Wood surfaces protected with transparent multilayer UV-cured coatings reinforced with nanosilica and nanoclay. Part I: morphological study and effect of relative humidity on adhesion strength

Multilayer coatings that are used to protect surfaces of wood components constituting ready to assemble furniture must have good adhesion to the substrate, particularly for wood furniture used in high humidity environments like bathrooms. It is not known whether adding nanoparticles to such coatings will change their properties. Surfaces of yellow birch wood (Betula alleghaniensis Britton) were protected with six different types of multilayer coatings (MCs). Prepared primer and topcoat UV-curable formulations constituting these MCs contained, respectively, hydrophobic-fumed silica (NS) and nanoclays (NC). They were added at 0 and 0.5 wt% for NS and at 0, 1, and 3 wt% for NC. The viscosity behavior of these formulations was evaluated as a function of shear rate. The morphology of the cross-section of coated wood samples was qualitatively studied by means of both scanning and transmission electron microscopy (SEM and TEM analysis). The adhesion strength of these MCs on wood surfaces was assessed as a function of relative humidity (RH). TEM images have revealed that topcoats reinforced with NC appear to have an intercalated (micro and nanoparticles) morphology. Statistical results have shown that NS and NS x NC do not significantly confer additional adhesion strength of MCs on wood surfaces, whereas the effect of NC is significant. The adhesion strength of coated wood samples conditioned at 80% RH was higher than that obtained on those conditioned at 40% RH. Multilayer coating # 5 ([0.5 wt% NS-1 wt% NC] combination) has been chosen as the ideal protection system for two reasons: (i) high adhesion strength at low RH and (ii) good barrier properties.