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Elastomeric Materials

Elastomeric Materials. Elastomeric Materials. Common characteristics; Large elastic elongation (i.e.200%) Can be stretched and then immediately return to their original length when the load was released Elastomers are sometimes called rubber or rubbery materials

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Elastomeric Materials

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  1. Elastomeric Materials

  2. Elastomeric Materials • Common characteristics; • Large elastic elongation (i.e.200%) • Can be stretched and then immediately return to their original length when the load was released • Elastomers are sometimes called rubber or rubbery materials • The term elastomer is often used interchangeably with the term rubber • Elastomers are usually thermosets (requiring vulcanization) but may also be thermoplastic (see thermoplastic elastomer).

  3. All materials have some elastic elongation • “elastic elongation = elongation of any material when that material is at its yield point” • Ceramic & metal- small elastic elongation; 2% • PE, elastic elongation; 50%

  4. Stress-strain diagram

  5. Idealized stress-strain curves for metals, conventional plastics and elastomer

  6. Diagram showing the random, natural state of elastomer when under no stress and when stressed

  7. A material may be elastomeric at room temp, however rigid at lower temp (why???) • They are amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible. • At ambient temperatures rubbers are thus relatively soft (E~3MPa) and deformable

  8. Most elastomers are crosslink. Atoms between crosslink can still move, uncoil and coil. • The long polymer chains cross-link during curing and account for the flexible nature of the material. • Without crosslink, an elastomer may be elongated beyond elastic limit, with crosslink, max. elongation is set safely within the elastic region • Crosslink density- total number of crosslink in the system (less elongation is desired, number of crosslink can be increased)

  9. Natural Rubber • Rubber tree (Hevea Braziliensis) • Natural rubber is obtained by drying a latex rubber (milk in which the butter fat component is suspended in water salution) • High temperature stability – cooking the crude natural rubber with sulphur (vulcanization) • Vulcanization creates crosslinking between rubber molecules • Natural rubber is highly elastomeric (elongation 1000% for vulcanized natural rubber) • Compared to other elastomeric materials, natural rubber shows higher tensile strength, high tear strength, high resilience, resistance to wear, etc

  10. Polymer repeating groups • Crude natural rubber was chiefly composed of cis-polyisoprena (a polymer chain with carbon carbon double bond with repeating unit) • Cis means that two pendent group (H and CH3) that are attached to the two carbons in the carbon carbon double bond • The alternate configuration where the two groups are located on the opposite side of the carbon carbon double bond is called trans • The presence of methyl group interfere the movement in polyisoprene polymer- restricted bending and twisting motion (increased stiffness, higher strength, and higher temperature stability

  11. Polyisoprene structure Cis-poliisoprena (Hevea rubber) Trans-poliisoprena (Gutta percha)

  12. Properties of cis and trans are quire different • Cis is highly elastomeric & sensitive to heat softening • Trans materials is called gutta percha, much harder than cis isoprena-used for golf balls • During vulcanization process, sulphur will react with carbon carbon double bond

  13. Synthetic Polyisoprena or Isoprene Rubber (IR) • Disruption of supplies of natural rubber during world war I and II & increase needs for elastomeric materials- needs for synthetic rubber • Synthetic polyisoprena made in early 1900s, used for tires for lightweight vehicles • Combination of cis and trans molecular forms- mixture of properties • Ziegler-Natta catalyst system was developed in 1950s, it was found that 90% pure cis-isoprena could be produced by this catalyst system • However, natural rubber is used mre extensively because of its low cost

  14. Butadiene Rubber (BR) • Synthetic rubber • Repeating units of both have a backbone of four carbon atoms including carbon carbon double bond • Polybutadiene has just two hidrogen attached to the carbon carbon double bond • Absence of methyl group in polybutadiene results in porrer strength & tear strength than would polyisoprena. Resilient is about the same. Polybutadiene has poor resistance to solvents

  15. Advantages of Polybutadiene; low cost, improvement in low temp. flexibility, compatibility with many other polymeric materials, good adhesion to metal • Butadiene monomer is added to the monomer of the other plastic – copolymer is created • Butadiene monomer + polystyrene = styrene butadiene rubber (SBR) • Bulky Styrene molecules add stiffness and intermolecular interference to butadiene while butadiene adds flexibility and toughness to styrene

  16. SBR

  17. BR

  18. Thermoplastic Elastomer (TPE) • These materials are not crosslinked, have some distinct processing advantages over traditional thermoset elastomers and physical properties of vulcanised elastomers • TPEs are able to be molded like thermoplastic (injection molding, extrusion, etc) • Thermoplastic elastomers are more temperature sensitive • Scrap and reject of these materials can be recycled-environmetal friendly behavior • Normal crosslinked polymers cannot be recycled because they don't melt. They don't melt because the crosslinks tie all the polymer chains together, making it impossible for the material to flow.

  19. Silicones, or polysiloxanes • Silicones, or polysiloxanes, are inorganic-organic polymers with the chemical formula [R2SiO]n, where R = organic groups such as methyl, ethyl, and phenyl. • These materials consist of an inorganic silicon-oxygen backbone (...-Si-O-Si-O-Si-O-...) with organic side groups attached to the silicon atoms, which are four-coordinate.

  20. Silicones, or polysiloxanes • In some cases organic side groups can be used to link two or more of these -Si-O- backbones together. By varying the -Si-O- chain lengths, side groups, and crosslinking, silicones can be synthesized with a wide variety of properties and compositions. • They can vary in consistency from liquid to gel to rubber to hard plastic. The most common type is linear polydimethylsiloxane or PDMS

  21. Silicones, or polysiloxanes • Service temperature to about 260C • Good chemical resistance, low water absorption, good electrical properties, & available in flame retardant grade • In the plumbing and automotive fields, silicone grease is often used as a lubricant. In plumbing, the grease is typically applied to O-rings in faucets and valves.

  22. In the automotive field, silicone grease is typically used as a lubricant for brake components since it is stable at high temperatures, is not water-soluble

  23. PROCESSING OF ELASTOMER Common machine used for rubber compounding:- • Banbury mixer • 2-roll mill

  24. APPLICATION OF ELASTOMER 1.0 Introduction Bearings - structural joints that are installed between a structure and its foundation. The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction.

  25. HOW THE BEARING WORKS Figure: Base-Isolated and Fixed-Base Buildings • A base isolated structure is supported by a series of bearing pads which are placed between the building and the building's foundation

  26. 2.0 How The Bearing work? As a result of an earthquake, the ground beneath each building begins to move. Each building responds with movement which tends toward the right. The building's displacement in the direction opposite the ground motion is actually due to inertia.

  27. 2.0 How The Bearing work? In addition to displacing toward the right, the un-isolated building is also shown to be changing its shape-from a rectangle to a parallelogram. –deforming The primary cause of earthquake damage to buildings is the deformation which the building undergoes as a result of the inertial forces acting upon it.

  28. 2.0 How The Bearing work? The base-isolated building retains its original, rectangular shape. It is the elastomeric bearings supporting the building that are deformed. It implies the inertial forces acting on the base-isolated building have been reduced.

  29. ELASTOMERIC BEARINGS Fig: Basic structure of rubber bearing 3.0 Elastomeric Bearings Consist of thin rubber sheets bonded onto thin steel plates and combined with an energy dissipation mechanism. The rubber sheets are vulcanized and bonded to the thin steel plates under pressure and heat. it is designed in such a way that bearing is very stiff and strong in vertical direction, but flexible in horizontal direction. Thick mounting steel plates are bonded to the bottom and top surfaces allowing the isolator to be firmly connected to the foundation below and the superstructure above.

  30. Processing Flow Chart - Seismic Rubber Bearings

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