Vacuum Web Coating

Will this improve the pumpdown speed?

Question

We have a metalliser that has two deposition zones. The volume is very large (15.53m 3). We are not use currently the second deposition source so we think to put up an empty airtight box in the empty space. Is there any benefit for the vacuum or is there any disadvantage in the sense of degassing of the metal surface?

Answer.

Essentially what you propose is possible and by reducing the volume you will reduce the pumpdown time.

There are a couple of points to bear in mind when making this type of change.  The box MUST be leak tight otherwise you will have a virtual leak permanently within the vacuum system and this will both reduce the pumpdown speed and the base pressure.  Depending where the volume is positioned will depend on if it collects any stray deposition. If it does it needs to be cleaned like any other surface to keep the surface area down to a minimum, otherwise you will be absorbing more water than before and this too could reduce the pumpdown speed, reducing the benefit. The increase in surface area of just the box surface is small compared to the surface area that is produced by any stray coating and so generally would not significantly change the amount of outgassing that would occur during the pumpdown. It is only if it collects stray deposition and it is difficult to clean and so gets left with the coated material on that it would affect the pumping.  Finally check the position with respect to any conductance gaps and the positioning of the pumps. You do not want the box to block any pumping orifice. If the box reduces the size of the pump throat then this would act as a constriction reducing the pumping speed of the orifice and slowing down the pumping of the vessel.

Assuming the box is leaktight, kept clean and suitably positioned then you can expect a pumping speed improvement.

High barrier performance Question.

Question.
I represent a converter who is trying to achieve good barrier properties (wvtr<0.2, otr<0.5) using metallized pet as part of the laminate. We have our own metallizer and the highest OD we can achieve on a single pass is 3.0. However, at this OD, we are unable to achieve the above noted barrier properties.  Are there other methodologies that are available to enable to hit these properties? We have heard of double side metallized films utilizing plasma and chemical treating? Are these possible solutions and have you had experience with these types of films. Secondly, we are also concerned with loss of properties with downstream processing like rewinding and slitting. Are there special processing requirements for finished metallized film in order not to degrade the barriers achieved during metallizing?

Answer

The biggest limitation to producing good barrier films is generally pinholes.

Pinholes are produced primarily by debris that remains on the film surface that is larger than the very thin metal coating you deposit so that after metallization if the debris is moved it leaves behind a pinhole.

A second source of pinholes is spitting from the boat. This can be caused by a combination of reasons such as a low purity aluminium wire or the wire having a thick oxide coating on as well as an unstable pool of molten metal that as the pool size changes encourages spitting from the ends of the collection of crud built up from the wire oxide and impurities.

Another source of pinholes is any pickoff. This is where any high spots on the reverse surface presses hard against the freshly deposited metal and in some cases it overcomes the adhesion and picks off the metal from one surface and it transfers to the second surface. Again leaving a pinhole.  Typically this is associated with large fillers and a hard wound roll.

If the incoming roll is cleaned to a high standard it is possible to reduce the number of pinholes and so improve the barrier performance.

This can be done by using techniques such as the tacky roll type method of removing debris.  All polymer film will be covered with debris. This is partly because the polymer film, as it is wound, generates an electrostatic charge that attracts airborne debris to the surface. This can include slitter dust. Thus if the roll is cleaned just before the final rewind before the roll then goes into the vacuum system this can help. If it is done earlier it can become recontaminated very easily. Ideally after cleaning the film should be in under a clean filtered air hood to limit recontamination. Also the film should be cleaned on both sides otherwise the debris from the back surface can become transferred onto the front surface as the film is rewound.

Similarly it may be possible to improve the wire purity and make sure the wire surface has a minimum of oxide present and may also be possible to improve the wire feed control to reduce some of the pool variations.  This will help reduce any spitting problems

Other possible things that can be looked at is to see if it is possible to wind the material between metallization and lamination with fewer or no front surface rolls in order to minimise the change of moving the debris and so limiting the number of pinholes that appear. Although it is better to clean the material and not have the debris present but this may not be easy to accomplish.

The use of adhesion promoting measures, such as plasma pre-treatment, if done well with the process carefully optimised, can improve the adhesion as well as the metal wetting. This has two advantages. The improved wetting means that the metal will spread out on the surface and so will produce a continuous coating at a reduced thickness and this can be seen as either the same OD at a thinner coating thickness or for the same thickness a higher OD. The second advantage is that the coating is less likely to have problems with pick off as a method of producing pinholes.

In addition there may be other changes that can show benefits. If the metallized film is laminated very soon after metallization it may lose more barrier performance compared to if any further processing were to be delayed by a day or more.  The aluminium coating is very soft and is more prone to damage if it is handled very soon after metallization. If the roll is stored for a short period of time the native aluminium oxide is allowed to build up on the surface of the aluminium and this is much stranger and any damage is likely to be less.

Where double side metallization wins is that there statistically it is unlikely that any of the pinholes on each of the two aluminium coatings lines up with each other and so the increased tortuous path reduces the gas or moisture transmission.

I hope this gives you some explanation about where the lack of barrier comes from and some possible routes forward.

Edge to edge deposition onto polymer webs.

We would like to metallized edge to edge. But we need to overcome the issue of metal deposition on drum.  It will difficult to remove. This is happen due to slippage of roll from un-winder at starting of the jumbo. We are running 56000 m / 12 micron pet / 2450mm width.

How can we reduce the initial slippage from un-winder?  Is there any specific Tape available in market to apply on both edges of drum to avoid excess deposition of aluminium on tape instead of drum?

Answer

Occasionally it is required to run film through a metallizer and to coat edge to edge on the web. To achieve this the normal edge shields are removed but this gives the problem that the deposition drum is no longer protected at the edges and the aluminium can coat the drum. This then needs very careful cleaning so that all the coating is removed but the highly polished drum surface is not damaged. 

An alternative arrangement is to wrap a protective film around the deposition drum at each edge. This protective film needs to be thermally stable and so it is common to use a polyimide film such as Upilex or Kapton that can withstand a temperature of 400 degrees centigrade.  This polyimide is also available as a tape that can be used to stick the film together around the drum. 

The key to this working well is for the unwind and rewind rolls to be accurately aligned, otherwise the film will move sideways during metallizing and will ride up one film edge and allow the metal deposit to reach the drum on the other side.

Where this slippage or telescoping can occur is if the unwind roll is wound soft and moves during pumpdown as the excessive air is removed and the roll collapses. During this collapse if there is any differential tension it will put a sideways force on the web and it will slip sideways. The amount of sideways movement will depend on the size of the differential tension and the quantity of the excess entrapped air.  So even if there is a large differential tension the film will not move sideways if the roll is wound sufficiently hard so that during pumpdown the roll does not collapse.  Thus with a well wound roll and assuming the rewind roll has been positioned well there should be no reason for the web to move out of the track provided by the masking film wrapped around the drum.  To be certain it is always helpful, if tension changes are required, to only make small adjustments slowly.  This is because the film is more likely to move sideways if the tension changes are large or made very rapidly.

In this way a standard metallizer should be capable of producing edge-to-edge metallized film without too much of a problem.   

A problem of bad odour.

We are facing the bad smell (odour) problem in our metallized OPP film.

What is the reason?

Answer

Without knowing the type of smell, i.e. rubber, polymer, paint, sulphurous, etc it is very hard to suggest what it might be. Below are some thoughts.

One common problem is if you are depositing zinc sulphide to make high refractive index coatings for transparent holograms by evaporation of the zinc sulphide. The zinc sulphide may undergo some dissociation, which releases some sulphur into the system.  Humans can detect sulphur at very low levels and so it is common to think that there is a huge problem but usually there is only a very small amount of dissociation that is insignificant to the coating refractive index and stoichiometry.

If you were simply depositing aluminium onto OPP then I would look carefully at the system. Is the system showing signs of excessive backstreaming causing oil to reach the evaporation boats and decomposing? Is the OPP a homopolymer or is it coextruded with another polymer that is a very low melting point polymer that is being damaged by the heat load during deposition? Is there a failure of one of the water-cooling lines causing something on the system to overheat?

Sometimes you can have an idea of where to look because of the type of smell. If the smell is of burning paint then look to things like a failure of the water cooling on items such as the diffusion pumps where the hot oil could be heating the body of the pump and causing the paint to char.  If the smell is a polymeric smell then look within the vacuum system and see if any water lines are getting too hot or if the web is showing signs of damage because of heat. Or if the smell is rubbery then again look to the water-cooling pipes getting hot. It depends on t he composition of the cooling water flexible pipes in the vacuum system. 

Also check what has changed between the system not smelling and the system now smelling.  If the system was serviced then check what was done during the servicing. What was disturbed, was everything put back where it was meant to go was anything difficult to put back? If a flexible pipe was hard to put back and got put back in the wrong position it may now be touching a hot spot in the system and gradually being burnt through and so soon will end in a water leak.

Within these suggestions may be enough information for you to find the problem. If not I would need more information to try to help further.

Plasma treatment

Could you please explain me about the plasma system in vacuum metallized machine? Now I have problem with the plasma system. The problem is corona level of mcpp film after finished from metallizer with 2.5 KW plasma was drop very quick or 1-2 week it's decrease from 40 dyne to 36 dyne.  I compare between use plasma and don't use plasma it's difference when used plasma corona drop faster than don't use plasma. Please help me and recommend me how to do?.....Thank you very much.

Answer

Polypropylene films often contain slip agents, these are present in the bulk of the polymer film and they migrate to the surface. If these slip agents are removed from the surface by any kind of surface treatment the slip agents will reappear as more migrate from the bulk to the surface.

When film is metallized it is important to remove the slip agent in order to improve the metal adhesion.

Generally an easy measure of the surface energy is to use either 'dyne pens' or to measure the contact angle of a water drop to the surface. This is used to check that the slip agent has been removed, as the surface energy of the polymer will rise as the low molecular weight material disappears.

The speed that the slip agent reappears depends on a number of factors including temperature with the low molecular weight slip agents being more mobile at higher temperatures.

The slip agent is added to the polymer to make it easier to handle. Thus it is common to only treat the surface that is to be coated so that the other surface still has the slip agent present.  This means that as the film is wound up the front and back surfaces come into contact. You then have a clean surface with a high surface energy in contact with one with a low surface energy that contains a mobile low molecular weight material. Nature tries to bring things to equilibrium and so will try to reach the lowest surface energy state. So the high surface energy side will attract the low energy material to it and so bring the high surface energy towards the low surface energy value.  The greater the difference between the high and low surface energy the greater the driving force and so the faster the high surface energy will reduce.

The metallized surface has a very high surface energy and so this will change more dramatically than an unmetallized surface.  As the film is rewound in vacuum these rolls will be hard and so each layer will be in intimate contact.  Both surfaces will tend towards the same value over time. The amount of time will depend on the storage temperature.

I hope this gives you some help in the processes that might be happening.

Distinguishing marks?

Can we check visually the type of adhesive used in the double Vacuum Metallized laminate?   Will there be a distinguishing mark for water-based or solvent-based adhesive?

Answer

I do not believe there is any mark included. I suspect you will need to ask the supplier which has been used.  There is always likely to be some residual solvent content in the lamination and so if you needed to check I would suspect that is a sample of the film were used any solvent could be extracted and measured by one of the mass
spectroscopy, or chromatography techniques to confirm that a solvent were present in the structure.  You would probably have to infer that water was used by the lack of solvent being present, as many polymers will have water present in the bulk.

More on telescoping problems.

Our company experiences telescoping inside the vacuum metallizer but on the rewinding side when the film is already metallized. We observe that telescoping occurs in the inner layers not on the outermost layer. What could have caused this and how can it be resolved? Thank you very much.

Answer.

Below are a couple of typical telescoping problems where the telescoping is close to the core.  The top one of the two is caused by the rewind not being aligned properly with the unwind.  If the core is mounted and the web treaded through and taped onto the core, it is most likely that the web has been taped on to one side of where the true alignment is and so when winding the rest of the web through it re-aligns to its true position. Winding slowly this may not be immediately noticeable but as the web speeds up this will become more noticeable but by then it would be under vacuum.  Sometimes this is easily remedied by making the operators take a little more time to pull a little material through and more carefully checking the alignment before taping to web to the core. 

The lower picture can be produced by either not having the tail of the web aligned properly or by having a problem with the acceleration of the web. If the web is not properly aligned or if the profile of the web is not good the tension on one edge of the web may be different to the other side and so when the web is accelerated the tension difference pulls the web over slightly and gives the telescoping effect. The larger the tension mismatch the greater the telescoping offset will be.

I hope this gives you some ideas as to what you might be seeing in your films.

Humidity and metallized film.

Sir, I need some information about Humidity factor for Metallized Film.

1- What value of Humidity is necessary for storage of Metallized Film?
2- Is Humidity also effects process of Metallization?
3- What possible defects can occur in Metallized Film?
4- What are the temperature requirements for the Process of Metallization and for Storage of Metallized Film?
5- What are the recommended conditions for storage of Metallizeable film after slitting process?

Sir kindly guide me if you have any useful information regarding Humidity.

ANSWER

As far as I am aware there are no standards or set levels for temperature or humidity levels for the storage of polymer films or metallized films.  (I am sure someone will write in and correct me if I am wrong)

The humidity affects some films more than others, i.e. PET can absorb moisture whereas OPP is not so affected.

Films that absorb moisture that are used for metallization will take longer to pump down and may have a slightly higher pressure during metallization.  This can lead to small differences in reflectivity and resistivity between films. Similarly if you have a 'wet' or 'dry' season there may be differences in films produced in each season. 

Humidity can affect the film winding. If the humidity is very low the electrostatic charging of the film will be greater and this can lead to winding difficulties. As the humidity increases the charge leakage is speeded up and so in high humidity conditions the static charge will be much lower and there will be no, or fewer, winding problems.

Following metallization high temperature and high humidity can accelerate the oxidation process of the aluminium. However this is still a self-limiting process.

Thus for storage the temperature and humidity should be low but for processing ambient temperature and slightly higher humidity will be necessary.   Also remember that in transferring rolls from one temperature area to another the rolls should be given enough time to equilibrate otherwise there is a very high risk of telescoping the rolls as the polymer changes dimensions.  Also as the layers of film slip over each other there is an increased risk of introducing micro-scratches into the surface.

I hope this helps even though there is not precise answer.

Telescoping problem.

Why would a plain polyester film roll in a vacuum metallizer fail by telescoping during metallizing?

Answer

The standard cause of telescoping in a vacuum system is a roll that has been wound with too much air between the layers so that as the roll collapses as the air is pumped out the layers loose friction and any source of differential tension will cause the roll to telescope.

If the roll has a gauge band, or one edge has been better slit than other or the web is curved or has none symmetric residual stress can all be causes of uneven tension causing telescoping.

The most likely time that this will occur is during the initial pumpdown time.

If the telescoping is occurring elsewhere, other than the unwind roll then this is more associated with the misalignment of a roll where the web is winding off axis and then corrects. This tends to be smaller and more regular corrections whereas the telescoping on t he unwind roll can be catastrophic as the web may slide all the way to the one side of the winding system and may prevent the process running without further damage to the web.

The standard corrective measure is to make sure the incoming rolls are wound with either a little more tension or using a lay-on roll that helps squeeze out air during winding the roll.  Sometimes the soft roll is used to hide some imperfection in the roll such as a heavy gauge band. Insisting on correctly tensioned rolls can help highlight this problem before it gets metallized as well as reducing the problem of telescoping.

Counting pinholes

You mentioned that we could count the pinholes by using a light box and counting method. We have tried to check for pinholes using a dark room and by putting the VM sample against a 2,000,000-candle power lamp and were able to detect these pinholes. However, our supplier just uses a light box with 4 fluorescent lamps and a glass top. With this kind of light box, pinholes cannot be detected. We also counted pinholes with an area of 17 cm x17 cm and then just converted it to count per square meter. Is that the proper was to count pinholes? I would also like to ask what is the standard or specifications of the light box and what counting method should we use, what is the procedure?

Answer

As far as I am aware there is not a standard for the measurement of pinholes.  As such everybody makes or buys their own light box and decides on the area for measurement and works accordingly.

In theory the number of pinholes should be the independent of the light source but this is not true. This is because high intensity lights will make areas that have had some coating but are of reduced optical density appear as pinholes that with lower intensity illumination are not classes as pinholes.

If you consider how pinholes are produced you will see how the various shapes and types of pinholes are created. The first and generally by far the largest cause of pinholes is the dust/debris on the surface of the film that is present during the metallization process.  It is impossible, even with the best cleaning techniques, to remove debris of less than 0.3 microns in size due to the Van der Waals forces present. If you consider that this debris of up to 300nm size is considerably larger than the thickness of the aluminium coating that is generally around the 15nm - 25nm type of thickness.  If the film has not been cleaned, which is often the case; there will be plenty of debris that is in the range 1-10 microns, considerably larger than the coating thickness.  If any of this debris is moved following deposition it will leave behind and area that is not coated with aluminium, which is seen as a pinhole.  If any of the debris is moved, or drops off, during deposition there may be a reduced deposition at that point, which may not be classed as a pinhole with a low illumination light but will be with a high intensity light.

The second major source of pinholes is due to 'pick-off'. This is where the adhesion may be lower that ideal between the aluminium and the substrate. Usually the substrate is either through filled or is a coextruded film with the back surface layer filled. The filler is used to reduce the coefficient of friction by allowing the filler to roughen the polymer surface. These peaks where the filler pushes out the polymer surface will be in hard contact with the freshly metallized front surface as the film is rewound. As the winding is in vacuum it will always be a hard wound roll. The high peaks press hardest against the coating and in some cases will stick to the coating batter that the aluminium adheres to the front surface of the substrate and so when the roll is next unwound the aluminium is picked off and remains on the back surface leaving a pinhole in the metallized film. This type of pinhole can generally be reduced in number by the appropriate optimised plasma treatment that gives the maximum aluminium adhesion of the metal to the front surface.

A source of thinner coating spots, which under high intensity illumination might be classed as pinholes, could have been created in the following way. Polymer films are not 100% polymerised and will often have a proportion of short chain oligomer that is mobile and this can migrate to the surface. This oligomer is low molecular weight and may also reduce the surface energy. In other materials it is common to add a slip agent, or many other additives, these too may be of low surface energy or low molecular weight.  The slip agents, as their title suggests, are added to improve the film handling characteristics by reducing the coefficient of friction. Even with a filled polymer where the filler reduces the coefficient of friction to some level, slip agents can be added to reduce the friction further.  This oligomer or the additives may have a lower melting point than the polymer and the material may be vaporized during the deposition process locally reducing the sticking coefficient of the aluminium and so locally reducing the thickness of the aluminium deposited.

High intensity sources will also tend to allow smaller diameter pinholes to become bright enough to be included. I did some work in counting defects and I used and image capture and image analysis to automate the process but this needed to have thresholds set for the minimum contrast difference that would count as a pinhole. Thus there is a margin of interpretation between what is a pinhole and what is not. This will change with intensity of illumination as the sensitivity on small diameter pinholes is improved.  It can also mean that the diameter of the larger pinholes is over estimated with high intensity lights as the detector can become saturated and there will be more of the light diffraction included in the measurement.

Hence suppliers may wish to use a lower intensity illumination because it will reduce the number and size of the pinholes counted. End users may wish to have higher intensity illumination because this gives a more realistic, if more frightening, count of the pinholes present.   At some point an agreement and a correlation between the supplier and customer measurement equipment and technique must be made. Otherwise there will be an ongoing battle over the number of pinholes and the film quality.