Physical properties.pdf

Physical properties Lecture objectives: By the end of the lecture, student will be able to: 1-Understand The Physical Properties Of Dental Materials. 2-Define different Thermal Properties. 3- Understand The Clinical Importance Of Each Property. Physical properties Density Is the mass per unit...

Physical properties Lecture objectives: By the end of the lecture, student will be able to: 1-Understand The Physical Properties Of Dental Materials. 2-Define different Thermal Properties. 3- Understand The Clinical Importance Of Each Property. Physical properties Density Is the mass per unit volume of the material. Its units are / gm/cm3 and pound/ inch3 . Density of the material Density of equal volume of HO2 Specific gravity= Density of water is 1gm/cm3 Specific gravity is numerically equal but without units. Density of some dental material A - Acrylic resin = 1.2 gm/ cm3 . B - Porcelain = 2.4 gm/ cm3. C- Gold alloys = 17.4 gm/ cm3. D - Cobalt chromium alloys= 8.3 gm/ cm3. Importance in dentistry 1-Upper denture must be made up of 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 properties When a patient drinks a cup of tea or eats an ice-cream, the temperature differences within the tooth can be quite pronounced. The pulp of the tooth would react severely if it was not protected from these temperatures. Therefore, the thermal properties of the materials must be considered. Thermal conductivity (k) It is the quantity of heat in calories per second passing through a body I cm thick with a cross section of I cm2 when the temperature difference is I 0C. The units are cal /sec /cm2 (0c /cm). Examples:a) Enamel = 0.0022 CAL/SEC/ cm2 /(0C). b) Amalgam = 0.055 CAL/SEC/ cm2 /(0C /cm). c) Gold alloys =0.710 CAL/SEC/ cm2 /(0C.) C) Zinc oxide Eugenol cement =0.0011 cal/sec/ cm2 /(0C /cm). Clinical importance 1)Metallic filling materials: The high thermal conductivity of amalgam is disadvantage so it needs insulators. Composite and ceramic restorations are non -conductive and do not need insulators. 2) Metallic denture base materials The high thermal conductivity of metallic denture base materials is advantage due to heat transmission causes tissue stimulation through vasodilatation and vasoconstriction of blood vessels of underling tissues keeping in the healthy state. Specific heat ( CP ) The amount of heat required to raise the temperature of a unit volume by 10c , its units are cal/gm-10c Clinical Importance: Specific heat of metal or alloy 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. Heat flow through a material Metals tend to be good conductors of heat, Dentin is a thermal insulator ( poor conductor of heat ) when a sufficient thickness of dentin is present, the patient feels, no sensitivity to heat and cold through a metallic restoration. When only a thin layer of dentin remains, some thermal protection must be provided for the pulp. The rate at which heat flows through a material, is expressed as thermal conductivity. Thermal diffusivity (h) It describes the rate at which a body with non uniform temperature approaches thermal equilibrium. Thermal Diffusivity = Thermal conductivity Specific heat x density For example 1gram of water requires 1.0 calorie to raise its temperature 1c0, where as 1gram of dentin requires only 0.28 calorie, and 1gram of gold requires only 0.03/ calorie to produce a 1c0 temperature increase. Thus, thermal conductivity alone will not tell us for instance, how rapidly the interior surface under a crown when its the Surface is heated. Exterior Thermal expansion (α) Thermal cycling of restorations with markedly different expansion coefficients from tooth structure can cause marginal percolation. In a composite restoration, the seal between the composite and tooth may be affected by the repeated temperature changes in the mouth. This difference may create gaps between tooth structure and restoration (marginal percolation) which leads to leakage of salivary components into the tooth leading to tooth discoloration, recurrent, caries, and pain. The porcelain and metal in porcelain fused to metal (PFM) restoration must contract at the same rate upon cooling from the porcelain firing temperature if the build up of large residual stress is to be avoided. The thermal expansion behavior of dental wax, gold alloy, investment denture base material and so on, are allimportant in producing properly fitting castings. Heat of fusion ( L ) Transitions from one state to another accompanied by absorption or liberation of heat usually by a change in volume . The amount of the heat in cal . or J. required to convert 1gm of a material from solid to liquid state. Q total heat absorbed 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 heat is liberated ( latent heat of fusion ) Viscoelasticity When a substance flows under the influence of an external force (gravity) the molecules or atoms come into contact with different neighbors. Thus bonds must be broken and remade and this gives rise to a resistance to flow, known as viscous Viscoelastic materials Materials such as amalgam and elastic impression materials, exhibit a combination of viscous ,elastic and anelastic behavior .This combined strain (Viscoelastic strain) is time dependent because elastic and anelastic portions are recovered but the viscous component is not. a) When the load is applied ,strain of the elastic part occurs immediately while strain of the viscous and anelastic parts are time dependent. b) When load is removed the elastic strain is immediately recovered . However, viscous strain is not recovered ,which results in some permanent deformation. References: 1-Van Noort R, Michele B., 2013. Introduction to Dental Materials (fourth edition); Elsevier Health. 2-Callister WD, Rethwisch DG., 2009. Materials science and engineering: An introduction (eight edition) Wiley, USA.