3.pysical properties 1 [Autosaved].pdf

 Physical properties Dr/ Eman Mohamed Mohamady Lecturer of Dental materials  Physical Properties of Dental Materials Definition: Properties that are not related to force application  Physical Properties of Dental Materials Mass-related properties: Densit...

 Physical properties Dr/ Eman Mohamed Mohamady Lecturer of Dental materials  Physical Properties of Dental Materials Definition: Properties that are not related to force application  Physical Properties of Dental Materials Mass-related properties: Density and specific gravity Thermal properties: 1. Melting temperature 2. Heat of fusion. 3. Specific heat 4. Thermal conductivity. 5. Thermal diffusivity 6. Coefficient of thermal expansion and contraction.  Electrical properties: Electrical conductivity and resistivity Less specific properties: 1. Water sorption. 2. Fluidity, viscosity and plasticity. Mass related properties  1. Mass related properties Density and specific gravity Density Is the mass per unit volume of the material Unit : gm/cm3  Specific gravity It is the ratio of the density of materials to the density of water at 4 °C Has no units Clinical Importance in density: Low density is an advantage in: a) Retention of the upper dentures. 1- Nonmetallic denture base material are lighter in weight compared to metallic denture bases. 2- base metals alloys are lighter than gold alloys b) Light weight affects the comfort Of the patient Clinical Importance in density: High density is an advantage in: Casting Low-density alloys require more casting forces to allow rapid filling of the mold Casting Casting Casting Casting Casting low density alloys require more casting forces  High Density Casting Upper dentures Thermal Properties  2. Thermal Properties 1-Melting and freezing temperature: It is the temperature at which the material melts or freezes  Importance in dentistry 1-In determining the melting machine or method of melting. 2-Avoid over heating of the molten metals to avoid valorization of lower melting metals leads to porosities. 3-Determine the qualities of investment materials. 2. Heat of fusion The amount of heat in calories or joules required to convert 1 gm of a material from solid to liquid state 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. Liberation of Latent heat of fusion  Latent Heat of Fusion: The temperature of the solidifying liquid doesn’t change with time due to liberation of the latent heat of fusion  Importance in dentistry: During casting , heating the molten alloy nearly 100 above the melting temp. will give time for the molten alloy to cool before solidification. To prevent incomplete casting (3)Specific Heat: It is the quantity of heat needed to raise the temperature of one gram of the substance 1ºC. Rate of heat rise. 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.  (4) Thermal Conductivity Amount of heat in calories, or joules, per second passing through a body 1 cm thick with a cross section of 1 cm when the temperature difference is 1°C. Rate of heat transfer 1-A metal base produces normal thermal stimulation in the supporting soft tissue. This leads to a physiological response of vasodilatation and vasoconstriction, which maintains the tissue in healthy condition. 2-Large Amalgam filling may lead to thermal irritation to the pulp. Cement base is placed between tooth and filling for insulation  High thermal conductivity Metallic denture base Metallic filling 5-Thermal diffusivity: It is the rate of diffusion of heat in the body Thermal diffusivity = thermal conductivity specific heat density  Importance in dentistry: I. Specific heat and Thermal conductivity of amalgam lead to thermal shock of the pulp. The diffusivity and thickness of an insulating material are important that govern the insulating efficiency. (5) Coefficient of Thermal Expansion: Change in length, per unit length for a 1°C change in temperature.  L final - L original α=   L original X ( C final C original) Example: -6 Enamel = 11 x 10 /°C -6 Acrylic resin = 90 x 10 /°C  Clinical importance in dentistry: 1. Marginal Percolation pumping in and out of saliva and food debris between the cavity walls and restoration due to great difference in the coefficient of thermal expansion between The tooth and the restorative material  Effect of Marginal percolation 1. Marginal leakage 2. Marginal discoloration 3. Recurrent caries 4. Hypersensitivity. Marginal leakage  Clinical importance in dentistry: 2. Porcelain and metal in Ceram metallic restorations 3. Matching the coefficient of thermal expansion (α) between: Porcelain artificial tooth and denture base to avoid crazing. 4. Distortion of the wax pattern due to high coefficient of expansion of wax. 4. Expansion of the cristobalite and quartz by displasive transformation is responsible for the thermal expansion of investment , to compensate for the solidification contraction of metals. 4 6 4 6 Water sorption  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. Very small amount of water sorption in the acrylic denture base may compensate for some of the polymerization shrinkages. 2-Imbibition of hydrocolloid if immersed in water leads to dimensional changes  Fluidity, viscosity and plasticity Fluidity is the tendency of liquid to flow. Viscosity is the resistance of fluid to flow. Plasticity is related to solid and semisolid. material which indicate that the materials easily and permanently deformed under force. Viscoelasticity and creep.  The rheologic behaviors of four types of fluids: Newtonian: constant viscosity Plastic : do not flow initially until reach am initial shear rate has been reached and then attain constant viscosity Pseudoplastic: the viscosity decrease with increasing shear stress (more liquid) easy to be mix Dilatant:the viscosity increase with increasing shear stress (more viscous) difficult to be mix