PP films have sometimes performed poorly in long term storage tests, although contradictory test results exist. PS Polystyrene films, like PP films, failed a test of ink transfer and migration of plastics to stamps, although polystyrol, a commercially engineered PP, subsequently passed a replication of the same test. They are flexible and have good stretch and ultraviolet resistance properties. All of the films discussed here have performed without major problems in the conditions required by stamp collectors. However, more testing with stamp protection in mind is certainly required.
Collectors face many problems in identifying plastics.
Even experienced polymer experts are often unable to distinguish one film from another without the aid of sophisticated laboratory tests. Collectors remain at the mercy of their philatelic product suppliers. They are well advised to become as informed as possible about philatelic plastics, and to use this information in asking penetrating and challenging questions. Avoid all products where answers consistent with this knowledge are not immediately forthcoming. In the natural evolution of knowledge and economic conditions, the current trend of replacing older plastics that damage stamps with safer ones will continue.