Physical Properties of Materials PDF

Physical Properties of Materials Dr. Mohamed Said/ Dr. Amr Saad References Phillips’ Science of Dental Materials - Saunders; 12th edition. Chapter 3 Introduction Atom is the basic unit of any material. All materials are built up from atoms and molecules....

Physical Properties of Materials Dr. Mohamed Said/ Dr. Amr Saad References Phillips’ Science of Dental Materials - Saunders; 12th edition. Chapter 3 Introduction Atom is the basic unit of any material. All materials are built up from atoms and molecules. There is a strong relationship between atomic basis of a material and its properties. To understand the properties of materials, we need to understand the way by which atoms can combine to make solids. Types of atomic bonding Atomic bonding Primary Secondary bonds bonds Van der Ionic Covalent Metallic Hydrogen Waals bond bond bond bond forces 1.a: Ionic bonds Are the result of an electron being given up by one atom and being accepted by another Example in dentistry: 1.b: Covalent bonds Between two atoms are the result of two atoms sharing a pair of electrons Sharing electrons with other atoms allows an atom to fill its outer shell with electrons and, thus, to lower its energy. Example: polymers, man made plastics, diamond, proteins and DNA. 1.c: Metallic bonds are similar to covalent bonds in that valence electrons are shared between atoms. The difference is that the electrons in a metal object are not shared by two atoms; instead, they are shared by all the atoms that make up that object Atoms in the metal crystals exist as clusters of Positive metal ions enclosed by a CLOUD of Electrons Ionic bonds are responsible for the excellent thermal and electric conductivity of metals 2.a: Hydrogen bond The hydrogen bond can be understood by studying a water molecule. Attached to the oxygen atom are two hydrogen atoms. These bonds are covalent. As a consequence, the protons of the hydrogen atoms pointing away from the oxygen atom are not shielded efficiently by the electrons. They become positively charged. On the opposite side of the water molecule, the electrons that fill the outer shell of the oxygen provide a negative charge. The positive hydrogen nucleus is attracted to the unshared electrons of neighboring water molecules. This type of bond is called a hydrogen bridge. 2.b: Vanderwall forces: In symmetrical molecule, the electron field constantly fluctuates. Its charge becomes momentarily positive and negative. Thus creates a dipole that attract other dipoles. These interatomic forces are quiet weak Physical properties What is physical property? Classification of physical properties 1. Thermal properties 2. Electrical properties 3. Optical properties 4. Mass properties What is physical properties? Physical property is a property that can change without involving a change in the chemical composition. Unlike mechanical, physical properties are not concerned with the application of force to a body. They are classified into thermal, electrical, optical, and mass properties Thermal properties Melting or freezing temperature: It is the temperature at which the material melts (transforms from solid to liquid) or freezes (transforms from liquid to solid) for crystalline materials. It is important to determine melting temperature to avoid overheating of molten metals. Heat of fusion It is the amount of heat in calories or joules that is required to convert 1 gram of a material from solid to liquid state at the melting temperature. Thermal properties Glass transition temperature (Tg) This property is characteristic to non-crystalline structures (as polymers & waxes) that do not have definite melting point but rather gradually soften as their temperature is raised. The temperature at which macromolecule molecular motion begins to force the polymer chains apart. Thus, polymeric materials soften when heated above this temperature When heated, they gradually soften in a wide temperature range and become viscous before transforming to a liquid and gradually harden when cooled. Therefore, they have no definite melting point but rather a Glass transition temperature″ ″Tg″. Thermal properties Thermal conductivity or coefficient of thermal conductivity It is the quantity 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.  In case of metallic restorations with high thermal conductivity as amalgam, extremes in temperature would produce pain & injury to the tooth pulp. Thus, an insulating base is needed under metallic restorations to prevent such thermal shocks. Low thermal conductivity of resin denture base materials tends to prevent heat exchange between the oral cavity and soft tissues. This exchange is important to keep vitality of soft tissues and allow the patient to have the normal sense of heat and cold while swallowing food and drinks. So metallic denture base would be better in this instance. Thermal properties Specific heat It is the quantity of heat needed to raise temperature of one gram of the substance 1°C Thermal diffusivity Measure of the speed with which a temperature change will proceed through an object when one surface is heated. Gold or amalgam restorations may cause high thermal shock due to their high thermal diffusivity Thermal properties Coefficient of thermal expansion (linear coefficient of expansion) Change in length per unit of the original length of a material when its temperature is raised by 1 °C. Ideally, filling materials should have similar CTE to that of the tooth structure to avoid breaking the marginal seal (and subsequent leakage) between the filling & the tooth during thermal changes. During denture construction, matching in CTE between denture base and artificial teeth is essential to avoid their separation during thermal changes or crazing (cracks at the junction between the porcelain teeth and acrylic resin base) Matching of CTE between Porcelain and metal in ceramo-metallic restorations (crowns & bridges) to provide proper metal ceramic bonding. 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: Hypersensitivity Recurrent caries Discoloration Electrical properties Electrical conductivity Ability of the material to conduct an electric current Electrical resistivity Resistance of material to conduct electric current Electrical properties Corrosion Chemical or electrochemical process in which a solid, usually a metal, is attacked by an environmental agent, resulting in partial or complete dissolution. Tarnish Process by which a metal surface is dulled or discolored when a reaction with a sulfide, oxide, chloride, or other chemical causes surface discoloration through formation of a thin oxidized film. Electrical properties Galvanic corrosion (electrogalvanism) Accelerated attack occurring on a less noble metal when electrochemically dissimilar metals are in electrical contact within a liquid corrosive environment. Galvanic shock Pain sensation caused by the electrical current generated when two dissimilar metals are brought into contact in the oral environment. Avoid using different metallic restorations (e.g. gold and amalgam) in opposing or adjacent teeth as they can form an electric cell, in conjunction with saliva, leading to galvanic corrosion.  Metal as Zn with, lower electrode potential, tends to oxidize, corrode or dissolve relatively easily in solutions while those such as Pt & Au, with higher electrode potential, resist oxidation. Cements to be used as insulating bases under metallic restorations should have high electrical resistivity to protect the underlying tooth tissues from galvanism. Optical properties Color Sensation induced from light of varying wavelengths reaching the eye. Light is an electromagnetic radiant energy of certain wavelength, and human eye is sensitive to wavelengths ranging between 400 nm (violet) to 700 nm (red). For an object to be visible, it must emit, reflect or transmit light. Color perception of an object is as a result of a physiological response to physical stimulus (light). Optical properties Color parameters The eye can distinguish between 3 color parameters: a. Hue It is the dominant wavelength of light (ex: red, blue or green). b. Value It is the lightness or darkness of color (lighter shade has higher value). c. Chroma It is the degree of saturation (intensity) of a particular hue. https://luminusdevices.zendesk.com/hc/en-us/articles/4419619543565-What-s-the-difference-between-color-terminology-like-Hue- Value-and-Saturation-Chroma- Two central incisors metal-ceramic crowns with porcelain margins. The value of these crowns is higher than that of the adjacent lateral incisor teeth. Optical properties Factors affecting color matching 1. Surface finish Rough surface appears lighter than smooth surface of same material as it reflects more white light. 2. Pigmentation Esthetic effects are sometimes produced by incorporating colored pigments in dental materials such as composite resin, denture acrylics. 3. Metamerism Phenomenon in which the color of an object under one type of light source appears to change when illuminated by a different light source. Color matching in dental clinic should be done under 2 light sources, one of which is sun light https://eclat-digital.com/rendering-metamerism/ 4. Fluorescence Ability of an object to absorb UV radiation and transfer it to blue light Natural teeth can absorb U.V. and near U.V. radiation (invisible light) and emit visible light of different wavelength, so teeth appear as a light source. Fluorescence contributes to brightness and vital appearance of vital teeth. Dental porcelains are formulated with fluorescing agents (as Cerium oxide) to produce the natural appearance of tooth structure, while acrylic resin doesn’t have the same property. 5. Opacity, translucency and transparency Opacity The extent to which light does not pass through a material. No image and no light can be seen through an opaque object. Translucency The quality of light passing through an object in a diffuse manner, only to reveal a distorted image that can be viewed through the material. Transparency The extent to which light passes through a material and to which an undistorted image can be seen through it. - Vital teeth are (opaque – translucent- transparent) ??? - Metals are (opaque – translucent- transparent) ???? - Dental ceramics are (opaque – translucent- transparent) ??? - Glass are (opaque – translucent- transparent) ???? Source: Imagine dental Zirconia Optical properties Color matching in dentistry The most common method used for color matching in dentistry is the “shade guide” Shade guides contain number of tapes with different hue, value and chroma They have groups that may be arranged according to hue or according to value Tab arrangements of the Vitapan classical shade guide. A, Manufacturer’s arrangement No. 1: group division according to hue. B, Manufacturer’s arrangement No. 1: “value scale,” no group division. C, Alternative arrangement: according to color difference in relation to the lightest tab, group division.  Disadvantage of shade guide is that color perception may vary from one individual to another. The VITA Easyshade V digital spectrophotometer was developed for precise, fast and reliable shade determination of natural teeth and ceramic restorations. Mass properties Density The mass per unit volume of a material Mass (gm/cm3) / Volume (gm/cc) Mass is the amount of matter an object contains While volume is how much space it takes up. Example: A bowling ball and a basketball are about the same volume as each other, but the bowling ball has much more mass. Mass properties Density Gold alloys is 14 – 17.5 Cobalt-Chromium alloys is 8.3 Acrylic resin is 1.2 Mass properties Specific gravity The ratio of density of material to the density of water at same temperature. It is numerically equal to density but without units. Density is important for retention of upper denture. Non-metallic denture bases are relatively lighter in weight (i.e. lower density) hence they have higher retention in patient mouth. Thank You !!!