Versatile nanocarbons
Since the early 1990s, automotive fuel-line components such as quick connectors and filters have used inner barrier layers consisting of nylon 12 and carbon nanotubes. Hyperion Catalysis now aims to introduce nanotubes into other resins used in auto fuel systems, such modified nylons and fluoropolymers. A new fluoropolymer/nanotube compound is being used to make O-rings for automotive fuel connectors.
In electronics, polycarbonate and polyetherimide (GE's Ultem) components of computer hard drives have been reinforced with nanotubes to render them conductive and very smooth.
Over the last three years, a major automotive OEM in Europe has been using carbon nanotubes in GE's Noryl GTX nylon/PPO alloy to mold exterior fenders. This conductive nanocomposite allows for electrostatic painting.
Michigan State University's Composite Materials and Structures Center in East Lansing developed a new surface-treated graphite nano-platelet. Graphite has a modulus several times that of clay and also has excellent electrical and thermal properties. When incorporated into an epoxy, it results in superior mechanical properties and excellent electrical conductivity compared with standard carbon fibers and nano-sized carbon black. MSU sees potential in ESD protection and EMI shielding. Plastic nano-graphite compounds are expected to sell for up to $5/lb, significantly less than compounds based on nanotubes or vapor-grown carbon fibers.
Carbon nanotubes have more going for them than just conductivity. Researchers at the National Institute of Standards and Technology (NIST), Gaithersburg, Md., report that carbon nanotubes in PP not only enhance the material's strength and properties, but also dramatically change how the molten polymer flows, virtually eliminating die swell.
Nano flame-retardants
Extensive research at NIST has established nanoclays' effectiveness as flame-retardant synergists. Nanoclay levels of 2% and 5% in nylon 6 reduced the rate of heat release by 32% and 63%, respectively, NIST found.
Specialty compounder Foster Corp. recently demonstrated that higher levels (13.9%) of nanoclay can be added to nylon 12 elastomers to achieve UL 94V-0 rating at 1/8-in. thickness. Used as a char former, the nanoclay allows the typical 50% loading of halogen/antimony oxide flame-retardant system to be cut in half, which significantly reduces detrimental effects on physical properties. The company first introduced nylon 12/nanoclay compounds for tubing and film in 2001.
Germany's Sud-Chemie (U.S. office in Louisville, Ky.) offers modified nanoclays called Nanofil as flame-retardants. It recently developed halogen-free EVA/PE wire and cable compounds containing 3% to 5% of new Nanofil SE 3000 plus 52% to 55% alumina trihydrate or magnesium hydroxide (typically used at 65% levels). The result is said to be improved mechanical properties, smoother cable, and higher extrusion speeds.
According to Hyperion Catalysis, two recent studies show that multi-walled carbon nanotubes may act as a flame retardant without use of halogen. In both EVA and maleic-anhydride-modified PP, 2.4% to 4.8% loadings of nanotubes show heat-release rates comparable to or better than those obtained with nanoclays.
Nano-nucleators
Among its many virtues, nanoclay can work as a nucleating agent to control foam cell structure and enhance properties of polymeric foams for applications from insulation to packaging. The University of Toronto's Dept. of Mechanical and Industrial Engineering studied extrusion of chemically foamed LDPE/wood-fiber compounds. Addition of 5% nanoclay to the mix decreased the cell size, increased the cell density and facilitated foam expansion. When burned, the foam showed good char formation. Similar results were obtained in LDPE/nanoclay foam blown with CO2 gas.
Researchers at Ohio State University's Dept. of Chemical Engineering (Columbus) found that small amounts of nanoclay surface-grafted with PMMA can reduce cell size and increase cell density in microcellular PS foamed with CO2. Another OSU study showed that smaller cell size and higher density can be achieved with 5% nanoclay in polyurethane foams blown with pentane or water.
Louisiana State University's Mechanical Engineering Dept. (Baton Rouge) reports that 4% to 5% nanoclay increases the flexural strength and elongation of epoxy syntactic foams used as core materials for sandwich composites in structural applications.
Conclusion:
Beside the organoclays principally also other nanostructured fillers like carbon nanotubes from Hyperion or Nanocyl, carbon nanofibres from Electrovac or POSS from Hybrid Plastics demonstrate new and improved properties when properly dispersed at a nanometer level in a polymer matrix. But here again the most restricting element for these new nanofillers is the very high price of these fillers. Dr. Beyer reported results on flame retardancy with these new nanofillers for the first time worldwide from Kabelwerk Eupen from Belgium at the BCC 2002 conference on Flame Retardancy of Polymeric Materials. Obviously many nanostructured nanofillers demonstrate very effective flame retardancy mainly by improvements of the char formation. He proposed that carbon nanotubes with its very high aspect ratio also strengthen the char and avoid its cracking. Improve in mechanical properties are also reported regularly on other international conferences. A first international conference "Nanotubes and Nanofibres" in june 2004 in Brussels organized by PRW from UK summarized the technical stand-of-art for these new nanofillers. One of the most important indications from this conference was that new synthesis methods will reduce the prices of the new nanofillers and then very soon real technical applications will come.
(Authors: Dr. Y.B.Vasudeo & Dr. R.Rangaprasad, Product Application & Research Center, Reliance Industries Limited, Mumbai)