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Classes of Polymeric Materials Chapter 3: Thermosets. Professor Joe Greene CSU, CHICO. Thermosetting Resins (thermosets). Introduction Thermoplastics are supplied as pellets, powders, or granules and do not undergo a chemical reaction.

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Classes of polymeric materials chapter 3 thermosets l.jpg
Classes of Polymeric MaterialsChapter 3: Thermosets

Professor Joe Greene


Thermosetting resins thermosets l.jpg
Thermosetting Resins (thermosets)

  • Introduction

    • Thermoplastics are supplied as pellets, powders, or granules and do not undergo a chemical reaction.

      • Thermoplastics have large molecular weights & long molecules

      • The high viscosities are reduced by high temperatures

    • Thermoset resins are supplied as liquid chemicals (low MW and low viscosity) and undergo a chemical reaction that features polymerization and crosslinking.

      • Liquid chemicals have short chains that polymerized into long chains and high molecular weights and high viscosity.

      • The chains are crosslinked (attached) to each other to make a stiff molecule

      • Rubbers involve cross-linking of already polymerized molecules to stiffen the molecules together in Vulcanization

      • Heat is needed to cause polymerization to build MW and to cause stiffening of molecule through cross-linking

      • Heat reduces the viscosity of the chemicals until the reaction occurs and then causes the viscosity to get very large during crosslinking.

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Thermosetting Resins (thermosets)

  • Types of thermosets

    • Temperature activated

    • Catalyst activated

    • Mixing-activated

  • Temperature activated Fig 3.84

    • All thermosets require heat to undergo chemical reaction

      • Lower temperature thermosets (room temperature cure) react to a more rubbery polymer that gets stiffer upon additional heat.

      • Pot life: time that it takes for the thermosets to react to a solid after mixed.

      • Gel time: time it takes for two liquid thermoset polymers that are mixed to form a gel or skin (and stop flowing)

    • Several thermosets are supplied as powder or granular form.

      • Heat reduces the viscosity and melts the polymer to allow it to flow & mold

      • Additional heat triggers a chemical reaction which forms a cross-linked 3D

    • Common heat activated polymers

      • Formaldehyde (FOR), phenoplasts (PF), amnioplasts (UF), polyester, vinyl ester, alkyd, allyl, furan, some epoxies, and polyimides

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Thermosetting Resins (thermosets)

  • Catalyst activated: Fig 3.85

    • Some thermosets supplied as stable liquid form

      • Small amount of liquid (catalyst) is added which starts a chemical reaction and leads to formation of 3D structure.

      • Chemical type and amount of catalyst controls the extent of reaction and the speed of polymerization.

      • Many systems can set at room temperature.

      • Useful for casting resins and for glass fiber reinforced composites.

      • Common polymer is unsaturated polyester resin (UPR)

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Thermosetting Resins (thermosets)

  • Mixing activated systems: Fig 3.86

    • Some thermosets supplied as two stable liquids.

      • When the two are added together, a chemical reaction starts and forms a 3D structure.

      • Ratio of the two chemicals and temperature controls the extent of reaction and the speed of polymerization.

      • Many systems can set at room temperature.

      • Useful for casting resins and for glass fiber reinforced composites.

      • Common polymers are polyurethane and epoxies.

      • Polyurethane can be mixed at high speeds in a Reaction Injection Molding (RIM) process.

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Commercial Thermosets


O = C




















  • Formaldehyde Systems: Functional Groups

    • Formaldehyde plus one of the three hydrogen containing chemicals to form a 3D molecular network

      • Phenol,

      • Melamine, or

      • Urea.

    • Condensation reaction involving the oxygen and two hydrogens from two different molecules, Phenol, Urea, or formaldehyde.

    • One stage systems with resols

    • Two stage systems with novolacs prepolymers, or precursers

    • Usually have large amounts of filler, e.g., wood flour, cellulose fibers and minerals.

    • Supplied as powder or granual form or pills (compacted preforms)

      • Molding temperatures (125°C – 200°C) and molding pressures of 2000 to 8,000 psi for compression molding and 18,000 psi for injection molding

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Commercial Thermosets

  • Formaldehyde Systems: Functional Groups

    • Phenoplasts (phenolics) are based on phenol and formaldehyde and were one of the first commercial polymers, Bakelite, and were used for billiard balls.

    • Used with other materials to act as a binder, adhesive, coatings, surface treatments, etc.

    • Applications

      • Temperature resistant insulating parts for appliances (handles, knobs), electrical components (connectors, distributor caps) and bottle closures.

      • Abrasive binder for grinding wheels and brakes.

      • Decorative laminates (counter tops or table tops)

      • Fire resistant rigid foams.

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Commercial Thermosets

  • Formaldehyde Systems: Functional Groups

    • Aminoplasts (amino resins) are based on urea and formaldehyde or melamine and formaldehyde.

      • Can be made translucent or in light colors for aesthetics

    • Urea-formaldehyde resins are used for many of the same applications as phenolics if have color requirements

      • Castable foam system is used for home insulation

    • Melamine-formaldehyde resins are based on melamine and formaldehyde

      • Noted for their excellent water resistance.

      • Used for dishwater safe dinner ware which can be decorated with molded-in paper overlays.

      • Form the surface layer for decorative laminates (Formica)

      • Used as an adhesives for water resistant plywood.

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Commercial Thermosets








  • Furan Systems

    • Feature a ring structure which can be opened cleaved to yield polymeric molecules which have 3-D molecular networks.

    • Combined with fomaldehyde related thermosets.

      • Used as binder for sand and foundry work or abrasive particles in grinding wheels.

      • Used as adhesives and matrix for reinforced plastics where corrosion resistance is important.

  • Allyl systems (Pg 171)

    • Manufacture involves the reaction of a monofunctional unsaturated alcohol, allyl alcohol (AA) with a difunctional acid.

      • Ester linkages are formed though not a polymer

      • 2 unsaturated C=C per monomer permits formation of 3-D molecule with the use of catalysts and elevated temperatures.

      • DAP (diallylphthalate) is most common allyl monomer

      • Thermoplastic pre-polymers are available that are cured with little shrinkage

      • Applications include high performance molding compounds for electrical

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Commercial Thermosets

  • Alkyd Systems

    • Alkyd comes from alcohol (alk) and acid (yd)

    • Reaction of difunctional alcohol and difunctional acids or anhydrides forms a polyester which is what alkyd is.

    • Used as coatings (paints, coatings, varnishes)

  • Unsaturated Polyesters

    • Thermoset reaction between a difunctional acid (or anhydride) and a difunctional alcohol (glycol)

    • At least some of the acid (or anhydride) features double bonds between adjacent carbon atoms for unsaturation.

    • Characteristic ester linkages are formed, hence the name Polyester

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Polyester Chemistry



  • Unsaturated Polyesters

    • Thermoset reaction between a difunctional acid (or anhydride) and a difunctional alcohol (glycol)

      C6H4(COOH)2 + (CH2)2(OH)2 -[(CH2)2 -O- C - C-O]-

      terephthalic acid + ethylene glycol Polyethylene terephthalate (PET)

    • Acids include: maleic, fumaric, isophthalic, terphthalic, adipic, etc.

    • Anhydrides include: maleic, phthalic

    • Glycols include ethylene glycol, diethylene glycol, propylene glycol

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Polyester Chemistry

  • Heat or radiation can trigger the cross linking reaction

    • Catalyst is used

      • Starts reaction but is not consumed and is retrieved at end of reaction.

    • Initiator

      • Methyl ethyl ketone (MEK) peroxide, benzoyl peroxide, and cumene hydroperoxide

      • Starts reaction, then is consumed in reaction.

    • Accelerators (or promoters) speed up the reaction.

    • Inhibitors extend shelf life (hydroquinone, tertiary butyl catechol)

  • Condensation Reaction results in CO2 and H2O

    • Monomer required to polymerize, e.g., Styrene, to react with the unsaturations in the polyester molecules to form 3-D network.

      • Styrene at 30% to 50% in commercial polyester systems for polyester

      • vinyl toluene for vinyl ester resins

      • methyl methacrylate

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Polyester Chemistry

  • Step 1: Create polymer and build MW of polymer chain

    • Condensation Polymerization of Di-ACID and Di-ALCOHOL

      • Fig 2.: Condensation reaction

        • Connects one end of acid with one end of alcohol to form polyester bond.

        • The opposite end of acid reacts with another free end of alcohol, and so on .

        • Have water as a by-product means condensation.

        • Still have unsaturated polymer. The Carbon atom has double bonds:

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Polyester Chemistry

  • Step 2: Crosslink polyester polymer with unsaturated styrene.

    • Addition (free radical) reaction to connect polyester with styrene

      • Use a peroxide (free radical) to open the unsaturated bond to form saturation

      • One reaction starts, the other unsaturated bonds open up and react with the styrene to form a saturated polymer.

      • The ends of the polyester-styrene crosslinked polymer has peroxide end-groups.

      • Peroxide is an initiator and not a catalyst since it is consumed in reaction. Catalysts are not consumed in the reaction and can be retrieved at the end of it.

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Sheet Molding Compound (SMC)

  • SMC is the paste that is compression molded

    • 33% polyester resin and stryrene, which polymerizes and crosslinks

    • 33% glass fibers (1” fibers)

    • 33% Calcium Carbonate

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Epoxy Chemistry

  • Epoxy: O H H

    C C H + H2N (C) N (C) NH2

    H H H H

    epoxide group + amines (DETA) epoxy

  • Other epoxy resins

    • diglycidyl ether of bisphenol A (DGEBRA)

    • tetraglycidyl methylene dianiline (TGMDA

    • epoxy phenol cresol novolac

    • cycloaliphatic epoxies (CA)

  • Curing agents (hardeners, catalysts, cross-linking agents)

    • aliphatic or aromatic amines (DETA, TETA, hexamethylene tetramine,etc.)

    • acid anhydrides (phthalic anhydride, pyromellitic dianhydride, etc.)

    • Active hydrogen react with epoxide groups.

    • As much as 15% hardener is needed

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Polyurethane Chemistry

  • Reaction between isocyanate and alcohol (polyol).

  • Crosslinking occurs between isocyanate groups (-NCO) and the polyol’s hydroxyl end-groups (-OH)

  • Thermoplastic PU (TPU) have some crosslinking, but purely by physical means.

    • These bonds can be broken reversibly by raising the material’s temperature, as in molding or extrusion.

    • Ratio between the two give a range of properties between a flexible foam (some crosslinking) to a rigid urethane (high degree of crosslinking).

    • In PUR foams density can range from 1 lb/ft3 to 70 lb/ft3.

    • Foams are produced by chemical blowing agents.

    • Catalyst are used to initiate reaction.

    • RIM process is used to produce fenders and bumper covers

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Other Thermosets

  • Polyimides

  • Bismaleimide

  • Polybenzimidazoles

  • Phenolics

  • Carbon Matrices

  • Thermoplastic matrices

    • Polyamides

    • Polypropylene

    • PEEK

    • Polysulfone

    • PPS

Polyimides l.jpg

  • For temperature stability up to 600 F

    • Polyimides or polybenzimidazole (PBI) rather than epoxy

    • Aerospace applications due to high cost

    • Chemical Structure

      • Polyimides

        • Characterized by cyclic group containing a nitrogen and two carbonyl groups (C with double bond with oxygen)

      • PBI

        • Characterized by a five member ring containing two nitrogens and is attached to a benzene ring.

      • Polyimids and PBI are structurally planar and very rigid. Large aromatic groups are added into polymer to make stiffer.

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  • Formed with two step condensation. Fig 2-5

    • First step: An aromatic dianhydride is reacted with an aromatic diamine to form polyamic (polamide) acid.

    • Second step: Curing of the polyamic acid.

      • Formation of imide group by closing of 5-member ring

      • Condensation step of solvent molecules: water, alcohol, solvents

      • Chain extension

      • Cross-linking

    • High viscosities of polyamid acids require use of prepregs.

      • Impregnating the fiber mat with monomer solutions of diamines and diester acids.

      • Long times and gradual increase in temperature are needed.

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  • Major condensation polyimids, Dupont’s Avimid N & K

    • are marketed as Prepreg polyimids

      • Avimid N Tg = 675F (360C), and

      • Avimid K: Tg = 490F (254C)

    • Linear polyimids are produced which have thermoplastic behavior above the Tg.

    • They process like thermoplastics for a few heat cycles.

    • Advantages of thermoplastic nature

      • Tractable nature of resins when hot facilitates the removal of volatiles.

      • Voids, formed as result of the evolution of gases, can be eliminated by applying pressure while heating the resins above Tg.

    • Applications

      • Wing skins for high performance aircraft.

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  • Addition Polyimides

    • Many polyimids are cross linked with an addition reaction

      • Two general cross-linking reactions are widely used

        • End group reactions

        • Bismaleimide reactions

      • Reactive End Group Resin Fig 2-6

        • First phase (imidization): results in the formation of the oligomeric (small polymer) imide

        • Second phase (consolation): is when the oligomer melts and flows to fill voids that were created from volatiles depart.

        • Third phase (crosslinking): oligomer builds MW & crosslinks

          • MW = 1500

        • Shorter polymer chains gave lower viscosity and better wet-out

          • Wet-out is defined as uniform coating and soaking of resin in fiber.

        • Commercial end group resin (PMR) is PMR 11, PMR 15 and PMR 20

          • PMR-11 has more end groups and higher cross-linking density and higher stiffness

          • PMR-20 gave better thermal stability.

          • PMR-15 has the best physical properties balanced.

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  • Second type of endgroup crosslinking has acetylene endgroups and is called Thermid 600

    • Crosslinking

      • First step: joining two polyimid oligomers to form a butadiene linkage which results in chain extension. Each double bond can react with double or triple bonds to form highly crosslinked.

      • Addition reaction

      • Problems is with too fast a cure and chain extension competing with cross-linking mechanism thus causing MW to build too fast.

        • Alleviated with proper solvents.

      • Disadvantage is the loss of tackiness in prepregs as the solvent evaporates.

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  • Bismaleimide (BMI) resins

    • Addition polymerization

      • Reactions involving bismaleimide (BMI) derivatives: Fig 2-8

      • Case 1

        • Carbon-Carbon double bond in the maleimide group reacts with the carbon-carbon double bond in the olefin co-reactant (similar to maleic acid is crosslinked with styrene in polyester)

      • Case 2

        • An aromatic diamine adds to the carbon-carbon double bond of the maleimide in what is called Michaels Reaction.

      • Both cases: the coreactants (olefin or diamine) form bridges between the imide molecules to form a crosslinked structure

    • Commericial products

      • Ciba-Geigy uses an olefinic compound with two olefins

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  • Bismaleimide (BMI) resins

    • Advantages

      • Low processing temperature versus polyimides (Cured at 350F)

      • Standard epoxy processing equipment can be used since same T.

      • Postcure of 475 F is required to complete polymerization.

      • BMI are fully formed polyimides when reacted to form composite

      • Thus, no volatiles are removed and no consolidation problems

      • Tack and drape are quite good because of the liquid component of the reactants

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  • Polybenzimidazole (PBI) resins

    • Less prevalent than the polyimides, PBI have equivalent and sometimes superior physical and thermal properties

    • Formation reaction- fig 2-9

      • Five member ring containing two nitrogens is formed with accompanying aromatic groups.

      • Groups are flat and stiff leading to good physical properties and aromatics result in high thermal.

    • Problems are expensive, difficult process, toxicity

      • Some have been alleviated and is commercially available

      • Resin is thermoplastic with a Tg over 800F (427C)

      • It does not burn, contribute fuel to flames or produce smoke

      • Forms a tough char

      • Resins are toxic and need to be handles with care.

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  • Phenolics is an old thermoset resin

    • Used for general purpose, unreinforced plastic

      • electrical switches

      • junction boxes

      • automotive molded parts

      • consumer appliance parts, handles, billiard balls

    • Fillers are required due to high shrinkage and brittle nature.

      • Sawdust, nut shells, talc, or carbon black

    • Fiber reinforced Phenolics have aerospace applications

      • Rocket nozzles, nose cones due to ablative nature (Goes from solid to gas during burning)

      • High temperature aircraft ducts, wings, fins, and muffler repair kits

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  • Phenolic chemical structure-

    • Formed by reaction between phenol and formaldehyde

      • Condensation reaction releases water as a byproduct.

      • Initially low molecular weight, soluble and fusible, A-Stage resin

      • Condensation reaction involves more and more phenol molecules that causes the resin to pass through a rubbery, thermoplastic state that is only partially soluble phase called B stage.

      • Resin is cured and cross-linked thermoset resin, C- Stage.

    • Other terms describing phenolic formation

      • Resole: If phenol/formaldehyde reaction is carried out in excess formaldehyde and base catalyst is called resole at low molecular weight stage. Requires just heat to convert to C-stage (1 step)

      • Novolac: If phenol/formaldehyde reaction is carried out in excess phenol with an acid catalyst is called novolac.

        • Requires addition of a hardener (hexamethylenen tetramine) to achieve C- Stage in 2 steps. It provides acid to both reactants which speeds up reaction.

        • Reinforcements are mixed with novolacs for composites. Bstaging is when any other resin is cured to an intermediate stageand cured by heating

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Thermoplastic Composites

  • Plastics are reinforced with glass and a few with carbon fiber

    • Nylon, PP, PBT, PEEK and PEK, and Polysulphone

  • Advantages

    • Requires less processing time since it is heated and not cured.

    • Thermoplastic pre-preg sheets have infinite shelf life versus thermoset

  • Disadvantages

    • Have lower thermal resistance than most thermoset composites

    • Have lower strength and modulus than some thermoset composites

    • Have difficulty wetting out high fiber loading composites.

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    Thermoplastic Matrices

    • Two types of thermoplastic composites: Discontinuous and continuous reinforcements

      • Discontinuous fiber- Conventional thermoplastics and short (3mm) or long fibers (6mm)

        • Polypropylene, nylon, PET, PBT, Polysulphone, PE, ABS, PC, HIPS, PPO

      • Short Glass or Carbon fiber increases

        • Tensile strength, modulus, impact strength, cost, thermal properties

      • Short Glass or carbon fiber decreases

        • Elongation,

        • CLTE,

        • Moisture


    Thermoplastic matrices31 l.jpg
    Thermoplastic Matrices










    • Several types of resin types

      • Conventional plastics: Less expensive (< $2 per pound)

        • Commodity plastics : PP, PE, PVC, PS, etc. (<$1 per pound)

        • Engineering resins: PC, PET, PBT, Nylon, ABS etc. (>$1pp)

      • High Performance Plastics: High Costs (> $10 per pound) and High Thermal Properties

        • PEEK, PEK, LCP, PPS, Polyaryle Sulfone, Polysulfone, Polyether sulfone, Polyimid

        • PEEK and PEK = $30 per pound

      • Polyarylesters

        • Repeat units feature only aromatic-type groups (phenyl or aryl groups) between ester linkages. Called wholly aromatic polyesters

    PolyEther-Ether-Ketone (PEEK)

    PolyEther-Ketone (PEK)

    Advantages and disadvantages of polyketones l.jpg
    Advantages and Disadvantages of Polyketones

    • Advantages

      • High continuous use temperature (480F)

      • High toughness, especially at high temperatures.

      • Outstanding wear resistance

      • Excellent water resistance and better than thermoset composites

      • Excellent mechanical properties

      • Very low flammability and smoke generation

      • Resistant to high levels of gamma radiation

      • Higher Elongation (30%-100%) versus thermosets (1%-10%)

    • Disadvantages

      • High material cost and long processing times

      • High processing temperatures due to high viscosities (1 Million poise) versus thermoset composites (Epoxy = 10 poise). Syrup = 1000 poise

      • Moderate or poor resistance to hot oils

      • Difficult to have high fiber loadings due to high viscosity

      • Need special processing techniques; comingle plastic powder with fiber sheet and consolidate (impregnate resin in fiber bundle) through heated rollers.

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    Thermoset Reacting Polymers

    • Process Window

      • Temperature and pressure must be set to produce chemical reaction without excess flash (too low a viscosity), short shot (too high a viscosity), degradation (too much heat)

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    Compression Molding of Polyesters

    • Compression molding was specifically developed for replacement of metal components with composite parts.

    • Materials can be either thermosets (SMC) or thermoplastics (GMT)

      • Most applications today use thermoset POLYESTER polymers, e.g., SMC or BMC. In fact,compression molding is the most common method of processing thermosets.

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    Resin Transfer Molding of Polyester or Epoxy

    • In the RTM process, dry (i.e.,unimpregnated ) reinforcement is pre-shaped and oriented into skeleton of the actual part known as the preform which is inserted into a matched die mold.

    • The heated mold is closed and the liquid resin is injected

    • The part is cured in mold.

    • The mold is opened and part is removed from mold.

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    Open Mold Processing of Composites

    • Open Mold processes of Polyester or Epoxy

      • Vacuum bag, pressure bag, SCRIMP

      • Autoclave: Apply Vacuum Pressure and Heat in an oven which can be 5 feet to 300 feet long

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    Polyurethane Processing

    • Polyurethane can be processed by

      • Casting, painting, foaming

      • Reaction Injection Molding (RIM)

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    Structural RIM for Urethanes (Fast RTM)

    • Fiber preform is placed into mold.

    • Polyol and Isocyanate liquids are injected into a closed mold and reacted to form a urethane.

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    Composite Reinforcement Classifications

    • Reinforcement Type

      • Discontinuous (fibers are chopped and dispersed in matrix resin)

        • Short fibers: fiber lengths 3mm or less (glass filled plastics, GF-Nylon)

        • Long fibers: fiber lengths greater than 6 mm. (Some injection molded materials with 6mm fibers, Sheet Molding Compound (SMC) with 1” fibers, DFP Directed Fiber Preforms for RTM and SRIM)

        • Particulates: fibers is forms as spheres, plates, ellipsoids (some injection molded materials reinforced with mineral fibers)

      • Continuous (fibers are throughout structure with no break points)

        • Glass roving: glass bundles are wound up in a packet similar to yarn.

        • Roving is woven into several weaves using a loom machine like in apparel.

          • Mat products: random swirl glass pattern.

          • Woven product: roving is woven into machine direction (warp) and cross direction (weft)

          • Uni product: roving is woven in one direction with a cross thread given to hold mat together.

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    Processing of Fiber Reinforcements

    • Carbon fiber or glass fiber

      • Hand lay-up and Spray-up

      • Filament winding

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    Injection Molding Glass Reinforced Composites

    Glass filled resin pellets

    • Plastic pellets with glass fibers are melted in screw, injected into a cold mold, and then ejected.

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    Composites Can Have a Fiber Preform

    • Fiber type

      • Roving form that can be sprayed into a 3-D preform

      • Roving form that is woven into a glass sheet and then formed to shape (preform)

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    Glass Fibers

    • Properties of Glass Fibers: (Table 3-1)

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    Carbon/Graphite Fibers

    • Need for reinforcement fibers with strength and moduli higher than those of glass fibers has led to development of carbon

    • Thomas Edison used carbon fibers as a filament for electric light bulb

    • High modulus carbon fibers first used in the 1950s

    • Carbon and graphite are based on layered structures of hexagonal rings of carbon

    • Graphite fibers are carbon fibers that

      • Have been heat treated to above 3000°F that causes 3 dimensional ordering of the atoms and

      • Have carbon contents GREATER than 99%

      • Have tensile modulus of 344 Gpa (50Mpsi)

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    Organic Fiber- Kevlar Properties

    • Properties- Table 3-3

      • Kevlar has high heat resistance, though less than carbon fiber.

      • Kevlar has exceptional exposure limits to temperature

        • No degradation in properties after 7 days at 300 F.

        • 50% reduction in properties after 7 days at 480F.

        • 50% reduction in properties after 12 months of sunlight exposure in Florida

      • Kevlar are hygroscopic and are susceptible to moisture and need to be dried

      • Aramids do not bond well to matrices as do glass and carbon fibers

        • The ILSS (interlaminar Short beam shear) values are lower.