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Thermoplastic resins, which are lighter than metals and ceramics, have many features such as excellent processability. They are used in widely various fields for daily necessities and for transportation equipment. Even for the same material, the elastic properties of the thermoplastic resin can vary depending on the molecular weight, molding temperature and conditions. Therefore, the elastic properties required for structural analysis must be evaluated each time. After this study derived a formula for yield initiation stress of injection-molded thermoplastics, the formula was applied for a method of obtaining the longitudinal elastic modulus and Poisson's ratio from results of a three-point bending test. From actual examination using polypropylene (PP) and polystyrene (PS), the calculated longitudinal elastic modulus and Poisson's ratio showed good agreement with literature values. Results clarified that these values depend on the injection molding temperature. Furthermore, the bulk modulus and shear modulus were obtained from the longitudinal modulus and Poisson's ratio. Their injection molding temperature dependences were investigated. The shear modulus showed the same tendency as that of the longitudinal elastic modulus, but results indicated the dependence of bulk modulus as small in PP: bulk modulus of PS increased with increasing injection molding temperature.

Effect of calcium carbonate (CaCO3) particle dispersion on the mechanical properties of polypropylene (PP)/montmorillonite (MMT) composites was investigated. These composites were fabricated by melt-mixing using a twin-screw extruder, and the dumbbell shaped specimen was also fabricated by injection molding. Tensile and notched Izod impact tests were conducted by using dumbbell shaped specimens. The results showed that CaCO3 particle dispersion improved the elongation at break and notched Izod impact strength of PP/organized MMT composites. This mechanism was discussed from MMT dispersion and fracture surface observations using a scanning electron microscopy. It was found that organized MMT was well-dispersed by dispersed CaCO3 particle. The reason was thought to be that localized shear stress near MMT was increased by increase of localized viscosity near MMT due to CaCO3 particle dispersion. Well-dispersed MMT improved the the stability of plastic deformation at quasi-static loading and energy dissipation at impact fracture, resulting in the improvement of the elongation at break and impact strength of PP/MMT composites. In addition, CaCO3 particle dispersion also improved the tensile modulus of PP/MMT composites. This mechanism was discussed from interparticle distance of MMT and CaCO3 calculated by Tyuzyo's equation. From the above results, it was suggested that CaCO3 particle dispersion was an effective way for improvement of mechanical properties and impact strength of PP/MMT composites.