Sci. Tech. Energetic Materials, Vol．７９,No．１,２０１８ 15 Research ４ paper １ ９ Study of the hygroscopic deterioration of ammonium perchlorate/magnesium mixture Yo su k e N ishiwaki＊,Takehiro Matsunaga＊＊,andMieko Kumasaki＊† ＊Graduate School of Environment and lnformation Sciences, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama-shi, Kanagawa, 240-8501 JAPAN Phone: +81-45-339-3994 †Corresponding author: [email protected] ＊＊National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba-shi, Ibaraki, 305-8565 JAPAN Received: December 21, 2016 Accepted: February 6, 2018 Abstract AStudy of hygroscopic deterioration is necessary because it can improve the safety and quality of pyrotechnics. For effective prevention of hygroscopic deterioration, this paper shows the deterioration mechanism of ammonium perchlorate (AP)/magnesium (Mg) mixture under the presence of moisture. The mixture has been used as Tenmetsuzai (Strobe composition in Japanese) and Mg has been known for its high hygroscopy. In this paper, the hygroscopic deterioration of strobe composition (AP/Mgmixture)wasmeasured by aging test which we were developed, and the deterioration products were analyzed with X-raydiffraction system and scanning electron microscope. The experiments identified two deterioration products : magnesium hydroxide and magnesium perchlorate. The influence of storage temperature and humidity to the rate of deterioration were investigated, and it revealed that the relative humidity of atmosphere has a significant impact. AP/Mg showeddrastic change at a point, and the point was considered to be Critical Relative Humidity. The rate equation which called Jander’s equation was applied to the deterioration behavior obtained in the experiments, and its coefficientswereobtained which enable to predict the degree of deterioration as function of time. Keywords: pyrotechnics, ammonium perchlorate, magnesium, aging, humidity 1. Introduction showed the effect of anti-deterioration agent for Pyrotechnics needs careful treatment because there pyrotechnics composition by using differential scanning have been fatal explosions caused by pyrotechnics１）. calorimetry11）.Boricacid and potassium dichromate were There have been a lot of study on the deterioration of effective for most of the pyrotechnics composition explosive, and some of them focused humidity as a cause. investigated in this study. However, they were ineffective Many humid areas inherently have been suffered from the to some pyrotechnics composition and caused reactions. influence of water molecule on materials, and researchers Another study showed minor impurities in water have investigated the deterioration products, change in influenced on the reaction behavior of antimony sulfide sensitivity, heat generated in spontaneous combustion, and Mg. Additionally, the reaction between antimony combustion velocity, and so on. The target explosives sulfide/Mg mixture and water exhibited induction were nitrocellulose (used for gunpowder)２）－４）,black period13）. powder (usedforpyrotechnics)５）－７）,explosive mixtures Pyrotechnics which contain Mg easily react to water, containing metal (used for fireworks)８）－16）,andother types but Mg were often used for pyrotechnics because of their of explosive mixtures (used for propellant)17）-19）. high performance. For example, Ammonium perchlorate The behavior of pyrotechnics in moisture have been (AP)/magnesium (Mg) mixture is one of the strobe also a subject of research in this field. A thermal analysis compositions. The mixture is considered to burn in two 16 Yosuke Nishiwaki et al. types of reactions occurred simultaneously or Table１ Sample states and storage conditions. alternatively, and each reaction emits lights in different Absolute 20）,21） Relative luminance. The phenomenon creates strobe effect .AP Sample humidity Temperature AP Mg humidity ο /Mg mixture exhibits strobe combustion which adds to name (Vaper pressure) [ C] [wt.%] [wt.%] [%RH] beautiful effect to the pyrotechnics performance, but it is [g m－３] ([kPa]) highly hygroscopic. Therefore the mixture poses a risk of A1 74.5 61.6 (9.19) 50 70.6 29.4 combustion performance change such as abnormal A2 74.7 38.1 (5.51) 40 70.6 29.4 combustion. In previous researches, reactions between AP A3 75.1 22.7 (3.18) 30 70.6 29.4 /Mg mixture in water were investigated10）,12）.Thestudies A4 50.9 43.1 (6.45) 50 70.6 29.4 suggested the reaction mechanism and elucidated the A5 53.2 28.6 (3.92) 40 70.6 29.4 effect of potassium dichromate as an anti-deterioration A6 56.0 17.0 (2.38) 30 70.6 29.4 A7 31.6 16.1 (2.33) 40 70.6 29.4 reagent. In those studies, the ion of magnesium and B 74.7 38.1 (5.51) 40 50.0 50.0 perchlorate were found in liquid phase and magnesium C 74.7 38.1 (5.51) 40 70.6 29.4 hydroxide in residue. D1 74.4 96.4 (14.8) 60 100 - For prevention of the hygroscopic deterioration of AP/ D2 74.7 38.1 (5.51) 40 100 - Mg mixture, the reaction mechanism between solid E 74.7 38.1 (5.51) 40 - 100 mixture and water vapor needs to be clarified. This paper aims to reveal the mechanism of the hygroscopic emits white strobe light22）.Equivalent mixture of AP and deterioration of AP/Mg mixture and the influential factors Mg was named as sample B. Sample C was pelletized AP / such as storage temperature and shape of mixture. In Mg mixture with a hand-press. Its density was 1.0~1.6 g contrast to previous studies focusing the reaction in water, cm－３ which is similar to those of stars in fireworks (1.6 g the study on the influence of humidity was scarce cm－３)23）. although the knowledge of the behavior under humidity is Aging test was carried out in the plastic container more practical than in bulk of water. The investigation which is in an incubator under the constant temperature leads pyrotechnics to increasing safety and improving and the constant humidity kept with the saturated salt quality through their lifecycle, and the result should method24）.Saturated aqueous solutions keeps equilibrium contribute to the safety of pyrotechnics. with moisture in atmosphere at a certain vapor pressure. In order to investigate deterioration mechanism in Half gram of powdery sample A, B, D, and E and pelletized humid environment, aging test in controlled environments sample C were placedoneach hexagonal balance tray. was conducted and the products were analyzed with The trays were placed in a sealed plastic container in powder X-ray diffraction (XRD). The deterioration which the humidity was controlled with saturated salt. progress was tracked by monitoring weight change in the The salts generate their characteristic humidity function of temperature and humidity as an analogous to depending on temperature. In this study, humidity was chemical reaction which is governed by temperature and kept at about 75%RH (Sodium chloride), 55%RH (Sodium the concentration of reactants. In the analysis, the relative bromide) and 30%RH (Magnesium Chloride), and ο ο ο ο humidity is paid attention as well as absolute humidity. temperature was kept at 30 C, 40 C, 50 Cand60C. Furthermore, the effect of physical contacts between Sample A which stored under about 75%RH of humidity metal and oxidizing agent to the deterioration rate was was named A1~A3. Sample A4~A6 was kept under about investigated with regard to moisture absorption. The 55%RH of humidity. Sample A7 was stored under about effect of density was measured by weight variation as 30%RH. As the figure increases, the ambient temperature well. The deterioration rate was analyzed and assumed was low. Sample D1 was stored in the same condition as that the diffusion of water molecules in deterioration that of sample D2, apart from temperature. All sample products on the Mg powder isamajorfactor, and an mixtures and the storage conditions are shown in Table 1. equation was obtained to predict the degree of deterioration as function of time. 2.2 Deterioration products analysis The deterioration product was investigated with 2. Experimental Powder XRD (RINT-2000 : Rigaku) which used CuK!! 2.1 Materials and samples radiation (1.5405 Å). The optics were a fixed divergence AP (Wako Special Grade : Wako Pure Chemical slit (1°), a fixed scatter slit (1°), and a fixed receiving slit Industries, Ltd.) and Mg (Mg-100 : Kanto Metal (0.3mm). The X-ray tube worked at 40 kV and 200 mA. Corporation) and magnesium hydroxide, Mg(OH)２ (99.9% : The components in the products were identified with Wako Pure Chemical Industries, Ltd.) were used without integrated X-ray powder diffraction software (PDXL). The further purification. Three types of sample mixture were deterioration surface was studied by the images obtained prepared since this study focused on the effects of with Scanning Electron Microscope (VE-8800 : keyence). atmosphere (Sample A), contacts between metal particles The accelerating voltage was 2 kV. and oxidizing agent particles (Sample B, D, E), and sample density (Sample C). The sample A and C were composed 2.3 Measurement of hygroscopic deterioration of AP and Mg with the weight ratio of 60 : 25, the ratio of behavior which was determined based on the one of mixture which In this study, the progress of hygroscopic deterioration Sci. Tech. Energetic Materials, Vol．７９,No．１,２０１８ 17 was expressed with weight variation. Each sample was ４ １ weighed regularly every 24 hours for 6 days with an 10000[cps] electronic balancer (PA64JP : Ohaus). ９ AP/Mg The weight variation rate, "! was defined as follows : "!$%&$#!$ !$#$## ٸ ٸ (1) # ٸA2 3 days ٸ Where $# is the sample weight after #hours storage, and $#is the initial weight of the sample. A2 6 days The samples stored for 1, 2, 3, and 6 days were taken out of the container and dried to remove water to separate the Intensity[cps] A2 24 days ٱ ٱ reversible water absorption from the irreversible reaction caused by humidity.