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NATURAL & SYNTHETIC RUBBER, Poly(styrene-butadiene-styrene),…
NATURAL & SYNTHETIC RUBBER
Natural Rubber and Latex
Natural rubber (NR) is an elastic hydrocarbon polymer that naturally occurs as a milky colloidal suspension (latex) in the sap of the rubber tree
(Hevea brasiliensis)
NR has elestic properties and it undergoes long range reversible extension even if relatively small force is applied to it.
NR is a natural polymer of isoprene (2-methyl-1,3-butadiene).
NR (cis-polyisoprene) does not have a strength chain but has a coiled structure.As a result it get a elastic properties.
Isoprene
cis-1,4-poluisoprene
cis-polyisoprene
Natural rubber (NR) Latex
NR latex in Hevea brasiliensis is located in latex vessels to be founded in various parts of the tree.
NR lates is a colloid system having the rubber particles dispersed in water.
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 an lipids, is surrounded by a surface of rubbery particles and gives them negative charge, which assures the latex stablity.
Product from latex: foam rubber, gloves, glues
Modification methods: centrifugation, sedimentation, water evaporation thickening and electrodecantation.
During modification process, dry rubber content is increased in latex and impurities and non-rubbery additives are removed from rubber.
Recovering rubber from latex
Method for recovering latex involves coagulation - adding an acid such as formic acid (HCOOH), coagulation takes about 12 hours.
The coagulum )soft solid slabs) is then squeezed through a series of rolls to remove the most water and reduce thickness to about 3mm.
The sheets are then draped over wooden frames and dried for several days.
Coagulation rubber on seperation and drying gives crude (raw) rubber.
Rubber at this stage id soft, sticky and thermoplastic. Low tensile strength and low elasticity.
The properties of rubber can be dramatically altered by crosslingking the polymer chains (vulcanisation process).
Vulcanization Process
One of the most important processes for most rubber technologies.
Vulcanization process for sulphur was discovered by Charles Goodyear in 1839.
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.
Crosslinking 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.
Vulcanization of rubbers by sulfur alone is an extremely slow and inefficient process.
C=C (double bonds) is where the chemical reaction between sulfur and the rubber hydrocarbon occurs.
Each crosslink requires 40-55 sulphur atoms (no accelerator) and it takes around 6 hours at 140 degree Celcius for completion.
Accelerator increases the speed of vulcanization and permit vulcanization to proceed at lower temperature and with greater efficiency.
It also decreases the quantity of sulphur necessary for vulcanization and thus improving 'aged' properties of the rubber vulcanizates.
Sulfur (vulcanizing agent) has a limitation. Elastomers must contain chemical unsaturation (C=C double bonds) for sulfur cross linking.
Amount of sulphur used determines the extent of hardness or toughness of the cured rubber.
Chemical such as organic peroxides and metallic oxides can be used for cross linking of polymers to further enhance the eventual properties of the finished rubber item.
Sulphur vulcanization technique comprises of mixing of crude rubber with about 5-30% of sulfur (cross-linking agent) and other additives, molding (shaping) the rubber mixture and heating the mixture to 120-200 degree Celcius.
Additives : activator, accelerator, coagulants, anti-oxidants, colour pigments, surfactants, softeners (oils), anti-foaming agents, anti-tack agents.
The rubber must be shaped prior to heating stage since cross-linking makes shaping impossible.
Heating where increased temperature speeds up the vulcanization process resulting in fast and complete cross-linking.
Properties of rubber improved by vulcanization : Tensile strength, elasticity, hardness, tear strength, abrasion resistance and resistance to solvents
Thermoset Rubber
Butyl Rubber (IIR)
Copolymer
isobutylene (98%)
isoprene (2%)
Process
Polymerisation
Advantages
Excellent impermeabillity
Good Flexibilty
SBS Rubber
Characteristic
Hard rubber
Trans-polyisoprene
Process
Free radical polymerisation
Has all trans-Configuration
Characteristic
Highly zig-zag chain
Cannot be stretched
Non-elastic
Neoprene
Process
Polymerisation of chloroprene
Advantage
High tensile strength
Resilience
Oil & flame resistance
Resistance to degradation by oxygen and ozone
Disadvantage
High cost limits to special-properties applications
Uses
Manufacture of hoses
Gaskets
Shoe heels
Stoppers
Conveyor belts
Printing roles
Insulator
Nitrile Rubber (BUNA-N)
Obtained
Copolymerisation
1,3 - Butadiene
Acrylonitrile
Presence of peroxide catalyst
Characteristic
Resistance
Petrol
Lubricating oils
Organic Solvents
Uses
Making oil seals
Hoses
Tank lining
Synthetic Rubber
Definition
Thermoplastic polymer that possesses the properties of rubber
TPEs processed like thermoplastic but the applications is like elastomers (elastic)
Advantages
Ability to stretch to moderate elongations and returns to its original shape
Longer life
Better physical range
Uses
Shoe soles
Athletic foorwear
Electrical wire
Coating
Medical
application
Tubing
Conveyor belts
sheet
Film stock
Example
Thermolast
Hipex
Santropene
Kraton
Pibiflex
Arnitel
Engage
Differences between vulcanized rubber and natural rubber
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
Insoluble in most organic solvents and oxidizing agents
Can be used over a wide temperature range (-40 to 100 degree Celcius)
Natural Rubber
Thermoplastic
Soft and sticky
Low tensile strength & low elasticity
Low wear and tear resistance
Large water absorption capacity
Low resistance to abrasion
Soluble in organic solvents like ether, CCl4, petrol, etc
Can be used over a narrow temperature range (10-60 degree Celcius)
Differences between Natural Rubber & Synthetic Rubber
Natural Rubber Vs Synthetic Rubber
Natural Rubber
Extracted from rubber trees and going though process vulcanation.
High tensile strength and is resistant to fatique 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, which means is can adhere to itself as well as other 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 natural rubber, 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.
Poly(styrene-butadiene-styrene)
Polystyerene
Tough hard plastic
Give Durability
Polybutadiene
Ruberry
Give rubber-like properties
Isobutylene isoprene rubber
Synthetic Rubber
Synthetic Rubber
