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The morphology and mechanical properties of iPP/SiO2 composites modified with metallocene propylene-based copolymers

The morphology of iPP/SiO2 96/4 composites modified with metallocene propylene-based copolymers m-EPR has been studied with optical and scanning electron microscopy while mechanical properties were investigated with tensile and impact measurements. The addition of four silica fillers differed in size (micro vs. nanosilica) and surface properties (hydrophilic vs. hydrophobic, e.g. polar vs. non-polar) enabled the comparison of different effects of filler (e.g. nucleation effect investigated with differential scanning calorimetry) on polymer matrix. Two elastomeric m-EPR1 and m-EPR2 polypropylene-based copolymers with different viscosity were added as impact modifiers and compatibilizers in range 0-20 vol% per hundred composite parts of iPP/SiO2 96/4 composites. Silica fillers and m-EPR elastomers affect the spherulite growth in the iPP matrix by two opposite, competetive effects: nucleation effect of filler and solidification effect of elastomer. Final morphology of ternary polymer composites is the result of filler and elastomers influence on polymer matrix but also the result of their mutual interactions and interactions with the matrix [1]. The addition of all silica grades affected crystallization and decreased the spherulite size in the iPP matrix as well as transformed well-developed spherulitic morphology to the morphology with irregular spherulitic grains. Although filler particle size played important role in spherulite growth, surface modification of considered silica fillers exhibited stronger influence on spherulite size than size of silica particles. The differential scanning calorimetry (DSC) investigations of binary iPP/SiO2 composites [2] showed that silica fillers with active polar surfaces (like S-120 and A-200 fillers with hydroxyl groups and A-R7200 with methacryl groups) have strong nucleation ability. Increased crystallization temperatures Tc and decreased supercooling effect ΔT = Tm - Tc are related to stronger nucleating effect of filler. Accordingly, nucleation ability of non-polar microsilica S-D17 is supposed to be lower due to better miscibility/compatibility of nonpolar S-D17 surfaces with iPP chains than silicas with polar surfaces. Analogous supercooling behavior was also observed in ternary iPP/SiO2/m-EPR composites what implies negligible effect of m-EPR elastomer on crystallization temperature Tc. This fact could be explained by prevailing of separated morphology of the iPP/SiO2/m-EPR composites observed by scanning electron microscopy (in which dispersed filler particles exhibited undisturbed nucleation ability despite of 20 vol% added m-EPR elastomers). The optical microscopy results showed that in ternary polymer composites solidification effect of both m-EPR elastomers did not overcome the nucleation effect of silica filler like in our previous investigation with styrenic-block copolymers [3, 4]. The spherulites in iPP/SiO2/m-EPR composites with microsilica S-120 and nanosilica A-200 are irregular with diffuse boundaries and grain-like morphology. Scanning electron microscopy revealed mainly separated morphology with rather rare interaction between dispersed filler and elastomers particles in these composites. This fact indicated better interaction between the propylene-based m-EPR copolymer and polypropylene matrix than m-EPR copolymers with silica fillers. The composites iPP/SiO2/m-EPR with treated surfaces (microsilica S-D17 and nanosilica A-R7200) formed combined morphology consisting of separated and multiple inclusion morphology observed in SEM micrographs. The m-EPR elastomers exhibited stronger recovering effect in spherulite grow (observed by polarizing optical microscopy) in these composites due to lesser nucleation effect of treated silica particles. Accordingly, solidification effect of both elastomers (especially m-EPR2 with lower viscosity) partly prevailed the nucleation effect of silicas with treated surfaces (microsilica S-D17 and nanosilica A-R7200). Tensile properties investigation of iPP/SiO2 composites modified with m-EPR elastomer showed that Young's modulus steadily rised with increased filler content due to reinforcing effect of filler [2, 5] while the addition of m-EPR elastomers to the iPP/SiO2 composite reduced composite stiffness. The addition of m-EPR elastomers to the iPP/SiO2 composites caused general decrease of tensile strength at break σb values. Ternary composites with treated filler S-D17 exhibit somewhat lower σb values than those ones with other fillers, probably as a result of the weakest nucleation ability and consequently larger spherulites. The m-EPR elastomers, especially m-EPR1 with longer macromolecules, improved significantly impact properties of all iPP/SiO2/m-EPR composites. The morphology with small spherulites or irregular grains in composites with hydrophilic silica fillers (S-120 and A-200) which exhibit strong nucleability seems to affect impact properties additionally. Presented results confirmed improved toughening of the iPP/SiO2 composites by m-EPR elastomers as very good impact modifiers but also indicated that the role of m-EPR as compatibilizers is not truly fulfilled is this composites like expected and predicted.

Polypropylene composites; Morphology; Mechanical properties; Structure-property relationship

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