Stabilization of nitrocellulose powder

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

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%. 

The lower stability of smokeless powder in comparison with that of nitrocellulose is accounted for by its content of residual solvent and of the oxidation products of this solvent. Since in a finer powder the ratio of the surface to weight is high, the oxidation processes are more intense. A larger amount of decomposition products of residual solvent is formed by oxidation, and their destructive effect... 

Strong bases have an adverse effect on the stability of smokeless powder as described above. Moreover, Angeli [59] found that pyridine and its homologues cause decomposition of nitrocellulose. (On the action of pyridine on other nitric esters see Yol. II.) At an elevated temperature (e.g. 110°C) pyridine can produce an intense denitration of esters which may even lead to an explosion. 

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. 

In addition to the testing methods common to nitric esters (nitrocellulose, nitroglycerine) and smokeless powder there are also methods used exclusively for testing the stability of smokeless powders. 

As early as 1867 Abel [73] realized that nitrocellulose tends to decompose in an acid medium, and suggested that sodium carbonate should be added to it to neutralize the acid products of the decomposition of the impurities in the powder or of nitrocellulose and nitroglycerine per se. However more than 2% sodium carbonate in the powder proved detrimental—due to its strongly alkaline reaction it impairs the stability of the powder. 

The systematic studies of T. Urbanski, Kwiatkowski and Miladowski [76] proved that the addition of an aromatic nitro compound distinctly enhances the stability of nitrocellulose and nitrocellulose powder. Thus, nitrocellulose containing 13.4% N which on heating for 5 hr at 120°C had pH=2.28 showed pH=2.89 on addition of 9 1% p-nitrotoluene, pH=3.17 on addition of 9.1 % 2,4-dinitrotoluene and pH=3.34 on addition of the same amount of a-trinitrotoluene. The same samples when heated in a constant volume (Tagliani test) gave at 134.5°C a pressure of decomposition... 

Powder produced with a volatile solvent should be freed from it as thoroughly as possible since too large a content of residual solvent is detrimental to the ballistic stability of the powder. In nitrocellulose powders the content of residual solvent should be lower than 1% in coarser powders (thicker flakes, strips or tubes) its content may be relatively higher, while it is relatively lower in finer ones. Powders... 

Ballistites initially consisted of equal amounts by weight of nitroglycerine and soluble nitrocellulose CP2 with the addition of aniline or diphenylamine as stabilizers. It was found, however, that the presence of aniline and diphenylamine is detrimental to the stability of the powder, and they were therefore omitted. The valuable properties of centralite as a solvent of low basicity were then recognized and it was used both for its ability to dissolve the nitrocellulose and for its stabilizing action. 

Loss of weight. One of the oldest and simplest quantitative methods is the determination of the loss of weight of a sample heated at a constant temperature. The original Sy-test [90] used in the U.S.A. for determining the stability of nitrocellulose and nitrocellulose powder ( U.S. Ordnance Department 115° Test ) consisted in heating a sample of the substance on a watch-glass at 115+0.5°C. 

Fig. 152. General view of digesters for stabilization of nitrocellulose (Hercules Powder Co., [3]).

Figure 65. Fred Olsen. Has done important work on cellulose and has made many improvements in detonating explosives, high explosives, and smokeless powder in particular, has invented processes for the quick stabilization of nitrocellulose and for the production of ball-grain powder. Chief of Chemical Research, Aetna Explosives Company, 1917-1919 Chemical Adviser, Picatinny Arsenal, 1919-1928 Technical Director, Western Cartridge Company, 1929—.

Diphenylnitrosamine, which is always present in powders made from diphenylamine, is decomposed at 110°, and that temperature therefore is not a suitable one for a study of the stability of smokeless powder. At 75° diphenylnitrosamine attacks nitrocellulose less rapidly than diphenylamine itself, but this is not true at lower temperatures (40° and 60°) at which there is no appreciable difference between the two substances. Carbazol at 110° is an excellent stabilizer but at 60° and 75° is so poor as to... 

The process for the manufacture of ball-grain powder which Olsen and his co-workers have devised 26 combines nicely with Olsen s process for the quick stabilization of nitrocellulose to form a sequence of operations by which a finished powder may be produced more rapidly and more safely than by the usual process. It supplies a convenient means of making up a powder which contains non-volatile solvents throughout the mass of the grains or deterrent or accelerant coatings upon their surface. 

For many years guanidine thiocyanate was the most easily prepared and the most commonly used of the salts of guanidine. Other salts were made from it by metathetical reactions. Nitro-guanidine, prepared from the thiocyanate by direct nitration with mixed acids, was found to contain traces of sulfur compounds which attacked nitrocellulose and affected the stability of smokeless powder, and this is one of the reasons why nitroguanidine powders did not come into early use. Guanidine thiocyanate is deliquescent. Strong solutions of it dissolve filter paper. 

A method, developed by Bergmann and Junk in 1904, for testing the chemical stability of nitrocellulose it was also subsequently employed for testing single-base powders. The test tube, which contains the specimen being tested, and which is equipped with a cup attachment, is heated at 132 °C= 270.4°F for two hours (nitrocellulose) or five hours (single base powders). At the end of the heating period the sample is extracted with water, and the test tube filled to the 50-ml mark with the water in the cup. The solution is filtered, and the content of nitrous oxides is determined by the Schulze-Tiemann method on an aliquot of the filtrate. 

Nitrocellulose powder is heated gradually from icx> upwards as in the deflagration test. A strip of iodide p ier is suspended over the powder and the temperature at which the test-paper gives a coloration is taken as an indication of the stability of the 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. 

Single-base casting powder the casting powder consists of nitrocellulose, stabilizer, solid additives for ballistic modification, and a small amount of plasticizer. The normal ratio of casting powder to casting solvent, 2 1 by volume, yields a final composition of approximately 60% nitrocellulose. 

Soon after the manufacture of nitrocellulose smokeless powder began it was established that the powder obtained by the partial dissolution of nitrocellulose in a mixture of alcohol and ether (partly colloidal powder) has a chemical stability inferior to that of the nitrocellulose from which it derived. Thus Vieille [11] reports that on heating to a temperature of 110°C CPj guncotton undergoes denitration with the evolution of 0.04 cm3 NO/hr/gramme whereas the powder obtained from these substances without a stabilizer undergoes denitration at more than twice the rate, namely 0.10-0.15 cm3 NO/hr/gramme of substance. 

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). 

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