Number of pinholes per unit area.

Is there a standard acceptable amount of pinholes per sq metre in metallized BOPP.

Also do you know anything about the possibility of 'welding' pinholes in metallized film by use of an electric charge?

Answer

As far as I am aware there is no standard that defines the grade of the film by the number of pinholes per unit area.  There are some figures quoted by aluminium foil manufacturers about the number of pinholes per unit are for foil and so the idea of counting pinholes is not unknown.  Possibly the reason for not bothering to count pinholes is that the barrier performance is not only affected by the number of pinholes but also by the size of the pinholes.  Thus a simple count of number of pinholes would not necessarily tell you what the barrier performance would be and films with the same number of pinholes could be radically different if one had large holes and the small ones.

Individual customers may have something in their acceptance specification about number of pinholes. This may be highlighted if the film is for an optical or graphic purpose but where it is for a packaging application the barrier performance will usually take precedent. Here the allowable number of pinholes tends to be either determined by the barrier performance as measured on a Mocon or similar instrument or something that is agreed between the metallizer and customer. 

There was a paper by Angelo Yializis that includes a bit about healing capacitor films.   If memory serves me correctly this is more about preventing shorting between layers than sealing pinholes.  If successive layers of metallized film are shorting out it is sometimes possible to pass a current through the film such that the current is too great for the short and so vaporising the metal and breaking the circuit at this point thus removing the short and recovering the full performance of the capacitor.

If you were thinking about closing the pinholes by welding a little bit of aluminium over the holes to block it up then 'no' I do not know of any process doing this.  If a fully opaque film is required then the usual approach is to double side metallize the film. The philosophy behind this is that the chance of two pinholes lining up to still show as a pinhole is negligible.  Double side metallizing is also used for high barrier films. Again the fact that holes do not line up means that and gas or moisture will have a tortuous path to diffuse through the film and hence the barrier improves.