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174
HRIIISH (CINEMATOGRAPHY
Vol. 22, No. 6
400
300
Fig. 3.
The time-temperature curves
when the specimen m a s
triacetate safety base.
tests the concentration is not high enough with the nitrate present in the body of the base to increase the rlainmability appreciably.
It can be seen from Table 1 that the untreated safety base did not continue to burn after the alcohol flame had burnt out. With a more flammable safety base a given nitrate content would result in a more flammable film than indicated in Fig. 1.
Flash Point Determinations
The flash point of a liquid or volatile solid is the temperature at which it gives off sufficient vapours to form an ignitable mixture with air near the surface of the material. The " flash point " of film is defined in this paper similarly, although the vapours given off by the film on heating are decomposition products. Materials in the neighbourhood of a fire are gradually heated and the spread of fire is often facilitated if these materials give off vapours which are ignitable. It was felt, therefore, that the " flash point " of the films was a better measure of the fire hazard than the ignition temperature.
To determine the flash point of the speci
mens of film under test a fixed weight of film (in this case 3.7 grammes), in the form of punchings \ cm. square, was placed in a test-tube and one junction of a platinumplatinum-rhodium thermocouple placed in the centre of the specimen. The other junction of the thermocouple was placed in a Dewar flask. The test-tube was supported in a small electric furnace, the temperature of which was raised at about 4C min. The temperature of the furnace could be measured by a mercury in glass thermometer in the furnace wall. A small pilot flame was mounted over the top of the test-tube to ignite the vapours when they were given off in a sufficient quantity.
A representative selection of the films, containing different amounts of cellulose nitrate, was tested in this manner, as well as a normal safety film base and a sample of cartridge paper. The specimens of film which contained cellulose nitrate, either as a surface coating or in the base, behaved similarly, and a typical graph of specimen and furnace temperature is shown in Fig. 2. In this case the flash point was 160 C. the vapours being
400
300
200
100
Fig. 4.
The time-temperature curves
when the specimen was
cartridge paper.