Plastic Transparencies

Synonyms

Overhead transparency
Film transparency
"Foil"

Description

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.

Overhead projector with plastic transparency used during lessons in a classroom. Image by Wikimedia Commons user mailer_diablo, available under a Creative Commons Attribution ShareAlike license (CC BY-SA 3.0).

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.

Deterioration

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.

Risk Level: Low to moderate, depending on type of material. Acetate, the more common, is at a significantly higher risk than polyester.