We came across one question answered by you ( dtd.Feb.2, 2009. "How are these pin holes formed ?") on internet regarding the pin holes in metallized PET film.
Is that pin holes are like sieve or unmetallized / metal removed portion of the film?
How to detect the same is only metal removed area or see through hole.
Pl. advice.
Answer
Pin holes or Pin windows is a catch-all term that is used to describe small areas that at first sight appear to have no metal coating present. On occasions it is possible to do some analysis on these areas and come up with some information about how they were formed.
If there was a particle of debris that was metallized and then removed after metallisation there is usually an area of uncoated polymer were the particle was positioned which has a small area of thinner metallisation surrounding this uncoated area. This thinner metal is due to the debris blocking the metal flux for part of the time in the deposition zone. As the debris first enters the deposition zone the area behind the debris is shadowed from the metal flux but as the debris traverses the zone this shadowing becomes less until the centre point where there is minimum shadowing. After the mid point the shadowing then is on the opposite side of the debris and this gets larger until the debris exits the deposition zone.
If the debris was not rolled away but slid along there would be an uncoated area that then had a scratch away from the pinhole. The scratch may have the metal completely removed or may just have the metal thinned.
By comparison a pinhole formed by pickoff is likely to have a pinhole with no graded metal thickness but generally has a step change in the metal thickness with either the full metal thickness or zero metal thickness where the metal has been removed.
If the pinhole is due to spitting there is likely to be a small particle left at the centre of the pinhole and also some heat damage to the polymer where the particle has impacted.
When the metallized film is held up to a strong light there may be many more pinholes visible than if the film is held up to a weaker light. The reason for this is that a number of the pinholes may be not really holes in the metal coating but simply areas of thinner metallisation that produce a significant difference in contrast. Areas where there is residual oligomer on the surface can result in these thinner coated areas. The oligomer can have a significantly lower melting temperature than the polymer. The oligomer can be heated by the evaporation source and the oligomer may be evaporated by the heat and as the oligomer evaporates it prevents the aluminium form depositing. If there is a lot of oligomer this may prevent all the aluminium from depositing or if there is a lower amount of oligomer then once the oligomer has evaporated there may be some metal deposited but thinner than the rest of the metal deposited on the clean surface.
With respect to the final performance of the coated polymer then all holes with no metal deposited will have no barrier performance whatsoever. It does not matter how the holes were formed the result is the same, no metal results in no barrier. Where the metal is thinner the barrier performance will be better than where there is no metal but less than where there is full metal.
The number of holes is dependant upon the amount of debris on the substrate before starting to metallize the web, the amount of oligomer or other low molecular weight material is present on the web surface, as well as being dependant upon the quality of adhesion and web surface roughness along with other factors such as roll tension and handling quality.
I hope this gives you a more complete knowledge of pinholes.
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