A new series of composite propellant compositions, based on ammonium perchlorate, hydroxy-terminated polybutadiene and having metallic fuel as aluminium powder, have been developed. Pressure cast-cum-cured compositions have also been developed with toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and a mixture of both curatives, i.e., TDI and IPDI, respectively, to study their effect on processibility, mechanical and ballistic properties of the compositions. The data indicate that the compositions based on bicurative have a pot life of 7 -8 h, viscosity build up is from 13280 poise to 14080 poise after 4 h, and the smooth processibility of the slurry is enhanced. Further, the mechanical properties are in the range 12.2 kg/cm 2 , 40.2 kg/cm 2 , and 40.2 per cent for tensile strength, E-modulus and elongation, respectively, and burn rate is almost the same, i.e., 16 ±0.5 mm/s. . INTRODUCTIONHydroxy-terminated polybutadiene (HTPB)-based solid rocket propellants are currently being used in space as well as in different ongoing missile programmes 1 . The low viscosity and low specific gravity along with high fuel value of HTPB makes it attractive to enable higher solid loading. The HTPB pre-polymer used for propellant slurry mixing is cured with a difunctional isocyanate curative. The urethane linkages -NH COO-formed by the -NCO with -OH reaction is chemically stable and is able to impart flexibility to the cured binder and makes the polymer an ideal elastomer 2 . A number of diisocyanates, like toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HMDI), diphenylmethane diisocyanate (MDI) and 4,4-dicyclohexylmethane diisocyanate (H 12 MDI) have been reported and used for HTPB Defence Science Journal, Vol. 57, No. 5, September 2007, pp. 669-675 2007, DESIDOC 669 system 3,4 . However, the most commonly used curative for the HTPB pre-polymer is TDI. The reactivity of TDI is comparatively more than other curatives except MDI and due to this, the pot life of TDIcured propellant is only in the order of 4-5 h. IPDI is another curative which is preferred to TDI mainly on merits of its slow reactivity, resulting in much extended pot life, i.e., 15-18 h as well as low toxicity [5][6][7] .Also, there is increasing trend to substitute TDI with other curatives in composite propellant based on HTPB. For structural integrity of grains, IPDI may not be a suitable curator for a casebonded motor. The thermal stresses in propellant grains are higher as the curing temperature is generally 70 o C. The curing temperature of a composition based on TDI is less as the reactivity is higher compared to IPDI. Owing to this, a new curative system has been conceived and utilised as bicurative
High burning rate composite propellants are achieved by incorporation of fine particles of oxidizer, transition metal oxides, and liquid ballistic modifiers. However, they pose processing problems, inertness to the composition and migration related issues. To overcome such problems, an attempt was made to incorporate ferrocenyl grafted HTPB as a burning rate modifier by partly replacing HTPB from 10 % to 50 % using TDI/ IPDI bicurative system and to study their processability in terms of viscosity, mechanical, thermal, sensitivity, and ballistic properties. The data on viscosity reveal that there is a marginal enhancement in end of mix viscosity as percentage of ferrocenyl grafted HTPB increases. The mechanical data reveal that tensile strength and elastic modulus increases, whereas percentage elongation decreases compared to base composition. The results on thermal properties infer that, as the percentage of ferrocenyl grafted HTPB increases, onset decomposition temperature decreases. The impact and friction sensitivity data also envisage that sensitivity increases in comparison to base composition. The data on ballistic properties revealed that there is ca. 53 % increase in burning rate, while decrease in “n” value from 0.39 to 0.2 was obtained compared to base composition.
Different compositions have been prepared by incorporating nano sized chromium oxide from 0.25 % to 1 % in HTPB/AP/Al based composite propellant formulation having 86% of solid loading and studied its effect on viscosity build-up, thermal, mechanical and ballistic properties. The findings reveal that on increasing the percentage of nano Cr 2 O 3 in the composition, there is an increase in end of mix viscosity, elastic modulus and tensile strength while elongation decreases accordingly. The data on thermal properties envisage the reduction in thermal decomposition temperature of ammonium perchlorate as well as formulations based on HTPB/AP/Al. The data on ballistic properties reveal that there is an enhancement in burning rate from 6.11 mm/s to 7.88 mm/s at 6.86 MPa, however, marginal increase in pressure exponent ('n' values) from 0.35 to 0.53 with 1 wt % of nano Cr 2 O 3 was observed in comparison to reference composition without chromium oxide.
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