Physical Properties of Dental Materials
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A comprehensive slideshow covering all the basics relating to dental materials and their physical properties. Based on standard text books - Phillips Science of Dental Materials (11th Edition).
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Physical Properties of Dental Materials
- 1. AARON SARWAL MDS 1ST PROF PHYSICAL PROPERTIES OF DENTAL MATERIALS
- 2. Index What are physical properties? Abrasion and Abrasion resistance. Viscosity Structural and Stress Relaxation Creep and Flow Color and Color Perception Thermo-physical Properties Tarnish and Corrosion
- 3. What are Physical Properties? Physical properties are based on the laws of mechanics, acoustics, optics, thermodynamics, elasticity, magnetism, radiation, atomic structure, or nuclear phenomena. Examples: color, creep, viscosity etc
- 4. Abrasion and Abrasion Resistance Abrasion is the process of scraping or wearing. (Phillips Science of Dental Materials, 11th edition) Hardness: Property of being difficult to indent, cut or scratch. (Phillips Science of Dental Materials, 11th edition) Simply resistance to plastic deformation or permanent deformation of material. Used within a given classification as an index of the ability of a material to resist abrasion or wear.
- 5. Abrasion and Abrasion Resistance contd… The surface hardness tests commonly used in dentistry are : •Brinell hardness test •Knoop hardness test •Vickers hardness test •Rockwell hardness test •Barcol hardness test •Shore hardness test
- 6. Brinell hardness test Oldest method used to test metals and alloys
- 7. Vickers Hardness Test Also known as 136 degree diamond pyramid test.
- 8. Knoop Hardness Test (a) Knoop Indenter (b) Vickers Indenter
- 11. Viscosity Resistance of a liquid to flow Viscosity is controlled by internal frictional forces and is measured in poise (Mpa/sec) or centipoise. Study of flow characteristics of materials and deformation is rheology. The term rheology was coined by Eugene C. Bingham.
- 12. Viscosity contd… To explain viscous nature of some materials, shear stress v/s shear strain rate curve can be
- 13. Structural and Stress Relaxation When force applied on a body, internal resistance to load is stress. The value of stress which causes an object to give way or break is called strength of material.
- 14. Strain can be defined as change in length per unit length of the body when subjected to stress. Has no definite unit. Strain may be elastic /reverts back to original form or plastic/ permanent deformation. Structural and Stress Relaxation Contd…
- 15. Structural and Stress Relaxation Contd…
- 16. Creep and Flow Creep is defined as the time-dependent plastic strain of a material under a static load or constant stress. The term creep implies a relatively small deformation produced by a relatively large stress over a
- 17. Creep and Flow contd… Flow implies a greater deformation produced more rapidly with a smaller applied stress. The term flow is generally been used in dentistry to describe the rheology of amorphous materials such as waxes.
- 18. Color and Color Perception Color is a sensation induced from light of varying wavelengths reaching eye. Cone cells of retina are responsible for spectral sensitivity
- 19. Color Perception The phenomenon of vision can be explained by considering the response of the human eye to light reflected from an object. Light from an object that is incident on the eye is focused in the retina and is converted into nerve impulses, which are transmitted to the brain.
- 20. The Three Dimensions of Color Color perception is described by three objective variables: Hue Value Chroma These three parameters constitute the three dimensions of “color space”.
- 21. HUE Hue is defined as the particular variety of a color, shade, or tint. E.g. RED, GREEN YELLOW
- 22. Chroma The intensity of the color, degree of saturation of a particular hue.
- 23. Value Value is defined as the relative lightness or darkness of a color or the brightness of an object • Value of 0 = black • Value of 10 = white
- 25. CIELAB Color System The Commission Internationale de l’Eclairage (CIE), an international color research group published the CIELAB color system in 1976.
- 26. Color Terminology Opacity - The measure of impenetrability of visible light. Translucency-Diffused transmission of visible light Transparency- capable of transmitting light
- 27. Color Terminology contd… Metamerism Fluorescenc e
- 28. Thermophysical Properties Thermophysical properties can be simply defined as material properties that vary with temperature without altering the material's chemical identity. However, it has become customary to limit the scope of the term to properties having a bearing on the transfer and storage of heat These are: Thermal Conductivity Thermal Diffusivity Coefficient of thermal expamsion
- 30. Thermal Diffusivity It is a measure of the rate at which a body with a no uniform temperature reaches a state of thermal equilibrium The thermal conductivity of zinc oxide-eugenol is slightly less than that of dentin, its thermal diffusivity is more than twice that of dentin.
- 31. Thermal Diffusivity contd… Material Thermal Diffusivity (in 10-4 cm2 /sec) Pure Gold 11,800 Amalgam 960 Composite 19 to 73 Water 14 Glass Ionomer 22 Dentin 18 to 26 Enamel 47 Zinc Phosphate 30 The low thermal conductivity of enamel and dentin aids in reducing thermal shock and pulpal pain when hot or cold foods are taken into the mouth.
- 32. Coefficient of Thermal Expansion The coefficient of thermal expansion describes how the size of an object changes with a change in temperature. It measures of the fractional change in size per degree change in temperature at a constant pressure.
- 33. Tarnish and Corrosion TARNISH: • Tarnish is a surface discoloration of the metal or even a slight loss of the lustre. • This surface discoloration is due to either, the formation of hard and soft deposits on the surface, or the formation of thin films of oxides, sulphides or chlorides. • Tarnish is often the first step of corrosion
- 34. Tarnish and Corrosion contd… CORROSION: Corrosion is not only a surface discoloration but is a disintegration of a metal by reaction with its environment.
- 35. Causes of Tarnish and Corrosion
- 36. Classification of corrosion DRY CORROSION METALS AND NON METALS WET CORROSION DISSIMILAR METALS HETEROGENOUS SURFACE COMPOSITION STRESS CORROSION CONCENTRATION CELL CORROSION
- 37. Dry Corrosion In which there is a direct combination of metallic and non-metallic elements Electrolytes are absent e.g. oxidation, halogenations, or sulfarization reaction.
- 38. Electrochemical Corrosion Corrosion occurs in presence of water or some other liquid electrolyte.
- 39. Dissimilar Metals Galvanism is results of difference in potential between dissimilar restoration in opposing or adjacent tooth.
- 40. Heterogeneous Surface Composition • A second type of electrolytic corrosion is that due to heterogeneous compositions of the metal surface. Example of this type may be the intectic and peritectic alloys. • When such an alloy is immersed is an electrolyte, the metallic grains with the less electrode potential are attacked and corrosion results. • A common situation for this type of corrosion would be an amalgam restorations with polished and unpolished area.
- 41. Stress Corrosion Degradation by the combined effect of mechanical stress and corrosive environment, usually in form of cracking.
- 42. Concentration Cell Corrosion Example: A metallic restoration which is partly covered by food debris will differ from that of saliva, and this can contribute to the corrosion of the restoration.
- 43. Protection Against Corrosion Avoid using dissimilar metals. The surface of any dental restoration should be smooth and lustrous. A polished, smooth surface provides easier cleaning and prevents accumulation of debris.
- 44. Clinical Significance of Galvanic Currents As long as metallic dental restoration materials are employed there seems to be little possibility that the galvanic currents can be eliminated. Galvanic Shock A pain sensation caused by electric current generated by a contact between two dissimilar metal forming a galvanic cell in oral environment. It can be a real source of discomfort to an occasional patient. Such post operative pain usually occurs immediately after insertion of a new restoration and generally is gradually subsides and disappears in a few days.
- 45. Conclusion A proper knowledge of physical properties of dental materials helps us in making correct choice for various clinical restorations. This in turn increases the durability and life span of the restoration. When tooth shade is selected using conventional means, knowledge & skill of practitioner comes into play. Technique based system provide dentist with distinct advantage in creating highly aesthetics, natural looking restoration.
- 46. References: CONTENT: Phillips science of dental material 10th & 11th edition Restorative dental materials – craig 13th edition Dental materials and their selection- 3rd edition by william j. O'brien IMAGES: Google Image Search www.pocketdentistry.com www.slideshare.net GIF and Animations: www.makeagif.com www.gifsearch.com Google Image Search
Editor's Notes
- Oldest methods used to test metals and alloys Small steel or tungsten carbide ball, typically 1.6 mm in diameter, subjected to a weight of 123 N and it remain in contact to specimen for 30 sec Smaller the area of indentation, harder the material and larger the BHN value
- The method is similar to BHN except that a 136 degree diamond pyramid shaped indenter is forced into the material with a definite load application. The indenter produces a square indentation, the diagonals of which are measured. It is suitable for brittle materials so it is used for the measurement of hardness of tooth structures and also of dental casting gold alloys. A significant advantage of this test is that it can be used for testing very small specimens because the indenter tip is small. Also, because the load applied to the specimen can be varied, the test can be used on materials that are soft as well as on those that are hard.
- Knoop Indenter: Highly polished, Rhombic-based, pyramidal diamond that produces a diamond shaped indentation. Ratio between the long and short diagonals of about 7:1. the Pyramid shape has an angle, Longitudinally of 172 and transverse of 130 •Used for thin plastic or metal sheets •Advantage of this method is that materials with a great range of hardness can be tested by varying the test load. •Disadvantage is that it needs very high polished and flat surface.
- This was developed as a rapid method for hardness determination. A ball or metal cone indenter is normally used and the depth of the indentation is measured with a sensitive dial micrometer. The indenter balls or cones are of different diameters and load applications ( 60 to 150 kg) The advantages of Rockwell system is that hardness is read directly and it is good for testing viscoelastic materials. The disadvantages are that a preload is needed, greater time required and the indentation may disappear immediately on removal of the load.
- This method is used to study the depth of cure of resin composites. The Barcol indenter is a spring loaded needle with a diameter of 1 mm that is pressed against the surface to be tested. If no penetration of the needle into the surface occurs the scale reads 100. The reading on the scale decreases as the indenter penetrates the surface. Depth of cure of a resin composite is tested by preparing specimens varying in thickness from 0.5 to 6.0 mm. Then the top surface of a specimen is activated by a light curing unit. The Barcol hardness of the top surface is compared with that of the bottom surface. 10% decrease in Barcol hardness of a resin composite results in a 20% decrease in the flexural strength.
- The success or failure of a given material may be as dependent on its properties in the liquid state as it is on its properties as a solid. Materials like cements and impression materials undergo a liquid-to-solid transformation in the mouth. Gypsum products used in the fabrication of models and dies are transformed from slurries into solid structures Amorphous materials such as waxes and resins appear solid but actually are supercooled liquids that can flow plastically under sustained loading or deform elastically under small stresses.
- NEWTONIAN FLUID: - an ideal fluid - Shear stress proportional to strain rate -Straight line on curve -Viscosity(η)= shear stress(τ)/strain rate (ε) -Constant velocity. PSEUDOPLASTIC FLUID: -viscosity decreases with increasing strain rate, until it reaches a nearly constant value. e.g. Ketchup, blood, nail-polish DILATENT FLUID Viscosity increase with increasing stress. The material become more rigid under stress(disadvantage) e.g.-Acrylic denture base material, sand in water PLASTIC FLUID Material behaves rigid until a minimum of stress is applied ,then it starts behaving like Newtonian fluid. e.g.- clay suspension, composite material
- Elastic strain is strain that totally disappears once the external load that caused it is removed. Plastic strain is strain that permanently remains once the external load that caused it is removed. It occurs when the force applied to the atoms moves them so far from their equilibrium position that they do not return to it once the force is removed.
- We can see here that the paper clip, made of metal has been permanently deformed, now there are trapped internal stresses. The atoms in the crystal structure are displaced and the system is not in equilibrium. In amorphous structures, some molecules are too close together and others too far apart when the substance is permanently deformed. The displaced atoms are in unstable positions. They can slowly return to their equilibrium positions by a solid-state diffusion which is dependent on thermal energy. The result is a change in the shape or contour of the solid. The material warps or distorts. Such stress relaxation can be a problem with elastomeric impression materials and can lead to distortions in the impression. The rate of relaxation increases with an increase in temperature. In this example, the clip tends to straighten out the moment it is dropped in hot water, but at room temperature, there is negligible change. There are many non-crystalline dental materials (such as waxes, resins) that, when manipulated and cooled, can undergo relaxation with consequent distortion at an elevated temperature. This may result in improper fit of prosthesis.
- Metal creep usually occurs as the temperature increases to within a few hundred degrees of the melting range. Metals used in dentistry for cast restorations or substrates for porcelain veneers have melting points (for pure metals) or melting ranges (for alloys) that are much higher than mouth temperatures, and they are not susceptible to creep deformation intraorally. Because of its low melting range, dental amalgam can slowly creep from a restored tooth site under periodic sustained stress, such as would be imposed by patients who clench their teeth.
- The flow of wax is a measure of its potential to deform under a small static load even that associated with its own mass.
- An important goal of dentistry is to restore or improve esthetics—the color and appearance of natural dentition. Esthetic considerations in restorative and prosthetic dentistry have received increasingly greater emphasis in recent times. For an object to be visible, it must reflect or transmit light incident on it from an external source. The incident light is usually polychromatic; that is, a mixture of the various wavelengths, commonly known as “white” light. Incident light is selectively absorbed or scattered (or both) at certain wavelengths.
- Cone-shaped cells in the retina are responsible for color vision. These cells have a threshold intensity required for color vision and respond to wavelengths for both normal color vision and color-deficient vision. Someone with normal vision has maximum sensitivity in the green-yellow region at about 550 nm and is least sensitive in the red and blue-violet regions of the spectrum. The signals from the retina are processed by the brain to produce the psychophysiological perception of color. Because a neural response is involved in color vision, constant stimulation by a single color may result in color fatigue and a decrease in the eye’s response to that color. Defects in certain portions of the color-sensing receptors result in the different types of color blindness; thus humans vary greatly in their ability to distinguish colors.
- Verbal descriptions of color are not precise enough to describe the appearance of teeth. These definitions are too variable, complex, and imprecise to describe a desired color of a dental crown to a laboratory technician.
- Professor Albert H. Munsell •Basic principles first published in 1905 •Hues are divide into 10 gradations: yellow, yellow-red, red, red-purple, purple, purple-blue, blue, blue-green, green
- •3-dimensional color system • L* refers to brightness (0 to 100) • a* represents red (+a*) vs. green (-a*) • b* indicates yellow (+b*) vs. blue (-b*). •When a* and b* are zero, the L value represents the continuum of black to white
- Metamerism -Phenomenon in which the color of an object under one type of light appears to change when illuminated by different light source. Different light sources interact differently with different materials. Fluorescence is the absorption of light by a material and the spontaneous emission of light in a longer wavelength. •In a natural tooth, it primarily occurs in the dentin because of the higher amount of organic material. •UV light is absorbed and fluoresced back as light primarily in the blue end of the spectrum.
- • Heat transfer through solid substances is most commonly occur by means of conduction. • The conduction of heat through metals occurs through the interactions of crystal lattice vibrations and by the motion of electrons and their interaction with atoms.
- In the oral environment, temperatures are not constant during the ingestion of foods and liquids. Under such conditions, thermal diffusivity is important.
- For a patient drinking ice water, the low specific heat of amalgam and its high thermal conductivity suggest that the higher thermal diffusivity favors a thermal shock situation more than that is likely to occur when only natural tooth structure is exposed to the cold liquid.
- Close matching of the coefficient of thermal expansion (α) is important between the tooth and the restorative materials to prevent marginal leakage. Opening and closing of gap results in breakage of marginal seal between the filling and the cavity wall, this breakage of seal leads to marginal leakage, discoloration & hypersensitivity.
- • Stain or discoloration arises from pigment – producing bacteria, drugs containing chemicals such as iron or mercury & absorbed food debris. • Although deposits are the main cause of the tarnish in the oral environment, surface discoloration may also arise on a metal from the formation of thin films, such as oxides, sulfides, or chlorides.
- • Corrosion may occur due to moisture, atmosphere, acid or alkaline solutions, & certain chemicals. • Common e.g. rusting of iron, a complex chemical reaction in which iron combines with oxygen in air & water to form hydrated oxide of iron.
- Eggs & other food contain significant amount of sulfur, various sulfides (hydrogen or ammonium sulfide) corrode silver, copper, mercury & similar metals present in dental alloys. Water, oxygen & chlorine ions present in saliva. Various acidic solutions such as phosphoric , acetic, & lactic acids at proper concentration & pH promote corrosion.
- This type of corrosion is less susceptible to occur in the mouth. Example, oxidation of metal surface during soldering and heat treatment procedures.
- Galvanic corrosion (also called bimetallic corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte.
- In a solid solution, any cored structure is less resistant to corrosion than the homogenized structure because of difference in electrode potential. Even an homogenized solid solution in some what susceptible to corrosion because of the difference in structure between the grains and their boundaries. The grains boundaries may act as anodes and the interior of the grains as the cathode. This results in the corrosion of the material in the anodic region at the grain boundaries.
- Example, a metal which has been stressed by burnishing produces the localized stress in some part of structure If stressed and under stressed metals are in contact in an electrolyte the stressed area will become anode and will corrode.
- A homogeneous metal or alloy can undergo electrolytic corrosion where there is a difference in electrolyte concentration across the specimen.
- • Certain metals develop a thin ,adherent , highly protective film by reaction with the environment, such a metal is said to be passive. • A thin surface oxide forms on chromium , a good e.g. of a passivating metal, & stainless steel contain sufficient amounts of chromium to passivate the alloy.
- The cement base itself although it is a good thermal insulator, has little effect in minimizing the current that is carried into the tooth and through the pulp. It is likely that the physiologic condition of the tooth is the primary factor responsible for the pain resulting from this current flow. Once the tooth has recovered from the injury of preparing the cavity and has returned to a more normal physiological condition the current flow then produces no response