Study On The Relationship Between Viscoelasticity Of Shape Memory Polyurethane And Restoring Force For Orthodontic Arch Wire

In orthodontics with fixed appliance, two problems exist with Ni-Ti arch wire, one is release of nickel ions which will cause anaphylactic; the other is rapid decrease of applied force. With the properties of a kind of biology security and the glass transition temperature of 35℃, the shape memory polyurethane can avoid above problems.First, material of polyurethane was formed into wire through extrusion. Elastic modulus and viscosity coefficient of the material at the temperatures of 24℃,35℃ and 50℃ were determined by matching the curves obtained from the results of tensile tests of the wires, and so the relationship between temperature and material properties under different temperatures were created. Then, based on the above results, in order to perform finite element analysis on the thermo-mechanical properties of SMP wire via ABAQUS, a three-dimensional thermo-mechanical constitutive equation of SMP was developed based on TOBUSHI’s one-dimensional equation. The thermo-mechanical procedure of PU was numerically simulated by ABAQUS coupled with the UMAT subroutine, which includes the deformation at a high temperature upon loading shape fixity due to cooling and the shape recovery through heating. The numerical results showed that the wire completed the shape memory procedure by simulation analysis, and under the condition of different strain, the wire was unloading at Tg -20℃ temperatures, the shape fixed rate was close to 80%. During temperature increasing from Tg-20℃ to Tg+20℃, the deformation of the wire recovered fully, and the stress decreased with the increase of temperature, and the size of the restoring force was within the scope of the 65g to 120g.Finally, according to the experiments and simulation, the viscoelasticity impact of shape memory polyurethane to restoring force for orthodontic arch wire was studied and the results were analyzed. The constraint of the orthodontic arch wire in the mouth was analyzed and simplified. The simply supported beam tes、the cantilever beam test and the three-point bending test were performed to SMP arch wire. In order to explore the slow release of orthodontic force, stress relaxation experiment was conducted and the wire was applied in the typodont model to verify the effect of orthodontic. The results showed that, due to the viscoelasticity impact for the wire and within a small deformation, the deformation curve fit with the standard deflection equation of elastic rod in simply supported beam test and cantilever beam test. The stress-strain curve of three point bending test showed that the stable mechanic performance of the wire was good for the visco-elasticity of the wire. The relaxing test showed that the release of mechanics was slow enough to meet with the force required in orthodontic, and the result of verification test in the typodont model was effective for orthodontic. All the tests proved that the polyurethane arch wire, together with properties of transparent and biocompatible, has a good future to apply in orthodontic.