Vacuum Web Coating

Question about retrofitting a glow discharge

Hi, We have a very old Dynavac Evaporative Metalliser and would like to obtain better adhesion in our substrates by building a Glow discharge. While the principle is simple (DC transformer connected to an Aluminum rod and the chamber, then keep the vacuum level low while energizing). Looking at your diagram on page 229 of your book the ideal setup would be to have Vacuum at 10 -1 with voltage at 900 DC, to achieve this vacuum we would install a bleed value at the opposite end of the chamber to the pumps. My questions are does this setup sound ok and can we use filtered air instead of an inert gas, or do you suggest some further reading before we jump into this?

Answer

The glow discharge sounds as if it would work however if I were to be modifying a system I would aim for a magnetically enhanced plasma to treat the surface. The power requirement can be the same but the plasma density with the magnetic enhancement will be greater as it can run at the lower voltage but will carry a greater current.  The design is very similar to a magnetron source with the magnets making a magnetic circuit for the electrons to race around.  The higher density plasma will give a faster treatment time than the glow discharge and so is more useful at the faster winding speeds.  It will also operate down to lower pressures and so can be more flexible in where it is sited in the system.

As to gases, any gas will allow a plasma to be struck but the surface treatment will be dependent upon the gases present.  Argon is a heavy atom and when it strikes a polymer surface it can break bonds but argon is inert and so cannot react with anything on the surface. Thus it can hit contaminants on the surface and may crosslink them to the substrate or may further fragment any short chain molecules or oligomers. To remove any hydrocarbons the plasma needs to contain a reactive gas, oxygen for preference which can react with the hydrocarbons to form volatile species that will desorb from the surface and can be pumped away.  If you have a roll of polymer in the system there will always be some oxygen around from the air trapped in the roll and also from any moisture in the air and absorbed into the polymer. The water will be cracked in the plasma and more oxygen released.  Thus you introducing dry, filtered air will provide the surface with two gases both of which are oxidising gases with oxygen being more active than the nitrogen.

Practically if the pumps are sited to the side of the winding system there will be a small pressure gradient from one side of the chamber to the other. When you introduce the gas there is a danger that you will exaggerate the pressure gradient. Sometimes I have seen a tube used with holes drilled in the tube to allow the gas out. By suitable positioning of the tube and varying the size of the holes the pressure can be balanced. There may still be some variation across the web from plasma generated by-products which will be present at a higher proportion towards the pump. 

If you use a magnetically enhanced plasma the system can be run at a lower pressure and this will reduce this effect somewhat. 

A question relating to plasma / sputter cleaning

Please go through the following abstract of a sputter engineer:

It is possible to get a plasma in your cathode with gas flow of 80-100 sccm of Argon and Oxygen. But if you use so little quantity of Oxygen gas, the target will go up with the sputter etch. That means you start with a sputter process. But this is not the goal of the pre-treatment system!!!! You will have a plasma for cleaning of the film and to get a better adhesion for the layer That means with minimum gas flow 1200 sccm Oxygen, the target do not go up with the sputter etch.The life time of the target will be longer than one or two years, depend of your production. I don’t now why you use this small quantity of oxygen gas but I know this quantity is too small, gas flow set points: Argon 400 - 500  sccm Oxygen 1200 - 1500 sccm.

Sir, based on above comments I would like to know:

How low gas flow of oxygen can increase sputtering process because I had read the sputtering will take place because of bombardment of heavier ions of argon gas to target surface

The goal of Plasma pre-treatment in metallizer is only the cleaning? I think we need both cleaning as well as sputtering

Answer

What you have is a magnetically enhanced sputtering source.  The way you use it will determine quite what happens to your substrate.  If the substrate passes through the plasma some distance away from the source it will be immersed in the plasma and bombarded by it. If the substrate is passed very closely over the surface of the source the polymer itself becomes the target material and the polymer surface is sputtered off.  If the polymer is passed through the plasma it may not only be bombarded by the plasma but also the plasma will be sputtering whatever target is used on the source.  If the pressure is low the sputtered material will undergo few collisions and will arrive at the substrate and may even deposit on the surface. Thus the polymer surface may contain a very low percentage of the sputter cleaning target material.  Often this target material is carbon and so if a chemical analysis is done of the surface it may well be that no contaminant is found because of the carbon present in the polymer masks the carbon deposited on the surface. To limit this deposition, the pressure of the plasma treatment zone is raised to a high pressure which causes the sputtered material to undergo many collisions causing the sputtered material to be scattered, some back to the cathode and the rest to all surfaces rather than just onto the web substrate.

Thus your engineer is suggesting that increasing the plasma cleaning zone pressure will limit the deposition of any target material.

Argon is a heavier atom and so will have more impact energy than the oxygen and this aids chain scission and crosslinking. The argon will not convert hydrocarbons into any volatile species that can then be pumped away and so it is usual to have a combination of argon and oxygen to be able to do this process.  The ratio of argon to oxygen is usually in the Argon 80% to 90% range and the oxygen 20% to 10% range.  This is safer than having very high levels of oxygen passing through the diffusion and rotary pumps. The rotary pumps have a thin film of hot oil over the moving metal surfaces and excessively high levels of oxygen can result in explosions. Thus if high levels of oxygen are to be used the roughing line needs to have nitrogen injected into the line to dilute the oxygen level back to atmospheric levels i.e. approximately 20%.

Cleaning of the surface includes all parts of the plasma process, sputtering, chain scission, crosslinking as well as chemical reactions.  The total pressure, as well as power, can affect the balance and speed of surface treatment.  This is part of the reason why, for any given substrate, the process needs to be optimised to provide the highest adhesion.  The variables you have available to change are the argon : oxygen ratio, the total flow and hence process pressure and the power to the magnetron cathode.  Most of then the gas ratio is fixed and the total flow is fixed and only the power is varied to optimise the adhesion. 

Lifetime of surface treatment

I don’t know if this is right, but as film price roar up to a level that converters will have to be in a dilemma in coming up with a decision. We have metallized cpp with dynes level of 32- 34. As per industry standards this level are considered in the lowest threshold in terms of good wettability during adhesive application. we get an acceptable adhesion with this dynes level initially but will this be a too risky move on our part especially with products that are not considered a commodity or fast moving(consumption).As you have mentioned the bond will definitely deteriorate as time past due to continuous contamination of the surface resulting to weak boundary layer. Temperature, time, pressure dependent as you said. Does this means there’s no guarantee that the initial bond reading will hold through before the shelf life of the package (6 months).Am I putting myself to a losing end? Thanks

Answer

The whole problem of film surface treatment lifetime is a difficult one.  The treatment can be dependent on the film type, quality and what additives have been included and so will vary from supplier to supplier. The lifetime also can depend on the storage time and conditions, particularly temperature with storage at a higher temperature reducing the lifetime of the treatment.

Personally I would be tempted to buy untreated film and to treat the surface immediately before processing. In this way I would hope to be sure of the level of surface treatment and as it was done at the time of further processing there would be minimal time for any degradation. I would expect this to give me more reproducibility than buying treated film and having a variable storage time before converting. 

Once you have converted the film either by coating or laminating the film the performance will be dependent on the quality of the adhesion you have achieved.  If you have the best possible adhesion the lifetime of the product will be longer than if the adhesion is poor.  The other factor that affects the lifetime would be the low molecular weight material that is contained within the bulk polymer. This can include slip agents. If the adhesion is poor these can migrate into the gaps between surfaces at the interface. If the adhesion is good there is little or no spaces for these slip agents to migrate and hence the adhesion remains higher for longer.

So you will know best what the quality of film you have and if it contains any slip agents, etc and if you have surface treated the film to get the best possible adhesion.  If you know these things you then need to assess if the film will be stored at a low temperature or will see a higher temperature.  This will not tell you how well the film will survive but it will allow you to estimate if there is a risk of the film degrading in performance.

This type of risk analysis may be helpful.  So if you have a film with a slip agent included the risk will be higher than without a slip agent included. If the film was surface treated to increase adhesion the risk would be lower than if the film was not treated and if the film was stored in cooled environment the risk would be lower than if it were stored hot.  You can then multiply these risks together to see the total risk.  Thus if you have a film with a slip agent included, no surface treatment and a hot storage environment you have all three as a higher risk and so the overall risk of degradation will be much greater than if the film was additive free, had been surface treated and stored cool.

I hope this helps.

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.