Critical Reviews in Oral Biology & Medicine, Vol 7, 23-35, Copyright © 1996 by International & American Associations for Dental Research
Setting reactions in dental amalgam. Part 2. The kinetics of amalgamation
T. Okabe and R. J. Mitchell
Department of Biomaterials Science, Baylor College of Dentistry, Dallas, Texas 75246, USA.
The literature on the setting mechanisms of dental amalgams made from
powders of silver-rich alloys of tin and/or copper has been critically
reviewed. Part 2 is a review of the kinetics of the reactions that convert
the mixture of alloy powder and liquid mercury to hardened amalgam
containing the phases and microstructures described in Part 1. It is
emphasized that amalgamation is a non-equilbrium process in which hardened
microstructures are determined as much by kinetics as by chemistry. The
setting reaction begins with dissolution of silver and tin into liquid
mercury; most of the product phases precipitate in the liquid mercury. The
processes that produce supersaturation in the liquid mercury and the
subsequent nucleation and growth of solid phases are considered. Mass
balance relationships that provide insight into the factors that control
the volume fraction of the undesirable gamma 2 Sn-Hg phase are described.
The nucleation and growth of eta' Cu-Sn crystals are also discussed; it is
found that these crystals nucleate on copper-rich phases and grow into the
liquid mercury. Finally, aspects of the setting reaction that are
controlled by intergranular and interphase diffusion in the solid are
discussed. These aspects include: the supersaturation of silver and tin
within the liquid mercury, nucleation and growth of the beta 1 Ag-Hg phase
in the surfaces of alloy particles, and the decomposition of initially
formed gamma 2 Sn-Hg.
