Anti-hygroscopicity of Ammonium Nitrate (AN) coated by surfactant materials

Anti-hygroscopicity of Ammonium Nitrate (AN) coated by surfactant materials
Abstract
The ammonium nitrate (AN) particles were coated by surfactant materials, using precipitation coating method. The hygroscopicity of AN – coated and AN without coating (used as control) were tested, and the samples of AN coated by surfactant materials, and were characterized by Fourier transform infrared (FTIR), and surface morphology of samples were observed by scanning electron microscopy (SEM). Mass friction of coating layer, and decline of absorption rate were calculated. The results indicate that cetylalcohol promising coating surfactant material for AN among the materials tested, namely, stearic acid, octadecylamine, alcohol, palmitic acid, and lauric acid. The mass friction of coating layer, and decline of absorption rate of AN coated by cetylalcohol were 0.98%, and 28.28%, respectively. The idea and approach presented in this paper have potential to made hydrophobic surface on the surface of AN particles to become anti-hygroscopicity. and also used in other energetic materials to reduce sensitivity.
Keywords: Ammonium nitrate (AN); Hygroscopicity; Anti-hygroscopicity; Surfactant materials, Precipitation coating
Introduction:
Ammonium nitrate (AN) (NH4NO3), is produced by the reaction of anhydrous ammonia gas(NH3), and aqueous nitric acid (HNO3) [1, 2]. Which is present as the major component in most industrial explosives, such as amatol, ANFO, and also used in fertilizers[3-6]. AN is of considerable interest as potential eco- friend oxidizing in solid propellants[2, 7-9]. The risks of used oxidizer ammonium perchlorate (AP) in solid propellants was realized with the hydrogen chloride (HCL) with water form hydrochloric acid which produces highly toxic smoke, this cause significant damage to the environment, due this is reason replaced by AN as oxidizer in solid propellants[10, 11]. It is one of the cheapest, and easily available oxidizer. However, its use in large rocket motors and as solid propellants is restricted due to the hygroscopicity[12]. The molecules of AN are polar it easy absorb the polar moisture from the surrounding, leads to dissolved AN[13].

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R.S. Damse used waterproofing materials mineral jelly and other to reduce hygroscopicity of AN[14]. Kun-Lun Hu et.at., used octadecane amine, resin wax and etc as coating agent for coating AN particles (20- 04 meshes), used melting process. Zhang et.al. used silane coupling agent KH792 for coating AN particles (50-75 meshes), used surface chemical coating[15]. The present work deals with a new approach to form the hydrophobic layer on the surface AN particles, by precipitation surfactant materials on the surface of AN molecules. to increase the decline of absorption rate, and decrease mass friction of coating layer, in the conditions of absorption rate measurement, the size of particles of AN are 70-140 meshes, temperature is 35 ℃, and the relative humidity is 67.5%.
Materials and Method:
Materials: AN supplied by Kecheng fine chemical CO. Ltd (+99.0% Shanghai, China ), Trichloromethane (+99.0%, Shanghai Ling Feng Chemical Reagent Co. LTD , China), and Cyclohexane (≥99.5% Industrial development zone, Chengdu, China). Surfactant materials used AlcoholC18, Stearic Acid, Cetylalcohol, Palmic acid, and Lauric acid (≥99.0% Chengdu Kelong chemical Reagent , China), Octadecylamine (≥99%Tianjin Fine Chemical Research Institute).
Method. 6g of AN coated by 2g of surfactant materials in 15ml chloroform and 15ml cyclohexane. The reaction with stirring at 60oC about 2 h stopped heat, let the reaction contents cooling, the temperature decreasing to 35oC in 5 h, at this stage the water of bath heating was changed, and when the temperature reach to 30oC, the end of reaction. AN coated was filtered by vacuum filter, and drying at 50oC, after drying 5g of AN coated tested at35℃, humidity is 67.5% absorption rate test 24h
Measurement of absorption rate. The samples were weighed 5g in 30×60 mm weighing bottle without cap, placed in a desiccator containing saturated solution of strontium chloride (relative humidity 68%) of the absorbent 35 ℃ 24 h, absorption rate of humidity HR was measured by the following equation:

Where: HR- absorption rate,%;
M0– absorbent sample before mass, g;
M – mass of the sample after the moisture, g.
Mass fraction of coating layer test. Mass friction of coating layer was determined by wetted the filtration paper, dried 1h in an oven weighting (WB). A sample of AN coated after absorption humidity was dissolved in water, and filtered through a tapered funnel, the filter paper was washed by water several times, and placed in an oven 1h, weighing again(WA). The mass fraction of the coating layer was calculated by the following equation:

Where: W- mass fraction of coating layer,%.
M0– mass of sample before absorbed moisture, g.
M1– mass of coating layer, g= WA– WB
Decline of moisture absorption rate was calculated by following equation:

Where: A- decline of moisture absorption rate,%.
HR1– absorption rate of AN sample,%.
HR2– absorption rate of coated AN sample,%.
Characterization. Spectra of the samples were recorded on a Fourier transform infrared (FTIR) spectrophotometer (Thermo Scientific Nicolet I S10, Thermo Fisher USA) by using KBr pellets. Surfaces modified were observed for their morphological study under scanning electron microscopy (SEM) (JEOL JSM 6380LV, Japan).
Results and Discussion
The absorption rate , mass friction of coating layer and decline of AN coated by different kind of surfactant materials are presented in table 1, in this table used cetylalcohol, alcohol C18, octadecylamine, stearic acid, Palmitic acid, and lauric acid to coated AN , the data from hygroscopicity test clearly indicate that cetylalcohol is the best surfactant materials to coating AN lower mass friction of coating layer was 0.98%, and high decline of absorption rate was 28.28%. Stearic acid and octadecylamine were found second and third best, respectively, while alcohol C18, Palmitic acid and lauric acid followed the order sequentially (table 1). It is seen that when change the concentration of solvents, the decline of absorption rate of AN coated by cetylalcohol, stearic acid, and octadecylamine were slightly decreased, and the decline of AN coated by alcohol C18, Palmitic acid, and Lauric acid were slightly increased (table 2). Also the best coating surfactant materials is cetylalcohol, has highly decline was 24.09%, and lower mass friction coating layer was 0.89%.
Table (1) 2g of surfactant materials, 6 g of AN, 15 mL of chloroform + 15 mL cyclohexane HR measurement conditions (70 mesh 140 mesh or more T = 35 ℃ humidity is 67.5%)

No

Surfactants

Absorption Rate %

Mass friction of coating layer%

Decline
%

1

Cetylalcohol

5.85

0.98

28.28

2

Stearic acid

6.45

1.20

19.89

3

Octadecylamine

7.71

1.18

19.62

4

Alcohol C18

6.42

0.39

13.13

5

Palmitic acid

6.11

1.14

3.52

6

Lauric acid

6.24

0.87

3.79

Table (2) 2g of surfactant materials, 6 g of AN, 30 mL of chloroform, at 60oC, HR measurement conditions (70 mesh 140 mesh or more T = 35 ℃ humidity is 67.5%).

No.

surfactants

Hygroscopicity
(HR) %

Mass friction of coating layer%

Decline
%

1

Cetylalcohol

5.61

0.89

24.09

2

Stearic acid

6.39

2.56

13.53

3

Octadecylamine

6.42

2.23

13.13

4

Alcohol C18

6.12

1.48

17.19

5

Palmitic acid

6.21

0.46

15.97

6

Lauric acid

6.17

0.48

16.51

In figure 1 observed the effect of time on hygroscopicity of AN coated by surfactant materials (cetylalcohol (a), alcohol C18(b), and stearic acid(c)) and AN without coating (d) were tested in the hygroscopicity test conditions were size of particles of AN were 70-140 meshes, temperature was 35 ℃, and the relative humidity was 67.5%. The absorption rate directly proportional with time. The lowest regression was obtained from AN coated by cetylalcohol (a), and the highest regression was obtained from AN without coating(d). The absorption rate of AN coated by cetylalcohol is significant than reported in Previous studies[16, 17].

Figure (1) effect of time on the hygroscopicity (Absorption rate) of AN coated by cetylalcohol (a), alcohol C18 (b), stearic acid (c), and AN without coated (d)
The influence of the amount of surfactant materials on the decline of absorption rate and mass friction of coating layer were studies on the AN coated by different amounts of cetylalcohol, (0.5, 1.0 1.5, 2.0. and 2.5 g), the regression of decline of absorption rate was increased to high value at 2.0 g of cetylalcohol, then decreased at 2.5g of cetylalcohol, this is due to polar head of surfactant not precipitated on the surface of AN particles, and this causes hydrophilic surface leads to increased hygroscopicity, thus, decreased decline of absorption rate. The mass friction of coating layer directly proportional with the amount of surfactant materials(figure 2). This results significant than results reported in literature[18, 19].

Figure (2) effect of amount of surfactants materials on the absorption rate and mass friction of coating layer.
In the measurement of absorption rate used 30×60 mm weighing bottle without cap, the effect of amount of sample in this bottle was studied, the weight of samples were tested 10, 15, 20, and 25 g. The regression of absorption rate is inversely proportional to the weight of the sample, in the both AN coated by cetylalcohol, and AN without coating (figure 3).
Figure (3) effect of weight of samples on the absorption rate
Characterization:
FTIR Absorption Spectra. The surface modified of AN and AN without modified were monitored by using FTIR measurement as depicted in figure 4. Alkanes C-H stretch peak around 3000-2850 cm-1was appeared after coating AN by cetylalcohol (a), higher than AN by alcohol C18, and steric acid in (b) and (d), respectively. Alkanes no appeared in AN without coating in (c). Hence, alkenes non polar has effected on hygroscopicity.

Figure 4 FTIR spectra of the ammonium nitrate coated by cetylalcohol (a); ammonium nitrate coated by alcohol C18 (b); ammonium nitrate without coating (c); ammonium nitrate coated by steric acid (d).
Scanning Electron Microscopy. The surface morphology in figure (5) presented the AN coated by cetylalcohol has a clear difference in shape particles and the surface smooth than, AN coated by stearic acid (figure 6), octadecylamine(figure 7) , and alcohol C18(figure 8), and ammonium nitrate without coating were observed in figure 9.
 

Figure 5. Scanning electron microscope of surface modification of AN coated by cetylalcohol.
The scale bar represents,100µm. Electron micrographs.

Figure 6. Scanning electron microscope of surface modification of AN coated by stearic acid.
The scale bar represents, 100µm. Electron micrographs.

Figure 7. Scanning electron microscope of surface modification of AN coated by Octadecylamine.
The scale bar represents, 100µm. Electron micrographs.

Figure 8. Scanning electron microscope of surface modification of AN coated by alcohol C18.
The scale bar represents, 100µm. Electron micrographs.

Figure 9. Scanning electron microscope of surface of AN without coating.
The scale bar represents, 100µm. Electron micrographs.
Conclusions.
In this study the cetylalcohol has been found to be the best surfactant materials for coating ammonium nitrate among the materials tested. The mass friction of coating layer of cetylalcohol was found 0.98%, and decline 28.28%, at the hygroscopicity test conditions temperature 30oC, 68% RH, and time 24h. the optimum amount of surfactant material 2g to coating 6g Ammonium nitrate. The samples were characterized by using FTIR spectra, and SEM. The weight of samples in hygroscopicity test effected significantly on the hygroscopicity of AN.