Influence of fillers on the water sorption of composites

The main objectives of this work were to characterize the water sorption of dental composites in terms of water uptake, diffusion coefficients (D), and polymer content, and to study how these parameters are influenced by the nature of the filler and the presence of 4-META (4-methacryloxy ethyl trimellitic anhydride). Four anhydrous composites--(1) tribasic calcium phosphate (TCP 50%) and triethylene glycol dimethacrylate (TEGDM), (2) silanated lithium aluminum silicate (SS, 75%) and TEGDM, (3) barium sulfate [BaSO4 (70%) and poly(methyl methacrylate) (PMMA)], and (4) silane-coated barium glass (SBG) (75%)--and PMMA were employed in the water-sorption studies at 37 degrees C. The effect of 5% 4-META on the diffusion and uptake of water was studied at 37 degrees C. The data conformed approximately to Fick's laws of diffusion. The values of D were found to be significantly smaller with SS, suggesting an effective coupling. Polymer contents in the latter two composites were determined by incineration of the samples to constant weight. The uptake of water by the filled specimens was about two-fold that which would be expected on the basis of the PMMA content. These filled specimens took nearly twice as much water as unfilled PMMA. An additional amount of water is perhaps accommodated at the interface between the filler and PMMA matrix. The D values for water in filled specimens were considerably larger than those in the unfilled specimens. It appears that the filler-matrix interface provides paths of facile diffusion similar to grain boundary diffusion.