A Study of Hydroxyl-Terminated Block Copolyether-Based Binder Curing Kinetics

In order to determine the curing reaction model and corresponding parameters of hydroxyl-terminated block copolyether (HTPE) and provide a theoretical reference for its practical application, the non-isothermal differential scanning calorimetry (DSC) method was used to analyze the curing processes of three curing systems with HTPE and N-100 (an aliphatic polyisocyanate curing agent), isophorone diisocyanate (IPDI), and a mixture of N-100 and IPDI as curing agents. The results show that the curing activation energy of N-100 and HTPE was about 69.37 kJ/mol, slightly lower than the curing activation energy of IPDI and HTPE (75.60 kJ/mol), and the curing activation energy of the mixed curing agent and HTPE was 69.79 kJ/mol. The curing process of HTPE conformed to the autocatalytic reaction model. The non-catalytic reaction order (n) of N-100 and HTPE was about 1.2, and the autocatalytic order (m) was about 0.3, both lower than those of IPDI and HTPE. The reaction kinetics parameters of the N-100 and IPDI mixed curing agent with HTPE were close to those of N-100 and HTPE. The verification results indicate a high degree of overlap between the experimental data and the calculated data.

Molecular Dynamic Simulations and Experiments Study on the Mechanical Properties of HTPE Binders

The mechanical properties of HTPE binders have been systemically studied through combining the microstructure molecular simulations with macroscopic experiments. In this study, the crosslinking structures of HTPE binders were established by a computational procedure. Based on the optimized crosslinking models, the mechanical properties and the glass transition temperatures (Tg) of HTPE/N-100, HTPE/HDI, HTPE/TDI, and HTPE/IPDI binder systems were simulated; specifically, the Tg were 245.758 K, 244.573 K, 254.877 K, and 240.588 K, respectively. Then the bond-length distributions, conformation properties, cohesive energy densities, and fraction free volume were investigated to analyze how the microstructures of the crosslinking models influenced the mechanical properties of HTPE binders. Simultaneously, FTIR-ATR spectra analysis of HTPE binders proved that the special peaks, such as -NH and -NCO, could be seen in the crosslinking polyurethane structures synthesized between prepolymers and curing agents. The dynamic mechanical analysis was carried out, and it found that the Tg of HTPE/N-100, HTPE/HDI, HTPE/TDI, and HTPE/IPDI binder systems were -68.18 °C, -68.63 °C, -65.67 °C, and -68.66 °C, respectively. In addition, the uniaxial tension verified that both the ultimate stress and Young's modulus of HTPE binder systems declined with the rising temperatures, while the strains at break presented a fluctuant variation. When it was closer to glass temperatures, especially -40 °C, the mechanical properties of HTPE binders were more prominent. The morphology of the fractured surface revealed that the failure modes of HTPE binders were mainly intermolecular slipping and molecular chain breakage. In a word, the experimental results were prospectively satisfied using the simulations, which confirmed the accuracy of the crosslinking models between prepolymers and curing agents. This study could provide a scientific option for the HTPE binder systems and guide the design of polyurethanes for composite solid propellant applications.

A novel route to improve the mechanical and rheological properties of HTPE/AP/Al propellant by adding a modified hyperbranched polyester

Both better mechanical and rheological properties are pursued for composite solid propellant. In this work, varying proportions of a modified hyperbranched polyester (MHBPE) were added to HTPE/AP/Al propellant. The static tensile property as one kind of mechanical properties of MHBPE/HTPE/AP/Al propellant were found to be superior to those of blank HTPE/AP/Al propellant as a result of the entanglement and interpenetration of molecular chains caused by the introduction of the hyperbranched structure. Evaluations on the related improved creep resistance and stress relaxation performance further demonstrated the advantages of introduction of MHBPE to HTPE/AP/Al propellant. Rheological properties of HTPE/AP/Al propellant with and without MHBPE during the casting process were investigated and compared and the results confirmed the improvement benefiting from low viscosity and loose void structure. Thus, modified hyperbranched polyester provided a novel route to potentially meet the requirements for propellant manufacturing and applications.

Investigation of a Low-Toxicity Energetic Binder for a Solid Propellant: Curing, Microstructures, and Performance

In this work, a promising propellant binder using the energetic branched glycidyl azide polymer (B-GAP) as a matrix and the low-toxic dimer acid diisocyanate (DDI) as a curing agent was prepared, under the catalysis of dibutyl tin dilaurate. The curing kinetics considering the thermal diffusion effect and the reaction endpoint of B-GAP/DDI were investigated by the thermal analysis method and a newly proposed variance method, respectively. Moreover, the buildup of microstructures during curing and the tensile and dynamic mechanical performance of this binder were respectively explored. Results show that there exists an obvious induction period in the beginning of the curing reaction, and the autocatalytic model shows that thermal diffusion can effectively describe the curing process. Shore A hardness of sample stabilizes around 40.78 in the end of curing, and the reaction endpoint of B-GAP/DDI is in the time range of 156-168 h. There exist cross-linking, suspension, and free chains during the whole curing process, and the cross-linking density of the binder reaches approximately 4.0 × 10^-4 mol·cm^-3 when the curing completes. Hydrogen bonding (H-bond) is found to be a strong binder: 53.3% of the carbonyls participates in forming the H-bond. Furthermore, this binder has better mechanical performance and lower glass-transition temperature than the GAP/N100 binder.

Improvement of mechanical properties of in situ-prepared HTPE binder in propellants

A new type of hydroxyl-terminal block copolymer (HTPE) binder with excellent mechanical properties was prepared using an in situ preparation method. Compared with traditional HTPE binder preparation, this method involves relatively simple operations, which not only reduces costs, but also does not require a complicated synthesis process to prepare the HTPE prepolymer intermediate. Thus, it is expected to replace the binder for HTPE propellants. The mechanical properties, crosslinking density, hydrogen bonding, and thermal performances of the prepared HTPE binders were investigated through tensile testing, low-field nuclear magnetic resonance (LF-NMR), Fourier-transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) analysis. The ultimate tensile strength (σ_m) of the in situ-prepared HTPE binder was 1.83 MPa, the fracture elongation (ε_b) was 371.61%, and the strength increased by 80% compared to the HTPE binders. The crosslink density (V_e) decreased with an increasing content of PEG and/or TDI. The proportion of H-bonds formed by the imino groups increased with the content of PEG and TDI and reached 81.49% at PEG and TDI contents of 50% and 80%, respectively, indicating a positive correlation between the H-bonds and σ_m. Based on the statistical theory of elasticity, the integrity of the curing networks showed that the contents of PEG and TDI affected the integrity of the curing networks. The DSC data of the in situ-prepared HTPE binder showed a lower glass transition temperature. Finally, compared to HTPE propellant, the strength and elongation of the in situ-prepared HTPE propellant increased by 206% and 135%, respectively. This exciting result greatly enhances the feasibility of the in situ HTPE preparation method.

HTPE binder for propellant synthesized by in situ preparation method has excellent mechanical properties and low glass transition temperature.