Thermal Analysis and Stability of Boron/Potassium Nitrate

applied sciences

Article Thermal Analysis and Stability of Boron/Potassium ◦ Nitrate Pyrotechnic Composition at 180 C

Chaozhen Li 1 , Nan Yan 1,*, Yaokun Ye 2, Zhixing Lv 3, Xiang He 1, Jinhong Huang 4 and Nan Zhang 4

1 State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China 2 Beijing Space Vehicle General Design Department, China Academy of Space Technology, Beijing 100094, China 3 Safety Technology Research Institute of Ordnance Industry, Beijing 100053, China 4 R & D Center, Liaoning North Huafeng Special Chemistry Co. Ltd., Fushun 113003, China * Correspondence: [email protected]; Tel.: +86-010-6891-2537

Received: 4 August 2019; Accepted: 29 August 2019; Published: 3 September 2019

Featured Application: This research aims to obtain the decomposition process of boron/potassium nitrate pyrotechnic composition, and verify its high temperature stability to meet the charge requirements of the separation device such as the cutter for the lunar probe.

Abstract: Aerospace missions require that pyrotechnic compositions are able to withstand 180 ◦C. Therefore, this paper studies the thermal stability and output performance of boron/potassium nitrate (abbreviated BPN) used in pyrotechnic devices. Firstly, differential scanning calorimetry (DSC) and thermogravimetric (TG) tests are used to analyze the thermal reaction process of KNO3, boron, and BPN to qualitatively judge their thermal stability. Then, apparent morphology analysis, component analysis, and the p-t curve test, which is the closed bomb test to measure the output power of the pyrotechnic composition, are carried out with BPN samples before and after the high-temperature test to verify BPN stability at 180 ◦C. The weight change of boron powder caused by chemical reactions occurs above 500 ◦C. When the temperature is lower than the peak exothermic temperature of decomposition, no obvious chemical reaction occurs with KNO3, and only physical changes (crystal transformation and melting) occur. Combined with a verification test at 180 ◦C for two days, it is concluded that boron and KNO3 components are stable at 180 ◦C. With an increase in boron content, the thermal stability of BPN is improved, with the best performance achieved when the ratio is 25:75 (B:KNO3). BPN samples without binder have the best thermal stability. In a test at 180 ◦C for five days, the binder affects the weight loss and p-t curve of BPN, and BPN with fluororubber binder is better than BPN with unsaturated polyester binder.

Keywords: boron/potassium nitrate; thermal analysis; high temperature stability; energetic material

1. Introduction

Boron/potassium nitrate (B/KNO3; abbreviated BPN) can be used for the output charge of an engine igniter, a ﬁre transfer charge, a small thrust dynamite output charge, a high temperature projectile charge, etc. BPN has the signiﬁcant characteristics of high combustion heat and low moisture absorption per unit weight. [1] BPN is also listed as a linear reference for the security of pyrotechnic agents [2]. At present, aerospace missions require that BPN composition is thermally stable. In some special launch missions [3], such as of deep space spacecraft, pyrotechnic devices must withstand a large temperature range, usually from 100 to +130 C[4,5]. Considering that there is a certain safety − ◦

Appl. Sci. 2019, 9, 3630; doi:10.3390/app9173630 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 3630 2 of 11 margin for temperature, it is of great significance to test and evaluate the thermal stability and output performance of BPN after exposure to the temperature of 180◦C, providing a basis for its application in the aerospace field. This research aims to obtain the decomposition process of boron/potassium nitrate pyrotechnic composition, and verify its high temperature stability to meet the charge requirements of the separation device such as the cutter for the lunar probe. K.R. Rani Krishnan et al. studied the effect on the ignition behavior of potassium boron nitrate pyrotechnic powder of adding RDX and HMX components which are two commonly used high energy explosives. In their paper, thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses which use Perkin Elmer TGA 7 and DSC 7 in flowing argon medium (30 mL min) at 5 ◦C/min, were carried out on the mixture of potassium boron nitrate and explosive components [6], which provided some inspiration for the present paper. R. Turcotte et al. researched the thermal analysis of black gunpowder in which KNO3 had a pre-ignition reaction under the action of sulfur and charcoal. The pre-ignition reaction occurred at a lower temperature than the decomposition temperature of KNO3 itself, and the decomposition temperature of KNO3 drifted under a complex system [7,8]. ES. Freeman, Seyed Ghorban Hosseini et al. [9,10] studied the thermal decomposition process of KNO3 by the TG-TDA method, and obtained the transformation, melting and decomposition process of KNO3. A. Eslami et al. [11] studied the thermal reaction characteristics of B+KNO3,B+Ba(NO3)2,B+PbO2 and other mixed systems, and the reaction process of a B+KNO3 mixed system was analyzed in stages. The stability of chemical agents refers to the ability of a sample to maintain its physical, chemical and explosive properties from changing beyond the permitted range under certain circumstances [12]. Stability analysis of physical properties mainly encompasses apparent morphology, color, weight, crystal shape, phase change (melting point, boiling point, volatilization), moisture absorption, particle size, charge density, crystal crack, volatilization, oil permeability, migration, and dielectric constant. Chemical stability encompasses purity, composition, valence state, oxidation state, slow/rapid thermal decomposition, weight loss, and composition of explosive products. The stability of explosion performance encompasses explosion point, detonation heat, detonation velocity, combustion velocity, p-t curve (which is the closed bomb test to measure the output power of the pyrotechnic composition), detonation pressure, detonation capacity, flame sensitivity, hot wire sensitivity, impact sensitivity, friction sensitivity, static inductance, initiation sensitivity, and shock wave sensitivity. In addition to stability, compatibility is also a consideration. This is particularly true of chemical compatibility, including the internal compatibility between the pyrotechnic charge and contact metal (bridge wire, tube shell, cover sheet, motion mechanism, etc.). At present, there are few reports on the thermal reaction process and high temperature stability. Existing studies of BPN mainly focus on the influence of the preparation process [13,14], particle size [15], ratio [16] and other factors related to ignition performance.

2. Materials and Methods

2.1. Materials

Laboratory reagent grade boron and KNO3 powder were all purchased for tests. The particle size of powdered boron was approximately 1 µm, and KNO3 was 99.5% pure. As shown in Table1, using boron powder and KNO3 as raw materials, BPN samples with diﬀerent proportions were prepared. Appl. Sci. Sci. 20192019,, 99,, x 3630 FOR PEER REVIEW 33 of 11

2 KNO3 Table 1. Samples of pure100 components and mixtures. ‐‐

3 1#BPNNo. Component Composition 15:85 (%) Binder ‐‐ 1 Boron 100 – 4 2#BPN 25:75 ‐‐ 2 KNO3 100 – 5 3#BPN3 1#BPN 33:67 15:85 – ‐‐ 4 2#BPN 25:75 – 6 4#BPN5 3#BPN 25:75 33:67 – Fluorine rubber 6 4#BPN 25:75 Fluorine rubber 7 5#BPN7 5#BPN 25:75 25:75 Unsaturated Unsaturated polyester polyester

2.2.Methods Methods The thermal stability of BPN after exposure at 180 ◦°CC was studied by methods such as thermogravimetric analysis and differential diﬀerential scanning calorimetry, and analysis of appearanceappearance morphology, weightweight loss,loss, content,content, andand outputoutput performance.performance. TheThe processprocess scheme is shown in Figure 11..

Samples

TG,DSC Appearance

Thermal exposed

Macroscopic Appearance physical changes

Thermochemical TG,DSC Weight loss ratio recation kinetics

Content analysis Content changes

Output P-t curve performance

Summary analysis

Figure 1. Experimental process scheme.

TG and DSC test data were applied to analyze the thermal reaction process of KNO3, boron TG and DSC test data were applied to analyze the thermal reaction process of KNO3, boron powder and BPN with different ratios, to qualitatively judge high-temperature stability. The apparent powder and BPN with different ratios, to qualitatively judge high‐temperature stability. The morphology analysis, thermogravimetric (TG) analysis, differential scanning calorimetry (DSC) analysis, apparent morphology analysis, thermogravimetric (TG) analysis, differential scanning calorimetry and output power test were carried out on BPN samples after the high-temperature test to verify the (DSC) analysis, and output power test were carried out on BPN samples after the high‐temperature stability of BPN under a high temperature of 180 C, thereby providing a basis for its application in the test to verify the stability of BPN under a high temperature◦ of 180 °C, thereby providing a basis for aerospace field. A summary of the analysis method is presented in Table2. its application in the aerospace field. A summary of the analysis method is presented in Table 2.

Table 2. Analysis methods for BPN thermal analysis and high temperature stability analysis.

Analysis Method content Measure the weight of loose sample before and after high temperature test with Thermogravimetry precision balance. Scanning microscope, optical digital microscope used to observe the color, particle Appearance size, etc. Appl. Sci. 2019, 9, 3630 4 of 11

Table 2. Analysis methods for BPN thermal analysis and high temperature stability analysis.

Analysis Method Content Measure the weight of loose sample before and after high temperature test with Thermogravimetry precision balance. Scanning microscope, optical digital microscope used to observe the color, Appearance Appl. Sci. 2019, 9, x FOR PEER REVIEWparticle size, etc. 4 of 11 Analyze the initial decomposition temperature and peak decomposition DSC analysis Analyzetemperature the initial ofdecomposition samples. temperature and peak decomposition temperature DSC analysis Ion exchange method for determination of KNO contentMannitol complex Content analysis of samples. 3 Ion exchangetitration method for for determination determination of of KNO boron.3 content Content analysis output performance MannitolP–t curve complex pressure titration peak method and rise for timedetermination to evaluate of theboron. power capacity of samples. output P–t curve pressure peak and rise time to evaluate the power capacity of samples. 3. Resultsperformance and Discussion

3.1.3. ThermalResults and Analysis Discussion of Single Component

3.1.The Thermal TG process Analysis of of boron Single was Component studied with a nitrogen atmosphere. As shown in Figure2, a weight loss of approximately 3.3% is observed from heating to 117.5 ◦C, which reﬂects the loss of moisture The TG process of boron was studied with a nitrogen atmosphere. As shown in Figure 2, a and volatiles in boron powder. Until 500 ◦C, the quality of boron powder samples increases gradually, weight loss of approximately 3.3% is observed from heating to 117.5 °C, which reflects the loss of and the speed increases after 650 ◦C. The total mass increases to approximately 118% at 1000 ◦C. It is moisture and volatiles in boron powder. Until 500 °C, the quality of boron powder samples increases speculated that the reaction in this stage is the slow reaction of boron powder and nitrogen, and the gradually, and the speed increases after 650 °C. The total mass increases to approximately 118% at ﬁnal product is BN. The reaction equation is 1000 °C. It is speculated that the reaction in this stage is the slow reaction of boron powder and nitrogen, and the final product is BN. The reaction equation is 2B + N2 = 2BN, (1) 2B + N2 = 2BN, (1)

1.2 118% 1.0 120

120 （545℃ (94.9%） 96.7% 1.0 80 0.8 500℃ 650℃ 80 0.8 40 40 0.6

0.6 0 0

0.4 112.6℃ 112.0℃

117.5℃ Weight/% Weight/% -40 0.4 -40 Heat Flow/mW Heat Flow/mW Heat -80 0.2 0.2 Potassium Nitrate -80 Boron (765℃ (6.8%( Weight Heat Flow -120 HeatFlow 0.0 Weight 0.0 -120 200 400 600 800 1000 200 400 600 800 1000 T/℃ T/℃

(a) (b)

FigureFigure 2. 2.(a ()a Thermogravimetric-Di) Thermogravimetric‐Differentialﬀerential Scanning Calorimetry Calorimetry (TG (TG-DSC)₋DSC) curve curve of ofboron boron powder; powder;

(b)(b TG-DSC) TG‐DSC curve curve of of KNO KNO3 3(in (in nitrogen nitrogen atmosphere).atmosphere).

TheThe DSC DSC curve curve of KNO of KNO3 has3 two has endothermic two endothermic peaks. Thepeaks. first The endothermic first endothermic peak (peak peak temperature (peak 132.6temperature◦C) corresponds 132.6 °C) to thecorresponds conversion to temperaturethe conversion of KNOtemperature3, which of is theKNO phase3, which transition is the ofphase KNO 3 fromtransition rhombic of toKNO trigonal.3 from rhombic The second to trigonal. endothermic The second peak (peakendothermic temperature peak (peak 332.0 temperature◦C) corresponds 332.0 to the°C) melting corresponds point of to KNO the melting3.[9] The point DSC of curve KNO3 shows. [9] The that DSC the curve decomposition shows that of the KNO decomposition3 occurs at moreof thanKNO 5003 occurs◦C. Combined at more than with 500 the °C. TG Combined curve, the with mass the TG change curve, of the KNO mass3 is change 5.1% at of 545KNO◦C,3 is indicating 5.1% at that545 KNO °C, indicating3 is melting that but KNO is not3 is melting decomposed but is ornot gasified decomposed from or the gasified melting from point the (332.0 melting◦C) point to the temperature(332.0 °C) to of the 500 temperature◦C. Therefore, of 500 the °C. physical Therefore, stability the ofphysical KNO3 stabilitychanges of within KNO3 the changes temperature within the range oftemperature 332–500 ◦C, range but the ofchemical 332–500 °C, stability but the is chemical not affected. stability is not affected. A weight loss of KNO3 of 88.1% mainly occurred between 545 °C and 765 °C. According to E.S. A weight loss of KNO3 of 88.1% mainly occurred between 545 ◦C and 765 ◦C. According to E.S. Freeman [17], the decomposition of KNO3 took place at this stage, generating nitrite, followed by the Freeman [17], the decomposition of KNO3 took place at this stage, generating nitrite, followed by the decomposition reaction of potassium nitrite, and the product K2O was reported to evaporate (m.p. 380 °C) [10]. The equation of weightlessness in this stage is [8] → 2KNO322 2KNO +O , (2) → 4KNO22 2K O+4NO+O 2, (3) Appl. Sci. 2019, 9, 3630 5 of 11

decomposition reaction of potassium nitrite, and the product K2O was reported to evaporate (m.p. 380 ◦C) [10]. The equation of weightlessness in this stage is [8]

2KNO3 2KNO2+O2, (2) Appl. Sci. 2019, 9, x FOR PEER REVIEW → 5 of 11 4KNO 2K O + 4NO + O , (3) 2 → 2 2 Compared with the operating condition of 180 °C, the melting temperature of KNO3 at 336.5 °C is aboutCompared 156 °C with higher. the operating No obvious condition chemical of 180 reac◦C,tion the melting occurs temperaturebefore the ofpeak KNO temperature3 at 336.5 ◦C of is aboutdecomposition 156 ◦C higher. (i.e., Nomore obvious than 545 chemical °C). It reaction can be qualitatively occurs before concluded the peak temperature that the boron of decomposition powder and (i.e., more than 545 ◦C). It can be qualitatively concluded that the boron powder and KNO3 components KNO3 components are stable at a high temperature of 180 °C. It should be noted that this result is areobtained stable ataccording a high temperature to the reaction of 180 temperature◦C. It should beof notedDSC, thatand thisthe resultstability is obtained of samples according at constant to the reactiontemperature temperature needs to of be DSC, verified. and the stability of samples at constant temperature needs to be verified. 3.2. Thermal Analysis of BPN with Different Ratios 3.2. Thermal Analysis of BPN with Different Ratios TG-DSC analysis of BPN samples with different ratios (heating rate 5 C/min) were carried out, TG‐DSC analysis of BPN samples with different ratios (heating rate 5 ◦°C/min) were carried out, and the curves are shown in Figure3. For BPN samples with di fferent ratios, the thermal reaction and the curves are shown in Figure 3. For BPN samples with different ratios, the thermal reaction process is similar to that of KNO3 before 500 ◦C. process is similar to that of KNO3 before 500 °C.

1#BPN 2#BPN 100 100 250℃ 2299.5% 250℃ ，97.7% 100 100 500℃ 292.7% 500℃ ，88.6% 80 550℃ 2277.5% 80

80 80 620℃ 2275.6% 60 60

635℃ ，58.0% 514.6℃ 60 60 40 40 Weight/%

Weight /% 40 40 20 20 547.4℃ Heat Flow/mW Subtracted_Data Y1 Subtracted_Data 20 0 20 0 111.6℃ 112.0℃ 335.8℃ 333.3℃ 593.8℃ 0 -20 0 -20 200 400 600 800 200 400 600 800 Temperature /°C Temperature/℃

(a) (b)

1#BPN 250 250℃ ，98.9% 100 500℃ ，95.2%

200 80 550℃ ，72.5% 528℃ ，72.8% 150

60 526.2℃

100

Weight/% 40

50 Subtracted_Data Y1 Subtracted_Data 20

0 116.5℃ 112.6℃ 0 200 400 600 800 Temperature /°C

(c)

FigureFigure 3.3. (a) TG–DSC curve of 1#BPN (15:85); (b) TG–DSC curvecurve ofof 2#BPN2#BPN (25:75);(25:75); ((c)) TG–DSCTG–DSC curvecurve ofof 3#BPN3#BPN (33:67) (heating(heating raterate 55 ◦°C/min).C/min).

As shown in Figure3, the ﬁrst endothermic peak (approx. 134 C) corresponds to the conversion As shown in Figure 3, the first endothermic peak (approx. 134 ◦°C) corresponds to the conversion temperature of KNO3, and the second endothermic peak (approx. 332 ◦C) corresponds to the melting temperature of KNO3, and the second endothermic peak (approx. 332 °C) corresponds to the melting point of KNO3. Before 500 ◦C, the mass change of sample 1# and 2# is about 9%, while that of sample point of KNO3. Before 500 °C, the mass change of sample 1# and 2# is about 9%, while that of sample 3#3# isis lessless thanthan 5%.5%. Between 500 ◦°CC and 650 ◦°C,C, there is aa drasticdrastic decompositiondecomposition reaction,reaction, andand thethe sample weight drops sharply. The temperature of the decomposition reaction is about 200 °C higher than the melting point. The main reaction at this stage is → B+KNO32 KBO +NO . (4)

The weight ratio results of BPN samples with different proportions at typical temperatures are shown in Table 3. At 200 °C, the mass variation of BPN samples with three ratios are in the range 0.9%–1.7%, and at 500 °C, 5.8%–11.4%. At 650 °C, the 2# (25:75) BPN sample (zero oxygen balance Appl. Sci. 2019, 9, 3630 6 of 11

sample weight drops sharply. The temperature of the decomposition reaction is about 200 ◦C higher than the melting point. The main reaction at this stage is

B + KNO KBO +NO. (4) 3 → 2 The weight ratio results of BPN samples with diﬀerent proportions at typical temperatures are shown in Table3. At 200 ◦C, the mass variation of BPN samples with three ratios are in the range 0.9%–1.7%, and at 500 ◦C, 5.8%–11.4%. At 650 ◦C, the 2# (25:75) BPN sample (zero oxygen balance ratio) has the least weight loss. Based on TG analysis of KNO3, the initial reaction temperature of BPN (500 ◦C) is about 50 ◦C lower than that of KNO3 (545 ◦C). However, when comparing the endothermic peak temperature of DSC curves of samples, the exothermic peak temperature showed a trend of rising with the increase of boron content, as shown in Table4.

Table 3. Weight statistics of potassium boron nitrate samples at diﬀerent temperatures.

Weight Ratio @ Weight Ratio @ Weight Ratio @ Weight Ratio @ Weight Ratio @ Samples 100 ◦C (%) 200 ◦C (%) 400 ◦C (%) 500 ◦C (%) 650 ◦C (%) 1#BPN (15:85) 99.3 98.3 94.8 88.6 57.5 2#BPN (25:75) 99.9 98.5 93.2 92.7 75.7 3#BPN (33:67) 100.0 99.1 97.4 94.2 72.5

Table 4. Peak temperature of DSC with diﬀerent ratios of BPN.

Samples First Endothermic Peak (◦C) Second Endothermic Peak (◦C) Exothermic Peak (◦C) 1#BPN (15:85) 133.6 332.0 547.4 2#BPN (25:75) 134.8 331.3 534.6 3#BPN (33:67) 136.5 332.6 526.2

The results show that with an increase of boron content, the exothermic peak is advanced, but the thermal stability of BPN samples are improved, with the 2#BPN sample (25:75) showing the best performance. Analysis at 180 ◦C shows that the DSC decomposition temperature of BPN is above 500 ◦C, and TG weight loss ranges from 5.8% to 11.4% at 500 ◦C. It can be qualitatively judged that BPN is stable at 180 ◦C.

3.3. Thermal Exposure Test at 180 ◦C and Analysis

After drying at 60 ◦C for 4 h, the samples of BPN, boron powder and KNO3 were placed in a temperature environment testing instrument for a thermal exposure test. The temperature settings were 180 ◦C for two and ﬁve days. The apparent morphology, DSC parameters, weight loss ratios and output performance of samples before and after the test were compared and analyzed. Apparent Morphology

The apparent morphologies of KNO3, boron and 4#BPN samples before and after diﬀerent temperature storage are shown in Figures4–6. Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 11 ratio) has the least weight loss. Based on TG analysis of KNO3, the initial reaction temperature of BPN (500 °C) is about 50 °C lower than that of KNO3 (545 °C). However, when comparing the endothermic peak temperature of DSC curves of samples, the exothermic peak temperature showed a trend of rising with the increase of boron content, as shown in Table 4.

Table 3. Weight statistics of potassium boron nitrate samples at different temperatures.

Weight Ratio Weight Ratio @ Weight Ratio @ Weight Ratio @ Weight Ratio @ Samples @ 100 °C (%) 200 °C (%) 400 °C (%) 500 °C (%) 650 °C (%)

1#BPN (15:85) 99.3 98.3 94.8 88.6 57.5

2#BPN (25:75) 99.9 98.5 93.2 92.7 75.7

3#BPN (33:67) 100.0 99.1 97.4 94.2 72.5

Table 4. Peak temperature of DSC with different ratios of BPN.

Samples First Endothermic Peak (°C) Second Endothermic Peak (°C) Exothermic Peak (°C)

1#BPN (15:85) 133.6 332.0 547.4

2#BPN (25:75) 134.8 331.3 534.6

3#BPN (33:67) 136.5 332.6 526.2

The results show that with an increase of boron content, the exothermic peak is advanced, but the thermal stability of BPN samples are improved, with the 2#BPN sample (25:75) showing the best performance. Analysis at 180 °C shows that the DSC decomposition temperature of BPN is above 500 °C, and TG weight loss ranges from 5.8% to 11.4% at 500 °C. It can be qualitatively judged that BPN is stable at 180 °C.

3.3. Thermal Exposure Test at 180 °C and Analysis

After drying at 60 °C for 4 h, the samples of BPN, boron powder and KNO3 were placed in a temperature environment testing instrument for a thermal exposure test. The temperature settings were 180 °C for two and five days. The apparent morphology, DSC parameters, weight loss ratios and output performance of samples before and after the test were compared and analyzed. Apparent Morphology

Appl.The Sci.apparent2019, 9, 3630morphologies of KNO3, boron and 4#BPN samples before and after different 7 of 11 temperature storage are shown in figure 4, figure 5 and figure 6.

60 °C, 4 h 180 °C, 2 60 °C, 4 h 180 °C, 2 days 60 °C, 4 h 180 °C, 2 days days

(a) (Original sample) (b) ((Original sample against (c) (enlarged 100 times) Appl. Sci. 2019, 9, x FOR PEER REVIEW white background) 7 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 11 FigureFigure 4. The 4. apparentThe apparent morphology morphology of KNO of3 compared KNO3 compared before and before after and different after ditemperatureﬀerent temperature tests. tests.

60 °C, 4 h 180 °C, 2 days 60 °C, 4 h 180 °C, 2 days 60 °C, 4 h 180 °C, 2 days 60 °C, 4 h 180 °C, 2 days

(a) (Original sample) (b) (enlarged 100 times) (a) (Original sample) (b) (enlarged 100 times) Figure 5. The apparent morphology of boron compared before and after different temperature tests. FigureFigure 5. 5. TheThe apparent apparent morphology morphology of of boron boron compared compared be beforefore and and after after different diﬀerent temperature temperature tests. tests.

60 °C, 4 h 180 °C, 2 days 60 °C, 4 h 180 °C, 2 days 60 °C, 4 h 180 °C, 2 days 60 °C, 4 h 180 °C, 2 days

(a) (Original sample against white (b) (enlarged 100 times) (a) (Original background)sample against white (b) (enlarged 100 times) background) FigureFigure 6. 6.The The apparentapparent morphology morphology of of 4#BPN 4#BPN compared compared before before and and after after di differentﬀerent temperature temperature tests. tests. Figure 6. The apparent morphology of 4#BPN compared before and after different temperature tests. AfterAfter observing observing the the boron boron powder, powder, KNO KNO3 3and and 4#BPN 4#BPNsamples samplesbefore beforeand andafter after highhigh temperature,temperature, After observing the boron powder,3 KNO3 and 4#BPN samples before and after high temperature, itit is is found found that that the the color color of of the the KNO KNO3 sample sample changes changesslightly slightlyfrom from white white to to yellow, yellow, and and the the particle particle itsize sizeis found and and shape shapethat the dodo color notnot change. of the KNO The The 3particle particlesample size, changes size, shape shape slightly and and color colorfrom of ofwhite boron boron to powder yellow, powder and and and 4#BPN the 4#BPN particle are are not sizenotobserved and observed shape to tochange.do change. not change. In Inaddition, addition, The particle there there issize, isno no shapechange change and in in colorappearance appearance of boron quality, powder suchsuch and asas4#BPN expansionexpansion are notof of observedfractures,fractures, to loose loose change. pits, pits, shrinkageIn shrinkage addition, or or meltingthere melting is residue.no residue. change in appearance quality, such as expansion of fractures, loose pits, shrinkage or melting residue. DSCDSC test test DSC test TheThe DSC DSC curves curves and and DSC DSC parameter parameter comparisoncomparison ofof 4#BPN4#BPN samplesample areare givengiven inin FigureFigure7 7and and TableTableThe5 respectively.5 DSCrespectively. curves and DSC parameter comparison of 4#BPN sample are given in Figure 7 and Table 5 respectively.

Appl. Sci. 2019, 9, 3630 8 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 11

2#BPN 2#BPN 25 a# normal sample 25 a# normal sample b# normal sample b# normal sample 20 c# 180℃2D c# 180℃2D # ℃ D 20 d 180 2 d# 180℃2D e# 180℃5D 15 e# 180℃5D f# 180℃5D 15 f# 180℃5D 10

5 10 Heat Flow/mW Heat

0 Flow/mW Heat 5

-5 0 -10

0 100 200 100 400 500 -5 180 400 420 440 460 480 T/℃ T/℃

0.8 2#BPN 2 a# normal sample b# normal sample 0 0.4 c# 180℃2D d# 180℃2D e# 180℃5D f# 180℃5D -2 0.0

-4 -0.4

Heat Flow/mW 2#BPN Heat Flow/mW Heat -6 a# normal sample b# normal sample -0.8 c# 180℃2D -8 d# 180℃2D e# 180℃5D -1.2 f# 180℃5D 120 125 110 115 140 145 -10 111 114 115 116 117 118 T/℃ T/℃

FigureFigure 7. 7.DSC DSC curves curves of of 4#BPN 4#BPN 60 60◦ °C,4C,4 hh samples and 180 180 °C◦C samples samples (heating (heating rate rate 5 °C/min). 5 ◦C/min).

Table 5. DSC parameter comparison of 4#BPN sample (◦C) (heating rate 5 ◦C/min). Table 5. DSC parameter comparison of 4#BPN sample (°C) (heating rate 5 °C/min). Parametric 60 C, 4 h 180 C, 2 days 180 C, 5 days Parametric ◦ 60 °C, 4 h 180 ◦°C, 2 days 180 ◦°C, 5 days InitialInitial temperature temperature of ﬁrstof first endothermic endothermic peak peak (◦C) (°C) 129.04 129.04 129.45 129.45 128.84 FirstFirst endothermic endothermic peak peak (◦C) (°C) 131.15 131.15 131.35 131.35 131.55 InitialInitial temperature temperature of secondof second endothermic endothermic peak peak (◦C) (°C) 334.95 334.95 335.12 335.12 335.35 SecondSecond endothermic endothermic peak peak (◦C) (°C) 336.02 336.02 336.06 336.06 336.23 Initial temperature of exothermic peak ( C) 405.34 406.37 404.33 Initial temperature of exothermic peak◦ (°C) 405.34 406.37 404.33 Exothermic peak ( C) 431.28 424.77 428.51 Exothermic peak◦ (°C) 431.28 424.77 428.51 Decomposition heat E (J g 1) 3447 3454 3367 Decomposition heat E· (J·g− −1) 3447 3454 3367 Energy loss rate 1 (%) 0 1.01 3.26 Energy loss rate 1 (%) 0 1.01 3.26 1 1Energy loss rate: (E60◦C4h–E180◦C, xd)/E60◦C4h. Energy loss rate: (E60°C4h–E180°C，xd)/E60°C4h.

AccordingAccording to theto the DSC DSC analysis analysis of the of variation the variatio of then of peak the temperature, peak temperature, the temperature the temperature difference betweendifference the firstbetween endothermic the first endothermic peak and the peak second and endothermic the second endothermic peak is very peak small, is comparing very small, the samplescomparing at 180 the◦ Csamples for two at days,180 °C 180 for ◦twoC for days, five 180 days, °C for and five 60 days,◦C for and 4h; 60∆ °CT

Table 6. Weight loss ratios of BPN samples in 180 ◦C test.

Weight Loss Ratio after Weight Loss Ratio after Weight Loss Ratio after Sample 1 60 ◦C, 4 h (%) 180 ◦C, 2days (%) 180 ◦C, 5days (%) 1#BPN (15:85) 0.097 0.12 0.11 4#BPN (25:75) 0.372 1.04 0.98 5#BPN (25:75) 0.095 1.19 1.59 KNO3 0.0083 0.044 – Boron 1.549 5.26 – 1 Binders of 4#BPN, 5#BPN are ﬂuorine rubber, and unsaturated polyester, respectively.

Table 7. Component variation of BPN samples in 180 ◦C test [18].

Sample Component 60 ◦C, 4 h (%) 180 ◦C, 2 days (%) 180 ◦C, 5 days (%) KNO (%) 85.80 85.66 85.90 1#BPN (15:85) 3 Boron (%) 14.87 14.88 14.85 KNO (%) 73.32 73.79 74.36 4#BPN (25:75) 3 Boron (%) 23.04 27.38 26.24

As shown in Table6, from the comparison of the weight loss ratios of BPN samples in the 180 ◦C test for different times, after five days, the weight loss ratio of 1# BPN without binder is approximately 0.1%, that of 4#BPN containing fluororubber binder is approximately 1%, and that of 5# BPN containing unsaturated polyester is approximately 2%. Combined with the analysis of component content, the change of KNO3 and boron in 1#BPN without binder in the 5-day 180 ◦C test is 0.5%, i.e., almost no change. Thus, it can be judged to some extent that the stability of BPN in which the main components are KNO3 and boron is well under 180 ◦C for two days and five days. The reason for the component variation of 4#BPN and 5#BPN is the binder (i.e., 4#BPN contains 3% fluororubber and 5#BPN contains 5.6% unsaturated polyester). P-t curve • P-t curve data of BPN samples in 180 ◦C test is given in Table8.

Table 8. P-t curve of BPN samples in 180 ◦C test [19,20].

Rate of Peak Pressure Sample Component Peak Pressure (MPa) Rise Time (ms) Change (%) 60 C, 4 h 2.11 3.09 1#BPN (15:85) ◦ 0.95 180 ◦C, 5 days 2.09 2.94 − 60 C, 4 h 2.57 1.82 4#BPN (25:85) ◦ 10.89 180 ◦C, 5 days 2.29 1.33 − 60 C, 4 h 3.34 4.09 5#BPN (25:75) ◦ 25.75 180 ◦C, 5 days 2.48 4.59 −

It can be seen from the data in Table8 that the peak pressure of the three kinds of BPN decreases with increasing time of the 180 ◦C temperature test. The thermal analysis of the single component shows that boron and KNO3 are relatively stable at 180 ◦C, and the peak pressure attenuation of the 1#BPN sample without binder is 0.95% (i.e., <1%), which also supports this argument. It is also known that the binder has a great impact on the high temperature stability of BPN. The high temperature stability of the ﬂuororubber binder is better than that of unsaturated polyester.

4. Conclusions

In conclusion, we studied the thermal decomposition process of KNO3, boron and BPN based on TG and DSC test data. The weight change of boron powder caused by chemical reaction occurred above 500 ◦C. When the temperature was lower than the peak exothermic temperature of decomposition, no Appl. Sci. 2019, 9, 3630 10 of 11

obvious chemical reaction occurred with KNO3, and only physical changes (crystal transformation and melting) occurred. Combined with a verification test at 180 ◦C for two days, it is concluded that boron and KNO3 components are stable at 180 ◦C. With the increase of boron content, the thermal stability of BPN sample improved, and the best performance was achieved when the ratio was 25:75 (B:KNO3). BPN samples without binder have the best thermal stability. In the 5 day 180 ◦C test, the binder affected the weight loss and p-t curve of BPN. Finally, BPN with fluororubber binder is better than BPN with unsaturated polyester binder.

Author Contributions: Conceptualization, C.L. and N.Y.; methodology, C.L. and Z.L.; formal analysis, N.Y.; data curation, J.H. and N.Z.; writing—original draft preparation, C.L.; writing—review and editing, N.Y. and Y.Y.; visualization, X.H. Funding: This research was funded by Beijing Space Vehicle General Design Department. Acknowledgments: Thanks for the support from State Key Laboratory of Explosive Science and Technology, Yunliang Lao’s guidance in the field of thermal analysis, and the funding and experimental support from Beijing Space Vehicle General Design Department. Conflicts of Interest: The authors declare no conflict of interest.

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