Preparation and Characterization of Eco-Friendly Hydrogen Peroxide Based Gel Oxidizer

Transcription

1 Proceedings of 7 th Asian-Pacific Conference on Aerospace Technology and Science May 23-26, 2013, Taiwan Preparation and Characterization of Eco-Friendly Hydrogen Peroxide Based Gel Oxidizer B.V.S.Jyoti 1 *, Dong-Gi Lee 1, Seung Wook Baek 1, Jae-Won Lee 2 1 Division of aerospace Engineering, School of Mechanical, Aerospace and Systems Engineering Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-Gu, Daejeon , Republic of Korea 2 Hanwha Corporation, Daejeon Plant, 52-1 Woisamdong, Yuseong-Gu, Daejeon , Republic of Korea *Corresponding author. Fax: , Tel.: jyotiv@kaist.ac.kr Abstract Gelled Hydrogen peroxide is an ideal oxidizer for such environmentally propulsion system where safety, storage, stability, transportation and handling are the main concern. An experimental investigation to prepare and study the thixotropic properties of hydrogen peroxide oxidizer gel using rotational rheometer and also the effect of temperature (283.15, and K) on these properties has been carried out. Rheological characterization was done at lower shear rate range (1 to 20 s -1 ). Oxidizer gel has been found to be thixotropic in nature with viscosity values drastically falling with shear rate. It was also observed that the gel apparent viscosity whole spectrum shifts to lower viscosity value with increase in temperature. However, the area enclose the hysteresis loop is less effected by the varying operating temperature. Keywords: Hydrogen peroxide, Gel, Thixotropy, Apparent viscosity, Rheological property 1. Introduction Generally gels are modified conventional liquids whose properties are altered by the addition of a suitable gelling agent and as a result their physico-chemical behavior undergoes a significant change [1-2]. Such a heterogeneous gel system must satisfy two criteria; the viscosity must be imparted by a colloidal structure that has a yield value upon deformation and it should be reversible when subjected to shear i.e. the primary modification of the parent liquid should be in a gel state which undergoes thinning when subjected to shear force. The last but the most significant characteristic desire is the modification of parent liquid by adding small concentration of gelling agent. The great advantages of gels are the fact that they combine the properties of both liquid and solid. In addition their rheological properties like thixotropic (time dependent shear thinningpseudoplastic) and viscoelastic properties help in ease of flow like in case of propulsion system, where gel propellants will flow like a liquid propellant on application of required shear, even help in ability to control the thrust by controlling the mass flow of propellant into 1

2 the combustion chamber, ease of handling, storage, stability and also reduce the risk of leakage. As a result a significant effort has been made worldwide in the past to formulate, prepare and study the rheological properties of gels and gel propellants for various applications such as rocket propulsion system [1-6]. In current study high test hydrogen peroxide based gel oxidizer is used because it is ecofriendly. Also hydrogen peroxide serves as an oxidizing agent in the combustion of several organic fuels. The major objectives of the present studies are to prepare the hydrogen peroxide gelled oxidizer. And to seek the effect of low shear rate and varying operating temperature on rheological properties of hydrogen peroxide gelled oxidizer system. 2. Materials and Methods 2.1 Preparation of Gel Oxidizer System Hydrogen peroxide (H 2 O 2 ) oxidizer system was selected as a base oxidizer. Conventional hydrogen peroxide has been used as a bipropellant or monopropellant in many liquid rocket propulsion systems. The first step towards the preparation of gel involves determination of critical concentration of gellant required to gel hydrogen peroxide. Moreover, the gellant should be compatible and non-reactive. However, the gelation process is not only affected by the type of gelling agent, but other parameters too play an important role, such as; gelation time, operating temperature, particle size etc. Fumed Silica (SiO 2 ) (6 wt%) was used in present experiment for gelation of hydrogen peroxide. Fumed Silica (SiO 2 ) is a colloidal particulate which is an amorphous form of silicon dioxide prepared by a flame hydrolysis. Fumed Silica forms a weak hydrogen bonding depending upon the type of solvent. In such cases silanols on adjacent silica particles are envisioned to interact directly by weak hydrogen bonding. This leads to particle flocculation through three dimensional network of particles and ultimately to Gelation [7]. Gel formation process is clearly seen in Fig.1. Fig. 1 SiO 2 Gel Network Formation Dispersion of SiO 2 particles in H 2 O 2 liquid oxidizer was prepared by mixing the component thoroughly. The suspension was stirred at room temperature to ensure complete dissolution of the gellant in the solvent. In order to form the gel, the SiO 2 has to be highly dispersed in the solvent to allow the network formation. The main driving force for silica gelation is expected to be the formation of weak hydrogen bond between silanol (Si-OH) groups on adjacent silica units. 2

3 During the process of gelation, the initial viscous liquid is converted into a self-supporting material which exhibits viscoelastic character. H 2 O 2 gel with SiO 2 show thixotropic behavior. Detail discussion is made in the later section of this paper. 2.2 Rheological Characterization Rheological study of freshly prepared gelled H 2 O 2 was carried out using a rotational rheometer (HAAKE RS600) and assesses gel viscosity at low shear rate range (1 to 20 s -1 ). This exercise is important to understand the flow properties of gel at low shear rate, ensure uniform quality of the gels for further studies and also to characterize the gels under a low shear rate range. A set of parallel plate geometry was used for the present investigation since it is good for non-newtonian linear viscoelastic fluids. A temperature control system inside the equipment ensured a constant fluid operating temperature during all rheological measurements. Fig. 2 Newtonian and Non-Newtonian Fluids Figure 2 depicts the behavior of viscosity as a function of shear rate for Newtonian and non- Newtonian fluids. 3. Results and Discussion In rheological study, the response of the sample is measured under flow conditions and the apparent viscosity is determined as a function of shear rate. Rheology study was conducted to investigate the shear-thinning thixotropic properties of hydrogen peroxide based gel oxidizer at lower shear rate region. Lower shear rate region study is important for quality control of the prepared gel along with characterization. 3.1 Appearance Hydrogen peroxide based gelled oxidizer with 6 wt% SiO 2 gellant concentration is depicted in Fig. 3. The much clear and stiffer characteristics and semi-solid like behavior of gel is 3

4 clearly visible. Moreover, the samples appear to be homogenous, transparent or slightly translucent gel with no evidence of phase separation. Even visual examination indicates that the gel is soft, free standing solid that can bear its own weight in a sample container. Hence this class of materials can be categorized as H 2 O 2 -SiO 2 gel. Fig. 3 Hydrogen peroxide oxidizer gel with gelling additive 6 wt % 3.2 Shear Rheology Test Shear rheology measurement is used to determine the flow properties under changing shear condition. In this case the curve shows the apparent viscosity as a function of the shear rate. The apparent viscosity in current experiment for H 2 O 2 -SiO 2 gel oxidizer was determined at low shear rate range from 1 to 20 s -1 for different operating temperatures (283.15, and K). The results on the variation of apparent viscosity at different operating temperature for H 2 O 2 SiO 2 gel are presented in Fig. 4. The result clearly indicates that the viscosity for gelled H 2 O 2 is decreasing significantly with increasing shear rate. In case of varying temperature, gel viscosity is not only decreasing significantly but also the whole viscosity spectrum is sifted to the lower viscosity region. It could be due to the energy obtained to overcome the resistance to flow, with increase in temperature. This is because with increasing shear rate and temperature the inter-particle weaker hydrogen bond gets weakened. This allows them to slip over one another more easily, offering less resistance to flow and there is a reduced apparent viscosity. Also it is, in general seen that apparent viscosity decreases linearly with shear rate, indicating that the gel exhibits a pseudoplastic character under the shear rate covered in the present study. This indicates that the formulated hydrogen peroxide gel oxidizer has shear-thinning-pseudoplastic property. The viscosity measurement indicates that the stationary gel has an apparent viscosity at the point the gel started flowing are 3614, 1246 and 735 Pa s at operating temperatures , and K respectively with some yield stress that may be brought to a level of 53.89, and Pa s at a nominal shear rate range of 1 to 20 s -1 and can be seen in Table 1. 4

5 Shear-thinning pseudoplastic behavior is very important to guarantee a safe handling and vehicle operation, because this property of gel provides a reasonable pressure drop when pumping the gel propellant system from storage tank to the combustion chamber. And for that reason gels for the application like propulsion or other pumpable system should show a shear-thinning behavior. Fig. 4 Hydrogen peroxide Gel Oxidizer Viscosity as a Function of Gellant Concentration Table 1. Apperant Viscosity of H 2 O 2 Gel at varying Temperature 3.3 Thixotropy Test In this test thixotropic character along with hysteresis Loop behavior of hydrogen peroxide oxidizer gel at different operating temperatures were studied. 5

6 In this experiment the shear rate was increased systematically to a maximum level to trace the up curve and it was decreased to get the down curve of the rheogram (Fig.5). The data reveal that the apparent viscosity of the gel decreases significantly with shear rate, and because the shear stress values do not exhibit proportionality with shear rate, both up and down curves do not superimpose each other and enclose an area between them, referred to as hysteresis loop. It can be defined as the reduction in structural strength during the shear load phase and the more or less rapid, but complete structural regeneration with time, with subsequent decrease in shear rate due to local spatial rearrangement of the molecules in the three dimensional network. And the presence of Hysteresis Loop indicates that a breakdown of structure has occurred and the area within the loop may be used as an index of the degree of breakdown. The same phenomenon can be observed in Fig. 5. This indicates that the gel exhibits a shear-thinning behavior with its thixotropic characteristics. And this also indicates a viscoelastic response too as seen in Fig. 5 and 6 [8]. Fig. 5 Rheological Test for Thixotropic Study of Hydrogen peroxide Gel Oxidizer 6

7 Fig. 6 Breakdown of Three Dimensional Network and Regain From Fig. 5 it is clearly seen that with increase in temperature, gel apparent viscosity (ramp up and down curve) spectrum is sifted to lower viscosity region. It can be defined as the reduction of structural strength due to break down of three dimensional network and in turn reduces the solution viscosity (i.e. the energy obtained to overcome the resistance to flow, with increase in temperature). However, the Hysteresis loop area is more or less the same. This indicates that the area enclose the hysteresis loop is less effected by the variation in operating temperature at low shear rate. And also that gel at low shear rate does not lead to a complete breakdown of gel structure. 4. Conclusion In present study, gellation of hydrogen peroxide oxidizer has been investigated experimentally. Fumed silica was used as a gelling agent to gel the conventional hydrogen peroxide oxidizer. The influence of low shear rate and varying operating temperature on apparent viscosity, shear-thinning-thixotropic property was investigated. Apparent viscosity of the gelled oxidizer system was found to decrease with increase in shear rate. However, the apparent viscosity whole spectrum shifted to lower viscosity region with increase in operating temperature. While no significant change in hysteresis loop has been observed. This shows that operating temperature and shear rate together plays an important role and significantly influences the rheological properties of H 2 O 2 gel system. Acknowledgment We are very grateful to the KAIST institute in providing the laboratory for experiments. 7

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