...He said the mylar was a different material than the melinex he recommends, and that it can't be totally deemed as "safe" for his standards...and I will trust what he says about the "archival-ness" of it.
Just to clarify...
If clear polyester film envelopes are your choice, I suggest buying them from a source of some known quality, such as the archival material suppliers; University Products, Light Impressions, Gaylord Bros., Talas, etc. A lot of the clear plastic envelopes available are made of polyester film, but not all of them. Polypropylene, styrene, and polyethylene might also be used, but those materials probably would not be mistaken for polyester film.
Also, not all polyester films are equal. There are at least two dozen types of Mylar and Melinex polyester clear films that look alike and, in some applications, perform interchangeably. Most of them are available in multiple thicknesses, too. The unique and desirable features of Melinex 516 and Mylar-D are high clarity and their lack of chemical coatings that could react inside a frame package.
I appreciate your confidence and endorsement of my suggestions, Val, but I can't take the credit you're giving me. I don't know enough about chemistry or testing procedures to deem any of those products "safe", so the standards are not mine, per se. Lacking the smarts to create my own standards, I can only seek the suggestions of authorities who know what they are talking about. In this case, Mylar Type D and Melinex 516 are among the few clear polyester films tested and approved for use in framing by the United States Library of Congress. It may be notable that many other brands and types of clear films were not approved.
The Library of Congress is quick to point out that their testing should not be taken as a recommendation for any purpose outside of the USLOC. Just the same, if it's good enough for framing documents and objects in the Library of Congress, it's good enough for me and my customers.