Transparent polyester polyol-based polyurethane coatings: the effect of alcohols

A series of polyester polyols were synthesized using 1,6-hexanediol (HDO), 1,4-cyclohexanedimethanol (1,4-CHDM) and trimethylol propane (TMP), alone or in combination with 1,4-cyclohexanedicarboxylic acid and adipic acid. The polyester polyols were reacted with isocyanate trimers to form polyurethane (PU) coatings. Physical properties of the polyesters such as hydroxyl value, acid value, molecular weight, viscosity, and glass transition temperature (T _g) had been determined, and the IR spectroscopic analyses of polyesters/PU were reported. The properties (impact resistance, film flexibility, hardness, optical transmittance, chemical resistance, water absorption, thermostability, and phase separation) of the PU coatings so prepared were characterized. The viscosities of polyester polyols were dependent on the structure of the alcohols. Polyester polyol containing only linear aliphatic diol (HDO) was a transparent liquid at room temperature. The viscosity was increased by raising the molar ratio of 1,4-CHDM and/or TMP. All PU coatings had excellent flexibility, impact resistance, and hardness. The coatings derived from diol CHDM had the highest hardness, and the PU derived from diol HDO had the lowest hardness. The chemical resistance and water absorption improved with greater molar ratios of 1,4-CHDM or TMP. Results of differential scanning calorimetry and wide-angle X-ray diffraction indicated that there was no obvious crystallinity in the PU networks. Dynamic mechanical analysis (DMA) results revealed that both CHDM and TMP can increase the T _g of PU, and the crosslinking density improved with increased molar ratio of TMP. Atomic force microscope (AFM) and DMA analysis revealed that the PU coatings had no obvious microphase separation, which enabled them to have excellent properties, and the different composition in polyols did not have significant influence on transmittance in the visible region. Results of thermogravimetric measurements indicated that all the PU coatings had good thermal stability.