A question relating to the cause of pinholes

How are these pin holes formed?

In the above discussion, it is mentioned that the barrier properties of a metallized film is affected mainly because of the pinholes and un-metallized areas which are caused due to the dust and debris attracted during the winding process of plain film. Is it the only reason or there could be any other reasons for the formation of pinholes?
In this reference I would like to put some additional points -

1. Pinholes could be formed as a defect in the metallizing process - In a vacuum physical vapour deposition process, material to be deposited is heated to a high vapour pressure by electrically resistive heating in vacuum.

 

A thermal evaporator uses an electric resistance heater to melt the material and raise its vapour pressure to a useful range. This is done in a high vacuum, both to allow the vapour to reach the substrate without reacting with or scattering against other gas-phase atoms in the chamber, and reduce the incorporation of impurities from the residual gas in the vacuum chamber.

 

In the above process the most common thermal evaporator used are ceramic boats, when the Aluminium wires get in contact with the boats it melts and evaporates. During this melting and evaporation process there is formation of bubbles at boiling point (@ 1200 to 1400 degree centigrade). When these bubbles burst it punctures the film leaving behind the defect known as pinholes.

 

Can this be the reason of pinholes formation?

Please advise.

 

ANSWER

When the polymer film is produced it is not in a clean environment, even in the highest quality lines. There will always be some dust and debris that is present in the environment and as the polymer film is wound there will be an electrostatic charge that is built up on the surfaces that will help attract debris to the surface.  Also in areas such as the casting drum and stenter oven the will be evaporated monomer that will condense as a fine white powder that may fall onto the web too.  The amount of debris will also depend on how much the film has been wound and handled before you receive it.  Most films will have been slit at least once from the mill roll down to smaller width rolls and the slitting process is another source of debris contamination. This is especially as the blade cutting the polymer generates debris and is adjacent to the winding film that has the electrostatic charge on the surface that acts like an electrophoretic pump.  This debris will be on both sides of the web and so any cleaning process needs to be on both sides of the web.  The best of the cleaning processes cannot remove debris of less than 300nm and these are still very much larger than the thickness of the metal coating. The number of debris particles on the surface can be in the thousands per square meter.  Most will not move and may be embedded or stuck to the surface and will not produce a pinhole. There will be a proportion that produces pinholes immediately and others that will appear after further downstream handling. 

This method of producing pinholes is by far and away the biggest source of pinholes.  ICI polyester films funded a student (E.H.H.Jameson) at Cambridge University who produced a Thesis on the analysis of the source of pinholes who determined this was the main source of the problem. 

 

You are correct that this is not the sole source of pinholes. There are pinholes produced from the evaporation source. Many of these are thought to be associated with the slag that forms on the surface of the molten pool rather than bubbles that burst.  The aluminium wire has the surface oxide that does not melt but forms a skin on the surface of the molten pool and some of the trace elements in the wire also can also add to the slag. This slag tends to accumulate towards the edges of the molten pool and so as the pool size changes, increasing or decreasing, there is a tendency to an increase in spitting from the pool edges. These spits are often seen as incandescent particles that may have enough energy to not just produce a pinhole in the coating but the more serious hole all the way through the polymer film. These can be minimised by stabilising the pool size. Thus in recent years the power to the resistance heated boats have been improved and smoothed as well as the wire feed made more consistent as well as the coating thickness monitoring improved so that the intervention to correct coating variations have been minimised.  All of these have reduced the variation in pool size and reduced the tendency of the sources to spit.

A further source of pinholes is due to pick-off.  If the coating is poorly adhered then when the web is re-wound any high spots that press into the metal layer may stick to the metal well enough that when the roll is next unwound the metal will be removed, picking off the metal.  This is quite common in some of the coarser filled films where the larger fillers may press hard into the web during re-winding particularly as in the vacuum system there is no air entrainment to enable a softer re-wind that reduces the contact pressure.  

All of these can potentially contribute to the total number of pinholes seen and it is very hard to identify which of the processes has caused any particular pinhole.  The debris type can sometimes be identified if the debris has been slid rather than rolled away as there will be a pinhole with a scratch associated with it and sometimes the debris will still be visible at the end of the scratch.

 

 

 

 

Comments

[Deshpande Sunil]

1.what is the low cost method to detect the pinhole in polyester film.
2. what is mimimum size of pinhole that can detect by human eyes.

[Deshpande Sunil]

Dear sir,

Can you enlighten us how to measure the resistance of evaporator boat at room temp. and at working temp. Please suggest the name of equipment. Please reply

Thanks and regards

Sunil Deshpande