Full Terms & Conditions of access and use can be found athttp://www.tandfonline.com/action/journalInformation?journalCode=uegm20

Download by: [Korea Advanced Institute of Science & Technology (KAIST)] Date: 28 January 2016, At: 07:39

Journal of Energetic Materials

ISSN: 0737-0652 (Print) 1545-8822 (Online) Journal homepage: http://www.tandfonline.com/loi/uegm20

Rheological Characterization of Ethanolamine GelPropellants

Botchu V.S Jyoti & Seung Wook Baek

To cite this article: Botchu V.S Jyoti & Seung Wook Baek (2016) Rheological Characterizationof Ethanolamine Gel Propellants, Journal of Energetic Materials, 34:3, 260-278, DOI:10.1080/07370652.2015.1061617

To link to this article: http://dx.doi.org/10.1080/07370652.2015.1061617

Published online: 28 Jan 2016.

Submit your article to this journal

View related articles

View Crossmark data

Rheological Characterization of Ethanolamine GelPropellants

Botchu V.S Jyoti and Seung Wook Baek

Korea Advanced Institute of Science and Technology, Aerospace Engineering, Yuseong-Gu,Daejeon, South Korea

Ethanolamine is considered to be an environmentally friendly propellant system because it has lowtoxicity and is noncarcinogenic in nature. In this article, efforts are made to formulate and prepareethanolamine gel systems, using pure agarose and hybrids of paired gelling agents (agarose +polyvinylpyrrolidine (PVP), agarose + SiO2, and PVP + SiO2), that exhibit a measurable yieldstress, thixotropic behavior under shear rate ranges of 1–1,000 s−1 and a viscoelastic nature. Toachieve these goals, multiple rheological experiments (including flow and dynamic studies) areperformed. In this article, results are presented from experiments measuring the apparent viscosity,yield stress, thixotropy, dynamic strain, frequency sweep, and tan δ behaviors, as well as the effectsof the test temperature, in the gel systems. The results show that the formulated ethanolamine gelsare thixotropic in nature with yield stress between 30 and 60 Pa. The apparent viscosity of the geldecreases as the test temperature increases, and the apparent activation energy is the lowest for theethanolamine-(PVP + SiO2) gel system. The dynamic rheology study shows that the type of gellant,choice of hybrid gelling materials and their concentration, applied frequencies, and strain all vitallyaffect the viscoelastic properties of the ethanolamine gel systems. In the frequency sweep experi-ment, the ethanolamine gels to which agarose, agarose + PVP, and agarose + SiO2 were addedbehave like linear frequency-dependent viscoelastic liquids, whereas the ethanolamine gel to whichPVP + SiO2 was added behaves like a nearly frequency-independent viscoelastic solid. Thevariation in the tan δ of these gelled propellants as a function of frequency is also discussed.

Keywords: ethanolamine, gel, propellant, rheology, thixotropy, viscoelasticity

INTRODUCTION

Gel propellants have been recognized as attractive candidates for future propulsion systems dueto their reduced tendency to spill and their energetic advantages over solid and liquid propel-lants. One of the benefits of gel propellants is their throttling capability, but accurate flowcontrol is more difficult than with conventional Newtonian propellants because of the uniquerheological behaviors of gels.

Address correspondence to Seung Wook Baek, Korea Advanced Institute of Science and Technology, AerospaceEngineering, 291 Daehak-ro, Yuseong-Gu, Daejeon 34141, South Korea. E-mail: swbaek@kaist.ac.kr

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/uegm.

Journal of Energetic Materials, 34: 260–278, 2016Copyright © Taylor & Francis Group, LLCISSN: 0737-0652 print/1545-8822 onlineDOI: 10.1080/07370652.2015.1061617

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

Gel propellants are in the form of soft solids or highly structured liquids, thus showing theproperties of yield stress materials. The most important characteristics of these materials are thatthey can behave as solids under small applied stresses and as liquids at high stresses. Thestrength of the gel skeleton is governed by the structure of the dispersed phase and itsinteractions. Normally, the continuous phase is low in viscosity; however, volume fractions ofa dispersed phase and/or strong interactions between components can increase the viscosity by athousand times or more and induce solid-like behavior at rest. This unique structure distin-guishes these fluids from Newtonian liquids and makes a gel exhibit a complex rheology(Munjal, Gupta, and Varma 1985; Gupta, Varma, and Munjal 1986). Most gel fuels can berheologically categorized as pure shear-thinning fluids. Rheological properties such as shearthinning and thixotropy (Natan and Rahimi 2002) and the viscoelastic properties of gels are ofconsiderable interest during processing and manufacturing. Consequently, rheological charac-terization is an essential step for the better understanding of gel systems (Verma, Gupta, andPandey 1996; Teipel and Forter-Barth 2005; Rahimi, Peretz, and Natan 2007).

Several rheological properties play an important role in determining how a material behavesas it moves from a static to a dynamic environment and vice versa (Barnes, 1997; Morrison2001; Irma and Mrinal 2002; Zhao et al. 2003; Santos et al. 2010; Shai and Arie 2010; Baek andKim 2011; Dennis et al. 2013; Victor and Benveniste 2013). For example, the force or stressrequired to initiate the flow of fluid and semisolid materials plays a significant role in thestorage, transportation, handling, sedimentation, leakage, and end-use performance of thosematerials. Likewise, the force or stress level at which appreciable flow stops once initiated mayalso be of interest. The stress level required to initiate flow is usually referred to as the yieldstress and is related to the level of internal structure in the material that must be destroyed orovercome before flow can occur. Conversely, the stoppage of flow once the stress is removed isrelated to the rebuilding of the structure (Moller, Mewis, and Bonn 2006).

The yield stress must be quantified carefully because the value obtained depends on theanalytical technique used. Consequently, the measured stress at which yielding occurs is usuallyreferred to as the apparent yield stress. Although this apparent yield stress may not represent thetheoretically correct value, it is still extremely useful in comparing and predicting materialbehavior. The yield stress and thixotropic behavior can be understood and modeled as two effectsof the same cause (Barnes 1999; Madlener and Ciezki 2009). Recently, considerable research anddevelopment work has been done in the areas of formulation and rheological characterization ofgel propellant systems. Gupta, Varma, and Munjal (1986) investigated the rheological behavior ofvirgin and metalized unsymmetrical dimethyl hydrazine/high substitution methyl cellulose gelledsystem. Natan and Rahimi (2002) reviewed the current knowledge base regarding gelled propel-lants from the rheological, atomization, and combustion perspectives. Rahimi, Peretz, and Natan(2007) presented viscosity vs. shear rate results for 2–4 wt% hydroxypropylcellulose/mono-methylhydrazine (MMH) and 3–5 wt% silica/inhibited red fuming nitric acid (IRFNA) gels.Santos et al. (2010) focused on the determination of an optimal mixing process and how thoseparameters affect the rheological properties and stability of gelled JP-8 and RP-1 hydrocarbonfuels. Jyoti and Baek (2014a, 2014b, 2015) investigated the flow and dynamic rheologicalbehavior of ethanol and hydrogen peroxide–based gel propellant. For rocket propulsion systems,thixotropic gels would allow an efficient feeding control process, feeding pressure reduction, andbetter atomization. In addition, the rheological properties of gelled propellant influence flow,injection, atomization, and combustion process (Varma and Jyoti 2011). This article addresses

ETHANOLAMINE GEL PROPELLANTS 261

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

phenomena such as thixotropy, yield stress, shear thinning, and structure rebuilding as well asviscoelastic properties of formulated ethanolamine gel systems. Ethanolamine can be consideredhas less toxic propellant system (Melof and Grubelich 2001), and its gelation can provideadditional advantages for propulsion application. Ethanolamine gives a respectable performancein terms of Isp (e.g., liquid ethanolamine with hydrogen peroxide have a calculated Isp of 245 scompared to 284 s for nitrogen tetroxide (NTO) and monomethyl hydrazine). Gelation will help inincreasing the density of ethanolamine and in turns the density Isp.

The purpose of this article is to examine the rheological properties of eco-friendly ethano-lamine gels suitable for use as rocket propellants. We investigated agarose and hybrids of pairedmaterials (agarose + polyvinylpyrrolidone [PVP], agarose + fumed silica [SiO2], and PVP +SiO2) used as gelling agents for ethanolamine gel. The selection of hybrids of the two gellingagents was motivated by an attempt to produce a substantial yield stress and viscoelasticity inthe ethanolamine gel propellant.

MATERIALS AND METHODS

Preparation of Nonmetalized Ethanolamine Gel Fuel Systems

In the present study, ethanolamine (99.5%) obtained from the Sigma Aldrich Corporation (SouthKorea, CAS: 14-14-35) was selected as the base fuel. The critical concentrations of a single gellant(also obtained from Sigma Aldrich) and of paired hybrids of gelling agents needed to gel ethano-lamine were determined by conducting several sets of experiments in the range of 1–10 (wt%) ofgellant. The formulations that resulted in gelation of ethanolamine were selected for the presentrheological study. The gelling agents used for the gelation process were compatible and nonreactivewith the base fuel. The gelation process is known to be affected not only by the type of gelling agentbut also by other parameters such as the processing temperature, mixing time, and type of gellingagent. A uniform process was maintained during all experiments. The requisite concentrations of thegelling agents for gelation of the ethanolamine were as follows: in the case of a single gellant, 4 wt%agarose (CAS: 9012-36-6, type: bioreagent, low electroendosmosis (EEO)) was used, and in thecase of hybrid gellants, 4 wt% agarose + 4 wt% PVP (CAS: 9003-39-8, average molecular weight:10,000), 4 wt% agarose + 4 wt% SiO2 (CAS: 112945-52-5, average particle size: 0.2–0.3 µm,powder form) and 6 wt% PVP + 6 wt% SiO2 were the concentrations used.

Dispersions of pure and hybrid gellant particles in ethanolamine fuel were prepared bymixing the ingredients thoroughly for 3 h. The suspensions were stirred in a sealed environmentat room temperature (298.15 K ± 1 K) at 3,000 rpm to ensure complete dissolution of the gellantin the fuel. The mix was then left undisturbed for 24 h to allow gel network formation. For thestability study, the formulated gels were stored for one year and acceleration stability tests wereperformed. In all cases, stable gels of fuels could be prepared (Figure 1).

Rheological Characterization

Rheological studies (including flow and dynamic studies) were performed on freshly preparedethanolamine gels using a rotational rheometer (HAAKE RS6000, Thermo Scientific,Germany). The rheometer was operated in controlled rate (CR) and controlled stress (CS)

262 B. V. S. JYOTI AND S. W. BAEK

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

modes to characterize the rheological behavior of the gelled propellants. The rheometer imposeda strain on the fluid and measured the resulting stress at shear rates up to 1,000 s−1. A 60-mmparallel plate configuration with a 1-mm gap height was used for all measurements in this study.A control system inside the rheometer ensured a constant temperature of 293 K during allrheological measurements. During the CR and CS flow studies, the gel apparent viscositymeasurement was performed with a 30 s sampling period. Each experiment was repeated atroom temperature at least three to four times in each condition.

Shear rate sweep and shear stress ramp experiments at up to 1,000 s−1 and 100 Pa wereperformed to characterize the shear-thinning behavior of each gel formulation. The shear raterange for these experiments was selected to provide insight into the behavior of the gels underconditions such as tank filling, storage, transportation, and low-speed propellant displacementwithin the injection feed system. The existence of yield stress and thixotropy in the ethanola-mine gel system was also investigated. The yield stress and thixotropy experimental methodswill be discussed in later sections. The effect of temperature and time on ethanolamine gelapparent viscosity was also investigated.

Knowledge of the viscoelastic properties of a gel is desirable to understand the microstruc-tural network of the gel system and its behavior upon impact with another gel, which is

FIGURE 1 Ethanolamine gels.

ETHANOLAMINE GEL PROPELLANTS 263

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

particularly important in the case of hypergolic propellants. The viscoelasticity of materials isoften determined under small strains (i.e., within the linear viscoelastic range). The linearviscoelastic region was identified by performing stress sweep at frequency of 5 Hz. From thisstudy, the elastic (storage modulus, G′) and viscous (loss modulus, G″) components of thesamples were estimated. The storage modulus (G′) corresponds to the amount of energy asample is able to store in the gel network during deformation and provides insight into thestrength of the sample. Further detailed study of the G′ and G″ properties of the samples wasperformed through frequency sweep and tan δ studies at 5 Pa strain.

RHEOLOGICAL RESULTS AND DISCUSSION

Flow Study

Determination of Apparent Viscosity (CR Mode)

A CR-mode flow study was conducted on the prepared gels to measure the apparentviscosity as a function of the shear rate. In a non-Newtonian fluid, the viscosity is dependenton the applied shear rate. This study was conducted to investigate the shear-thinning proper-ties of ethanolamine-based gel propellants at shear rates in the range of 1–1,000 s−1

(Figure 2). This measurement provides an understanding of the variation in the fluid flowbehavior with the yield stress, consistency, homogeneity, and overall quality of the gel. Theresults clearly indicate that the apparent viscosity of ethanolamine gels decreases signifi-cantly with increasing shear rate. This observation can be explained by the assumption thatas the shear rate is increased, the loosely associated three-dimensional network is disturbed.This may lead to an alignment of the network elements that results in the release of theliquid entrapped within, offering less resistance to flow and, as a result, a reduction in theapparent viscosity.

From Figure 2, it can be observed that when gels are disturbed at very low shear rates, theflow is initiated at the yield stress (except for the ethanolamine-[PVP + SiO2] gel at1–1,000 s−1), and a further increase in the shear rate leads to a thinning of the gels in theshear rate range 1–1,000 s−1. This yield stress phenomenon shows that the gels will not flowunless they have reached a critical stress level, called the yield stress. Whereas below the yieldstress the gels have solid-like behavior, above the yield stress, viscous flow (i.e., liquid-likebehavior) occurs due to the breakdown of the three-dimensional network structure of the gels. Inpractice, this means that the material has to be pumped to break down its structure and initiateits flow. However, the determination of the yield stress is challenging because the measuredvalue is dependent on the measurement apparatus, geometry, and experimental protocol.Moreover, the concentration of the gellant and the type of the hybrid gel affect the magnitudeof the apparent viscosity and the yield stress of the gel, as can be observed in Figure 2 andTables 1 and 2. The ethanolamine gel with the PVP + SiO2 gellant has a higher initial apparentviscosity and a lower yield stress in the shear rate range 1–1,000 s−1 than the other ethanolaminegel systems (Tables 1 and 2). This static yield stress is defined as the stress required to initiateflow and is often higher in value. It is usually better to measure the static yield stress wheninvestigating the initiating flow in a material, such as in pumping.

264 B. V. S. JYOTI AND S. W. BAEK

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

TABLE 1Apparent viscosity (η, Pa.s) for shear rate 1–1,000 s−1

Gel propellant ηinitial ηfinal

Ethanolamine + agarose 3.86 0.09Ethanolamine + (agarose + PVP) 2.40 0.11Ethanolamine + (agarose + SiO2) 3.82 0.14Ethanolamine + (PVP + SiO2) 32.25 0.07

FIGURE 2 Rheogram study of ethanolamine gel system at shear rates of 1–1,000 s−1.

TABLE 2Yield stress (Pa) (CR mode)

Gel propellant 1–1000 s−1

Ethanolamine + agarose 57.67Ethanolamine + (agarose + PVP) 33.97Ethanolamine + (agarose + SiO2) 49.12Ethanolamine + (PVP + SiO2) N/A

ETHANOLAMINE GEL PROPELLANTS 265

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

Determination of Thixotropic Behavior

In this experiment, the shear rate was increased systematically to a maximum level andthen decreased in similar steps to obtain the rheogram curve in both directions (Figure 2). Inall of the ethanolamine gels, there is a difference between the viscosity curves obtained fromthe increasing and decreasing runs. The presence of hysteresis is a strong indication that thegel is a thixotropic fluid. The area within the loop provides a direct measure of the extent ofthe thixotropic behavior of the gel. It also reflects the reduction in the structural strengthduring the shear load phase, the completeness of the structural regeneration with time, andthe subsequent decrease in the shear rate due to the local spatial rearrangement of themolecules in the three-dimensional network. Although this is a qualitative way to studythixotropy, it suggests that ethanolamine gels exhibit thixotropy in the shear rate range of1–1,000 s−1. Therefore, these results show that the gels’ apparent viscosity and degree ofthixotropy depend on the shear rate. The decrease in viscosity may be related to the shearthinning behavior of the gel, combined with its thixotropic characteristics. Additionally, thematerials demonstrating a yield stress are typically thixotropic, meaning that their propertiesare dependent on the shear history of the sample and are susceptible to aging. The yieldstress and thixotropic behaviors can be understood and modeled as two effects of the samecause.

Determination of Apparent Viscosity (CS Mode)

One of the quickest and easiest methods for measuring the apparent viscosity with yieldstress on a stress controlled rheometer is to perform a shear-stress ramp experiment anddetermine the stress at which an apparent viscosity peak is observed (Figure 3). Thismethod is sensitive and provides reproducible results for materials with both low and highyield stresses. However, as mentioned earlier, all of the methods for determining the yieldstress depend on the measurement apparatus, geometry, and experimental protocol.Figure 3 shows that, prior to the viscosity peak, the material is undergoing elasticdeformation. This peak in viscosity represents the point at which the elastic structurebreaks down (or yields) and the material begins to flow. This coincides with a rapidincrease in the shear rate and a consequent reduction in the apparent viscosity. The resultsclearly indicate that the apparent viscosity of the ethanolamine gels decreases significantlywith the yield stress as the shear stress increases. Moreover, the concentration of thegellant and the type of hybrid gel affect the magnitude of the apparent viscosity and theyield stress of the gel, as can be observed in Figure 3. The ethanolamine gel with the PVP+ SiO2 gellant has a higher initial apparent viscosity and yield stress than the otherethanolamine gel systems (Figure 3).

Time-Dependent Behavior of Ethanolamine Gels

In this test, the apparent viscosity of the ethanolamine gels with time was studied at aconstant shear rate (100 s−1).

In this experiment, a constant shear rate of 100 s−1 was applied on ethanolamine gelsamples for 60, 120, and 180 s (Figures 4–7). During this process, the apparent viscosity ofthe gels was measured as a function of time. From Figures 4–7, it can be observed that the

266 B. V. S. JYOTI AND S. W. BAEK

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

initial viscosity of all of the gel samples decreased over time for 60, 120, and 180 s. In allof the gels, the three-dimensional microstructure affects the time needed to move from onemicrostructural state to another; that is, from the rest state to the final state. The drivingforce for microstructural change during flow is the structural breakdown due to flowstresses. These effects determine the levels of viscosity and elasticity and the time to changefrom one state to another under the action of shear. These ethanolamine gel systems mighthave a more substantial microstructural network due to random alignment and spatialdistribution in the three-dimensional network, and there might also be a greater degree ofentanglement. Both of these conditions would result in a different viscous response at oneend of the flow curve and dramatically different at other end, which relates to the structuraldifferences in these ethanolamine gels for the 60, 120, and 180 s cases at a constant shearrate of 100 s−1. The present work is similar to the time-dependent study carried out forethanol-based metalized and nonmetalized gels (Jyoti and Baek, 2014b). With the constantshear rate (100 s−1) over time, the decreasing trend of apparent viscosity is observed and thevalue is decayed over time (Figures 4–7). The time-dependent behavior over the constant

FIGURE 3 Shear stress ramp study for the determination of ethanolamine gel apparent viscosity and yield stress.

ETHANOLAMINE GEL PROPELLANTS 267

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

applied shear rate shows the existence of the shear thinning thixotropic property of ethano-lamine gel fluids.

Effect of Temperature on Ethanolamine Gel Apparent Viscosity

In this experiment, at a constant stress of 10 Pa, the ambient test temperature was increasedfrom 0 to 60°C to study the effect of the test temperature on the ethanolamine gel apparentviscosity (Figure 8). During this process, the apparent viscosity of all of the gels wasmeasured as a function of the test temperature. The initial viscosity of all of the gel samplesdecreased with an increase in test temperature. However, in the ethanolamine gel with thePVP + SiO2 hybrid gelling agent, the apparent viscosity initially increased as the testtemperature increased from 0 to 10°C and then decreased as the temperature increased from10 to 60°C. This indicates that exposure to high temperatures provides the material compo-nents with more thermal energy, resulting in a decrease in the gel structural strength and adecrease in the apparent viscosity.

FIGURE 4 Time-dependent behavior of ethanolamine + agarose gel systems at a constant shear rate (100 s−1).

268 B. V. S. JYOTI AND S. W. BAEK

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

According to the Arrhenius equation, the apparent viscosity (ɳap) values obtained can becorrelated with temperature:

lnηap ¼ A:e�Ea=RT ;

where A is a constant related to molecular motion, Ea is the activation energy for viscous flow atconstant stress, R is the gas constant, and T is the absolute temperature in K.

Figure 9 shows the Arrhenius plots of the natural log of apparent viscosity (lnηap) versusthe inverse of temperature for all of the ethanolamine gel systems. The graph shows a nearlylinear relationship for all of the gel samples. The values of the apparent activation energyEa, which were calculated from the plot slopes, are shown in Table 3. Knowing theactivation energy is important in deducing the sensitivity of a gel to the test temperature:the higher the activation energy, the more sensitive the gel is to temperature. The activationenergy is also useful for determining the minimum amount of energy required to initiateflow in the gel. Lower values of Ea may indicate good suitability of the gelling agent asseen in Table 3.

FIGURE 5 Time-dependent behavior of ethanolamine-(agarose + PVP) gel systems at a constant shear rate (100 s−1).

ETHANOLAMINE GEL PROPELLANTS 269

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

Dynamic Strain Sweep Test (CS Mode)

Dynamic strain sweep measurements were used to determine the linear viscoelastic (LVE) rangeof the gel samples. This test involves monitoring the elastic modulus at a constant frequency. Itcan also be employed to determine the yield stress. Because the LVE range is frequencydependent, the test was performed at constant frequency of 5 Hz. When different gel samplesare compared, the limits of the LVE regions of the materials may be differentiated, unlike inshear experiments.

At small stress amplitudes, the elastic modulus (G′) is independent of the strain, whereasat higher strains, there is a sudden drop in the elastic modulus, indicating structuraldeformation and a transition from elastic to viscous behavior. This change can provideinformation about gel strength and stability. The strain value at the transition from the linearviscoelastic region to the viscous region represents the yield point (i.e., the critical strainpoint, γc: G0:γ ¼ τy, where G′ is the elastic modulus, γ is the strain, and τy is the yield point).The dynamic yield stress is defined as the minimum stress required to maintain flow.Consequently, the dynamic yield stress value may be most useful in applications in which

FIGURE 6 Time-dependent behavior of ethanolamine-(agarose + SiO2) gel systems at a constant shear rate (100 s−1).

270 B. V. S. JYOTI AND S. W. BAEK

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

the flow is maintained or stopped after initiation, where lower dynamic yield stress valuesmay be desirable (Figure 10). Figure 10 show the limits of the LVE range for constantfrequency of 5 Hz. These LVE plots enable one to distinguish network deformation in thegels. In Figure 10, it can be observed that the LVE of the ethanolamine gel with agarose andthe hybrid gels with agarose + PVP, agarose + SiO2, and PVP + SiO2 added are independentof strain from 0.1 to 10% in frequency 5 Hz condition. Below these strain levels, the gelstructure is intact, the material behaves like a solid, and G′ is greater than G″ (for constantfrequencies of 5 Hz), indicating that the material is highly structured. Increasing the strainabove the critical strain disrupts the network structure. The material becomes progressivelymore fluid-like, and the modulus declines.

Frequency Sweep Test (CS Mode)

Frequency sweep tests were performed below the critical strain of the ethanolamine gelsamples to investigate the viscoelastic properties of the gel network structure as a functionof the gelling agent and to determine the frequency dependence of the elastic (G′) and

FIGURE 7 Time-dependent behavior of ethanolamine-(PVP + SiO2) gel systems at a constant shear rate (100 s−1).

ETHANOLAMINE GEL PROPELLANTS 271

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

FIGURE 8 Effect of test temperature on the apparent viscosity of ethanolamine gel systems at 10 Pa stress.

FIGURE 9 Arrhenius plot showing the viscosity–temperature relationship of ethanolamine gel systems.

272 B. V. S. JYOTI AND S. W. BAEK

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

viscous (G″) moduli. This test provides information about unusual flow behavior. The shapeof the material function curves reveals the structural characteristics of the material. Thesecharacteristics are useful because they describe the gel in terms of its strengths andweaknesses. The shear storage modulus (G′) measures the stored energy, representing theelastic behavior, whereas the shear loss modulus (G″) measures the energy dissipated asheat, representing the viscous behavior. A high G′ in comparison to G″ represents strongintermolecular interactions and results from the ability to resist intermolecular slippage due

TABLE 3Apparent activation energy (Ea) (kJ/mol)

Gel propellant Ea

Ethanolamine + agarose 74.55Ethanolamine + (agarose + PVP) 76.49Ethanolamine + (agarose + SiO2) 49.81Ethanolamine + (PVP + SiO2) 21.27

FIGURE 10 Dynamic strain sweep study of ethanolamine gel system at 5 Hz.

ETHANOLAMINE GEL PROPELLANTS 273

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

to the relative strength of the loosely associated gel structures occurring in the three-dimensional network.

Figure 11 show the G′ and G″ values for the ethanolamine gel samples, measured in thefrequency range (ω) of 0.1–50 Hz. From Figure 11, it can be observed that for 5 Pa strain, the G′and G″ moduli of all of the gel samples (except the ethanolamine gel with PVP + SiO2 gellingagent) exhibit a linear dependence on frequency. Additionally, at low frequencies, the viscousbehavior reflected by G″ is dominant, whereas at high frequencies, the elastic behavior reflectedby G′ dominates. A possible explanation for these observations is that the formulated gels arestructurally weak gel-like materials, with a weak three-dimensional network structure due toweak physical forces, or viscoelastic liquids. Alternatively, the ethanolamine + agarose, etha-nolamine + (agarose + PVP), and ethanolamine + (agarose + SiO2) gels behave like a liquidviscoelastic material at low frequencies, but as the frequency increases, the gels behave like anelastic material. These physically weak gels possess intermediate rheological properties betweena solution and a strong gel. Therefore, the critical gel concentration could be less than the gellantconcentration used in the present study for the gelation of ethanolamine. The ethanolamine gelwith PVP + SiO2 added behaves consistently like a perfectly elastic gel, nearly independent of

FIGURE 11 Frequency sweep study of ethanolamine gel system at 5 Pa strain.

274 B. V. S. JYOTI AND S. W. BAEK

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

the frequency, in the 5 Pa strain case. These results provide key insights into the mechanisms atwork during propellant injection because at low frequencies, the strain simulates a disturbanceduring loading or storage, and at higher frequencies, the tests may provide information aboutfast phenomena such as injection.

Loss Factor (tan δ)

The loss tangent or damping factor, tan δ, where δ is the phase angle, is the ratio of loss toelasticity and is an indicator of the viscoelasticity of a sample. It is very important in oscillatoryanalysis. It provides comparative parameters that combine both the elastic and viscous con-tributions of a system. It also conveys information concerning the strength of the internalstructure of a system: if tan δ → ∞, the gel is viscous, and if tan δ → 0, the gel is elasticand its particles are highly associated and typical of a gel system.

Figure 12 show that for all ethanolamine gel samples (except the ethanolamine gel withPVP + SiO2 added), the tan δ values are above 1 at low frequencies (representing viscousbehavior) and below 1 at high frequencies (due to elastic behavior). However, in theethanolamine gel with PVP + SiO2 added, the tan δ value is between 1 and 0.1, whichcould be due to the predominantly elastic behavior of this gel system with highly associatedparticles. The ethanolamine gels with agarose, agarose + PVP, or agarose + SiO2 addedbehave like a viscoelastic liquid with frequency dependence, whereas the ethanolamine-PVP+ SiO2 gel behaves like a viscoelastic solid with frequency near-independence in the 5 Pastrain case.

FIGURE 12 Tan δ study of ethanolamine gel systems at 5 Pa strain.

ETHANOLAMINE GEL PROPELLANTS 275

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

CONCLUSION

In this article, efforts were made to understand the rheological properties required to make a gelsuitable for use as a propellant. To achieve this goal, multiple rheological experiments (includ-ing both flow and dynamic studies) were performed to evaluate the gels’ apparent viscosity,shear thinning, yield stress, thixotropy, dynamic strain, critical strain, LVE, storage modulus, tanδ, and apparent activation energy behaviors as well as the effect of temperature on the gels’apparent viscosity. The observed changes in the ethanolamine gel properties during the experi-ments were assumed to be solely due to the imposed conditions.

The gelation of ethanolamine fuel can be performed with pure agarose or with hybrid gellingagents such as agarose + PVP, agarose + SiO2, or PVP + SiO2 at reasonably low criticalconcentrations to obtain stable gels. All of the gels formulated exhibited shear-thinning,thixotropic behavior over the shear rate range of 1–1,000 s−1. The influence of the shear rateled to a reduction in the apparent viscosity with some yield stress behavior. Yield stress easeshandling, storage, and transportation and helps to avoid sedimentation. Thixotropic effectsprovide insight into the gel structural re-formation during the shear release phase and theconstant shear load phase. It was also observed that in all of the gels, the apparent viscositydecreased as the temperature increased, and the lowest apparent activation energy was requiredfor the ethanolamine-(PVP + SiO2) gel system.

The dynamic rheology analysis showed that the type of gellant and its concentrationplays a vital role in determining the viscoelastic properties of ethanolamine gel systems.For the ethanolamine gels with agarose, agarose + PVP, agarose + SiO2, or PVP + SiO2

added, at a frequency of 5 Hz, G′ was greater than G″ in the present study. Both the G′and G″ moduli of the ethanolamine + agarose, ethanolamine + (agarose + PVP), andethanolamine + (agarose + SiO2) samples exhibited a linear dependence on frequency,with a liquid viscoelastic material behavior at low frequencies and elastic behavior athigher frequencies. However, the ethanolamine gel with PVP + SiO2 added behavedconsistently like a perfectly elastic gel, independent of frequency, for the 5 Pa straincase. These results enable the distinction of gels and their network deformation and,consequently, the microscopic structure of the materials. Moreover, the tan δ resultsshowed that some gels behave as frequency-dependent viscoelastic liquids and somebehave as elastic gels with nearly frequency-independent behavior. The latter gels couldbe classified as weak physical gels with good cross-linking networks. The shear raterange, temperature, frequency, and strain, as well as the type of gellant and its concentra-tion, all play important roles and significantly influence the rheological properties of anethanolamine gel propellant system.

The required properties of a useful gelled propellant that we intended to address in thiswork were the presence of yield stress to prevent sedimentation, ability to flow duringstorage and handling, shear-thinning behavior for ease of flow in injection systems, thixo-tropic behavior for gel structural re-formation, and the gel’s three-dimensional network timeresponse at constant shear. The microscopic gel structure provided information about thegel’s strength, weakness, and stability. The results do not suggest an ideal gelled propellantbut instead provide insight into the required properties and potential directions for future gelpropellant development.

276 B. V. S. JYOTI AND S. W. BAEK

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

FUNDING

This work was supported by a National Research Foundation of Korea (NRF) grant funded bythe Korea government (MEST) (No. 2014R1A2A2A01007347).

REFERENCES

Baek, G. and C. Kim. 2011. Rheological properties of carbopol containing nanoparticles. Journal of Rheology 55:313–330. doi:10.1122/1.3538092.

Barnes, H. A. 1999. The yield stress—A review or “παντα ρει”—Everything flows? Journal of Non-Newtonian FluidMechanics 81:133–178. doi:10.1016/S0377-0257(98)00094-9.

Barnes, H. A. 1997. Thixotropy—A review. Journal of Non-Newtonian Fluid Mechanics 70:1–33. doi:10.1016/S0377-0257(97)00004-9.

Dennis, J. D., T. D. Kubal, O. Campanella, S. F. Son, and T. L. Pourpoint. 2013. Rheological characterization ofmonomethylhydrazine gels. Journal of Propulsion and Power 29:313–320. doi:10.2514/1.B34611.

Gupta, B. L., M. Varma, and N. L. Munjal. 1986. Rheological studies on virgin and metalized unsymmetrical dimethylhydrazine gelled systems. Propellants, Explosives, Pyrotechnics 11:45–52. doi:10.1002/prep.19860110204.

Irma, R. and B. Mrinal. 2002. Dynamic rheological measurments and analysis of starch gels. Carbohydrate Polymers48:191–202. doi:10.1016/S0144-8617(01)00235-1.

Jyoti, B. V. S. and S. W. Baek. 2014a. Rheological characterization of hydrogen peroxide gel propellant. InternationalJournal of Aeronautical and Space Sciences 15:199–204. doi:10.5139/IJASS.2014.15.2.199.

Jyoti, B. V. S. and S. W. Baek. 2014b. Rheological characterization of metalized and non-metalized ethanol gelpropellants. Propellants, Explosives, Pyrotechnics 39:866–873. doi:10.1002/prep.201400085.

Jyoti, B. V. S. and S. W. Baek. 2015. Formulation and comparative study of rheological properties of loaded andunloaded ethanol-based gel propellants. Journal of Energetic Materials 33:125–139. doi:10.1080/07370652.2014.939311.

Madlener, K. and H. K. Ciezki 2009. Some aspects of rheological and flow characteristics of gel fuels with regard topropulsion application. 45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Denver, CO, August 2–5.

Melof, B. M. and M. C. Grubelich. 2001. Investigation of hypergolic fuels with hydrogen peroxide. 37th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Salt Lake City, UT, July 8–11.

Moller, P. C. F., J. Mewis, and D. Bonn. 2006. Yield stress and thixotropy: On the difficulty of measuring yield stressesin practice. Soft Matter 2:274–283. doi:10.1039/b517840a.

Morrison, F. A. 2001. Understanding rheology. New York: Oxford University Press.Munjal, N. L., B. L. Gupta, and M. Varma. 1985. Preparative and mechanistic studies on unsymmetrical dimethylhydrazine-red fuming nitric acid liquid propellant gels. Propellants, Explosives, Pyrotechnics 10:111–117.doi:10.1002/(ISSN)1521-4087.

Natan, B. and S. Rahimi. 2002. The status of gel propellants in year 2000. International Journal of Energetic Materialsand Chemical Propulsion 5:172–194. doi:10.1615/IntJEnergeticMaterialsChemProp.v5.i1-6.

Rahimi, S., A. Peretz, and B. Natan. 2007. On shear rheology of gel propellants. Propellants, Explosives, Pyrotechnics32:165–174. doi:10.1002/(ISSN)1521-4087.

Santos, P. H. S., R. Arnold, W. E. Anderson, M. A. Carignano, and O. H. Campanella. 2010. Characterization of JP-8/SiO2 and RP-1/SiO2 gels. Engineering Letters 18:41–47.

Shai, R. and P. Arie. 2010. Rheological matching of gel propellants. Journal of Propulsion and Power 26:376–378.doi:10.2514/1.42904.

Teipel, U., and U. Forter-Barth. 2005. Rheological behavior of nitromethane gelled with nanoparticles. Journal ofPropulsion and Power 21:40–43. doi:10.2514/1.3471.

Varma, M. and B. V. S. Jyoti. 2011. Ignition and combustion studies of heterogeneous UDMH-RFNA gel propellants.International Journal of Energetic Materials and Chemical Propulsion 10:259–275. doi:10.1615/IntJEnergeticMaterialsChemProp.v10.i3.

Verma, M., B. L. Gupta, and M. Pandey. 1996. Formulation & storage studies on hydrazine-based gelled propellants.Defence Science Journal 46:435–442. doi:10.14429/dsj.46.4315.

ETHANOLAMINE GEL PROPELLANTS 277

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016

Victor, C. and N. Benveniste. 2013. Estimation of the rheological properties of a suspension in a gel fluid. AIAA Journal51:998–1003. doi:10.2514/1.J052042.

Zhao, Y., Y. Cao, Y. Yang, and C. Wu. 2003. Rheological study of the sol−gel transition of hybrid gels. Macromolecules36:855–859. doi:10.1021/ma020919y.

278 B. V. S. JYOTI AND S. W. BAEK

Dow

nloa

ded

by [

Kor

ea A

dvan

ced

Inst

itute

of

Scie

nce

& T

echn

olog

y (K

AIS

T)]

at 0

7:39

28

Janu

ary

2016