Transcript Classes of Polymeric Materials

Classes of Polymeric Materials
Chapter 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,3vinyl
ester, alkyd, allyl, furan, some epoxies, and polyimides
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
–
–
–
–
–
–
H
O=C
H
Formaldehyde plus one of the three hydrogen containing chemicals
to form a 3D molecular network
HNH
• Phenol,
OH
O
• Melamine, or
N CN
HN- C- NH
• Urea.
H C C H
H
H
N N N
H
H
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
6 to
8,000 psi for compression molding and 18,000 psi for injection molding
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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C C
H
C C
• Furan Systems
O
– Feature a ring structure which can be opened cleaved to yield
polymeric molecules which have 3-D molecular networks.
– Combined with fomaldehyde related thermosets.
H
Commercial Thermosets
H
H
• 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 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 prepolymers are available that are cured with little shrinkage
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• Applications include high performance molding compounds for electrical
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)
O
C6H4(COOH)2 + (CH2)2(OH)2
terephthalic acid + ethylene glycol
(PET)
O
-[(CH2)2 -O- C
- C-O]Polyethylene terephthalate
– 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
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Polyester Chemistry
• Step 1: Create polymer and build MW of polymer
chain
– Condensation Polymerization of Di-ACID and DiALCOHOL
• 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: O
Epoxy Chemistry
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.)
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– Active hydrogen react with epoxide groups.
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.
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– RIM process is used to produce fenders and bumper covers
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.
Glass filled resin pellets
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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,
Density, g/cc
• CLTE,
• Moisture Tensile Strength,
psi
sensitivity Tensile Modulus,
psi
Tensile
Elongation, %
Impact Strength
Nylon 6,6
1.13-1.15
Nylon 6,6 with
30% short glass
1.4
Nylon 6,6 with
30% long glass
1.4
Nylon 6,6 with
30% carbon fiber
1.06-1.10
14,000
28,000
28,000
32,000
230K – 550K
1,300K
1,400 K
3,300 K
15%-80%
3%
3%
4%
0.55 – 1.0
1.6-4.5
4.0
1.5
55
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18
15
1.0-2.8% (24h)
8.5% (Max)
0.7-1.1 (24h)
5.5-6.5 (Max)
0.9 (24h)
5.5-6.5 (Max)
0.7 (24h)
5 (Max)
$1.40
$1.70
$2.00
$2.70
ft-lb/in
CLTE (in/in/C
x10-6)
Moisture %
Cost $/lb
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•
Thermoplastic
Resins
Several types of resin types
– Conventional plastics: Less expensive (< $2.00 per pound)
• Commodity plastics : PP, PE, PVC, PS, ABS, etc.
• Engineering resins: PC, PET, PBT, Nylon, etc.
– 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)
O
O
O
C
n
PolyEther-Ketone (PEK)
O
O
C
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n
Properties of Reinforced PEEK
Mechanical Properties Reinforced
PEEK
Density, g/cc
Tensile Strength,
psi
Tensile Modulus,
psi
Tensile
Elongation, %
Impact Strength
1.30-1.32
PEEK 30%
glass fibers
1.52
PEEK with 30%
carbon fibers
1.43
10,000 – 15,000
23,000 – 29,000
31,000
500K
1,300K – 1,600K
1,900K – 3,500K
30% - 150%
2%-3%
1% - 4%
1.6
2.1 – 2.7
1.5 – 2.1
ft-lb/in
Hardness
CLTE
10-6 mm/mm/C
HDT 264 psi
R120
R120
40 - 47
12-22
15-22
320 F
550F -600F
550F -610F
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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
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to hold mat together.
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)
Property
Density
Tensile Strength, ksi
Tensile Modulus, Msi
CLTE (in/in/ F) x10-6
Specific Heat @72F
Softening Point, F
Dielectric Constant
Chemical Resistance
(% wt gain after
24hrs)
In H20
In 10%HCl
In 10% H2SO4
In 10% Na2CO3
Type of Glass
C
E
S
2.49
2.54
2.48
460
500
665
10
4
0.2
1381
0.008
10.5
2.8
0.195
1550
0.002
12.4
3.1
0.176
1778
0.003
1.1
4.1
2.2
2.4
0.7
4.2
3.9
2.1
0.7
3.8
4.1
2
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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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Carbon Fiber Mechanical Properties
Carbon Fiber Mechanical Properties
PAN Based
Tensile Modulus (Mpsi)
33 - 56
Tensile Strength (Msi)
0.48 - 0.35
Elongation (%)
1.4 - 0.6
Density (g/cc)
1.8 - 1.9
Carbon Assay (%)
92 - 100
PITCH Based Rayon Based
23 -55
5.9
0.2 - 0.25
0.15
0.9 - 0.4
25
1.9 - 2.0
1.6
97 - 99
99
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.
Properties of Kevlar
Tensile Mod
Tensile Strength
Elongation
Density
MPa
MPa
%
g/cc
Kevlar
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83
3.6
4
1.44
49
131
3.6
2.8
1.44
149
186
3.4
2
1.47
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