PETpla.net Insider 07+08 / 2011

BOTTLE MAKING 20 PET planet insider Vol. 12 No. 07+08/11 www.petpla.net As substrate PET bottles with a volume of 0.5l, uniform neck geom- etry and a standard PCO28mm thread were used. Results and discussion For the measurement of the depo- sition rate, an adhesive tape is applied on the bottle surface before the coating process permitting the determination of the layer thickness by means of atomic force microscopy (AFM). Therewith the deposition rate can be determined by measuring the step height between coated and uncoated regions (Figure 2 and Figure 3). Using this method, coating thickness was determined to 90nm resulting in a deposition rate of 18nm/s. It was found, that the deposi- tion rate is not influenced by a variation of RF bias. The surface morphology was ana- lyzed simultaneously with regard to the roughness of the coatings. Evaluations show, that there is no obvious change in the average surface roughness with R a values between 7.1nm for coated samples without the use of a substrate bias and 3.8nm with an 80V RF bias applied. This confirms the assump- tion, that an applied RF bias does not change the morphology of the coat- ings significantly. Furthermore it could be observed, that the coatings show a good adhesion to the substrate. This can be concluded as no obvious delamination could be observed during the investigations carried out. The barrier properties of the deposited coatings were evaluated by measuring the carbon dioxide permeation rates through both coated and uncoated PET bottles using a gravimetric method. The bottles were filled with a specific amount of dry ice and sealed with an aluminium closure. After evaporation of the dry ice, an internal pressure of 2bar was reached. The prepared bottles were stored at 23°C and a relative humidity of 40% rh. The barrier improvement factor (BIF) against carbon dioxide permeation is then calculated by the weight loss of the uncoated bottles compared to coated ones. Prelimi- nary results, due to limited number of samples, showing the best achieved barrier improvement are depicted in Figure 4. The achieved BIF values for a biased deposition seem to be higher than without. Although the results of the AFM measurements show, that the morphology of the coating surface does not change significantly. This clearly indicates changes in the layer structure itself. The results further- more suggest a linear dependency between the applied RF bias and the achieved BIF value. This has to be investigated more closely with a higher number of samples and a wider range of the RF bias variation, since the experiments carried out so far do not show any sign of a saturation level. Conclusions A new reactor for plasma coating of plastic bottles has been designed allowing a future investigation of the influence of an applied RF substrate bias on the performance of plasma polymer barrier coatings. The pre- sented results show that it is possible to achieve adhering coatings with enhanced CO 2 barrier properties on PET using a substrate bias. Besides the performance also the flexibility of the coating has to be taken into account. The influence on the strain tolerance is crucial for the success of this technology. For applications such as carbonated soft drinks a high strain tolerance is necessary, since the inter- nal bottle pressure leads to a stretch- ing of the bottle. Using coatings with a low strain tolerance, stretching of the bottle leads to a cracking of the bar- rier coating and thereby to a reduction of the barrier performance. In a next step, a transfer of the biased coating process to other materials such as polylactide (PLA) is targeted. This will show influences of different substrate materials on the plasma process and thereby on the structure of the plasma polymer layers and the result of the biased barrier coating. Acknowledgements The research project (No. 16306 N) of the Forschungsvereinigung Kunststoffverarbeitung has been sponsored as part of the “Industrielle Gemeinschaftsforschung” (IGF) by the German Bundesministerium für- Wirtschaft und Technologie (BMWi) due to an enactment of the German Bundestag through the AiF. Fig. 4: Best measured CO 2 -barrier improvement varying substrate bias *contributed by Ch. Hopmann, W. Michaeli, K. Bahroun, H. Behm Insti- tute of Plastics Processing (IKV) at RWTH Aachen University, Germany Fig. 2: AFM-image of a a-C:H coated PET bottle without substrate bias Fig. 3: AFM-image of a a-C:H coated PET bottle with substrate bias applied (80V) 2,5 2,0 1,5 1,0 0,5 0,0 0 40 80 60 RF Bias [V] CO 2 -BIF [-] References [1] N. N.: http://www.kiweb.de/Default. aspx?pageid=199&docid=219491. [2] S.-M. Baek, T. Shirafuji, S.-P. Cho, N. Saito, O. Takai, Japanese Journal of Applied Physics, 49, (2010), 08JF10-08JF10. [3] A. Grüniger, A. Bieder, A. Son- nenfeld, P. R. Von Rohr, U. Müller, R. Hauert, Surface and Coatings Technol- ogy, 200, (2006), 4564-4571. sensitive BEVERAGES