Determination of practical spreading rate by the volume shrinkage ratio of dry-film/wet-film of coatings (scraper method)

•A new test method for determination practical spreading of coatings is introduced.•Many deficiencies ISO 3233 have been resolved.•The operation is simple and fast, and the test results have high reliability.•The test method is applicable to the laboratory, paint shop or is also applicable. The practical spreading rate of coatings is very significant for industrial coatings. ISO 3233 is the current standard test method, it is very troublesome and have many factors affected the accuracy and stability of test results (such as mass solid fraction, liquid density, wettability of dry-film, adaptability of impregnation with dry-film, dry-film porosity, etc.). A new test method for the determination practical spreading of coatings by the volume shrinkage rate with dry/wet film is introduced in this paper. All the influencing factors of ISO 3233 have been avoided. Different from the traditional method of calculating the practical spreading by the ratio of dry and wet film thickness, the main factors affecting of the accuracy and stability can be avoided. The comparison between the test results of this method in this paper and ISO 3233, we found that the test results of this method in this paper have high accuracy, the difference between the test results is within ±2%, it is a fast, simple, intuitive test method with high accuracy, and can be used in laboratory or painting shop. This test method in this paper is applicable to almost all liquid coatings or resins on hard flat surfaces after drying, includes coatings formulated above materials which exceed the critical pigment volume concentration (CPVC) which ISO 3233 is not applicable. For coatings with 3D surfaces, larger roughness and elasticity coatings, the coating practical spreading rate can be calculated by appropriate measurement method to measure the dry-film thickness. The method in this paper is not applicable to expandable after dried coatings.