Polyolefi n nanocomposites based on metallocene catalysts

OPEN ACCESS In recent years polyole in nanocomposites are of great interest because of their high potential as materials with novel properties [1,2]. The properties of the nanocomposites are not only in luenced by the kind of illers but also by the microstructure of the polyole in, the distribution of the illers, and the preparation process. Nanocomposites prepared by extrusion moulding of mixed polyole in and nanoparticles show often less stability by agglomeration of the nanoparticles. A better distribution is obtained if the polymerization catalyst is absorbed on the surface of the nanoparticles. After adding an ole in a growing ilm of the polyole in is covering every nanoparticle (in situ polymerization).

In recent years polyole in nanocomposites are of great interest because of their high potential as materials with novel properties [1,2]. The properties of the nanocomposites are not only in luenced by the kind of illers but also by the microstructure of the polyole in, the distribution of the illers, and the preparation process. Nanocomposites prepared by extrusion moulding of mixed polyole in and nanoparticles show often less stability by agglomeration of the nanoparticles. A better distribution is obtained if the polymerization catalyst is absorbed on the surface of the nanoparticles. After adding an ole in a growing ilm of the polyole in is covering every nanoparticle (in situ polymerization).
Metallocene/(MAO) catalysts are soluble in hydrocarbons and can be easily adsorbed or anchored on the surface of the nano illers such as particles, ibers, layered silica, carbon nano iber (CNF), multi-walled carbon nanotubes (MWCNT), changing the surface to a hydrophobic one [6,7]. The MAO reacts, for example, with the OH-groups of silica or with carboxy groups of oxidized carbon nanotubes or is physically absorbed at the surface. Methane is formed by the chemical reaction of MAO with these groups.
Excess MAO is washed out. In a second step, the metallocene is added forming catalytically active polymerization sites on the nanosurface. The thickness of the polymer ilms, formed by addition of the ole in, depends on the polymerization conditions, especially the polymerization time, the kind of metallocene catalyst, and the pressure of the monomer. Using different metallocenes or ole ins a great variety of polymer matrixes can be obtained such as polyethylene, isotactic or syndiotactic polypropylene, ethylene 1-hexene or 1-octene copolymers [8,9]. The in-situ polymerization leads to composite materials where the nanoparticles or nano ibers are intensively covered with the polymer.
The composite materials show, for example, an improved stiffness with a negligible loss of impact strength, high gas barrier properties, signi icant lame retardant, better clarity, and gloss as well as high crystallization rates. Even low nanoparticle contents are already suf icient to obtain new or modi ied material characteristics, especially a faster crystallization rate and a higher crystallization temperature. Carbon nano ibers (CNF) or multiwalled carbon nanotubes (MWCNT) are an especially attractive class of illers for polymers because of their intriguing mechanical and thermal properties [10,11].
For the preparation the MWCNT were sonicated in a toluene suspension, treated by MAO stirred for 24 hours, iltrated, and washed with hot toluene [12]. After adding the chiral ansa zirconocene [(CH 3 ) 2 Si(2-CH 3 -4-Nap-Ind) 2 ] ZrCl 2 ( Figure 2) and propene isotactic high molecular weight polypropylene iPP/MWCNT composites with 0.9 -50 wt% iller content were obtained. The molecular weights of the polypropylene matrix in the nanocomposites were in the range of M w = 1,200,000 -1,700,000. The polymerization activity reached 5000 kg PP/mol Zr·h· [propene]. It was independent of the iller content. As expected for in-situ polymerization, the polymer grew directly on the nano iber surface and covered them with a thin PP layer. The dried polypropylene nanocomposites were obtained in powder form. By longer polymerization times, the thickness of the polyole in covering the nano iber increased. The nano iber/ MAO/zirconocene system worked like a supported catalyst. Filler contents between 0.5 up to 50 wt% were possible.
The morphology of the iPP/MWCNT nanocomposites was investigated by using transmission electron microscopy (TEM). Figure 3 shows the covered nanotubes at two different resolutions, right is the end of a tube by 20 times higher resolution than left.
The diameter of the MWCNT used (about 20 layers) is 20 nm and the thickness of the iPP coat is about 8 nm. Every nanotube is covered by a polymer ilm and no agglomeration can be seen. The adhesion of polymer and nanotube is excellent.
The main advantage of CNF or MWCNT illed PP is the change of mechanical properties. High molecular weight isotactic polypropylene illed with MWCNT is an exceptionally strong composite material. The tensile strength of a composite ilm increases by 20% if only 1 wt% of MWCNT is incorporated. Table 1 show the de lection and form stability of these composite materials measured by dynamic mechanical analysis. The de lection decreased to the half if the composite contains 3.7 wt% of MWCNT and the form stability increased by 25 °C. This shows a higher thermal performance of the nanocomposites of nearly 60%. The electrical conductivity starts at a MWCNT content of 5%. Also the crystallization rate from a melt, important for extrusion moulding processes increase strongly and make this composite material suitable for new applications such as in the automotive plastic industries [13].
Polyole in nanocomposites open up the approach to new classes of materials with great property combinations. A soft polyole in matrix can be combined with hard inorganic particles or strong layers of silicates or graphene or with ibers of extreme high tensile strength, such as carbon ibers, carbon nanotubes or polymer ibers. An easy way for the preparation of such polyole in nanocomposites is the in-situ polymerization using nanoparticles or ibers activated by metallocene/MAO or other single site catalysts. Materials with high gas barrier resistance, high thermal and electrical conductivity, and high form stability can be obtained as well as a good dispersion of the nano illers in the polymer matrix. The development and commercialization of metallocene/MAO based polyole in composites have just started and will help in the automotive industries to reduce the weight of a car and by this the energy consumption and is ine for the environmental.