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A
contribution
to
the
thermal
insulation
performance
characterization
of
corn
cob
particleboards
Anabela
Paiva
a,b,
Sandra
Pereira
a,b,
Ana
Sá
a,b,
Daniel
Cruz
a,
Humberto
Varum
c,∗,
Jorge
Pinto
a,daECT,Dep.deEngenharias,UniversidadedeTrás-os-MonteseAltoDouro,Portugal
bC-MADE,UniversidadedaBeiraInterior,Portugal
cDep.deEngenhariaCivil,UniversidadedeAveiro,Portugal
dLaboratórioAssociadoI3N,Aveiro,Portugal
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received2September2011
Receivedinrevisedform9November2011
Accepted14November2011
Keywords:
Corncob
Particleboard
Thermalinsulation
Agriculturalwastematerials
a
b
s
t
r
a
c
t
Analternativeexpediteexperimentalset-upisproposedtoevaluatethethermalinsulationperformance ofcorncobparticleboards.Testinginsituthermalinsulationperformanceunderrealthermaland hygro-metricconditions,usingmorerealisticsampledimensions,testingsimultaneouslyseveralsamplesand monitoringcontinuouslyforseveraldaysthethermalbehaviorofaproductaresomeadvantagesofthis proposedtechnique.Therefore,ithasshowntobeaccurateandversatile.Throughthisexperimental methodology,aparametricthermalinsulationstudyofthecorncobparticleboardinwhichtheimpact ofitsthicknessonitsthermalinsulationperformancewasalsopossibletoperform.
©2011ElsevierB.V.Allrightsreserved.
1. Introduction
The continuous search for better sustainable and economic processedsolutionshasbeenreceivingtheattentionofa broad researchcommunity worldwide. Theresulting solutions canbe thereforeadaptedbytheindustryleadingtoamoresustainable society.Thebuildingindustryisnotimmunetothisrealityandhuge effortshavebeendoneinordertofindalternativesustainable build-ingmaterialsandlowtechnologymethodswhichresultinamore sustainableandaffordableconstruction,complementedwiththe comfortstandardsrequirednowadays.CO2emissionstothe
atmo-sphere,energyandwaterconsumptionsaresomeparametersthat havesignificantimpactinthisequation.Reusing,optingforgreen buildingmaterials(whichmustberenewable,local,andabundant), retrofitting,choosinglowtechnologymethodsandtechniquesare somepracticesthathavegivengoodresultsinthiscontext.
Several authors[1–5]have already proposed usingdifferent agriculturalproductssuchasbagasse,cereal,straw,cornstalk,corn cob,cottonstalks,kenaf,ricehusks,rice,straw,sunflowerhullsand stalks,bananastalks,coconutcoir,bamboo,durianpeelandpalm leavesoil,amongothers,forproductprocessingsuchas particle-board,hardboardandfiberboard,andfocusingontheirthermal insulationcapacity.Otherauthorshavebeenstudyingthe tech-nicalpotentialofusingothertypesofresiduesuchasnewspaper
∗ Correspondingauthor.Tel.:+351234370938;fax:+351234370094.
E-mailaddress:[email protected](H.Varum).
[6],honeycomb[7]orpolymericwastes[8]intheprocessingof differentbuildingcomponents.
Amongtheaboveidentifiedagriculturalproductscorncobhas anadditionaladvantagewhenthoughtintermsofpossible applica-tionforalternativeprocessedproductsbecauseitdoesnotcollide withtheworldwidefoodstockanditisgenerallyconsideredas agri-culturalwaste.Recentresearchworks[9–14]havegivenparticular emphasistotheapplicationofcorncobintheindustry.
InthePortugueseagriculturalcontext,therecentfigures[15] indicatethatthereisadecreasingtendencyofthecornplantation intensityinthelastfewyears.However,thisactivityisstill rele-vantinthePortugueseagriculturalpanorama.Thecornplantation ismainlyprocessedin thenorthand centreregionsof Portugal mainland. Cornand cornstalkhave differentusesbut thecorn cobisingeneraltreatedasagriculturalwastewhichitis neces-sarytoburn.Unfortunately,thisoptiondoesnotcontributetoa betterenvironmentallyfriendlyworld.Findingfieldsinwhichit ispossibletousecorncobcanhelpsolvethisproblemandalso stimulatesmallindustrialactivitiesusinglowtechnology meth-odswhichmaycontributesimultaneouslytothesocialeconomical developmentofcertaininteriorregionsofthenorthofPortugal. Thebuildingindustryisoneofthosefieldsinwhichthecorncob canhaveanimportantroleasanalternativesustainablebuilding material.Meanwhile,asetofancientPortuguesebuildingslocated inthenorthofPortugalwererecentlyreportedtopresentcorn cobsin theirexternal walls[9].Thesefactshave inspiredusto studyacorncobparticleboardabletobeasustainableand afford-ablealternativeproductwhichmaybeappliedindifferentbuilding
0378-7788/$–seefrontmatter©2011ElsevierB.V.Allrightsreserved.
Fig.1.Corncobparticleboard.
applicationssuchasathermalinsulationproduct,anacoustic insu-lationproduct,alightweightpartitionwall,aceilingcoating,and indoordoors,amongothers.Inthesesuggestedpossiblebuilding applicationsthethermalinsulationperformanceofthecorncob particleboardisrelevantand,therefore,itisimportantto evalu-ateitsthermaltransmissioncoefficientandthermalconductivity. Themainobjectiveofthispaperconsistspreciselyinproposingan alternativeexpediteexperimentalset-upabletoevaluatethe ther-malinsulationperformanceofthecorncobparticleboardwhichis stillunderresearchandisbasicallymanufactured.
Thispaperisstructuredasfollows:firstly,thecorncob particle-boardisbrieflyputincontext;secondly,theproposedalternative expediteexperimentalset-upispresentedinwhichtheinherent facility,theequipmentandthesamplepreparationaredescribedin detail;thirdly,theobtainedexperimentalresultsarepresentedand thethermaltransmissioncoefficientoffourdifferentthicknessesof corncobparticleboardsarequantified;fifthly,thepotentialofthe proposedalternativeexpediteexperimentalset-upisdiscussed,as wellastheobtainedresults;finally,themainconclusionsofthis researchworkaredrawn.
2. Context
Asitwasmentionedacorncobparticleboardhasbeenunder researchby this team in order to assess it as a possible alter-nativesustainablesolutionfor differentbuildingapplications.A simplemanufacturingprocessconsistinginbindingcorncob par-ticles withwood glue wasadopted. This process includes four mainstageswhichare,respectively:mixingupofthecomponents (i.e.corncobparticlesandwoodglue);molding;curing(whichis natural)andunmolding.Theresultingcorncobparticleboardhas beentestedand,sofar,someofthephysical(e.g.density,water resistance,fireresistance,durability,acousticinsulationcapacity) andmechanical(e.g.compressionstrength,bendingstrengthand impactresistance)propertiesobtained,anditssustainability per-formancesuggestadequacyofthisproductfortheaboveidentified buildingapplicationsinanenvironmentallyfriendlyway.This engi-neeredsustainableproductispresentedinFig.1.
3. Proposedexperimentalset-up
In[12],aspecificexperimentaldevicewasdesignedby Dowl-ingandMathia.Theyproposedtocomparethethermalinsulating capacityofmaterials.However,thisdeviceislimitedintermsof materialsizesample.Ontheotherhand,Pintoetal.appliedthe thermographytocomparethethermalinsulationperformanceof corncobandextrudedpolystyrene(XPS)[9],butthistechnique isnotabletoevaluatethethermaltransmissioncoefficient(U)or
thethermalconductivity()ofmaterials.Therefore,analternative experimentalproceduretoevaluatethecorncob particleboards thermalinsulationperformanceisproposedinthisresearchwork whichisbasedontheapproachdescribedinISO9869[16].The facilities,theequipmentandthesamplepreparationarecrucialfor thesuccessoftheproposedexperimentalset-up.
3.1. Facility
Aconfinedroomwasusedasanalternativeexpeditesolution ofatestingthermalcell.Theroomhastohaveatleastadoorand a windowandtobeastightaspossible.Thewindowhastobe preferentiallyorientedtothenorthdirectioninordertoavoidthe directsunshineincidence,andalsoprotectedfromtheoutsideto avoidthesamplewettingbytherainwhichmaychangeitsmoister contentandconsequentlyitsthermalproperties.
InthePortuguesecontextand,inparticular,inthenorthregion of this country,it isconvenient toperform thistest duringthe winterorthesummerbecauseitiseasytoensurethedesirable uniform highthermalgradient between indoorand outdoor. A uniform highthermal gradient between indoor and outdoor is desirable becauseitistheidealconditiontoallowasignificant heat flowacrossthematerial.Duringthewinter, thiscondition canbeachievedbyusingasimpledomesticheatingdeviceplaced indoorthatenablesaconstanttemperatureinsidetheroomhigher than theexternaltemperature.In contrast,duringthesummer, an air conditioner can keep the roomconstantly cool. A room sized4.00m×3.00m×2.54m(length×width×height)wasused, asFig.2schematicallyillustrates.Inthiscase,thewindowsare ori-entedtothenortheastdirection,theyareprotectedfromtherain, thetestwasperformedduringtheendofthewinterof2011(i.e. from22/02/2011to22/03/2011)andunderdriedclimate condi-tions.
3.2. Equipment
Aheatfluxmetersystemwasadoptedaswellastwothermo hygrometric devices and a domestic heat device.The heat flux metersystemisformedbytwoheatfluxsensors(Fig.3a(I)),four superficialtemperaturesensors(Fig.3a(II)),adata-logger(Fig.3b) andacomputer,andinaccordanceto[16].Theheatfluxsensors enabletomeasuretheheatflowacrossanelementwhenthereis asignificantthermalgradientbetweenitstwosurfaces(i.e.indoor surfaceandoutdoorsurface).Thesuperficialtemperaturesensors wereusedasacomplementandasareferencetotheheatflux sen-sors,inordertomeasuretheinnersurfacetemperatureofthecorn cobparticleboardsample.Twosuperficialtemperaturesensorsby heatfluxsensorwereused,asshowninFig.3a.Meanwhile,inFig.3c
Fig.2.Facility.
Fig.3. Equipmentused.
theadoptedthermohygrometricdevicewhichincludesa temper-aturesensor(Fig.3c(I))andarelativehumiditysensor(Fig.3c(II)) isshown. Asitwasmentionedabove,two thermohygrometric deviceswereused,onetomeasurethetemperatureandhumidity indoorandtheothertomeasurethesameparametersoutdoor. 3.3. Samplepreparation
Corn cob particleboard samples (Fig. 4(I)) sized ecm×25cm×25cm(thickness×width×height)wereprocessed infourdifferentthicknessessuchas3cm,5cm,6cmand8cm.This rangeofthicknesscorrespondstothemostavailablethicknessesof
thecurrentlyappliedthermalinsulationproducts(e.g.XPS)inthe Portuguesebuildingindustrycontext.Thecuringprocessoccurred inthelaboratory(i.e.undercontrolledthermalandhygrometric conditions) and thesamples were completelydried during the tests.Theaveragedensityofthecorncobparticleboardsamples was334kg/m3.XPSpanels(Fig.4(II))sizedecm×76cm×64cm
(thickness×width×height) was used as a support of the corn cobparticleboardsamplesinwhichtherespectivethicknessesare similartotheaboveidentifiedones.ThesizeoftheXPSsupport panelissimilartothesizeofanexistingwindowoftheconfined room.AholewasdoneinthecentralareaoftheXPSpanelshaped andsizedsimilarlytothecorncobparticleboardsample.Thecorn
cobparticleboardsampleisthenplacedintheholeoftheXPSpanel andbothmaterialsareconnectedbypolyurethanefoamwhichis appliedfromtheouterface(Fig.4(III)).Theset(i.e.particleboard andXPS)replacesawindowanditisalsofixedbypolyurethane foamappliedfromtheinsidethis time asshown inFig.4c.All theseexperimentalprocedureshavetobedonecarefullyinorder toavoidinsulationvoids,thermalbridges,non-insulatedheaders andotherfaultswhichmaycompromisethereliabilityofthefinal results.InFig.5areshowna3cmanda5cmthicknesscorncob particleboardsamplesbeingtestedsimultaneouslyand alsocan befeaturedtheabovedescribedexperimentalset-up.
Theheatfluxsensorsandthesuperficialtemperaturesensors werefixedontheinnerfaceofthesamplesusingadhesivetape andthermalglueaccordingtoFig.5.Thisprocedureisextremely importanttakingintoaccounttherugosityofthesurfacesofthe corncobparticleboard.Itisimperioustoguaranteeaperfect con-tactbetweenthesurfacesoftheheatfluxsensorandtheinnerface ofthecorncobparticleboard,inordertoavoidanypossibilityofthe sensorscomingoffduringthetest.AccordingtoISO9869[16],the testdurationshouldberelatedtothethermalinertiaofthebuilding elementunderstudy.Forahighthermalinertiabuildingelementa fourteen-daytest,atleast,isrecommended.Ontheotherhand,for alowthermalinertiabuildingelementaminimumof72h(i.e.3 days)testisrecommended.Forinstance,acommonexternalbrick cavitywallwitha4cmairboxmaybeconsideredashavingahigh thermalinertia.Thecorncobparticleboardsamplemaybe consid-eredashavingalowthermalinertiaandthereforeaminimumfive daytestwasadoptedinthiscase.
4. Experimentalresults
Asitwasmentionedearliercorncobparticleboardsamplesof thicknesses3cm,5cm,6cmand8cmweretested.Thesesamples areshowninFig.6.
Fig.5.Twosamplesbeingtested.
Acontinuousdataacquisitionduringthetestswasperformed inwhichthevaluesoftheheatflowacrossthepanelmeasuredby thetwousedheatfluxsensors(q1(n)andq2(n)),theinteriorand theexteriortemperatures(Ti(n)andTe(n))andtherelative humid-itywereregisteredata10mintimingintervaleach(n).Figs.7–10 representgraphicallytheaboveidentifiedregistereddataforeach panelsampleexperimentallyanalyzed.Itcanbenoticedthatthe interiortemperatureoftheconfinedroomwaskeptalmost uni-form,at22.9◦Capproximately,inallthecases.Ontheotherhand, theexteriortemperatureshowedthenaturalandtheexpected vari-ationduringaday,inthisperiodoftheyearinthenorthregionof Portugal.Theexteriortemperaturewasalwayslowerthanthe inte-riortemperature,andthereforetherewasnoriskofoccurrenceof undesirableheatflowswappingacrossthesample.However,there wasamoment,on20thofMarch2011,whenthe6cmthickness
Fig.7.Temperaturesandheatflowrelatedtothe3cmpanel.March(Mar)of2011.
Fig.8.Temperaturesandheatflowrelatedtothe5cmpanel.February(Feb)and
March(Mar)of2011.
Fig.9.Temperaturesandheatflowrelatedtothe6cmpanel.March(Mar)of2011.
Fig.10. Temperaturesandheatflowrelatedtothe8cmpanel.March(Mar)of2011.
8cmpanel 0.95
corncobparticleboardsamplewasbeingtested,inwhichthe exte-riorandinteriortemperaturesalmostreachedthesamevalue.Itis alsoclearlynoticeablethattheheatflowpeaksoccurredin paral-lelwiththepeaksofthegradientoftheinteriorandtheexterior temperatures.
AccordingtoISO9869[16],thethermaltransmissioncoefficient (U)canbequantifiedapplyingExpression(1).
U(ntotal)=
ntotal n=1 q(n) ntotal n=1 (Ti(n)−Te(n)) (1) Inwhich:Uisthethermaltransmissioncoefficient;q(n)isthe heatflowacrossthesampleinthemomentn;Ti(n)andTe(n)are theinteriorandtheexteriortemperatureinthemomentn, respec-tively;ntotalisthetotalnumberofmomentsiswhichthedatawas registered.Takingintoaccountthattwoheatfluxsensorsareused corre-spondingtoq1(n)andq2(n),itispossibletoestimatetwothermal transmissioncoefficients, U1(ntotal)and U2(ntotal),byapplying Expression(1).Thus,thethermaltransmissioncoefficientofeach sample(U(ntotal))istheaveragevalueofU1(ntotal)andU2(ntotal), accordingtoExpression(2).
U(ntotal)=U1(ntotal)+U2(ntotal)
2 (2)
Inwhich:U(ntotal)isthethermaltransmissioncoefficientof thesample;U1(ntotal)andU2(ntotal)arethethermaltransmission coefficientrelatedtothedataregisteredbytheheatfluxsensors1 and2,respectively.
Basedontheabovepresentedexperimentaldata(Figs.7–10) andapplyingthepreviousexpressionsitispossibletoestimatethe thermaltransmissioncoefficientofthedifferentcorncob particle-boardsampleswhicharepresentedinTable1.
Theincreasingthicknessofacorncobparticleboardbenefitin termsofthermalinsulationperformance,whichisclearlyevident bythethermaltransmissioncoefficientspresentedinTable1andit corroboratesthesuggestionmadebyDowlingandMathia[12], con-cerningtheimprovementofthethermalinsulationperformanceof productsbasedoncorncobmaterial.
5. Discussion
The thermal conductivity () of a material can be quanti-fiedbyknowingitsthermaltransmissioncoefficient.Therefore,a 0.101W/m◦Cvalueofthethermalconductivityforthecorncob particleboardwasestimated.Takingintoaccountthatthe respec-tivevalueofXPSandexpandedpolystyrene(EPS)isapproximately 0.037W/m◦Cand0.045W/m◦C,respectivelyaccordingto[17],the studiedparticleboardhasshown a disappointinglylow thermal insulatingperformance. However,consideringthat XPSandEPS are already establishedcommercialindustrialized products,the particleboardwasmanufacturedusinganaturalorganicmaterial whoseprocessisstillinprogress,andtheobtainedthermal con-ductivityvalueforthecorncobparticleboardisstillacceptablein termsofthermalinsulatingbuildingpurposes,thereare undoubt-edlystrongreasonstobelievethattheanalyzedalternativeproduct ispromisinginathermalinsulatingperspective.
ThethermalinsulationperformanceofanXPSsample(i.e.(I) inFig.4)wasalsoexperimentallyanalyzedaccordingtotheabove proposedand describedprocedure,andduringthesameperiod oftheyear2011.Avalueof0.043W/m◦Cofthermal conductiv-itywasobtainedwhich isslightlyhigherthanthe0.037W/m◦C indicatedin[17](i.e.14%higher)butstillapproximated.This dif-ferencemayberelatedtooperationalerrorswhich mayinclude severalaspectssuchas:thefactthatthetestwasperformedinsitu rathertheninathermaltestingcell;theequipmentprecision;the dataacquisition;thatarepresentativeamountofXPSsampleswas notconsidered;amongotheraspects.Meanwhile,[16]considersa maximumadmissibleerrorof20%(i.e.higherthan14%).Thus,this factmayleadustotheconclusionthattheproposed experimen-talset-upwhichusesaconfinedroominsteadofathermaltesting cellforevaluatingthethermalinsulationperformanceofcorncob particleboardmaybeadequate.
Theconnectionbetweenthecorncobparticleboardsampleand XPSsupportpanel,thefixationoftheset(i.e.corncon particle-boardinsertedintheXPSsupportpanel)inthewindowframeand therespectivesealingup,theuncontrolledexteriortemperature variation,thecontactbetweenthesurfacesoftheheatingtransfer sensorandtheinnersurfaceofthecorncobparticleboardsample andtheequipmentcalibrationarealsoadditionaltechnicalaspects whichmayaffecttheaccuracyoftheproposedexperimentalset-up.
6. Conclusions
Analternativeexpediteexperimentalset-upbasedinISO9869 [16]wasproposedtoevaluatethethermalinsulationperformance ofcorncobparticleboard.Inbrief,itconsistsofusingaconfined roomabletoguaranteeaconstantinteriortemperatureand hav-ingawindow,workingasanalternativesolutionforathermaltest cell.TheparticleboardsampleisinsertedinanXPSsupportpanel andthesetreplacesawindow.Aheatfluxmetersystemformed bytwoheatfluxsensors,foursuperficialtemperaturesensors,a data-loggerandacomputerisadoptedtomeasuretheheatflow acrossthesampleandthesuperficialtemperatureoftheinner sur-faceofthesample.Inparallel,thermohygrometricsensorsplaced intheinteriorandtheexteriorallowthemeasuringofthe respec-tiveinteriorandexteriortemperaturesandrelativehumidity.Since thematerialunderstudywascorncobparticleboard,considered ashavingalowthermalinertia,afivedaystestwasadopted.This proposedexperimentalset-upprovedtobeaccurate,madeit pos-sibletoperformthermalinsulationperformancetestsunderreal thermalandhygrometricconditions,insitu,tousemore realis-ticsampledimensions,totestmorethanonesampleatthesame timeandtomonitorthethermalbehaviorofaproduct continu-ouslyforseveraldays.Technicalaspectsrelatedtotheconnection betweenthecorncobparticleboardsampleandtheXPSsupport panel,thefixationoftheset(i.e.corncobparticleboardinserted intheXPSsupportpanel)tothewindowframeandthe respec-tivesealingup,andtheperfectcontactbetweenthesurfacesofthe heatfluxsensorandtheinnersurfaceofthecorncobparticleboard sampleneedtobedonecarefullyinorder toavoidundesirable
thermalconditionfaults.InthePortuguesenorthregioncontext, thisexperimentalmethodologyismoreadequatetocarryout dur-ingthewinterorthesummerseasonsinwhichitismorereliableto ensurethedesirablesignificantthermalgradientbetweenindoors andoutdoors.Throughthisexperimentalmethodologyitwas pos-sible todoa parametric studyof thecorncobparticleboard in whichtheimpactofthethicknessofthepanelinitsthermal insu-lation performancewasverified.Thickercorncobparticleboard hassmallthermaltransmissioncoefficientcorroboratingthe sug-gestionsmadebyDowlingandMathia[12].Finally,theproposed alternativeexpediteexperimentalset-upcanbeeasilyadoptedfor thethermalinsulationperformancestudyofotheralternative sus-tainableengineeredmaterialsorproducts.
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