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A method to remove dedicated coatings using atmospheric plasma
 

Atmospheric plasma can be used to remove dedicated coatings as a contactless method. By regulating the power of the torch and the distance between the coating surface and the torch, the rate of removal can be controlled. These parameters thus influence the required treatment time for obtaining the complete removal of a coating. In this case study a HTS (high throughput screening) methodology is described for performing plasma removal of organic coatings on glass slides using a Tigres Plasma Blaster.

The Tigres Plasma Blaster torch is known to generate a lot of heat during a fixed torch one-spot prolonged treatment and coatings may suffer from burns rather than from chemical plasma removal. In order to exclude the effect of the excess of heat, the samples were positioned onto a rotating disc with a controlled speed of 45 rpm under the torch . For 1 min of plasma treatment the samples undergo an actual dwell time of about 1.1 sec under the plasma. By using spacers, the working distance – the distance between the plasma plume and the sample – can be adjusted. In total 11 sample spaces were created as can be seen in the following picture:

fig 1

Figure 1:Plasma treatment of rotating samples

The plasma torch is best used at full power and the 4 plumes are positioned in a way that we have 1 single plume, next to 2 plumes in-line, and again 1 single plume to treat the coatings. In this way the middle track of the treated surface gets a double dose of plasma treatment (2 x 200W/nozzle).

As an example a 4 um Paraloid B67 coating was applied on glass slides that were rotated under the plasma torch. The working distance was varied from 10 to 4 mm and the plasma treatment times varied from 1 to 10 min or 1.1 to 10.6 sec of actual dwell time. Each of the plasma treated samples was firstly visually evaluated by measuring the width of the middle removal track, and secondly by FTIR-analysis that allowed us to calculate the percentage of remaining Paraloid B67.

In Figure 1 the percentage of remaining coating is plotted as a function of treatment time for the varying working distances. Logically, as the treatment time increases more of the coating is removed.

fig 2

Figure 2: The % of remaining Paraloid B67 measured by FTIR analysis as a function of treatment time


In Figure 5 the width of the removed track was measured and plotted as a function of the plasma treatment time for the different working distances. The width of the removed area increases as the working distances decrease and the treatment time increases. The optimal combination of working distance and treatment time for this Paraloid B67 coating was found to be 4 mm of working distance and 6 min of treatment time (= 6.4 sec dwell time). Further increase of the treatment time did not further increase the width of the removed area. At a larger working distance the treatment time to obtain the same area of coating removed increased significantly.

fig 3

Figure 3: Width of the removed area as a function of treatment time