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Washed film is not normally used again for making fresh film, as it is too variable to meet the oxocodingly high quality standards of the industry, but it is sold for incorporation in collulose finishes or to the celluloid plastic industry.
(b) Storage properties of celluloid; decomposition and stability.
Nitrocellulose, the main constituent of nitrate film, is not a completely stable compound. It continually undergoes a chemical change akin to hydrolysis, whereby oxides of nitrogen are evolved. The reaction, under ordinary conditions, proceeds at an extremely slow rate, but may be accelerated by certain conditions.
For example, the reaction is accelerated by acids, and it is hore that one danger lies. For the oxides of nitrogen evolved in the reaction change with traces of moisture to nitric acid, and this acid, as we have said, acolorates the reaction, again producing more acid. The reaction is, in chemical language, suto-catalytic.
The second danger of the revotion is that it is exothermic; that is, hoat is evolved during the process. When the reaction is slow, this heat is of no significance; but as the reaction accelerates, particularly in a tight mass of celluloid or nitrocellulose, the host may build up to the stage where more is pronood than can be dissipated by conduction or convection through the material. In this case the mass will, quite sud-
denly, Turat into flame.
The third denger of this reaction is that it proceeds with great violace at temperaturos insufficient to inflame ordinary combus- tible reterial. Thus at 170-180 degrees Centigrade, celluloid almost immediately inflancs, and at much lower temperatures (130-140°C.) com- bustion will suddenly take place after a period measurable in hours rather than days.
It would appear from this that celluloid was too dangerous a commodity for more humanity. But these are laboratory results; tice a number of other considerations enter.
in prac- ically sealed vessel (such as a bomb) the oxides of nitrogen produced in
For example, in a hermet- the reaction are reduced to harmless nitrogen, and the reaction stops. In the presence of plenty of air, for example, in a box of celluloid toys, the oxides of nitrogen are dissipated and do no harm.
A constant shall supply of air is necessary for the reaction of decomposition; and, in passing, this condition is admirably fulfilled by a well-closed but not hermitically scaled drum.
But the most important consideration remains. days of the celluloid industry, these fires from self-ignition were very
In the early frequent; but careful research has now tramcndously reduced the hazard. Firstly, it was found that really pure nitrocellulose was far less liable to this decomposition than impure material. Secondly, a "stabilizer" was added to celluloid, which reacted chutically with the oxides of nit- rogen and rendcred then harmless.
The result of this work is to be scen to-day, when immense quantities of celluloid film circulate around the globe, and fires are comparatively rare. Nevertheless, the inherent vices of self-combustion, and of combustion at low temperatures, remain in celluloid, and the strin- gent precautions required by law in all countries attest to the continued
danger.
Only this year, at Richmond, in England, a disastrous fire broke out in a film-washing plant, and at the enquiry the probable cause was concluded to be this self-same reaction of decomposition of the cellu- loid, brought on by a spell of hot weather. It is needless to remark that under tropical storage such decomposition must be a greater hazard than in a temperate climato.
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