Hemp fibers/polypropylene composites by reactive compounding: Improvement of physical properties promoted by selective coupling chemistry

The elaboration of composites with natural fibers and polymeric matrices has to face two major difficulties: natural fibers present a hydrophilic character (absorption of water and possible deterioration of the properties) and thermal sensitivity (the fibers can be degraded during composite elaboration and/or in certain applications). In addition, common polymers are generally hydrophobic, which induces poor compatibility at the polymer/fiber interface. It has been demonstrated that the quality of the interface between the natural fibers and the polymeric matrix is a key issue which should be controlled in order to improve the mechanical properties, the durability and the recyclability of such composites. In this purpose, our approach is to develop a new method of fibers/polymer coupling consisting in a unique operation of reactive compounding, based on an original chemistry of organosilanes in the polymer melt. Our objective is to enhance the fibers/polymer interface by using two functionalized organosilanes simultaneously. The method is then to graft covalently one of these functions on the polymer chains and the other one onto hemp surface. In parallel, formation of Si-O-Si network between the two grafted organosilanes occurs during reactive extrusion. The achievement of this grafting of hemp fibers and thermoplastic matrix during a single compounding operation constitutes a new route for the elaboration of bio-based composites. Mechanical properties (tensile and impact) and thermal behavior of these new composites were compared to those of fibers/polymer composites without coupling agent or with a single classical one (Vinyltrimethoxysilane). Results showed that the presence of two organosilane coupling agents enhances considerably the tensile strength (+40%) of the newly developed composite. Their thermal decomposition, as shown by TGA, was shifted to higher temperatures and their water uptake at room temperature was decreased. Morphological characterizations by SEM suggest better adhesion at the interface, which can explain the properties enhancement. (C) 2012 Elsevier Ltd. All rights reserved.