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

Acrylic Denture Bases

Use - used to support artificial teeth

Classification
a. PMMA/MMA dough systems
b. PMMA/MMA pour resin systems

1. Components

a. Powder-PMMA polymer, peroxide initiator, and pigments
b. Liquid-MMA monomer, hydroquinone inhibitor, and cross-linking agents

2. Reaction
a. Heat (or chemicals) is used as an accelerator to decompose peroxide into free radicals
b. Free radicals initiate polymerization of MMA into PMMA
c. New PMMA is formed as a matrix around residual PMMA powder particles
d. Linear shrinkage is 5% to 7% of monomer on polymerization

3. Manipulation
a. P/L mixed to form dough or fluid resin to fill mold
b. Mold heated to start and control reaction

Suspension liners

Applications

o    Dentin lining under amalgam restorations
o    Stimulation of reparative dentin formation

Components

-Calcium hydroxide powder
-Water
-Modifiers

Manipulation

Used as W/P or pastes Paint thin film on dentin → Use forced air for 15 to 30 seconds to dry → Film is thicker (15 µm) than varnishes → Do not use on enamel or cavosurface margins

Properties

Physical

-Electrically insulating barrier
-Too thin to be thermally insulating

Chemical

-High basicity for calcium hydroxide (pH is II)
-Dissolves readily in water and should not be used at exposed cavosurface margins or gaps may form

Mechanical - weak film

Biologic - calcium hydroxide dissolves, diffuses, and stimulates odontoblasts to occlude dentin tubules below cavity preparation
 

FLUXING
To prevent oxidation of gold alloys during melting always use a reducing flux .
Boric acid & borax are used .

Acrylic Appliances

Use - space maintenance  or tooth movement for orthodontics and pediatric dentistry

1. Components

a. Powder-PMMA powder. peroxide initiator, and pigments

b. Liquid-MMA monomer, hydroquinone inhibitor, cross-linking agents, and chemical accelerators (N, N-dimethyl-p-toluidine)

2. 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

Applications

a. Dentulous impressions for casts for prosthodontics

b. Dentulous impressions for pedodontic appliances

c. Dentulous impressions for study models for orthodontics

d. Edentulous impressions for casts for denture construction