It’s hard to read anything these days without coming across a reference to nanotechnology. What some designers may not realize is that some nanotech products have already made it out of the lab and into the field. In the realm of engineered materials, plastics enhanced with nanometer-sized particles are now available that offer designers a range of new options and benefits such as optical clarity, strength, stiffness, thermal stability, reduced permeability, and flame retardancy.

These nanocomposites, as the plastics are known, are loaded with small amounts of nano-sized particles of inorganic materials that are homogenously dispersed in the polymer matrix. By evenly dispersing the particles throughout the polymer matrix, a greater interfacial area is created between the polymer and particles. It is this interfacial area that improves mechanical properties.

Depending on the materials used, different, and multiple, properties can be obtained. This mix-and-match, a la carte approach to selecting desired qualities is what often sets nanocomposites apart from standard plastic products. “When multiple attributes are needed, that is when nano materials are the right choice. Otherwise it might be better to use standard technology,” says David Jarus, R&D manager for PolyOne Corp, which offers both nano and standard materials. The company offers a range of Nanoblend compounds that are lighter, stiffer and tougher than standard products. Additional properties can be blended into the base material such as fire retardance and chemical resistance.

These pellets from PolyOne are used in the company’s NanoBlend nanocomposites.

Nanocomposites are generally considered to more easily flow and mold, and process faster. PolyOne’s Nanoblends, for instance, which have a low specific gravity of 0.9 to 0.96, flow easily for thin-wall parts and often cool faster than traditional materials.

RTP Co., Winona, Minn., also has a range of nanocomposites. Testing of its recently released compounds containing nanoclays demonstrated that they can cycle 25 percent faster in molding machines than many other standard materials.

The nano-based materials can come in many forms. Some recipes have been used for years such as adding nanosilver to plastics to inhibit microbial growth. But, the marketplace is set to explode with new nanocomposite technologies as applications are found for blends containing nanoclays and nanotubes.

Nandan Rao, technology director for DuPont Performance Materials, says nanocomposites offer tremendous benefits over standard materials. Initial testing of DuPont’s DNM product for instance illustrates the point. In one test, the addition of 1.5 percent by weight of DNM in glass-reinforced PET material produced improvements in heat deflection temperature (HDT) of 10 DegC to 15 DegC over typical glass-reinforced PET (see chart).

Since that introduction, Rao says that the company has made significant progress understanding the properties attainable as particle size is reduced and a “greater interfacial area” is generated. He added that significant improvements in crystallization, rheological, mechanical, and permeability properties can be achieved for improved processing, better structural performance, higher heat deflection temperatures, better electrical insulation and improved barrier to gas permeation.

Calling on clay

PolyOne offers a variety of nano and non-nano polymers that are used depending on the application.

To date, nanoclay technology has been more often discussed than nanotubes. A primary reason for this had been the high cost for nanotubes, but that reason is starting to wane as nanotube costs have dropped. Still, nanoclays have their own benefits such as improved barrier properties and flame retardancy.

Nanoclay materials are made with clay mixed at small percentages with plastic resins. In one application, the composite is used to produce a transparent film. The compounds offer improved mechanical and thermal properties compared to neat resins. And their low loading levels, in the 2 percent to 8 percent range, increase stiffness with minimal impact on specific gravity. In flame retardancy tests, the clay eliminates dripping and promotes the formation of a stable char. Testing by NIST and other groups has shown that the materials decrease peak heat release rates.

The barrier properties offered by nanoclay compounds is one reason that RTP Co. chose this type of material for fuel system applications, which have stringent barrier demands. “The new nanoclay compounds can provide a monolayer solution for small engine and fuel tank manufacturers to meet the new fuel emission standards for recreational vehicles and lawn and garden equipment,” says Sam Dahman, product development engineer at RTP. He says that the nanocomposites performance is due to “large aspect ratio layers creating an extremely tortuous path for diffusion.”

Enlarge this picture An example of the CNT- Percolation curve from RTP Co. As carbon-nanotube material is added, the resistivity is changed.

Another unique nanoclay material is called NanoVin produced by European-based SolVin. This nanocomposite combines polyvinyl chloride (PVC) and clay nanoparticles. According to Richard Thommeret, marketing manager for SolVin, the initial research was undertaken to improve the stiffness of PVC, but it was discovered that material had viscoelastic behavior. Thommeret says they are targeting coating applications such as mastics and inks where the viscosity behavior is critical. He adds that the material is meant for plastisol application, a mixture of paste PVC with plasticizer enabling the finished product to be soft.

The principle of the process is to apply a thin layer of plastisol on a support such as fabric, metal, and glass, Thommeret says. The plastisol material is applied as a fluid and as it contacts the surface to be coated it becomes viscous, sticking to the surface and not dripping. It is then ‘cooked’ to make it homogeneous and fire and impact resistant.

The company is working on new versions with barrier properties against moisture, oxygen and carbon dioxide, as the clay material has already demonstrated such improvement in polyamides.

Tumbling tube costs

The Iomega Corp. uses RTP’s carbon-nanotube compounds in part because of its consistent electrical resistivity and its cleanliness standards.

Nanotubes, most often made from carbon, also offer numerous benefits. They are infinitely small, but very strong and stiff, says PolyOne’s Jarus. He adds that they are appropriate for use in electrical applications because they are good at electrostatic dissipation.

They are also clean systems, says Ned Bryant, a senior product development engineer for the RTP Co. Clean means that they are free from ionic contamination and from out gassing from the plastic over time or from exposure to elevated temperatures.

To date, work with nanotubes has been limited by cost. In the past, prices have reached about $500 a pound, but prices are dropping. Bryant says some initial patents on nanotubes have expired and competition has increased. With dropping prices, many experts believe that nanotube technology is set to dramatically increase.

In May, RTP Company introduced several new carbon-nanotube-based compounds that are specifically engineered for electrical applications, specifically for dealing with electrostatic discharge (ESD) concerns. Seven different resin systems are available, including those for use with polycarbonate (PC), polyether-ether-ketone (PEEK) and polyetherimide (PEI).

Bryant describes the carbon nanotubes as resembling an extremely tiny steel wool pad that has been broken up. Measuring 10 nanometers or less in diameter, these fibers of conductive graphite, when properly dispersed, make up a fine network of wires inside the plastic to carry the electrical charges away from the part and keep it neutral.

And, like other nanocomposites, the loadings are small. In a typical, carbon-fiber material, the filler loading is often in the 10 percent to 20 percent weight range in order to get the electrical properties needed. With nanotubes, the same properties can be achieved at greatly reduced loadings. The actual weight percentages are proprietary, says Bryant, adding that they are under 5 percent by weight and most of them are significantly less. (See chart, Percolation Curve.)

At these loads, the nanotubes do not affect the plastics’ desired properties. By contrast, the adding of carbon fiber typically reduces the plastic’s flexibility, lowers the Izod impact strengths, and changes the shrinkage rate of the plastic.

“With the low loading of nanotubes, you still have easily molded properties and can fill very thin sections, and now it can be either electrically conductive or static dissipative,” Bryant says.

Eliminating residual voltage

A transmission-electron-microscope (TEM) image of SolVin NanoVin material showing the dispersion of nanoclay particles around the larger  PVC grains.

Bryant says the uniformity of the carbon nanotubes also helps to eliminate residual voltage, or “hot spots” as Bryant calls these areas of extreme electrical conductivity. Carbon fiber residing on the surface of the plastic can be conductive, almost metal like. They can build up charges and also bleed off the charges, but if the charge is bled off to quickly there can be a corona event, an electrical arcing. The size and uniformity of the nanotubes, says Bryant, eliminates this problem.

The ESD properties was one of the compound’s selling points for the Iomega Corp., San Diego. The company, which makes storage and network security solutions, uses the conductive material in its REV Backup Drive and removable REV disk that secures critical data for easy recovery.

The REV product family was initially introduced in 2004 and a second-generation REV drive was introduced in 2007. To date, Iomega has sold approximately 300,000 REV drives and 1.5 million REV disks. The RTP Co. customized a high-impact RTP 300 Series PC CNT compound for the original REV disk that met difficult application challenges, including consistent electrical resistivity. Now, with the new reformulated electrical grade compounds, RTP Company was able to deliver a CNT product for the second generation REV drive that was optimized to meet Iomega’s strict high purity cleanliness standards for low particle generation while simultaneously offering cost improvements.

Novel nano

Enlarge this picture Heat deflection is shown with the addition of DuPont nanomaterial to glass-reinforced PET.

Research and development of nanotubes is ongoing at many other companies and research facilities. NaturalNano, Rochester, N.Y., is moving forward with its own nanotube recipe. In June, the company says it successfully tested its Pleximer product using high loadings of halloysite nanotubes in a variety of polymers. At HNT concentrations as high as 20 percent, the material showed a storage modulus that was 10 times higher than that of the polymer alone, says Cathy Fleischer, president and CTO of NaturalNano. The company has entered into joint development agreements with some plastics’ manufacturers, most recently with Cascade Engineering, a developer and manufacturer of plastics products.

And, researchers at the University of Virginia have developed carbon nanotubes that they say will unite properties of plastics and metals in a new ultra-lightweight, conductive material that “may revolutionize electromagnetic shielding and more,” says engineering professor Mool C. Gupta.

The nanocomposite material is a mixture of plastic, carbon nanotubes and a foaming agent, making it extremely lightweight, corrosion-proof and cheaper to produce than metal. The carbon nanotubes play a key role in creating these unique properties, explained Gupta. Most notably, experiments revealed that only 1 percent to 2 percent of the material’s composition needed to be comprised of nanotubes to increase the electrical conductivity by 10 orders of magnitude. The addition of carbon nanotubes also increased the material’s thermal conductivity, improving its capacity to dissipate heat.

After experimenting with adding metal powder to plastics without impacting the weight of the material significantly, Gupta turned to carbon nanotubes. With their ultra-small diameter, high aspect ratio, high mechanical strength, good electrical and thermal conductivity and lightweight, Gupta and his team found that carbon nanotubes had all of the properties necessary to accomplish the objective.

“The long length and small diameter of carbon nanotubes forms an interconnectivity within the plastic that makes it electrically conductive,” says Gupta. “The structures work well for electromagnetic interference shielding. Once we reached this milestone, we began investigating ways to reduce the weight even further.”

For more information, email:
DuPont: carole.a.davies@usa.dupont.com
NaturalNano: fleischer@naturalnano.com
PolyOne Corp: david.honeycutt@polyone.com
RTP Co: nbryant@rtpcompany.com
Solvay: Richard.Thommeret@solvay.com