Vacuum Web Coating

Pinwindows in polypropylene

The question was as follows.

On our Metallizer (Valmet 2105) we experienced 'pin windows' with
fine scratches (1mm). Had been metallizing BOPP film with out problems, then
changed to CPP film. No problems with first 2 rolls, then pin windows/fine
scratching occurred. Have made adjustments to winding parameters, which we
had been using for the past 2yrs. At this stage still cannot completely
eliminate defect. Ran BOPP film thru Metallisation again, no pin windows - back to CPP pin windows appeared? Have been trying to eliminate since May.24 without
success. What can you suggest?

The answer is in the continuation.

Biodegradable metallized packaging

It is always a sign that a topic is starting to take off when there are small one or two day conferences just on that topic that start appearing.  This appears to now be the case with biodegradable packaging.  In early July there will be a two-day conference organised by PIRA in Europe on just this topic.

We also have in the press announcements that the first biodegradable transparent barrier film produced by Alcan using silica onto polylactic acid film will be announced in October at a conference in Germany.

A second ‘first’ that has been announced is the metallised biodegradable film produced by Innovia, suitable for both industrial and home composting, has been launched.  This is a metallised cellulose-based film has passed various tests and is shown in a twist wrap application.

Although the market will eventually be worldwide both look to be initially aimed at Europe where the cost of disposal of waste material is costly and is increasing.

Having seen this starting in Europe I wonder how soon before other metallizers follow suit and announce competing products?

Pattern metallization

I want to know the information about oil printing for pattern metallization. As what kind of oil we can use and is it possible by gravure . Whether it will not contaminate the roll on winding.

Please advice me.

In general you would use a low volatility oil such as the high vacuum oil in the vacuum pump.  It can be applied with gravure or with transfer rolls.  The idea is to use an amount which is just evaporated by the heat of the aluminum deposition so some trial and error will be necessary.  You can buy systems from the metallizer suppliers.

Eldridge M. Mount III

EMMOUNT Technologies

www.emmount-technologies.com

Latest question re. Static charge and surface energy

Query.

Could you please explain what is the difference between static charge & surface energy in polyester film? Generally it is clear that surface energy is inherent property of film but static charge that develops also because of the modification of surface structure although is vanish rapidly when comes in contact to a ground medium.

Surface Energy

All materials have an energy associated with the surface. 

The analogy has been made that the energy is related to the excess energy present from not having the next layer of atoms present and so having some bonds unmade.

The precise energy is then a function of the atoms present at the surface thus for polymers there are different amounts of Hydrogen, Carbon and Oxygen.  Of these the oxygen will produce more energy on the surface than carbon which produces more than hydrogen which is better than fluorine.  Thus to produce an increase in surface energy it is common to use an oxidising plasma to reduce the proportion of hydrogen and carbon and replace them with oxygen.  If a hydrophobic surface is required then the surface is fluorinated to remove any oxygen and increase the fluorine content of the surface.

In general the higher the surface energy the easier coatings will wet the surface this is because the coating will have a surface energy (known as surface tension) and the system will naturally produce the lowest combination of surface energies. Thus the substrate surface energy plus the substrate/coating interfacial energy will be have to be lower than the coating/air surface energy for the coating to wet.  If the combined energy is higher the coating will not wet and the coating will want to form beads on the surface.

Electrostatic charge.

When dissimilar materials are brought together and separated there is an electrostatic charge produced. If both materials are electrically conducting this will not have any significant effect. However, if one of the materials is an insulator a surface electrical charge will be produced. The rate of which this charge will be formed depends on both materials. There farther apart they are in the triboelectric series the faster the charge will be generated. If there is any conductivity at all the charge will leak away with time, the better the conductivity the faster the leak rate. Thus for Polyester Terephthalate (PET) which is used as an electrical insulator and usually has a low moisture content there is no appreciable electrical conductivity and so the surface will charge rapidly and will retain the charge for a long time.  In cases like this the mechanism for discharging the surface will relate to the ambient humidity making the air conductive. This is not very efficient and so it is still possible to charge the surface up to very high voltages often to the point where the surface will arc to the atmosphere as a method of discharging the surface.  This generally will only happen once the voltage is around 10,000V or greater. 

As the charged surface approaches a conducting roll the charge will reduce and may disappear whilst in intimate contact with the roll but as it leaves the roll the charge will reappear as the two surfaces separate. In very bad cases sparks can be seen between the approaching and departing web. Again it depends on the polymer conductivity and the maximum charge that appears on the web.

As the polymer is an insulator it is also possible to have charges of opposite polarity on the same side of the web that are not mobile enough to cancel each other out.

The surface charge can be tested using a hand held meter. This is held in close proximity to the surface and the average charge will be measured. In this way it is possible to follow the web around a winding system and chose the best site for any neutralisation system to be fitted.  Note that changing the polymer type will change the rate of charging and the capacity for retaining the charge and so an electrostatic audit needs to be performed for each different polymer.  It is also worth noting that the static charge will form on both sides of the web and it may be of the same polarity on both sides, it does not have to be of the opposite polarity. If it is of the opposite polarity there is again no guarantee that the charges will link around the edge of the web and neutralise each other.

It is also worth pointing out that holding a meter close to a fast winding web, possibly with a large electric charge on it that can discharge to the person, is a potentially hazardous measurement and so care does need to be taken.  Sticking the meter to the machine and measuring the voltages remotely has some advantages.

In coating machines run at atmospheric pressure it is typical to prefer to have a low ambient relative humidity to produce high quality coatings. This in turn can produce much higher electrostatic surface charges and so often where the humidity is controlled there will be a compromise in the set level.

It is also common to then see electrostatic neutralisation methods employed to reduce the winding and coating problems that static charge can produce.  This can be as simple as using a conducting metallised foil ‘tinsel’ in close proximity or even touching the web to provide multiple leakage paths to earth.  This can cause surface scratching in soft coatings or can cause micro-arcs that can also be seen as slight surface damage through to larger Litchenberg figures type damage.  This has led to powered electrostatic neutralisation bars where a charge of the opposite polarity is produced near to the charged polymer web and the air acts as a medium for charges to cancel each other out.  Using electrostatic neutralisation and a gas flow can direct the charge to the surface and can also help remove some larger debris from the surface.

Electrostatic charging does also occur in vacuum. It does not require air to charge the surface up. Thus in vacuum it is one of the hidden benefits of using a plasma cleaning process that the plasma also neutralises any electrostatic charge on the web surface.  As the plasma will fill a greater part of the vacuum system and not just the bright area of plasma that is easily visible this plasma can usually reduce the charge through much of the winding system.

Charles A. Bishop

C.A.Bishop Consulting Ltd.        www.cabuk1.co.uk

Unmetallized or light metallized spots.

Problem.

We are facing problem of unmetallized spots in vacuum metallization of aluminium on PET film. The spots are circular in shape and shows the density of metal deposition is very less and hence are visible like unmetallized spots.

Will you please suggest us the solution to overcome this problem during metallization or what are the causes for occurrence of this problem. We have checked suspected possibilities in this regard and are in search of appropriate solution.

Answer.

The spots are most likely related to oligomer on the surface of the polyester.  Some of the oligomers can be very low melting point low molecular weight material that will be evaporated by the heat depositing aluminium.  This works in the same way as the oil printing method for producing in-vacuum pattern metallisation. As the amount of oligomer is low then some metal is deposited otherwise there would be completely uncoated pinholes//pinwindows.

If this is happening each year the next question would be ‘does it appear at the same time or season each year?’  If so then one of the factors to consider recording is the humidity. 

Although the deposition spots look to be caused by residual oligomer on the surface of the polyester the humidity could be affecting the level of oligomer appearing by affecting the PET manufacturing process. High humidity can affect the polymerisation process and the times required to drive off water during the polymerisation this in turn can affect other parts of the polymerisation and potentially one side effect of this could be the level of residual oligomers.  If the polymer produced is then chipped and stored before being used in the film making process then the same reasoning would apply. Drying the polymer chip before extruding could be beneficial.  Ideally for each step in the process the starting material should be as constant as possible irrespective of the ambient temperature and humidity.

It could be that something as simple as drying the starting materials, or pre-heating the materials for longer before polymerisation or possibly later before extruding the film to make the web would control the percentage of residual oligomers more during high humidity times of the year. 

I hope this helps identify the source of the problem and enable a workable solution to be adopted.

Charles A. Bishop

C.A.Bishop Consulting Ltd.

www.cabuk1.co.uk

Decay of surface energy for metallized OPP films

There is always some form of additive in the films to prevent blocking and assist with slip. Commercial grades of OPP always contain these materials. Two aspects not covered in the discussion are the type of OPP film and process temperatures.

It is assumed it is a co-extruded film and not a coated PP grade. There are major differences between the two film types on their requirements when metallized, and this phenomena is more noted in co-extruded grades. Secondly, and more importantly, there has been no mention of process roll temperature.

When metallizing OPP it is critically important to maintain roll temperatures as low as possible. Outside reel temperature of the metallized BOPP should be less than 30 degrees Celsius. Good practice includes monitoring process drum temperature and maintain at a low level at all points on the drum. Bear in mind that the contact between film and drum is affected by tension and web profile, and that web surface temperature will be influenced by film thickness, by degree of wrap on the drum, and by web speed as well as the composition of the co-extruded and core layers in respect heat transmission.

There will be a build up of heat in the reel as metallizing proceeds, and the initial temperature of coated layers nearest the core will be lower, but the reel temperature will increase as the process continues resulting in relatively high levels toward the outside. This heat is retained in the reel and encourages oxidation of deposited aluminium, and of migration of materials in the tightly wound and warm/hot reel.

There are several ways of minimising the risk.

1) Maintain as low a level of process drum temperature as is practical with chillers

2) Check the process drum for hot spots and flush regularly to ensure efficient circulation of coolant

3) Check that deposition levels are optimised - heavier deposition means more latent heat of condensation onto web surface, and possible slower running

4) Consider length of reels and influence on temperature build up in reels

5) Look at web thickness/deposition level/web speed/web length and consider optimising to maintain a good temperature level and profile

6) Monitor reel temperatures immediately on removal from metallizer and build up a profile of the reel temperatures across the web. Fix maximum web temperatures.

6) Rewind/slit within 1 to 2 hour maximum after metallizing to cool web by passage through slitter/rewinder

7) If ambient temperatures are high - mid summer in Northern Hemisphere is sufficient - consider use of refrigerated storage before and between operations. I have seen refrigerated truck trailers used in UK to store reels between operations to minimise deterioration

Dr William Llewellyn
Senior Consultant
AWA Alexander Watson Associates

Web Site: www.awa-bv.com

Oil for pattern metallisation

Q

I want to know the information about oil printing for pattern metallization.As what kind of oil we can use and is it possible by gravure . Whether it will not contaiminate the roll on winding.

A

Most people use a diffusion pump oil for pattern metallization as it has been developed for vacuum use.  One of the controls that is an option is to heat the oil and vaporise it onto the gravure roll and use a doctor blade to limit the excess on the gravure.

The quantity of oil is the critical parameter. The process works by the aluminium vapour landing on the oil where the sticking coefficient is extremely low. As the sticking coefficient is so low the aluminium re-evaporates from the surface. As it re-evaporates the aluminium takes some oil with it. Thus, the aim is that all the oil is removed by the re-evaporated aluminium leaving behind a dry polymer. The reality is always slightly different.  The oil is not a on the surface as a flat parallel layer but has a curvature and so as the oil is evaporated the width of the line is reduced and so the line edges may not be as sharp as with an in-contact mask, particularly if almost all the oil has been removed. If, however oil is left on the surface it will transfer via the first front surface roll to other parts of the aluminium deposition contaminating the surface.

Thus the management of the quantity of oil deposited must be controlled quite precisely.

Some companies have a propriatory process that is designed to make sure that all the oil is removed or stabilised.

One company in the USA also claims to have changed the diffusion pump oil for a food contact compatible oil in order to make their product acceptable to the FDA for food packaging.

I hope this answers your question.

Loss of surface energy

We have few queries regarding Metallised BOPP Film.
Surface energy dropped on Metallised side from 50 dyne/cm to 33 dyne/cm within 48 hours of metallisation. The base BOPP film used for metallisation is free from any kind of migratory additive, like Slip / Antistat. Only non migratory antiblock additive is used on skin layer of the film. What are the possible reasons for the same? Whether impurity present in aluminium wire used for metallisation can be responsible for surface energy decay? According to you, how long the surface energy of 38dyne/cm retain on metallised surface of BOPP film after metallisation?

Dear Sir, I have never seen any other source of this problem other than contamination of the surface by the film while wound up.  In order to exclude it as the source you should do the following.

1                    1                     perform chemical test of the low energy metal surface with XPS (ESCA) and look for the absence of carbon or nitrogen

2                    2                     from a freshly metallized roll of film cut a slab of film and place it in a cold environment excluding moisture

3                    3                     take another slab and hold flat at room conditions of your roll storage

4                    4                     Separate several freshly metallized sheets of film and store individually separated by Mylar sheets.

Periodically measure surface energy and see which storage conditions are affecting the surface energy.

You should also check the reclaim you are using to insure it is free of additives and that it is not degraded giving low molecular weight contaminates

I suppose it might be possible that aluminum hydroxide is forming on the surface instead of aluminum oxide but I have never seen this myself and I do not know what the surface tension would be for this.  This surface would be very susceptible to corrosion of the aluminum

Eldridge M. Mount III

EMMOUNT Technologies

585 223-3996

emmount@earthlink.net

www.emmount-technologies.com

Included in this answer from Eldridge were the following two references

1.         Mount III, E.M., “CHEMICAL ANALYSIS OF THE POLYESTER/METAL SURFACE OF A DELAMINATION FAILURE”, Conference Proceedings, Society of Plastics Engineers, Annual Technical Conference, Charlotte, NC., May 2006

(a version of this paper will be presented at the AIMCAL Fall Conference in Reno later this year)

2.  Metallizing Technical Reference 3^rd Edn.   Published by AIMCAL

$15 each + shipping for members            see www.aimcal.org

$25 each + shipping for non-members   

In particular Section 7 ‘Surface treatment of webs for aluminium vapour deposition’

A second response to this question appears in the continuation section.

Delamination problems

I am facing delamination problem in few of the laminates.When I laminate Coated Polyester with polyethylene on solvenless laminate the bond strength is about nil.Polyethylene surface does not accept the adhesive and the ebtire adhesive remains on the polyester surface.This is almost happening with all the structures.Why the polyethylene is not accepting the adhesive? corona treatment of polyethylene is 42 dynes.
Can you suggest any solution?

Dear Sir, only a detailed chemical analysis of the PE surface can tell for sure but it is typical of PE films that they contain additives which bloom to the surface and create a weak boundary layer which could be the source of the poor bond strength.  It is also possible that the PE film surface was degraded during treatment or retreatment also creating a weak boundary layer.  Of course the adhesive could be at fault and this should be eliminated by contacting your supplier to check retained samples.

I have attached a paper on metal adhesion which may be helpful in looking at the potential factors as well as a section on treatment which might be helpful.

In general I would first assume that it is over treated as the dyne level is high but the adhesion is poor.  I would then next think that the PE film is old with a high surface concentration of additives or perhaps has to much due to a formulation problem.  Past that you have to do some analytical work to determine the source of the problem

Eldridge M. Mount III

EMMOUNT Technologies

585 223-3996

emmount@earthlink.net

www.emmount-technologies.com

A second response follows in the continuation.

HDPE problem with metal adhesion

Here is the problem that was presented.

We metallize HDPE film and we started to face metal adhesion problem since 2 month. Film is within specification, superficial tension on the side for metallization is 48-50 dynes, but metal adhesion is very bad: tape test gives 90% metal pick off. If we try to print on 50 dynes surface inks do not stick to it. There are few things to mention:

1. 48-50 dynes was never the case before (it used to be 36-38 going down quickly with the time and temperature). Producer is claiming technology was not changed: Gilles Nison from ExxonMobil who read this in copy can confirm this.

2. The best solution we could find till now is to treat the other side (originally untreated) up to the level of 38 dynes and metallize it right away. The problem with this method is that after metallization adhesion is very good, no blocking, but after 24 hrs adhesion is getting worse and reaches the level of 50% and then stays on this level. We take samples from beginning and the end of the roll and keep them. On these samples adhesion stays very good as it was right after metallization, but inside of the roll adhesion becomes bad.

There are generally 2 questions I wanted to ask you:

    - what type of chemicals could cause adhesion degradation from the inside (from the film) and from the outside (from the other layer of the film)?

    - how Al is making bonds on the treated surface of HDPE?

Thank you very much in advance,

Equipment we use:

            metalliser Galileo, 1650 mm wide

corona treater (two side) AFS, 2500 mm wide

The suggestions for tackling the problem follow.