Properties of Dental Materials – The staff working in a Dental Office must be fully acquainted with the physical, chemical and mechanical properties of the various materials to derive optimum benefit from their use. The dental materials are developed by manufacturers and selected by the Dentist for the use in Dental Office on the basis of certain characteristic properties.
These properties are for obvious reasons, considered as essential in producing restorations of desirable qualities. For the purpose of study, the properties of dental materials can be placed into four categories: strength and stress; ductility and malleability; thermal Properties of dental materials ; and adhesive properties.
Properties of dental materials – STRUCTURE OF MATTER
- All dental restorative materials, whether they be plastic or metal, are built from atoms. The study of dental materials requires a basic knowledge of matter, particularly solids, if the Properties of dental materials and reactions of the materials are to be understood.
- A space lattice or crystal is a three- dimensional arrangement of atoms in space such that every atom is situated similarly to every other atom.
- Molecular crystals occur when the regularity of a molecular chain allows close packing with neighboring chains. This might be localized or spade through a considerable extent of the solid.
- There are 14 possible types of space lattices for understanding the properties of dental materials.
- Most of the metals used in dentistry are formed by the cubic space lattice. The atoms are situated at the intersection of the three sets of plane. Each plane being perpendicular is called crystal plane.
Properties of dental materials – CHANGE OF STATE
Heat of vapourization: When water vapor condenses to form a liquid, energy in the form of heat is released, known as the heat of vapourization. It is defined as the amount of heat evolved when 1 gm of vapor is condensed to the liquid state. Conversely, it may also be defined as the amount of heat required to change 1 gm of liquid to a gas.
Latent heat of fusion: If the energy of the liquid decreases sufficiently by virtue of a decrease in temperature, a second transformation in state may occur and the liquid changes to a solid or freezes. Again energy is released in the form of heat. In this case, the energy evolved is known as the latent heat of fusion.
In as much as energy is required for a change from the solid to the liquid state, one can conclude that the attraction between the atoms (or molecules) in the solid state is greater than that in either the liquid or the gaseous state.
The force required to free the molecules from a liquid is related to the vapor pressure of the liquid.
Properties of dental materials – Interatomic Distance
Regardless of the type of structure in the solid state, there is a limiting factor that prevents the atoms or molecules from approaching each other too closely. If the atoms approach too closely, they are repelled from each other by their electron charges. On the other hand, the forces of attraction tend to draw the atoms together. The position at which these forces of repulsion and attraction become equal in magnitude (but opposite in direction) is the normal or equilibrium position of the atoms.
Properties of dental materials – Diffusion
All of the atoms possess certain amounts of energy, but some may have more energy (i.e., greater amplitude of vibration) than others. It is possible that the atoms with greater energy may be able to displace those with lower energy levels and thereby change their location in the space lattice. The energy required for such movement or diffusion is known as activation energy.
The diffusion of molecules in gases and liquids is well known. Diffusion rates for a given substance depend mainly on the temperature. The higher the temperature, the greater will be the rate of diffusion.
Properties of dental materials – STRESS AND STRAIN
When a specific force or load is applied to a body, a reaction of the same intensity and with opposite direction is produced. A stress is the force per unit area in a body which resists an external force.
It results in an internal tension. It should be noted that the two forces are equal in magnitude but differ in direction and, therefore, are not equal since they are vector quantities.
Depending on the applied load characteristics and consequent stress, different reactions from the tested material may occur. The stress can result in structural alteration of original dimensions. The rate between this alteration by the original dimension results in deformation, that is defined as strain.
Strain may be either
(i) Elastic strain: It is reversible; it disappears after the stress is removed.
(ii) Plastic strain : It is a permanent displacement of the atoms inside the material.
(iii) Combination of the two.
Properties of dental materials – Types of Stresses and Strains
By means of their directions, stresses can be classified under three types:
(i) Tensile stress : A tensile stress is any induced force that resists a deformation caused by a load that tends to stretch or elongate a body. A tensile stress is always accompanied by a tensile strain.
(ii) Compressive stress : If a body is placed under a load that tends to compress or shorten it, the internal forces that resist such a load are called compressive stresses. A compressive stress is always accompanied by a compressive strain.
(iii) Shear stress : A stress that tends to resist a twisting motion, or a sliding of one portion of a body over another, is a shear or shearing stress.
Complex stresses : It is extremely difficult to induce a pure stress of a single type in a body. Under almost all practical conditions, although one type of stress may be predominant in a structure, the other two types are present as well.
Properties of dental materials – STRENGTH
Strength is the maximal stress required to fracture a structure. It is called tensile strength, compressive strength (crushing strength) or shear strength, depending upon the predominant type of stress present. This is one of the properties of dental materials.
Properties of dental materials – CREEP
If a metal is held at a temperature near its melting point and subjected to a constant applied force, the resulting strain will be found to increase as a function of time. This time- dependent plastic deformation is referred to as Creep. This is one of the properties of dental materials.
Static creep is the time-dependent deformation produced in a completely set solid subjected to a constant stress.
Dynamic creep refers to this phenomenon when the applied stress is fluctuating, such as in a fatigue-type test.
Most metals used in dentistry have melting points that are much higher than mouth temperature and thus are not susceptible to the creep phenomenon. The most important exception is dental amalgam, which has components with melting points only slightly above room temperature.
Properties of dental materials – DENSITY
Density is the quality or condition of compactness of the matter. It varies with the composition and the amount of impurity or air space present inside the mass of a body. This is one of the properties of dental materials.
Properties of dental materials – ELASTICITY
Elasticity is the ability of a material that has been changed by a force to return to its original form. Materials such as rubber or a spring are specifically designed to return to their original form unless the force applied exceeds the material’s elastic limit. If that happens, the rubber or spring becomes permanently deformed. Materials that are rigid and break under force are termed inelastic or plastic. This is one of the properties of dental materials.
Properties of dental materials – DUCTILITY AND MALLEABILITY
Ductility and malleability are measurements of a metal’s ability to withstand permanent deformation by tensile or compressive forces. Metals that can be shaped into a wire or similar form are said to be ductile. Metals that can be rolled or hammered into a sheet are said to be malleable. This is one of the properties of dental materials.
Properties of dental materials – FLOW
If a material gradually deforms on application of continuous external force, it is said to flow. Many dental cements and some waxes are specifically designed to have good flow. This change, when considered undesirable in a material, is sometimes called creep or slump. Some materials such as dental waxes exhibit deformation in relation to temperature changes. In the presence of heat, the wax will slump and the dimensions will change. This action results in a distortion from the original shape. This is one of the properties of dental materials.
Properties of dental materials – SETTING TIME
Setting time is defined as the amount of time that passes between the initial point of mixing a material to the point of final hardness. Setting time of a material is associated with the chemical reaction rates between its constituents and affects their practical applications in restorative dentistry or during the various laboratory procedures. If the material is moved before the final set occurs, that movement will create distortion within the material, Setting time may be altered by manipulative procedures and this mode of behavior is of great practical significance. This is one of the properties of dental materials.
Properties of dental materials – SURFACE HARDNESS
A material’s ability to withstand indentation or abrasion is called surface hardness. Minerals are measured on the Mohs scale of hardness, with 1 being softest and 10 being the hardest. Properties of dental materials are commonly measured by the Brinell Hardness Number (BIlE) system. Softer materials that exhibit more indentation have smaller numbers. Harder materials with less indentation have a larger number. This is one of the properties of dental materials.
Properties of dental materials – THERMAL PROPERTIES
The ability of a material to transmit temperature changes is called thermal conductivity. The natural tooth structure is an excellent thermal insulator and transmits heat very poorly. This characteristic protects the pulpal tissues from sensing most thermal changes. Materials used for tooth restorations must be good thermal insulators. If not, other steps must be taken to protect the tooth’s nerve. Metal restorative materials, like amalgam or gold, are good thermal conductors, so such restorations require bases with good insulating properties of dental materials to be placed under them to avoid any thermal conductivity. This is one of the properties of dental materials.
Coefficient of Thermal Expansion
All materials respond to temperature changes by exhibiting molecular changes that alter their overall size. Materials expand or contract with temperature changes. The measurement of this change is called the coefficient of thermal expansion and it is measured in per centimeter per degree of temperature change. The ideal dental restorative would have exactly the same coefficient of thermal expansion as tooth structure. If it did, the dental restorative material would expand or contract just as the tooth does. Since most restorations differ from tooth structure, this property contributes to the creation of a microscopic space between the tooth and the restoration. The space may allow microleakage by which fluids, debris, microorganisms, etc., enter into the restoration and make their way to the pulp. This process causes sensitivity of the tooth and possibly even secondary caries.
Properties of dental materials – Viscosity
The ability of a material to flow is called viscosity. This is one of the properties of dental materials.
The higher a material’s viscosity, the thicker the material is and the less it flows.
Lower-viscosity materials will also exhibit better wet ability (the ability to spread out into a thin layer). This characteristic is determined by measuring the contact angle, the angle formed by a droplet placed on a surface. An ideal adhesive would flow out into a thin film and wet the total surface. The thickness of this film of adhesive is called the film thickness. Generally, the lower the film thickness, the stronger the adhesive.
Properties of dental materials – Surface Tension
Surface tension is caused by the cohesive attraction of surface molecules to each other as they are pulled inward by the other molecules. As a result, there is a tendency for the exposed surface to be reduced to the smallest possible area. Higher surface tensions provide greater wet ability. This is one of the properties of dental materials.
The conditions of the oral environment are far less than ideal for maximizing adhesion. The extremes of stress, temperature, and moisture place severe strains on dental materials, including any adhesive that might be utilized. However, advances in dental material research during the last 20 years have resulted in major advances in restoratives and operative procedures.
OTHER NON-MECHANICAL PROPERTIES
This term is applied to general deterioration and changes in the quality of materials during shipment from the manufacturing unit and storage in dental office. The factors affecting such changes are temperature, humidity, time length of storage, type of container, contamination, etc. Certain items, like, X-ray films, drugs etc., must show an expiry date.
Properties of dental materials – Tarnish
Tarnish may be considered as a surface discolouration of a metal with associated loss of luster. In tarnish the chemical reaction between the restoration and the oral cavity are restricted to the restoration surface only. Although tarnish may be undesirable esthetically, it will not often destroy the restoration. In certain cases it helps in preventing the subsequent corrosion, however on the other hand in certain cases it a forerunner of corrosion process. This is one of the properties of dental materials.
Causes of Tarnish
(1) Formation of the hard deposits (Calculus).
(2) Soft deposits (Plaque).
(3) Discoloring food or medicines, e.g., coffee, Betadine mouth wash, iron preparations, etc.
(4) Formation of Oxides, Sulphides and Chlorides (which may work in long run as protection against corrosion).
(6) Roughness of the restoration.
(7) Exposure to different temperature changes.
Properties of dental materials – Corrosion
Corrosion is a surface reaction of metals in the mouth from the components in saliva, food or oral atmosphere. Corrosion results from chemical reaction that penetrates into the body of the restoration. Corrosion usually eventually leads to the failure of the restoration. Both tarnish and corrosion occur more on the restoration with rough surface than on smooth surface. Corrosion around the margins of amalgam restoration is helpful as the corrosive products seal the margins. This is one of the properties of dental materials.
Factors Affecting the Corrosion
- Composition of Alloy: Mixture of more metals in alloy increases chances of corrosion, Single metal usually reduces the chances of corrosion.
- Homogenity of the alloy components.
- Nobelity of the metal.
- Stability of the molecular structures of the alloy.
- Environment (presence and composition of the medium).
- Fluctuation of the temperature.
- Exposure to chlorides, sulphides, oxides or acidic substance.
- Food habits.
- Roughness of the restoration.
- Oral hygiene maintenance.
Classification of Corrosion to understand the properties of dental materials
(1) Chemical or Dry Corrosion. In this type the metal reacts to form oxides, suffides in the absence of electrolytes, e.g., formation of Ag2S, oxidation of alloy particle in dental amalgam.
(2) Electrolytic or Electro Chemical or Wet Corrosion. This requires the presence of water or other fluid electrolytes. There is a formation of free electrons and the electrolyte provides the pathway for the transport of the electrons.
Types of corrosion in understanding the properties of dental materials
(i) Galvanic Corrosion. The galvanic corrosion can happen due to:
(a) Dissimilar Metals. Galvanic corrosion occurs when dissimilar metals lie in direct physical contact. Saliva with its salt provides a weak electrolyte, e.g., amalgam and gold restoration in opposing occluding teeth. For temporary relief varnish can be applied on amalgam, but for permanent relief or to avoid it to happen different metallic restorative materials should be avoided in oral cavity. This is one of the properties of dental materials.
(b) Heterogenous Compositions. In the same metallic restoration presence of impurity or solder joints, can also cause galvanic corrosion. This is one of the properties of dental materials.
(ii) Stress Corrosion. A metal which has been stressed by cold working, becomes more reactive at the site of maximum stress. If stressed and unstressed metals are in contact in an electrolyte, the stressed metal will become the anode of a galvanic cell and will corrode, e.g., in orthodontic wire that has been cold worked, stress corrosion may occur and cause the wire to break. This is one of the properties of dental materials.
(iii) Concentration Cell Corrosion or Crevice Corrosion
Electrolyte Concentration Cell. In a metallic restoration which is partly covered by food debris, the composition of the electrolyte under the debris will differ from that of saliva and this can contribute to the corrosion of the restoration.
Oxygen Concentration Cell. Differences in oxygen tension in between parts of the same restoration causes corrosion of the restoration. Greater corrosion occurs in the part of the restoration having lower concentration of the oxygen.
Prevention Against Corrosion
The corrosion can be avoided by the following methods or precautions:
(i) Passivation—Oxide films.
(ii) Increase in noble metal content.
(iii) Polishing or smother surface.
(iv) Avoid use of dissimilar metals.
(v) Greater oral hygiene maintenance.
VARIOUS IMPORTANT METALLURGICAL TERMS USED IN SUBJECT
(a) Cold Working. This is the process of changing the shape of a metal by rolling, pounding, bending, or twisting at normal room temperature.
(b) Strain Hardening. This occurs when a metal becomes stiffer and harder because of continued or repeated application of a load or force. At this point, no further slippage of the atoms of the metal can occur without fracture.
(c) Heat Softening Treatment (Annealing). This treatment is necessary in order to continue manipulating a metal after strain hardening to prevent it from fracturing. The process of annealing consists of heating the metal to the proper temperature (as indicated by the manufacturer’s instructions) and cooling it rapidly by immersing in cold water. Annealing relieves stresses and strains caused by cold working and restores slipped atoms within the metal to their regular arrangement.
(d) Heat Hardening Treatment (Tempering). This treatment is necessary to restore to metals properties of dental materials that are decreased by annealing and cold working. Metals to be heat hardened should first be heat softened (annealed) so that all strain hardening is relieved and the hardening process can be properly controlled. Heat hardening is accomplished in dental gold alloy by heating to 8400 Fahrenheit, allowing it to cool slowly over a 15 minute period to 480° Fahrenheit, and then immersing it in water.