The present invention relates to stabilized nitrocellulose-based propellant compositions. In particular it concerns nitrocellulose-based propellant stabilized with a stabilizer producing little to no carcinogenic and mutagenic by-products.
Smokeless powders have been developed since the 19th century to replace traditional gun powder or black powder, which generates substantial amounts of smoke when fired. The most widely used smokeless powders are nitrocellulose-based. Nitrocellulose is obtained by using nitric acid to convert cellulose into cellulose nitrate and water according to a general reaction:
3HNO3+C6H10O5→C5H7(NO2)3O5+3H2O
Nitrocellulose-based smokeless powder is then obtained by treating the thus obtained nitrocellulose by extrusion or spherical granulation, with or without solvent, two techniques which are well known to the persons skilled in the art.
Various improvements have been developed since the first discovery of nitrocellulose, by addition of further components, such as nitroglycerin and/or nitroguanadine allowing an increase of the detonation velocity. Pure nitrocellulose propellant is referred to as single-base propellant, and double- and triple-base propellants refer to compositions comprising nitrocellulose and one or two additional energetic bases, respectively, typically blasting oils such as nitroglycerin, nitroguanidine, or secondary explosives.
Nitrocellulose, as most nitrate esters, is prone to self-ignition as a result of thermal degradation due to the weakness of its O—N bond. When employed as an ingredient of propellants or other explosive compositions, the spontaneous ignition of nitrocellulose has caused serious accidents. It is obviously vital to inhibit or slow down this degradation for safety reasons but it is also important to retain the initial properties of the energetic composition. Degradation usually leads to gas emissions, heat generation and reduction of molecular mass affecting negatively the material structure and ballistic properties.
The decomposition of the nitrocellulose usually starts with a bond scission or hydrolysis, generating alkoxy radicals and nitrogen oxide (NOx) species (cf.
All conventional stabilisers used to date for nitrocellulose-based propellants belong to (a) aromatic amines (e.g., diphenylamine, 4-nitro-N-methylamine) or (b) aromatic urea derivatives (e.g., akardite, centralite) and are or produce toxic and/or potentially carcinogenic species at some point during the propellant's lifetime. For example, the most widely used stabilizers to date are diphenyl amine, akardite, and centralite. These compounds, however, form carcinogenic derivatives such as N-nitrosodiphenylamine (cf.
Hindered amines, such as triphenylamine, reduce the formation of N—NO groups, but fail to stabilize nitrocellulose satisfactorily. Conventional hindered phenols used in the plastics industry have been tested and at short term stabilize nitrocellulose with little to no N—NO formation. The phenols are able to trap the alkoxy radicals generated during the degradation of nitrocellulose and thus form new, relatively stable alkoxy radicals, by delocalisation of an electron at the foot of electron-rich, hindered groups as illustrated in
There thus remains in the field of solid propellants a need for stabilizers allowing long term stabilization of nitrocellulose-based propellants, fulfilling at least STANAG 4582 (Ed.1) and which do not produce carcinogenic and/or mutagenic by-products. The present invention proposes a family of stabilizers fulfilling both above requirements. These and other advantages of the present invention are presented in continuation.
The present invention is defined in the appended independent claims. Preferred embodiments are defined in the dependent claims. In particular, the present invention concerns a nitrocellulose-based propellant composition comprising:
(a) a nitrate ester based propellant comprising nitrocellulose; and
(b) a stabilizer consisting of a general formula (I):
(i) H;
(ii) unsaturated alkyl group;
R3 represents, H, alkyl substituted or not, or OR8;
Unless otherwise specified, the expression “substituted or not” is to be construed as any —H in a molecule may be substituted by any of an alkyl, alkene, or an aromatic ring. The alkyl or alkene is preferably C1-C9, more preferably C2-C5. A propellant composition is considered as being a “nitrocellulose-based propellant composition” if it comprises at least 40 wt. % nitrocellulose, based on the total weight of the composition.
The nitrate ester based propellant may be a single base propellant consisting of nitrocellulose alone or, alternatively, may be a double or higher base propellant comprising nitrocellulose in combination with at least one blasting oil and/or at least one energetic additive. As known by a person skilled in the art, a blasting oil is herein defined as an energetic compound obtained by nitration of a polyol such as glycerol, glycol, diethylene glycol, triethylene glycol, metriol . . . . The obtained nitrate is most of the time heavy, oily and presents explosive properties. Nitroglycerin is probably the most common blasting oil employed in the industry. The term “NOx” is used herein in its generally recognized sense, as a generic term for mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide).
In a preferred embodiment the blasting oil comprises at least a nitrated polyol, said nitrated polyol is obtained by nitration of polyol selected from a group consisting of glycerol, glycol, diethylene glycol, triethylene glycol and metriol, preferably glycerol.
An energetic additive according to the present invention; like blasting oils, are used to enhance the blasting power of nitrocellulose. Energetic additives can be an energetic plasticizer or an explosive. Examples of energetic plasticizers comprise nitramines such as butyl-NENA or dinitrodiazaalkane (DNDA). Examples of explosives suitable for use as energetic additives include RDX, HMX, FOX7, FOX12, CL20.
The preferred stabilizers of the present invention are capable of reacting with radical alkoxy groups formed by degradation of the nitrate ester by H-abstraction to form a first by-product capable of reacting with NOx formed by degradation of the nitrate ester to form a second by-product comprising no NNO groups. It is even more preferred if the second by-product is itself also capable of reacting with radical alkoxy groups or with NOx formed by degradation of the nitrate ester forming third by-products. Optimally, the third and subsequent by-products are also capable of reacting with such radical alkoxy groups or with NOx, thus substantially prolonging the efficacy of the stabilizer.
It is preferred that the blasting oil comprises at least a nitrated polyol, said nitrated polyol is obtained by nitration of polyol selected from a group consisting of glycerol, glycol, diethylene glycol, triethylene glycol and metriol, preferably glycerol
In a preferred embodiment, R1 in formula (I) represents C1-5 alkyl substituted or not, preferably CH3. It is preferred that R2 represents:
wherein R10 represents H, alkyl substituted or not, or aromatic ring substituted or not.
In one embodiment, the stabilizer is curcumin derivative of formula (II):
The stabilizer of formula (II) is preferably a curcumin derivative of formula (IIa), wherein R1 and R11 are both CH3; R2 and R12 are both OH; and R3 and R13 are both H.
The stabilizer may be present in the composition in an amount comprised between 0.1 and 5.0 wt. %, preferably between 0.2 and 2.0 wt. %, more preferably between 0.5 and 1.5 wt. %, with respect to the total weight of the composition. The nitrate ester-based propellant may comprise nitrocellulose only, thus defining a single base propellant or, alternatively, it may comprise a blasting oil, such as nitroglycerin, to define a double base propellant. A double base propellant according to the present invention preferably comprises not more than 60 wt. % nitroglycerin, and preferably comprises between 5 and 45 wt. %, more preferably between 7 and 22 wt. % nitroglycerin, with respect of the total weight of nitrate ester based propellant.
The propellant compositions of the present invention should fulfil the stability requirements defined in STANAG 4582 (Ed.1), namely generating less than 350 μW/g of heat flow for at least 3.43 days at a temperature of 90° C. Many propellant compositions of the present invention can achieve much better that this and may remain stable for over 30 days at 90° C.
Beside a nitrate ester based propellant and a stabilizer, the propellant compositions of the present invention may comprise additives. In particular, they may comprise one or more of the following additives:
The present invention also concerns the use of a stabilizer of formula (I) as defined above, for stabilizing a nitrocellulose-based propellant composition. The stabilizer is preferably of a formula (II),or (IIa) as defined supra.
For a fuller understanding of the nature of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings in which:
As illustrated in
A stabilizer as used in the present invention has a general formula (I)
(i) H;
(ii) unsaturated alkyl group;
Not wishing to be bound by any theory, it is believed that a stabilizer as defined in the present invention reacts as illustrated in
wherein R10 represents H, alkyl substituted or not, or aromatic ring substituted or not. For example, eugenol (III) or isoeugenol (IV) are suitable stabilizers according to the present invention as shown in
A most preferred embodiment of composition according to the present invention comprises a curcumin derivative of formula (II) as stabilizer.
wherein
In particular a stabilizer of formula (IIa) yields excellent stabilisation properties as illustrated in
The propellant composition may be a simple base propellant, wherein the nitrate ester propellant consists of nitrocellulose only or a double base propellant, wherein nitrocellulose is combined with a blasting oil and/or at least one energetic additive. The most common blasting oil is nitroglycerin.
A propellant composition according to the present invention comprises a stabilizer of formula (I), preferably in an amount comprised between 0.1 and 5.0 wt. %, more preferably between 0.2 and 2.0 wt. %, most preferably between 0.5 and 1.5 wt. %, with respect to the total weight of the composition.
Beside a nitrate ester based propellant and a stabilizer, a propellant composition according to the present invention may comprise additives. In particular, it may comprise one or more of the following additives:
Wherein the wt. % are expressed in terms of the total weight of the propellant composition.
An example of propellant composition according to the present invention is listed in Table 1.
STANAG 4582 (Ed. 1) of Mar. 9, 2007 entitled “Explosives, nitrocellulose-based propellants, stability test procedure and requirements using heat flow calorimetry”, defines an accelerated stability test procedure for single-, double-, and triple base propellants using heat flow calorimetry (HFC). The test is based on the measurement of the heat generated by a propellant composition at a high temperature. Fulfilment of the STANAG 4582 (Ed.1) test qualifies a propellant composition for a 10 year stability at 25° C.
A sample of propellant composition is enclosed in a hermetically sealed vial and positioned in a heat flow calorimeter having a measuring range corresponding to 10 to 500 μW/g. The sample is heated and maintained at a constant temperature of 90° C. for the whole duration of the test and the heat flow is measured and recorded. A heat flow not exceeding 350 μW/g for a period of 3.43 days at 90° C. is considered to be equivalent to at least 10 years of safe storage at 25° C. The graphs of
The propellant compositions of the present invention mark the beginning of the use of a new generation of stabilizers which can be referred to as “green stabilizers,” which combine efficient, long term stability of nitrocellulose-based propellants without formation of any detectable amounts of carcinogenic or mutagenic by-products.
Number | Date | Country | Kind |
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1317423.0 | Oct 2013 | GB | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2014/071041 | 10/1/2014 | WO | 00 |