1. Technical Field
The present invention relates to a smokeless high-energy solid propellant composition, which contains a bismuth-based compound as a combustion additive and exhibits a low pressure exponent, and to a method of preparing the same.
2. Description of the Related Art
Nitrate ester polyether (NEPE)-based propellants typically contain, as a nitramine-based oxidizer, hexanitro hexaazaisowurtzitane (HNIW), cyclotrimethylene trinitramine (RDX), or cyclotetramethylene tetranitramine (HMX) therein. These compounds are eco-friendly because little or no smoke and no toxic gases are emitted upon combustion, compared to conventional hydroxy-terminated polybutadiene (HTPB) propellants including ammonium perchlorate (AP) that emits toxic hydrochloric acid gas. However, NEPE-based propellants have a high burning rate change exponent (pressure exponent, n) of 0.6 or more depending on changes in pressure, and are thus subject to high load pressure upon combustion. Hence, in order to decrease the pressure exponent, lead compounds or bismuth compounds are used. Recently, because lead compounds are environmentally harmful, the use thereof is restricted. For this reason, there are recent reports in which the pressure exponent is decreased using bismuth subsalicylate, chosen from among bismuth-based compounds, instead of the lead compounds. However, when using acid compounds such as bismuth subsalicylate or bismuth salicylate, a urethane reaction which is a propellant curing reaction is accelerated, making it impossible to ensure the propellant processing time, that is, pot-life. In order to ensure sufficient pot-life. U.S. Patent (U.S. Pat. No. 6,168,677 B1) discloses propellant mixing at 15° C. or less and casting. However, an additional cooling device is required, and propellant mixing at 15° C. or less may decrease propellant material mixing efficiency, and additional mixing time is required to perform cooling up to a temperature equal to or less than 15° C. and the viscosity of the propellant is also increased, and thus the casting process become problematic.
Accordingly, Korean Unexamined Patent Publication No. 10-2011-0128736 discloses a method of mixing a propellant at room temperature while adjusting the casting time using a tertiary amine compound. However, the above method is also problematic because the propellant is mixed at 50˜60° C. upon initial propellant mixing, the temperature is gradually decreased, and a curing agent should be added at room temperature. Further, in the course of suppressing the accelerated curing reaction of bismuth subsalicylate using the tertiary amine compound, the tertiary amine is applied in different amounts every time, that is, reliability of the preparation processability may decrease. Moreover, because the amount of the added tertiary amine compound is very small, the propellant casting time may vary even by fine errors in the process of improving or adding materials.