Effects of Glass Fiber Content on the Cellular Morphology and Mechanical Properties of Microcellular Injection-Molded...

Glass-fiber-reinforced polypropylene (GF/PP) composite foams combine reinforcement with weight reduction, but how GF content governs the competition between cell refinement and foam expansion under fixed-cavity molding remains unclear. GF/PP composite foams containing 0-15 wt.% GF and a fixed 5 wt.% PP-g-MAH were prepared by fixed-cavity MuCell injection molding. Gas-free melt rheology, crystallization, cellular morphology, density, and mechanical properties were evaluated. Increasing GF content raised the apparent viscosity and shifted crystallization toward higher temperatures. The cell density increased from 6.12 × 10⁴ to 1.56 × 10⁷ cells cm^-3, whereas the average cell diameter decreased from 264.5 to 27.1 μm. However, the expansion ratio peaked at 1.27 for GF9 and then decreased to 1.12 for GF15, showing that continuous cell refinement did not produce a monotonic improvement in foam expansion. These trends suggest competing effects of GF on cellular development: at low-to-intermediate contents, GF interfaces may favor heterogeneous nucleation, while increased melt resistance may suppress excessive cell growth and coalescence; at higher contents, fiber crowding, reduced interfiber spacing, geometric confinement, and interfacial heterogeneity may increasingly restrict cell growth and foam expansion. Although the absolute tensile and flexural properties increased with GF content, the density-normalized properties and impact strength varied nonmonotonically. GF9 exhibited the highest measured mean specific tensile, flexural, and impact strengths, together with a mean impact strength of 24.4 kJ·m^-2, which was 32.6% higher than that of GF0. Thus, cell refinement alone does not ensure improved lightweight efficiency; an intermediate GF content provides the most favorable balance among cellular refinement, foam expansion, and fiber reinforcement.