Polyethylene terephthalate (PET) is the chemical name for polyester resin. It is a polymer of ethylene glycol and terephthalic acid. The resulting product in uncoated, biaxially oriented film form, without additives, is inert and has use as an "archival" quality container for paper, photographs and textiles. Polyester can be formed into both films and fibers (polyester shirts, dresses, and suits).
The original work in making PET film was done by ICI Ltd. in Great Britain in the 1930s. After World War II, DuPont purchased the "know-how" from ICI and began to make films in this country. DuPont's polyester film was named Mylar for trademark purposes . Over the years the market for PET films has grown, as many industrial and packaging applications became realities. Now, in America, there are four firms making biaxially oriented PET films--DuPont, ICI Americas, American Hoescht, and 3M. Films are also being imported from other parts of the world, particularly the Orient.
Over the years DuPont began expanding the range of Mylar types. What started out as an uncoated, biaxially oriented PET film has evolved into over 100 varieties. Some have coatings of PVDC, acrylics, wax, metal or even combinations of them. There are films that shrink at low temperatures and films with special surfaces. And they are all called Mylar!
Recently a form of Mylar called M-30 has been offered to museums on the retail market. This product is coated on both sides with a thin layer of polyvinylidene chloride (PVDC, an example of which is Saran). This coating was added to serve the food packaging industry which needed barrier properties and the ability to run the film on high speed packaging machines. The PVDC did both jobs well.
In food packaging it is not necessary to guard against non-archival ingredients in packaging materials . And PVDC is not archival. Given the right combination of time and temperature, it can break down and give off something akin to hydrochloric acid. PVDC is also applied to the base sheet with solvents, and traces of these solvents may remain from the coating process.
There are other forms of coated PET, none of which is considered to be of archival quality, for various reasons.
Even some of the uncoated versions of Mylar cannot be considered archival. The HS (heat shrinkable) version starts to deform near the boiling point of water and is used to package poultry and meat in heat shrink bags. The OL (ovenable lidding) version has an amorphous layer of polyester on the surface which allows the film to heat seal at temperatures lower than normal. It contains slip additives and there are antistatic treatments, built-in UV inhibitors, aluminum dispersions on the surface and particulates in the base sheet to roughen the surface. None of these varieties can be considered archival, because of additives in or on the film.
What kind of Mylar is right for conservation work? The sheet should be uncoated, biaxially oriented and with no chemicals or particulates added to the base sheet. The type recommended by DuPont and experienced users is Type D. While some of the industrial grades like Types A, AS, LB, and LBT could be used, film clarity could be lacking or there night be more than a desirable amount of surface roughness--particularly when working with photographic emulsions .
Ask your supplier for Type D Mylar (or its equivalent from ICI, 3M or American Hoescht) . Some day you or your museum or archives will be glad you chose the correct Mylar.