New Waterborne Resin for Direct-to-Metal Coatings Raises Performance Standards

There is good news for those who want to protect and preserve steel structures from environmental stress and also protect the environment and industrial workers. AVANSE™ MV-100, a revolutionary, high-performance, environmentally advanced waterborne acrylic latex polymer has been designed specifically for industrial maintenance and direct-to-metal (DTM) coating applications.

“We have been developing products to protect surfaces for a long time and have been pioneers in developing waterborne resins to protect against corrosion,” says Timothy Wood, Global Technology Director for Industrial and Construction products. “Now, Rohm and Haas developed AVANSE MV-100, a high-performance binder technology applicable in a variety of light- to medium-duty industrial maintenance segments, including direct-to-metal applications.” AVANSE MV-100 can be used either as a primer, midcoat or topcoat and allows paint manufacturers to use a single resin for multiple product categories.

Environmentally Advanced

“The new product represents not only a higher performing resin that should allow waterborne coatings to be used in more demanding applications where only solventborne paints were previously applied, but it’s also a more environmentally advanced product,” explains Wood.

By environmentally advanced, Wood means that coatings based on AVANSE MV-100, unlike solventborne coatings, emit very low levels of VOCs. Lowered VOCs benefit not only the environment, but are also less risky to the health of the workers applying the coating. In addition to having a better environmental profile, AVANSE MV-100 also offers an excellent alternative to traditional solventborne materials, such as alkyds, polyurethanes and epoxies, for high-performance coatings on steel and concrete.

Figure 1

Breaking Waterborne Perceptions

“We needed the binder to be high performing and at the same time help our customers comply with strict regulatory requirements,” confirms Shruti Singhal, market manager for industrial finishes. “At the same time, we also had to deal with the perception that waterborne coatings were inappropriate for metal surfaces; that they were light-duty products not suited for demanding industrial uses.”

Waterborne polymers made in the 1980s and 1990s did perform well for light- to medium-duty uses, such as on highway overpasses, or on non-metal surfaces not exposed to especially harsh elements or industrial abuses. But, to be successful for more demanding DTM uses, a new waterborne resin had to be developed – one with the guts of solventborne, one with superior barrier and weathering capabilities, and one that still had the one-component ease of use, and the safe and simple clean-up features of waterborne paint. The long-standing perception that a waterborne coating could not stand up to the harsher elements, especially water and the corrosive effects of salt, was a hurdle. “It seemed counter-intuitive to use a waterborne coating in DTM applications because metal’s interaction with water causes corrosion,” explains Wood. “But we have been long in the business of protecting metal surfaces and knew how to go about it.”

Figure 2

The secret for making waterborne coatings with the ability to withstand tough environments was to be found in the nature of the resin – the major component in the coating. While pigments give the coating its color and hiding, and can contribute to the performance, it is the resin that binds together the pigments, and that provides the coating with many of its important characteristics such as adhesion, durability and barrier properties.

Acrylic latex polymers, in which a high-molecular-weight resin is synthesized in the form of nanometer-scale particles (a nanometer is one-billionth of a meter) and dispersed in water, are an excellent choice for the paint’s binder because they allow the delivery of a high-molecular-weight resin in a low-viscosity medium. Molecular weight turns out to be one of the key differences between solventborne and waterborne products.

“Solventborne resins must start out at lower molecular weight,” says chemist Leo Procopio. “Otherwise, they would either not dissolve in the solvent, or their resulting viscosity would be too high for application as a paint.” For this reason, many solventborne resins, such as alkyds, must undergo a crosslinking process where their molecular weight increases after film formation via a chemical reaction. For several classes of solventborne coatings, such as epoxies and urethanes, the crosslinking involves mixing the resin with a hardener immediately prior to use. These systems are considered two-component, and once mixed the coating has a finite pot life before it must be discarded.

“Solventborne coatings are made by simply depositing a resin dissolved in solvent onto a surface and allowing an evaporation process to make the film,” says Procopio. “Film formation for waterborne products is more complicated because the resin in waterborne coatings is present as very small, spherical latex particles. When water evaporates from the film during the film formation process, the spherical latex particles form a closely packed layer.” The individual particles eventually meld together as the polymer chains in the particles diffuse across particle boundaries and become entangled, forming a continuous film. Unlike most solventborne coatings, however, the polymer chains in the acrylic latex particle are already a very high molecular weight and do not need to undergo a crosslinking reaction to build in properties such as durability. Therefore, they are one-component and easy to use – mixing with a hardener prior to use is not required, and they have no pot life issues to worry about.

Table 1

DTM Coating Properties

According to Procopio, better DTM coating properties with AVANSE MV-100 are the result of a unique interaction between latex polymer particles and pigment particles. The formation of composite particles through a tightly controlled process of absorbing latex particles onto the pigment surface leads to an improved distribution of pigment – such as titanium dioxide (TiO₂), the most common pigment in coatings – throughout the dried paint film. The more uniform pigment distribution provides a paint film with better barrier properties for corrosion resistance and higher gloss and durability.

To illustrate this, Figure 1 shows steel panels coated with 18 PVC DTM coatings based on either Avanse MV-100 or a commercial waterborne gloss acrylic polymer. The coating based on Avanse technology demonstrates considerably better protection against corrosion. A more detailed performance comparison is presented in Table 1.

The interaction between the latex polymer particle and the pigment particle is illustrated in Figure 2. The process begins when the fully dispersed polymer and pigment are mixed during the coating manufacturing process. The latex particles associate with the pigment particles and form a layer around each pigment particle. The adsorbed latex keeps the pigment particles apart and prevents agglomeration in the wet state. When the coating is applied and water begins evaporating, the binder maintains the separation of the pigment particles in the drying film. When the coating is dry, the pigment is optimally dispersed.

“Titanium dioxide plays two opposing roles in durability when a coating is exposed to the elements,” says Procopio. “TiO₂ acts as a catalyst in the presence of water, oxygen and light to generate free radicals that can break down the polymer base. But it also protects the polymer because it absorbs damaging ultraviolet light and converts it into harmless energy (heat). This prevents UV light from affecting the polymer’s backbone.” The improved distribution of TiO₂ more effectively shields the underlying polymer from the harmful UV light.

AVANSE Advantages

“One of the biggest pluses with AVANSE MV-100 comes with its enhanced performance properties,” says Singhal. “Better solvent resistance, corrosion resistance and weathering with outdoor exposure means the coating will last a long time. Its durability and lower usage of TiO₂is a strong value proposition for offering formulated cost savings to our customers.” The more efficient distribution of TiO₂in the film means less of the pigment is needed to obtain good hiding, and the low-VOC capability cuts down on the need for other film-forming agents, all of which could lead to lower manufacturing costs.

According to Singhal, AVANSE MV-100 stands up to solvents, chemicals, and adverse high temperature and humidity weather conditions. Earlier waterborne acrylics were known for their durability and resistance to weathering. As an _advanced waterborne polymer, AVANSE MV-100 has exceeded past performance. When tested, however, the new waterborne resin also answered the ongoing challenge of lowering VOCs.

Solventborne coatings release high levels of VOCs, and their release as chemical gasses can damage the environment and have a detrimental effect on human health. Concern about the short- and long-term adverse effects of VOCs has been rising for decades. A main concern is their role in the generation of ground level ozone and “smog” and its effect on the environment and human health, including health problems such as respiratory illnesses due to worsening air quality. As a result, federal and state agencies have been revising downward their regulations on acceptable levels of VOC emissions.

“Rohm and Haas is committed to developing products that are ahead of the regulatory curve and products that anticipate stricter regulations,” affirms Wood. “VOC emissions from coatings based on AVANSE MV-100 are well within the strictest safety standards, yet it is durable and protects metal surfaces.”