Process and performance of DAAF microspheres prepared by continuous integration from synthesis to spherical coating based on microfluidic system

Bidong Wu ,Jiahui Shi ,Mengsen Wei ,Rui Zhu ,Yi Liu ,Jinqiang Zhou ,c,Chongwei An , Jingyu Wang

a School of Environment and Safety Engineering, North University of China, Taiyuan 030051, China

b Shanxi Engineering Technology Research Centre for Ultrafine Powder, North University of China, Taiyuan 030051, China

c School of Mechatronical Engineering, Beijing Institute of Technology, Beijing 100081, China

Keywords:DAAF Micromixing technology Microdroplet technology Synthesis Spherical coating Continuousization

ABSTRACT In order to improve the energy output consistency of 3,3′-diamino-4,4′-azoxyfurazan(DAAF)in the new insensitive booster and the safety and efficiency in the preparation process, a continuous preparation system of DAAF from synthesis to spherical coating was designed and established in this paper, which combined ultrasonic micromixing reaction with microdroplet globular template.In the rapid micromixing stage, the microfluidic mixing technology with ultrasonic was used to synergistically strengthen the uniform and rapid mass transfer mixing reaction between raw materials to ensure the uniformity of DAAF particle nucleation-growth, and to prepare high-quality DAAF crystals with uniform structure and morphology and concentrated particle size distribution.In the microdroplet globular template stage,the microfluidic droplet technology was used to form a droplet globular template with uniform size under the shear action of the continuous phase of the dispersed phase solution containing DAAF particles and binder.The size of the droplet template was controlled by adjusting the flow rate ratio between the continuous phase and the dispersed phase.In the droplet globular template, with the diffusion of the solvent in the dispersed phase droplets, the binder precipitates to coat the DAAF into a ball, forming a DAAF microsphere with high sphericity, narrow particle size distribution and good monodispersity.The problem of discontinuity and DAAF particle suspension in the process was solved,and the coating theory under this process was studied.DAAF was coated with different binder formulations of fluororubber(F2604), nitrocellulose (NC) and NC/glycidyl azide polymer (GAP), and the process verification and evaluation of the system were carried out.The balling effects of large, medium and small droplet templates under different binder formulations were studied.The scanning electron microscope(SEM)results show that the three droplet templates under the three binder formulations exhibit good balling effect and narrow particle size distribution.The DAAF microspheres were characterized by powder X-ray diffraction(XRD), differential scanning calorimetry (DSC), thermo-gravimetric (TG) and sensitivity analyzer.The results showed that the crystal structure of DAAF did not change during the process, and the prepared DAAF microspheres had lower decomposition temperature and lower mechanical sensitivity than raw DAAF.The results of detonation parameters show that the coating of DAAF by using the above three binder formulations will not greatly reduce the energy output of DAAF, and has comparable detonation performance to raw DAAF.This study proves an efficient and safe continuous system from synthesis to spherical coating modification of explosives, which provides a new way for the continuous, safe and efficient preparation of spherical explosives.

1.Introduction

With the continuous development of the military industry, the destructive power of weapons systems is no longer the only pursuit, the safety of weapons and ammunition has become an important indicator of ammunition development [1].Improving the safety of explosive formulations is one of the methods to improve the safety of weapons and ammunition [2,3].The insensitive booster explosive plays a key role in this.The insensitive booster explosive is the main charge of the detonation weapon system.It is a metastable material with high sensitivity, high reliability and high reaction speed.It has the dual role of energy transfer and energy amplification in the weapon system.Its reliability and high energy output have always been the focus of attention [4].3,3′-diamino-4,4′-azoxyfurazan (DAAF) is a new insensitive booster explosive with great application prospect.Its crystal density is 1.747 g/cm3, heat of formation is 443 kJ/mol1,impact sensitivity is greater than 320 cm (2.5 kg drop hammer),safety performance is equivalent to that of wood explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), and it is greener than TATB synthesis process.The electrostatic spark sensitivity(0.0625 J), detonation velocity (8.3 km/s), detonation pressure(27.9 GPa) and critical diameter (1.25 mm) are comparable to typical pentaerythrite tetranitrate(PETN)explosives, and have the characteristics of insensitivity and energy amplification [5-9].Therefore, DAAF has great research and application value in the charge application of insensitive booster with good detonation performance, high output energy and high safety performance.Some studies have pointed out that in the process of synthesis and refinement of insensitive explosives,whether the reactants can be quickly and uniformly mixed directly affects the purity, particle morphology, crystal quality and particle size distribution of the products, and the crystal morphology of explosives has a great influence on its safety and detonation performance [10-15].Furthermore,coating the explosive particles into spherical particles is beneficial to reduce the mechanical sensitivity of explosives and improve the charge performance.At present,in the preparation of DAAF,the conventional stirring operation cannot achieve the effect of rapid mixing of reactants, and there are different concentration gradients at various places,so it is difficult to prepare products with uniform particle size distribution and high quality crystallization[16].Moreover,the sphericization of the product requires two steps of refinement and coating,which is inefficient and the quality of the coated ball is not high.In summary, it is necessary to carry out effective morphological regulation and efficient continuous preparation of DAAF in the synthesis and coating process to make it suitable for engineering applications.

Microfluidic technology is an effective way to synthesize energetic materials and prepare energetic materials with high sphericity in recent years, corresponding to micromixing technology and microdroplet technology, respectively [17-20].Zhu et al.developed an effective and safe micromixing reaction system to prepare trinitroresorcinol barium (BaTNR) and trinitroresorcinol lead(LTNR)particles with better crystal morphology,narrower size distribution and higher heat release, and a microfluidic platform combining a vortex microchannel reactor with a single circular chamber and a microfluidic oscillator was designed to prepare nanoscale hexanitrostilbene/octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazine (HNS/HMX) composite explosive particles [21,22].HAN et al.used microdroplet technology to prepare HNS energetic microspheres with high sphericity using NC as binder, which enhanced the fluidity and bulk density of HNS [23].Our research group used a droplet microfluidic platform to prepare high-quality HMX/TATB microspheres and one-step rapid preparation of hexanitrohexaazaisowurtzitane/2,4,6-Trinitrotoluene (CL-20/TNT) cocrystal assembly and spherical coating.The samples have a narrow particle size distribution and high sphericity, which improves the comprehensive performance and safety of explosives[24,25].In addition,the introduction of ultrasonic technology in micromixing technology can further improve the mass transfer effect and regulate the crystal morphology.Therefore, the micromixing technology with ultrasonic introduction has the characteristics of rapid mixing and rapid mass transfer and heat transfer,which can produce uniform supersaturation to generate energetic material particles with uniform particle size and morphology and improve the safety in the synthesis process of energetic materials [26].The characteristics of microdroplet technology that can produce droplet templates are suitable for the preparation of energetic material microspheres, and the preparation process parameters at the micro-scale are easy to control, which can regulate the size of microspheres and improve the physical and chemical properties of energetic materials [27].

Therefore,in this study,the ultrasonic micromixing technology and microdroplet technology were skillfully combined into the preparation process of DAAF, and the microfluidic process system was designed to realize the continuous controllable preparation of DAAF from synthesis to spherical coating.The high-throughput synthesis of raw DAAF with high quality and uniform morphology and particle size distribution was obtained.At the same time, DAAF spherical coating products with uniform particle size distribution and excellent performance level were continuously obtained.In the process, two high-quality continuous preparations of DAAF crystal and spherical coating DAAF products were realized to improve their preparation efficiency,direct application,energy output consistency and application safety.In this paper,the synthesis process of DAAF and the assembly process of coated spheres were studied, which solved the problems of discontinuity and uniform suspension of DAAF in the continuous process.DAAF spherical coated samples with different sizes under different binder formulations of fluororubber (F2604), nitrocellulose (NC) and NC/glycidyl azide polymer(GAP)were prepared[28].The morphology,crystal structure, thermodynamic parameters, bulk density, mechanical sensitivity and detonation parameters of the samples were studied.DAAF spherical coated samples with different sizes under different binder formulations of F2604, NC and NC/GAP were prepared.The morphology, crystal structure, thermodynamic parameters, bulk density, mechanical sensitivity and detonation parameters of the samples were studied.This technology provides a method for the continuous,efficient and high-quality preparation of DAAF from synthesis to spherical coating,with a view to applying it to the engineering application of new insensitive booster explosive.

2.Experiment

2.1.Material

3,4-diaminofurazan (DAF), self-made (purity 99.93%); NaHCO3,analytical purity, Guoyao Group Chemical Reagent Co., Ltd.Potassium persulfate composite salt (OXONE?), analytically pure,Shanghai McLean Biochemical Technology Co., Ltd.Suspension agent solution, self-made.NC (12% N, industrial grade), industrial grade, Shanxi North Xing 'an Chemical Industry Co., Ltd.GAP,number-average molecular weight(M X n=3502), hydroxyl value of 30.41 (mg KOH)/g, Luzhou North Chemical Industry Group Co.,Ltd.F2604was purchased from Foshan Junyuan Chemical Co, Ltd.Ethyl acetate (EA) was supplied by Tianjin Beichen Founder Chemical Co., Ltd.Sodium dodecyl benzene sulfonate (SDBS) was provided by Tianjin Kaitong Chemical Reagent Co, Ltd.

2.2.Design of continuous microfluidic system

Fig.1.(a) Schematic diagram of continuous microfluidic system: (a1) Ultrasonic micromixing reaction part; (a2) Droplet formation part; (b) RAW DAAF collection suspension device; (c) Micromixing chip and mixing reaction diagram; (d) Microdroplet chip and droplet template size control diagram.

Fig.1 shows the schematic diagram of the designed microfluidic system.The system was mainly composed of fluid drive unit,micro mixing reaction unit,micro droplet unit and auxiliary components(Fig.1(a)).The working process of the system was as follows: The raw material DAF/NaHCO3solution and oxidant solution are pumped into the ultrasonic micro-mixing reaction system by injection pump for reaction,and the synthesized DAAF is collected in a collecting cup with suction filtration function, and the DAAF is fully washed and filtered.Then use a silicone pad to seal the mouth of the filter bottle to ensure that there is no leakage when adding solvent.Add the solvent and suspending agent dissolved with adhesive into the collecting bottle, and use the stirrer to fully stir to ensure that DAAF particles are evenly suspended in the solvent(Fig.1(b)).Furthermore, the dispersed phase solution of DAAF and the continuous phase solution containing surfactant were pumped into the microdroplet chip through the distribution of peristaltic pump and injection pump to produce DAAF droplet microspheres.The droplets gradually spread with the solvent to solidify into microspheres, and DAAF microspheres were collected to obtain samples.

In the DAAF synthesis stage(Fig.1(a1)), the system was mainly divided into a raw material mixing reaction zone and a crystal nucleation growth zone for the micromixing reaction chip, and a constant temperature field and an ultrasonic field were provided for the two zones in an ultrasonic water bath atmosphere throughout the process.The raw material DAF buffer solution and the oxidant solution were input into the mixed reaction zone through a polytetrafluoroethylene(PTFE)tube(1 mm×1.6 mm)by an injection pump to uniformly mix and react.A passive vortex micromixing reaction chip was used to mix the raw materials in the system.The chip was made of glass, as shown in Fig.1(c).The mixing function in the chip was realized by four cylindrical chambers with a radius of 2.5 mm,each of which was arranged in a staggered manner.The cavities were connected by a microchannel with a width of 0.5 mm,and the microchannel inlet connecting the cavities was 3/4 circumference away from the outlet.The design aims to form a vortex chaotic flow in the circular cavity first after the collision of the two-phase fluid,so that the liquid can be folded and compounded.At the same time, the contact interface of the reactants was expanded, and then the fluid was sheared and split through a narrow microchannel and enters the next circular chamber for mixing.Ultrasonic was introduced in the mixing process.Ultrasonic cavitation gathers the dispersed ultrasonic energy near the tiny bubbles, resulting in severe acoustic flow and shock wave, causing the ultrasonic mechanical effect of fluid turbulence, thus promoting the mixing mass transfer and further strengthening the mixing effect between reactants.In this way,efficient and uniform mixing of two-phase microfluidics was achieved.Further, the fully mixed solution was continuously flowed and reacted in a PTFE tube driven by external pressure.As the reaction continues, the DAAF molecules continue to increase, and their saturation in the solution continues to increase.When it reaches the vicinity of the crystal nucleation growth zone, DAAF reaches the supersaturation of precipitation, which promotes the nucleation and precipitation of DAAF crystals in the coil(1 mm× 1.6 mm × 5 m PTFE tube) and' confined' growth.In the process of crystal nucleation and growth.If the length of coil tube is too short,DAAF cannot be fully crystallized in the microchannel.If it is too long,the pressure in the tube will increase and the conditions of flow reaction will be affected,and the adhesion of DAAF crystal in the tube will affect the yield.The effect of ultrasound will have a beneficial effect on the crystal morphology and particle size, and promote the formation of DAAF crystals with uniform particle size and morphology.

In the DAAF coating stage (Fig.1(a2)), the system was mainly divided into droplet generation unit of droplet chip and microsphere collection unit of thermostatic magnetic stirrer.The droplet generation unit was a fluid-focused microdroplet chip made of hydrophilic glass.The width and depth of the dispersed phase channel were 350 μm and 200 μm, respectively.The width and depth of the continuous phase channel and the cross-exit channel were 500 μm and 500 μm, respectively.When the fluid feed begins,the surfactant solution of the continuous phase and the DAAF suspension of the dispersed phase converge into the droplet chip through a PTFE tube.At the cross channel,the dispersed phase droplets were formed in the microchannel due to the shear force of the continuous phase relative to the dispersed phase.The size of the droplet template was controlled by adjusting the flow rate ratio between the continuous phase and the dispersed phase to control the size of the DAAF microspheres., as shown in Fig.1(d).The droplets flow along the direction of continuous phase flow,and the droplets gradually solidify into spheres during the flow.A magnetic stirrer in the collection unit was set at the appropriate temperature and speed to collect the microspheres for further collection and solidification in an aqueous solution in a beaker.

The modularization of the system and the independent control of the feed fluid provide a multifunctional and adjustable process characteristic.The system has the advantages of simple structure,convenient operation, high preparation efficiency, high product quality and easy continuous amplification production.The high specific surface area of the microchannel can quickly guide away the heat generated in the reaction, reduce the accumulation of reaction heat in the preparation process,facilitate human-machine isolation, reduce the intermittent operation of 'synthesis - refinement - coating' in the conventional operation, and significantly improve the safety of the energetic material manufacturing process.Therefore, this method has good economic benefits and application prospects.

2.3.Experimental steps

4 mmol DAF and 12 mmol NaHCO3were placed in the same beaker, and 20 mL deionized water was dissolved to no solid precipitation.6 mmol potassium monopersulfate (OXONE?) was weighed, and 20 mL deionized water was used to completely dissolve it.The above two solutions were pumped into two syringes and fixed on the injection pump.The micro-reaction system was connected.The flow rate of the injection pump was set so that the two solutions could be fed at the same time.The water bath temperature was set.The micromixing chip, outlet coil and collection bottle were placed in the water bath.After the temperature was raised to the required temperature, the ultrasonic and injection pump were started to feed.After the feed end,the collected bottle solution was kept warm and standing until no bubbles were generated.After filtration and washing,it was filtered for 30 min to obtain RAW-DAAF.The filter bottle was sealed, and the ethyl acetate solution dissolved with binder (F2604, NC, NC/GAP) and suspension agent (binder accounted for 5 wt% of the total mass,suspension agent accounted for 0.3 wt%of the solvent mass)were added to the collection bottle.The stirrer was fully stirred to ensure the uniform suspension of DAAF particles in the solution.After the DAAF was uniformly suspended, the peristaltic pump was connected to the suspension and the continuous phase solution was fixed to the injection pump(SDBS 0.2 g was weighed and dissolved in 100 mL deionized water to obtain the aqueous system with 0.2 wt%surfactant content).The two were connected to the droplet chip, and the flow rate ratio of the two phases was set.DAAF microspheres with flow rate ratios of 7, 12 and 17 were prepared under three different binder conditions.The microspheres obtained at a flow rate ratio of 7 were recorded as B-DAAF,the microspheres obtained at a flow rate ratio of 12 were recorded as M-DAAF, and the microspheres obtained at a flow rate ratio of 17 were recorded as S-DAAF.

2.4.Characterization methods

The surface morphology was characterized by scanning electron microscopy (SEM, SU-8020, Hitachi, Japan) working at an acceleration voltage of 5 kV.Samples were characterized after being coated through gold sputtering.The particle size distribution statistics were carried out according to method 403.2 of GJB770B-2005, where the maximum and minimum size of each particle was measured under a microscope to find the particle size and the particle size statistics were analyzed.The Brunauer Emmett Teller(BET) (ASAP2020, Micromeritics, shanghai) was tested under adsorbate N2,degassing at 120°C for 8 h,and the mesoporous mode was tested,sample pipe diameter was 9 mm.The crystal structures were determined by powder X-ray diffraction (XRD, DX-2700,Dandong Haoyuan Corporation, Liaoning, China).The analysis parameters were as follows: 2θ test angle 5°-50°, voltage 40 kV,current 30 mA, and Cu-Kα radiation.Thermal analysis was performed on a differential scanning calorimeter (DSC-131, Setaram,France,Shanghai,China),and the heating rate were 5°C/min,10°C/min,15°C/min,20°C/min.The thermal behavior of the sample was characterized by thermal gravimetric analysis (TGA METTLER TOLEDO).The sample (1-3 mg) was placed in a 70 μL ceramic crucible at a constant heating rate of 10°C/min from 40 to 400°C.The impact sensitivity was tested with a home-built Type 12 drop hammer apparatus.The special height (H50) represents the height from which 2.500 ± 0.002 kg drop hammer will result in an explosive event in 50%of the trials.In each determination,25 drop tests were made to calculate the H50.The test conditions were as follows: the weight of the pendulum was 1.50 kg, the mass of the sample was 20 mg, the relative pressure was 4.9 MPa, and the swing angle was 90°.The friction susceptibility of each test sample was represented by the probability of explosion,and 30 tests were carried out consecutively to characterize the friction susceptibility and probability of explosion of the samples.

3.Result discussion

3.1.Synthesis of high quality DAAF crystals and its microsphere assembly strategy

DAAF was obtained by one-step oxidative coupling of 3,4-diaminofurazan (DAF) in an alkaline environment through OXONE?.The chemical reaction formula is shown in Scheme 1[7].

In the mixed reaction stage of raw materials, the mixing effect plays a deterministic role in the reaction between substances[29].The mixing time can be used to describe the mixing between substances, and the mixing time can be calculated by Eq.(1)[21,30,31].

Scheme 1.The chemical reaction formula of DAAF.

where s is the diffusion distance of matter,half the diameter of the container; D is the molecular diffusion coefficient, and A is the shape factor.In the conventional method, when a jacket reactor with an inner diameter of 10 cm was used to prepare DAAF, its molecular diffusion distance was 5 cm (as shown in Fig.S1); The molecular diffusion distance of the micro-mixing reactor was 0.25 cm(as shown in Fig.S2).The same substance D was the same under the same reaction environment,and the shape of the circular cavity in the micro-mixing reactor can be seen as the same as the jacket reactor,so the shape factor A was also the same.According to the diffusion time equation, the mixing time of conventional method was 25A/D;The mixing time of micro-mixing method was 0.0625A/D.Compared with the conventional method, the mixing time of the material was reduced by 3 orders of magnitude when the micro-mixing method was used.It shows that the use of micromixing reactor can enhance the mixing effect between reactants and make the raw materials mix quickly.Furthermore, the mass transfer performance of the micromixing reaction stage in the microfluidic system was studied.The change rate of concentration in the chemical reaction can be expressed by Eqs.(2) and (3) [21].

where NAis the rate of change in the concentration; CAis the concentration of the reagent; CA,0is the concentration of the reagent at the wall of the channel;CA,∞is the interface concentration;and kLα is the overall mass transfer coefficient。According to the equality 1,2,3, kLα can be expressed as Eq.(4)

For certain reactions,A,CA,0,CA,∞,CA,and s can be approximated as constants, So θ can be regarded as a constant.It was found that the total mass transfer coefficient kLα is inversely proportional to the mixing time τ.Therefore, according to the above mixing time calculation results, because the total mass transfer coefficient was inversely proportional to the mixing time, the mass transfer efficiency of the micromixing method was 3 orders of magnitude higher than that of the conventional method under the above experimental conditions.In addition, in the process of preparing DAAF using a micro-mixing reactor, the ultrasonic action was started to enhance the effect of convection and diffusion between fluids, which will help improve the mixing efficiency of the material,thereby further improving the mixing and mass transfer effects in the micro-reactor[32,33].In summary,the micromixing reaction stage in the designed continuous microfluidic system exhibits excellent mixing performance and more prominent mass transfer performance between reactants.The morphology of DAAF obtained by ultrasonic micromixing method and conventional method was shown in Fig.2.The morphology of DAAF products obtained by ultrasonic micromixing was compared with that by conventional method.The DAAF obtained by ultrasonic micromixing method showed a regular uniform spindle shape,concentrated particle size,and a particle size distribution range of 16-23 μm; the DAAF obtained by the conventional method was irregular block, the morphology and particle size were uneven, and the particle size distribution range was 0-55 μm.This verifies the above calculation process, which shows that the ultrasonic micromixing method greatly improves the consistency of crystal morphology and particle size of DAAF in the synthesis process.This was because the rapid and strong mass transfer mixing process in the ultrasonic micromixing reaction was conducive to the generation of uniform supersaturation,and then the crystals with uniform morphology and concentrated particle size distribution were prepared, which will help to ensure the consistency of DAAF energy output.

Fig.2.(a)Morphology of DAAF synthesized by ultrasonic micromixing reaction;(b)Morphology of DAAF synthesized by conventional methods;(c)The particle size distribution of DAAF synthesized by ultrasonic micromixing reaction; (d) Particle size distribution of DAAF synthesized by conventional methods.

In the stage of DAAF forming droplet template,the flow of fluid was mainly affected by viscous force, inertial force, interfacial tension and shear stress.Reynolds number Reis an important dimensionless parameter in fluid mechanics,which represents the ratio of inertial force to viscous force, shown in Eq.(5).

where v represents the velocity of the fluid in the microchannel,d is the diameter of the microchannel, p represents the density of the fluid, and u represents the viscosity of the fluid.In microchannels,the value of Reynolds number is usually very small, and the influence of inertial force on liquid flow can be neglected.Viscous force will play a leading role,and the flow of liquid in microchannels was stable laminar flow.This stable laminar flow provides a stable environment for the formed droplet template,so that the dispersed phase forms a highly periodic and orderly flow of droplets under the shear force of the continuous phase.The size of the droplet template is mainly determined by the flow rate ratio of the continuous phase to the dispersed phase(the study of the influence factors of the droplet template in supporting materials).The shear force is controlled by controlling the flow rate ratio of the continuous phase to the dispersed phase, so as to prepare the droplet templates with different sizes.In the droplet template, there were solvent Ethyl acetate (EA), dissolved binder and suspended DAAF crystal particles.The droplet template was solidified into microspheres due to the gradual outward diffusion of EA in the droplet.Under flow conditions, the mass transfer in droplets was affected by convection and diffusion.The time scales in convection and diffusion can be expressed by Eqs.(6) and (7).

where V is the velocity of the fluid,u is the viscosity of the fluid,and L is the width of the microchannel.Due to the micro-scale size of the channel, the mass transfer time scale of convection and diffusion was also small,so the mass transfer process of EA in the droplet to the continuous phase was a relatively fast process, and as the droplet size decreases,the specific surface area increases,the mass transfer efficiency further increases,and the mass transfer process of EA was faster[25].Due to the diffusion of EA,the polymer binder dissolved in EA began to precipitate due to the increase of supersaturation.The suspended DAAF particles in the droplets were coated and bonded, and gradually formed a three-dimensional porous skeleton inside the droplets, which became the main structural support of the microspheres, so that the DAAF particles were coated inside the microspheres to form a spherical coating.

3.2.Problem handling in the process

3.2.1.Solution of DAAF crystal drying problem

DAAF particles after a long period of suction filtration after synthesis, the moisture in the particle gap was completely filtered to the collection container,the adsorption of water on the surface of the particles was difficult to be filtered down, it must be freezedried or dried to completely remove the operation, often for the drying of raw materials will take a lot of time, which reduces the efficiency of the process.Further, when the binder dissolved in EA encounters a solvent(such as H2O)that cannot dissolve the binder,it will polymerize and precipitate due to anti-solvent action, as shown in Fig.3.According to this principle, in the continuous coating process of DAAF, only a long time of suction filtration was performed on the synthesized DAAF raw material to ensure that the moisture between the particle gaps was completely discharged.Since the water adsorbed on the surface of the particles contacts with the EA dissolved with the binder (Fig.3(b)), the anti-solvent(water) action causes the binder in the EA to precipitate, thereby achieving the binder coating on the surface of the particles(Fig.3(c)).Due to the diffusion of EA in the droplets, the binder between the particles was precipitated, so that the particles were bonded and coated, and finally the DAAF microspheres coated by the binder were formed [34,35].As shown in Fig.3(d), it can be found that there was a binder coating on the surface of the DAAF crystal(at the red dotted line),which was due to the surface coating layer produced by the precipitation of the binder on the surface.At the yellow dotted line, it indicates that there was binder adhesion between particles.This proves the feasibility of this step in continuous process.

3.2.2.Uniform suspension of DAAF crystals in dispersed phase

The dispersed phase in the preparation of DAAF microspheres was a multiphase dispersion system,which was between colloidal dispersion system and coarse dispersion system.There was a large interface energy in this system.Due to the effect of van der Waals gravity, DAAF particles were easy to aggregate and sink, thus destroying the whole system.Therefore, the dispersed phase of DAAF was thermodynamically unstable, it tends to reduce the interface energy, so that the original dispersed particles appear coalescence,sedimentation and other phenomena,thus destroying the stability of the dispersed phase[36].The suspension agent is an additive with a special molecular structure.After being added to the suspension, part of it was adsorbed at the interface of DAAF particles, and the other part was toward the dispersion medium.Therefore, on the one hand, it can be adsorbed on the surface of DAAF particles,forming an electric double layer with Zeta potential around the particles and forming a protective film on the surface of the particles, so that the tendency of DAAF particles to merge was weakened, and the interaction of 'steric hindrance' makes the particles repel each other,thus preventing the re-agglomeration of dispersed particles and making them uniformly dispersed,forming a liquid-solid system with particle high suspension,flowability and stability.At the same time, it prevents DAAF particles from reagglomerating during long-term storage to maintain good suspension performance, dispersion performance and stability.The amount of suspending agent greatly affects the deposition process of DAAF.In order to prevent particle sedimentation, adjust the amount of suspension added,control the viscosity of the dispersion medium, reduce the specific gravity difference between the dispersion medium and the dispersion phase, and reduce the density difference between DAAF and the medium, so that the suspension agent forms a thixotropic structure with a certain strength.When the suspension was static, the suspension forms a gel network structure,which can support the DAAF crystal particles to hinder its sedimentation, so that the dispersed phase solution does not settle and delaminate during storage, maintaining its suspension stability;under the action of a certain external force,its structure was destroyed,and its fluidity can be restored and it was easy to flow.Therefore, this gel network structure must be appropriate, the structure was too weak to achieve the purpose of supporting particles hinder its settlement, poor stability of the dispersed phase,easy stratification after a period of time;structure was too strong,poor fluidity,dispersed phase was not easy to flow,hanging wall was serious,after shaking cannot be destroyed,it was difficult to make continuous phase shear into droplets [37,38].Therefore, the suspension effect of the suspension agent with different addition amounts on the DAAF dispersed phase solution under different binders was studied,as shown in Fig.4.Before the addition of the suspending agent, DAAF particles showed obvious sedimentation in the binder solution, with obvious stratification and poor suspension stability,which would be difficult to apply to the preparation of DAAF microspheres by microdroplet technology.After adding different contents of suspension, the suspension dispersion of DAAF particles was greatly improved, and the problem of particle sedimentation was solved.However, the dispersed phases with different binders showed different suspension conditions.The dispersed phase solution containing F2604still has small solution stratification when the suspension content was 0.2 wt%and 0.1 wt% (as shown in the red solid line).The dispersed phase solution containing NC showed good suspension effect under the three suspension agent contents,and no stratification occurred.At the same ratio,the dispersed phase solution containing NC/GAP has a layer height of about 2.3 mm at 0.3 wt%of the suspension content,which is lower than the layer height of 3.4 mm and 3.8 mm at 0.2 wt%and 0.1 wt%(as shown in the red solid line).The reason for this may be that NC is a high-viscosity binder, which itself has a structural support and promotion effect when DAAF was uniformly suspended, while GAP is a liquid binder, whose addition weakens the structural support of NC to DAAF particles.In general, after adding 0.3 wt% suspension agent, different binders showed better suspension effect, not easy to layer and settle, and the suspension had good fluidity, which was suitable for the preparation of DAAF microspheres by microdroplet technology.In order to ensure the consistency of the prepared microspheres, the use level of the suspension content of 0.3 wt% was unified.

Fig.3.Solution to RAW DAAF crystal drying problem.

Fig.4.(a) The suspension of dispersed phase before adding suspension agent; (b) The suspension of dispersed phase after adding different content of suspension agent.

3.3.Morphological analysis

In order to study the morphology of DAAF microspheres prepared continuously using a microfluidic system, the particle size distribution, surface morphology, and internal cross-section of microspheres coated with F2604, NC, and NC/GAP binders were studied using electron microscopy and scanning electron microscopy, as shown in Fig.5.It can be found from the electron microscope that DAAF coated with three binders can be uniformly balled, microscopic images show a high rate of ball, no obvious agglomeration between spherical particles, with good monodispersity, no significant difference between the samples.The morphology of a single microsphere was studied by SEM.The samples showed good balling effect, and at different flow rate ratios, different microsphere sizes were shown.This shows that the size of the microspheres can be effectively controlled by changing the process conditions during the continuous preparation process,reflecting the adaptability and flexibility of the system.Further studies have found that wrinkles appear on the surface of some of the formed microspheres.This was because the encapsulated DAAF was a micron-sized spindle-like structure, which was disorderly distributed when forming a droplet template.The irregular arrangement of this DAAF crystal leads to this surface wrinkle when the binder was precipitated and coated.In NC-coated medium and small spheres (Fig.5(bM2), Fig.5(bS2)) and NC/GAP coated large,medium and small spheres (Fig.5(cB2), Fig.5(cM2), Fig.5(cS2)),this surface wrinkle was weakened and shows a smoother surface.This was because the NC binder has a high viscosity.When forming the droplet template, due to the high viscosity of the dispersed phase, the DAAF crystal was tightly arranged inside the droplet template,and it was not easy to shift during the droplet movement.The surface of the droplet will precipitate the binder earlier due to the diffusion of EA,which will weaken the wrinkles on the surface of the particles.However, when the volume of the microspheres becomes larger, the particle arrangement in the unit volume appears relatively large voids,so there was no crystal particle support during the binder precipitation,and the related folds were formed(Fig.5(bB2)).Because GAP is a liquid binder in NC/GAP composite binder, it plays a supporting and repairing role in the process of droplet solidification, so the surface folds were less than those of F2604and NC binder coated microspheres.By studying the internal cross section of different microsphere sizes under different binders,it was found that the interior of the microspheres was mainly composed of three-dimensional skeleton structure formed by the binder and DAAF crystals.The red dotted line in the figure indicates that the binder forms a three-dimensional skeleton structure in the microspheres,the yellow dotted line was DAAF crystal particles.By comparison, it was found that compared with F2604binder, the microspheres formed by NC and NC/GAP composite binder showed more dense and complex three-dimensional porous structure,indicating that NC was more conducive to the establishment of three-dimensional skeleton in microspheres.By studying the particle size distribution of DAAF microspheres under different process conditions, it was found that the particle size distribution of three kinds of microspheres coated with NC binder was better than that of NC/GAP composite binder and better than that of F2604binder.Further,it can be seen from Fig.5(d)that the average particle size of the DAAF microspheres coated with the three binders decreases with the increase of the flow rate ratio,which further confirms that the system can control the size of the microspheres.At the same flow rate ratio, the average particle size of DAAF microspheres coated with NC binder was larger than that of NC/GAP binder and larger than that of F2604binder, which can be explained by the internal cross section of the microspheres: The DAAF microspheres coated with F2604binder have tight crystal structure between particles, and the three-dimensional skeleton structure was less than that of NC and NC/GAP binder.The formation of more threedimensional structure of NC binder supports the volume of microspheres, so that the average particle size was larger.Fig.5(e)shows the dispersion levels between different binder-coated microspheres relative to the average at different flow rates.A large standard deviation represents a large difference between most values and their average values; a smaller standard deviation represents that these values were closer to the average.It can be found that the DAAF microspheres coated with F2604binder have the narrowest particle size distribution when the flow rate ratio was 7 and 12.When the flow rate ratio was 17, the DAAF microspheres coated with NC and NC/GAP binders have similar and narrowest particle size distribution.This shows that F2604was suitable for the preparation of large size DAAF microspheres,NC and NC/GAP were suitable for the preparation of small size DAAF microspheres.

Fig.5.(a) DAAF microspheres coated with F2604 binder; (b) DAAF microspheres coated with NC binder; (c) DAAF microspheres coated with NC/GAP binder; (d) Line chart of average particle size of DAAF microspheres coated with different binders at different flow rates; (e) Standard deviation line chart of DAAF microspheres coated with different binders at different flow rate ratios.

Among them, in (a), (b), (c), B represents the large particle size microspheres prepared under the binder; M represents the medium particle size microspheres prepared under the binder; S represents the small particle size microspheres prepared under the binder; 1 is the overall morphology of the electron microscope of the sample;2 is the SEM morphology of the sample;3 is the crosssectional SEM image of the sample; 4 is the particle size distribution of the sample.

3.4.Porosity analysis of DAAF microspheres coated with F2604 binder

From the SEM images,it can be seen that the DAAF microspheres coated with F2604binder have narrower pores than those coated with NC and NC/GAP binders.In order to study the specific surface area and pore size distribution of microspheres when DAAF was coated with F2604binder.The Brunauer Emmett Teller (BET)method was used to test the N2adsorption-desorption curves of the three sizes of microspheres under the binder F2604to better characterize the assembly structure of the material,as shown in Fig.6.The specific surface area of the large, medium and small microspheres coated with F2604binder calculated by BET method is:1.334 m2/g,1.604 m2/g, 0.924 m2/g, respectively.The specific surface area of the medium microspheres was the largest, and the specific surface area of the small microspheres was the smallest.On the one hand,this was because compared with large microspheres,the particle size of medium microspheres decreases and its specific surface area increases.On the other hand,it can be found from the SEM images that the medium and large microspheres coated with F2604have wrinkled morphology, and the surface wrinkles of the small microspheres were weaker than those of the large and medium microspheres.At this time,this surface wrinkle becomes the dominant factor in the specific surface area measurement.Although the size of the small microspheres was the smallest, the specific surface area was relatively weak due to its smooth surface,which was consistent with the SEM characterization results.Further,the most concentrated mesopore size of the microspheres of different sizes and the average pore size of the sample can be determined from the BJH pore size distribution.The most concentrated mesoporous pore sizes of large, medium and small DAAF microspheres coated with F2604were 7.60 nm,10.52 nm and 17.18 nm, respectively, and the average pore sizes were 31.22 nm,48.13 nm and 63.97 nm, respectively.From the pore size distribution,it can be seen that in the case of binder F2604coating,with the decrease of microsphere size, the inner pore size of the microsphere gradually increases,which was also consistent with the SEM characterization results of the microsphere section.This was because the DAAF crystal particles were randomly and disorderly distributed in the microspheres when the microsphere template was formed.As the microsphere size decreases, the DAAF crystal particles in the microsphere region decrease and the degree of freedom of disorderly arrangement between crystals increases.At the same time, due to the decrease of the droplet template, the curing speed of the microspheres was accelerated, and the spontaneity of the close arrangement between the crystals was limited by the rapid binder curing process, resulting in an increase in the gap between the crystals and a gradual increase in the pore size.

3.5.Bulk density

According to ASTM873(test method for bulk density of biomass fuel with density particles),the bulk density of raw DAAF and DAAF with different sizes of large, medium and small microspheres coated with different binders under continuous process was measured [39,40].The results were shown in Fig.7.The results show that the bulk density of spherical DAAF prepared by continuous process was smaller than that of raw DAAF.This was because the binder-coated DAAF microspheres were three-dimensional porous structures.This structure reduces the amount of DAAF particles and the weight per unit volume,resulting in a decrease in the bulk density of the spherical coated microspheres.By comparing the bulk density of DAAF coated with different binders,it was found that DAAF coated with F2604binder had the highest bulk density under three sizes of microspheres.This phenomenon can be explained from the SEM image.This was because the F2604binder forms less three-dimensional porous structure inside the microsphere, which makes more DAAF particles per unit volume,resulting in higher volume density.Further, by comparing the volume density of different microsphere sizes under the same binder,it was found that when the flow rate ratio was 17,that is,the packing density of the microspheres was the highest when the microspheres were small.This was because in the packing, the smaller size of the sphere has less particle clearance than the larger size sphere under the same volume, so the packing density of the microspheres was higher under the small scale.

3.6.Crystal structure analysis

The crystal structures of raw DAAF and DAAF microspheres coated with different binders were analyzed by X-ray diffraction(XRD),as shown in Fig.8(the black vertical line in the abscissa was the standard characteristic peak position of DAAF).Through the diagram, it was found that the coated microspheres have characteristic peaks consistent with the raw material DAAF at 13.6°,18.5°,19.3°, 21.4°, 21.9°, 27.4°, 28.8°, 29.6°, etc.This shows that the spherical coating of DAAF using this continuous process has no effect on its crystal structure.By analyzing the XRD curve, it was found that the characteristic peak intensity and full width at half maxima (FWHM) of the sample microspheres changed compared with RAW DAAF.This was because the surface of the sample microsphere was coated with F2604, NC, NC/GAP polymers.The coating effect of these polymers on DAAF crystal makes the characteristic peak intensity of the sample microsphere at the crystal plane(11 0),(0 0 1),(2 1 0),(-2 0 1),(-2 11)and so on weaker than that of RAW DAAF during the XRD test[41,42].In addition,because DAAF was slightly soluble in the solvent EA used,the DAAF crystal particles were slightly dissolved in the EA before the sample microsphere was solidified, which makes the DAAF grains in the microsphere smaller than RAW DAAF,resulting in the widening of the XRD characteristic peak of the sample microsphere,that is,the FWHM changes.It was worth noting that the FWHM change of NCB DAAF was more obvious,because the average particle size of NC-B DAAF sample microspheres was larger than that of other samples,and the diffusion time of EA will be longer during its droplet curing process.The more DAAF was dissolved in the microspheres, the more the crystal size will decrease,which leads to the more obvious FWHM change.Therefore, more DAAF is dissolved by EA in the droplet templates, and the grain size will decrease more, which leads to more obvious changes in FWHM.

3.7.Thermal analysis

Fig.6.N2 adsorption-desorption curve and pore size distribution curve of DAAF microspheres coated with F2604 binder:(a)and(b)corresponds to large size microspheres;(c)and(d) Corresponds to medium size microspheres; (e) and (f) Microspheres of small size.

Fig.7.Columnar diagram of bulk density of RAW DAAF and sample microspheres under different process formulations.

Fig.8.XRD patterns of RAW DAAF and sample microspheres under different process formulations.

DSC was used to study the raw DAAF and DAAF Microspheres of coated with different binders thermal decomposition,as shown in Fig.9.From the DSC curve, it can be clearly seen that at different heating rates, the spherical coated DAAF has the same exothermic peak type as the raw material DAAF, but the exothermic peak temperature was ahead of the raw material DAAF, and the microspheres coated with NC/GAP binder were the most advanced, followed by NC binder, and F2604was the least advanced.The main reason for this phenomenon was that the uniform particle size distribution of the spherical coated microspheres was conducive to heat transfer,which can accelerate the process of mass transfer and heat transfer in the thermal decomposition process and promote the thermal decomposition process.In addition, since the threedimensional porous skeleton inside the microspheres was more conducive to convective heat transfer during thermal decomposition, the transferred heat can be effectively fed back to the DAAF crystal, thereby promoting the thermal decomposition process.Secondly, because the binder NC and GAP were energetic binders,their thermal decomposition temperature was lower than that of DAAF.During the heating process, they will first undergo thermal decomposition.Because they were uniformly coated on the DAAF crystal, the thermal decomposition process of DAAF was further promoted, and the thermal decomposition peak of DAAF microspheres was advanced.Because F2604binder was a substance with high thermal decomposition temperature, the thermal decomposition temperature of microspheres containing F2604binder was higher than that of microspheres containing NC and NC/GAP binder.However, the structure of the microspheres in this process was the dominant factor to promote the early decomposition of DAAF,so the thermal decomposition temperature of F2604was lower than that of RAW DAAF.In addition, due to the threedimensional skeleton structure inside the sample microspheres,the contact surface between DAAF was reduced, and the external heat can be better dispersed and transferred, which makes the sample microspheres have no melting endothermic phenomenon similar to that caused by heat accumulation before the thermal decomposition of RAW DAAF.Due to the addition of 5 wt% binder,the exothermic value of the sample microspheres was lower than that of RAW DAAF.However, by analyzing the DSC curve, it was found that the exothermic peak of the sample microspheres was weaker than that of RAW DAAF, but the exothermic range was wider than that of RAW DAAF,with a long exothermic process.This was because the coating of the binder makes the DAAF crystal have a buffer layer, which makes the sample microsphere not have a sharp thermal decomposition phenomenon, so that it shows the phenomenon of weakening the exothermic intensity and lengthening the exothermic process.

In order to further study the thermal decomposition kinetics and thermodynamic properties of DAAF microspheres coated with different binders,the thermal decomposition peak temperatures at different heating rates were obtained from DSC curves, and the thermodynamic parameters were calculated.The activation energy of each sample was calculated by Kissinger method (Eq.(S1)),Ozawa method (Eq.(S2)) and Starink method (Eq.(S3)) [43-46].The average activation energy (Ea) data under the three methods were used as the activation energy parameters of the sample (the fitting linear equation under the three methods was seen in the support material), so as to obtain other thermodynamic parameters.Combined with the apparent activation energy data,according to Eqs.(S4)-(S7), the activation enthalpy (ΔH≠) and Gibbs free energy(ΔG≠)of the sample and the critical temperature of thermal explosion(Tb) were calculated respectively.The above calculated results were shown in Table 1.According to the calculation results,the average apparent activation energies of DAAF microspheres coated with F2604,NC,NC/GAP and RAW DAAF were 156.68,168.75,147.57,168.70 kJ/mol,respectively.Compared with the RAW DAAF,the activation energy of F2604DAAF and NC/GAP DAAF decreased by 12.02 kJ/mol and 21.13 kJ/mol respectively.The activation energy of NC DAAF increased by 0.05 kJ/mol.It can be seen that the apparent activation energy of DAAF microspheres modified by spheroidization with F2604and NC/GAP binders was lower than that of raw material DAAF, indicating that the thermal decomposition activity of DAAF microspheres modified by spheroidization with these two binders was improved.It can also be seen from the calculation results that the ΔH≠value and ΔG≠values are all greater than zero,which indicates that the activation reaction of molecules is nonspontaneous, and substances need to absorb energy from the outside to produce chemical reactions.When the sample absorbs energy higher than ΔH≠, it can be excited.Therefore, the sample was stable under normal conditions.By comparing the thermodynamic data of different binder-coated microspheres, it was found that the DAAF microspheres coated with NC binder have better thermal stability than F2604and NC/GAP binders.This was because F2604and GAP contain -F and -N3based high-energy groups,which were more likely to cause intramolecular activation reactions when subjected to thermal stimulation, making their thermal stability lower than DAAF microspheres coated with energetic NC binder.

The TG curve and DTG curve of RAW DAAF and sample microspheres are shown in Fig.10.The results showed that both RAW DAAF and the sample microspheres were homogeneous decomposition process, and there was only one stage of thermal weight loss.Similar to the DSC analysis results, the sample microspheres were decomposed earlier than RAW DAAF.The temperature of the sample at the fastest thermos-gravimetric stage was from low to high: NC/GAP-DAAF (218.50°C), NC-DAAF (232.66°C), F2604-DAAF(235.16°C), RAW-DAAF (259.33°C), which was the same trend as that of DSC.

Fig.9.DSC diagram of RAW DAAF and DAAF microspheres coated with different binders at different heating rates.

Table 1 Thermodynamic parameters of RAW DAAF and DAAF microspheres coated with different binders.

3.8.Mechanical sensitivity

The safety of the samples was evaluated by testing the impact sensitivity and friction sensitivity of the raw DAAF and the spherical coated DAAF samples.The results were shown in Table 2.Compared with the impact sensitivity (H50) 110 cm of raw DAAF, the impact sensitivity (H50) of spherical DAAF samples coated with F2604, NCand NC/GAP binders was greater than 120 cm, and the friction sensitivity explosion percentage was 0%.This indicates that the preparation of spherical coated DAAF microspheres by this continuous process will reduce the mechanical sensitivity and improve the safety level of DAAF.This was because the DAAF was coated to modify the surface of the particles to make it more regular, so that the microspheres can be buffered under external mechanical stimulation, dissipate external forces, and the coating material can play a lubricating and endothermic effect, which can improve the safety of DAAF.At the same time, due to the uniform spherical structure of the microsphere,the sphere breaks up under the action of an external force, offsetting the effect of a portion of the force and suppressing the formation of hot spots, thereby further reducing the sensitivity of the DAAF [47].These results indicate that the DAAF microspheres prepared by continuous microfluidic technology have excellent mechanical sensitivity.

Table 2 Impact and friction sensitivity test results.

Table 3 Detonation performance of samples.

3.9.Detonation performance

Detonation velocity, detonation pressure and detonation heat were the main parameters of explosive detonation performance.The theoretical maximum detonation performance of raw DAAF and DAAF samples coated with different bonding formulations was calculated by EXPLO5 program, as shown in Table 3.It was found that the detonation parameters of DAAF coated with F2604,NC and NC/GAP binder formulations were not much different from those of raw DAAF, and the detonation pressure of DAAF coated with F2604binder formulation was higher than that of raw DAAF.Usually,when the energetic material was coated and modified by binder,the detonation parameters of the energetic material will be greatly lost due to the addition of binder, which improves its safety while reducing its detonation performance.However, in the continuous process,the three binders selected improve the safety level of DAAF without significantly reducing the detonation performance of the main material.Therefore, the one-step preparation of DAAF microspheres using this continuous process not only has a high detonation parameter level but also further improves the overall performance of DAAF through structural modification.

4.Conclusions

In this study,a microfluidic process system from DAAF synthesis to spherical coating continuous preparation was designed by combining the rapid mixing reaction in micromixing technology and the droplet template of microdroplet technology.The continuous one-step preparation of DAAF from synthesis to spherical coating was successfully realized by using this system and process.In the process, the coating binder was adjustable, the size of the product microspheres was controllable, and it had high flexibility and adaptability.The raw DAAF synthesized by the system has regular morphology and uniform particle size.The use of F2604,NC and NC/GAP binders has a good effect on the coating of DAAF into microspheres.The microspheres with different sizes both have a high level of sphericity and a narrow particle size distribution.This ensures consistent energy output.The three-dimensional porous structure of DAAF microspheres makes it have lower bulk density and lower thermal decomposition temperature than raw DAAF.This continuous process does not change the crystal structure of DAAF.Due to the coating and spherical structure of the binder,DAAF microspheres were less sensitive than DAAF.In addition,the detonation parameter data show that the detonation parameters of DAAF microspheres using three binder formulations will not be greatly reduced compared with raw DAAF, and their comprehensive performance was improved while maintaining their high detonation performance.Overall, the continuous and integrated preparation of DAAF from synthesis to spherical coating can be realized by the microfluidic system,which improves the safety and efficiency of DAAF preparation, provides a simple way for the spherical preparation of DAAF, and also provides an experimental theoretical reference for the rapid one-step product preparation of similar energetic materials.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to acknowledge National Natural Science Foundation of China (Grant No.22005275) to provide fund for conducting experiments.

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dt.2023.02.013.