Swelling, compression and tribological behaviors of bentonite-modified polyacrylate-type hydrogels

Hydrogel was synthesized from acrylamide and 2-acryloylamido-2-methylpropanesulfonic acid monomers (ratio: 50/50 wt %) and crosslinked with 0.25 wt % of methylene-bisacrylamide. This hydrogel was also modified by adding 4 wt % of sodium bentonite (NB). Selected properties of the hydrogels with and without NB were investigated and compared. Their water uptake was measured gravimetrically; the compression and compression creep were assessed by dynamic-mechanical and thermo-mechanical analysis (DMA and TMA, respectively) techniques. The friction and wear of the hydrogels were determined in a shaft(metal)-on-plate(hydrogel) type testing configuration under water lubrication. The hydrogel was transparent and exhibited very high equilibrium water content (>99 wt %). The latter was less affected; however, the hydrogel became slightly more hazy after NB incorporation. The crosslink density of the hydrogels was deduced from swelling and compression tests and compared with the theoretical values. Modification by NB enhanced the ultimate compression strength and reduced the related compression strain. The compression creep response under both loading and deloading strongly depended on the level of the initial load. A very low friction coefficient (similar to 0.003) and a relatively high specific wear rate (similar to 0.05 mm(3)/N m) were registered under water lubricated sliding wear using a metallic counterpart with high surface roughness. Scanning electron microscopy combined with energy dispersive spectroscopy delivered additional information on the NB dispersion and surface structure of the hydrogels.