Polypropylene (PP) is the most widely used thermoplastic. It has a good combination of physical and chemical properties, reduced cost per volume, great recyclability and good processability. PP nanocomposites have received a lot of attention in the past years because the addition of calcium carbonate nanoparticles to PP is expected to increase its stiffness and impact strength. The low concentration of nanofillers could be a cost effective alternate solution to engineering thermoplastics as long as good dispersion is achieved.
Numerous studies have been made to understand the influence of calcium carbonate (CaCO3) nanoparticles and their properties on the morphology and mechanical properties of the PP/CaCO3 nanocomposites. Some of those studies focused on median particle size and surface treatment of calcium carbonate. Particle size distribution is one of the most important characteristic of fillers. However, not much attention has been paid to studying the particle size distribution of nanoparticles after mixing.
In this work, polypropylene homopolymer composites were prepared using a Brabender internal mixer with CAM rotors (a medium shear-rate blade combining milling, mixing, and shearing forces against the test sample) at 190°C and 60rpm. Different compositions of calcium carbonate with different median particles sizes, including 0.07μm nanoparticles, with and without surface treatment were added to the polypropylene matrix.
SEM image analysis was used to obtain the particle size distribution of calcium carbonate in the polypropylene matrix. The Gamma Variate function was found to fit the particle size distribution of nanoparticles after mixing with a correlation coefficient above 0.99.
In addition to SEM image analysis, DSC and X-ray diffraction were used to investigate the morphology of the PP nanocomposites and its effect on their tensile and impact properties. Dynamic mechanical testing was also used to study the polymer melt behavior at low frequencies.
The X-Ray diffraction showed three strong α crystalline form peaks and one β crystalline form peak. The intensities of these peaks were used to calculate the k parameter to quantify the β-iPP in the PP composites. Based on this parameter, calcium carbonate nanoparticles seem to be more effective in promoting the formation of β-iPP than microparticles.
The results of the tensile tests showed that the elastic modulus increased up to 30% in comparison with that of neat PP with the addition of 20% of calcium carbonate nanoparticles. Tensile strength was also highest with the addition of 2wt% and 5wt% nanoparticles but the improvement was of only of 5% compared with neat PP. The highest tensile strength and elastic modulus were found with approximately 40% crystallinity.
Impact strength was higher with the addition of calcium carbonate microparticles. However, surface treated nanoparticles at 2wt% and 5wt% compositions and high k proved to increase the impact strength of PP above 40% compared with neat PP.
The best combination of mechanical properties was found for PP nanocomposites filled with 2wt% of 0.07μm treated particles and k=0.22.