Thinking Small Pays Big
Polymer nanocomposites are becoming increasingly popular as new developments take hold of the market.
By Geoff Giordano and Hope Inman
The polymer nanocomposites market is ready to take off, as recent usage has increased across a variety of applications in the plastics sectors, especially in the automotive and packaging industries. A study by The Freedonia Group Inc. shows that by 2020, the demand for nanocomposites in the U.S. will exceed 7 billion pounds with a value nearing $15 billion [1]. By 2025, production of nanocomposites will approach 5 million tons [2]. The recent surge in nanocomposite applications is fueled by the declining prices of nanocomposite materials and technical issues involving nanoadditives in compounds having been resolved. Plastics suppliers leading the way in commercializing nanocomposite materials include Bayer, Dow Chemical, Eastman Chemical, LyondellBasell, Mitsubishi Gas Chemical, Nanocor, Triton Systems, Honeywell, and RTP Co.
Automotives…and Beyond
The automotive industry pioneered the use of polymer nanocomposites. Nearly 80% of polymer nanocomposites are consumed by the automotive, aeronautics, and packaging industries; their use is driven by the decrease in weight of the structural parts they compose, which in turn reduces energy consumption. Nanocomposites also provide stiffness and strength, and they enhance thermal stability more than their metal counterparts. Polymer composite parts can be painted together with the rest of the automotive body and treated with the same processes used for metallic materials.
The first commercial polymer-nanocomposite product used by the automotive industry was a timing-belt cover; incorporating as little as 4 wt% nanostructured silicate clay into a polyamide-6 matrix improved rigidity and thermal stability in the product. Since then, new nanocomposite applications have been readily developed for the automotive industry and other areas, especially thermoplastic-based nanocomposites. In the past decade, General Motors and Montell have introduced thermoplastic olefin (TPO) nanocomposites that exhibit benefits for interior and exterior automotive parts by reducing weight and improving on low-temperature impact performance. Ashland Specialty Chemical began using nanoclays as an additive to toughen the resin used in sheet molding compound (SMC) while maintaining its physical properties.
In the packaging industry, the superior oxygen and carbon dioxide barrier properties of nanoclays in nylons have been used to produce multilayer PET bottles and films for packaging food and beverages. Commercial products in Europe and the U.S.—such as bottles for carbonated soft drinks and alcoholic beverages, and containers for “deli” meat and cheese—have employed nanocomposites technology to enhance flexibility in packaging and tear and puncture resistance, and to provide moisture control. Use of nanocomposites in packaging can also exponentially increase shelf life. Plastics supplier Nanocor produces Imperm, an MXD6 nylon/clay nanocomposite used as an oxygen barrier in bottles for beer and carbonated beverages, in packaging for processed meats and cheeses, and in extrusion-coating for paperboard packaging for juice or dairy products. Loading 5% Imperm in PET beer bottles increases the shelf life to six months and results in less than 10% loss of carbon dioxide [3].
In the energy industry, polymer nanocomposites have the potential to positively affect the creation of sustainable energy forms by offering new methods of extracting energy from inexpensive and environmentally benign resources. Developments in nanocomposites have produced membranes for fuel cells plus methods to make them smaller and more affordable. Polymer nanocomposites have also significantly improved battery technology by using nanostructured materials to create rechargeable batteries, both dry and wet.
The biomedical industry, also, has taken advantage of the flexibility afforded by the use of nanocomposite materials. Materials used in the biomedical field must meet certain criteria regarding biocompatibility, biodegradability, mechanical properties, and, sometimes, aesthetics [4]. In applications such as tissue engineering, bone replacement or repair, dental applications, and controlled drug delivery, biopolymer-based nanocomposites can be fine-tuned and perfected based on the needs of the particular product.
25 Years of Nanocomposites
Since researchers at Toyota created the first polymer clay nanocomposite in 1985, momentum has steadily built to create reinforced engineering materials on the nanometer scale. Research and development has proceeded feverishly in efforts to incorporate the three primary nanoadditives to polymers—clays, single-wall and multiwall carbon nanotubes, and metal or metal oxides—thereby improving performance of thermoplastics in various ways, from increasing polymer strength to enhancing inflammability.
“More materials and products will be made from the bottom up, that is, by building them from atoms, molecules, and the nanoscale powders, fibers and other small structural components made from them,” noted a 1999 report by the National Science and Technology Council Interagency Working Group on Nanoscience, Engineering and Technology. Twenty-five years prior, Professor Norio Taniguchi at Tokyo Science University coined the term “nanotechnology,” defining it as “processing of separation, consolidation, and deformation of materials by one atom or one molecule.”
When added to pristine polymers, nanoparticulates such as montmorillonite clay improve just about every performance characteristic. Such reinforced polymers are stronger, reduce flammability, enhance electrical conductivity, bolster the gas barrier, improve transparency, and reduce haze, thus making nanomodified polymers ideal for such diverse applications as food packaging, fuel containers, and wiring and insulation.
Increasingly, sentiment is growing in the plastics industry to harness what Dr. Donald Paul of the Cockrell School of Engineering at the University of Texas, Austin, terms “nanomagic.” Exfoliated clay-based nanocomposites are by far the leading nanofillers, he says. He began working with Southern Clay, outside Austin, several years ago to develop clay additives in an extruder.
But he points to Toyota’s 1985 breakthrough as the catalyst for the use of organoclays and other nanoscale polymer fillers.
Interest Builds
In a 2006 paper [5], Toyota engineers Akane Okada and Arimitsu Usuki recalled that company’s research into and creation of the first polymer clay nanocomposite. The pair began presenting their research in 1987 through 1989, when a company called Ube Inc. began manufacturing nanoclay-filled polymers and Toyota produced the Starlet passenger car, which featured a nanomodified timing-belt cover.
In 1997, researchers Jeffrey W. Gilman and Takashi Kashiwagi of the National Institute of Standards and Technology in Gaithersburg, Maryland, USA, revealed that nanomodified polymers could be more flame-retardant. Consequently, Dr. Paul notes, nanoclay-modified polymers have been used extensively in the wire and cable industry, “where insulation needs to come up to certain codes. Clays will help do this. There’s lots of interest in replacing more obnoxious flame retardants with clay.
“Using clay materials has been the most studied and utilized platform for [creating nanocomposites],” Dr. Paul continues. “They are readily commercially available, and, compared to some nanoparticles, quite inexpensive; but though they come from dirt, they’re not cheap as dirt. They do add cost by putting them in a polymer. But they can impart stiffness and strength and give you a boost in heat-distortion temperature.”
U.S. Auto Industry Adapts
In 2001, GM became the first U.S. automaker to use a nanocomposite part in its cars, according to Technical Fellow Will Rodgers. By using nanoreinforced polymers in the step-assist assembly of the M van, “we saved about two pounds of vehicle,” he notes.
“We didn’t use polyamide-based material; we used a polyolefin based material,” he continues. “Since 2001 we’ve moved into body-side molding applications and truck-bed surrounds. But it made more sense to take these materials and move into the interior of the vehicle. We were able to use nanocomposites on the center console of the Chevrolet HHR. Because of nano/organoclays, we moved away from glass-filled polypropylene-based material to this nanoclay-filled material that let us mold higher-quality parts. Fit and finish was better than what we could get with glass-filled material.”
Sometimes, parts made with nanocomposites are too good.
“You use so much less filler material when using nanoclays that the surface of the part is much higher quality,” Rodgers says. “On one implementation, the parts we were using looked too good and didn’t harmonize with the rest of vehicle, so we had to go back and modify tools to create a part that matched the rest of the vehicle better.” But prior to that, microparticles used to reinforce the part made up 20% of the total weight; nanoclays accounted for only 2.5% of the newer part’s weight.
Carbon Nanotubes Emerge
Carbon nanotubes—long cylinders of covalently bonded carbon atoms—discovered by Sumio Iijima in 1991, have promised a further boon in electrical applications because of how they improve polymer conductivity.
Some nanotubes are “stronger than steel, lighter than aluminum and more conductive than copper,” noted researchers Mohammad Moniruzzaman and Karen Winey of the Department of Material Science and Engineering at the University of Pennsylvania in 2006 [6]. As they note, research continues feverishly to harness the promise of carbon nanotubes. Like clays, carbon-modified polymers are also stronger, more impervious to penetration by gas and moisture, and more thermally stable.
Although Moniruzzaman and Winey note that nanotubes were first used as filler in 1994, they stress that only a few commercially available products employ carbon-filled polymers. “The only major commercial product based on nanotubes in the market for the past decade has been a nanotube/plastic composite with improved electrical conductivity that facilities electrostatic coating and is marketed by Hyperion Catalysis International,” they wrote.
Researchers are struggling to overcome the inconsistencies regularly encountered in attempts to prepare polymers reinforced with single-wall and multiwall carbon nanotubes. Patents have been steadily sought for uses of carbon-reinforced polymers in electromagnetic shielding, reinforced textiles, and other areas. But researchers have encountered roadblocks in achieving optimal dispersal of nanotubes when processing carbon-modified polymers because of their tendency to bundle. Experimentation continues to focus on how best to engineer nanotubes via melt blending, solution blending, or in-situ polymerization.
In the auto industry, some fuel lines incorporate nanotubes, but are “generally far too expensive to incorporate for any strength enhancement or properties like that,” GM’s Rodgers cautions.
Metals: The Next Nano Miracle?
Even less far along developmentally, polymers produced with metal or metal oxide are being studied as options in the production of reinforced plastics. Some industry experts say most research in the area has involved trying to capture the unique properties of nanosized metal particles—primarily optical, magnetic, and electrical—within a polymer matrix. Commercial application remains elusive, as chemists and physicists around the globe continue investigations that began as early as 1835 [7].
As with other nanoparticles, particularly carbon nanotubes, handling and achieving optimal dispersal of metal fillers are difficult.
“Manipulations of single nanoscopic objects (1–30 nanometers) by surface tunneling microscopy, spontaneous self-assembly and dielectrophoresis are the only available approaches for building functional devices using nanosized metals,” write Nicolais and Carotenuto of The Institute of Composite and Biomedical Materials at the National Research Council in Naples, Italy. They also note that nanosized metals are quite unstable, can aggregate in the polymer, and are susceptible to oxidization and contamination by air, moisture, and sulfur oxide [7].
But the promise of metal nanofillers more than warrants continued study, they say, noting that metals reduced in size undergo the most considerable property changes. Metal nanoparticles exhibit “quantum-size effects” like electron confinement that can be fine-tuned just by manipulating the size of the fillers. Furthermore, at smaller sizes, metal fillers become stronger thermal and electrical insulators, more chemically reactive and super-absorbent, and melt at lower temperatures.
Commercial expansion and development of the polymer nanocomposites market was a slow process for the past decade; progress was stalled by the inability of industry to move past pilot programs. Aversion to the technology because of cost and variability of quality has diminished, owing to further research and development and new innovations that are now on the market. The packaging and automotive industries will remain the dominant users of nanocomposites, but energy and electrical applications are expected to surge by 2025, owing to the emergence of carbon nanotube–based composite materials and the promise of metal nanofillers in current research and development.
References
1. Freedonia Group, “US polymer nanocomposites demand to exceed 7 billion pounds in 2020,” All Business, May 5, 2006, http://allbusiness.com/specialty-businesses/875082-1.html (accessed Oct. 16, 2009).
2. “Polymer nanocomposite market to grow well globally, particularly in USA,” Plastemart, 2003, http://www.plastemart.com/upload/Literature/Polymer-Nanocomposites-grow-... (accessed Sept. 25, 2009).
3. A.L. Brody, “Nanocomposite Technology in Food Packaging,” Food Technology, October 2007, pp. 80–83.
4. R.A. Hule and D.J. Pochan, “Polymer Nanocomposites for Biomedical Applications,” MRS Bulletin, 32, 354–58 (April 2007).
5. A. Okada and A. Usuki, “Twenty Years of Polymer-Clay Nanocomposites,” Macromolecular Materials and Engineering, 291, 1449–76 (2006).
6. M. Moniruzzaman and K.I. Winey, “Polymer Nanocomposites Containing Carbon Nanotubes,” Macromolecules, 39, 5194–5205 (2006).
7. L. Nicolais and G. Carotenuto, eds., Metal-Polymer Nanocomposites, John Wiley & Sons, Inc., Hoboken, N.J. (2005).