Physical Properties Lecture Notes PDF

Physical Properties  These are properties which are not related to force application. These are: 1- Mass-related properties: A.Density B.Specific Gravity 2- Thermal properties: A. Melting and freezing temperature B.Heat of fusion & latent heat of fusion C.Thermal conductivity D.Specific h...

Physical Properties  These are properties which are not related to force application. These are: 1- Mass-related properties: A.Density B.Specific Gravity 2- Thermal properties: A. Melting and freezing temperature B.Heat of fusion & latent heat of fusion C.Thermal conductivity D.Specific heat E.Thermal Diffusivity F.Coefficient of thermal expansion and contraction  4- Electrical Properties 3- Less specific properties: A.Water sorption. B. Fluidity, viscosity and plasticity. 4- Optical properties  Mass- Related Properties: Density It is the mass per unit volume of the material. Units are gm/cm3 or pound/in3  Mass- Related Properties: Specific Gravity It is the ratio of the density of a material to the density of water at 4°C.Since the density of one cc of water at 4°C = 1, therefore the specific gravity is numerically equal to density but without units.  Clinical importance in Dentistry 1-Complete denture Upper denture must be made of materials with low density to help its Retention. 2- During casting Molten metals with low density require higher pressure to fill the mold during casting  Thermal Properties 1. Melting and freezing temperature It is the temperature at which the material melts or freezes. 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  Thermal Properties 2a. Heat of fusion 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. As long as the mass is molten, the heat of fusion is retained by the liquid.  Dental Significance During casting metal must be heated 100°C above its melting temperature  Thermal Properties 2b. Latent Heat of fusion When the liquid is frozen, this heat is liberated Heat of Latent heat fusion of fusion  Thermal Properties: 3. Thermal Conductivity: It is the amount of heat in calories or joules passing per second through a body 1cm thick, 1cm2 cross sectional area when the temperature difference is 1°C  Clinical importance in Dentistry 1- Metallic filling materials 2- Metallic denture base materials  Thermal Properties 4. 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  Thermal Properties 5.Thermal Diffusivity Rate at which a body with a non uniform temperature approaches equilibrium. i.e. Rate of heat diffusion in the body Thermal diffusivity= Thermal conductivity Specific heat x density  Importance in dentistry Low specific heat combined with high thermal conductivity of a metallic restoration leads to thermal shock to the pulp. Adequate dentin thickness (density) should remain under the restoration  Thermal Properties 6. Coefficient of Thermal 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 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 ii. Discoloration iii. Recurrent caries 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. N.B. Thermal diffusivity is a more important property than thermal conductivity, Related to the thickness of the material (e.g. insulating base(non metallic) under metallic restoration should be of adequate thickness.)  Less specific properties 1- Water Sorption: It represents the amount of water adsorbed on the surface and absorbed into the body of the material.  Dental Significance: 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.  Less specific properties 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 Amalgum Composite Metallic bridge restoration restoration Ceramic crowns Composite restoration No light = No vision Light  What is 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.  Interaction of light and matter When a beam of light encounters or falls on a surface of a medium, the following may occur :  Interaction of light and matter 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  Dental Significance 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)  Interaction of light and matter 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.  Dental Importance Refractive index of the tooth and the esthetic restorative material should match each other, to avoid line of demarcation. Control of refractive index of the filler and matrix phases in composite resins and porcelain. A perfect match results in a transparent solid while large differences result in opaque materials.  Interaction of light and matter 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.  Importance in dentistry: 1- Opacifiers & pigments added to composite resins act as scattering centers giving opaque shades of the material. 2- Incorporated air bubbles in a restoration act as scattering centers.  Interaction of light and matter 4. Transmission Light passing through a medium without attenuation is completely transmitted.  Interaction of light and matter 4. Transmission Complete Transmission= Transparent Incomplete Transmission= Translucent No Transmission (absorption)=Opaque Properties of materials in relation to light transmission and absorption Transparency Translucency Opacity A property of a A property of the material, A property of the material that which allows the passage of material that allows the some light and scatters or passage of light reflects the rest so that prevents the so that objects objects cannot be clearly seen passage of light so can be clearly through them. that objects seen through Translucency decreases with cannot be seen them increasing the scattering e.g. glass, pure centers. through them. acrylic resin. e.g. tooth enamel, porcelain, e.g. wood and composite and pigmented metals acrylic resin.  Interaction of light and matter 5. Luminescence Immediate emission is called fluorescence, Delayed emission is called phosphorescence.  :Importance in dentistry Sound human teeth emit fluorescent light when excited by ultraviolet radiation giving bright and vital appearance Some anterior restorative materials and dental porcelains are formulated with fluorescing agents to reproduce the natural appearance.  Interaction of light and matter 6.Dispersion White light is a mixture of colors and its wavelength ranges from 350-700 nm. It gets dispersed by a prism to give the spectrum starting from the shortest wavelength, violet (350 nm) and ending by the longest wavelength, (700 nm).  LASER Light Characteristics of laser beam: Amplification Monochromatic: all photons have by the same wave length. Stimulated Coherent: all waves are in phase Emission of (have the same speed ). Radiation 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. Colors and Color Parameters The eye can distinguish differences between colors in only three parameters Hue Chroma Value  Color Parameters 1. Hue It is the color itself (the dominant wavelength)  Color Parameters 2. Chroma It is the intensity or saturation of the color  Color Parameters 3.Value The lightness or darkness of a color. 10 0  Justify VALUE is the most important color parameter Because it is related to the aspect of VITALITY in human teeth. A tooth of low value appears gray and non–vital A tooth of higher value appears vivid and translucent -VITAL  Justify VALUE is the most important color parameter A restoration with hue matching the adjacent dentition but its value is too high, we always have a result of a false look (too bright). When hue matches but the value is too low, → finished restoration that looks dead (darker or grayer or dull).  Colors Primary colors: Blue, green and red Secondary colors: Cyan ,magenta & yellow It results from the combination of two primary colors  Complementary colors Two colors are complementary to each other when their combination results in white e.g. yellow is the complementary color of blue Shade Selection  Factors affecting color appearance and selection 1- Source Different sources have different color content. i.e Incandesnt light has a color content different from that of fluorescent light.  Factors affecting color appearance and selection: Metamerism It is the change of color matching of two objects under different light sources. Thus, if possible, color matching should be done under two or more different light sources.  Factors affecting color appearance and selection: Metameric & Isomeric Pair  Factors affecting color appearance and selection: 2- Surrounding Colors of wall, lips or clothes of the patient modify the type of light reaching the object.  Factors affecting color appearance and selection: 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.  Factors affecting color appearance and selection: 3- Object: B- Surface texture (surface finish): This determines the relative amount of light reflected from the surface, smooth surface appears brighter than rough surface.  Factors affecting color appearance and selection: 3- Object: C- Presence of scattering centers as inclusions or voids: This increase opacity and lower the value (darker) D- Fluorescence: It makes the teeth bright and vital, as it increases the brightness.  Factors affecting color appearance and selection: 3- Object: E- Thickness: Increase in thickness, increase opacity, and lower the value. F- Metamerism Factors affecting color appearance and selection : 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.