Mechanical properties of wood adhesive bond lines tested in service classes according to Eurocode 5

Depending on the expected use of timber structures and wood-based panels jointed with adhesives, Eurocode/European standard defines various service classes for the design of buildings, where different moisture ranges might be expected. As a result, it can be assumed that the amount of moisture in the adherend has a significant impact on wood-adhesive bond lines. In-situ nanoindentation was used to investigate the influence of moisture on mechanical properties in service conditions for commonly used structural and non-structural wood adhesive systems (polyvinyl acetate adhesive PVAc, a flexible two-component melamine-urea-formaldehyde adhesive MUF, urea-formaldehyde adhesive intended for form pressing applications UF A, and particle board urea-formaldehyde adhesive UF B, and a phenol-resorcinol-formaldehyde adhesive PRF). The urea-formaldehyde adhesive intended for form pressing applications and PRF adhesives displayed better stability in elastic behaviour and very little change in creep behaviour at elevated relative humidities. The thermoplastic adhesive PVAc, particleboard urea-formaldehyde (UF A), and MUF adhesive systems with flexible thermoplastic components showed lower stability with increasing relative humidity levels above 66%. The two-component MUF adhesives showed increasing creep with increasing flexible second component at elevated relative humidities. The pre-polymerized adhesive PVAc turned out to be the most sensitive one to moisture.