Improving the properties of alkyd polyol-based waterborne polyurethane-amino baking films and coatings by castor oil as branched flexible oligomer

In this investigation, the alkyd polyol was synthesized via the fatty acid method using oleic acid, trimethylolpropane, neopentyl glycol, and phthalic anhydride as raw materials. A series of water-based polyurethane dispersions (WPUCOs) were prepared by incorporating different levels of branched flexible castor oil into the formulation. The hydroxyl content provided by castor oil accounted for 0%, 10%, 20%, 30%, 40%, and 50% of the total hydroxyl content provided by castor oil and the alkyd polyol, and these variations were designated as WPUCO-0, WPUCO-10, WPUCO-20, WPUCO-30, WPUCO-40, and WPUCO-50, respectively. The WPUCOs were synthesized using an alkyd polyol, castor oil, 4,4'-dicyclohexylmethane diisocyanate (H 12 MDI), and 2,2-dimethylolpropionic acid as key components. These WPUCOs were analyzed using Fourier transform infrared spectroscopy, laser diffraction particle size analysis, and rheological measurements. Waterborne polyurethane-amino resin curing films based on the above-mentioned WPUCOs were then fabricated and characterized through thermogravimetric analysis, differential scanning calorimetry, contact angle measurements, water absorption tests, and gel content determination. Furthermore, waterborne polyurethane-amino baking coatings were also formulated using WPUCOs to evaluate their performance. The results showed that yellowish WPUCOs were successfully prepared. The viscosity and particle size of WPUCOs are observed to rise as the proportion of castor oil in the formulation increases. Among all formulations tested, WPUCO-30 exhibited superior film properties and coating performances. For instance, the water absorption ratio of the resulting waterborne polyurethane-amino resin curing films based on WPUCO-30 was only 2.81%, while achieving a gel content of 93.85%. Additionally, the pencil hardness and impact resistance values for the waterborne polyurethane-amino baking coating based on WPUCO-30 reached 3H and 50 kg·cm, respectively, and glossiness measured at a 20° angle reached 89.6 GU. Moreover, the number of times the coating could withstand acetone scrubbing reached up to 12,408 times. Therefore, castor oil containing three-chain structures can be effectively utilized to enhance the performance characteristics of waterborne polyurethane-amino resin baking films/coatings.