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Study of a liquid insulation for the solid rocket motor
Materials Letters 61 (2007) 2406 – 2411 www.elsevier.com/locate/matlet
Study of a liquid insulation for the solid rocket motor Yalin Guo a,⁎, Guozheng Liang a , Zheming Qiu b , Aihua Liu b a
Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China b Xi'an Aerospace Composites Research Institute, Xi'an, Shaanxi 710025, China Received 26 August 2005; accepted 29 August 2006 Available online 29 September 2006
Abstract Formula of a liquid insulation for a solid rocket motor (SRM) was studied in this paper. By analyzing vulcanization mechanism and comparing mechanical and thermal properties of the rubber, composition of the insulation with liquid raw material was determined. Ignition tests of motors with liquid insulation were successful. Results indicated that the tensile strength, elongation at break, shear strength, specific heat and thermal conductivity of the liquid insulation were 6.25 MPa, 590%, 3.61 MPa, 3.4174 × 103 J/(kg K) and 0.421 W/(m K) respectively. The success of various ground ignition tests and flight test proved that this new liquid insulation can fulfill requirements of the SRM. © 2006 Published by Elsevier B.V. Keywords: Solid rocket motor; Liquid insulation; Formula
1. Introduction The temperature of gas in chamber produced by combustion of grain is about 3000 K–3900 K when a solid rocket motor (SRM) is ignited. Thermal protection measures must be taken to prevent the chamber from damaging by elevated gas and decreasing in strength by overheating. Strength decrease is dangerous to the integrality of the chamber. In general, insulation is adhered to the inner wall of the chamber. Meanwhile, the stress between case and grain can be buffered by insulation. Insulation can also seal the case which is fabricated by a filament wound process [1]. Insulations are usually produced by rubber matrix and filler such as asbestos, silica and carbon black. The matrix includes nitrile-butadiene rubber (NBR), styrene butadiene rubber (SBR), carboxyl-terminated liquid polybutadiene rubber (CTPB) and polybutadiene-acrylic acid (PBAA). NBR filled with asbestos is typical insulation [1]. Because of the harmfulness of asbestos, no-asbestos insulation is developed in recent years. For example, Kevlar pulp filled ethylene propylene diene monomer (KFE
⁎ Corresponding author. 0167-577X/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.matlet.2006.08.078
PDM) and Carbon fiber filled EPDM (CFEPDM) insulation are developed for use on the reusable solid rocket motor (RSRM) [2], carbon fiber/fumed silica filled silicone rubber insulation is developed for use on a ramjets motor [3], castable/sprayable insulation whose matrix is liquid polymers is developed for use on many SRM, and liquid polymers are used to produce such insulation that include polyetherurethane, carboxyl-terminated polybutadiene, hydroxyl-terminated polybutadiene, polybutadiene acrylonitrile and silicone etc [4]. Fig. 1 is the schematic diagram of a solid rocket motor chamber. Ablator and case of the chamber are bonded by insulation. The function of insulation here is binding, heat insulation and stress buffering. The purpose of this paper is to develop an insulation formula for this SRM. According to the function of the insulation, characteristics of the insulation are high binding strength, good thermal insulation, proper tensile strength and excellent toughness. Taking the forming process of the chamber into consideration, a hydroxyl-terminated liquid nitrile rubber (HTBN) is selected as matrix and fumed silica, which can improve strength and thermal insulation of the rubber, as filler of the liquid insulation. By analyzing vulcanization mechanism and comparing mechanical and thermal properties, the optimum contents of the fumed silica are obtained, then composition of the insulation is determined. The chamber with liquid insulation
Y. Guo et al. / Materials Letters 61 (2007) 2406–2411
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Fig. 1. Schematic diagram of the solid rocket motor chamber.
is produced. Ground igniting tests and flight test of the SRM are successful. Characteristics of the liquid insulation developed in this paper are summarized below. ① a HTBN with high acrylonitrile content is selected as matrix, which is helpful to bond properties of the insulation, ② insulation and chamber are formed simultaneously by the compression molding processing. The vulcanization mechanism is firstly analyzed in this paper, then the composition of the liquid insulation is determined according to the mechanical and thermal properties test, properties of the optimal formula are presented, and the performance of the insulation is finally proved by various ground igniting tests and flight test.
2.2.2. Samples for shear test Aluminium slices were firstly treated with acid solution, then the unvulcanized blend was applied to the surface of aluminium slices, thirdly aluminium slices were assembled on a mould and vulcanized in an oven, finally samples were taken out and sent to test.
2. Experimental
2.3. Physical properties
2.1. Materials
2.3.1. Mechanical properties Tensile properties and shear strength of the rubber were tested according to the People's Republic of China National Standard (GB528-1992 and GB7124-1986 respectively).
Hydroxyl-terminated liquid nitrile rubber (HTBN) was supplied by LAN Zhou Chemical and Industrial Corporation. Fumed silica was supplied by SHEN Yang Chemical and Industrial Factory. Di-n-butyl phthalate (DBP), which was used as diluent, was supplied by SHANG Hai Solvent Factory. Tolylene diisocyanate (TDI), the vulcanizer, was supplied by TAI Yuan Chemical and Industrial Factory. Glycerol (chemically pure), the cross-linking agent, was supplied by Xi'an Chemical Reagent Plant. 2.2. Preparation of samples 2.2.1. Samples for tensile (thermal) property measurement Firstly, unvulcanized blend was prepared by adding compositions of the formula into a container and stirring fully. Then, the blend was poured into a specially designed mould and compression molded. Finally, the sample was taken out.
2.2.3. Process of chamber with liquid insulation Steel case of the chamber was firstly cleaned with acetone, then the unvulcanized blend was applied to the surface of the ablator, thirdly case and ablator were assembled and compression molded on a mould, finally the chamber was fabricated and sent to test.
2.3.2. Thermal properties Specific heat, thermal diffusivity and thermal conductivity of the rubber were tested according to the People's Republic of China Military Standard (GJB1201.1-91). 2.4. Ignition tests Motors with liquid insulation were ignited at room temperature, − 40 °C and 60 °C on the ground. Table 1 shows some operation parameters of the motor. 2.5. Flight tests Two motors were selected to conduct flight tests.
3. Results and discussion 3.1. Vulcanization mechanism of the rubber [5] Hydroxyl, the active group of HTBN, can react with isocyanate to produce polyurethane elastomer, which is the chemical reaction during vulcanization of the rubber in this paper. The vulcanizer and cross-linking agent used in this experiment are tolylene diisocyanate (TDI) and glycerol which has three hydroxyls. The cross-linking agent can improve the strength of the rubber by increasing the cross-link density. According to the
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Table 1 Some operation parameters of the SRM Items
Value
Items
Value
Operation pressure (MPa) Temperature of chamber (K)
10–15 3000–3900
Operation time (s) Thickness of insulation (mm)
6–9 1–2
cross-link density, amounts of glycerol can be calculated by the mole number ratio of hydroxyls between glycerol and HTBN. Similarly, amounts of TDI can be obtained by the mole number ratio between the isocyanate of TDI and the total hydroxyl in formula. Isocyanate is very active as a result, it can react with many compounds with active hydrogen. The vulcanization reaction of the matrix composed of HTBN and TDI is between isocyanate and the active hydrogen of hydroxyl. Urethane, the product of the reaction between isocyanate and HTBN, can also react with isocyanate. The reaction process can be described below. (1) The chemical reaction between isocyanate and the active hydrogen of hydroxyl
(2) The chemical reaction between isocyanate and urethane
The reaction temperature between urethane and isocyanate is above 100 °C and this reaction can lead to branching and cross-linking. In addition, there is moisture in raw materials, hence, side reactions may take place between isocyanate and water. Main side reactions are summarized below. (3) The chemical reaction between isocyanate and water Isocyanate firstly reacts with water to produce unstable aminoformic acid which can decompose into carbon dioxide and amine, amine then reacts with isocyanate to produce urea. slowly
R
NCO H2 O Y R
R
NH2 R
quickly
NHCOOH
YR
NH2 CO2 z
quickly
NCO
YR
NHCONH
R
The reaction between R-NH2 and R-NCO is very quick, therefore, the reaction described above can be written below. 2R
NCO H2 OYR
NHCONH
R CO2 z
Fig. 2. Tensile properties of the rubber as a function of content of fumed silica.
Y. Guo et al. / Materials Letters 61 (2007) 2406–2411
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Fig. 3. Shear strength of the rubber as a function of content of fumed silica.
(4) The chemical reaction between isocyanate and urea
Surplus isocyanate can react with urea to produce biuret, the temperature of this reaction which can improve the strength of the rubber by branching and cross-linking is above 100 °C too. Conclusions can be drawn from the analyses above. The matrix is vulcanized by the reaction between isocyanate and active hydrogen of hydroxyl. The reaction between isocyanate and urethane can be controlled by temperature. Carbon dioxide which could lead to a harmful hole in insulation can be produced by the reaction between isocyanate and water, thus moisture content in raw materials should be strictly controlled. 3.2. Determination of the fumed silica contents 3.2.1. Influence of the fumed silica content on mechanical properties of the rubber Fig. 2 shows the relationship between tensile properties and fumed silica contents of the rubber. The figure demonstrates that tensile properties of the rubber are distinctly influenced by content of fumed silica. The tensile strength increases with the content of fumed silica, and after reaching the highest point of 6.0 MPa at a fumed silica content of 15.5 wt.%, it decreases with a further increase in the content of fumed silica. The tensile elongation at break of the rubber also increases with the content of fumed silica. The tensile elongation at break increases as the content of fumed silica increases to about 12.5 wt.%, and drops at higher fumed silica content. This phenomenon indicates that fumed silica can distinctly improve mechanical properties of the rubber when optimum amounts of fumed silica are added into the matrix. However, fumed silica, a thixotropic agent, can make the blend thick. When its content is small, the thick effect is not obvious, with its content increases, the blend becomes thicker, and more air, which could produce flaws in rubber, will be drawn into the blend during agitation, and hence, tensile properties of the rubber decrease with a
Fig. 4. Thermal properties of the rubber as a function of content of fumed silica.
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Table 2 Mechanical properties of the insulation No.
Tensile strength (MPa)
Elongation at break (%)
Shear strength (MPa)
1 2 3 4 5 Mid-value
6.25 6.91 6.22 6.46 6.05 6.25
590 590 620 550 550 590
3.37 3.13 3.27 3.96 4.32 3.61*
Note: Datum with an asterisk is an average.
further increase in the content of fumed silica. Analysis above demonstrates that tensile properties of the rubber can be improved by properly adding fumed silica, so it is suitable to select fumed silica as reinforcement material of the rubber. Fig. 2 also shows that in the fumed silica content range 12–15 wt.%, the tensile strength and elongation at break of the rubber/fumed silica composites are above 5.0 MPa and 500%, respectively. The maximum value of the tensile strength and elongation at break is 6.0 MPa and 570% at a fumed silica content of 15.5 wt.% and 12.5 wt.% respectively. This behavior indicates the rubber can fulfill tensile property requirements of insulation for its perfect toughness and good strength. The test of shear strength by tension loading is used to study the bond property of the rubber. Fig. 3 is the relationship between shear strength and content of fumed silica. It can be seen from Fig. 3 that the shear strength of the rubber increases gradually with the content of fumed silica. At a content of fumed silica of 12 wt.%, shear strength reaches the highest point. Then shear strength decreases sharply with a further increase in the content of fumed silica. It's obvious that the rubber can be reinforced by fumed silica to a certain degree. However, further increase of the content of fumed silica causes the thick effect, which will produce flaws in rubber, as a result, shear strength decreases with higher content of fumed silica. Fig. 3 also shows that the shear strength is above 3.0 MPa with the content of fumed silica ranging from 10.0 wt.% to 14.0 wt.%, that is to say, the bond properties of the rubber are good. 3.2.2. Influence of the fumed silica content on thermal properties of the rubber Both specific heat and thermal conductivity of the rubber strongly depend on the content of fumed silica as shown in Fig. 4. As the content of fumed silica increases from 10 wt.% to 17 wt.%, specific heat and thermal conductivity of the rubber decrease from 4.359 kJ/(kg K) to 2.279 kJ/(kg K)and from 0.543 W/(m K) to 0.307 W/(m K) respectively. This behavior can be attributed to the composition of fumed silica. The molecular composition of fumed silica is SiO2·H2O and the content of SiO2 which has a low specific heat and perfect thermal insulation is above 99%. As a result, both specific heat and thermal conductivity of the rubber decrease by adding fumed silica [6]. The figure also indicates that the average values of specific heat and thermal conductivity of the rubber with fumed silica are 3.333 kJ/(kg K) and 0.410 W/(m K) respectively, which means good thermal insulation. According to the analysis above, fumed silica has a similar effect on tensile properties and bond property of the rubber. All of them increase with the content of fumed silica, and after reaching the highest point, they decrease with a further increase in the content of fumed silica. The maximum value appears with the content of fumed silica ranging from 12.5 wt.% to 15.5 wt.%. The effect of fumed silica on thermal properties of the rubber is that both specific heat and thermal conductivity of the rubber decrease with the content of fumed silica. As a result of the analysis above, the content of fumed silica in the liquid insulation is determined. 3.3. Properties of the insulation 3.3.1. Mechanical properties of the insulation Table 2 shows the mechanical properties of the liquid insulation. The table shows that tensile strength, elongation at break and shear strength are 6.25 MPa, 590% and 3.61 MPa, respectively. It's obvious that the tensile strength of the insulation is good, the toughness and bond properties are perfect. 3.3.2. Thermal properties of the insulation Table 3 shows the thermal properties of the insulation at room temperature. Specific heat and thermal conductivity of the insulation are 3.4174 × 103 J/(kg K) and 0.421 W/(m K) respectively. These results indicate that insulating properties of the insulation are perfect. 3.4. Igniting tests The SRM chamber is fabricated as a result of the development on liquid insulation formula and used in ground igniting tests to testify the insulation. Table 3 Thermal properties of the insulation at room temperature No.
Specific heat (103 J/(kg K))
Thermal diffusivity (cm2/s)
Thermal conductivity (W/m K)
1 2 3 X Cv %
3.260 3.475 3.516 3.417 4.0
0.00101 0.00105 0.00101 0.00102 2.3
0.402 0.438 0.423 0.421 4.3
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Firstly, the ground ignition test is carried out at room temperature and the result is wonderful. The surface of the chamber is good and there is no overheat after ignition. Secondly, the SRM is stored at −40 °C and ignited at a low temperature. The test is successful. Thirdly, the SRM is stored at + 60 °C and ignited at a high temperature. There are overheats on the surface of the chamber to a degree, but no hole. Taking the aerodynamic heating effect produced during the flight of the SRM into account, the insulation structure of the SRM is adjusted properly. Then the SRM is ignited again and the test is successful. Finally, the SRM is cooled and heated alternately for several times, and then ignited. The test is successful. Based on the success of ground ignition tests, flight test is carried out. Two motors are selected to conduct the flight test. Results indicate that the design requirements of the motors are fulfilled. There are slight overheats on the surface of the recovery cases, but no hole. The flight test is successful. The success of the ground ignition tests and flight test proves that this new liquid insulation can fulfill the requirements of the SRM.
4. Conclusions
emphasis should be put on the development of liquid insulation which can substitute for the ablator.
Such conclusions can be drawn from the studies above. Acknowledgement 1. The insulation is vulcanized by the reaction between isocyanate and active hydrogen of hydroxyl. The moisture in raw materials should be strictly controlled. 2. By analyzing mechanical and thermal properties of the rubber, the optimum content of fumed silica is determined. 3. Tensile strength, elongation at break, shear strength, specific heat and thermal conductivity of the liquid insulation are 6.25 MPa, 590%, 3.61 MPa, 3.4174 × 103 J/(kg K) and 0.421 W/(m K), respectively. 4. The success of various ground ignition tests and flight test proves that the new insulation can completely fulfill the requirements of the SRM. A new liquid insulation is successfully developed and testified in this paper. However, an ablator is still needed for the SRM. In order to improve the performance of the motor,
Thanks are due to Mr. Cheng Chang-le for technical assistance during this research. References [1] Wang Zheng, Yongqiang Hu, Solid Rocket Motor, 1st ed., Astronautics Industry Press, Beijing, 1993, pp. 229–236. [2] Vernon W. Fitch, Norman F. Eddy, Space shuttle solid propellant rocket motors, asbestos filled insulation replacement, AIAA 97-2992. [3] Susumu Yamada, Chouji Serizawa, Kazushige Kato, Thermal and ablative properties of silicone insulation, AIAA 97-3259. [4] G.A. Zimmerman, Castable — sprayable insulations for rocket motors, AIAA TIS 3/28MF. [5] Shaoxiong Li, Yijun Liu, Polyurethane Adhesive, 1st ed., Chemical Industry Press, Beijing, 1988, pp. 14–53. [6] Mengjiao Wang, Huaiyao Gong, Guangzhi Xue, Handbook of Rubber Industry (II), 1st ed., Chemical Industry Press, Beijing, 1989, pp. 308–309.
Study of a liquid insulation for the solid rocket motor Yalin Guo a,⁎, Guozheng Liang a , Zheming Qiu b , Aihua Liu b a
Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China b Xi'an Aerospace Composites Research Institute, Xi'an, Shaanxi 710025, China Received 26 August 2005; accepted 29 August 2006 Available online 29 September 2006
Abstract Formula of a liquid insulation for a solid rocket motor (SRM) was studied in this paper. By analyzing vulcanization mechanism and comparing mechanical and thermal properties of the rubber, composition of the insulation with liquid raw material was determined. Ignition tests of motors with liquid insulation were successful. Results indicated that the tensile strength, elongation at break, shear strength, specific heat and thermal conductivity of the liquid insulation were 6.25 MPa, 590%, 3.61 MPa, 3.4174 × 103 J/(kg K) and 0.421 W/(m K) respectively. The success of various ground ignition tests and flight test proved that this new liquid insulation can fulfill requirements of the SRM. © 2006 Published by Elsevier B.V. Keywords: Solid rocket motor; Liquid insulation; Formula
1. Introduction The temperature of gas in chamber produced by combustion of grain is about 3000 K–3900 K when a solid rocket motor (SRM) is ignited. Thermal protection measures must be taken to prevent the chamber from damaging by elevated gas and decreasing in strength by overheating. Strength decrease is dangerous to the integrality of the chamber. In general, insulation is adhered to the inner wall of the chamber. Meanwhile, the stress between case and grain can be buffered by insulation. Insulation can also seal the case which is fabricated by a filament wound process [1]. Insulations are usually produced by rubber matrix and filler such as asbestos, silica and carbon black. The matrix includes nitrile-butadiene rubber (NBR), styrene butadiene rubber (SBR), carboxyl-terminated liquid polybutadiene rubber (CTPB) and polybutadiene-acrylic acid (PBAA). NBR filled with asbestos is typical insulation [1]. Because of the harmfulness of asbestos, no-asbestos insulation is developed in recent years. For example, Kevlar pulp filled ethylene propylene diene monomer (KFE
⁎ Corresponding author. 0167-577X/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.matlet.2006.08.078
PDM) and Carbon fiber filled EPDM (CFEPDM) insulation are developed for use on the reusable solid rocket motor (RSRM) [2], carbon fiber/fumed silica filled silicone rubber insulation is developed for use on a ramjets motor [3], castable/sprayable insulation whose matrix is liquid polymers is developed for use on many SRM, and liquid polymers are used to produce such insulation that include polyetherurethane, carboxyl-terminated polybutadiene, hydroxyl-terminated polybutadiene, polybutadiene acrylonitrile and silicone etc [4]. Fig. 1 is the schematic diagram of a solid rocket motor chamber. Ablator and case of the chamber are bonded by insulation. The function of insulation here is binding, heat insulation and stress buffering. The purpose of this paper is to develop an insulation formula for this SRM. According to the function of the insulation, characteristics of the insulation are high binding strength, good thermal insulation, proper tensile strength and excellent toughness. Taking the forming process of the chamber into consideration, a hydroxyl-terminated liquid nitrile rubber (HTBN) is selected as matrix and fumed silica, which can improve strength and thermal insulation of the rubber, as filler of the liquid insulation. By analyzing vulcanization mechanism and comparing mechanical and thermal properties, the optimum contents of the fumed silica are obtained, then composition of the insulation is determined. The chamber with liquid insulation
Y. Guo et al. / Materials Letters 61 (2007) 2406–2411
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Fig. 1. Schematic diagram of the solid rocket motor chamber.
is produced. Ground igniting tests and flight test of the SRM are successful. Characteristics of the liquid insulation developed in this paper are summarized below. ① a HTBN with high acrylonitrile content is selected as matrix, which is helpful to bond properties of the insulation, ② insulation and chamber are formed simultaneously by the compression molding processing. The vulcanization mechanism is firstly analyzed in this paper, then the composition of the liquid insulation is determined according to the mechanical and thermal properties test, properties of the optimal formula are presented, and the performance of the insulation is finally proved by various ground igniting tests and flight test.
2.2.2. Samples for shear test Aluminium slices were firstly treated with acid solution, then the unvulcanized blend was applied to the surface of aluminium slices, thirdly aluminium slices were assembled on a mould and vulcanized in an oven, finally samples were taken out and sent to test.
2. Experimental
2.3. Physical properties
2.1. Materials
2.3.1. Mechanical properties Tensile properties and shear strength of the rubber were tested according to the People's Republic of China National Standard (GB528-1992 and GB7124-1986 respectively).
Hydroxyl-terminated liquid nitrile rubber (HTBN) was supplied by LAN Zhou Chemical and Industrial Corporation. Fumed silica was supplied by SHEN Yang Chemical and Industrial Factory. Di-n-butyl phthalate (DBP), which was used as diluent, was supplied by SHANG Hai Solvent Factory. Tolylene diisocyanate (TDI), the vulcanizer, was supplied by TAI Yuan Chemical and Industrial Factory. Glycerol (chemically pure), the cross-linking agent, was supplied by Xi'an Chemical Reagent Plant. 2.2. Preparation of samples 2.2.1. Samples for tensile (thermal) property measurement Firstly, unvulcanized blend was prepared by adding compositions of the formula into a container and stirring fully. Then, the blend was poured into a specially designed mould and compression molded. Finally, the sample was taken out.
2.2.3. Process of chamber with liquid insulation Steel case of the chamber was firstly cleaned with acetone, then the unvulcanized blend was applied to the surface of the ablator, thirdly case and ablator were assembled and compression molded on a mould, finally the chamber was fabricated and sent to test.
2.3.2. Thermal properties Specific heat, thermal diffusivity and thermal conductivity of the rubber were tested according to the People's Republic of China Military Standard (GJB1201.1-91). 2.4. Ignition tests Motors with liquid insulation were ignited at room temperature, − 40 °C and 60 °C on the ground. Table 1 shows some operation parameters of the motor. 2.5. Flight tests Two motors were selected to conduct flight tests.
3. Results and discussion 3.1. Vulcanization mechanism of the rubber [5] Hydroxyl, the active group of HTBN, can react with isocyanate to produce polyurethane elastomer, which is the chemical reaction during vulcanization of the rubber in this paper. The vulcanizer and cross-linking agent used in this experiment are tolylene diisocyanate (TDI) and glycerol which has three hydroxyls. The cross-linking agent can improve the strength of the rubber by increasing the cross-link density. According to the
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Table 1 Some operation parameters of the SRM Items
Value
Items
Value
Operation pressure (MPa) Temperature of chamber (K)
10–15 3000–3900
Operation time (s) Thickness of insulation (mm)
6–9 1–2
cross-link density, amounts of glycerol can be calculated by the mole number ratio of hydroxyls between glycerol and HTBN. Similarly, amounts of TDI can be obtained by the mole number ratio between the isocyanate of TDI and the total hydroxyl in formula. Isocyanate is very active as a result, it can react with many compounds with active hydrogen. The vulcanization reaction of the matrix composed of HTBN and TDI is between isocyanate and the active hydrogen of hydroxyl. Urethane, the product of the reaction between isocyanate and HTBN, can also react with isocyanate. The reaction process can be described below. (1) The chemical reaction between isocyanate and the active hydrogen of hydroxyl
(2) The chemical reaction between isocyanate and urethane
The reaction temperature between urethane and isocyanate is above 100 °C and this reaction can lead to branching and cross-linking. In addition, there is moisture in raw materials, hence, side reactions may take place between isocyanate and water. Main side reactions are summarized below. (3) The chemical reaction between isocyanate and water Isocyanate firstly reacts with water to produce unstable aminoformic acid which can decompose into carbon dioxide and amine, amine then reacts with isocyanate to produce urea. slowly
R
NCO H2 O Y R
R
NH2 R
quickly
NHCOOH
YR
NH2 CO2 z
quickly
NCO
YR
NHCONH
R
The reaction between R-NH2 and R-NCO is very quick, therefore, the reaction described above can be written below. 2R
NCO H2 OYR
NHCONH
R CO2 z
Fig. 2. Tensile properties of the rubber as a function of content of fumed silica.
Y. Guo et al. / Materials Letters 61 (2007) 2406–2411
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Fig. 3. Shear strength of the rubber as a function of content of fumed silica.
(4) The chemical reaction between isocyanate and urea
Surplus isocyanate can react with urea to produce biuret, the temperature of this reaction which can improve the strength of the rubber by branching and cross-linking is above 100 °C too. Conclusions can be drawn from the analyses above. The matrix is vulcanized by the reaction between isocyanate and active hydrogen of hydroxyl. The reaction between isocyanate and urethane can be controlled by temperature. Carbon dioxide which could lead to a harmful hole in insulation can be produced by the reaction between isocyanate and water, thus moisture content in raw materials should be strictly controlled. 3.2. Determination of the fumed silica contents 3.2.1. Influence of the fumed silica content on mechanical properties of the rubber Fig. 2 shows the relationship between tensile properties and fumed silica contents of the rubber. The figure demonstrates that tensile properties of the rubber are distinctly influenced by content of fumed silica. The tensile strength increases with the content of fumed silica, and after reaching the highest point of 6.0 MPa at a fumed silica content of 15.5 wt.%, it decreases with a further increase in the content of fumed silica. The tensile elongation at break of the rubber also increases with the content of fumed silica. The tensile elongation at break increases as the content of fumed silica increases to about 12.5 wt.%, and drops at higher fumed silica content. This phenomenon indicates that fumed silica can distinctly improve mechanical properties of the rubber when optimum amounts of fumed silica are added into the matrix. However, fumed silica, a thixotropic agent, can make the blend thick. When its content is small, the thick effect is not obvious, with its content increases, the blend becomes thicker, and more air, which could produce flaws in rubber, will be drawn into the blend during agitation, and hence, tensile properties of the rubber decrease with a
Fig. 4. Thermal properties of the rubber as a function of content of fumed silica.
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Table 2 Mechanical properties of the insulation No.
Tensile strength (MPa)
Elongation at break (%)
Shear strength (MPa)
1 2 3 4 5 Mid-value
6.25 6.91 6.22 6.46 6.05 6.25
590 590 620 550 550 590
3.37 3.13 3.27 3.96 4.32 3.61*
Note: Datum with an asterisk is an average.
further increase in the content of fumed silica. Analysis above demonstrates that tensile properties of the rubber can be improved by properly adding fumed silica, so it is suitable to select fumed silica as reinforcement material of the rubber. Fig. 2 also shows that in the fumed silica content range 12–15 wt.%, the tensile strength and elongation at break of the rubber/fumed silica composites are above 5.0 MPa and 500%, respectively. The maximum value of the tensile strength and elongation at break is 6.0 MPa and 570% at a fumed silica content of 15.5 wt.% and 12.5 wt.% respectively. This behavior indicates the rubber can fulfill tensile property requirements of insulation for its perfect toughness and good strength. The test of shear strength by tension loading is used to study the bond property of the rubber. Fig. 3 is the relationship between shear strength and content of fumed silica. It can be seen from Fig. 3 that the shear strength of the rubber increases gradually with the content of fumed silica. At a content of fumed silica of 12 wt.%, shear strength reaches the highest point. Then shear strength decreases sharply with a further increase in the content of fumed silica. It's obvious that the rubber can be reinforced by fumed silica to a certain degree. However, further increase of the content of fumed silica causes the thick effect, which will produce flaws in rubber, as a result, shear strength decreases with higher content of fumed silica. Fig. 3 also shows that the shear strength is above 3.0 MPa with the content of fumed silica ranging from 10.0 wt.% to 14.0 wt.%, that is to say, the bond properties of the rubber are good. 3.2.2. Influence of the fumed silica content on thermal properties of the rubber Both specific heat and thermal conductivity of the rubber strongly depend on the content of fumed silica as shown in Fig. 4. As the content of fumed silica increases from 10 wt.% to 17 wt.%, specific heat and thermal conductivity of the rubber decrease from 4.359 kJ/(kg K) to 2.279 kJ/(kg K)and from 0.543 W/(m K) to 0.307 W/(m K) respectively. This behavior can be attributed to the composition of fumed silica. The molecular composition of fumed silica is SiO2·H2O and the content of SiO2 which has a low specific heat and perfect thermal insulation is above 99%. As a result, both specific heat and thermal conductivity of the rubber decrease by adding fumed silica [6]. The figure also indicates that the average values of specific heat and thermal conductivity of the rubber with fumed silica are 3.333 kJ/(kg K) and 0.410 W/(m K) respectively, which means good thermal insulation. According to the analysis above, fumed silica has a similar effect on tensile properties and bond property of the rubber. All of them increase with the content of fumed silica, and after reaching the highest point, they decrease with a further increase in the content of fumed silica. The maximum value appears with the content of fumed silica ranging from 12.5 wt.% to 15.5 wt.%. The effect of fumed silica on thermal properties of the rubber is that both specific heat and thermal conductivity of the rubber decrease with the content of fumed silica. As a result of the analysis above, the content of fumed silica in the liquid insulation is determined. 3.3. Properties of the insulation 3.3.1. Mechanical properties of the insulation Table 2 shows the mechanical properties of the liquid insulation. The table shows that tensile strength, elongation at break and shear strength are 6.25 MPa, 590% and 3.61 MPa, respectively. It's obvious that the tensile strength of the insulation is good, the toughness and bond properties are perfect. 3.3.2. Thermal properties of the insulation Table 3 shows the thermal properties of the insulation at room temperature. Specific heat and thermal conductivity of the insulation are 3.4174 × 103 J/(kg K) and 0.421 W/(m K) respectively. These results indicate that insulating properties of the insulation are perfect. 3.4. Igniting tests The SRM chamber is fabricated as a result of the development on liquid insulation formula and used in ground igniting tests to testify the insulation. Table 3 Thermal properties of the insulation at room temperature No.
Specific heat (103 J/(kg K))
Thermal diffusivity (cm2/s)
Thermal conductivity (W/m K)
1 2 3 X Cv %
3.260 3.475 3.516 3.417 4.0
0.00101 0.00105 0.00101 0.00102 2.3
0.402 0.438 0.423 0.421 4.3
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Firstly, the ground ignition test is carried out at room temperature and the result is wonderful. The surface of the chamber is good and there is no overheat after ignition. Secondly, the SRM is stored at −40 °C and ignited at a low temperature. The test is successful. Thirdly, the SRM is stored at + 60 °C and ignited at a high temperature. There are overheats on the surface of the chamber to a degree, but no hole. Taking the aerodynamic heating effect produced during the flight of the SRM into account, the insulation structure of the SRM is adjusted properly. Then the SRM is ignited again and the test is successful. Finally, the SRM is cooled and heated alternately for several times, and then ignited. The test is successful. Based on the success of ground ignition tests, flight test is carried out. Two motors are selected to conduct the flight test. Results indicate that the design requirements of the motors are fulfilled. There are slight overheats on the surface of the recovery cases, but no hole. The flight test is successful. The success of the ground ignition tests and flight test proves that this new liquid insulation can fulfill the requirements of the SRM.
4. Conclusions
emphasis should be put on the development of liquid insulation which can substitute for the ablator.
Such conclusions can be drawn from the studies above. Acknowledgement 1. The insulation is vulcanized by the reaction between isocyanate and active hydrogen of hydroxyl. The moisture in raw materials should be strictly controlled. 2. By analyzing mechanical and thermal properties of the rubber, the optimum content of fumed silica is determined. 3. Tensile strength, elongation at break, shear strength, specific heat and thermal conductivity of the liquid insulation are 6.25 MPa, 590%, 3.61 MPa, 3.4174 × 103 J/(kg K) and 0.421 W/(m K), respectively. 4. The success of various ground ignition tests and flight test proves that the new insulation can completely fulfill the requirements of the SRM. A new liquid insulation is successfully developed and testified in this paper. However, an ablator is still needed for the SRM. In order to improve the performance of the motor,
Thanks are due to Mr. Cheng Chang-le for technical assistance during this research. References [1] Wang Zheng, Yongqiang Hu, Solid Rocket Motor, 1st ed., Astronautics Industry Press, Beijing, 1993, pp. 229–236. [2] Vernon W. Fitch, Norman F. Eddy, Space shuttle solid propellant rocket motors, asbestos filled insulation replacement, AIAA 97-2992. [3] Susumu Yamada, Chouji Serizawa, Kazushige Kato, Thermal and ablative properties of silicone insulation, AIAA 97-3259. [4] G.A. Zimmerman, Castable — sprayable insulations for rocket motors, AIAA TIS 3/28MF. [5] Shaoxiong Li, Yijun Liu, Polyurethane Adhesive, 1st ed., Chemical Industry Press, Beijing, 1988, pp. 14–53. [6] Mengjiao Wang, Huaiyao Gong, Guangzhi Xue, Handbook of Rubber Industry (II), 1st ed., Chemical Industry Press, Beijing, 1989, pp. 308–309.