Defence Science Journal, Vol. 54 No. 4 October 2004 pp. 483-492 2004. DESIDOC Waterproof ing Materials for Ammonium Nitrate R.S. Damse High Energy Materials Research Laboratory, Pune-411 021 ABSTRACT This study explores the possibility of overcom ing the problem of hygroscopicity of ammonium nitrate by coating the particles with selected waterproofing materials. Gravimetric analysis of the samples of ammonium nitrate coated with eight different waterproofing materials, vis-a-v is, uncoated ammonium nitrate, were conducted at different relative hum idity and exposure time. The results indicate that mineral jelly is the promising waterproofing material for ammonium nitrate among the materials tested, viz, calcium stearate, dioctyl phthalate, kaoline, diethyl phthalate, dinitrotoluene, shelac varnish, and beeswax. Attempts were made to confirm the waterproofing ability of mineral jelly to ammonium nitrate using differential thermal analysis and x-ray diffraction patterns as an experimental tool. Suitab ility of mineral jelly as an additive for the gun propellant was also assessed on the basis of theoretical calculations using THERM program. Keywords: Hygroscopicity, phase transition, waterproo fing ability, mineral jel ly, amm onium nitrate. high energy materials, coating materials 1 INTRODUCTION The search for the new clean burning propellants is necessitated by the demand for smokeless and less-sensitive propellants, which can suit the ecological requirements calling for HCI-free burning. The solution of these problems requires the replacement of commonly used ammonium perchlorate by a different oxidiser'. Though the am monium dinitramide (ADN) and hydrazinium nitroformate (HNF) are energetic oxidisers, but these are no t available in the required quantities2. Hence, ammonium nitrate has received renewed attention as a possible eco-friendly oxidiser for use in solid propellants in the near future3. The innocuous nature of the combustion products of ammonium nitrate, its low c ost, and easy availability, makes it an attractive oxidiser to the propulsion community. However, the major drawback of ammonium Revised 13 August 2003 nitrate is its dimensional instability due to its polymorphic forms (Table I). A remarkable change in ammonium nitrate volume during transition from 32 practical point of view4. Its high hygroscopicity is also an undesirable factors. However, chemically pure and water-free am monium nitrate requires higher temperature for the transformation of phase IV. At 50 C, it directly transforms into tetragonal phase 11, whereas a trace of water favours the normal transition of phase IV to phase I11 at 32 C and causes cacking. Hence, hygroscopicity of ammonium nitrate has been pinpointed as the major cause for cacking. Cacking of ammonium nitrate is highly undesirable as far as the use of ammonium nitrate for solid propellant is concerned as it affects the structural integrity, energetics, and ballistic regularity of the propellant.
Transcript
8/13/2019 Waterproofing Materials for Ammonium Nitrate
To ove r com e t he se d r a wb ack s , va r i ous ( a ) Mi ner a l j ell y(MI) (JSS-8030-9)researchers have car r i ed out ex tens ive work toachieve the phase stabilisation of am mon ium nitrate, (b) alcium stearate CS) (AR-grade,
particularly to overco me the pro blem of transition6- ' bulk density 2)
f r om phase IV t o phase 11 i a t 32 C . Th e p re sen t (c) Diethyl phthalate DEP) (CP-grade)studv also deals with the sam e problem. How ever,the approach to resolvp the problem i s d i ffe rent . (d) ~ i ~ i ~ ~ ~ t ~ lDNT) ( ~ p - ~ ~ ~ d ~ )In the present s tudy, the a t t empts a re a imed a tovercoming the hygroscopicity of ammonium nitrate (e) Shelac varnish (SV) (CP-grade)
by coating its particles w ith wate rproofin g materials,whereas the al ready reported work is based on f) Kaoline (hydrated aluminium si l icate ,
the phase s tab i l i sa t ion of ammonium ni t ra te by white powder)
incorpora t ing the meta l oxide s and n i t ra tes , such (g) Dioctyl phthalate (DO P) (CP-grade)as C u O NiO KNO,, etc.
(h) Beeswax (BW)Var ious v iable methods and number of
waterproofing materials have been reported in theliterature to reduce the hygroscopicity of amm oniumnitratet '- . I t is also reported that ferti l iser-gradeammon ium nit rate is coated with fer t f low to avoidcacking and to improve th e free-flow characteristics
commerical lyt4.However, the quant i tat ive data onthe waterproofing abili ty of the coating materialsto ammon ium nitrate is not reported. In view ofthis, the results of gravimetric analysis obtainedon eight different waterproofing materials aimedat reducing the hygroscopicity, and thus, arrestingphase transit ion of ammonium ni t rate at 32 C soas to establ ish the ut i li ty of amm onium ni t rate intogun propellant, are presented in this paper.
2. E X P E R IM E N TA L P R O C E D U R E
2.2 P r e p a r a t i o n o f t h e S a m p l e s
Six different sample s of ammon ium nitrate coatedwith m ineral jelly, calcium stearate, Kao line, diocty lphthalate , diethyl phthalate and dini t rotoluene,
respectively were prepared using petroleum ether(60 40 ) as a solvent whereas ethyl alcohol wasused for the coating of she llac varnish to ammoniumnitrate. Selection of the solvent fo r the coating hasbeen based on the so lubi l i ty of the coa t ing(waterproofing) mater ial . Ammonium ni t rate wasinsoluble in the solvents used fo r the experiments .The samples were air dried initially and thereafterkept inside the desiccator. Complete evaporationof the solvent was confirm ed on the basis of constant-weight criteria.
2.1 Mat e r i a l s 2.3 Gravim et r ic Analys is of Coat ing Mater ia l s
Crystalline amm onium nitrate having an average The waterproofing abili ty of eight differentparticle size of 120 pm , purity 99 per cent, and specific coating materials to amm onium nitrate was assessedgravity 1.7wasused. Waterproofingmaterialsusedwere: on the basis of gravimetric analysis carried out at
8/13/2019 Waterproofing Materials for Ammonium Nitrate
Figure 1. Comparative data of moisture absorption for ammonium nitrate coated with waterproofing materials at 65 RH[AN: coating materials 10.0:0.1)1.
various levels of relat ive hum idity und er control ledcondit ions using appropriate saturated sal t solu-t ions [NaN 0,(65 RH ), K,S0,(75 RH ), SrNO,(85 RH), and H,O (95 RH )] and d i fferen texposure t ime intervals. Th e m aximum time intervalup to which the present experiments w ere conductedwas 96 h. Criteria for the assessm ent of w aterproofingabil i ty of the coating material w as worke d out onthe basis of percentage gain in weigh t of the sam pleas a function of relat ive humidity and exposuretime. The minimum gain in weight percentage indicatesthe superiority of the coating material in respectof waterproofing abil i ty, whereas the maximumgain in weight indicates the inferiority of the coatingmaterial. Mo isture absorption profiles were obtainedby plot t ing percentage moisture content againstthe exposure t ime. Th e results obtained by the
gravimetric analysis have been presented in theFig . 1 and a comparat ive data on m ois tu re con ten tfor the samples of ammonium nitrate coated withmineral jelly and uncoated amm onium n itrate obtainedat various relative humidity leve ls is given separa telyin the Fig.2. Gravim etric analysis was also carried
out fo r the samples o f ammonium n i t ra te coatedwith mineral jel ly to the extent of 0 .5 per cent ,1 .0 per cent , and 1.5 per cent , respectively, tooptimise the quanti ty of the coating material toammonium nitrate.
2.4 D i f f e r e n t i a l T h e r m a l A n a l y s i s o f S a m p l e s
Therm al behaviour of the samples was studiedusing NETZCH, a German make differential thermalanalyser (DTA). The DTA curves were recordedin an inert atmosphere using a 10 mg sam ple inalumin a crucibles at heat ing rate of 10 Clmin.
2.5 X - R a y D i f f r a c ti o n P a t t e r n s of S a m p l e s
Th e x-ray diffract ion pat terns of samples weredetermined using Philips analytical x-ray equipment.
Studies were carried out at ambient temperature35 C 2 'C) with F e target. The d values obtained
were converted to those for Cu and Cr targetsapplying the fol lowing relat ionship:
d(Fe)ld(x) = h(Fe) lh (x)
8/13/2019 Waterproofing Materials for Ammonium Nitrate
2 6 Evaluation of Mineral Jelly as an Additivefor Gun Propellant
Two series of the compositions were formulated.The first series comprised three differen t compositionsbased on standard tr iple-base (NQ) propellantscontaining additional ammon ium nitrate to the exten tof parts, 6 parts, and 8 parts, respectively designated
as amm onium nitrate-based compo sit ions(AN-1,AN-2, and AN-3) and the second ser ies comprisedthe same th ree composi t ions con tain ing add i t ionalmineral jelly to the e xtent of10 per cent to ammonium
nitrate in the composit ion designated asANMJ-based compositions(ANMJ-1, ANMJ-2, and ANMJ-3),respectively (Ta ble3). Th e theoret ical performanceof the comp osit ions was comp uted usingTHERMprogram and is presented in Table4
3 RESULTS & DISCUSSION
Th e data available from the gravimetric experimentsclear ly ind icate that mineral je l ly i s the bes twa te rp ro o f in g mate r i a l fo r am mo n iu m n i tr a te .Calcium stearate and kaoline were found to be the
AMMONIUM NITRATB COATED WITHI@ MINERAL JELLY
8 7 2 96
TIME (h )
Figure 2 Moisture absorption of ammonium n itrate (uncoated) and amm onium nitratecoated with mineral jelly (AN:MJ 10:O.l)(65 RH to 95 RH).
8/13/2019 Waterproofing Materials for Ammonium Nitrate
U A MS C: WA I CKYKUUFINU MA1 C K I A L S F U K A M M U N l U M Yl l K A I C
second and the third best , respectively, whereasd io c ty l p h th a l a t e , b eeswax , d i e th y l p h th a l a t e ,dini trotoluene, and shelac varnish fol lowed theorder sequential ly (Fig. 1) . I t is seen that mineraljel ly is the promising waterproofing material forammonium nitrate under a wide range of relat ivehumidity (65 R H t o 9 5 R H ) ( Fi g.2) and that
coating of ammonium nitrate with mineral jel ly(0.5 part) leads to relatively h igher m oisture content,whereas coating with mineral jel ly (1 .5 parts) doesnot show any appreciable difference than that ofthe coating with mineral jel ly (1 part) in respectof moisture content . Hence, the weight rat io foramm onium nitrate and mineral jel ly in the samplehas been worked o ut as amm oniu m nitrate:mineraljelly (1O.O:O.l).
Table 3. Chemical formulations of the propellants based onN Q containing additional ammonium nitrate andmineral jelly
Sr.No. Propellant composition
NQ +AN (5 parts)
NQ AN (6 pans)
NQ AN (8 pans)
Series 2
ANMJ-I NQ AN (5 pans) MJ (0.5 parts)
ANMJ3 NQ + AN (6 pans) MJ (0.6 pans)
ANMJ-3 NQ AN (8 pans) MJ (0.8 pans)
Various mechanisms have been proposed for Chemical composition of NQ propellant = NC (12.95 N )20.80 NG 20.60 P~cri te 5.00 Carbamite 3.60
the waterproofing act ion of the coating materials K SO 0.30
to amm onium nitrate by different scientists. Howev er,the phenomenon of the formation of hydrophobicfi lm over the granular surface of am mon ium nitrateis widely agreed upo n. It is postulated by Serizava15,e t al . that the mineral jel ly, a mixture of straightchain hydrocarbons (paraffinic), acts as a hydrophobicbarrier between ammonium nitrate crystals andingressing moisture. Thus, it reduces the hygroscopicityof ammonium nitrate significantly for a wide rangeof relative humidity. According to KustI6, the coatingso f h y d ro p h o b ic mate r i a l s , su ch a s p a ra ff in i chydrocarbons or polym eric resins increase the water
Table 2. The x-ray d ir ac ti on patterns (dvalucs) of ammoniumnitrate and ammonium nitratecoated with M J32-84 'C)
Ammonium nitrate Ammonium nitrateAN + AN
Cu-target I MJ Cr-target I MJ
Theor Exptl Theor Exptl
resistance of amm onium nitrate particles. The seconbest waterproofing material, calcium stearate, is substance very diff icul t to wet with water and shelps waterproofing by slowing down the ini t iawetting and acts as a sort of ba nie r to the hygroscopiciof amm onium nitrate as reported by Taylor anSillitto17.Dobrovfol SkiiI8attributed the waterproofingaction of calcium stearate to the physico-chemicapropert ies of amm onium nitrate. According to himthe additio n of calcium stearate up to per cent t
amm onium n itrate decreases the hygroscopici ty oammonium n i t ra te to a considerab le ex ten t . Thmicrostructu re becom es significantly com pact andense due t o the decrease o f vo ids and f ine spacewithin the powd er. It is also postulated by Phile niet al. that calcium stearate reacts with ammoniumni t ra te to fo rm a t igh t ly adher ing andmoisture-resista nt coa ting of Ca5NH,-(NO,), (OH),.3H20and thus reduces the hygroscopici ty of amm oniumnitrate. I t was experim ental ly shown that thinterfa ce of am m on ium n itrate and calcium stearahas both physical an d chemical stability. A ccordinto Ko rped ak and E dw ardm , calcium stearate acts aa parti t ioning agent to am mon ium nitrate granuleKaoline, in a f ine powder form, acts as an inercoating material , which adheres to the surface oamm onium n i t ra te crys ta l s o r g ranu les and i
8/13/2019 Waterproofing Materials for Ammonium Nitrate
s imul taneously capab le ' o f absorb ing mois tu re .Therefore, i t was real ised as the third promising
wate rp ro o fin g mate r i a l fo r amm o n iu m n i t r at e .Dimitr i jevic and Sikir ica2 ' postulated that 0 .1 percent to 5.0 per cent kaoline is more effective surfactantfor reducing hygroscopici ty of amm oniu m nitratedue to i ts abi l i ty to form the uniform dispersionenvelope around the ammonium nitrate crystals.Katayamaz2, t al have invest igated th at the act ionof kaoline as a readily dispe rsible slu dge coagulan tsuppresses the mois tu re absorp t ion and preven tsthe coalescence of amm onium n itrate crystals. Kust16,also postulated that the coating of f inely dividedkaol ine to amm onium n i t ra te acts as a condi t ion ingagent , leading to the formation of free-flowingparticulates.
The dioctyl phthalate and dieth yl phthalate arenon-polar in nature, and thus, creat e a sort of barrierbetween ammo nium nitrate and water droplets, resultingin hydrophobising effect . The dioctyl p hthalate hasmore non-polar nature than the diethyl phthalatedue to long carbon chain , resu l t ing in to morehydrophobic action. Therefore, the dioc tyl phthalatewas found to be bet ter waterproofing agent thanthe diethyl phthalate. Beeswax, an ester of longchain fatty acids, has shown com pnrable waterproofingabil i ty as dioctyl phthalate. Th e waterproo fingability of beeswax to am mo nium nitrate is attributedto the formation of water-insoluble soaps whichform thin f i lms around am mo nium nitrate particles
and decrease their h y g r o s ~ o p i c i t y ~ ~ .ccording toPagow ski and SubosZ', mixing of molten wax with
granulated am mon ium nitrate, intensifies the processof porosi ty, and thus, dec reases the hygroscopici ty.The d in i t ro to luene andshelac varnish were foundto be the non-promising waterproofing materialsfor amm onium n itrate a s compared to other materials.
Sam ples of amm onium nitrate coated with m ineraljel ly along with the uncoated ammonium nitratewere tested using DTA to confirm the phase stabilisationof ammonium nitrate. I t was found from the DTAresults that uncoated ammonium nitrate exhibi tedfour endotherms under ambient condit ions ( ini t ial
temperature 20 OC). The endotherms at3 C,84 C and 1 25 C can be at tr ibuted to phase IV tophase 111, phase I11 to phase 11, and phase I1 tophase I t ransi t ions, respectively. The endothermat 169 C corresponds to the melting point of amm oniumnitrate. In the samples where the amm onium nitratewas coated with the mineral jel ly, instead of theendotherm obtained a t 32OC in case of uncoatedamm onium nitrate, a new endo therm was observedat around 49 C (F ig .3 . This behaviour is similarto that exhibi ted by phase-stabil ised ammoniumnitrate (PS AN ) as reported in the literature6-'O.However,approach in both the cas es was different. From thepractical point of view, the effect of coating ofmineral jelly on moisture su sceptibility of ammoniumnitrate was evaluated un der controlled relative humiditycondition s. Th e moisture absorption profiles obtained
8/13/2019 Waterproofing Materials for Ammonium Nitrate
DAMSE: WATERPROOFING MATERIALS FOR A M M O N I U M N I l K A l C
at 65 RH, 75 RH, 85 RH, and 95 RH(Fig. 2) clearly bring out that coating of mineraljelly reduces the hydrophilic nature of ammoniumnitrate, which is an important attribute becausewater is reported to facilitate the formation of phase111 at 32 OC In view of these findings, mineral jellyis also considered to act as phase stabiliser to ammonium
nitrate by virtue of its waterproofing effect.
To obtain supporting evidences, samples ofammonium nitrate-coated with mineral jelly weresubjected to x-ray diffraction along with uncoatedammonium nitrate. The d values obtained on u
target for uncoated ammonium nitrate were 3.92,3.24, 3.19, 2.63, 2.45, and 2.28, which are closeto the values reported in the literaturei0. However,
samples of ammonium nitrate-coated with 1.0 per75
TEMPERATURE ( C)
Figure 3 DT hermograms
8/13/2019 Waterproofing Materials for Ammonium Nitrate
cent mineral je l ly gaved values of 3.99, 3.28, and ni t rate and a coolent for the gun propel lant . Hence,2.49. These resul ts are s imilar to those obtained i t is used as an addi t ive for the gun propel lantsfor ammonium nitrate incorporatingl0 uO d values containing amm onium ni t rate .3.91 and 2.47) and suggest acha ng e in the morphologicalstructure of am mon ium nitr ate-coa ted with mine:al A C K N O W L E D G E M E N T Sjelly. Similarly, the d values obtained on Cr targetalso conclude the same fact (Ta ble 2). This f indingis an evidence to the mechanisms for coat ing toammonium nitrate as proposed by differentscientist^^^ ^^.
Further evidence was obtained from the relat iveintensity calculated from the x-ray diffraction p atternsfor ammonium nitrate-coated with the mineral jellyand t he uncoa t ed am m oni um n i t r a t e f o r t hecorresponding 20 value.
The chemical composi t ions of the propel lantsbased on NQ containing additional amm onium nitrateand mineral jelly w ere formula ted to evaluate theballistic performance o f mineral jelly as an additivefor gun propellant (Table 3). Theoretically calculatedthcrmochemical param eters of the Propel lants aregiven in Table 4. I t has been observed from thedata that the additio n of mineral jelly (0.5-0.8 part)into propellant (ANM J-based propellant) decreasesthe f lame temperature (by 55 K) w ith only marginaldecrease in the force constant (10 Jlg). Th e marginaldecrease in force constant is attributed to th e decreaseof calorimetric value of the ANM J-based composition(15 cal lg) because of low er oxygen balance of themineral jelly. Hence, the mineral jelly plays an
addi t ional role of reducing the f lame temperatureof the propellant, and thus the gun erosion. It hasalso been reported that mineral jelly, to the extentof 5-6 weight per cent, was used as a stabiliser int h e e a r l y c o r d i t e ~ ~ ~ .
4 C O N C L U S I O N
The mineral je lly has been found to be thebest waterproofing mater ial f or amm onium n i t rateamong the materials tested in this study. Th e coatingof amm onium nitrate with 1 per cent mineral jellydecreases the hygroscopici ty of ammonium ni t ratenearly by f ive t imes at 65 RH , three t imes at75 RH, and two t imes a t 85 RH. However,beyond 95 RH level , the waterproofing effectof mineral jelly dec reased d rastically. Furth er, mineraljelly is found to act as a phase stabiliser to ammo nium
Th e author is grateful to Dr Haridwar Singh,
the then Director, High Energy M ater ials ResearchLaboratory (HEM RL), Pu ne, for his valuable guidanceand permission to publish this paper. He also wishesto thank Dr Harih ar Sing h, Director of Armament ,DRDO HQrs, New Delhi and Dr S.N. Asthana,Sc ient i s t F, HEMRL, Pune . f o r t he i r va l uab l esuggest ions, guidance, and encouragement .
R E F E R E N C E S
1. Quinn, Brewster M.; Scheridan, T.A.& Ishihara,A. Am monium ni t rate-magnesium combust ionand heat transfer mechanisms.J . Propul. Power1992, 8(4) , 760-67.
2. Borm an, S. Advanc ed energetic materials emergefor military and space applications.Chem. E n g g
N e w s 1994, 7 2 18-25.
3 . Enge l , W.; Ei sen r e i ch , N. & Dieml ing , A.Ammonium ni t rate , a less pol lut ing oxidiser,energetic materials-insensitivity to environm entalawareness. In Proc eedin gs of the 24Ih nternationalAnnual Conference of ICT, Germany, 1993.pp. 3.1-3.9.
4. Urbanski , T. Min ing explosives.In Chemistryand technology of explosives, Vol. 3, Ch. V.Pergamon Press , 1964. pp. 395-497.
5. Mclaren, A.C. Br ow n, R.N. On the mechanismof the thermal transformation in solid ammo niumnitrate. In Proceedings of the Royal Socei ty,London, 1962. Ser. 266A, 329-40.
7. Criado, A.J . ; Om ari , M.; Mart inej , J .A. ; Cobas,J . ; Calabres, R. M eri no , S. Stabi li ty of phaseIV of AN with Alt3. In Proceedings of the 241hInternational A nnual C onference of ICT, Germany,1993. pp. 75.1-7 5.1 1.
8/13/2019 Waterproofing Materials for Ammonium Nitrate
8. Engel , W. Phase stabilisation of AN by KEChemical Abstracts 1990, 113, 214884s.
9. Keenan, A.G.; Note, K Franco, N.B. Synergesticcatalysis of AN decomposition. J . Am. Chem.SOC. 1969, 91(12), 3168-171.
10. Eisenreich, N. Engel , W Kinetic study ofthe solid phase reaction of AN and u O by theuse of fast x-ray diffraction. J Thermal Anal.1989, 35(2), 577-84.
11. Urbasnski, T. Sal ts of nitric acid. In Chemistryand technology of explosives, Vol. 2, Ch. 19.Pergaman Press, Oxford, London, 1965.pp. 453-78.
12. Urbanski, T Commercial (mining) explosives.In Chemistry and technology of explosives,Vol. 4, Ch.19. Pergaman Press, Oxford, Poland,
14. Rao, K P Calculation of thermochemical constantsof propellant Def: Sci. J. 1979, 29(1), 21-26.
15. Serizawa, K.; Kazuya, 0 Koji, S. Compositionof gas-generating agent and process formanufacturing the agent for automobile airbaginflation. Nippon Oil and Fats Co Ltd, Japan.Japanese Patent No. 10, 120, 484 dated 12Ih May1998. 11 p
16. Kust, Roger N. Moisture-res istant calcium-containing particles for soil conditioners andfertilisers. Tetra-technologies, Inc, USA. USPatent No. 9,817,604, dated 30th April 1998.28 p.
Proceedings of the Third Symposium onCombustion and Flame. Wisconsin, London,1984. pp.572-80.
18. Dobrovol'Ski i, E. I.; Ganz, S.N.; Kuznetsov,I.E.Oshurko, A.U. Effect of water-soluble trace
element additives on the physicochemical propertiesof ammonium nitrate. Chemical Abstra cts 1967,6 8 23192 g.
19. Philen, Otis D ; Jr. Silverberg; Julius, Norton&Melvin, M (Tennesse Valley Authority), USA.Fertilizers containing micro and macro nutrients.US Patent No. 3, 503, 019, dated 4 ' Sept1969. 7p.
20. Korpedak, E .A . Vehicle traction material andbag therefore. Cope. Inc, USA. US Patent No.4,668,292, dated 26 ' May 1987. 4p.
21 Dimitrijevic, N Gordana, Sikirica. Preventionof cracking of ammonium nitrate. ChemicalAbstracts 1965, 6 7 92400q.
22. Katayamaa, A. Readily- dispersible sludgecoagulants. Chemical works Co Ltd, Japan.Japanese Pa tent No. 58, 198, 556, dated 14lMay 1982. 13p.
24. Pagowski, W Subos, B Method of obtainingporous ammonium nitrate for agricultural use.Chemical Abstracts 1995, 1 2 4 85795q.
25. Surjit Singh. Historical developments. In Hand-book on sol id propellants, Vol. 1, Mineral Jelly,Ch. VI. CI(ME) Report 1/76, India, 1976.pp. 11.1-10.
17. Taylor, J. Sillit to, G.P. Free-flowing moisture-resistant calcium-containing particles. In
