Substance class with versatile applications

Surface contamination by polysiloxanes (PDMS, silicone oil) is often the cause of discoloration, wetting and adhesion problems. Even low concentrations (coverage < 1 monolayer) can lead to massive disruptions in systems that are sensitive.  At the same time, however, polysiloxanes are also a desired ingredient in products such as paints, adhesives or cosmetics. For this reason, different questions arise with coating problems:

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Can a polysiloxane be detected on the surface?

ToF-SIMS is particularly suited to detect and analyze polysiloxanes. With a detection limit in the ppm range, the method provides significantly higher sensitivities than other detection techniques (e.g., IR spectroscopy). Thus, even small, yet destructive quantities can be detected.

How much polysiloxane does my process tolerate?

To answer this question, the semi-quantitative information from ToF-SIMS is combined with the results of suitable adhesion tests (e.g., cross-hatchings of paints). As an example, the bar diagram on the left shows the correlation of a semi-quantitative determination of the polysiloxane ­ coverage of steel surfaces and the color-coded results of an adhesion test after coating (good adhesion: green, poor adhesion: red). In the analyses, polysiloxanes were detected on all samples. By comparing the references (R.1 and 2) with the poor ­ patterns A to C, an poly ­ siloxane ­ limit value was determined that was harmless for the subsequent process. Based on this, the polysiloxane ­ coverage of new steel batches has since been determined and evaluated in a standardized procedure with a traffic light code.

PDMS mappingIn addition to the aspects already shown, ToF-SIMS is also able to distinguish between different classes of polysiloxane. This is illustrated by the lateral distribution image (imaging, chemical mapping) of various organic components which were detected in the area of a paint crater shown to the right. The surface of the varnish contains a modified polysiloxane B, which is displaced by a second polysiloxane A in the region of the crater. Apart from that, there is a teflon particle in the crater. Both Teflon and the incompatible polysiloxane A change the wetting properties of the surface and lead, therefore, to the formation of paint craters.

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