Stabilized nitrocellulose-based propellant composition

Abstract

The present invention concerns a nitrocellulose-based propellant composition comprising: (a) a nitrate ester based propellant consisting of nitrocellulose alone (single base) or in combination at least with a blasting oil (double or higher base); and (b) a stabilizer consisting of a general formula (I): ##STR00001##
Wherein the stabilizer combines efficient, long term stability of the nitrocellulose-based propellants composition without formation of any detectable amounts of carcinogenic or mutagenic by-products, such as NNO groups.

Claims

1. A nitrocellulose-based propellant composition comprising (a) a nitrate ester based propellant, and (b) a stabilizer consisting of a general formula (I): ##STR00014## wherein: R.sup.1 represents, alkyl substituted or not; R.sup.2 represents: (i) H; (ii) unsaturated alkyl group; ##STR00015## R.sup.3 represents, H, alkyl substituted or not, or OR.sup.8; R.sup.4 represents, alkyl substituted or not, aromatic ring substituted or not, or OR.sup.8; R.sup.5 represents, alkyl substituted or not, aromatic ring substituted or not, or OR.sup.9; R.sup.6 represents, aromatic ring substituted or not; R.sup.7 represents, alkyl substituted or not; R.sup.8 represents, alkyl substituted or not, or aromatic ring substituted; R.sup.9 represents, alkyl substituted or not, or aromatic ring substituted.

2. The propellant composition according to claim 1, wherein the nitrate ester based propellant is a single base propellant consisting of nitrocellulose alone or is a double or higher base propellant comprising nitrocellulose in combination with at least one blasting oil and/or at least one energetic additive.

3. The propellant composition according to claim 1, wherein the stabilizer is 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.

4. The propellant composition according to claim 3, wherein the second by-product is capable of reaction with radical alkoxy groups or with NOx formed by degradation of the nitrate ester for forming third and subsequent by-products capable of reacting with such radical alkoxy groups or with NOx.

5. The propellant composition according to claim 2, wherein the at least one 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, and wherein the at least one energetic additive is an energetic plasticizer selected from the group of nitramines or is an explosive.

6. The propellant composition according to claim 1, wherein R.sup.1 represents G-s alkyl substituted or not.

7. The propellant composition according to claim 1, wherein R.sup.2 represents: ##STR00016## wherein R.sup.10 represents H, alkyl substituted or not, or aromatic ring substituted or not.

8. The propellant composition according to claim 7, wherein the stabilizer is eugenol of formula (II) or isoeugenol of formula (IV): ##STR00017##

9. The propellant composition according to claim 1, wherein the stabilizer is of formula (II): ##STR00018## wherein R.sup.1 and R.sup.11 are same or different and represent alkyl substituted or not, preferably alkyl; R.sup.3 and R.sup.12 are same or different and represent H or alkyl substituted or not.

10. The propellant composition according to claim 9, wherein the stabilizer is of formula ##STR00019##

11. The propellant composition according to claim 1, wherein the stabilizer is present at an amount between 0.1 and 5.0 wt. %, with respect to the total weight of the propellant composition.

12. The propellant composition according to claim 1, wherein the nitrate ester based propellant comprises not more than 60 wt. % nitroglycerin, with respect of the total weight of nitrate ester based propellant.

13. The propellant composition according to claim 1, wherein the propellant composition has stability measured according to STANAC 4582 (Ed. 1) at a temperature of 90 C. without heat flow generation above 350 vv/g for at least 3.43 days.

14. The propellant composition according to claim 1, further comprising one or more of the following additives: (a) a potassium salt comprising potassium nitrate (KNO3) or sulphate (K2SO4), in an amount comprised between 0.01 and 1.5 wt. %; (b) combustion moderators comprising phthalates, Cl and citrate derivatives, in an amount comprised between 1.0 and 10.0 wt. %; (c) an anti-static agent comprising graphite, in an amount comprised between 0.01 and 0.5 wt. %; and (d) calcium carbonate 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.

15. The propellant composition according to claim 5, wherein the polyol consists of glycerol, and the energetic plasticizer is selected from the group consisting of butyl-NENA and dinitrodiazaalkane or the explosive is selected from the group consisting of RDX, HMX, FOX7, FOX12 and CL20.

16. The propellant composition according to claim 6, wherein R.sup.1 represents CH.sub.3.

17. The propellant composition according to claim 11, wherein the stabilizer is present at an amount between 0.5 and 1.0 wt. %, with respect to the total weight of the propellant composition.

18. The propellant composition according to claim 12, wherein the nitrate ester based propellant comprises nitroglycerin at an amount between 7 and 22 wt. %, with respect of the total weight of nitrate ester based propellant.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) 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:

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

(3) FIG. 2: shows assumed stabilization mechanisms of (a) akardite (AkII) and diphenylamine (DPA) (prior art), (b) a substituted trimethoxyphenol (prior art), and (c) and (d) stabilizers according to the present invention.

(4) FIG. 3: shows the normalized heat flow expressed in W/g generated by propellant compositions stabilized with various amounts of a stabilizer of formula (IIa) for (a) single base nitrocellulose propellants and (b) double base nitrocellulose/nitroglycerin (90/10 wt. %) propellants.

(5) FIG. 4: shows the normalized heat flow expressed in W/g generated by a double base propellant comprising 90 wt. % nitrocellulose and 10 wt. % nitroglycerin stabilized with 0.66 wt. % of a stabilizer of formula (IIa) according to the present invention (solid line) and with 0.70 wt. % diphenyl amine (DPA) of the prior art (dashed line).

(6) FIG. 5: shows the normalized heat flow expressed in W/g generated by a double base propellant comprising 80 wt. % nitrocellulose and 20 wt. % nitroglycerin stabilized with two stabilizers according to the present invention: eugenol (III) and isoeugenol (IV)

DETAILED DESCRIPTION OF THE INVENTION

(7) As illustrated in FIG. 1, degradation of nitrocellulose forms free oxide radicals (RO) 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 (AkII) and diphenyl amine (DPA) as illustrated in FIG. 2(a). Akardite (AkII) when exposed to NOx, forms carcinogenic NNO compounds as illustrated in reaction (A) of FIG. 2(a). Simultaneously or sequentially, it dissociates upon exposure to heat to form diphenyl amine (DPA) following reaction (B) of FIG. 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 {circle around (1)} of FIG. 2(a)). The radical formed on the amine can react with a NOx to form stable N-nitrosodiphenylamine (cf. reaction {circle around (2)} of FIG. 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 FIG. 2(b) effectively react with free oxide radicals (RO) but forming stable components which are unlikely to further react with NOx (cf. reaction {circle around (1)} of FIG. 2(b)). The efficacy of such stabilizers is limited to short periods of time only because of rapid phenols depletion.

(8) A stabilizer as used in the present invention has a general formula (I)

(9) ##STR00007##
Wherein:
R.sup.1 represents, alkyl substituted or not;
R.sup.2 represents:

(10) (i) H;

(11) (ii) unsaturated alkyl group;

(12) ##STR00008##
R.sup.3 represents, H, alkyl substituted or not, or OR.sup.8;
R.sup.4 represents, alkyl substituted or not, aromatic ring substituted or not, or OR.sup.8;
R.sup.5 represents, alkyl substituted or not, aromatic ring substituted or not, or OR.sup.9;
R.sup.6 represents, aromatic ring substituted or not;
R.sup.7 represents, alkyl substituted or not;
R.sup.8 represents, alkyl substituted or not, or aromatic ring substituted;
R.sup.9 represents, alkyl substituted or not, or aromatic ring substituted.

(13) Not wishing to be bound by any theory, it is believed that a stabilizer as defined in the present invention reacts as illustrated in FIG. 2(c) by first neutralising a radical alkoxy group by H-abstraction to form a radical capable of reacting with NOx by delocalization of the radical within the aryl ring (cf. reactions {circle around (1)}&{circle around (2)} of FIG. 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 according to a second and possibly further cycles with further radical alkoxy groups and NOx, thus substantially prolonging the stabilizing action of the stabilizers. For example, in case R.sup.2 represents a moiety of the type,

(14) ##STR00009##
FIG. 2(d) shows, after reaction {circle around (2)}, neutralisation of a further radical by H-abstraction to form a further radical (cf. reaction {circle around (3)} of FIG. 2(d), allowing further reaction with a NOx as illustrated in reaction {circle around (4)} of FIG. 2(d). Alternatively or concomitantly, the reaction may proceed with further reaction with a NOx molecule. The numerous reactions of neutralisation of NOx or radicals present in the composition allow a substantial reduction of the exothermic reaction represented in FIG. 1, so that the composition stability is substantially enhanced.

(15) In a preferred embodiment, R.sup.1 represents C.sub.1-5 alkyl substituted or not, preferably CH.sub.3; Further, it is preferred that R.sup.2 represents:

(16) ##STR00010##
wherein R.sup.10 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 FIG. 5:

(17) ##STR00011##

(18) A most preferred embodiment of composition according to the present invention comprises a curcumin derivative of formula (II) as stabilizer.

(19) ##STR00012##
wherein
R.sup.1 and R.sup.11 are same or different and represent alkyl substituted or not, preferably C.sub.1-5, more preferably CH.sub.3;
R.sup.3 and R.sup.12 are same or different and represent H or alkyl substituted or not (e.g., C.sub.1-5 alkyl), wherein each of R.sup.1 and R.sup.11, and R.sup.3 and R.sup.12, are preferably same, and more preferably both are H.

(20) In particular a stabilizer of formula (IIa) yields excellent stabilisation properties as illustrated in FIGS. 3 and 4 which are discussed in continuation.

(21) ##STR00013##

(22) 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. FIG. 3(a) illustrates the stability of a simple base propellant composition stabilized with various amounts of a stabilizer (IIa) according to the present invention. FIG. 3(b) illustrates the same for a double base propellant composition wherein the nitrate ester propellant comprises 90 wt. % nitrocellulose and 10 wt. % nitroglycerin, a commonly used blasting oil. Energetic additives, on the other hand, can be an energetic plasticizer selected from the group of nitramines such as butyl-NENA, dinitrodiazaalkane (DNDA), or an explosive such as RDX, HMX, FOX7, FOX12, 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. A most preferred blasting oil is nitroglycerin.

(23) 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. FIGS. 3(a) and 4(a) illustrate the stability of a single base and a double base propellant composition, respectively, stabilized with various amounts of a stabilizer according to formula (IIa). Although it is generally considered that a propellant composition cannot be satisfactorily stabilized with less than 1 wt. % stabilizer, it can be seen in FIGS. 3(a) and 4(a) that excellent stability results are already obtained with as little as 0.11 wt. % stabilizer of formula (IIa).

(24) 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 (KNO.sub.3) or sulphate (K.sub.2SO.sub.4), 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 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, 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.

(25) An example of propellant composition according to the present invention is listed in Table 1.

(26) TABLE-US-00001 TABLE 1 typical propellant compositions according to the present invention single base double base component wt. % wt. % nitrocellulose 89.0-96.0 82.0-86.0 nitroglycerin 0.0 7.0-11.0 K.sub.2SO.sub.4 0.5-1.0 0.5-1.0 dibuthylphthalate 3.0-7.0 3.0-7.0 graphite 0.2-0.4 0.2-0.4 calcium carbonate <0.7 <0.7 stabilizer of formula (I) 0.15-2.0 0.15-2.0
Experimental Tests

(27) 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. Fulfillment of the STANAG 4582 (Ed. 1) test qualifies a propellant composition for a 10 year stability at 25 C.

(28) 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 W/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 W/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 to 5 are plots of such measurements. The full scale of the ordinate (normalized heat flow) corresponds to a value of 350 W/g not to be exceeded according to STANAG 4582 (Ed. 1), and the vertical straight line indicates 3.43 days. The initial heat flow peak comprised within the shaded area of the graphs of FIGS. 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.

(29) FIGS. 3(a)&(b) show the results of the stability tests carried out on a single- and double-base nitrocellulose-based propellants, the latter comprising 10 wt. % nitroglycerin for various amounts of a stabilizer according to formula (IIa) comprised between 0.10 and 1.70 wt. %, with respect to total weight of the propellant composition. It can be seen that even with as little as 0.11 wt. % stabilizer the heat flow never exceeds 100 W/g for 3.43 days, when STANAG 4582 (Ed. 1) requires to maintain the heat flow below 350 W/g (full scale of the ordinate). The tests on single base propellants were carried out for a longer period, showing a prolonged stability of the compositions with a heat flow continuously lower than 150 W/g for over 20 days.

(30) FIG. 4 compares the stability of double-base propellant compositions stabilized with, on the one hand, 0.66 wt. % of the stabilizer of formula (IIa) according to the present invention (solid line) and, on the other hand, with diphenyl amine (DPA) of the prior art (dashed line). It can be seen that both stabilizers (Stabilizer (IIa) and DPA) fulfil the requirements of STANAG 4582 (Ed. 1), The stabilizer (IIa) according to the present invention is advantageous over DPA because, (a) Contrary to DPA, stabilizers according to the present invention do not generate any NNO carcinogenic by-product upon their stabilization activity, and (b) DPA curve (dashed line) shows a sharp peak stabilizing in a plateau at higher heat flow values, suggesting that all DPA was spent after only about two days (cf. reactions {circle around (1)}&{circle around (2)} in FIG. 2(a)) whence stabilization probably proceeds by reactions with by-products. By contrast, no discontinuity in the heat flow can be identified with stabilizer (IIa) over 3.5 days. and even for over 20 days, as revealed in FIG. 3(a) discussed supra with respect to single base nitrocellulose propellants. (c) As revealed in FIG. 3(a) discussed supra with respect to single base nitrocellulose propellants, the stabilizers of the present invention allow the maintenance of a heat flow substantially lower than 350 W/g at a temperature of 90 C. for periods well over 20 days. Longer term tests with DPA, however, are not easily performed because vials containing a composition stabilized with DP or AkII leaked earlier than the ones stabilized according to the present invention. It is assumed that gas generation by the reactions with DPA raises the pressure inside the vials above their limit of resistance, leading to the bursting open of the vials after a few days testing. Uncontrolled pressure rises must be avoided during transportation or storage of propellant compositions for obvious reasons.

(31) FIG. 5 shows the stability curves of two further embodiments of the present invention, eugenol of formula (III) (CAS: 97-53-0) and isoeugenol of formula (IV) (CAS: 97-54-1) which, like the curcumin derivative of formula (IIastabilizes well beyond 3.43 days a double base propellent composition composed of 80 wt. % nitrocellulose and 20 wt. % nitrocellulose maintained at a temperature of 90 C., thus fulfilling STANAG4582 without generating any NNO carcinogenic components.

(32) 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.