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.
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 a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Natural Rubber
Me
Me
n
• Raw material extracted from trees
• Poly-cis-isoprene (40%) in water
–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
cis
n
trans
cis polyisoprene
Tm = 28°C, Tg = -70°C
trans polyisoprene (gutta percha)
Tm = 68°C, Tg = -70°C
Natural Rubber
• Material is processed
Me
n
cis
Me
Natural Rubber
n
cis
• Latex is then dried, sorted and smoked
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)
Cl
• Typical shoe sole compound
n
– SBR (styrene-butadiene-rubber) (100) and clay (90)
polychloroprene
or Neoprene
• Typical electrical cable cover
– polychloroprene (100), kaolin (120), FEF carbon black (15)
and mineral oil (12), vulcanization agent
N
S
S
N
S
S
N
S
S
S
N
dibenzothiazyl disulphide (MTBS)
N
H
phenyl beta-naphthylamine (PNBA)
N
S
HN
N-(propynyl)-2-(thiazole) sulfenamide
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
S8 ZnO
accelerators
145C
Sx
• Peroxide Cures: can be used on most every polymer
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
m
n
Styrene-Butadiene Block Copolymer
Tensile Strength = 3 MPa
Tensile Modulus = 130 MPa
Elongation at break 550%
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
Cl
n
polychloroprene
or Neoprene
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
n
Cl
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
Me Me
Si
O
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
Me Me
Si
O
n
Polydimethylsiloxane
Tg = -123 °C
• Si-0 replaces C-C
backbone in
• Chemically Inert
• Low conductivity
• Heat/cold resistant
• Relatively expensive
• X-linking increases
stiffness and
strength.
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.
Silicones
Me Me
Si
O
n
Vulcanization of Silicones
Acetate-Cure
Me
Si OAc
Me Me MeMe AcO OAc
Si
Si
HO
O
OH
Si
n
Me
Me
Me
Me Me
Me
HOH
Si
Si
AcO
O
OAc
Si
n
Me
-AcOH
Me
Me Me
O Si Si O Si
n
Me
Me
Me Me
Me Si Si O
Me
O
Telechelic
Hydrosilation or Platinum cure
Me Me
Si
Me
O
Si
n
Me
Me
Me
Me Me
Si
O
Si
Me
Me
Si
Me
O
m
Pt
Me H
Si
n
O
m
Me
Si
O
Si
n
Me Me
Me Me
Me Si Si O
n
Me
Me
Si
O
m
Me
Me
Si
O
m
Me
Me Me
Me
Si
Si
O
Si O
n
O
O
Me
Si
Me
O
Me
Me
Me
Si
Me
n
Me
Thermoplastic Elastomers
• Five types
– Olefinics
– Fluoropolymers
– Styrenics
– Polyurethanes
– Polyesters
• Use physical cross-links to “vulcanize”
the polymer
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
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
Olefinic Thermoplastic Elastomers: EPDM
(Ethylene-Propylene-Diene Monomer)
• Many of the properties of vulcanized elastomers
– Resiliency
– Elasticity
• More easily processed
H3C
H3C
n
H3C
– Injection molding, extrusion and other standard
thermoplastic processes
Ziegler-Natta Polymers
– Highly compatible with polyolefins
– EPDM is crosslinked very lightly and may not be capable
of being melted
7-21 MPa Ultimate Tensile
Service range: -50 °C-150 °C
100-600% elongation
Diene 0-15wt%): norbornadiene,
cyclopentadiene
Roof liners
Ground liners
Thermoplastic Elastomers: EPDM
(Ethylene-Propylene-Diene Monomer)
H3C
H3C
n
H3C
Fluoropolymer elastomers
F3C F
FF
• Terpolymers
• Viton, Dynecon, Aflas, Kalrez, Chemraz
FF
• 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%
Styrene Butadiene Rubber (SBR)
•Developed during WWII
m
•Germany under the name of BUNA-S.
•North America as GR-S,Government rubber-styrene.
n
•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
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
NC
m
n
Thermoplastic Elastomers: Spandex
DuPont sells under the trade name Lycra
hard and soft blocks in its repeat structure
Polyurethane Processing
• Polyurethane can be processed by
– Slow process: Casting or foaming, or
– Fast process: Reaction Injection Molding (RIM)
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
Santoprene specialty thermoplastic-elastomer resin
Mass loss