Historically food could be kept for years if they were bottled or canned. The thick glass being impervious to oxygen and moisture was used for foods that could be degraded by either oxygen or moisture or both. Some foods are also degraded by light and these were protected by metal cans. Bottles and cans are rigid and heavy and are bulky to ship to the bottling or canning plant and so lighter weight equivalent materials have been developed of which one of the most widely used is packages that start out as polymer webs.
The polymer webs alone may not have sufficient barrier performance to meet the packaging requirements for shelf life and so improvement have been developed using filler in the polymer or coatings or laminates onto the polymers. As metal and glass have a proven barrier performance then theoretically very thin coatings of these materials should be capable of providing the necessary barrier performance. Unfortunately, there is a gap between the theoretical barrier performance and the practical barrier performance that is achieved. This paper is an attempt to explain why there is this gap and what might be done to reduce the gap.
Polymer webs can be produced by any of several different methods of which the main two methods are the stenter process or the bubble process. The stenter process starts with the extrusion of the thin strip of polymer that is first stretched forwards to give one direction of orientation and then stretched sideways to give the second direction of orientation, making the biaxial oriented film. The stretching process makes the thin strip wider and longer but thinner than the starting strip. The bubble process produces the biaxial orientation differently. In this process a tube of polymer is extruded where the end of the tube is nipped closed and air is pumped up the centre of the tube. This gas inflates the tube and produces the sideways stretch and the winding speed through the nip provides the forward stretch. The final mechanical performance depends on the process parameters and so polymer webs from different suppliers can perform differently even though they nominally meet the same specification.
This leads to an important fact that is often overlooked; polymer webs that are used as an input in vacuum metallizers are a variable and not a constant. All too often it is assumed that every roll will be the same, but this is far from reality. The polymer bulk characteristics such as residual stress, moisture content, crystallinity, thickness profile, web curvature and slit edge quality can all vary and can affect both the winding as well as the coating quality. In addition, the surface characteristics such as the surface chemistry and hence surface energy may vary. Also, the surface contamination both from exudates and particulates may vary. These can affect the coating wetting and adhesion as well affecting the coating growth and quality or being a potential source of pinholes in the coating.
It is difficult to fully polymerise a monomer. As the polymerisation process progresses it becomes more difficult for the remaining monomer to move to attach to the end of the forming polymer chains. The faster the polymerisation the less mobility takes place that results in more residual monomer or oligomer being presenting the polymer film. This oligomer will migrate to the film surfaces and as it is low molecular weight and mobile it can become a problem as it can prevent good adhesion of any coating being applied. The amount of oligomer contamination on the surfaces will depend on the time and temperature between film formation and examining the surface. Higher temperature enables a faster rate of migration of the oligomer from the bulk to the surface. Figure 1 shows an electron micrograph of the surface of a polyester terephthalate (PET) film that has been heated for an hour at a high temperature just to accelerate the process and demonstrate what this type of contamination can look like.
Figure 1. Oligomer growth on PET film
This problem is well known and the surface may be changed to try to combat the problem. This includes atmospheric plasma, corona or flame treatment of which corona is probably the most widely used. Many of these treatments have a limited lifetime as although the surface has been cleaned of the oligomer present at the time there is plenty more oligomer present in the bulk and this will continue to exude onto the surface. Often this process can be monitored by measuring the surface energy. In general, a clean film surface will have a higher surface energy than the oligomer contaminated surface. If we look at Figure 2 we can see a schematic of what can happen to a film surface over time. The rate of these changes can be affected by heating or cooling the film. The plasma treatment will usually be done only to the surface to be coated and then the film is wound up. This means the freshly treated surface will be brought into intimate contact with the back surface that still has plenty of mobile oligomer that can easily transfer from the back surface to the front surface. These two processes mean that the benefit of the treatment has a limited lifetime. Unless you know how old the film is from manufacture and at what temperature it has been stored and if it has been treated when any treatment has been applied and how long since the treatment we intend to coat it.
Figure 2. Polymer film surface energy change with time
Treatments of the surface are designed to either remove or stabilise the oligomer. The bombardment of the oligomer will break bonds and either the carbon can react to produce a gas that can be pumped away or the oligomer may be crosslinked to the film surface and so be made more stable. It is unlikely that all the oligomer will be removed or stabilized. The remaining oligomer may still represent a problem to any vacuum deposition process.
During vacuum deposition such as aluminium metallizing the heat from the deposition sources and the energy from the arriving aluminium atoms when they hit the oligomer may be sufficient to vapourise the low molecular weight oligomer. The vapourising oligomer will prevent the aluminium from nucleating and depositing in that area. If the oligomer is very thin then it may all be vapourised and then some aluminium will deposit in which case that area, when illuminated from behind, will show up as a bright spot, not unlike a pinhole. A pinhole being an area where there is no coating at all, these are sometimes referred to as pin-windows. Where the aluminium coats the whole surface, including over the oligomer, the adhesion of the aluminium will be higher when it bonds directly to the polymer film and will be poor where the aluminium covers the oligomer. When this coated film is wound up and unwound it is possible that more pinholes will appear due to coating pickoff. Pick off being where small areas of metal coating become attached to the back surface of the film in preference to the surface they were coated onto. Many polymer films have fillers included to produce a surface roughness that improves the handling characteristics. Fillers close to the surface or even protruding from the surface get pushed very hard into the freshly metallized coating and can result in pickoff.
Returning to the film substrate the other major contamination is from particulates. Polymer film as it is transported over rollers generates a triboelectric charge and as the polymer is insulating the charge can accumulate. This charges surface can attract particles present in the atmosphere and hold them onto the surface. The extruded film is heated to allow for the biaxial orientation and there are fumes escaping the film surface that contains oligomer which will condense as a white powder. This can fall back onto the film surface as it passes along the process. On the stenter the film edges are trimmed by slitting before winding up the film. The slitting process generates plenty of particulates and, despite the use of vacuum extract around the slitting process, there is always an increase in the particulates on the film following slitting. This means that all films have plenty of particulates present on the surface even as a full width mill roll. The amount of particulates increases each time the web is unwound/rewound such as for slitting into narrower widths or for plasma treating.
The particulates are important as they are a major source of pinholes in the vacuum coated layer. The schematic if Figure 3 shows how a pinhole can be produced in a metallized coating. It is to be noted that an aluminium metallized coating for barrier applications may only be of the order 30nm thick whereas the particulates that remain even after cleaning to remove particulates will be anywhere up to 300nm, ten times bigger than the coating thickness. When the web passes through the deposition zone the surface gets coated, where there are particles on the surface the coating may deposit on the particle. This is fine until the coated surface meets a roller when the particles may be dislodged and moved either by rolling away or by sliding or a combination of the two. Once the particle has been moved it leaves behind an area of polymer film not coated by the metal.
Figure 3. How pinholes may result from particulates on the film surface
There is a trend for producing vacuum coating systems where there are no front surface rollers following the deposition zone. When samples of the coated film are examined the number of pinholes is lower than when the same film is coated in a system where there are front surface rollers. This worries me as the next system the film is transported through, if it has front surface rollers, may well result in the number of pinholes increasing because the particles would still be present but the point where they got moved was just delayed until the next process. This makes the removal of front surface rolls a fix rather than a cure, the cure being to remove the particles so that there is nothing to shadow the film or be moved leaving behind a pinhole.
Figure 4. A photograph of pinholes in an aluminium metallized PET film.
Figure 4 shows a particularly bad example of the production of pinholes in an aluminium metallized PET film. Samples of the coated film were cut as a batch of approximately 20 layers of film cut out of the metallized roll using a scalpel and the carried around to use for demonstration of pinholes at various lectures. The microscopic movement of the sheets resulted in this high level of pinholes. This can be replicated by placing a sheet of metallized film, coated side upwards, on a light box, where the film is illuminated from behind, and then brushing the coating lightly with a soft brush (such as a photographic brush or a soft make-up brush). As the brush moves the particles on the surface the pinholes will appear. Brushing the surface a second time and more pinholes will appear and if more pressure is applied some scratches may appear too.
If the polymer film is examined before metallizing it is common for the film to appear to be clean but this is simply that the particles cannot be resolved by eye and it needs either a microscope using a technique such as dark field or differential interference contrast optical microscopy to enable the small particles to be observed and possibly sized or counted. Within the visible area there will be hundreds of particles ranging from very large numbers of ones below 1 micron and reducing numbers of larger particles from 5 microns to 100 microns.
Currently films used for food packaging are not cleaned prior to metallizing whereas barrier films used for electronic applications that require a vastly superior barrier performance are cleaned prior to vacuum deposition in order to try to minimize the possibility of pinholes. Often this cleaning is not sufficient as it is generally regarded that particles of 0.3 microns and smaller are difficult remove because the Van der Waals force holding them to the surface. Whilst these small particles cannot be removed they may still be moved over the surface leaving behind pinholes. For the very best barrier materials the polymer film may be cleaned and then an organic coating applied that is of sufficient thickness to cover over the remaining particles and so producing a near perfectly clean surface ready for vacuum coating. This does significantly increase the cost of the substrate which generally excludes it from the food packaging applications.
Cleaning can be done by either using tack rolls where an elastomeric roll is rotated at the same speed as the polymer film being wound. The particles stick to the tack toll and this roll then contacts a high adhesive roll where the particles are transferred to this high adhesive roll and accumulate until a layer of the adhesive roll is removed and the ability to keep the tack roll clean is refreshed. This process is hard to accomplish in metallizers where roll lengths can be in excess of 40km but for the electronic applications it has been possible to include this cleaning process immediately before the deposition drum inside vacuum systems. This is because the accumulation roll, as yet, cannot be refreshed automatically and so it progressively becomes saturated with particles. For the electronic applications the film quality is higher and the web length is often <5km such that the length to saturation is sufficient to not need to refresh the accumulation roll before the completion of the roll. An alternative cleaning process is to use neutralised, ultrasonically pulse air jets that are able to penetrate the boundary air layer and disturb the particles on the film surface that are then vacuumed away. This process has to be done at atmospheric pressure which requires it is included following a slitting process or has to be done on a separate winding step. Both of these processes are capable of removing particles down to ~0.3 microns.
During vacuum metallization there may be a problem of spitting from the resistance heated deposition sources that can also create pinholes. If the spits are large there may be sufficient thermal mass to enable the spit to not only result in a pinhole but also to burn through the polymer too making them even more detrimental to the barrier performance. Some metallizing systems include a defect monitoring system, but these usually are limited to identifying defects of 100 microns or greater. Whilst it is important to minimize these defects it may falsely give the impression that the coatings produced are defect free whereas they may still have a huge number of smaller pinholes which will still be the limiting factor in the barrier performance of the film. Figure 5 shows how even one small pinhole will drastically reduce the barrier performance. This can easily be a couple of orders of magnitude or more reduction in barrier just for the first pinhole and as there is usually so many pinholes. This is why there is such a large gap between the theoretical barrier and practical barrier that can be obtained for the vacuum deposited coatings.
Figure 5. A schematic showing how just a few pinholes can be detrimental to the barrier coating.
To make improvements it is helpful to know where on the curve shown in Figure 5 the coated film is positioned. Where the substrate has not been cleaned, before the vacuum coating process, pinholes will dominate the barrier performance and even reducing the size and number of particles may only result in a small barrier improvement. This is because there are still too many still present and they still dominate the barrier characteristics. This has been highlighted with the shaded area labelled ‘food packaging’. Only once the pinholes have been substantially eliminated will the barrier performance start to increase significantly. This is the area shaded and labelled ‘electronics encapsulation’. In this case the pinholes from particulate contamination are rare but the voids between the deposited grains/crystals are still present and there is still the atomic disorder at the grain boundaries and these have a higher permeation rate and begin to dominate the barrier performance. Once this point has been achieved it is worth then considering how the deposition process might be optimized to improve the coating nucleation and growth. This may be by changing the crystal size or even disrupting the growth to densify the coating and minimize the voids and reduce the permeation rate.
I hope that this has given some insight into why substrates are often a variable, rather than a constant, at the point where they are loaded into a roll to roll vacuum coater and why it should be no surprise that the output can also be variable. Barrier coatings are possible but to get the best possible barrier coatings the substrate needs to be carefully managed and specified as well as the manufacturing and storage history should be known. The surface needs to be clean both from exudates from the bulk polymer, that are generally a low molecular weight chemical contaminants that impairs adhesion, as well the particulate contaminants that lead to pinholes in the coating that severely limit the barrier performance. It should also be recognized that improving the barrier performance can rarely be done without some increase in cost. This will be due to either buying an upgraded substrate or in the additional processing needed to improve the existing substrate as well as an increase in measuring to confirm the quality of the incoming material.
To learn more about the author, Dr. Charles A. Bishop you can visit his contributor page or email him directly at firstname.lastname@example.org