Vacuum Web Coating

Good adhesion required for BOPP

Can you help me please to understand what are the main characteristics that a BOPP film must have in order to present a good UV ink adhesion, or good lamination forces with UV adhesives??

ANSWER

BOPP typically has a number of additives included and may also have a slip agent added to make the handling of the web easier by reducing the coefficient of friction. Unfortunately these additives and slip agents are often low surface energy chemicals and may be short chain molecules. Thus trying to bond anything to this surface will have only limited adhesion. 

A surface treatment that removes this low energy material is generally of great benefit in increasing adhesion.  However this needs to be done with some care. Too much surface treatment can damage the polymer by causing chain scission and this will also produce short chain molecules that are also associated with poor adhesion. Thus the surface treatment needs to be optimised.  There are a variety of surface treatments available, flame, corona, atmospheric plasma and vacuum plasma treatments to name the most common. 

There are some other things to be aware of such as humidity. Corona treaters can produce a different treatment from the same settings depending on the ambient humidity.  Once the treatment has been completed the time between treatment and use can affect the bond strength.  The slip agents, because they are added to the bulk, will reappear on the surface given a suitable time and/or temperature.   Also if the surface treatment is done on one side only and the film re-wound between surface treatment and coating there may be some of the slip agent from the back surface transferred to the front surface whilst the film was in the re-wound roll. Again time, temperature and also pressure will affect how rapidly this material is transferred.

Film suppliers often supply film with pre-treatments that are customised for particular inks adhesion.  Thus the pre-treatment will be different for aqueous inks compared to UV inks compared to solvent based inks. Many of these pre-treatments are proprietary processes and the supplier may not even tell you if it is a coating or a surface treatment such as corona.  This can cause a difficulty in that you effectively do not know the surface chemistry that you are trying to improve and so can only guess as to what treatment to apply. Generally a vacuum plasma treatment would use an oxygen and argon gas mixture because the argon provides some heavy atoms to bombard the surface with and the oxygen provides the atoms for any hydrocarbons to bond with, to form volatile molecules that can be pumped away. The exact percentages and the power needed can usually be juggled to give a good result.

Hence the comment that any surface treatment needs to be optimised. If you are obtaining film from different suppliers it will also need to be optimised for each supplier as each supplier may have a different pre-treatment, which could mean a different coating, as well as slight differences in additives.  All of this makes each surface different and potentially requiring a different level of surface treatment.

One area that you also need to consider is that if you remove all the slip agents from the front and back surface the coefficient will increase and so may cause an increased difficulty in winding well.

I hope this answer gives you something to work with.

Is there a relationship between OD and CoF?

ANSWER

Generally polyester films have some low molecular weight material on the surface.  This is most frequently oligomer that is residual material from the polymerisation process. There may also be other materials added that can migrate to the surface, such as slip agents in OPP, that may also reduce the coefficient of friction (CoF). 

   Often the surface of the polymer is treated to change the surface energy to improve the adhesion. This treatment to increase the surface energy can crosslink the low molecular weight material into the bulk polymer or can volatilise the material and remove it. Either way the low surface energy material is removed and the CoF will get worse (increase).   

    Even if there is no surface treatment the metallization itself will cover up the low molecular weight low surface energy surface and again the CoF will increase.

    Thus there is no fixed CoF that specifically relates to a particular Optical Density (OD) but rather the CoF relates to the history of the polymer web and any surface modification and surface treatment that has been carried out.  Pure PET films will readily block and so they are usually have fillers added to provide a controlled surface roughness. The surface roughness reduces the contact area and hence the CoF. The size, shape, type and distribution of these fillers controls the precise value of the CoF.  This is usually higher than is ideal for and easy to handle film. Coupled with the residuals this can make the ease of handling acceptable.

    Sometimes the back surface of the web is not treated and thus there may be the transfer of some of the low molecular weight low surface energy material from the back surface to the front surface once the metallized web has been re-wound.   This can cause a problem in reduced adhesion of any subsequent coatings to the metallized layer but the transferred material may also aid the ease of handling.

            Where a link may exist between OD and CoF is in the metallization nucleation and growth.  Any low molecular weight, low surface energy contaminant on the surface, that helps reduce the CoF, will stop the aluminium wetting the surface well. This will make the aluminium nucleate and grow as a series of hemispherical islands. This will make the coating quite thick before it completely covers the surface. Whereas it the surface has been treated to raise the surface energy and correspondingly the CoF rises the aluminium will wet the surface well and instead of the hemispherical-like growth the islands will be very flat and spread well over the surface this helps make the coating completely cover the film at a lower thickness. Thus the same OD may be achieved at a slightly lower thickness than for a non-wetting surface, lower CoF film.  However there will not be a direct correlation as the CoF can be varied by the filler or surface roughness as well as by any surface contaminant.

Thus for some materials there may appear to be a link between OD and CoF but this link may disappear depending on the film processing or with a change of film supplier and may never be apparent in other materials. 

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Q. re bond strength of laminates

I would like to know the standard procedure to check the bond strength of a laminate (ex: metpet & LDPE). it would better to give some reference article.

ANSWER

Many laminates are not tested for bond strength for the laminate but are instead tested to confirm they are ‘fit for purpose’. These tests can often be very specific to a particular product.  So that for a product that needs to be twist wrapped something like a ‘Gelbo’ test is performed whereas for a pouch the material may be made into a pouch and then tested.

A source of testing information is the American Society for Testing and Materials (ASTM) and they publish standard tests. These tests run into thousands published in over 20 large volumes and so the ASTM index needs to be searched carefully to identify the most suitable tests. 

The following are some of the ASTM tests that are used in the web coating industry but not necessarily a direct adhesion or lamination strength test.

e.g.

1.         the ‘sellotape’ or ‘Scotch tape’ test is

ASTM test D 3359-87             ‘Measuring adhesion by tape test’

2.         Polymer film surface energy can use test ASTM D 5946 – 96

‘Corona-treated polymer films using water contact angle measurements’

3.    Surface tension     ASTM D 2578 - 99a

'Standard test method for wetting tension of polyethylene and polypropylene films'

4.    Mullen Burst        ASTM D 774

Direct laminate testing can be done by peel tests. There are several of these such as 'T' peel, 180 Deg. peel, German wheel, etc.

Thus the most critical test may not be a standard peel test but an agreed 'fit for purpose' test agreed with your customer.

Question re. OD correlation with composition and thickness

Question

How to correlate the OPTICAL DENSITY measurements with thickness for an alloy coating of Aluminium and Zinc and then to have a relation between O.D and
resistivity ( surface )

ANSWER

There is no simple correlation method that I am aware of. Generally it requires a considerable amount of calibration work.

Most metallizers will have some type of monitoring system on-line for the purposes of deposition control. This will either be a measure of resistance or OD.  The primary control you will have is power to each source material.  The feedback control will be either from the OD or resistivity monitor and it will control only one of the sources, usually the aluminium.  Thus at one power setting of the zinc the aluminium can be varied to give a graph of power versus thickness which will also give a chart of changing composition.  This can be repeated for a number of different power levels to the zinc source.  Combining the information you should be able to extract information of coatings of the same thickness but different composition as well as varying thickness with the same composition. 

Once these graphs have been completed they can be used as a calibration for that machine.  If the same process is done on more than one machine it is worth producing a calibration graph for each machine as it is common for there to be slight differences between the performance of each machine.

Testing for the presence of primer

We need to check for the presence of primer on metalized film after the run. Can the same dye test we use on a PEI primer be used to test for an EAA primer on the finished metalized film? If not, what is the appropriate method in this application?

ANSWER

I am not familiar with the dye test you refer to.

Do you mean ‘dyne’ test that is a measure of the surface energy?

If so the answer is yes. It is worth checking the metallised film immediately and then again after some periods of time. These measurements after time ought to be done on material that is within a roll. Part of what you are looking to measure is if the metallised surface is becoming contaminated and part of the contamination could be from material transferred from the touching back surface whilst the film is in the roll. Thus testing a sheet of material immediately and leaving it as a sheet out in the atmosphere and testing it again a week later and again a month later would not be expected to have the same result as testing a surface that was still part of a roll, rewinding the roll, storing it for a week and then unwinding it and testing another part of the surface, rewinding it and repeating the same after a month.

The dyne pens will only tell of a change in the surface energy but it will not tell you anything about the surface chemistry.  If you want to find out if the surface has changed from a metal/metal oxide surface another method would be to use one of the surface analytical methods such as x-ray photoelectron spectroscopy (XPS) or secondary ion mass spectroscopy (SIMS) that can identify any additional material on the surface and give a chemical composition for the contaminant.

‘The Bernard Henry AIMCAL and SVC Scholarship Fund’

Press Release, 4 July, 2007

AIMCAL and the Society of Vacuum Coaters have agreed, using the resources of the Society of Vacuum Coaters Foundation, to establish the ‘The Bernard Henry AIMCAL and SVC Scholarship Fund’ in honor of the late Bernard Henry.  Members of AIMCAL and the SVC have shown their commitment to the education of students worldwide whose educational trajectory is likely to enhance science and engineering in general, and potentially vacuum and roll-coating technology in particular, and that it is appropriate to honor the memory of Bernard Henry by creating a scholarship fund in his name. 

These scholarships are open to students world wide to attend qualified institutions of higher learning.  By combining SVC and AIMCAL donations into a designated fund, it is believed that sufficient donations may be received to indeed create an endowed scholarship fund in Bernard Henry’s memory. If the total principal donated reaches $50,000, an endowment fund will be created, and this fund will fund an annual Bernard Henry AIMCAL-SVC Scholarship in the amount of 5% of the endowment fund assets, or a minimum of $2500.  If less than $50,000 but more than $5000 is collected by January 1, 2009, the principal of the fund will be used directly until exhaustion to fund an annual Bernard Henry AIMCAL-SVC Scholarship whose amount will be set by the SVC Foundation Board, but in no case will be less than $2500.  The first scholarship will be awarded in 2010.  (If the principal increases to more than $50,000 at any time, a permanent endowment will be created.) 

Donations to the fund should be addressed to the SVC Foundation, 71 Pinon Hill Place NE, Albuquerque, NM 87122-1914, and should specify the Bernard Henry AIMCAL-SVC Scholarship Fund.  Questions regarding this Fund or the SVC Foundation may be addressed to John Fenn (818-888-8649, johnfenn@earthlink.net) or James Seeser (314-918-0160, jseeser@aol.com).

The Society of Vacuum Coaters Foundation (SVCF) is a nonprofit corporation recognized by the Internal Revenue Service as tax-exempt under section 501(c)(3) of the Internal Revenue Code.

About Bernard Henry

After first degree in Materials Science and doctorate on the microscopy of semiconductor materials, both at Imperial College in London, UK,  Bernard moved to Oxford University in 1995.  He worked briefly on high performance cements and then on textured superconducting materials, but the major contributions for which he will be remembered as a scientist really started with his work on flexible gas barrier materials.   Bernard immersed himself in the question “what really determines the permeation barrier properties of polymer/inorganic composites?” Characteristically, he was unwilling to follow the path that others had taken, being much more interested in designing new research strategies - requiring measurements in laboratories on 3 continents.  The key ideas that came from this work - the importance of micron-scale defects, the role of chemical modifications in improving permeation properties, and practical solutions to old problems on how to fabricate new and better materials - have all contributed to a better understanding of transparent permeation barriers.  From 2000, Bernard started to work on polymer solar cells where his especial interest was in the transparent conducting materials like TiO2 and ZnO that are a key component in these complex composites, whilst maintaining a very active interest in the thin film barrier field, and in 2006 he became the project manager for the production-scale web coater facility in the Oxford Materials Department in which role he could combine his long-standing interest in the science of thin films with the development with his industrial colleagues of new commercial materials.

To those of us who collaborated with Bernard as a busy scientist, it is remarkable to learn that he also had huge commitment to sport; for himself at the highest possible level of performance (200 and 400 meters) as a long standing member of the Thames Valley Harriers, and also organising events for young people.  Bernard made friends all over the world both as a scientist and a sportsman, and we will remember him as a genuinely nice man with huge personal dignity who we were proud to know.

Dr Hazel Assender

University Lecturer

Department of Materials

Parks Road, Oxford

For questions about this release, contact James Seeser, Chair, SVC Foundation, 5 Old Westbury Lane, St. Louis, MO 63119, USA,  314-918-0160.

Differences between CPP & BOPP

I want to know the basic operating & quality difference between PET & CPP metallizing?  What are the factors affecting CPP metallization in addition to PET metallizing?  How one can improve the CPP metallization process?

ANSWER

I do not have any direct experience of metallizing CPP films but can possibly give some insight into the difference in the materials.

CPP stands for Cast PolyPropylene film.  This generally means that it has not been oriented and so the tensile performance is less than the oriented polypropylene and so it will withstand less tension being applied during the metallization process before the film distorts. Thus it is likely to require more care and a slower deposition rate and heat load than the equivalent oriented polypropylene. The PET will almost certainly be oriented and so this too will have enhanced tensile properties in the directions of the orientation making it much better for pulling tension around the deposition drum to help take out the temperature during deposition. Thus I would expect the PET to run through any metallizer faster and with fewer handling problems than any CPP film.

As the CPP is likely to be more heat sensitive I would try to make sure that any heat load is managed as well as possible. I would aim to be using the gas injection system between the web and deposition drum to make sure the heat transfer coefficient is as high as possible whatever the tension is set at.

Problem of ink adhesion.

Following is the structure:
INK/48 ga METPET(Metal Inside)/solventless adhesive/1.5 mil Poly
QUESTION: As a first step we laminate the METPET to Poly. As a second step we surface print. The ink adhesion is good but with slight twist of the film the ink is released from the PET showing miss-out. This happens mostly in shipping as well.
How to have 100% ink adhesion on METPET?
Please let us know. Thanks.

ANSWER

If I have understood the structure correctly the ink is printed directly onto the metal metallized surface.

One possibility relates to the PET. The back surface of the PET, the side not metallized may have some low molecular weight oligomer on the surface. This oligomer might be on the surface because it was never cleaned off and is still there from when the polymer was first made. Alternatively if it was cleaned off using a corona or plasma treatment there may be more oligomer that migrates to the surface especially during the metallization process when the temperature is raised and the oligomer mobility is increased.

If the back surface is contaminated with oligomer then as the metal surface is wound up the back surface contaminated with oligomer will be in contact with a freshly metallized metal surface that has a very high surface energy. As things in nature try to reach equilibrium the low surface energy material will transfer across to the high surface energy metal and thus will reduce the surface energy of the metal. If the PET has a filler included (possibly only in one surface for some co-extruded films) then where the fillers cause high spots on the surface will be prime contact areas where the transfer of such material is most likely to occur. As the oligomer is low surface energy where it contacts it can also be expected to spread out. Thus when the roll is unwound and printed the ink will have different (lower) adhesion wherever the oligomer is present.

It may be possible to check out these failures by sending a sample to a surface analytical laboratory and let them separate the ink from the surface and look at the surfaces that are produced in separating the ink from the metal. If both surfaces have the same surface composition then it will most likely also look like the composition of the PET oligomer.

Tape breaks.

We produce film for adhesive tapes with flame treatment.  One of our clients complained that after he produces hot melt tapes with our film he has big problem of breaks when the unwinding of the tapes is not constant.
Why is this happening?
What should we influence in order to overcome this problem?

ANSWER

I am no expert in hot melt coating or adhesives.

With respect to looking for possible problems of web breaks I would look at possible tear initiators such as the quality of the film slitting.  If the slitting blades are not sharp or are damaged then they can be putting damage into the tape edges. This damage can allow the film to tear if the tension is not applied evenly such as stopping and starting the winding in jerks. This can usually be examined by using an optical microscope or if particularly bad a magnifying glass.

Generally the flame treatment does not cause problems however things to check would be to make sure the gas flows are constant so that the gas mixture is always consistent and that the web is wound uniformly through the flame so that there is not any possibility of raising the film temperature too high.