Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB
Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB

Combined coatings

The barrier effect and enhanced emptying behavior of plastic containers

In numerous technical fields resource and energy efficiency is playing an increasingly important role. Optimization of resource usage and energy needs has a great impact on production processes as, alongside cost-efficiency, environmental aspects need to be taken into account more and more. A prime example of this is the replacement of glass with plastics which is aimed at for two reasons. For one, plastic packaging is lighter and thus contributes to a reduction of fuel costs during transportation. Furthermore, it is not susceptible to breakage. One disadvantage, however, is the poor barrier effect of such packaging. The latter can be augmented in a resource-efficient process by coating the plastic surface by using plasma technology.

Enhanced properties through plasma coating

Draining behavior of test oil.
Figure 1: Draining behavior of a test oil on untreated and plasma-coated polymer surfaces.

When attempting to improve the properties of plastic containers, coating and co-extrusion processes come into question. Considering coating processes, plasma technology is particularly promising and even environmentally friendly: with this technology it is possible to precisely attain a large variation of surface properties by varying the coating process parameters. This allows, for example, for a reduction of the permeation of substances and the adjustment of surface wetting. Plasma processes require only a minimal amount of coating substances since these are transformed into a coating in the plasma in a highly efficient manner. At the Fraunhofer IGB coatings distinctive for their excellent barrier effect against gasses and liquids and simultaneously displaying optimized drain behavior properties were developed for various materials and applications.

Plasma coating

Creation of reactive plasma gas in a special plasma reactor.
Figure 2: In a special plasma reactor the reactive plasma gas is created directly inside the canister and thus coats its inner walls.

In the plasma processes for the production of functional coatings (plasma polymerization) the necessary low molecular parent compounds are first partly fragmented by the energy input in the plasma. These fragments (radicals) subsequently react with the surface of the material to be treated and create a stably bound coating. In contrast to wet chemical processes plasma technology is advantageous because the coating steps can be carried out in direct succession – that is, without intermediate steps for drying or washing. Coatings which serve as barriers or conducive to better emptying behavior are, for example, fluorocarbon-based [1], methyl-based [2], high-density glass-like diffusion barriers [3] as well as adapted multi-layer systems.

Effective barriers against water vapor and oxygen

Multi-layer layer as a barrier against oxygen and water vapour.
Figure 3: Multi-layer layer as a barrier against oxygen and water vapour.

At the Fraunhofer IGB barrier coatings were produced which raised the barrier effect of the plastic polyethylene terephthalate (PET) against water vapor and oxygen by a factor of more than 1000 compared to untreated materials. In comparison to commercially available coatings based on ethylene vinyl alcohol copolymer (EVOH), this coating retains oxygen five times and water vapor even 50 times better.

Enhanced drain behavior

If an additional coating is applied in a subsequent step of the process, it is possible to adapt the surface to suit other needs relating, for example, to chemical and wetting properties. Oil and water-resistant fluorocarbon coatings, for example, create a Teflon-like surface. Test fluids drain remarkably better from such surfaces than from untreated ones. The improvement is noticeable not only for laboratory testing media such as water and oil but also for common industrial bulk goods like glues, varnishes, and paints. To give an example: it was possible to improve the emptying behavior of treated HDPE canisters for oil-containing bulk goods by 10 percent compared to untreated canisters. The multifunctional coatings have already successfully been applied to various flat materials as well as to moldings such as canisters and tanks.

Outlook

The coatings are suited to many applications, for example, containers for packaging and storage as well as for displays and inspection windows in damp rooms. Creating multifunctional layers or layer systems using plasma technology is particularly appealing since these can be designed to display a variety of chemical and physical material properties, and match the desired property profile while using only a single process.

Literature

[1] J. Barz, M. Haupt, U. Vohrer, H. Hilgers, C. Oehr, Ultrathin carbon-fluorine film processing, Surface and Coating Technology 2005, 200, 1-4, 453 ff.

[2] B. Jacoby, W. Bock, M. Haupt, H. Hilgers, M. Kopnarski, J. Molter, C. Oehr, T. Rühle, M. Wahl, Abscheidung, Charakterisierung und Anwendung von Plasma-Polymerschichten auf HMDSO-Basis, Vakuum in Forschung und Praxis 2006, 18, 4, 12 ff.

[3] M. Baier, Ultrabarriereschichten in R. Suchentrunk.: Jahrbuch Oberflächentechnik. vol. 65, Leuze Verlag Bad Saulgau, 2009, ISBN: 978-3-87480-253-6, 109 ff.

Funding

We would like to thank the German Federal Ministry of Education and Research (BMBF) for funding the project »Innofunk«, promotional reference 033R045A.