ElastomersElastomers are rubbers E < 1 GPa

1. The material must be macromolecular.

2. Must be amorphous (at least at low strains).

3. Tg must be below the operating temperature.

4. Must have low secondary forces between molecules

so as to obtain the requisite flexibility.

5. A moderate degree of crosslinking must exist to establish

an elastomeric network.

Polymers• World usage is 15 million metric tons (1000kg)

• Natural rubber is 35%• Synthetic rubber is 65%, (SBR –18%, rest is other elastomers)

• Natural rubber– 75% goes to tires, 5% automotive mechanical parts, 10% non-automotive

mechanical parts, 10% miscellaneous parts (medical and health related).

– Available as technically specified rubbers, visually inspected rubbers, and specialty rubbers.

– ASTM has 6 grades of rubber (Table I)• Six grades of coagulated technically specified natural rubber which is processed and

compacted into 34-kg blocks

– Rubber Manufacturers has further set of standards for 8 types of rubber Table II

Common Elastomers

Mechanical Behaviour of Elastomers

X-linked elastomer

QuickTime™ and aTIFF (LZW) decompressor

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QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Natural Rubber

• Raw material extracted from trees• Poly-cis-isoprene (40%) in water

cis polyisopreneTm = 28°C, Tg = -70°Ctrans polyisoprene (gutta percha)Tm = 68°C, Tg = -70°C –Natural rubber in unfilled form

• very large elastic deformations • very high resilience, •resistance to cold flow •resistance to abrasion, wear, and fatigue.

Natural rubber does not have good intrinsic resistance to sunlight, oxygen, ozone, heat aging, oils, or fuels (reactive double bond).

Vulcanizes with 4% sulfur

Me

n

Me

n

cis trans

Natural Rubber

• Material is processed

Me

n

cis

Natural Rubber

• Latex is then dried, sorted and smoked

Me

n

cis

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

Vulcanizable Rubber• Typical tire tread

– Natural rubber smoked sheet (100), – sulfur (2.5) sulfenamide (0.5), MBTS (0.1), steric 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

SS

S

N S

N

S

NS

N

dibenzothiazyl disulphide (MTBS)

NH

phenyl beta-naphthylamine (PNBA)

S

NSHN

N-(propynyl)-2-(thiazole) sulfenamide

Cl

n

polychloropreneor Neoprene

Vulcanization - Sulfur and Peroxide Chemistry

• Curative formulations are developed by trial and error. Sulfur cures provide a wide range of properties at low cost. Peroxides provide high-temperature stability and function on saturated polymers.

• Sulfur Cures: applied only to unsaturated materials

• Peroxide Cures: can be used on most every polymer

Sx S8 ZnOaccelerators 145C

ROOR 145C

Crosslinked Polymer Networks• Vulcanization, curing and crosslinking are equivalent terms

referring to the process by which individual polymer chains are transformed into a network.– Most vulcanizates have an average molecular weight of

about 4,000-10,000 in between crosslinks.

Elastomer Processing

• Compounding– Banbury mixer

Elastomer Processing

• Preforming

• Molding

• Dipping

Natural Rubber

• The difficulties with natural rubber– Strength– Availability– Bacterial breakdown– Creep– Residual proteins = immune response

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

Elastomers

Styrene-Butadiene Block CopolymerTensile Strength = 3 MPaTensile Modulus = 130 MPaElongation at break 550%

nm

Oil-Resistant Elastomers• 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– Slowly crystallizes & hardens below 10 °C– Copolymer with 2,3-dichlorobutadiene won’t crystallize

Cl

n

polychloropreneor Neoprene

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Cl

Cl

n

Cl

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

Me Me

n

Silicones Si O

Me Me

n

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.

Silicone Rubbers

• Si-0 replaces C-C backbone in

• Chemically Inert

• Low conductivity

• Heat/cold resistant

• Relatively expensive

• X-linking increases stiffness and strength.

Polydimethylsiloxane

Si O

Me Me

n

Tg = -123 °C

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.

Si O

Me Me

nSilicones

Vulcanization of Silicones

SiSi O Si

Me Me

nOH

MeMe

HO

MeMe

SiSi O Si

Me Me

nOAc

MeMe

AcO

MeMe

HOH

-AcOH

Me

SiAcO OAc

OAc

SiSi O Si

Me Me

nO

MeMe

O

MeMe

Telechelic SiSi O Si

Me Me

nMe

Me

O

Me

MeO

SiO

Me

SiSiOSi

MeMe

n

MeMe

O

MeMe

SiSi O Si

Me Me

nO

Me

Me

MeMe

SiSi O Si

Me Me

O

HMe

MeMe

m

mn

SiSi O Si

Me Me

nO

Me

Me

MeMe

SiSi O Si

Me Me

O

Me

MeMe

m

mnPt

Acetate-Cure

Hydrosilation or Platinum cure

Thermoplastic Elastomers

• Five types– Olefinics– Fluoropolymers– Styrenics– Polyurethanes – Polyesters

• Use physical cross-links to “vulcanize” the polymer

–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

Thermoplastic Elastomers

Thermoplastic Elastomers• Tri-block (or more) copolymers consisting of a ‘soft’ elastomeric

segment and two ‘hard’ amorphous blocks. – Under processing conditions, both segments are above Tg,

allowing the material to flow.– On cooling, separation of the phases into two domain types creates

physical crosslinks between molecules.

• Examples include:– polystyrene-block-polybutadiene-block-polystyrene– segmented polyurethanes - Spandex, Lycra

• Many of the properties of vulcanized elastomers– Resiliency

– Elasticity

• More easily processed– Injection molding, extrusion and other standard

thermoplastic processes

– Highly compatible with polyolefins

– EPDM is crosslinked very lightly and may not be capable of being melted

Olefinic Thermoplastic Elastomers: EPDM(Ethylene-Propylene-Diene Monomer)

7-21 MPa Ultimate TensileService range: -50 °C-150 °C100-600% elongation

Ground linersRoof linersDiene 0-15wt%): norbornadiene, cyclopentadiene

H3C

H3C

H3C

n

Ziegler-Natta Polymers

Thermoplastic Elastomers: EPDM(Ethylene-Propylene-Diene Monomer)

H3C

H3C

H3C

n

Fluoropolymer elastomers

• Terpolymers• Viton, Dynecon, Aflas, Kalrez, Chemraz• most chemically resistant of all elastomers

– resistant to acids, caustics, amines, aldehydes, steam, and salt water

• very expensive• Only available as o-rings and sheets• Amorphous

Viton: Hexaflouropropylene-vinylidene fluoride copolymerUse range: –40 to 200 °C  Excellent resistance to petroleum products and solvents. Very good high-temperature performance. Fluorocarbon elastomers make up the most widely used seals in the semiconductor industry.

Tensile Strength 12.1 MPa, Elongation 210%

F3C F F F

F F

•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

Styrene Butadiene Rubber (SBR)

nm

Oil-Resistant Elastomers

• NBR—Nitrile Butadiene Rubber– Copolymerization of butadiene and acrylonitrile– More expensive than SBR or BR– 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

n

NC

m

DuPont sells under the trade name Lycrahard and soft blocks in its repeat structure

Thermoplastic Elastomers: Spandex

Polyurethane Processing• Polyurethane can be processed by

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

Riteflex® MT9000 series of copolyester thermoplastic elastomers (TPE) are certified for use in drug delivery systems, medical devices, pharmaceutical and other healthcare applications, as well as in repeat-use, food-contact applications

Polyester thermoplastics

Santoprene specialty thermoplastic-elastomer resin

Mass loss