Polypropylene and Thermoplastic Olefin Nanocomposites
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The purpose of this chapter is to review the progress toward the preparation of polypropylene/clay and thermoplastic olefin/clay nanocomposites. Further, the structure and properties of these type of nanocomposites are discussed and, where possible, structure/property relationships are described. Lastly, the progress toward the achievement of commercially viable nanocomposites of this type is assessed.
Polyolefins account for about 185 billion pounds per year or about 50% of the total worldwide plastic's industry production. Polypropylene (PP) accounts for about 20% of this total or 74 billion pounds per year production. Therefore polypropylene is one of the most widely used thermoplastics not only because of its balance of physical and mechanical properties, but also because of its environmental friendliness (e.g., recyclability) and low cost. The most common type is isotactic PP, while syndiotactic PP is a minor contributor to the PP market. Polypropylene has an attractive combination of low density, high stiffness and toughness, and heat distortion temperature above 100°C, which endows it with extraordinary versatility of properties and applications ranging from molded parts to films and fibers. Innovative catalyst and process technologies have significantly simplified its production, resulting in the minimization of catalyst residues, waxy byproducts, and low stereoregularity components, by the use of high activity and highly stereoselective catalysts, such as Ziegler–Natta and metallocene catalysts. Highly energy-efficient processes, such as gas phase and liquid-pool processes, have minimized the need for solvents. Polypropylene offers ready means for recycling by remolding or feedstock recovery or by recovery of its energy content in power plants and steel mills. It has also continued to make inroads into the engineering plastics markets by replacing more expensive and less environmentally friendly resins. The addition of mineral fillers to polypropylenes has widened its sphere of applications by enhancing its stiffness properties. However, the concomitant effect is typically a decrease in toughness properties because the inorganic filler particles act as stress concentrators.
Montmorillonite (mmt) is the most common clay mineral used in polymer/clay hybrids and is composed of an octahedral alumina sheet sandwiched between two tetrahedral silica sheets. The anionically charged sheets, or clay platelets, are strongly held together by cations, such as Na+, Li+, Ca2 +, Fe2 +, and Mg2 +. The tightly bound stacks of clay platelets may be readily separated by simple dissolution in a polar solvent, such as water. In addition, the natural clay is miscible with polar polymers in which the platelets readily disperse. However, separation of the clay platelets in a nonpolar polymer is difficult. The thickness of the individual clay platelets is about 1 nm and the gallery spacing (spacing is defined in terms of d-spacing from X-ray diffraction and small-angle X-ray scattering, which is the distance from one clay platelet center to the next) of the platelets in natural clay is about 1 nm. Organic modification of the clay with alkyl ammonium chlorides, in which the alkyl chain is typically composed of 6 to 15 carbons, expands the gallery spacing to about 2.2 to 2.4 nm in typical commercial organoclays. Addition of maleic anhydride-modified PP often results in intercalated clay with gallery spacing of greater than 3 nm. The typically desired state is, upon addition of PP, the complete dispersion, i.e., exfoliation and dispersal, of the clay platelets so that there is no correlation between platelets and therefore no maxima in the X-ray scattering pattern. Considering that clay is composed of platelets that are about 1 nm in thickness and around 1000 nm in lateral dimensions and assuming a density of 2.65 g/cm3 yields an upper limit of about 750 m2/g surface area per gram for fully exfoliated clay. This is 5 to 10 times the surface area per gram achievable with ordinary fillers. Exfoliated clay in polymers was shown to endow unique properties to the system, such as increased Tg, increased stiffness, maintenance of impact strength, increased barrier to gases and liquids, increased flame retardancy, etc.