Biomaterial PDF

‫بسم اللة الرحمن الرحيم‬ Structure of Matter Presented by: Dr:Amr Sharawy Associate professor of dental materials  Introduction -The physical, mechanical and chemical properties of any material depend main...

‫بسم اللة الرحمن الرحيم‬ Structure of Matter Presented by: Dr:Amr Sharawy Associate professor of dental materials  Introduction -The physical, mechanical and chemical properties of any material depend mainly on: 1-Type of bond between atoms and molecules. 2-Inter-atomic distance. 3-Mannar of arrangement of atoms. 4-atomic packing.  Atomic Structure The atom is the basic unit of the internal structure of any material The atom consists of: 1- Central positive nucleus [positively charged protons and uncharged neutrons]. 2- Negatively charged particles [electrons] revolving around the nucleus in definite orbits (state of energy levels or shells). N.B; 1-Electrical state of atom: Neutral. 2-Atomic number: Number of electrons around the nucleus or number of protons inside the nucleus. 3-Valence electrons: Electrons in the outermost shell, which determines the chemical reactivity of the element. 4-The Atomic weight: Weight of Protons + Neutrons inside the nucleus  1-Inter-atomic Bonding (primary bonds): Atoms achieve a stable state by having eight electrons in their outer shell (as in inter gases). This can be obtained by: 1-Receiving extra electrons to complete the outer shell electrons (and the atom becomes negative ion). 2-Releasing electrons so that the outer shell has eight electrons (and the atom becomes positive ion). 3-Sharing of electrons so that the outer shells of two or more atoms are complete. -In solids, atoms are held together by either primary or secondary bonds.  Types of bonds Primary Bonds Secondary Bonds (Strong, Chemical) (weak , physical) Ionic Bond Covalent Bond Metallic Bond A. Primary Bonds: Def.: A bond that is formed between atoms and involves exchange or sharing of electrons. -Primary bond are strong chemical bond due involvement of valence electrons. -There are three types of primary bonds: 1) Covalent 2) Ionic 3)Metallic 1) Covalent Bonds: Def: It is sharing of electrons. - The hydrogen molecule, H2, is an example of covalent bonding. - The two atoms approach one another and the orbitals of the electrons begin to overlap, - The two electrons are shared between the two nuclei. The shared electrons will spend most of their time in the region where the orbitals overlap. - Double and triple covalent bonds are possible, where two or three pairs of electrons are shared between atoms. - Examples of covalent bonds are fluorine, hydrogen, diamond ,polymers and ethylene. Characteristics of covalent bonds: Very strong. Directional Insulators. Resist inorganic solvents. 2) Ionic Bonds: result from the electrostatic attraction between ions of unlike charge. -The classic example is sodium chloride (Na+ C1-), because the sodium atom contains one valence electron in its outer shell and the chlorine atom has seven electrons in its outer shell, the transfer of the sodium valence electron to the chlorine atom then the attraction of positive and negative ions results in the stable compound Na+ Cl-. Characteristics of ionic solids: 1-Heat resistant and insulator as solid. 2-Insoluble in organic solvents. 3-Easily dissolved in ionic solutions such as water, acids and alkalis, and dissociate into their constituent ions in solutions which in their turn can conduct an electric current. 4-Non directional 5-Basic bond for glasses and ceramics 6-Low coefficient of thermal expansion e.g In dentistry, ionic bonding exists in some dental materials, such as gypsum and Zinc phosphate cements. 3)Metallic Bonds: Def. : It is the attraction between +ve cores and free electrons or electron cloud". -It occurs in metals. Characteristics of the metallic bonds: The free mobility of electrons contributes to the following properties of metals: 1-High thermal and electrical conductivity due to the presence of free electrons. 2-Opaque due to absorption of light by free electrons. 3-Lustrous due to reflection of light by free electrons 4-High strength and hardness. 5-It leads to crystalline arrangement in metals 6- Low coefficient of thermal expansion B. Secondary Bonds (Van der Waals forces) "intermolecular bonds" "Dipole bonds": Def.: A bond that involves attraction between molecules. -These forces are physical, weak and arise from the polarization of molecules i.e. formation of electrical dipoles followed by physical attraction between the opposite poles. -Hydrogen bridges between water molecules are the most important example. Characteristics of secondary bonds: A solid whose molecules are bonded together by Vander Wall forces has: 1. Low strength and hardness. 2. Low thermal resistance.  Distinction should be made between: Atomic solids, such as diamond, and molecular solids, such as polymers, where in molecular solids, the covalently bonded molecules are held together by Van der Waals forces which control the properties. Molecular Atomic solids solids Primary Bonds Secondary forces hold No Secondary forces molecules High strength and Low strength and hardness hardness High melting temperature Low melting temperature Low coefficient of High coefficient of thermal thermal expansion expansion Example: Metals, diamond Example: Polymers, waxes Ceramics  2-Inter-atomic distance (I.A.D.): The space between atoms is caused by: 1- Inter-atomic repulsive force which results from the electrostatic fields of each atom. 2- Inter-atomic attractive forces which results from different types of bonding. The equilibrium distance is that distance at which the repulsive and attractive forces are equal. N.B. Application of an external force to a solid can displace atoms from the equilibrium position, and change the I.A.D.  State of Matter Matter can be classified into gas, liquid, and solid. A. Gases B. Liquids C. Solids The molecules in gases can In liquids, In solids, the atoms move freely and their with the have 1-largest interatomic distance 1-there is less inter- 1-the least I.A.D. in between atomic distance 2-the energy is high. 2-Less energy 2-the least energy.  3-Mannar of arrangement of atoms. The structure of solids Properties of materials depend on the arrangement of their atoms. Types of Solids: a) Crystalline solids. b) Amorphous solids. a) Crystalline Solids: Def.: Solids in which their is a regularity and repetition in the arrangement of their atoms i.e (atoms are regularly arranged in a space lattice). A space lattice :the arrangement of atoms in three dimensions such that every atom has a position similar to every other atom. The atoms may be held together by ionic bonds as in sodium chloride, covalent bonds as in diamond, or metallic bonds as in metals.  Types of crystal lattice or crystal systems: -Atomic packing in a crystal may take many configurations; the simplest way to study it is to consider a unit cell. Unit cell :which is the smallest repeating unit in the space lattice. -There are about 14 different types of space lattice but only few are of dental interest. The simplest way to study these types, is to consider a unit cell. -Unit cells are classified according to: a) The length of their axes (a ,b , c ). b) The interfacial angles( , ,  )  System Axes Axial angles Cubic a=b=c α=β=γ=90° Tetragonal a=b≠c α=β=γ=90° Orthorhombic a≠b≠c α=β=γ=90° Monoclinic a≠b≠c α=β= 90°≠γ Triclinic a≠b≠c α≠β≠γ≠90° Hexagonal a=b≠c α=β=90°;γ=120 Rhombohedral a=b=c α≠β≠γ≠90° Types of crystal lattice systems  1-Cubic system: The cubic space lattice is characterized by having axes that have equal lengths and they meet at right angle Cubic a=b=c α=β=γ=90° There are three types of the cubic system:  Hexagonal a=b≠c α=β=90°;γ=120 2-Hexagonal System: Simple hexagonal Close packed Hexagonal 19 b) Amorphous Solids: Amorphous means without specific form or shape. -Def.: Solids in which there is no regularity and no repetition in the arrangement of their atoms. -Gases and liquids are amorphous substances. -Some solids like glass and waxes are amorphous because of the random arrangement of their atoms, yet their atoms may form a short range order arrangement without repetition.  Comparison between crystalline and amorphous materials Crystalline solids Amorphous solids 1) Have definite melting temperature. 1) No definite melting temperature (gradually soften on heating and gradually harden on cooling).. The temperature at which they first form a rigid mass upon cooling or soften upon heating is called glass transition temperature Tg 2) No regular unit cell but may have a short range of regularity 2) Have regular unit cell with repetition. but no repetition. i.e. no repetition no regularity 3) Low internal energy. 3) Higher internal energy due to random arrangement of atoms.  4-Atomic Packing Factors: Definition: It is the fraction of the space of the structure unit occupied by the atoms and is calculated by: volume of atoms inside the unit cell Atomic packing factor= volume of unit cell A-For the simple cubic system = 0.54 which indicates that nearly 50% of the space is free. B-Body centered cubic (bcc) = 0.68 C-Face centered cubic (fcc) = 0.74 D-Closed packed hexagonal (cph) = 0.74 -Materials having higher atomic packing factor usually have higher density , stability and strength properties. -Most dental alloys crystallize in fcc or hcp space lattices. simple cubic Body centered cubic Face centered cubic Closed packed hexagonal  Imperfections in crystalline solids Theoretical strength Vs Actual strength Materials contain some defects or imperfections which decrease the actual strength  1- Point Defect: Vacancy: Interstitial Impurity: A missing atom due to imperfect Extra atom may be lodged packing during crystallization within the crystal Thermal vibrations when heated Strength Strength 2- Line Defect: e.g: Dislocation: displacement of a row of atoms from their normal positions in the lattice Strength 3- Plane Defect: e.g. Grain Boundaries in metals Strength  Polymorphism the existence of the material in different physical forms while having the same chemical structure For Organic materials → Isomerism e.g Gutta percha For Inorganic materials → Allotropy e.g Silica (SiO2) Polymorphism The existence of the same chemical compound in different crystalline (physical) forms. Polymorphism Allotropy Isomerism Occur in crystalline inorganic materials Occur in amorphous organic materials e.g. Silica in dental investment e.g. Natural rubber and gutta perca Silica (SiO2) -It is an important example for allotropy in dentistry. -It exists in nature in four different allotropic forms, which are; Quartz, Tridymite, Crystobalite and Fused quartz. -Each form has different physical properties due to their difference in crystalline structure but all are chemically SiO2. -On heating of the four forms ,two types of transformation take place:  * Polymorphism: e.g Silica 1-Reconstructive Transformation: 870ºC 1470ºC 1713ºC Fused Quartz Tridymite Crystobalite Quartz Hexagonal Rhombohedral Cubic Amorphous  2-Displacive transformation 870ºC 1470ºC 1713ºC β- β- Fused β- Quartz Quartz Tridymite Crystobalite Hexagonal Rhombohedral Cubic Amorphous 573ºC 160ºC 220ºC α-Quartz α-Tridymite α-crystobalite (SiO2) (SiO2) (SiO2)  Comparison between reconstructive and displacive transformation Reconstructive Transformation Displacive Transformation -Break down of atomic bonding -No break down of atomic bonding followed by reconstruction of only displacement of atoms giving new space lattice. the same space lattice but with larger volume. -Needs high thermal energy. -Needs less thermal energy. -Slow transformation. -Rapid transformation. --The manufacturer selects one -The selected type compensates of the types to be used in the for the solidification shrinkage of dental investment the metal during casting.  Correlation between atomic structure and materials properties: 1.Density is controlled by atomic weight, atomic radius, and the atomic packing factor. 2.Thermal expansions of materials with comparable atomic packing factors vary inversely with their melting temperature. 3.Electrical and thermal conductivity are very dependent on the nature of the atomic bonds [ionic/covalent/metallic]. 4.Melting and boiling temperatures can be correlated with the strength of the bond. Generally, materials with weaker bonds have low melting point 5.Strength can be correlated with the type of the bond. 6-The arrangement of atoms classify the materials into crystalline or amorphous structure. 7-Atomic structures are generally stronger than molecular structures because primary bonds control the properties.  Self assessment questions I. Choose the correct answer(s): 1-Fraction of space occupied by the atoms in a unit cell of a space lattice is: a. Interatomic distance. b. coordination number. c. Atomic packing factor. d. Atomic weight. 2-Amorphous solids are characterized by having: a. High internal energy. b. Random and short order arrangement c. Definite melting temperature. d. Specific shape and form in their structure. 3-Crystalline solids are characterized by having: a. Low internal energy. b. Random and short order arrangement c. Glass transition temperature. d. Specific shape and form in their structure 4-For secondary bond to occur, there must be: a. Electrons cloud. b. Electrons share. c. +ve core. d. Dipole. 5-Metals are electrical conductor due to presence of: a. Cloud of electrons. b. +ve & -ve ions. c. Shared electrons. d. All of the above. II. Give reasons for: (1) Theoretical strength of the materials is higher than the actual strength. (2) High electric conductivity of metals. (3) Molecular solids are weaker than atomic solids (4) Ceramics are heat resistant. (5) Ceramics have high melting point. Physical Properties of Dental Materials Presented by: Presented by dr. Amr Sharawy Associate professor of dental materials Cairo university physical properties are properties that are not related to force application. The physical properties described in this chapter include: I)Mass related properties: II)Thermal properties: III)Optical properties: VI)Other properties:  I) Mass related properties Density: Density: is the mass per unit volume of the material. Units : gm/cm3 Clinical Importance in dentistry: Retention of the upper denture: Dentures of lighter weight will help the retention of the upper dentures.(Non metallic denture base materials are relatively light in weight compared to metallic denture base.) 2. 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: Materials placed in the oral environment are constantly subjected to change in temperature; therefore, it is necessary to understand the thermal properties. These are: - Thermal conductivity - Coefficient of thermal expansion and contraction - Melting and freezing temperature - Heat of fusion & latent heat of fusion - Specific heat - Thermal diffusivity 1) Thermal Conductivity: Def:-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. Units: ( cal/sec/cm2/(°c/cm) Clinical Importance in Dentistry: 1) Metallic filling materials: e.g. The high thermal conductivity of amalgam is a disadvantage because if it is near the pulp it may cause patient discomfort as a result of temperature changes produced by hot or cold foods.A non-metallic substances are placed between the tooth and filling as a base for insulation. N.B.: 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 an advantage. A metal base is a good conductor and it produces normal thermal stimulation in the supporting soft tissue by having the heat readily conducted to and from the tissue by the denture base. 2) Thermal Coefficient of Expansion (): Def:. The change in length, per unit length for a 1°C change in temperature is called the linear coefficient of expansion. Units: /°C -If the tooth and the restoration have large difference in the coefficients of thermal expansion , so the tooth and the restoration will expand and contract at different rates in response to temperature changes. As large difference in (α) will leads to: -contraction with cold substances → Gap -expansion with hot substances → Closing of gap. Opening and closing of gap results in → breakage of marginal seal between the filling and the cavity wall. -The breakage of seal (marginal percolation) leads t0: i. Marginal leakage ii. Discoloration iii. Recurrent caries iv. Hypersensitivity. Clinical Importance in Dentistry: 1-Close matching of the coefficient of thermal expansion (α) between the tooth and the restorative materials is important to prevent marginal leakage 2-Porcelain and metal in ceramometallic restorations (crowns and bridges) to provide metal ceramic bonding. 3-Artificial tooth and denture base to avoid crazing. 3) Heat of fusion (L): Def.:The amount of heat in calories or joules needed to convert 1 gm of a material from solid to liquid state. Unit is (cal/g) Latent Heat of Fusion: def.: is the amount of heat in cal. or joule realesed to convert 1 gm of a material from liquid to solid state. Importance in dentistry: During casting, the metal must be heated 100 0C more than its melting temperature for proper melting. 4.Melting and freezing temperature: Units:(°C) Def.:It is the temperature at which the material melts or freezes. Importance in dentistry: For the fabrication of indirect metallic restorations (casting), the melting temperature of metals and alloys is important in determining the type of melting machine. 5. Specific heat: Def.: It is the quantity of heat needed to raise the temperature of one gram of the substance 1ºC. Therefore, metals have low specific heat, while non metals have high specific heat Unit is: (cal/g/ °C) Importance in dentistry: Because of the low specific heat of dental gold alloys , so prolonged heating is unnecessary during casting to avoid evaporation of the element of low melting points  6. Thermal Diffusivity Def. It is the rate at which a body with a non uniform temperature approaches equilibrium. i.e. (rate of heat diffusion in the body). Thermal diffusivity Δ = Thermal conductivity Specific heat x density Thermal diffusivity is more important property than thermal conductivity, as it is related to the thickness of the material (e.g. insulating base under metallic restoration should be of adequate thickness.)  III.Optical Properties: 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. Interaction of Light and Matter 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. When light falls on a smooth surface (perfectly smooth), the angle of incidence will be equal to the angle of reflection i.e. Smooch surfaces reflect light in one direction only and this is called specular reflection. Such surface appears shiny, e.g. mirrors. If light falls on a rough surface, it will be reflected in all directions, and this is called diffuse reflection. Rough surfaces undergo diffuse reflection and appear dull. Importance in dentistry: The restoration should have a highly smooth and polished surface, to simulate the tooth structure and match the color of the tooth. 2. Refraction: Def.: It is the change of the direction of a beam of light on entering second medium. -Refraction results from the difference in refractive indices of the two media. So for perfect matching the refractive index of the restoration should be equal to the refractive index of the tooth. Importance in dentistry: Refractive index of the tooth and the esthetic restorative material should match each other, to avoid line of demarcation 3. Scattering: If light rays passing through a medium are obstructed by any different inclusions it will be redirected in another direction and is attenuated. i.e. The original beam is weakened by scattering in a direction away from the observer eye. Importance in Dentistry: a) Rough composite fillings scatter more light thus appear opaque where as smooth highly polished fillings do not. b) Incorporated air bubbles in a restoration as well, act as scattering centers. c) Opacifires added to composite resins act as scattering centers that given rise to opaque shades of the material. 4. Luminescence (Fluorescence and Phosphorescence): It is a reemission of light of longer wavelength after absorption of energy of shorter wavelength from various sources. (i.e. after activation by light). Immediate emission is called fluorescence. Delayed emission is called phosphorescence. Importance in dentistry Anterior restorative materials and dental porcelains are formulated with fluorescing agents to reproduce the natural appearance of the tooth structure, as sound human teeth emit fluorescent light when excited by ultraviolet radiation. This fluorescing contributes to the brightness and vital appearance of human teeth. 5.Transmission: Light passing through an optical medium without attenuation, is said to be completely transmitted. Total transmission occurs in perfectly transparent materials. If part of the light is transmitted and part is reflected , the material appears translucent. 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. In such manner; the object cannot be clearly seen through them. e.g. Tooth enamel, porcelain, composite and pigmented acrylic resin. 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. e.g metals  Black colored White colored materials absorb materials reflects all light colors all light colors Blue colored material absorb all light colors except its own color i.e. reflects blue color Dispersion: Red Orange Yellow Green Blue Indigo violet 6. Absorption: As light passes through an optical medium, the electrons in the material may absorb the electromagnetic energy of that light. The more absorption of light, the more opaque is the material.  COLOR Color Parameters: Hue: It is the dominant wavelength. 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, i.e. measurement of color intensity. Value: It represents the lightness or darkness of color (the amount of grayness). A dark standard is assigned a value of 0 where a perfectly clear standard is assigned 10. -Value is the most important parameter of color in dentistry because it is intimately related to the aspect of vitality in human teeth. Examples: -Dead non-vital tooth have low value (more gray or dark). -Vital tooth have higher value (more vivid and translucent).  COLORs 1-Primary colors: Blue, green and red are primary colors. Combining suitable proportions of wave lengths of the three primary colors results in white. 2-Secondary colors: Each secondary color (cyan, magenta & yellow) results from the combination of two primary colors, e.g. green and red gives yellow, blue and red gives magenta, blue and green gives cyan. 3-Complementary colors : Two colors are complementary to each other when their combination results in white e.g. yellow is the complementary color of blue.  Factors affecting color appearance and selection: "Shade guide is used for color matching. So, it is important to match colors under appropriate conditions." 1.Source: Different light sources have different color content. i.e. Incandescent light has a color content different from that of fluorescent light. Metamerism: It is the change of color matching of two objects under different light sources. So, different illuminations between the clinic and the laboratory cause poor color matching. Metameric pairs: Two objects that are matched in color under one light source but are not matched under other light sources. Isomeric pair: They are color matched under all light sources Thus, if possible, color matching should be done under daylight. Daylight contains light of almost all visible wavelengths, Otherwise, color matching should be done under two or more different light sources. 2. Surrounding: Colors of wall, lips or clothes of the patient modify the type of light reaching the object. 3. Object: -Thickness: The thickness of a restoration can affect its appearance. Increase in thickness, increase opacity, and lower the value -Translucency: It controls the lightness or darkness of color. High translucency gives a lighter color appearance ((higher value))i.e. More vital tooth appearance. -Surface texture ((surface finish)): Smooth surface appear brighter than rough surface. -Presence of scattering centers as inclusions or voids, which increase opacity and lower the value ((more dark)). -Metamerism Two objects that are matched in color under one light source but are not matched under other light sources -Fluorescene: It makes the teeth bright and vital, as it increases the brightness. 4. Observer: Color fatigue: Constant stimulus of one color decreases the response to that color. A complementary color image persists after removal of the stimulus. Color Vision: Some individuals may have color blindness and inability to distinguish certain colors. Less specific properties 1- Water Sorption: It represents the amount of water adsorbed on the surface and absorbed into the body of the material. Importance: 1- Acrylic resin denture base materials have the tendency for water sorption. 2- Hydrocolloid impression materials will imbibe water if immersed in it leading to dimensional changes. 2-Fluidity, viscosity and plasticity: Fluidity :is the tendency of liquids to flow. Viscosity: is the resistance to flow. Plasticity: is a property related to solids or semisolids and indicates that the material is easily and permanently deformed under force. 3-Shelf Life: It is a term applied to the general deterioration and change in quality of materials during shipment and storage Choose the correct answer 1-The presence of scattering centers in composite restoration makes the restoration to appear: a. Translucent. b. Lustrous c. Opaque. d. Transparent. 2-It is the change of the direction of a beam of light on entering second medium: a. Reflection b. Refraction c. Scattering d. Transmission 3-Diffuse reflection makes the object appears: a. Less translucent. b. Dull. c. More translucent. d. Transparent. e. Shiny 4-All incident light colors with wave lengths different from that of the object surface color will be: a. Reflected. b. Refracted. c. Absorbed. d. Dispersed. 5-The Secondary colors are: a. Cyan, green and red. b. Magenta, cyan and green. c. Red, blue, green d. Cyan, yellow and magenta. II-Give reason(s): 1. Value is the most important color parameter in dentistry. 2. Aesthetic dental restorations should have smooth surfaces. 3. Inadequate polishing of an aesthetic restorative material yields a dull restoration. 4. Placing a metallic filling material in a deep cavity should be preceded by application of an insulating base. 5. A high casting pressure is generally required for base metal alloys. N  Presented by: Dr./ Amr Sharawy Assoc. professor of dental materials Force: What is force? It is the action applied on an object to change its position of rest or motion Force is defined by four characteristics: Speed determine if the force is static or dynamic Magnitude Point of application Direction determine if the force along the long axis or parallel to it  Units: N, kg, Ib  Stress (σ): Def:-Internal reaction of a structure to externally applied load; external load and internal stress are equal in magnitude and opposite in direction. σ = force / area Units: MN/m2 = MPa (M = 1000) Types:  1-Compressive stress 2-Tensile stress 3-Shear stress The body is subjected to 2 sets of The body is subjected to 2 sets of The body is subjected to 2 sets of forces: forces: forces: On the same straight line. On the same straight line. Parallel to each other. Directed toward each other. Directed away from each other. Directed toward or away from each other.  Complex stress if we stretch a wire, the observed stress will be tensile but the cross section of the wire will decrease indicating the presence of compressive stresses.  STRAIN (ε) : Def:-Change in length per unit length STRAIN (ε) = Lf – Lo Unit: dimensionless Lo Deformation: Change in length ( L final - L original ) Unit: cm, m Types of strain:- 1-Elastic strain (temporary ) Totally disappears upon removal of external load. 2-Plastic strain (permenant) Doesn’t disappear upon removal of external load.  Relation between stress & strain The stress-strain relationship of a dental material is studied by applying a load, measuring the deformation and calculating the corresponding stress and strain. Stress 1 2 4 8 16 / 18 20 24 32 Strain 0.5 1 2 4 8 / 8.1 8.5 9.1 10 Hook’s law: Stress is directly proportional to strain , until certain stress known as proportional limit Proportional limit Yield strength And Elastic limit Ultimate strength 400 – Stress (MPa) - 300 – Fracture strength - 200 – - 100 – - - - - - – – – – – – 0.0 0.01 0.02 0.03 0.04 0.05 - Elastic Plastic 0– Strain Strain Strain  The stress-strain curve consists of two portions Elastic portion (Linear) Plastic Portion (Non-linear) - It obeys Hook's law -It does not obey the Hook's law -The strain is directly proportional to the - The strain isn’t directly applied stress i.e. doubling the stress will proportional to the applied stress. double the strain. -When the stress is removed the original size -When the stress is removed, and shape is recovered. the original size and shape isn’t recovered.  Properties derived from the stress strain curve 1-Proportional limit (σPL) 2-Elastic limit (σEL) 3-Yield strength (σY) 4-Ultimate strength 5-Fracture strength (σF) It’s the maximum stress that It’s the maximum stress It is the stress at which the material begins It’s the maximum stress that a It’s the stress at which a a material can withstand that a material will withstand to function in a plastic manner material can withstand before material fracture without deviation from the without permenant faracture.. proportionality of stress to deformation It’s difficult to be measured than PL and strain. EL. Importance in dentistry: NB: Below PL, strain is -Yield stress is more elastic. Above PL, strain is important than the ultimate both elastic and plastic. stress, GRF because yield stress deformation. represents the clinical failure (functional failure). - N.B. Both proportional limit Dental consideration: Proportional limit Yield strength and elastic limit represent the And same stress. However they NB: It’s the most important point on the Elastic limit differ in fundamental concept, curve because beginning of undesired Ultimate strength in that permanent deformation is considered the beginning of functional failure of the 400 – the proportional limit deals material, with the proportionality of. - stress to strain in the structure Destructive deformation: permanent Stress (MPa) deformation of dental appliance under 300 – whereas masticatory forces Fracture strength the elastic limit describes the - elastic behavior of the Constructive deformation: material. -Shaping of orthodontic wires 200 – - 100 – - - - - - – – – – – – 0.0 0.01 0.02 0.03 0.04 0.05 -burnishing of crown margins - Elastic Plastic 0– Strain Strain Strain 6-Modulus of elasticity (E): (Elastic modulus, Young’s modulus) Def.: the constant of proportionality between stress and strain It measures the stiffness or rigidity of a material. It’s calculated from the equation: A σ B C Units: N/m2, MN/m2 (MPa) or kg/cm2 - It represented by slope of the linear portion of the curve ε -From the figure, the order of rigidity (stiffness) is A > B > C -It depends on the strength of interatomic bonds in a material, so it’s affected by the composition of the material. i.e.: Elastomers and polymers low modulus flexible material Metals and ceramics High modulus Stiff material Clinical Significance: 1-Denture base should be constructed of a rigid material To be used in thinner sections and allow proper stress distribution without the risk of bending to reduce the rate of bone resorption. 2-Rigid base should be used under restorative filling material to increase the fracture resistance of the filling. 7-Flexibility: Def:- is the strain which occurs in the material when the stress reaches the elastic limit. Units:-dimensionless -From the figure, the order of flexibility is C > B > A Clinical Significance: a-Endodontic files and reamers where considerable amount of elastic bending may take place with little stress b-Large strain or deformation may be needed with a moderate or slight stress as in elastic impression material, since flexibility represents the ease by which the impression can be removed from the mouth Stress Max Flexibility Strain  8- Ductility and malleability Ductility Malleability It’s the ability of a material to undergo plastic It’s the ability of a material to undergo plastic deformation under tensile force without rupture. deformation under compressive force without fracture. i.e. drawn into wires i.e. humored into sheets Both are represented on the stress – strain curve by the plastic strain portion. From the figure, material A has higher amount of plastic deformation the material B, so material A is said to be more ductile (malleable) than material B. Material C has no plastic deformation, i.e. fracture occurred at or just beyond PL of the material. These materials are described as brittle materials Dental application: Burnishability of the crown margins. Percentage elongation is the measure of ductility. σ A -Ductility & Malleability are demonstrated in the stress- strain curve by long plastic portion of the curve B -From the figure, the order of ductility is A > B > C C ε 9-Brittleness: -Brittle material : material showed no or very little plastic deformation on application of load -In other words, a brittle material fractures at or near its proportional limit. -This fracture occurs by crack propagation -Brittle materials are not tough and have low % elongation -Moreover, brittle materials are weak in tension than compression e.g dental amalgam has a compressive strength which is nearly six times higher than its tensile strength. Fracture toughness: Def.: the amount of energy required to fracture a material with crack. -Fracture toughness gives a relative value of a brittle material's ability to resist crack propagation. -Brittle materials have lower fracture toughness than ductile materials. Importance in dentistry: -The addition of 50% zirconia to porcelain and the presence of fillers in polymers increase their fracture toughness because they deflect the crack or obliterate the crack by this more energy will be needed to propagate the crack leading to higher fracture toughness. Fillers Polymer 10- Resilience and toughness: Resilience Toughness (Spring back action) Def: It represents the amount of energy absorbed by a material It is the energy required to stress the material to the point of fracture. when it’s stressed to the proportional limit. Opposite: Not resilient Not tough On the stress It represents the area under the linear portion of the curve. It represents the entire area under the curve. strain curve: σ σ PL PL ε ε Affected by: PL. and modulus of elasticity. Strength and ductility Units: mMN/m3 mMN/m3 (energy per unit volume) (energy per unit volume) Measurement: By calculating the area of the triangle under the elastic By calculating the area under the elastic and plastic portion of the stress-strain curve portion of the stress-strain curve. Dental -It represents energy stored in an orthodontic wire to be -Brittle materials are not tough. application: released during teeth movement over extended period of time.  Properties and Stress-Strain Curves Stress Stress Strain Strain Strong = P.L. Weak = P.L. Stress Stress Strain Strain Stiff = E Flexible = E Stress Stress Strain Strain Ductile = Plastic deformation Brittle = Plastic deformation Stress Stress Strain Strain Resilient = area of the Not Resilient = area of triangle below elastic the triangle below elastic slope slope Stress Stress Strain Strain Tough = area under the Not tough = area under the elastic and plastic (curve) elastic and plastic (curve) Other mechanical tests: 1- Diametral compression test for tension: Advantages: P It’s used to determine the tensile strength of brittle materials. Technique: A disk of the brittle material is compressed diametrically in a testing machine until it fractures. D T The applied compressive stress introduces tensile stress in the material perpendicular to the plane of the force application Tensile stress is calculated from the formula: D → diameter P T → thickness P → load 2- Transverse strength: (flexural strength, three point bending;3PB, Modulus of rupture) -The transverse strength of a material is obtained when a simple beam, supported at each end, is loaded with a load applied in the middle. Such a test is called a three point bending test. 3 PL 3 X load X length = or Stress = 2 bd2 2 X width X thichness2 Dental significance: 1) Denture base materials are subjected to such type of stress during mastication. 2) In long span bridges both the length and the thickness of the span are critical as evident from the equation for deformation. The deformation varies as the cube of these two dimensions. 3- Fatigue: Def:It’s the failure of the material under repeated cyclic loading below its P.L. Endurance lim Endurance limit: it’s maximum stress below which a material can withstand an infinite number of cyclic loading without failure by fatigue. -From such curve [ S-N ] we can see that; i)when the stress is high, the material will fracture at a low number of cycles. ii) As the stress is reduced, the fracture occurs at a high number of S-N curve: cycles -Therefore, Failure under cyclic loading dependent on: a. the magnitude of the load b. the number of loading repetitions. Mechanism of fatigue: Cyclic loading promotes crack propagation, Fatigue fracture occurs is by the developing of small microcracks, which coalesce to form macrocracks, which will propagate through the material. Clinical significance Structures such as complete dentures, implant and metal clasps of removable partial dentures, which are placed in the mouth by forcing the clasps over the teeth, are examples of restorations that undergo repeated loading, and may fracture by fatigue. 4- Impact strength test: Def: energy required to fracture a material under sudden force. *Two types of impact testers are available: 1-Charpy testing machine. the specimen is h supported horizontally at the two ends. h’ 2-Izod instrument, the material is clamped at one end and held vertically. Clinical Significance: dropping the complete denture on a floor may cause its fracture, for this reason high impact acrylic resin denture were developed. Izod Charpy Surface mechanical properties: 1) Hardness: Def:- is the resistance of the material to permenat indentation or penetration or scratching. Complete penetration i.e soft material No penetration i.e hard material -The most common methods for testing the hardness are the Brinell, Knoop, Vickers, Rockwell, and shore A -All methods used to measure the hardness, depend on the penetration of small indenter into the surface of the material. The smaller the indentation the higher is the number, the harder is the material and vice versa. Clinical Significance of surface hardness 1-Hardness is an important property to consider in order to avoid scratching structures like teeth or restorations e.g. Natural teeth should not be opposed by harder materials like porcelain. 2- Restorations made of hard material like cobalt chromium is very difficult to finish and polish but once it is polished it maintains its polished surface with No scratches. 2) Wear Def:- is the loss of material resulting from mechanical action. Wear is usually undesirable but during finishing and polishing procedures wear is highly desirable Causes of wear 1- Physiological e.g normal mastication 2- Pathological e.g bruxism. 3- Mechanical e.g improper use of tooth brushing Rheological Properties  Rheology: is the science which deals with flow and deformation of matter. Dentists are subjected to manipulate materials that flow or deform when subjected to stresses. Rheological properties includes: 1-Viscoelasticity 2-Creep  1-Viscoelasticity: Def: It describes materials that exhibit characteristics of both elastic solids & viscous fluids. -Viscoelastic materials are strain-rate dependent materials dependent on how fast they are stressed. -Increasing the rate of loading produces the higher value of their mechanical properties. At the beginning after 1 min. after 2 min. after 3 min. -Viscoelastic materials are combination of elastic, viscous and anelastic behaviors e.g. elastic impression materials, amalgam, dentin, gingiva and waxes. Mechanical models of viscoelasticity: 1-Ideal 2-Ideal viscous 3-Ideal elastic anelastic When load is Immediate strain Gradual strain Gradual strain applied The strain remains constant the strain increases linearly non linear (gradually) increase in with time. (gradually) with time. the strain with time When load is Immediate recovery No recovery gradual but complete recovery removed strain is time strain is time Strain is time independent dependent dependent Strain Strain Strain t0 t1 t0 t1 t0 t1 Time Time Time Viscoelastic behavior: It is a combination of elastic, anelastic and viscous behavior. This combination strain or viscoelastic strain is time dependent. -Upon load application i) Immediate strain will occur due to elastic portion and then followed by ii) gradual non linear increase in strain due to both viscous and anelastic parts. -Upon load removal i)The elastic strain is immediately recovered ii)The anelastic strain is gradually recovered. Strain iii)Viscous strain is not recovered which resultsin some permanent deformation from (1% -3%). t0 t2 t1 Time N.B : As viscoelastic strain is time dependent so rapid rate of loading (less time) will result in less permanent deformation Clinical Significance: Elastic impression materials must be removed rapidly from the mouth (snap removal) i.e. less time = high rate of loading) in order to: 1-Minimize the amount of permanent deformation in the impression 2-Increase the tear strength of the impression -On removal from the mouth they should be given time to recover before a model can be poured (to give time for gradual recovery of the anelastic part) Impression Tray Impression A-Before removal b-After removal) c-Slow removal d-sharp snap removal of the of the impression of the impression(ideal) of the impression impression(less permanent deformation) 2-Creep: Creep is: Time dependent permanent deformation occurs at stresses below the proportional limit and at temperature near the softening point of a material. wax wax At stress below proportional limit At stress below proportional limit and after certain time at elevated temperature No permanent deformation permanent deformation occurs -Since the softening temperature of most metals and ceramics is far above mouth temperatures, they do not creep in dental application. However, many polymers such as waxes and rubbers have softening point near mouth temperatures and can creep considerably. Clinical Significance: -In dental amalgam restorations, they contain components with melting temperature slightly above mouth temperature, thus they undergo creep which can be destructive to the restoration. N.B: -Flow: is the creep for amorphous materials, such as waxes. -Sag: is the creep at higher temperatures , such as metals.  Self assessment questions I-Give reasons for: 1-Amalgam and wax restorations undergo creep in patient mouth. 2-Structures subjected to bending fail on the surface that is increasing in convexity. 3-The yield strength is of greater importance than ultimate strength. 4-he high modulus of elasticity of ceramic solids. 5-The ability of pure gold to be easily (burnished) shaped into wires and sheets. II- Choose the correct answer: 1-The energy required to fracture a material under sudden force is …. a-Toughness b-Ultimate strength c-Resilience d-Impact strength 2- The greatest stress to which a material can be subjected and return to its original dimensions when the load is released is the: a. Elastic limit of the material b. Proportional limit of the material c. Compressive strength of the material d. Shear stress 3- The total work or energy required to rupture a material is: a-Resilience b-Toughness c-Brittleness d-Ultimate strength III. Draw and label the stress-strain curve of: 1-A stiff, ductile, strong and tough material. 2-A flexible, brittle and weak material. 3-Draw and label a strain-time relationship for a viscoelastic materials showing different behavior during and after load application  Polymers presented by Dr. Amr Sharawy Associate professor of dental materials  Polymers Millions of the polymer chains interlock and entanglement like the spaghetti structure. -Generally the mechanical properties of the polymers are Much lower than metals and ceramics. Polymer: A long chain molecule consisting of many repeating units called monomers or mers. Monomer (mer): Smallest repeating unit in a polymer. Mono = Single Mer term is derived from the Greek. Oligomer: Is a short polymer composed of less than 10 mers.  Polymerization reaction: The reaction by which the monomer units become chemically linked together to form(polymer). Homopolymer: One type of monomer Methylmethacrylate monomer PolyMethylmethacrylate polymer Copolymer: More than one type of monomer BUTYALmethacrylate & Methylmethacrylate Mers bonded together by covalent bonds within the polymer chain : Intra-molecular = covalent bond = 1ry bond Polymer chains bonded together by van deer +ve waal force due to the polarization and -ve -ve formation of dipoles along the polymer chain. +ve Intermolecular = 2ry -ve bond (polar bond) +ve -ve  Classification of Polymers 1. According to Origin: Natural polymers: e.g. a) proteins b) polysaccharides (starch) e.g. agar and alginate c) polynucleic acids (DNA) Synthetic polymers: They are produced in the laboratory by chemical reactions, e.g. Acrylic resin and elastic impression materials 2. According to their spatial configuration [arrangement in space]: 1.Linear: The monomer units are connected to one another in linear sequence.(chains are bonded by 2ry bonds) 2. Branched: In which side chains are connected to the main linear chain.(chains are bonded by 2ry bonds.). 3. Cross-linked polymers: In which the adjacent linear chains are joined one to another at various positions by covalent bonds.  3. According to Thermal Behavior Thermoplastic polymers Thermoset polymers They soften by heat when heated They undergo a and harden by cooling). chemical reaction and become permanently hard. *Upon heating, weak polar *They can not be re-melted or bonds are broken,& chains are separated, and it can be fused or reformed into other shaped. shape but degrade or decompose They are relatively soft and upon being heated to high their mechanical properties are sensitive to heat. temperature. The polymer chains are bonded *Linear Polymer chains are with primary covalent bonds. bonded to each other by (cross-linked acrylic resin). secondary bonds.  Polymerization reactions is the chemical reaction by which monomer units become chemically linked and form polymer chain Types of polymerization reactions: Condensation Addition polymerization polymerization  1. Condensation polymerization: Definition: reaction between two molecules to form a larger molecule, with the elimination of a smaller molecule (by-product)) -Change in composition takes place during polymerization. -Characterized by: 1- Elimination of Small molecular weight by-product. 2- Poor dimensional stability due to evaporation of the byproducts. Examples of condensation polymerization in dentistry are: -Polysulphides rubber impression materials Where water and lead sulfide are by-products of this reaction. -Condensation Silicones Where ethyl alcohol as by-products of this reaction.  2. Addition polymerization: Addition polymerization results in the formation of large molecules without the formation of by-products. No change in composition takes place. The structure of the monomer is repeated many times in the polymer, e.g. poly (ethylene), poly (acrylic acid), poly (methacrylic acid) and poly (methyl methacrylate). Ethylene C2H4 monomer ( H2C=CH2 ) + ( H2C=CH2 ) +( H2C=CH2)  Stages of addition polymerization: i- Activation and initiation ii- Propagation. iii- Termination. a. Activation and initiation : ***Requires the presence of free radicals. A free radical is a compound with an unpaired (unshared) electron. This unpaired electron makes the free radical very reactive. -To release the free radical initiator must be activated by either :- *Heat or *Light or *Chemical compound (dimethyl paratoluidine) R R heat or light 2R Initiator chemical compound free radical -Example of initiator is benzoyl peroxide -Addition polymerization could be inhibited by any material that reacts with the free radical When a free radical (R ) encounters a double bond → it pairs with one of the electrons leaving the other member of the pair free. Dot = single electron Now the monomer is a free radical b. Propagation: -Chain reactions should continue, with the evolution of heat until all the monomer has been changed to a polymer. -Linear growth of molecules →↑ Viscosity And so on until R1Mn. Where n is any integral number. c. Termination: The chain reactions can be terminated either by 1) Direct coupling or by the. 2) Exchange of a hydrogen atom from one growing chain to another Factors associated with polymerization: a. Evolution of heat as the reaction is strongly exothermic due to breakage of bonds. b. Reduction in volume on polymerization (polymerization shrinkage) c. Residual monomer is always left due to termination. Inhibition of polymerization: Any impurity in the monomer which can react with free radicals will inhibit or retard the polymerization reaction It can react either with: - the free radicals - any activated nucleus, - an activated growing chain to prevent further growth. Such inhibitors influences the length of the initiation period markedly, as well as the degree of polymerization.. e.g. Hydroquinone, Eugenol, large amounts of Oxygen Clinical significance: 1- The addition of small amount of inhibitor (hydroquinone) to the monomer will inhibit premature polymerization during storage increasing its shelf life. 2-Eugenol must not be placed in contact with any polymerizing material (e.g. resin composite), to avoid retardation of polymerization reaction.  General properties of polymers 1-Generally the polymers are molecular solids where: Strong primary covalent bonds exist between the mers along the whole length of the polymer chain (interatomic) Weak secondary Van der Waal forces exist between the chains of the polymer (intermolecular). Secondary Van der Waal forces (weak, polar bond) are responsible for; -- Lower strength, hardness and rigidity. -- Higher water sorption &dissolution in organic solvent (CRAZING). 2-Polymers are characterized by being amorphous and having glass transition temperature (Tg) (it is the temperature at which the polymer start to be soft) i.e. above Tg : the polymer is soft and rubber like and below Tg : the polymer will be very rigid 3. Visco-elastic materials: -The polymers are sensitive to the rate of loading. -At low rate of loading (slow forces) they behave in a ductile manner (more permanent deformation). -At high rate of loading (rapid forces) they respond in a brittle manner (fracture at or near the proportional limit). - During loading they undergo viscoelastic behavior and creep.  Factors affecting properties of polymers 1. Molecular weight and Degree of polymerization : -Molecular weight Molecular weight of the various mers multiplied by the number of the mers = M.W of one monomer X No. of monomer units The higher the molecular weight of the polymer, → the higher the degree of polymerization -Degree of polymerization defined as the total number of mers in a polymer chain. D.P.= M.W. of a polymer/ M.W. of a mer Effect on the properties; The longer the polymer chain → the greater the number of entanglement that can be formed among the polymer chain. i.e. The more difficult to distort the polymer. ***Strength, rigidity, and glass transition temperature increase with increasing chain length. Average molecular weight Not all the chains within the polymer will have the same length. Each chain will have its own molecular weight and degree of polymerization. In general the molecular weight of a polymer is reported as the average molecular weight because the number of repeating units may vary greatly from one chain to another.  The molecular weight distribution The molecular weight distribution is the fraction of low, medium, and high molecular weight molecules in a polymer. It has an important effect on the physical and mechanical properties as the average molecular weight does. Give Reason: Two polymers- having the same chemistry and same molecular weight- may have different properties. This may be because one is formed of long chains, while the other is formed mainly of shorter chains i.e. different molecular weight distribution. 2. Crystallization: Polymers may be crystalline or amorphous. Crystalline ones have their chains arranged in a regular geometric pattern with intervening amorphous regions. Effect on the properties: The higher the crystallinity of a polymer, the higher is its tensile strength as well as its Tg  3- Cross-linking: Adjacent linear chains are joined one to another at various positions by chemical bonds (covalent bonds). The effect of cross-1inking: A small degree of cross-linking limits the amount of movement of the polymer chains relative to each other when the material is stressed. This * increases : - strength, hardness, rigidity, -resistance to the action of solvents -and - glass transition temperature. * decreases water sorption. Extensive cross-linking may lead to brittleness of the material 4. Co-polymerization: Copolymers are polymer chains containing two or more different types of monomer units. Co-polymerization processes enable chemists to "tailor-make" molecules of predicted properties for special applications. Co-polymers are of three different types: random, block and graft. Linear co-polymer random Branched co-polymer random Block co-polymer Graft co-polymer 5. Plasticizers: Def: They are compounds which are added to partially neutralize the secondary bonds or intermolecular forces that normally prevent the resin molecules from slipping past one another when the material is stressed. There are mainly two types of plasticizers: a. External plasticizers: in which the plasticizer penetrates between the polymer chains. The polymer chains become further apart and the forces between them become less. b. Internal plasticizers: plasticizer of a resin can also be accomplished by copolymerization with a suitable co-monomer. In this case the plasticizer is a part of the polymer main chain. The effect of plasticizers: *They usually reduce the strength and hardness of the resin * Reduce the softening point (glass transition temperature) ***At room temperature the main difference between rigid and flexible polymers is that the former have their glass transition temperature above room temperature whereas the latter have theirs below room temperature. (the opposite action of cross-linking agent). 6. Sensitive to the rate of loading Polymers are sensitive to the rate of loading because they are viscoelastic materials.At slow rate of loading they behave in a ductile manner(more permanent deformation). At high rates of loading they respond in a brittle manner (fracture after elastic limit). 7. Sensitive to temperature. -Polymers are characterized by being amorphous and having glass transition temperature (it is the temperature at which the polymer start to be soft)i.e. above which the polymer is soft and rubber like material, and below which the polymer will be very rigid 8. strength can be increased by: a. Increasing the degree of polymerization and thus increasing M.W. b. Cross-linking. c. Copolymerization. d. Addition of inorganic fillers to form composite structures. e.g. composite restorative resins.  Applications in dentistry 1. Denture base materials. 2. Artificial teeth. 3. Impression materials. 4. Tooth restorative materials (e.g. resin composite). 5. Cements. 6. Crown and bridge facings. 7. Dies. 8. Endodontic fillings. 9. Implants. 10. Maxillofacial prosthesis  Application of polymers in dentistry Acrylic resin polymer Restorative Rubber impression materials materials Cement Endodontic filling materials THANK YOU ‫الر ِّح ِّيم ِِّ‬ ‫من َّ‬ ‫الر ْح ِّ‬ ‫ِّب ْاَس ِّم ِّ‬ ‫هللا َّ‬ ‫{‪}27‬‬ ‫اَسِ ِّي‬ ‫احل ُ َ ِّ ِّ َ‬ ‫ل‬ ‫ن‬ ‫م‬ ‫ً‬ ‫ة‬ ‫د‬‫ق‬‫ْ‬ ‫ع‬ ‫ل‬‫ْ‬ ‫ُ‬ ‫َو ْ‬ ‫{‪}28‬‬ ‫يَ ْفقَ ُهوا قَ ْو ِّل‬ ‫صدق اهللا الﻌﻅيﻢ‬  Applied surface phenomenon Presented by Dr. Amr Sharawy Associate professor of dental biomaterials. Definitions: Cohesion: Is bonding between two similar materials. Examples of cohesion: Bonding two pieces of pure gold together under pressure is an example of cohesion. The bonding in such case results from metallic bond and is called pressure welding. Adhesion: It bonding between two dissimilar materials through chemical reaction of their atoms and molecules. Adhesive: Is the liquid material used to produce adhesion. Adherend: Is the solid substance to which the adhesive is applied. For adhesion to take place: - Materials being joined must be in intimate contact. Examples of adhesion: Denture retention is accomplished by the adhesive action of a thin film of saliva between the soft tissue and the denture base.  Types of adhesion: 1-Chemical (true) adhesion 2-Mechanical interlocking (attachment) 3-Physical (polar) bond - Caused by chemical If a liquid flow into pores of a solid surface occurred through reaction (primary bonds). and is allowed to set, it will lead to physical attraction between mechanical interlocking. The bond, which denture and Thin film results between the hardened liquid and of saliva. the solid surface, may be strong and the phenomenon is called attachment. -Example: Example: Example: Chemical bond of Composite filling materials upper denture retention. polycarboxylate or glass ionomer cements with tooth structure, as both contain carboxylic group that reacts with the calcium of the tooth structure to form primary chemical bond. Adhesive Adherend  Adhesive Junction Def. It is the transition layer between the adhesive and the adherend Factors affecting the strength of adhesive junction: 1. Wetting 2. Thickness of adhesive film 3. Cleanliness of the adherend 4. Stresses due to setting contraction of the adhesive 5. Thermal stresses between the adhesive and the adherend 6. Type of bond formed  Factors affecting the strength of adhesive junction: 1) Wetting : -Is the ability of an adhesive to spread over the solid surface (the adherend) -The degree of wetting is measured by the contact angle. Contact angle: It is the angle between the surface of the liquid and the surface of solid. -The degree of wetting is measured by the contact angle. The smaller the contact angle (the more acute), the better the wettability. -For an adhesive to produce good wetting with the adherent, the contact angle must be zero or less than 90° -Good wetting promotes adhesion and indicates strong attraction between the liquid and solid surface molecules. Importance of wettability in dentistry: 1.Good wetting is important in soldering. 2.Good wetting is a factor in better denture retention. 3. A more natural appearance is achieved if restorative materials are wetted by a thin film of saliva. Factors affecting wetting: a. Surface tension of the adhesive b. Surface energy of the adherend c. The viscosity of the liquid adhesive d. The surface irregularities of the adherend (surface roughness) a.The surface energy of the adherend (S.E.) It is the surface free energy of the atoms at the solid surface. Increased surface energy of the solid, increases wettability. Examples: 1. Metals usually have a higher surface energy and therefore they are relatively easy to wet by suitable adhesive. 2. Waxes are not easily wetted because they have low surface energy. 3. Teflon used in non stick cooking utensils has low surface energy. S.E Contact angle Wetting. b. The surface tension of the adhesive (S.T.): The attraction force between the surface molecules of the liquid adhesive -Increasing surface tension of the adhesive, decreases the wettability. Therefore, using adhesive liquid of low surface tension will increase its wettability to a solid. -For good wetting, the surface tension of the liquid should be equal or less than the surface energy of the adherend (solid) S.T Contact angle Wetting  c. The surface irregularities of the adherend (surface roughness): Surface irregularities prevent an adhesive from completely wetting the adherend. Air pockets may be formed in small pit or crack and prevent the adhesive from penetrating into that area. S.R Wetting A. Improper irregularities : Irregular, deep and narrow surface roughness air pockets formation will prevent the adhesive from penetrating into that area, no intimate contact between adherend and adhesive will be formed Weak adhesive bond. B. Proper irregularities : Regular ,shallow and wide surface roughness no chance for air pockets formation intimate contact between adherend and adhesive Good adhesive bond. d. The viscosity of the liquid adhesive: Increasing the viscosity of the adhesive, decreases the wettability. Low viscosity of the adhesive is required to allow its easy flow on the surface of the adherend. This increases the strength of adhesion. viscosity Wetting 2) Thickness of the adhesive film: The thinner the adhesive film, the stronger is the adhesive junction Due to less air voids are present. The thinner the adhesive the stronger the adhesive joint 3)Cleanliness on the adherend: Any debris or surface contaminations prevent the adhesive from coming into the intimate contact which is necessary to produce adhesion. Clean surface Debris Clean surface------- better wetting 4)Stresses due to setting contraction of adhesive: Liquid adhesives undergo contraction during setting. This contraction results in the creation of stresses at the interface that severely decreases the strength of adhesion. setting stresses strength of adhesive junction 5) Thermal stresses (between the adhesive and the adherend): If the adhesive and adherend have different thermal coefficients of expansion, changes in temperature will produce stresses in the bond. -Close matching in coefficients of thermal expansion is required to minimize stresses and so increases the strength of adhesion. Thermal stresses strength of adhesive junction 6) The type of bond formed: No doubt that primary bonds between adhesive and adherend produce stronger adhesion than if secondary bonds are formed (Soldered Joint is stronger than glued junction).  Factors essential for proper adhesion Adherend should have Adhesive should have High S.E Low S.T Proper surface Low viscosity irregularities Low setting contraction Clean surface Thin film thickness - Close matching in coefficient of thermal expansion - Primary bond formed between both  Adhesion of a restoration to tooth structure is very difficult Obstacles: 1.The inhomogeneous composition of enamel and dentin. 2.Surface irregularities in the prepared cavity. 3.Debris in the prepared cavity. 4. Presence of water in the prepared cavity.  Failure of the adhesive junction 1- Adhesive Failure 2- Cohesive Failure between the adhesive and the within the adhesive or within the adherend adherend  Importance of adhesion in dentistry 1. Decrease marginal leakage between restoration and cavity walls. 2. The prevention of tooth decay by sealing pits and fissures. 3. Complete denture retention through thin film of saliva. 4. Soldering operation. 5. Ceramometallic restoration.  Bonding to tooth structure -Surface treatments should be performed in order to help bonding of materials to enamel and dentin. -Two mechanisms of adhesion (bonding) may be distinguished: chemical and mechanical. -The most widely used technique is acid etching of enamel and dentin to produce mechanical bonding between the tooth and composite filling materials. A- Bonding to enamel: ( acid etching technique ) - The most commonly used acid etch is 30-50 % phosphoric or citric acid -Etching the surface of enamel by applying the acid for 15-30 seconds. - The acid removes about 5 microns of the surface of enamel and produce micropores into which the adhesive (bonding) will penetrate, then resin composite (filling material) bond to the adhesive.-. -Acid etching helps bonding to enamel by:- 1. Removal of surface debris “produce clean surface”. 2. Producing pores in the surface into which resin penetrates to form tag-like extensions, giving mechanical interlocking. 3. Increase surface area of the enamel to the resin 4. Increasing the surface energy of the enamel, causing better wetting.. Enamel surface after acid etching B. Bonding to dentin: - Dentin poses greater obstacles to adhesive bonding than does enamel due to; i) Presence of higher amount of water so it is strongly hydrophilic. ii) Presence of smear layer which will prevent proper adhesion. N.B; Smear layer is a 5-10 microns thickness layer formed of a matrix of collagen containing tooth structure, blood, saliva and bacteria resulting from cavity preparation. Dentin bonding involves three distinct processes: 1.Etching (conditioning) 2.Priming 3. Bonding 1. Etchaning: -Dentin etching is done by applying the acid etchant for 10 to 15 seconds. -This will lead to: Partial or complete removal of the smear layer (debris layer). Demineralization of dentin surface. N.B; On the other hand, etching of dentine by acid will lead to reduction of surface energy of dentin, as the demineralization of dentin will lead to exposure of more collagen that have low surface energy. 2. Primer: It is used to elevate the surface energy of dentin to improve wetting. - Because composite resins are hydrophobic,the primer should contain both hydrophilic and hydrophobic materials. -The hydrophilic part should be designed to interact with the moist dentin surface, whereas the hydrophobic part bond to the restorative resin. 3.Dentin bonding agent: - Bond the primed dentin surface to resin composite restoration, and the bond results is micromechanical rather than true chemical adhesion. - The final successful bond that is aimed to be produced, should have a continuous layer along the dentin surface called hybrid layer (resin infiltrated dentinal layer), which is a resin reinforced layer part is tooth and part is resin. I. Give reason(s) for: Self assessment questions 1-It is difficult to obtain proper adhesion to tooth structure. 2-Acid etching is done to enamel surface before applying a composite filling material. 3-Thick adhesive film should be avoided. 4-Close matching in coefficient of thermal expansion between the adhesive and the adherend is essential. 5-Using of primer after etching the dentin surface. II. Choose the correct answer(s): 1. For stronger adhesive junction a. Surface energy of the adherend must be higher than surface tension of adhesive b. Surface energy of the adherend must be lower than surface tension of adhesive c. Deep irregularities must be made at the adherend surface d. High difference in coefficient of thermal expansion between adhesive and adherend. 2. Etching of dentine will lead to a. Increasing the surface energy of dentine surface b. Removal of smear layer c. Demineralization of dentin surface d. b&c 3. Which of the following is a desirable quality for a dental adhesive a. High viscosity b. High surface tension c. Many air bubbles d. Thin adhesive film 4. Stresses which may affect adhesion are a. Transverse stresses b. Setting stresses c. Compressive stresses d. Thermal stresses 5. Which of the following decreases the surface tension of a liquid a. Addition of detergents(impurities). b. Increase in temperature c. Increase in thickness d. a&b ‫بسم هللا الرحمن الرحيم‬ Metallurgy part 1 Metals Metallurgy: It is the science dealing with metals and alloys.  Metals Any element that Ionizes Positively in solution Metal is any element that ionizes positively in solution. About 91 of the 118 elements in the periodic table are metals. Non-metals occupy the right side in the periodic table. Metalloids transition group of elements or semiconductors are at the boundary between metals and nonmetals and they have properties midway between both. e.g. Carbon, Silicon and Boron.  Properties of metals Generally, the properties of metals result from both their Crystalline structure and Metallic bond where valence electrons are delocalized and free to move throughout the metal rather than the remaining bounded electrons. 1.Metals are crystalline Solids with the exception of mercury and gallium which are liquids at room temperature. Another exception is hydrogen, which is a very reactive metal; it exists as a gas at room temperature. 2.Ionize positively in solution 3.Luster: This arises from the response of the unbound electrons to reflect light which give the mirror-reflecting property. 4.Opacity results from the ability of the valence electrons to absorb light 5.Electrical and thermal conductors: mobility of the valence electrons are efficient carriers of thermal as well as electrical energy 6.High hardness, melting point and boiling point due to the strength of the primary interatomic bonding within the crystalline solid. 7.High density: is related to atomic weight, atomic radius and atomic packing factor. 8. Ductility and malleability: related to the crystalline imperfections which allow for plastic deformation.  9. On striking a metal surface, a metallic ring is given. 10. Most metals are white with slight differences in tint. Two metals are non white, gold (yellow) and copper (red).  Pure Metals used in Dentistry 1- Gold & Platinum Foil: As direct restoration. 2- Platinum Foil: In Porcelain Jacket crown. 3- Gold & Tin : Ceramo-metallic restorations. 4- Silver & Copper: In Electroplated Dies. 5- Mercury: In Amalgam restorations. 6- Titanium: In Implants & Crown & Bridge work.  Shaping of Metals 1- Casting ▪ Melting of the metal ▪ Pouring of the molten metal into a mold of the required shape. ▪ Solidification of the poured molten metal to take the shape of the mold.  2. Plastic forming (Cold Working) 1- At room temperature 2- Stresses should be applied ABOVE the YIELD strength?? Plastic deformation is through slip along crystal planes involving dislocation movements.  3. Powder metallurgy (sintering) Def. Bonding of solid particles by heat and pressure in the absence of any liquid → atomic diffusion It is an agglomeration process ** Bonding of powder particles ** Elimination of the initial porosity *** It is accompanied with *** shrinkage *** decreased porosity and increase density  4. Electroforming Process of electrolysis i.e. corrosion in reverse, A metal can be plated onto a conducting surface. Examples: Silver and copper electroplated dies.  Solidification of metals ‘Cooling curve of pure metal’ A-B’: Temperature of liquid metal decreases B’-B: Temperature increases B-C: Temperature remains constant C-D : Temperature of solidified metal decreases Initial cooling to B’ is called supercooling, where crystallization begins. Supercooling Once the crystals begin to form, latent heat of fusion causes the temperature to rise to temperature Tf Cooling or temperature time where it remains until the curve for pure metal crystallization is completed.  Solidification of metals ‘Cooling curve of pure metal’ 1. Temperature of liquid metal decreases 2. Temperature reaches the freezing/melting point ? 3. Temperature remains constant Due to liberation of latent heat of fusion as the molten metal changes to solid L L S 4. Temperature of solidified S metal decreases Rapid rate of cooling Slow rate of cooling  Mechanism of Solidification 1- Nucleus formation: Nuclei of crystallization. 2- Crystallization: Grains and grain boundary 1) Nucleus Formation: When a molten alloy is cooled and approaches its freezing temperature, the atoms try to aggregate forming initial starting points of crystallization [nuclei of crystallization] at supercooling point Nucleus formation can occur by a. Homogenous nucleation -The atoms of the metal itself form the nuclei of crystallization b. Heterogeneous nucleation - Foreign solid metallic particles e.g. iridium 2) Crystallization: As cooling continues the nuclei of crystallization grow independently in three dimensions [tree like structure (dendrites) ] to form crystals [grains] The growth is stopped when there is contact with adjacent growing crystals  Grain boundary Def.: It is a region of transition between two adjacent crystals [grains] -The atoms at the grain boundaries are located in distorted positions - It is the final sites to solidify -Grain boundaries appear as dark lines under the microscope  Effect of grain boundary on properties of metals: 1- Diffusion of atoms occurs more readily along the grain boundaries [Why?] Because of the more open structure found there. 2- Crystallization and formation of new nuclei in the solid phase started at the grain boundary. [Why?] Because there is enough surface energy to start the formation of new set of grains 3-Strength properties are higher for a cast metal having more grain boundaries i.e. smaller grains 4- They are the sites of corrosion attack [Why?]  Control of grain size Factors affecting grain size Time & Temperature controlled The greater the no. of nuclei of crystallization the greater the number of grains the smaller the grain size, the greater number of grain boundaries the better are the mechanical properties. Coarse grains Fine grains  Control of grain size 1- Rate of cooling from liquid state Rapid rate of cooling ** More number of nuclei of Crystallization & ** Smaller grain size ** the better the mechanical properties 2. Rate of nucleation and crystallization Rate of nucleation (control the number of nuclei) Rate of Crystallization (control the size of grains) Rate of nucleation > Rate of Crystallization Small grains 3- Nucleating agents (grain refiners, prefab. nuclei): May be added intentionally or may be found as impurity. Act as nuclei of crystallization *** Small grain size *** Increase the number of nuclei of crystallization.  Wrought Metals (by cold working and strain hardening) Definition: A wrought structure or fibrous structure is plastically formed structure that was subjected to stresses above its yield point at ambient temperature Procedure:- By hammering or rolling the metal at room temperature, plastic deformation occurs by dislocation movement along the slip planes. Microscopic picture: Fibrous Properties: A cold worked structure is highly stressed structure with -increased strength and hardness -lower ductility and corrosion resistance Elastic deformation: When a material is stressed under its elastic limit it will deform temporarily. Therefore, elastic strain in a metal is mainly due to stretching of the interatomic bonds. Plastic deformation:-The presence of dislocations plays an important role in the permanent deformation of metals. It involves the slip of layers of atoms over each other. The presence of dislocations allows the slip to occur by breakage of bonds only in a localized area at a time (dislocation movement) If the dislocation movement is easy, it means that atoms can move without obstruction, the metal is characterized by low strength, low hardness, and high ductility. If the dislocation movement is obstructed this will increase the strength and hardness of the metal and reduces the ductility due to consumption of slip planes. This may be achieved by: Grain boundaries Give reason: A metal with fine grain structure is characterized by high strength properties and vice versa  Heat treatment Annealing Def. : Heating the cold worked structure below the melting temperature of metal to Reverses the effect of cold working Stress-relief anneal recovery Recrystallization Grain growth  Effect of annealing on the mechanical properties 1) Stress-relief (Recovery): No visible change in the fibrous structure ** Any cold worked structure should be annealed before insertion in the oral cavity to provide: ** Relief of internal stresses to prevent warpage or fracture during service *** pronounced recovery of electrical conductivity *** Very slight decrease in strength *** No change in ductility Recovery 2) Recrystallization: If the temperature is held for longer time, the following will occur: Change from fibrous structure to fine cast structure, because new grains nucleate from the highly distorted grain boundaries ** Characterized by: low strength, low hardness, and high ductility -Recrystallization temperature (Tr) from 0.3 - 0.6 the melting temperature (Tm) Recrystalization 3) Grain growth: If the temperature is held for longer time, the following will occur: ** Further grain growth occurs ** Change from fine to coarse crystal [grain] structure ** Characterized by: -detrimental decrease in strength and hardness and -very high ductility Coarse crystal structure Grain growth Effect of annealing heat treatment on microstruc