**2. Experimental**

#### **2.1. Analytical equipments**

**NMR measurements:** The NMR spectra were recorded on a Bruker 800 MHz NMR spectrometer. The HTPB samples (10 % (w/v) for the 1H NMR and 30 % (w/v) for the 13C{1H} NMR analysis) were recorded in CDCl3 at room temperature. The 1H NMR acquisition parameters were: spectral width = 16 ppm, acquisition time =2 s, relaxation delay = 1 s, pulse width = 90 °, and number of scans =1000. Similarly, 13C{1H} NMR spectra were recorded using spectral width = 220 ppm, acquisition time = 2 s, relaxation delay = 10 s, pulse width = 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.

#### **2.2. Synthesis of PUs**

230 Polyurethane

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.

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

**NMR measurements:** The NMR spectra were recorded on a Bruker 800 MHz NMR spectrometer. The HTPB samples (10 % (w/v) for the 1H NMR and 30 % (w/v) for the 13C{1H} NMR analysis) were recorded in CDCl3 at room temperature. The 1H NMR acquisition parameters were: spectral width = 16 ppm, acquisition time =2 s, relaxation delay = 1 s, pulse width = 90 °, and number of scans =1000. Similarly, 13C{1H} NMR spectra were recorded using spectral width = 220 ppm, acquisition time = 2 s, relaxation delay = 10 s, pulse width =

It has introduced a new dimension to PU elastomeric technology.

the polymer and the PU systems.

**2.1. Analytical equipments** 

**2. Experimental** 

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

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 the hydroxyl value of 1242 mg of KOH/g.


HTPB-Polyurethane: A Versatile Fuel Binder for Composite Solid Propellant 233

), and thus, ܯഥ of the

NMR spectra. Although 13C NMR spectroscopy is good in terms of a wider range of chemical shifts and thus offering less possibility of overlapping peaks, problems associated with questionable assignments occasionally arise from steric-sensitive environments in the carbon skeleton. Additionally, the Nuclear Overhauser Enhancement (NOE) of different types of carbon is usually not equal and the wide spin-lattice relaxation time (t1) range makes quantitative measurements of carbon signals difficult. A combination of NMR techniques such as 1H, 13C{1H}, 13C{1H}-DEPT (Distortionless Enhancement By Polarization Transfer) and 1H/13C-HMQC (Hetero-nuclear Multiple Quantum Coherence) permit assignments of all 1H and 13C resonance peaks. To our knowledge, hitherto the actual physical characteristics of the HTPB are not precisely known, particularly its absolute number-average molecular weight (ܯഥ). In the current work, we have examined the 1H and 13C NMR spectra of HTPB in order to precisely

polymer. A typical 13C{1H} NMR spectrum (200MHz, CDCl3) of free radical HTPB prepolymer is shown in Fig.1. For convenience, resonances in the spectrum can be divided in to three distinct regions, *i.e.* (a) an olefinic region: δ 113-144, (b) a carbon bearing hydroxyl end group region: δ 56 – 65, and (c) an aliphatic region: δ 24-44. However, due to complex nature of the prepolymer, a complete assignment of all signals was not possible. The, methine and methylene carbons were distinguished by using the DEPT technique.

The 13C{1H}-DEPT spectrum of the polymer, recorded in CDCl3, is depicted in Fig.2.

**Figure 1.** 13C{1H} NMR (CDCl3, 200 MHz) spectrum of free radical HTPB prepolymer.

determine its number-average degree of polymerization (ܦܲതതതത

\*Hard segment content= ሼሾݓ்ூ ݓைା்ெሿΤ ሽ ݓ௧௧ ൈ ͳͲͲǡ ݓ ൌ ݓ݄݁݅݃ݐǤ

**Table 1.** The basic composition of the one step PUs.

The PUs were prepared in bulk by one step procedure. Mixing was carried out in a pilot mixer with facility for circulation of hot/cold water around the mixer jacket. The HTPB and BDO-TMP mixture were taken in the pilot mixer and stirred for 10 minutes. The calculated amount of TDI was added to the mixer, and the contents were stirred for 20 minutes at 40 ± 1 °C. The binder slurry was cast in to a Teflon coated mould and cured at 60 °C for 3 days.

#### *2.2.2. Filled PUs (propellant): PU-IIp*

The basic propellant composition that uses 68% ammonium perchlorate (AP) and 18% aluminum (Al) powder was taken up for study. AP (with purity > 99%) was used in bimodal distribution (3:1) having average particle size 280 m and 49 m, respectively. Particle size of AP and Al powder (mean diameter = 33.51 μm) were measured by a CILAS Particle Size Analyzer-1180 model. Dioctyl adipate (saponification value = 300 mg KOH/g) was used as a plasticizer. The mixing was carried out in two phases. In the first phase, all the ingredients, except the curing agent, were premixed thoroughly for about 3 h at 38 2 °C. Hot water was circulated through the jacket of the mixer bowl to keep a constant temperature throughout the mixing cycle. A homogeneous test of the slurry was carried out after completion of the premix to confirm the uniform dispersion of AP and Al powder. In the second phase of mixing, a calculated amount of curing agent, *i.e.* toluene diisocyanate (TDI) was added to the premixed slurry, and further mixed for 40 minutes at 40 1 °C. The propellant slurry was cast in to the Teflon coated mould and cured at 60 °C for 5 days.
