A number of gypsum products are used in dentistry as adjuncts to dental
operation.
1.
2.
3.
4.
5.
Type I: Impression plaster.
Type II: Dental plaster.
Type III: Dental stone (medium strength stone).
Type IV: Improved stone (high strength stone) (die stone).
Type V: high strength/high expansion stone.
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Impression plaster.
Mounting the casts to the articulation.
Form casts and dies.
Used as a binder for silica.
Used as a mold for processing dental polymers.
6- Used for bite registration (record centric jaw relation).
Properties of ideal model material (gypsum products):
Dimensional stability, no expansion or contraction during or after setting.
High compressive strength to withstand the force applied on it.
Hardness, soft material can be easily scratched.
Reproduce the fine details.
Produce smooth surface.
Reasonable setting time.
Compatible with the impression material.
Can be disinfected without damaging the surface.
Most gypsum products are obtained from natural gypsum rock. Because
gypsum is the dihydrate form of calcium sulfate (CaSO4. 2H2O), on
heating, it loses 1.5 g mol of its 2 g mol of H2O and is converted to
calcium sulfate hemihydrate (CaSO4. 0.5H2O). When calcium sulfate
hemihydrate is mixed with water, the reverse reaction takes place, and the
calcium sulfate hemihydrate is converted back to calcium sulfate
dihydrate.
1- Plasters are produced when the gypsum mineral is heated in an open
kettle at a temperature of about 110° to 120°C (dry calcination). The
hemihydrate produced is called β-calcium sulfate hemihydrate. Such
a powder is known to have a somewhat irregular shape and is porous
in nature. These plasters are used in formulating model and lab
plasters.
2- Stones are produced when the gypsum is dehydrated under pressure
and in the presence of water vapor at about 125°C (wet calcination),
the product is called hydrocal. The powder particles of this product
are more uniform in shape and denser than the particles of plaster.
Calcium sulfate hemihydrate produced in this manner is designated as
α-calcium sulfate hemihydrate. Hydrocal is used in making low- to
moderate-strength dental stones.
3- High-strength stones are produced when the gypsum rock is boiling
in a 30% calcium chloride solution, after which the chloride is
washed away with hot water (100°C), the product is called densite,
and the material is ground to the desired fineness. This variety is
made by gypsum The calcium sulfate hemihydrate in the presence of
100°C water does not react to form calcium sulfate dihydrate because
at this temperature their solubilities are the same. The powder
obtained by this process is the densest of the types.
1- Potassium sulfate, and terra alba (set calcium sulfate dihydrate) are
effective accelerators.
2- Sodium chloride in small amounts shortens the setting reaction but
increases the setting expansion of the gypsum mass.
3- Sodium citrate is a dependable retarder.
4- A mixture of calcium oxide (0.1%) and gum arabic (1%) reduces the
amount of water necessary to mix gypsum products, resulting in
improved properties.
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The setting reaction is explained on the basis of difference in the
solubilities of calcium sulfate dihydrate and hemihydrate. Hemihydrate is
four times more soluble than dihydrate.
When hemihydrate is mixed in water a suspension is formed which is
fluid and workable.
Hemihydrate dissolves until it forms a saturated solution. Some dihydrate
is formed due to the reaction.
Since solubility of dihydrate is much less than hemihydrate, the saturated
hemihydrate is supersaturated with respect to the dihydrate.
All supersaturated solutions are unstable. So the dihydrate crystals
precipitate out.
As the dihydrate precipitates out, the solution is no longer saturated with
hemihydrate and so it continues to dissolve. The process continues until
all hemihydrate converts to dihydrate.
Other theories include
.
The mixing process, called spatulation, has a definite effect on the setting
time and setting expansion of the material. Within practical limits an
increase in the amount of spatulation (either speed of spatulation or time
or both) shortens the setting time. Obviously when the powder is placed
in water, the chemical reaction starts, and some calcium sulfate dihydrate
is formed. During spatulation the newly formed calcium sulfate dihydrate
breaks down to smaller crystals and starts new centers of nucleation,
around which the calcium sulfate dihydrate can be precipitated. Because
an increased amount of spatulation causes more nuclei centers to be
formed, the conversion of calcium sulfate hemihydrate to dihydrate
requires somewhat less time.
The first effect of increasing temperature is a change in the relative
solubilities of calcium sulfate hemihydrate and calcium sulfate dihydrate,
which alters the rate of the reaction. As the temperature increases, the
solubility ratios decrease, until 100°C is reached and the ratio becomes
one. As the ratio of the solubilities becomes lower, the reaction is slowed,
and the setting time is increased.
The second effect is the change in ion mobility with temperature. In
general, as the temperature increases, the mobility of the calcium and
sulfate ions increases, which tends to increase the rate of the reaction and
shorten the setting time.
Practically, the effects of these two phenomena are superimposed, and the
total effect is observed.
Plaster can easily absorb water vapor from a humid atmosphere to form
calcium sulfate dihydrate. The presence of small amounts of calcium
sulfate dihydrate on the surface of the hemihydrate powder provides
additional nuclei for crystallization. Increased contamination by moisture
produces sufficient dihydrate on the hemihydrate powder to retard the
solution of the hemihydrate. Experience has shown that the common
overall effect of contamination of gypsum products with moisture from
the air during storage is a lengthening of the setting time.
Colloidal systems such as agar and alginate retard the setting of gypsum
products. Accelerators such as potassium sulfate are added to improve the
surface quality of the set CaSO4 .2H20 against agar or alginate.
Liquids with low pH, such as saliva, retard the setting reaction. Liquids
with high pH accelerate setting.
The operator also can change the setting time of model plaster to a certain
extent by changing the water/powder (W/P) ratio. The W/P ratio has a
pronounced effect on the setting time. The more water in the mix of
model; (plaster, dental stone, or high-strength dental stone); the longer the
setting time.
When set, gypsum products show relatively high values of compressive
strength. The compressive strength is inversely related to the W/P ratio of
the mix. The more water used to make the mix, the lower the compressive
strength. Model plaster has the greatest quantity of excess water, whereas
high-strength dental stone contains the least excess water. The set model
plaster is more porous than set dental stone, causing the apparent density
of model plaster to be lower.
After most excess water is evaporated from the surface, the hardness will
increase. Attempts have been made to increase the hardness of gypsum
products by impregnating the set gypsum with epoxy or methyl
methacrylate monomer that is allowed to polymerize.
The tensile strength of model plaster and dental stone is important in
structures in which bending tends to occur because of lateral force
applications, such as the removal of casts from flexible impressions.
Because of the brittle nature of gypsum materials, the teeth on the cast
may fracture rather than bend.
ANSI/ADA Specification No. 25 requires that types I and II reproduce a
groove 75 μm in width, whereas types III, IV, and V reproduce a groove
50 μm in width. Air bubbles are often formed at the interface of the
impression and gypsum cast because freshly mixed gypsum does not wet
some rubber impression materials (e.g., some silicone types). The use of
vibration during the pouring of a cast reduces the presence of air bubbles.
Contamination of the impression with saliva or blood can also affect the
detail reproduction.
When set, all gypsum products show a measurable linear expansion.
Under ordinary conditions, plasters have (0.2-0.3 %) setting expansion,
low to moderate strength dental stone about (0.15-0.25 %), and highstrength dental stone only (0.08-0.10 %). Typically, (over 75 %) of the
expansion observed at 24 hours occurs during the first hour of setting.
Increasing the W/P ratio; reducing the setting expansion. If during the
setting process, the gypsum materials are immersed in water, the setting
expansion increases slightly. This is called hygroscopic expansion.