Study On Design, Synthesis And Structure-properties Relationship Of Borate Ester Bonding Agents

With the replacement of the weapon model, a four-component hydroxy-terminated polybutadiene composite(AP/Al/RDX/HTPB) propellant has become the main variety of propellant charge in the solid rocket motor. The AP/Al/RDX/HTPB propellant with 89% solid content has high energy density.However, the high solid contents are prone to the weak leveling properties of the slurry and the poor mechanical properties of the propellant. What’s worse, the mechanical properties of the propellants are rapidly decreased in the process of accelerated aging, which it is difficult to meet the requirements of the solid rocket motors grain. In view of this background, the study on bonding agent is presented in this paper, aiming to develop the excellent bonding agent for the AP/Al/RDX/HTPB propellant, and solve the problems in engineering application.Based on the formula properties of the four-component hydroxy-terminated polybutadiene propellant, the design idea of borate ester bonging agent(BEBA) was presented in this paper. 13 kinds of BEBAs were synthesized and their physical and chemical properties were characterized. The influence of BEBAs on the interfacial interactions among the main components of the propellants was studied using the methods of inverse gas chromatography and contact angle measurement. Then, the effect of BEBAs on the network structure and mechanical properties of the HTPB-PU matrix were investigated, and the performances of BEBAs in the propellants were discussed. The excellent performance bonding agent used for improving the mechanical properties of the propellants was filtered. Furthermore, the effect mechanism of BEBA for AP/Al/RDX/HTPB propellants was proposed.(1) According to the formula properties of the four-component HTPB propellant,the design idea of BEBA was put forward. The preparation of BEBA is realized by an ester exchange reaction using borate ester precursors, interconnection monomer and external monomer as materials. The interconnected chain segments play a role of the molecular skeleton, which can regulate the solvent adsorption capacity of BEBA. Then,the coordination bonding interaction is produced by two centers of boron atoms and RDX. A plurality of amino groups on the external monomer of BEBA can form ionic bonds with AP. What’s more, the active hydrogen on the external monomer of BEBA can be involved in the crosslinking and curing reaction of the binder, thereby the fillerRDX, AP and the HTPB-PU matrix are bonded together to improve the mechanical properties of the four-component HTPB propellant.(2) The preparation of BEBA was proposed and established using the four-step synthesis method. The first step was the preparation of the borate ester precursors. 4kinds of hydroxy-terminated pre-polymers were synthesized as the interconnection monomers of BEBA in the second step. Then, 5 kinds of diethanolamine derivatives were synthesized using diethanolamine as raw material, which were used as the external monomers of BEBA in the third step. 13 kinds of BEBAs were synthesized by transesterification in the fourth step. The characteristic methods for physical and chemical properties of BEBA were established, such as boron content, amine value,hydroxyl value and p H value.(3) The interfacial interaction enthalpies were calculated from the acid-base parameters of AP, RDX, HTPB polyurethane(HTPB-PU) matrix and BEBAs, which obtained using inverse gas chromatography. The results indicated that the interfacial interaction enthalpies between AP or RDX and BEBAs were much higher than those of HTPB-PU and AP or RDX or BEBAs, so it could be that BEBA can be preferentially adsorbed on AP or RDX in the slurry mixing process. BEBA-4 prepared from poly(butylene adipate) as interconnection monomer had the strongest interfacial interaction, and the interfacial interactions between BEBA and AP or RDX are 133.86 k J·mol-1 and 121.06 k J·mol-1 respectively. The interfacial tension and interfacial adhesion work among the fillers, the HTPB-PU matrix and the fillers coated BEBAs were measured using the contact angle method. The results indicate that the interfacial tensions are decreased between the filler AP or RDX coated BEBAs and the HTPB-PU matrix, the interfacial adhesion works are increased. BEBA-1 and BEBA-4 show the best wetting interaction the interfaces of AP and RDX respectively. The interfacial tension is decreased from 11.39 m N·m-1 to 5.78 m N·m-1 between AP coated BEBA-1and the HTPB-PU matrix, the interfacial adhesion interaction is increased from 65.93 m N·m-1 to 71.90 m N·m-1, and the interfacial tension is decreased from 7.57 m N·m-1 to6.27 m N·m-1 between RDX coated BEBA-4 and the HTPB-PU matrix, the interfacial adhesion interaction is increased from 68.60 m N·m-1 to 73.09 m N·m-1.(4) The swelling properties and mechanical properties of HTPB-PU matrix film were tested to analyze the effects of BEBA content, curing parameters(RT), and the molecular structure of BEBA on the network structure and mechanical properties of the HTPB-PU matrix. The results showed that the addition of BEBA could significantly increase the crosslinking density of HTPB-PU matrix, the mechanical properties ofHTPB-PU matrix show that the maximum tensile strength(σm) increases and the elongation(εm) decreases. When the RT of curing system was increased, the crosslinking density of the network structure was increased, the σm of HTPB-PU film was increased and the elongation εm was decreased. The higher the active hydrogen contents of BEBA, the larger the σm of HTPB-PU matrix. The more polar of interconnected chain segments, the higher the σm of the matrix. The more polar the external groups, the higher the σm of the matrix.(5) BEBAs were in good compatibility with the main components of the propellants. 13 kinds of BEBAs have great improvement on the mechanical properties of the propellants in normal temperature, high temperature and low temperature.BEBA-4 prepared from poly(butylene adipate) as interconnection monomer can most significantly improve the mechanical properties of the HTPB propellant. BEBAs can improve the process performance by improving the flow of propellant slurry.Furthermore, the addition of BEBA-4 has little effect the burning rate and pressure index of the propellants, and can improve the interfacial adhesion strength and the aging resistance of the propellant. In summary, the BEBA-4 has excellent performance and good application prospects in the AP/Al/RDX/HTPB propellant.(6) The BEBA molecule is firmly combined with AP by the reaction of the amino groups in BEBA and AP, which are formed the ammonium ion bond. The coordination bonds between boron atoms of BEBA and nitrogen atoms or oxygen atoms of RDX are established, which refer to the strong interfacial bonding interaction between BEBA and RDX. Furthermore, the active hydrogen groups in BEBA can be involved in the crosslinking and curing reaction of the binder, which enhances the formation of the high modulus transition layer and led BEBA to enter the network structure of the HTPB-PU matrix. In summary, BEBA makes the filler and HTPB-PU matrix together by the effective bond interaction, and increases the interfacial adhesion bond strength,thus significantly improves the mechanical properties of the propellant.