Physical properties


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Physical properties

  1. 1. ABRASION Definition : it is a material removal process that can occur when ever surfaces slide against each other. Abrasion resistance : It is the ability of a material to resist abrasion or wear. Etiology : Factors that effect the wear of contacting enamel surfaces are - Hardness - Biting force - Frequency of chewing - Abrasiveness of the diet - Composition of intra oral liquids - Temperature changes - Surface roughness - Physical properties of the materials - Surface irregularities with hard impurity particles - Fine anatomic grooves - Pits or ridges Diagnosis : Hardness has often been used as an index of the ability of a material to resist abrasion or wear. Hardness may be useful for comparing materials within a given classifications, such as one brand of cast metal with another brand of same type of casting alloy. Knoop and Vicker’s hardness tests are based on indentation methods that quantify the hardness of materials. The tip of knoop diamond indenter has an elongated pyramid shape, where as the Vicker’s diamond indenter has an equilateral pyramid design. Both tests involve the application of indenter to test surface under a known load (usually 100N). The depth of surface penetration is reported as hardness, in units of force per unit area. 1
  2. 2. Silicon carbide and diamond abrasives will abrade dental porcelain more readily than does garnet. Treatment : Although dentists cannot control the bite force of the patient, they can adjust the occlusion to create broader contact areas in order to reduce localized stresses, and they can polish the abrading ceramic surface to reduce the rate of destructive enamel wear. Optical properties : Important goal of dentistry is to restore the function of damaged or missing natural tissues, color and appearance of natural dentition. Aesthetic dentistry considerations in restorative and prosthetic dentistry have received greater emphasis over the past several decades. The search for an ideal, general purpose, technique insensitive, direct-filling, tooth-colored restorative material is one of the continuing challenges of current dental materials research. Since aesthetic dentistry imposes severe demands on the artistic abilities of dentist and technician, knowledge of the underlying scientific principles of color is essential. Color and color perception : Principles of color : 1) Light is the form of electromagnetic energy visible to the human eye. Light is form of energy which propagates according to laws of physics. The energy spreads in the form of waves characterized by two different parameters – wavelength and amplitude. The wavelength is the distance between successive peaks (troughs). The amplitude is the wave height with relations to the directional axis of the wave. 2) In 1666, sir Isaac Newton observed that white light passing through a prism divided into an orderly pattern of colors now termed the spectrum. He also discovered that these colors would reproduce white 2
  3. 3. light when passed back through the prism proving that all spectral colors were in the original beam. 3) The electromagnetic spectrum ranges from 10-14 m (gamma rays) to 106 m (radio waves). It is only rays with a 380-760nm large that, through their action on specialized cells, elicit photochemical reactions from the retina, responsible for triggering visual perception of shapes and colors with in brain. That is why they cannot differentiate U-V or infrared rays. The perception of color of an object is the result of a physiological response to a physical stimulus. The sensation is a subjective experience, where as the beam of light, which is the physical stimulus that produces the sensation is entirely objective. The perceived color response results from either a reflected or a transmitted beam of white light or a portion of that beam. According to one of Grossman’s laws, the eye can distinguish differences in only three parameters of color. These parameters are i) Dominant wavelength ii) Luminous reflectance iii) Excitation purity 1) Dominant Wavelength. : light having short wave length is violet (400nm) and long wave length is red (700nm). Between these two wavelengths are those corresponding to blue, green, yellow and orange light. This attribute of color perceptions is also known as Hue. Hue is the name of the color. i.e. dominant color of an object. Ex. Red, Green, Blue Of all the visible colors and shades, there are only three primary colors: red, green and blue (or violet). Another color may be produced by 3
  4. 4. proper combination of these colors. For example yellow may be obtained by a mixture of green and red colors. Hue relationship : The relationship of primary, secondary and complementary hues are graphically explained by the color wheel. Primary Hues : The primary hues are - red, yellow and blue form basis of dental color system. In dentistry the metal oxide pigments used in coloring porcelains are limited in forming certain reds, therefore pink is substituted. The primary hues have a relationship to one another and form basic structure of color wheel. Secondary hue : Any two primary hues, when mixed form a secondary hue. When red and blue are mixed they create violet, blue and green ,yellow and red create orange. Altering the proportions of primary hues on a mixture will vary the hue of the secondary hue produced. Complementary hue : Colors directly opposite each other on the color wheel are termed complementary hues. A peculiarity of this system is that a primary hue is always opposite a secondary hue and vice versa. When a primary hue is mixed with a complementary secondary hue, the effect is to “cancel” out both colors and produce gray. This is the most important in dental color manipulations. Eg : when a portion of crown is too yellow, lightly washing with violet (complementary color) will produce an area that is no longer yellow. Hue sensitivity : After 5 seconds a staring at a tooth or shade guide, the eye accommodates and becomes biased. If one stares at any color for longer than 5 seconds, and then stares away at a white surface, or closes one’s eyes, 4
  5. 5. the image appears but in the complementary hue. This phenomenon is known as hue sensitivity, adversely effects shade selection. ii) Luminous Reflectance : The luminous reflectance of a color permits an object to be classified as equivalent to a number of series of a achromatic objects ranging from black to white for light diffusing objects and from black to perfectly clear or colorless for transmitting objects. The black standard is assigned a luminous reflectance of 0, where as white standard is assigned 100. This attribute of color perception is described as VALUE in one visual system of color measurement. Value : relative light ness or darkness of a color (whiteness / blackness) It is not the quantity of color but rather quality of brightness on a gray scale. Eg : a light tooth has a high value, a dark tooth has a low value. The use of value in restorative dentistry does not involve adding gray but rather manipulating colors to increase or decrease amounts of grayness. iii) Excitation Purity : The excitation purity or saturation of a color describes the degree of its difference from the achromatic color perception most resembling it. Number representing excitations purity range from 0 to 1. This attribute of color perception is also known as CHROMA. Chroma : degree of saturation of a particular hue Chroma can only present with hue. Eg : to increase the chroma of porcelain restoration more of that hue is added. @@@@@@@@@@Typical quantities for dominant wavelength, luminous reflectance, and excitation purity of materials and human tissues determined in reflected light are listed as follows – @@@@@@@@@@@@@@@ 5
  6. 6. Phenomenon of vision : It is related to the response of the human eye to light reflected from an object. Light from an object i.e. incident on the eye is focused in the retina and is converted into nerve impulses that are transmitted to brain. Cone shaped cells in the retina are responsible for the color vision. These cells have a threshold intensity required for color vision and exhibit a response curve related to the wavelength of the incident light. Cone cells function in hue and chroma interpretation. The rod cells are responsible for interpreting brightness differences and value. The curves in the figure illustrates individuals with normal color vision and individuals with color deficient vision. The normal observer curve indicates that the eye is most sensitive to light in the green-yellow region of wavelength 550nm and least sensitive at the red or blue regions of the color spectrum (700nm). Because a neural response is involved in cold vision, constant stimulation by a single color may result in color fatigue and a decrease in the eye’s response. The signals from retina are processed by the brain to produce the pscyhophysiological perception of color. Defects in certain portions of color-sensing receptors result in the different types of color blindness, and thus, human observers vary greatly in their ability to distinguish colors. Colorimeter a scientific instrument measures the intensity and wavelength of light. Although the colorimeter is more precise than the human eye in measuring slight differences in colored objects, it can be extremely inaccurate when used on rough or curved surfaces. But eye is able to 6
  7. 7. differentiate between two colors seen on side by side on smooth or irregular surfaces whether curved or flat. MEASUREMENT OF COLOUR : The color of dental restorative materials is most commonly measured in the reflected light by 1) Instrumental technique 2) Visual technique 1) Instrumental technique : Curves of spectral reflectance versus wavelengths can be obtained over the visible range. (405-700nm) with SPECTROPHOTOMETER and INTEGRATING SPHERE. Reflectance vs wave length : Curves ↓ Obtained from spectrophotometer ↓ From reflectance value obtained ↓ They get 3 stimulus values of particular light sources From the reflectance values and tabulated color matching functions → the tristimulus values (x, y, z) can be computed relatively to a particular light source that is source A → gas filled incandescent lamp (or) source C → average day light from sky. The ratios of each tristimulus value of a color value of a color to their sum are called the chromatically coordinates (x, y, z). X : x + y + z Y : x + y + z Z : x + y + z 7
  8. 8. Dominant wave length (Hue) and excitations purity (chroma) can be determined by referring its chromatically coordinates to a chromaticity diagram. X = standard observer Y = co-ordinate system (x, y, z) A = standard light source B = colour considered C = intersection of line AB with spectrum locus is dominant wavelength The excitation psoity (chroma) is the ratio of two lengths AB and AC. The (dominant wavelength) is ‘C’ that intersects line AB with spectrum locus is dominant wavelength i.e. hue. A diagram according to commission international de I’Eclairage (C.I.E) L*a*b* color space is characterized by uniform chromaticities. * In this diagram L = value (black to white) a*b* = chroma where + a = Red - a = Green + b = Yellow - b = Blue * The differences between two colors can be obtained from color difference formula. It is ∆E* (L*a*b*) = [(∆L*)2 + (∆a*)2 + (ab*)2 ]½ where L*, a*, b* depend on the tristimulus value of the specimen and a perfectly white object. * value of ∆E is 1 can be observed visually by half of the observers under standardized conditions. 8
  9. 9. * value of ∆E of 3.3 is considered perceptible clinically Visual technique : A popular system for visual determination of color is the Munsell color system. The parameters are presented in three dimensions. The color considered is compared with a larger set of color tabs. Value is determined first by the selection of a tab that most nearly corresponds with the lightness or darkness of the color. Value ranges from white (10/) to black (0/). Chroma is determined next with tabs that are close to the measured value but are of increasing saturation of color. Chroma ranges from achromatic or gray (/0) to highly saturated color (/18). The hue of color is determined last by matching with color tabs of value and chroma already determined. Hue is measured on a scale from 2.5- 10 in increments of 2.5 for each of the 10 color families. (Red-R, Yellow- red – YR, Yellow – Y, Green yellow – GY, Green – G, Blue-green, Blue-B, purple blue-PB, purple-P, red purple-RP). Example : The color of the attached gingiva of a healthy patient has been measured as 5R 6/4 Where 5R – hue 6 - value 4 – chroma Nickersons formula : Two similar color can also be compared in the Munsell for system by a color difference formula such as one derived by Nickersons. I = (C/5) (2∆H) + (6∆V) + (3∆C) Where C = average chroma 9
  10. 10. ∆H, ∆V, ∆C – differences hue, value + chroma Example : If color of attached gingiva of a patient with periodontal disease was 2.5 R 5/6 - 2.5 R 5/6 is derived from the Munsell color system. - 5R 6/4 is for healthy gingiva. Now to find the difference between the color of healthy gingiva in periodontal disease according to Nickerson’s formula. I = (C/5) (2∆H) + 6∆V + 3∆C C = average of chroma C in healthy gingiva is 4 C in periodontal disease is 6 Average of C = 4+6/2 = 5 ∆H = difference in hue Healthy – PDL gingiva hue 5 – 2.5 = 2.5 ∆V = difference in value Healthy – PDL gingiva value 6 – 4 = 2 ∆V = 1 ∆C = difference in chroma 6 – 4 = 2 So I = (5/5) (2) (2.5) + 6(1) + 3(2) = 5 + 6+6 = 17 Metamerism : 10
  11. 11. Metamerism can cause two color samples to appear as the same hue under on light source, but as un-matched hues under a different light source. They may have non-matching spectral analysis curves but appear to have identical colors under certain lighting conditions. There is more than one way to produce a color. I.e pure green versus a mix of blue and yellow. A pure green color will reflect light in green band, but green mixture color will reflect light in blue are exposed to a light with a full color spectrum they will appear similar. If they are exposed to a light source that does not contain light is blue band, the two colours will appear dissimilar. A spectral curve is a measure of wavelength of light reflected from a surface and reveals the actual component colors reflected from an object. The spectral curves of two metameric green surfaces that appeal identical but exhibit different reflections properties. Clinical relevance : Metamerism complicates the color matching of restorations. A shade button may match under incandescent lighting from the dental operatory lamp but not under fluorescent lighting in patients work place. How to reduce metamerism : - The best approach to color matching is to use three light sources. The color matches that holds up the best in these three lights is the best choice. - By lobbying manufacturers for ceramics with spectral curves as close as possible to those of natural tooth. - By having shade selected by some one else choice of two observers will often be the best match. - By testing over vision particularly color vision. - By shade selection using shade guides of the same material. - By limiting surface staining 11
  12. 12. Surface finish : When light strikes a smooth, flat, opaque, body, the reflected rays will all be parallel. If the body is rough, the reflected rays will no longer be parallel, a true scattering of these reflected light rays take place. When light strikes a smooth, flat, transparent body, the transmitted rays will all be parallel. If the body is rough, the transmitted rays are diverted in multiple directions or diffused. There may be surface fluctuations in the natural teeth by aging. So surface defects will have to be increased to reproduce a new, nontranslucent tooth in ceramics to reproduce an older tooth, the reverse applies. This problem is associated with the unpolished or worn glass ionomer and composite restorations. For example, as the resin matrix of composite material wears away, the restorations appears lighter and less chromatic (glayer). Surface thickness : The thickness of a restorations can affect its appearance. For example as the thickness of composite restoration placed against a white back ground increases the lightness and the chroma decreases. The most dramatic change observed is the increase in opacity as the thickness increases. Pigmentation : Esthetic effects are some times produced in a restoration by incorporating colored pigments in non-metallic materials such as resin composites, denture acrylics and dental ceramics. The colour observed when pigments are mixed results from the selective absorption by the pigments and reflection of certain colors. 12
  13. 13. Mercuric sulfide or vermilion is a red pigment because it absorbs all colors except red. The mixing of pigments therefore involves the process of substracting colors. For example a green color may be obtained by mixing pigment such as cadmium sulfide, which absorbs blue, violet, with ultramarine, red, orange and yellow. The only color reflected from such a mixture of pigments is green, which is color observed. Inorganic pigments rather than organic dyes are usually used because the pigments are more permanent and durable in their color qualities. When colors are combined with proper translucency, the restorative materials may be made to match closely the surrounding tooth structure or soft tissue. The color and translucency of human tissues shows a wide variation from patient to patient and from one tooth or area of the mouth to another. Fluorescence : Fluorescence is the emission of luminous energy by a material when a beam of light is share on it. The wavelength of the emitted light usually is longer than that of the exciting radiations. Typically, blue or U-V light produces fluorescence light that is in the visible range. Sound human teeth emit fluorescent light when excited by ultraviolet radiation (365 nm) the fluorescence being polychromatic with the greatest intensity in the blue region (450nm) of the spectrum. Some anterior restorative materials and dental porcelains are formulated with fluorescing agents (rare earths excluding uranium) to reproduce the natural appearance of tooth structure. Fluorescence makes a definite contribution to the brightness and vital appearance of tooth. For eg : a person with ceramic / composite restorations that lack a fluorescing agent appears to be missing teeth when viewed under a black light in a night club. Translucency : 13
  14. 14. Translucency is a property of substances that permits the passage of light but disperses the light so objects cannot be seen through the material. Translucency of teeth varies from one individual to the next. It can also be highly susceptible to changes with age. Dental enamel and dentin will likewise undergo a great many age related transformations. The enamel of a new tooth is not very translucent and dentin is very opaque. The enamel of an older tooth thins and grows more translucent or even transparent, the dentin becomes less opaque but more saturated. Sekiene et al (1975) describes three types of translucency – Type A : Little translucency : these are teeth giving no impression of transparency. The laboratory prescription form should state no transparency or barely translucent tooth. Type B : Translucency is found in incisal regions only, in the form of streaks. Type C : Translucency exists at incisal regions and proximal edges. It is often useful to record not just the extent of the translucent areas but also their hue. In view of wide range of possible shading, we use translucency rating scale ranging from 1 to 5 for the sake of simplicity with 1 representing low degree of translucency and 5 corresponding to highly transparent enamel. A photographic reference system is the best guide for conveying these essential data. To convey appearance and the extent of the translucent area, the dental practitioner lets the ceramic technician know that the tooth to be reproduced resembles photograph 5 or 6 say. The technician will have exact visual image of the color and extent of translucent area to be copied. Unfortunately, shade guides only offer standard translucency, generally at a lower level than natural teeth. They nerve give the right 14
  15. 15. information on the translucency of a tooth, which depends partly on the dental enamel and to a lesser extent on dentin. Some prefer the chromoscop (Ivoclar) guide at present, this offers 02 shades and a particular arrangement for shade selection. Ceramic color guides are most commonly manufactured from ceramic material differing from that used in powders. This increases the chances of occurrences of metamerism and hence errors in color matching. Shofer was the first company to offer a standard vita color guide made of same powders of ceramic material called crystal. Ivoclar’s chromoscop is offered with a full color guide system from same manufacturer. Transparency : Transparent materials allow the passage of light in such a manner that little distortion takes place and objects may be clearly seen through them. 15
  16. 16. Transparent materials such as glass may be coloured of they absorb certain wavelengths and transmit others. For example if a piece of glass absorbed all wavelengths except red, it would appear red by transmitted light. If a light beam does not contain red wavelengths glass would appear opaque because the remaining wavelengths would be absorbed. OPACITY : The color of an object is modified not only by the intensity and shade of pigment or coloring agent but also by translucency or opacity of the object. Opacity is the property of the materials that prevents the passage of light, when all colors of the spectrum from white light source such as sun light are reflected from an object with the same intensity as received, the objects appear white when all the spectrum colors are absorbed equally the object appears black. An opaque material may absorb some of the light and reflect the remainder. If for example red, orange, yellow, blue, violet are absorbed the material appears green in the reflected white light. (green not absorbed just reflected). 16
  17. 17. Opalescent effect : Teeth show opalescent. This opalescent effect is due to a particular type of light diffraction due presence of very fine and perfectly homogenous particles. In natural teeth, very fine particles occur, particularly in the enamel, in the form of the hydroxyapatite crystals, averaging 0.16µm long and 0.02- 0.04µm thick which is responsible for the opalescent effect. Teeth will show blue glints, especially at the incisal edges, with transmitted light, an orange- yellow shade will be observed however. A surface will reflect short wavelength (400nm) i.e. blue light due to fine particles, the other wavelengths 600-700nm of light spectrum will be absorbed. If the tissue composition alters as with heavily discolored (eg. tetracycline-stained), this opalescence may greatly diminish or even disappear, imparting some degree of opacity the teeth. This effect can now be recreated using modern ceramics such as Duceram-LFC (Ducera) lo fusing. To produce this effect artificially very five opaque particles with the refractive index differing from that of the ceramic paste should be mixed with the basic powder. Opalizers : All translucent materials dental ceramics as well as natural teeth contain so called opalizers. Opalizing materials most commonly take the form of fine or extra fine particles. The translucency created by these fine powders will depend on the amount, grain and composition of the opalizers. Dental enamel and incisal ceramic powders contain low amount of opalizing particles than natural dentin and dentin powders. 17
  18. 18. Opalizing particles produce a light scattering effect in tooth and dental ceramic which will vary in degree depending on their refractive index and the size and quantity of particles. The greatest the scattering, the more opaque will the material look, conversely, the less the scattering the more translucent will the material appear. The following opalizing substances may be used in ceramic powders titanium oxide, zirconium oxide, tin oxide. Counter-opalescence : This phenomenon is particularly noticeable on metal ceramic bridge. The incisal edge appears bluish where as proximal edges look dark and mainly orange-yellow, despite the use of opaline ceramics in these two areas. The explanation of counter opalescence is that light will be reflected because of opacity, and the transmitted light will give the tooth an orange shade. How to avoid counter-opalescent effect ? - By avoiding too shallow depth of ceramics - By using opaque dentins - By using darker opaque materials - By avoiding over-fired opaque materials Measurement of opacity / contrast ratio : The opacity of dental material can be determined instrumentally or by visual comparison with opal glass standards. Opacity is represented by a central ratio, which is the ratio between the day light apparent reflectance of a specimen of 1mm thick when backed by a black standard having a day light apparent reflectance of 70% relative to magnesium oxide. The contrast ratio (C0-70) for a resin composite should lie between the values of 0.55 and 0.70. The spectral reflectance curves of a composite resin backed by black and white standards may be noticed. The contrast ratio can also be calculated from optical constants. 18
  19. 19. Reflection, refraction and light transmission : When a light ray originating from an environment with refractive index 2, this results in a ray that is reflected in environment 1 and a ray undergoing refraction in environment 2. The angles of incidence and reflection will always be identical. However the angle of refraction will be proportional to the refractive indices of the materials crossed by the light rays. Under certain conditions all light is reflected by the surface and total reflection will take place where the angles of incidence exceed angle for refraction will be proportional to the refractive indices of the materials crossed by the light rays. Under certain conditions all light is reflected by the surface and total reflection will take place where the angle of incidence exceed angle for which all rays will be reflected. It is the critical angle. This explains why whitish areas appear tooth that is due to full reflection of the light rays. The same applies to the incisal edge where a very fine white edge accessionally appears breaking up the bluish look of the edge. Refractive index : The index of refraction (ξ) for any substance is the ratio of the velocity of light is vacuum (or air) to its velocity in the medium. When light enters a medium, it shows from its speed in air (300,000 km/sec) and may change direction. Eg : when a beam of light traveling in air strikes water surface at an oblique angle, the light rays are bent toward the normal. The normal is the line drawn perpendicular to the water surface at the point where the light contacts the water surface. And if the light is traveling through water and contacts water-air surface at an oblique angle, the beam of light is bent or refracted away from the normal. The refractive index is a characteristic property of the substance and is used extensively for identification. 19
  20. 20. One of the most important applications of refraction is the control of the refractive index of the dispersed and matrix phases in the materials such as resin composites and ceramics designed to have the translucent appearance of tooth tissue. a perfect match in the refractive indices results in a transparent solid, where as large differences result in opaque materials. Refractive index of various materials : Material Refractive index Feldspathic porcelain 1.504 Quartz 1.544 Synthetic hydroxyapatite 1.649 Tooth structure, enamel 1.655 Water 1.333 Optical constants : Esthetic dental materials such as ceramics, resin composites and human tooth structure are intensely light scattering and turbic materials. In a turbid material the intensity of incident light in diminished considerably when light passes through specimen. Kubelka Munk equations develop relations for monochromatic light between the reflection of an infinitely thick layer of a material and its absorption and scattering coefficients. Secondary optical constants ‘a’ and ‘b’ can be calculated as follows. where= RB is reflectance of dark backing (black standard) RW is reflectance of light backing (white standard) RB is reflectance of dark backing of specimen RW is reflectance of light backing of specimen b = (a2 – 1)½ 20 a = [R(B) – R(W) – RB + RW – R(B) R(W)RB + R(B) R(W) RW + R(B) RB RW – R(W) RB RW ] 2 [R(B) RW – R(W) RB ]
  21. 21. Scattering coefficient : The scattering co-efficient is the fraction of incident light flux lost by reversal of direction in an elementary layer. The scattering co-efficient, ‘s’ for a unit thickness of a material is defined as follows : Where S = scattering coefficient Ar ctgh = inverse hyperbolic cotangent R = light reflectance of specimen Rg = light reflectance of specimen with the backing A,b = calculated optical constants X = thickness of specimen The scattering coefficient varies with the wavelength of the incident light and the nature of the colorant layer. Composites with larger values of scattering coefficient are more opaque. Absorption coefficient : The absorption coefficient is the fraction of incident light flux lost by absorption in an elementary layer. The absorption coefficient ‘k’ for a unit thickness of a material is defined as follows : K = S (a –1) mm-1 Where K = absorption coefficient S = scattering coefficient a = optical constant The absorption coefficient also varies with the wavelength of the incident light and the nature of the colorant layer for several shades of resin 21 S = Ar ctgh 1 bx 1 – a (R +Rg) + RRg b (R-Rg) mm-1
  22. 22. composites. Composites with larger values of absorption coefficient are more opaque and more intensly colored. Light reflectivity : The light reflectivity RI is the light reflectance of a material of infinite thickness and is defined as follows : RI = a – b This property also varies with the wavelength of the incident light and nature of the colorant layer. Contrast ratio : 1) Reflectances :once a, b, and s are obtained, the light reflectance (R) for a specimen of any thickness (x) in contact with a backing of any reflectance (Rg) can be calculated by : ii) Opacity : Then opacity can be calculated from contrast ratio (C) : C = R0 /R When C = contrast ratio, R = reflectance R0 = computed light reflectance of specimen with black backing THERMAL PROPERTIES : Boiling and melting points : In laboratories, they can be used to help identify chemicals. Mixtures often have a melting or boiling range rather than a specific melting or boiling point. Some metals melt at high temperatures and are different to cast when an object melts or boils, the atomic bonds between atoms or molecules are broken by the thermal energy of the of the material. Some 22 R = [1 – Rg (a-b etgh bsx)] (a +b ctgh bsx – Rg)
  23. 23. materials like wood and cookie dough do not boil or melt but decomposed if heated sufficiently. Vapour pressure : Vapour pressure is the measure of liquids tendency to evaporate. As the temperature of liquid is increased and the boiling point is approached, the vapour pressure increases. The increased thermal energy allows more atoms or molecules to escape from the liquid and become gas. Materials with high vapour pressure at room temperature tend to evaporate readily. They are useful as solvents in the application of thin layers of viscous liquids (such as glue or point). In dentistry materials such as copal varnish primers are painted on the base material. The solvents evaporates, leaving behind a thin film of the desired substance. Methyl methacrylate, a component of dental acrylic resins (plastics) has a high vapour pressure and can evaporate easily when a denture is processed. Porosity may occur during processing, resulting in a weak denture. Denture processing techniques are designed to minimize porosity and the loss of methyl methacrylate. Heat flow through a material : Heat transfer through solid substances most commonly occurs by means of conduction. The conduction of heat through the interactions of crystal lattice vibrations and by the motion of electrons and their interactions with atoms. metals tend to be good conductors of heat, and this property must taken into considerations when placing metallic restorations. Dentin is a thermal insulator (poor conductor of heat) thus, when a sufficient thickness of dentin is present, the patient teeth no sensitivity to heat and cold through a metallic restorations. However when only a thin layer of dentin is present some thermal protection must be provided for the pulp. A 23
  24. 24. the rate at which heat flows through a material is expressed as thermal conductivity or thermal diffusivity. Thermal conductivity : Thermal conductivity (k) is a measure of the speed at which heat travels (in calories per second) through a given thickness of material (1cm) when one side of the material is maintained at a constant temperature that is 10 C higher than the other side. Thermal conductivity is expressed in units of cal cm/cm2 sec 0 c. According to the second law of thermodynamics heat flows from points of higher temperature to points of lower temperatures. If significant porosity exists in the structure, the area available for conduction is reduced and the rate of heat flow is reduced. Materials that have a high thermal conductivity are called conductors, where as materials of how thermal conductivity are called insulators. Compared with a resin-based composites that has a low thermal conductivity, heat is transferred more rapidly away from the tooth. When cold water contacts a metallic restorations because of its higher thermal conductivity. The increased conductivity of the metal compared with that of the resin composite induces greater pulpal sensitivity, which is experienced as a negligible, mild, moderate, or extreme discomfort, depending on previous tooth trauma and the pain response of the patient. Dental cements have a thermal conductivity similar to those of dentin and enamel. The thickness of the cement base and its thermal transfer to the pulp and the temperature differences across an insulator depends on the extent of the heating or cooling period and the magnitude of the temperature difference. Specific heat (Cp) : The specific heat (Cp) of a substance is the quality of heat required to raise the temperature of one gram of substance 10 C. 24
  25. 25. Water is usually chooses as standard substance and 1 gram as standard mass. The heat required to raise the temperature of 1gm of water from 150 C to 160 C is 1 calorie. Specific heat also depends on mass. For example 100gm of water requires more calories than 50gm of water to raise the temperature 10 C. Specific heat of liquids is higher than those of solids. Some metals have specific heat of less than 10% that of water. During the melting and casting process the specific heat of the metal or alloy is important because of total amount of heat that must be applied to the mass to raise the temperature to the melting point. The specific heat of both gold and metal used in gold in low, so prolonged heating is unnecessary. Thermal diffusivity : The value of thermal diffusivity of a material controls the time rate of temperature changes as heat passes through the material. The thermal diffusivity describes the rate at which a body with a nonuniform temperature approaches equilibrium. The thermal diffusivity ∆ is a measure of transient heat-flow and is defined as the thermal conductivity ‘k’ divided by the product of the specific heat ‘Cp’ times the density ‘p’. The units of thermal diffusivity are mm2 /sec. Eg : for a gold inlay or crown or a dental amalgam, the low specific heat combined with the high thermal conductivity creates a thermal shock more readily than normal tooth. Values of thermal diffusivity vary with composition of particular restorative material. For example, the thermal diffusivity of the zinc polyacrylate cement increases from 0.14 to 0.51 mm2 /sec as powder / liquid ratio increases from 0.5 to 5.0. 25 ∆ = K Cp xp
  26. 26. Coefficient of thermal expansion : If a balloon filled with room temperature air is brought outside on a cold January day in West Virginia, the balloon shrinks. This is visible example of that materials shrink or contract when cooled. When a material is heated, the extra energy absorbed causes the atoms or molecules to vibrate with an increased amplitude and expands. The most common way of measuring this expansion is by taking a length of material, heating it to certain temperature and then measuring the resultant change in length. It is the change in length per unit of the original length of the material when its temperature is raised 10 K. The units of α are typically expressed in units of µm/m0 K or ppm /0 K. In an ideal restorative material the coefficient of expansion would be identical to that of tooth tissues. if this is not the case the thermal mismatch can give rise to marginal gap formation and breakdown of adhesive bonds. Some materials such as sinus require only a small amount of heat energy to raise their temperature and readily expand or contract. Composite material have a low thermal diffusivity and provides some protection against thermal stimuli, as more heat energy is required to cause raise in the temperature and the corresponding expansion. The high co-efficient of thermal expansion of pattern waxes is an important factor in construction of properly fitting restorations. For example an accurate wax pattern than fits a prepared tooth contacts when it is removed from the tooth or die in the warmer area and then stored in a cooler 26 (1 final – 1 original) Loriginal x (0 K final – 0 C original) = α
  27. 27. area. This dimensional change in transferred to a cast restoration that is made from the cost-wax process. Thermal stresses produced from thermal expansion or contraction difference are important in the production of metal ceramic restorations. If we consider a porcelain veneer that is fired to a metal substrate if may contract to a greater extent than the metal during cooling and induce tensile stresses in the porcelain that may cause immediate or delayed crack formation. Although these stresses cannot be eliminated completely, they can be reduced appreciably by selection of materials whose expansion or contraction coefficients are matched fairly closely within 4%. Viscosity : Definition : viscosity is the resistance of liquid to flow. When placing materials, handling characteristics are important. Some materials should flow easily and wet the surface, while other materials need to be more like putty, which can be adapted or formed into a desired shape. The viscosity is the inability of the material to flow. Thick or viscous liquids flow poorly, while thin liquids flow easily. Viscosity is the temperature dependent property. The success or failure of a govern material may be an dependent on its properties in the liquid state as it on its properties as a solid. For example cements and impression materials undergo a liquid to solid transformation on the mouth. Gypsum products used in fabrication of models and dies are transformed from slurries into solids. Amorphous materials such as waxes and resins appear solid but actually are super cooled liquids that can flow under small stresses. The ways in which these materials flow or deform when subjected to stress are important to their use in dentistry. Although a liquid at rest cannot support a shear stress (shear force per unit area), most liquids, when placed in motion, resist imposed forces that cause them to move. The resistance to fluid flow is controlled by the 27
  28. 28. internal frictional forces of the liquid. Thus viscosity is the measure of the consistency of the fluid and its inability to flow. Dental materials have different viscosities depending on the preparation for their intended clinical applications. Zinc polycorboxylate and resin cements are more viscous when compared with zinc phosphate cement when properly mixed an luting cements. For example a liquid occupies the space between two metal plates. The lower plate is fixed. The upper plate is being moved to the right at a velocity ‘V’. A force ‘F’ is required to overcome the frictional resistance (viscosity) to fluid flow. Stress is the force per unit area that develops within a structure when an external force is applied. This stress causes a deformation or strain to develop. Strain is calculated as change in length divided by the initial reference length. If the plates have area ‘A’ in contact with a liquid, a shear stress can be defined as T = F/A The shear strain or rate of deformation is ______ Where v – velocity d – distance of top plate relative to fixed lower plate. As the force increases, V increases and a curve can be obtained for force versus velocity analogues to the load versus displacement curves that are derived from static measurements on solids. The slope of the curve is equal to the viscosity so that the exact scientific definition of viscosity ‘n’ is given by ___________ 28 V d ε = n = Shear stress Shear rate = F/A V/d
  29. 29. The rheological behaviour of four types of fluids may be shown on graph. In the ideal fluid i.e. Newtonian fluid stress is proportional to the strain and has a constant viscosity and exhibits a constant slope. In pseudoplastic behaviour the viscosity decreases with increasing strain rate until it reaches a nearly constant value. Liquids that show opposite tendency are dilatant that become more rigid as the rate of deformation increases. Some materials behave like a rigid body until some minimum value of shear stress is reached. They exhibit rigid behaviour initially and then attain constant viscosity are referred as plastic. The viscosity of most liquids decreases rapidly with increasing temperature. viscosity may also depend on previous deformation of the liquid. A liquid becomes less viscous and more fluid under repeated applications of pressure is referred as thixotropic. Eg : - Dental prophylaxis paste - Plaster of paris - Resin cements - Some impression materials The thixotropic nature of impression material is beneficial as the materials doesn’t flow out of a mandibular impression tray until placed over dental tissues. Velocity is measured in units of Mpa per second or centipose (Cp) Eg : Pure water at 200 C – 1.0 Cp Molasses – 300,000 Cp Agar at 450 C – 281,000 Cp Light body polysulfide at 360 C – 109,000 Cp Heavy body polysulpic – 1,360,000 Cp 29
  30. 30. Structural and stress relaxation : After a substance has been permanently deformed (plastic deformation) there are trapped internal stresses. For example, in a crystalline substance such as metal, the atoms in the crystal structure are displaced and the system is not in equilibrium. The permanent deformations situations are unstable. The displaced atoms are not in equilibrium positions. Through a solid-state diffusion process driven by their thermal energy, the atoms can move back slowly to their equilibrium positions. The result is a change in the shape or contour of the solid as the atoms or molecules change positions. The material distorts. The stress relaxation distorts the elastomeric impressions. The rate of relaxation increases with an increase in temperature. For example if a wire is bent, it may tend to strighten out of it is heated to a high temperature. but at room temperatures any such relaxation caused by rearrangement of metal atoms may negligible. Many other noncrystaline dental materials such as waxes, lesions and gels when manipulated and cooled can these undergo relaxations i.e. distortion at an elevated temperature. This may result in an inaccurate for of dental appliances. Creep and Flow : If a metal is held at a temperature near its melting point and is subjected to a constant applied stress, the resulting strain will increase over time. Creep is defined as the time dependent plastic strain of a material under a static load or constant stress. Metal creep usually occurs as the temperature increases to within a few hundred degrees of the wetting large. Metals used in dentistry for cast restorations or substrates for porcelains veneers have melting points that are much higher than mouth temperatures and they are not susceptible to creep deformation intraorally. 30
  31. 31. Dental amalgams contain 42-52 wt% Hg and began melting at temperatures only slightly above room temperature. Because of its bow 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. Because creep produces containing plastic deformation, the process can be destructive to a dental prosthesis. Creep may lead to an unacceptable fit of fixed partial denture frame works when a cost alloy with poor creep resistance is veneered with porcelain at relatively high temperatures (10000 C). The term “Flow” rather than creep describes the theology of amorphous materials such as waxes. The flow of wax is a measure of its potential to deform under a small static load, or with its own mass. Creep or flow can be measured under any type of stress, compression employed in testing of dental materials. A cylinder of prescribed dimensions is subjected to a given compressive stress for a specified time and temperature. The creep or flow is measured as the percentage decrease in length that occurs under these testing conditions. 31