Energetic Thermoplastic Elastomers Research Articles

A Novel Method of Improving the Mechanical Properties of Propellant Using Energetic Thermoplastic Elastomers with Bonding Groups.

The relatively poor mechanical properties of extruded modified double base (EMDB) propellants limit their range of applications. To overcome these drawbacks, a novel method was proposed to introduce glycidyl azide polymer-based energetic thermoplastic elastomers (GAP-ETPE) with bonding groups into the propellant adhesive. The influence of the molecular structure of three kinds of elastomers on the mechanical properties of the resultant propellant was analyzed. It was found that the mechanical properties of the propellant with 3% CBA-ETPE (a type of GAP-ETPE that features chain extensions using N-(2-Cyanoethyl) diethanolamine and 1,4-butanediol) were improved at both 50 °C and -40 °C compared to a control propellant without GAP-ETPE. The elongation and impact strength of the propellant at -40 °C were 7.49% and 6.58 MPa, respectively, while the impact strength and maximum tensile strength of the propellant at 50 °C reached 21.1 MPa and 1.19 MPa, respectively. In addition, all three types of GAP-ETPE improved the safety of EMDB propellants. The friction sensitivity of the propellant with 3% CBA-ETPE was found to be 0%, and its characteristic drop height H50 was found to be 39.0 cm; 126% higher than the traditional EMDB propellant. These results provide guidance for studies aiming to optimize the performance of EMDB propellants.

Preparation and Properties of a Novel High-Toughness Solid Propellant Adhesive System Based on Glycidyl Azide Polymer-Energetic Thermoplastic Elastomer/Nitrocellulose/Butyl Nitrate Ethyl Nitramine.

Glycidyl azide polymer (GAP)-energetic thermoplastic elastomer (GAP-ETPE) propellants have high development prospects as green solid propellants, but the preparation of GAP-ETPEs with excellent performance is still a challenge. Improving the performance of the adhesive system in a propellant by introducing a plasticizer is an effective approach to increasing the energy and toughness of the propellant. Herein, a novel high-strength solid propellant adhesive system was proposed with GAP-ETPEs as the adhesive skeleton, butyl nitrate ethyl nitramine (Bu-NENA) as the energetic plasticizer, and nitrocellulose (NC) as the reinforcing agent. The effects of the structural factors on its properties were studied. The results showed that the binder system would give the propellant better mechanical and safety properties. The results can provide a reference for the structure design, forming process, and parameter selection of high-performance GAP-based green solid propellants.

Non-Isothermal Simulation and Safety Analysis of Twin-Screw Extrusion Process for Synthetizing Glycidyl Azide Polymer-Based Energetic Thermoplastic Elastomer.

In order to study the temperature variation and flow characteristics in the twin-screw reactive extrusion process of synthetizing glycidyl azide polymer-based energetic thermoplastic elastomer (GAP-ETPE), a non-isothermal simulation and a safety analysis were carried out. Firstly, based on the synthesis principle of GAP-ETPE, a mechanical sensitivity test, viscosity test and differential scanning calorimetry (DSC) of GAP-ETPE were carried out. Secondly, a three-dimensional physical model of the intermeshing co-rotating conveying element was established by Gambit. A three-dimensional non-isothermal numerical simulation of the conveying and kneading elements was carried out using FLUENT 19.0 software. The temperature, pressure and shear stress field of conveying and kneading elements with different staggered angles were analyzed and compared. The results show that the maximum temperature of the kneading element is always slightly higher than that of the conveying element at the same rotational speed, but the average temperature in the flow channel is always slightly higher than that of the kneading element. The inlet and outlet pressure difference of the kneading elements with a 90° offset angle is the smallest and the safety is the highest. The shear stress in the flow channel of the conveying element is higher than that of the kneading element as a whole, but the shear stress near the outlet of the 90° kneading element is higher than that in the flow channel of the conveying element. Among the kneading elements, the 90° kneading element has the strongest dispersing and mixing ability, followed by the 60° and 45° kneading elements. According to the thermal and physical parameters of the material, the ignition response time is approximately 6 s, which provides a theoretical guide for the safety design of the GAP-ETPE twin-screw extruder.

Influence of Solid Filler on the Rheological Properties of Propellants Based on Energetic Thermoplastic Elastomer.

Glycidyl azide polymer-energetic thermoplastic elastomer propellant (GAP-ETPE) has high development prospects as a green solid propellant, although the preparation of GAP-ETPE with excellent performance is still a challenge. Focusing on the demand of high-strength solid propellants for free-loading rocket motors, a GAP-ETPE model propellant with excellent overall performance was prepared in this work, and the influence of adhesive structure characteristics on its fluidity was studied. Furthermore, the influence of filler on the rheological properties of the model propellant was investigated by introducing hexogen (RDX) and Al, and a corresponding two-phase model was established. The results may provide a reference for the structural design, molding process, and parameter selection of high-performance GAP-based green solid propellants.

Effect of azide polyether pyrolysis property on combustion and heat release of boron-based fuel-rich propellant

Overcoming the severe particle agglomeration of unreacted fuel boron (B) in combustion process to enhance the heat release efficiency of B-based fuel-rich propellants is still a great challenge. Glycidyl azide polyether (GAP) was introduced to synthesize GAP-based energetic thermoplastic elastomer (GAP-ETPE) binder, which was used to fabricate the B/GAP-ETPE propellant. High GAP content (85 wt%) can be used to synthesize 85%GAP-ETPE as the binder. With the increase of GAP content, the decomposition conversion rate of GAP-ETPE was increased and the apparent activation energy (Ea) of B/GAP-ETPE propellant was decreased. The heat of explosion and combustion of B/85%GAP-ETPE were increased from 4004.1 kJ/kg to 4546.2 kJ/kg and 24,056.5 kJ/kg to 24,938.3 kJ/kg, respectively. The combustion results showed that the burning rate of B/85%GAP-ETPE (3.1 mm/s) was higher than that of B/TPE (1.8 mm/s) using a thermoplastic elastomer (TPE) without GAP, and the particle injection effect was significantly improved. The pressurization rate of B/85%GAP-ETPE (85 wt% is maximal GAP content in binder in current study) was 2.6 MPa/s during combustion in a closed bomb, which was higher than that of other prepared B/GAP-ETPE, showing the propellant using GAP-ETPE with high GAP content exhibited better gas formation and higher heat release, which provided good dispersion and less particle agglomeration of B particles when burning. This strategy enables B to be ejected more effectively in the combustion process to achieve a better combustion performance and heat release.

BAMO-THF copolymer-based energetic thermoplastic polyurethanes with a decent bonding property

Despite the high enthalpy of formation of poly(3,3-bis(azidomethyl)oxetane) (PBAMO), it is not readily applicable as a polymeric binder for propellants owing to its low chain flexibility and poor mechanical properties. The aim of this study was to improve the chain flexibility and mechanical properties of PBAMO binders by copolymerizing BAMO with tetrahydrofuran (THF) to produce a BAMO-THF copolyether (PBT). We report the preparation of energetic thermoplastic polyurethane elastomers (ETPUEs) by using PBT (soft segment), dicyclohexylmethylmethane-4,4′-diisocyanate (HMDI) (curing agent), a mixture of diols: diethyl bis(hydroxymethyl)malonate (DBM) (chain extender), 2-cyanoethyl-bis(2-hydroxyethyl)amine (CBA) (chain extender), and 1,4-butanediol (BDO) (chain extender). By systematically varying the content of the components, a series of ETPUEs was prepared. The results showed that with a fixed hard segment content, ETPUEs with CBA/BDO as chain extenders showed a stronger bonding property, lower glass transition temperature; improved mechanical properties (σm ​= ​7.60 ​MPa, εb ​= ​570.0%), and a higher enthalpy of formation (4015 ​kJ⋅mol−1). Moreover, when the ratio between CBA and BDO was 1:1, the binding strength between the binders and filler RDX was optimized, yielding a simulated propellant with decent mechanical properties (σm ​= ​5.92 ​MPa, εb ​= ​16.5%).

Preparation, Thermal and Sensitivity Properties of Nano‐Sized Spherical Nitrocellulose Composite Crystal

AbstractThe sensitivity study of propellants has been the concern of many researchers, and many achievements have been gained in this field. Nitrocellulose was replaced by some energetic thermoplastic elastomers to prepare insensitive high energy propellants, which is an effective method for reducing the sensitivity of propellants. However, the propellants based on these binders still have some defects. Preparation and application of insensitive nitrocellulose may overcome these drawbacks and becomes a potential candidate to be used as an ingredient of insensitive high energy propellants without energetic thermoplastic elastomers. To reduce the sensitivity of nitrocellulose, a nano‐sized spherical nitrocellulose composite crystal was prepared by pretreatment, degradation and nitration of microcrystalline cellulose. The products were characterized by TEM, FT‐IR, XRD, XPS, elemental analysis, TGA, DSC and sensitivity tests. Results show that the prepared nano‐sized cellulose is spherical with a diameter in the range of 15–30 nm and displays a composite crystal with two kinds of crystalline structure. The nano‐sized spherical nitrocellulose composite crystal has a higher crystallinity and nitrogen content and less impact sensitivity in comparison with the common nitrocellulose. The improvement of crystal quality of nitrocellulose using composite crystal method is beneficial for reducing the impact sensitivity of nitrocellulose.

Effects of binder on molding properties of HATO-based explosives

The safety performance of HATO explosives coated by Fluororubber and energetic thermoplastic elastomer ETPE were discussed, and it was analyzed that the influence of pressure and temperature on the moldability of HATO-based explosives with different binders. The results showed: the mechanical sensitivity of HATO explosives coated by binder were low, which improved the safety of HATO explosives to a certain extent; Pressure and temperature have a greater impact on the mold-ability of HATO-based mixed explosives containing Fluororubber or ETPE binder, and the mold-ability of HATO-based mixed explosives containing ETPE binder is better, which relative density can reach more than 98% at room temperature.

Study of the enhanced burning rate during the plasma-propellant interaction process

Plasma ignition technology has been widely used for decades due to its great improvement in enhanced burning rate. However, the interaction process between the plasma jet and the solid propellant is still far from being elucidated. In this paper, coupling the flow model of plasma jet and the chemical reaction model of propellants, a more comprehensive model for predicting the plasma enhanced burning rate is presented. Three types of energetic thermoplastic elastomers propellant are involved. In addition, the closed bomb vessel test is conducted to investigate the burning characteristics of propellant under the action of plasma for comparison. It demonstrates that the enhanced combustion phenomenon of propellants in the experiment can be well described by an enhanced burning rate coefficient epsilon calculated with the model, which could reflect the effect of plasma jet to some extent. The results show that the composition of the energetic material and its surface temperature are the key factors influencing the plasma-enhanced combustion. This model would bring some insight on the interaction mechanisms between plasma and different propellants.

The study of mechanical and creep properties of glycidyl azide polyol energetic thermoplastic elastomer binder with bonding group with RDX and its interface reinforcement mechanism

Bonding agents as additional modifiers are used to improve the reinforcement effect between binder and high solid filler in propellant formula. However, polar bonding agents are easily soluble with polar plasticizer and weak bonding effect. Thus, grafting the bonding group into binder to form neutral polymeric is a novel approach to avoid the preceding shortcomings mentioned and is also meaningful to improve interface properties of propellant. In this work, glycidyl azide polyol energetic thermoplastic elastomer binder with –CN bonding group (named G-1binder) was synthesized and the influences of –CN bonding groups on the mechanical and creep resistance properties of G-1/RDX composites were studied. Then, the mechanism of interfacial interactions between binder with –CN group and cyclotrimethylenetrinitramine (RDX) were also illustrated through Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy and contact angle analyzer. Results demonstrated G-1/RDX propellants have stronger interfacial reinforcement effect and good mechanical and creep resistant properties. As well, the induced effect model was proposed, and the binding energy of interface was calculated by Materials Studio software. Computational results show that G-1/RDX propellant has a high binding energy and strong interactive force due to the existence of induced effect between –CN group and –NO2 group, which resulted in an increase in constrained regions around the RDX surface and led to the significant promotions of mechanical and creep resistance properties of G-1/RDX propellant.

Synthesis, Thermal Properties and Sensitivity of Ladder‐Like Nitrocellulose Grafted by Polyethylene Glycol

AbstractThe sensitivity study of propellants has been receiving heightened attentions from a lot of investigators. These propellants based on nitrocellulose are sensitive to accidental stimuli that result in catastrophic accidents. Thus energetic thermoplastic elastomers have been used as binders to replace some nitrocellulose to prepare insensitive high energy propellants. However, some defects are presented in the application of these binders. Based on the reason above, a novel binder named as step ladder‐like nitrocellulose was synthesized by isophorone diisocyanate reacting with free hydroxyl groups of nitrocellulose first, and then reacting with polyethylene glycol. The reaction product was characterized by FT‐IR, XRD, SEM, TGA, DSC and sensitivity tests. Results show that step ladder‐like nitrocellulose is successfully synthesized from nitrocellulose, and it has a quasi‐spherical shape and regular crystal structure. Furthermore, the thermal weight loss course of step ladder‐like nitrocellulose is relatively gentle in the range of 175–450 °C and its mechanical sensitivity is less than that of raw nitrocellulose.

Improvement of the creep resistance of glycidyl azide polyol energetic thermoplastic elastomer-based propellant by nitrocellulose filler and its mechanism

To investigate the effects of nitrocellulose (NC) content on the creep properties of glycidyl azide polyol energetic thermoplastic elastomer (GAP-ETPE)-based propellant, creep behaviour was studied by rotary rheometer under various shear stress and temperatures. Experimental results showed that creep strain and creep rate of samples increase with the increasing shear stress level, and similar creep behaviour tendency was observed under elevated temperature. When filled with 1 and 2 wt% NC, the creep strain and creep rate of GAP-ETPE/NC-based propellant were significantly less than the unfilled state, indicating that the NC contents partly inhibited the mobility of the binder segment and improved the creep resistance of GAP-ETPE-based propellant. Moreover, the constitutive equations of the creep curves were obtained from the four-element mechanical model, and the fitting results coincided well with the experimental data. Among the fitting parameters, the elastic modulus of high elastic deformation, E2, and the viscosity of Voigt units, η2, were sensitive and promising for the evaluation of creep strain of the GAP-ETPE-based propellant. Finally, the creep resistance model was established and verified by Fourier transform infrared spectroscopy, glass temperature and physical crossing point density of GAP-ETPE/NC-based propellant. Upon analysis of the experimental data, we concluded that the creep resistance mechanism of GAP-ETPE/NC-based propellant came from NC altering the flow lines of the continuous phase and restricted chain mobility in the propellant, which improved the creep resistance.

Effect of nitrocellulose (NC) on morphology, rheological and mechanical properties of glycidyl azide polymer based energetic thermoplastic elastomer/NC blends

As a new kind of propellants binder, energetic thermoplastic elastomer (ETPE) can make propellant recyclability and environment friendly disposal. The rheological behavior of ETPE binder could be beneficial to identify suitable and safe conditions for processing ETPE propellants. In this paper, ETPE/NC blends with different mass ratios of NC to ETPE were prepared by physical mixing method. The heat of explosion, morphological, thermal, mechanical and rheological properties of the resulted blends were systematically studied. It was found that the heat of explosion of ETPE/NC blends increased with the increasing of NC content. The scanning electron microscopic (SEM) images showed that the NC domains in the blends changed from tiny pieces to fibers with the NC mass ratio increasing, which indicates phase separation in the blends. The tensile mechanical properties of the blends had a peak value when the NC content was 10 wt.%, and then increased with the increasing addition of NC. The thermal behavior made clear that the ETPE and NC were partially miscible. Rheological studies on dynamic strain sweep and frequency sweep demonstrated that the content of NC of the blends had a monotonically effect on the rheological properties of the blends at 130 °C. Rheological studies also showed that the rheology of blends is dependent on temperature. The Cole-Cole and Han plots also confirmed phase separation in the blends.

A kind of bonding functional energetic thermoplastic elastomers based on glycidyl azide polymer

A kind of bonding functional energetic thermoplastic elastomers (ETPEs) were synthesized as the binder of solid propellants using the mixture of chain extenders including diethyl bis(hydroxymethyl) malonate (DBM). The results showed that the mechanical properties of ETPEs decreased with increasing percent of DBM in the chain extenders. On the contrary, the work of adhesion between solid ingredients hexogen (RDX) and ETPEs increased. In order to test the comprehensive impacts of two elements, the RDX/ETPE propellants were prepared. The results showed when the percent of DBM was 25%, the overall properties of the propellants were optimum.

Characterization of P(BAMO/AMMO) ETPE Prepared Using Different Diisocyanates

AbstractPoly(3,3‐bisazidomethyl oxetane/3‐azidomethyl‐3‐methyl oxetane) energetic thermoplastic elastomers (P(BAMO/AMMO) ETPEs) is one of the most valuable ETPEs in the field of energetic binders. P(BAMO/AMMO) ETPEs were prepared using different diisocyanates (TDI, HMDI, IPDI, and HDI) to investigate the influence of the diisocyanate on the performance of P(BAMO/AMMO) ETPEs. Mechanical properties and heats of formation were investigated. FT‐IR spectroscopy results showed that TDI‐based ETPE has the highest degree of hydrogen bonding with a value of 69.00 %. Mechanical test results showed that the TDI‐based ETPE has better mechanical property with maximum stress at 5.24 MPa and breaking elongation at 390 %. The order for degree of hydrogen bonding and mechanical property of different diisocyanate‐based ETPEs was TDI>HMDI>IPDI>HDI. The heats of formation were calculated by the group additivity method and by the heat of combustion method. The values of heats of formation for TDI‐based ETPE were 3.44 kJ g−1 and 3.75 kJ g−1 according to the two methods. Additionally, TDI‐based ETPE has a lager heat of formation than the other ETPEs.

Effect of energetic materials wettability on their outdoor effective elution rate

Energetic materials (EM) contained in military ammunitions have been found in the surface soil and water of training areas and may potentially represent a threat to human health and the environment. EM wettability is an essential physical parameter to characterize because it controls EM dissolution rate. This paper was conducted to determine the wettability of conventional and new EM formulations used in military ammunition. Wettability was estimated in the laboratory via contact angle measurements of water droplets on different EM surfaces. Results show that 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazinane (RDX), Octol and energetic thermoplastic elastomer (ETPE) 1000 are hydrophilic while Composition B, XRT, GIM, CX-85, ETPE 2000, and C4 are hydrophobic whereas HELOVA gun propellant has a mixed wettability oscillating between hydrophilic and hydrophobic. The present study demonstrates that wettability of EM formulation is generally controlled by their matrix constituents. Results indicate that hydrophobic formulations have a much slower outdoor environmental effective elution rate than hydrophilic ones, with the exception of the hydrophobic C4 formulation whose elution rate is extremely high. The addition of hydrophobic components into EM formulations is recommended to diminish the environmental impact on water, as it has already been done with XRT, GIM and CX-85 formulations.

Thermal decomposition kinetics of poly(3,3′-bisazidomethyl oxetane-3-azidomethyl-3′-methyl oxetane)

Poly(3,3′-bisazidomethyl oxetane-3-azidomethyl-3′-methyl oxetane) [P(BAMO/AMMO)] energetic thermoplastic elastomer (ETPE) is one of the most promising binders for the propellant and explosive formulations. It is synthesized with different content of hard segment and molar ratio of PBAMO and PAMMO. The results of mechanical test show that with higher content of hard segment and larger content of PBAMO, the ETPEs obtain higher tensile strength and lower breaking elongation. The thermal kinetics of the first decomposition stage of P(BAMO/AMMO) is investigated and the calculated apparent activation energy (E a) is about 169 kJ mol−1 by multi-heating rate method. In the single-heating rate study, f(α) = 1 − α is found to be the most probable mechanism function. Kinetic compensation effects are studied for the validation of the most probable mechanism function, and the results show that f(α) = 1 − α is quite suitable at a lower extent of conversion (α), but $$ f(\alpha ) = \frac{2}{3}\left( {1 - a} \right)\left[ { - { \ln }\left( {1 - a} \right)} \right]^{{ - \frac{1}{2}}} $$ is more fit when α is larger. P(BAMO/AMMO) ETPE was prepared, and the thermal decomposition kinetic of the first decomposition stage (mainly the thermal decomposition of –N3 group) was investigated. f(α) = 1 − α was a fitting mechanism function at a lower α, and $$ f(\alpha ) = \frac{2}{3}(1 - a)[ - \ln (1 - a)]^{{ - \frac{1}{2}}} $$ was more suitable when α was higher.

Synthesis and characterization of novel energetic thermoplastic elastomers based on glycidyl azide polymer (GAP) with bonding functions

The energetic thermoplastic elastomers (ETPE) of glycidyl azide polymer with bonding functions were synthesized by using mixture of chain extenders including 1,4-butanediol and N-(2-cyanoethyl) diethanolamine. From FTIR results, the ability of ETPEs to form hydrogen bond weakened with the percentage of N-(2-cyanoethyl) diethanolamine increasing, which further lead to the decrease of maximum stress of ETPEs and increase of elongation at break. On the other hand, the interfacial interactions between solid ingredients and ETPEs were enhanced owing to the more –CN content. With the percentage of N-(2-cyanoethyl) diethanolamine increasing, two factors affect the adhesion between binder and fillers at the same time. The RDX/ETPE propellants were synthesized and the mechanical properties of them showed that the ETPE-50 with bonding functions can effectively prevent the dewetting, and then improve the mechanical properties of propellants. That is, when the percentage of N-(2-cyanoethyl) diethanolamine was 50 %, the synthesized ETPEs had not only binding function but also bonding interaction.

Polyglycidyl nitrate (PGN)‐based energetic thermoplastic polyurethane elastomers with bonding functions

ABSTRACTPGN‐based ETPUEs were synthesized using mixture of chain extenders including 1, 4‐butanediol and Diethyl Bis(hydroxymethyl)malonate (DBM). Through the special chain extenders DBM, the –COOR was introduced into the energetic thermoplastic polyurethane elastomers (ETPUEs) and further enhances the adhesion between ETPUE and nitramine solid ingredients in propellants. From the analysis, with the percentage of DBM increasing, the work of adhesion (Wa) between nitramine solid ingredients and ETPUEs increased and the maximum stress (σm) of ETPUEs decreased on the other hand. In order to test the bonding functions of different ETPUEs, the RDX/ETPUE propellants were prepared and the stress–strain curves of all propellants were tested. The results showed that the ETPUE‐75 with 75% DBM can prevent the dewetting and improve the mechanical properties of propellants. The ETPUE prepared with chain extender including 1, 4‐butanediol and DBM were valuable for application in propellants. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2015,132, 42026.

Thermal decomposition of energetic thermoplastic elastomers of poly(glycidyl nitrate)

ABSTRACTEnergetic thermoplastic elastomers (ETPEs) are futuristic binders for propellant/explosive formulations. Poly(glycidyl nitrate) (PGN)‐based ETPEs have excellent performance, including a high energy and high oxygen content. PGN‐based ETPEs were synthesized on PGN as a soft segment and hexamethylene diisocyanate extended 1,4‐butanediol as a hard segment by a prepolymerization method. The thermal behavior of the PGN‐based ETPEs was investigated by thermogravimetric analysis (TGA) and derivative thermogravimetry. A fitting strategy was adopted to study every stage of decomposition. The results show that the ETPEs had four main decomposition processes, and the peaks of each stage were at 212, 262, 322, and 414°C. The gas products were tested by TGA/Fourier transform infrared spectroscopy/mass spectrometry, and the main gas products of the samples were N2O, CH2O, C2H4O, and CO2. The previous results indicate the proposed mechanism of thermal decomposition. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2014,131, 40965.