Classes of Polymeric Materials
Elastomers
Professor Joe Greene
CSU, CHICO
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Copyright Joseph Greene 2001
Elastomers
• Elastomers are rubber like polymers that are
thermoset or thermoplastic
– butyl rubber: natural rubber
– thermoset: polyurethane, silicone
– thermoplastic: thermoplastic urethanes (TPU),
thermoplastic elastomers (TPE), thermoplastic olefins
(TPO), thermoplastic rubbers (TPR)
• Elastomers exhibit more elastic properties versus
plastics which plastically deform and have a lower
elastic limit.
• Rubbers have the distinction of being stretched 2
200% and returnedCopyright
to original
shape. Elastic limit is
Joseph Greene 2001
Rubbers
• Rubbers have the distinction of being stretched 200%
and returned to original shape. Elastic limit is 200%
• Natural rubber (isoprene) is produced from gum resin
of certain trees and plants that grow in southeast Asia,
Ceylon, Liberia, and the Congo.
– The sap is an emulsion containing 40% water & 60% rubber particles
• Vulcanization occurs with the addition of sulfur (4%).
– Sulfur produces cross-links to make the rubber stiffer and harder.
– The cross-linkages reduce the slippage between chains and results in
higher elasticity.
– Some of the double covalent bonds between molecules are broken,
allowing the sulfur atoms to form cross-links.
– Soft rubber has 4% sulfur and is 10% cross-linked.
– Hard rubber (ebonite) has 45% sulfur and is highly cross-linked. 3
Copyright Joseph Greene 2001
Rubber Additives and Modifiers
• Fillers can comprise half of the volume of the rubber
–
–
–
–
–
–
–
Silica and carbon black.
Reduce cost of material.
Increase tensile strength and modulus.
Improve abrasion resistance.
Improve tear resistance.
Improve resistance to light and weathering.
Example,
• Tires produced from Latex contains 30% carbon black which improves the
body and abrasion resistance in tires.
• Additives
– Antioxidants, antiozonants, oil extenders to reduce cost
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and soften rubber, fillers, reinforcement
Copyright Joseph Greene 2001
Vulcanizable Elastomeric Compounds
• Rubbers are compounded into practical elastomers
– The rubber (elastomer) is the major component and other
components are given as weight per hundred weight rubber (phr)
• Sulfur is added in less than 10 phr
• Accelerators and activators with the sulfur
– hexamethylene tetramine (HMTA)
– zinc oxide as activators
• Protective agents are used to suppress the effects of oxygen and
ozone
– phenyl betabaphthylamine and alkyl paraphenylene diamine (APPD)
• Reinforcing filler
– carbon black
– silica when light colors are required
– calcium carbonate, clay, kaoilin
• Processing aids which reduce stiffness and cost
– Plasticizers, lubricants, mineral oils, paraffin waxes,
Copyright Joseph Greene 2001
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Vulcanizable Rubber
• Typical tire tread
– Natural rubber smoked sheet (100),
– sulfur (2.5) sulfenamide (0.5), MBTS (0.1), strearic acid (3), zinc
oxide (3), PNBA (2), HAF carbon black (45), and mineral oil (3)
• Typical shoe sole compound
– SBR (styrene-butadiene-rubber) (100) and clay (90)
• Typical electrical cable cover
– polychloroprene (100), kaolin (120), FEF carbon black (15) and
mineral oil (12), vulcanization agent
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Copyright Joseph Greene 2001
•
Synthetic
Rubber
Reactive system elastomers
– Low molecular weight monomers are reacted in a polymerization
step with very little cross-linking.
– Reaction is triggered by heat, catalyst, and mixing
• Urethanes processed with Reaction Injection Molding (RIM)
• Silicones processed with injection molding or extrusion
• Thermoplastic Elastomers
– Processing involves melting of polymers, not thermoset reaction
– Processed by injection molding, extrusion, blow molding, film
blowing, or rotational molding.
• Injection molded soles for footwear
– Advantages of thermoplastic elastomers
• Less expensive due to fast cycle times
• More complex designs are possible
• Wider range of properties due to copolymerization
– Disadvantage of thermoplastic elastomers
• Higher creep
Copyright Joseph Greene 2001
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Thermoplastic Elastomers
• Four types of elastomers
– Olefinics and Styrenics
– Polyurethanes and Polyesters
• Olefinics (TPOs are used for bumper covers on cars)
– Produced by
• Blending copolymers of ethylene and propylene (EPR) or ter polymer of
ethylene-propylene diene (EPDM) with
• PP in ratios that determine the stiffness of the elastomer
– A 80/20 EPDM/PP ratio gives a soft elastomer (TPO)
• Styrenic thermoplastic elastomers (STPE)
– Long triblock copolymer molecules with
• an elastomeric central block (butadiene, isoprene, ethylene-butene, etc.) and
• end blocks (styrene, etc.) which form hard segments
– Other elastomers have varying amounts of soft and hard blocks
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Copyright Joseph Greene 2001
Thermoplastic Elastomers
• Polyurethanes
– Have a hard block segment and soft block segment
• Soft block corresponds to polyol involved in polymerization in ether based
• Hard blocks involve the isocyanates and chain extenders
• Polyesters are etheresters or copolyester thermoplastic
elastomer
– Soft blocks contain ether groups are amorpous and flexible
– Hard blocks can consist of polybutylene terephthalate (PBT)
• Polyertheramide or polyetherblockamide elastomer
– Hard blocks consits of a crystallizing polyamide
Soft
Hard Soft
Hard Soft Hard
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Copyright Joseph Greene 2001
Commercial Elastomers
• Diene C=C double bonds and Related Elastomers
– Polyisoprene- (C5H8)20,000
•
•
•
•
•
Basic structure of natural rubber
Can be produced as a synthetic polymer
Capable of very slow crystallization
Tm = 28°C, Tg = -70°C for cis polyisoprene
Tm = 68°C, Tg = -70°C for trans polyisoprene
H H C H3H
Cis [C C C C ]
H
H
H H
H
Trans [C C C C ]
H
CH3 H
– Trans is major component of gutta percha, the first plastic
– Natural rubber was first crosslinked into highly elastic network by
Charles Goodyear (vulcanization with sulfur in 1837)
• Sulfur crosslinked with the unsaturations C=C
– Natural rubber in unfilled form is widely used for products with
• very large elastic deformations or very high resilience,
• resistance to cold flow (low compression set) and
• resistance to abrasion, wear, and fatigue.
– Natural rubber does not have good intrinsic resistance to sunlight,
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oxygen, ozone, heat aging, oils, or fuels.
Copyright Joseph Greene 2001
Commercial ElastomersH
H H H
[C C C C ]
• Polybutadiene
H
H
– Basis for synthetic rubber as a major component in copolymers
Styrene-Butadiene Rubber (SBR, NBR) or in
– Blends with other rubbers (NR, SBR)
– Can improve low-temperature properties, resilence, and abrasion or
wear resistance
H H Cl H
[C C C C ]
• Tg = -50°C
H
H
• Polychloroprene
– Polychloroprene or neoprene was the very first synthetic rubber
– Due to polar nature of molecule from Cl atom it has very good
resistance to oils and is flame resistant (Cl gas coats surface)
– Used for fuel lines, hoses, gaskets, cable covers, protective boots,
bridge pads, roofing materials, fabric coatings, and adhesives
– Tg = -65°C.
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Copyright Joseph Greene 2001
Commercial Elastomers
• Butyl rubber- addition polymer of isobutylene.
–
–
–
–
H CH3
[C C ]
H C H3
Copolymer with a few isoprene units, Tg =-65°C
Contains only a few percent double bonds from isoprene
Small extent of saturation are used for vulcanization
Good regularity of the polymer chain makes it possible for the
elastomer to crystallize on stretching
– Soft polymer is usually compounded with carbon black to increase
modulus
• Nitrile rubber
–
–
–
–
Copolymer of butadiene and acrylonitrile
Solvent resistant rubber due to nitrile C:::N
Irregular chain structure will not crystallize on stretching, like SBR
vulcanization is achieved with sulfur like SBR and natural rubber
• Thiokol- ethylene dichloride polymerized with sodium12
polysulfide. Sulfur makes
thiokol rubber self vulcanizing.
Copyright Joseph Greene 2001
Thermoplastic Elastomers
• Thermoplastic Elastomers result from copolymerization of
two or more monomers.
– One monomer is used to provide the hard, crystalline features, whereas
the other monomer produces the soft, amorphous features.
– Combined these form a thermoplastic material that exhibits properties
similar to the hard, vulcanized elastomers.
• Thermoplastic Urethanes (TPU)
– The first Thermoplastic Elastomer (TPE) used for seals gaskets,
etc.
• Other TPEs
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–
–
–
Copolyester for hydraulic hoses, couplings, and cable insulation.
Styrene copolymers are less expensive than TPU with lower strength
Styrene-butadiene (SBR) for medical products, tubing, packaging, etc.
Olefins (TPO) for tubing, seals, gaskets, electrical, and automotive.
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Copyright Joseph Greene 2001
Thermoplastic Elastomers
• Styrene-butadiene rubber (SBR)
– Developed during WWII
HH H H H H
C C [C C C C ]
H
H
H
n
• Germany under the name of BUNA-S.
• North America as GR-S,Government rubber-styrene.
– Random copolymer of butadiene (67-85%) and styrene (15-33%)
– Tg of typical 75/25 blend is –60°C
– Not capable of crystallizing under strain and thus requires
reinforcing filler, carbon black, to get good properties.
– One of the least expensive rubbers and generally processes easily.
– Inferior to natural rubber in mechanical properties
– Superior to natural rubber in wear, heat aging, ozone resistance,
and resistance to oils.
– Applications include tires, footwear, wire, cable insulation,
industrial rubber products, adhesives, paints (latex or emulsion)
• More than half of the world’s synthetic rubber is SBR
• World usage of SBR equals
natural
rubber
Copyright
Joseph Greene
2001
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Acrylonitrile-butadiene rubber (NBR)
H H
• Also called Nitrile rubber
– Developed as an oil resistant rubber due to
C C
H C:::Nn
H H H H
[C C C C ]
H
Hm
• the polar C:::N polar bond. Resistant to oils, fuels, and solvents.
–
–
–
–
–
–
Copolymer of acrylonitrile (20-50%) and butadiene(80-50%)
Moderate cost and a general purpose rubber.
Excellent properties for heat aging and abrasion resistance
Poor properties for ozone and weathering resistance.
Has high dielectric losses and limited low temperature flexibility
Applications include fuel and oil tubing; hose, gaskets, and seals; conveyer
belts, print rolls, and pads.
– Carboxylated nitrile rubbers (COX-NBR) has carboxyl side groups
(COOH)which improve
• Abrasion and wear resistance; ozone resistance; and low temperature flexibility
– NBR and PVC for miscible, but distinct polymer blend or polyalloy
• 30% addition of PVC improves ozone and fire resistance
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Copyright Joseph Greene 2001
Ethylene-propylene rubber (EPR)
• EPR and EPDM
– Form a noncrystallizing copolymer
• with a low Tg.
– The % PP and PE units determines properties
• Tg = -60°C for PE/PP of 67/33 to 50/50
H H
H H
H H
C C
C C
C C
H H
– Unsaturated polymer since PP and PE are saturated
• Resistant to ozone, weathering, and heat aging
• Does not allow for conventional vulcanization
H CH3
m
n
H CH2
CH m
CH
CH3
– Terpolymer with addition of small amount of third monomer (Diene D) has
unsaturations referred to as EPDM
• 1,4, hexadiene (HD); 5-ethylidene-2-norbornene (ENB); diclopentadiene (DCPB)
feature unsaturations in a side (pendant) group
• Feature excellent ozone and weathering resistance and good heat aging
– Limitations include poor resistance to oils and fuels, poor adhesion to many
substrates and reinforcements
– Applications include exterior automotive parts (TPO is PP/EPDM), construction
parts, weather strips, wire and cable insulation, hose and belt products, coated
fabrics.
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Copyright Joseph Greene 2001
Ethylene Related Elastomers
• Chlorosulfonated Polyethylene (CSPE)
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–
–
–
H H
Moderate random chlorination of PE (24-43%)
C C
Infrequent chlorosulfonic groups (SO2Cl)
Sulfur content is 1-1.5%.
H H
CSPE is noted for excellent weathering resistance
n
H
H
C
C
Cl
mO S Ok
Cl
• Good resistance to ozones, heat, chemicals, solvents.
• Good electrical properties, low gas permeability, good adhesion to substrates
– Applications include hose products, roll covers, tank linings, wire and cable
covers, footwear, and building products
• Chlorinated Polyethylene (CPE)
– Moderate random chlorination
• Suppresses crystallinity (rubber)
• Can be crosslinked with peroxides
• Cl range is 36-42% versus 56.8% for PVC
– Properties include good heat, oil, and ozone resistance
– Used as plasticizer for PVC
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Copyright Joseph Greene 2001
Ethylene Related Elastomers
• Ethylene-vinylacetate Copolymer (EVA)
– Random copolymer of E and VA
• Amorphous and thus elastomeric
• VA range is 40-60%
• Can be crosslinked through organic peroxides
H H
H H
C C
C C
H H
– Properties include
• Good heat, ozone, and weather resistance
n H O m
O=CCH3
• Ethylene-acrylate copolymer (EAR)
– Copolymer of Ethylene and methacrylate
• Contains carboxylic side groups (COOH)
– Properties include
• Excellent resistance to ozone and
• Excellent energy absorbers
– Better than butyl rubbers
H H
H H
C C
C C
H H
n H C OCH3
O m
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Copyright Joseph Greene 2001
FluoroElastomers
• Polyvinylidene fluoride (PVDF)
– Tg = -35°C
• Poly chloro tri fluoro ethylene (PCTFE)
– Tg = 40°C
H F
C C
H F
• Poly hexa fluoro propylene (PHFP)
– Tg = 11°C
• Poly tetra fluoro ethylene (PTFE)
– Tg = - 130°C
• Fluoroelastomers are produced by
F F
n
C C
F Cl
C C
n
F F
F F
C C
F F
n
– random copolymerization that
F CF3
n
– suppresses the crystallinity and
– provides a mechanism for cross linking by terpolymerization
• Monomers include VDF, CTFE, HFP, and TFE
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Copyright Joseph Greene 2001
FluoroElastomers
• Fluoroelastomers are expensive but have outstanding
properties
– Exceptional resistance to chemicals, especially oils, solvents
– High temperature resistance, weathering and ozone resistance.
– Good barrier properties with low permeability to gases and vapors
• Applications
– Mechanical seals, packaging, O-rings, gaskets, diaphrams,
expansion joints, connectors, hose liners, roll covers, wire and
cable insulation.
• Previous fluoroelastomners are referred to as
– Fluorohydrocarbon elastomers since they contain F, H, and C
atoms with O sometimes
• Two other classes of elastomers include fluorinated types
– Fluorosilicone elastomers remain flexible at low temperatures
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– Fluorinated polyorganophosphazenes have good fuel resistance
Copyright Joseph Greene 2001
Silicone Polymers
• Silicone polymers or polysiloxanes (PDMS)
– Polymeric chains featuring
CH3
Si O
CH
3
• Tg = -125°C
m
• Very stable alternating combination of
• Silicone and oxygen, and a variety of organic side groups attached to Si
– Two methyl, CH3, are very common side group generates
polydimethylsiloxane (PDMS)
• Unmodified PDMS has very flexible chains corresponding to low Tg
• Modified PDMS has substitution of bulky side groups (5-10%)
– Phenylmethlsiloxane or diphenylsiloxane suppress crystallization
• Substituted side groups, e.g., vinyl groups (.5%) featuring double bonds
(unsaturations ) enables crosslinking to form vinylmethylsiloxane (VMS)
• Degree of polymerization, DP, of polysiloxane = 200-1,000 for low
consistency chains to 3,000-10,000 for high consistency resins.
• Mechanism of crosslinking can be from a vinyl unsaturation or reactive end
groups (alkoxy, acetoxy)
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Copyright Joseph Greene 2001
Silicone Polymers
• Silicone polymers or polysiloxanes (PDMS)
– Properties
CH3
Si O
CH
3
• Mediocre tear properties
m
• High temperature resistance from -90C to 250C.
• Surface properties are characterized by very low surface energy (surface
tension) giving good slip, lubricity, and release properties (antistick) nand
water repellency.
• Excellent adhesion is obtained for curing compounds for caulk.
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Copyright Joseph Greene 2001
Silicones
• Unmodified PDMS has very flexible chains with a low Tg.
– Regular structure allows for crystallization below Tm
– Addition of small amount of bulky side groups are used to
suppress crystallization
• Trifluoropropyl side groups enhance the resistance to solvent swelling and
are called fluorosilicones
• Linear form (uncrosslinked) polysiloxane corresponds to DP of 200-1000 for
low consistency to 3,000-10,000 for high consistency resins
• Mechanism for crosslinking (vulcanization) can be based upon vinyl
unsaturations or reactive end groups (alkoxy)
– Silicone polymers are mostly elastomers with mediocre tear properties,
but with addition of silica can have outstanding properties unaffected by
a wide temp range from –90°C to 250°C
• Surface properties have low surface energy, giving good slip, lubricity,
release properties, water repellency, excellent adhesion for caulks
• Good chemical inertness but sensitive to swelling by hydrocarbons
• Good resistance to oils and solvents, UV radiation, temperature
• Electrical properties are excellent and stable for insulation and dielectric
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Copyright Joseph Greene 2001
• Properties
Silicones
– Low index of reflection gives silicone contains useful combination
of high transmission and low reflectance
– Can be biologically inert and with low toxicity are well tolerated
by body tissue
– Polymers are normally crosslinked in the vulcanization stage. Four
groups
•
•
•
•
Low consistency-room temperature curing resins (RTV)
Low consistency-high temperature curing resins (LIM,LSR)
High consistency-high temperature curing resins (HTV, HCE),
Rigid resins
– RTV elastomers involve low molecular weight polysiloxanes and
rely on reactive end groups for crosslinking at room temperature.
• One component, or one part, packages rely on atmospheric moisture for
curing and are used for thin parts or coatings
• Two component systems have a catalyst and require a mixing stage and
result in a small exotherm where heat is given off.
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Copyright Joseph Greene 2001
• Properties
Silicones
– LSR elastomers involve low molecular weight polysiloxanes but a
different curing system
• Relatively high temperature (150°C) for a faster cure (10-30s)
• Mixed system is largely unreactive at room temp (long pot life)
• Suitable for high speed liquid injection molding of small parts.
– HTV elastomers contain unsaturations that are suitable for
conventional rubber processing.
• Heat curable elastomers (HCE) are cross linked through high temperature
vulcanization (HTV) with the use of peroxides.
– Rigid silicones are cross linked into tight networks.
• Non-crosslinked systems are stable only in solutions that are limited to
paints, varnishes, coatings, and matrices for laminates
• Cross-linking takes place when the solvent evaporates.
• Post curing is recommended to complete reaction, e.g., silicone-epoxy
systems for electrical encapsulation.
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Copyright Joseph Greene 2001
Silicones Applications
• Most applications involve elastomeric form.
• Flexibility and hardness can be adjusted over a wide range
– Electrical applications high voltage and high or low temperatures
• Power cable insulation, high voltage leads and insulator boots, ignition
cables, spark plug boots, etc..
• Semi-conductors are encapsulated in silicone resins for potting.
– Mechanical applications requiring low and high-temperature
flexibility and chemical inertness
• ‘O-rings’, gaskets, seals for aircraft doors and windows, freezers, ovens, and
appliances, diaphragms flapper valves, protective boots and bellows.
– Casting molds and patterns for polyurethane, polyester, or epoxy
– Sealants and caulking agents
– Shock absorbers and vibration damping characteristics
• “Silly-Putty”: Non-crosslinked, high molecular weight PDMS-based
compound modified with fillers and plasticizers.
– Biomedical field for biological inertness include prosthetic devices
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Copyright Joseph Greene 2001
Miscellaneous Other Elastomers
• Acrylic Rubber (AR)
– Polyethylacrylate (PEA) copolymerized with a small amount (5%)
of 2-chloro-ethyl-vinyl-ether CEVE, which is a cure site.
– The Tg of PEA is about -27°C and acrylic rubber is not suitable for
low temperature applications.
– Polybutylacrylate (PBR) has a Tg of -45°C.
– Applications
• Resistant to high temperatures, lubricating oils, including
sulfur-bearing oils.
• Include seals, gaskets, and hoses.
• Epichlorohydrin Rubber (ECHR)
–
–
–
–
Polymerization of epichlorohydrin with a repeat unit of PECH.
Excellent resistant to oils, fuels and flame resistance. (Cl presence)
Copolymer with flexible ethyleneoxide (EO) provides Tg = -40C
Applications include seals,Copyright
gaskets,
diaphragms, wire covers
Joseph Greene 2001
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Miscellaneous Other Elastomers
H H
[C C S
• Polysulfide Rubbers (SR)
H H S
– One of the first synthetic rubbers. Tg =-27°C, PES Thiokol A
S ]
S
– Consists of adjacent ethylene and sulfide units giving a stiff chain.
– Flexibility is increased with addition of ethylene oxide for polyethylene-ethersulfide (PEES), Thiokol B
– Mechanical properties are not very good, but are used for outstanding resistance
to many oils, solvents and weathering.
H H
– Applications include caulking, mastics, and putty.
• Propylene rubber (PROR)
– Does not crystallize in its atactic form and has a low Tg = -72°C.
– Has excellent dynamic properties
C C O
H CH3 n
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Copyright Joseph Greene 2001
Miscellaneous Other Elastomers
• Polynorborene (PNB)
– Norborene polymerizes into highly molecular weight PNB.
– Tg = 35°C but can be plasticized with oils and vulcanized into an
elastomer with lower Tg = -65°C.
– Excellent damping properties that can be adjusted.
• Polyorgano-phosphazenes (PPZ)
– Form an example of a new class of polymeric materials involving
inorganic chains.
• Atoms of Nitrogen (azo) and Phosphorous form, the chain and a variety of
organic side groups, R1 and R2 can be attached to the phosphorous atom.
• Side groups include halo (Cl or F), amino (NH2 or NHR), alkoxy (methoxy,
ethoxy, etc.) and fluoroalkoxy groups.
• High molecular weight is flexible with a low Tg
• Excellent inherent fire resistance, weatherability, and water & oil repellency
• Applications
– coatings, fibers, and biomedical materials
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Copyright Joseph Greene 2001
Commercial Elastomers
• Characteristics
Name
Natural rubber
Polyisoprene
Polybutadiene
SBR
Nitrile
Butyl
EPR (EPDM)
Neoprene
Silicone
Thiokol
Urethanes
Chemical Name
cis polyisoprene
cis polyisoprene
Polybutadiene
Polybutadiene-styrene
Polybutadiene-acrylonitrile
Poly isobutylene-isoprene
Poly ethylene propylene- diene
Polychloroprene
Polydimethylsiloxane
Polyslkylenesulfide
Polyester or polyether urethanes
Vucanization agent
sulfur
sulfur
sulfur
sulfur
sulfur
sulfur
Peroxies or sulfur
MgO
peroxides
ZnO
Diisocycanates
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Copyright Joseph Greene 2001
Commercial Elastomers
• Costs
Name
Consumption 1983 (metric tons)
Natural rubber
676,267
Polyisoprene
Polybutadiene
335,541
SBR
887,005
Nitrile
57,239
Butyl
EPR (EPDM)
141,490
Neoprene
85,096
Silicone
Thiokol Psulfides
Urethanes
$/lb
$0.44
$0.72
$0.74
$0.66
$1.10
$0.76
$1.01
$1.29
$4.40
$1.83
$3.70
Type
General Purpose
General Purpose
General Purpose
General Purpose
Solvent Resistant
General Purpose
General Purpose
Solvent Resistant
Heat Resistant
Solvent Resistant
Solvent Resistant
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Copyright Joseph Greene 2001
Polymerization Mechanisms
• Step-wise (Condensation) Polymerization
– Monomers combine to form blocks 2 units long
– 2 unit blocks form 4, which intern form 8 and son on
until the process is terminated.
– Results in by-products (CO2, H2O, Acetic acid, HCl
etc.)
• Chain Growth (Addition) Polymerization
– Polymerization begins at one location on the monomer
by an initiator
– Instantaneously, the polymer chain forms with no byproducts
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Copyright Joseph Greene 2001
Condensation Polymerization Example
• Polyamides
– Condensation Polymerization
• Nylon 6/6 because both the acid and amine contain
6 carbon atoms
NH2(CH2)6NH2 + COOH(CH2)4COOH
Hexamethylene diamene
Adipic acid
n[NH2(CH2)6NH2 ·CO(CH2)4COOH] (heat)
Nylon salt
[NH(CH2)4NH · CO(CH2)4CO]n + nH2O
Nylon 6,6 polymer chain
Copyright Joseph Greene 2001
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Condensation Polymerization Example
• Polyurethane
– Reaction of isocyanate and polyether-alcohol (polyol)
• Polyester
– Polymerization of acid and and alcohol
• Polycarbonate
– Polycarbonates are linear, amorphous polyesters
because they contain esters of carbonic acid and an
aromatic bisphenol (C6H5OH)
OH
2
+ CH3
O
C CH2
Phenol + Acetone
CH2
OH
OH +
C
CH2
Bisphenol-A + water
Copyright Joseph Greene 2001
H2O
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Other Condensation Polymers
• Thermoplastic Polyesters
– Saturated polyesters (Dacron).
• Linear polymers with high MW and no
crosslinking.
• Polyethylene Terephthalate (PET). Controlled
crystallinity.
• Polybutylene Terephthalate (PBT).
O
– Aromatic polyesters (Mylar)
R
R
O C R
O
O
C
C
O
O C
R
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Copyright Joseph Greene 2001
Step-Growth Polymerization
Condensation Polymerizatio
• Main feature is that all molecular species in the
system can react with each other to form higher
molecular weight species.
– Step-growth polymerization reactions fall into two
classes
• A-R1-A + B-R2-B
• A-R1-A + B-R2-B
=> A-R1-R2-B + AB
=> A-R1-AB-R2-B
– where A and B are repeat polymer groups which react with each other;
» Example, for polyurethanes A = Isocyanate and B = Polyol and the
by-product is water.
– and R1 and R2 are long chain polymers
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Copyright Joseph Greene 2001
Formation of Polymers
• Condensation Polymerization
– Step-growth polymerization proceeds by several steps which result
in by-products.
• Step-wise (Condensation) Polymerization
– Monomers combine to form blocks 2 units long
– 2 unit blocks form 4, which intern form 8 and son on until the
process is terminated.
– Results in by-products (CO2, H2O, Acetic acid, HCl etc.)
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Copyright Joseph Greene 2001
Chain Growth (Addition)
Polymerization
• Chain Growth (Addition) Polymerization by
Free Radical Mechanism
– Involves three primary steps
• Initiation- formation of free radicals through homolytic
dissociation of weak bonds (e.g., peroxides). Results in opening
up unsaturated (C=C) bonds to saturated (C-C) bonds)
• Propagation- formation of long chain polymers of the now free
C-C bonds
• Termination- reactions at the ends of the polymer cause C-C to
terminate with a functional group that does not have any free
electrons to bond with and results in unsaturated end group (C38
C=CX)
Copyright Joseph Greene 2001
Chain Growth (Addition)
Polymerization
• Special case of Diene polymerization
– Very important in elastomers- mostly addition
– Polydienes are the backbone of the synthetic
rubber are produced by free radical
polymerization
– Early attempts of polymerization was slow and
produced low molecular weight polymers (oils)
– Emulsion polymerization (1930s) was introduced
to speed up polymerization and higher Molecular
weights
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Copyright Joseph Greene 2001
Polymerization Methods
• 4 Methods to produce polymers
– Some polymers have been produced by all four methods
• PE, PP and PVC are can be produced by several of these
methods
• The choice of method depends upon the final polymer form, the
intrinsic polymer arrangement (isotactic, atactic, etc), and the
yield and throughput of the polymer desired.
–
–
–
–
Bulk Polymerization
Solution Polymerization
Suspension Polymerization
Emulsion Polymerization
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Copyright Joseph Greene 2001
Formation of Polymers
• Polymers from Addition reaction
– LDPE
HDPE
PP
H H
H H
H H
C C
C C
C C
H H
n
– PVC
H H
H CH3
n
PS
H H
H H
C C
C C
H Cl
n
n
H
n
Copyright Joseph Greene 2001
41
Other Addition Polymers
• Vinyl- Large group of addition
polymers with the formula:
– Radicals (X,Y) may be attached to this
repeating vinyl group as side groups to form
several related polymers.
• Polyvinyls
– Polyvinyl chloride
– Polyvinyl dichloride
(polyvinylidene chloride)
– Polyvinyl Acetate (PVAc)
H
H
C
C
H
X
or
H
Y
C
C
H
X
H
H
H
Cl
C
C
C
C
H
Cl
H
Cl
H
H
C
C
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Copyright Joseph Greene 2001
H
OCOCH3
Manufacturing of Emulsion SBR
• Free-radical emulsion process
– Developed in 1930s and still in use
– Typical process (Figure)
• Soap stabilized water emulsion of two monomers is converted in a train of
10 continuous reactors (4000 gallons each)
• Water, butadiene, styrene, soaps, initiators, buffers, and modifier are fed
continuously
• Temp is 5 to 10°C and conversion proceeds until 60% of the reactants have
polymerized in the last reactor.
• Shortstop is added in the emulsion to stop the conversion at 60%
• Unreacted butadiene is flashed off with steam and recycled
• Unreacted styrene is stripped off in a distillation column that separates liquid
rubber emulsion from the gas styrene.
• Rubber is recovered from the latex in a series of operations.
– Introduction of antioxidants, blending with oils, dilution with brine,
coagulation, dewatering, drying, and packaging the rubber
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Copyright Joseph Greene 2001
Manufacturing of Emulsion SBR
• Polymerization
– Cold SBR: at 5 to 10°C is called the cold process,
• Better abrasion resistant, treadwear, and dynamic properties.
– Hot SBR: at about 50°C is called the hot process.
• Conversion is allowed to proceed to 70%
• Higher branching occurs and incipient gelation.
– Typical SBR recipes, Table from Morton’s Rubber
technology
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Copyright Joseph Greene 2001
Manufacturing of Emulsion SBR
• Compounding and Processing
– Similar to natural rubber
– Materials for large scale use, e.g., tires, based on
• Rubber, fillers (carbon black), extending oils, zinc oxide, sulfur,
accelerators, antioxidants, antiozonants, and waxes.
– Materials are mixed in a mill or twin rollers or calendered
– Processing into smooth compounds that can be quickly
pressed, sheeted, calendered, or extruded
• Recipes
– Large parts, e.g., tires and hoses, are given in Tables 7.6,
7.7, 7.8, and 7.9
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Copyright Joseph Greene 2001
Polymerization of Elastomers
• Butadiene-Acrylonitrile (Nitrile) Rubber
– Produced by emulsion polymerization
– Nitrile rubbers have nitrile contents from 10 to 40%.
• Chloroprene rubber
– Produced by emulsion polymerization
– Produced as a homopolymer that has a high trans 1,4
chain structure and is susceptible to strain-induced
crystallization, much like natural rubber.
• Leads to high tensile strength
– Does not lead itself to copolymerization
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Copyright Joseph Greene 2001
Polymerization of Elastomers
• Butyl Rubber– Only important commercial rubber prepared by cationic
polymerization
• Processes with AlCl at –98 to –90C
– Copolymer of isobutene and isoprene with isoprene used
in 1.5 % quantities
• The isoprene is introduced to provide sufficient unsaturations
for sulfur vulcanization.
– MW is in the range of 300,000 to 500,000
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Copyright Joseph Greene 2001
Processing of Elastomers
• Rubber Products
– 50% of all rubber produced goes into automobile tires;
– 50% goes into mechanical parts such as
• mountings, gaskets, belts, and hoses, as well as
• consumer products such as shoes, clothing, furniture, and toys
• Elastomers and Rubbers
– Thermoset rubbers
• Compounding the ingredients in recipe into the raw rubber with
a mill, calender, or Banbury (internal) mixer
• Compression molding of tires
– Thermoplastic elastomers
48
• Compression molding, extrusion, injection molding, casting.
Copyright Joseph Greene 2001
Processing of Elastomers
• Rubber Processors
– Mills and Banbury mixers
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Copyright Joseph Greene 2001
Compression Molding Process
• Materials
•Elastomers:
•Thermoplastic
•Thermoplastic Olefin (TPO), Thermoplastic Elastomer (TPE),
Thermoplastic Rubber (TPR)
•Thermoset rubbers
•Styrene Butadiene Rubber, isoprene
Thermoplastic:
Heat Plastic
prior to molding
Thermosets:
Heat Mold
during molding
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Copyright Joseph Greene 2001
Polyurethane Processing
• Polyurethane can be processed by
– Slow process: Casting or foaming, or
– Fast process: Reaction Injection Molding (RIM)
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Copyright Joseph Greene 2001
Injection Molding Glass Elastomers
• Plastic pellets with copolymer elastomers.
– Similar processing requirements as with injection
molding of commodity and engineering plastics
• Injection pressures, tonnage, pack pressure, shrinkage
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Copyright Joseph Greene 2001
Transfer Molding of Rubbers
• Transfer molding is a process by which uncured rubber
compound is transferred from a holding vessel (transfer pot) to
the mold cavities using a hydraulically operated piston. Transfer
molding is especially conducive to multicavity designs and can
produce nearly flashless parts.
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Copyright Joseph Greene 2001
Calendering of Rubbers
• Calendering is the process for producing long runs of uniform
thickness sheets of rubber either unsupported or on a fabric
backing. A standard 3 or 4 roll calender with linear speed range
of 2 to 10 feet/minute is typical for silicone rubber. Firm
compound with good green strength and resistance to
overmilling works the best for calendering.
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Copyright Joseph Greene 2001
Curing of Rubbers
• Extruded profile may be cured by hot air vulcanization (HAV),
steam vulcanization (CV) or liquid-medium cure. HAV consists
of a heated tunnel through which the profile is fed continuously
on a moving conveyor. Air temperature reaches 600°F to
1200°F, and cure times are usually short, on the order of 3 to 12
seconds. The recommended curing agents are DCBP-50 or
addition cure, both of which provide rapid cure with no
porosity.
• Steam cure commonly refers to the steam curing systems used
by the wire and cable industry and consists of chambers 4” –
6” in diameter and 100 – 150 feet in length. Steam pressure
varies from 50 psig to 225 psig depending on wall thickness of
the insulation.
• For liquid-medium cure, continuous lengths of extruded profile
are fed into a bath of moltenmaterial (salt or lead) which cures
the extrudate.
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Copyright Joseph Greene 2001
Polymer Length
• Polymer Length
– Polymer notation represents the repeating group
• Example, -[A]-n where A is the repeating monomer and n represents the
number of repeating units.
• Molecular Weight
– Way to measure the average chain length of the polymer
– Defined as sum of the atomic weights of each of the atoms in the
molecule.
• Example,
– Water (H2O) is 2 H (1g) and one O (16g) = 2*(1) + 1*(16)= 18g/mole
– Methane CH4 is 1 C (12g) and 4 H (1g)= 1*(12) + 4 *(1) = 16g/mole
– Polyethylene -(C2H4)-1000 = 2 C (12g) + 4H (1g) = 28g/mole * 1000 = 28,000
g/mole
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Copyright Joseph Greene 2001
Molecular Weight
• Average Molecular Weight
– Polymers are made up of many molecular weights or a
distribution of chain lengths.
• The polymer is comprised of a bag of worms of the same
repeating unit, ethylene (C2H4) with different lengths; some
longer than others.
• Example,
– Polyethylene -(C2H4)-1000 has some chains (worms) with 1001 repeating
ethylene units, some with 1010 ethylene units, some with 999 repeating
units, and some with 990 repeating units.
– The average number of repeating units or chain length is 1000 repeating
ethylene units for a molecular weight of 28*1000 or 28,000 g/mole .
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Copyright Joseph Greene 2001
Molecular Weight
• Average Molecular Weight
– Distribution of values is useful statistical way to
characterize polymers.
• For example,
– Value could be the heights of students in a room.
– Distribution is determined by counting the number of students in the
class of each height.
– The distribution can be visualized by plotting the number of students on
the x-axis and the various heights on the y-axis.
Frequency
Histogram of Heights of Students
25
20
15
10
5
0
Series1
60
70
80
Height, inches
Copyright Joseph Greene 2001
58
Molecular Weight
• Molecular Weight Distribution
– Count the number of molecules of each molecular weight
– The molecular weights are counted in values or groups that have similar lengths,
e.g., between 100,000 and 110,000
• For example,
– Group the heights of students between 65 and 70 inches in one group, 70 to 75
inches in another group, 75 and 80 inches in another group.
• The groups are on the x-axis and the frequency on the y-axis.
• The counting cells are rectangles with the width the spread of the cells and
the height is the frequency or number of molecules
• Figure 3.1
• A curve is drawn representing the overall shape of the plot by connecting the
tops of each of the cells at their midpoints.
• The curve is called the Molecular Weight Distribution (MWD)
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Copyright Joseph Greene 2001
Molecular Weight
• Average Molecular Weight
– Determined by summing the weights of all of the chains
and then dividing by the total number of chains.
– Average molecular weight is an important method of
characterizing polymers.
– 3 ways to represent Average molecular weight
• Number average molecular weight
• Weight average molecular weight
• Z-average molecular weight
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Copyright Joseph Greene 2001
Gel Permeation Chromatography
• GPC Used to measure Molecular Weights
– form of size-exclusion chromatography
– smallest molecules pass through bead pores, resulting in
a relatively long flow path
– largest molecules flow around beads, resulting in a
relatively short flow path
– chromatogram obtained shows intensity vs. elution
volume
– correct pore sizes and solvent critical
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Copyright Joseph Greene 2001
Gel Permeation Chromatography
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Copyright Joseph Greene 2001
•
Number Average Molecular Weight, Mn
Mn
NM
N M  N M  N M  ...



N  N  N  ...
 NMi is the molecular
• where
weight of that species (on the x-axis)
i
i
i
1
1
2
1
2
2
3
3
3
• where Ni is the number of molecules of a particular molecular species I (on
the y-axis).
– Number Average Molecular Weight gives the same weight to all polymer
lengths, long and short.
• Example, What is the molecular weight of a polymer sample in which the polymers
molecules are divided into 5 categories.
– Group Frequency
– 50,000
1
– 100,000
4
– 200,000
5
N i M i N1 M 1  N 2 M 2  N 3 M 3  ...

M


– 500,000
3
n
N1  N 2  N 3  ...
 Ni
– 700,000
1
1(50 K )  4(100 K )  5(200 K )  3(500 K )  1(700 K )
(1  4  5  3  1)
M n  260,000
Mn 
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Copyright Joseph Greene 2001
Molecular Weight
• Number Average Molecular Weight. Figure 3.2
– The data yields a nonsymmetrical curve (common)
– The curve is skewed with a tail towards the high MW
– The Mn is determined experimentally by analyzing the number of
end groups (which permit the determination of the number of
chains)
– The number of repeating units, n, can be found by the ratio of the
Mn and the molecualr weight of the repeating unit, M0, for
example for polyethylene, M0 = 28 g/mole
– The number of repeating units, n, is often called the degree of
polymerization, DP.
– DP relates the amount of
monomer that has been converted to polymer.
Mn
n
M0
Copyright Joseph Greene 2001
64
Weight Average Molecular Weight, Mw
2
N
M
 i i
N1 M 12  N 2 M 22  N 3 M 32  ...
Mw 

 N i M i N1 M 1  N 2 M 2  N 3 M 3  ...
• Weight Average Molecular Weight, Mw
– Favors large molecules versus small ones
– Useful for understanding polymer properties that relate to
the weight of the polymer, e.g., penetration through a
membrane or light scattering.
– Example,
• Same data as before would give a higher value for the
Molecular Weight. Or, Mw = 420,000 g/mole
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Copyright Joseph Greene 2001
Z- Average Molecular Weight
Mz 
3
N
M
 i i
2
N
M
 i i
N 1 M 13  N 2 M 23  N 3 M 33  ...

N 1 M 12  N 2 M 22  N 3 M 32  ...
– Emphasizes large molecules even more than Mw
– Useful for some calculations involving mechanical
properties.
– Method uses a centrifuge to separate the polymer
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Copyright Joseph Greene 2001
Molecular Weight Distribution
• Molecular Weight Distribution represents the
frequency of the polymer lengths
• The frequency can be Narrow or Broad, Fig 3.3
• Narrow distribution represents polymers of about
the same length.
• Broad distribution represents polymers with varying
lengths
• MW distribution is controlled by the conditions
during polymerization
• MW distributions can be symmetrical or skewed.
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Copyright Joseph Greene 2001
Physical and Mechanical Property
Implications of MW and MWD
• Higher MW increases
• Tensile Strength, impact toughness, creep resistance, and
melting temperature.
– Due to entanglement, which is wrapping of polymer
chains around each other.
– Higher MW implies higher entanglement which yields
higher mechanical properties.
– Entanglement results in similar forces as secondary or
hydrogen bonding, which require lower energy to break
than crosslinks.
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Copyright Joseph Greene 2001
Physical and Mechanical Property Implications
of MW and MWD
• Higher MW increases tensile strength
• Resistance to an applied load pulling in opposite directions
• Tension forces cause the polymers to align and reduce the number of
entanglements. If the polymer has many entanglements, the force would be
greater.
• Broader MW Distribution decreases tensile strength
• Broad MW distribution represents polymer with many shorter molecules
which are not as entangled and slide easily.
• Higher MW increases impact strength
• Impact toughness or impact strength are increased with longer polymer
chains because the energy is transmitted down chain.
• Broader MW Distribution decreases impact strength
• Shorter chains do not transmit as much energy during impact
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Copyright Joseph Greene 2001
Thermal Property Implications of MW & MWD
• Higher MW increases Melting Point
• Melting point is a measure of the amount of energy necessary to
have molecules slide freely past one another.
• If the polymer has many entanglements, the energy required
would be greater.
• Low molecular weights reduce melting point and increase ease
of processing.
• Broader MW Distribution decreases Melting Point
• Broad MW distribution represents polymer with many shorter
molecules which are not as entangled and melt sooner.
• Broad MW distribution yields an easier processed polymer
* Decomposition
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Copyright Joseph Greene 2001
Example of High Molecular Weight
• Ultra High Molecular Weight Polyethylene (UHWMPE)
• Modifying the MWD of Polyethylene yields a polymer with
– Extremely long polymer chains with narrow distribution
– Excellent strength
– Excellent toughness and high melting point.
• Material works well in injection molding (though high melt T)
• Does not work well in extrusion or blow molding, which
require high melt strength.
• Melt temperature range is narrow and tough to process.
• Properties improved if lower MW polyethylene
– Acts as a low-melting lubricant
– Provides bimodal distributions, Figure 3.5
– Provides a hybrid material with hybrid properties
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Copyright Joseph Greene 2001