Vacuum Web Coating

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

 

Quality issues – adhesion, substrates, cleaning – AIMCAL Fall Conference preview

            There are always quality issues that are important to everyone and at this coming AIMCAL Fall Conference were have some papers that will address some of these issues.

Adhesion is always a problem that we get lots of questions about and one of the most basic tests often appears in the discussions.  The ‘tape’ test. This test is cheap and easy to do and some at some time or other we have all probably used it and might even do so on a regular basis.  However this test is not really a test of adhesion but is a test of poor adhesion.  If the coating fails it shows that the adhesion strength is low.  BUT it the coating passes it DOES NOT mean that the coating has high adhesion. All it means is that it is better than the tape adhesive. This can mean that it is slightly better or considerably better but we do not know which.  The tape test can also be variable with it being sensitive to the operator doing the test, the methodology in carrying out the test, the quality and age of the tape, the humidity and many other variables.  Thus as a test it has a limited use but because it is so simple and cheap it is still by far and away the most widely used test for adhesion.  Don McClure will present a paper that describes the problems in using this test and warning of placing to much reliance on it and also what can be done to improve the quality of the test.

            Another of the perennial problems is that of pinholes.  Ever since work done back in the 1970’s at Cambridge University it has been known that debris left on the surface of the polymer films before they are metallized are the largest, but not the only, source of pinholes.  Despite this here we are, close to thirty years on, still tending to ignore this fact and using film that has not been cleaned and still complaining of pinholes in the coatings.   Sheila Hamilton of Teknek will be talking about cleaning surfaces using their tacky roll technology. This is where a tacky roll contacts the polymer surface and any loosely bound material is removed by becoming stuck to the tacky roll. As there is a lot of this loosely bound material the tacky roll can become clogged very quickly and so this roll contacts a higher tack roll to remove the debris and keep the film contact roll fresh. There are various elastomers available to control the level of tack, including being able to use rolls inside vacuum systems.  This technology is not only useful for reducing pinholes in vacuum deposited coatings but is also similarly useful for the ‘wet’ coating side of the converting industry and also were coatings are patterned, such as by laser scribing, can be cleaned after before further downstream processing.  Once the debris has been removed it can be looked at under a microscope and it can be possible to characterise some of the common sources of debris and the typical size range.

            To compliment these two papers there is also on from Mark Maloney of Oxford University that describes what could be described as a universal pre-treatment that can be used to graft a more active polymer onto the polymer substrate surface to promote adhesion.  This too is a technology that is equally applicable to the ‘wet’ as well as the vacuum coated products. 

Number of pinholes per unit area.

Is there a standard acceptable amount of pinholes per sq metre in metallized BOPP.

Also do you know anything about the possibility of 'welding' pinholes in metallized film by use of an electric charge?

Answer

As far as I am aware there is no standard that defines the grade of the film by the number of pinholes per unit area.  There are some figures quoted by aluminium foil manufacturers about the number of pinholes per unit are for foil and so the idea of counting pinholes is not unknown.  Possibly the reason for not bothering to count pinholes is that the barrier performance is not only affected by the number of pinholes but also by the size of the pinholes.  Thus a simple count of number of pinholes would not necessarily tell you what the barrier performance would be and films with the same number of pinholes could be radically different if one had large holes and the small ones.

Individual customers may have something in their acceptance specification about number of pinholes. This may be highlighted if the film is for an optical or graphic purpose but where it is for a packaging application the barrier performance will usually take precedent. Here the allowable number of pinholes tends to be either determined by the barrier performance as measured on a Mocon or similar instrument or something that is agreed between the metallizer and customer. 

There was a paper by Angelo Yializis that includes a bit about healing capacitor films.   If memory serves me correctly this is more about preventing shorting between layers than sealing pinholes.  If successive layers of metallized film are shorting out it is sometimes possible to pass a current through the film such that the current is too great for the short and so vaporising the metal and breaking the circuit at this point thus removing the short and recovering the full performance of the capacitor.

If you were thinking about closing the pinholes by welding a little bit of aluminium over the holes to block it up then 'no' I do not know of any process doing this.  If a fully opaque film is required then the usual approach is to double side metallize the film. The philosophy behind this is that the chance of two pinholes lining up to still show as a pinhole is negligible.  Double side metallizing is also used for high barrier films. Again the fact that holes do not line up means that and gas or moisture will have a tortuous path to diffuse through the film and hence the barrier improves.

Pinholes, cracks and barrier performance.

Are there typical industrial standards and referenced test methods for pinholes and gaps in metallized coatings on heat sealable multilayer barrier laminates? 

What is likely to cause gapped streaks (~ 1/4 inch wide) in the aluminium coating in such films?  (A supplier recently switched to a new bag which exhibited MVTR problems traced to gaps and pinholes).

Answer

As far as I am aware there is no standard for the number and size of pinholes per unit area.  I have searched for standards and found the following comments relating to pinholes in metal foils for blister packs.

Blood Compatible Materials and Devices: Perspectives Towards the ... –

by Chandra P. Sharma, Michael Szycher - 1991 - Medical

Therefore, although no standards exist which control the frequency or size of pinholes that can be tolerated in packaging materials, it is important to ...

books.google.com/books?isbn=0877627339...

Industrial Sterilization - by G. Briggs Phillips, William S. Miller - 1973 - Medical - 440 pages

Therefore, although no standards exist which control the frequency or size of pinholes that can be tolerated in packaging materials, it is important to ...

books.google.com/books?isbn=0822302993...

Admittedly these are no old references but I have not found anything to suggest there are now standards to work to.

There are some ASTM Test methods but these relate more to the testing to evaluate the level of leaks and they do not set any standard.

Eg. ACTIVE STANDARD: ASTM F2227-02(2007) Standard Test Method for Non-Destructive Detection of Leaks in Non-sealed and Empty Medical Packaging Trays by CO2 Tracer Gas Method

ACTIVE STANDARD: ASTM F2228-02(2007) Standard Test Method for Non-Destructive Detection of Leaks in Medical Packaging Which Incorporates Porous Barrier Material by CO2 Tracer Gas Method

Again these two are directed at medical packaging and are more aligned to metal foils and foil laminates for blister packs than for metallized laminates.

In metal foils pinholes would appear to mean larger diameter holes than pinholes seen in metallized films.  There are a couple of papers ‘Examining Defects of Various Sizes in Device Packages’ a report by Assistant Professor Laura Bix , Ondrea Kassarjian, Ronald A Iwaszkiewicz and Jane E Severin    Michigan State University (MSU)  & The Impact of Foil Pinholes and Flex Cracks on the Moisture and Oxygen Barrier of Flexible Packaging  By Lee Murray  Alcan Packaging, Neenah Technical Center  (as located by ‘Jimmy’ in his comment on pinholes) whereby in each of these papers they use lasers to mimic holes in foils.  This is done to compare what barrier performance should be achievable and the barrier performance that is being achieved in practice and then to suggest why there is a difference.  What is surprising is that the size of the holes they make are around 50 microns in size although the images they show of real pinholes in foils are closer to 10 microns or less. In this second paper they list the number of pinholes achievable in foil products they sell.  I have looked at other foil manufacturers and they have similar levels of pinhole numbers although none appear to define the diameter of the pinholes.  Thus from one manufacturer the following was in their published literature.

Aluminium foils sales literature for use in 'blister' packaging applications

Pin Holes : 0.025 and 0.03 mm foils are free from pin holes.

In 0.02 mm foil, 40% of foil is free from pin holes, rest less than 10 PSM (pinholes per sq m)

So relatively low numbers at 10 pinholes per sq m but these could be big pinholes or little ones and the difference in barrier performance could be huge.

Enough of foil if we turn to metallized film there does not appear to be any standards for this either and again there are only the standard test methods published for establishing the barrier performance of polymers, metallized polymers or laminates.

As far as testing is concerned a popular method is to use a photographic light box (a number of lights in a box with a translucent top plate to give a bright diffused back light) to put the metallized film on and to then simply count the bright spots.  This can sometimes be modified to using a cardboard sheet with a square of specific size to overlay on the film to then give a number of pinholes for a particular area.

This method only gives a value for the number of pinholes but no information about the size distribution.

Pinholes are also not always perfect in that they are not a cylindrical tube cut through the metal coating.  If you imaging the debris on the surface to be a sphere it will shadow the depositing metal flux differently as the web approaches, moves through and departs the deposition zone. Thus the pinhole with have a graded edge and if the debris falls off during the deposition it will have some coating across the surface but just have a different optical density to the full coating.  Thus there will always be some error in counting pinholes because of the brightness of the light passing through. The fully bright spots are easy to count but there will always be some that are questioned as to whether to include them or not.

It is possible to automate this process by using Image Analysis where a digital image of the area is taken and the contrast difference is used to distinguish the pinholes.  This does require setting thresholds for the contrast difference and so again will have an error.

By converting the captured image into a high contrast black and white image it is also possible to take the size of each pinhole. This too will have errors as the light tends to flare and so most pinholes will be sized too large but this is something that can be compensated for.

It is also possible to use microscopy to look for pinholes. This can use transmission microscopy (as per the light box) and the option here is to use different magnifications to look for the smaller pinholes.  Again the Image analysis can be used to count and size the pinholes.

It will depend on your applications as to what level of barrier you are requiring. This will determine how much of a problem you have with pinholes and how much effort you are having to spend on cleaning the web and how detailed the hygiene requirements are for the cleaning and management of the winding and deposition systems.  This will also determine how detail you need to be in evaluation the level of pinholes. It is obviously cheaper and quicker to use a light box with a cardboard template than it is use a microscope and image analysis system.

Pinholes and Gaps.

Pinholes are generated by several possible means. The most common being dust/debris on the surface that is metallized and then the pinhole appears when the debris is subsequently removed. But this is not the only method of producing pinholes. Spitting during the metallization process where crud from the molten pool may be ejected and damage the coating or web and result in a pinhole.  The pressure from fillers from the back surface of the web pressed hard onto the metallized coating can pick off the metal, particularly if the metal is not well adhered.

Looking at the shape and the edges of the pinholes can sometimes provide information about the source of the pinholes. Pick off is likely to have sharp edges whereas the shadowing of the debris is likely to have a graded edge and the spitting source is more likely to have evidence of the hot material landing on the polymer surface, such as a crater and maybe even the crud still in place.

Where there is debris on the surface it can be removed in two ways. Firstly it can simply drop off or rolled away leaving behind the pinhole. Secondly it can be pushed along the surface where if it slips on the metallized surface may leave a track from the pinhole in the direction of sliding. If the debris is sharp, or hard and the pressure high enough, the debris may scratch the metal and remove some of all of the metal producing a surface scratch. Again the direction of this damage can be useful as it can be an indicator of the quality of the winding.  If the scratching is in the machine direction then at least the winding is aligned. If the scratches are angled to the machine direction then it is an indication that the web is slipping sideways at some point in the winding process, which is adding further damage to the film besides the simple pinholes.

These scratches can be one possible start to the 'gaps' referred to.

Generally I do not hear any reference to gaps. What is common is that metallized (or transparent barrier coatings too) decrease in barrier performance when they are made into finished packages. Often there are specifications to the amount of loss that is permissible (i.e. no more than a 2x increase).

In evaluating coatings one of the tests used is the tensile test. If this is done under a microscope it is possible to watch the onset of cracking of the coating and the increase in cracking with increasing strain.  The onset and amount of cracking is dependent on the adhesion between the coating and the substrate. If the adhesion is poor less stress can be transmitted through the interface between the coating and substrate and so the point where the system has to find a way of relieving the stress is lower and cracking starts earlier.  The aluminium metal is less elastic than the polymer and it is only the fact that the coating is so thin and the adhesion reasonably good that the coating can deform without too much damage during packaging.  If the material is laminated the metal coating can also be near the neutral axis that may also help.

However where the package includes dead folds or creases there are likely to be regions of high strain that will lead to cracking of the coating and gaps will appear in these regions. If you then look at where these cracks might appear a good starting point will be areas of existing damage. Thus a pinhole with a scratch, particularly if the orientation is aligned, may already provide a gap that just expends and widens.

Thus if I were to have to look for areas of poor barrier I would look to the final package and look around the areas where the web has most likely been deformed the greatest during the packaging process.  This may also include temporary stresses, such as if the neck of a pouch were stretched open during filling and the stretch may have been too much for the type and thickness of the polymer and thus putting a permanent deformation into the film. Heat-sealing often uses both heat and pressure and can cause some film distortion. I would also test the metal adhesion to make sure the adhesion was being optimised. If the adhesion is lower than optimum I would check to see if the barrier improved on the existing package when the adhesion was improved to the optimum.

For the pinholes, if the cost and specification justifies it I would add a tacky roll cleaning process for both the front and back surfaces immediately before the rewind before the film enters the metallization process including a positive pressure filtered hood to prevent recontamination. Or consider how I could include the tacky roll inside the vacuum deposition process before a plasma pre-treatment. I know that a couple of companies have done this but it has been for much higher technology applications were the running speeds were lower and roll lengths shorter and so I am not sure the tacky roll technology is yet suitable for inclusion in vacuum for metallizing speeds and lengths.    

I hope these thoughts help.

Checking the resistivity of lacquered film.

Request: How do I check the Surface resistivity of 23-micron metallised and
both side lacquered coated film?

Answer.

The conductivity or resistivity of a metallized film does not require the film to be in intimate contact with any probes. Using an eddy current monitoring system the resistivity can be measured. The process uses a signal coil and a receiver coil, these can either be positioned one either side of the film or both concentrically on the same side of the film. The signal coil has a current passed though the coil that has a magnetic field that can induce a voltage in the metallised layer, which in turn can be detected by the receiver coil.  These coils have to be designed to be used for particular conductivity ranges and if they are used for coatings outside the designed range the system will lose some sensitivity.  This should not be a problem, as generally the system will read well for a resistivity over approximately 2 - 3 orders of magnitude.

I suspect that you may find that this type of system is used within your metallizers as part of the feed back control for getting the deposition from each metallizing source uniform.

If this is so then you can always check that the lacquered material resistivity is similar to when it was metallized.

Counting pinholes & type of illumination

1.You mentioned that we can count the pinholes by using a lightbox and counting method. We have tried to check for pinholes using a dark room and by putting the VM sample against a 2,000,000 candlepower lamp and were able to detect these pinholes. However, our supplier just uses a light box with 4 fluorescent lamps and a glass top. With this kind of light box, pinholes cannot be detected. We also counted pinholes with an area of 17 cm x17 cm and then just converted it to count per square meter. Is that the proper was to count pinholes? I would also like to ask what is the standard or specifications of the lightbox and what counting method should we use, what is the procedure?

Answer.

As far as I am aware there is not a standard for the measurement of pinholes.  As such everybody makes or buys their own light box and decides on the area for measurement and works accordingly.

In theory the number of pinholes should be the independent of the light source but this is not true. This is because high intensity lights will make areas that have had some
coating but are of reduced optical density appears as pinholes that with lower intensity illumination are not classes as pinholes.

If you consider how pinholes are produced you will see how the various shapes and types of pinholes are created.  The first and generally by far the largest cause of pinholes is the dust/debris on the surface of the film that is present during the
metallization process.  It is impossible, even with the best cleaning techniques, to remove debris of less than 0.3 microns in size due to the Van der Waals forces present. If you consider that this debris of up to 300nm size is considerably larger than the thickness of the aluminium coating that is generally around the 15nm - 25nm type of thickness.  If the film has not been cleaned, which is often the case; there will be plenty of debris that is in the range 1-10 microns, considerably larger than the coating
thickness.  If any of this debris is moved following deposition it will leave behind and area that is not coated with aluminium that is seen as a pinhole.  If any of the debris is moved, or drops off, during deposition there may be a reduced deposition at that point that may not be classed as a pinhole with a low illumination light but will be with a high intensity light.

The second major source of pinholes is due to 'pick-off'. This is where the adhesion may be lower that ideal between the aluminium and the substrate.  Usually the substrate is either through filled or is a coextruded film with the back surface layer filled. The filler is used to reduce the coefficient of friction by allowing the filler to roughen the polymer surface. These peaks where the filler pushes out the polymer surface will be in hard contact with the freshly metallized front surface as the film is rewound. As the winding is in vacuum it will always be a hard wound roll. The high peaks press hardest against the coating and in some cases will stick to the coating batter that the aluminium adheres to the front surface of the substrate and so when the roll is next unwound the aluminium is picked off and remains on the back surface leaving a pinhole in the metallized film.  This type of pinhole can generally be reduced in number by the appropriate optimised plasma treatment that gives the maximum aluminium adhesion of the metal to the front surface.

A source of thinner coating spots that with high intensity lights might be classed as pinholes can be created in the following manner. Polymer films are not 100%
polymerised and will often have a proportion of short chain oligomer that is mobile and this can migrate to the surface. This oligomer is low molecular weight and may also reduce the surface energy. In other materials it is common to add a slip agent, or many other additives, these too may be of low surface energy or low molecular weight.  The slip agents, as their title suggests, are added to improve the film handling characteristics by reducing the coefficient of friction. Even with a filled polymer where the filler reduces the coefficient of friction to some level, slip agents can be added to reduce the friction further.  This oligomer or the additives may have a lower melting point than the polymer and the material may be vaporized during the deposition process locally reducing the sticking coefficient of the aluminium and so locally reducing the thickness of the aluminium deposited.

High intensity sources will also tend to allow smaller diameter pinholes to become bright enough to be included.  I did some work in counting defects and I used and image capture and image analysis to automate the process but this needed to have thresholds set for the minimum contrast difference that would count as a pinhole. Thus there is a margin of interpretation between what is a pinhole and what is not. This will change with intensity of illumination as the sensitivity on small diameter pinholes is improved.  It can also mean that the diameter of the larger pinholes is over estimated with high intensity lights as the detector can become saturated and there will be more of the light diffraction included in the measurement.

Hence suppliers may wish to use a lower intensity illumination because it will reduce the number and size of the pinholes counted. End users may wish to have higher intensity illumination because this gives a more realistic, if more frightening, count of the pinholes present.   At some point an agreement and a correlation between the supplier and customer measurement equipment and technique must be made. Otherwise there will be an ongoing battle over the number of pinholes and the film quality.

How best to check crystal size or nucleation site density.

In an earlier answer you mentioned that changing the surface energy would change the wetting of the aluminium and change the nucleation site density and height of the nuclei. How would you characterize (analytical technique) the surface of the metalized polyester nucleation (island vs flat)?

Answer.

The simplest method of looking at the coating is to use a scanning electron microscope (SEM). This would allow you to look at the coating using high magnification.  This normally looks at the surface and the coating surface can be compared to the substrate surface.  If the coating is still non-conducting there may be a problem with the surface charging and so it may need a thin carbon coating to allow high quality images to be obtained.

Alternatively, for continuous coatings, if the coating is cut or the substrate folded the coating will have fractured and may then be possible to then look at the coating in cross section. This can let you look at any grain structure that is forming, any possible voids and grain sizes. 

It can often be difficult to interpret structures and this can be helped if you are comparing coatings, as then it can be easier to see the differences.

There can be problems in finding a suitable edge to look at the cross section, cutting can sometimes smear the metal at the point of cutting and folding may not produce a suitable fracture or distort the metal.  If this is the case then using a microtome to produce thin slivers of substrate with coating can be done.  This requires potting the sample in a polymer (usually some type of acrylic) and then using a glass or diamond knife to pare off thin slivers of material. If the polymer continues to distort then freezing the sample in liquid nitrogen and microtoming off thin slivers can be done, but the cold and fine nature of the work makes this a time consuming process.  The best of these samples can be looked at using transmission electron microscopy (TEM) and it may also be possible to obtain some structural information by diffraction techniques.

This too can be difficult to interpret as the work in cutting the slivers can produce fractures within the coating and it can be hard to determine if the cracking was native to the coating or produce in producing the sample.

It is possible to infer some information about the structure by comparing the coating thickness, conductivity, Optical Density (OD) or transmission and surface roughness.

Using a Talystep or other similar surface roughness or step height measuring technique the coating thickness and surface roughness of different samples can be compared.  For samples of the same thickness if the other measurements (resistivity or transmittance) are different it suggests structural differences.  For samples of the same OD or resistivity but different thickness again structural differences can be inferred.

If you want to identify why there are structural differences then it is worth going back to the start of the process and checking the surface energy of the substrate. If you then want to see what effect a change of surface energy has on the coating nucleation then you need to restrict the amount of deposition so that the coating thickness is around 3nm - 4nm so that you can then examine the surface using the SEM and count the number of nucleation sites per unit area and compare the number and size of nuclei.  If you are trying to do this on a standard metallizer then it may be worth stopping the wire feed whilst the shutter is still open and allowing the deposition rate to reduce. This will provide you with samples from the full thickness down to zero but it will have cost you many tens of meters of film but at least it will be under reasonably similar deposition conditions.  This would need to be done with or without surface treatment to give an idea of how you are changing the nucleation site density and hence the subsequent growth.

Hopefully this gives you a flavour for what can be done.

Counting pinholes

You mentioned that we could count the pinholes by using a light box and counting method. We have tried to check for pinholes using a dark room and by putting the VM sample against a 2,000,000-candle power lamp and were able to detect these pinholes. However, our supplier just uses a light box with 4 fluorescent lamps and a glass top. With this kind of light box, pinholes cannot be detected. We also counted pinholes with an area of 17 cm x17 cm and then just converted it to count per square meter. Is that the proper was to count pinholes? I would also like to ask what is the standard or specifications of the light box and what counting method should we use, what is the procedure?

Answer

As far as I am aware there is not a standard for the measurement of pinholes.  As such everybody makes or buys their own light box and decides on the area for measurement and works accordingly.

In theory the number of pinholes should be the independent of the light source but this is not true. This is because high intensity lights will make areas that have had some coating but are of reduced optical density appear as pinholes that with lower intensity illumination are not classes as pinholes.

If you consider how pinholes are produced you will see how the various shapes and types of pinholes are created. The first and generally by far the largest cause of pinholes is the dust/debris on the surface of the film that is present during the metallization process.  It is impossible, even with the best cleaning techniques, to remove debris of less than 0.3 microns in size due to the Van der Waals forces present. If you consider that this debris of up to 300nm size is considerably larger than the thickness of the aluminium coating that is generally around the 15nm - 25nm type of thickness.  If the film has not been cleaned, which is often the case; there will be plenty of debris that is in the range 1-10 microns, considerably larger than the coating thickness.  If any of this debris is moved following deposition it will leave behind and area that is not coated with aluminium, which is seen as a pinhole.  If any of the debris is moved, or drops off, during deposition there may be a reduced deposition at that point, which may not be classed as a pinhole with a low illumination light but will be with a high intensity light.

The second major source of pinholes is due to 'pick-off'. This is where the adhesion may be lower that ideal between the aluminium and the substrate. Usually the substrate is either through filled or is a coextruded film with the back surface layer filled. The filler is used to reduce the coefficient of friction by allowing the filler to roughen the polymer surface. These peaks where the filler pushes out the polymer surface will be in hard contact with the freshly metallized front surface as the film is rewound. As the winding is in vacuum it will always be a hard wound roll. The high peaks press hardest against the coating and in some cases will stick to the coating batter that the aluminium adheres to the front surface of the substrate and so when the roll is next unwound the aluminium is picked off and remains on the back surface leaving a pinhole in the metallized film. This type of pinhole can generally be reduced in number by the appropriate optimised plasma treatment that gives the maximum aluminium adhesion of the metal to the front surface.

A source of thinner coating spots, which under high intensity illumination might be classed as pinholes, could have been created in the following way. Polymer films are not 100% polymerised and will often have a proportion of short chain oligomer that is mobile and this can migrate to the surface. This oligomer is low molecular weight and may also reduce the surface energy. In other materials it is common to add a slip agent, or many other additives, these too may be of low surface energy or low molecular weight.  The slip agents, as their title suggests, are added to improve the film handling characteristics by reducing the coefficient of friction. Even with a filled polymer where the filler reduces the coefficient of friction to some level, slip agents can be added to reduce the friction further.  This oligomer or the additives may have a lower melting point than the polymer and the material may be vaporized during the deposition process locally reducing the sticking coefficient of the aluminium and so locally reducing the thickness of the aluminium deposited.

High intensity sources will also tend to allow smaller diameter pinholes to become bright enough to be included. I did some work in counting defects and I used and image capture and image analysis to automate the process but this needed to have thresholds set for the minimum contrast difference that would count as a pinhole. Thus there is a margin of interpretation between what is a pinhole and what is not. This will change with intensity of illumination as the sensitivity on small diameter pinholes is improved.  It can also mean that the diameter of the larger pinholes is over estimated with high intensity lights as the detector can become saturated and there will be more of the light diffraction included in the measurement.

Hence suppliers may wish to use a lower intensity illumination because it will reduce the number and size of the pinholes counted. End users may wish to have higher intensity illumination because this gives a more realistic, if more frightening, count of the pinholes present.   At some point an agreement and a correlation between the supplier and customer measurement equipment and technique must be made. Otherwise there will be an ongoing battle over the number of pinholes and the film quality.

Optical Density and barrier performance.

Question

'Will coated PET, treated PET, or plain PET metallized to same OD show
different MVTR & OTR?'

Answer
The answer to this is yes there will be differences.
The structure of the metallized coating at a microscopic level will be different for un-treated and treated PET. The wetting of the aluminium will be different and so the adhesion porosity will differ. Thus to get to the same OD the metal thickness are likely to be different.

Thus it is always very hard to replicate someone else's process and produce the same performance film. The source of the film can be different, as too can the debris level leading to differences in pinhole (pin window) size and distribution, as well as humidity levels, etc..

All of these factors can affect the metallization as well as the lamination process.
The samples quoted in the graph were probably produced in either the UK or US and so if nothing else the temperature and humidity of the material from manufacture through to lamination is likely to have been very different to that seen by films produced in more humid climates. Hence I would expect the results to be different.

If you are looking to improve the barrier performance then I would look to the standard problems in the material. Debris levels causing pinholes (pin-windows), surface energy causing poor adhesion which can lead to pick-off and more pinholes as well not delivering the optimum wetting and coating integrity.

The graphs in the AIMCAL Tech.Ref. are meant to be a guide and not a definitive measure.  The trends will be correct but the results would be expected to be different for materials produced from different machines (even in the same factory) let alone sited on different continents.

Is there a relationship between OD and CoF?

                      Is there a relationship between OD and CoF?

ANSWER

Generally polyester films have some low molecular weight material on the surface.  This is most frequently oligomer that is residual material from the polymerisation process. There may also be other materials added that can migrate to the surface, such as slip agents in OPP, that may also reduce the coefficient of friction (CoF). 

   Often the surface of the polymer is treated to change the surface energy to improve the adhesion. This treatment to increase the surface energy can crosslink the low molecular weight material into the bulk polymer or can volatilise the material and remove it. Either way the low surface energy material is removed and the CoF will get worse (increase).   

    Even if there is no surface treatment the metallization itself will cover up the low molecular weight low surface energy surface and again the CoF will increase.

    Thus there is no fixed CoF that specifically relates to a particular Optical Density (OD) but rather the CoF relates to the history of the polymer web and any surface modification and surface treatment that has been carried out.  Pure PET films will readily block and so they are usually have fillers added to provide a controlled surface roughness. The surface roughness reduces the contact area and hence the CoF. The size, shape, type and distribution of these fillers controls the precise value of the CoF.  This is usually higher than is ideal for and easy to handle film. Coupled with the residuals this can make the ease of handling acceptable.

    Sometimes the back surface of the web is not treated and thus there may be the transfer of some of the low molecular weight low surface energy material from the back surface to the front surface once the metallized web has been re-wound.   This can cause a problem in reduced adhesion of any subsequent coatings to the metallized layer but the transferred material may also aid the ease of handling.

            Where a link may exist between OD and CoF is in the metallization nucleation and growth.  Any low molecular weight, low surface energy contaminant on the surface, that helps reduce the CoF, will stop the aluminium wetting the surface well. This will make the aluminium nucleate and grow as a series of hemispherical islands. This will make the coating quite thick before it completely covers the surface. Whereas it the surface has been treated to raise the surface energy and correspondingly the CoF rises the aluminium will wet the surface well and instead of the hemispherical-like growth the islands will be very flat and spread well over the surface this helps make the coating completely cover the film at a lower thickness. Thus the same OD may be achieved at a slightly lower thickness than for a non-wetting surface, lower CoF film.  However there will not be a direct correlation as the CoF can be varied by the filler or surface roughness as well as by any surface contaminant.

Thus for some materials there may appear to be a link between OD and CoF but this link may disappear depending on the film processing or with a change of film supplier and may never be apparent in other materials.