Aesthetic enhancement of Ayous (Triplochiton scleroxylon) wood through biofunctional treatments combining plant-based dyes (Curcuma L., Iroko, Padauk) and polymer varnish modeled by RSM for increased durability in sunlight

The effects of plant-based dyes (Curcuma L., Iroko, Padouk) and an alkyd-urethane varnish on the color stability of Ayous wood (Triplochiton scleroxylon) exposed to UV light in a tropical environment were evaluated in this study. Natural aging tests, thermogravimetric analyses (TGA/DTG), and response surface modeling (RSM) were performed. The results show that Curcuma L. strongly intensifies the yellow component (Δb* = 31.68), with a noticeable color variation (ΔE* = 44.35 after 4 weeks), even in the presence of varnish. Varnished Padouk shows better color stability (ΔE* = 15.07), linked to high light absorption and a marked drop in the red component (a* from 29.25 to 6.44). Iroko stands out for its remarkable color stability even without varnish (ΔE* = 11.66), which shows how resistant it is to UV rays. The RSM model (R² = 99.8%) shows significant interactions between the type of extract, the presence of varnish, and exposure time. The varnish effectively protects the yellow component (F = 17.53), but this effect diminishes over time (β = −0.52 after 4 weeks for Curcuma L.). TGA/DTG analyses indicate that Padouk is more thermally stable (residue at 600 °C: 20%) than Iroko (15%), suggesting different responses to stress: photochemical for Curcuma, thermal for tropical woods. This method helps optimize finishing formulations and cut experimental testing by 40%, while promoting the development of sustainable bio-based coatings.