Dental Materials 211 Past Paper PDF 2024/2025

Dental Materials 211 (DMAT211) Dr. Ahmed Magdy Sayed Associate Professor of Dental Biomaterials Faculty of Dentistry Modern University for Technology and Information 2024/2025 Mission and Vision...

Dental Materials 211 (DMAT211) Dr. Ahmed Magdy Sayed Associate Professor of Dental Biomaterials Faculty of Dentistry Modern University for Technology and Information 2024/2025 Mission and Vision ‫الرؤية والرسالة‬ :‫رؤية الكلية‬ ‫تتطلع كلية طب الفم واالسنان – الجامعة الحديثة للتكنولوجيا والمعلومات – إىل أن تكون من ر‬ ‫أكث الكليات‬.‫واإلقليم ز يف مجال طب السنان‬ ‫ي‬ ‫المحىل‬ ‫ي‬ ً‫ز‬ ‫تمثا عىل المستوى‬ :‫رسالة الكلية‬ ‫ز‬ ‫ قادرون عىل التوافق مع متطلبات سوق العمل‬،‫يتمثون بالجدارة المهنية‬ ‫ز‬ ‫تلثم الكلية بإعداد أطباء أسنان‬ ‫العلم واإلسهام فيه بالنشطة البحثية مع تلبية احتياجات المجتمع المحيط ز يف إطار قيم‬ ‫ي‬ ‫ومواكبة التطور‬.‫أخالقية‬ Vision: The College of Oral and Dental Medicine - Modern University for Technology and Information aspires to be one of the most distinguished colleges at the local and regional levels in the field of dentistry. Mission: The college is committed to preparing dentists who are distinguished by professional merit and are able to comply with the requirements of the labor market and keep pace with scientific development and contribute to it through research activities while meeting the needs of the surrounding community within the framework of ethical values.  ‫‪Strategic goals and objectives‬‬ ‫الغايات والهداف‬ ‫ز ز‬ ‫متمية ف تعليم طب األسنان‬ ‫الغاية األوىل‪ :‬تحقيق قدرة تنافسية‬ ‫ر‬ ‫االسياتيجية‬ ‫األهداف‬ ‫الجامع‪.‬‬ ‫ي‬ ‫الهدف الول‪ :‬تطوير إسثاتيجيات التدريس والتعلم بما يتفق مع اتجاه الدولة المرصية لتطوير التعليم‬ ‫الجامع والنشطة الطالبية لتنمية مهارات‬ ‫للثنامج ونظم التقويم والكتاب‬ ‫ز‬ ‫الثان‪ :‬تطوير المحتوى‬ ‫ي‬ ‫العلم ر‬ ‫ي‬ ‫الهدف ي‬ ‫وخريج الكلية بما يتفق مع متغثات سوق العمل‪.‬‬ ‫ر ي‬ ‫طالب‬ ‫الهدف الثالث‪ :‬استيفاء أعداد أعضاء هيئة التدريس والهيئة المعاونة بما يتناسب مع أعداد الطالب‪.‬‬ ‫الهدف الرابع‪ :‬استخدام تكنولوجيا المعلومات وأساليب التعلم الحديثة‪.‬‬ ‫ز‬ ‫ز‬ ‫الغاية الثانية‪:‬‬ ‫التمي واإلبداع ف مجال البحث العلم‬ ‫ر‬ ‫األهداف االسياتيجية‬ ‫والدوىل‪.‬‬ ‫المحىل‬ ‫العلم بما يدعم تقديم خدمات بحثية وعالجية للمجتمع‬ ‫تحفث منظومة البحث‬ ‫ز‬ ‫الهدف الول‪:‬‬ ‫ي‬ ‫ي‬ ‫ي‬ ‫والشاكة البحثية محليا واقليميا وعالميا‪.‬‬‫الثان‪ :‬توسيع مجاالت التعاون ر‬ ‫ز‬ ‫الهدف ي‬ ‫الهدف الثالث‪ :‬تطوير البنية البحثية والتكنولوجية للكلية‪.‬‬ ‫العلم وضمان حقوق الملكية الفكرية‪.‬‬ ‫االلثام بأخالقيات البحث‬ ‫ز‬ ‫الهدف الرابع‪:‬‬ ‫ي‬ ‫نش البحاث العلمية المحلية والدولية‬ ‫الهدف الخامس‪ :‬تشجيع أعضاء هيئة التدريس والهيئة المعاونة عىل ر‬ ‫والحث عىل المشاركة العلمية ز يف المؤتمرات‪.‬‬ ‫الخريجي ز يف سوق العمل‪.‬‬ ‫ز‬ ‫تلب احتياجات‬‫الهدف السادس‪ :‬إنشاء برامج تعليمية لمرحلة الدراسات العليا ر ي‬ ‫ز‬ ‫ز‬ ‫المدن لتقديم خدمات عالجية ف طب األسنان‬ ‫الغاية الثالثة‪ :‬التكامل مع المجتمع‬ ‫ر‬ ‫االسياتيجية‬ ‫األهداف‬ ‫ز‬ ‫المدن المحيط لتلبية احتياجات المجتمع‪.‬‬ ‫ز‬ ‫الهدف الول‪ :‬التوسع يف التعاون مع مؤسسات المجتمع‬ ‫ي‬ ‫الهدف ز‬ ‫الثان‪ :‬التوعية التثقيفية المستمرة داخليا وخارجيا لتلبية احتياجات المجتمع المحيط بالرعاية الصحية‬ ‫ي‬ ‫لألسنان‪.‬‬ ‫الهدف الثالث‪ :‬التطوير المستمر للخدمات العالجية بالعيادات الخارجية للكلية‪.‬‬ ‫ز‬ ‫الخريجي‪.‬‬ ‫الهدف الرابع‪ :‬دعم برامج التواصل مع‬ ‫الغايــة ال ـرابعة‪ :‬التـ ُـم زي واإلبــداع الـمؤســس‬ ‫االسياتيجية‬ ‫ر‬ ‫األهداف‬ ‫الهدف الول‪ :‬تطوير البنية التحتية والتكنولوجية للكلية‪.‬‬ ‫الثان‪ :‬تنمية قدرات القيادات االكاديمية واالدارية الحالية والمستقبلية‪.‬‬ ‫ز‬ ‫الهدف ي‬ ‫الهدف الثالث‪ :‬تنمية قدرات اعضاء هيئة التدريس والهيئة المعاونة والجهاز اإلداري‪.‬‬ Strategic goals and objectives ‫الغايات والهداف‬ The first aim: Achieving distinct competitiveness in dental education First goal: Developing teaching and learning strategies in line with the Egyptian state’s direction to develop university education. Second goal: Developing the program’s scientific content, evaluation systems, university book, and student activities to develop the skills of college students and graduates in accordance with labor market variables. Third goal: Fulfilling the numbers of faculty members and supporting staff in proportion to the numbers of students. Fourth goal: Using information technology and modern learning methods. The second aim: Excellence and creativity in the field of scientific research First goal: Stimulating the scientific research system to support the provision of research services with therapeutic applications that meet the needs of the local and international community. Second goal: Expanding areas of cooperation and research partnerships locally, regionally and globally. Third goal: Developing the college’s research and technological infrastructure. Fourth goal: Commitment to scientific research ethics and ensuring intellectual property rights. Fifth goal: Encouraging faculty members and supporting staff to publish local and international scientific research and encouraging scientific participation in conferences. sixth goal: Establishing postgraduate educational programs that meet the needs of graduates in the labor market. The third aim: Integration with civil society to provide therapeutic services in dentistry First goal: Expanding cooperation with surrounding civil society institutions to meet community needs. Second goal: Continuous educational awareness, internally and externally, to meet the needs of the community surrounding dental health care Third goal: Supporting alumni communication programs. Fourth goal: Commitment to scientific research ethics and ensuring intellectual property rights. The fourth aim: Institutional excellence and creativity First goal: Developing the college’s infrastructure and technology. Second goal: Developing the capabilities of current and future academic and administrative leaders. Third goal: Developing the capabilities of faculty members, supporting staff, and the administrative staff.  Contents Structure of Matter............................................................................................ 1 Physical Properties........................................................................................... 11 Mechanical Properties.................................................................................... 22 Principles of adhesion...................................................................................... 38 polymers................................................................................................................ 46 Metallurgy............................................................................................................ 58 Tarnish and Corrosion..................................................................................... 73  Structure of Matter The Atomic Structure  The basic unit of any material is the atom  The atom consists of a nucleus and surrounding electrons Nucleus is the core of the atom and composed of: ✓ Positively charged protons. ✓ Uncharged neutrons. Electrons surround the nucleus in shells ✓ Negatively charged. ✓ The most outer electrons are called valance electrons ✓ Valence electrons affect the physical & chemical properties N.B: Atomic number = Number of electrons = number of protons. N.B: Atomic weight = weight of protons + neutrons.  Every element tries to reach the stable configuration by having 8 electrons in its outer shell by: Releasing Electrons ➔ become +ve charged Receiving extra electrons ➔ become –ve charged Sharing electrons with another atom Then forming the atomic bonds Chapter I Structure of Matter The atomic Bonds 1. Primary bonds a. Covalent bond Arises by sharing electrons between atoms, the atoms approach one another and orbital overlap happened. It May be the same element (e.g. H2) or between different elements (e.g. CH4) It may be single bond (e.g. CH3-CH3), double bond (e.g. CH2 = CH2) or triple bond (e.g. CH ≡ CH). Examples: Basic bond of diamond & polymer Characteristics of covalent bond: 1. Highly directional bond. 2. High strength and hardness. 3. High heat resistance. 4. Thermal and electrical insulators. 5. Dissolve in organic solvents. b. Ionic bond Occurs by electron transfer from one atom (become +ve ion) to another (Become –ve ion), then electrostatic attraction between them. Sodium gives his outer electron to Chlorine ➔ Na+ Cl-. Examples: Basic bond for Ceramics, Glasses. 2 Chapter I Structure of Matter Characteristics of Ionic bond: 1. Spherical in nature. 2. High strength and hardness. 3. High heat resistance. 4. Insulators as solids. 5. Electric conductors in solutions. 6. Dissolve in ionizing solvents (not in organic solvents). c. Metallic bond The metals have loosely held valence electrons, so they move freely between atoms forming cloud of electrons. So, the metals are consisting +ve ion cores surrounded by the cloud of free electrons. So, the metallic bond is the attraction between the +ve cores and –ve free electrons. Characteristics of Metallic bond: 1. High strength and hardness. 2. High thermal resistance. 3. High thermal conductivity (as free electrons conduct heat). 4. High electric conductivity (as free electrons carry the electric current). 5. Opaque (as free electrons absorb light). 6. Lustrous (as free electrons reflect the light). 7. Leads to crystalline arrangement in metals 8. Leads to easy of deformability of metals. 3 Chapter I Structure of Matter 2. Secondary bonds (Van der Waals Forces): a. Fluctuating Dipole: It developed between atoms due to asymmetry of electron distribution. This asymmetry gives the atom dipole character. It is a temporary bond. b. Hydrogen Bond: It developed between molecules when each molecule has a dipole. The oxygen atom has a negative charge in relation to the hydrogen atom inside the same molecule. Attraction occurs between the positive part of molecule and the negative part of another molecule. Characteristics of secondary bond: 1. Low strength and hardness. 2. Low heat resistance. 3. High thermal expansion. 4 Chapter I Structure of Matter Interatomic Distance (I.A.D):  It is the distance between atoms.  Resulted from 2 equal but opposite forces: i. Repulsive forces ➔ due to electrostatic field of each atom. ii. Attractive forces ➔ different types of atomic bonds.  Factors affecting interatomic Distance: 1. Temperature Heat increases I.A.D (as it increases energy of atoms) 2. Number of adjacent atoms: More adjacent atoms will increase I.A.D (as less specific attraction to any neighboring atom). 3. Type of the bond: Increase number of shared electrons in covalent bond will decrease I.A.D (as it means stronger bond). CH ≡ CH is stronger than CH2 = CH2, stronger than CH3-CH3. 4. Any external forces may displace atoms and change I.A.D. Classification of Solids: I. According to Intermolecular Bond: Atomic Solids Molecular Solids Bonds between atoms Primary Primary Bonds between molecules Primary Secondary Properties High strength and Low strength and hardness hardness Example Diamond Polymers 5 Chapter I Structure of Matter II. According to Arrangement of Atoms: Crystalline Solids Amorphous Solids Atoms are regularly arranged with Atoms are randomly distributed or repetition in 3D (called space lattice or with very short arrangement crystal lattice) Have low internal energy Have high internal energy Have definite melting temperature Have no definite melting temperature (gradually soften by heating and gradually harden by cooling) Glass Transition temperature: The temperature at which the amorphous solids start to soften or harden Crystalline Solids:  The atoms arrange themselves in a repeated manner.  The smallest repeated unit in a crystal lattice is called unit cell.  Unit cell may be one of 7 main patterns and subdivided to make 14 possible patterns. 1. Cubic System: Axes: a = b = c Angles between axes: 90o It is subdivided into: 6 Chapter I Structure of Matter a. Simple Cubic System (SC): Each atom at the corner of unit cell is associated with 8 surrounding 8 unit cells ➔ Each unit cell contains 8 X 1/8 = 1 atom b. Body Centered Cubic (BCC): Like SC but in contain an extra atom at the center of the cubic ➔ Each unit cell contains (8 X 1/8) + 1 = 2 atoms c. Face Centered Cubic (FCC): Like SC, but contain atom at the center of each face. N.B: The atom at the face is associated with 2-unit cells ➔ Each unit cell contains (8 X 1/8) + (6 X ½) = 4 atoms 2. Hexagonal Crystal Axes: a = b ≠ c Angles between axes: α = β = 90o,  =  a. Simple Hexagonal System: Contains 6 atoms at the top, 6 atoms at the bottoms (each associated with 6 unit cells) and one at the upper and lower faces ➔ Each unit cell contains (6 X 1/6) + (6 X1/6) + (2 X ½) = 3 atoms b. Hexagonal Closed Packed (HCP): Like simple hexagonal but it contains 3 unshared atoms at the same plane in the center of the unit cell. ➔ Each unit cell contains: (6 X 1/6) + (6 X1/6) + (2 X ½) + 3 = 6 atoms 7 Chapter I Structure of Matter Atomic Packing Factor: It is the fraction of space occupied by the atoms Volume of atoms inside unit cell APF = Volume of unit cell It is 0.54 in SC, BCC = 0.68, FCC = 0.74, 0.74 in HCP. Clinical importance: ↑ APF ➔ have ↑ densities and strength properties. Imperfection in Crystalline Solids:  Theoretical calculation of strength is much higher than actual strength. This is due to the presence of defects in the crystalline system.  Types of crystalline imperfections: a. Point defect: 1. Vacancy ➔ missing atom within the crystal 2. Self-interstitial atom ➔ extra atom from same metal is lodged within crystal. 3. Interstitial impurity ➔ extra atom from another metal is lodged within crystal. 4. Substitutional impurity ➔ atom from another metal replaced the space of the original atom. b. Line defect The most common type. Dislocation: it is the displacement of a row of atoms from their normal positions in the lattice. Clinical importance: plastic deformation in metals occurs by motion of dislocations c. Plane defect Such as grain boundaries in metals 8 Chapter I Structure of Matter Polymorphism  Materials that have the same chemical composition but found naturally in different physical forms.  If the material is inorganic, the polymorphism is called allotropy.  If the material is organic, the polymorphism is called isomerism.  The polymorphic forms have different physical properties as density, freezing point, optical properties, conductivities …etc  Dental application: Silica showed two types of transformation: 1. Displasive transformation: the same form just expanded at low temperature. 2. Reconstructive form: involves transformation from one form to another. 867 o C 1470 o C 1713 o C β-Quartz β-Tridymite β-Cristobalite Fused Silica (Hexagonal) (Rhombohedral) (Cubic) (Amorphous) 573 o C 160 o C 260 o C α-Quartz α-Tridymite α-Cristobalite N.B: The transformation form one form to another is called reconstructive transformation as it involves breakage of bond and reconstruct new bond. If Tridymite or cristobalite or fused quartz is rapidly cooled, there will be no time for slow transformation and thus all forms of silica can be found at room temperature. 9 Chapter I Structure of Matter Displacive transformation Reconstructive transformation No breakdown of atomic bonds Breakdown of atomic bonds followed by reconstruction of new space lattice Accompanied by expansion No Expansion Rapid transformation Slow transformation Occurs at lower temperatures Occurs at higher temperature Correlation Between Atomic Structure and Materials Properties:  Density: controlled by ➔ atomic weight, atomic radius, atomic packing factor.  Bond strength leads to: High strength and hardness. High melting and boiling temperature. Low coefficient of thermal expansion.  Electrical and Thermal conductivity ➔ depends on nature of atomic bond Metallic solids conduct heat and electricity. Ionic solution conduct electricity, while ionic solids are electrical insulators. Covalent solids are insulators.  Crystalline solids have low internal energy because they send their internal energy in arranging their atoms.  FCC is more ductile than BCC due to higher atomic packing factor.  FCC is more ductile than HCP due to symmetry of the cubic system. 10  Physical Properties I. Mass related Properties. Density II. Thermal Properties. 1. Thermal conductivity. 2. Specific heat. 3. Thermal diffusivity. 4. Coefficient of thermal expansion and contraction. 5. Melting and Freezing point. 6. Heat of Fusion. III. Rheological Properties. IV. Optical Properties. Mass Related Properties Density It is the mass per unit volume Unit: gm/cc OR gm/cm3 Importance in Dentistry: 1. Retention of upper denture: Denture with lighter weight will help in retention of the denture. i. Nonmetallic denture base is lighter than metallic one. ii. Base metal alloys are lighter than the gold alloys. 2. During Casting: Lighter alloys require more casting force to allow rapid filling of the mold Chapter II Physical Properties Thermal Properties: 1. Thermal Conductivity Definition: it is the quantity of heat in calories per second passing through a body of 1 cm thick with a cross section of 1 cm2 when temperature difference is 1o C. Unit: Cal/Sec/cm2 (oC/cm). Metals are better conductors than non-metals Importance in Dentistry: 1. Metallic denture base is preferred than non-metallic denture base, as they provide physiological stimulation (VC and VD) to oral tissue to maintain them in good health (thermal conductivity is an advantage). 2. Metallic restoration in deep cavities should be preceded by a protective base to protect the dental pulp form thermal shock (thermal conductivity is a disadvantage). 2. Specific heat Definition: It is the quantity of heat needed to raise the temperature of a 1 gm of the material 1o C. Importance in Dentistry: Prolonged heating of gold during casting is unnecessary because gold has low specific heat 3. Thermal Diffusivity: Definition: it is the rate at which a body with non-uniform temperature approaches thermal equilibrium. Unit: mm2/Sec. Thermal conductivity Equation: Specific heat X Density 12 Chapter II Physical Properties Importance in Dentistry: 1. Low specific heat combined with high thermal conductivity of dental amalgam ➔ create thermal shock 2. The thickness of remaining dentine is very important to prevent thermal pulp shock ➔ preserve the sound tooth structure. 4. Coefficient of Thermal Expansion and Contraction: Definition: it is the change in length per unit length of the material for 1o C change in temperature. It is called linear thermal coefficient of expansion and contraction (α). Lf - Lo Equation: α = Lo (Cof – Coo) Unit: / Co The value is small so it is expressed as part per million (10-6) Importance in Dentistry: 1. The high difference in α between tooth and restoration leads to marginal leakage at tooth-restoration interface then Marginal percolation (pumping in and out of food and saliva between tooth and restoration). This leads to Hypersensitivity, Recurrent carries and Marginal discoloration. 2. In porcelain fused to metal crown, slight difference in α between metal and porcelain is important to allow strong compressive bond. 3. The high difference in α between artificial teeth and acrylic resin denture base should be avoided to prevent crazing of the denture base. 4. The high α of wax used for wax pattern construction may lead to its distortion after cooling of the molten wax. 13 Chapter II Physical Properties 5. Thermal expansion of investment material is important to compensate the thermal shrinkage of molten metal. 5. Melting and Freezing Temperature: Definition: It is the temperature at which a material melts or freezes. Importance in Dentistry: 1. Determine the melting machine for melting dental casting alloys. 2. Determine the type of investment material. 3. Avoid over heating of materials to avoid evaporation of specific ingredients from it. 4. Waxes used in molten state inside the oral cavity should have low softening point to avoid burning of living tissues. 5. The solder should melt at temperature lower than soldered metals by 50- 100o C to avoid distortion of soldered structures. 6. Heat of Fusion: Definition: it is the amount of heat in calories required to convert 1 gm of a material from solid state to the liquid state at melting temperature. To liquefy a liquid, heat is applied. The temperature of solid increase until it reaches its melting temperature. The solid starts to liquefy and the temperature remain constant although the heat is applied (called heat of fusion) until all solid becomes liquid. Then the temperature elevated. 14 Chapter II Physical Properties When the liquid is cooled, it will liberate the same amount of heat and called Latent heat of fusion (It is the amount of heat liberated during converting 1 gm of the material from liquid to solid state at freezing temperature). Importance in Dentistry: Heating the molten alloys 100o C above its meting temperature to give time of molten alloy to completely fill the mold Rheological Properties:  It is the study of the flow of the material. 1. Fluidity: it is the tendency of the fluids to flow. 2. Viscosity: it is the resistance of fluids to flow. 3. Viscoelasticity: it the deformation of solids (will be discussed in chapter III).  Rheological properties of matter can be classified to: 1. Newtonian: The viscosity is constant with application of Shear stresses. 2. Dilatant: The Viscosity increased with increasing shear stresses. 3. Pseudo-plastic: The viscosity decreased with increasing shear stresses. Importance in Dentistry: ✓ Many impression materials and cements are subjected to shear stresses during their extrusion just before clinical use to increase their flow. 15 Chapter II Physical Properties Optical Properties: 1. Light  Definition: it is an electromagnetic radiation that can be detected by human eye.  Light at interface: When incident light travelling form one medium to another, one of the following results: 1. Reflection: If light falls on smooth surface, it reflects regularly, (where the angle of incidence = angle of reflection) and called specular reflection and the surface appears shiny. If light falls on rough surface, it reflects in all directions and called diffuse reflection. The surface appears dull. Importance in Dentistry: The surface of aesthetic restoration should be smooth to produce specular reflection so producing shiny surface. 2. Refraction: Definition: It is the change of light direction on entering second medium. It results from difference in refractive indices of 2 media. Importance in Dentistry: The refractive indices of aesthetic restorative material (composite resin, porcelain) should be matched with tooth structure. 16 Chapter II Physical Properties 3. Scattering: If a light beam (passing through a medium) found a scattering center (opacifiers or air bubbles), it will emerge in all directions and it will weaken. Therefore, the material will appear opaquer. Importance in Dentistry: 1. Opacifiers are added to esthetic restorative materials to obtain different shades. 2. Incorporation of air bubbles during mixing of restoration will give more opaque restoration. 4. Transmission: Properties of materials in Relation to Light Transmission: 1. Transparency: Most of light passed through the material, Object can be seen clearly through it. e.g: glass & acrylic resin. 2. Translucency: Some of light passed and the rest scattered or reflected, Object can’t be seen clearly. e.g: tooth enamel, porcelain, composite resin & pigmented acrylic resin. 3. Opacity: All of light is absorbed, the material prevents light passage through it. The objects can’t be seen through it. e.g: Metals, alloys, and gypsum products. 17 Chapter II Physical Properties 5. Fluorescence: The human teeth absorb light and emit it with longer wavelength. Importance in Dentistry: 1. Fluorescence gives the tooth brightness and vital appearance. 2. Tooth structure emits fluorescent light when exited by ultraviolet radiation. So, aesthetics restorative materials should simulate natural tooth structure. 3. Fluorescence helps in diagnosis of initial caries. 6. Opalescence: It is a light scattering effects that is occurs at the tooth surface especially at the incisal edges. Importance in Dentistry: This effect creates bluish-white color when the tooth is seen at different angles. 2. Color  Definition: It is a physiological response to a physical stimulus.  Color Description: Color can be described and systemically arranged in the three dimensions. Munsell color system described the color by three parameters: a. Hue: It is the dominant wave length. It represents the color of material (red, yellow, blue, …) b. Chroma: It represents the strength of color or degree of saturation or measurement of color intensity. Glass of water contains 10 ink drops have more chroma than a glass contains just 1 drop. 18 Chapter II Physical Properties c. Value: It represents the lightness or darkness of color. The lightest materials have a value = 10, while the darkest materials have value = 0 It is the most important color parameter as it represents vitality of tooth (non-vital tooth has low value and appears gray).  Color mixing: Additive color mixing Subtractive color mixing Occurs due to light mixing Occurs due to pigments mixing The eye is stimulated directly by a The eye is stimulated indirectly by visible light reflection and absorption. Primary colors are red, blue and green Primary colors are cyan, magenta and yellow Mixing for all primary colors leads to Mixing for all primary colors leads to white color black color Secondary colors are cyan, magenta Secondary colors are red, blue and and yellow green Complementary colors are pairs of Complementary colors are pairs of color, when mixed gives white color color, when mixed gives black color (red–cyan, green–magenta, and blue– (red–cyan, green–magenta, and blue– yellow). yellow). Dental Example: fluorescence Dental Example: colors of restorations 19 Chapter II Physical Properties  Factors Affecting Color Appearance and Selection: 1. Light Source: Different light sources have different color content Metamerism: The change of color matching of two objects under different light sources. Clinical Importance: Selection of tooth color should be done under day light, as it contains almost all visible wavelengths. 2. Surroundings: Color of surroundings (wall, lips, patient’s clothes,..) modify the type of light reaching the object. 3. Object: 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): Smooth surface appears brighter than rough surface. c. Presence of scattering centers: Increase opacity and lower the value (darker). d. Fluorescence: It makes the human teeth bright and vital, as it increases the brightness. e. Thickness: Increase in thickness ➔ increase opacity, and lower the value. f. Metamerism: 20 Chapter II Physical Properties 4. Observer: a. Color Response: Eye responds differently from one person to another due to difference in age, sex, memories, cultural and sociological background b. Color Blindness: Some people can’t distinguish between certain colors. c. Color Fatigue: Constant stimulation of one color decreases the response of the eye to that color. To avoid color fatigue: 1. Tooth shade should be recorded before tooth preparation to avoid prolonged staring at tooth. 2. If happened, looking to the complementary color (blue). Other Properties: 1. Water Sorption: Definition: it is the amount of water adsorbed on the surface and absorbed into the body of the material. Clinical importance: 1. Water sorption of acrylic resin denture base will compensate cooling shrinkage. 2. Hydrocolloids impression materials should not be immersed into water to avoid dimensional changes. 2. Shelf Life: It is a term applied to the general deterioration and change in quality of materials during shipment and storage. 21  Mechanical Properties Mechanical properties are a group of physical properties that describe the behavior of materials under force or load. Force:  On applying force on a body, it may cause: 1. Displacement 2. Acceleration 3. Deformation  Force is defined by: 1. Speed ➔ static or dynamic force 2. Magnitude 3. Point of application. Normal or Tangential 4. Direction Stress:  Definition: It is an internal reaction to the external applied force. It is equal in intensity and opposite in direction to external force. As both the applied force and stress are distributed over an area so Force F Stress = or σ = Area A As it is impractical to measure the stress, so we measure the external force over the area.  Unit: Pa = N/m2, MPa = MN/m2 = N/mm2, Ib/in2 Chapter III Mechanical Properties  Types of Stress: 1. Tensile Stress 2. Compressive Stress 3. Shear Stress 2 sets of force. 2 sets of force. 2 sets of force. On the same line On same line. Not on same line. (Parallel to each other). Away from each other Toward each other Toward each other Cause elongation to the Cause shortening of the Cause tearing or sliding body body of the body. 4. Complex Stresses: ✓ Combination between 2 or more types of stress ✓ The stresses in oral cavity are complex stresses. Strain: The stress causes distortion or deformation.  Definition: the change in length per unit length Deformation l final – l original Strain = or ε= Original length l original  Unit: mm/mm = dimensionless  Types of Strain: Each type of stress can produce corresponding strain 23 Chapter III Mechanical Properties Each type may be: 1. Elastic Strain (Temporary Strain): ✓ Can be recovered after load removal 2. Plastic Strain (Permanent Strain): ✓ Cannot be recovered after load removal Poisson’s ratio (μ):  During axial loading (tension or compression), there is a simultaneous axial and lateral strain.  i.e. in tension, the material elongates (in axial direction) also it is compressed (in lateral direction) Lateral strain  Within elastic range: μ= Axial strain  For most of dental materials, μ = 0.3 Stress-Strain curve: In order to study the properties of the material of dental interest, stress is applied over the material and the value of strain is recorded. Stress-strain curve is gained from the previous data by plotting stress in the vertical axis and strain in the horizontal axis. N.B: The engineering stress-strain curve1 will be discussed in this chapter. 1 In the engineering stress-strain curve, the cross-sectional area of the tested sample is assumed to be constant. If the change in cross sectional area is taken into consideration, it is called true stress-strain curve. 24 Chapter III Mechanical Properties From stress strain curve, the following mechanical properties can be derived: 1. Proportional limit: It the greatest stress the material can withstand without deviation from Hook’s law or the law of proportionality between stress and strain: If stress is doubled ➔ the strain is doubled 2. Elastic limit: It is the greatest stress the material can withstand without permanent deformation. The proportional limit and the elastic limit represent the same value. They differ in the fundamental concept. 3. Yield strength: The stress at which the material begins to function in a plastic manner (permanent deformation). The amount of permanent deformation is arbitrarily selected (may be 0.1%, 0.2% or 0.5%) (Called percent offset) 25 Chapter III Mechanical Properties Dental importance: It represents a functional failure (clinical failure) of restoration (The restoration can’t be used in patient mouth) 4. Ultimate strength: The maximum stress the material can withstand before fracture Yield strength is more important than ultimate strength. 5. Fracture strength: It is the stress at which the material will fracture. 6. Modulus of elasticity (Young’s Modulus) “E”: It is the constant of proportionality between stress and strain It represents the stiffness of the material within the elastic range It represents the slop of the elastic portion of the stress-strain curve 𝜎 Equation: E= kg/cm2 or MPa or Ib/in2 𝜀 It is not change either tested under tensile of compressive test It depends on: 1. Inter atomic or intermolecular forces of the material 2. Composition of the material. It is independent on: 1. Heat treatment 2. Mechanical treatment. Dental importance: a. Denture base should be constructed of a rigid material to be used in thinner sections without the risk of bending. b. Long span bridges are constructed of a rigid material to allow proper stress distribution. c. Rigid base should be used under restorative filling material to increase the fracture resistance of the filling. 26 Chapter III Mechanical Properties 7. Flexibility: The maximum strain that occurs when the material is stressed to its proportional limit. Clinical Importance: a. Flexibility of Elastic impression materials indicates easily removal from the mouth. b. Clasps of partial denture should be flexible to be easily removed from the tooth undercuts. c. Flexibility of endodontic files provides easily preparation of curved root canals. 8. Brittleness The brittle materials show no or very little plastic deformation. Brittle materials are weak in tension but strong in compression. Brittle materials fracture by crack and crack propagation. Dental amalgam compressive strength is 6 times higher than tension strength. 9. Malleability and ductility: Malleability ➔ The ability of metal or alloy to be hammered into thin sheets without fracture (withstand compressive stresses). Ductility ➔ The ability of metal or alloy to withstand tensile stresses (drawn into wire) (withstand tensile stresses). These are properties of metals and alloys They indicate the workability of the alloys. As burnishability of the metallic restorations. 27 Chapter III Mechanical Properties Brittle fracture Ductile fracture Occurs in brittle materials as composite Occurs in ductile materials as metals resin and ceramics The material shows no or little plastic The material shows great plastic deformation deformation Characterized by crack and crack Characterized by necking propagation N.B: % Elongation: It is the deformation resulted from application of tensile forces It is an indication of the workability of alloys 𝑖𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑖𝑛 𝑙𝑒𝑛𝑔𝑡ℎ % elongation = x 100 𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ Dental importance: ✓ Dental gold alloys % elongation = 20 % ➔ ductile ✓ Nickel-chromium alloys % elongation = 1 % ➔ brittle. 28 Chapter III Mechanical Properties 10.Resilience: It is the amount of energy needed to deform the material to its proportional limit. This energy is stored energy because when the load is removed the energy is released causing complete recovery of the deformed material. It is represented by area under the straight portion of the stress-strain curve Dental importance: a. It is important in orthodontic wires as they store energy and release it over a required time to move teeth. b. In removable dentures, acrylic teeth are more resilient than porcelain teeth. Therefore, they absorb masticatory forces and transmit less force over the residual ridges. c. Resilience of mouth guard allows absorption of undue energy and protection of teeth from damage. 11.Toughness: It is the amount of energy required to stress the material to fracture. It is represented by the area under the elastic and plastic portions of stress-strain curve. It is calculated graphically by calculating the number of squares X area of each square. 29 Chapter III Mechanical Properties 12.Fracture toughness: It is the amount of energy required to fracture the material in the presence of cracks or flaws. Crack act as a stress concentration factor ➔ less forces are needed to fracture the material. It is more obvious in the brittle materials as the ductile material can be plastically deform and redistribute the stresses. Dental importance: ✓ Presence of fillers in resin composites deflect cracks. ✓ Presence of crystalline phases of ceramics deflect cracks. ✓ Presence of zirconia particles heal cracks. Properties of stress-strain curve: Flexible Flexible Ductile Ductile Brittle Weak Strong Strong Flexible Resilient Resilient Brittle Ductile Brittle Flexible Stiff Stiff Weak Strong Strong Tough Weak Weak Ductile Brittle Stiff Stiff 30 Chapter III Mechanical Properties Other Mechanical Properties and Tests: 1. Diametral Compression Test (Brazilian Test) (Indirect Tensile Test): It is used to determine the tensile strength of brittle materials in an indirect way. A compression load is applied on a cylindrical specimen, so tensile stresses are introduced in a perpendicular plane on the applied load. 2𝑃 Tensile stress = 𝜋𝐷𝑇 P: load D: diameter T: thickness 2. Transverse strength (Modulus of rupture) (Flexure strength) (Three-point loading). Simple Beam is supported at both ends, and subjected to static load in the middle. The test gives the flexural strength and the accompanied deformation. 3PL S= 2bd2 S: Flexure strength P: load L: distance between supports B: breadth of the specimen (width) D: Depth of the specimen (thickness) The deformation is more important as it represent the functional failure of the restoration. PL3 Deformation = 4bd3 E E: Modulus of Elasticity 31 Chapter III Mechanical Properties N.B: The length and thickness are the most affecting factors in deformation. Deformation is related to the cube of length and thickness. Dental importance: ✓ It is important in denture base and long span bridges design as the deformation is varies with cube of length and thickness of the beam. ✓ The operator should choose a material with high modulus of elasticity to avoid its deformation during service. 3. Cantilever Bending: The sample is clamped at one end and load is moment Bending P applied on the other free end. As the force increase, the angle of bending Angular deflection increased Curve is drawn with bending moment versus angle of bending Dental importance: The bending properties is very important in endodontic files. 4. Fatigue strength: Definition: It is the stress at which the material fracture progressively under repeated cycling loading. Fatigue strength test is performed by subjecting the specimen to repeated cyclic stresses under the proportional limit until fracture. If the applied stress is high ➔ the material needs less cycles of load. If the applied stress is lowered ➔ the material needs more cycles of load to fracture until a stress the material will not fracture with infinite numbers of cycles and called endurance limit. 32 Chapter III Mechanical Properties Dental importance: The dental materials should have fatigue limit above the masticatory forces to withstand unlimited numbers of cyclic loading. 5. Impact strength: Definition: ✓ The energy required to fracture the material under sudden impact force. Test: ✓ The specimen is notched to standardize the fracture site. ✓ The specimen is supported, and is struck by sudden blow of weighted pendulum. ✓ The angle before striking the specimen and the angle after striking are measured, and the difference between them used for calculating the impact strength. The most commonly used tests: 1. Charpy test: The specimen is supported at both ends and struck at the middle. 2. Izod Test: The specimen is supported at one end and struck at the other end. 33 Chapter III Mechanical Properties Surface Mechanical Properties: 1. Hardness: Definition: It is the resistance to permanent indentation or penetration. Test: ✓ The principle of hardness tests is the hard materials are difficult to be penetrated, so they resist the action of the indenter. ✓ The hardness tests differ in the indenter shape and material. Dental Importance: a. Natural teeth should not be opposed by harder material as porcelain. b. Avoid scratch of soft materials as model and die. c. Restorative hard materials are difficult to polish, but they preserve their polished surface. 34  Chapter III Mechanical Properties Test Indenter Used for measurement Test limitation Brinell Steel Metal and Area of It is old method. Can’t be used for polymer as it ball alloys indentation Use large indenter so it can’t causes elastic deformation determine localized value (not indentation) Rockwell Steel Materials that Depth of Expressed by letter and number. Not suitable for brittle ball or show elastic indentation Letter expresses test condition. materials diamond recovery Number indicates the hardness. point Vickers Square Very hard Area of Microindentation test Can’t be used for viscoelastic based materials as indentation Indenter is called 136 diamond materials diamond gold alloys Pyramid pyramid Brittle materials, Tooth structure Knoop Diamond Metals and Area of Microindentaion test. Needs highly polished and flat pyramid alloys. indentation Can test the material with great surface. Enamel, range of hardness simply by Needs very long time. dentine. varying test load. Brittle The shape of pyramid allows materials. relaxation in the minor diagonal only not major so can be used in viscoelastic materials Shore A Blunt Rubbers Depth of Reads scale from 0:100 point indentation 0 = complete penetration indenter 100 = no penetration 35 Chapter III Mechanical Properties 2. Wear: Definition: It is loss of material resulting from mechanical action. Types: a. Physiologic: normal mastication leads to wear of the teeth. b. Mechanical: improper use of toothbrush. c. Pathologic: bruxism. Dental importance: It is undesirable process except in polishing. Viscoelasticity Some materials are affected by rate of loading. So different rate of loading leading to different stress-strain curves to same material. The viscoelastic materials behave in a ductile manner with slow rate of loading and behave in a brittle manner with high rate of loading. The viscoelastic material exhibit combination of three behaviors: Ideal elastic Anelastic Viscous When stress is applied Immediate deformation Deformation increases Deformation increases with time with time When stress is removed Complete recovery Complete recovery No recovery Immediate Needs time Strain not depends on Strain depends on time Strain depends on time time 36 Chapter III Mechanical Properties Viscoelastic behavior: When stress is applied: Immediate strain (elastic part). Gradual increase in strain (anelastic and viscous parts). When stress is removed: Immediate recovery (elastic part). Gradual recovery (anelastic part). Permanent deformation (viscous part). Dental application: a. Elastic impression materials ➔Should be removed rapidly from patient mouth (sharp snap removal) 1. To decrease the deformation produced by viscous part. 2. To increase the tear strength of the impression material. b. Elastic impression materials ➔ Should have time before pouring with gypsum ➔ To give time for recovery of the anelastic part. C. Creep: Definition: ✓ It is time dependent permanent deformation as a result of long stresses below yield strength. ✓ It occurs at temperature near softening temperature. Dental application: Polymers and dental amalgam are viscoelastic materials, have softening temperature near the mouth temperature, so they can creep if stresses are applied over them. 37  Principles of adhesion Definitions:  Cohesion: It is the bonding between similar materials. Examples of cohesion: Bonding between two pieces of pure gold under pressure. It results from metallic bond and called pressure welding.  Adhesion: It is the bonding between two dissimilar materials as a result of the attraction between their atoms, ions or molecules. Examples of adhesion: Denture retention with soft tissues due to presence of thin film of saliva.  Adhesive: It is a liquid material used to produce adhesion  Adherend (substrate): It is the substance to which the adhesive is applied. N.B: To produce adhesion, intimate contact between adhesive and adherend should be achieved. Types of Adhesion: 1. True adhesion: Caused by primary bond. Example: bonding of glass ionomer cement, zinc polycarboxylate cement and adhesive resin cement to tooth structure. 2. Mechanical interlocking: a. Macro-mechanical interlocking: It occurs by performing undercut inside the tooth structure. Example: amalgam restoration. Chapter IV Principles of Adhesion b. Micro-mechanical interlocking: It occurs as a liquid flow into pores of solid surface, then the liquid is hardened. The liquid is attached to the solid surface and the bond may be strong. Example: resin composite. Factors Affecting Strength of Adhesive Junction: I. Wetting It is the ability of the liquid to spread over a solid surface. It is measured by contact angle (smaller contact angle ➔ higher wetting) Contact angle (θ): It is the angle between the surface of the liquid and the surface of the solid. Factors affecting wetting: 1. Surface energy of adherend: It is the attraction force between the surface atoms of a solid. ↑ Surface energy ➔ ↑ wetting Metals have high surface energy than waxes. therefore, water spreads easily over metals more than wax. 39 Chapter IV Principles of Adhesion 2. Surface irregularities of the adherend: Shallow and regular surface irregularities provide mechanical interlocking with the adhesive ➔ good adhesion. Deep and narrow surface irregularities form air pockets which prevent wetting of the adherend with adhesive ➔ poor adhesion. Adhesive adherend 3. Surface tension of the adhesive: Air pocket Debris It is the attraction force between the surface atoms of a liquid. ↑ Surface tension ➔ ↓ wetting. Mercury has high surface tension. Therefore, it forms droplets rather than spreads over solid surfaces. Surfactant (surface active agent, wetting agent) reduces surface tension of liquids ➔ ↑ wetting. 4. Viscosity of the adhesive: Increase flow of the adhesive results in easily penetration of the surface irregularities. ↑ Viscosity ➔ ↓ Wetting. N.B: To produce adhesion, surface energy of adherend should be higher than surface tension of adhesive. 40 Chapter IV Principles of Adhesion II. Stresses due to setting contraction of adhesive. Most liquid adhesives undergo contraction when they hardened, so there are stresses at the interface. So, less contraction of adhesive ➔ stronger adhesive junction. III. Thermal stresses: The adhesive and adhered differ in coefficient of the thermal expansion and contraction (α). The change of temperature creates stresses at the interface. The great the difference in coefficient of the thermal expansion and contraction, the great the thermal stresses. IV. The Thickness of Adhesive Junction: The thinner the adhesive film, the stronger the adhesive junction due to: 1. More intimate contact between adherend surfaces. 2. Less stresses due to contraction. 3. Less thermal stresses. 4. Less air bubbles in the adhesive layer ➔ stronger. V. Type of bond formed: Adhesion by primary bond is stronger than adhesion by secondary bond. VI. Cleanliness of the adherend surface: Any surface debris or moisture prevents intimate contact between adherend and adhesive ➔ weaker bond. 41 Chapter IV Principles of Adhesion Summary of factors essential for proper adhesion Adherend Adhesive Adherend and adhesive 1. High surface energy. 1. Low surface tension. 1. Close matching in α. 2. Proper surface 2. Low viscosity. 2. Formation of primary irregularities. bond. 3. Clean surface. 3. Low setting contraction. 4. Thin film thickness. Types of adhesion Failure: Under tension, failure in adhesion junction may be: 1. Failure of adhesive bond. ➔ Adhesive failure 2. Failure of the adhesive. ➔ Cohesive failure 3. Failure of adherend. ➔ Cohesive failure Importance of adhesion in dentistry: 1. Decrease marginal leakage between restoration and cavity walls. 2. Prevention of tooth decay by sealing pits and fissures. 3. Complete denture retention through thin film of saliva. 4. Bonding agents. 5. Soldering operation. 6. Ceramo-metallic restoration. 42 Chapter IV Principles of Adhesion Obstacles for adhesion to tooth structure: 1. The heterogeneous composition of enamel and dentine. Enamel and dentine are formed from organic and inorganic components. Enamel differs from dentine in composition. 2. Surface irregularities in the prepared cavity. The irregularities of the prepared cavity are not uniform in dimensions. 3. Debris in the prepared cavity. Smear layer is formed from tooth and bacterial debris and covers dentine surface. 4. Presence of water in the prepared cavity. The dentinal tubules leach out water. Components Enamel Dentine Inorganic phase (mainly hydroxyapatite) (%) 94–96 50–70 Calcium phosphate ratio 1.64 1.56 Organic phase (mainly collagen) (%) 4–5 20–30 Water (%) 1–4 10–20 Bonding to tooth structure: Two techniques used mainly: 1. Chemical bond: Zinc polycarboxylate and glass ionomer cements bond to minerals of tooth structure by ionic bond due to presence of carboxylic group (COOH). Adhesive resin cement and recently introduced bonding agents bond chemically to tooth structure due to presence of 4-META and 10-MDP2. 2 4-META: 4-methacryloyloxyethyl trimellitate anhydride. 10-MDP, 10-methacryloyloxydecyl dihydrogenphosphate. 43 Chapter IV Principles of Adhesion 2. Mechanical interlocking: Used with composite resin filling material. It is called acid etching technique. a. Bonding to enamel: i. Acid etching by applying 37% phosphoric acid for 15 to 30 seconds. Importance of acid etching: 1. Increase surface energy of the enamel. 2. Cleaning enamel surface by removing of surface debris. 3. Increased surface area of the exposed enamel to the resin. 4. Forming micro-pores that penetrated by adhesive to form resin tags. ii. Adhesive (bonding agent) is applied to fulfill the micro-pores. b. Bonding to dentine: i. Acid conditioning (mild acid for less time than enamel). 1. Remove smear layer. 2. Demineralize dentine ➔ Collapse dentinal tubules ➔ decrease surface energy. ii. Primer: It is a molecule with two functional groups: 1. Hydrophilic group: interact with moist dentine ➔ reopen collapsed dentinal tubules ➔ increase surface energy. 2. Hydrophobic group: interact with adhesive. iii. Adhesive (bonding agent): bonds primer and composite resin. N.B: Newer generations of adhesive systems had been evolved with different techniques. 44 Chapter IV Principles of Adhesion Effect of phosphoric acid application; a) over enamel surface, b) over dentine surface. 45  polymers Definitions: Polymer: It is a high molecular weight molecule that is composed of many repeating units. (poly = many, mer = unit). Monomer: It is the smallest repeated building unit of the polymer chain. (mono = single). Polymerization reaction: It is the reaction by which the monomers form the polymeric chain. Classification of polymers: 1. According to origin: 1. Natural Polymers: Synthesized by living cells. eg: proteins, starch and polysaccharides (agar and alginate). 2. Synthetic polymers: Synthesized by chemical reactions inside the laboratory. e.g: Acrylic resin. 2. According to arrangement in space (spatial configuration): 1. Linear Polymers: The polymer molecule forms a thread without any branches. 2. Branched polymers: In the polymer molecule, a shorter side chains are attached to the main backbone chain. 3. Cross-linked polymers: The main polymer chains are bonded by covalent bond Chapter V Polymers 3. According to Thermal behavior: Thermoplastic polymers Thermoset polymers Linear Polymers Cross-linked Polymers Chains bonded by secondary bond and Chains bonded by Primary bond entanglement of chains Reversible reaction Irreversible reaction Heat ➔ softens the polymer then Heat ➔ decomposes (burn) the polymer harden by cooling Molded by heat and pressure ➔ Molded by heat and pressure during temporary destroy secondary bond initial stages ➔ forming primary bond Final product has the same chemical Final Product is chemically different than composition original substances Show Glass Transition Temperature (Tg) Not show Glass Transition Temperature 4. According to mechanical properties: 1. Elastomers (elastic polymers): They have wide range of elastic deformation and low modulus of elasticity. e.g: rubber base impression materials. 2. Plastic Polymers: They have moderate range of elastic deformation and moderate modulus of elasticity. e.g: acrylic resin. 47 Chapter V Polymers 3. Brittle Polymers: They have small range of elastic deformation and high modulus of elasticity. e.g: cross-linked acrylic resin. 5. According to chemical composition of monomers: 1. Homopolymer: It is a polymer consists of single type of monomers. 2. Copolymer: It is a polymer consists of two or more different types of monomers. a. Alternating copolymers: Different monomers are arranged regularly in an alternative pattern. b. Random copolymers: Different monomers are arranged randomly. c. Block copolymers: Each monomer forms a cluster. The clusters are arranged alternatively. d. Graft copolymers: One polymer forms a backbone. The other monomer is attached as side chains to the backbone. 48 Chapter V Polymers 6. According to Polymerization reaction: 1. Condensation Polymerization reaction: The monomers react together to form the polymer chains with elimination of a small molecule called by-product. eg: water, gas, …. etc. 2. Addition Polymerization reaction: The monomers react together to form the polymer chains without elimination of a by-product. The Addition polymerization reaction gives more accurate polymers than the condensation type as there is no elimination of by-products. Steps of Addition polymerization reaction: 1. Activation: It is the process of production of free radicals. Free radicals are the active form of the initiators. Free radical is highly reactive compound due to presence of unpaired electron. Initiator can be activated by: a. Heat. b. Light. c. Chemical compound. activation (by heat, light, or chemical compound) R—R 2R Initiator Free Radical 2. Initiation: It is the reaction of free radical with monomer. The free radical breaks the double bond of the monomer (C=C) and react with one carbon atom. 49 Chapter V Polymers The other carbon atom contains the unpaired electron making it very reactive. i.e: The reactivity of the free radical is transferred to the monomer (activated monomer) 3. Propagation: It is the process of addition of the monomers to the growing chains. Each activated monomer attacks C=C of unreacted monomers transferring the active part to it. Theoretically, the propagation step should be continued until all unreacted monomers are added. Otherwise, the propagation step is stopped by termination 4. Termination: The termination process stops the growth of growing chains either by: a. Direct coupling: Two growing chains reacts together to give one long stable chain. 50 Chapter V Polymers b. Hydrogen atom transfer: The Hydrogen atom transfers from active chain to another active chain to give two stable chains. N.B: Chain transfer is a transfer reactivity from active chain to inactive chain. 51 Chapter V Polymers Ring opening polymerization (ionic and cationic reactions): The terminal reactive group in the monomers are rings that open during initiation and propagation stages. Ring opening reactions showed less polymerization shrinkage and less heat evolution. Examples: polyether impression material, epoxy resin and siloraine composites (historical) Inhibition and retardation of polymerization reaction: The polymerization reaction could be lowered (retard) of stopped (inhibit) if any substance reacted with the active centers either of free radical or growing chains. Inhibitors such as hydroquinone are added to the monomers to inhibit the premature polymerization reaction of the monomers during storage or accidental exposure to heat or light. Eugenol inhibits the polymerization reaction of resin composite. So, bases containing eugenol should not be used as a base under composite resin restorations. Oxygen inhibits the polymerization reaction of resin composite. So, the surfaces of the restorations (in contact with oxygen during polymerization) should be polished to remove the unreacted surface layer or covered by a matrix band. 52 Chapter V Polymers Factors Associated with polymerization reaction: 1. Evolution of heat due to the breaking of the covalent bonds of the monomers. 2. Reduction in the volume (Polymerization shrinkage). 3. Presence of residual monomers. Because not all the monomers participate in the reaction. General Properties of the Polymers: Polymers are molecular solids (refer to chapter I). Polymers are amorphous solids (refer to chapter I). Factors affecting the properties of the polymers: 1. Molecular weight (M.W) and Degree of polymerization (DP):  M.W of polymer molecule = Weight of mers X Number of mers.  The higher the M.W, the higher the degree of polymerization  D.P = M.W of a Polymer M.W of a mer Effect on Properties: The longer the polymer chain, the more the entanglement between chains occurs ➔ more secondary bonds, therefore: a. Increase strength, rigidity and glass transition temperature. b. Decrease Water sorption, crazing and Ductility. N.B: Degree of conversion (Degree of polymerization): is the total number of monomers in the polymer chain. 53 Chapter V Polymers 2. Copolymerization:  Copolymer is a polymer consists of two or more different types of monomers. Effect on Properties: Copolymerization allows the chemists to synthetize a polymer with desired properties (Tailor-make effect). 3. Cross Linking:  It is the bonding of the polymer chains with primary covalent bond. This leads to forming a network structure.  The cross-linking agent is a chemical compound with two double bonds per molecule can bond covalently different chains.  Cross linking of rubbers is called vulcanization. Effect on Properties: Small degree of cross linking: It limits the movement of polymer chains past each other, therefore: a. Increase strength, rigidity and glass transition temperature. b. Decrease Water sorption, crazing and Ductility. Extensive cross linking: Leads to brittleness of the polymer. 54 Chapter V Polymers 4. Plasticizer:  It is the partially neutralization of the secondary Van der Waal forces between the polymer chains.  Plasticizer could be: 1. Internal plasticizer: Incorporated by copolymerization. Therefore, it is a part of the polymer. 2. External plasticizer: It is a compound that is added to the polymer and penetrates between the polymer chains to partially neutralize the secondary forces between the chains. Effect on Properties: It partially neutralizes the secondary forces between polymer, therefore: a. Decrease strength, rigidity and glass transition temperature. b. Increase Water sorption, crazing and Ductility. 5. Addition of Inorganic Fillers:  Addition of inorganic fillers to polymer to form composite structure will increase strength, rigidity and glass transition temperature. 6. Rate of Loading:  Polymers are viscoelastic materials. Therefore, they are sensitive to rate of loading: At slow rate of loading ➔ behave in a ductile manner. At high rate of loading ➔ behave in a brittle manner. 55 Chapter V Polymers 7. Degree of Crystallinity:  Polymers are generally amorphous solids especially if they are formed of large chains.  In certain polymers, the chains can be aligned to form different degrees of ordered crystalline structure.  Degree of crystallinity depends on: a) Chain configuration: (it should be simple) 1. Side branching and network ➔ decrease crystallinity 2. Copolymerization ➔ decrease crystallinity. b) Monomer chemistry: (it should be simple) 1. High molecular weight ➔ decrease crystallinity 2. Additives (plasticizers) ➔ decrease crystallinity. Effect on Properties: Increasing the degree of crystallinity: a. Increase strength, rigidity and glass transition temperature. b. Decrease Water sorption, crazing and Ductility. 8. Spatial Structure:  Cross linked polymers showed: a. Increase strength, rigidity and glass transition temperature. b. Decrease Water sorption, crazing and Ductility. More than the linear and branched polymers. 9. Temperature:  Polymers softens when they heated near their glass transition temperature. 56 Chapter V Polymers Elastomers:  They are polymers that display wide range of elastic deformation.  They are formed of highly coiled and twisted polymeric chains that are uncoiled and straightened with tensile load.  After removal of load, the chains spring back to their previous shape.  Elastomers are characterized by: a. Low modulus of elasticity. b. Their glass transition temperature is below room temperature. c. Have few cross linking. Uses of polymers in dentistry: 1. Denture base material. 5. Temporary crown materials. 2. Artificial teeth. 6. Endodontic filling and sealers. 3. Teeth restorative materials. 7. Maxillo-facial prosthesis. 4. Dental cements. 8. Impression materials. 57  Metallurgy It is the study of metals and alloy A.Metals Metal can be defined as any elements that ionized positively in solution. From the periodic table, it could be noticed that metals occupy the left side while the right side is occupied by non-metals. Moreover, metalloids (semiconductors) are arranged in between, e.g: carbon and silicon. Properties of metals: Metals gain their unique properties from their metallic bond and crystalline structure. 1. Ionized positively in solutions. 2. At normal conditions, they are crystalline solids. Except mercury and gallium ➔ liquids. Hydrogen ➔ gas. 3. Opaque ➔ because the free electrons absorb light. 4. Lusters ➔ because the free electrons reflect the light. 5. Good thermal and electrical conductors ➔ because free electrons carry heat and electricity. 6. High mechanical properties ➔ because the metallic bond is a primary bond. 7. High melting and boiling point ➔ because the metallic bond is a primary bond. 8. Malleable and ductile ➔ due to presence of crystalline imperfection (discussed latter). 9. Give metallic ring when striking. 10. Most metals are white in color with different tint with exception of gold which is yellow and copper which is red. Chapter VI Metallurgy Pure Metals in Dentistry: 1. Titanium in dental implants and framework of fixed partial dentures. 2. Mercury in dental amalgam restoration. 3. Silver and copper in electroplated dies. 4. Platinum foil in construction of porcelain crowns. 5. Gold foils used as direct restoration (Historical). Shaping of Metals: 1. Casting:  It is the process of pouring a liquid metal or alloy into a mold with desired shape.  Heat is applied above the melting point of metal or alloy. 2. Cold working:  It is the process of metal shaping by applying stresses above the yield strength of the material.  Plastic deformation occurs through slip of metal atoms through line crystalline defects (dislocations).  Application of heat below melting temperature facilitates cold working. 3. Powder metallurgy (Sintering):  It is the process of densification and agglomeration of metal powder.  It requires application of high temperature (below melting temperature) to allow atomic diffusion.  Sintering can be facilitated by application of pressure.  Sintering is accompanied by shrinkage and elimination of porosities. 59 Chapter VI Metallurgy 4. Electroplating:  It the process of electrolysis (corrosion in reverse) as in silver and copper electroplated dies. Solidification of Metals:  In order to understand metal solidification mechanism, a metal is melted then allowed to cool. Temperature during cooling is plotted as a function of time.  From the figure The temperature decreased from A to B ➔ the metal is purely liquid. The temperature remains constant from B to C (horizontal plateau) ➔ the metal is liquid and solid. The Temperature decreased from C to D (room temperature) ➔ the metal is purely solid.  The melting and freezing temperature (Tf) is indicated by the horizontal plateau.  The temperature remains constant at Tf due to the liberation of latent heat of fusion. N.B: Heat of fusion: the amount of heat required to convert 1 gm of the material from solid to liquid state at the melting temperature. N.B: Latent heat of fusion: the amount of heat liberated from convert 1 gm of the material from liquid to solid state at the melting temperature. N.B: Initial cooling to B’ is called supercooling. After the crystallization begins, the temperature is raised to Tf and remains until complete crystallization. 60 Chapter VI Metallurgy Structure during Solidification: Two-steps mechanism theory can explain the solidification of metal in two steps; nucleus formation and crystallization. 1. Nucleus formation:  When the molten metal cooled and reaches its freezing point, some atoms aggregate in the space lattice arrangement to form what is called “embryo”.  Once the supercooling reached, the formed embryos grow to form centers for solidifications called “nuclei of crystallization”  Nucleus originates by: a. Homogeneous nucleation: Nucleus arise from atoms of the metal itself. b. Heterogeneous nucleation: Nucleus arises from atoms of foreign metal such as: iridium. The foreign metal should have higher melting temperature than the original metal. Therefore, it would be solid before the solidification of the original metal starts. 2. Crystal Growth (Crystallization):  The molten atoms aggregate around the nuclei in three dimensions (tree like structure or dendrites) to form crystals.  The crystal growth continues until contact between adjacent crystals occurs.  Atoms between two adjacent grains are called grain boundaries. N.B: In very rare occasions, the molten metal solidifies in a single crystal (whisker). 61 Chapter VI Metallurgy Grain Structure of Solidified Metal:  The solidified metal is formed of crystals (Grains).  Each grain arises from a single nucleus.  Atoms inside each grain have the same orientation.  Atoms of different grains have different orientations because the nuclei of crystallization had different orientation.  Atoms at grain boundaries take up an intermediate position (relative to the adjacent grains) to fulfill the mismatch between the adjacent grains.  Due to their intermediate position, atoms at grain boundaries are strained and have higher energy than atoms inside the grains.  Grain boundaries affect the metal properties by: a. Formation of new nuclei in solid phase starts at grain boundaries. b. Atomic diffusion occurs more readily at grain boundaries. c. Impurities tend to accumulate at grain boundaries. d. Affects the mechanical properties. e. Affects the corrosion resistance. Factors Controlling Grain Size: 1. Rate of Cooling from liquid state:  Rapid rate of cooling produces more nuclei of crystallization ➔ more grains ➔ smaller grain size ➔ more grain boundaries. 2. Nucleating agents (Grain refiners):  Nucleating agent is a foreign metal with higher melting point than original metal (heterogeneous nucleation). 62 Chapter VI Metallurgy  Addition of nucleating agent produces more nuclei of crystallization ➔ more grains ➔ smaller grain size ➔ more grain boundaries. N.B: It is called grain refiner as they produce metal with smaller (finer) grain size. Relation Between Microstructure and Mechanical Properties: 1. Elastic deformation:  It is a temporary deformation, results from stretching of the interatomic bond.  It depends on chemical composition and not affected by microstructure or heat treatment. 2. Plastic deformation:  It is a permanent deformation, results from slip of atoms over each other.  Atomic slippage occurs at dislocations (line defect) and the movement (slippage) of atoms is called dislocation movement.  Casted metals contain numerous dislocations ➔ atoms slip (movement of dislocation) occurs easily. i.e: Permanent deformation requires less stress i.e: Metal characterized by low strength, low hardness and high ductility.  If dislocation movement is obstructed ➔ atoms slip become difficult. i.e Permanent deformation require more stress. i.e: Metal characterized by high strength, high hardness and low ductility.  Obstructing dislocation movement occurs by: 1. Grain boundaries: Therefore, metals with small grains have higher strength and hardness. 2. Other dislocations. 3. Other types of space lattice discontinuity (as impurities). 63 Chapter VI Metallurgy Wrought Metals:  They are the metals that subjected to cold working (Stresses above yield strength).  Cold worked structure characterized by: 1. Fibrous structure. 2. High strength, hardness and proportional limit. 3. Low ductility. 4. Low tarnish and corrosion resistance ➔ cannot be used inside patient mouth. Heat Treatment Annealing:  It is a heat treatment process that can reverse the effect of cold working.  It is done by heating the cold worked structure at temperature below its melting temperature.  It involves three stages according to time of heating: Cold working Recovery Recrystallization Grain growth Structure Fibrous Fibrous Fine cast grains Coarse grains structure structure Tensile strength Increased Very little Decreased (like Slight and hardness decrease cast metal) decrease Ductility Decreased No changes Increased (like Slight increase cast metal) Corrosion Low High High High resistance Internal High Relieved Strain free Strain free stresses 64 Chapter VI Metallurgy N.B: Any cold worked structure should be subjected to recovery to: a. Avoid its corrosion inside oral cavity. b. Avoid its fracture or warpage during service (due to high internal stresses). N.B: Recrystallization may be done if low strength and high ductility are required. 65 Chapter VI Metallurgy B. Alloys Alloy is the mixture to two or more metal. Alloys are more commonly used in dentistry because of their higher mechanical properties. Definitions: Phase: Any physically distinct, homogenous and mechanically separable portion of a system. Solution: A system in which the molecules of a solute diffuse and intermingle randomly within the atoms of solvent. Solvent: In the alloy system, the metal with persistent space lattice. If the two metals have the same space lattice, the solvent will be the metal occupy > 50% of the space lattice. Solute: The other metal in the alloy system. Classification of Alloys: 1. According to use: Alloy for inlays, crown and bridges, removable partial dentures and implant. 2. According to number of alloying elements: Binary alloys contain two metals, ternary alloys contain three metals etc… 3. According to major element: Gold alloy, silver alloy and etc… 4. According to nobility: High noble alloy: contains ≥40% gold and ≥60% other noble elements Nobile alloy: ≥25% noble metals Predominantly base metal alloys: