Vacuum Web Coating

Problem of Static

We are getting higher Static Charge development in the slit reels, more in lower thickness and smaller widths.  Please advice, How to get it reduced. Our slitting machines are equipped with mechanism of discharging. Yet it is not working properly in thinner films and lower widths.

Answer

Do you measure the static charge on the web?

When you mention a static neutralizer is this a simple carbon brush, tinsel wire, or an electrostatic discharge bar. If it is the latter is this a fixed voltage system or is there some kind of feedback or tuning?

The reason I ask these questions is that there are a variety of possibilities.  If the web material is the same for the thicker and thinner films it may be that all you are seeing is that the static charge is the same on both the films but the lower stiffness of the thinner film is showing the effects more visibly than the thicker film.  Typically if the webs are the same material then over the same rollers you would expect the triboelectric charge that is generated will be of the same magnitude. 

The charge generated can vary also with winding tension and well as humidity.  Humidity is a mechanism to dissipate charge and so on days of high humidity the charge generated will be lower than on days of low humidity. 

Static will be generated at every roller in the winding system. The static charge is cumulative and so it is important to have a measure of the charge on the web so that the positioning of the neutralization system can be optimized. It may even be that more than one neutralization system is needed.  The static charge will be generated on both sides of the web and so it is also important that the neutralization is done on both sides of the web. 

So I would suggest that you measure the static charge during winding at different positions along the web and look at how effective the neutralization is. This will also tell you if the position of the neutralization is in the correct position. In some cases it may be necessary to add a second neutralization station just before the rewind.

If the neutralization is an active one then it is also possible that it is set too high and instead of just neutralizing the surface it is in fact charging the surface but with the opposite polarity.

Seed layer for coating silver on glass

We are currently trying to deposit Ag on glass and have been told to deposit a layer of Nichrome first for better adhesion, maybe 5 to 20 angstroms. Is this necessary or should a glow discharge cleaning be good enough to get adhesion to the glass we want a nice bright coating, second surface, and the nichrome goes down a little dark. Any advice or experience would be greatly appreciated.

Answer

There is a tendency for the silver to not wet the glass well and so the coating will have to be quite thick before it becomes continuous and this also causes a higher surface roughness and this can reduce the reflectance.  A glow discharge clean will raise the surface energy of the glass and improve the wetting and hence produce a smoother coating as well as a continuous coating at a lower thickness. The adhesion between silver and glass may not necessarily be good and hence the recommendation to use a tie layer. The choice of tie layer is usually as a result of many peoples experience in what works well and what doesn't and hence the recommendation for the nichrome. However if you look at the work done with heat mirror coatings using titania/silver/titania you will find that to be able to use thinner silver coatings and keep them stable in the vacuum deposition environment some companies use either tie or protective thin layers on either side of the silver.  They too do not want the thin tie layer to become part of the optical performance and so the layers are kept very thin and often they try to match materials. So that if they are using a silica or silicon nitride layer next to the silver they will use a silicon thin tie layer but if they are using titania next to the silver they will use titanium. The thickness is also as has been recommended to you at less than 3nm. The expectation is that these tie layers will be discontinuous at this thickness and that they will also be easily oxidised and so optically they will disappear.

Thus for your silver coating I would plasma clean the glass, use the nichrome but aim for a thin layer of  1.5 - 2nm before depositing the silver.

One other thought that does cross my mind is, how are you determining the thickness of the nichrome layer? It is very easy to think you have a very thin layer but in reality to have quite a thick layer that then acts differently. How is the nichrome being deposited, by sputtering or some other method? If you are sputtering it will be easier to reproducibly deposit a thin layer but if you are using an electron beam gun there can be problems of deposition rate stability which can lead to thicker tie layers.   

So I would also check carefully that you are depositing a tie layer that is thin enough and not too thick.

I hope these thought help

Question re. Plasma treatment

I want to know how and up to what extent the distance of plasma target plates and substrate surface can affect treatment  & is there is any possibility of back treatment during plasma treatment because we are getting both side 70+i.e. water hold on both side of metallized film if yes then can it impact bond strength.

Answer

It depends on the geometry of the plasma treater and the positioning of the web as it passes the surface.  The pressure of the process defines the mean-free-path of the gases and if the gap between either surface is larger than the mean-free-path then it will be possible to generate a plasma on both sides of the web and hence yes it would be possible to be plasma treating both sides at the same time.

As to whether it can affect the bond strength the answer is yes. Any plasma treatment can affect the bond strength if the treatment is on the surface that is involved.  If you are asking can plasma treating the back surface affect the bond strength on the opposite surface the answer is no although it can have other effects that may cause problems. The back surface may have low molecular weight material present either by design as in the inclusion of slip agents or by default as in oligomers.  This low molecular weight material may aid the handling characteristics by lowering the coefficient of friction.  Plasma treating the back surface and removing this material may raise the coefficient of friction and cause the film to no longer slide over itself so easily.  This in turn can cause an increase in pick-off or scratching damage.  Neither of these effects will affect the bond strength.

The front surface treatment is the one that will affect bond strength and so long as this has been optimised you will get the best possible bond strength from this treatment. If the treatment has not been optimised the bond may be lower than ideal and it may also vary with varying conditions such as humidity of the air wound in the film used.  The plasma does not get diluted by being shared between the back surface and front surface. Essentially the plasma will have the same effect on all surfaces. 

I hope this helps with your problem.

Pattern metallization – AIMCAL Fall Conference preview

For those of you interested in pattern metallization there are three papers that will be of interest to you at this years AIMCAL Fall Conference in Myrtle Beach, USA, Oct 19-22. 

The first of these has been long awaited and is from Nick Copeland of Bobst General Vacuum and is about their ability to build in pattern metallization that can be kept in-register with existing patterning on the web.  This becomes important where webs have been either pre-printed or pre-embossed with a pattern.  Using a reference marker they are able to positioning of the web to keep the metallization pattern in register with these existing patterns to a high degree of precision.  This increases the flexibility of the process and allows it to be used in a number of the more challenging markets including security markets.

            The other two papers are both by Leybold Optics, the first by Christopher Schmitt on the measurement of patterned coatings and the second by Anye Chifen on modifications to the oil masking process to reduce the oil required and improve the quality of the patterning process.  The measurement of patterned materials can be a problem. Most measurement systems are taking an average measurement either optically they are averaging the light passing through an area or electrically they are averaging the conductivity based on an area of coating. In either case if the area is varying because of changes in the pattern shape the measurement will also be changing. If the measurement is changing it is then difficult to use this measurement as a feedback signal to control the deposition process and so control the coating thickness.  In this instance they have developed a method of taking Optical Density (OD) measurements from a very small area and so are more easily able to choose an area that is not fluctuating in size.  The second paper describes a different method of supplying the oil to the oil masking process. Typically this is done by heating the oil to vaporize it inside the vacuum system but in this case a metered flow of oil is injected into a hot evaporation box inside the system where the oil can be vaporized and via nozzles onto the web to provide masking.

            So for anyone either already involved in pattern metallization or those who are thinking about adding this to their skills there are three more good reasons for attending the coming conference.

Flake pigments – AIMCAL Fall Conference

Just to whet your appetite for the Fall Conference in some of the future posts I will highlight some of the papers that are expected to be given later this year at the AIMCAL Fall Conference in Myrtle Beach in the USA (Oct 19-22).

For this first tempter I will tell you a little about a paper being presented by Craig Engle of the South Western Research Institute (SWRI) on nano-platelets.  First let me say a little about the topic of nano-platelets.  This covers a wide range of materials that can vary in size range quite considerably. The platelets you are all probably most familiar with are those that are used in inks and paints.  Aluminium inks and paints have improved in reflectivity and brightness over the last few years.  The original pigments were balls of aluminium that were put in a ball mill with some very hard spheres that would flatten the softer aluminium into flakes. These flakes were not perfectly flat but were much more wrinkled and are often referred to as cornflakes because of the similar appearance. The next improvement was to polish some of these cornflakes and make them smoother. These too were not perfectly flat but were more lenticular in shape because the flakes would rock as they were being polished. The latest method of producing flakes is to coat a release layer onto a roll of substrate, often polyester film, and vacuum metallize aluminium onto the release layer. This metallized film is then passed through a liquid that dissolves away the release layer freeing the aluminium up. The aluminium is very thin and the film breaks up into flakes.  These flakes are as flat as the polymer film and so the reflectivity and brightness are far superior to the other methods of flake manufacture but the vacuum metallizing is a more expensive process that the simple ball milling.

            This paper from SWRI adds some new developments onto the manufacturing process.  The standard method of producing flakes in this way does nothing to control the size and shape of the flakes. The flakes produced will be random in shape and the sizes will be from hundreds of microns across to fines that are dust.  To make the flakes useful they have to be sized or milled and sized with the different size ranges giving different optical properties when they are used in the inks or paints.  Where these high aspect ratio flake materials can be used in other applications there is a preference that they are not only size more uniformly but also the shape is more regular than random.  What SWRI have done is to modify the surface that the metallization is done onto so that they can control the shape and size. Also as is implied by the title they have done this at a very fine level so that they can control the flakes down to the nanometre level.

Question about too much (?) cooling

I want to know the effect of over cooling during metallization, how does the cooling impact on bond strength and other quality parameters of metallized film.

Answer

When the substrate enters the deposition zone it is heated up by the radiant heat from the red hot evaporation boats and well as the heat of condensation of the depositing metal. What you are achieving with the cooling is to reduce the temperature rise that the film reaches.  Without any cooling the film would typically exceed 100 degrees Centigrade and for a thick coating at high speed could even exceed 150 degrees Centigrade.  By using a cooled deposition drum the substrate is pre-cooled often to subzero temperatures and even for the same rate of heating this will reduce the maximum temperature by the same difference as the minimum temperature is below room temperature. So if room temperature is 20 deg C and the deposition drum uses a glycol/water mix and allows cooling down to -20 deg C then the difference is 40 deg. So if the maximum temperature the film was reaching without a cooled deposition drum was 125 deg C then the maximum temperature with the cooled drum would be 85 deg C.   In reality this would be better than this as the film continues to be cooled throughout the deposition zone and not just pre-cooled.

The cooling continues after the deposition zone as the film is still in contact with the cooled deposition drum and this brings the temperature of the film back down to room temperature.

There is the possibility that the cooling available brings the temperature back down to below room temperature. Usually the film as it then passes over other rollers, which are at room temperature, will equilibrate to room temperature before it is re-wound into a roll.   If the film does not equilibrate and is wound cold then it will have the problem that as it rises in temperature on the roll it will loose some tension as the polymer expands and during the temperature change not all the layers will be at the same temperature and so moving it at this time can lead to slipping of the layers (telescoping).  This is also true of rolls have been stored in a cool warehouse and moved into a warmer area for deposition.

In terms of the deposition process the difference in temperature between an un-cooled film and a cooled film at the point of nucleation is only around 40 degrees and so there will only be a small difference in the nucleation size, with the hotter film having the slightly higher crystal size. Similarly with the continued growth the hotter film would tend towards a larger crystal size over the cooler film.  But this will be marginal compared to the difference that can be created by plasma treating the film surface to change the surface energy and hence the wetting of the film.

Adhesion, it could be argued, would be better for a cooled film as the differences in thermal contraction of the polymer and metal will be lower from the lower peak temperature.  The coefficient of thermal expansion for a metal is much lower than for a polymer and so as the temperature is reduced the polymer shrinks more than the metal coating does.  This can put a strain on the interface. If the temperature peak is reduced there is less contraction and so less strain on the interface. Adhesion failure occurs when this interfacial strain exceeds the adhesion and so minimising the strain is useful.

I hope this answers your question

Vacuum Technology & Coating.

I would like highlight the above journal which for those of you with internet access can be viewed at either www.vactechmag.com or www.vtcmag.com

In the March 2008 edition (Vol 9, No 3) there are a couple of articles that I think will interest many of you.

The first is one of the ‘Guides to vacuum technology’ and is ‘Outgassing – part 1 Desorption of molecules from surfaces’ by Luke Hinkle   This article starts by defining outgassing as being one of 4 phenomena comprising 1) surface desorption, 2) Contamination evaporation, 3) Diffusion from the bulk or 4) Sublimation.  The article then goes on to outline a ‘battle plan’ to minimise the problem which consists of two parts the first is to reduce the presence of water on surfaces and the second is to stimulate the molecules to assist them leaving the surface.    The article then goes on to describe the options for preventing the original absorption of material on the surfaces such as by venting using dry air or nitrogen.  To speed up the removal of any material there are various options such as baking the system, using ultra-violet radiation or bombarding the surface with energetic ions.  Water is held onto the surface by a binding energy approximately 100 times greater than its average thermal kinetic energy.  In the normal course of events the water molecule once attached to a surface can experience thermal agitation that is, in general, insufficient to free it from the surface.   The use of any of the above methods of adding energy increase the thermal agitation and more of the molecules will shake free of the surface. 

This gives a flavour of the article which goes into much more detail.

The second article is ‘Plasma Chemistry’ by Abe Belkind & Kurt Becker.  Virtually all vacuum deposition processes make use of plasma cleaning and in addition there are an increasing number of plasma enhanced deposition processes and plasma etching processes.  As such this article could well be of great interest to many readers for a wide variety of different reasons. The article gives a detailed description of the chemistry that occurs within a plasma including the formation of ions and radicals in all their different forms.  The article then proceeds to describe how the plasma can be used in a number of these different applications including some other non vacuum applications such as ozone generation for water treatment or as used for car exhaust cleaning.

Although this article may not change the way you use the plasma on your own system it does at least explain how the chemistry occurs and what the limitations might be.

Web Cleaning - Tacky rolls update

I was recently at the SVC annual technical conference where I listened to a paper given by Sheila Hamilton about the Teknek tacky roll technology.  I have long been enthusiastic about this technique for removing debris from webs or foils and think that it is underused within the vacuum coating industry. 

The process uses an elastomer roll that is in contact with the web surface where debris is removed from the web and sticks to the tacky roll. This process can remove debris of size down to 0.3 microns.  This is a simple process but effective.  As you can imaging webs have a high debris level and so it would be easy for the tacky roll to quickly become clogged with debris and thus lose the effectiveness as the tacky surface is covered in debris. To recover the tacky surface this roll is usually in contact with a roll with a higher tack that accumulates the debris. This roll will also become clogged but it is like an onion, built up of many layers that can be peeled off revealing successive layers of high tack surface.

As most pinholes are caused by debris being coated and moved after metallization the reduction in debris levels is an easy way of reducing the number of pinholes in the coatings.  As with any cleaning technique it is important to do the cleaning at the correct point in the process as well as to consider cleaning both sides and not just the side that is to be metallized.

If the roll of material is to be cleaned on a winder before it reaches the metallizer but if this is done it is important that the web is kept clean following the use of the tacky roll.  As polymers winding over rolls produces an electrostatic charge the web can attract airborne debris and so it is critical that between the tacky roll and rewinding the web is kept in a clean environment such as under a positive pressure clean air hood or, if the application demands it, in a cleanroom.

One approach I have been advocating for a number of years now is to incorporate the tacky rolls into the vacuum system. I have tried to encourage vacuum system suppliers to develop this process but the progress has been slow. I know that some companies (2 in the Far East & 1 in Europe) have incorporate the process and I now hear that Ulvac have started to offer systems that include the tacky roll. There are limitations though.  The systems they have added it too are all ones where the roll length is short and so they do not have to peel off layers from the high tack roll and this considerably simplifies the process.  Until this automated peeling process is developed to be robust for production so that the roll length can be increased to the long lengths used in modern metallizers I expect that these roll will only be used on the higher technology processes where roll lengths are short enough and the webs already clean enough that neither tacky roll becomes clogged by a single roll.

However I regard this as encouraging. Many people did not believe that the process would work well in vacuum. It is interesting that the elastomers were developed with the space programme in mind and so the problem of outgassing and loss of tack had already been addressed and was not the limiting factor that some expected.  Reports of some tack rolls ‘drying out’ are probably as a result of having them exposed to the plasma cleaning process, which because of the continual bombardment of the surface will break bonds on the surface and degrade the performance.  This is simple to prevent by suitably shielding the plasma from the tacky rolls.  I say simply but I suspect that if you want to add the tacky rolls as a retrofit item then space will be limited and shielding may be more difficult. New machines will be able to be designed with this in mind and suitable shielding should not be a problem.

Another piece of information that I was not aware of before is the high temperature use of the tacky rolls. The elastomers are stable o high temperatures and have also been used as a cleaning roll for hot embossing shims.  These embossing shims can pick up debris, oligomers, additives or fillers as the come in contact with the polymer surface and these can accumulate in the grooves of the metal shim reducing the quality of future embossing. Cleaning the shim is always an issue. Some use release agents to lower the shim surface energy and prevent this pickup but this can transfer onto the polymer and make the subsequent coatings hard to adhere to the surface. Thus the tacky roll method of cleaning has an advantage of cleaning the shim but not adversely affecting any subsequent adhesion.

So as you can see my enthusiasm for this tacky roll technology has not been diminished but has rather been endorsed with the news of the first machines being sold with this technology included.  I am still convinced that it is only a matter of time before this process is available for metallizers and for those wishing to produce pinhole free coating will be an essential part of any new metallizer specification.  I suspect that this process will be speeded up if any of you who are considering buying a new metallizer start to ask is the tacky roll cleaning available in the metallizer.  If the machine builders get enough people asking about this technology they will speed up their development to make sure it becomes available to meet their customer needs.

Until then watch this space & I will periodically report on progress.

A problem of un-metallized streaks

I would like to inform you that we are receiving metallized BOPP film from outside and facing the problem of unmetallized streaks on the surface which is more visual after the lamination to other printed substrate. This sort of defects we are getting normally and have made complaint and also claim for the rejection due to this defect. I am requesting you to pass the information why the unmetallized streaks appear on the surface to some part of the metallized BOPP film.

Answer

Unmetallized streaks.

It would be interesting to know more about the unmetallized 'streaks'.

Are they irregular in shape or parallel stripes?

What is the size of the streaks?

To me the word streaks suggest the streaks are anything from a few centimetres in length to a meter or more and irregular in shape.  Is this correct?

How wide are the rolls? 

What position are the streaks found, are the randomly anywhere across the whole width, and do they start immediately on the roll, are they found throughout the whole roll or are they only found part-way through a roll?

One possibility is that during metallization the heat load is close to causing the web to balloon off the cooled deposition drum. As the web sees the heat load from the deposition source the polymer wants to expand and if this expansion is too much the web buckles off the drum and if not controlled this will result in a 'tramline or railroad line'. This is seen as a parallel stripe of material that has a thinner coating on it. The thinner coating is because as the web lifts off the deposition drum the polymer expands and thus the surface area is greater than it was when on the drum and so the same depositing metal has to cover a larger surface and so the coating is thinner. Also the temperature is likely to be higher and so the sticking coefficient of the aluminium will be lower and so not all the metal will stick and this further reduces the coated thickness.

If these thinner coated sections appear and disappear this may be because the process is just on the knife edge of too much heat. A little more heat and the web would permanently lift off the drum and a parallel line would be established and possibly lead to a wrinkle in the roll.

All aluminium coatings oxidise. The thickness of the oxide layer can depend on many things including deposition rate and coating structure.  The thinner deposition area will have a lower deposition rate and so will have slightly higher oxygen content and also because of being thinner the surface oxide will potentially have a greater effect on transmittance than on the thicker areas.

Often the areas where wrinkles or tramlines appear are after the process has been running for some time and the heat load has gradually increased over time. Also they tend to be towards the centre of the roll. Near the edges of the polymer web it is possible that any expansion of the polymer web can overcome the polymer to drum friction and the web slips laterally on the drum and so relieves the lateral tension. However in the centre of the web the friction is too much and such expansion is less likely. Hence of ten the tramlines are only seen in the centre section.

Thus my questions about where the streaks are found within the roll help me to diagnose the probable cause of the defects. 

I hope this answer helps explain what might be occurring.

Moisture & metallizing

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.