Light weight HTPB-clay nanocomposites(HCN)with enhanced ablation performance as inhibition materials for composite propellant

Kvit Ghosh ,L.V.Gikwd ,R.K.Kll ,P.A.Kulkrni ,Arvind Kumr ,Shil Bnerjee ,Mnoj Gupt

a High Energy Materials Research Laboratory,Sutarwadi,Pune,411021,India

b Defence Institute of Advanced Technology,Girinagar,Pune,411025,India

Keywords:Nanocomposite Inhibitor Composite propellant Ablation

ABSTRACT In the present study,organically modified Montmorillonite clay with polar moiety,the Cloisite 30B,is used for preparation of Hydroxyl terminated polybutadiene(HTPB)-clay nanocomposites(HCN)by dispersion of nanoclay in polymer matrix under high shear mixing.The nanocomposites thus prepared are evaluated in composite propellants as inhibitor material for their functional utility.Several inhibition formulations containing 5 wt%-15 wt% of nanoclay,with or without the conventional filler Sb2O3,were prepared.All these formulations were evaluated for their physical,mechanical,thermal,and ablative properties.Ablation rate and density of the compositions containing Cloisite 30B is around 23% and 5%lower respectively in comparison of the base composition.Strain capability of these compositions is twofold higher than that of base composition.These compositions have also been evaluated for their smoke generation tendency by measuring infra red(IR)attenuation in the wavelength range 1.3μm-5.6μm and 8μm-13μm and thereby compared with the base composition.The corresponding results confirmed that the compositions containing Cloisite 30B as filler have much lower IR attenuation than compositions with conventional filler,Sb2O3.Replacement of 5%Sb2O3 by nanoclay showed 8%reduction in IR attenuation rate which further reduced to 16% on replacement of 15% of Sb2O3.Interfacial bonding of HCN based inhibitors is also comparable or even better than conventional inhibitors.Precisely,the nanoclay composites with Cloisite 30B as filler exhibit all desirable properties of an inhibitor.

1.Introduction

Composite propellants are most widely used class of solid rocket propellants for rockets and missile systems of recent times.Solid rocket motor typically includes an outer case or hardware which contains the solid propellant grains,a thermal insulation and inhibition system,an ignition system to initiate the propellant and a nozzle which expands combustion products and provide necessary thrust to the system.Composite propellants can be classified in to two categories mainly case bonded and cartridge loaded.While case bonded systems use a thermal insulation lined inside the motor which protects motor hardware to get expose to the high temperature of the chamber attained due to combustion of propellant,on the other hand cartridge loaded systems use inhibiting materials or inhibitors,the polymeric materials which are applied on the surface of the propellant to prevent burning of the propellant from that side.This helps to achieve the desirable pressure-time profile for a specific mission requirement.So,the performance of rocket or missile systems depends not only on the propellant but to an inhibition or insulation system also.

These studies focus mainly to enhance the integrated performance of the propulsion system by enhancing the capabilities of inhibition system used for cartridge loaded composite propellants.For the success of a propulsion system,an inhibitor should have following characteristics like

a)It should have excellent bonding with propellant over the entire range of working temperature(generally-40°C to+70°C)without any delamination,cracking or interface separation due to self stressing or shrinkage.

b)It should have good mechanical properties to withstand mechanical and thermal stresses during storage and handling.

c)It should have low density,low erosion rate,low thermal conductivity and high specific heat so that with less thickness of inhibitor and minimum weight penalty,desired burning profile can be achieved.

d)Additionally,inhibitors should have good aging characteristics with minimum 10 years-15 years of useful life and their smoke generation tendency on combustion should be very low.

Various inhibitor compositions are developed inhouse with different polymers like epoxy,polyurethane,siloxanes and butadienes with incorporation of different fillers like silica,antimony trioxide,kaolin etc for various applications[1-6].These inhibitions are used with different class of propellants like double base,composite modified double base(CMDB)and composite propellants for various rocket and missile systems[7-14].Out of these,a butadiene and antimony trioxide based inhibitor composition is selected with the aim to enhance the integrated performance of elastomeric inhibitor where Sb2O3is used as macroscopic filler,15%by weight in the inhibitor composition.

Polymer-clay nanocomposites have received much attention in the past decades because of their unique properties like enhanced mechanical strength,thermal stability and barrier properties with reduced flammability and ablation resistance[15-18].The ablation of a material depends on its inherent properties and also on extrinsic conditions such as thermal,chemical and mechanical factors related to theoretical or practical environment variables.Combustion of composite propellant generates a flame temperature of 3000 K-3500 K depending upon the content of metal powder used in composition.Since reinforcing nanoclays have very much higher thermal stability,so,efforts were made to replace conventional filler Sb2O3in inhibitor composition,partially and completely by organoclay(Cloisite 30B)to enhance the ablation resistance of the inhibitors.The aim of the present study was primarily to understand thermal and ablative properties under oxyacetylene torch exposure environment of hydroxyl terminated polybutadiene(HTPB)based nanocomposites.Effect of Cloisite 30B concentration on the thermal decomposition and ablation behavior of nanocomposites are herein investigated.Additionally,the attenuation performance of HTPB-clay nanocomposites to Infra Red radiations was also assessed in 2.25 m3smoke chamber as IR imaging guidance have been widely used on modern warfare.

2.Experimental

2.1.Materials

HTPB manufactured by free radical polymerization,with a number average molecular weight of around 2600 g/mol and hydroxyl value of around 42 mg of KOH per g,was procured from M/s Anabond Pvt Ltd,Chennai.4,4′-methylenebis cyclohexyl isocyanate(H12MDI)of purity＞98% was used as curator and procured from Sigma Aldrich.nButane diol(nBD)and 1,2,6 hexane triol(HT)of purity＞99% were procured from Merck.Antimony trioxide,Sb2O3confirms to purity＞99%and all passing through 200 BSS and retained on 240 BSS was used as conventional filler.Ferric acetyl acetonoate(FeAA)with melting point≈180°C-190°C was used as curing catalyst and procured through Merck.N-Phenylβnaphthylamine(PBNA)with melting point≈106°C-110°C was used as antioxidant and was procured from Sigma Aldrich.Montmorillonite clay organomodified with polar quaternary ammonium ion(methyl,tallow,bis-2-hydroxyethyl,quaternary ammonium in Closite 30B)were obtained from southern clay products,USA.

2.2.Methods

Preparation of HTPB-clay nanocomposite based inhibitors is carried out in three steps.First step is the preparation of nanocomposites by the established method and then in second these nanocomposites are used to prepare the inhibitor premix which is utilized to prepare the inhibitor final mix as and when required.

2.2.1.Preparation of nanocomposites

All samples were prepared via in-situ polymerization at 60°C by following the method described by M.Song et al.[19].Dispersion of organically modified nanoclay(Closite30B)in polymer matrix(HTPB)blended with nBD and HT was carried out with high shear mixing by homogenizer.After dispersion the samples were deareated by applying 1-2 torr of vacuum for 30 min and then these samples are used for preparation of inhibitor premix.

2.2.2.Preparation of inhibitor premix

Inhibitor binder ingredients(HTPB,nBD and HT)were mixed and deareated under vacuum at 60°C for 30 min.Solid ingredients,Sb2O3and nanoclay,were dried at 105°C for 2 h.PBNA was added and mixing was continued by mechanical agitator for 10 min.Other solid ingredients(Sb2O3,nanoclay or both)were added and mixing was continued for 30 min more without vacuum.In case of compositions where nanoclay is used as filler,dispersion of nanoclay was done by high shear mixing with homogenizer because shear force generated by mechanical agitator is not sufficient to delaminate the platelets of nanoclay.This method is well established for preparation of nanocomposites[20].Total six inhibitor compositions are processed.Initially,one third of Sb2O3quantity was replaced by nanoclay,then two third and finally complete Sb2O3was replaced by nanoclay.Two compositions with reduced quantity of total filler content i.e.5%and 10%of nanoclay only,were also processed.A base composition with 15% of Sb2O3was also processed for comparison purpose.Detailed compositions are presented in Table 1.

2.2.3.Preparation of inhibitor final mix

Curing agent,H12MDI and cure catalyst,FeAA were added to previously prepared,inhibitor premixes and mixing carried out without vacuum.This mix was allowed to cure at 60°C in required moulds for various sample preparation.

2.3.Characterization

2.3.1.FTIR analysis

FTIR data of nanoclay(Cloisite 30B)was recorded with FT-IR spectrometer Nicolet-iS 50 from Thermo Electron Corporation,USA.FTIR for all the samples are done by using attenuated total reflection(ATR)mode.In ATR-FTIR analysis,sample is spread completely over the diamond crystal for functional group investigation.

2.3.2.SAXS and EDX analysis

Cloisite 30B was characterized by small angle X-ray scattering analysis(SAXS)using instrument NANO-viewer,Rigaku,Japan,with Cu Kαradiation at a generator voltage of 40 kV and current of 40 mA for its intergallery spacing.The energy dispersive X-ray(EDX)Spectroscopic analysis was performed for Cloisite 30B with an EDX system from EDAX Inc.USA attached to Quanta 200 FEI make ESEM(environmental scanning electron microscope)for its chemical composition.In this technique,electron beam is used as source produced by a thermal emission,such as a heated tungsten filament.The electron beam interacts with the sample through a series of magnetic lenses.Several signal e.g.backscattering,secondary and Augur electrons,X-ray fluorescence photons are originated.The detector counts the number of low energy secondary electrons emitted from each point that showed the different elements present in the sample while characteristics X-rays are used for identifying elements present in to sample.

2.3.3.Physical and mechanical properties

Gel time for inhibitor compositions prepared with different fillers is measured with the help of a gel timer,model FGT-06 by TECHNE with a plunger diameter of 22 mm and 10 oscillations/min speeds.The density of cured inhibitor samples were determined by gas pycnometer,Thermo-scientific,USA,using helium gas as a medium at 30°C.Shore A hardness of inhibitor samples were estimated by durometer confirming the ASTM D2240 standard.The tensile properties like tensile strength and percentage elongation of inhibitor samples were evaluated using dumbbell shape specimens on a Hounsfield tensile testing machine,conforming to the ASTM D638 standards at a crosshead speed of 50 mm/min at ambient temperature.The testing instrument utilizes a highly sensitive and accurate load weighing system for detecting the load applied to the specimen under test,employing strain gauge load cells.

2.3.4.Thermo chemical analysis

Thermo-chemical properties of inhibitors are measured by technique called thermogravimetric analysis(TGA).TGA is a method of thermal analysis in which changes in physical and chemical properties of materials are observed as a function of increasing temperature at a constant heating rate or as a function of time at a constant temperature.These changes can be in the form of second order phase transitions,vaporization,sublimation,absorption,adsorption,desorption,etc.Additionally,this technique can also give information about the thermal stability of test sample.TGA is usually used to measure mass loss or gain due to decomposition,oxidation,volatilization etc.Weight loss analyses of inhibitor samples were carried out with a simultaneous thermal analyzer(SDTA,model SDT Q-600 of TA instrument,USA)over a temperature range of 30°C-600°C with a heating rate of 10°C/min.

2.3.5.Thermo physical analysis

Thermo physical properties like specific heat capacity,thermal diffusivity and thermal conductivity of inhibitor samples were determined by thermal properties analyzer,model FL-3000,Anter,USA by laser flash technique.This method involves uniform irradiation of a small disc shaped sample over its front face with a very short pulse of energy.The time temperature history of the rear face recorded through high speed data acquisition from a solid state optical sensor with very fast thermal response.Thermal diffusivity is determined from the time interval after the flash for the sample’s rear face to reach half of its ultimate temperature rise.Heat capacity is determined by the relative measurement of a known standard sample as a reference to the unknown sample.Thermal conductivity is computed from measured values of thermal diffusivity and specific heat capacity with the additional knowledge of density.This measurement is confirming to the standard ASTM E1461.

2.3.6.Ablation properties

To study the ablation behavior of the inhibitor samples,the severe hyperthermia environment was simulated by the oxyacetylene torch test the standard ASTM E285.In this set up,a mixture of oxygen and acetylene was fired and used as heat source of the experimental platform for ablation.The flow rates of oxygen and acetylene were 58 LPM(litres per minute)and 48 LPM respectively,which generates a flame temperature of around 2400°C and heat flux of about 300 W/cm2during the test.Prior to the test,sample thickness was monitored using digital gauge disc caliper micrometer.In order to measure the linear and mass ablation rates,samples were held for 10 s-20 s and then the flame was extinguished allowing the samples to cool down naturally.The thickness and mass change of all the samples during the test were divided from exposure time.The linear ablation rate(LAR)and mass ablation rate(MAR)and percentage char yield were calculated according to Eq.(1)-Eq.(3).

where T0,M0,Tfand Mfare the thickness and mass of the ablator specimen before and after ablation testing,respectively;t is the exact ablation time.Thickness measurements of ablated samples(Tf)were done using the same digital gauge disc caliper micrometer at many points(minimum 4-5)and average value is taken.

2.3.7.Interfacial bonding properties

Interface properties like tensile bond strength between propellant and inhibition were determined using ASTM D897 standard.Where cube shape samples of composite propellant with dimensions 25 mm×25 mm×25 mm are prepared and these samples are pasted on the fixtures available.Inhibition is poured in between the two propellant cubes and allowed to cure at specified curing temperature.Once the inhibition is cured,force is applied in opposite direction from both the propellant pieces and it is observed that at what force bond line fails,that value is reported as tensile bond strength.

2.3.8.Infrared attenuation performance

Smoke generation by inhibitor compositions is also determined by radiometric measurements in IR attenuation in a range of 2.0μm-2.4μm with a charge mass of 10 g.The size of smoke chamber used in the following experiments is 1.0 m×1.5 m×1.5 m with 2.25 m3effective volume.After passing the smoke chamber,the IR radiation signals are collected by IR radiometer SR-5000 from CI systems,Israel.Initially,background collection done without any smoke and then smoke produced in the chamber by burning of inhibitior samples and IR signals collected.After the detection,the door of smoke chamber was open and the exhaust fans were started to empty the smoke particles in the chamber.

3.Results and discussions

3.1.Characterization of ingredients

Initially,the organoclay Cloisite 30B and the conventional flame retardant filler antimony trioxide(Sb2O3)were characterized by FTIR,SAXS,EDX and TG-DTA.The basic characteristics of both the flame retardant fillers are given in Table 1.Fig.1(a)and Fig.1(b)represent the FTIR spectrums of Cloisite 30B and Sb2O3.In FTIR of Cloisite 30B,the broad band centred at 3397 cm-1is due to the presence of-OH stretching band for hydroxyl group.The band at 3632 cm-1is due to Al-OH bond in presence of intercalated water in Cloisite 30B.Bands present near 2852 cm-1and 2926 cm-1in Cloisite 30B is due to alkyl groups present in their organic modifications.It has also been reported that peak intensity at 1638 cm-1and 1469 cm-1is due to presence of water molecules from moisture.Si-O-Si bond is confirmed from 919 cm-1-1054 cm-1peak in the spectra.Two peaks at around 464 cm-1and 523 cm-1appears which are due to the presence of bonds such as Si-O-Al and Si-O-Mg,silica backbone of clay.These all peaks are well matched to the values reported in literature[19].Similarly,the FTIR spectrum of Sb2O3(Fig.1(b))exhibits specific and characteristic vibrational peaks at 1152 cm-1,1073 cm-1,954 cm-1,and 686 cm-1similar to that of reported values by Hazim et al.[20]along with a hydrate peak at 3656 cm-1.The low energy vibrational peaks mostly correspond to Sb-O-Sb and Sb＝O bonds.

Fig.1.(a)FTIR of cloisite 30B and(b)FTIR of Sb2O3.

Fig.2(a)and Fig.2(b)explains the weight loss patterns of Cloisite 30B and Sb2O3,respectively.SDTA analysis of Cloisite 30B shows three step weight loss as marked by the corresponding differential thermogravimetric analysis(DTGA).DTGA represents the first derivative of weight loss with respect of temperature.The maximum of each DTGA peak represents maximum reaction rate whereas,the minima or plateau represents least or no reaction.The observed minimas in the DTGA represents onset and end of decomposition reaction of respective weight loss stages.In case of Cloisite 30B,maximum of 29% weight loss takes place of total weight in a temperature range of 153°C-627°C.The relatively slow weight loss of about 2.5% in the temperature range 50°C-150°C is believed to be attributed by the loss of moisture and partial decomposition of organic modification on the clay[21].

TG-DTA analysis of Sb2O3shows single step weight loss in the temperature range of 462°C-666°C Orman et al.[22]also has previously reported similar sublimation behavior of Sb2O3in the temperature range 500°C-650°C.The corresponding DTA curve exhibit an endotherm with onset at 580°C and temperature of maximum weight loss(Tmax)645°C.These values corroborates to the melting and sublimation temperature as reported by Weast et al.[23].

The SAXS and EDX analysis of Cloisite 30B are shown in Fig.3 and Fig.4,respectively.The d-spacing of Cloisite 30B is calculated from Bragg’s Eq.(4)through SAXS analysis(Fig.3).This technique allows the determination of the spaces between clay platelets of the nanoclay.

Where,n is wavelength of the X-ray radiation used in the experiment,d is spacing between clay platelets andθis the measured diffraction angle.By monitoring of shape,intensity and position of basal reflection peaks from dispersed clay platelets,structure of nanocomposites can be identified[16,24]and dspacing can be calculated.For Cloisite 30B,d-spacing is found to be 18.5?.

EDX analysis was also carried out for the Cloisite 30B nanoclay.The powder clay sample was made into a thick film over the conductive adhesive tape with gentle compression and subjected to EDX analysis.Fig.4 shows the EDX of the nanoclay‘Cloisite 30B’,where the peaks for different constituent elements like C,O,Si,Mg are clearly seen.

Fig.2.(a)TG-DTA of cloisite 30B and(b)SDTA of Sb2O3.

Fig.3.SAXS analysis of Cloisite 30B.

Fig.4.EDX analysis of Cloisite 30B.

EDX of Antimony trioxide is carried out in similar manner as of Cloisite 30B by making a thick film of powder over the conductive adhesive tape with gentle compression.The sample prepared in this way is subjected to EDX analysis.Fig.5 shows the EDX of Antimony tri oxide‘Sb2O3’,where the peak of major constituent element antimony(Sb)is prominently seen.

Fig.5.EDX analysis of Antimony trioxide(Sb2O3).

After characterization,Cloisite 30B and Sb2O3are incorporated in inhibition formulation.Total five compositions are made in which initially Sb2O3is systematically replaced by Cloisite 30B partially and then completely.An inhibition composition where Cloisite 30B is solely used as filler by 10%of wt.is also prepared to find out the possibility of reducing the total filler content in inhibition compositions.A representative inhibitor composition with approximate ingredients percentage and details in terms of additives are given in Table 2.

3.2.Physical and mechanical characterization of inhibitors

Gel time of inhibitor compositions is a very important parameter which allows a propellant chemist to estimate the processing time available for application.After addition and homogenization of curing agent,samples are withdrawn for gel time measurement for each composition and detailed data is included in Table 3.Composition one has a gel time of 215 min which decreased to 194 min after replacement of 5%of filler with Cloisite 30B.Further replacement of antimony trioxide with Cloisite 30B reduced the gel time to 168 min and finally to 148 min.This is due to the much higher surface area of Cloisite 30B in comparison to Sb2O3.But 148 min of gel time for composition 4 is sufficient for processing of inhibitor composition with composite propellants.Further gel time can be improved by altering the curing agent or ratio of curing agent and binder as per application.After curing of inhibitor compositions,hardness Shore A measured on inhibitor sheets.Composition 1 had Shore A of around 75-80 which increased to 80-85 with partial replacement of Sb2O3with nanoclay.This is due to the very high specific surface area(≈800 m2/g)of clay platelet structure and greater reinforcement of nanoclay as tensile strength of these compositions is also high[25].However,with further enhancement of nanoclay,shore A comes down to 70-75 which may be due the fact that more amount of organoclay have more organic modification which give a plasticization effect to polymer chains which helps in reduction of hardness as well as increase of elongation in composition 4[26].Compositions 5 also have a Shore A of 70-75 which may be due to the least amount of filler in this composition.But all compositions meet the minimum requirement of hardness i.e.＞60 for inhibitor compositions hence can be utilized for their functional requirement.

Table 2Formulation details with fillers.

Density of compositions having Cloisite 30B as filler,are substantially lower than the base composition.Data presented in table-3 reveals that composition 5 has lowest density of 989 kg/m3,but this composition has least amount of filler also.Composition 4 which is having same amount of filler as of base composition(1)also has 4%-5% lower density than base composition which is highly desirable for inhibitors as it reduces the inert weight of missile systems which will finally results to enhanced performance of the system.

Mechanical properties of various inhibitor compositions are also presented in Table 3.This data reveals that compositions having nanoclay as filler have comparable tensile strength and much better elongation than base composition.Elongation of composition 4 is having 2 fold increases than the base composition with same amount of filler.High elongation is very much desirable for inhibitors as it helps to minimize the thermal stresses,developed near interface,due to differential thermal expansion of inhibitor and propellant and protects the interface to fail[27,28].

Table 3Physical and Mechanical properties of various inhibitor compositions.

3.3.Thermo-physical properties of inhibitors

Flame retardency or inhibition properties of an inhibitor or insulator depend upon its thermal properties like thermal conductivity,specific heat capacity and thermal diffusivity.These results are presented in Table 4 and Fig.6(a)and b.Data reveals that compositions with same % of filler and inclusion of Cloisite 30B(compositions 2,3 and 4)have substantially higher specific heat capacity which is desirable for inhibitor compositions.Composition 4 is having 95% higher specific heat capacity than composition 1 with same amount of filler.Thermal conductivity of these compositions is also low and that may be because of the low thermal conductivity of Cloisite 30B in comparison of Sb2O3[29,30].

3.4.Ablative performance and char yield percentage of various inhibitor compositions

The ablation properties of various inhibitors are summarized in Table 5.The LAR,MAR and the char yield of the ablated inhibitor were measured using Eq.(1)-Eq.(3)respectively and plotted in Fig.7.Figure illustrates that the increase of Cloisite 30B fraction in inhibitor compositions leads to the gradual decrease of linear and mass ablation rates.LAR decreases from 0.199 mm/s to 0.153 mm/s when Cloisite 30B fraction increases from 0 to 15 wt%,respectively.The same observation can be seen for MAR which decreases from 0.017 g/s to 0.015 g/s.Moreover,the char yield curve exhibits the similar behavior as LAR.The high char yield values(11.55%)are obtained for inhibitors without Cloisite 30B and only Sb2O3as a filler while char yield comes down to 10.57%with 15 wt%of Cloisite 30B as a filler.From Table 5 and Fig.7,it can be concluded that the incorporation of nanoclay,Cloisite 30B in inhibitor compositions enhances their ablation performance.This can result in to improved performance of the rocket and missile systems by reducing the inert weight of the inhibitors.

Literature explains that the formation of a thermal insulating and low permeability char on the outer surface of the nanocomposites during combustion is responsible for improvement in flame retardancy of nanocomposites.Heat transfer from an external source or a flame promotes thermal decomposition of organoclay and the polymer.This results in accumulation and reassembly of clay platelets on the surface of burning material.Therefore the carbonaceous char formed superficially during combustion is rich in silicates and act as a protective barrier by reducing the heat and mass transfer between the flame and polymer.That is the char who insulates the underlying polymer from heat and also slows oxygen uptake and the escapes of volatile gases produced by polymer degradation[31-38].

3.5.Back wall temperature measurement for ablated inhibition materials

Back wall temperature measurement is a method which expresses utility of prepared composite materials as inhibition for composite propellant significantly.This measurement is done in the same set up as of ablation rate measurement,only a thermocouple is placed on the back wall of metal sheet where composite sheet is pasted.The size of composite sheet taken for the measurement is 150 mm×200 mm×10 mm.After the exposure of around 40 s of oxy-acetylene flame,temperature rise with time is obtained with the help of data acquisition system.This experiment was performed at around 300 W/cm2of heat flux with composition 1 and 4 as these compositions are one to one replacement of each other and composition 4 seems to be best among five compositions.Detailed results are presented in Fig.8.

Table 4Thermo-physical Properties of various inhibitor compositions.

Table 5Ablation Properties of HCN based inhibitors.

Fig.6.(a)Thermal conductivity and specific heat capacity of HCN based inhibitors,(b)Thermal diffusivity of HCN based inhibitors.

Fig.7.Ablation performance parameters of HCN based inhibitors.

After 40 s of exposure to oxy-acetylene flame,temperature rise on the back surface,in case of composition 1 goes to 714°C where in case of composition 4,it goes to 317°C,only.These two compositions have only difference of filler nature;content of filler is same in both the cases.Composition 1 uses 15%by wt.of Sb2O3as filler,while composition 4 uses 15% by wt.of Cloisite 30B as filler.Thickness of both the composite sheets is same.Hence,the mechanism of ablation of Cloisite 30B is playing an important role here.

3.6.Thermo-chemical properties of inhibitors

Thermal decomposition behavior of composition 1 and composition 4 is studied with the help of SDTA to understand their ablation mechanism.Details are presented in Table 6 and Fig.9(a)and b.Table 6 shows temperature range of each wt.loss step with corresponding%wt.loss and%residue left for all five compositions.Tmaxof DTA for each step is also presented in Table 6.It is evident from the data that all inhibitor compositions have a three step weight loss.In first step,around 10 wt%-12 wt%.loss takes place over a temperature range of 205°C-392°C which may be attributed to the loss of water and volatile contents and may be initial decomposition is started but the polymer network majorly undamaged.In the second step,as temperature rises to 400°C,thermal decomposition of polymer matrix and organic modification of clay start via random chain scission reactions.At this step,maximum weight loss(around 60%-80%)takes place up to a temperature of 487°C.The final and third step involves the formation of carbonaceous char on the outer surface of composite which prevents heat transfer from external source to polymer matrix.Temperature goes up to 626°C during this step and wt%loss is around 9%-14%.

Table 6SDTA data of HCN based inhibitors.

Fig.8.Back wall temperature measurement for HCN based inhibitors,1 and 4.

This ablation mechanism is similar for both the fillers,Sb2O3as well as Cloisite 30B.But Fig.9(a)and(b)shows the TGA and DTA of composition 1 and composition 4 respectively,which reveals that during first step and second step of weight loss,initially rate of decomposition was higher for composition 4,where Cloisite 30B is used as filler.This may be because of the decomposition of quaternary ammonium group present as organic modification in Cloisite 30B but afterwards rate of decomposition of composition 4 become more than composition 1.Residue obtained during TGA for composition 4(12.94%)is much more than the composition 1(2.12%)which indicates more char formation in this case.

Fig.9.(a)TGA and(b)DTA results for HCN based inhibitors,1 and 4.

3.7.Morphological analysis

3.7.1.Visual observation

Visual observations were madeduring the ablation test and after ablation for ablated material.During ablation test it was observed that composition 1 had a lot of dripping on ignition while in case of composition 4,this dripping was strongly suppressed or eliminated.After ablation test,photographs of ablated composites were taken with the help of a digital camera.These photos are presented in Fig.10(a)and Fig.10(d).

Close observations revealed that ablated surface of composition 1 have a crumbly carbonaceous phase and it scraps off easily when tried.The thickness of virgin layer of inhibition in surroundings of centre of impingement is also lower in case of composition 1 in comparison of composition 4,however,this could not be measured due to uneven surface and carbonaceous char over it.

On the contrary,in case of composition 4,carbonaceous char present on the ablated surface was hard and difficult to scrap off manually.Thickness of virgin layer of inhibition in surrounding of centre of impingement was also more in this case(Fig.10(a)and(d)).However,small and shallow pits are observed on the ablated material surface in case of composition 4(Fig.10(b)and(e)).These results demonstrate that Cloisite 30B increases ablation resistance of inhibitor materials as flame retardant filler in comparison to Sb2O3.

Fig.10(c)and(f)show the cross section view of both ablated specimens for composition 1 or composition 4.These figures reveal that,the ablated surface of composition 4 is more porous than composition 1.Literature revealed that porous structure of ablated sample enhances transpiration and vaporization,which in turn reduces heat penetration vertically and leads to less sub surface decomposition and char formation during the ablation[39-42].

Fig.10.(a)-(c)Ablated surface images for inhibitor composition 1,(d)-(f)Ablated surface images for inhibitor composition 4.

3.7.2.SEM morphological analysis

The morphology of ablated surface of composition 1 and composition 4 is presented in Fig.11(a)-(d)respectively.SEM micrograph of composition 1 in Fig.11(a)and 11(b)show rougher surface with lots of pits and craters.This may be due to the escape of gases of antimony trioxide as a result of sublimation during combustion process.This leaves the ablated surface with lots of cracks on the surface and higher char formation due to greater exposure of polymer layer.

On the contrary,SEM micrographs of composition 4 in Fig.11(c)and 11(d)show relatively smooth surface with largely laminar topology.As we know that Cloisite 30B is used as flame retardant filler in this composition and dispersed Cloisite 30B in polymer matrix gives a card house structure on combustion at the surface of polymer composite which further prevents the exposure of virgin layer of polymer composite beneath it[30-34,43].This may be the reason for unchared surface seen in the micrograph of composition 4 and finally in reduced ablation rate of these composite materials.

3.8.Interfacial properties

Interfacial tensile strength of inhibitor compositions are determined with composite propellant and presented in Table 7 and Fig.12.This data elucidate that interfacial tensile strength of inhibitor compositions is increased by 19%from 1.47 MPa to 1.75 MPa by inclusion of Cloisite 30B as filler.This indicates that strong interfacial bonding is built up between the composite propellant and inhibitor with Cloisite 30B as filler.However,this enhancement in interfacial bonding was within experimental expectation as hydroxyl group present in the organic modification of Cloisite 30B will also take part in bond formation with unreacted isocyanate groups present at the surface of composite propellant,during curing process of inhibition.With this reaction proceeding,the three dimensional cross linking network structures are formed at the interface between the propellant and inhibitor[12,38].In conventional inhibitor systems where Sb2O3is used as filler,contribution towards cross linking is not there from filler.Hence,inhibitors with Cloisite 30B filler have better interfacial tensile strength than conventional inhibitor.

Fig.11.(a-b)SEM micrographs of ablated surface for inhibitor composition 1,(c-d)SEM micrographs of ablated surface for inhibitor composition 4.

Table 7Interfacial tensile bond strength and IR attenuation rate data of HCN based inhibitors.

Fig.12.Interfacial tensile bond strength of HCN based inhibitors with propellant.

3.9.IR attenuation performance of inhibition compositions

The energy of incident IR radiation is attenuated because of the scattering and absorbability of smoke particles.Transmittance which represents attenuating effect of the smoke can be calculated by the IR signal intensity before and after the smoke generation.

Transmittance and attenuation can be calculated by following equations-

Where,τis transmittance andβis attenuation rate,I0is transmitted intensity of IR radiation from background(without any smoke)and Itis transmitted intensity after smoke generated by burning of inhibition materials[44].

The signal processing system of radiometer gives intensity of incident and transmitted radiation.Experiments are performed in IR range of 1.3μm-5.6μm and 8μm-13μm.Then by Eq.(6),percent attenuation is calculated which is presented in Table 7,Fig.13(a)and Fig.13(b).This data reveals that inhibitor composition with Cloisite 30B as filler have less attenuation rate of incident IR radiation than the compositions with Sb2O3as filler.There is around 3%reduction in attenuation rate in 1.3μm-5.6μm IR range(Fig.13(a)),by replacing 5%of Sb2O3by Cloisite 30B,which further enhances up to 15.4% by replacement of 15% of Sb2O3by Cloisite 30B.Compositions having only Cloisite 30B as filler,10%by weight,have maximum reduction in attenuation rate of around 18%.Similar experiments are performed in IR range of 8μm-13μm(Fig.13(b)).Similar observation is made where maximum reduction in attenuation rate was with replacement of 15% of Sb2O3by Cloisite 30B.These results confirm the functional utility of Cloisite 30B containing inhibitor compositions in composite propellants.

Fig.13.(a)Attenuation rate curves of smoke produced by HCN based Inhibitors in 1.3μm-5.6μm IR range and(b)8μm-13μm IR range.

Fig.14.Schematic representation of Ablation mechanism.

3.10.Ablation mechanism

Ablation mechanism is not directly probed in the present study but on the basis of thermal and structural characterization of ablated inhibition samples and literature available in the form of previous studies,overall ablation behavior of HTPB-Cloisite 30B nanocomposites based inhibition materials is proposed as following steps and is schematically represented in Fig.14.

a)Rise of surface temperature and volatilization of water and unreacted species.

b)Decomposition and substantial weight loss of polymer matrix.

c)Formation of a thermally insulating and low permeability char.

In the last step,during formation of char,in the case of HTPBCloisite 30B nanocomposites,heat transfer from flame or gaseous phase promotes thermal decomposition of Cloisite 30B and HTPB matrix.These results in accumulation and reassembly of clay platelets on the surface of burning material[32]or in condensed phase and this arrangement is solely depend upon the dispersion of Cloisite 30B in polymer matrix.Few researchers have reported the similar microscopic structure of char as it was in dispersed form in polymer[32,33,43].Many researchers have reported that organoclay migrate to the surface of composites or condensed phase during combustion and form a rigid and effective char which prevents the heat transfer and slows the underlying composite material for oxygen uptake and the escape of volatile gases produced by polymer degradation[45-47].On the contrary,antimony trioxide volatilize at around 650°C,hence get combust in the gaseous phase and has less contribution towards char formation.This the reason for which Cloisite 30B acts as much better flame retardant filler in comparison to antimony tri oxide and improves the efficiency of inhibition materials.

4.Conclusion

In the present study,an attempt was made to utilize HTPB-clay nanocomposites in field of composite propellant.Nanoclay,Cloisite 30B is used as filler alone,as well as along with conventional filler antimony tri oxide in inhibitor compositions for composite propellant.The effect of nanofiller on thermo physical and ablative properties of inhibitor has been studied for its functional utility in propulsion systems.Study revealed the following conclusions-

·The linear ablation rate of inhibitors with 15 wt%Cloisite 30B as filler is almost 30%lower than the inhibitors with 15 wt%Sb2O3as filler in conventional inhibitor.

·Thermo-physical properties like thermal diffusivity,thermal conductivity and specific heat of these inhibitor compositions are also superior to the conventional inhibitors.

·Interfacial tensile strength of these inhibitors is also 19%higher than the conventional inhibitors.

·Inhibitor compositions with nanofiller have better mechanical and physical properties in terms of high elongation and low density,respectively,which can help in reducing the inert weight of the propulsion systems.

·IR attenuation rate of these inhibitors is also 15.4% less than conventional inhibitor which is one of the desired parameter of inhibitors.

All conclusions drawn above confirm the functional utility and superiority of inhibitors with nanofiller,Cloisite 30B,for composite propellants.Further,these inhibitor compositions can be modified for their use in other propulsion systems and with different kind of propellants.

Acknowledgement

The authors are thankful to Shri K P S Murthy,Director,HEMRL for valuable suggestions,constant encouragement and permission to publish this work.