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Elastomers

Elastomers. Elastomers. Definition- materials capable of reversible change in length at operating temperatures, that is, once a load is removed the material returns to its original dimensions; a rubbery compound;

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Elastomers

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  1. Elastomers

  2. Elastomers • Definition- materials capable of reversible change in length at operating temperatures, that is, once a load is removed the material returns to its original dimensions; • a rubbery compound; • Generally amorphous thermosets with Tg below room temperature to allow full chain mobility- the restoring force in entropic; • referred to as “memory”

  3. Types • Natural and synthetic rubber-Thiokol, Buna, Neoprene • Polyurethanes • Silicone-polysiloxanes • Natural proteins-elastin

  4. 5 1 0 F i b r e R i g i d 4 1 0 P l a s t i c S t r e s s F l e x i b l e P l a s t i c 2 N / c m 3 1 0 E l a s t o m e r 2 1 0 0 1 5 6 D Strain L / L Mechanical Properties B A

  5. Elastomers have properties that can be explained by a model that shares characteristics of thermosets and thermoplastics -have all of the domains shown below

  6. Principles of Polyurethane Chemistry High molecular weight polymers based isocyanate chemistry Polyisocyanates have the following general formula: R-(N=C=O)n, n= 2-4

  7. Addition of Nucleophilic Reactants The most important reactions of isocyanates is the formation of carbamic acid derivatives (7) by the addition of components with an acidic H-atom (6) across the C=N double bond (1).

  8. OH group-containing compounds (8) are by far the most important reactants for isocyanates. They are added under mild conditions, forming carbamic esters (9). Primary alcohols, secondary alcohols, and phenols show decreasing reactivity in that order. The trivial name urethane which is used for the compound ethyl carbamate, gave the whole polyurethane chemistry its name: polyisocyanates and polyols form polyurethanes.

  9. Intermolecular Bonds in Polymers

  10. The more nucleophilic primary and secondary amines react much more vigorously with isocyanates. In this reaction, ureas are formed (11).

  11. Important Building Blocks for Polyurethanes- Isocyanates MDI or aromatic polyisocyanate O C N C H N C O 2 C H 3 C H 3 N C O O C N N C O TDI-80 N C O 2 0 % 8 0 % H12MDI O C N C H N C O 2 HDI O C N ( C H ) N C O 2 6

  12. Polyols

  13. Synthetic Scheme of Polyurethanes

  14. Typical Chain-extenders

  15. Polyurethane Structural Model

  16. Schematic Model Domain Sturcture

  17. Hard Segment Interactions affect Mechanical Performance

  18. Effect of Polyurethane MW on Mechanical Properties

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