Benzoylation of microfibrillated cellulose–hydroxyapatite composites for green and water-resistant mechanical materials

Abstract

If composite materials comparable to bone-like bioceramics can be produced on an industrial scale, it would be expected to contribute to carbon neutrality, reduce environmental pollution caused by waste, and promote the realization of a sustainable society. We prepared a composite of microfibrillated cellulose and hydroxyapatite (HAP) with a hydroxyapatite weight fraction of 52% by crystallization of hydroxyapatite in a dispersion of microfibrillated cellulose in alkaline water at 50 °C. To improve water resistance of the composite, the composite was benzoylated with vinyl benzoate at 50–150 °C in DMF. Infrared absorption spectroscopy revealed that benzoylation proceeded above 110 °C. The benzoylated composites were uniaxially pressed at 120 °C at 300 MPa for 5 min to obtain the compacts. A three-point bending test revealed that the composite benzoylated at 110 °C exhibited ductile fracture, with an elastic modulus of 4.5 ± 0.2 GPa and a bending strength of 65.7 ± 1.2 MPa. When the compacts were immersed in water at room temperature for 24 h, the water absorption ratios of the composite benzoylated at 50–100 °C were 20–40%, while those benzoylated at 110–150 °C were less than 10%. The composite benzoylated at 110 °C had an elastic modulus of 3.3 ± 0.4 GPa and a bending strength of 38.4 ± 1.1 MPa. In contrast to the benzoylated TEMPO-oxidized cellulose nanofibers–HAP composite previously reported, whose modulus decreased to 2% of its original value after water immersion, the present composite retained 73% of its initial modulus, indicating a substantial improvement in water resistance.