Students Note Book On Model & Die Materials PDF

Students Note Book BIOMATERIALS DEPARTMENT School Of Oral & Dental Medicine BUA Content Chapter Topic Chapter I Model & Die Materials Chapter II Investment Material Chapter III Dental Alloys Chapter IV Casting Technology Chapter V Non-Metallic Denture Base Materials ...

Students Note Book BIOMATERIALS DEPARTMENT School Of Oral & Dental Medicine BUA Content Chapter Topic Chapter I Model & Die Materials Chapter II Investment Material Chapter III Dental Alloys Chapter IV Casting Technology Chapter V Non-Metallic Denture Base Materials  Chapter I Model and Die Materials: Gypsum Products A model or cast: It is the positive replica of the teeth and/or the associated supporting soft and hard tissues of the jaw. It is poured from an impression and is then used to construct an appliance, such as a full or a partial denture. A die: It is a model of a single tooth, generally used in the construction of crowns and bridges. Ideal requirement of the model and die materials: 1. Mechanical properties: a. Should have sufficient strength to avoid accidental breakage during use. b. Hard to resist scratching during carving of the wax pattern. 2. Produce fine details of the impression. 3. Should show little dimensional change on setting (Dimensional accuracy) 4. It should remain dimensionally stable on storage (Dimensional stability) 5. Compatible with the impression materials. (No need for separating medium and No interaction between the impression material and the model surface). 6. Good color contrast with the other materials being used. (e.g. impression materials). 7. Ease of use. 8. Cheap. N.B: Models and dies for dental use are mostly prepared from gypsum products. 1 Gypsum products Chemistry of gypsum Gypsum is naturally occurring mineral, which is chemically is calcium sulphate dihydrate, CaSO4. 2H2O. When the gypsum is heated to a temperature sufficiently high to drive off some of the water, it is converted into calcium sulphate hemihydrate, CaSO4. ½H2O. Types of gypsum products: Type I: Impression plaster. Type II: Model plaster. Type III: Dental stone. Type IV: High-strength stone/low-expansion dental stone. Type V: High-strength/high-expansion dental stone. It should be noted that all products are chemically the same, they are calcium sulphate hemihydrate (CaSO4. ½H2O) but they differ in: 1. The method of manufacture 2. Particle size and shape 3. Water/powder ratio 4. The physical and mechanical properties of set material 5. Their use. Manufacturing of the different types of gypsum products: Calcining of gypsum Commercially, gypsum is ground and subjected high temperature reaction to drive-off part of the water (one and half molecules of water). According to the conditions applied during the calcination reaction, different types of gypsum products are obtained: 2  Type II: Plaster of Paris (β Ca SO4 ½H2O): Manufactured by: Heating the gypsum in open vessel (open air) at 120°C. 120° C Ca SO4. 2 H2O Ca SO4 ½ H2O + 1 ½ H2O The powder particles obtained are: Irregular in shape, large and porous with Low density. Type III: Dental stone (α Ca SO4 ½H2O): (Autoclaved gypsum or hydrocal) Manufactured by: Heating the gypsum at 120 - 130 °C in an autoclave (under steam pressure). 120-130° C Steam pressure Ca SO4. 2 H2O Ca SO4 ½ H2O + 1 ½ H2O The powder particles obtained are: Smaller, more regular in shape, less porous and are denser than type II. Type IV: Improved stone: (High strength stone or Densite) Manufactured by: Boiling gypsum in 30% calcium chloride (CaCl2) or magnesium chloride (MgCl2) Boiling in 30 % CaCl2 Ca SO4. 2 H2O Ca SO4 ½ H2O + 1 ½ H2O The powder particles obtained are: The smallest, the most regular, the least porous and are highly dense. 3  Gypsum products Plaster of paris Dental stone Improved stone Chemical Ca SO4 ½ H2O Ca SO4 ½ H2O Ca SO4 ½ H2O structure Method of Heat to 120◦C in open air Heat to 120-130◦C in an boiling 30% in CaCl2 or production autoclave (steam pressure) MgCl2 solution Particle Large Smaller Smallest size & Irregular Regular Most regular shape Porosity Porous Less porous Least porous Strength weakest strong strongest W/P Ratio 50ml of water to 100gm 30ml of water to 100gm 22ml of water to 100gm powder powder powder 4 The setting process of gypsum products The setting reaction: N.B: Setting of a material means its transformation from the liquid state to the solid state. When calcium sulphate hemihydrate is mixed with water, a chemical reaction takes place, and the hemihydrate is converted back to calcium sulphate dihydrate. CaSO4 ½H2O + 1½H2O CaSO4. 2H2O + Heat Plaster Water Gypsum … The above reaction is called the setting reaction and it is an exothermic reaction. The amount of heat evolved is equivalent to the heat originally used for gypsum calcining. The amount of water required for setting (W/P ratio): It is important to know that chemically, each 100 g of CaSO4.½H2O requires only 18.61 ml of water to form CaSO4.2H2O. for all types of the gypsum products. However, practically, more water “excess water” is needed for each 100 g of powder in order to produce a homogenous workable mix. This is due to the difference in their particle size, shape and porosity. The minimum amount of water needed for different gypsum products: Gypsum product W/P ratio (g/ml) Plaster 50/100 Dental stone 30/100 Improved stone 22/100 Such excess water will be present in the final product as free water. This excess water will eventually evaporate, leaving the set material porous. It may take 7 days 5 to lose the excess water. Thus, the more the water used in mixing, the more porous the set product, the less hard and the weaker the set gypsum will be. Mechanism of setting (Crystalline theory): On mixing hemihydrate with water, the following is believed to occur: 1. Some of the hemihydrate dissolves in water, giving Ca++ and SO42- ions. 2. The dissolved hemihydrate reacts with water and forms calcium sulphate dihydrate in the solution. 3. The solubility of the hemihydrate in water is much higher than the dihydrate (0.2%). 4. As the reaction proceeds, the concentration of the dihydrate increases rapidly to render the solution super-saturated with dihydrate. 5. As super-saturation represents an unstable condition, the excess dihydrate starts to precipitate out of the solution in the form of nuclei of crystallization. 6. As the reaction continues, more dihydrate will be precipitated around the nuclei of crystallization, leading to crystal growth. 7. The reaction continues until all the hemihydrate is transformed into dihydrate. The following factors can be observed during the setting reaction: 1. The material becomes rigid, but not hard (initial set) at this stage can be carved but not shaped. 2. The so-called (final set) follows, when the material becomes hard and strong. 3. Heat is given out during setting, since the reaction is exothermic. 4. Dimensional change also takes place (setting expansion). 6 Properties of gypsum products 1. Setting time. 2. Dimensional changes. 3. Strength. 4. Surface hardness and abrasion resistance. 5. Reproduction of details. Setting time: It is time required for the reaction to be completed. OR It is the time elapsed from the beginning of mixing, until the material hardens. Stages of setting time: Setting time can be divided into various stages or phases as follows: 1. Mixing time: The time taken from addition of powder to the water until we obtain a homogeneous mix (not friable). 2. Working time (W. T): It is the time available to use a workable mix with even consistency. Working time = time available for mixing and pouring the gypsum product into the impression (pouring a cast) before initial setting. 3. Setting time: a. Initial setting time: It is the time since the beginning of mixing until until partial setting occurs. During this stage: At the point the material reaches the initial setting time it should no longer be manipulated. The material will not flow. It is possible to carve away the excess material. 7  b. Final setting time: It is the time elapsed from the beginning of mixing until complete setting takes place, i.e. the reaction is completed. N.B: Initial setting time: not all the hemi hydrate is converted to dihydrate. Final setting time: all the hemi hydrate is converted to dihydrate. Gypsum materials continue to develop strength after the initial setting. The model or die will be strong and hard to work upon. Final setting time takes several hours after mixing; however, it may be safe to use and handle the set gypsum “model or die” after 30 min. i.e. ready to use stage. Ready to use stage. It is the time at which the set material may be safely handled without fear of fracture. It may be considered as the time when the compressive strength is 80% of that obtained after one hour. Most modern products reach the ready for use stage in 30 minutes. Setting characteristics: 1. Loss of surface gloss. 2. Exothermic reaction. 3. Resistance of needle penetration. 4. Expansion during setting. Measuring the setting time: 1. Loss of the surface gloss: Indicates the initial setting time. Excess water on the surface is absorbed into the mass and at this time the surface does not reflect light and appears dull. 8  2. Penetration tests: The Vicat needle: is used to measure the initial setting time. It consists of a rod weighing 300gm with a needle of 1mm diameter. The Gillmore needle: has two weights and diameters. The initial setting time is measured by the rod weighting ¼ pound and the needle1/12 inch in diameter, while the final setting time is measured by the rod weighing 1 pound and the needle 1/24 inch in diameter. N.B.: Needles for measuring the final setting time are usually heavier and thinner than those used for measuring the initial setting time. Figure :Needles for measuring the setting time. 9 Factors affecting the setting time The setting time can be controlled by either: a) The manufacturer b) The operator A. Factors controlled by the operator: 1. Water/powder ratio: The higher the W/P ratio (a watery mix), the less the number of nuclei of crystallization present per unit volume of the mixture, and hence the longer is the setting time (retardation). 2. Mixing time and rate: An increase in the mixing time and/or rate accelerates the setting time because mixing can break up some of the formed dihydrate crystals, thus forming more nuclei of crystallization. 3. Temperature: The temperature of the water used for mixing, as well as the temperature of the environment, have an effect on the setting time of gypsum materials.  At 0ºC, the longest setting time.  From 20 °C to 50 °C, acceleration of setting time.  At 50 °C, no acceleration.  Above 50 °C, retardation of the setting time.  At 100ºC there is no setting (same rate of dissolution of hemihydrate and dihydrate). B. Factors controlled by the manufacture: 1. Fineness of powder: Grinding of the manufactured hemihydrate increases the surface area of the hemihydrate exposed to water, thus, increasing the rate of solution of the hemihydrate. This accelerates the setting. 10  i.e. The finer the particle size of hemihydrates, the faster they dissolve in water and the faster the mix will be hardened. 2. Impurities in the powder: Ca SO4. 2H2O (Gypsum) about 0.5% initially present as impurities (called Terra alba) will accelerate the setting (decrease setting time) by providing nuclei of crystallization. 3. Addition of accelerators and retarders:  Accelerators: Mechanism of action: - Addition of more nuclei of crystallization: nearly all acids and salts when added to plaster or stone, affect the setting. - Increasing the rate of solubility of the hemihydrate E.g., Potassium sulphate (K2SO4) up to 2% is the most commonly used accelerator.  Retarders: Mechanism of action: - Formation of an adsorbed layer on the hemihydrate “coating” thus, reducing its solubility OR by formation of an adsorbed layer on the growing dehydrate crystals, to inhibit their growth “poisoning the nuclei”. E.g., Organic materials such as glue, gelatine, some gums, dry blood or saliva. E.g., Borax is the most effective and commonly used type of retarders with gypsum products. 11  Dimensional changes: A. Setting expansion Theoretically, by mathematical calculations, the volume of the final dihydrate should be about 7% less than the total volume of hemihydrate and water entering the reaction. However, practically linear expansion is actually observed during setting to accompany the change from the hemihydrate to the dihydrate. Causes of setting expansion: This linear expansion is caused by “the outward thrust of the growing crystals of the dihydrate”. Factors controlling setting expansion: 1. W/P ratio: Increase W/ P ratio- increasing the amount of water in the mix- will decrease the nuclei of crystallization. On the other hand, this will increase the space between the particles and decreases the thrusting action. 2. Mixing time and rate: The increase in the mixing time and rate will lead to an increase in the setting expansion because more nuclei of crystallization will be formed that will interfere with growing leading to the outward thrust. 3. Chemicals “retarders and accelerators”: Chemicals decrease the setting expansion by regulating the shape of the growing crystals limiting its growth and reduce their ability to push each other apart thus decrease the thrusting action. e.g. K2SO4, NaCl. B. Hygroscopic expansion It is the expansion of gypsum when it is allowed to set under water during the initial stage of setting. The hygroscopic expansion may be more than double the normal setting expansion in air. This is because the additional water provides more room for crystal growth; therefore, the crystals can grow more freely resulting in a greater degree of expansion. 12  Immersion of plaster or stone casts in water during setting should be avoided because of the hygroscopic expansion (disadvantage). However, it is used to expand gypsum bonded investment materials to compensate for the casting shrinkage (advantage). Strength: Gypsum products are brittle materials. Their strength increases rapidly as the material hardens. Since the free water content “excess water” of the set product affects its strength, two types of strength of gypsum products are recognized. Types of strength: 1. Wet strength: “one hour strength or green strength”: It is the strength of the set gypsum containing the excess water that is required for hydration of the hemihydrate. 2. Dry strength “7 days strength”: It is the strength obtained after the set gypsum has been dried and lost the excess water. The dry strength is usually double the wet strength. As the last trace of water is lost, fine crystals of gypsum precipitate and serve as anchors between the larger crystals thus increasing inter-crystalline cohesion and strength. Factors affecting the strength: 1. The type of gypsum products: Improved and dental stone are stronger than model plaster. 2. W/P ratio: The higher the W/P ratio of the original mix, the weaker will be the set gypsum products. This is because more “excess water” will remain, which will eventually vaporize leaving more pores and weaker product. 3. Dryness: Dry strength is higher than the wet strength. 4. Mixing time & rate: Increase Mixing time and rate will increase the strength. 5. Addition of chemicals “accelerators + retarders”: They decrease the strength by reducing the inter-crystalline cohesion. 13  Surface hardness and abrasion resistance: In general, the surface hardness of gypsum is low. Therefore, it is susceptible to scratching. Improved and dental stone are harder than model plaster. Reproduction of details: Gypsum dies do not reproduce surface details, because: a. The surface of the set gypsum is porous on the microscopic level. b. Air bubbles are formed at the interface of the impression and gypsum cast, because freshly mixed gypsum does not wet some rubber impression materials. The use of vibration during pouring of the cast reduces the presence of air bubbles. 14 Recent developments Because the mechanical properties are not ideal, fracture of teeth from models can occur with careless handling. Two techniques are tried to produce dental stones with improvement in abrasion resistance and strength: 1. Impregnation of the gypsum by a polymer such as polyester, polystyrene, acrylics and epoxy resin. 2. Addition of wetting agent such as lignosulfonates. Such additives reduce the water/powder ratio and enable the production of a harder, stronger and more dense set gypsum. Manipulation a. Storage: In closed containers to prevent reaction with moisture from the atmosphere which can cause formation of the dihydrate that can accelerate the setting. b. Correct water/powder ratio should be used. If stone is mixed with too much water, the set material may be as weak as plaster. c. Incorporation of the air in the mix should be avoided. d. The powder is to be sifted on to the water in the rubber bowel and allowed to stay into the water for 30 seconds. This is technique minimizes the amount of the air incorporation into the mix during spatulation. To obtain a smooth mix, the powder and water should be mixed slightly with a spatula having a stiff blade then vigorously and at the same time wiping the inside surfaces of the bowel with the spatula to be sure that all powder is wet and mixed uniformly with water. e. Vibration helps the mix to flow well into impression and helps to eliminate air bubbles (increasing strength). N.B: Mechanical mixers under vacuum are available and produce gypsum with superior properties. 15