Nitrocellulose, decomposition

Nitrocellulose is among the least stable of common explosives. At 125°C it decomposes autocatalyticaHy to CO, CO2, H2O, N2, and NO, primarily as a result of hydrolysis of the ester and intermolecular oxidation of the anhydroglucose rings. At 50°C the rate of decomposition of purified nitrocellulose is about 4.5 x 10 %/h, increasing by a factor of about 3.5 for each 10°C rise in temperature. Many values have been reported for the activation energy, E, and Arrhenius frequency factor, Z, of nitrocellulose. Typical values foiE and Z are 205 kj/mol (49 kcal/mol) and 10.21, respectively. The addition of... 

The approach taken in the development of an analytical model for the combustion of double-base propellants has been based on the decomposition behavior of the two principal propellant ingredients, nitrocellulose and nitroglycerin. The results of several studies reviewed by Huggett (HI2) and Adams (Al) show that nitrocellulose undergoes exothermic decomposition between 90° and 175°C. In this temperature range, the rate of decomposition follows the simple first-order expression... 

Like nitrocellulose, nitroglycerin also undergoes a slow first-order exothermic decomposition at temperatures below 140°C. As the pressure is increased, this decomposition reaction is followed by a sudden explosive reaction. Evidence suggests that the explosive reaction is autocatalyzed by the accumulation of N02. The combined results of several studies indicate that... 

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. 

Eisenreich, N., and Pfeil, A., The Influence of Copper and Lead Compounds on the Thermal Decomposition of Nitrocellulose in Solid Propellants, Thermo-chimica Acta, Vol. 27, 1978, pp. 339-346. 

FIG. 2.5 Ballistite, a "smokeless powder" consisting of 60% nitrocellulose and 40% nitroglycerine, produces a thermogram with no transitions detectable prior to exothermic decomposition above 150 "C. 

The products of decomposition of smokeless powder resemble those formed by the decomposition of its ingredients, i.e. nitrocellulose or nitrocellulose with either nitroglycerine or dinitrodiglycol. 

J. Taylor [27] drew up the following table of the heat of explosive decomposition, the gas volume and rate of burning of British nitroglycerine (double base) and American nitrocellulose (single base) powders (Table 166). 

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... 

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... 

Indeed, it was found that amyl alcohol is converted into amyl nitrite and nitrate by the action of nitric oxides resulting from the decomposition of nitrocellulose. These substances are then oxidized to form valeric acid and amyl valerate products distinguishable by their characteristic smell. 

The decomposition of powder at an elevated temperature does not differ greatly from that of the nitric esters themselves, i.e. nitrocellulose and nitroglycerine (the decomposition of nitrocellulose at various temperatures was discussed earlier in Vol. II, p. 310). The higher the temperature, the more actively the decomposition of the powder proceeds, with total loss of nitrogen, as NO and N02, and carbon as CO and C02. Hydrogen is evolved chiefly as water, the amount of water decreasing with increase in the temperature of decomposition of the powder (Sapozhnikov s investigations). 

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. 

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. 

The majority of the stability tests for smokeless powder are much the same as the methods used to determine the stability of nitric esters, in particular nitrocellulose (Vol. II). They are based on heating samples of the powder, thus starting decomposition processes or hastening processes already initiated within the powder. The value of such methods is comparative, since at an elevated temperature different reactions occur than those which would arise under normal conditions of storage. Nevertheless experiments over many years have shown that certain interrelations may be established for the stability of powder at various temperatures. Vieille [61] reports that the heating of a sample of powder for 1 hr at a temperature of 110°C involves approximately the same decomposition as ... 

Investigations have shown that the basic properties of diphenylamine are so weak that it cannot hydrolyse nitrocellulose, but they are sufficiently strong to neutralize any acid product arising either from the decomposition of impurities in the nitrocellulose, from the oxidation of residual solvent or even from decomposition of the nitrocellulose itself. It was also demonstrated that the basic properties of diphenylamine may have a deleterious effect on the powder if the diphenylamine content exceeds 5%. The best stabilizing results are achieved by using 1.0-2.5% diphenylamine. 

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. 

Marqueyrol [74] reported experiments carried out in France over a period of 15 years to compare the efficiency of various stabilizers. The results are shown in Table 175. In addition to amyl alcohol and diphenylamine, the action of N-nitrosodiphenyl-amine (diphenylnitrosamine), carbazole, diphenylbenzamide, nitronaphthalene and naphthalene was also tested. The powder was stored at temperatures of 40, 60 or 75°C. The experiments were stopped when the powder showed signs of intense decomposition, giving off nitric oxides. This was also manifested by a sudden fall pf the nitrogen content in nitrocellulose isolated from the samples. 

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... 

---