Binder based on glycidyl azide polymer and solid propellant containing such binder

Abstract

The invention relates to a binder for solid propellants, wherein the binder comprises glycidyl azide polymer (GAP) and a plasticizer in the form of a sebacate or an adipate, where the plasticizer has a molecular weight below 314 g / mol. The invention is based on the insight that adipates and sebacates with a molecular weight below 314 g / mol are miscible with GAP and efficiently reduces the glass transition temperature (Tg) of the binder. The invention further relates to a solid propellant comprising said binder for use in rocket motors or gas generators.

Description

[0001] BINDER BASED ON GLYCIDYL AZIDE POLYMER AND SOLID PROPELLANT

[0002] CONTAINING SUCH BINDER

[0003] BACKGROUND OF THE INVENTION

[0004] The present invention is directed to a binder based on glycidyl azide polymer (GAP), a solid propellant comprising such binder and a rocket motor and a gas generator comprising the solid propellant.

[0005] Solid propellants, e.g. composite propellants, are used in many applications, e.g. military missiles and civilian space rockets. A solid propellant in the form of a composite propellant consists of a solid oxidizer agent in powder form, usually ammonium perchlorate (AP) embedded in a polymer-based binder. The propellant may also contain aluminum powder. During manufacturing, the binder is initially a liquid in which the powdery ingredients are mixed to form a viscous suspension. Finally, a curing agent is added and the suspension is cast in a mould and left to cure in an oven. In this way, a solid elastic propellant with high performance, high density (1500-1800 kg / m3) and desired geometry, is obtained. A high propellant density is desirable to house more propellant mass in a given volume. Typically, a composite propellant has a modulus of elasticity 3 - 7 MPa, tensile strength 0.5 - 1 MPa (true stress) and strain at maximum true stress in the range of 30 - 60% [K. O. Hartman, S. Morrow, Solid Propellants, in Encyclopedia of Physical Science and Technology, (Ed: R. A. Meyers), Academic Press, 2003, pp. 277-293, J. D. DeSain, et al., Tensile Tests of Paraffin Wax for Hybrid Rocket Fuel Grains, 45thAIAA / ASME / SAE / ASEE Joint Propulsion Conference, Denver, CO, USA, August 2- 5, 2009],

[0006] The solid propellant can be used, for instance, in a rocket motor or a gas generator. In both cases, the propellant is combusted in a pressure vessel and the generated gas is ejected through a nozzle. In a rocket motor, the gases are used to generate thrust, while in a gas generator the gases are used for to perform mechanical work, e.g. to inflate something or to drive pistons or turbines. The gas generator can also be used to generate fuel rich gases for use in a ramjet or scramjet engine. When ignited, the propellant burns on all free surfaces, and the amount of propellant combusted per time unit is proportional to the burning surface. By proper selection of the geometry of the solid propellant, i.e. the shape of the propellant, the combustion can be controlled to obtain a desired thrust profile of a rocket motor. It is of utmost importance that the propellant does not crack (e.g. during storage, transport or use), since this leads to an increase of the burning surface and the alteration of the thrust profile. An increased burning surface leads to an increase in the gas flow, which generates a higher pressure than expected and thereby an increased risk of motor failure. A motor failure might involve the motor not generating the expected thrust or that it explodes. Therefore, the propellant must be sufficiently elastic, also at low temperature. This requirement is particularly challenging for missiles intended for use at low temperatures, where the propellant must be elastic enough not to crack due to the mechanical strain it is subjected to during acceleration. For instance, it is desirable to use anti-tank missiles at temperatures as low as -40°C.

[0007] The binder commonly used in composite propellants is based on hydroxyl terminated polybutadiene (HTPB) which has a very low glass transition temperature (Tg) and is thus elastic at temperatures as low as -50°C. By using a binder based on the energetic polymer glycidyl azide polymer (GAP) instead of HTPB, the performance of the propellant can be increased, which means that e.g. the specific impulse and burn rate of a rocket motor can be increased. A rocket motor with higher specific impulse can make a rocket fly longer and with a higher burn rate, a rocket can accelerate faster. GAP has been studied during many years, mainly as binder for minimum smoke propellants. However, one problem with GAP is its poor mechanical properties, especially at low temperatures.

[0008] At temperatures below the glass transition temperature (Tg), a binder becomes brittle, which increases the risk of crack formation significantly. Cured GAP has a glass transition temperature (Tg) of approximately -37°C. During development of GAP in the USA, the requirement for cured GAP was a glass transition temperature (Tg) below -54°C, when combined with a plasticizer [Journal of Propulsion and Power, Vol. 8, No. 3, May-June 1992, M.B. Frankel et al: Historical development of glycidyl azide polymer]. This can be obtained using many different plasticizers. In general, plasticizers do not contribute to the tensile strength of polymers, which leads to decreased tensile strength with increasing content of plasticizer. For a binder based on GAP, it is therefore desirable to have as low a content of plasticizer as possible.

[0009] SE 503997 C2 describes an energetic, curable composition comprising hydroxyl terminated glycidyl azide polymer (GAP), a polyether containing hydroxyl groups with an average molecular weight of 1000-5000 g / mol, and a curing system consisting of one or more isocyanates. In this composition, it is the polyether mix combined with diisocyanate which reduces the glass transition temperature. In order to obtain a desired decrease of the glass transition temperature to -54°C or lower, at least 30% of the composition has to be polyether mix and diisocyanate, which means that the energetic properties of the composition are reduced since the polyether is a non- energetic substance.

[0010] It is previously known that the glass transition temperature of GAP can be reduced by addition of energetic plasticizers, such as 2-(butylnitroamino)ethyl nitrate (Bu- NENA) [Jensen et aL, Smokeless GAP-RDX Composite Rocket Propellants Containing Diaminodinitroethylene (FOX-7), Propellants, Explosives, Pyrotechnics, volume 42, issue 4 2017, pp. 381-385], where a binder containing 38% Bu-NENA was used to reduce the glass transition temperature of the binder to -55°C. Other energetic plasticizers that have been tested include butanetriol trinitrate (BTTN), diethylene glycol dinitrate (DEGDN), triethylene glycol dinitrate (TEGDN) and GAP- azide (GAP-A), but they have either not plasticized enough, been too volatile or yielded poor mechanical properties.

[0011] When using Bu-NENA as plasticizer, a large amount is required to obtain a sufficiently low glass transition temperature, which reduces the tensile strength of the binder. Another disadvantage is that Bu-NENA is an explosive and thus hazardous to handle. In addition, energetic plasticizers are produced on a relatively small scale, as they are explosive substances and thus have fewer areas of applications than non-energetic plasticizers. Energetic plasticizers are therefore much more expensive than non-energetic. It would therefore be advantageous to use non-energetic plasticizers in a binder based on GAP, as these are more widely available and can be procured and handled in a simpler and safer manner. The common non-energetic plasticizers used in HTPB-based binders for composite propellants are dioctyl sebacate (DOS), dioctyl adipate (DOA) and isodecyl pelargonate (IDP), but they are unfortunately not miscible with GAP. It is therefore desirable to find commercially available non-energetic substances that are miscible with GAP and efficiently reduces the glass transition temperature for GAP already at low amounts.

[0012] GB 987332 describes a solid fuel composition with a solid oxidizer and aluminum or magnesium powder embedded in a solid, rubbery gel comprising a homogenous mix of polyvinyl chloride in an organic plasticizer with a high boiling point. The plasticizer can be a sebacate or an adipate. However, GB 987332 A does not mention anything about adipates or sebacates being miscible with GAP or that they are used to give the binder a lower glass transition temperature and thus obtain better mechanical properties at low temperatures.

[0013] SUMMARY OF THE INVENTION

[0014] The invention is based on the insight that sebacates (R-OOC-(CH2)8-COO-R) and adipates (R-OOC-(CH2)4-COO-R) with a molecular weight below 314 g / mol are physically compatible with GAP and efficiently reduces the glass transition temperature of a binder based on GAP.

[0015] In accordance with different embodiments of the invention, the molecular weight of the plasticizer is higher than 170 g / mol, 200 g / mol, 225 g / mol or 250 g / mol. A higher molecular weight reduces the risk of migration, i.e. that molecules move through the solid material over time. A lower risk of migration means that the ageing properties of the material are improved, since its mechanical properties are not affected with time as a result of altered distribution of the plasticizer.

[0016] The plasticizer may consist of one of the sebacates dimethyl sebacate (DMS) and diethyl sebacate (DES) or one of the adipates dimethyl adipate (DMA), diethyl adipate (DEA), dipropyl adipate (DPA) and dibutyl adipate (DBA).

[0017] The invention also relates to a solid propellant comprising a binder based on GAP which contains a sebacate or an adipate as plasticizer, wherein the molecular weight of the plasticizer is below 314 g / mol.

[0018] The solid propellant may also comprise a salt based on nitrate, perchlorate or dinitramide, or alternatively an energetic substance such as hexogen (RDX), octogen (HMX) or diamino dinitroethylene (FOX-7). In addition to these substances, the solid propellant may also comprise aluminum powder.

[0019] Furthermore, the invention relates to a rocket motor and a gas generator comprising such a propellant.

[0020] BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 shows a graph of how the glass transition temperature of cured GAP varies with the level of plasticizer content.

[0022] Figure 2 shows results from tensile testing of a solid propellant where true stress is shown as a function of strain given in percent.

[0023] DETAILED DESCRIPTION OF THE INVENTION

[0024] Conventional non-energetic plasticizers used in binders based on HTPB have the disadvantage that they are not miscible with GAP. Many conventional energetic plasticizers are miscible with GAP, but those tried must be added at high amounts to achieve an acceptable glass transition temperature, resulting in low tensile strength. Energetic plasticizers are also hazardous to handle, significantly more expensive and of limited availability.

[0025] In order to solve the above-mentioned problems with known plasticizers for GAP, attempts were made to assess sebacates (R-OOC-(CH2)8-COO-R) and adipates (R- OOC-(CH2)4-COO-R) with molecular weights less than that of DOS (426.7 g / mol) and DOA (370.6 g / mol) as plasticizers for GAP. The sebacates and adipates evaluated were dimethyl sebacate (DMS), diethyl sebacate (DES), dibutyl sebacate (DBS), dimethyl adipate (DMA), diethyl adipate (DEA), dipropyl adipate (DPA) and dibutyl adipate (DBA). DBS, with a molecular weight of 314 g / mol, was excluded at an early stage since it was found to be physically incompatible with GAP. This means that DBS is not miscible with GAP. The other plasticizers tested have a molecular weight of 258 g / mol or lower, i.e. lower than 314 g / mol, and were found to be physically compatible with GAP and efficiently reduce the glass transition temperature. DMS and DES were found to work well but were dismissed since they are solid at room temperature, which complicates the mixing procedure.

[0026] Evaluation of the plasticizers was performed by first mixing GAP with the respective plasticizer and a curing system. The mixture was then poured into a mould and allowed to cure at 50°C for one week. The curing system may for example consist of an isocyanate or a mixture of isocyanates. In these tests, a mixture of an aliphatic poly isocyanate (Desmodur® N3300) and dicyclohexyl methane diisocyanate (Desmodur® W) was used. In this case, the ratio between the isocyanates Desmodur® N3300 and Desmodur® W was 70 / 30 and the molar ratio between isocyanate groups and hydroxyl groups, i.e. the NCO / OH ratio, was 0.9. Other ratios might be used to obtain desired mechanical properties of the binder, for instance lowering the NCO / OH ratio increases the elasticity but decreases the tensile strength. Other types of di-, tri or poly isocynates, or other types of curing systems, may also be used since the function of the plasticizer is independent of the curing system. Table 1 below shows the binder composition used in the plasticizer evaluation.

[0027] Table 1. Binder composition*.

[0028] *The mixtures also contained about 0.05 weight-% of the curing catalyst dibutyltin dilaurate, DBTDL (CAS no 77-58-7).

[0029] Table 2 below shows the glass transition temperature for cured GAP containing 18.2 weight-% of respective plasticizer (cured GAP without plasticizer is included for comparison). Table 2. Glass transition temperature for cured GAP containing 18.2 weight-% of respective plasticizer.

[0030] As shown in Table 2, the glass transition temperature (Tg) of the binders is below -55°C when using a content of 18.2 weight-% of either DES, DMA, DEA, DPA or DBA. In some cases, it may be desirable to prioritize the tensile strength of the binder, even though this will lead to a higher glass transition temperature (Tg). Therefore, a lower content of plasticizer may be of interest. The conditions may also be the opposite, i.e. that an even lower glass transition temperature (Tg) is desired. In this case, a higher content of plasticizer may be of interest in order to obtain a desired glass transition temperature, at the cost of tensile strength.

[0031] As shown in Table 2, DEA, DPA and DBA are the plasticizers giving the most efficient reduction of the glass transition temperature (Tg). These plasticizers were studied further in a series of tests in order to investigate how the glass transition temperature (Tg) varies with the amount of respective plasticizer. In this case, mixtures identical to those shown in Table 1 were prepared, except that the amount by weight of each respective plasticizer was varied. Also in these tests, GAP was mixed with respective plasticizer, after which the mixture was cured using a mixture of the isocyanates Desmodur® N3300 and Desmodur® W, in the proportions 70 / 30, at a NCO / OH ratio of 0.9.

[0032] The results of the further tests with DEA, DPA and DBA are presented in Figure 1 , where it is shown that the glass transition temperature (Tg) for the cured mixture of GAP and plasticizer decreases linearly with the amount by weight-% of plasticizer added. For comparison, the glass transition temperature (Tg) for a cured mixture of GAP and the energetic plasticizer Bu-NENA has been added to the graph in Figure 1. The graph in Figure 1 shows that DEA, DPA and DBA have similar properties and that a glass transition temperature (Tg) below -54°C can be obtained for the cured mixture by using as little as about 15 weight-%. In order to obtain such a low glass transition temperature (Tg) using Bu-NENA, approximately 30 weight-% plasticizer is required.

[0033] DBA has the highest molecular weight of the plasticizers shown in Figure 1 . A high molecular weight reduces the risk of migration, i.e. that molecules move through the solid material over time. A low risk of migration of the plasticizer in the solid propellant improves the ageing properties of the propellant, since its mechanical properties are not affected with time as a result of altered distribution of the plasticizer in the solid propellant. In order to obtain a desired level concerning risk of migration, an adipate or a sebacate with a molecular weight above 170 g / mol, 200 g / mol, 225 g / mol or 250 g / mol may be used.

[0034] Example

[0035] A total of 400 g of a solid propellant (a composite propellant) comprising a binder based on GAP, plasticized with dibutyl adipate (DBA), and a content of 70 weight-% filler in the form of ammonium dinitramide (ADN), was prepared as follows:

[0036] GAP was mixed with the plasticizer DBA and a bonding agent in the form of tetraethylene pentaamine acrylonitrile (TEPAN, CAS No 68412-45-3) and a very small amount of the curing catalyst DBTDL. In this case, 20 parts per weight of DBA for every 100 parts of GAP was used in the binder. Solid ADN particles were then added in several sequences. A curing system consisting of the isocyanates Desmodur® N3300 and Desmodur® W was finally added, after which the mixture was mixed. The mixing was performed in vacuum to prevent bubble formation in the propellant mass. The ratio between Desmodur® N3300 and Desmodur® W was 70 / 30, and the NCO / OH ratio was 0.9. The mixture was cast in a mould and left to cure for one week at 50°C. The detailed composition of the binder and the amount of respective substance for making 120 g of it is shown in Table 3 below. In addition to 120 g of the binder, the solid propellant also contained 280 g of a filler in the form of ADN. Table 3. Composition of the binder for solid propellant

[0037] Test samples of the solid propellant were produced for measuring density, glass transition temperature and mechanical testing. The density was measured to 1580 kg / m3and the glass transition temperature was found to be -53°C. Mechanical testing consisted of tensile testing performed according to STANAG 4506 Ed: 1 , at room temperature and a tensile rate of 50 mm / min. The strain was measured using a video camera. The graph in Figure 2 shows true stress as a function of strain given in percent for two test samples of the solid propellant. The stress-strain curves presented in Figure 2 show that the solid propellant has very good mechanical properties with a maximum true stress of 0.76 MPa and a strain at maximum true stress of 46%. In figure 2, the modulus of elasticity of the solid propellant can be read to 3 MPa.

[0038] When making a solid propellant, a binder based on GAP and any of the other effective plasticizers DES, DMA, DEA or DPA may be used, since their function as a plasticizer is independent of the filler used. Furthermore, other conventional fillers may be used instead of ADN, e.g. salts based on nitrate, perchlorate or dinitramide (e.g. ammonium perchlorate (AP), ammonium nitrate (AN) and guanylurea dinitramide (FOX-12)), or energetic substances (such as hexogen (RDX), octogen (HMX) or diamino dinitroethylene (FOX-7)). In addition to nitrate salts, perchlorate salts, dinitramide salts or energetic substances, aluminum powder may also constitute a part of the filler. The amount of filler in the solid propellant can also be varied between 50 and 90 weight %.

[0039] In one embodiment, the solid propellant is used in a rocket motor, either by placing the propellant in a pressure vessel of a rocket motor after casting, or by casting the propellant directly into said pressure vessel. The rocket motor includes said pressure vessel and a nozzle in connection with the pressure vessel. When the solid propellant is combusted, gas is formed, which is ejected through the nozzle to create thrust. In another embodiment, the solid propellant is used in a gas generator, either by placing the propellant in a pressure vessel of a gas generator after casting, or by casting the propellant directly into said pressure vessel. The gas generator includes said pressure vessel and a nozzle in connection with the pressure vessel. When the solid propellant is combusted, gas is formed, which is ejected through the nozzle and is used to perform mechanical work, to for example inflate something or to drive pistons or turbines. The gas generator can also be used to create fuel-rich gases for driving a ramjet or scramjet engine.

Claims

CLAIMS1. A binder for solid propellants, wherein the binder comprises glycidyl azide polymer (GAP), characterized in that the binder also comprises a plasticizer in the form of a sebacate or an adipate, wherein the molecular weight of the plasticizer is lower than 314 g / mol.

2. The binder according to claim 1 , characterized in that the plasticizer has a molecular weight higher than 170 g / mol, 200 g / mol, 225 g / mol or 250 g / mol.

3. The binder according to claim 1 , characterized in that the plasticizer comprises dimethyl sebacate (DMS).

4. The binder according to claim 1 , characterized in that the plasticizer comprises diethyl sebacate (DES).

5. The binder according to claim 1 , characterized in that the plasticizer comprises dimethyl adipate (DMA).

6. The binder according to claim 1 , characterized in that the plasticizer comprises diethyl adipate (DEA).

7. The binder according to claim 1 , characterized in that the plasticizer comprises dipropyl adipate (DPA).

8. The binder according to claim 1 , characterized in that the plasticizer comprises dibutyl adipate (DBA).

9. A solid propellant comprising a binder according to any of the preceding claims.

10. The solid propellant according to claim 9, wherein the propellant also comprises a salt based on nitrate.

11. The solid propellant according to claim 9, wherein the propellant also comprises a salt based on perchlorate.

12. The solid propellant according to claim 9, wherein the propellant also comprises a salt based on dinitramide.

13. The solid propellant according to claim 9, wherein the propellant also comprises an energetic substance, such as hexogen (RDX), octogen (HMX) or diamino dinitroethylene (FOX-7).

14. The solid propellant according to any of claims 10-13, wherein the propellant also comprises aluminum powder.

15. A rocket motor comprising a solid propellant according to any of claims 9-14.

16. A gas generator comprising a solid propellant according to any of claims 9-14.

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