Vacuum Web Coating

Troubleshooting adhesion – i.e. lack of adhesion.

So many times the question is asked ‘how can I improve the adhesion of my coating?’ or ‘I have uneven adhesion how can I improve this?’

Like most problems the first step is to check that the real problem has been identified. There may be a real lack of adhesion of between the coating & substrate but often the substrate has been contaminated & trying to apply solutions to improve the adhesion without first eliminating the source of the contamination may prove fruitless.

Another common problem is that the only test being used is the ‘tape test’.  I personally find this an appalling ‘test’; I hesitate to even call it a test.  There are so many variables to the test that make it questionable.   It is far better to have a ‘fit for purpose’ test where the coating is tested in the application it is to be used in.

However I know that many companies & operators will continue to use the ‘tape test’ as it is a quick, simple & low cost test and it is regarded as better than nothing.

Below I will give some suggestions of how to check if there has been some surface contamination and also what else might be done to improve the adhesion if something more is required, other than eliminating any local contamination.

As I am sceptical about the ‘tape test’ I would always advise checking out this first. This means checking if there have been changes between rolls of film with a coating with good adhesion & ones with poor adhesion.  It is possible that the roll of tape has been changed & the change in adhesion is reflecting this rather than any real change in the coating adhesion.  The properties of the adhesive can be affected by humidity & temperature including what has happened in the roll lifetime. Thus a roll of tape that was manufactured when the humidity was high will be different from ones manufactured when the humidity was low. The adhesives can age & so an old tape is likely to have a different level of stickiness than a newly manufactured tape.  Also the test can be operator sensitive along with method sensitive and so once again it is worth checking if the good & bad rolls have been checked by the same operator & it is not a difference in operators & methodology that is being measured.

Assuming that you are still satisfied that there is an adhesion problem then it is now worth checking out the substrate.

Start by establishing properties of the substrate by evaluating the surface energy of the not only the surface that the coating is to be deposited on but also the reverse surface.  The reason for checking the reverse surface is to see if the surface energy is similar or if a slip enhancing coating has been added to the reverse surface or an adhesion promoting coating added to the front surface. 

If a difference in adhesion has been observed then where there has been poor adhesion it is worth checking the surface energy in this area too.

The simple rule of thumb is that the lower the surface energy the worse the adhesion and so it is more difficult to stick metal to a polymer such as PTFE than to OPP or PET.

Hence what you are looking for are differences in surface energy to explain the poor adhesion. 

If the polymer surface has been wound on a slitting & coating machine that has also been used to coat some other polymers with a silicone release layer it would not be surprising to find that the surface energy was lower than expected. Silicones are notorious for migrating and cross contaminating other rolls and hence many companies isolate the systems that use silicones from all other systems to minimise this possibility.

If the poor adhesion is sporadic and in small spots it may be that the contamination is from something like an oil spot, possibly from an over-lubricated bearing. If this is the case the spacing of the spots should be in a regular pattern and the spacing should give an indication of the roll diameter and so might help identify the contaminated roll.

If the front surface is a significantly higher energy than the reverse surface it is worth checking if the reverse surface has a slip additive on it or if the front surface has an adhesion promoting coating added.  If there is a slip coating added to the reverse side check on the areas of poor adhesion the surface energy to see if it matches the lower energy reverse side.  It could be that the slip coating is contaminating the surface simply as it contacts the front surface when it is rolled up. It could be a supplier problem that the coating has not been fully cured or it was applied too thickly or it is unstable & not the best substrate for this use.   The other option if an adhesion promoting coating has been used is that there have been coating misses.

If after having been through all of these checks you are still convinced that you need something to increase the coating adhesion there are a number of options.  There are atmospheric processes available to increase the surface energy such as flame, corona & much more recently atmospheric plasma treatment.  All of these are aimed at breaking some polymer chain bonds at the surface making the surface more chemically active & increasing the direct bonding of the coating to the surface.  The other thing that the treatments can be aimed at is to stabilise the surface.  Often the surface will contain low molecular weight material that is not well bonded into the bulk polymer & so when the coating bonds to this it will only have the bond strength of the low molecular weight material either to itself or to the bulk polymer. In either case this will be much lower than the bond strength of the coating to the bulk polymer.  Unfortunately some of these treatments can activate the surface but not seal the surface to further low molecular weight material migrating back to the surface over time.

In the vacuum system there is the possibility of using a plasma treatment just before the deposition process.  This has the advantage of being immediately before the deposition & so the surface cannot age or become recontaminated & so it has the best chance of giving the optimum adhesion.

This is not to say it is without problems.  It is possible to under or over treat the surface. Under treating the surface will leave some low molecular weight material on the surface to act as a weak boundary layer. Over treating the surface can work in two ways one is to roughen the surface so that some of the specular reflection may be lost & the other is it is possible to break so many bonds that the surface has a layer of carbon left on the surface that too can have little bond strength.

What the aim is to do is to either remove the low molecular weight material or to crosslink it into the surface so that it has a better bond strength.

Argon is often the gas used for the plasma treatment however this can only be used to etch  & crosslink the surface it has no reactive gas component that would enable volatile compounds to be formed that could then be pumped away.  Hence any material sputtered off the surface may well fall back onto the surface & may add to the weak boundary layer.  A mixture of argon & oxygen is often much more successful because the oxygen can form the volatile compounds that get pumped away and also the oxygen may also bond to the surface & allow the metal to form an oxide on the surface giving the coating a good chemical bond.

If there is the suspicion of surface contamination this will be on both sides of the web surface. Thus it may be worth considering plasma treating both sides of the web so that the freshly metallised surface does not become recontaminated on rewinding & hence any downstream process does not also have an adhesion problem because of variable surface energy.

Raised edge problem.

Here is the question.

I would like to know if plasma treatment or arcs of plasma causes any raised edge on the metallised BOPP film. I am generally face with raised edge only one side (even applying with lowest rewind tension).

Anybody had this problem before?  Any comment about that?

Answer from CAB

There is nothing in the plasma treatment that would be causing a raised edge.

The cause of a raised edge is most often because the slitting blade used to cut the wide polymer web down to the narrower width that you purchase.  Polymer webs such as OPP or PET have to be made as wide and fast as possible to obtain the economies of scale. There are systems that can produce web 10 –12m wide.  Most metallizers and converters only use film in the 2 – 3m range and so the original ‘mill’ rolls have to be slit down to these narrower widths.

            The slitting is done by mechanical means. There are a variety of techniques all of which use sharpened metal blades. Over time these blades wear and as they become blunt the quality of the cut edge goes down.  The blade will also heat up and so a variety of edge defects can be produced.  These can be an increased amount of debris produced, which as the web will have an electrostatic surface charge will be attracted to the surface and be seen as an increase in the number of pinholes in metallized film. In other cases the cut may no longer be clean but can be leave odd strands or hairs of polymer on the edge. Similarly the defect you are seeing is the raised edge. This is a combination of the blade heating, the need for increased pressure as the blade is blunter and the result is instead of cutting cleanly the edge is a mixture of cutting and melting and it is the melting part that raises the edge.

            The advantage the manufacturers, converters or slitters have over anyone using a vacuum process is that as they wind the film back into a roll the polymer traps some air between successive layers of polymer and this air layer keeps the film apart enough to hide some manufacturing defects. This air moves with the surface and is related to the winding speed so that winding faster the air layer will be thicker. It is common to see a lay-on roll on the rewind roll and part of the benefit of this is to reduce the amount of air trapped between the layers.

            What becomes very noticeable is that, even with minimum tension, polymer rolls that have been wound in vacuum are always harder that those that have not been rewound in vacuum.  This is simply because there is nothing separating the layers and so each layer is in more intimate contact with the adjacent layers.

            Because of this more intimate contact any defect will become more noticeable. Gauge bands, caused by a slightly thicker profile of the web at that point will become more visible and be the hardest part of the roll. Any large trapped dirt trapped between layers will become much more noticeable and often can be seen to affect the roll through many tens of layers. Thus if the edge is not perfectly cut and there is a very slightly raised edge this will also become much more noticeable. At the edge the film is very slightly thicker than all the rest and so this problem becomes cumulative. I have seen very bad cases this raised edge eventually became so high that the edge folded over causing the edge to grow at twice the rate of the rest of the roll.

            If you re-wind the roll in air this problem should disappear. The defect is still present but the roll will become more presentable.

            This rewinding does nothing to remove the problem but only hide it.  The incoming material needs to be improved by finding a supplier who will increase the frequency of sharpening their slitting blades.

            One last thought. It is possible to raise the edge of polymer webs if as the web is moving the edge rubs against something. This too can heat the edge and along with the pressure can push up the polymer and thicken the edge.  This probably the next most common cause of a raised edge but is still regarded as uncommon compared to raised edges from slitting.

I hope this suitably answers your question.

Problems of delamination

The question

We had faced a problem of metal peel off during lamination with brown paper; following are the exact details of problem & adhesive used for lamination:

while laminating with paper using water based adhesive. Metal is peeling from metallized PET film(metallized on corona treated(56+) film with optical density 2.2 to 2.3). customer is using Metallised BOPP is past and want to shift to PET film.

Details of adhesive used are as follows:

Acrylic Ter-Polymer Emulsion for BOPP Film Lamination.

Physical Appearance : - A creamy white Ter-polymer emulsion.

Chemical Nature : - Lamination Adhesive is an excellent & proven Ter-polymer
emulsion for BOPP film lamination.

Non Volatiles : - 50 +\- 1%.

PH Value : - 4-6.

Acid Value : - 1 – 2.

Colloide : - Acrylic.

Particle Size : - 0.5 to 1.5 microns.

Viscosity by : - 30 – 60 CPS

Kindly suggest.

The answer.

There are two factors that probably contribute to this problem, one is the different thermal expansion coefficient between the paper and metallized polymer and the other is the adhesion between the metal and polymer.

Delamination is often due to the stress at the interface being greater than the adhesion between two materials.  An indication that there is high stress might be that the laminated film has some curl and that where there has been delamination there is no curl.  A source of curl could be the shrinkage of the adhesive as the moisture level in the adhesive is reduced.

There is a difference in the tensile performance of the PET and BOPP and so the ability to change dimensions will be different. It could be that this difference is putting an additional load onto the PET that is not there with the BOPP.  You would need to compare the elastic performance of each to evaluate this.

You mention that the film is corona treated to a surface energy of >56.  This in itself does not guarantee adhesion.  There are several possible problems that can occur.  Corona treatment is often dependent on the humidity present at the time of treatment. Unless the corona treater has some active control a change in humidity can cause a difference in treatment level.  The age of the roll can also be a factor.  Corona treatment can degrade with time and again this is related to temperature and humidity. Also if the back surface of the web was not also treated then any surface contamination from the reverse surface can be transferred from the back surface to the front surface as the two surfaces meet, as the roll is re-wound after corona treating. The longer the roll is left after corona treating the more material can be transferred.

The other aspect is that surface energy is not a measure of adhesion. It can be an indicator of a changing surface that may lead to an improvement in adhesion but it may also lead you to believe that the adhesion will be good when it will not be.  Initially the surface energy of the substrate may well be low and following corona treatment may well be higher but what you do not necessarily know is if the treatment has been optimised.  Generally with increasing power or time of treatment the surface energy will increase but this will reach a plateau.  Although the surface energy remains high the adhesion will be decreasing as the increased bombardment degrades the surface increasing the amount of short chain molecules creating a new weak boundary layer.

BOPP films commonly have slip agents included in the formulation and these migrate to the surface. The corona treatment is designed to reduce or eliminate these from the surface. However they still remain within the bulk and with time a fresh amount will migrate to the surface and return it to the lower energy state.  Hence it is also important to know if the surface energy was measured just after corona treating the surface or just before it was metallized or if it was measured at both times and if there was any change in value.

PET has low molecular weight oligomer that migrates to the surface also acts as a weak boundary layer. Again the corona treatment would be expected to reduce or eliminate the effects of the oligomer.  The oligomer is also within the polymer bulk and will also migrate out to re-contaminate the surface given time and/or temperature.   

Thus there are a few things to check out to determine the root cause of the delamination.

CAB

Non-web metallization question.

I would like to know what process is used to coat trophies etc., in my case coffin handles made from ABS plastic?

I am busy helping a non technical friend source a machine on a shoe-string and would like to know where to start.

I would like to know if it is possible to build our own machine? Is it merely a vacuum spray booth or much, much more complicated?

If it isn't viable to build we will search for surplus or auctions. I need to know what to look for.

Any info would be welcome.

Answer from CAB

This is along established Industry. Many Trophies, plaques, handles, etc. are required to be metallized to give the brightest possible finish.  In general the parts are loaded into a large vacuum vessel onto rotating racks that ensure that all parts and all surfaces see an equal amount of the evaporating metal. 

The golden colour is generally achieved by using a coloured lacquer on top of the silver coloured aluminium metallization.

There are various different processes but the stock method is to use resistance heated intermetallic boats with a wire feed of aluminium wire into the boat.  The boats run at a temperature of around 1400- 1450 Deg C.

This process is not without some problems.  The polymer substrates can be made of a variety of polymers and even with the same polymers different manufacturers may include different additives.  Part of the problem is to get good adhesion between the polymer and the metal.  Also required is the correct surface finish. This may require dipping the parts into a lacquer before the metallisation to get a very smooth surface finish.  This may also require a plasma treatment system within the coater to prime the surface before the metal goes down.

Too much metallisation and the surface of the metal can become matt or milky in appearance due to grain growth.

You will need a vacuum system that can pump down to 10 E-6 Torr with some kind of planetary motion for the substrates.  Depending upon the volume of parts you wish to do will depend on the size.  These systems can be a couple of metres in diameter and a 2 -3 metres long and depending on the part size can coat anywhere from 10's to 10,000 parts at a time. Often the deposition cycle can be less than 1 hour and so if you wish you can produce millions of parts per year.

If you want to do a distance learning basic appreciation course on vacuum deposition and resistance heated metallisation then try www.VacuumCoatingTraining.com and sign up for a couple of the relevant modules.

If you want to buy a second-hand machine there are plenty available that either are ready to go or can be readily modified.  Unfortunately my knowledge of the South African second-hand market is non-existent but I know they are easily come by in the UK and USA and so would assume they will be in South Africa too.

This one was in a recent batch I saw;

Edwards Aluminiser (photos available)

48in Chamber with carousel and jigs

Edwards 2000cc diffusion pumps

Glow discharge

Edwards 1000 Blower

Leybold Rotary and blower combination

System full working Price £9750

I assume you will not want to be shipping it out but would prefer to find one locally. At least this will give you an idea of the words you need to search for.

I hope this gives you a starting point.

Odd question needing an answer.

The unusual question.

This question may be a little off base but I will ask anyway and hope someone has an answer.

I would like to find a company that can wrap a printed adhesive paper or plastic around the sides of a surfboard shape picture frame.

What technique, machines and printed material would be able to evenly distribute a material around a substrate that is not straight without having to score the paper?

I'm thinking that a vacuum form technique would do the job. Am I correct in thinking this or is there other methods that will work?

Thank You

1^st stab at an answer.

Allan much of the material that is printed and destined for lamination is on oriented polymer web. This web is most usually biaxially oriented and to some extent heat stabilised.  The end result of all this is that the material is hopeless for lamination to anything other than completely flat surfaces.  Even trying to laminate to the gentle curve of a TV screen will result in creases at the corners.

There is an exception to all this and that is the shrink sleeves used at the tops of bottles.  This material has deliberately been processed differently to enable the polymer to be made into a tube and then the tube shrunk onto the top of the bottle so that it will conform to the bottle shape without any wrinkles.  They have even developed computer programmes to enable the film to be printed with a distorted image whilst the film is flat such that the distortion is removed when the film has been shrunk. 

Thus I would contact the film manufacturers and ask about their shrinkable films as used in the shrink sleeve industry.  Depending on the shape, size of distortion, temperature, etc will depend which material they recommend. 

The alternative approach if you are trying to custom image complex shapes could be to use in-mould labelling techniques or using something like the D2T2 (Dye Diffusion Thermal Transfer) printing technology which is used on the likes of mobile phones ceramic mugs, etc.

I know this is not a complete answer but it might prompt others to join in and offer some greater expertise.