Effect of addition of cellulose nanocrystals to wood coatings on color changes and surface roughness due to accelerated weathering

An aqueous ultraviolet-cured polyurethane acrylate transparent resin formulation was submitted for accelerated weathering for 1200 h, on wood substrate, with and without added cellulose nanocrystals (CNC) in coatings. In previous work, addition of CNC improved several mechanical properties of the coating without changing other desirable properties such as optical transparency, color, and gloss. Measurements of nanoroughness were performed with atomic force microscopy on coated weathered surfaces. Surface roughness increased 8–10 times following weathering. Color and lightness measurements were done periodically, each 100 h, during the weathering. Testing was also done for a multilayer coating, varnish on an opaque coating on wood. Addition of unmodified, i.e., hydrophilic, CNC to transparent coatings did not downgrade the color stability of coatings but actually increased their color stability, while the effect on hydrophobically modified CNC was somewhat less. Thus, addition of CNC to coatings not only increases mechanical properties but also increases color stability of coated wood.