Gypsum products /certified fixed orthodontic courses by Indian dental academy

GYPSUM PRODUCTS

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INDIAN DENTAL ACADEMY
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INTRODUCTION
GYPSUM
CLASSIFICATION
MANUFACTURE
SETTING PROCESS
PROPERTIES
MANIPULATION
COMPATIBILITY
INFECTION CONTROL
RECENT DEVELOPMENTS
CONCLUSION

REFERENCES

INTRODUCTION
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.

GYPSUM:
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.



KING SOLOMON’S TEMPLE

3. SELENITE: as transparent crystals.


4. SATIN SPAR: as fibrous crystals.


SYNTHETIC GYPSUM:
• Also produced as a by-product of manufacture of phosphoric acid.
• CHEMICAL GYPSUM.

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

2 H3PO4(s) + 3 CaSO4·2H2O(s)


PROPERTIES:


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



Specific gravity: 2.3



It cleaves perfectly in one direction.



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

USES:
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.
2. Grounded gypsum (land plaster) is sometimes used as a fertilizer for
soil that needs calcium.
3. Raw gypsum is also used to keep Portland cement from hardening
too quickly.
4. It is also used to make paint (as a filler), filters, insulation and wall
plaster.
5. Alabaster is used for carving ornaments and vases.
6. Selenite is sometimes used as an optical material.

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

DENTAL USES OF GYPSUM PRODUCTS:
• 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 nickelchromium alloys.

GYPSUM PRODUCTS:
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
IMPRESSION PLASTER

PLASTER

( ISO TYPE I )

HIGH STRENGTH PLASTER

( ISO TYPE II )

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

CHEMISTRY OF GYPSUM:
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.
o
o
40 – 50 C

1) CaSO4.2H2O

CaSO4.1/2H2O + Water
(Calcium Sulfate
Hemihydrate)
o
o
90 – 100 C

γ-CaSO4
+ Water
(Hexagonal Form)
(Soluble anhydrite)

2) CaSO4.1/2H2O

o

3) γ-CaSO4

o
300 – 400 C
4

CaSO
(Orthorhombic CaSO4)

MONOCLINIC
Calcium Sulfate Hemihydrate

HEXAGONAL
Soluble anhydrite

Calcium Sulfate Dihydrate


ORTHORHOMBIC
Insoluble anhydrite

MANUFACTURE:
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.
Mineral Gypsum

dehydration by heat
or other means

Plasters
Synthetic Gypsum

formulation

Dental plaster

Hydrocal

Dental Stone

Densite

High strength
dental stone


Low
High
expansion expansion

PLASTER OF PARIS:
 Traditional hemihydrate plaster.
 Produced by the dry calcination of ground gypsum in open containers
(pan, kettles or rotary kilns) at temperatures in the range of 120o to
180oC.


DIFFERENT FORMS OF HEMIHYDRATES:
 Depending on the method of calcination, different forms of
hemihydrate can be obtained.
α –hemihydrate
α modified hemihydrate
β – hemihydrate
 The α and β designations are retained because of tradition and
convenience. They are without chemical significance and their use is
solely to indicate the particular morphological appearance of the
crystals.
 The differences between the α and β hemihydrates are a result of
differences in the crystal size, surface area, and degree of lattice
perfection.

β− HEMIHYDRATE

α− HEMIHYDRATE

Crystal size LARGER

SMALLER

Shape

IRREGULAR

PRISMATIC

Packing

LOOSELY PACKED
AMPLE SPACE B/W
CRYSTALS

CLOSELY PACKED
LITTLE SPACE B/W
CRYSTALS

W/P ratio

MORE

LESS

Strength

LESS

MORE

Surface
area/wt.
Example

MORE

LESS

DENTAL PLASTER
DENTAL STONE
IMPRESSION PLASTER

The α−modified hemihydrate is made by boiling gypsum in a 30%
aqueous solution of calcium chloride and magnesium chloride. This
process yields the smoothest, most dense powder particles of the three
types, and the powder is used primarily for dies.



POWDER PARTICLES OF β-HEMIHYDRATE ( X 400.)


POWDER PARTICLES OF DENTAL STONE ( x 400.)


Crystals of α-hemihydrate powder before grinding ( X 3000.)

PARTICLES OF β−HEMIHYDRATE ( X 400.)


PARTICLES OF α-MODIFIED HEMIHYDRATE ( x 400 )

THE SETTING PROCESS:
THE SETTING REACTION:

Crystalline theory.
Gel theory.
Hydration theory


Crystalline theory:
Proposed in 1887 by Henry Louis Le Chatelier.
In 1907, it received the full support of Jacobus Hendricus vant’s
Hoff.
Also called as Dissolution-precipitation theory.
Based on the dissolution of hemihydrate powder and instant
recrystallization of gypsum, followed by interlocking of the crystals to
form the set solid.
The differences in the solubilities of calcium sulfate dihydrate and
hemihydrate causes the setting of these materials.


The setting mechanism may be viewed as follows:

Step 1. INDUCTION
CaSO4.½H2O

CaSO4(aq)

Step 2. CRYSTALLISATION
CaSO4(aq)

CaSO4.2H2O


STAGES:
DISSOLUTION
SUSPENSION
SATURATION
SUPERSATURATION
NUCLEI FORMATION
GROWTH OF NUCLEI
SPHERULITE FORMATION


 When the hemihydrate powder is mixed with water in the correct
proportions it forms a thick slurry.
 The hemihydrate is sparingly soluble in water (6.5g/L at 20o C), so
only a small amount can dissolve. Initially, therefore, the mix is a twophase suspension of hemihydrate particles in a saturated aqueous
solution.
 The stable hydrate at temperatures below 40o C is the dihydrate which
is even less soluble than the hemihydrate (2.4 g/L at 20o C).
 The aqueous solution is therefore supersaturated with respect to the
dihydrate, which crystallizes out at suitable nucleation sites in the
suspension.
 The gypsum crystals normally are acicular in habit, and often radiate
out from the nucleation centers in the form of spherulitic aggregates.

 As the dihydrate precipitates, the solution is no longer saturated with
the hemihydrate, so it continues to dissolves.
 Dissolution of the hemihydrate and the precipitation of the dihydrate
proceeds as either new crystals forms or further growth occurs on the
crystals already present.
 The process is continuous and continues until no further dihydrate
precipitates out of the solution.


NUCLEI FORMATION

GROWTH OF NUCLEI

SPHERULITE
FORMATION


EARLY STAGES IN SETTING OF A DIE STONE (X 400.)

The setting reaction is the reverse of the first stage of dehydration and so
is exothermic.

CaSO4. 1/2H2O + 11/2H2O

CaSO4.2H2O. + 3900cal/gmol.

This chemical reaction takes place regardless of whether the gypsum
product is used as impression material or a die material.


GEL THEORY:
 Also known as the colloidal theory.
 Proposes that when mixed with water, plaster enters into the colloidal
state through the sol-gel mechanism.
 In the sol state, hemihydrate particles are hydrated to form dihydrate,
thereby entering into an active state. As the measured amount of water
is consumed, the mass converts to a solid gel.

HYDRATION THEORY:
 Suggests that rehydrated plaster particles join together through
hydrogen bonding to the sulfate groups to form the set material.


STAGES IN SETTING:
The setting process is continuous, from the beginning of mixing until the
setting reaction is complete, by which time the material has reached its
full wet strength. However, important physical changes can be
recognized during this process.
The stages in setting may be designated as:
 Fluid
 Plastic
 Friable
 Carvable


Some X-ray diffraction studies suggests that hemihydrate particles
remain in the set product.
Approx. less than 50% gypsum is present in Type IV and Type V
stones.
Approx. 60 % in type III stones.
Approx. more than 90 % in Type II stones.
These results demonstrate a higher concentration of dihydrate in the
weaker set material or more conversion rate is seen in weaker set
materials.


 Initially, there is a continuous aqueous phase present, and the mix is a
viscous liquid, exhibiting pseudoplasticity so that it flows readily
under vibration; in this stage the mix has a glossy surface giving
specular reflections.
 As the setting reaction proceeds, gypsum crystals continue to grow at
the expense of the aqueous phase, and the viscosity of the mix
increases. When clumps of the growing gypsum crystals interact, the
mix becomes plastic; it will not flow under vibration but can be
readily molded. At this time the glossy surface disappears as the
aqueous phase is drawn into the pores formed when the growing
gypsum crystals thrust apart.
 Continued crystal growth converts the plastic mass into a rigid solid,
weak and friable at first but gaining strength as the relative amount of
solid phase increases.

WATER REQUIREMENTS:
CaSO4. 1/2H2O + 11/2H2O

CaSO4.2H2O. + Heat

1 g. mol

1.5 g. mol

1 g. mol.

145 gm

27 gm

172 gm

100 gm

18.6 gm

118 gm

 This 18.6 gm of water is called as Gauging water.
 In practice, powder cannot be mixed with such small amount of water
and still develop a mass suitable for manipulation.
 Some excess water is required for mixing to obtain the desired viscosity
of the mix, which can be easily manipulated.
 At the completion of reaction, the excess unreacted water remains in
the set mass. This residual water weakens the cast.

REQUIRED AND EXCESS WATER FOR
GYPSUM MATERIALS
GYPSUM

MIXING
WATER
( ml/ 100g
powder)

REQUIRED EXCESS
WATER WATER
( ml/100 g ( ml/100 g
powder )
powder)

MODEL
PLASTER

37 – 50

18.6

18 – 31

DENTAL
STONE

28 – 32

18.6

9 – 13

HIGH
STRENGTH
STONE

19 – 24

18.6

0–5


Different water requirements.

Mainly due to the differences in the crystal shape and surface area.
In case of β-Hemihydrate particles the factors that promote the
adhesiveness of the particles in the dry powder persist when they are
suspended in water. For this reason also the β-Hemihydrate powder
particles powder produces a flocculated suspension and needs a
relatively high proportion of mixing water to give a mix of workable
viscosity.


RATE OF SETTING REACTION:
Water/Powder ratio
Spatulation
Temperature
Colloidal system and PH
Additives
Accelerators
Retarders


W/P RATIO
Within wide limits, the rate of hydration during setting is independent
of the W/P ratio.
However, the rate at which the associated physical changes described
above occur is highly dependent on the W/P ratio of the mix, because
these changes occur from the interaction of clumps of gypsum crystals
growing from nucleation centers in the slurry.
Physical changes associated with the setting of the mix take place
more rapidly as the W/P ratio is decreased.
Thick mixes (low W/P ratios) harden more quickly because available
nucleation sites are concentrated in a smaller volume; interaction of
the growing solid phase occurs earlier.
Manipulation and setting times are thus directly proportional to W/P
ratio.

EFFECT OF WATER/PODWER RATIO ON
SETTING TIME
MATERIAL

MODEL
PLASTER
DENTAL
STONE
HIGH
STRENGTH
DENTAL
STONE

W/P
RATIO
( ml/g )

SPATULATION
TURNS

0.45
100
0.50
0.55
0.27
100
0.30
0.33
0.22
100
0.24
0.26www.indiandentalacademy.com

INTIAL (VICAT)
SETTING TIME
(min)

8
11
14
4
7
8
5
7
9

SPATULATION
The mixing process, called spatulation has a definite effect on the
setting time and setting expansion of the material.

An increase in the amount of spatulation (either speed of spatulation
or time or both) shortens the setting time.

When the powder is place 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 calcium
sulfate dihydrate can be precipitated.

MATERIAL

W/P
RATIO
( ml/g )

SPATULATION SETTING
TURNS
TIME
( min )

MODEL
PLASTER

0.50

20

14

0.50

100

11

DENTAL
STONE

0.30

20

10

0.30

100

8


TEMPERATURE:
Evidently the temperature has two main effects on the setting reaction
of gypsum products:

Change in the relative solubilities of calcium sulfate
hemihydrate and calcium sulfate dihydrate.

Change in the ion mobility


EFFECT ON SOLUBILITIES:
 The ratio of the solubilities of calcium sulfate dihydrate and calcium
sulfate hemihydrate at 20 oC is about 4.5.

 As the temperature increases, the solubility ratio decreases, until
100 oC is reached and the ratio becomes one.

 As the ratio of the solubilities become lower, the reaction is slowed,
and the setting time is increased.


SOLUBILITY OF CALCIUM SULFATE
HEMIHYDRATE AND CALCIUM SULFATE
DIHYDRATE AT DIFFERENT TEMPERATURES

Temperature CaSO4.1/2H2O CaSO4.2H2O
( g/100g
( g/100 g
( 0C )

RATIO

water)

water)

20

0.90

0.200

0.45

25

0.80

0.205

0.39

30

0.72

0.209

0.34

40

0.61

0.210

0.29

50

0.50

0.205

0.24

100

0.17

0.170

1


ION MOBILITY:
As the temperature increases, the mobility of the calcium and sulfate
ions increases, which tends to increase the rate of reaction and shorten
the setting time.

NET EFFECT:
Practically, the effects of these two phenomena are superimposed, and
the total effect is observed.


THE EFFECT OF TEMPERATURE ON SETTING
TIME OF DENTAL STONE
TEMPERATURE
SETTING TIME AT
(0C)
W/P RATIO 0.3
( min )
5
25
30
35
40
45
50

65
75
60
50
45
40
55

From E.C Combe: Notes on Dental Materials.

COLLOIDAL SYSTEMS AND PH:
Colloidal systems such as agar and alginate retard the setting of
gypsum products.
Retard the reaction by nuclei poisoning.
They get adsorbed on the CaSO4.2H2O nucleation sites or on the
CaSO4.1/2H2O and thus interfering in the setting reaction.
The adsorption of these materials on the nucleation sites retards the
setting reaction more effectively than the adsorption on the calcium
sulfate hemihydrate.
Liquids with low Ph, such as saliva, retard the setting reaction.
Liquids with high Ph accelerate setting.


ADDITIVES:
Used in the formulation on dental plasters and stones for many years,
mainly on an empirical basis, because at least in some cases their modes
of action are not completely understood.
ACCELERATORS

RETARDERS

Sodium Chloride(<2%)

Sodium Chloride(>20%)

Potassium sulfate

Sodium sulfate

Terra alba

Citrates
Tartrates
Acetates


SODIUM CHLORIDE:

 Provides additional sites for crystal formation. The increased number
of sites for nucleation also decreases the setting time of the material.

 Also increases the solubility of hemihydrate, so that it dissolves
rapidly, thereby also decreasing the setting time.

 If it is present in high concentrations( >20%), the sodium chloride will
deposit on the surface of crystals and prevents further growth. This
decreases the reaction rate.

POTASSIUM SULFATE:

 Reacts with water and hemihydrate to form.
Syngenite: K2 (CaSO4) 2.H2O.

 This compound crystallizes very rapidly and encourages the growth of
more crystals, thus decreasing the setting time.

 When present as a 2 % solution in water, the setting time is decreased
from 10 minutes to 4 minutes.


TERRA ALBA:
 Finely Powdered gypsum.

 In small amounts, it will provide additional sites for nucleation,
decreasing the working time and setting times.


SODIUM SULFATE:
 Salts of relatively low solubility, such as sodium sulfate, acts as
retarders in higher concentrations, by nuclei poisoning.
 As the setting proceeds, the amount of free water in the mix decreases
and the concentration of the additive increases.
 When the limit of solubility is exceeded, the salt precipitates on the
nuclei of crystallization, thus poisoning them.

BORAX:
 Sodium tetra borate decahydrate (Na2B4O5(OH)48H2O)
 When the plaster, or stone, powder is mixed with an appropriate
aqueous solution of borax (0.9%w/w), the powder particles become
coated with a thin layer of Ca2B6O115H2O (‘colemanite’) which is very
insoluble and delays the dissolution of the powder.

CITRATES AND TARTRATES:
Reaction of some additives with hemihydrate may occur; soluble
tartrates and citrates precipitate calcium tartrate and citrate
respectively.
These act by nuclei poisoning.
For a given anion, particular cation employed appears to affect the
setting reaction markedly.
Ca 2+ < K+ < H+

ACETATES:
May act by:
 nuclei poisoning.

 reducing the rate of solution of hemihydrate.

In formulating dental products, manufactures adjust the rate of setting or
raw hemihydrates by adding accelerators and retarders, often as a
balanced mixture.
Many accelerators and retarders reduce the setting expansion, in some
cases by changing the crystal habit of the growing gypsum crystals,
thereby reducing the effect of growth pressure. This is accompanied by a
reduction in the strength of the set material.


IMPURITES:
 If calcination is not complete and the gypsum particles remain, or if
the manufacturer adds gypsum, the setting time is shortened because
of the increase in the potential nuclei of crystallization.
 If orthorhombic anhydrite is present the setting reaction is delayed.
 If hexagonal anhydrite is present, the setting reaction is faster.

FINENESS:
 The finer the particle size, the faster the mix hardens as:
 Rate of hemihydrate dissolution is increased.
 Gypsum nuclei are more numerous.

THE MICROSTRUCTURE OF CAST GYPSUM:
The set material consists of a tangled aggregate of monoclinic gypsum
crystals, usually acicular in shape, with lengths in the range of 5 to
20µm.
The aggregate exhibits inherent porosity, on a microscopic scale, which
is of two distinct types:
1. Micro-porosity caused by the presence of residual unreacted water.
 Roughly spherical.
 Occur between clumps of gypsum crystals.
2. Micro-porosity resulting from the growth of gypsum crystals.
 Associated with the setting expansion.
 Smaller
 Appear as angular spaces between individual crystals in the


Micro-porostity due to presence of unreacted water


Micro-porosity caused due to growth of gypsum crystals

EFFECT OF W/P RATIO:
The Relative amounts of both types of porosity are affected by the W/P
ratio of the mix, but in two opposite ways:

1. A low W/P ratio leaves less residual water in the set mass and so
decreases the amount of the first type of porosity.

2. A low W/P ratio increases the effect of the crystal growth during
setting, because available nucleation sites are concentrated in a smaller
total volume of mix; interaction of growing gypsum crystals occur
earlier and is more effective so that the amount of the second type of
porosity is increased.


 In any W/P ratio, the total proportion of inherent porosity in the set
mass is the sum of these two types.

 The effect of the first type predominates, so for any given plaster or
stone there is always a decrease in the total inherent porosity of the set
mass (i.e. an increase in apparent density) as the W/P ratio of the mix
is reduced.

 Inherent porosity represents about 40% of the total cast volume at a
W/P ratio of 0.5. and about 20% at a W/P ratio of 0.25.
(Lautenschlager and Corbin, 1969).

VOIDS:
Areas of air in the mix. Two types:
•Internal Voids: weaken the material.
•External Voids: do not record impression anatomy in that area.
Ryerson NV(2000), investigated the effect of pressurized atmosphere on
the size and number of voids in dental stones while setting. Increased
atmospheric pressure reduced the size and number of voids. This method
produces improved cast surfaces and fewer, smaller voids. Several
pressure vessels are available for this purpose.


PICTURE SHOWING INTERNAL VOID

PROPERTIES:

MIXING, WORKING AND SETTING TIMES
SETTING EXPANSION
STRENGTH
SURFACE HARDNESS
ABRASION RESISTANCE
REPRODUCTION OF DETAIL
SOLUBILITY


The important properties requirements of gypsum products according to
ADA specification no. 25 are summarized in the table:
TYPE

SETTING
TIME
(min )

SETTING
EXPANSION
RANGE
( %)

COMPRESSIVE REPRODUCTION
STRENGTH
OF DETAIL
( MPa )
( µm )
Min.
Max.

2.5-5.0 0.0-0.15

4.0

8.0

75 + 8

+20 %

0.0-0.30

9.0

-

75 + 8

Type III +20 %

0.0-0.20

20.0

-

50 + 8

Type IV +20 %

0.0-0.15

35.0

-

50 + 8

Type V +20 % 0.16-0.30 35.0

-

50 + 8

Type I

Type II


MIXING, WORKING AND SETTING TIMES:
MIXING TIME:
Defined as the time from the addition of the powder to the water until the
mixing is completed.
Mechanical mixing of stones and plaster is usually completed in 20 to 30
seconds. Hand spatulation generally requires at least a minute to obtain a
smooth mix.
WORKING TIME:
The time available to use a workable mix, one that maintains a uniform
constituency to perform one or more tasks.
It is measured from the start of mixing to the point where the
consistency is no longer acceptable for the product’s intended purpose.
Generally, a 3-minute working time is adequate.

INDUCTION TIME:
Time from the beginning of mix till the exothermic heat is felt.

SETTING TIME:
The time that elapses from the beginning of mixing until the material
hardens is known as the setting time.
An arbitrary setting time can be determined by using suitable
penetrometers( e.g. Gillmore or Vicat needles).

LOSS OF GLOSS TEST FOR INITIAL SET:
As the material sets, the mix loses its gloss. This occurs as some of the
excess water is taken up in forming the dihydrate. This loss of the gloss
is also sometimes considered as a indication of initial set of the material.

GILLMORE NEEDLES:

 Two types of Gillmore needles.
 The lighter Gillmore needle is constructed from a brass cylinder, of
mass 0.25 lb (113.4 g), attached to a needle with a flat disk end of
diameter 1/12" (2.12 mm).
 The larger needle consists of a 1lb mass (453.6 g) on a diameter of
1/24" (1.06 mm).
 The corresponding stresses are 0.3 MPa and 5 MPa, respectively.

GILLMORE NEEDLES
USED TO DETERMINE THE
SETTING TIME OF
GYPSUM PRODUCTS


GILLMORE TEST FOR INITIAL SET:
 The mixture is spread out, and the smaller needle is lowered onto the
surface. The time at which it no longer leaves an impression is called
the initial set.
 This event is marked by a definite increase in strength.
 Acts as a guide to the time when the rigid material is strong enough to
handle and, in particular, when it can be carved or trimmed to the final
shape.
 The setting reaction continue for some time after this initial set.

GILLMORE TEST FOR FINAL SETTING TIME:
 Measured by the use of the heavier Gillmore needle.
 The elapsed time at which this needle leaves only a barely perceptible
mark on the surface is called the final setting time.

VICAT TEST FOR SETTING TIME:
 Used to measure the initial setting time of gypsum products.

 Consists of a rod weighing 300 gm with a needle of 1 mm diameter.

 A ring container is filled with the mix. The needle with a weighted
plunger rod is supported and held just in contact with the mix, then is
the needle is released and allowed to penetrate the mix. The time
elapsed until the needle no longer penetrates to the bottom of the mix
is known as Vicat setting time.

VICAT
PENETROMETER
USED TO
DETERMINE
THE SETTING
TIME OF
GYPSUM
PRODUCTS


In some cases, the Vicat and initial Gillmore measurements occur at the
same time, whereas in other instances, there is small difference.
If a dental manufacturer specifies a setting time, it will be a Gillmore or
Vicat initial setting time.


READY-FOR –USE CRITERION:
A subjective measure of the time at which the set material may be
safely handled in the usual manner.
Not determined by any designated test but by the ability to judge
readiness improves with experience.
Technically, the set material may be considered ready for use at the
time when the compressive strength is at least 80% of that which
would be attained at 1 hour.
Most modern products reach the ready-for-use state in approx. 30
minutes.


MANIPULATION TIME:
Recognition of the physical changes occurring in the mix during setting
is important in the manipulation of plaster and stone.
1. When casting (e.g. pouring casts or dies), manipulation must be
completed before the mix loses its fluidity. This change is marked by
the disappearance of the glossy surface from the mix.
2. When molding (e.g. taking impressions or jaw registrations,
articulating casts, flasking wax pattern dentures), manipulation must be
completed before the mix loses plasticity and enters friable stage. There
is no recognized objective method of measuring this time.


SETTING EXPANSION:
VOLUME CHANGES DURING SETTING:
Theoretically, calcium sulfate hemihydrate should contract
volumetrically during the setting process.
(CaSO4)2. H2O +

3 H2O

2 CaSO4.2H2O

Molecular
mass

290.284

54.048

344.332

Density

2.75

0.997

2.32

54.211

148.405

Eq. Volume 105.556
T. Volume

159.767

148.4.05

The volume of the calcium sulfate dihydrate formed is about 7% less
than the sum of the volumes of calcium sulfate hemihydrate and water.

However, experiments have determined that all gypsum products expand
linearly during setting.
Instead of 7% contraction, about 0.2% to 0.4 % linear expansion is
obtained.
The setting reaction causes a decrease in the true volume of the reactants
and under suitable conditions this contraction can be observed early in
the setting process, when the mix is still fluid.
However, once the mix begins to attain rigidity, marked by the loss of
surface gloss, an isotropic expansion is observed, resulting from growth
pressure of the gypsum crystals that are forming.


STAGES:
 INITIAL MIX
 INITIAL CRYSTAL GROWTH
 SOLID PHASE CONTACT
 EXPANSION
 TERMINATION


The initial contraction is unlikely to affect the important dimensions of a
gypsum cast, because in the still fluid mix it will occur mainly in the
vertical direction. Gravity will keep the mix adapted to the anatomical
portion of an impression.
The expansion that is observed after the mix attains rigidity takes place
in all directions and will affect the dimensions of the cast.
The point at which the initial contraction ceases is used as zero in the
laboratory measurements of effective setting expansion.
The observed expansion that occurs when plaster or stone sets is a
volumetric one.
In dental testing a value is determined for linear setting expansion and
the assumption is made that the expansion is isotropic. This assumption
is not always justified; if restraint is imposed in some directions and not
in others( e.g. by a rigid impression) setting expansion can be far from
uniform.

LINEAR SETTING EXPANSION OF GYPSUM
PRODUCTS SETTING IN AIR
TYPE

W/P
RATIO

SETTING
EXPANSION
(%)

IMPRSEEION
PLASTER

0.60

0.13

LABORATORY
PLASTER

0.50

0.30

DENTAL STONE

0.30

0.12

DIE STONE

0.23

0.10


From William J.O Brien: Dental materials- Properties and selection

EFFECT OF IMMERSION:
Gypsum products exposed to additional water while setting ( e.g. by
immersion) show a greater expansion than when setting in air, a
phenomenon commonly called Hygroscopic expansion.

The most well accepted reason for the increased expansion when the
hemihydrate reacts under water is the additional crystal growth permitted
by allowing the crystals to grow freely, rather than being constrained by
the surface tension when crystals form in air, also known as the Crystal
interlocking theory.


It follows, therefore, that the basic mechanism of crystal growth is the
same in both instances, and both phenomena are true setting
expansions.
The hygroscopic setting expansion is physical and is not caused by a
chemical reaction any more than is the normal setting expansion.
The reduction in W/P ratio increases the hygroscopic setting expansion
and the normal setting expansion in the same manner.
Increased spatulation results in increased hygroscopic setting
expansion as well.
The Hygroscopic setting expansion obtained during the setting of
dental plaster or stone is generally small in magnitude.
A dental stone may exhibit a normal setting expansion of 0.15%, with
a maximum hygroscopic setting expansion of not more than 0.30%.
Nevertheless, this difference may be sufficient to cause the misfit of
the denture or similar device on the cast.

FACTORS AFFECTING SETTING EXPANSION:


ADDITIVES



WATER/POWDER RATIO

ADDITIVES:
Manufactures can reduce the setting expansion and at the same time
control setting time by the addition of a balanced blend of accelerators
and retarders to the raw hemihydrate base plaster. Typical combinations
are:
•

Potassium sulfate- borax

•

Potassium sodium tartrate- sodium citrate.

Combinations of accelerators and retarders, in solution, to control the
setting expansion, are known as ANTI-EXPANSION SOLUTIONS.

.

The additives also reduce the strength of the set material. This is not a
disadvantage in impression plaster, but in stones and die stones strength
as well as dimension accuracy is important; formulation of the latter
materials therefore involves striking a compromise between a desirable
reduction in setting expansion and an undesirable reduction in strength


SODIUM CHLORIDE:
Provides additional sites for crystal formation.
The higher density of crystals limits the growth of crystals and hence
reduces their ability to push each other apart.
This results in decreased setting expansion.

POTASSIUM SULFATE:
Reacts with water and hemihydrate to form ‘ Syngenite’
This compound crystallizes very rapidly and encourages the growth of
more crystals.
Results in decreased setting expansion..

W/P RATIO:
Setting expansion is inversely proportional to the W/P ratio.
Reducing the relative amount of aqueous phase in the mix allows more
effective interaction of growing gypsum crystals during setting and so
increases the setting expansion.
Because of their lower water requirement, the raw hemihydrates used
to produce dental stones and die stones have a higher inherent setting
expansion in normal mixes than does plaster. This effect is masked,
however by the additives in their formulation.


STRENGTH:
Brittle material.
Weaker in tension than in compression.
For set plaster, the tensile strength is about 20% of the compressive
strength; for set die stone about 10%.
In practice, fracture of set gypsum typically occurs in tension, tensile
strength is a better guide to fracture resistance.
Compressive strength gives a better indication of surface hardness.

FACTORS AFFECTING STRENGTH:
•

WATER/POWDER RATIO

•

SPATULATION

•

ADDITIVES


WATER/POWDER RATIO:
Strength properties are inversely related to the W/P ratio and to the total
amount of inherent porosity.
When maximum strength is required, a given material should be mixed
with as low a W/P ratio as practicable.
The limiting factor is the viscosity of the mix because it increases with
decreasing W/p ratio and can become so high that the ability to pour
casts is prejudiced.
With any plaster or stone, using a low W/P ratio to obtain maximum
strength properties also gives an increased setting expansion, which must
be accepted. But in applications where dimensional accuracy is more
important than strength (e.g. impressions), higher W/P ratio can be used.


MATERIAL

MODEL
PLASTER
DENTAL
STONE
HIGH
STRENGTH
DENTAL
STONE

W/P
RATIO
( ml/g )
0.45
0.50
0.55
0.27
0.30
0.50
0.24
0.30
0.50

COMPRESSIVE
STRENGTH
(
Mpa )


12.5
11.0
9.0
31.0
20.5
10.5
38.0
21.5
10.5

EFFECT OF DRYING:
The free water content of the set product definitely affects its strength.
For this reason, two strength properties of gypsum are reported:
• The wet strength


also known as the green strength.

 It is the strength obtained when the water in excess of that
required for hydration of hemihydrate is left in the test specimen.
• The dry strength:
 When the excess water has been driven off by drying, the
strength obtained is dry strength.
 The dry strength may be two or more times as high as wet
strength.

The effect of drying on the compressive strength of set plaster is shown
in the graph:

A somewhat similar change in the surface hardness also takes place
during the drying process.

STRENGTH PROPERTIES OF GYPSUM PRODUCTS
TYPE

W/P
TENSILE
COMPRESSIVE
RATIO STRENGTH STRENGTH
( Mpa )
( Mpa )
Wet
Dry
Wet
Dry

Impression
Plaster

0.60

1.3

-

5.9

-

Dental
Plaster

0.50

2.3

4.1

12.4

24.9

Dental
Stone

0.30

3.5

7.6

25.5

63.5

Die Stone

0.23

4.3

9.9

40.7

80.7


From William J.O Brien: Dental Materials- Properties and Selection 5 th ed.

EFFECT OF SPATULATION:
With an increase in the mixing time, the strength is increased.
But this increase is only seen upto a hand mixing of 10 minutes.
If the mixture is overmixed, the gypsum crystals formed are broken up,
and less crystalline interlocking results in the final product.

EFFECT OF ADDITIVES:
The addition of an accelerator or retarder lowers both the wet and the dry
strengths of the gypsum product.
Such a decrease in strength can be partially attributed to the salt added
as an adulterant and to the reduction in intercrystalline cohesion.
Prombonas A, Vlissidis D(1994) measured the Compressive strength
and setting temperatures of mixes with various proportions of plaster to
stone. The mix made with 1:1 ratio of stone to plaster, may be the
material of choice for filling the upper half of the flask during complete

TENSILE STRENGTH:
Important in structures in which bending tends to occur because of
lateral force applications, such as removal of cast from impressions.
Because of the brittle nature of gypsum materials, the teeth on the cast
may fracture rather than bend. For brittle materials like gypsum
products, diametral compressive strength test is used to determine the
tensile strength of such products.


SURFACE HARDNESS AND ABRASION RESISTANCE:
In general, hardness is defined as the resistance to penetration.
For dental purposes, the surface hardness of a material is generally
measured in terms of its resistance to indentation.
The surface hardness of unmodified gypsum products is related in a
general way to their compressive strength. High compressive strength of
the hardened mass corresponds to high surface hardness.
After the final setting occurs, the surface hardness remains practically
constant until most excess water is evaporated from the surface, after
which its increase is similar to increase in compressive strength.
The surface hardness increases at a faster rate than the compressive
strength, because the surface of the hardened mass reaches a dry state
earlier than the inner portion of the mass.

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.
According to Craig, increase in hardness was obtained for model
plaster but not for dental stone or high-strength dental stone.
Generally, impregnating set gypsum with resin increases its
abrasion resistance, but decreases compressive strength and
surface hardness.
•

Soaking the gypsum dies or casts in glycerine or different oils
does not improve the surface hardness but rather makes the
surface smoother, so that a wax carver or other instrument will not
cut the stone as it slides over the surface.


•

Mixing high strength dental stone with a commercial hardening
solution containing colloidal silica (about 30%) improves the
surface hardness of set gypsum. The Knoop hardness numbers of
two commercial high-strength dental stone were 54 and 77
kg/mm2. When hardening solution was used, these values
increased to 62 and 79 kg/mm2 respectively.

The abrasion resistance of gypsum product is an important property in
certain dental procedures. For example, if a wax pattern is to be carved
and finished on a stone die, the metal instrument used to carve the wax
may abrade off and destroy adjacent areas of the stone.
Abrasion is a major concern when gypsum products are used for dies,
leading to the frequent recommendation that surface hardeners should be
used before waxing or scanning.

HARDENING SOLUTION:
Composed of:
Colloidal Silica: 30 %
Water
Modifiers
May be used in place of water to mix gypsum products.
Amount of solution used is less than if water were used alone because
surface-active modifiers allow the particles to be more easily wetted.
Affects the hardness and setting expansion of gypsum dies.


REPRODUCTION OF DETAIL:
ADA specification no. 25 requires that:



Type I and II reproduce a groove 75µm in width.



Type III, IV and V reproduce a groove 50µm in width.

Gypsum dies do not reproduce surface details very well because the
surface of the set gypsum is porous at microscopic level.


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