Life on earth began more than three billion years ago. Evolution had almost as much time to optimize biominerals like bone, dental enamel or mother-of-pearl. These composite materials produced by nature combine the hardness of crystalline minerals like hydroxylapatite or aragonite with the flexibility of organic substances such as collagen or chitin, making them some of nature's most stable materials. These properties are mainly the result of fine structures measuring only a few nanometers, in which inorganic material and organic "glue" bond together.
Col.9®, a novel nanobinder from BASF for facade coatings, emulates this example provided by nature. COL.9 is a dispersion of organic plastic polymer particles in which nanoscale particles of silica, the basic constituent of glass and quartz, are incorporated and evenly distributed. Thanks to this combination of elastic organic material and hard mineral, COL.9 based coatings combine the different advantages of conventional coating types. For example, unlike brittle, mineral based coatings, the widely used synthetic resin based dispersion paints are highly crack-resistant. But in summer, when dark house walls reach temperatures of 80C and above in the sun, these coatings betray their weakness: on exposure to heat the synthetic resin begins to soften, and particles of soot and other contaminants stick to their surface. Because of its high silica content, however, the nanocomposite of COL.9 doesn’t have this thermoplastic tackiness. At the same time, the mineral particles provide the coating with a hydrophilic i.e. water attracting surface on which raindrops are immediately dispersed. As regards cleanliness, this offers a dual benefit: in heavy rain, particles of dirt are washed off extensively from the facade surface. Also, the thin film of water remaining when the rain has stopped dries extremely quickly, which prevents mold formation. In contrast, the rain rolling off unevenly in thick droplets from water-repellent surfaces of fully synthetic resin coatings often leaves behind unattractive streaks of dirt.
"Significantly improved dirt pick-up resistance was the main objective in developing COL.9," says Oliver Wagner of BASF's Marketing Polymer Dispersions for Architectural Coatings. "Because especially in large conurbations, the high levels of carbon black and dust pollution rapidly lead to soiled building frontages and high renovation costs." Permanently incorporating the mineral nanoparticles in the much larger acrylate polymer particles, however, means that COL.9 based coatings offer even more advantages, explains Wagner. "The color tone remains stable and there is no surface chalking even after years of exposure to weather. And while COL.9 shares the properties of hardness and low dirt pick-up with silicate coatings, its flexible, organic content means it doesn't have their major drawback – their susceptibility to cracking that can rapidly lead to the entire coating flaking off".
COL.9 has demonstrated its outstanding properties in extensive hardness tests in the laboratory, but also under outdoor conditions. For these practical trials, weathering stations with a COL.9 coating were constructed in large cities in China and Indonesia and at other locations where a facade coating's two most resolute enemies – atmospheric pollution and the sun's UV radiation – are especially intense. In these trials, aligning the test surfaces towards the south at an exposure angle of 45° is known to accelerate the weathering processes 2.5 fold.
Nevertheless, a development period of almost ten years was required to progress COL.9 to market. "The biggest challenge was to combine the mineral nanoparticles homogeneously and inseparably with the organic dispersion to produce composite particles," explains Wagner. Because only this approach ensures that the nanoparticles will remain homogeneously fixed instead of agglomerating when the liquid coating is formulated with water and color pigments. This is the decisive factor for the formation of regular, stable nanostructures on the surface of a COL.9 coating after it dries: they reinforce its hydrophilic properties and in particular provide its high dirt repellency.
After several years spent passing through BASF's development laboratories, the first COL.9-based facade coating is now on the market: Herbol-Symbiotec® from Akzo Nobel. "This new flagship of our extensive range of coatings, freely available from specialized trade outlets since early 2008, is the result of successful cooperation between Akzo Nobel and BASF," says Andreas Götz, Head of the Deco Application Technology Laboratory at Akzo Nobel. Before the coating, which consists to about one half of COL.9, could be launched on the market, Herbol-Symbiotec again passed through an intensive development phase which was completed in 2007 with a broad-based practical trial by selected professional painters and an expert evaluation by the Max-Planck Institute of Colloids and Interfaces in Potsdam near Berlin. "Using electron microscopic techniques, we were able to fully confirm the composition stated by Akzo Nobel, especially the nanoparticles embedded in larger polymer particles," says Prof. Dr. Markus Antonietti, Director of the Institute's Department of Colloid Chemistry. "After drying, nanostructures known to be dirt repellent form on the surface of the coating."
Its exceptional properties guarantee the nanobinder COL.9 a great future: as well as being used in facade coatings, the product will in the foreseeable future also be included in water-based wood stains, for which its balanced ratio of hardness and elasticity make it ideally suited. BASF's scientists are currently focusing their efforts on optimizing the blocking resistance, i.e. achieving the lowest possible tendency of two coated wood components to stick together. This is especially important for window and door manufacturers to allow them to stack the coated parts as quickly and easily as possible in the warehouse. But house occupants also attach great value to knowing that windows and doors can be easily opened and closed even when outdoor temperatures are high in summer.
Floor coatings subject to heavy stress, for example in multi-story car parks, can also benefit from COL.9 – especially in exposed outdoor areas where weathering and dirt resistance are particularly important. The sales potential is high: the market for nanodispersions as binders for coatings is likely to reach about Euro 50 million in the next ten years.