1. Classes of Polymeric Materials Elastomers
Professor Joe Greene
CSU, CHICO
Copyright Joseph Greene 2001

2. 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 200% and returned to original shape. Elastic limit is 200%
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3. Copyright 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.
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4. 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 and soften rubber, fillers, reinforcement
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5. 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,
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6. Copyright Joseph Greene 2001
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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7. 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

8. 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
Copyright Joseph Greene 2001

9. 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
Copyright Joseph Greene 2001

10. 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
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, oxygen, ozone, heat aging, oils, or fuels. C H H3 ] [
Cis C H ] [
CH3
Trans
Copyright Joseph Greene 2001

11. Commercial ElastomersH C ] [
Polybutadiene
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
Tg = -50°C
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. H C Cl ] [
Copyright Joseph Greene 2001

12. Commercial ElastomersH H3 C
CH3 ] [
Butyl rubber- addition polymer of isobutylene.
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 sodium polysulfide. Sulfur makes thiokol rubber self vulcanizing.
Copyright Joseph Greene 2001

13. 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
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.
Copyright Joseph Greene 2001

14. Thermoplastic Elastomersn ] [
Styrene-butadiene rubber (SBR)
Developed during WWII
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

15. Acrylonitrile-butadiene rubber (NBR)H C ] [
C:::N n m
Also called Nitrile rubber
Developed as an oil resistant rubber due to
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
Copyright Joseph Greene 2001

16. 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
Unsaturated polymer since PP and PE are saturated
Resistant to ozone, weathering, and heat aging
Does not allow for conventional vulcanization
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. C H n
CH3 m
CH2 CH
Copyright Joseph Greene 2001

17. Ethylene Related ElastomersC H n Cl m S k O
Chlorosulfonated Polyethylene (CSPE)
Moderate random chlorination of PE (24-43%)
Infrequent chlorosulfonic groups (SO2Cl)
Sulfur content is 1-1.5%.
CSPE is noted for excellent weathering resistance
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
Copyright Joseph Greene 2001

18. Ethylene Related ElastomersC H n O
O=CCH3 m
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
Properties include
Good heat, ozone, and weather resistance
Ethylene-acrylate copolymer (EAR)
Copolymer of Ethylene and methacrylate
Contains carboxylic side groups (COOH)
Excellent resistance to ozone and
Excellent energy absorbers
Better than butyl rubbers C H n O
OCH3 m
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19. Copyright Joseph Greene 2001
FluoroElastomers C H F n
Polyvinylidene fluoride (PVDF)
Tg = -35°C
Poly chloro tri fluoro ethylene (PCTFE)
Tg = 40°C
Poly hexa fluoro propylene (PHFP)
Tg = 11°C
Poly tetra fluoro ethylene (PTFE)
Tg = - 130°C
Fluoroelastomers are produced by
random copolymerization that
suppresses the crystallinity and
provides a mechanism for cross linking by terpolymerization
Monomers include VDF, CTFE, HFP, and TFE C F Cl n C F n C F
CF3 n
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20. 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
Fluorinated polyorganophosphazenes have good fuel resistance
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21. Copyright Joseph Greene 2001
Silicone Polymers Si
CH3 m O
Silicone polymers or polysiloxanes (PDMS)
Polymeric chains featuring
Tg = -125°C
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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22. Copyright Joseph Greene 2001
Silicone Polymers Si
CH3 m O
Silicone polymers or polysiloxanes (PDMS)
Properties
Mediocre tear properties
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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23. 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 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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24. Copyright Joseph Greene 2001
Silicones
Properties
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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25. Copyright Joseph Greene 2001
Silicones
Properties
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.
Copyright Joseph Greene 2001

26. 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
Copyright Joseph Greene 2001

27. 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, gaskets, diaphragms, wire covers
Copyright Joseph Greene 2001

28. Miscellaneous Other Elastomers] [
Polysulfide Rubbers (SR)
One of the first synthetic rubbers. Tg =-27°C, PES Thiokol A
Consists of adjacent ethylene and sulfide units giving a stiff chain.
Flexibility is increased with addition of ethylene oxide for polyethylene-ether-sulfide (PEES), Thiokol B
Mechanical properties are not very good, but are used for outstanding resistance to many oils, solvents and weathering.
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 H
CH3 n O
Copyright Joseph Greene 2001

29. 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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30. Commercial Elastomers
Characteristics
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31. Commercial Elastomers
Costs
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32. 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 by-products
Copyright Joseph Greene 2001

33. 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

34. 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)
Phenol Acetone Bisphenol-A + water 2 OH +
H2O C
CH2
CH3 O
Copyright Joseph Greene 2001

35. Other Condensation Polymers
Thermoplastic Polyesters
Saturated polyesters (Dacron).
Linear polymers with high MW and no crosslinking.
Polyethylene Terephthalate (PET). Controlled crystallinity.
Polybutylene Terephthalate (PBT).
Aromatic polyesters (Mylar) O C R C O R
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36. 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-R2-B + AB
A-R1-A + B-R2-B => 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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37. 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.)
Copyright Joseph Greene 2001

38. 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 (C-C=CX)
Copyright Joseph Greene 2001

39. 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
Copyright Joseph Greene 2001

40. 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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41. Copyright Joseph Greene 2001
Formation of Polymers
Polymers from Addition reaction
LDPE HDPE PP
PVC PS C H n C H n C H
CH3 n C H Cl n C H n
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42. 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) C H X C H X Y or C H Cl C H Cl C C H
OCOCH3
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43. 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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44. 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
Copyright Joseph Greene 2001

45. 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
Copyright Joseph Greene 2001

46. Polymerization of Elastomers
Butadiene-Acrylonitrile (Nitrile) Rubber
Produced by emulsion polymerization
Nitrile rubbers have nitrile contents from 10 to 40%.
Chloroprene rubber
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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47. 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
Copyright Joseph Greene 2001

48. 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
Compression molding, extrusion, injection molding, casting.
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49. Processing of Elastomers
Rubber Processors
Mills and Banbury mixers
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50. 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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51. Polyurethane Processing
Polyurethane can be processed by
Slow process: Casting or foaming, or
Fast process: Reaction Injection Molding (RIM)
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52. 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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53. 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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54. 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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55. 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.
Copyright Joseph Greene 2001

56. 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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57. 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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58. 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.
Copyright Joseph Greene 2001

59. Copyright Joseph Greene 2001
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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60. 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
Copyright Joseph Greene 2001

61. 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
Copyright Joseph Greene 2001

62. Gel Permeation Chromatography
Copyright Joseph Greene 2001

63. Number Average Molecular Weight, Mn
where Mi is the molecular weight of that species (on the x-axis)
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
500,000 3
700,000 1
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64. 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.
Copyright Joseph Greene 2001

65. 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
Copyright Joseph Greene 2001

66. Z- Average Molecular Weight
Emphasizes large molecules even more than Mw
Useful for some calculations involving mechanical properties.
Method uses a centrifuge to separate the polymer
Copyright Joseph Greene 2001

67. 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.
Copyright Joseph Greene 2001

68. 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.
Copyright Joseph Greene 2001

69. 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
Copyright Joseph Greene 2001

70. 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
Mechanical
Properties
Melting
Point
* Decomposition MW MW
Copyright Joseph Greene 2001

71. 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
Copyright Joseph Greene 2001