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Elastomeric (Rubber) Material

Professor Joe Greene

CSU, CHICO

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Elastomer Definition • Polymers are classified as either

– thermoplastic or thermoset

• A special group of polymers can be grouped based upon a physical property rather than chemical nature – Elastomers can be stretched over and over again

• Elastomers have the distiction of being strenched 200% and returned to original shape. Elastic limit is 200%

• Figure 9.1

Strain

Stress Elastic Region

Elastic Limit

Yield Strength

Ultimate Strength

Plastic Region

Rupture

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Elastomers• Elastomers are rubber like polymers that are

thermoset or thermoplastic– Thermoset: butyl rubber (natural rubber), isoprene

rubber, Butadiene Rubber, Styrene Butadiene rubber, 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.

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Thermoset Elastomers• Natural is produced from gum resin of certain trees and

plants that grow in South America (Amazon)and Asia– The sap is an emulsion or latex containing 40% water & 60%

rubber particles.

– Crude natural rubber was made into shoes, balls, coatings, and used to rub out pencil markings, hence name rubber.

– Crude rubber has poor thermal properties. Needs vulcanization

– Crude natural rubber was composed of cis-polyisoprene. (Flexible)

– Trans polyisoprene has different properties. Trans is harder and is called gutta percha, used for golf ball covers or shoe soles

– Figure 9.4

C

CH3

C

H

H

C

H

H

C

H

C

CH3

C

H

H

C

H

H

C

HCis polyisoprene Trans polyisoprene

n n

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Thermoset Elastomers

• Vulcanization (Greek God of fire Vulcan) occurs with the addition of sulfur (4%).– Goodyear discovered the use of sulfur and heat for rubber

– 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.

• No longer and Elastomer because elongation drops below 200%

• Used for combs, insulation, and bowling balls

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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.

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Synthetic Rubber• Synthetic Isoprene- basic structure of rubber

– Mixture of cis and trans polyisoprene

– Catalyst can achieve between 10% cis and 90% cis

– More expensive than natural rubber

• Butadiene- basis for synthetic rubber – (BR) Butadiene rubber- no methyl group on chain

– Advantages are lower cost than isoprene, better low temp flexibility and compatible with other polymers, adhesion to metals

– Disadvantage is poor tensile strength, tear resistance and tack

C

H

C

H

H

C

H

H

C

H n

C C

CH3 H

CH3

n

Butadiene RubberButyl Rubber

C

Cl

C

H

H

C

H

H

C

H n

Neoprene Rubber

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Synthetic Rubber• Butadiene- basis for synthetic rubber

– (SBR) Styrene-Butadiene Rubber co-polymer• Copolymer is an improved butadiene rubber. Styrene stiffens the rubber.

• Buna-S or GSR.

• Used for tires, footwear, wire insulation, adhesives, gaskets, etc.

• Drawbackis poor oil resistance and oxidation and UV problems

• Used for making ABS

– Butyl rubber or polybutylene• Advantages are high damping capability, low gas permeability, and low UV

• Poor compatibility with other rubbers. Used for vibration pads, inner tubes.

C C

CH3 H

CH3

Butyl Rubber

C C

H

H H

kmn

C C

CH2CH2

H H

Styrene Butadiene Rubber

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Synthetic Rubber• Polychloroprene- neoprene rubber,

– Closest sythetic to natural rubber. – Used for fuel lines, hoses, gaskets.– Good oil resistance and good stiffness and strength

• Thiokol- ethylene dichloride polymerized with sodium polysulfide. Sulfur makes thiokol rubber self vulcanizing.

C

Cl

C

H

H

C

H

H

C

H n

Neoprene Rubber

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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) were 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

– Olefins (TPO) for tubing, seals, gaskets, electrical, and automotive.

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Copolymers of Polyethylene• Ethylene-ethyl acrylate (EEA)

– Properties range from rubbery to tough ethylene-like properties– Applications include hot melt adhesives, shrink wrap, produce bags,

bag-in-box products, and wire coating.

• Ethylene-methyl acrylate (EMA)– Produced by addition of methyl acrylate monomer (40% by

weight)with ethylene gas– Tough, thermally stable olefin with good elastomeric characteristics.– Applications include food packaging, disposable medical gloves, heat-

sealable layers, and coating for composite packaging

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Copolymers of Polyethylene• Ethylene-Vinyl Acetate (EVA)

– Repeating groups is ethylene with an acetate functional– Part of the pendent group are highly polar.– Vinyl acetate reduces crystallinity and increases chemical reactivity

because of high regions of polarity.– Result:flexible polymer that bonds well to other materials– Excellent adhesive (Elmers Glue)– Other applications include flexible packaging, shrink wrap, auto bumper

pads, flexible toys, and tubing

C C

H H

H H

C C

H OC = OC

H H

n m

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Copolymers of Polyethylene• Ethylene-Propylene (EPM)

– Ethylene and propylene are copolymerized in random manner and causes a delay in the crystallization.

– Thus, the copolymer is rubbery at room temp because the Tg is between HDPE (-110C) and PP (-20C).

– Ethylene and propylene can be copolymerized with small amounts of a monomer containing 2 C=C double bounds (dienes)

– Results in a ter polymer, EPDM, or thermoplastic rubber, TPO

C C

H H

H H

n

C C

CH3 H

H H

m

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Mechanical Properties of PE Blends

Mechanical Properties of PE Blends

Ethylene-vinylacetate

Ethylene-vinylalcohol

Ethylene-ethyl acrylate

Ethylene-methylacrylate

Density 0.922 – 0.943 1.14 – 1.19 0.93 0.942 – 0.945

TensileStrength, psi

2,200 – 4,000 8,520 – 11,600 1,600 – 2,100 1,650

TensileModulus, psi

7K – 29K 300 K – 385 K 4K – 7.5 K 12 K

TensileElongation, %

300% - 750% 180%- 280% 700% - 750% 740%

Impact Strengthft-lb/in

No break 1.0 – 1.7 No break

Hardness, Shore D17 – D45 D27 – D38

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Processing Properties of PE Blends

Processing PropertiesEthylene-vinylacetate

Ethylene-vinylalcohol

Ethylene-ethylacrylate

Ethylene-methylacrylate

Tmelt 103 – 108 C 142 – 181 C 83 C

Recommended TempRange (C: Compression) (I:Injection, E:Extrusion)

C: 200-300FI: 300F – 430FE: 300F – 380F

I: 365F – 480FE: 365F – 480F

C: 200 – 300FI: 250F – 500F

E: 300F – 620F

Molding Pressure 1 – 20 psi 1 – 20 psi

Mold (linear) shrinkage(in/in)

0.007 – 0.035 0.015 – 0.035

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Processing of Polymers

• Thermoplastics– injection molding, extrusion, blow molding,

thermoforming, rotational molding, compression molding

• Thermosets– compression molding, reaction injection molding, resin

transfer molding, casting, hand layup, etc.

• Elastomers– compression molding, extrusion, injection molding,

casting.

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Compression Molding Process• Materials

•Thermosets: Polyester, Vinyl ester, or Epoxy resins with glass fiber •Sheet Molding Compound (SMC), Bulk Molding Compound (BMC)

•Thermoplastics: Polypropylene, polyester, or others with glass fibers•Glass Mat Thermoplastic (GMT), thermoplastic BMC

•Elastomers: Thermoplastic or Thermoset rubbers•Thermoplastic Olefin (TPO), Thermoplastic Elastomer (TPE), Thermoplastic Rubber (TPR)•Thermoset Styrene Butidiene Rubber

Thermoplastic:Heat Plastic

prior to molding

Thermosets:Heat Mold

during molding

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Injection Molding Process and Cycle Time

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Extruder Equipment• Exit zone- die

– die imparts shape on the material, e.g., rod, tube, sheet, channel– exit material is called extrudate– extrudate swells at end of die due to normal forces from the polymer flow, called die swell

• Cooling zone– water bath or air cooled to lower the temperature below Tg

• Auxiliary equipment – puller– rollers for proper thickness– Wind-up or cut off

Die Swell