Preparation and properties of MDI/H₁₂MDI-based water-borne poly(urethane-urea)s-effects of MDI content and radiant exposure

Water-borne poly(urethane-urea)s (WBPUs) were prepared by a prepolymer mixing process using aromatic diisocyanate (MDI, 4,4′-diphenylmethane diisocyanate)/aliphatic diisocyanate (H₁₂MDI, 4,4′-methylenebis cyclohexyl isocyanate), polypropylene glycol (PPG, M_n = 1000), dimethylol propionic acid, and ethylene diamine as a chain extender, and triethylamine as a neutralizing agent. The effect of MDI on the molecular weight, chemical structure, dynamic thermo, and tensile properties of WBPUs was investigated. The yellowness index (YI, photo-oxidation behavior) change of WBPUs under accelerated weathering exposure was also investigated. The WBPUs containing higher MDI content showed lower molecular weight, which verified the participation of some high reactive isocyanate groups of MDI into side reaction instead of chain growing reaction. As the MDI content increased, the storage modulus and tensile modulus/strength of WBPUs film increased, and their glass transitions of soft segments (T_gs) and hard segments (T_gh) were shifted to higher temperature. The intensity of tan δ peak of all three samples increased with increasing radiant exposure. The YI of H ₁₂MDI-based WBPU sample (WBPU-0) was not occurred. The YI of WBPUs containing MDI increased with increasing MDI content and radiant exposure. However, the YI of sample WBPU-25 containing 25 mol % of MDI at 11.3 MJ/m ² (radiant exposure) was 6.6 which is a permissible level for exterior applications.