DENTAL ANATOMY AND MORPHOLOGY:
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Lecture: "Gypsum Products"
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CHEMISTRY OF GYPSUM PRODUCTS
A.
Introduction:
1.
2.
3.
B.
Calcium Sulfate Dihydrate
= CaSO4 • (2) H2O (Gypsum)
Calcium Sulfate Hemihydrate
= CaSO4 • (½) H2O (Impression Plaster,
Model Plaster, Cast Stone, Improved Stone, Die Stone, Binder for Investment)
Important Properties: Consistency, Setting Time, Setting Expansion, Crushing
Strength (compressive strength), and Porosity.
Chemistry of Gypsum Product Formation:
1.
Setting Reaction (Chemical Reaction):
[CaSO4 • (1/2)H2O] + [(3/2)H2O]
dental use
----------> CaSO4 • (2)H2O
<----------
+ HEAT
manufacture
2.
3.
Plaster,Stone,..... + Water
----------> Gypsum
Setting Mechanism:
a. Dissolution of hemihydrate crystals into the water.
b. Nucleation and growth of dihydrate cystals from solution.
c. Interlocking crystals during growth form hard set mass.
+ HEAT
Manipulation Stages:
a. Mixing Stage
b. Working Stage
c. Setting Stage
C. Manufacture of Gypsum Precursors:
Plaster Powder
1.
2.
3.
4.
5.
Chemical Name:
Formula:
Powder Shape:
Density:
Production Steps:
a. Temperature:
b. Process:
6.
Dental Products:
Stone Powder
Diestone Powder
"Plaster of Paris"
β-cal.sulf.hemi.
CaSO4• (½)H2O
Irregular
Porous
"Hydrocal"
α-cal.sulf.hemi.
CaSO4•(½)H2O
Uniform
Moderately Dense
"Densite"
α-cal.sulf.hemi.
CaSO4•(½)H2O
Uniform
Dense
Heat to 115 °C
Heat in air.
Heat to 125 °C
Heat under
steam pressure
Cast Stone,
Investment
Heat to 100 °C
Heat in 30%
CaCl2 solution.
Improved Stone
or Die Stone
Model Plaster,
Impress. Plaster
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PROPERTIES
A.
Important Properties for Gypsum Products:
1.
2.
3.
4.
5.
Consistency:
Setting Time:
Setting Expansion:
Crushing Strength:
Porosity:
Plaster
----11 min
0.20 %
1,600 psi
35 %
Cast Stone
----7 min
0.10 %
3,000 psi
20 %
Improved Stone
----6 min
0.05 %
≥5,500 psi
10 %
VARIABLES INFLUENCING PROPERTIES
A.
Manufacturing Variables:
1.
2.
3.
B.
User's Variables:
1.
2.
3.
4.
5.
C.
Specific Product Selection
Proportioning (W/P Ratio)
a. Slurry Water
Mixing (Spatulation)
a. Vigor
b. Duration
Temperature
Additives
Proportioning of Materials for Gypsum Products: (**Microstone, W/P = 28/100)
Plaster
Cast Stone
Improved Stone
2.
Water:
a. Reaction Water
b. Extra Water
c. TOTAL Water
Powder:
18 cc
32 cc
50 cc
100 gm
18 cc
12 cc
30 cc**
100 gm
18 cc
6 cc
24 cc
100 gm
3.
W/P Ratio:
[0.50]
[0.30]
[0.24]
1.
D.
Hemihydrate Type:
Particle Size:
Additives:
a. Accelerators: Gypsum Particles, Potassium Sulfate (K2SO4)
b. Retarders:
Borax (Na2B4O7.10H2O), Colloids
c. Fillers:
Silica
d. Pigments:
Effects of Increases in Variables on Final Properties:
(“+” = Increases length of time = postpones; “-“ = Decreases = shortens; “0” = No effect)
1.
2.
3.
4.
5.
6.
Accelerators:
Retarders:
Particle Size:
W/P Ratio:
Spatulation:
Temperature:
Setting
Time
+
+
+
-
Setting
Expansion
+
+
0
Strength
+
0
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Preparation for mixing of plaster,
stone, or diestone -- measuring
water (cylinder) and powder
(balance).
Dispensing of weighed powder
into premeasured water within a
mixing bowl -- allowing the
powder to sift into the water.
Blending the powder and liquid
together with a stropping action
using a stiff bladed spatula to push
the mix along the sides of the bowl.
Smooth blend of stone.
Moving the mixed stone into the
alginate impression while
vibrating the impression tray to
help remove bubbles and
encourage flow into impression
details.
Microscopic view of growing
gypsum crystals that produce
setting by pushing against one
another and become interlocked as
well.
Gilmore needels (small and large)
that are used to detect the onset
and completion of the setting
period.
Example of a stone working cast
with a removable die.
Working die that has been waxed to
produce an inlay pattern.
INFECTION CONTROL
A.
Infection Control Approaches:
(1. Additives in Impression Materials)
2. Immersion of Impression Materials
3. Additives in Gypsum Products
B.
Effects of Disinfectants on Impression Materials:
1. Dissolution or plasticization of surfaces of impression materials.
2. Swelling of impression materials.
3. May alter reaction rates of stone.
C.
Effects on Gypsum Products:
1. Very effective / Some influence on set gypsum properties
2. Less wear resistant surfaces
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MULTIPLE CHOICE STUDY QUESTIONS:
1.
The chemical name for GYPSUM is:
a. Calcium sulfate anhydrite
b. Calcium sulfate hemihydrate
c. Calcium sulfate monohydrate
d. Calcium sulfate sesquihydrate
e. Calcium sulfate dihydrate
2.
The chemical formula for GYPSUM is:
a. CaSO4 • (1/2) H2O
b. CaSO4 • (1) H2O
c. CaSO4 • (3/2) H2O
d. CaSO4 • (2) H2O
e. CaSO4 • (5/2) H2O
3.
The setting reaction of plaster with admixed water is:
a. Endothermic
b. Incapable of being chemically accelerated
c. Affected by exposure to light
d. Not sensitive to temperature
e. Accelerated by the addition of slurry water
4.
HYDROCAL is a commercial example of:
a. Calcium sulfate anhydrite
b. Calcium sulfate hemihydrate
c. Calcium sulfate monohydrate
d. Calcium sulfate sesquihydrate
e. Calcium sulfate dihydrate
5.
The production of STONE differs from PLASTER mainly in regard to:
a. Gypsum particle size
b. Gypsum particle shape
c. Processing atmosphere
d. Rate of heating
e. Time of heating
6.
The main ingredient of dental STONE is:
a. Alumina
b. Silica
c. Calcium sulfate hemihydrate
d. Starch
e. Colorants
7.
Compared to plaster, DIE STONE:
a. Requires much less water for mixing
b. Produces a more porous gypsum product
c. Expands much more on setting
d. Produces much less heat on setting
e. Requires much less spatulation for mixing
8.
Crushing strength of a set gypsum product is related primarily to:
a. Water temperature during mixing of the materials
b. Setting time of the mixture
c. Setting expansion of the mixture
d. Amount of porosity remaining in the set material
e. Mixing technique
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9.
The final porosity in set PLASTER is approximately:
a. 15%
b. 20%
c. 20%
d. 25%
e. 35%
10. The crushing strength of DIE STONE is at least:
a. 1,500 psi (10 MPa)
b. 2,500 psi (17 MPa)
c. 3,500 psi (24 MPa)
d. 4,500 psi (31 MPa)
e. 5,500 psi (38 MPa)
11. Which one of the following gypsum product mixtures will be the most fluid?
a. Plaster,
at W/P = 0.50
b. Stone,
at W/P = 0.40
c. Stone,
at W/P = 0.30
d. Improved Stone
at W/P = 0.27
e. Improved Stone
at W/P = 0.24
12. Which one of the following should have the highest crushing strength when it is
set and dry?
a. Plaster,
at W/P = 0.60
b. Plaster,
at W/P = 0.50
c. Stone,
at W/P = 0.50
d. Stone,
at W/P = 0.40
e. Improved Stone,
at W/P = 0.50
13. The best way to EXTEND the working time of a STONE mix in the laboratory is to:
a. Increase the W/P ratio, and increase the spatulation
b. Increase the spatulation, and decrease the temperature
c. Increase the temperature, and decrease the W/P ratio
d. Decrease the W/P ratio, and add a retarder
e. Decrease the temperature, and decrease the spatulation
14. The setting reaction of plaster, stone, and/or diestone:
a. Requires the dissolution of the powder particles in the water
b. Involves the adsorption of the water in the mixture into the powder particles
c. Depends on the presence of chemical modifiers to occur
d. Does not involve calcium sulfate hemihydrate
e. Produces a major change in volume of the mixture during setting
15. The setting expansion for improved stone (DIE STONE) is:
a. 0.05 %
b. 0.10 %
c. 0.20 %
d. 1.00 %
e. 2.00 %
16. The setting reaction of plaster is accelerated by all of the following EXCEPT:
a. Increased temperature
b. Rapid spatulation
c. Low relative humidity
d. Slurry water
e. Potassium sulfate
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17. Which of the following does NOT retard the setting of STONE?
a. Borax
b. Colloids
c. Blood
d. Reduced temperature
e. Slurry water
18. What is the theoretical amount of water required to react with 100 gms of calcium
sulfate hemihydrate to form calcium sulfate dihydrate?
a. 15 cc
b. 18 cc
c. 24 cc
d. 30 cc
e. 50 cc
19. Slurry water is:
a. Special accelerator solution sold with plaster, stone, and die stone
b. Dilute solution of calcium sulfate dihydrate crystals that are nucleating agents
c. 5% solution of potassium sulfate in water and glycerin
d. Dirty water
e. The name of the mixture of powder with liquid before setting occurs
20. Plaster powder exposed to air for long periods of time before use reacts differently
because:
a. Humidity is absorbed into the powder and pre-reacts with it
b. Oxygen is absorbed onto the particles and retards the reaction
c. Oxygen degrades calcium sulfate hemihydrate into a non-hydrated form
d. Dirt in the air becomes mixed with the powder and acts as nucleating agents
e. Oxygen degrades the accelerators and retarders in the mixture
© 2004-2005, Stephen C. Bayne, Chapel Hill, NC 27514