Nitrocellulose powder

Many methods have been proposed and are used to study the thermal stability of propellants and to ensure the absence of possible autocatalysed decompositions during storage. None are sufficiently reliable to merit individual description. In practice, stabilisers are added, the usual being diphenylamine for nitrocellulose powders and symmetrical diethyl diphenyl urea (carbamate or centralite) for double base propellants. Provided a reasonable proportion of stabiliser remains, the propellant can be assumed to be free from the possibility of autocatalytic decomposition. The best test of stability is therefore a chemical determination of the stabiliser present. 

The same author [27] gives some data on a propellant which is intermediate between composite and double base propellants. It is composed of ammonium perchlorate and a nitroglycerine-nitrocellulose powder (double base powder). 

In Russia, Mendeleyev [12] worked out a method for the manufacture of smokeless powder from pyrocellulose, i.e. relatively high-nitrated (12.5% N) nitrocellulose soluble in a mixture of ether and alcohol. In 1892 the manufacture of this powder was started for naval guns. Nitrocellulose powder of this type was soon adopted for military purposes in the U.S.A., where nitrocellulose powder became known as single base powder . 

The specific gravity of semicoiloidal nitrocellulose powder with a mixture of ether and alcohol as a solvent usually ranges from 1.54-1.63, although the specific gravity of nitrocellulose itself is 1.66. This indicates a certain porosity of the powder. The pores are filled either with air or with traces of residual solvent. According to Brunswig [17] 100 g of nitrocellulose rifle flake powder contains 4-8 cm3 of the air. 

Nitroglycerine powder—completely or almost completely wholly colloidal—is less porous, therefore more difficult to ignite than nitrocellulose powder and requires a stronger priming. 

Diagrams (Fig. 184) show the effect of the density of loading of nitroglycerine and nitrocellulose powder upon the amount of C02, CO, H2 and CH4 evolved (according to Andrew Noble [19]) in the explosion products. 

Fig. 184. Proportion of C02, CO, H2 and CH4 in products of explosion of cordite and nitrocellulose powder as a function of density, according to Brunswig [17].

The heat of explosion depends chiefly on the composition of the powder, e.g. in nitrocellulose powder on the content of nitrogen in the nitrocellulose and in nitroglycerine powder on the content of the nitroglycerine. 

Generally speaking, nitroglycerine powders give a higher heat of explosion, so that the temperature of their products is higher than that in nitrocellulose powders. 

Rideal and A. J. B. Robertson [23] suggest that initiation of the thermal explosion of nitrocellulose is preceded by liquefaction. Very likely the same would apply to nitrocellulose powder. 

Nitrocellulose powders (U.S.A.) Heat of explosive decomposition (water liquid) kcal/kg Gas volume (water vapour) l./kg Linear coefficient for rate of burning from one surface in./sec/in2... 

Smokeless powders have low sensitiveness to impact and friction. They do not ignite when hit by rifle bullets and are thus fairly safe to handle in war-time. Nevertheless there have been accidents caused by the sudden ignition of nitrocellulose powder brought about by the violent friction between the sharp edge of a heavy bin and powder scattered on the floor. 

Nitrocellulose powders is particularly sensitive to friction when hot. Accidents have been caused by the ignition of hot powder on removal from the drier, in all probability due to friction. Here electrification of the powder on drying also plays an important part (see below, pp. 542, 616) giving rise to the possibility of an electric discharge while it is being taken out. This had led to the regulation that the powder must not be taken out of the drier before it has been cooled. 

Nitrocellulose powder is more sensitive to friction than nitroglycerine powder, although the latter is more sensitive to impact (due to the presence of nitroglycerine). 

According to Kast [28] nitrocellulose powder does not detonate even when very strongly initiated (e.g. 50 g of picric acid or 100 g of tetryl), but may explode with a rate from 1000 to 1800 m/sec. 

Nitrocellulose powder Rifle flake non-polished powder loosely poured 0.79 3800... 

Medard [30] also described experiments which were carried out in France by Vieille as early as 1906, Dautriche (1913), Burlot (1920-26) and by himself in 1938. Dautriche found that nitrocellulose powder BM 17 D2 in bands of thickness ca. 44 mm detonated at the rate of 6560-7200 m/sec when initiated by 50 g of picric acid. 

Burlot found similar figures. He also examined the effect of the impact of a falling weight or of a rifle bullet. Only deflagration occurred — there was no detonation. However in his later experiments Burlot [31] has found that nitrocellulose powder (poudre BM9—in strips) can detonate under the shock produced by a rifle bullet D (caliber 8 mm, 7.5 g) having a velocity above 1200 m/sec (e.g. 1266 m/sec). 

The ignition temperature of nitroglycerine smokeless powder is approximately 180°C and that of nitrocellulose powder about 200°C. Ignition with a direct flame,... 

Fig. 189. Electric (static) charge of nitrocellulose powder as a function of moisture content, according to Nash [32],...

Langevin and Biquard [33] showed that the evaportion of a liquid (alcohol, ether, benzene) does not electrify the residual solvent. Hence evaporation of the solvent in driers cannot lead to electrification of the powder. In the opinion of these authors nitrocellulose powder may be ignited by the discharge of the condenser at a voltage of 3000 V, if the condenser charge is greater than 0.3 fiF. 

The following findings point to the harmful effect of air on the stability of green nitrocellulose powder, i.e. freshly pressed and containing a considerable quantity of solvent (alcohol and ether) (Table 171). 

Atmospheric humidity has a deleterious effect on the stability of powder. Storm [54] reports that a good nitrocellulose powder, which withstood heating at a temperature of 65.5°C for 400 days without marked decomposition, showed evident decomposition in 175 days when stored at the same temperature in an atmosphere saturated with water vapour. Powder which passed the first test subsequently withstood heating for 5 hr at a temperature of 135°C without explosion, whereas powder from the second test exploded at the same temperature after 10 min. 

Fig. 194. Stability of nitrocellulose powder at 110°C (measured as a reduction of weight)...

After World War I the influence of sea water on the stability of smokeless powder was examined. It was found that nitrocellulose powder submerged in the sea during military activities did not suffer any perceptible deterioration as a result of immersion in sea water for several years, neither in its colloidal properties nor in its stability. 

The stability test for nitrocellulose powder at a temperature of 134.5°C the sample will not be considered serviceable unless the time necessary to produce the evolution of nitrogen oxides is at least 45 min and the powder withstands this temperature without exploding for 5 hr. The test can be combined with methyl violet test decoloration of the test paper should not occur before 30 min or longer according to particular specifications. 

The test for nitroglycerine powders at a temperature of 120°C (nitroglycerine powders cannot withstand higher temperatures) here the same conditions apply as in the test for nitrocellulose powders, viz. appearance of the nitrogen oxides after a lapse of at least 45 min and no explosion for 5 hr. The same test can be carried out in the presence of methyl violet test papers. Decoloration of the paper should not occur before 30 min. 

Fig. 196. Effect of the addition of amyl alcohol and diphenylamine on the stability of nitrocellulose powder, according to Berger [66],...

More recently Demougin and Landon [67] examined the stability of nitrocellulose powder containing 1.02-7.8% diphenylamine at a temperature of 110°C. After 160 hr of heating they determined the nitrogen content in nitrocellulose isolated from powder (Table 174.). The initial content of diphenylamine in the sample was 7.8% on heating for 180 hr at a temperature of 110°C it was reduced to 1%. 

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