Vacuum Web Coating

Adhesion testing.

Is there any specified testing method for adhesion test on printed foil/film board so that everybody having the same method?

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Adhesion tests are notoriously difficult. I think the 'tape test' is not a good test because there are so many variables. However just about all of the other tests are much more time consuming and need multiple tests to become statistically significant. They also need a greater degree of operator training and usually cannot be performed next to the roll, as they require some laboratory space. Hence the 'tape test' is a cheap and cheerful
one that may be carried out by anybody.

The 'tape test' is not a measure of how good the adhesion is but is only a measure of how bad the adhesion is if shows any failure at all.

The other approach to adopt is to talk to the end user of the metallized
product and use a test that is appropriate to the end use of the metallized
film.  Thus if it is to be used in a twist wrap application a twisting test
would be most appropriate, if the film is to be used with a 'dead-fold' then
a folding test should be used.  In this way an exaggerated process can be
used as the test to prove the film is 'fit for purpose' rather than simply
has good adhesion.

Thus it is common that there is not a standard test.  It is also common that
there are disputes because what may be measured by the metallizer may be
different to what is measured by the customer.

It would be nice if someone could develop a standardised, universal,
quantitative adhesion test.

Metallized film storage

Request.

We know that metallised film gets affected by moisture.

Some customers want us to specify the life or safe period for

usage of uncoated metallized polyester film which tends to

get oxidized.

We know that it depends on storage conditions etc. If you

have any data/guidelines, can you help us, since the

customer is wanting this input from a recognized body and

I am sure they will accept AIMCAL's views.

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This is a very difficult subject as there is no one answer and any

answer depends so much on the region of the world and the season

where the rolls are stored in ambient conditions. 

Thus I personally would not be happy to post and put my name

to a set of storage conditions.

The rate of degradation of aluminium coatings not only depends

upon the temperature and humidity and rate of variation but it

also will depend upon the winding tension, the quality and type

of substrate as well as the type and quality of any pretreatments

as well as the aluminium deposition conditions.

For example, the pre-treatment will change the wetting

characteristics of the aluminium deposition and the number

of nucleation sites which in turn will affect the crystal structure

of the aluminium.  This in turn will affect the surface quality of

the aluminium and the number of grain boundaries and the

diffusion rate for oxygen and water vapour.

Thus for the same storage conditions the rate of oxidation and

corrosion of the aluminium could easily be different for two

rolls metallized by different companies but stored under

identical conditions.

What can be stated is that high temperature and high humidity

can speed up the process of oxidation. If the high humidity is

in an area of the World where there is a lot of atmospheric

pollution the moisture can be contaminated that can further
accelerate corrosion.

Thus the general aim in storage is to keep the temperature and

humidity low.

There are references to the expected rate of oxidation for bulk

aluminium but freshly deposited thin layers are often at the fast

end of the range of oxidation rates. So even with these published

figures there has to be some care when quoting the values.

Thus my recommendation would be to reach an agreement with

your customer as to what you both can accept as appropriate

storage contitions in terms of time, temperature and humidity.

Question about problems in induction sealing.

We are having some problem with the use of metallized polyester in induction sealing.   I am trying to reach you with the hope that you would help us in combating this problem.

                        

We are introducing ourselves as a packaging goods manufacturer having expertise in induction sealing.  We are doing lot of experimentation in combination with various substrates, considering the barrier properties of the product.

               

As required by the customer, we are going to use metallized holographic film [24 micron] as a tamper proof seal.  In this can you guide us to have right metallized film which can withstand 100 degree Celsius?

Some questions in this regard:

[1] What is the maximum temperature metallized film can withstand?

[2] Is the density of the film very important like 1.5 opd, 2.5 opd?

[3] Is any coating required to have the heat resistance power? [Since metallized film coated in both sides are available]

Since initial trials were failed and the findings are as follows:

[a] Curl-up from the edges.

[b] Shape contour was changed.

[c] Some shrinkages  evident.

[d] In some points a stain of burning appears.

[e] In this, 24 micron holographic film was used.

Is there any solution to have better product, where deformity and shrinkage can be eliminated/reduced?  Does CPP film being used for holographic film elsewhere?

We will be glad if you can kindly throw some light on the said problem.

The construction layer is like this

foil 30 micron+ holographic met. pet 24 micron+ wax coated with

Board.

Click on the continuation to see my attempt at an answer.

Relationship between WVTR and metallization

Please tell me what is relation between WVTR reading ( measurement)of a non metallised PET and Metallised PET and take as one example after lamination with a metallised film like a structure of PET/Met PET /Poly?

How to calculate the WVTR reading if u WVTR of PET and Poly and OD of metallisation?
thanks and regards

The answer.

The WVTR of the PET has some relationship to the crystallinity of the PET with the greater the crystallinity the better the barrier performance. Thus not all PET films are identical. 

The metallization ideally would have a direct relationship between the metal thickness and barrier performance with the thicker films having better barrier.  However this is not the whole story.  The biggest limiting factor of barrier performance of the metallised layer is the level of pinholes.  This is the dominant factor controlling the barrier performance, without any pinholes the barrier performance is much more directly related to the metal and the deposition process.  Ignoring the pinholes the deposition process can affect the barrier performance.  If the polymer has been plasma treated to give the highest surface energy the aluminium will wet out the surface well and so a continuous metal layer will form at a thinner level than if no plasma treatment were used. This will also mean there are likely to be fewer microscopic holes where the different crystals meet that also would contribute to a better barrier performance. 

Pinholes because they are related to cleanliness of the polymer web are subject to manufacturing changes.  Thus as a slitting wheel gets blunter the dust it generates increases and as the web is electrostatically charged it will attract some of this dust. Thus there will be different levels of dust on different rolls depending on the state of the slitting wheels, new or blunt.  Similarly there will be other factors that also may affect the debris levels on the rolls. Hence some rolls may have considerably more pinholes visible after metallization than others and this will give rise to variability in the barrier performance.

Thus the only thing I can suggest to you is that you build up your own calibration chart based on your own materials.  This will mean depositing the aluminium both thinner and thicker than you would normally do and use a variety of different substrates.  Ideally this would also include an example of a roll with a bad pinholing problem and another with the least number of pinholes. This would then give the calibration some spread in performance for the same conditions.

This is time consuming and is always a poor substitute to measuring a sample from the material produced.  If you rely on calibration charts without measuring a sample from the product you may miss a problem with one of the component materials.

In the ‘Metallizing Technical Reference’ 3^rd Edn. Published by AIMCAL (available from AIMCAL by visiting www.aimcal.org at a price of $15 + shipping to members and $25 + shipping for non-members) there are some graphs of Optical Density (OD) vs Barrier performance & OD vs thickness but these are from specific machines and may not directly match what you produce from your own machine.

Thus with the variability in the performance of the polymer film and also the metallized film it is hard to produce a formula that would predict the performance of a lamination.

I hope this suitably answers your question.

Charles A. Bishop

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

Metal pigment rationalisation?

Just an observation about the metallic flake pigment business over the last year or so.

Wolstenholme sold their ‘Metasheen’ business to Ciba

Wolstenholme also dissolved their joint venture activities in North America with Schlenk and with their distributors MD-Both Industries, preferring to go-it-alone in the bronze and aluminium pigment business.

Schlenk and MD-Both have continued their activities and opened a joint venture operation in the US.

Altana Chemie has announced it is to buy Ekart.

Silberline has joined forces with Taizhu of China

BASF have bought Engelhard

So what does all this mean?  Well the good news is that the aluminium flake pigment business is good and viewed by many of these large multinational businesses as having a future and hence they want to gain a larger market share.  The implications of this could be that if you are a smaller player it may mean that you could be gobbled up by one of the bigger players aiming to strengthen their position.

So should I be buying shares in……………………..??????

Watch this space.

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 on static / surface energy

Thanks for detailed explanation regarding static charge & surface energy.
Following are few more queries regarding the same:
1.what is the actual mechanism(physical changes develops over film surface)which takes place during charge up(static charge)& same for treatments like corona or plasma ?
2.if we check the dyne value or the surface treatment level of statically charged film & normal untreated film is we get any difference ?
3.same way if we metallize a statically charged film (charge not grounded before metallization)is we get better bond adhesion?
Waiting for your comments.

Answers

The electrostatic charge on the surface is simply an accumulation of electron or ions on the surface.  When two surfaces are brought together the electrons from the orbitals of each atom can interact and on separating the layers some of the electrons can be transferred from one surface to another thus charging up the surface. For a metal roller the electrons, if lost to a polymer surface passing over the roller, can be replaced easily from the bulk of the conducting metal. For the polymer, if it is an insulator, the charge will remain on the surface for some time. In the same way that electrostatic charge attracts dust to the surface it can also do the same for the depositing metal.

The modification of the surface energy by the use of plasma treatment to change the chemistry of the surface and in the electronic charge because of the different bonding and energies associated with the bonds that can be made.  For every atom bonded to the bulk polymer there will be some kind of binding energy associated with the bond.

When atoms are brought in close proximity to each other they will exhibit some attraction for each other from the asymmetric charge distributions in their respective electron clouds.  If the atom is removed because of the plasma bombardment then the net surface energy will change, similarly of a carbon atom is replaced by an oxygen atom there will be a change in bond strength and net surface energy.   The surface energy of the surface is made up of various components including surface area and the average of the energies associated with each of the atoms at the surface.  Thus by changing the type of atoms at the surface changes the surface energy. Also changing the surface roughness can change the wetting characteristics of the surface.

When metal atoms arrive at the surface they are very reactive and will easily bond to anything that is available to form bonds. Metal to oxygen is a more propitious bond that can be made rather than metal to carbon and the bond will be stronger.

In terms of the combined effects of surface energy and electrostatic charge it is believed that there can be some advantages to having some charge on the surface as this can attract material to the surface. There was also some work done many years ago to try to electrify the surface to improve the wetting and hence produce a conducting coating at a thinner thickness of metal deposition.  What makes this less useful is that controlling the level and sign of surface charge is difficult and so the most reproducible process is regarded as removing the change and producing a neutralised surface.

For more details of surfaces, surface energy please refer to Section 7 in the AIMCAL published book,  ‘Metallizing Technical Reference’ 3^rd Edn. Available from AIMCAL at www.aimcal.org at a cost of $15 + P&P for members or $25 + P&P for non members.

This does not refer to electrostatic charge generation only surface energy measurements and modifications.

I hope this suitably answers your questions.

Charles A. Bishop

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