Vacuum Web Coating

Pinholes, defects and barrier performance.

I have been reviewing the topic of barrier coatings and some of the finer details of why metallized, silica or alumina vacuum deposited coatings do not deliver the barrier performance expected.   Metal foil is used as an example of a perfect barrier and it has always been a complaint of metallized polymer films that they are not a complete replacement for foils because the barrier performance is not as good.  

 

Many years ago now there was a research programme to look at why the metallized films contained pinholes and this was proved to be due to debris on the polymer surface that was moved after metallization leaving behind a small uncoated area of film.  This uncoated film has no barrier improvement at all and it has been shown that these defects are the primary reason why any vacuum deposited coating does not produce a perfect coating and very high barrier performance.

 

What does not appear to be mentioned so frequently is how devastating pinholes can be to the barrier performance.  It only takes 5% of the metallized area to be missing in the form of pinholes and the barrier performance will be almost the same as for uncoated polymer film.

What is even more worrying is just how few and how small the pinholes need to be for them to reduce the barrier performance by at least an order of magnitude.

 

This means that even if we try to clean up the polymer films before vacuum coating it is likely that we will improve the coatings to some extent but not as much as if we could eliminate the pinholes altogether.  It seems unfair that those vacuum coaters who pay particular attention to keeping the vacuum system clean and substrate quality high may still end up with barrier coated films only slightly better than those who pay little attention to the hygiene issues.

 

In order to produce a surface with no contamination that can deliver a coating without any pinholes is hard to achieve.  If the polymer surface is cleaned by whatever means there will still be debris on the surface of a size below 1 micron or possibly as low as below 0.3 microns. Removing debris below this level is very difficult and so usually the surface is cleaned down to this level and then other measures are used to finish the cleaning process such as coating a polymer onto the substrate to cover up all the remaining particles.  This coating needs to be greater than the largest remaining particle in order to deliver a perfect surface for the barrier coating to be deposited onto.  If this polymer coating is deposited outside the vacuum system the coated surface needs to be protected from re-contamination. This includes from the atmosphere as well as from the back surface of the polymer substrate once the film is re-wound into the roll.  If the polymer is deposited in the vacuum system it too needs protecting from contamination as there can be debris contaminating rolls from previous cleaning activities.  Not only does the surface need protecting from re-contamination but it needs to be suitably prepared for deposition by plasma treating as the surface energy needs to be as high as possible to maximise wetting.  If the depositing species does not wet the polymer surface well the growing coating will be rougher and contain more holes and defects than if the depositing coating wets the surface well. 

 

The barrier performance is dominated by the pinholes bit once these are eliminated the defects in the growing coating become the next most critical target.  Thus wetting and producing a continuous coating at the thinnest possible coating thickness becomes the next most critical target. 

 

In the past the density of pinholes has been used as a measure of improvement but this is really a false target. The real target should be zero pinholes. Achieving zero pinholes will deliver the big change in barrier performance.  Anything other than zero pinholes will only achieve a more marginal barrier performance improvement.  

Base film production and problems with OTR

I have some questions about base film.

 

Can following parameters affect on OTR?

1.       Old Extruders(about 15 years old)

2.       High temperatures extrusion.

3.       High rpm of screw

4.       High filter pressure and filter mesh size

5.       Any contamination in water of cast machine

6.       Dust from TDO air blower

 

Because the film which is being produced from older plant giving higher OTR while from newer plant, its OTR is normal.

 

 

Answer

Yes the production of the polymer film can have affect on the substrate surface quality and hence on the OTR.  This starts with the polymerisation process which can determine the oligomer levels and residual moisture content.  The higher the oligomer levels the more will be present on the surface and this will make tend to make the OTR worse. 

 

Similarly dust is always a problem.  This can start as early as the polymer leaving the extruder.  Residual monomer can volatilise and condense on surfaces forming a white powder which can ‘snow’ onto the web.  The polymer web moving over rollers builds up a triboelectric charge that can attract dust to the surface.  Slitting at the end of stenters or to open up a bubble will produce dust and even with vacuum cleaners there will be some dust that reaches the web.  Thus in comparing production facilities you need to compare possible dust sources, both position and number of sources and number of rollers and position relative to dust sources.  Debris also comes from the atmosphere and so where the film line is sited is also important.  In some factories the cardboard core cutting machine is co-located with the film line and this one factor has a significant detrimental effect on the debris levels on the web. 

 

An examination of the surfaces of the webs from the different production lines can tell you if they are contaminated to the same levels.  But effectively you already know this as you have the information that the web from the older production system has a lower OTR than material from the newer production line.  What becomes more difficult is to identify the precise source of the debris as there will be contributions from many sources.  One method is to use a cleanroom type of particle monitor and sample the atmosphere down the production line and look for any regions of increased particle density. 

 

     

 

Question about protection of silver for mirrors

Cu is used to prevent the oxidation of silver in glass mirrors. Would it be possible to add a layer of Cu to the silver via PVD after the silver has been deposited on a polyester film in order to provide a similar protection mechanism? Could an acrylic film like Korad 05005 be used as the substrate instead of polyester to provide a tough yet clear surface as well as excellent UV protection? I have zero experience with PVD coatings so I am sorry if this is a silly suggestion.

 

Your suggestion is far from silly. In some cases it could be used but in most cases there are some reasons why it is not appropriate.

The silver onto PET for mirrors is generally used with the silver being used as the front surface whereas the silver on glass usually uses the glass as the front surface and so the silver is on the back and can be protected using copper and usually a lacquer too.

The use of reflectors on flexible substrates sound simple but there are more factors than are often talked about.  The cost of manufacture is one. Silver on glass is a good high performance system but it is perceived that it is much cheaper to coat a roll of polymer film than the heavier sheets of glass and the losses in transport due to breakages is much less with the polymer substrates.

The down side of this change in substrate is that the polymer is not a barrier for oxygen or moisture that glass is and so the silver is prone to degradation.  For some applications the high reflectivity of glass is not required and so aluminium can be used and the aluminium forms its own native oxide that is a hard protective layer.  Where the superior silver reflectivity is required other strategies have to be considered.  In some cases an anti-reflective over coating is used on the silver which also protects the silver from oxygen and moisture but this is a more difficult material to deposit and also is an additional material and so the manufacturing costs increase.  Alloys of silver have also been used to slow down the corrosion but this too increases the deposition cost as this needs to be a sputtered coating and the deposition rates are anywhere up to 1000x slower than evaporation.

Using different substrates has been done but generally PET is preferred for the all round performance including the film stability during the deposition process.  PET also have UV stabilised grades and optical grades that have high clarity. Again where the optical transmittance has to be maximised an anti-reflection coating on the polymer surface has been used.

Post metallisation - zero metal transfer coatings

There are various chemical coated polyester film in market which promises zero metal transfer post metallisation. They guarantee that chemical coat will not deteriorate up to 1 year. Now if we do metallising on these films, is it possible that -->

1)Metallising process affects/reduces the bond strength even if we metallize on coated side of such films?

2) If yes, then what parameters can affect the bond strength from metallisation prospective?

 

 

 

If you look at the reasons why you would get metal transfer and then think about how this could be reduced.  Metal will be more easily transferred if the adhesion is poor. Therefore the simplest method of reducing the metal transfer is to improve the metal adhesion.

 

This sounds as if the chemical coating is being sold as an adhesion promoter.  If they are selling it as an adhesion promoter they ought to have tested it immediately after production and after 1 year and be able to show you the results are the same. If they do not have the test result information then how could they guarantee the lifetime? So ask to see the information.

 

In answer to question 1, I would have assumed that to be effective you would have to metallize the coated side.  If you metallize on the opposite side then how could the coating possibly affect the metal transfer properties unless it was a very low surface energy coating to prevent metal transfer whilst stored as a metallized roll.   A simple test is to measure the surface energy of both sides of the film and to see if one side has a lower dyne level than raw polyester film.  If the coating is an adhesion promoter then yes the aim will be to increase the bond strength.

 

Improving the bond strength can be done by a variety of ways including increasing the number of bonding sites as well as increasing the bond strength of each bonding site.  Typically a vacuum plasma treatment increases the bond strength by removing hydrocarbon contamination, increasing the bonding sites by chain scission and increasing the binding energy by chemically substituting oxygen for the original carbon atoms.  Chemical coatings will be trying to do similar things. It will be the aim of the coating to get the coating to chemically bond to the polyester and on the new surface to have sufficient bonding sites that are chemically receptive to the aluminium that the bond strength will be higher than the original polyester.  This coating will often be applied during the polyester manufacture possibly even as an interdraw coating applied between the forward draw and sideways draw. This allows the coating to be applied before there has been too much time for oligomers or other additives to migrate to the surface which would be expected to lower the surface energy and act as contaminants. 

Tarnishing of vacuum deposited silver coatings

Silver has different wetting characteristic than aluminium which can give some odd effects.  If the silver coating is very thin and the substrate is not suitably treated to raise the surface energy the silver can reticulate and become patchy. This only tends to occur when the silver is very thin, probably less than 10nm, and semi-transparent.

 

Thicker silver coatings do have the problem of tarnishing. Tarnishing can be the colouring if the surface which can be everything from a light yellowing of the surface through to a deep yellow and at worst black. Tarnishing is due to the very high reactivity of the pure silver and various chemical reactions can occur.  The quality of the air in your locality can also have an effect as hydrogen sulphide reacts with silver and if the hydrogen sulphide content in the local air is high this will react quickly with the silver. Hydrogen sulphide is also present in many natural materials such as eggs, onions, wool, coals, etc. If operators have handled onions or eggs or even eaten them and perspired then touching the silver can contaminate the surface and lead to tarnishing. Similarly breathing on the silver surface can contaminate the silver and trigger tarnishing. Humans also produce chloride salts and this too can corrode the silver. Another reason to be carful not to touch or breath over the product.

 

The main product of silver tarnishing is silver sulphide. The reaction mechanisms are:
8Ag + 4HS^-  <--->  4Ag₂S + 2H₂ + 4e^-
0₂ + 2H₂O + 4e^- <---> 4OH^-

The first reaction is believed to occur in a thin film of water on the silver surface. In dry air, tarnishing does not take place. In the second reaction, oxygen acts as a cathodic species and consumes electrons as indicated in the equation. Higher concentrations of hydrogen sulphide increase tarnishing.  The humidity can be a critical factor as up to a relative humidity of 50% the rate of tarnishing is fairly constant and at a low rate. Above 70% relative humidity the rate of tarnishing accelerates rapidly.  Hence it is important that vacuum deposited roll are not over cooled and wound up cold so that when the system is vented to atmosphere there is no condensation onto the roll otherwise the silver will immediately be covered with a thin layer of moisture which will contain many chemicals some of which will attack the silver. Other things that affect the rate of reactions are heat and light. Keeping the rolls cool and in darkness will also slow down the rate of reaction.

 

This all becomes impractical for selling a plain silver coated yarn. It would be advisable to lacquer the surface to protect the surface from chemical attack. The choice of lacquer will need checking out that it contains nothing that will react with the silver.  If you are intending to lacquer the silver then the sooner it is done the better following vacuum deposition.  If you have to store the silver coated web then storing the roll in a black sealed bag and including in the bag some silica gel to keep the moisture levels down and placing it in a low temperature store would help slow down any surface reactions.

 

In some window film I know that to help stabilise the silver instead of using pure silver they use an alloy. This is usually deposited by sputtering.  If an alloy is used in evaporation then it may fractionate the different elements unless the alloy is a eutectic so if you think of trying this route the choice of the right alloy is critical.  

 

 

Aluminium Oxide transparent barrier coatings

Aluminium has long been the main vacuum coated barrier material but it does, of course, have the property of being opaque. So for those wanting a transparent barrier coating they have always looked to other materials of which the most popular has been some version of silica.  Most time this is a sub-stoichiometric version of the silica and in the case fo the plasma enhanced chemical vapour deposition (PECVD) process it also contains an amount of carbon, anywhere up to 20%, or more, depending on process conditions.

The cost of these alternative materials has been high with the lowest still being of the order 2x to 3x the cost of aluminium metallizing.  This is because of two significant costs, the capital cost of either a PECVD or an electron beam deposition system is a much higher cost than a resistance heated boat type metallizer and the deposition speed is often less than a half that of aluminium metallizing.

The one company that appeared to have solved this problem was Camvac who managed to produce an aluminium oxide coating from their modified standard metallizer.  It would appear that by them carefully introducing the oxygen at the right point and in the right quantity they could control the oxidation fo the coating but also not damage their resistance heated boats. 

Needless to say this technology has been patented and so for many years this has been the only low cost process that I have known to be operating.  However in recent times I have heard that there have been two others who have managed to develop a similar process. One I know has been checked out to make sure it was not infringing any patents and that they were clear to run production the other I do not know if it has been checked out but they are prepared to talk about the process and will be giving a paper on the process at the 2009 SVC conference in Santa Clara.  This process was developed by the Fraunhofer Institute in Dresden (FEP) in conjunction with a polypropylene converter and the Applied Materials built a machine using the technology for use in production.  The FEP specialise in using additional plasmas during deposition to activate any reactive gases and to also promote surface reactions and densification of the growing coating.  Thus I expect that this process uses that expertise to evaporate the aluminium quickly but then use the plasma to activate the oxygen to speed up the conversion of the metal to the oxide.

As aluminium oxide is less dense than the metal this conversion from metal to oxide can also help improve the barrier as it will add a compressive component to the coating, as it swells, that will close pores down and make diffusion through grain boundaries slightly harder.  If this technology is now available through one of the system builders then this could mean that the cost of obtaining transparent barriers could be due for a fall in the next couple of years as these coatings start to become available.

Comparison between CPP and BOPP

As we know CPP and BOPP exhibits different properties. Especially sealing properties are quite different. CPP exhibit better seal strength than BOPP. I request you to enlighten me on technical grounds why such difference is exhibited?

What will be the performance if we use same tool to seal CPP and BOPP?

Answer

The acronyms are as follows CPP = Cast PolyPropylene  and BOPP = Biaxial Oriented PolyPropylene.

Cast polypropylene is produced by extruding polypropylene and chilling it and so the polymer chains within the film are randomly distributed throughout the film in all three dimensions.

BOPP starts out in the same way, the polypropylene is extruded and chilled but then the polymer is re-heated and stretched in two directions hence the 'bi-axial orientation' in the description.  This stretching is done when the polymer is softened and so the polymer chains are able to rearrange themselves to some extent and so they become oriented into the two stretching directions.  This changes the mechanical performance of the polymer. The tensile performance is higher along the polymer chain length than it is across the polymer chains thus in either of the two stretched directions the tensile performance is improved but in the third axis the tensile performance is reduced.   In the ordering of the polymer chains many of them become ordered enough that they become crystalline. This too effects a change in the performance, as crystalline material is a better barrier than amorphous material.  Crystalline polymer is denser than amorphous polymer and the amorphous polymer will soften at a lower temperature than the crystalline.

Thus using the same heat sealing tool may well work OK but may need a higher temperature for the BOPP than for the CPP.

I hope this answer helps.

Question re. Aluminium degradation due to heat & light

If a vacuum metallized film has an adhesive applied to it and it is adhered to a car windshield (inside facing out) will the aluminium still oxidize? If so, how long will it take? Also, I have heard that sunlight will degrade aluminium, is this correct?

Answer

There is no straightforward answer.  For years aluminium metallized film has been available with an adhesive coating for laminating to both windows for housing as well as car rear windows.

The stability of the aluminium depends on many things including the thickness of the aluminium.  The regulations are that the front windshield has to have a high visible transmittance and so the aluminium has to be very thin and so other factors need to be well controlled for the aluminium to have a long life.  For rear side and rear windows the visible transmittance can be much lower and so the aluminium thickness can be much greater and the stability is generally greater too.

The aluminium when it is evaporated onto the polymer film needs to stick well and wet the polymer surface. If the aluminium does not do this it can have many microscopic holes through the aluminium metallic structure which make it easier for the aluminium to oxidise. Aluminium oxide is transparent and so the aluminium disappears with time.  High heat and humidity will accelerate any corrosion process.

As well as the aluminium deposition conditions and the surface quality and state of the polymer the quality and type of adhesive can also play a part.  Many of the adhesives are water based so that the film can be cut to shape dry and the fitting checked before the adhesive is activated by water and the film stuck and squeegeed to the window.   This water can be absorbed by the polymer as well as by the adhesive and if there are any defects in the aluminium it can work on these defects making them grow and become easily visible.

Sunlight can be a source of energy to accelerate various processes.  The polymer film will normally degrade with exposure to UV light but as the aluminium is a good UV filter the polymer will last longer because the aluminium is facing the sunlight and protects the polymer.

Aluminium is normally quite stable; it is when it comes in contact with moisture and possibly chemicals that corrosion can be accelerated.

Thus if you have a high quality aluminium metallized film suitable for rear windows then the film is likely to be stable for years.  Lower quality film could be expected to be reduced.  If you are considering a film for the front window then it will need to be a much thinner aluminium coating and this becomes much more difficult to get at very high quality and so is correspondingly more vulnerable to corrosion which because of the very thin coating thickness becomes much more visible.

Both sunlight and heat can speed up any degradation process.  Aluminium vacuum deposited onto polymer film is not pure aluminium. As part of the process the aluminium will contain at least 1% (and often a higher percentage) of oxide or in some cases hydroxide. The polymer is not a perfect barrier to oxygen or moisture diffusion and so more water and oxygen can reach the aluminium film from the air inside the vehicle.  Thus any existing defects are likely to grow in size with time.  Chemical reactions are speeded up with heat or other energy source.  Light contains UV energy and this can accelerate some chemical reactions.  So both light and heat can accelerate aluminium degradation.

Aluminium oxidises naturally. All aluminium has an aluminium oxide surface and it is the aluminium oxide that helps slow down further oxidation of the surface. The oxide is less dense than the metal and so wants to take up more space and so the oxide is under compression and provides a better oxygen barrier helping to slow down oxygen reaching the metallic aluminium.

With semi-transparent aluminium the thickness of the aluminium is only a few tens of nanometers thick and so small amounts of oxidation can become visible very quickly.  Thus any defects in the aluminium deposition can cause holes or cracks in the aluminium often too small to be visible to the naked eye but which can allow oxygen to pass through the aluminium.  The more defects there are the faster the oxidation of the aluminium.

Thus degradation depends on the quality of the aluminium deposition as well as the temperature the film will reach and the time it spends at an elevated temperature.

The UV part of the light can be absorbed to an extent providing energy which also can accelerate corrosion depending on the chemicals present.

Humidity also plays a critical role and high temperature and high humidity together are the worst combination.

Neither heat nor light will degrade aluminium by themselves, both require other components such as oxygen or moisture and can be further accelerated by other chemicals that maybe present within the polymer, adhesive or even migrating from the glass (such as sodium).

If comparing two films in the same situation of being laminated to the same type of glass and in the same environment the comparison can be done of the quality of the aluminium deposition.  If the same film is to be used in differing conditions then the aluminium structure and quality can be ignored and the differences in heating, and other materials (glass, adhesive, atmospheric contaminant content) need to be considered in comparing potential lifetimes.

A question about Aluminium oxidation

Would like to inquire if you have a study on vacuum metallized surfaces degradation? A case in point is what is the degradation of a layer of vacuum metallized material, if it was left uncovered, meaning to say there was no PE or LDPE layer applied over it to protect it from the elements. Also how long will the surface last and what is the typical reaction it would have. Will it breakdown into powder like substance as it turns into aluminium oxide?

Answer

Vacuum metallizing is generally achieved by using an aluminium coating.  Aluminium will oxidise very readily and so all aluminium films have an aluminium oxide surface.  The thickness of the oxide surface will depend on a number of factors such as the structure of the coating as well as the time and temperature history of the coating since the point of deposition.

Aluminium when it oxidises changes density and the lower density oxide takes up more space which puts the oxide into compression.  This oxide layer in compression helps reduce the rate at which oxygen can reach the metal and so although the first monolayer of oxide will form even whilst the aluminium is still in the vacuum system subsequent layers take increasingly longer to form.  Thus the first monolayer will form immediately, the first nanometer about 1 week, the second nanometer around 1 month whilst the third nanometer may take a year to form.

This is an idealised view of how the aluminium ages through oxidation.  The reality will be different and will also vary because of other factors.  The humidity can affect the corrosion rate particularly if the atmosphere is contaminated. Any airborne contamination will become attached to the moisture and this can form something more corrosive than simple water.

Aluminium coatings also have pinholes in the coatings and these pinholes are not simple holes pierced through a full thickness aluminium coating but are as a result of the aluminium depositing onto some debris and when the debris is moved a pinhole is produced.  The edges of the pinholes gradually thin from the full aluminium thickness to no thickness. Thus pinholes are a source of a more rapid observable corrosion as the thinner aluminium becomes more transparent more quickly than the full thickness aluminium.  This observable increase in pinholes size tends to attract attention but should be more of an incentive to reduce the number of pinholes rather than worry about the corrosion.

The structure of the aluminium can affect the corrosion rate because of the structural defects such as the grain boundaries and any voids.  These defects can be a method of wicking down moisture at an increased rate.  If the aluminium could be deposited at a single crystal the corrosion would only be possible from the front surface, however because of the crystal structure the corrosion can also take place from the crystal edges to. The surface oxide can still reduce these effects as the oxide layer builds up and the protective compressive layer is formed.  The wetting of the aluminium onto the substrate is important as too is getting high adhesion.  Good wetting and adhesion reduces any interfacial gaps where moisture can reside and corrode the aluminium from the interface.

The structure of the grown aluminium also affects the surface roughness and this can also affect the appearance of the aluminium.  If the crystal structure is coarse the surface roughness will be greater than if the crystal structure is fine. The finer structure is produced with faster deposition rates. Coarser structures are produced with lower deposition rates.  If the surface is rough then the oxide layer can start to affect the surface appearance more rapidly with the reflectance falling and the surface looking matt or even milky in colour.  Thickness also has an effect as thicker films generally have a rougher surface than thinner layers for the same deposition process.

So as you can see it is very hard to predict the lifetime of any particular coating.

It is possible to suggest some trends.  Such as higher temperature and/or higher humidity uses of metallized film are likely to show faster corrosion than for those used in lower temperature and humidity applications.

Web processing for Barrier

AIMCAL have another 2 day course in October as part of their Converting School that may be of interest to some of you.

Web Processing for Barrier
Dr. Charles Bishop

To register please contact AIMCAL via website www.aimcal.org

This course provides an overview of the technologies that can be applied to deliver barrier materials for markets such as food packaging, display, and photovoltaic (solar) applications. Initial discussion focuses on the fundamentals of diffusion and permeation to provide an understanding of the contributions and limitations of materials and processes. Advanced topics include newer technologies such as nanotechnology, scavengers, and indicators that result in "smart" materials.

Course Outline
Day 1: 8:30AM - 5PM
Day 2: 8:30AM - 4PM

• Introduction
  • The Basics of Barrier
  • Markets
• Terminology
• Techniques and standards for barrier measurement
• Materials
  • Performance
  • Blends and laminates
  • Nanomaterials
  • Selection
• Smart/active packaging
  • Indicators
  • Scavengers
• Technologies
  • Film making and extrusion
  • Coating
  • Lamination
  • Vacuum deposition
• Substrates, surfaces and quality
  • Relating materials and surfaces to barrier
  • Pre-treatment (wetting, adhesion)
  • Cleaning and barrier
  • Variation in barrier during processing

Who will benefit from this course?
Anyone working at facilities that convert or use barrier materials including engineers, designers, quality control, stability and production personnel.

Date for this course:
| October 9 - 10, Cleveland, Ohio |         contact AIMCAL at  www.aimcal.org to register