C05    Write the chemical reaction of the amalgamation process for low and high copper amalgams and discuss how the different reaction products affect the final properties of the amalgam. Contrast low and high copper amalgams.

The Setting Reaction

The above pictures (a - d) show that dental amalgam alloy particles are mixed with mercury. During the mixing process silver, tin and copper atoms are dissolved in the mercury where they start forming different precipitates. These precipitates form a matrix that retain the remaining partly dissolved amalgam alloy particles. The amount of mercury needed to wet all particles and occupy the space between the particles range from 40-60% mercury by weight. In other words, the final "silver filling" after it has been condensed contains around 40-50% mercury.

Traditional Amalgams (low copper amalgams)

Ag₃Sn + Hg    ->    Ag₂Hg₃    +      Sn_7-8Hg    +     Ag₃Sn (unreacted)

The Ag₃Sn phase is the gamma phase in the silver-tin phase diagram, the Ag₂Hg₃ phase is the gamma phase in the silver-tin phase diagram, and the Sn_7-8Hg phase is the gamma phase in the tin-mercury phase diagram. To avoid confusion, the Ag₃Sn is therefore called the gamma phase, the Ag₂Hg₃ the gamma-1 phase and the Sn_7-8 the gamma-2 phase.

Below is a picture showing how the set low-copper amalgam looks

Admixed High-Copper Amalgam

Ag₃Sn    +    AgCu(eutectic)    +     Hg    ->    Ag₂Hg₃    +     Cu₆Sn₅    +    Ag₃Sn (unreacted)     +    AgCu (unreacted)

As seen from this reaction the lathe-cut Ag₃Sn powder and the spherical Ag-Cu eutectic particles are mixed with mercury. When the amalgam set, the Ag₂Hg₃ (gamma-1 phase) forms as it did when the low-copper containing amalgam also set. However, rather than forming the tin-mercury phase, the tin reacts with copper and form the Cu₆Sn₅ phase (the eta phase of the Cu-Sn phase diagram). Some of the dental amalgam alloy particles remain partly unreacted after the amalgam has set. These unreacted particle parts strengthen the amalgam.

The reason the Cu₆Sn₅ crystals reduce the creep of the high-copper amalgam is that they act as small pins (see below) and hinder the gamma-1 (Ag₂Hg₃) grains to slip when the material is loaded.

Spherical High-Copper Amalgam

Ag₃Sn  - Cu₃Sn    +    Hg    ->     Ag₂Hg₃    +    Cu₆Sn₅    +     Ag₃Sn  - Cu₃Sn (unreacted) 

As seen from this reaction the spherical particles contain both Ag₃Sn (the gamma phase of the silver-tin phase diagram) and Cu₃Sn (epsilon phase of the copper-tin phase diagram) phases, and these particles are mixed with mercury. When the amalgam set, the Ag₂Hg₃ (gamma-1 phase) forms as it did when the low-copper containing amalgam also set. Again, rather than forming the tin-mercury phase, the tin reacts with copper and form the Cu₆Sn₅ phase (the eta phase of the Cu-Sn phase diagram). Some of the dental amalgam alloy particles remain partly unreacted after the amalgam has set. These unreacted particle parts strengthen the amalgam.

The key difference between the low copper and the high copper amalgams is that the low copper amalgams contain the gamma-2 phase, which is not present in the high-copper amalgams. Instead, the high copper amalgams contain the Cu₆Sn₅ phase. Since the gamma-2 phase corrodes faster than the Cu₆Sn₅ phase, the gamma-2 phase containing low copper amalgams develop microporosities due to corrosion faster than the high copper amalgams. These porosities weaken the amalgam margins and explain why marginal defects (chipped margins) are more often seen around low copper amalgams. This explains why high-copper amalgams should be used rather than the low-copper amalgams.