Occasionally, one or more solvents in a paint formulation need to be replaced due to VOC or HAPs concerns, toxicity, cost, odor, or lack of availability. Although a single solvent may be an adequate replacement, often a blend of solvents is a better choice. In addition, this may be an opportunity to upgrade the overall blend for cost, ease of application, wettability of surfaces, viscosity, and other properties.

When I am asked to find a replacement, I begin by considering evaporation rates. I use my own lists and supplier literature/websites to pick out several solvents with evaporation rates in the region of the solvent being removed from the formula. Then I look at the other properties of these solvents, such as viscosity and surface tension. Solvency is important, but most common resins are soluble in a wide range of solvents, particularly in high solids paints with low molecular weight resins. If the Hansen Solubility Parameters (HSP) of the replacement are within 2–3 MPa^1/2 units of those of the solvent being replaced, the solvency probably will be acceptable. [See the May, July, and August issues of CoatingsTech, 15, (5) 60; (7) 76; (9) 48 (2018) for articles on HSPs]. By comparing the various properties of the solvent being replaced with those of the candidate solvents (entering the data in a table is useful), the choice can be narrowed down to two or three solvents or solvent blends. These then can be tested in the formula. The major solvent suppliers offer software that can make the process considerably less complicated.

The major solvent suppliers offer software that can make the process considerably less complicated.

As an example, let us look at the results of a quest to replace 2-ethoxyethyl acetate (Cellosolve^TM acetate) in a high solids polyester-melamine coating. Table 1 shows properties of the solvent to be replaced, along with those chosen for initial consideration (as individual replacements or as parts of a blend).

Most of the solvents listed have evaporation rates very close to that of Cellosolve acetate. PM acetate has a higher evaporation rate of 32, but it can be blended with a slow solvent, such as butyl Cellosolve acetate, DPM acetate, or DBE to give a solvent with about the same average evaporation rate as the Cellosolve acetate. However, a blend is not identical to the solvent that it is replacing. The slow solvent in a blend may remain in a film, making it soft, even tacky. The slow solvents listed will not improve wetting as they have high surface tensions.

I like to use low surface tension solvents to help with paint application, wetting of surfaces, and minimization of surface defects. Amyl alcohol and DIBK have low surface tensions (< 25 dyn/cm), whereas cyclohexanone has a relatively high surface tension (35 dyn/cm). Other considerations include odor and cost. Many people find the odor of cyclohexanone objectionable, and some dislike the odor of PM acetate. Cyclohexanone and Pentoxone are expensive. Further analysis can be done by using an in-house or supplier solvent computer program to evaluate new solvent blends that include replacements and determine whether they have similar properties to the old blend. Based on the information collected, hexyl acetate, amyl alcohol, and DIBK looked best and were suggested for use in test formulations.

My recent articles on Hansen Solubility Parameters (CoatingsTech, May, July, and September 2018) have raised questions about what other solvent properties are important and about how to choose solvents. The main job of solvents is as carriers for resins, pigments, and other components. Other functions include controlling viscosity for application and flow-out, aiding in film formation, and wetting of pigments and substrates to help with dispersion and adhesion. Important solvent properties besides solvency include physical properties such as volatility, boiling point, surface tension, viscosity, and electrical resistance/conductivity. Other important criteria that I will not discuss here include cost, toxicity, flammability, and odor.

After the ability to dissolve or disperse resins, the volatility of solvents is the next key property in their selection and can be characterized in terms of rates of evaporation. However, absolute values of weight loss/time are not used. Rather, the coatings industry long ago decided to employ relative evaporation rates. These are dimensionless and are based on the ratio of the measured evaporation rate of a solvent to that of n-butyl acetate, the evaporation rate of which was arbitrarily set at 100. Tables of relative solvent evaporation rates may be found in suppliers’ literature. We tend to divide solvents into arbitrary and approximate groups of fast, medium, and slow. Fast solvents include acetone (1200), methyl ethyl ketone (700), methanol (600), and toluene (240). Those in the medium range include n-butyl acetate (100), i-butanol (80), xylene (63), and water (45 at 0% RH). Slow solvents include ethylene glycol monobutyl ether (6-10), 2-ethyl hexanol (1.35), benzyl alcohol (0.8), and diethylene glycol monobutyl ether (0.24). Specified values for solvents may vary slightly depending on which evaporation technique was used to measure the absolute rates. For example, rates listed above are based on evaporation from open dishes (measured by C. Hansen and W. French at PPG), whereas Shell data are based on results from the Shell Thin-Film Evaporometer (ASTM D3539, recently withdrawn, but still a viable method available from ASTM).

Evaporation rate tables are very useful for comparing solvents and in helping make solvent choices.

Evaporation rate tables are very useful for comparing solvents and in helping make solvent choices. Most paints contain several solvents. The choice of which ones to use depends on the mode of application, whether the coating is air-dried or baked, and its end use. Highly volatile solvents such as MEK may be included in a formulation for spray application to lower viscosity to improve spray-ability, yet be evaporated rapidly on spraying or soon afterward, thereby increasing sag resistance. One or more slow solvents often are included to enhance flowout and leveling after application, to aid in coalescence, or to allow the film surface to stay open longer (useful to prevent popping in baked coatings). The slowest solvents must be good solvents for the resins, i.e., have good solvency for them, otherwise poor appearance and inadequate film properties are likely to be the outcome.

The reader might think that boiling points of solvents would be a good way to evaluate volatility, but they are not. There is not a direct relationship between boiling point and evaporation rate. An example is that of cyclohexanone and cyclohexanol. Although they boil at nearly the same temperature (150–160°C), cyclohexanone evaporates 10 times as fast as cyclohexanol. However, the boiling points of solvents are important in resin synthesis, the properties of the resin being dependent on the temperature of the “cook,” which is controlled by the reflux temperature of the solvent blend. The higher the boiling point of the solvent, the higher the reflux temperature.

The surface tension of the solvent blend in a paint affects several properties, including the wetting of pigments and the quality of their dispersion, wetting of the substrate or undercoating, and sprayability. Low surface tension solvents can improve these properties and improve the dispersibility of additives or components that are difficult to dissolve or disperse such as epoxies. Some solvents such as alcohols, glycol ethers, and aliphatic hydrocarbons tend to migrate to the surface of a coating, thereby lowering the surface tension and making the surface more homogeneous. This improves leveling and reduces the tendency to form surface defects such as craters. This phenomenon is well known for waterborne coatings, but can occur in solventborne coatings as well (especially with C4—C8 alcohols). See JCT CoatingsTech, 3 (2), February 2006, p. 72, for more information on surface tension.

Some solvents such as alcohols, glycol ethers, and aliphatic hydrocarbons tend to migrate to the surface of a coating, thereby lowering the surface tension and making the surface more homogeneous.

The viscosity of a paint is dependent on the viscosity of the resin-solvent solution or dispersion and the interactions between the resin, solvent, and other components. With solventborne products, going to a solvent package that has half the viscosity of the current blend may not cut the paint viscosity in half, but it should reduce it considerably. This decrease is most noticeable as a lowering of the high shear viscosity “floor,” the relatively low viscosity seen at high shear, which is an indication of the ease of application. So, if you are fighting to improve the sprayability of a high solids solventborne coating without adding more solvent, consider using lower viscosity solvents.

Solvent electrical resistance becomes important when paints are sprayed electrostatically. If the paint has too low a resistance (is too conductive), overcharging will occur and the spray equipment may short out. Overcharging also can cause “wrap back” of paint onto the gun and even onto the person doing the spraying. If the paint has too high a resistance (conductivity is too low), transfer efficiency will be low, perhaps even lower than conventional air spray. See JCT CoatingsTech, 4 (6), June 2007, p. 80 and ASTM D5682 for more information.

CoatingsTech | Vol. 15, No. 11 | November-December 2018

Solvent-based varnishes and lacquers have been the coatings of choice for industrial wood applications for many years. These coatings can provide an attractive durable finish that is cost effective. Kitchen cabinet and furniture manufacturers choose these coatings because they are fast drying, they are easily repaired, they tolerate climate differences well, and they are extremely forgiving. Some of these coatings have good chemical and water resistance as well as good wear resistance.  The disadvantage of these chemistries is the high volatile organic compounds (VOC), the extreme flammability, the odor, which causes poor indoor air quality, the formaldehyde emissions, and the pot life incurred when the conversion varnish is catalyzed with an acid catalyst.

Due to increasing regulations, more environmentally friendly alternatives are now being considered. Waterborne (WB) acrylics, polyurethane dispersions (PUDs), and WB UV coatings are becoming more common for use in industrial wood applications because they have excellent resistance and mechanical properties, excellent application properties, and very low solvent emissions. Self-crosslinking acrylics have very good durability and moderately fast drying times. PUDs have very good abrasion and wear resistance. WB UV chemistry is gaining market share over traditional solvent-based chemistry because it enables the end user to increase production efficiency and maintain a smaller manufacturing footprint. WB acrylics, PUDs, and WB UV coatings can be formulated to pass Kitchen Cabinet Manufacturers Association (KCMA) and Architectural Woodworking Standards (AWS) specifications. WB chemistries can provide the appearance and resistance properties of solvent-based coatings with lower VOCs, lower flammability, and decreased toxicity.

Three types of solvent-based coatings are commonly used in industrial wood applications—nitrocellulose lacquers, pre-catalyzed lacquers, and conversion varnishes. Nitrocellulose lacquer is typically a low solids blend of nitrocellulose and oils or oil-based alkyds. These coatings are fast drying and have high gloss potential. They are typically used in residential furniture applications. Disadvantages include yellowing with time, becoming brittle, and poor chemical resistance. Nitrocellulose lacquers have very high VOCs—usually at 500 g/L or higher.

Pre-catalyzed lacquers are blends of nitrocellulose, oils or oil-based alkyds, plasticizers, and urea-formaldehyde. They use a weak acid catalyst such as butyl acid phosphate. These coatings have a shelf life of approximately four months. They are used in office, institutional, and residential furniture. Pre-catalyzed lacquers have better chemical resistances than nitrocellulose lacquers. They also have very high VOCs.

Conversion varnishes are blends of oil-based alkyds, urea formaldehyde, and melamine. They use a strong acid catalyst such as p-toluene sulfonic acid and have a pot life of 24 to 48 h. They are used in kitchen cabinet, office furniture, and residential furniture applications. Conversion varnishes have the best properties of the three types of solvent-based coatings typically used for industrial wood. However, they also have very high VOCs and formaldehyde emissions.

Water-based self-crosslinking acrylic emulsions and polyurethane dispersions can be excellent alternatives to solvent-based products for industrial wood applications. Acrylic emulsions offer very good chemical and block resistance, superior hardness values, outstanding durability and weatherability, and improved adhesion to nonporous surfaces. They have fast dry times enabling the cabinet or furniture manufacturer to handle the parts soon after application. PUDs offer excellent abrasion resistance, flexibility, and scratch and mar resistance. They are good blending partners with acrylic emulsions to improve mechanical properties. Both acrylic emulsions and PUDs can react with crosslinking chemistries such as polyisocyanates, polyaziridine, or carbodiimides to form 2K coatings with improved properties.

Waterborne UV-curable coatings have become popular choices for industrial wood applications. Kitchen cabinet and furniture manufacturers choose these coatings because they have excellent resistance and mechanical properties, excellent application properties, and very low solvent emissions. WB UV coatings have excellent block resistance immediately after cure, which allows the coated parts to be stacked, packaged, and shipped right off the production line with no dwell time for hardness development. The hardness development in the WB UV coating is dramatic and occurs in seconds. The chemical and stain resistance of WB UV coatings is superior to that of solvent-based conversion varnishes.

Experimental: Waterborne UV Coatings

A study was conducted to compare the properties of three WB UV coatings with commercially available solvent-based conversion varnish, water-based conversion varnish, and water-based pre-catalyzed lacquer. The project plan was to develop high performance WB UV resins and investigate their performance for industrial wood applications. These coatings were tested according to KCMA, AWS, and individual furniture manufacturer’s specifications.

Panel Preparation

UV Coatings

Approximately 3 wet mils of coating were sprayed over 18×18 in. stained birch plywood panel, air dried for 10 min, and force dried for 10 min at 50°C. The coating was cured with mercury bulb at 500 mJ/cm² and sanded with 3M Superfine Sanding Sponge. A second coat was applied at approximately 3 wet mils, air dried for 10 min, and then force dried for 10 min at 50°C. The coating was cured with mercury bulb at 500 mJ/cm². The coating was tested after 7 days. For edge soak, all sides of a 4×4 in. solid oak panel were coated and cured.

Other Coatings

Approximately 3 wet mils of coating were sprayed over 18×18 in. stained birch plywood panel, air dried for 10 min, and force dried for 30 min at 50°C. A second coat was applied at approximately 3 wet mils, air dried for 10 min, and then force dried for 30 min at 50°C. The coating was tested after 7 days. For edge soak, all sides of a 4×4 in. solid oak panel were coated.

Test Methods

Chemical/Stain Resistance

Enough chemical/stain was applied to create a 0.25 to 0.5-in. diameter spot on the test panel and covered with watch glass. After 16–24 h, the chemical/stain was removed, and the surface of the panel was washed with water. Each chemical/stain was rated on a scale of zero to five, with zero being complete destruction of the film and five being no effect on the film (see Figures 1 and 2).

Scrape Adhesion

A 4×4 in. piece was cut from each test panel. Adhesion was tested using a BYK Balanced Beam Scrape Adhesion and Mar Tester with 5000 g weight using the loop stylus. Adhesion was rated on a scale of zero to five, with zero being complete removal of the film and five being no effect on the film (see Figure 3).

Ball Point Pen Indentation

A 4×4 in. piece was cut from each test panel. Ball point pen indentation was tested with a BYK Balanced Beam Scrape Adhesion and Mar Tester with 300 g weight using the small pen #5785. The panel was tested after 1 h and rated on a scale of zero to five, with zero being complete removal of the film and five being no effect on the film (see Figure 3).

Plasticizer Resistance

A 2-in. square piece of red vinyl was applied to the test panel. A force of 0.5 lb/in.² was applied. The specimen was placed in an oven at 50°C for 72 h. After cooling at room temperature for 1 h, the vinyl square was removed and evaluated for softening and blistering. Results are shown in Figure 4.

Green Print Resistance

After the test panel cured for 1 h, a 2-in. square piece of # 10 cotton duck cloth was applied to the finish. A force of 2 lb/in.² was applied directly to the duck cloth. After 24 h, the cotton duck cloth was removed and evaluated for printing. See Figure 4 for results.

Hot Print Resistance

After the test panel cured for 14 days, a 2-in. square piece of # 10 cotton duck cloth was applied to the finish. A force of 1 lb/in.² was applied directly to the duck cloth. The specimen was placed in an oven at 60°C for 24 h. The duck cloth was removed, and the specimen was allowed to cool for 1 h and evaluated for printing (see Figure 4).

Boiling Water Resistance

Approximately 10 ml of boiling water was applied to the test panel. A ceramic coffee cup full of boiling water was placed on top of the 10 ml of water. After 1 h, the cup was removed and wiped with paper towel. After 24 h, whitening was evaluated. The results are depicted in Figure 5.

Hot and Cold Check Resistance

A 4×4 in. piece was cut from each panel. The panel was placed in humidity cabinet at 50°C and 70% humidity for 1 h. The panel was allowed to reach original room temperature and humidity. After 30 min, the panel was placed in a freezer at -10°C for 1 h and then removed and allowed to reach original room temperature and humidity. This cycle was repeated five times. Results are shown in Figure 6.

Edge Soak

A cellulose sponge was placed in a plastic container. The container was leveled and filled with detergent solution (1% Dawn® dish soap by weight in water) to 0.5 in. below top level of sponge. The panel was placed on sponge, cut side down, and permitted to stand for 24 h (see Figure 6).

Formulations

The formulations used for the WB UV coatings are presented in Table 1. Table 2 shows the data for the WB UV coatings.

Testing

Test results for WB UV coating formulations are presented in Figures 1-6.

Results

All of the WB UV coatings exhibited excellent chemical resistance. WB conversion varnish and SB conversion varnish had very good chemical resistance. WB pre-catalyzed lacquer had adequate chemical resistance for KCMA coatings. WB UV 2, WB conversion varnish, and SB conversion varnish had the best scrape adhesion. All of the coatings had excellent ball point pen indentation, plasticizer resistance, hot print and green print resistance, hot and cold check resistance, and edge soak. All of the WB UV coatings had superior boiling water resistance.

Experimental: Acrylic Emulsion

A multiphase self-crosslinking acrylic emulsion was developed and evaluated for use as both a one-component (1K) and two-component (2K) industrial wood coating. The performance was benchmarked against a competitive self-crosslinking acrylic that is promoted for KCMA/furniture finishes. Two-component formulations were crosslinked with 6% carbodiimide (by weight).

Formulations

The formulations used for the acrylic emulsion coatings are presented in Table 3.

Panel Preparation

Birch Plywood

Two coats at 200 microns were applied to birch plywood panels. The first coat was air dried for 1 h and sanded. The second coat air dried for 7 days and then tested.

Glass

A 200-micron drawdown was performed on glass panels. The panels were air dried for 7 days and were tested after 14 days.

Testing

Test results for the acrylic emulsion formulation are presented in Figures 7–13.

Results

The ABI emulsion is a viable product for industrial wood finishes, especially for lower VOC formulations. The ABI emulsion offered better processing, higher film build, and better wet clarity when compared with a competitive emulsion (see Figure 14). Carbodiimide crosslinking offers improved performance and is a potential alternative to pre-catalyzed lacquers.

Experimental: Polyurethane Dispersion

An amine-free PUD was developed and evaluated for use as a clear self-sealing topcoat in KCMA/furniture applications. Performance was benchmarked against a traditional PUD. Two-component products were crosslinked with 6% carbodiimide.

Formulations

The formulations used for the polyurethane dispersion testing are presented in Table 4.

Panel Preparation

3BH Leneta Cards

For testing using Leneta cards, 1.5 Bird drawdowns were conducted. The cards were air dried for 15 min, force dried for 15 min at 50°C, and aged for 7 days before testing.

Birch Plywood

On birch plywood, a first coat was sprayed at 4–5 wet mils and air dried for 15 min. The coating was then force dried for 15 min at 50°C, allowed to cool, and then sanded. A second coat was sprayed at 4–5 wet mils and air dried for 15 min, then force dried for 15 min at 50°C and aged 7 days prior to testing.

Testing

Chemical resistance results from testing on Leneta cards are shown in Figure 15 and the birch plywood results are shown in Figure 16. Boiling water resistance results are provided in Figure 17. Scrape adhesion, edge soak, Taber abrasion, and Koenig pendulum hardness results are shown in Figures 18–21, respectively.

Results

The amine-free PUD is a viable product for industrial wood coatings. It had excellent ethanol resistance, Taber abrasion, and water resistance. It atomized well, had very good build, and good wood warmth.

Conclusions

Water-based coatings made from WB UV resins, acrylic emulsions, and polyurethane dispersions all are good candidates for industrial wood coatings. They have very good chemical resistance and mechanical properties. They can be formulated at low VOCs and have low toxicity. They are viable alternatives to solvent-based chemistries.

This article was presented at the American Coatings CONFERENCE, April 9–11, in Indianapolis, IN.

CoatingsTech | Vol. 14, No. 6 | June 2018

Raw materials, in the form of individual chemical constituents that are incorporated into paints and coatings, represent an exceptionally diverse and vitally important sub-set of the overall specialty chemicals industry. They may either be added to a paint or coatings formulation during the product manufacturing process—or, in the case of multiple-component coatings systems (2K, 3K, etc.)—may be used to make coating components intended to be combined in the field to produce reactive, limited potlife products.

Coatings raw materials can be grouped into four broad categories of chemical constituents:

Resins: Polymers either dissolved—or carried—in water or organic solvents

Pigments: Both chromatic (“primary pigments”) and extender, filler, flattening, etc. (“secondary pigments”)

Solvents: Organic—water is not discussed in this article

Additives: Rheology modifiers; surfactants; dispersants; biocides; coalescents; catalysts; defoamers; adhesion promoters; and a host of other specialized chemical constituents typically used at very low levels, as a percentage of the formula weight

The global coatings raw material market is estimated to be valued at approximately $63.5 billion on 34.3 million metric tons of materials.

Raw Materials Market Analysis

Raw materials used in the manufacturing of paints and coatings represent a relatively small (~5%) but extremely important component of the $4.5 trillion global chemicals industry. All of the world’s leading chemical producers are active in the coatings market, and many coatings raw materials are used in other industries as well, including plastics; synthetic lubricants; adhesives; sealants; household, industrial and institutional cleaners (HI&I); personal care products; paper; plastics; water treatment and many others. The basic chemical components that are used to produce coatings chemical constituents can also be used to produce a wide array of other chemical compounds. This can be a problem at times, since this diversity of uses can create competitive situations for raw materials and their pre-cursors that are typically used in coatings, particularly during periods of tight supply.

Resins (“binders”), pigments, and fillers represent over 75% of the global coatings raw materials market.  Figure 1 shows the estimated distribution of coatings raw materials volume by type.

As might be readily anticipated, the distribution of the types of raw materials, based upon value, is somewhat different than distribution based on volume.  Figure 2 shows the estimated distribution of coatings’ raw materials value by type.

Resins

Total global sales of resins for use in coatings systems are estimated to be $31 billion on roughly 15.2 million metric tons.

Globally, acrylic resins are the most commonly used binder in paint and coatings systems. This is particularly true for decorative paints, and includes all acrylics, both pure and modified, such as styrene-acrylics and vinyl-acrylics. It is estimated that acrylic systems, both solventborne and waterborne, comprise approximately 27% of total coatings binder demand. It should be no surprise then, that acrylic resins tend to be the most susceptible to periodic disruptions in supply, accompanied by price fluctuations. In 2017, for example, shortages of methyl methacrylate (MMA), due to a variety of causes, including a shortage of acetone and two major U.S. suppliers of MMA being down (for different reasons) at the same time, led to nearly monthly increases, with MMA climbing roughly $0.50/lb, from “mid-$0.80s/lb” in February 2017 to “mid-$1.30s”/lb in January 2018. Prices on both the spot market and the black market during 2017 were reported to be “sky high.” While supplies are somewhat more stable going into 2018, price increases were announced for both January and February, and there are still likely to be additional increases.

On a global basis, alkyds are used to some degree in virtually every end-use coatings segment, and represent the second most common type of resin system used in coatings formulations. Alkyds comprise roughly 20% of resin demand in the global coatings market (≤10% in the United States). Although alkyd resins have been steadily declining in use, particularly in North America and EU, as VOC limits continue to drop, and the market has been moving to other resin types for water-based and higher-solids formulations, newer water-based alkyd systems are being introduced into the market, at least in part due to the increasing interest in resins made with higher renewable resource content.

Polyurethane coatings, either 1K, 2K (or occasionally 3K) are widely used in the automotive OEM, other transportation, automotive refinish, wood, industrial finishes, decorative coatings and even severe-service marine and high-performance industrial segments. Urethane resins currently comprise roughly 21% of the global demand for resins in coatings. Usage of polyurethane resins has been growing over the past several years due to their performance properties and their ability to be used in lower VOC formulations. An important, and growing, sub-segment of polyurethanes in the United States is 2K polyureas. To comply with increasingly stringent VOC requirements, polyurethane waterborne dispersions (PUDs) have been developed and used to formulate single-component coatings with improved abrasion resistance compared to waterborne acrylics. They can also be combined with other waterborne resins to meet cost targets and performance needs.

Approximately 16% of total demand for binders used in coatings is supplied by epoxy resins. Various resins in the epoxy family are widely used in electrodeposition (ED) coatings and in industrial coatings, particularly in the transportation, industrial maintenance and marine markets. Epoxy resins are also widely used in powder coatings. In recent years, high solids and ultra-high solids formulas using liquid epoxy resin dominate and continue to grow. Liquid epoxy resin is also used for 100% solids epoxy formulas applied as concrete surfacers, tank linings, and for other select applications, often augmented with phenoxy and novolac resins to enhance certain performance features. While the performance of waterborne epoxy resin technology has improved, even accounting for higher consumption due to its improved performance, it has only attained a small technology share, albeit with major usage in metal can coatings. These are, however, coming under increasingly close scrutiny in the United States, where BPA toxicity concerns continue.

Additional binders that are used as coatings raw materials include amino, polyester (with low-bake versions as the growth area), cellulosic, silicone/polysiloxane, silicate and vinyl resins. Fluoropolymers are another interesting type, with waterborne versions now being offered for high-end architectural exteriors and other applications. Also included in this sub-segment are hydrocarbon resins and natural resins such as rosins and shellacs. While technically not resins, linseed oil, tung oil and similar products are also included since they act as film formers. This sub-segment comprises approximately 16% of total resin demand in the global coatings markets. Another small but growing resin chemistry is that of radiation cure, with the current greatest volume used in wood and plastic coatings—where sometimes even dual WB/UV cure technology is used. See Figure 3 for a breakdown of the major resin types.

Pigments

The value and volume of pigments, both primary and secondary, for use in coatings formulations were estimated to be $16.5 billion on 11.0 million metric tons of product. On a volume basis, fillers and extenders are the largest sub-set of the pigments category and represent roughly 56% of all demand for pigments in the coatings market. Compounds that comprise this sub-segment include clay, calcium carbonate, talc, silica and other inorganic materials.

The second largest sub-set of the pigments category is titanium dioxide (TiO₂), which is the single highest-volume pigment used in coatings. Its largest use is in decorative coatings, but it is widely used in a variety of industrial OEM coatings and industrial maintenance/protective coatings, as well. Titanium dioxide represents approximately 31% of all pigments used as coatings raw materials, and this is likely to be somewhat problematic in 2018, with Q1 lead-times of 60–90 days, and scattered instances of allocation as well. The global economic climate currently favors all market segments that use TiO₂, most notably the building and construction segment. This will, therefore, be a year of continued price increases, which could easily take TiO₂ as high as $3100/mt, albeit still significantly short of its 2011–2012 historic high price of ~$4500/mt. Increases in Chinese chloride-process TiO₂ production are essentially being offset by continued closure of sulfate-process plants, and Huntsman’s plant in Finland is not scheduled to be back to full production until the end of 2018. If the acquisition of Cristal by Tronox is finalized (the U.S. Government filed a complaint aimed at stopping the deal on December 5, 2017), it is unlikely to unleash any additional pigment into the market during 2018, although might be reasonably be expected to add an additional 100Kmt to global output in subsequent years.

For certain end-use applications such as decorative, automotive OEM and automotive refinish, color is a primary driver of product selection. Hence, color pigments play a vitally important role in the coatings industry. Despite the importance of these materials, color pigments represent only a small component of pigment demand. Included in this segment are both inorganic and organic pigments. Inorganic color pigments such as iron oxide are the most frequently used and represent over 80% of the volume of color pigments. Organic color pigments are among the highest-priced raw materials and, thus, despite their relatively low volume, represent a significant portion of the market value. Organic color pigments are likely to increase 3–4% in 2018, as a result of competition for the basic chemicals from which they are built, and that largely come out of the AP region—production of which can be affected at almost any time as the Chinese government becomes increasingly proactive about shutting down chemical processes in an effort to improve air quality. Complex inorganic color pigments (often referred to as CICPs or ceramic pigments) are growing in importance because they meet the higher performance demands of chemical inertness and heat stability, along with lightfastness and excellent weathering properties. Moreover, with only a few exceptions (such as perylene black), CICPs comprise the majority of IR-reflective pigments that now enable formulation of various colors with energy efficiency properties. Color pigments represent >6% of the volume of all pigments used in coatings.

In addition to the pigments listed above, there is a wide array of other specialty pigments, such as anticorrosive pigments, metallic pigments, pearlescent pigments, carbon black and zinc oxide. While some of these pigments play an important role in coatings volume, none individually represents a significant volume. These other pigments represent roughly >6% of the total pigment demand. See Figure 4 for a breakdown of the major pigment groups.

Solvents

Solvents are the key contributors to the volatile organic content of paints and coatings emitted into the atmosphere and, as a result, are regulated by various local, regional, state and country regulatory agencies around the world.

Global revenues for solvents used in coatings formulations in 2017 were approximately $8 billion on 6.5 million metric tons.

Oxygenated solvents comprise over 60% of demand within coatings formulations, and include chemical components such as alcohols, ketones, esters, glycols and glycol ethers. Hydrocarbon solvents are either aliphatic or aromatic and comprise less than 40% of total usage within coatings formulations. As a result of the continuing shift in paint and coating formulations from solvent-based to water-based technologies, ultra-high solids and 100% solids, overall usage of solvents is declining as a percentage of the total coatings raw materials usage. While solvent usage as a percentage of total raw materials continues to decline, however, many end-use segments that use solvent-based coatings continue to grow. As a result, total solvent usage has been relatively flat over the past decade or so, and this trend is expected to extend into the foreseeable future. Similarly, within the solvents family, shifts are ongoing as formulators seek to find less toxic and more compliant, environmentally friendly solvents. Unfortunately, VOC regulations—and the concept of “what is exempt and what is not”—differ around the globe. For example, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (“Texanol™”), historically the most effective coalescing agent for latex paints, is listed as non-exempt by the U.S. Environmental Protection Agency, but in the EU it is listed as exempt. This can make formulating “global coatings formulations” anywhere from tricky to impossible, depending upon the coating, application, and performance requirements.

Additives

Additives comprise a broad category that covers a wide array of chemicals used as raw materials for coatings. Recent innovations include multi-functional additives to simplify the number of formula ingredients and also include those that help to achieve low- to zero-VOC formulations. Total revenue for additives used in coatings is estimated to be $8.4 billion on approximately 1.25 million metric tons.

Rheology modifiers are the largest sub-segment, representing over 30% of demand, and are used to control viscosity and to affect flow and leveling. Plasticizers are incorporated into formulations to improve the flexibility of the film, and may also be used at times for their coalescent properties. Biocides are added to formulations to prevent the growth of bacteria and other microorganisms while the coating is being stored, and also as a dry film preservative. Biocides, including special chemical components that are used to minimize marine fouling, comprise approximately 22% of total additives used in coatings. Surfactants represent approximately 19% of additive demand on a volume basis, with foam control additives at 15%.

The list of other additives is quite long. While none represents a significant component of coatings raw materials individually, “Other Additives” collectively comprise a significant portion of all additives and include adhesion promoters, antifoaming agents, anti-skinning agents, corrosion inhibitors, driers, flatting aids, flood control agents, sag control agents, slip aids and UV absorbers, to name a few. See Figure 5 for a breakdown of the major functional classes of additives.

Regional Distribution

Globally, regional distribution of coatings raw materials generally follows overall production of coatings. Figure 6 depicts total global demand for coatings by region.

Competitive Landscape

The competitive landscape is quite complicated. In general, there are three types of competitors that operate in this market:

The first type of competitor is large, multinational chemical companies serving many industries that compete across coatings raw material categories. Examples include:

• BASF
• Celanese
• Allnex
• DowDuPont
• Eastman Chemical
• Evonik
• Huntsman
• Lanxess
• Momentive
• W.R. Grace, et al.

Large, multinational chemical companies play a significant role in the coatings raw material market and command a considerable share of the raw material demand. The scale, degree of integration and broad product portfolios of these competitors are perceived as key advantages. Merger and acquisition activity among the large, multinational chemical companies has had a significant impact on the coatings raw materials market, and acquisitions are forecast to continue. Private equity firms continue to show an interest in acquiring raw materials suppliers.

The second type consists of multinational product specialists that focus on a limited product offering. These companies tend to specialize in a specific product or chemistry niche(s), and are frequently innovation drivers in the market. Competitors of this type vary widely in size, based upon geographic scope, product breadth and market focus. Examples include:

• Cathay (pigments)
• Heubach (pigments)
• Nubiola (pigments)
• Alberdingk Boley (W/B resins)
• Reichhold (resins)
• Worlee-Chemie (resins)
• ALTANA (BYK—additives)
• Troy Corporation (additives)
• Michelman (additives)

As is the case with multinational chemical companies, mergers and acquisitions are anticipated to continue, impacting the specialist competitors as they are acquired by either larger chemical companies to complement their portfolio, or by other specialists to gain scope and share.

The final type of competitor is the local/regional suppliers. These generally focus on a limited product offering. Examples among the numerous local/regional suppliers:

• Optimal Chemicals
•  Organik Kimya
• Silberline Synthopol
• Specialty Resins
• OPC Polymers
• Orion Engineered Carbons
• Many, many others

While none of the local and regional suppliers have significant share on their own, collectively they are an important source of coatings raw materials. Competitors in this group offer an assortment of value propositions tailored to their customer mix. In some cases, due to lower overheads, local/regional suppliers are able to provide lower cost alternatives to the major suppliers. In other cases, they are able to provide unique products or services that allow them to effectively compete. Significant consolidation is anticipated among this group.

Market Trends and Drivers

End-Use Markets

Demand for coatings raw materials is directly linked to coatings demand. Over the coming five years, demand for coatings is anticipated to grow at a rate of 4–5% annually. This would result in a 2022 demand for raw materials of approximately 42 million metric tons. Asia Pacific is forecast to experience the greatest volume growth (5–6%) to 2022. Europe is likewise forecast to experience moderate growth of perhaps 3–4%. North America is forecast to post somewhat more robust growth of 4–5%, but the exact mix of raw materials consumed within each region will depend on specific end-use market growth.

Economic Influences

Significant numbers of raw materials used in coatings formulations are either derived directly from oil for their chemical composition, or indirectly as a result of energy derived from oil for their mining and/or processing. The price of oil can be highly volatile and many factors can impact this forecast, driving the price/barrel either up or down. As a result, oil prices have a significant impact on the price of coatings raw materials. ChemQuest estimates that there is a “pass-through factor” of roughly 50%—i.e., if the price of oil doubles, raw material prices will increase by 50%. A realistic worst-case scenario might see prices of crude oil 50% higher than the forecast, which would increase raw materials for paints and coatings roughly 25%. Over the coming five years, the price of oil is forecast to remain relatively stable, within the range of $60–$70/barrel according to the U.S. Energy Information Administration (EIA). The implication for raw material suppliers and coatings formulators is that the price of the base materials that comprise coatings will likely rise at a rate similar to the rate of inflation to 2022.

This article was published in the May issue of European Coatings Journal and was reprinted with permission of the publisher.

CoatingsTech | Vol. 14, No. 5 | May 2018

A pre-publication read of Cynthia Challener’s excellent article in this issue on waterborne industrial coatings led me to reflect on my knowledge and experiences in this area. She notes that far fewer commercial waterborne than solventborne industrial products are being used in the United States. People who she interviewed point out many reasons for this, including cost, application difficulties, paint user inertia, and a general attitude that waterborne coatings are not as good as solventborne ones. It is ironic that when I entered the coatings industry almost exactly 43 years ago, I was told that waterborne coatings soon would replace solventbornes in all product areas—Industrial and Automotive as well Architectural. The last-named area certainly is heavily waterborne, and waterborne electrodeposition primers are universal in Automotive along with many waterborne basecoats and a few primer surfacers. However, Industrial has not yet joined the club. There are many paint and resin companies working to change that.

Considering the properties of water, it is not surprising that using it as a solvent or carrier in a coating leads to difficulties. For example, water has a surface tension of 73 mJ/m² compared to 20–40 mJ/m² for organic solvents. Low binder surface tension is needed for pigment and substrate wetting and to minimize surface defects. Therefore, additives must be used to lower the surface tension. Surfactant addition is a logical fix, but most surfactants stay behind and make the coating sensitive to water. Use of low surface tension solvents that disappear on drying or baking are a way to get around that, but may raise VOC too much. Most WB coatings need added solvents to compatibilize the aqueous and organic phases and enable coalescence to occur, whereas solventborne coatings are plasticized by the solvents that dissolved the resins and undergo steady solidification. However, newer WB formulations such as those with core-shell latexes with built-in coalescents allow reduced solvent levels.

Considering the properties of water, it is not surprising
that using it as a solvent or carrier in a coating leads to difficulties.

Humidity and temperature on application have considerable effects on film formation and quality of WB coatings. At high humidity, a waterborne film (especially one that is sprayed) is likely to remain wet and initially may be almost soupy, whereas at low humidity, the film may be too dry and have poor appearance. This is why automotive WB basecoats (and WB primer surfacers) are sprayed in booths with temperature and humidity control. This is too expensive for most industrial paint users.

In my experience, WB coatings tend to be more sensitive to oils and other contamination that cause dewetting, craters, and other defects. Therefore, they require better surface preparation and cleanliness than did the older low solids solventborne coatings. However, that is equally true of high solids coatings. Better cleaning and pretreatment is necessary in factory applications for both technologies. WB coatings are more likely to suffer from foam formation, air inclusion, and dissolution of air under pressure. Because of the last-named, airless spray should be avoided. I also have seen popping problems with baked WB coatings. In one case, popping could be prevented by use of a slow tail-end solvent that kept the film open, but, after the bake, enough of that solvent stayed behind that the film was too soft to meet the hardness specification. Regardless of all the difficulties noted here, there are many successful WB industrial coatings, and there will be many more in the future.

Regulations have been the biggest drivers of the adoption of WB industrial coatings, but in the current political climate, it is not clear what the future trend in regulations will be. The administration may move to relax them, but many states will wish to tighten them. Two other factors are the wish for some paint users to be better neighbors and use WB coatings, even if it costs a little more, and increasing customer demand for sustainability. I think that paint users and their customers will ask about sustainability. In addition, green building codes are likely to favor the lower emissions from WB coatings, especially as solvent levels are reduced. Another area that I have not heard mentioned for many years is lower fire insurance premiums for factories using only WB coatings. I recall that, years ago, this was one reason why some customers chose WB, especially when building new plants.

So, the prediction that all coatings would be WB coatings has not held, especially for industrial coatings. However, advances in resin technology and paint formulations plus other factors mentioned above are steadily increasing the market share of WB products in the industrial coating mix.

A waterborne two-pack polyurethane finish coating formulated for military aircraft provides extended durability performance suited for Australia’s harsh climate as well as substantial reductions in content of volatile organic compounds when compared with high-solids solvent-based aerospace coatings. A formulating practice using commercially available hydroxyl-functional polyurethane dispersions combined with hydrophilically modified polyisocyanate resins enables compositions to be made that are suitable for spray application, have drying properties useful for aircraft workers, and cure to form films with the specified resistance and flexibility properties.

INTRODUCTION

Two-pack solvent-based polyester polyurethane coatings have been the principal topcoat products used on military aircraft for several decades. These form the top layer of the coating system used over aluminum alloy substrates on military aircraft (Figure 1). The other layers include a pretreatment and a two-pack epoxy primer that provide anticorrosive protection for the substrate. The topcoat provides a range of important properties, such as color and camouflage, chemical resistance, and exterior durability. The topcoat must have good adhesion to the primer and also protect the primer from environmental degradation.

FIGURE 1—The layers of a typical aircraft paint system.

Most exterior topcoats currently used by the Royal Australian Air Force (RAAF) are covered by the U.S. military specification MIL-PRF-85285E,¹ which includes requirements for high-solids (HS) products and which limits content of volatile organic compounds (VOC) to a maximum of 420 g/L. HS products from the United States were introduced gradually over the period from 2000–2010 until they replaced the older two-pack topcoats completely. These older topcoats, which conformed to the U.S. specification MIL-C-83286B,² and which were also required to pass the performance testing in the RAAF paint system specification K62,³ typically contained VOC levels in the range 600 ± 50 g/L, and will be referred to here as “conventional solids” (CS) topcoats.

However, significant complaints were received by Defence Science and Technology (DST) Group about the poor durability of the HS technology topcoats, with faster changes occurring to gloss and color due to environmental exposure degradation than had been observed for the CS products, and substantial chalking that significantly affected their appearance. The U.S. specification MIL-PRF-85285D⁴ introduced categories of coatings allowing for two different classes: Class H for the HS solvent-based products and Class W for waterborne (WB). In addition, there were three different types classified according to VOC level. Types I, II, and III specified maximum VOC levels of 420, 340, and 50 g/L respectively, with only limited requirement for weatherability using a Xenon arc weatherometer for 500 h.

Type IV coatings first appeared in an amendment to MIL-PRF-85285D,⁵ and then finally in the fully revised specification MIL-PRF-85285E,¹ in recognition of the deficient weatherability of Types I, II, and III. This new category arose out of exterior durability studies undertaken by the Organic Coatings Team at Naval Air Systems Command (NAVAIR) in conjunction with paint companies formulating products for exterior durability testing on military aircraft.⁶ Type IV has a maximum VOC level of 420 g/L with the additional requirement for “extended weatherability” and bolsters the requirement for testing in a Xenon arc weatherometer to 3000 h.

Exterior durability testing of Type I and Type IV products from the major U.S. paint companies in three different gray colors has been undertaken by DST Group in Australia.⁷ These three colors are listed in Table 1 together with the tristimulus values obtained from the Federal Standard MIL-STD-595C,⁸ and the CIELAB color coordinates from the original standard MIL-STD-595A⁹.

Results of exposure testing, both in temperate Victoria in the south and the tropical conditions of northern Queensland, have shown that the Type IV products for each company are indeed more durable than their Type I counterparts, and some of these are in the process of being introduced into service by the RAAF. However, the performance of the different Type IV coatings of a particular color from different paint companies varies widely. Attempts to correlate the results of accelerated weathering testing with the exterior exposure results have met with limited success and will not be discussed here. Weathering results in this article will focus on exterior testing for one color only, this being FED-STD-595C-36375.⁸

When aiming to reduce VOC emissions from coatings as much as possible, a move to formulations that use water as the primary solvent is one logical route to consider. A high level of effort has been expended by chemical companies over the last few decades in developing new products suited for high-technology waterborne coatings, and the effort continues despite the slow uptake of these materials in the industrial and automotive sectors.

The ability to formulate waterborne products that cure to form finishes that perform the same as, or better than, the solvent-based products, continues to be an active field of endeavor for the paint formulator. To date, only one commercially available waterborne finish camouflage coating for military aircraft appears to have been qualified to MIL-PRF-85285E, and has been designated as a Class W Type III coating.¹⁰ In the present study, a waterborne coating technology is demonstrated that has excellent exterior durability performance, outlasting the best of the Type IV products, while meeting the other test requirements of MIL-PRF-85285E, except for reverse impact flexibility, which will be discussed.

EXPERIMENTAL

Approaches to Formulating

The particular waterborne formulations developed and the ingredients used, for which results are presented in this article, are proprietary and cannot be disclosed here; however, the general approach taken to making the paints is addressed in the following.

Raw materials were sourced from a variety of suppliers. Samples of hydroxyl-functional latexes, including certain polyurethane dispersions (PUDs), were offered as potential candidates providing the degree of chemical resistance and flexibility for the intended application when crosslinked into paint films. Several of these latexes were made up into Component A formulations, mixed with Component B (see below), drawn down, and allowed to cure under ambient conditions for at least seven days. Based on preliminary results, which included examining the best balance between rate of dry, chemical resistance, and flexibility, one particular PUD—a hydroxyl-functional aliphatic polyester polyurethane dispersion—was selected for all the subsequent tests undertaken in this work.

Latex-free pigment dispersions were made up into water-based pigment concentrates using durable pigments and a suitable dispersant system under high-speed dispersion followed by glass bead milling. The full Component A formulations were made up by stirring appropriate quantities of the pigment dispersions into the bulk latex, dispersing in the flatting agents, and then making up with a range of functional additives and adjusting with deionized (DI) water. The additives include surface additives such as substrate wetters and leveling agents, defoamers, adhesion promotors, and a hindered amine light stabilizer. In some cases, color adjustment was accomplished with commercial pigment concentrates containing lightfast and heat-stable pigments.

Two colors were made up and matched to two Federal Standard 595C colors: a light gray (FED-STD-595C-36375) and a mid blue-gray (FED-STD-595C-35237)—two of the colors employed on RAAF aircraft. Most of the testing reported here is for the light gray color 36375.

Polyisocyanates selected for this work included the hydrophilically modified aliphatic trimer types based on hexamethylene diisocyanate (HDI) such as Easaqua M 502 from Vencorex and Bayhydur 304 from Covestro. Component B formulations were made up by dispensing pre-dried organic solvents into screw-cap metal cans, which had been pre-dried in a warm oven. Weights were checked prior to adding the total quantity of water-dispersible polyisocyanate and then capped prior to mixing by gentle shaking of the cans. Component C is DI water.

Spray Application

Mixtures of the two parts were thinned with the minimum quantity of DI water needed to adjust spray application viscosity within the range of 30-40 s using a British Standard B4 viscosity cup, this being the most suitable range to achieve a balance between fluid flow from the spray gun and anti-sag capability on a vertical panel. The ready-to-spray (RTS) mixtures were filtered through a 190 micron fine mesh filter cone directly into the pot of the spray gun. Pre-primed panels were spray painted using a gravity-fed spray gun (1.4 mm nozzle) with pressure 30–35 psi adjusted for full fluid flow and a wide fan. Two coats of the topcoat were applied with a 10-min flash-off period in between, with the aim of achieving topcoat dry film thicknesses (DFTs) within the range 50 ± 10 µm. The coated panels were allowed to cure under ambient laboratory conditions [normally 20 ± 2°C and 35–65% relative humidity (RH)] for at least 14 days prior to testing.

Solvent-based Benchmarks

Solvent-based paint systems used for comparative purposes in most of the testing, including both anticorrosive primer and polyurethane topcoat, were obtained from three paint companies, referred to in this report as Companies A, B, and C. The primers and topcoats were mixed and applied according to the technical recommendations of the respective paint companies. The recommended primers used conformed to U.S. specification MIL-PRF-23377K.¹¹ The topcoat paints were designated Topcoats A, B, and C and have been selected as examples representing the following characteristics:

Topcoat A:   A “conventional solids” (CS) two-pack topcoat conforming to MIL-C-83286B with a measured VOC of 560 g/L

Topcoat B:   An HS “standard durability” topcoat conforming to MIL-PRF-85285E, Class H, Type I

Topcoat C:   An HS “extended weatherability” topcoat conforming to MIL-PRF-85285E, Class H, Type IV, described previously by Nickerson et al. as an “advanced performance topcoat” (APT).⁶

Testing

Testing was undertaken primarily in accordance with the requirements of MIL-PRF-85285E¹ and the test methods therein. Testing used industry-standard equipment and methods. Adhesion testing was conducted using the cross-cut tape procedure, Method B, in ASTM D 3359.¹² Some tests were specific to an Australian Defence Standard specification, DEF(AUST) 9001A, including tests for rate of dry.¹³ The substrate used for most tests was 1.2 mm thick aircraft aluminum alloy Al 2024-T3-Clad and was pretreated with Alodine 1200L prior to priming and topcoating. Al 2024-0-Clad, 0.5 mm thick, was used for reverse impact flexibility testing and 0.3 mm tinplate for mandrel bend testing. These flexibility tests are generally performed to determine the degree of flexibility of the topcoat and so do not require pretreatment or priming.

Panels prepared for testing the waterborne topcoat used MIL-PRF-23377K primer from Company A. Tests for topcoat flexibility were done without a primer as specified. Plain aluminum panels were used for some testing where the effect of substrate was considered to be negligible, particularly topcoat color, gloss, and durability.

An additional requirement for qualification as a Class W coating in the MIL-PRF-85285E specification is for the two components of the waterborne kit to pass all the specification requirements after being subjected to five cycles of freezing and thawing. Each cycle consists of a period of 16 h at –9 °C followed by a period of 8 h at room temperature.

The waterborne coating was tested and compared with Topcoat A to determine if there were any adverse effects on anticorrosive performance. The recommended anticorrosive primer from Company A was used as the primer for both topcoats in the salt spray and filiform corrosion tests, following the procedure in MIL-PRF-23377K.¹¹ Al 2024-T3-Clad was used as the substrate and the test panels were pretreated with Alodine 1200. Salt-spray corrosion testing was performed in accordance with ASTM B117.¹⁴ Panels that were scribed with two intersecting lines, exposing the bare substrate, were placed in a 5% salt-spray cabinet for 2,000 h and then examined for blistering and corrosion products in the scribes. Filiform corrosion on similarly scribed panels was initiated by exposing the panels to an atmosphere of hydrochloric acid fumes for 1 h in a dessicator containing 12N hydrochloric acid and then transferring the panels to a humidity cabinet maintained at 40 °C and 80% RH for 1,000 h. The panels were then examined by measuring the distances of filamentary corrosion extending out from the scribes.

Gloss and Color Measurements

Gloss measurements of the topcoats were made at 20°, 60°, and 85° angles using an Elcometer 407 glossmeter. Color measurements in CIELAB 1976 color space, L*, a* and b*, were performed with a Minolta 2500d spectrophotometer using daylight illuminant D65 and 10° standard observer with the specular component included. Total color differences ΔE_ab* were calculated from the differences in the component color coordinates using equation (1).¹⁵

(1) Exterior Weathering

Exterior durability testing was undertaken by measuring panels exposed at the DST Group Paint Exposure Test Facility at the Army Proving Ground at Monegeetta in central Victoria (latitude 37° 24’ S) and also at the Allunga Exposure Laboratory near Townsville in Queensland (latitude 19° 20’ S).¹⁶ Exposure angles at Monegeetta were adjusted to 20° facing north during the warmer months of the year (October to April) and to 55° facing north during winter. Exposure angles at Allunga were set to 5° facing north all year.

Quarterly measurements of gloss and color were made on unwashed (uw) and wet-wiped (ww) sides of each panel. The ww sides were prepared by gently wiping the prewetted surface with folded paper towels that had been soaked with water. Moderate pressure was applied to remove any deposited soil and loose chalking, and the panels were then allowed to drain and dry on a panel rack overnight. Figure 2 shows an example of an exterior test panel that has been exposed at Monegeetta for five years. The coating is a Type I topcoat from Company B. The extensive chalking that develops is readily apparent on the uw side of the panel.

Figure 2—An exterior test panel, coated with a light gray 36375 Type I topcoat from Company B, after a period of five years at Monegeetta, showing the extensive degree of chalking that develops on the uw side of the panel.

The degree of chalking was determined by the tape test performed on the uw sides of panels.¹⁷ For the light gray 36375 colored topcoats reported here, a good quantitative measure of chalking was calculated from the L* values on the ww and uw sides of panels using equation (2).

(2)���� Calculations of VOC Levels

Method A

The VOC level for the waterborne topcoat, based on the total RTS mixture, was calculated from the formulation. The first method of calculation defines the total VOC content (g/L), including exempt VOC, of one liter of RTS mixture [equation (3)].

(3)���� where m_v and m_w are the masses of the total volatiles (including water) and of the water only respectively, and V_RTS is the volume of the RTS mixture.

Method B

The VOC level calculated according to various regulatory procedures requires removal of water and exempt VOCs from both numerator and denominator according to equation (4) as per ASTM D 3960.¹⁸

(4)���� where m_ex is the mass of exempt volatile compounds, and V_w and V_ex are the volumes of water and exempt volatile compounds, respectively.

For solvent-based products (m_w = 0) that do not contain exempt volatile compounds, the VOCs obtained from equations (3) and (4) are identical. Information on the presence and wt% of individual exempt volatile compounds was obtained, where possible, from material safety data sheets for the individual components of the topcoat kits.

Method C

VOC levels (g/L) of the solvent-based topcoats were determined from measurements of solids (wt%) and density ρ_RTS (g cm^-3) of the RTS mixtures using equation (5).

(5)�� Results from these measurements were interpreted as being equal to VOC_a by equation (3), with m_w = 0.

Emissions per unit area of paint film

VOCs emitted from an RTS paint mixture were calculated as organic compound emissions (OCE) from a specified area of final paint film (gm^-2) at a specified DFT (µm) using equation (6).

(6)�� where VOC_RTS is the VOC content in grams of one liter of RTS mixture, DFT is the dry film thickness of the applied coating (µm), and V_solids is the volume occupied by the solids content of one liter of the RTS mixture (cm³). This gives the organic emissions from 1 m² of paint film at the specified DFT and ignores emissions arising from overspray due to reduced transfer efficiency in spray application. The calculation may exclude exempt volatile compounds.

In some cases, density of the solids content could be obtained from paint manufacturers to enable calculations of V_solids from the measured wt%. In other cases, estimates of the total volume of volatiles in the RTS mixtures were made from wt% data given in material safety data sheets and the densities for individual volatile compounds, then subtracting from the total RTS volume.

RESULTS

A summary of the results from testing the waterborne topcoat is presented in Table 2, which shows the tests undertaken, the requirements, and results, and the particular paint specifications for which the testing requirements were referred to. The individual test results, including comparisons with the solvent-based topcoats, are itemized and presented in more detail in the following.

In-can Properties

Component A: The waterborne Component A was examined after storing for 14 days without agitation. When the can was opened, the product displayed a small amount of syneresis; however, the product could be easily stirred by hand with a spatula to a smooth, homogeneous, and pourable composition. The coating was free of grit and displayed a smooth finish on the Hegman gauge.

Component B: Component B was a homogeneous, clear, very pale yellow liquid.

Accelerated Storage Stability

When stored at 57°C for 24 h, as per MIL-85285D,⁴ Component B displayed no gassing or pressure build-up, and there was no evidence of gelling or clouding. Storage stability testing in MIL-85285E requires both components to be stored at 60°C for 14 days. In this testing, Component B showed slight yellowing. Syneresis was evident in Component A and a thin layer of soft settlement was evident on the bottom of the can. After stirring to a uniform consistency with a spatula for 30 sec, the two components could be mixed and sprayed over a primed surface to give a coating that cured to produce a uniform, smooth paint film free of defects and with the same overall surface appearance as the control samples.

Mixing

Components A and B could be easily mixed by hand stirring with a spatula to a uniform consistency. The mixture was thinned with the minimum quantity of DI water needed to adjust the spray viscosity to 30–40 sec as determined by a British Standard B4 viscosity cup. Some care was needed to prevent the formation of bits during stirring, possibly due to some shocking out of the pigments or flatting agents on adding the Component B to Component A. Initial thinning of Component A with a portion of the DI water and continuous stirring by hand while adding the Component B prevented bit formation.

Spray Application

The mixed waterborne topcoat could be applied to primed surfaces by spray application, and dried and cured under ambient laboratory conditions to produce paint films with a uniform, smooth surface, free from a range of defects often characteristic of waterborne coatings, such as craters, fisheyes, and orange peel.

Potlife

A waterborne RTS mixture that had been made up and allowed to stand under ambient laboratory conditions for 5 h was spray applied and compared with a freshly made up RTS mixture. Both samples produced coating films that flashed off and dried at the same rate, cured to form films with the same smooth finish, appearance, and color, and had the same solvent resistance and adhesion properties

Freeze-thaw Stability

After being subjected to five freeze-thaw cycles, the two components of the waterborne coating remained stable. Component A displayed little signs of settling and could be restirred by hand to a uniform consistency. When mixed with Component B and thinned with DI water to spray viscosity, the coating could be spray applied over primed panels to form films that dried and cured to a smooth even finish free of defects and with unchanged color when compared with the control.

VOC Data

VOC data and emissions are presented in Table 3. The waterborne RTS composition contains a maximum of 100 g/L of organic volatiles. After removing the water and exempt volatiles from the calculation, as per equation (4), the waterborne mixture gives a VOC of 180 g/L. VOCs for the solvent-based topcoats were calculated from measurements of wt% and density using equation (5). As none of the topcoat mixtures A, B, or C contain exempt volatile compounds, the results corresponding to equations (3) and (4) are identical. Topcoat A gave a VOC of 560 g/L as expected for a CS topcoat, while the two HS topcoats gave VOCs conforming to the requirements of MIL-PRF-85285E for Types I and IV.

Figure 3 compares the organic compound emissions (OCE_50μm) calculated using equation (6) from one square meter of dried paint film at a set DFT of 50 µm, showing that emissions from the waterborne topcoat are reduced by approximately 69% when compared with the two HS products.

FIGURE 3—Total volatile emissions from 1 m2 of final paint film at 50 µm thickness.

Early Dry Properties

The spray-applied waterborne coating flashed off within 10 min, was set-to-touch in 1 h and dry-to-touch in less than 4 h when tested using the methods in ASTM D 1640.¹⁹ Both the waterborne topcoat and Topcoat A were print-free at 4 h whereas the HS Topcoats B and C remained tacky, and were noticeably slower to reach a print-free condition.

Surface Dry Condition

The surface dry condition test specified in DEF(AUST) 9001A requires that a small quantity of sand deposited on the coating, after 5 h under ambient laboratory conditions, will be easily removed with a fine brush. The waterborne coating and Topcoat A passed this test. Topcoat B just passed at 5 h, whereas Topcoat C failed.

Tape Resistance

After 12 h, the waterborne coating showed no evidence of any permanent marking, imprinting, or other visible defects caused by the masking tape. This contrasted with Topcoat C, which failed this test. Topcoat A passed easily, while Topcoat B passed marginally.

Hard-dry Condition

After 12 h, the waterborne coating was hard dry and showed no tackiness or any signs of distortion in the rotating thumb test. Topcoat A was also hard dry. Topcoat B was just acceptable, whereas Topcoat C remained tacky.

Cure

After seven days of cure, the waterborne coating withstood 25 double rubs using cotton wool soaked in methyl ethyl ketone (MEK) without rubbing through to the primer (as specified). In fact, the finish coating withstood 300 MEK double rubs with only very slight marring of the paint film being evident, as was also observed for the solvent-based coatings.

Opacity

The waterborne coating, formulated as the FED-STD-595 colors 36375 and 35237 gave contrast ratios > 0.99 (requirement: not < 0.95) at a DFT of 50 µm. The L* values measured over the black and white areas of the backing card were identical to each other within the uncertainties of the L* measurements of the coating color (± 0.03).

Surface Appearance

The waterborne coating dried to a uniform smooth surface free from runs, sags, bubbles, streaks, seeding, floating, mottling, and other defects. No orange peel was evident.

Color and Gloss

Gloss levels for the four topcoats (36375 only) and color differences with respect to the FED-595 color standard 36375 color chip are presented in Table 4. The requirements for camouflage topcoats are: 60° gloss < 5, 85° gloss < 10, and ΔE_ab* < 1. Topcoat C and the waterborne topcoat had very good color appearance with respect to the color standard. Topcoats A and B did not match the color standard particularly well, with color difference ΔE_ab* of 2.85 and 3.55, respectively. Topcoat B was also noticeably flatter in appearance than the other three topcoats, particularly at lower angles of viewing.

Dry Adhesion

Dry adhesion testing using the crosshatch test (ASTM D 3359, Method B) of the waterborne topcoat over the primer from Company A gave a rating of 5B (requirement: not < 4B). ¹²  Topcoats A, B, and C also gave adhesion ratings of 5B over their respective primers.

Water Resistance

After immersion in water at 50°C for seven days in accord with DEF(AUST) 9001A, the waterborne topcoat over the primer from Company A gave an adhesion rating of 5B (ASTM D 3359). The waterborne coating retained excellent appearance and was free of blisters and other defects. The color difference ΔE_ab* between the immersed and un-immersed portions of the panel was less than 0.1. Table 5 presents a comparison with the solvent-based topcoats.

Humidity Resistance

The waterborne coating and the three solvent-based topcoats over their respective primers showed no signs of blistering, softening, water whitening, or other film defects.

Heat Resistance

Results from heat resistance testing (120°C for 1 h) are given in Table 6. There was no effect on gloss of the waterborne topcoat, and the color difference ΔE_ab* after the test was less than 0.1 (requirement < 1.0). The solvent-based topcoats performed equally well.

Reverse Impact Flexibility

The waterborne coating, when tested over unprimed panels as specified, passed at 20% elongation, but just failed at 40% elongation as slight cracking was evident under 10X magnification. The MIL-PRF-85285E requirement is for a pass at 40% for Types I and IV coatings, whereas DEF(AUST) 9001A requires a pass at 20%. Topcoat A failed at 20% but passed at 10%. Topcoat B failed at 40% but passed at 20%. Topcoat C passed at 40%. These results are summarized in Table 7.

Low-Temperature Flexibility

The waterborne coating exhibited no cracking when bent over a 2-in. mandrel at –51°C as specified for camouflage colors. The solvent-based coatings A, B, and C also resisted cracking.

Mandrel Bend

The waterborne coating and the solvent-based coatings A and C resisted cracking when bent over a 1/16-in. mandrel. Coating B cracked but remained intact over a 1/8-in. mandrel.

Fluid Resistance

The coatings were tested by immersing panels in lubricating oil at 120°C for 24 h, hydraulic fluid at 66°C for 24 h, and jet fuel at room temperature for seven days as specified in MIL-PRF-85285E. Results are presented in Table 8. In all cases, the waterborne coating showed no signs of blistering, softening, or other defects. The only other allowance is that “slight staining of the coating is acceptable.”

Crosshatch adhesion testing of the waterborne coating 1 h after removing the panels from the fluids gave adhesion ratings of 5B. Very little color change was evident with jet fuel (ΔE_ab* = 0.37) and hydraulic fluid (ΔE_ab* = 0.50). Some staining was noticeable with lubricating oil (ΔE_ab* = 2.83), but this was comparable with the degree of staining from the solvent-based Topcoats A and B. Topcoat C was noticeably affected by the lubricating oil, with ΔE_ab* = 5.20 and being visibly yellowed below the immersion line.

Cleanability

The waterborne coating gave a cleaning efficiency of 98% (requirement 75% for camouflage colors). This compared favorably with the old technology Topcoat A, while being significantly better than the HS topcoats (Table 9). A particularly poor result was obtained for Topcoat B, a result of 60% constituting a fail.

Corrosion Resistance: Salt Spray

The use of the waterborne coating over the primer from Company A had no significant adverse effect on the degree of corrosion when compared with the recommended Topcoat A over the same primer. When tested in accord with MIL-PRF-23377K,¹¹ salt spray testing is required to show no blistering after 2000 h and the scribes must be free of corrosion products. The results of this testing showed no blisters with either topcoat after 2000 h. The scribes were free of corrosion products.

Corrosion Resistance: Filiform   

The requirement for filiform corrosion testing is that the coating system should not exhibit filiform growth extending beyond 0.25 in. (~ 6 mm) from the scribe, and that the majority of the filaments shall be less than 0.125 in. (~ 3 mm) from the scribe after 1,000 h. As for salt spray, the priming system from Company A was used with both the waterborne topcoat and Topcoat A. Photographs of these two panels are shown in Figure 4. The degree of filiform corrosion growth out from the scribe was monitored for a period up to 3000 h and was found to be similar in both number density and length for both topcoats (Figure 5). All filamentary growth was < 3 mm from the scribe, a result associated with the performance of the priming system from Company A.

FIGURE 4—Photographs of filiform corrosion test panels of waterborne topcoat (right) compared with Topcoat A (left) over the priming system from Company A after 3000 h.

FIGURE 5—Distribution of filiform filament distances from scribes after 3000 h for waterborne topcoat compared with Topcoat A over the same priming system.

Exterior Durability

Results of exterior exposure testing have been evaluated in a number of different ways. In the work reported in this paper, specifically for the Federal Standard light gray color FED-STD-595-36375,⁸ it was found that the development of chalking was manifested primarily as an increase in lightness value L*. Loose chalk produced a whitish appearance that gave rise to a large increase in the L* value and removal by gently wiping under running water generally returned an L* value that was more closely aligned with the unexposed surface.

The results of color measurements (Figure 6) show significant changes on the uw sides of panels, whereas the ww sides showed very little change. Most of the total color change occurring on the uw sides was due to an increase in lightness value L* and could be attributed to the loose chalk that developed and was easily wiped away. The relative stability of color underneath the chalked layer shows that there was little inherent color change arising from the pigmentations used in each coating.

The extent of difference in performance of the various topcoats discussed in this article is perhaps best demonstrated by comparing the appearance of the best and worst examples after exterior exposure for several years. Figure 7 compares the appearances of Topcoat B and the waterborne topcoat after five years of exposure at Monegeetta. The Type I topcoat from Company B shows a high degree of fading on the unwiped side, which has been shown to be a result of chalking. The waterborne topcoat shows very little change after five years.

FIGURE 6—Color changes on wiped and unwiped sides of panels of the four topcoats during exposure testing at Monegeetta; ΔEab* (open circles), ΔL* (dots), Δa* (crosses), and Δb* (triangles).

FIGURE 7—Photographs comparing the appearance of exposure panels of Topcoat B (left) with the waterborne topcoat (right) after five years at Monegeetta.

Gloss measurements could not be used successfully to determine the onset of chalking, primarily because of the effect of dirt pickup on gloss measurements of the unwiped sides of panels.

Results of chalking undertaken by the tape test at both Monegeetta and Allunga are shown in Figure 8. These results show that the development of chalking for each coating occurs at slightly faster rates at Allunga than at Monegeetta as expected, but the differences are not great.

For the particular color tested and reported in this article (FED-STD-595-36375), it was found that a most useful assessment of chalking could be determined by comparing the L* values on the uw sides of panels with the L* values on the ww sides. Differences in the L* values could therefore be used as a measure of extent of loose chalk development and usefully expressed as a fraction with respect to L*_ww [equation (2)]. These results are shown in Figure 9. The curves are characterized by a slight initial decline in Ch value attributable to a small amount of dirt pickup on the unwashed sides of panels. As chalking begins to take place, Ch values begin to rise and a point is reached where the effects of dirt retention and chalking just balance each other (Ch = 0). If chalking develops at a rapid rate, its magnitude becomes much larger due to the small amount of dirt retention.

FIGURE 8—Development of chalking by the tape test of the four different topcoats during exterior exposure testing at Monegeetta (dots) and Allunga (open circles).

FIGURE 9—Development of chalking by equation (2) of the four different topcoats during exterior exposure testing at Monegeetta (dots) and Allunga (open circles).

In all of these results, Topcoat A shows a sudden rise in chalking from about 15 months, as evidenced by the sudden increase in Ch value, and the degree of chalking after five years is indicated by a Ch value of 0.10. Topcoat B (Type I) shows that a Type I HS topcoat is no better than a conventional solids topcoat. While rapid chalking development commences at about the same time (15 months), the severity of the chalking after five years is much more visible to the eye with a Ch value of 0.14.

Topcoat C, which is qualified as a Type IV coating, shows a significant improvement over the others, with noticeable chalking development not occurring until about three years—a much slower rate of chalking development taking place than for Topcoats A and B and a much-reduced degree of severity after five years (Ch = 0.02). The waterborne coating also shows excellent results, with only very slight indications of chalking. The curve shows a very slow rate of increase in Ch value and no evidence of sudden increase in Ch over a period of five years.

DISCUSSION

The results of this work have demonstrated the capability of a new waterborne two-pack polyurethane topcoat formulation as a finish coating for military aircraft paint systems. The coating can be spray applied using conventional spray equipment and techniques to produce paint films that wet out military aircraft primers without producing film defects such as craters, fisheyes, and blistering. The coating flashed off and dried at rates faster than HS products and comparable with old CS technology topcoats. The coating dried to produce cured films with a smooth uniform finish and excellent opacity, and can be formulated to the requisite low-gloss level. The films have excellent degree of cure, as determined by MEK double rubs, and excellent adhesion to a chromated, solvent-based primer. The in-service performance properties include excellent resistance to water immersion, very good cleanability, and the required chemical resistance to jet fuel, hydraulic fluid, and lubricating oil, while having flexibility properties comparable with the solvent-based topcoats.

Dry adhesion was determined by the crosshatch test (ASTM D 3359, Method B¹²) over different primers, including chromated solvent-based primers from Companies A and B, a chromated waterbased primer from Company C, and a chromate-free waterbased primer from Company D. In all cases, the waterborne topcoat gave adhesion ratings greater than 4B (requirement: not < 4B).

The impact flexibility requirement for a pass of 40% elongation in MIL-PRF-85285E was found to be difficult to meet. Inspection of the four sets of impact panels for the different topcoats showed a distinct failure of Topcoat B at 40%, with a layer of coating breaking cleanly away from the substrate, whereas the others appeared to pass at 40%. Topcoat A, however, showed some concentric circular crazing at 40%, and slight cracking was also observed at 20% by examining with 10-power magnification. Topcoat C passed at 40%. This trend in improved flexibility in going from Topcoat A to B and then to C appears to be consistent with the observed downturn in chemical resistance properties and slower speed of dry of these three topcoats. Topcoat A, representing the old CS technology, dried and cured much faster than the two HS products and gave the best chemical resistance properties, so it might be expected to have the lowest flexibility, which it does (10%).

The HS products, starting off with Component A polymers, which are likely to be either lower molecular weight or lower glass transition temperature polymers than for Topcoat A, might be expected to produce more flexible films, but at the expense of speed of dry and chemical resistance. This correlates with the observation that Topcoat C gave very slow dry rate and the poorest chemical resistance results (Tables 5 and 8). In recognition of the observation that coatings with better flexibility are more likely to dry slower and have lower chemical resistance properties, the DEF(AUST) 9001A standard reduces the requirement for reverse impact flexibility to a pass at 20% elongation, which the waterborne topcoat meets. It was possible to reformulate the waterborne topcoat to be more flexible with passes at 40% and higher, but this led to poorer chemical resistance properties.

An examination of the safety data sheets for the solvent-based topcoats used in this study revealed that none of them used exempt solvents, hence VOCs calculated by equations (3) and (4) are the same (Table 3). The VOC_b value calculated for the waterborne topcoat is disadvantaged by the removal of water from both the numerator and denominator of the calculation [equation (4)]. Figure 3 produces a more useful comparison that shows the significant reduction in real organic compound emissions from a given area of paint film at a constant final thickness. At 50 µm DFT, the waterborne topcoat emits only 13 g of organic volatiles per square meter, in contrast to 41 g and 43 g from the HS products and 80 g from the CS topcoat. From a practical viewpoint, these calculations assume 100% transfer efficiency from the spray gun.

A number of approaches for reducing emissions of the current waterborne topcoat even further are currently being investigated. The OCE value of 13 gm^-2 reported above contains about 5 g of a solvent, N-methyl pyrrolidinone, present in the PUD latex as a result of the manufacturing process used to make it. By using an alternative latex that does not contain this solvent, it would be possible to reduce the emissions to 8 gm^-2. Furthermore, substituting the solvent used in Component B with an exempt volatile compound such as t-butyl acetate would enable the VOC calculated by equation (4) to be reduced to approximately 40 g/L.

The waterborne topcoat exhibits excellent resistance to exterior weathering when compared with the solvent-based products. The graphs in Figures 8 and 9 show that the Type I HS product from Company B, while offering the advantage of lower VOC and higher solids, has poorer durability as judged by the degree of chalking that develops. Topcoat B begins to develop chalking at about 15 months as does Topcoat A, but after five years, the degree of chalking is about 40% greater. The Type IV HS product from Company C shows much better results, with onset of chalking not taking place until about three years and, when developing, takes place at a slower rate than Topcoats A and B. The waterborne topcoat, however, has given by far the best results, showing very little sign of chalking after five years with a very slow rate of development.

Although the different products had measurably different color to start with, it is not expected that these differences would have significant impact on the other properties measured in this work. The minor adjustments needed to obtain good color matches are not expected to change the results from working properties such as chemical resistance, flexibility, adhesion to primer, or exterior chalking.

The differences between the chalking development curves for Monegeetta and Allunga are not particularly great. It was expected that exposure under the harsher conditions of tropical Queensland would produce significantly greater chalking rates and that the extent of chalking, as monitored by the Ch values calculated, might have been greater than those from Victoria. For each product, the time of onset of rapid chalking development and the extent of chalking in each case was about the same for the two sites. These results probably indicate that the weather and sunlight exposure conditions are more or less equally severe across the whole of Australia from the point of view of weathering.

One of the major issues for the RAAF is the effect of the severe climate conditions in Australia. There is a major emphasis to ensure that topcoat coatings that are not durable enough will not be accepted for use. None of the other results required by MIL-PRF-85285E testing is worthwhile if a product does not have good exterior performance. It is for this reason that the current study has ignored laboratory weather testing in favor of exterior exposure testing for several years. These studies are ongoing⁷ and will include verification of the latest waterborne topcoat coatings against the best performing Type IV topcoats.

In the present study, a waterborne topcoat technology has been formulated that demonstrates excellent resistance to chalking in exterior exposure testing, and when underway after several years, the rate and extent of chalking development is only minor compared with the solvent-based topcoats. In addition to this, it has been shown that the waterborne topcoat is capable of being formulated to the performance test requirements of MIL-PRF-85285E for military aircraft.

Some of the earlier waterborne prototypes in this work had very short potlives with respect to viscosity rise, resulting in gel times of less than one hour. The development work has established formulations for RTS mixtures that show no viscosity rise for many hours, and which maintain a sprayable consistency all day. For these types of formulations, potlife may potentially be governed more by the deterioration in working properties of the spray-applied coating as the RTS mixture ages. Checks on film properties of samples made up at different times, including drying rate, film appearance, gloss level, cure, and adhesion gave results unaffected by standing time after being made up RTS for up to five hours. The advantage of this is that a single mixture could be made up at the beginning of a working day without the need to remake several times throughout the day.

Although it is expected that corrosion performance would depend primarily on the pretreatment and primer layers of the paint system, it should be ensured that any change to a different topcoat does not compromise anticorrosive performance. Studies of the effect of the waterborne topcoat on corrosion are illustrated by the filiform results in Figures 4 and 5, showing that the extent of filiform filament growth is not greatly affected when compared with the recommended solvent-based topcoat. Further studies need to be undertaken to verify whether there is a significant difference or if there might be any benefit in using the waterborne topcoat.

The waterborne topcoat has been tested via spray application for wetting out and laydown properties over a number of different primers. In all cases, good application properties were observed, with the waterborne topcoat drying to form defect-free paint films with the same smooth finish and good adhesion to primer.

Ultimately, an important goal of this work is to offer the waterborne topcoat as part of a total water-based paint system that includes a waterborne chromate-free primer. This will require significant further work, particularly to obtain corrosion performance equal to the chromated paint systems.

CONCLUSION

A two-pack waterborne topcoat formulation has been developed that demonstrates the suitability of this technology for air force applications. This technology offers significantly reduced VOC emissions when compared with commercial HS solvent-based products. The waterborne topcoat exhibited excellent exterior durability performance under the harsh climatic conditions of Australia. After five years in Queensland, it exhibited very little chalking or color change. Tests showed that the waterborne topcoat passes all the requirements listed in Table 2 and can potentially be qualified as a Class W, Type IV topcoat in the MIL-PRF-85285E specification.

References

1. MIL-PRF-85285E, Performance Specification. Coating: Polyurethane, Aircraft and Support Equipment, 12 January, 2012.

2. MIL-C-83286B (USAF), Military Specification. Coating, Urethane, Aliphatic Isocyanate, for Aerospace Applications, 5 October, 1973.

3. RAAF Specification Engineering K62, Aircraft Finishing Systems–Epoxy/Polyurethane, Issue No.3, 2 April, 1982.

4. MIL-PRF-85285D, Performance Specification. Coating: Polyurethane, Aircraft and Support Equipment, 28 June, 2002.

5. MIL-PRF-85285D, Amendment 3, Performance Specification. Coating: Polyurethane, Aircraft and Support Equipment, 2 February, 2009.

6. Nickerson, W., Mera, A., and Lipnickas, E., Performance Comparison of Modified MIL-PRF-85285 Topcoats (Advanced Performance Topcoats), NAVAIR Public Release 09-775, 22 May, 2010; also presented at NACE 2009, Corrosion Paper #7997.

7. Clayton, T.P. and Danek, S.K., Environmental Durability of RAAF Topcoats Qualified to MIL-PRF-85285, ACW-2014 Aerospace Coatings Workshop, DSTO-Melbourne, 31 October, 2014.

8. MIL-STD-595C, Federal Standard. Colors Used in Government Procurement, 16 January, 2008.

9. MIL-STD-595A, Federal Standard. Colors Used in Government Procurement, 2 January, 1968.

10. QPL-85285 for MIL-PRF-85285E, Coating: Polyurethane, Aircraft and Support Equipment, 19 August, 2016.

11. MIL-PRF-23377K, Performance Specification. Primer Coatings: Epoxy, High-Solids, 7 June, 2012.

12. ASTM D 3359–97, Standard Test Methods for Measuring Adhesion by Tape Test, 1998.

13. DEF(AUST) 9001A, ADF Aircraft Epoxy/Polyurethane Paint Coating System, 5 June, 2009.

14. ASTM B 117–03, Standard Practice for Operating Salt Spray (Fog) Apparatus, 2003.

15. ASTM D 2244–93, Standard Test Method for Calculation of Color Differences from Instrumentally Measured Color Coordinates, 1993.

16. AS/NZS 1580.457.1, Paints and related materials–Methods of test. Resistance to natural weathering, 1996.

17. ISO 4628-6: 2011(E), Paints and varnishes–Evaluation of degradation of coatings–Designation of quantity and size of defects and of intensity of uniform changes in appearance–Part 6: Assessment of degree of chalking by tape method, 15 August, 2011.

18. ASTM D 3960–98, Standard Practice for Determining Volatile Organic Compound (VOC) Content of Paints and Related Coatings, 1999.

19. ASTM D 1640–95, Standard Test Methods for Drying, Curing, or Film Formation of Organic Coatings at Room Temperature, 1999.

20. AS/NZS 1580.401.1, Paints and related materials–Methods of test. Surface dry condition, 1999.

21. ASTM D 6905–03, Standard Test Method for Impact Flexibility of Organic Coatings, 2004.

22. ASTM D 5402–93, Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs, 1993.

This paper was awarded Honorable Mention at the 44th International Waterborne, High-Solids and Powder Coatings Symposium, February 22-24, 2017, in New Orleans, LA.

CoatingsTech | Vol. 14, No. 6 | June 2017

*This paper received the American Coatings Best Paper Award at the 2016 American Coatings CONFERENCE, April 11-13, in Indianapolis, IN.

For the past several decades, coating suppliers have struggled to develop waterborne (WB) with comparable performance to solventborne (SB) coatings. Alkyds, in particular, have seen very little conversion to WB or lower volatile organic compound (VOC) options, primarily due to performance gaps related to dry time, gloss, adhesion, and corrosion resistance even though versions of WB alkyds have been available for more than 50 years. In fact, as of 2012, less than 10% of the overall alkyd market was converted to WB options.^1�� Dow has developed novel technology that facilitates the dispersion of highly hydrophobic, high viscosity alkyd resins into water at near-zero VOC levels. These alkyd dispersions close the performance gaps of current WB alkyd options and offer formulators the opportunity to prepare high-performance WB alkyd paint at near-zero VOC.

Current Technology

Solventborne alkyds continue to remain popular due to their low cost, ease of application, and great versatility. Nevertheless, alkyd coating volume has been forecasted to decline by two percent per year mainly due to the loss of share to coating technologies that can offer higher performance at lower VOC.²  Paints based on waterborne acrylic emulsions have replaced certain SB alkyd coatings due to lower solvent content and faster dry. However, some coatings from acrylic emulsions can show worse adhesion, gloss, and corrosion resistance compared to SB alkyd coatings at thin film builds (Figure 1). Alkyds, which are typically applied at lower molecular weight than latex paints, display exceptional wetting of the substrate; consequently, their coating films offer higher gloss and corrosion resistance than those from latex paint. In addition, during air drying, alkyds crosslink via oxidation leading to very high molecular weight networks that further improves their resistance properties (Scheme 1). Another benefit of alkyds is their hydrophobicity, which contributes to excellent early water and humidity resistance.
Low-VOC alkyd options include high-solids alkyds, water-reducible alkyds, WB alkyd emulsions, and core-shell modified alkyd dispersions. The solids content of an SB alkyd can be practically increased by reducing the viscosity of the neat alkyd resin, thereby reducing the amount of solvent needed to bring the viscosity to a reasonable level. The viscosity can be reduced by decreasing the molecular weight of the alkyd resin by changing the dibasic acid/polyol ratio or by increasing the oil length. Although these changes result in VOC reduction, high-solids alkyds typically have longer dry times and reduced chemical/corrosion resistance properties compared to those from traditional SB alkyds.

Water-reducible alkyds can be formulated to 250–350 g/L VOC. They are often made by incorporating trimellitic anhydride or other raw materials, including carboxylic acid groups, into the structure. The additional acid is neutralized with ammonia or other amine to provide hydrophilicity (Figure 2). The alkyds are typically supplied at 70–75 wt% solids in a hydrophilic glycol ether solvent and are dispersed into water by the formulator. They exhibit excellent application characteristics and high gloss, but tend to have long drying times and high yellowing.¹ In addition, the increased hydrophilicity often leads to limited paint storage stability due to ester hydrolysis, which causes a noticeable drop in performance.

Alkyd emulsions can be prepared with little, if any, volatile solvent and have improved hydrolytic stability. The resins are dispersed through the addition of 5–10% anionic and/or nonionic surfactants and do not require the incorporation of excess carboxylic acid groups. In this process, a neat alkyd is heated to a temperature high enough to reduce the viscosity. A surfactant package is then added to the molten alkyd followed by gradual water addition. As water is added, the mixture forms a water-in-oil emulsion. As the water content increases, the alkyd inverts to an oil-in-water emulsion (Figure 3). In general, the shorter the oil length of the alkyd resin, the higher the temperature required; therefore, this technique is typically limited to medium- and long-oil alkyds. The emulsions typically have low storage stability and inferior performance due to the presence of high levels of surfactant, which can also migrate to the surface of the coating and lead to early water resistance or corrosion resistance failures.

To improve the hydrolytic stability of WB alkyds, core-shell alkyd-acrylic hybrids have been introduced. During manufacture, an alkyd of deliberately low molecular weight is copolymerized with a “hydrolysis-resistant” acrylic monomer, grafting the acrylic to the alkyd. The acrylic shell is made hydrophilic by neutralizing the acid groups with an amine. The alkyd is then dispersed into water. The acrylic shell extends into the water phase and helps protect the alkyd core from hydrolysis, making the hybrid more shelf stable than water-reducible alkyds (Figure 4). The hybrids also have fast drying time and high gloss at VOC levels that tend to be <100 g/L.¹ Unfortunately, during copolymerization, a considerable amount of acrylic homopolymerization (i.e., not grafted to alkyd) also occurs.³  High levels of ungrafted acrylic polymer detract from the properties of the modified alkyd and often result in considerable water sensitivity due to the high acid value acrylic polymers.

In all the examples mentioned above, low VOC was achieved through chemically or physically modifying the alkyd resin—reducing molecular weight, increasing the acid value and neutralizing, increasing the oil length, or adding a significant amount of surfactant—all of which cause a reduction in overall alkyd performance. In addition, modification steps or the use of specialty monomers/materials in the synthesis increases the cost of production of these resins.

So, the question remains, if a modified WB alkyd resin cannot offer the same performance as an SB system, why not disperse traditional SB resins in water? Until very recently, the answer to this question was that traditional “solventborne” alkyd resins are too hydrophobic to disperse in water or to remain stable as an alkyd emulsion. A novel, mechanical dispersion technology changes the rules and attacks the problem in a new way. The mechanical dispersion process enables the dispersion of very hydrophobic alkyd resins into water without polymer modification or significant amount of surfactant.

Novel Technology

When choosing to develop WB alkyd technology, a route to disperse hydrophobic, high-molecular weight/viscosity resins into water without any hydrophilic modification of the alkyd was implemented. It was hypothesized that these dispersions would yield WB alkyd coatings that maintained the excellent gloss, adhesion, and resistance properties associated with SB alkyd coatings. Thorough process studies demonstrated that mechanical dispersion could disperse unmodified alkyd resins of various compositions. Formulation work showed that these dispersions could be used to formulate WB alkyd coatings that mirrored the performance of SB alkyd coatings and decoupled performance from VOC level.

Mechanical dispersion is a continuous emulsification process in which a metered alkyd stream and a metered aqueous stream are combined under shear to create an emulsion of specified quality. The resulting dispersions are solvent-free (manufactured without added solvent), typically 50–65 wt% solids with a viscosity range of 500–5000 cP and a narrow particle size distribution between 100–300 nm. Figure 5 shows a schematic of the process. The process is well suited for quick product turnarounds, is easily adjustable, and economically scalable (1–100 kg/min resin flow). It effectively disperses neat alkyd resins with a range of acid values (2–30 mg KOH/g), oil lengths (short, chain-stopped, medium, and long), molecular weights (up to ~250 kg/mol), and viscosities (up to ~250,000 cP) with little to no surfactant (0–4%). Examples of the characteristics of short-oil (SO) and medium-oil (MO) alkyd resin dispersions are shown in Table 1.

Unlike the previous WB alkyds described where resin modification decreased coating performance, the performance of this enhanced class of WB alkyd dispersions is comparable to commercial SB alkyds (Figure 6).

In 2011, a Voice of the Market (VOM) study was conducted to help understand the growing trends in WB alkyd coatings. The major conclusions from the study were that (1) despite shortcomings in current WB alkyd technology, formulators still use it, and (2) a great majority of formulators desire to use WB alkyds provided that the performance gaps around gloss, adhesion, and corrosion resistance are closed.

To demonstrate the utility of this technology, the performance of an SO alkyd resin formulated at 560 g/L VOC in solvent was compared vs the same alkyd dispersed via mechanical dispersion and formulated at near-zero VOC (<5 g/L) (Table 2). Clear coatings were drawn down on cold rolled steel (CRS) targeting 1 mil dry film thickness (DFT) and cured for 14 days at 23˚C and 50% relative humidity (RH). Gloss, adhesion, hardness, and MEK resistance were measured after 14 days of curing and are reported in Table 3. The corrosion resistance of the coatings after 200 h salt spray exposure is shown in Figure 7. Notably, the two coatings had very similar performance, although the corrosion resistance of the WB alkyd coating was improved compared to the SB alkyd coating, even at lower VOC.

After validating the hypotheses, the next step was to develop an exceptional alkyd resin for metal protection in industrial applications. Both SO and MO alkyd resins were explored using design of experiments (DoE) methodology. The optimal composition of the MO alkyd resin was determined through the completion of DoEs varying the oil length, degree of branching, effects of molecular structure, and acid content (Table 4). Coatings were evaluated on resistance to yellowing, hardness, flexibility, gloss, adhesion, block resistance, scrub resistance, and flow and leveling. Based on the results, an MO alkyd resin showing the best balance of coating properties was chosen as the candidate to move forward.

The results from the first sets of DoEs led to extending the design into the SO alkyd space, hypothesizing that a SO alkyd would offer improved corrosion resistance. A Box-Behnken design for three variables at three levels was laid out to explore the effect of polyol type, fatty acid type, and aromatic acid type on coating performance in clear coatings (Figure 8).⁴ Coatings were evaluated on resistance to yellowing, hardness, flexibility, gloss, chemical resistance, adhesion, and resistance to corrosion. Based on our results, the MO and SO probes were scaled up and dispersed for additional formulation work.

Formulation Development

Early into formulation development of the mechanically dispersed alkyd resins, it was discovered that traditional WB alkyd coating formulation strategies did not give acceptable coating performance with this new class of WB alkyd dispersions. Since additives recommended for WB alkyds were designed to work with more hydrophilic resins, it was necessary to evaluate different additives that could be more compatible with the hydrophobic resin chemistry. For example, evaluation of five different dispersants in the grind led to dramatically different corrosion-resistance properties (Figure 9). To understand this observation, the paints were evaluated using top-down SEM (Figure 10). The results were striking. The more hydrophobic dispersants showed a much better distribution of TiO2 within the alkyd, which corresponded with improved corrosion protection. The necessity of proper additive selection was consistent across the other additives including rheology modifiers (RM) and defoamers. Figure 11 shows how RM 1 caused the formulation to become unstable and crash, whereas RM 2 thickened the paint to a usable viscosity without causing agglomeration.

Benchmark Study

Once satisfied with a near-zero VOC (<5 g/L) formulation (Table 5), we compared the performance of the SO and MO WB alkyd dispersions, formulated at 14% PVC, with six commercial solventborne alkyd paints (VOC range 330–600 g/L) and four commercial waterborne alkyd paints (VOC range <5–350 g/L).

All the paints were applied at a targeted 1 mil DFT on CRS panels. After curing for 14 days at 23°C and 50% RH, the paints were evaluated for dry time, gloss, hardness, adhesion, flexibility, and corrosion resistance.

As discussed earlier, the common techniques used to formulate low-VOC alkyd coatings often compromise coating performance such as reduced gloss, adhesion, and corrosion resistance.

Table 6 shows the performance of the paints based on the enhanced WB alkyd dispersions as compared to the commercial WB alkyd paints. Notably, all the commercial WB alkyd paints have lower gloss and worse adhesion compared to the paints derived from the enhanced alkyd dispersions. Hardness and impact resistance were comparable across the data set, with the exception of the paint formulated with the MO alkyd dispersion, which showed superior impact resistance. As expected, the corrosion resistance of paints from an unmodified, hydrophobic alkyd resin was higher than those from resins that have hydrophilic modification. This is evident in Figure 12, which compares the corrosion resistance of the paints derived from the enhanced WB alkyd dispersions to the commercial WB alkyd paints after 300 h salt spray. The paint based on the SO alkyd dispersion clearly outperforms all the commercial WB alkyd emulsion paints and matches the performance of the water-reducible alkyd which was formulated at >300 g/L VOC and enhanced for corrosion protection.

To demonstrate the distinct value of these WB alkyd dispersions, we also compared the performance of WB paints formulated from them to the performance of commercial SB alkyd paints. The same test protocols as described earlier were followed; all paints were drawn down on CRS, targeting 1 mil DFT. The biggest difference between the paints was VOC levels, although the PVC level for the paints was unknown. Compared with the commercial SB topcoat alkyd coatings, the enhanced WB alkyd dispersion coatings displayed very good gloss and adhesion (Table 7). However, the most impressive comparison is the corrosion resistance of the paints from these WB alkyd dispersions with that of the commercial SB alkyd paints at 1 mil DFT after 300 h salt spray exposure (Figure 13). The paint based on the enhanced SO alkyd dispersion matched the corrosion resistance performance of the best SB alkyd paint evaluated.

Short-Oil—Medium-Oil Alkyd Comparison

Clearly both paints formulated with the SO and MO alkyd dispersions performed very well compared to the commercial options available. In addition, the performance of the SO and MO alkyd dispersions were complementary. Both the SO and MO paints from the developmental dispersions had very high gloss (>85 at the 20˚ gloss reading) and excellent adhesion across a variety of substrates. However, the most noticeable difference between the coatings from the two resins was the balance of flexibility, MEK resistance, and corrosion resistance (Table 8). Paint formulated with the MO alkyd dispersion paint was significantly more flexible and had better solvent resistance (MEK double rubs) than the paint formulated with the SO alkyd dispersion. The paint formulated with the SO alkyd dispersion, however, provided extended corrosion resistance (>300 h) (Figure 14).

The next logical step was to evaluate the performance of blends of the SO and MO WB alkyd dispersions. The dispersions were pre-blended and then formulated according to the pigmented starting point formulas described earlier. The paints were drawn down on CRS panels targeting 1.5 mil DFT. The coatings were cured for 14 days at 21°C and 50% RH prior to evaluation. Table 9 shows the performance of the coatings. From the data, it is evident that the degree of flexibility and corrosion resistance can be tailored to meet specific application targets without sacrificing gloss or adhesion properties. The corrosion resistance of the paints to 200 h of salt spray exposure is excellent, slightly favoring blends with more SO alkyd dispersion (Figure 15).

Dry Time Improvements

One documented gap between SB and WB alkyd systems is the increased drying times of WB alkyd coatings. This was noticed in the SOA–MOA dispersion blend work. To improve the dry times of formulations containing these alkyd dispersions, an evaluation of drier package in clear coatings was completed using the SO alkyd dispersion. The mixture design included Co, Zr, Mn, and Fe driers at low to high levels. Interestingly, combinations of three or four driers gave the shortest drying times and the hardest films (Figure 16). Further work was done with a 14% PVC formulation, optimizing around Co/Zr as the primary driers. Inclusion of Ca as an auxiliary drier improved drying times. Table 10 highlights a few of the improvements realized upon reducing volume solids, solvent addition (<35 g/L VOC), and proper additive selection.

Conclusions

Mechanical dispersion technology facilitates the dispersion of hydrophobic, unmodified, alkyd resins. Formulation of these dispersions into waterborne alkyd paints at near-zero VOC can generate coatings with performance mirroring and, in some cases, exceeding that obtained from solventborne alkyd coatings. Short-oil and medium-oil alkyd dispersions can be blended to achieve one of the best balance of overall properties for a given application. These waterborne alkyd dispersions show that they can close the performance gap between commercial waterborne alkyd technology and commercial solventborne alkyd technologies with regards to gloss, adhesion, and corrosion resistance.

Acknowledgments

The authors would like to express our deep appreciation to Rebecca Ortiz and Ray Drumright for their technical contributions and review of this paper; Alan Piwowar for his analytical support; Daryoosh Beigzadeh, Guy Maslowski, Jun Yang, John Roper, Gary Spilman, Tim Young, Ahmad Madkour, Robert Sandoval, and Doug Hasso for design, synthesis, application, and processing support; and Jonathan Mason, David Carr, and Robert Mussell for their technical contributions and managerial support.

References

1.    ISH Chemical Estimates, SRI Alkyd Surface Coatings 2013.
2.    U.S. Paint & Coatings Industry 2011-2016, Kunsumgar, Nerlfi, & Growney.
3.    Hare, C.H. “Alkyd Resins,” Protective Coatings: Fundamentals of Chemistry and Composition, Technology Publishing Co., 1994.
4.    Ortiz, R., Romick, J., Young, T., Bills, R., Beebe, M., and Sullivan, J., “Bridging the Gap in Performance: WB and SB Short Oil Alkyd Coatings for Metal Protection,” Proc. 42nd Annual Intl. Waterborne, High Solids and Powder Coating Symp., The University of Southern Mississippi, 2015.