Vacuum Web Coating

A problem of poor adhesion

Sir we are having a PET film product that is having one side copolymer extruded layer & other side corona treated surface, we are metallizing this film on corona treated side but we are facing a problem of metal transfer after metallizing, after handling & after slitting ,in all three stages in first stage the metal transfer pattern is because of TD buckles because of loose winding & in second stage metal transfer because of pressing roll on foam sheet or finger marks & after slitting very fine unmetallized spots after & before winding. So over all metal transfer is there. In metallizer we have taken so many precautions like reduction of heat load by decreasing chill roll temp. to minimum ,increasing heat transfer rate by increasing gas inject flow, evaporators heating reduced to min., a special type of evaporators used to reduce heat load, Rollers cleaning rollers alignment to maintain constant traction.

Could you suggest where we are wrong & what else we can do to reduce metal transfer, Is there is some thing that we can do in base film to improve situation, however we have also conducted a trial in which this PET film is one side copolymer extruded layer & other side chemically coated & metallization done on chemical coated side to improve metal bond strength that can help to reduce metal transfer but results are same.

ANSWER

As with all problems of poor adhesion the first action is always to try to find out the precise plane of failure.  It is always assumed that the failure on adhesion is at the interface but it may not be it may be that the failure is cohesive and in the top layers of the polymer film. It will be difficult to do this as the polymer is transparent and if the failure is in the polymer the small amount left on the metal coating will be difficult to see.  Using a microscope to see if there is residual polymer or testing to see if the surface energy of the back of the metal is the same as the polymer surface may help give clues to where the failure is. Freshly deposited metal has a higher surface energy than the polymer and so if there are differences in energy it is likely that the failure is at the interface.

If the failure is at the interface this then is a problem of adhesion and this is dependent on the cleanliness and quality of the polymer surface and the type and amount of pre-treatment that is done to the surface before metallization.  Polymers can have proprietary treatments which may include coatings as well as include additives that help in the polymerisation process or modify the handling of the film. These treatments or additives can be present on the surface and may not help the adhesion.  Pre-treatments are often used to counteract or improve on any of these earlier surface modifications. Any residual low molecular weight material will be a cause of low adhesion and so either needs to be removed or needs to be crosslinked into the rest of the polymer surface to provide a better anchor to the deposited coating.  The pre-treatments such as flame, corona, atmospheric plasma or in vacuum plasma are all used not only to help remove or stabilise the low molecular weight material but also to change the chemistry of the polymer surface to help increase the amount of direct bonding of the depositing aluminium to the polymer.  The plasma bombardment will break some polymer chains and the oxygen in the plasma will replace some of the existing atoms and the aluminium will bond well to the oxygen.  If the surface energy of the polymer is measured before the plasma treatment and after the plasma treatment the surface energy will hopefully have been increased by something like 10 dynes above the untreated surface.

If the pre-treatments done outside the vacuum system such as a corona treatment it will be effective for some time after treatment but with time the surface energy will degrade back to the original level and the surface will need to be treated again. If the pre-treatment is corona there may also be some variability to the treatment depending on the humidity. If the corona treater is set at a fixed level it will be better at some humidity levels than others. The conductivity of air is dependent on the moisture level in the atmosphere and so the corona will reflect this and there will be a different effect if the power is fixed. Ideally the power will be adjusted to suit the conditions but this is not always the case.  Humidity also affects the surface of the polymer as on high humidity days the polymer will have more moisture on the surface than on dry days and this moisture has to be removed before any plasma can get to the surface to start the chemistry.  Thus if the surface treatment has been optimised on a dry day and then you have a high humidity day you may find the treatment less effective.

Pre-treatments will increase the surface energy but need to be optimised as it is possible to over treat the surface and although the surface energy is high the adhesion can have fallen off and become poor sometimes worse than if no treatment were used at all.  The plasma can be used to cause chain scission that is breaking the polymer chains to allow for direct bonding of the coating to the polymer. This is good in small amounts but if too much is done the amount of broken bonds means that the polymer chains are reduced to very small bits and this then becomes a weak boundary layer and will reduce the adhesion. The amount of oxygen at the surface will still be increased which is why the surface energy will still measure high but the strength of the bond will have fallen. This is why the process needs to be optimised and not simply assume that the highest surface energy will automatically give high adhesion.

As the plasma treatment starts the surface energy will rise and so too the adhesion. With an increase in power or time the surface energy will continue to rise and so too will the adhesion. At some level the surface energy will stop rising and will start to plateau out and at this point the adhesion will have reached a maximum and any additional treatment will result in a fall in the adhesion whilst the surface energy still is high and on the plateau.

So for your particular problem I would look at the failure and if it is an adhesive failure, which it does sound to be, I would then look back at the history of the film and any surface treatment that has been done and make sure that if there has been surface treatment that it has been optimised particularly making sure there is not over treatment.  If there is no pre-treatment then it would be worth considering using some pre-treatment.

I hope this helps.

Testing adhesion

Testing of adhesion : Is there a method ( other than scotch tape test ) to test the adhesion of the films after metallization. Sometimes we are getting some powdery feelings on the metallization and it seems the metallization bonding is weak due to less adhesion. Is there any Instrument available to estimate the adhesion? Or please suggest us how to proceed to ensure very good adhesion as a Quality gate.

Answer

If you have been reading the blog for any length of time or looked at some of my past posts you will see that I do not think much of the Tape Test.  It does not measure good adhesion but only give an indication of poor adhesion of any of the coating comes off during the test.

The great difficulty is finding any suitable alternative test.  All other tests take longer to prepare and require a higher level of operator skill as well as requiring a number of tests to be taken to give a statistically significant result. 

There are a number of different tests that can be done including German wheel peel tests, topple tests, as well as tensile tests.  The tensile test uses the fact that as the tensile load is applied cracks will appear across the sample. Once there are two cracks across the sample the only way the load can be applied to the coating is through the interface between the polymer web and the coating. If the adhesion is poor very little load can be transferred through the interface before the coating cracks again to release the tensile energy whereas if the adhesion is very high the loading is considerably higher before the next crack appears.

What none of these tests really gives any indication of is how appropriate the adhesion is for the purpose the coated film will be used for.  What may be more appropriate is to try to a use a ‘fit for use’ test for each coating.  So if the coated film is to be used with a process that requires good barrier performance in a package that includes a dead-fold then an appropriate test might be to do a dead-fold and examine the coating for delamination and cracking. The higher the adhesion the lower the amount of cracking and there will be no delamination.  Similarly if the material will be twisted an appropriate ‘fit for use’ test would include twisting of the coated film.

What you are trying to avoid is having a test that is much more aggressive than the use to which the film will be put. Conversely you are also looking for a test that will identify coated film that would fail during use. Hence there tends not to be a single test that is used to identify high adhesion and so these more specific ‘fit for use’ tests are used.

Adhesion is related to the surface of the polymer at the time the aluminium is deposited. The cleaner and higher the surface energy at the point of aluminium deposition the higher the adhesion will be.  As polymer films from different suppliers will have different proprietary recipes and surface treatments it can become important to make sure any plasma treatment included as part of the metallization process is optimised for each film.  There is no such thing as a standard plasma treatment.  It is also possible to over treat the polymer surface and not only get less than optimum adhesion but to reduce the adhesion to the level of untreated film or worse.

The surface energy of the treated polymer is an indicator of the surface condition but is not a foolproof method of optimising adhesion as the surface energy can be high but the adhesion poor.  The reason for this is that the plasma treatment will modify the surface by a mixture of processes including chain scission. As the plasma treatment starts the surface energy will rise and the adhesion will improve. The surface energy will reach a high value and remain almost constant at this high value despite continued plasma treatment. The adhesion is completely different it will rise as does the surface energy but it will reach a peak at the point where the surface energy first reaches the highest value. The adhesion will immediately then fall with any further plasma treatment and will not remain at a high plateau as the surface energy does. This is because the additional plasma bombardment continues to fragment the polymer by chain scission and this leads to a large amount of short polymer chains that reduce the adhesion. If the plasma treatment continues it will continue to fragment the surface and eventually the surface will become powdery.

Hence it is critical to have an optimised plasma treatment.  As film suppliers use proprietary recipes, treatments and coatings to modify the film properties the rate of the plasma treatment can vary from film to film and what may be optimised for one film may either over or under treat another.

I am sorry that there is no definitive answer to your questions regarding a specific adhesion test and how to guarantee adhesion.  I hope that the above answer allows you to understand what is happening so that you can choose the best ‘fit for purpose’ tests and also make sure any plasma treatment is optimised for each specific film used.

Low bond strength

We have facing low bond strength in PET copolymer coated film metallized at coated side with 2.2 OD. We found very low bond strength between pet & metal (about 20-30 gm. force) & some time we found 600-700 Gm.

Answer

            Where there is variable adhesion it is always good policy to make a start by confirming that you are trying to solve the correct problem.  The samples with low bond strength you are implying have a problem at the interface due to a poor adhesion between the metal and polymer surface. However this may not be true. The true failure could occur within either the metal layer or within the copolymer coating.  So the first action should be to determine the plane of failure.  This is not always easy to do as the polymer is likely to be transparent and looking for a very thin transparent layer on a bright metallic coating is a difficult procedure.   This may require looking at the surfaces in detail under a microscope. Another technique may be to test the failure surfaces to test for surface energy.  If both surfaces have identical surface energy then it is likely they both have the same chemical surface. In which case the failure is likely to be within one of the materials and not at the interface, or that something has migrated from the substrate to the surface that is contaminating the interface so that both surfaces have the contaminant present. If each surface has a different surface energy then it is likely that there is a simple poor adhesion that allows a separation at the interface.

If the failure is within the copolymer layer then it is important to look at the history of the copolymer coating. Has there been a change in the materials supply or a change in the coating process that has given rise to differences?

If the failure is at the interface then it is more likely that there is a low surface energy surface which contributes to the failure.  The low surface energy can be as a result of something in the polymer substrate or copolymer coating that migrates to the surface.  If nothing is done to change the low surface energy, before the metal is deposited, the wetting and adhesion will be poor.  Thus if the failure is really at the interface then it could be a problem of a contaminated surface or, if there is a surface treatment designed to improve the wetting and adhesion, it could be a failure of the surface treatment process.

It is also worth comparing the history of the samples that have low adhesion against those that have high adhesion, looking for differences in the whole supply and processing of the material and how these differences might relate to the adhesion differences.

Adhesion failure problem

We have 2 metallizer in our company. We have faced the metal peeling off problem. We have -20oC cooling in our chill drum and using the gas wedge also. After metallizing we measuring the temperature its 40oC . How to reduce the output roll temp? Waiting for your reply.

Answer

There are a number of aspects to your problem.

In general if you cool down the deposition drum from -20 Deg C to -30 Deg C the final temperature could be expected to reduce by a similar amount from 40 Deg C to 30 Deg C.

This reduction of the drum temperature may not be possible; it depends on the cooling capacity of the system.

A second method of improving the cooling is to increase the gas flow to the gas wedge. The heat transfer coefficient is dependent on the volume and pressure of gas trapped between the film and drum. The higher the trapped pressure the more gas collisions on both the hot film and cold deposition drum and so the higher the heat transfer coefficient.

Again this might not be possible in your system as there will always be a proportion of the gas the leaks out of the edge of the film as it passes around the deposition drum and this has to be pumped away.  IF you increase the gas into the wedge the leaking gas might take the chamber pressure higher that you would like for your metal deposition process.

A final possibility is to replace one of the standard rolls following deposition with a cooled roll.  This needs to be chosen well as it not only requires the roll to be cooled but also the film has to have sufficient wrap around the cooled roll to take benefit of the cooling. If the wrap is too short there will not be sufficient time to remove enough heat to make a significant difference to the temperature.

This final solution does require changing the winding system and requires an additional leadthrough for a further cooling liquid.

If the metal coating is flaking off this is more indicative of not having the right level of adhesion rather than it being a problem of overheating.  Adhesion can be poor because of too little or too much plasma treatment.  Have you optimised the plasma treatment on the film?

If not I would suggest that this might be more useful than reducing the temperature of the final roll alone. The roll is not very hot it is warm and so I would have thought it unlikely that reducing the temperature is going to make too much difference to the metal adhesion.  I would suspect that even after increasing the cooling the metal would still be prone to peeling off.  Check what type of plasma treatment is done, is it argon, argon/oxygen and has it been optimised for that grade of film?  If the process has not been optimised it could easily be either too little or too much and giving the poor adhesion. The treatment can be too much where the surface energy would be reading a high value but the surface damage would be enough to generate low molecular weight chain fragments that the metal would adhere to but where the low molecular weight material is no longer well bound to the polymer film.

I hope these suggestions help.

Adhesion problem

We use to produce metallized BOPP film usually with no problem, however recently we have suffering some metal adhesion problems. Each 5 meters BOPP master roll is slit at 2.5 meters and metallized with plasma treatment, however we have a kind of metal peeling in one of the edge. Do you have an explanation for this phenomenon? Thanks in advance.

Answer.

The immediate thing that springs to mind is to check the rollers for contamination on the side where the delamination occurs.

This type of poor adhesion can sometimes occur when the bearings have just been lubricated and there has been excess or oil/grease and some has migrated onto the rolls and this is transferred to the web causing poor adhesion.  This could be anywhere on any of the upstream machines. If the loss of adhesion is periodic the distance between patches may give an indication of the roll diameter where the problem starts from. If it is continuous it could be because the contamination has been transferred to several other rolls too.  This problem may reduce with time, if the lubrication was just a one off. If the lubrication is automatic it may be that it has been set too high and this could get progressively worse. 

If you determine that it is not a contamination problem then you would be looking for non-uniformity in any plasma treatment, whether this is flame, corona or vacuum plasma. Look for intensity differences, blocked flame burners, pressure differences across the width, etc.

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.

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.

Oligomers the problem?

Not long ago I was posed this question.

‘ recently I metalized a pet web both sides, when I finished the first side I checked it and it was ok, but the problem ocurred when I finished the other side, because I saw "white stains" on the first side.

My question is What happened?       Is impossible to metalize both sides?

And the other situation is the next, if I pass my finger in the metalized film, my finger becomes "silver" obviously the aluminium has not good adherence. Can you tell me what is the reason of it? ‘

As always it would be nice to be given more information such as if the polymer was pre-treated & if so by what process & was it done on both sides? Also was there any relationship between  the stained areas & the lack of adhesion or was the lack of adhesion everywhere and on both sides?

As this information was not available all I could do was speculate & include my assumptions in the reply.

What would your answer be?
To see my thoughts click on Continue.

Questions relating to adhesion.

            I am curious about what different processes are used to prepare metallized film.  In particular, webs are often flame, corona or plasma treated to improve adhesion but how many of these webs are treated also on the reverse side?  The reverse side of the web can also be contaminated and this contamination may be transferred to the front surface when the web is wound up. Thus the reverse surface could recontaminate the flame or corona treated front surface if the back surface were not also treated.  The back surface might have additional low molecular weight material exuded to the surface during the high temperature reached during the metallisation process. Thus the freshly metallised surface might also be contaminated as the roll is wound up in the vacuum system.  This may result in having to corona treat the metal surface prior to printing to improve the adhesion.  I thus have a second question of does anyone treat the back surface of the web in vacuum before the web is rewound to stop this low molecular weight material transfer?  This also leads to the more general question of how successful do you think any of these processes are?

Summary of the Questions.

1.                  Have any of you experience of also treating the reverse side of the web?

2.                  Do any of you plasma treat the back surface of the web in vacuum?

3.                  How successful do you think these treatments are?

Troubleshooting adhesion – i.e. lack of adhesion.

So many times the question is asked ‘how can I improve the adhesion of my coating?’ or ‘I have uneven adhesion how can I improve this?’

Like most problems the first step is to check that the real problem has been identified. There may be a real lack of adhesion of between the coating & substrate but often the substrate has been contaminated & trying to apply solutions to improve the adhesion without first eliminating the source of the contamination may prove fruitless.

Another common problem is that the only test being used is the ‘tape test’.  I personally find this an appalling ‘test’; I hesitate to even call it a test.  There are so many variables to the test that make it questionable.   It is far better to have a ‘fit for purpose’ test where the coating is tested in the application it is to be used in.

However I know that many companies & operators will continue to use the ‘tape test’ as it is a quick, simple & low cost test and it is regarded as better than nothing.

Below I will give some suggestions of how to check if there has been some surface contamination and also what else might be done to improve the adhesion if something more is required, other than eliminating any local contamination.