Recovery of scratch deformation formed on crosslinked polyorganosiloxane films

Abstract

As consumers generally desire longer lasting scratch-free coatings, scratch resistance has become one of the most important properties of automotive coatings. Surface scratches that deform a coating but do not break it can recover over time, eventually even becoming invisible to the human eye. This phenomenon arises from elastic-plastic behavior and is called "self-recovery". This paper mainly focuses on the recovery of scratch deformations. Conventional coatings with low glass transition temperature (Tg) have fast recovery characteristics, like rubbers, but cannot attain sufficient hardness. Therefore, it is difficult to simultaneously improve both scratch recovery characteristics and hardness in conventional coatings such as 2 K urethane coatings consisting of acrylic polyols because both arise from the same causes as the mobility of the main chain of the coating. Herein, we explore the remarkable improvement of scratch recovery characteristics without sacrificing hardness by using a base resin with a siloxane main chain instead of a carbon-carbon main chain such as that of an acrylic polyol.We demonstrate that crosslinked polyorganosilsesquioxane coatings (Si-films) achieve both hardness suitable for automobile coatings and improved scratch recovery characteristics superior to those of conventional coatings (C-films). Si-films demonstrated 20% better elastic recovery index than C-films, and the Si-films recovered about twice as fast as the C-films. Scratch deformations on the Si-films almost disappeared after 15 min, even at room temperature, while C-films were needed over 24 days. Thus, coatings based on a siloxane main chain may offer dramatically improved self-recovery.