**1. Introduction**

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One of the most promising applications of polyurethane (PU) polymers is as fuel-*cum*binder material in composite solid propellant. Since the last two decades, PU filled with oxidizer and metallic fuel is being widely used for rockets propulsion. Ariane boosters, shuttles Apogee motors, Peacekeeper (also called the MX-Missile Experimental) missile, Indian Augmented Satellite Launch Vehicle(ASLV) and Polar Satellite Launch Vehicle (PSLV) boosters are some of the motors that are fuelled by PU propellant. PU composite propellant (PCP) is a heterogeneous mixture of polymeric binder, inorganic oxidizer and metallic fuel as the major ingredients. It can be classified as a highly filled PU system in which the three dimensional elastomeric matrix binds the oxidizer and metallic fuel to form a rubbery material. It imparts necessary mechanical properties to the propellant grain to maintain its structural integrity. A PU propellant grain should have sufficient tensile strength and elongation to withstand various types of stresses experienced during handling and transportation, thermal cycling, sudden pressurization on ignition, and acceleration load during flight of the rocket motor. A tensile strength of approximately 7-8 kgf/cm2, an elongation of 40-50 % and initial modulus of 40-50 kgf/cm2 are reasonable for a typical case bonded rocket motor (Manjari et al., 1993). The PU binder accounts to 10-15 % of the composite propellant, and usually consists of three components: (1) a prepolymer (polyol), (2) an isocyanate curator, and (3) a chain extender (butan-1,4-diol) and cross-linking agent (trimethylol propane). The most commonly used polyol in recent time is the Hydroxyl Terminated Polybutadiene (HTPB). This liquid prepolymer has excellent physical properties such as low glass transition temperature, high tensile and tear strength, and good chemical resistance (Eroglu, 1998). The hydrocarbon nature of HTPB (98.6%) along with low viscosity (5000 mPas at 30 °C) and low specific gravity (0.90 g/cm3), makes it a promising fuel binder for PU propellant. It is capable of taking solid loading up to 86-88% without sacrificing the ease of processibility (Muthiah et al., 1992). In addition, it is also a major reducing agent and

© 2012 Mahanta and Pathak, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Mahanta and Pathak, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

gas producing fuel. It is physically and chemically compatible with the conventional oxidizers and other ingredients at normal storage conditions. As it contains mostly carbon and hydrogen, during combustion, it is decomposed to give large volume of stable molecules like carbon monoxide, carbon dioxide, and water vapours increasing the specific impulse of the rocket motor. Additionally, PU obtained from HTPB offers many advantages over conventional polyether and polyester based urethane systems. Properties exhibited by polyurethanes(PUs) prepared from HTPB include (a) excellent hydrolytic stability, (b) low water absorption, (c) excellent low temperature flexibility, (d) high compatibility with fillers and extenders, and (e) formulation flexibility (Sadeghi et al., 2006). Of late, there is a growing demand of segmented HTPB PUs as these PUs have a unique combination of toughness, durability and flexibility, biocompatibility and biostability that makes them suitable materials for use in a diverse range of biomedical applications (Poussard et al., 2004). HTPB based pervaporation membrane technology is the current wave of innovation. It has introduced a new dimension to PU elastomeric technology.

HTPB-Polyurethane: A Versatile Fuel Binder for Composite Solid Propellant 231

90 °, and number of scans = 300. HMQC spectra of HTPB samples (30 % (w/v) in CDCl3 at room temperature) were recorded on a Bruker 500 MHz with a 5 mm inverse Z-gradient probe. Spectral widths: F2 (1H)=8000 Hz, F1(13C)=27500 Hz. Time domains : (1H)=1024 and (13C) =515, acquisition time (1H) =0.23s, delay (1H) =2s. In processing, the FID was zero-filled to 32 K data points and the resulting 32 K time domain was Fourier transformed. Additionally, Gaussian apodization was also applied in both 1H and 13C domains. **Viscosity measurements:** A Brookfield HADV-II+ programmable rotational type viscometer equipped with a motorized stand (helipath stand) was used to perform isothermal viscosity measurement at different temperatures. The temperature was controlled by a thermostatic temperature control bath (Brookfield). The temperature control accuracy was ± 0.5 °C. Polymer samples were sheared at different shear rate (rpm). The spindle used for binder slurry was AB-4, whereas for propellant slurry, T-E was used. For each experiment, data was collected after one complete revolution. For each successive revolution, total 10 readings, each at an interval of one second were recorded at the set rotational speed by using Wingather Software. The average viscosity value was calculated and used for data analysis and modelling. **DSC experiments**: Mettler FP-900 thermal analysis system equipped with FP-85 standard cell and FP90 central processor was used for DSC measurement. The heat flow and temperature calibration of DSC were carried out using pure indium metal as per the procedure recommended by the manufacturer (ΔH = 26.7 J/g, MP = 158.9 °C). All experiments were carried out in an air atmosphere at different heating rates, ranging from 2-15 °C/min. Aluminum sample pans (40 μL) were used for the DSC experiments. Almost constant sample mass of 5 ± 1 mg was used. **Tensile properties**: The tensile stress-strain measurements were performed at room temperature, using samples previously kept at 23±2 °C and relative humidity of 50± 5% for 48 hrs, according to ASTM D 618. Elastomeric test specimens were punched from the cured slab using a die prepared in accordance with ASTM D 412-68. Tensile testing was performed in an Instron Universal Testing Machine (UTM) using dumb-bell shaped specimens of cured PUs as well as propellants. A 100 kg load was applied at a crosshead speed of 50 mm/min. Hardness was

measured by a Shore A Durometer as per the standard procedure.

The basic compositions that were studied in the present work are shown in Table 1. The binder system studied consists of PU formed by reacting mixture of alcohols [(HTPB, OH value = 42 mg KOH/g), Butanediol (BDO, OH value = 1232 mg KOH/g) as chain extender and trimethylol propane (TMP, OH value = 1227 mg KOH/g) as cross linking agent] with toluene diisocyanate (TDI, purity > 99 percent and a mixture of 2, 4 and 2, 6-isomers in 80:20 ratio). The BDO and TMP were mixed in a fixed ratio (2:1) and dried under vacuum to reduce the moisture content (< than 0.25%) of the mixture. The mixture thus obtained had

**2.2. Synthesis of PUs** 

*2.2.1. Unfilled PUs: PU-I and PU-II* 

the hydroxyl value of 1242 mg of KOH/g.

The polymer chemo-rheology and thermo-oxidative degradation are the two relevant key areas of interest, where in-depth knowledge is essential for the effective performance assessment of PU propellants. Chemo-rhelogy is related with the PU processiblity, whereas thermo-oxidation is related to the stability and combustion performance. The information of change on viscosity during the curing process is critical in modelling the PU flow behaviour. Though extensive works have been carried out on this topic in the last decade (Muthiah et al., 1992, Lakshmi & Athithan, 1999, Singh et al., 2002 & Mahanta et al., 2007), it is still a fascinating research area at present. The thermal decomposition of HTPB has been studied exclusively in inert atmosphere (Panicker & Ninan, 1997). However, thermo-oxidative degradation in air, which is the most relevant in view of combustion of the polymer, has not been studied thoroughly. Additionally, the HTPB prepolymer being the decisive component in HTPB PUs, characterization of this polymer (HTPB) at macro as well as micro levels has been of paramount importance in last decade. Two types of HTPB prepolymer are currently in use: i.e., free radical HTPB and anionic HTPB. The free radical grade HTPB is widely used in composite PU propellants because of its low cost and wide availability. The current chapter is focused on prepolymer characterization, rheology, and oxidative degradation of the polymer and the PU systems.
