>>19548724
I am not certain about this but I strongly suspect that the harder plastic flip holders are made out of polyethylene. The softer one are most likely made out of polypropylene. Both of those plastics will ultimately deteriorate over time, turn gooey, and damage a silver coin.
If you really don't want the protective cover to chemically interact with a silver coin, you'll need to get something of archival quality. The plastic capsules that are commonly used to protect coins are made of an acrylic, which *might* be suitable for archival purposes. It'll be better than polyethylene, polypropylene, or PVC for certain. If you can find capsules made out of Mylar, that would be the way to go as Mylar is certified for archival use.