Study On The Aging Properties And Storage Life Prediction Of HTPB Propellant

As a primary propellant, HTPB propellant is widely used in solid motor currently and in the future. Therefore, research on the storage properties of the HTPB propellant is significant for understanding the storage properties, analyzing its aging mechanisms and predicting its storage life.The thermal accelerated aging characteristics of the HTPB propellant were investigated in the following four aspects including the macroscopical mechanics properties, the microcosmic structure of binder matrix, interfacial adhesive property of the filler/binder matrix and the content of antioxidant H under thermal and thermal-mechanical coupling aging conditions. The influence of thermal and mechanics on the HTPB propellant were analyzed. The key factors and the aging mechanisms of HTPB propellant were reviewed combining the aging properties and the relationship of the components in HTPB propellant. The storage life of HTPB propellant in the two storage conditions was calculated. The reliability of the storage life was also analyzed.The results show that HTPB binder is easy to be oxidated by oxygen in the air during the storage. During the process, the oxidation crosslinking occurred and multiplicate oxidation products formed. The antioxidant H can effectively prohibit the oxidation of HTPB binder. The oxidizer AP can accelerate the oxidation and decomposition of HTPB. By theoretic calculation and experimental validation, the weak bond is C-N and C-O bond of polyurethane in HTPB curing system.The function mechanism and the constituents of antioxidant H were studied in HTPB propellant. It is found that the Antioxidant H is composed of N-N'-Diphenyl-p–phenylenediamine (DPPD) and its oxidation product N-N'-Diphenyl-p-benzoquinone diimines ( DPBQ). The changing rule of the content of them was obtained.It is approved that the antioxidant H can react with toluene diisocyanate(TDI) in HTPB propellant. The competitive reaction of antioxidant H, HTPB and TDI were studied. The results show that the reactive rate of HTPB/TDI is 12.9 times higher than that of antioxidant H / TDI at 50℃. The hydrogen in antioxidant H is less active than that in HTPB.The aging characterization method and parameters were determined under thermal and thermal-mechanical coupling aging conditions. The macroscopical mechanics properties were studied including tensile strength, elongation at maximal and break, surface hardness, the loss factor, etc. The microcosmic structures of binder matrix were studied including the cross-linking density, the gel fraction and the relative content of the C-N bond. The interfacial properties of the filler/binder matrix were studied including the interfacial tension, the work of adhesion, the critical debonding stress and the exponent of conglutination. The content of antioxidant H and its oxidation product were also studied. At last, elongation at maximal, the cross-linking density and the work of adhesion were selected as the main characteristic parameters of the macroscopical mechanics properties, the binder matrix and the interfacial adhesive properties of the filler/binder matrix respectively.The interfacial adhesive property of the filler/binder matrix in HTPB propellant was studied under thermal and thermal-mechanical coupling aging conditions. Results show that the work of adhesion and the critical debonding stress are decreased while the interfacial tension increased along with the aging time. It indicates that the aging makes the interfacial of the filler/binder matrix dewetting. The interfacial dewetting of the filler/binder matrix is more seriously under thermal-mechanical coupling condition than thermal aging condition. It indicates that the interfacial dewetting of the filler/binder matrix is one of the main mechanisms under thermal-mechanical coupling aging condition.Under thermal aging condition, the experimental results show that tensile strength, surface hardness, the gel fraction, the cross-linking density, the content of DPBQ and the exponent of conglutination are increased along with the aging time. The elongation at maximal and break, theαrelaxation peak value of the loss factor are decreased on time duration. The content of the C-N bond and DPPD are increased at first and then decreased. The higher the aging temperature, the larger the changing rate of the aging characteristics. There are three kinds of reaction under thermal aging condition including the continued-curing reaction, the oxidative crosslinking and the degradation. The effect of the three kinds of reaction is different during the aging process. The continued-curing reaction is dominating during the aging initial stages. During the aging metaphase, the oxidative crosslinking and the degradation are existent and they tend to balanceable. During the aging anaphase, the oxidative crosslinking exceed the degradation. Oxidative crosslinking is the most important aging factor of HTPB propellant under thermal aging condition.Under thermal-mechanical coupling condition, the experimental results show that tensile strength are decreased at first and then increased during the aging process. The gel fraction, the cross-linking density, the content of DPBQ and the relative content of the C-N bond are increased at first and then decreased. The elongation at maximal and break, the content of DPPD and theαrelaxation peak value of the loss factor are decreased on time duration. The surface hardness and the exponent of conglutination are increased along with the aging time.The higher the aging temperature, the larger the changing rate of the aging characteristics. The results show that the continued-curing reaction of HTPB binder matrix is dominating during the aging initial stages under thermal-mechanical coupling condition. During the aging metaphase, the oxidative crosslinking exceed the degradation. During the aging anaphase, the degradation is arisen gradually. Oxidative crosslinking is the main aging factor of HTPB binder matrix and the interfacial dewetting of the filler/binder matrix can not be slighting under thermal-mechanical coupling condition.Based on the grey correlation analysis, the main factor of macroscopical mechanical properties is the cross-linking density under thermal and thermal-mechanics coupling aging conditions. The study on the correlation of macroscopical-microcosmic properties shows that there is linear relationship between the maximal elongation, the cross-linking density and the content of the oxidation product of antioxidant H under thermal aging condition. The reduction of the maximal elongation is caused by the oxidative crosslinking. It is found that there is correlation between the elongation at maximal and the cross-linking density, the adhesion work under thermal-mechanical coupling aging condition. It shows that the oxidative crosslinking and the interfacial dewetting of the filler/binder matrix are dominant. The method is established to evaluate the macroscopical mechanical property by studying microcosmic property as a Non-Destructive Evaluation (NDE) or Micro-Destructive Evaluation (MDE) method.The aging mechanisms were obtained under thermal and thermal-mechanical coupling aging conditions. The reasons of HTPB propellant performances deterioration under the two storage conditions are the continued-curing reaction, the oxidative crosslinking , the degradation and interfacial dewetting between the filler and the binder matrix. Oxidative crosslinking is the most important aging factor of HTPB propellant under thermal aging condition. Oxidative crosslinking and interfacial dewetting are the main aging factors of HTPB propellant under thermal-mechanical coupling aging condition.Choosing the elongation at maximal as the key aging property,the storage life of HTPB Propellant was predicted. Selecting the changed 30% of the initial value of the maximal elongation as invalidation point, the predicted storage life of the HTPB propellant at room temperature (25℃) are 18.0 years and 13.8 years respectively under thermal and thermal-mechanical coupling aging conditions. The reliability is 0.9993, and 0.6217 respectively. If the reliability is 0.99, the storage life of the HTPB propellant are 20.7 years and 10.7 years respectively under thermal and thermal-mechanical coupling aging conditions.