Vacuum Web Coating

Request to improve barrier, adhesion & optical density

Sir I would like know about

 

1)    how can we improve Metal Bond Strength ?

      However we are applying plasma in process.

 

2)  Is metal bond strength have any technical relation with Optical Density,  as per our experience High OD have low Bond Strength and low OD have comparatively high Bond Strength.

What is tech. reason for it?

      

3)  How can we improve Barrier Properties of Metallize PET film and BOPP film.

 

 

Kindly suggest and oblige us.

 

Answer 

Let me start to answer your questions with some comments on the bond strength.

The surface of the polymer web is rarely, if ever, the same chemical composition as the bulk polymer that makes up the web.  This is because there will be present on the surface a variety of other materials. There are likely to be some monomer fragments or oligomers that are left over from the polymerisation process.  These will be of low molecular weight and very mobile and if left in place will form a weak boundary layer with any coating applied.  This weak boundary layer is where the adhesion is most likely to fail.  In addition if the polymer has had any additives included in the bulk polymer then these too may migrate to the surface further altering the surface chemistry.  In some cases this could include slip agents that are added to help reduce the surface energy of the polymer to reduce the coefficient of friction and so improve the polymer handling.  These slip agents are also low molecular weight materials and are very mobile. 

 

Surface treatment is often carried out on the polymer web in order to reduce the effects described above.  The surface treatment can be by flame, corona, atmospheric plasma or vacuum plasma.  These surface treatments are aimed at either crosslinking the contaminants to the bulk polymer or to convert the organic material into a volatile compound that can be removed more easily from the surface.  If the pre-treatment is done outside the vacuum system then the time between the treatment and the metallization becomes important.  As a number of the contaminants are contained within the bulk polymer then even if they have been removed from the surface they may return given time and temperature.  Also as the contaminants are removed from the surface the surface energy of the polymer may be increased and if the web is re-wound then the clean from surface will contact the still contaminated back surface. As the surface energies will try to equilibrate some of the low surface energy material will transfer across from the back surface to the front surface, re-contaminating the surface.

 

With the vacuum plasma treatment there is no opportunity for the surface to become re-contaminated before metallization which will improve adhesion.  Vacuum plasma also can include oxygen as one of the treatment gases which can substitute some of the carbon atoms with oxygen atoms.  This increase in oxygen content of the surface increases the surface energy of the polymer surface and this helps adhesion in two ways.  One is that the metal coating can bond directly to the oxygen which is a stronger bond that without the oxygen present. Also the higher surface energy means that the metal more easily wets the polymer surface. 

 

These pre-treatments need to be used with care as it is just as possible to over treat the surface as under treat the surface.  If the polymer surface is over treated the plasma can cause too much chain scission breaking the polymer chains up into ever smaller fragments.  Breaking some bonds is good as it allow additional bonding sites to be created but if this goes too far the surface is fragmented so much that even if the coating bonds well there is too much coating bonded to short chain fragments which themselves are no longer well bonded into the bulk polymer.    

 

Measuring the surface energy has to be done with care as it does not tell you if there has been over treatment.  The surface energy will go up as the plasma treatment increases. This increase will continue with either increasing power or treatment time up to a maximum value. Once the maximum value is reached the surface energy will stay at the plateau value irrespective of any increase in treatment time or power.  In comparison the adhesion will also increase with treatment time or power up to a maximum but once the maximum is reached any further increase in power or time will only result in a decrease in adhesion.

 

 

As different grades of polymer or different suppliers of polymer can contain different proportions or types of additives it means that the same plasma treatment cannot be used on every web.  Sometimes the same treatment can be used but in other cases the treatment will have to be changed to optimise the adhesion.

 

 

 

The metal adhesion and the optical density are both affected by the plasma treatment and so there appears to be some connection between adhesion and optical density although it can be hard to figure out the connection.   As the plasma treatment changes the surface energy of the polymer it also changes the wetting of the metal coating. If the surface energy is low the metal will not wet the surface and the metal will form islands on the surface which can be thought of as hemispheres.  Compare this to if the surface energy is increased and the metal better wets the surface the islands will spread out and the diameter of the islands will be much larger and the height lower.  Thus the higher energy surface will produce a continuous metal coating a lower thickness.  Thus a coating with poor adhesion and a given optical density will usually be thicker than a coating with high adhesion where for the same optical density the coating will be thinner. 

 

 

 

Barrier coatings will also be improved in the same way as the optical density by increasing the surface energy and adhesion.  Although this has an effect on the barrier performance it is usually much smaller that the effect caused by the number and size of pinholes in the coating. The pinholes are primarily caused by debris on the polymer surface. This debris is present on all polymer films and includes polymer powder debris from volatilised unpolymerised monomer that condenses and fall onto the surface, powder debris from the slitting of the web as well as any airborne debris attracted to the surface from the triboelectric charge generated by the polymer web winding over various rollers.  This debris gets metallized and if moved following metallization it leaves behind an unmetallized area known as a pinhole. If the debris rolls away the pinhole will be circular but if it slides across the surface the hole will have a tail to it which is a small scratch.  Some other pinholes are generated by pick-off as any hard contact with the back surface when the roll is re-wound can, on unwinding, pick off any poorly adhered metal leaving behind a pinhole.  So again good adhesion can help improve the coating properties, in this case reduce the amount of pick-off and so improve the barrier performance slightly.  The amount of pick-off is generally much smaller than the number of pinholes caused by debris left on the surface.

 

Removing debris is not easy.  Plasma treatment does not clean away the debris. The debris is held onto the surface by Van de Waals forces more than electrostatic charge and so the plasma although it eliminates any triboelectric charge does not release the debris from the surface as some people think.  To reduce the levels of debris takes some positive action such as a pulsed ultrasonic air jet cleaning system or probably more easily using a tacky roll system.  This is where an adhesive roll is brought into contact with the polymer surface and the debris sticks to the tacky roll leaving behind a clean polymer web. 

 

It is only by reducing the number and size of these pinholes that the barrier performance can be significantly improved.

 

 

AIMCAL Webinar "Utilizing Mayer Rod Coating Technology"

"Utilizing Mayer Rod Coating Technology" by Doug Krasucki, R.D. Specialties & Dr. Ed Cohen, Edward Cohen Consulting

Join us for a Webinar on July 14

Space is limited.
Reserve your Webinar seat now at:
https://www1.gotomeeting.com/register/349884224

Join us for a webinar on July 14th, the first in a coating method series by Dr. Ed Cohen and Friends

The Mayer rod is one of the most widely used coating methods, because it is an effective versatile inexpensive coating technique, which gives good coating quality and uniformity. This webinar will discuss the fundamental operating principles of the Mayer rod and the hardware and technology needed to effectively run the process.

Specific topics to be discussed are:

• Why it is (and should) be used
• How does it function?
• Hardware configurations
• Operating Range
• What are key variables?
• Products
• How to use effectively

Title: "Utilizing Mayer Rod Coating Technology" by Doug Krasucki, R.D. Specialties & Dr. Ed Cohen, Edward Cohen Consulting

Date:Tuesday, July 14, 2009

Time:11:00 AM - 12:00 PM EDT

After registering you will receive a confirmation email containing information about joining the Webinar.

 
System Requirements
PC-based attendees
Required: Windows® 2000, XP Home, XP Pro, 2003 Server, Vista

Macintosh®-based attendees
Required: Mac OS® X 10.4 (Tiger®) or newer

Optical density test method

Question

Is there any international test method to measure optical density of metallized films?

 

Answer

In the AIMCAL Metallizing Technical Reference 4^th Edn. The opening section (1.1) is the AIMCAL test procedure for determining Optical Density. Included in this there is reference made to USA Standard Number PH2-19-1959 that provides the basis for the concept of diffuse optical density.

Copies of the AIMCAL Met.Tech.Ref. 4^th Edn is available from AIMCAL via the website www.aimcal.org

 

Below is the abstract for the ASTM D1003 - 07e1 Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics.

As you compare transmittance between un-coated  and coated material the basic measurement is one of transmittance and so this might be of interest to you too.

 

ASTM D1003

Significance and Use

Light that is scattered upon passing through a film or sheet of a material can produce a hazy or smoky field when objects are viewed through the material. Another effect can be veiling glare, as occurs in an automobile windshield when driving into the sun.

Although haze measurements are made most commonly by the use of a hazemeter, a spectrophotometer may be used, provided that it meets the geometric and spectral requirements of Section 5. The use of a spectrophotometer for haze measurement of plastics can provide valuable diagnostic data on the origin of the haze, and Procedure B is devoted to the use of a spectrophotometer.

Procedure A (hazemeter) test values are normally slightly higher and less variable than Procedure B (spectrophotometer) test values.

Regular luminous transmittance is obtained by placing a clear specimen at some distance from the entrance port of the integrating sphere. However, when the specimen is hazy, the total hemispherical luminous transmittance must be measured by placing the specimen at the entrance port of the sphere. The measured total hemispherical luminous transmittance will be greater than the regular luminous transmittance, depending on the optical properties of the sample. With this test method, the specimen is necessarily placed at the entrance port of the sphere in order to measure haze and total hemispherical luminous transmittance.

Haze data representative of the material may be obtained by avoiding heterogeneous surface or internal defects not characteristic of the material.

Haze and luminous-transmittance data are especially useful for quality control and specification purposes.

Before proceeding with this test method, reference should be made to the specification of the material being tested. Any test specimen preparation, conditioning, dimensions, or testing parameters, or combination thereof, covered in the materials specification shall take precedence over those mentioned in this test method. If there are no material specifications, then the default conditions apply.

1. Scope

1.1 This test method covers the evaluation of specific light-transmitting and wide-angle-light-scattering properties of planar sections of materials such as essentially transparent plastic. Two procedures are provided for the measurement of luminous transmittance and haze. Procedure A uses a hazemeter as described in Section 5 and Procedure B uses a spectrophotometer as described in Section 8. Material having a haze value greater than 30 % is considered diffusing and should be tested in accordance with Practice E 167.

1.2 The values stated in SI units are to be regarded as standard.

Note 1—For greater discrimination among materials that scatter a high percent of light within a narrow forward angle, such as is the case with abraded transparent plastics, adjust the hazemeter and perform measurements in accordance with Test Method D 1044.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

Note 2—This test method is not equivalent to ISO 13468–1 and ISO/DIS 14782.

2. Referenced Documents

D1044 Test Method for Resistance of Transparent Plastics to Surface Abrasion
D1898 Practice for Sampling of Plastics
D618 Practice for Conditioning Plastics for Testing
D883 Terminology Relating to Plastics
E167 Practice for Goniophotometry of Objects and Materials
E259 Practice for Preparation of Pressed Powder White Reflectance Factor Transfer Standards for Hemispherical and Bi-Directional Geometries
E284 Terminology of Appearance
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
ISO 13468-1 Plastics-Determination of the Total Luminous Transmittance of Transparent Materials
ISO/DIS 14782 Plastics-Determination of Haze of Transparent Materials

 

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.  

 

 

A problem with adhesion to CPP containing additives

Just a few comments about polymer web blocking first.  Polymer films block (stick together and are hard to unwind) if the surfaces are too smooth and the contact area is too high and this blocking is worse and more easily achieved when the web is wound in vacuum where there is no air entrained between the polymer layers.  This polymer web can also be hard to wind through converting machines too and this can relate to the coefficient of friction of the polymer.  So it is common to find polymer films with a variety of additives to make the handling easier and to reduce the possibility of blocking.  Additives used can include fillers to change the surface roughness as well as slip agents that are low molecular weight materials that can migrate out of the bulk polymer to the surface and reduce the coefficient of friction. Often adding fillers to increase the surface roughness cannot reduce the coefficient of friction enough to make handling as easy as the customer’s desire and so a combination of fillers and slip agents are often used in combination. The fillers increase the surface roughness and reduce the contact area which reduces the friction and the slip agents reduce the surface energy making the surface increasingly slippy which can cause a problem in obtaining high adhesion for coatings.

 

 

As the additives can cause a problem with adhesion.  Even if the CPP is pre-treated such as by corona treatment this can still be a problem. If any pre-treatment is done before the substrate reaches the vacuum system then when the roll is re-wound the slip agent present on the reverse surface can transfer to the front surface and negate the effect of the pre-treatment. If you are plasma treating the substrate in vacuum then it may be possible to only treat the surface to be metallized and so improve the metal adhesion.  Any plasma treatment needs to be customised for your particular polymer surface.  Too little plasma treatment may improve the adhesion but not maximise the adhesion whereas too much plasma treatment will cause a fall in maximum adhesion because of too much chain scission and the production of short chain carbon species that form a weak boundary layer.

 

If the web has been pre-treated the storage time and conditions between treatment and metallization is important.  The longer and higher the storage temperature the more slip agent will have returned to re-contaminate the surface and so reduce the adhesion.

 

Note the slip agent will also contaminate the metallized surface as the metal surface is re-wound against the polymer back surface.  The low molecular weight slip agent is in contact with the very high surface energy freshly metallized surface and the surface energy tries to come to equilibrium by transferring some of the low surface energy mobile slip agent across to reduce the energy of the metallized surface.

 

The erucamide slip agent is designed to migrate to the surface and even if removed there is still plenty more within the polymer bulk that will migrate and repopulate the surfaces.  The antistatic and antiblocking agents are also likely to be able to migrate to the surface which would suggest they are relatively low molecular weight and are likely to have the same effect as the slip agent in terms of affecting the adhesion.  These different additives will have different chemical compositions and so the precise chemical composition of the polymer surface will depend on these materials and the quantities of each that have been added to the polymer.  Thus if you change supplier it is possible that each supplier will add different levels and proportions of each additive and so what is the optimal plasma treatment for the web from one supplier may not be optimised for material from another supplier even though they are meant to be equivalent grades.

 

I hope this helps.

 

 

 

Question about source to substrate distance

dear sir,  please giving the answer .
In the new metalliser I’m listen there is moving source of evaporation for drum distance we can change according to the product specification . is this system is successful or not?
please reply or thanks in advance.

 

Answer

In general the source to substrate distance is set at a distance to optimise the collection efficiency of the evaporant onto the drum at the deposition pressure.

Some machines do allow you to alter the source height. If you move the source closer to the deposition drum the evaporant will deposit onto a shorter distance and so the heat load will be higher.  Also as the boat separation is the same the thickness variation is likely to be larger than at the set source to substrate distance. If you move the source further away the evaporant will deposit over a larger area and so the evaporation shields will need to be repositioned. It is likely that the yield may fall as the collection efficiency may fall. The heat load will be reduced as the depositing material is spread over a larger area as well as possibly a slightly lower flux.  If the source to substrate distance is altered then any uniformity shields will need to be changed to compensate.

 

Unless you have a very specific reason for altering the source to substrate distance it always appears, to me, like a lot of work for little advantage.

 

I hope this answers your question.

Question re. Resistance heated aluminium evaporation boats

Question

In the evaporation boats certain manufactures use MOC Value and others use resistivity values. Is there a relationship between them? Also Is there a relationship between boats resistivity/MOC values and boats life?

 

Answer

Just to clarify MOC refers to micro-ohm cm and is the working temperature resistivity.

The boat behaves as a metallic resistor, i.e. its resistance increases as the temperature rises. Therefore, it is additionally necessary to know the ratio between resistance at operating (high) temperature (R_HT) and the resistance at room temperature (R_RT) in order to accurately characterize the electrical properties. For ceramic boats, the ratio R _HT /R _Rt is between 2.3 and 6.0 and will depend on the chemical composition as well as the powder size distribution for the sintered powder compact which will influence the proportion of grain boundaries.  Grain boundaries are areas of high resistance and hence if the grain size is different between different boats because of different processing the room temperature resistance will be different but also the ratio to high temperature resistance may also be different too.

 

If you look in the AIMCAL Metallizing Technical Reference 4^th Edn. Page 80 there is a table and a graph of the R_HT for different boat operating temperatures showing the expected service life in hours.  It shows that the service life decreases if the boats are driven harder by increasing the boat temperature.  The high temperature resistivity changes with time which is to be expected as there is erosion of the boat with time. This causes the cross sectional area to change. In addition the chemical composition changes because of some materials being leached by the molten aluminium preferentially. As the different components have a different individual resistivity the varying composition leads to varying total resistivity.

This Metallizing Technical Reference 4^th Edn. is available directly from AIMCAL at www.aimcal.org 

 

I hope this answers your question.

Roller Performance Workshop

Another offering from AIMCAL in their Converting School programme is due soon.  If you want to participate go to www.convertingschool.com  or www.aimcal.org

 

Roller Performance Workshop
Timothy J. Walker
Dr. Kevin A. Cole

June 15 - 16
Rochester, NY

Hotel: Extended StayAmerica Rochester - Greece
Room Rate: $64.99

Description:

Abstract: The goal of this workshop is to present techniques to reduce tension variations, slip, and scratching associated with poor roller performance. This workshop will provide one-of-a-kind training in web handling technology where equipment demonstrations are an integral part of the learning process.   Instructors: The workshop will be presented by Timothy J. Walker of TJWalker & Associates and Dr. Kevin A. Cole of Optimation Technology, Inc. Mr. Walker, CEO of TJWalker & Associates, has a BS in Mechanical Engineering from the University of Iowa and a MS in Management of Technology from the University of Minnesota. Nationally recognized as a leader in web technology, he developed his experience through progressively responsible positions at 3M, culminating in his role as Corporate Web Handling Technology Leader. Dr. Cole is a Senior Web Handling Development Engineer with a PhD in Mechanical Engineering from the University of Rochester. He is a corporate expert in web conveyance and winding. In addition to his many patents, Dr. Cole is a welcomed keynote presenter and published author for various trades journals and web handling conferences. Who will benefit from this workshop? This workshop is directed towards the technical person responsible for roller conveyance and winding process understanding relating to operational excellence from the perspective of the interaction between the machine components, systems and web features. Personnel responsible for existing and new machines and process performance will especially benefit from the combination of class room training reinforced by laboratory experiments and demonstrations. These would include process engineers, capital engineers, mechanical and electrical maintenance personnel, and anyone else with an interest in how conveyance and winding processes can be characterized and optimized. The only prerequisite is an aptitude for technology with a pragmatic interest in the linkage between theory and practice.   BONUS: Test your web on your rollers: For the ultimate hands-on experience, arrange to evaluate your webs and your rollers during the workshop.   Roller Conveyance Training: Roller conveyance introduction; purpose of roller conveyance Physical properties important to roller conveyance Right angle rule of roller tracking Roller traction Factors influencing traction Roller surfaces Traction measuring techniques Traction prediction techniques Roller Traction/Tension Measurement: Demonstrated on two development conveyance machines operating in both endless band and unwinder/winder modes Measuring bearing drag Measuring web-roller traction Measuring traction as speed-to-tension ratio increases Measuring increased traction with surface roughness and grooving Calibrating load-cell rollers Measuring float roller hysteresis

 

Elastomer spreader rolls

I have had a few questions recently regarding wrinkles and aside from the usual comments on the importance of adequate web cooling and tension control the item that seemed to get overlooked most often was the aging of the elastomer spreader rolls.

Elastomer rolls age for a number of reasons.  The most common are due to a slow loss of volatiles within the elastomer composition. This may be at a very low rate but simply being in vacuum some will lose some content and this can result in the roll slowly hardening, possibly cracking and losing performance.  If the roll is heated the heat can accelerate any volatile losses.   This may still be slow and may take many months to become noticeable. If there is a plasma the ultraviolet light from the plasma can damage the elastomer and cause it to degrade also hardening the elastomer. Another possible cause of elastomer degradation is the use of solvents to clean the elastomer.  If the wrong solvent is used this can react with the elastomer and damage it.

It is easy to overlook these changes because they are so slow to occur and visually there may be nothing that draws attention to the change.  It is worth taking a measurement of the hardness of the elastomer when it is new and periodically checking the value.  As the elastomer hardens it will no longer deform as easily and thus will not spread the web as much as when it was new and soft.  If the web is not spread as much before it meets the deposition drum it will take less heat to reach the point where wrinkles start. Hence for some materials that could be run without wrinkling they progressively become more difficult to run even though the process and materials are the same. 

Transparent conducting coatings

The most widely used transparent conducting oxide is probably indium tin oxide. However this is expected to change in the future.  There are fears that the supply of indium is limited and so will become a bottleneck for many applications as well as becoming cost prohibitive. The result of this is that there has been a renewed interest in some of the alternative materials such as doped tin oxide, doped zinc oxide as well as newer materials such as the new class of transparent conductors based on doped titania.  Niobium and tantalum doped titania have been deposited onto wafers or glass substrates and shown reasonable conductivity.  It appears that anatase titania is preferred to rutile titania.  I have not yet seen any reports of these materials being deposited roll to roll onto flexible substrates where the deposition temperature tends to be more limited than on the rigid substrates. 

The development of transparent conducting materials also includes some combined material solutions.  Using one of the less conducting transparent conductors for widely available materials can be enhanced by depositing a multilayer of transparent conducting layer, very thin transparent silver (or gold) and a second transparent conducting layer.  The transparent conducting layer can be optimised as an antireflecting layer for the thin silver and so the transmittance in the visible can be maximised.   An alternative approach is to deposit or print very fine high conducting parallel tracks over the moderate transparent conducting layer. Although the fine tracks may be opaque the overall effect is of a transparent coating.  This has been done before where for heated windscreens in cars a very fine wire was laminated between glass layers and drivers often were not even aware of the fine wire being present.  This part-printed material has been demonstrated for backlit displays.

Also continuing to demonstrate improvements are the organic transparent conductors which have been used in simple all organic displays.  New dopants are being tried to improve the conductivity of the basic organic materials.

Although indium tin oxide will remain the easiest starting material for many development programmes many will switch to the newer materials as the alternative materials start to increase in availability.