Vacuum Web Coating

Roller problem creating ‘starry-sky’ effect

We are facing a problem in our metallizer, the chrome coating of aluminium roller has been vanished and metallized side of film is directly touching aluminium and creating star-sky effect (metal chip-off / unmetallized spots), we have around 60 days to get new rollers so mean while could you suggest some option to cover aluminium roll surface to avoid this problem, we have tried to cover aluminium roll by cork tape, PP tape & Teflon cloth but we don’t get satisfactory results.

Answer

The 'starry sky' is caused by dirt on the web being coated and then moved following metallization.  The best way to improve this is to remove the dirt before metallizing. This will give the best solution and will not depend on the quality of the rolls after metallization.

If you cannot clean the film well enough then there are a couple of things to look at and consider. With the loss of the chromium coating to the roll the diameter of the roll will have decreased.  Do you know, by how much the diameter has changed?

How is the chrome roll driven?

If the roll is driven by a motor the speed of the roll will be determined by the winding speed of the web and the diameter of the roll. If the roll diameter has changed then it is possible the speed of the roll is now slightly wrong compared to the web speed. This will mean that instead of the web being in contact with the roll but not moving relative to the roll surface it will instead be moving at a slightly different speed to the web and so will be much more likely to slide or roll any of the surface debris away from its original position leaving behind an unmetallized hole that you see as a 'star'.

If you know precisely the diameter of the roll then you should be aiming with any replacement covering to also bring the roll to the identical diameter.  If the covering makes the diameter too small or too larger it will still have this slipping action and will generate 'stars' by moving the debris around. The closer you get to matching the original diameter the less slip there will be and so fewer 'stars'.

If the roll is not positively driven but is tendency driven by the web then part of the problem will be the differences in contact pressure and coefficient of friction. Materials such as Teflon will most likely have a much lower coefficient of friction and so will tend to slip against the web more than the chromium with the higher coefficient of friction. In slipping over the surface it will move the debris around and create the 'stars'.  So in this case what you are really looking for is a material that is similar to the chromium in terms of surface finish and coefficient of friction. 

A starting point would be to look at the damaged film under a microscope.  Look at the 'stars' that have been formed.  Check to see if they are similar around the whole circumference or if there is any evidence of some debris being moved. If there is evidence of the debris being moved make a note of the amount of movement and the direction.  If the debris has slid rather than been rolled there will be a slide mark and this will give you an idea of the distance moved.  If you measure a number of these from different positions in the roll it will give you some indication if the slipping is happening throughout the roll or only at speed changes.  A driven roll of the wrong diameter is likely to produce damage throughout the roll; a tendency driven roll may only produce damage where there are speed or possibly tension changes.

If the damage is throughout the roll then if all the damage is of a similar length it will also give an indication of how short the circumference is and so what diameter the roll needs to be increased to to prevent the relative movement.

I hope this helps.

Adhesion failure problem

We have 2 metallizer in our company. We have faced the metal peeling off problem. We have -20oC cooling in our chill drum and using the gas wedge also. After metallizing we measuring the temperature its 40oC . How to reduce the output roll temp? Waiting for your reply.

Answer

There are a number of aspects to your problem.

In general if you cool down the deposition drum from -20 Deg C to -30 Deg C the final temperature could be expected to reduce by a similar amount from 40 Deg C to 30 Deg C.

This reduction of the drum temperature may not be possible; it depends on the cooling capacity of the system.

A second method of improving the cooling is to increase the gas flow to the gas wedge. The heat transfer coefficient is dependent on the volume and pressure of gas trapped between the film and drum. The higher the trapped pressure the more gas collisions on both the hot film and cold deposition drum and so the higher the heat transfer coefficient.

Again this might not be possible in your system as there will always be a proportion of the gas the leaks out of the edge of the film as it passes around the deposition drum and this has to be pumped away.  IF you increase the gas into the wedge the leaking gas might take the chamber pressure higher that you would like for your metal deposition process.

A final possibility is to replace one of the standard rolls following deposition with a cooled roll.  This needs to be chosen well as it not only requires the roll to be cooled but also the film has to have sufficient wrap around the cooled roll to take benefit of the cooling. If the wrap is too short there will not be sufficient time to remove enough heat to make a significant difference to the temperature.

This final solution does require changing the winding system and requires an additional leadthrough for a further cooling liquid.

If the metal coating is flaking off this is more indicative of not having the right level of adhesion rather than it being a problem of overheating.  Adhesion can be poor because of too little or too much plasma treatment.  Have you optimised the plasma treatment on the film?

If not I would suggest that this might be more useful than reducing the temperature of the final roll alone. The roll is not very hot it is warm and so I would have thought it unlikely that reducing the temperature is going to make too much difference to the metal adhesion.  I would suspect that even after increasing the cooling the metal would still be prone to peeling off.  Check what type of plasma treatment is done, is it argon, argon/oxygen and has it been optimised for that grade of film?  If the process has not been optimised it could easily be either too little or too much and giving the poor adhesion. The treatment can be too much where the surface energy would be reading a high value but the surface damage would be enough to generate low molecular weight chain fragments that the metal would adhere to but where the low molecular weight material is no longer well bound to the polymer film.

I hope these suggestions help.

Low adhesion on film that passes tape test

We are coating & embossing on 12 micron polyester film. After that tape test ok.
After metallization called is holography film (with plasma treatment) tape test also pass & dyne value 58+.  When we are laminate with reverse printing pet film with holography film after that found the bond strength is very poor.

Answer

The tape adhesion test does not measure adhesion it only measures lack of adhesion.  If a coating comes off with the tape test it has very bad adhesion.

If the tape brings off the coating all it proves is the adhesion is anything from just a little better than the tape adhesive all the way through to adhesion such that the failure plane will be a cohesive failure within one of the layers and not at the interface.

I would check your deposition process to see if there has been any type of pre-treatment to the polymer film before metallization. If there is no pre-treatment then using a pre-treatment could improve the adhesion. If you are using a pre-treatment then there are two possibilities. One is that the treatment is not enough to maximise the adhesion and the second is that the treatment is too much and the surface has been damaged such that you have gone past the peak adhesion to a lower level off adhesion.

If you have a pre-treatment then review how the level of treatment was optimised. If it wasn't optimised then start from a low level and increase the treatment to see where the optimum level is. This will probably require using the lamination to test the adhesion level as the tape test does not have the range of adhesion to give any meaningful answers.

Lifetime of surface treatment

I don’t know if this is right, but as film price roar up to a level that converters will have to be in a dilemma in coming up with a decision. We have metallized cpp with dynes level of 32- 34. As per industry standards this level are considered in the lowest threshold in terms of good wettability during adhesive application. we get an acceptable adhesion with this dynes level initially but will this be a too risky move on our part especially with products that are not considered a commodity or fast moving(consumption).As you have mentioned the bond will definitely deteriorate as time past due to continuous contamination of the surface resulting to weak boundary layer. Temperature, time, pressure dependent as you said. Does this means there’s no guarantee that the initial bond reading will hold through before the shelf life of the package (6 months).Am I putting myself to a losing end? Thanks

Answer

The whole problem of film surface treatment lifetime is a difficult one.  The treatment can be dependent on the film type, quality and what additives have been included and so will vary from supplier to supplier. The lifetime also can depend on the storage time and conditions, particularly temperature with storage at a higher temperature reducing the lifetime of the treatment.

Personally I would be tempted to buy untreated film and to treat the surface immediately before processing. In this way I would hope to be sure of the level of surface treatment and as it was done at the time of further processing there would be minimal time for any degradation. I would expect this to give me more reproducibility than buying treated film and having a variable storage time before converting. 

Once you have converted the film either by coating or laminating the film the performance will be dependent on the quality of the adhesion you have achieved.  If you have the best possible adhesion the lifetime of the product will be longer than if the adhesion is poor.  The other factor that affects the lifetime would be the low molecular weight material that is contained within the bulk polymer. This can include slip agents. If the adhesion is poor these can migrate into the gaps between surfaces at the interface. If the adhesion is good there is little or no spaces for these slip agents to migrate and hence the adhesion remains higher for longer.

So you will know best what the quality of film you have and if it contains any slip agents, etc and if you have surface treated the film to get the best possible adhesion.  If you know these things you then need to assess if the film will be stored at a low temperature or will see a higher temperature.  This will not tell you how well the film will survive but it will allow you to estimate if there is a risk of the film degrading in performance.

This type of risk analysis may be helpful.  So if you have a film with a slip agent included the risk will be higher than without a slip agent included. If the film was surface treated to increase adhesion the risk would be lower than if the film was not treated and if the film was stored in cooled environment the risk would be lower than if it were stored hot.  You can then multiply these risks together to see the total risk.  Thus if you have a film with a slip agent included, no surface treatment and a hot storage environment you have all three as a higher risk and so the overall risk of degradation will be much greater than if the film was additive free, had been surface treated and stored cool.

I hope this helps.

Health And Safety information request

I'm looking for information on experience concerning health and safety in the domain of vacuum coatings like PVD or PACVD and their possible production of nanoparticles (and subsequent risk for health and safety). It has been years now since the first vacuum coater started his plant and I think there should be data on the subject as there is longer and longer experience. However, all I found until now was "there is nothing sure". Do you know how this matter is addressed in industrial size facilities? Which protection means are used? And what is the return of experience? (Like "are the workers in good health?" "Are the protection means adequate?" or “was it much noise for nothing?"). Thank you very much in advance.

Answer

This is a subject that never goes away as the technology changes and the nature of the coatings change.

Vacuum metallization has been done for more than a century but with different materials and processes than for some of the newer applications.

As in the roll to roll coating industry more than 90% of the machines that have been sold worldwide are for aluminium evaporation there is the most information collected about this process. With some of the newer materials the process may have only been run using laboratory equipment and scale and there may only be one or at best a few production machines that might provide more information but also might not be involved in that type of data gathering.  Also it might be several years before any adverse effect show up in any employees.

Thus it is common to find safeguards based upon conservative estimates of the risks involved.  The working premise is that is better to err on the side of caution than to be sued in some future time for not taking suitable care.

The Heath and Safety Executives in many countries around the world will take information and may read the risks differently and there can be variations between countries about the risks and personal protective precautions that need to be taken.  There are a number of published books on exposure limits to various metals, polymer, ceramics, compounds, etc.  However a number of these will be for particle sizes greater than are now being produced, often in laboratories, is varying quantities.  These books are regularly updated as newer information becomes available and so it is always worth checking the latest edition to see if the information you have is up-to-date.

My own experience of working on new materials was that we would check on the exposure levels and we would then suit up in full protective clothing (including masks and filtered breathing source) and wear pumped collection badges that were designed to accumulate material over a fixed period of time that would later be analysed to monitor the expected exposure to dust and to also monitor the range of size of dust that could be produced.  This did entail trying to produce a lot of dust by sung practises that normally we would not use as well as following our normal procedure so that we had a range of possible exposure levels.  We would then compare this to the allowed exposure limits and see how they compared.  Where we had a lot of fine particles these were always deemed to be a higher risk and so where no data existed we tended to move the classification into the next most severe category until the data was upgraded.

I know of other companies who did not take this view and only did the bare minimum required.

What has changed since then is that the size of nanoparticles has reduced still further and it has been proven that the chemical reactivity of these finer particles is much more that the coarser particles previously produced.  What is not yet clear for many of these particles is what they will do to the human body and at what levels. It is often not possible to use humans to test these things out other than by accident and with hindsight and hence it can take many years for the true recommendations to appear.

Many of these nanoparticles can be freely taken into the lungs and into the smallest of the alveoli where as they are in a moist environment they may stick and cause an irritation. Even an irritation is a problem as the lungs will secrete liquid in defence of this irritation and if this persists the liquid can increase in viscosity and granulate and possibly even form scar tissue. This is bad news as the scar tissue is less permeable to oxygen than the original surface and so there will be lung function impairment.  This process may take many years to manifest itself.  What can be worse is it the fine particles cause an allergic reaction where this can all be accelerated or worse still if the reaction triggers the formation of cancerous cells in some way. Now in other areas it has been proven that moving to finer particles increases the reactivity dramatically and so the fear is that will also be the case in reactions with the human body. Materials that in larger form were benign may in their finer form trigger some form of adverse reaction. However as this might take a long time to show up and there are so many different variations of each material it is taking a very long time to accumulate enough data to publish reliable recommendations.  Hence the general recommendation as treat every nanomaterial as a possible problem until proved otherwise, a little extreme but understandable.

I hope this helps and suitably answers your question.

I invite others who may have different experiences to post a response to add to this answer.

Questions re. backsurface treatment & starry film

1.         We are getting back treatment in our PET film, for 52+ corona treated film we are getting 48+ dyne value on non treated side, what can the possible reasons for this problem?

2.         How this back treatment can affect metal bond strength as we are using this film for metallization?

3.         For such corona treated film after metallization we are seeing Star-Sky defect under light box, is this problem is related to back treatment or what other reasons possible for this problem.

Answer

I would suggest we separate out the different problems.

The star sky sounds to be a simple case of many pinholes. Pinholes are primarily caused by dust or debris on the polymer surface that is metallized and then as it gets moved away it leaves a small area that is not metallized behind that is seen as one of your stars when the film is placed on a light box.

A secondary source of these demetallized areas can be through pickoff. This can be where the filler protruding from the back surface is pressed hard against the front surface and when next unwound the pressure has cause the metal to transfer from the front surface to the back surface causing the loss of a small amount of metal that is also see as a pinhole of star on your light box.  If the back surface of your web has been corona treated to raise the surface energy it will also raise the adhesion and may make this back surface pickoff more likely than if the back surface were left untreated.

Corona treatment is generally carried out on the front surface only but there are circumstances where the resistance between the roll and ground or film and the roll are altered sufficiently that not only is there a corona plasma produced on the front surface of the web but also another one is produced on the back surface of the web too. It is unlikely to be as powerful as the front surface corona as is possibly being demonstrated by the slightly lower surface energy of the back surface.  This back surface treatment can lead to some winding problems because the higher surface energy tends to make the back surface less slippy and so slightly harder to wind and so this can lead to more stick-slip behaviour during re-winding and this can encourage some of the movement of debris which you see as an increased number of stars.

Thus I would go back to the corona treatment station and check on the set up, paying particular attention to the connections and also the conductivity of the roll to make sure it has been set up correctly to provide a front surface corona only.

Crystallinity in BOPP films

What is the role of crystallinity in BOPP films, how can we measure crystallinity. What process parameters can influence crystallinity in BOPP films?

Answer

Crystallinity changes many things in the polymer such as density, tensile performance, optical transmittance, barrier performance, etc, and can be changed with orientation and temperature.

Sometimes there are trade offs of an improvement in one parameter and a decline in another.

With the change in density it is possible to estimate the crystallinity by measuring the density but more often  Differential Scanning Calorimetry (DSC) is used to determine the crystallinity of the polymer.

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

Problem of surface ‘shininess’

We metallize a cavitated BOPP film and are told that the metal 'shininess' of our film is inferior to the competition. Is there an ASTM standard for measuring the shininess/brightness/reflectivity of metallized surfaces? Is gloss measurement an option? If so what angle.

Answers.

Here are some comments on ASTM gloss measurement. A low angle measurement (20 degrees) is used for high gloss surfaces and high angles (85 or 60 degrees) for low gloss surfaces. But often the issue is not measurement technique but perception. So in this case it may be important to come to an understanding with the customer about what “shininess” means to them. It could be the specular reflectance (20 degrees) or the “sheen” (85 degrees). If the surfaces are not identically smooth, the smoother one could appear to be shinier with higher specular reflectance while it may have lower sheen. And if either supplier’s process modifies the surface smoothness, the customer may be reacting to that, rather than the reflectance of the metal coating.

Gloss - ASTM D2457

Gloss is a measure of how shiny or reflective a material is at a specified angle based on refractive index.

Gloss Measurement

An incandescent light source is directed at the test specimen at a specified incidence angle. A receptor is located at the mirror reflection of the incident beam. Polished black glass with a refractive index of 1.567 is used as a standard and is assigned a gloss of 100 at all geometries. Measurements are made using a gloss meter.

Gloss Geometries
Angle	Use	Note
20°	high-gloss films	Equivalent to ASTM D523 at 20°.
45°	intermediate and low gloss films
60°	Intermediate-gloss films	Equivalent to ASTM D523 at 60°.

Comparison of gloss data can only be made between similar materials and test procedures. Gloss values for transparent and opaque materials are not comparable. Gloss varies with smoothness and flatness and is sometimes used to compare these attributes.

Gardner Gloss - ASTM D523

Gloss is a measure of how shiny or reflective a material is at a specified angle based on refractive index.

An incandescent light source is directed at the test specimen at a specified incidence angle. A receptor is located at the mirror reflection of the incident beam. Polished black glass with a refractive index of 1.567 is used as a standard and is assigned a gloss of 100 at all geometries. Measurements are made using a gloss meter.

Gloss Geometries
Angle	Use	Note
20°	Good for comparing specimens having 60° gloss values higher than 70.	Equivalent to ASTM D2457 at 20°.
60°	Comparing specimens and determining when 20° gloss is more applicable.	Equivalent to ASTM D2457 at 60°.
85°	Good for comparing specimens for sheen. Most often used when specimens have 60° gloss values lower than 10.	Commonly known as "Sheen".

Don

Donald J. McClure, Ph D, President
Acuity Consulting and Training
23002 Dunham Lake Road
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715-689-2902 (land line)
651-470-6939 (cell)
acuityct@hotmail.com

Second Answer

A simple measure of how shiny a metal surface is to measure the reflectivity.

If you wish to measure the polymer surface before metallization it is possible to measure the Haze of the surface. Haze is a measure of the off axis reflection.  If a light is directed at the surface normal to the surface and a detector also placed normal to the surface (where the light passes through the centre of the detector) it will measure the reflected light.  If another detector is placed at a small angle away from normal, usually 2.5 degrees or 5 degrees to normal, this will measure the scattered light or 'haze'.  The larger the haze the lower the specular reflectance and so it is possible to estimate the metal coating performance from the uncoated surface.  Some of the polymer manufacturers quote a Haze measurement and the angle they measure at.

Other measures that might be informative would include a measure of the surface roughness as the specular reflection is affected by the surface roughness. The smoother and flatter the surface the higher the specular reflectance.  Thus on a cast film it is possible to tell the difference between the air side of the film and the casting drum side of the film as the side that contacted the casting drum will usually have the same roughness as the casting drum.

Thus measure of surface roughness is now done using Atomic Force Microscopy or one of the variants.

There are two possibilities that you might consider.  One is that the quality of your film is as good as the competitors but that your metallization is not as good. The other is that the metallization is equally as good but the film surface is not as good.

The metallization can be a variable.  How the metal nucleates and grows on the surface can affect the surface roughness of the metal coating and hence the final reflectance.  If the metal wets the surface poorly the nucleation sites will be able to grow quite high islands of metal but with gaps between the islands. This will continue and give rise to a rougher coating than if the metal wets the surface well. If the metal wets the surface well the nucleation sites spread out and the islands touch each other even whilst the coating is still quite thin. This leads to a smoother coating.  If the deposition rate is low the crystal size will be bigger than if the deposition rate is very high. The larger crystal size also leads to a rougher surface.  As the coating grows in thickness the differences in growth rates of the different crystal orientations becomes exaggerated and this too is seen as an increase in the roughness of the metal coating.

Thus by having poor wetting, a slow deposition rate and a thick coating it is possible to create a much rougher surface than if the process maximised the substrate surface energy and hence maximised the metal wetting, used the fastest possible deposition rate and limited the coating thickness to only what was required and not an excess.  Often it is thought that a little bit extra coating thickness will help and often it does exactly the opposite.

A final thought is the if you are comparing reflectivity measurements and possibly looking as differences of only a few percent it is important that either the samples are all measured on the same instrument and by the same person using the same procedure and calibration or that the different instruments are capable of measuring some sample materials to give the same results and at the same accuracy.

It is common for the same material to measure different values on different instruments for a variety of reasons.  The illumination may be different, some lamps may have filters to bring the spectral response closer to daylight others do not. Some calibrate the instruments differently. Some use a silver mirror for the 100% reflectivity calibration others will use an aluminium mirror. These mirrors will age and so even using the same mirror the results may be different over the timescale of a few months.

Hence when discussing the reflectance it is worth making sure that the different samples were at lest all measured on the same machine and ideally with the same calibration.  It would not be the first time that the reflectivity has been disputed when in reality it is the differences in measurement that were the variable and not the substrate or metallization.

I hope these thoughts help you.

Charles Bishop     www.cabuk1.co.uk