LONG UNSATURATED ALIPHATIC CHAINS AS STABILISERS FOR NITRATE ESTERS AND NITROCELLULOSE-BASED APPLICATIONS

Abstract

A nitrocellulose-based composition for use as a propellant or as a combustible item is provided. The nitrocellulose-based composition includes a nitrate ester-based component including nitrocellulose; and a stabiliser (I) in the form of a long aliphatic chain having at least two unsaturation sites, the compound having a molecular weight of at least (120) and an iodine number of at least (25).

Description

TECHNICAL FIELD

The present invention relates to stabilised nitrate ester-based combustible compositions for use as propellants and combustible items. In particular, it concerns nitrocellulose-based compositions stabilised with a novel stabiliser producing little to no carcinogenic and mutagenic by-products.

BACKGROUND FOR THE INVENTION

Nitrocellulose is the most widely used nitrate ester for the production of energetic materials such as propellants, combustible items, nitro-films or components inherent to these applications. Nitrocellulose is obtained by using nitric acid to convert cellulose into cellulose nitrate and water according to a general reaction:

3HNO₃+C₆H₁₀O₅→C₆H₇(NO₂)₃O₅+3H₂O

Nitrocellulose-based propellants are subsequently obtained by treating the hitherto obtained nitrocellulose by extrusion, spherical or mechanical granulation, with or without solvent, three techniques that are well known to the persons skilled in the art. Combustible items such as modular charges or combustible cases are made using the blistering technique, also well known to the persons skilled in the art.

Various improvements have been made since the first discovery of nitrocellulose, by addition of further components, such as nitroglycerine (another nitrate ester) and/or nitroguanidine allowing an increase of the energetic content. Pure nitrocellulose propellants are referred to as single-base propellants, and double- and triple-base propellants refer to compositions comprising nitrocellulose and one or two additional energetic bases, respectively; typically blasting oils such as nitroglycerine, 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 a component 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 negatively affecting the material structure and ballistic properties.

The decomposition of nitrate esters usually starts with a bond scission or hydrolysis, generating alkoxy radicals and nitrogen oxide (NOx) species (cf. FIG. 1). The radicals further react, generating more radicals, speeding up the degradation process, and ultimately leading to chain scission accompanied by heat generation. In order to prolong the service life of the propellants, stabilisers are added to the energetic mixture to scavenge these radical species and slow down the degradation pattern.

Most conventional stabilisers used to date for nitrocellulose-based propellants and combustible items belong to (a) aromatic amines (e.g., diphenylamine, 4-nitro-N-methylamine) or (b) aromatic urea derivatives (e.g., akardite, centralite). They are/or produce toxic and/or potentially carcinogenic species at some point during a propellant's lifetime. For example, the most widely used stabilisers to date are diphenylamine, Akardite, and centralite. These compounds, however, form carcinogenic derivatives such as N-nitrosodiphenylamine (cf. FIG. 2) or N-nitrosoethylphenylamine.

Hindered amines, such as triphenylamine, reduce the formation of N—NO groups, but fail to stabilise nitrate esters satisfactorily. Conventional hindered phenols used in the plastics industry have been tested and for short amounts of time stabilise 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. Long term stability is, however, not always guaranteed, probably due to rapid phenol depletion and relative stability of the newly formed alkoxy radicals.

Investigating a new generation of non-carcinogenic stabilisers, a nitrocellulose-based propellant composition stabilised with a tocopherol was described in WO2016135228, with a substituted phenol in WO201 5049284, with a trialkoxy benzene in WO201 5049285, with a non-aromatic ionone in WO2016135227.

There thus remains in the field of solid propellants and combustible items much room for the development of stabilisers allowing long term stabilisation 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 novel family of stabilisers fulfilling both of the above requirements. These and other advantages of the present invention are presented in continuation.

SUMMARY OF THE INVENTION

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 composition for use as a propellant or as a combustible item, said nitrocellulose-based composition comprising:

• • (a) a nitrate ester-based component comprising nitrocellulose; and
  • (b) a stabiliser (I) in the form of a long aliphatic chain having at least two unsaturation sites, the compound having a molecular weight of at least 120 and an iodine number of at least 25.

In the present invention, a “long aliphatic chain” is understood as an aliphatic chain comprising at least 7 CH_i groups, with i=0 to 3.

In a preferred embodiment, the nitrate ester-based component consists of nitrocellulose alone thus defining a single base composition. Alternatively, the nitrate ester-based component consists of a mixture comprising nitrocellulose in combination with at least a blasting oil and/or at least one energetic additive, thus defining a double or higher base composition. The blasting oil can comprise at least a nitrated polyol, which is obtained by nitration of a polyol selected from a group consisting of glycerol, glycol, diethylene glycol, triethylene glycol and metriol, preferably glycerol. At least one energetic additive can be an energetic plasticizer selected from the group of nitramines, such as butyl-NENA, dinitrodiazaalkane (DNDA), or is an explosive such as RDX, HMX, FOX-7, FOX-12, CL20, SMX. For example, the nitrate ester-based component may comprise up to 60 wt. % nitroglycerine, such as between 5 and 45 wt. %, preferably between 7 and 22 wt. % nitroglycerine, with respect to the total weight of nitrate-ester based propellant.

It is preferred that the stabiliser be a component capable of reacting with both degradation products of the nitrate ester, namely alkoxy radicals and NOx, mainly by hydrogen abstraction of one labile proton of the stabiliser, located in the alpha-position of an unsaturation generating another free radical species able, e.g. to further react with the NOx counterpart. For example, the stabiliser can be selected from one or more of the following components and their derivatives:

• • terpenes or terpenoids, including squalene, farnesol, myrcene, thymol, citral, limonene, lycopene,
  • carotenoids,
  • omega 3-6-9 unsaturated fatty acids including one or more of arachidonic acid, linolenic acid, mead acid, herring acid
  • triglycerides (i.e. oils) derived from the above-mentioned fatty acids including linseed oil,
  • (co-)polymers including polyenes, unsaturated polyester resins, homopolymers or copolymers of one or more of butadiene, isoprene, or pentadiene.

The stabiliser preferably consists of a component selected from a group consisting of polybutadiene, polyisoprene, squalene, farnesol, limonene, myrcene, lycopene and citral, preferably the stabiliser is polybutadiene.

The stabiliser is preferably 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 1.0 wt. %, with respect to the total weight of the composition.

The nitrocellulose-based composition may comprise one or more complementary stabilisers. For example, the complementary stabiliser can be selected from one or more of the following components:

(a) a substituted phenol compound (2) having the general formula (2-I):

wherein: R⁶ represents: (i) H, (ii) alkyl substituted or not, or (iii) an alkoxy group; and R⁷ and R⁸ are same or different, and represent (i) alkyl substituted or not, or (ii) alkoxy group;

(b) a trialkoxy benzene (3) having the general formulae (3-I) or (3-II):

wherein R⁹, R¹⁰ and R¹¹ are same or different and represent C_1-5 alkyl unsubstituted or substituted with an alkoxy group; and

(c) an aromatic compound (4) having a general formula (4-I):

wherein: R¹² represents, alkyl-substituted or not; R¹³ represent (i) H, (ii) unsaturated alkyl group,

• R¹⁴ represents, H, alkyl-substituted or not, or OR¹⁸;
• R¹⁵ represents, alkyl-substituted or not, aromatic ring-substituted or not, or OR¹⁸;
• R¹⁶ represents, alkyl-substituted or not, aromatic ring-substituted or not, or OR¹⁹;
• R¹⁷ represents, aromatic ring-substituted or not;
• R¹⁸ represents, alkyl-substituted or not, or aromatic ring-substituted;
• R¹⁹ represents, alkyl-substituted or not, or aromatic ring-substituted.

(d) a substituted phenol compound (5) having the general formula (5-I):

wherein: R²⁰, R²¹ and R²² are the same or different and represent: (i) alkyl-substituted or not, (ii) alkoxy group.

(e) a substituted phenol compound (6) having the general formula (6-I):

wherein: R²³, R²⁴, R²⁵ and R²⁶ are the same or different and represent: (i) alkyl-substituted or not, (ii) alkoxy group.

The nitrocellulose-based composition may also comprise one or more of the following additives:

• • (a) a potassium salt, preferably potassium nitrate (KNO₃), and more preferably in an amount comprised between 0.001 and 1.5 wt. %;
  • (b) combustion moderators, such as phthalates, CI and citrate derivatives, preferably in an amount comprised between 1.0 and 10.0 wt. %
  • (c) an anti-static agent, preferably graphite, and more 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. %,wherein the wt. % are expressed in terms of the total weight of the nitrocellulose-based composition.

The nitrocellulose-based composition according to the present invention, preferably has a stability measured according to STANAG 4582 (Ed. 1) at a temperature of 90° C. without heat generation above 350 μW/g of at least 3.43 days, preferably of at least 5 days, more preferably of at least 10 days.

The present invention also concerns the use of a component (I) consisting of a long aliphatic chain having at least two unsaturation sites for stabilising a nitrate-ester based composition comprising nitrocellulose for applications including propellants and combustible items.

BRIEF DESCRIPTION OF THE FIGURES

For a more complete 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:

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

FIG. 2: shows the assumed stabilisation mechanisms of akardite (AkII) and diphenylamine (DPA) (prior art).

FIG. 3: shows the normalised heat flow expressed in μW/g generated by nitrocellulose-based compositions at 90° C. with no stabiliser, for comparison purposes (1B=single base, and 2B=10 wt. % NGL double base compositions). The fine dashed lines represent the minimum requirements of STANAG 4582.

FIG. 4: shows the normalised heat flow expressed in μW/g generated by propellant compositions stabilised with various stabilisers of the present invention: polybutadiene (la), polyisoprene (Ib) and polystyrene (Ic), as well as the conventional stabiliser DPA (prior art) for double base nitrocellulose/nitroglycerine (60/40 wt. %) propellants at 90° C.

FIG. 5: shows the normalised heat flow expressed in μW/g generated by propellant compositions stabilised with 1 wt. % of various terpenoid stabilisers: squalene (Id) and limonene (le), for double base nitrocellulose/nitroglycerine (90/10 wt. %) propellants.

FIG. 6: illustrates the chemical pathways of (a) a 1,5-diene moiety (e.g. squalene, polybutadiene) and (b) of a 1,4-diene moiety (e.g., arachidonic acid).

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1, the degradation of nitrocellulose forms free alkoxy radicals 2 (R—O) and NOx 3. These degradation products are capable of reacting further with nitrocellulose 1, which can rapidly lead to an explosion of the nitrate ester-based propellant due to excess heat generation. The most commonly used stabilisers are Akardite (AkII) 4 and diphenylamine (DPA) 5 as illustrated in FIG. 2(a)&(b). Akardite (AkII) 4 when exposed to NOx, forms carcinogenic N—NO compounds 6 as illustrated in reaction (B) of FIG. 2(a). Simultaneously or sequentially, it dissociates upon exposure to heat to form diphenylamine (DPA) 5. Whether used directly as a stabiliser, or present in the composition following heat dissociation of Akardite 4, diphenylamine (DPA) 5 stabilises 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 5 to form a stable compound (ROH, 9) (cf. reaction (C) of FIG. 2(b)). The radical formed on the amine 8 can react with a NOx to form stable N-nitrosodiphenylamine 10 (cf. reaction (D) of FIG. 2(b)). The NNO group of N-nitrosodiphenylamine 10 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 stabilisation properties. Hindered phenols effectively react with free oxide radicals (R—O) but form stable components which are unlikely to further react with NOx. The efficiency of such stabilisers is limited to short periods of time only because of rapid phenol depletion.

A nitrocellulose-based composition according to the present invention can be used as a propellant or as a combustible item, such as modular charges or combustible cases. The composition comprises a nitrate ester-based component comprising nitrocellulose, and a stabiliser (I) which does not produce carcinogenic components. The stabiliser (I) as used in the present invention is a compound presenting a long aliphatic chain having at least two unsaturation sites, the compound having a molecular weight of at least 120 and an iodine number of at least 25. The long aliphatic chain comprises at least 7 CH_i groups, with i=0 to 3, preferably at least 12 groups. The iodine number is determined according to ISO 3961:2013, as follows: mass of iodine in grams that is consumed by 100 grams of the tested chemical substance and is used to determine the amount of unsaturation of that chemical substance.

The nitrate ester-based component may consist of nitrocellulose alone thus defining a single base composition, or of a mixture comprising nitrocellulose in combination with at least a blasting oil and/or at least one energetic additive, thus defining a double or higher base composition. The blasting oil may comprise at least a nitrated polyol, which is obtained by nitration of a polyol selected from a group consisting of glycerol, glycol, diethylene glycol, triethylene glycol and metriol, preferably glycerol, and wherein the at least one energetic additive is an energetic plasticizer selected from the group of nitramines, such as butyl-NENA, dinitrodiazaalkane (DNDA), or is an explosive such as RDX, HMX, FOX-7, FOX-12, CL20, SMX. Nitroglycerol is a particularly preferred blasting oil for forming a double base composition. Beside nitrocellulose, a double base propellant composition according to the present invention preferably comprises not more than 60 wt. % blasting oil (such as nitroglycerine) 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 energetic additive, with respect to the total weight of nitrate ester-based propellant. A preferred blasting oil is nitroglycerine (NGL).

STANAG 4582 requires that a propellant composition yields a stability defined by a generation of energy not exceeding 350 μW/g during at least 3.43 days at a temperature of 90° C. FIG. 3 illustrates the stability of a single base (1 B, dashed line) and a double base (2B, solid line) propellant compositions without any stabiliser at a temperature of 90° C. The fine dashed lines illustrate the minimum requirements of STANAG 4582. It can be seen that if the single base composition 1B is rather stable in time, fulfilling the requirements of STANAG 4582 without a stabiliser, this is not the case of a double base composition comprising 10 wt. % nitroglycerine (NGL), which goes into autocatalyisis after two days at 90° C. Double base compositions must necessarily be stabilised to be commercialized in countries imposing STANAG 4582.

Preferred examples of aliphatic, unsaturated stabilisers (I) according to the present invention, include:

• • terpenes/terpenoids such as squalene, farnesol, myrcene, thymol, citral, limonene, and the like,
  • carotenoids such as lycopene,
  • omega 3-6-9 unsaturated fatty acids such as arachidonic acid, linolenic acid, mead acid, herring acid, and the like) and their derivatives (oils e.g. linseed oil or condensed products)
  • polymeric compounds such as polyenes, unsaturated polyester resins and particularly preferred polymeric compounds include homopolymers or copolymers of butadiene, isoprene, pentadiene, and the likeMany of these compounds are commercially available.

The stabiliser is preferably a component capable of reacting with both degradation products of the nitrate ester, namely alkoxy radicals and NOx, mainly by hydrogen abstraction of one labile proton of the stabiliser, located in the alpha-position of an unsaturation. This reaction converts, for example, the unstable alkoxy nitrocellulose derivative into a stable alcohol compound (and therefore terminating the degradation process of the nitrocellulose) and a first by-product able to trap the NOx species or delocalise its free radical using the adjacent double bond. It can subsequently react with, for example, a NOx species or possibly undergo another hydrogen abstraction of another proton located in alpha-position of another unsaturation, yielding a diradical moiety that can eventually form a conjugated system or a bridged molecule. The thus formed successive by-products are capable of further reacting with the degradation products of the nitrate ester, following the hitherto described scheme, producing no harmful NNO groups and ensuring a good chemical stability of the energetic material.

Not wishing to be bound by any theory, it is believed that a stabiliser (I), as defined in the present invention works by repetitive sequences of H-abstractions of labile hydrogen in the alpha positions of unsaturations of the stabiliser by unstable alkoxy radicals generated during the degradation of the nitrate ester (cf. FIG. 1, molecule 2) and subsequent recombinations either with other carbon-based free-radicals generated similarly or NOx species (cf. FIG. 1, molecule 3) arising from the ageing of the nitrate ester. This series of reactions converts unstable alkoxy nitrocellulose 2 derivatives into stable alcohol compounds and converts NOx species 3 into stable nitrated compounds and thus preventing the unwanted decomposition process of the nitrocellulose. The successive by-products thus formed are capable of further reacting with the degradation products of the nitrate ester, following the hitherto described scheme, producing no harmful NNO groups and providing a good chemical stability of the energetic material. This is particularly true for polymeric stabilisers such as polybutadiene or polyisoprene.

Two examples of chemical pathways are given in FIG. 6. FIG. 6(a) illustrates the chemical pathways of a 1,5-diene moiety (e.g. squalene, polybutadiene) and FIG. 6(b) illustrate the pathways of a 1,4-diene moiety (e.g., arachidonic acid). In FIG. 6, R can take any value and can be same or different at both ends of a chain. The nature of the final products depends on the nature of the substituents (e.g. an electron-withdrawing group would favour the hydrogen abstraction whilst a bulky substituent would rather favour the delocalisation of the charge). In the case of 1,5-diene moiety illustrated in FIG. 6(a), the unstable alkoxy nitrocellulose 2 derivative abstracts the labile hydrogen of the stabiliser (I) converting it into a stable alcohol compound (and therefore preventing the unwanted decomposition process of the nitrocellulose) and a first by-product 12, able to trap the NOx species generating 17, or delocalise its free radical using the adjacent double bond, yielding 13. It can subsequently react with e.g. a NOx species giving 16, or with another carbon-based free radical arising from the same or another stabiliser molecule giving bridged or cross-linked molecules 18, or possibly undergo another hydrogen abstraction of another proton located in alpha position of another unsaturation, yielding a diradical moiety 14 that can eventually form a conjugated system 15. The same applies to the 1,4-diene moiety as shown in FIG. 6b with the exception that conjugated systems cannot be formed.

The successive by-products 12 to 24 illustrated in FIG. 6(a)&(b) are capable of reacting with more NOx and alkoxy radicals from the degradation of the nitrate ester 1, and thus substantially increase the efficiency of the stabiliser function. Since no harmful NNO groups are formed due to the lack of nitrogen atoms in compound (I) structure, the stabiliser according to the present invention produces little to no carcinogenic and mutagenic N-nitroso by-products.

In a preferred embodiment, the stabiliser is selected from a group consisting of polybutadiene, polyisoprene, squalene, farnesol, limonene, myrcene, lycopene, citral, polystyrene.

A preferred stabiliser is polybutadiene (Ia) (any type/combination)

Another preferred stabiliser is polyisoprene (Ib)

Another preferred stabiliser is polystyrene (Ic):

Another preferred stabiliser is squalene (Id):

A propellant composition according to the present invention comprises a stabiliser (I), in the form of a long aliphatic chain having at least two unsaturation sites, the compound having a molecular weight of at least 120 and an iodine number of at least 25. The stabiliser is preferably present 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. FIG. 4 illustrates the stability of a double base propellant composition (60 wt. % nitrocellulose and 40 wt. % NGL) stabilised with 1 wt. % of various polymeric stabilisers (I) according to the present invention: polybutadiene (Ia), polyisoprene (Ib) and polystyrene (Ic), as well as the conventional stabiliser DPA (prior art). Compared with the non-stabilised double base composition (2B) of FIG. 3, blasting after two days at 90° C. with only 10 wt. % NGL, it can be seen that all the stabilised compositions of FIG. 4 pass the STANAG 4582 minimum requirement of 3.43 day below 350 mW/g heat flow. Prior art stabiliser DNA, however, becomes unstable after 7 days and releases carcinogenic components. The polymeric stabilisers (Ia-c) according to the present invention, however, are perfectly stable after 9 days at 90° C., without generation of any identified carcinogenic components.

FIGS. 5 illustrates the stability of a double base propellant composition (90 wt. % nitrocellulose and 10 wt. % NGL) stabilised with 1 wt. % of various terpene stabilisers (I) according to the present invention: squalene (Id) and limonene (Ie).

It can be seen in FIGS. 4 and 5 that a double base propellant composition stabilised with 1 wt. % of a stabiliser of according to the present invention maintains the level of energy released at 90° C. perfectly stable below 200 μW/g for 9 days and longer.

Double base propellants compositions containing nitroglycerine (NGL) are particularly less stable than single base propellants. As shown in FIG. 3, with as little as 10 wt. % NGL, a double base propellant not containing any stabiliser enters into autocatalysis after only 2 days. The stabilising properties of a stabiliser according to the present invention are at least as good—and often substantially better—than those of DPA, without forming any carcinogenic NNO-components. As shown in FIGS. 4 and 5, a double base propellant composition stabilised with a stabiliser (I) according to the present invention has a stability measured according to STANAG 4582 (Ed. 1) at a temperature of 90° C. without heat generation above 350 μW/g of at least 3.43 days, preferably of at least 5 days, more preferably of at least 10 days, and more.

Even longer stabilisation times can be obtained by combining a stabiliser (I) according to the present invention with a complementary stabiliser in the form of an aromatic compound. This second stabiliser has a different, complementary stabilisation mechanism and is believed to provide a synergistic effect. The complementary stabiliser is preferably selected from the following group:

(a) a substituted phenol compound (2) having the general formula (2-I):

wherein: R⁶ represents: (i) H, (ii) alkyl substituted or not, or (iii) an alkoxy group; and R⁷ and R⁸ are same or different, and represent (i) alkyl substituted or not, or (ii) alkoxy group;

(b) a trialkoxy benzene (3) having the general formulae (3-I) or (3-II):

wherein R⁹, R¹⁰ and R¹¹ are same or different and represent C_1-5 alkyl unsubstituted or substituted with an alkoxy group; and

(c) an aromatic compound (4) having a general formula (4-I):

Wherein: R¹² represents, alkyl substituted or not; R¹³ represent (i) H, (ii) unsaturated alkyl group,

• R¹⁴ represents, H, alkyl-substituted or not, or OR¹⁸;
• R¹⁵ represents, alkyl-substituted or not, aromatic ring-substituted or not, or OR¹⁸;
• R¹⁶ represents, alkyl-substituted or not, aromatic ring-substituted or not, or OR¹⁹;
• R¹⁷ represents, aromatic ring-substituted or not;
• R¹⁸ represents, alkyl-substituted or not, or aromatic ring-substituted;
• R¹⁹ represents, alkyl-substituted or not, or aromatic ring-substituted.In a preferred embodiment, R¹² represents C_1-5 alkyl-substituted or not, preferably CH₃; further, it is preferred that R¹³ represents:

Wherein R28 represents H, alkyl-substituted or not, or aromatic ring, substituted or not. For example, eugenol (4-III) or isoeugenol (4-IV) are suitable complementary stabilisers according to the present invention.

A more preferred embodiment of composition according to the present invention comprises a curcumin derivative of formula (4-II) as a complementary stabiliser,

wherein

• R¹² and R²⁹ are the same or different and represent alkyl substituted or not, preferably C_1-5, more preferably CH₃; R¹⁴ and R³⁰ are the same or different and represent H or alkyl substituted or not (e.g., C_1-5 alkyl), wherein each of R¹² and R²⁹, and R¹⁴ and R³⁰, are preferably the same, and more preferably both are H.

(d) a substituted phenol compound (5) having the general formula (5-I):

wherein: R²⁰, R²¹ and R²² are the same or different and represent: (i) alkyl-substituted or not, (ii) alkoxy group.

(e) a substituted phenol compound (6) having the general formula (6-I):

wherein: R²³, R²⁴, R²⁵ and R²⁶ are the same or different and represent: (i) alkyl-substituted or not, (ii) alkoxy group.

Besides a nitrate ester-based component and a stabiliser, 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. %, acting as a muzzle flash reducer;
  • (b) combustion moderants, to slow the burning rate of the propellant, such as phthalates, centralite and citrate derivatives, preferably in an amount comprised between 1.0 and 10.0 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, a base to neutralize residual acidity in the nitrocellulose, preferably in an amount comprised between 0.01 and 0.7 wt. %,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.

TABLE 1
typical propellant compositions according to the present invention
		single base	double base
	component	wt. %	wt. %
	nitrocellulose	89.0-95.0	82.0-84.0
	nitroglycerine	0.0	7.0-11.0
	stabiliser of formula (I)	0.15-2.0	0.15-2.0
	KNO3	0.5-1.0	0.5-1.0
	dibuthylphthalate	2.0-7.0	2.0-7.0
	graphite	0.2-0.4	0.2-0.4
	calcium carbonate	<0.7	<0.7

EXPERIMENTAL TESTS

STANAG 4582 (Ed. 1) of Mar. 9, 2007 entitled “Explosives, nitrocellulose based propellants, stability test procedure and requirements using heat flow calorimtry”, 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 FIGS. 3, 4 and 5 show the stability of compositions as a function of time measured as defined above over a period of 9 days. The fine horizontal dashed line corresponds to a value of 350 μW/g not to be exceeded according to STANAG 4582 (Ed.1), and the fine vertical dashed line indicates 3.43 days. The initial heat flow peaks of graphs of FIGS. 3, 4 and 5 are ignored as they are not representative of any specific reaction or phase transformation of the propellant composition, provided they do not exceed an exotherm of 5 J.

As discussed above, FIG. 3 shows the stability of unstabilised single base (1 B) and double base (2B) compositions. After two days at 90° C. a double base composition comprising 10 wt. % NGL becomes unstable and goes to autocatalysis. FIGS. 4 and 5 show the results of the stability tests carried out on double base nitrocellulose based propellants comprising 40 wt. % and 10 wt. % nitroglycerine, respectively, and stabilised with 1 wt. % of a stabiliser (la-e)) according to the present invention. It can be seen that the heat flow never exceeds 200 μW/g for 9 days, when STANAG 4582 (Ed.1) requires to maintain the heat flow below 350 μW/g for 3.43 days. For comparison, FIG. 4 also shows the stability of a corresponding composition stabilised with 1 wt. % of a state of the art diphenylamine (DPA) stabiliser. The composition remains stable for a week and becomes unstable.

As can be seen in FIG. 4, both DPA and stabiliser (I) fulfil the requirements of STANAG 4582 (Ed.1). Stabiliser (I) according to the present invention is, however, advantageous over DPA and Akardite because,

• • (a) there is significant increase in performance and therefore a longer use and storage in safety: DPA curve (solid line) shows a thermal runaway for a double base propellant containing 40% of nitroglycerine after less than 7 days where stabilisers according to the present invention (la, lb and Ic remain below 200 μW/g for over 9 days, as shown in FIG. 4.
  • (b) contrary to DPA, stabilisers according to the present invention do not generate any N—NO carcinogenic by-products from their stabilisation activities.
  • (c) DPA curve (solid line) shows a sharp peak stabilising in a plateau at higher heat flow values, suggesting that all DPA was spent after only about two days (cf. reactions (C) & (D) in FIG. 2) whence stabilisation probably proceeds by reactions with by-products. By contrast, no discontinuity in the heat flow can be identified with stabilisers (I) over 3.43 days. and even for over 9 days, as revealed in FIG. 4 for propellants containing up to 40 wt. % of nitroglycerine.

The propellant compositions of the present invention consolidates the development and use of a new generation of stabilisers which can be referred to as “green or environmentally-friendly stabilisers,” which combine efficient, long term stability of nitrocellulose-based propellants without the formation of any detectable amounts of carcinogenic or mutagenic by-products.

Claims

1. A nitrocellulose-based composition for use as propellant or as combustible item, said nitrocellulose-based composition comprising: (a) a nitrate ester-based component comprising nitrocellulose; and(b) a stabiliser (I) in the form of a long aliphatic chain having at least two unsaturation sites, the compound having a molecular weight of at least 120 and an iodine number of at least 25.

2. The nitrocellulose-based composition according to claim 1, wherein the nitrate ester-based propellant component consists of nitrocellulose alone (single base) or of a mixture comprising nitrocellulose in combination with at least a blasting oil and/or at least one energetic additive, thus defining a double or higher base composition.

3. The nitrocellulose-based composition according to claim 2, wherein the blasting oil comprises at least a nitrated polyol, which is obtained by nitration of a polyol selected from a group consisting of glycerol, glycol, diethylene glycol, triethylene glycol and metriol.

4. The nitrocellulose-based composition according to claim 1, wherein the stabiliser is a component capable of reacting with both degradation products of the nitrate ester, namely alkoxy radicals and NOx, mainly by a series of hydrogen abstraction of one labile proton of the stabiliser, located in the alpha-position of an unsaturation and recombinations with similar carbon-based free radicals or the NOx species from the ageing of nitrocellulose.

5. The nitrocellulose-based composition according to claim 5, wherein the stabiliser is selected froth one or more of the following components and their derivatives: terpenes or terpenoids, including squalene, farnesol, myrcene, thymol, linolenic, carotenoids,omega 3-6-9 unsaturated fatty acids including one or more of arachidonic acid, linolenic acid, mead acid, herring acid,triglycerides derived from the above mentioned fatty acids including linseed oil; or(co-)polymers including polyenes, unsaturated polyester resins, homopolymers copolymers of one or more of butadiene, isoprene, or pentadiene.

6. The nitrocellulose-based composition according to claim 5, wherein the stabiliser consists of a component selected from a group consisting of polybutadiene, polyisoprene, squalene, farnesol, limonene, myrcene, lycopene and citral.

7. The nitrocellulose-based composition according to claim 1, wherein the stabiliser is present in the composition in an amount comprised between 0.1 and 5.0 wt. % with respect to the total weight of the composition.

8. The nitrocellulose-based composition according to claim 1, wherein the nitrate ester-based component comprises not more than 60 wt. % nitroglycerine with respect to the total weight of nitrate ester based propellant.

9. The nitrocellulose-based composition according to claim 1, having a stability measured according to STANAC 4582 (Ed. 1) at a temperature of 90° C. without heat generation above 350 pW/g of at least 3.43 days.

10. The nitrocellulose-based composition according to claim 1, further comprising one or more of the following compounds as complementary stabilisers: (a) a substituted phenol compound (2) having the general formula (2-1):

11. The nitrocellulose-based composition according to claim 1 further comprising one or more of the following additives: (a) a potassium salt;(b) combustion moderators, such as phthalates, Cl and citrate derivatives;(c) an anti-static agent;and(d) calcium carbonate

12. Use of a component (I) consisting of a long aliphatic chain having at least unsaturation sites for the stabilization of a nitrate-ester based composition comprising nitrocellulose for applications including propellants and combustible items.

13. The nitrocellulose-based composition according to claim 3, wherein the blasting oil comprises at least a nitrated polyol, which is obtained by nitration of the glycerol.

14. The nitrocellulose-based composition according to claim 3, wherein the at least one energetic additive is an energetic plasticizer selected from a group consisting of: butyl-NENA, and dinitrodiazaalkane (DNDA), or is an explosive selected from a group consisting of: RDX, HMX, FOX-7, FOX-12, CL20, and SMX.

15. The nitrocellulose-based composition according to claim 5, wherein the stabiliser is polybutadiene.

16. The nitrocellulose-based composition according claim 1, wherein the stabiliser is present between 0.2 and 2.0 wt. % with respect to the total weight of the composition.

17. The nitrocellulose-based composition according claim 1, wherein the nitrate ester-based component comprises between 5 and 45 wt nitroglycerine, with respect to the total weight of nitrate ester based propellant.

18. The nitrocellulose-based composition according claim 1, having a stability measured according to STANAC 4582 (Ed. 1) at a temperature of 90° C. without heat generation above 350 pW/g of at least 5 days.

19. The nitrocellulose-based composition according claim 11, wherein the potassium salt is present and in an amount comprised between 0.001 and 1.5 wt. %.

20. The nitrocellulose-based composition according claim 11., wherein the anti-static agent is present and is graphite.