Coins should be stored in inert plastic such as polyethylene teraphthelate (PET) and not reactive plastics like polyvinyl chloride (PVC). Polyethylene teraphthelate (PET) is a rigid plastic that is used to make soft drink bottles for example, and it is good for coin storage because it is unreactive. Polyvinyl chloride (PVC) is a soft plastic (though it can be hard and brittle if it's old) and it is bad for coin storage because it is reactive: in a hot, humid environment it can produce a sticky residue.
Coins are best stored in low humidity environments, though different alloys react to humidity in different ways.
Saflip brand 2x2s are made of polyethylene teraphthelate (PET) and are safe for coin storage. Polyvinyl chloride (PVC) 2x2s or other products are often sold without stating what they are made of so buyer beware.