Synthesis and characterization of transparent and high impact resistance polyurethane coatings based on polyester polyols and isocyanate trimers

Abstract

A series of polyester polyols were synthesized by polycondensation reaction using adipic acid (AA), 1,4-cyclohexanedicarboxylic acid (1,4-CHDA), and 1,6-hexanediol (HDO), 1,4-cyclohexanedimethanol (1,4-CHDM) and trimethylol propane (TMP), in which the molar ratio of the reactants AA/1,4-CHDA was varied. These series of polyols were reacted with isophorone diisocyanate (IPDI) and hexamethylene diisocyanate (HDI), alone or in combination, to form polyurethane (PU) coatings.The physicochemical properties (chemical structure, hydroxyl value, acid value, and molecular weight) of the polyols so synthesized and the mechanical and optical properties (impact resistance, film flexibility, optical transmittance and phase separation) of the PU coatings so prepared were characterized with FTIR spectroscopy, wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and atomic force microscopy (AFM).The results of this study show that the AA/1,4-CHDA molar ratio, the isocyanate used, and the NCO/OH ratio all have a significant impact on the properties of the PU coatings. While AA/1,4-CHDA molar ratio does not seem to affect the film flexibility and optical transmittance of the PU coatings, it has a significant effect on the impact resistance of the coatings. Impact resistance of the PU coatings increases as the AA/1,4-CHDA molar ratio decreases (i.e. more 1,4-CHDA used). PU coatings based on IPDI and combination of IPDI and HDI exhibit higher impact resistance and optical transmittance than HDI-based coatings. While changing NCO/OH ratio has only a slight impact on the optical transmittance and little effect on film flexibility, impact resistance of the PU coatings first increase, then decline with an increase in NCO/OH ratio.