Gypsum edentulous by jennyyingdi

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									                                    Gypsum
                                        TNT

                    Compare between gypsum products

   Difference             Plaster                dental stone       High strength stone

   Method of             Open air                 Autoclave             30%Cacl2
   production

 Particles size &   Irregular, porous            Regular, less         Regular, less
      shape                                        porous                porous

   W/P Ratio            50-60/100                   30/100                22/100

Setting expansion       0.2-0.3 %                   0.08 %             0.05-0.07 %

  Compressive          12.5 Mn/m2                31.5 Mn/m2            38.5 Mn/m2
    strength
   Hardness         15 R.H.N                      60 R.H-N              80 R.H-N

Porosity % after           45 %                     15 %                  10 %
    setting
     Uses            -Primary cast(for          -Secondary cast     -Secondary cast &
                    edentulous patient)       -Secondary models      models for crown
                        -impression               for partially       & bridge work
                          material            edentulous patients        -binder in
                          -flasking           -binder investment        investment
                       -articulating                material




                                          1
Account on setting time
def.
-it is the time required for the reaction to be completed
A-initial setting time
def.
-it is the time from the beginning of mixing till the mix reach a random stage of
 strength
 properties
-the material is weak and will not flow
-it is possible to carve away the excess material with a knife
-it takes about 12 minutes to reach this stage
B-final setting time
def.
-it is the time by which the material is completely set
-the material continue to develop strength for some time after the initial setting .
properties
-the model or die will be strong and hard to work upon
-it may take several hours after mixing
MEASURING SETTING TIME
1-loss of gloss indicates the initial setting of the mass
2-the temp. rise also indicates setting , since the reaction is exothermic
3-penetration test
-initial setting time can be measured by:
              Vicate needle                 or                      Gillmore needle
          rod weighting 300g                                  rod weighting ¼ pound
        needle of 1 mm diameter                          needle of 1/12 inch in diameter

-final setting time can be measured by :
              Gillmore needle
          Rod weighting 1 pound
            needle of 1/24 inch




                                            2
FACTORS AFFECTING SETTING TIME
A-FACTORS CONTOLLED BY THE MANUFACTURE
1-impurities:
-CaSO4.2H2O “gypsum” about 0.5% present as impurities “terra alba” …will
 accelerate the setting time by providing nuclei of crystallization
2-fineness
-the smaller the particle size of hemihydrate , the faster the mix will harden
-if the powder is small in size, this will lead to increase rate of dissolution of
 hemihydrate and add more nuclei of crystallization
3-chemicals
-accelerators :
 NaCl2  up to 2% , but higher concentration retard the setting time
 Tartarates ,sodium sulphate and potassium sulphate are also accelerators
 Na2SO4 …….3.4%
 K2 SO4 ……..2%             “most commonly used

-they act by adding more nuclei of crystallization and also increase the rate of
 hemihydrate solubility without affecting the solubility of dihydrate
-if their conc. Is increased above certain limits , their action is reversed and they
 become readers ….because they can deposit on the surface of crystals

-retarders : “ Baag”
- e.g. borax , agar , alginate , gums
-they form a coat around the hemihydrate particles , this decreases the rate of
 solubility

B-FACTORS CONTROLLED BY THE OPERATOR
1-mixing time and rate :
 -increasing hand or mechanical mixing , breaks up some of the formed dihydrate
  which increases the nuclei of crystallization and accelerates the setting time
2-w/p ratio
 -increasing w/p ratio , decreases nuclei of crystallization and this will retard S.T.
3-water temperature :
 -25°C-50°C accelerates S.T.
 -50°C has little effect
 -above 50°C the reaction is retarded
 -at 100°C no setting and the reaction is reversed




                                            3
WATER/POWDER RATIO

CaSO4 ½ H2O + 1 ½ H2O                      CaSO4.2 H2O + HEAT
M.W: 145.15 + 27.02                         172.17

From this equation : the amount of water required for each 100gm powder can be
Calculated
Powder/gm                                                water/ml
100                                                        ?
145.15                                                    27.02

amount of water = (27.02 )(100)/145.15 =18.61 ml.water

-theoretically , each 100 gm of plaster or dental stone requires only 18.61 ml water
 to complete the chemical reaction .
-any excess water will lead to reduction in strength
-however water in excess is required to make the mix workable .
-this excess water evaporates leaving pores
-due to the difference in the particle size ,shape , and porosity of different gypsum
 products …different amounts of water is required to make the mix workable


         Products               W/p ratio
         Plaster                50-60/100

         Dental stone           30/100

         High strength stone    22/100




                                            4
DIMENSIONAL CHANGES OF GYPSUM

A-setting expansion
-setting expansion may be considered as an advantage or disadvantage since it
 may be a source of error
-so , it is necessary to control it to obtain the desired accuracy
-it is due to the thrusting action of crystal during growth
-percentage of setting expansion and percentage of porosity :
      Gypsum product            Linear setting expansion % % porosity
      Plaster of paris          0.2-0.3                        45
    Dental stone               0.08                           15
    High strength stone        0.05-0.07                      10
Setting expansion can be decreased by :
A-adding chemicals
 -chemicals which accelerate the S.T. , decreases the setting expansion because they
  regulate the shape of the crystals and limit its growth and reduce their ability to
  push each other apart
B-increasing w/p ratio
 -this will decrease the nuclei of crystallization , on the other hand this will increase
  the space between the particles and decreases the thrusting action
C-mixing time and rate
  -decreasing mixing time and rate decreases the nuclei of crystallization

B-hygroscopic expansion
-it occurs when the gypsum is allowed to set under water in the initial stage of setting
-this is due to additional crystal growth
-it is double the setting expansion
-water usually present around the particles and not inside them
-hygroscopic expansion can be affected by immersion time and amount of water
 added and w/p ratio of the original mix
-decrease w/p ratio increases the hygroscopic expansion “this is important in casting”
-the more water added at the initial setting time ..the more the H.E.

STRENGTH OF GYPSUM
-the free water content of the set product affects its strength , for this reason there
 are 2 types of gypsum strength
1-wet strength : “one hour strength “
  -the strength when the excess water “more than the amount required to hydrate
   the hemi hydrate” is left in the test specimen
2-dry strength : “7 days strength”
  -the strength obtained when the specimen has been dried “free of excess water “



                                             5
strength of gypsum depends on:
a-dryness of the set material : the material should be left to dry at least for 1 hour
  “wet strength”
 -the dry strength can be obtained after 7 days , it is usually double or more than the
  wet strength
b-the material : that is used . for example, autoclave or calcined hemihydrate
c-the additive present : the strength of the material is usually reduced by the
  presence of additives
 -these additives may reduce the intercrystalline cohesion
d-w/p ratio : increasing the w/p ratio decreases strength
              -the set product is porous in nature and increased w/p increases
               porosity and decreases strength




                                           6

								
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