Effect of coupling agent and silica particle size on the rheological, structural, and interfacial properties of polypropylene-based porous composites by batch foaming

Polypropylene (PP) porous composites reinforced with micro- and nano-sized silica fillers have emerged as promising materials for lightweight structural and functional applications due to their enhanced mechanical and morphological properties. This study investigates the influence of micro and nano silica fillers, with and without a coupling agent, on the porous structure and rheological properties of PP composites. Composites were prepared using an extrusion process incorporating various filler contents (3% and 10% micro silica, and 3% nano silica) both with and without surface treatment using a coupling agent. The samples were foamed using a physical foaming process to create porous structures, with conditions optimized to promote uniform pore formation. The melt flow rate (MFR) was measured to assess the impact of filler type and surface treatment on the flow behavior. Fourier Transform Infrared Spectroscopy (FTIR) was used to evaluate chemical interactions between the fillers and the PP matrix in the presence of a coupling agent. Scanning Electron Microscopy (SEM) was conducted on the foamed samples to analyze the morphology and pore architecture. Tested samples included neat PP, PP with coupling agent, and various micro/nano silica compositions. The results showed that both the size of silica particles and the presence of a coupling agent significantly influenced the pore size, distribution, and uniformity after foaming. Nano silica and micro silica with a coupling agent produced finer, more homogeneous pores, whereas uncoupled fillers led to larger and irregular pores. The study highlights the critical role of filler characteristics and interfacial compatibility in tailoring porous PP composites for lightweight and functional applications. • Micro- and nano-silica reinforcement significantly modified PP rheology , reducing melt flow rate due to increased melt resistance, while PP-g-MA partially restored flowability by improving filler dispersion. • FTIR confirmed enhanced interfacial interactions between PP and silica in the presence of PP-g-MA, with reduced free Si–OH signals and improved organization of the Si–O network. • Foam morphology strongly depended on filler size and compatibilization : micro-silica caused coarse, irregular cells, whereas nano-silica promoted finer cellular structures due to higher nucleation efficiency. • Compatibilizer dramatically improved pore uniformity , reducing cell coalescence, refining core morphology, and stabilizing cell walls across all composite formulations. • The PP–g-MA/nano-silica (3 wt%) composite produced the most uniform microcellular structure , yielding the smallest core–wall gradient and the finest pore sizes among all systems. • EDS elemental mapping demonstrated superior silica dispersion when PP-g-MA was used, especially for nanosilica, confirming the role of compatibilizer in eliminating agglomeration. • Combined rheological, structural, and morphological analyses identified 3 wt% nanosilica + PP-g-MA as the optimal formulation for lightweight, uniform, and stable PP microcellular foams.