Comparison of 5- and 6-membered cyclic carbonate-polyisocyanate adducts for high performance coatings

Nowadays, coatings need to fulfill a variety of requirements such as having excellent mechanical, chemical, and optical properties at low baking temperatures. On a large scale, polyisocyanates, amines or melamines are used as crosslinking agents in the coatings industry. In this work, a new self-crosslinking agent based on a hydroxy functional 6-membered carbonate with high ring tension and thus presumably lower baking temperature was synthesized and the behavior as self-crosslinking agent was compared to the crosslinking agent derived from the commercially available 5-membered glycerol carbonate. The hydroxy functional 6-membered carbonate monomer was synthesized enzymatically under mild reaction conditions from commercially available substances, linked to a hexamethylene diisocyanate trimer and self-polymerized afterward. NMR- and IR-spectroscopy and GC-MS analysis were found to be suitable techniques to characterize monomers and crosslinking agents. DSC measurements were performed to evaluate appropriate reaction parameters for the attachment reaction of the 6-membered cyclic carbonate to the polyisocyanate without ring opening. The progress of self-crosslinking has been followed by characteristic changes in IR spectra as well as time and temperature-dependent changes of storage and loss modulus while oscillating rheological crosslinking. Furthermore, glass transition temperatures of the resulting coating films are determined, and sol gel analysis was performed to estimate the degree of crosslinking. After application on steel, aluminum and glass plates application tests were performed. In addition to excellent mechanical and chemical properties, the coating film showed good adhesion to the surface and was colorless. Combining these properties with relatively low baking temperatures, 6-membered cyclic carbonate crosslinking agents could represent a new technology for the coatings industry.