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Plasmacoated PLA

In order to pursue plasma coating on PLA films, contacts were established to leading research groups in Europe. Different coating methods were applied:

Method 1:
The trials revealed a problem when using specific siloxanes as precursors for deposition of SiOx. In particular, the requirement for a certain density of activated oxygen in the plasma in order to fully oxidize the siloxanes led to problems with film degradation. Since the plasma is in direct contact with the surface that is being coated, the activated oxygen species in the plasma can damage the film. Past experience using lower plasma input power had shown that films deposited from similar precursors under these conditions had not been effective barriers as they retained significant organic components. Therefore, it was concluded that this option was not likely to provide a solution. Plasma deposition of SiOx on polylactide films might still be possible, either by a series of experiments in which input power is optimised in a trade-off between barrier deposition and film damage, or alternatively through use of remote-source plasma technology in which the sample to be coated is not directly exposed to the plasma source. However, the latter option required equipment not available to the project.

Method 2:
Attempts were made to deposit SiOx coatings using plasma containing a mixture of hexamethyldisiloxane (HMDSO) and oxygen (O2) on flexible PLA films. Surface analysis subsequently showed that polylactide film surface modification had occurred. In particular, X-ray photoelectron spectroscopy (XPS) revealed that the carbon percentage on the film surface could be reduced from 60% in uncoated polylactide to 11% in the case of plasma-treated films. This finding indicates that a pure SiOx is not formed, however, some remaining organic component exists in the surface coating (i.e. chemically the film may be described as RSiOx). This is consistent with the suggestions from other researchers. Atomic force microscopy (AFM) also proved a surface structure similar to that expected on HMDSO plasma-treated films. Despite these promising results, permeability values for both oxygen and water vapor remained well above the target levels required. A possible explanation of this might be the film surface roughness, which could present problems in terms of placing a uniform SiOx coating of approx. 20nm thickness. Thus, it has become clear that a considerable amount of further process optimisation is needed, which should be pursued in a new project.

Danish Polymer Center, Risø hopes to continue the collaboration with Aachen (method 2) under another project with a view to optimising the technology.

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Reported by

Danish Polymer Centre - Risø Research Centre
Frederiksborgvej 399
4000 Roskilde
Denmark
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