Vacuum Web Coating

The differences are between primer and plasma metallization

Question

'What the differences are between primer and plasma metallization? Which type is recommend to HDPE film in order to receive the best adhesion?'

ANSWER

I have not come across the phrases 'primer metallization' or 'plasma metallization' before.

I suspect that it refers to the treatments the films have seen.  Some grades of polymer film can be purchased that have a primer on one surface that is designed to improve the metal adhesion.  The film manufacturers sell this film at a premium over grades without the primer. Films without the primer would be expected to have worse adhesion. However it is possible to improve this adhesion by plasma treating the film prior to metallization. I would expect that this might be the different that is being referred to by the phrases 'primer metallization' and 'plasma metallization'.

I have no recommendation to make on film.  The choice of film will depend on many things. There are many manufacturers of film they have different standards of cleanliness and quality.  Thus you need to choose a film that will meet your requirements mechanical and optical performance at the right quality and consistency that suits you and at a price you can afford.    

'There is a method to slow down the decreasing of the surface tension on HDPE film after the metallization?'

I know that additives, temperature and moisture can have effect to the surface tension but what about raw materials and metallization process?

If the film has any additives or and unpolymerised monomer residues these can often migrate to the film surface. If these materials are of a lower surface energy than the PE surface they will try to bring the surface to the lowest energy state by covering the surface. This will cause a problem with metal adhesion as so the film is often pre-treated to improve the adhesion, typically flame, corona or plasma treatment. 

A freshly metallized surface has a phenomenally high surface energy but this is not long lived and it decays. Even so it should remain a high surface energy for some time.  However this metallized surface, when the film is rewound, comes into contact with the back surface of the film.  This means this phenomenally high surface energy surface is in intimate contact with a surface that is covered with low surface energy, low molecular weight and very mobile material. Nature always tries to bring surfaces to equilibrium and so it is energetically favourable for this material to migrate to the high-energy metal surface.  This process is speeded up with increased temperature, also the amount of surface contact is important. Thus if the roll is rewound hot and very tightly in the metallizing machine the rate of decay is likely to be faster than if the roll is cooled fully to ambient temperature and is would with a low tension.  As there is no interleaving air in the vacuum system it is likely that all rolls will be wound hard and so this may only have a limited benefit.

An alternative approach is to clean the back surface of the film to remove the contaminant.  The problem with this approach is that if the surface energy of the backside of the film is increased the web handling becomes much more difficult. Raising the surface energy generally increases the coefficient of friction (CoF). If the layers of film cannot easily slip over each other this can lead to poor shaped rolls, or even blocked rolls, or if the rolls change between slip & stick the metallized surface can be damaged and show signs of micro scratching.

Another option is to corona treat the metallized surface immediately before any further coating or lamination is done. In this way the contamination can be allowed to occur and the handling remain acceptable but the contamination removed and the surface energy increased immediately prior to it being needed.

Finally in some cases where the surface contamination was critical an interleaving was included. This was thin clean material that was inserted between the front and back surfaces. This tends not to be possible with most metallizers and so is not a common technique and as it uses an additional material it is also more expensive.

What plastic films can be metallized?

Question

What characteristics must a plastic film have in order to be metallized?

Answer

Essentially more or less any polymer film can be vacuum coated. Different substrates have different problems and so sometimes the vacuum system has to be operated differently and the deposition speeds may be slower than usual.

I have samples of foam, non-woven and woven substrate, also materials as different as PE, PP, PET, PA, PI, PTFE.....

The other consideration is the film thickness. The foam films although the substrate is quite thick it is compressible and very stretchy and so cannot take the same tension or heat as a solid film that is much thinner.

The type of substrate will also determine some of the optical and mechanical properties. The different polymers have different smoothness, different optical clarity, as well as different surface energies and so metallizing has to be customized for each to be able to get the best adhesion for the process.

Another difference that can affect the metallization quality is the moisture content of the polymer. Some polymers absorb little or no water, others can absorb several percent of water and this has to be suitable managed if the water is to not affect the aluminium deposition quality. If the water reaches the deposition zone it can oxidise the aluminium and make the coating yellow, golden, milky, or otherwise different to the bright metallic coating usually desired.  The other aspect of water content can be that the polymers with a higher water content may also have a higher heat transfer coefficient and to may be less affected by heat load problems and may be wound at higher speeds.  Again the system operation and design can be different for different materials depending on the level of water that is expected to be present. Higher water levels will require more cryopumping to remove the water. 

Thus almost any polymeric web material can be vacuum coated but it may require some system modification.

Why use a pre-drum spreader?

What is the purpose of the pre-drum spreader roll?

Answer

The pre-drum spreader is to put some lateral tension into the web before the web sees the deposition zone where the rise in temperature will make the film want to expand. The initial expansion relaxes this tension and then as the expansion continues the web does not move on the drum and so a lateral compressive builds up.  If the lateral compressive force is too much the web will buckle off the drum and you see this as the formation of a tramline.

This pre-spreader roll usually has a soft polymer surface and it relies on the softness of the polymer to deflect which results in the lateral spreading of the web to take place.  Over time these rolls age and become harder and so lose some of their performance.  The roll can have a rubber, elastomer or polymer material as the covering, depending on the material will depend on the aging process. Some age more quickly with heat others can be simply be dried out slowly by the vacuum and others can be hardened by an interaction with solvents that may be used to clean the rolls as part of routine maintenance.

Another complimentary method of reducing the wrinkling is to inject a gas between the web and the deposition drum. Part of the problem is that the web cannot easily move on the deposition drum hence as the web comes in contact with the cooled deposition drum it wants to shrink as it cools to reach the same temperature as the drum.  The tension is applied to the web to hold it hard against the deposition drum to help maximise the heat removed from the film during the deposition process. The combination of the coefficient of friction and the tension applied to the web not only holds the web tight against the drum but also stops the web moving relative to the drum even though it is contracting and then expanding and finally contracting again with the changes of temperature.  Injecting gas behind the web lubricates the back surface enabling the web to move relative to the drum and so reducing the chance of the web buckling. The trapped gas also increases the conduction part of the heat transfer coefficient and so reduces the film temperature too. Hence there are two mechanisms that help stop buckling.

A further point on buckling. Some work done by Mike McCann showed that any dirt on the drum (even down to a micron or more) is sufficient to lift the web off the drum and cause a local overheating and adding to the buckling forces. Thus wrinkles will happen more frequently on dirty machines than on immaculately clean machines.

The positioning of the wrinkles can also be affected by the residual stress in the web as too can any profile variations.

Relationship between extractables and delamination.

I have added following my comment to your article named Oligomers problem? If you can answer my question, I will be appreciate.
We lived same kind of Oligomers problem on the polyethylene film. The corona treatment level of the polyethylene film that we produced was 44 and its COF
value was 0.24. After one day when we laminated it with PET, the lamination can be delaminated easily and the surface of the polyethylene was like hazy. We firstly doubt from excessive slip rate, but the film surface was not slippery. We checked our solventless adhesive component rates and we didn't find anything.
We laminated the PET with another different polyethylene film and the lamination process was successful this time.
We decided that our problem was the polyethylene film and we checked all of polymer blend. We sent them to migration and extraction test. At the result one of
them was responsible and we found it. Its xylene extraction rate was % 2,63 instead of % 0,23 and hexane extraction rate was % 3,85 instead of 0,53. Same time its migration limit 13,85 gr/dm2 and it was not used as a food contact polymer because of limit is 10 gr/dm2.
At the result the residual unpolymerised short chain monomers and oligomers in the polyethylene can be migrated to the surface of the polyethylene and can cause the delamination problem easly.

What I want to learn is that, what is the critic oligomer level in the polyethylene for
lamination. According to Food Grade Codex, xylene extractable rate is 13,2% and hexane extractable rate is 5,5%, but what is the boundary for lamination process?

Answer.

I have information about migration tests and acceptable levels for food contact but surprisingly little or nothing on the levels that are acceptable for lamination. 

I suspect that this is arrived at from experience and differs with different systems.

I do have a couple of comments that may be worth considering.

Corona treatment needs to be optimised for any given film. What works well for one grade may not be as good for another grade. Similarly the treatment can be affected by changes in humidity and so if the process has been optimised on a dry day it may no longer be optimised on a high humidity day. The measurement of surface energy needs to be used with care. Surface treatment will in general raise the surface energy. This increase will continue up to a plateau and remain at this high plateau for some time before eventually declining.  However the adhesion works differently. The adhesion will also rise similar to the surface energy but once it reaches a maximum it immediately declines, it does not stay at a high plateau. This is because the balance of the improvement versus damage is no longer favourable
and the treatment is damaging more of the surface than it can improve. The chemistry stays the same and so the surface energy will remain high but the surface damage will be creating short chain molecules, which will act as a weak boundary layer and cause the adhesion to decline.  Corona treatment will raise the surface energy on the treated side but as soon as it is rewound any low molecular weight, low surface energy material from the untreated back surface will want to migrate to the front surface. Nature drives surfaces to the lowest available equilibrium energy.  This process can be time, temperature and pressure dependent.  If the roll has been hard wound and is stored at high temperature for some time the front surface will have been significantly contaminated.

If there is a large amount of organic contaminant on the surface Corona treatment may not have been able to remove it all. If the surface has been surface treated well with the process optimised for the specific grade of film and atmospheric conditions and has been treated immediately before lamination this is likely to give the maximum adhesion available for that particular pair of materials. If the two materials have good adhesion then the migration of additional material into the interface will be low and the lamination will not be expected to degrade with time.  If the adhesion is not good then it can be expected to have further problems of a reduction in the lamination strength with time due to additional material migrating into the 'gap' between the two materials at the interface where the two materials are only adjacent but without any direct bonding.


So my answer is no, I do not know of any published information that defines the level of extractables below which lamination will be good.  But hopefully I have described where adhesion may be lost or improved in your process that may help you overcome the problem.  Also there is always the possibility that someone else reading the Blog will be able to help with an answer.

If someone is reading this and can help I would encourage you to post a reply to the blog.  I am sure that everyone would benefit from any additional information that might be out there.

Question regarding infra-red coatings

What is the infared coating & what is process for this, night vision glass is the part for this, the process is patented by any ?

Answer

Infra-red (IR) coatings covers every or any coating that has some performance in the IR.  Aluminium rises from being transparent in the ultraviolet to being reflective in the visible and this reflectivity continues into the IR.  For some solar control filters the plasma edge (where the reflectivity goes from low value to high value) is chosen to be in the near IR. Choosing where this plasma edge is allows different amounts of solar rejection.  Choosing the solar performance allows products to be produced for different areas of the world.

Some of the colour shifting products have a military performance in the IR and are used to change the IR reflectivity of military vehicles. 

Some other IR materials are used as hard protective surfaces that allow IR transparency but are opaque in the visible. 

Many optical coatings combinations of high and low refractive index materials with each layer deposited with a specific thickness in this way band pass and edge filters, dichroic mirrors and anti-reflection coatings can be produced.

Most of these types of coatings have been patented at some time. Some are such well known designs that they are out of time on patents but may have been superseded with other materials or designs to give a higher performance.

You would need to carry out a patent search on each specific type of coating to find what has been published. Bear in mind that some of the military materials although they may be patented they may not be found by some search engines, or copies be available.

Lamination / delamination problem

We laminate PET +Foil+LDPE. We make sachets from this laminate. PET delaminates.

Please give me corrective measure to control this problem.

How it occurred?

Answer.

Where there is a delamination my first advice is always to check the plane of the failure to make sure that you have correctly identified where the failure is occurring.  It is very easy to assume where the failure is occurring without investigating in detail.  If the failure is at the PET/foil interface then you have a number of options.

The failure is in the bulk of the PET

The failure is at the PET/adhesive interface

The failure is in the bulk of the adhesive

The failure is at the adhesive/foil interface

If the failure is in the bulk of the PET then this would suggest a weak boundary layer within the PET and this probably relates to the manufacturing process.

If the failure is at the PET/adhesive interface it is probably due to the PET being contaminated, usually by having low molecular weight material left on the surface from the polymerization process. Additionally there may also be additives that migrate out of the surface.  Any or all of these can reduce the surface energy of the PET and limit the adhesion. If the PET is suitably cleaned the surface energy can be raised and the adhesion will improve.  Surface treatment needs to be optimized, as it is possible to under or over treat the surface. Also the treatment needs to be done at the appropriate time as many treatments can degrade over time.

If the failure is in the adhesive it means that the adhesive choice may not be appropriate for the two materials.

If the failure is at the Foil/adhesive interface it can be that the foil is contaminated. This would mean checking with your foil supplier about their process with regard to use of lubricants and cleaning processes.

PET and moisture

Does plain pet 12 Micron can absorb moisture at ambient conditions?

Answer

It is the way of nature to try to bring everything to equilibrium.  Hence if the PET film has been produced, or wound in conditions of low humidity, or has been wound through a vacuum system that will have reduced the moisture the water content, then if to roll is stored in ambient conditions were the humidity is higher than the roll content the roll will absorb moisture.  The rate that this will occur is dependent on the size of the difference and the temperature. The greater the difference and the higher the temperature the faster the absorbtion will be.  Similarly if the film has been produced or rewound in high humidity conditions and then it is stored in low humidity conditions the film will lose moisture.  Again the rate will depend on the difference and temperature.

Barrier coatings on BOPP

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 has been called a dyad and by depositing several dyads onto a film the barrier performance increases enormously as the coating defects are reduced and any defects left are remote from each other and so the diffusion path is extremely tortuous.

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

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

Rootes pumps and Booster pumps

What is the difference between the roots pump and the booster pump?

ANSWER

Booster pumps are usually a similar type of pump to a diffusion pump in that some oil is vaporised and directed down a reducing volume to help knock the gas down the tapered tube to compress the gas and so increasing the pressure to make it easier to pump away by the backing pumps.

The rootes blower to some extent does the same job but it is usually classed as a backing pump and is used to improve the crossover as the backing pumps (often rotary vane pumps) run out of pumping speed and before the diffusion pump becomes effective. Often the rootes pumps are place in-line with the roughing line. The rootes pump is often left to freewheel without being driven when the chamber is initially roughed down. Once the pressure falls sufficiently the rootes pump starts to be driven where the rotating lobes can cause a reducing volume forcing the pressure up and making the rotary pumps more effective.

Hence using either can make the operation of the total system more efficient.  Modern vacuum systems appear to favour the rootes plus backing pump combination over the combined diffusion pump plus booster pump combination.  I must confess I have never checked on why, it could be that the cost benefits or speed is better with the rootes pump than with the booster. 

Bubbles in dry lamination

We reversely print PET with white and yellow ink background. When we laminate (dry lamination) it with Alfoil and MBOPP we observe bubbles(spots) appears. I mean small holes type appearance occurred.  What might be the cause?  If you can’t understand our problem you may ask relevant questions from us.

Answer

When printing the ink needs to wet the surface to form a uniform coating across the whole surface. If there is any area of low surface energy the ink will pull back (reticulation) and leave an area without any ink on it. Thus if there is any surface contamination this could result in these 'spots'.  This will only occur while the ink is liquid and mobile enough to move under surface tension. 

When you laminate surfaces together you can again have problems with surface contamination. The two surfaces need to be compatible and if there is a contamination on one surface it can mean that the two surfaces are incompatible at that point. Thus for most of the surface the two surfaces will be in intimate contact and exclude any air whereas where the surface is contaminated and does not adhere there may be an air interface that will show up more clearly as a spot. 

Wherever you see a spot/bubble I would examine the film under a high power microscope and look at the defect in detail. Often somewhere towards the centre of the spot will be a specific defect. It depends on which layers are being laminated together as to which side the defect originates from. The type of defect can vary, from being a large piece of debris that is holding the layers apart, like a tent, with the debris as the post around which is an air pocket. Alternatively on something like a metallized surface it could be a pinhole where some low molecular weight material has migrated through and started to spread out from the hole and the low surface energy has prevented adhesion and there is a thin air interface.

On both these it may be that if you apply an increase in pressure the spots reduce in size or disappear. If they remain disappeared it may indicate some debris, if it reappears with time it may be the surfaces are separating away and this could be because the debris is so large or because of the low surface energy.

If the ink is missing altogether then it is an ink wetting problem and not a lamination problem.

Without seeing the samples and being able to investigate I hope this has given you some indications as to the possible cause of your problems.