“Nano, nano.” I always thought that was the signature theme of Robin Williams’ character Mork from the television program “Mork and Mindy.” But, it seems, 10–9 —“nano”—might be a dimension teetering on the high-wire between science fiction and real technology.
Since Nobel Laureate Richard Feynman’s seminal 1960 book There’s a Lot of Room at the Bottom—or was it Eric Drexler’s 1986 Engines of Creation or his 1997 Nanosystems?—the scientific world has been dazzled and intrigued by this quantum leap into mysterious miniaturization. Boldly venturing where no man or woman has gone before, Drexler and his supporters have proposed molecular manipulation to manufacture, sense, control, and otherwise profoundly alter our lives.
Obviously, controlling the structure of materials down to a few atoms or molecules should exert a great impact on everything from computing to medicine to human behavior. The ability to manipulate matter one atom at a time has been the stuff of science fiction for years. But the recent development of high-technology tools—especially probes sensitive enough to both image and move individual atoms and molecules—has begun to convert fantasies into reality.
Glowing stories on nanotechnology fail to address crucial issues that will disturb our beliefs, culture, and even the definition of what we consider human.
Recently, some researchers have fabricated a transistor from a single carbon molecule and built prototype information storage devices with data bits as small as 50 nm across. At present, these devices are, of course, laboratory novelties. No one knows what will result from these deep probes. Although scientists can fabricate nanoengines or nanosensors one at a time in the laboratory, they—or someone—still must find commercially feasible means to make millions of them at speed. These first baby steps suggest that it might be possible to build working nanodevices—and the benchtop successes have begun to generate considerable hope, to accompany the hype. The boundaries of this new realm are still being drawn in often contentious debate.
Nanospace is a mysterious territory that appears to operate according to its own rules. Scientists are just beginning to think about trying to understand the physics of the teeny-weeny—when we leap from micro to nano. Purists must think much smaller before entering the nanoworld.
Below about 50 nm is where the new arena is located, where forces such as gravity that govern the everyday world lose their classical definitions and possibly new equations must be postulated.
Food Packaging and Nanotechnology
Some of the food packaging applications postulated, such as intelligence, sensing and signaling microbiological and/or biochemical change; or active, dispensing antimicrobials when the need is detected, are far, far in the future. A few, such as modifying permeation behavior, are at least in the laboratory or even exiting the laboratory portal. But are these within the realm of new nanotechnology, or merely extensions of long-known barrier-altering microtechnology?
Permeability of Plastics for Food Protection. Plastic food package structural requirements and basic principles that have prompted the proposals to plunge into nanocomposite technologies are fairly straightforward: inorganic—or even organic—fillers increase or decrease gas permeabilities of polymer matrices.
High Gas Permeability. Polyolefin resin melt-blended with inert inorganic fillers and stabilizers can be converted into film that may be uniaxially oriented. The result can be a plastic microstructure with a sufficient number of elongated, narrow-shaped, microporous voids that allow permeation of oxygen and carbon dioxide in the range of about 5,000–10,000 cc/100 in2/atm/day, deferring respiratory anaerobiosis in modified-atmosphere-packaged fresh fruit and vegetables. Whoops! What happens if such voids are generated in orienting films, sheets, or bottles that are intended as materials with low gas/water vapor permeability?
Low Gas Permeability. Enhanced barrier, i.e., reduced gas permeability, can be obtained through the incorporation of nanometer-sized platelets in the polymer matrix—sort of a micro version of bricks and mortar in a wall.
A mixed matrix barrier containing either inorganic flakes such as the silicate mica or semipermeable polymer flakes such as nylon—or nanometer-size particles—depends strongly on the so-called aspect ratio of the dispersed phase particles. The aspect ratio of a typical square platelet filler is defined as one lateral dimension divided by the thickness dimension. As the aspect ratio for an impermeable flake increases, a permeant such as oxygen experiences a large increase in effective path length. This increased path length can markedly reduce the net rate of movement across the barrier layer. An example is incorporating low-permeability nylon flakes with large aspect ratios into a polyethylene layer to enhance hydrocarbon barrier—not a food application, but suggestive of benefits.
Aspect Ratios of Inclusions. Much larger effective aspect-ratio flakes can be incorporated using this method with smeared-out polymer flakes than what is practical with brittle inorganic flakes like mica. A large increase (40–50%) in barrier properties is achieved.
Large-aspect-ratio platelet fillers are superior to spherical ones (such as conventional strength-enhancing mineral fillers) for adding barrier efficacy due to tortuous-path theory. The gas must travel a longer pathway through the polymer to bypass the inclusions. With large-aspect-ratio flakes, essentially series resistance contributions occur from both the dispersed and continuous phases.
A further step in the development of advanced barriers might be the development of flake that is impermeable to one agent but highly permeable to water vapor to produce a truly “breathable” barrier so desirable for packaging of some fresh foods.
Complex behavior occurs with dispersed spherical particle limits for “impermeable” fillers with intermediate-aspect-ratio fillers. This behavior applies for gases permeating through the mixed matrix with high-permeability-selective fillers that allow water vapor to pass. For aspect ratios above 50–100, major barrier enhancements are feasible. The effective permeability due to the increased tortuous path length a molecule must traverse in the filled polymer depends on two variables: the volume fraction of the flakes and the aspect ratio.
The governing Nielson equation for permeability as a function of aspect ratio is:
P = Po(1–c/1+ c/2)
where P = permeability of the filled polymer, Po = intrinsic permeability of the polymer, c = volume fraction of filler, and = aspect ratio (width/height). Large reductions in effective permeability as a function of volume fraction for various aspect ratios have been demonstrated.
Nanocomposites in Plastic Structures
Having retreated to the comfortable world of macro and micro inclusions to achieve tortuous paths, it is time to return to the new world of nanocomposites. RTP Co., Winona, Minn. (www.rtpcompany.com) and Nanocor (now Mitsubishi), Arlington Heights, Ill. (www.nanocor.com) have produced package structures incorporating these materials.
Polymer–Silicate Nanocomposites. These represent an emerging class of specialty plastic blends because of the “nano”-scale dispersion. High aspect ratio and surface area are achieved that result in much better properties than those of conventional mineral fillers: higher heat resistance, increased flexural modulus, lower gas permeation rate, easy flow, dimensional stability, and good surface appearance.
Typically, the type of silicate used in polymer–silicate nanocomposites is montmorillonite, a 2:1 layered smectite clay. This clay has a natural platy structure with individual platelets having thicknesses of 1 nm and surface lengths on the order of 100–1,000 nm—one rationale for classifying the materials as nanocomposites. montmorillonite is hydrophilic, which makes proper exfoliation and dispersion into conventional polymers difficult. Thus, it is usually modified through substitution of its sodium ions with organic ammonium ions, resulting in an organo-clay complex. The intercalation procedure expands the gallery and improves the compatibility of the complex with the polymer so that individual platelets can be more easily separated in a polymer matrix.
Incorporation of Platelets into Plastic. Among the methods for production of the polymer–silicate nanocom-posite are synthesis of the compound in the polymerization stage with the complex in-situ with the liquid monomer. Nanocomposites could be prepared from solvents in which both polymer and complex are dissolved.
Another method involves the preparation of polymer–silicate nanocomposites by melt processing. In this process, the application of shear during compounding assists exfoliation and dispersion. The melt-compounding route would be an approach in the production of polymer–silicate nanocomposites, since existing technologies and equipment could be utilized and scaled to commercial quantities.
Nylon-6 Nanocomposites. Compared to unfilled matrix nylon-6, a typical nylon-6 nanocomposite generates a 52% increase in flexural modulus and deflection temperature increase of over 35°C. Why nylon-6? Because it is a polar polymer—a requisite for combining with the inorganic material. Restating the issue: no other commercial polymer such as polyolefin can combine with montmorillonite.
Furthermore, any orientation such as that common for polypropylene generates voids that, of course, allow gas to pass.
Polyester, always oriented in film, sheet, or bottles, additionally suffers from the potential from residual mineral-catalyzed degradation reaction of the polymer.
Montmorillonite. Why montmorillonite? Because it is available, because it naturally breaks down into nano-size particles, and because its proponents have been working with it for decades.
Properties of Nanocomposite Packaging Plastics
Besides improvements in mechanical and heat-resistance properties, nanocomposite compounds can exhibit dramatically enhanced barrier properties, as represented by lower oxygen transmission rates (OTRs).
Nylon-6 nanocomposites can achieve an OTR almost four times lower than unfilled nylon-6. The reduction is typically attributed to the increase in effective diffusion distance, as solutes must travel a tortuous path around well-dispersed platelets of high aspect ratio. As a result of the nanometer-length scale, excellent transparency is retained in sheets and films formed from nylon-6 nanocomposites.
Research on direct incorporation of nanocomposite clays into polymer is being performed under sponsorship of the Soldier Systems group in Natick, Mass..
Surface Coating. Research on dispersing nanocomposite materials on the surface of plastic films is simultaneously underway. MicroCoating Technologies is employing chemical vapor deposition to coat a nanometer-thick layer of silicon oxide to enhance barrier. Whether this is nanotechnology or plasma deposition technology is a semantic question, regardless of the merits of the technology.
Avery Dennison Corp., Concord, Ohio (www.averydennison.com) has developed a patent-pending, ultra-high gas barrier coating for films by using nanoparticles. Unlike other techniques that disperse the nanoparticles into the film resin, this proprietary process coats the film substrate with nanoparticles. This yields a highly controlled and consistent coating that is believed to provide ultra-high gas barrier, flex-crack resistance, clarity, and a very thin coating, less than 1 µ thick.
The company has observed OTRs below 0.005 cc/m2/day at 23°C and dry conditions. A range of substrates used in flexible packaging applications have been coated and tested. Many have generated impressively low OTRs, although coating optimization is still required for other flexible packaging films.
Initial testing of OTRs after Gelbo flexing have been very promising. These tests, performed on unsupported film samples, indicate that the coating performs better than metal oxides with respect to flex cracking. Additional Gelbo tests on a 2- to 3-layer laminated structure are in process. The company expects more promising results with a laminated structure, as the additional layers provide greater support for the coating.
With regard to clarity, light transmission after coating exceeds 93%, and haze is less than 2%.
As with all technologies, there are limitations to the coating. It is not an effective moisture barrier, and its gas barrier properties decrease with increasing relative humidity. As a result, in most cases the total film structure must include a component with low water vapor permeability. The coating’s sensitivity to moisture is reversible—its barrier protection completely recovers once the humidity is reduced—analogous to that of polymer barrier ethylene vinyl alcohol.
The film’s high gas barrier, flex crack resistance, and clarity may make it an ideal choice for flexible packaging applications requiring these properties.
Approaching an Elusive Goal
To twist a paraphrase into a vision of tomorrow, oh, what a tortuous path we blaze when first we disturb our molecules, atoms, and quantum mechanics. But tortuous paths, in this context, are an objective—to retard the passage of those gases and vapors that can harm our packaged foods. And while macro-minerals can effect benefits—increasing the gas permeability for modified-atmosphere packaging—nanocomposites appear capable of approaching the elusive goal of converting plastic into a superbarrier—the equivalent of glass or metal—without upsetting regulators.
Perhaps in food packaging, as passive benefactors, nanocomposites will generate their initial beachhead. But, of course, when barrier is the objective, aspect ratio appears to be crucial—can we maintain uniformity of dispersion and aspect ratio in this emerging family of plastics when we blow, thermoform, or orient them?
The question is how—and when—can we scale up to commercial package structures?
And so, let us watch this wonderful world of nanotubes, nanorobots, and re-runs of “Mork and Mindy” and participate pensively in nanopackaging.
Koros, W.J. 1999. Permeation processes in barriers and membranes: complementary differences. In Proceedings of Polymers, Laminations and Coatings Conference, TAPPI, Atlanta, Ga.
Rotman D. 1999. Will the real nanotech please stand up? Technol. Rev., March-April, pp. 46-53.
Voss, D. 1999. Moses of the nanoworld. Technol. Rev., March-April, pp. 60-62.
by AARON L. BRODY
President and CEO, Packaging/Brody, Inc.