Vacuum Web Coating

Question re. retrofitting a ZnS source High Index coatings.

**** Q ****

We currently metallise PET with aluminium. We are considering retro-fitting our coater so we can manufacture HRI products.

Firstly, what are the pros and cons of zinc suplide compared to titanium dioxide (or any other alternatives you suggest we consider).

Second, do we need a plasma treater ? We have heard conflicting information - some stating a plasma treater is essential and some stating it isn't. Please help.

**** Answers *****

Zinc Sulphide tends to be a soft porous coating but can be deposited at metallizing speeds (100's m/min).  The ZnS can be thermally evaporated from a slot source that spans the whole web width.

The ZnS can dissociate and recombine easily and so stoichiometry is generally not an issue.  Operators may not be so happy with the material as it can smell when the system is vented because of the residual Sulphur.

Titania tends to be denser, harder and is usually deposited by slower techniques ( 1 - 10's m/min).

If you are after a hard wearing, highest quality hologram then titania will probably be the preferred material.

Many companies using the coating for holograms want to deposit the coating and emboss later.

It is possible to emboss into the polymer through the ZnS and still get a good hologram. ZnS has the advantage that it does less wear on the holographic shims than the harder titania and so production costs can be further reduced.

Part of the problem of depositing titania is getting the stoichiometry right. Sputtering from a titanium target and reacting the coating with oxygen to produce titania is not neceaasrily easy.  The oxygen cannot be controlled selectively to reach the growing coating and not the sputtering cathode & so the cathode gets poisoned. The sputtering rate of titania is 20x slower than for the metal and so the process tends to avalanche to sputtering slowly from an oxide target.  There are methods of correcting for this runaway process but they all push up the deposition costs. Ideally if the coating is to be deposited by sputtering a dual cathode with an AC power supply including  arc control would be preferred.

It is possible to deposit titania from electron beam deposition sources but getting a consistent stoichiometry is not trivial.  Often an additional oxygen plasma is used both to densify the coating to make it more like a sputtered coating but also as a method of reducing the excess oxygen that is required to convert the metal to titania.

Uniformity can also be an issue.  Sputtering has a deposition rate fall off towards the ends of the cathode so that to get high uniformity across the whole web width requires a cathode length wider than the web width.   For electron beam sources it depends if you have a series of individual sources that have their deposition flux integrated across the width or if there is a single sweeping electron beam with a single crucible that spans the web width.

Critical to both will be the position of the pumping system in the system you are planning to convert and the method of feeding in the oxygen gas.  Ideally the pumping will be symmetric about the web centreline (including any cryopumps) if this is not the case there are additional problems if achieving uniformity.  Moisture from the webs can be a source of oxygen to the coating as well as the oxygen to controllable add to the process.  If there is asymmetric pumping then there will be a pressure gradient across the web and it then becomes difficult to deliver sufficient oxygen, an no more, to all parts of the sputtering cathode or vapour flux from the e-beam source.  Any imbalance will lead to non-stoichiometric coatings.

Plasma cleaning.  There are many different plasma cleaning sources available. Some work better than others.  The choice of power, time and gas composition can all affect the effectiveness of any plasma treatment.  In general it is better to plasma clean than not.  Most polymer film has contamination on the surface. This contamination is a source of poor adhesion. It either needs to be better bound into the polymer web or volatilised and pumped away leaving behind the polymer web surface.  If the surface is under treated there the adhesion will not be optimised. If the surface is over treated it may still have a high surface energy but it will also have a weak boundary layer on the surface caused by too much polymer chain scission by the over treatment.  Thus plasma treatment is a balance that has to be optimised for each polymer film. Changing supplier can be it is best to re-optimise as different film suppliers will use different proprietary formulations and so the surfaces may well be different.

Argon plasma can roughen the surface and sputter efficiently but there is no mechanism for converting any hydrocarbons into gaseous species that can be pumped away and so what ever is sputtered from the surface may well fall back and still be a contaminant to the surface even though it has been plasma treated.  Thus using an oxygen/argon plasma is probably the most favoured  plasma treatment gas mixture for the widest range of polymer webs.  There are exceptions and sometimes a different bonding site is required that makes oxygen less favoured.

My advice would always to be to keep the process as simple as possible.  Hence do not plasma treat if you do not have to.  Thus I would do some trials first to check if the adhesion of the coating is good enough i.e. fit-for-purpose. 

For ZnS my expectation is that the adhesion will be a problem if you do not use any plasma treatment. I would expect a plasma treatment to be a critical part of the process. 

If you elect to go with titania then it may be much less critical, depending on the process you adopt.  If you are using an oxygen additional plasma this may aid your adhesion, if you are using a sputtering source this will already be bombarding your substrate to some extent and will have a better adhesion than from an evaporation source.

I think this just about answers your question as far as I can.  The choices will depend on the substrate material, the system you are wanting to convert, the cost you have to produce the coating coupled to the performance of the coating.

Question – correlation thickness to resistivity

**** Question ****

Is there a way to equate ångströms to a surface ohms value for deposited aluminium on Polyester?   Currently we have been trying to determine the thickness of the deposited aluminium on the polyester for my company. We have an old correlation but no one know where it came from of Å to ohms (surface measurement only).

***** Answer *****

The conversion can be made so long as there is a calibration for your machine at similar conditions.  The structure of the deposited coating is dependant upon the surface energy of the substrate, the surface roughness of the substrate, the temperature of the substrate and rate of cooling available as well as the rate of deposition, the energy of the depositing material, the system pressure and other factors such as system and substrate outgassing.

Thus any calibration ought to be for a particular system derived from typical deposition conditions.

If you are unable to generate a calibration curve then any conversion from some other calibration curve will contain a potential error. So long as the same calibration is used every time then the error will be consistent and reproducible.

A common source of a calibration curve is from the 'Metallizing Technical Reference' 3rd Edn.  This is published by AIMCAL & costs $25 + shipping (to non-members $15 + shipping to members).

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.

Tolerance of OD variation in metallized film.

**** Question  ****

What is the tolerance limit for Optical density variation in case of metallized BOPET film? How can this variation be reduced?

**** Answer  ****

Any tolerance is whatever you chose to put on the optical density measured values. A tolerance is what limits you put on what is tolerable as a product.  In general the reasons for quality or uniformity variations are independent of the substrate material.

Metallizers have a series of evaporation boats and each boat is powered separately and also has a separate wire feed and so each one can have a different output.  The uniformity is thus dependant upon the quality of the controls to each boat and the quality of the measurment of the thickness of the aluminium coated.

The coating uniformity across the web will depend on the number of boats, the boat spacing, the boat arrangement (staggered or not), the operating pressure, how hard the boats are being driven as well as if any corrective profile shields are used.

Modern metallizers often use the electrical conductivity as a measure of the coating thickness. After calibration this can also be used to produce coatings of specific Optical Density.

The measurement is taken using the Eddy Current method.  This system needs to be designed with the expected thickness range in mind as working outside the designed range can lead to reduced sensitivity and larger errors.

With all of these above factors in mind then 20 years ago it was common to find coatings that were produced to a tolerence of +/- 10%.  Over the years this has improved and using the best practice on a modern machine it is now expected that +/- 2% can be achieved.

This requires an eddy current monitor measuring the conductivity of the coating in-line with each evaporation boat. Ideally with the conductivity close to the middle of the monitors conductivity range.

Boats to be in a staggered pattern with wide boats at minimum boat spacing with the boats being driven moderately ( not at maximum power ) and with a profile shield that has been designed for the operational speed & deposition rate bing used.  This will give the best coating uniformity and coating reproducibility.  Variations from this can make the coating variations greater and hence either the coating thickness tolerence has to be greater or the amount of material produced that fails to meet specification will increase.

Hence the choice of tolerence is yours but it needs to be set to match the age and systems available on your machines.  Tightening the tolerence without machine improvements is likely to increase the amount of out-of-tolerence material the machine produces.

Questions relating to quality of metallized PET films

Request:

1) Which side of metallised pet film is best suited for Lamination and reverse printing.

2) How the WVTR & OTR of metallised pet can be improved.

3) How the oligomers affect the barrier properties of metallised PET.

Answering your questions in order.

1.    Films can be printed and/or laminated in a variety of different
constructions.  There are different grades of polymers some of which have print receptive surfaces; others have surfaces prepared for lamination or
metallisation.  The preparations are usually aimed at improving adhesion and
are chemically modified to suit the particular materials to be used.  Thus
the preparation for aqueous ink is different to that for solvent based ink
and both may well be different to the preparation for lamination.  In
addition to this some films are co-extruded with the reverse side being
filled for improved film handling and the front surface is without filler to
have a better surface smoothness. In this case the smooth side is usually
used for metallizing to get the best specular reflectivity.

Thus without the full specification for the material it is impossible to
state categorically which side of a polymer film is best for which purpose.
The best course of action is to talk to your film supplier to confirm what
the two surfaces are designed for.

2.    The barrier performance of the metallised film is primarily governed
by the number of pinholes and coating thickness. If there were no pinholes
the barrier performance would be governed by the coating thickness and the
microstructure of the metal thin film. The surface energy of the polymer can
affect the barrier performance as the higher the surface energy the better
the metal will wet the polymer surface. This in turn will produce a
continuous coating at a thinner thickness than for a polymer surface that
has a lower surface energy and where the metal does not wet the surface as
well. The more continuous the coating the fewer the gaps between the metal
crystals which are a further source of oxygen and moisture migration.

Thus the best things that can be done to improve the barrier performance
would be to improve the surface cleanliness of the polymer web to reduce the
number of pinholes. An extreme version of this has been demonstrated by the
process of pre-cleaning the polymer web but then depositing a polymer layer
in the vacuum system immediately prior to the metal deposition to produce
the cleanest, flattest surface possible. This polymer surface needed to be plasma treated to then improve the wetting of the metal and it was also overcoated with another thin polymer layer to protect the deposited metal to
prevent post metallization damage.

Another method of improving the barrier performance that has been used was
to metallise both sides of the polymer web. This works because the pinholes
are unlikely to be directly opposite to each other and so the diffusion path
is so long that there is significant reduction in the oxygen and moisture
transmission.

3.    Oligomers can affect the barrier performance because they can be a
cause of poor adhesion allowing for metal pickoff or metal delamination.
They may also be of low melting point and may be vaporised by the depositing
metal causing thin metal deposition where the oligomer is vaporised. These
have the appearance similar to pinholes except that instead of the holes
being completely clean of metal they have a much thinner layer of metal that
the surrounding metal thus when backlit they appear almost the same as
pinholes.