Properties of natural rubber composites with structurally different clay intercalable surfactants

Three different surface modifications of montmorillonite (MMT) have been investigated regarding the modifier effect on the ultimate properties of natural rubber-clay nanocomposites. The reference filler was modified by natrification of the native MMT, and was subsequently modified by two alkylammonium salts to achieve exchange of Na⁺ with selected organic cations, either octadecyltrimethylammonium (ODTMA) or oleyltrimethylammonium (OLEYL). Consequently, varying concentrations of MMT fillers (1, 3, 5 and 10 phr) were mixed with the elastomeric matrix natural rubber SMR 20 aimed to melt intercalation. For both composites containing organo-modified fillers, vulcanization characteristics revealed an increase of curing rate with rising filler concentrations, while XRD diffraction indicated the formation of intercalated as well as exfoliated structures. The morphology of the natural rubber-clay composites examined by electron microscopy (TEM and SEM) supported the conclusions of the XRD results regarding the intercalation and exfoliation of the surface-modified fillers to smaller nano-size aggregates, particularly tactoids of a few layers or a few thin clay layers. Tensile strength and elongation at break, increased with rising mass content of MMT fillers in the composites. The dependences indicate certain differences in the reinforcement mechanism depending on the surface modifier; while the effect of ODTMA consists in substantial hydrophobization of the surface, it is suggested that OLEYL filler apparently participates also in vulcanization reaction by the double bond in the middle of the aliphatic chain of the ligand.