Feb 22 2007
Waterborne polyurethane coatings are increasingly gaining acceptance in the market, in particular for the trade coating of parquet floors. They emit only very small amounts of volatile organic compounds (VOC), are nearly odorless and have since achieved a property level nearly identical to that of their solventborne counterparts. However, there are almost no polyurethane dispersions available today that do not contain N-methyl pyrrolidone (NMP). The responsible working group of the European Union has decided that formulations containing more than five percent of this substance must be classified as toxic in the future. The coating raw materials experts at Bayer MaterialScience went looking for alternatives, and they were successful: Thanks to the waterborne binders of the latest generation, Bayhydrol® XP 2557, XP 2558 and XP 2593/1 it is now possible to formulate NMP-free wood coatings having properties that equal or surpass those of conventional systems.
"Particularly noteworthy is that the new polyurethane dispersions require less cosolvent to form excellent coatings than do the products containing NMP," said Dr. Christoph Irle, a wood coatings expert at Bayer MaterialScience. The resultant coatings are characterized by excellent resistance to abrasion and high mechanical resistance. The Bayer developers created the new polyurethane dispersions by replacing the usual prepolymer mixing process, in which NMP serves as a solvent, with the acetone process.
This required adjusting the recipes for the dispersions to account for the lower solvent power of the acetone, which is distilled off following formation of the prepolymer to yield a solvent-free product.
Another innovation for parquet applications is the new NMP-free, low-molecular-weight polyacrylate dispersion Bayhydrol® XP 2651. Waterborne two-component systems formulated with it offer outstanding chemical resistance and high gloss. The product can be used alone or in combination with new NMP-free polyurethane dispersions as the A component in two-component systems. The mechanical resistance and appearance of the coating can be adjusted as desired by varying the mixing ratio of these components.
The new polyacrylate is a secondary dispersion created by dispersing a resin that is produced by polymerization in an organic phase. Because its molecular weight is similar to that of solventborne acrylic resins, two-component systems formulated with it offer the same advantages as the corresponding solventborne coatings: outstanding chemical resistance, good transparency and high gloss. "It can be used to create the ‘wet look’ - you get the impression that you are looking at shiny wet parquet," said Irle.