Effect of isocyanate to hydroxyl index on the properties of clear polyurethane films

Clear polyurethane films were made from aqueous polyurethane pre-polymer dispersions and water-dispersible aliphatic isocyanate pre-polymer, varying the amount of isocyanate used to cure the films. Film series were made from one of two polyol dispersions; the control XP-7110 having a higher molecular weight and containing 10 wt.% n-methylpyrrolidone (nmp) was compared to a zero-volatile organic compound (VOC) lower molecular weight alternate polyol XP-2591. Increasing the relative number of isocyanate groups to hydroxyl groups in these films provided an increase in glass transition temperature (Tg), tensile strength, elastic modulus and improved barrier properties to dimethyl methylphosphonate (DMMP) for both polyols in this study. At all indices in this study XP-7110 films achieved slightly higher Tg and slightly better resistance to permeation by DMMP than the XP-2591 films. Water resistance of the films was not improved at higher indices nor was there any difference between the polyols. Yet, the experimental results suggest that it is possible to produce polyurethane binders with acceptable film properties while reducing the VOC content of the formulation by using t-butyl acetate as the isocyanate reducing solvent. The zero-VOC polyol achieved better tensile properties with higher indexing than the current polyol containing 10 wt.% VOC. While the zero-VOC polyol did not perform as well as the baseline polyol in some tests, the differences in performance were small and thus there may be an opportunity to use the zero-VOC polyol as a partial or even a complete replacement of the current material.

► Higher indexing increased Tg, strength, modulus and improved barrier properties.
► Tensile strength and modulus are affected by more urea linkages at higher indexing.
► Urea character increases with indexing resulting in decreased permeability of DMMP.
► VOC reduction can be achieved by thinning the isocyanate with t-butyl acetate.
► Blending XP-7110 and zero-VOC XP-2591 may optimize properties and lower total VOCs.