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CHAPTER 7: POLYMER BLENDS AND COMPOSITES

Polymer Science and Technology IINovember 18, 2015

• The major problem in the application of polymers in engineering is their low stiffnessand strength.

• When compared to metals; the moduli are ~ 100 times lower

and strengths ~5 times lower

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• Homopolymer Moduli: 0.3-20 GN/m2

Strength: 12-140 MN/m2

• Steel Moduli: 200 GN/m2

Strength: 200 MN/m2

• Aluminium Moduli: 70 GN/m2

Strength: 200 MN/m2

Commodity Plastics

The «Big Four» Commodity Thermoplastics• PE• PP• PVC• PSMore than 85 % (by volume) of world plasticconsumption belongs to the «Big Four» plastics.

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Engineering Polymers (Plastics)The polymers that are used in the manufacture of premium plastic products where

high-T resistance,high impact strength, chemical resistance andother special properties

are required.

Engineering Polymers (Plastics)• Aliphatic polyamides (Nylon 6, Nylon 66)• ABS resin• Polycarbonate• Polysulfones• Poly (phenylene oxide)• Polyacetals• Engineering polyesters and fluoroplastics

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Methods to increase the stiffness andstrength of polymers

• Novel homopolymer design• Crystallization• Crosslinking• Copolymerization• Radiation• IPN (Interpenetrating networks) structures• Polymer blends and alloys• Reinforced polymers

POLYMER BLENDS

Blending may be used• to reduce the cost of an expensive engineering

thermoplastic• to improve the processability of a high-T or

heat sensitive thermoplastic• to improve impact resistance

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BLENDING MISCIBILITY

Commercial blends may be

• homogenous• phase-separated• a bit of both

Type of polymer blend (homogenous or phase-separated) will depend upon many factors suchas;• kinetics of the mixing process• processing temperature• presence of solvent or other additivesHowever, the primary consideration fordetermining miscibility of two polymers is aTHERMODYNAMIC ISSUE that is governed by theGIBBS free-energy considerations.

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If is positive at a given temperature, polymers in the blend will separate into phases.For complete miscibility, two considerations arenecessary:• must be negative• 2nd derivative of with respect to volume fraction

of component 2 ( ) must be greater than zero

mmm STHG

mG

mG

mG

2

0)G( P,T22

m2

In general, polymer blends, which will separate atequilibrium into two mixed-composition, can exhibit awide range of phase behaviour, including upper andlower critical solution temperatures.

Idealized phase diagram for a polymer blend

Two phases coexist

The blend is miscible at all compositions

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• LCST behaviour is quite common for polymerblends compared to UCTS behaviour.

• LCST=240oC. This means, if the blend is meltprocessed above 240oC, phase separationoccurs.

• UCST behaviour may be observed only in a solution which is a low MW solvent used.

• Recently, there has been interest in blendscontaining three-component polymers.

(TERNARY BLENDS)

PMMA-PEMA-Poly (styrene-co-acrylonitrile)(SAN)

• SAN is compatible (miscible)with PMMA andPEMA.

• But, PMMA and PEMA are immiscible.

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Commercial Polymer Blends:Examples of Miscible Polymer Blends

Polymer 1 Polymer 2

PS Poly (methyl vinyl ether)

Poly (2,6-dimethyl, 1,4-phenylene oxide)

PVC PCL

Nitrile rubber

PVF(poly vinylidene fluoride)

PEMAPMMA

Properties of Blends

-Properties of miscible polymer blends may be intermediate between those of the individualcomponents (i.e. additive behavior)-In other cases blend properties may exhibit eitherpositive or negative deviation from additivity.

(Positive deviation from additivity)

Dependence of miscible blend properties on composition

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CompatibilizersMechanical properties of immiscible blends areoften poor due to the inadequate interfacialstrength between the dispersed phase and matrix.Additives to promote miscibility by reducinginterfacial tension are called «compatibilizers»• Reactive compatibilizers: They chemically react

with blend components.• Nonreactive compatibilizers: Block or graft

copolymers of the blend homopolymers.

Interpenetrating Networks (IPNs)

IPNs are combinations of two or more polymersin network form.

IPNs include PUs, PS, PEA, PMMA.

Earliest commercialized IPNs used in manyautomative applications, consists of PP andEPDM (ethylene-propylene-diene terpolymer)

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Sequential IPN

swollen with styrene+DVB

If no crosslinking agent is used for second polymerin the formation of sequential IPN

Polymerizing the secondmonomer before theequilibrium sorptionoccurs

When both polymers aresynthesized andcrosslinkedsimultaneously

Semi-IPN (singlenetwork of theinitial polymer)

Gradient IPN

Simultaneous IPN (SIN)First or second polymer step polymerizationOther chain polymer

To eliminate copolymerization

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IPN structures are used for• Soft contact lenses• Ion exchange resins• Pressure sensitive adhesives• Controlled release of drugs• Preparation of novel membranes

Reinforced Polymers Polymer Composites• Commercialization of composites began;

Cellulose Fibers + Phenolic ResinUrea ResinMelamine Resin

• The most familiar composite material isFiberglass R (1940) unsaturated polyester+ glass fiber matrix

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Reinforced Polymers:• High strength• Low weight

Uses of polymer composites:• Automotive• Marine• Construction• Electrical and electronics• Aerospace and military

Examples:1) Competition kayakEpoxy resin: thermoset polymer matrix

– Lightness– Excellent corrosion resistance due to water– Economical construction of small batches

Kevlar fibers + carbon fibres–Essential strength and rigidity–Very little cost in extra weight

– Very fast– Manoeuvrable– Light

In small boats performance is criticallow price more importance

Glass fibre reinforced plastic (GRP)

polyester

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2) Tennis racquet (Nylon matrix+carbon fiber)Nylon66 matrix Low density

Economical construction by injection moldingin large batches

Polymer is moulded around the low Tmmetal alloy core75 km/h

3) Rubber car tyreReinforced at several different levels.• at microscobic level

- carbon black mixed with polymer increasedstiffness, strength and wear resistance

• at macroscobic level- rigid cords (polyester fibres and/or steel wires)-to provide strength and stiffness in radial and

circumferential direction

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Reinforcing agents have followingabilities/functions

• Must be stiffer and stronger than the polymermatrix

• It has good particle size, shape and surfacecharacter for effective mechanical coupling tothe matrix

• It preserves the desirable qualities of thepolymer matrix

Mechanism of Reinforcement

Stress StrainIn the presence of reinforcing agent

Total strain in thematrix

undeformed state under a tensile load

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The strength of the composite depends on thestrength of the bond between particle and matrix.

The more interface effective reinforcementEffectiveness of a reinforcement A/V ratioA: surface area of a particleV: its volumeA/V to be as high as possible

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- a>>1 fibre- a <<1 platelet

Therefore two main classes of reinforcingagents;- fibres (glass fibres, carbon fibers)- platelets (mica and talk)

m: mass of compositev= volume of compositemf: mass of fibres occupying a volume vf

mm: mass of matrix occupying a volume vm

Assuming there are no voids;m= mf + mm

v= vf + vm

fibre

matrix

vv f

f

vvm

m

fm 1 or

mfff )1( density of composites

f density of fibre

density of matrixm

Reinforced polymersgenerally have lowdensities.

Epoxy reinforced with70% carbon fibres is only 1700 kg/m3

(Density of water 1000 kg/m3).

proportions of fibre in thematrix by volume

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• In practice, composite materials contain voids whichcomprise trapped air or solvents, etc.

• A void is source of weakness.

• A void content greater than 2 % poor fabrication• A void % < 0.5 % high class «aircraft quality»

fabrication

POLYMER MATRICES• Initially thermoset polymers

1) Thermoset polyesters- Inexpensive polymers- Used with glass-fibre reinforcements

2) EpoxiesPreferred to polyesters

- Superior mechanical performance- But higher cost than polyesters

• TODAY Reinforced thermoplastic materialsPopular matrices: Semi-crystalline polymers PP and NylonMajor advantage: Forming is possible by normal injectionmoulding or extrusion.

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Fibrous Reinforcement

• Glass fibers

• Oriental polymeric fibers(Aramid) Kevlar-Du Pont Company

• Carbon fibers

E-glassS-glass

Glass fibre is widely used one.E-glass

SiO2 54.0 %CaO 17.5 %

Al2O3 14.0 %B2O3 8.0 %MgO 4.5 %

S-glass: higher modulusand strengthC-glass: improvedresistance to water andacids

Glass fibers are manufactured by extruding molten glass at high linearvelocity through a large number (100-1000) of holes in a platinium plateknown as a «bushing».Then they cooled and solidify.

Si

O

O

O

O

Structure of silica glass

tetrahedrom

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Advantages of glass fibres:• Resistance to high temperatures - softening

point is 850oC.• Transparency to visible light – takes the colour

of matrix• Isotropy – such as thermal expansion is

identical in axial and radial directions.Disadvantages:• Susceptible to surface damage

Carbon and Kevlar fibres are less widely usedthan glass fibers due to their relatively high cost.

Best carbon fibers are produced from PAN (polyacrylonitrile)

PAN is converted to graphite by controlledheating process.

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Advantages of C fibers:• Chemical inertness: resistance to moisture

and common chemicals• High electrical and thermal conductivity along

the fibre axis• Dimensional stability: thermal expansion is

low and negative

Disadvantage:• Colour: black

Oriented Polymer Fibres

Aramid (Aromatic Polyamid) Fibres• Aromatic groups• Amide groups

C

O

C

O

N

H

O O N

H

n

Kevlar 49 : Poly (paraphenylene terephthalamide)(mostly used one)

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The order of excellencecarbon > aramid > glass

Platelet reinforcementTalc: 3MgO.4SiO2.H2OMica: K2O.3Al2O3.6SiO2.2H2O• Both are crystalline10-1000 μm1-5 μm thickness• Low price• Stiffness and strength

Naturally-occuringmaterials, never obtained in pure form

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Interfacial adhesion and coupling agents:Coupling agents low MW organic-inorganiccompounds that promote adhesion betweenfiller and matrix.

Engineering thermoplastic: PEEK (Tm=334oC)30 % fiber loading

Generally organofunctional silanes

Mechanical properties of composites are stronglyinfluenced by the• size• type• concentration• dispersion of reinforcing agent (filler)• interfacial tension between the matrix and filler

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Composites are produced by a number of methods:• Compression moulding• Resin-transfer moulding• Filament winding and pultrusion

Filament winding operation

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Pultrusion line

Nanocomposites• End of 1980s. Toyota researches developed first

nanocomposites from Nylon-6• Clay nanofillers

thickness: ~1nmaspect ratio (D/L): 10:1

1000:1• Commonly used clay nanofiller is

montmorrilonite (mmt): naturally-occuringsilicate

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