**Question**
How to determine the best plasma combination for metallised PET. Normally which combination of gas is best suited for metallised PET films and in what percentage. What is generally the best colour for plasma when seen during vacuum metallisation.
**Answer**
There is no set proportion of gases for plasma treatment. Ideally there will be both argon and oxygen present. The oxygen can be derived from pure oxygen or from water vapour and so some systems locate the plasma treatment in the winding zone to get both a higher pressure and to make use of the water vapour extracted from the polymer as it unwinds. The water is broken down in the plasma to provide the plasma with the oxygen. Systems that have the plasma treatment zone in the deposition zone area usually rely more on the oxygen being provided from bottled gas.
Another consideration in using oxygen is that the excess oxygen that does not react with any surface contamination to form carbon monoxide or carbon dioxide has to be pumped away through pumps that often contain thin films of hot oil. This can be a potential explosion risk. This is more of a risk as the gas is compressed at the rotary pumps. Hence it is important to have something like a nitrogen purge, added to the pumping line before the backing pumps, to protect the pumps from too much oxygen. The aim is to never have an oxygen concentration greater than ambient air (20%).
So most typically there will be a gas feed into the system of argon and oxygen with the oxygen being of the order 10% - 20%. The heavier argon provides the physical bombardment of the surface and the oxygen provides the chemical recombination that converts the hydrocarbons into volatile species that can be pumped away. Without the oxygen these would be sputtered from the surface but most likely would fall back onto the surface and recontaminate the surface.
There is a big problem in talking about the colour of plasmas. Around 8% of the population has some problem or other with colour vision. On top of this if you have ever looked at a plasma for some time your colour perception will be changed as you eye sensors become saturated with some wavelengths and de-tune the sensitivity and so when you look away all the colours in the room will be different to when you viewed them before looking at the plasma. Another problem is people’s different description of the same colour. What is pink to one person may be lilac to another or red to someone else. Thus the only true method of describing colour is using a scanning spectrometer.
I would normally use a combination of mass flow controllers and/or pressure monitoring along with the voltage and current information from the power supply to control the plasma and trust the instruments rather than my perception of the colour.
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