Gypsum Products Notes PDF

GYPSUM Strength Less More Rocks or solid particles which were Surface More Less ground to powder form and then Surface More Less...

GYPSUM Strength Less More Rocks or solid particles which were Surface More Less ground to powder form and then Surface More Less area/wt. heated. Example Dental plaster Dental stone Chemically, the gypsum produced for & Impression dental applications is nearly pure plaster CALCIUM SULPHATE DIHYDRATE (CaSO4⋅2H2O) CHEMICAL EQUATION Calcination – process of heating (Manufacturing of Gypsum) gypsum o Open Calcination 110-120°C o Closed Calcination CaSO4⋅2H2O -------- CaSO4⋅1⁄2 H2O + 11⁄2 H2O Calcium Sulfate Calcium Sulphate Hemihydrate Dihydrate Open Calcination - Heating is done in a kettle or an open bath while stirring at 110° to 120°C Improved Stones (230° to 250°F) Gypsum + 30% Calcium Chloride - Product: β-hemihydrate/Plaster of Solution at 100°C Paris Characteristics: - Plaster of Paris consists of large, 1. Very dense crystals irregularly shaped orthorhombic 2. Cubic or rectangular crystals crystal particles with capillary 3. Large crystals pores 2 Forms - Microscopically, crystals are spongy o Type IV Gypsum = Dental Stone and irregular in shape High Strength o Type V Gypsum = Dental Stone Closed Calcination High Strength High Expansion - Heating is done in autoclave at 16PSI, 120° to130°C ADA CLASSIFICATION NO.25 OF GYPSUM - It produces α-hemihydrate/Dental PRODUCTS Stone - Consists of smaller, regularly Type 1: Impression/Soluble Plaster shaped crystalline particles in the - Used to take impression form of rods of prisms Type 2: Plaster Model/Plaster of Paris - Crystals are prismatic and more - Used to make study cast/Model for Records regular in shape Purposes β- α- Type 3: Dental Stone (Class 1 Dental Stone) HEMIHYDRATE HEMIHYDRATE I. Stronger than Plaster of Paris Crystal Larger Smaller II. Used as working cast that can Size Shape Irregular Prismatic withstand forces applied Packing Loosely packed Closely packed Type IV: Die Stone ample space little space b/w - High strength, lower expansion b/w crystals crystals Type V: Dental Stone High Strength High W/P More Less Expansion (Investment Model) ratio - Used as investing medium to compensate the o Vicat needle – measure initial high shrinkage of metals such as chrome setting time (1/4 lb.) cobalt ion. o Gillmore needle – measures final setting time (1/4 LB for IST & OTHER USES IF GYPSUM PRODUCTS 1 LB for FST) Used for ceramics Used for medical purposes = cast for EXOTHERMIC SETTING REACTION broken bones The maximum temperature is reached Used to stabilize articulation between during the stage when final hardening upper and lower cast occurs. Temperature rise depends on the bulk of material used and can reach TERMINOLOGIES 30°C at the center of a mass of setting Mixing Time (MT) material. This may be maintained for Working Time (WT) several minutes due to the thermal Setting Time (ST) insulating characteristics of the materials. MIXING TIME Addition of the powder to the water until SETTING TIME the mixing is completed. Initial Setting Time Gypsum should be soaked in water for - Measured from the start of mixing until 20 seconds it reaches a semi-hard stage Mechanical mixing is usually - Record the reading/temperature of completed in 20 to 30 seconds gypsum every two minutes - The 2nd increase of temperature Hand spatulation generally requires at - 7-13 minutes least a minute to obtain a smooth mix WORKING TIME (INITIAL SETTING TIME) Final Setting Time - Measured from the start of mixing until Time available to use a workable mix the 2nd decrease of temperature A uniform consistency is maintained to - 20 minutes perform one or more tasks Measured from the start of mixing to the SIGNIFICANCE OF FINDING I.S.T AND F.S.T point where the consistency is no longer I.S.T – Time to trim off extra plaster or acceptable for the product’s intended stone that overflowed purpose - To prevent the breakage of the cast Generally, a 3-minute working time is F.S.T – Cast separation is done after 45 adequate mins. – 1 hour - To prevent premature separation; SETTING TIME prone to breakage Time that elapses from the beginning of - To prevent powdery casts mixing until the material hardens This is usually measured by some type of penetration test, using instruments like:  WATER POWDER RATIO (W/P) FACTORS AFFECTING SETTING EXPANSION 1) W/P Ratio – Lesser w/p ratio, longer mixing Type WP Ratio C.S. (psi) Setting Exp. time, greater SE I. Plaster.50 -.75 290 - 870 0 -.15% 2) Spatulation/Mixing Procedure – increase in Impression amount of spatulation, shorter ST, increased II. Plaster.45 -.50 1300 0 -.30% SE of Paris III. Dental.28 -.30 3000 0 -.20% 3) Use of Chemical Modifiers – Generally Stone reduce setting expansion, makes crystals flat, IV. Die.22 -.24 5000 0 -.10% less effective on outward thrust of crystals. Stone (potassium sulfate, borax) V. Dental.18 -.22 7000.10 -.30 % Stone high strength, PRACTICAL FACTORS AFFECTING SETTING high exp. TIME Presence of impurities like set plaster or stone CHEMICAL EQUATION - Decrease setting time: Faster setting (SETTING OF GYPSUM) Fineness: Particle size and shape of crystals WATER - Smaller crystals, more soluble, faster H2O+ CaSO4⋅1⁄2 H2O ----- CaSO4⋅2H2O + HEAT Calcium Sulphate Calcium Sulphate W:P Ratio Hemihydrate Dihydrate o More water, less powder: Longer ST - Too thin: Prone to air bubbles Type I: Impression Plaster (0.13%) o Less water, more powder: Type II: Model Plaster (0.3%) Shorter ST Type III: Dental Stone (0.15-0.2%) - Too thick: Not all particles Type IV: Die Stone (0.08%) (powder) will be mixed with water Type V: High Strength, High Expansion Mixing time – 2 minutes Dental Stone (0.3%) - Longer time, faster mixing RPM, faster ST Temperature SETTING EXPANSION - Up to 50° C: Not much change in Two types: setting reaction 1. Normal Setting Expansion - Above 50°C: Retarder - Refers to the setting expansion when a - 100°C: NO reaction gypsum product is allowed to expand in air. Accelerators and Retarders o Accelerators – tend to remove 2. Hygroscopic Setting Expansion the surface coating of crystals - Refers to the expansion of a gypsum product making them more soluble to when it is allowed to set, immersed in water. water: Faster setting time - More expansion - Low concentration of NaCl and - Increase the setting expansion of gypsum- Na2 SO4 bonded investment materials. - K2 SO4 – best accelerator at concentrations greater than 2%  o Retarders – tend to create SUMMARY surface coating of crystals Setting Setting Strength making them less soluble to Time Expansion water: Slower setting time 1. Accelerators - - - - Borax, nitrates, citrates - Higher concentration of NaCl 2. Retarders + - - and Na2 SO4 3. Particle Size + + + STRENGTH AND HARDNESS 4. W/P Ratio + - - - Is the result of the interaction and 5. Spatulation - + - entanglement of the crystals - Measured in terms of compressive strength 6. Temperature - 0 0 Laboratory plaster (27.5 MPA/ 4000 PSI) Dental Stone (62 MPA/ 9000 PSI) CARE OF GYPSUM PRODUCTS Die Stone (79 MPA/ 11,500 PSI) A. Store in an airtight container to prolong shelf-life. Shelf-life is the property to preserve *Note: Dry strength or gypsum (after 24 hours) life w/o deterioration of product. is twice the wet strength B. Shake bottle first before getting powder C. Use a dry spoon or scoop when getting 2 TYPES OF STRENGTH powder 1) Wet strength = when water, in excess of D. Always follow correct W/P ratio what is required for the hydration of hemihydrate is left in the specimen PROPERTIES OF A GOOD CAST 2) Dry strength = when the specimen is dried A. Must have high strength – reduces the free of excess water. The dry strength is twice chance of fracture the wet strength B. Should be hard – so that surface will have no scratches during carving FACTORS CONTROLLING STRENGTH C. Accurately measured and stable – setting 1. W/P Ratio = greater w/p ratio, greater expansion should be controlled porosity, the weaker the cast; the greater w/p D. Compatible with impression material – no ratio the lesser the dry strength reaction 2. Spatulation = increase in mixing time, E. Good color contrast with other materials increase strength, if w/in 1 minute. Overmixing F. Should be smooth and free of bubbles has the reverse effect. G. Cheap and easy to use 3. Chemical Modifiers = addition of both accelerator and retarder lower both the wet DISINFECTING SOLUTION FOR CASTS and dry strength of gypsum products. Iodophore used ACCDG. To manufacturer’s instructions or by immersion in 1:10 dilution of sodium hypochlorite solution for 30 minutes. In a hospital dental clinic, a model poured from a non-disinfected impression can be aseptically wrapped and sterilized in ethylene oxide.