Rheological and mechanical properties of polypropylene composites containing microfibrillated cellulose (MFC) with improved compatibility through surface silylation

We investigated the rheological and mechanical properties of a polypropylene (PP) composite reinforced by microfibrillated cellulose (MFC) fibers, of which the size was fractionated at ca. 24 µm by removing substances such as lignin or hemicellulose out of natural plants to exert the maximum mechanical performance of cellulose fibers. Due to the poor compatibility of MFC with a non-polar hydrophobic PP matrix, the MFC surface was modified through silylation and acetylation to increase its wetting and dispersion characteristics. The content of silylated-MFC could increase up to 30 wt% in the PP composite through large-volume process such as extrusion and injection molding. In addition, all mechanical properties of the silylated-MFC/PP composite were improved compared to untreated MFC/PP, acetylated MFC/PP composite. It was ensured by a strong shear thinning characteristic of the PP composite with silylated MFC content 30 wt% exhibiting a 69% lower viscosity (337 Pa·s at 4.7 s⁻¹) than the neat PP at 180 °C. The same 30 wt% silylated specimens gave a well-dispersed fibrous MFC in PP, providing a tensile modulus (3.09 GPa) and a flexural modulus (3133 MPa), which were 64 and 81% higher than the neat PP, and 16 and 15% higher than the untreated-MFC composites, respectively. The flexural strength and the heat distortion temperature were also increased by 10 and 22%, compared to neat PP, respectively. In the acetylation reaction, the fibril structure of MFC was damaged due to full substitution by acetyl groups, and the reinforcing effect was insignificant as a filler in PP matrix. The developed silylated-MFC/PP composites provided excellent mechanical properties and high-loading processability that would be difficult to achieve by other eco-friendly composite systems.