Different Methods Of Dispersing Carbon Nanotubes In Epoxy Resin And .

Materials 2019, 12, 29983 of 23This result can help to design composite materials able to fulfill some of the strongly desiredrequirements for their application in aeronautics, which is a very complex study. In fact, the developmentof a carbon fiber reinforced composite (CFRC) and the preservation of its structural integrity requiresthe investigation and control of different interacting factors: critical aspects concerning the application,accessibility, and ability for the inspection of vital parts and components; studies on the consequencesof impact, fatigue, temperature, and hostile environment; nature of inherent flaws; etc. [36]. Capezzutoet al. proposed an interesting strategy for the detection of low-velocity impact damage on compositestructures [37]. The study of this aspect is of relevant interest because damages due to low-velocityimpact events not only weaken the structure undergoing a continuous service load but also maygenerate different types of flaws before full perforation, i.e., sub-surface delamination, matrix cracks,fiber debonding or fracture, indentation, and barely visible impact damage (BVID) [37]. The possibilityto confer the ability to decrease fatigue crack growth rates in CFRCs is a current challenge that may besolved using a combination of CNTs and GPOSS [35].However, one of the most difficult challenges to face when CNTs are incorporated in epoxyresins is related to the difficulty of obtaining acceptable levels of dispersion. In fact, carbonnanotubes tend to form bundles because of intense intertubular van der Waals attractive forces,which can prevent obtainment of materials with high reproducibility in all the microzones of the bulkmaterial. Hence, a dispersion assessment is of primary importance for the manufacturing process ofcomposites [8–10]. The work described in this paper regards the manufacturing processing of couponsof epoxy multifunctional composites based on carbon fiber reinforced epoxy resin nanofilled withCNTs. More specifically a study of the optimization of the dispersion methods was performed.The crucial stage of nanoparticle dispersion was studied through a series of experiments.In particular, several mixing methods and some of their combinations, namely (1) ultrasound;(2) stirring; (3) by roller machine; (4) by gears machine; and (5) ultrasound radiation high stirringwere investigated to disperse MWCNT in an epoxy formulation containing solubilized GPOSSnanocages. The chemical composition of the epoxy matrix was chosen to obtain high mechanicalperformance suitable for manufacturing load-bearing structures.It is well known that the combination of epoxy resin and hardener defines the matrix materialproperties and the various possible combinations allow one to tailor material properties accordingto the desired requirements [1]. In this work, the tetrafunctional epoxy precursor TGMDA was usedin combination with a reactive diluent to decrease the viscosity of the resin and facilitate the step ofnanofiller dispersion in the matrix; 4,4 diamminodioheynil sulfone (DDS) was used as a hardeneragent due to the high mechanical performance of the resin solidified with this class of curing agent.2. Materials and Methods2.1. MaterialsEpoxy resin. The epoxy matrix was prepared by mixing the epoxy precursor TGMDA (epoxyequivalent weight 117–133 g/eq) with the epoxy reactive monomer 1,4-butanedioldiglycidylether (BDE)that acted as a reactive diluent. These resins, both containing epoxy functionality, were obtainedfrom Sigma-Aldrich. The epoxy mixture was made by mixing TGMDA with BDE monomer at aconcentration ratio of 75:25 wt% epoxide to flexibilizer. In particular, the use of a percentage of 25%of reactive diluent was chosen to reduce the viscosity of the epoxy resin and hence to improve thenanofiller dispersion.Carbon nanotubes. The MWCNTs (3100 grade) were obtained from Nanocyl S.A (Sambreville,Belgium). The average diameter and the average length, evaluated by high-resolution transmissionelectron microscopy (HRTEM), were 9.5 10 9 m and 1.5 µm, respectively. In particular, an outerdiameter ranging from the minimum of 10 nm to the maximum of 30 nm was measured, whereas thelength of MWCNTs was from hundreds of nm to a few micrometers. The number of walls varied from4 to 20 in most nanotubes. The specific surface area of MWCNTs estimated by the Brunauer Emmett

Materials 2019, 12, 29984 of 23Teller (BET) method was around 250–300 m2 /g; The carbon purity and the metal oxide percentage,Materials 2019,12,thermogravimetricx FOR PEER REVIEW analysis (TGA) were 95.0 and 5.0, respectively. An amount4 ofof22calculatedby0.5 wt% of MWCNT was used for blend preparation.POSS omericsilsesquioxanes(GPOSS),functionalizedwith eightoxiraneoxiraneforgroupseach molecule,were dispersedin the matrix.epoxy matrix.The POSS/epoxycompositesgroupseach formolecule,were dispersedin the epoxyThe .GPOSSwaspurchasedfromHybridPlastic(USA).prepared with 5 wt% of POSS. GPOSS was purchased from Hybrid Plastic (USA).Curing agent.agent. DDSDDS (4,4(4,4′0 diaminodiphenylSigma-Aldrich (Milan,(Milan, Italy)Italy)Curingdiaminodiphenyl sulfone),sulfone), purchasedpurchased fromfrom ichiometricconcentrationwithrespecttoallthewas used as a hardener agent and added at a stoichiometric concentration with respect to all the epoxyepoxyringsarisingfromTGMDA,BDE,andPOSS.rings arising from TGMDA, BDE, and POSS.Carbon fibers.fibers. Thermofixedused forfor thethe preparationpreparation ofof thetheCarbonThermofixed unidirectionalunidirectional carboncarbon fabricfabric waswas s. In particular, thermofixed UD carbon–GV 501 U TFX (G. Angeloni s.r.l., Quarto d’Altino(VE), Italy),Italy), withwith anan arealareal densitydensity ofof 0.5160.516 kg/mkg/m22 andand fibersfibers composedcomposed ofof 24,00024,000 individualindividual carboncarbon(VE),filaments, waswas used.used. TheThe thicknessthickness waswas 0.50.5 mm.mm.filaments,2.2. Methods2.2.2.2.1.2.2.1. Study of the DispersionDispersion ofof CarbonCarbon NanotubesNanotubes inin thethe EpoxyEpoxy MatrixMatrix andand PanelPanel PreparationPreparationForFor thethe studystudy ofof thethe dispersiondispersion ofof carboncarbon nanotubes,nanotubes, differentdifferent techniquestechniques reconsidered.TheTheuse useof themechanical stirring, gearbox milling, and calendaring (three-roll mill) were considered.ofcombinationof theofdifferenttechniques,such as ultrasound-assistedwith high withstirring,or combining,the combinationthe differenttechniques,such as ultrasound-assistedhighstirring, orforinstance, forthe instance,ultrasound-assistedwith a high stirringwith thecalendaring,werealso takencombining,the ultrasound-assistedwith amethodhigh stirringmethodwith thecalendaring,intoaccount.were also taken into account.Ultrasonicationperformed elscherHielschermodelUP200S(200Ultrasonication was performedmodelUP200S(200W, odifferentsonotrodeswereusedaccordingtokHz) (Hielscher Ultrasonics, Teltow, Germany). Two different sonotrodes were used according to thethevolumeof thedispersion:3 mmsonotrodeforvolumesvolumesfromfrom55mLmLupuptoto 200200 mLmL and a 22 mmvolumeof thedispersion:a 3ammtiptipsonotrodeformmtiptip sonotrodesonotrode forfor .mL.AHeidolph h,Germany)formechanicalthe mechanicalstirring.A HeidolphGermany)waswasusedusedfor thestirring.ThreeThreedifferentmixingelementsin orderto evaluateeffectofofthetheflowflow generatedgenerated by thedifferentmixingelementswerewereusedusedin orderto twiththreeimpeller on the particle dispersion: helix blade, a viscojet with two cones, and a viscojet with threecones.cones. The main differencedifference betweenbetween aa helixhelix bladeblade stirstir barbar andand aa viscojetviscojet isis thethe flowflow generatedgenerated intointo thethematrix,matrix, andand re1).1).(a)(b)Figure 1.1. Flows generatedgenerated byby usingusing aa helixhelix bladeblade stirstir barbar (a)(a) andand viscojetviscojet stirstir barbar (b).(b).FigureTheThe gearboxgearbox millermiller waswas shownshownininFigureFigure2.2.

Materials 2019, 12, 2998Materials 2019, 12, x FOR PEER REVIEW(a)5 of 235 of te(Milan)(Milan)Italy.Italy.Theof ofthetheMWCNTin theepoxyresinresinon thewas eMWCNTin theepoxyonmicroscalethe microscalewas Japan).Threeimageswereobtainedforeachmaterialwith an optical microscope (Olympus BX51, Tokyo, Japan). Three images were obtained for eachtype,and type,theseandwereusedfor usedthe analysis.Althoughthis methodis a low-accuracymethodandmaterialthesewerefor the analysis.Althoughthis methodis a ewhichand rudimentary, the reliability and measurement accuracy of aggregate size is sufficient to decidemethodthe bestnanofillersto measuresizetheof theagglomeratesofwhich hadmethodhaddispersionthe best averagesize of theparticlesin the ofsystemin question.agglomeratesparticlesin the system in question.StandardconditionssetStandard conditions llowingMany values of the setting parameters were analyzed in order to obtain a wide range of tween allbetweenmethodsallandthe optimizationof the process.a representativemethodsand the optimizationof the o theepoxyDDS in a stoichiometric amount with respect to the total oxirane rings wasaddedto precursorthe epoxy C until complete hardener solubilization.blendconstitutedTGMDA 75and BDE25 wt%120 25precursorblend byconstitutedby wt%TGMDA75 wt%andatBDEwt% at 120 C until complete hardener C. At this temperature, an amount of carbon nanotubes andThenthemixturewascooledto90solubilization. Then the mixture was cooled to 90 C. At this temperature, an amount of carbonGPOSScompounds,equalto 0.5% and5% inof theof ofthetheepoxyprecursornanotubesand GPOSScompounds,equalto weight0.5% and5% amountin weightamountof cursor blend, respectively, were added using the dispersion method providing the bestdispersionSectionof ciencySection 3.1.dispersion methods).2.2.3. Laminate Manufacturing2.2.3. Laminate ManufacturingFlat panels were prepared using the epoxy matrix containing carbon nanotubes. A proper amountFlat panels were prepared using the epoxy matrix containing carbon nanotubes. A properof resin was used in order to obtain a panel with a 50:50 fiber to matrix ratio.amount of resin was used in order to obtain a panel with a 50:50 fiber to matrix ratio.The manufacturing process consisted of three main steps:The manufacturing process consisted of three main steps: Pre-impregnation Pre-impregnationHand lay-upof lay-upthe prepregpreparationof the bagofvacuumHandof theandprepregand preparationthe bagmoldingvacuum moldingCuringautoclave inCuringin sswaswascarefullydoneso thatthe fabricwas onpre-impregnationprocesscarefullydoneso thatthe fabricwas perfectlyaligned,theresindistributed.andthe wasresinhomogeneouslywas homogeneouslydistributed.Atthe edto evenlydistributethe byresinAtthisthis stage,stage, thepressurerollers,designedto evenlydistributethe resinthebythe pressureexertedon thefabric,a crucialaspect.aspect.AfterAfterfeedingfeeding thethe material forpressureexertedon thefabric,is ais crucialfor ticallythrougha seriesof severalrolls to llythrougha seriesof severalrollstheto prepreggive thepreviouslystudied. studied.prepreg previouslyFor the hand lay-up of the prepreg, the release agent, Marbocote (UK), was applied to the mold.A laminate 0/90/0/90/0/90 was prepared with a tolerance of 5 on the orientation. During the

Materials 2019, 12, 29986 of 23Materials 2019, 12, x FOR PEER REVIEWFor2019,the 12,handlay-upthe prepreg,Materialsx FORPEER ofREVIEW6 of 22the release agent, Marbocote (UK), was applied to the mold.6 of 22 on the orientation. During the leranceof 5placement of fabrics, air entrapment and wrinkling were avoided by applying pressure to the fibersplacementairof entrapmentfabrics, air entrapmentand avoidedby avoidedapplyingpressureto thefibersin the fiberswarpin fabrics,the warp direction.Then,vacuum werebag waspreparedaccordingto the followingscheme:in the warpdirection.Then, thevacuumbag waspreparedaccordingto thefollowing dingtothefollowingscheme:The bag was kept under vacuum until the internal pressure was between 0.1 and 0.8 bar (76–610The bagkeptvacuumuntilthe theinternalpressurewas between0.1 and 0.8 bar (76–610Thebag waswaskeptunderundervacuumuntilinternalpressuremm Hg)beforeits introductioninto the autoclaveforthe curingcycle.was between 0.1 and 0.8 ecuringcycle.(76–610Hg)conditionsbefore its introductionintotheatautoclavethe curing cycle.Themmcuringused an initialstepmoderatefortemperature(125 C for 1 h) followed byThe curingcuring (125 C Cforfor1 1h)h)followedThefollowedbybyaa second one at higher temperature (180 C for 3 h). Figure 3 shows a graphicalregisteredautoclave asecondoneathighertemperature(180 econdat higher temperature(180 C for 3 h). Figure 3 shows a graphical registered autoclavecycle foronea manufacturingdemonstrator.cycle forfor aa manufacturingmanufacturing demonstrator.demonstrator.cycleFigure 3. Scheme of the prepared vacuum bag.Figure 3. Scheme of the prepared vacuum bag.AA picturepicture ofof oneone ofof thethe preparedprepared panelspanels isis shownshown inin FigureFigure 4.4.A picture of one of the prepared panels is shown in Figure 4.Figure 4. Picture of the manufactured panel.Figure 4. Picture of the manufactured panel.Figure 4. Picture of the manufactured panel.2.3. Characterization Methods2.3. Characterization Methods2.3. Characterization Methods2.3.1. Laser Scanning Vibrometry Test2.3.1. Laser Scanning Vibrometry Testmainpurposeof the vibrometrytest was to estimate the operative deflection shapes (ODS)2.3.1.TheLaserScanningVibrometryTestThe mainpurposeof thevibrometrytest wasto estimatethedeterminationoperative deflectionshapes(ODS)that underthe whitenoiseexcitationconditiontraducedinto theof modalfrequenciesThe main purpose of the vibrometry test was to estimate the operative deflection shapes ucedintothedeterminationofmodalfrequenciesand relative damping coefficients. The test facility consisted of a reverberation box in which a speakerthat under the white noise excitation condition traduced into the determination of modal frequenciesand relativeThe testfacilityconsistedof a simply-supportedreverberation box ina speakerservedas andampingacoustic coefficients.loading elementof thepanel,which wasonwhichthe fouredgesand relative damping coefficients. The test facility consisted of a reverberation box in which a speakerservedas anacousticloadingelementthe panel,whichwas simply-supportedon the fouredgesofthe box(Figure5). Theseedgeswereofbondedon softmaterialsheets (i.e., polystyrene)in ordertoserved as an acoustic loading element of the panel, which was simply-supported on the four aterialsheets(i.e.,polystyrene)inordertoavoid any coupling mechanism among the plunge rigid motion and the interested elastic mode shapes,of the box (Figure 5). These edges were bonded on soft material sheets (i.e., polystyrene) in order toavoidamongtheTheplungerigidofmotionand thepositionedinterestedinelasticmodeandat anyoncecouplingavoiding mechanismacoustic energylosses.presencea microphoneproximitytoavoid any coupling mechanism among the plunge rigid motion and the interested elastic hepresenceofamicrophonepositionedinthe sample served to measure the sound pressure level (SPL) of the incident sound waves. Next, toshapes, and at once avoiding acoustic energy losses. The presence of a microphone positioned inproximityto the sampleservedto headmeasurethe soundpressurelevel (SPL)of the incidentsoundtheoutlet surface,a scanninglaser(PolytecPSV 400)was positionedto measurethe vibrationproximity to the sample served to measure the sound pressure level (SPL) of the incident soundwaves. Next,thearticle.outlet surface,a scanninglaser head(PolytecPSV 400) wasto measurevelocityof thetotestThe wholemeasurementchainwas characterizedbypositioneda class 1 level.waves. Next, to the outlet surface, a scanning laser head (Polytec PSV 400) was positioned to measurethe vibration velocity of the test article. The whole measurement chain was characterized by a class 1the vibration velocity of the test article. The whole measurement chain was characterized by a class 1level.level.

Materials 2019, 12, 2998Materials 2019, 12, x FOR PEER REVIEW7 of 237 of 22FigureLaser vibrometryvibrometry testFigure 5.5. Lasertest set-up.set-up.AtAt thethe basebase ofof thisthis acquisition,acquisition, therethere isis aa densedense theoreticaltheoretical backgroundbackground thatthat permitspermits simulatingsimulatingtheSpecifically, underthe vibratingvibrating behaviorbehavior ofof thethe samples,samples, particularlyparticularly itsits ownown naturalnatural frequencies.frequencies. well-defined constrained conditions, i.e., a simply-supported sample, the empirical formula forforcalculatingcalculating thesethese frequenciesfrequencies isis asas follows:follows:r " #D r1 π 2 r2 π 2 ,(1)ωr ωr ,(1)mabwhere r1 and r2 are the modal indices of its modes rth, m is the mass per unit of area, D the bendingwherer1 andarebtheindices of quantitiesits modes ofrththe, m sampleis the massperandunitthickness,of area, Drespectively).the bendingstiffness,and ar2andaremodalthe dimensional(lengthstiffness,and aitandb arethe dimensionalquantitiesof ortheresonancesample (lengthand thickness,respectively).Furthermore,is alsopossibleto calculatethe criticalfrequencyof the singlesample alorresonancefrequencyofthesinglesamplemeans of a second relationship, a function of the speed of sound c (340 m/s) and of the arameters:r2 mωc c(2)2ωc c(2)DFurthermore, associated with these formulas, the law for its associated modes calculation isFurthermore, associated with these formulas, the law for its associated modes calculation isdefined as:defined as: r πy r πx1) 2 sin 1sin(3)ϕr (x,r(x,y) 2sinφy(3)a sin b2.3.2. Sound Transmission Loss Test2.3.2. Sound Transmission Loss TestSound reduction index (SRI) or sound transmission loss (STL) is the most usual product-relatedSound reduction index (SRI) or sound transmission loss (STL) is the most usual product-relatedacoustical quantity determined in laboratory or field conditions. The sound insulation performance inacoustical quantity determined in laboratory or field conditions. The sound insulation performanceterms of sound transmission loss (STL) was assessed according to the ISO 15186 standard, because ofin terms of sound transmission loss (STL) was assessed according to the ISO 15186 standard, becausethe dimension of the test article. Figure 6 shows a schematization of the experimental layout. In thisof the dimensio

Among thermosetting matrices, the use of epoxy resins is increasingly growing. Carbon fiber reinforced composites (CFRCs), manufactured using epoxy matrices, have attracted . Hence, a dispersion assessment is of primary importance for the manufacturing process of composites [8-10]. The work described in this paper regards the manufacturing .