PVC flips were popular decades ago before anyone knew any better. However, overtime, the damage to coins became apparent.
These Chile 1968So coins were shipped from Santiago and some coins are still sold in their original holders. I was fortune to purchase a set that did not have any signs of damage, however, this set currently for sale on Ebay is not so fortunate. Notice how the 5 Peso coin is discolored where it made contact with the flip:
Photo below shows the upper half of the PVC flip used to ship the Ecuador 2006 coin, now in the square hard plastic capsule, that shows the most significant damage I've encountered thus far (flip labelled 240A):
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Modern soft plastic flips don't use PVC, however, they are rumored to bleed other damaging chemicals on to coins over time.
See Recognizing Coin Holders That Contain PVC.
The hard plastic Mylar flips and capsules don't outgas harmful chemicals.