Karim Bahroun

Surface pre-treatment for barrier coatings on polyethylene terephthalate

Polymers have favourable properties such as light weight, flexibility and transparency. Consequently, this makes them suitable for food packaging, organic light-emitting diodes and flexible solar cells. Nonetheless, raw plastics do not possess sufficient barrier functionality against oxygen and water vapour, which is of paramount importance for most applications. A widespread solution is to deposit thin silicon oxide layers using plasma processes. However, silicon oxide layers do not always fulfil the requirements concerning adhesion and barrier performance when deposited on films. Thus, plasma pre-treatment is often necessary. To analyse the influence of a plasma-based pre-treatment on barrier performance, different plasma pre-treatments on three reactor setups were applied to a very smooth polyethylene terephthalate film before depositing a silicon oxide barrier layer. In this paper, the influence of oxygen and argon plasma pre-treatments towards the barrier performance is discussed examining the chemical and topological change of the film.It was observed that a short one-to-ten-second plasma treatment can reduce the oxygen transmission rate by a factor of five. The surface chemistry and the surface topography change significantly for these short treatment times, leading to an increased surface energy. The surface roughness rises slowly due to the development of small spots in the nanometre range. For very long treatment times, surface roughness of the order of the barrier layers thickness results in a complete loss of barrier properties. During plasma pre-treatment, the trade-off between surface activation and roughening of the surface has to be carefully considered.

Influence of layer type and order on barrier properties of multilayer PECVD barrier coatings

Due to their macromolecular structure, plastics are limited in their scope of application whenever high barrier functionality against oxygen and water vapour permeation is required. One solution is the deposition of thin silicon oxide coatings in plasma-enhanced chemical vapour deposition (PECVD) processes. A way to improve performance of barrier coatings is the use of multilayer structures built from dyad layers, which combine an inorganic barrier layer and an organic intermediate layer. In order to investigate the influence of type and number of dyads on the barrier performance of coated 23 µm PET films, different dyad setups are chosen. The setups include SiOCH interlayers and SiOx-barrier layers deposited using the precursor hexamethyldisiloxane (HMDSO). A single reactor setup driven in pulsed microwave plasma (MW) mode as well as capacitively coupled plasma (CCP) mode is chosen. In this paper the effects of a variation in intermediate layer recipe and stacking order using dyad setups on the oxygen barrier properties of multilayer coatings are discussed with regard to the chemical structure, morphology and activation energy of the permeation process.Changes in surface nano-morphology of intermediate layers have a strong impact on the barrier properties of subsequent glass-like coatings. Even a complete failure of the barrier is observed. Therefore, when depositing multilayer barrier coatings, stacking order has to be considered.

Erhöhung der Flexibilität von Barrierebeschichtungen

A ihrer Verarbeitungsbedingungen und der geringen Materialdicke stellen Kunststo folien besondere Anforderungen an eine Veredelung im Plasma. In vielen Fällen soll dabei die durch die makromolekulare Struktur des Kunststo s bedingte, hohe Durchlässigkeit der Folien gegenüber üchtigen Medien, wie zum Beispiel Sauersto oder Wasserdampf, verringert werden. Der Einsatzbereich plasmapolymerer Barrierebeschichtungen zur Verringerung der Permeation wird jedoch zumeist durch die hohe Flexibilität der Kunststofffolie beschränkt. Die Entstehung dehnungsinduzierter Risse in der Beschichtung ist für verschiedene plasmapolymere Schichten unter Belastung untersucht und beschrieben worden [ALF03, HMGW09, YHRG01]. Abgeschieden auf Polyethylenterephthalat (PET) verlieren silizium-basierte Sperrschichten (SiOx) zumeist ab einer geringen Substratdehnung von circa 2 % ihre Sperrwirkung. Kohlenwasserstoffbasierte (a-C:H) beziehungsweise siliziumorganische (SiOCH) Schichtsysteme ertragen Dehnungen > 4 %. Der Ansatz ist daher, durch Kombination der verschiedenen Schichtarten unterschiedlicher Sprödheit plasmapolymere Mehrschichtsysteme zu entwickeln, welche bei Dehnungen von 4 % noch eine deutliche Barrierewirkung aufweisen. In alternierend aufgebauten Mehrschichtsystemen sollen spröde, anorganische SiOx-artige Schichten die Barrierefunktion übernehmen sowie weiche, organische Schichten die Flexibilität des Verbundes sicherstellen. Hierzu werden an den zwei Standorten verschiedene Niederdruck-Anlagen zur plasmagestützten Beschichtung von PET-Folien (Hostaphan RN36, Dicke: 36 μm, OTR: circa 45 cm3/(m2 day bar)) der Firma Mitsubishi Polyester Films GmbH, Wiesbaden, eingesetzt. Die Anlagen erlauben die Beschichtung von Folien im Rollezu-Rolle-Verfahren mit Hilfe von Radiofrequenz(RF, IFAM) beziehungsweise Mikrowellen-Plasmen (MW, IKV). Im Hinblick auf eine mögliche spätere industrielle Umsetzung des Verfahrens soll durch den Vergleich der Ergebnisse der Ein uss der Anregungsart auf die Eigenscha en der Beschichtung untersucht werden.

Expanding the Range of Applications for the Plasma-Enhanced Deposition of Barrier Coatings

Having worked intensely on plasma-assisted barrier coating (plasma enhanced chemical vapor deposition, PECVD) of polymers, a technique already used commercially to enhance shelf-life performance of PET bottles, for the last two decades, it seemed to be obvious to investigate the transfer of this existing technique to novel beverage materials such as PLA and PP. As a result of their high oxygen and carbon dioxide permeability the market potential of PP and PLA is still restricted in the beverage packaging sector. On the one hand, the use of plasma coatings to improve the barrier properties of these materials shows advantages. For example, these coatings are less likely to interfere with PLA-bottle biodegradability. On the other hand, materials such as PP and PLA put higher demands on plasma coating processes due to their different surface properties that make it much more difficult to establish an adequate adhesion between coating and bulk material and their increased sensitivity to high temperature. By using a pre-treatment specific to the materials, we succeeded in applying effective barrier coatings by plasma polymerization using both hydrocarbons as well as silicon organic monomers. For PLA the pre-treatment process can be integrated directly into the plasma polymerization process, whereas for PP it was necessary to use a LF-plasma in an external facility to enable a good coating adhesion. The barrier properties against oxygen could be increased by a factor of about 12 for PP and 4 for PLA. Hydrocarbon-based coatings proved to be more effective compared to HMDSO-based coatings. These first evaluations show that it is possible to raise the barrier performance of PP and PLA bottles to match the levels obtained with uncoated monolayer PET types. Our current research concentrates on improving the processes in order to increase the barrier performance and the efficiency of the process.