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Review ReviewCoating Methods for Surface Modification of Coating Methods for Surface Modification of : A Mini-Review : A Mini-Review Baha I. Elzaki 1,2 and Yue Jun Zhang 1,* Baha I. Elzaki 1,2 and Yue Jun Zhang 1,* 1 School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, 1 China;School [email protected] of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, 2 DepartmentChina; [email protected] of Chemical Engineering, College of Engineering, Karary University, Omdurman 12304, Sudan 2 * Correspondence:Department of Chemical [email protected]; Engineering, College Tel.: +86-25-8431-5083 of Engineering, Karary University, Omdurman 12304, Sudan * Correspondence: [email protected]; Tel.: +86-25-8431-5083 Academic Editor: Jiyoung Kim AcademicReceived: Editor:20 April Jiyoung 2016; Accepted: Kim 15 June 2016; Published: date Received: 20 April 2016; Accepted: 15 June 2016; Published: 23 June 2016 Abstract:Abstract: Using ammonium nitrate (AN) as a oxidizer is limited limited due due to to its its hygroscopicity. hygroscopicity. ThisThis reviewreview consolidatedconsolidated thethe availableavailable informationinformation ofof variousvarious issuesissues pertainingpertaining toto thethe coatingcoating methodsmethods ofof thethe surfacesurface modificationmodification of ammonium nitratenitrate for reducing its hygroscopicity. Moreover this review summarizessummarizes thethe recentrecent advancesadvances andand issuesissues involvedinvolved inin ammoniumammonium nitratenitrate surfacesurface modificationmodification byby physical,physical, chemicalchemical andand encapsulationencapsulation coating methodsmethods to reducereduce thethe hygroscopicity.hygroscopicity. Furthermore,Furthermore, coatingcoating materials,materials, processprocess conditions,conditions, andand thethe hygroscopicityhygroscopicity testtest conditionsconditions areare extensivelyextensively discusseddiscussed along,along, withwith summariessummaries ofof thethe advantagesadvantages andand disadvantagesdisadvantages ofof eacheach coatingcoating method.method. OurOur findingsfindings indicatedindicated thatthat thethe investigationinvestigation andand developmentdevelopment ofof anti-hygroscopicityanti-hygroscopicity ofof AN,AN, andand thethe mechanismsmechanisms ofof surfacesurface modificationmodification by coatingcoating urgentlyurgently require further research in orderorder toto furtherfurther reducereduce thethe hygroscopicity.hygroscopicity. Therefore, this review is usefuluseful toto researchersresearchers concernedconcerned withwith thethe improvementimprovement ofof ammoniumammonium salts’ anti-hygroscopicity.

Keywords:Keywords: ammonium nitrate; chemical coating; encapsulationencapsulation coating; hygroscopicity; modificationmodification surface;surface; physicalphysical coatingcoating

1.1. Introduction Introduction

AmmoniumAmmonium nitratenitrate (AN)(AN) NHNH44NO3 isis an an inorganic (Figure 11).). ItIt isis lowlow cost,cost, safesafeand and has has a a simplesimple manufacturingmanufacturing processprocess [[1,2].1,2].

FigureFigure 1.1. FormulaFormula ofof ammoniumammonium nitrate.nitrate.

PurePure AN is is a a colorless colorless salt salt with with relative relative density density equal equal to 1.725 to 1.725 g/cm g/cm3 and 3a andmelting a of point 169 °C. of 169The ˝decompositionC. The decomposition process is process significantly is significantly promoted promotedat a slightly at higher a slightly temperature higher temperature of 210 °C [3–5]. of 210AN˝ isC[ widely3–5]. AN used is widelyin , used inand fertilizers, as an active and component as an active in component mixtures. in explosive It is mixtures. a strong Itoxidizer is a strong and oxidizeran insensitive and an explosive insensitive [4,6,7]. explosive AN has [4 ,a6 ,positive7]. AN hasoxygen a positive balance, oxygen and the balance, nitrate and theacts nitrate as the source ion acts of asoxygen, the source resulting of oxygen, in a promis resultinging compound in a promising as a propella compoundnt and asgas a generator propellant [8–10]. and gasThe generatorphase transition [8–10]. in The pure phase AN transitioncan also occur in pure at different AN can temperatures also occur at as different demonstrated temperatures in Figure as demonstrated2 [11,12]. in Figure2[11,12].

Materials 2016, 9, 502; doi:10.3390/ma9070502 www.mdpi.com/journal/materials Materials 2016, 9, 502; doi:10.3390/ma9070502 www.mdpi.com/journal/materials Materials 2016, 9, 502 2 of 8 Materials 2016, 9, 502 2 of 8

FigureFigure 2. 2.Phase Phase transitionstransitions of of AN. AN.

Recently,Recently, there there has has been been renewed renewed interest interest inin a clean-burning propella propellant,nt, aiming aiming at green at green propellantspropellants with with HCl-free HCl-free burning. burning. Ammonium Ammonium perchlorateperchlorate (AP) (AP) was was replaced replaced by bydifferent different oxidizers oxidizers becausebecause it releases it releases toxictoxic . gases. There There are ammonium are dinitramide (ADN) and hydrazinium (ADN) and nitroformate hydrazinium (HNF) energetic oxidizers, but these are not available in adequate quantities [13–16]. AN-based nitroformate (HNF) energetic oxidizers, but these are not available in adequate quantities [13–16]. composite are primarily attractive due to the combustion products without toxic gases AN-based composite propellants are primarily attractive due to the combustion products without (HCl-free) [17]. Although ammonium nitrate is suitable as an oxidizer in large rocket motors, it is, toxichowever, gases (HCl-free) restricted [17due]. to Although the hygroscopicity ammonium [18, nitrate19]. Furthermore, is suitable the as surfaces an oxidizer of the in AN large solid rocket motors,particles it is, however,are strongly restricted polar, which due tocause the hygroscopicityhygroscopicity [20,21]. [18,19 ].Hygroscopicity Furthermore, of the AN surfaces has been of the AN solidpinpointed particles as the are major strongly cause polar, for caking, which and cause has beco hygroscopicityme the biggest [ 20obstacle,21]. Hygroscopicity for its usage of AN has beenand pinpointed propellants as the[22,23]. major Generally, cause forreducing caking, the and hygroscopicity has become of theAN biggesthas been obstacle studied by for many its usage explosivesresearchers and propellantsusing surface [22 modification,23]. Generally, techniques, reducing by thecoating hygroscopicity AN particlesof with AN different has been types studied of by manycoating researchers methods, using in particular surface modification physical, chemical techniques, and encapsulation by coating coating AN particles methods. with To differentthe best of types of coatingour knowledge, methods, the in particularavailable coating physical, methods chemical that mo anddify encapsulation the surface of AN coating have methods.not been aggregated To the best of and thoroughly reviewed. Therefore, the objective of this paper is to review the recent studies for the our knowledge, the available coating methods that modify the surface of AN have not been aggregated surface modification of AN to reduce the hygroscopicity, with an attempt to summarize the relevant and thoroughlyinformation with reviewed. emphasis Therefore, on the main the coating objective methods of this such paper as physical is to review coating, the chemical recent coating, studies for the surfaceand encapsulation modification coating of AN methods. to reduce This consequent the hygroscopicity,ly increases withthe effectiveness an attempt of to the summarize use of AN the relevantas a information solid propellant with oxidizer. emphasis The on theabove-ment main coatingioned methods methods and such concerns as physical have coating, been deeply chemical coating,discussed and encapsulation and compared coating to each methods.other. The Thisrecent consequently advances and increases development the effectivenessin this regard ofhave the use of ANbeen as athoroughly solid propellant addressed. oxidizer. The above-mentioned methods and concerns have been deeply discussed and compared to each other. The recent advances and development in this regard have been thoroughly2. Coating addressed. Methods Surface modification of AN with several coating methods was applied in order to reduce 2. Coatinghygroscopicity, Methods which has been tackled by several previous studies. This section will discuss these methods as follows: Surface modification of AN with several coating methods was applied in order to reduce hygroscopicity,2.1. Physical whichCoating hasMethod been tackled by several previous studies. This section will discuss these methods as follows: The physical coating method is used to modify the surface of AN particles to reduce the 2.1. Physicalhygroscopicity. Coating In Method this method the interaction between the surface of AN particles and the coating agents occurs by physical absorption; for example, the polar head of the surfactant has an affinity to Thethe polar physical surface coating of AN particles method and is usedmakes to a hy modifydrophobic the uniform surface membrane of AN particles on the surface to reduce with the hygroscopicity.the hydrophobic In this tails method of the surfactant the interaction materials between [24]. In addition, the surface the ofsuccess AN particlesof this process and themainly coating agentsdepends occurs on by the physical solubility absorption; (the solubility for example, of the surfactant the polar is headcontrolled of the by surfactant the temperature) has an of affinity the to the polarsurfactant surface and of the AN insolubility particles of and AN, makes as well a hydrophobicas on the suspension uniform of membraneparticles in aqueous on the surfacesolution, with the hydrophobicbecause the soluble tails ofsurfactant the surfactant can easily materials deposit on [in24soluble]. In addition, particles of the AN, success with the of suspension this process making mainly a uniform coating layer on the surface of the particles [25]. Moreover, the stirring and densities of solvents depends on the solubility (the solubility of the surfactant is controlled by the temperature) of the have significant roles in making the suspension system [26]. In fact, the mechanism of coating by surfactant and the insolubility of AN, as well as on the suspension of particles in aqueous solution, surfactant materials probably varies from surfactant group to surfactant group and even within the becausesame the surfactant soluble surfactantgroups due canto the easily different deposit leng onths insolubleof the tails particles [27]. In 1996, of AN, Chattopadhyay with the suspension used makingstearic a uniform to coatingcoat the layersurface on of the the surface AN partic ofles, the and particles hydrophobic [25]. Moreover, film was formed the stirring to achieve and densities the of solventspurpose have of preventing significant moisture. roles in This making study the provided suspension a worthy system explanation [26]. In on fact, coating the mechanism AN with of coatingstearic by surfactantacid; however, materials no details probably were mentioned varies from about surfactant the coating group process to surfactantand the hygroscopic group and test even within the same surfactant groups due to the different lengths of the tails [27]. In 1996, Chattopadhyay used stearic acid to coat the surface of the AN particles, and hydrophobic film was formed to achieve the purpose of preventing moisture. This study provided a worthy explanation on coating AN with stearic acid; however, no details were mentioned about the coating process and the hygroscopic test Materials 2016, 9, 502 3 of 8

Materials 2016, 9, 502 3 of 8 conditions [28]. Furthermore, there was a water-resistant base of an AN explosive composition made by blendingconditions the [28]. particles Furthermore, of AN there with was different a water-resistant groups, such base as of alkalinean AN explosive earth metals composition and earth made metal salts ofby aliphaticblending the carboxylic particles of AN [29 with]. AN different particles groups, were such modified as alkaline by aeart compositeh metals and consisting earth metal of anion, salts of aliphatic carboxylic acids [29]. AN particles were modified by a composite consisting of anion, cationic, non-ion, and amphoteric surfactants. Therefore, the dispersion improved by adding a quantity cationic, non-ion, and amphoteric surfactants. Therefore, the dispersion improved by adding a quantity of a dispersant agent such as diatomate or vermiculite; in addition the anti-water mechanism is shown of a dispersant agent such as diatomate or vermiculite; in addition the anti-water mechanism is shown in Figurein Figure3. To 3. our To knowledgeour knowledge this this is is the the first first study study onon the coating coating of of AN AN with with a composite a composite surfactant, surfactant, whichwhich can becan used be used as the as the base base for for the the surface surface modification modification of of AN AN with with composite composite surfactant surfactant [30]. [ 30].

FigureFigure 3. Micro-structure 3. Micro-structure of of composite-modified composite-modified water-resistant water-resistant powdery powdery AN. AN.

Another piece of research showed that the AN particles were coated with mineral oil. Under a Anothertemperature piece of 20 of °C, research 65% relative showed humidity, that the and AN the particlesabsorption were moisture coated time with of 96 mineral h, the decline oil. Under of a temperaturethe moisture of 20 absorption˝C, 65% relativerate increased humidity, to 65% and with the the absorption mass ratio moistureof the coating time layer of 96 at h, 1%. the It decline is of theworth moisture mentioning absorption that this rate study increased was classified to 65% as with the thehighest mass level ratio in ofthe the literature coating for layer the current at 1%. It is worthrate mentioning of decline in that the thisphysical study coating was for classified a decrease as thein moisture highest [31]. level Furthermore, in the literature Kun-lun for Hu the et currental. rate ofused decline stearic in acid, the physicalresin, wax, coating etc., as forcoating a decrease materials; in moisturethe particle [31 sizes]. Furthermore, of AN were 20–40Kun-lun mesh Hu in et al. usedthe stearic melting acid, process. resin, Under wax, the etc., conditions as coating of a materials; temperature the at 25 particle °C, a relative sizes ofhumidity AN were of 90%, 20–40 and mesh an absorption moisture time of 8 h, the mass ratio of the coating layer was decreased from 6.25% to in the melting process. Under the conditions of a temperature at 25 ˝C, a relative humidity of 90%, 2.17%, and the decline was 65.28%. This study used a short duration of the absorption moisture test and anwith absorption a high mass moisture ratio of the time coating of 8 h,layer the [3 mass2]. Moreover, ratio of the the coupling coating cationic layer was surfactant decreased (N,N- from 6.25%Dimethyldodecylamine) to 2.17%, and the decline has been was used 65.28%. to improve This the study anti-hygroscopicity, used a short durationanti-caking ofand the explosion absorption moistureperformance test with of AN a highparticles mass [33]. ratio For reducing of the coatinghygroscopicity layer the [32 heating]. Moreover, melt method the couplingwith different cationic surfactantamounts (N, Nof-Dimethyldodecylamine) paraffin was used, resulting has in a been modification used to improveof AN particles the anti-hygroscopicity, [34]. Octadecylamine anti-caking was and explosionalso used to performance modify ammonium of AN particlesnitrate and [33 characterized]. For reducing the contact hygroscopicity angle to investigate the heating the melt methodexplosive withdifferent performance amounts [35]. ofThe paraffin mechanism was used, of interaction resulting inbetween a modification ammonium of AN nitrate particles and [34]. Octadecylamineoctadecylamine was was also studied used by to modifyusing SEM, ammonium XRD, XPS nitrateand IR, andwhich characterized resulted in the the coating contact of anglethe to investigateammonium the explosive nitrate surface performance with octadecylamine [35]. The mechanism through physical of interaction absorption, between which ammonium improved the nitrate explosion performance [36]. The performances of modified AN using physical coating methods are and octadecylamine was studied by using SEM, XRD, XPS and IR, which resulted in the coating of the summarized in Table 1. ammonium nitrate surface with octadecylamine through physical absorption, which improved the explosion performanceTable [36 1.]. Performance The performances of coated AN of modifiedusing the physical AN using coating physical method. coating methods are summarized in Table1. Absorption Mass Ratio of Absorption Moisture Decline of Absorption No. Year Ref. Moisture Rate (%) Coating Layer (%) Test Condition Moisture (%) 1 2003 Table1.62 1.%Performance of4% coated ANT = using 20 °C, RH the = physical83%, t = 48 h coating method.64.78% [30] 2 2004 7% 1% T = 20 °C, RH = 65%, t = 96 h 80% [31] 3 2006 Absorption2.174% Mass2.17 Ratio% of T = 25Absorption °C, RH = 90%, Moisture t = 8 h Decline65.28 of% Absorption [32] No. Year Ref. 4 2007 Moisture5.27 Rate% (%) Coating-Layer (%) T = 25 °C,Test RH Condition= 90%, t = 30 h Moisture69.02% (%) [33] 15 20032013 1.62%3.04% 2.5 4%% T T= =30 20 °C,˝ C,RH RH = 90%, = 83%, t = 24 t = h 48 h88 64.78%% [34] [30 ] 2 2004 7% 1% T = 20 ˝C, RH = 65%, t = 96 h 80% [31] 3 2006 2.174% 2.17% T = 25 ˝C, RH = 90%, t = 8 h 65.28% [32] 4 2007 5.27% - T = 25 ˝C, RH = 90%, t = 30 h 69.02% [33] 5 2013 3.04% 2.5% T = 30 ˝C, RH = 90%, t = 24 h 88% [34] Materials 2016, 9, 502 4 of 8

2.2. Chemical Coating Method The chemical coating is a method used for AN particle modification to reduce hygroscopicity. In thisMaterials method 2016 the, 9, 502 adsorption of coating materials on the surface of the AN particles causes a chemical4 of 8 reaction resulting in the formation of a coating layer on the particles. AN particles with a size of 60–802.2. Chemical mesh were Coating coated Method by silane coupling agents KH550 and KM602; the hygroscopicity test conditionsThe were chemical a temperature coating is of a 35method˝C, a used relative for AN humidity particle of modification 92%, and a to time reduce of 24 hygroscopicity. h, and the decline of absorbedIn this method moisture the adsorption was 38.7% ofin coating the AN materials modified on the with surface KH550. of the AN However, particles the causes mass a chemical ratio of the coatingreaction layer resulting was not determinedin the formation [37]. of In a contrastcoating laye ther decline on the ofparticles. the moisture AN particles absorption with a increased size of to 56.25%60–80 when mesh the were AN particlescoated by were silane coated coupling with agents an anionic KH550 surfactant and KM602; mixture the hygroscopicity of lignosulfonate test and conditions were a temperature of 35 °C, a relative humidity of 92%, and a time of 24 h, and the decline , at a temperature of 35 ˝C, a relative humidity of 55%, and a moisture absorption time of 24 h. of absorbed moisture was 38.7% in the AN modified with KH550. However, the mass ratio of the However,coating the layer mass was ratio not of determined the coating [37] layer. In contrast was not the mentioned, decline of the nor moisture was the absorption relative humidity increased near to the AN critical56.25% relative when the humidity AN particles [38]. were Another coated piece with ofan research anionic surfactant showed themixture modification of lignosulfonate of AN particlesand sizedurea, 50–75 at mesha temperature with silane of 35 coupling°C, a relative agent humidity KH792 of 55%, as the and coating a moisture material. absorption The time hygroscopicity of 24 h. was testedHowever, under the conditionsmass ratio of of the a coating temperature layer was of 20not˝ mentioned,C, a relative nor humidity was the relative of 88%, humidity and a moisturenear absorptionthe AN time critical of8 relative h, and thehumidity absorption [38]. Another rate of thepiece moisture of research was showed decreased the modification from 12.86% of to AN 10.92%, with aparticles decline sized of 15.08%. 50–75 In mesh this study,with silane the mass coup ratiolingof agent thecoating KH792 layeras the was coating not determined, material. The and the declinehygroscopicity of the moisture was absorptiontested under was conditions low [39 ].of Tablea temp2 showserature theof 20 performance °C, a relative of humidity modified of AN 88%, using and a moisture absorption time of 8 h, and the absorption rate of the moisture was decreased from a chemical coating method. 12.86% to 10.92%, with a decline of 15.08%. In this study, the mass ratio of the coating layer was not determined, and the decline of the moisture absorption was low [39]. Table 2 shows the performance Table 2. Performance of coated AN using the chemical coating method. of modified AN using a chemical coating method.

Absorption Mass Ratio of Absorption Moisture Decline of No. Year Ref. MoistureTable Rate2. PerformanceCoating of Layer coated AN usingTest Conditionthe chemical coatingAbsorption method. Moisture ˝ 1 2003Absorption 11.4% Mass - Ratio of T = 35AbsorptionC, RH = 92%, Moisture t = 24 hDecline 38.7% of [37] No. Year ˝ Ref. 2 2005Moisture 0.21% Rate Coating 0.3% Layer T = 35 C,Test RH Condition = 55%, t = 24 hAbsorption 56.25% Moisture [38] 3 2009 10.92% - T = 20 ˝C, RH = 88%, t = 8 h 15.08% [39] 1 2003 11.4% - T = 35 °C, RH = 92%, t = 24 h 38.7% [37] 2 2005 0.21% 0.3% T = 35 °C, RH = 55%, t = 24 h 56.25% [38] 2.3. Encapsulation3 2009 Methods10.92% - T = 20 °C, RH = 88%, t = 8 h 15.08% [39]

Encapsulation2.3. Encapsulation is Methods a new coating method. The capsule-like coating layer is formed on the surface of AN particles, which are homogeneous and of a certain thickness. The mechanism of encapsulation Encapsulation is a new coating method. The capsule-like coating layer is formed on the surface of is shownAN particles, in Figure which4. The are process homogeneous of encapsulation and of a cert coatingain thickness. involves The dispersingmechanism of the encapsulation AN particles is in a solutionshown of anin encapsulatingFigure 4. The process material of encapsulation dissolved in co anating organic involves solvent, dispersing suspending the AN the particles whole in system, a and thensolution evaporating of an encapsulating away the mate organicrial dissolved solvent [ 40in, 41an]. organic Based solv on theent, reportedsuspending literature, the whole the system, materials formingand capsulesthen evaporating are divided away intothe organic three categories: solvent [40,41]. Based on the reported literature, the materials forming capsules are divided into three categories: (1) Cellulose derivatives, such as , ethyl cellulose. (2) Resin(1) Cellulose and plastics, derivatives, such such as poly as nitrocellulose, methyl methacrylate, ethyl cellulose. acrylic acid resin, polyvinyl acetylate, (2) Resin and plastics, such as poly methyl methacrylate, acrylic acid resin, polyvinyl acetylate, polystyrene, etc. polystyrene, etc. (3) Elastomeric(3) Elastomeric polymer polymer such such as as polyurethane polyurethane elastomer.elastomer.

FigureFigure 4. Monomer 4. Monomer and and polymeric polymeric particles particles adsorbed surface-coating surface-coating schematic. schematic.

DependingDepending on the on the different different processes processes and and approaches approaches of of the the encapsulation encapsulation coating coating process, process, there there are three major categories: are three major categories: (1) Mechanical and physical methods such as the spray-drying method. Materials 2016, 9, 502 5 of 8

(1) Mechanical and physical methods such as the spray-drying method. (2) Physical methods such as the phase separation method. (3) Chemical methods such as the precipitation polymerization method.

Many studies were done to deal with the anti-moisture of ammonium nitrate. The study showed the modification of AN particles by the polymerization action in the interface of xylene/AN; the odctadecylamine forms a monolayer with self-assembling through hydrogen bonding on the surface of AN particles [42]. Meanwhile, the structure of the self-assembled monolayer film on the surface modified with octadecylamine and dimethyl benzene was characterized. This study has an explanation of the interaction between the monolayer and the AN [43]. The microencapsulation technology has been used for reducing the hygroscopicity and agglomeration of AN by coating it with polystyrene (PS) [44]. The AN particles were modified to improve the hygroscopicity and caking by coating particles using encapsulation through a physico-chemical process technique [45]. In addition, the AN particles were modified by liquid phase separation techniques using polyvinyl butyral (PVB), in order to reduce the hygroscopicity and caking capability [46]. The AN particles were modified by liquid phase separation technology, using polyvinyl acetate (PVAC), polyacrylamide (PAM), and polyethylene glycol (PEG) as coating materials. In this study, the modification effects of PVAC and PAM in the AN decreased the hygroscopicity and the coated particles, which have a better spherical shape [47]. Moreover, Yue (2003) used 10% methyl methacrylate monomer as a coating material and 0.7% azobisisobutyeonitrile as an initiator in the reaction medium of petroleum ether, cyclohexane and toluene, with a reaction time of 8 h at 60 ˝C. Under the conditions, at temperature of 32 ˝C, a relative humidity of 90% and an absorption moisture time of 24 h, the absorption rate of moisture was decreased from 12.80% to 9.91%, and the decline was 22.50%. The absorption moisture test conditions were good, but the mass ratio of the coating layer and the AN particle sizes were not determined [37]. The surface polarity of AN was decreased by using a spray-drying technique, and a surfactant, a coupling agent and polyvinyl butyral as coating materials. In this study, the duration of the hygroscopicity test was short [48]. Furthermore, AN was coated with 4% polystyrene monomer, and 1% azobisisobutyeonitrile as an initiator was used in the polymerization reaction medium chloroform. Under the conditions of a temperature of 20 ˝C, a relative humidity of 75%, and an absorption moisture time of 12 h, the mass ratio of the coating layer was decreased from 1.24% to 0.623%, and the decline was 88.8%. The reported hygroscopicity was also affected by the particle size of the uncoated AN [49]. In 2013, Zhicheng et al. used methyl methacrylate as a monomer in precipitation polymerization, and azobisisobutyeonitrile as an initiator, and chloroform as a solvent, and the reaction time was 8 h at 65 ˝C. Under the conditions of a temperature of 35 ˝C, a relative humidity of 92%, and an absorption moisture time of 24 h, the absorption rate of the moisture was decreased from 12.26% to 8.78%, the decline was 28.35%, and the mass ratio of the coating layer was 0.48, which is the highest level in literature reported until now. In this study, the absorption moisture test conditions were high [50]. The performance of modified AN using encapsulation coating is summarized in Table3. Although the surface modification of AN using physical coating, chemical coating and encapsulation coating methods had advantages in reducing the hygroscopicity, these methods also had disadvantages as shown in Table4.

Table 3. Performance of coated AN using the encapsulation coating method.

Absorption Mass Ratio of Absorption Moisture Decline of No. Year Ref. Moisture Rate Coating Layer Test Condition Absorption Moisture 1 2003 9.91% - T = 32 ˝C, RH = 90%, t = 24 h 22.50% [37] 2 2004 2.8427% 1.5% T = 30 ˝C, RH = 94%, t = 8 h 49% [48] 3 2008 0.623% - T = 20 ˝C, RH = 75%, t = 12 h 88% [49] 4 2011 0.33% - T = 30 ˝C, RH = 75%, t = 24 h 30.6% [47] 5 2013 8.78% 0.48% T = 35 ˝C, RH = 92%, t = 24 h 28.35% [50] Materials 2016, 9, 502 6 of 8

Table 4. Advantages and disadvantages of coating methods.

Method Advantages Disadvantages

(1) Simple. (1) Uses a large amount of coating agent. (2) Convenient and easy to manufacture. (2) Thickness of coating layer not easy to control. (3) Improves the stability of the particles. Physical coating (3) Large difference of interfacial tension (4) Safety. between the surface coating layer and the (5) Enhances the compatibility of particles with polarity of AN. other materials.

(1) Small dosage of coating agent. (1) The surfactant and coupling agent have low (2) Strong binding force. molecular weight. Chemical coating (3) The hydrophobic group made the thin layer (2) The surfactant has a small solubility in water. on the surface prevent hygroscopicity. (3) Low hygroscopicity properties.

(1) Improves the physical properties of coated (1) The brittle polymer is susceptible to cracking particles on the surface. during the drying process. (2) There are sticky polymer adhesives Encapsulation coating (2) Protects the particles from external moisture. (3) Polymer hygroscopicity was zero. not dispersed. (4) Coating layer is thin. (3) Polymer polarity low is difficult to stick on surface of AN particles.

3. Conclusions The hygroscopicity of AN has restricted its application in explosives and propellants. This mini-review clearly summarized the surface modification of AN particles using physical, chemical and encapsulation coating methods to reasonably reduce the hygroscopicity. Our findings indicated that further exploration and improvement of the anti-hygroscopicity of AN and the mechanisms of surface modifications are urgently required for a greater reduction in hygroscopicity.

Conflicts of Interest: The authors declare no conflict of interest.

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