young call girls in Rajiv Chowk🔝 9953056974 🔝 Delhi escort Service
4448933.ppt
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.
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.
2. 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
8. Natural Rubber
• 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
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
– 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)
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
Sx
S8 ZnO
accelerators
145C
ROOR
145C
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.
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
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.
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.
29. Thermoplastic Elastomers
• Five types
– Olefinics
– Fluoropolymers
– Styrenics
– Polyurethanes
– Polyesters
• Use physical cross-links to “vulcanize”
the polymer
30. –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
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. • 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 Tensile
Service range: -50 °C-150 °C
100-600% elongation
Ground liners
Roof liners
Diene 0-15wt%): norbornadiene,
cyclopentadiene
Ziegler-Natta Polymers
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. •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)
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. DuPont sells under the trade name Lycra
hard and soft blocks in its repeat structure
Thermoplastic Elastomers: Spandex
40. 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
