Which plastic, and why it matters more here
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In every other craft in this estate a box holds the product. Here it takes part in what happens to it, and the wrong plastic is a slow solvent.
The container is chemically active
A specimen sits in a closed box for decades. Whatever that box gives off, the specimen breathes.
This is not a marginal effect. Acid vapor from storage materials is a named, documented cause of damage in geological collections, and it is the cause of efflorescence on carbonate specimens far more often than anything the specimen brought with it.
What to avoid
PVC. The important one. As it degrades it emits acidic products: hydrochloric acid, and formic and acetic acid, with acetic acid identified as the main compound responsible for damage in heritage collections.
Soft, flexible, glossy clear plastics are the ones to suspect, and PVC was the default for clear boxes and sleeves for a long time.
Cardboard and untreated paperboard, which are acidic to begin with and feed pests.
Rubber and many foams, which degrade and off-gas.
Raw wood, especially oak, which emits acetic and other carboxylic acids.
What is safe
Polypropylene and polyethylene, which are chemically inert and the standard answer for specimen boxes.
Polystyrene for rigid clear boxes, which is stable, and brittle.
Acid-free, lignin-free board where card is wanted.
Glass, which is inert, and heavy.
Inert is the requirement. Attractive is a separate question and it is the one that usually wins, which is how collections end up in the wrong boxes.
How to tell them apart
Ask the supplier for the resin. A packaging supplier can answer this for any item they sell, and it is a useful question a collector can ask one.
Resin identification codes where they appear: 2 and 4 are polyethylene, 5 is polypropylene, 6 is polystyrene, 3 is PVC.
Smell is a crude but real indicator: a sharp chemical smell when a box is opened after being closed is something coming off it.
And when it cannot be established, assume it is not safe for long-term storage of anything reactive. The cost of being wrong is the collection rather than the box.
The resins we stock, and what each one is
