(kJ mol-1) at 40 °C 89.5 88.3 93.3

PU-I PU-II PU-IIp

where �� is the viscosity at t = 0 and �� is the rate constant for viscosity build up. This exponential model was applied to the experimental data. The initial viscosity and the rate constants at each temperature were calculated from the intercept and slope of the straight line of �� � ��� � plots before the gel point and their values at each isothermal temperature are listed in Table 6.


**Table 6.** Values of viscosity (��� and rate constants���� for various PU systems.

The results indicated that the rate constants increased with increase in temperature from 40 to 70 °C, while the �� decreased. However, the filled PU (PU-IIp) has shown a very slow build up as it is evident from the very low reaction rate constants. This could be due to the effect of various fillers molecules, which restrict the mobility of the reacting molecules, hence slow down the reaction rate. Further, the relationship of rate constant and viscosity (��� with temperature followed the Arrhenius exponential relationship, *i.e.* ����� � ��exp �� ��⁄���, and ����� � �� exp���⁄ � �� , where �� and �� are the apparent rate constant, and initial viscosity at � =∞, �� and �� are the kinetic activation energy, and the viscous flow activation energy, respectively. The values of ��, ��, �� , and �� of the PU reaction on different systems were determined from the Arrhenius plots and listed in Table 7. Further, unlike unfilled PUs (PU-I and PU-II), the filled PU (PU-IIp) showed the shear thinning behaviour. The effect of shear rate on viscosity is shown in Fig.14. For non-Newtonial material, if the viscosity decreases with shear, the rate of decrease is the measure of pseudoplasticity of the material. The flow of highly loaded propellant slurry (86 % solid loading) can be more closely approximated by the Power Law fluid model (Mahanta et al., 2007). The pseudoplasticity index (PI) and viscosity index were calculated from the curve by fitting to a Power Law equation i.e. ���� � ���, where � is the apparent viscosity, � is the shear rate in rpm, � is the pseudoplasticity index, and � is the viscosity index. Newtonian fluid are the special case of Power Law fluid, when � = 0, viscosity is independent of shear rate. For dilatent fluid � is positive, while for pseudoplastics � varies from 0 and -1. In the current work, for the purpose of characterizing the PU-IIp, the minus sign of the � was excluded and reported in percentage.

**Figure 14.** Viscosity (at various intervals) versus shear rate of PU-IIp at 40 °C.

form:

are listed in Table 6.

excluded and reported in percentage.

Temp. (°C)

The model representing the change of viscosity (�) with reaction time (t) has the following

where �� is the viscosity at t = 0 and �� is the rate constant for viscosity build up. This exponential model was applied to the experimental data. The initial viscosity and the rate constants at each temperature were calculated from the intercept and slope of the straight line of �� � ��� � plots before the gel point and their values at each isothermal temperature

Unfilled polyurethane Filled polyurethane

PU-I PU-II PU-IIp ��(mPas) ��(min-1) ��(mPas) ��(min-1) �� (mPas)x10-2 ��(min-1) 40 4349 9.76 x 10-3 6320 12.08 x10-3 10869 1.80 x10-3 50 2647 14.62 x10-3 4142 17.70 x10-3 5226 2.80 x10-3 60 1352 20.70 x10-3 2590 27.40 x10-3 3150 4.11 x10-3 70 606 32.49 x10-3 2008 34.80 x10-3 --- ---

The results indicated that the rate constants increased with increase in temperature from 40 to 70 °C, while the �� decreased. However, the filled PU (PU-IIp) has shown a very slow build up as it is evident from the very low reaction rate constants. This could be due to the effect of various fillers molecules, which restrict the mobility of the reacting molecules, hence slow down the reaction rate. Further, the relationship of rate constant and viscosity (��� with temperature followed the Arrhenius exponential relationship, *i.e.* ����� � ��exp �� ��⁄���, and ����� � �� exp���⁄ � �� , where �� and �� are the apparent rate constant, and initial viscosity at � =∞, �� and �� are the kinetic activation energy, and the viscous flow activation energy, respectively. The values of ��, ��, �� , and �� of the PU reaction on different systems were determined from the Arrhenius plots and listed in Table 7. Further, unlike unfilled PUs (PU-I and PU-II), the filled PU (PU-IIp) showed the shear thinning behaviour. The effect of shear rate on viscosity is shown in Fig.14. For non-Newtonial material, if the viscosity decreases with shear, the rate of decrease is the measure of pseudoplasticity of the material. The flow of highly loaded propellant slurry (86 % solid loading) can be more closely approximated by the Power Law fluid model (Mahanta et al., 2007). The pseudoplasticity index (PI) and viscosity index were calculated from the curve by fitting to a Power Law equation i.e. ���� � ���, where � is the apparent viscosity, � is the shear rate in rpm, � is the pseudoplasticity index, and � is the viscosity index. Newtonian fluid are the special case of Power Law fluid, when � = 0, viscosity is independent of shear rate. For dilatent fluid � is positive, while for pseudoplastics � varies from 0 and -1. In the current work, for the purpose of characterizing the PU-IIp, the minus sign of the � was

**Table 6.** Values of viscosity (��� and rate constants���� for various PU systems.

���� � ������ (5)


**Table 7.** Kinetic and thermodynamic parameters for different PU-systems.

The pseudoplasticity indexes calculated from the Power Law equation are plotted as a function of cure time (Fig.15).

It is observed that the PI is higher at higher temperature. This indicates that at higher temperature the PU-IIp becomes more non-Newtonian. Interestingly, the PI decreases at 40°C and 60 °C with cure time, whereas at 50 °C, it is almost consistent within the pot life of 3 hours, usually required for casting of the propellant slurry into the rocket case. However, the viscosity index decreased initially with temperature, and afterwards, it increased with the cure time. This is attributed to the increase in cross linking, caused by PU reaction. The flow behaviour of HTPB propellant slurry assumes to have great importance as this is the cause of many grain defects in large scale motor. To make a logical decision regarding propellant mixing and casting, not only the effect of temperature and time on viscosity of the propellant slurry should be thoroughly studied, but pseudoplasticity of the slurry should also be equally emphasised. This study has indicated that at 50 °C, the PI remains consistent within the required pot life, so it is assumed that propellant mixing and casting at this temperature may result in a better quality grain.

HTPB-Polyurethane: A Versatile Fuel Binder for Composite Solid Propellant 249

**3.4. Thermo-oxidative degradation of prepolymers (HTPB) and PU-II** 

rate escalates initially resulting in the disappearance of the peak.

**Figure 16.** Dynamic DSC scans of HTPB prepolymers and PU-II at the heating rate of 10 °C/min.

The HTPB polymers are vulnerable to oxidative degradation due to it reactive carbon-carbon double bonds and hydroxyl functionality. These prepolymers are exposed to air, humidity, increased temperature and a lot of shear, during processing for PUs manufacturing. Oxygen and water can ingress into the system by several ways during storage, handling as well as processing, leading to oxidative degradation of the polymer. Oxidative degradation is due to reaction with oxygen from air, which can lead to deterioration of the polymer properties. As discussed earlier, the olefinic groups of HTPB may be present in three configurations namely, *cis*-1,4-; *trans*-1,4-; and *vinyl*-1,2-units. The content of these units varies from polymer to polymer. Generally, these olefinic groups are of different reactivity in the oxidation reaction (Duh et al., 2010). As a result, the per centage of *cis*-1,4-; *trans*-1,4-; and *vinyl*-1,2- olefinic groups in the HTPB may have great effect on oxidation rates and product composition. The typical DSC curves obtained for free radical HTPB as well as PU-II are shown in Fig.16. The DSC thermogram of Krasol LBH-3000 is also given for comparison. It is seen that the HTPB prepolymer degrades in two distinct stages, i.e*.* (1) 170-260 °C, and (2) 290-400 °C when heated up to 400 °C. The first exotherm is attributed to the thermal oxidation reaction of HTPB prepolymer. Upon heating in air atmosphere, HTPB and oxygen are involved in a variety of free-radical reactions as shown in Fig.17. The oxidation reactions, as indicated by the first stage exothermic peak in the DSC thermogram, are attributed to oxygen uptake *via* (a) peroxidation, (b) hydroperoxidation, and (c) crosslinking by peroxide linkage. In the first exotherm, the DSC thermogram of free radical HTPB depicted two peaks, one at 205.0 °C and the other at 244.3 °C, which clearly established that two different oxidative paths ((*i.e*., peroxidation, and hydroperoxidation) were involved in the oxidation process. In contrary to this, a single peak was observed for Krasol LBH-3000 at 234.5 °C. The plausible explanation for this anomaly could be that the per centage of *vinyl*-1, 2-units in the sample of LBH-3000 was higher as compared to free radical HTPB. Owing to the higher reactivity of *vinyl*-1,2 content, the reaction

**Figure 15.** Pseodplasticity index (PI) of PU-IIp as a function of cure time at different temperatures.

A quantitative study of thermodynamic parameters (��#, ��#, and ��#) helps in understanding the reaction mechanism. It is also used to optimise the cure cycle of the PU reaction, both in terms of time and energy. Wynne-Jones-Eyring–Evans theory (Arlas et al., 2007) presents the temperature dependent pre-exponential factor, and the kinetic constant is given as:

$$k\_{\mu} = \frac{k\_B T}{h} e^{\left[N + \left(\Delta S^\dagger/R\right)\right]} e^{\left[-\left(\Delta H^\dagger + NRT\right)/RT\right]} \tag{6}$$

where � is the temperature (K), � = 8.314 Jmol –1K-1 is the universal gas constant, �� is the kinetic rate constant, � is called molecularity, � = 6.62 x 10–34 Js is the Planck's constant, �� is the Boltzmann constant, ��# is the activation enthalpy, and ��� is the activation entropy. The classical Arrhenius constant have ��� and � equals to 1 for reactions occurring in liquid state. Thus, assuming � � �, plotting �� ���⁄�� ��� � �⁄ , the values of ��# & ��# were calculated from the slope and the intercept of the straight line obtained. Also, the ��# value can be calculated from the fundamental thermodynamic relation, *i.e*. ��# � ��# � ���#. The results thus obtained are listed in Table 7.

It is observed that the activation entropy is negative and quite low. This suggests that the polymerization path is more ordered, that makes the reaction thermodynamically disfavoured. Negative values for activation entropy also indicate the association of reactants prior to chemical reaction.

#### **3.4. Thermo-oxidative degradation of prepolymers (HTPB) and PU-II**

248 Polyurethane

given as:

the propellant slurry should be thoroughly studied, but pseudoplasticity of the slurry should also be equally emphasised. This study has indicated that at 50 °C, the PI remains consistent within the required pot life, so it is assumed that propellant mixing and casting at

**Figure 15.** Pseodplasticity index (PI) of PU-IIp as a function of cure time at different temperatures.

�������#⁄���

where � is the temperature (K), � = 8.314 Jmol –1K-1 is the universal gas constant, �� is the kinetic rate constant, � is called molecularity, � = 6.62 x 10–34 Js is the Planck's constant, �� is the Boltzmann constant, ��# is the activation enthalpy, and ��� is the activation entropy. The classical Arrhenius constant have ��� and � equals to 1 for reactions occurring in liquid state. Thus, assuming � � �, plotting �� ���⁄�� ��� � �⁄ , the values of ��# & ��# were calculated from the slope and the intercept of the straight line obtained. Also, the ��# value can be calculated from the fundamental thermodynamic relation, *i.e*. ��# � ��# � ���#.

It is observed that the activation entropy is negative and quite low. This suggests that the polymerization path is more ordered, that makes the reaction thermodynamically disfavoured. Negative values for activation entropy also indicate the association of reactants

������#�����⁄���

(6)

�� � ���� �

The results thus obtained are listed in Table 7.

prior to chemical reaction.

A quantitative study of thermodynamic parameters (��#, ��#, and ��#) helps in understanding the reaction mechanism. It is also used to optimise the cure cycle of the PU reaction, both in terms of time and energy. Wynne-Jones-Eyring–Evans theory (Arlas et al., 2007) presents the temperature dependent pre-exponential factor, and the kinetic constant is

1 3 5 7 9 11

40 °C 50 °C 60 °C

**Cure time x 10-3 (s)**

this temperature may result in a better quality grain.

25.0

27.0

29.0

31.0

**Pseudoplasticity index (%)**

33.0

35.0

The HTPB polymers are vulnerable to oxidative degradation due to it reactive carbon-carbon double bonds and hydroxyl functionality. These prepolymers are exposed to air, humidity, increased temperature and a lot of shear, during processing for PUs manufacturing. Oxygen and water can ingress into the system by several ways during storage, handling as well as processing, leading to oxidative degradation of the polymer. Oxidative degradation is due to reaction with oxygen from air, which can lead to deterioration of the polymer properties. As discussed earlier, the olefinic groups of HTPB may be present in three configurations namely, *cis*-1,4-; *trans*-1,4-; and *vinyl*-1,2-units. The content of these units varies from polymer to polymer. Generally, these olefinic groups are of different reactivity in the oxidation reaction (Duh et al., 2010). As a result, the per centage of *cis*-1,4-; *trans*-1,4-; and *vinyl*-1,2- olefinic groups in the HTPB may have great effect on oxidation rates and product composition. The typical DSC curves obtained for free radical HTPB as well as PU-II are shown in Fig.16. The DSC thermogram of Krasol LBH-3000 is also given for comparison. It is seen that the HTPB prepolymer degrades in two distinct stages, i.e*.* (1) 170-260 °C, and (2) 290-400 °C when heated up to 400 °C. The first exotherm is attributed to the thermal oxidation reaction of HTPB prepolymer. Upon heating in air atmosphere, HTPB and oxygen are involved in a variety of free-radical reactions as shown in Fig.17. The oxidation reactions, as indicated by the first stage exothermic peak in the DSC thermogram, are attributed to oxygen uptake *via* (a) peroxidation, (b) hydroperoxidation, and (c) crosslinking by peroxide linkage. In the first exotherm, the DSC thermogram of free radical HTPB depicted two peaks, one at 205.0 °C and the other at 244.3 °C, which clearly established that two different oxidative paths ((*i.e*., peroxidation, and hydroperoxidation) were involved in the oxidation process. In contrary to this, a single peak was observed for Krasol LBH-3000 at 234.5 °C. The plausible explanation for this anomaly could be that the per centage of *vinyl*-1, 2-units in the sample of LBH-3000 was higher as compared to free radical HTPB. Owing to the higher reactivity of *vinyl*-1,2 content, the reaction rate escalates initially resulting in the disappearance of the peak.

**Figure 16.** Dynamic DSC scans of HTPB prepolymers and PU-II at the heating rate of 10 °C/min.

HTPB-Polyurethane: A Versatile Fuel Binder for Composite Solid Propellant 251

We observed from Fig.18 that in both the cases, the thermograms shifted towards higher temperatures as the heating rate ሺߚሻ increased. This shift of thermograms to higher temperature with increasing heating rate is anticipated since a shorter time is required for the samples to reach a given temperature at a faster heating rate. However, the shapes of the exothermic curves at all heating rates are similar. It indicates that similar reaction mechanisms are involved in oxidative degradation, irrespective of heating rates. The measured values of the onset temperature (ܶ), peak temperature (ܶ), final temperature (ܶ), and oxidation enthalpy (οܪ௫) for HTPB prepolymers and PU-II are listed in Table 8 and 9,

**Figure 18.** DSC thermogrames for decomposition of free radical HTPB at various heating rates (the

ܶ ( °C)

**Table 8.** Thermo-oxidative properties of HTPB prepolymer at various heating rates (ߚ(.

Thermo-oxidative properties of substrate polymers Free radical HTPB Krasol LBH-3000

> ܶ ( °C)

ܶ ( °C)

ܶ ( °C) οܪ௫ (Jg-1)

οܪ௫ (Jg-1)

2 159.6 204.1 233.4 896 147.0 188.9 212.4 510 4 177.0 225.4 252.4 653 160.7 206.8 236.1 454 6 186.5 235.7 270.4 595 166.0 216.0 238.9 426 8 187.0 239.1 271.0 649 170.1 222.8 247.1 443 10 187.7 244.3 279.0 628 173.9 234.5 264.4 369

respectively.

insert Fig. is for PU-II).

ܶ ( °C)

ܶ ( °C)

ߚ (°C min-1)

**Figure 17.** Thermo-oxidative reactions of HTPB prepolymers: (a) peroxidation, (b) hydroperoxidation, and (c) cross-linking by peroxide linkage.

The second exothermic peak occurred at 290-400 °C. The broad exothermic peak is attributed to the major oxidative degradations of HTPB prepolymer involving chain unzipping. It results from the endothermic depolymerisation, exothermic cyclization, and oxidative cross-linking processes of the HTPB prepolymer. The exothermicity is due to the energy released in the formation of new bonds during cross-linking and cyclization, which is greater than the absorbed energy for bond scission during depolymerisation. For PU-II, it is seen that the thermo-oxidative profile has a pattern very similar to that of HTPB prepolymer. This is expected as the PU-II constitutes HTPB more than 92 % of its weight. However, the most important difference is that the thermo-oxidative peak (first exotherm) is slightly less pronounced and occurs somewhat earlier than HTPB prepoymer. The peak temperature of PU-II is 203.8 °C which is 40 °C less as compared to HTPB prepolymer. In second stage i.e. between 290-400 °C, a small elevation is observed around 315 °C, which could be attributed to the cleavage of urethane linkages and subsequent loss of toluene diisocyanate, followed by depolymerization, cyclization, and crosslinking of HTPB prepolymer giving a broad exotherm with peak temperature of 374.6 °C, which is slightly less than its prepolymer peak temperature. This finding is in well agreement with the fact that cleavage of urethane linkages in HTPB PUs is the first step during thermal decomposition (Chen &Brill, 1991). As our objective was to study the thermo-oxidative behaviour of the polymer, we restricted only to the first exothermic peak of the DSC thermogram. The influence of different heating rates (ߚ (on the thermooxidative behaviour of free radical HTPB prepolymer is illustrated in Fig.18.The insert in Fig.18 shows the influence of different heating rates (ߚ (on the thermo-oxidative behaviour of PU-II.

We observed from Fig.18 that in both the cases, the thermograms shifted towards higher temperatures as the heating rate ሺߚሻ increased. This shift of thermograms to higher temperature with increasing heating rate is anticipated since a shorter time is required for the samples to reach a given temperature at a faster heating rate. However, the shapes of the exothermic curves at all heating rates are similar. It indicates that similar reaction mechanisms are involved in oxidative degradation, irrespective of heating rates. The measured values of the onset temperature (ܶ), peak temperature (ܶ), final temperature (ܶ), and oxidation enthalpy (οܪ௫) for HTPB prepolymers and PU-II are listed in Table 8 and 9, respectively.

250 Polyurethane

HO CH2 CH CH CH2 OH

and (c) cross-linking by peroxide linkage.

behaviour of PU-II.

x

+ p O2 (b)

O2 (c)

+ p O2 (a)

CH2 CH CH CH CH2 CH CH CH2

CH2 CH CH CH2 CH2 CH CH CH2

CH2 CH CH CH2 CH2 CH CH CH2

(crosslinking by peroxide linkage)

CH2 CH CH CH2 CH2 CH CH CH2

p x-p

p x-p

p x-p

p x-p

HOO

O O

(peroxidation)

(hydroperoxidation)

O O

**Figure 17.** Thermo-oxidative reactions of HTPB prepolymers: (a) peroxidation, (b) hydroperoxidation,

The second exothermic peak occurred at 290-400 °C. The broad exothermic peak is attributed to the major oxidative degradations of HTPB prepolymer involving chain unzipping. It results from the endothermic depolymerisation, exothermic cyclization, and oxidative cross-linking processes of the HTPB prepolymer. The exothermicity is due to the energy released in the formation of new bonds during cross-linking and cyclization, which is greater than the absorbed energy for bond scission during depolymerisation. For PU-II, it is seen that the thermo-oxidative profile has a pattern very similar to that of HTPB prepolymer. This is expected as the PU-II constitutes HTPB more than 92 % of its weight. However, the most important difference is that the thermo-oxidative peak (first exotherm) is slightly less pronounced and occurs somewhat earlier than HTPB prepoymer. The peak temperature of PU-II is 203.8 °C which is 40 °C less as compared to HTPB prepolymer. In second stage i.e. between 290-400 °C, a small elevation is observed around 315 °C, which could be attributed to the cleavage of urethane linkages and subsequent loss of toluene diisocyanate, followed by depolymerization, cyclization, and crosslinking of HTPB prepolymer giving a broad exotherm with peak temperature of 374.6 °C, which is slightly less than its prepolymer peak temperature. This finding is in well agreement with the fact that cleavage of urethane linkages in HTPB PUs is the first step during thermal decomposition (Chen &Brill, 1991). As our objective was to study the thermo-oxidative behaviour of the polymer, we restricted only to the first exothermic peak of the DSC thermogram. The influence of different heating rates (ߚ (on the thermooxidative behaviour of free radical HTPB prepolymer is illustrated in Fig.18.The insert in Fig.18 shows the influence of different heating rates (ߚ (on the thermo-oxidative

**Figure 18.** DSC thermogrames for decomposition of free radical HTPB at various heating rates (the insert Fig. is for PU-II).


**Table 8.** Thermo-oxidative properties of HTPB prepolymer at various heating rates (ߚ(.



$$\frac{da}{dt} = k \langle T \rangle f(a) \tag{7}$$

$$\frac{d\alpha}{f(a)} = \frac{\mathcal{A}}{\beta} \exp\left(-\frac{E\_a}{RT}\right)dT\tag{8}$$

$$\ln\left(\frac{\beta}{T\_p^2}\right) = \ln\left(\frac{AR}{E\_a}\right) - \frac{E\_a}{RT\_p} \tag{9}$$

$$\log(a) = \frac{\text{A}}{\text{\(\beta\)}} \int\_0^\text{T} \exp\left(-\frac{E\_a}{RT}\right) dT = \frac{\text{AE}\_\text{a}}{\text{\(\beta\)}} \text{p} \text{(x)}\tag{10}$$

$$
\ln g(a) = \ln \frac{A E\_a}{\beta R} - 5.330 - 1.052 \frac{E\_a}{R T} \tag{11}
$$

$$
\ln \beta = \mathcal{C} - 1.052 \frac{E\_a}{\mathcal{R} \mathcal{T}},
\text{where } \mathcal{C} = \ln \frac{A E\_a}{g(a) \mathcal{R}} - 5.330 \tag{12}
$$

$$\ln\left(\frac{\beta}{T^2}\right) \cong \ln(\frac{AR}{g(a)E\_a}) - \frac{E\_a}{RT} \tag{13}$$



$$\ln \frac{g(a)}{\tau^2} = \ln \left[ \frac{AR}{\beta E\_a} \left( 1 - \frac{2RT}{E\_a} \right) \right] - \frac{E\_a}{RT} \tag{14}$$


HTPB-Polyurethane: A Versatile Fuel Binder for Composite Solid Propellant 259

The chapter provides an insight into the microstructure and sequence distribution of the substrate polymer obtained from analysis of 1D and 2D 13C and 1H NMR techniques. The absolute molecular weight of the prepolymer has been determined by high field NMR method. This study pointed out that the HTPB prepolymer was a Newtonian fluid and viscosity decreased exponentially with temperature. The activation energy for viscous flow for free radical HTPB was less than that of anonic prepolymer. The chemorheological analysis concludes that the shear rate has no significant effect on the viscosity of the PU reaction within the cure time. The viscosity of various PU systems rises exponentially with cure time. The rate of viscosity build up for filled PU (propellant) is quite low as compared to the unfilled PU systems. Unlike the unfilled PUs, the filled PU slurry showed pseudoplastic behavior, *i.e*. the shear rate had significant effect on viscosity of the propellant slurry. For a typical composition with 86% solid loading, the pseudoplasticity index was found to be higher at higher temperature. It shows that at higher temperature, it becomes more non-Newtonian. Additionally, it also revealed that the pseudoplasticity index remained unchanged within the cure time studied (*i.e.*, 3 h), when maintained at 50 °C, which is desirable in view of propellant flow during casting of the propellant slurry. Further, the filled PU (propellant) gave excellent elastomeric properties, which were apt for solid rocket motor requirement. Additionally, the desired properties can be easily accentuated by simply tailoring the hard segment content of the PU composition. Thermo-oxidative behavior, as studied by DSC of the substrate polymer and the PU elastomers, confirms that PU elastomers are more resistant to thermooxidation as compared to the substrate polymer. The thermo-oxidative degradation could be modeled well by an empirical equation given by Avrami-Erofeev. Endowed with so many advantages, HTPB PUs is undoubtedly a versatile and ubiquitous fuel binder for solid rocket motors. However, in order to gain an in depth insight into the multi-step reaction mechanism, further analysis of the DSC data is warranted. Future studies aim at the simulation of the thermo-oxidative profile of HTPB PUs by using a suitable Computer

**4. Conclusion** 

Software in order to understand its complexity.

*Defence Research & Development Organization, SF Complex, Jagdalpur, India* 

Authors are thankful to the General Manager SF Complex, Jagdalpur for his kind

*Department of Applied Chemistry, Indian School of Mines, Dhanbad, India* 

**Author details** 

Abhay K. Mahanta

Devendra D. Pathak

**Acknowledgement** 

permission to publish the article.

**Table 12.** Kinetic parameters for no-isothermal oxidation by Coats -Redfern equation.

## **3.5. HTPB polyurethanes: Stress-strain properties**

PU elastomers exhibit good elasticity in a wide range of hard segment contents. This is due to the change of soft or hard segments in different proportion and structure. PUs are composed of short alternating hard and soft segments. The hard segment of PUs usually consists of diisocyanate linked to a low molecular weight chain extender such as butanediol. Meanwhile, the thermodynamic incompatibility between hard and soft segments can lead to the micro-phase separation and hence make a significant contribution to elastomeric properties. Basically, soft segments provide the elasticity, while hard segments play a role in reinforcing the filler and physical cross-linking. In a condensed structure, hard segments usually exist in glassy state or crystalline state. Because of the strong hydrogen bonds of hard segments, their domains can be formed and distributed in the soft segments. The PU elastomeric properties obtained for different systems are reported in Table 13. As a generic trend, it was observed that increase in hard segment content corresponded to higher values of hardness, tensile strength and modulus. The increase in mechanical properties with hard segment content was attributed to the progressive effect of hydrogen bonds within the hard domains of the cross-linked PUs.


**Table 13.** Elastomric properties of different PU systems.

#### **4. Conclusion**

258 Polyurethane

ߚ

ܣ min-1

domains of the cross-linked PUs.

Elastomeric properties:

**Table 13.** Elastomric properties of different PU systems.

ܧ

**3.5. HTPB polyurethanes: Stress-strain properties** 

kJmol-1 r2 ܣ

°Cmin-1 Free radical HTPB Krasol LBH-3000 Polyurethane: PU-II

2 21.1 91.8 0.994 19.3 82.5 0.997 --- --- --- 4 19.4 86.6 0.995 16.3 71.7 0.997 21.0 87.7 0.957 6 16.1 73.7 0.998 15.7 69.0 0.996 22.0 91.0 0.966 8 17.1 77.2 0.995 15.0 66.3 0.996 19.0 79.4 0.955 10 14.4 66.7 0.995 12.4 56.3 0.994 17.9 75.1 0.957

PU elastomers exhibit good elasticity in a wide range of hard segment contents. This is due to the change of soft or hard segments in different proportion and structure. PUs are composed of short alternating hard and soft segments. The hard segment of PUs usually consists of diisocyanate linked to a low molecular weight chain extender such as butanediol. Meanwhile, the thermodynamic incompatibility between hard and soft segments can lead to the micro-phase separation and hence make a significant contribution to elastomeric properties. Basically, soft segments provide the elasticity, while hard segments play a role in reinforcing the filler and physical cross-linking. In a condensed structure, hard segments usually exist in glassy state or crystalline state. Because of the strong hydrogen bonds of hard segments, their domains can be formed and distributed in the soft segments. The PU elastomeric properties obtained for different systems are reported in Table 13. As a generic trend, it was observed that increase in hard segment content corresponded to higher values of hardness, tensile strength and modulus. The increase in mechanical properties with hard segment content was attributed to the progressive effect of hydrogen bonds within the hard

Parameters Unfilled polyurethanes Filled polyurethanes

Hard segment (% w/w) 4.34 7.25/7.34/7.43/7.52/7.61 7.25/7.34/7.43/7.52/7.61

TS (kgf/cm2) 2.4 3.5/3.6/4.0/4.2/4.4 7.3/8.6/8.9/10.8/11.8

Elong. (%) 350 759/631/627/520/437 44/42/39/35/33 Mod.(kgf/cm2) --- --- 45/52/59/78/83 Hardness (Shore-A) 10 10/14/15/18/20 65/79/80/83/85

PU-I PU-II PU-IIp

ܧ

kJmol-1 r2 ܣ

min-1

min-1

**Table 12.** Kinetic parameters for no-isothermal oxidation by Coats -Redfern equation.

(Free radical HTPB)

ܧ kJmol-1 r2 The chapter provides an insight into the microstructure and sequence distribution of the substrate polymer obtained from analysis of 1D and 2D 13C and 1H NMR techniques. The absolute molecular weight of the prepolymer has been determined by high field NMR method. This study pointed out that the HTPB prepolymer was a Newtonian fluid and viscosity decreased exponentially with temperature. The activation energy for viscous flow for free radical HTPB was less than that of anonic prepolymer. The chemorheological analysis concludes that the shear rate has no significant effect on the viscosity of the PU reaction within the cure time. The viscosity of various PU systems rises exponentially with cure time. The rate of viscosity build up for filled PU (propellant) is quite low as compared to the unfilled PU systems. Unlike the unfilled PUs, the filled PU slurry showed pseudoplastic behavior, *i.e*. the shear rate had significant effect on viscosity of the propellant slurry. For a typical composition with 86% solid loading, the pseudoplasticity index was found to be higher at higher temperature. It shows that at higher temperature, it becomes more non-Newtonian. Additionally, it also revealed that the pseudoplasticity index remained unchanged within the cure time studied (*i.e.*, 3 h), when maintained at 50 °C, which is desirable in view of propellant flow during casting of the propellant slurry. Further, the filled PU (propellant) gave excellent elastomeric properties, which were apt for solid rocket motor requirement. Additionally, the desired properties can be easily accentuated by simply tailoring the hard segment content of the PU composition. Thermo-oxidative behavior, as studied by DSC of the substrate polymer and the PU elastomers, confirms that PU elastomers are more resistant to thermooxidation as compared to the substrate polymer. The thermo-oxidative degradation could be modeled well by an empirical equation given by Avrami-Erofeev. Endowed with so many advantages, HTPB PUs is undoubtedly a versatile and ubiquitous fuel binder for solid rocket motors. However, in order to gain an in depth insight into the multi-step reaction mechanism, further analysis of the DSC data is warranted. Future studies aim at the simulation of the thermo-oxidative profile of HTPB PUs by using a suitable Computer Software in order to understand its complexity.

#### **Author details**

Abhay K. Mahanta *Defence Research & Development Organization, SF Complex, Jagdalpur, India* 

Devendra D. Pathak *Department of Applied Chemistry, Indian School of Mines, Dhanbad, India* 
