Paints, Inks and Coatings

Consumer Solutions All Silicones Are Not Created Equal

By Gerald L. Witucki

Introduction Used as the primary binder in a formulation or as an additive, these technologies improve thermal, chemical, water and For decades, Dow and other members of the silicone industry weather resistance of coatings, along with flexibility, foam have promoted silicon-based technologies impacting many control, wetting and adhesion promotion. Paint scientists have aspects of everyday life, including the paint and coatings observed these enhanced attributes for high-performance markets. These materials provide benefits to applications as far applications, including: primers, heat-resistant coatings, ranging as food processing and medical devices to electronics industrial maintenance coatings, hygienic coatings, marine and hair care, but for some paint formulators, issues relevant to coatings, anti-fouling coatings, abrasion-resistant coatings, one particular material class create reluctance and doubt about automotive clear coats and architectural coatings. the entire chemistry set of silicon-based technologies. Concerns related to the incompatibility of silicones, or the recoatability of Understanding the scope of silicon-based technologies and the paints containing materials broadly described as “silicones,” can differing physical attributes associated to this diverse material impede potential solutions to coatings market demands. But set allows a coating formulator to move beyond misconception more often, formulators have learned to differentiate silicon- and gain access to a versatile and robust class of problem-solving based technologies and recognize the advantages and efficacy technologies. This paper will cover the Si-based technologies with of these useful technologies. broad utility in the coating industry, including silanes, resins, fluids and copolymers, but let’s start with the most notorious: In fact, nearly every year for the last three decades, coatings polydimethyl siloxanes. formulators who invest in silicon-based technologies have patented a growing number of inventions citing silicon-based materials as Polydimethylsiloxane (PDMS) the enabling technology (see Chart 1). As of 2013, approximately 10% of all U.S. patents issued cite a silicon-based technology (not including silica) in the primary claim of invention. Si Si Si Si O O O n Chart 1. U.S. Patents Issued (per year) for “Paint and Siloxane” 600 PDMS is a colorless silicon-based polymer (see above) used primarily for surface treatment. This class of material is most 500 closely associated with the term “Silicone.” These polymers are linear chains of alternating silicon and oxygen atoms, with each 400 silicon atom bearing two covalently bonded methyl (-CH3) groups. Depending on the degree of polymerization (n), PDMS can range 300 from a volatile fluid to a gum-like substance. The use of PDMS 200 is prevalent across many industries including personal care and machine maintenance lubrication, which ultimately, for factory- 100 applied coatings, may lead to surface contamination and surface defects, such as fish-eyes and craters. 0 2001 2004 2007 2010 2013 1983 1986 1989 1992 1995 1998 A common remedy to surface contamination is to add the • Non-reactive groups include alkyl, phenyl and offending agent into the paint, thereby equalizing the surface trifluoropropyl, which provide moisture resistance, tensions of the surface and the paint. As a result, though often organic compatibility and chemical resistance. reviled, PDMS is one of the most common remedies to silicone • Organic reactive groups include epoxy, methacrylate, contamination. The active ingredient in many “Fish-eye killing” amino, vinyl, sulfido and others. These groups offer good additives is simply low viscosity PDMS. Therein lies a potential compatibility and reactivity with organic resins, thereby area of confusion and the root of many of the concerns serving as coupling agents between organic coatings and associated with “silicones.” inorganic substrates. Low-viscosity PDMS is fairly miscible in organic systems. In In the paint industry, the most common use of silanes are fact, silicone fluids (<10 centistokes (cS)) have been found to tetraethoxysilane hydrolyzates (tetraethylortho silicate (TEOS)) be reasonable diluents, with a Kauri-butanol rating similar to for use as a binder in zinc-rich maintenance primers. This that of mineral spirits. In the electronics industry, due to low application, with excellent recoatability and adhesion, high- residual deposits, volatile PDMS is used as a cleaning solvent lights the distinct differences between silanes and PDMS. As for electronic circuitry. But as the chain length of the PDMS additives into coatings, silanes improve adhesion, filler and extends, volatility, solubility, recoatability and compatibility pigment incorporation and bulk properties. In applications such with organic resins decreases. as automotive coatings, numerous patents cite the benefits of In coatings, PDMS is used to provide wetting, leveling, foam silane modification, including controlled hydrophobicity, UV and control and gloss, as well as mar resistance and slip angle thermal stability, chemical resistance and corrosion protection. (coefficient of friction) reduction. Low miscibility and surface Also, the inherent weatherability and moisture resistance of energy along with high polymer mobility allow PDMS to alkyl silanes make them suitable for use in water-repellent migrate to the coating surface, improving wetting or levelling, formulations that meet industry specifications and customer or providing a desired layer of lubricity, but bearing with it the demands for greater protection of wood and masonry unfortunate potential of creating a weak boundary layer on the substrates, including concrete, sandstone, granite, limestone, surface and reducing recoatability and intercoat adhesion. This marble, brick, tile, wood, gypsum and perlite. issue is particularly relevant to high viscosity PDMS. The effectiveness of PDMS can itself create problems. Used Polyorganosiloxane Copolymers at very low levels (loading levels of 0.05 to 0.1% are not Polyorganosiloxane copolymers possess both the methyl- uncommon), PDMS is a valuable problem solver, but improper rich structure of PDMS and a variety of longer alkyl and aryl dosage can result in well-known surface defects. With materials groups which provide compatibility with organic materials and available on the market ranging from less than one centistoke, mitigate many of the problems associated with PDMS while to greater than 100,000 (cS), and with nothing more than a providing benefits such as wetting, leveling, gloss, foam control, viscosity measure-ment to differentiate, the opportunity for slip and mar resistance. They are also used as deaerators for misapplication is high. microfoam. Polyorganosiloxane copolymers allow for good recoatability, but depending on the PDMS/organic ratio and the Silanes particular resin system, problems of intercoat adhesion could 1 2 arise. Within families of polyorganosiloxane copolymers, the SiR R largest group is silicone polyethers. X(0-4) (4-x) Silicone Polyethers Silanes are monomers, containing just one silicon atom. As X monomers, silanes do not possess the characteristic siloxane (RO)n (-Si-O-Si-) polymer backbone, and as such, cannot be classified Si as silicones. These materials have viscosities less than water O Si O Si O Si O Si n and are often completely miscible in organic polymers, acting as strong polar solvents. Silanes can possess a wide range of (RO)n substituents, both reactive and non-reactive. Silanes react to form Telechelic Silicone Polyether X siloxane polymers. The ratio between the reactive (including the hydrolyzable chlorine and alkoxy) and non-reactive (e.g., alkyl or R Si O Si O Si O Si O Si R aryl) groups will determine the crosslink density of the resulting a b c polymer and the associated physical properties. • Inorganic reactive groups include chlorine, alkoxy, (Hydrophilic)w (Oleophilic)x hydroxyl and others. These groups react well and provide Y z good adhesion with inorganic substrates such as glass, Pendant, Rake Polyether metals and minerals.

pg 2 With polyether functionality attached to either end of the fixtures, but improved chemical resistance is often also silicone backbone (telechelic) or along the polymer chain observed. Silicone resins can be developed with a wide range (pendant or rake), silicone polyethers are used in coatings of flexibility – from soft to hard and brittle – and a range of where formulators are looking for many of the benefits organic compatibility. associated with silicon-based technology, including mar/slip In the uncured state, silicone resins already possess significant resistance, wetting, leveling and foam control, joined with levels of crosslinking. These crosslinks improve the thermal, organic characteristics. The organic portion of the silicone chemical and radiation resistance. In addition, many silicone polyether copolymer improves the compatibility of silicone resins are formulated to contain high levels of aromatic with organic components, allowing for good incorporation substituents (phenyl), which provide compatibility with organic and improving the hand-feel and performance of coating polymers. As a result, with the exception of the pure methyl formulations. The wide variety of potential raw materials and variants, silicone resins do not cause the defects associated with the myriad of possible combinations allows this technology to PDMS. In addition, the crosslinking and compatibility restrict offer application-specific performance. the mobility of the polymer, preventing migration, keeping the silicone within the coating. As a result, silicone resins have Silicone Antifoams minimal impact on recoatability and intercoat adhesion. While some silicone foam control agents are single component polymers (e.g., silicone polyethers), most antifoams are Silicone Elastomers compounded mixtures (e.g., silica reacted with fluids) specifically designed for the coatings industry which requires the delicate balance between foam disruption and defect-free films. These problem solvers are available as neat fluids and emulsions. Variations of silicone antifoams include organic modification of silicone fluid, degree of crosslinking of the silicone fluid and others. By varying the type of silicone used, the degree of crosslinking, the type of silicone, etc., antifoams can be tailored to perform in many different types of coatings, but defects can occur with some antifoam/coating combinations. Owing to low use levels (0.05 to 1.0%), recoatability, or intercoat adhesion are rarely issues. Silicone elastomers are rubber-like materials (think silicone Performance is very system-dependent. bathtub caulk) in emulsion (analogous to organic latex) or dried powder form. These materials are used in coatings to provide Silicone Resins water resistance, mar resistance and matte, textured finishes. R R R In either form, the elastomer is fully crosslinked as supplied. O O While initial surfactant compatibility should be evaluated, no O Si Si Si O free-flowing PDMS exists within the elastomer to migrate from the coating and create subsequent issues. O O O O O Si Si Si O Siliconates O R R R - + Silicone resins can be characterized by possessing a high level RSi3/2 Metal of crosslinking across polymer chains. A typical silicone resin Siliconates are reaction products of alkyl alkoxy silanes (e.g., building block consists of a silicon atom bearing a single methyltrimethoxy silane) and metal hydroxides (e.g., sodium or organic substituent (R = organic moiety, most commonly potassium hydroxide). These are high pH (11) water-based solutions methyl or phenyl) and three oxygens shared with adjacent used as low-VOC, penetrating water-repellent treatments for silicon atoms. These units can be homopolymerized, or damp-proof exterior façade or masonry applications. Similar to copolymerized with other siloxane units to provide specific silicates (which do not possess alkyl substitution), siliconates performance attributes. exist in monomeric or dimer form until exposed to atmospheric Silicone resins can be cold-blended, reacted with many organic carbon dioxide. Upon application, these materials form water- resins, or even used as the sole binder for a wide range of insoluble silicone resins. Though siliconates might impact coatings formulations (use level: 15-100% of resin binder system). substrate wetting by water-based topcoats, the high solubility They are primarily used to impart UV and thermal resistance and subsequent crosslinking provide excellent dispersibility in in applications such as exhaust stacks, cookware and lighting aqueous resins without the issues associated with PDMS.

pg 3 Conclusion Silicon-based technologies have advanced coatings innovation. Dow is leading these efforts through existing solutions and new research efforts. Leveraging six decades of experience, Dow has worked with the industry to identify select needs and applications where the company’s expertise and products can make a difference. Silicon-based materials provide many of the performance needs in the coatings industry, enabling new coating qualities. Coatings formulators leveraging the traditional benefits provided by silicon-based technology, such as slip and mar resistance need be aware of the potential issues associated with polydimethylsiloxane (PDMS) materials. While the reputation of PDMS is understandable (but controlled by proper viscosity and dosage selection), other silicon-based technologies are designed to be inherently more compatible, with less propensity to migrate, eliminating, or at least minimizing, the issues surrounding PDMS “silicones.”

This article was originally published in the March 2016 issue of the Journal of Protective Coatings & Linings.

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