Novel Phosphorous-Based Deep Eutectic Solvents for the Production of Recyclable Macadamia Nutshell–Polymer Biocomposites with Improved Mechanical and Fire Safety Performances

Deep eutectic solvents (DESs), a new class of ionic liquid analogue solvents, provide excellent opportunities to chemically solubilize and functionalize wood components. Here, we synthesized a novel phosphorous-based DES (p-DES) using glycidyltrimethylammonium chloride and sodium diphenyl phosphate, confirming by Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies. Then, p-DES was used for phosphorylation of macadamia nutshell powder (p-wood) under mild reaction conditions along with partial lignin removal, resulting in a higher crystallinity degree in the p-wood. Using scanning electron microscopy, transmission electron microscopy, FTIR, thermogravimetric analysis, particle size, and X-ray diffraction analyses, it was found that treatment using p-DES significantly changes the morphology, structure, and properties of the macadamia nutshell powder. This treatment made p-wood a highly phosphorus-functionalized powder suitable to be used as a high-performance fire-retardant filler. The epoxy resin biocomposites were fabricated and it was found that the biocomposite containing 10 wt % p-wood shows significant improvements in flexural strength and modulus. Meanwhile, the mechanical properties of the biocomposite containing 20 wt % p-wood are relatively comparable with those of neat epoxy resin without compromising on flexural strength and modulus. Moreover, the UL-94 and cone calorimetry results indicate that the biocomposite containing 20 wt % p-wood has a V-1 rating along with 74% and 344 °C reduction in peak heat release rate and maximum smoke temperature, compared to the neat epoxy resin, and shows a significant increase in char yield and limiting oxygen index value. In contrast to the neat epoxy resin, the presence of the macadamia nutshell powder in the developed biocomposites resulted in recyclability through the wood structure under mild conditions.