Overcoming the Performance Cliff at High Loadings: Enhanced Mechanical, Dielectric, and Rheological Properties of PBT Composites Reinforced with Flat Glass Fibers

High fiber loadings often cause a performance cliff in short-glass-fiber-reinforced thermoplastics because of fiber agglomeration. Herein, polybutylene terephthalate (PBT) composites reinforced with flat glass fibers (FGFR) and conventional circular glass fibers (CGFR) are systematically compared at loadings of 20–50 wt%. The flat fibers exhibit a specific surface area of 0.50 μm−1, approximately 1.61 times that of the circular fibers (0.31 μm−1). At 50 wt% loading, FGFR achieves a tensile strength of 115.92 ± 1.29 MPa and a notched impact strength of 13.07 ± 2.06 kJ/m2, which are 11.4% and 29.7% higher than those of CGFR, respectively (p < 0.001 and p < 0.05, Welch’s t-test). In contrast, CGFR reaches its maximum tensile strength of 107.45 ± 3.84 MPa at 40 wt% and then decreases by 3.2% at 50 wt%; together with a significant quadratic (inverted-U) trend of the CGFR strength–loading curve (p < 0.001) and the significant FGFR–CGFR divergence at 50 wt%, this reversal marks the onset of the performance cliff. At 10 GHz and 50 wt% loading, FGFR exhibits a dielectric constant of 4.30 and a loss tangent of 0.014, which are 17.8% and 22.2% lower than those of CGFR, respectively, owing to suppressed interfacial polarization associated with the preferential layered arrangement of the flat fibers. Rheologically, as the fiber content increases from 20 to 50 wt%, the melt flow rate of FGFR decreases by approximately 35.5%, whereas that of CGFR decreases by approximately 60.5%. The modified Halpin–Tsai, Kelly–Tyson, Maxwell–Garnett, and Krieger–Dougherty models, incorporating fiber geometry and a unified agglomeration correction, semi-quantitatively capture the tensile, impact, dielectric, and rheological properties. These results demonstrate that flat-fiber cross-section engineering is an effective strategy for developing high-performance wave-transparent composites under high filler loadings.