Use of coupling agents to enhance the interfacial interactions in starch–EVOH/hydroxylapatite composites

Different zirconate, titanate and silane coupling agents were selected in an effort to improve the mechanical properties of starch and ethylene-vinyl alcohol copolymer (EVOH)/hydroxylapatite (HA) composites, through the enhancement of the filler particles–polymer matrix interactions and the promotion of the interfacial adhesion between these two phases. The mechanical performance was assessed by tensile tests and discussed on the basis of the respective interfacial morphology (evaluated by scanning electron microscopy). The main relevant parameters were found to be the surface properties and reactivity of the filler (non-sintered HA) and the chemical nature (pH and type of metallic centre) of the added coupling agent. Significant improvements in the stiffness were achieved (about 30% increase in the modulus) when using the acidic zirconate coupling agents. The acidic zirconate combined the capability of crosslinking the polymer matrix with the establishment of donor–acceptor interactions and hydrogen bonding between it and the ceramic particles, leading to very good interfacial adhesion. The optimization of these coupling processes associated with the introduction of higher amounts of filler, may be an effective way to produce composites with mechanical properties analogous to those of the human cortical bone.