Strange residue on cardboard flips?

I keep nearly all my coins in these cardboard and plastic 2x2 flips, and I've noticed that on some of them, there's an odd residue that shows up and clouds the plastic windows on them with something of an impression of the coin's relief.

I've noticed this happens pretty much only on copper or bronze coins (though I also saw a more minor example with a sterling silver coin, so I think any copper content at all is a factor), mostly older ones that have lost their shine, and happens maybe on one out of maybe every hundred or so coins with copper in them, so its not too common. It also seems to be more frequent with coins that have been in there a long time, judging by my handwriting on the example I took a picture of, that particular coin has probably been in this flip for around a decade, though some coins I've had for even longer have none of this gunk at all, even if the coin itself is older and has more of a patina.

I keep the flips unstapled (so I can pull the coins out and examine them closely), so I was easily able to take some out and get a good look at this stuff. I ran a clean cotton swab around the plastic on a couple of these and it came back with a greenish substance unsurprisingly reminiscent of copper oxidation. The coins themselves were perfectly clean of any similar residue as far as I could tell, although many of them would stick to the plastic when I pulled them, so thankfully only the flips themselves seem to be affected.

Does anyone here know what causes this, what exactly it is, and if there's any way to prevent it? Is there any danger to the coins from this? 

Hi colleague.

       A coin in a closed space breathes-excretes from the metal due to the action of moisture -the powerful influence of the environment on her- air, carbon dioxide, and this substance is imprinted on the plastic, it is a "thin layer of green alkaline copper carbonate (copper that protects it against corrosion //so-called passivation//

https://cs.m.wikipedia.org/wiki/M%C4%9B%C4%8F

Maybe a chemist would explain it better.  After finishing the army, I worked for 15 years as a foreman in a multinational Japanese-American company in the production of electronic components, a copper-nickel-silver-lead electroplating workshop, as well as a laboratory and tests.  And so I learned something about those metals.

       this is the professional formula of that substance:   Cu(OH)2·CuCO3.

This is natural for all metals, they secrete substances - which also protect them, but on the other hand, they can be damaged by the action of their surroundings.  Copper has this green mass as a protection against oxidation.  That's why roofs are made of copper, etc. they last a long time coated with a green deposit.

Ivan

Hi, may I suggest to use selfadhesive flips from a well-known brand, like Hartberger?
And I also advise to close the flips, if you want to take the coin out, you have to destroy indeed the flip.
I have coins here who are already locked up for 40+ years and I never saw this problem.

Hopefully you will be able to solve this.
Have a nice day.

I would advice to find out if those are cheap PVC flips, which are really not ideal for storing copper coins and if the answer is yes, replace them. If you are intend on taking coins out you could switch to plastic flips like THESE

Idolenz

I would advice to find out if those are cheap PVC flips, which are really not ideal for storing copper coins and if the answer is yes, replace them. If you are intend on taking coins out you could switch to plastic flips like THESE

I actually have considered getting some of those, but unfortunatley, getting enough to replace all my cardboard flips would cost me well over $1500

That said though, I'm certain the cardboard ones don't have PVC 

You can put two coins in one flip if you don't need a paper insert halving your cost 😉 there are also probably cheaper ones (but don't get ones out of PVC 😅). I would have said maybe you life in a warm moist climate but no Canada should be far of that. But like MIMAEl said this is a natural reaction but not always the benign passivating kind of basic carbonates. If the coin is contaminated with organic material or other sulphur compounds and more important chlorides "bronze disease" can form and with the moisture of the atmosphere it can damage copper and its alloys.