Sunday, August 25, 2013

gypsum products



*         GYPSUM




*         PROPERTIES





*         CONCLUSION

*         REFERENCES

*       Auxiliary dental material.
*       Glossary of  Prosthodontic terms:
*      Model: Facsimile used for display purposes, a miniature                    representation of something.
*     Dental Cast:  A positive life size reproduction of a part or parts of                  the oral cavity.
*      Die: The positive reproduction of the form of a prepared tooth in               any suitable substance.
*       Direct link between the clinical phase of treatment and the technical  
     laboratory procedures.
*       Contributes significantly to the ultimate success of the prosthesis.

*      Originates from the Greek word ‘Gypos’ which means chalk.
*      Dihydrate of calcium sulfate.
*      Chemical formula CaSO4.2H2O.
*      Sulfate mineral most commonly found.
*      Usually white to yellowish white in color.
*      Large beds of gypsum were formed when seawater evaporated, leaving dissolved Calcium and Sulfate ions to form deposits of gypsum.  
*      United States is the largest producer as well as the biggest consumer of gypsum. Others are Canada, France, Japan and Iran.

Gypsum mineral can be found in various forms:
  1. ROCK–GYPSUM: widely occurring massive dull coloured rocks.
      2.   ALABASTER: Large fine-grained white stones. Often used for carving into vases and ornaments. Also used in building of King Solomon’s Temple
      3.   SELENITE: as transparent crystals
      4.   SATIN SPAR: as fibrous crystals.


           Also produced as a by-product of manufacture of phosphoric acid.

3 H2SO4(l) + Ca3(PO4)2(s) + 6 H2O(l)      2 H3PO4(s) + 3 CaSO4·2H2O(s)


ü  Found as prismatic, curved or twisted monoclinic crystals of   vitreous luster.       LATTICE TYPES

ü  Specific gravity: 2.3

ü  It cleaves perfectly in one direction.                                                      

ü  Moh hardness no: 2, which means that it can be scratched by a fingernail.

  1. As a raw material for making Plaster of Paris. Plaster of Paris is called so, because the gypsum that was used to manufacture it came from a village called Montamarte, near Paris. It is used extensively in construction purposes. 
  1. Grounded gypsum (land plaster) is sometimes used as a fertilizer for soil that needs calcium.
  2. Raw gypsum is also used to keep Portland cement from hardening too quickly. 
  1. It is also used to make paint (as a filler), filters, insulation and wall plaster. 
  2. Alabaster is used for carving ornaments and vases.
  1. Selenite is sometimes used as an optical material.

Gypsum produced for dental application is nearly pure calcium sulfate dihydrate. 

         Impression plaster is used to make the impression of the edentulous     mouth.
         For preparation of study models of oral and maxillofacial structures.
         To form cast and dies on which dental prosthesis are constructed.
         As a mold material for processing complete dentures.
         For mounting of casts on the articulator.
         Also used as a binder for silica, gold alloy casting investment, soldering investment and investment for low melting point nickel-chromium alloys.


*      Refers to the various forms of calcium sulfate, hydrous and anhydrous.
*      Manufactured by the calcination of calcium sulfate dihydrate.
*      Calcination can be controlled to produce partial or complete dehydration.
     ADA No: 25.                                             ISO  No: 6873.

Gypsum products can be classified into five types:

                                          GYPSUM PRODUCTS

 ( ISO TYPE I )                        ( ISO TYPE II )               

                                                                                           STONE                                                                                                                                  ( ISO TYPE III )
                                                                                          STONE, HIGH STRENGTH,                                                                                                LOW EXPANSION                                                                                                        ( ISO TYPE IV )
                                                                                          STONE, HIGH STRENGTH,                                                                                                HIGH EXPANSION                                                                                                            ( ISO TYPE V )


In the temperature range of 20º to 700ºC that is important in dental manipulation of gypsum products, three phase changes occur in the CaSO4-H2O system.
                                  40º – 50ºC
  1) CaSO4.2H2O                            CaSO4.1/2H2O   +    Water
                                                      (Calcium Sulfate
                                 90º – 100ºC
 2) CaSO4.1/2H2O                          γ-CaSO4            +    Water              
                                                    (Hexagonal Form)
                                                    (Soluble anhydrite)
                                 300º – 400ºC
 3)  γ-CaSO4                                  CaSO4
                                                      (Orthorhombic CaSO4)
                                                      (Insoluble anhydrite)


Calcium Sulfate Hemihydrate   Soluble anhydrite  Insoluble anhydrite
Calcium Sulfate Dihydrate


From the conversion temperature given before, it can be seen that calcium sulfate hemihydrate would be produced by heating gypsum to temperatures in the range of 50o to 90 oC. However, at these temperatures the reaction is slow; even at 90oC substantially complete conversion takes about 12 hours (Khalil et. al., 1971). Therefore, in commercial processes temperatures higher than this are used, for shorter times.

The stable phase at these higher temperatures is hexagonal calcium sulfate, so the initial product of calcination is partly of very largely this anhydrous form. However, on cooling to temperatures below 85 oC and exposure to atmospheric moisture, the hexagonal calcium sulfate rehydrates to form the hemihydrate.

In the production of plaster, the gypsum is ground to a fine powder, impurities such as sulphur (S) and quartz (SiO2) are removed, and then it is subjected to calcination.

                                      dehydration by heat
                                         or other means
 Mineral Gypsum                                              formulation
                                                             Plasters                 Dental plaster
Synthetic Gypsum                              Hydrocal                Dental Stone       
                                                            Densite                   High strength
                                                                                           dental stone