By Cynthia Challener, CoatingsTech Contributing Writer
Silicones have unique physical properties and impart a range of properties to paints and coatings. When used as additives at very low levels, they can dramatically improve processing conditions and the final appearance of paints. Silicone crosslinkers are also useful for improving the performance of alkyd and acrylic paints. Protective coatings with silicone-based binders have enhanced properties over traditional epoxy resins. For example, inorganic silicone coatings are used in high-temperature applications because they are more heat stable than organic resins. Choosing the right silicone chemistry for a given application and achieving compatibility with other coating ingredients are both essential for maximizing the benefits afforded by silicone additives, crosslinkers, and resins. Advances in silicone chemistry are helping to overcome the initial hesitance of formulators to use silicones in a growing number of paint and coating applications.
According to Grand View Research, the global silicone market is expanding at a compound annual growth rate of 4.6% and is predicted to reach a value of $18.73 billion by 2025. The market includes pure silicone, hybrid and inorganic resins, rubber compounds, and oligomers used in building and construction, plastics and composites, paints and coatings, chemicals, personal care, food and beverage, electrical, automotive, energy, and other applications. In the silicone resins segment, paint and coating applications account for a significant and increasing share of the market as formulators look to leverage the hydrophobicity and water vapor permeability, heat and UV resistance, flexibility, anti-corrosion, and electrical insulation properties they offer. Silicone additives in the form of fluids, gels, resins, and elastomers act as defoamers, wetting and dispersing agents, rheology modifiers, surfactants, and lubricating agents.
Silicones used in paints and coatings can be completely inorganic or inorganic/organic blends. Inorganic (100% silicone) resins are currently used as additives in architectural coatings and non-high-heat industrial applications. The high-heat property is derived from 100% inorganic-based resins. Fully inorganic resins with their excellent heat stability also provide corrosion protection and weather resistance in coatings designed for very high-temperature applications. Polymethyl siloxanes with low organic content find use in some high-temperature coatings as well. Hybrid resins prepared by copolymerizing silicones with organic resins are also used in coating systems to most commonly improve weather- and chemical-resistance in architectural coatings. Silicone-based additives impart desirable flow and leveling properties, reduce foam, aid in water repellency, and impart release and surface slip characteristics. Silicone curing agents are also being designed to improve coating performance and to potentially replace traditional, less desirable materials.
Silicone resins used in resin blends typically replace 5–15% of the existing resin. In high-temperature coatings, silicones account for 100% of the resin content. When used as additives to reduce foam, for instance, they typically account for less than 0.5% by weight of the total formula. Aminopolysiloxanes are generally incorporated at 1–3% by weight of the total formula.
Advances in Silicone Resin Chemistry
Many silicone resins used in paints and coatings are hybrid resins such as silicone-containing acrylics and polyesters. In architectural coatings, siliconized polyester resins are increasing in popularity due to the superior durability they provide over traditional non-silicone-containing resins, and at a decreased cost compared to fluoropolymers, according to Alex Yahkind, AkzoNobel R&D senior scientist, Global Technology Group. Many silicone-based resins are utilized in coil coatings that are applied in the factory to metal, and later used to manufacture roofing and siding. “Incorporation of silicone increases UV-resistance of these coatings, which is essential in these applications,” Yahkind says.
Recent advances in silicone-based crosslinkers (polysiloxanes) designed to reduce post cure have been successful and are also helping to grow the protective coatings segment.
In the field of high-temperature coatings, previous silicone resins were either heat cured or air dried, according to Jon Fedders, North America marketing director for Dow Coating Materials at The Dow Chemical Company. “Air-dried resins suffered from post-cure embrittlement and would often become rigid. More recent silicone materials show better room-temperature curing and considerable flexibility,” he observes. As an example, he points to DOWSIL™ 2405 Resin, which provides formulators with the option to significantly extend the formulating space for room-temperature-curing in addition to providing high-temperature-resistant coatings that are flexible, impact resistant, and offer a solventless delivery. “Due to these types of advances, there will likely be an increase in the use of silicones in the protective coatings market,” Fedders adds.
The increasing complexity of global protective coating projects for which materials are sourced from around the world is also driving the use of more silicone-based resins. “Protective coatings must provide corrosion protection not only once the coating is applied, but during the construction and assembly phases, which often occur in different places,” remarks Sioned Ferriday, AkzoNobel Anticorrosives and Finishes Development lab manager. AkzoNobel has developed novel ambient-cure coatings designed to address this challenge. These formulated coatings are easy to apply and provide protection throughout their lifetime, including excellent performance at ambient and high temperatures, according to Ferriday.
Recent advances in silicone-based crosslinkers (polysiloxanes) designed to reduce post cure have been successful and are also helping to grow the protective coatings segment, according to Fedders. He also notes that although work is still in the preliminary stage, siloxanes are increasingly becoming a possible alternative to isocyanate crosslinkers for paints and coatings. “The number of inquiries related to the development of silicone as a non-isocyanate solution is increasing and suggests we’ll see further developments in this space,” Fedders remarks. The type of reaction depends on the chemical structure of the crosslinker and the resin system used, but the overall result is the improvement of the chemical resistance, durability, and often the adhesion of coatings to various substrates, according to Yahkind. In addition, low- and medium-molecular-weight vinyl and hydride reactive polymers, such as those manufactured by AB Specialty Silicones (ABSS), impart additional functionality, such as phenyl groups for heat stability or fluoro groups for surface modification. Their availability has increased the range of coating formulations that can benefit from the improved properties, according to director of Business Development Charles Olsen. His company offers Andisil FS vinyl and Andisil SF diphenyl dimethyl vinyl copolymers in this category.
It is also worth noting that the hydrophobicity and moisture vapor permeability of silicones continue to drive their use in many types of paints and coatings. “Silicones keep water out but allow substrate moisture to evaporate to the atmosphere. The result is increased service life of the coating and substrate alike, with the added benefit of reduced maintenance costs,” says Mark Westfall, marketing manager with Wacker Chemical Corporation. He adds that additional advancements have related to controlling the glass transition temperature (Tg) of silicone resins, which affects many properties such as adhesion, abrasion resistance, and dirt pick-up resistance.
Advances in Silicone Additive Chemistry
All types of silicones, including resins, aminopolysiloxanes, pH modifiers, foam control products, etc., are used as additives in paints and coatings, according to Westfall. “The active silicone atoms in these various compounds alter a number of different coating properties,” he explains. For instance, he notes that aminopolysiloxanes alter open-time to prevent lap marks and improve color acceptance, early water resistance, flow and leveling control, and dirt pick-up reduction. “Reducing dirt pick-up and staining are major focus areas for WACKER,” he adds.
One important attribute of silicone additives for Ferriday is their flexibility. “Coatings formulated with silicone-based additives have the flexibility needed to provide them with more options in creating custom products,” she comments. Silicone additives also help formulators control the flow, leveling, and anti-foaming properties of their coatings, according to Yahkind. For Fedders, beyond antifoam applications, the most important property for silicone chemistry as an additive is the role of water repellant. “Water repellants are known for delivering low water uptake while providing a high degree of breathability, thus allowing water in the substrate to be released throughout the lifetime of the coating,” he says.
Recent advances in hydrophobes are now providing not only water repellency and breathability, but are also tackling the so-called problem of “snail trails” or surfactant leaching, according to Fedders. He notes that DOWSIL 904H Coating Additive is a silicone additive for architectural facade coatings that supports enhanced aesthetics for building exteriors by reducing the snail trail effect commonly seen when water-soluble ingredients leach to the surface. By increasing the surface hydrophobicity, it can also improve water resistance, repellency, and beading.
Vinyl functional polymers, according to Olsen, can also be used as additives. When used in alkyd formulations for incorporation of dimethyl, methyl phenyl, or fluoro functionality, they can impart additional high-temperature resistance, low surface energy (from dimethylsiloxane groups), or ultra-low surface energy (from trifluoropropyl methyl siloxane groups) to coatings.
Inorganic Coating Advances
Inorganic silicone coatings, often called Inert Multipolymeric Matrix (IMM) coatings, are extremely flexible and resistant to temperatures up to 650°C (1202°F). As a result, they are widely used within the oil and gas industry, according to Ferriday. “It is important to consider the ambient temperature conditions to which steelwork might be exposed when specifying coatings for service at higher temperatures. The resin package in a coating allows for coating cure at ambient temperatures. Coating formulators have to consider the pigment package as well as how these materials influence the final coatings’ performance. Customization is highly technical, and it requires knowledge of all raw materials in the coating to optimize the performance of a finished formulation,” she observes.
To address these issues, AkzoNobel developed Interbond 1202UPC, which complies with the ISO12944-9:2018 international standard for performance of coatings in an offshore CX environment. “Unlike traditional IMM coatings, ambient-cure, anti-corrosive properties are attained prior to post cure during high-heat service. The coating can withstand the often-significant period of time and severe conditions between coating application and the start of high heat service. In addition, it has a corrosion creep of less than 3mm—a result usually only seen from inorganic zinc silicate-based systems,” Ferriday says.
As with most coating ingredients, the use of silicone compounds in coating formulations presents a unique set of challenges. Finding the correct amount is one of them. “When using silicone in coatings, you have to be very careful—a little bit can be great to improve the coating’s properties, but too much may yield problems with adhesion and recoatability, and result in surface defects such as so-called ‘fish eyes’,” Yahkind observes. Achieving proper reaction chemistry is also crucial, according to Olsen. “ABSS technical assistance is offered to customers to help ensure successful formulation development projects,” he notes. On the other hand, the high rates of efficiency when using silicones means that dosages can be reduced. “Reduced dosages result in a reduced cost of use, which is typically a big concern of paints and coatings formulators and can be a major benefit during the formulation process,” Fedders asserts.
Selecting the right resin type for a given protective coating application is a third issue that must be carefully addressed. For very complex oil and gas projects, for instance, Ferriday notes that different systems are required for the broad range of operating temperatures, steel types, and insulation conditions. She further points out that temperature-resistant coatings often rely on some degree of post-cure at higher temperatures to become fully anti-corrosive and damage-resistant. There have been examples of IMM coatings corroding before being put to use because of deficiency in ambient cure. “Although inorganic zinc silicate primers can be used to protect IMM coatings against ambient cure breakdown, they are difficult to apply correctly, and the requirement for an extra coat in the system reduces productivity. They are also not recommended for use under insulation due to the rapid consumption of zinc in the harsh environment on the steel surface,” Ferriday comments. A coating is therefore required that combines excellent ambient cure and high-heat performance without the use of an inorganic zinc silicate primer. Interbond 1202 UPC has been developed by AkzoNobel to satisfy these requirements.
Silicones keep water out but allow substrate moisture to evaporate to the atmosphere. The result is increased service life of the coating and substrate alike, with the added benefit of reduced maintenance costs.
Keeping projects on track can sometimes be an issue when using silicone chemistry in coatings. “Silicones are like many other leading-edge technologies. There are enough unexplored properties related to silicones that project creep can often affect the development timeline,” says Westfall. Establishing a goal and staying on project is essential to bringing a product to market.
Incompatibility with organics is an equally important issue when using silicone-based resins and additives in coating formulations. Westfall mentions ionic and cationic conflicts during coating production as examples. This challenge can, however, be converted into a great strength when considering antifoams, according to Fedders. “Silicones are known for providing excellent defoaming performance, with compatibility managed through organic modification,” he says. “In fact,” Fedders adds, “the organic modification of silicone materials has also overcome the previous incompatibility problems in nearly every other application, including automotive-related applications. Silicone additive chemistry can now be considered very versatile and easy to use in most types of solventborne and waterborne formulations.”
The higher cost of silicone ingredients can also be an issue for some coating formulators. Their increased efficiency, particularly for silicone-based additives, can overcome this problem and in some cases even offer cost savings over traditional additives. The superior performance properties obtained with silicone resins in terms of durability and corrosion, chemical, and weather resistance also balance the higher costs of these materials.
New environmental regulations in China, the major supplier of silicone metal, have led to the shut down and/or closure of many production sites, affecting global supply.
Recently, however, prices of silicone compounds have been rising due to raw material supply limitations. These issues are expected to continue through 2019. New environmental regulations in China, the major supplier of silicone metal, have led to the shut down and/or closure of many production sites, affecting global supply.
Moving Beyond Limitations
Coatings can be highly engineered, according to Ferriday, and the level of engineering is generally reflected in the price point and what the market will accept. A cost/benefit analysis helps manufacturers decide the type of coating and how it will be used. “For instance, while the use of epoxy materials, which are less expensive than silicone resins, in coatings is common, silicones provide greater options in building custom coatings outcomes,” she asserts. The question of cost vs value of a properly formulated coating continues to be a challenge, however, according to Westfall. “Silicones improve specific performance properties in paints. With proper formulation, I believe we can achieve most of the identified goals within our performance purview. Most of our new developments are focused on performance property improvements. We’d like to improve paints and coatings beyond the properties we encounter today,” he explains.
Most high-temperature silicone coatings are solvent-based, however, which is becoming an issue as demand for water-based materials in this space increases, according to Fedders. “The first generation of water-based products do not yet meet market needs, often still contain high levels of co-solvents, and exhibit poor drying performance. There is certainly much room for improvement in waterborne high-temperature materials, and we anticipate many more developments as the demand for waterborne products across the paint and coatings industry continues to grow,” he observes.
“It’s very exciting to see silicone additives moving from simply being invisible problem solvers, like antifoams, into a space where they are truly adding consumer value to formulations. As we look to the future, there will certainly be a lot of new advances to watch out for,” Fedders asserts. Westfall agrees, but extends this to all aspects of silicone chemistry, not just additives: “We see a generational change in how silicones are consumed by the formulator. While customers used to ask for assistance with how to incorporate silicones into their paints, they are asking for help improving the properties of their paints by using silicones.”
CoatingsTech | Vol. 16, No. 2 | February 2019