This page is a preliminary effort to define the historic base films that have been used in the manufacture of tape, especially audio tape.
From an historic perspective, magnetic recording started with audio and then branched into instrumentation, data, and video. We anticipate posting some historic articles in a new history section of this Web site in the near future. This page is meant as a short guide to basefilms as an aide to assisting in restoring tapes.
The earliest work in Germany and later in the U.S. focused on coated paper. I have never seen a paper German tape and the circa 1935 carbonyl iron tape that I determined was blank save for some test tones was an acetate base. In North America, paper tape was sold at the end of World War II into the 1950s. The Brush Soundmirror tapes that I have seen have been paper on metal cine-type reels.
In late 1943, there was an explosion and fire at the IG Farben tape plant in Germany (not war-related) that destroyed the acetate tape production facility. From late 1943/1944 on, the German tape was homogeneous (i.e. the magnetic particles embedded in the base film, not coated on it like all other tapes) and it was a PVC carrier. This was called Magnetophonband Typ L (L for the tradename Luvitherm).
Acetate base film became the standard early in the production of tape in the U.S. 3M started producing type 111 tape in 1948. For an interesting view into the history of 3M tapes, please look here. There is an almost-complete list of all audio tapes manufactured by 3M from 1947 through 1992.
Polyester tapes (also known as Mylar™ and Tenzar™ [TENsilized mylAR]) were made as early as 1953 and became mainstream with the introduction of the 2-inch quadruplex video recorder in 1956. 3M’s last introduced acetate tape was type 201 in 1962 and I believe it was kept in manufacture into the 1970s. I also believe that 111 was manufactured into the late 1960s or early 1970s, but the end dates are not in the published list.
Other than a few experiments by 3M and some BASF tapes in the 1950s and 1960s, PVC was not widely used. Most tapes were either acetate or polyester.
The good news about acetate tape is that for almost all post WWII acetate tapes, you can hold them up to the light and see light through the edges of the tape pack. You can also tell how it breaks under tension. Acetate will break without significant elongation, while polyester will turn into round drinking straws before breaking in most cases.
Never, ever bake an acetate tape.
Summary:
1/4-inch reels:
Paper (1946-195?)
Acetate (1935-1943 in Germany & 1948-197?)
Polyester (1953-today)
PVC (1943-196?, mostly in Germany)
Cassettes are believed to be all polyester, although some of it is very thin (especially in micro cassettes).
Tapes of 1/2-inch and wider are mostly polyester, although there may be significant quantities of 1/2-inch acetate.
All video, data, and digital audio tapes are presumed to be polyester.
Some early instrumentation tapes may be acetate, but most are polyester.
There is more discussion of failure modes here. Additional specific articles can be found in the Tape Aging category.