Vacuum Web Coating

Transparent metallizing & what are Zones?

ANSWER

Transparent metallizing can refer to a number of different products. This includes depositing aluminium metallizing that is subsequently oxidised to form transparent aluminium oxide such as by the process patented by Amcor (Camvac) a number of years ago.  It can also refer to the deposition of zinc sulphide (ZnS) that is a transparent high refractive index material that is used for transparent holograms where the high index ZnS on the low index polymer gives rise to some reflectance at the interface. Moving away from the resistance heated boat technology and using electron beam deposition technology it is possible to reactively deposit many materials such as silica, titania or alumina of which the alumina and silica are used for transparent barrier applications.

Zones.  A zone is just an area or volume. Typically metallizers are separated into the 'winding zone' and the 'deposition zone' and the pressures maintained within these zones are allowed to be different. The zones have to be separated by plates but as the web has to pass from one zone to the other and back again there has to be small gaps known as conductance gaps. The gas in the higher-pressure zone will leak into the lower pressure zone through these conductance gaps.   Generally in the winding zone there will be the roughing pumps and cryogenic (Polycold or equivalent) pumping. This is because most of the water vapour can be expected to be released into the vacuum system at the point the web is unwound.  However experiments have shown that it is worth including some of any cryopanel in the low-pressure deposition zone. Typically the pressure difference will be one to two orders of magnitude difference between zones.  Where a larger pressure difference is required an intermediate buffer zone may be included.

Zones are also used where multilayer deposition is used around the same deposition drum. The zones may be used to keep apart the deposition of pure metals from compounds such as oxides where leakage of the oxygen from the oxide can contaminate the metal deposition.

Thus the use of the word zone is not necessarily very specific.  I would take the term double zone metallizer to indicate the winding is separated from the deposition area. The intention being to minimise the contamination of the depositing aluminium with any water vapour which could cause unwanted oxidation or the formation of some hydrate on the aluminium metal surface.  The lower the contamination level of the aluminium will give a greater reflectivity and would be expected to improve the barrier performance and electrical conductivity.  So it is not necessarily for a special product but just for a higher quality product.

Double side metallized film and problem with scratching

I want to understand the product both side metallized film application.
How both side metallized film of 2.2 OD can be produced without any scratches?

What is the roll configuration for making both side metallized film of 2.2 OD?

Suggest a method how to produce the both side metallized film of OD2.2 each
side in present metallizer, in which we only produce one side metallized film?

ANSWER

Sometimes it is hard to achieve the degree of barrier that is required by the customer. Pinholes are the most common problem in achieving high barrier performance and a simple method of improving the barrier performance is to double side metallize the web. In this case there are still a high number of pinholes but the statistics are that few, if any, of the pinholes will be directly opposite each other across the film and so any gas leaking through one pinhole will not have an easy exit route.  This tortuous path is the basis of many different barrier structures including the so called ‘ultra-barrier’ coatings for the barrier materials for the display or solar cell industry where the barrier requirements are around 6 orders of magnitude better than those required for food barrier packaging.

Aluminium is a soft coating and can easily be scratched.  However aluminium also oxidises easily and aluminium oxide is a much tougher material.  Normally there is time between metallizing and any further downstream process and so there is time for a very thin, but still tough, aluminium oxide layer to form.  When a film is double side metallized it is common to want to aluminise the second side immediately after the first side however when this is done it is common to find the first side has a problem of scratching.  A number of companies have found that delaying metallizing the second side for a couple of days, or more, allowed the oxide layer to form on the first metallized side and the scratching problem disappeared.

Unless the machine was configured with 2 deposition drums and the right drive controls there is no way to complete double side metallizing in the same process. It will always have to be done as two separate metallizing runs with the web repositioned between each run. 

If you are monitoring the coating thickness by optical transmittance or resistivity using an eddy current monitor then an easy way of setting the new levels for the second side coating is to calibrate the monitor using two sheets of single side metallized film. As the film transparency without any coating is high there will be little difference in having two layers of polymer included in the measurement. By using two sheets if single side metallized film the measured transmittance will be equivalent to a double side metallized film. The resistivity measurement is also going to be similar for two single side metallized films and a double side metallized film as in both cases the total metal quantity is the same and both have the metal separated into two layers.

Be aware that running the second side metallization may not be as simple as running the material through at identical process conditions as the first side.  In the deposition zone some of the heat passes through the web directly to the cooled deposition drum. In the case of the second side deposition the heat passes through the polymer web until it reaches the metal deposited on the first side and then it is reflected back through the web.  The heat transfer coefficient is also different between the web and deposition drum and metallised surface and deposition drum. Also once the polymer film is metallized it is impossible for the water to diffuse out of the back surface and assist the heat transfer between film and drum. Thus the heat transfer coefficient is lower than for direct film to drum contact.  So it is common to have to run the second side metallization through more slowly because of the limited cooling available.  This can be improved by the gas injection system where some gas is forced into the gap between web and drum to compensate for the lack of water vapour in the gap.

Problem with spreader roll and wrinkles.

We have metalliser and are facing problem of crease formation at rewinder. In the rewind assembly there are two idle rolls. One is rubber spreader roll and other is lay on roll. We had tried by altering the winding mode of rewinder under wind to over wind and vice versa but of no use as the crease continues to form. Afterward we had interchanged the position of spreader roll and lay-on roll, now spreader roll is in contact with rewinder roll. In this fashion no crease formation is seen.

My question

1)         Is there any impact on metallised film with this arrangement.

2)         Can lower micron(10µ)run with out physical defect.

3)         What are the possible defects can arise with this arrangement.

4)         why crease formation is not there in altered position of rolls.

5)         is the life of rubber spreader is over.

6)         what are the problems can arise if the pre drum spreader roll is having imbalances.

7)         Can imbalance of pre drum spreader roll contribute to the formation of tramlines and creases

8)         How the banana roll bow can be adjusted and what is optimum angle can be kept for almost all type of width can run without any change in the angle.

ANSWER

The pre-drum spreader is to put some lateral tension into the web before the web sees the deposition zone where the rise in temperature will make the film want to expand. The initial expansion relaxes this tension and then as the expansion continues the web does not move on the drum and so a lateral compressive builds up.  If the lateral compressive force is too much the web will buckle off the drum and you see this as the formation of a tramline.

This pre-spreader roll usually has a soft polymer surface and it relies on the softness of the polymer to deflect which results in the lateral spreading of the web to take place.  Over time these rolls age and become harder and so lose some of their performance.  The roll can have a rubber, elastomer or polymer material as the covering, depending on the material will depend on the aging process. Some age more quickly with heat others can be simply be dried out slowly by the vacuum and others can be hardened by an interaction with solvents that may be used to clean the rolls as part of routine maintenance.

Another complimentary method of reducing the wrinkling is to inject a gas between the web and the deposition drum. Part of the problem is that the web cannot easily move on the deposition drum hence as the web comes in contact with the cooled deposition drum it wants to shrink as it cools to reach the same temperature as the  drum.  The tension is applied to the web to hold it hard against the deposition drum to help maximise the heat removed from the film during the deposition process. The combination of the coefficient of friction and the tension applied to the web not only holds the web tight against the drum but also stops the web moving relative to the drum even though it is contracting and then expanding and finally contracting again with the changes of temperature.  Injecting gas behind the web lubricates the back surface enabling the web to move relative to the drum and so reducing the chance of the web buckling. The trapped gas also increases the conduction part of the heat transfer coefficient and so reduces the film temperature too. Hence there are two mechanisms that help stop buckling.

A further point on buckling relates to cleanliness. Some work done by Mike McCann showed that any dirt on the drum (even down to a micron or more) is sufficient to lift the web off the drum and cause a local overheating and adding to the buckling forces. Thus wrinkles will happen more frequently on dirty machines than on immaculately clean machines.

The positioning of the wrinkles can also be affected by the residual stress in the web as too any profile variations can affect the position of wrinkles.  The spreader is always set assuming the tension will be uniformly distributed across the web. So if the web has a high spot that takes most of the tension the rest of the web will be under less tension and will wrinkle more easily.  This also means that rolls of film from the edges of larger rolls, that have an angled stress profile, can more easily crease than rolls slit from the centre of larger rolls.

Many of the spreader rolls are made from a soft elastomer and these can harden with time. The spreader action occurs because of web tension that act on the angled slits and that in turn causes the roll to defect giving the spreading action. This spreading action is reduced as the roll gets harder because the elastomer deflects less.  Rolls harden due to a number of reasons such as drying out because of being in vacuum or heat or solvent attack during cleaning. 

I am not sure of a 'universal' position for any banana roll, I have set them to work for a series of similar films but always expect to make adjustments for lager thickness or material differences. However I am no expert in using this type of roll and so I hope other will post additional information on this. 

Down-gauging is possible but there can be limits to how thin a web can be used.  Thinner webs need to be handled much more gently and so the tensions need to be reduced and also the sensitivity and accuracy to monitoring the tension needs to be improved. Moving down from 12 microns to 10 microns may be possible for good quality film but some of the problems of gauge variations and residual stress non-uniformity will be more exaggerated.  Down-gauging can also be more of a problem for wider width machines. Again this is a reflection of film variations across the width of the web.

Regarding the lay-on roll and spreader roll at the rewind I would suggest talking to the machine supplier and asking their for their comments and advice. The spreader roll stretches the web laterally but this effect only lasts for a short distance following the roll.  So it depends on the distance between the spreader roll and the point the web touches the re-wind roll. If this distance is too large the spreading effect will be lost. If this process was working originally but has become less effective then I would suspect that this may again be a problem with the elastomer material aging.

I think that this covers most if not all the questions but not necessarily in the original order.   Others may have different experiences and so please feel free to post comments.

The perennial problem of tramlines

Initial Question

a)During metallisation excessive tramlines formation is observed.

There is not any effect of change in tension values of draw rollers on tram lines.
What are possible reasons of tramlines formation and how they can be controlled. What are the points on which we act upon.

ANSWER

The main cause of tramlines is too much heat during metallization.

It may be possible to reduce the problem by either using a spreader roll to lay the film on the deposition drum.  The web dimensions will try to change whilst on the deposition drum. Initially the spreading will put a tensile force transversely across the web. This will be increased as the film is reduced in temperature whilst in contact with the cold drum.  Then when the film is in the deposition zone the web will expand and will be put under compression. If the transverse compression force is too much the film will buckle off the deposition drum and the tramlines are formed.

Thus increasing the spread of the film initially, pulling more tension, reducing the deposition drum temperature will all help resist the buckling.

Another thing that can cause the film to buckle off the deposition drum is defects in the deposition drum, dirt or debris on the deposition drum or the back surface of the web.  Any defect or dirt will keep the film from laying flat on the drum and this will prevent the transfer of heat between the web and drum and cause the film to buckle off the drum more easily.  Thus making sure the deposition drum is smooth and clean is essential.

The final method of reducing tramlines is too instead of trying to resist the buckling by preventing the web form moving across the drum surface is to do the exact opposite. This is to force some gas between the web and drum and this acts as a lubricant and allows the web to slip across the drum and instead of the film buckling it slides over the drum and does not buckle off the drum surface.

Follow-up question

One more thing that is required to be explained to you is that from analysis we found that, tramlines formation is only in trim side rolls of film line. From the film line we get jumbo roll in 8.4mts width, same is slit at primary slitter. the center position slit roll runs very smooth in metalliser, but both of the side slit rolls of jumbo forms tramlines. Tramlines came initially up to 4000 to 10000mts.Can you explain why this is happening in side rolls only.

ANSWER

I think you have already identified the problem in an earlier e-mail. The bow angle produced in the film manufacturing process is most uneven at the edges of the mill
roll and so the rolls slit from the edges can be expected to have more winding problems as one edge is likely to take most of the applied tension. This difference in tension will means that within the deposition zone the heat transfer coefficient is likely to be different across the web with it being highest where the tension is highest.

The reason the tramlines do not appear immediately can be that it takes time for the heat to build up in the deposition zone. The heat load during deposition is not constant. The deposition drum will be coolest before deposition starts once deposition starts the recycled coolant will be likely to increase in temperature.  Many chillers do not keep the liquid at a constant input temperature but allow some temperature increase. This slight increase in temperature will adversely affect the heat transfer coefficient and hence it will take ea few minutes for the web temperature to reach the temperature at which the tramlines appear.  It may be that if you monitor the temperature of the coolant input liquid you will be able to see this temperature increase.

Internally, in the deposition zone, as the aluminium is also deposited onto the deposition shields these shields will increase in temperature and these will radiate heat which adds to the heat load on the web. Hence these too will contribute towards the time constant between the deposition process starting and the tramlines starting.

As ever it is the combination of several things that each contribute to the tramlines.  Increasing the cooling of the web, either by increased cooling to the deposition drum or improving the heat transfer coefficient by use of the gas injection technique, if the gas injection is already used increasing the gas volume injected will increase the gas pressure between the web and drum and can increase the heat transfer coefficient further. Also an increase in tension on the web, if possible, can also help increase the heat transfer coefficient.

Reflectors on a white substrate

When we try to metallize a white substrate in our vacuum metallizer it always turns out gray. I am looking for a bright silver look. Any ideas would be appreciated.

ANSWER

The simple answer is that you are probably not depositing enough aluminium and
so you are seeing mostly the aluminium but also a proportion of the white
background & so your eye averages the two and sees grey.  Check out the percentage
reflection or transmission that you are depositing.  You need to be depositing a truly opaque aluminium.

A second possibility that produces a matt surface rather than a bright specular reflecting surface is that the method used to get the white film is to pack the polymer with fillers which may have increased the surface roughness substantially.  So if what you are seeing is a matt surface more than a grey but specular reflecting surface it could be this that is the cause.

The next danger you will run into is that if the reason for the grey is simply the aluminium thickness is too thin then as the aluminium thickness is increased there may be preferred growth planes to the crystals & these can result in an increased surface roughness which results in a decrease in the specular reflectance and an increase in the diffuse reflectance.  This can have a number of different appearances from a milky surface that is semi-reflective with a white sheen over the reflector or it can be more matt and grey.

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Barrier / CoF ??

Is barrier related to the Coefficient of Friction (CoF) of the film?
Plus I am seeing a large variation in the mechanical properties of film after metallisation...ie shrinkage..

After metallising 2 rolls from the same extrusion we get a lot of variation.

Answer.

The barrier performance is not generally directly related to the CoF.  However changing the CoF can result is handling problems that can then affect the barrier performance.  Barrier coatings are invariably thin and can be easily damaged and so if the film does not handle well the stick-slip behaviour can result in an increase in surface scratching which in turn will reduce the barrier performance.  The barrier performance in most vacuum deposited coatings is determined by the number of pinholes in the coating and the number of pinholes is related to the film surface quality. The dirtier the film or the rougher the surface the greater the number of pinholes and the worse the barrier performance.  The coating will cover the dust/debris but if this dust/debris is subsequently moved there is an uncoated pinhole left that has no barrier performance at all. If the film is handling badly any slip can also slide the debris along giving not just a pinhole but also a scratch in the direction of sliding away from the pinhole worsening the barrier performance.

Thus measuring the CoF cannot give you a measure of barrier performance, nor measuring barrier performance give you a measure of the CoF. They are independent from each other but a high CoF can lead to winding problems that can adversely affect the barrier performance reducing it from its initial value.

The metallizing process puts heat into the film in the deposition zone. In this heated zone the polymer will want to expand and in doing so will reduce the tension applied. If the temperature goes high enough the polymer will relax and any locked in residual tension will be released.  If the film was produced by extruding and then biaxially orienting the film by using a forward draw process followed by a sideways draw process the resultant film will have a stress profile. The most uniform stress will be along the centreline of the full width manufactured web and the most uneven stress will be at the edges of the web. No most metallizers cannot take full width rolls that can be 10m wide and so they buy rolls slit from this 'Mill' roll. Thus they may get 5 metallizer width rolls out of a single mill roll. Of these 5 rolls the best would always be the centre roll, the next best the two adjacent to the centre roll and the two worst rolls would be the two from the edges. 

Thus what you are likely to be experiencing is a difference in shrinkage that reflects the position of the rolls relative to the manufacturers mill roll.   There are some manufacturers that offer low shrinkage rolls that have had the final polymer passed through an additional oven at low tension to try to remove as much of this residual stress as possible but this is a slow and hence expensive process and they charge a premium for the film.

It may be that your rolls fall into a series of groups. Ones that have no problems and these may well be from the centre of the mill roll. Others may tend to shrink more on one edge and other may shrink more on the other edge and these would suggest they were taken from the edges of the mill roll.

If you are able to reduce the heat load during deposition by reducing the deposition drum temperature or improving the heat transfer coefficient,etc this might improve things.