ABSTRACT
Polyolefins such as polyethylene (LDPE, HDPE, LLDPE) and polypropylene are the most used thermoplastic polymers. In 2006, the production of these polyolefins reached 105 million tons worldwide [1]. They are often filled with organic or inorganic components to increase their strength, impact resistance, or conductivity and to reduce permeability of gases [2]. Within the last years, much research in academic and industrial laboratories has focused on the field of polyolefin nanocomposites because of their high potential as materials with novel properties such as improved mechanical properties, increased heat distortion temperature (HDT), reduced permeability and flammability [3-5]. Exceptionally strong materials could be synthesized by the soft polyolefin matrix with nanosized, rigid filler particles. The properties of the nanocomposites are not only influenced by the kind of fillers, but also by the microstructure of the polyolefins and the preparation process. A lot of work has been carried out to use layered silica and metal oxides [3], clay, self-assembled nanoboehmites [6], or montmorillonite (MMT) [7-9] as fillers to obtain composite materials, which show a lower permeability for gases such as oxygen, nitrogen, steam. In the past, most composites are commonly prepared by mechanical blending of the particles or fibers above the melting. Blending or melt compounding of polyolefins with nanoparticles is hard to achieve, especially at high filler content and lead to aggregation, intercalation which decreases the mechanical properties [10]. Above 60 wt%, filler results in highly particle aggregation. For mixing in solution, the solubility of polyethylene (HDPE) and polypropylene is too low. Another problem is the hydrophilic nature of most inorganic fillers and the
hydrophobic nature of the polyolefins. The differences result in weak interfacial adhesion between filler and polymer matrix and low mechanical properties. Therefore fillers must be modified by surface active agents. Both disadvantages can be solved by in-situ generation where the catalyst is absorbed on the surface of the nanofillers, changing the surface to a hydrophobic one. In a second step, the activated fillers are used as catalysts for olefin polymerization. Each particle or fiber is covered by a polyolefin film. For bigger particles also heterogeneous nanosized Ziegler-Natta catalysts can be used [11]. For nanoparticles, homogeneous catalysts are preferred to cover the surface with active sites.
