Physical Properties of Dental Materials PDF

Physical Properties of Dental Materials These are properties that are not related to force application. The physical properties described in this chapter include: I) Mass related properties: Density II) Thermal properties: 1. Thermal conductivity. 2. )Coefficient of thermal expansi...

Physical Properties of Dental Materials These are properties that are not related to force application. The physical properties described in this chapter include: I) Mass related properties: Density II) Thermal properties: 1. Thermal conductivity. 2. )Coefficient of thermal expansion and contraction. III) Less specific properties: a. Water sorption. b. Fluidity, viscosity and plasticity. IV) Optical properties: Optics is the science that deals with light, vision and sight. (which includes color) I-Mass related properties: Density: is the mass per unit volume of the material. Its units are gm/cm3 and pound/in3 #Clinical Importance in dentistry: 1.Retention of the upper denture: #Dentures of lighter weight will help the retention of the upper dentures. #Non metallic denture base materials are lighter in weight compared to metallic denture base). 2.Light weight complete or partial denture: affects the comfort of the patient. 3.During casting: low density alloys require more casting forces to allow rapid filling of the mold cavity (cobalt chromium alloys need special casting machine II) Thermal Properties: 1 Thermal Conductivity: The thermal conductivity of a substance is the amount of heat in calories, or joules, per second passing through a body 1 cm thick with a cross section of 1 cm2 when the temperature difference is 1°C. #Clinical Importance in Dentistry: 1.Disadvantage: In Metallic filling materials: eg: large amalgam filling close to the pulp may cause patient discomfort because it conduct heat to the pulp causing pulp shock. Therefore, a cement base or insulator (non-metallic substance) is placed under the filling material for pulp protection. N.B.: Composite and ceramic restorations are non-conductive and do not need insulators. 2.Advantage: Metallic denture base materials: Metallic denture base should conduct heat to keep the underlying tissues in a healthy state by allowing better tissue response due to vasodilatation and vasoconstriction. Therefore, metallic denture base better than non metallic denture base. 2 Thermal Coefficient of Expansion This change in length, per unit length for a 1°C change in temperature is called the linear coefficient of expansion Examples: Enamel = 11 x 10-6°/C Acrylic resin = 90 x 10-6°/C Composite = 22 X 106°/C #Clinical Importance in Dentistry: 1.Close matching of the coefficient of thermal expansion (α) between the tooth and the restorative materials: -All filling materials expand by heat and contract by cold.  -An ideal filling material should have coefficient of thermal expansion the same or near that of the tooth to avoid Marginal Leakage. Opening and closing of gap results in → breakage of marginal seal between the filling and the cavity wall, this breakage of seal leads to Marginal Percolation. -Marginal percolation: Is the in and out movement of saliva , food and bacteria in the space created between the filling material and the cavity wall due to high difference in coefficient of thermal expansion leading to : i. Discoloration ii. Recurrent caries iii. Hypersensitivity 2.Porcelain and metal in ceramo-metallic restorations (crowns & bridges) to provide metal ceramic bonding. 3.Artificial tooth and denture base to avoid crazing (cracks at the junction between the porcelain teeth and acrylic resin base. 3. Melting and freezing temperature: It is the temperature at which the material melts or freezes. #Clinical Importance in Dentistry: 1.For the fabrication of indirect metallic restorations (casting), the melting temperature of metals and alloys is important in determining the melting machine. 2.Determine the type of investment material. III) Other Properties: 1.Water sorption: It represents the amount of water adsorbed on the surface and absorbed into the body of the material. #Clinical Importance in Dentistry: 1. Plastic denture base materials have the tendency for water sorption. High percentage of water sorption may lead to dimensional change and warpage. 2.The tendency for hydrocolloid impression to imbibe water if immersed in it will lead to dimensional changes. IV) Optical Properties: -The observation of color is a physiological response to 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 light When a beam of light falls on a surface of a medium, the following may occur: 1.Reflection: -Most objects we see are visible, because they reflect light to our eyes. -Smooth Surface: Smooch surfaces reflect light in one direction only and this is called specular reflection. The angle of incidence will be equal to the angle of reflection. The surface appears shiny , e.g. mirrors -Rough Surface: It will be reflected in all directions, and this is called diffuse reflection. Rough surfaces appear dull. #Clinical Importance in Dentistry: The anterior restoration should be smooth and polished.. 2. Transmission: Light passing through an optical medium without attenuation, is said to be completely transmitted. Total transmission occurs in perfectly transparent materials. 3. Fluorescence and phosphorescence -Electrons may be activated to higher energy levels when exposed to electromagnetic waves of light of high energy. -As the electron returns to its lower energy level energy is released. -Immediate emission ie release of energy is called fluorescencewhile delayed emission is called phosphorescence #Clinical Importance in dentistry 1.Sound human teeth absorb light of short wave length and emit light of longer wave length. This gives the brightness and vital appearance of human teeth. 2.Some anterior restorative materials and dental porcelains contain fluorescing agents to reproduce the natural appearance. Properties of materials in relation to light transmission and absorption i. Transparency: A property of the material, that allows the passage of light so that an object can be clearly seen through them. e.g. glass and acrylic resin. ii. Translucency: A property of the material which allows the passage of some light and scatters or reflects the rest. Therefore, the object cannot be clearly seen through them. e.g. Tooth enamel, porcelain, composite and pigmented acrylic resin. Translucency decreases with increasing the scattering centers. iii. Opacity: The 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. COLOR Color Parameters: Hue: It represents the color of the material, i.e. blue, green, red and yellow. Chroma: It represents the strength of color or degree of saturation of the color. A beaker of water containing one drop of a color is lower in chroma than a beaker containing ten drops of the same color. Value: It represents the lightness or darkness of color (the amount of grayness).A dark standard has a value of 0 where white standard has a value of 100.A tooth of low value appears gray and non-vital. Value is the most important parameter of color in dentistry because it is intimately related to the vitality in human teeth. Examples: a. Dead tooth have low value (more gray or dark). b. Vital tooth have higher value. Colors: Primary colors: Blue, green and red are three primary colors. Combining three primary colors result in white color. Combination of two primary colors will give the secondary colors: yellow, cyan and magenta. Green + red =yellow Blue + red =magenta Blue + green= cyan Complementary colors: Two colors are complementary to each other when their combination gives rise to white color. Yellow is the complementary of blue.