Synthesis and properties of β-CD/RDX microcapsules by host-guest inclusion complexation①

JIA Xinlei,WANG Jingyu,TAN Yingxin,HOU Conghua,ZHANG Yuanping

(School of Environmental and Safety Engineering,North University of China,Taiyuan 030051,China)

Abstract：A new coating method in the field of energetic materials,host-guest inclusion complexation was introduced.Based on the mature microcapsule technique,β-CD/RDX microcapsules were prepared by inclusion complexation.The properties of the particles were characterized by scanning electron microscopy (SEM),X-ray diffraction (XRD),and differential scanning calorimetry (DSC).XRD analysis show that the crystal structure of β-CD/RDX microcapsule does change in comparison to the pure RDX.The SEM results show that β-CD/RDX microcapsules are smooth,solid spherical particles with almost no coating defects.The DSC results show that compared with RDX,the activation energy of β-CD/RDX microspheres has been increased by 10.88 kJ/mol,and the thermal critical temperature has been improved by 9.21 ℃,whereas the self-accelerating decomposition temperature TSDAT is enhanced by 9.22 ℃.Therefore,the thermal decomposition performance has been improved significantly.

Key words： inclusion complexation;β-CD/RDX;microcapsules;thermal stability

0 Introduction

Microcapsule technology is a kind of encapsulation technique,that is,the solid,liquid or gas particles are encapsulated in a capsule with a diameter of 5～200 μm by means of various technical means[1].It plays a role in delayed release[2-5],anti-oxidation[6],anti-moisture[7-8]improving the dispersivity[9-10]and enhancing the compatibility with other substances to improve the mechanical properties of composite materials and so on.With small size and large surface area,microcapsule particle is conducive to dispersion and complete combustion.The inorganic core material is covered within the organic wall,which prevents its accumulation in the explosive combustion process.Although microcapsule technology is a mature technology,its research and application in energetic materials are still relatively scarce.

By inclusion complexation,microcapsules were formed on molecular water withβ-cyclodextrin used as the microcapsule coating material.Sinceβ-CD polyhydroxy is present on the outer side of the cyclodextrin molecular cavity,while the inner surface of the cyclodextrin molecule has only hydrogen atoms that cover the solitary electrons to the glycosyl oxygen atoms,so the whole cyclodextrin molecules have amphipathic performance,i,e.,cavity lipophilic and extracorporeal hydrophilic.The hydrophobic cavity can be combined with many inorganic and organic molecules to form host-guest inclusion[11].In the field of explosives,many of the single compound explosives mostly have high mechanical sensitivity and poor stability,and there is a large risk in the use of security.Due to the cavity effect ofβ-cyclodextrin,these disadvantages can be greatly improved by using it as a binder,which greatly improves the performance of the mixed explosive.

In recent years,microcapsule technology has been applied to the field of energetic materials,YANG Zhijian[12]prepared the energetic composites by using Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) as core material and thermoplastic polyester-ether elastomer (TPEE) as shell material via an emulsion solvent evaporation (ESV) method.Theβ-cyclodextrin is encapsulated with RDX and HMX through saturated solution method by Shi Yongwen et al[13].The solid inclusion products ofβ-cyclodextrin and RDX and HMX are obtained.However,this study only confirms the formation of inclusion complex by measuring the X-ray diffraction of the solid product,which don't further study the morphology and properties of the obtained clathrates.Using dextrin as crystal control agent,Hu Shuzhi et al[14]obtained a higher purity HMX with the crystal particles more regular and smooth,nearly spherical.The above researches have provided some reference value for the experiment of this paper.

Mainly based onβ-CD hydrophobic internal cavity structure,usingβ-CD hydrophobic lipophilic effect and space volume matching effect,β-CD/RDX microcapsules interact with RDX molecules to form stable clathrates through covalent interactions.In our paper,β-CD/RDX microcapsules are prepared by host-guest inclusion complexation.The performance of microcapsules is characterized by scanning electron microscopy (SEM),X-ray diffraction (XRD),differential scanning calorimetry (DSC).The safety performance of the prepared microcapsule particles is tested by the impact sensitivity.It is helpful to explore new desensitization technology for explosive coating.Also,further enrichment of the theoretical knowledge of the coating technology is conducive to provide a certain reference value for the development of nitrification explosives in the low-sensitivity and high-energy direction.

1 Experiment

1.1 Reagents and instruments

JRJ300-S digital high speed shear emulsifier was provided by Shanghai more magnetic Electronic Technology Co.,Ltd.VOSHIN-650W Ultrasonic Cell Crusher was provided by Wuxi Woxin Instrument Co.,Ltd.S4700 Cold Field Emission Scanning Electron Microscope was from Hitachi,Japan. DSC-131 differential scanning calorimeter was purchased from Setaram company,France.DX-2700 ray powder diffraction system was purchased from Dandong Hao yuan company.

1.2 Expriment process

Firstly,β-CD saturated solution is prepared in the 65 ℃ distilled water,and 5 g RDX is dissolved in acetone solution as a standby.Secondly,the RDX solution is slowly added dropwise to theβ-CD saturated solution by using a gel dropper.keep the stirring speed 500 r/min,stir for 30 min.At the same time,the temperature in the flask is slowly lowered to -10 ℃ in the low temperature thermostat.Afterβ-CD forms the saturated solution,its annular central voids are also occupied by water molecules.Since hydrophobic voids are more easily combined with foreign RDX molecules,when the RDX solution is slowly added, these water molecules would soon be replaced by RDX molecules to form relatively stable inclusion compounds.As the temperature of the distilled water slowly decreases,the inclusion complex gradually precipitate out of the aqueous solution.β-CD/RDX microcapsules are obtained after filtering and freeze-drying.(β-CD molecular structure diagram shown in Fig.1)

2 Results and analysis

2.1 Crystal structure analysis

The raw material RDX,refined RDX andβ-CD/RDX microcapsules are analyzed by XRD.The results are presented as follows.

It can be seen from Fig.2,diffraction peaks of X-ray diffraction patterns ofβ-CD/RDX microcapsules can correspond to those of the raw material RDX,indicating that the inclusion ofβ-cyclodextrin does not alter the lattice constant of RDX.From Fig.2 (b),the diffraction peak position of the refined RDX is basically the same as that of the raw material.But at the same diffraction angle,th40e diffraction peak intensity of the two is different.Compared with the raw material RDX,the intensity of the diffraction peak of the refined RDX becomes weaker,which is be cause the X-ray diffraction peak gradually decreases as the particle size decreases[15].From Fig.2(c),compared with the diffraction peak of the sample before coating,the diffraction peak of the coated sample is broadened and the intensity has declined.This is because theβ-CD is non-stereotyped (amorphous),and the coating of RDX will weaken the diffraction peak intensity of the sample,resulting in a decrease in the diffraction intensity of theβ-CD/RDX microcapsule sample.However,the diffraction peak position of the sample after coating is basically the same as that of the raw material,which is becauseβ-CD/RDX microcapsules are inclusion complex based on the cavity effect ofβ-cyclodextrin.The whole inclusion process is formed by the interaction between theβ-CD molecule and the RDX molecule,and the formation process does not affect the crystalline structure of RDX.

2.2 Morphology analysis

The raw material RDX,the refined RDX and β-CD/RDX microcapsules are subjected to scanning electron microscopy (SEM) testing.The results are shown in Fig.3.

As shown in Fig.3,(c1),(c2),(c3),β-CD/RDX microcapsules are regular solid spherical particles with smooth surface,uniform dispersion,dense coating and no leakage or mutual bonding phenomenon.It can be seen from Fig.4 that most of the particles ofβ-CD / RDX microcapsules are distributed between 4.1 and 4.6 μm,and have a narrow particle size distribution with a median particle size of 4.2 μm.This can be explained by wetting theory[16-17].The cohesive force betweenβ-CD and RDX is mainly generated by the molecular action in the bond system,and there are two main stages in the process.The first phase is that after the surface of the RDX particles is contacted by theβ-CD molecule,and theβ-CD molecules wet the RDX surface through the Brownian motion.As the temperature of the solution reaction system decreases,β-CD molecules will be attached to the RDX surface.The second stage is mainly to produce adsorption force.When the molecular distance betweenβ-CD and RDX is between 0.9 and 1.0 mm,adsorption occurs between the two molecules.RDX surface will attractβ-CD,until their spacing reaches the maximum steady state.With low surface tension,β-CD can match with RDX surface to achieve a better degree.So the coating effect of the preparedβ-CD/RDX microcapsules is more perfect.

2.3 Thermal analysis

2.3.1 Effect ofβ-CD on thermal decomposition kinetics of RDX

The thermal properties of the raw material RDX,refined RDX andβ-CD/RDX microcapsules are analyzed by DSC and the activation energy is calculated.The DSC curves are shown in Fig.5.

As shown in Fig.5,for different heating rates,the decomposition peak temperatures of the raw material RDX and the preparedβ-CD/RDX microcapsules increase with the increase of heating rate.At the same heating rate,the peak temperature (Tp) of the refined RDX andβ-CD/RDX microcapsules varied from 0.64 to 1.14.This is because the addition ofβ-CD affects the decomposition activity of the activating molecule,and the stability of the explosive molecules has been improved to some extent,resulting in the hysteresis of decomposition peak temperature.

According to the heating rate at 5,10,20 K/min under the exothermic peak,the apparent activation energy,pre-exponential factor and thermal critical temperatureTp[18-19]can be determined respectively by Kissinger (1) and (3).

(1)

(2)

(3)

(4)

whereβiis the heating rate, K/min;Tpis the decomposition peak temperature of the explosive at different heating rates;Tbis the critical temperature of thermal explosion;Ais the pre-exponential factor,min-1;Ris the gas constant,8.314 J/(mol·K);Eais the apparent activation energy;TSDATis the self-accelerating decomposition temperature,K.The thermal decomposition kinetics parameters of different RDX particles are presented in Table 1.

Table 1 Thermal decomposition kinetic parametersof different RDX samples

It can be seen from Table 1,activation energy,A,TbandTSDATof the refined RDX andβ-CD/RDX microcapsules all have been improved compared with the raw material RDX.The activation energies of refined RDX andβ-CD/RDX microcapsules increase from 140.57 kJ/mol to 147.96 kJ/mol and 151.45 kJ/mol.The thermal critical temperatureTbincreases from 217.63 ℃ to 221.40 ℃ and 226.84 ℃,respectively.The self-accelerating decomposition temperatureTSDATincreases from 214.82 ℃ to 2218.65 ℃ and 224.04 ℃,respectively.The thermal properties ofβ-CD/RDX microcapsules significantly are improved,indicating that the addition ofβ-CD effectively can improve the thermal stability of RDX.

2.3.2 Effect ofβ-CD on thermodynamic parameters of RDX

In order to further discuss the catalytic effect of nano-combustible agent on HMX,according to the formulas (5),(6) and (7),activation enthalpy ΔH,activation free energy ΔGof the raw material RDX,the refined RDX andβ-CD/RDX microcapsules respectively.The results are shown in Table 2.

(5)

(6)

ΔG=ΔH-TΔs

(7)

whereT=Tp0;E,Ais given in Table 1;KBis the Boltzmann constant,1.380 7×10-23J/K;his the Planck constant,6.626×10-34J·s.

Table 2 Thermodynamic parameters ofdifferent RDX particles

It can be seen from Table 2,compared with RDX and the refined RDX,ΔHofβ-CD / RDX microcapsules is increased by 10.88 kJ/mol and 3.49 kJ/mol,respectively,and ΔGis increased by 7.3 kJ/mol and 2.74 kJ/mol.This indicates that when turning into active RDX ions,the coated RDX particles need to absorb more energy,that is to say,the thermodynamic properties ofβ-CD/RDX microcapsules are greatly improved.

3 Conclusions

(1) Inclusion complexation madeβ-CD successfully coat on the RDX surface.XRD analysis shows that the structure ofβ-CD/RDX microcapsules cannot change.And the preparedβ-CD/RDX microspheres are spherical solid particles with a median particle size of 4.2 μm.Also,the dispersion effect is good and the coating is dense.

(2) Through analysis and calculation,the activation energyEa,Tb,TSDAT, ΔHand ΔGofβ-CD/RDX microspheres are improved obviously,which indicates thatβ-CD/RDX particles prepared by inclusion complexation has good thermal properties.