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Abrasive and polishing agents Abrasive and polishing agents Document Transcript

  • INTRODUCTION Before any dental restoration or appliance is placed in the mouth is should be highly polished. Not only a rough surface on restoration, denture, orthodontic appliance and so forth uncomfortable, but also food and other debris cling to it. Such a restoration or appliance becomes dirty and in some cases tarnish or corrosion may occur. Rough surface are likely to occur unavoidably during the fabrication of an appliance. The finished restoration has three benefits the oral health, function and esthetics, the well contoured and polished restoration promotes oral health by resisting the accumulation of food debris and pathologic bacteria. This is accomplished through a reduction in total surface area and reduced roughness of restoration surface. Another surfaces are easer to maintain in a hygienic state when preventive home care is practiced. Oral function is enhanced with a well polished restoration, because food glides more freely over occlusal and embrasure surface during mastication. 1
  • Smooth restoration contacts minimize wear rates on opposing and adjacent teeth. This is particularly true for restorative materials such as ceramics that contain phases that are harder than tooth enamel and dentin. Rough material surfaces lead to the development of high contact stresses that can cause the loss of functional and stabilizing contacts between teeth. Finally, esthetics demands may require the dentist to handle highly visible surfaces of restorations differently than those that are not accessible. Although a high mirror like polish is preferred for previously mentioned reasons this type of surface may not be aesthetically compatible with adjacent teeth in highly visible areas such as labial surfaces of maxillary anterior teeth. Fortunately, these surfaces are not subject to high contact stresses, and they are easily accessible for cleaning. Finishing, cutting, grinding and polishing process: The finishing process usually removes material such that: a) The surface finishes and imperfections are removed. The material is shaped to an ideal form and the outermost surface of the material is developed to a desired state. 2
  • The particles of substrate material are recovered the action of hard material that comes into frictional contact with the substrate. In this process the tensile and shear stress are induced within both the substrate and abrasive instrument but for the abrasive to cut the substrate, the stresses induced within the abrasive should not exceed the stresses induced into the substrate. If the stress in the instrument exceeds which developed in the substrate, than in such cases the instrument fails to cut, grind or polish and the blade edges will become dull and abrasive will fracture or tear away from their binder. A cutting operation usually refers to the use of bladed instrument or use of any instrument in a blade like fashion. The substrate may divided into large separate pieces or may sustain large deep grooves by a cutting operation. The cutting blades are regularly arranged. A separating wheel can be used in a blade like a fashion. A separating wheel does not contain individual blades but its shape allows it to be used in a rotating blade like fashion, to slice through the sprue and die stone materials. A grinding operation remove small particles of a substrate through the action of bounded or coated abrasive instrument. Each particle may contain several sharp points that run along the substrate surface and remove particles of the material. 3
  • These grinding instruments contain many randomly arranged abrasive particles, because these particles are randomly arranged innumerable unidirectional scratches are produced on material surface. Cutting and grinding are both considered to be predominantly unidirectional in their course of action. A polishing operation is considered at the most refined of finishing process, produces the finest of the particles. It acts on an extremely thin region of the substrate surface. The polishing is considered to be multidirectional in its course of action. The ideally polished surface should be atomically smooth with no surface imperfections. This condition is virtually impossible to achieve because most restorative materials are brittle and easily acquire surface flaws during cutting and grinding procedures. Types of abrasives: Abrasives can be broadly divided into natural abrasives and manufactured abrasives. Natural abrasives include : Arkansas stone, chalk, corundum, diamond, Emery, garnet, pumice, quartz sand, Tripoli and zirconium silicate cuttle and kieselguhr are derived from the remnants of living organisms. 4
  • Manufactured abrasives are synthesized materials that are generally preferred because and their predictable physical properties. Silicon, carbide, aluminum oxide, synthetic diamond, rough and tin oxide are example of manufactured abrasives. Arkansas stone: Arkansas stone is semi translucent, light gray, siliceous sedimentary rock mixed in Arkansas. It contains microcrystalline quartz and is dense hard and uniformly textured. Small pieces of this material are attached to metal shanks and trued to various shapes for fine grinding of tooth enamel and metal alloys. Chalks : One of the mineral forms of calcite is called chalk. Chalk is white abrasive composed of calcium carbonate. It is used as a mild abrasive paste to polish tooth enamel gold foil, amalgam and plastic materials. Corundum : This mineral form of aluminum oxide is usually white, its physical properties are inferior to those of manufactured aluminium oxide, which has largely replaced corundum in dental applications, corundum is used primarily for grinding metal alloys and is available as a bonded abrasive is several shapes. It is most commonly used in an instrument knows as a white stone. Diamond : Diamond is transparent, colorless mineral composed of carbon. It is the hardest substance known. Diamond is called a super abrasive, 5
  • because and its ability to abrade any other diamond abrasives are supplied in several forms, including bonded abrasive rotary instruments, flexible metal backed abrasive strips, and diamond polishing pastes. They are used in ceramic and resin-based composite materials. Emery : Thus abrasive is a grayish black corundum that is prepared in a fine ground forms. Emery is used predominantly in the form of coated abrasive disks and is available in a variety of grit size. It may be used for finishing metal alloys or plastic material. Garnet : The term garnet includes a number of different minerals that possess similar physical properties and crystalline forms. These minerals are the silicates of aluminium, cobalt, iron, magnesium and manganese. The garnet abrasive used in dentistry is usually dark red. Garnet is extremely hard and when fractured during the grinding operation, forms sharp, chisel-shaped plates, making garnet a highly effective abrasive. Garnet is available on coated disks and arbor bands. It is used in grinding metal alloys and plastic materials. Pumice : Volcanic activity produces this light gray, highly siliceous material. It is used mainly in grit form but can be found in some rubber- bonded abrasives. Both forms are used on plastic materials. Flour of pumice is an extremely fine grained volcanic rock derivative from Italy and 6
  • is used in polishing tooth enamel, gold foil, dental amalgam, and acrylic resins. Quartz : The most commonly used form of quartz is very hard, colorless and transparent. It is the most abundant and widespread of minerals. Quartz crystalline particles are pulverized to form sharp, angular particles that are useful in making coated abrasive disks. Quartz abrasives are used mostly to finish metal alloys and may be used to grind dental enamel. Sand : Sand is a mixture of small mineral particles predominantly composed of silica. The particles represent a mixture of colors, making sand abrasives distinct in appearance. Sand particles have a rounded to angular shape. They are applied under the air pressure to remove refractory investment materials from base metal alloy castings. They are also coated onto paper disks for grinding of metal alloys and plastic materials. Tripoli : This abrasive is derived from a light weight, friable siliceous sedimentary. It can be white, gray, pink, red or yellow. The gray and red type are most frequently used in dentistry. The rock is ground into very fine particles and is formed with soft binders into very fine particles and is formed with soft binders into bars of polishing compound. It is used for polishing metal alloys and some plastic materials. Zirconium silicate : Zircon or zirconium silicates is supplied as an off- white mineral. This material is ground to various particle sizes and is used 7
  • to make coated abrasives disks and strips. It is frequently used as a component of dental prophylaxis pastes. Cuttle : Cuttle fish, cuttle bone, or cuttle are the common names for this abrasive. It is a white, calcareous powder made from the pulverized internal shell of a Mediterranean marine mollusk of the genus sepia. It is available as a coated abrasive and is useful for delicate abrasion operations such as polishing of metal margins and dental amalgam restorations. Kieselguhr : This material is composed of the siliceous remains of minute aquatic plants known as diatoms. The coarser form is called diatomaccous earth, which is used as a filler in many dental materials, such as the hydrocolloid impression materials. It is excellent mild abrasive. The risk of respiratory silicosis caused by chronic exposures to airborne particles of this material is significant and appropriate precautions should always be taken. Silicon carbide : This extremely hard abrasive was the first of the synthetic abrasives to be made. Both green and blue black types are produced and have equivalent physical properties. The green form is often preferred because substrates are more visible against the green color. Silicon carbide is extremely hard and brittle. Particles are sharp and break to form new sharp particles. This results in highly efficient cutting of a wide variety of materials, including metal alloys ceramics, and plastic 8
  • materials. Silicon carbide is available as an abrasive in coated disks and vitreous bonded and rubber bonded instruments. Aluminium oxide : Fused aluminium oxide was the second synthetic abrasive to be developed after silicon carbide. Synthetic aluminium oxide (alumina) is made as a white powder. It can be much harder than corundum (neutral alumina) because of its purity. Alumina can be processed with different properties by slight alteration of the reactants in the manufacturing process. Several grain sizes are available and alumina has largely replaced emery for several abrasive uses. Aluminium oxide is widely used in dentistry. It is used to make bonded abrasives, coated abrasives, and air-propelled grit abrasives. White stones are made of sintered aluminium oxide and are popular for adjusting dental enamel and for finishing both metal alloys and ceramic material. Pink and ruby variations of aluminium oxide abrasives are made by adding chromium compound to the original melt. Those variations are sold in a vitreous bonded form as non-contaminating mounted stones for the preparation of metal-ceramic alloys to receive porcelain. Any remnants of these abrasives should not interfere with porcelain bonding to the metal alloy. A review by Yamamoto (1985) suggested that carbide burs are the most effective instrument for finishing this type of alloy because they do not contaminates the metal surface with entrapped abrasive particles. 9
  • Synthetic diamond abrasives : Manufactured diamond is used almost exclusively as an abrasive and is produced at five times the level of natural diamond abrasive. This abrasive is used in the manufacture of diamond saws, wheels, and burs. Block with embedded diamond particles are used to true other types of bonded abrasives. Diamond polishing pastes are also produced from particles smaller than 5mm in diameter and are useful in polishing ceramic materials. Synthetic diamond abrasive are used primarily on tooth structure, ceramic materials and resin-based composite materials. Rouge : Iron oxide is the fine, red abrasive component of rouge. It is blended, like Tripoli, with various soft binders into a cake form. It is used to polish high noble metal alloys. Tin oxide : This extremely fine abrasive is used extensively as a polishing agent for polishing teeth and metallic restorations in the mouth. It is mixed with water, alcohol, or glycerin to form a mildly abrasive paste. Factors influencing the efficiency of abrasives: 1. The hardness of the abrasive particles. To be effective abrasive and polishing agents should possess certain characteristic. First of all the cutting or abrading particle must be able to scratch or cut grooves on the surface of the material to be treated. It must therefore be harder than the surface against which it is to be used. It must also be sufficiently hard to retain an effective cutting edge. 10
  • The harder the material within the particle limits the longer the agent remains effective as an abrading agent. 2. The shape of the abrasive particles. Particles with sharp edges are more efficient than those with abture angle. 3. The particle size of the abrasive the particle size must be enough to remove sufficient amount of mateials. 4. Mechanical properties of the particles. Brittleness can be an advantage. If the material breaks in the process, then it should be able from another sharp edge. 5. The pressure applied to the abrasive. The pressure should be light, because great pressure may fracture an abrasive cutting instrument and also causes frictional heat. 6. Properties of the material that is being abraded. Brittle material can be abraded rapidly where as the mollable and ductile material (e.g. pure gold), will flow and instead of being removed by abrasives. 7. Rate of movement. Slower the movement of the abrasive instrument deeper will be the scratches. The ideal abrading and polishing speeds respectively are for acrylic resin, it is 2000 and 3000 rpm for gold alloy – 1500 rpm to 3000 rpm. 11
  • During abrading polymeric materials excessive heat must be avoided as it can cause stress relief and warpage. Finally, the range of particle size as well as distribution and position of the abrading particles on the holding surface plays an important role in effectiveness of an abrasive agent. For example if the particles are to close to each other and the debris from the cutting can’t escape and becomes entrapped between the abrasive particles and an it loses its effectiveness. ABRASIVE AND EROSIVE WEAR Abrasive wear, wear is a material removal process that can occur whenever surfaces slide against each other. The process of finishing a restoration involves abrasive wear through the use of hard particles. The dentistry the outermost particles or surface material of an abrading instrument is referred to as the abrasive. The material being finished is called as substrate. Substrate wear is further divided into process of two body and three body wear. The body occurs when abrasive particles are firmly bonded to the surface and abrasive instrument and no other abrasive particles are used. E.g. diamond bur. 12
  • Three body wear occurs when abrasive particles are free to translate and rotate between two surfaces. E.g.: Dental prophylaxis, which involves the use of a rotating rubber cup and abrasive paste on tooth or material surface. These two process are not mutually exclusive. The diamond particles may debond from a diamond bur and cause three body wear. Likewise some abrasive particles in the abrasive paste can become trapped in the surface of a rubber cup and cause two body wear. Lubricants are often used to minimize the risk for these unintestional shifts from the body wear and vice versa. Erosive wear : Erosive wear is caused by hard particles impacting a substrate surface, carried either by a sit of ream of air or liquid. Most dental laboratories have air driven grit blasting units that employ hard particle erosion to remove surface material. A distinction must be made between this type of erosion and chemical erosion, which involves chemicals such as acids and alkalis instead of hard particles to remove substrate material. Acid etching is a familiar term that is used, more commonly chemical erosion, chemical erosion is not used as a method of finishing dental materials it is used primarily to prepare surfaces to enhance bounding or coating. 13
  • Hardness of abrasive: A stated previously the inherent strength of cutting blades or abrasive particles on a dental instrument must be great enough is remove particles of substrate material without becoming dull or fracturing too rapidly. The strength of an abrasive is often measured by the hardness of its particles or surface material to plastic deformation. The first hardness was published by Eriedrich Mohs- A German microbiologist in 1820 the ranked 10 minerals by their relative earth resistance, mineral received a score of 1 and most scratch resistance. Mineral received the score of 10 (diamond). The Mohs scale was later exposed in 1930s to accommodate several new abrasive materials that received scores in a 9 to 10 range. Knoop and Vickers hardness tests are based on indentation methods that quantity the hardness of materials. The tip of a knoop diamond indents had an elongated pyramid shape. Where as the Vickers diamond indenter has an equilateral pyramid design. Both tests involve the application and indenter to a test surface under a known load (usually 100W). The depth of surface penetration is reported as hardness in units of force per unit area. 14
  • Hardness values for dental materials and abrasives Material Mohs hardness Knoop Hardness (kg/mm2 ) Vickers hardness (Kg/mm2 ) Diamond Silicon carbide Aluminium oxide Porcelain Pumice Denture resins 10 9-10 9 6-7 6-7 - 7,000-10000 25,000 21,000 560 460 20 - 430 - - Abrasive Instrument Design Abrasive grits – abrasive grits are derived from materials that have been crushed and passed through a series of mesh screens to obtain different 15
  • particle size ranges. Dental abrasive grits are classified as course, medium and superfine according to particle size ranges. In finishing process rate of materials is not the only important factor. The surface finish obtained with each abrasive is as important. If too hard abrasive is used, or the grain size is too course for use on a given material, deep acratches result in substrate that can’t be removed easily in subsequent finishing process. Additionally. If an abrasive does not have the proper particle shape of does not break down in a manner that creates or exposure new sharp edges particle, it will tend to gauge the substrate. 1. Bonded abrasive : Bonded abrasive consists of abrasive particles that are incorporated through a binder to form grinding tools such as points, wheels, separating discs and wide variety and other abrasive shapes. Particles are bonded by four general methods. 1. Sintering. 2. Vitreous bonding (glass or ceramic.) 3. Resinold bonding (usually phenolic resin). 4. Rubber bonding (usually silicone rubber). Sintered abrasives are the strongest type because the abrasive particles are fused. The vitreous bonded abrasives are mixed a glass or ceramic matrix material, cold pressed and then are heated to cure the resin. 16
  • Rubber bonded abrasives are made in a manner similar to that for resin bonded abrasives. The type of bounding method employed for the abrasive greatly affects the grinding behaviour of the tool on the substrate, bounded abrasives that lead to disintegrate rapidly against a substrate are too weak and result in increased abrasive costs because the reduced instrument life. Those that tend to degrade too slowly clog with grinding debris and result in the loss of abrasive efficiency increased heat generation and increased finishing time. An ideal binder holds the abrasive particles in tool sufficiently long enough to cut, grind, or polish the substrate, yet release the either before its cutting efficiency is lost or before heat build up causes thermal damage to the substrate. Besides are specially formulated for substrate specific applications. A bounded abrasive instrument should always be treed and dressed before its use. Truing is a procedure through which the abrasive instrument is run against a harder abrasive block until the abrasive instrument rotates in the handpiece without accealricty or ran out when placed on substrate. The dressing procedure like is used to shape the instrument but accomplishes two different purposes. First dressing procedure reduces the instrument to its correct working size and shape. Second it is used to 17
  • remove clogged debris from the abrasive instrument to restore grinding efficiency during the finishing operation. The clogging of the abrasive instrument with debris is called ‘abrasive blinding’. Abrasive binding occurs when the debris generated from grinding or polishing occludes the small spaces between the abrasive particles on the tool and reduces the depth that particles can penetrate into the substrate. As a result, abrasive efficiency is lost and greater heat is generated. A blinded abrasive appears to have a coating of the substrate material on its surface. Frequent dressing of the abrasive instrument during the finishing operation on truing instrument, maintain the efficiency of the abrasive in removing the substrate material. Blinders for diamond abrasive are manufactured specifically to resist abrasive particle loss rather than no degrade at a certain point and release particles. One reason for this is that diamond is the hardest material known, so that diamond abrasive particles do not loose their cutting efficacy against substrate. There is no need for new abrasive particles to be exposed during the grinding process. Another reason is that diamond grits are expensive and must be used in limited quantities for instrument manufacture special bonding processes have been designed to allow for extended instrument life by keeping the 18
  • abrasive particles firmly bound to the instrument shank yet with maximum particle exposure. Diamond particles are bonded to metal wheel and to bur with special heat resistant resins such as polymides. The supercourse plated with a refractory metal film such as nickel titanium nitride coatings are used as an additional layer on some of the recent diamond abrasive instruments to further extend their longevity. Finishing diamond used for resin based composites contain diamond particles 40mm or less in diameter, and many are not nickel plated. Therefore they are highly susceptible to abuse and should always be used with light force and copious water spray to preserve the very fine diamond coatings. Coated abrasive disks and strips Coated abrasives are fabricated by securing abrasive particles to a flexible backing material (heavy weight proper or mylar) with a suitable adhesive material. These abrasives typically are supplied as disks and finishing strips disks are available in different diameter and with thin and very thin backing thickness. Finishing and polishing for cast restoration Finishing and polishing for cast restoration are finished and polished using rotary instruments. The speed of the instruments (revolutions) differs 19
  • form one alloy to another, generally increasing, going from Class I to Class IV alloys. This should be done on the die. According to the following sequences: 1. Gross finishing This should remove roughness impacted on the casting from the investment mold walls, eliminate surface discolouration and eliminate fine excess and obliterate minute defects. This step is always done using abrasive stones and discs, ranging from sand and silicone carbide to alumina, until no surface discrepancies are detected. 2. Removal of scratches and irregularities This will eliminate undetected fine scratches by removing the alloy substance between these scratches or locally melting the alloy at the surface, obliterating these irregularities. Also, the conversion stage of the finishing process is initiated. This step is done using wire metal brushes, usually brass or steel, to be followed by rubber wheels and cones. There is a large amount of heat produced in this stage, and it may be a factor in fulfilling is objective. The alloy surface should have a satin like surface by the end of this step. 20
  • 3. Conversion Conversion will create the most biologically compatible surface. This step is done using a rubber wheel or cone, followed by brushes carrying extremely fine abrasives, such as Tripoli or specifically compounded abrasive pastes. The alloy surface should be shiny by the end of this stage. 4. This creates a reflective, non-adhesive surface. It is done using rouge on a felt cone or wheel. The alloy surface should be extremely shiny by the end of this stage. The entire oral surface of cast alloys should be finished and polished. FINISHING AND POLISHING THE PORCELAIN JACKET AND CERAMIC CROWN Porcelain adjustment kits are used for contour reduction of porcelain restorations. These kits have a variety of stones and polishing wheels of different abrasiveness. Since, there is some vibration produced as the rotary instrument abrades from the surface, the porcelain crown should be adjusted while being held between the thumb and fingers, not on the die. A porcelain surface adjusted with disks of stones has had the surface glaze removed. The resultant surface is rough and dull in appearance. Restoring the polished porcelain surface is accomplished by employing one of a variety of polishing sequences. 21
  • There may be a perforation for a polished porcelain surfaces, rather than a highly glazed surface. Polished porcelain has a thin like surface than in some cases simulates the surface of natural tooth structure. A highly glazed porcelain surface at times will appear to reflective. In some patients whose natural tooth surfaces are highly reflective, a glazed surface is, naturally, indicated. The surface finish is a matter of careful evaluation for each patient. From the standpoint of soft tissue compatibility, the porcelain surfaces in the gingival crevices are best allowed to retain the natural glaze produced by baking. Porcelain surfaces that have been adjusted can be reglazed in a porcelain oven, polished, by using a series of porcelain polishing instruments, or polished using a series of rubber impregnated abrasives followed by a creamy mix of the leviated alumina and water. More recently specially compounded porcelain polishing pastes have gained popularity for final polishing of a porcelain surface. Reglazing the porcelain does require an oven, whereas the other polishing procedures can be accomplished with rotary instruments. In order to avoid fracture of a margin by vibration, the porcelain jacket is polished also while being held between the fingers rather tan the die. No matter which polishing procedure is used, it is important to follow the recommended sequence to obtain the optimal surface finish on the 22
  • porcelain. The levigated alumina is used with a lead-center unstiched muslin wheel on a lathe motor. A stitched wheel is undesirable since it tends to tear the crown out of the finger grasp. Further protection of the crown is secured by lining the polishing cubicle with dental towels. The crown and adjacent thumb surfaces are loaded with the creamy levigated alumina and the crown is pressed firmly against the rotating wheel, turning the crown constantly in many directions assures the production of a good luster. In the areas where cervical excess has been removed, a final polish is also achieved with the lead center wheel and creamy levigated alumina. The wheel should turn away from the cervical margin, not into it. This is sequence with the lead center wheel is completed in 3 to 4 minutes. Polishing material is removed by scrubbing the crown with a soft bristled tooth brush under a stream of water. All moisture is removed with compressed air. The crown is cleansed with alcohol, redried and the tested in the patient’s mouth. The patient is asked to wet the anterior teeth, including the crown, with saliva. An evaluation of the finish can now be made by the patient and the operator. If a high reflective luster is desired, the crown is returned to the laboratory and polished with a creamy mix of tin oxide. This tends to add even further highlight effect to the porcelain surface. 23
  • CONTENTS  Introduction  Finishing, Cutting, Grinding and Polishing Process  Factors Influencing the Efficiency of Abrasives  Hardness of Abrasive  Abrasive Instrument Design  Finishing and Polishing of Cast Restoration  Finishing and Polishing of the Porcelain Jacket and Ceramic Crown  References 24