Ying-Jun Xu

Epoxidized soybean oil cured with tannic acid for fully bio-based epoxy resin

The construction of fully bio-based epoxy resins (EP) has been of particular interest in both academia and industrial circles for years; among these, epoxidized soybean oil (ESO) derived thermosets have received the most attention, but they usually exhibit poor performance due to their flexible fatty chains. Herein, tannic acid (TA), with its great degree of functionality and massive aromatic structures, was chosen as the multi-phenol curing agent for ESO to prepare fully bio-based EP thermosets with a high relaxation temperature and satisfactory mechanical properties. As a natural 2-substituted imidazole-containing substance, histidine (H) was used as the curing accelerator under moderate curing conditions (120–180 °C). This EP system showed high curing activity and a good curing degree while operating. The cured thermosets were found to be thermally stable (T5% > 270 °C) and displayed a high relaxation temperature (77 °C) with a tensile strength of 23 MPa. Preliminary adhesion tests showed that the cured product exhibited a high lap-shear strength of about 19 MPa in adhesion failure mode. Taking these advantages into account, this kind of fully bio-based EP could introduce more chances for versatile applications, such as being used in structural materials and construction adhesives.

Carbon Fibers Decorated by Polyelectrolyte Complexes Toward Their Epoxy Resin Composites with High Fire Safety

The achievement of both robust fire-safety and mechanical properties is of vital requirement for carbon fiber (CF) composites. To this end, a facile interfacial strategy for fabricating flame-retardant carbon fibers decorated by bio-based polyelectrolyte complexes (PEC) consisting of chitosan (CH) and ammonium polyphosphate (APP) was developed, and its corresponding fire-retarded epoxy resin composites (EP/(PEC@CF)) without any other additional flame retardants were prepared. The decorated CFs were characterized by SEMEDX, XPS and XRD, indicating that the flame-retardant PEC coating was successfully constructed on the surface of CF. Thanks to the nitrogen- and phosphorous-containing PEC, the resulting composites exhibited excellent flame retardancy as the limiting oxygen index (LOI) increased from 31.0% of EP/CF to 40.5% and UL-94 V-0 rating was achieved with only 8.1 wt% PEC. EP/(PEC8.1@CF) also performed well in cone calorimetry with the decrease of peak-heat release rate (PHRR) and smoke production rate (SPR) by 50.0% and 30.4%, respectively, and the value of fire growth rate (FIGRA) was also reduced to 3.41 kW·m–2·s–1 from 4.84 kW·m–2·s–1, suggesting a considerably enhanced fire safety. Furthermore, SEM images of the burning residues revealed that the PEC coating exhibited the dominant flame-retardant activity in condensed phase via the formation of compact phosphorus-rich char. In addition, the impact strength of the composite was improved, together with no obvious deterioration of flexural properties and glass transition temperature. Taking advantage of the features, the PEC-decorated carbon fibers and the relevant composites fabricated by the cost-effective and facile strategy would bring more chances for widespread applications.