Engineers with problems to solve right now might not give a second thought to nanotechnology since so much of it still seems more like science fiction than fact. But nanotech has, in fact, already boosted the performance of commonly used materials and it promises to do even more in the near future.
While a handful of adhesives and thermoplastic compounds already contain nanoscale additives to boost bulk mechanical properties such as tensile or impact strength, nanotech's has made a stronger impact on coatings and surface enhancements for metal, plastic, and ceramic substrates.
The reason why comes partly down to economics. Nanoscale additives don't come cheap; even some commercial ones cost hundreds dollars per pound. So putting them in a coating and concentrating them on the surface of a part makes a lot of sense. "It's where you get the most bang for your buck," says Bob Kumpf, head of future business for Bayer MaterialScience LLC. And putting a tiny amount of nanoparticles in a coating can have a tremendous impact on important surface properties, such as resistance to wear, chemicals, and dirt. Nanotech coatings can influence thermal, optical, and electrical properties too. "The surface of a part is really where a lot of things happen," says Kumpf.
Nanotechnology may also have a unique ability to balance all this added functionality with aesthetic requirements. "Nanotech is, by definition, invisible to the human eye," says Kumpf. And invisibility means nanotech can often work its functional magic without changing the way surfaces look. Consider the case of titanium dioxide. "It's well known for its UV resistance. But when you put it in paint, everything turns white," says Bob Matheson, a technical manager for strategic technologies at DuPont Performance Coatings. So the company has been using nanoscale titanium dioxide particles that still offer the UV protection while remaining invisible in colored paint formulations.
Despite all the buzz it gets today, remember that nanotechnology really amounts to business as usual for the kinds of companies that develop materials, paints, and coatings. "We're very good at dispersing nano particles, which can be tricky to handle," says Matheson. As an example, he cites the paint industry's use of carbon black and silica. "Both are true nanoparticles, and we've used them for thirty years." Or to take another example, BASF has for decades offered products that incorporated nano-sized dispersions of one polymer within another. "We're already experts in this," says Volker Warzelhan, senior vice president for polymer research.
Still, the really exciting work in nanotech coatings has really just begun as materials scientists intentionally take advantage of the properties that emerge when things get very small. Sally Ramsey, chief chemist and cofounder of Ecology Coatings, notes that her nanotech coatings sometimes make use of additives that have been used for years in larger, micron sizes. "What's happening now is that we're using nanoparticles to produce properties that wouldn't exist with larger particles," she explains.
This intentional reliance on smallness takes a few different forms in coatings and paints. Mostly, it involves the use of nanoscale additives that fall into one of two categories—particles that measure between a few and about 100 nm or nanoscale structures on larger particles and even part surfaces.
DuPont, for example, has started to incorporate both approaches in its paints and coatings. Matheson reports that the company has developmental products and research efforts based on various metal oxides, nanoclays, carbon, silica, crystalline materials, and exotic nanomaterials like silsesquioxane oligimers. The company has also come up with a nanosized "pulp" version of Kevlar. Usually a Kevlar fiber has a diameter of 12 microns and a length measured in kilometers. "The nanoparticle Kevlar has a length of 12 microns and roughly the same aspect ratio," he says. That translates to a fiber roughly with a two nanometer diameter.
Other companies likewise use several types of nanoparticles for different formulations—even within the same formulation. Ecology Coatings sometimes puts two different nanoparticles in the UV-cure coatings it has developed for plastics, metals, and papers. "Everything is very application specific and proprietary," says Ramsey. In general, though, she says the formulations contain oxides or other nanoparticles, oftentimes used in conjunction with micron-sized additives that have traditionally been used in urethane and epoxy coatings.
Looking further down the road, other types of particles will play a role too, especially as materials suppliers come up with more efficient manufacturing processes. Bayer MaterialScience, for instance, last month announced that it has come up with a new way to make multi-wall, five-nanometer carbon nanotubes and can now offer sampling quantities to prospective users. Other companies already supply this type of nanotube too, but Kumpf believes Bayer's process may have a cost edge, though it's too early to say how much of one.
Beyond the use of nanoscale particles in coating formulations, another promising route for nanotech involves nanoscale surface enhancements—such as those offered by by Surface Engineering Associates. The company uses vapor deposition techniques to deposit nanoscale layers on the surface of ceramic, elastomer, and metal substrates. "Think of our system as an atomic spray can," says Todd Schlesinger, a material scientist who founded Surface Engineering. Depending on the application requirements, these nanolayer coatings range from ten to several hundred nanometers thicks. "The nice thing about nanolayers is that they improve a part's surface characteristics without changing its dimensions or underlying mechanical properties in any measurable way," notes Schlesinger. This ability matters the most with small, precision parts—such as the nitinol stents that the company coats for one medical customer.
Calling Surface Engineering's technology a coating, though, may be somewhat misleading. Schlesinger notes that the nanoscale layers on the part surface don't so much form a distinct coating as they embed themselves in microscopic voids on the substrate surfaces. "There's no distinct boundary between the coating and the surface," he says.
Once incorporated into a paint or coating, nanoparticles or thin coatings can target a variety of properties. In the case of Surface Engineering, Schlesinger has focused on improving the coefficient of friction and release properties of elastomers and engineered ceramics such as zirconia and aluminum oxide. These nano-thin layers of fluoropolymers can reduce ceramic-on-ceramic coefficient of friction from 0.7 to 0.1 or below, Schlesinger reports.
Nanotech paint products, meanwhile, have targeted scratch resistance. PPG, for example, already has an automotive clearcoat based on nanotech. DuPont and BASF also have nanotech clearcoat products nearly ready to go.
What does nanotech do for scratches? Matheson lays out two mechanisms by which nanoscale silica particles can fight scratches. In one, the coating is formulated so the silica rises to the top of the coating, where these hard particles directly ward off scratches. The other, the one now favored by DuPont, involves dispersing the silica evenly through the coating, where it acts to disrupt scratches before they become visible. "The scratches are there, but the human eye can't see them," says Matheson.
Nanoparticles can also serve as a means for increasing a cured coating's crosslink density, which makes for harder coatings. GE Silicones has for more than a decade used colloidal silica, which measures less than 100 nm, to improve crosslinking and thus the scratch resistance of its hardcoats for plastics. According to Haroula Reitz, global technology manager, these nanotech hardcoats improve abrasion resistance in tabor haze tests by a factor of two.
In upcoming products, she says GE Silicones will use different nanotech particles to balance the need for hardness with other desirable properties for optical media—like optical clarity and a low coating modulus that won't cause plastic substrates to warp. "The goal is to achieve just the right amount of crosslinking without going overboard," Reitz says.
Ecology Coatings likewise makes use of nanoparticles to increase hardness as its UV coatings cure. Ramsey additionally describes the role of the nano particles as "filling in the interstices" between the larger particles that also go into the company's coatings. Nanoparticles, which are small enough not to block the UV light, may also allow more complete cure through the thickness of the coating than formulations containing only micron-sized particles.
More to Come
In the future, nanoparticles will address a host of other properties as well. DuPont is working with nano versions of titanium dioxide and zinc oxide to improve the UV performance and crack resistance of undercoats—and again neither type of particle effects appearance because of their size. "It's probably the best example I have of how nanoscale particles help us achieve a balance of properties that wouldn't be available with larger additives," says Matheson.
Carbon nanotubes promise coatings with beneficial electrical and optical properties. Unlike traditional spherical additives, nanotubes "lay down like pickup sticks," forming conductive pathways through a material, Kumpf explains.
And many researchers see nanotech as a way to create "self-cleaning" surfaces with hydrophobic or even superhydrophobic properties. BASF has just such a research effort going. So does GE. According to Reitz, GE Silicones and the parent company's Global Research Center each have programs underway to develop low-surface energy surfaces—the former using nano-additives and the latter through nanostructured surfaces that qualify as superhydrophobic.
Ramsey adds that "Nanoparticles sometimes impart properties we weren't expecting." She's found that some of Ecology's nanotech coatings, while initially formulated for scratch resistance, ended up exhibiting some of these emergent properties. For one, she has found that some of the coatings exhibit antimicrobial properties. For another, the company has come up with a coating that makes pulp-based paper waterproof.
Other researchers are looking into nanotech to create "smart" materials capable of acting as sensors. GE's Global Research Center has come up with a moisture-sensing gold salt—both it and the superhydrophobic surface capability were revealed during a presentation from GE Plastics on surfaces that have both aesthetic and enhanced functional attributes. DuPont, meanwhile, has been looking at the optical filtering potential of quantum dots based on zinc sulphide crystals. These materials absorb light with the color of light they absorb varying with the size of the crystal. "They're interesting materials," says Matheson. "We just haven't figured out any way to make money from them yet."
Even when spread thin in coatings, nanotech still has to clear some economic hurdles. "The economies of scale, almost by definition, just aren't there for many nanotechnologies," says Matheson. He and others involved in the development of nanotech coatings point to the fact that nanotech coatings not only contain expensive ingredients but need a lot of custom formulation effort. For these reasons, don't expect to see some of the more exotic nanotech products anytime soon. The consensus from those working on self-cleaning surfaces, for example, is that they won't be widely commercial for another five years.
Engineers can, however, take advantage of a few nanotech products with enhanced wear and friction properties right now.