Two-component polyurethane systems (2K PU) are the most widely used topcoats for high-quality protective applications. These coatings combine outstanding chemical and weathering resistance with excellent mechanical properties, but environmental, market and cost pressures have led to the development of alternatives to these conventional topcoats, including isocyanate-free binders. Besides conventional acrylic monomers, polymer chemists have at their disposal various versatile building blocks, such as neoacid derivatives. Neoacid esters are widely used to upgrade solvent and waterborne resins for both decorative and protective coatings. Among other benefits, these very hydrophobic monomers reduce surface tension and improve flow, durability, chemical and UV resistance.

This article presents a new family of polymers, combining neoacid esters and alkoxysilane monomers. Variation of process parameters, monomer composition and level of alkoxysilane yielded a series of polymers with a range of properties. Polymers with solids content ranging from 70-to-100 percent were produced using this technology and formulated into one-component (1K) and 2K protective coatings — both clear and pigmented. These coatings were then compared to 2K polyurethane and acrylic silane systems for typical performance parameters.

2K PU Coating Systems

In these systems, it is usual to cross-link polyester and/or acrylic polyol resins with aliphatic isocyanates based on hexamethylene or isophorone backbones. These coatings combine outstanding chemical and weathering resistance with excellent mechanical properties.

However, 2K PUs also have shortcomings. Handling of the isocyanate cross-linkers presents potential exposure of workers to these hazardous chemicals, especially during spray application. In addition, isocyanates must be mixed in correct proportion with the polyols shortly before application — typically 30 minutes to two hours. After this period, the product’s viscosity increases too much to be sprayed and any unused mix must be disposed of safely. This disposal results in material waste and thus, in overall increased usage cost.

Alkoxysilanes in Coatings

High-durability coatings increasingly take advantage of resins based on silane chemistry. The combination of silane and organic monomers offers the synthetic chemist an infinity of possibilities to optimize resin performance. Such copolymers are now used for protective applications including marine, railways, chemical plants and bridge coatings. Organo-silane polymers are mostly appreciated for their very high durability and have replaced polyurethanes in some cases. In particular, acrylic-silane systems have exhibited excellent weathering resistance¹.

Acrylic-silane resins can be manufactured via radical copolymerization of silane-functional acrylate and methacrylate monomers. Most commonly, γ-methacryloxypropyl-trimethoxysilane (MPTMS) is reacted with methyl methacrylate, butyl acrylate and styrene or other vinyl monomers to form linear copolymers with pendant alkoxysilane groups. In theory, these resins can be formulated as 1K systems because they contain moisture-curable silane groups. Thus, ambient moisture would be a catalyst for the cross-linking reaction (Fig. 1). Park et al. observed excellent UV and weathering resistance in moisture-cured coatings based on alkoxysilane-acrylic resins with a high level of MPTMS, up to 30-weight percent².

Fig. 1: Moisture curing of alkoxysilane-based coatings. All figures courtesy of the authors.

Despite the high potential of siloxane-based resins, so far, 1K moisture-cure coatings based on this chemistry have only reached limited market penetration. The high price of these resins, mostly due to the high cost of siloxane monomers, is probably partly responsible for this. Aside from price, these systems present an additional number of drawbacks. For ease of application (in particular spraying), especially when formulated into pigmented systems, acrylic-silane systems must use polymers with relatively low molecular weights. Polymers with such low molecular weights often require long curing times to develop mechanical properties, more so when applied at low temperatures^3,4.

Neocarboxylic Acid Monomers for Coatings

In 1955, Koch, Gilfert and Huisken of the Max Planck Institute in Germany, described a three-step chemical reaction between mono-olefins, carbon monoxide and water leading to synthetic monocarboxylic acids⁵. Acids prepared by this process are known as Koch acids or neocarboxylic acids. There are various grades of neocarboxylic acids available commercially, most notably neopentanoic and neodecanoic acid. Because of their bulky, branched structures, these acids lack reactivity to achieve high conversion in conventional esterification reactions. Therefore, neocarboxylic acids are converted into derivatives, of which the vinyl or glycidyl esters find broad use in polymer synthesis.

Nowadays, monomers based on neoacids find most of their applications in the world of coatings, where they are well-known for upgrading the performance of vinyl, acrylic, polyester and other resins. The most common monomers used in these applications, based on C10 acids, are glycidyl neodecanoate and vinyl neodecanoate, with three to six methyl groups per molecule.

The highly branched chemical structure, the strong aliphatic character and the molecular bulkiness confer a number of very attractive properties to these vinyl and glycidyl monomers, reflected in the properties of the coatings in which they are used. The homopolymer of vinyl neodecanoate has a critical surface tension of 24 dyn/cm² and the presence of this monomer brings high contact angles to derived coatings (Fig. 2). Moreover, the tertiary structure of the acid chain ensures excellent chemical stability⁶ and the absence of aromatic structures explains excellent UV resistance.

Fig. 2: Contact angle of acrylic copolymers with 30-weight percent vinyl neodecanoate (VV) or 2-ethylhexyl acrylate (2-EHA), respectively.

The Experimental Part

In the present work, vinyl neoesters and alkoxysilane monomers were copolymerized to produce a series of resins. These silane-vinyl resins were then formulated into 1K moisture-cured coatings. Variables evaluated in the synthesis of resins include the following.

• Vinyl neoester monomer type and concentration.
• Fox-predicted glass transition temperature.
• Nature and concentration of alkoxysilane.
• Nature and concentration of other monomers (if present).
• Process parameters (for example, reaction temperature and time, initiator type and concentration).

A number of variables were considered in the moisture-cured coatings, including the following.

• The nature and concentration of solvents and catalysts.
• The use of moisture scavengers and additives to stabilize the coatings.

While the study focused on hardness development of the coatings, other performance properties were evaluated, such as accelerated weathering resistance, water contact angle, gloss, abrasion resistance, volatile emissions and viscosity. For the sake of brevity, this text discusses only the effect of a few selected variables (percentage of alkoxysilane monomer and resin glass transition temperature [Tg]) on the most important performance parameters (pot life, hardness development, and solvent and weathering resistance). The work was aimed at developing a versatile binder system that could bridge several application areas currently covered by other types of coatings, offering a more attractive cost-performance balance, as indicated in Figure 3.

Fig. 3: Proposed space for the new binder technology.

Resin Preparation

Resins discussed here combine methoxysilane monomers, vinyl neodecanoate and/or vinyl neononanoate. Monomer choice resulted from a desire to combine the unique properties of neoacid derivatives, such as hydrophobicity and durability, with the moisture-curing mechanism of methoxysilanes. The glass transition temperature of the polymers was manipulated by varying the ratio between vinyl neodecanoate and vinyl neononanoate (homopolymer Tgs of -3 and +70 C, respectively). The molecular weight of the polymers was varied by modification of the process conditions and the use of low levels of some additional monomers. All polymers were prepared in n-Butyl acetate.

Coating Preparation

The various resins were mixed with catalysts and diluted with butyl acetate to the application viscosity (100 mPas) before application at 150 µm wet with a bar coater. Films were then dried at 23 C and 50-percent relative humidity (RH). Typical solids content of the clearcoats ranged from 65-to-70-weight percent. A commercial 60-weight-percent solid 2K PU system was included for reference purposes. White coatings were also formulated and tested against commercial references (2K PU and acrylic-silane).

Pot Life of the Coatings

A quick way to assess pot life of ready-to-apply coatings is to determine the time at which the viscosity has doubled from its original value. After that time, the coating may no longer perform as initially designed and should be tested prior to field application. All clearcoats prepared according to the proprietary route had pot lives of several months, despite being fully catalyzed and ready for application. In some cases, skin formation was observed on the top of the product in cans that had been open several times, indicating reaction with moisture from the air. In all cases, the bulk viscosity of the coatings remained constant. This important observation indicates that vinyl silane resins are suitable for truly 1K coatings. For comparison, a lab-made clearcoat based on acrylic-silane resins and used as reference had a pot life of less than one hour. The reference polyurethane had a pot life of two hours.

Clearcoat Properties

Table 1 shows the properties of a typical clearcoat based on a vinyl-neoester/alkoxysilane polymer, compared to those of a commercial 2K PU clearcoat. It is easy to see that the vinyl-silane coating exhibits overall properties very close to those of the commercial 2K PU, with the exception of the following.

Table 1: Comparison Between a Vinyl-Silane and a 2K PU Clearcoat.

• Being a 1K system.
• Having a much longer pot life.
• Showing a much faster Koenig hardness development (Fig. 4) and reaching a higher level of final hardness.

Fig. 4: Hardness development of vinyl-silane and 2K PU clearcoat.

Figure 5 shows the results of accelerated weathering (QUV-B) of the two clearcoats, indicating that the vinyl-silane system can achieve similar performance to the 2K PU.

Fig. 5: Accelerated weathering of vinyl-silane and 2K PU clearcoat.

White Pigmented Coating Properties

Table 2 shows the properties of a simple white topcoat based on a vinyl-neoester/alkoxysilane polymer compared to those of a commercial 2K PU clearcoat and a commercial acrylic-silane. Also, in a white pigmented system, the vinyl-silane coating exhibits overall properties very close to those of the commercial references.

Table 2: Comparison Between a Vinyl-Silane, a 2K PU and an Acrylic-Silane White Topcoat.

Not a Single Resin, Yet a Family of Possibilities

The graphs and tables shown thus far display single-point performance data, i.e., they refer to a specific vinyl-silane binder system. In order to present these results, this study performed a scan of a broad range of variables, which allowed the authors to build a toolbox, linking resin composition and coating performance.

The Effect of the Level of Silane Monomer

The level of silane monomer was varied from zero to 37.5 percent in a series of resins with similar composition and Tg (60 C). Figure 6 shows the Koenig hardness development of clearcoats based on these resins, compared to the commercial 2K polyurethane benchmark. In addition to the room temperature curing (23 C, 50 percent RH), some films were also force-cured for four days at 40 C and 90 percent RH. After one day, all silane-based systems were significantly harder than the polyurethane benchmark. Increasing the silane level up to about 12.5 percent resulted in higher hardness due to cross-linking as expected.

Fig. 6: Koenig hardness development of clearcoats with increasing level of silane monomers compared to a commercial 2K PU clearcoat.

The Effect of Resin Glass Transition Temperature

The Tg of the polymers was decreased in a second series of resins from 60 C to -3 C by using vinyl neodecanoate rather than vinyl neononanoate, with a fixed concentration of 25 percent vinyl trimethoxy silane monomer. Figure 7 shows that, as expected, the rate of hardness development and the final hardness of the derived coatings was significantly reduced with decreasing Tg.

Fig. 7: Hardness evolution of clearcoats from resins with different estimated Tg  (25 weight percent silane monomer).

Solvent Resistance

Figure 8 shows the solvent resistance of a series of clearcoats as a function of the level of silane monomer after seven days of drying at room temperature. Additionally, resins with 25-weight-percent silane and Tg from +60 C to -3 C were included in this evaluation. As expected, solvent resistance increases with the level of crosslinking monomers.

Fig. 8: The effect of the level of silane and resin Tg on solvent resistance of clearcoats.

Weathering Performance

Cold-rolled steel panels coated with clearcoats were placed in a QUV chamber for 2,000 hours. The accelerated weathering tests consisted of cycles of four hours of QUV-B at 60 C followed by four hours of condensation at 50 C. Durability of the coatings was assessed by gloss retention after 500, 1,000 and 2,000 hours of exposure. Figure 9 shows the gloss retention of these coatings. Gloss retention increased with the concentration of silane up to a level of 12 to 25 percent. At this level, gloss retention after 2,000 hours is similar to that of the reference polyurethane system.

Fig. 9: The effect of silane level on gloss retention.

Conclusions

Resins combining vinyl neoesters and alkoxyysilane monomers can be formulated into 1K coatings with pot lives of at least several months. Unlike with 2K systems, end users would not need to mix two components shortly before use. Also, vinyl silane systems are free of toxic isocyanates. High-solids coatings based on vinyl silane binders potentially dry faster, can be formulated at higher solids than 2K PU systems and can offer similar weathering performance. With an adequate choice of alkoxysilane monomer, vinyl silane systems exhibit lower costs than alternative chemistries, while offering overall improved performance to the coatings formulator and end-user.

About the Authors

Denis Heymans holds an engineering degree from the Institut Meurice Chimie and a master’s degree in polymer science from the Université Catholique de Louvain in Belgium. He has 27 years of experience in the development of acrylic, vinyl and polyester binders for decorative and protective coatings. Heymans is now senior technology leader at Hexion EMEAI in charge of research and technical service for VeoVa, Cardura and Versatic products.

Catherine Romanowska holds a bachelor’s degree in chemistry and biotechnology from Institute Paul Lambin in Brussels. She has five years of experience in the development of acrylic and vinyl binders for protective coatings and decorative paints.

Marcelo Herszenhaut received his Bachelor of Science degree in chemical engineering from the Military Engineering Institute in Rio de Janeiro. He’s held technical, sales and management positions in the chemical industry. Herszenhaut has broad experience in lubricants, basic petrochemicals, polymers, high-performance synthetic fibers and coatings. He is currently the marketing manager for Hexion’s Versatic business unit Americas and is also responsible for the Versatic Acids platform.

References

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2. Hong-Soo Park et al., Journal of Applied Polymer Science, 2001, 81, 1614-1623.
3. R. Iezzi, J. Martin, J. Tagert, P. Selbodnick, J. Wegand and E. Lemieux, NRL review, featured research, 2013, 1, 88-98.
4. S. Nixon, European Patent EP 1 292 650 B1.
5. J. Falbe, New Synthesis with Carbon Monoxide, Springer-Verlag, 1980, ISBN 3-540-09674-4.
6. D. Basset, Journal of Coatings Technology, 2001, 73, 43-55.