CREATING TOMORROW S SOLUTIONS REPRINT I EUROPEAN COATINGS JOURNAL I JANUARY 2017 BUILDING BRIGHTER FAÇADES Composite Dispersion Binder Reduces Fading and Dirt Pick-up
28 ARCHITECTURAL COATINGS TECHNICAL PAPER Source: Brad Pict - Fotolia BUILDING BRIGHTER FAÇADES Composite dispersion binder reduces fading and dirt pick-up. By Lenine de Sousa Gomes, Wacker Chemie AG. A new form of hybrid binder has been developed in which the silica particles are more homogeneously distributed than in previous technologies. The binder reduces dirt pick-up and can also improve colour retention on exposure. Leaching of soluble components, giving rise to efflorescence and snail trails, is also reduced. The choice of binders plays a key role in determining the properties and functionalities of the resulting paint and coating formulations ranging from dirt pick-up resistance to durability and overall appearance of the end product. A novel dispersion developed by the Wacker chemical group yields optimum results by combining the unique properties of both organic and mineral materials. The high pigment stability of this binder makes for vibrant exterior paints that retain their colour for much longer and are less prone to fading. Paints formulated with the new binder also reduce the amount of dirt that the façade surface picks up and reduce or even eliminate snail trails (due to leaching) on the façade. COLOUR LENDS CHARACTER TO FAÇADES WHILE IT LASTS Figure 1: Schematic model of a polymer particle of Primis AF 1000, which combines a silica core and an organic shell with silane groups (yellow dots) and surfactants (red dots) to yield unique binder performance. Inorganic polymer (silica) Organic polymer and silane chemistry Primis AF 1000 binder Inorganic core: silica particle Organic shell: acrylic copolymers The purpose of exterior coatings is to protect buildings from external influences such as cold, heat, rain, ice and UV radiation. At the
TECHNICAL PAPER 29 RESULTS AT A GLANCE Hybrid binders have been developed that incorporate inorganic materials (generally silica) with an organic emulsion binder, to increase hardness while maintaining easy coalescence. However, agglomeration of silica particles can cause a range of problems from increased hydrophilicity to cracking. A new form of hybrid binder has been developed in which acrylic polymers are functionalised with silanes, which bind to fine silica sol particles and so ensure they are homogenously dispersed with much less tendency to agglomerate. Exposure tests show that the new binder gives lower dirt pick-up than standard emulsion binders and can also improve colour retention. Another appearance problem known as snail trails arises when soluble components in paints are leached out under moist conditions. Several tests confirmed that the new binder reduces leaching and the formation of efflorescence salts on the surface of paints. same time, they function as design elements, lending visual charm to a building s walls by adding colour, shape or surface details. With the rise of postmodern architecture in the 1980s, more and more architects found the courage to explore bright colours. Colours and surface texture are what turn exterior coatings into a design element, defining the look both of individual buildings and of entire regions. When thinking of Sweden, for example, most people picture red wooden buildings, while the baroque structures of Central Europe stand out in bright pastels, and the old quarters of Paris are adorned with sandy, champagne-coloured plasters. The main problem with colourful façades, however, is that over time the brightly coloured paints tend to fade. Due to the influence of Figure 3: Comparison of paints formulated with a standard composite binder (left) and with the new binder (right). The silica particles in the standard paint tend to agglomerate more in the standard paint. Figure 2: Schematic overview of technologies that combine organic and inorganic components. Left, mixing silica sol with polymer dispersions; centre, encapsulation based on emulsion polymerisation; right, new composite technology treating the surface of silica sol with reactive organic polymers. 100 nm 100 nm
30 ARCHITECTURAL COATINGS TECHNICAL PAPER weather, wall coatings often lose their shining brilliance. A key element for paint formulations is the choice of binders they allow for a durable protective film to be formed and enable the coating to cure quickly, while remaining elastic and flexible at the same time. Furthermore, fillers and pigments need to coalesce with the help of the right binder for the façade to keep its appearance over a long time. That is why a novel binder system marketed as Primis AF 1000 has been developed, designed to make brilliant colours last a long time. The dispersion effectively stabilises coating pigments, creating long-lasting façades that remain attractive for years to come, soil less and avoid snail trails. HYBRID BINDER HAS MORE HOMOGENEOUS SILICA DISTRIBUTION The hybrid binder combines organic and mineral components, using silane chemistry to deliver a binder with a completely novel property profile. It is an aqueous polymer dispersion combining two phases one based on acrylate copolymers and the other on silica sol particles (see Figure 1). Inorganic silica is typically hard, thermostable and UV stable. Thus, it lends paints very low tackiness and, Figure 4: Dirt pick-up resistance of paints based on the new binder and standard dispersions based on styrene acrylic and vinyl acetate/veova binders, at an outdoor weathering station in the United Arab Emirates (60 SE / Dubai). SA therefore, low dirt pick-up and long-lasting colour retention. The organic polymer phase is based on acrylic esters and performs a binder s traditional role producing excellent adhesion to the substrate and a high degree of cohesion, while at the same time offering flexibility, good mechanical properties and hydrophobicity. The construction industry has been combining organic and inorganic components in binders for some time. This can be done in different ways. Sometimes, the mineral particles are simply mixed into an organic dispersion, or encapsulated by means of emulsion polymerisation (see Figure 2). However, these two technologies only barely achieve sufficient homogeneity of the silica in the polymer phase. This poor homogeneity can lead to the dispersion having storage stability problems, as well as crack formation and high water uptake in coatings due to silica agglomeration. Agglomerated silica in the dispersion results in large domains of silica, which are distributed in the polymer film once the dispersion has dried. In this case, cracks form easily throughout the whole polymer film due to different internal forces between the silica domains and the polymer particles surrounding them. Another problem is that if the silica is not well dispersed in the polymer phase, it will not be protected and its hydrophilicity will allow water to easily pass through the polymer film, increasing the water uptake. These phenomena can be extrapolated to paints. The new binder s development process provides excellent homogeneity of silica in the polymer phase (see Figure 2). In this mineralised binder, the acrylic polymer components are functionalised with silanes, which in turn react with the silica. In this way, the mineral and organic components are chemically bound together in a controlled way, preventing the silica from agglomerating. Transmission electron microscopy (TEM) pictures clearly show how the silica is dispersed better throughout the coating in comparison to the same paint based on a conventional composite binder (see Figure 3). EXPOSURE TEST SHOWS GREATLY REDUCED DIRT PICK-UP Primis AF 1000 VAc-VeoVa The high thermal conductivity of the mineralised dispersion allows paint and façade surfaces to dry rapidly. In addition to producing ideal barrier properties, the binder reduces the amount of dirt that coatings pick up. Thermoplasticity is one of the hallmarks of traditional acrylate dispersions, but it causes the façade to soil quickly in hot environments. The new binder technology, however, provides good elasticity at an optimum level of surface hardness, prevent- Figure 5: Colour retention of plasters containing iron oxide pigment based on the new binder and a standard styrene-acrylic dispersion, at an outdoor weathering station in Germany (60 SW / Burghausen). 3.0 2.5 2.0 1.5 1.0 0.5 0 50 60 70 80 90 100 110 120 Weeks Primis AF 1000 SA Primis AF 1000 SA
TECHNICAL PAPER 31 ing soiling by making it very difficult for dirt particles to adhere to the surface. Many outdoor weathering tests have been performed with this binder at different locations, for example at the company s technical centre in Dubai, United Arab Emirates. Figure 4 illustrates how white and coloured exterior paints based on the new binder behave in comparison to the same paints based on conventional polymer binders (styrene acrylic and vinyl acetate/ VeoVa binders). The paints contain 32% binder (50% solids content) and 18% titanium dioxide with a pigment volume concentration (PVC) of approximately 47%. Dirt pick-up is clearly lower with the new composite binder after just one year of exposure in Dubai facing at 60 to the south-east. For this test a primer for plaster, which presents a rough surface, was used in order to make the test more challenging. Sand observed on the panels after the test came from the well-known Dubai sandstorms. PIGMENT STABILITY CAN BE IMPROVED Pigments are blended into exterior paints in order to add rich, vivid colour to the walls of homes and other buildings. While organic pigments are known for their wide variety of shades, they are not always particularly stable, especially when exposed to UV radiation. As a result, they fade and usually become paler over time due to the sun and weather. Inorganic pigments, by contrast, are better able to resist outdoor conditions, but do not offer the same variety of shades and hues. The hybrid dispersion significantly increases the stability of organic pigments in paint formulations. This is because mineralisation protects organic pigments more efficiently against UV radiation the pigments lose their brilliance and intensity considerably more slowly as a result, allowing exterior paints to retain their original colour for longer. Figure 6: Contact angle test (in-house method) of binder technologies. Primis AF 1000 SA VAE-A 67.7 62.2 59.0 Untreated paint Treated paint 67.2 61.6 52.1 Figure 7: Scanning electron microscope (SEM) pictures of paints based on the new binder (left), styrene-acrylic (needle-like precipitates middle) and vinyl acetate-ethylene-acrylate binders (spongiform precipitates right).
32 ARCHITECTURAL COATINGS TECHNICAL PAPER Enhanced colour consistency and paint longevity has been verified through outdoor exposure at different locations. Figure 5 shows an example of a test run at an outdoor weathering station in Burghausen, Germany, with plasters containing 1.5 to 2.0 mm fillers. The plasters depicted in this picture contained 13% binder (50% solids content), 2% titanium dioxide and 2% Bayferrox brown 645T pigment. The Delta E values correspond to the difference in colour over time based on the original colour of the plasters and show enhanced colour retention in comparison to the same plaster based on a conventional styrene acrylic. Colour development can also be observed on the panels. The plaster based on the new binder shows excellent colour development with a more vivid brown than the plaster based on the comparison grade. SNAIL TRAILS DUE TO LEACHING ARE REDUCED What are sometimes called snail trails are a phenomenon that occurs with painted exterior walls vertical run marks seen on dry paint films. They are seen, in particular, if it has rained or after a heavy dewfall, disturbing the visual appearance of the façade paint. Snail trails can originate when water-soluble salts in the dried paint migrate to the wall surface be it via gentle rain, dew or irrigation sprinklers and then dry on the surface. This migration of salts (leaching) causes streaks that are usually randomly spaced and follow the gravitational direction. Snail trails appear not only on exterior coatings, but can also be seen indoors in wet rooms, such as kitchens and bathrooms, due to high humidity. Figure 6 shows the results of a contact angle test run in the laboratory to confirm that paint surfaces change following exposure to water. Two well-known binders for exterior paints were selected for comparison with the new binder: one styrene acrylic and the other vinyl acetate-ethylene-acrylate. For the test, paints with 42% binder (50% solids content) and 4% Colanyl Blue A2R 131 (phthalocyanine blue dispersion) were applied to Leneta test cards and cured for 24 hours at 50% relative humidity and 23 C. A droplet of water was put on the cured paint, and the contact angle between the paint surface and the water droplet was measured. The same procedure was performed after treatment of the cured paints with water for one minute and allowing the panels to dry for 24 hours. In general, a decrease of the contact angle after water treatment was observed, which confirms that the surface of the paints showed increased hydrophilicity after water treatment. This is very likely due to the migration of water-soluble salts in the paint to the surface. The smallest decrease of contact angle in this test was seen with the paint based on the hybrid binder, which suggests that it contributes less to leaching than the other binders that were tested. The change in the surface of these paints was also analysed using scanning electron microscopy (SEM). Figure 7 shows pictures taken of paint surfaces that were treated with water as previously described during contact angle measurement. The paint based on the new binder showed no change to the surface, while both paints based on other market technologies showed precipitates on their surfaces. The paint based on the styrene-acrylic binder featured needle-like precipitates, while the one based on the vinyl acetate-ethyleneacrylate binder displayed spongiform precipitates. These precipitates change the appearance of the surface of wall paints, displaying the visible marks which are known as snail trails. NMR ANALYSIS CONFIRMS LEACHING IS REDUCED In order to obtain a better comparison of binders, water extracts from the binders were analysed using nuclear magnetic resonance (NMR). For this test, polymer films of the dispersions were prepared and cured for 14 days at 23 C and 50% relative humidity. These polymer films were immersed in water for 24 hours at 23 C. Water-soluble salts coming from the different polymer films were obtained after water evaporation at 50 C. The remaining extract was dissolved in a polar solvent (deuterium oxide) and a non-polar one (trichloromethane) and analysed using NMR. Figure 8 shows NMR charts of the extracts obtained from the different polymers dissolved in the polar solvent and the chart of the polar solvent itself. Peaks on the charts correspond to substances Figure 8: Evaluation of leaching using nuclear magnetic resonance of water extracts of paints based on different binder technologies with a polar solvent. 9 8 7 6 5 4 3 2 1 0-1 ppm Primis AF 1000 VAE-A SA Polar solvent
TECHNICAL PAPER 33 You can achieve great results by using the right raw materials. Lenine de Sousa Gomes Technical Service Manager for Dispersions in Paints & Coatings Wacker Polymers lenine.desousagomes@wacker.com 3 questions to Lenine de Sousa Gomes What has to be considered when formulating with the new binder? Actually, there are no special constrains formulating with this new binder. Perhaps you should keep in mind that a part of the binder is inorganic when calculating the PVC, but just contact us if you face any problems in formulating, plus, guide formulations are also available. Which organic pigments are most problematic regarding fading? It is difficult to give a specific answer since results are very recipe-dependent. However, difficult pigments are for instance Red 254 or Violet 23. Yet, I think you can achieve great results by using the right raw materials in your formulation, including our binder. What locations and durations were chosen for the weathering tests? We have plenty of possibilities for testing in our different technical centers around the world, but in this case, we decided to choose Burghausen (Germany) and Dubai. Burghausen is our standard outdoor weathering station for Europe, and Dubai was considered in order to have a better evaluation of dirt-pick-up. Pictures from Dubai shown in this article were taken after one year exposure. that were washed out from the polymer films (leaching) or are related to the solvent used. The new binder shows excellent results with regard to leaching after this test, since peaks seen on the chart come from the solvent, while the other polymers under consideration clearly showed more peaks in addition to those coming from the solvent. Charts with a non-polar solvent were also considered, but will not be discussed here the results are comparable with those of the polar solvent already discussed. The new hybrid binder thus combines the unique properties of inorganic and mineral components. In comparison to other composite technologies, a more homogeneous distribution of silica in the dried paint film can be achieved through the combination of acrylic polymers, silane chemistry and silica sol. As a result, paints formulated with it have high dirt pick-up resistance, show reduced cracks or water uptake and minimise the formation of snail trails. The dispersion s good pigment compatibility and colour consistency can create richly coloured façades, even with critical organic pigments, that retain their brilliance for an unusually long time.
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