Dependence of Particle Size and Size Distribution on Mechanical Sensitivity and Thermal Stability of Hexahydro-1, 3, 5-trinitro-1, 3, 5-triazine

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1 Defence Science Journl, Vol. 59, No. 1, Jnury 2009, pp Ó 2009, DESIDOC Dependence of Prticle Size nd Size Distribution on Mechnicl Sensitivity nd Therml Stbility of Hexhydro, 3, 5-trinitro, 3, 5-trizine Xioln Song nd Fengsheng Li Nnjing University of Science & Technology, Nnjing , Chin ABSTRACT Two kinds of RDX smples, with brod nd nrrow prticle size distribution, hve been fbricted by wet riddling nd solvent/non-solvent methods, respectively. By controlling the technicl condition, the RDX powders with different prticle sizes were obtined for ech smple. All smples were chrcterised by lser grnulrity mesurement nd scnning electron microscope (SEM. Using mechnicl sensitivity tests, slow cook-off test nd differentil scnning clorimetry (DSC, the mechnicl sfety nd therml stbility of RDX smples, depending on the prticle sizes nd size distribution, were studied. Results indicted tht, for ech kind of RDX prticles, the mechnicl sensitivity nd therml stbility of smples chnged ccording to the prticle size. However, lthough two smples hd lmost the sme verge prticle size, their sfety chnged when two prticle size distributions differed. Concretely, the mechnicl sensitivity of RDX reduced nd their therml stbility incresed grdully long with the decresing of prticle size. Menwhile, RDX with brod size distribution hd higher mechnicl sensitivity nd therml stbility thn smples with nrrow size distribution. Keywords: RDX, prticle sizes, size distribution, mechnicl sensitivity, therml stbility, kinetics 1. INTRODUCTION The wide ppliction of hexhydro, 3, 5-trinitro, 3, 5-trizine (RDX hs prompted vigorous efforts to understnd nd improve its sfety 1-5. Among the fctors influencing the sfety of explosives (especilly RDX, such s physicl nd chemicl structures, chrge dimeters nd density, etc, size nd size distribution of explosives prticles ply significnt role, but the specific influences of size nd size distribution on sfety properties re uncler 6. Liu et l. reported tht the friction sensitivity of RDX decresed linerly s the prticle size ws reduced from 154 mm to 10 mm 7. However, Yng et l. investigted the friction sensitivity of RDX with verge prticle size =8.95 mm, mm, mm nd 640 mm, respectively, but filed to find ny specific reltionship between prticle size nd friction sensitivity 8. Chen 9 even indicted the mechnicl sensitivity of explosives cn be incresed by reducing their prticle sizes. The possible resons for the bove controversy re s follows: First the verge prticle size ws dopted in the bove studies for chrcterising the prticle size effects on sensitivity. However, the ctul size distribution of prticles round the verge vlue, s well s prticle morphologic properties, my hve certin effects on the experimentl results. As the results show in this study, the RDX sfety of two smples with lmost the sme verge prticle size my be significntly different if these hve different prticle size distributions. Therefore, considertion of the prticle size distribution is necessry to clrify the reltionships between RDX sfety nd the prticle size. However, few Received 26 December 2007 reserchers ddressed the effect of size distribution, which my be the cuse of the discrepncy noted bove. The second reson for the bove controversy my be the limited by dynmic rnges of the prticle sizes dopted in those studies. To completely understnd the effects of prticle size on RDX sfety properties, brod dynmic rnge of prticle sizes should be studied. In this study, the rnge of verge prticle size of RDX chnge is from 490 mm to mm, which ws much wider rnge thn the rnge used in mny other previous studies. The sfety of RDX smples, with lmost the sme verge prticle size but different size distribution nd morphologies, re compred. In ddition, the dependence of prticle size nd size distribution on mechnicl sensitivity, therml stbility, nd decomposition of RDX re discussed. 2. EXPERIMENTAL Rw RDX powder ( =41.8 mm, d 90 =271.8 mm ws obtined from Yingung Chemicl Plnt of Chin. Emulsifier (OP (A.R. nd Acetone (A.R. were purchsed from Shnghi Chemistry Regent Ltd. Using wet riddling nd solvent/ non-solvent methods, two kinds of RDX smples were fbricted. By controlling the technicl prmeters, the RDX prticles with different verge prticle sizes ( were obtined within ech of the two kinds of smples (shown s Tble 1. Prticle size nd size distribution of smples were mesured by Mster Sizer Instrument. The morphologies of smples were exmined by scnning electron microscope 37

2 DEF SCI J, VOL. 59, NO. 1, JANUARY 2009 Tble 1 Fbriction of RDX smples with different prticle sizes nd size distribution Method Mteril Medium Technicl prmeters (ìm Wet riddling Rw RDX Alcohol (95 Wt.% Size of sieves Solvent/non-solvent Corse RDX Acetone (solvent, queous Stirring rte, temperture difference solution of emulsifier (non-solvent between solvent nd non-solvent 0.49, 2.86, 5.6, 10.4, 16.6, 41.8, 92.5, , 4.86, 19.4, 62.8 (S HGZ impct instrument ws used to test the impct sensitivity of RDX smples. Ech smple (35 mg ws tested for 25 time to obtin H 50 (The H 50 vlue represents the height from which dropping 5 kg weight results in n explosive event in 50 per cent of the trils.. With 4 peering tests, n verge vlue of H 50 ws clculted. WM friction instrument (90, 3.92 MP ws employed to test the friction sensitivity of smples. Ech smple (20 mg ws tested 25 time nd n explosive probbility P (% ws obtined. An verge vlue of P ws estimted with 4 peering tests. In slow cook-off test, the heting rte of ech smple ws t 3 o C min. The self-ccelerted temperture of ech explosive chrge in the course of heting ws logged to estimte the therml sensitivity of RDX smples. Differentil scnning clorimetry (DSC of smples ws performed on TA Model Q600 differentil scnning clorimeter under floting N 2 tmosphere (10 ml min. The heting rtes of ech smple were t 5 C min, 10 C min nd 20 C min. 3. CHARACTERISATION OF SAMPLES Figure 1 shows the prticle size nd size distribution of rw RDX nd prt of prepred smples tht hve lmost the sme but different size distribution. Figure 1 ( indictes tht rw RDX with =41.8 µm hs very brod size distribution rnging from 300 nm to 300 µm. Figure 1 (b lso shows brod size distribution smple with three distribution peks. However, the smple prepred by solvent/non-solvent exhibits more nrrow size distribution curve in Figure 1(c. It is distinct tht lthough two smples Figure 1. Size distribution of RDX smples: - rw RDX powders, =41.8 µm; b-prepred by riddling, =2.86 µm; c-prepred by solvent/non-solvent method, =2.29µm. Figure 2. SEM imges of RDX smples: -rw RDX powders, =41.8 µm; b-prepred by riddling, =2.86 µm; c-prepred by solvent/ non-solvent method, =2.29 µm. 38 ( (b (c

3 SONG & LI: MECHANICAL SENSITIVITY AND THERMAL STABILITY OF RDX (shown s Figs 1 (b nd 1 (c hve lmost the sme, their size distribution re considerbly different. Figure 2 provides the SEM imges of the bove smples. The differences of microstructure mong these kinds of prticles re obvious. The morphology of rw RDX is nonuniform nd shows irregulr polyhedron shpes with very corse surfces. The morphology of smple shown in Fig. 2 (b is similr to the rw powders, in which there re mny little prticles mong lrge ones. Peculirly, unlike the other two kinds of smples, the microstructure of prticles prepred by solvent/non-solvent method is homogeneous nd hs cosh or spherule shpes. 4. MECHANICAL SENSITIVITY ANALYSES Smll-scle mechnicl sensitivity tests were performed on ll the RDX smples prepred, the results re shown in Fig. 3. Every plot of Fig. 3 contins two curves corresponding to the trend of impct or friction sensitivity dt chnged s function of prticle size. For the smples with brod size distribution, the H 50 increses s the prticle size decreses. Especilly within the scle of 0.49~41.8 µm, this trend is while the externl force cts on the smller RDX prticles, relesed hets will dissipte fster nd the force cting on unit re of prticles surfces becomes lower due to the lrger contcting re mong these smller prticles. Therefore, hot spot is hrd to form to enble detontion. Besides, hot spot is more likely to be formed t corser surfces becuse of their lrger friction coefficient. Therefore, RDX with brod size distribution smples cn generte more hets thn those with nrrow distribution when the two kinds of powders undergo the sme mechnicl stimultion. 5. THERMAL STABLITIES ANALYSES 5.1 Therml Sensitivity Tests Figure 4 is mde by two sub-plots of therml sensitivity to prticle size, corresponding to two kinds of RDX smples respectively. Plots (, b illustrte tht the T brek of both kinds of smples decrese s their prticle sizes become lrger, which implies tht the smller RDX prticles hve lower therml sensitivity. Compring the experimentl results between two kinds of smples, one finds tht the influence of prticle H Smples with brod size distribution Impct sensitivity, H 50 Friction sensitivity, P (% P (% H Smples with nrrow size distribution Impct sensitivity, H 50 Friction sensitivity, P (% ( (b Figure 3. Impct nd friction sensitivity of RDX smples s function of prticle size: -prepred by riddling; b-prepred by solvent/ non-solvent method. The error brs re respectively the verge nd stndrd devition, of the verge vlue obtined from four peering tests. b much clerer, indicting tht these kind of prticles with smller size re more pssive to impct force. In friction test, this kind of trend lso exists, i.e., smller prticles hve lower explosive probbility. In Fig. 3(b, for smples with nrrow size distribution, their impct nd friction sensitivity both rise lmost linerly long with increse of prticle size from 2.29 µm to 62.8 µm. On the other hnd, ccording to different prticle sizes, the verge vlue of H 50 for nrrow distribution smples ( H 50 =52 cm is slightly higher thn tht of brod distribution prticles (=46.49 cm, suggesting tht the former is more pssive to impct stimuli. Menwhile, the verge vlue of explosive probbility ( P of nrrow distribution smples equls to 82 per cent, nd is bit lower thn tht of brod distribution smples ( P =85 per cent. Hot spot theory cn be employed to explin the bove experimentl results 10. In the mechnicl sensitivity tests, T brek ( o C T brek ( o C T brek =211.9 o C b T brek =210.2 o C RDX with brod size distribution RDX with nrrow size distribution Figure 4. Self-ccelerted temperture of RDX s function of prticle size: ( -prepred by riddling; (b b- prepred by solvent/non-solvent method 39

4 DEF SCI J, VOL. 59, NO. 1, JANUARY 2009 size distribution on the therml sensitivity of RDX is not tht cler in terms of the little difference of verge T brek vlues obtined from different prticle sizes, in which the T brek of brod size distribution smples is higher thn tht of nrrow size distribution smples by only 1.7 C. 5.2 Therml Decomposition Tests Figure 5 is the DSC curves of RDX smples with different size distribution in N 2 tmosphere t the heting rtes of 5 C min, 10 C min nd 20 C min. In ech cse, the temperture of the exothermic pek nd the decomposition het (determined by the re of the exothermic pek in DSC curve decrese with decresing heting rte. However, the results for the different kinds of smples do not generlly where E is finl pprent ctive energy of therml decomposition for smple, E (5-10K min, E (5-20K min nd E (10-20 K min re the ctive energies clculted from Eqn. (1 by Strink method. Figure 6 shows the plots of pprent ctive energy ( E of therml decomposition to the prticle size nd size distribution of RDX smples. In Fig. 6, for ech kind of smple, no reltionship is observes between nd prticle size. However, on compring two kinds of smples, the verge vlue (clculted with the dt t different of for nrrow size distribution smples (=109.6 kj mol is lower thn tht for brod size distribution smples (=124.5 kj mol. As result, it implies tht smples with nrrow size distribution will decompose first in the course of heting, which is in Het Flow (W g Exo ln(t p 1.8 f R 2 = E = kj mol T p (K 5 o C min 10 o C min 20 o C min Temperture ( o C Het Flow (W g b Exo ln(t p 1.8 f R 2 =0.997 E = kj mol T p (K 5 o C min 10 o C min 20 o C min Temperture ( o C ( (b (c Het Flow (W g c Exo ln(t p 1.8 f R 2 = E = kj mol T p (K 5 o C min 10 o C min 20 o C min Temperture ( o C Figure 5. DSC trces of RDX with different prticle size nd size distribution: ( rw RDX powders, =41.8 µm; (b prepred by riddling, =2.86 µm; (c prepred by solvent/non-solvent method, =2.29 µm. Ech inset is Strink plot for the therml decomposition pek of DSC curves. Symbol R 2 is used to identify the liner coefficient of ln (T p 1.8 f to 1000 T p-. shift to the sme extent t ny given heting rte. To study the effects of prticle size nd size distribution on the pprent ctive energies of therml decomposition of RDX, Strink method is used in kinetic evlution. It is n order of mgnitude more ccurte thn others nd complies with the following eqution 11,12. S æt ö p E ln ç = A + C ç è f ø RT (1 p where T p is the temperture of exothermic pek in DSC curve, K; f the heting rte, K min ; E is the ctive energy, J mol ; s, constnt, nd A is constnt depending on the choice of s. In the cse of Kissinger method s=2 nd A=1, the Ozw method s=0 nd A=1.0518, while the Strink method s=1.8 nd A= E. The lst method is employed, nd E of smples re determined. Becuse there re differences mong vlues of R 2 in the inserted grphs of Fig. 5, the finl pprent ctive energy of ech smple is expressed s n verge vlue of E clculted from Strink s formul with DSC dt collected t every two heting rtes. [ E + E 1 1 (5 10 min - + E ] K (5 20 K min (10-20 K min E = (2 3 E (kj mol Rw RDX RDX with brod size distribution RDX with nrrow size distribution Figure 6. Plots of E for RDX to prticle size. The error brs re respectively the verge nd stndrd devition of the verge vlue of E. ccordnce with the experimentl results in Fig. 4. As to therml stbility, het conductivity of explosive plys significnt role. In generl, s the prticle size of explosive decreses, the specific surfce re nd the mount of toms locted on the prticle surfce increse, which 40

5 SONG & LI: MECHANICAL SENSITIVITY AND THERMAL STABILITY OF RDX mens tht the outer electronic orbit extends nd the toms vibrting spce expnds. These chnges result in n improvement of the therml conductivity mong explosive prticles 135. As result of higher therml conductivity, the hets from therml decomposition cn be dissipted in time nd further decomposition is restrined. Accordingly, smller prticles led to higher therml stbility. Furthermore, the contcting res mong the prticles with smooth surfces re lesser thn those of prticles with corse surfces. Therefore, therml conductivity of nrrow size distribution smples is lower, thus fcilitting het ccumultion in the course of heting. If the hets generted re more thn the hets which re rdited, temperture of explosive system increses continuously, thereby ccelerting therml decomposition of explosive gin. Likely becuse of such self-ctlytic rection, the RDX smples with nrrow size distribution exhibit lower vlues of T brek nd ( E betweem two kinds of smples. 6. SUMMARY In the introduction, not only mny reported results bout the influence of prticle size on sfety of explosives were summuised but lso the discrepncies mong them ws shown. Therein, it ws speculted tht the size distribution of explosive prticles directed the disunity. To investigte the effects of size distribution on mechnicl sensitivity nd therml stbility of explosives, two kinds of RDX smples with nrrow nd brod size distribution were fbricted by wet riddling nd solvent/non-solvent methods. Menwhile, through controlling the size of sieves nd temperture differences between solvent nd non-solvent etc, RDX with different prticle sizes were obtined for ech kind of smple. The results of the tests indicte tht the prticle size hs direct influence on sfety properties of RDX, but such influence depends to lrge extent on the size distribution s expected. For both kinds of smples, the mechnicl sensitivity decrese long with the decres of the prticle size. Moreover, in slow cook-off test, smller RDX prticles hve lower therml sensitivity. However, RDX with brod size distribution is more sensitive to mechnicl ction thn the nrrow size distribution smples when their vlues re close. In ddition, the verge vlue of ctive energy for RDX smples with nrrow size distribution is lower thn smples of brod size distribution. ACKNOWLEDGEMENTS This work is the combined output of severl reserch groups in our centre. The uthors would like to thnk Chengfng Che of Ntionl Specil Superfine Powders Engineering Reserch Centre in Nnjing University of Science &Technology for her enthusistic supports in grnulrity mesurement nd DSC nlysis, nd Dr Hridwr Singh of High Energy Mterils Reserch Lb (HEMRL, Indi, for providing mny helpful suggestions for our experiments. REFERENCES 1. Siviour, C.R.; Gifford, M.J. & Wlley, S.M. Prticle size effects on the mechnicl properties of polymer bonded explosive. J. Mter. Sci., 2004, 39, Xu, S.Y.; Zho, F.Q. & Li, S.W. Impct nd friction sensitivity of composite modified double bse propellnt contining hexnitrohexzisowurtzitne (CL-20. Chinese J. Prop. Technol., 2006, 27, Horst, J.H. ter; Geertmn, J.H. & Rosmlen, G.M. vn. The effect of solvent on crystl morphology. J. Crystl Growth, 2001, 230, Krky, P.; Sidhoum, M. & Christodoultos, C. Aqueous solubility nd lkline hydrolysis of the novel high explosive hexnitrohexzisowurtzitne (CL-20. J. Hzrd. Mter., 2005, B120, Lumn, J.R.; Wehrmn, B. & Kuo, K.K. Development nd chrcteriztion of high performnce solid propellnts contining nno-sized energetic ingredients. In Proceedings Comb. Inst., 2007, 31, Liu, Y.C.; Wng, Z.S. & Lv, C.L. Influence of prticlesize nd grdtion of RDX on the shock sensitivity of polymer bonded explosive. Act Armmentrii, 2005, 26, Liu, Y.C.; Wng, J.H. & An, C.W. Effect of prticle size of RDX on mechnicl sensitivity. Chinese J. Explos. Prop., 2004, 27, Yng, B.L.; Chen, R.Y. & Co, X.H. Influence of prticle size of RDX on the detontion properties. Chinese Inititors Pyrotech., 2004, 3, Chen, T.S.; Zhng, Y.R. & Zhng, Y.H. Study on the influence of prticle size on the mechnicl sensitivity of HMX. Chinese Sichun Ordn. J., 2006, 5, Armstrong, R.W.; Ammon, H.L. & Elbn, W.L. Investigtion of hot spot chrcteristics in energetic crystls. Thermochim. Act, 2002, 384, Fn, R.H.; Lu, H.L. & Sun, K.N. Kinetics of thermite rection in Al-Fe 2 O 3 system. Thermochim. Act, 2006, 440, Vyzovkin, S. & Wight, C.A. Kinetics in solids. Annu. Rev. Phys. Chem., 1997, 48, Ye, Shuji; Tonokur, Kenichi & Kosh, Mitsuo. Energy trnsfer rtes nd impct sensitivity of crystlline explosives. Comb. Flme, 2003, 132, Feng, X.J.; Wng, X.F. & Hn, Z.L. The study of chrging size influence on the response of explosives in slow cook-off test. Expl. Shock Wves, 2005, 25, Tillotson, T.M.; Gsh, A.E. & Simpson, R.L. Nnostructured energetic mterils using sol-gel methodologies. J. Non-Cryst. Solids, 2001, 285,

6 DEF SCI J, VOL. 59, NO. 1, JANUARY 2009 Contributors Ms Xioln Song is doctor cndidte studying t Ntionl Specil Superfine Powder Engineering Reserch Center of Chin (NSSPERCC. Menwhile, she works s reserch scientist in the field of synthesis nd sfety estimte of superfine energetic mterils in North University of Chin. In NSSPERCC, she is engged to mstering nd improving the sfety of new explosives, pyrotechnics nd solid propellnts, nd synthesizing the nnometer nd nnocomposites energetic mterils. Prof Fengsheng Li is Professor t Nnjing University of Science & Technology, Chin. He hs been working on the design nd mnufcture technology for new propellnt nd superfine mterils. He invented mny techniques for mnufcture of nno nd micron mterils nd mny techniques for mnufcture of micron-nno composites. For yers, he involved himself with reserch on the effects of nnometer mterils nd micron-nno composites on the burning rtes, pressure exponents nd mnufcturing technology of solid propellnts. He hs invented severl new nd effective techniques for incresing the burning rtes nd decresing pressure exponents of solid propellnts. He hs compiled 10 books nd more thn 200 scientific ppers hve been published in vrious reputed journls. He ws conferred the title of outstnding scientist with prominent contribution by the Government of Chin. Presently, he works s the Director of Ntionl Specil Superfine Powder Engineering Reserch Center of Chin (NSSPERCC, Director of Science Committee for Zoology nd Environment Mterils in Jingsu province of Chin, Administrtive Director of Prticles Technology Society of Chin, nd the Committeemn of Fine Chemicl Engineering Committee of Chemistry Engineering Society in Chin. 42