Research Article Synthesis and Characterization of Fibre...

Research ArticleSynthesis and Characterization of Fibre Reinforced SilicaAerogel Blankets for Thermal Protection

S Chakraborty1 A A Pisal2 V K Kothari1 and A Venkateswara Rao2

1Department of Textile Technology IIT New Delhi 110016 India2Air Glass Laboratory Department of Physics Shivaji University Kolhapur Maharashtra 416 004 India

Correspondence should be addressed to A Venkateswara Rao avrao2012gmailcom

Received 24 November 2015 Revised 10 February 2016 Accepted 17 February 2016

Academic Editor Antonio Riveiro

Copyright copy 2016 S Chakraborty et alThis is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Using tetraethoxysilane (TEOS) as the source of silica fibre reinforced silica aerogels were synthesized via fast ambient pressuredrying using methanol (MeOH) trimethylchlorosilane (TMCS) ammonium fluoride (NH

4F) and hexane The molar ratio of

TEOSMeOH(COOH)2NH4F was kept constant at 1 38 373 times 10minus5 0023 and the gel was allowed to form inside the highly

porous meta-aramid fibrous battingThe wet gel surface was chemically modified (silylation process) using various concentrationsof TMCS in hexane in the range of 1 to 20by volumeThefibre reinforced silica aerogel blanket was obtained subsequently throughatmospheric pressure drying The aerogel blanket samples were characterized by density thermal conductivity hydrophobicity(contact angle) and Scanning ElectronMicroscopyThe radiant heat resistance of the aerogel blankets was examined and comparedwith nonaerogel blankets It has been observed that compared to the ordinary nonaerogel blankets the aerogel blankets showed a58 increase in the estimated burn injury time and thus ensure a much better protection from heat and fire hazards The effect ofvarying the concentration of TMCS on the estimated protection time has been examined The improved thermal stability and thesuperior thermal insulation of the flexible aerogel blankets lead to applications being used for occupations that involve exposure tohazards of thermal radiation

1 Introduction

Aerogels are cellular solids that feature very low densityhigh specific surface area and consist of a coherent openporous network of loosely packed bonded particles or fibreswhose voids are filled with gas They comprise particle andpore dimensions in the range of 1 to 1000 nm Aerogelshave a wide range of exceptional properties such as lowestthermal conductivity (sim001WmsdotK) that a solid can everhave high porosity (sim95 to 98) high optical transmission(around 90) in the visible region high specific surface area(sim1000m2g) low sound velocity (100ms) low dielectricconstant (2 to 4) and low refractive index (101 to 11)[1 2] Due to very low thermal conductivity low densityhigh porosity and small pore sizes (1 to 1000 nm) aerogelsare regarded as one of the best insulating materials [3ndash7]However from the point of view of applications aerogels havethe drawback that they absorbmoisture from the atmosphere

they are fragile they cannot be easily handled and they can-not be used to insulate complex shaped bodies [8]Thereforefibre reinforced aerogel fabric has been produced as analternative to achieve better flexible drapable and durableinsulation materials Fibrous materials that may be used asthe reinforcing material can be a nonwoven fabric [9 10]

Fibre reinforced aerogel blankets find applications in theconstruction of heat and flame resistant protective clothingfor industrial workers for protection against thermal hazardsof an electrical arc protective clothing for workers exposedto molten substances and related hazards and firefighterprotective clothing Such protective gear requires light weightclothing which offers high thermal protection and comfortProtective clothing such as that used by a structural fire-fighter is usually a multilayered one with an outer layerthermal insulation layer and an inner layer In which formand how the aerogel can be utilized to effectively enhance

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2016 Article ID 2495623 8 pageshttpdxdoiorg10115520162495623

2 Advances in Materials Science and Engineering

Table 1 Details of the fabrics used for making three layered assembly for evaluation

Fabric Cover () GSM (gm2) Thickness (mm) Bulk density (kgm3)Nomex IIIA fabric 094 220 044 500Nomex nonwoven felt mdash sim150 285 52Modacryliccotton fabric 086 196 041 478

protective performance of such a clothing assembly are yetto be explored properly Aerogel nanoparticle coated fabricsfor firefighting applications were found to have increasedthermal resistance good air permeability and moisturemanagement properties [11] Results of the superior thermalprotection of lightweight firefighter protective clothing usingaerogel fillers have been reported [12 13] Fire penetrationand burn testing of fire blanket materials showed aerogelblankets to be a top performing insulation material [14]

Nonwoven felt produced from inherently heat retardantfibrous materials is used as component of protective clothingfor protection against high heat exposure In the presentstudy meta-aramid fibre namely Nomex has been usedfor making nonwoven felt We have produced low den-sity hydrophobic flexible tetraethoxysilane (TEOS) basedsilica aerogel blankets via sol-gel formation inside meta-aramid (Nomex) needle punched fabric silylation withtrimethylchlorosilane (TMCS) and ambient pressure dryingSubsequently the prepared aerogel-fabric samples were char-acterized by density thermal conductivity hydrophobicityorganic and inorganic bond formation and protective eval-uation to radiant heat fluxes

2 Experimental Procedures

21 Materials The chemicals used for the preparation of sil-ica alcosols and the subsequent silylation of silica alco-gel blankets were tetraethoxysilane (TEOS 98 (SigmaAldrich)) oxalic acid ((COOH)

2 2H2O (Qualigenic Fine

Chemicals India)) ammonium fluoride (NH4F (Loba

Chemie)) methanol (MeOH (Merck India)) and trimethyl-chlorosilane (TMCS (Fluka Puriss grade Switzerland)) andhexane (Merck India) Double distilled water was used forthe preparation of all the solutions Nomex (DuPont India)m-aramid fibre matrix was used for making the nonwovenfelt which was subsequently used to produce aerogel blanketCommercially availableNomex IIIA (Nomex 93Kevlar 5and 2 antistatic fibre) woven fabric and modacrylic cotton(6040) blended woven fabric were used in combinationwith aerogel blanket for thermal protective evaluationThree-layer assembly was prepared using a Nomex IIIA wovenfabric as outer layer aerogel blanket as middle layer and amodacryliccotton fabric (blend 6040) as the inner layer forevaluation of the radiant heat protection Particulars of thefabrics used for the present study are given in Table 1

22 Methods of Preparation Nomex (DuPont m-aramid)fibres of 22 Denier (denier defined as gram9 km) and50mm length were converted into fibrous nonwoven batting

on a laboratory model needle punching machine (DiloGermany) The needle punched fabric was prepared with150 punchesm2 10mm needle penetration depth and setto have areal density of approximately 150 (gm2) Silicaaerogel blanket was prepared by forming silica based gelon the Nomex fibrous nonwoven fabric and subsequentlytransformed into aerogel by suitable drying technique Theaerogel was produced through a two-step acid-base sol-gelprocess followed by ambient pressure drying Figure 1 showsa flowchart for the preparation of aerogel-nonwoven battingThe sol was prepared using TEOS precursor diluted inMeOH The acidic catalyst oxalic acid ((COOH)

2 0001M)

was added to the sol stirred for 1 hour and kept atroom temperature for 12 hours for hydrolysis The molarratio of TEOS MeOH (COOH)

2 NH4F was kept constant

at 1 38 373 times 10minus5 0023 respectively After 12 hours ofhydrolysis the base catalyst ammonium fluoride (NH

4F

05M) was added dropwise to the sol while stirring andstirred for further 5min Immediately after that the sol waspoured onto the Nomex fibrous batting (65 cm times 65 cmdensity 52 kgm3 thickness 385mm) kept on Petri dish andallowed to form gel at room temperature The complete gelformation took place within 5min Petri dishes containingthe alcogel samples in methanol were kept wrapped withaluminum foil and aged for 1 hour After aging methanol wasdecanted and the samples were transferred from Petri dishesto a separate container and surplus of hexane was addedMethanol in the gel was exchanged with hexane at 50∘C ina shaker (Remi Instrument India) at 120 rpm for 3 hoursTo make the gels hydrophobic and to have the spring-backeffect subsequently the gels were immersed in a silylatingagent (TMCS in hexane) and kept in an oven for 16 hours at50∘C and shaken at 120 rpmThe concentration of TMCS wasvaried from 2 to 10 in the solvent (hexane) The unreactedTMCS was exchanged with the hexane solvent again in thesame shaker at 50∘C for 4 hours at 120 rpm After this step thehexane was decanted and the resulting silylated gelled fabricwas dried in an oven at 50∘C for 1 hour and 100∘C for 2 hoursto get the hydrophobic aerogel composite fabric

23 Reaction Mechanism In the present study the surfacechemical modification of the wet gelled fabric was made withTMCS in hexane It took place according to the mechanismshown in Scheme 1

In order to obtain low density and better hydrophobicsilica aerogel fabric the Hrsquos of the surface hydroxyl groupsformed during hydrolysis and condensationmust be replacedwith the organosilicon groups [(CH

3)3-Si-] Therefore the

surface modification of TEOS based wet gel blankets was

Advances in Materials Science and Engineering 3

Si

Si

SiO

Si

Si

O+ 2

OH

OH

Cl +

CH3

CH3

CH3 2HCl

O-Si-(CH3)3

O-Si-(CH3)3

997888rarr

Scheme 1

Wetting of Nomex fibre

Gelation

Alcogel based Nomex fibre

Surface modification

Retrieved transparent and hydrophobic silica aerogel blanket

Addition of base catalyst NH4F = 0023

1hr stirring and 12hr for hydrolysis

Exchanged with hexane for 4hr shakingat 50∘C on 120 rpm

Aging at 50∘C in methanol

Exchanged with hexane for 4hr shaking

38 373 times 10minus5Molar ratio of TEOS MeOH oxalic acid = 1

Drying of modified aerogel blanket [1hr at 50∘C and 2hr for 150∘C]

Silylation with TMCS for 16hr at 50∘C

Figure 1 Flow chart of the experimental procedure for the preparation of the aerogel blanket

carried out with TMCS in hexane The governing chemicalreactions are shown as follows

2(CH3)3-Si-Cl + H2O

(CH3)3-Si

(CH3)3-Si

O + 2HCl (1)

(CH3)3-Si-Cl + CH3OH (CH3)3-Si-O-CH3 + HCl (2)

(CH3)3-Si

(CH3)3-Si

O + 2CH3OH-Si-O-CH3 + H2O2(CH3)3 (3)

(CH3)3-Si-O-CH3 + HCl (CH3)3-Si-Cl + CH3OH (4)

(CH3)3-Si-Cl + equivSi-OH equivSi-O-Si(CH3)3 + HCl (5)

(CH3)3-Si-O-CH3 + equivSi-OH equivSi-O-Si(CH3)3 + CH3OH (6)

As shown in reaction (1) the reaction between trimeth-ylchlorosilane (TMCS) and pore water yields hexamethyldis-iloxane (HMDSO) and HCl and as it happens very rapidly itmay cause cracking of the gel In order to avoid the reactionbetween TMCS and pore water directly prior to the surfacemodification the pore water of silica hydrogel was exchangedwith hexaneThe hexane solvent used in the reactionmixturehelps to decrease both the reaction rate of TMCS with thepore water and the capillary stress during drying due to itslow surface tension

24 Methods of Characterization The apparent density of theaerogel-fabric composites was obtained from the measure-ment of weight of the samples (in grams) using a microbal-ance with an accuracy of up to 10minus4 g with the knownaerogel-fabric composites volume The Fourier TransformInfrared (FTIR) spectroscopy studies were carried out usingPerkin Elmer (Model number 760) IR spectrophotometerin the range of 400ndash4000 cmminus1 The spectroscopic analysisof the samples was carried out using attenuated total inter-nal reflection technique (ATR) Microstructural studies of


(a) (b)

(c) (d)

Figure 2 Microscopic view and SEM of an aerogel blanket (a) ordinary microscopic image (30x) (b) Nomex fibrous batting (15Kx)(c) aerogel-fibre blanket (30Kx) and (d) aerogel in between the fibres (300Kx)

the silica aerogel blankets were carried out using scanningelectron microscope (SEM) The hydrophobicity of the aero-gel blankets was determined using a contact angle meter(Kruss DSA 100) The thermal conductivity (120582) of the silicaaerogel blankets was measured using thermal conductivitymeter Alambeta [15] whichmeasures the thermal resistancethermal conductivity with an accuracy of 00001WmsdotKTheradiant heat resistance of the aerogel blanket was evaluatedon an instrument developed based on the principle outlinedin ASTM F 1939 (Standard Test Method for Radiant HeatResistance of Flame Resistant Clothing Materials with Con-tinuousHeating) where the aerogel blankets were exposed toa radiant heat flux of very high intensity and the temperaturewas measured at the other side of the fabric using a coppercalorimeter of known mass and thickness Cumulative heat(Jcm2) was plotted with time and the time taken to crossStoll 2nd degree burn injury the curve [16] has been used asa standard reference to compare the thermal insulation andheat resistant properties of the aerogel blankets

3 Results and Discussion

31 Scanning Electron Microscopy The silica aerogel fibrouscomposite blankets were examined under ordinary Nikonmicroscope at 30x magnification Figure 2(a) and it has beenobserved that a semitransparent aerogel formed in the fibrousmatrix The SEM structure of a single unmodified Nomexfibre (15x) is shown in Figure 2(b) and it can be compared to

a fibre after aerogel was formed as shown in Figure 2(c) TheSEM image of the aerogel-fibre blanket at 3K magnificationshows granular appearance of the aerogel deposited on thefibre indicating the change in the nature of the surface of thefibres The SEM image of the aerogel at 30000x is shown inFigure 2(d) The SEM images shown in Figures 2(c) and 2(d)show a three-dimensional interconnected aerogel and itsmacroporous structures Thus the fine macroporous struc-tures of the aerogel blanket have been observedWith smallersize of the particles more discontinuities are created andresulted in increased thermal contact resistance and discon-tinuous solid backbones which impedes heat transfer

32 Density and Thermal Conductivity Measurements Thedensity of the silica aerogel blankets was determined bytakingweight of the samples on an electronicmicrobalance ofan accuracy up to 10minus4 g and from volume of the sample basedon its length width and thickness The thermal conductivitydetermines both the steady state and transient state thermalproperties of thin insulationmaterials like a textile fabricThethermal conductivity (WmsdotK) was determined at the end ofthe transient phase The measured bulk density and thermalconductivity of the aerogel blankets with the variation ofthe molar ratio of silylating agent TMCSTEOS are shownthe Figure 3 It has been observed that the bulk densityof the parent Nomex blanket increases as they are madefrom the aerogel blankets but it gradually decreases with anincrease of the TMCSTEOS molar ratio The initial increase


0020

0025

0030

0035

0040

0045

0

20

40

60

80

100

120

140

0 2 4 6 8 10 12

Only nonwoven fabric


Concentration of TMCS in hexane ()

Den

sity

(kg

m3)

Ther

mal

cond

uctiv

ity (W

mmiddotK

)

Density (kgm3)Thermal conductivity (WmmiddotK)

Figure 3 Density and thermal conductivity of the aerogel blankets

in the density is due to the formation of solid hydrophobicaerogels inside the pores and around the fibres of the fibrousmaterials Higher density at lower TMCS concentration canbe attributed to the fact that lower TMCS concentrationleads to cluster surfaces formed with a few Si-(CH

3)3formed

unmodified surface silanols and the formation of densesilica aerogels due to the condensation of silanol groups Theincrease in volume percentage of TMCS (in hexane) increasesthe hydrophobization more number of trimethylsilyl groupsget attached to the silica surfaceminimizing the condensationreaction leading to an increase in pore volume and hencea decrease in the density of the aerogels The aerogels withhigher pore volume are obtained as more hydrophobizationprevents the condensation of the silanol groups and henceprevents the structural collapse of the silica gel networkwhich occurs due to capillary forces developed during thedrying process The aerogel consists of macropores havingcharacteristic diameter of 1ndash1000 nm depending upon thedensity of the composite aerogels [17] The mean free pathof gas molecules in air at a pressure of 1 bar is 70 nm whichis comparable to the average aerogel pore size [18] Hencethough there is a weight gain and increase in bulk densityfor all the aerogel-fabric samples the overall thermal conduc-tivity decreased as the aerogel material partially suppressesthe gas phase conduction and also the aerogel matrix formedin between the fibrous contact points decreases solid phaseconductivity The initial increase in thermal conductivitycan be due to increase in bulk density and the furtherdecrease can be due to decrease in bulk density and decreasein the solid phase conduction In the case of the aerogel-fibre composite blankets fibres reduce the thermal radiationtransport and at the same time enhances the strength andflexibility of the aerogel blankets [17]

33 Fourier Transform Infrared Spectroscopy Figure 4 showsthe FTIR spectra of silica aerogel blankets with varyingamounts of the silylating agent It is clearly seen from

600 1100 1600 2100 2600 3100 3600

Refle

ctan

ce

(au

)

-OH

Si-O-Si

-OHSi-C C-HC-H

Si-C

Wave number (cmminus1)

2 TMCS4 TMCS6 TMCS

8 TMCS10 TMCS

Figure 4 FTIR spectra of the aerogel blanket

the figure that the peak at around 1600 cmminus1 belongs toadsorbed H

2O and the broad peak at around 3500 cmminus1 is

due to Si-OH groups With increasing the TMCS concen-tration it can be noticed that the intensity of the absorptionpeaks related to Si-OH decreases confirming the increasedhydrophobicity of the aerogel blankets This has been furthercorrelated with the finding of the contact angle studies (themore the TMCS the more the contact angle) The peaksoccurring at around 1250 860 cmminus1 are due to Si-CH

3bonds

and the peaks around 2980 and 1450 cmminus1 respectively arerelated to C-H bonds Absorption peaks due to bendingand stretching of C-H bonds at 1450 cmminus1 and 2980 cmminus1appear to be prominent The intensity of the peaks at around860 and 1250 cmminus1 due to Si-C bonds becomes sharp withincreased TMCS concentration confirming the hydrophobicmodification of the aerogel blanket surfaces [19ndash21]

34 Contact AngleMeasurements A contact anglemeasuringinstrument (Kruss DSA 100) has been used to measure thecontact angle of different aerogel blanket samples A 10120583Lwater droplet was used to measure the contact angle Theimage captured by the high resolution camera was collectedfor all the samples for comparison The contact angleswere measured directly by enlarging the images obtained bythe instrument (Figure 5) All the aerogel blanket samplesshowed excellent hydrophobicity and the sample with thehighest proportion of TMCS used showed the contact angleas high as sim165∘

35 Thermogravimetric Analysis The thermogravimetric(TGA) analysis has been carried out to investigate the thermalstability of the silica aerogel blankets in air atmosphere(Figure 6) The initial weight loss around 100∘C is due to theevaporation of moisture absorbed by the aerogel blanketswhich is approximately 4 In the TGA curve for pure


4 TMCS 6 TMCS 8 TMCS 10 TMCS

Figure 5 Photographs showing a water droplet on the surface of silica aerogel blankets obtained at different concentrations of TMCS

0

20

40

60

80

100

120

50 150 250 350 450 550 650 750

Wei

ght (

)

100 NomexAerogel blanket

Temperature (∘C)

Figure 6 Thermogravimetric analysis of 100 Nomex-III andaerogel-fibre blanket (6 TMCS)

Nomex there are two distinct regions of rapid weight lossone at around 450∘C (corresponds to a DTA endotherm)and the second at around 550∘C (oxidation followed by arapid exothermic weight loss) [22] In the case of TGA ofthe hydrophobic aerogel obtained through silylation of silicagels with TMCS and subsequently dried atmosphericallya very little weight loss is experienced at around 450∘Calong with a strong exothermic DTA peak correspondingto the oxidation of surface methyl groups [23] This is notclearly observed in the present case which may be becauseof the presence of Nomex fibres in combination with theaerogel The temperature of terminal decomposition for pureNomex fibre was observed at around 700∘CThe aerogel-fibrecomposite blankets retainedmore than 40 of their weight atthis temperature which is only the weight of the aerogel afterpyrolysis of the fibrousmass After heating beyond 470∘C thehydrophobicity of the aerogel samples was lost

36 Protective Evaluation of the Aerogel Blankets The threelayered combinations with the 100 Nomex IIIA (DuPont)woven fabric as the outer layer the aerogel blankets withdifferent levels of silylation (TMCS concentration in hex-ane) used in the middle layer and an inner layer of the

35

40

45

50

55

60

65

Withoutaerogel

2 4 6 8 10


Estim

ated

pro

tect

ion

time (

s)

Figure 7 Estimated protection time of the aerogel blankets versusthe concentration of the TMCS in hexane

modacryliccotton (60 40) fabric were prepared and testedagainst the radiant heat A combination of fabrics whichcontains a Nomex nonwoven blanket (without aerogel) inthe middle layer has been used as a control sample forcomparison All fabric combinations were exposed to a heatflux of 35 kWm2 and time (seconds) required to cross theStollrsquos curve for burn injury is noted After each experimentthe face of the copper calorimeter usedwas cleaned to removethe oil and deposits due to charringThe heat flux calibrationwas repeated from time to time and adjusted The estimatedprotection time for all the fabrics is graphically shown inFigure 7 With the use of aerogel blanket in the middle layerthe protective value of the fabric combinations increasedsignificantly An increase in protection time of 36 has beenobserved comparing the first aerogel blanket combination(2 TMCS) with the control fabric A maximum protectiontime of 634 seconds (585 increase) was observed for thethird combination (6 TMCS) As it has been observed thatthe thermal conductivity in all aerogel blankets decreasedcontinually with increased TMCSTEOS ratios it can beone of the factors responsible for the increased thermal


insulation of the aerogel blankets The decreasing densityof the aerogel blankets can be another reason of increasedthermal protection It appears from the experimental datathat a further increase in the concentration of TMCS doesnot effectively increase insulation Increased number of Si-CH3groups developed in such samples may not contribute

to increase in pore volume and actually supported oxidationat higher temperature and degradation of Nomex fibrousstructure of the blankets and did not produce expectedreduction in heat transfer

Considering all the samples superior insulation andincreased thermal protection have been experienced usingthe aerogel blankets Fibrous layers absorb most of theinfrared radiation and dominant mode of heat transfer isconduction only In the case of aromatic polyamide fabrics(namely Nomex outer layer) total transmittance for a heatingsource at 2000∘K can be less than 10 only and no trans-mission in the infrared region [24] Heat transfer throughthese aerogel blankets can be described as for opticallythick aerogels For a given temperature gradient within anaerogel heat is transferred through silica particle networkwhere the mean free path of phonons is far below thedimensions of amorphous primary dielectric particles Thesolid conductivity is also proportional to a density dependentgeometrical factor that considers the effect of ineffective deadends of solid backbone [25] Characteristic pore size withinthe aerogels causes gas phase heat transfer to be reducedcompared to free air In conjunction with the reduced solidthermal conductivity and suppressed gas phase heat transferaerogel blankets act as excellent thermal insulating materialsIn addition the thermal stability of the aerogel blanketscontributes to a great extent to the protection as it hasbeen experimentally observed At high heat exposure theNomex nonwoven felt used as control fabric broke open at thecenter though degraded aerogel blankets did not lose theirintegrity However on exposure to extreme heat (sim470∘C)the hydrophobicity of aerogels blankets was lost due to theoxidation of the -CH

3groups as it has been found from the

TGA data analysis [23]

4 Conclusions

The TEOS based fibre reinforced silica aerogel blanketsproduced via gelation silylation and atmospheric pressuredrying route appear to be a very useful thermal insulatingmaterial in the case of extreme heat exposureThe hydropho-bicity of the aerogel blankets has been found to be increas-ing with increased silylating agent (trimethylchlorosilane(TMCS)) concentration which has been confirmed from theFTIR and contact angle measurements The bulk densityand the thermal conductivity of the aerogel blankets werefound to reduce with increase in the silylation treatmentSuccessful trials taken by altering the concentrations of thesilylating agent (TMCS) showed great improvement in theestimated protection time from 2nd degree burn injury Theeffectiveness of the silica aerogel blankets as protective shield-ing has been found to increase with the increasing TMCSconcentration The aerogel blankets have the applications inthe area of firefighting systems

Competing Interests

The authors declare that there are no competing interestsregarding the publication of this paper

Authorsrsquo Contributions

S Chakraborty and A A Pisal contributed equally

Acknowledgments

Thecorresponding author (ProfessorAVenkateswaraRao) ishighly thankful to the University Grant Commission (UGC)New Delhi India for funding this work under UGC-BSR-Faculty Fellowship Letter no F 18-12011 (BSR) datedMay 212013 One of the authors Abhijit A Pisal is highly gratefulto the UGC New Delhi for the Stipendiary Candidateshipunder the UGC-BSR-Faculty Fellowship

References

[1] N Husing and U Schubert ldquoAerogelsmdashairy materialschemistry structure and propertiesrdquo Angewandte ChemiemdashInternational Edition vol 37 no 1-2 pp 22ndash45 1998

[2] J Fricke and T Tillotson ldquoAerogels production characteriza-tion and applicationsrdquo Thin Solid Films vol 297 no 1-2 pp212ndash223 1997

[3] E R Bardy J C Mollendorf and D R Pendergast ldquoTher-mal conductivity and compressive strain of aerogel insulationblankets under applied hydrostatic pressurerdquo Journal of HeatTransfer vol 129 no 2 pp 232ndash235 2007

[4] R Caps and J Fricke ldquoAerogels for thermal insulationrdquo in Sol-Gel Technologies for Glass Producers and Users pp 349ndash353Springer New York NY USA 2004

[5] D M Smith A Maskara and U Boes ldquoAerogel-based thermalinsulationrdquo Journal of Non-Crystalline Solids vol 225 pp 254ndash259 1998

[6] M Schmidt and F Schwertfeger ldquoApplications for silica aerogelproductsrdquo Journal of Non-Crystalline Solids vol 225 no 1ndash3 pp364ndash368 1998

[7] H L Paul and K R Diller ldquoComparison of thermal insulationperformance of fibrous materials for the advanced space suitrdquoJournal of Biomechanical Engineering vol 125 no 5 pp 639ndash647 2003

[8] A Katti N Shimpi S Roy et al ldquoChemical physical andmechanical characterization of isocyanate cross-Linked amine-modified silica aerogelsrdquo Chemistry of Materials vol 18 no 2pp 285ndash296 2006

[9] R Baetens B P Jelle and A Gustavsen ldquoAerogel insulationfor building applications a state-of-the-art reviewrdquo Energy andBuildings vol 43 no 4 pp 761ndash769 2011

[10] Aerogel composite with fibrous batting US 7078359 B2[11] Abu Shaid M Furgusson and L Wang ldquoThermophysiological

comfort analysis of aerogel nanoparticle incorporated fabricfor fire fighterrsquos protective clothingrdquo Chemical and MaterialsEngineering vol 2 no 2 pp 37ndash43 2014

[12] Z Qi D Huang S He et al ldquoThermal protective performanceof aerogel embedded firefighterrsquos protective clothingrdquo Journal ofEngineered Fibers and Fabrics vol 8 no 2 pp 134ndash139 2013


[13] L Jin K Hong and K Yoon ldquoEffect of aerogel on thermalprotective performance of firefighter clothingrdquo Journal of FiberBioengineering and Informatics vol 6 no 3 pp 315ndash324 2013

[14] J G R Hansen and B J Frame ldquoFlame penetration and burntesting of fire blanket materialsrdquo Fire and Materials vol 32 no8 pp 457ndash483 2008

[15] L Hes M De Araujo and V V Djulay ldquoEffect of mutual bond-ing of textile layers on thermal insulation and thermal contactproperties of fabric assembliesrdquoTextile Research Journal vol 66no 4 pp 245ndash250 1996

[16] A M Stoll andM A Chianta ldquoHeat transfer through fabrics asrelated to thermal injuryrdquoTransactions of theNewYorkAcademyof Sciences vol 33 no 7 pp 649ndash670 1971

[17] S-C Lee and G R Cunnington ldquoConduction and radiationheat transfer in high-porosity fiber thermal insulationrdquo Journalof Thermophysics and Heat Transfer vol 14 no 2 pp 121ndash1362000

[18] X Lu R Caps J Fricke C T Alviso and R W PekalaldquoCorrelation between structure and thermal conductivity oforganic aerogelsrdquo Journal of Non-Crystalline Solids vol 188 no3 pp 226ndash234 1995

[19] A P Rao A V Rao and GM Pajonk ldquoHydrophobic and phys-ical properties of the ambient pressure dried silica aerogels withsodium silicate precursor using various surface modificationagentsrdquo Applied Surface Science vol 253 no 14 pp 6032ndash60402007

[20] A Venkateswara Rao S D Bhagat H Hirashima andG M Pajonk ldquoSynthesis of flexible silica aerogels usingmethyltrimethoxysilane (MTMS) precursorrdquo Journal of Colloidand Interface Science vol 300 no 1 pp 279ndash285 2006

[21] D Y Nadargi S S Latthe HHirashima andA V Rao ldquoStudieson rheological properties of methyltriethoxysilane (MTES)based flexible superhydrophobic silica aerogelsrdquo Microporousand Mesoporous Materials vol 117 no 3 pp 617ndash626 2009

[22] J R Brown and B C Ennis ldquoThermal analysis of Nomex andKevlar fibersrdquoTextile Research Journal vol 47 no 1 pp 62ndash661977

[23] P M Shewale A V Rao J L Gurav and A P Rao ldquoSynthesisand characterization of lowdensity andhydrophobic silica aero-gels dried at ambient pressure using sodium silicate precursorrdquoJournal of Porous Materials vol 16 no 1 pp 101ndash108 2009

[24] J Quintiere ldquoRadiative characteristics of fire fightersrsquo coatfabricsrdquo Fire Technology vol 10 no 2 pp 153ndash161 1974

[25] H-P Ebert ldquoThermal properties of aerogelsrdquo inAerogels Hand-book Advances in Sol-Gel DerivedMaterials and Technologiespp 537ndash564 Springer New York NY USA 2011

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Table 1 Details of the fabrics used for making three layered assembly for evaluation

Fabric Cover () GSM (gm2) Thickness (mm) Bulk density (kgm3)Nomex IIIA fabric 094 220 044 500Nomex nonwoven felt mdash sim150 285 52Modacryliccotton fabric 086 196 041 478

protective performance of such a clothing assembly are yetto be explored properly Aerogel nanoparticle coated fabricsfor firefighting applications were found to have increasedthermal resistance good air permeability and moisturemanagement properties [11] Results of the superior thermalprotection of lightweight firefighter protective clothing usingaerogel fillers have been reported [12 13] Fire penetrationand burn testing of fire blanket materials showed aerogelblankets to be a top performing insulation material [14]

Nonwoven felt produced from inherently heat retardantfibrous materials is used as component of protective clothingfor protection against high heat exposure In the presentstudy meta-aramid fibre namely Nomex has been usedfor making nonwoven felt We have produced low den-sity hydrophobic flexible tetraethoxysilane (TEOS) basedsilica aerogel blankets via sol-gel formation inside meta-aramid (Nomex) needle punched fabric silylation withtrimethylchlorosilane (TMCS) and ambient pressure dryingSubsequently the prepared aerogel-fabric samples were char-acterized by density thermal conductivity hydrophobicityorganic and inorganic bond formation and protective eval-uation to radiant heat fluxes

2 Experimental Procedures

21 Materials The chemicals used for the preparation of sil-ica alcosols and the subsequent silylation of silica alco-gel blankets were tetraethoxysilane (TEOS 98 (SigmaAldrich)) oxalic acid ((COOH)

2 2H2O (Qualigenic Fine

Chemicals India)) ammonium fluoride (NH4F (Loba

Chemie)) methanol (MeOH (Merck India)) and trimethyl-chlorosilane (TMCS (Fluka Puriss grade Switzerland)) andhexane (Merck India) Double distilled water was used forthe preparation of all the solutions Nomex (DuPont India)m-aramid fibre matrix was used for making the nonwovenfelt which was subsequently used to produce aerogel blanketCommercially availableNomex IIIA (Nomex 93Kevlar 5and 2 antistatic fibre) woven fabric and modacrylic cotton(6040) blended woven fabric were used in combinationwith aerogel blanket for thermal protective evaluationThree-layer assembly was prepared using a Nomex IIIA wovenfabric as outer layer aerogel blanket as middle layer and amodacryliccotton fabric (blend 6040) as the inner layer forevaluation of the radiant heat protection Particulars of thefabrics used for the present study are given in Table 1

22 Methods of Preparation Nomex (DuPont m-aramid)fibres of 22 Denier (denier defined as gram9 km) and50mm length were converted into fibrous nonwoven batting

on a laboratory model needle punching machine (DiloGermany) The needle punched fabric was prepared with150 punchesm2 10mm needle penetration depth and setto have areal density of approximately 150 (gm2) Silicaaerogel blanket was prepared by forming silica based gelon the Nomex fibrous nonwoven fabric and subsequentlytransformed into aerogel by suitable drying technique Theaerogel was produced through a two-step acid-base sol-gelprocess followed by ambient pressure drying Figure 1 showsa flowchart for the preparation of aerogel-nonwoven battingThe sol was prepared using TEOS precursor diluted inMeOH The acidic catalyst oxalic acid ((COOH)

2 0001M)

was added to the sol stirred for 1 hour and kept atroom temperature for 12 hours for hydrolysis The molarratio of TEOS MeOH (COOH)

2 NH4F was kept constant

at 1 38 373 times 10minus5 0023 respectively After 12 hours ofhydrolysis the base catalyst ammonium fluoride (NH

4F

05M) was added dropwise to the sol while stirring andstirred for further 5min Immediately after that the sol waspoured onto the Nomex fibrous batting (65 cm times 65 cmdensity 52 kgm3 thickness 385mm) kept on Petri dish andallowed to form gel at room temperature The complete gelformation took place within 5min Petri dishes containingthe alcogel samples in methanol were kept wrapped withaluminum foil and aged for 1 hour After aging methanol wasdecanted and the samples were transferred from Petri dishesto a separate container and surplus of hexane was addedMethanol in the gel was exchanged with hexane at 50∘C ina shaker (Remi Instrument India) at 120 rpm for 3 hoursTo make the gels hydrophobic and to have the spring-backeffect subsequently the gels were immersed in a silylatingagent (TMCS in hexane) and kept in an oven for 16 hours at50∘C and shaken at 120 rpmThe concentration of TMCS wasvaried from 2 to 10 in the solvent (hexane) The unreactedTMCS was exchanged with the hexane solvent again in thesame shaker at 50∘C for 4 hours at 120 rpm After this step thehexane was decanted and the resulting silylated gelled fabricwas dried in an oven at 50∘C for 1 hour and 100∘C for 2 hoursto get the hydrophobic aerogel composite fabric

23 Reaction Mechanism In the present study the surfacechemical modification of the wet gelled fabric was made withTMCS in hexane It took place according to the mechanismshown in Scheme 1

In order to obtain low density and better hydrophobicsilica aerogel fabric the Hrsquos of the surface hydroxyl groupsformed during hydrolysis and condensationmust be replacedwith the organosilicon groups [(CH

3)3-Si-] Therefore the

surface modification of TEOS based wet gel blankets was


Si

Si

SiO

Si

Si

O+ 2

OH

OH

Cl +

CH3

CH3

CH3 2HCl

O-Si-(CH3)3

O-Si-(CH3)3

997888rarr

Scheme 1


Gelation














2(CH3)3-Si-Cl + H2O

(CH3)3-Si

(CH3)3-Si

O + 2HCl (1)


(CH3)3-Si

(CH3)3-Si

O + 2CH3OH-Si-O-CH3 + H2O2(CH3)3 (3)







(a) (b)

(c) (d)








0020

0025

0030

0035

0040

0045

0

20

40

60

80

100

120

140

0 2 4 6 8 10 12




Den

sity

(kg

m3)

Ther

mal

cond

uctiv

ity (W

mmiddotK

)




3)3formed



600 1100 1600 2100 2600 3100 3600

Refle

ctan

ce

(au

)

-OH

Si-O-Si

-OHSi-C C-HC-H

Si-C


2 TMCS4 TMCS6 TMCS

8 TMCS10 TMCS





3bonds







0

20

40

60

80

100

120

50 150 250 350 450 550 650 750

Wei

ght (

)


Temperature (∘C)




35

40

45

50

55

60

65

Withoutaerogel

2 4 6 8 10


Estim

ated

pro

tect

ion

time (

s)









4 Conclusions


Competing Interests




Acknowledgments


References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




Si

Si

SiO

Si

Si

O+ 2

OH

OH

Cl +

CH3

CH3

CH3 2HCl

O-Si-(CH3)3

O-Si-(CH3)3

997888rarr

Scheme 1


Gelation














2(CH3)3-Si-Cl + H2O

(CH3)3-Si

(CH3)3-Si

O + 2HCl (1)


(CH3)3-Si

(CH3)3-Si

O + 2CH3OH-Si-O-CH3 + H2O2(CH3)3 (3)







(a) (b)

(c) (d)








0020

0025

0030

0035

0040

0045

0

20

40

60

80

100

120

140

0 2 4 6 8 10 12




Den

sity

(kg

m3)

Ther

mal

cond

uctiv

ity (W

mmiddotK

)




3)3formed



600 1100 1600 2100 2600 3100 3600

Refle

ctan

ce

(au

)

-OH

Si-O-Si

-OHSi-C C-HC-H

Si-C


2 TMCS4 TMCS6 TMCS

8 TMCS10 TMCS





3bonds







0

20

40

60

80

100

120

50 150 250 350 450 550 650 750

Wei

ght (

)


Temperature (∘C)




35

40

45

50

55

60

65

Withoutaerogel

2 4 6 8 10


Estim

ated

pro

tect

ion

time (

s)









4 Conclusions


Competing Interests




Acknowledgments


References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)

(c) (d)








0020

0025

0030

0035

0040

0045

0

20

40

60

80

100

120

140

0 2 4 6 8 10 12




Den

sity

(kg

m3)

Ther

mal

cond

uctiv

ity (W

mmiddotK

)




3)3formed



600 1100 1600 2100 2600 3100 3600

Refle

ctan

ce

(au

)

-OH

Si-O-Si

-OHSi-C C-HC-H

Si-C


2 TMCS4 TMCS6 TMCS

8 TMCS10 TMCS





3bonds







0

20

40

60

80

100

120

50 150 250 350 450 550 650 750

Wei

ght (

)


Temperature (∘C)




35

40

45

50

55

60

65

Withoutaerogel

2 4 6 8 10


Estim

ated

pro

tect

ion

time (

s)









4 Conclusions


Competing Interests




Acknowledgments


References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




0020

0025

0030

0035

0040

0045

0

20

40

60

80

100

120

140

0 2 4 6 8 10 12




Den

sity

(kg

m3)

Ther

mal

cond

uctiv

ity (W

mmiddotK

)




3)3formed



600 1100 1600 2100 2600 3100 3600

Refle

ctan

ce

(au

)

-OH

Si-O-Si

-OHSi-C C-HC-H

Si-C


2 TMCS4 TMCS6 TMCS

8 TMCS10 TMCS





3bonds







0

20

40

60

80

100

120

50 150 250 350 450 550 650 750

Wei

ght (

)


Temperature (∘C)




35

40

45

50

55

60

65

Withoutaerogel

2 4 6 8 10


Estim

ated

pro

tect

ion

time (

s)









4 Conclusions


Competing Interests




Acknowledgments


References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






0

20

40

60

80

100

120

50 150 250 350 450 550 650 750

Wei

ght (

)


Temperature (∘C)




35

40

45

50

55

60

65

Withoutaerogel

2 4 6 8 10


Estim

ated

pro

tect

ion

time (

s)









4 Conclusions


Competing Interests




Acknowledgments


References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials









4 Conclusions


Competing Interests




Acknowledgments


References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Research Article Synthesis and Characterization of Fibre...

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