The Effect of Residual Solvent N,N′‐Dimethylformamide on the Curing Reaction and Mechanical Properties of Epoxy and Lignin Epoxy Composites

The effect of residual N,N′-dimethylformamide (DMF) on curing behavior and mechanical properties of neat epoxy and lignin epoxy composites is studied. The significant enhancement in their mechanical properties is found to be induced by residual DMF for the first time. The spectra and mechanical analysis indicates that the entrapped DMF is likely to decompose at high temperature to yield dimethylamine, which should act like excess amine and take part in the formation of network structure, lower the crosslink density, increase the chain packing, improve the Van der Waal's interaction, and eventually enhance their mechanical performances. Lignin epoxy composites prepared with the aid of tetrahydrofuran or DMF show significant difference in their mechanical properties. It is therefore important to distinguish the filler effect from the solvent effect when a solvent is used in epoxy composites preparation.