Vacuum Web Coating

Problem re. high sagginess

We are getting high sagginess (20 mm in drive and operating side) in polyester film

Please advise what could be the probable reason & remedies.

ANSWER

Generally where there is a sag in the film it indicates one of two things. One is that during the manufacture the web has been produced with a differing lengths at the edges and the center of the mill roll so that when the mill roll is slit it shows up as rolls of film that have one edge longer than the other.  This can be tested by cutting off a length of film (~25m) and laying it out on a flat floor with no tension on it. If the two edges are of a different length the film will lay down with a curvature with the center of curvature on the side with the shortest edge. 

The second reason could be related to the film profile. If the profile is poor and there is a thicker part of the film, this will take the most tension. If the film has been wound with a very high tension, especially if extra tension was being used to remove any signs of bagginess, the film could have been over stressed and a permanent set put into the film that too could be seen as bagginess once the tension had been removed.

The more common problem is the first of these.  Once the film has been made it is difficult to correct this problem.  It may be possible to rewind the film through an oven that takes the film up to a high temperature under a very low tension, the same as is used for producing high stability film, and in this way allowing the internal stresses to relax and hopefully reduce the asymmetric stresses and reduce the bow angle and so reduce the curvature and bagginess.  However this process would be a time consuming and hence significantly increase the film costs. It may be possible to pull a little extra tension to hide the problem in the metallizer but this may also require the film to run slower as it may be more sensitive to heat stress during the heating as the web passes through the deposition zone.

My first step would be to cut some film and lay it on the floor to check for any curvature and if it is curved to go back to the manufacturer and make sure it is understood why it has occurred and to make sure it does not happen again either by improving the manufacturing control or by adding an additional specification to your existing film specification excluding film with a curvature more than what you consider acceptable.

Boat lifetime and tension imbalance

1,How does the length of rolls affect the life and performance of boats.
2,What actually happens to the boat, if the metallisation time kept too long.
3,Why the boat life (hrs) is more in the metallisation of smaller length rolls.
4,Why the Torque (%) values of DTR-1 and DTR-2 (Drum tensioning roller) is not kept same, what will happen to the film and process if it is kept same.

ANSWER

In general there are benefits to running boats continuously. Primarily there are fewer heating and cooling cycles and so one ought to expect a slightly better lifetime.  How do you assess the lifetime?  The critical time is the time spent at the deposition temperature with the molten aluminium in contact with the boat. If the time is set at some other point such as from the start of the pumpdown, or first switching on the boats and so includes the ramp-up and similarly the ramp-down then the shorter rolls will include a higher proportion of this unproductive time.  I would check how you time the boat lifetime as it does seem strange that the shorter rolls give a longer boat lifetime.  It could be that taking out the time spent at lower temperatures on both cases will reverse the view of the boat lifetime.

Where this differs is if the boat temperature is higher than optimum then the higher operating temperature will be expected to lead to a reduced lifetime.  Boats generally decline in performance due to the corrosive nature of the aluminium whilst it is in the molten state. This can be coupled to the boat quality and can depend on the manufacturer, the method of manufacture and the materials used.  The boats are made up of a powder compact that is then sintered and cut to shape (except one Japanese manufacturer that makes each boat individually).  The grain size and the porosity of the boat are one of the critical features. Coarse grain powders give greater porosity and shorter lifetimes. The molten aluminium is able to penetrate the grain boundaries and degrade the material.  Again the higher the temperature the easier it becomes for this migration of molten aluminium to take place.

Other reasons for a shorter lifetime can be to position of the wire and the rate of feed of the wire. I have seen grooves that appear to have been eroded through the boat because of a high wire feed rate where it almost appears as if the wire is still rigid enough to scratch a groove into the boat and this acts as a weakness and leads to a premature failure.  The aim is to get the wire feed so that the wire just becomes molten at the point of meeting the molten pool.  It does not want to melt early and drip into the pool, nor does it want to rub through the pool and erode the boat.

Generally the tension wants to be stable and well controlled around the deposition drum  If the tension is not isolated it will either follow the unwind tension or the rewind tension.  If large rolls are being used the tension may be changing with the roll diameter.  This would mean that the tension around the deposition drum would also change. As the tension of the web around the deposition drum affects the heat transfer coefficient and hence can affect the cooling of the web it is important that the tension is optimised and maintained at a constant value. If the tension varies there can be problems, if the deposition rate is high and the tension is too low the web can start to wrinkle. The other problem is that if the tension is too high then the film can be permanently distorted as the yield strength is exceeded as the film temperature is raised and the yield strength of the film consequently falls.  The other consideration is that even though there are different tensions applied in different zones it is preferable the tensions are not hugely different. Large differences in tension can be hard to isolate as the tension isolation is often achieved using a wrap around a roll and if the wrap angle is not sufficient some tension will transfer from one side to the other. This can also be a cause of the web slipping over the roll surface and so can put micro-scratches into the web surface.  So simply because the tensions are isolated it does not mean that the tensions have to be set very differently.  The isolation is purely to keep the tension around the deposition drum constant.

Does COF value have an effect on lamination?

Original question

Does COF value effect on lamination?

Answer

It depends on how the CoF has been achieved.  There are two components that contribute to the CoF one is the surface roughness and the other is the surface energy.  Many films use fillers to increase the film surface roughness, which reduces the contact area, which reduces the CoF. Usually this is not sufficient to make the film easily handled and so a slip agent is added to reduce the surface energy and so further reducing the CoF.  Unfortunately the slip agents are low molecular weight as well as low surface energy. They are added to the bulk polymer and are expected to migrate to the surface. 

Thus if a slip agent has been included to help deliver a particular CoF it can be a problem to coating or lamination adhesion because of the low surface energy, low molecular weight material present on the surface that will fail relatively easily.

Follow-up question

I want to know if COF value is high then delamination strength will be low and if COF value is less delamination strength will be high. What is recommended standard of COF in LDPE.HDPE and nylon Coex LDPE.

Answer

There is not a definitive answer.  Take a film where the surface of the film is smooth, the CoF can be high and the adhesion good then if a filler is added to the same film the CoF may fall because the contact area has been reduced but the adhesion may have increased because the surface area at the interface has also increased because of the increased surface roughness.   Thus adhesion is not directly related to the CoF in every case.

The CoF is usually defined by what converters find easy to work with.

Typically adding fillers can take the CoF down to levels of around 0.3 - 0.4

This may be higher that is regarded as acceptable in which case slip agents are usually added to reduce this level down further.  Fillers may also increase the Haze of the film because of the increasing roughness and so it is not simply a matter of using fillers and increasing surface roughness alone.

CoF of LDPE can be up as high as 0.7 but with 0.1% slip additive it can be reduced to below 0.2 where 0.16 – 0.2 CoF is regarded as acceptable for easy processing.

So first I may be worth establishing from your end users what is an acceptable range of CoF for processing and compare this figure to the what films are available using different levels of fillers or slip agents.

Keep in mind that once you metallize the surface the CoF is likely to rise, particularly if you have used a surface treatment to clean the surface and increase the adhesion.

Moisture, aluminium, oxidation, white spots and adhesion.

A couple of the questions received appear to relate to the same problem and so I have put the questions together and given a single answer.

Does moisture affect the metallization of Aluminium? 

We laminate Metalized polyester with PEG coated paper and metal peel off.  Once the oxidation starts in metallization will it destroy the whole metal with passage of time or not?

Will metalized MPET will be oxidised if we laminate it with PEG coated Paper.   The ratio of PEG coated paper is 80% PEG and 20% water.  We observe white spots in metalized paper.  Please try to find root cause and suggest us what measures can safe us.

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

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

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

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

Loss of gloss in a metallized BOPP film.

We are facing GLOSS problem of BOPP Metallized film at Galileo Metallizer. Look of film is dull. How we can improve the GLOSS of BOPP Metallized Film? What are the factors effecting the GLOSS of BOPP and Metallized BOPP.  Kindly suggest me the best solution to increase the GLOSS.

ANSWER

There are several possible problems that can give rise to gloss problems.

One is that your vacuum system has a slight leak or is operating at a too high pressure. (if cleaning has been delayed the base pressure may be more difficult to achieve and the water captured by this very high surface area coating can also look like a leak). The moisture can oxidise the surface and lead to a grey/milky appearance.

If the coating thickness is greater than normal the surface roughness can increase and this surface scattering can also give the appearance of a loss of gloss.

BOPP films can also have additives such as slip agents included and these can migrate and contaminate the surface. This may be particularly true if the rolls after metallizing are rolled up whilst still hot where the speed of migration is higher.

It has also been shown that plasma treating the film can reduce the surface roughness, which can be measured as an increase the reflectivity.

Thus it is impossible to say what is causing your problem but hopefully this gives you one or two things to look for.

PS.       This is your final reminder about the 1st AIMCAL India Summer School which takes place next week 20th & 21st Aug.   Details at www.aimcal.org

Gold colour metallization

If you are metallizing Aluminium and you wanted to change the colour of the metallized web from the "normal shiny" aluminium colour to a hint of  "gold" colour, is there a way to do that by running at vacuum levels less than say 10 –4 mbar?

I know you will get lanes, and such, but I was wondering if just running at poor vacuum you could generate the colour change?

ANSWER

It is possible to work at higher pressures but this method is fraught with problems.  The production of a gold colour has always been an indicator of a leak, either water or air. The biggest problem is that I do not think anybody has recorded the precise level of leak that will give the gold colour effect consistently.  There are lots of other effects that the leak can produce, such as a completely transparent film as all the coating is converted to aluminium oxide (as per the Camvac patent) or it can turn the surface milky.  Part of the difficulty is that if you are intending depositing a reactively deposited coating the pumps are usually hung off one end of the vacuum system and so the gas flow will be different near to the pumps compared to the pumping at the opposite side of the web and so the colour would be different on one side to the other. Hence it could require extensive development time and never be of the highest quality.

If all you are wanting is a gold effect there are several ways of achieving the effect. Using a dyed film as developed by Martin Processing (now CPF Films), using a coated dyed lacquer or by using an alloyed wire.

The dyed film transforms the usually water white transparent PET film into a golden coloured film. This dying is achieved using a heated oil bath. This then simply requires metallization of the golden substrate to provide the effect. The dyed lacquer requires an additional coating compared to the surface oxidation of dyed substrate but is probably the simplest solution.

The final solution is to use an alloy wire such as supplied by  Speedmet A.S. Ltd who sell a wire specifically for depositing a gold effect metal layer.  This wire has a slightly different wetting characteristic for the wire on the existing aluminium evaporation boats but once the boat surface has been used once it works the same as aluminium wire.

.
If you want more details then contact John Coupland on e-mail
john.coupland@speedmet.com

These other solutions have all proven to be easier than trying to reactively deposit a controlled oxide thickness onto the aluminium.

Coefficient of Friction - CoF

After the lamination of a co-extruded film with the metallized film the product work very bad on the packaging machine we measure the CoF of the co-extruded film we find it good but when we compare the CoF of our metalized film with that of another good working sample we find it higher does the CoF of metalized film affect on the runability in vertical packaging machines ?

ANSWER

Many films have a naturally high CoF and are modified to lower the CoF.   There are two things usually done to modify the CoF, the first is to add fillers so that the surface becomes rougher and although this does not lower the surface energy it does reduce the surface area. The second modification is often a chemical modification that lowers the surface energy and so lowers the CoF.  This combination gives the final CoF that
makes the film easily handlable.

For metallization it is common to treat the polymer surface in order to raise the surface energy to improve the aluminium wetting and adhesion.  It depends on the original method of modification of the CoF as well as the subsequent surface treatment as to the final metallized film CoF and this is likely to be different for the metallized side and for the un-metallized side.   Some metallized films using fillers are through filled and so the front and back surface are roughened whereas some better optical performance films are co-extruded and the fillers added only to one of the co-extruded layers giving a roughened back surface and a smooth front surface for
metallizing.  This modification remains the same before and after metallizing.  The chemical modification is what produces the significant change.   The chemical modification may be added to the polymer, in which case the chemical additive can be expected to appear on all surfaces as it migrates out from the bulk.  If the chemical modification is added as a coating during the film manufacture then it may only be added to the smooth surface where there are no filler to lower the CoF.

To improve the metal adhesion during metallization the surface energy of the surface has to be high to encourage the wetting of the aluminium.  This requires that any surface modification used to lower the surface energy be removed.  This may be done by corona treating the film before the vacuum process, or it may be done by plasma treating the film once in the vacuum system or even both.  If the film has a chemical treatment on both sides then if the front side only is treated it is possible that the front surface will become re-contaminated by contact with the back surface and so it may
be that both sides are cleaned of any treatment.  If the chemical treatment is contained within the bulk then it is possible that given time some of the chemical will migrate back to the surface and so the CoF may reduce with time and/or an increased temperature.  The metallized surface acts as a barrier coating to any chemical contained within the bulk of the polymer and so the metal surface will tend to have a smooth and high surface energy giving the surface a higher CoF than the original film.  The un-metallized surface may well have an increased surface energy but this may fall back with time depending on the chemistry.

The CoF can affect how the film handles.  Depending on the roll arrangement and drive system will depend on how much the affect has on the handling. Some rolls use the friction between the film and roll and the wrap angle to control the tension or tension isolation. Either raising or lowering the surface energy too much may affect the performance of such rolls as well as affecting the adhesion and wetting of other coatings or inks.   Winding the film may be particularly different if the back surface and front surface have very different values of CoF.

So the answer would be yes I would expect changes to the CoF to affect the ease of running of film on any machine but it may be more noticeable on some machines more than others.  Keep in mind that changing (lowering) the surface to improve the easy running of the film may cause problems of reduced adhesion of any additional layers.

I hope this gives you some explanation of what might be the reason for the changes in CoF of the film.

AIMCAL India SUmmer School

AIMCAL Summer School India 2007 August 20-21, 2007 | Pragati Maidan Exhibition Centre | New Delhi, India  Web Handling & Converting · Vacuum Coating · Web Coating and Drying
	For eight years, AIMCAL has offered these highly rated courses in the United States and Europe. Now in the ninth year, for the first time, the popular AIMCAL Summer School is coming to India! AIMCAL Summer School India assembles the best consultants and educators in the industry for two days of focused technical sessions. Our courses are designed to provide "practical, usable solutions to converting challenges". The courses selected provide job-enhancing, practical information for engineers, operators, technicians, technical managers and sales people involved in coating, laminating and metallizing. Courses and instructors are carefully selected to ensure maximum value for attendees. VISIT www.AIMCAL.org/association/India2007/intro.asp  for full agenda and to register securely online. In association with Print-Packaging.com and the India Converting Show. General Information  Be comfortable! Dress for AIMCAL Summer School India is business casual. ASK OUR EXPERTS! Technical Questions Welcome! Session Tracks  Web Handling & Converting Vacuum Coating Web Coating and Drying Full Schedule --> Instructors:   Dr. Charles Bishop Dr. Dilwyn Jones Dr. Ed Cohen Fees  $500.00 USD The fee for attending AIMCAL Summer School India is $500 USD (Rs.20,500 INR) per person. Currency Note: Credit Card registrations will be processed and charged in USD. If you wish to use a company check and pay in INR, please use the amount Rs.20,500 INR VISIT www.AIMCAL.org/association/India2007/intro.asp for full agenda and to register securely online.

A problem of oil backstreaming

The initial question was we think we have a problem of backstreaming, what might be the cause?

Following the first answer the problem was elaborated on and so the second answer was produced.

We have vacuum coating unit for aluminisation of Kapton films by physical vapour deposition. Two numbers of films of 25 microns thickness is pasted on the walls of the chamber and aluminium foils are loaded on the bus bar with tungsten filaments which is introduced in between the films. It consists of a rotary pump, rootes pump and diffusion pump and the vacuum level is 10-5 Torr. The plant is 10 years old. First step is roughing followed by backing again roughing and then high tension plasma treatment for cleaning for 15 minutes. Then diffusion pump is switched on with high vacuum and finally vaporizing aluminium by applying low tension for 30 seconds.  Now the problem is aluminium from the top most portion of the Kapton film in semi circle with some I foot diameter where the vacuum duct is coming, actual size is 2.5m x 2m, is peeling off on tape peel test. We have changed all oils, cleaned all internal parts possible and still the problem is there.

1^st  ANSWER

There can be a few causes of oil backstreaming. One is that the roughing pump has stopped working as efficiently as previously. In this case it is worth checking the oil level and also the water content of the oil, it may be that the rotary pump needs to be ballasted.   If the oil in the rotary pump looks to be a milky and opaque it needs to be ballasted to raise the temperature and encourage the water to be evaporated out of the oil.  If you have changed the oil this would solve this problem, unless you have an internal water leak that is contaminating the rotary pump oil rapidly. So even after changing the oil it is worth checking the colour of the oil and making sure it remains a transparent liquid of a light straw colour.

The other common problem is that the system has been left pumping for a prolonged period of time at the base pressure of the system. It is often believed that the best way to remove the water from a vacuum system is to pump down to a very low base pressure. This is not so. Water is removed from the surface by gas bombardment or some other technique for adding some energy such as heat or ultra-violet light.  What was found in the semi-conductor industry was that it was better to feed gas in at quite a high vacuum pressure and restricts the falling pressure so that the gas bombardment of the surfaces is maintained. This has proved to be more effective at removing the water. This larger gas flow makes it harder for the diffusion pump to start backstreaming because of the continued gas load.

Another common problem is that the backing pump does not pump the vacuum vessel down far enough to safely change to the diffusion pump and the diffusion pump stalls as the gas pressure in the diffusion pump rises so high that the oil vapour collides with so much gas that it never reaches the cooled walls to condense and drain back down to the heater of the diffusion pump to be re-vaporised.  Instead the oil vapour collides and is redirected up into the vacuum system. 

I would suggest a close review of the system including how the system is operated paying particular attention to the order and pressures used for the opening of valves and switching pumps.

2^nd ANSWER – following the additional information.

Going back to the problem and working through the possibilities.

The starting point with any problem is to firstly check that we are trying to solve the right problem.

Firstly I would check that the failure is really at the interface and no a cohesive failure within the substrate.  This is unlikely but it is worth checking as if the failure is cohesive then we could spend months trying to solve the wrong problem.

If you have access to surface analytical machines then checking a fresh failure to see what elements are on each surface will tell you if the failure is at the interface or within one of the layers.

Assuming the failure is at the interface there are several possibilities.

From your description of the position and shape of the area of failure I would agree that it looks as if it is a failure related to the geometry within the vacuum coating system and not a contamination that has been brought into the system on the film.

There are several possibilities. You mention using a plasma clean. This may not be optimised.  The plasma density will be affected by the pressure and positioning of the film within the plasma.  The ideal would be to have a regular shaped plasma that all the film sees uniformly for an equal amount of time.  Your geometry does not lend itself to this.  There can be problems with plasma cleaning.  Because some plasma treatment is good thee is temptation to think that more will be better. This is not always true.  The plasma treatment can remove any low molecular weight contamination (assuming there is some oxygen gas available) and may also do some chain scission to enable direct metal bonding to the polymer.  How ever if the plasma treatment does not contain any oxygen the organics may not be removed and the chain scission may be too great creating a powdery surface, that is often referred to as a weak boundary layer, which will fail.  If the surface energy is plotted the surface energy will increase with an increase in plasma treatment time up to a plateau and beyond this time the surface energy will remain constant for some time before it finally declines.  If the adhesion is measured this is not the same, the adhesion will initially rise but as the surface energy reaches the plateau the adhesion will only peak and then decline even though the surface energy remains high.

Thus optimising the plasma treatment needs to be done but if there is a difficult geometry then this may be hard to do for the whole surface.

Oil contamination.

It is possible to get oil backstreaming from roughing pumps as well as from diffusion pumps.  It is not good policy to run pumps at the low end of their pressure range for long periods of time.  

There are two aspects to pumping down a vacuum chamber, one is to reduce the system contaminants and the other is to produce a low pressure to give a long mean free path so that the evaporating material is not scattered between leaving the deposition source and reaching the substrate. 

The main contamination for any vacuum system tends to be water vapour that is absorbed on the surfaces and which takes some time to desorb.  Often systems are pumped to try to achieve a very low base pressure in the belief that this is the best way of removing the moisture. This is not so. Most water is removed by gas bombardment of the surfaces and so as the pressure falls the amount of gas is reduced and so the number of collisions with the surfaces goes down and the effective rate of removing the water also falls.  Thus it is better to add a dry gas during the pumpdown to increase this surface bombardment, which would be seen as limiting the pressure the system is pumped down to. Also adding some other energy can be beneficial in removing the moisture. This can be as simple as using a UV lamp or by using a plasma which also has a very high UV content.

Thus I would review how you pump the system down and where you have the crossover points for changing from roughing to pumping with the diffusion pump.   If you look to the semiconductor industry they have also looked at the problem of pumping and contamination. They use this higher pressure type of operation and get cleaner coatings than when they try to pump directly to a very low base pressure. 

A simple test to see if you are getting backstreaming is to add to the system a cooled plate where the adhesion to the substrate is poor. With this cooled plate installed, run the process but without igniting the plasma cleaning and without depositing the aluminium. So the plate is pumped for the same lengths of time as the substrate would normally see.  When the system is vented check the surface of the plate for any oil contamination. If there is any oil contamination you may need to repeat the process just as far as the end of the plasma clean time, but again without the plasma, just as a method of checking to see if it is the roughing line that is backstreaming or the diffusion pump that is causing the problem.

If it is the roughing pump then you need to revise the pressure and how you achieve the plasma cleaning.  Either you need to switch to the diffusion pump and use an input gas to provide the correct operational pressure or you need to increase the roughing pump gas load to reduce the backstreaming or you need to consider adding a cooled baffle into t he pumping throat to collect the backstreaming oil. These cooled are commonly used but be aware that adding one will constrict the pumping area and so the pumpdown time will be much longer and also it does not stop the backstreaming but only limits the amount of oil that gets past the baffle.

I hope this gives you something to try to solve the problem.  Please let me know when you have some success. It is always difficult for me to judge if the suggestions I have made are relevant or not and it is only if people let me know afterwards that I can judge if I have helped or not.

Vacuum International - Year 1, Issue No.1. Jly/Sept 2007

The journal Glass Coatings has ended publication and there has appeared a new publication, Vacuum International.  This new publication covers technologies and high-tech applications of the vacuum coating industry.  This includes coating, plasma, micro- and nano-technologies and photonics as well as a special section that covers the vacuum coating for the glass industry.

            For those interested in finding out further details there is a website  www.vacuumtech.eu 

            This first edition include sections on product news, business news, followed by a number of articles:-

Thin film PV – Glass and photovoltaics: two worlds continuously moving closer.

Production techniques – Thin films: Italy leads the way.

Sun-Tech – The development of LEDs and SMD electronics on transparent polyester film.

Singulus Technologies – Vacuum coating: a technology with potential.

Pfeiffer Vacuum – From individual components to complete complex vacuum systems.

Brazing – VLT: complete supply and service for high & ultra-high vacuum applications.

GNB Corporation – Baffles improve performance.

Science & Research – Plasma treatment for protection of metals.

            A number of the articles are company specific and expand on the company expertise and products or services.   Others articles give an overview of a particular current technology.  One of these is on the use of PECVD for the deposition of silica as a protective layer on metals for corrosion resistance. This provides an interesting comparison to the use of silica transparent barrier coatings for food packaging applications.  Another is about photovoltaics (PV).  This article highlights the glass substrate based PVs that, although they currently have a significant share of the market, can be expected to be superseded by the lighter PVs on flexible substrates.

            

            I look forward to the next edition.