CURING BEHAVIOUR OF STRUCTURAL WOOD ADHESIVES - PARALLEL PLATE RHEOMETER RESULTS

The curing speed of a wood adhesive directly determines the required clamping time. Therefore, the curing time has a considerable influence on the production time and consequently on the efficiency / productivity of wood gluing factories. Conventionally, the curing speed is laboriously determined via lap shear tests according to EN 302-6 and EN 15416-5 for phenolic and aminoplastic and one-component polyurethane (1K-PU) adhesives, respectively. Due to moisture exchange between adhesive systems and wood adherends, only plate rheometers that are equipped with wooden plates offer the possibility to monitor the visco-dynamic curing process of a wooden bond realistically. This paper compares the results of rheological measurements that were performed during the curing of four different adhesive types with shear strength results. Epoxy and – with limitations – moisture curing 1KPU adhesives corresponded well with the theoretical curing behaviour, with the occurrence of a gel point at the crossing of the loss modulus G’’ and storage modulus G’. In contrast, Polycondensation and EPI adhesives displayed a completely different curing behaviour, most notably that G’ was larger than G’’ throughout the entire curing time. The rheological curing data showed a good correlation with regard to the shear strengths for the Epoxy, 1K-PU and MUF adhesives but not for the EPI adhesive. The difference in behaviour of the EPI adhesive could potentially be a result of its special chemistry.