An experimental study was carried out to investigate the effects of coupling agents on the rheological properties, processability, and mechanical properties of highly filed polypropylenes (PP). Inorganic fillers used were CaCO₃ and glass beads, and coupling agents used were two silane coupling agents, N‐octyl triethoxy silane and γ‐aminopropyl triethoxy silane, and one titanate coupling agent, isopropyl triisostearoyl titanate. It was found that the addition of the coupling agents to the PP‐CaCO₃ (50 wt percent) decreased the melt viscosity and increased the melt elasticity (first normal stress difference). However, the addition of the silane coupling agents to the PP‐glass beads (50 wt percent) affected the rheological properties of the melts quite differently. The N‐octyl triethoxy silane had relatively little effect on either the melt viscosity or the melt elasticity, whereas the γ‐aminopropyl triethoxy silane increased the melt viscosity and decreased the melt elasticity. The CaCO₃‐ or glass bead‐filled polypropylenes, with and without coupling agents, were injection‐molded and the mechanical properties of the molded specimens were measured by the use of an Instron testing machine. It was found that the effect on the tensile strength and percent elongation of the filled polypropylenes depended upon the specific coupling agent utilized. A melt‐spinning study was also carried out to investigate the effect of coupling agents on the spinnability (defined as the maximum draw‐down ratio) of the PP‐CaCO₃ system, and on the mechanical properties of the melt‐spun fibers. It was found that the spinnability of the PP‐CaCO₃ was enhanced considerably by the addition of the coupling agents, and that the tensile strength of the melt‐spun fibers was also improved by their addition. Scanning electron micrographs were taken of the fracture surface of injection‐molded specimens and an attempt was made, with the aid of photomicrographs, to explain the mechanical properties of molded specimens observed experimentally.

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