Determination and Calculations of Mercury Vapor Concentration and Energy Released from Freshly Condensed Dental Amalgams Having Various Copper Percentages within the Alloy

Dental amalgam is an alloy consisting of a mixture of fine metallic powder of silver, tin, zinc, copper, and a trace amount of palladium in combination with about fifty percent elemental mercury that forms a matrix phase. Dental amalgams consisting of a high-copper content are the most common types of alloys currently utilized for the restoration of decayed, broken, and fractured posterior human teeth. The present research objective was primarily to improve the material properties by determining and analyzing the amount of mercury vapor released from dental amalgam received from eight different commercial brands. The mechanical hardness of the alloys was found to increase with an increase in copper content in the amalgam. The effect of copper addition on material aging was also studied. During the release of mercury vapor, the corresponding energies associated with the release of mercury vapor from each sample were determined for each successive measurement. The results indicated that increasing the copper content of the amalgam counters the release of mercury vapor from posterior teeth and improves the hardness properties.


Introduction
Dental amalgam material has been utilized by dentists for the restoration of mainly posterior teeth in humans since about 1833, when it was introduced in the United States from England [1]. There are other dental restorative materials than amalgams being used today, but dentists choose to use dental amalgam for posterior teeth restoration due to its higher relative strength and cost-effectiveness compared to composite resin [1,2]. In 2008, the use of dental amalgam was banned in countries such as Norway, Sweden, and Denmark due to concerns regarding mercury vapor toxicity [3]. In the United States, the American Dental Association (ADA) and the Government approved amalgam's use in dentistry, as major manufacturers are motivated by sheer profit and dental amalgam's affordability to customers [4]. Dental restorative material has been known to release mercury vapor at very high levels of concentration when triturated and during subsequent condensation as part of cavity preparation in human teeth [3]. The silver in the alloy is responsible for the setting expansion and creates good atomic bonding with tin, enabling an increase in strength and resistance to corrosion within the alloy [4]. More fundamentally, the silver renders the material adapted to high surface shine when polished after placement in the tooth-prepared cavity by the dentist. The tin usually causes setting contraction and gives the alloy its malleability property. Copper improves strength, minimizes corrosion and tarnishing, reduces creep, and improves marginal leakages. Zinc is the scavenger element that reduces oxidation and enables the removal of impurities in the alloy [5,6].
In low-copper alloys, the anodic potential of the Sn 8 Hg (γ 2 phase) is greater than that of the Ag 2 Hg 3 (γ 1 phase), and this leads to corrosion and resultant distribution of the γ 2 phase; thus, forming the reaction products that develop a protective layer of oxide and hydroxide on the outer surface of the amalgam due to aging [12,13]. It has been established conclusively that this founding phenomenon helps in the deceleration of the rate of oxidation. Mercury in the Sn 8 Hg (γ 2 phase) is released either as mercury vapor or as solutes in the saliva. Since the Sn 8 Hg (γ 2 phase) occupies about 15% vol. of the mixed alloy as a continuous or near-continuous structural component, the corrosion loss of this component weakens the matrix and renders it more susceptible to either marginal breakdown or possible failure, including brittleness. Porosity, which usually exists to some extent in most amalgams, accelerates the corrosion of the material [14].
High-copper amalgam has a composition of 40-60 wt. % silver, 12-30 wt. % tin, 8.5-33 wt. % copper, 0.5-9 wt. % indium, and 1-3.5 wt. % zinc, and in some amalgam alloys, it has a trace amount of palladium (0.7 wt. %) [15]. Researchers have suggested that high-copper amalgam must have a minimum of 12 wt. % copper for the elimination of the γ 2 phase, which causes an emission of a high concentration of mercury vapor from the alloy [16][17][18][19]. The development of high-copper amalgam by adding silver-copper eutectic particles to traditional silver-tin lathe-cut particles while dispersion hardening the alloy is reported to produce improved physical properties [20]. However, these are not the result of the dispersion hardening, as the silver-copper eutectic particles were big and too spaced out to inhibit dislocation movement. Instead, these improved properties were due to the formation of Cu 6 Sn 5 η-phase [20][21][22]. The great affinity of tin for copper ensures that the γ 2 -phase is significantly reduced or partially eliminated in addition to generating visible improvements in physical properties such as increased strength, less tarnishing, corrosion resistance, and creep reduction.
Increased levels of weight percent copper present in dental amalgam alloy [23] enable the interdiffusion of copper into the elemental mercury upon trituration, thus initiating a reaction of Sn 8 Hg (γ 2 -phase). This γ 2 -phase is eliminated through the addition of copper to the alloy. Such methods of transformation occurred through changes in the Cu 3 Sn (ε-phase) [24,25]. The concept of enthalpy and the Gibbs free energy of the reaction and phase product are considered and utilized for the calculation of the energies released at each time interval during mercury vapor measurements. The two main phases that precipitate subsequent to trituration are Ag 2 Hg 3 (γ 1 -phase) and Sn 8 Hg (γ 2 -phase) [24]. Such precipitates initially coexist with the liquid mercury for a short period of time, approximately 10-15 min, and the mixture maintains a plastic consistency, which allows for the placement and shaping of the amalgam during its condensation into the tooth-prepared cavity. The metallic powder is made to be mixed in correct proportion with liquid mercury (consisting of 40-50 wt. %). During trituration, the mercury becomes supersaturated with the metallic atoms, thus leading to nucleation and the growth of the distinct phases which eventually precipitate from the alloy solution [26][27][28].
The alloy particles are manufactured in micro-cut, fine-cut, and coarse-cut particle sizes. The alloy generally develops phases in the form of binary phases, Ag-Sn (γ-phase), Ag-Sn-Cu (ternary phase), and Ag-tin-Cu-Zn (quaternary phase) [29]. The solubility of copper in the Ag 3 Sn (γ-phase) is only 1 wt. %. Therefore, excess copper forms the copper-rich phase. The amount and type of phase may vary due to the thermal processing, including the Cu 3 Sn (ε-phase) and the Cu 6 Sn 5 (ζ-phase). High-copper admixed alloy contains a eutectic microstructure of an Ag-rich γ-phase and a copper-rich γ-phase [30]. Zinc is present in the low-copper amalgam powder, and its concentration exceeds the solubility limits of 1.6 wt. % Ag-Sn (β-phase), and 5.9% subsequently forms Cu 5 Zn 8 [31,32].
During the amalgam aging process, changes in the composition and microstructural phases occur. The reaction of mercury and mercury-rich amalgam continues during the setting. High-copper amalgam produces a lesser amount of γ 2 -phase and transforms into the Cu 6 Sn 5 phase. Amalgams that have been in place for many years usually present some transformation of the Ag 2 Hg 3 (γ 1 -phase), subsequently resulting in the Ag 9 Hg 11 (β 1 -phase) [33]. This loss can occur due to three possible mechanisms:

1.
Dissolution of mercury at the amalgam surface.

2.
Evaporation from the exposed surface of the amalgam.

3.
Migration of mercury to the interface of the remaining portion of the unreacted high-copper particles [34].
Two other phase changes generally seem to manifest and are likely to occur during the aging process besides those associated with corrosion, one of which is the reaction of the deleterious Sn 8 Hg (γ 2 -phase) phase with unreacted Cu 3 Sn to form the Cu 6 Sn eta-phase [24]. Such will occur in the copper alloy if there is any Sn 8 Hg (γ 2 -phase). The other likely change is that of the Ag 2 Hg 3 (γ 1 -phase) into Ag 9 Hg 11 (β 1 -phase) [24]. This phenomenon has been observed in high-copper alloy surfaces where corrosion occurred. Diffusion of mercury frequently occurred during transformation of the alloy at room temperature during setting process. Diffusion of mercury vapor originates from the Ag 2 Hg 3 γ 1 -rich mercury phase, which regularly transforms into the Ag 9 Hg 11 β 1 phase [34][35][36][37].
This study examines the microstructural changes of the material with advanced aging. Previous research has shown that when the material slowly undergoes oxidation, the mercury vapor levels decrease as a function of time and the oxidative process [37,38]. Such factors control the material properties, such as compressive strength, ductility, hardness, corrosion resistance, creep, and mercury vapor emission [39]. The research objective was primarily to determine and analyze the amount of mercury vapor released from each brand of dental amalgam, having different copper contents, with equal time duration of vapor measurement.
In order to measure the concentration of mercury vapor from freshly condensed dental amalgams of various brands and the energy released at each stage of measurement, a Jerome J505 Mercury Vapor Analyzer (Arizona Instrument, Chandler, AZ, USA) was used during the experimental procedure. The Jerome J505 Instrument works on the principle of drawing an inflow of saturated air with particulates via a built-in pump. The air is made to flow over a gold-metallic strip at a warm temperature. Since gold has a high affinity for mercury atoms, the mercury atoms in the saturated air enter the instrument through an orifice by way of a 12-inch plastic tubing attached to the instrument [40].
The standard unit range for the Jerome J505 Mercury Vapor Analyzer is ng/m 3 (50 to 500,000), µg/m 3 (0.05 to 500), and mg/m 3 (0.00005 to 0.50000). The instrument automatically computes statistical parameters such as the variance, standard deviations, and percentage error in the data obtained. Percentage error indicated ±0.05%, each successive measurement demonstrated similarities in results, and each measurement was obtained under normal conditions [40]. In order to understand the phase purity and orientation of the Dispersalloy and Sybralloy, X-ray diffraction analysis was performed using the Bruker D8 (Billerica, MA, USA).
Each sample was polished using rough, medium, and smooth-grade Emory polishing paper and was measured under similar environmental conditions and operating speed (temperature between 20-30 • C at 1 atm). These amalgams were distributed in capsules, and inside this capsule was a thin plastic/polymer membrane that separates the metallic powder on one side from the liquid mercury on the other side. Prior to trituration, the amalgam capsule was inserted between the two vibrating prongs of the Zenith dental variable speed amalgamator, which features a torque motor, having the choice of three speed settings: high (4800 rpm), medium (4200 rpm), and low (3600 rpm). The amalgamator was connected to a 120-vold power supply. At the onset of electric power, the amalgam capsule was allowed to vibrate at this high speed for 15 s for the trituration process of mixing.
Prior to initiating measurement, the Mercury Vapor Analyzer was warmed-up for about 10 min, and a test sample reading was obtained in order to verify the accuracy of the measurements. Amalgam capsules were then condensed and prepared in uniform size. The samples were inserted into a Stony Lab 250 mL borosilicate glass (Nesconset, NY, USA). A 12-inch length of plastic tubing with a diameter of 4 mm was attached to the Stony Lab glass, while the other end was attached to the J505 instrument. At the onset of operations, saturated ambient air containing mercury atoms was drawn through a 12-inch plastic tube with a diameter of 4 mm attached to the 250 mL flask. Saturated air samples containing mercury atoms were drawn into the instrument by means of a built-in electrical pump located inside the instrument. The normal flow rate of saturated air is 1 L/min. The sample air then flows through a scrubber filter and directly into the sample cell located inside the instrument; the entrance of the sample cell is controlled by a one-way valve to prevent back-flow.
Six weeks subsequent to sample preparation, the hardness of each sample was measured using the Microvickers Hardness Tester, Model M-400-H1 (Mitutoyo, Kawasaki, Japan).
Thermodynamic calculation methods were applied in determining the energies released from each sample's stage of concentration measurement (see Equation (1) [29]. All of the measurements were obtained at room temperature during the research experiment (temperature between 20-30 • C at 1 atm).
The study also examined the microstructural changes of the material with advanced aging by analyzing the X-ray diffraction pattern obtained from both the Dispersalloy and the Sybralloy using the D8 Bruker. Table 1 below shows the results recorded from the Mercury vapor analyzer. In accordance with the results, and as demonstrated by Figure 1 below, the amalgam samples released a significant amount of mercury vapor. The Dispersalloy (11.8% Cu) released the highest concentration of mercury vapor, while Sybralloy (33% Cu) released the lowest amount of vapor. The vapor release measurement was conducted starting at zero seconds followed by an interval of 20 s up to 2800 s.

Mercury Vapour Concentration
The results from Table 1 show the decrease in mercury vapor released with the passage of time and with the increase in weight percent copper in each sample.
At the starting time (zero seconds) of the mercury vapor measurement of each alloy, the mercury vapor level for the Dispersalloy (11.8% Cu) gave a value of 846 kg/m 3 compared to the concentration value of 796 kg/m 3 for the Permite C/SDI (15.4% Cu) brand. For the same starting time for the Permite C/SDI (15.4% copper), the mercury released was much lower in value than that of the Dispersalloy amalgam. This is a verification that the amalgam alloy with the lowest percentage of copper will release the greatest concentration of mercury vapor, because the increased amount of copper added to the alloy will suppress the release of mercury vapor. Such evidence of the mercury vapor released from each amalgam alloy can be seen in the plots of Figures 1 and 2

Mercury Vapour Concentration
The results from Table 1 show the decrease in mercury vapor release passage of time and with the increase in weight percent copper in each sampl At the starting time (zero seconds) of the mercury vapor measurement o the mercury vapor level for the Dispersalloy (11.8% Cu) gave a value of 846 pared to the concentration value of 796 kg/m 3 for the Permite C/SDI (15.4% Cu the same starting time for the Permite C/SDI (15.4% copper), the mercury r much lower in value than that of the Dispersalloy amalgam. This is a verificat amalgam alloy with the lowest percentage of copper will release the greates tion of mercury vapor, because the increased amount of copper added to th suppress the release of mercury vapor. Such evidence of the mercury vapor re each amalgam alloy can be seen in the plots of Figures 1 and 2  As observed from the plot of Figure 2, both the mercury vapor levels released from the Contour (31% Cu) amalgam and the Sybralloy (33%) amalgam appear to be about the same. Both alloys have approximately the same amount of copper content in their compositions, thus, each alloy releases that amount of mercury vapor in very close proximity. For the high-copper amalgam alloys such as Contour and Sybralloy, the much-increased copper content serves to eliminate the γ 2 phase which is responsible for inducing corrosion and tarnishing within the alloy [41,42].

X-ray Diffraction Analysis
The X-ray diffraction pattern of Dispersalloy (11.8% Cu) and Sybralloy (33% Cu) shows the relative comparison of the Sn8Hg (γ 2 -phase), of which a greater amount is present in the Dispersalloy plot, as is demonstrated in Figure 1. The major phase in each the Dispersalloy (11.8% Cu) and Sybralloy (33% Cu) brands is the silver-tin (γ-phase), the strongest phase having the tallest peak. The mercury vapor is released during the γ 1 -phase (Ag 2 Hg 3 ) and mostly from Sn 8 Hg (γ 2 -phase), which is predominant in the low-copper alloy. The Sybralloy (33% Cu) from Figure 1 showed a reduction in the γ 2 -phase due to the higher weight percent copper in that alloy. Interestingly, 15% vol. of the matrix phase is composed of Sn 8 Hg (γ 2 phase), which is usually a stable phase [32]. This means that the lesser amount of copper enables strong bonding and is brooched in the Cu 3 Sn ε-phase or Cu 6 Sn 5 η-prime phase [32,33].
The X-ray diffraction image for Dispersalloy as shown in Figure 3 shows a large accumulation of the γ 2 -phase, and the X-ray diffraction for Sybralloy showed a lesser amount of the γ 2 -phase, thus indicating a lesser amount of mercury vapor released from Sybralloy than from Dispersalloy and for the other amalgam samples. Previous research has shown that the material slowly undergoes oxidation, and the mercury vapor levels decrease as a function of time and the oxidative process [37]. Such factors control the material properties, such as compressive strength, ductility, hardness, corrosion resistance, creep, and mercury vapor emission [39]. As observed from the plot of Figure 2, both the mercury vapor levels released from the Contour (31% Cu) amalgam and the Sybralloy (33%) amalgam appear to be about the same. Both alloys have approximately the same amount of copper content in their compositions, thus, each alloy releases that amount of mercury vapor in very close proximity. For the high-copper amalgam alloys such as Contour and Sybralloy, the much-increased copper content serves to eliminate the γ2 phase which is responsible for inducing corrosion and tarnishing within the alloy [41,42].

X-ray Diffraction Analysis
The X-ray diffraction pattern of Dispersalloy (11.8% Cu) and Sybralloy (33% Cu) shows the relative comparison of the Sn8Hg (γ2-phase), of which a greater amount is present in the Dispersalloy plot, as is demonstrated in Figure 1. The major phase in each the Dispersalloy (11.8% Cu) and Sybralloy (33% Cu) brands is the silver-tin (γ-phase), the strongest phase having the tallest peak. The mercury vapor is released during the γ1-phase (Ag2Hg3) and mostly from Sn8Hg (γ2-phase), which is predominant in the low-copper alloy. The Sybralloy (33% Cu) from Figure 1 showed a reduction in the γ2-phase due to the Attempts have been made to reduce the γ 2 phase by increasing the copper content in the alloy, effectively above 13% [1]. It is established that the early full strength of the amalgam is achieved within one hour of placement in the prepared tooth cavity [5]. The setting reaction of this alloy is the same as the reaction for the conventional alloy [10]. After the formation of the γ 2 phase, there is a reaction between this and the silver-copper component, leading to the formation of the copper-tin phase and γ 1 phase [10,18].
The results of the Vickers hardness measurement as shown in Table 2 above for each of the amalgam samples, showed that the hardness of the amalgam increased with the increase in copper content within each of the samples. Such phenomena are, however, independent of time. amount of the γ2phase, thus indicating a lesser amount of mercury vapor released fro Sybralloy than from Dispersalloy and for the other amalgam samples. Previous resear has shown that the material slowly undergoes oxidation, and the mercury vapor lev decrease as a function of time and the oxidative process [37]. Such factors control t material properties, such as compressive strength, ductility, hardness, corrosi resistance, creep, and mercury vapor emission [39]. Attempts have been made to reduce the γ2 phase by increasing the copper content the alloy, effectively above 13% [1]. It is established that the early full strength of the am gam is achieved within one hour of placement in the prepared tooth cavity [5]. The setti reaction of this alloy is the same as the reaction for the conventional alloy [10]. After t formation of the γ2 phase, there is a reaction between this and the silver-copper comp nent, leading to the formation of the copper-tin phase and γ1 phase [10,18].
The results of the Vickers hardness measurement as shown in Table 2 above for ea of the amalgam samples, showed that the hardness of the amalgam increased with t increase in copper content within each of the samples. Such phenomena are, howev independent of time.

Analysis of Energy Released
The energy of formation given off or generated due to the release of mercury vapor can be determined by the following thermodynamic method (Equation (1)): where: The energies given off from each of the amalgams at each stage of measurement are determined from the calculations as indicated in Equation (1). A decrease in energy is observed with time; therefore, it can be concluded that the energy is proportional to the concentration of mercury vapor released from the amalgam, regardless of the brand. Also, the higher the copper content present in the alloy, the less mercury vapor is released from the amalgam.
In accordance with the results generated from the plots, it is determined that the amalgam brands with higher copper percentages tend to release a lower concentration of mercury vapor within a shorter time interval, accounting for the steeper gradient of higher copper levels within the alloy. The energy of formation is indeed lower for the alloy having the higher copper percentage in the alloy composition.
The results of the experimental data, as listed in Table 1, give the values of the concentrations of mercury vapor as given off by various amalgam alloys. From the Dispersalloy (11.8% copper) brand of dental amalgam to the Sybralloy (33% copper), the measurement of mercury vapor shows a decrease, indicating that vapor release is directly proportional to the copper content of the alloy. Additionally, the mercury vapor decreases with an increase in time and attains a somewhat steady-state value. Such steady-state values will be further achieved due to the oxidation of the alloy with the passage of time.
Both the Contour (31% Cu) and Sybralloy (33% Cu) brands of amalgam demonstrate the lowest energy of formation, which supports the conclusion that the highest copper content in the alloy produces the least concentration of mercury vapor released from the amalgams. This research has proven the desired results of the experiment. As shown, the hardness of the amalgams increases with increasing copper percentage. The corresponding plot is shown in Figure 4, which shows a graphical representation of the phenomena.  The energy released versus the time plot was tabulated as shown in Figure 5. The energies given off from each of the amalgams were determined from calculations as shown in Equation (1). Further investigation regarding the amalgam alloy is required for designing alloys with optimum compositions. In recent years, the American public has become more concerned and ambivalent about the continued mercury vapor release from dental amalgam. Presumably, as more knowledge of this material propagates, there is a high chance that amalgam could become obsolete due to the concerns of the mercury released from such material.
The energy released versus the time plot was tabulated as shown in Figure 5. The energies given off from each of the amalgams were determined from calculations as shown in Equation (1). The energy released versus the time plot was tabulated as shown in Figure 5. The energies given off from each of the amalgams were determined from calculations as shown in Equation (1).

1.
A systematic study has been performed to determine the release of mercury vapor from the eight most common brands of dental amalgam. The release of Hg vapor in order of decreasing amount is found to be as follows: Disperesalloy Brand, Permite C/SDI, Valliant PHD, Megalloy EZ, Tytin FC, Tytin/kerr, Contour, and Sybralloy. 2. The hardness of the amalgam is inversely proportional to the mercury vapor released from the alloy (i.e., hardness increases with decreasing copper percentage).

1.
A systematic study has been performed to determine the release of mercury vapor from the eight most common brands of dental amalgam. The release of Hg vapor in order of decreasing amount is found to be as follows: Disperesalloy Brand, Permite C/SDI, Valliant PHD, Megalloy EZ, Tytin FC, Tytin/kerr, Contour, and Sybralloy.

2.
The hardness of the amalgam is inversely proportional to the mercury vapor released from the alloy (i.e., hardness increases with decreasing copper percentage). 3.
The amount of energy required for the removal of mercury atoms during vapor release decreases with increasing time duration.

5.
A new dental amalgam alloy can be achieved by possibly introducing a new metal to the existing alloy, such as titanium powder.
In brief, this study concludes that the copper content in amalgam is correlated with the amount of Hg vapor released from the alloy. The low-copper amalgam showed higher releases of Hg vapor. In order to manufacture an improved amalgam alloy, either a reduced quantity of mercury or an increased quantity of copper should be considered when designing the alloy composition, along with the introduction of new material if possible. More research and investigations need to be made into the modification of this alloy without significantly affecting its properties.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author upon reasonable request.