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Natural and Synthetic Rubber, Elastomer :, prepared from latex - Colloidal…
Natural and Synthetic Rubber
Introduction
Rubber
The original name given to latex (natural product harvested from the rubber tree).
The term “rubber” originated from ‘rub away"
Thermoset Elastomer
(thermoset rubber)
Undergo a chemical change during processing
(vulcanization) to become permanently
insoluble and infusible.
Crosslinked rubber
Thermoplastic Elastomer
(TPE)
Rubbery materials that combine the characteristics
of rubber with the recyclability
and processing advantages of plastics.
Do not required crosslinking
Natural Rubber & Latex
Introduction
Has elastic properties - undergoes long range reversible extension
natural polymer of isoprene (2-Methyl -1, 3 – Butadiene)
Natural rubber (NR) - elastic hydrocarbon polymer
Natural rubber (cis-polyisoprene) does not have a straight chain but has a coiled structure
Natural rubber (NR) Latex
Hevea Brasiliensis - latex vessels - tapping based on cutting of the tree bark -
colloid system having the rubber particles dispersed in water
Small amounts of proteins
Resinous matters (including lipids)
Hydrocarbons and mineral substances are present in NR latex
non-rubbery matters - surrounded by a surface of rubbery particles - negative charge - latex stability.
Products (e.g. foam rubber, gloves, condoms, glues) are produced directly from latex
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
coagulation - adding an acid such as formic acid (HCOOH) - takes about 12 hours.
coagulum (soft solid slabs) - squeezed through a series of rolls which drive out water - reduce thickness to about 3 mm.
sheets are draped over wooden frames - dried in smokehouses for several days.
Coagulated rubber on separation and drying gives crude (raw) rubber.
Rubber, at this stage, is soft, sticky and thermoplastic. It has low tensile strength and low elasticity.
The properties of rubber can be dramatically altered by crosslinking the polymer chains.
Vulcanization process
Introduction
Vulcanization is a cross linking - individual molecules of rubber (polymer) - converted into a three dimensional network of interconnected polymer chains through chemical cross links.
This crosslinking produces a net-like structure - more stable elasticity than the purely electrostatic nature of the pre-vulcanization bonds.
Sulphur, the original crosslinking agent (curative), remains the most successful and economical cross linking agent even today.
Sulphur
has a limitation. The elastomers must contain chemical unsaturation (C=C double bonds) for sulfur cross linking.
Amount of sulphur - 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.
without accelerator
Vulcanization of rubbers by sulfur alone is an extremely slow and inefficient process.
The chemical reaction between sulfur and the Rubber Hydrocarbon occurs mainly at the C=C (double bonds) and each crosslink requires 40 to 55 sulphur atoms
The process takes around 6 hours at 140°C for completion, which is uneconomical by any production standards.
with accelerator
It also decreases the quantity of sulphur necessary for vulcanization and thus improving 'aged' properties of the rubber vulcanizates.
increases the speed of vulcanization and permit vulcanization to proceed at lower temperature and with greater efficiency.
Vulcanization technique
Mixing of crude rubber with about 5-30% of sulfur (cross-linking agent) and other additives such as: activator - accelerator - coagulants - anti-oxidants - color pigments, Surfactant - softeners - anti-foaming agents - anti-tack agents
Molding (shaping) the rubber mixture. The rubber must be shaped prior to heating stage since cross-linking makes shaping impossible.
Shaping processes for rubber products can be divided into four basic categories: extrusion, calendering, coating and molding & casting.
Heating the mixture to 250-400ºF (120-200ºC) - C-S bonds replace C-H bonds linking chain polyisoprene molecules. Each link is formed by one to seven sulfur atoms
Synthetic Rubber
Thermoplastic Elastomers (TPE)
Thermoplastic polymer - possesses the properties of a rubber.
processed like thermoplastics - applications of an elastomer.
the ability to stretch to moderate elongations and return to its near original shape, creating a longer life and better physical range than other materials.
athletic footwear, automotive components, 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.
Thermoset Rubber
Butyl Rubber (IIR)
Copolymer of isobutylene and isoprene - isobutylene isoprene rubber
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) - 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 = SBS its durable. Polybutadiene is rubbery = SBS its rubber-like properties.
polystyrene chains tend to clump together - the different clumps become tied together with rubbery polybutadiene chains - the ability 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
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
Elastomer :
Polymer that displays elastic properties.
Rubbery materials.
The term “elastomer” derived from ‘elastic polymer’
prepared from latex - Colloidal Solution of Rubber in Water.