Vacuum Web Coating

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.

To buy new or to buy second-hand and refurbish.

The purchase of a machine is a large event for many companies as the capital investment is high and it is important to make the right decision.  There has always been a discussion about reducing the risks.  One viewpoint is that by reducing the cost by buying a second-hand machine the business risk is reduced. There is an opposing view that by reducing the cost in buying a second-hand machine the business risk is actually increased as the performance may not match that of a new machine and the time for refurbishment may exceed that of delivery of a new machine as well as the risk of a premature system problem from an age related failure. 

            The decision can depend on many factors such as the availability of suitable second-hand machines, the availability of a company skilled in refurbishing vacuum systems, the process being compatible with the system or what compromises might have to be made as well as the cost and delivery comparison against buying from new. 

Metallizers are easier to refurbish than some of the multilayer deposition systems that tend to be more customised.   The changes and up-grades to metallizers tend to be small.  If you compare metallizers of twenty years ago to current ones they are remarkably similar.  There have been more changes in the electronics to power, monitor and control the system than significant large mechanical changes.  In fact in the future it is likely that everyone will have to regularly upgrade the electronics or more specifically the computer control of the systems as the chips & operating systems become obsolete. This would suggest that more companies will be more familiar with refurbishing systems and be less worried by the process.

            Where the risks appear in buying an old metallizer and up-dating it can be in things such as welds that may be flexed as the system is pumped down and brought back up to atmosphere.  These welds can fail due to fatigue failure over time and it is almost impossible to predict when one of these failures will occur. What is certain is that older machines are more prone to this type of failure than newer machines and therefore the risk of this type of failure goes up.  If this type of problem occurs it will appear as a leak and this will take time to trace and once found will require a skilled welder to rectify the problem.  This can cost the loss of time and possibly an amount of film produced with a lower reflectivity before the problem is diagnosed.  This risk has to be estimated and added to the rest of the factors. Often it is almost an indication of how optimistic the person is in estimating this risk as to how significant it is judged to be.

            Another factor tends to be the delivery time.  Often it is expected that a refurbishment will take less time than building a system from new. However this is not always the case as the system may need to be shipped to whoever is doing the refurbishment, fully stripped down, cleaned and re-built with appropriate changes. Until the system is stripped it may not be apparent how much needs to be replaced or repaired.  The shipping, stripping and assessment may take longer that simply getting acceptance of the drawings for a new machine and ordering the parts.  As metallizers are regular items many have fairly standard sets of drawings with only minor variations and so do not have to be designed from scratch thus saving a large amount of time.  I personally have been involved in a project where a number of people in one company had a preference for buying a second-hand machine and refurbishing but eventually were persuaded that buying new would be quicker and also cheaper.  In this case it proved to be the correct decision.  The system was so old that the refurbishment estimate came to more than the quote for a new machine and the delivery time for then new machine was also more than one month quicker.

            Vacuum coating machines other than metallizers that use other deposition technology such as magnetron sputtering, electron beam deposition or chemical vapour deposition have other considerations.  These machines are generally much more expensive that metallizers and so the accountants always are interested in opportunities to save money.  It is a great temptation to compromise on the performance and even the process to allow the purchase of a cheap second-hand machine.  Usually this is a very poor decision.  Unless the machine is meant to be a research machine, where there are few expectations of productivity and reliability, it is usually found that there are large problems in operating the system well and productivity is poor compared to a properly designed optimised system. Again I have had experience of having to run a process on a system never designed to run such a process and it was always a struggle to keep on top of the process and produce products within tolerance. The system was also much more difficult to maintain, both in routine cleaning as well as routine maintenance. Part of the problem was that there was not enough space to get easy access to different areas and components making every task slower and more difficult.  In the long term this was a much more expensive system because of the poor productivity, longer down time, and variable product.  The rule-of-thumb learning from this was that, more often than not, compromise costs money not saves money.

            It will be interesting to see how things change in the future.  As the prince of energy is rising rapidly it could be that the design of systems will change to save energy in the operation of the system.  Already companies are looking at different sources to change the collection efficiency from ~50% to > 95% which in effect halves the cost of metallizing as well as reducing the scrap from cleaning shields.  This type of change, when it becomes available, may not be easy to retrofit into existing systems and so the economics of refurbishing or buying new will be very definitely in favour of buying new.   Materials costs are also rising as it costs energy and hence more to mine and refine materials and so using materials in a more sustained way will become more important.  This would suggest that refurbishing would be preferred. One way of achieving this is for the system design to start with the expectation that the system will be continually upgraded every few years. This would require a better and closer collaboration between manufacturers and customers with long term contracts for the periodic upgrades.  This would however give the customers a route to any technology and process improvements so that they remain competitive for longer before each new system purchase. This will obviously change the business for the system manufacturers as they will have less income from system sales but interim income from system upgrades.

            What is very clear is that the costs of buying new will never be as cheap as it is now.  As energy costs continue to rise, which has the knock-on effect of pushing up materials costs and so increasing inflation and hence a rise in labour costs, so too will the cost of new machines. So if any of you are considering buying a new system the sooner the cheaper.

            As ever this is just my opinion formed from my experiences. I am sure others will have different experiences which give them a different perspective. Please feel free to post these experiences and your purchase preferences. It would be good to hear different views. 

Gas and Moisture Permeation Measurement for High Barrier Films

You may be interested in participating in this one-day workshop being organised by the Printing and Graphics Science Group of the Institute of Physics and the UK Displays and Lighting Network.  Please pass the information on to your co-workers if it is relevant.

Gas and Moisture Permeation Measurement for High Barrier Films
at
Institute of Physics, London, UK
on
Thursday 25th September 2008

There are many instances where flexible films are being used because of their conformal nature or transparency. While many materials provide near ideal optical and mechanical properties, their barrier properties often need to be improved to provide useable product life. For example in plastic electronics, many of the promises of flexible OLEDs and organic PV can only be realised by having a substrate with superior barrier properties. This raises the issue of barrier measurement since any measurement systems needs to have at least 1 order of magnitude better resolution than the barrier properties being measured.

This joint workshop aims to improve understanding in the area of barriers and their measurement.  It will be of interest to scientists developing barrier layers for films, and using or improving permeability testing equipment.

The meeting will include presentations from:

* Holgor Norenberg, Technolox

* Hazel Assender, University of Oxford

* Stephane Cros, CEA, France

* Sharon Louch, CPI

* Jasper Michels, Holst Centre, Netherlands

The programme will have space for short presentations or comments from delegates.  If you wish to contribute in this way, please e-mail Eifion Jewell of UKDL at eifion@ukdisplaylighting.net

We hope to provide an opportunity for delegates to get together informally on the evening of Wednesday September 24th.  Further details will be announced later.

For details regarding the event please see the workshop web page, or contact Eifion Jewell of UKDL at eifion@ukdisplaylighting.net 

Registration

To encourage the widest possible attendance the cost of the event has been kept as low as possible at £50 +VAT for Academic delegates and Institute members, £75 +VAT for Industrial delegates and £30 +VAT for students.  The fee includes lunch and refreshments.

To register contact the UKDL Team on info@ukdisplaylighting.net or call +44(0)1908 276665.

Please note the attendance cost on the registration form is an early bird discount. An additional surcharge of £25 will be made to bookings made within 5 working days of the event.

The venue

Institute of Physics, 76 Portland Place, London, W1B 1NT

For directions, see http://www.76portlandplace.com/location/

We look forward to seeing you there.

Kind regards

Dilwyn Jones, Printing and Graphics Science Group Chair

Eifion Jewell, UKDL

System cleaning

System cleaning as a topic is raised from time to time but often for different reasons.  One of the common problems is the accumulation of stray deposition that is very porous and so absorbs moisture and because of the very high surface area of the porous material is easily capable of slowing down the pump down times.  In fact some use the slowing pump down time as the trigger to tell them when to clean out all of this stray deposition.  Other work differently and clean the system down as a regular event so that they are always minimising the pump down time.

There are other aspects to cleaning not least of which is the relationship between pinholes and cleaning.  If the stray deposition is cleaned off in-situ there will be a large amount of very fine debris produced.  This debris can settle onto surfaces and if left these will be stirred back up again during pump down and can become the cause of additional pinholes by contaminating the web and the rolls.  Even if the cleaning is not done in-situ the detachment of the shields can produce some debris and this too can end up as contaminant and result in pinholes. Using vacuum cleaners to reduce the debris can help but a physical wipe of rolls is advisable as a final action.  It is worth noting that even small details can affect the cleaning process. I have often seen operators use a cloth that they wet with a solvent by bringing the rag to the solvent bottle top and upending the bottle to wet the rag.  The effect of this is that the debris already collected on the rag gets washed into the bottle and the whole bottle of solvent becomes contaminated.  With time the level of contaminant in the solvent increases from the repeated wetting of the contaminated cleaning rag.  The better method of wetting a cleaning cloth is to use a dispenser that squirts some solvent onto the cloth thus preventing anything getting back into the bulk solvent.  Also using disposable wipes rather than prolonged use of the same cleaning cloth can be beneficial.

This might sound like a fine detail but not having clean roll can have a major effect on the metallization process.  Not only can the debris be transferred onto the web and result in pinholes but also if the debris is transferred onto the web but on the back side it can result in the web being lifted from intimate contact with the cooled deposition drum which can result in the start of tramlines (railroad tracks).  Work done by Mike McCann and Dilwyn Jones showed that it only took debris of a diameter of 5 microns to start a tramline. As the web is prevented from contacting the cooled drum it has a localised higher temperature which increases the thermal expansion which results in the web buckling and the formation of the tramline.  As it is impossible to see debris of such a small size it means that if you wipe any surface or roll and can see any contaminant on the cleaning wipe at all it means that there will be a huge number of smaller debris that you cannot see all of which potentially can cause problems.

So you can see how cleaning becomes important both for the removal at both the gross level as in the stray deposition but also at the fines level as described above.

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

Low bond strength

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

Answer

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

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

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

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

A problem of curl in metallized paper

We are facing a problem of curling in the metallized paper.

After top coating the metallized paper, the paper curls.

We have tried to give steam on the back side of the paper just before the rewind station.

At that time, it seems that the paper is not curling.

But after a couple of days, after slitting the paper if starts curling.

1.What is the best way of measuring the curl in a paper?

2.What is the sureshot way of producing curl free paper?

3.Does giving steam helps in reducing the curl in the paper?  If so, how much moisture should be added in the paper? Should we add some other material in the steam?

4.What can be the different causes of curling in the paper?

Please guide us on these issues.

Answer

A simple way of measuring curl is to cut a thin strip of material and let it rest on its edge on a flat surface and measure the radius of curvature.  This can mean cutting strips at an odd angle to get a sample that only has a curve and not also a twist in the strip. The curvature allows you to calculate the bending stress on the strip.

Curl is a measure of a difference in stress between two layers.  This is most commonly caused by the swelling and drying of one or more layers of material.

The first thing to do is to establish the sense of the stress.  If the metal surface is on the inside of the curve then the coating added on top of the metal would appear to have contracted.

If the curvature is such that the metal surface is on the outside of the curve then it is the paper that has contracted most causing the curvature.

If it is the coating on top of the metal that has contracted then this might be caused by the coating drying too rapidly so that the coating cures from the top surface first and then through the coating thickness. This solidifies the top surface as a skin and then the solvent has to be extracted from below this skin and as it is removed the lower levels want to shrink but as the top surface is fixed it puts stress into the coating.  If the drying is slower the solvent has time to migrate out from the depth of the coating with the solids settling down to the surface and this reduces the stress in the coating.

If the problem is a problem of shrinkage of the coating then adding water to the paper could be helping and is something that is worth checking out.

If you reduce the water does the problem get worse and start sooner?  If so then it might be that adding more water will swell the paper more before the coating is added so that as the coating shrinks the paper shrinks a similar amount.

If the curvature is the opposite direction with the metal surface outside the curvature it may be that the addition of moisture is the problem. As the water is dried off possibly more slowly than the coating is dried it contracts more than the coating and thus the curvature is the opposite direction. Again the test is to reduce the water addition and see what happens to the curvature.  What you are looking for is a balance to the shrinkage over the longer timescale.

Bear in mind that the drying rates for the paper and the coating may well be different, which is why you see no curl early on and curl later. What you want is no curl when the material is used later and so it may be that you will see curl when you test immediately after coating and drying but have to trust that the additional time and drying will improve the film to bring it too a neutral stress point with no curl.  So when you do your changes in moisture level you need to not only check the curl immediately but also plot any changes to the curl over time.

Where stress is concerned and curl has been a continuing problem at least one company solved the problem by coating both sides of the substrate with the same material so that the stress remained balanced. This was a drastic solution but in their case cost was not an issue and it did stop the problem. 

I hope these suggestion help.

New type of deposition source.

In the past I have advocated using a slot deposition source instead of the more standard resistance heated evaporation boats.  The reason for this is that the deposition efficiency of the standard boat sources can be anywhere from 35% - 60% depending on the geometry adopted.  This is poor compared to using a slot source where the deposition efficiency can easily be >95%.  This high efficiency deposition improves costs not least because the stray deposition is minimised and so the system cleaning can be reduced and the scrap is minimised.

The lack of interest in developing slot sources has been for many reasons. The crucible material has always been put forward as a problem.  Molten aluminium is very corrosive and is part of the reason why resistance heated boats only last 10 – 15 hours.  I have never been convinced about this argument as Mitsubishi and Hitachi both developed air-to-air systems that could operate for >100 hours using alumina coated boats.  Another argument for not developing the source was that problem of feeding the source.  In an enclosed source any hole is a potential exit route for aluminium vapour. If the source is fed by an aluminium wire then where the wire enters the source the vapour could escape and condense and potentially weld the hole shut. The alternative approach is to have the whole aluminium inventory in the crucible from the start of the process. This significantly increases the thermal mass and slightly slows down the heat up time but more significantly slows down the cooling time. This is particularly slow if there is an emergency stop and so venting the chamber can be delayed making the recovery time significantly longer.

These excuses have been enough for machine manufacturers not to take much interest in developing a slot source. However things are now changing.  The cost of energy is rising and so machine efficiency is of increasing importance. Reducing system wastes is also becoming more important as the environmental issues such as disposal is becoming more costly and recycling is preferred.  Also the high energy process for manufacturing aluminium will increase the materials costs and so minimising aluminium cost will become of more interest as costs continue to rise.

This brings be on to the new source.  This source uses the novel idea of having magnetic levitation instead of a crucible. Hence all the problems of materials choice are dispensed with by having no crucible at all.   A couple of coils are used to produce magnetic fields that pin the metal in place.  A slug of metal can be fixed in place to start the process off.  A high frequency alternating current can be used to heat and melt the metal whilst it is pinned by the magnetic field. The vapour is not constrained by the magnetic field and so can escape. By enclosing the source and leaving an escape slot the vapour can be directed towards the substrate.  The molten metal can be replenished by a wire feed. As with the evaporation boats the wire feed has to be balanced against the evaporation rate.  The wire feed still has to be managed to allow the wire to enter and yet restrict the vapour escaping. This source has been used on web substrates and the material efficiency has been demonstrated to be well in excess of 95%. 

Unfortunately this source has been developed by a user company rather than one of the machine manufacturers and so at this time does not look like becoming widely available as a replacement for resistance heated boats.  However at least one manufacturer is developing a jet vapour source, also a slot source, and this holds out more hope that this, when they finally start to publicise it more widely, will become available to those who want it.