The subject of polymer hybrid composites based on layered inorganic compounds such as clays has been the subject of intense industrial and academic study for more than a decade. Polymer materials may be filled with several inorganic, synthetic and/or natural minerals in order to increase properties like heat resistance, mechanical strength and impact resistance. However, there is little or no interaction between the two mixed components. In order to increase interaction between the polymer and reinforcing material, the dimensions of the reinforcing elements on a molecular level must be decreased.
Layered silicates or clays are one material class of reinforcing elements on a nano-scale. Such elements have dimensions of the order of one billionth of a meter, or one nanometer. Such a composite is called a nanocomposite. There are many types of nanocomposites, whereby the defining characteristic is that the reinforcing component has the dimensions mentioned above.
The most heavily researched type of nanocomposite uses layered mineral clays as the reinforcing component. Polymer-clay nanocomposite materials have attracted a great deal of attention because they provide enhanced mechanical and thermal properties as compared to conventional materials. Specifically, these hybrid composites exhibit increased tensile strength, modulus, and heat distortion resistance, as compared with a pure polymer. The composites also have lower water sensitivity, reduced permeability to gases, and a similar thermal coefficient of expansion. Further, it has been found that nanocomposites impart a level of flame retardance and UV resistance not present in the pure polymer component. Because of these enhanced properties, such nanocomposites are useful in the electronics, automobile, aircraft, and aerospace industries, among others.
The type of clay, the purity of the clay, and the choice of clay pre-treatment are all controllable variables that influence the nature and properties of the final nanocomposite. Many clays are aluminosilicates, which have a sheet-like or layered structure, and consist of silica SiO4 tetrahedra bonded to alumina AlO6 octahedra in a variety of ways. A 2:1 ratio of the tetrahedra to the octahedra results in smectite clays, the most common of which is montmorillonite. Smectite clays are clays that can be swollen with small molecules. Other metals such as magnesium may replace the aluminum in the crystal structure.
Montmorillonite clay is the most common type of clay used for nanocomposite formation. Montmorillonite naturally forms stacks of plate-like structures, or platelets. Each platelet is less than 10 Angstroms thick, but over 200 times that in width. The spaces between these platelets are called gallery spaces. Each platelet is composed of silicon oxide and aluminum oxide. Under the proper conditions, the gallery spaces can be swollen and filled with monomer, oligomer, or polymer. Specifically, a swelling of the gallery spaces increases the distance between platelets in the stacks. Clay platelets swollen with polymer are said to be intercalated. If the clay swells so much that it is no longer organized into stacks, it is said to be completely exfoliated.
In addition to montmorillonite clay, other types of clay may also be used, including hectorites, which contain very small platelets, and synthetic clays. Synthetic clays can be produced in a very pure form and can carry a positive charge on the platelets, in contrast to the negative charge found in montmorillonites.
Depending on the precise chemical composition of the clay, the layers generally bear a charge on the surface and edges of the platelets. This charge is balanced by counter-ions, which reside in part in the gallery spaces between the layers. Thus, the stacks of clay platelets are held tightly together by electrostatic forces. Because of the charged nature of pure clay, the clay is generally highly hydrophilic. For example, montmorillonite is naturally hydrophillic. This makes pure clay poorly suited to mixing and interacting with most polymer matrices. As such, a necessary prerequisite for successful formation of polymer-clay nanocomposites is the alteration of the clay polarity to make the clay organophilic. An organophilic clay can be produced from a normally hydrophilic clay by ion exchange with an organic cation such as an alkylammonium ion. The particular pre-treatment process used may have an effect on the formation of the nanocomposite product formed. Such pre-treatment processes are well known in the art.
As noted above, a nanocomposite may be in the form of an intercalated or exfoliated hybrid. In an intercalate hybrid, the polymer component is inserted between the clay layers or platelets such that the gallery spaces are expanded, but the platelets still bear a well-defined spatial relationship to each other. In an exfoliated hybrid, the layers or platelets comprising the clay stacks have been completely separated and the individual platelets are distributed throughout the polymer matrix. Various factors affect this delamination of the clay stacks, including the exchange capacity of the clay, the polarity of the reaction medium and the chemical nature of the interlayer cations (e.g. onium ions).
Levels of exfoliation may be determined by an x-ray scattering test. An absence of scattering peaks at a characteristic scattering angle indicates high levels of exfoliation. Conversely, a large scattering peak indicates decreased (or poor) exfoliation. The scattering angle is inversely correlated with interlayer or gallery spacing. Specifically, scattering angle (theta) is linearly proportional to l/d, where d equals interlayer spacing. Therefore, the level of exfoliation is measured by analyzing the level of scattering intensity at the expected scattering angle (based on the interlayer spacing). Thus, the interlayer or gallery spacing is a function of the particular clay. Complete exfoliation, wherein all stacks are delaminated into single platelets surrounded by polymer, may not be required to attain optimal nanocomposite properties. However, substantial exfoliation is generally desired in order to attain the above noted enhanced properties in the resulting product. Substantial or high levels of exfoliation is defined herein as an exfoliation level that lacks any significant scattering peak in an x-ray scattering test.
In addition to an x-ray scattering test, electron microscopy can provide a pictorial perspective of the exfoliation level in a nanocomposite. However, it is does not provide as quantitative a measure of the exfoliation level of the entire sample compared to an x-ray scattering test. Dispersion of the clay or other reinforcing material throughout the polymer matrix may also be seen with electron microscopy. Generally, a consistent dispersal of the reinforcing material is desirable, wherein the reinforcing material is not clumped in certain areas of the matrix.
Various attempts have been made to produce a substantially exfoliated nanocomposite. The most successful method has been in situ polymerization, wherein a monomer is polymerized in situ to give the corresponding polymer-clay nanocomposite. In the late 1980's and early 1990's, researchers from Toyota™'s Central Research and Development Laboratories demonstrated an in situ polymerization method of monomer to nylon, producing nylon-6 (a clay-nylon hybrid). Montmorillonite clay was exchanged with an acid, replacing the cations on the clay surface and swelling the clay. The modified clay was dispersed in the polar monomer, epsilon-caprolactam, which was then polymerized into the polymer commonly known as nylon-6.
Such in situ polymerization methods rely on chemical processes to achieve the desired exfoliated product, and there success has been limited. A polar monomer can intercalate between clay sheets, and polymerization leads to exfoliation. However, these chemical methods have not been successful for other polymer components, particularly nonpolar polymers. Certain polymers, such as polypropylene, require a much more complicated synthetic approach than other monomers, such as those involved in producing nylon-6. As such, polypropylene nanocomposites, or other such polymers, have not been produced by in situ polymerization (i.e. polymerizing propylene in the presence of clay).
Attempts to produce well-exfoliated polymer-clay nanocomposites by non-chemical methods (e.g. melt blending or solution blending of polymer-clay systems) have not been successful. Twin-screw extrusion of polymer-clay mixtures has not yielded well-exfoliated clays in the polymer, though it has resulted in relatively good polymer-clay contact. Twin-screw extrusion, or melt extrusion, is well known in the art, wherein a mixture or compound is processed through a twin-screw extruder or compounder, or an intensive mixer, such as a Farrel Continuous Mixer™.
In addition to melt-extrusion, the method of solid-state shear pulverization has been developed for preparing polymer materials, as disclosed in U.S. Pat. No. 5,814,673 to Khait and U.S. Pat. No. 6,180,685 also to Khait, the disclosures of which are incorporated herein by reference. As disclosed in the Khait patents, a chemical change to a polymer material is effected by application of mechanical energy through solid-state shear pulverization in the presence of cooling sufficient to maintain the material in the solid state during pulverization. However, the Khait patents do not disclose or suggest a method of producing highly exfoliated polymer-clay nanocomposites.
The present invention is directed to a method of producing highly exfoliated polymer-clay nanocomposites through a non-chemical process, whereby the resulting product exhibits a high level of exfoliation and dispersion. The polymer-clay nanocomposites produced therefrom may be used in a wide variety of applications, including barrier films for reduced permeability, and applications requiring a polymer with improved solvent resistance, reduced flammability, increased strength and/or hardness, light weight, and resistance to heat distortion. Industrial applications for such nanocomposites include, inter alia, the automotive, aircraft and aerospace industries.