Copyright Joseph Greene 2001 1 Classes of Polymeric Materials Chapter 3: Elastomers Professor Joe Greene CSU, CHICO

Copyright Joseph Greene 2001

1

Classes of Polymeric MaterialsChapter 3: Elastomers

Professor Joe Greene

CSU, CHICO


2

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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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.


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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Vulcanizable Elastomeric Compounds• Rubbers are compounded into practical elastomers

– The rubber (elastomer) is the major component and other components are given as weight pre 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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Vulcanizable Rubber• Typical tire tread

– Natural rubber smoked sheet (100),

– sulfeur (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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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


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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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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 HardHard Hard

Soft Soft


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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.

CH

CC

H

H3HH

HC C ][

CH

CH

HH

HC C ][

CH3

Cis

Trans


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Commercial Elastomers• 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 HCH

CH HHC C ][

H HCH

CCl HHC C ][


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Commercial Elastomers• 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.

H H3

CCH3HCC

][


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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.


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Thermoplastic Elastomers• 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

C C

H

H H

n

H HCH

CH HHC C ][


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Acrylonitrile-butadiene rubber (NBR)

• 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

H HCH

CH HHC C ][C C

H C:::N

H H

n


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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 C

H H

H H

n

C C

H CH3

H H

m

C C

H CH2

H H

mCH

CHCH3


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Ethylene Related Elastomers• 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

C C

H H

H H

n

C

Cl

H

m

C

S

H

kClO O


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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

– Properties include• Good heat, ozone, and weather resistance

• 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

C C

H H

H H

n

C C

H O

H H

O=CCH3

m

C C

H H

H H

n

C C

H C

H H

OOCH3

m


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FluoroElastomers• 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 C

H F

H F

n C C

F Cl

F F

n

C C

F CF3

F F

n

C C

F F

F F

n


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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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Silicone Polymers• 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)

Si

CH3

CH3

m

O


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Silicone Polymers• 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.

Si

CH3

CH3

m

O


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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 aree 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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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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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.


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Silicones Applications• Most applications involve elastomeric form.

• Flexiblity 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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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


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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 polyehtylene-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

HCH

CH

S S ][H S S

C C

H CH3

H H

n

O


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Miscellaneous Other Elastomers• Polynorbornene (PNB)

– Norborene polymerizes into highly molecular weight PNB.

– Tg = 35°C but can be plasticised 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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Commercial Elastomers• Characteristics

Name Chemical Name Vucanization agentNatural rubber cis polyisoprene sulfurPolyisoprene cis polyisoprene sulfurPolybutadiene Polybutadiene sulfurSBR Polybutadiene-styrene sulfurNitrile Polybutadiene-acrylonitrile sulfurButyl Poly isobutylene-isoprene sulfurEPR (EPDM) Poly ethylene propylene- diene Peroxies or sulfurNeoprene Polychloroprene MgOSilicone Polydimethylsiloxane peroxidesThiokol Polyslkylenesulfide ZnOUrethanes Polyester or polyether urethanes Diisocycanates


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Commercial Elastomers• Costs

Name Consumption 1983 (metric tons) $/lb TypeNatural rubber 676,267 $0.44 General PurposePolyisoprene $0.72 General PurposePolybutadiene 335,541 $0.74 General PurposeSBR 887,005 $0.66 General PurposeNitrile 57,239 $1.10 Solvent ResistantButyl $0.76 General PurposeEPR (EPDM) 141,490 $1.01 General PurposeNeoprene 85,096 $1.29 Solvent ResistantSilicone $4.40 Heat ResistantThiokol Psulfides $1.83 Solvent ResistantUrethanes $3.70 Solvent Resistant


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Manufacturing of Emulsion SBR• Free-radical emulsion process

– Developed before 1950 and still in use– Typical process (Figure 7.3)

• Soap stabilized water emulsion of two monomers is converted in a train of 10 continuous reactors (4000 gallons each)

• Water, butadiene, styrene, soaps, initiaors, buffers, and modifer 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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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 7.5


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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 calenedered

– 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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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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Processing of Elastomers• Rubber Processors

– Mills and Banbury mixers


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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


38

Polyurethane Processing• Polyurethane can be processed by

– Slow process: Casting or foaming, or– Fast process: Reaction Injection Molding (RIM)


39

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


40

Carbon Black• Reference: Rubber Technology, Chapter 3• Phenomenon of carbon black reinforcement was discovered in early 1900s

– Physical and chemical attachments are capable of giving reinforcement effects by increasing the tensile strength and modulus of the rubbery phase

– Carbon black and vulcanization generates a 3-D network

• Carbon black– Range of physical and chemical attributes

• Particle size, surface area, structure, surface activity

– Gas-furnace blacks: Thermal black process: 3% of current carbon black• Initially made using gas as the source of carbon and the fuel source• Carbon black had small particles and were acidic• Worked well with natural rubber• Large amounts of air pollution was generated and expensive

– Oil furnace black (1943) is the current manufacturing method: 97% of black• Low grade petroleum feedstock was cheaper, less polluting, and flexible process• Higher structure and more alkaline than gas furnace (channel) blacks• Improved significantly the properties of SBR polymers


41

Carbon Black Manufacture• Manufacture and Morphology

– Typical oil furnace reactor, Figure 3.1• Refractory lined tube that can be horizontal or vertical.• Feedstock oil, natural gas, or other fuel, and air are preheated and injected

into the combustion zone at specific rates for the carbon black– Burning generates a very hot, turbulent atmosphere for cracking the feedstock

oil.– 90% of the feedstock is based on refinery heavy bottom oils.– Chemical reactions to convert the aromatic feedstock to elemental carbon are

not well understood and complex– Collision of particles in a liquid-like state produces aggregates of spherical

particles fused together in a random grape-cluster configuration, Fig 3.2– The carbon is formed in aggregates with a distribution of sizes

• Water quench is used to rapidly reduce the temperature and terminate the reaction.

– The smoke exiting the reactor is a mixture of carbon black aggregates, combustion gases, and moist air.

– The smoke preheats the feedstock and air, and generates steam for plant use.– Fluffy black and gases (tail gas) are separated by filtration, and the loose black

is pulverized to a 325 mesh and then pelletized


42

Carbon Black Manufacture• Manufacture and Morphology

– Wet-pelleting process is used• A rotating, pin-studded shaft mixes the loose black with water and binder to

produce small beads or pellets.

– Wet pellets are fed into a rotary drier heated by combustion of the tailgas from the earlier step in the process.

• Steam that is generated is removed and replaced with air that oxidizes the carbon black, which influences the chemical properties of the carbon black and, in turn, the cure rate and properties of the vulcanizates.

– The pelleted black is screened for uniformity and passed over magnetic separators to remove metallic contamination that may have gotten in the product stream.

– Finished product is packaged and shipped– Furnace black categories

• Reinforcing: hard, tread. Have a smaller particle size and lower yields and more expensive than semi-reinforcing.

• Semi-reinforcing: soft, carcass.


43

Carbon Black Properties• Physical and Chemical Properties

– Particle size can be measured by electron micrographs, Figure 3.3• Average diameter is 19 to 95 nm (nanometers or 10-9 m)• Particles are measured manually or with image analysis software

– Particle size can be measured by tint strength test (ASTM D3265)• Carbon black sample is mixed with zinc oxide and a soybean oil epoxide to

produce a black or gray paste.• Paste is spread to produce a suitable surface for measuring the reflectance of

the mixture with a photoelectric reflectance meter.– Reflectance is compared to the reflectance of paste containing the

Industry Tint Reference Black (ITRB) prepared in the same manner.– Tint test is affected by the structure as well as the particle size of the

black.» For a given particle size, the higher structure blacks have a lower

tinting strengths.» Average particle size can be estimated from statistical equations

that relate tint strength and structure to particle size as measured from electron micrographs.


44

Carbon Black Surface Area• Surface Area

– Very important in carbon black because it defines how much surface is available for interactions with other materials present in a rubber compound.

• Small particle-size black will have higher surface area, but the texture or nature of the surface area can also influence the surface area.

– BET method (ASTM D3037) to determine surface area » Adsorption of a gas, usually nitrogen, on the surface.

– Surface area can be measured from electron micrographs

• Standard rubber grade black (nitrogen surface area of less than 130 m2/g) are nonporous

• Non-specialty furnace blacks give good inverse correlation between nitrogen surface area and the particle size measurements.

• Specialty furnace blacks require a devolatilization step to remove residual oils present on the surface of the blacks

– Volume of void space between aggregates per unit weight of carbon black increases with the number of particles per aggregate

• Non-spherical particles pack differently from spheres


45

Carbon Black Chemical Properties• Chemical Properties

– Chemical nature of a carbon black is variable• Evidence for the presence on the surface of at least four oxygen containing

groups, carboxyl, phenol, quinone, and lactone.

– Elastomers are polar in nature, neoprene or nitrile rubber• Will react more strongly with fillers with dipoles, OH, COOH, or Cl

– Chemical surface groups affect the rate of cure with many vulcanization systems

• Physical adsorption activity of the filler surface is of much greater overall importance for the mechanical properties of the general-purpose rubbers than the chemical nature.

– Oxygen content influences the cure rate• Increased oxygen gives longer scorch period, a slower rate of cure, and a

lower modulus at optimum cure.• Amount of oxidation during the pellet drying operation can affect the cure

rate and modulus of rubber compounds.

– Carbon blacks are generally electrically conductive because of the highly conjugated bonding scheme in crystalline regions


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Carbon Black Nomenclature• Nomenclature

– First digit following the letter indicates the particle size range• Lower numbers for smaller particle-size blacks

– Last two digits are arbitrarily assigned by ASTM– Table 3.1– Properties

• ASTM D1765

ASTM Old NameN110 SAF (Super-abrasion furnace)N220 ISAF (Intermediate Super-abrasion furnace)N330 HAF (High abrasion furnace)N358 SPF (Super processing furnace)N660 GPF (General-purpose furnace)N762 SRF (Semireinforcing furnace)


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Carbon Black Properties• Properties

– High surface area and high structure carbon blacks are associated with increased reinforcement

• Particle size affects abrasion resistance, heat build-up (resilience), tensile strength, and tear strength.

• Structure has more of an effect on modulus, hardness, and extrudate swell.

– Four carbon blacks are shown to demonstrate the effects of varying surface area, structure, and black loadings on various compound properties.

– Structure Differences• N339 vs N356 • N650 vs N660

– Both pairs have• Equivalent surface N2 surface area• Large differences in structure from

– DBP absorption and void volume data

– N339 and N356 vs N650 and N660 shows large difference in surface area

Nitrogen Surface area m2/g

DBP Absorption cc/100g

Void Volume cc/100g

Tint Strength, %ITRB

N339 98.9 118.4 69.2 108.8N356 100.1 160.2 77.7 103.2N650 38.8 123.7 57.4 52.7N660 39.3 89.7 44.5 59.2


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Carbon Black Properties• Three compound recipes based upon different

polymers enable the observation of changes in carbon black effects from one polymer to another– Table 3.3

– Figures 4 through 12• Mechanical properties for Different concentrations (loading

levels) of carbon black

EPDM CompoundEPDM 100 phrNaphthenic oil 12Zinc oxide 5Stearic acid 1Processing aid 2Sulfur 1.5MBT 0.5TMTD 3Carbon black 0 to 80phr

SBR CompoundSBR-1500 100 phrAromatic oil 5Zinc oxide 3Stearic acid 1.5Sulfur 1.75CBS 0.85DPG 0.28Carbon black 0 to 80phr

NR CompoundNatural Rubber 100 phrHighly aromatic oil 15Zinc oxide 5Stearic acid 2.5Antioxidant 2Antiozonant 2Sulfur 1.5CBS 1.6Carbon black 0 to 80phr


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Carbon Black Properties• Compound Property Group 1

– Viscosity, modulus, hardness, extrudate swell• Measures the degree of stiffening that carbon contributes• High structure and an increase in the amount of carbon black surface

available for attachment to the polymer result in the rubber compound to be more viscous and less elastic

– Viscosity, modulus, hardness, extrudate swell, Figures 3.4, 3.5,3.6• Increases with increased amount of carbon black for all three recipes, SBR,

EPDM, and NR– The N356 carbon black (highest N2 surface area) had the highest viscosity,

modulus at 200% elongation, and hardness; and the least amount of extrudate swell.

– The higher the N2 surface area the higher the viscosity, modulus at 200% elongation, and hardness; and the lower amount of extrudate swell.

– The N660 carbon black (lowest N2 surface area and lowest void volume) had the lowest viscosity, modulus at 200% elongation, and hardness; and the most amount of extrudate swell.


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– Abrasion resistance, tear strength, and tensile strength• Measures the resistance to failure under several types of stress• Strength related properties enhanced by carbon black surface area and

increased black loading up to a limiting value that is dependent on the packing characteristics (morphology) of the carbon black aggregates.

– High structure and an increase in the amount of carbon black surface available for attachment to the polymer in the rubber compound.

– Asblack loading in increased to maximum level, the carbon aggregates are no longer adequately separated by polymer which weakens the rubber composite

– Abrasion Resistance, Figures 3.8a, 3.8b, 3.8c• Abrasion resistance is most affected by surface area and loading• Lower surface area GPF blacks (N650 and N660) contribute small

improvements in abrasion, regardless of carbon black loadings• Higher surface area HAF blacks (N339 and N356) contribute better

improvements in abrasion, depending on carbon black loadings.• Higher structure N356 black reach maximum abrasion resistance at lower

loadings than N339, but N339 ultimately gives higher abrasion resistance


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– Tear-strength, Figures 3.9a and 3.9b• As carbon black is increased, the tear strength increases up to a peak, then

decreases after that.• Structure causes a shift in the strength curve to the left (lower limiting value

for strength because of the effect of higher structure on packing)

– Tensile strength, Fig 3.10a• Unfilled EPDM rubber compound has very low tensile strength.• Tensile strength is increased dramatically as carbon black is added until a

maximum tensile strength is attained.• Higher surface area HAF blacks give improved tensile strength compared to

GPF blacks, but not significantly difference due to structure.• NR compound has inherently higher tensile strength in the unfilled natural

rubber due to its crystallizing ability.– Carbon black causes less of a change in NR– Tensile strength reaches a maximum at relatively low carbon black loadings (2-

40 phr) and shows a decreasing tendency as the black loading is increased.

Documents

Copyright Joseph Greene 2001 1 Classes of Polymeric Materials Chapter 3: Elastomers Professor Joe Greene CSU, CHICO