The improvement of mechanical properties of spherical silica particle-filled polyolefin were investigated using di-block polymer consisting of polyolefin unit and ethyl acrylate-acrylic acid copolymer unit. The silica particles were treated with a silane coupling agent having amino group. The amino group introduced on the particle surface was expected to react with the carboxyl group in the di-block polymer. The compounds were prepared by melt blending and the mechanical properties were evaluated by a tensile test. As a result, both fracture and yield stresses were improved by additioning of di-block polymer. However, it seemed that the effect was strongly dependent upon the additional order of the components during melt blending process. The higher effect was obtained in the procedure of which the silica particles were dispersed in the matrix previously, then di-block polymer was added in the system. And there was no change in the procedure by the opposite additional order. The polyolefin units in di-block polymers tend to assemble strongly each other strongly in the polyolefin matrix. Therefore, they hardly moved and localized at the particle/matrix. This seems to be the reason why the mechanical properties of the composite modified modified with di-block polymer were strongly affected by additional order during the melt blending process.

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The effect of stress ratio on the fatigue failure of short-fiber reinforced polypropylene (PP) was investigated. An amino silane coupling agent was employed to improve the interfacial adhesion between PP and glass fiber, and a urethane type sizing agent was applied to deteriorate the adhesion. Fatigue tests were carried out at stress ratios of 0, -1, and ∞. The stress-strain relationship and the surface temperature of the specimens were measured during fatigue. The fatigue strength for R=∞ was 50% lager than that for R=0 for the same number of cycles to failure, and the temperature increase for R=0 was greater than that at other stress ratios. The surface temperature at R=∞ of the composite was almost identical of that of the resin. It must have come from the difference in the fatigue damage mechanism between composites and the resin. Temperature increase of the composite during cyclic loading at R=0 may have come from the damping effect in high-tension stressed resin around the ends of fibers. However, at R=∞ stress around them may have been much smaller than that for R=0 because of the buckling of fibers under compressive stress. To remove the effect of temperature increase, the fatigue test was carried out under the constant specimen temperature of 25 °C, and it was found that the fatigue life was 100 times longer than that obtained from the uncontrolled test where the specimen temperature increase during fatigue. For the fatigue test at 25 °C, the brittle fracture surface was observed, while ductile fracture surface appeared for the temperature increase fatigue test.

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