Propellant Binder Research Articles

Noncovalent Interactions Define Global Properties of HTPB Isomers.

Commercially available hydroxyl-terminated polybutadiene (HTPB) variants [containing various percentages of its isomers (cis, trans, and vinyl) and utilized in numerous applications ranging from rocket propellant binders to automobile tires] exhibit differences in their thermomechanical properties. Observed thermomechanical response variation between two isomers of HTPB (based upon simulation work at the molecular dynamic level) has been shown to be associated with noncovalent interaction between the isomers. Even though the study has been presented for a specific polymeric system, it could be of interest to the broader polymer chemistry community concerned with the reasons behind the property variation of different isomers of a specific molecule.

Synthesis of propellant grade HHTPB by hydrogenation of HTPB using Pd-activated charcoal as catalyst

To derive more performance from the conventional composite propellant based on hydroxyl-terminated polybutadiene (HTPB) and ammonium perchlorate (AP), it is envisaged to hydrogenate HTPB, which increases the H/C (hydrogen to carbon ratio) of the base polymer. This paper attempts to describe the partial hydrogenation of HTPB using a catalytic method that uses palladium supported by activated charcoal as a catalyst and HTPB polymer as a precursor. HTPB has a hydroxyl value of 41.0 mg KOH/g with a number-average molecular weight (Mn) of 6150, polydispersity (PD) of 2.25 was used as a precursor. Iso-propyl alcohol (IPA) and toluene were used as a solvent media to disperse the HTPB polymer during hydrogenation. The estimate revealed an increase in specific impulse (ISP) up to 1.2 s with ∼37 % hydrogenated HTPB (HHTPB) as binder in composite solid propellants. Partial hydrogenation is only attempted to retain other process capabilities essential for realizing a defect-free solid propellant grain with good structural integrity. In contrast, others attempted to reach near saturation. A reactor capacity of 100 ml, operating at 60 bar pressure, was used to synthesize HHTPB, and the extent of hydrogenation was controlled based on the numerous experiments varying the reactor parameters like operating temperature, operating pressure, and solvent-to-polymer ratio for a given catalyst concentration. Detailed characterization of the end product by FTIR, 1H NMR and 13C NMR to reveal the degree of hydrogenation along with generic polymer characteristics. The measured hydroxyl value of HHTPB is comparable with HTPB; however, a marginal increase in molecular weight and polydispersity was noticed at 37 % conversion, wherein other researchers found a loss in -OH value. Thermogravimetry analysis revealed that no significant change in the gasification rate. A slight increase in the H/C ratio and calorific value was observed for HHTPB compared to HTPB. Despite the increase in viscosity of HHTPB, the increase in the vinyl-type functional distribution of HHTPB aids for better process-ability during propellant processing. However, the propellant formulation should be optimized with the help of plasticizers, solid loading, etc., to achieve the required properties.

Properties of high cis-1,4 content hydroxyl-terminated polybutadiene and its application in composite solid propellants

In this paper, high cis-1,4 content hydroxyl-terminated polybutadiene (cis-HTPB) with different molecular weights was prepared through the oxidative cracking process using cis-butadiene rubber as raw material. Firstly, this article comprehensively compared the differences between cis-HTPB and conventional I-HTPB in terms of molecular weight distribution, functionality, viscosity, molecular polarity, and other physicochemical properties, which provided effective data support for its subsequent application. In addition, the reaction kinetics study showed that cis-HTPB with isocyanate curing agent has high reactivity, allowing it to be rapidly cured at low temperatures, and the cured elastomers had excellent mechanical properties, with tensile strength and elongation up to 1.89 MPa and 1100%, respectively. It was also found that cis-HTPB has extremely excellent low-temperature resistance, and the glass transition temperature (Tg) of its cured elastomer is as low as −101 °C. Based on the above studies, cis-HTPB is applied as a binder in composite solid propellants for the first time to investigate its practical performance, and the results indicated that cis-HTPB-based propellants have excellent process and mechanical properties.

HTPB propellant binder supplemented with nitro potato starch: Formulation, characterization, and thermal decomposition behavior

This study conducted a detailed examination of the characteristics of hydroxyl‑terminated polybutadiene (HTPB) doped with potato starch nitrate (NPS) as a high-energy-density binder. The investigation specifically focused on evaluating the impact of NPS doping on the chemical structure, energetic attributes, and thermokinetic behavior of HTPB-NC binder systems. The findings demonstrated advantageous features, including favorable chemical compatibility, improved material density accompanied by remarkable thermal stability, substantial energy release, and high chemical reactivity. These findings contribute to the understanding of HTPB-NPS composite and offer evidence for its remarkable features, indicating promising prospects for its utilization in the development of advanced formulations for high-energy-density propellants in future applications.

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.

Study on compatibility of common propellant binders with ADN

In this article, we use molecular dynamics simulation and thermal decomposition analysis to determine the physical and chemical compatibility of ADN/binders systems. The experimental results show that the ADN/inert binder systems do not have good physical compatibility, but HTPB and CTPB can stabilize the thermal decomposition of ADN, improve safety of use, while PEG severely affects safety of use and belongs to a chemically highly incompatible system. The components of the ADN/azide binder systems have strong interactions between the components, among which the |Δδ| of the ADN/PBT system is 0.643 < 3 Mpa1/2, and PBT can stabilize the thermal decomposition of ADN, with great application potential. The interactions between the components of the ADN/nitrate ester-binder systems are smaller than that of the ADN/azide binder systems, but there are hydrogen bonding between the components. Theoretically, the ADN/nitrate ester binder systems have better physical compatibility. The comprehensive assessment of both physical and chemical compatibility assists in selecting binder systems that are more compatible with ADN propellants in practical applications.

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.

Isocyanate-free Carboxyl-terminated Polybutadiene and Epoxidised Hydroxyl-terminated Polybutadiene Cross-linking System as a Binder in Solid Heterogeneous Propellants

Isocyanate-free Carboxyl-terminated Polybutadiene and Epoxidised Hydroxyl-terminated Polybutadiene Cross-linking System as a Binder in Solid Heterogeneous Propellants

Synthesis of multifunctional additives for solid propellants: Structure, properties and mechanism

To simplify the composite propellant formulation and address the current issue of the single-functionality present in existing additives, the multi-cyano, amine-based polybutadiene (AEHTPB-CN) was prepared based on AEHTPB by adopting appropriate synthesis strategies. By replacing 10% of HTPB binder in the propellant formulation, it can effectively enhance the interfacial bond strength between the propellant binder matrix and solid fillers (AP (ammonium perchlorate) and RDX (cyclotrimethylene-trinitramine)), the mechanical properties of the HTPB/AP/RDX/Al propellant were superior to blank control propellant with an improvement of 35.4% in tensile strength, 62.0% enhancement in elongation at break, and reduce the propellant burn rate by 10.7% with any energy loss. The function mechanism of AEHTPB-CN was systematically elucidated through experiments and computer simulation techniques. The results show that the tertiary amine group in AEHTPB-CN can react with AP to form ammonium ionic bonds, and the hydroxyl and cyano groups can form hydrogen bonding interactions with AP, which enables AEHTPB-CN to be firmly adsorbed on the AP surface through chemical and physical interactions. For RDX, the interfacial bonding effect of AEHTPB-CN is attributed to their ability to form C–H⋅⋅⋅NC weak hydrogen bonding interaction between the cyano group and RDX methylene group.

A study on the decomposition pathways of HTPB and HTPE pyrolysis by mass spectrometric analysis

Hydroxyl-terminated polybutadiene (HTPB) and hydroxyl-terminated polyether (HTPE) are used as binders for rocket propellants, while the mechanisms for their pyrolysis processes are still not clear. In this work, the slow and fast pyrolysis behavior of HTPB and HTPE were studied with a combination of single photoionization mass spectrometry (SPI-MS) and GC-MS, respectively. SPI-MS allowed on-line studying the dynamics of volatile formation as a function of the pyrolysis process, and GC-MS was utilized to characterize the condensed products. Furthermore, the heavy products (m/z > 300) in the condensed liquids were also characterized by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS). Based on the experimental results, a full picture of the product distributions from the HTPB and HTPE pyrolysis was described. Meanwhile, the decomposition pathways were proposed and discussed in detail. The results showed that the radical-addition reaction dominated the decomposition pathways of HTPB, in which the six-membered ring products were readily formed. HTPE was mainly transformed into ethers or aldehydes during the pyrolysis process, and the cyclic products were mainly composed of tetrahydrofuran and its derivatives.

Efficient metal-free crosslinking of common propellant binders using nitrile oxide–alkene click ligation

A promising tritopic nitrile oxide with high stability and high reactivity was synthesized easily and applied in efficient metal-free crosslinking of alkenyl-terminated binders as a click ligation reagent.

Alkyl Vinyl Imidazolium Ionic Liquids as Fuel Binders for Photo-curable Energetic Propellants

Alkyl Vinyl Imidazolium Ionic Liquids as Fuel Binders for Photo-curable Energetic Propellants

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.

Examining the Effect of Trimethylol Propane Crosslinker on Butanetriol Trinitrate Plasticized Polycaprolactone Polymer Networks of Propellant Binder System

AbstractThe knowledge of polymer network characteristics is important to tune the properties of advanced energetic propellants based on nitrate ester plasticized polyol binder system. In this article, plasticized polyurethane networks of polycaprolactone (PCP) and hydroxyl‐terminated polybutadiene (HTPB) with different crosslinker content [0 to 0.20 parts] at various stoichiometric ratios (r‐value) [0.8 to 1.0] were studied. Five numbers of polyurethane networks of 1,2,4‐butanediol trinitrate (BTTN) plasticised PCP were formulated at each r value 0.8, 0.9 and 1.0 using 0, 0.05, 0.10, 0.15 and 0.20 parts of trimethylolpropane (TMP) crosslinker. Similarly, five numbers of dioctyl adipate (DOA) plasticized HTPB based polyurethane networks were formulated at r‐value of 0.8. FT‐IR spectra of these polyurethane networks (PUNs) were recorded for functional group identification. Stress‐strain behavior of PUNs was studied for understanding the effect of TMP content and r value on mechanical properties. Hard segment content of PUNs were calculated and its influence on network characteristics and mechanical properties was evaluated. CLD values (ν) increased with increase of hard segment content of PUN and the values quantified by different approaches followed the trend νe (by solvent swelling method) &gt; νne2(by Initial modulus method)&gt;νne1 (by Mooney Rivlin method).

Properties of Hydroxyl‐Terminal Polybutadiene (HTPB) and Its Use as a Liner and Binder for Composite Propellants: A Review of Recent Advances

AbstractThis paper reviews hydroxyl‐terminal polybutadiene (HTPB) used as liner and binder for composite propellants, applied in rocket motors studied throughout the last decades, emphasizing the recent advances. The contribution aims to show the importance of HTPB in the propellant composition and on the liner coating of the rocket motor, which is in contact with the propellant grain. This paper also shows that many researchers have thoroughly studied several properties of HTPB, which are relevant to its role in liners and propellants. We have also reviewed analytical techniques such as Fourier Transform Infrared Spectroscopy (FTIR) as a characterization technique for HTPB and HTPB‐based polyurethanes. Reflection and transfectants FTIR may be a future trend in studies about polymeric matrices for liners and propellants, and in the area of propellant formulations that use energetic binders, graphene and nanomaterials.

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).

Molecular dynamic simulations and experimental study on pBAMO-b-GAP copolymer/energetic plasticizer mixed systems

In order to investigate the effects of plasticizers on the binding energy, solubility parameter and mechanical properties of pBAMO-b-GAP copolymer (the copolymer of 3,3′-bis(azidomethy) oxetane and glycidyl azide polymer), the related copolymer/plasticizer mixed systems were established by molecular dynamics simulations. Besides, the mechanical properties of the formulas based on pBAMO-b-GAP containing the three energetic plasticizers were also studied experimentally. The results showed that all involved energetic plasticizers could be miscible with the pBAMO-b-GAP. The order of binding energy was theoretically found to be pBAMO-b-GAP / BuNENA > pBAMO-b-GAP/ TMETN > pBAMO-b-GAP/ DIANP which was consistent with the trends of the experimental tensile strength values of three mixed materials. Excitedly, the DIANP showed strongest effects among the three plasticizers on improving the toughness of the copolymer according to the calculation of Cauchy pressure and experimental value of elongation. For the pBAMO-b-GAP copolymer, the DIANP could be a promising plasticizer and compolymer/plasticizer mixed system was suitable used as binder in azido polyether-based propellants.

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.