Thermo-hygro-mechanical characterization and continuum-mechanical modeling of fast curing polyurethane adhesives

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In the industry, polyurethane adhesives are frequently used. They exhibit elastic behaviour at room temperature and can tolerate large deformations. In addition to moisture-curing one-component systems, two-component polyurethanes can also be found. The former ones are often boosted with blended moisture and, thus, are also two-component adhesives. In moisture-curing systems, an isocyanate-terminated prepolymer reacts with water (from the environment or from the booster) over the carbamic acid and the elimination of carbon dioxide to amine, which reacts with other isocyanate groups to form urea groups. 2K systems without moisture-cure often consist of polyol mixtures in the resin component and a liquid isocyanate-terminated prepolymer in the hardener component. The cross-linking of pure moisture-based systems is controlled by diffusion; the curing takes place from the surface of the bond to the bulk and the rate of cure decreases with time, especially after the formation of a skin of cross-linked polymer. Due to the introduction of boosters the reaction starts, similar to all 2K systems, after mixing of the components throughout the entire adhesive bulk. Then, the working life in which all technical operations related to the bonding must be done begins. Crosslinking and thus the working life depend on the temperature such that the curing can be controlled via temperature control. In the industry, there is considerable interest to increase the speed of curing processes. By the reduced working life and the acceleration which is caused by increasing the temperature, the joining process must be completed in a very short time which can cause problems during the positioning of the parts to be joined. It is desired, after a long open time (for moisture-curing 1K systems, before the formation of the skin) or a long pot life (2K systems), only after positioning of the components to increase the reaction rate by targeted heat input. It is possible to heat the metallic parts to be joined inductively in very short times to the required temperature. At the boundaries of the bond, heating rates between 100 K/min and 100 K/min can be reached. This approach is referred to as rapid curing. Due to the poor thermal conductivity of the adhesives, temperature gradients develop in the adhesive layers which are about 5 mm in thickness.The goal of the project is both the experimental characterization and the continuum mechanical modelling of rapidly curable polyurethane adhesives to optimize curing processes. To this end, all relevant time dependences and inhomogeneities are taken into account. New experimental techniques and material models must be developed that can represent cross-linking effects under the influence of both moisture and heat. The validation of the model is performed on the basis of the model which has to be implemented into a finite element program. Experimental tests uning tensile-shear specimen and practice-relevant material design are providewd.