Microbial contamination on surfaces is a problematic and potentially deadly issue, particularly in high-risk settings such as healthcare, aged care, and food and beverage. In this research, we present an antimicrobial waterborne polyurethane additive that will be of interest to the coatings community due to its high compatibility with a variety of coating systems, which results in a range of highly diverse and useful applications.

This next-generation antimicrobial uses a nano-composite approach to create a silver-based active ingredient that is non-toxic and truly non-leaching. The presented technology produces strong binding between silver and the polymer backbone. This prevents leaching while retaining high activity with proven results against a range of pathogens.

Introduction

Each day, approximately 1 in 31 U.S. hospital patients and 1 in 43 nursing home residents contract at least one infection associated with their healthcare.¹ Healthcare-associated infections accrue direct costs to U.S. hospitals of at least $28.4 billion each year.² Healthcare settings are just one example of a high-risk area where the transmission of disease is dangerous and expensive.

Silver is well known for its excellent antimicrobial properties, and many products have been produced that exploit this.^3-5 These products typically work via the controlled release of silver ions or nanoparticles, resulting in concerns over the release of silver into the environment and the interactions it subsequently undergoes.^6-8

Countering this risk, a novel process has been developed that binds the silver active ingredient directly to a polymer backbone, creating a nanocomposite from which the silver does not leach.^9,10 This produces an antimicrobial material that is not only sustainable and environmentally friendly, but also displays incredible longevity as the active ingredient is not depleted.

In this research, a water-based silver-polyurethane nanocomposite material has been developed that displays antimicrobial activity against bacteria, viruses, and fungi. Its high potency means it can be utilized as an additive in other coating systems, and its compatibility with a range of materials has been shown, including acrylates, polyurethanes, and other waterborne systems.

As a result, this antimicrobial additive can be used effectively in a range of highly diverse applications (textiles, walls and floors, furniture, high-touch surfaces, filters, etc.).

Experimental

The silver-polyurethane composite was produced using Inhibit Coatings’ proprietary functionalization process.

Characterization

X-ray diffraction (XRD) was used to confirm the crystalline structure and composition of the silver particles within the polymer. A PANalytical X’Pert PRO diffractometer was utilized with a copper K-alpha X-ray source at a wavelength of 1.5405 Å, operating at 45 kV and 40 mA.

A JEOL 2010 transmission electron microscope (TEM), operated at 200 kV, was used to visualize the silver particles within the polymer. The silver-polyurethane composite was diluted and then drop cast onto 200 mesh copper grids and plasma treated with a JEOL EC-52000IC ion cleaner prior to TEM analysis.

Independent Testing

Antimicrobial Testing

Antibacterial testing of the silver-polyurethane composite applied to a polyester textile was conducted according with the AATCC 100-2019 standard¹¹ by Microbe Investigations Switzerland (MIS). The sample was tested against Escherichia coli (E. coli) (ATCC 8739) for a contact time of 24 hours.

Antiviral testing of the silver-polyurethane composite in a polyurethane coating on acrylic was conducted according to the Japanese Industrial Standard JIS Z 2801 standard (modified for viruses)¹² by Microchem Laboratory. The sample was tested against Human coronavirus (ATCC VR-740, Strain: 229E) and Influenza A virus (H1N1) (ATCC VR-1469, Strain: A/PR/8/34), for a contact time of 2 hours.

Antifungal testing of the silver-polyurethane composite in a polyurethane coating on acrylic was conducted according to the ASTM G21 standard¹³ by Microbe Investigations Switzerland (MIS). The test details utilised are provided in Table 1.

Leach Testing

Leaching of silver from the silver-polyurethane composite was conducted by Nanosafe, Inc. Testing of the composite applied to fabric samples followed U.S. EPA Method 1311 “Toxicity Characteristic Leaching Procedure,”¹⁴ adapted for the assessment of silver-leaching potential from textiles.¹⁵ Testing of the composite applied to wood samples was conducted under guidance of OECD 313 “Estimation of Emissions from Preservative-Treated Wood to the Environment.”¹⁶

Toxicology Testing

Toxicology testing, as outlined in Table 2, was conducted by STILLMEADOW, Inc., on the silver-polyurethane composite.

Preservative Testing

Preservative testing was run using a Difco Paddle Tester over three weeks, using three concentrations of the silver-polyurethane composite in a polyurethane dispersion, and testing weekly for bacterial growth.

Results and Discussion

Silver Particle Characterization

Characterization techniques have verified that metallic silver nanoparticles have formed.

The XRD pattern of the silver-polyurethane composite has shown peaks with 2θ values of 38.2°, 44.3°, 64.5°, and 77.6° (Figure 1), which can be attributed to the (111), (200), (220), and (311) crystallographic planes of face-centered cubic (fcc) silver crystals, respectively.¹⁷

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

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

Introduction

Siloxane Chemistry

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

Siloxane-based wetting agents are generally smaller molecules with relatively short siloxane backbones. As shown in Figure 2, they are typically either trisiloxanes (n = 0, m = 1) or have comb structures with very few –S��(���₃)₂–O– units (n) and only a very few organo-modifications (m); generally, n + m is less than 5. Hydrophilicity can be tuned by varying the nature and length of the pendant organic groups and by modifying the ratio of hydrophilic and hydrophobic moieties present in the molecule. In aqueous systems, these amphiphilic siloxanes are driven to the air–water interface and can lower surface tension within a time frame consistent with their molecular mobility. Molecules with longer siloxane backbones can also be rendered oleophobic and may also be driven to the air–liquid interface in nonaqueous, organic-based systems. Siloxane-based wetting agents are most often used to lower the surface tension of aqueous and solventborne, as well as high-solids and 100% solids, formulations to improve substrate wetting. Occasionally, these wetting agents are also used to compatibilize insoluble oils—particularly silicone contaminants—to prevent problems such as dewetting and defects.

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

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

Siloxane-based Surface Control Additives in Coating Applications

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

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

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

The Need for Recoatability: The Balancing Act

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

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

Results and Discussion

Experimental

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

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

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

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

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

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

Design of the New Surface Control Additives

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

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

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

Evaluations of New Surface Control Additives in Waterborne Wood Coatings

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

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

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

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

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

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

Discussion

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

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

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

Conclusions

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

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

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

References

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

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

Introduction

Two-component waterborne polyurethane (2KWB PU) coatings have been recognized as coatings with excellent performance properties, and they are used in a wide variety of applications.¹ However, they require the introduction of a curing agent as a second component to provide the crosslinking needed to enhance coating performance. Several challenges for 2KWB coatings are well known, including limited pot life, increased waste, potential risk for mixing failure, and additional process steps before application. Therefore, in certain markets, paint formulators continue to look for one-component waterborne (1KWB) coatings that perform comparably to 2KWB coatings; for example, coatings for the residential vinyl window market. Vinyl windows represent more than 70% of windows sold in the United States residential window market² and nearly all of them are white in color. Consumers are demanding more color options and, to meet that demand, window manufacturers are painting more and more windows today. Currently, 2KWB coatings are the market standard, but window manufacturers and applicators are seeking high-performance 1KWB coatings that are easy to apply and provide comparable performance to the 2KWB coatings being used today. This article discusses the physical properties, as defined in American Architectural Manufacturers Association (AAMA) 615-13 specification,³ of a newly developed 1KPUR dispersion (hereinafter called “Next-gen 1KPUD”) that demonstrates comparable performance properties to 2KWB coatings used to paint residential vinyl windows today.

Experimental

To increase the performance of a 1KWB PU to the level of a 2KWB PU, an intensive development program was undertaken. Its goal was to develop a 1KWB PU with higher film hardness (e.g., >2H pencil gouge hardness) and with superior chemical resistance (e.g., detergent, nitric acid, and mortar). The study examined various kinds of high molecular weight polyol, different isocyanate monomer, various levels of hard block content, chain extension, neutralization, and solubilizing agents.

The development program succeeded, identifying a 1KWB PU dispersion with comparable performance to a 2KWB PU. The novel 1KWB PU dispersion, Next-gen 1KPUD, was synthesized using a prepolymer mixing process.⁴ Key properties of this PUD are listed in Table 1.

Physical Properties Comparison with High-Performance PUD and 2KWB PU System

Physical properties of the Next-gen 1KPUD were compared with commercially available high-performance PUDs and 2KWB PU systems. To that end, each of the PUDs and 2KWB PU systems was formulated as clear to avoid effects from other paint ingredients. Characteristics of high-performance PUDs and 2KWB PU systems used in this study are compiled in Table 2.

All systems were drawn down on glass and chromated aluminum panels at a wet-film thickness of 5 mils. Panels were dried at ambient conditions. Aside from dry times, all testing was conducted after a seven-day rest at ambient conditions. Dry times were measured by a dry-time recorder (DT-5040) manufactured by the Paul N. Gardner Co., Inc. For assessing film hardness, pendulum hardness was measured by a pendulum damping tester (Model 299/300) manufactured by Erichsen GmbH & Co. KG. Chemical resistance was measured by a spot test of the indicated chemical on glass. Stain resistance was also measured by a spot test for the indicated period. Humidity resistance was measured by exposing the film to 38°C at 100% relative humidity (RH) for 168 h and assessing for blisters.

Physical Properties for American ��Architectural Manufacturer Association 615-13 Performance Evaluation

Physical properties for American Architectural Manufacturer Association (AAMA) 615-13 of paint formulated with the Next-gen 1KPUD were compared to three different commercially available paints for vinyl windows. Two of them were 1KWB systems and the other was a 2KWB system. Physical property testing on each system was conducted according to AAMA 615-13 specifications. Table 3 shows a summary of all tests that were conducted. All systems were spray applied on PVC panels at a target dry-film thickness of 1.5 mils. All panels were initially wiped with acetone prior to coating application for performance testing.

Panels were allowed to flash dry at ambient conditions and dried at 50°C for 10 min. All testing was conducted after an additional seven-day rest at ambient temperature. Table 4 shows a generic formulation used as the base formulation for this study.

Accelerated Weathering

To assess the weathering performance of the paint formulated with the Next-gen 1KPUD, we compared it to a paint formulated with commercially available PUDs and 2KWB PU. Two studies were performed. Initially, the Next-gen 1KPUD and the commercially available PUDs (PUD-A and PUD-D) were compared using a clear formulation containing UVA and HALS. Secondly, the Next-gen 1KPUD was formulated into a white pigmented 1KWB PU coating and compared to a white pigmented 2KWB PU coating. Table 5 shows these white pigmented formulations used as the base formulation.

All systems were spray applied on chromated aluminum panels at a target dry-film thickness of 1.5 mils. Panels were allowed to flash dry at ambient conditions and then dried at 50°C for 10 min. Testing began after an additional seven-day rest at ambient temperature. The accelerated weathering tests were performed in a Xenon arc Weather-Ometer Ci-65 (Atlas Material Testing Technology, USA) according to ASTM D6695 (Cycle 1) and in a QUV/se (Q-Lab Corporation, USA) according to ASTM D4587 (Cycle 2). Gloss retention and color retention were measured for a total time of 2,000 h. Gloss retention was determined using a gloss meter (Micro-TRIgloss) manufactured by BYK Gardner Corp., and Delta E was determined using a spectrophotometer (Color i7) manufactured by X-Rite, Inc., as described in ASTM D 2244.

Results And Discussion

Physical Properties Comparison with High-Performance PUD and 2KWB PU

Physical properties of the Next-gen 1KPUD were compared to commercially available 1K PUDs and 2KWB PU systems. The results of chemical resistance and stain resistance testing are summarized in Table 6. As compared with commercially available 1K PUDs, the Next-gen 1KPUD showed superior chemical and stain resistance. It outperformed even PUD-D, a polycarbonate-based resin, known for high hardness and physical performance. Furthermore, its chemical and solvent resistance matched even those of the crosslinked 2KWB PU coating (PAC-A+PIC-B).

Fast-drying properties and film hardness development are required at most coating applicators to ensure high productivity; therefore, each formulation’s dry time and König pendulum hardness development were evaluated. These results are summarized in Table 7. The Next-gen 1KPUD showed the fastest dry-hard time of all systems tested. This result is consistent with the PUD’s higher T_g��(Table 2) and its high molecular weight polyol in its backbone. The 2KWB systems, which rely on the chemical reaction between its isocyanate and polyol components, showed expectedly slow dry times at an ambient drying condition.

Humidity resistance is another key physical property in understanding the degradation of coatings due to water. To assess water resistance, blister formation on the film was measured after exposure to moisture. The result is summarized in Table 8. PUDs based on polycarbonate diols are known to have good water resistance due to better hydrolytic stability. The Next-gen 1KPUD, even though it is not polycarbonate-based, matched the humidity resistance of the polycarbonate-based PUD that was tested, PUD-D. It even outperformed the two 2KWB PU systems that were evaluated.

Physical Properties for AAMA 615-13 Performance Evaluation

Since the Next-gen 1KPUD showed better physical performance than different types of commercially available 1K PUDs and matched the performance of the 2KWB systems during our initial study, the testing standards were increased. A brown pigmented 1KWB coating formulated with the Next-gen 1KPUD was compared with three different commercial vinyl window paints under the stringent AAMA 615-13 testing protocol. The results of physical property testing according to AAMA 615-13 are summarized in Table 9. Overall, the paint formulated with the Next-gen 1KPUD passed all AAMA 615-13 physical property requirements and performed just as well as the 2KWB commercial system.

The most noteworthy result of the coating formulated with the Next-gen 1KPUD is its high pencil gouge hardness. The need for increased film hardness compared to the AAMA specification (min. B) is highly desirable by coatings applicators and window manufacturers for vinyl window applications. They would like to avoid potential damage during packaging, transportation, and installation of windows on site. As expected, the commercial 2KWB paint had much higher pencil hardness than other commercial 1KWB paints. The 2KWB PU’s crosslinked structure imparts toughness to the film which a 1KWB PU lacks. Surprisingly, the Next-gen 1KPUD matched the film hardness of the 2KWB system, while also lacking this crosslinked structure.

Accelerated Weathering Performance

To understand long-term weathering performance, the Next-gen 1KPUD was compared to both commercially available, high-performance 1K PUDs and a 2KWB PU system using two protocols. First, to compare the Next-gen 1KPUD’s weathering to commercially available, high-performance 1K PUDs, the films were exposed to QUV-A following ASTM D4587 C2. The results are shown in Figure 1. The Next-gen 1KPUD maintained 100% gloss retention after 2000 h, while PUD-D (polycarbonate-based PUD) started to reduce gloss after 1000 h and PUD-A (polyester-based PUD) started to reduce gloss even sooner, after only 750 h.

Second, to compare the Next-gen 1KPUD’s weathering performance to a 2KWB PU coating, it was formulated with a white pigment to match the 2KWB PU coating. The results of the Xenon arc weathering testing are summarized in Figure 2.

A white pigmented coating formulated with the Next-gen 1KPUD weathered better than the 2KWB PU coating. The formulated Next-gen 1KPUD coating retained its gloss and color beyond 2000 h, while the 2KWB PU coating gradually reduced gloss after just 250 h. Overall, the Next-gen 1KPUD had better weathering performance than both the high-performance commercial PUDs and the 2KWB PU coating that was evaluated.

Conclusions

The Next-gen 1K PUD, a novel 1K PU dispersion, outperformed commercially available 1K PUDs by exceeding their chemical and stain resistance and matched even the performance of crosslinked 2KWB coatings. Moreover, the Next-gen 1KPUD demonstrated faster dry times than the 2KWB coatings, which will increase productivity during the coating application. A coating formulated with the Next-gen 1KPUD passed all the AAMA 615-13 physical performance requirements. Further, its film hardness matched that of a crosslinked 2KWB coating. Finally, accelerated weathering tests under QUV-A and Xenon arc exposure showed that a coating formulated with Next-gen 1KPUD will outperform commercially available 1K PUDs and a 2KWB coating that were evaluated.

Acknowledgments

The author would like to acknowledge the contributions of Lyubov Gindin, Stephanie Goldfein, Natalee Smith, Tom Durkin, and Phil Jones to this paper and thank Ron Konitsney, Tina Kasardo, and Derick Henderson for preparing paint formulations and conducting the studies described in this article.

References

1. Wicks, Z.W., Wicks, D.A., and Rosthauser, J.W., “Two Package Waterborne Urethane Systems,” Prog. Org. Coat., 44, 161-183 (2002).
2. American Architectural Manufacturers Association (AAMA) 2016/2017 U.S. National Statistical Review and Forecast, March 2017.
3. American Architectural Manufacturers Association (AAMA) 615-13 Voluntary Specification, Performance Requirements and Test Procedures for Superior Performing Organic Coatings on Plastic Profiles.
4. Dieterich, D., “Aqueous Emulsions, Dispersions and Solutions of Polyurethanes; Synthesis and Properties,” Prog. Org. Coat., 9, 281-340 (1981).

Presented at the American Coatings Conference, April 9–11, 2018, in Indianapolis, IN.

CoatingsTech | Vol. 16, No. 2 | February 2019

The industrial wood coatings segment of the paint and coatings industry comprises a wide range of coatings applied at the factory to a broad array of wood and wood composite substrates. Overall segments of this market sector include joinery, kitchen cabinets, furniture, flooring, millwork, specialty wood products (musical instruments, for instance), and exterior building products. Some of these segments can be delineated into further subcategories. Furniture, for instance, includes both general/residential furniture, children’s furniture, office and hotel furniture, and outdoor furniture. Each of these furniture subsegments has its own requirements with respect to coating performance. In addition, kitchen cabinets and kitchen furniture typically have stricter requirements in terms of scratch, abrasion, and chemical resistance. Millwork includes architectural doors and decorative plywood, which are typically finished with high-performing coatings. Exterior building products include doors, windows, structural panels, and siding.

A growing number of wood species are used in these various applications. High-end products are constructed from maple, walnut, and a host of exotic woods. Lower cost species include African mahogany, pine, oak, and poplar. Composite wood products such as medium density fiberboard (MDF), high density fiberboard (HDF), medium density overlays (MDO), softwood plywood (SWPW), and particleboard are also part of the industrial wood segment and are coated with sealers, primers, and paints. In addition, wood-like substrates such as fiber cement and vinyl are often considered to be application targets for wood coatings.

Many Resin Technologies

With so many different substrates and end-use applications, it is not surprising that the number of resin chemistries and curing mechanisms is quite large. “The choice of resin and curing mechanism is dependent of the customer requirements for a given application,” says Michael Law, global business director for Waterborne Technologies at allnex. Some suppliers are very specific. Ikea, for instance, allows only the use of coatings that contain no solvents (water-based or 100% solids), he notes. In all cases, local environmental, health, and safety regulations dictate certain aspects of coating choices and, in particular, levels of volatile organic compounds (VOCs) and hazardous substances.

The most common resins used in industrial wood coatings include alkyds, acrylics, nitrocellulose, and polyurethanes (PUs), according to Nick Bartoszek, global marketing director for New Product Development with Sherwin-Williams’ Industrial Wood Division. Unsaturated polyesters, urea, and melamine systems are also used. Solventborne, waterborne, powder, and 100% solids options are available. One- and two-component systems are both common. PUs are typically cured with isocyanates and polyesters, and alkyds with acid catalysts. Two-component PUs have typically been used for high-build applications. UV-cured coatings (urethane or polyester acrylates) find widespread use for pre-finished flooring and flatstock applications such as doors, kitchen cabinets, and furniture due to their low VOCs, rapid cure, and high durability, according to Shane Carter, technical director with Axalta’s Industrial Wood Coatings business.

Waterborne acrylic and polyurethanes systems are predominately used in primers and sealers, but are growing in topcoat use as performance has greatly improved over the years.

Water-based coatings are largely used in building products, primarily flatboard and particularly products that are primed. “Waterborne acrylic and polyurethanes systems are predominately used in primers and sealers, but are growing in topcoat use as performance has greatly improved over the years,” James Monroe, market segment manager for Furniture and Flooring at BASF, observes. They accounted for 39% of the wet pounds and 28% of the dollar value of industrial wood coatings sold in 2016, according to Steven Nerlfi, a director with consulting firm Kusumgar, Nerlfi & Growney (KNG). Solvent-based coatings, including traditional and high solids formulations, are largely used for wood furniture and cabinetry. Radiation-cured coatings, meanwhile, accounted for 5% of the wet pounds and 16% of the dollar value for industrial wood coatings in 2016.

Solventborne coatings, in fact, account for 60% of industrial wood coating consumption worldwide, according to Susan M. Anderson, a director with The ChemQuest Group. Traditional nitrocellulose-based lacquers and conversion varnish systems remain dominant due to their lower cost, forgiveness in application, and excellent performance, according to Monroe. The importance of solventborne coatings can be attributed to the prominence of China in the industrial wood market, according to Nerlfi. On its own, the country accounted for approximately 53% of the wet pounds of coatings consumed in 2016, with nearly 85% of that volume being solvent-based.

There is a definite shift to waterborne systems, however. This shift has been occurring for some time in mature markets such as Europe and the United States. Over time, the majority of building products customers have moved to waterborne or radiation-cure technologies due to environmental regulations and a desire to be more environmentally friendly, according to Carl Gaynor, market segment manager with Axalta’s Industrial Wood Coatings business. “The cabinetry and furniture industries still use solventborne products, but regions such as southern California, the Mountain States, Texas, and the Northeast are increasingly focused on lowering VOCs, which we anticipate impacting business decisions in those regions,” he adds. The shift is also underway in China, where the central government and provincial regulations are focused on transitioning to water-based solutions to address environment, health, and safety issues facing the country, according to Law. Meanwhile, the fastest growing chemistries are UV-curable systems, both 100% solids and waterborne acrylates, according to Monroe. “UV systems provide the highest performance and the fastest cure profile, allowing manufacturers to shorten finishing times. These coating systems do cost more, but faster finishing cycles improve throughput and shorten lead times, driving down the total manufacturing costs,” he comments.

Growing Market for Industrial Wood Coatings

Not surprisingly, the diversity of the industrial wood coatings market also leads to variations in estimates of its size. The volume of the global industrial wood coatings market in 2016 is estimated by KNG to be 5.8 billion formulated wet pounds, which carried a value of nearly $8 billion. The consulting firm puts the global annual growth rate at 4%. Geographically, Asia Pacific accounted for 65% of the wet pounds and 55% of the dollars, with China accounting for 80% of both (see above). Europe, including EU countries along with Russia and Turkey, accounted for 20% and 27% of global consumption in pounds and dollars, respectively. North America had 8% and 11% shares of the global market in volume and value terms, respectively. South and Central America combined accounted for 6% of both global volume and value in 2016.

The ChemQuest Group, meanwhile, pegs the value of the global wood coatings market in 2018 to be higher—at $11 billion and growing slightly faster at 6% per year. Anderson notes that the compound annual growth rate for the Asia Pacific region is in the double digits, compared to a growth rate in North America and Europe of under 5% per year.

Economic Growth Is Main Driver

Demand for industrial wood coatings in general tracks with growth of the overall economy and, in particular, on housing starts and remodeling, according to Anderson. For that reason, growth has recovered since the recession, particularly in North America and Europe. Not surprisingly, it is strongest in emerging markets. Manufacturers supplying the commercial sector are doing well, with architectural doors and millwork for hospitality, health care, and service buildings a strong point, according to Monroe. He also notes that residential needs are stable and improving, but there still is some hesitation with consumers. Anderson does caution that the preferences of younger generations for ready-to-assemble furniture and refinished/refurbished furniture has tempered future growth forecasts in these regions. Demand for coatings for specialty wood products, such as sporting goods and musical instruments, meanwhile, is tied to consumer confidence in the economy, according to Bartoszek.

The Role of Regulations

Technology development in the industrial wood coatings sector is largely driven by the introduction of increasingly strict environmental regulations and growing expectations from consumers for more sustainable products. “Consumer awareness and education are driving regulatory changes that are intended to make products safer and more sustainable,” Monroe observes. He notes that new developments in materials that go into coatings face more scrutiny, which can have an impact on innovation and time to market with new products.

“In the United States, one of top drivers for product development throughout the industry is California Proposition 65 and other new and more rigorous standards for air and water quality that are emerging around the country,” according to Bernie Ackerman, PPG Building Product segment manager. “These mandates are dictating the use of raw materials that do not include benzophenone, toluene, n-methylpyrrolidone (NMP), and other substances on the Proposition 65 chemical list. The need for compliance with these mandates is driving up the cost for coatings in the industrial wood coatings market, which is pushing companies like ours to explore effective and affordable alternative materials that provide the same benefits these listed substances do,” he explains. In Europe, demand for resins and coatings derived from bio-based materials is rising in response to interest from some brand owners and end-users, according to Law.

In China, he notes, that less demanding furniture subsegments are transitioning to water-based coatings as manufacturers face the threat of closure. “In many instances, this transition has not been without problems, so the sector is going through a steep learning curve, which is also driving R&D efforts throughout the supply chain to achieve greater performance from waterborne solutions,” Law comments. In addition, it is possible in some cases that production will shift from China to less-regulated countries where solvent-based coatings can still be freely used without restriction. Such a trend would be short-term, however, because most countries have at least begun the process of strengthening legislation regarding environmental protection.

Regulatory trends towards limiting formaldehyde content and emissions in industrial wood applications are also driving formulation development in multiple regions.

Improvements have been made in waterborne coating technologies, and wood finishers are beginning to appreciate them, according to Bartoszek, “Production and finish quality have significantly improved in the past 10 years. Waterborne provides a better work environment in the finishing department, with minimal use of solvents leading to better air quality. One caveat is that finishing department housekeeping and environmental controls need to be tighter.�� If the finisher meets this need, then water will produce desired results,” he adds.

There are many solutions available, and the choice is often dependent on the level of investment that wood product manufacturers are willing to make to support new low-VOC technologies, agrees Law. “Of course, some applications have less demanding specifications that enable a quicker transition to low-VOC coatings, while for those with higher performance requirements, the transition is more difficult,” he adds.

In today’s digital world, Monroe adds that consumers expect immediate feedback, and manufacturers need to adapt to the times. “Manufacturing lead times are shortening and companies are becoming leaner, and the coatings used by wood finishers must support these changes,” Monroe asserts.

Looking Ahead

Increasing regulations and consumer expectations will continue to drive the further development of higher-performance, water-based solutions. “Regulation and policy changes will impact what and how coatings are used. In addition, consumers are more aware and want to know more about the things they buy and how they impact the environment. They, however, want products that satisfy their concerns and perform, which will drive innovation,” Monroe says.

He expects that advances in the equipment used to apply UV coatings and in UV coating technology will have the most impact on the types of industrial wood coatings that are used and for which applications. Allnex, meanwhile, expects to see development of multi-layer coating solutions based on a hybrid of low-VOC technologies driven by the level of investment that the furniture coaters are prepared to make to support these changes, according to Law. “It always comes down to a balance of cost and performance.” He notes as an example the replacement of solventborne systems with a combination of UV-curable and one-component acrylic coatings, with the choices being controlled by the level of investment made.

Resin systems will also likely be developed that can be crosslinked via two or three pathways, giving finishes a choice of crosslinking mechanism to match the needs of different applications and wood surfaces/finishers, according to Law. There will also be continued interest in increasing the speed of cure and rapid property development, particularly at low temperature with reduced energy consumption, he notes. “There is continued demand to go faster: more production, less coats, improved durability, lower VOC,” agrees Bartoszek. “Chemistry is allowing us to meet those challenges,” he says. For instance, the company recently introduced a second-generation near-zero-VOC universal primer for sealing MDF. It applies smooth, dries fast, and can be coated with virtually any topcoat, giving finishers numerous options to lower VOC while maintaining their topcoat of choice, according to Bartoszek.

Resin systems will also likely be developed that can be crosslinked via two or three pathways, giving finishes a choice of crosslinking mechanism to match the needs of different applications and wood surfaces/finishers.

Sherwin-Williams is driving towards higher-performance single-pack waterborne coatings and working to improve formaldehyde-free alternatives to acid cure technologies, according to Bartoszek.�� “Acid cure technologies have long been the performance benchmark in many wood finishing industries in North America due to their excellent durability, application flexibility, and cost effectiveness.�� We will be introducing new formaldehyde-free coating technology that we feel will raise the standard in performance and application and will satisfy performance expectations,” he says. The company also continues to improve its UV LED curable systems and expand into electron-beam (EB) curing.

Axalta has also reinforced its waterborne offerings with commercial waterborne UV systems, a variety of low-VOC solventborne systems, and its Zenith Waterborne line, which is designed to be a substitution for solventborne kitchen and furniture coatings, according to Gaynor. “Axalta will continue to develop and offer a wide array of custom coating options for this market and all of its OEM customers using its state-of-the-art design center in North Carolina to remain on the cutting edge of wood coatings trends,” he states.

In the flooring segment, there has been increased demand for wood finishes that offer excellent durability and protection while providing greater grain definition and low gloss, according to Gaynor. PPG is also seeing a huge push for stain- and scratch-resistant flooring, which has shifted the market to a better overall flooring performance. “Water-resistant hardwood flooring and scratch-, mar-, and stain-resistant coatings are always in need of improved performance. We’ve developed and formulated a number of additives into our coatings that deter scuffs from footwear, dogs, and other hazards that are improving the appearance and longevity of in-home flooring,” he comments.

Both PPG and Axalta have observed growth in demand for luxury vinyl tile (LVT) in the flooring market, with customers increasingly preferring this material over hardwood. “Vinyl has become a major player in America’s flooring market, growing approximately 20% per year over the last five years,” says Gaynor. Vinyl flooring is naturally more water resistant, but still requires high performing, durable finishes that complement the natural aesthetic, similar to organic wood substrates, he notes. The impact on coatings is minimal because the coatings formulations for both types of products are similar, but there is a need for resin and additive suppliers to focus on more flexible products for these types of floors, because they “give” more than wood floors do, according to Ackerman. Axalta has engineered a full line of coating systems to meet the specific needs of vinyl flooring manufacturers, according to Carter. Ackerman notes that the need for increased flexibility in the LVT market has had a positive technological impact on the formulation of wood coatings as well.

In addition to gloss, durability, and other physical performance aspects, color is an important characteristic of coatings for industrial wood products. “Consumer demand is dictating an increased range of colors and styles, especially the development of custom colors,” Ackerman observes. In addition, earth tone colors and coatings that achieve a natural, wood grain look have become increasingly popular with customers, according to Gaynor. The cabinet industry has been trending more towards opaque colors, specifically light grays and blue dry finishes, he adds. To meet the growing expectations for a wide color choice, Sherwin-Williams recently introduced its Color Express™ program, which allows customers to dispense custom colors from an automated machine to obtain accurate and repeatable color quickly, according to Bartoszek. They can access the system through select Sherwin-Williams facilities or have it placed within their production operations.

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

*This paper was presented at the American Coatings Conference, April 11-13, 2016, in Indianapolis, IN.

Government agencies have fostered the change from fossil fuel-based coatings to those with a reduced environmental footprint. Foremost among these are AIM VOC limits that necessitate the use of water in place of hydrocarbon solvents. While a current coatings formulator will certainly consider these aspects in formulating new coatings, they will not be allowed to compromise performance. Using a combination of creative chemistry and advanced process technologies, vegetable oils have been incorporated into self-crosslinking polymers supplied in an aqueous continuous phase that allows the formulator to design the requisite dry film performance into coatings for multiple markets. Oil-modified polyurethane dispersions produce films with the exceptional abrasion and chemical resistance needed for gymnasium floor coatings.

Alkyd latex polymers achieve a rich wet-look appearance on concrete/stone pavers and extended corrosion resistance over metal—all in shelf-stable formulations. Data comparing these technologies to their non-biobased counterparts are presented. Additionally, the status of these coatings through the U.S. Department of Agriculture’s BioPreferred® Program is considered.

U.S. DEPARTMENT OF AGRICULTURE BIOPREFERRED® PROGRAM

The United States Congress established the foundation for the BioPreferred Program (“Program”) in Title IX section 9002 of the Farm Security and Rural Investment Act of 2002, commonly known as the 2002 Farm Bill.^1,2 The legislative history of section 9002 suggested that Congress had three main objectives:�� improve the demand for biobased products, spur rural economic development, and enhance the�� energy security of the United States by reducing its dependency on fossil fuels.³ Moreover, Congress expressed that its intent for including section 9002 was “to stimulate the production of new biobased products and to energize emerging markets for those products.”⁴ Thus, Congress introduced the Program’s two major initiatives: (1) procurement of biobased products by Federal agencies and their contractors and (2) the voluntary labeling of biobased products, now referred to as mandatory Federal purchasing and voluntary labeling, respectively.¹ Under the mandatory Federal purchasing initiative, Federal purchasers and contractors are required to buy biobased products that meet the criteria for certain product categories such as Interior Paints and Coatings—Latex and Waterborne Alkyd; Interior Paints and Coatings—Oil-based and Solvent-borne Alkyd; Multipurpose Lubricants; Roof Coatings; and Disposable Tableware. The Program was reauthorized and expanded by the Food, Conservation, and Energy Act of 2008 (2008 Farm Bill) and the Agricultural Act of 2014 (2014 Farm Bill).^5,6

Using a combination of creative chemistry and advanced process technologies,
vegetable oils have been incorporated into self-crosslinking polymers
supplied in an aqueous continuous phase that allows the formulator
to design the requisite dry film performance into coatings for multiple markets.

The U.S. Department of Agriculture (USDA), the manager of the Program, oversees the Program’s Guidelines for Designating Biobased Products for Federal Procurement as directed by law. Currently, the Program has identified 97 product categories for mandatory Federal purchasing, each with its own minimum biobased content requirement, and has awarded certifications for over 2,500 biobased products to display the USDA Certified Biobased Product label through the voluntary labeling initiative. Biobased products are derived from plants and other renewable agricultural, marine, and forestry materials and typically provide an alternative to conventional petroleum-derived products.² For the purposes of the Program, biobased products exclude food, animal feed, and motor vehicle fuels. Products that meet the requirements for one or both initiatives are shown in the BioPreferred Catalog, available on the Program’s website (www.biopreferred.gov). Although users may not directly purchase products from the Catalog, sourcing information such as product details and the manufacturer or vendor’s website is provided. To participate, manufacturers and vendors may visit the Program’s website and select “How To Participate” in the left-hand navigation panel for instructions.

Mandatory Federal Purchasing

If a product’s biobased content meets or exceeds the minimum biobased content requirement for one or more of the 97 product categories that have been identified for mandatory Federal purchasing, it is considered a “qualified biobased product.”² Federal agencies and their contractors are required to purchase this product instead of the traditional, non-biobased alternative. To identify a product category for mandatory Federal purchasing, manufacturers and vendors provide the USDA with technical information for the products that comprise it. The USDA reviews this information to determine if the product category is appropriate to add to the Program’s Guidelines. The USDA expects to identify product categories for intermediate ingredients and feedstocks and complex assemblies in 2016.

Voluntary Labeling

While the mandatory Federal purchasing initiative focuses on Federal acquisition and support of biobased products, the voluntary labeling initiative aims to “promote the purchase of biobased products by commercial entities and the general public” through a customer-facing certification mark, the USDA Certified Biobased Product label.⁷ Manufacturers and vendors may apply for certification to display the label, which ensures consumers that the product or the product’s packaging has been third-party tested and verified for biobased content through ASTM D6866 Standard Test Methods for Determining the Biobased Content of Solid, Liquid, and Gaseous Samples Using Radiocarbon Analysis.² If, after testing through this method, a product meets or exceeds the minimum percent biobased content requirement for its particular product category, the product is considered a USDA Certified Biobased Product. If a product falls under a category that is not among one of the identified 97 product categories for mandatory Federal purchasing, the minimum percent biobased content requirement for that product is 25% by default. An example of the label is shown in Figure 1; the graphic is meant to represent the sun, soil and farm, and aquatic environments. In this example, the product is 57% biobased, its packaging is 32% biobased, and the “FP” indicates that it qualifies for mandatory Federal purchasing. All USDA Certified Biobased Products are subject to the USDA’s periodic auditing activities.

Achievements

After the Program was authorized by law, it received backing and attention from other branches of the U.S. Government. In particular, the Federal Acquisition Regulation, which directs all Federal purchasing, was updated in April 2012 to implement Farm Bill requirements for the procurement of biobased products.⁸ Additionally, on March 19, 2015, President Obama issued Executive Order (EO) 13693, “Planning for Federal Sustainability in the Next Decade.”⁹ The EO establishes an annual target for the number of contracts to be awarded with BioPreferred and biobased criteria and the dollar value of these products to be delivered and reported under those contracts in the following fiscal year. Federal agencies are to ensure contractors submit timely annual reports of their BioPreferred and biobased purchases as well.

In June 2015, the Program submitted to Congress an in-depth economic impact study on the biobased products industry in the United States, An Economic Impact Analysis of the U.S. Biobased Products Industry.¹⁰ As mandated by the 2014 Farm Bill, this report was the first federally sponsored economic report of its kind for the biobased products industry. The report’s key findings are that, in 2013, the U.S. biobased products industry contributed four million jobs, $369 billion value added to the U.S. economy, and, for every one biobased products job, 1.64 more jobs were created in the United States.

Prior to the release of the June 2015 study, the Program also released Why Biobased?, a report that fuses the recent literature exploring the “opportunities in the emerging bioeconomy”¹¹ in October 2014. Why Biobased? also examines how “government policies and industry business-to-business sustainability programs are driving the biobased economy.” The report states that “the biobased economy is, in fact, growing” and that it “offers great potential for increased job creation in numerous sectors across the U.S.”¹²

Summary

The USDA BioPreferred Program plays an important role in expanding market opportunities for biobased products such as biobased paints and coatings, and the intermediates ingredients that are used to make them. Through the implementation of congressionally mandated purchasing requirements that harness the purchasing power of the U.S. Government and its voluntary, consumer-facing USDA Certified Biobased Product label, the Program continues to reduce our nation’s reliance on petroleum, create new jobs, and increase the innovative use of renewable agricultural resources.

BIOBASED POLYMERS

In this article, the term “biobased” refers to products derived from biologic sources, mainly plants, and as such may be described as biorenewable. Today’s polymer scientist has a number of biobased raw materials to utilize as new coating resins are conceived and developed. A recent notable addition to this list is succinic acid, augmenting age-old glycerin in polyester synthesis. Seed-stock derived fatty acids are now being modified to serve as pigment dispersants and surfactants, taking a lead from the historic use of soya lecithin for these purposes. Oil-seed products are among the oldest film-formers known, having been in use in forms similar to those available today since around the year 1400.¹³ That some of these have survived for many millennia is testament to the durability of these film-formers. For the past century, alkyds have taken advantage of the ambient cure capability of those seed oils containing sufficient quantities of unsaturated fatty acids to provide an autoxidizing crosslinked film and remain perhaps the most used biobased product in the coatings industry.

Regulations limiting the VOC of organic coatings have been implemented by various government entities over the last two decades and continue their trend to ever-lower numbers. For example, witness the recent changes in VOC limits for the Ozone Transport Commission (OTC) as well as several Air Quality Management Districts in California and Utah.¹⁴ To enjoy the benefits of both biobased resources and self-crosslinking polymers, these regulations necessitated the development of water-dispersed polymers that satisfy the shelf-life requirements of the architectural and industrial maintenance markets. It is the performance of this class of polymers as compared to their fossil fuel-based (e.g., crude oil and natural gas, non-crosslinked counterparts) that is described in this article. The performance advantages are found across a multitude of coatings applications.

POLYURETHANE POLYMERS

Very few coatings are subject to the daily abuse of floor coatings. These coatings must withstand foot traffic, often with shoe soles embedded with sand, sliding chairs, endless spills of food and household chemicals, and the movement of the wood substrate due to changes in humidity. The toughness imparted to the film by inter-molecular hydrogen bonding between the urethane amine proton and carbonyl oxygen atom makes polyurethane chemistry ideally suited for this application.15

To obtain suitable performance in a water-dispersed form requires very specific chemical structures and processing of which a generic description is offered. First, a low molecular weight hydroxyl-terminated polymer, chosen from the class polyether, polyester, or polycarbonate, is mixed with dimethylol propionic acid (DMPA). A stoichiometric addition of an isocyanate (usually aliphatic) is then made to synthesize an isocyanate terminated “pre-polymer” that preserves the acid functionality of the DMPA. Processing is carried out in an aprotic, water-soluble solvent like methyl ethyl ketone (MEK) or n-methyl pyrrolidinone. (NMP). This isocyanate terminated pre-polymer is added to a reaction mixture of water, a di-amine, and a tertiary amine. The di-amine will chain-extend the pre-polymer, reacting preferentially with the isocyanate groups; this reaction is an order of magnitude faster than the isocyanate-water reaction. It is the competing reaction with water that favors the use of aliphatic over aromatic isocyanates. Simultaneously, the tertiary amine will de-protonate the carboxylic acid on the DMPA, thus generating negative charges along the length of the newly chain-extended polymer. These negative charges provide the requisite polarity for dispersion in water; this class of polymers is commonly called polyurethane dispersion, or PUD. To reduce the VOC of the formulated coating made with this PUD to below 275 g/L as mandated by the South Coast Air Quality Management District (SCAQMD), the process solvent must be removed from some products via vacuum, which will add cost to the final product.

Incorporation of oxidizable fatty acids into this water-dispersed polymer utilizes an intermediate product. An oil such as soya or linseed is reacted with a polyhydric alcohol such as pentaerythritol via trans-esterification. Careful control of stoichiometry and process conditions yields a reaction mixture that is largely di-esters with two hydroxyl groups. This is introduced into the urethane synthesis with the hydroxyl end-capped polymer, DMPA and di-isocyanate where it is reacted into the backbone via the urethane reaction. The unsaturation on the fatty acid chains is nonreactive in this process and thus is preserved for oxidative cure in the applied film. These polymers are referred to as waterborne oil-modified urethanes (WBOMU). The lower solution viscosity, T_g, and film formation characteristics of this class allow for manufacture in reduced levels of solvent that produce formulated varnishes well below 275 g/L— thus precluding the need for the added expense of vacuum stripping.

For both classes of polymer, PUD and WBOMU, the molecular weight is significantly higher than the traditional aromatic isocyanate-based solventborne oil-modified urethane (OMU). As a result, the viscosity is much higher; thus, these products are offered to the market at solids ranging from 30–40% by weight so as to provide a reasonable viscosity. The composition is considerably higher in isocyanate content than traditional solventborne OMU and, as noted, is largely based on aliphatic isocyanate. This results in a much higher cost. Consequently, many of the floor-coating products utilizing PUD technology are mixtures of PUD and (styrene) acrylic latex polymers. The (styrene) acrylic serves to reduce cost at the expense of performance. A number of these hybrid PUD/styrene-acrylic (PUD Acrylic) blended products are supplied as a two-component material; an aziridine is added on-site to enhance the performance, particularly the stain resistance, which was sacrificed for lower cost. To their detriment, aziridine chemistries often contain hazardous components. As demonstrated in Table 1, the self-crosslink of the WBOMU enjoys the ease and reduced toxicity of being one component and does not suffer performance deficiencies.

Three key performance attributes of floor coatings are mar resistance, stain resistance, and abrasion resistance. Mar resistance was assessed using several in-house methods and is reported subjectively. Measurement of stain resistance is a requirement set forth by the Maple Flooring Manufacturers Association (MFMA), and their protocol was used in this study.¹⁶ A total of nine reagents were used: water, 1% Spic and Span®, Wesson® Oil, 50% ethanol, 70% isopropanol, Parson’s Sudsy Ammonia®, yellow mustard, Windex®, and Formula 409®. Stain resistance for each reagent was scored on a 1 to 5 basis; with 5 equaling no effect and 1 equaling total film destruction. A maximum summed value of 45 is possible. Relative abrasion resistance is assessed with a Taber Abraser per ASTM D4060. The reader will understand that considerable variation is seen in data generated by this instrument; to minimize testing variation, the results presented here were run side-by-side. Note that while the data presented are for in-house products, we recorded similar performance by category for commercial floor finishes.

The self-crosslink of the WBOMU provides excellent mar and stain resistance in a single component finish. Its wear properties are superior to standard solventborne OMU, which will typically lose 100+ mg of film in this test. While pure PUD has excellent abrasion resistance, the mar is not acceptable unless the film is cured with an aziridine (data not shown). The deleterious effects of adding an acrylic to the coating are obvious in the mar, stain, and wear resistance properties. A two-component composition compensates for stain and mar resistance but does little for wear.

ALKYD POLYMERS

Alkyds based on drying oils have been popular as architectural and industrial coatings vehicles for well over a century. Henry Reichhold was the first merchant supplier of this technology starting in 1927. In today’s regulatory environment, the use of solvent-cut alkyds in architectural finishes is limited. Creative process engineering and sophisticated surfactant combinations have allowed these chemistries to be supplied as an emulsion at very low to zero VOC in a composition that resists hydrolysis for years.¹⁷ Products based on classic polyester (alkyd) chemistry ranging from 34–68% biobased content are commercially available.

A key aspect of alkyds based on drying oils relative to their fossil fuel-based acrylic counterparts is the fact that the alkyd is a liquid polymer which, as an applied film, will react with molecular oxygen and increase in glass transition temperature and molecular weight. The very low initial Tg precludes the need for coalescing solvents. Polymers based on acrylate monomers are essentially solid at room temperature. This can limit flow into porous substrates with a consequent lack of aesthetics in some applications. The higher T_g of acrylic polymers may also necessitate the use of VOC solvents for proper film formation.

This differential in substrate penetration is perhaps best demonstrated in a “wet-look” coating applied to porous concrete paver stones. This application dictates that the substrate appears dark without building a high gloss finish and is accomplished in part by the polymer penetrating deeply into the very small voids in the substrate. Quantifying this optical change is effectively done with a colorimeter by comparing the “L” values (CIE L.a.b color space) of the coated and uncoated substrate. As the substrate becomes darker from coating application, the “L” value will decrease. Thus, a greatly enhanced wet-look appearance will be reported as a large negative Δ L.

This coating application was long-held by solvent acrylic lacquers at well over 500 g/L VOC. With the advent of VOC limits, this market transitioned to acrylic latex with the subsequent loss of the desired appearance. Introduction of alkyd latex polymers provided a vehicle that would both meet VOC limits and provide the desired deep, rich, wet-look.¹⁸ Table 2 provides data for an alkyd latex, a commercial solvent acrylic, and commercial acrylic latex applied to several popular cementitious and stone surfaces. Note that no chemical analysis of these commercial products was undertaken; the descriptions provided here are taken from the product labels.

The fluid nature of the alkyd polymer allows the penetration necessary to alter the direction of reflected incident radiation and thus provide an enhanced “wet-look.” Post-application oxidation builds molecular weight and crosslink density that function to provide the improved chemical resistance relative to the high VOC acrylic lacquer. The exceptionally high molecular weight of the acrylic latex precludes penetration and development of the desired “wet-look” but does provide chemical resistance.

Air quality regulators monitor the usage of AIM coatings by category and use the associated tons of emitted solvent(s) to guide their changes in VOC limits. Such is the case with the new asphalt driveway sealer category added by SCAQMD¹⁹ and OTC¹⁴ member states. The 50 g/L limit imposed by these agencies limits this coating category to emulsion polymers. A comparison of emulsions based on alkyd, acrylic, and asphaltic binders was performed in the Reichhold laboratories.²⁰ In this work, we used commercial paint based on acrylic and asphaltic binders (label analysis), and formulated an in-house alkyd latex polymer in a similar fashion based on label and SDS analysis of the commercial paints. An alkyd latex with 50% biobased content was utilized. This polymer has an acrylic modification to impart some lacquer dry character. Weathering is a function of both pigmentation and binder selection; one formulation using the alkyd latex was tailored for improved durability by utilizing a pigment combination well outside those seen in commercial products.

Commercial asphalt sealers are supplied in two basic formulations that we have designated as unfilled and filled. In this vernacular, filled coatings contain a significant volume fraction of large-particle silica sand in conjunction with a traditional paint extender. Typically, this silica is 270 mesh (50 microns) in top particle size and may provide some anti-slip properties to the applied coating. Other extender pigments noted in commercial products include Nepheline Syenite, Kaolin, calcium carbonate, and, while not a traditional coating extender, ground tire rubber. For comparison purposes, we have tested alkyd emulsions in both filled and unfilled pigmentations.

Filled and unfilled formulations along with the pertinent physical properties are presented in Table 3. Note that λ = PVC/CPVC, where PVC is the pigment volume concentration and CPVC is the critical pigment volume concentration calculated from oil absorption. Filled formulations maintain the same relative carbon black loading as the unfilled formulation. The weight ratio of silica sand to Nepheline Syenite was maintained as the PVC varied. The unfilled formulation is, as noted previously, designed for improved weathering and for stir-in production; the treated pyrogenic silica does not require aggressive high speed dispersion. Weights have been rounded to the nearest decimal place.

Weathering data is presented here for alkyd latex formulations using both pyrogenic silica and Nepheline Syenite due to its durability under both acid and alkaline conditions. Calcium carbonate weathered well in a QUV exposure at slightly alkaline pH but frosted in exposure to the slightly acid conditions required in cyclic prohesion. Also included is a formulation utilizing treated pyrogenic silica, chosen for durability, as the extender.

These coatings were exposed to accelerated weathering in both a QUV and a Xenon Arc Weather-Ometer. Trends were similar for both instruments; only the QUV data are presented (see Table 4). Coating degradation results in visible whitening of the film, which is evidenced by a large change in the L (light-dark) value. Accordingly, presenting color change as Δ L is adequately descriptive. Exposure hours are rounded to the nearest 100, Δ L is rounded to the nearest tenth. For the reported QUV exposure, the instrument settings used were:

• UV lamps: UVA 340
• UV cycle: 8 h at 60°C
• Humidity cycle: 4 h at 50°C

The pyrogenic silica used in the unfilled coatings is among the most hydrophobic extenders available to the coatings chemist. With an oil absorption of 290, a very low PVC is required to reach a sufficient λ to provide the desired matte finish. This combination develops a film surface that is quite resistant to moisture ingress and subsequent hydrolysis. The UV absorption of the carbon black pigment further limits film degradation and this alkyd latex survives well over 4000 h of exposure before a significant color change is noted.

The filled formulations use larger particle size and much more hydrophilic pigmentation. The resultant ingress of water during exposure results in a more rapid surface degradation and the consequential whitening of the film. At 500 h of exposure, the color change is visible and becomes quite a bit more so at 1200 h. Note that in the early periods of exposure, the higher filler loading shows less color change. This is reversed beyond 4000 h and continued through 9000 h of exposure. Exposure to accelerated weathering of the commercial products resulted in color change similar to the filled alkyd latex formulation. Note that our accelerated weathering evaluations of these and other commercial products (all data not presented) did not necessarily show a correlation between warranty period and performance.

Table 5 shows the chemical resistance for a number of test films. This testing was done as covered spot tests using a 24-h duration unless otherwise noted. The industry has not established a standard set of chemicals to be used in evaluating this class of coatings, and the full gamut of products tested is more extensive than reported here. The data is limited to the more pertinent and common reagents. Ratings are on a subjective 0–5 scale, with 5 being no noticeable effect and 0 being film destruction.

The data offers several interesting points. First, all commercial products are not created equally. Acrylic 1 is clearly inferior in chemical resistance, failing badly upon short-term exposure to brake fluid, in this case in a much less severe uncovered spot test. Second, as seen in previous data, the crosslinking of oil-based polymers imparts improved chemical resistance; the alkyd latex in both pigmentations is superior to non-crosslinked commercial acrylic products. Third, that the alkyd is liquid allows for excellent pigment binding and performance over a wide range of pigment volume concentrations. Finally, note that the filled alkyd latex formulation is under 5 g/L VOC and the unfilled formulation, minus the propylene glycol that is not needed for film formation, would be 15 g/L VOC. Fatty acids grafted to the polymer backbone reduce T_g thus often preclude the need for coalescing solvents.

The performance benefit of the self-crosslinking of biobased alkyd latex polymers formulated as blacktop sealers relative to their non-renewable counterparts is further demonstrated via ASTM D 5402 Resistance to Color Tracking and ASTM D 1308 Resistance to Re-emulsification. Resistance to color tracking was tested at one-day and seven-day dry times with 50 double rubs using water-wet cheesecloth. Re-emulsification is done with the same dry times and uses a one-hour water spot followed by 100 double rubs with a water-wet cheesecloth. A commercial asphalt emulsion offered as a seven-year product and a commercial product labeled as an acrylic were included in these tests. As part of our testing of pigmentation the extender pigment used in the filled alkyd latex is calcium carbonate rather than Nepheline Syenite. The data are presented in Table 6. Pictures of the Re-emulsification test are included.

We see that with overnight dry, the alkyd latex consistently resists the transfer of coating to a wet cloth. Lacquer dry polymers lack a crosslink mechanism and do not appear to develop the requisite toughness to pass this testing regimen after a week’s cure. That the more heavily filled alkyd latex was slightly inferior to the unfilled formulation may suggest that a difference in formulation relative to the non-film-forming products is also a factor in their reduced performance.

The efficient pigment binding of alkyd polymers noted when testing asphalt sealer formulations was also demonstrated in the development of a 34% biobased content alkyd latex (Alkyd Latex 2) designed for metal primer applications. The rapid method of Asbeck²¹ uses a salt water spot test to identify coatings above critical PVC. Utilizing this method in evaluating this alkyd latex primer vehicle proved insightful. In Table 7, data are presented showing PVC and lambda (λ) for this alkyd latex and Styrene Acrylic Latex 1, which was developed for use in metal coatings. Pigmentation is a fixed loading of red iron oxide and anticorrosive pigment. Calcium carbonate was used to adjust λ (PVC). Permeability was determined by the wet cup method and is reported simply as mg of water transmitted per hour at steady-state conditions.

Alkyd Latex Polymer 2 was successfully formulated into a red oxide metal primer that passed 500 h of salt fog and humidity testing at 1.5 mils DFT. The formulations are below 15 g/L VOC as no coalescing solvent was needed in this 34% biobased polymer. Efficient pigment binding allowed performance at λ values as high as 0.82.¹⁷ Stability of alkyd latex formulations is of paramount importance and is particularly difficult to achieve in formulations containing reactive anticorrosive pigments. To demonstrate the remarkable stability of this red oxide primer, heat-aged studies of the latex and the formulated paint made from that heat-aged latex were undertaken. In addition to measuring the tested products for viscosity and pH change, aged materials were coated on cold-rolled steel and subjected to salt fog (ASTM B117) and humidity (ASTM D2247) testing. Data presented in Table 8 show minimal changes in viscosity or performance through this vigorous test regimen. Pictures of the panels after 336 h of salt fog are included.

As a final demonstration, the performance of a low-VOC alkyd latex polymer with measured biobased content of 40% was evaluated via the National Transportation Product Evaluation Program (NTPEP). In annual tests alternating between roadways in the southern and northern area of the United States, coatings and the associated retro-reflective glass beads are applied transverse to roadbeds with typically 15,000 average daily traffic counts. The applied stripe is monitored for durability (wear resistance, amount of substrate showing) on a 0–10 scale and for retro-reflectivity. Retro-reflectivity is measured in milli-candellas (mcd) and is a function of the retention of the glass beads used to top-dress the applied coating: these beads are intended to (retro-) reflect a car’s headlight back to the driver. Much of the road-marking market has been captured by the so-called “quick dry” acrylic latex polymers meeting Federal Specification TT-P-1952F, Type II. Alkyd latex with 40% biobased content using essentially identical pigmentation to this acrylic was applied to NTPEP test decks in Florida in 2012 and Pennsylvania in 2014. Alkyd latex had a durability of 10 (perfect), after 24 months on the test deck in Florida. Test decks on northern roadways are typically more severe due to the salt, sand, and snow plows used to maintain safe travel. Typically, the test results on concrete, a nonflexible surface, are more severe than testing on the thermoplastic asphalt. Durability and retro-reflectivity data for several white products as recorded for the skip-line section on the Pennsylvania concrete deck from 2014 are presented in Table 9. Polymers based on biobased materials require fewer VOC grams and are comparable in performance to their fossil fuel-based counterparts.

CONCLUSIONS

Incorporating significant biobased content into coating binders is best accomplished via vegetable oil-modified polyester or polyurethane polymers. This allows a biobased content of 34–68% for current commercially available products designed for various end uses. Providing these polymers as a latex or dispersion in water allows for formulations at VOC content well below the lowest established AIM limits worldwide. That these unsaturated oils crosslink to form thermoset films provides a level of performance that easily competes with thermoplastic fossil fuel-based binders. In addition to the low VOC and desirable performance, these polymers can be used in coatings that are favored in government purchases through the USDA BioPreferred® Program. They enjoy the marketing advantage of displaying the USDA Certified Biobased Product label.

ACKNOWLEDGMENTS

The writer would like to gratefully acknowledge the technical contributions of Alicia Albrecht and Jennifer Hall. Additional appreciation is expressed to Ms. Albrecht for her invaluable assistance in the preparation of this manuscript.
Reichhold also expresses its appreciation to Jennifer Chu for preparing the section regarding the USDA BioPreferred® Program.

References

1. Farm Security and Rural Investment Act of 2002. Public Law 107-171, May 13, 2002.
2. USDA. BioPreferred®. https://www.biopreferred.gov/BioPreferred.
3. USDA. Guidelines for Designating Biobased Products for Federal Procurement. Fed. Regist., 68, 70730-70746, 2003.
4. USDA. Guidelines for Designating Biobased Products for Federal Procurement. Fed. Regist., 70, 1792-1812, 2005.
5. Food, Conservation, and Energy Act of 2008. Public Law 110-234, January 3, 2008.
6. Agricultural Act of 2014. Public Law 113-79, February 2, 2014
7. USDA. Voluntary Labeling Program for Biobased Products. Fed. Regist., 74, 38297-38317, 2009.
8. Department of Defense, General Services Administration, National Aeronautics and Space Administration. Federal Acquisition Regulation; Biobased Procurements. Fed. Regist., 77, 23365-23368, 2012.
9. The President. Executive Order 13696 Planning for Sustainability in the Next Decade. Federal Register website. https://www.federalregister.gov/articles/2015/03/25/2015-07016/planning-for-federal-sustainability-in-the-next-decade.
10. Golden, J.S., Handfield, R.B., Daystar, J., and McConnell, T.E., An Economic Impact Analysis of the U.S. Biobased Products Industry: A Report to Congress of the United States of America. A Joint Publication of the Duke Center for Sustainability & Commerce and the Supply Chain Resource Cooperative at North Carolina State University. USDA BioPreferred® Program website www.biopreferred.gov/BPResources/files/EconomicReport_6_12_2015.pdf.
11. Buckhalt, Ron B., USDA Report Outlines Opportunities in the Emerging Bioeconomy. USDA website. www.usda.gov/wps/portal/usda/usdahome?contentid=2014/10/0224.xml.
12. Golden, J.S. and R.B. Handfield, Why Biobased? USDA BioPreferred® Program website. www.biopreferred.gov/BPResources/files/WhyBiobased.pdf.
13. Church, A.H., The Chemistry of Paints and Coatings, Seeley & Co., London, 1915.
14. (a) Ozone Transport Commission Draft Model Rule Architectural and Industrial Maintenance Coatings OPP V4, June 4, 2010; (b) South Coast Air Quality Management District Board Meeting June 3, 2011, Amending Rule 1113; (c) Utah Administrative Code 63G-3-102(5) Rule 307-631, October-1-2015.
15. Wicks, Z., Jones, F., Pappas, S.P., and Wicks, Z.W., Organic Coatings Science and Technology, John Wiley and Sons, 2007.
16. Maple Flooring Manufacturers Association Specification for Gymnasium Finishes and Sealers for Maple, Beech and Birch Floors, 2015.
17. Danneman, J., “Optimizing Anticorrosion,” Euro. Coat. J., l, #2, December P82 (2014).
18. Hall, J., “Novel Waterborne Technology for Wet Look Sealers,” Proc. Eastern Coatings Show, Atlantic City, NJ, May 2, 2013.
19. South Coast Air Quality Management District amended Rule 1113 September 6, 2013.
20. Albrecht, A., “Unique Biobased Alkyd Latex Polymer Designed for Blacktop Sealer Applications,” Proc. Western Coatings Symp., October 25-28, 2015.
21. Asbeck, W.K., “Critical Pigment Volume Concentration Measurements, a Very Fast Method,” JCT CoatingsTech, 2 (12) (March 2005).

• This paper was presented at the American Coatings Conference, April 11-13, 2016, in Indianapolis, IN.

As a result of requirements to lower VOC, polymers with low minimum film formation temperature (MFFT) are increasingly being utilized in waterborne architectural coatings. However, coating compositions based on these emulsion polymers suffer from poor durability, contributing to soft and tacky coating films. Performance compromises, including lower scrub resistance, high soiling tendency, poor wash, and burnish resistance are often the reported deficiencies of low-VOC paints.

The VOC reduction has raised significant technical challenges but also driven the development of durable, low-VOC waterborne coatings. This success has been enabled by innovative polymer and paint compositions. The progress towards greener products and higher performance standards is clearly demonstrated by the new paint products on the market, a strong testament to the coatings industry’s continued commitment to better product quality and lower environmental impact. A recent survey on commercial semi-gloss (SG) and flat paints confirms that almost all interior paints are now formulated at VOC <50 g/L, with some products even claiming to be zero VOC and odorless.

A benchmarking study examining the interior wall paints
recently introduced into the market was conducted to understand
the general trend and to baseline the stain resistance
and washability performance of these new products.

For premium interior paints, the most sought-after attributes are stain resistance or easy clean, scrub durability, maximum hiding, and desired application characteristics. The latter two properties can be optimized by appropriate selections of TiO2, polymeric opacifier, and rheology modifiers. This article will focus more on balancing stain removal or washability and scrub durability. Washability or stain removal is a coating’s ability to withstand scrubbing to remove the staining materials with no changes to the coating’s appearance or its protective functions. One mechanism of stain removal is mechanical erosion of paint films. Hence, a latex paint with good stain removal often exhibits poor scrub resistance. Adding to the challenge are the wide variations in chemical and physical characteristics of the stains encountered. A coating formulation is often optimized for hydrophobic stain washability at the expense of hydrophilic stain removal and vice versa. Stain resistance or easy clean is, therefore, a desirable property that is lacking in most latex paints and is more challenging to attain in flat interior paints as the types and amounts of pigments increase.

A benchmarking study examining the interior wall paints recently introduced into the market was conducted to understand the general trend and to baseline the stain resistance and washability performance of these new products. New acrylic polymers were also evaluated against the commercial interior SG and flat paints.

EXPERIMENTAL

Commercial Paints

The commercial interior paints selected in this study are summarized in Table 1, including three SG and seven flat paints. The three premium SG paints were chosen because they represented the highest price points in the category, with SG1 and SG2 being the top product lines from the same manufacturer. Flat paints covering a wide price range were purchased from the “big-box” and independent company stores. The seven flat paints were produced by five different paint companies. The information on volume solids (VS) and VOC was taken from each product’s technical data sheet.

Paint Formulations

The SG and flat formulations used to evaluate the acrylic polymers are given in Tables 2 and 3, respectively. The VS and pigment volume concentration (PVC) were 39.6% and 23.6%, respectively, for the SG formulation and 40.5% and 47.4%, respectively, for the flat formulation. Neither the SG nor flat paints contained any VOC, owing to the use of Optifilm™ 400 co-solvent.

Gloss and Contrast Ratio of Dry Film

The test paints were prepared on the Leneta 3B opacity charts using a 3-mil bird drawdown bar. The films were allowed to dry overnight in a controlled temperature and controlled humidity (CT/CH at 77ºF and 50% relative humidity) chamber. Gloss readings were taken after one-day dry using a BYK-Gardner Micro-tri-gloss meter. Three measurements were collected, and the average gloss values were reported. A BYK-Gardner Colorguide colorimeter was used to measure Y%-reflectance over the white and black parts of the opacity chart. Opacity or contrast ratio was calculated as the ratio of Y% reflectance over the black section to Y% reflectance over the white section.

Block Resistance

The test paints were prepared on the Leneta 3B opacity charts using a 3-mil bird drawdown bar. The films were dried in the CT/CH chamber for one day. In the room temperature (RT) block test, two square strips of 2.54 cm x 2.54 cm were placed together with paint film against paint film under a 454-g weight. After 24 h, the strips were separated and evaluated. For the elevated temperature (ET) block test, the paint strips after one-day drying at CT/CH were placed in a 120ºF oven for 30 min. The weight load of 1000 g was transferred to the paint films via a 2.54-cm diameter rubber stopper. The films were allowed to cool for 30 min before the block ratings were given. One-day room (1d-RT) and elevated temperature (1d-ET) block were rated on the ASTM D-4946 scale from 0 (poorest) to 10 (best). The test was run in triplicate and the average value was reported.

Wet Adhesion

Wet adhesion tests measure the adhesion of a coating under wet conditions to an aged alkyd substrate. Among the numerous wet adhesion tests, ASTM D-6900 was employed in this study. The gloss alkyd panels were prepared by casting 7-mil of Duvoe Duvguard 4308 Medium Green (4308-6650) Gloss Enamel paint onto a Leneta scrub chart and allowing it to cure for three to six weeks at CT/CH.

The test and control paints were drawn down in parallel on the same alkyd panel with a 7-mil Dow bar. After the panels were dried for 4 h at CT/CH, the films were cross hatched through to the gloss alkyd substrate layer. The test panels were then soaked in water for 30 min. Size and density of blisters were reported. Before scrubbing, 20 mL of 50% Lava soap solution and 5 mL of water were added to the paint panels on the scrub machine. The number of the cross hatched squares not removed after 1000 scrub cycles was reported as percent film remaining.

Scrub Resistance

Relative scrub resistance was evaluated on the Garner Straight Line Washability and Wear Abrasion Machine. The coatings were applied at a wet film thickness of 7 mil over Leneta black plastic charts and allowed to dry for seven days in the CT/CH chamber. The nylon bristle brushes were conditioned by running 400 cycles before the test began. A standardized abrasive scrub media (#SC-2 from the Leneta Company) was used. The test included the addition of 7 mL of scrub media and 5 mL of water at the beginning and after every 400 cycles. The experimental latex was drawn down and scrubbed side-by-side with an internal scrub control. The test was done in triplicate and the number of cycles to failure was recorded.

Tannin Blocking

The tannin stain solution was prepared by neutralizing the 10% aqueous tannic acid to pH of 7.0 using ammonium hydroxide and then equilibrated overnight. A coat of a commercial control paint was applied 6 in. wide on the sealed portion of the Leneta WB card using a 6-mil drawdown bar and allowed to dry overnight in the CT/CH chamber. Approximately 10 mL of the tannin solution was deposited from a dropper onto the paper towel covering the dried control paint. A foam brush after dipping into tannin sample (excess tannin was wiped off) was used to evenly distribute the tannin over the paper towel. The WB chart with the control paint in contact with tannin solution was placed back in the CT/CH chamber to dry overnight. The test paint along with the control paint was prepared side-by-side using a 10-mil square bar, dried overnight at CT/CH, and then coated with the control paint on top. After overnight drying of the control topcoat, a BYK-Gardner Colorguide colorimeter was used to measure ΔE as an indication of color change due to tannin bleeding through.

Stain Resistance and Washability

A quantitative, multi-stain test method was employed to assess the removal of both hydrophilic and hydrophobic stains. The hydrophilic soilants are common household stains: mustard, ketchup, hot coffee, grape juice, and blue fountain ink. The hydrophobic stains include ball point pen, #2 pencil, blue crayon, red grease pencil, and two brands of red lipsticks (lipsticks #1 and #2). The paint films were prepared on white Leneta Scrub Test Panels (P122-10N, B#4311) using a 7-mil Dow bar and allowed to dry for a minimum of three days in the CT/CH chamber. The test paints formulated with experimental latexes were drawn down side-by-side with a control paint.

Mustard and ketchup were applied using a 20-mil square drawdown bar. For liquid stains such as coffee, grape juice, and fountain ink, a strip of single-ply paper towel was used to hold the liquid stains in place. Hydrophobic, solid stains were directly marked on the white panels. All stains were allowed to sit on the paint film for two hours. The films were washed for 100 cycles using ASTM standard sponges and Leneta standardized non-abrasive scrub medium as the cleaning solution. The panels contacted by hydrophilic stains were gently rinsed under running tap water to remove excess stains before sponge wash.

Degree of staining was determined using the ΔE values of unstained versus stained and then washed portions of the paint film measured by a BYK-Gardner colorimeter. Resistance against hydrophilic stains was visually assessed by comparing the water-rinsed section to the sponge-washed section.

RESULTS AND DISCUSSION

Commercial Interior SG and Flat Paints

Table 1 shows that all commercial products selected in this study were formulated at VOC less than 50 g/L. Figure 1 displays the tiered advertisements for the commercial SG and flat paints. Common themes can be extracted from the frequency analysis of product features highlighted by paint manufacturers:

• Stain resistance and easy clean (3/3 of SG paints and 7/7 of flat paints)
• Exceptional hide and self-priming (3/3 of SG paints and 6/7 of flat paints)
• Scrub/scuff/mar resistance (2/3 of SG paints and 5/7 of flat paints)
• Antimicrobial-mildew resistant (2/3 of SG paints and 4/7 of flat paints)

These key performance characteristics reflect consumers’ expectations and, therefore, define the quality standards for premium, low-VOC interior paints.

Properties of SG Commercial Paints

Table 4 summarizes the lab evaluation of the three SG paints. All three paints formed relatively glossy films yielding high gloss readings. For SG3, its gloss readings of 33.8 at 20° and 71.4 at 60° approached those of high gloss products.¹ Substrate adhesion, although not promoted on the product feature list, is a basic but critical requirement. The three SG paints, as expected, all passed the four-hour wet adhesion test with 100% film remaining. SG1 and SG2, formulated for the highest quality by the same paint manufacturer, provided durable finishes. The scrub resistance of these two paint samples exceeded 2500 cycles. Comparatively, SG3 was weak in scrub durability, and its paint film failed before reaching 1000 cycles. Among the three paints, only SG2 exhibited acceptable one-day block resistance. Block resistance was not one of the featured properties shown in Figure 1, which was confirmed by the test results.

Premium paints are expected to deliver a smooth, uniform appearance with a minimum number of coats. This is why hiding ranked so high in Figure 1. Contrast ratio of the drawdown film is a simple measure of intrinsic hiding. The three commercial SG paints spanned the normal range for contrast ratio, from 97.1% to 99.4%. Tannin blocking was also tested to assess these products for the claimed attribute of “self-priming” or “primer and paint in one.” Varying degrees of tannin staining were observed, as indicated by the DE values, from ~2.4 for SG2 and SG3 to 5.5 for SG1. A lower DE value is desired, indicating good tannin blocking performance of the test paint. Among the three products, SG2 provided the lowest contrast ratio but the best tannin blocking performance, suggesting that there is no direct correlation between the contrast ratio and tannin blocking.

Properties of Flat Commercial Paints

A larger number of flat paints are included in this study since many paint properties deteriorate as the PVC increases and polymer binder usage decreases. New binder technologies and formulation approaches are required to deliver the promise of durable and washable flat finishes. Table 5 compiles the performance properties of the seven commercial flat paints. Block resistance is not listed because it is less important given the fact that block resistance is generally enhanced by the extender pigments used in flat paint formulations.

The low 60°and 85° gloss values confirmed that all seven paints produce matte finishes. All except FL4 exhibited good wet adhesion to the aged gloss alkyd substrate. The flat paints display smaller variations in scrub resistance than the SG paints. However, they can be divided into two performance groups: four products scrubbing greater than 1400 cycles while the other three failing between 700~1000 scrub cycles. Similar to the SG paints, the flat paints yielded contrast ratios in a typical range, from 97.5% to 99.7%. The DE values measured after tannin blocking tests ranged from 0.88 to 6.15. FL3 and FL4 were not the best in class for scrub durability. They did, however, deliver the maximum intrinsic hiding (highest contrast ratios) and tannin blocking (least color change or DE) as self-priming paints. The applied hiding by using other application tools such as brushes and rollers were not evaluated in this work.

Stain Resistance and Washability of Commercial Interior Paints

As shown in Figure 1, stain resistance and/or easy clean is clearly the number one claimed property of the interior paints across the sheen or PVC range. Five hydrophilic stains and six hydrophobic stains were used to test the washability of commercial interior paints. Residual color from each stain was measured by DE. Small DE values are desirable, denoting slight or no staining of the paint surface. The total DE value for the hydrophilic stains and the hydrophobic stains are plotted in Figures 2 and 3, respectively.

Stain resistance and washability are affected by the composition and surface characteristics of the paint film in addition to the chemical and physical properties of the stain. Wetting, adhesion, and penetration of stains on the coating surface are influenced by surface tension and viscosity of the stain, as well as the surface energy and porosity of the coating film. The data in Figure 2 shows that mustard tends to cause the heaviest discoloration on the paint surface. Depending on the paint formulation, grape juice and blue ink sometimes left high amounts of staining compounds behind (SG3, FL2, and FL4). Visual inspection of the stained and water-rinsed sections revealed that good washability or low DE value generally corresponded well to the paints possessing high resistance to the water-based household stains. Most stains were removed during the water-rinse step, and the subsequent sponge wash did not further reduce the color of the stained area significantly.

Porosity of paint films generally increases as the PVC increases from SG to flat formulations. Higher porosity allows greater stain penetration and consequently “easy-clean” is more difficult to attain with flat paints. However, this expected decline was not observed for the penetrating liquid stains shown in Figure 2. The group of flat paints exhibited similar overall washability of hydrophilic stains. In some cases, the flat paints even rendered improved removal of hydrophilic stains compared to the SG paints. This is presumably because the paint manufacturers optimized the surface characteristics to minimize the negative effect of increasing film porosity on staining and stain removal.

However, the formulation strategy that enabled good hydrophilic stain removal did not yield similar results for the hydrophobic stains on flat paint surfaces. Figure 3 shows that most hydrophobic stains were completely removable from the SG finishes. The flat paints as a product category recorded higher total DE values in general. Red grease pencil once marked on the painted surfaces seemed to be tenacious and difficult to remove even from SG paint films. This appears to be the case for blue crayon on the flat paint films as well. Additionally, ballpoint pen and #2 pencil were not the biggest contributors to the total DE value and their removability was almost unaffected by different flat paint formulations that most likely were based on different binder technologies. The two red lipsticks were the biggest contributors and differentiators of washability performance. For these reasons, the five hydrophilic stains and the two lipsticks were considered in the evaluation of experimental acrylic polymers.

High Performance Acrylic Polymers

Three acrylic polymers, denoted as API, APII, and APIII, were evaluated in SG and flat paints using the formulations shown in Tables 2 and 3. The following designation was used to describe the experimental paint samples, for example, API-SG indicating the SG paint sample based on API and API-FL indicating API in the flat formulation.

Figure 4 compares the overall washability of the experimental SG paints with the three commercial SG paints. API offered slightly cleaner removal of five hydrophilic stains than the best performing commercial paint SG1. However, API displayed extremely poor washability for lipstick #2. The DE reading of 30.1 caused by the staining of lipstick #2 alone accounted for more than half of the total DE value. The compositions of API were modified in the development of APII to specifically enhance the removal of lipstick #2. The goal was successfully achieved with APII: a dramatic reduction of residual color (DE value from to 6.6) from lipstick #2 resulted in an acrylic polymer that delivered comparable washability to the premium commercial paints (Figure 4). The most remarkable improvement in overall stain washability was realized with APIII. The seven stains left minimal residual colors on the APIII-SG paint film. The total DE value combining seven stains was 15, which was less than the single DE value corresponding to the mustard discoloration on the commercial paints.

Figure 5 presents the comparison between the experimental and commercial flat paints. The stain removal property of the three experimental acrylic polymers did not differ as much as seen in the SG formulation or as the wide spread demonstrated by the commercial flat paints. In the flat formulation, all three acrylic polymers offered outstanding overall washability performance, more competitive than all the commercial flat paints. Without any special additive for the “easy-clean” paint property,² APIII provided a step-change in washability performance and excellent balance of hydrophilic and hydrophobic stain removal. Moreover, the data in Figures 4 and 5 demonstrate that APIII can afford the best stain resistance and washability for a wide PVC range from SG to flat formulations.

Other performance properties of the three acrylic polymers are summarized in Table 6. All three polymers demonstrated good four-hour wet adhesion. The experimental paints provided comparable intrinsic hiding or contrast ratios to the commercial paints. AP2-SG exhibited exceptional block resistance, a desired property for SG paints. AP1 and AP2 delivered excellent scrub durability in both SG and flat formulations. All three acrylic polymers maintained over 1000 cycles of scrub durability even in the flat formulation, suggesting that good washability was not the result of surface erosion due to poor scrub resistance. It should be noted that the two paint formulations are screening tools for the experimental polymers and it is therefore possible to optimize a given performance property with changes of paint formulations.

CONCLUSIONS

The coatings industry has made substantial progress in reducing VOC and enhancing performance of waterborne coatings. This article examines the performance of commercial semi-gloss and flat paints recently introduced into the market. For interior applications, the most valued product attributes are stain resistance or easy clean and hiding followed by durability, antimicrobial, and mildew resistance. The benchmarking results have validated the successful development of higher performance, greener products. Three acrylic polymers are highlighted in this article. They offer improved stain washability while providing comparable scrub resistance and hiding to the state-of-the-art commercial products. These new acrylic polymers can thus enable coatings formulators to develop durable and washable, high performance flat to non-flat interior paints.

REFERENCES

1.���� ��Wu, W., “Structured Nano-Acrylic Polymers, Low-VOC, Excellent Block Resistance, Hardness and Film Formation,” Paint & Coat. Ind., 20-27 (November 2013).

2.���� ��Meng, J., Guo, C., Zhou, W., and Liao, R., “Novel Application of Fluorosurfactants in�������� Easy-Clean Architectural Paints,” Paint & Coat. Ind., 30-37 (July 2013).