1. Elastomers Elastomers 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.
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.

2. Polymers World usage is 15 million metric tons (1000kg) Natural rubber
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

4. Common Elastomers

5. Mechanical Behaviour of Elastomers
X-linked elastomer

8. Natural Rubber Natural rubber in unfilled form
Raw material extracted from trees
Poly-cis-isoprene (40%) in water
cis polyisoprene
Tm = 28°C, Tg = -70°C
trans 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

9. Natural Rubber
Material is processed

10. Natural Rubber
Latex is then dried, sorted and smoked

11. 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

12. Vulcanizable Rubber Typical tire tread Typical shoe sole compound
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
dibenzothiazyl disulphide (MTBS)
dibenzothiazyl disulphide (MTBS)

13. 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

14. 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.

15. Elastomer Processing
Compounding
Banbury mixer

17. Elastomer Processing
Preforming
Molding
Dipping

18. Natural Rubber The difficulties with natural rubber Strength
Availability
Bacterial breakdown
Creep
Residual proteins = immune response

19. 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

20. Elastomers Styrene-Butadiene Block Copolymer Tensile Strength = 3 MPa
Tensile Modulus = 130 MPa
Elongation at break 550%

21. 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

22. 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

23. Silicones

24. 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.

25. Polydimethylsiloxane
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
Tg = -123 °C

26. 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.

27. Silicones

28. Vulcanization of Silicones

29. Thermoplastic Elastomers
Five types
Olefinics
Fluoropolymers
Styrenics
Polyurethanes
Polyesters
Use physical cross-links to “vulcanize” the polymer

30. 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

31. 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

32. Olefinic Thermoplastic Elastomers: EPDM (Ethylene-Propylene-Diene Monomer)
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
Ziegler-Natta Polymers
7-21 MPa Ultimate Tensile
Service range: -50 °C-150 °C
% elongation
Diene 0-15wt%): norbornadiene, cyclopentadiene
Roof liners
Ground liners

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

34. 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 copolymer
Use 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%

35. 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

36. 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

37. Thermoplastic Elastomers: Spandex
DuPont sells under the trade name Lycra
hard and soft blocks in its repeat structure

39. Polyurethane Processing
Polyurethane can be processed by
Slow process: Casting or foaming, or
Fast process: Reaction Injection Molding (RIM)

40. Polyester thermoplastics
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

41. Santoprene specialty thermoplastic-elastomer resin

42. Mass loss