Topic 2_Properties Of Dental Materials_AC 2 PDF

PROPERTIES OF DENTAL MATERIALS Arief Cahyanto, DDS., M.Eng., PhD. Associate Professor Department of Restorative Dentistry Ajman University UAE Material used to replace missing portions of teeth are exposed to environmental challenges: Attack by the oral environment Subjected to biting fo...

PROPERTIES OF DENTAL MATERIALS Arief Cahyanto, DDS., M.Eng., PhD. Associate Professor Department of Restorative Dentistry Ajman University UAE Material used to replace missing portions of teeth are exposed to environmental challenges: Attack by the oral environment Subjected to biting forces (180-580N). Cleaning and polishing prophylactic procedures. Consequently, it is of tremendous importance to understand the physical, electrical, and mechanical properties of materials used in dentistry. PROPERTIES OF DENTAL MATERIALS THERMAL CONDUCTIVITY DIMENSIONAL CHANGE Chemical and thermal ELECTRICAL PROPERTIES Galvanism Corrosion SOLUBILITY & SORPTION WETTABILITY ADHESION MECHANICAL PROPERTIES Stress and strain Elastic modulus Proportional limit and Yield Strength Ultimate strength Elongation and Compression Resilience and Toughness Hardness Thermal conductivity Thermal conductivity has been used to measure heat transfer and is related to the rate of heat flow. Materials have different rates of conducting heat, with metals having higher values than plastics and ceramics. When a portion of a tooth is replaced by a metal restoration such as amalgam or gold alloy, the tooth may be temporarily sensitive to temperature changes in the mouth. Materials with high thermal conductivity values are good conductors of heat and cold.  Human enamel and dentin are poor thermal conductors compared with gold alloys and dental amalgam. When a portion of a tooth is replaced by a metal restoration such as amalgam or gold alloy, the tooth may be temporarily sensitive to temperature changes in the mouth. The reason for using cements as thermal insulating bases in deep cavity preparations is that although dentin is a poor thermal conductor a thin layer of it does not provide enough thermal insulation for the pulp unless a cement base is used under the metal restoration. Thermal Conductivity Thermal conductivity has been used as a measure of the heat transferred and is defined as the number of calories per second flowing through an area of 1 cm 2 in which the temperature drop along the length of the specimen is 1°C/cm. Dimensional Change Maintaining dimensions during dental procedures such as preparing impressions and models is important in the accuracy of dental restorations. Dimensional change is the percent shrinkage or expansion of a material. (Expressed as linear dimensional change or volumetric dimensional change) TWO TYPES OF DIMENSIONAL CHANGE 1. Chemical Dimensional Change Dimensional changes may occur during setting due to a chemical reaction, such as with elastomeric impression materials, resin composite restorative materials, or from the cooling of wax patterns or gold restorations during fabrication. 2. Thermal Dimensional changes: Restorative dental materials are subjected to temperature changes in the mouth. These changes result in dimensional changes in the materials and to the neighbouring tooth structure. Because the Thermal expansion of the restorative material usually does not match that of the tooth structure, differential expansion occurs, resulting in leakage of oral fluids between the restoration and the tooth.  Thermal expansion is expressed as the coefficient of thermal expansion The linear thermal coefficient of expansion of a material is a measure of how much it expands per unit length if heated 1 degree higher.  The relationship between the coefficient of thermal expansion of human teeth and restorative materials is important. The values for amalgam and composites are about three to five times those of human teeth, while the values for gold alloys are approximately the same as for human teeth. The clinical effect of this difference is presented in the phenomenon of percolation. Percolation (micro-leakage) If a tooth is filled with a restoration-like composite, the seal between the composite and the tooth may be compromised over time by repeated temperature changes in the mouth, which causes different amounts of expansion and contraction of the composite and tooth. It causes a gap between the composite and tooth structure, which leads to the leakage of salivary components into the tooth toward the pulp. When the temperature returns to normal, the oral fluids are forced out of this space. Percolation (micro-leakage) This phenomenon is called percolation and is undesirable because of possible irritation to the pulp and recurrent decay. Dental amalgam is unusual in that percolation decreases with time after insertion as a result of the space being filled with corrosion products from the amalgam. Electrical properties Two electrical properties of interest are: Galvanism Corrosion Galvanism Is the generation of electrical currents from the presence of dissimilar metals in the mouth. When two different metals (e.g. aluminum and gold) are opposed in the oral cavity. Saliva functions as an electrolyte; when the restorations touch, an electrical current flows, causing pain to the patient with a persistent metallic taste. Corrosion Is the dissolution of metals in the mouth. Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual destruction of materials (usually a metal) by chemical or electrochemical reactions with their environment. It means electrochemical oxidation of metal in reaction with an oxidant such as oxygen, hydrogen or hydroxide. Many structural alloys corrode merely from exposure to moisture in air, but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area, more or less uniformly corroding the surface. Corrosion Is the dissolution of metals in the mouth. It can result from a galvanic action. It may result from a chemical attack of metals by components in food or saliva. Dental amalgam, for example, reacts with sulphides and chlorides in the mouth, as shown by polished amalgams becoming dull and discolored with time. This effect is sometimes called tarnish. “it is a surface reaction of metals in the mouth from components in saliva or foods”. Tarnish is a thin layer of corrosion that forms over copper, brass, aluminum, magnesium, neodymium and other similar metals as their outermost layer undergoes a chemical reaction. Solubility and Sorption The solubility of materials in the mouth and the sorption (adsorption plus absorption) of oral fluids by the material are important criteria in their selection. The solubility of a material results in its dissolution, followed by disintegration. An example of the importance of dissolution in dentistry is the loss of zinc phosphate cement retaining gold crown, which results from dissolution followed by chemical disintegration. Sorption includes absorption and adsorption. Absorption refers to the uptake of a liquid by a bulk solid (e.g. acrylic). Adsorption indicates the concentration of molecules at the surface of a liquid or solid (saliva on the tooth). Wettability Is a measure of the affinity of a liquid for a solid as indicated by the spreading of a drop. Hydrophilic, (wetted by liquid). Hydrophobic, (poor wetting). If the contact angle is>90 degrees that liquid cannot wet the solid surface.  Wetting of denture base plastics by saliva. Wetting of tooth enamel by pits and fissure sealants. Wetting of rubber impressions by gypsum material. Adhesion The force that binds two dissimilar materials together when they are brought into intimate contact. Adhesion depends on Wetting Solid B Example of application of adhesion in dentistry is the adhesion between denture and saliva, adhesion between saliva and soft tissues. Mechanical properties Knowledge of the magnitude of biting forces is essential in understanding the importance of the mechanical properties of dental materials. Maximum biting forces decrease from the molar to the incisor region, and the average biting forces on the first and second molars are about 580 Newtons (N), whereas the average forces on bicuspids, cuspids, and incisors are about 310, 220, and 180 N, respectively.  Patients exert lower biting forces on bridges and dentures than on their normal dentition. For example, when a first molar is replaced by a fixed bridge, the biting force on the restored side is approximately 220 N compared with 580 N when the patient has natural dentition. The average biting force on partial and complete dentures has been measured to be about 111 N; therefore, patients with dentures can apply only approximately 19% of the force of those with normal dentition. Stress Stress is the force per unit area, that is acting on a material. Stress is measured in Mega-pascals (MPa). The stress is shown to be inversely proportional to the area of application. Stress The stress is shown to be inversely proportional to the area of application. Several types of stress may result when a force is applied to a material. These are referred to as : Compressive stress - when the material is squeezed together, or compressed Tensile stress - when pulled apart Shear stress - one portion of the material is forced to slide by when another portion. Flexure stress (bending moment) Flexural loads are a combination of tensile and compression when bending. Flexural strength is measured by applying a load in the middle of a beam that is simply supported (not fixed) at each end. Biting Forces Whenever a stress is present, deformation or strain is induced. Strain is the change in length per unit length of a material produced by stress. Strain is easier to visualize than stress, since it can be observed directly. Stress-strain curve A convenient means of comparing the mechanical properties of materials is to apply various forces to a material and to determine the corresponding values of stress and strain. For each material, there is a stress-strain proportional relationship, establishing a stress-strain curve. A typical stress-strain graph is obtained from a simple compressive or tensile test.  Proportional limit and Yield strength The proportional limit: is the maximum stress that a material will withstand without deviation from the linear proportionality of stress to strain. Yield strength: the stress at which a material exhibits a specific limiting deviation from linear proportionality of stress to strain. “stress at which the material begins to function in a plastic manner.”  Proportional limit and yield strength indicate the stress at which the material no longer functions as an elastic solid. The strain recovers below these values if the stress is removed, and permanent deformation of the material occurs above these values. Materials are said to be elastic in their function below the proportional limit or yield strength(return to its original dimension when the force is removed), and to function in a plastic manner (permanent deformation) above these stresses. These two properties are particularly important because a restoration can be considered as a clinical failure when significant amount of permanent deformation takes place even though the material does not fracture. Ultimate Strength The maximum stress that a material can withstand before fracture. Stress-strain curve in tension for dental gold alloy. Ultimate Strength If a fracture occurs from tensile stress, the property is called the tensile strength; if in compression, the compression strength; and if in shear, the shear strength. The tensile and compressive strength of a material may be significantly different. Human enamel, amalgam, and composites, have large differences and are stronger in compression than in tension. Elastic modulus (Young’s modulus) Is a measure of elasticity of the material: how stiff the material is in the elastic range Elastic modulus= Stress/Strain The slope of the straight -line region (elastic range )of the stress- strain curve is the measure of the relative rigidity or stiffness of a material. Material 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 stress to produce the same amount of strain. Elastic modulus (Young’s modulus) A low modulus value indicates flexible material. Whereas it may be advantageous for an impression material to be flexible, it is essential for a restorative material to be rigid. Elongation and compression The amount of deformation that a material can withstand before rupture is reported as the percent elongation when the material is under tensile stress or the percent compression when it is under compressive stress. The percent's elongation and compression are measures of ductility and malleability.  Ductility: is the maximum plastic deformation a material can withstand when is stretched before it fractures. (under tensile) For example, a metal that can be readily drawn into a long, thin wire is considered to be ductile. Malleability: the ability of a material to sustain a considerable permanent deformation without rupture under compression. These two properties indicate the amount of plastic strain, or deformation, that can occur before the material fractures and, as such, they indicate the brittleness of the material. The gold alloy with 19% elongation can be deformed considerably before fracture (ductile alloy). Composite with 2-3% compression considered as Brittle material. Resilience and toughness Resilience: is the energy absorbed up to a proportional limit and relates to the resistance to deformation under impact. Modulus of resilience:- is the maximum amount of energy a material can absorb without undergoing permanent deformation. It is represented by the area under the elastic, straight-line portion of the stress-strain curve. Ex: Acrylic resin denture teeth are more resilient than porcelain teeth and consequently absorb most masticatory forces and transmit less to the underlying bone, preserving it.  Toughness: is the energy absorbed up to ultimate strength and relate to the resistance to fracture under impact. Toughness is indicated as the total area under the stress-strain curve from zero stress to fracture stress. Areas indicating resilience (A) and toughness (A+B) of a material Areas indicating resilience (A) and toughness (A+B) of a material Stress-strain curves for composite (material A) and unfilled acrylic (material B). The two materials have approximately the same resilience, but material B is considerably tougher. The elastic modulus of enamel is about three times greater than that of dentin. Enamel is stiffer and more brittle material than dentin.conversely, dentin is more flexible & tougher.  The elastic modulus of enamel is about three times greater than that of dentin. Enamel is stiffer and more brittle material than dentin. Conversely, dentin is more resilient and tougher. Stress-strain plot for enamel and dentin that have been subjected to compression. The ultimate compressive strength (CS), proportional limit (PL), and elastic modulus (E) values are shown. Hardness is the resistance of a material to indentation. Surface hardness is a parameter frequently used to evaluate material surface resistance to plastic deformation by penetration. Surface hardness test The usual method to achieve hardness value is to measure the depth or area of an indentation left by an indenter of a specific shape with a specific force applied for a specific time.  There are four common standard test methods for expressing the hardness of a material: Brinell, Rockwell, Vickers, and Knoop. Ex: human enamel-343KHN., dentin-68KHN., amalgam-110KHN., porcalin-460KHN., gold alloy-85KHN, unfilled acrylic resin-20KHN. “i.e. Enamel and ceramic are two of the hardest, and unfilled acrylic is the softest material.” Substance Hardness is based on its ability to resist scratching (resistance to indentation). Vickers Hardness Test: A hardened square-based pyramid is pressed under a specified load into the polished surface of a material  An evaluation of the hardness of the dental material is important. Denture base materials, for example, have low hardness and patients must take care not to aggressively brush their denture during cleaning to avoid abrasion of the denture. Also, hardness is important with model and die materials on which crown and bridge patterns are made because a soft surface may become scratched or chipped, compromising the accuracy of the final restoration. Dental materials minimum standards or specifications. The American National Standards Institute (ANSI) and the American Dental Association (ADA), in conjunction with the International Organization for Standardization (ISO) and federal organizations (FDI), have established more than 100 standards or specifications for dental materials. These are helpful for selecting materials for dental practice and ensuring their quality control.