Impact of temperature and humidity on the performance of thermally expandable particles modified adhesives

Fiber reinforced composites offe great lightweight potential, but need to be joined in a way to avoid damaging the fiers and to maintain the strength. Structural adhesives can be advant-ageous for this purpose, since they offe a high degree of design freedom, enable non-destructivejoining of differet materials and also have increased lightweight construction potential. Onechallenge of adhesive bonding occurs at the end of the product life cycle, as debonding is usuallyimpossible or can only be achieved at great effor and expense. For this reason, there is anincreasing demand for adhesives that can be easily debonded after use. In this master thesis,an epoxy-based and a polyurethane-based structural adhesives were modifie with two different thermally expandable particles in differet weight concentrations (4wt%, 8wt%, 12wt%,16wt%, 20wt%, 25wt%) and investigated with regard to the influenc of the particles on thedebonding ability (debonding on demand). Differet mechanical and thermal analyses were usedfor this purpose. It was demonstrated that the polymer network density, the adhesive strength,its elongation at break and its thermal properties have a significat influenc on the expansion ofthe particles. In experiments performed with the epoxy adhesive, no expansion of the particlescould be observed. The inflence on the thermal behavior determined with thermomechanicalanalysis, thermogravimetric analysis and differetial scanning calorimetry was also minor. Themechanical tests, in contrast, showed an increase in tensile strength after temperature stimulus. Thus, there is no debonding on demand potential for this adhesive-particle combination.However, thanks to the low density of the particles some potential applications, like lightweightdesign, can be further investigated. The combination of the polyurethane with the particlesshowed an effec on the polymer matrix in the thermal analysis even at a low weight concentration of 4wt%. This effec increased with increasing particle content. Effect on tensile propertiesbecame significat at a concentration of 12wt%. The firs potential for debonding on demandwas shown by the addition of particles and the application of a thermal stimulus, as tensilestrength decreased. Subsequently, lap shear tests were performed using fier reinforced compos-ite substrates and the polyurethane adhesive with differet weight concentrations of thermallyexpandable particles. These tests were conducted to analyze the influenc of the particles onthe mechanical properties of the bonded joint. A decrease of the lap shear strength was observed after application of the thermal stimulus only for concentrations higher than 16wt%. Inaddition, specimens were exposed to differnt relative humidities to investigate a possible agingeffec caused by water absorption. The water absorption of the polyurethane with and withoutthermally expandable particles had no significat effec on the lap shear strength.