The effects of soft segments on the physical properties and water vapor permeability of H12MDI-PU cast films

This research was based on the study of the effects of H₁₂MDI-1, 4BD PU soft segments on the physical properties and water vapor permeability of films cast from solvent evaporation or wet coagulation method. The soft segments studied included polyether, polyester, and polycaprolactone polydiols. The NCO/OH mol ratios of prepolymer were prepared by 2, 3, 4, 5, and 8, respectively. The chain lengths of the soft segments used were: PTMG of molecular weights 650, 1000, 2000, and 2900; PBA of 1000, 2000, and 3000. The results revealed that the polyether-based PU cast films had lower T_gs than the polyester-based PU films. In general, the polyether-based PU films shows the characters of higher water vapor permeability, lower breaking elongation, and higher breaking strength. Films with higher molecular weight soft segments in the polymer chains exhibited lower T_gs, lower breaking strength, higher breaking elongation, and higher water vapor permeability. As the hard segment contents were increased, the films exhibited higher T_gs. Films with higher hard-segment ratios had the highest breaking strength but the water vapor permeability, on the other hand, became lower. Films cast from the solvent evaporation method had higher breaking strength and higher breaking elongation but lower water vapor permeability than films cast from the wet coagulation method. © 1994 John Wiley & Sons, Inc.