In dealing with you question, I have decided to see how archivists who
are involved in the conservation and preservation of historical
materials approach this problem, and to see their recommendations.
From my research it would seem that there are the problems of
preservation against access, and cost to protect your collection
against the value of collection.
This article from Havard College Library sets out in great detail the
preservation of postcards. On the specific topic of plastic sleeves
they state the following:
Postcards: Navigating the Preservation Options
?Plastic sleeves are an alternative. Polyester is the preferred
plastic (Mylar Type D and ICI Melinex 516 are two frequently
recommended brands). Uncoated polyester is extremely stable, strong,
and clear; it lies flat (so does not promote curling); and is rigid
enough to provide good support. Some forms of polyethylene are also
stable and safe to use, but selection is difficult. Low-density types
can contain anti-block and slip agents that cause the plastic to
adhere readily to the surface of photographs (and thus, perhaps, to
certain kinds of postcards). High-density types are naturally
slippery, so do not require anti-block or slip agents, and thus have
less tendency to stick. If one were to ask a vendor whether a
particular polyethylene product were additive free, however, it?s
likely that he or she would be unable to answer the question. Buy from
a reliable dealer (again, see footnote 14). Polyethylene is
translucent rather than clear, soft (so less supportive), and
scratches easily. It provides good protection from handling, however,
which is a primary objective.?
The Museums, Libraries and Archives Council in the UK also produce a
useful discussion of preservation materials.
?Museum Conservation Materials
Polyester is the most generally useful of all plastic fims used for
long term storage. Frequently appearing under its common trade names
of Melinex or Mylar, polyester film is usually the material of choice
for preserving documents and photographs. It is crystal clear and
chemically stable, and contains none of the fillers, plasticisers or
other additives present in many other plastics. It is these additives
that deteriorate or leach out, causing damage to sensitive objects.?
They always provide a list of suppliers of these materials, but they
add it is not an endorsement, however, I suspect they would not refer
to them unless they were reputable. The web site you referred me to is
not one of them. Perhaps you would wish to order some of these
companies? catalogues and obtain further information on specifications
and pricing. Some of these companies are:
Conservation by Design
The Museum Association also produce a list of suppliers.
You will have noted in the Harvard article reference to some US
companies. I have not linked to these as I believe you are in the UK
and wish to source UK suppliers. If you want links to them ask by way
?Companies that carry a broad range of preservation-related supplies include:
University Products (517 Main Street, P.O. Box 101, Holyoke, MA
01041-0101; telephone 1-800-762-1165);
Gaylord Brothers (P.O. Box 4901, Syracuse, NY 13221-4901; telephone
Light Impressions (439 Monroe Avenue, P.O. Cox 940, Rochester, NY
14603-0940; telephone 1-800-828-6216.
Companies that carry a narrower range of (mostly storage) supplies include
Archival Products (2134 E. Grand Ave., Des Moines, IA 50317; telephone
Conservation Resources International; (8000-H Forbes Place,
Springfield, VA 22151; telephone 1-800-634-6932);
Hollinger Corporation (9401 Northeast Drive, P.O. Box 8360,
Fredericksburg, VA 22404; telephone 1-800-634-0491); and
Conservation Materials (1275 Keppy Lane #10, P.O. Box 2884, Sparks, NV
89431; telephone 702-331-0582).?
Other archives recommending Mylar or Melinex.
Idaho State Historical Society
?Suitable enclosures are made of polypropylene, Mylar? polyester, or
polyethylene [not polyvinyl chloride (PVC)], which are chemically
stable materials free of additives and surface coatings.?
Essex Record Office
?Individual packaging ranges from simply wrapping each item in
acid-free paper/tissue, or polyester (Melinex or Mylar) sleeves to
more elaborate enclosures such as acid-free boxes.?
I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.