I've seen two types of these pages - acetate and polypropylene. The acetate type is similar to the film base itself, and it should be very stable unless it is glued together. If glue is used, the composition of the glue is a factor to be considered. Polypropylene is very inert, but there may be plasticizers and other additives remaining in the (polypropylene) film that could cause problems. Unfortunately, you can't tell. These additives are proprietary, and what is used depends on the brand and lot of the bulk film. Almost all of the harmful stuff is almost always removed in processing and aging, however. Your best bet is to get sleeves from a big, old company who has experience in the area. I have had polypropylene sleeves stick to the film base. This is more annoying than anything else. I suspect it is physical rather than chemical. It makes the negatives hard to remove, but otherwise it seems to do no harm.