Tailored mechanical properties of soybean oil-based non-isocyanate polyurethanes by copolymer integration†

Abstract

Stiff thermoset polyurethane (PU) plays a crucial role in high-performance applications, particularly in industries requiring exceptional mechanical integrity, chemical resistance, and thermal stability. To reduce the environmental impact of PU production, (i) soybean oil has emerged as a renewable and abundant alternative to petroleum-based feedstocks, offering biodegradability and a reduced carbon footprint, while (ii) non-isocyanate polyurethane (NIPU) provides a greener approach by eliminating hazardous isocyanate compounds and avoiding isocyanate-functionalized chemicals. However, the development of soybean oil-based NIPU faces challenges in achieving the desired stiffness and resistance against fracture due to the large molecular size and inconsistent structure of soybean oil, which result in low crosslinking density and a lack of short-range ordering. To address the limitations of soybean oil-based NIPU, we developed a method that restricts polymer network relaxation by incorporating short-range ordered polymer segments using a copolymer with ethyl methacrylate (EMA) segments. Surpassing the highest mechanical properties reported for soybean oil-based NIPU to date, co-NIPU-x derived from copolymers with higher EMA content exhibits improved mechanical properties, demonstrating a four-fold increase in Young's modulus and a two-fold increase in tensile stress. The adjustable poly(2-aminoethylmethacrylate-ran-ethylmethacryate) (poly(AEMA-ran-EMA)) composition ratio allows for a wide range of mechanical properties, with Young's modulus ranging from 60 to 1030 MPa and tensile stress from 2.1 to 25 MPa. Furthermore, these NIPU samples exhibited enhanced adhesion properties with lap shear strength exceeding 7 MPa, significantly higher than those of traditional formulations. The thermal stability was improved with the NIPU samples resisting structural degradation, and chemical resistance was confirmed by sufficient swelling ratios in both hydrophilic and hydrophobic solvents, underscoring their suitability for a broader range of industrial applications.