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
Color
Color is the wavelength of light
Light
Is an electromagnetic radiation beam of photons
Visible spectrum has wavelengths range from 400 violet- 700 red nm
Hue
It is dominant wavelength of light red , blue or green
Value
It is the lightness or darkness of color lighter shade has higher value
Chroma
It is the degree of saturation ( intensity) of a particular hue
Factors affecting color matching
- Surface finish
- Pigmentation
- Metamerism
- Fluorescence
- Opacity, translucency and transparency
Surface finish
1-Surface finish
smooth surface reflect Light in one direction while rough surface reflect in more than one direction
Rough surface appears lighter than smooth surface of same material. reflect in one direction.
Explanation
Some of the light reflected from the rough surface remains white, mixes with light reflected from the body and dilutes its color
Pigmentation
Esthetic effects produced by incorporating colored pigments (metal oxides) in dental materials such as composite resin , denture acrylics
Metamerism
A phenomenon of different color matching under different light sources.
Clinical significance of metamerism
✓Color matching should be done under 2 light sources, one of which is sun light or white light to avoid metamerism
Fluorescence
Reemission of light of longer wavelength after activation by light
This material has the ability to absorb light of certain wave length and remit light of a different wave length (usually longer)
Example
Natural teeth can emit visible fluorescence light (a blue-white color 400- 450 nm) when exited by U.V or near U.V radiation (300-400 nm)_(ex: photoflash lamps, vapor lamps, sunlight contain near UV)
Clinical significance of fluorescence
Fluorescence contributes to brightness and vital appearance of teeth.
✓ Ceramic crowns or composite restorations that lack a fluorescing agent appear as missing teeth when viewed under a black light (UV).
✓ Dental porcelains are formulated with fluorescing agents (as Cerium oxide) to produce natural appearance, while acrylic resin doesn’t
Opacity
The property of a material that prevents passage of light.
✔ Objects cannot be seen through it.
Translucency
The property of a material that permits passage of some light and scatter or reflect the rest.
✔ Objects cannot be seen clearly through it, e.g. enameldental ceramics.
Transparency
The property of a material that allows passage of light without or with only little distortion.
✓ Objects can be seen clearly through ite.g. Glass
Color matching in dentistry
The most common method used for color matching is the “shade guide”.
Shade guides contain No of tapes with different hue, value & chroma.
✔ Arranged acc to hue (VITA classic) or chroma (VITA Pan 3D Master).
✓ Disadvantage of shade guide is that color perception may vary from one individual to anther
