Post on 17-Dec-2015
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
are needed to see this picture.
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
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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