Transparency sheets are typically acetate or sometimes polyester (Mylar), and they are found in many common document dimensions. They are printed on through an electrostatic toner process (e.g. laser printer or Xerox copier). Plastic transparency sheets contain no cellulose fibers, which means that they are not technically paper. Legal-sized plastic film transparencies (often those used with overhead projectors) are, however, frequently lumped in with paper files.
Polyester, or Mylar (c. 1950–present): Debuting in the 1950s, polyester became the standard film material in the 1970s. Drafting film is commonly referred to by the trade name Mylar or Melinex.
Depending on the age and type of plastic used, transparency sheets may become yellow or embrittled. In general, polyester sheets are more stable than acetate.
Acetate (1939–present; significant decrease in 1960s): Early acetates are especially prone to shrinking, and they may exhibit bubbling or "channeling" between layers due to disparate warping. Acetate with "vinegar syndrome" can infect other acetate-based materials stored nearby, particularly in a poorly ventilated storage area. Deterioration is hastened by high relative humidity (RH). If it is suffering from decay, acetate film will emit an vinegar odor. Segregate acetate film in unbuffered paper sleeves and do not enclose it in polyester sleeves. Reformat if possible.
Polyester, or Mylar (c. 1950–present): Polyester cannot be easily torn and, despite some reported reaction to the diazo process, is considered chemically inert. If stored in appropriate conditions, polyester may last over 500 years. Some early drafting polyester matte coatings (1950s–70s) are known to flake; reformat these as soon as possible. Store all polyester film flat in acid-free folders. Interleaving with unbuffered paper may help prevent media transfer or loss.