Study On Reduction Methods And Insensitive Mechanism Of Mechanical Sensitivity Of Nitrocellulose

The traditional gun propellant and solid propellant based on nitrocellulose（NC）/nitroglycerin（NG）are easily exploded when subjected to accidental stimuli（e.g.,flame,impact and shock waves,etc）,which results in a catastrophic accident.Thus,the sensitivity reduction of gun propellant and solid propellant has been the objective of numerous efforts for researchers.At present,the low-vulnerability（LOVA）propellants,insensitive high-energy propellants and insensitive solid propellant have been researched by replacing some NC with energetic thermoplastic elastomers,which is to reduce the sensitivity of ammunition to accidental stimuli.However,these propellants with new formulations still have some defects and shortages,such as poor mechanical properties.NC,the binder and main energy component of gun propellant and solid propellant formulas,has played an irreplaceable role since the invention of smokeless propellants.Thus,if the NC is directly treated with desensitization,it will provide a new idea for the development of insensitive high-energy propellant and insensivive solid propellant.The purpose of this paper is to reveal the relationship between NC mechanical sensitivity and its molecular structure,and study the desensitization method and mechanism of NC.The followings are research contents and results of this paper.（1）The mechanical mechanism and influencing factors of NC sensitivity were studied by the theoretical and experimental methods.The density functional theory（DFT）,thermal analysis technique,molar activity index and oxygen balance method were used to study the relationship between the mechanical sensitivity of NC and its molecular structure and thermal decomposition performance.The cellulose samples with different crystal structures,morphologies and sizes were prepared from the microcrystalline cellulose.The NC products were prepared by nitrification of these cellulose samples,and then their mechanical sensitivity were tested.The results show that the impact sensitivity of NC is related to the dissociation energy of O-NO₂ bonds and the ability of releasing NO₂ gas,and the higher the nitrogen content,the greater the impact sensitivity of NC.The crystal structure,morphology and size have different effects on the impact sensitivity of NC,and the effects of morphology and size on its impact sensitivity are more significant,but they have no effect on the friction sensitivity.The friction sensitivity of NC is affected by the internal friction among its molecular chains.The research results reveal the generation mechanism of mechanical sensitivity of NC,which provides a theoretical basis for the desensitization methods of NC.（2）The crystal quality of NC was improved by chemical method.The nano-sized spherical cellulose composite crystal was prepared by hydrolysis of microcrystalline cellulose with mixed acid solution,and then it was nitrated to prepare the nano-sized spherical NC composite crystal.The results show that the hydrolyzed product of microcrystalline cellulose is a nano-sized spherical cellulose composite crystal of 15-30 nm.Its nitrification product is nano-sized spherical NC that is a composite crystal of I-type NC and II-type NC,and the crystallinity of nano-sized spherical NC composite crystal is 70.5%that is higher than that of common NC（66.2%）.The nitrogen content of nano-sized spherical NC composite crystal is14.89%that is higher than the nitrogen content of common NC.Thus,the thermal decomposition temperature of nano-sized spherical NC composite crystal is decreased by40°C and its decomposition heat is increased by 66.0%.However,the impact sensitivity of the nano-sized spherical NC composite crystal is 44.6%lower than that of common NC,and its friction sensitivity dose not change significantly.Thus,the quality improvement of NC crystal can reduce its sensitivity to the impact actions.（3）The silica gel coating layer was grafted onto the surface of NC by a condensation reaction.A kind of NC/Si O₂ gel composite was prepared by grafting silica gel coating on the surface of NC with the sol-gel method and ultrasonic assisted dispersion method.The results show that silica gel is an amorphous state,and it is bonded with the NC surface layer by an C-O-Si bond.The decomposition ratio of thermal decomposition reaction of composite is reduced and the capacity of heat release and thermal stability of composite are improved after grafting the silica gel on the surface layer of NC.The impact sensitivity of the NC/Si O₂composites prepared in this paper decreases by more than 35.0%,but the main molecular structure of NC is not affected,which do not change their friction sensitivities.Therefore,the surface coating of NC can reduce its impact sensitivity.（4）The internal molecule composition of NC was changed by the cocrystal technique.The nitrocellulose/cellulose acetate butylate（NC/CAB）cocrystal compound was prepared by solvent-nonsolvent method under non-energetic binder CAB as a co-crystallization ligand.The results show that the NC and CAB molecules are combined by the hydrogen bonds.A new crystalline phase is formed in the NC/CAB cocrystal composite,and its morphology is a sheet-like structure.There is a hydrogen interaction between NC and CAB.The thermal decomposition reaction temperature of the NC molecule in the cocrystal compound does not change,and the thermal decomposition reaction temperature of the CAB molecule decreases by 27°C.Compared with the NC,the energy of NC/CAB cocrystal compounds are decreased by 35.0%,50.1%and 60.8%,respectively.However,their impact sensitivities are decreased by 40.2%,75.7%and 114.5%,and their friction sensitivities are decreased to 0.The mechanical sensitivity of the NC suffer a steep decline.Thus,the change of internal molecule composition of NC by the cocrystal technique can decrease its mechanical sensitivity.（5）The NC molecule was branched with acetyl side chains.Cellulose nitrate acetate was prepared by the esterification between acetic anhydride and residual hydroxyl groups of NC in a dichloromethane solvent.The results show that the active solvent can change the morphological structure of cellulose nitrate acetate,and the acetylation reaction must be carried out in an inert solvent.The thermal stability of cellulose nitrate acetate is slightly higher than that of NC,and its decomposition heat does not change significantly.Compared with the raw NC,the energy of the cellulose nitrate acetate is decreased by 13.4%,and its impact sensitivity and friction sensitivity are decreased by 13.2%and 8.0%,respectively,and the impact sensitivity has obviously decreased.Thus,the modification of NC molecule is an effective method to reduce its impact sensitivity and friction sensitivity.（6）The molecular structure of NC was changed by an inert binder polyethylene glycol（PEG）.The step ladder-like NC was synthesized by modifying NC under isophorone diisocyanate as a coupling agent and PEG as a branch chain.The results show that the step ladder-like NC is a near-spherical article,and has a more perfect crystal phase structure.However,the nitrogen content,thermal stability and decomposition heat of the step ladder-like NC are decreased in different degrees.Therefore,compared with the raw NC,the energy of the step ladder-like NC is decreased by 47.1%,but its impact sensitivity and friction sensitivity are decreased by 93.2%and 76.0%,respectively.The decrease degree in mechanical sensitivity is significant.Thus,the modification of the molecular structure of NC can effectively reduce its sensitivity.According to the research results,the mechanical sensitivity of NC is related to its molecular structure and other internal essential factors,and the qualities such as morphology and size affect the impact sensitivity of NC.The desensitization methods and techniques of NC surface grafted silica gel,NC/CAB cocrystal,NC side chains linked acetyl groups and step ladder-like NC are to reduce the mechanical sensitivity of NC at the expense of the little energy of NC.The desensitization method of nano-sized spherical NC composite crystal can significantly reduce the impact sensitivity of NC while increasing its energy.The desensitization methods of NC surface grafted silica gel and nano-sized spherical NC composite crystal can only reduce the impact sensitivity of NC,and the preparation time of nano-sized spherical NC composite crystal is longer.The decrease degree for the mechanical sensitivity of cellulose nitrate aceate is less.The decrease degree for the mechanical sensitivity of the NC/CAB cocrystal and the step ladder-like NC is significant,and their preparation methods are relatively simple,which are the more suitable methods for reducing the sensitivity of NC.The research achievements would provide a technical support for the formulations research of high-energy and insensitive propellant and solid propellant.