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RUBBER INDUSTRIES

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History• Priestley gave the name “rubber” after

observing the ability of material to rub out a pencil mark

• World War I, inferior grades of rubber produced in Germany or Russia

• 1839: Goodyear discover the cure or vulcanization of rubber with sulfur

• 1941: Japanese invasion stimulate research and manufacture of various synthetic rubbers

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Natural Rubber (NR)• Established on plantation in Malaya, Indonesia,

Liberia etc. due to freedom from insect and fungus diseases which beset the tree in its native land

• Trees require 7 yrs to reach bearing age and continue to yield for a no. of yrs

• Collection & processing of latex requires considerable labor

• Latex obtained by tapping tree to allow liquid to accumulate in small cups collected frequently to avoid putrefaction or contamination

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• Latex carried to collection station where it is strained and preservative (NH3) added

• Rubber separated usg. coagulation process which occurs when various acids and salts added together

• Rubber separates from liquid as white, doughlike mass, milled and sheeted to remove contamination and dryed

• New method to form coagulated latex into granules by cutting with rotating knives or by shear between two rollers running at different speeds

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• Granules dried in mechanical dryers in hours instead of days that air or wood-smoke dryers required

• Rubber softened before compounded with various additives

• After softening, compounding materials eg. Carbon black, sulfur or sulfur compounds, vulcanizing acelerator, antioxidant and oil are mixed

• Then rubber is shaped into desired product by extruding or molding and vulcanised

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• Vulcanization leads to cross-linked thermoset polymer that cannot be softened or melted by reheating to original melting point

• Important to control ht. of vulcanization (140oC) to obtain optimum properties in the finished rubber

• Vulcanization – chemical rxn., hence speed increases with temp. and care to ensure rxn. does not take place prematurely

• Natural rubber – cis-1,4-polyisoprene when stretched, molecule crystallize producing superior form of reinforcement

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• Both NR&SR have long threadlike molecules

• Reversible extensibility property results from randomly coiled arrangement of long polymer chains

• When extended, chains distorted but like a spring, revert to kinked arrangement upon removal of stress.

• NR contains 6 to 8% nonrubber materials and outstanding heat-buildup resistance

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Synthetic Rubber (SR)• Classified into vulcanizable and

nonvulcanizable and by chemical composition of polymer chain

• Table 36.4 – various classes and types together with accepted abbreviations

• Most widely used SR – styrene butadiene rubber (SBR)

• Rubber monomers – butadiene, styrene, acrylonitrile, chloroprene, isobutylene, isoprene, ethylene and propylene

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Monomer production• Butadiene : (Fig.36.1 & 36.2)- obtained fr. coprodt of steam cracking of

petroleum for ethylene manufacture- alternative routes - dehydrogenation of butane

or butenes in one/two steps process- Houdry process (1 step): n-butane (feedstock),

catalyst (aluminium&chromium oxides), 1,4-butadiene yield (57-63%)

- 2 steps process converts n-butane to n-butenes, undergoes oxidation-dehydrogenation catalyst plus compress air or steam converts n-butenes to 1,4-butadiene

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• Styrene : (Fig. 36.1 & 36.3)- Rubber monomer for polystyrene plastics- Predominant route via ethylbenzene made from

alkylation of C6H6 and C2H4

- Then ethylbenzene dehydrogenate to styrene over Al2Cl3, solid HPO3 or Si-Al catalyst

• Acrylonitrile :- Made from Sohio process that treats propylene

with air and ammonia in fluidised bed catalytic reactor

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Synthetic – Rubber Polymerization

• Controlling factor: dissipation of polymerization heat

• HOW???- Adjustment of reaction rate to distribute heat

generation over time- Use of refrigerants eg. Ammonia- Operation in dilute media eg. emulsion or

solutions• Quality factor: molecular weight & molecular

configuration

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Monomer Recovery

• Rubber quality and reaction rate both fall off as polymerization proceeds

• Normal to stop rxn short of complete conversion to rubber

• Recovery of unrxted monomers & purification is important

• Recovery methods: steam strippipng from aqueous latices or by distillation from solvent systems

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Coagulation And Drying

• Finishing process consists of precipitating rubber from latex emulsion or solvent solution in crumb form, dried and compressed into bale

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Packaging

• Important factor since problems involving adherence to package and contamination resulting from inadequate protection and tendency of rubber to flow and fail to retain its shape

• Most rubber wrapped in polyethylene film

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Rubber Compounding

• Pure rubber (natural/synthetic) not suitable for use

• Desirable properties of plasticity, elasticity, toughness, hardness or softness, abrasion resistance, impermeability achieved by art of compounding

• Table 36.7 – typical rubber compound• Table 36.8 – rubber processing chemicals

according to their uses

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• Vulcanizing agents usu. Sulfur compounds react with polymer to prod. Cross-linked material

• Accelerators reduce the time required for vulcanization from several hrs to few mins & contain nitrogen and sulfur

• Age resistors or antioxidants protect rubber goods from attack by oxygen and ozone in atmosphere

• Catalytic plasticizers or peptizers serve to reduce viscosity of rubber to permit easier processing

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Rubber Fabrication• Rubber may be spread, cemented, calendered,

molded, extruded, caulked, puttied, or wrapped into virtually any shape: coated on cloth, plastic or metal and sandwiched or forced into cracks

• Rubber extremely tough material• Calendering/coating: coating fabric to make

waterproof ie. rolling rubber compound into fabric on multiroll calender machines

• Molding: doughlike rubber compounds molded into virtually any shape, retained by curing compound in the mold, eg. tennis ball

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• Extruding: weatherstrip, hose, inner tubes, tire treads, gaskets, channels and many other rubber articles are fashioned by extrusion of plastic compound, automobile tire serves as an example