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Message:: [QUOTE="clayirving, post: 722808, member: 18867"]I received a reply from Tom Denly:<blockquote>Mylar D is a Brand name owned by Dupont plastics.  It is truly a  true polyester with an alignment of the molocules in long straight runs.   The true reason Mylar D is so good is that it is made to an American Society of  Testing Materials standard of purity that is extremely tight.  Many  plastics are mixed with other materials which will lower its melting point to a  point that it can be heat sealed, these additions are in some cases sulfuric  acid, hydrocloric acid, many oils, and occasionally esters.  If not mixed  perfectly these additives can leach out of the polyester into the paper  money.   You will find that our holders are made to  the Standards of Mylar D for purity and therefore can not be heat sealed but are  welded to form a perfect seal.  Our holders are manufactured by computer  driven machines such that each holder is exactly like the next. When I first got into the holder business  Dupont made Mylar D, ICI Corp made Melinex 121, and Three M made a third  brand of polyester each to the same A. S. T. M standard.  Three M stopped  making the material as there was too much competition for the limited need for  the material, Then Dupont bought ICI Corp and realized that why compete and ICI  could make it all which it does now.  So truly it is a Melinex 121 make to  Dupont Standards and Dupont ownership.  I do not type real well but I hope  I have answered your question. </blockquote>I wrote back to Tom:<blockquote>I have no problem at all    understanding your typing. I do have a question about Melinex 121 -- I can't    find any information about this particular product, but I did find there are    more than 930 basic grade types of Melinex! Most references for archival    sleeves indicate Melinex 516 or Melinex 456 are used. The Library of    Congress Preservation Web site indicates: ---------------------------------------------------------- </blockquote> <blockquote>Update on Acceptance of Polyester Film Products for Use      in Encapsulation of Paper Artifacts (Revised October      22, 2004) Over the past several months, we have tested      several biaxially oriented polyethylene terephthalate (PET) polyester films      and found them to be satisfactory for use in encapsulation of paper      artifacts. The acceptable films are:    <ul> <li>      [*]DuPont Teijin Films Melinex® film types 455, 456, 515, 516, made by        DuPont Teijin Films (see <a href="http://www.dupontteijinfilms.com/" target="_blank" class="externalLink ProxyLink" data-proxy-href="http://www.dupontteijinfilms.com/" rel="nofollow">http://www.dupontteijinfilms.com/</a>        for international distributors) [*]DuPont Teijin Films Mylar® type LJP181 and KL, made by DuPont Teijin        Films (see <a href="http://www.dupontteijinfilms.com/" target="_blank" class="externalLink ProxyLink" data-proxy-href="http://www.dupontteijinfilms.com/" rel="nofollow">http://www.dupontteijinfilms.com/</a>        for international distributors) [*]DuPont Hongji Films Type 8511 and Melinex®516, made by DuPont Hongji        Films Foshan Co. Ltd., China [*]Hostaphan® 43SM film, made by Mitsubishi Polyester Film, LLC, of        Greer, SC 29652. [*]SKC Inc.'s films SH71S®, SKC SH72S® and SKC SH18S®. These films are        available from a number of distributors. [*]Tairlin CH28® film, manufactured by Nan Ya Plastics Corporation,        Taipei, Taiwan, and available from Granwell Products, Inc., 185 Fairfield        Avenue, West Caldwell, NJ 07039.</li> </ul>To the best of our knowledge,      all of the tested films, with the exception of Melinex type KL and type KM      films, are coated for slip on one or both sides for ease of handling. The      two uncoated films are available in very small thicknesses, 92 gauge for      Melinex® type KL and 48-200 gauge for type KM, which conservators may find      limiting for many applications (100 gauge equals approximately 1 mil or one      thousandth of an inch). While we would generally prefer uncoated stock to      eliminate any chance of an unforeseen interaction with the artifacts being      encapsulated, we do realize that most processors find slip coating to be      essential for ease of handling. Moreover, most such coatings are highly      stable.------------------------------------------------------------------------------------------------------------- </blockquote>Tom replied:<blockquote>121 was the old, old, old number and now my distributer says he uses  516.  I need to kep up with things </blockquote>[/QUOTE]

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