Dispersion And Properties Of Polypropylene/Micro-and Nanosized Silica Composites

Ultrafine silica particles, including micro- and nanosized silica, have been employed in a variety of areas because of their optical transparency, electrical insulation, chemical and physical stabilities, biocompatibility and tunable surface properties. Polypropylene (PP) is an important general plastic and PP-silica composites have attracted more and more attention owing to their excellent processability, unique mechanical and thermal stability, etc. However, to enhance the dispersion of nano-fillers is a major challenge due to their strong tendency to agglomerate. In the dissertation, modified nano-silica and silica-polymer hybrid-particles were respectively compounded with polypropylene (PP) by melt mixing to prepare PP/SiO2 composites. The dispersion of SiO2 in PP, as well as structure and properties of PP/SiO2 composites were studied. The main contents and results were drawn as following:(1) Maleic anhydride grafted styrene-ethylene-butylene-styrene (MA-SEBS) was used as compatibilizer of PP/SiO2 nanocomposites. The effects of MA-SEBS on dispersion, mechanical properties, crystallization and melting properties were stedied. MA-SEBS enhances the dispersion of silica in PP and improves the interface adhesion between silica and PP matrix, which leads to improving the impact strength of composites.(2) Nano-sized silica particles were modified with methyl methacrylate (MMA) and butyl acrylate (BA) by in situ emulsion copolymerization. These modified nanoparticles were compounded with PP to prepare PP/SiO2 nanocomposites. The effects of modification on dispersion of SiO2 in PP, mechanical properties, crystallization were studied.PMMA and PBA are successfully grafted onto nano-SiO2. The solubility parameter and modulus of the copolymer covering on SiO2 surface are controlled by means of changing the copolymer composition. PMMA grafted on nano-silica enhances the dispersion of the nanoparticles in PP and interfacial adhesion between SiO2 and PP, decreases the size of PP spherulites in nanocomposites, which increase the Young's modulus and toughness of PP/SiO2 nanocomposites. PBA grafted on nano-silica further improves the dispersion and the interfacial interaction, decreases the size of PP spherulites in PP/SiO2 nanocomposite, which leads to further toughening the PP/silica nanocomposite. Howerever, its low modulus decreases the modulus of the PP/silica nanocomposite. The nanocomposites containing PP and nano-silica particles modified with P(MMA-co-BA) have balanced stiffness and toughness due to the moderate modulus and solubility parameter of P(MMA-co-BA).(3) A series of core-shell silica hybrid-particles with nanometer P(MMA-co-BA) shells were fabricated by the sol-gel process and emulsion copolymerization of methyl methacrylate (MMA) and butyl acrylate (BA), which were subsequently compounded with PP in the molten state to prepare homogeneously dispersed PP/SiO2 composites. The effects of silica hybrid-particles on dispersion of SiO2 in PP, mechanical properties and crystallization behave were studied.Upon changing the sizes of silica and the feed silica/co-monomer ratio, the sizes of silica hybrid-particles P(MMA-co-BA) and shell thickness on the silica core were controled. Owing to the existence of the nanometer P(MMA-co-BA) shells, the silica hybrid-particles were almost mono-dispersed in the PP matrix and improve the the interfacial interaction between PP and silica, which leads to improve the impact strength of composites. The sizes of silica hybrid-particles and shell thickness have great effects on mechanical properties.That is, when the polymer shell thickness was 15 nm, compared to pure PP, the impact toughness of the PP/silica composite was more than doubled with little degradation of tensile strength. The homogeneous debonding at the PP/silica interface, plastic void expansion and plastic matrix shearing during impact fracture greatly toughened the PP/silica composites. The incorporation of well-dispersed silica hybrid-particles acts as nucleating agents to increase the Tcp of PP phase in the PP/SiO2. The non-isothermal crystallization kinetics of PP/SiO2 composites is studied using Mo approach.