Relationship between viscoelasticity and scratch morphology of coating films

In order to make clear the relationship between viscoelasticity of coating film and its scratch resistance, a number of clear coatings were examined by surface observation and measurements of their mechanical properties. The scratches can be classified into two types by means of optical microscopic observation with attached polarizing filter, scraped fracture-type and plastic deformation-type. The ratios of the damaged areas to the whole observed areas were determined by image analysis as a measure of scratch resistance. The comparison between the relaxation spectrum of the coatings and the ratio of the scratched area showed that the degree of surface damage caused by a scratch tester which simulated car washing machine correlated with the storage modulus of the coatings G(st), at a relaxation time of 1 s. Moreover, the degree of plastic deformation by scratches depended also on S-pls the minimum strain which could cause plastic deformation in the film. (C) 2000 Elsevier Science S.A. All rights reserved.