The clay–polymer layer-by-layer process (a) and an illustration of the quadlayer nano brick wall structure of PEI (blue), PAA (green), PEI and MMT clay (red). Credit: M.A. Priolo, et al; Nano Letters.
It’s easy to imagine that the makers of food products have a wish list for packaging that, besides the obvious need to be inexpensive and easy to use, 1) indicates freshness; 2) is antimicrobial; 3) is microwaveable; 4) is transparent; and 5) creates a strong barrier to air.
I don’t know how the progress is going on #1 and #2, but a group of researchers from Texas A&M University seem to have a good handle on addressing wishes #3, 4 and 5. In two papers (one about a year old and one more recent), a nanocomposites team led by Jaime Grunlan, a TAMU mechanical engineering professor who did his Ph.D. work in materials science, reports on work it has been doing with applying layers of a special clay to polymer film to create highly effective gas barriers.
In the first paper, published in early 2010 Applied Materials & Interfaces (doi:10.102.1021/am900820k), Grunlan’s group reported on a transparent and super air-tight “brick and mortar”-like nanostructure construction method, based on water processing. This construction is accomplished via a layer-by-layer assembly method of depositing alternating thin films of sodium montmorillonite clay and branched polyethylenimine onto a 179 µm substrate of poly(ethylene terephthalate). After putting 70 of these PEI-MMT bilayers together on the PET substrate (resulting in a film thickness of 231 nanometers), the permeability to oxygen becomes 0.002 X 10-6 cc/(m2 day atm), the lowest permeability ever reported for a polymer-clay composite.
The researchers attributed the lack of permeability of the brick wall nanostructure to “alternate adsorption of polymeric mortar and highly oriented, exfoliated clay platelets.” Essentially, oxygen or other gas molecules have to travel such a “tortuous” bricks-and-mortar path through the film that they seldom succeed in getting through.
They also predicted that films made in this way, because of their high level of transparency and ability to be a gas barrier, “would be good candidates for a variety of flexible electronics, food and pharmaceutical packaging.”
The interest in this type of superbarrier film performance to food and pharmaceuticals seems pretty obvious, but the interest to the electronics industry is rooted in the development of flexible devices (think flexible organic LEDs) for which other thin film materials, such as SiOx, Al2O3 and EVOH are problematic because of cracking or expensive fabrication processes.
TEM cross-sectional image of film with five quadlayers. Credit: M.A. Priolo; Nano Letters.
In the most recent paper, published in Nano Letters (doi:10:1021:nl103047k) the TAMU group reports they have been able to create an even thinner superbarrier, composed of only 12 polymer and four clay layers (51 nanometers), that are far less permeable than SiOx and Al2O3.
Actually, in this new work, they have added one more component — poly(acrylic acid) — and alternated the layering sequence in an approach that produces a “quadlayer” of PEI-clay-PAA-clay. When put on a PET substrate, four of these quad layers are enough to better the performance of SiOx and EVOH, and that even five layers have an average light transmission of 95 percent across the visible light spectrum.
The researchers note that, “When combined with ambient processing from water, high transparency and flexibility, these quadlayer coatings are among the best barrier films of any type ever reported.” Another feature is that, unlike metalized plastics, food wrapped in these films can be heated in a microwave oven.
Comparison of PET versus polymer—clay thin film. Credit: M.A. Priolo.
There is one weakness, however, to these films: Their performance lessens as humidity increases. They have learned that some of this can be offset through thermal cross-linking in the PEI and PAA layers. Nevertheless, they report that the humid oxygen transmission rate of the quadlayer film is much less than for untreated PET. Where humidity performance is a concern, the group suggests that quadlayer assemblies could be deposited directly onto a film, such as poly(chlorotrifluoroethylene) that is known to have high moisture barrier properties.
Grunlan recently did a presentation on this work at a meeting of the American Chemical Society, and in an ACS news release he puts his group’s work in perspective, noting “Others have added clay to polymer to reduce (gas) permeability, but they are thousands of times more permeable than our film. We have the most organized structure — a nano-brick wall — which is the source of this exceptional barrier. This is truly the most oxygen impermeable film in existence.”
His group also notes that because the new film is about 70 percent clay and contains a small amount of polymer, it is more eco-friendly than current plastics and may be able to replace some foil packaging.