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Emulsion polymerised styrene- butadiene rubber (E- SBR) is one of the most widely used polymers in the world today. Emulsion polymerised styrene- butadiene rubber is employed in many demanding applications, which enhance the quality of life and contribute significantly to our economy and standards of living.
Effect of Viscosity on Properties of E-SBR
Emulsion polymerised styrene- butadiene rubber is commercially available in Mooney viscosities ranging from 30 to about 120 ( ML1+ 4@ 125C ). Lower Mooney viscosity emulsion polymerised styrene- butadiene rubber grades band more easily on the mill, incorporate fillers and oil more readily, show less heat generation during mixing, are calendered more easily, shrink less, give higher extrusion rates and have superior extrudate appearance than the higher Mooney viscosity grades. On the other hand, the high Mooney viscosity styrene- butadiene rubber’s have better green strength, less porosity in the vulcanizate, and accept higher filler and oil loadings.
Effect of Molecular Weight on Properties of E-SBR
As the molecular weight of the styrene- butadiene rubber increases, the vulcanizate resilience and the mechanical properties, particularly tensile strength and compression set, improve. The processability of styrene- butadiene rubber improves as its molecular weight distribution broadens. Formation of high molecular weight fractions with the increase in the average molecular weight can however, prevent improvements in the processability. This is due to the fact that the tendency for gel formation also increases at higher molecular weights.
Effect of Polymerisation Temperature on Properties of E-SBR
In addition to the polymer viscosity, polymerisation temperature also plays an important role in shaping the processability. Emulsion styrene- butadiene rubbers produced at low polymerisation temperatures have less chain branching than those produced at higher temperature. At an equivalent viscosity, cold polymerised emulsion polymerised styrene- butadiene rubber is normally easier to process than hot polymerised emulsion polymerised styrene- butadiene rubber, and this applies particularly to a better banding on mills, less shrinkage after calendering, and a superior surface of green tire compounds. Hot rubbers give better green strength because they have more chain branching.
Effect of Styrene Content on Properties of E-SBR
The styrene content of most emulsion styrene- butadiene rubber varies from 0% to 50%. The percent styrene of most commercially available grades of emulsion polymerised styrene- butadiene rubber is 23.5%. In vulcanizates of styrene- butadiene rubber, as styrene content increases, dynamic properties and abrasion resistance decrease while traction and hardness increase.
Effect of Glass Transition Temperature (Tg) on Properties of E-SBR
Polymerisation temperature also affects the microstructure of emulsion polymerised styrene- butadiene rubber. In the cold polymerised emulsion polymerised styrene- butadiene rubbers, the butadiene component has, on average, about 9% cis- 1.4, 54.5% trans- 1.4, and 13% of vinyl- 1.2 structure. At a 23.5% bound styrene level, the glass transition temperature, Tg, of styrene- butadiene rubber is about –50° C. As the styrene content in the styrene- butadiene rubber increases, the glass transition temperature also increases. Rubbers with very low Tg values are characterised by a high resilience and good abrasion resistance, but have poor wet traction. By contrast, those rubbers with high Tg, as, for instance, styrene- butadiene rubber 1721, exhibit a low resilience and poor abrasion resistance with an excellent wet traction.
Effect of Emulsifiers on Properties of E-SBR
The emulsifier remains in the rubber after coagulation can also have an influence on the processability. Rosin acid emulsifiers impart better knitting, tack and adhesion to the styrene- butadiene rubber polymer. Generally, polymers emulsified with rosin acid have better extrusion rates, slower cure rates, poorer heat resistance and can cause mould fouling and polymer discoloration. Fatty acid emulsified styrene- butadiene rubber polymers generally have less tack, faster curing, and high tensile properties. A compromise of the above properties is obtained by using a mixed rosin acid/ fatty acid emulsifier system.
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