Comparative analysis of Shore D and pendulum hardness tests for thin varnish coatings on wood materials

Accurate evaluation of varnish hardness, particularly considering the polymer structure of the coating, is essential for predicting the mechanical performance, durability, and scratch resistance of wood coatings used in the furniture industry. Furthermore, the adoption of sustainable testing methods that minimize material consumption and environmental impact is important in ensuring efficient and responsible characterization processes. This study aimed to determine the most appropriate hardness testing method for thin varnish films applied to wood by comparing two standardized techniques: the Shore D hardness test (ASTM D2240-15) and the Pendulum (König) hardness test (ASTM D4366-16). Four types of varnishes (acrylic, polyurethane, water-based (non-acrylic), and cellulosic) were applied to Eastern beech (Fagus orientalis) panels under controlled laboratory conditions. Hardness values obtained from both methods were compared with scratch resistance data (ASTM D7027-20) to assess their validity. Statistical analyses (ANOVA and Pearson correlation) indicated that the Pendulum method showed a strong positive Pearson correlation with scratch resistance (r = 0.77), whereas the Shore D method exhibited a weak correlation (r = 0.22). Overall, the findings indicate that the pendulum method provides a more reliable evaluation of thin varnish coatings, as Shore D measurements are affected by indenter penetration into the wood substrate, leading to misinterpretation of coating hardness.