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JOURNAL OF COMPOSITE M AT E R I A L S
Article
The effect of nano-additive reinforcements on thermoplastic microballoon epoxy syntactic foam mechanical properties
Journal of Composite Materials 0(0) 1–10 ! The Author(s) 2017 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021998317716267 journals.sagepub.com/home/jcm
Kerrick R Dando1,2 and David R Salem1,2,3
Abstract Syntactic foams comprising glass microballoons have gained considerable attention over the past several years due to mechanical and thermal properties that are advantageous for use as a core material in naval and aerospace applications. Recent advancements in the production of thermoplastic microballoon syntactic foams have allowed for an increase in microballoon volume fraction (up to 0.9 volume fraction), with correspondingly lower densities but reduced mechanical properties. In this work, carbon nanofibers and halloysite nanotubes were incorporated in thermoplastic microballoonbased syntactic foam to enhance the mechanical properties and the relative effects of these two nanoscale reinforcements were compared. X-ray micro-computed tomography was employed to analyze the microstructure of the materials produced, and scanning electron microscopy was used to assess the dispersion of nano-additives within the resin. Compressive strength and modulus enhancements as large as 180% and 250% respectively were achieved with a 0.25 wt% addition of carbon nanofiber and increases of 165% and 244% respectively were achieved with a 0.5 wt% addition of halloysite nanotube. Tensile strength and modulus enhancements as large as 110% and 165% respectively were achieved with a 0.125 wt% addition of carbon nanofiber and increases of 133% and 173% respectively were achieved with a 0.125 wt% addition of halloysite nanotube.
Keywords Syntactic foam, carbon nanofiber, halloysite, nano-reinforcement
Introduction Syntactic foams consisting of 0.3–0.74 volume fraction of glass microballoons in epoxy resin matrices have gained considerable attention in recent years due to their low density combined with high compressive strength,1 low moisture absorption,2 and high-energy absorption.3 Consequently, there has been extensive evaluation of the tensile,4,5 compressive,1,6,7 and hygrothermal2 properties of these materials. The uniquely tunable properties of syntactic foams have led to their increased commercial use, primarily in aerospace8 and marine9 applications. A significant limitation of glass microballoon epoxy syntactic foams is the density ceiling (0.35 g/cc) due to packing limitations of the rigid, polydisperse glass microspheres.1,3 To overcome this limitation, expandable hollow thermoplastic microballoons have been used to enable the production of syntactic foams with
microballoon volume fractions up to 0.95 and densities as low as 0.067 g/cc.10 However, while density is reduced with the incorporation of large volume fractions of thermoplastic microballoons, mechanical properties decrease significantly owing to the large amount of purposely placed voids within the composite. 1 Composites and Polymer Engineering (CAPE) Laboratory, South Dakota School of Mines & Technology, USA 2 Nanoscience and Nanoengineering Program, South Dakota School of Mines & Technology, USA 3 Department of Materials and Metallurgical Engineering, South Dakota School of Mines & Technology, USA
Corresponding author: David R Salem, Composite and Polymer Engineering (CAPE) Laboratory, South Dakota School of Mines & Technology, 501 East Saint Joseph St., Rapid City, SD 57701, USA. Email: david.salem@sdsmt.edu
2 A potential use for these highly loaded syntactic foams and a motivation for the present study is to develop these materials for use in cryogenic storage vessels, lunar and Martian habitats, and other extraterrestrial construction materials.11–13 For these performance goals to be reached, syntactic foams must be produced yielding optimized low thermal conductivity, low density and strong mechanical properties. To counteract the reduced strength properties, reinforcements, through embedded fibers, have been investigated and published as a possible means to improve strength and stiffness.14–17 The primary drawbacks to fiber reinforcement include limitations in microballoon volume fraction levels, detrimental impact on density and difficulty in adequately dispersing fibers to produce uniform syntactic foams. One alternative to remedy this mechanical property deficiency and avoiding the use of fiber reinforcement is to incorporate nano-additives. Nano-additives lend mechanical property enhancements without impacting microballoon volume fraction limitations and have a negligible impact on composite density.18–21 Previous work22 has shown that small additions of carbon nanofibers (CNFs) in glass microballoon epoxy syntactic foams led to improvement in tensile strength properties. This is primarily due to the large aspect ratio of the CNFs, bridging cracks, and reducing crack propagation under tensile loading.5 CNF additions improved compressive properties by increasing strength and modulus by 7.3% and 15.5%, respectively, compared to the neat epoxy binder.23 Zhang et al.24 reported that, while the compressive strength properties of carbon fiber reinforced syntactic foams remained unchanged, flexural strength and fracture toughness increased with increasing weight percentage additions up to 1.5 wt%, after which fiber agglomeration became a detrimental effect.24 One potential drawback to incorporating CNFs is their detrimental impact on thermal conductivity.25 Due to the CNF’s high aspect ratios and high thermal conductivity, even small weight percentage additions can greatly increase thermal conductivity.26 This is an important factor to keep in mind when trying to produce low thermal conductivity insulation materials. Another potential nano-additive that can lend mechanical property enhancements with minimal thermal conductivity impact is halloysite. Halloysite is a naturally occurring mineral that has been heavily researched for use as an efficient drug delivery vehicle.27–29 Halloysite has a unique tubular structure somewhat resembling multi-walled carbon nanotubes (MWCNT), making them a potentially cheaper alternative for mechanical property enhancement than MWCNTs.30 Halloysite nanotubes (HNTs) are discrete nanoparticles, having little to no surface charge. This means
Journal of Composite Materials 0(0) intercalation and exfoliating processing steps, used in dispersion of nano-additives (montmorillonites), can possibly be removed when dispersing HNTs in polymer systems. HNTs have recently been incorporated for strength/toughness enhancements in resin systems, showing promise as a potential nano-additive in syntactic foam materials.31–33 While halloysite has been shown to increase mechanical properties, their hollow-tubular structure, sometimes utilized for drug delivery, could potentially reduce the thermal conductivity increase observed in composites enhanced with CNFs.34 The present work compares the structure and mechanical properties of thermoplastic microballoon epoxy syntactic foams loaded with varying weight percentage of either CNFs or HNTs.
Experimental Materials SC-15 resin, manufactured by Applied Poleramic Inc. was selected as the epoxy resin system. SC-15 is a lowviscosity (550 45 cP), two-phase toughened epoxy cured with a cycloaliphatic amine.35 Acetone was used as a solvent to reduce the viscosity of the epoxy resin to 57 12 cP, for improved wetting and increased working time. D15 hollow-thermoplastic spheres (‘‘microballoons’’) produced by Akzo Nobel were used to produce the highly loaded syntactic foams. The D15 microballoons have a true particle density of 0.015 g/cc 0.001 g/cc and diameter range of 60–90 mm. The carbon nanofibers (PR-19-XT-PS) were produced by Pyrograf Products, having a density of 1.95 g/cc with diameter and length of 100–200 nm and 30–100 mm, respectively. The halloysite nanotubes (NN-HNT200) were produced by Naturalnano, with a density of 2.3 g/cc and diameter and length of 40–200 nm and 0.5–1.2 mm, respectively.
Sample fabrication Preparation of resin. Dispersion of the CNF and halloysite nanotube (HNT) additives in epoxy resin followed identical preparation methods. Master batches containing 1.25 wt% of either additive were prepared to allow for production of the subsequent six syntactic foam sample types. First, part A of the epoxy resin was weighed out, followed by the weighing and addition of either additive. This modified part A, containing the additive, was then speed mixed using a speed mixing unit (Hauschild Speedmixer DAC 1100 FVZ) to distribute the nanomaterial into the resin. The resin was mixed for 10 min at 1500 r/min followed by 15 min at 2500 r/min and finally 15 min at 4500 r/min using a mechanical mixer with a high-shear impeller. This modified resin was then
Dando and Salem sonicated for 1 h at an amplitude of 4, followed by 30 min at an amplitude of 20. Syntactic foam fabrication. For epoxy syntactic foam sample formation, SC-15 resin and curing agent were speed mixed (130:100, resin: curing agent by weight ratio) to allow for adequate dispersion of the curing agent and ensure uniform cure of the material. Varying amounts of the epoxy part A master batch (described above) were added depending on the desired weight percentage of nano-additive targeted for that material. Acetone was then added to the epoxy and speed mixed. Next, D15 hollow thermoplastic spheres were added to the epoxy solution and speed mixed. This mixture was then transferred into a 114.3 mm 114.3 mm 12.7 mm mold and packed down using a tongue depressor. The mold, containing the mixture, but without a lid, was placed in an oven at 30 C for 30 min to facilitate evaporation of the acetone diluent. After 30 min, the mold-lid was clamped in place for the remainder of the cure period. Excess resin exited through the clearance space between mold parts during the 24-hour cure period at 60 C. Once cured, the syntactic foam plaque was de-molded, and the density of the syntactic foam was evaluated (gravimetrically) and compared to the theoretical density and material mass-input values to verify that the intended loadings were achieved.
Property characterization Microstructure analysis. Syntactic foam specimens were imaged using X-ray micro-computed tomography (Xradia MicroXCT-400), a nondestructive method allowing 3D morphological characterization. X-ray micro-computed tomography (X-ray micro-CT) utilizes X-rays to capture sample image ‘‘slices’’ that can be analyzed in two-dimensional form, or compiled together to produce a three-dimensional image or video. Specimen dimensions employed for analyses were: 6.35 mm 6.35 mm 12.7 mm. Ninety cross-sectional image slices through the material were acquired using X-ray micro-CT in the X, Y, and Z directions (total of 270 images). These image slices were analyzed using ImageJß software to determine microballoon morphology and volume fraction. The volume fraction calculated from image analysis was compared to the empirically measured sample density as a crossreference. Epoxy specimens containing weight percentage additions of either HNTs or CNF were analyzed to determine if the nano-additives were adequately dispersed during the dispersion method. The tensile fracture surfaces of syntactic foams containing halloysite or CNFs were analyzed to determine failure mode(s) and
3 dispersion performance of the nano-additive within the composite. The fracture surfaces were analyzed using scanning electron microscopy (Zeiss Supra 40 variable-pressure field emission SEM), where all images were acquired using a secondary electron detector. Compressive strength. Specimens (25.4 mm 25.4 mm 19.05 mm) of epoxy syntactic foams were prepared for compression analysis, and tested using a MTS Q-Test 10 Elite Controller (5 kN load cell) in accordance with ASTM D 695.36 Samples were compressed to 80% strain at a rate of 0.5 mm/min to analyze initial compression, yield point, and densification region. Load–displacement data obtained from these tests were then employed for the calculation of compressive yield strength and modulus. Compressive yield strength is denoted as the first point on the stress–strain diagram at which an increase in strain occurs without an increase in stress. For each sample type, 15 specimens were tested (5 each, cut from 3 molded plaques) to determine the mean compressive strength values. Tensile strength. Tensile analyses were performed on 25.4 mm 25.4 mm 12.7 mm specimens using a MTS Q-Test 10 Elite Controller (5 kN load cell) in accordance with ASTM D 1623.37 Sample tests were carried out until yield, denoted as the point where failure occurs, where strain rates were adjusted to produce failure in 2–5 min. Load–displacement data obtained from these machines were then used for the calculation of Young’s modulus and yield strength. For each sample type, 15 specimens were tested (cut from 3 molded plaques) and the mean yield strength and strength value determined.
Results and discussion Structure and morphology Once the master batches were produced, epoxy specimens containing 1 wt% of either halloysite or CNFs were cast, fractured, and analyzed using scanning electron microscopy, with a neat epoxy specimen used for comparative analysis (Figure 1). The fracture surface was analyzed to determine if the mixing method performed adequately in dispersing nano-additives and preventing or minimizing agglomerations that are detrimental to composite performance properties. From Figure 1, we can see images of the fracture surfaces for both nano-additive reinforced epoxy samples. The images are representative of what was observed throughout the analysis of the fracture surfaces of both types of materials. Prominent protrusion of CNFs or HNTs (examples highlighted in figures) was
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Figure 1. Scanning electron microscope images of: (a) 1 wt% CNF epoxy, (b) 1 wt% HNT epoxy, and (c) neat epoxy samples.
observed across the material fracture surface, with no agglomerations noted. Analysis of the neat epoxy fracture surface shows numerous sub-micron specks, representing the rubbery toughening component, a characteristic feature of SC-15.35,38,39 Syntactic foam samples containing different weight percentages of either nano-additive were then produced and characterized using X-ray micro-CT to comparatively analyze the foam microstructures (Figure 2). This analysis method allows for artifact-free characterization of thermoplastic microballoon morphology, the microballoon dispersion in single-filler and mixedfiller systems, and the volume fraction microballoon loading (by two- or three-dimensional image analysis) for comparison with values obtained by density measurements. Due to instrument resolution limitations, the nanoadditives could not be directly visualized using this analysis method; however, the internal microstructure of the syntactic foam structure was well defined. Utilizing this tool, we were able to get an understanding of how these high microballoon loadings could be achieved.10 During the curing cycle, the heat produced from the exothermic reaction (190 C) of the epoxy matrix causes the thermoplastic microballoon shells to soften and expand. In so doing, excess resin is expelled and the microballoon shells deform to contours that efficiently fill the mold volume, and increase the microballoon volume fraction in the process. ImageJß software was used to analyze the epoxy syntactic foams, which had nominal 0.90 microballoon volume fractions based on gravimetric density. Syntactic foam microballoon volume fractions were calculated from
Figure 2. X-ray micro-CT cross-sectional image of a 0.9 volume fraction thermoplastic microballoon epoxy syntactic foam containing 1 wt% HNTs.
the analyses of 90 individual image slices (of the type shown in Figure 2). The measured syntactic foam microballoon volume fractions were virtually identical to values calculated from gravimetric analyses thereby validating that the correct densities and loadings were achieved.
Compressive properties Following microstructural analysis of the syntactic foam specimens, samples were prepared and tested for their compressive properties. Figure 3 shows the characteristic compressive stress versus strain curves for these syntactic foam samples under compressive loading, with the base 0.9Vmb thermoplastic microballoon epoxy syntactic foam shown for comparison. The stress versus strain curves for thermoplastic microballoon syntactic foam loaded with 0.25 wt% CNF and thermoplastic microballoon syntactic foam loaded with
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Figure 3. Compressive stress vs. strain curve comparison of the base thermoplastic microballoon epoxy syntactic foam to foams loaded with carbon nanofibers or halloysite nanotubes.
Figure 4. (a) Compressive strength and (b) compressive modulus of thermoplastic microballoon syntactic foams reinforced with varying weight percentages of carbon nanofibers.
0.5 wt% HNT are shown for comparative analysis, and exhibit similar compressive response properties. In the first region, initial compression occurs; signified by a linear region corresponding to the elastic behavior of the foam. This region ends at the compressive yield point, which is followed by a plateau region or region of slightly increasing stress (region 2). Region 2 involves energy absorbing deformation, and possibly some rupturing, of the microballoons under the compressive load. Region 3 begins at the inflection point in the curve and is characterized by an exponential increase in compressive stress, resulting from densification of the syntactic foam as the thermoplastic microballoons continue to deform under increasing compressive strain.
Figure 4(a) and (b) respectively shows the compressive strength and modulus properties of thermoplastic microballoon syntactic foams loaded with varying weight percentage of CNFs, where error bars shown in all graphs represent the 95% confidence interval. A dramatic increase in both compressive strength and modulus (150% and 200%, respectively) are noted with the addition of 0.125 wt% CNFs. There is an additional increase (15% and 20%, respectively) in strength and modulus properties as CNF loading is further increased to 0.25 wt%. Thereafter, the compressive strength and modulus of these materials remained essentially unchanged up to CNF loadings of 0.75 wt%. At CNF loadings of about 1 wt% the strength and modulus properties appear to drop off and continue this trend
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Figure 5. (a) Compressive strength and (b) compressive modulus of thermoplastic microballoon syntactic foams reinforced with varying weight percentages of halloysite nanotubes.
at the 1.25 wt% loading. Overall, strength and modulus enhancements as large as 180% and 250% respectively can be achieved with a 0.25 wt% addition of CNF. Figure 5(a) and (b) respectively shows the compressive strength and modulus properties of thermoplastic microballoon syntactic foams loaded with varying weight percentages of HNTs. Dramatic increases in compressive strength and modulus (135% and 170% respectively) were noted with the addition of 0.125 wt% HNTs. A further overall increasing trend in strength properties with increasing HNT wt% up until 0.75 wt% is apparent, with a subsequent drop off at 1 wt% and continued decreasing trend with 1.25 wt%. A similar trend was noted for modulus, where the reduction in modulus enhancement was observed at the 0.75 wt% HNT loading. Overall, strength enhancement as large as 183% was achieved with a 0.75 wt% addition of HNT and a modulus enhancement as large as 244% was achieved with a 0.5 wt% addition of HNT. These results show that the compressive property enhancements with HNTs are equivalent to the enhancement seen with CNFs. It is interesting to note that these results show large increases in compressive strength properties from the baseline for thermoplastic microballoon syntactic foams, while previous work with nano-reinforced glass microballoon epoxy syntactic foams indicated that nano-additives lend minimal compressive strength property enhancements.24 The syntactic foams produced in this work are at a much higher microballoon volume content (Vmb ¼ 0.9) as compared to glass microballoon syntactic foams (Vmb ¼ 0.7), and the thermoplastic microballoons have a much lower crush strength than the glass counterpart.10 The nano-additives incorporated into the syntactic foams in this work (HNT or CNF) lend increased rigidity and yield strength properties to the highly loaded thermoplastic microballoon syntactic, which can be observed due to the lower stiffness and strength of the baseline material.
Tensile properties Figure 6(a) and (b) respectively shows the tensile strength and modulus properties of thermoplastic microballoons loaded with varying weight percentages of CNFs. As with the compression analysis, a dramatic increase in both tensile strength and modulus (110% and 165% respectively) was noted with the initial addition of 0.125 wt% CNFs. These material properties remained essentially invariant with increasing CNF loading up to 0.5 wt%, after which a statistically significant drop in tensile strength is observed. Overall, tensile strength and modulus enhancements as large as 119% and 168% respectively were achieved with a 0.5 wt% addition of CNF. Figure 7(a) and (b) respectively shows the tensile strength and modulus properties of thermoplastic microballoons loaded with varying weight percentages of HNTs. A dramatic increase in tensile strength and modulus (133% and 173% respectively) was noted with the initial addition of 0.125 wt% HNTs. Tensile modulus properties remained essentially unchanged with increasing HNT loadings, while there is a statistically significant drop in tensile strength after 0.125 wt% (0.75 wt% discounted). Tensile strength and modulus enhancements as large as 133% and 173% respectively were achieved with a 0.125 wt% addition of HNT. When comparing tensile properties of both nanoadditive reinforced syntactic foams, it can be seen that both HNTs and CNFs give essentially equivalent tensile property enhancements; however, a 14% greater tensile strength improvement was observed for HNTs over CNFs at 0.125% loading. This observation can be explained by the fact that while halloysites have a 15% greater density over CNFs, they have smaller diameter (40–200 nm) and length (0.5–1.2 mm). This means that the HNT packing density can be greater than that of CNFs, for a given mass loading, thereby allowing for greater resistance to crack propagation and higher
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Figure 6. (a) Tensile strength and (b) tensile modulus of thermoplastic microballoon syntactic foams reinforced with varying weight percentages of carbon nanofibers.
Figure 7. (a) Tensile strength and (b) tensile modulus of thermoplastic microballoon syntactic foams reinforced with varying weight percentage of halloysite nanotubes.
tensile properties. These findings are supported by previous work showing fibers with smaller diameters exhibit greater potential for limiting crack propagation and increasing tensile properties.40 Once produced and tested, the fracture surfaces of thermoplastic microballoon syntactic foams containing varied weight percentage additions of CNFs were analyzed. The failure modes of the thermoplastic microballoon syntactic foam containing 1 wt% addition of CNFs (Figure 8) are representative of all samples, and are similar to fracture surfaces analyzed in the previous work.10 As stated earlier, during the production of 0.9 volume fraction syntactic foam, excess space is filled by distorted microballoons, leaving sphere shells coated with a thin layer of resin. Because of this morphology, mechanical properties are primarily dictated by the microballoons and under tensile forces, exhibit a tearing response, exposing the honeycomb-like microstructure in the process. While CNFs show potential for use in tensile property enhancement (Figure 6), 90% of the syntactic foam volume is occupied by the
thermoplastic microballoons, dictating the failure mode of the material. These fracture surfaces were also analyzed to understand nano-additive dispersion quality in the two-filler system. The CNFs appeared to be adequately dispersed and consistent across all samples, with noticeable fiber pullout holes or exposed nanofibers (Figure 8 expanded view) and little to no agglomerations noted. As with the CNF reinforced syntactic foams, the fracture surfaces of syntactic foams containing varied weight percentage additions of HNTs were analyzed. The failure modes of the thermoplastic microballoon syntactic foam containing 1 wt% addition of HNTs (Figure 9) is shown and representative of all samples. The dispersion quality of the HNTs was evaluated to determine if the inclusion of microballoons had an effect on nanotube agglomeration (Figure 9 expanded view). It was found that halloysite dispersion was identical to that seen in the initial epoxy specimens analyzed (Figure 1). HNTs are seen protruding from the fracture surfaces with no agglomeration observed.
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Figure 8. Scanning electron microscope image of a thermoplastic microballoon syntactic foam loaded with 1 wt% loading of carbon nanofibers tensile fracture surface with expanded view showing nanofibers present at the fracture surface.
Figure 9. Scanning electron microscope image of thermoplastic microballoon syntactic foam loaded with 1 wt% loading of halloysite nanotube tensile fracture surface with expanded view showing a nanotube present at the fracture surface.
Conclusions This work highlights a consistent and repeatable method for the production of highly loaded thermoplastic microballoon syntactic foams utilizing CNFs or HNTs as reinforcements. A consistent method for mixing of the nano-additives in epoxy resin was determined, showing adequate dispersion of each respective nanomaterial with little to no agglomeration. From mechanical analysis, it was observed that small weight percentage inclusions of each nano-additive into the syntactic foam matrix can lead to large enhancements in strength and modulus with negligible impacts on composite foam density. Compressive strength and modulus enhancements as large as 180% and 250% respectively can be achieved with a 0.25 wt% addition of CNF and increases of 165% and 244% respectively can be achieved with a 0.5 wt% addition of HNT. Tensile strength and modulus enhancements as large
as 110% and 165% respectively can be achieved with a 0.125 wt% addition of CNF and increases of 133% and 173% respectively can be achieved with a 0.125 wt% addition of HNT. Acknowledgement The authors acknowledge the National Aeronautics & Space Administration’s Experimental Program to Stimulate Competitive Research (EPSCoR) for the financial support of our research through grant NNX12AB17G. Acknowledgements are also due to the Composite and Polymer Engineering (CAPE) Laboratory and staff for equipment usage, guidance and technical assistance with experimentation.
Declaration of Conflicting Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Dando and Salem Funding The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was funded by NASA EPSCoR major research grant #NNX12AB17G.
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