Carbon Nanofiber Multifunctional Mat

CHAPTER

11

Carbon Nanofiber Multifunctional Mat

Carla L. Lake, Patrick D. Lake Applied Sciences Inc, Cedarville, OH, USA

CHAPTER OUTLINE 11.1 Introduction ................................................................................................ 11.2 Development of Carbon Nanofiber Mat .......................................................... 11.2.1 Carbon nanofiber ...................................................................... 11.2.2 Carbon nanofiber mat................................................................ 11.2.3 Fabrication of CNF mat-reinforced composites ............................ 11.2.4 Short beam shear testing........................................................... 11.3 Conclusion.................................................................................................. Acknowledgments ................................................................................................ References ..........................................................................................................

313 315 315 317 321 322 328 328 328

11.1 INTRODUCTION Multifunctional structural systems that utilize nanoscale reinforcing materials have been of interest to the scientific community for the past 20 years [1e8]. Structural components built using polymer matrix composites (PMCs) are attractive for a wide range of applications due to their combination of light weight and high stiffness. PMCs allow for the design of complex unitized structures, which can significantly reduce the number of parts and fasteners in an aerospace structure. PMCs when combined with carbon nanomaterials (CNMs) are regarded as ideal candidates to replace conventional metallic components to reduce weight and potentially impart multifunctionality to the component. Various CNMs have been incorporated into polymers to form nanocomposites [4e13]. The potential benefits of having carbon nanotubes (CNT) and carbon nanofibers (CNFs) in nanocomposites include weight reduction, improved mechanical properties (modulus, strength, fracture toughness, fatigue resistance [14e18], delamination resistance [19,20], impact strength [21,22], and structural damping [23e27]), improved electrical [28] and thermal conductivity, increased thermal stability [29], improved flame retardancy [30e32], enhanced barrier properties [33], and reduced environmental effects such as moisture absorption [34] and degradation by irradiation [35]. Nanotube Superfiber Materials. http://dx.doi.org/10.1016/B978-1-4557-7863-8.00011-6 Copyright Ó 2014 Elsevier Inc. All rights reserved.

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Recent results show significant improvements of PMC fracture toughness with the addition of CNFs, as a result of crack deflection by the dispersed nanostructures [36e38]. Small quantities of tailored CNFs can efficiently reinforce the matrix without any adverse impact on the overall composite. Improvements in tensile strength and tensile modulus by 11% and 22.3%, respectively, have been reported in carbon fabriceepoxy composites with incorporation of 2 wt.% of CNF in the matrix, serving to significantly increase the interphase volume and result in microcrack mitigation [39e41]. Figure 11.1 shows an example of CNF bridging a crack that has formed in epoxy. This ability to hinder crack propagation has been identified as the primary mechanism by which CNF is able to improve interlaminar shear strength (ILSS) and resin mechanical properties in general. CNMs are a new class of multifunctional additives that are becoming commercially available at practical production volumes and prices. This class of materials includes single-walled carbon nanotubes (SWNT), multiwalled carbon nanotubes (MWNT), nanographene platelets (NGP), and stacked-cup carbon nanotubes, also known as CNFs. The advantage of using CNMs in applications that require multifunctionality comes from their extremely large surface area, which enables them to reinforce and interact with the polymer matrix at the molecular level. By being able to control the loading and the dispersion of CNMs, several functionalities can be translated into lighter composite parts. As in all cases where nanosized additives are involved, the development of high-performance composites requires homogeneous dispersion of their nano

FIGURE 11.1 ASI’s carbon nanofibers can bridge cracks in epoxy resin composites providing structural support.

11.2 Development of Carbon Nanofiber Mat

counterparts. Well-documented success stories, in both theoretical and experimental levels, show clear benefits of nanotechnology in PMCs to improve their structural performance and impart multifunctionality. Typically these findings have been limited to small laboratory-scale environments, which have not proceeded to commercially reality. This has much to do with the wide spectrum of nanoparticle species, qualities and purity grades available in the market today; the absence of quality control; and the nonapplication of standard manufacturing approaches. There are a large number of raw material manufacturers using different composite synthesis methods, and their standards for product quality and consistency are not uniform. Thus design engineers for spacecraft and aircraft are faced with comparing a multitude of nanoparticles that respond differently to processing schemes and are often difficult to handle and qualify. Filtration of nanoparticles by larger carbon fibers/glass fibers used as the primary reinforcing phase is typically a problem when incorporating nanofibers or nanotubes into laminated composites. A larger loading of nanoparticles occurs at the resin injection site and a lower loading occurs at the outlet side. Miller et al. showed evidence of nanoparticle filtration on a resin transfer molding-processed epoxy/ carbon fiber composite [35]. Factors that contributed to nanoparticle filtration were poor dispersion prior to infiltration and the braid architecture of the structural reinforcement. Ultimately, achieving multifunctional properties in composites depends on a proper choice of fiber type and/or geometry, mastering the composite processing techniques, effective placement of the nanofillers in the polymeric continuous phase, and developing innovative product designs that overcome processing and interface challenges. Nanomaterials hold extraordinary promise for improving composite structures. Considering the technical and processing hurdles associated with these materials, there is need for development of a continuous sheet good based on carbon nanoparticles. The sheet material should be easy to handle and integrate into traditional composite processing techniques. A useful form of a continuous sheet good would be a mat or veil. Continuous sheets of nonwoven veils of various materials have been widely used in industry for surface engineering of composite properties ranging from surface quality to adding functionality. CNF mat, or nanomat, produced in high volumes could serve as a new veil material with a wide range of applications including lightning strike protection [42], electromagnetic interference shielding [43], electrostatic dissipation, thermal management [44] and vibration damping [25,26]. This chapter describes fabrication of CNF mat, new processing techniques developed, and initial composite property results.

11.2 DEVELOPMENT OF CARBON NANOFIBER MAT 11.2.1 Carbon nanofiber Applied Sciences, Inc.’s (ASI’s) “PyrografÒ III” CNFs, shown in Fig. 11.2, have demonstrated their value as additives to improve the performance of polymer

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FIGURE 11.2 Scanning electron microscope image of Pyrograf III material.

composites in various studies. Pyrograf III CNFs are graphitic filaments having a diameter of approximately 100 nm and an aspect ratio greater than 1000. In terms of physical size, and production cost, CNF completes a continuum bounded by carbon black, fullerenes, and SWNT to MWNT on one end and continuous carbon fiber on the other end. Relative to other CNMs, CNFs are more easily dispersible than MWNT. The van der Waals forces are lower due to the larger diameter of CNFs, which means that less energy is required to disperse them into individual fibers. Although MWCNTs are becoming commercially available at low prices, the raw materials require extensive processing steps, such as purification and functionalization, to make the materials effective for use. These postprocessing steps have yet to be demonstrated at a level higher than the laboratory scale and pose significant challenges for scale-up efforts associated with commercialization. CNF can be manufactured with a distinctly different structure than CNT. As shown in Fig. 11.3, CNF can show a “stacked-cup” morphology where graphene appears to be a stack of conic sections or a scroll structure with edges that terminate on the surface of the fiber. The stacked-cup structure is markedly different than nanotubes, which are composed of sheets of graphene arranged in a structure resembling concentric cylinders. The CNF also has a larger average outer diameter (w100 nm), a resultant lower specific surface area (w20 m2/g), and a longer average length (w100 mm) than CNT. Despite these differences in scale and morphology, composites produced from CNF exhibit properties similar to

11.2 Development of Carbon Nanofiber Mat

(a)

(b)

Stacked cup Turbostratic carbon

Carbon

Hollow core Stacked cup

Hollow core

FIGURE 11.3 (a) The transmission electron microscope (TEM) image represents a typical PR-19 carbon nanofiber with a large fraction of turbostatic carbon deposited on the catalytic layer (stackedcup carbon). The catalytic carbon layer is carbon precipitated from the catalyst particle, while the turbostatic layer is added through chemical vapor deposition techniques. (b) The image is typical of PR-25 fiber that only has the catalytic carbon layer. (For color version of this figure, the reader is referred to the online version of this book.) Source: Figures courtesy of Jane Howe, Oak Ridge National Laboratory, TN, USA.

composites made from MWNT. The difference is that CNF composites often cost less and there are fewer processing issues.

11.2.2 Carbon nanofiber mat CNF mat was produced by integrating CNF with polyacrylonitrile (PAN)-derived carbon fiber veil to form a continuous nanomaterial sheet good that can be incorporated into composite systems using traditional composite processing methods and equipment. CNF and carbon fiber veil are commercially available at low cost and high volume. This is a proprietary process developed by ASI and there can be a wide variety of applications for this material. CNF used in the process is a highly graphitic multifunctional nanomaterial and is available in tons at less than $0.75/g. Several product forms of CNF are available with an index of graphitization, surface state, and other properties that can be tailored for specific applications. The CNF mat can be produced using carbon veils from suppliers such as Hollingsworth & Vose Company, Technical Fibre Products, Inc., or Southeast Nonwovens, Inc. However, other noncarbon veils, such as glass veils or polymer veils, can also be used in the fabrication of CNF mat. Figures 11.4 and 11.5 compare optical and scanning electron micrographs of CNF mat produced according to the traditional method of paper making (Fig. 11.4) and ASI. CNF mat produced following its proprietary method (Fig. 11.5).

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FIGURE 11.4 (a) Optical and (b) SEM images of nanofiber mat produced via filtration, a process typically used in papermaking. The filtration process generates a distribution of agglomerated nanofiber within the body of the veil and greatly limits the strength and durability of the sheet good. (For color version of this figure, the reader is referred to the online version of this book.)

FIGURE 11.5 (a) Optical and (b) SEM images of nanofiber mat produced via ASI’s proprietary process. The process developed by ASI generates a uniform sheet good with a well-developed network of nanofiber supported by the carbon fiber veil that is well dispersed and is durable enough to process on reel-to-reel processing operations. (For color version of this figure, the reader is referred to the online version of this book.)

The veil shown in Fig. 11.4 was produced using a traditional wet-laid papermaking process. One can readily see that the papermaking process yields a poor dispersion of the nanomaterials and contains agglomerates of CNF nested between PAN carbon fibers. These sandwiched agglomerates are virtually impossible to penetrate with resin during processing to incorporate the sheets into a composite.

11.2 Development of Carbon Nanofiber Mat

Furthermore, the sandwiched agglomerates are inefficient in forming a conductive network. In contrast, the veil produced using ASI’s proprietary process results in an excellent dispersion of CNFs. For this study, long lengths of CNF mat were produced, with widths from 12 in. to 30 in. Blends of different CNF types, lengths, and surface treatments were used to fabricate the mats. The areal weight of the mat material can be tailored based on application requirements. The CNF mat was produced without a backing paper since the material has sufficient strength to be handled without damage. In addition, the process used can incorporate other nanomaterials including CNT and NGP. Figure 11.6 shows a 14 in. wide by 80 ft long section of a CNF mat ready for prepreg processing. The CNF mat is easy to handle and overcomes all the primary difficulties derived from dispersion, processing, and handling typically associated with CNMs. Small-scale impregnation trials were performed on CNF mat samples to determine the material’s wettability and durability. Cytec 5250-4 bismaleimide (BMI) resin film (prepreg grade) with an areal weight of 34 gsm was used to impregnate the CNF mat samples. The small-scale tests showed the CNF mat samples impregnated well. The successfully impregnated veil samples were fully cured. A sample piece of cured CNF mat laminate is shown in Fig. 11.7. In addition, over 100 linear ft of CNF nanomat in two rolls were produced for prepregging trials in Cytec’s Anaheim facility (Fig. 11.8). Prepregging trials were conducted at Cytec by sandwiching the nanomat between two 150 gsm films of BMI resin 5250-4 provided by Cytec. Cytec’s choice of the 150 gsm weight for the films was based on the estimated quantity of resin required to wet out the assumed weight of the CNF nanomat. Cytec reported that the nanomat exhibited good strength and was easy to pull from the roll.

FIGURE 11.6 Image of 80-ft-long roll of nanofiber mat. (For color version of this figure, the reader is referred to the online version of this book.)

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FIGURE 11.7 Image of fully cured laminate. The laminate was produced by impregnating a CNF mat using BMI resin film (Cytec Engineered Materials, Anaheim, CA). The CNF mat impregnated well and is fully consolidated. (For color version of this figure, the reader is referred to the online version of this book.)

FIGURE 11.8 CNF mat being processed on a 1200 pilot line at Cytec.

The nanomat material was prepregged at 210  F with 100 psi for all nips and with a sled speed of 5 fpm. The resulting material was flexible and had a small amount of tack. The color of the material was consistently black and showed no sign of significant imperfections. Cytec reported that, overall, this material was

11.2 Development of Carbon Nanofiber Mat

relatively easy to prepreg. Impregnation trials were performed using handsheets and laminates that were cocured to a glass fabric using Cytec 5240-4 BMI resin film. The material was processed with no difficulty. Approximately 100 ft2, in a continuous length, of CNF nanomat was processed on Renegade Materials industrial-scale prepreg line. A roll of 35 gsm BMI film was used in the trial. The nanomat was impregnated with resin using films on either side of the nanomat. The material processed well with no special handling or equipment required. Figure 11.9 shows images of the CNF nanomat processed at Renegade Materials facility in Springboro, OH, USA.

11.2.3 Fabrication of CNF mat-reinforced composites Laminates were produced via the vacuum-assisted resin transfer molding (VARTM) technique. A filtration effect has occasionally been reported when manufacturing nanostructured composite laminates by VARTMdespecially at filler concentrations above 1 wt%. Low-viscosity resin and proper use of resin distribution media has been shown to greatly reduce the negative effects of this phenomenon. ASI avoided the problems associated with filtration by utilizing novel CNF product forms, namely, CNF mat, a continuous sheet good composed mainly of CNF, as interleaves for the woven carbon fiber fabric in the composite. In other composites, ASI applied CNF directly to the woven carbon fiber reinforcement and proceeded with the composite layup. Test coupons for each mode of testing were subsequently cut by water jet from the panels. The water-lubrication provided smooth specimen surfaces and prevented an undesirable temperature increase. The following four composite configurations were fabricated:

FIGURE 11.9 CNF nanomat processing on industrial-scale prepreg line at Renegade Materials. (For color version of this figure, the reader is referred to the online version of this book.)

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1. A baseline two-dimensional woven laminated composite consisting of six layers of the Qiso triaxial braided fiber architecture of 0  60 (from A&P Technology) with an SC-79 epoxy matrix (from Applied Poleramics). The layers were stacked in a (0 /90 ) sequence. This composite configuration will be referred as BL in the subsequent discussions. 2. A laminated composite panel consisting of five layers of the Qiso triaxial braided fiber interleaved with four layers of CNF mat; this configuration represents a 16.6% weight reduction in comparison to the BL panel. This reduced weight composite configuration, designated as RW, was fabricated to determine if the panel weight could be reduced while maintaining the same mechanical performance as the BL, by interleaving the CNF mat plies. 3. A laminated composite panel with the same layup as the BL, to which a 5 wt% CNF-reinforced resin was applied to each of the woven fabric plies. The presence of the CNF in the fiberematrix interface of the composite laminates is expected to enhance material shear strength properties. This composite panel is designated as 5CNF. 4. A laminated composite with the same layup as the BL, to which CNF mat was added to outer surfaces and interleaved between each ply. This composite configuration, designated as CNF mat, is expected to add mechanical reinforcement to the composite and surface electrical conductivity. Figure 11.10 shows schematic drawings representing each of the laminated composite configurations.

11.2.4 Short beam shear testing In order to determine the effect of adding the CNF, CNF mat, and nanocapsules, short beam shear (SBS) tests were performed as per American Society for Testing and Materials (ASTM) D2344, at two different temperatures, 23  C and 200  C. The effect of different material configurations on the interlaminar shear behavior of the composite laminates was also studied. As shown in Fig. 11.11, the composite specimen is placed on two cylindrical supports and a cylindrical head is moved down to apply a force at the center and generate an increasing transverse load until the first failure is recorded. The load at failure is then used to determine the apparent interlaminar shear strength of the composite. The results are average values of five individually tested specimens for each sample. The experiment measures the effectiveness of the CNFs to reinforce the interface of the laminated composites. In the SBS test, the determination of ILSS is based on classical (Bernoullie Euler) beam theory. For a beam of rectangular cross-section loaded in three-point bending, the maximum interlaminar shear stress occurs at the midthickness of the beam between the center and end supports and is calculated according to Eqn (11.1). ILSS ¼

0:75  Pm bh

(11.1)

11.2 Development of Carbon Nanofiber Mat

(a) Baseline composite panel 6 QISO braided carbon fiber/SC79 epoxy resin 0 /90

(b) Reduced weight composite panel 5 plies QISO braided carbon fiber/SC79 epoxy resin

4 Interleaved CNF Nanomat

(c) 5 wt% CNF resin reinforced composite panel 6 plies QISO braided carbon fiber/ CNF reinforced SC79 epoxy resin

(d) Conductive CNF Nanomat composite panel CNF Nanomat

6 plies QISO braided carbon fiber 5 interleaved CNF Nanomat

CNF Nanomat

FIGURE 11.10 (a) Baseline two-dimensional woven laminated composite (BL) consisting of six layers of triaxial braided fiber architecture and SP-79 epoxy matrix; (b) laminated composite panel consisting of five layers of triaxial braided fiber interleaved with four layers of CNF mat (RW); (c) laminated composite panel with the same layup as (a), to which a 5 wt% CNF-reinforced resin was applied to each of the woven fabric plies (5CNF); (d) laminated composite with the same layup as (a), to which the CNF mat was added to outer surfaces and interleaved between each ply.

where Pm is the maximum load during test (lbf ) b is the measured specimen width (in.) h is the measured specimen thickness (in.) The ILSS is an important material property associated with composite laminates and defines when individual plies fail in shear. Conventional laminated composites

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FIGURE 11.11 Short beam shear test loading configuration (ASTM D2344).

FIGURE 11.12 Representative loadedisplacement curves for all the composites tested at 23  C. (For color version of this figure, the reader is referred to the online version of this book.)

11.2 Development of Carbon Nanofiber Mat

have an inherent weakness that manifests itself in poor resistance to interlaminar shear. Because CNFs have exceptional stiffness and tensile strength, it is anticipated that adding them to the fiberematrix interface of composite laminates will enhance material shear strength properties. For each composite configuration tested, a loadedisplacement curve was selected that best represents the average of five separate test specimens. The typical loadedisplacement curves for each of the five composite panels tested at room and high temperature are shown in Figs 11.12 and 11.13. The material response at room temperature shows a typical brittle failure mode. As anticipated, all the samples containing CNF (5CNF) or CNF mat (RW and CNF mat), show failure at higher loads when compared to the BL specimen. The RW configuration shows a higher load to failure when compared to the BL, even though this specific configuration has less one ply of carbon fiber, totaling a reduction of over 16% in the weight of the composite panel. This result alone shows the great promise of CNF-reinforced composites for lightweight structural composites. The material response at elevated temperature shows a nearly linear elastic trend during the early stage of loading. This regime continues until an apparent elastic

FIGURE 11.13 Representative loadedisplacement curves for all the composites tested at 200  C. (For color version of this figure, the reader is referred to the online version of this book.)

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FIGURE 11.14 ILSS average values for the five different composite configurations tested by SBS at room temperature (23  C). (For color version of this figure, the reader is referred to the online version of this book.)

limit is reached. At this point, the behavior of the material follows one of the two trends: the load decreases or increases. By increasing the temperature, the failure behavior changes from “almost brittle” to “almost plastic”. The BL composite reaches the elastic limit at the lowest load among the five composites. The CNF mat composite achieved the highest peak load among all the composites and maintained a higher load at 0.100 displacement compared to all others. All the composites, with exception of RW, showed a typical behavior of load decrease after the peak load was reached. The RW reaches a peak load higher than that of the BL and lower than that of the rest of the composites, but after this point, the load continues to increase, with the highest peak load observed at a displacement of 0.074. This result indicates that while the damage initiation occurred at a lower load, the damage tolerance of the material increases with the presence of the CNF mat interleaved between each ply. From the load histories, the SBS strength was calculated for each composite using Eqn (11.1). The average ILSS values were determined from five tests for each of the composite configurations tested at room and elevated temperatures and are shown below in Figs 11.14 and 11.15, respectively. Replacing one of the braided Qiso plies by four interleaved lightweight CNF mats (RW) resulted in a composite panel with 16.6% lower weight and a similar ILSS as the unreinforced laminate (BL). This result shows the potential of adding nanomaterials to decrease the overall weight of Carbon Fiber Reinforced Plastic (CFRP) composites. At room temperature, the highest increase in ILSS, 14.5%, was recorded for the composite laminated containing 5 wt% of CNFs dispersed in the resin. The increased ILSS suggests that the improved shear performance may

11.2 Development of Carbon Nanofiber Mat

FIGURE 11.15 ILSS average values for the five different composite configurations tested by SBS at elevated temperature (200  C). (For color version of this figure, the reader is referred to the online version of this book.)

Table 11.1 Summary of ILSS of the Different Composite Configurations Tested at Room Temperature (23  C) Specimen

Average (psi)

Standard Deviation (psi)

Coefficient of Variation (%)

Improvement (%)

BL RW 5CNF CNF mat

2259.2 2281.3 2587.8 2452.6

137.8 217.1 102.4 102.6

6.1 9.5 3.96 4.18

– 1.0 14.5 8.56

Table 11.2 Summary of Short Beam Shear Strength of the Different Composite Configurations Tested at High Temperature (200  C) Specimen

Average (psi)

Standard Deviation (psi)

Coefficient of Variation (%)

Improvement (%)

BL RW 5CNF CNF mat

397.7 438.4 509.3 575.8

44.8 29.1 88.8 44.9

11.3 6.6 17.4 7.8

– 10.2 28.1 44.8

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be related to the ability of the high aspect ratio CNFs to effectively ‘‘anchor’’ the microcracks together, delaying their growth, coalescence and thus laminate failure. Also, CNFs are able to deflect and stop the growth of the microcracks. At high temperatures, the ILSS values decrease substantially, which is associated with the softening of the matrix. This was evidenced as well by the “almost plastic” failure behavior. Also, the presence of the CNFs, either dispersed in the resin (5CNF) or in the nanomat (RWand CNF mat), leads to higher improvement when compared to the room-temperature test results. Another interesting aspect of these results is the fact that the presence of the interleaved CNF mat, in the CNF mat sample, increases the ILSS by almost 45%, compared to BL. This was expected, since in a plastic regime, the presence of the nanomaterials at the fiber/matrix interface prevents delamination, which allows better performance of the composite. The presence of well-dispersed CNFs in resin prevents microcrack formation and/or propagation, thus showing the highest ILSS improvement. A summary of the ILSS results for both room- and elevated temperature testing are reported in Tables 11.1 and 11.2.

11.3 CONCLUSION A method of producing a nanofiber mat composed of highly graphitic CNFs in an isotropic array embedded in a carbon fiber veil has been developed. Areal weight can be tailored using combinations of CNF having different aspect ratios and degrees of graphitization. An optimum formulation was used to generate over 250 ft of nanomat for use in prepregging trials. Sufficient strength to provide handling ease and straightforward production of prepreg rolls using conventional methods has been observed from the nanofiber mat. CNF mat is useful for imparting electrical conductivity to structural composites and can be produced with conventional commercial materials. The most significant finding from this study is that through use of low loadings of CNFs, traditional composites can be made stronger and more resistant to damage and fatigue. This study demonstrated the feasibility of use of CNFs and carbon nanomats in composite laminates to either increase the envelope of mechanical properties at a given weight or reduce the number of plies of conventional carbon fiber reinforcement while retaining the mechanical properties of the composite. Interlaminar shear stress (ILSS) results at both room temperature and elevated temperature (200  C) showed that the addition of well-dispersed CNFs improves the shear properties of the composite.

Acknowledgments This work was supported by USAF SBIR Contract FA8650-09-M-5021.

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