Advanced Powder Technology 21 (2010) 443–447

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Advanced Powder Technology

journal homepage: www.elsevier .com/locate /apt

Original Research Paper

Crystal habit modification of ammonium perchlorate by ethylene glycol

Makoto Kohga *, Hirotatsu TsuzukiDepartment of Applied Chemistry, National Defense Academy, Japan

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 October 2009Received in revised form 14 January 2010Accepted 20 January 2010

Keywords:Ammonium perchlorateCrystal habitPropellant oxidizer

0921-8831/$ - see front matter � 2010 The Society ofdoi:10.1016/j.apt.2010.01.004

* Corresponding author. Address: Department ofDefense Academy, Hashirimizu 1-10-20, Yokosuka,Tel.: +81 046 841 3810; fax: +81 046 844 5901.

E-mail address: kohga@nda.ac.jp (M. Kohga).

Ammonium perchlorate (AP)-based composite propellant has been widely used as a solid propellant, buthigher burning rates are in demand. We report that crystal-habit-modified AP is an effective oxidizer thatincreases the burning rates of propellants. The modified APs have a dendritic shape. However, dendriticAP is not convenient to mix and cast in an AP/binder slurry because of its high viscosity. We attempted toprepare a crystal-habit-modified AP with polygon or sphere form. The crystal habit of AP was modified byethylene glycol such that the intensity of the lattice plane (2 1 0) of the modified AP was remarkably high.The shape was almost hexahedral, and the mean particle diameter was approximately 150 lm.� 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder

Technology Japan. All rights reserved.

1. Introduction

Solid propellant is a kind of fuel for rockets. High-burning-ratepropellants that generate a large quantity of combustion gases in ashort time are in demand to enable rockets to fly at faster speeds.Ammonium perchlorate (AP)-based composite propellant has beenwidely used as a solid propellant. The burning rate of AP-basedpropellant increases with increasing AP content and also increaseswith decreasing AP particle diameter. AP-based composite propel-lants prepared with fine AP and with higher AP contents are re-quired to obtain a high burning rate.

AP particles about 20 lm in diameter can be prepared by a stan-dard grinding method such as a vibration ball mill. However, it isdifficult to prepare finer AP sizes using this method [1,2]. Jet mills,which use compressed air or gas to produce fine particles, are ableto produce AP particles finer than 20 lm in diameter. However, thejet mill technique is dangerous because fine AP can ignite and ex-plode easily by slight impact or friction. To prepare fine AP using ajet mill, special facilities are needed that have an anti-explosiveroom, provisions for a nitrogen atmosphere, impurity removalequipment, etc. While it is not easy to safely prepare fine AP parti-cles by a mechanical grinding method, some methods of preparingfine AP particles have been based on spray-drying and freeze-dry-ing [3–8].

Ito and Hagihara et al. [9–11] reported that the crystal habit ofAP was modified by the use of certain organic solvents and surfac-tants as habit modifiers and that the modified APs were effective

Powder Technology Japan. Publish

Applied Chemistry, NationalKanagawa 239-8686, Japan.

oxidizers for preparing high-burning-rate composite propellants[12]. Most of the crystal-habit-modified AP particles are dendritic.

During the preparation of the AP-based composite propellant, alarge amount of AP was sufficiently mixed with a small amount offuel binder. This AP/binder slurry was then cast into the rocket mo-tor. Therefore, the viscosity of the AP/binder slurry needed to besuitable for mixing and casting. The viscosity of the slurry basedon the modified APs was high during mixing and casting becausethe APs were dendritic in shape. Therefore, a propellant couldnot be prepared above 73.5% AP when the dendritic-modified APwas used as an oxidizer [12].

When polygon or spherical AP particles were used as the oxi-dizer, the AP/HTPB slurry had a favorable viscosity for mixingand casting into the rocket motor. Therefore, the upper limit con-tent of the AP that can be incorporated in the propellant with poly-gon or spherical AP will be larger than that with the dendritic AP.We expected that a polygon or spherical crystal-habit-modified APwill be an effective oxidizer for enhancing the burning rate of anAP-based composite propellant. In this study, we attempted to pre-pare polygon or spherical crystal-habit-modified AP, as describedbelow.

2. Experimental

2.1. Preparation of crystal-habit-modified AP

First, a polygon or spherical crystal-habit-modified AP wasneeded for this study. Hagihara and Ito [10] prepared some den-dritic-modified APs according to the following procedure: a satu-rated aqueous solution of AP at 333 K was poured into an organicsolvent, e.g., ethanol, 1-butanol, or 1-propanol, cooled to 273–275 K, and the resulting mixture of saturated AP aqueous solution

ed by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Table 1Compositions of saturated solutions of AP in aqueous ethylene glycol and yield forone batch.

Symbol Ethylene glycol/distilledwater mass ratio (–)

Yield for one batch (g)

recryAP 0/10 182EGAP02 20/80 168EGAP04 40/60 157EGAP06 60/40 140EGAP08 80/20 101EGAP10 10/0 28

444 M. Kohga, H. Tsuzuki / Advanced Powder Technology 21 (2010) 443–447

and organic solvent was quickly cooled to 273 K. The AP was thenrecrystallized from the cooled solution. The crystal habit of therecrystallized AP had been modified.

In a preliminary experiment, an almost hexahedral crystal-ha-bit-modified AP was obtained by pouring saturated aqueous APsolution at 333 K into ethylene glycol cooled to 273 K. The yieldof AP recrystallized by this method was low because of the moder-ate solubility of AP in ethylene glycol at 273 K (5.5 g AP/100 g eth-ylene glycol). A large quantity of AP was required to prepare apropellant; therefore, a large amount of ethylene glycol would beneeded to prepare the modified AP by this method. Consequently,this method was not pursued in our study.

To increase the yield, the modified AP was prepared by recrys-tallization from an aqueous ethylene glycol solution saturated withAP. The yield of AP prepared by this method exceeded that pre-pared by Hagihara’s method [10] described above, and the aqueousethylene glycol solution could be repeatedly recycled.

2.2. Measurement of particle characteristics

Scanning electron microscopy (SEM) was used to observe anexternal view of the prepared AP. The crystallographic propertiesof the AP were examined by X-ray diffractometry (XRD). The char-acteristic X-ray wavelength used was that of Cu Ka (k =1.5418 � 10�10 m). Fourier transform infrared (FTIR) spectra wererecorded in the range 400–4600 cm�1. The sensitivity to impactwas investigated by a drop hammer test on the basis of JapaneseIndustrial Standard K4810 [13]. The drop hammer test involveddropping a 5 kg iron hammer onto the test sample gripped be-tween two cylindrical rollers placed on the anvil of the test rig,and sensitivity was investigated from the relationship betweenthe height from which the hammer was dropped and whether ornot there was an explosion.

3. Results and discussion

3.1. Preparation of crystal-habit-modified AP

Fig. 1 shows the preparation process of the crystal-habit-modi-fied AP. Ethylene glycol was used as a habit modifier for AP in thisstudy. Mixtures of ethylene glycol and distilled water were pre-pared at six different mass ratios, as indicated in Table 1. The aque-ous ethylene glycol solutions at 273 and 333 K were prepared with

Distilled water Ethylene glycol

Ethylene glycol aqueous solution

AP

Filter by suction

Saturated AP solution at 333 K Saturated AP solution

at 273 K333 K 338 K

Dry at 333 K, less than 20 Pa, and for 6 hours

200 cm3

2000 cm3Iced water

Distilled water Ethylene glycol

Ethylene glycol aqueous solution

AP

Filter by suction

Saturated AP solution at 333 K Saturated AP solution

at 273 K333 K 338 K

Dry at 333 K, less than 20 Pa, and for 6 hours

200 cm3200 cm3

2000 cm32000 cm3Iced water

Fig. 1. Preparation process of crystal-habit-modified AP.

iced water and hot bath. AP was then dissolved in these mixturesto obtain a saturated solution at each temperature. Then 200 cm3

of the saturated AP solution at 333 K was heated to 338 K in ahot bath and was immediately poured into 2000 cm3 of the solu-tion at 273 K.

The time taken to remove the heated solution from the hot bathand pour it into the cooled solution at 273 K was about 10 s. Duringthis period, the AP recrystallized from the saturated AP solution at333 K because the temperature decreased. The reason for heatingthe saturated AP solution at 333–338 K was to prevent recrystalli-zation of AP during this pouring process. AP was recrystallizedfrom the mixed solution upon cooling. The solution was stirredat 300 rpm with a mixer equipped with three propellers (/ 4 cm)and cooled to 273 K.

The recrystallized AP was collected by suction filtration anddried in a vacuum thermostat at 333 K and <20 Pa for 6 h. The boil-ing point of the ethylene glycol at 20 Pa was 291 K. Thus, any resid-ual ethylene glycol on the AP particles would have been removedby the drying process. The AP samples prepared with ethylene gly-col were designated herein as EGAP samples.

For one batch, 200 cm3 of the hot saturated AP solution waspoured into 2000 cm3 of the cooled solution and AP recrystallized.The yield of recrystallized AP for one batch is shown in Table 1. Theyield decreases with increasing proportion of ethylene glycol, andremarkably so above 60%. The yield of the EGAP10 sample wasone-ninth of that of the recryAP sample.

The EGAP samples were dried in a vacuum thermostat at333 K and <20 Pa for 6 h. The FTIR spectra were recorded after

1000200030004000

(c) ethylene glycol

4600 400

(a) recryAP

NH4+

Stretching

NH4+

Bending ClO4-

Stretching

ClO4-

Bending

Wavenumber (cm-1)

Tra

nsm

ittan

ce

(b) EGAP80

CH Stretching

Fig. 2. FTIR spectra of recryAP, EGAP08, and ethylene glycol.

M. Kohga, H. Tsuzuki / Advanced Powder Technology 21 (2010) 443–447 445

samples were dried to confirm that residual ethylene glycol onthe EGAP particles was removed by this drying process. Fig. 2shows the FTIR spectra of recryAP, EGAP08, and ethylene glycol.The FTIR spectrum of ethylene glycol featured a band at around2800–3000 cm�1, attributable to the CH group. The FTIR spec-trum of EGAP08 was almost the same as that of the recryAPand did not feature the band at around 2800–3000 cm�1,indicating that the ethylene glycol could be removed from theEGAP.

Fig. 3. SEM photographs of A

3.2. SEM

Fig. 3 shows SEM images of EGAP samples. The recryAP sampleswere not uniform in shape. On the other hand, EGAP appeared tobe made up of polygons, and in particular, EGAP08 and EGAP10were almost entirely hexahedral. We found that the formation ofpolygonal AP particles was favored with the ethylene glycol. Theparticle diameters of the AP samples were measured from theseSEM images. Fig. 4 shows the particle size distributions for recryAP

P prepared in this study.

100 200 300 400

5

10

15

20

25

0

Particle size (µm)

Num

ber

basi

s di

stri

butio

n (%

)

recryAPEGAP02EGAP04EGAP06EGAP08EGAP10

Fig. 4. Particle distributions.

Table 2Dw of AP.

Sample Dw (lm)

recryAP 213EGAP02 198EGAP04 204EGAP06 158EGAP08 142EGAP10 145

15 20 25 30 35

(f) EGAP10

Inte

nsity

(ar

b. u

nit)

2 (deg.) 2 (deg.)

(e) EGAP08

(d) EGAP06

15 20 25 30 35

(c) EGAP04

(b) EGAP02

(a) recryAP

Fig. 5. XRD patterns.

0.4

0.6

0.8

1

r (-

)I

446 M. Kohga, H. Tsuzuki / Advanced Powder Technology 21 (2010) 443–447

and EGAP samples. The peak of the size distribution of recryAPsamples was at 60 lm and that of EGAP was between 40 lm and120 lm. The width of the distribution gradually decreased withincreasing proportion of ethylene glycol. The weight mean particlediameters, Dw, were shown in Table 2. The Dw values for recryAPand EGAP02-EGAP04 were about 200 lm, whereas those ofEGAP06-EGAP10 were about 150 lm. The Dw values for EGAP06-EGAP10 were thus approximately 50 lm smaller than those ofrecryAP and EGAP02-EGAP04.

20 40 60 80 100

0.2

0

Concentration of ethylene glycol (%)

Fig. 6. Influence of concentration of ethylene glycol on Ir.

3.3. XRD patterns

Fig. 5 illustrates the XRD patterns of the recryAP and EGAP sam-ples. The XRD pattern of the recryAP sample was consistent withthe typical pattern of AP. The lattice planes of the EGAP sampleswere broadly similar to those of the recryAP. The XRD pattern ofthe EGAP02 sample was almost the same as that of the recryAP.For EGAP04-EGAP10, the relative intensity of the (2 1 0) plane,i.e., 2h = 24.6�, was obviously high and increased with increasingproportion of ethylene glycol. In particular, the XRD patterns ofthe EGAP06-EGAP10 displayed remarkably high intensity of the(2 1 0) plane.

According to the ASTM record of AP, the main peaks appear at15.3�, 19.4�, 22.7�, 23.9�, 24.6�, 27.4�, 30.1�, 30.8�, and 34.6� of2h. The relative intensity of the (2 1 0) plane, i.e., 2h = 24.6�, ofthe EGAP sample was remarkably high. The ratio of the intensityof the lattice plane (2 1 0) to total amount of intensity of each mainpeak, Ir, was calculated from the XRD patterns of each AP sample.The crystal habit modification magnitude was represented by thevalue of Ir.

Fig. 6 shows the influence of the concentration of ethylene gly-col on Ir. The value of Ir increased with increasing concentration of

ethylene glycol. The increase in Ir was great below 60%, andremarkably between 40% and 60%. Above 60% it was small. This re-sult suggested that the crystal habit modification magnitudewould be enhanced as the concentration of ethylene glycol in-creased and the crystal habits of EGAP06-EGAP10 were sufficientlymodified by the ethylene glycol.

The value of Ir increases with increasing concentration of ethyl-ene glycol, indicating that the concentration of ethylene glycolwould be an outstanding factor to modify the crystal habit of AP.Many researchers have been concerned with the crystal habit mod-ification of materials. A reason for the crystal habit modification isthat an element of the ingredients in the solution adsorbs on a par-ticular face of the crystal and hence the rate of growth of that faceis changed by the adsorption [14–16]. Hagihara and Ito [10]reported that the critical habit of the AP modified with organic

20 40 60 80 100

0.5

1

1.5

2

0

Concentration of ethylene glycol (%)

Vis

cosi

ty (

mPa

s)

Fig. 7. Viscosity of saturated AP ethylene glycol/water solution at 200 s�1 and 293K.

M. Kohga, H. Tsuzuki / Advanced Powder Technology 21 (2010) 443–447 447

solvents may be attributed to adsorption upon specific crystalforms. The adsorption of a molecule of ethylene glycol on the crys-tal face of the AP would lead to accelerated growth of the latticeplane (2 1 0) or suppress the growth of the faces other than the lat-tice plane (2 1 0), and consequently, the crystal habit of the APwould be modified by ethylene glycol.

The small amount of the hot solution was poured into the largequantity of the cooled solution, and the AP was recrystallized bythe rapid cooling. The solution was a rapid, temporary supersatu-ration at the beginning of the mixing, and the recrystallization ofAP would be quick. This rapid recrystallization during the supersat-uration step was also factor in the crystal habit modification.

The viscosity of ethylene glycol was larger than that of water.Fig. 7 shows the viscosity of the saturated AP ethylene glycol/watersolution at 200 s�1 and 293 K. The viscosity increased with increas-ing concentration of ethylene glycol. The solution was stirred withthe mixer during the cooling. Viscosity influences the mixing con-dition of the solution. That is to say, the mechanism of the solidformation in the solution sometimes depends on the solution’s vis-cosity. Different viscosity levels would alter AP’s crystallizationprocess; therefore, the crystal habit modification would dependon the concentration of ethylene glycol.

3.4. Drop hammer test

AP is an oxidizer and is explosive upon impact. The impactsafety can be estimated by a drop hammer test. Table 3 showsthe impact sensitivities of the recryAP and EGAP08 samples. TherecryAP sample exploded in each of six drops from above a heightof 45 cm and in two drops from 40 cm, but it did not explode in anydrops from less than 35 cm. The EGAP08 sample exploded in each

Table 3Sensitivity on the basis of drop hammer tests.

Drop height (cm) Judgement

recryAP EGAP08

45 ssssss ssssss

40 ����ss ssssss

35 ������ ssssss

30 ������ �s��s�25 ������ ������Sensitivity class 6 5

s: Explosion; �: no explosion.

of six drops from above a height of 35 cm and in two drops from30 cm, but it did not explode in any drops from less than 25 cm.The impact sensitivity classes of the recryAP and EGAP08 samplesare thus 6th and 5th, respectively. In other words, the impact sen-sitivity of EGAP08 is higher than that of recryAP.

Explosive materials such as RDX and HMX are added to solidpropellants for high-performance purposes [17]. The sensitivityclasses of RDX and HMX are 2nd and 1st, respectively. The sensitiv-ity of EGAP08 is thus considerably lower than those of RDX andHMX. The EGAP08 can be used as an oxidizer in a composite pro-pellant provided that sufficient care is taken to avoid impacts.

4. Conclusions

We attempted to modify the crystal habit of AP by using ethyl-ene glycol and investigated the particle properties of the crystal-habit-modified AP. The AP was recrystallized from a saturatedsolution in water/ethylene glycol. The crystal habit of AP was mod-ified when the ethylene glycol content in the solution exceeded60%. The intensity of lattice plane (2 1 0) of the modified AP wasremarkably high. It is clear that the crystal habit of AP is modifiedby ethylene glycol. The shape of the modified AP was almost hexa-hedral, and the mean particle diameter was approximately150 lm. It is expected that a high-burning-rate propellant couldbe obtained by using this modified AP. We also found that thehexahedral modified AP can be prepared with ethylene glycol.The burning rate characteristics of the propellant using this AP par-ticle will be the focus of further study.

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