6. Amalgam Flashcards
Dental amalgam is an alloy formed by the reaction on (2)
Mercury (liquid)
Powder (silver, tin, copper, other metals)
Classifications of amalgam are based on (2)
Composition
Particle shape and size
Types of compositions of amalgam (2)
Traditional
Copper-enriched
Function of silver and tin in amalgam powder (2)
Intermetallic
Gamma phase reacts with mercury liquid to form amalgam
Function of copper in amalgam powder
Increases strength and hardness
Function of zinc in amalgam powder
Scavenger during production - oxidises preferentially and slag formed/removed
Most products are now zinc free
Function of mercury in amalgam powder
Few particles - pre-amalgamated alloys - increases reaction speed
Function of mercury in amalgam liquid
Reacts with other metals Triple distilled (very pure)
Types of particles in amalgam (2)
Lathe cut
Spherical, spheroidal
Features of lathe cut amalgam particles (2)
Coarse, medium, fine
Formed by filling ingots
Features of spherical amalgam particles (2)
Range of particle sizes
Formed by spraying molten metal into inert atmosphere
Amalgam setting reaction
Ag3Sn + Hg –> Ag3Sn + Ag2Hg3 + Sn7Hg9
Amalgam setting reaction gamma
Gamma –> gamma + gamma 1 + gamma 2
Amalgam setting reaction particle types
Powder + liquid –> unreacted particles + amalgam matrix
Gamma phase shows (2)
Good strength
Good corrosion resistance
Gamma 1 shows
Good corrosion resistance
Gamma 2 shows (2)
Weak strength
Weak corrosion resistance
Effect of voids on strength and corrosion (2)
Decrease strength
Increase corrosion
Effect of removing gamma 2 on amalgam
Amalgam will be made stronger
Tensile strengths of amalgam components (4)
Gamma - 170MPa
Amalgam - 60MPa
Gamma 1 - 30MPa
Gamma 2 - 20MPa
Types of setting dimensional changes of amalgam (2)
Traditional
Modern
Features of traditional amalgam setting changes (2)
Initial contraction - solution of alloy particles in Hg
Expansion - gamma 1 crystallisation
Features of modern amalgam setting changes (2)
sSmall contraction
Solid solution of Hg in Ag3Sn
Why are amalgam materials now usually zinc free
Due to reaction of zinc with saliva/blood
Reaction of zinc with saliva/blood
Zn + H2O –> ZnO + H2