Vacuum Web Coating

High barrier coatings for BOPET

Which High barrier coatings on BOPET film are available?

Upto which extent Barrier properties will improve?

Answer

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 BOPET 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.

Embossing & holography - differences

Could you please help me to understand the difference between Holography & Embossing?

Answer.

Embossing is the process of pressing a surface with some surface relief into a softer surface so that the softer surface will conform to the surface relief and take on the complimentary shape.

If the surface relief happens to be a holographic device the embossing process will transfer that holographic device into the softer material.

This softer material can be a thermodeformable polymer such that the surface relief hard surface is heated and so will more easily deform the thermoformable surface. The problem of the heat is that once the embossing surface is removed the thermoformable surface can relax and lose some of the definition. Alternatives are to have a surface that can be cured in some way, either by light or heat, so that once the surface relief pattern has be produced it can be fixed more permanently.

Thus embossed holographic surfaces can be metallized but may lose some of the image sharpness because the metallization process is hot through the deposition zone. Some companies prevent this by embossing the metallized polymer after the metallization so that they have to deform the thin metal layer as well as the polymer. 

I hope this explains the difference for you

Printing on which side?

a/ When printing on Metallized BOPP (treated on outside only), what side should be printing on, Polymer or Metallized ?
b/ Should primer be applied and will different primer/ink types effect ink retention?

Answer.

I have seen printing done on either side.  There are differences that it is worth being aware of.

The metallized surface, when freshly deposited, has a very high surface energy and so it is easy to wet out the metal surface and get good adhesion. This can change with time.

BOPP can have additives such as slip agents added. These are contained within the bulk and migrate to the surface to reduce the coefficient of friction by reducing the surface energy. These low molecular weight slip agents can be removed by treating the surface such as by corona.  This has only a temporary beneficial effect as more of the slip agent will migrate out of the bulk to the surface given time. The exact time is also affected by temperature with higher temperatures reducing the time for the slip agents to migrate to the surface.   The slip agent is indiscriminate and will migrate to all surfaces.  Once a surface has been metallized this can prevent the slip agent migrating to the metal surface. However this does not mean that the slip agent will not appear on the metal surface.  Once the web has been metallized and re-wound the metal surface will be in contact with the reverse surface which may have some slip agent present. The metal surface will be in direct contact with the slip agent and the high surface energy of the metal will make it energetically advantageous for the slip agent to stay on the metal surface.  Thus the metal surface will age as more slip agent migrates out of the bulk to the reverse surface and is transferred across to the front surface in the wound roll. 

The metal surface will also be changing with time as the aluminium oxidises. The initial oxidation will be immediate but the oxidation process does not stop but will continue albeit at an ever slowing rate.  Once a nanometer or so of oxide is reached the surface energy will plateau out and will not change any further except by the contamination route as described above.

The net result of all this is that there can be problems of adhesion that can relate back to the time between the corona treatment and metallization and the printed coatings.  Generally adhesion will be higher if the printing is carried out with the minimal time between the surface treatment and metallization and the printing.  Similarly the metal adhesion can be affected by the time between corona treatment and metal deposition. Frequently the metallizers will also use an in-vacuum plasma treatment to make sure the surface is freshly cleaned prior to metal deposition.

The choice of which side to print onto can be affected by the time lag between corona and metallization and printing.  If the time is short then it is less likely to matter and the choice can be made purely of aesthetic and functional grounds. If the time lag is great it may be that only one side will work without delamination because of the self contamination process.

Although this may not answer your question directly I hope it gives you an insight into the processes that are happening and it will allow you to make an informed choice.

Problems relating to surface energy.

This topic has also caused several questions to be asked about surface energy, decay of surface energy and the mechanisms involved; such as……

What is the normal surface tension of metallised film after the metallising process? What would be the ideal surface tension of the metal side of VMCPP if we are to laminate by a solvent free process?

I wonder if there is an effect on the dyne level of standard PET when metallized? A none treated standard PET has a dyne level of 40 to 42, is it possible that this level gets down to 32 when metallized (metallized side of course)?    If so, how this loss of treatment occurs (mechanism)?

We being a converter was wondering whether there is a decay curve for surface energy for BOPP, CPP, films and their metallised counterparts?

ANSWER

Most film manufacturers will give a surface energy if asked.

Generally it PET is 41-44 & PP is 29-31 before any treatment.

The reason why PP (OPP or CPP) tends to deteriorate faster is that PP films often have more additives included and these migrate to the surface and reduce the surface energy.   PET films not only include fewer additives but also any surface treatment is retained much longer.

OPP can be retreated but the re-treatment will also have a limited lifetime.  The additives are within the bulk and will begin to migrate out as soon as the treatment is completed.  Contamination of the surface by the additives can also occur by transfer of them from the back surface onto the front surface.  Thus if the roll has only had one side treated and then been left stored for some time there will have been plenty of time for the additives present on the back surface to transfer onto the front surface reducing the effect of any surface treatment. This is in addition to the migration problem.

Metallized surfaces can initially have a contact angle of, as low as, ~5 degrees indicating a very high surface energy but this declines and is possibly an indication of the growth of the oxide layer from the monolayer at the time of metallization through to the 1nm within an hour or so through to 2nm in a week to 3nm in a month or so.  After a couple of days the contact angle will have risen significantly and will continue to rise towards a more stable level of around 55 degrees which corresponds to a surface energy of ~ 45 dynes/cm.   If the surface energy level is falling to much below this value it is likely that the surface has become contaminated such as from any low molecular weight material that is present on the back surface which can be transferred to the front surface during the time the web is rolled up.  The higher the surface energy of the metallized layer the more energetically favourable it will be for the low energy material to transfer from the back side to the high energy metal surface thus significantly reducing the surface energy.  This low molecular weight material can easily reduce the surface energy down to the equivalent of an untreated film before metallization.

Problems with tramlines

There have been several questions posted recently all asking to solve the problem of the formation of tramlines during metallization.

A)        We are facing problem of tramlines during metallization. Could you please suggest to me how can remove of this problem?

B)        The tramlines problem we face is only in side rolls. We have a 8.5mtr film line. The following work has been done to avoid tramlines-

1) reduce the child drum temp.
2) reduce the boats heating ( boats temp.)
3) gas wedge open ( inject) during metallization time.

We still face the problem of tramlines in the side rolls from the film line do you have any other suggestions?

C)        Apart from the gauge variation in the base film what other factors can cause tramlines in the PET film during metallization process?  Any suggestions to overcome the problem?

Initial answer.

The mechanism for the formation of tramlines is a function of the heat load during deposition.  The heat load is from the radiation heat emitted by the resistance heated evaporation boats and also from the latent heat of condensation of the aluminium metal as it condenses on the polymer web.  The result of this heat is that the polymer wants to expand. This is easily done on the machine direction as the web is in tension and it results in less tension applied to the web around the section of the deposition drum where the web is being heated.  The large wrap angle often means that this is not seen by the winding system as at the point the web arrives at and departs from the drum the web is cool and the tension is as set.  In the heated zone the main problem is the polymer trying to expand in the across-the-web (transverse) direction. There is already a compressive force on the web because of the tension being pulled. This is sometimes counteracted by use of a spreader roll just as the web is laid on the deposition drum. The effect of this spreader roll is to try to put some transverse tension into the web. Once the web is on the deposition drum it is cooled and this will increase this transverse tension. Then as the web passes into the deposition zone and the heat is applied the web wants to expand. Initially this expansion is taken up by the transverse tension which is reduced to zero and then if the polymer continues expanding the polymer will have a transverse compression applied as the web does not slide over the deposition drum but is held in place by a combination of the applied tension and the coefficient of friction between the polymer and drum.  If this compressive force is too much the polymer will lift off the drum surface and start to balloon to relieve the compressive force. The part of the web then not in contact with the deposition drum will then heat up further and expand more, increasing the ballooning.  The web not in contact with the drum now has a greater surface area than when it was in contact with the drum and so the same aluminium deposition is seen as a thinner coating in this area. Thus the light and dark stripes when the web is looked at following deposition.  Debris on the deposition drum can sometimes trigger the onset of tramlines. Debris may only be a few microns in diameter but it can be sufficient to lift some of the polymer off the surface causing it to lose some of the heat transfer performance and so locally heating that part of the web more than the rest. Thus it is always worth keeping the drum as clean as possible.

That describes the basic mechanism and so we have to look at what can be changed.  The heat transfer coefficient between the web and the cooled deposition drum is a critical parameter. This can be improved by increasing the tension. Often this is not possible as the tensions are already at the maximum.  Where the tension cannot be increased other things need to be considered.  If the web is not spread, as it is meets the deposition drum it would be worth looking to do this.  If the system has been working well up until now but the performance is starting to drop it could be that the spreader roll is aging and hardening and may need to be recovered to bring the performance back to the original.  Once the web is on the deposition drum it can be an advantage to reduce the temperature of the cooled drum. In this way the initial cooling is increased and the thermal expansion in the deposition zone reduced.  Finally there is a completely different approach, which is to use the air-wedge technology where a gas is inserted between the web and deposition drum. This gas is trapped by the web because of the applied tension and is compressed between the web and drum and it increases the heat transfer coefficient by using the gas to transfer the heat by convection from the hot web surface to the cooled drum surface. The gas also acts as a lubricant allowing the web to expand and contract more easily thus reducing the stress within the polymer and reducing the propensity to buckle off the surface.

Supplementary answer.

The factors that cause the tramlines are heat load to the film, heat transfer coefficient between film and drum, coefficient of friction between film and drum, distribution of tension on the film as well as quality of the winding system. SO you need to look at anything that can affect one or more of these factors.

Examples would be filler type and content. Films can be through filled or can be co-extruded with usually the filled co-extruded layer providing the back surface and the smooth surface being the one metallized.  The size, shape and distribution of the fillers will affect the coefficient of friction. The fillers are added to reduce the contact area between successive layers of polymer when the film is wound into a roll. Without the filler the coefficient of friction can be too high and the pure polymer surfaces can easily block.

Fillers can range in type and there is a cost/performance choice that has to be taken.  Even with films from the same source there may be occasional inconsistencies in the filler distribution and hence coefficient of friction. This is why any tests of films needs to be repeated many times to be certain that the results are typical and not just a one off result that is higher or lower than the normal.

Anything that is done to the film once manufactured that can also have a detrimental effect ought to also be included in your review.

This would include things such as slitting or pretreatments such as corona.  What you are looking for is any source of non-uniformity.   Slitting is a mechanical process and has an effect on the film. A badly worn slitting blade can damage the edge of the polymer web. The blunt blade will cut with more friction and will heat up and can cause a raised edge on the polymer web. In bad cases this raised edge can cause winding problems and will locally change the coefficient of friction as well as being the equivalent to a gauge band at the film edge taking more tension and hence reducing the tension applied across the rest of the web.

So if you are having a problem with tramlines start by checking that the supplied film is as uniform and reproducible as possible.

Within the vacuum system what is happening to cause tramlines is that the polymer film is trying to expand under the influence of the radiant heat from the boats and the condensation heat from the depositing aluminium. The tension applied to the film holds the film against the cooled deposition drum preventing the polymer film from moving. If the film heats up too much the expansion will put too much compressive load into the film and it will buckle off the surface of the drum. As the film is no longer constrained or cooled this will rapidly expand and balloon off the deposition drum surface producing the tramlines.

Standard techniques to overcome this are to try to increase the cooling to the deposition drum, try to improve the heat transfer coefficient between film and drum, try to put some lateral pretension into the film by spreading the film as it is laid on the drum so that the expansion initially is taken up by this tension thus reducing the maximum lateral compression the film sees, increasing the tension applied to the film to hold it harder against the deposition drum and finally even consider running the process more slowly.  An alternative approach is also done and that is to inject a gas wedge between film and drum so that the web floats on this gas layer and the compressed gas provides a conduction medium that will significantly improve the heat transfer coefficient thus extracting more heat from the film and reducing the maximum temperature the film will reach.  If the gas wedge technology is already used it may be possible to inject more gas to increase the gas compression and further increase the heat transfer coefficient. This will also increase the gas loss at the edges, which, in turn, may result in an increase in the total chamber pressure. This side effect may have other detrimental effects on the film quality and so it may not be preferred.

Another cause of tramlines, that is often overlooked, is contamination of the deposition drum.  When the vacuum system is cleaned there will be a huge amount of dust generated. If you consider that some of this will fall onto the top surface of the exposed deposition drum and will be present between the deposition drum and the film it will be lifting the film out of intimate contact with the drum and so at that point the film will be overheating. This can be the start of a tramline. This dirt may stay stuck to the drum and so potentially every revolution of the drum will allow the same dirt particle to start a new tramline, or help continue an existing one. If the dirt becomes stuck to the web it may be carried away and the problem reduced with time. As the removal of heat is so critical to the process it makes keeping the deposition drum as clean as possible equally critical.

I hope these answers help to give you some pointers of areas to investigate.

Please let me know of your progress.  It always helps to know if the comments have been helpful or not.

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ATTENTION: The Vacuum Coating Blog is being updated with a new layout. This design will serve as a temporary layout during the transition. Thank you for your patience.

AIMCAL

Roll hardness testing

Question.

I am looking for a portable hardness tester for film rolls and all I find on Internet is for paper and it does not seem to be efficient for plastic film. Durometers are not very useful either. Does someone know a device that could give us a good hardness reading ?

Answer.

            The hardness of rolls can be quite important. If a roll arrives too loosely wound it can easily telescope on being handled or during the initial vacuum pump down process.  If the rolls are wound hard they can highlight any profile variations and so a soft wound roll may immediately raise suspicions of either poor handling or poor quality film.   If there is any profile variation such as an area of over thickness then this will show up as a gauge band. If the thickness variation is large and the hardness too high this can stretch the film along the gauge band and put some permanent deformation into the film.

            No matter what state of hardness the film arrives in by the time it has been re-wound in the vacuum system it will be a hard roll. This is because there is no air interleaving between the film layers as the film is re-wound.  The hardness is in effect a measure of the quantity of air that is wound into the roll.

            It is common to want to assess the roll hardness and often operators can be seen to knock the rolls with a knuckle to get a feel and listen to the sound as a method of testing the roll.  In fact this method has been improved upon by use of a stick (Billy Club) to do the knocking. These methods are subjective and so can be variable from operator to operator and depend on their experience.

It is possible to purchase a simple test meter that standardises the same process.  There are several products on the market that can be calibrated and give a reproducible measure of roll hardness. The method of testing can vary from having a spring loaded plunger that for a given force will press into the roll and a measure of the depth will give a reading of the hardness. Another test is to use a projectile, such as a ball bearing, that is dropped from a standard height and the height of the bounce is measured. The harder the roll the higher will be the bounce.  This same principle can have the projectile mechanically fired so that the test can be carried out at any angle and not just vertically downward.

In the book by Good & Roisun (1) the following methods or instruments are described, the Billy Club and its Variants, RhoMeter and RhoHammer (used to be the Beloit RhoMeter), Backtender's Friend, Schmidt (Concrete) Hammer, Parotester and TAPIO RQP (2 – 5)

            It is best to measure the roll several times across the whole width so that any variations are picked up.  Some of the above meters now have the facility to store many data readings and to be able to transfer the data to computers to make recording the data easy.

            One word of caution.  Measuring the hardness whether by the knuckle or Billy stick or by automatic machines that bounce a projectile off the surface or press a probe into the surface, all can cause damage to the film.  The indentation can penetrate more than 25 layers down into the roll. The harder the roll the greater the depth the damage can penetrate.  This damage may only be of consequence to some of the more demanding products such as transparent conducting oxides or similar ceramic coatings as used in optical or electronic applications.  Decorative metal coatings may be completely unaffected by this type of testing.      

1.         Good J.K.G & Roisum D.R. ‘Winding: Machines, mechanics & measurements’             Pub. DEStech Publications Inc.   2007

            ISBN: 978-1-932078-69-5

2.         www.testingmachines.com            Schmidt Hammer

3.         www.millassist.com/rhometer.htm     Rhometer

4.         www.tapiotechnologies.fi/paper_roll_hardness_more.html  TAPIO RPQ

5.         www.proceq.com                     PAROtester

Questions from students - update.

            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 easily look up elsewhere. 

            I edit this blog as an unpaid activity and as everybody will see from the list of questions below it could easily become a full time job. I do run a Consultancy business and if your companies do have so many problems then perhaps they would care to employ me as a Consultant and thus enable me to spend the time to answer these questions in full.

Thus, for the benefit of the readership in general, please forgive me if some of these questions do not get answered in full by me on the blog 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.

To all the questions that have been asked I have replied to the questioner by e-mail with an answer that is aimed at giving a suggestion about how to go about answering the question.  For those questioners who have not received an e-mail from me I would suggest you check out your ‘spam’ filters as when replying some of the e-mails have been returned as undeliverable having been ‘rejected for policy reasons’.

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 benificial 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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I am working on a project were by i have to design a laminate for hygroscopic powder. The MVTR of laminate should be 0.3g/sq.m/24hrs@38c & 90R.H. Can you Kindly suggest some laminate structure with their thickness. As such i am looking for 2 or 3 layer structure. what thickness of MET-PET/LDPE structure should i use to have MVTR of 0.3.

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1.what are different surface treatment carried out?
2.What is effect of surface treatment on properties of polymer & which are the parameter that are effected by surface treatment?
3.In one of your answer i read that surface treatment is temporary process again the film get contaminated what do you mean by that?
4.what are different material that can be used for metallisation other than alluminium?
5.what is moisture barrier property of metallised BON?
6.what should be the range of surface energy in any film?
7.what is reaction taking place during metallisation?
8.can metallistion be done on printed film?

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I want to pack hygroscopic product whose IMC is 8.3 & CMC is 10.3. then what should be the MVTR of laminate. total area of pouch is (21.6*13.0)cm. can you pls tell me how the MVTR of laminate could be decided.

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pls tell me how can we get metallic glossier effect on printing. can it be done on PET/MET-PET/POLYETHYLENE. or we have to metallise the PRINTED PET. is their any other option

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