Sheet molding compound resins from soybean oil: Thickening behavior and mechanical properties

Thermosetting resins for sheet molding compound (SMC) and bulk molding compound applications were synthesized from soybean oil. The SMC resins were prepared from maleated hydroxylated soybean oil (MHSO) and maleated acrylated epoxidized soybean oil (MAESO) with styrene. When thickened with divalent cations such as MgO, these resins exhibited a substantial rise in viscosity at room temperature because of complexation of MgO with the terminal acid groups on maleic anhydride. The resulting high viscosity sheet upon heating rapidly reduced its viscosity as the labile MgO‐acid groups thermally disassociated. The flexural strength σ and moduli E of these polymers varied from 61 to 87 MPa and from 1.6 to 2.4 GPa, respectively and the tensile strength and moduli varied from 27 to 44 MPa and from 1.6 to 2.5 GPa, respectively. The mechanical and fracture properties of these polymers were related to the amount of functional groups f, on the fatty acid backbone, and followed the Rigidity Percolation and Twinkling Fractal Theories, σ ～ [Eν]^1/2, where ν ～ [f ? 1]. The fracture energy G_IC decreased significantly with increasing crosslink density, as G_IC ～ ν^?2 and the plastic zone r_p decreased as r_p ～ ν^?3. The new bio‐based resins possessed mechanical and thermal properties comparable with petroleum‐based unsaturated polyesters. POLYM. ENG. SCI., 47:1469–1479, 2007. © 2007 Society of Plastics Engineers