Vacuum Web Coating

Dear sir,

We laminted the BOPP with MCPP (A supplier) and we encounter the problem that the metal from MCPP was 100% transfer to BOPP. We used tape test before lamination and it was passed.
The AL adhesion test on MCPP was 4.1-6.2N/18mm which is within the acceptance level.
At the same time, we used MCPP from another supplier to laminate with same BOPP at same parameter setting and we did not found metal transfer. Please advise what is the cause of the metal transfer on MCPP of A supplier?
We appreciate your comment on problem we faced. Thank you

From: Cheong

There are several things relating to this problem.

First check carefully that the plane of failure is really between the metal and the polymer and is an interfacial problem and not in the polymer and so is a cohesion problem.

Assuming it is a problem of adhesion. The different suppliers of MCPP can be using different manufacturers of CPP and so may have differences in composition regarding amount of polymerisation, residucal oligomer content, additives, etc, Any residual oligomer or slip additives will find its way to the surface and will become a problem for metal adhesion. The amount present on the surface can depend on the time and temperature the roll of material sees between manufacture and metallization as well as any pre-treatments such as corona or vacuum plasma treatment.

The tape test for adhesion is a poor test and can be variable for many reasons. I do not like it as a test but recognise it is commonly used because of the ease of use. It is not a measure of adhesion but is usually an indication of poor adhesion However once the coating is not removed by the tape test there is no way of discriminating between coated films that all pass the test. What some companies do is to buy tapes of increasing adhesion so that they have an extended range of tape test. They will have the recommended tape as their everyday tape test but then for some applications they will have specific tapes that have a higher tack strength. This enables then to test for a higher metal coating adhesive strength, still without knowing what it is other than better than the standard tape test.

If both metallized film suppliers are sourcing the uncoated polymer from the same source then you need to look at any differences in pretreatments. If they both use corona treatment then look at the time and temperature differences between treatment and metallization. THe longer the time and higher the temperature the more likely the film surface has become recontaminated with oligomer or slip agents, either from the bulk of the polymer and migrating out or from transfer from the back surface back onto the front surface.
In the case of vacuum plasma treatment then you need to look at the plasma cleaning gas composition, power and residence time. It is possible to increase the surface energy but also damage the surface such that the adhesion is not imporved. This is a case where a little plasma treatment can be useful but too much can be damaging. The surface energy may continue increasing and finally reaching a plateau but the adhesion will rise initially then reach a peak and with continuing treatment fall again. This is because the chain scission produces too amny short chain molecules that are no longer attached to the bulk polymer. THe gas composition is generally better if it contains some oxygen as argon alone cannot remove any hydrocarbons and only with oxygen present can you both bond to the polymer and the depositing metal thus giving some very highly bonded sites which imporve the adhesion significanty.

I hope this helps.
CAB

Dear sir,

Thank you for your valueable advise. We have another doubt, we refer to the AL Adhesion test result on MCPP film, it show the MCPP have higher strength of AL adhesion and no metal trasnfer reported. value is apporox 4.1-6.2N/18mm. But for finished product, adhesion become very poor, does this mean that the effect come from lanimation not MCPP?

Thank you
Cheong

The final product will always fail at the weakest point and so if the adhesive is of a higher adhesiuon than either the metal to polymer interface or the cohesion in the BOPP polymer then the failure will be at one of these planes and not within the adhesive.

So in all cases, either testing or final products, a knowledge of the individual adhesion strengths can help predict which plane of failure could be expected.

CAB

Dear sir,

We have laminated printed BOPP with metallized BOPP by solvent less systemfor several years without problems.
Now a days we have a serious delamination problem.
The structure is BOPP / Ink / solvent less adhesive / metallized BOPP
The faliure is : the adhesive migrates from the metallized BOPP to printing ink.
This happens when we use metallized BOPP. If we use transparent BOPP the delamination do not occur.
We treat the metallized film in line during the lamination process and apply the adhesive on the metal.
The delamination beggins after 3 or more weeks after the lamination process, so is our customers that are perceving the problem .
Have you ever seen a problem like this?
Could you help us ?

If I understand the problem correctly the plane of failure is between the adhesive, which was applied to the metallized OPP, and the metal as follows;

BOPP / ink / solvent less adhesive / PLANE OF FAILURE / metallized BOPP


It was mentioned that the metallized film was in-line treated during the lamination process.

Is this a corona treatment of the metallized surface before the solvent less adhesive is applied?

If the above is correct there are a couple of things I would check out to start with. Firstly would be to confirm the plane of failure. This can be difficult but also can be important. Confirm that on the metal surface there is only metal and not a thin layer of adhesive.

If there is a thin layer of adhesive it would imply that the failure is in the adhesive as a cohesive failure and not a delamination occurring at the interface. This would then suggest that there is some difference in the adhesive between the good and the failing materials.

If there is no adhesive and it is an interfacial failure then we need to look at the metal surface and the adhesion of the adhesive to the surface. There are a couple of possible reasons for the change in the adhesion of the adhesive to the metal, one is the metal surface contamination has increased and the surface treatment is less effective and the other is that the surface treatment itself has changed. If we first take a look at now a metal surface can become contaminated. The metal immediately after metallization has a very high surface energy and when the roll is wound up in the metallizer it will be in hard contact with the BOPP back surface where there may be plenty of low molecular weight material that is quite mobile. Nature tries to bring surfaces to equilibrium and so where a high surface energy surface meets a low surface energy surface it tries to reduce the difference and so the mobile low molecular weight material will migrate to the fresh metal surface and reduce the surface energy. The longer the roll has before being unwound for the next processing step the more of this low molecular weight material will have been transferred and the more similar the surface energies will become. If the BOPP contains slip agents this material is designed to be mobile and it will easily transfer to the metal surface. This process will also be speeded up if the temperature is increased. So if your rolls that are giving problems have been stored for longer or have been exposed to a higher temperature than usual it could mean the contamination levels on the metal surface are higher.
The alternative part of this same problem is the corona treatment (assuming the surface treatment is a corona treatment). Is the corona treatment identical between the good rolls of film and the ones with the delamination problem? Has the corona treatment system been cleaned and replaced? Has the weather changed between the good and bad rolls? Typically the corona treatment is set up to give maximum adhesion. If this is changed then the surface may be under or over treated. Changes in humidity change the conductivity of the air and so the corona characteristics change and the surface treatment will also change. If maintenance to the treater has been carried out, is the gap the same as it was previously, as this too will affect the plasma characteristics of the corona and so the treatment level.

One possible tool for looking at the various surfaces might be to use surface energy measurements. I do not know if you routinely look at the surface energy of the various surfaces before and after any treatments but if there are sample of the different films before and after surface treatment and where they are failing it might enable some answers to be found. If the metallized film has a greater level of contamination on the surface than previously, you might expect the surface energy to be lower than with the earlier lower amount of contamination.

As you will see from the above the search is to find out what has changed from the rolls being good to the rolls having the delamination problem. I hope this helps in your search.

Dear Sir,

we are coating BOPP film on gravure with methanol base ink. But not getting the best result so plz can you tell me the best method for preparing ink

I am no expert on inks, formulation or coating conditions. I would suggest that you put your question to Dr Edward Cohen on the Web Coating Blog.

If you go to the AIMCAL website at www.aimcal.org and go to the blogs there is one run by Ed.

Alternatively if you go to the Ask AIMCAL part of the site your question will be sent to a number of consultants which includes Ed Cohen and you should get some help from them.

Sorry I could not answer your question directly.