Synthesis and Mechanical Properties of Magadiite/SBR Composites

This work investigates mechanical reinforcement in styrene-butadiene rubber (SBR) composites containing sulfur-functional, organosilane-modified magadiite, a layered silicate (Na₂Si₁₄O₂₉.9H₂O).  Use of magadiite (MGD) enables us to vary the filler shape, surface area loading (filler area per mass of SBR), and silane graft density in MGD/SBR composites.  The goal is to rationalize the composites’ crosslink density and mechanical properties in terms of these variables to help us understand reinforcement mechanisms in platelet-filled elastomers.  Because Si69 does not graft onto as-prepared Na-MGD, we first treat MGD with cetyl trimethylammonium bromide (CTAB), resulting in CTA-MGD with expanded interlayer spacing.  The interlayer surfaces of CTA-MGD platelets are silylated with either triethoxysilylpropyl-tetrasulfide (TESPT or Si69) or 3-mercaptopropyltriethoxysilane (MPTES).  Surface-modified MGD materials (CTA-MGD, Si69-MGD, and MPTES-MGD) are mixed with SBR and cured to prepare elastomer composites.  For composites prepared with CTA-MGD (with Si69 added during the mixing stage) or Si69-MGD pre-functionalized with low SI69 graft density (l-Si69-MGD), XRD reveals significant expansion of the MGD interlayer spacing due to intercalation of elastomer.  Composites based on CTA-MGD and l-Si69-MGD have crosslink density values and mechanical properties comparable to or greater than that of silica/SBR with equivalent filler loading.  Overall, this work demonstrates that we can synthesize composites with formulations substituting MGD for silica, producing MGD/SBR composites with mechanical properties comparable to those of similar silica/SBR composites.  These results are rationalized in terms of a proposed reinforcement mechanism involving expansion of the MGD interlayer gallery and intercalation of silane coupling agent and SBR molecules.