Polyoxazolidones for High Temperature Applications

Polyoxazolidones of different crosslink densities were prepared by reacting NCO-terminated oxazolidone quasi-prepolymers with diglycidyl ether of bisphenol A. The NCO-oxazolidone quasi-prepolymers were prepared at a 5/1 NCO/epoxy ratio, utilizing three different types of isocyanates: 4,4'-methylene bis(phenyl isocyanate) (MDI), polymeric MDI (PMDI) and 4,4'-methylene bis(cyclohexyl isocyanate) (H₁₂MDI). These resins were prepared at isocyanate indices of 105 and 120, and in the case of PMDI, with an additional index of 200. The viscoelastic properties of these resins during curing were studied using rheometric dynamic spectroscopy. The chemical structure of the NCO-terminated oxazolidone quasi-prepolymers and of the resulting poly-oxazolidones were determined using infrared spectroscopy. Stress-strain response, flexural strength and modulus, compressive strength and modulus and shear strength were evaluated, as well as adhesion properties (lap shear strength). The measured properties were found to be very good for all types of the polyoxazolidones. Practically all polyoxazolidones retained over 50% of the room temperature tensile strength at 150°C. Room temperature lap shear strength of polyoxazolidone adhesives changed slightly at 150°C, and in the case of PMDI-based polyoxazolidone adhesive at an isocyanate index 120 did not change after immersion in boiling water for 3 hours. The high temperature performance of the polyoxazolidones was also confirmed by the excellent retention of mechanical properties after aging for seven days at 150°C as well as by dynamic mechanical analysis (DMA) and thermo-mechanical analysis (TMA).