Moisture related elastic and viscoelastic behaviour of wood adhesives by means of in-situ nanoindentation

The adhered wood in multi-layered panels is subjected to different stress and strain states as a result of moisture induced swelling and shrinking. Consequently, it is expected that the adhesive layer has to compensate these stresses and responds beside elastic with viscoelastic deformation. To better understand the humidity dependent mechanical behaviour of commonly used cured wood adhesives, different adhesive systems that are currently used in furniture and flooring industry (emulsion polymer isocyanate EPI, polyvinyl acetate PVAc, urea formaldehyde UF and ultra-low emitting formaldehyde amino adhesive ULEF) were investigated by means of in-situ nanoindentation. Therefore, spruce (Picea abies) lamellas were adhesively bound under laboratory conditions. Micro-mechanical properties of adhesive layers, such as elastic modulus, creep and relaxation were investigated in quasi-static mode at constant temperature and different controlled humidity levels to monitor the mechanical response of the cured adhesive polymers in the corresponding ambient. As a general trend, all adhesives respond with decreasing elastic modulus to increased humidity. EPI revealed the most stable elastic behaviour to moisture. In contrast PVAc, UF and ULEF show significant differences with changing relative humidity levels. Lowest creep and relaxation was observed for UF and ULEF, followed by EPI. The most sensitive behaviour to moisture was observed for PVAc adhesive polymer.