Properties of Amalgam.

The most important physical properties of amalgam are

• Coefficient of thermal expansion = 25-1 >ppm/ C (thus amalgams allow percolation during temperature changes)

• Thermal conductivity-high (therefore, amalgams need insulating liner or base in deep restorations)
• Flow and creep. Flow and creep are characteristics that deal with an amalgam undergoing deformation when stressed. The lower the creep value of an amalgam, the better the marginal integrity of the restoration. Alloys with high copper content usually have lower creep values than the conventional silver-tin alloys.

 Dimensional change. An amalgam can expand or contract depending upon its usage. Dimensional change can be minimized by proper usage of alloy and mercury. Dimensional change on setting, less than ± 20 (excessive expansion can produce post operative pain)

•  Compression strength. Sufficient strength to resist fracture is an important requirement for any restorative material. At a 50 percent mercury content, the compression strength is approximately 52,000 psi. In comparison, the compressive strength of dentin and enamel is 30,000 psi and 100,000 psi, respectively. The strength of an amalgam is determined primarily by the composition of the alloy, the amount of residual mercury remaining after condensation, and the degree of porosity in the amalgam restoration.
• Electrochemical corrosion produces penetrating corrosion of low-copper amalgams but only produces superficial corrosion of high copper amalgams, so they last longer

• Because of low tensile strength, enamel support is needed at margins

• Spherical high-copper alloys develop high tensile strength faster and can be polished sooner
• Excessive creep is associated with silver mercury phase of low-copper amalgams and contributes to early marginal fracture
• Marginal fracture correlated with creep and electrochemical corrosion in low-copper amalgams
• Bulk fracture (isthmus fracture) occurs across thinnest portions of amalgam restorations because  of high stresses during traumatic occlusion and/or the accumulated effects of fatigue
• Dental amalgam is very resistant to abrasion

Cement liners

Applications (if remaining dentin thickness is <0.5 mm)

o    Used for thermal insulation where cavity preparation is close to the pulp
o    Used for delivering medicaments to the pulp

•    Calcium hydroxide stimulates reparative dentin or
•    Eugenol relieves pain by desensitizing nerves
•    Used to deliver F ion to enamel and dentin

Components

o    Paste of calcium hydroxide reactant powder, ethyl toluene sulfonamide dispersant, zinc oxide filler, and zinc stearate radiopacifier
o    Paste of glycol salicylate reactant liquid, titanium dioxide filler powder, and calcium tungstenate radiopacifier

Reaction

Chemical reaction of calcium ions with salicylate to form methylsalicylate salts Moisture absorbed to allow calcium hydroxide to dissociate into ions to react with salicylate Mixture sets from outside surface to inside as water diffuses

Manipulation

Dentin should not be dehydrated or material will not setMix drop of each paste together for 5 secondsApply material to dentin and allow I to 2 minutes to set

Properties

o    Physical-good thermal and electrical insulator
o    Chemical-poor resistance to water solubility and may dissolve
o    Mechanical-low compressive strength (100 to 500 psi)
o    Biologic-releases calcium hydroxide constituents, which diffuse toward the pulp and stimulate
o    reparative dentin formation

PFM Alloys

Applications-substructures for porcelain-fused-to-metal crowns and bridges
 
Classification
o    High-gold alloys
o    Palladium-silver alloys
o    Nickel-chromium alloys

Structure

Composition
o    High-gold alloys are 98% gold. platinum. And palladium
o    Palladium-silver alloys are 50% to 60% palladium and 30 to 40% silver
o    Nickel-chromium alloys are 70% to 80% nickel and 15% chromium with other metals

Manipulation
o    Must have melting temperatures above that of porcelains to be bonded to their surface
o    More difficult to cast (see section on chromium alloys)

Properties - Physical

Except for high-gold alloys, others are less dense alloys
Alloys are designed to have low thermal expansion coefficients that must be matched to the overlying porcelain

Chemical-high-gold alloys are immune, but others passivate

Mechanical-high modulus and hardness
 

Investment Techniques 

Single step investing technique:
The investing procedure is carried out in one step either by brush technique or by vacuum technique.

a). Brush technique:
The accurate water-powder ratio is mixed under vacuum. A brush is then used to paint the wax pattern with mix then the casting ring is applied over the crucible former and the ring is filled under vibration until it is completely filled.

b). vacuum technique:
• The mix in first hand spatulated, and then with the crucible former and pattern is place, then ring is attached to the mixing bowl.
• The vacuum hose is then attached to the assembly. The bowel is inverted and the ring is filled under vacuum and vibration

Two-step investing technique:

The investing procedure is carried out in two steps:

• First, the wax pattern is painted with a thick mix andis left till complete setting, the set investment block(first cost) is immersed in water for about tenminutes . the casting ring is then applied over the crucible former and filled with the properly mixedinvestment (second coat) till the ring is completely filled and the mix is left to set.The two-step investing technique is recommendedwhenever greater amount of expansion is required. Thistechnique also minimizes the distortion of the waxpattern and provides castings with smoother surfaces.

• The investment is allowed to set for the recommendedtime (usually one-hour) then the crucible former is removed. If a metal sprue former is used, it is removedby heating over a flame to loosen it from the wax pattern. Any loose particles of investment should beblown off with compressed air should be placed in a humidor if stored overnight.
 

Impression Material

Reaction

PMMA powder makes mixture viscous for manipulation before curing. Chemical accelerators cause decomposition of benzoyl peroxide into free radicals that initiate polymerization of monomer

New PMMA is formed into a matrix that surrounds PMMA powder. Linear shrinkage of 5% to 7% during setting. but dimensions of appliances are not critical