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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 orhardeners 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 temperatureapplication as they have higher softeningtemperatures and better creep resistance thanthermoplastics

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Polymeric Matrices for composites

Thermoplastics Thermosettingg

EpoxiesPolyestersPhenolicsPolyimides

NylonsPolyester (PET, PBT)PolycarbonatePolyacetalsPolyamide-imide (PAI)Polyether-ether ketone (PEEK)Polysulfone (PSUL)Polyphenylene sulfide (PPS)Polyether imide (PEI)

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DSCCrystallization or reaction

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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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• 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 thepolyester 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.

Epoxies

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Polyesters

• Developed in 1833, consisted of unsaturated linearpolyester molecules dissolved in styrene (styrene isa 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 which occurs on curing is around 4-8% (pretty high).

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Definition: These addition-type polyimides or maleimide-based 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.

Bismaleimide: an uncrosslinked polymer that behaves as a termosetting

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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 goodbalance 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 andmanufacture. PP is cheaper than PE. PP is harder, morerigid and has a higher stress cracking resistance than PE.•Polycarbonate is an amorphous, transparent material, hasgood impact resistance, can be used up to 140°C.• Polyamide (e.g. Nylon) has a high m.p. (260°C) andmaintains its properties to about 150°C.• Polyetheretherketone (PEEK) is a semi-crystalline polymerhaving 20-40% crystallinity. PEEK has a high Tg (143°C)and m.p. (343°C), good toughness (6 MPa▪m1/2) and goodsolvent 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 circuitry also has a high demand for PEEK's large temperature range.

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Density 1300 kg/m3Young's modulus (E) 3700 MPaTensile strength (σt) 90 MPaElongation @ break 50%notch test 55 kJ/m2Glass temperature 130-150 °C and 260-290 ºCmelting point ~350 °Cheat transfer coefficient (λ) 0.25 W/m.Klinear expansion coefficient (α) 1.7 10-5 /K

Price 40-90 €/kg

PEEK

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Structure of some thermoplastic polymers

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Elastomers

• Rubber-like elasticity – can be stretched more than200%.• In general the suitable temperature range for anelastomer 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-linkchains using sulfur.

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• Elastomers

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