I have been reviewing the topic of barrier coatings and some of the finer details of why metallized, silica or alumina vacuum deposited coatings do not deliver the barrier performance expected. Metal foil is used as an example of a perfect barrier and it has always been a complaint of metallized polymer films that they are not a complete replacement for foils because the barrier performance is not as good.
Many years ago now there was a research programme to look at why the metallized films contained pinholes and this was proved to be due to debris on the polymer surface that was moved after metallization leaving behind a small uncoated area of film. This uncoated film has no barrier improvement at all and it has been shown that these defects are the primary reason why any vacuum deposited coating does not produce a perfect coating and very high barrier performance.
What does not appear to be mentioned so frequently is how devastating pinholes can be to the barrier performance. It only takes 5% of the metallized area to be missing in the form of pinholes and the barrier performance will be almost the same as for uncoated polymer film.
What is even more worrying is just how few and how small the pinholes need to be for them to reduce the barrier performance by at least an order of magnitude.
This means that even if we try to clean up the polymer films before vacuum coating it is likely that we will improve the coatings to some extent but not as much as if we could eliminate the pinholes altogether. It seems unfair that those vacuum coaters who pay particular attention to keeping the vacuum system clean and substrate quality high may still end up with barrier coated films only slightly better than those who pay little attention to the hygiene issues.
In order to produce a surface with no contamination that can deliver a coating without any pinholes is hard to achieve. If the polymer surface is cleaned by whatever means there will still be debris on the surface of a size below 1 micron or possibly as low as below 0.3 microns. Removing debris below this level is very difficult and so usually the surface is cleaned down to this level and then other measures are used to finish the cleaning process such as coating a polymer onto the substrate to cover up all the remaining particles. This coating needs to be greater than the largest remaining particle in order to deliver a perfect surface for the barrier coating to be deposited onto. If this polymer coating is deposited outside the vacuum system the coated surface needs to be protected from re-contamination. This includes from the atmosphere as well as from the back surface of the polymer substrate once the film is re-wound into the roll. If the polymer is deposited in the vacuum system it too needs protecting from contamination as there can be debris contaminating rolls from previous cleaning activities. Not only does the surface need protecting from re-contamination but it needs to be suitably prepared for deposition by plasma treating as the surface energy needs to be as high as possible to maximise wetting. If the depositing species does not wet the polymer surface well the growing coating will be rougher and contain more holes and defects than if the depositing coating wets the surface well.
The barrier performance is dominated by the pinholes bit once these are eliminated the defects in the growing coating become the next most critical target. Thus wetting and producing a continuous coating at the thinnest possible coating thickness becomes the next most critical target.
In the past the density of pinholes has been used as a measure of improvement but this is really a false target. The real target should be zero pinholes. Achieving zero pinholes will deliver the big change in barrier performance. Anything other than zero pinholes will only achieve a more marginal barrier performance improvement.


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