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Green composites: A review of adequate materials for automotive applicationsGeorgios Koronis, Arlindo Silva, Mihail FontulInstituto Superior Tecnico, Mechanical Engineering Department, Lisbon, Portugalarti cle i nfoArticle history:Received 27 December 2011Received in revised form 7 May 2012Accepted 3 July 2012Available online 24 July 2012Keywords:A. Polymermatrix composites (PMCs)B. Mechanical propertiesNatural bersabstractThis study provides a bibliographic review in the broad eld of green composites seeking-out for mate-rials with a potential to be applied in the near future on automotive body panels. Hereupon, materialsderiving from renewable resources will be preferred as opposed to the exhaustible fossil products. Withthe technical information of bio-polymers and natural reinforcements a database was created with themechanical performanceofseveral possiblecomponentsfortheprospectgreencomposite. Followingthe review, an assessment is performed where aspects of suitability for the candidate elements in termsof mechanical properties are analyzed. In that section, renewable materials for matrix and reinforcementarescreenedaccordinglyinordertoidentifywhichholdbothadequatestrengthandstiffnessperfor-mance along with affordable cost so as to be a promising proposal for a green composite. 2012 Elsevier Ltd. All rights reserved.1. IntroductionGreen composites deriving fromrenewable resources bring verypromising potential to provide benets to companies, natural envi-ronment and end-customers due to dwindling petroleumre-sources. The shift to more sustainable constructions inautomotive industry is not only an initiative towards a more viableenvironmentandcostefciencybutalsoademandofEuropeanregulations. Thelatter areplaying animportant roleasadrivingforce toward sustainable materials use. According to the EuropeanGuideline 2000/53/EG issued by the European Commission, 85% ofthe weight of a vehicle had to be recyclable by 2005. This recycla-ble percentage will be increased to 95% by 2015 [1]. Another waytobalancesustainabilityandcostiswiththeuseofcompositesinautomobilepanels, asintroducedbyanumberofautomakerswhichuserenewablematerialsincomposites. Compositesmadeof renewable materials have been rampantly used in interior andexterior body parts. Similar components are used as trim parts indashboards, doorpanels, parcel shelves, seatcushions, backrestsand cabin linings. In recent years there has been increasing interestin the replacement of berglass in reinforced plastic composites bynatural plant bers such as jute, ax, hemp, sisal and ramie [24].A natural based material can be dened as a product made fromrenewable agricultural and forestry feedstock, including crops andcrop by-products and its residues. Although end-of-life directivesandregulationswill askforcomponentsof higherrecyclability,theuseof renewablematerials has not beendictated. Furthermarket penetration of green composite will occur only when theirproduction can be rendered cost effective and competitive to thepresentinjection-moldedthermoplasticsusedonmanyvehicles.Materials experts from various automakers estimate that an all-ad-vanced-composite auto-body could be 5067% lighter than a cur-rentsimilarlysizedsteelauto-bodyascomparedwitha4055%massreductionforanaluminumauto-bodyanda2530%massreductionfor anoptimizedsteel auto-body[5]. Specicallyforthe future electrical vehicles chassis, the light weighting materialsapproachisvitalinordertooffsettheaddedweightofbatterieswhile atthesametime lowering thecurbweightand increasingtheirmaximumrange. Suchanauto-bodycouldbeevenlighterwith the addition of natural bers in the composite because theseare less dense than the synthetic types.1.1. Green interior composites in the automotive industryIn recent years, attempts have been observed to reduce the useof expensive glass, aramid or carbon bers and also lighten consid-erably the cars body by taking advantage of the lower density andcost that some natural bers provide. In that sense, renewable -bers as reinforcements were vastly used in composites of interiorparts for a number of passenger and commercial vehicles.Mercedes-Benz used an epoxy matrix with the addition of jutein the door panels in its E-class vehicles back in 1996 [6]. Anotherparadigm of green composites application appeared commerciallyin 2000, when Audi launched the A2 midrange car: the door trimpanels were made of polyurethane reinforced with a mixed ax/si-sal material [7]. Toyota on its turn claims to be the leading brand inadoption of environmentally friendly materials as 100% bioplastics.The natural ber reinforcedgreencomposite was usedinthe1359-8368/$ - see front matter 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2012.07.004Corresponding author. Address: Instituto Superior Tecnico, Mechanical Building2, Room 1.45, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal. Tel.: +351 926177071;fax: +351 218474045.E-mail address: [email protected] (G. Koronis).Composites: Part B 44 (2013) 120127ContentslistsavailableatSciVerseScienceDirectComposites: Part Bj our nal homepage: www. el sevi er . com/ l ocat e/ composi t esbRAUM 2003 model in the spare tire cover. The part made of a PLAmatrixfromsugarcaneandsweetpotatoanditwasreinforcedwith kenaf bers [8]. Later examples are the interior componentswhich combine bamboo bers and a plant-based resin polybutyl-ene succinate (PBS), and oor mats made fromPLA and nylon bersfor Mitsubishi motors [9]. ToyotaaddedtheMatrixandRAV4modelstothelistofvehiclesusingsoy-basedseatfoamsinthesummer of 2008 [10]. Recently, Ford selected wheat straw as rein-forcement for a storage bin and inner lid in its 2010 Flex crossovervehicle while BMW, for the 7 Series sedan used prepreg natural -bermatsandauniquethermosettingacryliccopolymerforthelower door panel [11]. Lately, Toyotadevelopedaneco-plasticmade fromsugar caneandwilluseittolinetheinteriors ofthecars. In fact, its rst use will be on the new CT 200 for its luggagecompartment as announced at Automotive World Congress in Jan-uary 2011 [12].1.2. Green exterior composites in the automotive industryThe concept of natural ber incorporation in exterior car parts isnot new. Dealing with an exterior part though is a more complex incomparison to the interiors cousin parts which are protected fromweather conditions. The exterior components must be able towithstandextremeconditionssuchasexposuretowetnessandchipping (not splinter due to mechanical impacts) [6]. The rst re-leaseofexteriorsgreencompositesappearedin2000, whentheMercedes-Benz Travego travel coach model, was equipped with apolyester/ax-reinforcedengineandtransmissionenclosuresforsound insulation [13]. These are the rst samples of natural bersuse for standard exterior components in a production vehicle andrepresents a milestone in the application of natural bers [6]. How-ever, thesepartswereunderthehoodandthereforeitisunderquestion their classication as exteriors. Some years later, DaimlerChrysler AG (Stuttgart, Germany) started using bers of the abacaplant in place of berglass for the production of the spare tire wellcoversoftheMercedes-BenzA-Class, two-doorcoupevehiclein2004. Theypatentedthisnovel mixtureof polypropylene(PP)-thermoplastic and abaca bers back in 2002 [14]. That was the rstlarge-scale application (about 40 metric tons/88,000 lb per year) ofnatural ber composites in an exterior part [15].Inanotherresearchproject, bio-basedmaterialswereusedinhighratiocontent whilepresentingverynoteworthystructuralperformance. A homogenous part made of thermoset resin (PTPprepreg) andhempbers replacedsuccessfullya conventionalpolyester-berglass reinforced component. The novel bio-based re-sin consisted of 90% renewable content materials and the rest de-rived from petrochemicals. The green composite was placed in themiddle section between theheadlights above thefender ofMANpassengers bus and was tested for its resistance to weather condi-tions [16]. In the ECO Elise concept car launched in July 2008, Lotusswapped out its typical berglass reinforcements for hemp bersin the composite body panels, the double-curvature xed hardtopand the spoiler [17]. Sustainable hemp technical fabrics have beenused as the primary constituent in the high quality A class com-posite body of polyester base. Exposed hemp bers in an unpaintedstripe from the bumper to the spoiler made a striking eco-contrasttothemetallicnishwhichsignalsimmediatelythatthiscarisdifferent.Some of the aforementioned concepts are indeed taking advan-tageofnaturalbers intobio-resinsandarestrivingtocombinetheoptimummaterialsforalightweightcompositeproductionof high renewable content. Nevertheless, these composites eitheremployonlypartiallyrenewableconstituentsorarenotappliedin large surfaces of the vehicles body. Therefore, on one hand theyare not considered fully green solutions and on the other hand theycannot contributetogreat materialssavingandtonoteworthyweight reduction.2. Constituent materials for a green compositeIt is presented concisely in this paragraph, an integrated proce-duretoidentifythemostadequateconstituents(resinandrein-forcement)fortheproductionofaprospectivegreencomposite.By comparing values adopted by studies regarding the mechanicalperformances of bers, matrices and identical composites, it is de-picted which combination holds the best potential for a compositeof fair structural performance.2.1. The reinforcement elementNatural bers are renewable bers that grow in crop elds andcan be used as laments or reinforcements in composites manufac-turing in the same way as the synthetic ones of glass for instance.Throughout the bibliographic research it was observed that a lot ofinterest for composite in automotive applications has been given tobers like abaca [14], kenaf [18], hemp [16,17] and ax [19]. That ispartially because of their present application in other automotiveenclosurespartsandconsumerplasticproducts. Recently, note-worthy attention has been given tothe abundant jute [3] and tothe stiff ramie ber [20]. Fig. 1, contains data on the annual volumeproduction per plant for many kinds of bers that were found onthe aforementioned studies. The data was adopted from the FAO-STAT information bulletin of the food and agriculture organizationfrom the United Nations.Attheoutset, itisclearlyunderstoodwhylotsofstudiesaretesting jute bers in composites aiming at automobile applications[3,21,22]. Infact, theyarebyfaroneofthemostabundant berplants being cultivated worldwide and with fair mechanical perfor-mance. On the other hand, ax is one of the most important anddemandable bast ber in Europe. About 80% of the total world axcrop is grown in France, Belgium, Spain, UK and Holland. Flax is rel-atively stronger, crisper and stiffer to handle [23]. Ramie bers arehighlighted by numerous studies because of its valuable mechani-cal properties. From the early years of biocomposites research, ithas been proven to provide good performance when compared tothe other bers as seen in thestudy ofHermann et al. [2]. Moreclearly ramie is the longest and one of the strongest ne textile -bers and therefore demonstrates high potential as a reinforcementin polymer composites [7]. Currently, Yu et al. [20] showed that ra-mie has higher values than ax and jute and its tensile strength isapproximately that of berglass. Abaca ber (or manilla hemp) hasbeen proven another good exemplar for reinforcing exterior auto-motivepartsaspreviouslymentionedforthecaseofMercedes-Benz cars. On the contrary, considering a recent study ofBledzkiand Jaszkiewicz [24] whereas abaca bers were tested in a com-posite system, they were characterized by lower mechanicalparameters in comparison to jute. The explanation for this untyp-ical behavior of the composite could be the ber processingFig. 1. Annual volume grown per ber plants in world production.G. Koronis et al. / Composites: Part B 44 (2013) 120127 121method which differed from the one of jute. Similar results wereshowninanotherstudyofthesameauthor[25]wherejuteandabaca were tested in PP matrices. In that study, the coupling agentdetermined the overall performance.2.1.1. Mechanical performance of natural bersIn order to have a broader view of the mechanical and physicalpropertiesofdifferentnaturalbers, availabledatafromseveralauthors was compiled in Table 1. Indicative prices (USD/kg) whichare included in that table are adopted from several nonconcurringsources and thus may not represent the present state.With the values from Table 1, two graphs are created below inFig. 2 depicting the mechanical performance of the bers reviewed.Average specic stiffness and specic strength were calculated asthey are important indicators of structural performance for auto-mobile panels. The former two values happen to be the most crit-ical engineering characteristics of automobile design over the pastyears[26]. Specically, materialswithhighspecicstiffnessandspecicstrengtharelikelytohavespecial meritinapplicationsinwhichweight will beacritical factor. BecausethevaluesofYoungs modulus and tensile strength used for the charts calcula-tions were found to be different in every study, the extreme values(of specic stiffness and strength) were marked in ranges. In paral-lel to that occurrence, the variation of values in the physical prop-ertiesofthebersisattributabletodifferentharvestingseasonsand/or regions of the planet.It can be observed from Fig. 2, that there is no optimum berthatoutperformsinvaluesall therestinbothcharts. E-glassisclearlybetterintermsof specicstrength, butisoutperformedbykenaf, hempandramieinspecicstiffness. Inanattempttohave anaverage performance similar to E-glass, a reasonablechoice could be to select hemp which is stronger than ramie andstill stiffer thanE-glass. Denitelymorefactors areneededtochoose the optimum material besides its mechanical performance.Onefactorthatwasnottakenintoaccountistherawmaterialscost asit varieswidelywiththeregionof harvest andseason.The selection mechanism will be further discussed and presentedin the next chapters.2.1.2. Major concerns regarding the use of natural bers asreinforcementsParallel to the advantages natural bers bring with their use incomposites they have also drawbacks regarding their performance,their behavior in polymeric matrix systems and their processing.First of all, natural bers have an inability to provide a consistentpatternof physical propertiesinagivenyear; thosepropertiescan vary from every harvesting season and/or from harvesting re-gion based on interchangeable sun, rain and soil conditions. Addi-tionally, these variations can be surprisingly observed even in thesamecultivations populationinbetweenthecrops. Morepre-cisely, theirpropertiesareessentiallydependentonthelocality,onthepartoftheplanttheyareharvestedfrom(leaforstem),the maturity of the plant and how the bers are harvested and pre-conditioned in the form of mats or chopped bers, woven or unwo-ven. All thesefactorsresultinsignicantvariationinpropertiescompared to their synthetic ber counterparts (glass) [27]. More-overimportantparametersarethetypeofgroundonwhichtheplant grows, the amount of water the plant receives during growth,the year of the harvest, and most importantly the kind of process-ing and production route. An approach to address this problem istomixbatchesof bersfromdifferentharvests. Blendingbersprovidesahedgeagainstvariabilityinanysinglebercrop. Byhaving multiple suppliers of ber and harvests, the ratio of bersensures relatively consistent performance in the nished part[15]. Alternatively, itisintroducedtothemarketthatagenetictransformedvarietymayguaranteeproductsofconstantquality[28].Oneothermajornegativeissueofnaturalbersistheirpoorcompatibility with several polymeric matrices. That may result innon-uniformdispersionof bers withinthematrix. Their highTable 1Properties of several natural bers and E-glass. The values are adopted from the studies and database of [7,19,4753]. References inside the table are for price only.Fibers Density (g/cm3) Diameter (mm) Tensile strength (MPa) Young modulus (GPa) Elongation at brake (%) Price (USD/kilo)Flax 1.5 40600 3451500 2739 2.73.2 3.11 [54]Hemp 1.47 25250 550900 3870 1.64 1.55 [54]Jute 1.31.49 25250 393800 1326.5 1.161.5 0.925 [54]Kenaf 1.51.6 2.64 350930 4053 1.6 0.378 [54]Ramie 1.51.6 0.049 400938 61.4128 1.23.8 2 [54]Sisal 1.45 50200 468700 9.422 37 0.65 [54]Curaua 1.4 710 5001100 11.830 3.74.3 0.45 [55]Abaca 1.5 1030 430813 31.133.6 2.9 0.345 [56]E-glass 2.55 1525 20003500 7073 2.53.7 2 [54]Fig. 2. Mechanical performance of several bers.122 G. Koronis et al. / Composites: Part B 44 (2013) 120127moisture sensitivity leads to severe reduction of mechanical prop-erties anddelaminating. Furthermore, lowmicrobial resistanceand susceptibility to rotting can act as restriction factors particu-larlyduringshipment andlong-termstorage, aswell asduringcomposite processing [29]. Similar to the case of wood composites,naturalbersandplasticarelikeoilandwater, anddonotmixwell. Asmostpolymers, especiallythermoplastics, arenon-polar(hydrophobic, repellingwater) substancesandnotcompatiblewith polar (hydrophilic, absorbing water) wood bers and, there-fore, poor adhesion between polymer and ber may result [30]. Inorder toimprovetheafnityandadhesionbetweenreinforce-ments and thermoplastic matrices in production, chemicalcoupling or compatibilising agents have to be employed[20,29,31]. Chemical coupling agents are substances, typicallypolymers that are used in small quantities to treat a surface in sucha way that increased bonding occurs between the treated surfaceand other surfaces.Another primary drawback of the use of bers is the low pro-cessing temperature required (limited thermal stability). The per-mittedtemperatureisupto200 C, abovethislimit thebersstarttodegradeandshrinkwhichsubsequentlyresultsinlowerperformanceof thecomposite. Ingeneral, whenbersaresub-jected to heat, the physical and/or chemical structural changes thatoccur are depolymerization, hydrolysis, oxidation, dehydration,decarboxylation, andrecrystallization[32], andthusconnethevariety of resins they can be blended with [33]. In order to avoidthis processing defect, the range of temperatures has to be limitedas well as the processing time [34].All the aforementioned aspects render the natural bers incor-porationinexterior surfacesof vehiclescomplicated, especiallywhen legislations in force and requirements of safety demand cer-tain levels of performance to be fullled. For that reason, car mak-ers are skeptical for their use in theexterior body panels even ifthey are widely used for interiors or hidden parts of the vehicleschassis. On the other hand, when composites containing natural -bers are used, there are added benets achieved as enhanced envi-ronmental performance due to the lower density of natural ber incomparison to glass. Those results were presented in the study ofAlves et al. [3] where simulation tests were done on a jute ber/polyester hood part compared with a conventional berglass/poly-ester component.2.2. The matrix materialSeveral matrix materials deriving fromrenewable resources maywell represent promisingcandidates for applicationinagreencompositeeitherbeingbiodegradableornon-biodegradable. Theemerging issue henceforth is the level of recyclability and/or decomposition when they are disposed of. In the case of a hypothetical100%bio-basedcomposite, evenif the material couldnot be recycleddirectly there are ways to be opted out through incineration for en-ergy recovery. In the case of incineration, there are no emissions oftoxic gases [35] and by decomposition there are no gases at all.On one hand, traditional thermosets render the overall productnot easily recyclable. On the other hand, traditional thermoplasticshave processing limitations as high melt viscosity, a serious prob-lemin the case of injection molding processing. The novel bio-basedthermosets (plant oil-based resins) resembling the synthetic ther-mosets (phenolics, polyesters, epoxies, etc.) are indeed difcult torecycle and reuse but can be later decomposed in most cases. Also,some, but not all, soybean resins or other plant oils can be manufac-tured inawaytobebiodegradable [36,37]. Thermosetpolymerscoming fromvegetable oils are usually formed by cationic polymer-ization with other monomers, such as styrene, divinyl benzene, andcyclopentadiene. Inothercasesepoxidizedoilsareconverteddi-rectly, either in the presence of thermally latent catalysts to initiatethe polymerization, or inthe presence of anhydrides as curing agent.Some of these interpenetrating polymer networks are also poten-tially (bio) degradable in soil [38,39]. All these additives are syn-thetic derivatives and non-renewable and thus they are notcontributingtoatotalgreencompositemanufacturing. Itwouldbe preferable then to opt for materials which are bio-thermoplasticsthat do not need the polymerization process and may combine bothbenets of recyclability and prospect disposal.2.2.1. Mechanical performance of natural resinsTable2showsinformationfromseveralstudiesonbio-resinsforgreencompositesproduction. Onceagain, becausethevaluesTable 2Properties of natural polymers in relation with polypropylene. The values are adopted from other studies [7,5764]. References inside the table are for price only.Polymer Density (g/cm3) Melting point (Tm C) Tensile strength (MPa) Young modulus (GPa) Elongation at brake (%) Price (USD/kilo)Thermoplastic starch 11.39 110115 56 0.1250.85 3144 5.5 [54]PLA 1.211.25 150162 2160 0.353.5 2.56 2.42 [54]PLLA 1.251.29 170190 15.565.5 0.832.7 34 4.5 [59]PHB 1.181.26 168182 2440 3.54 58 4 [65]PHBV 1.231.25 144172 2025 0.51.5 17.525 3.5 [66]PP 0.91.16 161170 3040 1.11.6 20400 1.65 [54]Fig. 3. Mechanical performance of several polymer resins.G. Koronis et al. / Composites: Part B 44 (2013) 120127 123were differing in each study, the extreme values were marked inranges. Contrarytonaturalbersthough, thesebio-basedresinsprovide reproducible properties since they are industrialized prod-ucts designed specically for a number of applications in the con-sumer market. The acronyms listed in the table are the following:PLA represents the poly(lactic acid) and PLLA is the poly-L-lactide,theyareboththermoplasticaliphaticpolyester. PHBstandsforpolyhydroxybutyrate another aliphatic polyester, and PHBV is thecopolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate). FinallyPP acronym represents the conventional polypropylene polymer.Raw materials in each study are provided from several supplierswhich may provide different bids regarding the ordered quantityper year or per shipment. Furthermore, the price of each polymerdoesnotrepresentadirectperformancemeasure. Whileforin-stancePHBhasalmostthesamestrengthofPP, itspriceisveryhigh comparing to PLA, which makes it inefcient in cost for largescale applications. With the data provided by Table 2, two differentgraphs were created as seen on Fig. 3 which are in accordance tothe same modeling that was followed in the charts in the previouschapter.Focusing on Fig. 3, it is again observed that there is no optimumresin that outperforms all the rest in performance in both charts.PPoutperformstherestinstrengthbutitfallsbehindPLAandPHB in the average stiffness chart to the right. What is importantto mention here is that PP has a short range of variations and atthe same time high performance values, for that reason its averageperformance is higher than the other resins presented here. Alter-natively, if only maximum values were considered, PP would haveheld lower ranks in both graphs.2.2.2. Major concerns regarding the use of bio-based resins as matricesBio-resins are resin or resin formulations derived from a biolog-icalsourceandcanbebiodegradableorcompostable, hypotheti-cally after their use they can be disposed and decomposed.Insofar as the decomposition nature, their use on A-class nish sur-faces is rather problematic considering long-life applications with-out delicatetreatmentsand/or coating. That alsomayoccur innatural bersastheymaydegradeeveninsyntheticresinsdueto the inevitable void contend of the composite.Another major drawback for those kinds of resins is their highcost which makes them unaffordable even for large scale produc-tions. An example of this is the polylactic acid (PLA) resin, a com-monlyusedbio-resinthat isat least 1.5timesmoreexpensivethan the extensively used synthetic resin PP (while PLA is the lessexpensiveof thebiopolymersasseenonTable2). Someotherdrawbacks of bio-based resins include brittleness, low heat distor-tion temperature, high gas permeability, low melt viscosity for fur-ther processing which restrict their use in a wide-range ofapplications [40].Finally there is a grand debate for whether or not these materi-als represent a real sustainable alternative to conventional plastics.Consideringafutureshiftfromthecurrentsynthetic-basedtoabio-baseddominant plasticseconomy, it isratherpossiblethattheeconomicstabilityrelations betweensocieties will betorndown. Suchashift requiressubstitutionof manycommonrawmaterialsthatarecurrentlyproducedinvastfromfossil (petro-chemical) or mineral resources, by products produced from renew-able (plant-based) resource [41]. The sensitive point in theadequateselectionof materialsisthat theoccurringcompositeshould not contain materials from edible sources for instance. Edi-ble crops or any kind of edible raw material can subtract a part offood quantity from the human food chain and may result in socialupheaval in the global balance of the food supply. Additionally, theupcomingbioplasticsindustryhastodeal withthedilemmaofwhether the bioplastics will be likely to decrease the fertile lands,or increase the incentive to cut down forested areas to create morearable land.One way to tackle one of the forthcoming problems is by pro-ducing the desirable quantity of materials inthe labthroughmicrobial production (e.g. biotechnological fermentation pro-cesses). Renewablepolyesters baseduponbiotechnological fer-mentationprocesses have beensuccessfully producedandarecurrently being introduced to the market as well as about 90% ofthe literature on lactic acid production is focused on the same pro-cess [41]. However, in such processes cheap raw materials shouldbeimprovedfurthertomakethemcompetitivewiththechemi-cally derived ones [42]. The question then arises if these laboratoryproduced green materials can be considered natural and what theirenvironmental impact is during production. This paper will not ad-dress these issues.2.3. Mechanical performance of green compositesGreen composites fabricated using plant bers (cellulose) andresinssuchasmodiedstarchesandproteinshavealreadybeendemonstrated in the interiors of automobiles while few exampleshave been shown for exteriors. Novel green composites have beentested in numerous studies in an attempt to explore their perfor-mance in several applications. Table 3, illustrates a number of stud-ies that tested several types of reinforced bio-resins with differentkinds of natural bers. The traditional composite of PP berglassreinforcement is referred below so as to have a comparison to thenovel green candidates.With the data provided in Table 3, two different graphs are pre-sented in Fig. 4, following the same graph modeling as Figs. 2 and3. It is observable that in most cases green composites made of PLA,PLLA and natural bers like ax, ramie or jute resemble the perfor-mance of the traditional PP-berglass reinforced composites. Note-worthy performance is representing the ax reinforced PLLAcomposite comparing to PP-berglass as seen in the study of Oks-man et al. [43] (instance 8, 9 in Table 3) where ax shows betterperformancewhenblendedwithPLLAratherthanwithPP. Fur-thermore, juteberappearstohavehighercompatibilitytoPLAthanthePP, PHBorstarchmatricesjudgingbytheirmechanicalperformance. Onthestudyof Bledzki andJaszkiewicz[24] (in-stance 5 in Table 3), jute bers show the highest tensile strengthwithin the natural ber group, although the jute ber compositesare characterized by lower mechanical values comparing to abacareinforced ones. This could be an outcome of the ber processingmethod used [24].Therefore, only the knowledge of mechanical properties of thetestedcompositesisnot sufcientforassessingthefull perfor-manceof theresultingcomposite. Thosecompositeshavebeenproducedindifferentmoldsandwithdifferentbertreatments.Each manufacturing method can result in different performancesof the produced composites and subsequently it is impractical tocomparestudieswithdissimilarprocessing. But sincethereareno akin studies in all levels indentied so far in order to have a fullcomparative review for green composites with different bers, thisstudy will suggest an intermediate way to qualify the compositesconstituent elements.3. Selection using ternary diagramsCost/unit tensile strength ($$/MPa) is regularly one of the mostimportant criteria and materials with lower cost/unit strength arepreferable. However, the main limitation of this explanation is thatit considers only one property as the most critical and ignores theothers [44].124 G. Koronis et al. / Composites: Part B 44 (2013) 120127In order to avoid that practice; the authors will enrich the depthof theone-dimensional factors byconsideringthreebi-dimen-sional factors: specic strength, specic stiffness andcost perweight. These factors are considered to be orthogonal as they areuncorrelated and thus fulll their purpose and pertinence of use.Moreover, regarding the materials that are screened, the syntheticones which were presented in Table 1 and Table 2 (PP for resin andE-glassforreinforcement)arenotrenewableandthereforetheyareoutofthescopeofthisselectionmethod. Theirpresenceinthe above tables of the previous chapters was only for the purposeof making direct performance comparisons between them and therenewables. Fig. 5presentstwoternarydiagramswhichallowaglobal comparison of the candidate materials for matrices and rein-forcements which are intended for a 100% green composite.3.1. The evaluation methodThe ternary diagram of Fig. 5 illustrates the best materials fordifferent criteria weights as materials show up in different regionsof the triangles area. With the aid of this tool, the decision makerTable 3Mechanical properties of several green composites bers and PP + GFR composites.Elongation to break (%) Tensile strength (MPa) Young modulus (GPa) Processing Reference1. Starch + 30% jute 2 0.2 26.3 0.55 2.5 0.23 Thermoplastic injection molding [67]2. PLA + 30% ramie 4.8 0.2 66.8 1.7 n.s Hot pressing sheet molding [20]3. PLA + 30% jute 1.8 0 81.9 2.9 9.6 0.36 Thermoplastic injection molding [50]4. PTP+ 25% hemp n.s 62 2 7.2 0.3 Compression molding [16]5. PHBV + 30% jute 0.8 0 35.2 1.3 7 0.26 Thermoplastic injection molding [24]6. PLLA + 30% ax 2.3 0.2 98 12 9.5 0.5 Film stacking compression molding [68]7. PHB + 30% ax 7 1.5 40 2.5 4.7 0.3 Film stacking compression molding [68]8. PLA + 30% ax 1 0.2 53 3.1 8.3 0.6 Twin-screw extruder + compression moldinga[43]9. PP + 30% ax 2.7 1.5 29.1 4.2 5 0.4 Twin-screw extruder + compression moldinga[43]10. PP + 30% jute 1.4 0.1 47.9 2.7 5.8 0.47 Thermoplastic injection molding [50]11. PP + 30% berglass 3.01 0.22 82.8 4.0 4.62 0.11 Compression molding [54]n.s: Non-studied.aLong bers composite.Fig. 4. Mechanical performance of several bers.Fig. 5. Ternary diagrams of the resin for matrix, on the left, and bers for reinforcement, on the right.G. Koronis et al. / Composites: Part B 44 (2013) 120127 125canbe ina positiontoselect themost appropriatecandidateregarding thepercentage of importance giventoeach dimensionofthethreeaxes. Ananalogousapproachwasusedinthestudyof Ribeiro et al. [45] for the constitution of the life cycle engineer-ingmethodology, thoughinthepresentstudyonlyuncorrelatedfactors were considered. The weight applied in this study was alsoused in another similar study [46].Theaveragespecicstiffnessandspecicstrengthvaluesforresins andbers werenormalizeddeliberatelyinapercentagescale. The same modeling was followed for the cost values but inaninversescalewhichindicatesthatthelesscostlymaterial isthemostfavorable. Thematerialswhichdidnotshowupinthediagramsarenotrepresentingthebestcombinationofcost/vol-ume, specicstrengthandstiffnessinanypartitioningof theseproperties. Thecalculationmethodtodetectthematerialregionborders inside these ternary diagrams is manual. In a series of va-lue tests the areas that each material holds inside the triangle aretracked down. Giving an example regarding a weighted decision onber selection(Fig. 5onthe right), considering a decisionof305020% for cost-stiffness-strength, the best selection is hemp.Followingthechoices presentedfor thetwobasicelementsregarding the green composites composition, another familiardiagramwascreatedbut thistimecontainingaprospect greencomposite which is presented in Fig. 6. Taking the three dominantresins (Fig. 5 on the left) and combining them with the bers thatoccupy similar areas in the bers chart on the right of Fig. 5, vedifferent composites were compared. Consequently, the combina-tions were: PLA-ax, PLA-kenaf, PLLA-curaua, PLLA-hemp andPHB-ramie. Thevaluesof themechanical performancesof eachcompositewerecalculatedbytheruleofmixturesadoptingthevaluesfromTables1and2, likewisethecostofeachcompositewas calculated by the percentages of the materials that it incorpo-rates(30%reinforcementand70%resin). Theresultsarenotex-pected to be accurate in absolute terms but are consideredaccurate enough to have a quick snapshot of parallel comparisonin relative performance.Once more, the possible composites with low overall ranks didnotappearonthediagram. Specicallyinthatcomparison, PLA-ax ranked rst both in average specic strength and cost/volumewhile PHB-Ramie was the stiffest of all composites and thereforethese two dominated all the other candidates. When the relativeimportanceofspecicstiffnessintheselectionprocessishigherthan 30% PHB-ramie is the best selection, regardless of the otherfactors. Therest of possiblegreencompositeshadperformanceandcostvaluesmuchlowerthanthosetwowhilenotshowingappraisable values. The nal diagram could have been different ifothersetsofconstituentmaterialswerechosen, howeveritwaspreferred to combine those that were emerging as better choicesin the same regions of both diagrams of Fig. 5.It must be noted once again that both stiffness and strength arehighly affected by the interface bonding between ber and matrix,and that this is especially true when natural bers are considered,with different possibilities for ber treatment. The authors consid-ered that, all things being equal, the ternary diagrams are a gooddecision making tool when three properties are considered impor-tant in the selection process. When ber treatments and compositeprocessing parameters are established in a relatively standard wayfor these types of composites, it will be possible to build ternaryselection diagrams similar to the ones in Figs. 5 and 6.The present study does not consider yet environmental data inthe selection criteria. However, given that weight is implicitly con-sidered and that weight is one of the most important factors whenthe environmental impact of an automobile is computed, it is ouropinion that, until more accurate data is obtained on these naturalcomposites, the more general approach taken in the present studyis still valid.4. Discussion and conclusionsThe application of green composites in automobile body panelsseems to be feasible as far as green composites have comparablemechanical performance with the synthetic ones. Conversely, greencomposites seem to be rather problematic due to their decompos-able nature. The biodegradability issue is one problem that needsto be addressed when aiming to 100% bio-based composites appli-cation, especiallywhendealingwithstructural partsof exteriorpanels for future vehicles. More aspects have to be considered suchas reproducibility of these composites properties and their long lifecycle as parts of the exterior body parts. Unfortunately, to the pres-ent the bio-thermoplastics cost is a major barrier for their general-izeduseintheautomotiveindustrybutitisexpectedthatsoonmanufacturers of these materials will turn up affordable solutionsas their demand in industrial scale applications will no doubt tendto decrease their prices to more affordable levels. The trend can alsobe reversed in the sense that the necessity for environmentally con-scious solutions can overturn the value chain and put a premiumprice on environmental impact of current solutions.An essential point is whether these materials can be combinedin the best way to reach the level of performance of their predeces-sors while having the lowest possible cost. The methodology pre-sented above could bearststepin thevast areaof multifactordecision making. Aspects thathavetodowith manufacturabilityand/or supply chainwere nottaken intoaccount while stillverycritical and will be included in future studies.AcknowledgementsThe rst author gratefully acknowledges the support of the Por-tuguese FCT foundation Fundao para a Cincia e a Tecnologia;for granting him with a PhD scholarship under reference No. SFRH/BD/33971/2009.References[1] Greening transport, European commission. European parliament and thecouncil, SEC/2008/2206 FIN [08.07.08].[2] Herrmann AS, Nickel J, Riedel U. Construction materials based uponbiologically renewable resources from components to nished parts. PolymDegrad Stab 1998;59:25161. Fig. 6. Ternary diagram of green composites.126 G. Koronis et al. / Composites: Part B 44 (2013) 120127[3] AlvesC, FerroPMC, SilvaAJ, ReisLG, FreitasM, RodriguesLB. Ecodesignofautomotive components making use of natural jute ber composites. J CleanerProd 2011;18:31327.[4] Jayaraman K. Manufacturing sisalpolypropylene composites with minimumbre degradation. Compos Sci Technol 2001;63:36774.[5] Davies G. Materials for automobile bodies. Oxford: Replika Press Pvt. Ltd.; 2003.[6] Suddell B, Evans W. Natural ber composites in automotive applications. In:MohantyAK, MisraM, Drzal TL, editors. Natural bers, biopolymers, andbiocomposites. CRC Press; 2005.[7] MohantyAK, MisraM, Drzal TL, SelkeSE, HarteBR, HinrichsenG. Naturalbers, biopolymers, andbiocomposites: anintroduction. In: MohantyAK,Misra M, Drzal TL, editors. Natural bers, biopolymers, and biocomposites. Boca Raton: Crc Press-Taylor & Francis Group; 2005. p. 136.[8] Anonymous. Bioplastics inautomotiveapplications. Bioplastics Mag; 2007[cited 2 (1): 148] .[9] mitsubishi-motors.com. Mitsubishi Motors develops plant-based green plasticoor mat. Tokyo: Mitsubishi Motors Co. [20.06.06]. .[10] North America enviromental report, report recycling use. .[11] Stewart R. Automotive composites offer lighter solutions. Reinf Plast2010;54:228.[12] toyota.com. Automotive news worldcongress. Detroit [11.01.11]. .[13] Anonymous. Daimler Chrysler turns to natural bres. Reinf Plast 2000;44:21.[14] media.daimler.com. DaimlerChryslerusesanatural-bercomponentintheexterior of the Mercedes-Benz A-Class. Stuttgart. [29.06.05]. .[15] Brosius D. Natural ber composites slowly take root. Compos Technol 2006.[16] Mssig J, Schmehl M, von Buttlar HB, Schnfeld U, Arndt K. Exteriorcomponents based on renewable resources produced with SMC technology considering a bus component as example. Ind Crops Prod 2006;24:13245.[17] Malnati P. ECO Elise concept: lean, speedy and green composites technology;2009;15(4):46-8[citedAugust]. .[18] ChenY, SunLF, ChiparusO, NegulescuI, YachmenevV, WarnockM. Kenaf/ramie composite for automotive headliner. J Polym Environ 2005;13:10714.[19] ArbelaizA, Ferna ndezB, CanteroG, Llano-PonteR, ValeaA, MondragonI.Mechanical properties of axbre/polypropylenecomposites. Inuenceofbre/matrix modication and glass bre hybridization. Compos, Part A: ApplSci Manuf 2005;36:163744.[20] YuT, RenJ, Li S, YuanH, Li Y. Effect of ber surface-treatments onthepropertiesof poly(lacticacid)/ramiecomposites. Compos, Part A: Appl SciManuf 2010;41:499505.[21] Fatima S, Mohanty AR. Acoustical and re-retardant properties of jutecomposite materials. Appl Acoust 2011;72:10814.[22] Kumar A, MohanA, DiwanRK, MalikA, Khandal RK. Studies onEuphorbiacoagulummodiedpolystyrenewastejutecomposites: Part II. J PolymMater 2009;26:6779.[23] Munder F, Frll C, Hempel H. Processing of natural ber plants for industrialapplication. In: Mohanty AK, Misra M, Drzal TL, editors. Natural bers, biopolymers, and biocomposites. Boca Raton: Crc Press-Taylor & Francis Group; 2005.[24] Bledzki AK, Jaszkiewicz A. Mechanical performance of biocomposites based onPLAandPHBVreinforcedwithnatural bresacomparativestudytoPP.Compos Sci Technol 2010;70:168796.[25] BledzkiAK. AbacabrereinforcedPPcompositesandcomparisonwithjuteand ax bre PP composites. EPRESS Polym Lett 2007;1:75562.[26] Advanced automotive technology: visions of a super-efcient family carelectric vehicle information. Washington, D.C: Ofce of TechnologyAssessment (OTA); 1995.[27] ODonnell A, DweibMA, Wool RP. Natural bercompositeswithplantoil-based resin. Compos Sci Technol 2004;64:113545.[28] MouginG. Naturalbrecompositesproblemsandsolutions. JECcompositesmagazine; 2006 [cited (25)]. .[29] BismarckA, MishraS, LampkeT. Plant bers as reinforcement for greencomposites.In: MohantyAK, MisraM, Drzal TL, editors. Natural bers, biopolymers, andbiocomposites. Boca Raton: Crc Press-Taylor & Francis Group; 2005.[30] Ashori A. Wood-plastic composites as promising green-composites forautomotive industries! Bioresour Technol 2008;99:46617.[31] KimJ, YoonT, MunS, RheeJ, LeeJ. Woodpolyethylenecompositesusingethylenevinyl alcohol copolymer asadhesionpromote. Bioresour Technol2006;97:4949.[32] Gassan J, Bledzki AK. Thermal degradation of ax and jute bers. J Appl PolymSci 2001;82:141722.[33] MoXQ, WangKH, SunXZS. Straw-basedbiomass andbiocomposites. In:MohantyAK, MisraM, Drzal TL, editors. Natural bers, biopolymers, andbiocomposites. Boca Raton: Crc Press-Taylor & Francis Group; 2005. p. 47395.[34] WielageB, LampkeT, MarxG, NestlerK, StarkeD. Thermogravimetricanddifferential scanning calorimetric analysis of natural bres and polypropylene.Thermochim Acta 1999;337:16977.[35] Ochi S. Development of high strength biodegradable composites using Manilahempberandstarch-basedbiodegradableresin. Compos, PartA:Appl SciManuf 2006;37:187983.[36] MohantyAK, MisraM, HinrichsenG. Biobres, biodegradablepolymersandbiocomposites: an overview. Macromol Mater Eng 2000;276277:124.[37] Wool R, Khot SN, University of Delaware. ASM International; Bio-based resinsand natural bers; 2000. .[38] UyamaH, KuwabaraM, TsujimotoT, KobayashiS. Enzymaticsynthesisandcuring of biodegradable epoxide-containing polyesters from renewableresources. Biomacromolecules 2003;4:2115.[39] ShogrenRL, PetrovicZ, LiuZS, ErhanSZ. Biodegradationbehaviorof somevegetable oil-based polymers. J Polym Environ 2004;12:1738.[40] Suprakas SR, Mosto B. Biodegradable polymers andtheir layeredsilicatenanocomposites: in greening the 21st century materials world. Prog Mater Sci2005;50:9621079.[41] vanDamJEG, deKlerk-EngelsB, StruikPC, RabbingeR. Securingrenewableresource supplies for changing market demands in a bio-based economy. IndCrops Prod 2005;21:12944.[42] John R, Nampoothiri K, Pandey A. Fermentative production of lactic acid frombiomass: an overview on process developments and future perspectives. ApplMicrobiol Biotechnol 2007;74:52434.[43] Oksman K, Skrifvarsb M, Selinc JF. Natural bres as reinforcement in polylacticacid (PLA) composites. Compos Sci Technol 2003;63:131724.[44] Jahan A, Ismail MY, Sapuan SM, Mustapha F. Material screening and choosingmethods a review. Mater Des 2010;31:696705.[45] Ribeiro I, Pecas P, Silva A, Henriques E. Life cycle engineering methodology appliedto material selection, a fender case study. J Cleaner Prod 2008;16:188799.[46] Saur K, Fava JA, Spatari S. Life cycle engineering case study: automobile fenderdesigns. Environ Prog 2000;19:7282.[47] Wambua P. Natural bres: can they replace glass in bre reinforced plastics?Compos Sci Technol 2003;63:125964.[48] Sedan D, Pagnoux C, Smith A, Chotard T. Mechanical properties of hemp brereinforced cement: Inuence of the bre/matrix interaction. J Eur Ceram Soc2008;28:18392.[49] GeorgeJ, SreekalaMS, ThomasS. Areviewoninterfacial modicationandcharacterization of natural ber reinforced plastic composites. Polym Eng Sci2001;41:147185.[50] Bledzki AK, Jaszkiewicz A, Scherzer D. Mechanical properties of PLAcomposites with man-made cellulose and abaca bres. Compos, Part A: ApplSci Manuf 2009;40:40412.[51] Bledzki AK, Mamun AA, Jaszkiewicz A, Erdmann K. Polypropylene compositeswith enzyme modied abaca bre. Compos Sci Technol 2010;70:85460.[52] GodaK, SreekalaMS, GomesA, Kaji T, Ohgi J. Improvementofplantbasednatural bersfortougheninggreencompositeseffectof loadapplicationduring mercerization of ramie bers. Compos, Part A: Appl Sci Manuf2006;37:221320.[53] kenaf-ber.com. Technical data and information about thermal insulation andsoundproong of kenaf natural bers. Mantova: KEFI, S.p.A. [03.07.10]. .[54]Design G. CES EduPack Ver.6.2.0 materials selectionsoftware. Cambridge: GrantaDesign Limited; 2010.[55] Satyanarayana KG, Guimares JL, Wypych F. Studies on lignocellulosic bers ofBrazil. Part I: source, production, morphology, propertiesandapplications.Compos, Part A: Appl Sci Manuf 2007;38:1694709.[56] Foodandagricultureorganizationof theUnitedNations. Agricultural pricestatistics; 2010[upated10.09.10; accesed15.11.10]. .[57] Van deVelde K, KiekensP. Biopolymers: overviewofseveral propertiesandconsequences on their applications. Polym Test 2002;21:43342.[58] AverousL, MoroL, DoleP, Fringant C. Propertiesof thermoplasticblends:starch-polycaprolactone. Polymer 2000;41:415767.[59] Clarinval AM. Classication and comparison of thermal and mechanical propertiesofcommercializedpolymers. In: IENICA, editor. Internationalcongress&tradeshow, industrial applications of bioplastics. York, UK; 2002, February 35.[60]Tsuji H. In vitro hydrolysis of blends fromenantiomeric poly(lactide)s Part 1. Well-stereo-complexed blend and non-blended lms. Polymer 2000;41:362130.[61] PhongL, HanESC, XiongS, PanJ, LooSCJ. Properties andhydrolysisof PLGAandPLLAcross-linked with electron beamradiation. PolymDegrad Stab 2010;95:7717.[62] Ahankari SS, MohantyAK, MisraM. Mechanical behaviour of agro-residuereinforced poly(3-hydroxybutyrate-co-3-hydroxyvalerate), (PHBV) greencomposites: A comparison with traditional polypropylene composites.Compos Sci Technol 2011;71:6537.[63] Li Y, Shimizu H. Improvement in toughness of poly(l-lactide) (PLLA) throughreactive blending with acrylonitrile-butadiene-styrene copolymer (ABS):morphology and properties. Eur Polym J 2009;45:73846.[64] CurveloAAS, deCarvalhoAJF, Agnelli JAM. Thermoplasticstarch-cellulosicbers composites: preliminary results. Carbohydr Polym 2001;45:1838.[65] Shiroma E, Drumond S, Blazek G, Souza MR, Lachtermacher M, Wang SH. PHBtoughening by blending with polyethyl glycol. In: World polymer congress and41st international symposiumonmacromolecules. Brasil: Riode Janeiro;2006. p. 12 [July 1621].[66] Dent A, H. Breakdown: BiodegradablePlastics. MATTERMagazine; 2008;5(2)[cited 05.09.11]. .[67] Vilaseca F, Mendez J, Pelach A, Llop M, Canigueral N, Girones J, et al. Compositematerials derived from biodegradable starch polymer and jute strands. ProcessBiochem 2007;42:32934.[68] Bodros E, PillinI, MontrelayN, BaleyC. Couldbiopolymers reinforcedbyrandomly scattered ax bre be used in structural applications? Compos SciTechnol 2007;67:46270.G. Koronis et al. / Composites: Part B 44 (2013) 120127 127