Vacuum Web Coating

Question following on from ‘Tramlines’ question.

Today, I read your answer in Aimcal's Q&A regarding curvature/flatness. (Tramlines)

I have been working in polyester film industry. Polyester film has both the curvature

and the flatness problems due to many causes.

But I do not have experience in other films industry such as BOPP, BOPA and the like.

Are there the same problems in other films? Which film is most serious?

My answer was as follows

Most, if not all, films have some problems that are a result of the method of manufacture. The films that are produced by extruding a polymer & then stretching the cast film to get bi-axial oriented film, like PET, will have similar problems. The quality of the film is likely to be a reflection of the expertise of the supplier & where the supplier wants to be ranked in the level of quality of the films they produce.  Some of the problems can be exaggerated in the vacuum system and these may be worse for the likes of polyethylene than for polyester simply because of the difference in things like tensile performance.

Other films such as cast cellulose will also have some problems but the source of these will be slightly different because of the different manufacturing process.

The thing they all have in common is that it is very difficult to produce a uniform thickness film to within better than 1% & this will give uneven tension on the web with the thickest part of the web taking more tension than the rest & if this happens to be near one edge it is likely that the web will form wrinkles somewhere in the winding process.

Confusing Terminology – Barrier performance

The Question was;

How does a perm rating differ or compare to MVTR. If a customer asks for a perm rating of .02, can I convert this to an MVTR rate?

Thanks,   Dave Wilson

 

My simple answer was that I did not know for certain.  Personally I would take 'perm' to be an abbreviation of 'permeation' & hence that they are talking about the same thing.

Thus I would take the 0.02 to mean 0.02 g/sq m/day MVTR

However, I consulted an expert in Barrier Coatings, Dr. Bernard Henry & got the following definitive answer.

The term Perm refers to the unit of measurement of permeance. A metric unit perm measures 1 g per 24 h per m2 per mm Hg. The perm is used to express the resistance of a material to the penetration of moisture.

Permeance is the ratio of the transmission rate to the difference between the partial pressure of the gas on the two sides of the film. Metric unit: g/(m2.day.mmHg).

Whereas water vapour transmission rate is the time rate of water vapour flow normal to the surfaces, under steady state conditions per unit area. Metric unit is g (STP) per m2 per day.

Sometimes though people refer to Perm and are talking about water vapour transmission in which case your answer above is correct. Best to find out the units and then you know for sure.

This answer was supplied by Dr Bernard Henry of Oxford University who will be running the half-day short course on Barrier Coatings at the forthcoming AIMCAL Fall Conference & who is regarded as a world expert on barrier coatings.  There is still time to register for the short courses which run on Sunday 16^th Oct. Check out details at www.aimcal.org

Request: What is Plasma?

Request: What is Plasma

Why it is important?
How it is effective when done with PET?

Many of us use a plasma to treat our polymer films whether we know it or not. A plasma includes such treatments as flame, corona as well as vacuum plasma treatment.

For a more detailed explanation read on.

Question re. oxidation of aluminium.

Question.

On multiple passes through the metallizer we notice various levels of oxidation on the metallized (aluminium) side of 92 gauge PET film. There appears to be little oxidation after the first pass but significant oxidation after the second. The amount of metal is a target of 2.5 optical density for each pass.
Is there a way to measure the oxidation? Is there a way to control the amount of oxidation that occurs? Can it be eliminated from the web prior to lamination?

I have put my answer into the continuation. Just click on the continue button to see if it makes sense.

A question relating to tramlines

Question. - Reasons of Tramlines in polyester film during metallisation
and actions taken to avoid them?

Answer.

Tramlines are a heat related problem.  As soon as the polymer web touches the deposition drum it starts to contract as the temperature is reduced from ambient to the temperature of the cooled drum. At there is tension on the web the frictional force between the web & the drum are high enough to stop the web moving & so it will be in tension. Then the web reaches the deposition zone where there is a high heat load and all of this is reversed. The web wants to expand quickly & soon passes through ambient temperature & gets hotter still. Again the web tension is high & the friction stops the web moving over the surface & thus the web will be under a high compressive load. If the heat load is too high this compressive load gets too great & the web will buckle off the drum surface. Where the web is then unconstrained the surface a rea will be greater than if it stayed in contact with the deposition drum & so the same amount of coating is spread over a greater surface area & so is actually thinner & this appears as a tramline.

So there are several possible solutions. One is to use a slightly higher tension on the web around the deposition drum. This requires some care as if the tension is too high the web can neck if the elastic limit is exceeded. This is possible as the elastic limit falls as the temperature rises.

Another possible solution is to use a spreader roll as the polymer web is laid onto the deposition drum. This will prestretch the polymer web & put it into tension that will offset some of the compressive force produced during the deposition process.

A third possibility is to use a gas injection between the drum & the polymer web. This works in two ways. One is that it will improve the heat transfer coefficient & so lower the polymer temperature reducing the stress. It also floats the polymer web and reduces the frictional force allowing the polymer to change dimensions more easily & also reducing the tendency to balloon off the drum surface.

Incidentally some recent work has also shown that debris trapped between the web & drum will also lift the polymer off the surface & cause tramlines &/or ballooning. Thus using clean polymer film & paying particular attention to cleaning the system is also helpful.

Should we be satisfied with a boat life of ~15 hours?

            Systems can be driven hard and the boat life suffers.  There are low cost sources of boats but this too tends to lead to shorter boat life as the refining of the powers for the ceramic composite tend to be coarser & less densely packed.

            Compared to this is a small trickle of information that occasionally leaks out of Japan.  A company wished to use their existing induction heated crucibles for the deposition of aluminium but were advised against trying. The reasons given were that molten aluminium is extremely corrosive & the crucibles would not survive. Despite the warnings they tried & sure enough the crucible lifetime was short, less than 10 hours.  Rather than this being a deterrent they decided to improve the crucibles to increase the lifetime until they became acceptable.  Thus over the years they have improved to around 40 hours.

            I was trawling through some references looking for something else & I cam across an article about using air-to-air metallizers. In this article it referred to using an induction heated linear crucible of 75" length for depositing aluminium where they had increased the lifetime up to & beyond 120 hours.

            Now having read that I am asking myself why should the crucibles for an induction-heated source last for 120 hours & for resistance heated sources only last around 15 hours. Are the materials so radically different?

            The answer appears to be, no, the basic crucible material can be the same.  What is different is they spend time in making sure they have produced the densest material possible & then where the aluminium contacts the surface they have deposited alumina to fill the pores.  Thus the aluminium cannot penetrate the surface & corrode the crucible.

            So I now ask myself who is going to be the first to offer this same technology for resistance-heated boats?  Perhaps they are already available from Japan & I just do not know where to buy them.  If anyone out there knows of a source of this material or a supplier of these boats I would be interested to know more. 

More books – American Vacuum Society Monographs.

It did occur to me that I had missed out a large cannon of useful information because I was only itemising books that have an ISBN number.

Below is a listing of the articles available as AVS Monographs on different aspects of vacuum technology.

If you are interested in any of these books just go to the AVS website to order them direct.