Physical Properties Flashcards
Def of physical properties
Properties that can be changed without involving a change in the chemical composition
Don’t deal with force applications
Mass relating properties
Density
Mass per unit volume of material
Mass / volume gm/cm3
Clinical significance of density
Density is important for retention of upper denture
Non metallic denture bases are relatively lighter in weight lower in density they have higher retention in patient mouth
Melting or freezing
The temperature at which the material melts or freezes for crystalline materials.
Clinical significance of melting or freezing temperature
The melting temperature of metals and alloys is important in determining the melting machine in fabrication of indirect metallic restoration
Glass transition temperature
This property is characteristic to non-crystalline structures as polymers & waxes (gradually soften as their temperature is raised) melting range.
The temp at which a sharp increase in the coefficient of thermal expansion occurs
Temp at which the material begins to soften on heating or solidify upon cooling
Thermal conductivity
It is the quantity of heat in calories or joules, per sec passing through a body 1 cm thick with a cross section of 1 cm² when the temp difference is 1° C.
Clinical significance of thermal conductivity
High thermal conductivity of large deep metallic restoration may cause thermal pulp shock and this can be avoided by:
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Adequate dentin thickness should remain under restoration.
If not, an insulating cement base should be placed under the restoration.
2- High thermal conductivity of metallic denture base is an advantage, as it activates blood circulation maintaining soft tissue vitality, opposite to acrylic denture base (thermal insulators).
Specific heat
It is the quantity of heat needed to raise temperature of one gram of the material 1° C.
Materials with low specific heat heats up easily.
Thermal diffusivity
It is the rate at which a body of non-uniform temperature approaches thermal equilibrium or a measure of transient heat flow.
Clinical significance of thermal diffusivity
Gold or amalgam restorations may cause high thermal shock due to their high thermal diffusivity (have high thermal conductivity and low specific heat).
Thermal diffusivity Thermal conductivity/
Specific heat * Density
↑ thermal conductivity ⬇️specific heat
⬆️ thermal diffusivity
.
Coefficient of thermal expansion (CTE)
It is the change in length per unit length of a material for 1° C change in temperature.
Clinical significance coefficient of thermal expansion
Ideally, filling materials should have similar CTE to that of the tooth structure to avoid breaking the marginal seal microleakage.
Matching in CTE between denture base and artificial teeth is essential to avoid their separation during thermal changes.
Electrical conductivity
The ability of material to conduct an electric current
Clinical significance of electrical properties
An insulating cement base should be placed under metallic restorations
