Effect of Hardener on Properties of Biobased Epoxy Thermosets From Polymerization of Epoxidized Soybean Oil With Citric and Itaconic Acids

The effect of hardener on crosslink density and other properties of biobased epoxy thermosets derived from epoxidized soybean oil was studied. Curing reactions were self‐catalyzed by itaconic and citric acids at 70°C and 120°C for 5 h to produce cross‐linked polymer networks. Conversion and gel content ranged from 75.4% to 94.2% and 93.2% to 95.6%, respectively, and were enhanced by higher reaction temperature and stoichiometric ratio of crosslinker to epoxide. Citrate‐based polymers exhibited higher crosslink densities ( υ c ; 242.9–338.5 mol/m 3 ) than the corresponding itaconate‐based polymers (32.8–54.1 mol/m 3 ). Additionally, υ c increased with cure temperature and molar ratio of carboxylic acid to epoxide. Storage modulus ( G ′) and glass transition temperature ( T g ) closely mimicked the behavior of υ c , with citrates, higher curing temperature, and a higher molar ratio of acid affording increased T g and G ′ versus itaconates, lower temperature, and lower amount of acid. The sub‐ambient T g (−17.1°C to 9.5°C) and low G ′ (0.3–11.1 MPa) suggested elastomeric applications. Temperatures corresponding to 10% mass loss ( T 10 ) and the rate of maximum degradation (DTG) ranged from 254.5°C to 356.2°C and 391.0°C to 404.0°C, respectively. Lastly, chemical degradation studies demonstrated nearly complete hydrolysis (> 94%) under alkaline (> 94%) and acidic (> 92%) conditions, highlighting the potential of these materials as cleavable biobased epoxy networks for sustainable applications where high flexibility, low mechanical strength, and end‐of‐life degradability are advantageous.