Understanding the effect of nanofillers on the properties of polypropylene and glass fiber/polypropylene multiscale composites

In this study, polypropylene (PP) based nanocomposites were prepared by incorporating different kinds and amounts of silica nanoparticles and graphite nanoplatelets (GNP). The role of various percentages of compatibilizer polypropylene grafted with maleic anhydride (PPgMA) into PP nanocomposites was also investigated. In order to analyze the effect of the manufacturing process on the material’s properties, the samples were produced by (i) melt compounding and compression molding and (ii) extrusion and injection molding. It was found that injection molding provides significantly greater stiffness and strength compared to compression molding for all types of PP nanocomposites. Several characterization techniques were used in order to correlate the microstructure to the physical and mechanical properties of the materials. Both silica and GNP were found to be effective nucleating agents, significantly increasing the crystallization rate during isothermal crystallization and favoring the nucleation of the the β- phase, which manifests superior impact strength and toughness compared to the most common α-form crystals. Graphite nanoplatelets were found more efficient in inducing polymorphism and favoring the formation of a transcrystalline phase on the filler surface. A significant correlation between the tensile modulus, glass transition temperature and the amount of constrained phase, as assessed through tensile and DMA analyses, revealed the presence of a secondary reinforcing mechanisms, which, concurrently to the primary stiffening effect of the high modulus filler, contributes to the enhancement of the bulk properties. A complex constrained phase, responsible for providing a secondary reinforcing mechanism, was modeled as immobilized amorphous and transcrystalline regions located at the filler surface. The non-linear viscoelastic creep of the composites, successfully studied by the application of the time strain superposition principle (TSSP), showed a considerable enhancement of the creep stability in nanocomposites with respect to unfilled PP, especially for higher creep stresses. The study of creep dependance on the temperature showed that the stabilizing effect provided by the nanoparticles was more effective at high temperatures and, considering the time temperature superposition principle (TTSP), at long loading times. The equivalence between the time strain- and time temperature- superposition principle was substantiated by comparing the correspondent superimposed master curves. The nanofilled PP matrices have also been used for the preparation of microcomposites reinforced with short glass fibers (GF). Interfacial shear strength (ISS) was measured by means of the single fiber fragmentation test on various PP/GF microcomposites. Results show that the strength at the fiber/matrix interface can be remarkably increased when using nanocomposite systems, especially in the case of dimethyldichlorosilane-functionalized silica nanoparticles and GNP platelets, and that the improvement is further increased when the nanoparticles are used in combination with PPgMA. The thermodynamic fiber/matrix work of adhesion, estimated by contact angle measurements, showed a good correlation with the ISS values. Hybrid composites reinforced with short glass fibers and nanofillers were produced and characterized in order to investigate how the morphology and the mechanical properties of the composites were affected by the combined effect of two fillers of rather different size scales (i.e. micro- and nano- scale). The stronger fiber/matrix adhesion combined with the enhancement of the matrix properties resulted in superior tensile properties and impact resistance and improved viscoelastic behavior. As means of comparison, thermosetting hybrid composites based on epoxy resin were also produced by incorporation of GNP and short GF.