Natural rubber (NR) composites highly filled with nano--alumina (nano--Al2O3) modified in situ by the silane coupling agent bis-(3-triethoxysilylpropyl)-tetrasulfide (Si69) were prepared. The effects of various modification conditions and filler loading on the properties of the nano--Al2O3/NR composites were investigated. The results indicated that the preparation conditions for optimum mechanical (both static and dynamic) properties and thermal conductivity were as follows: 100 phr of nano--Al2O3, 6 phr of Si69, heat-treatment time of 5 min at 150 degrees C. Furthermore, two other types of fillers were also investigated as thermally conductive reinforcing fillers for the NR systems: (1) hybrid fillers composed of 100 phr of nano--Al2O3 and various amounts of the carbon black (CB) N330 and (2) nano--Al2O3, the particles of which are smaller than those of nano--Al2O3. The hybrid fillers had better mechanical properties and dynamic performance with higher thermal conductivity, which means that it can be expected to endow the rubber products serving under dynamic conditions with much longer service life. The smaller sized nano--Al2O3 particles performed better than the larger-sized nano--Al2O3 particles in reinforcing NR. However, the composites filled with nano--Al2O3 had lower thermal conductivity than those filled with nano--Al2O3 and badly deteriorated dynamic properties at loadings higher than 50 phr, both indicating that nano--Al2O3 is not a good candidate for novel thermally conductive reinforcing filler. POLYM. COMPOS., 37:771-781, 2016. (c) 2014 Society of Plastics Engineers
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