Vacuum Web Coating

Latest 2 questions ----- this time with answers. Part 2.

Q2

What is the shelf life of printed metallized paper? What is the effect of high humidity (90%) to printed metallized paper? Are metallized paper attacked by fungus?

Answer

As with everything it depends on the type & manufacturer of the paper.

Paper is basically hygroscopic & so can vary in water content with the surrounding humidity. When the humidity is around 40% -50% RH the moisture content of the paper may be around about 7% but at >95% RH the moisture content can rise to as much as 20%.

These values will vary depending upon the type of paper. Some papers have a polymeric type coating which will help stabilise the moisture content in the 5%-10 % range.  It can be problematic passing the paper through the vacuum coating process as the paper can lose too much water & if the water content falls below 5% it  will become stiff & brittle and much harder to wind.

Any material that can absorb and desorb moisture from the atmosphere may get spores lodged in the surface & so may get fungus or moulds growing on the surface.  The metallisation is usually only on one side of the paper & so it is more likely that any growth will be on the un-metallized side.  However the metallization is unlikely to be perfect and any pinholes and/or holes or cracks in the metal coating will allow moisture into the paper & again the spores may be present.

Some spores/fungi do not like metal & so they may not grow even of present in the atmosphere however the reverse side may still be an acceptable site. If the paper contains a whitener it may depend on the whitener used. Titanium dioxide is an anti-bacterial material & so it can promote the properties of self-cleaning & self-sterilization.  This is because the titamium dioxide is activated by light and by photocatalysis can neutralise contamination.  There are many other fillers & whiteners that do not have this property & so will not prevent grows appearing.

A simple test that you can carry out is to run a finger across one of the white spots. Does the white material stick to your finger?  If the white material does wipe off on your finger does the bright aluminium reappear?

If the answer is that either the white powder does not wipe off nor does the bright metallic aluminium reappear then it is most likely that the aluminium has corroded to aluminium oxide which is transparent &/or white depending upon the crystal size & light scattering.

If the white material does wipe off & the metal surface reappear then it would appear to be a mould growth.  If you then really want to know what the white material is surface analysis would be the route to defining the material.

The first step is to wipe the white areas to determine the most likely source of the problem.

If nothing wipes off it is unlikely to be a mould it is more likely to be the aluminium is converting to transparent aluminium oxide & your are seeing the paper through the coating.  You can see if corrosion is a problem by taking some salt water & putting a spot on the aluminium film & watching to see the results. Heating the sample slightly could speed up the results. It is likely there will be corrosion of the aluminium.   Compare the results with what you have. Are they similar?

If so you likely have corrosion of the aluminium. The fact that you have people breathing near to the rolls of material you will have salts in the moisture in the air & so it is possible these will act as corrosion initiators for the corrosion.

A second method is to use ellipseometry to look at the 'white' areas. Try to measure the thickness & refractive index (RI) using ellipseometry. If it can reproducibly find a thickness & RI it is likely it will be for aluminium oxide.

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So there you have my answer to the question.  My question to you all is – have I missed anything out?   

If you think I have hit the ‘Comments’ below & let me know what I’ve missed.

Latest 2 questions ----- this time with answers. Part 1.

Q1

Does the temperature gained on the surface of the BOPP film after metallising, cause a rapid reduction in treatment? Is it necessary to have a minimum dyne level of 36 on metallised BOPP to laminate it to polyester?

Answer

Yes it is possible that the heat can reduce the surface energy of the OPP.  It will depend on the pre-treatment & level of oligomer on the surfaces, both front & back.  If there has been a plasma treatment on the front surface only it will have good adhesion to the aluminium but the back surface is likely to have plenty of low molecular weight material on it and a lower surface energy. On rewinding it is possible to contaminate the front surface of the aluminium with some of the low molecular weight material from the back surface.

Lamination is like any other deposition process, the higher the surface energy the better the wetting of the adhesive & the better the bond.

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So there you have my answer to the question.  My question to you all is – have I missed anything out?   

If you think I have hit the ‘Comments’ below & let me know what I’ve missed.

Barrier – short courses at AIMCAL Fall Conference

On the Sunday before the AIMCAL Fall Technical Conference starts there will be a couple of short courses relating to barrier coatings.

In the Sunday morning Bernard Henry will be taking everyone through the principles of barrier coatings.

This will include a description of the new more efficient barrier coatings that includes:

A)                Inorganic/polymer multilayer coatings  (dyads)

B)                Ion assisted sputtered layer deposition

C)                Atomic layer deposition

D)                Hybrid PECVD barriers.

This course is designed specifically for those who wish to learn about the different approaches being adopted. The course will provide an insight of the principle behind each barrier technology; a review of current barrier performances & groups involved in developing these materials as well as an overview of techniques used to measure the permeability of high barrier films.

This will be followed in the afternoon by a course designed to compliment the morning course that covers the following topics & relates them to how they affect the barrier performance that could be expected.

1.    Substrates & cleaning
2.    Pre-treatments & effects
3.    Nucleation film growth & modification (nucleation modification)
4.    Structural modification (bulk film modification)
5.    System design & effects.

It is common that we have little or no control over the substrates before we receive them but we are still expected to be able to deliver high quality barrier coatings. 

This course is aimed at taking participants through the whole process from receiving the substrate roll & possibly having to evaluate the quality through to how to improve the surface. This includes cleaning, plasma pre-treats, polymer coating & curing and their effect on the nucleation & growth of the subsequent coatings.  Also included will be details of what techniques are used to modify the growth & density of the coatings & what implications this may have.  Finally there will be a review of the system design issues such as front surface rolls, sources of contamination, shielding types & options and deposition source types and the effect of deposition rates on the coating cleanliness.   

If any of you are considering taking the course & have specific questions that you want to be sure are addressed during the course then please send them to me & I will try to make sure I cover them.   Hopefully I already have them included in the course. 

Alternatively if you have a problem that you wish to share with the class then feel free to bring them along.

Worried by backstreaming?

Backstreaming is a common concern and yet many operators still insist that vacuum systems need to be pumped to a very low base pressure before starting the deposition process.  These two facts seem to be contrary, as we all know that backstreaming becomes increases with reducing pressure & time spent at the low pressure.

Therefore it seems only sensible not to pump down to a low base pressure.

However if I suggest this method the immediate response is what about the water vapour absorbed on the walls?  The belief is that pumping down to a low base pressure helps remove the water from the walls. 

            This is not true; the ability to pump to a low pressure is an indication that the water has been removed. The lower pressure does not actually remove the water.

There are several ways of speeding up the removal of the water. One is to use heat and this is commonly used on ultra-high vacuum systems with trace heating elements wrapped around all the external pipe-work. The second method is to apply energy by some other means such as ultra-violet lamps that excite the water & assist in breaking the bond to the surfaces.

            The final method, that has been little used, but works is to only pump to a set pressure which can either be where the roughing pumps are still working or to a point just below the stalling point of the diffusion pump.  At the elevated pressure there is increased bombardment of all surfaces because of the higher gas pressure.  The greater number of gas molecules means that there are more molecules bouncing off the walls & the kinetic energy is sufficient to break the bonds holding the water to the surfaces & hence it speeds up the release of the water.

            Thus feeding the system with a small leak of dry nitrogen for a set time is better at reducing the water than trying to pump to a lower pressure. If there is dry nitrogen in the system it may be possible to strike a plasma which will further increase the energy & so will further increase the water evolution rate.

            The nitrogen, or other inert gas, needs to be dry otherwise the process is counterproductive.

            By this method it is possible to reduce the pumpdown time slightly as well as never having to pump to the ultimate base pressure and risk backstreaming.  In turn this will them improve the system productivity & the film quality.

  If in doubt try it out   -    & let me know how you get on.

Latest two questions

Q1

Does the temperature gained on the surface of the BOPP film after metallising ,cause a rapid reduction in treatment? Is it necessary to have a minimum dyne level of 36 on metallised BOPP to laminate it to polyester?

Q2

What is the shelf life of printed metallized paper? What is the
effect of high humidity (90%) to printed metallized paper? Are metallized
paper attacked by fungus?

Let me know your thoughts on these latest two questions.

I have answered them & will post my responses in a couple of days. Lets see if the answers match.

HELP - overflowing boat problem

            I was recently sent a question.  A problem of the molten pool overflowing one end of the evaporation boat and reaching the water-cooled copper contact.  The result of this is that the level of spitting increases.

            My first response was to suggest that the boat was checked to make sure it was fitted level.  The response back was that the boats were level & that was not the problem.

            Whilst I know that hot spots on boats can sometimes change the shape of the molten pool I have never come across a pool variation such that the liquid all collects at one end of the boat to the point of overflowing the recess & reaching the copper blocks.

            My suspicion is that despite the protests that the boats are level they are, in fact, not as level as believed.

            What do you think?   

Are there any other reasons for this behaviour?

Chemometrics – or, what to do with your data to make more profit.

Modern metallizers usually come with some data-logging capability. Along with this data storage and computing power has never been as great.  Yet, despite this, what do most of us do with the data, other than fill up the available storage?  The answer is probably nothing.

            This is such a waste, as cumulatively this data can provide information on the process and also, if there are complaints, can provide details of not just why the roll was different but what monitoring can be used to prevent this happening again.

            Once the data has been analysed it can provide information on where tightening up the control would give the best improvement, minimising rejects and hence increasing quality & profits.

            Want to know more?    Then hit continue & read on.

Residual gas analysers (RGAs) - an under-utilised tool by metallizers.

            I suppose my views are coloured by the fact that the times when I get to work on vacuum systems are generally when the system has problems or a new process is being developed.  In both these cases it is important to understand what is happening inside the vacuum system.

            What is useful, particularly when troubleshooting the process, is to have some information of what the system was like when operating well.  Thus, although helpful, adding an RGA only after a problem has occurred does not provide as much information as could be obtained by having an RGA permanently attached.

            Using the RGA on a system when it is working well gives information about the performance of the system and the normal partial pressures.  This can be stored & used for comparison to help identify any changing proportions as well as total gas load differences.    

            Although pressure gauges can be used to monitor the process health, by comparing the time to reach a set base pressure, they cannot discriminate between different sources of problem anywhere near as well as an RGA. 

            The other advantage is that often when it is decided to add an RGA to help solve a problem the system has already incurred considerable downtime.  In many cases the RGA would have saved most of this downtime & thus paid for itself.  Over the lifetime of a system this could be expected to happen enough times that it always looks, at least to me, to be a false economy not to have an RGA included in the original system specification.

I am sure others will have experiences that would support this view and maybe some that would dispute it.  Feel free to pass comment on this view? 

Winding in vacuum – Asking for suggestions for books that specifically address the problems of winding in vacuum

Below is a copy of some correspondence between myself & Dr David Roisum who is the Editor of the AIMCAL Web handling / Converting Blog  @ http://www.webhandlingblog.com/  who is a recognised world expert on the subject.

Q

David,  which books would you recommend for those wanting to learn more about winding in vacuum?

Winding in vacuum systems have the added problems of not having any entrained air to help roll conformation, (they are always hard) there is the added problem of the deposition zone where the web is cooled, often to sub-zero temperatures, then heated to around 100 deg.C, re-cooled to a little above ambient. This all happens inside a box that is inaccessible when it is under vacuum.

Most books I have seen do not have any reference to what special requirements there are for winding in vacuum.

Any suggestions?

A

Charles,

There are thousands of articles and a half dozen books on the subject of winding.  However, I do not recall ever seeing even a brief mention of winding in a vacuum.  Perhaps someone in the cybersphere knows of something.  There are, however, a couple of recent articles on the subject of heat wrinkles on the metallizer.

David Roisum, Ph.D.
Finishing Technologies, Inc.
920-725-7671
drroisum@aol.com
roisum.com

Comment by CAB

David, thanks for the reply..

I am aware of the papers by Dilwyn Jones & Mike McCann on heat & wrinkles & the earlier work they refer to.

I had been hoping that somewhere there was a book with at least a chapter devoted to winding in vacuum.

It looks like a hole in the market if you want to write another book

best regards

CAB

Throwing the question out to the world –

Does anyone know of such a Book that specifically includes a chapter on Winding in vacuum?

Metallizing - have we driven all the costs out of the process?

            Over the last few years there has been concern about the influx of low cost material from countries where labour costs are minimal.  One argument has been that the labour costs are a small fraction of the metallizing costs & so the shipping costs would balance the total cost & it was not really the threat that everyone was expecting.

            This view may be a little blinkered as if the labour costs are low for the metallization step they are also low for the film manufacturing & converting steps too. Similarly for the mining & materials processing.  This could mean that any or all of the materials, i.e. polymer web, aluminium wire, boats, electricity, could be considerably cheaper. This would make much more impact on the cost of metallization and so even with the shipping costs it would be economic to sell into the world market.

            With this thought in mind my thinking then turned to what could be done to compete with this threat.

Click on continue to read more.