Carbon nanofiber reinforced syntactic foams: Degradation mechanism for long term moisture exposure and residual compressive properties Ronald L. Poveda 1 , Gleb Dorogokupets 1 , Nikhil Gupta * Department of Mechanical and Aerospace Engineering, Polytechnic Institute of New York University, Brooklyn, NY 11201, USA article info Article history: Received 19 June 2013 Received in revised form 1 July 2013 Accepted 6 July 2013 Available online 13 July 2013 Keywords: Carbon nanofiber Syntactic foam Moisture Compressive strength High strain rate abstract Hollow particle filled polymer matrix composites, called syntactic foams, are challenging for studies related to environmental exposure and degradation mechanisms due to the possible role of matrix, particleematrix interface, and particle material and wall thickness. In the current study, syntactic foams reinforced with 1 wt.% vapor grown carbon nanofibers (CNFs) are subjected to water immersion and characterized for residual compressive properties under quasi-static and high strain rates. The testing is conducted on four different types of syntactic foams, fabricated with glass hollow particles of two different densities: 220 and 460 kg/m 3 in 30 and 50 vol.% quantities. After a period of 6 months, a maximum of 7% weight gain is observed in the worst performing syntactic foam. The exposed specimens are tested for residual compressive properties and the results are compared with the properties of dry specimens. The quasi-static compressive strength of CNF reinforced syntactic foams is found to decrease and the modulus remained unaffected due to the moisture exposure. The high strain rate compressive strength was 1.3e2.2 times higher for wet and dry specimens compared to the quasi-static strength of the same type of syntactic foams. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Multiscale reinforcement can result in development of a unique set of properties in composites. Carbon-based nanomaterials, such as carbon nanotubes and nanofibers, have been widely used with other microscale reinforcements such as fibers in composite ma- terials [1,2]. In particular, carbon nanofibers (CNFs) are gaining attention in recent times for the same purpose as summarized in the review articles [3,4]. CNFs can improve the strength and modulus of polymer composites [3e6]. Reinforcement of hollow particle filled composites, known as syntactic foams, with CNFs is of great interest. A small quantity of CNFs can significantly alter the properties of syntactic foams, without adversely affecting the density [7,8]. Evaluation of the environmental performance of polymer ma- trix composites is very important for marine and aerospace appli- cations [9,10]. These structures are exposed to severe moisture and temperature conditions [11,12]. The adoption of syntactic foams in such applications can be accelerated by determining the effect of environmental conditions on their properties. Environmental effects have been studied for polymer matrix composites [13,14], including syntactic foams [15]. In the present work, the effects of moisture absorption on the compressive properties of multiscale syntactic foams, reinforced with 1 wt.% CNFs, is studied. The CNFs are dispersed in four different grades of syntactic foams, which are distinguished by the difference in glass hollow particle (glass microballoon (GMB)) density and volume content. Cylindrical specimens used for compression testing are cut and immersed in deionized water until saturation. Periodic weight measurement of the specimens is car- ried out to quantify the moisture absorption over time. Specimens are then tested under quasi-static and high strain rate (in the range 700e1700 s 1 ) compression to observe any variation in the compressive strength and modulus between wet and dry speci- mens. Extensive scanning electron microscopy is conducted to determine the moisture related degradation mechanisms of syn- tactic foams. 2. Literature review A comprehensive literature survey of environmental effects on syntactic foams is presented in Table 1 . Plain (containing only matrix polymer and hollow particles) and reinforced (containing an additional reinforcing phase) syntactic foams are found * Corresponding author. Tel.: þ1 718 260 3080; fax: þ1 718 260 3532. E-mail address: [email protected](N. Gupta). 1 Tel.: þ1 718 260 3080; fax: þ1 718 260 3532. Contents lists available at SciVerse ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab 0141-3910/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2013.07.007 Polymer Degradation and Stability 98 (2013) 2041e2053
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Polymer Degradation and Stability 98 (2013) 2041e2053
0141-3910/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymdegradstab.2013.07.0
a b s t r a c t
Hollow particle filled polymer matrix composites, called syntactic foams, are challenging for studiesrelated to environmental exposure and degradation mechanisms due to the possible role of matrix,particleematrix interface, and particle material and wall thickness. In the current study, syntactic foamsreinforced with 1 wt.% vapor grown carbon nanofibers (CNFs) are subjected to water immersion andcharacterized for residual compressive properties under quasi-static and high strain rates. The testing isconducted on four different types of syntactic foams, fabricated with glass hollow particles of twodifferent densities: 220 and 460 kg/m3 in 30 and 50 vol.% quantities. After a period of 6 months, amaximum of 7% weight gain is observed in the worst performing syntactic foam. The exposed specimensare tested for residual compressive properties and the results are compared with the properties of dryspecimens. The quasi-static compressive strength of CNF reinforced syntactic foams is found to decreaseand the modulus remained unaffected due to the moisture exposure. The high strain rate compressivestrength was 1.3e2.2 times higher for wet and dry specimens compared to the quasi-static strength ofthe same type of syntactic foams.
� 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Multiscale reinforcement can result in development of a uniqueset of properties in composites. Carbon-based nanomaterials, suchas carbon nanotubes and nanofibers, have been widely used withother microscale reinforcements such as fibers in composite ma-terials [1,2]. In particular, carbon nanofibers (CNFs) are gainingattention in recent times for the same purpose as summarized inthe review articles [3,4]. CNFs can improve the strength andmodulus of polymer composites [3e6]. Reinforcement of hollowparticle filled composites, known as syntactic foams, with CNFs is ofgreat interest. A small quantity of CNFs can significantly alter theproperties of syntactic foams, without adversely affecting thedensity [7,8].
Evaluation of the environmental performance of polymer ma-trix composites is very important for marine and aerospace appli-cations [9,10]. These structures are exposed to severe moisture andtemperature conditions [11,12]. The adoption of syntactic foams insuch applications can be accelerated by determining the effect ofenvironmental conditions on their properties. Environmental
: þ1 718 260 3532.
All rights reserved.07
effects have been studied for polymer matrix composites [13,14],including syntactic foams [15].
In the present work, the effects of moisture absorption on thecompressive properties of multiscale syntactic foams, reinforcedwith 1 wt.% CNFs, is studied. The CNFs are dispersed in fourdifferent grades of syntactic foams, which are distinguished by thedifference in glass hollow particle (glass microballoon (GMB))density and volume content. Cylindrical specimens used forcompression testing are cut and immersed in deionized water untilsaturation. Periodic weight measurement of the specimens is car-ried out to quantify the moisture absorption over time. Specimensare then tested under quasi-static and high strain rate (in the range700e1700 s�1) compression to observe any variation in thecompressive strength and modulus between wet and dry speci-mens. Extensive scanning electron microscopy is conducted todetermine the moisture related degradation mechanisms of syn-tactic foams.
2. Literature review
A comprehensive literature survey of environmental effects onsyntactic foams is presented in Table 1. Plain (containing onlymatrix polymer and hollow particles) and reinforced (containingan additional reinforcing phase) syntactic foams are found
Table 1Available studies on environmental exposure of syntactic foams.
Refs. Syntactic foam type Testing conditions Results
Ishai et al., 1995 [18] Syntac� 350Matrix: epoxy resinParticles: GMBs (vol.% unspecified)Sandwich skin: carbon or glassfiber reinforced laminate
- Moisture absorption up to 2 years inwater at 25e74 �C
- Compression, tension, low-velocityimpact, post-impact flexural test
Compressive and tensile strength decrease asmoisture absorption increases, significantdrop in tensile strength between 0 and3 wt.% moisture content, damage size increaseswith impact energy, weight gain: 0.22e1.01 wt.%
Karthikeyan et al., 2001 [17] Matrix: epoxy resinParticles: Ecospheres SI GMBs(47.42 vol.%)Reinforcement: short E-glass fibers
- Moisture absorption for 30 days inwater vapor (70 �C), saline waterand sea water (room temperature)
- Compression test
Max weight gain: 7.36%, compressive strengthrange: 40e62 MPa, largest percentage strengthincrease/decrease: 21%/21%
Gupta, Woldesenbet,2003 [19]
Matrix: epoxy resinParticles: 3M GMB (65 vol.%)
- Moisture absorption between w900and 1500 h, salt and deionized water
between room temperature and 70 �C- Compression test
Compressive modulus decrease: 48%e68%,negligible changes in peak compressive strength,maximum weight gain at 70 �C: 3.9%e6.7% indeionized water and 1.9%e2.5% in salt water
- Moisture absorption over 6 monthsin deionized water at 100 �C
- Impedance analysis, thermalconductivity, compression test
Syntactic foam weight gain >50% over 6 months,decreases in strength and modulus due to aging,thermal and electrical conductivities increasewith mass gain
- Moisture absorption for over 2 yearswith fresh water between 50and 100 �C
- Thermal conductivity analysis
Water uptake does not level off at alltemperatures tested after over 2 years, longertime interval needed, thermal conductivityincreases linearly with water uptake
Permittivity difference between wet and normalGMB reinforcement was negligible due to similarhumidity conditions, permittivity increased withdry GMB reinforcement
- Moisture absorption for over3 months in deionized water at60 and 100 �C
- Compression test
Mass gain <5% at 60 �C and between 10% and50% at 100 �C after 3 months, mass gain increaseswith microsphere content, capability to resistload decreases with aging
- Moisture absorption for 90 days insalt and rain water at room temp.
- Compression and tensile test
Compressive strength decreased up to 14.5%,modulus decreased up to 41.6% and maximumweight gain was in the range 0.48%e0.97%
Roggendorf, Schnettler,2012 [16]
Matrix: aromatic epoxy resinParticles: acrylonitrile copolymermicroballoons with CaCO3 coating(0e40 vol.%)
- Moisture absorption for 1500 h at85 �C (climatic chamber) and 100 hat 120 �C (pressure cooker) at highrel. humidity
- Dielectric analysis
Increase in density up to 35 mg/cm3 in pressurecooker and up to 15 mg/cm3 in climatic chamber,dielectric constant increased less than 7% for allcompositions
- Moisture absorption ranging from1900 to 4550 h in salt and deionizedwater at room temp.
- Flexural test
Flexural modulus decrease: 6%e35%, maximumsyntactic foam weight gain at roomtemperature: 25%
R.L. Poveda et al. / Polymer Degradation and Stability 98 (2013) 2041e20532042
Fig. 1. Micrograph of a CNF/syntactic foam containing 50 vol.% GMBs. The inset on thetop right side shows a higher magnification view where CNFs can be seen dispersed inthe matrix resin between microballoons.
R.L. Poveda et al. / Polymer Degradation and Stability 98 (2013) 2041e2053 2043
subjected to various environmental conditions. The exposureperiod varies from a few days [16,17] to over two years [18]. Themoisture conditions include water vapor [17], salt water [19,20],rain water [21], sea water [15,22,23], and deionized water [24e26].The weight gain depends on the immersion time [16], temperature[18,27], applied pressure [15], foam composition [20], and particlesurface treatment [28]. Temperature has a strong effect on themoisture absorption in syntactic foams because diffusivity in-creases with temperature. Mismatch in the coefficient of thermalexpansion of hollow particles and matrix resin can cause in-terfaces to fail and lead to increase in moisture absorption. Thepresence of large ions in salt and sea water causes slower diffusionrates and saturation at lower moisture levels. The post-immersionmechanical properties of syntactic foams have been evaluatedthrough compression [17,19,22,23], tension [18,21], and flexuretesting [29,30]. Degradation of the mechanical properties of syn-tactic foam has been observed in most of the noted studies, wherethe modulus and the strength decrease by up to 68% [19] and26.9% [23], respectively.
Thermal properties such as conductivity [26,27] and electricalproperties such as impedance [26,31] and permittivity [16,28,32]have also been analyzed for environmentally exposed specimens.The thermal and electrical conductivities generally increase withmoisture uptake [26]. The thermal conductivity is found to increaselinearly with moisture uptake [27], and electrical conductivity isfound to vary between 10�9 and 10�8 W�1 cm�1 [31], which is anorder of magnitude of variation. Analysis of studies summarized inTable 1 shows that there is a great variability in the test conditionsand the follow up tests that are used to determine the extent ofmoisture-induced damage in syntactic foams. It can also be notedthat CNF reinforced syntactic foams are not yet investigated formoisture effects, which is the focus of the present work.
3. Materials and methods
3.1. Constituent materials
Epoxy resin DER 332 and hardener DEH 24 (DOW Chemical Co.,NY) are used as the matrix resin system. In all of the composites,1 wt.% CNFs are randomly dispersed. PR-24 CNFs (Pyrograf ProductsInc., OH) are used as the nanoscale reinforcement. The density ofCNFs is 1950 kg/m3 according to the manufacturer’s datasheet.Glass hollow particles (3M, MN) of nominal true particle density of220 and 460 kg/m3 are used in 30 and 50 vol.% to fabricate fourgrades of syntactic foams. The average diameter of these particles is35 and 40 mm, respectively, and the calculated nominal wallthickness is 0.52 and 1.29 mm, respectively [33].
For comparison with syntactic foams, one slab of 1 wt.% CNFreinforced epoxy matrix composite (referred to as CNF/epoxycomposite) is also fabricated. Table 2 provides compositions of CNFreinforced syntactic foams (referred to as CNF/syntactic foams)fabricated for the study.
3.2. Composite fabrication
A high shear impeller fitted onto a mechanical mixer is used todisperse CNFs in the epoxy resin at 650 rpm as per the previously
Table 2Composition of syntactic foams with 1 wt.% CNFs.
Microballoons(vol.%)
Carbon nanofibers(vol.%)
Epoxy resin(vol.%)
Hardener(vol.%)
30 0.42 61.03 8.5550 0.30 43.60 6.10
optimized procedure [8]. For the CNF/syntactic foams, GMBs areadded in addition to the initial CNF/epoxymixture, and the slurry ismixed slowly for an additional 15 min. The hardener is then addedand the slurry is stirred for an additional 5 min. The slurry is thenpoured into aluminum molds and then placed on a mechanicalshaker for 5 min to degas. The mixture is cured at room tempera-ture for 48 h, and further post-cured in an oven at 100 �C for 2 h.
The specimen nomenclature includes GMB density followed bytheir volume fraction; for example, N220-30 represents CNF rein-forced syntactic foams containing GMBs of 220 kg/m3 density in30 vol.%. A micrograph of a typical CNF reinforced syntactic foam isshown in Fig. 1. GMBs are observed to be uniformly dispersed in thesyntactic foam structure. CNFs are not visible at lower magnifica-tion but they can be seen dispersed in the matrix resin at highmagnification. The difference in the length scale of GMBs and CNFsis useful in obtaining high loading of GMBs to reduce the weight ofthe composite and reinforcing of matrix in the interparticle regionby means of CNFs.
3.3. Moisture absorption testing
Cylindrical specimens of 5 mm thickness and 10 mm diameterare machined from the fabricated slabs using a diamond core drill.The specimens are dried in a convection oven for 2 h at a temper-ature of 100 �C, and thenweighted in order to calculate the averagedensity of each composite and determine the weight gain duringenvironmental testing. For moisture absorption testing, a periodicweighing procedure is used, similar to previous studies [19,20].
The specimens are completely immersed in deionized water atroom temperature for environmental exposure. Specimen floata-tion due to the buoyancy behavior of syntactic foams is impeded inorder to ensure that all surfaces of the specimens are in contactwith water. The specimens are periodically taken out for weighing.Before weighing, the excess surface water is wiped and the speci-mens are left in air for about 5 min. Fig. 2 shows a three-dimensional model of syntactic foam microstructure, illustratingthat GMBs on the surface are cut and will be filled with water assoon as the specimen is immersed in water. The first measurementis taken after 10min of immersion to characterize thewater filled inthe surface GMBs. The specimen weight measurements are takentwice a day for the first seven days and then once a day until the
Fig. 2. A 3D model representing syntactic foam microstructure. The microballoons onthe surface may be cut and their cavities may be filled with water as soon as thespecimen is immersed in water.
Table 3Theoretical and experimental density values for various composites fabricated in thepresent study with 1 wt.% CNFs.
Material type Experimental density (kg/m3) Theoretical density (kg/m3)
R.L. Poveda et al. / Polymer Degradation and Stability 98 (2013) 2041e20532044
total immersion time of six months. The specimens are then sub-jected to compression testing.
3.4. Quasi-static compression
The quasi-static compression tests are conducted using an Ins-tron 4469 mechanical test system with Bluehill 2 software. Thecompression testing is conducted in the displacement controlmode at a constant crosshead velocity of 1 mm/min. Lubricant isapplied between the specimen and the compression platens toprevent specimen barreling. The compressive modulus andstrength of each tested specimen are calculated from the resultingloadedisplacement data. The compressive strength is defined bythe peak stress at the end of the elastic region. In the absence of aclear stress peak, tangents are drawn in the elastic and plastic re-gions and the stress corresponding to the intersection of thesetangents is taken as the compressive strength.
0
1.5
3
4.5
6
7.5
0 25 50 75
Mo
istu
re a
bso
rb
ed
(%
)
√t (√h)
CNF/epoxyN220-30N220-50N460-30N460-50
Fig. 3. Profile of moisture absorption in CNF/epoxy and CNF/syntactic foam compositesover time. All composites contain 1 wt.% CNF. Multiple specimens of each materialwere tested but the data are shown only for one specimen for clarity.
3.5. High-strain rate compression
High strain rate compression testing was conducted using an in-house developed split-Hopkinson pressure bar (SHPB) test system.An in-depth description of the SHPB can be found in publishedstudies [34,35]. Inconel 718 alloy incident and transmitter bars areused in the SHPB setup. Strain gages placed on the incident and thetransmitter bars are used to record the strain pulses propagating inthe bar during the test. The time dependent stress sðtÞ, strain εðtÞ,and strain rate _εðtÞ are calculated by Gray [36]
_εðtÞ ¼ 2cbεrðtÞl0
(1)
sðtÞ ¼ AEεtðtÞA0
(2)
εðtÞ ¼Zt
0
_εðsÞds (3)
where cb is the sound wave velocity in the bars, εrðtÞ is the reflectedstrain recorded from the incident bar, εtðtÞ is the transmitted strain
recorded from the transmitter bar, A and E are the cross-sectionalarea and the elastic modulus of the bars, respectively, and A0 andl0 are the cross-sectional area and the length of the specimen,respectively.
3.6. Microscopy
A Hitachi 3400N variable pressure scanning electron micro-scope (SEM) equipped with secondary and backscattered electrondetectors is used for imaging the failure surfaces. Specimens arecoated with a conductive layer of gold before imaging.
4. Results and discussion
The experimental and theoretical densities of each compositeare presented in Table 3. The theoretical density is calculated usingthe rule of mixtures with an epoxy resin density of 1160 kg/m3. Thecalculated and measured density values differ from each other byonly 0.7%e3.7%, indicating that there is no significant air voidentrapment in the matrix during the composite fabrication. Thematrix air voids may significantly affect the moisture absorptionand retention characteristics of syntactic foams.
4.1. Moisture absorption behavior
The compression test specimens are used for the moisture ab-sorption study, followed by the measurement of residualcompressive properties at quasi-static and high strain rates. Theprofile of moisture uptake over time for each composite is pre-sented in Fig. 3. One representative specimen of each syntacticfoam composition is selected for display in this figure to avoid
R.L. Poveda et al. / Polymer Degradation and Stability 98 (2013) 2041e2053 2045
clutter. It can be observed in Fig. 3 that maximum weight gain inmost of the syntactic foams plateaus between 0.75% and 2% mois-ture absorption, with the only exception being the N220-50 syn-tactic foam. For comparison purposes CNF/epoxy composites arealso tested under the same conditions. These composites exhibit alower level of moisture absorption compared to the CNF/syntacticfoams likely due to the absence of GMBs in their structure. Thefollowing trends can be observed in Fig. 3:
� The moisture uptake of CNF/epoxy is the lowest compared toany syntactic foam.
� The moisture uptake of syntactic foams containing thin walledGMBs is higher than that of thick walled GMBs.
� The syntactic foams containing 50 vol.% GMBs absorb a higheramount of moisture than those containing 30 vol.% GMBs atlong term moisture exposure.
Several water uptake mechanisms are possible in composites[15], such as diffusion in the polymer resinmatrix, infiltration of theparticleematrix interface, and degradation and filling of the GMBs.Considering that the GMB volume fraction is high in the presentsyntactic foams, the epoxy film between GMBs may be thin, whichcan facilitate diffusion of water molecules to the particleematrixinterface and degradation of interface leading to debonding. If theglass material was inert to the presence of water, there would havebeen no effect of GMB wall thickness on the moisture absorptionprofile of syntactic foams containing the same volume fraction ofGMBs. However, Fig. 3 shows that the syntactic foams containingthin walled particles absorb a higher amount of water, which im-plies that there is an interaction betweenmoisture and GMBs. CNFsare several orders of magnitude smaller than GMBs in dimensions,so no visible role of CNFs in found in the moisture absorptionbehavior of syntactic foams. The level of moisture absorption inCNF/syntactic foams is similar to that in plain syntactic foamsobserved in previous studies [19,20].
The moisture absorption curve shown in Fig. 3 can be dividedinto five regions as schematically represented in Fig. 4. Region 1corresponds to the water that fills in the cut GMBs that are presenton the surface of syntactic foam specimens as illustrated in Fig. 2.This is not the water uptake due to diffusion and such surface watercan be easily removed by drying procedures. In the present case, itis assumed that the first weight gain reading after immersioncorresponds to the water that is filled in the cavities of the surfaceGMBs. This weight gain reading is subtracted from all subsequentreadings to identify the moisture uptake due to diffusion in thepolymer or at the GMBematrix interface. Region 2 corresponds tomoisture diffusion in the polymer matrix and at the particle matrixinterface. The rate of diffusion depends on the temperature of thetesting and the properties of the polymer. Over a period of time, themoisture content in the polymer and at the interface saturates andit appears in the form of Region 3. The composites are shown to
Fig. 4. Schematic representation of specimen weight gain over time. Five stages areidentified in the graph. The figure is not drawn to scale.
exhibit Fickian diffusion behavior when the moisture absorption isprimarily in the polymer matrix [37], which corresponds to Region2 in Fig. 4, and the weight gain in most composites is shown toeventually plateau. The linear trend of weight gain is observed tooccur within the first 625 h for all CNF/epoxy and CNF/syntacticfoams.
If the particles were inert to the moisture attack then thesaturation level of Region 3 would be maintained. However,appearance of a second region of increasing moisture absorptionindicates that the particles degrade over time and absorb additionalmoisture, which is represented by Region 4. When the particles arecompletely degraded and their cavities are filled with water, thefinal saturation level, Region 5, is achieved in the syntactic foamspecimens. All GMBs present in a syntactic foam specimen do nothave the same size and wall thickness [38]. In addition, the thinwalled particles and the particles present near the surface maydegrade earlier than those present deep inside the material.Therefore, in a real syntactic foam specimen, where particles of awide range of sizes and wall thicknesses may exist, the stagesindicated by Fig. 4 overlap with each other and may not be wellresolved. The following discussion will be focused on obtainingexperimental evidence of degradation of particles and particleematrix interface.
Based on their composition, different types of glasses havedifferent interactions with moisture. In the present case, sodalimeborosilicate GMBs are used. The environmental characteristics ofGMBs are determined by conducting a separate experiment. Abeaker was filled with water, which was maintained at 95 �C usinga hot plate. A small quantity of thick walled GMBs (460 kg/m3
density) was poured into the beaker and allowed to age for fivedays. The high temperature is expected to accelerate the degrada-tion process; similar procedures have been previously used withborosilicate glass [39]. GMBs were then extracted, dried for 24 hand then observed using a SEM to see any signs of degradation.
Scanning electron micrographs of GMBs subjected to acceler-ated weathering are shown in Fig. 5. Degradation can be attributedto the presence of sodium, which is a prominent element in thecomposition of GMBs. Alkali elements such as sodium leach out incontact with water through dealkalization of the glass, causing theoverall structure to degrade. The outer layer of GMBs becomesporous due to dealkalization, as observed by previous studies onglasses [40,41]. Flakes of porous material, fine granular particles,and scales on the GMB surface can be observed in SEM imagesshown in Fig. 5. Based on the degradation observed in Fig. 5, it islikely that the GMBs have degraded in syntactic foams over the longterm exposure and moisture saturation of syntactic foam speci-mens. Fig. 5 shows degradation of thick walled (1.29 mm) GMBs; thethinwalled (0.52 mm) GMBs may take considerably less time beforetheir walls degrade enough to open up the void for water to seepinside and lead to the appearance of Regions 4 and 5 in the mois-ture absorption trends. Fig. 3 corroborates with these observationsand shows that the moisture absorption in syntactic foams con-taining thin walled GMBs is higher and possible early degradationof GMB wall leads to higher moisture content for saturation.
Based on the information about particle degradation, the overallmechanism of moisture absorption and degradation in syntacticfoams is summarized in Fig. 6. Initially, the moisture uptake isthrough the polymer matrix and the interface of the particlesexposed on the cut surface of the specimens. Over time the mois-ture diffuses inside the specimen and the attack on particleematrixinterface may lead to partial or complete debonding. The moisturepresent in the interface starts to degrade GMBs by a dealkalizationprocess. The GMB degradation leads to formation of scales, granularfine particles, and pores in the GMB wall. Sufficient degradation ofTHE particle wall opens up the cavity in the GMB, which is filled
Fig. 5. (a) Degradation of GMBs having 460 kg/m3 density as evidenced by the formation of pores and appearance of outer layers, (b) formation of particulates due to significantdegradation of a microballoon, (c) flaking of porous outer layer on microballoons, (d) microballoon displaying surface damage and formation of a porous layer.
R.L. Poveda et al. / Polymer Degradation and Stability 98 (2013) 2041e20532046
with water over time. In obtaining experimental evidence of thismechanism, one of the concerns is that only the surface layer ofsyntactic foam may be affected and the moisture may not diffusedeep inside the specimen. Some previous studies assumed theentire specimen to be saturated with moisture but no direct evi-dence was presented [15,20]. Hence, a weathered specimen of
Void
Microballoon Epoxy (Matrix)
Water
Increasing moisture e
Syntactic foam immersed in water
Water diffuses in the polymer matrix and attacks interface
Fig. 6. Illustration of moisture dama
N220-50 CNF/syntactic foam was randomly selected for micro-scopic observation throughout its thickness to detect the depth ofmoisture damage and the extent of GMB degradation. The spec-imen was split in half by lightly hammering a sharp chisel and thefracture surface was observed through SEM. The specimen is 5 mmthick; the microscopic observations are taken at the subsurface
xposure time
Water progressively breaks down GMB structure to form
fine particles and flakes
GMB Cavity filled with water and residual
glass particles
ge sequence in syntactic foams.
Fig. 8. Micrograph of GMBs located at about 750 mm depth from the specimen surface. Degradation due to moisture can be seen in the GMBs.
Fig. 7. Micrographs of GMB damage inside an N220-50 specimen. The observation is taken at about 50 mm depth from the outer surface of the specimen.
R.L. Poveda et al. / Polymer Degradation and Stability 98 (2013) 2041e2053 2047
Fig. 9. Surface damage of a GMB located in the mid-section of the specimen (about2.5 mm through the thickness).
R.L. Poveda et al. / Polymer Degradation and Stability 98 (2013) 2041e20532048
level, at 50 mm depth, and in the internal regions, at 750 mm and2.5 mm depth, from the surface.
Examples of damage in the syntactic foam microstructure atdifferent depths are shown in Figs. 7e9. The GMB damage is
(a)
(c)
0
30
60
90
120
0 0.1 0.2 0.3 0.4
Stress (
MP
a)
Strain (mm/mm)
Dry WetDry Wet
N220-30
N460-30
0
50
100
150
200
250
0 0.1
Stress (
MP
a)
Strain
Fig. 10. Typical stressestrain curves of (a) N220-30 and N460-30, (b) N220-50 and N460-5material type were tested but results for only one specimen of each type are shown for cla
severely close to the specimen surface and progressively de-creases through the thickness. The GMBs located at 50 and750 mm depth from the specimen surface show signs of dealkal-ization leading to flaking and partial disintegration of the GMBwall. The GMB located in the middle of the specimen also showssigns of surface degradation but the wall has not yet disintegratedenough to open the cavity for moisture accumulation, which mayoccur over time. The GMB degradation at all depths of the syn-tactic foam specimen confirms that the moisture has penetratedthroughout the specimen thickness and is not limited to only thesurface layer. It is noted that not all the GMBs present in thespecimen degraded due to moisture. It is possible that somepreferential diffusion paths developed in the specimen. However,it is expected that the signs of GMB degradation become moreand more severe over time. The debonding and surface degra-dation caused by moisture would affect the mechanical propertiesof syntactic foams.
4.2. Quasi-static compressive properties
The stressestrain curves of representative wet and dry CNF/epoxy composites and CNF/syntactic foams are presented in Fig. 10.These graphs show trends similar to those observed in previous
(b)
0
30
60
90
120
0 0.1 0.2 0.3 0.4
Stress (
MP
a)
Strain (mm/mm)
Dry WetDry Wet
N220-50N460-50
0.2 0.3 0.4, mm/mm
Dry Wet
0, (c) CNF/epoxy composite under wet and dry conditions. Multiple specimens of eachrity.
(a) (b)
0
0.5
1
1.5
2
2.5
Co
mp
ressive m
od
ulu
s (
GP
a)
Foam type
Wet Dry
0
30
60
90
120
Co
mp
ressive s
tren
gth
(M
Pa)
Foam type
Wet Dry
Fig. 11. Comparison of (a) compressive modulus and (b) compressive strength of moisture exposed and dry CNF/syntactic foams containing 1 wt.% CNFs.
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studies for plain syntactic foams that did not contain any additionalreinforcement [19,42]. The characteristic stress plateau is obtainedin the compressive stressestrain graphs of CNT/syntactic foams.Compressive modulus and strength are calculated from thesegraphs and are presented in Fig. 11. CNF/epoxy composites showhigher modulus in wet condition than in dry condition. Themodulus of epoxy resins is found to increase by long termmoistureexposure as observed by several previous studies listed in a reviewarticle [43]. Nanoclay/epoxy composites have also shown a similartrend [44]. It is also likely that any microcracks or voids present inthe material are filled with the infused moisture and resulted inincreased modulus in the wet condition.
Fig. 12. Micrographs of (a) N220-30, (b) N220-50, (c) N460-30, and (d) N460-50 CNF/syntactobserved in the microstructures of all specimens. Microballoon crushing is not prominentl
There is no significant difference in the modulus of wet and dryCNF/syntactic foam specimens. In some cases, the standard devia-tion is large and there is no conclusive evidence that the moistureabsorption decreases or increases the modulus of CNF/syntacticfoams. Increase in the modulus of matrix and decrease in themodulus of GMBs due to degradation can offset each other toprovide nearly the same level of modulus inwet and dry specimens.It appears that CNFs helped in retaining the modulus of syntacticfoams because several available studies on plain syntactic foamsshowed decrease in modulus due to moisture absorption. Thecompressive strength of wet CNF/epoxy composites is 28% lowerthan that of dry specimens. The strength of all syntactic foams is
ic foams tested under quasi-static compression. Debonded and intact microballoons arey observed in these micrographs.
Fig. 13. GMB debonding in (a) N220-50 and (b) N460-50 CNF/syntactic foams. Fig. 14. Micrographs of CNFs on the failure surface in (a) N220-30 and (b) N460-30CNF/syntactic foams.
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decreased by 25%e35% due to the moisture exposure, which issimilar to CNF/epoxy composites.
The microstructural features of CNF/syntactic foams failed un-der quasi-static compression are shown in Fig. 12. The compres-sive failure of syntactic foams usually shows a large amount ofcrushed GMBs [38,45], but a large number of particles are seenintact in Fig. 12 due to debonding. The debonded particles areshown in Fig. 13, where a clean interface is observed between theparticle and the matrix. It is likely that the matrix cracking startsat the onset of the specimen failure due to the increased modulusof the matrix after moisture exposure. The debonded GMBs canmigrate to these cracks and are preserved from crushing. A rela-tively larger amount of GMBs seem to survive in syntactic foamscontaining thicker walled particles as observed in Fig. 12. Thematrix failure is shown in Fig. 14, where CNFs can be observed onthe failure surface. The crack bridging and deflection caused byCNFs is helpful in retaining the strength of syntactic foams. Allfracture surfaces observed using SEM show a large number ofCNFs on the surface.
4.3. Effect of strain rate on compressive properties
The high strain rate compressive stressestrain graphs for arepresentative material, N220-30 CNF/syntactic foam, in dry andwet condition are shown in Fig. 15. Similar profiles are observedfor other material types. Specimen failure is obtained in the formof fragmentation at the end of these tests. The characteristics ofthese curves are different from those observed under quasi-staticcompression (Fig. 10). The stress plateau region is considerably
shorter and the specimen failure takes place at lower strain thanthat obtained under quasi-static compression. The densification atthe end of stress plateau under quasi-static compression is notobserved at high strain rates. The strain rate is not directlycontrolled in SHPB testing; it is recovered from the test results.Hence, testing multiple specimens at the same strain rate is notpossible in the SHPB technique. In addition, it has been previouslydiscussed that reliable modulus values are not obtained from SHPBtesting because of the steep rise in strain rate in the initial part ofthe testing [34]. Therefore, only strength values are calculated andcompared in Figs. 16 and 17 for N220 and N460 type syntacticfoams, respectively, in wet and dry conditions. The specimens aretested in the strain rate range of 700e1700 s�1. It can be observedin Figs. 16a and 17a that within the high strain rate region, CNF/syntactic foams do not show any clear indication of strain ratesensitivity. Wet specimens containing 50 vol.% GMBs have thelowest strength for both particle types. The specimens exposed tomoisture show lower strength at high strain rates compared to dryspecimens of the same type. These trends are similar for syntacticfoams containing the same GMB volume fraction, irrespective ofGMB wall thickness. Figs. 16b and 17b compare the high strain ratestrength with respect to the quasi-static compressive strength. It isfound that the strength at high strain rates is 1.5e2.2 and 1.3e1.8times higher for syntactic foams containing thin and thick walledGMBs, respectively. For both GMB types, the wet specimens showa higher strength increase than the dry specimens. The largerfailure strain in wet specimens allows greater deformation andsuppresses fragmentation of specimens, which leads to higherstrength.
(a) (b)
30
60
90
120
150
180
700 1000 1300 1600 1900
Peak stress (
MP
a)
Strain rate (s-1
)
N460-30 DryN460-30 WetN460-50 DryN460-50 Wet
0.5
1
1.5
2
2.5
700 1000 1300 1600 1900
σH
SR/σ
QS
Strain rate (s-1
)
N460-30 DryN460-30 WetN460-50 DryN460-50 Wet
Fig. 17. Comparison of (a) peak stress and (b) normalized peak stress of dry and moisture exposed specimens for N460-30 and N460-50 specimens.
(a) (b)
30
60
90
120
150
180
700 1000 1300 1600 1900
Peak stress (
MP
a)
Strain rate (s-1
)
N220-30 DryN220-30 WetN220-50 DryN220-50 Wet
1
1.5
2
2.5
3
700 1000 1300 1600 1900
σH
SR/σ
QS
Strain rate (s-1
)
N220-30 DryN220-30 WetN220-50 DryN220-50 Wet
Fig. 16. Comparison of (a) peak stress and (b) normalized peak stress of dry and moisture exposed specimens for N220-30 and N220-50 specimens.
(a) (b)
0
30
60
90
120
150
0 0.05 0.1 0.15
Stress (
MP
a)
Strain (mm/mm)
880 /s
1340/s
1670/s
1700/s0
30
60
90
120
150
0 0.05 0.1 0.15
Stress (
MP
a)
Strain (mm/mm)
820 /s
1390 /s
1480 /s
1600 /s
Fig. 15. High strain rate compressive stressestrain graphs for N220-30 specimens at (a) dry and (b) moisture exposed conditions.
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R.L. Poveda et al. / Polymer Degradation and Stability 98 (2013) 2041e20532052
5. Conclusions
The environmental degradation under water immersion con-ditions is studied for carbon nanofiber (CNF) reinforced syntacticfoams containing glass microballoons (GMBs). The immersion isconducted for over six-months, until saturation in weight gain isobtained. Quasi-static and high strain rate compressive propertiesare characterized for the as-fabricated and moisture exposedspecimens. Most compositions in the study exhibit a moistureabsorption percentage between 0.75% and 2%, which the exceptionbeing N220-50 syntactic foam. It is found that the initial moistureuptake in syntactic foams takes place through the epoxy resinmatrix and particle/matrix interface and the specimen reaches asaturation level. Over time, degradation of GMBs is observedbecause of dealkalization. After sufficient degradation of GMBwalls, water may penetrate into the GMB cavity and provide thesecond saturation level. The moisture exposure does not affect thequasi-static compressive modulus of CNF/syntactic foams butdecreased the strength by about 30%. The high strain rate strengthwas found to be 1.3e2.2 times higher for both wet and dry syn-tactic foams depending on the GMB wall thickness and volumefraction.
Acknowledgments
This work is supported by the Office of Naval Research throughthe grant N00014-07-1-0419 and the National Science FoundationGK-12 Fellows grant 0741714. The authors thank 3M, MN forproviding glass microballoons and relevant technical information.Dr. Dung D. Luong and Níckolas Gonçalves Dutra are thanked forhelpwith some images. Authors alsowish to thank Dr. Gary Gladyszfor useful discussions about particle degradation.
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