Haven't looked into the others, but I did do a quick scan of SGC's site, and found statements that the flip insert and the black framing insert were both acid free. I did not see any explicit statements that the plastic holder itself was acid free. That of course doesn't mean that it isn't, but it does seem odd to specifically state that the other materials in there are acid free but not say anything about the case itself if it too is acid free.
As I said, I haven't looked at the others, but I do find it surprising that any of these companies would either 1) use acid free materials and not have a big bold explicit statement to that effect or 2) skimp on the cost and not use acid free materials. Every collecting supply that I have purchased for paper collectibles in recent memory have all had some statement to the effect of "certified acid free". We can debate what that means exactly (acid free at the time of manufacture, truly inert, etc), but the bottom line is that all of the major manufacturers of these supplies recognize that being "acid free" is a major concern for collectors, particularly where paper is involved. To just gloss over (or, perish the thought, skimp on) that part of your product seems negligent to me. Just sayin'.
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