. Organic Coatings: Science and Technology, Third Edition, John Wiley & Sons, 2007.
4. Goldschmidt A.; Streitberger H.J. Automotive Refinishing. In BASF Handbook on Basics of Coatings Technology; Vincentz, 2003; pp 710-717.
5. Webster O.W. Group Transfer Polymerization: A Critical Review of Its Mechanism and Comparison with Other Methods for Controlled Polymerization of Acrylic Monomers. In Advances in Polymer Science; Springer, 2004, 167; pp 1-34.
6. Coessens V.; Matyjaszewski K. Fundamentals of Atom Transfer Radical Polymerization Chem. Edu. 2010, 87(9); pp 916-919.
7. Semsarila, M.; Perrier S. ‘Green’ reversible addition-fragmentation chain-transfer (RAFT) polymerization. Nature Chem. 2010, 2(10); pp 811-820.
8. Bzowej E.; Shalati M.; Haldankar G.; Brinkhuis R.; Elfrink P. Controlled molecular structure polyols. In Proceedings of the Annual Meeting Program of the FSCT, 2004, 82nd, 12/1-12/17.
9. Mestach D.; Gaans A.; Vandevoorde P.; Buser T.; Haldankar G.; Shalati M. Optimization of the pot life / drying time balance for polyurethane coatings based on high solids acrylic polyols. In Proceedings of the Annual Meeting Program of the FSCT, 2004, 82nd, 11/1-11/18.
10. Haldankar G.; Shalati M.; DeGooyer W.; Gessner M.; Bosma M.; Brinkhuis R.; Vijerberg C. Novel rheology control agents. JCT CoatingsTech 2008, 5(6); pp 38-43.
11. Bosma M.; Haldankar G.; DeGooyer W.; Shalati M. Microgels as additives for controlling sag-leveling properties. In Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium, 2002, 29th, pp 395-408.
12. Hill L. Calculation of crosslink density in short chain networks. Progress in Organic Coatings 1997, 31(3); pp 235-243.

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

Specialty chemicals impact the performance of products they are used to manufacture, either during processing or when in use. Whether single compounds or mixtures of chemicals, specialty chemicals exhibit unique functionality that can benefit many different types of end uses, such as biocides, or are designed for use in a particular market segment, such as paints and coatings. In 2018, the five largest specialty chemicals segments—specialty polymers, industrial and institutional cleaners, electronic chemicals, surfactants, and flavors and fragrances—accounted for 37% of the global market, while the 10 largest segments accounted for 63% of total annual specialty chemicals sales, according to IHS Markit. Overall, Grand View Research estimates the global specialty chemicals market is expanding at a compound annual growth rate (CAGR) of 3.7% (2016–2027) and predicts it to reach a value of $844.2 billion in the forecast period.

More specifically, the American Coatings Association-published��Global Market Analysis for the Paint and Coatings Industry (2019–2024),��prepared by the ChemQuest Group, anticipates specialty polymers and additives to be among the fastest-growing specialty chemical segments��through 2022.��One of its more than 125 figures charts��the five-year (2017–2022) value growth of 13 specialty chemicals markets, including specialty coatings valued at ~$32 billion with a CAGR of 3.8% (2017–2022). In contrast, the same chart shows specialty polymers for high-performance thermoplastics (valued at $6 billion)��will potentially outpace��growth of��all other specialty chemical markets at a��CAGR of��5.6%��(2017–2022).

Top Trends in Coatings: Complexity, Sustainability, Performance, and Cost

Specialty chemicals are added to paint and coating formulations to bring functionality to applied films. “It is amazing how many things are required to happen in such a thin layer,” notes Rosanna Telesca, leader of the market center coatings at SONGWON Industrial Group. “A key trend, therefore, is to increase the functionality of coatings while simplifying the raw materials and the formulations,” she says. “In addition to being more sustainable and safer, specialty chemicals for coatings today need to be more versatile and support their use in a large number of different formulations used in a broad range of applications.”

The top trends driving innovation remain performance, simplicity, and sustainability, all of which are aligned with the delivery of benefits that paint and coating manufacturers demand as they formulate advanced products to meet end-user needs, agrees Dan Latas, Lubrizol performance coatings marketing manager. Performance relates to both protection and appearance, while simplicity involves reducing the amount of surface preparation required and the number of coats and level of maintenance required, as well as making coatings easier to apply. Sustainability goes beyond lower volatile organic compounds (VOCs) to include the elimination of other harmful chemicals, use of biobased raw materials, and the production of compostable, biodegradable, and recyclable products.

Both the industry and its customer base continue to demand higher performing products that provide market differentiation while being less harmful to consumers, workers, and the environment, and exceeding all government regulations, asserts James Bowler, senior industry marketing manager for W. R. GRACE Coatings Additives.

Specific performance attributes receiving attention today, according to Scott Grace, head of regional technical services for Covestro LLC, include improved application and curing efficiency (e.g., faster drying, elimination of paint layers, and faster curing and property development).

Ruediger Mertsch, head of applied research and technology for automotive/ transportation at Evonik Coating Additives points out that reduction of complexity can also take the form of one-component (1K) rather than two-component (2K) systems and use of just one hardener for both the primer and topcoat.

For industrial applications, for example, there is growing interest in direct-to-metal (DTM) coatings, because they reduce time and complexity for lower labor costs and energy intensity, notes Diana Rowe, North America marketing manager for industrial coatings with BASF. “Although a small percentage of the industrial coatings market, DTM coatings comprise one of the fastest-growing segments due to the cost savings for the final system,” she explains.

Reductions in the energy levels required to flash-off, dry,
and fully cure coating layers are driving innovations like
lower-temperature curing coatings for automotive
OEM paint build-ups.

All of these issues are being tackled as the trend to increase environmentally friendly systems occurs, including solvent-free applications such as UV or powder coatings, high-solids solvent-based systems, and 100% waterborne products. Consumers still expect premium paints with eco-friendly attributes to be coupled with excellent performance, such as hardness, scrub resistance, UV stability, low blocking, and dirt pick-up resistance, according to Julie Vaughn Biege, global business development director from Emerald Kalama Chemical.

This situation created significant challenges because many lower-volatility technologies are associated with performance trade-offs. High-performing additives are needed to adjust the final properties in the liquid and cured coating, says Stefan Moessmer, head of business line paint additives at BYK. Grace adds that reductions in the energy levels required to flash-off, dry, and fully cure coating layers are driving
innovations like lower-temperature
curing coatings for automotive OEM paint build-ups. “In addition, formulators look for cost savings with comparable or improved performance, but continue to drive towards greater coatings performance and improved durability,” he says.

Ultimately, says Shiona Stewart, North America director for additives at BASF, a fine balance must be achieved between meeting a customer’s needs for performance improvements while also delivering the cost savings required to incentivize adoption.

Resins and Additives

Innovation in resin and additive technologies, according to Geoff Webster, technical associate at Eastman, is also taking place to meet high-solids requirements and the push to lower costs and carbon footprint. “Together, these trends are driving new technologies to extend service life with enhanced properties (i.e., weathering, corrosion resistance, etc.) while lowering solvent emissions for industrial coatings,” adds Olivia Clinard, market development manager at Eastman.

Customers are more often considering the total application costs, looking at the entire lifecycle of the coating instead of simply the upfront cost, according to David McNeece, principal product group specialist for amines with Ascend Performance Materials. “This shift is an acknowledgement that there is value in systems that perform well over extended use, as well as coating formulations that reduce downtime by being easier to install, cure faster, or in general go into service faster.”

Of course, new technologies that enable lower upfront costs are welcomed as well, observes Webster, such as new resins for ultra-high-performance weathering coatings systems that provide similar performance to fluorinated resins but at lower cost. In addition to sustainability, the automotive coating sector is buffeted by trends in urbanization and mobility, which are defining the future needs of specialty chemicals and coatings, according to Dan Kersting, global business director for additives with allnex. “Application needs today are being driven by mobile and digital consumers—drivers that were not considered at all in coatings development cycles just five to years ago,” he asserts.

Macro trends like ACES (Autonomous, Connected, Electrification, Shared) result in several key areas of innovation, from the need for lower temperature bake coatings for electric vehicles (EVs) to demand for self-cleaning coatings to enhance LIDAR camera systems, according to Eastman market development manager, Tom Klug.

The growing shared vehicle network also means cars are on the road for more miles with more exposure to the elements. “When next-generation vehicle coatings hold their OEM factory-applied appearance longer, the shared driving experience is improved for passengers and trust is established with vehicle owners,” Webster explains.

That is true for the interior as well, according to Sebastian Prock, global marketing segment leader for industrial applications in Clariant’s Industrial and Consumer Specialties business. “Continuous improvement of scratch resistance for interior coated plastic parts is becoming increasingly relevant as, particularly with the quick development of self-driving technology vehicles, expectations are for interiors to provide more of a living space experience for passengers,” he says.

There is continued regulatory, consumer and brand pressure to move away from Bisphenol-A (BPA) in food-contact applications.

In the food packaging sector, recyclability and sustainability are top trends leading to greater use of metal cans, particularly for beverages. “Cans are recycled 2.5 times more than any other packaging option,” notes Lin Feng, principal scientist at Eastman. She adds that the COVID-19 pandemic has driven a significant increase in metal can use for food due to the storability and long shelf-life afforded by metal containers.

Meanwhile, in construction, environmental friendliness and increased durability remain important, with a clear focus on renovation and restoration vs new construction, according to Eric Dumain, global marketing director for liquid resins at Arkema. “For all jobs, though, productivity tops the list of trends. Customers are looking for coatings that require fewer layers, reduce material usage and waste, and enable faster job completions, whether in the factory or in the field,” he insists.

Marc Chan, North American marketing manager for Clariant’s Industrial and Consumer Specialties business, says one example is exterior architectural coatings, where there is a strong trend towards easy-to-clean, dirt-repellant coatings that have a long-term effect.

Overcoming Risk Aversion

Adoption of new technologies in risk-averse industries can be a real challenge, and the paint and coatings industry is no exception, particularly when it comes to industrial and protective coatings. “It is doubtless that the paint and coating market is capable of extremely innovative leaps, yet we have to recognize that this industry can also be very conservative when it comes to the introduction of new products and even more of new chemistries,” Telesca says.

Change, adds Webster, means risk of potential early failure.��“New materials in coatings and coating formulations must demonstrate meaningful differential performance and be rigorously tested during development,” he comments.

It is critical for manufacturers to protect the value and reputation of their brand, so extended evaluation and adoption timelines are necessary to incorporate new innovations, especially for exterior applications, Vaughn from Emerald agrees. For instance, the testing phase for introducing a new additive can be very long depending on the added functionality and, in some cases, go beyond two to three years even before presenting a new product to the first customer, according to Telesca.

Getting those new products tested by customers can also be challenging. “Formulators are incredibly busy today, with competing priorities and limited resources, and thus typically will only screen the additives they feel will have the highest success,” Bowler observes. However, Telesca believes that while introducing new products and chemistries has become increasingly difficult and demanding in terms of cost, time, and resources, customers are still very keen on trying improved, more environmentally friendly, safer, higher-
performing products.

To be successful, R&D departments in the specialty chemical sector must have a good understanding of the target market, the application process, end-consumer needs, and likely demand. Solutions can then be developed to fill the gaps for coating manufacturers and thereby increase acceptance, according to Mouhcine Kanouni, technical marketing manager for performance additives with Clariant.

Given the long qualification cycles, some specialty chemical suppliers such as Eastman are looking to accelerate testing and ultimately advance innovation to the market more quickly, according to Feng. Close cooperation between specialty chemical suppliers and formulators is often fundamental to driving innovation at a pace the market needs, Webster asserts. Klug adds that launching new material innovations requires that all key stakeholders benefit, from formulators to end consumers. “We are embracing ways to accelerate testing, partnerships for developing solutions, and ultimately ways our capabilities can help formulators advance innovation to the market more quickly,” he says. For instance, Eastman uses a test method initially developed by Ford to chemically monitor the rate of photo-oxidation in new resins, enabling the development of more robust resins and additives while also accelerating testing and development with customers that are eager to bring superior solutions to market.

Addressing Formulation Variability

Coatings formulations are incredibly varied across and within coatings formulators. Each component of the formulation offers some functionality and some trade-off, according to McNeece. “The biggest challenge is seeing what functionality and trade-off a specialty chemical brings to a particular formulation, then trying to replicate that into as many formulations as possible. It takes a lot of research and development, and testing,” he comments.

There are very few “like for like” drop-ins that address the major challenges faced, agrees Stan Cook, architectural coatings marketing director at Dow. Here again, he notes that collaboration with coatings formulators is required to fully leverage the unique materials that Dow develops using High-Throughput Research (HTR) and various modeling capabilities in actual products.

There is an added challenge for raw material suppliers, according to Stewart, that many specialty chemical ingredients are not a 1:1 ratio of resin to formulated coating. “Many of our customers are looking to reduce complexity, which can often mean streamlining the total number of raw materials they source. Making changes can, therefore, be complicated for customers with multiple formulations dependent on certain ingredients,” she explains.

Different requirements for new resins and additives may also be defined by regional and/or country-specific needs. “Regionalization allows the development cycle to truly be regional, while the interactions regarding technology tend to be defined globally. As a result, some suppliers, such as allnex, have global, regional, and country-specific development labs that can focus on the needs defined by the regional/country regulatory environments for the application. “With this approach, we are able to accommodate individual demands with global technologies made in our global manufacturing footprint,” Kersting says.

Meeting Numerous Registration Requirements

Increasing regulation threats in various regions globally are leading to less freedom to create new products, according to Sascha Herrwerth, head of global applied research and technology for industrial coatings with Evonik Coating Additives. “The challenges of innovating and commercializing new coating technologies to meet the demand of formulators are many, and they are quite dynamic as global regulations, regional market needs, and other external drivers continue to evolve,” agrees Latas.

Two of the key issues relate to choosing materials that are unique enough to fill a new space and meet stringent environmental requirements and streamlining the design process to get to the best product, notes Kanouni. Latas specifically mentions that maintaining global registry has become more difficult. New materials may indeed need specific regulatory registrations depending on the region in which they will be used/marketed, their chemistry class, or their function.

Preservatives are a good example, says Kelly Pippine, VP of marketing and technology with Emerald. Europe recently implemented further restrictions on certain biocide use levels due to tighter health and safety classifications, and other geographies may consider similar actions.��“Further,” she notes, “new biocidal registrations can be long, complex processes, limiting the number of new preservative agents becoming available.”

In the United States, the challenge always begins with getting new chemical substances registered (i.e., under the Toxic Substances Control Act, or TSCA) without Significant New Use Rules (SNURs), which have the potential to be less accepted by customers, according to Grace.

“Launching new products in the era of TSCA reform and increased listings on Prop 65 remains a significant challenge for all companies bringing new technology to the market,” adds Stewart. “Since TSCA reform in 2016, companies have had to re-learn how to register products to limit undesirable use restrictions and allow broad use of the new technology across multiple market segments and application methods. At present, longer periods of time for [Premanufacture Notice, or PMN] review, higher fees, unexpected use restrictions, and added warning recommendations on safety data sheets have all required scientists and product registration stewards to spend more time and resources working through the PMN process,” she explains. Once again, collaboration between specialty suppliers and formulators is needed to bring new technologies to the market. “It’s important that suppliers are aware of end-use applications to provide guidance and training on safe handling of products and work together to manage any perceived limitations of use. If not, the availability of new technology in the United States could be limited with respect to other regions,” Stewart states.

There has been tremendous innovation in most coating component technologies, but particularly for additives.

In the architectural space, Dumain notes that U/L certifications are cost prohibitive, and some companies want global health, environmental, and safety registrations, which also can create a challenge, particularly due to the high costs for registration according to the European Union REACH legislation. “That can limit the introduction of innovations to multi-national companies, while regulatory constraints that are still quite regionalized continue to limit wide-spread adoption of certain materials,” he says.

In food packaging, meanwhile, there is continued regulatory, consumer and brand pressure to move away from Bisphenol-A (BPA) in food-contact applications, where it is commonly used in plastics and coatings applications, such as the BPA-epoxy-based linings of metal cans, according to Feng. “Since 2015, BPA has been banned in all food-contact applications by France and has been listed on California Proposition 65. Consumers also now expect canned foods to be free of substances perceived to have negative health impacts, while maintaining current shelf life and flavor characteristics. From a technical standpoint, it is challenging to find the right alternatives due to the rigorous performance requirements for the coatings,” she remarks.

On the other hand, modern regulations significantly drive product development, according to Moessmer. He points to new silicone additives free of cyclic silicones and replacements for organotin catalysts traditionally used to accelerate the formation of polyurethane bonds.

There have also been developments in preservative technologies. “As the preservative palette has continued to shrink, innovative methods have changed the way formulations are developed and preserved. Hurdle technologies, multifunctionals, and boosters are effective solutions that do not inhibit microbes on their own, but they can enhance the antimicrobial efficacy of a registered preservative,” Pippine observes.

COVID-19 Challenges

The emergence of the COVID-19 pandemic has created some additional challenges for developers of new specialty chemical solutions for the coatings industry. “Currently, due to the effects that COVID-19 has had on the market, it’s been difficult to get customers excited to evaluate innovative, future-facing technologies; limited time in labs is prioritized to taking care of today’s business, not necessarily looking towards tomorrow’s next generation of products,” Grace explains.

In addition, he notes that because of the negative effects that COVID-19 has had on bottom lines, many companies have elected to focus more on maintaining financial stability rather than on investing in changes that might be difficult to initiate. Social distancing and safety protocols have inherent limitations as well, not only within lab environments, but also when trying to work closely with customers to collaboratively innovate and test new ideas, Latas adds.

With COVID-19 impacting the ways business is done, companies have also had to take a more digitally focused approach to product launches, Kanouni observes. “Webinars, social media, Skype, etc. have all given us new avenues to engage with the market more effectively, which is important, because providing the offer of technical assistance to support customers as they become familiar with a product remains as important as ever,” he says.

On a positive note, in the midst of COVID-19, there has been a frenzy of activity in the academic and industrial world addressing sanitation and working to crack the code on anti-viral coatings, according to Rowe. “With increased use of alcohols and other disinfectants, we will need our architectural, furniture, and flooring coatings to withstand new, harsh sanitation practices,” she comments.

Furthermore, Vaughn observes that many companies that participate in the DIY, packaging, and hygiene product sectors have reported strong sales in their quarterly financial reports. The increase in demand for DIY products reverses an ongoing trend toward the do-it-for-me/contract approach.

Resins and Additives Most Important Specialty Coating Chemicals

Most performance and appearance parameters are dictated by resin, additive, and pigment chemistries, and thus specialty chemicals play a significant role in these coating components. “There is a lot of opportunity in paints and coatings to offer value,” asserts McNeece.

In fact, each space in the formulation palette benefits from specialty chemicals—from novel monomers that enhance resin performance, to solvents that reduce VOC, to additives that enhance rheology and pigment efficiency, to the pigments that allow for improved exposure resistance, according to Dumain. The main driver in the promotion of specialty chemicals, Grace emphasizes, is to be able to demonstrate the value proposition that these specialty products bring, i.e., what is the benefit versus cost position of the specialty product?

Resins often serve as the driver of performance; however, it is a complex interplay between all the components in a can of paint, Stewart says. She adds that the use of more sophisticated specialty chemicals as coating additives can heighten overall performance. “For industrial coatings, the resin provides the long-term durability, but it’s the defoamers, dispersing agents, and rheology modifiers that ensure good appearance and processing,” she explains.

Kersting agrees that newer formulations designed for high-performance applications rely heavily on specific additive chemistries that provide needed flexibility, durability, and long-term performance. One example of sophisticated additive chemistry from Moessmer is controlled polymerization technology, such as group transfer polymerization, which is used to produce well-defined, highly effective dispersing additives for optimum pigment stabilization.

Overall, Vaughn believes there has been tremendous innovation in most coating component technologies, but particularly for additives. “Multifunctionals, modifiers, performance enhancers, low-VOC technologies, and functional ingredients can make paints and coatings perform in new and exciting ways, while also improving the [health, safety, and environmental, or HS&E] profile and value,” she says.

In some cases, the line between resin and additive is blurring, with functionality typically addressed with additives now being built into resin backbones. “The decision of whether a new technology will be built into the binder or be supplied as an additive is both a complex and multi-faceted decision that requires input from R&D, marketing, EH&S, and manufacturing teams,” Cook remarks. It is also certain, according to Webster, that the wide-ranging and demanding needs of the paint and coatings market will not be met by any one supplier independently.��“Cooperation is critical to enable coating material innovators to go to coating manufacturers with more complete solutions than in the past,” he asserts.

Enabling Technologies

Specialty chemicals have contributed greatly to the advancement of coating technologies over the last several decades. One fundamental example highlighted by Cook is acrylic binders that enabled the waterborne revolution and ongoing development of unique binder morphologies that allow for further VOC reductions and reduction in the need for TiO₂. Improving the performance of water-based coatings, according to Latas, continues to be the most interesting area for exploration. “For instance, waterborne polyamide polyurethane dispersion technology is an exciting opportunity to bring new levels of performance for water-based coatings. The capabilities and potential applications continue to evolve, bringing new opportunities throughout the coatings industry,” he says.

Hybrid products that combine different curing technologies—for example, polyisocyanates that also contain silane curing groups—are of great interest as well, according to Grace. Reactive co-binders for various resins that allow greater crosslinking density and, thus, more durability for ready-cured coatings are important new specialty chemicals too, Mertsch notes.

Nanoparticle, encapsulation, and self-repairing technologies for use in “smart coatings” also offer novel solutions for performance enhancement, observes Bowler. “Many of these technologies are still in the growth phase, but they continue to gain acceptance in the market,” he says.��������

In the additives space, the greatest focus has been on increasing functionality while reducing VOC content, according to McNeece. Multifunctional additives, in particular, are of interest because they can simplify paint formulations and reduce the risk of potential ingredient interactions and performance trade-offs, adds Vaughn. Examples include new low-VOC coalescents that also enhance hardness, efficiency, in-can stability, open time, and resistance to block, dirt pick-up, and scrubbing and multifunctional additives that enhance varying combinations of flow, leveling, defoaming, substrate wetting, gloss, emulsification, dispersion, rheology, and drying performance. There is also growing use of additives based on renewable raw materials to meet increasing demands for enhanced sustainability, Moessmer comments.

Specialty chemicals have also been developed to enhance the performance of coatings targeting specific end-use applications. Prock points to improved anti-graffiti paints, improved self-healing paint systems, and further enhanced flame retardant paints. In the architectural segment, Kersting observes that products targeted toward the DIY market leverage interesting new technologies that have brought performance to the forefront. The automotive market, meanwhile, is focused on reducing the energy intensity of paint shops with the introduction of lower bake temperatures and technologies that enable faster curing. In the general industrial and protective markets, novel new monomers are real game changers, according to Webster, enabling polyesters to exceed performance of many standard main resins on the market.��Within the transportation coatings market, Ansurkar says that synergistic interactions of multiple chemistries are delivering differentiated performance with respect to low-bake temperatures, UV curing, energy savings, and other enhancements, all without sacrificing process efficiency. For packaging coatings, novel monomers enabling BPA-NI polyester resins to perform equivalently as BPA-epoxy are an exciting development for Feng. These coatings provide excellent performance even when exposed to “hard-to-hold” foodstuffs that create hydrolytic and corrosive environments.

Many New Solutions with Real Potential

Given the importance of specialty chemicals in the development of existing coating technologies, it is not surprising that they continue to play a key role in new solutions that are under development or just being introduced to the market. Some examples include:

• Smart coatings with anti-corrosion, anti-bacterial, anti-virus, water-repellant, self-cleaning, and other properties that have been adopted in niche markets but not yet into general coatings;
• Biocide-free anti-microbial coatings;
• Thermal/electric conductive coatings;
• Reflective coatings;
• Coating products that can be applied in unconventional ways, such as inside a mold for a plastic part or as a paint film laminate;
• Products with increased adhesion to low surface energy substrates;
• High-solids waterborne 1K polyurethanes;
• Fast-curing polyaspartic technology (slower adoption due to the need to invest in new application equipment);
• Hybrid systems that provide unique synergies through the combination of acrylic and silicone technologies;
• Instant-set resins that reduce downtime and material and labor costs;
• Non-isocyanate curing systems;
• Organo-tin-free catalysts that enable rapid curing of 2K solventborne high-performance coatings at room temperature;
• Newer multifunctional additives;
• PTFE-free wax additive alternatives that can help formulators prepare in advance for potential new regulations that restrict the use of PTFE;
• Dispersants that allow significantly increased pigment loadings while exhibiting drastic decreases in foaming, improved shelf life of the concentrate, and excellent color stability;
• Multifunctional additives for more cost-effective powder coatings that provide heat stability during baking, an electrostatic charge during application, and light stability in the applied film;
• Peelable additive technologies for removable coatings that provide a temporary coating while protecting the substrate and base coating; and
• Bio-based products that have the performance levels to be able to compete with conventional products.

In many of these examples, the challenge is to commercialize these technologies that bring tremendous value to customers and consumers through enhanced durability, appearance, chemical resistance, anti-microbial, and other properties in a more sustainable manner at an acceptable cost. “Cost remains a constant barrier for many green innovations,” Klug states. “Many new low-VOC solutions, as well as bio-based and/or recyclable materials, have not made it to market due to potential increases in raw material costs. Over time, however, we expect customers to demand a more meaningful green footprint from their products and expect the position of these innovations to improve relative to where they are today,” he continues.

Gaps in Coating Performance Waiting for Solutions

Specialty chemicals will also be crucial to the successful development of new technologies that address ongoing gaps in coating performance. Many of the needs are focused around water-based systems. Latas points to the desire for improved DTM performance from waterborne coatings, including 1K waterborne polyurethanes. For Rowe, the focus is on pushing the performance boundaries of waterborne, 1K solutions into corrosion protection categories of C4 and C5. There is also general room for innovation with regard to film performance to address common challenges and potentially enable step changes in key features, adds Vaughn. For instance, in the construction sector, formulators are looking for solutions that can handle vapor pressure coming from concrete substrates, which would reduce application downtime and overall cost and make these coatings more durable, according to McNeece. The demand for more anti-blocking technologies for deep base formulations, which could potentially be remedied through the use of multifunctional coalescents, and solutions for enhanced early water resistance to reduce issues related to spotting or moisture susceptibility before the film has a chance to fully set are other examples noted by Vaughn.

“We are looking to help customers increase the efficiency of sustainable processes that still perform at a high level technically, ideally achieving their goals using fewer, more versatile additives,” Telesca says. Controlling cost is another driver.

“In line with the need for lower lifetime costs, we believe that new specialty chemicals in coatings will address long-term color stability, even in extreme environments,” McNeece comments.

Emphasis is, of course, also being placed on achieving greater sustainability. Moessmer states that because modern coating systems need to be carbon-dioxide-neutral to avoid contributing to the greenhouse effect, many new developments are directed towards bio-based and renewable systems. Successful bio-based technologies, however, must provide equal or superior performance at a similar cost to conventional products, according to Grace. In the automotive sector, Ansurkar notes that lightweighting has created the need for coatings that can work on multiple complex substrates and the desire for energy savings, and a smaller environmental footprint continues to result in processing changes and coating layer-reduction without sacrificing performance. The packaging industry is also looking for renewable and/or bio-degradable materials, enabling process efficiency, and improving lifecycle and recyclability, according to Feng. Overall, Webster expects emissions, carbon footprint, and VOC regulations to continue to be more restrictive, which will create the need for new coating materials and new application equipment.

Longer-term, for autonomous vehicles, coatings on car cameras will need to be self-cleaning and available in a diverse paint color palette that can be recognized by the various types of cameras and sensors used to enable autonomous driving, according to Klug. Shared vehicles, adds Grace, will require interior coatings that afford soft, luxurious surfaces while still maintaining chemical and stain resistance.

Bowler is exploring paints that act as solar panels on houses or cars, coatings that automatically clean windows or solar panels, and coatings that change color by turning a knob on the wall or in a car. “The possibilities are endless,” he asserts.

Much of the new developments, says Kersting, will be defined by the needs of emerging regions as their economic power overtakes that of the established regions of North America and Europe. “The needs in China, Asia, and Latin/South America are different based on the different rates at which urbanization is taking place. In addition, these regions have a large percentage of new consumers that are far more mobile and flexible in their needs and use of technology,” he observes.

Specialty Chemicals Suppliers are Important

Clearly, specialty chemicals are key to providing beautiful surfaces with brilliant colors that perform multiple functions. “Specialty chemicals manufacturers are in the front line for contributing towards a more sustainable society for our generation and the future of our planet since these products are eventually used in thousands of different applications,” Chan asserts.

With their unique characteristics, they provide solutions to challenges that cannot be solved with commodity-type raw materials. “The need for high-performance paints and coatings will only grow over time,” states McNeece. “Specialty chemicals play a vital role in performance, but require technical expertise on the part of specialty chemicals manufactures and close partnerships with formulators,” he adds.

As regulations and other demand influencers continue to drive complexity throughout the paints and coatings industry, it becomes particularly valuable for coating formulators and suppliers to collaborate, Latas agrees. “Specifically for product safety and compliance concerns, it is critical to define early in the process what the governmental and NGO requirements are for diverse markets and to clearly identify the required regulatory approvals and registry listings. Working with the right specialty chemical supplier can help ensure unmet needs are addressed and optimal specialty chemical performance is achieved while also assuring compliance with all relevant regulations,” he concludes.

Through collaboration with partners across markets like paints and coatings, Chan agrees that specialty chemical suppliers will be best placed to support the industry players in the future creation of coatings that are all around more sustainable and more efficient to manufacture with less environmental impact.

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

Increasing awareness of environmental, health, and food safety issues, as well as increasing regulations on substances with potential health effects, are major driving forces for consumer behavioral changes. These behavioral changes propel the shift toward development of alternative products free from substances of concern. One such substance is Bisphenol-A (BPA), a chemical commonly found in the protective linings of food or beverage metal cans. BPA has undoubtedly become a major cause of concern, prompting serious examinations from various regulatory bodies across the world connecting BPA to adverse health effects.^1-5

Since 2015, France has issued a ban on BPA in all packaging, containers, and utensils intended to come into contact with food.1 As recently as 2018, the European Union published a regulation that further restricts the presence or use of BPA-containing substances in plastic food contact materials. The new regulation reduces the specific migration limit for BPA in varnishes and coatings for food contact applications by an order of magnitude (from 0.6 mg/kg to 0.05 mg/kg) over previous regulations.^2,3 Although the U.S. Food and Drug Administration has not issued a complete ban on the use of BPA in food can lining applications, several states such as Maryland, Connecticut, and California have placed further restrictions on its presence in food cans. Given the increasing regulatory pressure, the demand for Bisphenol-A Non-Intent (BPA-NI) metal packaging coatings will continue to increase in years to come.⁴

Resin manufacturers, can coating formulators, can makers, and brand owners are responding to this shifting market demand by actively innovating alternative solutions that can meet or exceed the performance of BPA epoxy coatings while minimizing the cost of these transitions. The performance requirements that BPA-NI can linings need to satisfy are high. The coatings must exhibit enough flexibility and adhesion to withstand the rigorous mechanical demands of can forming processes. They also need to survive high temperatures and high-pressure food sterilization processes in the presence of hydrolytic and corrosive environments such as low pH, acids, sulfur, and salts. Not only does the coating need to meet these high requirements pertaining to can forming and sterilization processes, it must also survive the storage test or pack test as part of the routine assessment of long-term shelf stability.^6,7

Polyester coatings are one of the most promising BPA-NI alternatives to BPA epoxy-based coatings. The modularity of building blocks in polyesters allow for fine-tuning of its compositions with a wide variety of monomers, enabling the achievement of balanced properties including high-glass transition temperature, flexibility, and toughness. Several notable monomers, namely 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD), 1,4-cyclohexanedimethanol (CHDM), isosorbide, and tricyclodecane dimethanol (TCDDM), have emerged as promising building blocks for superior polyester performance. In particular, TMCD is known to impart a variety of excellent properties such as good temperature resistance, toughness, chemical resistance, and hydrolytic stability. TMCD is used in BPA-free specialty plastics prevalent in sports bottles. TMCD-containing protective resin systems have demonstrated good resistance to corrosion and chemical attack while simultaneously demonstrating good flexibility and adhesion in a variety of applications, including metal packaging applications.^7,8 New product qualification in storage or pack tests is a vital step in the development and evaluation process of new metal packaging coatings. Pack tests may last from several months to several years and may involve filling and storing goods in cans at elevated temperatures. At specified time intervals, cans are opened and visually inspected for defects. Food and beverage cans come in various sizes and shapes, and coatings formulations vary across the industry. As the combination of can types and food products are numerous, this coating qualification effort can take significant time and financial investment, as well as collaboration across value chains. Given the cost, complexity, and time involved in qualifying new coatings, reducing qualification time has the potential to accelerate development cycles. A predictive analytical technique that can reduce the need for time-consuming pack tests would be highly desirable across the metal packaging value chain. Though the goal seems lofty, it is in the best interest of the metal packaging industry to develop techniques that may predict long-term performance for new coatings.

Electrochemical impedance spectroscopy (EIS) is a useful analytical technique that has been widely employed for numerous applications in industry and academia over several decades. This technique assesses the barrier properties of organic coatings and their time-dependent, long-term durability and performance in response to various experimental conditions.^9-22 EIS can reveal a wealth of information on the state of corrosion in coating prior to the visual appearance of corrosion. With EIS, a small sinusoidal alternating current (AC) wave is applied and the resulting resistance (impedance) to current flow is assessed as a function of frequency. Impedance data are collected and analyzed with relevant electrical circuit models to understand the corrosion state of the coating. Kern and colleagues employed EIS to study the performance of cans coated with various formulas and filled with different food contents. They used direct current (DC) treatments and electrolyte aging treatments to understand and predict corrosion during storage.⁹ Charge transfer capacitance, related to the delaminated metal area, and electrolyte aging, respectively, were found to be reliable parameters and procedures for quality evaluation and performance ranking of various cans. De Vooys and colleagues also employed EIS as a screening tool to evaluate the suitability of coated metal packaging cans with new food products by measuring their impedance over a two-week period.¹⁰ The evolution of complex impedance at low frequency over a two-week period with threshold impedance values was used as a pass/fail criterion for product/container combinations. Others have employed EIS as a means to quantify the adhesion of different types of lacquers for food packaging,¹¹ to measure the delamination of coatings over time when exposed to various conditions,¹² to study the water and electrolyte uptake of organic coatings,^13,14 and many other applications.^15-22

In this study, we employed an EIS technique in conjunction with aging procedures involving concentrated food simulant and elevated temperatures for a series of coatings. The aging procedure acts to accelerate the corrosion process, while EIS provides quantitative measurements that enable in-depth understanding of corrosion mechanisms. This combination enables faster coating quality evaluations and rapid selection of formulations for further evaluation. The performance evaluation from EIS is further validated using an industrially relevant enamel rater test. This work provides fundamental insight into the relative performance of polyester resins in various formulations as part of a resin development strategy.

Experimental Procedures

EMN-MP resin is a mid-molecular weight TMCD-based polyester with a number average molecular weight (M_n) in the range of 6000–8000 g/mole and T_g��~90 °C. Commercial Control resin is a non-TMCD-based, high-molecular weight resin with an M_n of > 10,000 g/mole and T_g��100–110 ºC. The two resins were formulated into white polyurethane (PU) and gold phenolic PU formulations as representative solventborne BPA-NI 3-piece food can interior lacquers. The formulation details for white PU coatings and gold phenolic PU coatings are listed in Tables 1 and 2, respectively. All resins were reduced with Aromatic 100 (A-100) solvent to 50 wt% solids. Fascat® 9102 catalyst solution was diluted with A-100 to 10% of the original supplied concentration.

The formulations were drawn down onto electro tin plate (ETP) substrate panels supplied by Lakeside Metals Inc. (0.25# Bright T-1 0.009-0.010 x 4.0 x 12.0 in.). All formulations were applied with appropriate wire-wound rods to target film weights of 10 gram per square meter (gsm) and 6 gsm for white PU and gold phenolic isocyanate coatings, respectively. The coated panels were cured at 200 ºC for 12 min in a forced air convection oven.

Electrochemical impedance spectroscopy was performed using Gamry Instrument “Reference 600” Potentiostat equipped with Gamry framework and Echem Analyst. The samples were held in a cylindrical glass cell by a clamp fixture attached to an O-ring gasket affixed to the grooved bottom of the cell. A nickel electrode and a graphite electrode were used as reference and auxiliary electrodes, respectively. The potential was applied in a range of 5 mV from open circuit potential and the frequency was varied from 10⁵ to 0.1 Hz.

Food simulants used in this study were 3% acetic acid (3 wt% acetic acid in 97 wt% DI water), 2% lactic acid (2 wt% lactic acid in 98 wt% DI water), and 5% acetic acid (5 wt% acetic acid, in 95 wt% DI water).

To perform EIS measurements for white PU panels, the drawn down panels were first retorted in the presence of 3% acetic acid solution. The EIS cell apparatus was fixed on top of the area of interest, and this area only was exposed to the electrolyte during the retort process at 131 ºC for one hour. Upon completion of retort and cooling, the first EIS measurements were performed, and subsequent measurements were taken at defined time points. To perform EIS measurements for gold phenolic PU panels, the area of interest in the drawn down panels was exposed to 2% lactic acid by fixing the EIS cell apparatus on top of the panels. Following 36 days of exposure to 2% lactic acid, the EIS measurements were obtained. All EIS measurements and aging were performed at room temperature. Enamel rater measurement was performed using WACO Digital Enamel Rater II. Test voltage was set at 6.3 V. Can lids were stamped from white PU flat panels and retorted in the presence of 5% acetic acid. Upon retort, the can lids were exposed to 5% acetic acid, and the can lids were placed in an oven maintained at 50 ºC. At defined time points, the can lids were taken out of oven and enamel rating was obtained.

Results and Discussion

The electrochemical impedance spectroscopy technique has been employed for decades for measuring and monitoring of time-dependent changes of organic coatings.^9-22 The measured impedance values are presented in Nyquist or Bode plots and fitted to relevant and appropriate electrical equivalent circuits consisting of resistive and capacitive elements in various configurations that help with the physical interpretation of the impedance data.^15-17 With this technique, the penetration of electrolyte into the coating can be followed and the initiation of corrosion at the metal/coating interface can be detected. Figure 1 shows the stages of corrosion, relevant electrical circuits, and the expected shape of Bode plots at different stages of corrosion when a coated panel is exposed to electrolyte. In Stage 0, at the start of immersion, the coatings are dry and behave like a pure capacitance. The appropriate circuit configuration is that of electrolyte resistance and coating capacitance in series. The coating capacitance depends on coatings thickness. In Stage 1, the coatings begin to absorb the electrolyte until the film micropores become saturated with the electrolyte. In this phase, the coatings can be represented by a resistance and a capacitance component in parallel. As the film continuously absorbs electrolyte, the coatings capacitance increases with increasing electrolyte content, whereas the resistance component decreases until the film becomes saturated. The film affinity towards the electrolyte and crosslinking density influence the speed of electrolyte penetration in the film. Coatings capacitance can be used to determine the change in coatings porosity or electrolyte diffusion coefficient according to the Brasher-Kingsbury equation.²² In Stage 2, as the electrolyte reaches the coatings/metal interface, corrosion is initiated and blisters in the coating begin to develop. The newly formed oxidized layer can be represented using a double layer capacitance and a charge transfer resistance in parallel, or the “two time-constants” as commonly used in EIS. There might be no visible sign of corrosion at this stage, but the magnitude of the complex impedance at low frequency continues to drop. In the subsequent stage, delamination starts, and pore breakthrough occurs along with a diffusion-limited corrosion reaction as the reactant gets delivered to the surface. A Warburg impedance component is added to the equivalent circuit in this stage.

There are many ways to interpret or use the results of resistive and capacitive elements from relevant electrochemical circuits. McIntyre and Pham¹⁸ used R_po (pore resistance) and C_dl (double layer capacitance) terms to estimate the degree of delamination in can coatings. They also used C_c (coating capacitance) to estimate the coating porosity and correlate it with flavor scalping performance. Hu et al. used a coating capacitance term to monitor water uptake and diffusion of chlorine ions in epoxy-coated aluminum alloys in the presence of NaCl solutions.¹³ Kern et al. used charge transfer capacitance in a slightly more complicated circuit configuration to estimate the delaminated area after exposure to food simulants.⁹ De Vooys et al. simply relied on the evolution of the shape of Bode plots over time and assigned relevant electrical circuits to fit the data. The magnitude of overall impedance of the coated metals exposed to food simulant over two weeks was used as a pass/fail criterion. McIntyre and Pham also used EIS to track the low-frequency impedance over several weeks and showed that the rate of decrease in impedance has some correlation with the coating porosity.¹⁸

In this study, we compare EIS responses to characterize the progress of corrosion as a proxy for long-term performance in a pack test. The magnitude of impedance at low frequency (0.1 Hz) is used to compare the relative performance of different coatings as appropriate. EMN-MP resin and Commercial Control resin were each formulated into white PU formulations (Table 1) and drawn down on tinplate. The coatings were then subjected to a retort procedure in an autoclave at 131 ºC for one hour in the presence of 3% acetic acid in a custom-made set up that exposes only the intended area to the food simulant.

Figure 2 shows the frequency-dependent impedance values immediately after retort and after a subsequent 48 h of immersion in 3% acetic acid. At the beginning of the test, both coatings were at the same corrosion stage (Stage 1, electrolyte penetration). However, after the subsequent 48 h of exposure, the Commercial Control coating had progressed into Stage 2 corrosion, as apparent by its Bode plot shape indicating initiation of corrosion, whereas the coating based on EMN-MP resin was still in the stage of electrolyte penetration (Stage 1). This observation is supported by the general shapes of the Nyquist plots shown in Figure 3.

At zero hours, both coatings showed one semi-circle indicating Stage 1 corrosion. After 48 h of exposure, two semi-circles began to emerge in Nyquist plot of Commercial Control coating indicative of Stage 2 corrosion, while only a semi-circle was observed for EMN-MP coating, indicative of Stage 1 corrosion. The decrease of impedance at low frequency (0.1 Hz) for both formulas is traced in Figure 4a and consistently shows that Commercial Control coating has lower corrosion resistance over the exposure time. As the impedance values plateau at longer times, it is apparent that the corrosion slows after the initial retort exposure. EMN-MP coating demonstrates better corrosion resistance of 2.56 Mega Ohms as compared to 0.44 Mega Ohms conferred by Commercial Control coating in this post-retort plateau region (Figure 4b).

Both resins were also formulated into gold phenolic PU formulations (Table 2) and drawn down onto tin plates. Upon curing, the panels were exposed/soaked in 2% lactic acid solution and aged for 36 days at room temperature. Figure 5 shows the frequency-dependent impedance measurement of both panels after 36 days of exposure in 2% lactic acid. Figure 6 compares the Nyquist plots of both panels at the end of 36 days of exposure in 2% lactic acid. Comparing the curve shape from the Nyquist plots, it is evident that EMN-MP coating was still in Stage 1 corrosion as indicated by one semi-circle, whereas Commercial Control coating has entered into Stage 2 corrosion as evidenced from the appearance of two semi-circles. The low frequency (0.1 Hz) impedance values over 48 h of observation are shown in Figure 7a, and the corrosion resistance of both coatings are compared in Figure 7b. Again, the EMN-MP coating exhibited significantly better corrosion resistance of 53.7 Mega Ohms as compared to 2.47 Mega Ohms with Commercial Control coating in gold, phenolic isocyanate formulation at the same dry film thickness. Perhaps the higher hydrophobicity of EMN-MP resin and, therefore, its lower propensity to absorb 2% lactic acid food simulant, might contribute to the delay in corrosion development. The superior corrosion resistance observed with EMN-MP resin can be, at least in part, attributed to the hydrophobic contribution of TMCD. As evident from the two examples above, EIS techniques coupled with different aging procedures can produce quantitative and qualitative assessments that guide the development and evaluation of resin and coating performance.

To validate the EIS results, we also compare the performance of each of these resins with enamel rater testing commonly used in industry, as documented in various patents and articles.^23-27 The test included an accelerated aging approach using 5% acetic acid (2% higher than the concentration of acetic acid used for EIS experiment on white coatings) at 50 ºC over several days of exposure. This experiment was performed on stamped can lids, a notably harsher mechanical treatment than flat metal panels. It was expected that the high concentration of simulant and higher temperature would accelerate the corrosion process. Enamel rater is a DC-based technique that applies a constant 6.3-volt potential across the coating during four seconds and measures the resulting current. The enamel rating is an index of the amount of metal exposed or delaminated from the coating during exposure to food simulant. A higher current flow indicates a worse coating performance. Figure 8 tracks the current flow of can lids coated with white PU coatings exposed to 5% acetic acid at 50 ºC over seven days. The coating based on EMN-MP resin demonstrates a better corrosion resistance than Commercial Control coating at the end of seven days of exposure as evidenced by its lower current flow relative to that of Commercial Control coating.

At the end of the experiment, the exposed can lids were subjected to a tape-pull test for qualitative assessment of coating adhesion. Figure 9 shows the pictures of stamped can lids based at the seven-day exposure interval, before and after the tape-pull test. It is evident that the coating based on EMN-MP resin exhibits a qualitatively better coatings adhesion at the end of this accelerated aging test. It is also worth noting that signs of underfilm corrosion were already visible in the case of Commercial Control even prior to the tape-pull test. The tape-pull test further revealed underfilm corrosion in Commercial Control case. Signs of underfilm corrosion were also present in the exposed surface of EMN-MP coating after the tape-pull test, but the exposed area is qualitatively smaller than in Commercial Control coating, owing to better coating adhesion although quantitative assessment was not performed. The results observed in the enamel rating test are, thus, in agreement with the results of the EIS evaluation.

Conclusions

Electrochemical impedance spectroscopy is employed as a screening tool in the innovation process of new BPA-NI metal packaging resins. The resins in this study were formulated into coatings, which were subjected to aging treatments with several food simulants at relatively high concentrations. Time-dependent impedance measurements by EIS reveal the progress of corrosion in the coatings and provide quantitative measurements of corrosion resistance. EMN-MP resin in white PU coatings exhibited better barrier properties with slower corrosion kinetics and higher corrosion resistance relative to Commercial Control resin in white PU formulation. Similarly, in the gold phenolic PU formulations EMN-MP coating exhibits better barrier properties relative to Commercial Control coating. The EMN-MP resin system also shows better barrier properties in the enamel rater test on white PU coatings. The results from the enamel rater test are consistent with results observed with EIS. This work demonstrates the utility of EIS and the more commonly available enamel rater tests as powerful tools in understanding the barrier properties and corrosion kinetics of coatings as part of a resin design strategy. The data provided by these techniques can help to enable quantitative data-based decision making and faster coatings development cycles.

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19. Da Silva, L.R.R., Avelino, F., Diogenes, O.B.F., Sales, V. de O.F., da Silva, K.T., Araujo, W.S., Mazetto, and S.E., Lomonaco, D. “Development of BPA-free Anticorrosive Epoxy Coatings from Agroindustrial Waste,” Prog. Org. Coat., doi.org/10.1016/j.porgcoat.2019.105449.
20. Romano, A.P. and Olivier, M.G. “Investigation by Electrochemical Impedance Spectroscopy of Filiform Corrosion of Electrocoated Steel Substrate,” Prog. Org. Coat., 89, 1-7 (2015).
21. Amirudin, A. and Thierry, D. “Application of Electrochemical Impedance Spectroscopy to Study the Degradation of Polymer-Coated Metals,” Prog. Org. Coat., 26, 1-28 (1995).
22. Brasher, D.M. and Kingsbury, A.H. J. Appl. Chem., 62, 4 (1954).
23. Patel, H.K. US Patent 4,963,602. Aqueous Epoxy Resin-Acrylic Resin Coating. Compositions Containing Also Phenoxy, Novolac and Resole Resin Composition.
24. Westerof, W., Kolk, D.J., Van Riggelen, W.J.P.S.M. US Patent 5,739,215. Use of a Polyester in the Preparation of Coatings for the Interior of Can Ends.
25. McVay, R.L. US Patent 7,475,786. Can Coatings, Methods for Coating Can and Cans Coated Thereby.
26. Heyes, P.J. US Patent 5,238,517. Production of Laminated Materials.
27. Kojima, S., Watanabe, Y. “Flexibility of Epoxy Coatings. Part 2: The Relationship with the Degree of Cure,” J. Appl. Polym. Sci., 36, 224-228 (1996).

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

Despite growing interest around the world in the use of concrete and composite materials, wood remains widely used in exterior architectural applications, including siding, decking, fencing, and similar applications. Coatings continue to play an important role in maintaining the appearance and protecting the integrity of wood. “Exterior wood has conveyed quality and beauty for centuries, but that beauty requires significant maintenance.�� Architectural wood coatings continue to reduce that maintenance cycle while further enhancing its allure, thus supporting a healthy market,” asserts Jon Fedders, Dow Coating Materials, North America business director. There are differences in the use of wood on a regional basis and, therefore, different requirements and preferences for types of wood coatings. Developments in technology, however, are largely driven by three key goals: to improve sustainability, to enhance durability and extend painting cycles, and to reduce the time and effort required to complete coating projects.

Regional differences in the use of exterior wood are caused mainly by differences in preferred building materials and varying economic growth rates, according to Erik Pras, global marketing director for Versatic™ Acids & Derivatives at Hexion. For example, he notes that in North America and Northern Europe, the use of wood as an exterior building material is popular, while in Central Europe and high-growth parts of Asia, concrete is mainly used. Similarly, wood is not a common substrate in architectural exterior applications in South America. The use of wood decking is growing in China, however, according to Melanie Bauer, global marketing director for Coatings with Michelman. “Affluent Asian buyers are purchasing larger homes with decks made out of wood and pavers,” she says. She adds that in Southern and Central Europe, homeowners who would typically use pine are switching to more tropical hardwoods that do not need as much protection, but are coated to improve their visual appearance. Also, in Europe and the United States, there is a move away from traditional wood decking and siding towards stamped concrete and wood composites, according to Bauer. Increasing popularity of the “weathered” look is also reducing the use of coatings for protection, but creating greater demand for stains that produce this appearance while still protecting the wood.

Approximately 40% of exterior wood coatings used today are solvent-based.

Despite these various shifts in the market, wood remains the primary choice for commercial and residential applications due to its wide availability, according to Ravi Ravichandran, vice president of R&D, King Industries. “While composite decking materials have been available in the marketplace for a while and even though manufacturers have greatly improved the look and feel of these types of materials (i.e., more like natural wood), natural wood is still largely the substrate of choice for consumers looking for exterior wood surfaces,” agrees Rick Bautista, director of product marketing at The Behr Paint Company. And strong housing markets continue to drive demand for wood coatings. In the United States, according to Ravichandran, baby boomers are downsizing to updated, smaller homes. There is also a trend to extend interior stain colors to exterior spaces such as decks and patios to facilitate more connectedness to nature and a seamless transition that allows extension of the indoor living space to the outdoors. Fedders adds that in North America, in particular, many people are in fact expanding their interior living spaces by adding decks. “Decks not only allow people to enjoy the outdoors, they often provide a return on investment comparable to updating a kitchen,” Fedders observes.

Optimized Performance: The Shift from DIY to DIFM

Overall, the global market for wood coatings in exterior architectural application is a healthy business with strong growth opportunities, asserts Gerjan van Laar, market segment manager for Decorative Coatings at DSM Coating Resins. Rising disposable income is��increasing the demand for premium wood coatings. “People are willing to pay more for unique appearances and functional products that help them fulfill their needs. There is, for instance, a huge movement towards matted coatings, because consumers really admire the natural beauty of surfaces like concrete and wood,” he comments. “In addition, due to the movement from Do It Yourself (DIY) to Do It For Me (DIFM) in both Europe and North America, we have seen an increasing demand for renovation in the professional paint market,” he remarks.

Architectural wood coatings continue to reduce the maintenance cycle while further enhancing its allure.

Contractor labor shortages are complicating this situation. “People are spending their time differently because consumer behavior is changing. For example, millennials and generation Xers are outsourcing the practical jobs more than the baby boom generation did. At the same time, there is a shortage of professional painters in mature economies, which is leading to increasing demand and a relatively small offering,” says van Laar. AkzoNobel has observed that the increasing cost and lack of availability of contractor labor is driving a shift from in-field finished to in-factory finished products, according to Anthony Woods, segment marketing director for Wood Coatings in the company’s Industrial Coatings business. “The quality and durability of in-factory finishes are allowing the transition from factory-primed to fully factory-finished products,” he asserts. The increasing use of other substrates and the factory-applied coating of certain construction components are additional factors contributing to greater opportunities in the renovation and maintenance areas, adds van Laar.�� “Efficiency and ease of use are increasingly driving product development in our industry, as the shortage of professional painters around the world is pushing the demand for their time and for easier solutions,” he adds. With respect to cost, Pras notes that the binder is an important factor, but in some regions, labor is much more important. In these cases, increased coating lifetimes for reduced recoating frequency are of great interest. “End users and contractors want more ‘bang for their buck,’ so cost-performance continues to be an important balance,” he notes. Simplifying stain projects and solving wood-related problems continue to drive innovation in this category, according to Bautista, with quicker return to service an important focus for both DIY and professional customers alike.

There is a huge movement towards matted coatings, because consumers really admire the natural beauty of surfaces like concrete and wood.

Optimized performance for the construction industry and DIY consumers is driving the market, agrees Bauer. She identifies flame retardant coatings and coatings that offer ease of application (fewer coats needed for protection and visual appeal) as two examples. There is also a push for multipurpose products that can be used in interior and exterior applications and on both wood and metal, coatings that perform well on both wood and wood-plastic composite materials, waterborne coatings with performance equal to that of solvent- based systems, and solventborne coatings that provide better penetration into softwoods and cedar at similar or lower solvent levels, according to Pras. Due to climate change, van Laar observes that the weather has become very unpredictable, and coatings on exterior surfaces have more to endure, increasing the importance of performance indicators such as dirt pick-up, drying time, weather resistance, and durability/longevity.

Meeting the Demand for Sustainable Products

Increasing the sustainability of exterior architectural wood coatings is also essential today. Regulations and labeling requirements are both being fine-tuned, and toxic��ingredients��are being banned from paint formulations, according to van Laar. The specifics of regulations, much like preferences, tend to be regional. The Ecolabel requirements in Europe are driving products to more waterborne systems, according to Bauer. The biocidal product regulation (BPR) in the region has also made it increasingly difficult to register primers and impregnants to protect exterior wood, observes Wood.�� The demand for waterborne wood coatings is also on the rise in China due to the new tax implemented by the government to reduce pollution, according to Ravichandran. There is also interest in high-solids solventborne coatings around the world. In the United States, in fact, Bauer points out that approximately 40% of exterior wood coatings used today are solvent-based because professional contractors prefer the performance of solvent-based products. “Although U.S. producers would prefer the switch to waterborne systems for low VOC, that will only happen when waterborne products perform the same as, or better than, solvent-based products,” she states.

Despite this performance issue, van Laar notes that painters and applicators are more proactively aware of their safety and health, and companies are making different choices and choosing paints that prevent health issues��for��their employees. “The sustainability of products and application techniques is still a high priority, and customers want to see efficiency improvements in their production,” Woods says. Building owners and managers and home owners are taking sustainability into account in their purchasing decisions, seeking contractors that know how to reduce the carbon footprint of their building assets at a maintenance or renovation level, van Laar adds. “The sustainability value of coating technology is fast becoming a key product differentiator, with more painters focusing on the environmental and health and safety impact of their consumer choices and professional activities,” he concludes.

Selection of Optimal Products

The choice of wood coating is based on several factors, including the type of wood, its condition, the application, preference for solvent or water-based systems, and, of course, cost. Typically, clear to semi-transparent coatings are used on new wood, semi-solid to solid color coatings are applied to slightly aged or stained wood, and thicker restoration coatings are used on wood that has been neglected but is still structurally sound, says Fedders.

With respect to resin choice, the most common technologies used in the exterior wood coatings market are alkyds, acrylics, and urethanes, and selection depends on the specific performance requirements. “Each of these technologies has specific benefits for end users, and it very much differs per region which technology is preferred by the painters or end users,” van Laar comments. The top waterborne systems include acrylics, alkyds, and acrylic hybrids, while alkyds are the main resin type used in solvent-based systems, according to Bauer. Going forward, van Laar expects that hybrid and tribrid solutions will be needed to meet evolving design and market needs.

The top waterborne systems include acrylics, alkyds, and acrylic hybrids, while alkyds are the main resin type used in solvent-based systems . . . hybrid and tribrid solutions will be needed to meet evolving design and market needs.

Solvent-based alkyd stains are fast-drying, cost-effective, and can provide a matte to glossy appearance. “Alkyd is still the leading chemistry because it works, is economic, and these applications are well-tested and accepted,” asserts Pras. He also notes that contractors want systems that are tried and true and will not cause headaches after application (i.e., claims, return visits, defects), while the DIY market wants easy to apply and clean systems. Alkyds can meet all of these requirements. For decks and fencing in Europe and the United States, high penetration coatings rather than film formers are generally used, according to Bauer. Options include linseed oil, tung oil, teak oil, and wood oils because they allow for a more natural appearance of the wood. She adds that professionals in the U.S. market also prefer oil-based products because they offer re-coating opportunities and a sustainable revenue stream without film forming or sheen. For siding, however, while a small segment of consumers desires traditional oil-based stains, water-based/acrylic stains and finishes continue to become more popular in the industry due to their ease of use, clean-up, and durability, according to Bautista. Behr is also seeing a shift toward solid color stains, as opposed to less opaque stains, due to their durability and longer warranties.

Moving from Solvent-Based To Waterborne

The switch from solventborne to waterborne systems is one of the key drivers of new technology development. “As more countries implement stricter regulations and ecolabeling requirements, formulators are challenged to develop waterborne systems that maintain the same functionality as traditional��solvent-based coatings,” says Sara Mårlind, marketing manager for Levasil Colloidal Silica at Nouryon. The performance of a coating in exterior architectural applications is influenced by the type and intensity of exposure, species, and quality of wood, and the coating properties such as glass transition temperature, thickness, permeability, light stabilization, etc., notes Ravichandran. While the resin is the most important component, Fedders notes that the entire formulation must work together to achieve the desired performance profile. He gives as one example the importance of UV absorbers for exterior applications. Exterior architectural coatings must also be fast drying, have low odor, and offer enhanced durability and protection for extended product lifetimes, according to Pras.

Technologies that are enabling optimization of durability and penetration capabilities include water-reducible alkyds and/or acrylic hybrids . . . . Other technologies being investigated include sol-gel chemistry, waterborne micelles, micro-emulsion reactions, self-assembled structures, nano materials, and nano technology.

Once again, different members of the value chain have different performance requirements that impact coating development. Overall, Bauer says that ease of application/formulation with improved brushability, non-drip coatings, recoatability, and longevity drive innovation, but longevity is more important in Europe where some manufacturers offer 10-year warranties, while in the United States, DIY consumers continue to look for water-repellency and non-drip coatings. Paint producers, meanwhile, want to formulate with fewer additives and are looking to resin manufacturers to help with multiple performance properties like matting, abrasion resistance, gloss, etc.�� “Technologies that are enabling optimization of durability and penetration capabilities include water-reducible alkyds and/or acrylic hybrids,” Bauer notes. She adds that there is research underway to determine if improvement in rheology leads to improved brushability and if coatings can be developed that can be successfully applied under non-deal conditions, such as to wet wood. Other technologies being investigated, according to Ravichandran, include sol-gel chemistry, waterborne micelles, micro-emulsion reactions, self-assembled structures, nano materials, and nano technology, with a focus on the synergistic action of these techniques for the development of coatings that afford scratch resistant, antibacterial, dust-repellent, and/or self-healing surfaces.

Improving Efficiency

Margin compression across the value chain has led to a widespread introduction of cost and production efficiency measures to add value to the total cost of ownership and formulation, van Laar says. Higher levels of mechanical and chemical resistance have also been achieved in recent years, extending the maintenance intervals for exterior architectural wood coatings. Use of additives like colloidal silica has made it possible to achieve the same results in waterborne coatings, such as dirt pick-up resistance and excellent durability, that have been achieved only with solventborne coating formulations, facilitating the move to more sustainable solutions, according to Mårlind. The incorporation of higher levels of bio-based content into coating solutions, without affecting functional performance, has also enabled lower carbon footprints and stronger environmental profiles, according to van Laar. As an example, he points to DSM’s Decovery® SP-2022 XP resin, a self-matting bio-based solution for protecting the natural beauty of surfaces. “Coatings formulated with this resin offer outstanding ultra-matt aesthetics and high performance that customers and consumers demand, while greatly improving production efficiency and ensuring respect for the health of people and the planet,” van Laar asserts.

Behr focuses on the development of products that reduce the return-to-service time and help mitigate environmental elements. One example is BEHR Premium Quick Dry Oil Base Wood Finish, which can be applied to damp wood; dries in 60 minutes (dramatically faster than other oil-based stains); resists rain after 60 minutes; and allows the user to prep, stain, and entertain all in the same day (as opposed to the two- to three-day turnaround time of a typical stain project), according to Bautista. Water-based acrylic coatings with erosion failures have been another important development, says Fedders, because they provide acrylic durability without undesirable flaking failures that require sanding. ��He also points to the development of next-generation restoration coatings, now on the market, that minimize flaking failures and greatly extend the lifetime of decks. “Cool” deck coatings that do not attract heat are also adding value in this market, according to Bauer.

Use of additives like colloidal silica has made it possible to achieve the same results in waterborne coatings, such as dirt pick-up resistance and excellent durability, that have been achieved only with solventborne coating formulations.

Hexion, meanwhile, is seeing increased interest in affordable truly 1K moisture-curable systems for use in exterior architectural applications. In response, the company has developed VeoVa™ silane technology comprising VeoVa vinyl ester and silane monomers. “This isocyanate-free platform allows customers to tailor their products for the desired end uses, balancing hardness and flexibility and pot life and cure speed, while also delivering similar performance to that of 2K polyurethane coatings,” says Pras.

Technology Trends

Current research covers many different areas. Nouryon, for instance, is focused on achieving anti-soiling properties with its Levasil Colloidal Silica product across different waterborne coatings applications. Hexion continues to develop technologies that close the gap in performance between waterborne and solvent-based exterior wood coatings, including acrylic emulsions based on extremely hydrophobic monomers for improved water repellence and durability, waterborne dispersions that can be formulated totally solvent-free, and OH-emulsions with improved appearance and high crosslink densities. Michelman is also working on improving the performance, ease of application, and penetration of waterborne products, as well as enhancing the long-term mildew and mold resistance of wood coatings to eliminate the need for use of cleaning solutions that damage the wood and potentially the landscaping around it. AkzoNobel continues to refine its global weathering approach for exterior industrial wood coatings, which involves the use of accelerated weathering testing and external weathering all over the world in some of the harshest climates. The goal is to ensure the company gathers the appropriate data and provides the highest confidence in the exterior durability performance of its coatings.

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

Many of the wood substrates incorporated into new residential and commercial buildings are coated at the factory. While the performance requirements are the same for field-applied coatings, the dynamics of the market are wholly separate.

In Europe, waterborne systems including one-component, core-shell acrylics are often used. There is also some use of waterborne, two-component polyurethane topcoats and primers for bridging the gap between waterborne and solventborne coatings, according to Anthony Woods, segment marketing director for Wood Coatings at AkzoNobel. These systems are also widely used in North America. While demand for UV and waterborne UV-cured coatings for exterior protection is currently low in Europe, with new formulations and increased durability, AkzoNobel expects to see a move towards UV in the future, enabling customers to benefit from production efficiencies. “100% UV coatings allow for very fast curing, with many lines running at over 100 feet/minute, and also provide excellent chemical resistance and toughness,” Woods explains. He does note, though, that the health and safety profile of reactive UV oligomers means this technology is not generally suitable for spraying. Instead, solventborne or, more commonly, waterborne UV is used when low-VOCs, speed, and chemical performance are required. Waterborne UV is already growing in popularity in North America due to its similar application characteristics and tougher, more durable finish.

“Overall, innovations in formulated coatings for exterior architectural wood products are being driven by the desire for higher productivity through faster lines and shorter cure times, coupled with the need to resist blocking and provide good crush resistance. There is also a constant drive to improve the durability of exterior products. The design of exterior coatings is always a balance between developing a coating flexible enough to deal with the dimensional instability of the substrate, and the durability demands of dirt pick-up resistance, toughness, and gloss retention. The recent development of 100% UV products is probably the most significant step change in industrial wood coatings. These coatings deliver improved durability combined with a step-change in production costs,” Woods states. Within the waterborne field, he observes that the constant improvement of core-shell and gradient latex technology is also improving durability. In particular, he points to nano-core shell lattices, which give excellent wood penetration to improve aesthetics while also delivering the required performance.

Woods also observes that the use of the chemically modified wood accoya (acetylation via pressure impregnation for mold resistance, longer life, and increased durability) in joinery, decking and cladding applications is creating opportunities for the development of new coating systems that match the advances in the material itself. Finally, AkzoNobel has also observed an increasing use of renewable materials for resin manufacturing and more non-biocidal products that provide strong anti-
fouling protection while avoiding the leaching of biocides from the wood substrate/coating.

Consumer and regulatory pressure to replace bisphenol-A (BPA)-based materials in food contact metal packaging coatings has increased in recent years. Regardless of the controversy around BPA, consumers expect canned foods to be free of substances perceived to have negative health impacts while maintaining current shelf life and flavor characteristics. To address the market needs, formulators must innovate to deliver BPA-non-intent (BPA-NI) solutions that can meet or exceed the performance of BPA-based materials. This presents a challenge with regard to improving the resistance to food sterilization and stability during pack testing, and simultaneously balancing mechanical performance that allows the BPA-NI coating to withstand the aggressive canning process.

One response to these technical challenges has been the development of BPA-NI polyester resin technology through innovation on a monomer basis. This monomer innovation provides protective performance attributes such as resistance to corrosion and chemical attack, while enabling flexibility and adhesion through innovative resin and formulation design. Fundamental techniques such as electrochemical impedance spectroscopy (EIS) and cathodic disbonding were employed in combination with industrial fitness-for-use evaluations to demonstrate the improved protective barrier properties of novel non-BPA resins in formulated coatings. In addition, hydrophobicity and interfacial properties were studied to understand the impact of resin structure on coating performance from both experimental and computational perspectives. Applying this suite of methods and analysis builds strong structure-property correlations as part of a resin development strategy for novel non-BPA resins in metal packaging coating applications.

INTRODUCTION

Bisphenol-A (BPA) is a chemical commonly used in food contact plastics and coatings applications such as the BPA-epoxy-based linings of metal cans containing food or beverages. In recent years, the use of BPA in food contact applications has come under scrutiny. The 2003–2004 National Health and Nutrition Examination Survey (NHANES III) conducted by the Centers for Disease Control and Prevention (CDC) found detectable levels of BPA in 93% of 2517 urine samples from people six years and older.¹ In 2008, the National Toxicology Program of National Institute of Health (NIH) determined that BPA may pose risks to human development, raising concerns for early puberty, prostate effects, breast cancer, and behavioral impacts from early-life exposures.² Due to the potential health concerns, France has banned the use of BPA in all packaging, containers, and utensils intended to come into direct contact with food since 2015.³

With increasing pressure from food brands, formulators and can makers are actively looking for alternative solutions that can meet or exceed the performance of BPA-based coatings. From a technical standpoint, it is challenging to find the right alternatives due to the rigorous performance requirements for the coatings as well as the low price of BPA-epoxy resins. For example, the coating must be able to endure high temperatures, high pressure food sterilization, and long-time direct contact while exposed to the food materials, which include hydrolytic and corrosive environments such as low pH, acids, sulfur, and salt. Adhesion of the coating to the metal can is also crucial for both preventing corrosion and withstanding the can forming process. To respond to the technical challenges, coating scientists and chemists must innovate to develop new resin technologies.

Among all the new resin technologies today, polyesters with a balance of key performance attributes have emerged as one of the most promising alternative solutions. For polyester resins in this application, it should be noted that enabling high glass transition temperature (T_g) in combination with good mechanical properties, such as flexibility and toughness, is critical to the final film performance. These considerations are often applied when selecting monomers for resin design. Therefore, several qualified specialty glycol monomers such as 1, 4–cyclohexanedimethanol (CHDM), isosorbide, tricyclodecane dimethanol (TCDDM), and 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD) have received broad attention as building blocks. TMCD also demonstrated superior hydrolytic stability to other specialty monomers in a degradation kinetics study based on model compounds (Figure 1). Through previous work, polyesters containing TMCD are also known to demonstrate a variety of excellent properties such as good temperature resistance, toughness, chemical resistance, and hydrolytic stability. They have been successfully used in BPA-free specialty plastics applications such as durable water bottles.^4-6 Applying the monomer innovation to resin development, TMCD-based resin systems have shown attractive performance attributes such as resistance to corrosion and chemical attack, while enabling flexibility and adhesion through innovative resin and formulation design.

To better understand the structure–property relationships of polyester resins, a series of fundamental methodologies including electrochemical impedance spectroscopies (EIS) and cathodic disbonding tests were developed to study barrier properties, interfaces, and adhesion of metal packaging coatings. EIS is a widely used corrosion evaluation tool based on an electrical analogue of corrosion processes. It uses simple electrical circuits typically comprising of resistive and capacitive elements. For polymer coated metal systems, the EIS test is sensitive to the electrochemical changes at metallic interfaces as well as the resistive properties of organic coatings in a variety of aggressive or corrosive environments. In the past decades, both academia and industry have been using this technique to characterize coating barrier properties including diffusivity and polarity,⁷ to characterize water uptake,⁸ to detect formation of blisters and pinholes, and to recognize the loss of adhesion.⁹ The cathodic disbonding test is also an effective electrochemical technique to evaluate adhesion performance. Cathodic disbonding is an important delamination mechanism associated with interfacial corrosion of organic coatings on metal substrates that lead to an exposure of bare metal to the aqueous food environment.¹⁰

In this study, the electrochemical techniques have been employed in combination with industrial fitness-for-use (FFU) evaluations to better understand the barrier properties, interface, and adhesion of metal packaging coatings. This work provides fundamental insights on the relative performance of polyester resins in a variety of formulations where improved barrier properties and enhanced coating-
metal interfacial strength is observed with the TMCD-based resins.

Experimental Procedures

Materials and Sample Preparation

In this study, BPA-NI Resin A and Resin B were TMCD-based polyester resins developed and produced by Eastman Chemical Company, while Control A and Control B are the benchmark resins based on commercial BPA-NI polyesters. All of the polyester resins were classified by molecular weight: both Resin A and Control A (Category A) have absolute-number average molecular (Mn) greater than 10,000 g/mol while Resin B and Control B (Category B) have absolute-Mn in the range of 4,000–7,000 g/mol. Formulation components in this study were chosen for the purpose of representing BPA-NI interior lacquers. All coatings based on these polyester resins were formulated and applied in the Eastman Chemical coating development laboratories. The details of resins and formulation components are given in Table 1 and Table 2.

Prior to formulating, Resin A and Control A were reduced with Aromatic 100 to achieve 50% solids; Resin B was reduced with Aromatic 100 to 55% solids; Fascat 9102 catalyst was diluted to 10% of the original supplied concentration with Aromatic 100; likewise, Nacure 5925 catalyst was also diluted to 20% of the original supplied concentration with Aromatic 100. The formulation details of gold benzoguanamine phenolic formulation, clear PU formulation, and white PU formulation are provided in Table 3 and Table 4, respectively.

Electro tin plate (ETP) substrate panels were described by the vendor, Lakeside Metals Inc. as 0.25 # Bright T-1 0.009–0.010 x 4.0 x 12.0 in. All formulated paints were drawn down onto 4.0 x 12.0 in. (10.16 cm x 30.48 cm) tinplates (provided by Lakeside Materials Inc.) using an appropriate wire wound rod targeting a dry film thickness (DFT) of 0.4 mils (10.2 μm) for clear and gold formulations, or 0.6 mils (15.2 μm) for white formulations. Following a 15 min room temperature flash-off at constant temperature and humidity conditions (73°F ± 2, 50% RH ± 5), panels were baked at 200°C for 12 min in an air oven.

Testing and Evaluations

Electrochemical Impedance Spectroscopy (EIS)

A Gamry Instrument “Reference 600” Potentiostat, equipped with Gamry framework and Echem Analyst, was used in electrochemical impedance measurements. The flat coated panels (3.0 x 4.0 in.) were masked by a Gamry designated masking tape with a hollow circle at the center. A cylindrical glass cell with a rubber O-ring attached to the grooved bottom of the cylindrical cell and a clamp fixture was used to hold the samples. A nickel electrode and a graphite electrode were used as reference and auxiliary electrodes, respectively. To simulate food, several food simulants were used as the electrolyte and corrosive environments. The potential was applied in a range of ± 5 mV from open circuit potential and the frequency was varied from 10⁵ to 10^-1 Hz.

Cathodic Disbonding Test

The cathodic disbonding test is an internally developed coating disbonding test modified from a number of standard test methods.¹¹ It helps to differentiate coatings on the basis of their susceptibility to adhesion failure in the presence of a defect. The scheme of the cathodic disbonding experimental setup and the lab setup are shown in Figure 2a and 2b. The flat coated panels (3.0 x 4.0 in.) were masked by a Gamry designated masking tape with a hollow circle at the center. In the unmasked area at the center, the coatings were scribed with an “X” mark (as shown in Figure 2b using a knife. A cylindrical glass cell with a rubber O-ring attached to the grooved bottom of it and a clamp fixture was used to hold the samples. A coated panel with X scribe (cathode) was connected with the negative electrode of the voltage supply while a graphite (anode) was connected with the positive electrode of a direct current (DC) voltage supplier. Between cathode and anode, the cylindrical glass cell was filled with electrolyte made by 3.5 wt% NaCl solution with 150 ppm Manoxol OT solution to ensure both parallel electrodes were submerged in the electrolyte solution. The DC voltage applied between the parallel cathode and anode was 5 volts (V) for 60 sec. After this electrochemical process was completed, the sample was rinsed with DI water and followed by air drying.

Under the applied DC electric field, the hydrogen bubbles were initially generated at the X scribe and then propagated to the coating-tinplate interface following the cathode reaction as shown in equation (1). In addition to the hydrogen bubbles generated at the metal surface lifting up the coating from the tinplate, the generation of ����¯ also weakens the coating–tinplate adhesion, resulting in coating delamination.

(1)

Food Simulants and Retort

The recipes of food simulants used in this study are shown below:

(i) Lactic acid–Acetic acid-Salt (LAS) food simulant: 1 wt% lactic acid, 1 wt% acetic acid, and 1 wt% NaCl in 97 wt% DI water;

(ii) 3% Acetic acid food simulant: 3% acetic acid in 97% DI water; and

(iii)�� 2% Lactic acid food simulant: 2% lactic acid in 98% DI water.

A coupon measuring 2.5 x 4.0 in. was cut from the coated panel for retort testing. The coupons were scribed by a knife with an X mark on the bottom-half of the panel and then placed in 250 mL closed-cap glass jars half filled with 3% acetic acid food simulant where half the coupon is out of the food simulant and the other half is submerged in food simulant. The retort test was conducted on a coated panel with an X scribe at 131°C for 60 min in an autoclave.

Computational Modeling

Calculated LogP values which estimate the value of the octanol-water partitioning coefficient were determined by using ACD/Labs Chemsketch software. The calculated logP values were also validated by ChemBioDraw Ultra 13.0, Molinspiration and Accelrys Materials Studio 5.5. Molecular structures of trimer model compounds represented as glycol1-terephthalate-glycol2 (G1-T-G2) with hydroxyl end groups were used in the experiments and calculations.¹²

Solubility parameters were calculated for 30x30x30 ų amorphous cells with 1 g/cm³ cell density for 3 terephthalate and glycol units that were constructed by using Accelrys Materials Studio 5.5 software. PCFF forcefield was used to build and minimize cells and calculate cohesive energies, which led to the calculation of cohesive energy densities followed by calculation of solubility parameters.¹³ Average solubility parameters were calculated for 500 different amorphous cells followed by 5000 steps of molecular mechanics geometry optimization with Ewald Summation method¹⁴ and 12.5 Å vdW cut-off distance for each composition.

Results and Discussion

EIS and Corrosion Mechanisms

EIS is a powerful technique to understand corrosion mechanisms and barrier properties of coatings by providing an accurate in-situ measurement for characterizing polymer-coated metals and changes in coating performance during exposure in corrosive environments. The term “impedance” refers to the frequency-dependent resistance to current flow of circuit elements such as resistors, capacitors, inductors, etc. In practice, the corrosion resistance is integrated from all types of resistance involved and can be approximately estimated by using |Z(w®0)|, the impedance value at low frequencies. In this study, corrosion resistance is identified by the impedance value at 0.1 Hz. However, to describe the corrosion process quantitatively, Bode plots are fitted using electrical equivalent circuits corresponding to the appropriate stage of corrosion.

Stage Zero: Dry Film

Like a dielectric layer, before immersion in an electrolyte solution a dry polymer film often plays a role like a pure capacitor as shown in Figure 3.�� When the film responds to the frequency, the overall impedance can be presented as shown in equation (2):

(2) therein ω = 2πf

where j is the complex number (  j ²=-1); R_sis the solution resistance which is the resistance of the food simulant; C_cis the coating capacitance; w is the angular frequency; and_�� f _����is the frequency. The Bode plot for Stage zero is simply demonstrated in Figure 3.

��

Stage I: Food Simulant Absorption

Before the corrosion process starts, the dry coating must absorb the electrolyte until the polymer film gets fully saturated by the electrolyte as shown in Figure 4. The electrolyte is the food simulant in this case. In Stage I, the coating is no longer a pure capacitance due to the presence of water and ions. Instead, the combination of the capacitance component and the resistance component of the coating contribute to the overall impedance. With higher water and ion uptake, the value of the capacitance component increases while the value of the resistance component decreases. They change independently as a function of time before the film gets fully saturated by the food simulant. In general, the diffusion kinetics are highly dependent on the physical properties of the polymer film such as crosslinking density. When the film responds to the frequency, the overall impedance can be presented as shown in equation (3):

(3) where j is the complex number (  j ²=-1); R_sis the solution resistance which is the resistance of the food simulant; C_c�� is the coating capacitance; w is the frequency; and R_c��is the coating resistance. As shown in Figure 4, a single time-constant can be indicated by a frequency–independent impedance plateau at low frequency followed by a frequency–dependent impedance plot in the medium frequency region. The increase of C_cas a function of exposure can be used to determine the diffusion coefficient as well as the food simulant uptake.

Stage II: Corrosion Initiation

After the film gets fully saturated, water and ions in the food simulant start to be delivered to the coating-tinplate interface and initiate the corrosion process. In this process, the redox reactions between the metal and food simulant (H⁺��either from food simulant or from hydrolyzed water) lead to corrosion. At this stage, the newly formed oxidized layer with semi-dielectric character exists under the polymer film, playing a role as the combination of a double layer capacitance and a charge transfer resistance, as presented in equation (4):

(4)
where R_s��is the solution resistance which is the resistance of the food simulant; C_c�� is the coating capacitance; w is the angular frequency; C_dl is the double layer capacitance; R_ct�� is the charge transfer resistance; and R_c�� is the coating resistance.

Stage III ~ IV: Pore/breakthrough Formation and Delamination

A process that depends on diffusion of reactants toward or away from the surface has a particular low-frequency character. The impedance with this characteristic is usually described as “Warburg” impedance (as shown in Figure 7), which indicates the breakthrough of a barrier and localized disbonding. At this stage, the overall impedance can be presented as shown in equation (4) (pore/breakthrough formation) and equation (5), (delamination), respectively:

(5)

(6) where R_s�� is the solution resistance which is the resistance of the food simulant; C_c�� is the coating capacitance;�� w is the frequency; C_dl�� is the double layer capacitance; R_ct�� is the charge transfer resistance; R_po�� is the pore resistance; k is the redox reaction rate; and D is the diffusion coefficient at this stage. At Stage III ~ IV, the barrier has been damaged locally, even though the defects might not be able to be detected by the naked eye. In this case, R_ct�� representing pore resistance is used to describe the concept of “coating resistance” because this value now is highly dependent on the number of pores in the film or capillary channels resulting from the formation of ionically conducting paths through the coating, instead of the intrinsic physical properties of the coating barrier.

To demonstrate the progression of coating failure as a function of LAS food simulant exposure time, Control B was chosen to formulate a clear coating, then subjected to the EIS test in LAS food simulant. As shown in Figure 7, Control B in a clear PU formulation starts to show Stage I corrosion (one time-constant) after 4 min of exposure, indicating that the food simulant absorption has begun. After 15 min, the characteristics of Stage II corrosion (e.g., two time-constants) have been observed, and this is followed by the Bode plot with Warburg character (Stage III ~ IV) after 5 h of exposure in LAS food simulant.

To compare BPA-NI polyester resins in clear PU formulations, Resin A and commercial Control A were selected to formulate the paints followed by appropriate baking. A comparison of the EIS spectrum of Resin A and Control A in these two coatings in Figure 8a shows similar corrosion resistance and barrier properties after 5 h of exposure. Resin A-based clear PU shows a corrosion resistance of 53.7 mega Ohms, which is slightly higher than that of the Control A-based coating (39.8 mega Ohms). Both coatings demonstrate excellent barrier properties and almost two orders of magnitude improvement on corrosion resistance after 5 h of exposure as compared to the low T_g polyester control in the same formulation (0.69 mega Ohms, as shown in Figure 7).

In Figure 8, Resin A was formulated in both clear PU and gold benzoguanamine phenolic formulations. Through the comparison of EIS data after 5 h of exposure (Stage I corrosion), it has been found that the gold benzoguanamine phenolic formulation exhibits significantly better corrosion resistance (170 mega Ohms) as compared to the corrosion resistance of clear PU formulation (53.7 mega Ohms) at the same dry film thickness. The authors believe that the presence of triazine and aromatic structures in benzoguanamine-formaldehyde and phenolic-formaldehyde crosslinkers may provide better hydrophobicity and barrier properties as compared to IPDI trimer-based PU structures.

Time-based Corrosion Resistance

In many cases, corrosion is an electrochemical process that requires multiple steps, and each step is associated with a different mechanism and kinetics. The estimation or prediction of long-term corrosion performance, such as the corrosion observed in a pack test, often relies on continuous time-based corrosion observations over a relatively long interval of testing instead of a single data point at a short exposure time. As part of a resin design strategy, a continuous in-situ EIS test has been conducted on white PU coatings based on four BPA-NI polyester resins including Control A (high-T_g high Mn polyester), Control B (low-T_g low Mn polyester), Resin A (high-T_g high Mn polymer), and Resin B (med-T_g low Mn polyester) in 2% lactic acid food simulant. The EIS test in one testing period was set up to continuously run for 48 h. Ten hours of relaxation time was given prior to the next testing period. Figure 9a and 9b demonstrate a decay of corrosion resistance as a function of exposure time for each sample during the 1st and 2nd 48-h test intervals. After a total of 106 h of exposure, all the white PU samples still remain in the Stage I corrosion process. During the 2nd 48-h test period, the decay of corrosion resistance for each sample becomes significantly slower, followed by a plateau of impedance at longer times. The two high Mn polyesters seem to be separated from the other two low Mn polyesters after the 2nd 48-h interval, where the high Mn polyester-based coatings show higher values of corrosion resistance. By the end of the test, the comparison of all four white PU coatings shows a ranking on corrosion resistance: Resin A > Control A > Resin B > Control B in white PU formulations (Figure 10). Since all of the samples are still in Stage I corrosion, the decay kinetics indicate the diffusion coefficient while the films absorb the food simulant, whereas the plateau level reflects the solubility of the electrolyte solution in the coating film. Therefore, higher corrosion resistance correlates with higher hydrophobicity or lower solubility in 2% lactic acid food simulant in this case. In Figure 11, LogP and Hildebrand solubility parameters were calculated for glycol (G1)-terephthalic acid (T)-glycol (G2) trimer model compounds through computational modeling. In general, a higher LogP value or lower Hildebrand solubility parameter indicates better hydrophobicity of a polymer. With the same molecular weight (trimers) and acid composition (terephthalic acid) in the model compounds, it has been hypothesized that the glycols with higher LogP values or lower Hildebrand solubility parameter lead to a lower concentration of the aqueous food simulant in the bulk of the film, thus reducing the rate of corrosion.¹² Considering the molecular weight contributions in the resins, the comparison between a TMCD-based resin and a control polyester resin at a similar molecular weight and T_g range (Resin A vs Control A and Resin B vs Control B) indicates that the improvement of corrosion resistance in coatings formulated with Resin A and Resin B is primarily due to the hydrophobicity contributions from TMCD (Figure 11).

Interface and Adhesion

To better understand what happened during late stage corrosion, the EIS comparison between Resin A- and Control A-based clear PU coatings (shown in Figure 8a) was extended to longer exposure times. After 12 days in LAS food simulant, it has been observed that both Resin A- and Control A-based clear PU coatings are in Stage III ~ IV corrosion with clear Warburg impedance in the Bode plots (Figure 12). Although the pores and capillary channels that provide conducting paths (e.g., Warburg impedance) through a coating have already formed in both Control A- and Resin A-based clear PU coatings, the interface, with its excellent double-layer capacitance and charge transfer resistance, can still provide excellent corrosion prevention.

In Figure 12, Resin A shows a higher plateau in the middle—frequency range, which indicates the value of charge transfer resistance corresponding to equation (6). When a redox reaction occurs, electrons enter the metal and metal ions diffuse into the electrolyte. Thus, charge is being transferred. The current density of the charge transfer process at the applied potential follows Faradays Law [equation (7)]:

(7) where i_o��is exchange current density;�� C_o is the concentration of oxidant at the electrode surface; C_o*����is the concentration of oxidant in the bulk; C_R�� is the concentration of reductant at the electrode surface; C_R*������is the concentration of reductant in the bulk; h is the overpotential (difference between applied potential and open circuits potential, OCP); F is Faradays constant; T is absolute temperature; R is the ideal gas constant; a is the reaction order; and n is the number of electrons involved. When the overpotential is very small (± 5 mV vs OCP in this experiment) and the electrochemical system is at equilibrium (C_o�� =�� C_o*���� and�� C_{R����}=  C_R*����), charge transfer resistance can be represented, as shown in equation (8):

(8) With the same experimental conditions, the difference between the two clear PU coatings on charge transfer resistance is believed to be due to the number of electrons involved, which correlates to the percentage of area without polymeric barrier due to the loss of adhesion. Several studies^15-17 for different applications have found that charge transfer resistance is correlated to adhesion experimentally. For the comparison shown in Figure 12, the authors believe that the Resin A-based clear PU with significantly higher charge transfer resistance indicates that Resin A provides a clear PU coating with a stronger coating-tinplate interface and better adhesion as compared to Control A.

Besides the EIS tests, gloss loss after a 3% acetic acid retort test was measured as a way to evaluate the barrier performance of the coatings. As the retort test was conducted on a coated panel at 131°C for 60 min in an autoclave, the presence of high temperature and pressure has significantly accelerated the corrosion formation at the coating-tinplate interface, causing the development of “blisters” as the result of under-film corrosion. This process often leads to gloss loss due to (i) the changes in surface and interface smoothness caused by under-film corrosion; (ii) rust stains the coating surface caused by broken “blisters”; and (iii) a change in coating refractive index caused by water retention. Similar to what occurs during the late stage corrosion process, the corrosion development and coating delamination at X scribes could be much faster than that in other areas because the barrier layer has been broken through manually. In this scenario, adhesion performance can be evaluated based on a visual observation. As shown in Figure 13a, no significant difference can be observed between Resin A- and Control A-based clear PU coatings from gloss reduction, indicating that Resin A provides similar barrier properties as Control A in the clear PU formulation. This retort result is consistent with the insights obtained from EIS data (Figure 8a).

In Figure 13b, when the aggressive cathodic disbonding test was applied under 5V on X scribed panels, the Control A-based clear PU coating with a large disbonded area (less desirable) is differentiated from the Resin A-based coating with very little disbonded area (more desirable). The disbonded area was identified by a high-resolution camera and the areas of bare tinplate were quantified by pixel-counting image analysis software (Figure 14). In this case, the disbonded area, in percentage of the total area, of Control A-based clear PU coating was determined as approximately 52%, while only a minor disbonded area (< 5% disbonded area) was found for Resin A-based clear PU coating. In this case, a smaller disbonded area indicates better adhesion or interfacial strength. The cathodic disbonding result in Figure 13b is consistent with the EIS results (Figure 12) regarding adhesion performance. The combination of EIS and cathodic disbonding results indicates that Resin A provides a stronger interface to tinplate as compared to Control A. Considering the molecular weight and T_g effects, the comparison between TMCD-based Resin A and Control A polyester resin at a similar molecular weight and T_g range indicates that the improvement of adhesion in coatings containing Resin A could be due to the hydrophobicity contributions from TMCD.

CONCLUSIONS

In this study, fundamental methodologies based on EIS and cathodic disbonding tests were successfully developed and applied to understand the barrier properties, interfaces, and adhesion of BPA-NI metal packaging coatings. Corresponding to EIS Bode plot characteristics, a series of equivalent circuit models indicating the stages of corrosion process were developed to demonstrate the degradation mechanisms including (i) food simulant absorption, (ii) corrosion initiation, and (iii) pore/breakthrough formation. These circuit models also enable quantitative analysis of coating performance to design resins that are in tune with coating properties in metal packaging applications.

A combination of electrochemical techniques and industrially relevant FFU evaluations were conducted to differentiate the corrosion resistance of TMCD containing vs non-TMCD containing clear and white PU coatings. In the early stage corrosion process, Resin A exhibited a slightly improved barrier performance in the coatings as compared to Control A due to the low permeability after being fully saturated by the food simulants. This is believed to be due to the hydrophobicity contribution from TMCD in polyester Resin A. A similar conclusion has been obtained when comparing the TMCD-containing Resin B to the Control B resin in both white and clear PU coatings.

In addition to PU formulations, TMCD containing Resin A and Control A resin were also formulated with benzoguanamines and phenolic crosslinkers to evaluate these resins in gold lacquer applications. A comparable barrier performance was observed during the early stage corrosion processes in a clear PU coating with Resin A in and Control A. However, the comparison between Resin A in the clear PU formulation and gold benzoguanamine phenolic formulations indicates that the gold formulation is significantly better on barrier performance with the same resin and film thickness.

EIS results for a late-stage corrosion process based on Resin A- and Control A-based clear PU coatings are consistent with the results obtained from cathodic disbonding tests. Analysis of these results demonstrates that the coating made with TMCD-containing Resin A provides a significantly improved coating-tinplate interface that leads to superior adhesion performance as compared to Control A resin in a clear PU formulation.

ACKNOWLEDGMENT

Technical expertise provided by Damiano Beccaria, Sandra Case, Peter Chapman, Alain Cagnard, Samuel Puaud, John Maddox and Carlos Carvajal from Eastman Chemical Company is gratefully acknowledged. The Computational Modeling results presented in this paper were generated in collaboration with Erol Yildirim, Harold Freeman, and Melissa Pasquinelli, Fiber and Polymer Science Program, North Carolina State University. The contribution made by Dr. Yildirim for this paper is highly appreciated. The authors also appreciate the technical support from Dr. Li-piin Sung through the Polymer Surface Interface Consortium, Engineering Laboratory, National Institute of Standards and Technology.

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*This paper received the Best Paper Award at the 2019 CoatingsTech Conference, sponsored by the American Coatings Association, April 8–10, in Cleveland, OH.

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