Optimising plasma treatment

Hi, how can we optimise the effectiveness of plasma treatment in a vacuum process with a constant gas composition being of 80% Argon and 20% oxygen in one cylinder?

Answer

You have three process controls that can be varied, gas flow which gives the plasma pressure, power and web winding speed.  Assuming that your web winding speed is defined by the required coating thickness and aluminium deposition rate this only leaves the gas flow and power to control.

The then is to fix the gas flow and then to progressively increase the plasma power and evaluate the coating adhesion that is produced. This can then be repeated for different gas flows.  Probably the biggest problem is the evaluation of the results. Measuring adhesion cannot be done using the simple tape test because the tape test is so variable and does not give any real measurement of adhesion.  Using something like the tensile test where the amount of cracking is monitored which can be converted into a measure of interfacial adhesion.

A possible alternative is to do the plasma treatment and not deposit the aluminium onto the web. In this way the surface energy can be measured. The surface energy can be expected to increase with increasing power up to a point where the surface energy reaches a plateau.  It is important that the web is not over treated and the plasma treatment is done only to the point where the surface energy just reaches the maximum value.  Treating beyond this point only introduces additional surface damage that will then begin to make the adhesion fall from the maximum.

Note that the plasma treatment that you optimise will be for that particular substrate material and may no longer be optimised for other grades or the same nominal material but from other suppliers.  This is because other grades may not have the same surface as the one you are using for the optimisation trials.  This is just a warning to be careful about assuming that optimisation on one substrate will be good for every grade of material.  It may be, but if you find later that the adhesion is not as good for a different grade of material it may not be a problem of the plasma other than it is not optimised for that particular grade.

However I recognise that it is impractical to have to go through an optimisation test for every type of substrate and grade and so a reasonable action is to optimise the process for one polymer type and grade and then use the same process for all other films. It would be worth optimising the process again if the polymer type is changed as different polymers contain different additives.  If the optimisation ends up with the same plasma gas flow and power then it is more likely that the plasma treatment will be useful for most grades even if it is not optimised.

I hope this helps.