Microstructure of injected natural fiber-polymer composite
Résumé
Using short natural fibers in thermoplastic composites not only improves polymer mechanical properties, but also allows making materials that are lightweight compared to composites reinforced with glass fibers. The main applications are plastic parts for automobiles,packaging and construction.Natural fibers come from renewable resources, are cheap and easily available. However, there are a lot of morphology and properties differences between glass and natural fibers. Glass fibers have a homogeneous composition and diameter, are straight, stiff and brittle, but natural fibers usually have a various chemical compositions and appear as thick and stiff bundles and/or as thin flexible elementary fibers.During injection molding process, fibers are flowing and then, after material solidification, their dispersion and orientation are "frozen". While the microstructure of glass fibers reinforced thermoplastic is well known, natural fibers case is still not studied in details. Will natural fibers take the same orientation in the injected parts as glass fibers? How natural fibers flexibility influences their orientation? Do natural fibers remain curved in composites?The work objective is to analyze and to understand the microstructure of flax fiber-polymer-composite. Fibers were mixed with polypropylene in a twin screw extruder, pelletized and injected in a rectangular-shaped mold. By optical microscopy in transmitted and reflected light accompanied by image analysis, fiber orientation and curvature were quantified, at different depths and locations in the injected parts.Results were related with shear and elongation flows experienced by the material during processing.