Cold SBR

However, the final properties of the tire show some poiats of superiority over natural mbber, ie, iu higher abrasion resistance of cold SBR. Siace the polymer has a T of —45° C compared with the —72 " C of aatural mbber, it shows poorer low temperature properties. Also, siace the resdieace of SBR is only about 50%, compared to at least 70% for the aatural mbber, there is more heat budd-up with SBR. Ia fact, although it is eatirely possible to produce an ad-synthetic automobde tire, this is not the case for tmck tires because their greater mass leads to an unacceptable degree of heat budd-up (13). 

Polymerization proceeds stepwise through a train of reactors. This reactor system contributes significantly to the high degree of flexibility of the overall plant in producing different grades of rubber. The reactor train is capable of producing either cold (277-280 K, 103-206 kPa) or hot (323 K, 380-517 kPa) rubber. The cold SBR polymers, produced at the... 

SBR is the most widely used synthetic elastomer. It is an amorphous random copolymer consisting of a mixture of l.2, cis and trans isomers. Cold SBR produced at —20 C consists of 17% 1,2. 6% cis and 77% trans isomers of polybutadiene. This commercial product has a Tt of -60 C, an index of refraction of 1.534S, and a coefficient of linear expansion of 66 X 10 s cm/ cm C. Because of the high percentage of the trans isomer, it is less flexible and has a higher heat buildup, when flexed, than Hevea rubber. Although carbon black-filled or amorphous silica-filled SBR has useful physical and mechanical properties, the SBR gum rubber is inferior to Hevea rubber. 

Polymerization of emulsion SBR is started by free radicals generated by the redox system in cold SBR and by persulfate or other initiator in hot SBR. The initiators are not involved in the molecular structure of the polymers. Almost all molecules are terminated by fragments of the chain transfer agent (a mercaptan). Schematically, the molecules are RSM H. where RS is the C H S pan of a dodccyl mercaptan molecule M is the monomer involved n is the degree of polymerization, and H is a hydrogen atom formerly attached to the sulfur of a mercaptan. In the case of free-radical-initiated polymerization of butadiene, by itself to form homopolymers or with other monomers for fonn copolymers, the butadiene will be about 18% 16% fix-1.4 and 66% trms-1,4-... 

The target polymerization temperature will usually be chosen to optimize production rates or product quality. Cold SBR, which is made near S C, is an interesting case in this regard. The cold product is superior as a rubber to hot (60°C emulsion polymerization) SBR, because it contains less low-molecular-weight polymer which cannot be reinforced with carbon black. There is also less branching and more tra/) -l,4 units in the cold SBR. Hot SBR is easier to mill and extrude because of its low-molecular-weight fraction and is used mostly for adhesive applications while cold SBR, which is made mainly for tires, accounts for about 90% of all production of this polymer. 

A typical redox system for cold SBR production employs sodium formaldehyde sulfoxylate (reducing agent), a hydroperoxide (oxidant), and ferrous sulfate, plus a chelating agent or a chelated iron salt. A simplified kinetic mechanism for this redox couple follows (Wright and Tucker, 1977). 

Example 5.1 Generally, hot SBR is better suited to adhesive formulation than cold SBR. Explain. 

Styrene-butadiene rubber is the largest volume synthetic elastomer commercially available. It ean be produced by free-radical emulsion polymerization of styrene and butadiene either at 50 to 60°C (hot emulsion SBR) or at about 5°C (cold emulsion SBR). The two kinds of SBR have sigmfieantly different properties. The hot emulsion SBR process, which was developed st, leads to a more branehed polymer than the cold emulsion process. Cold SBR has a better abrasion resistance and, eonsequently, provides better tread wear and dynamic properties. 

EMULSION POLYMERIZATION Used for standard SBR. Monomer is emulsified in water with emulsifying agents. Polymerization is initiated by either decomposition of a peroxide or a peroxydisulfate. Hot SBR is initiated by free radicals generated by thermal decomposition of initiators at 50°C or higher. Cold SBR is initiated by oxidation-reduction reactions (redox) at temperatures as low as —40°C. Stjrrene content normally is 23%. Copolymer is randomly distributed. Structure of butadiene contents is about 18% ds-1,4, 65% frans-1,4, and 15-20% vinyl. 

Shortly after Word War II, the American synthetic rubber industry began production of cold SBR, from which, it was found, superior tire rubber, especially as regards tread wear, could be prepared. Subsequent studies showed that the reduction in temperature from 50 to 5°C had little or no effect on the microstructure of the polydiene units (cis-1,4 versus trans-1,4 versus 1,2), or on the comonomer composition, but did exert a marked influence on the molecular weight distribution (Table VI). It was also shown [70] that the... 

After the second world war, the United States became aware of the new developments in cold SBR production, which was superior to Bima S type rubbers, and subsequently most of the production facilities in the United States were converted to rim the cold SBR process. Cold SBR has become the standard process for emulsion-based SBR production across the globe. 

Developments in the anionic polymerization of butadiene were adopted for manufacture of solution SBR. While the emulsion process gave primarily 1,4-cis microstructure in the final product, the solution process gave a lower level of 1,4-cis level, typically around 45%. Furthermore the cis content as well as 1,2-vinyl content could be modified. In addition, better control of branching and molecular weight distribution attainable with anionic process made solution SBR suitable for tire applications, challenging the established use of cold SBR. Developments in the anionic process also led to new copolymer structures in which blocks of polybutadiene can be coupled to blocks of polystyrene, generating a imique class of polymers. Developments in SB block copolymers led to new materials which were thermoplastic in character, unlike SBR which is an elastomer. Solution-processes-based thermoplastic SB block copolymers form the basis of the transparent impact polystyrene (TIPS) as well as the other block copolymers used in plastics modification. The block copolymers of styrene and butadiene are the subject of the second part of this article. 

Cold SBR. Cold SBR is produced by the emulsion process (3-12). In the emulsion process an emulsion of monomers (styrene and butadiene) is formed in water by the help of an emulsifying agent, usually a soap (13). The monomers in the emulsion are polymerized by a water-soluble initiator fragment, a free radical generated from a hydroperoxide (3) or from an oxidation reduction process. As the free radical enters the emulsion droplet, a micelle, it polymerizes the monomers present. 

A simplified process diagram of cold SBR production facility is given in Figure 2 (19,20). The process can be divided into four imits ... 

The polymerization section consists of a series of polsrmerization reactors jacketed to remove the heat of polymerization. Initially the polsrmerization reactors are charged with deaerated water and the reactants fed to the reactor. As the pol5unerization reaction takes place, the heat generated is removed to maintain a reaction temperature of around 5°C, and hence the term cold SBR process. Once the desired level of conversion is reached in the displacement reactors, a short stopper is added to terminate the reaction. Then the imreacted monomers are removed by flashing the heated latex. All the imreacted monomers are fed into a recycle tank and returned to the start of the process (13). The latex is then fed to blending tanks to obtain the desired specification and to make the product imiform. Also, at this stage, slightly off-spec material can be blended in. 

The resultant latex contains about lO particles/cm and each particle is about 60 nm in diameter. The content of solids is typically between 20 and 25 %. The basic technology of emulsion polymerisation has remained more or less unchanged since the 1940s, when the introduction of the redox catalyst system saw the production of the so-called cold SBR . The redox system allows the production of free radicals at a low temperature of 5 °C instead of 50 C (hot SBR), resulting in a better controlled reaction and a rubber with improved mixing characteristics and better final properties. 

The second generation of SBRs appeared about 1950. These were polymerized at about 5°C using a redox initiating system and by reference to the low polymerization temperature became known as cold SBRs . Typical polymerization recipes are given in Table 6.1. 

SBR is produced by two basically different processes emulsion (ESBR) and solution (SSBR). The emulsion process can be carried out at two temperatures 5 °C (cold-emulsion) and 50-60 °C (hot-emulsion). The resulting materials have significantly different properties. Thus, cold SBR process offers better abrasion resistance and provides better head wear, while the hot emulsion SBR process gives a more... 

Tire Cord Dip. Tire cord adhesives is another important outlet for latexes. A basic formulation is given in Table 4. The latex is mainly a vinyl pyridine (VP) type. Depending on the fiber to be bonded, the VP latex can be diluted with hot SBR or nonagglomerated cold SBR latex. The dilution depends on the difficulty of bonding the different fibers. Rayon was easy to bond to the carcass and did not require VP latex. Substitution of rayon by nylon and polyester tire cords necessitated the development and use of VP latex. 

SBR is made by emulsion copolymerization of butadiene and styrene imder two different temperature conditions, at 122°F (50°C Hot SBR) and at 41°F (5 C Cold SBR). The lower molecular weight and broader molecular weight range of hot SBR favors its use in adhesives. The low molecular weight fraction provides quick stick, while the high molecular weight fraction provides shear strength. 

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