PRS101 Fall 2024 Bio Materials PDF

Bio Materials PRS 101 First Year (2024-2025)  Faculty of Oral and Dental Medicine ‫رؤية ورسالة الكلية‬...

Bio Materials PRS 101 First Year (2024-2025)  Faculty of Oral and Dental Medicine ‫رؤية ورسالة الكلية‬ ‫الرؤية‬ ‫تتطلع الكلية أن تكون في مصاف المؤسسات التعليمية المعترف بها إقليميا وعالميا من خالل برامج تعليمية متطورة وأبحاث‬.‫تطبيقية مبتكرة وتنمية مجتمعية مستدامة‬ Vision The Faculty of Oral and Dental Medicine aspires to be a recognized educational institution, regionally and internationally, by providing advanced educational programs, innovative applied research, and sustainable community development. ‫الرسالة‬ ‫ ذو كفاءة معرفية وتطبيقية من خالل برامج تعليمية متطورة تتوافق مع االحتياجات الفعلية لسوق العمل المحلي‬،‫إعداد طبيب أسنان ملتزم بالقيم االنسانية واألخالق المهنية‬ ‫ كما تلتزم الكلية بإعداد بحوث تطبيقية متوافقة مع االستراتيجيات القومية‬.‫والعالمي‬.‫وكذلك تقديم خدمة مجتمعية مستدامة وفقا لمعايير الجودة العالمية‬ Mission The mission of the Faculty of Oral and Dental Medicine is to prepare knowledgeable and well- trained dentists committed to human values and professional ethics, by developing advanced educational programs that correspond to the actual needs of the local and global labor market. The Faculty is also committed to preparing applied research in line with national strategies, as well as providing sustainable community service following international quality standards.  Basic Biomaterials Fall 2024 Topic ILOs Structure of Matter Explain the atomic relations and space lattices of different types of dental materials. Physical Properties Recognize the physical properties of dental materials Mechanical Properties Discuss the mechanical behavior of materials used in the dental laboratory or clinic Adhesion Outline the basic principles of adhesion to oral soft and hard structures Metallurgy List the properties of metals and alloys estimate their uses in the dental field. Tarnish & Corrosion Outline the basic types and causes of tarnish and corrosion of metallic restorations in the oral cavity. Polymers Describe polymeric materials and report their specific properties Dental Casting Alloys Recite types and properties of noble and base metal alloys used as metallic denture bases. Introduction to Nanotechnology Define Nanotechnology, Classify the materials at nanoscale and list the fabrication techniques of nanostructures.  Appendix Chapter Topic Page ** Introduction a-f Chapter 1 Structure of Matter 1 - 12 Chapter 2 Physical Properties 13 - 31 Chapter 3 Mechanical Properties 32 - 52 Chapter 4 Polymers 53 - 65 Chapter 5 Surface Phenomena & Adhesion 66 - 75 Chapter 6 Metallurgy 76 - 93 Chapter 7 Tarnish & Corrosion 94 – 101 Chapter 8 Dental casting alloys 102-103 Chapter 9 Introduction to Nanotechnology 104-106 Chapter 10 Introduction to Tissue engineering  Intended Learning Outcomes of The Course (ILOs) a- Knowledge and Understanding: a1- Explain the space lattices of different types of dental materials in relation to their atomic relations and energy responses. a2 - Recognize the physical thermal and optical properties of dental materials a3- Discuss the mechanical behavior of materials used in the dental laboratory or clinic a4- Describe polymeric materials and report their specific properties a5- Outline the basic principles of adhesion to oral tissues and artificial structures a6- List the properties of metals and alloys; and estimate their uses in the dental field a7- Recite types and causes of tarnish and corrosion of noble and base metals. b- Intellectual Skills: b1- Sort materials and compare their properties b2- Categorize the materials as regards their physical, mechanical and biological properties to suit each clinical case c- Professional and Practical Skills: c1- Master selection of dental materials for different application techniques. c2- Realize the possible resulting clinical problems and correlate them with materials’ errors d- General and Transferable Skills: d1- Identify the value and role of lifelong learning, self-assessment and critical thinking in maintaining competency. d2- Communicate effectively in a scientific environment using material science terms.  d3- Search in texts and websites relevant to the course content, to collect data that help to assure their lifelong learning. d4- Appreciate team work in the lab. d5- Show respect to assisting personnel.  Introduction What is Dentistry? It is the art and science dealing with hard and soft oral and dental tissue (structures) in health and disease by diagnosis, treatment and prevention. *Hard: Teeth and Bone *Soft: Lips, Cheeks, Tongue, Gingiva, Oral mucosa Aim : 1) To relief Pain 2) Diagnosis and treatment of oral diseases 3) To restore function → Maintain masticatory efficiency → Speech 4) To restore esthetics → Simulate the natural appearance of the patient → Beautify the existing condition Departments Academic Clinical ↓ ↓ Departments not dealing Departments dealing directly with patients with patients ↓ ↓ 1) Biomaterials (Dental Materials) 1) Diagnosis, Oral Medicine and Periodontology 2) Oral Biology 2) X-ray a) Human Dentition b) Oral Histology 3) Operative 3) Oral Pathology 4) Endodontics 5) Oral Surgery 6) Crown and Bridge 7) Prosthesis 8) Pedodontics 9) Orthodontics Academic Departments: 1) Biomaterials (Dental Materials): The materials placed in the oral cavity come in contact and interact with living tissues Therefore, they must be biocompatible i.e.: They should not be toxic, irritant or harmful and they should not undergo tarnish and corrosion. What is Dental Materials? It is an academic science through which we will understand composition, manipulation, properties, behavior and applications of all the materials used in clinics and in the lab a  2) Oral Biology: a) Human Dentition ( Dental Anatomy)  Tooth→ Crown & Root  No. of teeth→ Adults(Permanent): 32 teeth, 16 upper and 16 lower, 8 in each quadrant → Children (Deciduous): 20 teeth, 10 upper and 10 lower  Arrangement: Anterior teeth→ Incisors →Central Incisor → Lateral Incisor → Canines →Tearing of teeth Posterior teeth → Premolars →2 (1st premolar & 2nd premolar) → Molars →3 (1st molar, 2nd molar, 3rd molar" wisdom tooth") b) Oral Histology: It deals with the study of soft and hard Normal tissue on Microscopic level Enamel Dentin Dental Pulp: The tooth is embedded in the bone (alveolar bone) attached to it by Periodontal ligament 3) Oral Pathology: It deals with the study of soft and hard oral and dental tissue but in diseased conditions Clinical Departments 1) Diagnosis, Oral Medicine and Periodontology a. Diagnosis → to diagnose the patient & detect the problem. b. Oral Medicine→ to detect & treat diseases affecting oral cavity (tumor, cyst, etc.). c. Periodontology→ treats gingival diseases. 2) Oral Radiology o It is the dentist's 3rd eye o It helps to examine embedded structures e.g. Tooth Root number and form, the dental pulp, number of root canals, periapical area, the relation of the tooth to the adjacent anatomical structure b  3) Operative Dentistry: Deals with treatment of dental caries Dental Caries: It is an irreversible bacterial disease of hard tissue (Enamel, Dentin and Cementum) leading to decalcification/ demineralization Food Debris+ Bacteria → acids which will act on the tooth structure→ Caries Once caries occurs, it will proceed further.The tooth won't be able to regenerate. Caries has to be removed by preparing a Cavity (Cavity→ prepared by the dentist to remove caries) This cavity is then filled with a Filling Material/ Restorative Material Filling materials maybe →Metallic e.g. Amalgam →Non-metallic e.g. Composite Selection of the suitable restorative material depends on the amount of forces falling on the restoration, the esthetic demand of the patient,………………. Eg: Restored with Amalgam Restored with Composite 4) Endodontics: If the the pulp is involved by a) traumatic exposure Or b) caries extension into the pulp Therefore, endodontics deals with treatment of pulp and periapical diseases The pulp is removed using a serrated needle like instrument of different sizes called Endo File and Reamers After the canals are cleaned, the canals have to be filled with endodontic filling material The filling material is either →Metallic: Silver Points →Non-metallic: Polymer called Gutta Percha together with Sealer *Gutta Percha is present in different sizes to conform to the size of the canal *Sealer seals the microscopic voids found in between the circular Gutta Percha c  5) Oral Surgery: Department dealing with Tooth extraction, tumors, fractures,……………… 6) Prosthodontics: Prosthodontics may be Fixed (Crown and Bridge) or Removable (Prosthesis) According to the number of teeth present in the patient's mouth, patients may be divided into  Dentulous → all teeth are present. (Operative, crown, periodontology…)  Edentulous → all teeth are missing (Complete denture, oral surgery)  Partially edentulous → some teeth are present & some are missing.( Bridge, partial denture) Prosthesis: artificial appliance restoring missing part of the body ex. Limb, nose, ear, etc. A) Crown & Bridge (fixed) Crown: Fixed artificial restoration of the coronal portion of the natural tooth restoring function & esthetics. Indications: 1. Endodontically treated tooth. 2. Badly broken down tooth. Bridge: Fixed artificial restoration replacing one or more of the missing teeth. d Indications: To replace one or more missing teeth. B) Removable Prosthodontics :( Prosthesis) 1. Complete Denture: artificial restoration restoring the loss of all natural teeth. Components: 1) Artificial teeth →Porcelain →Acrylic Resin 2) Denture Base → Metallic (Co-Cr alloy) →Non Metallic (Acrylic Resin) 2. Partial Denture (P.D.): artificial restoration replacing the missing natural teeth. Components: 1) Denture base: that part of P.D. that supports artificial teeth & help in transferring occlusal stresses to supporting oral structures. 2) Clasp: is that part of P.D. that engage undercut area of natural tooth to provide retention of partial denture. 3) Artificial teeth. e N.B: The use of dental implants has developed recently as a treatment modality for partially edentulous patients Implants are made from Titanium (Ti) or Titanium alloys as they are biocompatible materials 7) Pedodontics: The branch of dentistry that deals with the care and treatment of children's teeth Child; children: pedo 8) Orthodontics: It is the specialty of dentistry that is concerned with the study and treatment of malocclusions, which may be a result of tooth irregularity, disproportionate jaw relationships, or both. This can be achieved either by removable appliances ( made of Acrylic resin) or by fixed appliances (brackets, bands, wires) f CHAPTER 1 Structure of Matter -1- CHAPTER 1 Structure of Matter Learning objectives By the end of this chapter, the student should be able to: Explain the atomic relations and space lattices of different types of dental materials. Introduction: 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 I.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. 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 -2- 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. I.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 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-). -3- 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 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. II. 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. -4- A- Fluctuating Dipole: Instantaneous location of more electrons on one side of the molecule than the other results in asymmetry in their electron distribution.. 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. -5- The Structure of Solids: The properties of materials depend on the arrangement of their atoms. Such arrangements may be classified as: 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 (,,). -6- 1) The Cubic System: - The length of the axes a,b,c are equal. - The interfacial angles =  =  = 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 = 2atoms. 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. 2. Hexagonal structure -7- The axis a = b but # c. The angle  =  = 90° but the angle  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. 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 -8- 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 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 a with repetition. short range of regularity but no 3) Low internal energy. repetition. 3) Higher internal energy. -9- 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 substitutional b. Line defects Dislocation: It is a displacement of a row of atoms from their normal position in the lattice - 10 -  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. Isomerism Allotropy Polymorphism Is the existence of an element in Is the existence of a compound in more than two different crystalline more than two different crystalline forms. forms. e.g. Iron below and above 910⁰C e.g. Silica in dental investment Silica (SiO2) It is an important example for Polymorphism in dentistry. It exists in nature in four different Polymorphic 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: - 11 -  Reconstructive Transformation Displacive Transformation -Break down of atomic bonding -No break down of atomic bonding followed by reconstruction of new only displacement of atoms giving space lattice. the same space lattice but with larger volume. - Irreversible transformation - Reversible transformation -Slow transformation. -Rapid transformation. -Needs high thermal energy -Needs less thermal energy. 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). Activity Answer the following question: Discuss the correlation between atomic structure and materials properties. - 12 - CHAPTER 2 Physical Properties - 13 - CHAPTER 2 Physical Properties Learning objectives By the end of this chapter, the student should be able to: Recognize the physical properties of dental materials. 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) Less specific properties: 1) Water sorption. 2) Shelf life. V) Optical properties Optics is the science that deals with light, vision and sight, which includes color. - 14 - 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). - 15 - 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. N.B.: Composite and ceramic restorations are non-conductive and do not need insulators. - 16 - 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 (): This change in length, per unit length for a 1°C change in temperature is called the linear coefficient of expansion, . 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. ** 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: - 17 -  i. Marginal leakage ii. Discoloration iii. Recurrent caries iv. Hypersensitivity. 2. Porcelain and metal in ceramometallic restorations (crowns & bridges) to provide metal ceramic bonding. 3. Artificial tooth and denture base to avoid crazing. 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. - 18 - 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. 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 - 19 -  thickness of an insulating base material are the important factors controlling the insulating efficiency. 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. - 20 - IV) Other Properties: 1) Water Sorption: It represents the amount of water adsorbed on the surface and absorbed into the body of the material. Importance: 1. Plastic denture base materials have the tendency for water sorption. High percentage of water sorption may lead to dimensional change and warpage. Very small amount of water sorption in the acrylic denture base may compensate for some of the polymerization shrinkage. 2. The tendency for hydrocolloid impression to imbibe water if immersed in it will lead to dimensional changes. 2) Shelf Life: It is a term applied to the general deterioration and change in quality of materials during shipment and storage. V) 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. - 21 - Properties of materials in relation to light transmission and absorption: i. Transparency: A property of the material, that allows the passage of light so that an object can be clearly seen through them. e.g. glass and acrylic resin. ii. Translucency: A property of the material, which allows the passage of some light and scatters or reflects the rest. In such manner; the object cannot be clearly seen through them. e.g. Tooth enamel, porcelain, composite and pigmented acrylic resin. iii. Opacity: The property of the material that prevents the passage of light. Opaque material absorbs all of the light. Objects cannot be seen through them. 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 - 22 - When a beam of light encounters or falls on a surface of a medium, the following may occur: 1. Reflection: Most objects we see are visible, because they reflect light to our eyes. When light falls on a smooth surface (perfectly smooth), the angle of incidence will be equal to the angle of reflection. i.e. 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 difference in refractive - 23 - 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. 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 - 24 - 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. 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. - 25 - 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. Cone-shaped cells in the retina are responsible for color vision. They are especially sensitive to red, green and blue. - 26 - 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 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 - 27 -  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 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. - 28 - Factors affecting color appearance and selection: The appearance of the tooth or an object is a sum total of all its optical properties. 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: 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)): - 29 -  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. Physical Properties PROBLEM 1: During casting, cobalt chromium alloys need special casting machine. Give reason. PROBLEM 2: In the clinical evaluation of a 2-years old acrylic resin filling material, recurrent caries and marginal discoloration are noted. What most likely caused this problem? - 30 - 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? - 31 - CHAPTER 3 Mechanical Properties - 32 - CHAPTER 3 Mechanical Properties Learning objectives By the end of this chapter, the student should be able to: Discuss the mechanical behavior of materials used in the dental laboratory or clinic. 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. - 33 -  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 It is in male > female It is in adult > children It is in natural dentition > restoration Stress (): 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  = 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. Units of stress are: Ib/in 2 or Kg/cm2 or MN/m2 (MPa) Types of Stress: Generally, there are two types of stresss: - 34 -  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 Molecules making up the body must resist sliding past one another. - 35 - 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 (): Strain is the change in length, or deformation per unit length, when a material is subjected to a force. Deformation (L) Strain  Length (L) Unit: it is dimensionless; therefore, 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. - 36 - 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 proportional The strain isn’t linearly proportional to the applied stress i.e. doubling to the applied stress. the stress will double the strain. -When the stress is removed the -When the stress is removed, the original size and shape is original size and shape isn’t recovered. recovered. 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.): - 37 -  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: 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. - 38 - 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. 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. - 39 -  - 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. 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. - 40 -  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. 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 Stress strain curve (triangle) (MPa) A B Clinical Significance: Strain Resilience has a particular importance in the evaluation orthodontic (m/m) wires, since the amount of work expected from a spring wire for moving a tooth is of interest. - 41 - 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] 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, - 42 - 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. - 43 - 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 () , but also the amount of deformation expected. Stress or the transverse strength is calculated from the equation: 3 PL 3 X load X length = or Stress = 2 2 bd 2 X width X thichness2 Clinical Significance: The transverse strength test and the accompanying deformation are very important in comparing: 1) Denture base materials & 2) Long span bridge s 3) 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. - 44 -  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: 2P Tensile stress   DT Where P = Load D = Diameter T = Thickness Dynamic Mechanical Tests: 1) 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. - 45 -  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. 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. 2) 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. - 46 -  Fatigue tests are performed by subjecting the specimen to alternating stresses below the P.L until fracture occurs. 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. - 47 -  -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. -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. - 48 - Therefore, hard material is considered both advantage and disadvantage. 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. 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 t0 t1 * 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. - 49 - 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 (t 1 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) Nonlinear increase of the strain with time On load removal → t0 t1 strain will nonlinear decrease to zero with time (gradual but complete recovery) Therefore, the strain is time dependent Viscoelastic behaviour: 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 - 50 - ii) the anelastic strain is gradually recovered. t0 t1 iii) viscous strain is not recovered which results in some permanent deformation (1% -3%). 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. 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. Activity In dentistry yield strength is more important than ultimate strength. - 51 - CHAPTER 4 Polymers - 53 - CHAPTER 4 Polymers Learning objectives By the end of this chapter, the student should be able to: Describe polymeric materials and report their specific properties. 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. Classification of polymers: 1. Classification according to the origin Natural Polymer Synthetic Polymer - 54 -  1- Proteins They are produced industrially 2- Polyisoprenes e.g. rubber or in the laboratory by gutta percha chemical reactions 3- Polysaccahrides e.g. agar Dental example: acrylic resin, alginate elastomeric impression 4- Polynucleic acids materials 5- 2. Classification according to arrangement in space: Linear Branched Cross –linked polymers The structural units are Side branch chains are Adjacent linear chains connected to one connected to the main are joined at various another in adjacent ones positions by covalent sequence 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. - 55 -  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 set maintain their shape by a chemical reaction [physical change, with evolution of heat reversible reaction] [irreversible reaction] Soft Hard -Cross-linked structure -Structure -Linear chains bonded where the chains are with Van der Waal forces bonded with primary covalent bond -Effect of heat -The mechanical - They decompose upon properties are sensitive to 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. Polymeriza tion 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. condensetion Monomer  Polymer  by - product polymeriza tion - 56 -  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: A) 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. - 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. Benzoyl peroxide initiator heat  free radical tertiary amine activator Benzoyl peroxide initiator  free radical Diketone photo initiator Tertiary amine  free radical (reducing agent) blue vis ible l ight 470 nm - 57 -  2) Propagation: R + M "Stable monomer"  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: This occurs when two free radicals react to form a stable molecule: R  Rn  M + R  R  Mn+1  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. B) Addition polymerization by ring opening mechanism: - 58 -  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. - 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 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. - 59 - 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. b. Degree of polymerization (D.P): The degree of polymerization (D.P.) is defined as the total number of mers in a polymer chain. M.W. of a polymer e.g. D.P. = 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. - 60 -  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. 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; - 61 -  - 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. 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-polymer Block co-polymer Graft co-polymer Effect on the properties; - 62 - 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: 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. - 63 -  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. 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. - 64 - Activity What are the factors affecting the properties of polymers? CHAPTER 5 Surface PHENOMENA and Adhesion - 66 - CHAPTER 5 Surface Phenomena and Adhesion Learning objectives By the end of this chapter, the student should be able to: Outline the basic principles of adhesion to oral soft and hard structures. 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. - 67 -  Types of adhesion: True adhesion Mechanical attachment Definition Bonding between dissimilar Bonding between materials through bonding dissimilar materials between their atoms and through mechanical molecules "chemical reaction". interlocking" i.e. no actual bond Examples Glass ionomer and zinc Amalgam, composite, zinc polycarboxylate with the tooth. phosphate cement with the tooth. Mechanism Both glass ionomer and A liquid flows into pores polycarboxylate contain CooH or irregularities in the group that react chemically with surface of a solid and set calcium of the tooth. (harden) forming a strong 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 - 68 - 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. 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. - 69 - 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. 4. The viscosity of the liquid adhesive: Increasing the viscosity of the adhesive decreases the wettability. Viscosity wetting - 70 - 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. 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). - 71 - 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. 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. - 72 - 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 adhes