The bending of wooden bilayers in response to the change of relative humidity can be utilized to build autonomous actuation devices. For bending, the properties of the adhesive may play a crucial role in terms of humidity transfer and mechanical performance. Hence, the type of the adhesive may have an impact on the bilayer responsiveness upon humidity changes. In this study, bilayers with various sizes were fabricated and glued with five different adhesives. The same samples were exposed at first to a single desorption phase for 48 h and in a second experiment to ten de-/adsorption cycles with a cycle length of 24 h. The initial moisture contents (MC) of the bilayers differed depending on the type of adhesive. Due to the additional water uptake of the wood layers bonded with water introducing adhesives (PVAC, MUF and PRF), these bilayers had a higher initial MC than those bonded with non-water introducing adhesives (PUR and Epoxy). During the de-/adsorption cycles, the overall amplitude of bending stayed relatively constant. However, the start- and the endpoints of the movement were shifted in the first three cycles, since here, the cycle length was not sufficiently long to reach an equilibrated MC state. However, after this initial adjustment phase, the bending of the wooden bilayers was reversible and repeatable. No significant differences in the kinetics of bilayers manufactured with the different adhesive systems could be observed. Thus, the specific adhesive properties do not seem to influence the responsiveness and performance of the wooden bilayers.
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This work was funded by the Deutsche Forschungsgemeinschaft (DFG) priority program SPP 1420: “Biomimetic Materials Research: Functionality by Hierarchical Structuring of Materials” and by the Swiss National Foundation (SNF) Project 163191 “Smart shape-changing wood elements for improved energy efficiency of buildings” which is gratefully acknowledged. We would like to thank Sanja Kostic for the suggestion of the epoxy adhesive components ratio. Furthermore, the following persons are acknowledged as well: Thomas Schnider for cutting the spruce and beech samples, Marie Claire Graf for the help during the sample manufacturing and Erik Bachtiar for support during writing of the MATLAB code.
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