Mylar is the best plastic sleeve out there. Dupont isn't using the Mylar name anymore if I recall it right, as they have a "new" version of polyester that's supposedly better.
Some cardboard can deteriorate a bit, but the process is pretty slow in most cases. Light, heat and being too dry or too moist are the big culprits in deterioration of anything. All that can be hard to manage, excessive cycling of heat is probably worse than a consistently high heat.
I used to have concerns about cards in slabs. And still have some slight concern. As the wood fibers break down which happens with time, one byproduct is acid. (Varying types and degrees depending on the wood pulp content and exact chemistry. ) Newsprint is a great example of a paper that degrades somewhat rapidly. In a confined space with little ventilation that acid can remain, and accelerate the degradation.
Some cards will be fine for a long time, many T and E cards are on stock that I believe doesn't have much if any wood pulp, so it's not particularly acidic. And most modern cards since the change to white cardstock, right from the restart in 81 for most companies or whenever they started, and 93 for Topps are probably not acidic if not acid free.
On the good side, there are cards that I'd consider the most vulnerable that have been slabbed for a long enough time now to get a feel for whether it's ok or not. And they seem fine.
I'd check with an archival supply place to see if the Mylar sleeves are available. They should be, as they're made in a lot of sizes for the archival market. Polypropylene is nearly as good, not quite as clear, and flexible rather than slightly stiff. But usually about half the price.
One caution. Putting Mylar sleeves into toploaders while very effective is challenging if you want the card out after a few years. a couple of mine I had to cut the toploader to get the sleeve out. The mylar is smooth enough that over time pressure forces the air out from in between them and then they're stuck. The force sticking them is around 130Lbs. They'll slide eventually, but it's like trying to pull a sleeved card out from under someone's foot.
Steve B
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