Fluoropolymer / Tatb-based Pbx Interface Structure

Fluoropolymer/TATB(triaminotrinitrobenzene) based plastic bonded explosive (PBX) was chosen as the object of study. The structure and surface properties of TATB and vinylidene fluoride-chlorotrifiuorethylene copolymers were fully characterized and studied. Mechanism of interface interaction between TATB and fluoropolymers, the effect of the process course on the explosive crystal and the distribution of binder in the molding powders and in the charges were studied by multi-methods. The influence of the sequence structure of fluoropolymers on the adsorption behavior was simulated by molecular dynamics(MD). It can provide some theoretical and experimental basis for the interface modification and formulation design of highly filled Particulate PBX composites by the systematic study of TATB, fluoropolymer and their mutual interaction.The surface properties and structure of four types of TATB with different particle size were studied by scanning electric microscopy(SEM), inverse gas chromatography (IGC) and contact angle method, X-ray photoelectric spectroscopy(XPS), Fourier transformed infrared-photoacoustic spectroscopy (FTIR-PAS). The results show that when the particle size is reduced, the holes, cavities and internal defects of TATB are gradually decreased and the surface appear smoother, the bend vibration and C=C vibration of TATB benzene ring shift at higher wave number. The disperse component of TATB surface rises as the temperature rises, and the higher the rise rate is when the larger the particle size is, which have not be reported before. The surface acid-base properties of four types of TATB show obvious difference. Fine TATB have positive lewis acidity(K_a) and basicity(K_b), but the other three types of TATB behaves endothermal adsorption at the interaction with polar probe molecules The study of the surface properties of vinylidene fluoride(VDF)-chlorotrifluorethylene(CTFE) random copolymer shows that the disperse component of surface free energy decreases as the temperature increases and fluoropolymer with more CTFE chain unites has a higher disperse component and electrophilic ability.The mechanical properties and safeties of PBX relate to its interface properties. So the TATB/fluoropolymer interface properties were studied by XPS and FTIR-PAS. The results show that there is no strong hydrogen band and no chemical band, but only weak van der Waals interaction or analogous hydrogen band between TATB and fluoropolymer binders. As the CTFE content in polymer decreases, accompanied by the reduction of the polarity of polymer, the interaction between H element in fluoropolymers and NO₂ on TATB is enhanced, the adsorption amount is increased, the phenomena induces that the interaction of NO₂ of TATB and H in polymer is stronger than that of NH₂ of TATB and F, Cl in polymer. Coarse TATB with more defects has the best ability to adsorb fluoropolymers.The effect of the distribution of the binder in the charge on the properties of PBX is worth being investigated, but has not been understood now. Therefore the internal structures of the molding powders and the charges of TATB/fluoropolymer based PBXs were observed by SEM. The results show that TATB crystals were smashed, accompanied by the creation of cracks, twin crystals and the debonding of TATB/fluoropolymer interface under the action of stress in the pressing process of PBX. At the same time the fracture of more particles is avoided due to the restrain of the binder to the spreading of cracks. By observation of the charge cross section by back scattered electron (BSE) and energy-dispersive X-ray(EDX) analysis, it is found that fluoropolymer binder isn't homogeneous distribution and mainly concentrates at the border of the molding powders, which shows that the plastic flow in the charge is restricted under the condition of high temperature and high pressure.In order to study the interface interaction mechanism between fluoropolymer and TATB in molecular scale, the adsorption of fluoropolymers including PVDF, PCTFE, VDF/CTFE alternative and random copolymers with the molar ratio 1:1,1:2,1:3,1:4 on the surface of TATB was simulated by Molecular dynamics with compass forcefield and NVT ensemble. From the analysis of the adsorption energy, the conformation of the adsorbed chains and the variation of end-to-end distance, it is found that alternative copolymers have stronger adsorption ability on TATB surface. The alternative copolymer of monomer ratio 1:1 has the best adsorption effect, the chain adsorbs on TATB surface as train conformation. As the increase of CTFE content in the copolymer, the rigidity of polymer is increased, the adsorption ability is reduced and the adsorption energy is lower, more tails or loops is formed, which coincide with the results that there is stronger interaction between H and NO₂.. At the same time, the simulate results show the adsorption ability and the conformation of the adsorbed chain of the VDF/CTFE alternative copolymer have strong dependence on the temperature and the best adsorption temperature interval is 300～ 320K.The adsorbed copolymers show more trains and tails conformation at multi-chains adsorption. In the presence of solvent molecules, the adsorbed copolymer shows more tails and loops.In the study of the macro-properties of PBX, it is found that F2311, which has better adsorption ability, has better process property in the course of preparing the molding powders. F2314 binder/TATB based PBX has better mechanical properties, which show that the strength of binder has evident influence on the mechanical properties of PBX charge besides the interface interaction.