Modern 2K PU waterborne coatings: Efficiently and with high-performance - Sustainability from megatrend to business Dipl. Ing. Robert Reyer*, Dr. Martin Melchiors, Dr. Thomas Stingl, Bayer MaterialScience AG, Leverkusen, Germany (*author & speaker) Introduction The world today poses new demands. Economic and ecological advantages are one of the dictates of responsible care for a company on its way towards sustainable business practices. A steadily rising number of our customers have also come to this realization. Furthermore, analysts, investors and the general public call for it. People and the media are thus rapidly making waterborne technology a megatrend in the global marketplace. [1] Two decades ago, the economy had already recognized the growing importance of both ecology and the principle of responsible care, which is reflected so concisely in Bayer s current international slogan: Bayer: Science For A Better Life. Research began into the opportunities offered by 2K polyurethane technology for reducing solvent content. This led to the development of very high solids 2K PU (polyaspartic) coatings and the likewise low-emissions waterborne 2K PU systems, which represent an alternative to existing conventional systems. [2] Waterborne 2K polyurethane coating systems based on Bayhydrol & Bayhydur & Desmodur offer users and coating manufacturers numerous options for fulfilling the demands on low VOC, resistance and long service life of coated objects. Another positive aspect of waterborne 2K systems is that the water contained in the dispersions and the waterborne 2K PU spray coating systems is returned completely to the ecosystem by natural means, i.e. by evaporation during the drying process followed by rain. To put it another way, nature merely lends us the water and recycles it. Furthermore, in the case of waterborne systems with a correspondingly low VOC content, subsequent thermal afterburning can be eliminated depending on the field of application and the maximum annual solvent consumption. This in turn reduces CO 2 emissions as well as operating and energy costs, because additional firing with natural gas is no longer necessary.
Internal activation cuts energy costs and reduces CO 2 emissions Waterborne 2K polyurethane coatings based on a new generation of raw materials now offer even better performance. On par with solventborne systems in terms of film appearance and resistance, they have much lower solvent content, and in addition to significantly reducing greenhouse gas emissions, they can also save time and energy during drying because of the lower oven temperature, thus offering end users another cost advantage. This is achieved through the use of what is known as internal activation. But what is it? When users want high productivity in the coating step and therefore short curing/drying times, conventional 2K PU coating systems usually contain DBTL or other similar external catalysts to accelerate crosslinking/drying. But these have one major disadvantage: They significantly shorten pot life. Plus, organic tin compounds are considered ecologically harmful. Internally activated dispersions present an entirely different picture. In this case, harmless activation centers are incorporated virtually directly in the dispersion. Compared to conventional catalysts, they offer the advantage of not shortening pot life despite significantly accelerating drying. [1] Depending on the application and object, the oven temperature can therefore be lowered significantly compared to a solventborne coating system, thereby reducing both the end user s energy costs for drying and CO 2 emissions. Thanks to their good compatibility, internally activated dispersions can be combined with conventional PAC and PU dispersions, making it possible to specifically set the desired drying rate. The fact that this involves a chemical reaction and not physical drying is demonstrated by measuring the remaining free NCO groups from the low-viscosity aliphatic polyisocyanate hardener after forced drying (30 min. at 60 C) using infrared spectroscopy. In a comparison of a waterborne 2K PU system based on a standard PAC dispersion with a coating based on an internally activated PAC dispersion, IR analysis shows that in the coating based on the standard PAC dispersion, 70% of the NCO groups are still not crosslinked. In other words, the resistance characteristics to aggressive media or water have not yet developed at this point in time. In contrast to the coating based on the internally activated PAC dispersion, all OH groups available for crosslinking are already fully crosslinked with the aliphatic polyisocyanate hardener, and only 30% of the NCO groups are not crosslinked. They stem from the 30% overcrosslinking of the coating system (NCO:OH ratio 1.3:1) and, as the process continues, they react completely with moisture in the air to form polyurea [Fig.1].
[Fig. 1 - NCO reduction during baking (IR measurements)] Self-healing effect makes microscratches disappear New production processes can be used to manufacture solvent-free, OH-bearing polyurethane dispersions. With the proper choice of raw materials for the dispersion design, these dispersions produce coating films with very advantageous characteristics. One possibility, for example, is a self-healing effect. After scratching, the coating film exhibits reflow, meaning that the surface, previously dulled by microscratches, repairs itself, regaining almost completely its original gloss and eliminating the microscratches as a function of ambient temperature and time (2h at 60 C, e.g. from exposure to the sun, can restore up to 98% of the original gloss) [Fig. 2]. [Fig. 2 - Self-healing comparison test]
Furthermore, coating films of this kind are hard but highly flexible, even at very low temperatures. This special set of characteristics makes the use of this dispersion (PU 1) particularly attractive for applications such as coating smart phones [Fig. 3], [Fig. 3 Smart phone] laptops and tablet PCs. Interest among owners of such upscale and correspondingly expensive devices in a self-healing coating is very great, because the coating preserves the elegant appearance of a device and thus helps to maintain its value. Another application is automotive and transportation coatings, microscratches caused by using the car wash can no longer worry you. When a vehicle is cleaned in a car wash, microscratches can be caused, for instance, by brushes that are damaged or contaminated with fine sand, or when dried with dirty fabric or chamois cloths [Fig. 4].
[Fig. 4 - Amtec Kistler test/one test cycle simulates 80 to 100 normal car washes] The coatings can be applied to vehicle parts made of either metal or plastic. Self-healing is achieved by a targeted combination of a densely crosslinked network (required for the necessary resistance to environmental influences, such as tree resin, acid rain, insect secretions and aggressive bird droppings), flexible hard/soft segments between the linkage points and a high fraction of hydrogen bridge bonds in the crosslinked film. These ensure that in the event of plastic deformation (microscratching) of the coating film, the hydrogen bridge bonds release and the film can yield flexibly. At a moderate temperature or when heated to between 30 C and 60 C (motor vehicle surfaces reach these temperatures even in the temperate climate zones due to exposure to the sun), this process is reversible on a molecular level; the hydrogen bridges reform and the microscratches heal themselves. The process can also be triggered or accelerated simply by holding a household hair dryer or infrared lamp to the scratched area for a few minutes [Fig. 5].
[Fig. 5 Self-healing test with an infrared lamp on a model car] High gloss and high resistance Modifying the molecular design of the polyurethane dispersion produces another polyurethane dispersion (PU 2) that not only exhibits very fast hardening and drying in the coating film, but also possesses very good resistance to thermal yellowing under extended exposure. Also worth to mention is the exceptional high gloss of such clearcoats and topcoats, resulting from excellent pigment wetting and good leveling of the films. Thanks to its good compatibility, this new polyurethane dispersion is also very well suited as a combination partner for polyacrylate dispersions. This can significantly increase the gloss of coating systems based on the PAC dispersion [Fig. 6], and achieve good gloss retention in short-term and outdoor weathering [Fig. 7 and Fig. 8]. [3]
[Fig. 6 - Gloss of waterborne 2K topcoat, white] [Fig. 7 - Xenon test, waterborne 2K clearcoat]
[Fig. 8 - Outdoor weathering, waterborne 2K topcoat]. The field of application of this PU dispersion ranges from classical metal coating to wood and furniture coating, as it also offers high chemical resistance in correspondingly formulated clearcoats and topcoats. We tested for example the 16-hour resistance of a waterborne 2K clearcoat formulated with this PU dispersion and dried at room temperature against red wine, coffee, ketchup, mustard and mashed carrots (the parents of toddlers will be all too familiar with the coloring effect of mashed carrots!) [Tab.1] [Fig. 9]. [Tab.1]. 16-hour resistance test, waterborne two-component PU clearcoat Test substance Result Red wine No discoloration /no defects Coffee No discoloration /no defects Ketchup No discoloration /no defects Mustard No discoloration /no defects Mashed carrots No discoloration /no defects
[Fig. 9 - Resistance test] Summary Waterborne 2K polyurethane coatings based on the new generation of cosolvent-free, OHfunctional polyurethane dispersions and internally activated polyacrylate dispersions are excellently suited for the use in transportation (e.g. trains, trucks, buses and ACE market) clearcoats and topcoats as well as for metal, plastic and wood substrates. Depending on the coating formulation, they prove to be hard, yet still flexible, even in cold weather. Additional values to mention are the corresponding coatings are high-gloss, weather resistant, highly chemical resistant and, depending on the dispersion used, characterized by a pronounced selfhealing effect after scratching or are very fast-drying, meaning the oven temperature for drying coated objects can be reduced to cut energy costs and CO 2 emissions. These new PU and PAC dispersions are already used in the field of general industrial, wood and furniture coating. The dispersions presented here make it possible using waterborne products to fulfill or even exceed the requirements imposed on coatings in market segments previously dominated by conventional coating systems but with a much lower VOC content. We expect a significant expansion in the use of waterborne polyurethane coatings in the medium term. These new building blocks therefore will add a new chapter to the overall success story of aliphatic 2K polyurethane coatings.
References [1] BMS Toolbox for waterborne 2K polyurethane coating systems 03/2010 [2] R. Reyer VOC reduction at the cost of efficiency? Proceedings of the VILF Annual Meeting 11/2004 [3] M. Melchiors, T. Stingl, R. Reyer New components for a green paint technology Phänomen Farbe 06/2010 Acknowledgement I would like to thank my colleagues Dr. Martin Melchiors, Dr. Thomas Stingl and the whole lab team for providing the experiment results and the good discussion.