Prospects and challenges of nanomaterial engineered prepregs for improving interlaminar properties of laminated composites––a review

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In the recent years, several investigators are incorporating nanotechnology, one of the most powerful trendsetters in material research, to conventional polymer prepregs to enhance mechanical properties of composite strucutures. The current paper

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  Prospective Article Prospects and challenges of nanomaterial engineered prepregs forimproving interlaminar properties of laminated composites –– a review  A.B.M. Iftekharul Islam  and  Ajit D. Kelkar ,  Department of Nanoengineering, JSNN, North Carolina A&T State University, 2907 E Gate City Blvd,Greensboro, NC-27401, USAAddress all correspondence to A.D. Kelkar at  kelkar@ncat.edu (Received 12 January 2017; accepted 6 March 2017)  Abstract In the recent years, several investigators are incorporating nanotechnology, one of the most powerful trendsetters in material research, toconventional polymer prepregs to enhance mechanical properties of composite strucutures. The current paper outlines the role of nanotech-nology in reinforcing resin and challenges for fabricating nanomaterial reinforced prepregs. As delamination is the most critical problem forcomposite materials, the current study only focuses the application nanotechnology as a possible solution to alleviate delamination problemsin laminated composites. The importance of nanoengineered prepregs is discussed in aviewpoint of improvement in interlaminar properties ofthe laminated composite materials. Introduction For the past three decades use of lightweight composite mate-rials in aerospace, automotive, and for alternate energy appli-cations like wind turbine blades has increased dramatically.However, composite structures fabricated by using prepregseither autoclave or out of autoclave manufacturing technique,is not an easy task and usually involves complicated tempera-ture and/or pressure control. Moreover, most of the prepregmanufacturing process involves resin impregnation into the 󿬁  ber preform. Improper impregnation of resin causes void,resin-rich areas and may affect the performance of the com- posite structure signi 󿬁 cantly. Typically, complicated compos-ite structures are manufactured using prepregs and involvesvery little joining or machining and most of the times resultsinto near neat shape components. Prepregs are usually consid-ered as the best manufacturing option for high-performance polymer composite structures compared with other manufac-turing processes like vacuum assisted resin transfer moldingetc. The  󿬁  ber content achievable using composite prepregis typically very high and is around 65% and void content istypically <0.5% in an out of autoclave prepreg. [1] The trend in current composite research indicates that the further modi 󿬁 -cation of material system with nanomaterials may help inimproving other properties such as interlaminar strengthof lam-inated composites manufactures using prepregs. However, add-ing an extra phase of nanomaterials pose new challenges in thefabrication of laminated composite structures. This paper pro-vides detail review of these challenges and possible solutionsto overcome these challenges. In the future, the nanoengineered  prepregs are likely to play a key role in the composite industry, particularly in the applications where the failures are due tolimitation in the interlaminar strength. The role of nanomaterial for improvinginterlaminar properties of thelaminated composites Literature review indicates that there are three distinct types of resin reinforcement mechanisms that are utilized to improve theinterlaminar properties of the laminated polymer composites(Fig. 1). The most common nanomaterials used are oxide nano- particles. Different oxide nanoparticles mixed with polymer resin have shown signi 󿬁 cantly higher strength as compared with the neat resin interface.Table I shows various nanoparticles that are used to improvethe interlaminar properties. Sometimes, a combination of onemore nanoparticles can give more fracture energy than their additive increment. Hsieh et al. [2] obtained up to 24.5% moreinterlaminar fracture energy with a combination of rubber microparticles and silica nanoparticles as compared with their individual effects on the fracture energy. Additional surfacetreatment of nanoparticles may also exhibit improvement inan interlaminar fracture toughness. [3] One-dimensional nanomaterials are also used to enhanceother mechanical properties of the resin (Table II). The nano 󿬁  bers have larger surface area compared with nano particleswhen comparison is based on the same volume fraction.Hence, the possibility to consume crack initiation energy and crack de 󿬂 ection energy is more in nano 󿬁  bers than nanoparti-cles. Another bene 󿬁 t of nano 󿬁  bers is that its large surface  MRS Communications (2017) , 1 of 7© Materials Research Society, 2017doi:10.1557/mrc.2017.13 MRS COMMUNICATIONS  •  www.mrs.org/mrc  ▪ 1 https://doi.org/10.1557/mrc.2017.13Downloaded from https:/www.cambridge.org/core. IP address: 98.26.206.124, on 01 Apr 2017 at 13:14:21, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms.  area maximizes adhesion or bonding with the matrix [11] and creates  󿬁  ber bridging.Among the nanomaterials, carbon nanotube (CNT) has thehighest mechanical strength. The adaptation of CNT reinforc-ing polymer matrix has been successfully achieved by severalresearchers (Table III). The only limiting factor for CNT iscost and availability of large quantities from the scale up production point of view. Fabrication attempts for nanoengineered prepregs Literature review related to prepregs indicate that very littlework is reported in the area of manufacturing techniques for nanoengineered prepregs. The research efforts for nanoengi-neered hybrid prepregs fabrication are limited to four categories(Fig. 2):1. Nanomaterials with resin (two-phase, nanomaterial, and resin);2. Nanomaterials, mixed with resin and impregnated intoconventional fabric (three phase);3. Nano 󿬁  berlayerssandwichedbetweenconventional prepregs-hybrid prepregs;4. Nanomaterials modi 󿬁 ed conventional prepregs.The most common practice for fabricating nanoengineered  prepreg is to mix nanoparticles with the resin and infuse inthe regular fabric. However, achieving high-volume fractionof nanomaterials for fabricating prepregs is challenging.Liang et al. [26] developed a method for epoxy-infused Bucky paper prepregs. Ogasawara et al. [27] used hot melt method for the prepreg preparation and obtained up to 21 vol% of multi-wall CNT for an epoxy-based prepreg and achieved 2.9 timesultimate tensile strength than that of neat epoxy. Zhouet al. [28] utilized a solution impregnation with a combinationof   󿬁 lament winding method to fabricate unidirectional siliconcarbide (1.5 wt%) nanophase epoxy prepreg and obtained 20% gain in the  󿬂 exural modulus. Chen et al. [29] used solutionimpregnation method for surface modi 󿬁 ed CNT nano prepregand observed better bonding characteristics of CNTs withepoxy. Asarco et al. [30] fabricated prepregs using carbonnanoclay-modi 󿬁 ed carbon  󿬁  ber-phenolic resin by hand-layupand using vacuum bagging technique. Zhou et al. [28] used SiC nanoparticles mixed with epoxy and developed a solutionimpregnation method to develop prepregs using conventionalcarbon fabric. Rahman et al. [12] used carbon nano 󿬁  bersmixed epoxy resin and impregnated the regular carbon  󿬁  bers. No additional literature where semi-curing techniques areused to manufacture solid prepregs is reported. Conventional prepregs are generally fabricated in a continuous style by hot melt or solution impregnation method and both methodsrequire continuous  󿬁  ber or fabric. Nano 󿬁  bers are usually soft and it is dif  󿬁 cult to fabricate prepregs by using a conventional prepreg manufacturing process. Therefore, researchers devel-oped hybrid prepregs by placing nano 󿬁  bers in conjunctionwith regular   󿬁  ber preform. [31,32] . Li et al. [19] used polysulfonenano 󿬁  ber membrane and sandwiched the membrane betweentwo epoxy prepregs. Magniez et al. [18] grew phenoxy nano 󿬁  berson prepregs by electrospinning. Joshi and Dikshit (2011) [23] sprayed nano 󿬁  bers on CRFP prepregs. Garcia et al. [21] rolled conventional prepregs on CNT forest (Fig. 3). Chen et al. [29] dis-solved CNTs by acetone and soaked it on the epoxy prepregs. Figure 1.  Types of nanomaterials used for enhancing inter laminar region of composites. Table I.  Nanoparticles for improving interlaminar properties.Nano/ microparticle typeProperty improvement AuthorAlumina G IC  -74% Kelkar et al. [4] Silica K IC  140%, G IC  300% Johnsen et al. [5] Silicate/glass 40% K IC  Yao [3] Vanadium/ molybdenumDebonding Kotoul andDlouhy [6] Graphene G IC  25% Kamar et al. [7] Rubber G IC  150% Zeng et al. [8] Elastomer 30% interlaminarfracture toughnessHillermeier andSeferis [9] Alumina K IC  100%, G IC  450% Wetzel et al. [10] TiO 2  K IC  70%, G IC  200% 2 ▪  MRS COMMUNICATIONS  •  www.mrs.org/mrc https://doi.org/10.1557/mrc.2017.13Downloaded from https:/www.cambridge.org/core. IP address: 98.26.206.124, on 01 Apr 2017 at 13:14:21, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms.   Nano enhanced prepreg SE 84 Nano ™ marketed by Gurit,as reported, increases 20% compressive strength than their similar product without nano reinforced. [33] Other companieslike Nano-tech, Nanocyl, Zyvex ™ technologies sell nano-engineered resins for improved performance. Most of theattempts of prepreg fabrication reported in the literaturelacksprocessoptimization, thermal,chemical,andmechanicalcharacterizations. Challenges involved in fabrication of nanoengineered prepregs  Fabrication of nanomaterials The  󿬁 rst challenge prior to the fabrication of the nanoengi-neered prepreg is to manufacture suf  󿬁 cient amount of nanoma-terials for commercial production. Use of small diameter high-strength  󿬁  bers obtained using electrospinning has great  potential in the fabrication of nanoengineered prepregs. [34] Other alternatives include use of CNTs. CNTs typically havethe highest strength, but they are relatively very expensivecompared with the electrospun nano 󿬁  bers and therefor thecost. Limits as an alternate material in the fabrication of nano-engineered prepregs. [35]  Incorporation of nanomaterial in to composite system A common approach to incorporate nanomaterials into thecomposite material system is mixing them with the resin.However, this approach may increase the resin viscosity and the proper penetration through the  󿬁  ber becomes dif  󿬁 cult toachieve due to the strong tendency of   󿬁 ne particles to agglom-erate. [36] Mostof the time the agglomerated particles are 󿬁 ltered out by the micro 󿬁  bers of the fabric [34,37] , which leads to poor dispersion. [38] The nanoscale additives may result in lower strengths and lower dynamic stiffness because of improper dis- persion of metal oxides at a nanometer scale. [35] To avoid mixing problem, Falzon et al. [39] grew CNT by chemical vapor deposi-tion method on silicon substrates and transferred to prepregs.Differenttechniqueofapplyingnanomaterialshasbeendescribed in Tables II and III. Chen et al. [17] andZhang et al. [40] used spray-ing technique to deposit CNTs onto micro 󿬁  ber performs toimprove out-of-plane mechanical properties. Researchers alsoworked on depositing nanomaterials directly on the surface of the fabric. [25,41  –  44] The choice of nanomaterial-mixing techniquemay be responsible for different thermal and mechanical proper-ties. [45] Chemical compatibility with matrix materials is also animportant issue [46] for nanomaterials and corresponding resinsystem. The viscosity and resin gel time are two other important factors [47] of liquid resin transfer molding process during the fab-rication of composite materials. Both of these factors regulate theinjectability of the resin and the 󿬁  ber preform wetting process. [48] Seyhan et al. [49] observed a signi 󿬁 cant increase of viscosity for speci 󿬁 c shear rate when the multi-walled carbon nanotubeswere mixed with polyester resin. Therefore, resin with nanomate-rialtechniquemaycreateadditionalcomplexityduringimpregna-tion while fabricating prepregs. Table II.  Nanofibers for improving interlaminar properties.Application method Nanofiber Improvement (%) AuthorMixing with resin CNF Damage initiation energy 52 Rahman et al. [12] Vapor-grown CNF G IC  100 Sadeghian et al. [13] Tetra Ethyl Ortho Silicate (TEOS) ILSS 15 Shinde and Kelkar [14] Using nanofiber layer CNF Fracture toughness 200 Wei [15] CNF ILSS 190 Dhakate et al. [16] CNF ILSS 86 Chen et al. [17] Phenoxy poly (hydroxyetherof bisphenol A) FracturetoughnessModeI150%andModeII30 Magniez et al. [18] Polysulfone G IC  281 Li et al. [19] Direct deposition Polyether ketone cardo (PEK-C) G IC  100 Zhang et al. [20] Table III.  CNTs for improving interlaminar properties.ApplicationmethodProperties improvement AuthorMechanicalattachmentG IC  up to 2.5 timesG IIC  up to 3 times Garcia et al. [21] Spray on fabric/ prepregG 1C  46% Zhang et al. [22] load 26% in Mode-I,38% in mode IIJoshi andDikshit [23] Directly grown Peak strength 85% Carley et al. [24] G IC  more than 300%G IIC  more than 400% Veedu et al. [25] Prospective Article MRS COMMUNICATIONS  •  www.mrs.org/mrc  ▪ 3 https://doi.org/10.1557/mrc.2017.13Downloaded from https:/www.cambridge.org/core. IP address: 98.26.206.124, on 01 Apr 2017 at 13:14:21, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms.  Figure 2.  Different approaches for getting nanoengineered prepreg. Figure 3.  Transfer of CNT on conventional prepreg (Reprinted from Ref. 21 with permission from Elsevier). Table IV.  Change in thermal behavior of resin by addition of nanomaterials.Nanomaterial Resin system Effect of nanomaterial AuthorCarbon Nanofiber mat Epon815C-Epikure3290 Epoxy(TGDDM-DDS)Decreased  Δ  H, Increased Tg,Decreased EaAussawasathien andSancaktar [59] CNT Epoxy SC-1 Increased Tg, Decreased CTEMore thermal stability (TGA)Hosur et al. [60] CNT Epoxy (DGEBA) Increased storage modulus, Increasedloss modulus, Increased Tg,Decreased CTERahman et al. [61] Nanoclay (Cobaltethylhexanoate)Unsaturated polyester (R937-DPE24) Decreased heat of reaction Bensadoun et al. [45] Organoclay, Montmorillonite(MMT)Diglycidyl ether of bisphenol-A withtriethylene tetramine (TETA)Increased thermal stability (TGA)Increased storage modulusVelmurugan andMohan [62] Vapor-grown CarbonnanofiberEpoxy (TGDDM-DDS) Increased Ea, Hindrance effectof carbon fiberXie et al. [58] Organo-montmorillonite(Org-MMT)Diglycidyl ether of bisphenyl A(E-51) with ImidazoleDecreased cure rate Xu et al. [63] 4 ▪  MRS COMMUNICATIONS  •  www.mrs.org/mrc https://doi.org/10.1557/mrc.2017.13Downloaded from https:/www.cambridge.org/core. IP address: 98.26.206.124, on 01 Apr 2017 at 13:14:21, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms.   Presence of residual solvent  The residual solvent, used for dissolving the nanomaterials,may cause void in prepregs. [50] The choice of appropriate sol-vent is also important. Zhang et al. [22] used methanol for CNT dispersion and sprayed it over the fabric. However, for vapor-grown carbon nano 󿬁  ber composites researchers Choiet al. [51] , Patton et al. [52] , Siddiqui et al. [53] reported undesirablechemical effects of ethanol and dimethylformamide [54] on the properties of the fabricated composites. Therefore, a detailstudy pertaining use of solvent during prepregging process isimportant. Optimizing the dispersion density of  interlaminar nano 󿬁   bers to achieve the maximum fracture toughness in laminated composites Along with the many successful attempts, there are plenty of researchers who did not get the optimum interlaminar proper-ties by using nanoparticles and nano 󿬁  bers. The utilization of optimum quantity and dispersion of nano 󿬁  bers or nanoparticlesto improve the fracture toughness of laminated composites is animportant objective during the nanoengineered prepreg fabrica-tion process. White and Sue [55] observed that the use of MWCNT mat improved interlaminar fracture toughness dueto stress redistribution in resin-rich area rather than load trans-fer. Dzenis [56] argued that entangled nano 󿬁  bers can improveinterlaminar fracture resistance much like the hooks and loops in Velcro. Other mechanisms [57] such as de 󿬂 ection, bridging, debonding, etc. may be studied to observe the perfor-mance of nanomaterials reducing delaminations. Thermal characterization of resinsystem with nanomaterials As prepreg comprises a partially cured resin, the curing kineticsis a very important parameter during the prepregging process.The modi 󿬁 cation of epoxy resins with nanoparticles can result in a signi 󿬁 cant shift in thecure kinetics curve involving heat of reaction,  Δ   H  , and activation energy (  E  a ) (Table IV). Even for  the nanomaterial such as carbon nano 󿬁  ber, which is similar to the parent   󿬁  ber material (carbon fabric), the cure kineticsof the resin system can be entirely different. [58] The glass tran-sition temperature ( T  g ) and coef  󿬁 cient of thermal expansion aresome of the other important parameters that can be affected dueto introduction of nano 󿬁  bers. The nano modi 󿬁 ed resin usuallyshows higher glass transition temperatures and higher thermalstability compared with conventional resin. Conclusions This review paper provided state of the art information related to some of the challenges that are involved in the development of nanoengineered prepregs. The literature review clearly indi-catesthat traditional two phase composite material system com- prising  󿬁  ber mats and resin may not be adequate to prevent delaminations in the laminated composites. Obviously, theaddition of nanolayer between the conventional prepregs mayhelp in reducing delaminations. However, this additional nano-layer in between conventional prepregs involves additionalcomplexity. Developing processing technologies to success-fully accommodate nanolayer between the conventional pre- pregs remains to be one of the biggest challenges for asuccessful nano-engineered prepreg product development.Furthermore maintaining the quality of prepreg with an out-standing control of parameters is also very important. All the properties like rheological, thermal and chemical should becharacterized for any material system for designing the fabrica-tion process for the prepreg. The characterization of curingkinetics can play a key role in the development of nanoengi-neered prepregs. For the fabrication of nanoengineered pre- pregs, researchers will have to work over a large domain,from nano to macro. Process improvements, which includeseries of optimization techniques and product characterizationcomprising various mechanical, chemical, and physical testing,would act as a bridge from nanoscale to bulk level to produce asuccessful nanoengineered prepregs.  Acknowledgments This work was performed at the Joint School of Nanoscience and  Nanoengineering, a member of Southeastern NanotechnologyInfrastructure Corridor (SENIC) and National NanotechnologyCoordinated Infrastructure (NNCI), which is supported by National Science foundation (ECCS-1542174). References 1. V. Michaud, S.S. Tavares, A. Sigg, S. Lavanchy, and J.-A.E. Månson: Lowpressure processing of high  󿬁 ber content composites. FPCM8 (2006).Available at http://scholar.googleusercontent.com/scholar?q=cache:ajEMgNTg0ncJ:scholar.google.com/&hl=en&as_sdt=0,34 (accessed April16, 2015).2. T.H. Hsieh, A.J. Kinloch, K. Masania, J. Sohn Lee, A.C. Taylor, andS. Sprenger: The toughness of epoxy polymers and  󿬁 bre compositesmodi 󿬁 ed with rubber microparticles and silica nanoparticles.  J. Mater.Sci  .  45 , 1193 – 1210 (2009).3. X.F. Yao: Dynamic response and fracture characterization of polymer-claynanocomposites with Mode-I crack.  J. Compos. Mater  .  39 , 1487 – 1496(2005).4. A.D. Kelkar, R. Mohan, R. Bolick, and S. Shendokar: Effect of nanoparti-cles andnano 󿬁 bers onModeIfracture toughnessof 󿬁 berglassreinforcedpolymeric matrix composites.  Mater. Sci. Eng. B, Solid-State Mater. Adv.Technol  .  168 , 85 – 89 (2010).5. A.B.Johnsen,A.J.Kinloch,R.D.Mohammed,A.C.Taylor,andS.Sprenger:Toughening mechanisms of nanoparticle-modi 󿬁 ed epoxy polymers. Polymer (Guildf)   48 , 530 – 541 (2007).6. M. Kotoul and I. Dlouhy: Metal particles constraint in glass matrixcomposites and its impact on fracture toughness enhancement.  Mater.Sci. Eng. A  387 – 389 , 404 – 408 (2004).7. N.T. Kamar, M.M. Hossain, A. Khomenko, M. Haq, L.T. Drzal, andA. Loos: Interlaminar reinforcement of glass  󿬁 ber/epoxy compositeswith graphene nanoplatelets.  Compos. A, Appl. Sci. Manuf  .  70 , 82 – 92(2015).8. Y. Zeng, H.Y. Liu, Y.W. Mai, and X.S. Du: Improving interlaminar fracturetoughness of carbon  󿬁 bre/epoxy laminates by incorporation of nano-particles.  Compos. B Eng  .  43 , 90 – 94 (2012).9. R.W. Hillermeier and J.C. Seferis: Interlayer toughening of resin transfermolding composites.  Compos. A, Appl. Sci. Manuf  .  32 , 721 – 729 (2001). Prospective Article MRS COMMUNICATIONS  •  www.mrs.org/mrc  ▪ 5 https://doi.org/10.1557/mrc.2017.13Downloaded from https:/www.cambridge.org/core. IP address: 98.26.206.124, on 01 Apr 2017 at 13:14:21, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms.
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