Preparation And Properties Of Polylactic Composite Reinforced With Magnesium Alloy Flakes

In order to overcome the shortcomings of polylactide (PLA) or magnesium alloy (Mg) when they are used alone as bone fracture fixation materials, a new PLA-based composite reinforced with magnesium alloy flakes was developed by hot-pressing method. Mechanical properties of this composite including bending strength, shearing strength and impact toughness as well as the influence of micro-arc oxidation (MAO) treatment were investigated. Finaly the degradation properties of composite in simulated body fluid (SBF) were explored.The results showed that with increasing the content of Mg, the bending strength, shearing strength and impact toughness of the composite with the untreated Mg alloy (un-MAO-composite) increase gradually. When the volume content of Mg alloy is 60%, they are 229.3MPa,84.5MPa and 222.4kJ/m2, respectively. MAO treatment of Mg alloy could effectively improve the interfacial bonding properties of the composite, and the interfacial debonding phenomenon was not found. Accordingly, the impact toughness increases significantly. During the preparation process, PLA at viscous flow state penetrated into the pores of the MAO coating surface, thus the mechanical interlocking was formed after PLA cooled down. In addition, the thermal expansion properties of PLA and Mg are quite different with each other, and the residual thermal stress formed during the cooling process would further enhance mechanical interlocking effect.Theoretical calculation shows that during the three-point bending process, the maximum tensile stress of PLA on the surface of the composite is about 35 MPa, which is close to the tensile strength of pure PLA (about 40MPa). Thus the tensile stress is considered as the main reason for bending failure of surface PLA. Theoretically, the maximum shear stress at the interface of two phases in the composite is about 0.4MPa during the three-point bending process, while the measured value between PLA and untreated Mg alloy is 1.08MPa, which illustrates that the shear stress at the interface only was one of the reasons for interfacial debonding phenomenon.In vitro degradation test shows that the bending strength of un-MAO-composite declined rapidly with increasing the immersion time. After 28 days immersion, the Mg flakes separated with PLA. Bending strength of MAO-composite decreased gradually, and the composite still kept intact after 28 days immersion and the bending strength was 64.9MPa, which shows that MAO treatment could significantly increase the maintain ability of the strength of the composite in SBF. The pH value of the immersed solution for pure PLA decreased gradually, while the one for MAO-composite began to increase gradually after 5 days immersion. After 7 days immersion at room temperature, H2 gas started to release for un-MAO-composite, while no gas for MAO-composite even immersed for 28 days, which indicated that the MAO ceramic coating could protect the magnesium alloy in the composite from corrosion in SBF for a long time. The molecular weight of the PLA in MAO-composite gradually decreased during immersion and after 3 weeks immersion, corrosion cracks formed on the Mg surface.