materials Review ReviewCoating Methods for Surface Modification of Coating Methods for Surface Modification of Ammonium Nitrate: A Mini-Review Ammonium Nitrate: 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 propellant 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 salt (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 melting point 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 fertilizers, used inand fertilizers, as an active and component as an active in component explosive 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 ion 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. gases. There There are ammonium are ammonium dinitramide 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 propellants 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 explosives 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