Vacuum Web Coating

Question re. aluminium wire purity.

Does a difference in purity level of the aluminium wire effect the characterstics of a web coated with aluminium. If yes, then which characteristics do get affected and what ingredients and in what% difference affect those characteristics?

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Answer

The purity of the aluminium wire can affect both the process and the metallization quality.  Included in the question of purity ought to also include the diameter of the wire as for a larger diameter wire there is a lower oxide to metal ratio than for a small diameter wire.  Impurities (silicon, iron and others) can contribute to the slag that forms on the surface of the molten pool. There may also be an additional contaminant to any wire and that is any residuals from the lubricants that are used on the wire drawing process. An excess of this lubricant left on the wire surface can lead to spitting as the lubricant vaporizes.   Anything that builds up the slag reduces the evaporation performance. As the slag builds up it reduces the available evaporation area and so the spread of the evaporant reduces and the uniformity gets worse. Also as the slag builds up the spitting is likely to get worse.  Thus, in general, the higher the aluminium wire purity the better the coating performance.  This has been the rule-of-thumb for many years but more recently it has been suggested that some impurities can actually help the barrier performance. However the amount of detailed information to prove the claim is limited and may not take account of other variables such as debris levels on the film surface that is directly related to the number of pinholes in the final coating and which also directly affect the barrier performance.

Hence until there is a more definitive study I would continue to use high purity wire to get the best possible coatings. 

Not only does the wire purity affect the slag build-up but also the boat type, brand, quality and surface design affect the evaporation performance. The molten pool only forms over the part of the surface that is wet by the aluminium and so if the aluminium does not wet the boat well the slag will form a greater proportion of the molten pool than if the aluminium wets the boat well and covers the whole surface.

I hope this answers your question.

CAB

Researchers unveil anti-fog glass coating

Fogged-up windshields could soon be a thing of the past, as a new lacquer has been developed to ensure better visibility in cars of the future.

The electrically conductive coating uses nanotechnology to heat the windshield across its entire surface - with no wires to obstruct the view.

On cold winter mornings, a driver's vision is often blurred by moisture precipitating on the inside of the windshield. This happens when warm, humid air comes into contact with a cold surface.

At a particular temperature, known as the dew point, the moisture in the air condenses and forms a layer on the colder surface. Cold air is not able to contain as much moisture as warm air and this fact is much more noticeable in small spaces - in a car, for example.

Condensation can be prevented by increasing the volume of air (opening the windows), by heating the whole of the vehicle's interior, or by heating at least the windshield to a temperature above the dew point.

Ivica Kolaric of the Fraunhofer Technology Development Group TEG in Stuttgart, Germany, favours the third option. His new process warms up the windscreen - though not with costly copper heating elements, rather it uses a transparent coating of carbon lacquer in the form of carbon nanotubes (CNT).

Kolaric and his team are currently working on a bonding system which, in a year or two from now, could keep not only windshields but also bathroom mirrors free from condensation. When attached to an electricity supply, the lacquer coating is transformed into a wide, flat heater that exactly covers the surface to be heated and continues to function even when it is damaged in places.

If a heated windshield containing wire heating elements is chipped by a stone, for example, and one of the wires is severed, the entire heater could very well cease to function because of the interruption to the current.

For the CNT heater, however, a few small defects in the coating are not a problem because the current flows over the whole surface.

A further advantage of the 'flat' conductor is its uniform heat distribution. Every single point on the surface of the windshield is heated evenly, rather than the warmth radiating outwards from the heating elements.

However, the CNT coating itself does not store any heat, as Kolaric explains: “The lacquer converts the electricity almost entirely into warmth and transfers this to the windshield. The windshield is clear in a very short time with minimal power consumption. What's more, the CNT resistance heater can be integrated in the vehicle's standard 12-volt power supply.”

For more information, visit www.fraunhofer.de

Question re. metallizing carbon fibre reinforced polymers

Question

Suggest metallizing on non metal/CFRP components.If any rep. in India,Kindly inform.

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Answer

Generally the carbon fibre is encapsulated in the polymer matrix and as such the metallization process is the same as onto the unfilled polymer.  So if the polymer matrix is polypropylene or polyethylene it would be wise to consider a pre-treatment to increase the metal adhesion. Many polymers have additives of which some may, either by design or not, migrate to the surface and these can prevent good adhesion. Hence the preference to pre-treat the surface to increase the surface energy.

Question re. loss of dyne level

Question

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A customer has a dyne problem. After metallizing their paper, their dyne values fall off dramatically. Within 6 hours they have to seal the surface or they cannot print on it. In years past, they typically had 6-8 days.

They claim that the humidity is regulated in the plant. This phenomenon is worst in the spring and fall. What might be the causes of this surface tension problem?

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Answer.

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I would put this down as a classic problem for asking the question ‘what has changed?’

You mention that ‘in years past’ which suggests you have plenty of data on this particular product.  In which case look carefully at when the product had the longer lifetime at the higher dyne level and the current material with the problem.  What I would start looking for would be changes in the substrate material.  The way that paper is manufactured is that it sees a variety of chemicals and often at least one surface is treated to improve the smoothness for metallization and this may also include coating as well as calendaring. If the paper is from the same manufacturer it is worth checking if they have changed their process in any way. They may have changed the composition of something during their process or the thickness of any surface coating or both.  Alternatively it could be that the supplier has changed and hence it would be expected that the processing would be different from each supplier.

If the substrates prove to be different then this is potentially the source of the problem.

If the substrates are the same then the next step is to start looking at your own process in fine detail.  This starts with the incoming material and any storage time.  Is the storage time different such as short on one and longer on the other? During storage was the humidity and temperature held constant?   Is there a seasonal difference between the good and bad rolls?  You mention that it is worse in the Spring and Fall this would suggest that there is an effect probably relating to atmosphere. Often the humidity is regulated in the plant where coating and re-winding is done but I have seen factories where the rolls when they go into storage leave the humidity controlled area. So it is worth checking the roll history all the way from manufacture to use to see if there are any uncontrolled times. 

After this has been checked out, assuming there is no difference, start to check out any pre-treatment and the deposition process. Look at any/all information that is available. Were the pumpdown times the same? If not, why not?  The pumpdown times can be an indication of other problems such as system leaks or insufficient cleaning or substrate differences.

This type of approach is the methodical but time consuming one.  An alternative, but likely much more expensive, approach is to use surface analysis to diagnose any potential contaminant that is leading to a loss of dyne energy.  Using x-ray photoelectron spectroscopy (XPS) or secondary ion mass spectroscopy SIMS it is possible to look for differences between the high dyne and low dyne surfaces. If there is an additional material on the lower energy surface then the next problem is to identify where it comes from.  These surface analytical machines can be expensive to use but so too can the loss of production and so there is a trade off to be made.  The bigger the problem causing greater loss of production gives a greater justification for using any tool that might help solve the problem.

I hope this has given you some process to use to work to a solution to your problem.

AIMCAL Fall Conference review of papers No 2.

There were two interesting papers on resistance heated evaporation boats.  Both had aimed at increasing the lifetime and wetting of the boats whilst also reducing the corrosion.

            The first of the papers was from GE Advanced Materials and their claim was to have produced a boat with the corrosion performance similar to a 2 component boat but with the wettability of a 3 component boat.  The way they have improved the boat design has been to pay increased attention to the purity of the raw materials, chemical composition, homogeneity, particle size and morphology and processing methods.  The new processing method of the improved source materials creates a boat with enhanced chemical resistance of the grain boundary phase.  This reduces the rate of corrosion that is a primary cause of shortened boat lifetimes.  The chemical modification also leads to improved wettability of aluminium over the boat surface that also improves the puddle stability.  These factors increase the deposition rate and/or line speed and with longer boat operation lifetimes results in significantly higher productivity.

            The down side of this is that the boats cost 15% higher than conventional boats. The cost models shown still show an increased profitability even when using the higher cost new boats.

            The second paper by Denki Kagaku Kogyo K.K. aims to do exactly the same things but they take a completely different approach. Using the capillary action of a molten metal on the surface and the basic mechanism they have modified the surface to optimise the wetting and spreading of the molten pool.  The surface was modified by putting grooves into the surface and by using a specific groove width, depth and spacing the wetting is optimised.  Where the grooves are present the wetting occurs and conversely where the grooves are not present the molten pool does not wet.  The improved wetting also means that there is reduced local corrosion that leads to extended lifetimes and the larger area molten pool leads to reduced spitting.

            I have since found out more about these boats both more about the operation of the boats in production metallizers as well as some details about the method of production. The grooves are across cut across most of the width of the boat and the way the aluminium spreads is by wicking across the width of the boat down each groove. Once the aluminium has filled a groove the pool depth will grow slightly and then the aluminium will spread round the end of the groove and wet the beginning of the next groove at which point the aluminium will wick down this groove and repeat the process.  Thus if the wire feed is increased the molten pool that initially will appear as a rectangle in the centre of the boat will progressively increase groove by groove but will retain the rectangular shape.  This will continue until there are no further grooves and/or the boat depression is filled.

            Users I have talked to have all stated that there is less spitting from this type of boat.

            Typically boats are made by mixing the appropriate powders followed by compressing the powder mix to form a billet. This billet is heat-treated and then the boats are cut and machined out of this billet.  There are problems of uniformity of powder mixing and heat treatment such that it is typical to have a range of boat resistivity produced depending where each boat originated within the billet. Refining this process with better mixing and more closely controlled heating can narrow the range to some extent.   Denki have taken a different approach and they mix the powder but then form and heat-treat each boat individually.  The result of this is that they can produce every boat with nominally the same resistivity within a few micro-ohms.  Thus the days of sorting through boxes of old boats to get a matched set of boats ready for a boat change ought to be numbered.

            This is the typical Japanese approach to manufacturing consistency and although this is still early days this boat does appear to have raised the bar on boat quality.  The great news that I have been hearing is that this has been priced similarly to the competitors so they do not appearing to be charging a premium for the added quality.

            As I have stated this is early days and so if any of you can add to this information I am sure we would all like to hear the details. 

Copies of the AIMCAL Fall Conference Proceedings are available from AIMCAL at www.aimcal.org. 

AIMCAL Summer School - India

Hot off the press.    AIMCAL has found a partner and will be running their first Summer School in India later this year.

It is proposed that the courses will run in parallel with the India Converting Show that is scheduled from 21- 24 August (Tuesday through Friday).  The aim is to run 3 parallel sessions on winding, coating and vacuum metallization and to run the courses over the first 2 days of the Show.

This is only provisional and I will post more details as they become known.

If you want to be added to the mailing list for the Summer School either send me your details or contact the organizers at AIMCAL via their website www.aimcal.org

Book review – Metallic Effect Pigments

Metallic pigments are used in paints, inks and plastics for a variety of effects.

The market for metallic pigments has been growing for many years. The requirements for increasing reflectivity to make the inks and paints produce a more mirror-like effect has led to an increasing use and hence manufacturing of vacuum deposited metallic effect pigments.  This book is dedicated to the topic of metallic effect pigments and compliments another book in the same European Coatings Literature series on ‘Special Effect Pigments’.  Both of these books are written by groups of authors. In the case of ‘Special Effect Pigments’ the book is written by 5 people all employed by Merck KgaA and the main theme of the book is the history, manufacture, measurement and use of pearl lustre pigments.  In the case of this new book on ‘Metallic Effect Pigments’ the book has been written by no less than 21 contributing authors all employed by Eckert GmbH & Co KG. 

The format of the book is to review the history of metallic pigments. This is followed by a section on the fundamentals that covers the definition of metallic effect pigments, the manufacturing techniques and test methods.  The next section then details what are referred to as special effect pigments. This is taken to mean the pigments produced by the physical vapour deposition (PVD) manufacturing process and secondly coloured aluminium pigments.  The next half of the book is dedicated to the applications of the metallic effect pigments. Different coating types (solvent, aqueous, powder & UV) are described and well as specific applications such as automotive, decorative and industrial coatings. The various printing techniques are also described as well as the inclusion of metallic pigments into plastics, cosmetics and personal care products as well as for corrosion protection products.  The book concludes by describing the effect on people and the environment as well as a succinct chapter on safety advice.

As would be expected of a team effort from one of the leading manufacturers of metallic pigments this book provides a solid grounding in the manufacture and use of metallic effect pigments.  With respect to vacuum web coaters this book describes all the basic information required for any metallizer to make a start in making PVD metallic flake pigment.      

For those interested in getting a copy of the book the details are as follows

‘Metallic Effect Pigments’

Peter Wissling et al                              

Pub. Vincentz Network 2006                         

ISBN   3-87870-171-3

Available from William Andrew Publishing at sales@williamandrew.com

Question re. polymer substrate surface quality.

Question.

What is the polymer used to coat a plastic to improve the surface to an optical reflecting standard for aluminium vacuum depositing. Where would we be able to obtain the polymer.

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Answer

As you might expect there are many answers to this question.

Polyester webs have been used as substrates for aluminium reflectors for many years.  The performance of the polyester can be changed depending upon the size, shape and amount of filler used in the polymer.  Thus for the best optical performance it is preferable to have no filler included. This is sometimes referred to as optical grade. Unfortunately this adds a problem to the metallization process as when the very smooth, flat surface is re-wound with no entrapped air (as it is under vacuum) the film will block and the winding will be poor and the roll may even stick together such that it cannot be unwound.  Hence most optical grades of polyester will either have some filler included or will be of a co-extruded type where the back surface has a layer with filler included but the front surface is an unfilled layer.

This type of material is regularly used for high quality reflector applications.

Beyond this it is possible to deposit a polymer layer within the vacuum system immediately prior to the metallization. This has the added effect of any dust or debris present on the surface becomes covered by the polymer coating and so the smoothest, flattest and cleanest surface is metallized.  This polymer coating is usually an acrylate (eg. tripropylene glycol diacrylate (TPGDA)) that can be electron beam (or possibly) cured. Even though this polymer surface is clean it may still require a plasma treatment to optimise the aluminium adhesion.

As aluminium oxidises the reflectivity can decline with time. Where the reflectivity is required to be high and remain high some coat the aluminium with silica as a transparent protective layer.  As the bare aluminium is fragile until the aluminium oxide layer builds up and so other also deposit a polymer layer on top of the aluminium as a protective layer. This may reduce the reflectivity over the bare aluminium film but over time the protected aluminium reflectivity can stay at a higher level for longer.

I hope this gives answer gives you a choice of options of which at least one will meet your needs.

Question re. condensing gases onto cryocold surfaces

Question.

At the moment we are using a PolyCold and cryogenic panel to remove retained solvents on 12μ polyester in our metallizer.

This works ok on a web of 700mm, but will not work on a web of 1400. Would it be better to use liquid nitrogen as a cooling medium instead. (not sure as to what temp. solvent condenses at).

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Answer.

When you talk about polyester film and using a cryogenic panel to remove retained solvents I am curious about your substrate material.  Usually polyester does not have much if anything by way of retained solvents. However it does contain moisture and depending upon the manufacturing process and then the subsequent winding and storage conditions this can be as much as 2% by weight of moisture.  Most Polycold cryopanels are used specifically to pump water vapour and nothing else.

The way any cryopanel works is to lower the temperature of the panel to the point where any atoms hitting the surface have a high sticking coefficient. The temperature governs what atoms will stick and which will not. The Polycold type cryopanels have the temperature reduced enough such that they have a very high sticking coefficient for water molecules but little else.  Gases such as oxygen or hydrogen are generally only captured if the water vapour traps the gas as it condenses and solidifies into ice.

The performance of the cryopanel depends upon the capacity of the system. As the gases condense and solidify an ice layer builds up. Ice is not as good a thermal conductor as the copper tube or panel surface. The ice may not form as a dense layer as the density depends on the rate of arrival of the gas and if the rate is very high the layer is likely to be porous. A porous ice layer is an even worse thermal conductor. The arriving gas is at a higher temperature than the cryopanel and so the cryo panel will warm up. The amount it warms is dependent upon the gas load and the capacity of the cryopump to keep up with the gas load.  The capacity of the cryopump has to be sized to pump the last amount of water vapour from a roll every bit as well as the first few meters otherwise there will be a progressive pressure rise through the process as the pumping performance declines and the ice layer builds up and the porosity gets progressively worse.

Thus on your system if the cryopanel has been sized to cope with the gas load from a 700mm wide roll of polyester and it then has in effect double the load applied then it is likely that the cryopanel temperature will rise to a higher level during the process and the ice layer formed is also likely to be less dense because of more gas arriving which means the pumping performance of the cryopanel is likely to decline.  If the cryopumping system has the capacity it may be that simply doubling the surface area of the cryopanel will cure the problem.  If the cryopanel surface is already at a maximum it may be that you need a second cryopump.