Polypropylene layered silicate compounding

Lee, S. H., Cho, E., and Youn, J. R., Rheological behavior of polypropylene/layered silicate nanocomposites prepared by melt compounding in shear and elongational flows, J. Appl. Polym. Sci, 103, 3506-3515 (2006). 

Even after organic modification of the clays, polypropylene does not wet the surface of clays because it is nonpolar. It is necessary to blend in a functionalized polymer such as maleated polypropylene (PP-g-MA) that wets the modified clay surface more readily and is also miscible with the bulk polymer. Okada and coworkers were the first to produce polypropylene layered silicate nanocomposites by melt compounding the modified elay with PP-g-MA and PP. The progress made since then in preparing and characterizing polypropylene layered silicate nanocomposites is reviewed in this chapter. We discuss advances in formulations, preparation methods and characterization then proceed to effects of the dispersion state (intercalated vs. exfoliated) and of silicate loading on crystallinity, mechanical performance and other properties, and end with a summary of progress to date with these composites. All the results presented in this chapter refer to isotactic polypropylene nanocomposites with layered or smectite clays. 

Polypropylene (PP) is one of the most widely used plastics in large volume. To overcome the disadvantages of PP, such as low toughness and low service temperature, researchers have tried to improve the properties with the addition of nanoparticles that contains p>olar functional groups. An alkylammonium surfactant has been adequate to modify the clay surfaces and promote the formation of nanocomposite structure. Until now, two major methods, i.e., in-situ polymerization( Ma et al., 2001 Pirmavaia, 2000) and melt intercalation ( Manias et al.,2001) have been the techniques to prepare clay/PP nanocomposites. In the former method, the clay is used as a catalyst carrier, propylene monomer intercalates into the interlayer space of the clay and then polymerizes there. The macromolecule chains exfoliate the silicate layers and make them disperse in the polymer matrix evenly. In melt intercalation, PP and organoclay are compounded in the molten state to form nanocomposites. 

With UlOlO-type C18-Mt and C18-Mc, a peak associated with the layered structure of the silicate was observed around 20 = 0.8°. It is inferred from this that silicate layers with UlOlO intercalated are dispersed and that they maintain their layered structure. The interlayer distance for UlOlO-type PPCN (PP+U1010+C18-Mt) (Fig. 30f) is equivalent to that of the U1010/C18-Mt interlayer compound (Fig. 30b) which was fabricated using UlOlO and C18-Mt in the same proportions. Therefore, it is thought that UlOlO is selectively-intercalated between the layers of C18-Mt during mixing, even if polypropylene is mixed simultaneously. 

The intercalation of polymers (PS, PEO, polypropylene, " etc.) from their melts into a silicate lattice is also a promising procedure. Molecular dynamics calculations and the kinetics of formation of these intercalates were considered in this study. A new line of investigation involves direct hydrothermal crystalhzation of sihcate layers from a polymer gel. This approach extends the range of intercalated polymers because it allows one to use compounds that do not contain functional groups. 

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