I've had doubts about the slabs myself.
I believe most are polycarbonate (lexan) which is halfway decent, and has some UV blocking ability.
The sleeves when used are either Mylar - excellent. Or polypropylene, also a fine choice.
While I like SGC for looks, I'm not so sure about the gasket. Acid free is possible but the details aren't known.
The idea I had early on was that for cards on acidic paper like strip cards, and most Topps and Bowman, the slab might seal in the acid from the cardstock degrading. And that would accelerate the process.
I actually asked SGC, and their slabs are not airtight, so there may be enough transfer of air to prevent that.
I do think some study and/or some transparency about materials from the grading companies would be a fine thing.
Steve B
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