Question: I have a query on the metallised films. Does the pinhole count vary with the thickness of the film, for eg. what would be the rough estimation (in case if the studies are available) of pinhole count in a 15 micron, 20 micron and 30 micron vacuum metallised BOPP film? Is it true the higher the thickness, the lesser the pinhole count or is it vice-versa?
Answer: The pinholes are primarily as a result of contamination of the film during manufacture. The amount of dust that falls on the film is related to the exposure time between the extruder and the winding up of the roll and the amount of static charge that is generated by the film. Thus it is possible for there to be different levels depending on film thickness as the exposure times will differ and the amount of static charge will also differ. The amount of static charge may also be affected by antistatic treatments too.
What I do not think can be predicted is the level of pinholes for any given thickness of film.
However as thicker films are generally produced at a slower winding speed than thinner films there will be more time for dust to settle on thicker films than thinner films produced on the same film line in the same factory location. Unless the film is made by the same manufacturer and on the same film line then there is always likely to be differences because of different location and local atmospheric conditions. These differences can be expected to be even greater between different manufacturers, even of nominally the same film.
Another thing that can affect the pinhole count is the tension and rewinding of the film in the vacuum system and downstream of the metallization process. Once the film has the metal coating applied it is the movement of the debris that exposes the pinholes and so any slip between layers of film within the roll can help the debris move and so show up as pinholes. Thinner films have to be wound using lower tensions and are generally more difficult to wind than thicker films and so may expect more pinholes to be exposed. However this does not mean the cleanliness of the film is necessarily any different but only that a smaller number of the debris particles present have been displaced following metallization.
The thickness of metallization can have an effect although it might not be a large effect. Thicker metal coatings can have fewer pinholes and other defects simply because any pinholes formed during metallization may get some metal coating even if not the full amount and as the metal coating is thicker it is a little tougher and so displacing the debris may require a little more force than for the thinner coatings. Also some pinholes are not really pinholes but are areas where the coating is incomplete and with the thicker coating there are fewer of these voids.
Although thicker coatings may have fewer pinholes the thickness variation is likely to be larger as even at the last of the deposition occurs there will still be some holes that are not filled and so the variation is between no coating and full thickness. Even where there has been nucleation the orientation of the crystal structure can vary with some growing faster than others and so as time increases so too does the difference in the slower growing crystals and faster growing crystals.
So the film substrate can have an effect on pinholes either from the slower rate of manufacture of thicker film or between different manufacturers. Also the metal coating can can change the number of pinholes as well as the roughness can increase with coating thickness.