|M.Sc Student||Anna Faingold|
|Subject||Hybrid Fibril/Nanoparticle Injection Molded Polymer|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Siegmann Arnon (Deceased)|
|Full Thesis text|
Polymer composites containing fibrilar or particulate fillers have been intensively studied; however, systems containing both have been just rarely investigated. Hybrid composites containing both fibril and nanoparticle fillers are of potential interest when each of them is expressed in the composite and synergetic effects are exhibited.
The objective of the present study was to investigate hybrid carbon fiber (CF)/nanoparticle polypropylene injection molded composites. Microscopic and macroscopic properties of the hybrid composites, which include various contents and sizes of TiO2, Al2O3 or carbon black (CB) nanoparticles, were to be characterized.
The composites compounding was preformed in an extruder and then injection molded into a standard ASTM mold. The morphology of the composites was studied via electron microscopy to analyze the fiber orientation. The fiber length and its distribution were investigated via optical microscopy. The electrical conductivity, mechanical, rheological and thermal properties of the composites were also studied.
Morphology studies of the composites revealed that the presence of nanoparticles strongly affected the CF orientation. Skin/core structure, typical for fiber reinforced injection molded composites, becomes undetectable and fibers are mainly disoriented through the sample volume in composites reinforced with 8%wt CF and 5%wt of any studied nanoparticles smaller than 40nm in diameter.
The nanoparticle type and size incorporated in the hybrid composites, surprisingly, were found to affect the fiber attrition. Mean fiber lengths significantly shorter than 200µm, typical for injection molded brittle fibers reinforced composites, were found in the hybrid composites containing TiO2 30-40nm nanoparticles. Replacing the TiO2 nanoparticles by Al2O3 or CB of the same size and content, results in higher fiber length values.
The electrical conductivity of the hybrid composites also exhibits unexpected behavior. While 8%wt of CF is above the percolation threshold for polypropylene injection molded composites, the addition of 5%wt of insulating nanoparticles, smaller than 40nm in diameter, results in insulating hybrid composites.
The mechanical properties of the studied composites are affected by the changes in the fiber length and orientation.
The principle conclusion of the present study is that the injection molding of hybrid composites results in significantly shorter fiber length, possibly due to surface defects of the fibers induced by the nanoparticles during the flow process, enabling further fiber breakage under the existing stresses. The reduction of fiber length depends on the nanoparticles type and size and seams to affect the fiber orientation during the injection molding. Both effects determine the composites mechanical and electrical properties.