Silicone Rubber Based Insulation Material And Its Thermochemical Ablation Mechanism

In the secondary combustion chamber of the integral solid rocket ramjet, mix andcombustion are taken place between the hot fuel-rich gas from primary combustion of thefuel-rich propellant and the excessive ducted air stream. A lot of reaction heats are releasedduring the combustion process. Therefore, there are many requirements, such as, anti-oxidation,anti-ablation and anti-erosion, for the insulation material to be used in the secondary combustionchamber of solid ducted rocket. Silicone rubber based insulation material, a kind of elastomericmaterial with excellent anti-oxidative and mechanical properties, can meet these requirements.However, the technologies of the formulation design and performance adjustment about thesilicone rubber based insulation material has not been fully developed in our country, and theunderstanding about the ablation structure and thermochemical ablation mechanism of thematerials is quite lacked in China. Therefore, it is very important to disclose the correlationsbetween the insulation material formulation and the ablation rate, and to realize the ablationstructure and chemical reactions during the ablation process. The results in this dissertation arevery usefull in establishment of the ablation theory and decreaseing the ablation rate of thisinsulation material.Based on the analysis of the ablative environment in the secondary combustion chamber ofthe integral solid rocket ramjet, the studies in this paper were focused on the ablation structures,the chemical reactions and their reaction products of the silicone rubber based insulation materialin the pyrolysis, ceramization and oxidation stages under the ablation process to realize theablative characteristics of the material systemically. Oxygen-acetylene ablation test and ramjetmotor test were employed to investigate the effects of ingredients and environmental parameterson the ablation rates of the insulation material. The reactions in the pyrolysis, ceramization andoxidation processes of the insulation material under the ablation test environments were studied.The thermodynamic and kinetic data of these reactions was obtained. The correlations betweenthe ablation structure, composition and reactions of the insulation material with environmentaltemperatures were deduced. Therefore, the thermochemical ablation mechanism of the siliconerubber based insulation material was established. An ablative model, which was based on theablation structure with virgin layer, pyrolysis layer, ceramic layer and surface layer, wasestablished. Finally, the effective factors on the ablation rates of the insulation material werenumerically studied.The ablation rates of the insulation material from the oxygen-acetylene flame test werereduced by increasing the contents of phenyl silicone rubber and carbon fibers, or decreasing thecontent of silica. The optimum composition of silicone rubber based insulation material isconsist of50parts per hundred grams of rubber (phr) methylvinyl silicone rubber,50phr phenylsilicone rubber,30phr fumed silica and15phr carbon fibers, of which the ablation rate is0.049mm·s-1. After ablation, virgin layer, pyrolysis layer, ceramic layer and surface layer were observed along the ablation direction. The experimental results showed that the insulationmaterial with low ablation-rate generally formed a thick and firm ceramic layer. Siliconcarbide was in-situ produced in the ceramic layer. The content of silicon carbide in-situproduced in the ceramic layer of the optimum insulation material composition from theoxygen-acetylene ablation test is about8.1%. The surface layer is mainly consisted of silica.The insulation material samples expanded under the ablation environment of the ducted rocketsecondary combustion chamber. Therefore, the porosity of the ceramic layer is greater thanthat from oxygen-acetylene ablation test. Because of the low temperature and short ablationtime, the silicon carbide content of the ceramic layer formed in the secondary combustionchamber is lower than that produced under the oxygen-acetylene flame. In the front part of thesecondary combustion chamber, where the ablative environment is high temperature and lowoxidized gas level, a high carbon content was detected in the surface layer of the ablatedinsulation material sample, and a thin virgin layer was remained. In the back part of thesecondary combustion chamber, where the ablative environment is low temperature and highoxidized gas level, the ablated insulation material sample showed high silica content on thesurface and a thick virgin layer.Under the oxygen-acetylene flame ablation condition, the temperature range of pyrolysisprocess for the typical silicone rubber based insulation material is765-943K. In thistemperature range, silicone rubbers decomposed, but carbon fibers and silica kept steady, so aporous pyrolysis layer of the insulation material is formed. The thermal stability of phenylsilicone rubber is better than that of the methylvinyl silicone rubber because of the blockingeffects of the phenyl groups. Circular type decomposition of silicone rubber took place.Dimethyl siloxane cycles are the main decomposition products of the silicone rubber with methyland vinyl groups, but diphenyl-multimethyl siloxane cycles are found in the liquiddecomposition products of the phenyl silicone rubber. Meanwhile, some other thermaldecomposition reactions, such as, side groups of silicone rubbers are ruptured from the polymerchain, and molecular redistribution reactions, take place to produce low molecular-weightcompounds. Polymerization of the aromatic groups from the themal decomposition of thesilicone rubber leads to produce hydrogen gas. The molecular formulas of the solid thermaldecompostion products pf the two silicone rubbers are SiC0.90O1.11and SiC1.65O1.26respectively.The pyrolysis layer sample of the typical silicone rubber based insulation material is consisted ofcarbon fibers, fumed silica and solid decomposition products of the two silicone rubbers. Thedecomposition kinetic equations of the typical silicone rubber based insulation material and thatof the two silicone rubbers were also obtained.Under the oxygen-acetylene flame ablation condition, the temperature range of theceramization process from the pyrolysis layer is between943K and2073K. The reactions ofceramization process include the polymerization of aromatic structure, the transformation ofsiliconoxycarbide to carbide silicon, the reactions between carbon and silica, melting of silica,the reaction between silicon carbide and silica. The ceramic layer sample, which produced by the ceramization reaction from the pyrolysis layer, is mainly consisted of carbon, silica andsilicon carbide. The density and conductibility of the ceramic layer is less than half of these ofthe insulation material because of its porous structure. However, the changes of physical andstructure parameters of the ceramic layer are relatively low between943K and1873K. Whenthe temperature is more than1673K, the reactions of ceramization process to produced in-situsilicon carbide, such as, the siliconoxycarbide transformation, carbothermal reaction betweencarbon and silica, will take place. At1873K, the content of silicon carbide in the ceramic layermade from tube furnace is as high as30.4%, and nano silicon carbide whiskers are also observedin the sample. The reaction between reductive silicon carbide and oxidative silica could occurto produce silicon monoxide and carbon monoxide. The kinetic equation of the ceramizationreaction of the typical silicone rubber based insulation material pyrolysis layer was obtained, aswell as the kinetic equations of the carbon and silica reaction, silicon carbide and silica reaction.During the oxidation process of ceramic layer sample, the reactions of C/O2, C/H2O, C/CO2.SiC/O2, SiC/H2O, and SiC/CO2will take place. The kinetic equations of these oxidationreactions of ceramic layer sample, carbon and silicon carbide with oxygen, water or carbondioxide were gained. Carbon could be quickly oxidized between800K and1400K, which leadsto the weight loss of ceramic layer. The oxidation rate of carbon became faster according to thesequence of carbon dioxide, water, oxygen. Silicon carbon showed good anti-oxidationproperties because silica was produced by the oxidation and obstructed silicon carbide fromoxidized gas. Besides its nice oxidation resistance, the in-situ produced silicon carbides have acapability of bonding the ingredients of the ceramic layer together to make a whole structure inthe ceramic layer. So the thermooxidative stability of the ceramic layer is better than that of themixture of carbon, silica and silicon carbide. When the carbon and silicon carbide in theceramic layer were oxidized by the oxidative gas under the high temperature environment, anablation surface layer mainly composited by silica was formed.Physical and numerical ablation models of the silicone rubber based insulation material,which considered four layers ablation structure (virgin layer, pyrolysis layer, ceramic layer andablation surface layer), were established. The calculation results were validated by theexperiment data. The effects of ingredients and ablative environment on the ablative rates ofsilicone rubber based insulation material were numerically studied.