Propellant Binder Research Articles

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.

Synergistically catalysed pyrolysis of hydroxyl terminated polybutadiene binder in composite propellants and burn rate enhancement by free-standing CuO nanoparticles

Free-standing nanoparticles are a ubiquitous requirement for high-efficiency catalytic applications. Here, we report the catalytic pyrolysis of hydroxyl terminated polybutadiene (HTPB) binder in composite propellant by free-standing CuO nanoparticles synthesized through a novel aqueous thermolysis synthetic route. The formation mechanism of CuO nanocatalyst and the reactions between precursors were followed up by analyzing the thermal events occurred and residual analysis. Composite solid propellant samples were prepared with 0.5%, 1% and 2% CuO nanocatalyst and the catalytic pyrolysis as well as burn rate studies were conducted. By the addition of 2% nanocatalyst, complete pyrolysis of the HTPB binder occurs and the burn rate increases by > 6% at ambient pressure. The catalytic effect of the CuO nanocatalyst on the high-temperature decomposition products of AP resulting in the formation of reactive intermediates and subsequent synergistic effect leads to the complete pyrolysis of the binder leading to the increase in the propellant burn rate.

Effect of various alcohols and reaction time on the properties of hydroxyl terminated natural rubber synthesized via oxidative degradation

The oxidative degradation of deproteinized natural rubber (DPNR) using cobalt bis(acetyl acetonate) in the presence of different alcohols and various reaction time; followed by reduction with sodium borohydride (NaBH4) for the synthesis of hydroxyl terminated liquid natural rubbers (HTNRs) was carried out. The synthesis of HTNRs from natural rubber is particularly important as an alternative to hydroxyl terminated polybutadiene (HTPB); one of the current leading synthetic rubber for the production of rubber binders for composite solid propellants. Four different alcohols (i.e. methanol, ethanol, propanol and hexanol) were investigated in order to achieve degraded DPNR with lowest molecular weight. The structure of the obtained HTNRs were analysed using gel permeation chromatography (GPC) and fourier transform infrared (FTIR). Based on GPC analysis, it was found that the use of ethanol at 1 h reaction time resulted in lowest molecular weight of approximately 6,690 g/mol. Analysis of chain scission of the HTNRs were consistent with the molecular weight attained. FTIR analysis, on the other hand confirmed the presence of hydroxyl functional group in the synthesized HTNRs with the highest amount of hydroxyl group presence in the HTNR prepared using ethanol. Overall, the properties of synthesized HTNRs (molecular weight and functional groups) displays a potential material for the production of propellant binder and are comparable to the properties of commercially available HTPB.

Pot life extension of hydroxyl terminated polybutadiene based solid propellant binder system by tailoring the binder polymer microstructure

ABSTRACTA possibility of extending the pot life of the HTPB-TDI based propellant binder system without adverse modification of mechanical properties is explored in the present study. It is proposed that by tailoring functionality distribution of the base HTPB polymer and changing the binder composition concurrently, the pot life of the binder system can be extended while the mechanical characteristics are kept within the acceptability window. Using an existing empirical relationship between fraction of high molecular weight (Fh) in HTPB and r-value corresponding to a more optimized set of mechanical properties, the r-values were calculated for different HTPB resins. HTPB resins with widely varying fractions of high molecular weight (Fh) were chosen and binder networks were prepared at different r values. Viscosity build up and chemical kinetics were studied for different formulations. From the studies, it is shown that the extension of pot-life is achievable by about 150 minutes without sacrificing the mechanical characteristics.

Structural and thermal degradation properties of novel metallocene-polyurethanes

Thermal degradation of two series of chemically well-defined polyurethane (PU) elastomers have been studied using thermogravimetry coupled with a mass spectrometer (TG-MS), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The samples were synthesised such that they contained two different soft segments (SSs) that are of great interest for aerospace applications. At a given soft fragment, four different hard segments (HSs) based on single diisocyanate molecules (i.e., isophorone diisocyanate (IPDI), toluene-2,4-diisocyanate (TDI), 4,4′-diphenyl methane diisocyanate (MDI) and hexamethylene diisocyanate (HMDI)) were used with no chain extenders. Each segmented PU family was prepared by stoichiometric reactions of macrodiol hydroxyl-terminated polybutadiene (HTPB) and a metallocene-derived one (i.e., (ferrocenylbutyl)dimethylsilane grafted on HTPB, which is also known as Butacene). The structure of the macroglycols and the prepared PUs were studied using Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies. Both the catalytic effect of the ferrocenyl group and the influence of the HS nature on the thermal behaviour of these PUs polymers have been elucidated. In addition, the gaseous volatile products released during the thermal decomposition process have been investigated, and these products are of interest in propulsion chemistry. Understanding the structure-property relationships of these advanced organometallic PUs is relevant for application as a solid composite propellant binder.

Preparation and curing behavior of high-stress solid propellant binder based on polydicyclopentadiene

The ring-opening metathesis polymerization reaction of dicyclopentadiene (DCPD) was carried out using Grubbs first generation catalyst. Fourier transform infrared (FTIR), dynamic-thermo mechanical analysis (DMA), and Raman spectroscopy were used to investigate the curing behavior of this polymer. The FTIR results showed that DCPD had not cured completely and the polymers were composed of linear and cross-linked polydicyclopentadiene (PDCPD). The DMA test showed that the polymer possesses the glass transition temperature of linear PDCPD and cross-linked PDCPD, which had also proved the FTIR result. Furthermore, in order to explain the strange phenomenon that the band at 3004 cm−1 should have been detected in infrared spectrum, the Raman spectrum of PDCPD was applied to analyze the bonding mechanism of =C–H bond in the process of polymerization. Moreover, the real-time FTIR result cure formula showed that the cure degree increases first then constants trend with cure time of increasing, the cure degree reached the maximum value (96.76%) at 60°C for 192 h.

Comparative Assessment of Stabilised Polybutadiene Binder under Accelerated Ageing

Polybutadiene elastomers are versatile materials, being employed at several applications from rocket propellant binder to adhesives and sealants. The elastomers derived from hydroxyl-terminated polybutadiene are usually stabilised with antioxidants to prevent degradation. In this study, a comparative assessment among 2,2'-methylene-bis (4-methyl-6-tert- butylphenol) (AO2246), 2,6-di-tert-butyl-4-methylphenol (BHT), p-phenylenediamine (pPDA), and triphenylphosphine (TPP) regarding stabilisation of hydroxyl-terminated polybutadiene binder under accelerated ageing (six months at 65 °C) was carried out. Evaluation of antioxidants effectiveness was exa- mined through Oxidation Induction Time, sol/gel extraction, swelling and mechanical testing, dynamic mechanical analy- sis, and mass variation measurement. AO2246 yielded the best performance, meanwhile BHT was poorly protective. TPP acted as prooxidant, causing a severe degradation of the binder, and pPDA was not manageable to be assessed due to the lower curing degree of the resulted polyurethane.

A Fast and Effective Pyridine-Free Method for the Determination of Hydroxyl Value of Hydroxyl-Terminated Polybutadiene and Other Hydroxy Compounds

ABSTRACTHydroxyl-terminated polybutadiene (HTPB) is the workhorse propellant binder for launch vehicle and missile applications. Accurate determination of the hydroxyl value (OHV) of HTPB is crucial for tailoring the ultimate mechanical and ballistic properties of the propellant derived. This article describes a fast and effective methodology free of pyridine based on acetic anhydride, N-methyl imidazole, and toluene for the determination of OHV of nonpolar polymers like HTPB and other hydroxyl compounds. This method gives accurate and reproducible results comparable to standard methods and is superior to existing methods in terms of user friendliness, efficiency, and time requirement.

Polybutadiene crosslinked by 1,3-dipolar cycloaddition: Pyrolysis mechanism, DFT studies and propellant burning rate characteristics

In a composite solid propellant, it is desirable to have energetic groups in a binder that contribute to the combustion characteristics. For this the conventional hydrocarbon binder hydroxyl terminated polybutadiene (HTPB) was chemically modified via a urethane route to derive azide and propargyl end capped polybutadiene namely azidoethoxy oxy carbonyl amine terminated polybutadiene (AzTPB) and propargyl oxy carbonyl amine terminated polybutadiene (PrTPB). The blend of the polymers underwent curing to form triazole–triazoline networks by 1,3-dipolar addition reaction.Thermal decomposition of the cured triazoline–triazole crosslinked network was investigated. The decomposition occurs in two stages. The first stage is attributed to urethane cleavage along with decomposition of triazoline–triazole. The second is due to polybutadiene backbone decomposition. Pyrolysis gas chromatography–mass spectrometry (GC–MS) studies at 300°C indicated that, the major products of decomposition for the first stage are nitrogen, N2, CO2, 3-isocyanato-4-methylbenzenamine, tolylenediisocyanate (TDI), propargyl alcohol and 2-(vinyloxy)ethanimine). The nature of the products reveals that urethane cleavage follows two different pathways. In the first case, the thermal dissociation of polyurethanes yielding alcohol and isocyanate occurs. The theoretical activation barrier for this reaction computed by density functional theory (DFT) is 197kJ/mol. The second pathway is through an internal elimination mechanism wherein an amine and alkene are formed with the liberation of carbon dioxide. The computed activation barrier for this step is 217kJ/mol. During cleavage of urethane, triazole–triazoline groups also undergo ring rupture. The triazole ring cleavage takes place through a two step process initiating with N–N cleavage with an activation energy of 144kJ/mol.The burn rate of the propellant was evaluated and a lower pressure exponent is exhibited. This could be directly correlated to the thermal decomposition pattern of the cured binder and propellant. This study provides insight for processing energetic solid propellants wherein the basic decomposition mechanism of the new triazole–triazoline crosslinked polybutadiene based propellant binder and ballistics of the solid propellant processed using the new binder.

Azidated Ether-Butadiene-Ether Block Copolymers as Binders for Solid Propellants

Polymeric binders for solid propellants are usually based on hydroxyl-terminated polybutadiene (HTPB), which does not contribute to the overall energy output. Azidic polyethers represent an interesting alternative but may have poorer mechanical properties. Polybutadiene–polyether copolymers may combine the advantages of both. Four different ether-butadiene-ether triblock copolymers were prepared and azidated starting from halogenated and/or tosylated monomers using HTPB as initiator. The presence of the butadiene block complicates the azidation step and reduces the storage stability of the azidic polymer. Nevertheless, the procedure allows modifying the binder properties by varying the type and lengths of the energetic blocks.

High Strain Rate and Shock Properties of Hydroxyl-Terminated Polybutadiene (HTPB) with Varying Amounts of Plasticizer

Hydroxyl-terminated polybutadiene (HTPB) has long been used as a binder in propellants and explosives. However, cured HTPB polyurethanes have not been characterized in a systematic fashion as a function of plasticizer content. In this study, three isocyanate-cured HTPB variants with different amounts of plasticizer were formulated. The materials were characterized across a range of strain rates from 10−3 to 106 s−1. Group interaction modeling (GIM) was used to predict the material behavior based on the underlying structure of the polymer. Increasing the amount of plasticizer was found to reduce the strength of the material across all strain rates. GIM was found to overpredict the modulus but predicted the shock response very well.

Viscoelastic behavior of carboxyl-terminated (butadiene-co-acrylonitrile)-based composite propellant binder containing polyglycidyl-type bonding agent

The influence of tris(2,3-epoxypropyl) isocyanurate as bonding agent on the physico-chemical, viscoelastic and uniaxial tensile mechanical properties of carboxyl-terminated (butadiene-co-acrylonitrile) cured with the polyglycidyl ether of glycerol and epichlorhydrin was investigated. Cross-link density values were estimated by swelling measurement. Temperature and frequency dependencies of rheological behaviour parameters (storage modulus, loss modulus, loss factor and glass-rubber transition temperature) were also analyzed. Based on the frequency dependencies of storage modulus, put in the range of temperature from -50 ?C to 20 ?C by the experiment, master curves (logG' vs log ?) were generated, reaching broader frequency interval in comparison to that used in the measurements. By choosing the glass-rubber transition temperature to be the reference one, Williams-Landel-Ferry equation constants were determined. Further, material constants, fractional free volume at the glass-rubber transition temperature and thermal coefficient of free volume expansion were calculated. Although small quantity of tris(2,3-epoxypropyl) affects the network density, the dynamic mechanical analysis showed that the bonding agent content did not affect the glass-rubber transition temperature of the tested materials.

Synthesis of GAP and PAMMO Homopolymers from Mesylate Polymeric Precursors

In azidic binders for solid propellants, the N3 functionality is introduced by substitution of a halogen or tosyl group, but recently the mesyl group has been suggested as an alternative. The mesylate group has two advantages, mainly related to its small dimensions and low cost. Poly(glycidyl azide) and poly 3-azidomethyl-3-methyl oxetane were prepared by using both tosylate and mesylate precursors. The azidation kinetics were studied at three different temperatures while keeping all other operating parameters the same. The results confirmed the good potential of the mesylate precursors for the production of azidic binders.

On the Thermal Stability of Partially Decomposed Ammonium Perchlorate

AbstractThe response of propellants and explosives to thermal stimuli is an important aspect of their behavior, especially for their safe storage and handling. In the case of ammonium perchlorate (AP), the principal oxidizer used in solid composite propellants, exposure to elevated temperatures causes various levels of decomposition and morphological changes. These changes occur in both neat AP particles and AP based propellants. The hazards associated with thermally damaged, or partially decomposed AP are investigated. It is found that AP is more thermally stable after partial low temperature decomposition. Also, it is found that the extent of decomposition of the AP particles is strongly influenced by prior thermal damage. This dependency is attributed to the exhaustion of nucleation sites on certain crystal planes. Specific surface area measurements of the thermally damaged particles show that the particles recrystallize before they can decompose to a further extent in low temperature decomposition. The behavior of partially decomposed AP in the presence of a propellant binder is also examined.

아자이드와 알킨의 1,3-쌍극자 고리첨가반응에서 고압이 반응속도에 미치는 영향에 대한 연구

The effect of pressure on 1,3-dipolar cycloaddtion has been studied by means of FT-IR and NMR spectroscopy. Pressure accelerates 1,3-dipolar cycloaddition without solvent or catalyst. This simple and inexpensive method eliminates the need for work-up or purification. The method is expected to be applied to the synthesis of binders for solid rocket propellants.

Synthesis and characterization of hydroxyl-terminated triblock copolymer of poly(glycidyl nitrate-block-butadiene-block-glycidyl nitrate) as potential energetic binder

Hydroxyl-terminated triblock copolymer poly(glycidyl nitrate-block-butadiene-block-glycidylnitrate) an energetic binder was synthesized by ring opening polymerization of glycidylnitrate in the presence of hydroxyl-terminated poly(butadiene)catalyzed by BF3 · OEt2. The different conditions of synthesis were used to improve the molecular weight and mechanical properties of binder. The resulted triblock copolymer was characterized by FTIR, 1H NMR, GPC and DSC. According to DSC, the glass transition temperature of synthesized block polymer is –27°C, which is suitable for use as an energetic propellant binder.

Molecular Dynamics Simulation on the Binder of Ethylene Oxide–Tetrahydrofuran Copolyether Cross-Linked with N100

Ethylene oxide–tetrahydrofuran copolyether (P(E-co-T)) crosslinked with isocyanate Desmodur (N100) is widely used as the binder system in solid energetic propellant. The effect of its cross-linking degree on the physical properties is important for the evaluation of the propellant binders. In this work, an efficient method was presented for simulating the cross-linking process, predicting the microscopic behaviors and macroperformances of cross-linked P(E-co-T)–N100 binder systems. During the simulation of cross-linking network forming, the initial physical mixture model of P(E-co-T)/N100 was firstly constructed and optimized through molecular dynamics. Then the possible cross-linking topology was generated by means of the identification of the reactive site pairs. In this way, the P(E-co-T)–N100 cross-linking pathway was realized by alternate structure optimization and junction reaction. The cross-linking intermediate models were analyzed, and the density and mechanical property profiles have revealed th...

Thermal decomposition properties and compatibility of CL-20 with binders HTPB, PBAN, GAP and polyNIMMO

The compatibility of filler 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) with rocket propellant binders: hydroxyl-terminated polybutadiene (HTPB), butadiene-acrylonitrile-acrylic acid terpolymer (PBAN), glycidyl azide polymer (GAP) and poly(3-nitratomethyl-3-methyloxetane) (polyNIMMO), has been examined. The compatibility of the compounds has been tested in accordance with the STANAG 4147 standard and its modification consisting in the change of the heating rate. As it arises from STANAG 4147 standard criterion, CL-20 is not compatible with polyNIMMO, PBAN and GAP and possibly incompatible with HTPB. Changes of relative position of peaks between measurements performed in hermetical pans and pans with a pinhole and with different heating rate were observed. In case of polyNIMMO and HTPB, changes of measurement parameters lead to estimated compatibility change. The analysis of the thermal decomposition of CL-20 revealed that it is a two-phase process. The first phase is associated with decomposition in solid phase; the second phase is associated with decomposition of volatile intermediate products of CL-20 decomposition. Due to the complex process of decomposition of tested samples, the apparent activation energy was used for the assessment of the compatibility. The apparent activation energy of the initial phase of decomposition CL-20 and its mixtures with binders are compatible with one another within the limits of measurement error. Results of measurement of apparent activation energy do not indicate a destabilizing effect of binders on the initial phase of decomposition of CL-20.

Influence of 1,4-butanediol on hydroxyl-terminated poly(butadiene) based composite propellant binder characteristics

1.4-butanediol, as a chain extender, was incorporated into a hydroxyl-terminated poly(butadiene) based composite rocket propellant binder composition. As curing agents, isophorone diisocyanate (IPDI) and toluene diisocyanate (TDI) were used. Composite propellant binder network and mechanical properties, influenced by a presence of 1,4-butanediol, were examined. Network characteristics, sol-gel content and crosslink density have been calculated and successfully correlated to the mechanical uniaxial tensile properties of the tested propellant binders. Differential scanning calorimetry studies showed that 1.4-butanediol content did not influence the glass transition temperature, however the uniaxial tensile properties were shown to be a function of the crosslink density.

Issues Related with Pot Life Extension for Hydroxyl‐Terminated Polybutadiene‐Based Solid Propellant Binder System

AbstractExtension of the pot life for hydroxyl‐terminated polybutadiene (HTPB) based propellant binder systems was attempted through employing a bicurative system comprising of toluene‐di‐isocyanate (TDI) and isophorone‐di‐isocyanate (IPDI). Pot life characteristics were studied through evaluating curing kinetics for the HTPB‐bicurative binder formulations. Curing kinetics were studied through viscosity buildup during the cure reaction and estimation of the rate constants for viscosity build‐up. Introduction of IPDI slowed down the curing process, but at the same time, has not adversely affected the mechanical characteristics. One of the discouraging factors in adopting IPDI as curative is the poor interface characteristics between liner and insulator of the solid propellant grains, and it has been proposed that the root cause for the same is the permeation of IPDI through the insulator. Quantification of such permeation was attempted from the numerical values for cross‐link density evaluated for various binder compositions. Remedy for poor interface characteristics is suggested in this study.