Fundamentals of Dental Materials 1 PDF 2019-2020

Fundamentals of DENTAL MATERIALS 1 PRS 121 Dental Materials Team Department of Dental Materials College of Dentistry October University for Modern Sciences and Arts (MSA) 2019- 2020...

Fundamentals of DENTAL MATERIALS 1 PRS 121 Dental Materials Team Department of Dental Materials College of Dentistry October University for Modern Sciences and Arts (MSA) 2019- 2020 1  CONTENTS Page Course Objectives 4 Introduction. …………..…………………….. 5 Chapter (1) Structure of Matter. ………………………….. 15 Chapter (2) Physical Properties. ………………………..... 29 Chapter (3) Mechanical Properties. ………………………. 51 Chapter (4) Polymers. ……………………………………... 75 Chapter (5) Surface Phenomena & Adhesion. …………… 89 Chapter (6) Metallurgy. ……………………………..…… 99 Chapter (7) Tarnish & Corrosion. …………………………... 119 References. ……………………………..……… 129 4  Chapter (1) STRUCTURE OF MATTER All materials are built up from atoms and molecules, so it is not really surprising that there is a close relationship between the atomic basis of a material and its properties. Therefore, if we are to understand the properties of materials we need to have an understanding of the way atoms can combine to make solids. Generally, the physical, mechanical and chemical properties of any material depend mainly on: 1- Types of bonds between atoms and molecules. 2- Manner of arrangement of atoms in space. 3- Atomic packing. I. Structure of Atoms and Atomic Bonding 1. Structure of Atoms The atom is the basic unit of the internal structure of any material. 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). Electrons follow certain arrangement in the order of 2n2; where n is the principle number. 3- Valence electrons: Electrons in the outermost shell, which determines the chemical reactivity of the element. 17 4- Electrical state of any atom should be Neutral, therefore the number of protons should be equal to the number of electrons; this is called “Atomic number”. The Atomic weight is nearly proportional to the number of Protons + Neutrons. It affects density and specific heat with little effect on mechanical properties. 2. Types of Bonds * Inter-atomic Bonds (Primary bonds) * Inter-molecular bonds (Secondary bonds) II. Inter-atomic Bonding (Primary bonds) In any element, except inert gases, atoms achieve a stable state by having eight electrons in their outer shell. This can be obtained by: a) Receiving extra electrons to complete the outer shell electrons (and the atom becomes negative ion). b) Releasing electrons so that the outer shell has eight electrons (and the atom becomes positive ion). c) Sharing of electrons so that the outer shells of two or more atoms are complete. The electronic configuration which results from the three previous methods will give rise to strong atomic attraction or bonding called ‘primary bond’. Primary bonds are strong chemical bonds since they involve the outer most valence electrons. 1. Primary Bonds: Characteristics: 1- High strength & hardness 2- High thermal resistance 3- High chemical resistance 18 1) Ionic Bonds: It results from the electrostatic attraction between ions of unlike charges, (attraction of positive and negative ions). The classic example is sodium chloride (Na+ C1-). 2) Covalent Bonds: "It is sharing of electrons". In many chemical compounds, two valence electrons are shared by adjacent atoms. The hydrogen molecule, H2, is an example of covalent bonding. The single valence electron in each hydrogen atom is shared with that of the other combining atom, and the valence shells become stable. Covalent bonding occurs in many organic compounds, such as hydrocarbons (CH4) and acrylic resin. 3) Metallic Bonds: "It is the attraction between +ve cores and free electrons or electron cloud". It occurs in metals, because they easily give up the electrons in their valence shells giving positives cores. The electrons move freely through the 19 metal from atom to atom and form electron cloud. There is attraction between free electrons and the positive charged cores. Characteristics of the metallic bonds: The free mobility of electrons contributes to the following properties of metals: 1- High thermal and electrical conductivity. 2- Opacity (due to absorption of light by free electrons). 3- High strength and hardness. III. Secondary Bonds (Van Der Waal Forces) These forces are physical, weak, less heat resistant and arise from the polarization of molecules i.e. formation of electrical dipoles. A- Fluctuating Dipole: Instantaneous location of more electrons on one side of the nucleus than the other results in asymmetry in their electron distribution. 20 B- Permanent Dipole: The hydrogen bond is an important example. In H2O there is a covalent bond because oxygen and hydrogen atoms share electrons. However, the electrons around oxygen nucleus are more than those around the hydrogen nucleus and as a result the hydrogen portion of the water molecule is positive in relation to the oxygen portion. Therefore "attraction will take place between the positive hydrogen portion of one water molecule and the negative oxygen portion of another water molecule". 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 weak physical forces which control the properties. The Structure of Solids: The properties of materials depend on the arrangement of their atoms. Such arrangements may be classified as: 21 a. Crystalline solids: structures with regular repetition of atoms. b. Amorphous solids: structures without a specific form. a) Crystalline Solids: Solid dental materials are termed crystalline when their atoms are regularly arranged in a space lattice. A space lattice is the regular arrangement of atoms in the space so that every atom is situated similarly to every other atom. Types of space lattices: 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 which is the smallest repeating unit in the space lattice. Unit cells are classified according to: a) The length of their axes (a,b,c). b) The interfacial angles (a,b,g). 22 1) The Cubic System: - The length of the axes a,b,c are equal. - The interfacial angles a= b = g = 90° a. Simple cubic system: This structure is hypothetical for pure metals but provides us with good starting point for understanding. Thus number of atoms per unit cell in S.C. structure = 8 atoms at each corner X 1/8 = 1 atom. b. Body Centered Cubic (B.C.C.): As in iron below 910ºC In body centered cubic the unit cell has an atom at each corner of the cube and another atom at the center of the unit cell. There are two atoms per unit cell in B.C.C. structure where each atom at each of the eight corners of the cube is participating in eight unit cells and one atom at the center. Thus number of atoms per unit cell in B.C.C. structure = 8 atoms at each corner X 1/8 + one atom in the center = 2 atoms. c. Face Centered Cubic (F.C.C.): As in gold, silver, copper palladium, platinum and iron above 910ºC. The unit cell is a cube with an atom at each corner and one in the center of each of the six faces, but none of the center of the cube. Therefore the number of atoms in F.C.C. = (8X 1/8) + (6X 1/2) = 4 atoms. 23 2) Hexagonal structure The axis a = b but # c. The angle a = b = 90° but the angle g equals 120°. a. Simple Hexagonal structure At each corner of hexagonal cell, the atom value can be considered 1/6 atom and at the face as 1/2 atom Therefore, the number of atoms per unit cell = (2X6X 1/6) + (2X 1/2) = 3 atoms. b. Hexagonal close-packed structure (H.C.P.): as in Zn, magnesium and titanium. In H.C.P., the unit cell is hexagonal At each corner of hexagonal cell, the atom value can be considered 1/6 atom and at the face as 1/2 atom and in the center 3 unshared atoms. Therefore, the number of atoms per unit cell = (2X 6X 1/6) + (2X 1/2) + 3 atoms = 6 atoms. 24 Atomic Packing Factors: Definition: It is the fraction of the space of the structure unit occupied by the atoms and is calculated by: Atomic packing factor = Volume of atoms inside the unit cell ÷ Volume of unit cell Materials having higher atomic packing factor usually have higher stability, higher densities and higher strength properties. For the simple cubic system sc it equals 0.54 which indicates that nearly 50% of the space is free. Other arrangements (commonly occur in metals) Body centered cubic (bcc), Face centered cubic (fcc) have 0.68 and 0.74 atomic packing factor respectively. Materials having higher atomic packing factor usually have higher densities and strength properties. b) Amorphous Solids: Amorphous means without shape. Gases and liquids are amorphous substances. Some solids like glass and some polymers are amorphous because of the random arrangement of their atoms, yet their atoms may form a 25 short localized range of order lattice with a considerable number of disordered units in between. Crystalline solids Amorphous solids 1) Have definite melting 1) No definite melting temperature temperature. (gradually soften on heating and gradually harden on cooling). 2) Have regular unit cell 2) No regular unit cell but may have with repetition. a short range of regularity but no repetition. 3) Low internal energy. 3) Higher internal energy. Imperfections in Crystalline Solids: Ideal crystalline structure allows computing their theoretical strength, which is different and much higher than their actual strength; this is because nature is not perfect and materials are found to contain some defects or imperfections. Types of Crystalline Imperfections a. Point defects Vacancy Interstitial Impurities 26 b. Line defects Dislocation: It is a displacement of a row of atoms from their normal position in the lattice c. Plane defects: grain boundaries in metals. These crystalline imperfections play an important role in the mechanical behavior of the crystalline material (strength & deformation) Polymorphism Polymorphic materials are these that can exist with more than one crystal structure by changing the surrounding physical condition. The polymorphic forms have the same chemical reactions but different physical properties. 27  Polymorphism Allotropy Isomerism Occur in crystalline Occur in amorphous organic inorganic materials materials e.g. Silica in dental e.g. Natural rubber and gutta investment perhca 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 but all are chemically SiO2. With the application of heat to silica, two types of transformations can take place: Reconstructive Displacive Transformation Transformation - Break down of atomic - No break down of atomic bonding followed by bonding only displacement of reconstruction of new space atoms giving the same space lattice. lattice but with larger volume. - Irreversible transformation - Reversible transformation - Slow transformation. - Rapid transformation. - Needs high thermal energy - Needs less thermal energy. 28 Correlation between atomic structure and materials properties: 1. Density is controlled by atomic weight, atomic radius, and the atomic packing factor. 2. Melting and boiling temperatures can be correlated with the strength of the bond. 3. Thermal expansions of materials with comparable atomic packing factors vary inversely with their melting temperature. 4. Hardness: Generally, materials with weaker bonds have a decreased hardness and low melting point. 5. Electrical and thermal conductivity are very dependent on the nature of the atomic bonds [ionic/covalent/metallic]. 6. Strength can be primarily governed by the type of bond, although the arrangement of atoms controls the deformation and resistance to stresses. 7. Crystalline structures and amorphous structures. (As mentioned before). 29  2 Chapter (2) Physical Properties 30  Chapter (2) PHYSICAL PROPERTIES These are properties that are not related to force application on a body. Some of the physical properties of dental interest include: I) Mass related properties: Density and specific gravity. II) Thermal properties: 1. Heat of fusion. 2. Thermal conductivity. 3. Coefficient of thermal expansion and contraction. 4. Melting and freezing temperatures 5. Specific heat 6. Thermal diffusivity. III)Electrical properties: 1. Electrical conductivity and resistivity 2. Electromotive force 3. Electrochemical corrosion. IV) Optical properties Optics is the science that deals with light, vision and sight, which includes color. 31  I) Density and Specific gravity 1) Density: It is defined as the mass per unit volume of the material. Mass/Volume; Its units are gm/cm3 and pound/in3. Material Density (g/cm3) Gold alloy 14 - 15 Chromium cobalt Alloy 7-8 Acrylic Resin 1.2 2) Specific gravity: It is the ratio of the density of a material to the density of water at 4ºC. Since the density of one cc of water at 4ºC = 1, therefore the specific gravity is numerically equal to density but without units. Clinical Importance in dentistry: 1. Retention of the upper denture: Dentures of lighter weight will help the retention of the upper dentures. 2. Light weight complete or partial denture affects the comfort of the patient. 3. During casting low density alloys require more casting forces to allow rapid filling of the mold cavity (cobalt chromium alloys need special casting machine). 32  II) Thermal Properties When a patient drinks a cup of tea or eats an ice-cream, the temperature differences within the tooth can be quite pronounced. The pulp of the tooth would react severely if it was not protected from these temperatures. When placing a filling, crown, bridge or denture, care must be taken for the need to protect the pulp from extremes of temperature. Therefore, the thermal properties of the dental materials must be considered. 1) Thermal Conductivity: The thermal conductivity of a substance 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. 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 and beverages unless adequate tooth tissue remains or non-metallic substances are placed between the tooth and filling as a base for insulation. 33 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. This leads to a physiological response of vasodilatation and vasoconstriction, which maintains the tissue in a good and healthy condition. 2) Thermal Coefficient of Expansion (a): This change in length, per unit length for a 1°C change in temperature is called the linear coefficient of expansion, a. L final - L original α= L original X ( ! C final ! C original) Examples: Enemal = 11 x 10-6/°C Acrylic resin = 90 x 10-6/°C Composite = 22 X 106/°C Clinical Importance in Dentistry: We can assume that objects with different coefficients of thermal expansion will expand and contract at different rates in response to temperature changes. 34 Close matching of the coefficient of thermal expansion(α) between: 1. The tooth and the restorative materials to prevent marginal leakage, as large difference in (α) will lead 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, 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 (a) (crowns & bridges) to provide metal ceramic bonding. 3. Artificial tooth and denture base (b) to avoid crazing. (a) (b) 35 3) Heat of fusion (L): Transitions from one state to another are accompanied by absorption or liberation of heat and usually by a change in volume. The amount of heat in calories or joules required to convert 1 gm of a material from solid to liquid state. As long as the mass is molten, the heat of fusion is retained by the liquid. When the liquid is frozen, this heat is liberated (Latent heat of fusion). Latent Heat of Fusion: Is the amount of heat in cal. or joule liberated to convert 1 gm of a material from liquid to solid state. 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: 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 melting machine. 36 5) Specific Heat: 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. Importance in dentistry: Because of the low specific heat of dental gold alloys, prolonged heating is unnecessary, during casting. 6) Thermal Diffusivity: It describes the rate at which a body with a non uniform temperature approaches equilibrium. i.e. it is a measure of transient heat flow (rate of heat diffusion in the body). Thermal diffusivity = Thermal conductivity Specific heat x Density Importance in dentistry: 1- The low specific heat of a metallic restoration combined with its high thermal conductivity leads to thermal shock to the pulp. 2- Thermal diffusivity gives a clear indication of the rate of temperature rise at one point of a body as a result to increased temperature at another point. Therefore, the diffusivity and thickness of an insulating base material are the important factors controlling the insulating efficiency. 37  In such a case, adequate dentin thickness (density) should remain under the restoration BUT if not possible, an insulating cement base (non metallic) should be placed with adequate thickness (density) between the tooth and restoration for insulation. N.B.: Thermal diffusivity is a 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) Electrical Properties Electrical conductivity and resistivity: The ability of a material to conduct an electrical current may be stated as either specific conductance or conversely as specific resistance or resistivity. Resistivity is the more common term. Importance in dentistry: 1- The conductivity of restorative materials used to replace lost tooth structures is of great importance. 2- Electrical resistivity is an important consideration in the investigation of pain perception. Less resistance is offered by by carious lesions. 38  IV) Optical Properties One of the important goals of restorative dentistry is to restore the color and appearance of natural teeth. Thus, esthetic consideration in restorative and prosthetic dentistry has assumed a high priority and makes severe demands on the artistic abilities of the dentist and technician. The perception of color is a physiological response to physical stimulus (light). For anything to be visible and to detect its color, the object must reflect, transmit or emit light. So, it is very important to know something about light. 1. LIGHT Light is usually polychromatic, i.e. a mixture of various wavelengths. Visible light is a small part of the electromagnetic radiation that can be detected by the human eye in the range from 400-700 nanometers. 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. 39  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. Black color materials absorb all light colors. White color materials reflect all light colors. Blue color materials absorb all light colors but reflect its color. Interaction of Light and Matter When a beam of light encounters or falls on a surface of a medium, the following may occur: 40 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. Smooth 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. 2. Refraction: It is the change of the direction of a beam of light on entering second medium. Refraction results from the 41 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. Control of refractive index of the filler and matrix phases in composite resins and porcelain. A perfect match results in a transparent solid while large differences result in opaque materials. 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) Opacifiers added to composite resins act as scattering centers that given rise to opaque shades of the material. 42 b) Incorporated air bubbles in a restoration as well, act as scattering centers. 4. 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 (i.e. diffuse transmission), the material appears translucent. 5. Luminescence (Fluorescence and Phosphorescence): Electrons may be activated to higher energy levels when exposed to electromagnetic waves of light of high energy. As the electron returns to its lower energy position, energy is released, producing luminescence. The wavelength of the emitted light is longer than that of the exciting waves. * Immediate emission is called fluorescence, * Delayed emission is called phosphorescence. 43 Importance in dentistry: 1. Sound human teeth emit fluorescent light when excited by ultraviolet radiation. This fluorescence contributes to the brightness and vital appearance of human teeth. 2. Some anterior restorative materials and dental porcelains are formulated with fluorescing agents to reproduce the natural appearance. 6. Dispersion: White light is composed of a mixture of colors, which is dispersed to its component colors by passing it through a prism. Since the wavelengths of white light ranges from 350 to 700 nm, it gets dispersed by a prism to give the spectrum starting from the shortest wavelength, violet (350 nm) and ending by the longest wavelength, (700 nm). 2. COLOR Color is the quality of an object with respect to light that is reflected or transmitted by it. The object reflects light of certain wavelength producing a certain color. Perception of Color: Light from an object which is incident on an eye is focused onto the retina and converted into nerve impulses, which are transmitted to the brain. 44  Cone-shaped cells in the retina are responsible for color vision. They are especially sensitive to red, green and blue. Color Definition and Dimensions (Parameters): Color of most objects is due to the selectivity of that object for light of certain wavelength. The object reflects light of certain wavelength from its surface and absorbs the remainder of colors producing a characteristic color. This phenomenon is known as selective absorption and reflection. According to one of Grossman’s laws, the eye can distinguish differences in only three parameters of color; described as three dimensions of color, namely hue, value and chroma. Color Dimensions: 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. A beaker of water containing one drop of colorant is lower in chroma than a beaker containing ten drops of the same colorant. 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. A tooth of 45 low value appears gray and non-vital; therefore, it is the most important parameter. Value is the most important parameter of color in dentistry because it is intimately related to the aspect of vitality in human teeth. Examples: a. Dead tooth have low value (more gray or dark). b. Vital tooth have higher value (more vivid and translucent). c. When we make a restoration and the hue (color) matches the adjacent dentition but its value is too high, we always have a result of a false look (too bright). Conversely, when hue matches but the value is too low, we end up with a finished restoration that looks dead (darker or grayer or dull). Types of Colors: Primary colors: Blue, green and red are three primary colors. Combining suitable portion of wavelengths of three primary colors result in white color. Secondary colors: Combination of two primary colors will give the secondary colors: yellow, cyan and magenta. Green + red yellow Blue + red magenta Blue + green cyan 46 Complementary colors: Two colors are complementary to each other when their combination gives rise to white color. Yellow is the complementary of blue. Color Matching: In dental practice, color matching is most often performed with the use of a "Shade guide". More recently, an intra oral spectrophotometer could be used for color selection. So, it is important to match colors under appropriate conditions. Factors affecting color appearance and selection: The appearance of the tooth or an object is a sum total of all its optical properties. 47 1. Source: Different 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 pair: Two objects that are matched in color under one light source but are not matched under other light sources form metameric pair. Isomeric pair: They are color matched under all light sources. Thus, if possible, color matching should be done under two or more different light sources. 2. Surroundings: Colors of wall, lips or clothes of the patient modify the type of light reaching the object. 3. Object: 48  a. Translucency: It controls the lightness or darkness of color. High translucency gives a lighter color appearance ((higher value)) i.e. More vital tooth appearance. b. Surface texture ((surface finish)): This determines the relative amount of light reflected from the surface. Smooth surface appear brighter than rough surface. c. Presence of scattering centers as inclusions or voids, which increase opacity and lower the value ((more dark)). d. Fluorescence: It makes the human teeth bright and vital, as it increases the brightness. e. Thickness: The thickness of a restoration can affect its appearance. Increase in thickness, increase opacity, and lower the value. f. Metamerism: Two objects that are matched in color under one light source but are not matched under other light sources 4. Observer: a. Color response: eye responds differently among individuals. b. Color vision: Some individuals may have color blindness and inability to distinguish certain colors. c. Color fatigue: Constant stimulus of one color decreases the response to that color. A complementary color image persists after removal of the stimulus. 49  PHYSICAL PROPERTIES PROBLEM 1: During casting, cobalt chromium alloys need special casting machine. PROBLEM 2: In the clinical evaluation of a 2-years old acrylic resin, recurrent caries and marginal discoloration are noted. What most likely caused this problem? PROBLEM 3: Large amalgam filling or gold filling, which is in proximity to the pulp may cause the patient considerable discomfort. What might caused this problem, and how to solve it? PROBLEM 4: A ceramic veneer to be bonded on an anterior tooth matches the color of the shade guide but not the adjacent tooth. What most likely caused this problem, and how can it be avoided? 50 3 Chapter (3) Mechanical Properties 51  Chapter (3) MECHANICAL PROPERTIES Mechanical properties are defined as the group of properties that describes the behavior of materials under forces or loads. They are important because most restorative materials must withstand forces in service. Because no single mechanical property can give a true measure of quality, understanding the principles involved in a variety of mechanical properties is essential to obtain the maximum service. In general, the properties of a solid are determined by the nature of its atomic bonding forces. Force: The result of an applied force on a body is a change in its position of rest or motion; if the body remains at rest the forces will cause its deformation. i.e. force is the action which causes one of the following reactions or all of them: 1) Displacement 2) Acceleration 3) Deformation A force is defined by the following characteristics; speed, magnitude, point of application, and direction. The speed of the force determines whether the force is static or dynamic. The point of application determines whether the force is normal or tangential. Units: Newton (N), Pound (Ib), Importance in dentistry: Average biting force in the molar region is 560N 52 It is in male > female It is in adult > children It is in natural dentition > restoration Equilibrium Statement: Generally, any body is in equilibrium: 1) External equilibrium 2) Internal equilibrium - Any type of load should be balanced by an external reaction to be in external equilibrium, otherwise the body will accelerate, e.g. a body placed on a table will be balanced by the table. - The body on which the force is applied; is in an internal balance i.e. an internal reaction balances the external force. This internal reaction is known as “Stress”. Stress (s): It is the internal reaction to the external applied force. It is equal in magnitude and opposite in direction to the external force. The external force and the internal resistance are distributed over a given area of the body. Consequently, the stress in a structure is designated as the force per unit area. Stress, therefore, can also be defined as the force per unit area. Force Stress = s= F/A Area The stress in a structure varies directly with the external force and inversely with the area over which it is applied. Since the internal resistance to the applied force is impractical to be measured, the more convenient way is to measure the external applied force to the cross sectional area, this will be described as the applied stress. 53 Units of stress are: Ib/in 2 or Kg/cm 2 or MN/m 2 (MPa) Types of Stress: Generally, there are two types of stresss: A) Normal or axial: * Tensile Stress * Compressive Stress B) Tangential: * Shear Stress Tensile Stress: Tension results in a body when it is subjected to two sets of forces directed away from each other in the same straight line. Tensile Stress ® Elongation Molecules making up the body must resist being pulled apart. Compressive Stress: Compression results when the body is subjected to two sets of forces directed towards each other on the same straight line. Compressive stress ® Shortening Molecules making up the body must resist being forced more closely together. Shear Stress: Shear is the result of two sets of forces directed towards each other but not in the same straight line. Shear stress ® Tearing Or Sliding 54  Molecules making up the body must resist sliding past one another. Complex Stresses: A single type of stress is extremely difficult to be induced in a structure e.g. if we stretch a wire, the observed stress will be predominantly tensile but the cross section of the wire will decrease indicating the presence of compressive stresses. Also, complex stresses are produced by 3 point loading. Importance of stress in dentistry: 1- Dental restorations are subjected to extremely great stresses because the area over which the forces are applied is extremely small. 2- The forces applied to a dental restoration are resolved as a combination of compressive, tensile, and shear stresses (complex stresses) rather than pure single stress. Strain ( e ): Strain is the change in length, or deformation per unit length, when a material is subjected to a force. 55  Deformation (DL) Strain = Length (L) Unit: it is reported as absolute value or as a percentage. Types of strain: 1. Temporary or elastic strain which disappears on removal of the external force. The material will return to its original shape. 2. Permanent or plastic strain which will not disappear on removal of the external force. The material will not return to its original shape. Stress-strain curve: If we plot a graph for the stress and the corresponding strain of a dental material subjected to load we find that the strain is proportional to the induced stress. This part of the curve obeys Hook’s law which states that “stress is directly proportional to strain until a stress value known as proportional limit”. The shape and magnitude of the stress-strain curve are important in the selection of dental materials. The stress-strain curve consists of two portions Elastic Portion (Linear) Plastic Portion (Non-linear) - It obeys Hook's law -It doesn’t obey the Hook's law The strain is linearly The strain isn’t linearly proportional to the applied proportional to the applied stress. stress i.e. doubling the stress will double the strain. -When the stress is removed -When the stress is removed, the the original size and shape is original size and shape isn’t recovered. recovered. 56 1) Proportional limit (P.L.): It is defined as the maximum stress that a material will withstand without deviation from the law of proportionality of stress to strain. I. therefore, describes the relation between stress and strain. 2) Elastic limit (E.L.): It is defined as the maximum stress that a material will withstand without permanent deformation resulting. It, therefore describes the elastic behavior of the material. For all practical purposes, the elastic limit and the proportional limit represent the same stress within the structure and the terms are often used interchangeably in referring to the stress involved. 3) Yield strength (Y.S.): It is the stress at which the material begins to function in a plastic manner. At this stress a limited permanent strain usually 0.1% or 0.2% of the total permanent strain occurs in the material. The yield strength may also be defined as the stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain. Clinical Significance: 57  Practically it is more important for the dental restoration to have high Y.S. than U.S. so that, during mastication no permanent deformation takes place after load removal. For example, a bridge that is permanently deformed in service due to excessive biting force is shifted out of contact with the opposing natural teeth. The bridge is permanently deformed because a stress equal to or greater than the yield strength and greater than the elastic limit was developed in the structure. N.B.: A restoration exhibiting a permanent deformation will be no more fitting the purpose, in spite of the fact that it didn’t break. 4) Ultimate Strength (U.S.): If higher and higher forces are applied to a material, a stress will be reached at which the material will fracture. If the fracture occurs from tensile stress, the property is called the tensile strength and, if in compression, the compressive strength. Importance in dentistry: Yield stress is more important than the ultimate stress, because yield stress represents the clinical failure (functional failure). 5) Fracture stress: It is defined as the stress at which the material will fracture. 6) Modulus of Elasticity or (Young's Modulus): It is the constant of proportionality between stress and strain. It represents the slope of the elastic straight line portion of the stress-strain curve. 58  It is a measure of rigidity or stiffness of a material within the elastic range. Materials with higher Young's modulus values are said to be stiffer or more rigid than those of low Young's modulus values because they require much more stresses to produce the same amount of strain. The modulus of elasticity of a material does not change either tested in tension or compression. It is a fundamental property of a material; it mainly depends on the inter-atomic or inter-molecular forces of the material. It is quite dependent on the composition of the material. Material (A) has a higher modulus of elasticity i.e. stiffer than material (B) The stronger the inter-atomic bonding, the greater the values of the elastic modulus and the more rigid the material. Clinical Significance: - Denture base should be constructed of a rigid material for two reasons: a. To allow load distribution on the whole design. b. To be used in thinner sections without the risk of bending. This gives comfort the patient. Denture Base Long Span Bridge 59 - High modulus of elasticity is required to allow proper stress distribution in case of long span bridges. 7) Flexibility: Maximum flexibility is the strain resulting in the material when the stress reaches the elastic limit. This is very important for impression materials, which often must be severely deformed to be removed from undercuts, but must have the ability to spring back without suffering any permanent change in shape. Clinical Significance: a) Flexibility is the amount of bending an alloy will withstand and still return to its original shape. Clasps are flexed during mastication, therefore-e it is necessary to fabricate them with alloy of high flexibility. b) Flexibility is also an important property in elastic in impression materials, since it represents the ease by which the impression can be removed from the mouth. 60 8) Ductility and Malleability: They are two significant properties of metals and alloys, which indicate the workability of the metals and alloy. Malleability is defined as the ability of the material to be plastically deformed under compression without fracture (hammered into thin sheets). Ductility is defined as the ability of the material to be plastically deformed under tension without fracture (drawn into wire) Percentage elongation is the measure of ductility It represents the maximum amount of permanent deformation l final - l original %Elongation = ´ 100 l original Importance in dentistry: It gives an indication of the workability of the alloy. Clasps can be adjusted, orthodontics appliances can be prepared, crowns or inlays can be burnished if they are prepared from alloys of high values of percentage elongation. 9) Brittleness: If a material showed no or very little plastic deformation on application of load it is described as being brittle. 61  In other words, a brittle material fractures at or near its proportional limit. Moreover, brittle materials are weak in tension; For example, dental amalgam has a compressive strength which is higher than its tensile strength. 10) Resilience: It is defined as the amount of energy required to deform the material to its proportional limit. It represents the resistance of the material to permanent deformation, when the load is released complete recovery of the material will occur. It is measured by the area under the straight portion of the stress strain curve (triangle) Stress (MPa) A B Strain (m/m) 62 Clinical Significance: Resilience has a particular importance in the evaluation orthodontic wires, since the amount of work expected from a spring wire for moving a tooth is of interest. 11) Toughness: It is the energy required to stress the material to the point of fracture. It is represented by the area under the elastic and plastic portion of the stress-strain curve. Therefore, toughness of a material is the ability to absorb energy up to the point of fracture. The toughest materials are those with high proportional limits and good ductility. However two highly different materials can have the same toughness. 12) Fracture Toughness: Cracks may arise naturally in a material after a time in use. Any defect usually weakens the material, and sudden fractures may happen at stresses below the yield stress. Sudden fractures occur in brittle materials that don’t have the ability to plastically deform and redistribute stresses like ductile materials. Fracture toughness is defined as the amount of energy required to fracture a sample with crack [i.e. The stress intensity at fracture] 63  It is a material property that is proportional to the energy consumed in plastic deformation. Accordingly brittle materials have lower fracture toughness than ductile materials. Properties and Stress-Strain Curves The shape of a stress-strain curve and the magnitudes of the stress and strain allow the classification of materials with respect to their general properties. The idealized stress- strain curves represent materials with various combinations of mechanical properties. For example, 64 Bending mechanical properties: 1) Cantilever bending: The bending properties of many materials are equally or more important than their tensile or compressive properties. Bending properties are usually measured by clamping a sample at one end and applying a force at a fixed distance from the face of the clamping. Clinical significance: The bending properties of wires, endodontic files and reamers and hypodermic needles are especially important. 2) Transverse bending (Transverse strength): (modulus of rupture, or flexure strength). 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. In practice, the stresses within in a material are complex. Thus if a beam is bent; the lower portion of the beam is in tension, and the top is in compression. Shear stresses are also present. This test determines not only the strength of the material (s) , but also the amount of deformation expected. Stress or the transverse strength is calculated from the equation: 65 Clinical Significance: The transverse strength test and the accompanying deformation are very important in comparing: 1) Denture base materials & 2) Long span bridges. Dynamic Mechanical Tests: 1) Diametral compression test (Indirect tensile test): The diametral compression test or the indirect tensile test is used to measure the tensile strength of brittle materials. These brittle materials include dental amalgam, cements, ceramics and gypsum products. These materials are much weaker in tension than in compression thus this contributes to their failure in service. In this test a disk of the brittle material is compressed diametrically in a testing machine until fracture occurs, the compressive stress applied to the specimen introduces tensile stress in the material, the tensile stress is calculated by: 66  2P Tensile stress = p DT Where P = Load D = Diameter T = Thickness 2) Impact strength and Impact Test: A material may have high static strength values such as compressive, tensile and shear strengths but may fracture when loaded under impact i.e. Subjected to dynamic loading Impact strength: "Is the amount of energy absorbed by the material when subjected to sudden force". The Impact strength is measured by clamping a specimen of known dimensions firmly in position and breaking it with a swinging pendulum. Because specimens break in different places, they are notched in order to ensure consistent results. The material fractures at the notch, since this is its weakest part. The values are usually reported in joules. ** Two types of impact testers are available: Charpy tester, and Izod instrument. 67 Clinical Significance: A sudden blow might correspond to the energy of impact resulting from an accident to a person wearing a restoration or from dropping the denture on a floor, for this reason high impact acrylic resin denture were developed. 3) Fatigue Strength and Fatigue Test: The repeated application of small stresses (below the P.L.) to an object causes tiny small cracks to be generated within its structure. These tiny cracks do not cause failure immediately. With each application of stress, the cracks grow until the material breaks. Metals, plastics and ceramics can all fail by fatigue. Fatigue is the, fracture of a material when subjected to repeated (cyclic) small stresses below the P.L. Failure under repeated or cyclic loading is dependent on the magnitude of the load and the number of loading repetitions. It is one of the most difficult properties to test, but possibly the most important for predicting clinical longevity. Fatigue tests are performed by subjecting the specimen to alternating stresses below the P.L until fracture occurs. 68  Fatigue data are often represented by an S.N. curve, a curve of the stress at which a material will fail as a function of the number of loading cycles. From this curve we see that when the stress is sufficiently high, the specimen will fracture at a relatively low number of cycles. As the stress is reduced, the number of cycles required to cause failure increases. From the graph the fatigue limit can be found. Stresses below this limit will not cause fracture. The restoration should be designed so that the fatigue stresses are below the fatigue limit. Clinical significance The determination of fatigue properties is of considerable importance for certain types of dental restorations subjected to alternating forces during mastication. Structures such as complete dentures, implants, 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. Surface mechanical properties: 1) Hardness: - Hardness is the resistance of the material to permanat indentation or penetration or scraching. - It is surface property which can not be determined from stress strain curve. - 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. 69  - Hardness is measured as a force per unit area of indentation. - Some of the most common methods of testing the hardness of restorative materials are the Brinell, Knoop, Vickers, Rockwell, and shore A Load Load Material (A) is harder than (B) Clinical Significance: 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. - Restorations made of hard material like cobalt chromium is: Very difficult to finish and polish. Once it is polished it maintains its polished surface with no scratches. Therefore, hard material is considered both advantage and disadvantage. 70 2) Wear: Wear is the loss of material resulting from mechanical action. Wear of tooth structure and restorative materials may result from mechanical, physiological and pathological condition; - Normal mastication may cause attrition of tooth structure Physiological form of wear - Bruxism may cause pathological form of wear. - Improper use of tooth brushing may cause mechanical (abrasive) form of wear. Viscoelasticity and Creep (Strain- rate sensitive materials) Viscoelastic materials are strain rate sensitive materials dependent on how fast they are stressed. Increasing the rate of loading produces higher value of their mechanical properties. Viscoelastic materials are combination of elastic, viscous and anelastic behaviors. e.g. elastic impression materials, amalgam and waxes. t0 t1 Starin-time relationship: I) Ideal elastic material: If a material behaves as an ideal elastic solid; When stress is applied below the proportional limit * Immediate amount of strain will result 71  * The strain remains constant with time. When the load is removed (at the time t1) the strain Immediately decreases to zero. Therefore, the strain is independent of the rate of loading or time in which the load was applied. ii) Ideal viscous material: When stress is applied below the proportional limit at (t0 time) the strain increases uniformly until the stress is removed at (t1 time) the strain will not recover after stress removal. Therefore, the strain is directly proportional to the time of load application strain t0 t1 iii) Anelastic material (delayed elasticity): When stress is applied at (t0 time) Non linear increase of the strain with time On load removal → strain will non linear decrease to zero with time (gradual but complete recovery) Therefore, the strain is time dependent 72 t0 t1 Viscoelastic behavior: It is a combination of elastic, anelastic and viscous behavior. This combination strain or viscoelastic strain is time dependent. The elastic and anelastic portions are recovered but the purely viscous components are not. - 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 release of stress i) the elastic strain is immediately recovered and ii) the anelastic strain is gradually recovered. iii) viscous strain is not recovered which results in some permanent deformation (1-3%). t0 t1 N.B: As viscoelastic strain is time dependent so rapid rate of loading (less time) will result in less permanent deformation Clinical Significance: i) Elastic impression materials must be removed rapidly from the mouth (snap removal i.e. less time = high rate of loading) in order to; a. Minimize the permanent deformation as a result of viscous deformation during removal. b. Increase tear strength i.e. less chance to tear. 73  Furthermore, 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) ii) Dental amalgam undergoes creep. Creep: Creep is - Time dependent permanent deformation. - At stresses well below their yield strengths. - And at temperatures near the softening point of a material. Clinical Significance: Metals tend to creep (slowly deform) when stressed near its melting temperature. In dental amalgam restorations, they contain components with melting temperature slightly above room temperature. Thus they undergo creep. 74  Chapter 5 POLYMERS 75  Chapter (4) POLYMERS Polymers are long chain molecules (macromolecules) consisting of many repeated units called mer units. The molecules within each chain are bonded by strong covalent bonds i.e. intra-molecular covalent bonding. The long chain molecules are bonded together by Van der Waal forces (inter-molecular bonds). Polymers are molecular structures consisting of millions of polymer chains that interlock and exhibit a spaghetti-like structure. Definitions: * Polymer is a macromolecule that is made up of many units (poly = many; mer = unit) * Monomer is the smallest repeated unit in the polymer chain (mono = single). It is often the basis of naming the material. Thus polystyrene is a polymer composed of styrene units. * Polymerization reaction is the reaction by which the monmerunits are chemically linked together by a strong covalent bond to form a polymer chain. * Homopolymer is formed from one type of monomer * Copolymer is formed when two or more different types of monomers are joined. * Oligomer is a short polymer chain composed of 2-8 mers. 76 Classification of polymers: 1. Classification according to the origin Natural Polymer Synthetic Polymer 1- Proteins They are produced industrially 2- Polyisoprenes e.g. or in the laboratory by rubber gutta percha chemical reactions 3- Polysaccahrides e.g. Dental example: acrylic resin, agar alginate elastomeric impression 4- Polynucleic acids. materials 2. Classification according to arrangement in space: Linear Branched Cross –linked polymers The structural units Side branch chains Adjacent linear are connected to one are connected to the chains are joined at another in adjacent main ones various positions sequence by covalent bonds In both linear and branched polymers, the polymer chains are bonded to each other by weak secondary Van der Waal forces which contribute to their properties. In general, they have low mechanical properties, high water sorption and low thermal and chemical resistance. In cross-linked polymers, the adjacent linear chains are joined to one another at various positions by strong primary covalent bonds. Thus, they have high mechanical properties, low water sorption and high thermal and chemical resistance. 77 3. Classification according to their thermal behaviour: Thermoplastic resin Thermoset resin - Method of - They are shaped by heat -They are formed into a shaping and after cooling they permanent shape and maintain their shape set by a chemical [physical change, reaction with evolution reversible reaction] of heat [irreversible Soft Hard reaction] - Structure - Linear chains bonded -Cross-linked structure with Van der Waal forces where the chains are bonded with primary covalent bond - Effect of - The mechanical - They decompose heat properties are sensitive to upon heating to high temperature. temperature In thermo sets both intra-molecular and inter-molecular bonds are strong primary covalent bonds. The polymer chain forms a network, with cross links between them. Preparation of polymers: Polymers are prepared by a process called polymerization which is the chemical reaction by which monomer units become chemically linked together to form the polymer chains. Polymerization Monomer by ® Polymer reaction Types of Polymerization Reactions: 1) Condensation polymerization (Step Growth Polymerization): The reaction between two molecules to form a larger molecule with the elimination of a smaller molecule such as water as a by-product. 78  An example for condensation polymerization is the polysulphide rubber impression material. The low molecular weight polysulphide paste is converted to high molecular weight material by condensation reaction. Water and lead sulphide are byproducts of this reaction. 2) Addition polymerization reactions: I. Addition polymerization by free radical mechanism: The reaction between two molecules to give a larger molecule without the elimination of a smaller molecule (no by-product). There is no change in the composition; the structure of the monomer is only repeated many times. Addition Monomer = Polymer polymerization Stages of addition polymerization: The addition polymerization reaction passes through 3 stages: 1) Activation and initiation: - The addition polymerization requires the presence of free radical. - A free radical is a compound having chemical group of an unpaired electron. This unpaired electron makes the radical very reactive. - The free radical attacks the double bond of the monomer molecule resulting in the shift of the unpaired electron to the end of the monomer and the formation of activated monomer molecule. Thus the monomer itself becomes a free radical. 79  -The initiator is the substance used to generate free radicals. ** Before initiation occurs, the initiator has to be activated Activation of the initiator is done by heat or chemical compound or light. heat Benzoyl peroxide initiator ¾¾¾® free radical tertiary amine activator Benzoyl peroxide initiator ¾¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾¾® free radical Diketone photo initiator ¾Tertiary ¾ ¾amine blue ¾¾ ¾ ¾¾® free radical (reducing agent) visible l ight 470 nm 2) Propagation: R + M "Stable monomer" ¾¾¾® RM "active monomer" The chain reactions continue with the evolution of heat, until all the monomer is changed to polymer. This second stage "propagation" continues as the chain grows in length and so on until RMn , where n is any integral number. 3) Termination: The chain reactions can be terminated either by: a. Direct coupling: 80  This occurs when two free radicals react to form a stable molecule: R ¾ Rn ¾ M + R ¾¾® R ¾ Mn+1 ¾ R b. Exchange of a hydrogen atom from one growing chain to another N.B. Under normal conditions and due to termination, the polymerization is incomplete with a residual monomer always left. Factors accompanied with polymerization: a. Evolution of heat as the reaction is strongly exothermic. b. Reduction in volume (polymerization shrinkage). c. Presence of residual monomer because the polymerization reaction is practically never complete. II. Addition polymerization by Ring Opening mechanism: The ring opening reaction is the second type of addition polymerization used with current dental product. In ring opening the terminal reactive groups in the monomers are rings. 81 - The reactive terminal rings open under the influence of a cationic initiator and can then act as a cation itself, attack and open additional rings. - Whenever the ring is opened, the cation function remains attached→ lengthening the chain and forming the polymer. Examples - Polyether rubber impression material - Siloranes (low shrinkage composite) Advantages of materials polymerized by ring opening polymerization: 1- Less polymerization shrinkage. 2- Less heat evolution. Inhibition and Retardation 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 activated initiator or any activated nucleus, or with an activated growing chain to prevent further growth. The presence of such inhibitors influences the length of the initiation period, as well as the degree of polymerization. For example, the addition of a small amount of hydroquinone to the monomer will inhibit polymerization if 82 no chemical initiator is present, and it will retard the polymerization in the presence of an initiator. N.B. Eugenol, or large amounts of oxygen will inhibit polymerization. General properties of polymers: * Generally the polymers are molecular solids where: a. Strong primary covalent bonds exist between the mers along the whole length of the polymer chain (interatomic or intramolecular) b. Weak secondary Van der Waal forces exist between the chains of the polymer (intermolecular). N.B. These secondary Van der Waal forces (weak, polar bond) are responsible for; - Reduced strength, hardness and rigidity. - More water sorption. * 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) Factors affecting the properties of polymers: 1) Molecular weight and Degree of polymerization: a. Molecular weight (M.W): The molecular weight of a polymer molecule equals the molecular weight of the various mers multiplied by the number of the mers and may range from thousands to millions of M.W. units, depending on the preparation conditions. The higher the molecular weight of the polymer, the higher the degree of polymerization. 83  b. Degree of polymerization (D.P): The degree of polymerization (D.P.) is defined as the total number of mers in a polymer chain. e.g. D.P. = M.W. of a polymer M.W. of a mer In general those polymers made up of large molecules are stronger and more resistant to thermal and mechanical stresses than those composed of small molecules. The growth of polymer chains is a random process. Some chains grow faster than others. Some are terminated before others. Thus, 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 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. Effect on the properties; The longer the polymer chain, the greater the number of entanglement (temporary connection) 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. 84 2) Cross-linking: Definition Cross-linking occurs where adjacent linear chains of a polymer are joined together at various positions by primary covalent bonds (permanent connections) forming a 3- dimensional network cross-linked structure. A chemical compound with two double bonds per molecule can act as a cross-linking agent, since each C = C can react with a different chain. Effect on the properties; - 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 increase the strength, hardness, rigidity, and glass transition temperature. - It also decreases the water sorption and crazing (separation between the polymer chains leading to minute cracks in the surface). N.B.: Extensive cross-linking may lead to brittleness of the polymer. 85 3) Copolymerization Definition Copolymers are polymer chains containing two or more different types of monomers. Types Co-polymers are of three different types: random, block and graft. In the random type of co-polymer, the different mers are randomly distributed along the chain. In a block co-polymer, identical polymer units occur in relatively long sequence along the main polymer chain. In graft co-polymer, sequence of one of the monomers is grafted onto a "back bone" of the second monomer species. Random co- Block co-polymer polymer Graft co-polymer Effect on the properties; Copolymerization enables chemists to "tailor-make" molecules of predicted properties for special applications. N.B. Block and graft polymers often show improved impact strength 4) Plasticizers: Definition They are compounds which are added to partially neutralize the secondary Van der Waal forces (intermolecular forces) that normally prevent the polymer chains from slipping past one another when the polymer is stressed. Plasticizers may be: 86 1- External plasticizers: Addition of the plasticizer which penetrates between the polymer chains thus the polymer chains become further apart and the forces between them become less. 2- Internal plasticizers: It is accomplished by copolymerization with a suitable co-monomer in this case the plasticizer is a part of the polymer chain Effect on the properties; They usually reduce strength, hardness, rigidity, and glass transition temperature of the polymer (the opposite action of cross-linking agent). This principle is used to produce soft lining polymers. 5) Addition of inorganic fillers: Addition of inorganic filler to the polymer forms composite structure Effect on the properties It increases strength, hardness, and rigidity of a polymer. Rate of loading: Polymers are sensitive to the rate of loading (rate of deformation) because they are viscoelastic materials: a. At slow rate of loading, they behave in a ductile manner. i.e. more permanent deformation. b. At high rates of loading, they respond in a brittle manner. Temperature: Polymers are so sensitive to temperature. They soften as they are heated near their glass transition temperature. 87 Physical state of polymers: Dental polymers exist at room temperature either as rubbers (elastomers) or hard. Rubbers consist of long chain molecules that are coiled. When the material is stretched the only work done is uncoiling of the molecules. Thus such materials are easy to deform. This deformation is largely reversible. Although rubbers are rubbery at room temperature, the effect of intermolecular forces will increase as temperature decreases, and at a certain temperature (the glass transition temperature Tg) the intermolecular forces become so large as to inhibit uncoiling. Thus below the glass transition temperature the material will be rigid. Likewise if a hard polymer is heated, it looses rigidity at a certain temperature and becomes rubbery. Uses of polymers in dentistry: 1) Denture base materials. 2) Acrylic teeth for partial and complete dentures. 3) Impression materials: agar, alginates and rubbers. 4) Implants 5) Crown and bridge facings. 6) Endodontic fillings. 88  CHAPTER 5 SURFACE PHENOMENON and ADHESION 89  Chapter (6) SURFACE PHENOMENA AND ADHESION The surface phenomenon is increasingly playing a major role in the understanding of the behavior of biomaterials. Particularly important is the characterization of solid surfaces, the wetting of solids by liquids and adhesion. Obtaining adhesion between dental restoratives and tooth is a highly important prerequisite. Definitions: a. Adhesion: It bonding between dissimilar materials through chemical reaction of their atoms and molecules. 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. b. Cohesion: Is bonding between 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. c. Adhesive: Is the liquid material used to produce adhesion. d. Adherend: Is the solid substance to which the adhesive is applied. For adhesion to take place: - Materials being joined must be in close contact (intimate contact). - The adhesive must be applied in the liquid state to produce a thin layer. 90 Types of adhesion: True adhesion Mechanical attachment Definition Bonding between Bonding between dissimilar materials dissimilar materials through bonding between through mechanical their atoms and molecules interlocking" i.e. no actual "chemical reaction". bond Examples Glass ionomer and zinc Amalgam, composite, zinc polycarboxylate with the phosphate cement with the tooth. tooth. Mechanism Both glass ionomer and A liquid flows into pores polycarboxylate contain or irregularities in the CooH group that react surface of a solid and set chemically with calcium of (harden) forming a strong the tooth. mechanical bond. Chemical Adhesion Mechanical Attachment Restorative Material Restorative Material Chemically Reacting Penetrating Irregularities Wetting or Wettability: Is the ability of an adhesive to wet the surface of the adherend. It is measured by the contact angle. Contact angle: It is the angle between the surface of file 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° i.e. forces of adhesion between them are more than the forces of cohesion between adhesive molecules together. Good wetting promotes adhesion and indicates strong attraction between the liquid and solid surface molecules. 91 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. 4. To produce a smooth surface of casting, the wax pattern is coated by surface acting agent (wetting agent or debubblizer) before investing. This will improve wax wettability thus, producing smoother surface. Surface Tension and Surface Energy: Atoms and molecules at the surfaces of liquids and solids possess more energy than do those in the interior or bulk of the structure. In the case of liquids, this energy is called surface tension. N.B.: The surface tension is decreased by increasing the temperature and impurities. In case of solids, this energy is called the surface energy. It is greater than the internal energy, because the outermost atoms are not equally attracted in all directions. 92 Factors affecting Wetting: 1. The surface energy of the adherend (S.E.) (reactivity of the solid surface): Increased S.E. of the solid, increases wettability. S.E. of the solid wetting 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. 2. The surface tension of the adhesive (S.T.): Increasing S.T. of the adhesive, decreases tile wettability. S.T. of the adhesive wetting Therefore, using adhesive liquid of low surface tension will increase its wettability to a solid. N.B. For good wetting, the surface tension of the liquid should be equal or less than the surface energy of the adherend (solid). 3. The surface irregularities of the adherend (surface roughness): - If surface roughness is regular and shallow, This results in intimate contact between adherend and adhesive Good adhesive bond. - But if surface roughness is irregular and deep, so air pockets formation is more common, where air pockets will prevent the adhesive from penetrating into that area, therefore no intimate contact between adherend and adhesive will be formed Weak adhesive bond. 93 4. The viscosity of the liquid adhesive: Increasing the viscosity of the adhesive decreases the wettability. Viscosity wetting Factors affecting the strength of adhesive junction: 1) Cleanliness on the adherend: Any debris or surface contaminations prevent the adhesive from coming into the intimate contact which is necessary to produce adhesion. ** Adhesion to clean and dry surface of enamel and dentin is better than adhesion to wet contaminated one. 2) Thickness of the adhesive: The thinner the adhesive film, the stronger is the adhesive junction, with less air voids are present. ** Thin adhesive film allows: - more intimate contact - Less thermal stresses - Less stresses due to setting contraction of adhesive. 3) 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. 4) Thermal stresses: 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. 94 5) 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 joint). Failure of adhesive junction: If an adhesive bond is tested in tension, one of three things may happen: i. Adhesive failure (adhesive-adherend separation). ii. Cohesive failure of the adhesive. iii. Cohesive failure of the adherend. Importance of adhesion in dentistry: 1. Decrease marginal leakage between restorations and cavity walls. 2. Complete denture retention through the thin film of saliva. 3. Bonding agents. 4. Ceramo-metallic restorations. 95 Conditions which prevent ideal adhesion in the oral cavity: 1) The inhomogeneous composition of enamel and dentin: Enamel and dentin are inhomogeneous in their composition, being partly organic and partly inorganic. An adhesive which would adhere to the organic component would not be able to adhere to the inorganic portion. At the same time materials that would adhere to enamel would not be able to adhere to dentin. Thus adhesion would not be uniform over the entire surface. 2) Surface irregularities in the prepared cavity: The surface of the prepared cavity is full of pits and fissures. These morphologic roughnesses are further increased by the scratches which are produced by the dental burs used in preparing the cavity. It is difficult to design an adhesive that flows into these minute irregularities and wets the entire surface area of the cavity preparation. 3) Debris in the prepared cavity: Microscopically, the tooth surface is covered with debris that is formed when the dentist prepares the cavity (smear layer). Thorough cleaning of the cavity cannot remove this debris completely. This debris prevents adhesive from complete wetting of an adherent. 4) Presence of water in the prepared cavity: Of major importance is the presence of water. This is not water from saliva, but a microscopic single molecule layer of water which is always on the tooth surface. This film prevents the adhesive from coming into intimate contact with the tooth. 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. 96  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 - Etch 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 microtags into which the adhesive 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- Increasing the surface energy of the enamel, causing better wetting. 4- Exposure of greater surface area of the enamel to the resin. 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. 97 Dentin bonding involves three distinct processes: 1. Etching: Dentin etching is done by applying the acid etchant for 10 to 15 seconds. This will lead to: i. Partial or complete removal of the smear layer (debris layer). ii. 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. Priming: - It is used to elevate the surface energy of dentin to improve wetting. - Because composite resins are hydrophobic, 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. 98  Total etch versus self-etch: Choices about dental materials and techniques are based on a variety of factors that include effectiveness, longevity, efficiency, and patient satisfaction. For these reasons, when using total-etch or self-etch depends on the clinical situation. The purpose of etching is to remove the smear layer that is present after tooth preparation. In addition it opens the dentinal tubules, demineralizing enough of the dentin to allow the formation of resin tags within the dentin structure. This process in combination with dentin adhesives result in bond strengths adequate to place and retain restorations. Both techniques accomplish these goals, and come with risks and benefits. Total etching: Total etching is the classic technique of utilizing a 30% to 40% phosphoric acid gel to prepare both the enamel and the dentin for adhesive procedures. Advantages: 1. The ability to prepare enamel, dentin, and sclerotic dentin for bonding, resulting in high bond strengths. 2. Total-etch systems do not interfere with the polymerization of dual-cure resin products, so they can be used universally. Disadvantages: 1. Utilizing a total-etch system can be technique- 99 sensitive. First we have the challenge of adequately etching the enamel without over-etching the dentin. Enamel surfaces require 25 seconds of exposure to phosphoric acid. Dentin on the other hand should not be exposed to the gel for more than 15 seconds. 2. Over-etching dentin results in postoperative sensitivity as well as decreases in bond strength due to the demineralization penetrating further into the tubules than the resin tags will go, and formation of a gap. 3. Rinsing adds the next challenge as we have to understand how moist to leave the preparation before placing the dentin adhesive. The concept is to rinse the gel away and dry to remove pooling of water, yet leave water in the dentinal tubules. The water is important because when the dentin is too dry, the collagen matrix of the dentin tubules collapses and an adequate hybrid zone is not formed. This collapse prevents penetration of the resin and decreases bond strengths. Primer is hydrophilic and designed to chase water, so having moisture is inherent in a successful bond. Therefore it’s fundamental to overcome both the risk of postoperative sensitivity and over-drying. Self-etching: Self-etching systems rely on 10% maleic acid or acidic monomers to remove the smear layer and demineralize the tooth structure. 100 Advantages: 1. The main advantage of this system near absence of postoperative sensitivity. The absence of postoperative sensitivity comes from: Not over-etching or over- drying the dentin as part of the protocol. Not having to manage the timing of the material against enamel vs. dentin. We don’t have to worry about rinsing and drying Disadvantages: 1. Self-etching systems are less effective at preparing enamel surfaces and sclerotic dentin resulting in lower bond strengths and the concern about adhesive failures. Many clinicians are utilizing a hybrid technique to overcome this challenge. They place phosphoric acid gel on only the enamel margins for 25 seconds. They rinse, thoroughly dry, and then use a self-etching dentin adhesive. 101  CHAPTER 6 METALLURGY 102  Chapter (6) METALLURGY It is the study of metals and alloys. I. Metals A metal is any element that ionizes positively in solution. About 80 of the 103 elements currently listed in the periodic table of elements (Fig. 1), could be classed as metals. The basis for a lot of metals properties is the fact that valence electrons are delocalized (unbound) in metallic solids and are free to move throughout the metal rather than remaining bound to individual atoms. From the periodic chart of elements (Fig. 1) non-metals occupy the right side with a stepwise transition group of elements called metalloids. Metalloids or semiconductors are at the boundary between metals and nonmetals and they have properties incoming with both or midway between both. e.g. Carbon, Silicon and Boron. Properties of metals: 1. Ionize positively in solution. 2. In a normal environment, they are crystalline solids with the exception of mercury and gallium which are liquids at room temperature. 3. A metallic surface exhibits a luster that is difficult to duplicate in other types of solid matter. This arises from the response of the unbound electrons to electromagnetic vibrations at light frequencies, which give the mirror-reflecting property. 103 104 4. Metals are good electrical and thermal conductors. This is because free electrons are efficient carriers of thermal as well as electrical energy along a potential gradient. 5. Metals are characterized by high hardness, melting and boiling points. These properties are due to the strength of the primary interatomic bonding within the crystalline solid. 6. Metals are characterized by their ductility and malleability and this is related to the crystal structure and imperfections which allow for plastic deformation as will be discussed latter. 7. On striking a metal surface, a metallic ring is given. 8. Most metals are white with slight differences in tint. Two metals are non white, gold and copper, both happen to be rather important in dentistry. Shaping of metals: a. Casting This involves melting the metal or alloy and shaping it in a mold of the required shape. 105 b. Cold Working: A solidified block of a cast metal can be formed by mechanical working to produce a rod, wire, tube or other shapes. During the process of cold working, stresses are applied above the yield strength of the material where the mechanism of plastic deformation is through slip along crystal planes involving dislocation movements. c. Powder metallurgy (sintering): Sintering is the process of bonding of solid particles by heat in the absence of any liquid. It is an agglomeration process that involves not only bonding of powder particles but also the elimination of the initial porosity to give a denser product. 106  Sintering occurs naturally if the temperature is high enough to allow for significant number of atoms to diffuse. The atomic diffusion can be aided by applying pressure. It is accompanied by an appreciable amount of shrinkage and decreased porosity when temperature, time and pressure are applied. d. Electroforming: Using the process of electrolysis i.e. corrosion in reverse, a metal can be plated onto a conducting surface e.g. silver and copper electroplating. Solidification of metals: If a metal is melted and then allowed to cool, its temperature during cooling can be plotted as a function of time as shown in the figure. From the figure, temperature decreases from A to B then temperature is constant until time C. After C, the temperature decreases steadily to room temperature. Temperature Tf indicated by the straight or plateau portion BC, is the freezing point or fusion temperature. During freezing, heat is evolved as the metal changes from the liquid to the solid state; this heat is the latent heat of fusion. Cooling or temperature time curve for pure metal. 107  It is equal to the heat of fusion and equals to the number of calories of heat liberated from one gram of a substance when it changes from the liquid to the solid state. At all temperatures above Tf, the metal is molten and at all temperatures below Tf, it is a solid. Structure during solidification: The most accepted theoretical model proposed for solidification of metals is a two-steps mechanism involving nucleus formation and crystallization. a. Nuclear 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 supper cooling point (homogeneous nucleation). Foreign solid metallic particles e.g. iridium, which has a higher melting temperature than that of the liquid metal are added to the liquid metal (Heterogeneous nucleation) b. Crystallization: The metals can solidify in single crystal [grain] which is very rare, or polycrystalline. As cooling continues the nuclei of crystallization grow independently in three dimensions [tree like structure] to form crystals [grains]. The growth is stopped when there is contact with adjacent growing crystals c. Grain boundary: It is a region of transition between different oriented crystal lattice of the two adjacent crystals [grains]. At the grain boundaries, the atoms take up position intermediate 108 between those of the atoms in the adjacent space lattices thus have higher energy. The atoms at the grain boundaries are located in distorted position to bridge the mismatch in the lattice orientation of adjacent crystals [grains]. Because of their irregular arrangement, grain boundaries affect the properties of polycrystalline solids in various ways: 1. Crystallization and formation of new nuclei in solid phase usually start at the grain boundaries, where there is enough surface energy to start the formation of new set of grains. 2. Diffusion of atoms occurs more readily along grain boundaries. 3. Impurities in metals tend to accumulate at grain boundaries. 4. Grain boundaries also play an important role in the mechanical behavior of metals and affect their corrosion resistance. Control of grain size: In general, the smaller the grain size, the better are the mechanical properties. The factors affecting grain size are: 1. Rate of cooling from the liquid state: As the number of grains is proportional to the number of nuclei of crystallization at the time of solidification, the more rapid the rate of cooling, the more the number of nuclei of crystallization, and the smaller is the grain size. Rapid cooling can be done by using molds of high thermal conductivity, with small sized casting and by heating the metal to just above its melting temperature. 109 2. Nucleating agents "grain refiners": Addition of certain nucleating agents, during solidification will act as nuclei of crystallization producing castings of small grain size i.e. increase the number of nuclei of crystallization. These are called grain refiners which may be added intentionally or may be found as impurity. Effect of Stress on Micro-Structure of Metals: 1. When a material is stressed under its elastic limit, it will deform temporarily. So elastic deformation or strain in a metal is mainly due to stretching of the interatomic bonds. Since the modulus of elasticity is the resistance for elastic deformation and it depends on the chemical composition of the material "nature of the atomic bond" it is not affected by microstructure. 2. Plastic deformation involves the slip of layers of atoms over each other in certain planes in the metal crystals. Slip dose not occur by the movement of an entire plane of atoms over the next layer in a single movement, which requires enormous stress. Instead, the slip occurs by a localized region of shear, which passes progressively through the length of the slip plane. This localized shear zone is called a dislocation and the movement of the localized zones is called movement of dislocation. Wrought metals: These are metals that had been formed from cast or grain structure by cold working to attain a microscopically fibrous structure. Hammering, rolling or drawing into a wire transforms the grain structure into fibrous structure. 110 Cold working and strain hardening: A wrought structure or fibrous structure is plastically formed structure that was subjected to stresses above its yield point at ambient temperature. As mentioned previously plastic deformation occurs by moving of a dislocation through slip planes and become difficult if it meets other type of lattice discontinuity. Greater stress is required to produce further slip. Therefore, the metal becomes stronger and harder by cold work. With further increase in cold working, fracture occurs. In conclusion, cold worked structures are highly stressed structures with increased hardness, strength and proportional limit. On the other hand, these structures have lower ductility and corrosion resistance. The effect of cold working can be reversed simply by heating the cold worked structure and this is termed heat treatment annealing. The heating of a cold worked metal may lead to the following three stages: i. Stress-relief anneal or recovery. ii. Recrystallization. iii. Grain growth. i. Stress-relief recovery: It involves No visible change in the fibrous structure Very slight decrease in strength No change in ductility