WHAT ARE THE PHYSICAL PROPERTIES OF DENTAL MATERIAL?
The physical properties that will be discussed are
- Optical properties
- Thermal properties
- Electrical properties
- Other physical properties
About colour and light…
The perception of the Colour of an object is the result of a physiological response to a physical stimulus (light), either a reflected or a transmitted beam of white light or a portion of that beam.
It is an electromagnetic radiation that can be detected by the human eye. It can be seen that the visible electromagnetic radiation is in the range from 400-700 nanometers.
A) Color parameters
i) Hue: It is the dominant wavelength. It represents the color of the material, i.e green, red and blue.
ii) Chroma: or excitation purity. It represents the strength of the color or degree of saturation of the color (color intensity).
iii) Value: or luminous reflectance. It represents the lightness or darkness of color. BLACK = 0 ; WHITE = 100
B) Properties of materials in relation to light transmission and absorption
i) Opacity: is a property of the material that prevents the passage of light. Opaque material = absorbs all light = black
ii) Translucency: is a property of the material, which allows the passage of some light but disperses the light. So objects cannot be clearly seen through them. E.g. Ceramics, resin composites, denture plastics
iii) Transparency: is a property of a material, that allows the passage of light in such a manner that little distortion takes place. So that objects can be clearly seen through them. E.g. Glass, pure acrylic resin
C) Factors affecting color appearance and selection.
i) Surface finish and thickness: This determines the relative amount of light reflected from the surface. rough surface appears grayer than a smooth surface ; thickness of a restoration can affect its appearance. Opacity increases as the thickness increases.
ii) Metamerism: It is the change of color matching of two objects under different light sources. if possible, color matching should be done under two or more different light sources.
iii) Fluorescene: It is the emission of light by a material when a beam of light is shone on it. It makes the teeth bright and vital, as it increases the brightness. *Anterior teeth restoration*
A) Thermal Conductivity
- It is the amount of heat in (calories or joules) per second passing through a body 1cm thick, with a cross section 1cm2 when the temperature difference is 1°C.
Clinical importance in Dentistry: * Metallic filling materials.
- Gold or amalgam filling or crown in proximity to the pulp may cause the patient considerable discomfort when hot or cold foods produce temperature changes, cements are relatively poor conductors and insulate the pulp area.
B) Thermal Diffusivity
- It is a measure of transient heat-flow. Denture-base material should have a high value of thermal diffusivity in order that the patient keep a satisfactory response to hot and cold stimuli in the mouth.
WHAT IS THE COEFFICIENT OF THERMAL EXPANSION?
The change in length per unit length of the material for a 1°C change in temperature is called the linear coefficient of thermal expansion(α).
Clinical importance in Dentistry:
- Close matching of the coefficient of thermal expansion (α) is important between: the tooth and the restorative materials to prevent marginal leakage.
This can lead to: Recurrent caries. Discoloration. Hypersensitivity.
- Such expansions and contractions may break the marginal seal of the filling in the tooth, particularly if the difference between the coefficient of expansion of the tooth and the restorative material is great.
- For the filling material, the most ideal combinations of properties would be a low value of thermal diffusivity combined with a coefficient of thermal expansion value similar to that for tooth substance.
A) Electrical Conductivity and Resistivity
- The ability of a material to conduct an electric current may be stated either as specific conductance or conductivity, or, conversely, as the specific resistance or resistivity.
- Resistivity is important in the investigation of the pain perception threshold resulting from applied electrical stimuli and of displacement of dentinal fluid in teeth caused by ionic movements.
- The electrical resistance of normal and carious teeth has been observed to differ, with less resistance offered by the carious tissue. Sound enamel is a relatively poor conductor of electricity, whereas dentin is somewhat better.
- The presence of metallic restorations in the mouth, results from a difference in potential between dissimilar fillings in opposing or adjacent teeth.
- When two opposing fillings contact each other, the cell is short-circuited, and if the flow of current occurs through the pulp, the patient experiences pain.
- The galvanic currents developed from the contact of two metallic restorations depend on their composition and surface area.
C) Electrochemical Corrosion
- It is also referred to as wet corrosion, since it requires the presence of water or some other fluid electrolyte.
- Metals undergo chemical or electrochemical reactions with the environment resulting in dissolution or formation of chemical compounds.
- In most cases corrosion is undesirable. However, in dental practice, a limited amount of corrosion around the margins of dental amalgam restorations may be beneficial, since the corrosion products tend to seal the marginal gap and inhibit the ingress of oral fluids and bacteria.
Other Physical Properties
A) Tarnish and discolouration – is a surface discoloration on a metal , or a slight loss or alteration of the surface finish or luster.
B) Water sorption – represents the amount of water adsorbed on the surface and absorbed into the body of the material during fabrication or while the restoration
C) Abrasion resistance – is the ability of a material to withstand mechanical action that tends progressively to remove material from its surface.
D) Creep & Flow – Creep is the time-dependant plastic strain of a material under a static load or constant stress. Flow is a measure of material’s potential to deform under small elastic load.
E) Dimensional stability
- Syneresis: It refers to a loss of water by evaporation due to exposure to the air. The result is shrinkage in dimension.
- Imbibition: It may occur when a substance takes on additional water. The result is swelling in the material.
- Viscosity: The ability to flow. Thick or viscous liquids flow poorly, whereas thin liquids flow easily. gypsum product (plaster).
- Adhesion: It is the interaction between two materials at an interface where they are in contact.
What is the difference between adhesive and cohesive?
F) Mechanism of bonding
- Physical bonding: The forces involved may be primary (ionic and covalent) or secondary (hydrogen bonds, dipole interaction, or van der Waals) forces.
- Chemical bonding: chemical bonding to the inorganic component (hydroxyapatite) or organic components (mainly Type I collagen) of tooth structure.
- Mechanical bonding: Mechanical interlocking of the adhesive with irregularities in the surface of the adherent.
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