Water diffusion in a crosslinked polyether-based polyurethane adhesive

Crosslinked polyurethanes (PU) are employed as structural adhesives. Research has been done extensively on the interaction of water with thermoplastic PU but is still largely unexplored in the case of PU networks. A chemically simple but well-defined PU is utilized here as a starting point to understand the influence of water in this kind of polymer. For up to 15 days of immersion in water, no changes in the chemical composition of the PU were observed in the IR spectra. Depending on temperature, water saturates the PU samples much earlier. Plasticization is the result as a significant decrease of the mechanical modulus and a glass transition shift to lower temperature is observed in the polymer. Interestingly, only some 70% of the water saturation is sufficient for maximum plasticization. It is concluded from the IR spectra that water debilitates the interactions between polymer chains within the network. The bridging of at least a part of the originally hydrogen-bonded urethane groups is substituted by hydrogen bonds between water and the urethane moieties.