Download Polymers: Natural and Synthetic Rubbers - Properties, Vulcanization, and Applications and more Summaries Chemistry in PDF only on Docsity!
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Rubbers
High molecular weight polymers, which can be stretched at least twice
their length and return to their original form after removing the stretched force. Because the carbon chains are not in straight chained they are in the form of coils. Rubber is used to describe a material with elastic properties. It can be both natural and synthetic. Elastomers due to inter molecular forces attains a coiled amorphous structure, when a stress is applied, deformation starts by breaking the weak inter molecular force and attains a linear ordered structure. When the stress is taken back, the material regains the coiled structure and becomes amorphous once again.
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Rubbers
Synthetic rubbers were generally referred as elastomers.
Basically an elastomer is a polymer with very weak inter molecular network structure / polymers that are joined by chemical bonds with slightly crosslinked structure. Natural rubber is thermoplastic and it becomes thermoset and/or elastomeric depending on the extent of crosslinking (Vulcanisation).
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Natural Rubbers
•Commercial source of natural rubber is from the latex of para rubber tree – Hevea Brasiliensis. •The main constituent of the rubber latex will be poly cis-1,4 isoprene (2-methlybuta-1,3- diene) which will be around 30%-40% in the form of colloidal solution in water. •It is an addition polymer, addition between molecules of isoprene takes place by 1, addition and one double bond shifts between 2&3 positions.
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Natural Rubbers
- The rubber from Havea Brazilians tree is cis isomer
- In nature, the trans form of isoprene is obtained from Gutta-percha tree (trans-1,4-polyisoprene). CIS POLY ISOPRENE n TRANS POLY ISOPRENE
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Natural Rubbers
•Latex is a suspension of microscopic natural rubber particles in aqueous medium. The surface of the latex particles are charged and this creates a force of repulsion which keeps them away from coalescing. •Rubber is from the latex mixture, by coagulating with acid (Acetic acid or Formic acid). Acid neutralizes the charge between the particles and forms a homogeneous precipitate.
- When an acid is added, the charged particles will be neutralized and the force of repulsion between the particles will be eliminated and the whole rubber particles will coalesce homogeneously.
- Strong acids will form non homogeneous material as the pH
drops too fast. Whereas weak acids will reduce the pH slowly and
so it forms homogeneous material.
- The coagulated rubber will be a soft white mass, which
undergoes creeping process to be transformed into sheets of
rubber with specific dimension.
- These sheets will be dried under smoke at a temp of 40-50°C or
dried under sun, which transform the color of the sheet from
white to amber.
- This process also makes the rubber to have long life and
degradation against microorganism
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Drawbacks of Natural Rubbers
- It is plastic in nature: It becomes soft at high temp and is too brittle at low temp so it can be used in the temperature range of 10- 0 C only
- It is weak: its tensile strength is only 200kg/cm 2
- It has large water – absorption capacity
- It is not resistant to non-polar solvents like vegetable oils, benzene
- It has little durability.
- It is attacked by oxidizing agents like nitric acid, concentrated sulphuric acid.
- When stretched to greater extent it suffers from permanent deformation
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Vulcanization of Natural Rubbers
- To overcome the drawbacks of natural rubber a process called Vulcanisation was introduced by Goodyear.
- It is a process in which raw rubber is treated with Sulfur. •Technically, the sulfur reacts with the double bond of rubber and forms crosslinked structures, which prevents the deformation at high elasticity, gives high tensile strength and increases the resistance to abrasion.
Vulcanization of Natural Rubbers
•Raw dry rubber is heated with sulfur (5-8% based on the requirement), zinc oxide (a filler, 5%) and accelerator (0.5% -1%) at 100-140°C depending on the application.
- As the sulphur quantity increases, the rubber becomes tougher. •>30% sulphur gives ebonite (vulcanite).
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Vulcanization of Natural Rubbers
- When rubber is heated with Sulphur. Its tensile strength, and resistance to swelling are Increased tremendously.
- The Sulphur atoms forms cross covalent bond with the double bond of the isoprene units bringing about excellent resistance to temperature chemical, oxidation process. •The extent of stiffness of vulcanized rubber depends on the amount of Sulphur added for example a tyre rubber may contain 3- 5% Sulphur, but a battery case rubber may contain as much as 30% Sulphur.
- Vulcanite or ebonite or Hard rubber: Contains about 32% Sulphur. It possesses no elasticity because of the cross – links. Vulcanite is tough, high abrasion-resistant and has extremely good electrical insulation properties.
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Vulcanization of Natural Rubbers
Advantages of vulcanization:
- Possesses low water absorption capacity.
- Good tensile strength and extensibility-2000kg/cm^2
- Has higher resistance to oxidation and to abrasion
- Is better electrical insulator
- Resistant to organic solvents
- Useful temperature range-40 0 to 100 0 C
- Possess low elasticity.
- It has good resilience
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Raw Natural Rubber Vulcanized Natural Rubber Soft and sticky Comparatively hard and non-sticky Low tensile strength and not very strong (200Kg/cm 2 ) High tensile strength and very strong (2000Kg/cm 2 ) Can be used over a narrow range of temperature from 10 to 60 °C Can be used over a wide range of temperature from -40 to 100 °C Low abrasion resistance High abrasion resistance Absorbs a large amount of water Absorbs a small amount of water Soluble in solvents like ether, carbon disulphide, carbon tetrachloride, petrol and turpentine Insoluble in all the usual solvents Chemical resistance is low Chemical resistance is high Tackiness is high Tackiness is low.
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Synthetic Rubber
Styrene rubber (Buna-S or Styrene Butadiene rubber SBR or GR-S)
- It’s a random copolymer of Butadiene and Styrene in presence of Na as catalyst. •The DP of Butadiene and Styrene will be 75 and 25 respectively. •With increasing styrene ratio, the stiffness gets increased and becomes harder and less rubbery.
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Synthetic Rubber
Styrene rubber (Buna-S/SBR or GR-S)
- Butadiene can be polymerized by two techniques, Emulsion and Solution polymerisation.
- Emulsion polymerisation, proceeds by free radical polymerisation.
- Solution polymerisation, the mechanism of polymerisation will be by anionic initiated by alkyl lithium compound.
- The carbanion formed will initiate the polymerisation and propagates until monomers are there.
