Influences of molecular weight of PDDA dispersing agent on mechanical and thermal properties of natural rubber/nanosilica composites

Abstract

The aim of this thesis is to study the influence of dispersing agent on the properties of the NR/silica nanocomposite. The factor focused here were the molecular weight, chemical structure and loading of dispersing agent. There were two different types of dispersing agent including homo-PDDA and co-PDDA. For homo-PDDA, there were three different MW PDDAs: l-PDDA (MW =100,000-200,000), m-PDDA (MW = 200,000-350,000) and h-PDDA (MW = 400,000-500,000). The silica loadings were varied from 1 to 2, 3 and 4 wt% while the concentrations of dispersing agent were fixed at 1 and 3 wt% of silica. In the beginning preparation, nanosilica surface was modified by dispersing agent in aqueous suspension form. The silica-PDDA structure was generated via self-assembly electrostatic adsorption, and then blended with the NR in order to form NR/silica nanocomposite sheet. The effects of the dispersing agent types on precipitation were investigated here. The results revealed that both homo- and co-PDDA reduced the precipitation and improved the stabilization of the silica/dispersing agent colloid. However, homo-PDDA with all three molecular weights gave nearly the same median particle size and much smaller than that of co-PDDA. Compared with the pure NR, the result showed that the addition of dispersing agent clearly enhance the mechanical properties of the NR/silica nanocomposite. Comparing amongst the dispersing agent, the level of enhancement tensile and tear strength was in the order of l-PDDA > m-PDDA > h-PDDA > co-PDDA. The reason for this was not just only the reduction of silica agglomeration and increment of filler-rubber interaction but also the increase of crosslink density of the NR/silica nanocomposites. However, the onset and endset decomposition temperature of the nanocomposites was not affected by the presence of the dispersing agent. Moreover, the onset decomposition temperature was independent of the silica loading while the endset one slightly increased with increasing silica loading. The increasing amount of dispersing agent from 1 wt% to 3 wt% of silica did not enhance any properties significantly as expected. In fact, this seemed to deteriorate the properties of NR/silica nanocomposites. This was due to the formation of larger silica cluster indicated by the larger median particle size.