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In the development of thermoplastic products, it is necessary to conduct the necessary mechanical tests and evaluate the reliability of thermoplastics in each case, because the mechanical properties of the same material vary depending on the molding process conditions and product shape. In order to build a sustainable society, it is expected that the evaluation of the mechanical properties of thermoplastics, which are resource and energy saving, will be required. In this paper, the glass transition temperature and melting point of injection-molded thermoplastics were evaluated by differential scanning calorimetry, and the correlation between Poisson’s ratio and free volume was obtained by applying the theory proposed by Flory et al. A certain correlation was found between the Poisson’s ratio of polymers and the change in free volume determined by the glass transition temperature. It is also clear that this relationship can be approximated by orders of magnitude. The Poisson’s ratio of the core layer tended to be smaller than that of the skin layer. It has also been found that there is a negative correlation between the Young’s modulus and the free volume of the polymer material.

The mechanical recycling of discarded plastic products as resources for environmental preservation has recently gained research attention. In this context, it is necessary to use waste materials for fiber-reinforced thermoplastics (FRTP). Glass and carbon fibers are often damaged by shear and compression during melt-forming processes. To achieve a sustainable society, it is necessary for thermal recycling to produce minimal to no residue and for mechanical recycling to maintain the length of fibers used in FRTP to preserve their performance as a reinforcing agent. Aramid fibers (AFs) do not shorten during the melt-molding process, and their composites have excellent impact strength. On the other hand, plastics reinforced with glass or carbon fibers are reported to have a superior strength and modulus of elasticity compared to aramid fibers. This study investigates the dispersion of a carbon nanofiber (CNF), a whisker, as the third component in aramid-fiber-reinforced polypropylene (PP/AF). The results and discussion sections demonstrate how the dispersion of CNF in PP/AF can enhance the mechanical properties of injection-molded products without compromising their impact resistance. The proposed composition will have excellent material recyclability and initial mechanical properties compared to glass-fiber-reinforced thermoplastics.

Short fiber-reinforced thermoplastic polymers (SFRTPs) are commonly used in various molding methods due to their high specific elasticity and strength. To evaluate the interfacial strength, several determination methods have been proposed, including the interfacial shear strength (IFSS). In previous research, an IFSS evaluation method based on the short beam shear method was proposed. However, this method is only applicable to micrometer-sized fibers with high stiffness levels that are not easily bent. When utilizing cellulose fiber, the interfacial shear strength (IFSS) results frequently exhibit significant deviations. To tackle this issue, we suggest an enhanced experimental technique that employs beam-shaped specimens with welding points based on the short beam shear test. Furthermore, we conducted a three-dimensional analysis of the original method to determine the fiber orientation angle and IFSS. The outcomes were compared with previously reported determinations. The IFSS achieved through the novel method proposed in this paper exhibits high precision and reliability, rendering it suitable for use with soft and flexible fibers.

The surface mechanical properties of thermoplastics are crucially important for evaluating molded products’ vulnerability to scratching. Because surface mechanical properties reflect material performance directly in terms of durability and frictional behavior, understanding and modeling them is important for industry and research. This emphasizes the surface mechanical properties of Vickers hardness and the static friction coefficient, with attempts to model them as functions of stress at yield initiation. Vickers hardness can be related to the compressive stress at yield initiation. The static friction coefficient can be modeled as a function of the surface shear strength and Vickers hardness. This research has improved our understanding of thermoplastics’ surface mechanical properties and has enabled the prediction of the scratch performance of molded products and the provision of effective indicators for material design.

Thermoplastics, which are used in widely various applications from consumer goods to automobiles, are lighter than metals and ceramics and have excellent moldability. Injection molding is often used as a thermoplastic molding process because it can achieve near-net-shape molding and because it has excellent mass production capabilities. Injection molded products have a hierarchical structure, with the surface area contacting the mold designated as the skin layer and the interior of the molded product broadly identified as the core layer. The molecular orientation formed by the melt molding process is strongly dependent on the flow direction. Quantifying anisotropy is extremely important when discussing the mechanical properties of thermoplastic molded products. This study clarifies whether the mechanical anisotropy of polymer injection molded products can be evaluated quantitatively using short-beam shear testing. This study also elucidates the quantitative significance of the quantitative values.Results show that the shear stress components in each direction obtained from the short-beam shear test are components of the yield initiation shear stress under uniaxial tensile loading resolved in each direction. To quantify the degree of stress concentration attributable to the internal structure of the molded product, the ratio of the yield initiation shear stress to the shear stress components in each direction was divided by √3. Comparison of the degree of stress concentration with the shear stress components generated in the short beam shear test showed the thickness direction as weakest against shear stress and showed the width direction as the strongest.

A method for evaluating the surface shear strength of thermoplastics is proposed and validated. Thermoplastics are commonly used for their mass production and solid properties, but their surface properties, specifically friction and wear, are also required in some cases. The surface strength against shear loading, which has not been evaluated quantitatively as a material mechanical property, must be ascertained to elucidate these properties. We propose the overlapped short-beam shear tests to evaluate the surface shear strength of injection-molded thermoplastics. We also perform ball on disk testing to validate the results obtained from the overlapped short beam shear tests. Results show that the surface shear strength obtained from the proposed test is related to the frictional force obtained from the ball on disk tests, thereby validating the proposed method.

Glass short fiber-reinforced thermoplastics (SGFRTP) are used to reduce carbon dioxide emissions from transportation equipment, especially household vehicles. The mechanical properties required for SGFRTP include flexural strength, impact resistance, etc. In particular, impact resistance is an important indicator of the use of SGFRTP. For this study, a mechanical model was developed to explain the notched impact strength of SGFRTP injection molded products in terms of their interfacial shear strength. The values obtained from the model show good agreement with the experimentally obtained results (R2 > 0.95). Results also suggest that the model applies to different fiber orientation angle and a range of fiber lengths in the molded product that are sufficiently shorter than the critical fiber length.

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The molecular orientation formed by melt-forming processes depends strongly on the flow direction. Quantifying this anisotropy, which is more pronounced in polymer blends, is important for assessing the mechanical properties of thermoplastic molded products. For injection-molded polymer blends, this study used short-beam shear testing to evaluate the mechanical anisotropy as a stress concentration factor, and clarified the correlation between the evaluation results and the phase structure. Furthermore, because only shear yielding occurs with short-beam shear testing, the yielding conditions related to uniaxial tensile loading were identified by comparing the results with those of three-point bending tests. For continuous-phase PP, the phase structure formed a sea-island structure. The yield condition under uniaxial tensile loading was interface debonding. For continuous-phase PS, the phase structure was dispersed and elongated in the flow direction. The addition of styrene–ethylene–butadiene–styrene (SEBS) altered this structure. The yielding condition under uniaxial tensile loading was shear yielding. The aspect ratio of the dispersed phase was found to correlate with the stress concentration factor. When the PP forming the sea-island structure was of continuous phase, the log-complex law was sufficient to explain the shear yield initiation stress without consideration of the interfacial interaction stress.

Polycarbonate (PC), an engineering plastic, has excellent mechanical strength and toughness. Moreover, this transparent polymer material can be used in fields where materials require mechanical properties and transparency. Nevertheless, PC is known to have a high melt viscosity. Moreover, blending with polystyrene (PS), an inherently brittle material, has been used to adjust its melt viscosity. As a result, the PS makes PC/PS polymer blends more brittle than PC alone. As described herein, after attempting to achieve compatibility with inorganic particles, the results show that the dispersion of small amounts of inorganic clay and silica particles in PC/PS polymer blends maintained transparency while improving the impact strength to a level comparable to that of polycarbonate. Apparently, the inorganic particles promote the fine dispersion of PS. Moreover, the spherical morphology of the inorganic particles is more effective at compatibilizing the polymer blend because the inorganic particles can apply isotropic interaction forces.

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.