The elements of study Physical properties include: 1-density 2-thermal properties 3-electrical properties 4-optical properties
Density: Density is the mass per unite volume of the material .its units are gm/cm³ and pound/in³.
Examples of density of some dental materials: a) Acrylic resin = 1.2 gm/cm³ b) Porcelain = 2.4 gm/cm³ c) Gold alloys = 17 gm/cm³ d) Cobalt – chromium alloys = 8 gm/cm³
Importance in dentistry: 1) Upper denture must be made up of the materials that are of low density to help its retention. 2) Molten alloys of low density require higher pressure to fill the mold cavity during casting procedures.
Thermal conductivity (K): The thermal conductivity of substance is the amount of heat in calories, or joules, per second passing through a body 1cm thick with a cross section of 1 cm² when the temperature difference is 1°C . The unites are cal/sec/cm²/(°C/cm).
Examples of (K): a) Enamel = 0.0022 cal/sec/cm²/(°C/cm). b) Amalgam = 0.055 cal/sec/cm²/(°C/cm). c) Gold alloys = 0.710 cal/sec/cm²/(°C/cm).
Importance in dentistry 1- Metallic filling material: e.g. The high thermal conductivity of amalgam is a disadvantage because if it is near the pulp it may cause patient discomfort as a result of temperature changes produced by hot or cold foods and beverages unless adequate tooth tissue remains or non-metallic substances are placed between the tooth and filling as a base for insulation. N.B. composite and ceramic restorations are non- conductive and do not need insulators.
2) Metallic denture base materials: Metallic denture base material is an advantage because heat transmission causes tissue stimulation through vasodilatation and vasoconstriction of blood vessels of underlying tissues keeping it in a healthy state.
Specific heat (CP): How rapidly the temperature increases in a material depends upon the specific heat of the material, which is defined as the amount of heat required to raise the temperature of a unite volume by 1°C. Thus , its units are Cal.gm-1.°C-1
Importance in dentistry: Specific heat of metal or alloys is important during the melting process because it indicates the amount of heat that must be applied to the mass to raise the temperature to the melting point.
3) Thermal diffusivity (h) The transfer of heat to a cold source is dependent on both the thermal conductivity and the specific heat, with the former regulating the rate at which the heat enters and passes through the material and the latter determining the rate at which the temperature will rise as heat enters the material.
This is presented by the thermal diffusivity, h, such that: K h = --------------- Cp P P = density of material . Cp = specific heat K = thermal conductivity. The thermal diffusivity gives a clear indication of the rate of rise of temperature at one point due to a heat source at another point, and may be considered the most important in dental applications. An interesting feature is the low diffusivity of water, showing it to be an excellent thermal insulator. For this reason, Eskimos can be quite warm when sheltering in their igloos.
4) Thermal expansion (α) : When a material is heated, the energy absorbed causes the atoms or molecules to vibrate and the material expands. the most common way of measuring this expansion is by taking a length of material, heating it to a certain temperature and then measuring the resultant change in length. This change in length, per unit length for a 1°C change in temperature, is called the linear coefficient of expansion,α.
L final – L original α =--------------------------------------- L original x (C final – C original) Examples: Enamel = 11 x 10-6/°C Acrylic resin = 77 x 10-6/°C Composite = 22 x 10-6/°C
Importance in dentistry: In a composite restoration, the seal between the composite and tooth may be affected by the repeated temperature changes in the mouth which cause different amounts of expansion and contraction of the composite and the tooth. This difference may create gaps between the two (marginal percolation) which leads to leakage of salivary components into the tooth leading to tooth discoloration, recurrent caries, and pain
5) Heat of fusion (L) : Transitions from one state to another are accompanied by absorption or liberation of heat and usually by a change in volume. The amount of heat in cal or J required to convert 1 gm of a material from solid to liquid state. Q (total heat absorbed) L = ------------------------------- m (mass of substance)
As long as the mass is molten, the heat of fusion is retained by the liquid. When the liquid is frozen, this is liberated (latent heat of fusion). Latent heat of fusion: is the amount of heat in cal. Or joule liberated to convert 1 gm of a material from liquid to solid state.
Electrical properties: (electrical conductivity and resistively) The ability of a material to conduct an electric current may be stated as either specific conductivity or conversely as the specific resistitvity, where resistivity is the resistance of a material electric current of one cm length and a cross – sectional area of one cm².
the resistance of a homogenous conductor of uniform cross section at a constant temperature varies directly with the length and inversely with the cross sectional area of the sample according to the following equation: L R = P -------- A R is the resistance in ohms: P is the resistivity. L is the length. A is the sectional area.
Importance in dentistry: Zinc oxide – eugenol cements have the highest resistivity followed by zinc polyacrylate and zinc phosphate cements. The glass ionomer cements are the most conductive.
4) Optical propertied: The restoration of teeth has moved more and more from the purely functional towards the aesthetic. Consequently, the optical properties of the materials that are selected and used by the dentist have become of great importance. The perception of color is a physiological response to a physical stimulus (light). For example, the choice of color differs from person to person, and presents a real problem for those who suffer from color blindness.
Light : is electromagnetic radiation that can be detected by the human eye. It can be seen that the visible electromagnetic radiation is the range from 400 – 700 nanometers.
Color parameters: a) Hue : this represents the color of the material . E.g. blue, green, yellow , orange,… b) Chroma : it is strength of color, or it is a measurement of color intensity. A beaker of water containing one drop of colorant is lower in chroma than a beaker of water containing ten drops of the same colorant. c) Value : it represents the lightness or darkness of a color. A black standard is assigned a value of 0, where as a white standard is assigned 100. a tooth of low value appears gray and non-vital therefore it is the most important parameter.
Primary colors: The primary color are red, green, and blue, from which all other colors can be produced. This fact is used in TV sets to create a full color picture from only three colored sets of dots. Combining the three primary colors results in white. Secondary colors: each secondary color- cyan, magenta, and yellow – results from the combination of two primary colors, e.g green and red gives yellow.
Complementary colors: two colors are complementary to each other when their combination results in white e.g. yellow is the complementary color of blue. Green Cyan Yellow White Blue Red Magenta
Transparency, translucency, and opacity: Transparency: is the property of materials that allow the passage of light in such a manner that objects can be clearly seen through them. e,.g. glass and clear acrylic resin. translucency: is the property of materials that allow the passage of some light but disperses ( scatters or reflects) the rest so that objects cannot be clearly seen through them e.g. dental porcelain, composite resin and pigmented acrylic resin .
Opacity: is the property pf materials that prevents the passage of light. An opaque material either absorbs or reflects all of the light. When all light is absorbed the material appears black.Examples of opaque materials are metals.
Metamerism : the color of objects under one type of light may appear different under another light source. This phenomenon is called metamerism. Fluorescence: natural tooth structure absorbs light at wavelengths too short i.e. not visible to the human eye between 300 and 400 nm (near-ultraviolet radiation). Natural sun light and ultraviolet lights used in decorative lighting are sources containing amounts of near ultraviolet radiation.
The energy that the tooth absorbs is converted into light with longer wavelengths, in this case the tooth actually becomes a light source. This phenomenon is called fluorescence. Fluorescence contributes to the brightness and vital appearance of a human tooth. A person with porcelain crowns or composite restorations that lack a fluorescing agent appears to be missing teeth when viewed under a black light . Therefore, some anterior restorative materials and dental porcelains are recently formulated with fluorescing agent to reproduce the natural appearance of tooth structure.
Propagation of light: 1) Reflection : light is reflected in all directions, this is called diffuse reflection. Rough surfaces undergo diffuse reflection when the roughness have dimensional larger than the wavelength of the reflected wave. Smooth surfaces reflect light in one direction only, this is called specular reflection . Highly polished surfaces (mirrors) reflect all light in one direction where the angle of incidence equals the angle of reflection.
2) Refraction : As light becomes incident on a surface separating two different media, a part is reflected and a part is refracted( i.e. there is a change in the direction of light as it enters the other medium). The ratio of the angle of incidence and sine angle of refraction is a constant called “refractive index”.
3) Scattering : this occurs when light passing through an optical medium redirects some of the radiation into direction other than that of the beam. Thus, the original beam is weakened by scattering in a direction away from the observer’s eye. As scattering of light increases, the body appears more dull and opaque. 4) transmission: light passing through an optical medium without attenuation is said to be completely transmitted. Total transmission is undergone by perfectly transparent material