Vacuum Web Coating

Tarnishing of vacuum deposited silver coatings

Silver has different wetting characteristic than aluminium which can give some odd effects.  If the silver coating is very thin and the substrate is not suitably treated to raise the surface energy the silver can reticulate and become patchy. This only tends to occur when the silver is very thin, probably less than 10nm, and semi-transparent.

 

Thicker silver coatings do have the problem of tarnishing. Tarnishing can be the colouring if the surface which can be everything from a light yellowing of the surface through to a deep yellow and at worst black. Tarnishing is due to the very high reactivity of the pure silver and various chemical reactions can occur.  The quality of the air in your locality can also have an effect as hydrogen sulphide reacts with silver and if the hydrogen sulphide content in the local air is high this will react quickly with the silver. Hydrogen sulphide is also present in many natural materials such as eggs, onions, wool, coals, etc. If operators have handled onions or eggs or even eaten them and perspired then touching the silver can contaminate the surface and lead to tarnishing. Similarly breathing on the silver surface can contaminate the silver and trigger tarnishing. Humans also produce chloride salts and this too can corrode the silver. Another reason to be carful not to touch or breath over the product.

 

The main product of silver tarnishing is silver sulphide. The reaction mechanisms are:
8Ag + 4HS^-  <--->  4Ag₂S + 2H₂ + 4e^-
0₂ + 2H₂O + 4e^- <---> 4OH^-

The first reaction is believed to occur in a thin film of water on the silver surface. In dry air, tarnishing does not take place. In the second reaction, oxygen acts as a cathodic species and consumes electrons as indicated in the equation. Higher concentrations of hydrogen sulphide increase tarnishing.  The humidity can be a critical factor as up to a relative humidity of 50% the rate of tarnishing is fairly constant and at a low rate. Above 70% relative humidity the rate of tarnishing accelerates rapidly.  Hence it is important that vacuum deposited roll are not over cooled and wound up cold so that when the system is vented to atmosphere there is no condensation onto the roll otherwise the silver will immediately be covered with a thin layer of moisture which will contain many chemicals some of which will attack the silver. Other things that affect the rate of reactions are heat and light. Keeping the rolls cool and in darkness will also slow down the rate of reaction.

 

This all becomes impractical for selling a plain silver coated yarn. It would be advisable to lacquer the surface to protect the surface from chemical attack. The choice of lacquer will need checking out that it contains nothing that will react with the silver.  If you are intending to lacquer the silver then the sooner it is done the better following vacuum deposition.  If you have to store the silver coated web then storing the roll in a black sealed bag and including in the bag some silica gel to keep the moisture levels down and placing it in a low temperature store would help slow down any surface reactions.

 

In some window film I know that to help stabilise the silver instead of using pure silver they use an alloy. This is usually deposited by sputtering.  If an alloy is used in evaporation then it may fractionate the different elements unless the alloy is a eutectic so if you think of trying this route the choice of the right alloy is critical.  

 

 

Aluminium Oxide transparent barrier coatings

Aluminium has long been the main vacuum coated barrier material but it does, of course, have the property of being opaque. So for those wanting a transparent barrier coating they have always looked to other materials of which the most popular has been some version of silica.  Most time this is a sub-stoichiometric version of the silica and in the case fo the plasma enhanced chemical vapour deposition (PECVD) process it also contains an amount of carbon, anywhere up to 20%, or more, depending on process conditions.

The cost of these alternative materials has been high with the lowest still being of the order 2x to 3x the cost of aluminium metallizing.  This is because of two significant costs, the capital cost of either a PECVD or an electron beam deposition system is a much higher cost than a resistance heated boat type metallizer and the deposition speed is often less than a half that of aluminium metallizing.

The one company that appeared to have solved this problem was Camvac who managed to produce an aluminium oxide coating from their modified standard metallizer.  It would appear that by them carefully introducing the oxygen at the right point and in the right quantity they could control the oxidation fo the coating but also not damage their resistance heated boats. 

Needless to say this technology has been patented and so for many years this has been the only low cost process that I have known to be operating.  However in recent times I have heard that there have been two others who have managed to develop a similar process. One I know has been checked out to make sure it was not infringing any patents and that they were clear to run production the other I do not know if it has been checked out but they are prepared to talk about the process and will be giving a paper on the process at the 2009 SVC conference in Santa Clara.  This process was developed by the Fraunhofer Institute in Dresden (FEP) in conjunction with a polypropylene converter and the Applied Materials built a machine using the technology for use in production.  The FEP specialise in using additional plasmas during deposition to activate any reactive gases and to also promote surface reactions and densification of the growing coating.  Thus I expect that this process uses that expertise to evaporate the aluminium quickly but then use the plasma to activate the oxygen to speed up the conversion of the metal to the oxide.

As aluminium oxide is less dense than the metal this conversion from metal to oxide can also help improve the barrier as it will add a compressive component to the coating, as it swells, that will close pores down and make diffusion through grain boundaries slightly harder.  If this technology is now available through one of the system builders then this could mean that the cost of obtaining transparent barriers could be due for a fall in the next couple of years as these coatings start to become available.

Comparison between CPP and BOPP

As we know CPP and BOPP exhibits different properties. Especially sealing properties are quite different. CPP exhibit better seal strength than BOPP. I request you to enlighten me on technical grounds why such difference is exhibited?

What will be the performance if we use same tool to seal CPP and BOPP?

Answer

The acronyms are as follows CPP = Cast PolyPropylene  and BOPP = Biaxial Oriented PolyPropylene.

Cast polypropylene is produced by extruding polypropylene and chilling it and so the polymer chains within the film are randomly distributed throughout the film in all three dimensions.

BOPP starts out in the same way, the polypropylene is extruded and chilled but then the polymer is re-heated and stretched in two directions hence the 'bi-axial orientation' in the description.  This stretching is done when the polymer is softened and so the polymer chains are able to rearrange themselves to some extent and so they become oriented into the two stretching directions.  This changes the mechanical performance of the polymer. The tensile performance is higher along the polymer chain length than it is across the polymer chains thus in either of the two stretched directions the tensile performance is improved but in the third axis the tensile performance is reduced.   In the ordering of the polymer chains many of them become ordered enough that they become crystalline. This too effects a change in the performance, as crystalline material is a better barrier than amorphous material.  Crystalline polymer is denser than amorphous polymer and the amorphous polymer will soften at a lower temperature than the crystalline.

Thus using the same heat sealing tool may well work OK but may need a higher temperature for the BOPP than for the CPP.

I hope this answer helps.

Question re. Aluminium degradation due to heat & light

If a vacuum metallized film has an adhesive applied to it and it is adhered to a car windshield (inside facing out) will the aluminium still oxidize? If so, how long will it take? Also, I have heard that sunlight will degrade aluminium, is this correct?

Answer

There is no straightforward answer.  For years aluminium metallized film has been available with an adhesive coating for laminating to both windows for housing as well as car rear windows.

The stability of the aluminium depends on many things including the thickness of the aluminium.  The regulations are that the front windshield has to have a high visible transmittance and so the aluminium has to be very thin and so other factors need to be well controlled for the aluminium to have a long life.  For rear side and rear windows the visible transmittance can be much lower and so the aluminium thickness can be much greater and the stability is generally greater too.

The aluminium when it is evaporated onto the polymer film needs to stick well and wet the polymer surface. If the aluminium does not do this it can have many microscopic holes through the aluminium metallic structure which make it easier for the aluminium to oxidise. Aluminium oxide is transparent and so the aluminium disappears with time.  High heat and humidity will accelerate any corrosion process.

As well as the aluminium deposition conditions and the surface quality and state of the polymer the quality and type of adhesive can also play a part.  Many of the adhesives are water based so that the film can be cut to shape dry and the fitting checked before the adhesive is activated by water and the film stuck and squeegeed to the window.   This water can be absorbed by the polymer as well as by the adhesive and if there are any defects in the aluminium it can work on these defects making them grow and become easily visible.

Sunlight can be a source of energy to accelerate various processes.  The polymer film will normally degrade with exposure to UV light but as the aluminium is a good UV filter the polymer will last longer because the aluminium is facing the sunlight and protects the polymer.

Aluminium is normally quite stable; it is when it comes in contact with moisture and possibly chemicals that corrosion can be accelerated.

Thus if you have a high quality aluminium metallized film suitable for rear windows then the film is likely to be stable for years.  Lower quality film could be expected to be reduced.  If you are considering a film for the front window then it will need to be a much thinner aluminium coating and this becomes much more difficult to get at very high quality and so is correspondingly more vulnerable to corrosion which because of the very thin coating thickness becomes much more visible.

Both sunlight and heat can speed up any degradation process.  Aluminium vacuum deposited onto polymer film is not pure aluminium. As part of the process the aluminium will contain at least 1% (and often a higher percentage) of oxide or in some cases hydroxide. The polymer is not a perfect barrier to oxygen or moisture diffusion and so more water and oxygen can reach the aluminium film from the air inside the vehicle.  Thus any existing defects are likely to grow in size with time.  Chemical reactions are speeded up with heat or other energy source.  Light contains UV energy and this can accelerate some chemical reactions.  So both light and heat can accelerate aluminium degradation.

Aluminium oxidises naturally. All aluminium has an aluminium oxide surface and it is the aluminium oxide that helps slow down further oxidation of the surface. The oxide is less dense than the metal and so wants to take up more space and so the oxide is under compression and provides a better oxygen barrier helping to slow down oxygen reaching the metallic aluminium.

With semi-transparent aluminium the thickness of the aluminium is only a few tens of nanometers thick and so small amounts of oxidation can become visible very quickly.  Thus any defects in the aluminium deposition can cause holes or cracks in the aluminium often too small to be visible to the naked eye but which can allow oxygen to pass through the aluminium.  The more defects there are the faster the oxidation of the aluminium.

Thus degradation depends on the quality of the aluminium deposition as well as the temperature the film will reach and the time it spends at an elevated temperature.

The UV part of the light can be absorbed to an extent providing energy which also can accelerate corrosion depending on the chemicals present.

Humidity also plays a critical role and high temperature and high humidity together are the worst combination.

Neither heat nor light will degrade aluminium by themselves, both require other components such as oxygen or moisture and can be further accelerated by other chemicals that maybe present within the polymer, adhesive or even migrating from the glass (such as sodium).

If comparing two films in the same situation of being laminated to the same type of glass and in the same environment the comparison can be done of the quality of the aluminium deposition.  If the same film is to be used in differing conditions then the aluminium structure and quality can be ignored and the differences in heating, and other materials (glass, adhesive, atmospheric contaminant content) need to be considered in comparing potential lifetimes.

A question about Aluminium oxidation

Would like to inquire if you have a study on vacuum metallized surfaces degradation? A case in point is what is the degradation of a layer of vacuum metallized material, if it was left uncovered, meaning to say there was no PE or LDPE layer applied over it to protect it from the elements. Also how long will the surface last and what is the typical reaction it would have. Will it breakdown into powder like substance as it turns into aluminium oxide?

Answer

Vacuum metallizing is generally achieved by using an aluminium coating.  Aluminium will oxidise very readily and so all aluminium films have an aluminium oxide surface.  The thickness of the oxide surface will depend on a number of factors such as the structure of the coating as well as the time and temperature history of the coating since the point of deposition.

Aluminium when it oxidises changes density and the lower density oxide takes up more space which puts the oxide into compression.  This oxide layer in compression helps reduce the rate at which oxygen can reach the metal and so although the first monolayer of oxide will form even whilst the aluminium is still in the vacuum system subsequent layers take increasingly longer to form.  Thus the first monolayer will form immediately, the first nanometer about 1 week, the second nanometer around 1 month whilst the third nanometer may take a year to form.

This is an idealised view of how the aluminium ages through oxidation.  The reality will be different and will also vary because of other factors.  The humidity can affect the corrosion rate particularly if the atmosphere is contaminated. Any airborne contamination will become attached to the moisture and this can form something more corrosive than simple water.

Aluminium coatings also have pinholes in the coatings and these pinholes are not simple holes pierced through a full thickness aluminium coating but are as a result of the aluminium depositing onto some debris and when the debris is moved a pinhole is produced.  The edges of the pinholes gradually thin from the full aluminium thickness to no thickness. Thus pinholes are a source of a more rapid observable corrosion as the thinner aluminium becomes more transparent more quickly than the full thickness aluminium.  This observable increase in pinholes size tends to attract attention but should be more of an incentive to reduce the number of pinholes rather than worry about the corrosion.

The structure of the aluminium can affect the corrosion rate because of the structural defects such as the grain boundaries and any voids.  These defects can be a method of wicking down moisture at an increased rate.  If the aluminium could be deposited at a single crystal the corrosion would only be possible from the front surface, however because of the crystal structure the corrosion can also take place from the crystal edges to. The surface oxide can still reduce these effects as the oxide layer builds up and the protective compressive layer is formed.  The wetting of the aluminium onto the substrate is important as too is getting high adhesion.  Good wetting and adhesion reduces any interfacial gaps where moisture can reside and corrode the aluminium from the interface.

The structure of the grown aluminium also affects the surface roughness and this can also affect the appearance of the aluminium.  If the crystal structure is coarse the surface roughness will be greater than if the crystal structure is fine. The finer structure is produced with faster deposition rates. Coarser structures are produced with lower deposition rates.  If the surface is rough then the oxide layer can start to affect the surface appearance more rapidly with the reflectance falling and the surface looking matt or even milky in colour.  Thickness also has an effect as thicker films generally have a rougher surface than thinner layers for the same deposition process.

So as you can see it is very hard to predict the lifetime of any particular coating.

It is possible to suggest some trends.  Such as higher temperature and/or higher humidity uses of metallized film are likely to show faster corrosion than for those used in lower temperature and humidity applications.

Web processing for Barrier

AIMCAL have another 2 day course in October as part of their Converting School that may be of interest to some of you.

Web Processing for Barrier
Dr. Charles Bishop

To register please contact AIMCAL via website www.aimcal.org

This course provides an overview of the technologies that can be applied to deliver barrier materials for markets such as food packaging, display, and photovoltaic (solar) applications. Initial discussion focuses on the fundamentals of diffusion and permeation to provide an understanding of the contributions and limitations of materials and processes. Advanced topics include newer technologies such as nanotechnology, scavengers, and indicators that result in "smart" materials.

Course Outline
Day 1: 8:30AM - 5PM
Day 2: 8:30AM - 4PM

• Introduction
  • The Basics of Barrier
  • Markets
• Terminology
• Techniques and standards for barrier measurement
• Materials
  • Performance
  • Blends and laminates
  • Nanomaterials
  • Selection
• Smart/active packaging
  • Indicators
  • Scavengers
• Technologies
  • Film making and extrusion
  • Coating
  • Lamination
  • Vacuum deposition
• Substrates, surfaces and quality
  • Relating materials and surfaces to barrier
  • Pre-treatment (wetting, adhesion)
  • Cleaning and barrier
  • Variation in barrier during processing

Who will benefit from this course?
Anyone working at facilities that convert or use barrier materials including engineers, designers, quality control, stability and production personnel.

Date for this course:
| October 9 - 10, Cleveland, Ohio |         contact AIMCAL at  www.aimcal.org to register

Health And Safety information request

I'm looking for information on experience concerning health and safety in the domain of vacuum coatings like PVD or PACVD and their possible production of nanoparticles (and subsequent risk for health and safety). It has been years now since the first vacuum coater started his plant and I think there should be data on the subject as there is longer and longer experience. However, all I found until now was "there is nothing sure". Do you know how this matter is addressed in industrial size facilities? Which protection means are used? And what is the return of experience? (Like "are the workers in good health?" "Are the protection means adequate?" or “was it much noise for nothing?"). Thank you very much in advance.

Answer

This is a subject that never goes away as the technology changes and the nature of the coatings change.

Vacuum metallization has been done for more than a century but with different materials and processes than for some of the newer applications.

As in the roll to roll coating industry more than 90% of the machines that have been sold worldwide are for aluminium evaporation there is the most information collected about this process. With some of the newer materials the process may have only been run using laboratory equipment and scale and there may only be one or at best a few production machines that might provide more information but also might not be involved in that type of data gathering.  Also it might be several years before any adverse effect show up in any employees.

Thus it is common to find safeguards based upon conservative estimates of the risks involved.  The working premise is that is better to err on the side of caution than to be sued in some future time for not taking suitable care.

The Heath and Safety Executives in many countries around the world will take information and may read the risks differently and there can be variations between countries about the risks and personal protective precautions that need to be taken.  There are a number of published books on exposure limits to various metals, polymer, ceramics, compounds, etc.  However a number of these will be for particle sizes greater than are now being produced, often in laboratories, is varying quantities.  These books are regularly updated as newer information becomes available and so it is always worth checking the latest edition to see if the information you have is up-to-date.

My own experience of working on new materials was that we would check on the exposure levels and we would then suit up in full protective clothing (including masks and filtered breathing source) and wear pumped collection badges that were designed to accumulate material over a fixed period of time that would later be analysed to monitor the expected exposure to dust and to also monitor the range of size of dust that could be produced.  This did entail trying to produce a lot of dust by sung practises that normally we would not use as well as following our normal procedure so that we had a range of possible exposure levels.  We would then compare this to the allowed exposure limits and see how they compared.  Where we had a lot of fine particles these were always deemed to be a higher risk and so where no data existed we tended to move the classification into the next most severe category until the data was upgraded.

I know of other companies who did not take this view and only did the bare minimum required.

What has changed since then is that the size of nanoparticles has reduced still further and it has been proven that the chemical reactivity of these finer particles is much more that the coarser particles previously produced.  What is not yet clear for many of these particles is what they will do to the human body and at what levels. It is often not possible to use humans to test these things out other than by accident and with hindsight and hence it can take many years for the true recommendations to appear.

Many of these nanoparticles can be freely taken into the lungs and into the smallest of the alveoli where as they are in a moist environment they may stick and cause an irritation. Even an irritation is a problem as the lungs will secrete liquid in defence of this irritation and if this persists the liquid can increase in viscosity and granulate and possibly even form scar tissue. This is bad news as the scar tissue is less permeable to oxygen than the original surface and so there will be lung function impairment.  This process may take many years to manifest itself.  What can be worse is it the fine particles cause an allergic reaction where this can all be accelerated or worse still if the reaction triggers the formation of cancerous cells in some way. Now in other areas it has been proven that moving to finer particles increases the reactivity dramatically and so the fear is that will also be the case in reactions with the human body. Materials that in larger form were benign may in their finer form trigger some form of adverse reaction. However as this might take a long time to show up and there are so many different variations of each material it is taking a very long time to accumulate enough data to publish reliable recommendations.  Hence the general recommendation as treat every nanomaterial as a possible problem until proved otherwise, a little extreme but understandable.

I hope this helps and suitably answers your question.

I invite others who may have different experiences to post a response to add to this answer.

Crystallinity in BOPP films

What is the role of crystallinity in BOPP films, how can we measure crystallinity. What process parameters can influence crystallinity in BOPP films?

Answer

Crystallinity changes many things in the polymer such as density, tensile performance, optical transmittance, barrier performance, etc, and can be changed with orientation and temperature.

Sometimes there are trade offs of an improvement in one parameter and a decline in another.

With the change in density it is possible to estimate the crystallinity by measuring the density but more often  Differential Scanning Calorimetry (DSC) is used to determine the crystallinity of the polymer.

Problem of surface ‘shininess’

We metallize a cavitated BOPP film and are told that the metal 'shininess' of our film is inferior to the competition. Is there an ASTM standard for measuring the shininess/brightness/reflectivity of metallized surfaces? Is gloss measurement an option? If so what angle.

Answers.

Here are some comments on ASTM gloss measurement. A low angle measurement (20 degrees) is used for high gloss surfaces and high angles (85 or 60 degrees) for low gloss surfaces. But often the issue is not measurement technique but perception. So in this case it may be important to come to an understanding with the customer about what “shininess” means to them. It could be the specular reflectance (20 degrees) or the “sheen” (85 degrees). If the surfaces are not identically smooth, the smoother one could appear to be shinier with higher specular reflectance while it may have lower sheen. And if either supplier’s process modifies the surface smoothness, the customer may be reacting to that, rather than the reflectance of the metal coating.

Gloss - ASTM D2457

Gloss is a measure of how shiny or reflective a material is at a specified angle based on refractive index.

Gloss Measurement

An incandescent light source is directed at the test specimen at a specified incidence angle. A receptor is located at the mirror reflection of the incident beam. Polished black glass with a refractive index of 1.567 is used as a standard and is assigned a gloss of 100 at all geometries. Measurements are made using a gloss meter.

Gloss Geometries
Angle	Use	Note
20°	high-gloss films	Equivalent to ASTM D523 at 20°.
45°	intermediate and low gloss films
60°	Intermediate-gloss films	Equivalent to ASTM D523 at 60°.

Comparison of gloss data can only be made between similar materials and test procedures. Gloss values for transparent and opaque materials are not comparable. Gloss varies with smoothness and flatness and is sometimes used to compare these attributes.

Gardner Gloss - ASTM D523

Gloss is a measure of how shiny or reflective a material is at a specified angle based on refractive index.

An incandescent light source is directed at the test specimen at a specified incidence angle. A receptor is located at the mirror reflection of the incident beam. Polished black glass with a refractive index of 1.567 is used as a standard and is assigned a gloss of 100 at all geometries. Measurements are made using a gloss meter.

Gloss Geometries
Angle	Use	Note
20°	Good for comparing specimens having 60° gloss values higher than 70.	Equivalent to ASTM D2457 at 20°.
60°	Comparing specimens and determining when 20° gloss is more applicable.	Equivalent to ASTM D2457 at 60°.
85°	Good for comparing specimens for sheen. Most often used when specimens have 60° gloss values lower than 10.	Commonly known as "Sheen".

Don

Donald J. McClure, Ph D, President
Acuity Consulting and Training
23002 Dunham Lake Road
Siren, WI 54872-8819
715-689-2902 (land line)
651-470-6939 (cell)
acuityct@hotmail.com

Second Answer

A simple measure of how shiny a metal surface is to measure the reflectivity.

If you wish to measure the polymer surface before metallization it is possible to measure the Haze of the surface. Haze is a measure of the off axis reflection.  If a light is directed at the surface normal to the surface and a detector also placed normal to the surface (where the light passes through the centre of the detector) it will measure the reflected light.  If another detector is placed at a small angle away from normal, usually 2.5 degrees or 5 degrees to normal, this will measure the scattered light or 'haze'.  The larger the haze the lower the specular reflectance and so it is possible to estimate the metal coating performance from the uncoated surface.  Some of the polymer manufacturers quote a Haze measurement and the angle they measure at.

Other measures that might be informative would include a measure of the surface roughness as the specular reflection is affected by the surface roughness. The smoother and flatter the surface the higher the specular reflectance.  Thus on a cast film it is possible to tell the difference between the air side of the film and the casting drum side of the film as the side that contacted the casting drum will usually have the same roughness as the casting drum.

Thus measure of surface roughness is now done using Atomic Force Microscopy or one of the variants.

There are two possibilities that you might consider.  One is that the quality of your film is as good as the competitors but that your metallization is not as good. The other is that the metallization is equally as good but the film surface is not as good.

The metallization can be a variable.  How the metal nucleates and grows on the surface can affect the surface roughness of the metal coating and hence the final reflectance.  If the metal wets the surface poorly the nucleation sites will be able to grow quite high islands of metal but with gaps between the islands. This will continue and give rise to a rougher coating than if the metal wets the surface well. If the metal wets the surface well the nucleation sites spread out and the islands touch each other even whilst the coating is still quite thin. This leads to a smoother coating.  If the deposition rate is low the crystal size will be bigger than if the deposition rate is very high. The larger crystal size also leads to a rougher surface.  As the coating grows in thickness the differences in growth rates of the different crystal orientations becomes exaggerated and this too is seen as an increase in the roughness of the metal coating.

Thus by having poor wetting, a slow deposition rate and a thick coating it is possible to create a much rougher surface than if the process maximised the substrate surface energy and hence maximised the metal wetting, used the fastest possible deposition rate and limited the coating thickness to only what was required and not an excess.  Often it is thought that a little bit extra coating thickness will help and often it does exactly the opposite.

A final thought is the if you are comparing reflectivity measurements and possibly looking as differences of only a few percent it is important that either the samples are all measured on the same instrument and by the same person using the same procedure and calibration or that the different instruments are capable of measuring some sample materials to give the same results and at the same accuracy.

It is common for the same material to measure different values on different instruments for a variety of reasons.  The illumination may be different, some lamps may have filters to bring the spectral response closer to daylight others do not. Some calibrate the instruments differently. Some use a silver mirror for the 100% reflectivity calibration others will use an aluminium mirror. These mirrors will age and so even using the same mirror the results may be different over the timescale of a few months.

Hence when discussing the reflectance it is worth making sure that the different samples were at lest all measured on the same machine and ideally with the same calibration.  It would not be the first time that the reflectivity has been disputed when in reality it is the differences in measurement that were the variable and not the substrate or metallization.

I hope these thoughts help you.

Charles Bishop     www.cabuk1.co.uk

Coefficient of Friction (CoF) of plain & metallized films

Need to understand the factors influencing the COF values Plain: Plain of Metallized Polyester films. The observation is that after metallization the COF values change (generally increase) as compared to the pre metallization values.
1.Can this value be controlled?
2.Does this also depend on the OD to which the film is metallized?
3.How significant is the Film: Film cof of the Metallized Polyester in the subsequent Converting operation?

Answer

The Coefficient of Friction (CoF) is dependent on the materials in contact, the surface energy and the contact area.  So for your polyester film you will have a CoF of the polyester in its natural state which is likely to be high (probably >0.5) and in this state the polyester is likely to ‘block’ (stick together) when rolled up.  Blocking is where the film sticks together and is difficult or impossible to unwind. This can be made worse by electrostatic charging of the film and surface cleaning which increases the surface energy.

To reduce the propensity of the film to block and to make the film more easily handled there are a couple of methods used.  This first is to add fillers which will change (increase) the surface roughness which, on a microscopic scale, reduces the amount of two surfaces that are in contact. This use of fillers can reduce the CoF down to 0.3 – 0.4 which is still regarded as too high for optimal film handling.  The fillers are not without problems. Typically the haze increases by 0.4-1.0% per 1000ppm of silica filler used.  To minimise the haze the film can be made as a co-extruded film with only one thin layer being filled which reduces this effect although because it changes the surface roughness does not eliminate it.  Also changing the type and size distribution can reduce the haze.  The danger is than in reducing the haze the CoF will once again begin to rise.

The other common method of reducing the CoF is to add a slip agent to the polymer. These slip agents migrate out of the bulk of the polymer to the surface and reduce the surface energy and CoF. As little as 0.1% slip agent into the bulk polymer may reduce the CoF down to less than 0.2 where 0.16 – 0.2 CoF is regarded as providing easy handling.  The slip agents make the handling better but can be a disaster for obtaining good adhesion.  One of the reasons for treating the surface by flame, corona, atmospheric plasma or vacuum plasma is to remove or crosslink in these low molecular weight low surface energy materials to both increase the surface energy to obtain better coating wetting as well as remove the weak boundary layer that would become the failure plane at the interface.

The surface treatments do not change the CoF that is produced by the use of fillers resulting in the change in surface roughness.  How ever as mentioned above the use of fillers alone usually does not produce a low enough CoF to make handling the film easy.

When the film is metallized it is most likely to have been surface treated to improve the wetting and adhesion of the metal coating and so the metal surface will no longer have any low energy material present and so the CoF is likely to have increased. 

Even if a slip agent is not included in the bulk polymer there will be unpolymerised monomer that will appear as oligomers on the surface as low molecular weight material that can have a similar effect. These oligomers will also be removed by any surface treatment.

What you do have is the back surface of the film that is most commonly untreated and so there will still be the low molecular weight material on the back surface, either slip agent or oligomer.  As the freshly metallized surface has a very high surface energy the film when rewound will bring together these two different surfaces.  The low surface energy material will want to migrate to the high surface energy metal surface and so over time you can plot the reduction of the metal surface energy as more of the low molecular weight material, which is very mobile, is transferred across to the high surface energy metal surface. Given enough time these two surfaces will approach the same surface energy.  

In general the optical density (OD) of the metal coating does not affect the CoF. The metal coating is nominally conformal and so will have the same surface roughness as the surface it has just covered.  This is true for most coatings but if you deposit an extremely thick coating ~1 micron thick or thicker it is possible to get selective crystal growth that can increase the surface roughness which will cause the metal surface to appear matt. Thus if you coating still appears to be of mirror quality then it is likely to be thin enough that there is no significant crystal growth and hence the surface roughness of the metal should match the polymer surface roughness. 

Finally the question of significance of the CoF to film handling relating to downstream converting processes.  This is a one of those questions that it is virtually impossible to answer.  There will be a different answer depending on whether the process is a simple lamination process or it is a wrapping or filling process.  In the lamination there will probably be fewer rollers and less contact with the film whereas with the latter process there will be more rolls to help make folds and change the direction of the film movement which will make the film more sensitive to the CoF of the film.  This may be one where you have to try the film and see if there is a problem.

Please note that in checking the film you also need to time the gap between metallization and use on the next process.  Ideally you would also record the temperature too.  The reason for this is that if the film is used the same day as metallization it may have a handling problem but the same film if left for a week, possibly at an elevated temperature, may no longer have a problem on the same machine.  The reason for this is the reduction of the surface energy of the metal.   This is done by the transfer of low molecular weight material onto the metal surface.  The rate of transfer of material from the bulk or across from one surface to the other is increased at elevated temperatures as the low molecular weight material becomes more mobile.

One word of warning, if you try to lower the surface energy be careful, as this could cause problems of adhesion to other surfaces.  There can be a trade-off between handling performance and adhesion.

I hope this helps.