In this work, a no. of chain extended moisture-curable urethane prepolymers were synthesized in order to develop isocyanate terminated urethane prepolymer formulations that would simultaneously display both high adhesive strength and low viscosity. Proton NMR spectroscopy (1H-NMR), size exclusion chromatog. (SEC), differential scanning calorimetry (DSC), and Brookfield viscometry were utilized for characterizing the prepd. urethane prepolymers. In addn., the adhesion strength of the cured prepolymers was detd. by tensile shear strength test according to the DIN EN (Deutsches Institut fuer Normung, the German Institute for Standardization) 1465 std. Esp., the role of different types of linear (butanediol, pentanediol) and branched chain extenders (dipropyleneglycol (di-PPG)), tripropyleneglycol (tri-PPG) and the influence of their dosage on the degree of microphase sepn. between hard segments (HS) and soft segments (SS) in urethane prepolymers were studied. Furthermore, the benefits of utilizing either a one-step vs. a two-step polymn. process were investigated. The results revealed that the extent of phase sepn. of different urethane prepolymers was dependent on the extent of hydrogen bonding interactions which was extensively studied by attenuated total reflectance IR spectroscopy (ATR-FTIR). The incorporation of branched chain extenders (di-PPG and tri-PPG) did not result in notable phase sepn. between hard segments and soft segments, while linear chain extenders (pentanediol and butanediol) readily promoted phase sepn. The degree of phase sepn. was particularly pronounced for butanediol, and when the linear chain extender ratio was higher than or equal to 0.74. Compared with a two-stage process, one-stage process produced more randomly distributed polymer chains with highly dispersed hard segments. Thus, urethane prepolymers exhibiting strong adhesive strength with simultaneously low viscosity were successfully developed by systematic adjustment of structural parameters.
- isocyanate-terminated urethane prepolymers
- segmented polyurethane
- soft and hard segments