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POLYMER MATRIX COMPOSITE

Polymer Matrix Composite (PMC) is thematerialconsistingof a polymer (resin) matrix combined

with a fibrousreinforcingdispersedphase. Polymer Matrix

Compositesareverypopularduetotheirlowcostandsimplefabricationmethods.

Useofnon-

reinforcedpolymersasstructurematerialsislimitedbylowleveloftheirmechanicalproperties:tensilestr

engthofoneofthestrongestpolymers-

epoxyresinis20000psi(140MPa).Inadditiontorelativelylowstrength,polymermaterialspossesslowi

mpactresistance.

ReinforcementofpolymersbystrongfibrousnetworkpermitsfabricationofPolymerMatrixComposite

s(PMC)characterizedbythefollowingproperties: H ightensilestrength;

H ighstiffness;

H ighFractureToughness;

G oodabrasionresistance;

G oodpunctureresistance;

G oodcorrosionresistance;

L owcost.

ThemaindisadvantagesofPolymerMatrixComposites(PMC)are:

L owthermalresistance;

H ighcoefficientofthermalexpansion.

TYPESOFPOLYMERMATRIX

Twotypesofpolymersareusedasmatrixmaterialsforfabricationcomposites:

y Thermosets

Thermosetresins require addition of acuring agentor hardener and impregnation onto a

reinforcing material, followed by acuring stepto produce a cured or finished part. Once

cured, the part cannot be changed or reformed, except for finishing. Some of the more

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common thermosets includeepoxies,polyurethanes, phenolic andamino

resins,bismaleimides(BMI, polyimides),polyamides.

y

Thermoplastics The thermoplastics can usually be reheated and reformed into another shape and its

mechanical characteristics are preserved. Examples of thermoplastic polymer used are

PEI (Polyethyleneimine), PPS (Polyphenylene Sulfide), PEEK (Polyetheretherketone)

and PEKK (Polyetherketoneketone), L ow Density Polyethylene ( L DPE), H igh Density

Polyethylene ( H DPE), polypropylene, nylon, acrylics.

AccordingtothereinforcementmaterialthefollowinggroupsofPolymerMatrixComposites(PMC)areused:

Fiberglasses G lassFiberReinforcedPolymers;

CarbonFiberReinforcedPolymerComposites;

Kevlar(aramid)fiberreinforcedpolymers.

Reinforcingfibersmaybearrangedindifferentforms:

Unidirectionalfibers;

Rovings;

V eilmat:thinpileofrandomlyorientatedandloopedcontinuousfibers;

Choppedstrands:thinpileofrandomlyorientatedandloopedshort(3-4inches)fibers;

W ovenfabric.

PropertiesofPolymerMatrixCompositesaredeterminedby:

Propertiesofthefibers;

Orientationofthefibers;

Concentrationofthefibers; Propertiesofthematrix.

PropertiesofPolymerMatrixCompositesmaybeestimatedbytheRuleofMixtures.

PolymerMatrixComposites(PMC)areusedformanufacturing:secondaryload-

bearingaerospacestructures,boatbodies,canoes,kayaks,automotiveparts,radiocontrolledvehicles,sp

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ortgoods(golfclubs,skis,tennisracquets,fishingrods),bullet-

proofvestsandotherarmorparts,brakeandclutchlinings.

Thermosets

Ofthese,epoxiesarethemostcommonlyusedintheindustry.EpoxyresinshavebeeninuseinU.S.industr

yforover40years.Epoxycompoundsarealsoreferredtoasglycidylcompounds.Theepoxymoleculecan

alsobeexpandedorcross-

linkedwithothermoleculestoformawidevarietyofresinproducts,eachwithdistinctperformancecharac

teristics.Theseresinsrangefromlow-viscosityliquidstohigh-

molecularweightsolids.Typicallytheyarehigh-viscosityliquids.

Thesecondoftheessentialingredientsofanadvancedcompositesystemisthecuringagentorhardener.Thesecompoundsareveryimportantbecausetheycontrolthereactionrateanddeterminetheperformance

characteristicsofthefinishedpart.Sincethesecompoundsactascatalystsforthereaction,theymustconta

inactivesitesontheirmolecules.Someofthemostcommonlyusedcuringagentsintheadvancedcomposi

teindustryarethearomaticamines.Twoofthemostcommonaremethylene-

dianiline(MDA)andsulfonyldianiline(DDS).

Severalothertypesofcuringagentsarealsousedintheadvancedcompositeindustry.Theseincludealipha

ticandcycloaliphaticamines,polyaminoamides,amides,andanhydrides.Again,thechoiceofcuringagentdependsonthecureandperformancecharacteristicsdesiredforthefinishedpart.Polyurethanesareano

thergroupofresinsusedinadvancedcompositeprocesses.Thesecompoundsareformedbyreactingthep

olyolcomponentwithanisocyanatecompound,typicallytoluenediisocyanate(TDI);methylenediisoc

yanate(MDI)andhexamethylenediisocyanate( H DI)arealsowidelyused.Phenolicandaminoresinsare

anothergroupofPMCresins.Thebismaleimidesandpolyamidesarerelativenewcomerstotheadvanced

compositeindustryandhavenotbeenstudiedtotheextentoftheotherresins. [1][2][3]

Thermoplastics

Thermoplastics currentlyrepresentarelativelysmallpartoftheACMindustry.Theyaretypicallysupplie

dasnonreactivesolids(no chemicalreaction occursduringprocessing)andrequireonly heatandpressure

toformthefinishedpart.Unlikethethermosets,thethermoplasticscanusuallybereheatedandreformedi

ntoanothershape,ifdesired. [

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Fiberreinforcments

Fiberreinforcementmaterialsareaddedtotheresinsystemtoprovidestrengthtothefinishedpart.Thesele

ctionofreinforcementmaterialisbasedonthepropertiesdesiredinthefinishedproduct.Thesematerialsd

onotreactwiththeresinbutareanintegralpartoftheadvancedcompositesystem.

Thethreebasictypesoffiberreinforcementmaterialsinuseintheadvancedcompositeindustryarecarbon

/graphite,aramid,andglassfibers.Fibersusedinadvancedcompositemanufacturecomeinvariousform

s,includingyarns,rovings,choppedstrands,andwovenfabricmats.Eachofthesehasitsownspecialappli

cation. W henprepregmaterialsareusedinpartsmanufacture,wovenfabricormatsarerequired.Inproces

sessuchasfilamentwetwindingorpultrusion,yarnsandrovingsareused.

L imitations

Despitetheirstrengthandlowweight,compositeshavenotbeenamiraclesolutionforaircraftstructures.

Compositesarehardtoinspectforflaws.Someofthemabsorbmoisture.Mostimportantly,theycanbeexp

ensive,primarilybecausetheyarelaborintensiveandoftenrequirecomplexandexpensivefabricationm

achines.Aluminum,bycontrast,iseasytomanufactureandrepair.Anyonewhohasevergottenintoamino

rcaraccidenthaslearnedthatdentedmetalcanbehammeredbackintoshape,butacrunchedfiberglassbu

mperhastobecompletelyreplaced.Thesameistrueformanycompositematerialsusedinaviation. [4]

Aluminumisaverytolerantmaterialandcantakeagreatdealofpunishmentbeforeitfails.Itcanbedentedo

rpuncturedandstillholdtogether.Compositesarenotlikethis.Iftheyaredamaged,theyrequireimmediat

erepair,whichisdifficultandexpensive.Anairplanemadeentirelyfromaluminumcanberepairedalmost

anywhere.Thisisnotthecaseforcompositematerials,particularlyastheyusedifferentandmoreexoticm

aterials.Becauseofthis,compositeswillprobablyalwaysbeusedmoreinmilitaryaircraft,whicharecons

tantlybeingmaintained,thanincommercialaircraft,whichhavetorequirelessmaintenance. [4]

Aluminumstillremainsaremarkablyusefulmaterialforaircraftstructuresandmetallurgistshaveworke

dhardtodevelopbetteraluminumalloys(amixtureofaluminumandothermaterials).Inparticular,alumi

num-

lithiumisthemostsuccessfulofthesealloys.Itisapproximatelytenpercentlighterthanstandardaluminu

m.Beginninginthelater1990sitwasusedfortheSpaceShuttle'slargeExternalTankinordertoreducewei

ghtandenabletheshuttletocarrymorepayload.Itsadoptionbycommercialaircraftmanufacturershasbe

enslower,however,duetotheexpenseoflithiumandthegreaterdifficultyofusingaluminum-

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lithium(inparticular,itrequiresmuchcareduringwelding).Butitislikelythataluminum-

lithiumwilleventuallybecomeawidelyusedmaterialforbothcommercialandmilitaryaircraft.

Mostcommerciallyproducedcompositesusea polymer matrixmaterialoftencalledaresinsolution.Thereareman

ydifferentpolymersavailabledependinguponthestartingrawingredients.Thereareseveralbroadcategories,ea

chwithnumerousvariations.Themostcommonareknownas polyester ,vinylester ,epoxy ,phenolic ,polyimide ,pol

yamide ,polypropylene ,PEEK ,andothers.Thereinforcementmaterialsareoftenfibresbutalsocommonlyground

minerals.Thevariousmethodsdescribedbelowhavebeendevelopedtoreducetheresincontentofthefinalproduc

t,orthefibrecontentisincreased.Asaruleofthumb,layupresultsinaproductcontaining60%resinand40%fibre,wh

ereasvacuuminfusiongivesafinalproductwith40%resinand60%fibrecontent.Thestrengthoftheproductisgreatl

ydependentonthisratio.

Resins

Typically,mostcommoncompositematerials,includingfiberglass,carbonfiber,and Kevlar ,includeatleasttwopar

ts,thesubstrateandtheresin.

Polyesterresintendstohaveyellowishtint,andissuitableformostbackyardprojects.ItsweaknessesarethatitisUV

sensitiveandcantendtodegradeovertime,andthusgenerallyisalsocoatedtohelppreserveit.Itisoftenusedinthe

makingofsurfboardsandformarineapplications.ItshardenerisaMEKP,andismixedat14dropsperoz.MEKPisco

mposedofmethylethylketoneperoxide,acatalyst.WhenMEKPismixedwiththeresin,theresultingchemicalreact

ioncausesheattobuildupandcureorhardentheresin.

Vinylesterresintendstohaveapurplishtobluishtogreenishtint.Thisresinhaslowerviscositythanpolyesterresin,andismoretransparent.Thisresinisoftenbilledasbeingfuelresistant,butwillmeltincontactwithgasoline.Thisresint

endstobemoreresistantovertimetodegradationthanpolyesterresin,andismoreflexible.Itusesthesameharden

eraspolyesterresin(atthesamemixratio)andthecostisapproximatelythesame.

Epoxyresinisalmosttotallytransparentwhencured.Intheaerospaceindustry,epoxyisusedasastructuralmatrix

materialorasstructuralglue.

Shapememorypolymer (SMP)resinshavevaryingvisualcharacteristicsdependingontheirformulation.Thesere

sinsmaybeepoxy-based,whichcanbeusedforautobodyandoutdoorequipmentrepairs;cyanate-ester-

based,whichareusedinspaceapplications;andacrylate-

based,whichcanbeusedinverycoldtemperatureapplications,suchasforsensorsthatindicatewhetherperishabl

egoodshavewarmedaboveacertainmaximumtemperature. [5]Theseresinsareuniqueinthattheirshapecanbere

peatedlychangedbyheatingabovetheir glasstransition temperature(T g ).Whenheated,theybecomeflexibleand

elastic,allowingforeasyconfiguration.Oncetheyarecooled,theywillmaintaintheirnewshape.Theresinswillretur

ntotheiroriginalshapeswhentheyarereheatedabovetheirT g .[6]Theadvantageofshapememorypolymerresinsis

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thattheycanbeshapedandreshapedrepeatedlywithoutlosingtheirmaterialproperties,andtheseresinscanbeus

edinfabricatingshapememorycomposites. [7]