From Science & Research
From Science & Research Plasma-Assisted Barrier Coating The continuing replacement of metal and glass packaging has lead to a constant increase in the use of plastics like polyethylene terephthalate (PET) for packaging. In 2009, 885 billion liters of beverages were expected to be bottled in PET, which account for 33.6 % of the overall bottled volume [1]. The protection of sensitive goods like soft drinks (CSD), teas or mineral water poses high demands on the packaging regarding CO 2 loss, flavor loss and O 2 ingress. The permeation of these gases limits the shelf life of the beverages. Besides the use of PET as source material for the production of beverage bottles, a trend towards alternative materials as polypropylene (PP) [2] and polylactide (PLA) can be observed. When speech comes to the sustainability of packaging materials, bio-based and biodegradable materials such as the biopolymer polylactide (PLA) represent an attractive alternative to PET. Disadvantages are its softening point of around 60ºC and its relatively poor barrier properties to water vapour, oxygen and CO 2 permeation. Table 1 shows typical permeation coefficients for PET, PP and PLA. oxygen (DIN 53380) (cm³∙mm / m²∙d∙bar) carbon dioxide (DIN 53380) (cm³∙mm / m²∙d∙bar) water vapor (DIN 53122) (g∙mm / m²∙d) PET 1.000 - 2.500 5.000 - 10.000 100 - 200 PP 40.000 - 60.000 132.000 - 212.000 30 - 50 PLA 6.250 - 15.000 25.000 - 80.000 6.000 - 45.000 Table 1: Typical permeation coefficients of oriented PET-, PP- and PLA-films at 23 °C A well known and industrially used technology for barrier improvement of PET-bottles is the plasma enhanced chemical vapor deposition (PECVD) [3]. Therefore, the Institute of Plastics Processing at RWTH Aachen University (IKV) has investigated the transfer of existing plasma barrier coating technologies used for PET to novel materials, such as PLA and PP. Experimental For the experiments, a microwave-induced plasma reactor for inner coatings of hollow bodies was used (principle shown in Fig. 1). In the apparatus, the inner volume of the bottle is evacuated to 20 Pa while the outer volume of the bottle is evacuated to a much lower pressure. Process gases such as hexamethyldisiloxane (HMDSO) or acetylene (C 2 H 2 ) in combination with argon or oxygen are being induced for the deposition of plasma-polymerized permeation barriers. The pulsed microwave radiation is guided from four surrounding slot antennas into the bottle inside igniting the plasma as a confined thin skin on the inner bottle wall. In order to reduce the influence of different bottle geometries on the deposition process, similar bottle shapes with a volume of 0.5 liter, uniform neck geometry and a standard PCO 28 mm thread were used. Results and Discussion First studies are done with HMDSO/oxygen (prefix H) mixtures and acetylene with the addition of argon (prefix E) varying microwave power, gas flow and gas composition. The studies are carried out with a total coating time of 2.5 sec. The first qualitative tests to investigate the layer adhesion on the different substrates, using a tape test, show that good adhesion to the substrate can only be achieved on PET. Further tests were made to increase the layer adhesion by means of a pre-treatment in a plasma using non-layer forming gases. The tests show that for PLA a pre-treatment in an O 2 plasma leads to good results and can be incorporated in the deposition process. For a further investigation of the layer adhesion on the different substrates, scanning electron microscopy technique is being used on bent sample pieces of coated bottles. Fig. 2 shows the surface of acetylene and HMDSO/O 2 -coated PET and PLA samples after bending. It was found that under stress straight parallel cracks appear on the coating surfaces. Both coatings are deposited smoothly with no obvious delamination of the coatings thus indicating a high adhesion strength between the substrate and the deposited layer. Unlike the observed behaviour on PET, it was found that the surface of the coatings seems to be much rougher on PLA with a completely different crack propagation. Therefore, the results lead to the conclusion that the use of different substrate materials has an influence on the plasma process itself and especially on the structure of the plasmapolymer layers and their behavior to an exposition to stress. In order to evaluate the barrier performance of deposited barrier coatings oxygen permeance measurements were carried out at constant temperature of 23°C and constant relative humidity. The achieved barrier improvement (“BIF” = barrier improvement factor, which can be seen in Fig. 3) related 24 bioplastics MAGAZINE [04/10] Vol. 5
From Science & Research Article contributed by Slot Antenna Bottle Microwave Shield Grid Quartz Glass Walter Michaeli, Karim Bahroun, Friederike v. Fragstein, Henrik Behm Institute of Plastics Processing (IKV) RWTH Aachen, Germany to oxygen permeability compared to uncoated bottles is higher in tendency for acetylene based coatings with a high gas flow and microwave radiation. The results show that BIF values can be reached that reduce oxygen permeability levels of PP and PLA bottles, approaching those of an uncoated PET bottle. Conclusions The presented results show that it is possible to achieve adhering coatings with good O 2 barrier properties on PET, PP and PLA. In order to deposit effectively firm-bonded plasma polymer layers on polymer substrates a good adhesion to the substrate is important. For PP and PLA the adhesion can be improved by means of a plasma-assisted pretreatment using non-layer forming gases like O 2 or N 2 . For PLA, the surface treatment can be integrated into the coating process. The results clearly indicate an influence of different substrate materials on the plasma process and thereby on the structure of the plasmapolymer layers and their behavior under strain. In a next step, possibilities to improve the barrier performance of plasma-polymerized layers on polymers are under investigation. One possible technique is the combined interior and consecutive exterior coating of bottles. Acknowledgements The research project (No. 16306 N) of the Forschungsvereinigung Kunststoffverarbeitung was sponsored as part of the „Industrielle Gemeinschaftsforschung“ (IGF) by the German Bundesministerium für Wirtschaft und Technologie (BMWi) through the AiF, to whom we extend our thanks. References [1] N.N., ‘Glas, PET oder Karton?“ Verpackungs-Rundschau, 9, 14-26 (2009) [2] Gahleitner, M., Wachholder, M., ‘Auf der Überholspur“, Kunststoffe, 12, 47-49 (2005) [3] Grill, A.: Cold Plasmas in Materials Technology, Institute of Electrical and Electronics Engineers, Inc., New York (1994) www.ikv-aachen.de Fig. 1: Interior coating of bottles by microwave plasma polymerization Fig. 2: SEM images of bent plasmapolymer coatings on PET and PLA O 2 permeance [cm³ / (pkg·d·bar)] 3,0 2,5 2,0 1,5 1,0 0,5 0,0 Microwave Generators BIF 5,0 BIF 11,7 0,10 PET uncoated coated 0,02 2,68 PP Gas Feed Throttle Valve 0,23 1,28 BIF 3,8 PLA Ethyne based coating Temperature: 23°C BIF = Barrier Improvement Factor 0,34 Fig. 3: Oxygen permeance of coated bottles (acetylene based coatings) bioplastics MAGAZINE [04/10] Vol. 5 25
