Water-Resistant Tough Composites of Cellulose Nanofibers and Hydroxyapatite

To develop carbon-neutral tough mechanical materials, we prepared the composite of TEMPO-oxidized cellulose nanofibers (TCNF) and hydroxyapatite (HAP) by coprecipitation and acylated it with vinyl acetate, vinyl hexanoate, vinyl octanoate, vinyl laurate, and vinyl benzoate in dimethyl sulfoxide to endow the composite with water resistance. The acylated composites were densified in a mold at 120 °C and 300 MPa to obtain rectangular compacts. The bending strengths of the acylated compacts were smaller than those of the parent TCNF-HAP compact: 72% of the parent compact for acetate, 55% for hexanoate, 62% for octanoate, 56% for laurate, and 80% for benzoate. When the compacts were immersed in water at room temperature for 24 h, the weight increase due to water absorption was 63% for the nonacylated composite compact, 88% for acetate, 11% for hexanoate, 12% for octanoate, 16% for laurate, and 28% for benzoate. The bending strengths after immersion in water were 0% of that of the dry compact for acetate, 28% for hexanoate, 29% for octanoate, 17% for laurate, and 8% for benzoate. Hydrophobic acyl groups, particularly hexanoyl and octanoyl groups, successfully enhanced the water resistance of the composites.