Multiscale evaluation of water-based organic coatings modified with medicinal and aromatic plant-derived additives using design of experiments

In this study, medicinal and aromatic plant-derived additives were utilized into water-based organic coatings to investigate their effects on film formation, surface characteristics, adhesion, and thermal behavior. Nettle (Urtica dioica), turpentine oil, cellulose ether, and camphor were added to the coating formulation at coded concentrations corresponding to 0–6 wt% total additive loading. A Design of Experiments (DOE)-guided approach was used to evaluate the formulation-dependent variation in coating thickness and to identify an optimized additive combination. The optimized and reference coatings were evaluated by combining microscale SEM morphology observations with macroscale measurements of coating thickness, FT-IR spectroscopy, contact angle, opacity, and cross-cut adhesion, together with TG-DTA analysis of thermal degradation behavior. The quadratic model provided a statistically significant fit for coating thickness, with an R2 of 0.9004, adjusted R2 of 0.8007, predicted R2 of 0.6607, and an insignificant lack of fit, indicating that the model adequately represented the experimental data (p < 0.05). The optimized formulation reduced coating removal in the cross-cut adhesion test from 55.6% for the reference coating to 13.3%, indicating a substantial improvement in adhesion performance. TG-DTA analysis showed that the total mass loss decreased from 88.45% for the reference coating to 42.71% for the optimized formulation, while the degradation temperature remained nearly unchanged at approximately 408 °C. Contact angle measurements showed a decrease from 65° to 54° after additive incorporation, indicating increased surface wettability rather than improved hydrophobicity. SEM observations further supported the formation of a more continuous and homogeneous coating morphology in the optimized formulation.