Latest 2 questions ----- this time with answers. Part 2.
Q2
What is the shelf life of printed metallized paper? What is the effect of high humidity (90%) to printed metallized paper? Are metallized paper attacked by fungus?
Answer
As with everything it depends on the type & manufacturer of the paper.
Paper is basically hygroscopic & so can vary in water content with the surrounding humidity. When the humidity is around 40% -50% RH the moisture content of the paper may be around about 7% but at >95% RH the moisture content can rise to as much as 20%.
These values will vary depending upon the type of paper. Some papers have a polymeric type coating which will help stabilise the moisture content in the 5%-10 % range. It can be problematic passing the paper through the vacuum coating process as the paper can lose too much water & if the water content falls below 5% it will become stiff & brittle and much harder to wind.
Any material that can absorb and desorb moisture from the atmosphere may get spores lodged in the surface & so may get fungus or moulds growing on the surface. The metallisation is usually only on one side of the paper & so it is more likely that any growth will be on the un-metallized side. However the metallization is unlikely to be perfect and any pinholes and/or holes or cracks in the metal coating will allow moisture into the paper & again the spores may be present.
Some spores/fungi do not like metal & so they may not grow even of present in the atmosphere however the reverse side may still be an acceptable site. If the paper contains a whitener it may depend on the whitener used. Titanium dioxide is an anti-bacterial material & so it can promote the properties of self-cleaning & self-sterilization. This is because the titamium dioxide is activated by light and by photocatalysis can neutralise contamination. There are many other fillers & whiteners that do not have this property & so will not prevent grows appearing.
A simple test that you can carry out is to run a finger across one of the white spots. Does the white material stick to your finger? If the white material does wipe off on your finger does the bright aluminium reappear?
If the answer is that either the white powder does not wipe off nor does the bright metallic aluminium reappear then it is most likely that the aluminium has corroded to aluminium oxide which is transparent &/or white depending upon the crystal size & light scattering.
If the white material does wipe off & the metal surface reappear then it would appear to be a mould growth. If you then really want to know what the white material is surface analysis would be the route to defining the material.
The first step is to wipe the white areas to determine the most likely source of the problem.
If nothing wipes off it is unlikely to be a mould it is more likely to be the aluminium is converting to transparent aluminium oxide & your are seeing the paper through the coating. You can see if corrosion is a problem by taking some salt water & putting a spot on the aluminium film & watching to see the results. Heating the sample slightly could speed up the results. It is likely there will be corrosion of the aluminium. Compare the results with what you have. Are they similar?
If so you likely have corrosion of the aluminium. The fact that you have people breathing near to the rolls of material you will have salts in the moisture in the air & so it is possible these will act as corrosion initiators for the corrosion.
A second method is to use ellipseometry to look at the 'white' areas. Try to measure the thickness & refractive index (RI) using ellipseometry. If it can reproducibly find a thickness & RI it is likely it will be for aluminium oxide.
So there you have my answer to the question. My question to you all is – have I missed anything out?
If you think I have hit the ‘Comments’ below & let me know what I’ve missed.
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