Active behavior of poplar wood bilayers in response to changes in relative humidity

Hygromorphic wood bilayers are innovative active systems consisting of two thin wood layers bonded together with grain perpendicular to each other. The active layer, highly responsive to changes in relative humidity, induces curvature; the passive layer, less responsive, regulates the deformation. Until now, research has focused on active layers made of mid-high density woods in search of high responsiveness. However, low-density woods can be suitable for applications that require limited curvature, or for varying the composition of bilayer panelling. In this study, rotary cut veneers of poplar (Populus x canadensis clone ‘I-214’) wood, an important low-density wood for the plywood industry, were used for the active layers. Beech (Fagus sylvatica L.) veneers were also tested for comparison. Cherry (Prunus avium L.) quarter cut sliced veneers were always used for the passive layers. Bonding was realized by one-component polyurethane (PUR) or two-component polyvinyl acetate (PVA) adhesive. The specimens were subjected to 24 h adsorption and desorption, and to 144 h cyclic testing, monitoring moisture content and curvature. Two-way ANOVA showed that curvature change of beech bilayers was significantly higher; nonetheless, the degree of curvature of poplar ones can be considered suitable for various applications. Cherry veneers proved suitable for passive layers. The PUR adhesive enabled greater curvature than the PVA one, which can be attributed to the stiffer bonding formed. Overall, this study offers new knowledge on the behaviour of active layers made with low-density woods, which can support future modelling and development of active products.