Presidential Green Chemistry Challenge: 2000 Greener Reaction Conditions Award
Bayer Corporation (now Covestro)
Two-Component Waterborne Polyurethane Coatings
Innovation and Benefits: Bayer developed a series of high-performance, water-based, two-component polyurethane coatings that eliminate most or all of the organic solvents used in conventional polyurethane coatings. Bayer's water-based polyurethane coatings reduce volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions by 50–90 percent.
Summary of Technology: Two-component (2K) waterborne polyurethane coatings are an outstanding example of the use of alternative reaction conditions for green chemistry. This technology is achieved by replacing most or all of the volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) used in conventional 2K solventborne polyurethane coatings with water as the carrier, without significant reduction in performance of the resulting coatings. This may seem an obvious substitution, but, due to the particular chemistry of the reactive components of polyurethane, it is not that straightforward.
Two-component solventborne polyurethane coatings have long been considered in many application areas to be the benchmark for high-performance coatings systems. The attributes that make these systems so attractive are fast cure under ambient or bake conditions, high-gloss and mirror-like finishes, hardness or flexibility as desired, chemical and solvent resistance, and excellent weathering. The traditional carrier, however, has been organic solvent that, upon cure, is freed to the atmosphere as VOC and HAP material. High-solids systems and aqueous polyurethane dispersions ameliorate this problem but do not go far enough.
An obvious solution to the deficiencies of 2K solventborne polyurethanes and aqueous polyurethane dispersions is a reactive 2K polyurethane system with water as the carrier. In order to bring 2K waterborne polyurethane coatings to the U.S. market, new waterborne and water-reducible resins had to be developed. To overcome some application difficulties, new mixing/spraying equipment was also developed. For the technology to be commercially viable, an undesired reaction of a polyisocyanate cross-linker with water had to be addressed, as well as problems with the chemical and film appearance resulting from this side reaction. The work done on the 2K waterborne polyurethanes over the past several years has resulted in a technology that will provide several health and environmental benefits. VOCs will be reduced by 50–90 percent and HAPs by 50–99 percent. The amount of chemical byproducts evolved from films in interior applications will also be reduced, and rugged interior coatings with no solvent smell will now be available.
Today, 2K waterborne polyurethane is being applied on industrial lines where good properties and fast cure rates are required for such varied products as metal containers and shelving, sporting equipment, metal- and fiberglass-reinforced utility poles, agricultural equipment, and paper products. In flooring coatings applications where the market-driving force is elimination of solvent odor, 2K waterborne polyurethane floor coatings provide a quick dry, high abrasion resistance, and lack of solvent smell (<0.1 pound organic solvent per gallon). In wood applications, 2K waterborne polyurethane coatings meet the high-performance wood finishes requirements for kitchen cabinet, office, and laboratory furniture manufacturers while releasing minimal organic solvents in the workplace or to the atmosphere. In the United States, the greatest market acceptance of 2K waterborne polyurethane is in the area of special-effect coatings in automotive applications. These coatings provide the soft, luxurious look and feel of leather to hard plastic interior automobile surfaces, such as instrument panels and air bag covers. Finally, in military applications, 2K waterborne polyurethane coatings are being selected because they meet the demanding military performance criteria that include flat coatings with camouflage requirements, corrosion protection, chemical and chemical agent protection, flexibility, and exterior durability, along with VOC reductions of approximately 50 percent.