Influence of moisture on stress–strain behaviour of adhesives used for structural bonding of wood

Tensile strength, Young's modulus and stress–strain behaviour of adhesive films were investigated by means of tensile tests at 23 °C under various ambient moisture conditions (5–95% RH, water exposure and redrying). The adhesive films were produced from two one-component moisture-curing polyurethane adhesives (1C PUR), one phenol–resorcinol–formaldehyde resin (PRF) and one melamine–urea–formaldehyde resin (MUF). These four adhesive films are commonly used for structural bonding of wood. In addition, films were made from three non-commercial 1C PUR prepolymers, all of which had their ethylene oxide (EO) proportions specifically modified. For all the tested adhesives other than PRF, the findings of the tensile tests revealed a linear dependency of tensile strength and Young's modulus on the relative humidity (RH). Both parameters decreased significantly with increasing RH. The redried samples illustrate the reversibility of this effect. These observations are mainly attributed to physical bonds like hydrogen bonds, which are disrupted by water molecules entering the polymer film and re-established whilst re-drying. No evidence was found for an influence of the EO content of the prepolymers on their tensile strength or Young's modulus at high RH. Regarding the 1C PURs at high RH the findings revealed an influence of hydrophilic catalyst on tensile strength, but not on Young's modulus. Under all tested ambient conditions, the fracture strain of PRF and MUF specimens remained below 5%, whereas that of the 1C PURs and the prepolymers reached at least 20%. This illustrates the ductility of the tested 1C PUR polymers on all tested climate stages in contrast to the brittleness of MUF and PRF polymers.