Clear nanocomposite barrier films for brand impact
A transparent nanocomposite film, composed of clay and polyethylenimine (PEI), has been produced as a potential alternative gas barrier to metal foil for food packaging.
Jaime Grunlan of Texas A&M University, USA, who headed the research, explains, ‘The goal was to produce a simple water-based coating that could achieve properties similar to ceramic thin films for a gas and moisture barrier for plastic substrates and to compete with metallised and silicon oxide (SiOx)-based films’.
Grunlan notes that the film's main appeal is its optical transparency of 86%, which make it ideal for branding purposes compared to metal foil-based films.
The barriers are produced by dipping or spraying plastic packaging with dilute aqueous mixtures of both sodium montmorillonite and PEI alternately. The protective layers are then built up via a layer-by-layer self assembly method.
The solution typically contains 0.1-1wt% of either clay or polymer. The recommended solution exposure time ranges from 10-15 seconds, with no longer than a minute for any one layer required. Each single layer is typically 1-100nm.
Grunlan says, ‘After 40 composite layers have been deposited, the resulting oxygen transmission rate (OTR) is below 0.35cm3/m2day when the pH of PEI is 10.
This low permeability is due to a brick wall nanostructure created by the alternate adsorption of polymeric mortar and highly orientated exfoliated clay platelets. More recently, we have produced a 51nmthick coating that has an OTR that is undetectable.
He claims, ‘When multiplied by thickness, the calculated permeability is better than both SiOx and metallised plastic film, which are its closest competitors.’
Currently, the film has optimum barrier properties at 20-40ºC, says Grunlan, and is suitable for microwaveable temperatures. Yet the team are unable to confirm the highest temperature the film can withstand. He admits however, that at very high temperatures the material properties are expected to degrade slightly.
Furthermore, he believes that the work has implications for flexible electronics.
‘There is no adequate technology that is transparent and flexible and offers high barrier performance all at the same time. With this composite you can bend the film many times and its barrier properties will be intact. With SiOx, for example, it is difficult to keep those barrier properties with heavy flexing.’
The coating can also be upscaled easily at minimal cost, says Grunlan, using spray machinery, or running a line of plastic films that can be dipped in and out of containers, or by moving films on a line to be automatically sprayed by the solution and vacuum or air dried.
Christopher Roberts, Director of Versaperm Ltd in Berkshire, UK, comments ‘The main reasons for considering alternatives to foil are cosmetic/marketing considerations, where the vendor wants the product to be visible and technical, where the requirements are not as high as foil would produce.
‘However, one of the major difficulties in using plateletshaped fillers to improve barrier [performance] is ensuring that they are laid down in parallel layers, ideally with as little interstitial spacing horizontally and vertically as possible. It may be that the process described in these papers has been successful in controlling this layering.
He adds, ‘Whether the cost of producing the multiple layers will provide a competitive process to alternative chemistries and processes will determine the future success’.
The films have also been made with polyacrylic acid, in addition to PEI.