SSBR Chain End Structure Determination & Cured Silica Formulation Properties

The continuously increasing world wide energy consumption is accompanied with a decreasing acceptance of currently dominating energy generating processes including energy derived from nuclear power, coal, oil or gas. Because there is no short or even medium term solution to replace secondary energy sources by alternative less environmental harmful solutions, energy saving technologies need to be implemented quickly.

Increasingly, national countries legislatives are enforced, which are directed towards safe and less CO₂ producing processes. Among others the steadily expanding automobile sector is committed to improve energy efficiency, e.g. to achieve specific national countries CO₂ emission targets for automobiles. Tire design and performance were proven to impact automobile fuel consumption, safety, durability and noise characteristics. Accordingly, the European Union enforced challenging targets with respect to tire rolling resistance, but also to wet grip and noise, which tire manufacturers need to comply with starting as early as 2012.

Functionalized SSBR was reported to enhance the interaction of polymer chain ends with silica and/or carbon black leading to a significant decrease of rolling resistance and heat build up in tires, correlating to a reduced vulcanizate hysteretic energy loss.

Though some remarkable improvements of rolling resistance were reported performance improvements are often the result of combinatorial experiments rather than of a systematic investigation of polymer-filler interaction on a molecular level.

Any systematic approach requires a precise understanding of the structure of those polymer bound moieties, which are formed upon SSBR modification and are identified to react with selected fillers or with other polymer chains during compounding or vulcanizate formation.

Model reactions of n-butyl lithium and of low molecular weight polybuadienyl lithium oligomers with alkyl-pyrrolidinones provided a detailed understanding of both the corresponding modified SSBR chain-end structure and of the nature of side products formed. The evaluation of the model experiments under varying reaction conditions enabled the optimization of the polymer functionalization degree. Furthermore, based on the detected polymer chain-end structure the preferred reactions of the functional moieties can be postulated under vulcanization conditions. In addition, the knowledge of the polymer chain-end structure is essential for attempts to multiply the efficient functionality at the polymer molecule, e.g. polymer backbone. Therefore, structure determination proved to be very helpful for optimization of the efficiency of Styron functionalized SSBR in tire formulations.

As one result of developmental work at Styron R&D, modified polymers are available which enable an improvement of the rolling resistance property in silica and in carbon black compound vulcanizates by up to 35% compared with 1^st generation functionalized SSBR grades at a processing index in the range of nun-functionalized SSBR in test formulations.