The chemistry and technology of transparent flexible polyurethanes

Thermoset and thermoplastic polyurethanes have been prepared which are transparent and flexible. A synthesis technique and reaction and curing conditions were investigated to establish a reproducible method of obtaining these transparent urethanes: this is based on a polycaprolactone diol/4,4 1 -methylene biscyclohexyl diisocyanate (H12MDI) prepolymer system. Only certain chain extenders will give transparent polyurethanes and these are aliphatic diols, cycloaliphatic diols and triols. Their relative reactivities with diisocyanates were studied. The way in which the various chemical constituents control light transmission characteristics in polyurethane elastomers has been investigated by studying the morphology using differential scanning calorimetry (DSC), infrared spectroscopy and dynamic mechanical thermal analysis techniques. Physical and mechanical properties of the prepared materials were used as the criteria of their quality and measurementswere made of modulus, ultimate tensile strength, elongation at break, hardness, tear strength, tension set and compression set. The stability of these materials toward thermooxidative degradation, UV exposure, hydrolysis and gamma radiation have been quantfied. In Northern Australia outdoor weathering trials for retention of transparency in these polyurethanes have been organised and these are still progressing, but no firm conclusions can be drawn at this date.