Scratch/mar, surface structure and rheology measurements

This contribution yields a brief review of intensively discussed topics over the years in the field of physical characterization of coatings with focus on: scratch resistance, surface structure and rheology measurements. Scratch/mar is one of the most important physical properties to characterize the mechanical quality of a coating system. The brushes and dirt in a car wash, for example, produce scratches measuring only a few micrometers in width and up to several hundred nanometers in depth. With this background several measuring methods have been discussed over the last 15 to 20 years in the automotive and coatings industry. Procedures that create a single scratch have been developed and improved at the end of the 1990s (micro or nano scratch method). These methods are different from more practically oriented procedures that are based on relatively simple methods to try to test or even come close to reality (e.g. car wash brush method). The methods are reviewed briefly considering also other physical properties (e.g. cross-linking density). In addition to color, effect and gloss, the visual impression of a painted surface is influenced especially by the surface structure (leveling, waviness, orange peel, appearance). To characterize the structure of a surface, mainly two different measuring methods (profilometry and 'wave-scan') have been established in the automotive and coatings industry over the years. For example, the mechanical profilometry allows detailed investigations concerning the surface topography from substrate to topcoat. Basic relations and relevant application examples from the areas 'metal, plastics, coating', investigated with profilometry and 'wave-scan' in the course of time, will be summarized. Many application and technical properties of coatings are influenced by their flow behavior. High product quality can only be guaranteed by an exact knowledge of the rheological behavior of the coating and the used raw materials, respectively. In view of the increasing use of waterborne systems flow anomalies as thixotropy, yield points or also viscoelastic behavior can be observed more often. Yield point and thixotropy influence important materials properties as storage stability, pumping behavior or leveling and flowing. Against this background the measuring possibilities of characterizing the rheological properties with rotational rheometers concerning yield point and thixotropy will be presented.