by Cynthia Challener, CoatingsTech Contributing Writer
OCTOBER 2016 | Vol. 13, No. 10
The shift to waterborne and 100% solids coatings to reduce VOCs and growing end-user expectations for greater coating performance and functionality are placing pressure on resin, additive, and pigment suppliers to develop novel materials that facilitate the formulation of low-VOC systems while imparting unique functionality. Additive manufacturers are taking on this challenge. Although used in minor quantities, additives can have a significant impact on the properties of coatings during processing, application, and after film formation. New products are being introduced regularly to help coating formulators achieve new performance attributes.
GROWING DEMAND FOR ADDITIVES
Coating additives are traditionally used to improve pigment wetting and reduce foaming during processing; improve flow and leveling and substrate wetting during application; and improve UV durability, reduce surface defects, and enhance substrate adhesion in applied coatings. They include surfactants, dispersants, rheology modifiers, adhesion promoters, UV absorbers, and many other types of compounds, such as nanoparticles and waxes. Newer functional additives are designed with unique chemistries to achieve these goals and more.
The global market for coating additives is projected to grow at a compound annual growth rate of 6.1% between 2016 and 2021 to reach $9.19 billion, according to market research firm Markets and Markets. This growth is largely being driven by expansion of the construction and automotive industries in Asia (China, Korea, Japan, and India). Additive formulations that help reduce the VOC content of paints and coatings are in highest demand, particularly in the architectural segment, which accounts for the largest share of additive sales. Demand will be greatest in the construction segment, however, where additives are used to improve the ability of coatings to protect surfaces from moisture, microbial growth, UV light, heat, corrosion, and other chemical attack. Regionally, Asia-Pacific is the largest consumer of coating additives.
NOVEL SURFACTANT STRATEGIES
Surfactants are one of the essential components in the synthesis of emulsion polymers, providing colloidal stability and control of the particle size of the latex, according to Steven Chan, applications manager with Croda. They have also become a critical component of successful waterborne paint formulations as solvents, which allow paint to spread easily during application to a surface, are removed from paints to meet regulatory requirements for allowable VOC levels, and as paint manufacturing processes demand faster and better wetting and foam control as line speeds increase, notes Tara Conley, technical services manager with Dow Coating Materials.
Conventional surfactants adsorb onto the monomer/polymer droplet surface, form micelles in the aqueous phase, migrate to the air/water interface, or can be free molecules; but this equilibrium can be disrupted by environmental factors such as temperature or pH and external forces like mechanical mixing, Chan observes. “The surfactants can then desorb from the polymer droplet surface, causing particle flocculation and instability. In addition, during film formation, the adsorbed surfactant is released, after which point it can accumulate in hydrophilic domains in the dried film that result in the formation of porous films with reduced water resistance, blocking resistance, and mechanical properties,” he adds.
To address these issues, Croda has developed Maxemul™ non-migratory surfactants (NMS), reactive compounds that graft onto the surfaces of polymer particles during emulsion polymerization. Final latex stability is enhanced with positive effects on shelf life and mechanical and barrier properties, according to Chan. Maxemul 6106 and 6112 are modified alcohol ether phosphates (anionic), and Maxemul 5010 and 5011 are alkenyl ethoxylates with carboxyl functional groups (non-ionic). The surfactants copolymerize with unsaturated monomers at the vinylic positions during emulsion polymerization and as a result, unlike conventional surfactants that desorb during drying and film formation, become part of the polymer and remain evenly distributed in the dry film, thus improving water resistance and other film properties.
Importantly, Chan notes that Maxemul surfactants can be used in the same manner as conventional emulsifiers during polymerization. Typically, Maxemul 6106 and 6112 are added in the seeded stage as anionic surfactants, while Maxemul 5010 and 5011 are added in the pre-emulsion or particle growth stages for stability. Normal usage levels range from 1.5%–3% based on monomer content. These non-migratory surfactants can be used in emulsions wherever enhanced film properties are required, according to Chan. “Use of reactive surfactants is relevant for various application areas. In architectural coatings, better pigment binding properties, water resistance, and scrub resistance can be achieved. In protective coatings, superior barrier properties, better corrosion resistance, and lower water diffusion rates are observed,” he explains.
Dow Coating Materials offers TRITON™ HW-1000 Surfactant as a high performing alternative to standard Gemini surfactant chemistries at a lower cost. The non-ionic surfactant chemistry on which the surfactant is based is, according to Conley, unique to Dow and designed to improve the final appearance of water-based, low- and zero-VOC paints and ensure their protective qualities by improving surface wetting performance. “TRITON HW-1000 Surfactant reduces surface tension and foam during application, thus leading to a better paint film build with fewer imperfections, which in turn results in greater chemical and corrosion resistance properties,” she says. With these characteristics, the new surfactant is particularly beneficial for thin-film metal coatings used in industrial applications and commercial architecture and coatings used in industrial applications where moisture and salt may be present.
Air Products, meanwhile, has focused on the development of unique surfactants that are both highly effective substrate wetting agents and help to lower the minimum film forming temperature (MFFT) of many water-based formulations. “Reduction of MFFT allows the formulator to use these products to prevent surface defects, ensure consistent wet-out of the substrate and reduce the amount of coalescing solvents needed for film formation,” states Asaf Korkut, coatings marketing manager with Air Products. “As a result, the formulation VOC content can be lowered to meet regulatory targets without sacrificing coating performance,” he adds.
Dynol 360 is a hydroxyl thio-ether-based superwetter that is inherently biodegradable and zero-VOC according to the methyl palmitate standard. Surfynol AD01 and Surfynol 107L are alkane diol-based surfactants that combine good substrate wetting with strong anti-foaming properties in many different formulations. All three are typically used at 0.1–1.0% depending on the formulation and substrate being coated, according to Korkut. They can be added to both the pigment and filler dispersion stages of paint production or via post addition to the letdown or final paint. All three materials are relatively hydrophobic and thus should be mixed for at least 20 minutes to ensure complete dissolution.
Reduction of MFFT allows the formulator to use these products
to prevent surface defects, ensure consistent wet-out of the substrate
and reduce the amount of coalescing solvents needed for film formation.
The three new surfactants are suitable for all water-based paints and coating formulations (as well as inks and adhesives), including architectural coatings (interior and exterior), wood, concrete, plastic, and direct-to-metal formulations, according to Korkut. “Because they are effective in lowering both equilibrium and dynamic surface tension, these new materials are particularly suited for industrial coating application methods, such as curtain, spray, and roll coating. They are also effective with most common binder types, including acrylics, styrene acrylics, alkyds, epoxies, and polyurethanes,” Korkut comments.
RHEOLOGY MODIFIERS MAKING NEWS
Rheology modifiers are added to paint and coating formulations to moderate their flow and leveling properties, which can be a challenge in simple water-based formulations and particularly difficult for solvent-free systems and coatings applied under stressful conditions. Arkema has developed two novel rheology modifiers that meet the specific needs of more complex coating products: easier activation and effective performance in hot-spray applications.
CRAYVALLAC® LV is a 100% active, third-generation polyamide rheology modifier with easy activation and high performance in solvent-free epoxy and polyurethane coatings for marine, protective, general industrial, and wood coatings, according to Mark Piggott, senior account manager for Arkema Coating Resins. “The ease of activation and higher performance of this product compared to earlier generations of polyamide rheology modifiers affords higher film builds, resulting in excellent sag resistance and very good shear thinning for the spray application of coatings,” he notes. The additive is used at 0.5–2% wt. on total formulation and requires heating (45–65°C) and shear (ideally high speed dispersion) for activation, which is typically achieved by incorporating the rheology modifier during the pigment/filler mill base grind.
CRAYVALLAC OPTIMA is also a 100% active, third-generation polyamide rheology modifier designed for hot spray applications. It has a wider activation window in high solids epoxy primer coatings for marine, protective, and general industrial applications requiring good sag during hot spray application or application onto hot substrates, according to Piggott. It is incorporated at 0.2–1.5% wt. on total formulation and also required heat and shear for activation.
Dow Coating Materials, meanwhile, has focused on the development of rheology modifiers that improve the performance of contractor-quality waterborne architectural paints. ACRYSOL™ DR-180 Rheology Modifier is a versatile hydrophobically modified alkali soluble emulsion (HASE) thickener that can partially or completely replace cellulosic thickeners (hydroxyethylcellulose (HEC) technology), according North America Additives marketing manager Tom Polaski. “This alternative achieves the desired coverage and touch-up performance that professional painters need, but with added cost savings. It also offers exceptional spray, roller, and brush application; improved spatter resistance compared to cellulosic thickeners; and improved water resistance compared to conventional HASEs,” he asserts. The water resistance of the new rheology modifier is of particular interest because water blushing—the effect that leaves white rings on coated surfaces when wet objects are removed after a period of time—can occur in paints formulated with HASE or HEC rheology modifiers. “We have found that ACRYSOL DR-180 Rheology Modifier reduces the water blush effect. Clear coatings formulated with the new and other available additives were applied to glass panels and submerged in water for two hours. The results showed a reduced water blush effect on the glass panel coated with this rheology modifier. This characteristic may make ACRYSOL DR-180 Rheology Modifier a HASE rheology modifier candidate for clear coating formulations, particularly wood finishes,” Polaski explains.
The second rheology modifier from Dow Coating Materials, ACRYSOL RM-725 Rheology Modifier, is a next-generation, easy-pouring hydrophobically modified ethylene oxide-based urethane (HEUR) that optimizes hiding and surface smoothness in waterborne contractor and high quality DIY architectural coatings, according to Conley. “In addition to being compatible with a broad range of binder types including polyvinyl acetate (PVA), vinyl acetate ethylene (VAE), and full acrylic binder systems, this new rheology modifier provides sag-level balance with the added benefit of utilizing proprietary ACID SUPPRESSION™ Technology, which employs pH-triggered association versus requiring the use of solvent or surfactant to improve ease of handling,” she observes.
DISPERSANT DEVELOPMENTS
Waterborne coatings also pose challenges in terms of pigment dispersion and color development. Not surprisingly, then, additive manufactures have been working to develop novel technologies to overcome the limitations of current products. “Automotive and performance industrial coatings are shifting from solvent to waterborne basecoats, both in the refinish (2K PU) and OEM (PE/melamine or alkyd/melamine) segments. The limiting or slowing factor to a complete shift is the poor color development achieved with certain pigments (lower chroma, less vibrancy, long grind times),” observes Loulou Rozek, technical service manager, Americas with Borchers, a business unit of OMG Americas.
For automotive waterborne coatings, Borchers has developed Borchi® Gen 0851, a novel polyurethane dispersant that enables improved color development for organic pigments and high-jetness with carbon black pigments. The novel dispersant has a unique interaction profile with these difficult pigments that is the result of specific engineering and the incorporation of a combination of polar functional groups and aromatic functionality (for interaction with aromatic, low polarity pigments) that provide enhanced pigment surface wetting, according to Rozek. “The result is faster grinding times, lower pigment dispersion particle sizes, lower grind viscosities, and stronger colors with the enhanced undertones that are demanded by the automotive industry,” she says. Dosages vary from 20% (most pigments) to 100% (high jetness carbon black), and the dispersant must be added in the grind stage. Rozek adds that the dispersant has the added benefit of stabilizing pigments in resin-free dispersions. “Most importantly, automotive spray professionals have indicated that waterborne basecoats formulated with this dispersant spray just like solventborne basecoats, specifically with black and red pigments. In addition, coatings containing certain difficult pigments also have chroma and vibrancy properties matching those of comparable solventborne basecoats. These results suggest that it is now possible to produce quality waterborne basecoats without any compromise,” states Rozek.
TAMOL™ 2011 from Dow Coating Materials is manufactured without ammonia, leading to low-odor properties, which are desirable for interior architectural applications. The hydrophobic polyacid copolymer dispersant enhances the long-term hiding, tint strength, and gloss of premium interior architectural paints by surrounding pigment particles, effectively keeping them separated over time for long-term paint performance and stability, according to Conley. Its hydrophobic nature also repels water on a paint’s surface, increasing its stain removal capability and allows higher levels of dispersant use for greater gloss and hardness.
SLIP-MAR ADDITIVES FOR INDIRECT FOOD CONTACT
Many slip-mar additives are not approved for indirect food contact use. Emerald Specialties Group, a business group of Emerald Performance Materials, has developed new Masil® Ultra SL1000, a water-based, zero-VOC, slip-mar additive and the first product in the new Masil Ultra platform, which is highly effective for decreasing the coefficient of friction, maintaining gloss, and preventing haziness at very low use levels, according to Andrew Recker, technical director of Additives with Emerald Specialties Group. “Notably, this additive is also cleared under a wide variety of U.S. FDA 21 CFR citations for indirect food contact, including many that are not typically applicable to other available slip-mar additives, such as FDA 21 CFR 175.300. As a result, the additive can achieve performance that may not have been previously possible in applications such as flexographic printing and waterborne coatings used in food packaging,” he adds.
IMPROVING UV DURABILITY AND FIGHTING CORROSION
Protective coatings are widely used to extend the useful life of many products. Exterior coatings are taking on increasing importance for the protection of metal, plastic, and wooden structures globally, according to Mervin Wood, technical expert of Performance Additives with BASF. “The rising cost of raw materials, energy, and labor make replacement costs prohibitive and necessitate the preservation of a product’s useful lifetime for as long as possible,” he says. In the automotive industry, the thinner gauge steel used to reduce costs and meet ever-increasing fuel economy requirements is more susceptible to the effects of corrosion and rust through, yet consumers are demanding longer lifetimes for automobiles. “The need for more durable coatings is clear,” Wood observes.
A reliable protective coating provides a barrier to the effects of water, oxygen, sunlight, pollutants, and biological attack and is the first line of defense against the elements that cause structural corrosion and degradation. Therefore, maintenance of the coating’s properties such as flexibility, adhesion, water repellency, and impermeability are of utmost importance, according to Wood. In the automotive, architectural, and appliance industries, he adds that the decorative aspects of these coatings are also very important, and properties such as gloss, distinction of image (DOI), color, and resistance to color change, and resistance to biological attack take on added importance. Specialized additives are used to protect polymer systems from the damaging effects of ultraviolet light, moisture, heat, and pollutants.
In automotive clearcoats, sterically hindered amines, which do not absorb UV light at wavelengths above 250 nm, are added to prevent pigment discoloration and photochemical degradation and thus improve gloss retention and crack prevention, according to Wood. These substances are almost exclusively derivatives of 2,2,6,6-tetramethylpiperidine and are referred to as hindered amine light stabilizers or HALS. BASF has introduced Tinuvin® 249, a new hindered amine light stabilizer that is a derivative of 2,2,6,6-tetramethylpiperidine. Within acid catalyzed acrylic-melamine clear coats, Tinuvin 249 enables superior cure and hardness development compared to basic N-H and N-Alkyl HALS, while in polar epoxy-carboxy clearcoats, the new stabilizer offers excellent compatibility by delivering superior gloss and reduced exudation during over bake conditions versus traditional non-interacting HALS, according to Wood. “Whether the clearcoat formulation is acid-catalyzed acrylic-melamine, 2K polyurethane, epoxy-carboxy, or thermoset acrylic-melamine, Tinuvin 249 has been shown to be an extremely versatile and broadly applicable HALS when compared to other commercially available HALS technologies. When used in conjunction with UV absorbers based on hydroxyphenyl-triazine or hydroxyphenyl-benzotriazole, Tinuvin 249 is well suited to achieve superior weathering results over a variety of substrates,” he asserts. The product is incorporated in the outermost coating layer at approximately 0.75-2 wt% relative to total resin solids intended for automotive and industrial applications.
Emerald Specialties Group is also developing a hyperfunctional siloxane as an anticorrosive additive that dramatically increases adhesion to steel and the critical crosslinking and hardness properties of direct-to-metal films, according to Recker. He notes that the designation of “hyperfunctional” is based on the significantly greater degree of functionality added to the polymer compared to other functional additives. “This secondary crosslinking resin modifier increases the crosslink density of the coating independent to the resin chemistry, and due to its higher functionality, the modifier greatly increases corrosion resistance to metallic substrates through both increased adhesion and lower permeation,” he comments. Recker also notes that the modifier has been tailored so the type of polymer is essentially irrelevant, making it useful in combination with any resin and in a wide variety of formulations, including 100% solids systems. Current work has shown significantly increased stability, particularly for water-based systems. “Corrosion is a widespread challenge for formulators; the new modifier’s hyperfunctionality and broad compatibility make it extremely valuable for markets such as direct-to-metal, industrial, and water-based coatings,” observes Recker.
REACTIVE APPROACH TO SUPERHYDROPHOBICITY
Surfactants are not the only additives that can be reacted with coating resins to provide improved coating performance. Emerald Specialties is pursuing this tactic for the development of a functional, hydrophobic siloxane additive that crosslinks with polyaspartics, polyurethanes, and polysiloxanes. “Unlike other hydrophobic additives, the reactivity binds the additive to the resin, resulting in higher permanence and durability, plus superior hydrophobicity with very high contact angles and surface areas. The resulting coating surfaces have significantly lower energy surfaces and better performance with respect to ice adhesion elimination, stain resistance, water repellency, and antifouling,” Recker states. This unique chemistry has so far proven to be particularly valuable for marine, aerospace, and industrial, and high-end, hydrophobic architectural coatings,” he adds.
PROACTIVE ADDITIVES
Certain technologies from Emerald are also increasingly being used as reactive additives to replace other chemistries that may be less safe, less stable, or have a greater environmental impact, according to Hitesh Soni, product manager, Epoxies, CVC Thermoset Specialties. For example, he notes that ERISYS® GA-240 is a tetra-glycidyl m-xylene diamine increasingly used as a drop-in replacement for polyfunctional aziridine crosslinkers in adhesive and coating formulations. “Not only is GA-240 a safer alternative that does not require reformulation to incorporate, it is also economical because it will remain active in formulations for several days,” Soni explains. In contrast, polyfunctional aziridine crosslinkers have a short pot life and require re-inoculation due to hydrolysis-related decomposition of the aziridine. As a crosslinker, ERISYS GA-240 works with functionalized acrylic resins in many coating and adhesive formulations, controlling tack and adhesion, and preventing cold flow, according to Soni. It is added to the formulation just prior to coating the substrate. The crosslinking reaction results in increased polymer molecular weight for improved cohesion, higher shear strength, lower peel adhesion, less tack, higher temperature performance, and improved chemical resistance—properties similar to those seen with polyfunctional aziridine crosslinkers.
CREATING TEXTURE
A new coating additive from Evonik allows formulators to create stone or ceramic finishes in different colors that serve as decorative or functional structured elements. VESTAMELT®, a polyamide 12 (PA 12) copolyamide powder, can be used in both waterborne and solvent-based systems that are hot- or cold-cured. Surfaces treated with coatings based on VESTAMELT have high flexural strength and elasticity and excellent abrasion resistance and are less susceptible to soiling and easier to clean, according Peter D. Colburn, director, engineered products for Americas High Performance Polymers for Evonik. He adds that the powder particles have a shock-absorbing effect and protect the surface from cracking. “VESTAMELT acts similarly to impact-resistance modifiers in plastics, and as a result is ideally suited for use in coil coatings, where the coatings are initially applied to metal sheets that are then shaped as desired. In this application the coatings have to withstand enormous stresses during processing,” Colburn observes. Structure is achieved by using a combination of different powder particle sizes, which range from 5 to 60 micrometers. “With this approach there are virtually no design limitations on structured surfaces,” he asserts.
ANTIBACTERIAL DRY-FILM PROTECTION
The switch to water-based architectural coatings has driven the development of waterborne in-can and dry-film preservatives. While the focus of dry-film preservatives has traditionally been to protect coatings against fungal and algal growth, Troy Corporation has introduced Troysan® 1050, a novel dry-film preservative that offers antibacterial protection for interior paints, coatings, caulks, and sealants. The new aqueous, VOC-free, dry-film preservative is formulated to impart fast-acting, long-lasting bacterial resistance to interior coated surfaces, according to Troy’s vice president of R&D and Technology Izzy Colon. The preservative is also effective at very low use levels, enabling formulators to achieve low cost-in-use. “In addition, when used at optimized use levels, Troysan 1050 enables protected systems to pass the JIS Z2801 test for antibacterial coatings,” Colon notes. He adds that with its VOC-free composition, Troysan 1050 is an excellent choice for today’s “green” hygienic coatings.