Theoretical Notes² (1) - Dental Materials II - 2019-2020

Dental Materials II PRS 231 Prof. Dr. Heba Shalaby Head of Dental Materials Department With Great Acknowledgement to Prof. Dr. Dina Mostafa Prof. Dr. Hala A. Bahgat Dr. Ahmed Zaki Zidan Dental Materials Team...

Dental Materials II PRS 231 Prof. Dr. Heba Shalaby Head of Dental Materials Department With Great Acknowledgement to Prof. Dr. Dina Mostafa Prof. Dr. Hala A. Bahgat Dr. Ahmed Zaki Zidan Dental Materials Team Department of Dental Materials College of Dentistry October University for Modern Sciences and Arts (MSA) 2019 - 2020 1  ‫رؤﯾﺔ اﻟﻜﻠﯿﮫ‬ ‫ﻛﻠﯿﺔ طﺐ اﻷﺳﻨﺎن ﻓﻲ ﺟﺎﻣﻌﺔ أﻛﺘﻮﺑﺮ ﻟﻠﻌﻠﻮم اﻟﺤﺪﯾﺜﺔ واﻵداب راﺋﺪة ﻓﻲ ﻣﺠﺎل‬ ،‫ وﻣﺮﻛﺰا ﻟﻠﺘﻤﯿﺰ ﻓﻲ اﻟﺘﻌﻠﯿﻢ واﻟﺒﺤﺚ اﻟﻌﻠﻤﻲ وﺧﺪﻣﺔ اﻟﻤﺠﺘﻤﻊ‬،‫ﺻﺤﺔ اﻟﻔﻢ واﻷﺳﻨﺎن‬.ً ‫ﻣﺤﻠﯿﺎ ً وإﻗﻠﯿﻤﯿﺎ ً ودوﻟﯿﺎ‬ Faculty Vision Faculty of Dentistry at October University for Modern Sciences and Arts leading in the field of oral health and center of excellence in education, scientific research and community services; locally, regionally and internationally. ‫رﺳﺎﻟﺔ اﻟﻜﻠﯿﮫ‬ ‫ﻛـﻠﯿﺔ طـﺐ اﻷﺳـﻨﺎن ﺑـﺠﺎﻣـﻌﺔ أﻛـﺘﻮﺑـﺮ ﻟـﻠﻌﻠﻮم اﻟﺤـﺪﯾـﺜﺔ واﻵداب ﺗـﻘﺪم ﺑـﺮاﻣـﺞ ﺗـﻌﻠﯿﻤﯿﺔ‬ ‫ﻟـﻠﺒﻜﺎﻟـﻮرﯾـﻮس واﻟـﺪراﺳـﺎت اﻟـﻌﻠﯿﺎ ﺗـﺴﮭﻢ ﻓـﻲ إﻋـﺪاد أطـﺒﺎء أﺳـﻨﺎن ذوي ﻛـﻔﺎءة ﻋـﺎﻟـﯿﺔ‬ ،‫ وﺗـﻘﻮم ﺑـﺎﻟـﺒﺤﺚ اﻟـﻌﻠﻤﻲ‬،‫ﯾـﻠﺒﻮن ﻣـﺘﻄﻠﺒﺎت ﺳـﻮق اﻟـﻌﻤﻞ وﯾـﻄﺒﻘﻮن اﻟـﻤﻌﺎﯾـﯿﺮ اﻷﺧـﻼﻗـﯿﺔ‬ ‫وﺗﺸﺎرك ﻓﻲ ﺧﺪﻣﺔ اﻟﻤﺠﺘﻤﻊ وﺗﻨﻤﯿﺔ اﻟﺒﯿﺌﺔ‬ Faculty Mission Faculty of Dentistry October University for Modern Sciences and Arts offers educational programs for bachelor and graduate studies contribute to the preparation of dentists with high efficiency meet the requirements of the job market and apply ethical standards, and conduct scientific research and participate in community service. 2 Core values ‫اﻟﻘﯿﻢ اﻟﺤﺎﻛﻤﮫ‬ ‫ﻟﻠﻜﻠﯿﮫ‬ Honesty ‫✓ اﻷﻣﺎﻧﺔ‬ Excellency ‫✓ اﻟﺘﻤﯿﺰ‬ Credibility ‫اﻟﻤﺼﺪاﻗﯿﺔ‬ ✓ Innovation ‫✓ اﻻﺑﺘﻜﺎر‬ Integrity ‫✓ اﻟﻨﺰاھﺔ‬ Team Work ‫✓ اﻟﻌﻤﻞ اﻟﺠﻤﺎﻋﻲ‬ 3  CONTENTS Page Course Objectives 5 Chapter (1) Impression Materials. 7 ………………………….. Chapter (2) Model & Die Materials. 35 ………...….………….. Chapter (3) Investment Materials. ………..……………..... 46 Chapter (4) Casting Technology. ……….…..……………. 52 Chapter (5) Dental Casting Alloys. ……………………….. 60 References. ……….……………………..……… 75 4  OBJECTIVES of the COURSE Course Aim: This course is designed to teach students how to apply the laboratory materials used in dentistry. During the course the student will get knowledge on biocompatibility and tissue reactions to biomaterials. The students will also learn how to choose the suitable material according to their properties and limitations. The course includes both didactic lectures and laboratory sessions. Course Intended Learning Outcomes (ILOs) At the completion of this course successful candidates will be able to achieve the following learning outcomes: A) Knowledge & Understanding A.1- List the different impression materials that could be used for constructing complete and partial dentures. A.2- List the types and explain the properties of gypsum products used to construct primary and study casts A.3- Name the types and discuss the properties and uses of investment materials A.4- List the steps of casting procedures of metallic appliances A.5- Outline and describe types of dental casting alloys A.6- Identify and describe non metallic materials for denture construction B) Intellectual Skills: B.1- Categorize materials and compare their properties 5  B.2- Distinguish different types of gypsum and investment materials to combine the types with specific uses B.3- Correlate the resulting casting defects with manipulation errors C) Professional & Practical Skills C.1- Master different application and manipulation techniques. C.2- Manage proper selection of dental materials. C.3- Diagnose the possible resulting defects. C.4- Correlate the resulting defects with manipulative errors. 6  Chapter (1) IMPRESSION MATERIALS An impression material is a substance used for recording the form and dimensions of the oral tissues. The material is introduced into the mouth in a plastic condition to set against the oral tissues. It is then removed from the mouth. The set impression is a negative reproduction of the oral tissues. A positive reproduction is obtained by pouring a suitable model or die material into it. Impressions may be taken of a part or the entire dental arch. 7 Desirable qualities of impression materials: 1. Accuracy: An impression material should be accurate. This is very important since a restoration or an appliance fabricated in the laboratory cannot be more accurate than the impression from which the used model is prepared. 2. Biologically acceptable: An impression material should be non-toxic, non-irritant and of acceptable odour and taste. 3. It should not be affected by the oral fluids. 4. Easily manipulated, with minimal equipment. 5. It should have a suitable working time to allow for placement of the impression material in the tray and then insertion of the tray into the mouth before beginning of hardening. 6. It should have a suitable setting time to avoid fatigue to both operator and patient. Three minutes is considered reasonable time for setting. 7. It should accept addition and correction after taking the impression. 8. Readily disinfected without loss of accuracy. 9. It should have a good shelf life. Factors affecting accuracy of the impression materials: 1. The impression material should have high flow at the beginning, i.e. the material must be in a fluid or plastic state on insertion into the mouth, in order to have the ability to record fine details. 2. The impression material should be dimensionally accurate: The dimensional changes associated with its setting reaction should be negligible, i.e. should not expand, contract or warp during setting. 8 3. The impression material should be dimensionally stable: There should be negligible dimensional changes on storage of the impression in the dental laboratory before pouring the cast i.e. it should be dimensionally stable after removal from the mouth. 4. The impression material should be elastic on removal from the mouth so that undercuts can be recorded without distortion of the impression. 5. During removal from the mouth, it should be adherent to the tray. 6. The impression material should be compatible with the gypsum products: - Should not need a separating medium when poured (it would occupy a space or may affect the accuracy). - Should not affect the setting or surface qualities of the model e.g. should not react with the cast material or material which affects the surface of the model. Classifications of Impression Materials: Impression materials may be classified according to: I. The manner they harden into: 1. Impression materials hardening by chemical reaction: e.g. Plaster of Paris, impression pastes, alginates, rubber impression materials.....etc. 2. Impression materials softening and hardening by heat: Thermoplastic substances e.g. wax, impression compound and reversible hydrocolloids (though the latter is not strictly a thermoplastic material). II. Their use: 1. Impression materials used in complete denture prosthesis. 9  2. Impression materials used in partial denture prosthesis and for individual teeth (for inlays, crowns, bridges …etc). III. Their behavior after setting: Depending on their ability to regain their original shape after removal from the undercuts. 1. Inelastic impression materials e.g. wax, Plaster of Paris, ZnO/eugenol and impression compound. They are used only when no undercut exists. 2. Elastic impression materials e.g. hydrocolloids and rubber impression materials. They can be used when undercut exists. Impression Materials Non-elastic Impression Materials These are materials that lack elasticity after setting. When removed from the undercuts they either break like plaster or distort like wax, impression compound and zinc oxide-eugenol. 10  I. Plaster of Paris It was introduced in 1887, although it has great merits, it is rarely used now because it is rigid and fractures easily. Application: Used to make impression for edentulous patients. Composition: The material is supplied in the form of powder, to which water is added. The powder is composed of: 1. β-CaSO4.1/2H2O; not α and/or improved stone because: i. It is weaker: So it will break during removal from the undercut and thus avoids injury to the patient and the fractured part can be reassembled accurately. ii. It requires higher W/P ratio, which increases the flow at the beginning and thus records fine details. iii. The higher W/P ratio will reduce the exothermic heat evolved during taking of the impression. 2. Modifiers: Chemicals used to regulate the setting time as well as the setting expansion: The use of an accelerator; 4% potassium sulphate decreases the setting expansion and the setting time. If the setting time is unduly shortened, a retarder; 1% borax is used, which will further decrease the setting expansion. Properties of plaster impression materials: Accuracy of plaster impression materials: a. Since the mixed material is semi-fluid when inserted into the mouth it has excellent recording of the fine details. b. A plaster impression fractures on removal from undercuts and can be reassembled. c. When pouring a plaster impression in plaster or in stone, a separating medium should be used. 11  II. Impression Compound One of the oldest impression materials in dentistry is a thermoplastic material known as impression compound, modeling compound or modeling composition. Application: 1. Full jaws impression in edentulous mouth. 2. Impression trays in which a final impression is taken with another material. Composition: * It is supplied in the form of sheets, stick cylinder or cones of different sizes and colors. * There are different formulas for impression compounds. They are essentially a mixture of: - Thermoplastic material: e.g. Natural resins and waxes. They soften by heating and give the qualities of flow and cohesion. - Fillers: e.g. Talc, soapstone or diatomaceous earth. They add body and give a suitable working consistency. - Plasticizers: e.g. Stearic acid or stearin. These act as lubricants and together with fillers control the flow and consistency of the material. Classification: The available materials may be classified into: (ADA classification) 1. Type I (Lower fusing materials): For recording prosthetic impressions: Such as preliminary impressions of edentulous patients, supplied in sheets about 4-5 mm thick. 12 2. Type II (Higher fusing materials): They are used as tray compound in wash technique, where the aluminum stock tray containing the impression of the dental compound is used to hold a second impression material as zinc oxide- eugenol, which will record the final impression (corrective wash). 13 Setting mechanism: These materials are thermoplastic, they are used warm at 45°C and then cooled to mouth temperature 37°C at which they are fairly rigid. The setting mechanism is therefore reversible physical process rather than a chemical reaction. Hard Heating Soft Cooling Properties: 1. Accuracy of impression compound: a. Flow: They are not sufficiently fluid to record all the fine details. b. Dimensional accuracy and dimensional stability: i. The dimensional changes during hardening are great since they have high coefficient of thermal expansion. On cooling during setting, there is considerable cooling shrinkage. Shrinkage also occurs on cooling from mouth temperature to room temperature. ii. Distortion occurs on removal from undercuts since they are plastic and have certain percentage of flow after setting in the mouth. Distortion also occurs due to dimensional changes during storage of an impression. Stresses can be set up within the material, subsequently distortion can occur due to relief' of these stresses. Therefore, the impression should be cast maximum within an hour. iii. The material is non-elastic the most serious drawback of this material is that it drags and distorts when being removed from an undercut area. iv. They are compatible with model and die materials and do not need a separating medium. 2. Other properties: 14  a. Conductivity: Dental compound has poor thermal conductivity that is why when being softened, its outside will soften first and the inside will soften later. Time must be allowed during either heating or cooling to allow the dental compound to come to a uniform temperature. b. These impression can be removed from the mouth, re-softened and reinserted for any corrections required. As many time as necessary a satisfactory impression is obtained. c. Can be electroplated with copper. III. Zinc Oxide/ Eugenol Applications: Full mouth edentulous impressions. Composition (chemistry): Two paste system: (most common presentation) Supplied as soft or hard-set type usually supplied in two metallic collapsible tubes containing the pastes. One paste called the base paste contains: a. Zinc oxide 80%. b. Inert oil 15% to form a paste. The second paste called the accelerator contains: 15 a. Oil of cloves or eugenol 12-15%. Oil of cloves: Sometimes used instead of the eugenol, since it is less irritant than eugenol. b. Filler talk or kaolin or diatomaceous earth to form a paste. c. Moisture. A very slight amount of moisture water is added to the zinc oxide parts or accelerator, since it is essential for the hydrolysis of the zinc oxide to its hydroxide. Manipulation and setting mechanism: The two pastes: are provided in contrasting colors and usually equal lengths of two are mixed over a glass slab or special oil-resistant paper pad, until a homogenous color is obtained. - Zinc oxide in the presence of moisture reacts with eugenol to form a chelate of zinc eugenolate, which forms an amorphous matrix holding the unreacted particles of zinc oxide. ZnO + Eugenol H2O Zinc eugenolate + ZnO Chelation Matrix Unreacted N.B.: Water is essential for the reaction since the dehydrated ZnO cannot react with eugenol. Properties: 1. Accuracy of zinc oxide and eugenol: a. Flow: Very accurate as they are sufficiently fluid to record the fine details in the mouth. b. Dimensional accuracy and stability: There is no or very little dimensional change on setting. There is probably no dimensional change during storage of the impression. 16  c. Elasticity: Being inelastic, it will not record the form of the undercut. d. Compatibility with the die material: It is removed from the model by softening in hot water. 2. Other properties: a. Eugenol can he irritant, giving a tingling or burning sensation to the patient and leaves a persistent taste, which may be unpleasant to some patients. N.B. Non-eugenol zinc oxide impressions are now available to avoid such effect. b. The impression paste can adhere to tissues, so the lips of the patient are usually coated with petroleum jelly (Vaseline) before making the impression. Elastic Impression Materials I. Hydrocolloids General properties: - A colloid must be distinguished from a solution and suspension or emulsion. - Colloids fall between these two extremes. They are heterogeneous two-phase system, like suspension but the particle size of the dispersed phase is smaller. In the colloid, the particles in the dispersed phase consist of molecules that are held together either by primary or secondary forces. Colloids are termed hydrocolloids when the dispersion medium is water. - Colloids may exist in the sol and gel state. In the sol state, the material exists as a viscous liquid. A sol can be converted into a gel, due to agglomeration of the molecules of the dispersed phase, to form fibrils or chains of molecules, in a network pattern, called brush heap structure. 17 - Agglomeration of the molecules of the dispersed phase to form fibrils or chains of molecules in a network pattern, is caused by, either a reduction in temperature (in which case it is a reversible process as in agar), or a chemical reaction (where the reaction is irreversible as in alginate). 1. Gel Strength: The strength or toughness of a gel depends on: a. The concentration of the fibrils: The greater the concentration, the stronger the material. b. The concentration of fillers: Which are inert powders added to a gel to render it less flexible. 18 c. In reversible gels: The lower the temperature, the stronger the gel and vice versa. 2. Dimensional changes after setting (dimensional stability) { synersis and imbibition}: A gel can either loose or take up water. - The loss of water is termed synersis and occurs either by evaporation. - The uptake of water is called imbibition, clearly synersis and imbibition of the gel should be avoided as the former causes shrinkage, and the latter results in swelling and expansion. - Various storage media, are suggested to reduce the dimensional changes. The best results are obtained with 100% relative humidity which results in minimum amount of shrinkage). 3. Disinfection: Because the hydrocolloid impression must be poured within a short time after removal from the mouth, the disinfection procedure should be relatively rapid to prevent dimensional changes. After the impression is thoroughly rinsed, the disinfectant is sprayed on the exposed surface. The impression should not be soaked in the disinfectant solution. 4. Compatibility with gypsum: Every precaution must be taken to ensure maximum surface hardness of the gypsum cast. This may be due to the presence of the water on the surface of the hydrocolloid which affects the setting and surface properties of the gypsum cast. 19  The cast should not be left too long in contact with the impression. The set stone absorbs water from the water- filled impression. The result is a chalky surface with poor details. Reversible Hydrocolloids: [Agar impression materials]: Agar hydrocolloid was the first successful elastic impression material used in dentistry. The elasticity of the material at the time that it is removed from the mouth allows impressions of undercut areas. Applications: 1. Full mouth impression. 2. Quadrant impression. Composition: The material is supplied as a gel in sealed containers to prevent evaporation of water. The gel is formed of: 1. Agar 13-15%: As dispersed phase. This is a polysaccharides extracted from certain types of seaweeds. 2. Borax 0.2%: Strengthens the gel and increases the viscosity of the sol but retards the setting of gypsum products. 3. Potassium sulfate 2%: To counteract the retarding effect of borax on the gypsum cast materials. 4. Water 83%: As the dispersion medium. 5. Some products contain certain amount of fillers: Which control strength, viscosity and rigidity. E.g. Diatomaceous earth. Iary, silica or wax. 20 Manipulation and setting mechanism: The material is supplied as gel, which can be converted to sol by heating, cooling it again converts it to gel in the mouth: Agar hydrocolloid cool at 43ºC Agar hydrocolloid hot sol heat at 100ºC cold gel The liquefaction and gelation temperatures are different; the latter being lower and the effect is called Hysteresis. Agar impression needs special apparatus that consists of three compartments for heating, storage and tempering. 21 The following sequence is used during manipulation: 1. Heating and storage: The material is brought to the fluid state by heating the tube in boiling water for about 10 minutes, and then stored in water at 65 ºC. 2. Conditioning or tempering: Tempering (at 46±1 ºC) is necessary to cool the material to a temperature that is compatible with the oral tissues. 3. Setting & Gelation: After insertion and seating of the perforated tray, the material is allowed to gel in mouth. 4. Removal of the impression: The tray should be removed with a sharp snap to avoid inaccuracy created by the visco-elastic nature of the material. 5. Pouring the impression: Immediate casting of the impression is recommended. It is however possible to store it in 100% humidity if needs be. Properties: 1. Accuracy of Agar-Agar: a. Flow and dimensional accuracy: Agar is sufficiently fluid to record fine details. The first material to gel is that which is in contact with the tray, since this is cooler than the tissues. b. Dimensional stability: Avoid synersis or imbibition by immediate casting of the impression; it should be stored in 100 % humidity. c. Elasticity: It is visco-elastic, with an elastic recovery of 98.8%. This visco-elastic behavior has considerable clinical importance. It demonstrates the necessity of removing the impression rapidly when it is removed from the mouth, which reduces the amount of permanent deformation (or distortion) at the end. d. Compatibility with die materials: avoid soft surface of the stone model. e. Retention of the gel to the tray: Is achieved by using a perforated tray. 22 2. Other properties: a. Mechanical properties: Proper thickness not less than 4to 6mm should be used as their tear strength is lower than rubbers. b. The need for special apparatus for heating, storage and tempering of the impression make the manipulation time consuming. c. It cannot be electroplated, due to imbibition. Irreversible hydrocolloids: [Alginate impression materials]: Alginate is the most widely used material. Applications: 1. Full mouth impression. 2. Quadrant impression. Composition: The material is supplied as powder to which water is added. 1. Soluble salt of alginic acid (12%): e.g. Potassium or ammonium alginate. It reacts with calcium ions to give calcium alginate gel. 2. Calcium sulfate (12%): It releases Calcium ions to react with alginate (reactor). 3. Trisodium phosphate (2%): It reacts with Ca++ to give CaPO4, to delay the gel formation (retarder). 4. Filler (70%): e.g. Diatomaceous earth. It strengthens the gel. Setting mechanism: - On mixing the powder with water, a sol is formed. The calcium, and the phosphate salts begin to dissolve. 1- Retardation Step: 23  Gel formation is delayed by the following reaction, which is responsible for the working time (reaction responsible by working time). Trisodium phosphate + Calcium sulfate Sodium sulfate (retarder) (reactor) + Calcium phosphate 2- Gelation Step: Then the following reaction occurs to form an elastic gel of calcium alginate (gelation reaction). Potassium alginate + Calcium sulfate Potassium sulfate (sol) (reactor) + Calcium alginate (gel) No substantial quantity of calcium alginate gel is formed until the trisodium phosphate is used up i.e. when the 24 retarder is consumed the calcium salt begins to react with the potassium alginate. Properties: 1. Accuracy: a. Flow: It has high flow, so it can record fine details in the mouth. b. Dimensional accuracy: During gelation it is important that the impression should not be moved. c. Dimensional stability: Alginates are not dimensionally stable on storage due to synersis and imbibition, therefore the cast should be poured immediately to obtain best results. It can be stored in 100% humidity. d. Compatibility with die material: Easy removed from die and do not need separating medium. The impression should be removed from the cast and not left for long time. If left it will affect the surface of the gypsum due to synersis. e. Elasticity: It is visco-elastic; they are (strain-rate) dependent. Sufficiently elastic to be withdrawn over undercuts with an elastic recovery of 97.3%. f. Flexibility: It is the most flexible of all elastic impression materials, which represents the ease with which the material can be removed from the mouth. g. Retention to the tray: Is achieved by using perforated tray. 2. Other properties: a. Mechanical properties: The tear strength of alginate is low; therefore the thickness of the impression should not be less than 4 mm. Also rapid rate of removal increases its tear strength. b. It cannot be electroplated, due to imbibition. N.B.: Rapid rate of removal of the impression from the mouth increases the tear strength and decreases the permanent deformation. 25  II. Rubber Impression Materials Non-aqueous elastomeric impression materials: These are elastic impression materials, which are soft and rubber like technically known as elastomers. Elastomers consist of long chain molecules that are coiled. The specific feature of a rubbery polymer is that when the material is stretched the only work done is in uncoiling the molecules, thus such materials are easy to deform and the deformation is largely reversible. The available types of rubber base materials are basically three (polysulfide, silicones and polyether); the different consistencies of each type are governed by the molecular weight of the polymers and the amount of fillers present. 1. Polysulfide: i. Light consistency. ii. Regular consistency. iii. Heavy consistency. 2. Silicones: a. Condensation: i. Light consistency. ii. Regular consistency. b. Addition: iii. Heavy consistency. iv. Putty consistency. 3. Polyether: Regular consistency. 26 In general: - Rubber materials possess a much greater dimensional stability than the hydrocolloid impression materials. - They can be electroplated (except polyether). - It is preferable to cast the impression within the first hour after its removal from the mouth. General applications: - Inlays. - Crowns and bridges. 27  1. Polysulfide Rubber Alternative names: Rubber-base, mercaptan. Composition: These materials are supplied as two pastes: 1. The base paste contains: a. Polysulfide polymer; with terminal SH (mercaptan) groups as well as pendant SH groups. b. Filler: Between 1 to 54% (e.g. titanium dioxide) to provide the required strength. The percentage of fillers controls the consistency from light regular, heavy, to putty. 2. The reactor paste (catalyst or accelerator): a. Lead dioxide (PbO2): This causes polymerization and cross-linking oxidation of SH groups and gives the brown color of the paste. b. Plasticizers. Setting reaction of condensation types: Condensation polymerization with the formation of water as by-product. Properties: 1. Accuracy: a. Flow: The material is highly accurate, it can record fine details. b. Dimensional accuracy and dimensional stability: Polymerization shrinkage of 0.25% occurs for 24 hours. Also After setting the by-product (water) of the condensation is lost causing shrinkage. N.B.: To obtain accurate impression, it should be cast within one hour. c. Elasticity: It is visco-elastic with elastic recovery of 98% if removed rapidly. 28  d. Flexibility: The most flexible of all rubbers. This flexibility allows the set material to release from undercut areas and be removed from the mouth with minimum of stresses. e. Compatibility with die material: Compatible with the die materials and the use of surface active agents (surfactants) improve their wettability 2. Other properties: a. Tear strength: Polysulfides have the highest resistance to tearing. b. Electroplating: It can be electroplated by silver plating. 2. Silicone Rubber Composition: There are two types of silicone rubber. 1. Condensation type. 2. Addition type. 1. Condensation Type: The condensation silicone rubbers are usually supplied as a base paste and a liquid catalyst or paste. a. Base paste: i. Polydimethyl siloxane polymer: With terminal hydroxyl groups (OH group). ii. Fillers: Which controls the consistency. b. Catalyst: (Usually liquid and sometimes paste) i. Metal organic ester: Such as tin octoate or dibutyl tin dilaurate. Setting reaction of condensation types: Condensation polymerization with the formation of a alcohol as by-product. 2. Addition type: 29  a. Base paste: i. Moderately low molecular weight polymer with silane groups. ii. Filler. b. The catalyst: i. Moderately low molecular weight polymer with vinyl terminal groups. ii. Filler and chloroplatinic acid catalyst. The addition reaction during setting does not produce by-products. Polysiloxane with silane + vinyl terminated silicone + platinic acid catalyst cross-linked silicone rubber Properties: 1. Accuracy: a. Flow: Both types of silicone are highly accurate provided that they have been manipulated properly. b. Dimensional stability and accuracy: - The condensation types have polymerization shrinkage of 0.6% in 24 hours, half of this amount occurs, in the first hour. The vaporization of alcohol accounts for some of this shrinkage. - The addition types that polymerize without by- products have polymerization shrinkage of only 0.05%. c. Elasticity: Silicone rubbers are more ideally elastic than polysulfides. They exhibit minimal permanent deformation and recover rapidly when strained. The elastic recovery is 99.5%. d. Flexibility: Less flexible than polysulfide but more than polyether. e. Compatibility with die material: Most available types produce smooth surface stones. Some types of 30  addition silicone may produce hydrogen during setting, but this effect has been eliminated by the addition of gas absorbers in the composition. The use of surfactants to improve the wettability of vinyl type impression (as they are hydrophobic) improves the quality of cast and dies which are produced. 2. Other properties: a. Tear strength: The tear strength is low for silicone impression materials when compared to polysulfide or polyether. Rapid rate of removal increase the tear strength. b. Disinfection: They can be disinfected without affecting the properties. c. Electroplating: They can be electroplated as polysulfide. N.B.: - One of the disadvantages of vinyl silicone impression materials is their inherent hydrophobicity. This decreases the ability of the material to wet properly the soft wet tissues and at the same time decreases the wetting of the soft die materials to its surface, which may affect accuracy. To render the surface of the impression hydrophilic, a surfactant is added to the paste. This surfactant then allows the impression to wet soft tissues better and to be poured in stone more effectively. - Sulfur contamination from natural latex gloves inhibits the setting of the addition silicone impression materials. 31  3. Polyether Rubbers Composition: The polyether rubbers are also supplied as two pastes: 1. Base paste: a. A polyether polymer with terminal ethylene-imine group. b. Filler. 2. Catalyst paste: a. An aromatic sulfonic acid ester (aromatic sulfate). b. Filler. Manipulation and setting mechanism: Addition polymerization reaction. Properties: 1. Accuracy: a. Flow: It is highly accurate. b. Dimensional stability and accuracy: Polymerization shrinkage of 0.25% occurs in 24 hours. c. Elasticity: It is visco-elastic with elastic recovery of 98.9%. d. Flexibility: The least flexible of all rubbers, which represents difficulty in removal from the mouth. e. Compatibility with die material: They are compatible with die material producing excellent surface. 2. Other properties: a. Mechanical properties: Tear resistance is better than that of silicone impression but less than polysulfide. b. Disinfection: Polyethers are susceptible to dimensional changes if the immersion time is longer than 10 minutes because of their hydrophilic nature. c. Electroplating: It is hydrophilic, thus it cannot be electroplated. 32 General factors affecting accuracy of rubber base impression materials: 1. They can record fine details. Their fluidity largely depends on their composition. They are supplied in different viscosities e.g. light bodies for injection by syringe and medium and heavy for use on a tray. Silicones are also supplied as putty type materials. 2. There is small contraction on setting in the mouth due to polymerization shrinkage. Contraction also occurs on cooling the impression from mouth to room temperature. 3. They are sufficiently elastic to be withdrawn over undercuts and are usually tougher and less likely to tear than the alginates. 4. An adhesive should be used to assure adherence of the material with the acrylic tray. 5. Rubber impressions can be electroplated with silver, to form electroplated dies (polyether is hydrophilic, thus it cannot be electroplated). N.B.: Thickness of the elastic impression: The thickness of an impression material in a tray is a factor contributing to its accuracy. A hydrocolloid impression should be 4-5 mm to increase the tear strength, whereas a rubber base impression should be 2-3 mm thickness except for polyether it should not be less than 4 mm to facilitate removal of the impression from the mouth (due to its low flexibility i.e. stiff). 33  Chapter (3) MODEL AND DIE MATERIALS Definition: A model [cast]: is a replica of the teeth and/or the associated supporting soft and hard tissues of the jaw, which is prepared from an impression. A die; is a model of a single tooth. Model/ Cast Die Requirements for model and die materials: 1. Mechanical properties: a. Should have high strength to resist breakage during use. b. Should be hard to resist scratching during use. 2. It should be able to reproduce fine details of the impression. 34 3. It should have little dimensional change on setting, and should remain dimensionally stable during storage. 4. Compatibility with impression materials. There should be no interaction between the surface of the impression and the model or die 5. Good color contrast with other materials being used Types of model and die materials: Gypsum products. Alternatives to gypsum products: Silicophosphate cement, amalgam, polymers and filled polymers, metal sprayed dies, electroplated dies, and ceramic dies. Gypsum Products Gypsum is a natural mineral, which is found in compact mass in nature. Chemically, it is calcium sulphate dihydrate (CaSO4.2H2O). It is usually white to milky yellowish in color. Types of Gypsum Products According to ADA specification No. 25, pure gypsum products, used in dentistry, are classified as, model plaster, dental stone, high-strength stone. ** All forms of gypsum products are chemically the same 1 → calcium sulphate hemihydrate “CaSO4. 2 H2O”, but they differ in: 1. The method of manufacture 2. This leads to difference in particle size, shape, and form 3. This further leads to different W/P ratio 4. Physical properties 5. Their use in dentistry 35 Manufacturing of Different Types Commercially, the gypsum is ground and subjected to temperature, to drive off part of the water crystallization. heat 110-130° C CaSO 4.2H 2 O ⎯⎯ ⎯ ⎯⎯→ CaSO 4. 1 2 H2O + 11 2 H 2O (Ca sulphate dihydrate) hemi hydrate water 1) Model plaster type II (Plaster of Paris, ß- hemihydrate): Model plaster got its term Plaster of Paris because it was obtained by burning the gypsum from deposits near Paris, France. CaSO 4.2H 2 O ⎯open ⎯⎯ air → CaSO 4. 1 2 H2O + 11 2 H 2 O (Ca sulphate dihydrate) 110 −130° C βhemihydrate water The powder particles are irregular in shape, large, porous (45%). 2) Dental stone type III (α-hemihydrate): steam pressure CaSO 4.2H 2 O ⎯⎯ ⎯ ⎯⎯→ CaSO 4. 1 2 H 2 O+ 11 2 H 2 O Ca sulphate dihydrate autoclave 120 -130° C αhemihydrate The particles are smaller, more regular in shape, less porous (15%) i.e. dense 3) High strength stone type IV (Improved stone): CaSO 4.2H 2 O ⎯boiling ⎯ ⎯30% ⎯CaCl ⎯⎯2 → CaSO 4. 1 2 H 2 O+ 11 2 H 2 O Ca sulphate dihydrate αhemihydrate Then the chlorides are washed by hot water. The particles are the smallest, most regular “prismatic in shape”, and the least porous (10%) i.e. densest. N.B.: All gypsum products are supplied in the form of powder to be mixed with water. 36 Setting Reaction (hardening reaction): The crystalline theory According to the crystalline theory the difference in the solubility of calcium sulphate dihydrate and hemihydrate causes the setting of these materials. On mixing hemihydrate with water the following will occur: - Some of the hemihydrate dissolves in water, giving Ca++ and SO4- ions, which in turn forms the dihydrate in the solution. These are considered nuclei of crystallization. - The solubility of the hemihydrate in water is much higher than the formed dihydrate. - As the reaction proceeds, the concentration of the dihydrate increases rapidly to render the solution super saturated with dihydrate. - More dihydrate will be precipitated around the nuclei of crystallization, leading to crystal growth. - Reaction continues until all the hemihydrate is transformed into dihydrate. Setting Reaction: Ca SO4. 1/2 H2O + 1 1/2 H2O → Ca SO4. 2H2O + Heat Water/Powder ratio 1 ** Theoretically, 100 gm of CaSO4. 2 H2O requires only 18.6 ml of water for the CaSO4.2H2O to be formed. 37 This water is known as water of crystallization or bonded water. - However, more water is needed practically for each 100 gm of powder in order to produce a homogenous workable mix. - This excess water will be present in the final product as free water. - It will evaporate leaving the set material porous. - It may take 7 days to loose the excess water. Product W/P ratio gm/ml Model Plaster 50 /100 Dental stone 30 /100 High strength stone 22 /100 The difference in the W/P ratio is due to: - the difference in the particle size, shape, and - the porosity of each type of gypsum product. Setting time - It is the time elapsed from the beginning of mixing, until setting or hardening occurs. - This time is divided into: Mixing time The time taken from addition of powder to the water until we obtain a homogeneous mix (not friable) - Hand mixing 1 minute - Mechanical mixing 20-30 seconds Working time (3 minutes) Working time = time available for mixing, and its use before initial setting. Initial setting time (12 minutes) It is the time elapsed from the beginning of the mixing until partial setting occurs. 38  During this stage, the material will not flow, it is rigid but not hard. It is possible to carve away the excess material. Final setting time (Several hours) It is the time elapsed from the beginning of mixing until complete setting takes place. The model or die will be strong and hard. Measuring of Setting Time: i. Loss of gloss: - To judge proper working time. - It indicates partial setting ii. Temperature rise: Since the reaction is exothermic, it indicates setting. iii. Penetration tests: This is an accurate method; they depend on resistance of the set gypsum material for penetration by needles, which have a specific weight and definite tip diameter. a. Vicat needle for measuring initial setting time b. Gillmore needles for measuring initial and final setting times. 39 Factors Controlling the Setting Time: Theoretically, according to the crystalline theory, the following will control: 1. The solubility of the hemihydrate 2. The number of nuclei of crystallization 3. The rate of crystal growth. a. Factors Controlled by the Manufacturer: 1. Fineness of the powder - The smaller the particles, the more is the wetting and the faster the rate of dissolution of the hemihydrate This increases rapidly the number of nuclei of crystallization and consequently the growing CaSO4.2H2O in a given volume - Thus a more rapid growth rate is obtained Acceleration or shorter setting time will be obtained. 2. Impurities A small amount of calcium sulphate dihydrate 0.5-1% Terra Alba acts as nucleating agents thus increase the number of nuclei of crystallization and a more rapid growth rate is obtained ∴ they decrease the setting time 3. Chemicals “Retarders and accelerators” * Accelerators (e.g. 2% K2SO4) They increase the rate of solubility of the hemihydrate in a given volume ∴ increasing the nuclei of crystallization in this volume → more Rapid growth rate. ∴ they accelerate the S.T. i.e. shorter S.T. or reduced S.T. * Retarders (Borax, blood, saliva, or hydrocolloid) - By coating hemihydrate particles thus reducing their solubility. 40  Or - By coating the growing crystals thus inhibiting the growth. → Slow growth rate ∴ They retard the S.T. i.e. longer S.T. or increased S.T. N.B.: ** The impression should be washed before pouring the model To wash the blood and saliva that act as retarder for gypsum (model material). b. Factors Controlled by the Operator: 1. W/P ratio: High W/P (thin mix) will lead to less nuclei of crystallization in a given volume, so few CaSO4.2H2O crystals will be formed ∴ slow growth rate ∴ retardation i.e. longer S.T. or increased S.T. 2. Mixing time and rate: Increasing mixing time and rate within limits → provide more nuclei of crystallization in a given volume ∴ more growing CaSO4.2H2O crystals will occur ∴ rapid growth rate will be obtained ∴ acceleration of S.T. i.e. shorter S.T. or reduced S.T. 3. Temperature: - From 20-50 °C acceleration of setting time. (as heat accelerate any chemical reaction). - Above 50 °C retardation of the setting time. - At 100 °C no reaction takes place. because the solubility of hemihydrate = the solubility of dehydrate. 41  Properties of Gypsum Products 1) Dimensional Changes: a. By calculations, a volumetric contraction should occur during the setting reaction (7%). b. Setting expansion is actually observed, This can be explained on the basis of crystalline theory. It is due to the thrust action of the growing crystals N.B.: Because the set gypsum is greater in external volume, than its crystalline volume → the set material is porous. The final structure is composed of interlocking crystals between which pores containing the excess water. Linear Setting expansion Porosity Model plaster 0.2-0.3 45% Dental stone 0.08 15% High strength stone 0.05-0.07 10% Factors Affecting the Setting Expansion: i. W/P ratio: High W/P ratio High w/p (thin mix) will lead to less nuclei of crystallization in a given volume ∴ few growing CaSO4.2H2O, so their outward growing thrust is decreased. ∴ less setting expansion i.e. setting expansion is reduced or decreased N.B.: The same effect on hygroscopic expansion ii. Mixing time and rate: Increasing the mixing time and rate within limits will provide more nuclei of crystallization in a given volume. ∴ more growing CaSO4.2H2O crystals in this volume ∴ increasing the outward thrust of the growing crystals 42 ∴ more setting expansion (S.E.) is increased iii. Chemicals: In general, chemicals regulate the shape of the growing crystals thus decreasing their thrusting action, thus decreasing the setting expansion. c. Hygroscopic expansion: - It is the expansion of gypsum when it is allowed to set under water during initial stage of setting. This additional water provides more room for crystal growth. i.e. Hygroscopic expansion is a physical reaction. - Hygroscopic expansion may be more than double the normal setting expansion in air. 2) Compressive strength: The strength of the gypsum products increases rapidly, as the material hardens after the initial setting time. There are two types of strength of gypsum products: a. Wet strength (green strength or 1 hour strength): It is the strength of the set of gypsum containing the excess water b. Dry strength: It is the strength obtained after the gypsum has been dried and lost the excess water. Dry strength is usually double the wet strength. Factors Affecting the Strength: 1. W/P ratio: The higher the W/P ratio the more excess water will remain, which eventually vaporize leaving more pores and weaker product. 2. Mixing time and rate: - Increasing the mixing time and rate within limits will provide more nuclei of crystallization in a given volume. 43  ∴ more growing calcium sulphate dihydrate crystals will be present in this volume, increasing the crystalline interlocking, ∴ strength will be increased - But over-mixing results in decreasing the strength, because the formed gypsum crystals will be broken up and less crystalline interlocking will be obtained. 3. Chemicals: The chemicals regulate the shape of the growing CaSO4.2H2O crystals thus reducing the intercrystalline cohesion and decreasing the strength. 4. Type of gypsum products: Strength of Improved stone > Strength of dental stone > Strength of model plaster. N.B.: Improved stone can be weak as model plaster if it is mixed with excess water than the required W/P ratio. 5. Dryness: Dry strength is higher than the wet strength. 3) Surface Hardness and Abrasion Resistance: - Generally, the surface hardness is low, so the material is easily susceptible to scratching - Addition of resin [polymer] to the surface of the set gypsum will increase hardness and abrasion resistance. 4) Reproduction of Detail: They do not reproduce surface details accurately, because: a. The surface of the set gypsum is porous on microscopic level. b. Air bubbles are formed at the interface of certain impressions and gypsum cast. 44  Chapter (3) INVESTMENT MATERIALS Definition: Investment is a ceramic material that is used for making a mould into which the metal or alloy is cast. Requirements of investment materials: 1. Should withstand high temperature without decomposition during casting. 2. Should have sufficient expansion to compensate for the casting shrinkage. 3. Should have sufficient strength at: - Room temperature, to withstand manipulation without fracture. - High temperature, to withstand the force of molten alloy entering the mold. 45 4. Should be porous to allow escape of air or gases during casting. 5. Should produce a smooth surface and fine details of the casting. 6. Should be broken away easily after casting. 7. Should be easily manipulated. 8. Should be cheap. Components of Investment Materials: All the investment materials are composed of: i. Refractory material (65%) ii. Binder (30%) iii. Modifiers (5%). i. Refractory Material Definition: - It is a material that withstands high temperature. - It is usually a form of Silicon dioxide (silica), e.g. quartz, tridymite, cristobalite or a mixture of these, i.e. one of the polymorphic forms of silica. Function: 1) It withstands high temperature. 2) It produces thermal expansion of the investment which is caused by the displacive transformation of silica from a to b form during casting. On heating, the refractory material undergoes two types of transformations: 1. Reconstructive transformation: Quartz 870º Tridymite 1470º Cristobatite 1710ºC Fused silica (Hexagonal) (Rhombohedral) (Cubic) (Amorphous) 46 2. Displacive transformation: It is the transformation from α form of silica to b form; This is associated with an expansion. - Cristobalite has the highest thermal expansion followed by quartz and then by tridymite. - The type of refractory material is determined by the amount of casting shrinkage of metals and alloys. As cristobalite gives more expansion than quartz so it is used in the crown and bridge work, while quartz is used in soldering. ii. Binder Definition: It binds the refractory particles (silica) together to form a coherent solid mass. Function: 1. Binds the silica particles. 2. Provides more strength to the material. 3. Gives, in some types of investment, setting and hygroscopic expansion, which share in the compensation of the casting shrinkage of the metal. iii. Modifiers Definition: These are chemicals added in small amount to modify various physical properties. Function: 1. They prevent oxidation of the molten alloy during casting e.g. graphite. 2. They produce smooth surface mould. Types of Investment Materials: 47  There are three types of investment according to the type of binder used: 1. Gypsum-bonded investment. 2. Phosphate-bonded investment. 3. Silicate-bonded investment. Effect of temperature on gypsum bonded investment: 1. When it is heated from 200-400 °C it contracts due to the dehydration of the binder. This can be eliminated by addition of chemicals (boric acid). 2. From 400-700 °C, it expands again due to the transformation of the α form to the b form of silica. 3. Above 700 °C, CaSO4 will react with carbon in the investment leading to evolution of sulfur trioxide gas (SiO3) causing contamination and porosity of the casting. 4. Above 1200 °C, during melting of high fusing dental alloys (base metal), this will lead to decomposition of the binder (gypsum). So gypsum bonded investment is never used in casting of base metal alloys, this limited its use to gold alloys (a low fusing dental alloys). 48 49 50  Chapter (4) CASTING TECHNOLOGY Casting procedure is the process of converting the wax pattern into metallic restoration by pouring the molten metal into a mold. - The indirect metallic restorations (crown, bridge, partial denture framework, inlay……..) are usually constructed either from gold or base metal alloys. - After tooth preparation is completed and impression is taken inside patient mouth, a stone cast will be obtained with removable die onto which wax pattern will be constructed. 1. Wax Pattern Construction: - Direct technique: Formation of the wax pattern in the patient’s mouth on the prepared tooth - Indirect technique: Formation of the wax pattern in the laboratory on a cast or a die - Indirect and direct technique: The pattern is prepared outside the patient mouth, but the final adjustment is made inside the patient mouth. - When the wax pattern has been completed, a sprue is attached to it. 51 2. Spruing the Pattern: Functions: - Proper handling of the wax pattern to avoid its distortion. - Sprue will create a channel: i. To allow the molten wax to escape. ii. To enable the molten metal to flow into this mold. - Act as a reservoir for the molten alloy, to compensate for solidification shrinkage. a) Materials: The sprue can be made of: Wax, Plastic or Metal. - Wax sprues are preferred for most castings because they melt at the same rate as the wax pattern, allowing easy escape of the molten wax. - While plastic or metallic sprue softens at higher temperature than the wax pattern. Therefore it must be removed after investing before wax elimination 52 b) Attachment and direction: - Sprue should be attached to the thickest part of the wax pattern with a 45° angle to the proximal wall surface. - This orientation avoids turbulence, necking and casting porosity. c) Diameter (Thickness): - Thickness of sprue should be slightly thicker than the thickest part of the wax pattern because: - This improves the flow of the molten alloy into the mold. - This also provides a reservoir during solidification. d) Length: The length of the sprue should be adjusted to keep the mounted wax pattern at: ** 6-8 mm from the end of the casting ring. - If this distance is increased (short sprue), this may prevent escape of air leading to back pressure porosity. - If this distance is decreased (long sprue), this may lead to fracture of the investment, as mold will not withstand the impact force of the entering molten alloy. e) Number: It is determined according to the size of the wax pattern. Sprue may be: - Single - Double - Multiple Crucible former: The sprue is attached to a crucible former usually made of rubber which form the base of the casting ring during investing 53 Casting ring and liner: The casting ring may be; i. Rubber ring ii. Metallic ring lined with wet asbestos free liner Functions of the liner: a. It creates a space to allow for investment b. It allows setting of the investment under water to give hygroscopic expansion c. It facilitates removal of the investment from the casting ring after casting. The asbestos liner should be shorter 3 mm from both end of the ring, to provide retention of the investment to the ring. 3. Investing Wax Pattern: Objective: Investing the wax pattern is performed to produce a mold after wax elimination. 54 Wettability: The wax is hydrophobic → mixed investment can not be properly adapted to the wax pattern. This will lead to rough surface. Application of thin uniform layer of wetting agent on the wax pattern. → - remove any debris on the wax pattern - reduce the contact angle between the wax and the investment Time for investing: The wax pattern should be invested immediately to avoid its distortion due to release of the internal stresses. Two factors determine the release of internal stresses: - Time: The larger the time interval before investing wax pattern, the more distortion due to release of internal stresses - Temperature: The nearer the softening point of the wax is approached, the more distortion of the wax pattern due to release of internal stresses 55 Investing procedures: - Use correct W/P - There are two different methods for investing the wax pattern: ▪ Hand investing ▪ Vacuum investing, which is highly recommended to obtain casting with minimal surface defects. - After the ring is filled to the rim with the mixed investment, allow the investment to set for ≈ 1 hour. 4. Wax Elimination: Heating the investment in a thermally controlled furnace is performed to achieve: Complete wax elimination: - Thermal expansion of the investment by displacive transformation from α - β - Adequate time/temperature should be provided for complete wax elimination, otherwise residues are retained in the mold preventing formation of complete casting. 5. Casting: - Proper melting of the metal or alloy. - Proper application of the casting force to force the molten metal or alloy into the mold. a) Melting: It is either performed by; - Flame ▪ Blow pipe flame ▪ Oxyacetylene flame Use always the middle zone, because it is the hottest and reducing zone. - Electric Melting The electric units are heated either by induction or resistance heating system. 56  b) Casting machines [with or without vacuum system]: All casting machines accelerate the molten metal into the mold either by: - Air pressure - Centrifugal - Centrifugal and air pressure. 6. Devesting, Finishing and Polishing of the Casting Casting Defects Unless every step in the casting procedures is performed properly, the casting may not fit the preparation accurately 1. Distorted Casting: Distorted casting may be due to distorted wax pattern as result of; - Improper handling of the wax pattern - Delayed investing of the wax pattern. 2. Dimensional inaccurate casting: Dimensionally inaccurate castings may be due to improper mold expansion to compensate accurately for the solidification of the molten metal or alloy. This maybe due to improper selection of the type of the investment. 3. Incomplete casting: It is due to the resistance of the flow of the molten alloy into the mold: - Using thin sprue causing premature solidification of molten metal. 57  - Using short sprue. - Insufficient venting of the investment. - Incomplete wax elimination. - Under heating of the alloy causing premature solidification of molten metal. - Insufficient casting pressure to drive the molten alloy into the mold. 4. Porosity: a. Shrinkage spot porosity: It may be due to improper feeding of the mold with the molten alloy during solidification. This may be related to: - Using too thin sprue. - Incorrect attachment of the sprue. b. Back pressure porosity It is caused by the failure of the air or gases to escape from the mold thus creating back pressure during flow of the molten alloy into the mold. This may be due to: - Using short sprue. - Insufficient venting of the investment. - Incomplete wax elimination. 5. Surface Roughness: The whole surface texture of the casting is roughened by: - Improper use of wetting agent on the wax pattern - Improper vacuum investing - Too rapid Or Over heating of the investment during wax elimination. 58  Chapter (5) DENTAL CASTING ALLOYS Definition: Dental casting alloys are those used for the construction of indirect metallic restoration e.g. inlays, onlays, crowns, bridges, endodontic post and core, and removable partial denture framework. Requirements: a. Functional Requirements 1. Stiffness It is the resistance to elastic deformation, determined by the modulus of elasticity. High stiffness means minimum deflection under stresses within the elastic range of the stress strain curve. 59  This allows equal stress distribution under the restoration. Metals, with high modulus of elasticity can be consequently constructed in thin section. 2. Resilience It is the ability to absorb energy without plastic deformation so that the stresses are not transmitted to the underlying supporting tissue 3. High yield stress High yield stress is required to resist permanent deformation under the masticatory stresses in the mouth. 4. Fatigue Resistance The fatigue strength and the fatigue limit should be high to resist cyclic loading. 5. High ductility 6. High sag resistance Sag resistance is the ability of the alloy to resist plastic flow under its own weight at high temperature, this is useful during soldering. 7. High tarnish and corrosion resistance The metal or alloy must not dissolve in the oral fluids or liberate toxic corrosion products. 8. Fit It is the ability of the casting to reproduce accurately the wax pattern from which it is constructed. 9. Compatibility with porcelain The following factors influence the compatibility of the metal with the porcelain: ▪ The melting temperature of the metal must be higher than the firing temperature of the porcelain to have high sag resistance ▪ The coefficient of thermal expansion and contraction should be slightly higher than that of porcelain to obtain compressive bonding ▪ The metal must have high modulus of elasticity and yield stress to provide stiff and strong substrate 60  under the porcelain thus avoiding cracking of porcelain ▪ The metal must not discolour the porcelain. b. Working requirements 1. Ease of casting: The alloy must be easily melted with minimum slag formation It must also possess sufficient fluidity to rapidly fill the mold 2. Ease of soldering: The liquid solder must wet the alloy surface readily forming a true adhesive bond. 3. Ease of burnishabilit:y Marginal adaptation is usually improved by burnishing. Ductility is an important factor in the burnishability. Classification of dental casting alloys Metals can be divided into: a. Noble metals Noble metals are those which are resistant to tarnish and corrosion in dry air, during casting, soldering or use in the mouth. 61  Noble metals are gold, platinum, palladium, iridium, and rhodium. The noble metals together with silver are sometimes called precious metals. b. Base metals Other metals than noble metals are base metals Noble Metal Casting Alloys 1. Direct restoration Gold was first used in dentistry as pure metal in the form of gold foil Gold foil is placed directly in the prepared cavity as direct filling 2. Indirect restoration (Noble metal casting alloys) General Constituents of Gold Alloys: Gold is alloyed with several metals in order to modify the properties. A) Noble metals and their effect on properties: 1. Gold: a high noble metal that resists tarnish and corrosion. It has low strength, hardness and high ductility. Its melting temperature is 1063°C, and specific gravity 19.3. 2. Platinum: a noble metal that resists tarnish and corrosion. It increases strength and hardness of gold; it is also tough and ductile. Its melting temperature is 1755°C, and specific gravity 21.37, which increases the weight of gold alloys. 3. Palladium: cheaper than platinum, and is used as a replacement for platinum since it imparts many of its properties to dental alloys. Its specific gravity is half that of platinum 11.4. It is a malleable and ductile metal. 62  Palladium as low as 5% have a pronounced effect on whitening the color of gold alloy, and it decreases the greening effect of silver and the red color of copper. It is also effective in preventing corrosion of silver in the oral cavity. 4. Silver: malleable, ductile, stronger and harder than gold. It may be the whitest of all metals. Its melting temperature is 960°C, and specific gravity 10.4. It improves the mechanical properties of gold alloys. Pure silver occludes oxygen in the molten, which is evolved during solidification. As a result, small pits, porosity and rough casting surface develop. This tendency is reduced when 5 to 10% Cu is added to Ag. 5. Iridium, Ruthenium, and Rhodium: iridium is more commonly used in dentistry than Ruthenium, and Rhodium, all have the same effect; small amounts are added or they are present as impurities in the alloy to modify the properties. As little as 0.005% is effective in refining the grain size of gold alloy. B) Base metals: 1. Copper: malleable and ductile metal with a characteristic red color. It imparts strength and hardness to gold alloys. It is an important factor in heat treatment. Its melting temperature is 1083°C. 2. Zinc: acts as a scavenger (deoxidizing agent) during melting and casting of gold alloys. It improves the castability and fluidity of the alloy. 3. Indium: a soft and grey metal with a low melting point of 156°C, it is not tarnished by air or water. It is used in some gold alloys as replacement of zinc. It produces oxides in porcelain alloys, which help bonding with porcelain. 63  4. Tin: lustrous white metal with melting temperature 232°C. It is added in small amounts to produce oxides, which help bonding with porcelain, and also combine with platinum and palladium producing hardening effect. 5. Iron: added in very small amounts (1%) with porcelain alloys in order to form precipitation hardening (Pt3Fe). Classification of Gold Alloys: 1. According to gold content a. Carat specification The carat of an alloy is the parts of pure gold in 24 parts of alloys 24 carat gold = pure gold 18 carat gold = 18 parts pure gold 6 parts other metals b. Fineness specification The fineness of an alloy is the parts of pure gold in 1000 parts of alloy 1000 fine = pure gold 750 fine = 750 parts pure gold 250 parts other metals The fineness rating maybe used in denoting the gold solder according to gold. 2. According to mechanical properties The dental gold alloys are classified into four types. Yield % Vicker’s Gold Copper strength Elongation Hardness Type I 87 4 140 18% 35 Type II 76 8 140-200 18% 30 Type III 70 10 200-340 12% 20 (hard condition) Type IV 65 15 340-500 10% 8 (hard condition) 64 N.B.: Only Type III and Type IV respond to heat treatment because their gold copper ratio allows the precipitation of the ordered structure (AuCu) in the parent alloy. This is not the case in Type I and Type II, therefore they don’t respond to heat treatment. The modulus of elasticity of cast dental gold alloys ranges between 75 – 105 × 103 MPa, depending on the composition of the alloy Density The density of cast dental gold alloys ranges from 15.2-16.8 gm/cm3 3. According to alloy description: A) Description by color a. White gold alloys: The white gold alloys are predominantly gold in composition but are whitened with platinum, palladium (>5%) and silver. b. Yellow gold alloys: Gold and copper percentage compensates for the decrease in platinum, palladium, and silver content thus the yellow gold color is prominent B) Description based on low gold content (economy gold) - The ADA requirements for dental gold alloys require that the noble metal content should not be less than 75%. - As the price of gold is increased, the gold content in dental gold alloys was decreased to 42-58% (economy gold alloys). - The reduction in gold was replaced by palladium, silver and other metals. N.B.: Palladium, silver, copper ratio becomes very critical because: 65 ▪ Silver rather than copper in dental gold alloys produces tarnish in the oral cavity, Palladium was specific to make silver in dental gold alloys tarnish resistant. For economy gold alloys 1% palladium was required for every 3% silver to offset the tarnish tendency of the silver. ▪ If the silver copper ratio was not carefully balanced, the alloy would also tarnish, even with correct palladium silver content this is due to the limited solubility of silver in copper leading to the precipitation of silver rich phase in the microstructure. The properties of economy gold alloys are comparable to Type III and Type IV gold alloys. Base Metal Cast Alloys Definition: These are alloys which do not contain noble metals. Because of the increased cost of gold all over the world, base metal alloys have been introduced in dentistry.They are substitutes for gold alloys Type III and IV. Types: 1. Cobalt chromium alloy introduced in 1928. 2. Nickel chromium alloy introduced later. 3. Titanium and titanium alloys introduced recently. Composition: a. Composition of: Cobalt chromium Similar Nickel chromium 66 Major elements ( ≈ 90% by weight): ▪ Cobalt: - It is used interchangeably with nickel - It is responsible for the increase of modulus of elasticity, strength, and hardness. ▪ Chromium: - It is responsible for resistance to tarnish and corrosion by formation of passive layer which is a thin uniform non porous and adherent layer of chromium oxide. - A minimum of 12% chromium is required to form this passive layer. - Meanwhile, a maximum of 30% chromium is the limit of solubility of chromium in cobalt. Additional chromium would produce a highly brittle phase known as σ -phase. ▪ Nickel: - It is used interchangeably with cobalt. - It is responsible for the increase in the modulus of elasticity, strength, and hardness, but to lesser extent than cobalt. - It is also responsible for the ductility of the alloy. - It is a well known metal to produce allergic reaction in some patients where nickel free cobalt chromium are used Minor elements ( ≈ 10% by weight) The effect of these elements on the properties of cobalt chromium and nickel chromium alloys is much more pronounced than that of the major elements. ▪ Molybdenum: - It acts as a grain refiner → ∴ it increases the strength. ▪ Silicon and manganese: - They act as deoxidizers 67  - They are responsible for increasing the fluidity of the molten alloy, thus improving its castability. ▪ Carbon (0.4%): Carbon can combine with any of other alloying elements to form carbides. These solidify last during cooling after casting → ∴ carbides appear at the grain boundaries - The precipitation of these carbides at the grain boundaries increase the strength and hardness of the alloy (Discontinuous carbide precipitation at the grain boundaries) - A change in the carbon content in the order of ± 0.2% changes the properties to such extent that the alloy can not be acceptable in dentistry. ** A decrease by 0.2% than the desired percentage gives an alloy with low strength, and hardness. ** An increase by 2% over the desired percentage gives a too hard and brittle alloy (continuous carbide formation at the grain boundaries) ∴ Precaution should be taken that no excess carbon is absorbed during casting of the alloy. ▪ Aluminum (specifically exists in nickel chromium alloy): Aluminum and nickel form Nickel Aluminite The precipitation of this compound is responsible for the increase of strength (precipitation hardening) ▪ Beryllium (specifically exists in nickel chromium alloy): It lowers the melting range of the alloy by 100 oC It improves the fluidity of the molten alloy, thus improving its castability Beryllium vapor is carcinogenic to the technician, leading to the fibrosis of the lungs ∴ free beryllium alloys are introduced 68  b. Composition of commercial pure titanium [Cp Ti] and titanium alloys: Commercially pure titanium is available in four grades according to oxygen content (0.18-4%) and iron content (0.2-0.5%). Titanium has two polymorphic forms α -phase and β -ph Comparison between Base Metal Alloys and Gold Alloys (Type III and Type IV) Property/Clinical Significance Base metal Alloys Gold Alloys CoCr NiCr Ti + Ti alloy Type III Type IV I. Biocompatibility The passive layer in the base The nobility of the Identalloy program metal alloys may help in their gold alloys may help Each alloy has a certificate that biocompatibility in their lists biocompatibility ▪ The ADA composition However ▪ Nickel is allergic classification [women>men] ▪ Complete composition of the ▪ Beryllium vapor alloy is carcinogenic to ▪ Its commercial name the technician ▪ Its manufacturer When the dental prosthesis is delivered by the laboratory to the dental office, this certificate is placed in the patient’s chart. This information is useful when: ▪ There are problems with the restoration e.g. allergy ▪ Planning additional restorations II. Physical Properties Excellent Excellent a. Resistance to tarnish and Due to the presence of passive Due to presence of corrosion layer noble metals ▪ CroO2 in Co/Cr and Ni/Cr ▪ Gold [noble metal] alloys Type III 70% Type IV ▪ TiO In Cp Ti and Ti alloys 65% Ti will repassivate in ▪ Platinum [noble nanoseconds, if the passive layer metal] is scratched Type III 1% Type IV 2% ▪ Palladium Type III 3% Type IV 3% 69 P r o p e r t y / C l i n i c a l Base metal Alloys Gold Alloys Significance CoCr NiCr Ti + Ti alloy Type III Type IV b. Color Lustrous silvery white of properly Yellow or white finished and polished - Yellow color is due to presence of ▪ Gold ▪ Copper: it deepens the yellow color Yellow color yields a warmer and more esthetic appearance with no blue or gray line subgingivally in ceramometallic restorations - White color is due to the presence of ▪ Silver ▪ Palladium > 10 ▪ Platinum c. Melting range ▪ Co/Cr 800 – 1050 0C ≈ 1400 – 1500 °C ▪ Ni/Cr ≈ 1300 °C ▪ Cp/Ti ≈ 1700 °C Its alloying decreases its melting range d. Density Co/Cr 15 – 18 gm/cm3 Density affects the castability of the alloy Ni/Cr Generally, high density alloys will accelerate faster into the Cp Ti + Ti alloys ≈ 4.5 gm/cm3 mold forming complete casting more easily. N.B. Which alloy is better in the construction of bulky maxillary restoration 70  Base metal Alloys Gold Alloys Property/Clinical Significance CoCr NiCr Ti + Ti alloy Type III Type IV e. Coefficient of thermal Co/Cr 14 × 10 − 6 / °C ≈ 14.8 × 10 − 6 / °C expansion and contraction Ni/Cr It affects the compatibility with porcelain −6 Cp Ti + Ti alloys ≈ 9 × 10 / °C f. Casting shrinkage ≈ 2.3% ≈ 1.6% It affects the selection of the type of the investment which will expand to compensate for it III. Mechanical properties Cp Ti is similar a. Modulus of elasticity ≈ 250 × 10 3 MPa in mechanical ≈ 100 × 10 3 MPa It affects the stress distribution ∴ They are used in thin section properties to ∴ They are used in on the supporting tissue gold alloys thick section to give the It also affects the thickness of While Ti alloys same effects as base the restoration are similar in metal alloys [i.e. to flex mechanical the same amount as properties to base metal alloys] base metal alloys b. Yield strength 600 – 700 MPa ≈ 300 − 500 MPa c. Ultimate tensile strength ≈ 800 MPa ≈ 700 MPa d. % Elongation 1 - 2% 8 – 20% N.B Increasing Nickel contents Gold alloys are more increases ductility tough e. Hardness ≈ 350 VHN ≈ 250 VHN ∴ difficult in finishing and ∴ easy in finishing and polishing polishing but they retain but they retain surface for longer surface finish for shorter time time 71 Property/Clinical Base metal Alloys Gold Alloys Significance CoCr NiCr Ti + Ti alloy Type III Type IV a. Melting Co/Cr ▪ Flame Flame Ni/Cr Oxygen acetylene Gas air touch [middle flame zone] It should be properly adjusted Too much oxygen → oxidation Too much acetylene → carbide precipitation ▪ Electric melting Cp Ti + Ti alloys electric melting Casting machine Co/Cr Air pressure casting Ni/Cr machine Cp Ti + Ti alloys → Special designed casting machine using centrifugal and air pressure casting force under well controlled vacuum atmosphere Cooling Bench cooling Rapid cooling F i n i s h i n g a n d ▪ Sand blasting: ▪ Acid Pickling: polishing Mechanical smoothness of the Place the gold casting casting in warm HCL using ▪ Electrolytic polishing: Teflon tweezers, to The restoration is placed as anode remove surface where rough surface is depleted oxides expressing smooth shiny surface ▪ Polishing is then done by rubber cups and paste Give reason ▪ Acid pickling should not be performed for cast base metal This is not performed to avoid acid attack to the passive layer of base metal alloys. 73