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Natural and Synthetic Rubber - Coggle Diagram
Natural and Synthetic Rubber
Natural Rubber
What is Natural Rubber (NR)?
NR is an elastic hydrocarbon polymer that derived from latex, a milky colloidal suspension in the sap of the rubber tree (hevea brasiliensis).
NR undergoes long range reversible extension extension even if relatively small force is applied to it.
NR (cis-polyisoprene) does not have a straight chain but has a coiled structure. As a result of this, it gets elastic property.
NR is a natural polymer of isoprene
(2-Methyl -1, 3 – Butadiene)
Natural Rubber (NR) Latex
NR latex is obtained from in latex vessels to be founded in various parts of the tree by tapping based on cutting of the tree bark by special knife under approximate angle of 30°.
Small amounts of proteins, resinous matters (including lipids), hydrocarbons and mineral substances are also present in NR latex
Part of these non-rubbery matters, mainly proteins and lipids, is surrounded by a surface of rubbery particles and gives them negative charge, which assures the latex stability.
Rubber products: foam rubber, gloves, condoms, glues
The latex is modified for these reasons to have higher dry rubber content, DRC values (minimum 60 – 65 % of rubber).
Modification methods: centrifugation, sedimentation, water evaporation thickening and electro-decantation.
During this modification process, dry rubber content is increased in latex and impurities and non-rubbery additives are removed from rubber
Recovering Rubber from Latex
NR is commonly processed through coagulation, where an acid such as formic acid is added to the latex to separate the rubber particles that takes about 12 hours.
The coagulum (soft solid slabs) is then squeezed through a series of rolls which drive out most of the water and reduce thickness to about 3 mm.
This process helps separate the rubber from latex, allowing for the recovery of rubber in a usable form.
The sheets are then draped over wooden frames and dried in smokehouses for several days. Coagulated rubber on separation and drying gives crude (raw) rubber.
Vulcanisation Process
Vulcanization also known as curing or cross-linking, is one of the most important processes for most of rubber technologies.
Vulcanization is a cross-linking process in which individual molecules of rubber (polymer) are converted into a three dimensional network of interconnected polymer chains through chemical cross links.
produces a net-like structure that gives a more stable elasticity than the purely electrostatic nature of the pre-vulcanisation bonds.
Sulphur, the original crosslinking agent (curative), remains the most successful and economical cross linking agent even today
Sulfur as vulcanizing agent has a limitation. The elastomers must contain chemical unsaturation (C=C double bonds) for sulfur cross linking.
Amount of sulphur used for vulcanization process determines the extent of hardness or toughness of the cured rubber.
Other chemicals used for cross linking of polymers are Sulfur Monochloride, Tellurium, Selenium, Thiuram accelerators, Polysulphide polymers, p-Quinonedioximes, Metallic Oxides, Organic Peroxides, Di-isocyanates, etc.
The addition of other chemicals and fillers can further enhance the eventual properties of the finished rubber item.
Sulphur Vulcanization Technique
Mixing of crude rubber with about 5-30% of sulfur (cross-linking agent) and other additives such as activator (zinc oxide, stearic acid), color pigments, softeners (oils), etc.
Molding (shaping) the rubber mixture. The rubber must be shaped prior to heating stage since cross-linking makes shaping impossible.
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Properties of Rubber Improved by Vulcanization
Tensile strength
Elasticity
Hardness
Tear strength
Abrasion resistance
Resistance to solvents
Rubber, at this stage, is soft, sticky and thermoplastic. It has low tensile strength and low elasticity.
Differences Between Vulcanized Rubber & Natural Rubber
Natural Rubber
Thermoplastic
Soft and sticky
Low tensile strength & low elasticity
Low wear and tear resistance
Large water absorption capacity
Low resistance to abrasion
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Vulcanized Rubber
Thermoset
Hard and non-sticky
High tensile strength & high elasticity
High wear and tear resistance
Low water absorption tendency
High resistance to abrasion
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Synthetic Rubber
What is Synthetic Rubber?
Synthetic rubber is produced through the polymerization of various monomers. It can be tailored to have specific properties such as resistance to heat, aging and abrasion.
Thermoplastic Elastomers (TPE)
That possesses the properties of a rubber.
TPEs are processed like thermoplastics, but their applications are those of an elastomer.
Ability to stretch to moderate elongations and return to its near original shape, creating a longer life and better physical range than other materials.
Shoe soles, athletic footwear, automotive components such as fender extensions and corner panels, insulation coating for electrical wire, tubing for medical applications, conveyor belts, sheet and film stock.
Example of TPE: Thermolast, Hipex, Copec, For Tec E, Santoprene, Termoton, Arnitel, Solprene, Engage, Hytrel, Dryflex, Mediprene, Kraton, and Pibiflex.
No tires of TPE
Thermoset Rubber
Butyl Rubber: Isobutylene Isoprene Rubber(IIR)
Copolymer of isobutylene and isoprene
Butyl rubber is produced by polymerization of ~98% of isobutylene with ~2% of isoprene.
Excellent impermeability, good flexibility
Rubber stopper for medicine vials & bottles, chewing gum, tire inner tube, used in speakers, sealant for roof repair, insulating glass window, gas masks and other protective clothing.
SBS Rubber
Poly(styrene-butadiene-styrene), or SBS, is a hard rubber that is used to make soles of shoes, tire treads, and other products where durability is important.
Polystyrene is a tough hard plastic, and this gives SBS its durability. Polybutadiene is rubbery, and this gives SBS its rubber-like properties.
The clumping of polystyrene chains in SBS, joined by rubbery polybutadiene chains, allows the material to retain its shape after being stretched.
Trans-polyisoprene
Synthetic rubber (trans-polyisoprene) was obtained by the free radical polymerization of isoprene.
The rubber so formed has all trans- Configuration.
As a result of this, synthetic rubber has a highly regular zig-zag chain which cannot be stretched. This accounts for non-elasticity of Synthetic Rubber.
Neoprene
Neoprene or polychloroprene is a synthetic rubber that are produced by polymerization of chloroprene.
DuPont marketed polychloroprene as Neoprene, a trademarked name that has since become generic
Neoprene is valued for its high tensile strength, resilience, oil and flame resistance, and resistance to degradation by oxygen and ozone; however, its high cost limits its use to special-properties applications.
Neoprene is used in the manufacture of hoses, gaskets, shoe heels, stoppers, conveyor belts and printing rollers etc. It is also used as an insulator.
Nitrile Rubber (BUNA-N)
BUNA–N is obtained by copolymerization of 1, 3 – Butadiene and acrylonitrile in presence of a peroxide catalyst.
The name BUNA–N is made up of Bu which indicates 1,3 – Butadiene, NA is for Sodium (Na) and N indicates acrylonitrile.
BUNA-N is resistant to the action of petrol, lubricating oils and organic solvents.
It is used in making oil seals, hoses, tank linings etc
Differences between Natural Rubber & Synthetic Rubber
Natural Rubber
Extracted from rubber trees and going through processing of vulcanization.
High tensile strength and is resistant to fatigue from wear such as chipping , cutting or tearing.
Moderate resistance to damage from exposure to heat, light and the ozone in the air
NR has tack, it can adhere to itself as well as others materials. It adheres particularly well to steel cord, which makes it an excellent material for use in tires.
Synthetic Rubber
Made of various monomers after the polymerization.
Better resistance to abrasion than NR, as well as superior resistance to heat and the effects of aging.
Many types of synthetic rubber are flame-resistant, so it can be used as insulation for electrical devices.
It also remains flexible at low temperatures and is resistant to grease and oil.
Natural Rubber vs Synthetic Rubber
Overall, the combined properties of natural rubber outweigh synthetic rubbers or combinations of synthetic rubbers available.
However, synthetic rubber is advantageous because it is easier to produce.
Natural rubber is a crop able to grow only in tropical climates and it doesn't age well, so for many countries it is easier to use synthetic rubber.
Synthetics may also be more useful in certain applications because of their resistance to extreme temperatures and corrosive environments.