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Types of polymer matrices
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•Most of the polymer matrices are thermosets (75%)
• Thermosets are cured using curing agents or
hardeners to form a network structure (cross-linked)
• Thermosets are brittle at room temperature and
have low fracture toughness values
(KIC = 0.5 -1.0 MPa▪m1/2)
• Thermosets are suitable for high temperature
application as they have higher softening
temperatures and better creep resistance than
thermoplastics
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Polymeric Matrices for composites
Thermoplastics
Nylons
Polyester (PET, PBT)
Polycarbonate
Polyacetals
Polyamide-imide (PAI)
Polyether-ether ketone (PEEK)
Polysulfone (PSUL)
Polyphenylene sulfide (PPS)
Polyether imide (PEI)
Thermosettingg
Epoxies
Polyesters
Phenolics
Polyimides
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Crystallization or reaction
DSC
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Terminology
• Shelf life or storage life – the length of time that unmixed resins
can be stored
• Pot life – resins into which the initiator has been mixed
• Inhibitor – molecules which absorb free radicals are added to
resin mixture to slow down or prevent further cross-linking
• A-stage – referring to resole and nonvolac resins
– Resole: a low molecular mass material, only heat is
needed to covert the resin to the C-stage
– Nonvolac: hardener is needed to achieve the C-stage
• B-stage – a rubbery phase, the resin mixture is partially
soluble and partially cured
– Prepreg: pre-impregnated tape (B-stage)
• C-stage – resin mixture is cured to a fully cross-linked
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Epoxies
• Developed in 1939, was mainly for coatings and adhesives.
• Many different structures available today are derived from
bisphenol acetate and epichlorohydrin.
• Epoxy is more expensive and is more viscous than the
polyester resin making it very difficult to process
• A higher curing temperature (up to 180°C) with two to three
stages of curing will be required.
• The shrinkage is much smaller
than for polyesters (1-4%)
• In general, epoxies are stiffer
and stronger, but more brittle
than polyesters. Epoxies also
retain their properties better in
high temperatures than
polyesters do.
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Polyesters
• Developed in 1833, consisted of unsaturated linear
polyester molecules dissolved in styrene (styrene is
a cross-linking monomer).
• Curing can take place when an organic peroxide
(e.g. MEKP) is added to the polyester resin.
Free radicals are then created
during the chemical reaction
which leads to a formation of
a 3-dimensional network structure.
• Polyesters are fairly easy
to process as they are relatively
inexpensive and have low
viscosities. The shrinkage
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which occurs on curing is around 4-8% (pretty high).
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Bismaleimide: an uncrosslinked polymer that behaves as a
termosetting
Definition: These addition-type polyimides or maleimidebased polyimides are used in high performance
structural composites requiring higher temperature use
and increased toughness. Monomers are usually
synthesized from maleic anhydride and an aromatic
diamine; the bismaleamic acid formed is
cyclodehydrated to a bismaleimide resin.
The double bond of the maleimide
is very reactive and can undergo
chain extension reactions.
Epoxy blends of BMI have
exhibited use temperatures
of 205° to 245 °C and
increased toughness.
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Phenolics
• Developed in 1872, known as phenolformaldehyde.
• A resole is produced by reacting a phenol with an
access amount of aldehyde in the presence of a
basic catalyst. (one-stage resin)
• A nonvolac is generated when excessive phenol is
reacting with an acid catalyst.
(two-stage resin)
• Low cost, excellent heat
resistance and good
balance of properties
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Phenolic Resins - -
These materials are much like a thermoplastic
material at room temperature but change to a
thermosetting material at elevated temperature.
They are cheaper and stronger then epoxy and
many polyester resins. They are not as popular as
composite matrix material as are the epoxy resins.
As with the other plastic matrix materials, phenolics
are relatively poor conductors of heat and electricity.
They are non-magnetic. A disadvantage is their
relatively high shrinkage during curing.
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Thermoplastics
• Are linear polymers, don’t cross-link, might be branched
• Have superior toughness to thermosets
• Polypropylene and polyethylene are similar in origin and
manufacture. PP is cheaper than PE. PP is harder, more
rigid and has a higher stress cracking resistance than PE.
•Polycarbonate is an amorphous, transparent material, has
good impact resistance, can be used up to 140°C.
• Polyamide (e.g. Nylon) has a high m.p. (260°C) and
maintains its properties to about 150°C.
• Polyetheretherketone (PEEK) is a semi-crystalline polymer
having 20-40% crystallinity. PEEK has a high Tg (143°C)
and m.p. (343°C), good toughness (6 MPa▪m1/2) and good
solvent resistance
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Polyetheretherketone (PEEK), also referred to as polyketones, is obtained from
aromatic dihalides and bisphenolate salts by nucleophilic substitution.
PEEK is a semicrystalline thermoplastic with extraordinary mechanical
properties. The Young's modulus is 3.6 GPa and its tensile strength 90 MPa.
PEEK exhibits two glass transition temperatures at around 140°C (284°F)
and around 275ºC (527°F), depending on cure cycle and precise formulation.
PEEK melts at around 350°C (662°F) and is highly resistant to thermal
degradation. The material is also resistant to both organic and aqueous
environments, and is used in bearings, piston parts, pumps, compressor plate
valves, and cable insulation applications. It is one of the few plastics compatible
with ultra-high vacuum applications.
PEEK is considered an advanced biomaterial used in medical implants, often in
reinforced format using biocompatible fibre fillers such as carbon. Also in
carbon fibre reinforced form, PEEK has come under consideration as an
aerospace structural material due to its high strength-to-weight ratio. Electronic
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circuitry also has a high demand for PEEK's large temperature range.
PEEK
Density
1300 kg/m3
Young's modulus (E)
3700 MPa
Tensile strength (σt)
90 MPa
Elongation @ break
50%
notch test
55 kJ/m2
Glass temperature
130-150 °C and 260-290 ºC
melting point
~350 °C
heat transfer coefficient (λ)
0.25 W/m.K
linear expansion coefficient (α) 1.7 10-5 /K
Price 40-90 €/kg
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Structure of some thermoplastic polymers
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Elastomers
• Rubber-like elasticity – can be stretched more than
200%.
• In general the suitable temperature range for an
elastomer is between -50oC and 120oC. However,
Silicone has a wider range (-115oC and 315oC)
• Natural rubber –cis-polyisoprene
• Styrene-butadiene rubber (SBR)
• Acrylonitrile-butadiene rubber (NBR)
• Chloropreene (CR)
• Polysiloxane (Silicone)
• Vulcanization – a chemical reaction to cross-link
chains using sulfur.
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