Polymer Blends and Composites -...

PolymerBlendsandComposites

KMU407PolymerScienceandTechnologyII

2016-2017FallSemester

MethodstoIncreasetheS/ffnessandStrengthofPolymers

1.  Novelhomo-polymerdesign2.  CrystallizaGon3.  Crosslinking4.  CopolymerizaGon5.  RadiaGon6.  Polymerblends

ApolymerblendisamixtureoftwoormorepolymersthathavebeenblendedtogethertocreateanewmaterialwithdifferentphysicalproperGes.Generally,therearefivemaintypesofpolymerblend:1.  thermoplasGc–thermoplasGcblends;2.  thermoplasGc–rubberblends;3.  thermoplasGc–thermoseYngblends;4.  rubber–thermoseYngblends;and5.  polymer–fillerblends

PolymerblendinghasaZractedmuchaZenGonasaneasyandcost-effecGvemethodofdevelopingpolymericmaterialsthathaveversaGlityforcommercialapplicaGons.Inotherwords,theproperGesoftheblendscanbemanipulatedaccordingtotheirendusebycorrectselecGonofthecomponentpolymers.

MiscibleandImmisciblePolymerBlendsGenerally,polymerblendsareclassifiedintoeither

•  homogeneous(miscibleonamolecularlevel)or

•  heterogeneous(immiscible)blends.•  miscibleblends:poly(styrene)(PS)–poly(phenyleneoxide)(PPO)andpoly(styrene-acrylonitrile)(SAN)–poly(methylmethacrylate)(PMMA)are,•  immiscibleblends:poly(propylene)(PP)–PSandpoly(propylene)–poly(ethylene)(PE).

Miscible(singlephase)blendsareusuallyopGcallytransparentandarehomogeneoustothepolymersegmentallevel.Single-phaseblendsalsoundergophaseseparaGonthatisusuallybroughtaboutbyvariaGonsintemperature,pressure,orinthecomposiGonofthemixture.Since,ulGmately,theproperGesofapolymerblendwilldependonthefinalmorphology,variousresearchgroupshaverecentlyundertakenextensivestudiesofthemiscibilityandphasebehaviorofpolymerblends.Generally,polymerblendscanbecompletelymiscible,parGallymiscibleorimmiscible,dependingonthevalueof∆Gm.Thefreeenergyofmixingisgivenby

polymer blends, increasing market pressure now determines that, for specificapplications, polymer blends must perform under some specific conditions (e.g.,mechanical, chemical, thermal, electrical). This presents a major challenge as thematerials must often function at the limit of the properties that can be achieved;consequently, in-depth studies of the properties and performance of polymerblends are essential.

1.2Miscible and Immiscible Polymer Blends

Generally, polymer blends are classified into either homogeneous (miscible on amolecular level) or heterogeneous (immiscible) blends. For example, poly(styrene)(PS)–poly(phenylene oxide) (PPO) and poly(styrene-acrylonitrile) (SAN)–poly(methyl methacrylate) (PMMA) are miscible blends, while poly(propylene) (PP)–PS and poly(propylene)–poly(ethylene) (PE) are immiscible blends. Miscible (single-phase) blends are usually optically transparent and are homogeneous to the polymersegmental level. Single-phase blends also undergo phase separation that is usuallybrought about by variations in temperature, pressure, or in the composition of themixture.Since, ultimately, the properties of a polymer blend will depend on the final mor-

phology, various research groups have recently undertaken extensive studies of themiscibility and phase behavior of polymer blends. In practice, the physical propert-ies of interest are found either by miscible pairs or by a heterogeneous system,depending on the type of application. Generally, polymer blends can be completelymiscible, partially miscible or immiscible, depending on the value of DGm [4].The free energy of mixing is given by

DGm ¼ DHm " TDSm ð1:1Þ

For miscibility (binary blend), the following two conditions must be satisfied:the first condition DGm< 0; and the second condition

@2ðDGmÞ@w2

i

! "

T ;p

> 0 ð1:2Þ

where DGm is the Gibbs energy of mixing, w is the composition, where w is usu-ally taken as the volume fraction of one of the components.DSm is the entropy factor and is a measure of disorder or randomness, is always

positive and, therefore, is favorable for mixing or miscibility especially for low-molecular-weight solutions. In contrast, polymer solutions have monomers with ahigh molecular weight and hence the enthalpy of mixing (DHm) is also a decidingfactor for miscibility. DHm is the heat that is either consumed (endothermic) orgenerated (exothermic) during mixing. If the mixing is exothermic then thesystem is driven towards miscibility. The mixing is exothermic only whenstrong specific interactions occur between the blend components. The mostcommon specific interactions found in polymer blends are hydrogen bonding,

2 1 Polymer Blends: State of the Art, New Challenges, and Opportunities

∆GmistheGibbsenergyofmixing,∆Hmistheenthalpyofmixing,∆Smistheentropyfactor

Formiscibility(binaryblend),thefollowingtwocondiGonsmustbesaGsfied:thefirstcondiGon∆Gm<0;andthesecondcondiGonwhere∆GmistheGibbsenergyofmixing,ΦisthecomposiGon,whereΦisusuallytakenasthevolumefracGonofoneofthecomponents.







@2ðDGmÞ@w2

i

! "

T ;p

> 0 ð1:2Þ




∆Smistheentropyfactorandisameasureofdisorderorrandomness,isalwaysposiGveand,therefore,isfavorableformixingormiscibilityespeciallyforlowmolecular-weightsoluGons.Incontrast,polymersoluGonshavemonomerswithahighmolecularweightandhencetheenthalpyofmixing(∆Hm)isalsoadecidingfactorformiscibility.∆Hmistheheatthatiseitherconsumed(endothermic)orgenerated(exothermic)duringmixing.Ifthemixingisexothermicthenthesystemisdriventowardsmiscibility.ThemixingisexothermiconlywhenstrongspecificinteracGonsoccurbetweentheblendcomponents.ThemostcommonspecificinteracGonsfoundinpolymerblendsarehydrogenbonding,dipole–dipole,andionicinteracGons.







@2ðDGmÞ@w2

i

! "

T ;p

> 0 ð1:2Þ




ExperimentallyobservedphasediagramsinpolymerblendsystemsmaybelowercriGcalsoluGontemperature(LCST),uppercriGcalsoluGontemperature(UCST),combinedUCSTandLCST,hourglass-,and/orclosed-loop-shaped.Themostcommonlyobservedphasediagramsare•  LCST(phaseseparaGonofamiscibleblendduringheaGng)and•  UCST(phaseseparaGonofamiscibleblendduringcooling).

PhaseseparaGoninpolymersoluGonsmayproceedeitherby•  nucleaGonandgrowth(NG)•  spinoidaldecomposiGon(SD)•  thecombinaGonofboth.

UppercriGcalsoluGontemperature,UCST LowercriGcalsoluGontemperature,LCST

TheblendingoftwoormorepolymersalwaysaffectstheproperGesoftheresulGngmaterial.ThreedifferenteffectsonthoseproperGescanbedisGnguished:1.  addiGveeffect2.  antagonisGceffect3.  synergisGceffect

Effects of Blends on Tg

Compa/bilityinPolymerBlends•  Ingeneral,thecompaGbilitybetweenthepolymerphasesdecidesthe

properGesofaheterogeneouspolymerblend.

•  Theinterfacebetweenthepolymerphasesinapolymersystemischaracterizedbytheinterfacialtensionwhich,whenapproachingzero,causestheblendtobecomemiscible.

•  Inotherwords,iftherearestronginteracGonsbetweenthephasesthenthepolymerblendwillbemiscibleinnature.

•  LargeinterfacialtensionsleadtophaseseparaGon,withthephaseseparatedparGclesperhapsundergoingcoalescence;thiswillresultinanincreasedparGclesizeand,inturn,decreasedmechanicalproperGes.

•  TheinterfacialtensioncanbereducedbytheaddiGonofinterfacialagentsknownascompatabilizers.

•  Thesearegenerallymoleculeswithhydrophobicandhydrophilicregionsthatcanbealignedalongtheinterfacesbetweenthetwopolymerphases,causingtheinterfacialtensiontobereducedandthecompaGbilityofthepolymerblendstobeincreased.

•  CompaGbilityresultsinareducGonofthedispersedparGclesize,anenhancedphasestability,andincreasedmechanicalproperGes.

Useofablockcopolymerforcompa/biliza/on:Theblockcopolymerwillprefertomigratetotheinterfacetoreducetheinterfacialtension.RedblocksarecompaGblewithPolymerA(matrix).BlueblocksarecompaGblewithPolymerB(dispersedphase).Theconsequencewillbelowerinterfacialtension,beZerinterfacialadhesionandbeZerdispersion

ThethreemajorclassesofcompaGbilizerscanbedisGnguishedfromeachotherintermsoftheprimarymechanismbywhichtheyreducetheinterfacialtensionbetweenincompaGblepolymersandthusfavorfinerdispersionwithmoreregularandstableequilibriummorphologies:

Useofareac/vefunc/onalpolymerforcompa/biliza/on:ReacGonattheinterfacebetweenfuncGonalgroupsonthedifferentpolymerscreates,"in-situ",agraiedblockcopolymer.ThefuncGonalizedcopolymerismisciblewiththematrixandcanreactwithfuncGonalgroupsofthedispersedphase.

Useofannonreac/vepolymercontainingpolargroupsforcompa/biliza/on:Ingeneral,thecompaGbilizermustbecompaGblewithonephase(generallywiththenonpolarphase)andmustcreatespecificinteracGonswiththeotherphase

Company Product Tradename

MODIFIED POLYOLEFINS

DuPont

Ethylene-VAc-CO (CO denotes carbon monoxide), ethylene-BA-CO and

ethylene-BA-GMA terpolymers; ethylene-MA, ethylene-EA and ethylene-BA

copolymers.

• Use of CO as a comonomer results in the incorporation of -C(O)-

(ketone) groups along the chain backbone.

Elvaloy

DuPont A very broad range of MAH-grafted polyolefins. Fusabond

DuPont

Ethylene-methacrylic acid (MAA) ionomers. Zn2+ or Na+ is used as the counterion

in the different product grades.

• MAA repeat unit: -CH2-C(CH3)(COOH)-.

• Anionic MAA repeat unit: -CH2-C(CH3)(COO-)-.

Surlyn

DuPont Poly(vinyl alcohol), repeat unit: -CH2-CH(OH)-. Elvanol

STYRENIC BLOCK COPOLYMERS

BASF

Styrene-butadiene (SB) diblock copolymers.

• B repeat unit: -CH2-CH=CH-CH2-. Styrolux

BASF Styrene-butadiene-styrene (SBS) triblock copolymers. Styroflex

Dexco Polymers

Styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene (SIS) triblock

copolymers.

• I repeat unit: -CH2-CH=C(CH3)-CH2-.

VECTOR

Kraton Polymers SBS and SIS triblock copolymers, their hydrogenated midblock versions and their

hydrogenated midblock versions grafted with functional groups such as MAH. . KRATON

Kuraray

SBS and SIS triblock copolymers (hydrogenated B or I block).

• See Figure 22 for the chemical structures. SEPTON

OTHER TYPES OF COMPATIBILIZERS

Degussa Methacylate-based polymeric compatibilizers. DEGALAN

Dow Chemical

Polycaprolactone (PCL) polyesters, PCL polyester / poly(tetramethylene glycol)

(PTMEG) block polyols.

• PCL repeat unit: -(CH2)5-COO-.

• PTMEG repeat unit: -(CH2)4-O-.

TONE




BASF




Dexco Polymers

Styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene

(SIS) triblock copolymers.


VECTOR

Kraton Polymers

SBS and SIS triblock copolymers, their hydrogenated midblock

versions and their hydrogenated midblock versions grafted with

functional groups such as MAH. .

KRATON

Kuraray





Dow Chemical

Polycaprolactone (PCL) polyesters, PCL polyester /

poly(tetramethylene glycol) (PTMEG) block polyols.



TONE

Polymer

Chemistry

Innovations

Methacrylate-terminated reactive polystyrene.

• See Figure 27 for the chemical structure. Methacromer

Struktol Mixture of aliphatic resins with a molecular weight below 2000

g/mole, blend of medium molecular weight resins. STRUKTOL

Table 2: A representative (but not comprehensive) selection of polymeric compatibilizer suppliers and

their products, some acronyms used in this report, and trade names for the products. The products

listed below will be discussed further in Section 4.

Table 2 lists the companies and products that will be discussed further in providing examples of the use of polymeric additive technologies. The information provided in Table 2 is intended




BASF




Dexco Polymers

Styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene

(SIS) triblock copolymers.


VECTOR

Kraton Polymers

SBS and SIS triblock copolymers, their hydrogenated midblock

versions and their hydrogenated midblock versions grafted with

functional groups such as MAH. .

KRATON

Kuraray





Dow Chemical

Polycaprolactone (PCL) polyesters, PCL polyester /

poly(tetramethylene glycol) (PTMEG) block polyols.



TONE

Polymer

Chemistry

Innovations

Methacrylate-terminated reactive polystyrene.

• See Figure 27 for the chemical structure. Methacromer

Struktol Mixture of aliphatic resins with a molecular weight below 2000

g/mole, blend of medium molecular weight resins. STRUKTOL

Table 2: A representative (but not comprehensive) selection of polymeric compatibilizer suppliers and

their products, some acronyms used in this report, and trade names for the products. The products

listed below will be discussed further in Section 4.

Table 2 lists the companies and products that will be discussed further in providing examples of the use of polymeric additive technologies. The information provided in Table 2 is intended

AninterpenetraGngpolymernetwork(IPN)isdenedasablendoftwoormorepolymersinanetworkwithatleastoneofthesystemssynthesizedinthepresenceofanother.EachindividualnetworkretainsitsindividualproperGessosynergisGcimprovementsinproperGeslikestrengthortoughnesscanbeseen.AnIPNcanbedisGnguishedfrompolymerblendinthewaythatanIPNswellsbutdoesnotdissolveinsolventsandcreepandflowaresuppressed.

Interpenetrating networks (IPNs)

• Twolightlycrosslinkednetworks• Cannotbeuntangled.• Eachnetworkhasdifferentmechanicalproper/es.

•Togethertheyarestronger,tougherthansumoftheindividualpolymers=synergis/c

Classifica/onofIPNsBasedonChemicalBonding•  CovalentSemiIPN:AcovalentsemiIPNcontainstwoseparatepolymersystemsthatarecrosslinkedtoformasinglepolymernetwork.•  NonCovalentSemiIPN:AnoncovalentsemiIPNisoneinwhichonlyoneofthepolymersystemiscrosslinked.•  NonCovalentFullIPN:AnoncovalentfullIPNisaoneinwhichthetwoseparatepolymersareindependentlycrosslinked.

Journal of Drug Delivery 3

(a) (b) (c)

Cross-links

Polymer ΙPolymer ΙΙ

(d)

Cross-links


(e)

Cross-links


(f)

Figure 1: (a) A polymer blend; (b) a graft copolymer; (c) a block copolymer; (d) semi-IPN; (e) full IPN; F- cross-linked copolymer.

1 1 1 1 1

1 1 1 1 1

2 2 2 2 2

2

2

22

2

2

2

2 2 2 2

Monomer 1

Cross-linker 1

Monomer 2

Cross-linker 2

Sequential IPN

Polymerize

Polymerize

IPNSwell in monomer 2

and cross-linker 2

Polymer network 1Po

2

22222

2

22

2

Sw

Figure 2: Sequential IPN formation.

Classifica/onofIPNsBasedonArrangementPaLern•  SequenGalIPN-InsequenGalIPNthesecondpolymericcomponent

networkispolymerizedfollowingthecompleGonofpolymerizaGonofthefirstcomponentnetwork.

•  SimultaneouslyIPN-AnIPNisformedbypolymerizaGonoftwodifferent

monomerandcross-linkingagentpairstogetherinonestep.

•  TermoplasGcIPN:TheseIPNshavecompletelyerasedtheideaofchemicalcross-linkersandusephysicalcross-linkers.ThermoplasGcIPNsarecombinaGonoftwophysicallycross-linkedpolymers.

•  GradientIPN:GradientIPNshavecomposiGonswhichvaryasafuncGonofposiGoninthesample.InthistypeofsystemtheconcentraGonofsecondmonomernetworkhasagradientoverthefirstmonomernetwork.

Journal of Drug Delivery 3

(a) (b) (c)

Cross-links


(d)

Cross-links


(e)

Cross-links


(f)

Figure 1: (a) A polymer blend; (b) a graft copolymer; (c) a block copolymer; (d) semi-IPN; (e) full IPN; F- cross-linked copolymer.

1 1 1 1 1

1 1 1 1 1

2 2 2 2 2

2

2

22

2

2

2

2 2 2 2

Monomer 1

Cross-linker 1

Monomer 2

Cross-linker 2

Sequential IPN

Polymerize

Polymerize

IPNSwell in monomer 2

and cross-linker 2

Polymer network 1Po

2

22222

2

22

2

Sw

Figure 2: Sequential IPN formation.

4 Journal of Drug Delivery

1 1 1 1 1

1 1 1 1 1

2 2 2 2 2

2 2 2 2 2

Monomer 1

Cross-linker 1

Monomer 2

Cross-linker 2

Stepwise and

chain polymerization

Simultaneous IPN

Figure 3: Formation of simultaneous IPN.

CostablizerInitiator

Monomer(s)

SurfactantWater

Aqueous andnonaqueous phase

Sonication

Hydrophobic monomerdispersed in water

Polymerization

IPN particlesdispersed in water

Synthesis of IPN particles from hydrophobic monomers via direct miniemulsion process

Figure 4: Synthesis of IPN particles by miniemulsion polymerization.

form homogenous solution by stirring. This aqueous phaseis added to oil phase to prepare w/o emulsion [33] but inw/w emulsion technique an aqueous solution of water solublepolymers is emulsified as a dispersed phase in an aqueoussolution of another polymer that acts as continuous phase.Then the dispersed polymer phase is cross-linked to form IPNnetwork [32].

3.3. Miniemulsion/Inverse Miniemulsion Technique. Thistechnique allows one to create small stable droplets in acontinuous phase by the application of high shear stress [34].The idea of miniemulsion polymerization is to initiate thepolymer in each of the small stabilized droplets. To preventthe degradation of miniemulsion through coalescence, asurfactant and a costablizer are added that are soluble indispersed phase but insoluble in continuous phase. Thisprocess of IPN formation can be divided into three steps.In the first step, constituent polymers are obtained bysonication using specific initiator. In the second step, one ofthe constituent polymers is polymerized and cross-linkedusing a cross-linking agent. As a result a semi-IPN is formedtill the second stage. In the third step, a full IPN is formedpolymerizing and cross-linking the second constituentpolymer by the addition of second cross-linker. Figure 4

represents the formation of IPN particles by the process ofdirect (oil in water) miniemulsion polymerization.

In case of inverseminiemulsion (water in oil), hydrophilicmonomers can be easily polymerized. In this case themonomer solution is miniemulsified in a continuoushydrophobic phase. The polymerization process canbe initiated either from the continuous phase or fromthe droplet. Koul et al. synthesized novel IPN nanogelscomposed of poly(acrylic acid) and gelatin by inverseminiemulsion technique. Acrylic acid monomer stabilizedaround the gelatin macromolecules in each droplet waspolymerized using ammonium persulfate and tetramethylethylene diamine and cross-linked with N, N-methylenebisacrylamide (BIS) to form semi-IPN nanogels, which weresequentially cross-linked using glutaraldehyde to form IPNs[35].

4. Factors That Affect IPN Morphology

Most IPN materials that have been investigated show phaseseparation.The phase however varies in amount, size, shape,and sharpness of their interfaces and degree of continuity.These aspects together constitute the morphology of IPN

Classification based on Matrices

Composite materials

Matrices

Polymer Matrix Composites (PMC)

Metal Matrix Composites MMC)

Ceramic Matrix Composites (CMC)

Thermoset Thermoplastic Rubber

Polymer Matrix Composite (PMC) is the material consisting of a polymer (resin) matrix combined with a fibrous reinforcing dispersed phase. Polymer Matrix Composites are very popular due to their low cost and simple fabrication methods. The reinforcement in a PMC provides high strength and stiffness. The PMC is designed so that the mechanical loads to which the structure is subjected in service are supported by the reinforcement. The function of the matrix is to bond the fibers together and to transfer loads between them.

Polymer matrix composites are often divided into two categories: •  reinforced plastics •  advanced composites

The distinction is based on the level of mechanical properties (usually strength and stiffness) Reinforced plastics, which are relatively inexpensive, typically consist of polyester resins reinforced with low-stiffness glass fibers (E-glass). They have been in use for 30 to 40 years in applications such as boat hulls, corrugated sheet, pipe, automotive panels, and sporting goods.

Advanced composites, which have been in use for only about 15 years, primarily in the aerospace industry, consist of fiber and matrix combinations that yield superior strength and stiffness. They are relatively expensive and typically contain a large percentage of high-performance continuous fibers, such as high-stiffness glass (S-glass), graphite, aramid, or other organic fibers.

•  Whatarethemostcommonadvancedcomposites?–  Graphite/Epoxy–  Kevlar/Epoxy–  Boron/Epoxy

Discontinuous phase - Reinforcement Continuous phase - Matrix

Polymer(Matrix) Composite (Matrix + Reinforcement)

•  Reinforcements –  Principal load bearing member.

•  Matrix –  provides a medium for binding and holding the reinforcements

together into a solid. –  protects the reinforcement from environmental degradation. –  serves to transfer load from one insert (fiber, flake or particles) to

the other. –  Provides finish, colour, texture, durability and other functional

properties.

Functions of Matrix

•  Holds the fibres together. •  Protects the fibres from environment.

•  Distributes the loads evenly between fibres so that all fibres are subjected to the same amount of strain.

•  Enhances transverse properties of a laminate. •  Improves impact and fracture resistance of a component.

•  Helps to avoid propagation of crack growth through the fibres by providing alternate failure path along the interface between the fibres and the matrix.

•  Carry inter-laminar shear.

Desired Properties of a Matrix

The matrix properties determine the resistance of the PMC to most of the degradative processes that eventually cause failure of the structure.

•  Reduced moisture absorption.

•  Low shrinkage.

•  Low coefficient of thermal expansion.

•  Good flow characteristics so that it penetrates the fiber bundles completely and eliminates voids during the compacting/curing process.

•  Must be elastic to transfer load to fibres.

•  Reasonable strength, modulus and elongation (elongations hould be greater than fibre).

•  Strength at elevated temperature (depending on application).

•  Low temperature capability (depending on application).

•  Excellent chemical resistance (depending on application).

•  Should be easily processable into the final composite shape.

•  Dimensional stability (maintains its shape).

•  ThematrixphaseofcommercialPMCscanbeclassifiedaseitherthermosetorthermoplasGc.

ThermoseMngresins•  ThermoseYngpolyestersarecommonlyusedinfiber-reinforcedplasGcs,and

epoxiesmakeupmostofthecurrentmarketforadvancedcompositesresins.

•  Theviscosityoftheseresinsislow.

•  ThermosetresinsundergochemicalreacGonsthatcrosslinkthepolymerchainsandthusconnecttheenGrematrixtogetherinathree-dimensionalnetwork.Thisprocessiscalledcuring.

•  Thermosets,becauseoftheirthree-dimensionalcrosslinkedstructure,tendtohavehighdimensionalstability,high-temperatureresistance,andgoodresistancetosolvents.Recently,considerableprogresshasbeenmadeinimprovingthetoughnessandmaximumoperaGngtemperaturesofthermosets.

Common Thermoset Resin Types •  Polyester: Lowest Cost •  Vinyl ester: Industry Standard •  Polyurethane: Premium Cost •  Epoxy: Highest Cost &Commonly used in aerospace applications

� Polyesters 9 Advantages:

- Easy to use - Lowest cost of resins available

9 Disadvantages: - Sensitive to UV degradation- Only moderate mechanical properties

FRP Overview: Resins

8

� Vinyl esters 9 Advantages:

- Very high chemical/environmental resistance- Higher mechanical properties than polyesters

9 Disadvantages: - Sensitive to heat- Higher cost than polyesters


9

� Polyurethanes 9 Advantages:

- Higher strength and flexibility than vinyl esters- Very high chemical/environmental resistance- Higher mechanical properties than vinyl esters

9 Disadvantages: - Higher cost than vinyl esters (about 1.5 x)


10

Common Thermoset Resin Types

� Epoxies9 Advantages:

- High mechanical and thermal properties - High moisture resistance - Long working times available - High temperature resistance

9 Disadvantages: - More expensive than polyurethanes- Critical mixing/Consistency- Corrosive handling


11

Common Thermoset Resin Types

Thermoplas/cresins•  ThermoplasGcresins,someGmescalledengineeringplasGcs,includesome

polyesters,polyetherimide,polyamideimide,polyphenylenesulfide,polyether-etherketone(PEEK),andliquidcrystalpolymers.

•  Theyconsistoflong,discretemoleculesthatmelttoaviscousliquidattheprocessingtemperature,typically260to371°C,and,aierforming,arecooledtoanamorphous,semicrystalline,orcrystallinesolid.

•  ThedegreeofcrystallinityhasastrongeffectonthefinalmatrixproperGes.•  ThermoplasGcsoffergreatpromiseforthefuturefromamanufacturingpointof

view,becauseitiseasierandfastertoheatandcoolamaterialthanitistocureit.ThismakesthermoplasGcmatricesaZracGvetohigh-volumeindustriessuchastheautomoGveindustry.Currently,thermoplasGcsareusedprimarilywithdisconGnuous-fiberreinforcementssuchaschoppedglassorcarbon/graphite.

•  ThermoplasGcs,althoughgenerallyinferiortothermosesinhigh-temperaturestrengthandchemicalstability,aremoreresistanttocrackingandimpactdamage.

ComparisonofGeneralCharacteris/csofThermosetandThermoplas/cMatrices

76 . Advanced Materials by Design

CONSTITUENTS OF POLYMER MATRIX COMPOSITESMatrix

The matrix p roperties determine the resistanc eof the PMC to most of the degrada tive p roc essestha t eventua lly c ause fa ilure of the struc ture.These p roc esses inc lude impac t damage, delami-na tion, wa ter absorp tion, c hemic a l a ttac k, andhigh-tempera ture c reep . Thus, the matrix is typ i-c a lly the weak link in the PMC struc ture.

The matrix phase of c ommerc ia l PMCs c an bec lassified as either thermoset or thermop lastic .The genera l c ha rac teristic s of eac h matrix typeare shown in figure 3-2; however, rec ently de-veloped matrix resins have begun to c hange thisp ic ture, as noted below.

ThermosesThermosetting resins inc lude polyesters, vinyl-

esters, epoxies, b isma leimides, and polyamides.Thermosetting polyesters a re c ommonly used infiber-reinforc ed p lastic s, and epoxies make upmost of the c urrent market for advanc ed c om-posites resins. Initia lly, the visc osity of these re-sins is low; however, thermoset resins undergoc hemic a l reac tions tha t c rosslink the polymerc ha ins and thus c onnec t the entire ma trix to-gether in a three-d imensiona l network. This p roc -ess is c a lled c uring . Thermoses, bec ause of theirthree-d imensiona l c rosslinked struc ture, tend tohave high d imensiona l stab ility, high-tempera tureresistanc e, and good resistanc e to solvents. Re-c ently, c onsiderab le p rogress has been made inimproving the toughness and maximum opera t-ing tempera tures of thermosets. A

4See, for instance, Aerospace America, May 1986, p. 22.

ThermoplasticsThermop lastic resins, sometimes c a lled eng i-

neering p lastic s, inc lude some polyesters, poly -etherimide, polyamide imide, polyphenylene sul-fide, polyether-etherketone (PEEK), and liquidc rysta l polymers. They c onsist of long, d isc retemolec ules tha t melt to a visc ous liquid a t theproc essing tempera ture, typ ic a lly 500” to 700”F (260° to 3710 C), and , a fter forming, a re c ooledto an amorphous, semic rysta lline, or c rysta llinesolid . The degree of c rysta llinity has a strong ef-fec t on the fina l ma trix p roperties. Unlike the c ur-ing p roc ess of thermosetting resins, the p roc ess-ing of thermop lastic s is reversib le, and , by simp lyrehea ting to the p roc ess tempera ture, the resinc an be formed into another shape if desired .Thermop lastic s, a lthough genera lly inferior tothermoses in high-tempera ture strength andc hemic a l stab ility, a re more resistant to c rac kingand impac t damage. However, it should be notedtha t rec ently developed high-performanc e ther-mop lastic s, suc h as PEEK, whic h have a semi-c rysta lline mic rostruc ture, exhib it exc ellent high-tempera ture strength and solvent resistanc e.

Thermop lastic s offer g rea t p romise for the fu-ture from a manufac turing point of view, bec auseit is easier and faster to hea t and c ool a materia lthan it is to c ure it. This makes thermop lastic ma-tric es a ttrac tive to high-volume industries suc has the automotive industry. Currently, thermo-p lastic s a re used p rimarily with d isc ontinuous-fiber reinforc ements suc h as c hopped g lass or c a r-bon/ graphite. However, there is g rea t potentia lfor high-performanc e thermop lastic s reinforc edwith c ontinuous fibers. For examp le, thermop las-

Figure 3-2.—Comparison of General Characteristics of Thermoset and Thermoplastic Matrices

Process Process Use SolventResin type temperature time temperature resistance Toughness

Thermoset . . . . . . . . . . . . . . . . . . . . . . . . . Low I High I High I I High 1 LowToughened thermoset . . . . . . . . . . . . . . .Lightly crosslinked thermoplastic. . . . . t 1 t t 1Thermoplastic. . . . . . . . . . . . . . . . . . . . . . High 1 Low I Low Low I High ISOURCE: Darrel R. Tenney, NASA Langley Research Center.

Reinforcement•  TheconGnuousreinforcingfibersofadvancedcompositesareresponsiblefor

theirhighstrengthandsGffness.•  Themostimportantfibersincurrentuseareglass,graphite,andaramid.•  Otherorganicfibers,suchasorientedpolyethylene,arealsobecoming

important.•  PMCscontainabout60percentreinforcingfiberbyvolume.

—

Ch. 3—Polymer Matrix Composites ● 7 7

tic s c ould be used in p lac e of epoxies in the c om-posite struc ture of the next genera tion of fightera irc ra ft.

ReinforcementThe c ontinuous reinforc ing fibers of advanc ed

c omposites a re responsib le for their high strengthand stiffness. The most important fibers in c ur-rent use a re g lass, g raphite, and a ramid . Otherorganic fibers, suc h as oriented polyethylene, a rea lso bec oming important. PMCs c onta in about60 perc ent reinforc ing fiber by volume. Thestrength and stiffness of some c ontinuous fiber-reinforc ed PMCs a re c ompared with those ofsheet mold ing c ompound and various meta ls infigure 3-3. For instanc e, unid irec tiona l, high-strength graphite/ epoxy has over three times thespec ific strength and stiffness (spec ific p ropertiesa re ord ina ry p roperties d ivided by density) ofc ommon meta l a lloys.

Of the c ontinuous fibers, g lass has a rela tivelylow stiffness; however, its tensile strength is c om-petitive with the other fibers and its c ost is d ra -matic a lly lower. This c omb ina tion of p ropertiesis likely to ensure tha t g lass fibers rema in the mostwidely used reinforc ement for high-volume c om-merc ia l PMC app lic a tions. Only when stiffnessor weight a re a t a p remium would a ramid andgraphite fibers be used .

InterphaseThe interphase of PMCs is the reg ion in whic h

loads a re transmitted between the reinforc ementand the matrix. The extent of interac tion betweenthe reinforc ement and the matrix is a design va ri-ab le, and it may vary from strong c hemic a l bond-ing to weak fric tiona l forc es. This c an often bec ontrolled by using an appropria te c oa ting on thereinforc ing fibers.

Figure 3-3.—Comparison of the Specific Strength and Stiffness of Various Composites and Metalsa

.

.

/

(0°) Graphite/epoxy

(0°) Kevlar/epoxy

+ (0° ,900)Graphi te /epoxy/

+(0°) S-glass/epoxy

Graphite/epoxy

+ Sheet molding compound (SMC)

.

1 2 3 4

Specific tensile strength (relative units)Specific properties are ordinary properties divided by density; angles refer to the directions of fiber reinforcementa Steel: AlSl 4340; Alumlnum: 7075-T6; Titanium: Ti-6Al-4V.

SOURCE: Carl Zweben, General Electric Co.

Forinstance,unidirecGonal,high-strengthgraphite/epoxyhasoverthreeGmesthespecificstrengthandsGffness(specificproperGesareordinaryproperGesdividedbydensity)ofcommonmetalalloys.

Interphase•  TheinterphaseofPMCsistheregioninwhichloadsaretransmiZedbetween

thereinforcementandthematrix.

•  TheextentofinteracGonbetweenthereinforcementandthematrixisadesignvariable,anditmayvaryfromstrongchemicalbondingtoweakfricGonalforces.

•  ThiscanoienbecontrolledbyusinganappropriatecoaGngonthereinforcingfibers.

•  Generally,astronginterracialbondmakesthePMCmorerigid,butbriZle.•  AweakbonddecreasessGffness,butenhancestoughness.•  Iftheinterracialbondisnotatleastasstrongasthematrix,debondingcan

occurattheinterphaseundercertainloadingcondiGons.TomaximizethefracturetoughnessofthePMC,themostdesirablecouplingisoienintermediatebetweenthestrongandweaklimits.

Limitations of PMC

–  Low maximum working temperature. –  High coefficient of thermal expansion-

dimensional instability –  Sensitivity to radiation and moisture. –  Processing temperature are generally

higher than those with thermosets. –  Low elastic properties in certain directions

PROPERTIESOFPOLYMERMATRIXCOMPOSITES

•  TheproperGesofthePMCdependonthematrix,thereinforcement,andtheinterphase.

•  Consequently,therearemanyvariablestoconsiderwhendesigningaPMC.–  Thetypesofmatrixandreinforcement–  TheirrelaGveproporGons,–  Thegeometryofthereinforcement,and–  Thenatureoftheinterphase.

•  EachofthesevariablesmustbecarefullycontrolledtoproduceastructuralmaterialopGmizedforthecondiGonsforwhichitistobeused.

CompositeReinforcementTypes


FRP Overview: Fibers� Used in many different forms:

Short Fibers

Long Fibers

Chopped Fibers

Woven Fibers5


Fibersareusedinmanydifferentforms


FiberTypes•  GlassFiber(firstsyntheGcfiber,themostwidelyused,subjecttocreepunderhighsustainedloading-subjecttodegradaGoninalkalineenvironment)

•  Boron(firstadvancedfiber)•  Carbon(premiumcost)•  SiliconCarbide•  Aramid(extremelysensiGvetoenvironmentalcondiGons)•  Basalt(thefutureofFRPfibers?)

GlassFiberTypes

•  E-glass(fiberglass)-electricalapplicaGons•  S-glass-strengthapplicaGons•  C-glass-Corrosionresistant•  D-glass-LowdielectricapplicaGons•  A-glass-AppearanceapplicaGons•  AR-glass-Alkaliresistant

Chemical Composition of E-Glass and S-glass Fibers

% Weight Material E-glass S-glass Silicon Oxide Aluminum Oxide Calcium Oxide Magnesium Oxide Boron Oxide Others

54

15

17

4.5 8

1.5

64

25

0.01

10

0.01

.8

•  TheuseofconGnuous-fiberreinforcementconfersadirecGonalcharacter,calledanisotropy,totheproperGesofPMCs.PMCsarestrongestwhenstressedparalleltothedirecGonofthefibers(0°)andweakestwhenstressedperpendiculartothefibers(90°).

•  WhendisconGnuousfibersorparGclesareusedforreinforcement,theproperGestendtobemoreisotropicbecausethesereinforcementstendtoberandomlyoriented.SuchPMCslacktheoutstandingstrengthofconGnuous-fiberPMCs,buttheycanbeproducedmorecheaply,usingthetechnologiesdevelopedforunreinforcedplasGcs,suchasextrusion,injecGonmolding,andcompressionmolding.

Polymer Processing

FormingProcessesforThermoseYngmatrixcomposites:o  Handlay-upandspray-uptechniques.o  Filamentwinding.o  Pultrusion.o  Resintransfermolding.o  Autoclavemolding.FormingProcessesforThermoplasGcmatrixcomposites:o  InjecGonmolding.o  Filmstacking.o  Diaphragmforming.o  ThermoplasGctapelaying.

51

Filament Winding Filament Winding method involves a continuous filament of reinforcing material wound onto a rotating mandrel in layers at different layers. If a liquid thermosetting resin is applied on the filament prior to winding process is called Wet Filament Winding. If the resin is sprayed onto the mandrel with wound filament, the process is called Dry Filament Winding.

Besides conventional curing of molded parts at room temperature, autoclave curing may be used.

52

Filament Winding

Filament Winding Process •  For Round or Cylindrical parts •  A tape of resin impregnated fibers is wrapped over a rotating mandrel to form a part. •  These windings can be helical or hooped. •  There are also processes that use dry fibers with resin application later, or prepregs are

used. •  Winding direction

•  Hoop/helical layers •  Layers of different material

•  High strengths are possible due to winding designs in various direction

Demolding

•  To remove the mandrel, the ends of the parts are cut off when appropriate, or a collapsible mandrel (e.g., low melt temperature alloys ) is used.

•  Curing in done in an Autoclave for thermoset resins (polyester, epoxy, phenolic, silicone) and some thermoplastics (PEEK)

•  Fibers are E-glass, S-glass, carbon fiber and aramids (toughness and lightweight).

Filament winding - winding patterns •  hoop (90º) - girth or circumferential

winding –  angle is normally just below 90°

degrees –  each complete rotation of the

mandrel shifts the fibre band to lie alongside the previous band.

•  helical –  complete fiber coverage without

the band having to lie adjacent to that previously laid.

•  polar –  domed ends or spherical

components –  fibres constrained by bosses on

each pole of the component. •  axial (0º)

–  beware: difficult to maintain fibre tension

Filament winding - applications

•  pressure vessels, storage tanks and pipes

•  rocket motors, launch tubes

•  drive shafts

•  wind turbine blades

55

Filament Winding

•  Advantages –  Good for wide variety of part sizes –  Parts can be made with strength in several different directions –  Very low scrap rate –  Non-cylindrical parts can be formed after winding –  Flexible mandrels can be left in as tank liners –  Reinforcement panels, and fittings can be inserted during

winding

•  Disadvantages –  Viscosity and pot life of resin must be carefully chosen –  Programming can be difficult –  Some shapes can't be made with filament winding –  Factors such as filament tension must be controlled

Pultrusion Pultrusion is a process where composite parts are manufactured by pulling layers of fibres/fabrics, impregnated with resin, through a heated die, thus forming the desired cross-sectional shape with no part length limitation.

•  Pultrusion is an automated, highly productive process of fabrication of Polymer Matrix Composites in form of continuous long products of constant cross-section.

Pultrusion process involves the following operations: 1.  Reinforcing fibers are pulled from the creels. Fiber (roving) creels

may be followed by rolled mat or fabric creels. Pulling action is controlled by the pulling system.

2.  Guide plates collect the fibers into a bundle and direct it to the resin bath.

3.  Fibers enter the resin bath where they are wetted and impregnated with liquid resin. Liquid resin contains thermosetting polymer, pigment, fillers, catalyst and other additives.

4.  The wet fibers exit the bath and enter preformer where the excessive resin is squeezed out from fibers and the material is shaped.

5.  The preformed fibers pass through the heated die where the final cross-section dimensions are determined and the resin curing occurs.

6.  The cured product is cut on the desired length by the cut-off saw.

Pultrusion process is characterized by the following features:

•  High productivity.

•  The process parameters are easily controllable.

•  Low manual labor component.

•  Precise cross-section dimensions of the products.

•  Good surface quality of the products.

•  Homogeneous distribution and high concentration of the reinforcing fibers in the material is achieved (up to 80% of roving reinforcement,

up to 50% of mixed mat + roving reinforcement).

•  Pultrusion is used for fabrication of Fiber glass and Carbon fiber

reinforced polymer composites and Kevlar (aramid) fiber reinforced

polymers.

Pultrusion

•  Manufacturing –  Fibers are brought

together over rollers, dipped in resin and drawn through a heated die. A continuous cross section composite part emerges on the other side.

Pultrusion

Advantages:

Ø Minimal kinking of fibres/

fabrics

Ø Rapid processing

Ø  Low material scrap rate

Ø Good quality control

Potential Problems:

Ø  Improper fiber wet-out

Ø  Fiber breakage

Ø  Inadequate cure

Ø Die jamming

Ø Complex die design

•  Shapes such as rods, channels, angle and flat stocks can be easily produced.

•  Production rate is 10 to 200 cm/min.

•  Profiles as wide as 1.25 m with more than 60% fiber volume fraction can be made routinely.

•  No bends or tapers allowed (continuous molding cycle)

Pultrusion - Applications

•  panels – beams – gratings – ladders •  tool handles - ski poles – kites •  electrical insulators and enclosures •  light poles - hand rails – roll-up doors •  450 km of cable trays in the Channel Tunnel

Produc/onTechniquesforPolymerComposites

Ch. 3—Polymer Matrix Composites . 79

DESIGN, PROCESSING, AND TESTINGDesign

Advanc ed c omposites a re designed materia ls.This is rea lly the fac t tha t underlies their useful-ness. Given the spec trum of matrix and reinforc e-ment materia ls ava ilab le, p roperties c an be op-timized for a spec ific app lic a tion. An advanc edc omposite c an be designed to have zero c oeffi-c ient of therma l expansion. It c an be reinforc edwith c ombina tions of fiber materia ls (hybrid PMCs)and geometries to maximize performanc e andminimize c ost. The design opportunities of PMCmateria ls a re only beg inning to be rea lized .

The enormous design flexib ility of advanc edc omposites is ob ta ined a t the c ost of a la rge num-ber of unfamilia r design va riab les. In fac t, c om-posites a re more ac c ura tely c harac terized as c us-tomized struc tures, ra ther than materia ls. Althoughthe eng ineering p roperties of the homogeneousresins and fibers c an be determined , the p rop-erties of eac h c omposite depend on the c ompo-sition, fiber geometry, and the na ture of the in-terphase. However, the c a tegories of mec hanic a land physic a l p roperties used to c harac terize PMCsare c a rried over from long eng ineering experi-enc e with meta ls.

A ma jor need in advanc ed c omposites tec hnol-ogy is a better c apab ility for modeling struc ture-p roperty rela tionships (d isc ussed in more dep thin c h. 5). In sp ite of this lac k, however, experi-enc e to da te has shown tha t designers and man-ufac turers c an p roduc e reliab le PMC struc tures.This is p robab ly due to two fac tors. First, in thefac e of unc erta inty, designers tend to overdesign;tha t is, they a re c onserva tive in their use of ma-teria l, to avoid any possib ility of materia l fa ilure.Sec ond , PMC struc tures a re extensively testedbefore use, ensuring tha t any potentia l p rob lemsshow up during the tests. Thus, the PMC materi-a ls themselves have been p roven, in the sensetha t struc tures c an be fab ric a ted tha t a re relia -b le and meet a ll design c riteria . However, bothoverdesign and emp iric a l testing a re c ostly anddrive up the p ric es of PMCs. Thus, a p rinc ipa lbenefit of enhanc ed modeling c apab ility will beto help make advanc ed c omposites more c ost-c ompetitive.

ManufacturingGiven the many d ifferent fibers and matric es

from whic h PMCs c an be made, the sub jec t ofPMC manufac turing is an extremely b road one.However, more than any other sing le a rea , Iow-c ost manufac turing tec hnolog ies a re requiredbefore advanc ed c omposites c an be used morewidely. The basic steps inc lude: 1 ) impregna tionof the fiber with the resin, 2) forming of the struc -ture, 3) c uring (thermoset matric es) or therma lp roc essing (thermop lastic ma tric es), and 4) fin-ishing.

Depend ing on the p roc ess, these steps mayoc c ur separa tely or c ontinuously. For instanc e,the sta rting materia l for many PMCs is a p repreg;i.e., a fiber tape or c loth tha t has been p reimpreg-na ted with resin and partia lly c ured . In pultru-sion, by c ontrast, impregna tion, forming, and c ur-ing a re done in one c ontinuous p roc ess. Someof the more important fab ric a tion p roc esses forPMCs a re listed in tab le 3-1.

In the aerospac e sec tor, advanc ed c ompositestruc tures a re c ommonly fab ric a ted by the slowand labor-intensive p roc ess of hand lay-up of

Table 3-1 .—Production Techniques forPolymer Composites

Technique Characteristics ExamplesSheet molding Fast, flexible, 1-2” SMC automotive body

fiber panelsInjection molding Fast, high volume Gears, fan blades

very short fibers,thermoplastics

Resin transfer Fast, complex parts, Automotive structuralmolding good control of fiber panels

orientationPrepreg tape lay-up Slow, laborious, Aerospace structures

reliable, expensive(speed improved byautomation)

Pultrusion Continuous, constant l-beams, columnscross-section parts

Filament winding Moderate speed, Aircraft fuselage,complex geometries, pipes, drive shaftshollow parts

Thermal forming Reinforced thermoplastic All of above(future) matrices; fast, easy

repair, joiningSOURCE: Office of Technology Assessment, 1988.

APPLICATIONS OF PMCs

•  Polymer composites are used to make –  very light bicycles that are faster and easier to handle than standard ones, –  fishing boats that are resistant to corrosive seawater and –  lightweight turbine blades that generate wind power efficiently. –  New commercial aircraft also contain more composites than their predecessors.

A 555-passenger plane recently built by Airbus, for example, consists of 25 percent composite material, while Boeing is designing a new jumbo aircraft that is planned to be more than half polymer composites.

•  Polymer Matrix Composites (PMCs) are used for manufacturing:

–  secondary load-bearing aerospace structures, –  boat bodies, canoes, kayaks, –  automotive parts, radio controlled vehicles, –  sport goods (golf clubs, skis, tennis racquets), –  fishing rods, bullet-proof vests and other armor parts, brake and clutch linings.

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