Physical Properties of Dental Materials These are properties

Physical Properties of Dental Materials

These are properties which are not related to force application. These are: 1 - Mass-related properties: Density 2 - Thermal properties: - Thermal conductivity - Coefficient of thermal expansion and contraction - Heat of fusion & latent heat of fusion - Melting and freezing temperature - Specific heat 3 - Less specific properties: - Water sorption. - Fluidity, viscosity and plasticity. 4 - Optical properties

Mass- Related Properties: Density It is the mass per unit volume of the material. Units are gm/cm 3 or pound/in 2 Clinical importance in Dentistry: 1 - Retention of the upper denture. 2 - Weight of complete or partial denture. 3 - During casting.

Thermal Properties: 1 - Thermal Conductivity: It is the amount of heat in calories or joules passing per second through a body 1 cm thick, 1 cm 2 cross sectional area when the temperature difference is 1°C Clinical importance in Dentistry: 1 - Metallic filling materials.

2 - Metallic denture base materials.

2 - Thermal Coefficient of expansion( α) 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) α= L final- L original x (°C final- °C original)

Clinical importance in Dentistry: Close matching of the coefficient of thermal expansion (α) is important between: 1 -The tooth and the restorative materials to prevent marginal leakage. Opening and closing of gap results in → breakage of marginal seal between the filling and the cavity wall, this breakage of seal (marginal percolation) leads to: i. Marginal leakage iii. Recurrent caries ii. Discoloration iv. Hypersensitivity.

2 -Porcelain and metal in ceramometallic restorations (crowns and bridges) to provide metal ceramic bonding. 3 -Artificial tooth and denture base to avoid crazing.

3 - Heat of fusion (L) is the amount of heat in calories or joules required to convert l gm of a material from the solid to the liquid state at the melting temperature. It is calculated as follows: Q Where L: is the heat of fusion. L = --------Q: is the total heat absorbed. m m: mass of the substance melted As long as the mass is molten, the heat of fusion is retained by the liquid. When the liquid is frozen or solidified, this heat is liberated. It is called "Latent heat of fusion".

Latent heat of fusion It is the amount of heat in calories or joules liberated when 1 gm of a material is converted from liquid to solid state. Importance in dentistry: During casting, the metal must be heated 100 °C more than its melting temperature for proper melting

Heat of fusion Latent heat of fusion

Dental significance During casting metal must be heated 100°C above its melting temperature L Tm L+S S Time

4 -Melting and freezing temperature Dental Importance: For the fabrication of indirect metallic restorations (casting), the melting temperature of metals and alloys is important in determining the melting machine used for casting Tm Time

5 -Specific heat It is the quantity of heat needed to raise the temperature of one gram of the substance 1°C Therefore Metals have low specific heat, while non metals have high specific heat Importance in dentistry: - Because of the low specific heat of dental gold alloys, prolonged heating is unnecessary, during casting

Less specific properties 1 - Water Sorption: It represents the amount of water adsorbed on the surface and absorbed into the body of the material. Importance: 1 - Acrylic resin denture base materials have the tendency for water sorption. 2 - Hydrocolloid impression materials will imbibe water if immersed in it leading to dimensional changes.

2 -Fluidity, viscosity and plasticity: Fluidity is the tendency of liquids to flow. Viscosity is the resistance to flow. Plasticity is a property related to solids or semisolids and indicates that the material is easily and permanently deformed under force.

Optical Properties The perception of the Color of an object is the result of a physiological response to a physical stimulus (light). Light 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.

Properties of materials in relation to light transmission and absorption 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. Translucency is a property of the material, which allows the passage of some light and scatters or reflects the rest. In such manner, the object cannot be clearly seen through them Translucency decreases with increasing the scattering centers. e. g. tooth enamel, porcelain, composite and pigmented acrylic resin.

Opacity is a property of the material that prevents the passage of light. Opaque material absorbs all of the light. Objects cannot be seen through them. -Black color materials absorb all light colors. -White color materials reflect all light colors. -Blue color materials absorb all light colors but reflect its color.

Interaction of light and matter When a beam of light encounters or falls on a surface of a medium, the following may occur :

1 - Reflection: A- Smooth surface: Reflections on a smooth surface give a glossy appearance to the surface → Specular Reflection Smooth surface: angle of incidence = angle of reflection The restoration should have a highly smooth and polished surface to simulate the tooth structure and match it. B- Rough surface: Reflections of light on a rough surface are diffused (i. e: in all directions). → Diffuse Reflection The surface appears to have little gloss (i. e dull)

2 - Refraction: It is the change of the direction of a beam of light on entering second medium. Refraction results from the difference in refractive indices of the two media. For perfect matching the refractive index of the restoration should be **equal to the refractive index of the tooth. → transparent solid while ** large differences result in opaque materials. Example: Control of refractive index of the filler and matrix phases in composite resins and porcelain.

3 -Scattering: If light rays passing through a medium are obstructed by any different inclusions it will be redirected in another direction and is attenuated. i. e The original beam is weakened by scattering in a direction away from the observer eye → ↑ OPACITY ↓ TRANSLUCENCY. Importance in dentistry: 1 - Opacifiers & pigments added to composite resins act as scattering centers that give rise to opaque shades of the material. 2 - Incorporated air bubbles in a restoration act as scattering centers.

4. Transmission: Light passing through an optical medium without attenuation → completely transmitted. Total transmission occurs in perfectly transparent materials. If part of the light is transmitted and part is reflected ( i. e. diffuse transmission), the material appears translucent. Complete Transmission Incomplete Transmission No Transmission (absorption) Transparent Translucent Opaque

COLOR Color parameters: A- Hue: It is the dominant wave length. It represents the color of the material, i. e yellow, green, red and blue. B- Chroma: It represents the strength of the color or degree of saturation of the color (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 color (the amount of grayness). A black standard is assigned a value of O, whereas a white standard is assigned 10. ***A tooth of low value appears gray and non–vital = DEAD, therefore, it is the most important parameter. Because it is intimately related to the aspect of vitality in human teeth.

Factors affecting color appearance and selection Shade guide is used for color matching. So, it is important to match colors under appropriate conditions. 1 - Source: Different sources have different color content. i. e Incandesnt light has a color content different from that of fluorescent light.

Metamerism: It is the change of color matching of two objects under different light sources. Metameric pairs: Two objects that are matched in color under one light source but are not matched under other light sources form metameric pair. Isomeric pair: They are color matched under all light sources. Thus, if possible, color matching should be done under two or more different light sources.

2 - Surrounding: Colors of wall, lips or clothes of the patient modify the type of light reaching the object. 3 - Object: A- Translucency: It controls lightness or darkness of color. High translucency gives a lighter color appearance (higher value) i. e more vital tooth appearance

B- Surface texture (surface finish): This determines the relative amount of light reflected from the surface, smooth surface appears brighter than rough surface. C- Presence of scattering centers as inclusions or voids: This increase opacity and lower the value (more dark)

D- Fluorescene: It makes the teeth bright and vital, as it increases the brightness. E- Thickness: The thickness of a restoration can affect its appearance. Increase in thickness, increase opacity, and lower the value. F- Metamerism

4 - Observer: A- Color response: Eye responds differently among individuals. B- Color Vision: Some individuals may have color blindness and inability to distinguish certain colors. C- Color Fatigue: Constant stimulus of one color decreases the response to that color.

Laser Light Amplification by Stimulated Emission of Radiation. The principle of laser production is simply that an element or compound (medium) can be excited by high energy to produce a special type of light called laser Characteristics of laser beam: a- Monochromatic: all photons have the same wave length. b- Coherent : all waves are bin phase (have the same speed ). c- Collimated: all waves are parallel ( minimum divergence )

Clinical applications of laser a- Surgery for removal of soft tissues. b- Removal of initial carious lesions. c- Curing of composite resin.

C OLOR s Primary colors: Blue, green and red are primary colors. Combining suitable proportions of wave lengths of the three primary colors results in white. Secondary colors: Each secondary color (cyan , magenta & yellow) results from the combination of two primary colors, e. g. green and red gives yellow, blue and red gives magenta, blue and green gives cyan. Complementary colors : Two colors are complementary to each other when their combination results in white e. g. yellow is the complementary color of blue.