Mechanically Robust Flexible Polyurethane Foams Formulated with Polyols Comprising Soybean Protein

Abstract

Soybean protein is a compelling raw material for the preparation of high-performance biobased polyurethane (PU) foams in light of its rich polyfunctional moieties containing active hydrogens. However, soybean protein is immiscible with polyols owing to the presence of numerous carboxylic groups of strong polarity, which in turn results in below-par mechanical performance when it is simply applied as a spherical physical filler in a target matrix. In this study, the solubility of soybean protein in polyols is realized through esterification to enable a designated hydroxyl functionality. The esterification process not only converts the original carboxylic group into an ester moiety but also correspondingly introduces hydroxyl groups into the macromolecules. This increases the content of active hydrogens, which ultimately facilitates the subsequent reactions with isocyanates. As a result, the esterified soybean protein (ESP)-derived flexible PU foam exhibits dual excellence in tensile strength and toughness, and its compressive strength and modulus have been improved by 3.2 and 2.5 times, respectively, when compared to the unmodified SP-reinforced PU. Moreover, the incorporation of ESP into the PU framework on a molecular level enables the formation of a secondary interpenetrating network that improves the resilience of the matrix. The as-fabricated foam delivers a superior compressive recovery rate of up to 98.6% even after 150 cycles, which stands out prominently among reports on biobased PU. This work demonstrates a simple and green method to transform soybean protein into biomass-derived multifunctional polyols, which could spur innovations from the community toward high-performance biobased polymeric materials.