Solid Propellant Binder Research Articles

Mapping oft the optimal mechanical responses in commercial‐grade HTPB‐based binders

AbstractAlternative binders for composite solid propellants (CSP) have always been a concern. One of the closest alternative to the broadly used military‐grade hydroxyl‐terminated polybutadiene (HTPB) would be its commercial‐grade. Factorial design of experiments (DoE) was applied to optimize the mechanical properties of the binder. Commercial‐grade HTPB was used, in addition to 1,4‐butanediol (BDO) as chain extender, and either isophorone diisocyanate (IPDI) or 4,4’‐methylenedicyclohexyl diisocyanate (H12MDI) as curing agents. The effect of simultaneously varying two factors: [NCO]/[OH] molar ratio, and BDO:HTPB mass fraction on the mechanical properties of the binder was evaluated. DoE gave the optimal conditions at 0.68 molar ratio, and 0.71 % BDO:HTPB mass fraction for IPDI; and at 0.66 and 0.57 %, respectively, for H12MDI. Higher strength was observed for binders formulated with H12MDI. At the optimal conditions, CSP formulated with commercial HTPB showed higher strength than CSP formulated with military HTPB. Moreover, with H12MDI higher elongation was observed (28±4 %) when compared to IPDI (19±1%). Thus, the former is a promising diisocyanate for tailoring CSP formulations with commercial‐grade HTPB.

Inserting urethane into 1,2,3‐triazole click‐cured solid propellant binder to form microphase separation and enhance mechanical and combustible performances

AbstractHuisgen‐click curing systems are widely used in solid propellant binders due to their low humidity sensitivity and mild reaction conditions. However, the rigid triazole rings from the Huisgen‐click reaction decrease the molecular moving ability and increase decomposition temperature, lowering the mechanical and combustion performance. Thus, we bonded the Huisgen‐click system and polyester binders with urethane, forming microphase separation and improving chemical stability. The results show that the elongation at break, tensile strength, and Young's modulus of the binders increase by 533.3%, 201.2%, and 1263.9%, respectively. Furthermore, the urethane reduces the thermal resistance index of the binders from 191.2 to 179.2°C, which means better combustion performance. Further mechanism study reveals that the urethane leads to microphase separation, which aggregates the hard segments and weakens their restriction effect on the molecular motion of polyether. Such a microphase separation brings comprehensive improvement in the mechanical properties of the solid propellants and probably can be used in other multi‐component systems of energetic materials.

Review on energetic copolymer binders for propulsion applications: Synthesis and properties

AbstractThis article aims to outline the current progress in the field of energetic copolymer binders for composite solid propellants (CSPs). Propellants, which are a type of energetic material, are used to generate thrust in rockets and missiles, and they are generally less sensitive than explosives. The common formulations of CSPs contain several different chemical compounds that are typically bound together by a polymeric matrix to form a continuous solid. The use of inert polymers, however, does not enhance the overall specific impulse of the propellant. Energetic copolymers have emerged as a compelling category of binders for CSPs in recent years, offering potential advantages over traditional binders such as improved performance, enhanced safety, and increased manufacturing efficiency. The paper reviews the various types of energetic copolymer binders that have been developed, their potential advantages and drawbacks, and the current challenges and opportunities in the field. It suggests directions for future research and development and aims to provide a useful resource for researchers and practitioners interested in the use of energetic copolymer binders toward CSPs.

Synthesis and characterization of a novel hydroxy telechelic polyfluoroether to enhance the properties of HTPB solid propellant binders

As the most commonly used polymer binder for solid propellants, it is of great significance to modify hydroxyl-terminated polybutadiene (HTPB) to obtain improved low-temperature behavior and mechanical properties. A novel hydroxy telechelic polyfluoroether (HTPF) was synthesized by cationic ring-opening polymerization of fluorine-containing epoxides (FPO). HTPF was introduced into HTPB polyurethane to enhance the mechanical behavior and thermodynamic properties of the elastomer. As a result, the microphase-separated structure of the polyurethane was optimized and the bonding effect with the aluminum powder became tighter. The tensile strength and elongation at break were increased by 94.69% and 197.49% after adding aluminum powder, respectively. For thermodynamic properties, the stability property in a wider temperature range was maintained up to 467.2 ℃, and the glass transition temperature (Tg) was lowered by 5.6 ℃. Therefore, block polymerization of fluorinated polyethers with HTPB proved to be a promising scheme to obtain advanced propellant binders.

Preparation and Characterization of Polyurethane/Nitrocellulose Blends as Binder for Composite Solid Propellants

AbstractPolyurethane (PU) elastomers are largely used in the field of high‐energy composites such as composite solid propellants (CSPs) and high‐energy polymer‐bonded explosives (PBXs) due to their distinguished characteristics. Conventional PU binders are mostly non‐energetic materials, and consequently reduce the energy performance significantly. Nitrocellulose (NC), is an energetic polymer widely used as an ingredient in propellants, explosives, fireworks, and gas generators, may be introduced in PU‐based compositions to overcome their performance drawback. In this context, PU/NC polymer blends at different mass ratios were prepared in the present work using hydroxyl‐terminated polyester prepolymer (Desmophen® 1200) and nitrocellulose (NC) by solution blending process. The physico‐chemical structure of the prepared PU/NC polymer composites were characterized by Fourier transform infrared spectroscopy (FTIR), X‐ray diffraction (XRD) and density measurements. The thermal decomposition behavior was investigated by differential scanning calorimetry (DSC). Based on the obtained DSC results, the Arrhenius parameters were computed by different isoconversional kinetic approaches, namely, iterative Kissinger‐Akahira‐Sunose (It‐KAS), iterative Flynn‐Wall‐Ozawa (It‐FWO) and Vyazovkin's nonlinear integral method coupled with compensation effect (VYA/CE). Additionally, in order to highlight the influence of the introduction of the NC to the binder composition on the performance of a composite propellant, the theoretical performances, namely, theoretical specific impulse, the adiabatic flame temperature, as well as the ideal exhaust gaseous species were determined based on NASA Lewis Code, Chemical Equilibrium with Application (CEA).

Styrene‐Ethylene/Butylene‐Styrene (SEBS) Block Copolymer Binder for Solid Propellants

AbstractCurrently solid composite propellants are manufactured using bespoke, specially manufactured binders. This adds significant cost and obsolescence risks to their development and manufacture. This paper reports on the production of two composite propellants made from a commercially‐available‐off‐the‐self polymer as binder by means of a resonant acoustic mixer (RAM) slurry process. The binder consisted of thermoplastic elastomer styrene‐ethylene/butylene‐styrene (SEBS) block copolymer and the solid filler, either ammonium perchlorate (AP) or 1,3,5‐Trinitro‐1,3,5‐triazinane (RDX). Thermodynamic code demonstrated that SEBS‐based propellants exhibited similar theoretical ballistic performance to conventional HTPB‐based propellants. Chemical compatibility between SEBS and the solid fillers was shown by the differential scanning calorimetry (DSC) and vacuum stability methods. The novel combination of RAM and slurry coating process produced a propellant moulding powder that was then consolidated by remote hot pressing to simulate an industrial extrusion or rolling process. The propellants exhibited as expected explosive hazard properties and their thermal analysis by DSC‐TGA showed typical oxidizer behaviour between AP and SEBS, characterised by two decomposition exotherms (also reported in AP/HTPB compositions), and little or no interaction between RDX and SEBS. The apparent activation energy for decomposition for AP/SEBS (84/16 % w/w) propellant was similar to that for AP/HTPB propellants. This work demonstrates the viability of using SEBS polymer as a binder for solid propellants. The development of propellants using commercially off the shelf, thermoplastic elastomeric binders could add significant benefits in waste reduction through recycling, security of supply, and manufacturing costs.

Facile synthesis of three diazido compounds and their application in polyether polytriazido elastomers as solid propellant binders

AbstractThree linear diazido compounds as novel binders have been prepared via a one‐step substitution of corresponding bromo‐ended halide. The structures of these novel diazido compounds and their reactivity with alkyne have been characterized by FTIR, 1H NMR, 13C NMR, GPC, and element analysis. Polytriazolepolyethyelene oxide‐tetrahydrofuran (PTPET) elastomers are prepared by curing alkyne terminated polyethyleneoxide‐tetrahydrofuran (ATPET) with glycidyl azide polymer (GAP) and three novel diazido compounds at different molar ratios of ‐N3 to alkyne. The properties of the PTPET elastomers are measured by TGA, DSC and mechanical apparatus. Plasticizer A3 [1:1 mixture of bis‐(2,2‐dinitropropy) acetal (BDNPA) and bis‐(2,2‐dinitropropyl)] is added thereafter as a comparison. The results revealed that PTPET elastomers synthesized with the novel compounds that have longer ‐CH2 chain and ether group could have better properties and serve as promising binders for propellant components.

Enthalpy of Formation of p(BAMO)‐b‐GAP and Energetic Characteristics of Solid Propellants Based on the Copolymer

AbstractThe block copolymer of 3,3’‐bis(azidomethy) oxetane and glycidyl azide polymer (p(BAMO)‐b‐GAP) is an ideal energetic binder for solid propellants. In this work, the enthalpy of formation of p(BAMO)‐b‐GAP was first investigated by the heat of combustion experimental method and group additivity method. Meanwhile, the energetic characteristics of solid propellants based on the copolymer were systematically calculated. The results proved that the ΔfH of p(BAMO)‐b‐GAP could be estimated accurately by two methods and the positive ΔfH values were obtained. Dual‐oxidant formulas were designed in which AP was replaced gradually by some chlorine‐free high‐energy materials. The calculation results showed when AP was gradually replaced by RDX or HMX, the energy level of propellants based on p(BAMO)‐b‐GAP was improved first but then decreased with AP content further decreased, indicating there was an optimum ratio between AP and its alternative. However, when AP was replaced by CL‐20 or ADN, the values of Isp were improved steadily. When less than 40 % AP was replaced, the contribution to the energy level of propellant by RDX or HMX was even more than that by CL‐20.

Hyperbranched polyamide modified graphene oxide-reinforced polyurethane nanocomposites with enhanced mechanical properties.

As is well known, it is difficult to simultaneously improve both the strength and elongation at break of polymers filled with nanomaterials. This work obtained high-performance composites with enhanced strength and elongation at break via cross-linking hydroxyl-terminated polybutadiene (HTPB) chains with hyperbranched-polyamide-modified graphene oxide (HGO), and the preparation, characterization, and mechanical properties of the composites serving as a composite solid-propellant binder have been described in detail. Compared with pure HTPB polyurethane (P-HTPB), the tensile strength and elastic modulus of the composite containing 0.1 wt% HGO (H-0.1/HTPB) increase by 57.8% and 65.3%, respectively. Notably, the elongation at break of the H-0.1/HTPB composite can reach up to 1292.6%, which is even higher than that of P-HTPB. Moreover, the capabilities of the composites to resist deformation have also been enhanced significantly. The glass transition temperatures of the composites are still extremely low (∼−73 °C), which is beneficial for their applications. It can be expected that this study can provide an effective fabrication approach and strategy for preparing high-performance polyurethane composites.

Poly(methoxy-NNO-azoxyethene), a new polymer for active binders in solid composite propellants

Radical polymerization of unsaturated alkoxy-NNO-azoxy compounds was explored. Polymethoxy-NNO-azoxyethene, the first energetic polymer based on alkoxy-NNO-azoxy compounds, was obtained, and its major characteristics were studied. Thermodynamic calculations show that poly(methoxy-NNO-azoxyethene) may be of interest as a polymer component of active binders for solid composite rocket propellants.

Assessment of antioxidant resistance to thermal - oxidative degradation of stabilized polybutadiene binder

polybutadiene is the most common polymeric binder for composite solid propellants. This polymer suffers from oxidative degradation reactions on storing with deterioration in mechanical properties. Certain anti-oxidant materials can retard such reaction offering enhanced aging characteristics. In this study, two different primary antioxidants including phenol-based (AO2246) and amine-based (Flexzone 6H) were employed to stabilize polybutadiene polymer. Oxidation induction time (OIT) was evaluated using isothermal DSC. High OIT value means high resistance to oxidative degradation. The impact of anti-oxidant on mechanical properties was evaluated using shore A. Flexzone 6H significantly improved binder oxidative stability with an increase in OIT value by 20 folds with significant decrease in oxidation enthalpy. Flexzone 6H can offer enhanced resistance toward oxidative reactions; as it can act as efficient H atom donor to break down chain degradation reaction. Additionally, the three stabilized aromatic rings can capture free radicals. AO2246 enhanced the binder mechanical characteristics during aging at 80 0C using Shore A test. In the meantime, AO2246 could enhance the mechanical properties due to its integration into the polymeric matrix due to –OH functional groups. The two anti-oxidants demonstrated controversy effects. Whereas Flexzone 6H offered enhanced resistance to oxidative degradation; AO2246 offered enhanced mechanical properties.

Inside Cover: Development and Characterization of a Photopolymeric Binder for Additively Manufactured Composite Solid Propellant Using Vibration Assisted Printing (Prop., Explos., Pyrotech. 6/2020)

Inside Cover: Development and Characterization of a Photopolymeric Binder for Additively Manufactured Composite Solid Propellant Using Vibration Assisted Printing (Prop., Explos., Pyrotech. 6/2020)

Curing Behaviors of Alkynyl-Terminated Copolyether with Glycidyl Azide Polymer in Energetic Plasticizers.

Alkynyl-terminated polyethylene oxide−tetrahydrofuran (ATPET) and glycidyl azide polymer (GAP) could be linked through click-chemistry between the alkynyl and azide, and the product may serve a binder for solid propellants. The effects of the energetic plasticizers A3 [1:1 mixture of bis-(2,2-dinitropropy) acetal (BDNPA) and bis-(2,2-dinitropropyl) formal(BDNPN)] and Bu-NENA [N-butyl-N-(2nitroxyethyl) nitramine] on the curing reaction between ATPET and GAP have been studied. A diffusion-ordered nuclear magnetic resonance spectroscopy (DOSY-NMR) approach has been used to monitor the change in the diffusion coefficient of cross-linked polytriazole polyethylene oxide−tetrahydrofuran (PTPET). The change in the diffusion coefficient of PTPET with A3 plasticizer is significantly higher than that of PTPET with Bu-NENA. Viscosity analysis further highlighted the difference between A3 and Bu-NENA in the curing process—the curing curve of PTPET (A3) with time can be divided into two stages, with an inflection point being observed on the fourth day. For PTPET (Bu-NENA), in contrast, only one stage is seen. The above methods, together with gel permeation chromatography (GPC) analysis, revealed distinct effects of A3 and Bu-NENA on the curing process of PTPET. X-ray Photoelectron Spectroscopy (XPS) analysis showed that Bu-NENA has little effect on the valence oxidation of copper in the catalyst. Thermogravimetric (TG) analysis indicated that Bu-NENA helps to improve the thermal stability of the catalyst. After analysis of several possible factors by means of XPS, modeling with Material Studio and TG, the formation of molecular cages between Bu-NENA and copper is considered to be the reason for the above differences. In this article, GAP (Mn = 4000 g/mol) was used to replace GAP (Mn = 427 g/mol) to successfully synthesize the PTPET elastomer with Bu-NENA plasticizer. Mechanical data measured for the PTPET (Bu-NENA) sample included ε = 34.26 ± 2.98%, and σ = 0.198 ± 0.015 MPa.

Development and Characterization of a Photopolymeric Binder for Additively Manufactured Composite Solid Propellant Using Vibration Assisted Printing

AbstractAn emerging area of research in the energetic materials community is the development of new manufacturing methods, such as additive manufacturing (AM), that can be used to selectively deposit energetic materials into complex geometries, which provides more control over how the energetics perform during combustion. Although ammonium perchlorate (AP) composite propellant has been 3D printed at solids loadings that are comparable to traditional formulations (85 wt.%) with vibration assisted printing (VAP), traditional propellant binders such as hydroxyl terminated polybutadiene (HTPB) are poorly suited for 3D printing large or complex structures because propellant made with HTPB deforms easily under its own weight and isocyanates do not crosslink HTPB fast enough to facilitate rapid polymerization after deposition. Ultraviolet (UV) curable photopolymers have been used for many types of AM processes, such as stereolithography, and there is a variety of commercially available binders with customizable chemical and mechanical properties. However, little work has been done to characterize the optimal cure characteristics of UV curable binders that are compatible with composite propellants. Furthermore, it has also been speculated that aluminized propellants may not be amenable to UV curing since opaque particles impede UV transmission. In this work, the curing properties of a photopolymer that has similar characteristics to HTPB with AP and aluminum were quantified. The ingredients consisted of a polybutadiene urethane acrylate and hexanediol diacrylate (HDDA) polymer binder, which could be mixed in various ratios to control properties such as adhesion to particle surfaces and viscosity, as well as bisacylphosphine oxide (BAPO) which is a well‐known photoinitiator for deep curing in the coatings industry. The effect of wavelength, exposure time, and intensity on cure depth were quantified on the neat photopolymer. In addition, the effect of aluminum content (up to 20 %) on the cure depth of propellant with a solids loading of 85 wt.% was measured. It was shown that at higher intensities (∼20 mW/cm2), aluminized propellants could be cured to depths on the order of 1–2 mm, which is greater than the typical layer thickness of propellants printed via VAP (∼0.25 mm). In addition, it was shown that there were no visible interfaces in aluminized propellant (15 % aluminum) that was cured layer‐by‐layer. The approach taken could be applied to a wide range of other granular, opaque composite materials, such as metal, ceramic, and fibrous mixtures.

Cross-linking network structures and mechanical properties of novel HTPE/PCL binder for solid propellant

A series of cross-linked hydroxyl-terminated polyether and poly(e-caprolactone) HTPE/PCL binders were prepared by varying the relative mass contents of PCL to HTPE. The correlations between the microstructures and macroscopic mechanical properties of HTPE/PCL binders at a wide temperature range of − 50 to 70 °C were investigated. The cross-linking network structures of HTPE/PCL binders were analyzed by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and low-field nuclear magnetic resonance (LF-NMR), and the universal testing machine was used to characterize the mechanical properties of HTPE/PCL binders. It is found that the influence of adding PCL on the physical cross-linking network structures is not obvious, but it reduces the degree of microphase separation. For the chemical cross-linking network structures of HTPE/PCL binders, the cross-linking density (Ve) demonstrates an increasing tendency with enhancing PCL mass content, but the molecular weight between the cross-linking points (Mc) shows an opposite change trend. The addition of PCL can improve the maximum tensile strength (σm) of HTPE/PCL binders under the wide temperature range of − 50 to 70 °C. The HTPE/PCL binder with 40 wt% PCL possesses the better mechanical properties; its σm increases from 0.63 to 0.81 MPa at 20 °C, 1.64 to 3.06 MPa at − 50 °C and 0.57 to 0.68 MPa at 70 °C separately comparing with the HTPE binder.

Network regulation and properties optimization of glycidyl azide polymer‐based materials as a candidate of solid propellant binder via alternating the functionality of propargyl‐terminated polyether

ABSTRACTA kind of glycidyl azide polymer (GAP)‐based composite has been fabricated using propargyl‐terminated ethylene oxide‐tetrahydrofuran copolymer (PPET) with two (p‐) and three (t‐) alkyne functionalities via Huisgen reaction. Independent upon the PPET functionality, both crosslink densities and mechanical properties for two GAP/PPET systems showed a positive‐interrelation changes of initial increase and subsequent decrease with an increase of azide/alkyne molar ratios. At equivalent of azide/alkyne molar ratios, the composites containing t‐PPET with higher alkyne functionality exhibited better mechanical properties, while those with two alkyne functionality presented lower glass transition. Under the regulation of alkyne functionality as 3 and azide/alkyne molar ratio as 3:1, the tensile strength, Young's modulus and breaking elongation could simultaneously reach the maximum values of 1.38 MPa, 4.07 MPa, and 122.5%, which was ascribed to optimal participation of azide/alkyne reaction into network construction. Overall, this study provides an additional optimization route for network‐structured binders in solid propellant system. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48016.

Studies on PBFMO-b-PNMMO alternative block thermoplastic elastomers as potential binders for solid propellants

A novel energetic polymeric binder PBFMO-b-PNMMO alternative block thermoplastic elastomer was developed for metal-rich solid propellants.

High dimensional stability and low viscous response solid propellant binder based on graphene oxide nanosheets and dual cross-linked polyurethane

This work describes the fabrication, characterization, and deformation behavior of graphene oxide (GO) filled dual cross-linked polyurethane (PU) nanocomposites serving as solid propellant binder. The composites were synthesized through attaching isophorone diisocyanate onto GO nanosheets and cross-linking the modified GO (MGO) with PU chains. This methodology leads to uniform dispersion of MGO and its strong interface adhesion with matrix. The coefficient of thermal expansion (CTE) of the composites is tuned to the value of 150 ppm/°C, and reduced by 38.8% compared to neat PU. Moreover, both coefficient of energy dissipation and residual strain are linearly reduced, and the deformation recovery capability is specifically increased. Loss tangent (tan δ) shows the low dynamic viscous behavior, suggesting strong dependence on MGO content. Besides, the composite with 1.0 wt% MGO exhibits a critical point, where the transition from solid-like to liquid-like behavior nearly disappears. This study provides an achievable and important way to prepare the high-performance polymeric composite with prominent deformation properties.

Synthesis, Characterization and Thermal Properties of Poly(glycidyl azide‐r‐3‐azidotetrahydrofuran) as Azido Binder for Solid Rocket Propellants

AbstractAzido polymers have been investigated as energetic binders in the area of solid rocket propellants. However, the low temperature mechanical properties of them are not comparable with the traditional propellant binders. In this work, a new kind of azido binder named poly (glycidyl azide‐r‐3‐azidotetrahydrofuran) (PGAAT) was successfully synthesized. The molecular structures of monomers and copolymers were characterized. The sensitivity and thermal properties of the azido binder were studied. The cationic copolymerization of 3‐methylsulfonyloxytetrahydrofuran with ternary cyclic ethers was confirmed. The PGAAT azido binder exhibited attractive features like low glass transition temperature (Tg, −60 °C) and high energy (1798 J/g). The results indicate that the polymer is a suitable candidate binder for the solid rocket propellants.

Glycidyl Azide‐butadiene Block Copolymers: 2 Synthesis from a Mesylated Precursor

AbstractGlycidyl azide‐butadiene block copolymers are potential binders for solid propellants. The azidic part contributes to the overall energy outputs, while the butadiene block improves the mechanical properties. Synthetic strategies proposed in literature are not appropriate for production of large‐scale quantities. The mesylation of one homopolymer, followed by reaction with the OH terminal groups of the second is proposed here. The reaction is simple and does not show the drawbacks of the previously suggested alternatives. The block copolymers can be used as “partially” energetic binders. Moreover, they work well as compatibilizer in mechanical blends of the two homopolymers, that otherwise rapidly segregate in two distinct phases.