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Amalgam Alloy Constituents Effects

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Amalgam Alloy Constituents Effects --Amalgam Alloy Constituents Effects Amalgam alloy consists of different elements depending on the type of amalgam, but almost the three basic elements (Ag, Sn, Cu) exist, and sometimes Zn, Pd, and In. 1. Silver Silver reacts with mercury forming (Ag2Hg3) (γ1) phase which is the matrix of the dental amalgam structure and has a strong influence on both the mechanical behavior of the amalgam and its interaction with the environment. Silver increases strength and expansion, and has good corrosion resistance. 2. Tin In low copper amalgam tin reacts with mercury forming a network of (Sn7Hg) phase (γ2), the γ2 phase in the structure of the set amalgam was the principal phase undergoing corrosive breakdown. Because the γ2 phase formed a coherent network in the amalgam, the corrosion continued throughout the amalgam structure with time. γ2 phase have been eliminated by increasing the copper content, where tin reacts with copper forming η phase (Cu6Sn5) which has better properties than γ2 phase. Tin decreases strength and expansion and lengthens the setting time. 3. Copper Copper increases strength, reduces tarnish and corrosion, and reduces creep and, therefore, marginal deterioration. Copper accomplishes these effects by tying up tin, preventing the formation of gamma 2(γ2), the weakest, most tarnish- and corrosion-prone phase, and the phase with the highest creep values. In addition, it reduces creep by tying up tin and forming copper-tin (Cu6Sn5), the eta phase( η), whose crystals interlock to prevent slippage and dislocations at the grain boundaries of gamma-1( γ1) particles which is a major cause of creep in amalgam. Is added at the expense of the silver, because if Cu is substituted for Sn, the amalgam will expand excessively, and if Cu was substituted for Ag, no significant changes in dimensional change occurred. 4. Zinc Zinc is added for the benefit of the manufacturer because it prevents oxidation of the other metals in the alloy during the manufacturing process; in so doing, it keeps the alloy from turning dark. Zinc accomplishes this by combining readily with oxygen to form zinc oxide. If a zinc-containing alloy is moisture contaminated, it will result in surface blistering, internal corrosion, and a delayed expansion of up to 4% by volume beginning 3 to 5 days after the contamination and continuing for up to six months. This can lead to a reduction in strength of up to 24%. Zn + H2O ZnO + H2 high-copper amalgam alloys that contain zinc in a 1% concentration exhibit lower rates of margin fracture than do zinc-free alloys. This is due to zinc's behavior as a sacrificial anode which delays corrosion of tin in the Cu6Sn5 phase. 5. Indium The effect of indium on amalgam reduce in creep and an increase strength. Marginal breakdown was significantly less for the indium-containing alloy. The addition of indium lead to improvement of corrosion resistance. 6. Palladium Palladium-containing alloys in vitro have been found to exhibit reduced corrosion rates and in vivo have a slightly greater luster than non-palladium alloys. (in a 0.5% concentration reduce tarnish and corrosion). --Setting Reaction This is a process by which liquid Hg reacts with dental amalgam alloy particles to produce a matrix with intermetallic compounds of Hg with constituents of the alloy. 1. Low – Copper Alloys Reaction *When the low-copper is triturated, mercury diffuses into the silver-tin particles and silver and tin dissolve, to a very limited extent, into mercury. As this occurs, the particles become smaller. * Because the solubility of both silver and tin in mercury is limited (0.035 and 0.6wt% respectively), and silver is much less soluble in mercury than tin, when mercury becomes saturated with silver and tin, silver precipitates out first as silver-mercury (γ1) followed by tin in the form of tin-mercury (γ2). *The set amalgam consists of core gamma particles surrounded by a matrix of gamma 1 (γ1) and gamma 2 (γ2). Unreacted γ phase (which is the highest strength) is bound by a matrix of γ1 and γ2 phases. The above reaction is as follow: Ag3Sn + Hg Ag2Hg3 + Sn7Hg + Ag3Sn Excess γ phase γ1 Phase γ2 Phase Unreacted γ Phase 2. High-Copper Alloys Reactions The setting reaction of high copper amalgams is a little more complex than in low-copper amalgam. Its notable feature is the lack of a γ2 (Sn7Hg) phase. a. Admixed Alloys Reactions As admixed high copper amalgam is triturated, mercury diffuses into the silver-tin particles and silver and tin dissolve, to a very limited extent, into the mercury. Silver from the silver-copper eutectic particles also enters the mercury. Because of the fact that tin has a greater affinity for copper than for mercury, copper combines with tin forming a ring of Cu6Sn5 around the eutectic particles while the silver precipitates out as γ1 this means that the gamma-2 phase is reduced or eliminated. The final set amalgam consists of core gamma and silver-copper eutectic particles in a matrix of γ1. The eutectic particles are surrounded by the eta (η) phase. These reactions are illustrated in the equations (1 and 2). Ag3Sn + Ag3Cu2 + Hg Ag2Hg3 + Sn7Hg + Ag3Sn + Ag3Cu2 --(1) Excess γ Silver-Copper γ1 Phase γ2 Phase Unreacted Unreacted Phase Eutectic γ Phase Eutectic phase Ag3Cu2 + Sn7Hg Ag2Hg3 + Cu6Sn5 -------------------------(2) η (eta) phase The second reaction (equation 2) occurs at mouth temperature for 1-2 weeks and γ2 phase is thus eliminated. b. Single Composition Alloy Reaction *Since the solubility of Hg is more in Sn, the Sn on the surface of the alloy particles will be depleted by the formation of γ2 phase, while the percentage of Cu will relatively increase as a result of limited reaction with Hg. *Therefore, alloy particles are surrounded by γ1 and γ2 phases, whereas the periphery of the alloy particle becomes an eutectic alloy of Ag and Cu. As with admixed alloys, this Ag-Cu phase reacts with γ2 phase to form η phase and more γ1 phase, eliminating γ2 phase. So here the alloy particles function like Ag-Sn alloy initially providing sufficient working time and ease in manipulation. This reaction is shown in equation (3). Ag3Sn + Cu3Sn + Hg Ag2Hg3 + Cu6Sn5 + Ag3Sn + Cu3Sn ----------------(3) Excess γ Excess ε γ1 Phase η (eta) Unreacted Unreacted phase phase phase γ phase ε phase -- Properties Of Dental Amalgam Dental amalgam must have several properties to withstand the stresses and corrosive environment where it is used. 1. Strength *The strength of an amalgam restoration must be high enough to resist the biting forces of occlusion. *The strength of the amalgam depends on the phases that are present. Having more of the stronger phases results in a stronger material. *There are two types of strength are compressive strength and tensile strength. However, the tensile and shear strengths are comparatively low. Therefore, amalgam should be supported by tooth structures for clinical success in the long term. * Spherical particle alloys and copper-enriched alloys develop strength more rapidly than conventional lathe-cut materials. Fine-grain, lathe-cut products develop strength more rapidly than coarse-grain products. If the amalgam restoration is subjected to chewing or other oral forces before sufficient strength develops, it is at risk for fracture. There are many factors affecting strength of dental amalgam, which are :- 1. Particle size 2. Particle shape 3. Microstructure of amalgam 4. Porosities and voids in amalgam 5. Hg/Alloy ratio 6. Trituration time 7. Condensation pressure 8. Corrosion activity 2. Creep *Creep is a slow change in shape caused by compression due to dynamic intra-oral stresses. Creep causes amalgam to flow, such that unsupported amalgam protrudes from the margin of the cavity. These unsupported edges are weak and may be further weakened by corrosion. Creep also creates overhangs on fillings leading to food trapping and secondary decay. The gamma-2 phase in low copper amalgam is primarily responsible for the relatively high values of creep. The ADA(American Dental Association) specification No.1 allow creep up to 3%. *Creep occurs because of grain boundary sliding. η crystals on γ1 grains prevent grain boundary sliding and therefore are responsible for decreased creep values of high copper alloys. Higher creep is associated with flow of amalgam over cavity margins which are thin and easily fracture under occlusal stress ("ditched amalgam"). There are several factors affecting the creep resistance which are: 1. Microstructure of amalgam 2. Hg/Alloy ratio 3. Trituration 4. Condensation pressure 5. Delay between trituration and condensation 3. Dimensional Change *The net contraction or expansion of an amalgam is called its dimensional change. Dimensional change is negative if the amalgam contracts and positive if it expands during setting. The ADA specification No.1 requires the dimensional change be an expansion or a contraction of no more than 20 micrometers /cm. *Expansion could result in post-placement sensitivity or protrusion from the cavity, whereas contraction would leave gaps prone to leakage between the restoration and the tooth. Dimensional change is affected by many factors, such as:- 1. Components 2. Moisture contamination: In case of alloys containing Zn, if contaminated with moisture before amalgam is set, may evince delayed (or) secondary expansion. This is due to release of H2 gas within the restoration. Since the gas cannot escape out, it causes expansion of the restoration. The gas is formed as follows: Zn + H2O ZnO + H2 -------------------------(2.1) 3. Corrosion Resistance Corrosion resistance of dental amalgam is very important property, because the applied environment (human body) is very aggressive, and during the reaction between the amalgam and its environment, different corrosion products will be released (especially mercury) and enter the human body which may affect negatively the health after certain time.
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