Low-temperature curing of adhesives – Large-scale experiments

In contrast to mechanical joining techniques, which can be manufactured under any ambient conditions, adhesively bonded joints always require some minimum temperatures – typically above 10 °C – in order to cure safely. Due to this limitation, either many technical applications exclude adhesives right from the start or practitioners at high costs are forced to adapt their production processes to the specific environmental requirements of the polymers to be cured. As a response to these manufacturing challenges, a new process for accelerated curing – induction heating with Curie particles (CP) – was studied to cure three different structural adhesives (two epoxy resins and one polyurethane) at low temperatures. The CP are added to the adhesives as a typical filler material and can only be heated by an externally applied electromagnetic field up to their material-specific Curie temperature Tc. Thus, a self-regulating process is created, which effectively prevents the adhesives from overheating while simultaneously eliminating the need to control curing temperatures by external measures like IR cameras. Recently, the authors published a series of papers in which CP were used to cure at ambient temperatures (+23 °C); the results of which are now to be extended towards low-temperature curing. For that, an experimental campaign was carried out using the exemplary connection type of Glued-in Rods (GiR), which are frequently used in various timber engineering applications. In detail, the heating behaviour of inductively cured GiR conditioned at –10 °C and –30 °C was compared to GiR inductively cured starting from +23 °C. In addition, reference sets with and without added CP were produced, whereby all GiR were subjected to tensile tests in the following. Afterwards, the series were compared on the level of load–displacement curves, failure loads, as well as observed fracture patterns. The results proved that CP-curing is capable of curing adhesives in temperature ranges currently not intended for structural adhesives by technical regulations. When cured under low-temperature conditions, the GiR involving epoxy adhesives showed a fracture behaviour that proved to be equal to that of the references. The failure loads of the 2 K-PUR dropped to 50–75% of the reference values, which, however, is strongly assumed to be related to moisture on the cold adherends and not to a general unsuitability of polyurethane adhesives for the process – an assumption that will be validated by low-temperature experiments using single lap shear joints presented in a subsequent study.