Clay-filled 'nanocomposites' offer extraordinary properties.

They are generating a lot of excitement despite the secrecy surrounding their development. They are said to exhibit dramatic improvements in mechanical, thermal, barrier, and flame-retardant properties, all without significant loss of toughness or clarity. These new reinforced plastics, known as nanocomposites, contain very tiny platelets of a special type of surface-modified clay called montmorillonite, which is dispersed in a thermoplastic or a reactive liquid resin. The clay particles are one nanometer thick (or 40 billionths of an inch) - hence the term nanocomposites. Only 2-5% of the "nanoclay" is required to obtain a remarkable boost in properties.

Nano-clays have been used in paints and various organic fluids. Nylon-based nanocomposites are now being brought to commercial fruition. Work with nano-clays in polypropylene and liquid thermosets is still in various stages of R&D. Potential market opportunities run the gamut from automotive applications to barrier packaging - where there is keen interest in a transparent material with reduced permeability to oxygen and other gases. Automotive applications range from underhood parts requiring higher heat-distortion temperatures to instrument panels and bumpers, which can benefit from improved dimensional stability and surface finish that allow for molded-in color.

NYLON/CLAY HYBRID VERSUS STANDARD NYLON IN PACKAGING

                                                       Effect of
Properties                                             Nano-Clay

Mechanical      Stiffness (Modulus)                     + +
                Heat resistance (HDT)                   + +
                Tensile strength                        +
                Tensile elongation                        -
                Puncture strength                       +
Barrier         Gas barrier                             + +
                Solvent barrier                         + +
                Aroma barrier                           +
                UV barrier                              +
Optical         Haze                                    +
                Gloss                                   +

+ + much improved, + slightly improved, - not improved

Source: Ube

NANO-CLAY PLUS NYLON

A significant player in the nanocomposites field is Nanocor Inc. of Arlington Heights, Ill., a subsidiary of AMCOL International Corp., a supplier of specialty chemicals and minerals. Formed about two years ago, Nanocor's mission is to make and market a family of nano-clays These clays can be chemically modified with various surface treatments to make clay complexes (which the company calls nanomers) that are compatible with organic monomers and polymers.

Nano-clays are based on montmorillonite, a layered alumino-silicate whose individual platelets measure on the order of one micron, giving them an aspect ratio of about 1000:1. Montmorillonite is hydrophilic, able to absorb 20-30 times its volume in water, and therefore must be chemically modified to render the surfaces organophilic.

Nanocor recently started up its first commercial-scale facility in Aberdeen, Miss., to produce 10 million lb/yr of purified, polymer-grade montmorillonite clay. The plant is readily expandable to 20 million lb/yr. Besides Nanocor, Southern Clay Products, Gonzalez, Texas, appears be the only other domestic supplier of montmorillonite.

In the late 1980s, Toyota's Central R&D Laboratories in Japan were the principal birthplace of nanocomposites, according to Karl Kamena, Nanocor's manager of sales & marketing. Toyota researchers found that commonly used polymer reinforcements (including glass fibers and conventional clays) were not homogeneously dispersed at the microscopic level. The researchers hypothesized that surface-modified nanometer-thick silicate sheets might achieve more uniform dispersion and thus have remarkably improved properties. Toyota developed proprietary, patented technology for the production of nylon nanocomposites.

Most nanocomposite performance data currently available is based on nylon 6 compositions in which the clay is reacted into the polymer during polymerization - such as are being produced by Japan's Ube Industries under license from Toyota. According to Kamena, Unitika of Japan has also been producing nylon nanocomposites (though not under a Toyota license) utilizing synthetic mica for about two years.

Ube is in the initial stages of marketing its "nylon-day hybrid" (NCH) compounds in the U.S. through its N.Y.C. office. Marketing manager Masaru Tabata says the series includes nylon 6 and 6/66 copolymer grades. Barrier films for food packaging are a key target, but Tabata says nylon 6 NCH grades are also aimed at automotive under-hood uses.

Recently released data from Ube show that cast films of NCH-2 grade (2% clay in nylon 6) have about half the oxygen permeability of straight nylon 6 films. Improvements in aroma and uv barriers have also been demonstrated. Nylon nanocomposites also show improved transparency in air-cooled blown films.

Considerable improvements in HDT and other mechanical properties also have been shown. Compared with untilled nylon 6, NCH has 40% higher tensile strength, 6896 higher tensile modulus, 60% higher flexural strength, and 126% higher flexural modulus, Ube says.

Meanwhile, Nanocor recently acquired a license for the Toyota technology and aims to sublicense it to firms wishing to make nylon nanocomposites. "We think these will initially be resin manufacturers, and compounders could well follow," says Kamena. He hints that a global producer of nanocomposites with North American facilities could emerge in the next several months.

NANO-CLAY FIREBLOCK

At the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., fire researchers have demonstrated that the heat-release rate of a nylon 6 nanocomposite with 5% clay content is 63% lower than that of pure nylon 6. Unlike many conventional flame-retardant nylons, nanocomposites produce no increase in carbon monoxide or soot during combustion. "These clays form a very high-performance char very rapidly," says fire science researcher Jeffrey Gilman.

In November, NIST hosted a conference for scientists and plastics industry representatives. The conference was aimed at forming a consortium to explore the potential for lowering the flammability of commodity thermoplastics through the use of nanocomposites. (For more information, Gilman can be reached by phone at 301-975-6573 or via e-mail at [e-mail].)

GOING BEYOND NYLON

According to Nanocor, different organic surface treatments can modify nano-clays to make them hydrophylic, hydrophobic, or anything in between. (Nanocor has a computer model to predict the interaction of different agents with the clay surface.) Kamena says this extends the nanocomposite concept to virtually any thermoplastic or thermoset polymer matrix. Like nylons, some other nanocomposites could be prepared in the reactor during polymerization. For other polymer systems, melt-compounding processes are being developed.

Kamena says Nanocor and 20 or so industry partners are focusing on thermoplastic applications like PET barrier packaging and PP automotive parts. For example, Ford Motor Co. reportedly has expressed strong interest in PP nanocomposites for uses ranging from exterior body parts to interior instrument panels.

Recently, Nanocor has also started to examine liquid thermosets, including epoxies, unsaturated polyesters, and polyurethanes. Potential applications range from epoxy coatings to adhesives, laminates, and elastomers.

Meanwhile, nanocomposites of various polymers, including PET and polystyrene, have been prepared by melt extrusion at Cornell University's Materials Science and Engineering Department in Ithaca, N.Y. A group led by associate professor Emmanuel Giannelis is modifying organic and synthetic clays to match particular thermoplastics. Giannelis says, "The reason we don't yet have a PP nanocomposite is that we are still trying to fine-tune the clay and the process. For PP, we are looking at compounding rather than the reactor route."

The potential of nanocomposites as low-cost, high-strength materials for automotive parts was among the 64 new research projects announced last fall by the U.S. Commerce Dept.'s Advanced Technology Program, which provides partial funding for innovative industrial R&D. ATP funds for nanocomposite research went to Dow Plastics, Midland, Mich., which has a joint-venture with automotive processor Magna International, Southfield, Mich. The five-year project will develop methods for dispersing the nano-clay particles in resins (including commodity TPs such as PP) and demonstrating cost-effectiveness of nanocomposites for exterior front and rear parts.