The mechanical behavior of microcellular open-cell foams prepared by a thermally induced phase separation process are investigated. The foams studied were prepared from isotactic polystyrene, polyacrylonitrile, and poly(4-methyl-1-pentene) (rigid foams), and polyurethane and Lycra (elastomeric foams). Their densities were in the range 0.04--0.27 g/cm3. Conventional polystyrene foams were used for comparison. The moduli and collapse stresses of these foams were measured in compression and compared with the current constitutive laws which relate mechanical properties to densities. A reinforcement technique based on the in-situ precipitation of silica was used to improve the mechanical properties. 13 refs., 4 figs., 3 tabs

Aubert, J.H. (Sandia National Labs., Albuquerque, NM (USA))

Mark, J.E. (Cincinnati Univ., OH (USA))

Ozkul, M.H.

UNT Digital Library

CERTAINDATACONTAINEDIN THISDOCUMENTMAYBEDIFFICULTTO'READIN IVilCROFICHEPRODUCTS.• THE MECHANICALBEHAVIOROF MICROCELLULARFOAMSM. H. Ozkul*, J. E. Mark*, and J. H. Aubert***Departmentof Chemistryand the Polymer Research Center SAND--91-0125CUniversityof Cincinnati,Cincinnati,OH 45221-0172**Sandia National Laboratories,Albuquerque,NM 87185 _ PP.91 00677 5ABSTRACTThe mechanical behaviorof microcellularopen-cell foams prepared by a thermallyinducedphase separation processare investigated.The foams studiedwere preparedfrom isotactic polystyrene, polyacrylonitrile, and poly(4-methyl-l-pentene) (rigidfoams), and polyurethaneand Lycra (elastomeric foams). Their densities were in therange 0.04-0.27 g/cre3. Conventional polystyrenefoams were used for comparison.The moduliand collapse stressesof these foams were measured in compressionandcompared with the current constitutive laws which relate mechanical properties todensities. A reinforcementtechnique based on the in-situ precipitationof silica was 'used to improvethe mechanicalproperties.iNTRODUCTIONMicrocoilular foams have primarily been developed for their use in inertialconfinement fusion in high energy physics laboratories [1]. Recently, however, thepossibilityof using these foams in biomedical and drug release applicationshas alsobeen discussed [2]. The preparation technique and pore structure of microcellularfoamsare differentthan those of conventionalfoams.These differencesare a resultofusinga thermally-inducedphase separationtechnique.First, the polymerand solventare heated above their critical point to form a homogenous solution, then phaseseparationis induced by loweringthe temperature, and finally the solvent is removedby either extraction or by vacuum sublimation, to produce a foam. The cells thusformed are open and have dimensions of 0.1 to 20 pm which are 10 to 100 timessmallerthan that of conventionalfoamshavingthe same density.The constitutivelawsfor conventionalfoams have been previouslyformulated [3,4]and are well understood.Under small strains, the w,_llelements perpendicularto theapplied force bend and the relative modulus depends on the relative densityaccordingto the followingequationfor open-cell foams:MASTER <,i.._.= C1 2where Ef and Es are the Young's moduli and pf and p=are the densities of the foamand cell wall polymer, respectively. Here C1 is a constant which was found Eexpenmentally to be nearly equal to 1 [4]. A theoretical model [5] predicted C1=0.91.When the applied force increases, the walls parallel to the force begin to buckle .creversiblyin the case of the elastomericfoams. The collapse mechanismand the __O,ISTRIBUTIONFTHISDOCUMENTISUNLIMITED.. IIDISCLAIMERThis report was prepared as an account of work sponsored by an agency of theUnited States Government. Neither the United States Government nor any agencythereof, nor any of their employees, makes any warranty, express or implied, orassumes any legal liability or responsibility for the accuracy, completeness, or use-fulness of any information, apparatus, product, or process disclosed, or representsthat its use would not infringe privately owned rights. Reference herein to any spe-cific commercial product, process, or service by trade name, trademark, manufac-turer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof.The views and opinions of authors expressed herein do not necessarily state orreflect those of the United States Government or any agency thereof.i '1resultingcollapsestressare differentfor the rigidpolymers;plastichingesare formedat the adjoiningpointsof verticaland horizontalwall elements and the collapse stressapi isgiven by;Opl P_j.f_3,2.... oy--;"c2(Ps' (2)i..where oysis the yieldingstrengthof the polymerand C2 is a constantwhichwas foundexperimentallyto be 0.30 [4]. For the closed-cellfoamsthe membrane forces and thepressureof the gas inthe cells shouldalso be considered.The elastic mechanical properties of microcellular foams have been previouslystudied [6,7] by the modelsgiven above, and the constant C1 in eq.(1) was foundto besmallerthan the predictedvalue. In thisstudy,an attempt was made to understandthediscrepancy between the theoretical and experimental results by introducingdefectsinto the structure of the foams by compressingthe rigid conventional ones in theplastic region. Large decreases in the modulus were observed during the reloadingexperiment.The polymers used to make the foams are isotactic polystyrene (IPS), poly-acrylonitrile (PAN), and poly(4-methyl-l-pentene) (TPX) (rigid foams), andpolyurethane (PU) and Lycra (elastomeric foams). The densities of foams are in theran_e 0.04-0.27 g/cre3.A techniquebased on the in-situprecipitationof silica,which has been widely usedfor the reinforcementof elastomers[8], was appliedandthe effect of the reinforcementon the mechanicalpropertieswas also investigated.EXPERIMENTALThe foamswere prepared by usingthe solventslisted in Table I. The test sampleswere preparedby puttingthem between two parallel plates and cuttingthe part of thesample protrudingwith a blade perpendicularto the plates. The irregularitieson thesurface were removed by rubbing the sample between similar plates where eachsurface of the plateswere covered with sand paper. The dimensionsof samples wereapproximetly 10X10XSmm.Compressiontesting of foams was carried out using an Instron Testing Machine(1122 model) with a 1000 Ib load cell. The crosshead speed was 0.02 inch/rain,andthe strainwas calculatedfromthe displacementof cross-head.The tests were made ata temperal!ureof 20°C +1°C. ,The effect of sample height/lateraldimensionratio on the results was tested andnone was observed. The samples were also tested in different directions and noobvious differencewas obtained, indicatingthat the foams are isotropic.Althoughthesamples showed longterm relaxation, the effect of strain rate on the modulus over a p.range of an order of magnitudewas notseen. P.Extruded closed-cell polystyrene foams (provided by the E and C Company, "Cardena CA) were compressed to produce defects in the structure. For eachcompression ratio a different sample was used, and after preloading the sample wasleft for recovery, and then reloaded. The moduli and collapse stresses weredetermined from the loading and reloadingexperiments. _:ot(1l, -j, i"BSilica ReinforcementThe TPX, PU, and Lycra samples were held immersed in tetraethoxysilane (TEOS)for 24 hours. The other samples were placed in TEOS vapor for the same periodoftime. The hydrolysis of TEOS (with diethyl-amine as catalyst) was carried outaccordingto the reactionSi(OC2H5)4+ 2H20 _ SiO2+ 4C2H5OH (3)(mllrwhich results in the precipitationof silica particles into and onto the polymer. Theamount of silica precipitatedwas calculatedfrom the difference between the densitiesbefore and after the treatment.Table.IFoam and polymerpropertiesFoam PolymerFoam pf(gcrrP) Solvent Method ps(gerrY) E=(MPa) Gys(MPa)IPS 350 0.162 1-chlorodecane Gelation/with 1.11 [9] 5600 [10] 148 [11]IPS 351 0.155 extractionIPS 352 0.168IPS 353 0.094IPS 354 0.099IPS 444 0.137 1-chlorodecane Gelation/extrac.PAN 221A 0.040 Maleicanhyride Sublimation 1.18 [7] 3400 [7] ' 83 [1"2]PAN 247 0.092PAN 389 0.081 Dimcthylformamide Gelation/PAN 391 0.058 and ethyleneslyrol extractionPAN 444 0.052_l-'X 101 0.046 Decalinand Gelation/ 0.83 [7] 1250 [7]1-dodecanol extractionLycra-23 0.225 DimethylaCetamide Gelation/ i.20 [13,4] 45 [13,4]PU 0.234 /Hi0 extraction 1.20 [4] 45 [4]PU 0.274,r--- i ii iiTEST RESULTS and DISCUSSIONA double logarithmic plot of relative modulus versus relative density for themicrocellularfoams is given in Fig.l, together with the predictionof the theory (eq.1)for conventional foams. A least squares linear regressionanalysis gives a slope of2.29 and the constant C1=0.38, which are predicted by the theory to be 2 and 1,respectively.Thesimilar plot of the relative collapse stressversus relative densityforrigidfoams is given in Fig.2. Linear regressiongives a slope of 1.85 and C2=0.15. Thetheoretical curve of eq.(2), which has slope of 1.5 and C2-0.3, is also given in thisfigure, i=The experimentaldata liewell belowthe theoreticalcurves in both Figures 1 and 2. "The Esand Osvaluesgiven inTable II were taken fromthe literature.The importanceofthe value of Es chosen has been previouslymentioned[7], where the higherthe value E|_oCP°_'_-3-- . " , .9, , , ,-" 4 -4-1.6 -1.4 -1.2 ,,1.0 -0.8log (pf/ps) log (pt/ps_Fig.1.Variationof relativemoduluswith Fig.21Variationof relativecollapsestressrelativedensity.The solidlineis eq.(1), with relativedensity.Themlidlineis eq.(2).also true for the Osvalues, but it appears that choosingeven the lowestvalues for Esand os is not enough to approach the predicted curves. Williams [6] measured the .moduliof microcellularTPX foamSby usingthe penetrationmethod,and he explainedthe lower values of the modulusby a fraction of non-contributingmaterial, and theineffiencyof the contributingmass. He also observed that the foams having a looseand randomlydistributedstructurehave lowervalues of the modulusthan those whichhave an orderly distributed polymerstructure. Jackson et al. [7], using the dynamictension-compressionmeasurements,found the constant C1 to be 0.16 in eq.(1). Theyconcludedthat the imperfectcell geometryand the inefficientuse of polymerin makingup the cell microstructurewere responsiblefor the discrepancyfrom the theory.Theyalso added that the friable foams might have been damaged during the cuttingandmountingprocess.The moduliand collapse stresses of preloaded extruded polystyrene foams aregiven in Table II, together with the prestrainvalues and permanent strains obtai_."Jafter the preloading. Permanent strains appear to be small enough not to cause asignificantdensification.When the amountof prestrainincreases,so does the numberof defectsproducedin the structure,which resultsin a decrease in the modulus. For aprestrain of 65%, the modulus drops to 13% of its original value. The initial linearportion of the stress-straincurve shows a break before reaching the collapse stress,indicating a kind of buckling, and under large prestrains, the collapse stressdisappears. Under moderate prestrains, the collapse stresses were not influencedvery muchby thisprocess.In Fig. 3 the stress-straincurvesforthe microcellularfoamsare given. For both rigidand elastomericfoams, the plateau regions of the curves show no horizontal or flatportion. Instead, they increase continuouslywiththe strain. Similar behavior has beenobserved for the highlypreloadedextruded polystyrenefoamsdescribed in Fig. 4. Thelength of the horizontal portionsof the curves decrease when the applied prestrainincreases, and finallyfor highervalues of prestrainit disappearsentirely.The dramatic decrease in the modulus of preloaded conventional foams and thesimilaritybetween the stress-straincurvesof these foamsand the microceilularonesI,kTable II. The Change of Modulusand Collapse Stresswith PrestrainPrestraJn(%)• 11.0 15.2 20.2 30.1 39.9 51.4 65.0 cPermanentstrain(%) • 5.0 7.4 9.1 9.8 15.3 16.1 16.8 (IEfinal/Einitial" 0.86 0.60 0.49 0.22 0.19 0.16 0.13Ofrel/Gnil_" 0.99 1.00 0.98 1.03 1.03 1.13 - ._$0('4"' J' rtD.IU Iu¢ ¢I'- k-t/) CnI " ! " | " I0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.2 0.4 0.6 0.8,COMPRESSION COMPRESSIONFig.3. The stress-straincurvesof microcellu- Fig.4.The stress-straincurves oflarfoams.A:PU; B:Lycra; C:TPX101;D:PAN391; preloaded extrudedpolystyrenefoam -E:IPS351.Thescaleoftheverticalaxisisarbitrary Prestratn(%):AC0;Bcl1;C:20.2;D;30.1;E:65,0indicate that the lower values obtained in the mechanical response of microcellularfoams mightbe due to the defects producedin the structure during production.Thesedefects might appearas nonuniformwall elements,crackedand brokenelements, orplasticaly deformed elements which are formed because of the non-uniformtemperature distributionpresent in the foam during the cooling process. Also anyshrinkage that occurredduring the removal of the solvent mightbe another reason.The explanationbased on the damage occurringduring the shaping of samples mightbe true for only the lower density and friable foams, but it is not the case for theelastomeric or highdensity rigid ones which are tough enough to be shaped withoutcausing any damage.The modulusand collapsestressesof the microcellularfoamswhich are reinforcedby the in-situ silica precipitation method are given in Table II1. lt appears that bothmoduli and collapse stresses increase by the treatment, indicatingthat the modulusand yield strengthof rigid polymers,and the modulusof elastomeric onesarei_bythis method.However, the ratio of the increase in the mechanical propertieswith theincrease in the densities is greater for the elastomers than for the rigid ones. Thesamples showed shrinkage in differentextents during the treatment process, and theincrease in thedensities after the treatmentis partiallydue to thisshrinkage.CONCLUSIONSThe moduliand collapse stresses of microcellularfoams were compared with thecurrent constitutivelaws of conventionalfoams. Although the square dependence ofthe modulus and the 1.5 power dependence of the collapse stress on relativedensities predicted by the theory appear satisfactory, the experimental data lie wellbelow the predicted curves, lt is concluded that the defects formed during the i;production are responsible for the _;iscrepancies from theory. In-situ silica _¢reinforcement is a cunvenient method to increase both the moduli and collapsestressesof microcellularfoams. _:CKCtbLp• REFERENCES1. J.H. Aubertand R.L. Clough,Polymer,26 2047 (1985)2. J.H. Aubert,A.P. Sylwester,and P. Rand, PolymerPrepfints,=_.0.,447 (1989)3. L.J. Gibsonand M.F, Ashby, R.Soc.Lond.A, A382, 43 (1982)4. L.J. Gibsonand M.F. Ashby,Cellular Solids.Structureand Prooert_e_,(Pergammon Press,Oxford,England,1988)5. W.E. Warrenand A.M. Kraynik,J. Appl. Mech,55, 341 (1988)6. J.M. Williams,J. Mat. Sci._,_ 900 (1988)7. C.L. Jackson,M.T. Shaw,andJ.H. Aubert(acceptedby Polymer)_8. J£ Mark and D.W. Schaefer in Polymer Based MolecularComoosite_,editedbyD. W. SchaeferandJ.E.Mark (M ater.Res.Soc.Proc. 171. Pi t t sburg,PA 1990)9. J. Brandrupand E.H. Immergut,PolymerHandbook(3rd Ed., JohnWileyand SonsInc, NewYork,NY 1989) p.V/82.10.H.A. Lanceley,J. Mann,andG. Pagany in ComoositeMaterials,edited byL, Holliday(ElsevierPub. Comp.1966) p.22411.The ratio of yieldstress/modulusis calculatedfor the polystyrenesgiven inPlasticsEncyclopedia,editedby J.Agranoff,(McGraw-HillInc., 61, NewYork,NY1984-1985) p.453, andan average ratio of 0.0265 is obtained;by usingthis ratio,and the modulusgiven in Ref:10, the yieldstressof IPS is ¢aJculaledto be 146 MPa.12.Ref,11, p.353.13.The Lycra has been mentionedas a polyurethanein W.M, Phillips,W.S. Pierce,G. Rosenberg and J.H.DonachyinSyntheticBiomedicalPolymers.ConceDtsandADDlication,editedby M. Szychen and W.J.Robinson(Technomic PublishingCo.Company,Inc.,Westport,CT 1980) p.41,therefore, the valuesgiven in Ref.4 forthepolyurethaneare also usedfor the Lycra.Table III. Propertiesof SilicaReinforcedFoamsFoam Density Shrinkage SiO2 Et*/Ef Oct'la cIPS350 0.296 43.0 5.2 4.2 _ 4.90.271 36.4 5.2 3.7 4.6IPS351 0.244 34.3 4.9 1.9 2.50.175 7.7 5.1 1.5 1.5IPS352 0.284 31.0 13.0 3.3 -IPS353 0.120 20.1 2.3 1.5 1.50.119 17.4 2.9 1.8 2.5IPS354 0.145 24.3 15.2 4.4 3.6PAN 389 0.257 60.0 26.7 9.9 17.00.142 31.0 23.3 3.1 3.1' 0.113 13.8 11.0 1.9 1.70.107 8.0 7.5 2.0 1.9PAN391 0.122 45.0 19.1 2.4 2.2TPX101 0.267 57.0 174.0 12.6 13.3PU 0.474 33.6 13.9 5.4 6.40.345 17.7 16.3 3.6 4.00.332 23.3 4.3 2.6 2.60.270 10.0 2.2 1.9 1.5LYCRA 0.344 21.0 29.2 6.1 5.30.330 8.0 43.5 5.70.303 10.4 25.0 2.8 3.2 !0.269 11.1 7.0 2.¢ 2.7"Substcript "t'shows the treated foam propertiesand Ocshowsthe collapsestress. ':=O_(-

The mechanical behavior of microcellular foams

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The mechanical behavior of microcellular foams

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