Lecture 5: Physical Properties of Dental Materials PDF

Dental biomaterials I course # DEN-DBM201-1 Lecture 5: P H Y S I C A L P RO P E RT I E S O F D E N TA L M AT E R I A L S DR/ Fahiem M. El-Shamy Lecturer in Dental Biomaterials Faculty of Dentistry, Mansoura & Menoufia National Un...

Dental biomaterials I course # DEN-DBM201-1 Lecture 5: P H Y S I C A L P RO P E RT I E S O F D E N TA L M AT E R I A L S DR/ Fahiem M. El-Shamy Lecturer in Dental Biomaterials Faculty of Dentistry, Mansoura & Menoufia National University, Egypt Former Ass. professor in Dental Biomaterials Faculty of Dentistry, Jazan University, K.S.A. This chapter provides brief descriptions of physical properties as background and preparation for more detailed discussion in later in which these properties are used to describe the characteristics of specific dental materials. ❑Physical properties are based on the laws of physics that describe mass, heat , light, electricity, energy, force, and other physical phenomena. ❑Mechanical properties are a subgroup of physical properties that deal with force. In this chapter, we will discuss the following properties: I- Mass-related properties: Density II- Thermal properties: - Thermal conductivity - Co-efficient of thermal conductivity (K) - Thermal Coefficient of expansion( α) - Melting and freezing temperature - Heat of fusion & latent heat of fusion - Specific heat - Thermal diffusivity III- Optical properties I- Mass related properties DENSITY: Density is the mass per unit volume of the material. Its unit is: gm/cm3 and pound/in3 Clinical importance in dentistry 1. Retention of the upper denture. Upper denture must be made up of materials that are of low density. 2. During casting. Molten alloys of low density require higher pressure to fill the mold cavity during casting procedures. II- Thermal properties  II.1 Thermal conductivity : It is the ability of the material to transfer heat. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity. Heat transfer occurs at a Heat transfer occurs at a lower rate in materials of higher rate in materials of low thermal conductivity high thermal conductivity II.2 Co-efficient of thermal conductivity (K): The amount of heat transported in one second across the unit cube when one face is hotter than the other. Importance: 1- Metallic filling material: Amalgam filling in proximity to the pulp may cause the patient considerable discomfort with hot or cold foods, an insulating cement base (poor conductors) applied to protect the pulp. e.g. Zinc phosphate cement base 2- Metallic denture base materials with high thermal conductivity help to maintain tissue health by stimulating blood vessels through vasoconstriction and vasodilation in response to thermal changes, while also ensuring that patients do not swallow substances that are too hot II.3- Thermal Coefficient of expansion( α) Change in temperature either by increasing or reduction leads to change in the interatomic distance. Co-efficient of thermal expansion:- 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) L final – L original L original x (0C final –0C original )  The clinical significance of co-efficient of thermal expansion: Close matching of the coefficient of thermal expansion (α) is important between: 1-The tooth and the restorative materials to prevent marginal leakage or microleakage (is the penetration of fluids, bacteria and ions into the space existing between all restorative materials and cavity walls). 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 ceramo-metallic restorations (crowns and bridges) to provide metal ceramic bonding. 3-Artificial tooth and denture base to avoid crazing. II.4 -Melting and freezing temperature It is the temperature at which the material melts (liquid state) or freezes (solid state). Importance in dentistry: In casting the melting temperature of metals and alloys is important in determining the melting machine and selection of type of investment materials. II.5- Heat of fusion : Amount of heat in joules or calories required to convert 1 gm of the substance from the solid state to liquid state.  II.6- Specific heat : It is the amount of heat in calories used to increase 1 gram of substance 1°C. Specific heat of H2O to increase its temp. From 15 to 16 is 1 calorie in one second. 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  II.7- Thermal diffusivity : It is the rate by which the material converted from non-uniform temperature approaches thermal equilibrium (uniform temperature ). Clinical Significance Gold or amalgam restorations, due to their high thermal diffusivity, may cause high thermal shock (as they have high thermal conductivity and low specific heat). III- Optics  Optics is the science of studying light, sight (how well the eyes capture images), and vision (how the brain processes the information coming from the eyes) (including color). One of the important goals of restorative dentistry is to restore the color and appearance of natural teeth. Thus, knowledge of some basic scientific principles of light and color is essential. I) Nature of light Light is abeam of photons Color is the wavelength of light.  II) Production of light: i)Thermal: e.g.: a) The sun b) hot metal ii) Electrical: By passing of electric current through an inert gas like neon. iii) Chemical (fluorescence and phosphorescence). III) Interaction of light and matter  1- Reflection: Specular reflection Diffuse reflection When light falls on a smooth surface If light falls on a rough ((perfectly smooth)), surface, it will be reflected The angle of incidence will be equal to in all directions. the angle of reflection. i.e. Smooth surfaces reflect light in one direction only. Such surfaces appear shiny, e.g. mirrors.  Importance in dentistry: The restoration should have a highly smooth and polished surface to obtain Specular reflection of light.  2- Refraction: Change in the direction of light beams ((bending)) as they enter a different medium. Refractive index = velocity of light in vacuum velocity of light in the medium Importance of refraction: - Refractive index of the dispersed phase and matrix phase should be matched  3-Scattering If light rays passing through a medium are obstructed by inclusions, it will be redirected in another direction and is attenuated. Scattered light Opacifier + Air bubbles Importance: Presence of scattering centres causes light to emerge in all directions. e.g. a) Opacifiers added to composite - opaquer shades of the materials. b) Incorporation of air bubbles in the restoration.  4- Properties of materials in relation to light transmission and absorption: 1- Transparency. (Transparent material) is the property of a material, that allows the passage of light in such a manner that little distortion takes place so that objects can be clearly seen through it e.g. glass. 2-Translucency. (Translucent material) is the property of the material, which allows the passage of some light and scatters or reflects the rest. In such a manner, the object cannot be clearly seen through them e.g. tooth enamel, porcelain, composite, and pigmented acrylic resin. 3- Opacity: (opaque material) is a property of the material that prevents the passage of light. Objects cannot be seen through them. Opaque material absorbs the light. A little of the light is reflected back. Each object reflects some light waves of a certain color. A Black color materials absorb all light colors. White color materials reflect all light colors. Blue color materials absorb all light colors and reflect its own color.  5)Luminescence [Fluorescence and Phosphorescence] Occurs when the wavelength of the emitted light is longer than that of the exciting waves. Ultraviolet light (365 nm) produce fluorescent light in the visible range (400--700 nm). Immediate emission is called fluorescence. objects can glow when excited and ceases immediately after excitation. Delayed emission is called phosphorescence. Objects can glow in the dark and emit light for some time after being exposed to light  Importance in dentistry 1) Sound human teeth emit fluorescent light when excited by ultraviolet radiation (365 nm). 2) The fluorescence contributes to the brightness and vital appearance of human teeth. 3) Restorative materials and dental porcelains are formulated with fluorescing agents  LASER Light Amplification by Stimulated Emission of Radiation. Production of laser light: Emission Process An element or compound "medium“ SOLID, LIQUID OR GAS can be excited by high energy to produce a special type of light called laser Characteristics of laser light a. Monochromatic:. all photons have the same wavelength. b. Coherent:. all waves are in phase (have the same speed ). c. Collimated:. all waves are parallel Laser is named according to the medium used. e.g. CO2 laser.., Argon laser , Diode laser.,.... etc. Clinical application of laser: a- In Surgery for removal of soft tissues, tissue regeneration and ulcer healing without pain. It also help coagulation of blood. b- Removal of initial carious lesions. c- Curing of composite. d- Endodontics e- Filling of pits and fissure with a mixture of hydroxyapatite. A Laboratory application of laser: a- Welding "joining of high fusing alloys. b- Sintering of ceramics.  2) Color Vision and color perception: For an object to be visible, it must reflect or transmit light incident on it from an external source. Light from an object that is incident on an eye, is focused on the retina and is converted into nerve impulses that are transmitted to the brain. Cone shaped cells in the retina are responsible for color vision. Defect in certain portion of color sensing receptors results in different kind of color blindness.  Color definition and dimensions: The perception of color is often described by three dimensions of the color is called Munsell color system. Color dimensions are: Albert H. Munsell in 1917 1- Hue: ((Dominant wavelength)) name of the colour and is related to the dominant wavelength e.g. blue, red and green etc. 2- Chroma: ((Excitation or spectral purity)) Degree of saturation or strength and intensity of a particular hue ((colour)). 3- Value: ((Luminous reflectance)) Amount of greyness in a given color ((lightness or darkness of color).  A restoration with hue ((color)) matches the adjacent dentition, but its value is too high, giving a false look ((too bright)). When hue matches but the value is too low, restoration looks dead (darker, greyer, or dull)). looks dead (darker,greyer, or dull)). false look ((too bright)).  light Source and color appearance: If possible, color matching should be done under two or more different light sources one of them is day light. and the laboratory steps should be performed under the same lighting conditions. Metamerism The change in color matching of two objects under different light sources is called metamerism. i- Metameric pair: Two objects that are matched in color under one light source but are not matched under other light sources form metameric pair. ii- Isomeric pair They are color matched under all light sources  Shade color matching Used in determining the color of natural teeth so that artificial substitute restorations will posses similar color and esthetics. 1- Dental Shade guide Shade guides used in dentistry are comprised of a set of standard color standards, which mimic the color of dental structures. Used to visually determine a color match with natural teeth. Dental Shade guide The most popular shade guides are: A- VITA Classical B- Chromoscop C-Vitapan 3D Master shade guide. A- VITA Classical shade guide consists of 16 tabs, arranged into four groups A, B, C and D B- Chromoscop consists of 20 tabs, The 16 basic shades in the shade groups A, B, C and D, Plus four Bleach shades BL 1 to BL 4. C- VITA SYSTEM 3D-MASTER introduced in 1998. It has 26 shades, divided into group 1to5. Tabs are marked using a number-letter-number combination. First number i.e. 1-5 represent Value Letter L, M, R represent Hue from yellowish to reddish The second number designates Chroma Mujtaba Ashraf 41 2- Photocolorimetry: Steps: ❖ The clinician or assistant holds the 3 or 4 closest- matching shade guide teeth next to patient teeth and a photograph is taken. ❖ Photograph is sent to dental lab. along with impression. ❖ Then the technician measure the color with a computer program. 3- Chair side colorimeter: A potable device used to determine tooth shade by direct application to patient tooth. HAVE FUN ! THANK YOU  EVALUATION SHEET # 6 1. Define and discuss the clinical significance of the following: a. Density b. Co-efficient of thermal conductivity (k). c. Thermal coefficient of expansion( α). d. Melting and freezing temperature. e. Specific heat. f. Thermal diffusivity. 2. What does laser mean, how is it produce, its characteristics and what is the clinical application of laser? 3. What are properties of materials in relation to light transmission and absorption? 4. What are color dimensions, isomerism and metamerism? 5. What is shade color matching, mention its different methods? 45