Effect of organo-modified montmorillonite nanoclay on mechanical,thermal and ablation behavior of carbon fiber/phenolic resin composites

Golla Rama Rao ,Ivautri Srikanth *,K.Laxma Reddy

a AdvancedSystems Laboratory,DRDO,Hyderabad,India

b NationalInstitute ofTechnology,Warangal,India

Keywords:Carbon/phenolic Nanoclay Mechanical properties Thermal stability Ablation rate

ABSTRACT The mechanical,thermal and ablation properties of carbon phenolic(C-Ph)composites(Type-I)reinforced with different weight percentages of organo-modified montmorillonite(o-MMT)nanoclay have been studied experimentally.Ball milling was used to disperse different weight(wt)percentages(0,1,2,4,6 wt.%)of nanoclay into phenolic resin.Viscosity changes to resin due to nanoclay was studied.On the other hand,nanoclay added phenolic matrix composites(Type-II)were prepared to study the dispersion of nanoclay in phenolic matrix by small angle X-ray scattering and thermal stability changes to the matrix by thermogravimetric analyser(TGA).This data was used to understand the mechanical,thermal and ablation properties of Type-I composites.Inter laminar shear strength(ILSS),flexural strength and flexural modulus of Type I composites increased by about 29%,12% and 7% respectively at 2 wt.% addition of nanoclay beyond which these properties decreased.This was attributed to reduced fiber volume fraction(%Vf)of Type-I composites due to nanoclay addition at such high loadings.Mass ablation rate of Type-I composites was evaluated using oxy acetylene torch test at low heat flux(125 W/cm2)and high heat flux levels(500 W/cm2).Mass ablation rates have increased at both flux levels marginally up to 2 wt.% addition of nanoclay beyond which it has increased significantly.This is in contrast to increased thermal stability observed for Type-I and Type-II composites up to 2 wt.% addition of nanoclay.Increased ablation rates due to nanoclay addition was attributed to higher insulation efficiency of nanolcay,which accumulates more heat energy in limited area behind the ablation front and self-propagating ablation mechanisms triggered by thermal decomposition of organic part of nanoclay.

1.Introduction

Thermal protection systems(TPS)are essential sub systems of rockets and ballistic re-entry vehicles.Increased shear strength and flexural strength with reduced thermal conductivity are the main requirements for the TPS used under ballistic conditions.This is because,the liners of the rocket nozzles and thermal insulating shields of the re-entry vehicle structures encounters aerodynamic shear forces due to high velocity gases from the plume of the propulsion systems.Hence,TPS should withstand such harsh aerodynamic shear loads while simultaneously acting as heat insulator to protect the subsystems that are behind them.Carbon fiber reinforced phenolic(C-Ph)composites are widely used in fabrication of TPS.Though C-Ph composites have established themselves as the best TPS materials,there is always an urge to further improve their properties to realize relatively thinner TPS which can reduce the overall weight of aerospace systems[1].

Researchers have explored addition of various nano materials like,nano silica,carbon nanotube,carbon black,nanoclay,polyhedral oligomer silsesquioxane(POSS)to C-Ph composites for enhancing their thermo mechanical performance[2-8].Among all these,nanoclay is the most versatile nano filler due to its low cost and ease of dispersion in the resin systems.Many researchers have reported significant improvements in stiffness and strength of pure polymeric matrices with the addition of a small amount of nanoclay particles[9-16].This is due to the high aspect ratio(200-1000)and modulus(170 GPa)of nanoclay and its ability to offer high amount of surface area(750 m2/g)as a filler which restricts the mobility of polymer chains under stress.However,exfoliation of the nanoclay platelets in the polymer matrix is the primary condition to get improved thermo mechanical properties for the composites[17].On the other hand,nanoclay addition to the polymeric systems,results in improved thermal stability and reduced thermal conductivity which are intended for ablative applications[18].Thus nanoclay has become a preferred nano filler to improve the performance of C-Ph based TPS.However,there are contradicting reports on the utility of the nanoclay as filler in fiber reinforced polymeric composites in improving their thermo mechanical properties.

For instance,T.M.Robert et al.prepared short silica fiber-Phenolic composite with different clay loadings(0,1,3,5,and10 wt.%)and reported that the resultant composites displayed optimum non flammability and mechanical properties at around 3 wt.% nanoclay loading[19].L Asaro et al.reported fabrication of composites with modified bentonite clay as fillers and reported decreased flexural strength,whereas Suresha et al.& J J Karippal et al.reported increased flexural strength with nanoclay for polymeric composites up to 5 wt.%loading[20-22].Joseph Koo et al.reported reduced ablation performance up to 5 wt% addition of nanoclay to C-Ph beyond which increased ablation performance was reported[23,24].

Thus varying reports are there in the literature on the utility of nanoclay as a filler in fiber reinforced phenolic based composites in improving mechanical and ablation properties.Present work latently addresses the reasons for such a scatter in the reported property variation due to nanoclay as a filler.

It is well known that,the clay addition enhances the viscosity of the phenolic resin significantly which in turn should influence fiber volume fraction of the composites[20].This should result in change in the thermo mechanical properties of the resultant composites.This aspect was not studied so far by any research group.On the other hand,to enhance the compatibility of the nanoclay with the polymeric matrix,generally they are treated with organic modifiers.These organic modifiers(for example o-MMT:Organic modified montmorillonite clay)are generally present on clay surface up to 30%of the weight of the clay.As the thermal stability of organic modifiers is low,they may degrade during the ablation of CPh composites at an accelerated rate releasing various gaseous products.They may influence the ablation performance of the C-Ph composites.This aspect was not studied so far by any research group.On the other hand,thermal insulation property imparted by the nanoclay to C-Ph,can significantly influence the advancement of the heat front during ablation.This can result in variation in the ablation or erosion performance of composites.These aspects were not studied so far any research group.

Therefore,the objectives of the current work are.

(i)To find optimum concentration of nanoclay as filler in the CPh composites for increased mechanical,thermal and ablative properties.

(ii)To understand the ablation mechanisms of nanoclay added C-Ph composites in the light of oxidative degradation of organic part of nanoclay during ablation and the thermal insulation properties imparted by it to the host composite.

(iii)To understand the changes in the ablation properties of nanoclay C-Ph composites at low heat flux and high heat flux.

2.Materials

Resole type phenolic resin was selected as matrix material.It has solid content of about 62 wt.% and char yield of about 54 wt.% at 1000°C in inert atmosphere.PAN based carbon fabric(Torayca Carbon Fiber T 300,3K)was selected as the primary reinforcement because it has high tensile strength and tensile modulus.Nanoclay used in this study(Nanomer 1.31 PS,M/s Sigma-Aldrich)was surface-modified Montmorillonite(o-MMT)with 0.5-5 wt.% Aminopropyltriethoxysilane,15-35 wt.% Octadecyl amine.

3.Experimental work

3.1.Thermal stability study of nanoclay

Thermogravimetric analysis(TGA:M/s TA instruments,Q500)for the as received nanoclay was carried out up to 900°C in inert atmosphere to study the percentage of organic modifier present in O-MMT.

3.2.Preparation of nanoclay modified carbon fiber/phenolic composites

3.2.1.Preparation of nanoclay added phenolic resin

Nanoclay was dispersed in phenolic resin using a planetary ball mill(M/s Insmart Systems)by taking approximately 200 g of resin in a 500 ml stainless steel(SS)jar.This mixture was ball milled for 2 h at 250 rpm using SS balls.Ball to charge ratio was maintained at 0.5:1.Thus four different nanoclay phenolic resin compositions were prepared by adding 1%,2%,and 4%and 6 wt.%of nanoclay to phenolic resin.Viscosity changes to the phenolic resin due to the nanoclay addition were measured using Brookfield Viscometer at 30°C.Details on fabrication of composites are given below.

3.2.2.Fabrication of type I composites

Twelve carbon fabric layers of 300×300 mm size were cut and impregnated with nanoclay dispersed phenolic resin and were stacked on a flat metal plate by hand lay-up process.Stacked layers were subjected to vacuum bagging,followed by curing in the autoclave.Cure was carried at 90°C for 1 h followed by 120°C for 1.5 h,150°C for 1 h and 170°C for 2 h under a pressure of 5 bar and vacuum of 1 bar.Thus five different variants of the Type I composites were fabricated along with the blank carbon-phenolic composite.Details of the prepared Type I composites are given in Table 1.

3.2.3.Fabrication of type II composites

Nanoclay added phenolic resin was poured into a petri dish and the volatiles were allowed to evaporate by heating the mixture to 50°C for 1 h.Subsequently the mixture was cured in a hot air oven at 90°C for 1 h followed by 120°C for 1.5 h,150°C for 1 h and 170°C for 2 h.Thus five different variants of type II composites were prepared along with a blank phenolic matrix sample.Details of the prepared type II composites are given in Table 1.

Table 1Details of Type I and Type II composites prepared.

3.3.Characterisation and testing

3.3.1.Dispersion of nanoclay

Type II composites were evaluated for the dispersion of the nanoclay in phenolic resin using small angle X-ray(SAXs:Antonpaar,Model:SAXS Point 2.0)by scanning the samples from 1°to 10°.

3.3.2.Density,fiber volume fraction(%Vf)

Type-II composites were tested for density as per ASTM D792 using Archimedes method.The fiber content of composites was measured by acid digestion technique as per ASTM D3171.The test involves taking known weight of the fabricated composite,and digesting the matrix part of it by refluxing in concentrated nitric acid.Subsequently,the left over fabric was washed,dried and weighed.The fiber volume fraction was calculated from the weight of the fiber,density of fiber and density of the composite.

3.3.3.Thermal stability test

Prior to the thermal stability studies,thermogravimetric analyser(TGA:Make TA Instruments,Model Q-500)was calibrated for temperature,weight accuracy using standard reference samples(Calcium oxalate monohydrate),standard weights and standard nickel(for curie temperature).After ensuring the repeatability of instrument,thermal stability of Type I& Type II composites in nitrogen atmosphere were evaluated.A sample mass of 10±2 mg was collected from the fabricated composites.TGA runs were carried out for these samples,from room temperature to 900°C at a heating rate of 10°C/min under continuous flow of nitrogen(flow rate 60 ml/min).Weight of the carbonaceous char(CR)and temperature at which 5%weight loss(T5%/°C)observed for the samples was recorded.Thus one sample each from the Type I and Type II composites were subjected to thermal stability tests.

FromCR,the oxidation index(OI)of the Type I and Type II composites was calculated using empirical Eq.(1)[25-27].

3.3.4.Thermal conductivity

Thermal conductivity of Type I composites were measured by hot disc thermal constants analyser(M/s Hot disc AB,Sweden;Model:TPS 2500S)at room temperature as per ISO 22007-2.Two samples having approximate size of 80 mm×80 mm×4 mm were collected from each of the fabricated composite.These samples were used to sandwich a sensor which acts as both heat source for increasing the temperature of the sample and as a resistance thermometer.The sensor was electrically heated,and the increase in temperature of the sensor surface is monitored as a function of time.Thermal conductivity and diffusivity of the material that is used to sandwich the sensor,strongly influences the temperature raise of the sensor.From this,the thermal conductivity of the test samples are calculated.

3.3.5.Interlaminar shear strength(ILSS)& flexural strength

Samples having size of 40 mm×10 mm×4 mm and 100 mm×10 mm×4 mm for were collected for ILSS and flexural strength tests respectively from the Type-I composites.ILSS test was carried out as per ASTM D 2344 using three-point bending fixture of UTM(M/s ADMET Model 2505).Support span to thickness ratio of 4:1 and crosshead speed of 1.0 mm/min were used for ILSS testing.Flexural strength was carried out as per ASTM D790 by using three-point bending fixture.Support span to thickness ratio of 16:1 and crosshead speed of 1.0 mm/min were used for flexural testing.Six number of samples were tested for each test.The average value and standard deviation were calculated for each of the composition.Standard deviation is shown as error bar in the results.

3.3.6.Oxyacetyleneflame test at different heatflux levels

Type I composites were subjected to oxyacetylene flame test as per ASTM E285.Test specimens having a size of 100 mm×100 mm×4.5 mm were used.Heat flux of oxyacetylene flame was calibrated with heat flux gauge by adjusting distance between sample and the source of flame.One set of Type I composite specimens were subjected to a constant heat flux of 500 W/cm2for a period of 60 s,while the other set of Type I composites were subjected to a constant heat flux of 125 W/cm2for a period of 60 s.Mass ablation rate for Type I composites were calculated as mass loss per second.Two samples were tested under each category at each flux and the average ablation rate is reported.

3.3.7.Morphology and composition characterisation

Morphology and compositional changes in Type I composites due to exposure to the oxyacetylene flame were studied using Scanning Electron Microscopy(ESEM:FEI Quanta 400)and X-ray diffractometer(Philips PWD,Model:1830 The Netherland)respectively.

4.Results and discussions

4.1.Dispersion of nanoclay in type II composites

SAXS patterns of as received nanoclay and Type II composite are shown in Fig.1.The d-spacing of the pure nanoclay before dispersion in the phenolic resin was found to be 3.2 nm.It can be seen from Fig.1,that,characteristic peak observed for the pure nanoclay(O-MMT nanoclay)got subsided up to 6 wt.% of nanoclay addition in Type II composites,indicating that,clay platelets are singular and got exfoliated in the phenolic resin.This result indicates that,the shearing force exerted during ball milling of clay-phenolic mixture is sufficient enough to exfoliate the clay platelets.This is essential to improve the mechanical and thermal properties of the host matrix[28].

4.2.Effect of nanoclay loading on viscosity of phenolic resin

Fig.1.SAXS scans of Nanoclay(NC),Resin+1 wt.%NC,Resin+2 wt.%NC,Resin+4 wt.% NC and Resin+6 wt.% NC.

Fig.2.Effect of nanoclay content loading(wt%)on viscosity of phenolic resin.

The viscosity changes to the phenolic resin due to different loadings of nanoclay is shown in Fig.2.The viscosity of pure phenolic was 280 cP.Viscosity increased up to five times as compared to the pure phenolic at 6 wt.%of nanoclay addition.It is reported that,when the nanoclay interacts with the phenolic resin,significant swelling of the clay galleries takes place due to monomer entrapment in to the clay platelets[29,30].These swollen platelets lead to formation of weak gels with increased viscosity.Resin viscosity is a very important parameter in processing of the composite material.Higher viscosity of the nanoclay modified phenolic resin,can reduce the resin extraction during the processing,resulting in a higher resin content in the fabricated composites[20].

4.3.Density,fiber volume fraction of type I composites

Density,fiber volume fraction(%Vf)of fabricated composites are shown in Table 2.It can be seen that,as the loading of nanoclay in resin increased,%Vfof the composite is coming down.This effect is maximum for composites processed with 4 wt.% and 6 wt% nanoclay added resin systems.This could be attributed to the fact that,at these nanoclay loadings,viscosity of the resin increased significantly,which restricted its squeeze out during processing of composites[20].This has reduced the fiber volume fraction of composites processed with 4 wt.% and 6 wt.% nanoclay added phenolic.

Above changes in the fiber volume fraction of the composites,should have significant effect on mechanical,thermal properties of these composites which is discussed in detail in subsequent sections.

4.4.Thermal stability of the type I and type II composites

Thermo grams of Type I and Type II composites along with the as received nanoclay is shown in Fig.3.Temperature at which 5%weight loss(T5%/°C)was observed,char residue at 900°C,and oxidation index are shown in Table 3.The observed trends were found to be matching with the previously reported data of other researchers[18,19].Higher value of oxidation index indicates,higher thermal stability for the material under the study[25].Increased thermal stability can be attributed to the strong interaction of the nanoclay layers with the matrix thereby hindering the interaction of oxygen.Temperature at 5 wt.% loss of the sample,percentage residue at 900°C,indicates that,though thermal stability has increased for both Type I and Type II composites up to 2 wt.% percentage addition of nanoclay,beyond that,thermal stability is showing a downward trend.Increased thermal stability up to 2 wt.% of nanoclay addition is due to hindrance offered by nanoclay in oxygen and matrix interactions[31,32].Decreasing thermal stability beyond 2 wt%nanoclay addition can be attributed to the reasons discussed below.

From TGA plot(Fig.3),it was observed that,the thermal degradation of organic modified Montmorillonite(o-MMT)occursbetween 250°C and 450°C.This low temperature degradation of nanoclay is primarily due to degradation of organic modifier part of clay.The residual mass of clay at 900°C is about 70%.This indicates around 30%of the initial weight of the o-MMT is organic in nature.

Table 2Density,fiber volume fraction of Type I composites.

Fig.3.(a)TGA of Nanoclay and Type II composites,(b)TGA of Nanoclay and Type I composites.

Table 3Thermal stability and Oxidation Index of Type I and Type II composites.

During the high temperature degradation of the nanoclay added composites under inert atmosphere,two possible reactions can occur.First possible reaction is pyrolytic degradation of phenolic matrix as shown in Eq.(2)[33].

At higher heating rate or with more input heat flux,the rate of reaction(2)increases significantly[34].

Other possible reaction is that,beyond 250°C,organic part of the nanoclay degrades as shown in Eq.(3)and the resultant oxygen species can react with pristine matrix as shown in Eq.(4).

In absence of nanoclay,limited amount of oxygen species that have evolved from the reaction(2)can interact with the phenolic matrix(as shown in reaction 4)and oxidise it leading to marginal weight loss.However,when nanoclay is present,additional oxygen released reacts with pristine matrix there by increasing the rate of matrix degradation.

Thus there are two competing mechanisms that are operating in nanoclay added composites during high temperature exposure.Up to certain critical weight percentage(2 wt% in presence case)the hindrance offered by the nanoclay in blocking the interaction of the matrix with the oxygen dominates which results in increased thermal stability,whereas beyond the critical amount,additional degradation mechanisms triggered by reaction(3)&(4)dominates leading to reduction in the thermal stability of the composite.As the SAXS data is showing the nanoclay got exfoliated indicating good dispersion of nanoclay throughout the matrix,and that itself was acting as a source for generating oxidising species,the matrix degradation rate increased for both Type I and Type I composites at higher loadings of the nanoclay.

4.5.Thermal conductivity of type I composites

Thermal conductivity of the Type I composites is shown in Table 4.It can be seen that the thermal conductivity of the composite is decreasing as the nanoclay content in composite was increasing.This can be attributed to the fact that,nanoclay acts as scattering centres for the heat energy carrying phonons[2].This results in energy loss at these inclusions,thereby reducing the overall thermal conductivity of the composite.

4.6.ILSS of type I composites

The effect of weight percentage of nanoclay on ILSS of Type I composite is shown in Fig.4.ILSS value increased by 29%at 2 wt%addition of nanoclay as compared to the blank sample(C-Ph composites).Beyond,2 wt% addition of nanoclay,ILSS values have started to comedown even though there is a good dispersion of nanoclay(inferred from SAXS studies).This can be understood as below.

ILSS of the layered composites is due two factors namely.

Fig.4.ILSS of Nanoclay modified C-Ph composites(Type-I composite).

Table 4Thermal conductivity data of Nanoclay added C-Ph composites.

(i)Chemical binding force of matrix in keeping adjacent fabric interfaces together,

(ii)Mechanical locking between the two fabric interfaces due to overlap of the troughs and peaks of the fabric weaves of two adjacent layers.

In case of nanoclay addition up to 2 wt.%,matrix content increased marginally(Table 2)while the interface area between the two adjacent layers increased significantly.This is because of the higher surface area of nanoclay.Besides,this,the chemical functional groups present in o-MMT are known to enhance the chemical bonding between the matrix to the nanoclay[35-37].This strengthens the matrix further.Increased interfacial area between the adjacent layers,coupled with the increased matrix strength,lead to higher interfacial strength.As the matrix content has not increased up to 2 wt.% nanoclay addition,the mechanical locking between the adjacent layers is intact.This resulted in higher ILSS with the nanoclay addition.

However,beyond 2 wt.% loading of nanoclay,the inter layer mechanical locking has come down as the fiber volume fraction of the composites got reduced.This resulted in reduced ILSS.On the other hand,higher nanoclay may degrade the matrix strength itself as the higher amount of additives can hinder the crosslinking of the resin system,there by leading poor strength for the matrix[38].

4.7.Flexural strength and modulus of type I composites

The effect of nanoclay loading on flexural strength and modulus of Type I composites are shown in Fig.5.Flexural strength increased up to 2 wt.% nanoclay addition by about 12%.Flexural failure is a combination of shear,compression,tensile failure.In general,tensile strength is not known to increase significantly with nanoclay addition for the fiber reinforced composites[22].Hence the improved flexural strength up to 2 wt.% clay addition can be ascribed to the increased shear strength of the layers.As the shear strength is coming down beyond 2 wt.%,flexural strength is also coming down for the composites.Besides this,reduction in the fiber volume fraction at higher loading of the nanoclay,is also contributed to the reduced flexural properties of the composites.

Flexural modulus also increased up to 2 wt.% addition of nanoclay to C-Ph composites even though there is a slight reduction in the fiber volume fraction as compared to the blank C-Ph composite.As the carbon fiber is the main contributor for the strength/modulus,increase in these properties even when the carbon fiber volume faction is reducing is indicating that,clay galleries present in the composite are acting as source for higher and efficient stress transfer between the fiber to matrix.This enabled in harnessing maximum ability of reinforcement against the applied bending forces which resulted in higher flexuralmodulus for the composites added with limited amount of nanoclay(up to 2 wt.%).As there is a huge reduction in the fiber volume fraction of the Type I composites beyond 2 wt.% addition of nanoclay,the positive contribution of the nanoclay towards the modulus improvement got nullified and thus there is a net reduction of modulus was observed.

Table 5Mass ablation rate of Nanoclay modified C-Ph composites.

4.8.Effect of nanoclay content on the ablative performance of type I composites

The results of the mass ablation rate of blank carbon/phenolic resin composite and nanoclay modified carbon fiber/phenolic composites are presented in Table 5.In case of blank C-Ph,at low heat flux,fibers are intact after ablation whereas at high heat flux fiber breakage can be seen(as indicated with arrows in Fig.6).This resulted in slightly increased ablation rate(Fig.6a&b)for blank CPh composite.Mass ablation rates have increased marginally up to 2 wt.%nanoclay addition as compared to the blank C-Ph at low heat flux.Microstructure of the ablated surfaces indicates that,there is no appreciable damage to the composite surface as compared to the C-Ph composites loaded with high wt.%of nanoclay.However,there is a marginal increase in ablation rates even at 2 wt.% nanoclay loading(Fig.6c to i).This could be attributed to the fact that,char formed in presence of nanoclay is found to be brittle with many cracks,which flies off leading to increased mass ablation rate.Beyond 2 wt.% nanoclay loading,mass ablation rates increased drastically even when the heat flux for ablation testing was low.

On the other hand,at higher heat flux,mass ablation rates have increased drastically at all levels of nanoclay loading.One possible explanation given by previous researchers for increased ablation rate for the nano silica added C-Ph composites is the reaction of the char with silica forming silicon carbide as shown in reaction 5[2,5].Since nanoclay contains significant amount of silica,similar reactions are possible.However,XRD data of the ablated samples shows formation of SiC only at 6 wt.%addition of nanoclay to C-Ph composites that too at a very low concentration(as inferred from the intensities of the XRD peaks of Fig.7)

Fig.6.Showing SEM images of the ablated samples(a)Blank C-Ph after low flux ablation test(b)Blank C-Ph after high flux ablation test(c)2%nanoclay added C-Ph showing loosely held char after low flux ablation test(d)4%nanoclay C-Ph showing patches of char free zones after testing at low flux(e)&(f)2%,4%nanoclay C-Ph showing porous char after low flux ablation testing respectively(g)2%nanoclay C-Ph showing severe damage to fibers after high flux ablation testing(h)&4%and 6%nanoclay added C-Ph composites showing deep pits formed after high flux ablation testing due to self propagating ablation zones.

Hence,the observed trends in the increased ablation rates with the nanoclay addition are to be understood based on the mechanisms as given below.

Mechanism I:As the nanoclay amount is increasing in the Type I composites,thermal conductivity is coming down(Table 4).This results in poor propagation of heat energy through the thickness of the composite.As the heat energy is localised,effective conversion of matrix in to char is higher in the nanoclay added composites.Thus by the time ablation front advances,the conversion of the matrix in to char is complete with no traces of pristine matrix present.As the matrix that is binding the composite is absent in the ablation front,large chunks of the ablated surface were flying off leading to higher ablation rate(Fig.6,Table 5).At higher flux and higher nanoclay content this effect was more pronounced.This is because,at higher nanoclay content,lower thermal conductivity of the composite confines more heat energy in the vicinity of the advancing ablation front there by creating fully charred matrix.

Mechanism II:During the ablation,from the surface of the ablating material towards the core char front and heat front will form with the later moving ahead of the former.In the heat front zones,with the raise in the temperature,organic part of the nanoclay starts degrading as per reaction(3).Oxygen that was available due to nanoclay decomposition,oxidises the matrix in the char front as shown in reaction(4).Due to the removal of the matrix,large craters/perforations forms in the char front(Fig.6).These perforation allows non uniform exposure of the buried layers(char front)present under the ablation front to the flame,leading to selective acceleration of the reaction(3)and(4)at these zones.This allows more heat to reach to these zones directly from ablation front.This results in one more cycle of decomposition of the organic part of the nanoclay,reaction of resultant oxygen with char and its transformation to a gaseous product leading to increase in the depth of the crater.This repeated cycles of self-propagating mechanism leads to removal of large chunks of matrix and fibers which forms pits thus leading to increased ablation rate.This mechanism becomes more severe at high clay content and higher flux.This can be evidenced from the fact that,the ablated surface of 2 wt.%nanoclay got severely damaged with large pits in case of high heat flux as compared to the minimum damage without surface pits in case of low heat flux ablation(Fig.6).

Fig.7.XRD of ablated surface of 6 wt%Nanoclay modified carbon-phenolic composite.

Thus,as the amount of the nanoclay was increasing the degradation rate of matrix was increasing.On the other hand,when the heat flux increased,energy required for the reactions 1 and 2 were supplied at a faster rate there by increasing overall ablation rate.Thus both higher nanoclay content as well as higher flux were imparting higher ablation rate to the Type I composites.

5.Conclusions

This study has been carried out using O-MMT(organic modified)nanoclay as filler in carbon-phenolic(Type I)composites.The influence of parameters such as amount of loading of nanoclay in Type I composites was studied on mechanical,thermal and ablation properties.Type II composites were prepared by adding different amount of nanoclay to pure phenolic matrix.Changes in the viscosity of the phenolic resin due to nanoclay addition was correlated with the mechanical properties of the Type I composites.Thermal stability of O-MMT nanoclay as well as Type II composites was correlated with the thermal and ablative properties of Type I composites.The results obtained in this research work are summarised as below.

·2 wt.% nanoclay as filler is optimum in C-Ph composites for increased mechanical properties,thermal stability without much compromise on ablative properties.

·Thermal insulation of C-Ph composites increases due to nanoclay addition which is intended attribute for thermal protection systems.However,oxidative degradation of organic part of nanoclay accelerates the ablative degradation of host composite.

·Ablative performance of nanoclay added C-Ph composites varies significantly as a function of heat flux used for ablation testing.Higher heat flux(500 W/cm2in present study)and higher loading of nanoclay(beyond 2 wt.% in present study)results in increased ablation rate for nanoclay added Carbon-phenolic composites.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.