A crumb rubber modified syntactic foam

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  • Materials Science and Engineering A 474 (2008) 390399

    A crumb rubber modified sya,b, u Jo


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    In this stu ifiedcrumb rubbe overglass beads y mahybrid foam ct tesconducted o ur-pospecimens a foamwas evaluate singsyntactic foa endinthe hollow g inter 2007 Else

    Keywords: Fo eleme

    1. Introduction

    Syntactic foams are particulate-filled composite materialsconsistingThey havetures due tmatrix usuepoxies. Thhigh strengity, and crecross-linksin a brittleof the epoxweak and thtoughness ostrength an

    An effecticles [31impact ene

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    ing to higher toughness of the matrix. In addition, the lowerstiffness rubber particles serve as stress concentrators. Once thestress exceeds the strength of the materials, microcracks will

    0921-5093/$doi:10.1016/jof hollow glass spheres embedded in a resin matrix.been used as the core in composite sandwich struc-o their light weight and water tightness. The resinally used in manufacturing the foam materials areey are preferred to other matrix systems due to theirth and stiffness, thermal and environmental stabil-ep resistance [1]. These epoxy systems tend to formwhen curing. This cross-linking mechanism results

    behavior of the epoxies [2]. Owing to the brittlenessies, the impact tolerance of the epoxy-based foam ise residual strength is low. It is desired to improve thef the epoxy matrix without considerably sacrificing

    d stiffness.tive way of toughening epoxies is to add rubber par-

    0]. Evidently, the rubber particles can absorb morergy through elastic deformation of the particles, lead-

    ding author at: Department of Mechanical Engineering, Louisianaity, Baton Rouge, LA 70803, USA. Tel.: +1 225 578 5302;78 5924.dress: guoli@me.lsu.edu (G. Li).

    initiate. Accompanying the creation of microcracks, a consid-erable amount of impact energy will be consumed, resultingin higher energy absorption capacity. However, these microc-racks will not easily develop into macrocracks. The propagationof microcracks will be blunted, stopped, and arrested by therubber particles through mechanisms like rubber pinning andrubber bridging-over. Therefore, the addition of rubber parti-cles provides a way of absorbing impact energy; it also providesa mechanism of preventing the microcracks from developinginto macrocracks or catastrophic structural failure. In summary,the rubber particles may enhance the impact tolerance of theepoxy matrix through micro length-scale damage and elasticdeformation.

    Azimi et al. [11] investigated the fatigue crack propagationand fracture toughness of a modified syntactic foam containingboth glass microballoons and reactive liquid rubber. They con-tributed the enhanced crack propagation resistance and fracturetoughness to a synergistic action between the microballoon andthe rubber modified epoxy matrix. Gupta et al. [12] investigatedthe static compressive toughness of glass microballoon basedsyntactic foams containing a small amount (2% by volume) of

    see front matter 2007 Elsevier B.V. All rights reserved..msea.2007.04.029Guoqiang Li , Mana Department of Mechanical Engineering, Louisiana State U

    b Department of Mechanical Engineering, Southern UniveReceived 27 January 2007; received in revised form 5 A

    dy, the impact response and residual strength of a crumb rubber modrs, were investigated. The foam had a hybrid microstructure bridgingand crumb rubber particles into a microfiber and nanoclay filled epoxas core and fiber reinforced epoxy as facings. A low velocity impa

    n the sandwich beams and control beams made of the foam only. Fond control specimens without impact damage. The effect of the hybridd based on the test results. The stress field interaction was evaluated um possessed a higher capacity to dissipate impact energy and to retain blass bead particles and crumb rubber particles by means of stress fieldvier B.V. All rights reserved.

    am; Crumb rubber; Low velocity impact; Sandwich; Residual strength; Finitentactic foamhn a

    ity, Baton Rouge, LA 70803, USABaton Rouge, LA 70813, USA007; accepted 6 April 2007

    syntactic foam, which contained up to 20% by volume ofseveral length scales. It was formed by dispersing hollowtrix. Sandwich beam specimens were prepared using thet using an instrumented drop tower impact machine wasint bending tests were conducted on the impact damagedon the low velocity impact response and residual strengtha finite element analysis. It was found that the rubberizedg strength. There was a positive composite action betweenaction and reduction in stress concentration.

    nt analysis

  • G. Li, M. John / Materials Science and Engineering A 474 (2008) 390399 391

    crumb rubber particles. They found a significant increase in com-pressive toughness and energy absorption. However, they alsoobserved about 50% decrease in Youngs modulus and about10% reduction in compressive strength. It is believed that thesmall amount of crumb rubber particles may only serve as inclu-sions in the foam. They may not fully display their synergy orpositive composite action with microballoons. Also, as a sand-wich core, it is desired to know its behavior when subjectedto transverse bending loads and dynamic impact loads becausetransverse bending loads are more important than uniaxial com-pressive loads for composite sandwich structures and foreignobject impacts, in particular low velocity impacts, which cannotbe avoided during manufacturing, transportation, and installa-tion.

    In this study, a similar rubberized syntactic foam was inves-tigated. Th(microballoticles into aexpected thticular struwould servabsorbing idebonding;ness; the mor sites forthe matrix iused. Hereume fractio[13]; whileonly on theof the nanocalated, orlighter, stroimpact tole

    The objits impactmechanismstrength. Tbending tewich specmicroscopyaged specimthe toughesis was con

    reduction in stress concentration due to the presence of bothmicroballoons and rubber particles.

    2. Specimens preparation and experimentation

    2.1. Raw materials

    The epoxy, DER 332 and the hardener DEH 24 were obtainedfrom DOW Chemical. The mixing ratio for the epoxy and thehardener was 17:3 by volume. Nanomer I.28E was supplied byNanocor Incorporation. The 1.6-mm long milled glass fiberswere provided by Fiberglast Developments Corporation. Q-cel6048 hollow glass particles were received from Potters Indus-tries Incorporation and crumb rubber GF 170 was obtainedfrom Rouse Polymerics. E-glass 7715 style plain woven fabric

    ed frparinme



    prin ofed wing acorpto foionsixednic pic wplitwa

    n, i.eas e

    r, thexynex

    rticlens. PepoxEHwas

    Table 1Physical and

    Material ateation

    DER 332DEH 24DER 332 + DNanomer I.28Milled fibersCrumb rubbeGlass beadsE-glass 7715is foam was formed by dispersing hollow glass beadsons) and a considerable amount of crumb rubber par-microfiber and nanoclay filled epoxy matrix. It wasat each component would be responsible for a par-

    ctural or functional property. The hollow glass beadse to reduce the weight and provide a mechanism formpact energy by glass beads crushing and interfacialthe crumb rubber particles would enhance the tough-icrofiber and nanoclay would increase mechanismsenergy absorption and would also serve to reinforcef a sufficient amount of microfiber and nanoclay werea sufficient amount of microfiber means the fiber vol-n must be larger than the critical fiber volume fractiona sufficient amount of nanoclay required depends notvolume fraction of nanoclays but also on the statusclay in the polymer matrix, phase separated, inter-exfoliated. It was expected that this foam would benger, stiffer, and tougher. It would have an increasedrance and structural capacity.ective of this study was to experimentally evaluatetolerance and residual strength, and understand thes for improvement in impact response and residualo this end, both low velocity impact and four-pointsts were conducted on foam specimens and sand-imens with the foam as core. Scanning electron

    (SEM) studies were performed on the impact dam-ens to examine the fracture surface and also study

    ning mechanisms involved. A finite element analy-ducted to understand the stress field interaction and

    obtainfor precal andproper

    2.2. F

    Thepersioachievoperatrials Inwaves

    cavitatthen multrasoacoustand ammixingpersioepoxy,Furthethe epo

    Theber pafractioto theener Dslurry

    mechanical properties of raw materials

    Flexural strength(MPa)

    Flexuralmodulus (MPa)

    Tensile strength(MPa)


    EH 24 108 2793 66 4.4E

    3448 4.8r

    fabric 3000 om Fiberglast Developments Corporation was usedg the sandwich skins. Table 1 summarizes the physi-

    chanical properties of the epoxy resin system and theof the constituents added.


    mary step of the fabrication process involved dis-nanoclay I.28E in epoxy DER 332. Dispersion wasith the help of a Sonics Vibracell ultrasonic probet a power of 750 W obtained from Sonics and Mate-oration. Ultrasonic mixing uses high-energy sonicrce intrinsic mixing of particles and matrix via sonic. The required amount of nanoclay was measured andmanually with epoxy in a beaker for 34 min. Therobe was immersed in the mixture, tuned to produce

    aves that resonate at a frequency of 20 kHz 50 Hzude of 40% of the maximum amplitude. Ultrasonics continued for 20 min, which ensured a proper dis-., without clustering, of the nanoclay particles in thevidenced by the SEM observations in Figs. 1113.1.6 mm long milled glass microfibers were added to

    nanoclay mixture and mixed for 34 min.t step of fabrication involved premixing of crumb rub-s and the hollow glass beads in the required volumeremixed rubber and glass particles were then addedynanoclaymicrofiber mixture along with the hard-24 and mixed with a mechanical blender till a thickobtained. This slurry was then cast into a wooden

    (%)Viscosity (cps) Average particle

    size (m)Density (g/cm3)

    40006000 1.1619.522.5 1.07

    900 1015 1.9 15.8 2.5 89 1.15 50 0.48 2.54

  • 392 G. Li, M. John / Materials Science and Engineering A 474 (2008) 390399

    Table 2Volume fractions of each batch (%)

    Batch no.

    1 2 3 4

    Epoxy 100 40 40 40Microballoon 0 57.6 47.6 37.6Crumb rubber 0 0 10 20Nanoclay 0 1.6 1.6 1.6Microfiber 0 0.8 0.8 0.8

    mold and ccuring in aspecimen wlong, 50.8-

    Four bation of eachObviously,with Batchand residuation of Batwith crumbBatch 2, thBatch 4 waadditionalber particlerubber add20% for Ba

    Each baspecimens.of E-glassa sandwichto be usedcontainedimens werwere left ubatch in Btested andsandwich swere firstlyto determin3 and 4, thwere four-pwere four-pand then fostrength.

    After thbatch were

    the volume was calculated. Thus the nominal density of eachbatch of specimens could be evaluated. This was compared withthe theoretical density obtained by the rule-of-mixtures method[13].

    Table 3 summarizes the density calculations for differentbatches of foam core specimens. From Table 3, there is a closeagreement between the nominal density and the theoretical den-sity obtained by the rule-of-mixtures method. It is also seenthat the Batch 2 specimen has the highest reduction in den-sity compared with the Batch 1 (pure resin) specimen. This can

    ibuted to the highest concentration of hollow glass parti-n theightimenmicres





    /s, 3

    Fourderngthg tesS 81engthenst be

    tructg tes


    ed ustigae ti

    Table 3Density calcu


    1234ured for 24 h at room temperature followed by postn oven at 100 C for 3 h. After post curing, the slabas demolded and cut to beam specimens 304.8-mm

    mm wide and 15.2-mm thick.tches of specimens were prepared. The volume frac-

    constituent in each batch is summarized in Table 2.Batch 1 was the control batch. By comparing Batch 21, the effect of microballoon on the impact responsel strength can be identified. Batch 3 was a modifica-ch 2 by replacing 10% by volume of microballoonsrubber particles. Therefore, comparing Batch 3 withe effect of rubber incorporation can be identified.s a further modification of Batch 3 by replacing an10% by volume of microballoons with crumb rub-s. Therefore, from Batch 2 to Batch 4, the effect of

    ition can be evaluated (rubber content 0%, 10%, andtch 2, Batch 3, and Batch 4, respectively).tch in Batches 1 and 2 contained twelve identicalSix of these specimens were wrapped with two layers

    7715 plain woven fabric reinforced epoxy to preparestructure, while the others were left unwrapped,

    as core specimens. Each batch in Batches 3 and 427 identical specimens. Twenty-one of these spec-e wrapped as mentioned above, while the othersnwrapped, to be used as core specimens. For eachatches 1 and 2, three core specimens were impactthree were four-point bending tested; while threepecimens were four-point bending tested and three

    impact tested and then four-point bending testede their residual strength. For each batch in Batchesree core specimens were impact tested and threeoint bending tested; while three sandwich specimensoint bending tested and 18 were firstly impact testedur-point bending tested to determine their residual

    e fabrication process, the core specimens from each

    be attrcles. Oin a sl2 specthat of1, pure27%, fBatch

    2.3. E


    impacttigatepropagof theof theof 2 mused.

    2.3.2.In o

    ual strebendinan MTspan lspecimconducwich sbendin


    observto invethe samweighed. Based on the dimensions of each specimen, involved in


    Weight (g) Volume (cm3) Density (g/cm3)264 235.35 1.12192 235.35 0.82196 235.35 0.83208 235.35 0.88other hand, the addition of rubber particles resultedincrease in density when compared with the Batch, due to the higher density of rubber particles than

    roballoons. Compared with the control batch (Batchin), the reduction of density for Batch 2 was aboutwed by about 26% for Batch 3 and about 21% for


    velocity impact (LVI) testsI tests were conducted using a DynaTup 8250HVing machine according to ASTM D2444 to inves-impact resistance by measuring the initiation andn energies. The loadtime and energytime responsesimens were measured by the instrumented feature

    naTup. Experiments were conducted at velocitiesm/s, and 4 m/s. A hammer weight of 3.4 kg was

    r-point bending teststo evaluate the inherent bending strength and resid-of the sandwich and foam core structures, four-pointts were performed according to ASTM C393 using0 machine. The cross-head speed was 4 mm/min and

    was 254 mm. After impact testing the foam corefailed completely, thereby making it impossible tonding tests on them. On the other hand, the sand-ures after impact testing were subjected to four-pointts to investigate the residual strength.

    phologyrface morphology of the tested specimens wassing a Hitachi S 3600N VP SEM. This was conductedte the crack pattern at the fracture surfaces and atme get an insight into the toughening mechanismsthe rubberized foam material.

    Reduction in density (%) Theoretical density (g/cm3) 1.15

    26.79 0.7925.89 0.8521.43 0.92

  • G. Li, M. John / Materials Science and Engineering A 474 (2008) 390399 393

    Table 4Impact test results of core specimens

    Initiation eneAverageStandard d



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