An essential structure of the intermediate generated from zinc oxide and stearic acid during sulfur cross-linking reaction of isoprene rubber is proposed using time-resolved zinc K-edge X-ray absorption fine structure and infrared spectroscopies in situ. The structure is dominantly a bridging bidentate zinc/stearate complex, the molar ratio of the zinc ion to stearate and the coordination number of which are unexpectedly 1:1 and 4, respectively. Combination with a density functional calculation for identifying the intermediate predominantly suggests that its most possible structure is (Zn2(μ-O2CC17H35)2)2+(OH–)2·XY, where X and Y are water and/or a rubber segment. This intermediate has been unknown despite the long history of rubber science and technology. The newly observed zinc/stearate complex may play a role to accelerate the sulfur cross-linking reaction of rubber like an enzyme because of the high Lewis activity of the zinc ion.

High performance eco-friendly natural rubber biocomposites with various contents up to 40 parts per one hundred rubber by weight of lignin were successfully prepared from sodium lignosulfonate and natural rubber latex using the soft process.

Synchrotron in situ zinc K‐edge X‐ray absorption near‐edge structure measurements are conducted during sulfur crosslinking reactions of isoprene rubber at 140 °C. A two‐phase network formation via two different reactions is revealed in a benzothiazolesulfenamide‐accelerated curing system with zinc oxide and stearic acid. DSC measurements support this observation. The results are in good agreement with previous work using small‐angle neutron scattering, where network domains of high network‐chain density are proposed to be embedded in a mesh network. A formation of the mesh network is found to occur a little earlier than that of the network domains, and its fraction is unexpectedly observed to be larger than that calculated on the basis of a concentration of zinc stearate. The result is ascribable to amounts of zinc intermediates forming the mesh network. This study may be the first in situ evaluation for the inhomogeneous network formation during vulcanization.

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