Mechanical performance of starch based bioactive composite biomaterials molded with preferred orientation

Abstract

Composites of blends of starch with ethylene vinyl alcohol copolymer (SEVA-C) filled with 10, 30 and 50% by weight (wt.) of hydroxyapatite (HA–the major inorganic constituent of human bone) were produced by twin-screw extrusion (TSE) compounding. These composites were molded into tensile test bars using two molding techniques: i) conventional injection molding and ii) shear controlled orientation in injection molding (SCORIM). The bars produced were mechanically characterized by means of tensile testing and dynamical mechanical analysis (DMA). The structure of the moldings was assessed by wide-angle X-ray diffraction (WAXD) and the failure surfaces of the moldings analyzed by scanning electron microscopy (SEM). The enhancement of stiffness observed with HA reinforcement results partially from the stiffening effect of the blend associated with the decrease in plasticizer content during the compounding stage. SCORIM was able to further increase the stiffness of SEVA-C/HA composites, allowing a maximum improvement of 12% for 30% wt. HA as compared to conventional molding. DMA results showed that the reinforcement of SEVA-C causes the broadening of the relaxation peak of the polymer, suggesting a structural change within the starch fraction that may be related with thermal degradation of the polymer. The addition of HA particles reduces the preferred orientation exhibited by the SEVA-C matrix, which is believed to limit the maximum mechanical performance that can be attained. Nevertheless, composites based on a biodegradable matrix with modulus above 7 GPa (in the bounds of the lower limit for human cortical bone) could be successfully produced.