Aufskalierung plasmapolymerer Beschichtungsverfahren (English shop)

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Plasma polymerisation is a technically interesting and accepted coating technique. In an environmentally friendly manner, the process produces a wide variety of thin films with the most diverse surface functions. One of the obstacles to its broad and economically successful introduction is the uncertainty regarding the scale-up of such plasma polymerisation processes to large-volume reactors.

Scientific studies have so far dealt mainly with increasing the deposition rate and have thus contributed to the efficient design of plasma processes. Furthermore, characteristic parameters have been developed, in particular the Yasuda parameter, in order to describe and compare a plasma process on different reactors with possibly different types of excitation. In all of these studies, the main focus is on energy-intensive plasma processes and their deposition rate. Low-energy processes, by contrast, are considered only very rarely, since, among other things, they permit only low deposition rates. Nevertheless, they offer technically interesting application possibilities, so that their scale-up in particular is of special importance.

This gave rise to the motivation to investigate the scale-up of low-energy plasma processes. It is precisely such processes that require large-volume reactors, so that they can compensate for their disadvantage of a low deposition rate by large numbers of parts per batch.

The starting point for this work was a special plasma-polymer coating process that had already been developed on a small-volume laboratory reactor. This is a special embodiment of the so-called plasma-polymer release layer (DE 10034737). The selected coating process belongs to the category of structure-retaining plasma polymerisation processes; it operates with an excess of precursor.

During the development of this coating, it had become apparent how sensitively the properties of the layer system react to changes in the gas composition and in the coupled-in electrical power during the coating process. Thus, with the selection of this process, a coating system was available that has a narrow process window and whose properties can be clearly described by application tests as well as by physico-chemical analysis. The layer properties are independent of the selected substrate.

With the aid of this coating process, it was investigated whether and in what way such a plasma polymerisation process can be scaled up to large-volume reactors and whether general rules or procedures can be derived from this.

For this purpose, the selected process and the coating produced with it were first characterised by means of FTIR and XPS spectroscopy as well as mass spectroscopy. From this description, a layer model was developed.

Subsequently, the plasma process was scaled up by applying a procedure which is essentially based on a mass-spectrometric comparison and which thereby differs distinctly from the previously known volume-proportional scale-up strategy according to Yasuda.

After the scale-up, known reaction parameters were applied. It became apparent that they cannot describe the scale-up situation found and do not respond to all external process parameters.

For this reason, a new characteristic parameter V was developed on the basis of theoretical considerations. The newly developed parameter V was able to describe the process scaling for the selected structure-retaining plasma polymerisation process.