Vacuum Web Coating

Another problem with sparking.

Question

In a previous post it was mentioned that sparks could occur when a non-conductive web (PET film) is passing a conductive roll (grounded in that case). I have seen a similar phenomenon however with aluminium metalized PET film when winding trough a plasma cleaner.

When the foil is passing by a grounded roll, with the metalized side in contact with the roll, small sparks occur continuously over the complete width of the roll.

When the same foil is passing an isolated roll, the sparks are not showing up continuously, but their intensity is much stronger (you could start to call it lightning in a box).

The higher the winding speed the bigger the problem, and when the speed is below a few cm per minute the problem disapears.

Is there any explanation of what is happening or any known methods to solve this problem?

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ANSWER

The problem is essentially the same as with the uncoated PET.   There are a number of factors to bear in mind such as the following.

Metallized PET is used to make capacitors that are charge storage devices. Aluminium will oxidise very rapidly even in a vacuum system and aluminium oxide is an insulator.

Most aluminium metallizers do not metallize to the very edges of the web.

Thus when winding metallized web one would think that the aluminium metal surface when it contacts metal rolls would completely discharge but this is often not true. The metal-to-metal contact is not necessarily very good. The metal roll may also not be well grounded because the bearings may have a resistance that may be dependent on the lubrication used.  Thus the electrostatic charge can still build up on the web.

Electrostatic charge will dissipate with time. Thus if you do have a leakage path with a finite resistance there will be a time associated with reducing the charge to a level where sparking does not occur.

Thus at higher speeds the contact time for charge dissipation will have been reduced and so the charge remaining will be higher and so the sparks will be brighter or more frequent.  As the metal/PET is a capacitor the stored charge can be high. As the metal is a conductor the charge from a very large surface area can be discharged at a single point hence some very large arcs can be possible.  On unmetallized PET because the PET is an insulator only a limited surface area will contribute to the sparks and so these will usually be smaller sparks with lower energy.

Using insulated rolls just increases the storage of charge until it reaches a level where it will has sufficient energy to bridge the gap to the next nearest earthing point or where it sees a poorly conducting surface as a good enough earth to short to.

If the webs are being wound at atmospheric pressure then there is an additional insulator between the aluminised metal surface and any metal roll and that is the entrained air that is carried along with the web.  This air layer varies in thickness with speed and so at slow speeds the gap may only be a few tens of microns but at higher speeds this may be considerably more.  Thus the amount of arcing will also be dependent upon the humidity and tension during winding.

Air is conducting but the conductivity is dependent upon the moisture content. Thus the arcing will be worse with dry air with the air being more of an insulator and hence the charge left on the web able to reach higher levels. Also because the dry air is non-conducting the time constant for charge dissipation will be longer.

I hope this sufficiently explains what you are experiencing.

A problem of sparking

We are facing problem in Metallized in edge to edge. As soon as metal start deposit on drum edge, it's giving heavy sparking near edge which is resulting bunch of holes in the film edges.  We are applying slight graphite paint ( Release agent) on film edge to remove deposited aluminium after completion of cycle. Even if do not apply release agent then also we are facing the same problem.

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ANSWER

Most commonly electricity is the cause of sparking.  The source of the electricity can vary.  One possible source of sparking is if the system is not properly earthed and differential charges build up. The metallized film is, in effect, a large capacitor.  The source of charge can be from triboelectric charging of the polymer web running over metal rolls.  The amount of charge is dependent upon the polymer conductivity, moisture content, polymer type, roller type and speed.

One method of removing any triboelectric charge is to plasma treat the polymer. The plasma will spread through the chamber and provides a mechanism for neutralising the polymer surface.  If the polymer is not plasma cleaned this may allow high surface charges to build up and result in t he discharge at the edges.

Another possibility is if the system is not well earthed and there is a plasma treater included. The plasma will require a cathode and anode. Generally the system is at earth potential and is used as the anode. If the system is not well earthed it can be that various components within the vacuum system end up at some floating potential. Where there is a potential difference this can also lead to sparking.

The use of the graphite paint may also contribute to the problem. If the carrier for the graphite is not completely dried there may be outgassing causing localised high pressure regions that would enable sparking to occur at lower voltages.

If you wish to check out the triboelectric charging there are meters that will measure the voltage. Although they are not designed for use in vacuum I have used one in a vacuum to check the level of charging under real conditions rather than at atmosphere.  Often the surface charge can rise to several thousand volts which is more than enough to create arcs (sparks).

Checking out the system earth requires measuring any potential difference between the system and a good reliable earth.  If there is a difference it is possible that there could possibly be charges induced from the high currents used to power the resistance heated boats.

Hopefully this answer gives you enough ideas to uncover the source of the sparking.

Breathing aluminium dust.

We would like to know if there is any study  made on the effect of breathing aluminium dust for health (long term) regarding the exposure level we normally have in our industry. Is there any case of disease registered or known and if so, what are the details?

In fact, we always try to improve conditions in which our employees work and we are now looking for new methods or procedures to increase the security for that point. Are there special equipments used over a good air system to eliminate aluminium dust or smart work instructions to avoid breathing aluminium dust?

Please, give us your suggestion or point us out contacts who could help us:

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ANSWER

I am assuming that Canada has a similar body to the Health and Safety Executive (HSE) that we have here in the UK.  It is this body that researches the information available and defines the working limits for breathing dusts/fumes etc.

The HSE produce a book that tabulates the maximum exposure limits and these are usually presented as either as a Long-term exposure limit that uses a Time Weighted Average reference period and is given in parts per million &/or mg.cu m and may also give  a Short-term exposure limit that uses a 15 minute reference exposure time also expresses as in parts per million or mg.cu m

The HSE can be accessed via the website  www.open.gov.uk/hse/hsehome.htm


Cleaning vacuum metallizers can be done in a variety of ways and so there is no single answer as to what protective clothing/precautions are required. Some metallizers only clean by chipping off the excess metal deposits in-situ and this can generate a huge amount of dust and also can lead to explosions or fires where the dust has accumulated and the exothermic oxidation leads to a fire that if the dust level is high can lead to a flash fire/explosion.  In this case the operators should be well protected with, at a minimum filter masks, and preferably clean air hoods, protective overalls, gloves and hard hat.  Often injuries occur not from the fire or
flash but because it is a surprise event the operator reacts and in reacting bangs themselves on some other part of the system which can cause bruising or cuts.

Other systems have been built with cleaning in mind where the deposition shields are interchangeable and in this case the old shields are taken off for cleaning elsewhere and new shields are installed. This leads to a faster turnaround and allows the dirty and potentially hazardous cleaning to be carried out in a safer environment.

Shields can have the boron nitride or carbon 'paint' applied to encourage the release of the aluminium in large pieces reducing the dust generated. Once a coarse clean of the shields has been done some use a sandblasting unit to give the shield a final clean before re-painting with release agents.  In some industries the shields are cleaned using a chemical cleaner. In the case of aluminium this would be something like potassium or sodium hydroxide. Again most of the aluminium would be removed by a coarse technique (hammer & chisel) followed by dipping into the chemical solution. This tends to be a higher cost cleaning technique because of the cost of chemicals and the increasing cost of chemical disposal.

This final method of reducing the dust is to use disposable shields where a heavy gauge aluminium foil is used over a permanent shield and this is then disposed of and as it is of the same material as the deposited aluminium it can be recycled.

Questions from students.

            For those of you who check out this blog on a regular basis will have noticed there has been a rash of questions over the last few weeks.  I have the suspicion that the word is being passed around students attending one particular Institutes packaging course that this might be a good source of answers.  Although this might be true I do not believe it is necessarily helping those students.  I have always believed that training courses are designed to help the students to think and develop a methodology to enable them to solve problems.  In the future they will probably have to answer questions without being able to ask for outside help and if they have not developed the necessary expertise they may not be able to produce acceptable answers.  With this in mind I will be adopting the approach of suggesting some questions that once answered might allow the problem to be solved or making some other suggestions but will aim to not give a complete answer. If you check down the list of questions below it becomes apparent that some students appear to be using my time to answer questions that they could look up elsewhere. 

Thus, for the benefit of the readership in general, please forgive me if some of these questions do not get answered by me either immediately or even at all.  The opportunity is still available for any reader of this blog to answer any or all of the questions. For those of you who feel disposed to help some students out I have collected a number of the most recent questions below.

QUESTIONS.

I am packaging student following are my questions.

1.how can we decide the void space or headspace in any package is their any formula ?
2.what are test performed on package for food grade certification ?
3.till what time a laminate can be stored?
4.in side gusset pouch what is the gusset given ?

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WHAT ARE DIFFERENT COATING GIVEN ON PET LAMINATE ?

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I have few more questions related to adhesive, can you tell me which adhesive can be used in carton so that while opening the flap it should not tear off. also lowering the viscosity can solve the problem. if grammage of board is increased can it help. or we have to use particular adhesive?

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In one of our pack the batch number which is printed is rubbed off what can be the problem, the printing is done on chemically coated pet, can any surface treatment on printing area can solve the problem

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What is pearlised BOPP? What is difference between metallised & pearlised BOPP? How it is manufactured? How it differ from normal BOPP in barrier properties? Is it only in BOPP or it comes in other material also?

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In one of the fatty acid cake which has 23% moisture content. over period of time the cake looses the moisture this moisture is unable to go outside & condenses & forms white mass over the cake. The film used is PVC. What could be the reason? Is their any material which can provide zero barrier so that moisture goes out & does not condenses

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If a laminates having same MVTR but different composition one is 2 layered structure with higher thickness (higher micron) & other is 3 layered structure but of lower thickness (less micron).which is beneficial in terms of stiffness, machinability ,shelf life of laminate, cost, etc.

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I am a packaging student.
I was doubt but not regarding any coating, I am working on project anti-counterfeiting in pharmaceuticals, following are my doubts.
* what are the technology used for stopping counterfeiting, i want the details of each technology?
* how costing is calculated before incorporating any technology?
* what is the future of anti-counterfeiting?

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I am student of packaging institute. Right now i am working as trainee in confectionary industries. my industry is using 12mic Pet/20mic METBOPP'(seat strength=1.2and bond strength=120)
they have given me 2 project.
1. in first project company is trying to convert 12mic Pet into 10mic Pet
2.in second project , company is planning to convert 2 layer laminate into single layer laminate i.e30mic BOPP(printed BOPP)
Aim of the company is cost saving.
so kindly suggest me few thing in both the project. and in second project what type of grade of BOPP should be used

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1) Why PVC and PVDC is most popularly used then PP in blister packs?
2)The web material before loading on to machine is stored at Low temp (A.C) and when the material is loaded to m/c the temperature is room temperature so there is vast difference in temp within a minute, what problem will the material face? the materials are Aluminium, PVC, PVDC and Nylon

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Air-to-air vacuum coating machines - design issues

This was the question asked.

I have a job interview coming up. I am a mechanical engineer and would be
designing the mechanical equipment for the web. I am comfortable with this
part however I have never pulled a web through a vacuum chamber before. I would
a least like to study up on the mechanisms of sealing a web passing into and
out of a vacuum. Do you know of a place on the WWW where I could see some
pictures and pros and cons explaining these mechanisms. Perhaps some links
to manufacturers of seals might do for now. This is a semi-conductor
conversion for solar cells on .005 thick stainless steel. I would be
grateful for any other information you think might be helpful. Thank you in
advance for helping.

The answer.

There are some papers that it would be worth checking out.

Hartwig E.K. et al  ‘Air-to-air metallizer – suitability & profitability.’

            Proc. 34^th Ann. Tech. Conf. SVC  1991   pp 152 – 161

Taguchi T et al  ‘Air-to-air metallizer: design & operational data.’

            Proc. 35^th Ann. Tech. Conf. SVC  1992   pp 135 – 140

Taguchi T et al  ‘Air-to-air metallizer.’

            Proc. 35^th Ann. Tech. Conf. SVC  1992   pp 424 – 426

There are also some patents, The one I have to hand is a UK patent assigned to Hitachi.  GB 2084264.

As I recall Mitsubishi Heavy Industry & Hitachi both had patents on air-to-air systems for vacuum metallization.

Neither appears to have been a great success.  Their target was aluminium metallization which needs to be done at 600m/min or greater.

They wanted to take polymer film directly from the film line through an in-lime metallizer.  Unfortunately the metal deposition efficiency was poor and so the stray deposition collected by the shielding inside became the limiting part of the process. This was reduced by using heated shields but this then increased the heat load on the polymer and so the coating speed had to be reduced thus becoming the bottleneck to the film production.  Net result was that using batch coaters was more effective.

Thus most systems that have been built have been batch coaters.  ECD would be one of the suppliers of PV deposition systems and there are many pictures of these machines.  Currently with the huge current interest in photvoltaics most of the machine manufacturers have been approached to build systems. Most sell machines that are to a functional specification but do not offer a process.  ECD are different in that they also offer a complete process.

Problems that need to be looked at carefully are contamination and surface scratching due to the high number of surface contact rolls.  The conductance gap between chambers needs to be small to minimise the number of chambers used between air and the vacuum required for the process. If the gap is close fewer chambers will be required but the risks of surface damage due to the web catching the edges of the gap increase. As the air is drawn in through these gaps there can be occasions where the air howls in and sets up a flutter on the web (polymer or metal) these oscillations can be large particularly if a resonance is set up. This flutter can make the gap that works well at atmospheric pressure appear too narrow when used in action.

Any debris brought into the system on the surface of the web will have to pass through the nip rolls. This can cause dents and/or scratches to the surface. Thus the process does not start with the machine but with the quality of the incoming material.

Generally the metal webs are relatively narrow and so some suppliers think the winding system is easier. However the stiffness of the web means that the issues of having a good quality winding system do not disappear. Similarly the quality of the profile of the incoming material is critical to good winding.

A question relating to film curl

Can you please help me to understand what are the possible sources of making the Polyester film curled, how can we prevent/minimize to get it curled. and what is standard method to measure it, one of our customer has complained for it recently in our recent supply of plain polyester film.

Answer

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Curl is usually measured by cutting out strips of film out of the roll of approximately 150mm x 5mm. If it curls naturally it is easy to rest the curled strip on its edge and also to measure the radius of the curl and from this to calculate the stress in the film.

However it is rare to find curl on a plain uncoated polyester. Curl usually occurs where there is a differential stress either through the thickness of the film or between the substrate film and any deposited coating on the substrate. Typically if a polymer coating has been added to the PET substrate and when the coating is cured it shrinks the contraction of the coating puts a compressive stress into that side of the film compared to the other uncoated side. Thus to reduce the total stress the coated film will bend (curl) so reduce the stress in the coated side. This is because the uncoated side extends and has an increase in tensile stress that offsets some of the compressive stress in the coated side. The amount of curl (radius) is a measure of when the two sides are in balance.

A source of temporary curl can be in applying something to the surface of the web that can be absorbed. As the liquid (moisture) is absorbed the surface will swell and extend causing curl. As the liquid diffuses through the bulk of the polymer the curl will disappear again as both sides become extended by similar amounts.

This type of curl is unusual on a plain PET film.

However if what you are meaning by curl is the film is bending laterally this is due to one side of the web being shorter down its length than the other side. This simple way for checking this is to cut off a length of film around 25m long and laying it out on a smooth flat floor. Usually a simple view down the length will tell you if the web is bending away from being straight. If a straight line is made between the two ends of the shorter side at the mid point the distance between the straight edge and the web can be measured. This chord can be used to calculate the radius of curvature.

This type of bend is common in PET film manufacture. PET usually has a stress profile that is related to the ‘Bow’ angle. This Bow angle is produced during the drawing process. As PET is produced in very wide rolls, wider than most metallizers can manage, it is slit down from the Mill, full width, roll to narrower rolls to suit the metallizer size. Unfortunately this means that only the centre roll will have a uniform bow angle and hence uniform stress across the roll width. For the rolls nearest the Mill roll edge the difference in stress between the two edges will be greatest and this is likely to result in the largest curvature when a length of film is laid out on the floor without any applied stress.

Once the roll has been threaded through a winding system the tension can be applied. As the stress is applied it will all be taken by the short edge first and only when this has been stretched to be as long as the longer side will the rest of the stress be shared across the whole width of the film.

I hope this suitably explains what can cause differential stress within a film and hence curl.

High barrier coatings on BOPET film

Questions.

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Which High barrier coatings on BOPET film are available?

Up to which extent Barrier properties will improve?

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Answer.

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There are many coatings that are used to produce high barrier
performance.  Some of these are changing such as PVdC that was a favourite
for many years but as concerns about PVC increased the PVdC coatings were
banned in some countries and thus its use has been falling.   Some of the
barrier coatings are used where the packaging needs to be retortable but are
expensive and so are not used where the products are not going to be heated
to the same extent.

On the vacuum deposited coatings the standard barrier coating is aluminium
and this gives a barrier performance of <1 gm/sq m/day of moisture and < 1
cc/sq m/day for oxygen coupled to a good light barrier. This is important
where fats are concerned as light can turn fats rancid.  However for other
foods such as coffee there is not the same requirement for a light barrier
and so transparent barrier coating such as silica or alumina can be
deposited and these aim for a similar level of barrier to the aluminium but
are completely transparent.  These transparent barrier coatings are more
difficult and slower to deposit and so are more expensive also they often do
not have equally good moisture and oxygen barrier performance and so a
decision has to be taken which has to have the higher performance so the
deposition process can be adjusted accordingly.

The latest barrier coatings to be developed have been the ultra barrier
coatings at have a barrier performance several orders of magnitude better
than the food packaging coatings and have been developed for the electronics
packaging industry.  These consist of the oxide barrier coatings but to
improve the performance they are deposited onto a polymer smoothing layer.
This pair of layers have been called a dyad and by depositing several dyads
onto a film the barrier performance increases enormously as the coating
defects are reduced and any defects left are remote from each other and so
the diffusion path is extremely tortuous.

Thus you have the choice of many polymer coatings such as EVOH as well as
the vacuum coatings to produce a variety of barrier materials with a range
of barrier performance and costs.  To help define what coatings you can use
you need to find out what requirements you need to have satisfied.  For
example do you need only moisture barrier or do you need only oxygen barrier
or some other gas barrier such as for modified gas packaging, or do you need
both moisture and gas barrier?  Do you need light barrier or not?  Is the
BOPP to be laminated or not? etc.

From this you can then eliminate some materials and of those left you can
then look at the costs and performance and make a choice.