and sometimes mylar
flips can
crack through normal use especially at the "hinge" between the two pockets. Mylar can scratch gold if you're not careful. But, I do use mylar
flips for my coins, but have my electro-types and replicas in small envelopes which is how I got them. I have never
had a problems with envelopes, but mine are not 40 years old. Again, it is important to get acid-free. If you want to
handle the coin (physically) each time you get it out, it might be easier to use envelopes. If you want to just look at the coin, then mylar
flips are probably easier. And there is of course, mahogany box and trays, or the often despised slabbing, which I mention only to be
complete on the list.
I have comic book bags that are called mylar, a soft plastic but unlike a mylar
flip. If you could get the soft rigidity of a
PVC flip, with the inertness of a mylar
flip, that would be best.
For small coins in mylar
flips, it is often useful to keep them in kointains, or little polypropylene bags. Kointains can be useful if you are concerned with a thin coin snapping in two. For larger, thin coins, something like micro-membrane boxes can be useful.
I have seen people keep coins in notebooks, but it seemed to me that the 'pages' were soft
PVC.