Double Yielding of Polypropylene/Calcium Carbonate Composites: Influence of Filler Content and Test Conditions

ABSTRACT The double yielding phenomenon of homopolymer polypropylene (H‐PP) is reported for the first time, along with a systematic investigation of PP/CaCO 3 composites. The tensile deformation process is divided into four stages, with double yielding occurring in stages II and III. Differential scanning calorimetry (DSC), polarized light microscopy (PLM), scanning electron microscopy (SEM), and thermal infrared imaging indicate that CaCO 3 's effect is filler content dependent: an appropriate filler content (10%) induces β‐crystal formation that promotes uniform deformation, whereas high levels (20%) lead to particle agglomeration and premature second yielding. A mechanism involving two synergistic plastic processes, namely intraspherical and interspherical deformation, is proposed. In HPP, the first yield is governed by amorphous phase deformation and the second by‐ α‐crystal transformation, while in PP/CaCO 3 composites, interspherical deformation drives the first yield and loose βcrystals yield before necking, delaying the‐ α‐crystal‐dominated second yield. Quantitative analysis using the Ree‐Eyring, linear, and quadratic regression models provides key insights into yield stress, local temperature rise, softening slopes, and stress drops, offering a scientific basis for optimizing these composites in high‐performance engineering applications.