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How are hot and cold emulsion polymerization of SBR different?

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The primary difference between hot and cold emulsion polymerization of styrene-butadiene rubber (SBR) lies in the temperature at which the process is conducted, which significantly alters the structure and performance of the resulting polymer molecules.1

Hot Polymerization: Conducted at approximately 50 °C (120 °F), this method results in polymer molecules that are more branched.1
Cold Polymerization: Conducted at approximately 5 °C (40 °F), this process produces molecules that are more linear and generally possess a higher molecular weight.1

These structural differences are critical for industrial applications, particularly in the manufacturing of tires. The more linear and higher molecular weight chains produced through cold polymerization improve the rolling resistance and wear resistance of the final rubber product. In some instances, cold polymerization is extended to create very high molecular weight products, which are then mixed with heavy oil to produce "oil-extended" elastomers that offer superior wear resistance.1