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

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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

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    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 rubbe