SUBSTITUTED PHENOL STABILIZER

Technical Field

[0001] The present invention relates to stabilized nitrocellulose-based propellant compositions. In particular it concerns nitrocellulose-based propellants stabilized with a stabilizer producing little to no carcinogenic and mutagenic by-products.

Background for the invention

[0002] Smokeless powders have been developed since the 1 9 ^th 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:

3HN0 ₃ + C ₆ Hio0 ₅ → C ₆ H ₇ (N02)305 + 3H ₂ 0 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.

[0003] 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. A pure nitrocellulose propellant is referred to as a 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. [0004] 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. [0005] The decomposition of the nitrocellulose usually starts with a bond scission or hydrolysis, generating alkoxy radicals and nitrogen oxide (NOx) species (cf. Figure 1 ). The radicals further react generating more radicals, speeding up the degradation process, and ultimately lead to chain scission accompanied by heat generation. In order to prolong the service life of the propellants, stabilizers are added to the energetic mixture in order to scavenge these radical species and slow down the degradation pattern.

[0006] 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. Figure 2(a)) or N-nitrosoethylphenylamine.

[0007] 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 in the 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 Figure 2(b). The long term stability is, however, not always guaranteed, probably due to rapid phenol depletion and relative stability of the newly formed alkoxy radicals.

[0008] US322881 5 discloses propellant compositions comprising benzophenone derivatives, indicating that the burning rates of the compositions comprising benzophenone were higher than the one of unmodified propellant (cf. c.3, 1.65-72 and Examples VI-XI). A higher burning rate does not suggest that benzophenone may be used as a stabilizer for propellant compositions.

[0009] There thus remains in the field of solid propellants a need for stabilizers allowing long term stabilization of nitrocellulose-based propellants, fulfilling at least STANAC 4582 (Ed. l ) 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.

Summary of the invention

[0010] 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 substituted phenol of general formula (I):

Wherein R ¹ represents, (i) H, (ii) alkyl substituted or not, or (iii) an alkoxy group substituted or not, but not an aromatic group bounded to the phenol through a ketone, and wherein R ² and R ³ are same or different, and represent (i) alkyl substituted or not, or (ii) alkoxy group;

[001 1] Unless otherwise specified, the expression "substituted" and derivatives thereof 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 G -Cg, more preferably C2-C4. 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. An "alkoxy group" refers to any alkyl group, R, singular bonded to oxygen as -R-0-. [0012] 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.

[001 3] 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, FOX1 2, CL20

[0014] The substituted phenol stabilizer of the present invention is preferably a substance capable of reacting by H-abstraction with radical alkoxy groups formed by degradation of the nitrate ester 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. Optimally the second by-product is capable of reaction with radical alkoxy groups or with NOx formed by degradation of the nitrate ester, preferably forming third and subsequent by-products capable of reacting with such radical alkoxy groups or with NOx. [001 5] 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. [0016] In a preferred embodiment, R ¹ represents CH ₃ . It is further preferred that R ² and R ³ represent CH ₃ , yielding a stabilizer of formula (la):

[001 7] Alternatively, R ¹ in formula (I) represents H. It is further preferred that R ² and R ³ are same and represent OR ⁴ , wherein R ⁴ represents an alkyl unsubstituted or substituted with alkoxy group. In particular, R4 can represent CH ₃ , yielding a stabilizer of formula (lb):

[0018] Alternatively, R ² and R ³ may represent

[0019] The substituted phenol 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.

[0020] The propellant compositions of the present invention should fulfil the stability requirements defined in STANAC 4582 (Ed. l ), namely generating less than 350 μ\Λ/ / 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.

[0021] 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:

(a) a potassium salt, such as potassium nitrate (KNO3) or sulphate (K2SO4), preferably in an amount comprised between 0.01 and 1 .5 wt. ;

(b) combustion moderators such as phthalates, CI and citrate derivatives,

preferably in an amount comprised between 1 .0 and 1 0.0 wt. ;

(c) an anti-static agent such as graphite, preferably in an amount comprised between 0.01 and 0.5 wt. ; and

(d) calcium carbonate, preferably in an amount comprised between 0.01 and 0.7 wt. ,

(e) anti-coppering agent such as bismuth or tin oxides, 0.01 to 1 .5 wt. .

Wherein the wt.% are expressed in terms of the total weight of the propellant composition.

[0022] The present invention also concerns the use of a substituted phenol stabilizer of formula (I), preferably of formula (lb) or (la) as defined above, for stabilizing a nitrocellulose-based propellant composition.

Brief description of the Figures

[0023] 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:

Figure 1 : shows a reaction of spontaneous decomposition of nitrocellulose with formation of free radicals and NOx.

Figure 2: shows assumed stabilization mechanisms of (a) akardite (Akll) and diphenylamine (DPA) (prior art), (b) a substituted trimethoxyphenol (prior art), and (c) substituted phenol stabilizer according to the present invention. Figure 3:. shows the normalized heat flow expressed in μ\Λ/ / g generated by a (a) single base nitrocellulose propellant and (b) a double base 90 -nitrocellulose / 1 0 -nitroglycerin propellant, each stabilized with various amounts of a substituted phenol of formula (la). Figure 4: compares the normalized heat flow curves generated by a 90%-nitrocellulose / 1 0%-nitroglycerin double base propellant composition stabilized with a substituted phenol of formula (la) and with DPA according to prior art.

Figure 5 : shows the normalized heat flow expressed in μ\Λ/ / g generated by various nitrocellulose / nitroglycerin double base propellant compositions stabilized with a substituted phenol of formula (la). Detailed description of the invention

[0024] As illustrated in Figure 1 , degradation of nitrocellulose forms free oxide radicals (R-O ) and NOx. These degradation products are capable of reacting further with nitrocellulose, which can rapidly lead to an explosion of the nitrate ester based propellant due to excess heat generation. The most commonly used stabilizers are certainly akardite (Akll) and diphenyl amine (DPA) as illustrated in Figure 2(a). Akardite (Akll) when exposed to NOx, forms carcinogenic N-NO compounds as illustrated in reaction (A) of Figure 2(a). Simultaneously or sequentially, it dissociates upon exposure to heat to form diphenyl amine (DPA) following reaction (B) of Figure 2(a). Whether used directly as stabilizer, or present in the composition following heat dissociation (B) of akardite, diphenyl amine (DPA) stabilizes a propellant composition by the following mechanism. A free radical alkoxy group generated by the propellant abstracts the hydrogen of the amine group of DPA to form a stable compound (ROH) (cf. reaction ® of Figure 2(a)). The radical formed on the amine can react with a NOx to form stable N-nitrosodiphenylamine (cf. reaction @ of Figure 2(a)). The NNO group of N-nitrosodiphenylamine is, however, carcinogenic and should be avoided for safety reasons. Triphenylamine has been tested in the past in order to prevent formation of NNO groups, but with little success in stabilization properties. Hindered phenols as illustrated in Figure 2(b) effectively react with free oxide radicals (R-O ) but forming stable components which are unlikely to further react with NOx (cf. reaction ® of Figure 2(b)). The efficacy of such stabilizers is limited to short periods of time only because of rapid phenols depletion.

[0025] A substituted phenol stabilizer as used in the present invention has the following general formula (I):

Wherein R ¹ represents: (i) H, (ii) alkyl substituted or not, or (iii) an alkoxy group, but not an aromatic group bound to the phenol through a ketone; and R ² and R ³ are same or different, and represent (i) alkyl substituted or not, or (ii) alkoxy group.

[0026] In a preferred embodiment, R ¹ represents CH ₃ . Again, R ² and R ³ can be same, and preferably represent CH ₃ , yielding a stabilizer of formula (la):

[0027] In another embodiment, R ¹ represents H. R ² and R ³ are then preferably same. R ² and R ³ can be alkyls, in particular tert-C4H ₉ . Alternatively, R ² and R ³ can be an alkoxy group, advantageously, methoxy, yielding a stabilizer of formula (lb):

[0028] Alternatively, R ² and R ³ may represent tert-C4H ₉ [0029] Not wishing to be bound by any theory, it is believed that a stabilizer of formula (I) as defined in the present invention reacts as illustrated in Figure 2(c) by first neutralising a radical alkoxy group by H-abstraction to form a radical capable of reacting with NOx by derealization of the radical within the aryl ring (cf. reactions ®&@ of Figure 2(c)). At this stage, the invention has already solved a first problem of providing a stabilizer capable of stabilizing a nitrocellulose-based propellant at least as efficiently as diphenylamine, without generating NNO-groups. It is believed, however, that the stabilizers of the present invention yield by-products capable, after tautomerization, of further reacting in second cycles and even in third cycles with further radical alkoxy groups and NOx, thus substantially prolonging the stabilizing action of the stabilizers (cf. reactions ©&0 in Figure 2(c)).

[0030] The propellant composition may be a simple base propellant, wherein the nitrate ester propellant consists of nitrocellulose only. Figures 3(a) illustrates the stability of a simple base propellant composition stabilized with various amounts of a stabilizer (la) according to the present invention.

[0031] Alternatively, the propellant composition may be 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. Figure 5 illustrates the stability of various double base propellant compositions wherein nitrocellulose is combined with 1 0, 20, and 40 wt.% nitroglycerin, a commonly used blasting oil, and stabilized with 2.0 wt.% of a stabilizer of formula (la). Blasting oils preferably comprise at least a nitrated polyol, said nitrated polyol being obtained by& nitration of polyol selected from a group consisting of glycerol, glycol, diethylene glycol, triethylene glycol and metriol, preferably glycerol. Nitroglycerin is a preferred blasting oil. Energetic additives, on the other hand, can be an energetic plasticizer selected from the group of nitramines such as butyl-NENA, dinitrodiazaalcane (DNDA), or an explosive such as RDX, HMX, FOX7, FOX1 2, CL20. A double base propellant composition according to the present invention preferably comprises a nitrate ester based propellant comprising not more than 60 wt.% blasting oil (such as nitroglycerin) or energetic additive with respect to the total weight of nitrate ester based propellant. More preferably, it comprises between 5 and 45 wt.%, most preferably between 7 and 22 wt.% blasting oil or energy additive, with respect of the total weight of nitrate ester based propellant. [0032] A propellant composition according to the present invention comprises a substituted phenol 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. Figure 3 illustrates the stability of (a) a single base propellant composition and (b) a 90wt.%- nitrocellulose / 1 0 wt.%-nitroglycerin double base propellant composition, wherein in both cases the compositions are stabilized with various amounts of a stabilizer according to formula (la) comprised between 0.25 and 2.00 wt.% Figure 5 compares the stability curves of propellant compositions stabilized with 2.00 wt.% stabilizer of formula (la) for different amounts of nitroglycerin (NCL) in the compositions, ranging from 0 wt.% NCL (= single base composition) to 40 wt.% NCL.

[0033] 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:

(a) a potassium salt, such as potassium nitrate (KNO3) or sulphate (K2SO4),

preferably in an amount comprised between 0.01 and 1 .5 wt. %;

(b) combustion moderators such as phthalates, centralite and citrate derivatives, preferably in an amount comprised between 1 .0 and 1 0.0 wt. %;

(c) an anti-static agent such as graphite, preferably in an amount comprised

between 0.01 and 0.5 wt. %; and

(d) calcium carbonate, preferably in an amount comprised between 0.01 and 0.7 wt. %,

(e) anti-coppering agent such as bismuth or tin oxides, 0.01 to 1 .5 wt.%.

Wherein the wt.% are expressed in terms of the total weight of the propellant composition. [0034] Table 1 lists an example of propellant composition according to the present invention.

Table 1 : typical propellant compositions according to the present Invention

EXPERIMENTAL TESTS

[0035] STANAC 4582 (Ed. 1 ) of March 9, 2007 entitled "Explosives, nitrocellulose-based propel lants, 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 STANAC 4582 (Ed. l ) test qualifies a propellant composition for a 1 0 year stability at 25°C.

[0036] 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 1 0 to 500 μνν/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 μ\Λ/ / g for a period of 3.43 days at 90°C is considered to be equivalent to at least 1 0 years of safe storage at 25°C. The graphs of Figures 3 to 5 are plots of such measurements. The full scale of the ordinate (normalized heat flow) corresponds to a value of 350 }\N / g not to be exceeded according to STANAC 4582 (Ed. l ), and the vertical straight line indicates 3.43 days. The initial heat flow peak comprised within the shaded area of the graphs of Figures 3 to 5 is ignored as it is not representative of any specific reaction or phase transformation of the propellant composition, provided it does not exceed an exotherm of 5 J.

[0037] Figure 3(a) shows the results of the stability tests carried out on a single-base nitrocellulose propellant stabilized with various amounts of a stabilizer according to formula (la) comprised between 0.25 and 2.00 wt.%. It can be seen that even with as little as 0.25 wt.% stabilizer the heat flow never exceeds 1 50 μ\Λ/ / g for 3.43 days, when STANAC 4582 (Ed. l ) requires to maintain the heat flow below 350 μ\Λ/ / g (full scale of the ordinate). The time scale of the graph of Figure 3 extends to 20 days, and it can be seen that after a first exotherm of up to about 1 50 μ\Λ/ / g, the heat flow curves of all the samples level at a value of less than 1 00 μ\Λ/ / g for a long period, extending up to over 20 days for the composition comprising 2.0 wt.% stabilizer (solid line). Such long term stability is quite exceptional, considering that maintaining an exotherm below 350 μ\Λ/ / g during 3.43 days at 90°C is considered according to STANAC 4582 to be equivalent to a safe storage at 25°C of a propellant composition for ten years. [0038] Figure 3(b) shows the results of the same stability tests as reported in Figure 3(a), but carried out on a 90wt.%-nitrocellulose / 1 0 wt.%-nitroglycerin double base propellant composition. It can be seen by comparing Figures 3(a) and (b) that the presence of nitroglycerin increases slightly the reactivity of the propellant composition, in particular for the sample comprising 2.0 wt.% stabilizer (solid line), with a normalized heat flow below Ι ΟΟ μνν / g for a single base propellant (Figure 3(a)), and with values thereof at about 1 30 μ\Λ/ / g agter 8 days for a double base propellant comprising 1 0 wt.% nitroglycerin. All the other curves level out at a value close to about 1 00 μ\Λ/ / g after a first peak at about 1 50-1 60 μνν / g. In all cases, the curves in Figure 3(b) are well within the boundaries defined by STANAC 4582 (i.e., time up to 3.43 days (cf. vertical line) and full scale ordinate). [0039] Figure 4 compares the stability of a double-base propellant composition (90 wt.% nitrocellulose and 1 0 wt.% nitroglycerin) stabilized with, on the one hand, 0.5 wt.% of a stabilizer of formula (la) according to the present invention (short dashed line) and, on the other hand, with 0.7 wt.% of diphenyl amine (DPA) of the prior art (long dashed line). It can be seen that both stabilizers (Stabilizer (la) and DPA) fulfil the requirements of STANAC 4582 (Ed. l ), The stabilizer (la) according to the present invention is, however, advantageous over DPA because,

(a) Contrary to DPA, stabilizers according to the present invention do not generate any N-NO containing carcinogenic by-product upon their stabilization activity.

(b) DPA curve (dashed line) shows after two days a sharp peak stabilizing in a plateau at higher heat flow values (about 1 5C^W / g),. By contrast, no discontinuity in the heat flow can be identified with stabilizer (la) over 3.5 days, (and even over 8 days when referring to Figure 3(b) (short dashed line). [0040] Figure 5 shows the results of the stability tests carried out on double-base nitrocellulose propellants containing various amounts of nitroglycerin (1 0, 20, and 40 wt.%) stabilized with 2.0 wt.% of a stabilizer of formula (la). For comparison, the stability curve of a single base propellant containing 2.0 wt.% stabilizer (la) is also illustrated (solid line). As discussed above in reference to Figures 3(a)&(b), it is obvious from Figure 5 that the presence of nitroglycerin in the composition increases the activity of thereof, as indicated by the higher values of the maximum exothermic peak for double base compositions cumminating at about 200 μ\Λ/ / g for nitroglycerin contents higher than 30 wt.%. In all cases, however, the exothermic curves drop after the first peak to values below 1 50 μ\Λ/ / g, characterizing stable compositions. The drop of the exothermic curves is indicative of long term efficient reactions involving the stabilizer.

[0041] 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.