interfacial properties of nano-silica modified cfrps smithersinformationltd.,2014 1. introduction...

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1. INTRODUCTION

Duetothehighemphasisputonreducevehicle weight for space transportation systems, advanced polymermatrixcomposite materials are primary candidates for structures. Such as carbon fiber composite cryogenic pressure vessel have wildly application prospect in launch vehicle because of light construction1-4. So, applications of carbon fiber reinforced polymercomposites (CFRPs) are rapidlyexpandingincryogenicenvironments.

The composite structure of RLV must undergosevereshrinkageduring thechangefromroomtemperature(RT)to cryogenic conditions. For CFRPs, the carbon fiber (CF) has a lowercoefficient of thermal expansionthan thematrix during cooling.Theresulting thermal residual stresses are ofcompressivenatureinthefiberandtensile nature in the matrix. Undersuch extreme conditions, decreasing

differenceinthelinearcoefficientofthermalexpansion(CTE)betweenthefibersandthematrixcanincreasethemechanical properties of composites5-7. In addition, the mechanical behavior of CFRPs are highly depends not only onthefiberandmatrixpropertiesbutalso on the interfacial properties in cryogenic conditions7-10. The variation of interphase properties with changing temperature can affect the properties of the whole CFRPs. There are almost not any papers about the IFSS performance offiberreinforcedpolymercomposites(FRPs)undercryogenicconditions.

Thoughepoxyresinispredominantlyemployedasamatrixduetoitsexcellentheat resistance and good chemical resistance. However the CFRPs fabrication from neat epoxy resincannot meet industrial requirementdue to its low mechanical properties and high isotropic coefficient of thermal expansion. Nano-silicamodifiedresinmatrixcanimprovethe

mechanical properties of CFRPs, such as dimensional stability, strength and toughness, have been in on a wide range of applications. In this paper, the ILSS and interfacial fracture properties of carbon-fiberreinforcednanomodifiedepoxycompositeswereinvestigatedbymicro-droplet debond test at the room temperature and 77 K respectively.

2. MATERIALS PREPARATION AND TESTING APPARATUS

2.1 MaterialsNano scale silica particles weresynthesized by sol-gel reaction as follows.Tetraethoxy-silane(TEOS)asalkoxide,wasperformedinn-methyl-2-pyrrolidone(NMP)andethylalcoholsolution cosolvent systems. The molar ratio TEOS:H2O=1:3 was used inall experiments. The silica sol weremodifiedbysilanecouplingagentKH-560.Finally, silicananoparticlesbeobtainedbydryingsilicasolat60°C.

Nano-silica fillers of 1 weightpercentage (wt.%), 3 wt.% weredispersed in the epoxy resin

InterfacialPropertiesofNano-silicaModifiedCFRPsUnderCryogenic Condition

Rongguo Wang1, Chengwei Zhang2, Lifeng Hao1, Weicheng Jiao1, Fan Yang1, and Wenbo Liu3*1Center for Composite Materials and Structures, 2Harbin Institute of Technology,3School of Materials Science and Engineering,HarbinInstituteofTechnology,Harbin,150080,P.R.China

SUMMARYThechangeofinterfaceperformancebetweencarbon-fiberandmatrixresininfluencethepropertiesofcarbon-fiberreinforcepolymercomposites(CFRPs)whentemperaturewaschangedfromroomtemperaturetocryogenicconditions. In order to find the interfacial properties of epoxy/nano-silica matrix CFRPs under cryogenicconditions,theinterfacialshearstrength(IFSS)andtheinterlaminarshearstrength(ILSS)wereinvestigatedatroom temperatures and 77K respectively. The morphology of fractured interface was investigated by scanning electronmicroscope(SEM).Theresultsshowthattherearehighermechanicalpropertiesofinterfaceat77Kcondition.

Keywords: Cryogenic,Composites,Interface,Carbonfiber,Nanomodified

*Correspondingauthor:[email protected]

269Polymers & Polymer Composites, Vol. 22, No. 3, 2014

Interfacial Properties of Nano-silica Modified CFRPs Under Cryogenic Condition

(Bisphenol-A) with bisphenol Acyanate ester as curing agent by high-speedmixedonhighsheardispersingemulsifier(FlukoFA25).Theprocessconditions are as follows: rotation speedof19,000r/min,timeof5min,mixedtemperatureat25~45°C.Hereinafter,nano-hybridsepoxy/cyanateestercompositesarenamedNEC.

Commercially available T700/12kcarbonfiberswithadiameterofabout7 µm, purchased from Japan Toray, was usedinthepresentwork.DiglycidyletherofbisphenolAepoxyresin(E51)wasprovidedbyWuxiResinFactory(China).Thenano-silicate/epoxywith0,1,3wt.%nano-fillerscontentinresincompositionwasdippedontotheT700unidirectional carbonfiber to obtain thetotalfiberlayerof12plies,fiberorientationof0°andfibercontentinCFRPSof70±2vol.%.Specimensofall systems were obtained by curing at 130°Cfor3h,150°Cfor3h,190°Cfor 6 h to reach full cure.

2.2 Testing of Interfacial Shear StrengthofNano-modifiedCFRPsMicro-droplet debond test was performed to determine the interfacial shearstrength(IFSS)betweenCFandnano-modifiedresinbypullingoutafiberfromcuredepoxyresindroplet.The resin was wetted onto the single carbonfiber toformamicro-dropletdue to the function of surface tension. Thesinglefiberwasfixedonametalframe. The embedded length of single micro-droplet was measured through opticalmicroscopic.Theknivescamein contact with the solid resin droplet andtheforcerequiredtodebondthedroplet from thefiberwas recorded.TheIFSSwascalculatedfromEq.(1):

τ IFSS =FMax

π ⋅ d ⋅ 2r (1)

where t IFSS is interfacial shear strength, Fmaxisthemaximumload,disthefiberdiameterand2r is the embedded length. Theexperimentshouldberepeatedatleast20timesforeachkindofsample.

Aschematicofancarbon-fiberattachedto the plate along with micro-droplet is shown in Figure 1. Micro-droplet testfiguresbeforeandafterdebondisshown in Figure 2a and b.

Fiber reinforced polymer composite materials for cryogenic properties used to be studied in laminated board tensile, bending, compression, interlaminar shear, no one study the interface shear strength (IFSS) of CFRPS at 77K.The cryogenic conditions of IFSS test were acquired with liquid nitrogencontinuing sprayed onto the resin droplet. Experimental configurationof themicro-droplet test isMODELHM410(Japan).

2.3 Testing of ILSS of Nano-silicaModifiedCFRPsTheinterlaminarshearstrength(ILSS)of nano-silica composites was tested on anuniversaltestingmachine(WD-1,Changchun, China) using a three point short-beamshear (SBS) testmethodaccordingtoASTMD2344-76.ThesketchmapisshowninFigure 3.

2.4 Structure of Nano-silica ModifiedCompositesToinvestigatetheeffectsofthefillednano-silicaparticles,highmagnificationSEM was used to observe the areas filledwithnano-silica.TheelementaryanalysisisperformedbyEDSspectrumscanning.

Figure1.Micro-dropletonT700carbon-fiberattachedtotheplatewithglue

Figure2.Opticalmicrographofmicro-dropletsoncarbonfiberinmicro-bondtesting. (a) Before debonding, (b) After debonding, (c) Liquid nitrogen was injected onto microdroplet

270 Polymers & Polymer Composites, Vol. 22, No. 3, 2014

Rongguo Wang, Chengwei Zhang, Lifeng Hao, Weicheng Jiao, Fan Yang, and Wenbo Liu

3. RESULTS AND DISCUSSIONS

3.1 Results of IFSS Under RT and Cryogenic ConditionsThe influenceof testing temperatureon the properties of IFSS was studied at RT and 77K respectively, and 0,1, 3, 5 wt.% nano-silica modifiedepoxyexampleswere tested (shownin Figure 4). Fifteen specimens were tested for each resin system. The IFSSofneatresin,1wt.%and3wt.%nanomodifiedresinwassignificantlyimproved under cryogenic condition. But IFSS of 5 wt.% system almosthas no difference between RT and 77K. The higher IFSS at 77K was obtained for 0 wt.%, 1 wt.% and3wt.%duetothedifferenceinCTEbetweencarbonfiberandresinmatrix.Carbonfibershrinkageratearelowerthan those of resins possible when temperature decrease from RT to 77K. So there are the radial pressure stressesdueto thermalshrinkageonthe carbonfiber surface,which leadto the IFSS increased. In addition, higher mechanical properties of resin matrix can improve the IFSS. Highmechanical strength can be obtained throughnanomodifiedresinwhentheweight content is controlled in a certain range. Moreover the poor performance ofthe5wt.%specimenmightbedueto agglomeration of the nano-silica, which might hinder the adhesion of the interface during cure.

HighmagnificationSEMimagesandEDS results after debond bymicro-droplet testing are shown in Figure 5. Thefigureshowsthatthenano-silicawasuniformlydispersedintheepoxyresinwithasiliconcontentof1.4wt.%.There are almost no silicon in the surfaceofcarbonfiber.Through theresults above, we can conclude the micro-droplet debond mainly occur in theinterphasebetweenthecarbon-fiberandmatrix.

Figure 3. Schematic of ILSS testing device on 77K

Figure4.ComparisonofIFSSofCFRPswiththeneatandnano-silicamodifiedepoxy samples by micro-droplet testing at RT and 77K

Figure 5. SEM and EDS image of sample after micro-droplet testing



3.2 Interfacial Shear Strength Properties Under 77kThe effect of nano-silica modifiedon the composites interlaminar was carried out using interlaminar shear strength (ILSS) test of CFRPScomposites. Figure 6 shows the results of ILSS of different nano-silica hybrid weight content samples at RT and77K.Theresultsshowed1wt.%sample has the highest ILSS and all results at 77K are higher than at RT conditions. Contrasting Figure 4 with Figure 6, we can conclude the resultsof IFSS(shown inFigure 4) consistent with the trend of the test resultsofILSS(showninFigure 6). Nano-silica modified can improveCFRPs ILSS on 77K, but almost not improveCFRPspropertiesatRT.Asarecognized conclusion, ILSS is related tothemechanicalpropertiesofmatrixresin and interface/interphase. IFSS wasobviouslyimprovedwith1wt.%nano-silicamodifiedmatrixatRTand77K,duetothenanomodifiedmatrixresin and interphase have highest mechanical properties. The ILSS of all systems CFRPS are successfully improved on 77K due to increase of mechanical properties of the resin.

4. CONCLUSIONS

The IFSS and ILSS of nano-silica modifiedCFRPsaremeasuredatRTand 77K by micro-droplet debond testing and short-beam shear (SBS)tests. The results showed that the properties of IFSS and ILSS can be improvedat77K,and1wt.%sampleshave the highest values of IFSS and ILSS. ILSS of CFRPS are effectively improved by nano-silica modified epoxyresin,butILSSalmostnotbeincreased with nano content rise at room temperature. The SEM images andEDSresultsshowedthatthemicro-droplet debond mainly occur in the interphase between the carbon-fiberandmatrix.

ACKNOWLEDGEMENTS

The authors gratefully acknowledgethe financial support from theMajorState Basic Research DevelopmentProgram of China (973 Program,No.2011CB605605), SpecializedResearch Fund for the DoctoralProgram of Higher Education No.20122302120033), SpecializedResearch Fund for the Doctoral

Program of Higher Education (No.20122302120034) and ResearchFund ofNationalKeyLaboratory ofScienceandTechnologyonAdvancedComposites in Special environments (9140C490107130C4901).

Thisworkwassupportedbyfundingfrom the National Natural ScienceFund Program of China (Grant No.50803002)andNationalProgramonKeyBasicResearchProject(GrantNo.2011CB605605).

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Figure6.ComparisonofILSSofCFRPswiththeneatandnano-silicamodifiedepoxy samples at RT and 77K



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interfacial properties of nano-silica modified cfrps smithersinformationltd.,2014 1. introduction...

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