Classes of Polymeric Materials Chapter 3: Thermosets

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Classes of Polymeric MaterialsChapter 3
Thermosets
Professor Joe Greene CSU, CHICO
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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

• 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 (125C 200C) 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 raction of a
  monofunctional unsaturated alcohol, allyl alcohol
  (AA) with a difunctional acid.
• Ester linkages are formed though not a polymer
• 2 unsaturated CC per monomer permits formation
  of 3-D molecule with the use of catalysts and
  elevated temperatures.
• DAP (diallylphthalate) is most common allyl
  monomer
• Thermoplastic prepolymers 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 (or initiator) is used. Methyl ethyl
  ketone (MEK) peroxide, benzoyl peroxide, and
  cumene hydroperoxide
• 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 polyols hydroxyl end-groups (-OH)
• Thermoplastic PU (TPU) have some crosslinking,
  but purely by physical means.
• These bonds can be broken reversibly by raising
  the materials 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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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. T
• 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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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

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

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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
• sensitivity

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Thermoplastic Resins

• Several types of resin types
• Conventional plastics Less expensive (lt 2.00
  per pound)
• Commodity plastics PP, PE, PVC, PS, ABS, etc.
• Engineering resins PC, PET, PBT, Nylon, etc.
• High Performance Plastics High Costs (gt 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)
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Properties of Reinforced PEEK

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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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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 3000F 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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Carbon Fiber Mechanical Properties
Note 1Mpsi Mpa
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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.