Thermal destruction of carbon black network structure in natural rubber vulcanizate

摘要

To investigate thermal destruction and rearrangement of the carbon black (CB) network consisting of CB aggregates in rubbery matrix, the proper heat-treatment temperature without thermal decomposition of rubber matrix were examined by using differential scanning calorimetry, dynamic mechanical measurements, and thermal expansion measurements. 383 K was chosen as the heat-treatment temperature under vacuum. The volume resistivity (P_v) of 50 phr CB-filled natural rubber vulcanizate (CB-50) increased rapidly up to a heat-treatment time of 24 h and it decreased by further heat-treatment time, whereas the P_v of CB-80 remained almost constant without depending on heating time. Three-dimensional electron microscope (3D-TEM) observations revealed that after the heat-treatment for 75 h, the average lengths of the crosslinked and the branched chains and the crosslinked points density (D _(cross)) of the CB network decreased, whereas the branched points density (D_(branch)) increased with decrease of D_(cross). After the heat-treatment, their fractions (F_(cross′s)) of crosslinked chains decreased, whereas their fractions (Fbranch′s) of the branched ones increased. Especially, F_(branch) of CB-50 became larger than that of CB-80, while the decrease of Fcross of CB-50 was almost the same as that of CB-80 by the heat-treatment. And, F_(cross) and F_(branch) of the heat-treated CB-50 were the same compositions (F _(cross) and F_(branch) = ca. 0.7 and ca. 0.3, respectively) as those of the heat-treated CB-80. It is suggested that the CB network of CB-80 is more thermal stable than that of CB-50. These results directly indicate that CB network is broken and is rearranged by a heat-treatment.