Date of Award
Campus Access Dissertation
Harry J Ploehn
This research seeks a better understanding of mechanical reinforcement and energy dissipation in elastomers containing dispersed platelet filler particles. Specifically, we developed, validated, and characterized styrene-butadiene rubber (SBR) composites containing sulfur-functional, organosilane-modified magadiite platelets. Use of magadiite (unit cell formula Na2Si14O29,,O9H2O, abbreviated MGD), with surface chemistry similar to that of silica, gives us the opportunity to vary the filler particle shape and size while independently controlling particle/elastomer crosslinking. Rationalizing the composites¡¦ mechanical properties in terms of these variables will hopefully lead to a better understanding of reinforcement mechanisms and energy dissipation in platelet-filled elastomers. In the future, this knowledge may help us formulate platelet/elastomer composites that can circumvent some of the well-known trade-offs in tire performance.
The spacing between platelets in MGD particles is relatively small (1.54 nm) and the interlayer is hydrophilic, making MGD unreactive with Si69, the most commonly used sulfur-functional organosilane. For this reason, we first treated MGD with cetyl trimethylammonium bromide (CTAB) to make CTA-MGD. The ion exchange of CTA+ cations for Na+ expanded the interlayer spacing to 3.10 nm and rendered the interlayer more hydrophobic, making it more accessible for surface modification by organosilanes and subsequent intercalation by elastomer chains.
Starting with CTA-MGD, two organosilanes ¡V triethoxysilylpropyltetrasulfide (Si69 or TESPT) and 3-mercaptopropyltriethoxysilane (MPTES) ¡V were used to silylate the interlayer surfaces of the fully expanded CTA-MGD platelets, producing organosilane-modified MGD (OS-MGD). Pre-functionalization of MGD with silane enables us to independently control the silane graft density on MGD. High concentrations of organosilane molecules displaced most of the CTA+ cations, leading to collapse of the CTA-MGD interlayer from 3.10 nm to 2.26 nm for Si69-MGD, and to 2.15 nm for MPTES-MGD due to the replacement of bulky CTA+ with the more compact organosilanes. However, using a low initial Si69 concentration resulted in partial displacement of CTA+ cations; sufficient CTA+ cations remained in the MGD to support the interlayer at 2.88 nm.
OS-MGD samples were re-treated with CTA+ cations to evaluate the interlayer accessibility to elastomer chains. OS-MGD prepared with high initial Si69 concentrations did not intercalate very much additional CTA+, and the interlayer spacing did not increase significantly. This result suggests that Si69 effectively crosslinked the MGD platelets via a bridging configuration. However, MPTES-MGD re-treated with CTAB showed an increase in interlayer spacing from 2.15 nm to 2.66 nm. Thus MPTES-MGD platelets were not crosslinked.
Finally, these surface-modified MGD materials (CTA-MGD, Si69-MGD, and MPTES-MGD) were mixed with SBR and cured to prepare elastomer composites. For OS-MGD prepared with higher concentrations of Si69 or MPTES, SBR did not enter the MGD interlayer. These composites did not manifest significant changes in crosslink density or increased mechanical reinforcement.
However, for composites prepared with CTA-MGD (with Si69 added during the mixing stage) or Si69-MGD (prepared with low Si69 concentration, denoted l-Si69-MGD without any additional added Si69), we observed significant additional expansion of the MGD interlayer spacing due to intercalation of elastomer. These results suggest that the initial OS-MGD must be expanded by some threshold amount to permit elastomer intercalation. The grafted Si69 crosslinked with the intercalated elastomer chains, leading to an increase in effective filler concentration. Composites based on CTA-MGD and l-Si69-MGD had crosslink density values comparable or greater than that of silica/SBR. Also CTA-MGD/SBR and l-Si69-MGD/SBR composites manifested improved mechanical properties compared to silica/SBR, including both the tensile modulus at low strain (0.05%), shear modulus at moderate strain, and tensile modulus at strains up to break. The l-Si69-MGD/SBR composite appears to have a filler network similar to that of silica/SBR, as indicated by similar magnitudes of the Payne effect (decreasing complex modulus with increasing strain amplitude).
Overall, this work shows that sulfur-functionalized MGD has promise as an active filler for elastomer composites. Use of platelet shape and surface chemistry as formulation variables may give formulators additional options for achieving mechanical reinforcement, controlling energy dissipation, and circumventing performance tradeoffs.
LI, S.(2012). Reinforcement and Energy Dissipation In Platelet-Filled Elastomers. (Doctoral dissertation). Retrieved from http://scholarcommons.sc.edu/etd/581