Lecture 2 - Gypsum Products.ppt

Gypsum Products the gypsum produced for dental applications is nearly pure Calcium Sulfate Dihydrate (CaS04•2H2O)  Calcination – process of heating gypsum   Open Calcination Closed Calcination .Gypsum  Rock or solid particles which have been grounded into powdered form and then heated.  Chemically. irregularly shaped orthorhombic crystal particles with capillary pores  Microscopically. crystals are spongy and irregular in shape .Open Calcination  Heating is done in a kettle or an open bath while stirring at 110° to 120°C (230° to 250°F)  Produce ß-hemihydrate/Plaster of Paris  Plaster of Paris consists of large.  Crystals are prismatic and more regular in shape .Closed Calcination  Heating is done in autoclave at 120° to 130°C  It produces hemihydrate/Dental stone  Consists of smaller. regularly shaped crystalline particles in the form of rods or prisms. ADA Classification of Gypsum Products  Type I:Impression/Soluble Plaster   TypeII:Plaster Model/Plaster of Paris    stronger than plaster of paris Used as working cast that can withstand forces applied Type IV:Dental Stone High Strength (Class II Dental Stone)     Used to make study cast/model for record purposes TypeIII:Dental Stone (Class 1 Dental Stone)   Used to take impression Stronger than Type III Used as working die for crown restoration and other treatments needing accurate measurements Die material-refers to single tooth/several teeth Type V:Dental Stone High Strength High Expansion  Used as investing medium to compensate the high shrinkage of metals=chrome cobalt ion . Other Uses of Gypsum Products  Used for ceramics  Used for medical purposes=cast for broken bones  Used to stabilize articulation between upper and lower cast . Chemical Equation CaSO4•2H2O + heat CaSO4•½H2O+1½H2O (CaSO4)2•H2O Calcim Sulfate Hemihydrate . Setting and Final Setting Times  Mixing Time (MT)  Working Time (WT)  Setting Time (ST) .Test for Working.  Hand spatulation generally requires at least a minute to obtain a smooth mix.  Mechanical mixing of stones and plasters is usually completed in 20 to 30 seconds.Mixing Time  Defined as the time from the addition of the powder to the water until the mixing is completed. . one that maintains a uniform consistency to perform one or more tasks. a 3 minute working time is adequate .Working Time  Is the time available to use a workable mix.  It is measured from the start of mixing to the point where the consistency is no longer acceptable for the product’s intended purpose.  Generally. measure initial setting time Gillmore needle:   ¼ lb – measures initial setting time 1 lb – measures final setting time .  This is usually measured by some type of penetration test.Setting Time  The powder is mixed with water. using instruments like:   Vicat needle . and the time that elapses from the beginning of mixing until the material hardens. Setting Time  Initial Setting Time    Measured from the start of mixing until the 2nd increase of temperature 7-13 minutes Final Setting Time    Measured from the start of mixing until the 2nd decrease of temperature 20 minutes Record the reading/temperautre of gypsum every two minutes .  To prevent the breakage of the cast.T.  F.S. – cast separation is done after 30 mins.T. – 1 hour.S. – time to trim off extra plaster or stone that overflowed.  I.Significance of Finding I.S. and F.T.   To prevent premature separation.T.S. prone to breakage To prevent powdery casts . Manipulation  Know the W:P ratio  Put the water then powder in the bowl  Let the powder settle for 30 seconds to remove air  Mix it using rotatory motion at 120 rpm for 1½ minutes  Tap the mixture to release entrapped air . the longer  Solubility  More soluble.Theoretical Factors Affecting Setting Time  Number   of nuclei crystallization Greater number of nuclei. decrease ST. the shorter Few number of nuclei. faster setting time . the faster the setting time  Rate  of the hemihydrate of crystal growth Faster rate of crystallization. increase ST. faster W:P Ratio     Too thin : prone to air bubbles Too thick : not all particles (powder) will be mixed with water More water. more powder : shorter ST . more soluble.Practical Factors Affecting Setting Time  Presence of impurities like set plaster or stone   Fineness-particle size and shape of crystals   Decrease setting time. less powder : longer ST Less water. Smaller crystals.    Mixing Time – 2 minutes  Longer time. faster mixing RPM. higher concentration of NaCl and Na2SO$ . faster the setting time Temperature  Up to 50°C : accelerator  Above 50°C : retarder Accelerators and Retarders  Accelerators – tend to remove the surface coating of crystals making them more soluble to water : faster setting time  Low concentration of NaCl and Na2SO4  K2SO4 = best accelerator regardless the concentration  Retarders – tend to create surface coating of crystals making them less soluble to water : slower setting time  Borax. citrates. nitrates. 30 3000 IV.S.Water Powder Ratio Type WP Ratio C. Plaster of Paris .18 . Plaster Impression ..45 .24 5000 V. Dental Stone .. Die Stone . high expansion ..870 II. Dental Stone high strength.75 290 .50 -..50 1300 III.28 . (psi) I.22 .22 7000 Example: For every 100grams of powder 50=70 cc of water . Factors Affecting Compressive Strength  Shape and size of the particles  W:P ratio  Mixing technique (undermixing/overmixing)  Use of chemical modifier .