Edit: I’ve now muted this thread and won’t be able to respond, thanks for reading tho!
Keywords for further reading: electrospinning, thermoplastic, thermoset, copolymer, electrospun polymer nanofibers.
Material scientist here.
PSA:
The filter material of N95 masks is electrospun polypropylene. This material is virtually indestructible under normal indoor storage conditions.
The expiry dates on the masks are there not because the filter material will degrade, but because the elastic bands, nose seal and the glue that holds the nose seal in place will deteriorate. The reason for this is too much detail to put here but it’s got to do with the way elastomers, emulsifiers, solvents etc behave.
As long as you can obtain good sealing around the edges, an expired N95 mask fresh out of the package will be virtually as effective as an unexpired one.