Effect of difunctional acids on the physicochemical, thermal, and mechanical properties of polyester polyol-based polyurethane coatings

Polyester polyol (PP)-based polyurethanes (PUs) consisting of two difunctional acids [1,4-cyclohexanedicarboxylic acid (CHDA) and 1,6-adipic acid (AA)] and also two diols [1,4-cyclohexanedimethanol (CHDM) and 1,6-hexanediol (HDO)] were synthesized by a two-step procedure with a variable feed ratio of CHDA to AA but fixed ratio of CHDM and HDO. The prepared PPs and/or PUs were characterized by Fourier transform infrared spectroscopy, X-ray diffraction spectroscopy, and atomic force microscopy. The effects of difunctional acids on the thermal, mechanical, and dynamic mechanical thermal properties of PPs or PU films were investigated by thermogravimetry analysis, differential thermogravimetry and dynamic mechanical thermal analysis. The results show that PP exhibits a lowest viscosity with the mole fraction of CHDA and AA at 3 : 7 whereas it delivers a lowest melting point with the mole fraction at 9 : 1. After PPs being cross-linked by isocyanate trimers, the impact resistance, shear strength and glass transition temperature increase the mixed-acid formulations with increasing the content of CHDA. In detail, the resultant PU almost simultaneously exhibits the best mechanical and thermal properties when the mole fraction of CHDA and AA is kept constant at 9 : 1, thus giving rise to a high glass transition temperature of 56.4°C and a onset decomposition temperature of 350°C, and also delivering a balanced toughness and hardness with an impact resistance of 100 J/g and storage modulus as high as 10⁹Pa. This path for synthesis of PP-based PU provides a design tool for high performance polymer coatings.