The dependence of pendulum hardness on the thickness of acrylic coating

Pendulum hardness, which is widely used in the characterization of organic coatings, depends greatly on the thickness of the coating. However, it is still unclear whether a qualitative or quantitative relationship exists between pendulum hardness and coating thickness. In the present article, the pendulum hardness values of acrylic coatings with different thicknesses are measured using a König pendulum hardness tester to clarify the dependence of pendulum hardness on thickness of coating. The results show that the pendulum hardness of acrylic coatings decreases gradually with the increasing thickness of coating within a thickness range, and the sensitivity of pendulum hardness to thickness of coating depends greatly on the glass transition temperature of the coatings. An equation suitable for describing the relationship between logarithmic decrement of the amplitude of pendulum's oscillation and thickness of coating is presented, which can separate the contributions of substrate and coating on the logarithmic decrement. This study demonstrates that the measured value of pendulum hardness is not the bulk hardness value for the coating but the representative value of the system consisting of substrate and coating. An excellent correlation between pendulum hardness and thickness of acrylic coatings is obtained, which is fairly supported by the experimental data.