Vacuum Web Coating

Question re. lamination problems.

We are doing water-based lamination with metallized BOPP film we also do it with metallized CPP film. However when we do adhesive lamination with BOPP we find problem of corrosion i.e. water attacks metallization layer. We have a doubt that the composition of metallized layer being coated on CPP is different than that of BOPP as they are from two different suppliers. Could you please guide us that what could be there in the metallized layer that is so hydrophilic that it does not allow moisture to evaporate?

Answer.

Cast PP and Biaxially Oriented PP differ where the PP is a mixture of amorphous and crystalline material. The orientation process can result in some alignment within the polymer of the crystalline material. This alignment changes the performance of the polymer is all sorts of ways including tensile performance and also permeation performance.  It is likely that the moisture barrier of the BOPP is better than that of CPP.  This might be a contributing factor.

What you do not mention is what the thickness is of each of the materials and this can also be a factor.  The barrier performance of any material is thickness dependent. Thus, if the CPP and BOPP are of different thickness then this too could affect the performance.

Similarly if the metallization thickness is different in the two samples then this too could result in a differing barrier performance.  Keep in mind that if the materials are metallized by different suppliers there may also be other differences in the metallized layer.  The faster the rate of aluminium deposition the smaller will be the crystal size of the aluminium coating. If the metallization is done at different pressures this too will result in differences within the coating. Higher pressure will result in a less dense coating and lower permeability for the same thickness.  Measuring the coating thickness and also the Optical Density (OD) of each coating will give an indication of how similar the two metallization processes are.  If the thickness were the same but the OD different, or the OD the same but the thickness different it would indicate there are differences in the metallization process. If both thickness and OD are the same, or very similar, it would indicate the metallization processes are also similar.  Another difference could also be in the quality of the coating as described by the number of pinholes per unit area.  The higher the level of pinholes the greater the permeability of the coating as well as the greater the number of starting points for any corrosion.  The surface roughness of the substrate can also affect the nucleation and growth of the aluminium and hence the barrier performance. The higher the surface roughness the worse the barrier performance is likely to be.  Other factors that could affect the corrosion of the metallized film are the temperature and humidity conditions seen by the roll of material.  If the metallized film is re-wound hot, as in greater than 30 deg C, in the metallizer it is likely that the aluminium coating will grow a thicker oxide more rapidly than if the temperature at the re-wind is closer to ambient, generally less than 30 deg C.  After metallization if the rolls are stored in a high humidity atmosphere they will hydrate the aluminised film and lead to more rapid oxidation than if stored in a low humidity atmosphere. Adhesion is another possible factor that could have an effect. It would be expected that the system with the lower adhesion to suffer from higher permeation and more likely to have a greater propensity to corrosion.   I would normally expect the metal to CPP adhesion to be lower than that of metal to BOPP.   

I would suggest that measuring the barrier performance, OD, metal thickness and adhesion of each of the metallized materials before lamination would be a useful comparison. 

I would expect that the barrier performance of the BOPP would be higher than that of the CPP thus making the removal of the water from the adhesive slower than for the CPP.

I hope that this gives you some points that you find useful in helping solve this problem.

Problem with low bond strength

We are metallized film with plasma by applied a 3.0 to 4 kW power. We are

using 1200 sccm of oxygen & 400 sccm of argon combination.

With above combination we are facing problem of low metal bond strength in

metallization done on corona treated side film.

If we try to increased the power of plasma more then 5 or 6 kW, plasma get

tripped & unable to start for next 15-20 min.

I would like to understand that at what power range will give best metal

adhesion?

We are doing metallization on 12 mic with 2.65 optical density. Maximum

width of metallization is 2450mm.

Pl reply with possible cause of low metal bonds, cleaning frequency of

plasma, gas combination & rate of gas, power etc.

ANSWER
pre-treatments are one of the most frustrating parts of the process because it can be so variable, often varying with changes in the weather.

Corona treatment is aimed at increasing the surface energy of the polymer film. So too is the in-vacuum plasma treatment.

Using either or both of these processes there are three possible outcomes.

1.    The treatment has little or no effect and the adhesion is little better than with no treatment.

2.    The treatment delivers a higher adhesion.

3.    The treatment has some effect but sometimes the adhesion is worse than using no treatment.

The pre-treatments are used to do a variety of different things to the.  It can be used to help remove loosely bound material from the surface, also low molecular weight material that has migrated to the polymer surface can also be removed or carbonised or cross-linked into the polymer surface, also the polymer surface can be chemically modified to enhance both the wetting and adhesion.

What you are trying to achieve is a maximum value for the surface energy. However a simple measure of the surface energy can be misleading for the following reason.  There will be a surface energy associated with the untreated polymer film. Now as the pre-treatment is done the surface energy should be higher than the untreated film.  As you have a roll of material optimising this process should be done quite easily by winding material through and progressively increasing the power to the corona treater or in-vacuum plasma treater.  It is then possible to measure the surface energy at each power level.  What you should see is a progressive increase in the surface energy with power to a point where the surface energy levels off at some maximum value.

       It is this maximum value that is misleading.  If you also plot the adhesion of a coating or metallization it will follow a similar curve except that where the surface energy levels off and continues with higher powers (or longer treatment time) at an almost constant high value, the adhesion instead of levelling off it reaches a peak and then almost immediately falls off with increasing power or treatment time.

    The reason for this fall off of the adhesion is that the surface of the polymer has been overtreated. The treatment is often a balance between chain scission that generates new bonding sites and aids adhesion and the scission that creates new short chain molecules that are short enough to be weakly bonded into the bulk polymer and hence form a new weak boundary layer. If the power is further increased these short chain molecules tend towards carbon molecules, thus the weak interface is made up of an excess of carbon even if high oxygen content plasma is used.

The use of corona and in-vacuum plasma can make it easier to overtreat the surface in some circumstances.

Often there is some time between the corona treatment and metallizing process. During this time it is common for low molecular weight materials to migrate back to the surface and re-contaminate the surface and so the in-vacuum plasma treatment is essential.  However if the same material is both corona treated and vacuum plasma treated in the same day it may be that the surface is overtreated.   Thus it becomes important not only to know what the treatments are but the time between the treatments can become equally important.

There are other variables that also need to be monitored and understood. The temperature and humidity can affect both the process and the polymer.  The corona treatment at the same power can produce different results when the humidity is high compared to when the humidity is low.

Winding the polymer in high humidity will trap more water in the roll than winding in low humidity. This is released in vacuum and will also vary the gas content of the plasma treatment process.

I am slightly surprised that the oxygen flow is so much higher than the argon.  Mostly I see argon being the larger flow and oxygen being somewhere in the 10% - 20% range.  The argon provides the heavy ions for doing the chain scission and the oxygen provides the bonding to the carbon by-products making then volatile and capable of being pumped away and also the oxygen will bond onto the polymer where fresh chain ends have been created which can also improve the bonding to the aluminium.

So for your problem I would start by checking the optimisation of the process. Making sure that the surface energy has been maximised but also making sure that the polymer has not been overtreated and that the surface has not been carbonised.

I would also look at the variability in the process. What are the variations in the humidity and differences in time between corona treatment and metallizing?  Look at the optimisation for a constant corona power but with highs and lows of humidity and time between processes. This should give you some idea how much the process can vary and what might be done with the vacuum plasma treatment to compensate for the variations.

I hope this gives you something to work with.

PET film haze

Is there any relation between Pet Film Haze with orientation of film or crystallinity or amorphousity?  Can we correlate HAZE with anyone or with all?

Answers

The answer is yes things such as crystallinity and haze are linked. The draw ratio of can affect the amount of crystallinity and in general the greater the draw ratio the clearer the film (lower haze) but this can then be worsened by the thermal setting process. The thermal setting allows the film to relax and helps reduce the problem of shrinkage but the longer the time or higher the temperature the more the film will relax and some clarity will be lost. (Bear in mind these may be quite small differences). If the draw is not equal in both orientations the crystalline regions within the film can be oriented and so there can be optical differences with orientation. This is most easily measured by plotting the refractive index with orientation. 

Similarly the polymerisation process, which affects things like the molecular weight can also have an effect on tensile performance which then affects the draw and so can affect crystallinity and haze.

Crystalline regions within the polymer are denser and harder and the amorphous material flows around these crystallites. It can be noticeable that cast film may be very clear but after the initial forward draw the film has more haze but after the sideways draw the haze reduces again.  This may be more noticeable with filled film. This is thought to be from crystallites or filler begin slower to re-orientate and so protruding through the surface and roughening the surface and increasing the haze. With the sideways draw this helps flatten then back into the film and so the haze reduces but possibly not to the low level of the cast film.   

I hope this helps.

Additional answer from Dilwyn Jones (AIMCAL Instructor in web handling)

Most thin gauge PET film (including that for metallizing) has a small amount (<0.5%) of inorganic filler, such as calcium carbonate, silica, china clay and glass bead, added to improve the handling behaviour both during manufacture and subsequent processing.  Without it, reels would telescope if wound at realistic speeds, and block during storage.  The composition, particle size and volume fraction of the filler are the main factors influencing haze.  Haze has contributions from both the bulk and the surface, as the filler particles near the surface increase the surface roughness.  There may also be particles that are so large they give individual optical effects, such as Newton's Rings, or visible marks in the metal layer.  Finally, voids form around some particles during the stretching steps of film manufacture, increasing the haze.

Stretching ratios, temperatures during crystallisation, and PET molecule factors such as glycol and IV have a smaller effect.

Without inorganic filler, there is still catalyst residue and internal contamination to scatter light and cause haze.  This is a mechanism in clear, thicker film.

A lot of the detailed knowledge on this is of course proprietary to the major manufacturers.  However, there may be examples in the patent literature especially, and the open scientific literature also.

Number of pinholes per unit area.

Is there a standard acceptable amount of pinholes per sq metre in metallized BOPP.

Also do you know anything about the possibility of 'welding' pinholes in metallized film by use of an electric charge?

Answer

As far as I am aware there is no standard that defines the grade of the film by the number of pinholes per unit area.  There are some figures quoted by aluminium foil manufacturers about the number of pinholes per unit are for foil and so the idea of counting pinholes is not unknown.  Possibly the reason for not bothering to count pinholes is that the barrier performance is not only affected by the number of pinholes but also by the size of the pinholes.  Thus a simple count of number of pinholes would not necessarily tell you what the barrier performance would be and films with the same number of pinholes could be radically different if one had large holes and the small ones.

Individual customers may have something in their acceptance specification about number of pinholes. This may be highlighted if the film is for an optical or graphic purpose but where it is for a packaging application the barrier performance will usually take precedent. Here the allowable number of pinholes tends to be either determined by the barrier performance as measured on a Mocon or similar instrument or something that is agreed between the metallizer and customer. 

There was a paper by Angelo Yializis that includes a bit about healing capacitor films.   If memory serves me correctly this is more about preventing shorting between layers than sealing pinholes.  If successive layers of metallized film are shorting out it is sometimes possible to pass a current through the film such that the current is too great for the short and so vaporising the metal and breaking the circuit at this point thus removing the short and recovering the full performance of the capacitor.

If you were thinking about closing the pinholes by welding a little bit of aluminium over the holes to block it up then 'no' I do not know of any process doing this.  If a fully opaque film is required then the usual approach is to double side metallize the film. The philosophy behind this is that the chance of two pinholes lining up to still show as a pinhole is negligible.  Double side metallizing is also used for high barrier films. Again the fact that holes do not line up means that and gas or moisture will have a tortuous path to diffuse through the film and hence the barrier improves.

Question re. Static.

Will the presence of static charge on the face material affect its surface energy? Since static charge is generated through friction, this becomes an unavoidable problem. But will this have an effect on the printability of the face material? Will the presence of dust particles cause the material to lose surface energy thereby causing poor ink adhesion? If so, is there a relationship between surface energy and static?

Answer

Static charge can be caused by friction, separation or induction. 

Of these, separation is the one that most affects winding webs.  As the two dissimilar materials are brought together the surface electrons are brought into close proximity to each other and it is possible for electrons to cross from on surface to the other and on separation for the electrons to stay with one surface rather than the other. In this way the polymer, which is negative on the triboelectric series can collect charge on leaving each roller. The larger the difference between materials in the Triboelectric Series, the greater the static charge that can be created on the material surface. In addition the faster the winding speed the greater the charge that can be built up.  The charge can be cumulative so that over a series of rolls the charge will increase after each one.  If winding in air the humidity will have an effect as water in the atmosphere affects the conductivity of the air. Higher humidity increases the conductivity and so will speed up the decay of any charge on a surface. It will also allow arcing to occur at lower charge levels.  Conversely a dry atmosphere is less conducting and the polymer will reach a higher charge before it can arc to discharge the surface charge.

When you wind film an electrostatic charge is already building as the film approaches each roller.  When the film winds around a roller it does not, in theory, move against the roller and so friction is less relevant.  As most rollers in your system will be metal and hence conducting they will be able to dissipate any static charge to earth and thus you only find the charge on the polymer film. 

Dust particles will be attracted to the surface because of the static charge on the film.

For those of you who disbelieve this have a look at any TV screen and wipe your finger across the surface and you will usually find a layer of dust on the surface that has been attracted to the vertical surface by the static charge on the screen.  Dust does not affect the surface charge directly. Dust cannot provide a leakage path to earth to dissipate the charge. It is possible that dust can form a slight electrical charge concentrator and so if the surface is going to arc to atmosphere it could be initiated at a dust particle rather than from the flat surface of the film.

Static charge and surface energy are separate factors.  The same static charge can be built up on the same polymer irrespective of whether the surface has been plasma treated to raise the surface energy or not.  However the presence of a static charge can affect ink adhesion and wetting.  If you look at ink jet printing the ink droplets are often electrostatically charged as part of the print control process. Also if you look at many modern painting processes these too are electrostatically charged, to improve the surface coverage. Both of these show that liquids can be affected by static charges. It depends on the liquid chemistry as to precisely what effect the charge will have. Liquids may be conducting, or not, each of which will be affected differently.

Thus the ideal would be to use static eliminators to neutralise the film immediately before printing but to have also treated the surface to increase the surface energy to improve the wetting and adhesion of the ink.

If you use a corona treatment before printing this will serve both purposes as the corona plasma has both electrons and ions and so will naturally neutralise the surface whilst also increasing the surface energy.

If the corona treatment is positioned on the machine well before the printing such that the film winds around rollers between the corona and printing stations then there will still some charge on the film at the point of printing. 

Alternatively static neutralizers can be used just prior to printing to reduce the effects of any built up charge.

I hope this helps.

CAB

Just to give you a flavour of the triboelectric series here is a list of materials starting with the electropositive materials and moving down to neutral and then to electronegative.

The farther apart the two materials (film and roller) are the greater the charge that can be produced.

TRIBOELECTRIC SERIES

Glass                                                  Electro positive

Nylon

Wool

Silk

Aluminium

Paper

Cotton

Steel         ------------------------------- Electro neutral

Wood

Hard Rubber

Nickel, Copper

Brass, Silver

Gold, Platinum

Acetate Fibre (rayon)

Polyester

Cling Film

Polythene

PVC

Silicon

Teflon                                                 Electro negative

 

Moisture and aluminium performance

Can moisture affect the performance of aluminium metallized films?

Answer.

Yes moisture can affect the performance of aluminium metallized films. This starts with the film before metallization. Most polymers contain moisture as well as the water in the air that is trapped between layers as it is wound into a roll.  This is all carried into the vacuum system. Even when the system is pumped out to a low base pressure there will still be enough oxygen and water in the system that a monolayer of oxygen can form on a surface in less than 1 second.  Thus all aluminium metallized films have a proportion of oxide in them, usually of the order 1% - 2%.  If the vacuum system has a leak this can be somewhat worse.

The aluminium is a metal that forms an oxide on the surface that acts as a good barrier layer and prevents further rapid oxidation.

The adhesion of the aluminium to the polymer web is dependent on a number of things. This can include the polymer quality, contamination, the storage conditions (humidity and temperature), any surface treatment, the type of treatment, the age of any treatment as well as the process conditions such as deposition pressure.  When laminating another layer to the surface it will depend on the relative adhesion strengths and the residual stress following the lamination. If the residual stress is large the adhesion of the aluminium needs to be higher than if the residual stress is low.

The speed of oxidation and amount of oxidation depends on the thickness of the aluminium layer. If the aluminium is very thin and the adhesion is poor the there will be very little aluminium left once the surface has been oxidised. If the aluminium is thicker then even after the surface oxidation there will be sufficient metal left to give a long lifetime for the rest of the aluminium.

AIMCAL Fall Technical Conference

AIMCAL Fall Technical Conference 
and
22nd International Conference on Vacuum Web Coating
October 19 – 22, 2008
Myrtle Beach Marriott Resort at Grande Dunes
Myrtle Beach, South Carolina

   

CALL FOR PAPERS
DEADLINE: To assure consideration, abstracts must be received by May 31, 2008.

The AIMCAL Fall Technical Conference is an Industry forum for the global technical community involved in web coating processes.

Visit the AIMCAL Website to submit your paper.

Topics of interest for this year’s conference include, but not limited to the following:

Vacuum Web Coating

• Process optimization and preventive maintenance in the vacuum web coating processes
• Leading edge roll to roll technologies, products and markets
  including flexible displays, flexible semiconductors, superconductors, thin film battery, flexible solar cells, super barriers, film sensors, anti-counterfeiting films and papers, OLED, holography, and solar control 
• Machine and equipment accessories upgrade cost / benefit
• Advances in substrate technology
• Advances in process control and measurement
• Patterning and edging of vacuum coated films and papers
• New coating processes, tools and equipment
• Advances in barrier performance, process and measurement

Atmospheric Web Coating and Laminating

• Web Coating Technology related to applications and fluid rheology
• Process Measurement including coating weight, defects, viscosity, process modeling
• Laminating processes including alternatives to laminations, process optimization, web variability, modulus, guiding and tracking
• Coating Operations: An overview that compares/contrasts various coating techniques, roll coating systems, slot die/curtain coating techniques, extrusion coating, coating thin on metallized films, toll coating
• Substrates and liners technology including new technology and performance, innovations, and applications
• Web coated product markets - technology and market overviews

Specialty Web Coating

• Novel or developing web coating technologies
• Nano coating technology
• Photovoltaics technology
• Developing web coating technologies and markets
• Specialty substrates and coatings technology
• RFID technology

A session on Market Trends in the Converting Industry is being scheduled. Topics of interest are global market and technology trends, future role of converting process in growth industries and integrating product development through the supply chain.

Presentations for the Fall Technical Conference are selected upon the following criteria:

• Applicability to the advancement of the manufacturing process
• Uniqueness and innovativeness of new technology and its contribution to the Industry
• Value to the technical/production community

The audience for the Fall Conference is Technical Professional personnel in R&D, Manufacturing, and Marketing. Presentations should be at a technical level of interest to this audience. Papers covering related subjects, not specifically addressed in the call for papers should still be sent for committee review.

Visit the AIMCAL Website to submit your paper.

www.aimcal.org

Applied Web Handling Conference 2008

Applied Web Handling Conference 2008
May 6 -9 | Radisson University Hotel  | Minneapolis, MN

AIMCAL, the Association of Industrial Metallizers, Coaters and Laminators and CEMA, the Converting Equipment Manufacturers Association are pleased to announce the second annual technical conference devoted to applied web handling technologies within the Converting, Paper, and Plastics industries.

FOR MORE INFORMATION AND SECURE REGISTRATION VISIT THE AIMCAL WEBSITE   www.aimcal.org

Uniformity of pumping, system symmetry.

For those of you depositing metals this is going to be of less interest but for those of you either reactively depositing coatings or thinking about developing a reactive process this should be of great interest.

I recently went to work on a vacuum system that had been designed as a research and development machine capable of depositing coatings by reactive deposition. Typical coatings that the machine was designed for included titania, alumina and the transparent conducting coatings such as indium tin oxide (ITO).

The system had been designed with coating uniformity in mind and the pumping was arranged around the chamber along the centreline of the web path. The gas input was through manifolds that were similarly positioned to give a uniform flow across the width of the web. Thus the general arrangement of the sputtering and other deposition sources, pumping and gas input could be expected to give uniform films.

Needless to say the reason I am writing this note is that the coatings were anything but uniform.  Originally some ITO coatings were deposited from ceramic sputtering targets and these were reasonably uniformly conducting across the width and fairly easily optimised. However when it was tried to deposit ITO coatings from a metal target it was impossible to optimise the conductivity and when the resistivity was measured in more detail it became apparent that there was a different resistivity on each edge. When a minimum was reached on one edge the other was more than 20x higher and vice versa.

On a detailed review of the system, checking the symmetry of all the equipment it became obvious what was the cause of the problem.  The original design included a cryocoil in the winding zone to getter the water vapour from the roll of polymer and this too was uniformly distributed across the web to give uniform pumping. There was some work done on the best positioning of cryocoils/cryopanels in the vacuum system and having cryocoils in both the winding and deposition zones was shown to be better than having a single cryocoil in either the winding or deposition zone.  To take advantage of this information additional cryopumping was added to the deposition zones. Unfortunately at this point the symmetry was lost as the cryocoil was positioned on only on one side between the original pumps and the chamber wall. To make this worse as the coils were behind the sputtering sources and so could be expected to see some heat they were protected by being protected by a metal box.  The net result of this was that the conductance to the original pump was restricted on the side that had the cryocoil.

We know the end result was skewed pumping to the deposition zone such that the pressure at each end of the sputtering source would be different and so the requirement for reactive gas would be different. Hence, as the reactive gas was being uniformly introduced across the whole width, the optimum point to give the correct stoichiometry for a coating with minimum resistivity would be different at each end of the sputtering target. 

The solution to the problem is simply to change the shape of the cryocoil in the deposition zone to have an equal surface area on either side of the existing pumping orifice. This would reduce the volume the existing coil takes and so the protective radiation shield could be moved further back and so most of the restriction in conductance would be removed. Thus the uniformity would be returned and the pumping speed increased at the same time.

This has yet to be done to prove the above solution is correct as the work has to be planned and fitted into programmed downtime. However we are in no doubt that we have found the source of the non-uniformity and how to correct it.

I think this offers an excellent lesson on why it is essential to have symmetry about the web centreline for all aspects of the system and process.  Adding equipment that restricts the conductance only on one side of the web or adding additional pumping such as a cryocoil and placing in a convenient space in the chamber rather than symmetrically across the web can result in non-uniform coatings.  I emphasise that this will only be a significant problem for reactive processes. Although minor variations may be measured across the web in other coatings for most applications these are negligible.  When depositing ITO from a ceramic target, where some reactive gas is included to compensate for any dissociation during the sputtering process, the dynamics of the process are such that working with an excess of reactive gas is not as critical as it is with the full reactive process converting a metal target to the desired compound.  Hence the non-uniformity of the coating will be similar to plain metal coatings rather than full reactive deposition.

Although it is less important to have symmetry of pumping in metal deposition it will become increasingly important as the width of deposition systems continues to increase. Already we have machines of 4.45m wide. If these are pumped from one end only imaging what the pressure difference will be from one end to the other. This pressure difference will become greater and greater as the width continues to increase and this will be seen as producing different deposition rates from resistance heated boats at each end of the system.  A simple solution is to pump from each end although there will still be a pressure gradient from the web centreline to each edge.  A better solution would be to distribute the pumping across the whole width. It is worth noting that as customer specifications continue to increase, particularly for high technology applications, it can be expected that there will be an increasingly tight specification for resistivity which may require an improvement in system symmetry.

As you might have guessed this is a hobbyhorse of mine. I have worked on too many systems that were never designed with symmetry in mind but were expected to produce coatings by a reactive process that have been a major challenge. Mostly the difficulties could have been eliminated by better design making the processing much easier and the productivity of the system much higher. Thus I find it disappointing that even on new machines I still see some of the same design errors.

Problem re. Ink adhesion to metallized film

In fact we first laminate the metal side of METPET in to 1.5 mil Polyethylene film using solventless adhesive of Liofol 7980/7275.
Once it is cured, in second process we surface print on PET side of the structure.
Following is the final structure from outside to inside:
                                        OPV (OVER PRINT VARNISH)
                                        INK
                                        PET SIDE (OF METPET)
                                        METAL SIDE (OF METPET)
                                        ADHESIVE (SOLVENTLESS)
                                        POLY SEALANT FILM
DEFECT:
        The ink adhesion is good but with slight twist of the film the ink is released from the PET showing Missout and a kind of pinholing showing metal.
        This happens mostly in shipping and in handling the package at customer plant.
       
        If we try to separate the layers, the metal is released from the PET but the do not delaminate with Poly sealant layer.

       How to have 100% ink adhesion on PET of METPET?

Forgot to mention one important information, sorry.
The metal on METPET is on the corona treated side.

Answer

It sounds as if the possible problem is the same for the ink as for the metal.

Corona treatment is helpful to a certain extent but is a variable treatment.  When PET is manufactured there is always some residual unpolymerised material left in the polymer. This oligomer is low molecular weight and is easily able to migrate through the polymer and it will appear on the surface.  The amount on the surface will depend on the age and storage conditions of the polymer roll.

If untreated and left on the surface both the ink and the metal will bond to this oligomer, which forms a weak boundary layer, and will have a poor bond strength.

Corona treatment will improve this bond but has limitations.  The corona treatment needs to be optimised and also it needs to be done immediately before coatings are applied.  The corona treatment varies with humidity and so what might be optimised on a dry day with low humidity may not be optimised for a very humid day.  The treatment is also not a permanent treatment.  The oligomer will still be present within the bulk of the polymer and with time and/or temperature it will migrate back out to recontaminate the surface. 

Thus if the corona treatment is done before the metallization on both sides of the PET the metal adhesion may be at one level but as the film will have been heated during the metallization process the back surface treatment may already be recontaminated by the time the film leaves the metallizer. Similarly if the lamination is done using heat there will be a further migration of material to the PET surface.

The indication you have given is that the ink is printed onto a surface that has not been corona treated. Thus the ink adhesion could be improved using a corona treatment before the printing.

I personally would not rely on the corona treatment for the metallization but would consider using a plasma treatment within the metallizer, if at all possible, so that the treatment is immediately before the metallization and there is less chance of re-contamination. I would also use an oxygen containing plasma to make sure that I removed as much organic contamination as possible. An argon plasma has no mechanism for removing organic material. It just knocks material off the surface which can re-deposit on the surface. Oxygen will convert organics into volatile species that can be pumped away by the vacuum pumps. Also the oxygen will form bonds on the surface that will also form bonds with the aluminium and increase the bond strength.

If you cannot carry out a plasma treatment but only have corona treatment as an option I would make sure the process has been optimised and would check that this has been optimised at different humidity levels.  I would also make sure that the time the roll is stored between corona treatment and metallization is minimised again to minimise the time for recontamination. 

Re-contamination occurs by two processes one is the migration of material from the bulk as mentioned above and the other is by transfer of material from one surface to another. Bear in mind that, as the back surface has not been treated, the low molecular weight, and often low surface energy material on the back surface will be brought in contact with the high surface energy front surface as the film is re-wound. As nature tries to equilibrate energies it is preferable for some of this low energy material to transfer from the back surface to the front surface.

This is the reason for minimising the time the film is standing between corona treatment and metallizing.

Hopefully this explains a possible source of variable adhesion and some options to correct the problem.