Physical Properties of Dental Material
Dental materials are specially fabricated materials, designed for use in dentistry. Physical properties include
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Viscosity is the resistance of a liquid to flow, this property comes into play when we deal with materials in the fluid state and depending upon their viscous behavior, materials can be broadly classified into 4 groups.
- Newtonian– Newtonian is the ideal fluid, that demonstrates shear stress that is proportional to the strain rate. It has a constant viscosity.
- Pseudoplastic– Viscosity decreases with increasing strain rate till a certain value after that it becomes constant.
- Dilatant– In dilatant, viscosity increases with an increase in strain rate, means more you try to deform it, more rigid it becomes. For a dilatant fluid, if you try to make it flow more by applying more pressure to deform it, it will become more rigid and viscous and difficult to manipulate.
- Plastic– Such materials exhibit rigid behavior initially, but once their threshold value is crossed, they attain a constant viscosity.
- Thixotropic– This is a special category in which viscosity of materials decreases when the pressure is applied on the material. For example plaster of paris, prophylaxis paste and some impression materials. Viscosity of thixotropic decreases upon repeated loading, like while stirring or when we load alginate in an impression tray, it is viscous enough to be loaded into tray to fill it up and does not flow out of tray, but when you press the alginate filled tray against the teeth, it thins out, flow increases so that it can easily flow into all the surfaces and crevices to capture maximum detail. Units of viscosity is Mpa per sec, centipoise(cP) and viscosity of most liquids decrease with increase in temperature.
Creep is the physical property of dental material, it is time-dependent plastic deformation of a material under static or dynamic loading. A sag is a form of creep that occurs in metal at high temperature under its own weight. For example, metals used for long-span bridges should have high sag resistance, so that when porcelain is fired onto them, which requires high temperature for fusing porcelain, these metals should not sag.
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This term is used for amorphous materials like waxes. It is a measure of its potential to deform under small static load.
Color and Color Perception
You should know the eye is sensitive to a wavelength between 400-700 nm, most sensitive to the green yellow region (550nm) and least sensitive to the red or blue region (500-700 nm). There are 3 dimensions of color.
- Hue– Dominant color
- Value-Lightness or brightness of color
- Chroma– Degree of saturation
Look hue is the basic color, If I say this object is yellow, I am talking about hue, If I say it is a bright yellow or dark yellow, I am describing the lightness or darkness, like the brightness, so I am describing value. If I say that this yellow is a deep yellow, like how rich is this yellow, I am talking about the saturation of color.
Then phenomenon to remember
Metamerism– Objects matched under one type of light do not match under other light sources.
Bezold Brucke Effect– Color appears to change with an increase in the intensity of light.
Fluorescence– When an object absorbs light of a shorter wavelength and emits light of higher wavelength, it exhibits a property of fluorescence, the natural tooth has this property and this contributes to brightness and vital appearance of the human tooth.
Coefficient of thermal expansion(COTE)
It is the change in length per unit of original length of a material when it’s temperature is raised by 1K. This high COTE of dental waxes is their biggest drawback, whenever temperature fluctuations occur, dimensional changes occur due to the release of residual stresses that are induced by a change in temperature of the material.
Also this property in metal-ceramic restorations, where COTE of metal used for coping and the overlying veneering porcelain has to be closely matched, it should not vary by more than 4%, in this COTE of metal should be equal to or slightly more than porcelain, so that when metal cools to room temperature after firing of porcelain, it contracts more than porcelain. This puts porcelain into compression and strengthens the porcelain as well as the metal ceramic bond.
No residual stresses are produced within a material due to any kind of disturbance, this disturbance can be external-temperature change, external deformation causing strain or internal- due to polymerization shrinkage in resins.
Tarnish and Corrosion
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Lastly, in physical properties, we have tarnish and corrosion. Tarnish is surface discoloration whereas corrosion is actual deterioration of the metal. Corrosion can be dry or wet (electrochemical). Wet corrosion is one that occurs in presence of an electrolyte, and dry corrosion is chemical corrosion by oxidation, halogenation, and sulfurization. The electrolyte can be water or some other electrolyte, like saliva in oral environment.
In wet corrosion, there is an anode (metal with lower electrode potential), that corrodes away, that produces electrons, undergoes oxidation. Cathode undergoes reduction, consumes free electrons.
Types of wet corrosion are-
- Galvanic corrosion
- Heterogeneous surface composition corrosion
- Stress corrosion
- Concentration cell corrosion/crevice corrosion
Gold alloys are corrosion resistant and to the base metal, chromium is added to provide corrosion resistance, chromium undergoes passivation to form chromium oxide and form a thin layer over the surface and makes the metal passive to corrosion. Titanium oxide is another example that renders commercially pure titanium passive to be used for dental implants.