Vacuum Web Coating

Evaporation boats / crucibles

Question

Can you explain me the difference between the boat type evaporation system and the crucible type evaporation system used in the vacuum metallizers?

Which is better and how does it affect the barrier properties of the metallized film?

 

Answer

            Most metallizers use resistance heated evaporation boats. The use of crucibles is rare.

On occasions the boats are also referred to as crucibles and the two words used interchangeably.  Crucibles tend to have a deep depression in the material to contain the liquid whereas the boats only have a shallow depression or, in some cases, no depression at all. 

 

Where I have seen crucibles used has been for the deposition of silica transparent barrier coatings where the material in the crucible has been heated by induction heating and not resistance heating of the crucible.  In this case the crucible holds enough material that it does not need to be replenished during the deposition run and so there is no wire feed to the crucible.

 

Aluminium is very corrosive when molten and it attacks the boat/crucible material. The wire feed into the boat does create cold and hot spots in different areas of the boat and there is a flow to the molten pool. The wire feed is often not uniform and this also adds to the changing shape of the molten pool and it is this changing pool size that can lead to spitting and this leads to pinholes and reduced barrier.  The crucible because it does not have a feed and because the molten pool is a constant shape does not have this type of problem but it does have a reducing height of liquid which can affect the deposition profile slightly over time.

 

It is worth noting that the reduction of barrier from spitting is a minor factor and that barrier is more usually dependent upon the quality of the web surface. Dust or debris on the surface that is moved following metallizing leading to pinholes is by far and away the largest source of loss of barrier performance.

 

I hope this answers your question.

 

Barrier measurements – some causes for variation

Barrier measurements can be a source of dispute and often can exhibit a wide variation in values.  This is not surprising and hopefully I can shed some light on why there may be such a wide range in the measured values.

 

It is common to get a wide range of variation in barrier measurements even from the same substrates.  Particularly as the major source of loss of barrier is pinholes and the major source of pinholes is surface debris. 

 

I am hoping that whenever any of you measure WVTR that you are taking a number of samples from the roll and taking an average.  If not then the variation will be more extreme than if you are taking average values.

 

If you track back to see where and how the surface contamination might occur you will find that it most likely starts all the way back at the extruder and builds up all the way through the converting process to the metallizer.  The polymer will have some monomer vaporising from the hot extruder and in the stenter oven more will be vaporised. This vapour will condense into powder and will collect on surfaces and small piles of powder will accumulate until the pile becomes unstable and slips and a snow storm of powder will be released.  In the stenter this snow like powder will fall onto the moving web. As the build up and release of powder is not continuous it will produce patches of more contaminated web which will have a correspondingly worse barrier performance.   After the stenter the web will be slit which is another source of debris, particularly towards the edges.  The web is also actively attracting airborne debris because of the electrostatic charge that is builds up as it repeatedly passes over metallic rollers.  Thus you web will all be contaminated to some level but there will be areas of higher contamination and possibly isolated areas of very high contamination. 

 

Another source of contamination that commonly causes problems is from the cores. Typically cardboard cores are used. These may be plain cardboard or impregnated cardboard. The way the cardboard cores are gripped may cause damage and the cardboard to shed debris. Even impregnated cardboard cores once damaged will shed large quantities of debris. This debris, depending on the quantity shed will primarily affect the ends of the rolls.  This may only affect a few tens of metres of the roll for a core that sheds little debris but the effect can extend well beyond a hundred metres for a badly damaged core that is shedding a large quantity of debris.

 

Thus it would not surprise me if any of you found that the WVTR towards the edges of the web were always worse than in the centre although this will depend on the width and the unifying effect of the electrostatic charge.  Also if you take samples from fairly near to the end of the roll then in cases where the core is of the best quality you may be clear of any end effects but for a badly damaged core you may still be seeing the effects of a core that is shedding large quantities of debris.

 

 This is why it becomes important to take a number of measurements and average out the results for each roll.  Also it may require that you enter further into the roll to get more consistent results as well as taking the average of many samples.

 

 

As to the question of ‘what can be done?’

 

There are some things that are simple too difficult and expensive to do such as change the stenter design or adopt single pass hot air with filtration to try to remove all airborne contaminants.  Thus the web will always have some debris on it. 

 

There are some areas that can be relatively cheaply addresses such as making sure that any slitting process uses vacuum extract to try to minimise the dust escaping from the slitting area as well as electrostatic neutralisation to reduce the polymer static attraction for the dust, possibly even a positive pressure clean air hood to further help keep out the contaminated slitter air.

 

Examine the rest of your production process prior to the metallization and check to see if you have any other process that could potentially be a source of contamination and again look to protect the web wherever possible.  In one factory cardboard core cutting was carried out next to the film line so that every time the saw was operating the contamination of the web was worse. Simply moving the core cutting away from the film line improved the surface quality enormously.

 

As it is impossible to eliminate all contamination then to remove some of this contamination the web can be cleaned. This needs to be a physical clean but the use of brushes will only put scratches into the surface. The only process I have seen that does not damage the film but has a significant effect has been the use of tacky rolls.  Teknek appear to be leading the world in this technology and are being used as part of the process to produce the so called ultra barrier materials.  The tack rolls do not remove all the contamination as the adhesion of debris of 0.3 microns and smaller is through Van der Waals forces and this is high enough that it is virtually impossible to pull of the debris. Thus this process will remove debris down to 0.3 microns in size but not everything.   This may not sound to be good but it is usually a very great improvement over not cleaning at all and it should bring all the surfaces to a consistent level.   www.teknek.com  is the website if you wish to check it out.

 

Any web cleaning needs to be done as the last thing before metallization. Also after cleaning the web needs to be protected from re-contamination.  It is no use cleaning the surface and then doing a slitting process as the surface will be re-contaminated immediately.  Cleaning the surface and then using a clean air positive pressure hood from the cleaner to the re-winder helps retain the surface quality.

  

Although the debris dominates the number of pinholes and hence the barrier performance it is not the only possible source of loss of barrier.  There are also the production of pinholes through coating pick-off which can be related to poor adhesion, filler levels and winding tension.  If you are changing the surfaces by adding coatings prior to metallization it may be that any plasma treatment needs to be optimised for the different chemistry of the coating compared to the bare film in order to maximise the adhesion for both materials.  Higher deposition rates can result in the aluminium molten pool being less stable and this can produce more spitting than at lower deposition rates where it is easier to keep the wire feed and power more stable. This increase in spitting can also result in pinholes.

 

I do stress that these are generally minor effects compared to the variations in debris.

 

 

Thus unless you clear the roll end effects caused by debris from the cores and take a number of measurements from across and down the web and average the results there are always likely to be a wide variation in individual barrier measurements.

 

AIMCAL Converting School Brussels - 1. Web processing for Barrier & 2. Roll-to-roll Vacuum Deposition

CONVERTING SCHOOL

Web Processing for Barrier
Dr. Charles Bishop

March 10 - 11, Brussels, Belgium

The Web Processing for Barrier course with
Dr. Bishop is intended to give an insight into the variety of technologies that can be applied to deliver barrier packaging and the markets that they are designed to service. This would include barrier packaging for food applications through to the more challenging so called "Ultra" barriers required for display and solar applications. The introduction starts with the definitions that are used for barrier materials and gives an overview of the range of uses and the market areas where there is currently high sales growth.

For course outline and description as well as registration, visit the website.
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.

Roll-to-Roll Vacuum Deposition
Dr. Charles Bishop

March 12 - 13, Brussels, Belgium

The Roll-to-Roll Vacuum Deposition course is aimed at providing an overview of roll to roll vacuum deposition. There are different aspects to producing good vacuum deposited coatings. It requires a good vacuum combined with well controlled deposition sources depositing coatings onto equally well defined web surfaces. This course will help operators put each of these different aspects into context and help them to understand where and how improvements might be made.

For course outline and description as well as registration, visit the website.
Who will benefit from this course?
Anyone who deposits, uses or converts vacuum deposited coatings including engineers, designers, quality control, stability and production personnel.

Hotel Information:
Bestern Western Premier Carrefour de l`Europe
Rue du Marché aux Herbes 110,
B-1000, Belgium
Phone: 32 (0)2 5049400
To receive discount room rate, reference group code: AIMCAL

More information at www.convertingschool.com or www.aimcal.org

Some information about OTR

I required some information about OTR.

What is OTR?
Is Vacuum level effects OTR results?
What are the factors effecting OTR results?
What is the recommended OTR for Metallized BOPP film?
What steps are required to improve OTR of Metallized BOPP film?

I shall be thankful for early response.

ANSWER

OTR stands for Oxygen Transmission Rate.

 

Yes the vacuum level can affect the OTR as the vacuum level can affect the nucleation and growth of the coating as well as the contamination rate of the coating.

 

The lower the base pressure the faster the deposition rate, the denser the coating and the cleaner the coating. An increased base pressure or process pressure will indicate an increased level of water vapour and this can oxidise the aluminium deposition. A higher pressure can also cause a lower density coating as more gas can be included in the growing coating.

 

There is no recommended OTR as this depends on the coating thickness and quality and so will be substrate, manufacturer and process dependant.

 

OTR is dependent upon the coating perfection the same as all barrier performance.  If the substrate is very rough the barrier performance will be lower than if the substrate surface is smooth. If the polymer surface is dirty the barrier performance will be worse than if the polymer surface is clean because the dirt on the surface will lead to pinholes which have no barrier performance. The adhesion of the coating needs to be as high as possible. If the adhesion is poor the chances of coating pickoff increases and any pickoff also leads to pinholes and reduced barrier performance. Also poor adhesion reduces the coatings ability to withstand flexing and will result in cracking. This is important as most film used in packaging will be flexed, folded, creased or otherwise handled during the conversion processing and the final barrier will be often be worse than that measured immediately after metallization.   

 

Thus if you wish to improve the OTR of any coated film it starts with the incoming film quality.  Ideally the coated surface will be smooth and clean. The film will then be optimally plasma treated immediately before metallization to maximise the adhesion. The deposition will be done at the lowest possible practical process pressure and once metallized the film will be stored for a day or so the ensure the aluminium coating surface has time to oxidize to ensure that the surface is hardened making it tougher and less inclined to be damaged by subsequent handling.

 

I hope this helps.

AIMCAL Fall Technical Conference

We are now starting to collect papers for this years Fall Technical Conference.  In these tough business times I believe it to be even more important that research and development is not stopped.  New products are always a way to help companies through tough economic conditions as well as placing the company in an ideal position to fully exploit the economic growth when it begins.

As such I urge all who can to submit papers for the conference to make it the best ever.  Good quality technical papers always help stimulate ideas and broaden everyones knowledge which are the tools that are needed for process and product development. As such it is in everybodies interest to make sure we have enough high quality papers and so this is a plea for readers to either submit a paper or recruit someone else to submit a paper.  The details of the conference are as follows;

AIMCAL Fall Technical Conference
and
23rd International Conference on Vacuum Web Coating
October 17 - 22, 2009
Amelia Island Plantation
Amelia Island, Florida

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

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

Web Coating and Laminating

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

Vacuum Web Coating

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

Specialty Web Coating

Novel or developing web coating technologies
Nano coating technology
Photovoltaics technology
Developing web coating technologies and markets
Specialty substrates and coatings technology
RFID technology
A session on Market Trends in the Converting Industry is being scheduled. Topics of interest are global market and technology trends, future role of converting process in growth industries and integrating product development through the supply chain.

Presentations for the Fall Technical Conference are selected upon the following criteria:
Applicability to the advancement of the manufacturing process
Uniqueness and innovativeness of new technology and its contribution to the Industry
Value to the technical/production community
The audience for the Fall Conference is Technical Professional personnel in R&D, Manufacturing, and Marketing. Presentations should be at a technical level of interest to this audience. Papers covering related subjects, not specifically addressed in the call for papers should still be sent for committee review.

Vacuum deposition onto ‘different’ substrates.

 

            Generally when we talk about roll-to-roll vacuum deposition we think of deposition onto rolls of polymer films.  This does not have to be the case.  I have seen vacuum deposited coatings onto metal foils, woven and non-woven fabrics, as well as foams and non forgetting paper.   Each of these substrates tends to offer a different challenge when depositing coatings.  The most obvious problem for the fabrics and foams is that they may not be opaque and so the depositing coating may also reach the deposition drum which will need to be protected. This protection may simple be a length of polymer film wrapped around the drum than can be allowed to collect the deposit and then discarded after each deposition run. If the temperature of deposition is high the polymer chosen may be from one of the high temperature polymers such as PEEK (polyetheretherketone).  Wrapping a polymer film around the deposition drum will impair the heat transfer performance and so the deposition rate may have to be reduced to limit the heat load the fabric is subjected to.  Fabrics will also have higher air and water vapour trapped in the rolls than for solid films and so pumping may be slower and as there is a significantly higher surface area this may also be seen as a higher pressure rise as the thermal load hits the fabric and releases more surface absorbed water vapour.  In this respect the fabrics and foams can be regarded as similar to paper that is also a very porous material. However paper contains much more water than most polymers do with up to 20% moisture in high humidity conditions.  Also paper, if it is dried out too much, can become fragile if the water content falls much below 5% whereas the polymer strength remains constant so long as the temperature remains constant.  Metal foils offer a different set of challenges. The metal contains much less moisture but may contain oils that were used during the foil manufacture that can penetrate into the metal grain boundaries and which can exude back out particularly when the foil is heated.  This can often be seen as stains on any coatings and may also result in poor adhesion where the oil is present.

Thus vacuum roll-to-roll coating should be regarded as a very versatile process capable of depositing coatings onto almost any material that can be made into a continuous strip that is capable of being wound up.  

 

 

X-rays and metal detection question

Question

WHAT IS CURRENT THINKING ON A METALLIZED LAYER IN POUCHES GOING
THROUGH A XRAY METAL DETECTOR IN A PACKAGING LINE

 Answer

 My understanding is that the x-rays will be attenuated by the continuous metal coating. The amount of attenuation depends on the thickness of the metal coating. This attenuation will affect the ability to detect small contaminants where the amount of attenuation will determine the minimum size of contaminant that can be detected although this will also depend on the mass and hence material type of the contaminant.  The thicker the metal the more the attenuation and hence the larger and more dense the contaminant will have to be before it can be detected.  This may also affect the line speed as the system sensitivity can be improved if the process is slowed down and hence some of the loss due to attenuation can be compensated for by reducing the line speed but with the concomitant increase in cost.

A question relating to the cause of pinholes

How are these pin holes formed?

In the above discussion, it is mentioned that the barrier properties of a metallized film is affected mainly because of the pinholes and un-metallized areas which are caused due to the dust and debris attracted during the winding process of plain film. Is it the only reason or there could be any other reasons for the formation of pinholes?
In this reference I would like to put some additional points -

1. Pinholes could be formed as a defect in the metallizing process - In a vacuum physical vapour deposition process, material to be deposited is heated to a high vapour pressure by electrically resistive heating in vacuum.

 

A thermal evaporator uses an electric resistance heater to melt the material and raise its vapour pressure to a useful range. This is done in a high vacuum, both to allow the vapour to reach the substrate without reacting with or scattering against other gas-phase atoms in the chamber, and reduce the incorporation of impurities from the residual gas in the vacuum chamber.

 

In the above process the most common thermal evaporator used are ceramic boats, when the Aluminium wires get in contact with the boats it melts and evaporates. During this melting and evaporation process there is formation of bubbles at boiling point (@ 1200 to 1400 degree centigrade). When these bubbles burst it punctures the film leaving behind the defect known as pinholes.

 

Can this be the reason of pinholes formation?

Please advise.

 

ANSWER

When the polymer film is produced it is not in a clean environment, even in the highest quality lines. There will always be some dust and debris that is present in the environment and as the polymer film is wound there will be an electrostatic charge that is built up on the surfaces that will help attract debris to the surface.  Also in areas such as the casting drum and stenter oven the will be evaporated monomer that will condense as a fine white powder that may fall onto the web too.  The amount of debris will also depend on how much the film has been wound and handled before you receive it.  Most films will have been slit at least once from the mill roll down to smaller width rolls and the slitting process is another source of debris contamination. This is especially as the blade cutting the polymer generates debris and is adjacent to the winding film that has the electrostatic charge on the surface that acts like an electrophoretic pump.  This debris will be on both sides of the web and so any cleaning process needs to be on both sides of the web.  The best of the cleaning processes cannot remove debris of less than 300nm and these are still very much larger than the thickness of the metal coating. The number of debris particles on the surface can be in the thousands per square meter.  Most will not move and may be embedded or stuck to the surface and will not produce a pinhole. There will be a proportion that produces pinholes immediately and others that will appear after further downstream handling. 

This method of producing pinholes is by far and away the biggest source of pinholes.  ICI polyester films funded a student (E.H.H.Jameson) at Cambridge University who produced a Thesis on the analysis of the source of pinholes who determined this was the main source of the problem. 

 

You are correct that this is not the sole source of pinholes. There are pinholes produced from the evaporation source. Many of these are thought to be associated with the slag that forms on the surface of the molten pool rather than bubbles that burst.  The aluminium wire has the surface oxide that does not melt but forms a skin on the surface of the molten pool and some of the trace elements in the wire also can also add to the slag. This slag tends to accumulate towards the edges of the molten pool and so as the pool size changes, increasing or decreasing, there is a tendency to an increase in spitting from the pool edges. These spits are often seen as incandescent particles that may have enough energy to not just produce a pinhole in the coating but the more serious hole all the way through the polymer film. These can be minimised by stabilising the pool size. Thus in recent years the power to the resistance heated boats have been improved and smoothed as well as the wire feed made more consistent as well as the coating thickness monitoring improved so that the intervention to correct coating variations have been minimised.  All of these have reduced the variation in pool size and reduced the tendency of the sources to spit.

A further source of pinholes is due to pick-off.  If the coating is poorly adhered then when the web is re-wound any high spots that press into the metal layer may stick to the metal well enough that when the roll is next unwound the metal will be removed, picking off the metal.  This is quite common in some of the coarser filled films where the larger fillers may press hard into the web during re-winding particularly as in the vacuum system there is no air entrainment to enable a softer re-wind that reduces the contact pressure.  

All of these can potentially contribute to the total number of pinholes seen and it is very hard to identify which of the processes has caused any particular pinhole.  The debris type can sometimes be identified if the debris has been slid rather than rolled away as there will be a pinhole with a scratch associated with it and sometimes the debris will still be visible at the end of the scratch.