Relationship between reagents molar ratio and dispersion stability and film properties of waterborne polyurethanes

Abstract

Environmentally-friendly waterborne polyurethanes showing a broad range of properties have been synthesized by the prepolymer method. A macrodiol based on poly(ϵ-caprolactone) diol (PCL) was used as soft segment (SS) and isophorone diisocyanate (IPDI), 2-bis(hydroxymethyl) propionic acid (DMPA) and 1,4-butanediol (BD) as hard segment (HS). The IPDI/(PCL + DMPA) and PCL/DMPA molar ratio were varied in order to determine the influence of these variables in particle size and stability of dispersions and also in final properties of polymer films. Particle size of the obtained dispersions, determined by means of dynamic light scattering, showed a narrow distribution with small particle diameters. Isocyanate content increase leads to bigger particles due to urethane linkages which restrict chain mobility, whereas DMPA content increase promotes small particles due to higher density of ionic groups. The stable dispersions have been used for films preparation, which have been characterized from the view point of their physicochemical, thermal and mechanical properties, as well as morphology. The increase of IPDI/(PCL + DMPA) molar ratio leads to higher yield stress, stress at break and modulus, maintaining high elongation at break values. Nevertheless, the increase of DMPA content promotes less crystalline soft domains achieving soft segment strain induced crystallization under stress and thus, obtaining higher stress at break and improving elongation at break. Films surface hydrophilicity is predominantly affected by IPDI/(PCL + DMPA) molar ratio, whereas water diffusion depends on DMPA content.