Vacuum Web Coating

Adhesion problem

We use to produce metallized BOPP film usually with no problem, however recently we have suffering some metal adhesion problems. Each 5 meters BOPP master roll is slit at 2.5 meters and metallized with plasma treatment, however we have a kind of metal peeling in one of the edge. Do you have an explanation for this phenomenon? Thanks in advance.

Answer.

The immediate thing that springs to mind is to check the rollers for contamination on the side where the delamination occurs.

This type of poor adhesion can sometimes occur when the bearings have just been lubricated and there has been excess or oil/grease and some has migrated onto the rolls and this is transferred to the web causing poor adhesion.  This could be anywhere on any of the upstream machines. If the loss of adhesion is periodic the distance between patches may give an indication of the roll diameter where the problem starts from. If it is continuous it could be because the contamination has been transferred to several other rolls too.  This problem may reduce with time, if the lubrication was just a one off. If the lubrication is automatic it may be that it has been set too high and this could get progressively worse. 

If you determine that it is not a contamination problem then you would be looking for non-uniformity in any plasma treatment, whether this is flame, corona or vacuum plasma. Look for intensity differences, blocked flame burners, pressure differences across the width, etc.

Adding nitrogen as third gas in plasma treater.

Sir, here in our Metallizer we have put a third gas line (N2) for plasma, now we are going to take trails with combination of three gases (Oxy, Argon and Nitrogen). But I don’t have any experience of three gas combination, so can you tell me how I have to start, should I reduce the flow of oxygen and put nitrogen by keeping Ar same OR should I reduce the flow of Argon and keeping oxygen the same and add Nitrogen, please mail me how this Nitrogen work in this plasma system, should I increase current value also?

Answer.

Re. Nitrogen plasma content/treatment, the following papers might have some information that you may find helpful.

1.    Shi M.K. et al

Plasma treatment of PET and acrylic coating surfaces - 1.  In-situ XPS measurements

Journal of Adhesion Science & technology  Vol.14, No 12, 2000  pp 1485-1498

2.    Poncin-Epaillare F. et al

Reactivity of surface groups formed onto a plasma treated polypropylene film

Macromoecular Chemistry & Physics  Vol 200, No.5 May 1999  pp 989-996

3.    Vallon S. et al

Adhesion improvement of silica layers on polypropylene and polycarbonate induced by plasma treatments

Euradh '96. Adhesion '96.  Vol 1 Conference proceedings.

Cambridge, 3-6th Sept 1996 pp 325-330  9(12)4

4.    Nowak S. et al

Surface characterisation & adhesion of plasma treated PP

Polymeric materials science and engineering. Vol 62  Conference proceedings. Boston Mass.  Spring 1990 pp 437-441  012

5.    Andre V. et al

In-situ metallization of PP films pretreated in a nitrogen or ammonia low-pressure plasma

Thin Solid Films  vol 181,  No 1/2 , 10th Dec 1989  pp 451 - 460

There are always a number of options that you can take.

One would to be keep the total pressure constant and also the argon flow constant and then use the reactive gas flow to maintain the total pressure constant. In using two reactive gases these can be added as a fixed ratio.

Alternatively the gasses can be added as fixed flows and the total pressure can be allowed to vary.

I personally would tend to use the first method.  Although the flow of the reactive gas can vary I believe that, as the supply of reactive gas is always expected to be in excess of what is required for the desired effect, small variations of gas flow are not significant.

Room temperature and humidity – Air conditioning

What is the effect of Metallised Room temperature on metallised film, because here we don’t have air conditioner for our Metalliser Room, some times temperature in room reaches 39 to 40 degree, will it effect the quality of Metallised Film? What other advantages we can get if we put Air conditioner there?

Answer.

The temperature is of secondary importance to the control of humidity.  As you know humidity can seriously affect the speed of the system pumpdown, particularly if the system has accumulated stray deposition to massively increase the surface area and hence the water absorbed from the atmosphere during venting and whilst at atmospheric pressure.

The important things relating to temperature are more relating to the rolls of film. If the room temperature is controlled and is cooler that the outside temperature then it is important that the rolls of film are brought into the room and allowed time to stabilize to the same temperature. Changes of temperature will make rolls change dimensions and this can lead to the rolls telescoping when moving if they have not had sufficient time for all the roll to reach the same temperature.

Other temperature related effects ought to be minor. If the air conditioning is keeping the room at a lower temperature than normal then the whole system will be at a lower starting temperature and so will run slightly cooler. This has two effects, one is that any water absorbed on the surface will be slightly slower to be desorbed and the other is that the system will radiate less heat to the film. So one effect is bad and the other good but in the general scale of things you may find it hard to even see these effects.

The greater benefit would be an improved consistency to the humidity content of the room irrespective of the season.

Another benefit may be that the operators are more comfortable.

How best to check crystal size or nucleation site density.

In an earlier answer you mentioned that changing the surface energy would change the wetting of the aluminium and change the nucleation site density and height of the nuclei. How would you characterize (analytical technique) the surface of the metalized polyester nucleation (island vs flat)?

Answer.

The simplest method of looking at the coating is to use a scanning electron microscope (SEM). This would allow you to look at the coating using high magnification.  This normally looks at the surface and the coating surface can be compared to the substrate surface.  If the coating is still non-conducting there may be a problem with the surface charging and so it may need a thin carbon coating to allow high quality images to be obtained.

Alternatively, for continuous coatings, if the coating is cut or the substrate folded the coating will have fractured and may then be possible to then look at the coating in cross section. This can let you look at any grain structure that is forming, any possible voids and grain sizes. 

It can often be difficult to interpret structures and this can be helped if you are comparing coatings, as then it can be easier to see the differences.

There can be problems in finding a suitable edge to look at the cross section, cutting can sometimes smear the metal at the point of cutting and folding may not produce a suitable fracture or distort the metal.  If this is the case then using a microtome to produce thin slivers of substrate with coating can be done.  This requires potting the sample in a polymer (usually some type of acrylic) and then using a glass or diamond knife to pare off thin slivers of material. If the polymer continues to distort then freezing the sample in liquid nitrogen and microtoming off thin slivers can be done, but the cold and fine nature of the work makes this a time consuming process.  The best of these samples can be looked at using transmission electron microscopy (TEM) and it may also be possible to obtain some structural information by diffraction techniques.

This too can be difficult to interpret as the work in cutting the slivers can produce fractures within the coating and it can be hard to determine if the cracking was native to the coating or produce in producing the sample.

It is possible to infer some information about the structure by comparing the coating thickness, conductivity, Optical Density (OD) or transmission and surface roughness.

Using a Talystep or other similar surface roughness or step height measuring technique the coating thickness and surface roughness of different samples can be compared.  For samples of the same thickness if the other measurements (resistivity or transmittance) are different it suggests structural differences.  For samples of the same OD or resistivity but different thickness again structural differences can be inferred.

If you want to identify why there are structural differences then it is worth going back to the start of the process and checking the surface energy of the substrate. If you then want to see what effect a change of surface energy has on the coating nucleation then you need to restrict the amount of deposition so that the coating thickness is around 3nm - 4nm so that you can then examine the surface using the SEM and count the number of nucleation sites per unit area and compare the number and size of nuclei.  If you are trying to do this on a standard metallizer then it may be worth stopping the wire feed whilst the shutter is still open and allowing the deposition rate to reduce. This will provide you with samples from the full thickness down to zero but it will have cost you many tens of meters of film but at least it will be under reasonably similar deposition conditions.  This would need to be done with or without surface treatment to give an idea of how you are changing the nucleation site density and hence the subsequent growth.

Hopefully this gives you a flavour for what can be done.

Adhesion problem with metallized CPP

Good day to you!  I just have few inquiries regarding metallized films characteristics.  Hope you would find time answering my inquiries.  These are the details:

    1.  I have a customer who's having difficulties in producing good bonds with this structure:  12u PET' inks / Dry Lamination Adhesive / 25u VMCPP.  It seems that the failure is between metal and CPP layer, so they are experiencing 100% metal transfer and thus resulting to poor adhesion.  Is there a way they can QC check their incoming VMCPP to avoid the problem?  Can their VMCPP supplier increase the metal adhesion (how)?  They also have a job with this structure:  20u OPP' inks / Dry lamination adhesive / 25u VMCPP, they don't experience problems in this structure, what do you think is the main reason? 

    2.  Is there a general rule in metallizing in Polyethylene film? 

Answer.

there are a number of possibilities. PP films are generally difficult to handle and so often have fillers and slip agents added to reduce the coefficient of friction and make the film more easily handled.  The slip agents are low molecular weight materials that are added to the bulk polymer and are expected to migrate to the film surface and reduce the coefficient of friction.  These low molecular weight additives not only reduce the friction but also can reduce the adhesion of any coating that is added. It is common to try to clean the surface to raise the level of adhesion. This can be done by flame, corona, atmospheric plasma or vacuum plasma processes. However these treatments may be both variable and also short lived for a variety of reasons.  Some of these treatments are 'automatic' processes which sometimes means either the treatment is on or off. The treatment may be a standard treatment irrespective of the process conditions.   Moisture can affect some of the processes. Films will have varying amounts of moisture in them depending on when they were wound and the humidity level present during re-winding.  Thus if the treatment is not optimised the adhesion may also not be optimised.  If the power is turned up it is possible to over treat the film surface equally as easily as under treat the surface. Over treating the surface will carbonise the surface and can also produce a weak boundary layer which will also be seen as reduced adhesion.

Once the surface has been treated the surface energy will have been increased. The film will then be re-wound and the back surface and front surface will be in intimate contact. This means the back surface with a mobile low molecular weight material present will be in contact with the front surface that has been freshly treated and with a significantly higher surface energy. Nature always wants to bring surfaces to equilibrium and so there will be a tendency for the low molecular weight material to migrate to the front surface and bring the surface energy back down. On top of this the slip agent (or other additives) will also want to migrate to the surface. Thus the higher the temperature or the longer the time the roll sits on the shelf the more likely it is that any pre-treatment will have returned to the pre-treatment surface energy levels and consequently the lower adhesion levels.

A further variation will be the differential stress of any laminate.  The coefficient of thermal expansion is likely to be different for the different polymers.  When laminated if the lamination is done at an elevated temperature then as the laminate cools the polymers will shrink at different rates and so the film may curl. If held flat there will be a stress on the interface between the layers and if the adhesion is poor the material will delaminate at this weak interface.

This will also change with film thickness. What can work with one layer at 12 microns thickness may fail at 25 microns thickness simply because of the differences in stresses seen at the interface.

One check they make on the CPP surface is to check the surface energy of the film before metallization. Look to see if the surface energy is varying which would indicate the films have been pre-treated but either the treatment is not well controlled or the shelf life is critical and the time between treatment and testing is varying. 

Any treatment needs to be optimised for the materials and the system being used. Generally a vacuum plasma treatment using an Argon/Oxygen mixture delivers good results but this also needs to be optimised to give maximum adhesion.

Also consider that different material suppliers will often use different additives and so equivalent films may be chemically different and so require separate optimisation.

Polyethylene usually has a lower tensile performance and so the tension used within the metallizer has to be lower and this means the deposition rate needs to be lower as too the winding speed slowed down.  The lower tensile performance means the film cannot be held as hard against the deposition drum and so less heat is removed and so the film will run hotter for the same deposition rate and hence the rate has to be reduced and the web speed reduced.

I hope this answers your questions.

Process cooling options.

What type of the cooling is most beneficial for the BOPP metallization?

Options appear to be pre cooling of the film, post cooling of the film & the main deposition drum cooling.

Answer.

As with most films the cooling relates to the coating properties. If you were having problems with wrinkling then additional cooling would help.  If you have the 'gas wedge' type cooling available it is worth using as not only does it reduce the coefficient of friction but it also improves the heat transfer coefficient. Otherwise you usually have enough wrap on the deposition drum between the lay-on roll and deposition zone for the web to reach the same temperature as the cooled drum and so adding a pre-deposition cooled roll may have little effect. If the rolls of film are coming out of the metallizer hot (above ambient temperature) then it may be preferable to use a post-deposition cooling roll.  If you do not have the 'gas wedge' type technology then if you are suffering from wrinkles then reducing the deposition drum temperature may help, as too might increasing the tension of the web around the deposition drum. However if you are already at a high tension around the deposition drum it may not be possible to increase the tension any further without putting a permanent extension into the web.

Question re. large roll size

1200mm UW Diameter metallising machines has been manufacturing for the years.  Do you think if that much big diameter is useful for vacuum coating?  There will be too many layers and there wasn’t enough air between them so do you think it'll cause sagging or other problems after metallising?  Do you know anybody can use this full diameter for it's whole production period?

Answer.

There have been several machines built with larger diameters some were for metallizing paper that had a thicker substrate and the length was similar to those of polymer films but the roll diameter needed to be larger.

The polymer film lengths have been increasing partly through increasing the roll diameter but also through down gauging giving longer rolls but with only a small increase in roll diameter.  What some manufacturers have found is that the longer deposition time has only a limited benefit in productivity. In some cases the longer time at the metallizing temperature caused a reduced boat lifetime, which is counter intuitive.  There is intrinsically not a problem with increasing the roll diameter, there are options of increasing the core diameter and changing materials to counter any tendency to distort.  If the edges of a roll are becoming proud it may not be because of the roll sagging so much as the edge slitting being done by slightly blunt blades that is raising the edges.

Bear in mind that the paper industry has much larger and heavier rolls and often winding at higher speeds and so the mechanics of high speed wide and large rolls has already been solved.  Also as many of the film manufacturers become involved with metallization I fully expect the widths to increase to half mill roll size and the lengths to increase too. In this way they will continue to obtain the economies of scale.

I hope this answers your question.

What are the basics of PVD

I’d like to know the basics of PVD; do you have any literature that explains the theory?

I understand that aluminium can be vaporized and adhere to a substrate, but I can’t understand how some oxides adhere to them. I imagine that the metallic element is evaporated and reacts with the oxygen, correct?  How does it work?

Answer.

William Andrew Publishing has a series of books that cover many aspects of PVD.   Below is a link that takes you to the one I wrote that specifically deals with deposition using roll-to-roll coating systems. This does have information about the different types of vacuum sources and reactive deposition but is aimed at web processing. If you want to deposit hard coatings onto rigid substrates there are other books in the series that would have more details of this type of hard coating (TiC, etc)

http://www.williamandrew.com/title.php?id=3

You are essentially correct that you can evaporate one material and the combine it with another to form a compound in a reactive process. Exciting the gas by using plasma to excite the gas so making it more reactive can speed up this process.

However it is also possible to deposit compounds directly by magnetron sputtering where the target material is of the final desired stoichiometry and once the target has been bedded in it will deposit the same compound directly onto the substrate. This is a slower process than the likes of evaporation but is generally stoichiometrically more precise and so the likes of transparent conducting coatings can be deposited where there is a controlled oxygen substoichiometry of a few parts per million of oxygen that creates the conductivity.

The way the materials are heated to be evaporated can vary from using resistance, induction, radiant, arc or electron beam heated evaporation sources. Additional plasma can be used to help react or densify the coating, known as ion plating, and these plasmas can be generated in a variety of ways such as ion beam, triode, microwave, cold cathode, arc, etc.  All of which makes the choice and design of systems an interesting proposition and why there can be very different system designs from different manufacturers for producing the same coatings.

Finally there is also the Chemical Vapour Deposition route to depositing coatings where the starting materials are either liquids or gases and these are mixed and decomposed using either heat, light, plasma or a combination of these to deposit the desired coating and pump away the exhaust gases.  This can be competitive for some processes such as some oxide barrier coatings or some of the semiconductor photovoltaic coatings but for many others the management of the chemicals can be more expensive than the higher cost of the other process types. 

Below are the books that I generally list as being useful.

Added to this would be the book by R.F Bunshah of PVD for which I do not have the ISBN number to hand (sorry about that).

•Books

1.  C.A.Bishop             ISBN 0-8155-1535-9

     Vacuum deposition onto webs, films and foils

2.  Handbook of Thin Film Processes  Vol 1 & 2

     Eds David A. Glocker S. Ismat Shah       ISBN 0 7503 0311 5

3.  Semiconducting transparent thin films – H.L. Hartnagel, A.L.Dawar, A.K. Jain, C.Jagadish               ISBN 0-7503 0322 0

4.  Thin-film optical filters – H.A. MacLeod      ISBN 0-7503 0688 2

5.  Coated glass – applications & markets

            Russell J. Hill & Steven J. Nadel           ISBN 0-914289 01 2

6.   Modern vacuum practice – N.Harris    ISBN 0-07 707099 2

7.   Coatings on glass       H.K.Pulker               ISBN 0 444 42360 5

8.   Thin film processes  -  J.L.Vossen & W.Kern  ISBN 0-12-728250 5

9.   The physical properties of thin metal films

       Ed. G.P. Zhigal’skii & B.K.Jones  ISBN 0-415-28390-6

10.  Industrial plasma engineering –Vol 1 (Principles) & 2(Applications)

       J.Reece Roth    ISBN 0 7503 0318 2

11.  Chemical vapor deposition  -

        Ed Jong-Hee Park    ISBN 0 87170 731 4  

I hope that you find this information of use.

Differences between metallized PET & BOPP

1.         How do MET PET and MET BOPP differ from each other?

2.         Any specific difference in property?

3.         For Flexible packaging Applications - will MET PET or MET BOPP will be better?

Answer.

The basic differences in the mechanical properties will still be present following metallization. Thus PET generally has a higher tensile strength than BOPP and so where packages require this property PET would be preferred.

It used to be that BOPP was cheaper than PET and so a driving force to replacing PET with BOPP was cost and some compromise was made on the mechanical differences.  PET was optically slightly better with a lower haze.  Over the years BOPP has been improved and so the gap between properties has reduced but with process improvements I think the price differential has also been reduced and so the driving force to reduce cost by moving from PET to BOPP is not as great as it has been in the past. 

In the same way that for some products it was possible to move from PET to BOPP then in some applications it has also been possible to move from BOPP to PE, again as a cost saving process.

In terms of metallizing these same mechanical property differences can affect the metallizing process. PET has a higher yield point and tensile performance and so it is possible to pull the web harder against the cooled deposition drum and so for the same thickness web it is possible to metallize the PET at either a higher speed fro the same thickness or, for the same speed, a greater thickness.

The higher specification PET often appears to have fewer process problems, a higher throughput with higher adhesion and better optical performance and so has many advantages but the costs are higher and so it has to be justified by requiring a superior application specification for which a premium price can be charged.

I hope this highlights the generic differences and where the different materials fit in the scheme of things.

Transfer metallization Question

We need to make a transfer of vacuum deposited aluminium onto 30 gr/sq/m/ paper in order to avoid putting the paper into the vacuum system. Could you please advise which material would be the best substrate to carry aluminium? I was thinking about siliconized PET (to deposit metal on siliconized side), but have some concern if metal would then migrate into plain side of PET? Or if we rewind strait after metallization there is no risk of flake off?

Answer.

PET is certainly used for this process. I am not sure about using silicone on the substrate. Yes it does provide a poor adhesion level but what you do not want is the metal coming off before it reaches the transfer process. Also silicone is notorious for migrating to places where you do not want it to be. I would be concerned that the silicone would transfer to the back surface of the web and so then be transferred to the metal surface and so giving adhesion problems in the transfer process.  Often the transfer process relies on the adhesive used to attach the metal to the new substrate (paper in this case) being stronger than the adhesion to the original substrate. Thus you do not necessarily need to have an ultra-low adhesion on the PET just so long as it is lower than the high strength adhesive on the paper. The adhesive you chose can be either a hot or cold adhesive depending on whether you want a hot or cold stamping process.

If you want to also transfer some of the polymer release coating then it is possible to use a UV curable polymer and partially curing the coating so that it is sure to fail within the polymer thus transferring some of the polymer with the metal.