Effects of Hydration on Mechanical Properties of Acylated Hydroxyapatite–Starch Composites

Transesterification of vinyl acetate, vinyl benzoate, and vinyl laurate with hydroxyapatite–starch composites was carried out in dimethyl sulfoxide at 25–120 °C for 8 h. The composites of acylated starch and hydroxyapatite were compressed by hot pressing at 60 °C or 120 °C at 120 MPa to obtain the compacts. The three-point bending test of the compressed compacts revealed that acetylation and benzoylation did not alter the mechanical properties of the composites much, while lauroylation softened the composites. Bending elastic moduli of the acetylated and benzoylated composites were 2–3 GPa, whereas those of the lauroylated composites were 0.1–0.3 GPa. Both hydroxyapatite–starch compacts and their acetylated ones readily collapsed in the bending test after immersion in water, while the benzoylated and lauroylated composite compacts were water-resistant. The bending strengths of the compacts of benzoylated and lauroylated composites after immersion in water were ca. 60–75% of those of the pristine compacts. The stain at break of the lauroylated composites after immersion in water was 1.1–1.75 times larger than that of the as-prepared onestrends similar to natural bone.