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June, 1953
FLAMMABILITY AND FLASH POINT OF SAFETY FILM
175
first ignitable and the subsequent flame travelling down the test-tube at this temperature. After some 4 or 5 minutes the temperature of the film increased rapidly, denoting an exothermic reaction, and on ignition the vapours burnt with a self-supporting flame. Soon after this the flame died out, the vapours were no longer ignitable, and the temperature of the specimen fell to below the temperature of the furnace again. The tests showed that the severity of the exothermic reaction increased as the percentage of cellulose nitrate increased. As the temperature of the furnace was raised further ignitable vapours were again given off. This occurred at about 280°C, although in this case there was no exothermic reaction. Some 8 to 10 minutes after this second flash point the flames became self-supporting.
Fig. 3 shows the temperature graphs for the triacetate base used in the film samples containing cellulose nitrate, and Fig. 4 those for cartridge paper. The flash points which were determined are shown in Table I. In those films containing any cellulose nitrate, the nitrate decomposed at a lower temperature than the rest of the base giving ignitable vapours at temperatures between 160 and 220°C. The second flash point of these films was due to the decomposition of the base, at temperatures in the range 270 to 300°C.
It is known that nitrate film is unstable to some degree under normal conditions.2 As the film deteriorates with age it is likely that the flash point will be lowered. It would seem
probable therefore that with films containing any cellulose nitrate there will be some slight risk of instability and deterioration which might cause a lowering of the flash point.
Conclusions
Within the limits of concentration used in these tests, the inclusion of cellulose nitrate in the body of the safety film base did not appreciably increase the flammability. With the cellulose nitrate present as a surface coating, however, the flammability increased as the percentage of nitrate present increased.
The presence of cellulose nitrate in either form leads to a considerable reduction in the flash point. Thus under certain conditions, for instance in a closed or poorly ventilated space, a flammable mixture of gases would be obtained at a lower temperature than with film containing no cellulose nitrate.
Acknowledgments
The work described in this paper forms part of the programme of the Joint Fire Research Organization of the Department of Scientific and Industrial Research and Fire Offices' Committee ; the paper is published by permission of the Director of Fire Research.
The authors wish to express their indebtedness to Ilford, Ltd., for specially preparing and supplying the samples of film used in these tests. Thanks are also due to Mr. P. C. Bowes for many helpful suggestions during the flash point determinations.
REFERENCES.
1 . Pickard, R. W., and Hird, D., " A test to measure 2. Cummings, J. W., Hutton, A. C, and Silfin, H
the flammability of kinematograph safety film." Brit. Kine., 21, 61, 1952.
' Spontaneous ignition of decomposing cellulose nitrate films." J. Soc. Mot. Pic. Eng., 54, 268, 1950.