The Flexible Plastics
Nitrate, Acetate and Polyester: the flexible plastics that made it possible
Everyone who is at least tangentially interested in the conservation of the audiovisual heritage has some sort of knowledge about the basic characteristics of the plastic bases, their justification and incidence in the film industry worldwide.
Maybe the most famous of the anecdotes regarding the subject is the high flammability of the nitrate base, and all the romanticism involved in the various stories about archives, laboratories, studios and projection booths that perished as a consequence of fires caused by combusted nitrate films (a scene that was immortalized in Cinema Paradiso, Giuseppe Tornatore’s 1988 film). Another famous story that surprises cinephiles is the film massacre that occurred during the Second World War in order to reuse the cellulose for the manufacturing of other objects.
That said, besides the dissemination of these well-known events, we have prepared a compared guide that describes the three plastics from various perspectives. It is worth mentioning that in this post we are focusing only on the base or film support, without taking into consideration the emulsion of the motion picture film. We hope you find it useful and interesting!
How did we get to nitrate? In order to achieve a motion picture film, a support was needed with certain qualities: strength, optical clarity and flexibility.
Nitrate
Chemical compound — full name: Nitrocellulose or Cellulose Nitrate:
How is it obtained: This support was made with a mixture of cotton linters, nitric acid and sulfuric acid. This was dissolved in solvents to produce a clear and viscous “dope”. Then a plasticizer was added (such as camphor), and it was cast on a highly polished glass surface or metal drum. Finally, the solvents were allowed to evaporate. Nitrocellulose is a compound formed by nitrating cellulose through exposure to sulfuric acid or another powerful nitrating agent.
When was it used: Many scientists collaborated throughout the XX century in the development of the formula that enabled the use of cellulose nitrate for the manufacturing of motion picture film. George Eastman starts marketing the nitrate roll film in 1889. Kodak stops the production of nitrate film in the United States in 1951, although the manufacturing continued in other countries until the sixties.
Why was it discontinued: The main reason was the extreme flammability of this material, and the fact that this is fast and it does not need the oxygen in the air to keep burning (once it is burning it is extremely difficult to put out. Water, fire-extinguishers, sand, etc. are useless).
What makes it different: Despite its high flammability, it is an excellent film base, praised for its almost “luminous” images. It is much stronger and has a longer projection life than the early safety bases. It has good resistance to humidity and it is very clear, because although it has a slightly yellow hue, the thin sheets used in motion picture (around 140 micron) were almost completely transparent allowing the passage of almost a 95% of the white light.
Challenges for nitrate conservation: Due to the high risk of fire ignition, those nitrate films still conserved should be stored in isolated vaults, specially designed for this use, in order to avoid disasters in case of fire. On the other side, films should be stored at a maximum temperature of 2Cº/36Fº with a relative humidity between 20% and 20% (in ideal conditions). When temperature and humidity are above the desired levels nitrate is prone to hydrolysis, a reaction in which, through the breaking of the nitrocellulose structural bonds, nitrous oxide is released, and in the presence of further humidity, produces nitric acid which will act as catalyst agent in the consecutive states of degradation. Thus, the nitrate film will soon lose all of its qualities of transparency, elasticity and flexibility.
What gauges can we found in nitrate: Kodak brand only produced nitrate film in 35mm and never used it for amateur gauges (8mm, S8mm and 16mm). However, outside the US, there might be cases of other brands that produced 16mm film with nitrate base. There are also 17.5mm nitrate films that were the result of a 35mm split in two.
Keys for nitrate identification: You should first pay attention to the year of the film (or the film material). Also, films may be marked ‘NITRATE FILM’, ‘NITRATE’ or ’N’ along its edge and/or have horizontal dashes (tick marks) between every fourth sprocket hole. The laboratory method is to put the piece of film into a tube of trichloroethylene. Nitrate film sinks, safety floats. When none of those techniques work or are available, some people chose to burn a small sample in the open air and see the reaction. However it is a very destructive technique and we do not recommend it!
Acetate
Chemical compound — Full name: Cellulose Acetate, that includes the variants cellulose diacete and cellulos triactete (and to a lesser extent cellulose acetate propionate).
How is it obtained: Cellulose acetate film was the first safety film manufactured as an option to nitrate film. It was produced by treating cellulose with a mixture of acetic anhydride, glacial acetic acid and sulfuric acid (triacetate). Diacetate: This initially formed triacetate, but that plastic could not yet be used as a motion picture base because a necessary practical coating solvent was not available in commercial quantities and acceptable price. Consequently the triacetate formed was partially hydrolyzed with acetic acid and water, creating cellulose diacetate. Triacetate: It is achieved through a mixture of wood pulp, acetic acid, acetic anhydride, solvents and triphenyl phosphate (which acts as plasticizer and fire retardant).
When was/is it used: Diacetate: This acetate type was tried commercially in 1909, but due to its lack of resistance to moisture and some deficient physical properties, 35mm film continued to be manufactured with cellulose nitrate during the first half of the XXth century. Cellulose diacetate’s use only spread out after 1922, with the introduction of 16mm amateur movie film. It was also used for the 28mm gauge introduced by Pathé in 1912 and the 9.5mm in 1922. However, both nitrate and diacetate were eventually replaced by other non-flammable cellulose plastics, with high tensile and tear strength. Triacetate: Used for all film gauges since 1948.
What makes it different: Diacetate: It was the first safety motion picture film. It was mostly used for amateur formats such as 9.5 and 16mm (although some 35mm film rolls can be found). It has a characteristic camphor smell. Triacetate: This type of acetate has improved strength and toughness compared with cellulose diacetate. It provided for the first time a safety base with sufficient toughness for 35mm motion picture film for theatrical use. Its introduction permitted the manufacture of nitrate to be discontinued. Even though since 1999, it is not used for color prints, triacetate is still used for camera negatives and some intermediates. This is mainly due to the fact that polyester films are so resistant to breakage that they are often more likely to break the film equipment should a jam or extra tension occur, and because acetate is easily spliced using solvent cement, making the editing process easier.
Challenges for acetate conservation: Acetate is subject to vinegar syndrome, especially when it is poorly processed and stored. In ideal conditions, it should be stored at a maximum temperature of 4Cº/40Fº with a relative humidity of 30–50%.
What gauges can we found in acetate: All gauges.
Keys for acetate identification: Usually acetate films have an edge print that says “safety film”. When diacetate, it may smell like camphor. Also, if it smells like vinegar we can affirm it is decomposing acetate, since it is the smell of acetic acid being released.
Polyester
Chemical compound — Full name: polyethylene terephthalate (PET). Kodak’s trade-name is ESTAR.
How is it obtained: Made from ethylene glycol and terepththalic acid. No solvents are used in the manufacturing.
When was/is it used: Its development was almost parallel to that of triacetate (40’s), however, during many years it wasn’t used for the manufacturing of motion picture film because a desired level of transparency wasn’t being achieved nor a good adherence between base and emulsion. Towards the 1960’s it was already widely used as support for magnetic tracks and for S8mm films. Since the 1990’s to date it is used for almost every copy, sound negative, intermediate, panchromatic separations and other BW preservation elements.
What makes it different: It is a very tough support. Cement splicing is not possible, hence splicing needs to be done by ultrasonic or tape splicers.
Challenges for polyester conservation: It has an excellent dimensional stability (it does not shrink). It is also chemically very stable (it does not easily deteriorates and resist room temperature). Challenges will be linked to the preservation of the film emulsion.
Keys for polyester identification: It is not always easy to distinguish acetate from polyester. One option is to hold the film roll against a light source. Acetate is usually opaque while polyester is translucent. Another option it to perform the tear test, given that polyester is much stronger and difficult to tear than acetate, but this is a very destructive test that should be performed only on the extremes of the protection leaders. Finally, if you are patient enough, you can perform the birefringence test, with polarizer filters. Find a guide here.
Sources:
- Adelstein, Peter Z., IPI Media Storage Quick Reference, Image Permanence Institute, 2009.
- AMIA Film Technology Workshop.
- Del Amo García, Alfonso, Inspección técnica de materiales de archivo de una filmoteca, Edición Electrónica, 2007.
- Hidalgo Maestro, Luis, Los polímeros en el arte.
- National Media Museum, Nitrate Film.
- The dangers of Cellulose Nitrate Film.
