Azide polymers

Gaseous fragments are eliminated on heating, and numerous chemical species are formed at the reacting surface of the azide polymer. The heat transfer process from the high-temperature zone to the reacting surface determines the burning rate of an azide polymer. 

GAP is synthesized by replacing C-Cl bonds of polyepichlorohydrin with C-N3 bonds.The three nitrogen atoms of the N3 moiety are attached linearly with ionic and covalent bonds in every GAP monomer unit, as shown in Fig. 4.6. The bond energy of N3 is reported to be 378 kj mol per azide group. Since GAP is a liquid at room temperature, it is polymerized by allowing the terminal -OH groups to react with hexamethylene diisocyanate (HMDl) so as to formulate GAP copolymer, as shown in Fig. 4.7, and crosslinked with trimethylolpropane (TMP) as shown in Fig. 4.8. The physicochemical properhes of GAP prepolymer and GAP copolymer are shown in Table 4.4 and Table 4.5, respectively.I ] 

Chemical formula Molecular mass Heat of formation Adiabatic flame temperature 

The adiabatic flame temperature of GAP copolymer is 1370 K at 5 MPa and large amounts of C(g), H2, and Nj are formed as initial combustion products. Fuel components such as C(s), CO, and Hj predominate, with only very small amounts of CO2 and H2O being formed. 

---

Gun Propellents. Low sensitivity gun propeUants, often referred to as LOVA (low vulnerabUity ammunition), use RDX or HMX as the principal energy components, and desensitizing binders such as ceUulose acetate butyrate or thermoplastic elastomers (TPE) including poly acetal—polyurethane block copolymers, polystyrene—polyacrjiate copolymers, and glycidyl azide polymers (GAP) to provide the required mechanical... 

Typical crystalline materials used as oxidizers are perchlorates, nitrates, nitro compounds, nitramines, and metal azides. The polymeric materials used as fuel components are divided into nitrate esters, inert polymers, and azide polymers. Optimized combinations of these oxidizer and fuel components yield the desired ballistic characteristics of propellants or explosives. 

Three types of polymeric materials are used inert polymers, active polymers, and azide polymers. No exothermic heat is produced when inert polymers are decomposed thermally. On the other hand, exothermic reactions occur when active polymers and azide polymers are decomposed. Self-sustaining burning is possible when active polymers and azide polymers are ignited. 

Double-base propellants containing azide polymers are termed nitro-azide polymer propellants. DEP used as a plasticizer of double-base propellants is replaced with azide polymers in order to increase energy density. The compatibility of GAP prepolymer with NG serves to suitably desensitize the mechanical sensitivity of NG and gives superior mechanical properties in the formulation of rocket propellant grains. 

Azide polymers such as GAP and BAMO are also used to formulate AP composite propellants in order to give improved specific impulses compared with those of the above-mentioned AP-HTPB propellants. Since azide polymers are energetic materials that burn by themselves, the use of azide polymers as binders of AP particles, with or without aluminum particles, increases the specific impulse compared to those of AP-HTPB propellants. As shown in Fig. 4.15, the maximum of 260 s is obtained at (AP) = 0.80 and is approximately 12 % higher than that of an AP-HTPB propellant because the maximum loading density of AP particles is obtained at about (AP) = 0.86 in the formulation of AP composite propellants. Since the molecular mass of the combustion products. Mg, remains relatively unchanged in the region above (AP) = 0.8, decreases rapidly as (AP) increases. 

Niehaus, M., Compounding of Glycidyl Azide Polymer with Nitrocellulose and its Influence on the Properties of Propellants, Propellants, Explosives, Pyrotechnics, Vol. 25, 2000, pp. 236-240. 

Komai, L, Kobayashi, K., and Kato, K., Burning Rate Characteristics of Glycidyl Azide Polymer (GAP) Fuels and Propellants, Solid Propellant Chemistry, Combustion, and Motor Interior Ballistics (Ed. Yang, V, Brill, T. B., and Ren, W.-Z.J, Progress in Astronautics and Aeronautics, Vol. 185, Chapter 2.9, AlAA, Virginia, 2000. 

Kubota, N., and Sonobe, T., Combustion Mechanism of Azide Polymer, Propellants, Explosives, Pyrotechnics, Vol. 13, 1988, pp. 172-177. 

Polymeric materials used as fuel components of pyrolants are classified into two types active polymers and inert polymers. Typical active polymers are nitropoly-mers, composed of nitrate esters containing hydrocarbon and oxidizer structures, and azide polymers, containing azide chemical bonds. Hydrocarbon polymers such as polybutadiene and polyurethane are inert polymers. When both active and inert polymers are mixed with crystalline oxidizers, polymeric pyrolants are formed. 

Azide polymers contain -N3 bonds within their molecular structures and burn by themselves to produce heat and nitrogen gas. Energetic azide polymers burn very rapidly without any oxidation reaction by oxygen atoms. GAP, BAMO, and AM-MOare typical energetic azide polymers. The appropriate monomers are cross-Hnked and co-polymerized with other polymeric materials in order to obtain optimized properties, such as viscosity, mechanical strength and elongation, and temperature sensitivities. The physicochemical properties GAP and GAP copolymers are described in Section 4.2.4. 

The combustion performance of a rocket motor is dependent on various physicochemical processes that occur during propellant burning. Since the free volume of a rocket motor is limited for practical reasons, the residence time of the reactive materials that produce the high temperature and high pressure for propulsion is too short to allow completion of the reaction within the limited volume of the motor as a reactor. Though rocket motor performance is increased by the addition of energetic materials such as nitramine particles or azide polymers, sufficient reaction time for the main oxidizer and fuel components is required. 

The azide chemical bond, represented by -N3, contains thermal energy, which is released when the bond is broken without oxidation. Typical chemicals containing azide bonds are glycidyl azide polymer, designated as GAP, BAMO, and AMMO. These polymers are copolymerized with hydrocarbon polymers to formulate fuel-... 

---