I was at the ProFlex 2019 conference in Dresden, Germany, last fall and listened to a paper where the defects in the coating were measured on-line in the vacuum system, and these results were correlated with barrier measurements taken at a later stage. The results of this correlation were then used to predict the barrier performance of film as it was being vacuum-coated. Speaking to the author of the paper afterwards, it became clear that this process was at an early stage, and the accuracy of the predictions had not been sufficiently tested yet.
This method is reasonable but, in my opinion, is really too late as the measurement of the defects is taken after the substrate has been coated, and so the cost of coating has already been incurred. The next step must surely be to measure the defects before the coating is added so that, if it is predicted to fail to meet the desired barrier requirements, the roll of film can be rejected before any coating is applied.
Multiple sources of problem defects
This leads to the question of what is the source of the defects that result in the loss of barrier. There are papers that demonstrate that the resistance-heated boats evaporating aluminum can produce defects in the coating because of spitting from the boats. The spitting is often being caused by variations in the deposition process that cause the molten pool of aluminum to vary, and spits from the changing conditions at the edges of the pool get ejected as spitting that can result in pinholes and even damage to the polymer web from the larger spits.
However, this is only a part of the range of defects that are the problem. The major source of pinholes in the metallized coating is from particulate contaminants on the substrate surface that are metallized and then moved after metallization, leaving behind pinholes in the coating where they were positioned during coating. These particulates are present when the roll is received and ready to be coated, therefore measuring the particulate contamination of the substrate before metallization could tell you whether the film is even worth coating at all. If the particulate coating is too bad, then potentially there will be too many pinholes generated, and the resultant loss of barrier will mean a coated film that fails to meet the required barrier performance. This early measurement gives the opportunity to take corrective action to improve the roll, such as by cleaning the surfaces, so that it can meet the performance requirements after metallization.
This is potentially useful, but the trickier questions are who performs the measurements, when are they done, and who pays for this. There appears to be a requirement for continuously improving barrier films, but the expectation, for many applications, seems to be that this will be at no additional cost. In my experience, it is easy to improve the barrier performance, but it will always be at some additional cost.