Dental Materials

(Dental Gypsum Products)

Lecture 8
9 Pages

Prof. Siham Sayes

‫ ﺳﮭﺎم ﺳﺎﯾﺲ‬.‫أد‬

‫ﻗﺴﻢ طﺐ اﻷﺳﻨﺎن‬
‫اﻟﻤﺮﺣﻠﺔ اﻟﺜﺎﻧﯿﺔ‬

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 Dental Gypsum Products
Many dental restorations and appliances are constructed outside the
patient’s mouth using models which should be accurate replicas of the
patient’s hard and soft tissues.
The morphology of the hard and soft tissues is recorded in an impression and models
are prepared using materials which are initially fluid and can be poured into the
impression, then harden to form a rigid replica. These replicas are called study models,
casts, or dies. Each of these replicas has a specific purpose in dental practice.

Study models: A study purpose cast which has a positive likeness of an oral
cavity or any other object
Cast: Positive replica of an oral cavity or any other object. Casts of teeth are used
to make orthodontic retainers, and to make removable dentures for the patient.
Die: Positive replica of a prepared tooth (for crown and bridges) made out of gypsum
products, epoxy resins, a metal or a refractive material.

Properties of ideal model material

1. Dimensional stability (no expansion or contraction during or after setting).
2. High compressive strength to withstand the force applied on it.
3. Hardness (soft material can easily scratch).
4. Reproduce the fine details.
5. Produce smooth surface.
6. Resenable setting time.
7. Compatible with impression materials.
8. Can be disinfected without damaging the surface.

Dental Gypsum Products

Gypsum is a naturally occurring. Chemically, the mineral gypsum is a calcium sulphate
dihydrate (CaSO4. 2 H2O).
Gypsum products used in dentistry are based on calcium sulphate hemihydrate
(CaSO4. 1/2 H2O). Heat removes water and converts dihydrate to hemihydrate.

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There is difference in solubility of hemihydrate and dihydrate. Calcium sulphate
dihydrate is less soluble than calcium sulphate hemihydrate.

Application in Dentistry
1. Impression plaster was used for impressions of the mouth.
2. Casts and dies over which dental prostheses and restorations are made.
3. Mounting to the articulator.
4. Molds for processing dental polymers.
5. Dental investments: when plaster is mixed with silica it is known as dental
investment. They are used to form molds into which molten metal is cast.

Classification
The current ISO standard ((1998) for dental gypsum products identifies five
types of material as follows:
I. Type 1: Impression plaster
II. Type 2: Plaster
III. Type 3: Cast stone
IV. Type 4: Die stone, high-strength, low expansion
V. Type 5: Die stone, high-strength, high expansion

Composition
Gypsum products used in dentistry are formed by driving off part of the water from
gypsum to form calcium sulphate hemihydrate.
[Gypsum Powder] + [H2O] [Gypsum] + [HEAT]
[CaSO4-(1/2) H2O] + [(3/2) H2O] [CaSO4-(2) H2O] + [Heat]
Calcium sulphate hemihydrate calcium sulphate dihydrate
The hemihydrate is mixed with water and reacts to form the dihydrate.

Manufacture of dental gypsum products

The various types of gypsum product used in dentistry are chemically identical, in that
they consist of calcium sulphate hemihydrate, but they may differ in physical form
depending upon the method used for their manufacture.
The physical form of model plaster, stone, and die stone are different, but they are
made of the same chemical, calcium sulfate hemihydrate (CaSO2.1/2 H2O).

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 A. Manufacture of dental plaster
Plaster crystals are produced by heating gypsum to 115°C in air. Crystals are irregular,
porous, small, and do not pack well together. They are called β -calcium sulfate
hemihydrate. Therefore, making a useful mixture requires more water than for stone.
B. Manufacture of dental stone
Stone crystals are created by heating gypsum to 125°C under pressure in the presence of
steam. The hemihydrate produced under these conditions is called α -calcium sulfate
hemihydrate. This hydrate has larger, denser, more regular crystals than the β -calcium
sulfate hemihydrate of plaster. Their more regular shape allows better packing, requiring
less water for mixing.

C. Manufacture of die stone

Die stone crystals are the most uniform of all, produced by heating at 100°C in a 30%
solution of calcium chloride (CaCl2). The hemihydrate crystals produced in this case are
the least porous, most regular, and largest of all of the hemihydrates. The hemihydrate
crystals produced under these conditions is called α -calcium sulfate hemihydrate.
These require the least water for mixing. The strength of the set material is strongly
related to the residual porosity of the solid.

Setting Reaction
When plaster and stone are mixed with water it takes up one and a half molecules of
water, i.e., it regains its water and becomes calcium sulphate dihydrate.
CaSO4 ½H2O + 3/2H2O CaSO4-2H2O + HEAT
The reaction is exothermic and is the same for all gypsum products.

Theories of setting
There are two theories: Crystalline theory and Gel theory.
Crystalline theory is more widely accepted. The setting reaction is explained on the basis
of difference in solubility of hemihydrates and dihydydrate. Hemihydrate is four times
more soluble than dihydrate. The chemical reaction is relatively simple but the actual
microstructural events are more complicated.
Plaster (hemihydrate) dissolves in water. Gypsum (dihydrate) also is soluble in water,
but it dissolves only approximately one fourth as much as does hemihydrate particles.

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 - Once the hemihydrate particles contact water, they begin to dissolve. The
hemihydrate enters into solution and reacts with the water to form the dihydrate.
- Because gypsum is less soluble than plaster or stone, the solution soon becomes
saturated, and the dihydrate begins to precipitate throughout the mix as needle-like
crystals of gypsum.
- The loss of hemihydrate from the solution by the formation of gypsum allows more
hemihydrate to dissolve into solution. Soon, more dihydrate forms on the growing
gypsum crystals, and the process continues to repeat itself.
- Each crystal as it forms becomes nucleus for crystallization. During this process part
of gypsum form a gel which acts as a cementing medium between the crystals. Then
the rigid mass is formed by the interlocking network formed by the long needle- like
gypsum crystals.
CaSO4.1/2H2O + H2O CaSO4. 2H2O + Heat (exothermic reaction)

Water/powder ratio
The W/P ratio is a very important factor in deciding the physical and chemical properties
of the final product. The size distribution (porous) and shape of the powder particles
determines the relative amount of water that is required to make a suitable mixture (a
workable mix). Only a small amount of water is required to complete the reaction. For
chemical reaction of 100 g of calcium sulfate hemihydrate to dihydrate, 18 ml of water is
required. Additional water is required for mixing.

Reaction Water 18 cc 18 cc 18 cc
Extra Water 32 cc 12 cc 6 cc
TOTAL Water =
50 cc 30 cc 24 cc
Powder = 100 gm 100 gm 100 gm
W/P Ratio = 0.50 0.30 0.24
Excess water:
o The actual amount of water necessary to mix the calcium sulphate hemihydrate (to
make a workable mixture) is greater the amount required for the chemical
reaction (18 ml of water per 100 gm for hemihydrate).

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 o The excess water itself does not react with the hemihydrate crystals. It is
eventually lost by evaporation once the gypsum is set. The excess water serves
only to aid in mixing the powder particles and is replaced by voids.
The use of excessive water:
• Lengthens the setting time
• Reduces the strength
• Reduces the setting expansion of the set gypsum materials.
o As the amount of water decreased, the amount of residual porosity is
smaller, and the final strength is greater. Therefore, following the
recommendations of the manufacture is important in a decision regarding the
water-powder ratio.

Properties
Setting time
The total setting time for gypsum materials can be divided into:
I. The initial setting times
II. The final setting time.
Initial setting time is the interval between the time the water and powder are mixed
and the time that the mix can no longer be poured into a mold or impression.
Therefore, the initial setting time is equal to the working time of the material.
Clinically, the initial setting time can be watched when the freshly mixed material
loses its gloss.
Chemically, this loss of gloss occurs because the surface water is drawn into the bulk of
the material.
Gypsum should not be manipulated after the initial setting time (8 to 16 minutes).
The final setting time is defined as the time at which the conversion of the hemihydrate
to dihydrate is complete.
Clinically, this means that the gypsum then can be removed from its impression without
distortion or fracture. The final setting time is 45 to 60 minutes.
Factors affecting setting time:
Several factors can affect the setting time of gypsum products:
A. Mixing and spatulation: Within limits, the longer and faster the dental gypsum is
mixed, the faster it will set because nuclei of crystallization are broken and well
distributed within the mass.
B. Water/powder ratio: Increasing the amount of water ensures that less gypsum
crystals will be formed per unit volume of the mix. This increases the setting time
by prolonging the time required for the crystals to grow enough.

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 Reducing the W/P ratio cause a quickening of the setting reaction by increasing
the number of crystals per unit volume.
C. Temperature: Making a mix with warm water (37.5 ᵒC) would likely hasten the
setting time. If the water temperature is raised above 37.5 ᵒC, the setting time
will increase.
D. Humidity: Gypsum materials are hygroscopic (absorb water from the air). If
plaster or stone is left in an open container for several days, it absorbs water from
the air and surface of the particles will convert to the dihydrate. The effect during
mixing is an increased setting time, why?
Because the surface dihydrate have low solubility.
To avoid changes in the rate of the setting reaction, gypsum materials should be
kept in a closed container to protect them from humidity.
E. Modifiers (accelerators and retarders):
• Accelerators are chemicals that increase the rate of setting, and retarders
have the opposite effect.
• A 2% water solution of potassium sulfate substituted for pure water
reduces the setting time.
• Borax is a chemical that will extent the setting time of some gypsum
products to several hours if added to the powder at a concentration of 2%.
• Set gypsum particles (dihydrate) accelerate the reaction by acting as
nucleation sites upon (as nucleating agents) which new dihydrate particles
can form. These particles also are called terra alba, and concentrations of
0.5% to 1% added to the water are effective. A practical way of using terra
alba is to use the slurry water (water containing particles of dihydrate)
from the model trimmer to mix the gypsum.
• Colloidal particles such as blood, saliva, agar, or unset alginate retard the
setting reaction of gypsum. These colloids bind (cover) to the hemihydrates
and interfere with the addition of water to form the dihydrate. The result is
soft, easily abraded surface on the gypsum cast.
• A variety of things can act as retarders, including blood and saliva (that
might still be present on the surfaces of impression materials). To avoid
this problem, impression should be rinsed in cold water to remove traces
of blood and saliva before the impression is poured.
Setting expansion
Setting expansion is of two types:

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 I. Normal setting expansion
II. Hygroscopic setting expansion
Regardless of the type of gypsum product an expansion of the mass can be detected
during the change from the hemihydrate to the dihydrate after mixing with water.
I. Normal setting expansion:
There is an outward growth of crystals from nuclei of crystallization, as a result of the
growth there is an entanglement and there is interception between the crystals. If one
crystal intercepts another crystal there will be stress at the point of interception in the
direction of the growth the impinging crystals. If the process is repeated by thousands
of the crystals during growth, it is possible that the outward stress or thrust could
produce an expansion of the mass.
The final structure immediately after setting is composed of interlocking crystals
between which are micro pores containing excess water. On drying the excess water is
lost and the total empty space is greatly increased.
Factors affecting the setting expansion:
o W/P ratio: The higher the W/P ratio the less the expansion because of fewer
nuclei of crystallization per unit volume are present than the thicker mixes and
since it can be assumed that the space between the nuclei will be greater in such
case, it allows that there will be less growth interaction of the dehydrate crystals
with less outward thrust resulting.
o Addition of chemicals (accelerators and retarders): both will reduce the setting
expansion.
o Increased spatulation increases setting expansion.
II. Hygroscopic expansion:
If the setting process is allowed to occur under water the setting expansion may be more
than double in magnitude this is called hygroscopic expansion.

Strength
Gypsum is a brittle material, thus weaker in tension than in compression.
Factors affecting strength:
1. The effect of drying: The strength of the gypsum increases rapidly as the material
hardens after the initial setting time. The excess water presents in the set mass
affect the strength. Therefore, there is wet strength and dry strength. The wet
strength is when there is excess left in the mass. The dry strength is that when the
excess water has been dried in air or in oven in warm temperature. The dry strength
may be two times greater than the wet strength.

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 2. The effect of W/P ratio: Strength are related to W/P ratio and so the greater the
amount of free water in the set stone, the less the strength, the higher porosity.
3. Mixing time also effect the strength. Increase mixing increase the strength but over
mixing will reduce strength because it will break up the crystals which are formed
and will result in less crystal interlocking.
4. The addition of accelerators and retarders lowers both the wet and dry strength.

Hardness and abrasion resistance

This is related to the compressive strength. The higher the compressive strength of the
hardening mass, the higher the surface hardness. After the final setting occurs, the surface
hardness remains constant until most of the excess water is dried, after which it increases.

Flow
The flow of freshly mixed gypsum would depend on the amount of water used (W/P ratio).
The greater the amount of water used, the greater would be the flow. However, a
correctly proportioned mix has sufficient flow. Vibrating the mix greatly improves the flow.

Reproduction of detail
Gypsum products reproduce detail accurately. Significance this property is important for:
- Impression plaster because it has to accurately duplicate oral tissues.
- Cast material has to duplicate all the detail recorded by the impression.

Advantages and disadvantages

o Gypsum model and die materials have the advantages of being inexpensive and
easy to use.
o The accuracy and dimensional stability are good and are able to reproduce fine
detail from the impression.
o The mechanical properties are not ideal and the brittle nature of gypsum
occasionally leads to fracture- particularly through the teeth, which form the
weakest part of any model.
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