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Effect of nanoparticles and nanofibers on Mode I fracture toughness of fiber glass reinforced polymeric matrix composites
Materials Science and Engineering B 168 (2010) 85–89
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Effect of nanoparticles and nanofibers on Mode I fracture toughness of fiber glass reinforced polymeric matrix composites Ajit D. Kelkar a,b,∗ , Ram Mohan a,b , Ronnie Bolick a,b , Sachin Shendokar b,c a b c
Computational Science and Engineering, North Carolina A&T State University, Greensboro, NC 27411, USA Center for Advanced Materials and Smart Structures, North Carolina A&T State University, Greensboro, NC 27411, USA Mechanical Engineering Department, North Carolina A&T State University, Greensboro, NC 27411, USA
a r t i c l e
i n f o
Article history: Received 28 October 2009 Received in revised form 21 December 2009 Accepted 7 January 2010 Keywords: Nanocomposites Electrospinning Alumina nanoparticles Tetra ethyl orthosilicate (TEOS) nanofibers Interlaminar properties
a b s t r a c t In the recent past, the research involving the fabrication and processing of reinforced polymer nanocomposites has increased significantly. These new materials are enabling in the discovery, development and incorporation of improved nanocomposite materials with effective manufacturing methodologies for several defense and industrial applications. These materials eventually will allow the full utilization of nanocomposites in not only reinforcing applications but also in multifunctional applications where sensing and the unique optical, thermal, electrical and magnetic properties of nanoparticles can be combined with mechanical reinforcement to offer the greatest opportunities for significant advances in material design and function. This paper presents two methods and material systems for processing and integration of the nanomaterial constituents, namely: (a) dispersing alumina nanoparticles using high energy mixing (using ultrasonication, high shear mixing and pulverization) and (b) electrospinning technique to manufacture nanofibers. These reinforced polymer nanocomposites and the processing methodologies are likely to provide effective means of improving the interlaminar properties of woven fiber glass composites compared to the traditional methods such as stitching and Z-pinning. The electrospinning technology relies on the creation of nanofibers with improved molecular orientation with reduced concentration of fiber imperfections and crystal defects. Electrospinning process utilizes surface tension effects created by electrostatic forces acting on liquid droplets, creating numerous nanofibers. These nanofibers thus have potential to serve as through-the-thickness reinforcing agents in woven composites. While the electrospun nanofibers provide bridging through-the-thickness reinforcement, the use of the nanoparticles influences the thermo-physical properties and provides an effective means from commercially available nanolevel material configurations to form reinforced polymer nanocomposites. Studies indicate that their mechanical behavior and performance however depends on incorporation, and functionalization of these alumina nanoparticles. Both these methods and material systems provide effective means for integrating the nanomaterial constituents into traditional fiber composite systems. In particular, this paper discusses the experimental study of the processing and delamination (interlaminar failure) characteristics (via Mode I fracture toughness assessment) of reinforced composite systems. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Glass fiber reinforced composites have high temperature resistance and low coefficient of hygrothermal expansion. The multitude of fabric architectures of glass fibers such as the unidirectional (UD), multidirectional (MD), woven (WF), knitted, stitched, Z-pinned and braided fabrics are suitable for various applications in the area of aerospace, naval, defense and energy structures.
∗ Corresponding author at: Computational Science and Engineering, North Carolina A&T State University, 301 Fort IRC Building, Greensboro, NC 27411, USA. Tel.: +1 336 334 7255/7437; fax: +1 336 256 1247. E-mail addresses: [email protected], [email protected] (A.D. Kelkar). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.01.015
Different processing technologies such as the hand lay-up, filament winding, compression molding, pultrusion, autoclave, Resin Transfer Molding (RTM), Vacuum Assisted Resin Transfer Molding (VARTM), and Sheet Molding Compound (SMC) are regularly used for the manufacturing of high quality fiber glass epoxy composites suitable for variety of applications [1]. These manufacturing processes have an integral problem of delamination or interlaminar failure. Delamination [2] occurs in laminated composite structures when debonding of plies occurs. In polymer matrix laminated composites, bonding between the plies of glass fibers is provided by epoxy resin which is a weaker component. Woven composites with through-the-thickness reinforcements such as stitching or Z-pinning are expected to provide good properties not only in mutually orthogonal directions but also in the transverse direction
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A.D. Kelkar et al. / Materials Science and Engineering B 168 (2010) 85–89
[3]. It is also expected to have better fatigue and impact resistance due to interlacing in the case of stitched or Z-pinned composites. But through-the-thickness interlacing complicates the stress analysis and makes it more difficult to design composites with confidence. More over, stitching and Z-pinning results in the damage of glass fiber tows, which degrades the tensile properties in composite [4]. A detailed analysis of relationship between failure mechanisms and physical properties of stitched and unstitched laminates is discussed by Cox and Mouritz [5]. Other methods to resist and enhance the delamination characteristics are by matrix toughening and modification of interlaminar interfaces. There is an excellent research effort that refers to improvement in the mechanical properties of conventional fiber reinforced plastics (FRP) using secondary interlaminar reinforcement of Non-Woven Tissue (NWT) [6]. In addition, our prior work on the performance of the composites with secondary reinforcement using TEOS electrospun nanofibers at ply interfaces has resulted in 30% improvement in short beam shear strength compared to neat composites without the nanofiber preform interface layers [7]. The present paper investigates the use of alumina nanoparticles and secondary reinforcement using electrospun nanofibers at ply interfaces. VARTM based processes have been effectively demonstrated for the manufacture of polymer matrix based glass fiber reinforced composites integrated with nanomaterials at the ply interfaces. This paper focuses on the experimental research efforts for the fabrication and performance characteristics of these composite systems. In particular, the focus is on composite systems with the inclusion of (i) Alumina nanoparticles at ply interfaces using fiber modification. (ii) Tetra ethyl orthosilicate (TEOS) nanofibers at ply interfaces as secondary reinforcement. Fig. 1. Electrospinning: (a) schematic and (b) laboratory setup.
In both of these approaches some form of epoxy resin with reinforcement of S2 glass fibers was used. The same method for manufacturing based on VARTM and performance evaluation was utilized. Thus the common factors that forms the basis of comparison for the above-mentioned composites are (i) (ii) (iii) (iv)
Use of epoxy polymers. Glass fiber as primary reinforcement. Manufacturing of composite using VARTM. Characterization using ASTM 5528 Mode I fracture toughness test standard.
spinneret and collector plate from Glassman High Voltage Inc. with a voltage range 0–30 kV; and the solution dispensing pump manufactured by New Era Pump Systems Inc. with a capacity to hold 30 ml syringe with a controllable flow rate within the range of 0.001–10 ml/h. Dispensing pump controls the rate of TEOS sol–gel discharge at the spinneret tip. Discharge rate is adjusted depending upon the viscosity of the solution and voltage applied during the process. At low viscosity and voltage, discharge rate is low usually kept at 0.01 ml/min. 2.2. Materials system
2. Experimental work 2.1. Electrospinning setup for spinning TEOS nanofibers Electrospinning is a non-contact drawing process in which a solution droplet emanating from the tip of spinneret is attracted towards a grounded collector under the action of electrical potential difference applied [8–11]. The electro-hydrodynamic forces cause the droplet to elongate under bending instability and whipping to produce fibers of nano-scale diameter (nanofibers) with exceptionally long lengths. Evaporation of solvents takes place as the nanofibers are deposited on a grounded collector. Fig. 1(a) shows the schematics of the electrospinning setup whereas Fig. 1(b) shows the actual laboratory electrospinning setup. This setup has four operating components involved actively during the process of electrospinning: the spinneret which is kept at positive potential; the collector plate that is grounded; the high voltage supply that maintains the potential difference between
2.2.1. TEOS sol–gel formulation The TEOS sol–gel formulation has been adopted from Ref. [12], which has combination of TEOS, ethanol, deionized water and HCl. The discharge rate and the success of the electrospinning process were found to depend heavily on the ambient conditions. The ambient temperature and humidity conditions influence the viscosity and discharge rate of the sol–gel. It is observed that if aging of TEOS sol–gel is performed under appropriate ambient conditions of temperature and humidity (a temperature of 22.2 ◦ C/72 F; humidity 44%), the low discharge rate of 0.01 ml/min was found to be sufficient to get a full deposition of electrospun fibers on to a 9 in. × 16 in. fiber glass sheet in about an hour. As the solution from dispensing pump approaches the tip of spinneret, solution droplet gets charged on its surface. The forces acting on the droplet at the tip are hydrostatic surface tension and surface charge due to potential difference applied [13]. As the tip is at a positive potential, the corresponding surface charge is positive, and this charge causes repulsive force counter acting the surface tension. Due to applied electric field, this
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Fig. 3. Schematic of the fiber modification process for alumina nanoparticle integration.
2.3. Processing of composites
Fig. 2. SEM micrograph of TEOS nanofibers.
interaction of surface charge and surface tension causes the spherical droplet to stretch into a conical shape called “Taylor Cone” [14]. The tip of Taylor cone has the lesser surface area but is under the influence of same voltage potential. Thus the tip of cone is elongated into a charged jet which on further increase in length and reduction of diameter experiences “bending instability” [15]. Bending instability is caused by the non-linear characteristics of electric charge and dynamics of fiber jet. Under the action of bending instability, the charged jet undergoes whipping that causes further elongation and reduction in diameter of fiber with evaporation of solvents. Fig. 2 shows representative SEM micrograph of the TEOS nanofibers produced by the electrospinning process. A voltage of 18 kV and a distance of 100 mm between the tip of the spinneret and the collector plate were found to produce the minimum diameter fibers [16–18].
2.2.2. Alumina nanoparticle composite system The three constituent materials used in the fabrication of the composites integrated with alumina nanoparticles are polymeric resin (including the curing agent), woven fiber glass and 110 nm diameter alumina nanoparticles. A silane coupling agent is used to treat the alumina nanoparticles to improve the adhesive potential of the alumina nanoparticles with the epoxy resin and fiber glass. In the past, the use of alumina nanoparticles for improvement in mechanical properties of vinyl ester resin has been demonstrated using functionalization [19]. The resin used is a proprietary EponTM 9504 epoxy resin system (proprietary modified bisphenol-A resin) and curing agent EpikureTM 9554 formulation manufactured by Hexion Specialty Chemicals for the matrix material component. The formulation is a low viscosity (about 350 cP at room temperature) system with a gel time of about 4 h for ease of impregnation of fiber plies and a fast infusion time. The fiber fabric used is a high tow-density 10 K plain bidirectional woven fabric S2 fiber glass (supplied by BGF Industries Inc.) with a volumetric density of 2.15 g/cm3 and aerial density of 0.082 g/cm2 . Alumina particles used are Dispal D23 sized at 110 nm manufactured by Sasol Inc. It is boehmite-alumina (80% Al2 O3 , 1.6% NO3 , ∼18.4% H2 O with traces of (<0.002%) Na2 O) and has a crystallite lattice structure {1 2 0} with a crystallite size of 9 nm. It is hydrophilic and dispersible grade with a water dispersibility of 98%. The silane functionalizing agent used is Tris-2-metoxyethoxy vinyl silane (T2MEVS).
2.3.1. Processing of alumina interlaced fiber glass composites The dispersed deposition of nanomaterial constituents provides an effective means of their integration to form advanced composite systems. Two different methodologies based on resin and fiber modification that utilized alumina nanoparticles have been recently employed in conjunction with conventional VARTM process for the composite laminates. Both resin and fiber modification has been shown to influence the interlaminar delamination characteristics of the formed composites. Further discussions are available in Refs. [20,21]. Fiber modification introduces an interface layer of alumina nanoparticles. This is similar to the presence of interface layer of electrospun fibers that is focused in this paper. Processing of these fiber modified alumina nanoparticulate composite system is discussed next.
2.3.2. Interface alumina nanoparticle integration by fiber modification The fiber modification involves the dispersion of alumina onto the surface of the fabric. Alumina nanoparticles in the amount of 2% by weight of the targeted infusion resin are dispersed on to the interfacing plies of the fiber glass fabric. The alumina particles are dissolved in a solvent of water and methanol (50:50 mix ratios) before it is sprayed by an atomizer on to the fabric surface. Alumina particles are also treated with silane agent before dispersion onto the fabric surface by a similar process with the addition of the silane agent to the polar solvent mixture of water and methanol. The solvent/transfer agent is removed by heating the coated fabric in an oven to a temperature of 200 F, prior to the fabrication of the composite laminates. Fig. 3 presents schematically the steps involved in the fiber modification process during the processing of glass fiber–polymer matrix composites with alumina nanoparticulates at the ply interface. Composite panel manufacturing is performed using the traditional low cost VARTM technique using epoxy resin without alumina modification. Composite panels for three different material configurations were fabricated for all the characterization tests. The three material configurations are based on the type of alumina particle interface layer (with and without functionalization) and composite without alumina nanoparticle interface layer as shown in Table 1.
Table 1 Material nomenclature for alumina hybrid composite material.a Material
Configuration
MAT 1 MAT 2 MAT 3
Baseline: EponTM /S2 glass Fiber modification with pristine alumina Fiber modification with functionalized alumina
a Resin System is EponTM 9504 (proprietary modified bisphenol-A resin) and curing agent EpikureTM 9554.
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2.3.3. Processing of fiber glass Epon composite with TEOS nanofibers The effect of Ultrathin Fibrous Sheets (UFS) on Mode I fracture toughness has been studied by incorporating UFS at various layers in addition to mid-plane [22]. The present work focuses on the use of TEOS nanofibers as interface layers. In the present work, material system for composite panels was 10 layers of glass fiber with TEOS nanofibers only at the critical ply interface (mid-plane) as this is the delamination and crack propagation interface during the double cantilever beam tests. TEOS nanofibers weighing nearly 1 g were placed at this mid-plane. Epon 862 with curing agent W in the ratio of 100:26.4 was used as the resin system. Resin was flown through ply lay-up using a H-VARTM setup [23]. The use of the conventional manufacturing process with the additional electrospun nanofiber interface layers was found to require substantial modifications and refinement. Comprehensive discussion on methodology for incorporating electrospun fibers into prepeg laminates to improve delamination resistance of composite is described by Dzenis [24]. Various polymers considered by Dzenis for electrospinning does not include TEOS nanofibers. Also, the use of VARTM processing with S2 glass fibers poses different set of problems. Few problems those were rectified after series of empirical experimentation are listed here. One of the foremost problems encountered during the composite processing by VARTM process when electrospun nanofibers are added at the ply interfaces is that of dry areas or non-wetting of the nanofibers. If nanofibers do not get sufficient resin to soak, the dry spots created at the interfaces will act as microcracks degrading the interlaminar properties. This dryness of the interface nanolayers is due to the difficulty during resin infusion in regular VARTM caused by the additional permeation resistance created by compressed electrospun nanofibers. To eliminate these problems, interlaminar electrospun fibers were presoaked in Epon 862 and curing agent W solution when they are laid up. In addition, the use of H-VARTM setup enables to increase temperature of the mold to 110 ◦ F during resin infusion. At 110 ◦ F the viscosity of Epon 862 was found to be low enough to ensure complete soaking of electrospun fibers and removal of excessive resin. One more problem that is typically associated with secondary reinforcement of ply interfaces is the potential increase in the thickness of the composite panels. This problem was eliminated by selecting thin layers of TEOS electrospun nanofiber sheet for interfacial reinforcement. Typical diameters of the TEOS nanofibers on the selected sheets for reinforcement were on an average 500 nm. TEOS electrospun fiber sheets were sintered at 900 ◦ C before they are laid up at glass fiber sheet interfaces. Sintering of TEOS nanofibers resulted in the reduction in fiber diameter as well as improvement in the porosity in the random electrospun fabric [25]. 2.4. Characterization 2.4.1. Fracture toughness characterization The double cantilever beam test coupons were prepared according to ASTM 5528-94a specimen specifications from the composites
Table 2 Average GIC values for composites with electrospun nanofibers.b Specimen
Neat resin
Espun
1 2 3 4 5
796.85 1177.86 1150.54 1144.25 1184.18
987.42 935.47 1038.33 972.56 889.89
Avg. GIC Std. Dev.
1090.74 147.73
964.73 49.9
GIC values are in J/m2 . b Resin System is Epon 862 with curing agent W in the ratio of 100:26.4.
in both the cases. The schematic of DCB specimen with electrospun nanofibers at mid-plane interface is shown in Fig. 4. Double cantilever beam (DCB) ASTM D5528 Mode I interlaminar fracture test was conducted for all the composite material specimens using a 445 N maximum load MTS machine in conjunction with displacement control mode. The maximum displacement was 63.5 mm, which was the test limit. Test displacement rate is 0.1016 mm/min (0.04 in./min). A Teflon insert 0.0005 in thick was used to emulate the initial crack size during the fabrication. The Mode I fracture toughness GIC was obtained using the Modified Beam Theory (MBT) as shown below in Eq. (1). GIC =
3Pı 3Pı = 2ba∗ 2b(a + ||)
(1)
where P = load; ı = displacement; b = width; a = delamination length; = rotation and shear correction. 3. Results and discussions The average Mode I fracture toughness GIC values in J/m2 computed for the specimen with and without nanofibers referred as neat and electrospun, respectively are presented in Table 2. The corresponding Mode I fracture toughness GIC values for the composites with alumina nanoparticles formed with fiber modification are presented in Table 3. It should be noted here that resin system Epon 862 is used for making DCB specimens with electrospun nanofibers at the critical interface of S2 glass fiber plies, where as in case of alumina nanoparticle reinforcement with fiber modification uses Epon 9504 resin. As these two different resin systems are used, there is large difference in the GIC values for the two systems. The test data presented in Table 3 indicates that the presence of interface alumina particulate layers resulted in an increase in the Mode I fracture toughness values thus improving the delamination resistance characteristics of the composite as compared to the two phase composite without alumina nanoparticles. The Mode I fracture toughness values presented are based on specimens that did not show any fiber bridging. As seen in Table 3, functionalized alumina (MAT 3) showed significant improvement compared to the non-functionalized form. When neat Epon and S2 glass fiber specimens are compared with the alumina interface layered composites, the test data indicates that there has been over 51% improvement in Table 3 Average GIC values for hybrid composites with alumina nanoparticles.c
Fig. 4. ASTM 5525-94a double cantilever beam test specimen with Teflon inserts and electrospun nanofibers at mid-plane.
Specimen
Avg. GIC
Mat 1 Mat 2 Mat 3
290.0 ± 5.61% 440.0 ± 10.78% 506.0 ± 7.96%
GIC values are in J/m2 . c Resin System is EponTM 9504 (proprietary modified bisphenol-A resin) and curing agent EpikureTM 9554.
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GIC for the case of non-functionalized alumina. When it is compared with functionalized alumina interface layers, the improvement of GIC is found to be above 74%. This clearly indicates that the use of alumina nanoparticles leads to significant improvement of Mode I fracture toughness of glass fiber epoxy composite laminates. For glass fiber composites with electrospun nanofibers, there is however a small reduction in the GIC value by about 5% than the neat resin specimens without the electrospun interface layers. Though this is a small change, these results indicate a reduction in Mode I fracture toughness values contrary to the expectation. This could potentially be due to the fact that the amount of electrospun nanofibers layers was not sufficient to make any significant difference in the Mode I fracture toughness characteristics. Weight of secondary reinforcement of TEOS nanofiber used at mid-plane was nearly 1 g only (roughly about 0.2 weight percentage of the composite panel), significantly less than the resin and glass fiber content. Load sensitivity for the small amount and thickness of TEOS electrospun nanofibers layer present at the critical ply interface is planned to be further investigated using less S2 glass layers or by using thin S2 glass prepeg material to give a higher weight percentage of nanofiber reinforcement without significantly increasing the thickness. 4. Conclusions Composites formed with integration of nanomaterial constituents via conventional manufacturing processes have been demonstrated for two material systems. They involved the use of alumina nanoparticles and electrospun nanofibers. Though two different sources of nanomaterials have been employed, both the forms have provided an effective means of nanomaterial integration for the advanced composite material systems. The addition of alumina nanoparticles at the interface layers was demonstrated to have significant effect on the Mode I fracture toughness characteristics. The results indicate a 51% increase in the Mode I fracture toughness values of non-functionalized alumina interlaced composite over the conventional two phase composite system. This increase was nearly 74% with the functionalization of the alumina nanoparticles as demonstrated in the present paper. The electrospinning process provides an effective means of processing nanofibers and the associated advanced composites through the conventional VARTM process and its variant H-VARTM. The double cantilever beam test results for the advanced composites with the electrospun nanofiber interface layers showed a slight decrease in the Mode I fracture toughness values compared to the baseline two phase composite. This could potentially be attributed
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to the very low quantity of the nanofiber layers that is acting as impediment for the interface bonding due to improper wetting compaction. Further improvements in Mode I fracture toughness can be achieved by improving wettability and bonding characteristics through the use of B-staged nanofiber film layers. Acknowledgement This research was sponsored by Office of Naval Research. References [1] A. Haque, M. Shamsuzzoha, F. Hussain, D. Dean, J. Comp. Mater. 37 (20) (2003). [2] C.E. Browning, H.S. Schwartz, in: J.M. Whitney (Ed.), Composite Materials: Testing and Design (Seventh Conference), ASTM STP 893, Philadelphia, 1986, pp. 256–265. [3] M.V. Hosur, A. Abraham, S. Jeelani, U.K. Vaidya, J. Comp. Mater. 35 (12) (2001). [4] S. Mall, D.W. Katwy, R.L. Bolick, A.D. Kelkar, D.C. Davis, J. Comp. Struct. 90 (2009) 201. [5] B.N. Cox, A.P. Mouritz, Composites: Part A 31 (2000) 1–27. [6] S.H. Lee, J.H. Lee, S.K. Cheong, H. Noguchid, J. Mater. Process. Technol. 207 (2008) 21–29. [7] A.D. Kelkar, R.V. Mohan, R. Bolick, S.M. Shendokar, Adv. Mater. Res. 47–50 (2008) 1031. [8] A. Formhals, US Patent 1,975,504. (1934). [9] D.H. Rheneker, J. Doshi, J. Electrostat. 35 (1995) 151–160. [10] Z.M. Huang, Y.Z. Zhangb, M. Kotaki, S. Ramakrishna, J. Comp. Sci. Technol. 63 (2003) 2223–2253. [11] D. Li, Y. Xia, Adv. Mater. 16 (14) (2004) 1151–1170. [12] C.J. Brinker, G.W. Scherer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, 1990, 97–233. [13] Y.M. Shin, M.M. Hohman, M.P. Brenner, G.C. Rutledge, Polymer 42 (2001) 9955–9967. [14] G.I. Taylor, Proc. R. Soc. Lond., Ser. A (1964) 280. [15] A.L. Yarin, S. Koombhongse, D.H. Reneker, J. Appl. Phys. 89 (5) (2001) 3018. [16] A.D. Kelkar, R.V. Mohan, R. Bolick, S.M. Shendokar, IMECE 2009-12188 2009 ASME International Mechanical Engineering Congress and Exposition IMECE, Orlando, 2009. [17] J.M. Deitzel, J. Kleinmeyer, D. Harris, N.C. Tan, Polymer 42 (2001) 261–272. [18] X. Zong, K. Kim, D. Fang, S. Ran, Polymer 43 (2001) 4403–4412. [19] Z. Guo, T. Pereira, O. Choi, Y. Wang, H.T. Hahn, J. Mater. Chem 16 (2006) 2800–2808. [20] O. Akinyede, R.V. Mohan, A.D. Kelkar, J. Sankar, J. Comp. Mater. 43 (7) (2009) 769–781. [21] O. Akinyede, R.V. Mohan, A.D. Kelkar, J. Sankar, Proceedings of International SAMPE Symposium and Exhibition, SAMPE, 50, 2005, pp. 440-448. [22] L. Liu, Y.M. Liang, G.Y. Xu, H.S. Zhang, Z.M. Huang, J. Reinforced Plast. Comp. (2008). [23] A.D. Kelkar, J.S. Tate, R. Bolick, Proceedings of the 2003 American Society of Engineering Education Annual Conference and Exposition Session, 2003, p. 1482. [24] Y. Dzenis, US Patent 6,265,333 (2001). [25] A.D. Kelkar, R.V. Mohan, R. Bolick, S.M. Shendokar, IMECE 2008-68033 2008 ASME International Mechanical Engineering Congress and Exposition IMECE, Boston, 2008.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Effect of nanoparticles and nanofibers on Mode I fracture toughness of fiber glass reinforced polymeric matrix composites Ajit D. Kelkar a,b,∗ , Ram Mohan a,b , Ronnie Bolick a,b , Sachin Shendokar b,c a b c
Computational Science and Engineering, North Carolina A&T State University, Greensboro, NC 27411, USA Center for Advanced Materials and Smart Structures, North Carolina A&T State University, Greensboro, NC 27411, USA Mechanical Engineering Department, North Carolina A&T State University, Greensboro, NC 27411, USA
a r t i c l e
i n f o
Article history: Received 28 October 2009 Received in revised form 21 December 2009 Accepted 7 January 2010 Keywords: Nanocomposites Electrospinning Alumina nanoparticles Tetra ethyl orthosilicate (TEOS) nanofibers Interlaminar properties
a b s t r a c t In the recent past, the research involving the fabrication and processing of reinforced polymer nanocomposites has increased significantly. These new materials are enabling in the discovery, development and incorporation of improved nanocomposite materials with effective manufacturing methodologies for several defense and industrial applications. These materials eventually will allow the full utilization of nanocomposites in not only reinforcing applications but also in multifunctional applications where sensing and the unique optical, thermal, electrical and magnetic properties of nanoparticles can be combined with mechanical reinforcement to offer the greatest opportunities for significant advances in material design and function. This paper presents two methods and material systems for processing and integration of the nanomaterial constituents, namely: (a) dispersing alumina nanoparticles using high energy mixing (using ultrasonication, high shear mixing and pulverization) and (b) electrospinning technique to manufacture nanofibers. These reinforced polymer nanocomposites and the processing methodologies are likely to provide effective means of improving the interlaminar properties of woven fiber glass composites compared to the traditional methods such as stitching and Z-pinning. The electrospinning technology relies on the creation of nanofibers with improved molecular orientation with reduced concentration of fiber imperfections and crystal defects. Electrospinning process utilizes surface tension effects created by electrostatic forces acting on liquid droplets, creating numerous nanofibers. These nanofibers thus have potential to serve as through-the-thickness reinforcing agents in woven composites. While the electrospun nanofibers provide bridging through-the-thickness reinforcement, the use of the nanoparticles influences the thermo-physical properties and provides an effective means from commercially available nanolevel material configurations to form reinforced polymer nanocomposites. Studies indicate that their mechanical behavior and performance however depends on incorporation, and functionalization of these alumina nanoparticles. Both these methods and material systems provide effective means for integrating the nanomaterial constituents into traditional fiber composite systems. In particular, this paper discusses the experimental study of the processing and delamination (interlaminar failure) characteristics (via Mode I fracture toughness assessment) of reinforced composite systems. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Glass fiber reinforced composites have high temperature resistance and low coefficient of hygrothermal expansion. The multitude of fabric architectures of glass fibers such as the unidirectional (UD), multidirectional (MD), woven (WF), knitted, stitched, Z-pinned and braided fabrics are suitable for various applications in the area of aerospace, naval, defense and energy structures.
∗ Corresponding author at: Computational Science and Engineering, North Carolina A&T State University, 301 Fort IRC Building, Greensboro, NC 27411, USA. Tel.: +1 336 334 7255/7437; fax: +1 336 256 1247. E-mail addresses: [email protected], [email protected] (A.D. Kelkar). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.01.015
Different processing technologies such as the hand lay-up, filament winding, compression molding, pultrusion, autoclave, Resin Transfer Molding (RTM), Vacuum Assisted Resin Transfer Molding (VARTM), and Sheet Molding Compound (SMC) are regularly used for the manufacturing of high quality fiber glass epoxy composites suitable for variety of applications [1]. These manufacturing processes have an integral problem of delamination or interlaminar failure. Delamination [2] occurs in laminated composite structures when debonding of plies occurs. In polymer matrix laminated composites, bonding between the plies of glass fibers is provided by epoxy resin which is a weaker component. Woven composites with through-the-thickness reinforcements such as stitching or Z-pinning are expected to provide good properties not only in mutually orthogonal directions but also in the transverse direction
86
A.D. Kelkar et al. / Materials Science and Engineering B 168 (2010) 85–89
[3]. It is also expected to have better fatigue and impact resistance due to interlacing in the case of stitched or Z-pinned composites. But through-the-thickness interlacing complicates the stress analysis and makes it more difficult to design composites with confidence. More over, stitching and Z-pinning results in the damage of glass fiber tows, which degrades the tensile properties in composite [4]. A detailed analysis of relationship between failure mechanisms and physical properties of stitched and unstitched laminates is discussed by Cox and Mouritz [5]. Other methods to resist and enhance the delamination characteristics are by matrix toughening and modification of interlaminar interfaces. There is an excellent research effort that refers to improvement in the mechanical properties of conventional fiber reinforced plastics (FRP) using secondary interlaminar reinforcement of Non-Woven Tissue (NWT) [6]. In addition, our prior work on the performance of the composites with secondary reinforcement using TEOS electrospun nanofibers at ply interfaces has resulted in 30% improvement in short beam shear strength compared to neat composites without the nanofiber preform interface layers [7]. The present paper investigates the use of alumina nanoparticles and secondary reinforcement using electrospun nanofibers at ply interfaces. VARTM based processes have been effectively demonstrated for the manufacture of polymer matrix based glass fiber reinforced composites integrated with nanomaterials at the ply interfaces. This paper focuses on the experimental research efforts for the fabrication and performance characteristics of these composite systems. In particular, the focus is on composite systems with the inclusion of (i) Alumina nanoparticles at ply interfaces using fiber modification. (ii) Tetra ethyl orthosilicate (TEOS) nanofibers at ply interfaces as secondary reinforcement. Fig. 1. Electrospinning: (a) schematic and (b) laboratory setup.
In both of these approaches some form of epoxy resin with reinforcement of S2 glass fibers was used. The same method for manufacturing based on VARTM and performance evaluation was utilized. Thus the common factors that forms the basis of comparison for the above-mentioned composites are (i) (ii) (iii) (iv)
Use of epoxy polymers. Glass fiber as primary reinforcement. Manufacturing of composite using VARTM. Characterization using ASTM 5528 Mode I fracture toughness test standard.
spinneret and collector plate from Glassman High Voltage Inc. with a voltage range 0–30 kV; and the solution dispensing pump manufactured by New Era Pump Systems Inc. with a capacity to hold 30 ml syringe with a controllable flow rate within the range of 0.001–10 ml/h. Dispensing pump controls the rate of TEOS sol–gel discharge at the spinneret tip. Discharge rate is adjusted depending upon the viscosity of the solution and voltage applied during the process. At low viscosity and voltage, discharge rate is low usually kept at 0.01 ml/min. 2.2. Materials system
2. Experimental work 2.1. Electrospinning setup for spinning TEOS nanofibers Electrospinning is a non-contact drawing process in which a solution droplet emanating from the tip of spinneret is attracted towards a grounded collector under the action of electrical potential difference applied [8–11]. The electro-hydrodynamic forces cause the droplet to elongate under bending instability and whipping to produce fibers of nano-scale diameter (nanofibers) with exceptionally long lengths. Evaporation of solvents takes place as the nanofibers are deposited on a grounded collector. Fig. 1(a) shows the schematics of the electrospinning setup whereas Fig. 1(b) shows the actual laboratory electrospinning setup. This setup has four operating components involved actively during the process of electrospinning: the spinneret which is kept at positive potential; the collector plate that is grounded; the high voltage supply that maintains the potential difference between
2.2.1. TEOS sol–gel formulation The TEOS sol–gel formulation has been adopted from Ref. [12], which has combination of TEOS, ethanol, deionized water and HCl. The discharge rate and the success of the electrospinning process were found to depend heavily on the ambient conditions. The ambient temperature and humidity conditions influence the viscosity and discharge rate of the sol–gel. It is observed that if aging of TEOS sol–gel is performed under appropriate ambient conditions of temperature and humidity (a temperature of 22.2 ◦ C/72 F; humidity 44%), the low discharge rate of 0.01 ml/min was found to be sufficient to get a full deposition of electrospun fibers on to a 9 in. × 16 in. fiber glass sheet in about an hour. As the solution from dispensing pump approaches the tip of spinneret, solution droplet gets charged on its surface. The forces acting on the droplet at the tip are hydrostatic surface tension and surface charge due to potential difference applied [13]. As the tip is at a positive potential, the corresponding surface charge is positive, and this charge causes repulsive force counter acting the surface tension. Due to applied electric field, this
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Fig. 3. Schematic of the fiber modification process for alumina nanoparticle integration.
2.3. Processing of composites
Fig. 2. SEM micrograph of TEOS nanofibers.
interaction of surface charge and surface tension causes the spherical droplet to stretch into a conical shape called “Taylor Cone” [14]. The tip of Taylor cone has the lesser surface area but is under the influence of same voltage potential. Thus the tip of cone is elongated into a charged jet which on further increase in length and reduction of diameter experiences “bending instability” [15]. Bending instability is caused by the non-linear characteristics of electric charge and dynamics of fiber jet. Under the action of bending instability, the charged jet undergoes whipping that causes further elongation and reduction in diameter of fiber with evaporation of solvents. Fig. 2 shows representative SEM micrograph of the TEOS nanofibers produced by the electrospinning process. A voltage of 18 kV and a distance of 100 mm between the tip of the spinneret and the collector plate were found to produce the minimum diameter fibers [16–18].
2.2.2. Alumina nanoparticle composite system The three constituent materials used in the fabrication of the composites integrated with alumina nanoparticles are polymeric resin (including the curing agent), woven fiber glass and 110 nm diameter alumina nanoparticles. A silane coupling agent is used to treat the alumina nanoparticles to improve the adhesive potential of the alumina nanoparticles with the epoxy resin and fiber glass. In the past, the use of alumina nanoparticles for improvement in mechanical properties of vinyl ester resin has been demonstrated using functionalization [19]. The resin used is a proprietary EponTM 9504 epoxy resin system (proprietary modified bisphenol-A resin) and curing agent EpikureTM 9554 formulation manufactured by Hexion Specialty Chemicals for the matrix material component. The formulation is a low viscosity (about 350 cP at room temperature) system with a gel time of about 4 h for ease of impregnation of fiber plies and a fast infusion time. The fiber fabric used is a high tow-density 10 K plain bidirectional woven fabric S2 fiber glass (supplied by BGF Industries Inc.) with a volumetric density of 2.15 g/cm3 and aerial density of 0.082 g/cm2 . Alumina particles used are Dispal D23 sized at 110 nm manufactured by Sasol Inc. It is boehmite-alumina (80% Al2 O3 , 1.6% NO3 , ∼18.4% H2 O with traces of (<0.002%) Na2 O) and has a crystallite lattice structure {1 2 0} with a crystallite size of 9 nm. It is hydrophilic and dispersible grade with a water dispersibility of 98%. The silane functionalizing agent used is Tris-2-metoxyethoxy vinyl silane (T2MEVS).
2.3.1. Processing of alumina interlaced fiber glass composites The dispersed deposition of nanomaterial constituents provides an effective means of their integration to form advanced composite systems. Two different methodologies based on resin and fiber modification that utilized alumina nanoparticles have been recently employed in conjunction with conventional VARTM process for the composite laminates. Both resin and fiber modification has been shown to influence the interlaminar delamination characteristics of the formed composites. Further discussions are available in Refs. [20,21]. Fiber modification introduces an interface layer of alumina nanoparticles. This is similar to the presence of interface layer of electrospun fibers that is focused in this paper. Processing of these fiber modified alumina nanoparticulate composite system is discussed next.
2.3.2. Interface alumina nanoparticle integration by fiber modification The fiber modification involves the dispersion of alumina onto the surface of the fabric. Alumina nanoparticles in the amount of 2% by weight of the targeted infusion resin are dispersed on to the interfacing plies of the fiber glass fabric. The alumina particles are dissolved in a solvent of water and methanol (50:50 mix ratios) before it is sprayed by an atomizer on to the fabric surface. Alumina particles are also treated with silane agent before dispersion onto the fabric surface by a similar process with the addition of the silane agent to the polar solvent mixture of water and methanol. The solvent/transfer agent is removed by heating the coated fabric in an oven to a temperature of 200 F, prior to the fabrication of the composite laminates. Fig. 3 presents schematically the steps involved in the fiber modification process during the processing of glass fiber–polymer matrix composites with alumina nanoparticulates at the ply interface. Composite panel manufacturing is performed using the traditional low cost VARTM technique using epoxy resin without alumina modification. Composite panels for three different material configurations were fabricated for all the characterization tests. The three material configurations are based on the type of alumina particle interface layer (with and without functionalization) and composite without alumina nanoparticle interface layer as shown in Table 1.
Table 1 Material nomenclature for alumina hybrid composite material.a Material
Configuration
MAT 1 MAT 2 MAT 3
Baseline: EponTM /S2 glass Fiber modification with pristine alumina Fiber modification with functionalized alumina
a Resin System is EponTM 9504 (proprietary modified bisphenol-A resin) and curing agent EpikureTM 9554.
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2.3.3. Processing of fiber glass Epon composite with TEOS nanofibers The effect of Ultrathin Fibrous Sheets (UFS) on Mode I fracture toughness has been studied by incorporating UFS at various layers in addition to mid-plane [22]. The present work focuses on the use of TEOS nanofibers as interface layers. In the present work, material system for composite panels was 10 layers of glass fiber with TEOS nanofibers only at the critical ply interface (mid-plane) as this is the delamination and crack propagation interface during the double cantilever beam tests. TEOS nanofibers weighing nearly 1 g were placed at this mid-plane. Epon 862 with curing agent W in the ratio of 100:26.4 was used as the resin system. Resin was flown through ply lay-up using a H-VARTM setup [23]. The use of the conventional manufacturing process with the additional electrospun nanofiber interface layers was found to require substantial modifications and refinement. Comprehensive discussion on methodology for incorporating electrospun fibers into prepeg laminates to improve delamination resistance of composite is described by Dzenis [24]. Various polymers considered by Dzenis for electrospinning does not include TEOS nanofibers. Also, the use of VARTM processing with S2 glass fibers poses different set of problems. Few problems those were rectified after series of empirical experimentation are listed here. One of the foremost problems encountered during the composite processing by VARTM process when electrospun nanofibers are added at the ply interfaces is that of dry areas or non-wetting of the nanofibers. If nanofibers do not get sufficient resin to soak, the dry spots created at the interfaces will act as microcracks degrading the interlaminar properties. This dryness of the interface nanolayers is due to the difficulty during resin infusion in regular VARTM caused by the additional permeation resistance created by compressed electrospun nanofibers. To eliminate these problems, interlaminar electrospun fibers were presoaked in Epon 862 and curing agent W solution when they are laid up. In addition, the use of H-VARTM setup enables to increase temperature of the mold to 110 ◦ F during resin infusion. At 110 ◦ F the viscosity of Epon 862 was found to be low enough to ensure complete soaking of electrospun fibers and removal of excessive resin. One more problem that is typically associated with secondary reinforcement of ply interfaces is the potential increase in the thickness of the composite panels. This problem was eliminated by selecting thin layers of TEOS electrospun nanofiber sheet for interfacial reinforcement. Typical diameters of the TEOS nanofibers on the selected sheets for reinforcement were on an average 500 nm. TEOS electrospun fiber sheets were sintered at 900 ◦ C before they are laid up at glass fiber sheet interfaces. Sintering of TEOS nanofibers resulted in the reduction in fiber diameter as well as improvement in the porosity in the random electrospun fabric [25]. 2.4. Characterization 2.4.1. Fracture toughness characterization The double cantilever beam test coupons were prepared according to ASTM 5528-94a specimen specifications from the composites
Table 2 Average GIC values for composites with electrospun nanofibers.b Specimen
Neat resin
Espun
1 2 3 4 5
796.85 1177.86 1150.54 1144.25 1184.18
987.42 935.47 1038.33 972.56 889.89
Avg. GIC Std. Dev.
1090.74 147.73
964.73 49.9
GIC values are in J/m2 . b Resin System is Epon 862 with curing agent W in the ratio of 100:26.4.
in both the cases. The schematic of DCB specimen with electrospun nanofibers at mid-plane interface is shown in Fig. 4. Double cantilever beam (DCB) ASTM D5528 Mode I interlaminar fracture test was conducted for all the composite material specimens using a 445 N maximum load MTS machine in conjunction with displacement control mode. The maximum displacement was 63.5 mm, which was the test limit. Test displacement rate is 0.1016 mm/min (0.04 in./min). A Teflon insert 0.0005 in thick was used to emulate the initial crack size during the fabrication. The Mode I fracture toughness GIC was obtained using the Modified Beam Theory (MBT) as shown below in Eq. (1). GIC =
3Pı 3Pı = 2ba∗ 2b(a + ||)
(1)
where P = load; ı = displacement; b = width; a = delamination length; = rotation and shear correction. 3. Results and discussions The average Mode I fracture toughness GIC values in J/m2 computed for the specimen with and without nanofibers referred as neat and electrospun, respectively are presented in Table 2. The corresponding Mode I fracture toughness GIC values for the composites with alumina nanoparticles formed with fiber modification are presented in Table 3. It should be noted here that resin system Epon 862 is used for making DCB specimens with electrospun nanofibers at the critical interface of S2 glass fiber plies, where as in case of alumina nanoparticle reinforcement with fiber modification uses Epon 9504 resin. As these two different resin systems are used, there is large difference in the GIC values for the two systems. The test data presented in Table 3 indicates that the presence of interface alumina particulate layers resulted in an increase in the Mode I fracture toughness values thus improving the delamination resistance characteristics of the composite as compared to the two phase composite without alumina nanoparticles. The Mode I fracture toughness values presented are based on specimens that did not show any fiber bridging. As seen in Table 3, functionalized alumina (MAT 3) showed significant improvement compared to the non-functionalized form. When neat Epon and S2 glass fiber specimens are compared with the alumina interface layered composites, the test data indicates that there has been over 51% improvement in Table 3 Average GIC values for hybrid composites with alumina nanoparticles.c
Fig. 4. ASTM 5525-94a double cantilever beam test specimen with Teflon inserts and electrospun nanofibers at mid-plane.
Specimen
Avg. GIC
Mat 1 Mat 2 Mat 3
290.0 ± 5.61% 440.0 ± 10.78% 506.0 ± 7.96%
GIC values are in J/m2 . c Resin System is EponTM 9504 (proprietary modified bisphenol-A resin) and curing agent EpikureTM 9554.
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GIC for the case of non-functionalized alumina. When it is compared with functionalized alumina interface layers, the improvement of GIC is found to be above 74%. This clearly indicates that the use of alumina nanoparticles leads to significant improvement of Mode I fracture toughness of glass fiber epoxy composite laminates. For glass fiber composites with electrospun nanofibers, there is however a small reduction in the GIC value by about 5% than the neat resin specimens without the electrospun interface layers. Though this is a small change, these results indicate a reduction in Mode I fracture toughness values contrary to the expectation. This could potentially be due to the fact that the amount of electrospun nanofibers layers was not sufficient to make any significant difference in the Mode I fracture toughness characteristics. Weight of secondary reinforcement of TEOS nanofiber used at mid-plane was nearly 1 g only (roughly about 0.2 weight percentage of the composite panel), significantly less than the resin and glass fiber content. Load sensitivity for the small amount and thickness of TEOS electrospun nanofibers layer present at the critical ply interface is planned to be further investigated using less S2 glass layers or by using thin S2 glass prepeg material to give a higher weight percentage of nanofiber reinforcement without significantly increasing the thickness. 4. Conclusions Composites formed with integration of nanomaterial constituents via conventional manufacturing processes have been demonstrated for two material systems. They involved the use of alumina nanoparticles and electrospun nanofibers. Though two different sources of nanomaterials have been employed, both the forms have provided an effective means of nanomaterial integration for the advanced composite material systems. The addition of alumina nanoparticles at the interface layers was demonstrated to have significant effect on the Mode I fracture toughness characteristics. The results indicate a 51% increase in the Mode I fracture toughness values of non-functionalized alumina interlaced composite over the conventional two phase composite system. This increase was nearly 74% with the functionalization of the alumina nanoparticles as demonstrated in the present paper. The electrospinning process provides an effective means of processing nanofibers and the associated advanced composites through the conventional VARTM process and its variant H-VARTM. The double cantilever beam test results for the advanced composites with the electrospun nanofiber interface layers showed a slight decrease in the Mode I fracture toughness values compared to the baseline two phase composite. This could potentially be attributed
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