Vacuum Web Coating

Plasma treatment

Could you please explain me about the plasma system in vacuum metallized machine? Now I have problem with the plasma system. The problem is corona level of mcpp film after finished from metallizer with 2.5 KW plasma was drop very quick or 1-2 week it's decrease from 40 dyne to 36 dyne.  I compare between use plasma and don't use plasma it's difference when used plasma corona drop faster than don't use plasma. Please help me and recommend me how to do?.....Thank you very much.

Answer

Polypropylene films often contain slip agents, these are present in the bulk of the polymer film and they migrate to the surface. If these slip agents are removed from the surface by any kind of surface treatment the slip agents will reappear as more migrate from the bulk to the surface.

When film is metallized it is important to remove the slip agent in order to improve the metal adhesion.

Generally an easy measure of the surface energy is to use either 'dyne pens' or to measure the contact angle of a water drop to the surface. This is used to check that the slip agent has been removed, as the surface energy of the polymer will rise as the low molecular weight material disappears.

The speed that the slip agent reappears depends on a number of factors including temperature with the low molecular weight slip agents being more mobile at higher temperatures.

The slip agent is added to the polymer to make it easier to handle. Thus it is common to only treat the surface that is to be coated so that the other surface still has the slip agent present.  This means that as the film is wound up the front and back surfaces come into contact. You then have a clean surface with a high surface energy in contact with one with a low surface energy that contains a mobile low molecular weight material. Nature tries to bring things to equilibrium and so will try to reach the lowest surface energy state. So the high surface energy side will attract the low energy material to it and so bring the high surface energy towards the low surface energy value.  The greater the difference between the high and low surface energy the greater the driving force and so the faster the high surface energy will reduce.

The metallized surface has a very high surface energy and so this will change more dramatically than an unmetallized surface.  As the film is rewound in vacuum these rolls will be hard and so each layer will be in intimate contact.  Both surfaces will tend towards the same value over time. The amount of time will depend on the storage temperature.

I hope this gives you some help in the processes that might be happening.

Choice of gases for vacuum plasma treatment.

**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.

Plasma treatment of PET

The questions

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?

The response.

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 that is 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 is 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 us 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.  If the process can be shown to be particularly sensitive to variations in reactive gas flow then Plasma Emission Monitors (PEM) can be used. Here it is common to use a ratio of one of the reactive gas peaks to one of the inert gas peaks and keep the ratio constant by controlling the reactive gas flow.  This is rarely necessary for plasma treatment but is common for reactive deposition processes.

I hope this answers your questions.

Charles A. Bishop

C.A.Bishop Consulting Ltd                         www.cabuk1.co.uk

More details re. Plasma treatments.

Question

In general, which of the following processes in low pressure plasma treatment are accomplished by inert gases, reactive gases, or electrons.

Ablation (Etching)

Deposition

Cross-linking

Functionalisation (Surface Activation)

Answer

(my apologies to Sam for the slow response to this question)

The specific processes involved with any plasma treatment will depend upon the power, pressure and type of plasma that is being used.  At very low powers there may be very little that happens at all.  There has to be sufficient energy to break bonds, after which chemistry can start to occur. 

            The mechanism for cleaning when using a plasma is that ion and electrons physically bombard the surface.  Under this bombardment the weakly bonded atoms can be sputtered from the surface or they are converted into easily volatilised species that do not polymerise or re-deposit onto the web/foil. In addition to cleaning the plasma will chemically change the surface usually this is done specifically to allow the depositing material to be better bonded to the surface. It can also be done specifically to produce non-wetting surfaces but this is much less common.

            Oxygen, air, water or N₂O can all be used to remove organics by oxidation.  These gases also can leave oxygen bonded into polymer surfaces that can act as a tie layer.    Adding oxygen to the process can increase the effectiveness by increasing the production of ozone and atomic oxygen.  This process can be particularly good at removing hydrocarbons.

            Hydrogen by itself or in mixtures may be used in some cases where the contaminant is sensitive to oxidation.  The contaminants are converted to low molecular weight volatile species that do not polymerise and hence are more easily evaporated or sputtered from the surface into the plasma and thence pumped away.

            Noble gases usually means that argon gas is used to give a more physical etch.  The problem with noble gases is that there is no mechanism to convert the fragments into permanently volatile compounds and hence they tend to redeposit on the surface or be polymerised.  The physical bombardment by heavier noble gases either causes more bond breaking to leave active sites or increases the amount of crosslinking both of which can lead to a more stable and higher adhesion interface.

            Noble gases are also used as carrier or diluent gases.  The noble gases increase the vacuum ultra-violet (VUV) output of the plasma significantly. This can aid the dissociation of the other gas increasing reactivity and thus speeding up the process.

Surface treatment of polymers.

There have been a number of questions asked recently regarding surface treatment, specifically of BOPP, such as these below.

Can you kindly advise the different methods of surface treatment we can use for treating BOPP film (e.g.: flame treatment, plasma treatment in vacuum....)?
What are the advantages and disadvantages of each of the treatments? At the right conditions (can you pls also advise the conditions), what is the rate of improvement for different plasma powers, gas flow rates, and web speeds when we compare it with no
plasma treatment BOPP film

1)What do we try to achieve with plasma treatment (in vacuum) on BOPP film.

2)How is BOPP film affected when treated by plasma treatment in vacuum? In other words, how is the structure of the surface altered?

3)If there are any disadvantages of plasma treatment (in vacuum) what are they?

I will briefly run through the different surface treatments that have been used and comment on any advantages or disadvantages.  What I cannot do is give process details such as power & speed for a particular treatment.  This depends on so many factors that are system and substrate specific.

Firstly I will explain in more detail why I cannot give these details.

Read on for the complete answer.

Question re. plasma treater

I am a student attending the University of Waterloo. I am now on my work term working for a company producing BOPP.

This company has a plasma treatment unit (in a vacuum)(within a metallizer [for 250 cm rolls]) that does not function well. Due to some software problems, we can only restrict the Plasma Power up to around 2kW (maximum when fixed: around 11kW). Plasma Power exceeding around 2.5kW will cause the arc value to increase significantly (which is undesirable).

My job is to optimize the effectiveness of plasma treatment (in a vacuum) process with a constant gas composition being of 80% Argon and 20% oxygen (in one cylinder). Are the only other parameters from which I can alter, record and graph against optical density, OTR, and WVTR values:

gas flow rate
plasma power (within the limit)
vacuum level (If so, how will it effect the process?)

Moreover, to test for the effect of the plasma treatment process, what properties, other than optical density, OTR, and WVTR can we measure to get QUANTATIVE results?

Any other tips?   Please advise.

My response is in the continuation, please feel free to add your comments too.

Request: What is Plasma?

Request: What is Plasma

Why it is important?
How it is effective when done with PET?

Many of us use a plasma to treat our polymer films whether we know it or not. A plasma includes such treatments as flame, corona as well as vacuum plasma treatment.

For a more detailed explanation read on.