Tailoring the Mechanical Properties of Bio-Based Rigid Polyurethane for Orthopedic Educational Models Using Castor Oil-Polyether Polyol Blends

The incorporation of castor oil as a bio-based polyol offers a sustainable alternative to petroleum-derived polyols in rigid polyurethane (PU) production, supporting efforts to reduce fossil resource dependency. In this study, rigid PU was synthesized from a blend of castor oil and polyether polyol for orthopedic educational model applications. The effect of varying the NCO/OH molar ratio (1.3–2.5) on the resulting foam’s physical and mechanical properties was evaluated. Additives including silicone glycol, triethylenediamine and stannous octoate, and calcium carbonate were used to control foam characteristics. The materials were processed via a one-shot method and characterized through density measurement, compressive strength testing, Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Results showed that increasing the NCO/OH ratio improved density and compressive strength up to an optimal value at 1.9, reaching 484.09 kg/m³ and 22.74 MPa, respectively, aligning with the standards applicable for orthopedic applications. Higher NCO/OH ratios led to excess unreacted isocyanate, acting as a reactive diluent, reduced crosslink density, and caused greater void formation ultimately compromising mechanical performance. FTIR confirmed urethane formation with minimal residual isocyanate, while SEM analysis revealed that PU1.9 samples had smaller, uniform cell structures with thicker walls. These findings highlight the need to optimize the NCO/OH ratio and show that blending castor oil with the formulation can produce bio-based rigid polyurethane that meets orthopedic performance requirements, thereby improving the sustainability of orthopedic simulation materials.