K.V.Suresh Bu,P.Knk Rju,C.R.Thoms,A.Syed Hmed,K.N.Ninn

aSolid Propellant Space Booster Plant,Satish Dhawan Space Centre,Sriharikota,524124,India

bRocket Propellant Plant,Vikram Sarabhai Space Centre,Thiruvananthapuram,695 022,India

cDepartment of Chemistry,Indian Institute of Space Science and Technology,Thiruvananthapuram,695 547,India

Studies on composite solid propellant with tri-modal ammonium perchlorate containing an ultra fine fraction

K.V.Suresh Babua,*,P.Kanaka Rajua,C.R.Thomasb,A.Syed Hameda,K.N.Ninanc

aSolid Propellant Space Booster Plant,Satish Dhawan Space Centre,Sriharikota,524124,India

bRocket Propellant Plant,Vikram Sarabhai Space Centre,Thiruvananthapuram,695 022,India

cDepartment of Chemistry,Indian Institute of Space Science and Technology,Thiruvananthapuram,695 547,India

A R T I C L E I N F O

Article history:

19 May 2017

Accepted 7 June 2017

Available online 9 June 2017

Ammonium perchlorate

Composite propellant

Burn rate

Copper chromite

Iron oxide

Composite solid propellant is prepared using tri-modal Ammonium perchlorate(AP)containing coarse, fine and ultra fine fractions of AP with average particle size(APS)340,40 and 5μm respectively,in various compositions and their rheological,mechanical and burn rate characteristics are evaluated.The optimum combination of AP coarse to fine to ultra fine weight fraction was obtained by testing of series of propellant samples by varying the AP fractions at fixed solid loading.The concentration of aluminium was maintained constant throughout the experiments for ballistics requirement.The propellant formulation prepared using AP with coarse to fine to ultra fine ratio of 67:24:9 has lowest viscosity for the propellant paste and highest tensile strength due to dense packing as supported by the literature.A minimum modulus value was also observed at 9 wt.%of ultra fine AP concentration indicates the maximum solids packing density at this ratio of AP fractions.The burn rate is evaluated at different pressures to obtain pressure exponent.Incorporation of ultra fine fraction of AP in propellant increased burn rate without adversely affecting the pressure exponent.Higher solid loading propellants are prepared by increased AP concentration from 67 to 71 wt.%using AP with coarse to fine to ultra fine ratio of 67:24:9.Higher solid content up to 89 wt.%was achieved and hence increased solid motor performance. The unloading viscosity showed a trend with increased AP content and the propellant couldn't able to cast beyond 71 wt.%of AP.Mechanical properties were also studied and from the experiments noticed that%elongation decreased with increased AP content from 67 to 71 wt.%,whereas tensile strength and modulus increased.Burn rate increased with increased AP content and observed that pressure exponent also increased and it is high for the propellant containing with 71 wt.%of AP due to increased oxidiser to fuel ratio.Catalysed composite solid propellant is prepared by using burn rate modi fiers Copper chromite and Iron oxide.Addition of Copper chromite and Iron oxide has enhanced the burn rate of tri-modal AP based composite solid propellant.The catalytic propensity of copper chromite is higher than that of iron oxide.The pressure exponent increased with the catalyst concentration and the values obtained are compatible for solid rocket motor applications.

?2017 The Authors.Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

The composite solid propellants(CSP)based on hydroxyl terminated polybutadiene binder fuel,Aluminium powder fuel(Al) and Ammonium perchlorate oxidizer(AP)are widely used in currentsolid rocket motors[1].Solid propellants with high densityand speci fic impulse are desirable for booster stage application and it needs high levels of solid loading in a given polymer matrix.The mechanicaland burningcharacteristicsofthesepropellants depend to a large extent on the composition,average particle size (APS)and particle size distribution of the AP powder in the propellant[2-15].In most of the practical applications bi-modal AP containing coarse and fine fraction are used for better packing and tailoring of burn rate.The burn rate of the propellants prepared using the bimodal AP increases with increase in AP concentration in propellant and decreases with increase in APS[10].Propellant formulations based on bi-modal AP having viscosity in acceptablerange for mixing and casting can be obtained at solid loading (combined proportion of AP and Al)of around 86%(w/w)).Higher solid loading can be obtained with the optimized fractions of solid ingredients reaches the maximum packing density.Packing density is achieved bya combination of parameters such as particlesize and size distribution,number of sizes(modality),weight fractions of components and the shape and surface characteristics of particles.

Buckmaster and co-workers[16,17]reported using theoretical calculations that the fine AP particles are necessary in CSP to achievethe requiredratio of ammonium perchlorate tofuel binder.The rheological characteristics of the propellant paste,mechanical and burning characteristics of the CSP based on bimodal AP powder,of various coarse to fine ratios have been reported by many authors [9-14].Many of the reports show optimum rheological and mechanical properties at coarse to fine weight ratio of approximately 3:1[13].The theoretical model developed by Iqbal and Liang[4] reported the advantages of AP with wide particle size distribution in a CSP.They have demonstrated that the burn rate of CSP can be increased by decreasing the APS of AP,which in turn can be decreased by increasing relative composition of AP fine fraction. CSP based on tri-modal AP with fractions of APS 400,200 and 80μm are prepared and rheological and mechanical properties are studied by Rodic and Bajlovski[7].The incorporation of ultra fine fraction of APS less than 10μm substantially decreases the APS of AP.However,studies on propellant formulations using AP of particle size less than 10μm(ultra fine)are scanty[4,8],though its positive effect on burn rate is well known.The present work explores the possibility of using an ultra fine AP in CSP and obtaining an optimised tri-modal distribution,incorporation of high AP content bythe optimised tri-modal distribution of APand exploring the catalytic effect.

2.Materials and methodology

Hydroxy terminated Polybutadiene(HTPB,Binder fuel)of average molecular weight 2640 g/mol and hydroxyl value of 41.0 mgKOH/g and Ammonium perchlorate(AP,Oxidizer)are obtained from in-house manufacturing plants.DioctylAdipate(DOA,plasticizer),Toluene diisocyanate(TDI,curative),trimethylolpropane (TMP,crosslinker),1,4 butane diol(BDO,binder additive),Phenyl--β-naphthylamine(PBNA,antioxidant)and atomized Aluminum powder(Al,metal fuel)are obtained from open market and used. AP is processed into coarse, fine and ultra fine fractions of APS of 340,40 and 5μm respectively.All the compositions were processed at 2.5 kg mix level in a horizontal sigma mixer.The temperature during mixing is kept around 40°C by circulating hot water in the mixer jacket.The propellant slurry after the mixing was casted under vacuum in a carton by passing through a grill valve in order to eliminate the air pockets during the mixing operation.The propellants cartons after casting were cured at 50°C for 8 days.

The PSD of AP is measured using Tyler sieves and Malvern particle size analyser.Surface area of catalyst is measured using Monosorb Surface area analyser.Viscosity of propellant paste is measured using Brooke field Viscosmeter using a T spindle at 2.5 RPM,40OC.The tensile strength measurements of the CSP slabs is measured using Instron 4466 UTM at cross head speed of 50 mm/ min as per ASTM D 412.Five dumbbell slabs were tested and average values were reported for mechanical properties.The burn rate is measured using acoustic emission strand burn rate tester.i.e. Crawford bomb as per MIL standard 286 C and average value of five strands was reported.Pure nitrogen is used as pressurant for the burn rate measurement at different pressures and electrically heated nichrome wire is used for ignition of strand.The microstructure of propellant slices is analysed by Hitachi 2403 A SEM instrument.

First set of trials explores the effect of ultra fine AP on properties of CSP.Ultra fine AP used from 0 to 12 wt.%and AP coarse and fine fractions were compensated accordingly as mentioned in Table 1. AP and Al.powder loading at 68 and 18 wt.%were kept constant respectively.The total solid loading is kept constant at 86 wt.%.The viscosity of propellant paste,mechanical properties and burn rate of CSP is obtained.The burn rate is evaluated at different pressure to determine the pressure exponent.

Second set of trials deals with the effect of AP content on properties of CSP.AP content increased from 67 to 71 wt.%and correspondingly liquid part is compensated.Tri-modal AP of coarse to fine to ultra fine ratio of 67:24:9 is used and Al.powder is kept at 18 wt.%for all trials.The composition details are presented in Table 2.The viscosity of propellant paste,mechanical properties and burn rate at different pressures are evaluated.Pressure exponent also obtained.

Third&fourth set explores the catalytic propensity(i.e.copper chromite and iron oxide)of CSP.Trials are carried out by adding burn rate modi fier Copper chromite(CC)and Iron oxide(IO)to CSP formulation with solid loading of 86%(.i.e.68%AP+18%Al.powder).Tri-modal AP of coarse to fine to ultra fine ratio of 67:24:9 is used for all trials.The burn rate is evaluated at different pressures and the pressure exponent is calculated.The trial details are presented inTable 3.The characteristics of burn rate modi fier are given in Table 4.

3.Results and discussion

3.1.Effect of ultrafine AP

3.1.1.Viscosity of propellant paste vs.ultrafine AP

The propellant prepared with 86%(w/w)of solids with bi-modal AP powder containing the coarse and the fine AP has a viscosity of 9000 P at the end of mixing.Partial substitution of coarse and fine AP with the ultra fine decreased the viscosity of the propellant slurry in spite of the increase in surface area of the AP in the mix. Thus,the viscosity of the propellant slurry decreased from 9000 to 7700 P when the concentration of ultra fine AP increased from 0 to 9 wt%.However,further increase in the concentration of ultra fine AP to 12%increased the viscosity of the propellant slurry substantially.Fig.1 shows the effect of concentration of ultra fine AP on the viscosity of the propellant slurry at 86%solid loading.

The maximum packing possible in a bi-modal distribution of particles is 86%[16,18].That is,14%void space will be available between the coarse and fine particles which will be occupied by the liquid HTPB and other additives binder in the propellant formulation.In a propellant with tri-modal AP powder containing an ultra fine fraction,the ultra fine particles go to the void space created between the coarse and fine particles.This displaces the liquid polymer from the void space thereby making more liquid resin available for wetting the AP particles,resulting in the decrease of viscosity of the slurry[14].Literature show that the maximum packing in tri-modal distribution of particle is possible at coarse tofine to ultra fine ratio of 67:24:9[18].Thus,the minimum viscosity achieved in the propellant formulation prepared with the AP containing 9 wt.%ultra fine is due to the maximum packing.Increase in the viscosity above 9 wt.%ultra fine AP is due to the presence of the excess ultra fine particles beyond the volume of the void space.The microstructure of the cross-sections of the CSP shows uniform distribution of ultra fine AP particles.Fig.2 a&b shows the SEM images of the cross-sections of CSP prepared with the AP powder containing 0 and 9 wt.%ultra fine respectively.

Table 1 Composition details of first set of trials(1A-bi-modal AP&1B to 1E-tri-modal AP).

Table 2 Composition details of second set of trials(2A to 2E-tri-modal AP).

Table 3 Composition details of third&fourth set of trials(3A to 3D and 4A to 4D tri-modal AP).

3.1.2.Mechanical properties vs.ultrafine AP

Partial substitution of coarse and fine AP with ultra fine AP at 86 wt.%total solids loading showed a trend in mechanical properties analogues to that of the viscosity of the propellant slurry. Thus,the tensile strength increased with the increase in concentration of ultra fine in the AP,up to 9%;the increase being marginal up to 6%and rapid from 6 to 9%.The tensile strength decreased when AP ultra fine content increased to 12%.On the other hand,the %elongation and the resultant initial modulus showed maximum and minimum,respectively,at 9 wt.%of ultra fine.The maximum tensile strength and elongation values obtained are 0.72 MPa and 51.1%,respectively and the minimum modulus value is 2.70 MPa. Fig.3 and Fig.4 show the variation of tensile strength,elongation and modulus of the composite propellants.The trend in tensile strength,elongation and modulus observed is in agreement with the theoretical calculation for optimum tri modal packing[18]and the results for composite propellants with bimodal AP containing various concentrations of fine particles,reported by Ahmet et al. [12].The effectof packing densityon initial modulus is explained by Eilers and Van Dyck equation[19],

Fig.1.Variation of viscosity with the concentration of ultra fine AP.

where,EandE0are the initial modulus of propellant and polymer matrix respectively,φis the weight fraction of fillers(solid loading) andφpis the packing fraction of fillers for a given composition. ConsideringE0andφkept constant and E shall have a minimum value whenφpis attained maximum value(φpranges from 0.6 to 0.9 based on type of packing and method of packingof particles in a given volume.First set of trials showed that propellant got optimum packing density at 67:24:9 ratio of AP coarse to fine to ultra fine resulted in minimum initial modulus value.

3.1.3.Burn rate&pressure exponent vs.ultrafine AP

The burn rate increased with increase in the concentration of ultra fine AP and～10%increase in burn rate was observed when 12 wt.%of ultra fine was substituted with AP coarse and fine AP shown in Fig.5.This can be attributed to the increase in the surface area of the AP powder from 0.027 to 0.116 m2/g.The pressure exponent of the burn rate law(n)was obtained from the slope of the linear plots of ln P versus ln r.The variation of n value with AP ultra fine content is in the range of 0.36-0.38,except for 9 wt.%of ultra fine AP containing propellant whose value is 0.324 as shownin Fig.6.The low pressure exponent at 9 wt.%could be due to the better dispersion of ultra fine AP particles at this composition.

3.2.Effect of AP content

關(guān)于旅游產(chǎn)業(yè)概念的界定，由于研究的角度不同，所得出的結(jié)論也自然不同，但共識(shí)性的因素主要是圍繞著“旅游資源”、“旅游設(shè)施”、“旅游服務(wù)”等。所以對(duì)旅游產(chǎn)業(yè)概念可以表述為：“以旅游者為對(duì)象，借助旅游資源和旅游設(shè)施，滿足旅游者在旅游活動(dòng)中的吃、住、行、游、娛、購(gòu)等需求，來(lái)實(shí)現(xiàn)旅游者精神和物質(zhì)追求的綜合性產(chǎn)業(yè)”[3]。

3.2.1.Viscosity of propellant paste vs.AP content

The propellant prepared with the 67 wt.%of AP powder has a viscosity of 7680 P at the end of mixing.Viscosity of the propellant slurry increased from 7680 to 20800 P when the AP content increased from67 to 71 wt%.Fig.7 showstheeffectof AP contenton the viscosity of the propellant slurry at optimised coarse, fine and ultra fine content.The figure shows that there is 37.5%increase in viscosity of the paste when AP loading increased from 68 to 69% and continues with further solid loading.The increase in the total surface area of the solid part and corresponding decrease in the liquids results in increase of the viscosity.Fig.8 a&b show the SEM images of the cross-sections of composite propellants prepared with the AP powder containing 67 and 71 wt.%respectively.

3.2.2.Mechanical properties vs.AP content

Increase in AP content from 67 to 71 wt.%showed increase in tensile strength from 0.53 to 0.85 MPa as shown in Fig.9,on the other hand%elongation decreased from 49.31 to 28.29%.Modulus also increased from 1.87 to 5.32 MPa.Density of the propellantincreased from 1.75 to 1.82 when the AP content increased from 67 to 71 wt.%.This result is in agreement with previous studies[13]as the tensile strength increases with increasing total solid content. Fig.10 showed that 17.3%of elongation decreased when the AP content increased from 68 to 69%and the%elongation continuously decreased and reached to 42.6%of original value when AP loading increased from 67 to 71%.

Table 4 Properties of burn rate modi fiers.

Fig.2.SEM image of the cross section of composite propellant prepared with 0 and 9 wt.%ultra fine AP.

Fig.3.Variation of tensile strength with the concentration of ultra fine AP.

Fig.4.Variation of elongation and modulus with the concentration of ultra fine AP.

Fig.5.Burn rate at various concentration of ultra fine AP.

Fig.6.Effect of concentration of ultra fine AP on pressure exponent.

Fig.7.Variation of viscosity with the concentration of AP.

3.2.3.Burn rate&pressure exponent vs.AP content

The burn rate increased with increase in the AP content and～20%increase in burn rate was observed when AP content increased from 67 to 71 wt.%as shown in Fig.11.The increase in burn rate can be attributed to the increase in the surface area of the AP powder and increase in oxidiser to fuel ratio.The pressure exponent of burn rate law(n)was obtained from the slope of the linear plots of ln P versus ln r.The n value increased from 0.326 to0.435 when the AP content increased from 67 to 71 wt.%as shown in Fig.12.This can be attributed to the increase in the oxidiser to fuel ratio when solid content increased from 85 to 89 wt.%,which is in line with the earlier reports[20].

Fig.8.SEM images of the cross section of composite propellant prepared with 67 and 71 wt.%of AP.

Fig.9.Variation of tensile strength with the concentration of AP.

Fig.10.Variation of elongation and modulus with the concentration of AP.

Fig.11.Effect of AP content on burn rate.

Fig.12.Effect of AP content on pressure exponent.

3.3.Effect of catalyst on CSP containing ultrafine AP

Copper chromite(Cuo.Cr2O3)and iron oxide(Fe2O3)are well known catalysts which enhance the burn rate of the AP based composite propellants[21-25].The burn rate of the propellants prepared at 86 wt.%solids with the optimized tri-modal AP and various concentrations of the catalysts measured at 2.26,3.24 and 4.22 MPa pressures is shown in Fig.13.The burn rate increased rapidly with catalyst concentration up to 0.1 wt.%and then slowly up to 0.6 wt.%.The catalytic propensity i.e.the ratio of change in burn rate to the change in concentration of the catalyst was comparable for iron oxide at low concentration range(up to 0.1%) compared to the copper chromite,whereas copper chromite showed higher catalyst propensity beyond 0.2%concentration.The increase in burn rate with catalyst concentration depends on the pressure also.Burn rate increase observed by the addition of 0.6 wt.%copper chromite was 44,51 and 55%at pressures of 2.26, 3.24 and 4.22 MPa,respectively.The increase in burn rate observed with the corresponding concentrations of iron oxide was 34,41 and 43%.The better catalytic activity of copper chromite at higher concentration can be attributed to its higher surface area and higher number of Lewis acid sites(Table 4).The pressure exponent calculated against the concentration of the catalysts is shown in Fig.14.The pressure exponent rapidly increased with the catalyst concentration up to 0.2 wt.%and then remains more or less steady. The composite propellants prepared with copper chromite showed higher pressure exponent values compared to the propellants prepared with the corresponding concentrations of iron oxide.The pressure exponent obtained was in the range of 0.324-0.434, which is suitable for rocket motor applications.

Fig.13.Effect of catalyst on the burn rate.

Fig.14.Effect of catalyst concentration on pressure exponent.

4.Conclusion

Acknowledgement

The authors thank Director,Vikram Sarabhai Space Centre for the experimental support and Director,Satish Dhawan Space Centre for the permission to publish the article.

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14 January 2017

*Corresponding author.Tel.:+918623223287;fax:+918623225154.

E-mail addresses: suresh32kv@rediffmail.com, suresh.babu@shar.gov.in (K.V.Suresh Babu).

Peer review under responsibility of China Ordnance Society.

http://dx.doi.org/10.1016/j.dt.2017.06.001

2214-9147/?2017 The Authors.Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

in revised form