Micro-seeding and soft template effects on the control of polymorph and morphology of HMX micro particles in solvent-antisolvent process

Sajjad Damiri,Shahriar Namvar,Hadi Panahi

Department of Applied Chemistry,Maleke-ashtar University of Technology,Shahin-shahr,Esfahan,Iran

Micro-seeding and soft template effects on the control of polymorph and morphology of HMX micro particles in solvent-antisolvent process

Sajjad Damiri*,Shahriar Namvar,Hadi Panahi

Department of Applied Chemistry,Maleke-ashtar University of Technology,Shahin-shahr,Esfahan,Iran

Micro seeding Polymorph HMX Explosive Micro-particle Solvent-antisolvent process

A seeding strategy was developed in the preparation of cyclotetramethylenetetranitramine(HMX)explosive micro-particles by solvent-antisolvent method,to control their polymorphs from dangerous gamma(γ)type to the desired and standard beta(β)form with the size distribution of＜10.0 μm,by using a low concentration ofβ-HMX fine particles as micro-seed in the antisolvent medium.All products were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),and dynamic light scattering particle size analyzer.In the next step,the effective factors on the sizes and morphologies of micro-particles in the presence and absence of two soft templates of poly(ethylene glycol)-400(PEG-400)polymer and coconut fatty acid diethanolamide(lauramide)surfactant were investigated.The results of experiments showed that using of water-soluble PEG-400 in the low antisolvent temperatures leads to the production of very spherical particles.Also non-ionic surfactant of lauramide,direct the crystal growth to needle-like structures.The advantages of this method are its capability for the simple production ofβ-HMX micro-particles in the large scale production process,with the various crystal structures and particles size distributions.

1.Introduction

1,3,5,7-Tetranitro-1,3,5,7-tetraazacyclooctane commonly known as Octogen or HMX(high melting explosive)is the most powerful military explosive in current military applications[1].HMX explosive has four different crystalline polymorphs ofα,β,γ,andδwith orthorhombic,monoclinic,monoclinic,and hexagonal crystal lattice system,respectively.It is generally agreed that the friction and impact sensitivities areδ＞γ＞α＞β,but it is dependent on the size and morphology of crystals[2-5].According to military standard specifications[6],various types and classes of HMX crystals should be produced and used as beta polymorph in the ammunitions.Because,β-HMX is the most dense,insensitive and the thermodynamically stable form at room temperature[2,3,7,8].

Nowadays,productions of micronized particles and nanoparticles of various explosives,including HMX[9-12],are widely considered for using in explosive formulations[13-15]and solid propellants[16,17].Production processes by solvent-anti-solvent method[10,18,19]are considered to be one of the simple,safe and economical techniques for the production of micronized particles of explosives on the industrial production scale[20].But the important subject about the preparation of micronizedβ-HMX high explosive by solvent-anti-solvent method is that unlike the other explosives such as cyclotrimethylenetrinitramine(RDX)and pentaerythritol tetranitrate(PETN),HMX has four different polymorphs,and proper experimental conditions for the preparation of very fine particles ofβ-HMX are inconsistent to the necessary conditions for the creation of stableβpolymorph.In other words,for the preparation of micronized particles,it needs to very fast precipitation and super-saturation;with increasing in the solvent and anti-solvent temperature difference,the concentration of HMX in organic solvent,etc.But this rapid precipitation conditions in the case of HMX high explosive lead to creation of unstable and unsafe polymorphs such as gamma type[10,12,21,22],so that the final product does not meet the necessary safety and standard requirements.Of course,some other methods have also been proposed for the production of fineβ-HMX particles,such as jet-mil[20],supercritical carbon dioxide[23,24],ultrasonic assistedmethod[25],or others[18].But in practice,the widespread application of these techniques are limited due to the scale up diff iculties,high sensitivity of dry explosives to impact,friction and static electricity stimuli[18,20].

Practically,in the manufacturing process of micronized particles by solvent-anti-solvent process,the rate of nucleation formation in comparison to the growth rate is dominant.Here the external seeding and nucleation can be used to control the formation of desired polymorphs.Nucleation can be either homogeneous,without the influence of foreign particles,or heterogeneous,with the influence of foreign particles.Seeding is a powerful tool for the separation of nucleation and growth[26-28].In this technique,previously nucleated crystals are used as seeds and introduced into new solution equilibrated at lower levels of super saturation.The micro seeding particles can act as a scaffold for the crystal to grow on,thus eliminating the necessity of creating a new surface and the incipient surface energy requirements[26,29].

This research introduce a simple,safe and low-cost procedure,based on the using ofβ-HMX micro-seeding in solvent-antisolvent process for the production of HMX micro particles,to control HMX polymorph from gamma to beta type.Also,in other section of this work,the effect of two soft templates of polyethylene glycol 400(PEG-400)and coconut fatty acid diethanolamide(lauramide)surfactant on the morphology of these fine particles were investigated to achieve various spherical,rod-like or other particle shapes.The template techniques are suitable approaches to fabricate micro and nanostructures with desired shape and size by inducing the target materials to grow according to the patterns of the templates.Soft templates can be dissolved in the liquid phase,mostly including surfactant micelles and copolymers[30-32].It is wellknown that the surfactant micelles and inverse micelles with different shapes such as spherical and rod-like are formed in the solutions when their concentrations reach the critical micelle concentration(CMC)[18].Thus,these micelles can then be used as the soft templates for the fabrication of organic and inorganic microstructures.

2.Experimental

2.1.Materials and instrumentations

Theβ-HMX,type B containing 0.5 wt.%RDX,with size distribution in the range of 100.0-200.0μM purchased from Iranian defense industries.Water soluble polymer of poly(ethylene glycol)-400 and acetone solvent were purchased from Sigma--Aldrich,and coconut fatty acid diethanolamide surfactant was purchased from Iranian Chemical Process Company(Tehran,Iran).Particle size distributions of products were characterized using dynamic light scattering particle size analyzer by Fritsch Analysette 22 MicroTec plus instrument.The sensitivity to impact stimuli was determined by BAM fall hammer apparatus(Reichel&Partner GmbH)using a 2 kg drop weight and the results are reported in terms of height for 50%probability of explosion(h50%)of the sample with the uncertainty limits of±5 cm.The polymorphs of HMX micro particles were characterized by an X-ray spectrometer Bruker,D8 ADVANCE(Germany)equipped with Cu Kα radiation,and also their morphologies were recorded using scanning electron microscope(SEM)instrument of Philips(FEI XL-30 model).The wet samples of g-HMX were dried in room temperature,and were stored in dry,dark bottles.XRD studies shows that these samples are stable for higher than one week.Conversion of gamma to beta polymorphs in the ambient temperatures are relatively slow[1],and in the higher temperature it was accelerated[22].

2.2.Seeding and micronization procedure

For the production of beta-HMX micro-seeds,a certain amount of coarseβ-HMX in water(1:5 weight ratio)was carefully milled in a clean porcelain crucibles until to reach the least particle size.The grinded suspension was poured into a glass beaker to settle coarse particles.After 20 min,the very small suspended particles were separated and used as antisolvent havingβ-HMX micro-seeds.The particle size of micro-seeds was＜5.0μm,with the concentration of 0.2 wt.%in the water antisolvent.For the continuous production process,the first micro-seeding is sufficient to control the polymorphs of micronized HMX particles.

For the preparation of HMX micro-particles in laboratory scale,4.0 gβ-HMX was dissolved into 250.0 mL of acetone solvent at the temperatureof 52.0°C and the solutionwas added gradually,with a flow rate of 250.0-1000.0 mL/min to water antisolvent with or without of 0.2 wt.%β-HMX micro-seeds.The temperature of the antisolvent was maintained in the temperature of 0.0-5.0°C.After that,the desired products were vacuum filtered and washed with suf ficient water to separate the residual solvent in the samples.Also,in other cases,the effects of 0.5 wt.%PEG-400 polymer and lauramide surfactant into the antisolvent on the morphology of products were investigated.

3.Results and discussion

3.1.Effect of micro-seeding on the polymorph of HMX microparticles

The using of rapid precipitation processes for the preparation of HMX fine particles,as stated in section(2.2),leads tothe creation of unsafe and unstable gamma polymorphs.The XRD patterns and particle size distributions of micro-seeds,and product with and without seeding are provided in Fig.1.The main peaks of XRD patterns of un-seeded product in Fig.1B were evaluated by X'Pert High Score Plus software and were indexed according to powder diffraction file(PDF)number 00-044-1621 for gamma-HMX with monoclinic crystal system.On the other hands,in accordance to the XRD patterns provided in Fig.1C,using of 0.2 wt.%β-HMX microseeds in the water,having relatively symmetrical particle size distribution,can easilycontrol the polymorphs of micro-particles from gamma to beta type.This product was indexed according to PDF number 00-042-1768 forβ-HMX with monoclinic crystal lattice.Size distribution curves of the modified product(see Fig.1C)shows that approximately 75%of the particle sizes are below 5.0μm and the 25.0%are between 5.0 and 10.0μm.Here,the cumulative distributionQr（xi）represents the concentration of particles equal to or smaller than a given particle sizexi.Values of the cumulative distributions range from 0 to 1.Also,the density distributionqr（xi）represents the amountof particles of a givenparticle sizexi,relative to the entire particle size distribution.

Therefore,micro-seed additives can be used to control and modifythepolymorphsofHMX fineparticlesin solventantisolvent process.In unseeded systems,the formation of crystals is via a spontaneous nucleation that will occur at a certain and usually high super saturation.A metastable polymorph formed via spontaneous nucleation or via seeding will eventually undergo a phase transformation to a more stable one,and finally to the thermodynamic stable form[26,29].The crystallizations controlled by seeding are effectively applied during the nucleation step by adding seeds of the desired form and thus overriding spontaneous nucleation.The theory behind the polymorph control ability of micro-seeding can be thought to derive from the physical intermolecular interaction that occurs between compounds in a supersaturated solution.In solution,liberated soluble molecules are free to move about in random flow.This random flow permits for the possibility of two or more molecular compounds to interact.This interaction can potentiate intermolecular forces between the separate molecules and form a basis for a crystal lattice.The placement of a seed crystal into solution allows the recrystallization process to accelerate by eliminating the need for random molecular collision or interaction.By introducing an already preformed basis of the target crystal to act upon,the intermolecular interactions are formed much more easily than relying on random flow.Often,this phase transition from solute in a solution to a crystal lattice will be referred to as nucleation.Seeding is therefore said to decrease the necessary amount of energy and time needed for nucleation to occur in a recrystallization process[26,27].

Some safety speci fications such as sensitivity to impact stimulus show the amount near to 5.4 Nm.for gamma products,and 7.5 Nm.for fine beta-HMX micro-particles,respectively.In other words,the quality and safety of the manufactured products in the presence of micro-seeds are improved.Because polymorphs change from gamma to standard beta type,and also sensitivity to impact are improved.

An interesting point in the mentioned micro-seeded method is that beta micro-particles produced during the process,can also act as newβ-HMX nuclei,and the production process go on continuously.The main experimental conditions in the production process of HMX microparticles,with the size distribution of＜10.0 μm,are practically fixed.Because size and morphology of particles can change with the some factors reported in section 3.2.The most important factor in controlling of polymorphs of HMX fine particles,is rapid precipitation or super-saturation process.The lower precipitation kinetics,can lead to increasing of particle sizes.Usually other process factors are constant and flow rate of HMX solution in the acetone controls the size of particles.Here we studied effect of two highest and lowest flow rate range,250.0 and 1000.0 mL/min,on the polymorphs of HMX by XRD method.In both conditions the polymorphs of products were gamma type.The important point by using this seeding method is that with increasing of kinetics of fine particles formation,by increasing of HMX solution flow rate to water antisolvent,the mentioned controlled effect can also be observed on the formation of beta polymorphs.This can increase the production capacity of process in the industrial scale.XRD spectrum recorded for the samples prepared by two flow rate of 250.0 and 1000.0 mL/min verify this subject(figure not shown).Of course,SEM graphs provided in Fig.2 offer that in the lower flow rate,the particle size become more coarse and asymmetric.Here,the sufficient time exists for crystal growth process,but in the higher flow rate the nucleation is dominant and the particles are symmetric.

3.2.Control of size and morphology of particles

Control of the particle size of HMX micropartcles in solvent/non-solvent methods is reported by Bayat et al.[10].In this research,it was also studied the effects of some main operational parametersand determining parameterson the sizeand morphology ofβ-HMX micro-particles,such as HMX concentrations in the solvent,solvent and antisolvent temperature,the amount of solution flow rate,and discharge ratio of solvent and water,were investigated and optimized.

The investigations showed that the temperature difference between the solvent and water is very effective on the size and morphology of products.In the higher temperature difference,high solution temperature and low water temperature,the microparticlesaresphericalwith thesmallestparticlesize.The maximum temperature of HMX solution in this study was selected 52.0°C,near to the boiling point of acetone solvent.When,the temperature of water antisolvent change from 5.0 to 30.0°C,the mean particle sizes change from 3.0 to 25.0 mm,respectively.When,the water temperature increase to 30°C,some asymmetric and largerparticles will be produced( figure notshown).The reason for this behavior is that the particle growth kinetics in higher temperatures is slower,and the crystal growth can continue with various external defects.The results of other experiments shows that whenever the HMX concentrations are lower and flow rate are higher,the mean particle size are smaller.To prepare HMX microparticles with mean particle size＜10.0μm,It is better to control the production process in the conditions of:250.0 mL of acetone solvent containing 4.0 g HMX at the temperature of 52.0°C and flow rate of 500.0-1000.0 mL/min,water as antisolvent with the temperature of 0.0-5.0°C,with solution-antisolvent volume ratio from 1:5 to 1:10.

3.2.1.Effect of soft templates on the morphology of particles

As observed in Fig.3,the results of other experiments show that the use of the water-soluble polymer of PEG-400 in the low antisolvent temperatures,leading to the production of more symmetrical and spherical particles.Also the presence of non-ionic surfactant of lauramide soft template,direct the crystal growth to irregular morphologies.Increasing of antisolvent temperature in the presence of lauramide lead to the formation of needle-like crystal,with the diameter near to 3.0-5.0μm and the length up to 10.0-40.0μm.In this condition,PEG-400 template direct crystal habits to very asymmetrical shape.The growth mechanism of spherical and needle-like HMX micro-particles can be proposed on the basis of time-dependent morphology evolution behavior[33]and nucleation kinetics[34].Here,the temperature difference between HMX solution and water is seen to be one of the most important parameters that determines the state of aggregation,size,and morphology of the particles.In the higher antisolvent temperature,the rate of super-saturation is lower and there are a sufficient time to grow particles on the rod-like or inhomogeneous structures.When the temperature of antisolvent are lower than 5.0°C,nucleation is dominant compared to crystal growth,and the morphology of particles become symmetric and spherical.

4.Conclusions

The results showed that the micro-seeding strategy,by using a low concentration ofβ-HMX fine particles in the antisolvent medium,can control the polymorphs of HMX micro-particles from dangerous gamma(γ)type to the desired and standard beta(β)with mean particle size distribution of＜10.0 μM.The unique advantages of this method are its low cost and safe conditions for the scale up production HMX micro-particles,and it can be developed to other explosive materials having various polymorphs.Also in the next steps,the effective factors on the sizes and morphologies of micro-particles in the presence of two soft templates of PEG-400 polymer and coconut fatty acid diethanolamide surfactant in the water antisolvent showed that PEG-400 in the low antisolvent temperatures,leading to the production of very spherical particles,and also non-ionic surfactant of lauramide,direct the crystal growth to needle-like structures.

Acknowledgements

The authors are grateful for the financial support of this work by Malek-ashtar University of Technology(I.R.Iran)Grant No.1395-064..

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30 December 2016

in revised form 21 April 2017 Accepted 24 May 2017 Available online 27 May 2017

?2017 Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

*Corresponding author.

E-mail address:s_damiri@mut-es.ac.ir(S.Damiri).

Peer review under responsibility of China Ordnance Society.