Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, June, 1930 Page 25 their Immediate combustion, a potential explosion hazard is introduced, inasmuch as these gases may be ignited later on when they reach a supply of air (oxygen). The possibility of employing some gas or chemical to neutralize the poisonous gases evolved by the decomposition of nitrate film has been suggested. Gas to be suitable for this purpose evidently should be non-flammable and non-poisonous, and should have the property of forming non-flammable and non-poisonous neutralization products when combined with the poisonous gases from the film. This gas should be automatically liberated in case of a film fire, or the gas might be evolved by a chemical compound which could be stored with the film, and which on heating would evolve the neutralizing gas. It appears that the only common gas which could be employed in the above manner to neutralize the oxides of nitrogen is ammonia, but ammonia is itself a poison even in very small concentrations (0.25 to 0.45 per cent by volume in air). It causes acute inflammation of the respiratory organs and edema of the lungs. Further, ammonia gas in high concentrations with air forms explosive mixtures. It would therefore be impractical to store and liberate ammonia gas in sufficient quantity to neutralize the poisonous oxides of nitrogen from a film fire without danger of having present an excess of the ammonia gas. I think it is sufficient, without going into this phase of the subject more in detail, to point out that at the present time no gases or chemicals are commercially available for safely and effectively neutralizing the poisonous gases evolved by decomposing film. Of course, when ammonia water is available this may be used to some extent in neutralizing the oxides of nitrogen. It is generally understood that fire extinguishing systems employing water serve only to extinguish or control film fires, but as a matter of fact, the application of water to decomposing film serves a double purpose inasmuch as water reacts chemically with the poisonous oxides of nitrogen, changing them to nitric acid, which is soluble in water. In a well-sprinklered room in which films are stored, the bulk of the poisonous oxides of nitrogen evolved by decomposition of the film would toe chemically acted upon and dissolved by the water. Unfortunately the poisonous carbon monoxide gas is only slightly soluble in water and cannot be effectively removed by this means. It is apparent, from the foregoing considerations that when cellulose nitrate film is used adequate provision should be made for its safe handling and storage, having in mind the low ignition and decomposition temperature of the film, its extremely rapid rate of combustion, and its ability to decompose with the evolution of explosive and poisonous gases. Cellulose Acetate Film Manufacturers of slow-burning film having a cellulose acetate base first submitted samples to the Laboratories for test about twenty years ago. Following an extended investigation, the acetate base film in the form of ribbon for motion pictures was listed by the Laboratories as slow-burning, the fire hazard being somewhat less than that of common newsprint paper in the same form and quantity. This type of film, however, did not come into general use immediately, probably on account of the superior qualities of the cellulose nitrate film for photographic purposes. Recently, however, the manufacturers of acetate film have succeeded in improving its qualities to a marked degree, and this type of film is now quite generally used particularly in projectors of the non-professional or miniature type, which may be operated in public without a standard booth if the slow-burning acetate film is used. Ignition Temperature The ignition temperature of cellulose acetate is between 700 and 800° F., as compared with about 300° F. for cellulose nitrate. Decomposition A temperature of about 500° F. is required to produce decomposition of cellulose acetate film. In the neighborhood of this temperature the evolution of fumes in material quantity occurs. The decomposition of cellulose acetate film, however, is not exothermic, as is the case with cellulose nitrate. In other words, the decomposition of cellulose acetate film once started does not continue except under conditions where there is an external source of heat. It will be noted that in the case of cellulose nitrate film the decomposition continues when once started even in the absence of an external source of heat. This difference between the decomposition of cellulose nitrate film and that of cellulose acetate film is therefore of great importance from the fire and life hazard standpoint. Combustion The cellulose acetate film continues to burn when once ignited if the supply of air is sufficient to freely support combustion. The combustion, however, will cease in a restricted supply of air. The rate of combustion of cellulose acetate film is relatively slow, and the amount of heat evolved is of a low order being much less than that of paper or wood. Evolution of Gas Like all cellulose products, cellulose acetate on combustion or decomposition gives off irritating and suffocating fumes. During the last ten years engineers of the Laboratories' staff have frequently had occasion to make decomposition and combustion testis with cellulose acetate films. On several occasions workers have been exposed to these fumes without any ill effects. Of course, in high concentrations the fumes would be so irritating and suffocating as to cause workers to leave the room. In tests on cellulose nitrate films it is, of course, necessary to avoid breathing the oxides of nitrogen and carbon monoxide gases evolved. Under practical storage conditions the acetate film will, in case of a fire, be subjected to combustion in a more or less restricted supply of air (oxygen). In this event the combustion will be accompanied by some decomposition. Under these conditions most of the hydrogen, methane and carbon monoxide will undergo oxidation or combustion, but sufficient quantities of carbon monoxide (about 1 per cent) may be evolved to render the air surrounding the fire, particularly if in a closed room, dangerous to breathe, as would be the case in the combustion of paper under similar conditions. The acetic fumes, aldehydes, and ketones are irritating and suffocating, but are not comparable in their poisonous effects with the deadly oxides of nitrogen evolved by the decomposition of nitrate film. I think that the most important safety factor with reference to cellulose acetate film is its slow combustion, which can he easily extinguished by application of water or by smothering the fire. It is therefore possible to readily provide measures for controlling film fires involving cellulose acetate. In small quantities the film can be safely used for purposes of amusement without any special safeguards, but where large quantities of cellulose acetate film are to be stored in hospitals, I think it would be advisable to provide a cabinet or vault, preferably of fire-resistive construction, for the protection of the patients from the fumes in case of a fire. BRITISH-GERMAN PICTURE TELEGRAPHY A new service of picture telegraphy between Great Britain and Germany was opened January 7, with a simultaneous exchange of photographs and written greetings between the Postmaster General of England and the German Minister of Posts and Telegraphs. This service has been opened to the public, and a charge has been fixed for sending picture telegrams from England to Germany at the rate of 2%d ($0.05) a square centimeter, with a minimum charge of £1, for which about fifteen square inches may be sent. These rates have been fixed provisionally, and it is possible that they will be revised when experience has thrown light upon the cost and revenue of the service.