Sustainable Vanillin-Based Epoxy Resin with Excellent Flame Retardancy and Mechanical Properties

Nowadays, traditional petroleum-based diglyceryl ether of bisphenol A epoxy resins (DGEBA) have prominent problems such as nonrenewable raw materials, flammability, and poor toughness. Herein, a vanillin-based epoxy resin (VH-HDA-EP) containing a rigid Schiff base structure and flexible alkane segment was synthesized from vanillin, 1,6-hexanediamine, and epichlorohydrin through a facile and highly efficient method, and it was cured with 4,4′-diaminodiphenylmethane (DDM) to prepare VH-HDA-EP/DDM thermosets. Curing kinetics analysis demonstrated that the activation energy of VH-HDA-EP was much higher than that of DGEBA. Thermogravimetric analyses illustrated that the char yield of VH-HDA-EP/DDM was 30.8 wt % at a nitrogen atmosphere, which was nearly two times that of DGEBA/DDM (17.9 wt %). Moreover, VH-HDA-EP/DDM obtained a limiting oxygen index as high as 38.5% and passed the UL-94 V-0 rating test. Compared with DGEBA/DDM, the total heat release, total smoke production, and maximal smoke density of VH-HDA-EP/DDM were significantly reduced by 45.1, 73.4, and 36.2%, respectively. The char residue analysis results revealed that the Schiff base structure of VH-HDA-EP/DDM could contribute to the formation of continuous and dense char residues in the condensed phase. The storage modulus and glass transition temperature of VH-HDA-EP/DDM were 2821 MPa and 186 °C, which were higher than those of DGEBA/DDM (1990 MPa and 168 °C). The tensile strength and elongation at break of VH-HDA-EP/DDM were 55.6 MPa and 8.3%, respectively, and it showed obvious strain strengthening characteristic. Furthermore, the enhancement mechanism of flame retardancy and mechanical properties was proposed, which is mainly due to the complementary effect of the rigid Schiff base structure and flexible alkane segment. This work provides insights for preparing high-performance bio-based epoxy resins with excellent flame retardancy and mechanical properties.