Amalgam Flashcards
What is amalgam?
- An alloy formed by reaction of Mercury (liquid)
Silver, ting, copper and other metals (powder) in a 50:50 ratio
Give the Amalgam composition
70% Ag (Silver)
25% Sn (Tin)
3 % Cu (Copper)
1% Zn (Zinc)
1% (Mercury)
Function of Silver, Tin in powder constituents
- Intermetallic compound Ag3Sn
- Y phase
- Reacts with Hg liquid to form amalgam
Role of copper in powder to create amlagam
- increases strength and hardness
Role of Zinc in amalgam
deoxidiser, scavenger for oxygen to strengthen the amalgam
delays expansion
What are the two type of particle types amalgam can be found in? describe each of them
Lathe-cut
- Produced by machining solid ingot of alloy on a lathe
- Available as course, medium or fine grained powder
- The chippings are very stressed and very reactive to mercury - need to be heat treated in boiling water to slow the setting reaction
Spherical Particles
- Ingredients of alloy melted together and sprayed into inert atmosphere
- droplets solidify as small spherical pellets of various sizes
What needs to occur to mercury used in amalgam and why?
- Needs to be triple distilled so that surface layer of contaminants is not formed that can interact with reaction
- If dull surface then contaminated
What are the 3 phases of amalgam setting reaction?
a mix of three different phases of the amalgam left after mixing.
Y/Gamma phase = mercury + silver-tin
- silver tin reacts with liquid mercury to form amalgam. This holds gamma1 and 2 together
y has good strength and corrosion resistance
Y1/Gamma 1 phase = mercury + silver alone
- extremely strong and good corrosion resistance
Y2 = mercury + tin alone
very weak and poor corrosion resistance
however contributes to setting reaction
What can occur if there is an interaction fo Zinc with saliva or blood?
- Bubbles of H2 formed within amalgam
- pressure build up causes expansion
- Downward P cause pulpal pain
- upward P restn sits proud of surface
Give some advantages to spherical particles
less Hg required
higher tensile strength
higher early compressive strength
less sensitive to condensation
easier to carve
Give some good amalgam properties
corrosion can help to seal restoration margins
high abrasion resistance - good for posterior teeth
ok compressive strength
good handling properties and relatively fast mixing and setting time
Give some negative amalgam properties
poor biocompatibility
- toxic mercury hard to dispose of
no chemical bond to tooth surfaces
creep and viscoelasticity
poor aesthetics
high thermal conductivity and diffusivity (may need a liner)
Define Creep
Material experiences low level stress (below elastic limit stress) applied repeatedly over prolonged time period , it may flow resulting in permanent deformation
Give some benefits of new copper enriched amalgam
Higher early strength
Less creep
Higher corrosion resistance
Increased durability of margins
