Amalgam - Dental amalgam
Posted by John Doe at Dental Assistant on February 4, 2012.
Categories: Dental Materials
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AMALGAMATION PROCESSES
LOW-COPPER ALLOYS
The amalgam alloy is intimately mixed with liquid mercury to wet the surface of the particles so the reaction between liquid mercury and alloy can proceed at a reasonable rate. This mixing is called trituration. During this process, mercury diffuses into the y phase of the alloy particles and begins to react with the silver and tin portions of the particles, forming various compounds, predominantly silver-mercury and tin-mercury compounds, which depend on the exact composition of the alloy. The silver-mercury compound is Ag2Hg3 and is known as the gamma one (yt) phase, and the tin-mercury compound is Sn7.8Hg and is known as the gamma two (y^) phase, However, the silver-tin, silver-mercury, and tin-mercury phases are not pure. For example,
Ag3Sn always contains some copper and occasionally small amounts of zinc. The Ag2Hg3 dissolves small amounts (1% to 3%) of tin and Cu6Sn5 (T)'), and, similarly, Cu6Sn5 could dissolve various elements present. Therefore y, ylt and y2 are better descriptive terms of these three phases formed in dental amalgam than are the pure compounds.
While crystals of the y1 and y2 phases are being formed, the amalgam is relatively soft and easily condensable and carvable. As time progresses, more crystals of y1 and y2 are formed; the amalgam becomes harder and stronger, and is no longer condensable or carvable. The lapse of time between the end of the trituration and when the amalgam hardens and is no longer workable is called working time.
The amount of liquid mercury used to amalgamate the alloy particles is not sufficient to react with the particles completely. Therefore the set mass of amalgam contains unreacted particles. About 27% of the original Ag3Sn compound remains as unreacted particles. A simplified reaction of a low-copper amalgam alloy with mercury can be summarized in the following manner:
y (Ag3Sn) + Hg H > Y (Ag3Sn) + Y1 (Ag2Hg3) +Y2 (Sn7-8Hg)
excess unreacted
The dominating phase in a well-condensed, low-copper dental amalgam is the Ag2Hg3 (Yi) phase, which is about 54% to 56% by volume. The percentages of the y and Y2 phases are 27% to 35% and 11%) to 13%), respectively.
HIGH-COPPER ALLOYS
The main difference between the low- and high-copper amalgam alloys is not merely the percentage of copper but the effect that the higher copper content has on the amalgam reaction. The copper in these alloys is in either the silver-copper eutectic or ? (Cu3Sn) form. The proper amount of copper causes most, if not all, of the y2 phase to be eliminated within a few hours after its formation, or prevents its formation entirely. The y2 phase in amalgam is the weakest and is the most susceptible to corrosion; therefore restorations using amalgam made with insufficient copper have a shorter period of serviceability, whereas high-copper amalgams tend to have superior physical and mechanical properties.
Reaction of Mercury in an Admixed High-Copper Amalgam Alloy During trituration, mercury diffuses into the amalgam particles and dissolves. The solubility of mercury in silver, tin, and copper differs considerably. Whereas 1 mg of mercury dissolves in copper, 10 mg can dissolve in silver and 170 mg in tin, all at the same temperature. Therefore particles composed mainly of silver and tin dissolve almost all the mercury, whereas very little mercury is dissolved by the silver-copper eutectic particles. The mercury dissolved in the silver-tin particles reacts as in low-copper alloys and forms the Yi and y2 phases, leaving some silver-tin particles unreacted. In a relatively short time, however, the newly formed y2 phase (Sn7_8Hg) around the silver-tin particles reacts with silver-copper particles, forming Cu6Sn5, the eta prime Cn') phase of the copper-tin system, along with some of the y1 phase (Ag2Hg3) around the silver-copper particles. The amalgamation reaction may be simplified as follows.
The initial reaction is the same as for low-copper dental amalgam:
y (Ag3Sn) + Ag-Cu (eutectic) + Hg > Y (Ag3Sn) + yl (Ag2Hg3) + y2 (Sn7-8Hg) + Ag-Cu
And the secondary, slow solid-state reaction is:
y2 (Sn,,Hg) + Ag-Cu (eutectic) > n' (Cu6Sn5) + y, (Ag,Hg3) + Ag-Cu (eutectic)
Reaction of Mercury in a Unicomposi-tional Alloy In unicompositional alloys, too, the difference in solubility of mercury in tin, silver, and copper plays an important role. Because the solubility of mercury in tin is 170 times more than in copper and 17 times more than in silver, much more mercury dissolves and reacts
with tin than with copper or silver. Thus tin in the periphery of the particle is depleted by the formation of the Y2 phase, whereas the percentage of copper increases as a result of the limited reaction with mercury. As a result, particles of unicompositional alloys in the very early stages of setting are surrounded by Yi and y2 phases, whereas the periphery of a unicompositional alloy becomes an alloy of silver and copper. As with the admixed type of alloy, the y2 phase reacts with the silver-copper phase, forming Cu6Sn5 Cn.') and more Ag2Hg3 (y^.
The difference in the elimination of the y2 phase in an admixed and unicompositional alloy is that, in the admixed type, the y2 forms around the silver-tin particles and is eliminated around the silver-copper particles. In unicompositional alloys the particles at the beginning of the reaction function like silver-tin particles of the admixed type, providing proper working time and ease of manipulation. Later, the same particles function like the silver-copper particles of the admixed type, eliminating the y2 phase.
The unicompositional particle is composed of a very fine distribution of Ag3Sn (y) and Cu3Sn (e). The overall simplified reaction with Hg is:
y (Ag3Sn) + e (Cu3Sn) + Hg > n' (Cu6Sn5)+Yi(Ag2Hg3)
Thus the reaction of mercury with either the high-copper admixed or the unicompositional alloys results in a final reaction, with Cu6Sn5 CnO being produced rather than Sn7.8Hg (y2).
In some high-copper alloys, there may be residual y2 of less than 1%. Note that there is no definitive proof that the y2 phase ever forms, even temporarily. By the time electron micro-probe analyses can be performed, the reaction will have reached equilibrium, and the final reaction products of T)' and y1 will have already formed.
MICROSTRUCTURE OF AMALGAM
In dental applications the amount of liquid mercury used to amalgamate with the alloy particles is less than that required to complete the reaction. Thus the set amalgam mass consists of unreacted particles surrounded by a matrix of the reaction products. The reaction is principally a surface reaction, and the matrix bonds the unreacted particles together. The initial diffusion and reaction of mercury and alloy are relatively rapid, and the mass changes rapidly from a plastic consistency to a hard mass. Completion of the reaction may take several days to several weeks, which is reflected by the change in mechanical properties over this time.
Fig. 11-3 Microstructure of set dental amalsam, etched with iodine etch. A, Lathe-cut particles: A, unreacted original particle. B, Spherical alloy particles: A, original particle; B, y. (From Allen FC, Asgar K, Peyton FA: J Dent Res 44:1002,
1965.)
The microstructures of set amalgam of the low-copper, lathe-cut, and spherical types are shown in Fig. 11-3. The outlines of the unreacted alloy particles (y) are visible (A). The y1 and y2 phases in the matrix are identified by the letters B and C, respectively. Voids in each of the two samples are identified by the letter D. After the completion of the solid-state reaction in the high-copper admixed and unicompositional alloys, the microstructures show no y2 phase (Fig. 11-4).
Fig. 11-4 Microstructure of high-copper admixed (A) and spherical unicompositional (B, C) alloys. A, A is an unreacted portion of y; S is the y, phase; C is the reaction zone around the Ag-Cu eutectic particle; D is an unreacted portion of an Ag-Cu particle. C, A is an unreacted portion of a spherical unicompositional Ag-Sn-Cu particle; B is the y, phase; C is the reaction zone around an original particle.
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