Introduction:

Styrene butadiene rubber (SBR) explains families of synthetic rubbers derived from the styrene and butadiene. Such materials have good abrasion resistance and good aging stability whenever protected by additives.  In the year 1929, E. Tchunkur and A. Bock discovered that mixtures of butadiene and styrene in a ratio of 75:25 can be copolymerized in emulsion. The styrene or butadiene ratio affects the properties of the polymer; having high styrene content, the rubbers are harder and less rubbery. In the year 1930, the first emulsion polymerized SBR termed as Buna S was made by I. G. Farben, a leading chemical industry in Germany. Buna S derives Bu from butadiene and Na from sodium and S from Styrene. 

SBR is very cost-effective and possesses no unique chemical resistance properties. This can be compounded to give very fine abrasion, wear and tensile qualities. This can be replaced for natural rubber in most of the applications having significant cost savings. Its resilience is approximately similar as natural rubber. It is a good choice whenever deciding to select a material that consists of great mechanical properties and flexibility. 

Structural formula of Styrene Butadiene Rubber:

Fig: Structural formula of Styrene Butadiene Rubber

Types of Styrene butadiene rubber (SBR):  

There are two main kinds of SBR, Emulsion SBR (E-SBR) and Solution SBR (S-SBR), based on the various manufacturing procedure. 

1) Emulsion SBR: 

Emulsion-SBR (E-SBR) produced via emulsion polymerization is initiated through free radicals. The emulsion polymerization procedure has some benefits. It is generally employed under mild reaction conditions which are tolerant to water and needs merely the absence of oxygen. The procedure is relatively robust to impurities and amenable to employing a range of functionalized and non-functionalized monomers. Additional advantages comprise the fact that emulsion polymerization provides high solids contents having low reaction viscosity and is a cost-effective method. The physical state of the emulsion (or colloidal) system makes it simple to control the procedure. Thermal and viscosity problems are much less important than in bulk polymerization. 

E-SBRs formed at low polymerization temperatures have less chain branching than those produced at higher temperature. The styrene content of most emulsion SBR differs from 0% to 50%. The percent styrene of most commercially obtainable grades of E- SBR is 23.5%.

2) Solution SBR:

Solution-SBR is produced via an anionic polymerization procedure. Polymerization is initiated through alkyl lithium compounds. Water is strictly excluded. The procedure is homogeneous (that is, all components are dissolved) that gives greater control over the procedure, allowing tailoring of the polymer.  The organolithium compound adds to one of the monomers, producing a carbanion that then adds to the other monomer, and so forth. Relative to E-SBR, S-SBR is increasingly favored as it offers enhanced wet grip and rolling resistance, which translates to greater safety and better fuel economy. SBR is produced via the copolymerization of butadiene having styrene in the approximate proportion of 3:1 by weight.

3) Emulsion SBR Polymerization process:

The raw materials needed in the polymerization of E-SBR comprise the monomers (that is, styrene and butadiene), emulsifier, water, initiator system, modifier, shortstop and a stabilizer system. The original polymerization reactions are charged out in batch reactors in which all the ingredients are loaded to the reactor and the reaction is shortstopped after it has reached the desired conversion. The present commercial productions are run constantly by feeding reactants and polymerizing via a chain of reactors before shortstopping at the desired monomers conversion.  The monomers are continuously metered to the reactor chains and emulsified by the emulsifiers and catalyst agents.

In cold polymerization, the most broadly employed initiator system is the redox reaction between chelated iron and organic peroxide by using sodium formaldehyde sulphoxide (SFS) as reducing agent (see the two equations below).

In hot polymerization, potassium Peroxodisulphate is employed as an initiator. 

Fe (II) EDTA + ROOH →Fe (III) EDTA + RO+ OH        (i) 

Fe (III) EDTA + SFS → Fe (II) EDTA                           (ii)

Mercaptan is added to furnish free radicals and to control the molecular weight distribution via terminating existing growing chains even as initiating a new chain. The thiol group acts as a chain transfer agent to prevent the molecular weight from attaining the extremely high values possible in emulsion systems. The sulphur-hydrogen bond in the thiol group is very susceptible to attack by the growing polymer radical and therefore loses a hydrogen atom via reacting with polymer radicals (see equation (iii)). The RS formed will carry on initiating the growth of a new chain (see equation (iv)). The thiol prevents gel formation and enhances the processability of the rubber. 

P+ RSH →P-H + RS              (iii)

RS+ M →RS-M                      (iv) 

Throughout polymerization, parameters like temperature, flow rate and agitation are controlled to obtain the right conversion. Polymerization is generally allowed to proceed to around 60% conversion in cold polymerization and 70% in hot polymerization before it is stopped by a shortstop agent which reacts rapidly by the free radicals. Illustrations of common shortstopping agents are sodium dimethyldithio carbamate and diethyl hydroxylamine. Once the latex is correctly shortstopped, the unreacted monomers are stripped off the latex. Butadiene is stripped via degassing the latex by means of flash distillation and reduction of system pressure. Styrene is eliminated by steam stripping the latex in a column. The latex is then stabilized by the suitable antioxidant and transferred to blend tanks. In case of oil-extended polymers or carbon black master batches, such materials are added as dispersions to the stripped latex. The latex is then transferred to finishing lines to be coagulated by sulphuric acid, sulphuric acid/sodium chloride, glue/ sulphuric acid, aluminum sulphate, or amine coagulation aid. The kind of coagulation system is chosen based on the end-use of the product. Sulphuric acid/sodium chloride is utilized for general purpose. Glue/sulphuric acid are employed for electrical grade and low water sensitivity SBR. Sulphuric acid is employed for coagulations where low-ash-polymer is needed. Amine coagulating aids are employed to enhance the coagulation efficiency and reduce production plant pollution. The coagulated crumb is then washed, dried, dewatered, baled and packaged.

Solution SBR Polymerization process:

Solution polymerized styrene-butadiene rubber is acquired by the termination-free, anionic living polymerization of styrene and butadiene, taken in both batch or continuous procedure enabling production of various dry and oil extended polymers. The procedure is first based on the purification of solvent and monomers via distillation and adsorption operations and also blanketing by dry nitrogen of all chemical mix and feed tanks, in order to make sure the lowest  level of poisons detrimental to the polymerization reaction.

Dry solvent (that is, cyclohexane or cyclopentane), styrene, initiator, butadiene and other reactants are constantly loaded to the polymerization reactor train or charged batch wise in a particular sequence to the batch polymerization reactors, based on grades to be produced. Reaction temperature control is allowed by the use of boiling reactor while employing of proper randomizing agent makes sure a complete arbitrariness of styrene by the desired level of vinyl unit.  The polymerization conditions lead to a practically complete depletion of monomers; at the end of polymerization the living chain ends are terminated via addition of substances that modify the polymer structure; therefore radial or branched or linear rubber can be obtained in order to match the needed properties.

After polymerization completion the solution is then pumped to the blend tank operating at slight pressure. Residual traces of unconverted monomers, altogether by a part of the solvent, are flash vaporized, condensed and then recycled to the wet solvent tank, whereas the concentrated polymer solution is blended in the blends tanks. The blended solution by the antioxidant agents is fed to the stripping section where the solvent is eliminated by steam distillation in the presence of a dispersing agent aimed to control the crumb size in the slurry. The crumb slurry is then pumped to the finishing unit, where the crumb is dewatered on a shaker screen, being the water partially re-circulated to the strippers and partially sent to waste water treatment. The vapors obtained from the stripping part are or else condensed and the solvent, separated from water through a decanter, is sent to the wet solvent tank. The dewatered crumbs are dried in two mechanical extruders in series, cooled by air, weighed and baled.

Applications of Styrene Butadiene Rubber:

1) The elastomer is widely employed in pneumatic tires, shoe heels and soles, gaskets and even in chewing gum. 

2) Emulsion SBR is broadly employed in coated papers, being one of the most cost-effective resins to bind pigmented coatings. 

3) It is as well employed in building applications, as a sealing and binding agent.

4) SBR can be employed to 'tank' damp rooms or surfaces.

5) It is as well employed by speaker driver manufacturers as the material for low damping rubber surrounds.

6) It is utilized in great quantities in automobile and truck tires.

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