Study On Solid Propellant Based On Modified Nitrocellulose And Its Gel/Composite Gel

Nitrocellulose(NC), an important derivative of natural cellulose, is widely used in both military and civilian areas according to its nitrogen content. For the inherent characteristics of the molecular structure of cellulose, NC has limited energy performance(the theoretical maximum nitrogen content is 14.14 %) and poor mechanical properties(semi-rigid molecular chains) which affect its application in a new generation weapon area as a propellant energetic skeleton material—adhesive. Therefore, to improve the properties of NC has important significance for improving the overall performance of gunpowder and solid propellant. This paper is mainly aimed at improving the quality and application performances of NC. The study is focused on the quality and morphology control of NC, modification of NC molecular structure, and nano-composite of NC. The main points of this paper are as follows:1) The military NC qualified to ―eight standard‖ showed strong instability in molding process or the preparation of NC base propellant. The quality of NC affected its plasticizing properties and processability, and the uniformity of nitration which affected the quality of NC was effected by the component of nitration system, nitrification temperature and morphology of the raw material. The nitrogen content of NC obviously decreased with increasing water content in HNO3/H2SO4/H2 O while the uniformity of NC would improve if the water content increases appropriately. Mixed acids with high sulphuric acid content was employed to give a material of high nitrogen content but bad uniformity. With increase of HNO3 content in HNO3/CH2Cl2, the uniformity of NC improved firstly, then declined before unchanged. The uniformity of NC improved with increasing nitration temperature which was conducive to the diffusion of esterification reagents. The role of the keratinization during the drying process makes the nitrogen content and nitrogen distribution uniformity of NC worse than that NC prepared from cotton linters without being dried.2) According to different sources of cellulose, the raw material for preparation of NC mainly divided into refined cotton cellulose and wood pulp cellulose. Compared with cotton cellulose, there are some advantages on wood cellulose such as lower price, higher yield, better structural stability, more economical and environmental. Due to differences in the secondary structure, aggregation and material forms, wood pulp cellulose has lower crystallinity which has a higher degree of reactivity and availability in theory. We used wood pulp as raw material, and study its reaction characteristics in HNO3/CH2Cl2 system. By controlling the composition of HNO3/CH2Cl2, nitrification time, nitrification temperature and stirring method, NC with nitrogen content range from 11.5% to 13.5% can be prepared. Various indicators of nitration wood pulp meet the requirements of military nitration cotton. In addition, nitration wood pulp has fewer impurities and better stability than nitration cotton. N itrate glycerol ether cellulose(NGEC) can be obtained from refined cotton after etherification by Glycidol and then be nitrificated in HNO3/CH2Cl2 system. NGEC has good compatibility with most conventional propellant additives and components such as RDX, HMX, DINA, and 13μm Al etc. A recipe modified double base propellant was prepared using nitrification wood pulp or NGEC replacing part of 3 # NC. Its tensile strength and elongation have certain level of increase. The tensile strength of double base propellant which contains the nitration of wood pulp increased from 9.09 MPa to 10.5MPa(increased by 15.5%), as well as its elongation increased from 13.4 % to 15.6%(increased by 16.4%). The tensile strength of propellant which contains NGEC increased from 6.36 MPa to 7.81MPa(increased by 22.8%), while its elongation increased from 8.87% to 15.95%(increased by 79.8%). In addition, due to the higher nitrogen content of NGEC, the burning rate of propellant including NGEC under 8~18MPa pressure range increase around 0.5mm/s compared to conventional double base propellant, while the burning rate pressure exponent nearly unchanged.3) Nanocomposite technology provides a new method for plastics toughening and reinforcing. The effect of nano-scale fillers on the properties of na no- modified composites is influenced by the dispersion of nano-fillers in the matrix and the compatibility of the nanoparticles with the matrix. In this paper water-dispersible graphene oxide(GO) and cellulose nanofibers(CNFs) is used to modify the propeties of double-base propellant. We focused on the infulences of GO/CNFs on mechanical and combustion performances of modified double-base propellant. The results of tensile test and b urning test of propellant showed that the mechanical properties and combustion performances of CNFs modified double base propellant had been improved significantly compared to conventional modified double base propellant. The tensile strength and elongatio n at break can be increased by nearly 34%, 45%, respectively. The addition of 2wt% of GO increases the tensile strength by 22%, 30% and 11% at 20 oC, 50 oC and-40 oC respectively as well as increases the elongation at break by 47%, 24% and 24% respectively. The burning rate of propellant can be increased by 22% while the pressure coefficient decreased by 20%. The addition of 1wt% of CNFs increases the tensile strength by 18%, 34% and 27% at 20 oC, 50 oC and-40 oC respectively as well as increases the elongation at break by 45%, 35% and 18% respectively. The burning rate of propellant can be increased by 27.5% while the pressure coefficient decreased by 20%.4) Morphology of energetic materials significantly affects its combustion performance. NC‘s morphology structure also has a greater impact on its application performance. In this paper, by non-solvent induced phase separation, NC wet gels were formed in ternary NC/acetone/alcohol system. The dynamic viscoelastic properties of NC gel were influenced by non-solvent species, solution configurations, and ratio of solvent/ non-solvent etc. It needed shorter time to get a gel with a larger volume in a kind of weak solvent. NC gel with a higher modulus was obtained by adding non-solvent into the NC solvent. NC gel prepared in ternary system with higher non-solvent content had larger volume, smaller NC concentrations and smaller modulus. NC gels formed in initial solvent with different solvent/ non-solvent rate were strong- linked. By increasing initial NC concentration, NC gel changed from strong-linked to weak- linked, and G′ and G′′ of NC gel were not always increasing with increasing the NC concentration of gel. NC gel is relatively stable, and is without temperature sensitive. NC alcogels(surface areas between 141 to 204 m2g-1) were prepared after ethanol exchange and drying under supercritical CO 2. These aerogels showed a small shrinkage during drying which was found to be extremely limited at only 8%. The thermal decomposition of NC aerogel became more accelerated and more acute compared to NC powders. NC aerogels formed in the system with higher ethanol content had larger surface areas, larger pore size distribution range, larger average pore diameter, and larger meso-pore and macro-pore volume.5) There are small molecule branch chains in the big molecule main chain of NGEC which can increase its molecular flexibility. In order to study the impact of molecular structure on gel performance contrastively. NGEC gels were formed in ternary NGEC/acetone/ethanol system by non-solvent- induced phase separation. Similar to NC gels, NGEC gels formed in initial solvent with different solvent/ non-solvent rate were strong- linked; NGEC gel changed from strong- linked to weak- linked as increasing initial NGEC concentrations. Different from NC gels, the storage modulus(G′) and loss modulus(G′′) of N GEC gels rapidly increased with increasing N GEC concentration of gels. In addition, the NGEC gels were temperature sensitive. NGEC aerogels with high porosity, low density, and specific surface area are prepared from NGEC alcogels after supercritical CO2 drying. Compared to NC aerogels, NGEC gels had a smaller specific surface area and crystallinity but larger shrinkage. NGEC aerogels formed in the system with higher ethanol content had larger surface areas, larger pore size distribution range, larger average pore diameter, and larger meso-pore and macro-pore volume.6) Due to the porous structure of aerogel, aerogels can be used in catalysis, electrochemistry, separation and sensing fields. Nanocomposites with special functions can be prepared by using polymer gel as a matrix or templates, while functional nanoparticle as additives. In this paper, cellulose nitrate derivative aerogel used as a matrix, NC(NGEC)/Al composite gel had prepared with different content of nano Al powder which used as representative of energetic particles. With increasing Al content, the dynamic modulus of composite gels increased at first and then decreased. The modulus of composite gel decreased as the scan temperature increased. After supercritical drying, NC(NGEC)/Al composite aerogel with three-dimensional network structure including large pore space were successfully prepared. With the increase of Al content, the specific surface area of composite aerogel was significantly reduced. The pore size distribution of composite aerogel was ranging from several nanometers to several microns. The XRD results showed that the composite aerogel restained the nano Al crystal. DSC thermal analysis results indicated that the peak of DSC slightly moved toward the high temperature with increasing Al content, and the heat content of NC(NGEC)/Al aerogel were gradually reduced.