Stephen:
I cannot directly answer your question, but perhaps someone at Light
Impressions could tell you more about the safety (or dangers) of
encapsulation. They are a leading supplier of archival materials and
do offer mylar polyester film for encapsulation.
Here is their information:
Light Impressions
P.O. Box 940
Rochester, NY 14603-0940
1-800-828-6216 (technical assistance, M-F, 9-5 E.T.)
www.lightimpressionsdirect.com
Good luck,
Christy Sinksen
Assistant Registrar
The Georgia Museum of Art