Vacuum Web Coating

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.

Coefficient of Friction (CoF) of plain & metallized films

Need to understand the factors influencing the COF values Plain: Plain of Metallized Polyester films. The observation is that after metallization the COF values change (generally increase) as compared to the pre metallization values.
1.Can this value be controlled?
2.Does this also depend on the OD to which the film is metallized?
3.How significant is the Film: Film cof of the Metallized Polyester in the subsequent Converting operation?

Answer

The Coefficient of Friction (CoF) is dependent on the materials in contact, the surface energy and the contact area.  So for your polyester film you will have a CoF of the polyester in its natural state which is likely to be high (probably >0.5) and in this state the polyester is likely to ‘block’ (stick together) when rolled up.  Blocking is where the film sticks together and is difficult or impossible to unwind. This can be made worse by electrostatic charging of the film and surface cleaning which increases the surface energy.

To reduce the propensity of the film to block and to make the film more easily handled there are a couple of methods used.  This first is to add fillers which will change (increase) the surface roughness which, on a microscopic scale, reduces the amount of two surfaces that are in contact. This use of fillers can reduce the CoF down to 0.3 – 0.4 which is still regarded as too high for optimal film handling.  The fillers are not without problems. Typically the haze increases by 0.4-1.0% per 1000ppm of silica filler used.  To minimise the haze the film can be made as a co-extruded film with only one thin layer being filled which reduces this effect although because it changes the surface roughness does not eliminate it.  Also changing the type and size distribution can reduce the haze.  The danger is than in reducing the haze the CoF will once again begin to rise.

The other common method of reducing the CoF is to add a slip agent to the polymer. These slip agents migrate out of the bulk of the polymer to the surface and reduce the surface energy and CoF. As little as 0.1% slip agent into the bulk polymer may reduce the CoF down to less than 0.2 where 0.16 – 0.2 CoF is regarded as providing easy handling.  The slip agents make the handling better but can be a disaster for obtaining good adhesion.  One of the reasons for treating the surface by flame, corona, atmospheric plasma or vacuum plasma is to remove or crosslink in these low molecular weight low surface energy materials to both increase the surface energy to obtain better coating wetting as well as remove the weak boundary layer that would become the failure plane at the interface.

The surface treatments do not change the CoF that is produced by the use of fillers resulting in the change in surface roughness.  How ever as mentioned above the use of fillers alone usually does not produce a low enough CoF to make handling the film easy.

When the film is metallized it is most likely to have been surface treated to improve the wetting and adhesion of the metal coating and so the metal surface will no longer have any low energy material present and so the CoF is likely to have increased. 

Even if a slip agent is not included in the bulk polymer there will be unpolymerised monomer that will appear as oligomers on the surface as low molecular weight material that can have a similar effect. These oligomers will also be removed by any surface treatment.

What you do have is the back surface of the film that is most commonly untreated and so there will still be the low molecular weight material on the back surface, either slip agent or oligomer.  As the freshly metallized surface has a very high surface energy the film when rewound will bring together these two different surfaces.  The low surface energy material will want to migrate to the high surface energy metal surface and so over time you can plot the reduction of the metal surface energy as more of the low molecular weight material, which is very mobile, is transferred across to the high surface energy metal surface. Given enough time these two surfaces will approach the same surface energy.  

In general the optical density (OD) of the metal coating does not affect the CoF. The metal coating is nominally conformal and so will have the same surface roughness as the surface it has just covered.  This is true for most coatings but if you deposit an extremely thick coating ~1 micron thick or thicker it is possible to get selective crystal growth that can increase the surface roughness which will cause the metal surface to appear matt. Thus if you coating still appears to be of mirror quality then it is likely to be thin enough that there is no significant crystal growth and hence the surface roughness of the metal should match the polymer surface roughness. 

Finally the question of significance of the CoF to film handling relating to downstream converting processes.  This is a one of those questions that it is virtually impossible to answer.  There will be a different answer depending on whether the process is a simple lamination process or it is a wrapping or filling process.  In the lamination there will probably be fewer rollers and less contact with the film whereas with the latter process there will be more rolls to help make folds and change the direction of the film movement which will make the film more sensitive to the CoF of the film.  This may be one where you have to try the film and see if there is a problem.

Please note that in checking the film you also need to time the gap between metallization and use on the next process.  Ideally you would also record the temperature too.  The reason for this is that if the film is used the same day as metallization it may have a handling problem but the same film if left for a week, possibly at an elevated temperature, may no longer have a problem on the same machine.  The reason for this is the reduction of the surface energy of the metal.   This is done by the transfer of low molecular weight material onto the metal surface.  The rate of transfer of material from the bulk or across from one surface to the other is increased at elevated temperatures as the low molecular weight material becomes more mobile.

One word of warning, if you try to lower the surface energy be careful, as this could cause problems of adhesion to other surfaces.  There can be a trade-off between handling performance and adhesion.

I hope this helps.

Quality issues – adhesion, substrates, cleaning – AIMCAL Fall Conference preview

            There are always quality issues that are important to everyone and at this coming AIMCAL Fall Conference were have some papers that will address some of these issues.

Adhesion is always a problem that we get lots of questions about and one of the most basic tests often appears in the discussions.  The ‘tape’ test. This test is cheap and easy to do and some at some time or other we have all probably used it and might even do so on a regular basis.  However this test is not really a test of adhesion but is a test of poor adhesion.  If the coating fails it shows that the adhesion strength is low.  BUT it the coating passes it DOES NOT mean that the coating has high adhesion. All it means is that it is better than the tape adhesive. This can mean that it is slightly better or considerably better but we do not know which.  The tape test can also be variable with it being sensitive to the operator doing the test, the methodology in carrying out the test, the quality and age of the tape, the humidity and many other variables.  Thus as a test it has a limited use but because it is so simple and cheap it is still by far and away the most widely used test for adhesion.  Don McClure will present a paper that describes the problems in using this test and warning of placing to much reliance on it and also what can be done to improve the quality of the test.

            Another of the perennial problems is that of pinholes.  Ever since work done back in the 1970’s at Cambridge University it has been known that debris left on the surface of the polymer films before they are metallized are the largest, but not the only, source of pinholes.  Despite this here we are, close to thirty years on, still tending to ignore this fact and using film that has not been cleaned and still complaining of pinholes in the coatings.   Sheila Hamilton of Teknek will be talking about cleaning surfaces using their tacky roll technology. This is where a tacky roll contacts the polymer surface and any loosely bound material is removed by becoming stuck to the tacky roll. As there is a lot of this loosely bound material the tacky roll can become clogged very quickly and so this roll contacts a higher tack roll to remove the debris and keep the film contact roll fresh. There are various elastomers available to control the level of tack, including being able to use rolls inside vacuum systems.  This technology is not only useful for reducing pinholes in vacuum deposited coatings but is also similarly useful for the ‘wet’ coating side of the converting industry and also were coatings are patterned, such as by laser scribing, can be cleaned after before further downstream processing.  Once the debris has been removed it can be looked at under a microscope and it can be possible to characterise some of the common sources of debris and the typical size range.

            To compliment these two papers there is also on from Mark Maloney of Oxford University that describes what could be described as a universal pre-treatment that can be used to graft a more active polymer onto the polymer substrate surface to promote adhesion.  This too is a technology that is equally applicable to the ‘wet’ as well as the vacuum coated products. 

Barrier coatings – AIMCAL Fall Conference preview

            There is a growth in the use of barrier coatings, in particular transparent barrier coatings. This is not just in the more established food packaging markets but also in the electronics markets where a very much higher performance transparent barrier coating is required for protecting the organic coatings for light emitting devices as well as for protecting photovoltaic cells.

            There are a number of papers that will be presented at the forthcoming conference that will look at different aspects of both depositing the coatings and also measuring what has been produced.  The theory is simple, if you can deposit a perfect thin film of a good transparent barrier material you will have a perfect good barrier coated polymer substrate. Unfortunately it is extremely difficult to deposit a perfect coating.  The substrate is invariably not clean enough or smooth enough and any surface defects usually end up as defects in the coating which allow gases and vapours to flood through. So there are various methods that are used to try to overcome the shortcomings of the substrates to help reach the ultimate goal of a perfect coating.  Even with a perfect substrate in terms of cleanliness and smoothness it may not be a suitable match for the coating and the adhesion might be poor and so it is common to have to treat the substrate surface to encourage wetting and adhesion. More care is needed as this same treatment can also damage the smoothness of the substrate.  Even then as the coating grows there will be growth defects such as grain boundaries or voids that are also a source of loss of barrier performance.  Hence it is not surprising that each year we have a number of papers that describe the techniques and efforts that are being worked on to overcome these challenges.  This year is no exception with papers from as far afield as Singapore, Netherlands and UK as well as from the USA.

Pattern metallization – AIMCAL Fall Conference preview

For those of you interested in pattern metallization there are three papers that will be of interest to you at this years AIMCAL Fall Conference in Myrtle Beach, USA, Oct 19-22. 

The first of these has been long awaited and is from Nick Copeland of Bobst General Vacuum and is about their ability to build in pattern metallization that can be kept in-register with existing patterning on the web.  This becomes important where webs have been either pre-printed or pre-embossed with a pattern.  Using a reference marker they are able to positioning of the web to keep the metallization pattern in register with these existing patterns to a high degree of precision.  This increases the flexibility of the process and allows it to be used in a number of the more challenging markets including security markets.

            The other two papers are both by Leybold Optics, the first by Christopher Schmitt on the measurement of patterned coatings and the second by Anye Chifen on modifications to the oil masking process to reduce the oil required and improve the quality of the patterning process.  The measurement of patterned materials can be a problem. Most measurement systems are taking an average measurement either optically they are averaging the light passing through an area or electrically they are averaging the conductivity based on an area of coating. In either case if the area is varying because of changes in the pattern shape the measurement will also be changing. If the measurement is changing it is then difficult to use this measurement as a feedback signal to control the deposition process and so control the coating thickness.  In this instance they have developed a method of taking Optical Density (OD) measurements from a very small area and so are more easily able to choose an area that is not fluctuating in size.  The second paper describes a different method of supplying the oil to the oil masking process. Typically this is done by heating the oil to vaporize it inside the vacuum system but in this case a metered flow of oil is injected into a hot evaporation box inside the system where the oil can be vaporized and via nozzles onto the web to provide masking.

            So for anyone either already involved in pattern metallization or those who are thinking about adding this to their skills there are three more good reasons for attending the coming conference.

Flake pigments – AIMCAL Fall Conference

Just to whet your appetite for the Fall Conference in some of the future posts I will highlight some of the papers that are expected to be given later this year at the AIMCAL Fall Conference in Myrtle Beach in the USA (Oct 19-22).

For this first tempter I will tell you a little about a paper being presented by Craig Engle of the South Western Research Institute (SWRI) on nano-platelets.  First let me say a little about the topic of nano-platelets.  This covers a wide range of materials that can vary in size range quite considerably. The platelets you are all probably most familiar with are those that are used in inks and paints.  Aluminium inks and paints have improved in reflectivity and brightness over the last few years.  The original pigments were balls of aluminium that were put in a ball mill with some very hard spheres that would flatten the softer aluminium into flakes. These flakes were not perfectly flat but were much more wrinkled and are often referred to as cornflakes because of the similar appearance. The next improvement was to polish some of these cornflakes and make them smoother. These too were not perfectly flat but were more lenticular in shape because the flakes would rock as they were being polished. The latest method of producing flakes is to coat a release layer onto a roll of substrate, often polyester film, and vacuum metallize aluminium onto the release layer. This metallized film is then passed through a liquid that dissolves away the release layer freeing the aluminium up. The aluminium is very thin and the film breaks up into flakes.  These flakes are as flat as the polymer film and so the reflectivity and brightness are far superior to the other methods of flake manufacture but the vacuum metallizing is a more expensive process that the simple ball milling.

            This paper from SWRI adds some new developments onto the manufacturing process.  The standard method of producing flakes in this way does nothing to control the size and shape of the flakes. The flakes produced will be random in shape and the sizes will be from hundreds of microns across to fines that are dust.  To make the flakes useful they have to be sized or milled and sized with the different size ranges giving different optical properties when they are used in the inks or paints.  Where these high aspect ratio flake materials can be used in other applications there is a preference that they are not only size more uniformly but also the shape is more regular than random.  What SWRI have done is to modify the surface that the metallization is done onto so that they can control the shape and size. Also as is implied by the title they have done this at a very fine level so that they can control the flakes down to the nanometre level.

Delamination of holographic film.

We are manufacture solvent bais holography film.
After holography tape test is 95% pass but we are facing the continuous delamination problem

Answer

The tape adhesion test does not measure adhesion it only measures lack of adhesion.  If a coating comes off with the tape test it has very bad adhesion.

If the tape does not bring off the coating all it proves is the adhesion is anything from just a little better than the tape adhesive all the way through to adhesion such that the failure plane will be a cohesive failure within one of the layers and not at the interface.

I would check your deposition process to see if there has been any type of pre-treatment to the polymer film before metallization. If there is no pre-treatment then using a pre-treatment could improve the adhesion. If you are using a pre-treatment then there are two possibilities. One is that the treatment is not enough to maximise the adhesion and the second is that the treatment is too much and the surface has been damaged such that you have gone past the peak adhesion to a lower level off adhesion.

If you have a pre-treatment then review how the level of treatment was optimised. If it wasn't optimised then start from a low level and increase the treatment to see where the optimum level is. This will probably require using the lamination to test the adhesion level as the tape test does not have the range of adhesion to give any meaningful answers.

Solvents are often a problem in causing delamination but it often only happens where the adhesion is already not as good as it could be. The better the coating is bonded to the substrate the more difficult it is for the solvent to migrate into gaps in the interface.  If the adhesion is poor the solvent can migrate into the interface and any swelling will put a force on the interface encouraging delamination. Defects in the substrate surface or coating can often be traced back as the source of delamination. The defects have poor adhesion associated with them which allows in the solvent and the delamination spreads from there.

The same can be true of pinholes where after lamination the air in the pinhole can expand and the pressure of the volume change can start a delamination if the adhesion is not good enough.

Question about too much (?) cooling

I want to know the effect of over cooling during metallization, how does the cooling impact on bond strength and other quality parameters of metallized film.

Answer

When the substrate enters the deposition zone it is heated up by the radiant heat from the red hot evaporation boats and well as the heat of condensation of the depositing metal. What you are achieving with the cooling is to reduce the temperature rise that the film reaches.  Without any cooling the film would typically exceed 100 degrees Centigrade and for a thick coating at high speed could even exceed 150 degrees Centigrade.  By using a cooled deposition drum the substrate is pre-cooled often to subzero temperatures and even for the same rate of heating this will reduce the maximum temperature by the same difference as the minimum temperature is below room temperature. So if room temperature is 20 deg C and the deposition drum uses a glycol/water mix and allows cooling down to -20 deg C then the difference is 40 deg. So if the maximum temperature the film was reaching without a cooled deposition drum was 125 deg C then the maximum temperature with the cooled drum would be 85 deg C.   In reality this would be better than this as the film continues to be cooled throughout the deposition zone and not just pre-cooled.

The cooling continues after the deposition zone as the film is still in contact with the cooled deposition drum and this brings the temperature of the film back down to room temperature.

There is the possibility that the cooling available brings the temperature back down to below room temperature. Usually the film as it then passes over other rollers, which are at room temperature, will equilibrate to room temperature before it is re-wound into a roll.   If the film does not equilibrate and is wound cold then it will have the problem that as it rises in temperature on the roll it will loose some tension as the polymer expands and during the temperature change not all the layers will be at the same temperature and so moving it at this time can lead to slipping of the layers (telescoping).  This is also true of rolls have been stored in a cool warehouse and moved into a warmer area for deposition.

In terms of the deposition process the difference in temperature between an un-cooled film and a cooled film at the point of nucleation is only around 40 degrees and so there will only be a small difference in the nucleation size, with the hotter film having the slightly higher crystal size. Similarly with the continued growth the hotter film would tend towards a larger crystal size over the cooler film.  But this will be marginal compared to the difference that can be created by plasma treating the film surface to change the surface energy and hence the wetting of the film.

Adhesion, it could be argued, would be better for a cooled film as the differences in thermal contraction of the polymer and metal will be lower from the lower peak temperature.  The coefficient of thermal expansion for a metal is much lower than for a polymer and so as the temperature is reduced the polymer shrinks more than the metal coating does.  This can put a strain on the interface. If the temperature peak is reduced there is less contraction and so less strain on the interface. Adhesion failure occurs when this interfacial strain exceeds the adhesion and so minimising the strain is useful.

I hope this answers your question

Coefficient of Friction (CoF) of plain & metallized films

Need to understand the factors influencing the COF values Plain: Plain of Metallized Polyester films. The observation is that after metallization the COF values change (generally increase) as compared to the pre metallization values.
1.Can this value be controlled?
2.Does this also depend on the OD to which the film is metallized?
3.How significant is the Film: Film cof of the Metallized Polyester in the subsequent Converting operation?

Answer

The Coefficient of Friction (CoF) is dependent on the materials in contact, the surface energy and the contact area.  So for your polyester film you will have a CoF of the polyester in its natural state which is likely to be high (probably >0.5) and in this state the polyester is likely to ‘block’ (stick together) when rolled up.  Blocking is where the film sticks together and is difficult or impossible to unwind. This can be made worse by electrostatic charging of the film and surface cleaning which increases the surface energy.

To reduce the propensity of the film to block and to make the film more easily handled there are a couple of methods used.  This first is to add fillers which will change (increase) the surface roughness which, on a microscopic scale, reduces the amount of two surfaces that are in contact. This use of fillers can reduce the CoF down to 0.3 – 0.4 which is still regarded as too high for optimal film handling.  The fillers are not without problems. Typically the haze increases by 0.4-1.0% per 1000ppm of silica filler used.  To minimise the haze the film can be made as a co-extruded film with only one thin layer being filled which reduces this effect although because it changes the surface roughness does not eliminate it.  Also changing the type and size distribution can reduce the haze.  The danger is than in reducing the haze the CoF will once again begin to rise.

The other common method of reducing the CoF is to add a slip agent to the polymer. These slip agents migrate out of the bulk of the polymer to the surface and reduce the surface energy and CoF. As little as 0.1% slip agent into the bulk polymer may reduce the CoF down to less than 0.2 where 0.16 – 0.2 CoF is regarded as providing easy handling.  The slip agents make the handling better but can be a disaster for obtaining good adhesion.  One of the reasons for treating the surface by flame, corona, atmospheric plasma or vacuum plasma is to remove or crosslink in these low molecular weight low surface energy materials to both increase the surface energy to obtain better coating wetting as well as remove the weak boundary layer that would become the failure plane at the interface.

The surface treatments do not change the CoF that is produced by the use of fillers resulting in the change in surface roughness.  How ever as mentioned above the use of fillers alone usually does not produce a low enough CoF to make handling the film easy.

When the film is metallized it is most likely to have been surface treated to improve the wetting and adhesion of the metal coating and so the metal surface will no longer have any low energy material present and so the CoF is likely to have increased. 

Even if a slip agent is not included in the bulk polymer there will be unpolymerised monomer that will appear as oligomers on the surface as low molecular weight material that can have a similar effect. These oligomers will also be removed by any surface treatment.

What you do have is the back surface of the film that is most commonly untreated and so there will still be the low molecular weight material on the back surface, either slip agent or oligomer.  As the freshly metallized surface has a very high surface energy the film when rewound will bring together these two different surfaces.  The low surface energy material will want to migrate to the high surface energy metal surface and so over time you can plot the reduction of the metal surface energy as more of the low molecular weight material, which is very mobile, is transferred across to the high surface energy metal surface. Given enough time these two surfaces will approach the same surface energy.

In general the optical density (OD) of the metal coating does not affect the CoF. The metal coating is nominally conformal and so will have the same surface roughness as the surface it has just covered.  This is true for most coatings but if you deposit an extremely thick coating ~1 micron thick or thicker it is possible to get selective crystal growth that can increase the surface roughness which will cause the metal surface to appear matt. Thus if you coating still appears to be of mirror quality then it is likely to be thin enough that there is no significant crystal growth and hence the surface roughness of the metal should match the polymer surface roughness.

Finally the question of significance of the CoF to film handling relating to downstream converting processes.  This is a one of those questions that it is virtually impossible to answer.  There will be a different answer depending on whether the process is a simple lamination process or it is a wrapping or filling process.  In the lamination there will probably be fewer rollers and less contact with the film whereas with the latter process there will be more rolls to help make folds and change the direction of the film movement which will make the film more sensitive to the CoF of the film. This may be one where you have to try the film and see if there is a problem.

Please note that in checking the film you also need to time the gap between metallization and use on the next process.  Ideally you would also record the temperature too.  The reason for this is that if the film is used the same day as metallization it may have a handling problem but the same film if left for a week, possibly at an elevated temperature, may no longer have a problem on the same machine.  The reason for this is the reduction of the surface energy of the metal.   This is done by the transfer of low molecular weight material onto the metal surface.  The rate of transfer of material from the bulk or across from one surface to the other is increased at elevated temperatures as the low molecular weight material becomes more mobile.

One word of warning, if you try to lower the surface energy be careful, as this could cause problems of adhesion to other surfaces.  There can be a trade-off between handling performance and adhesion.

I hope this helps.

Water Vapour Transmission Rates

I am from Venezuela and I work with Packages. I am very interested to find information about Water Vapor Transmission Rates for different materials like PET, BOPP, PETmet, BOPPmet & Alu foils & compare their WVTR depending on their calliper & environmental conditions such temperature (30°C and 40°C for example). Do you have any kind of info related with that?

ANSWER

A wide subject to deal with.

Let’s start with the polymer. 

The same polymer can have widely varying properties depending on the polymer processing.

The composition of the polymer can vary from company to company and even within the processing company depending on what the end application is meant to be. The polymer can be free of filler or can include fillers and this will change the barrier performance. The fillers themselves can affect the barrier performance depending on the type of filler the shape and size of the filler. Some fillers are almost spherical whilst others are more flake-like. These flake fillers if oriented will provide a much higher barrier than the spherical fillers even of the same material.  If the polymer forms crystalline regions during processing the crystalline material will be a higher barrier than the amorphous regions and so the degree of crystallinity will affect the barrier performance. 

Thus it is not uncommon to find many different values for the barrier performance of the same material.  All that this means is that there is insufficient detail of the material to discriminate between them.

Coatings

The same kind of variability can also apply to coatings.  The performance of the coatings starts with the substrate. Initially there is the question of how clean the substrate surface is, as debris can lead to pinholes which are a source of reducing the barrier performance of the coatings.  The use of additives in the polymer is another source of problems. Many additives migrate to the surface and adversely affect the adhesion which can lead to adhesion failure or delamination problems. Common ones are slip agents that are used to reduce the coefficient of the polymer to improve the handling characteristics.  Next there is the use of pre-treatments which covers everything from proprietary coatings, added by the film manufacturers that are sometimes used for adhesion promotion, anti-static or handling improvements, through to corona, flame, atmospheric plasma and vacuum plasma treatments as well as in-vacuum polymer deposited planarising layers.  Most of these will have an effect on the nucleation, growth and adhesion of any coating that is deposited.  The quality of the coating deposited can be hugely affected by the surface that they are deposited onto. If the surface energy is low then the coating will not wet the surface poorly and the coating will have to be thicker to become continuous and so this is likely to be a worse barrier than if the surface energy was high which would encourage the depositing coating to wet the surface well and hence produce a continuous coating at a reduced thickness.

In addition to the vacuum deposited coatings there is also to possibility to add a polymer coating afterwards. If this is done in vacuum as part of the same process than there are two ways in which this coating can work. One is that the polymer will wick down the grain boundaries and any pinholes and fill them up. The polymer will have a lower diffusion rate than if they were left as voids which could be equated to having the same diffusion rate as air. The second way the polymer works is that it helps to lock in any remaining debris that if moved would produce new pinholes and also, in the case of aluminium, it protects the soft aluminium from scratches.

Materials

There are two basic classes of materials that are used to provide barrier organic and inorganic. The simplest structure is a single layer of material. If we look at the organic materials most polymers do not provide enough barrier performance to meet the requirements.  The inorganic materials can provide suitable barrier but are not suitable for other reasons such as metal foil is not transparent, it may be too stiff for the packaging type and it does not allow for product screening or allow food to be cooked using microwave ovens.  This then directs us towards a multilayer structure which provides us with a Pandora’s Box of options.  We can produce multilayer materials by co-extrusion, extrusion coating, lamination, coating or vacuum deposition.  Also we are not limited to simply a second layer but can have multiple layers such as the seven layer co-extruded barrier polymers.  This does not necessarily mean seven different materials it could be as few as two materials or as many as four.  The use of filler in the polymer allows us to combine both organic and inorganic materials.  These fillers can be of various types and shapes (more variables) and can be added into the polymer as part of the film extrusion process or into the co-extruded layer or to extrusion coating or other coating processes.  The addition of vacuum deposition coatings allows very thin flexible inorganic coatings to be deposited directly onto polymer substrates. Here too there become various options. It is possible to not only deposit the inorganic layer but also to deposit organic materials in the vacuum.  This in-vacuum organic layer can be used as a planarising layer to clean and smooth the substrate surface helping to improve the quality of the inorganic barrier material.  A second organic layer on top of the inorganic layer can protect and seal the inorganic layer and also improve the barrier performance of the basic inorganic layer.  Many of these structures are then added to by lamination before the final barrier structure is achieved.

            Although this final structure achieves the desired barrier and other properties it does have one drawback and that is that it becomes difficult if not impossible to recycle.  In the past this has not been an issue but in many countries this is becoming significant and packaging that cannot be recycled is under pressure to change and in some cases financial penalties can be imposed.  Thus there is now a trend to either down gauge to minimise the weight of non-recyclable packaging or a change in materials to make the structure either recyclable or contain a high proportion of recycled material.

            The inorganic materials that are used currently tend to be from a relatively small range of materials.  The most widely used material is aluminium that is used for opaque barrier products. This is deposited by thermal evaporation from resistance heated aluminium wire fed boats.  This material has been developed over the last fifty years and provides the combination of low cost relatively low deposition temperature, high deposition rate and a stable coating in thin layers.   When choosing a transparent material it was thought that as glass is used as a transparent barrier material why not use it in thin film form for the same purpose. Thus methods of depositing silica were developed for transparent barrier coating onto polymer substrates. This included resistance, radiant or induction heated thermal evaporation, electron beam evaporation or plasma enhanced chemical vapour deposition.  All of these processes work but have different cost structures.  Electron beam deposition and plasma enhanced chemical vapour deposition appear to be the two most favoured processes. Other transparent inorganic materials have been used such as alumina where in one case a standard aluminium metallizer was modified to oxidise the aluminium post deposition thus converting the opaque aluminium to the transparent alumina. 

            This biggest challenge to inorganic barrier coatings is the cleanliness of the substrate polymers.  All polymers attract dust to the surface due to the generation of electrostatic charging of the surface as the polymer travels over rolls of dissimilar materials.  This debris is very hard to remove and although the size of the debris is too small to be visible it is still much larger than the thickness of the inorganic coatings that are deposited. This debris is often moved after the coating has been deposited and this results in a pinhole on the coating being exposed. These pinholes are the reason why few of the inorganic coatings meet the theoretical barrier performance.

Silica and alumina are not necessarily the best barrier materials but until the problem of pinholes has been overcome there is no benefit in changing to a possibly more expensive inorganic material.  Where planarising layers have been used other inorganic materials have been used such as compounds of silica and silicon nitride that are graded into each other or indium tin oxide or tin oxide.  These have all been shown to have superior barrier performance but only if the pinholes are eliminated.

Silica and various silicates are probably the largest class of material that is used as a flake as a filler to produce filled organic/inorganic coatings.  These fillers instead of being spherical naturally form flakes and as flake aspect ratio is large (often more than 100:1) and the coating thickness small the flakes are forces to lay down parallel to the substrate surface. These flakes will overlap each other and so and gas or vapour that wants to migrate through the coating has a very tortuous path to go round each of these flakes. It is this tortuous path that slows down the diffusion rate and is seen as a barrier improvement. 

This tortuous path idea is used in other ways.  If a metallized substrate does not have sufficient barrier performance it is possible to metallize the other side and further improves the barrier performance.  The improvement may be more than the expected. The reason for this is that although each of the metallized coatings may have pinholes in them statistically the pinholes are virtually never directly opposite each other and so the path between pinholes is tortuous.  This can be maximised if the layer between the metal layers is minimised and so if two metallized films are laminated to each other with the metal coating facing each other the performance can be expected to be better than if the same substrate is metallized on both sides.       

Barrier performance

            Below is a table of the barrier performance of a variety of materials from single polymers to coated, coated/filled, and vacuum deposited coatings.

            Note that these are guidelines only as these values can vary depending on the manufacturer or converter and the quality of the substrate and coatings.

Film grade		Thickness	Oxygen transmission	Water Vapour	Deposition Process	Film Composition
		microns	cc/sqm/day.atm	g/sqm/day
XM 1123	1 side coated PVOH   (chlorine free)	13	0.2	31
D 866	1 side aqueous PVdC coated  other side added print adhesion(813 type) pasturisable	14	7	7
D 887	1 side aqueous PVdC coated   pasturisable	14	7	7
D 888	1 side clear barrier (chlorine free) - other side added print adhesion(813 type)	13	1	25
D 889	1 side clear barrier (chlorine free)	13	1	25
M 30	2 side solvent coated PVdC.   Coating heat sealable	14	8	8
M 34	1 side solvent coated PVdC.   Coating heat sealable	14 or 21	9	9
M 44	1 side aqueous coated PVdC	14	8	8
M 45	1 side solvent coated PVdC film based on P25 thermoformable base	14 or 21	6	6
MC 2	1 side metallized film overcoated with PVdC on both sides. Coating heat sealable	14	0.15	0.6
CS	1 side PVdC coated special film for packaging individual slices of cheese	13	16	8
200RSBL300	Special PET barrier laminate for vacuum  insulation panels	61	0.00062	0.005
	Mylar M series are typically adhesive laminated to PE
	D series are the platelet filled coatings based on mica type materials
	Melinex 813 has chemically modified surface for adhesion to nitro-cellulose based inks
	PET / Blank	12	100	64.64
	PVDC	24	8	0.3
	EVOH	24	0.16 - 1.86 *	N/A
	m-OPA	15	30
	Aluminized PET (single)	~ 30 nm	0.31 - 1.55	0.31 - 1.55	Evaporation	Al
	Aluminized PET (double)	~ 30 nm each	0.03	N/A	Evaporation	Al
	Aluminum on PE	7 microns Al	0.001	N/A	Laminated	Al
	SiOx on PET	10 - 80 nm	0.35 - 10**	0.46 - 1.24	Evaporation	SiOx
	SiOx on PET	10 - 80 nm	0.08 - 1.55	0.5 - 5.0	PECVD	SiOx
	Al2O3 on PET	20 nm	1.5	5	Evaporation, Reactive Evaporation	Al2O3
	Al2O3/SiOx on PET	50 nm	2.0 - 3.0	1	Evaporation	Al2O3/ SiOx
	Diamond-like Carbon on PET	20 nm	2	1.5	PECVD	Diamond
	* depending on relative humidity & ethylene content  - ** depending on used process
	Bare PET (Toray, 12mm)	-	175	41.4
	AlOx (eb)	17	4.7	1.04
	AlOx (sputt.)	17	3.75	0.78
	SiOx (eb)	40	3.35	1.06
	ITO (sputt)	24	0.65	0.58
	Bare PET (Melinex, 50mm)	-	36.33	3.86
	AlOx (sppa)	31	1.91	0.57
	AlOxNy (sputt)	57	1.96	0.11
	Bare PET (RNK, 12mm)	-	180	36.7
	AlOx (PAPVD)	10	4.8	1.61