Polyurethane technology is based on the combination of polyisocyanate crosslinkers with hydroxy-functional polymers, such as polyesters, polyacrylates, polyethers and increasingly also polycarbonates.
Fig. 6: Polyurethane formation
Hydrogen bond formation in polyurethane coating
The polyols are first converted into the stock coating and, due to the high reactivity of the isocyanate groups, are only mixed with the polyisocyanate crosslinker just before processing, i.e. they are processed as a two-component coating. Depending on the polyol used and the crosslinker selected, the coating properties, such as gloss, flow, scratch resistance, chemical resistance (e.g. to cleaning agents) and light and weather stability, can be adjusted as desired. A tailored formulation results in, for example, the production of films with "easy-to-clean" or "soft-touch" properties. A crucial factor for the property profile is the polyurethane network formed. The urethane bonds are stable to hydrolysis and saponification. The formation of hydrogen bonds between the urethane groups is particularly important. These bonds contribute to the enhancement of the resistance properties and result in so-called reflow properties, i.e. plastic scratches of the polyurethane coating are substantially reversible ("self-healing"), initiated by the recovery forces of these bonds, above the glass transition temperature (Tg). This principle is utilized, for example, in two-component polyurethane automobile clear coats for improved scratch resistance. An example for self-healing products is Desmodur® XP 2679. Find out more.
Fig. 8: A self-healing two-component polyurethane clear coat with high reflow ability