Hydrolytic stability of polyurethane/polyhydroxyurethane hybrid adhesives

Polyurethane/polyhydroxyurethane (PU/PHU) hybrid materials are potential alternatives to PU materials in adhesive use, when the user needs to be protected from free di-isocyanates. PU/PHU materials have promising mechanical and adhesive properties, but they are susceptible to a higher water uptake than corresponding PU materials. In adhesive use, water and temperature are typically responsible for environmental ageing, which can cause unexpected bond failure. Therefore, the effect of water uptake on mechanical and adhesive strength is crucial. In this study, the water uptake of PU/PHU at various temperatures was studied and the contribution of absorbed water to material properties was investigated. The highly crosslinked amorphous PU/PHU was synthesized from a multifunctional polyethyleneimine (PEI) and a cyclic carbonate terminated prepolymer made from a hexamethylene di-isocyanate (HDI) based isocyanate prepolymer. High water uptake of PU/PHU originated from the hydrophilic amine used as well as the pendant hydroxyl groups in the hydroxy urethane. After the high initial water absorption, a quasi-equilibrium was reached at room temperature. Water immersion at elevated temperature introduced a second water absorption step which eventually caused PU/PHU to become a water-soluble gel. Considering the potential applications of PU/PHU, the cohesive strength had increased after water immersion cycles, but the adhesive strength was irreversibly reduced resulting in total bond failure after five water immersion cycles. In contrast, longer storage at dry elevated temperature improved the cohesive and adhesive strength. Thus, a careful design of the application is required for PU/PHU adhesives to fully utilize the advantage of the high lap shear strength in dry condition.