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June, 1953
FLAMMABILITY AND FLASH POINT OF SAFETY FILM
71
* O *r^ o "
Fig. 1. Distance of spread of flame as a function of the cellulose nitrate content. (O — cellulose nitrate as mixture with base, • — cellulose nitrate as surface coating on base.)
4 8 12
PERCENTAGE OF CELLULOSE NITRATF
the supports by two thin steel strips, and ignited at one end. The distance over which the film burnt was taken as a measure of its flammability. In previous work on the development of this flammability test many different types of safety film as well as samples of paper, cotton, rayon and nitrate film were tested. None of the samples of safety film burnt over a distance greater than 16 in., whereas samples of nitrate film, some newsprints, cotton and rayon were still burning at 21 in.
In the present series of tests six samples of each type of film were tested on the apparatus and the results are shown in Table I.
Fig. 1 shows the mean distance of spread of flame plotted as a function of the cellulose nitrate content. A flame spread of 4 in. indicates that the film did not continue to burn after the alcohol flame had burnt out.
It would appear from this that a cellulose nitrate content of up to 4 per cent, either as a surface coating or as a mixture with the base, does not increase the flammability of the film appreciably. Higher cellulose nitrate contents, up to concentrations of 16 per cent, in the body of the base cause little increase in its flammability. However, if higher cellulose nitrate contents are present as a surface coating then a marked increase in the flammability is observed. With 14.5 per cent cellulose nitrate present as a surface coating, the film was still burning after a distance of 21 in., and was thus more flammable than any type of safety film. A probable explanation of the difference in behaviour of the film with the nitrate in the base and as a surface coating may be that if the cellulose nitrate surface coating is thick enough, the film assumes the highly flammable characteristics of cellulose nitrate. However, within the limits of these
Fig. 2 The time-temperature curves when the specimen was triacetate base with 14.5 per cent cellulose nitrate as surface coating.