Enhancing mechanical properties of epoxy resin using waste lignin and salicylate alumoxane nanoparticles

Extracted lignin from wastewater of Kraft process and lab-made salicylate alumoxane (Sal-A) nanoparticles were used as toughening agents in epoxy matrix. Epoxy/lignin composite, epoxy/Sal-A and epoxy/lignin/Sal-Al nanocomposites with various toughening agent loadings were cured with an aromatic diamine hardener. Lignin as an available cheap material and Sal-A, as multifunctional structures, both containing numerous phenolic hydroxyls on their surfaces, were incorporated into epoxy matrix with the aim of improving thermal and some mechanical properties of the resulting composites. Both particles interacted physically (directly) and chemically (indirectly) with the epoxy chains. Simplex lattice mixture design of experiment was applied for formulation development and optimization. Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the extracted lignin, Sal-A nanoparticles and synthesized composites. Differential scanning calorimetry (DSC) was used to interpret thermal curing process. The presence of lignin and Sal-A nanoparticles in the epoxy matrix decreased the exothermic peak temperature and total heat of curing reaction. In the presence of 2.5 wt% lignin and 1.875 wt% Sal-A nanoparticles, tensile strength of epoxy composites was 22.23% and 30.92% higher than that of reference (pure) epoxy resin, respectively. Vickers hardness of epoxy composites in the presence of 2.5 wt% lignin and 2.5 wt% Sal-A nanoparticles was increased by 17.41% and 15.39%, accordingly.