chapter 7: polymer blends and...
TRANSCRIPT
15.12.2015
1
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
15.12.2015
2
• 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.
15.12.2015
3
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
15.12.2015
4
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
15.12.2015
5
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.
15.12.2015
6
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
15.12.2015
7
• 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.
15.12.2015
8
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
15.12.2015
9
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)
15.12.2015
10
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
15.12.2015
11
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
15.12.2015
12
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
15.12.2015
13
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
15.12.2015
14
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
15.12.2015
15
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
15.12.2015
16
- 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
15.12.2015
17
• 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.
15.12.2015
18
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
15.12.2015
19
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.
15.12.2015
20
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)
15.12.2015
21
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
15.12.2015
22
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
15.12.2015
23
Composites are produced by a number of methods:• Compression moulding• Resin-transfer moulding• Filament winding and pultrusion
Filament winding operation
15.12.2015
24
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
15.12.2015
25