International projectionist (Jan-Dec 1948)

Nitrate Film Symposium The article on nitrocellulose film by Robert A. Mitchell which appeared in IP for February* elicited widespread interest and numerous comments from the field, the most interesting and comprehensive of which came from the assistant director of an independent research laboratory which prefers to remain unidentified. This commentary, and Mr. Mitchell's reaction thereto, are presented here. DESPITE the over-all real merit of Mr. Mitchell's article on nitrocellulose film, it appears to us that it contains enough errors to be dangerously misleading. For example : it is stated that "film base is made by mixing two parts pyroxylin with about one part of camphor, adding traces of plasticizers and stabilizers." The article later brings out the point that the camphor is very important in the reduction of explosibility and fire hazards in the film. In actual fact no film is manufactured with anything like this quantity of camphor. Many film bases use none whatever, relying on other types of plasticizers to make the film sufficiently flexible. The chief reason why film base is less explosive or inflammable than guncotton is due to its lower degree of nitration. "Stabilizers" are rarely or never used in film base. The stability is obtained by special purification of the nitrocellulose and not by added chemicals. Is Film Base Explosive? Under the heading "Rate of Combustion" it is stated that "celluloid does not and cannot explode either by ignition in a confined space or by percussion." The question as to whether celluloid or film base can or cannot explode has sometimes been debated. These debates, generally, revolve about the definition of the word "explodes." If we confine the term to the results obtained with nitro-glycerine, dynamite, etc., film base does not explode. However, since it does contain sufficient oxygen for its own combustion, and since its rate of burning increases rapidly as the pressure is increased, film base confined tightly in a chamber and ignited will rapidly generate enough pressure from the gases liberated to burst almost any container. In this sense film base is explosive. Celluloid Degradation Gases In general, however, as Mr. Mitchell points out, the reports of film "explosions" may be attributed to a different phenomenon, namely, the rapid combustion of mixtures of the decomposition fumes and air. The article offers a typical analysis showing in what proportions the cellu * "The Anatomy of Nitrocellulose Film: Its Import to the Projectionist" by Robert A. Mitchell; IP for February, 1948, p. 5. loid degradation gases are usually present. This table is incorrect and leads the author to a particularly dangerous conclusion. It probably was obtained from results on the gases from exploded guncotton which show very small amounts of oxides of nitrogen, A better table is one given by the Chemical Warfare Service in their report on the Cleveland Clinic disaster, as follows: % by Volume N02 (N,04) 6.9—8.9 NO 1.4— 8.2 CO 47.4 — 59.1 CO, 21.3 — 24.5 02 " None H0 0.9— 3.2 CH, 1.0— 2.7 Mr. Mitchell also states that "of the celluloid degradation gases themselves, only carbon monoxide is a deadly poison." Although this is true for the analysis given in the original proof, it is by no means true for the correct analysis. In this case although carbon monoxide is a deadly poison, it is far less dangerous than the nitric oxide which is present. Nitric oxide has been the cause of most of the deaths which have occurred from nitrocellulose fumes. Its poisonous action is especially insidious in that the immediate effects may not indicate to the injured person that he is in serious danger and it may be several hours before severe symptoms develop. These symptoms are extreme shortness of breath and weakness caused by the presence of liquid in the lungs. For this reason it is extremely important that anyone who has been exposed to the fumes of film decomposition or film fires be hospitalized at once even though he may feel no symptoms whatever. This precaution may well be a matter of life or death. Old Film Inflammabilitv The article also points out that "old film has lost some of its camphor and is therefore more dangerously inflammable than new film." Although it is often true that very old film may be more dangerous than new film, this increased hazard would be almost entirely due to increased likelihood of breaks in the film rather than loss of camphor. The section on fire extinguishers is good, although we believe it places undue confidence in carbon dioxide fire extinguishers. They are excellent in the projector itself if the carbon dioxide can be released instantly. They are practically useless, as are all other forms of fire extinguishers, if a reel of film has really started to blaze. R. A. Mitchell Buttresses His Position I T APPEARS that the foregoing esti the score of the camphor content of nitrocellulose film base, but even this furnishes an interesting sidelight on the tendency of film manufacturers to skimp on an expensive and very necessary ingredient of their product. It is well worth pointing out that the large-scale production of synthetic camphor was stimulated by the acute need for this product by film manufacturers. Of course, substitutes are being used, to the acute distress of projectionists who have to "nurse" brittle film. Composition of Celluloid When I stated that celluloid consists of two parts nitrocellulose to one part camphor, I had in mind only the highest quality standard celluloid. That these proportions are not far wrong may be gathered from the following: "A celluloid of normal composition contains about two parts of nitrocellulose to one of camphor." (Masselon, Roberts, and Cillard, Celluloid, J. B. Lippincott Co.) "Nitrocellulose plastics are made by taking 70 to 80 parts by weight of nitrocellulose (11 percent nitrogen), mixing with nonvolatile solvents and plasticizers . . . and 20 to 30 parts by weight of camphor" (Ralph K. Strong, Van Nostrand's Scientific Encyclopedia.) These formulas, however, are general. Since camphor is combined with the nitrocellulose differently in film base than in celluloid intended for molding, it is possible that nothing like so high a percentage of camphor is present in film base. The general procedure for combining these two ingredients is given by the following quote: "The dehydrated nitro-cotton is . . . dissolved in tumbling barrels or mixers in suitable solvents, those commonly employed being acetone and methyl alcohol, and at the same time the so-called softeners, such as camphor, are added, these resulting in a flexible film." (George Eastman's article under the heading "Photography" in Encyclopaedia Britannica.) The aforementioned work by Masselon, Roberts, and Cillard states that the sol 10 INTERNATIONAL PROJECTIONIST • April 1948