Co-precipitation of tapioca starch and hydroxyapatite. Effects of phosphorylation of starch on mechanical properties of the composites

Abstract

Organic-inorganic composites were prepared by co-precipitation of tapioca starch and hydroxyapatite (HAP) with polymer-hydroxyapatite weight ratios of 70/30, 50/50, and 30/70. Both tapioca starch (TS) and phosphorylated tapioca starch (PTS) were employed as starting materials and effects of phosphate groups of starch on hybridization were examined. Uniaxial hot press of the precipitated powder at 120 ​°C at 120 ​MPa afforded rectangular test blocks. The bending strength of TS-HAP (70:30), (50:50), and (30:70), and PTS-HAP (70:30), (50:50), and (30:70) were 19, 21, 18, 26, 37, and 36 ​MPa, respectively, and their strain at break were 2.6, 2.3, 1.7, 4.3, 3.5, and 2.7%, respectively. Both of the bending strength and strain at break were larger for the composites prepared from phosphorylated tapioca starch. The bending elastic modulus of the PTS-HAP composites increased with increasing the volume fraction of HAP in the composites, showing that rule of mixture is applicable to the PTS-HAP composites. Crystallite sizes of HAP were smaller for the composites prepared from phosphorylated tapioca starch than those prepared from tapioca starch. Therefore, phosphate groups in starch assisted nucleation of HAP and/or inhibited crystal growth of HAP, and stabilized the organic-inorganic interface by ionic interactions. Composites prepared from nongelatinized tapioca starch showed poorer mechanical properties than those prepared from gelatinized one. These carbon neutral composites should be promising as versatile light and rigid mechanical materials.

Keywords

• Hybrid materials
• Hydroxyapatite
• Starch
• Bending strength