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Reducing VOCs without Sacrificing Coating Performance

SUNDAY, JULY 15, 2018

By Mohsen Soleimani, Ph.D., and Vince Goldman, BASF Corporation


Introduction

Decreasing the VOC emissions of coatings has been a progressive effort for scientists in the industry. Compared to solutions, waterborne dispersions generally afford a lower viscosity at equal solids content; therefore, replacing solventborne systems with waterborne dispersions is an effective strategy for reducing VOC emissions. 

Dispersions, however, often contain processing aids, such as initiator fragments and surfactants, that can decrease coating performance. To facilitate the replacement of solventborne coating systems, the industry has sought to improve the performance of waterborne dispersions. Significant challenges remain, especially in industrial coatings, which demand high performance in corrosion and chemical resistance, UV stability and gloss retention.

Performance in industrial coatings is strongly associated with the presence of a thermoset network formed via a crosslinking mechanism. Reactions with low-activation energy, such as isocyanate-hydroxyl or epoxy-amine, are used to produce thermoset coatings at room temperature and to avoid lengthy and energy-intensive bake schedules.

Two-Component Polyurethane Coatings

Solventborne, two-component polyurethane (2KPU) systems are used in several industrial applications — from automotive refinishes to corrosion protection. Coatings are prepared by combining a solventborne polyol with a polyisocyanate (PIC) to form a thermoset coating after application. Ideally, all the hydroxyl groups would react with isocyanates after application, but the reaction starts in the pot prior to application, which leads to higher molecular weight species and an increase in solution viscosity. This allows for identification of a viscosity threshold, after which the paint should not be applied (Figure 1A). Solventborne 2KPU coatings are relatively simple to use in formulation and application, and they have become a market standard for making thermoset coatings with robust physiochemical performance and weatherability.


Figure 1. An overview of a solventborne two-pack coating with polyisocyanate (A) is shown next to its waterborne counterpart (B). A common challenge with the waterborne dispersion is lack of a discernible end-of-pot-life signal.

Waterborne Two-Component Polyurethane Coatings

For a polymer dispersion, the solids and viscosity correlate in a much different way, compared to a polymer solution. In a polyol dispersion, the polymer is confined in nanoparticles. Unlike polyol solutions, molecular weight does not impact the viscosity of dispersions; therefore, significantly higher molecular weight polyols can be used. Such dispersions can be combined with PICs to produce thermoset coatings with superior performance. 

Traditional PICs are too hydrophobic and are not suitable for waterborne applications. Hydrophilically modified PICs are used, instead, modified with emulsifier groups to impart water dispersibility. This approach invariably decreases the average functionality of the PIC. While the preferred reaction is that of the isocyanate with the polymer hydroxyl groups, other strong side reactions can consume isocyanates groups, such as reaction with water. 

Although the reaction of isocyanate with hydroxyls does occur in the pot, similar to solventborne systems, there is no clear impact on dispersion viscosity (Figure 1B). As a result, it is not possible to identify when application should be stopped (end of usable pot life) to avoid applying paint that cannot provide the ultimate performance.

New Approach to Polyol Dispersions for 2K Polyurethane Coatings

We have followed several approaches to develop waterborne resins that can be formulated into high performance coatings with properties similar to that of the solventborne rivals. In waterborne 2KPU systems, many challenges described above can be addressed by appropriate design of waterborne resins. To conduct emulsion polymerization, amphiphilic molecules, such as salts and surfactants, are often used to initiate polymerization and bring colloidal stability to the nanoparticles formed during the reaction. These ingredients act as processing aids and enable synthesis of relatively high-solids dispersions but are often harmful to coating performance attributes such as rain fastness, humidity and corrosion resistance. Furthermore, in a typical emulsion polymerization, formation of some water-phase polymer is unavoidable. These water-soluble molecules can interact with polymer particles and other formulation ingredients, detract from the coating performance and complicate its rheology and film formation.

Figure 2. New synthetic capability allows for a clean-water phase and, therefore, control over viscosity via flocculation (A) that results in a distinct pot-life marker via viscosity increase (B).

To address these shortcomings, the BASF team developed a new technology that provides control of the composition and amount of water-phase polymer in a way that is not attainable by normal emulsion polymerization. Moreover, dispersions are synthesized without addition of surfactant and ionic molecules; therefore, dispersions with a very clean-water phase can be made. Additionally, using this technology makes it possible to create electrostatically charged hairy layers and tune them to balance steric and electrostatic contributions to the stability of the dispersion. This is one of the key aspects that allows us to introduce hydroxyl-functional dispersions with a discernible end-of-pot-life marker similar to solventborne counterparts. Figure 2A shows the flocculation of particles that results in an increase in viscosity, triggered by a change in the pH of a dispersion. This change in pH is a common feature in waterborne 2KPUR coatings due to side reactions of PICs as described previously.

New Waterborne Acrylic Technology for 2K PUR Coatings

As described above, dispersions made using this new technology are electro-sterically stabilized. Although creating a charged hairy layer during a typical emulsion polymer is possible, it is often accompanied by the creation of water-phase polymer. The water-phase polymers interfere with colloidal stability and may contribute to aggregation due to depletion flocculation. They can also complicate formulation and detract from intended final properties of the coating. 

The new technology completely decouples creation of a hairy layer from formation of water-phase polymers and, thus, allows formation of dispersions with very clean-water phase.

The designed hairy layer results in particles for which the stability potential is a function of pH in such a way that they can form higher-order fractals upon a drop in pH.In colloid chemistry, flocculation is a process in which colloid particles come together to form fractal-shaped space-filling structures. Such three-dimensional structures have more excluded volume compared to the sum of the precursor particles, and therefore, they can strongly contribute to the viscosity of dispersions. In essence, this scenario resembles solventborne systems where the PIC reaction with a polyol results in higher-molecular-weight polymer molecules and creates a viscosity raise.

Here, PIC reaction with water, an indispensable part of a dispersion, results in a pH drop that affects colloidal stability and triggers flocculation once a specific pH is reached. Figure 2B depicts changes in pH and viscosity of a dispersion as a function of time after the addition of PIC. As one can observe, pH drop results in a reproducible viscosity increase (a triplicate is shown). Therefore, although these dispersions are ultimately stable at basic pHs, the engineered stabilization mechanism enables tuning the onset of viscosity increase in 2K applications with PIC as pH drops to the acidic range.

Polyol Dispersion with End-of-Pot-Life Marker

Polymer composition, as well as colloidal stability of the dispersion, were systematically varied to optimize paint rheology and performance of the coating. This work resulted in a lead candidate named Joncryl OH 8314, which is a dispersion with milky appearance, solids of 44- to 46-wt percent, viscosity of 200-1000 cps, OH value of 70 mg KOH/g, and 2-hour pot-life.

Table 1. Performance of white DTM formulation

Joncryl OH 8314 can be formulated for myriad applications, such as interior/exterior industrial, institutional maintenance, automotive interior, and furniture and flooring. As seen in Table 1, the white direct-to-metal (DTM) formulation results in films with high gloss (> 85 at 60 degrees Centigrade), high hardness (65 pendulum swings, F-H pencil hardness), good flexibility and good weathering (1000 hours in QUV-A resulted in ΔE<2 and gloss retention >90 percent).

As the goal of waterborne dispersion is to match that of solventborne, a commercial solventborne 2K DTM was used as a benchmark for comparison. Excellent chemical resistance was obtained against harsh chemicals such as gasoline, brake fluid and 10-percent NaOH, matching that of the solventborne system.

Chemical Resistance
Spot Tests (1 hour) per ASTM D1308


Figure 3. Chemical Resistance Testing

Due to the clean-water phase afforded by our new synthetic capability, Joncryl OH 8314 performs well in single-coat DTM applications, providing good adhesion and corrosion resistance to metallic substrates such as cold-rolled steel (CRS), hot-dip galvanized (HDG), and electro-galvanized (EZG), and aluminum (Figure 4). The coating also exhibits superior humidity and water-soak resistance.

Joncryl OH 8314 vs. Commercial Solventborne
DFT = 2.5 mils, 432 hours ASTM B-117

Figure 4. Salt Spray Results

While hardness of Joncryl OH 8314 is matched by the commercial solventborne, it outperforms in taber abrasion and both forward- and reverse-impact resistance.

Conclusion

Traditional waterborne 2KPU systems suffer from lack of discernible end-of-pot-life marker, as well as reduced performance, when compared to solventborne counterparts. BASF advancement in producing waterborne resins enables commercial-scale synthesis of dispersions with engineered colloidal stability and with clean-water phase (free of salts and surfactants) which is hard to achieve with traditional technologies.

This technology was used to produce polyol dispersion suitable for waterborne two-pack applications with polyisocyanate. Such dispersions exhibit a discernible end-of-pot-life signal, via viscosity increase, similar to solventborne systems. Moreover, engineering the polymer allows achieving superior resistance attributes as low isocyanate demand. The absence of hydrophilic moieties in the dispersion reflects in superior coating performance in attributes such as humidity resistance and corrosion resistance as a DTM binder. The hydrophobic nature of the binder is beneficial in a variety of applications, such as interior/exterior industrial, institutional maintenance, automotive interior, and furniture and flooring.

About the product:

*Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, PaintSquare or its editors.

ABOUT THE AUTHOR
Mohsen Soleimani, Ph.D., and Vince Goldman, BASF Corporation

Mohsen Soleimani, Ph.D., Advanced Materials & Systems Research, BASF Corporation; Vince Goldman, Senior Formulation Scientist, Dispersions & Resins, BASF Corporation

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