The science of dental materials

The science of dental materials involves
a study of the composition and properties
of materials and the way in which they
interact with the environment in which
they are placed.
B.D.S., M.Sc. (Prosth.)
FOURTH EDITION 2015-2016
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The science of dental materials involves a study of the composition and
properties of materials and the way in which they interact with the
environment in which they are placed.
1- Prevention.
2- Restoration.
3- Rehabilitation.
Prevention
a- Materials of brushing and flossing.
b- Fluoride therapy.
c- Fissure sealants.
Restoration
a- Filling materials (temporary filling, silver filling, tooth colored filling, gold
inlay, ceramic inlay).
b- Materials have soothing and promote healing of the pulp (calcium
hydroxide).
c- Root canal treatment (solutions used to clean the canal, materials which fill
the canal like gutta percha, silver cone, and sealing paste)
d- Crown and onlay.
e- Post and core.
f- Cements.
g- Model of teeth to fabricate restorations (gypsum products).
h- Impression materials.
Rehabilitation
a- Artificial teeth (acrylic, porcelain).
b- Implant (titanium screw).
c- Fixed partial denture materials.
d- Cements.
e- Removable partial denture materials (metal framework and plastic (acrylic)
denture base, or made entirely of plastic).

Forms of matter
Change of state
Matter exists in three forms (solid, liquid, and gas). The difference in form
is mainly due to different in force that held the atoms together (bonds).
Atoms are held together by some forces. These interatomic bonding forces
that hold atoms together are cohesive forces. Interatomic bonds may be
classified as:
1- Primary bonds.
2- Secondary bonds.
These are chemical in nature.
a- Ionic bonds: these are simple chemical bonds, resulting from mutual
attraction of positive and negative charges; the classic example is
sodium chloride.
b- Covalent bonds: in many chemical compounds, two valence electrons
are shared. The hydrogen molecule is an example of this bond.
c- Metallic bonds: The third type of primary atomic interaction is the
metallic bond which results from the increased spatial extension of
valence-electron wave functions when an aggregate of metal atoms is
brought close together. This type of bonding can be understood best
by studying a metallic crystal such as pure gold. Such a crystal
consists only of gold atoms. Like all other metals, gold atoms can
easily donate electrons from their outer shell and form a "cloud" of
free electrons. The contribution of free electrons to this cloud results
in the formation of positive ions that can be neutralized by acquiring
new valence electrons from adjacent atoms.

In contrast with primary bonds, secondary bonds weaker bonds may be said
to be more physical than chemical, they do not share electrons. Instead,
charge variations among molecules or atomic groups induce polar forces
that attract the molecules. Since there are no primary bonds between water
and glass, it is initially difficult to understand how water drops can bond to
an automobile windshield when they freeze to ice crystals. However, the
concepts of hydrogen bonding and Van Der Waals forces (two types of
bonds that exist between water and glass) allow us to explain this adhesion
phenomenon.
Van Der Waals forces: this is due to the formation of dipole. In the
symmetric atoms (e.g. inert gas) a fluctuating dipole is formed, i.e. within
an atom there is accumulation of electrons in one half leading to a negative
polarity and on the other half a positive polarity. This attracts other similar
dipoles. A permanent dipole is formed within asymmetrical molecules, e.g.
water molecule.
Figure (1-1): (A) Ionic bond formation
characterized by electron transfer from
one element (positive) to another
(negative). (B) Covalent bond formation
characterized by electron sharing. (C)
Metallic bond formation characterized by
electron sharing and formation of a gas
or cloud of electrons.

Figure (1-2): Hydrogen bond formation between water molecules. The polar
water molecule ties up adjacent water molecule via an H…O interaction
between molecules.
Physical and Mechanical Properties of Dental Materials
Stress
When a force (external) acts on the body, tending to produce deformation, a
resistance is developed within the body to this external force.
Stress: it is the internal resistance of the body to the external force. Stress is
equal in magnitude but opposite in direction to the external force applied. The
external force is also known as load. For simple tensile or compressive the
stress is given by the expression:
F: the applied force, and A: the cross-sectional area.
Types of stresses
1- Tensile stress.
2- Compressive stress.
3- Shear stress.

Tensile stress is a result in a body when it is subjected to two sets of forces
that are directed away from each other in the same straight line. The load
tends to stretch or elongate a body.
Compressive stress is a result in a body when it is subjected to two sets of
forces in the same straight line but directed towards each other. The load
tends to compress or shorten a body.
Figure (1-3).
Shear stress is a result of two forces directed parallel to each other. The load
tends to twist or slide of one portion of a body over another.
Figure (1-4).

Strain
The application of an external force to a body results in a change in dimension
(shape) of that body (deformation).
For example, when a tensile force is applied the body undergoes an extension,
the magnitude of which depends on the applied force and the properties of the
material.
The numerical value of strain is given by the expression:
Figure (1-5): Diagram indicating how
the magnitudes of (a) compressive and
(b) tensile stresses and strains.
Figure (1-6): Universal
testing machine

It is difficult to induce just a single type of stress in a body. Whenever force is
applied over a body, complex or multiple stresses are produced. These may be
a combination of tensile, compressive, or shear stresses.
If we take a cylinder and subject it to a tensile or compressive stress, there is
simultaneous axial and lateral strain. Within the elastic range the ratio of the
lateral to the axial strain is called Poisson's ratio.
A tensile load is applied to a wire in small increments until it break. If each
stress is plotted on a vertical coordinate and the corresponding strain (change
in length) is plotted on the horizontal coordinate a curve is obtained. This is
known as stress strain curve. It is useful to study some of the mechanical
properties.
Figure (1-7): Complex stress
pattern developed in cylinder
subjected to compressive stress

Figure (1-8).
ductile
brittle
R
stiff flexible
R: Resilience.
T: Toughness.
Figure (1-9).
Tweak
strong

The stress strain curve is a straight line up to point P after which it curves.
The point P is the proportional limit, i.e. up to point P the stress is
proportional to strain. Beyond P the strain is no longer elastic and so stress is
no longer proportional to strain. Thus proportional stress can be defined as
the greatest stress that may be produced in a material such that the stress is
directly proportional to strain. The proportional limit deals with
proportionality of strain to stress in the structure.
Below the proportional limit (point P) the material is elastic in nature, that is,
if the load is removed the material will return to its original shape. Thus
elastic limit may define as the maximum stress that a material will withstand
without permanent deformation. The elastic limit describes the elastic
behavior of the material.
Figure (1-10).

It is defined as the stress at which a material exhibits a specified limiting
deviation from proportionality of stress to strain.
Yield strength often is a property that represents the stress value at which a
small amount (0.l % or 0.2 %) of plastic strain has occurred. A value of either
0.1 % or 0.2 % of the plastic strain is often selected and is referred to as the
percent offset. The yield strength is the stress required to produce the
particular offset strain (0.1 % or 0.2 %) that has been chosen. As seen in
Figure (1-11); the yield strength for 0.2 % offset is greater than that associated
with an offset of 0.1 %. If yield strength values for two materials tested under
the same conditions are to be compared, identical offset values should be
used. To determine the yield strength for a material at 0.2 % offset, a line is
drawn parallel to the straight-line region (see Figure 1-11), starting at a value
of 0.002, or 0.2 % of the plastic strain, along the strain axis, and is extended
until it intersects the stress-strain curve. The stress corresponding to this point
is the yield strength. Although the term strength implies that the material has
fractured, it actually is intact, but it has sustained a specific amount of plastic
strain (deformation).
Ultimate tensile strength: It is the maximum stress that a material can
withstand before failure in tension.
Ultimate compressive strength: It is the maximum stress that a material can
withstand before failure in compression.
UTS

Figure (1-9): Stress strain plot for stainless steel orthodontic wire that
has been subjected to tension. The proportional limit (PL) is 1020 MPa.
Figure (1-11): Although not shown, the elastic limit is
approximately equal to this value. The yield strength (YS) at a 0.2 %
strain offset from the origin (O) is 1536 MPa and the ultimate
tensile strength (UTS) is 1625 MPa. An elastic modulus value (E) of
192.000 MPa (192 GPa) was calculated from the slope of the
elastic region.

Once the elastic limit of a material is crossed by a specific amount of
stress, the further increase in strain is called permanent deformation, i.e.
the resulting change in dimension is permanent. If the material is
deformed by a stress at a point above the proportional limit before
fracture, the removal of the applied force will reduce the stress to zero,
but the strain does not decrease to zero because plastic deformation has
occurred. Thus, the object does not return to its original dimension when
the force is removed. It remains bent, stretched, compressed, or otherwise
plastically deformed.
As shown in figure (1-11); the stress-strain graph is no longer a straight
line above the proportional limit (PL), but rather it curves until the
structure fractures. The stress strain graph shown in figure (1-11) is more
typical of actual stress-strain curves for ductile materials. Unlike the
linear portion of the graph at stresses below the proportional limit, the
shape of the curve above (P) is not possible to extrapolate because stress
is no longer proportional to strain.
An elastic impression material deforms as it is removed from the mouth.
However, due to its elastic nature it recovers its shape and little
permanent deformation occurs.
It represents the relative stiffness or rigidity of the material within the
elastic range. It is the ratio of stress to strain (up to the proportional limit),
so the stress to strain ratio would be constant.

It therefore follows that the less the strain for a given stress, the greater will
be the stiffness, e.g. if a wire is difficult to bend, considerable stress must
be placed before a notable strain or deformation results. Such a material
would possess a comparatively high modulus of elasticity.
The metal frame of metal-ceramic bridge should have high stiffness. If the
metal flexes, the porcelain veneer on it might crack or separate.
Generally in dental practice, the material used as a restoration should
withstand high stresses and show minimum deformation. However, there
are instances where a large strain is needed with a moderate or slight stress.
For example in orthodontic appliance, a spring is often bent a considerable
distance under the influence of a small stress. In such a case, the structure
is said to be flexible and it possesses the property of flexibility. The
maximum flexibility is defined as the strain that occurs when the material
is stressed to its proportional limit.
It is useful to know the flexibility of elastic impression materials to
determine how easily they may be withdrawn over undercuts in the mouth.
It is the amount of energy absorbed by a structure when it is stressed not to
exceed its proportional limit.
Resilience can be measured by calculating the area under the elastic portion
(straight line portion) of the stress strain curve calculating (the area of the
triangle=1/2 bh).
Resilience has particular importance in the evaluation of orthodontic wires.
An example: The amount of work expected from a spring to move a tooth.

It is the energy required to fracture a material. It is also measured as the
total area under the stress strain curve (elastic and plastic portions of
stress strain curve). Toughness is not as easy to calculate as resilience.
It is the relative inability of a material to sustain plastic deformation
before fracture of a material occurs.
Brittleness is generally considered as the opposite of toughness, glass is
brittle at room temperature. It will not bend appreciably without breaking.
It should not be wrongly understood that a brittle material is lacking in
strength, from the above example of glass we see that its shear strength is
low, but its tensile strength is very high, if glass is drawn into a fiber, its
tensile strength may be as high as 2800 MPa.
Many dental materials are brittle, e.g. porcelain,
acrylic, cements, gypsum products.
Figure (1-12): The area under stress strain graph may be used to calculate
either (a) resilience or (b) toughness.
Ductile Brittle
Nylon Acrylic

It is the ability of a material to withstand a permanent deformation under
a tensile load without rupture. A metal that can be drawn readily into a
wire is said to be ductile. It is dependent on tensile strength. Ductility
decrease as the temperature increased.
Figure (1-14): Stress strain plots of materials that exhibit different mechanical properties.
(UTS) ultimate tensile stress, (PL) proportional limit.
Figure (1-13): Schematic of
different type of deformation in
brittle (glass, steel file) and
ductile (copper) materials of
the same diameter and having
a notch of the same dimension.

It is the ability of a material to withstand considerable permanent
deformation without rupture under compression as in hammering or rolling
into a sheet. It is not dependent on strength as is ductility. It increases with
raise in temperature.
Gold is the most ductile and malleable metal. This enables
manufacturer to beat it into thin foils. Silver is the second.
It is the reaction of a stationary object to a collision with a moving object.
Impact strength: it is the energy required to fracture a material under an
impact force.
Dentures should have high impact strength to prevent it from breaking if
accidentally dropped by patient.
A structure subjected to repeated or cyclic stress below its proportional limit
can produce abrupt failure of the structure. This type of failure is called
fatigue.
Restorations (filling, crown, denture) in the mouth are subjected to cyclic
forces of mastication, so these restorations should be able to resist fatigue.
Pendulum

The hardness is the resistance to permanent surface indentation or
penetration.
The value of hardness, often referred to as the hardness number, depends
on the method used for its evaluation. Generally, low values of hardness
number indicate a soft material and vice versa.
Used for measuring hardness of metal and plastic materials.
Figure (1-15): Shapes of hardness indenter points (upper row
and the indentation depressions left in material surfaces (lower row).
The measured dimension M that is shown for each test is used to
calculate hardness. The following tests are shown:
Brinell test: A steel ball is used, and the diameter of the indentation is
measured after removal of the indenter.
Rockwell test: A conical indenter is impressed into the surface. Under a
minor load (dashed line) anti a major load (solid line), and M is the
difference between the two penetration depths.
Vickers test: A pyramidal point is used, and the diagonal length of the
indentation is measured.
Knoop test: A rhombohedral pyramid diamond tip is used, and the long
axis of the indentation is measured.

Figure (1-17): Vickers indentation.
Figure (1-18): Vicat penetrometer used to
determine initial setting time of gypsum
products.

After a substance has been permanently deformed (plastic deformation),
there are trapped internal stresses; such situations are unstable. The
displaced atoms are not in equilibrium positions through a solid-state
diffusion process driven by thermal energy, the atoms can move back
slowly to their equilibrium positions, the result is a change in the shape or
contour of the solid as the atoms or molecules change positions. The
material warps or distorts.
This stress relaxation leads to distortion of elastomeric impressions.
Waxes and other thermoplastic materials like compound undergo
relaxation after they are manipulated.
It is the maximal stress required to fracture a structure.
The three basic types of strength are:
1- Tensile strength.
2- Compressive strength.
3- Shear strength.
It is the maximal tensile stress the structure will
withstand before rupture.
Tensile strength is measured by
subjecting a rod, wire or dumbbell
shaped specimen to a tensile loading
(unilateral tension test).

Brittle materials are difficult to test by using the unilateral tension test.
Instead, an indirect tensile test called diametral compression test is used.
In this method, a compressive load is placed on the diameter of a short
cylindrical specimen.
( )
It is the deformation that results from the application of a tensile force.

The flexural strength of a material is obtained when one
loads a simple beam, simply supported (not fixed) at each
end, with a load applied in the middle, such a test is
called (three-point bending test).
Flexural strength test is especially useful in comparing denture base
materials in which a stress of this type is applied to a specimen of denture
acrylic with masticatory loads.
It is the resistance to motion of one material body over another. If an
attempt is made to move one body over the surface of another, a
restraining force to resist motion is produced; this restraining force is the
(static) frictional force and results from the molecules of the two objects
bonding where their surfaces are in close contact.
Figure (1-20): Microscopic area of contact between two objects. The
frictional force, which resists motion, is proportional to the normal force
and the coefficient of friction.

It is a loss of material resulting from removal and relocation of materials
through the contact of two or more materials
Tooth brushing with a dentifrice may cause wear of teeth.
Adhesion is the force which causes two different substances to attach
when they are brought in contact with one another. When the molecules of
the same substance hold together; the forces are said to be cohesion.

Rheology
Rheology is the study of flow of matter. In dentistry, study of rheology is
necessary because many dental materials are liquids at some stage of their
use, e.g. molten alloy and freshly mixed impression materials and
cements. Other materials appear to be solids but flow over a period of
time.
It is the resistance offered by a liquid when placed in
motion, e.g. honey has greater viscosity than water. It is
measured in poise (p) or centipoise (cp).
It is the increase in strain in a material under constant stress. It is time
dependent plastic deformation or change of shape that occurs when a
metal is subjected to a constant load near its melting point. The term flow
has been used rather than creep to describe rheology of amorphous
materials such as waxes.
Dental amalgam has components with melting
points that are slightly above room temperature
and the creep produced can be very destructive to
the restoration; e.g. glass tube fractures under a
sudden blow but bends gradually if leaned
against a wall.
These materials exhibit a different viscosity
after it is deformed, e.g. zinc oxide eugenol
cements show reduced viscosity after
vigorous mixing.

Thermal Properties of Dental Materials
It is the quantity of heat in calories or joules, per second passing through a
body 1 cm thick with a cross section of 1 cm2
, when the temperature
difference is 1°C.
It is the quantity of heat needed to raise the temperature of 1 g of the
substance 1°C.
It describes the rate at which a body with nonuniform temperature
approaches equilibrium.
It is the change in length per unit length of a material for a 1°C change in
temperature.
TIME

Restorative materials may change in dimension upto 4.4 times more than
enamel for every degree temperature change, when there is cooling
contraction and on heating there is expansion of materials, which may
eventually lead to marginal leakage adjacent to restoration.
It is the heat in calories or joules required to convert 1g of a material from
solid to liquid state at the melting temperature.

Optical Properties of Dental Materials
Esthetic effects are sometimes produced in a restoration by incorporating
colored pigments in nonmetallic materials such as resin composites,
denture acrylics, silicone maxillofacial materials, and dental ceramics.
The color observed when pigments are mixed results from the selective
absorption by the pigments and the reflection of certain colors.
Opacity is a property of materials that prevents the passage of light. When
all of the colors of the spectrum from a white light source such as sunlight
are reflected from an object with the same intensity as received, the object
appears white. When all the spectrum colors are absorbed equally, the
object appears black. An opaque material may absorb some of the light
and reflect the remainder. If, for example, red, orange, yellow, blue, and
violet are absorbed, the material appears green in reflected white light.
Translucency is a property of substances that permits the passage of light,
but disperses the light, so objects cannot be seen through the material.
Some translucent materials used in dentistry are ceramics, resin
composites, and denture plastics.
opacity translucency

Transparency is a property of material allows the passage of light in such
a manner that little distortion takes place and objects may be clearly seen
through them.
Transparent substances such as glass may be colored if they absorb certain
wavelengths and transmit others. For example, if a piece of glass absorbed
all wavelengths except red, it would appear red by transmitted light. If a
light beam containing no red wavelengths were shone on the glass, it
would appear opaque, because the remaining wavelengths would be
absorbed.
The index of refraction for any substance is the ratio of the velocity of
light in a vacuum (or air) to its velocity in the medium.

Other Properties
Water sorption of a material represents the amount of water adsorbed on
the surface and absorbed onto the body of material during fabrication and
usage. Usually warpage and dimensional change are associated with high
percentage of water sorption.
It is the time required for the reaction to be completed. If the rate of the
reaction is too fast, the material has a short setting time.
The setting time does not indicate the completion of the
reaction which may continue for much longer time.
It is the term applied to the general deterioration and
change in quality of materials depending on particular
application.

The presence of metallic restorations in the mouth may cause a
phenomenon called galvanic action, or galvanism. This results from a
difference in potential between dissimilar fillings in opposing or adjacent
teeth. These fillings, in conjunction with saliva or bone fluids such as
electrolytes, make up an electric cell. This cell short-circuited, and if the
flow of current occurs through the pulp, the patient experiences pain and
the more anodic restoration may corrode, like gold with amalgam.

R-phrases Hazard symbols/ R-phrases
F: Highly flammable substances.
Highly flammable
Xn: Harmful substances which may
cause death or acute or chronic damage to
health when inhaled, swallowed, or
absorbed via the skin.
Harmful
T: Toxic substances which in low
quantities cause death or acute or chronic
damage to health when inhaled,
swallowed or absorbed via the skin.
Toxic
C: Corrosive substances which may, on
contact with living tissues, destroy them.
Corrosive
Xi: Irritant noncorrosive substances
which, through immediate, prolonged or
repeated contact with the skin or mucous
membrane, may cause inflammation.
Irritant
N: Dangerous for the environment
substances which, where they enter the
environment, could present an immediate
or delayed danger for one or more
components of the environment. Dangerous for the
environment

A number of gypsum products are used in dentistry as adjuncts to dental
operation.
1. Type I: Impression plaster.
2. Type II: Dental plaster.
3. Type III: Dental stone (medium strength stone).
4. Type IV: Improved stone (high strength stone) (die stone).
5. Type V: high strength/high expansion stone.
1- Impression plaster.
2- Mounting the casts to the articulation.
3- Form casts and dies.
4- Used as a binder for silica.
5- 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.
Potassium sulfate, and terra alba (set calcium sulfate dihydrate) are1-
effective accelerators.
Sodium chloride in small amounts shortens the setting reaction but2-
increases the setting expansion of the gypsum mass.
Sodium citrate is a dependable retarder.3-
A mixture of calcium oxide (0.1%) and gum arabic (1%) reduces the4-
amount of water necessary to mix gypsum products, resulting in
improved properties.

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 high-
strength 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.

When any of the gypsum products is mixed with water, it should be
spatulated properly to obtain a smooth mix. Water is dispensed into a
mixing bowl of an appropriate size and design. The powder is added and
allowed to settle into the water for about 30 seconds. This technique
minimizes the amount of air incorporated into the mix.
The spatulation can be continued either by:
1- Hand using a spatula.
2- Hand-mechanical spatulator.
3- Power-driven mechanical spatulator.
Spatulation by hand involves stirring the mixture vigorously while wiping
the inside surfaces of the bowl with the spatula. Spatulation to wet and
mix the powder uniformly with the water requires about 1 minute at 2
revolutions per second.
Vacuuming during mixing reduces the air entrapped in the mix. Vibration
immediately after mixing and during pouring of the gypsum minimizes
air bubbles in the set mass.
Pouring an impression with gypsum requires care to avoid trapping air in
critical areas. The mixed gypsum should be poured slowly or added to the
impression with a small instrument such as a wax spatula. Once poured,
the gypsum material should be allowed to harden for 45 to 60 minutes
before the impression and cast are separated.
Figure (2-1): Flexible rubber
mixing bowl and spatula

Figure (2-2): Power-driven
mechanical spatulator with a
vacuum attachment
Figure (2-3): Vibrator is designed to
promote the release of bubbles in the
gypsum mix and to facilitate pouring of
the impression

Easily manipulated.1-
Sufficient strength at room temperature: To permit ease in handling and2-
provide enough strength at higher temperatures to withstand the impact
force of the molten metal.
Stability at higher temperatures: Investment must not decompose to give3-
off gases that could damage the surface of the alloy.
Sufficient expansion: Enough to compensate for shrinkage of the wax4-
pattern and metal that takes place during the casting procedure.
Beneficial casting temperatures: Preferably the thermal expansion versus5-
temperature curve should have a plateau of the thermal expansion over a
range of casting temperatures.
Porosity: Porous enough to permit the air or other gases.6-
Smooth surface.7-
Ease of divestment8-
Inexpensive.9-
In general, an investment is a mixture of three distinct types of materials:
1- Refractory Material: This material is usually a form of silicon dioxide,
such as quartz, tridymite, or cristobalite, or a mixture of these.
2- Binder Material: Because the refractory materials alone do not form a
coherent solid mass, some kind of binder is needed.
3- Other Chemicals: Usually a mixture of refractory materials and a binder
alone is not enough to produce all the desirable properties required of an
investment material.

The investments suitable for casting gold alloys contain (65-75 %) quartz
or cristobalite, or a blend of the two, in varying proportions, (25-35 %) of
α-calcium sulfate hemihydrate, and about (2-3 %) chemical modifiers.
The calcium sulfate-bonded investment is usually limited to gold
castings, and is not heated above 700°C. The calcium sulfate portion of
the investment decomposes into sulfur dioxide and sulfur trioxide at
temperatures over 700°C, tending to embrittle the casting metal.
Therefore, the calcium sulfate type of binder is usually not used in
investments for making castings of palladium or base metal alloys.
It is the most common type of investment for casting high-melting point
alloys. This type of investment consists of three different components.
One component contains a water-soluble phosphate ion. The second
component reacts with phosphate ions at room temperature. The third
component is a refractory, such as silica. Different materials can be used
in each component to develop different physical properties.
Another type of binding material for investments used with casting high-
melting point alloys is a silica bonding ingredient. This type of
investment may derive its silica bond from ethyl silicate, an aqueous
dispersion of colloidal silica, or from sodium silicate. One such
investment consists of a silica refractory, which is bonded by the
hydrolysis of ethyl silicate in the presence of hydrochloric acid.

The term polymer denotes a molecule that is made up of many (poly)
parts (mers). The mer ending represents the simplest repeating chemical
structural unit from which the polymer is composed. Thus poly (methy1
methacrylate) is a polymer having chemical structural units derived from
methyl methacrylate.
Monomer (one part): It is a molecule that forms the basic unit for
polymers, and can combine with others of the same kind to form a
polymer.
Polymer: It is a substance which has a molecular structure built up
completely from a large number of similar units bonded together.
Copolymer: It is a polymer made by reaction of two different monomers.
Terpolymer: It is a polymer synthesized from three different monomers.

The molecular weight of the polymer molecule equals the molecular
weight of the various mers multiplied by the number of the mers. The
higher the molecular weight of the polymer, the higher the degree of
polymerization.
The term is the process by which the monomers
convert into polymers, but the is defined
as the total number of mers in a polymer molecule.

Figure (3-3): Linear, branched, and cross-linked homopolymers and copolymers

Denture base, special tray, record base.
Artificial teeth.
Obturators for cleft palate.
Composite tooth restoration.
Orthodontic space maintainer.
Crown and bridge.
Endodontic filling.
Impressions.
Maxillofacial prosthesis.
Dies.
Endodontic filling material.
Splints and stents.
Athletic mouth protectors.
Cements.
Polymerization reactions fall into two basic types:
1- Addition polymerization.
2- Condensation polymerization.
(Free-Radical Polymerization)
Most dental resins are polymerized by addition polymerization which
simply involves the joining together of monomer molecules to form
polymer chain. In this type of reaction, no byproduct is obtained.
The reaction takes place in three :
1- Initiation stage.
2- Propagation stage.
3- Termination stage.

1- Activation and initiation stage
To start the addition polymerization process a free radicals must be
present. (Free radicals are very reactive chemical species that have an
unpaired electron).
The free radicals are produced by reactive agents called initiators.
(Initiators are molecules which contain one relatively weak bond which
is able to undergo decomposition to form two reactive species (free
radical), the decomposition of bond of initiator need source of energy
(activator) such as heat, chemical compound, light, electromagnetic
radiation).
Initiator is used extensively in dental polymers is (Benzoyl peroxide).
Addition polymerization reaction is initiated when the free radical reacts
with monomer molecules producing another active free radical species
which is capable of further reaction.
2- Propagation stage
The initiation stage is followed by the rapid addition of other monomer
molecules to the free radical and the shifting of the free electron to the
end of the growing chain.

3- Termination stage
This propagation reaction continues until the growing free radical is
terminated either by:
a- Reaction of two growing chains to form one dead chain
b- Reaction of growing chains with materials as (hydroquinone, eugenol,
impurities, or large amounts of oxygen).
A condensation reaction involves two molecules reacting together to form
a third, large molecule with production of by-product such as water,
halogen, acid, and ammonia. Condensation reaction progresses by the
same mechanism of chemical reaction between two or more simple
molecules.
Factors control the structure and the properties of polymers:
1- The molecular structure of repeating units including the use of copolymer.
2- Molecular weight or chain length.
3- The degree of chain branching (Linear, network, 3D).
4- The presence of cross-linking agent.
5- Presence of plasticizers or fillers.
The following list indicates the requirements for a clinically
acceptable denture base material:
1- High strength, stiffness, hardness, toughness, and durability.
2- Good thermal conductivity.
3- Processing accuracy and dimensional stability.
4- Chemical stability (unprocessed as well as processed material).
5- Insolubility in and low sorption of oral fluids.
6- Absence of taste and odor.

7- Biocompatible.
8- Natural appearance.
9- Color stability.
10- Adhesion to plastics, metals, and porcelain.
11- Ease of fabrication and repair.
12- Moderate cost.
13- Accurate reproduction of surface detail.
14- Resistance to bacterial growth.
15- Radiopaque.
16- Easy to clean.
1- Heat cured resin.
2- Cold cured resin.
3- Visible light cured resin.
4- Microwave activated resin.
Figure (3-4): Chest radiographs in
which a segment of denture base has
been placed over the lower right half
of the chest.

1- Poly (methy1 methacrylate), (prepolymerized phase) it may be modified
with small amounts of ethyl, butyl, or other alkyl methacrylates to
produce a polymer somewhat more resistant to fracture by impact.
2- Initiator such as benzoyl peroxide to initiate the polymerization of the
monomer liquid after being added to the powder.
3- The pigment such as cadmium sulfate is used to obtain the various tissue-
like shades.
4- Titanium oxides are used as opacifiers.
5- Nylon or acrylic fibers are usually added to simulate the minute blood
vessels of oral mucosa.
Figure (3-5): Denture base acrylic

1- Methyl methacrylate monomer: it is clear, colorless, low viscosity liquid,
boiling point is 100.3°C, and distinct odor exaggerated by a high vapor
pressure at room temperature Care should be taken to avoid breathing the
monomer vapor. Animal studies have shown that the monomer can affect
respiration, cardiac function, and blood pressure.
2- Hydroquinone inhibitors are added to give the liquid adequate shelf life.
The inhibitor is a chemical material added to prevent polymerization
during storage and in order to provide enough working time.
3- Plasticizers are sometimes added to produce a softer, more resilient
polymer. They are relatively low-molecular weight esters, such as dibutyl
phthalate.
4- If a cross-linked polymer is desired, organic compounds such as Ethylene
glycol dimethacrylate (EGDMA) are added to the monomer, using cross-
linking agents (chemical bonds between different chains) provides greater
resistance to minute surface cracking, termed crazing, and may decrease
solubility and water sorption.
5- With chemical cured acrylic an accelerator is included in the liquid.
These accelerators are tertiary amines (N,N-dimethyl-para-toluidine).
These acrylics also called self-curing, cold-curing, or autopolymerizing
resins.

 3:1 by volume.
 2.5:1 by weight.
By use this ratio the volume shrinkage is (6 %) and linear shrinkage is (0.5 %).

The liquid placed in clean, dry mixing jar followed by slow addition of
powder, allowing each powder particle to become wetted by monomer.
After mixing the powder with liquid the mixture is left until it reaches a
consistency suitable for packing. During this period, a lid should be
placed on the mixing jar to prevent evaporation of monomer.
The polymer-monomer mixture, on standing, goes through several
, which may be qualitatively described as:
The polymer gradually settles into the monomer forming a fluid,
incoherent mass.
The monomer attacks the polymer by penetrating into the polymer. The
mass is sticky and stringy (cobweb like) when touched or pulled apart.
As the monomer diffuses into the polymer, it becomes smooth and dough
like. It does not adhere to the wall of the jar. It consists of undissolved
polymer particles suspended in a plastic matrix of monomer and
dissolved polymer. The mass is plastic and homogenous and can be
packed into the mold at this stage.
The monomer disappears by further penetration into the polymer and/or
evaporation. The mass is rubber like, non-plastic, and cannot be molded.
The curing temperature must be maintained close to 74° C, because the
polymerization reaction is strongly exothermic. The heat of reaction will
be added to the heat used to raise the material to the polymerization
temperature.

Because of the excessive temperature rise, porosity will more likely occur
in thick sections of the denture. Porosity also results when insufficient
pressure is maintained on the flask during processing.
:
: heat the flask in water at 60-70 °C for 9 hours.
: heat the flask in water at 74 °C for 90 minutes, then
boil for 1 hour for adequate polymerization of the thinner portions.
Other problems associated with rapid initial heating of the acrylic dough
above 74°C is production of internal stresses, warpage of the denture after
deflasking, and checking or crazing around the necks of the artificial teeth.
1- Urethane dimethacrylate matrix.
2- Acrylic copolymer.
3- Microfine silica filler.
4- Camphoroquinone-amine photo initiator system.
It is supplied in premixed sheets having clay like consistency. It is
provided in opaque light-tight packages to avoid premature
polymerization. The denture base material is adapted to the cast while it
is in a plastic state. It is polymerized in a light chamber (curing unit) with
blue light of 400-500 nm from high intensity quartz-halogen bulbs. The
denture is rotated continuously in the chamber to provide uniform
exposure to the light source.

Completely polymerized acrylic resin is tasteless and odorless. Denture
with porosity can absorb food and bacteria, resulting in an unpleasant
odor and taste.
The esthetic of acrylic is acceptable, because it is a clear transparent resin
which can be easily pigmented and it is compatible with dyed synthetic
fibers.
The polymer has a density of 1.19 gm/cm3
.
They have adequate compressive and tensile strength for complete or
partial denture applications. Ideally denture base resins should have high
impact strength to prevent breakage when it is accidentally dropped. Cold
cured acrylic has lower impact strength, but addition of plasticizers
increase the impact strength.
The strength is affected by:
a- Composition of the resin.
b- Technique of the processing.
c- Degree of polymerization.
d- Water sorption.
e- Subsequent environment of the denture.
Acrylic resins have low hardness; they can be easily scratched and
abraded.
Heat cured acrylic resin: 18-20 KHN.
Cold cured acrylic resin: 16-18 KHN.

They have sufficient stiffness (2400 MPa) for use in complete dentures.
However, when compared with metal denture bases they are low. Self-
cured acrylic has slightly lower values.
A well processed acrylic denture has good dimensional stability.
Acrylic resins shrink during processing due to:
a- Thermal shrinkage on cooling.
b- Polymerization shrinkage.
However, in spite of the high shrinkage, the fit of the denture is not
affected because the shrinkage is uniformly distributed over all surfaces
of the denture; the processing shrinkage is balanced by the expansion due
to water sorption.
 Volume shrinkage: 8 %.
 Linear shrinkage: 0.53 %.
Self-cured type has a lower shrinkage (linear shrinkage: 0.26 %).
Acrylic resin absorbs water and expands. This partially compensates for
its processing shrinkage. This process is reversible. Thus, on drying they
lose water and shrinkage. However, repeated wetting and drying should
be avoided as it may cause warpage of the denture.
Acrylic is virtually insoluble in water and oral fluids. They are soluble in
ketones, esters, and aromatic and chlorinated hydrocarbons. Alcohol
causes crazing in some resins.

Stability to heat: poly methyl methacrylate is chemically stable to heat up
to a point. It softens at 125 °C.
Thermal conductivity: they are poor conductors of heat and electricity.
Coefficient of thermal expansion: acrylics have a high coefficient of
thermal expansion.
Heat cured acrylics have good color stability. Cold cured has lower color
stability, due to oxidation of amine accelerator.
Completely polymerized acrylic resins are biocompatible. True allergic
reaction to acrylic resins is rarely seen in the oral cavity. Direct contact of
the monomer over a period of time may provoke dermatitis.
The highest residual monomer level is observed with cold cured acrylic.
The adhesion of acrylic to metal and porcelain is poor, and mechanical
retention is required. Adhesion to plastic denture teeth is good (chemical
adhesion).
Acrylic resins dispensed as powder/liquid have the best shelf life. The gel
type has a lower shelf life and has to be stored in a refrigerator.

Dental impression: It is a negative record of tissue of the mouth. It is
used to reproduce the form of the teeth and surrounding tissues. A
positive reproduction is obtained by pouring dental stone or other suitable
material into the impression and allowing it to harden.
The positive reproduction of a single tooth is described as die, and when
several teeth or a whole arch is reproduced, it is called cast or model. The
impression material is carried to the mouth in a tray, which either stock
tray or special tray.
Accurate reproduction of surface details.
A pleasant odor, taste, and esthetic color.
Absence of toxic or irritant constituents.
Adequate shelf life for requirements of storage and distribution.
Reasonable cost.
Easy to use with the minimum of equipment.
Setting characteristics that meet clinical requirements.
Satisfactory consistency and texture.
Readily wets oral tissues.
Elastic properties with freedom from permanent deformation after
strain.
Adequate strength so it will not break or tear on removal from the
mouth.
Dimensional stability over temperature and humidity ranges normally
found in clinical and laboratory procedures for a period long enough
to permit the production of a cast or die.
Compatibility with cast and die materials.
Readily disinfected without loss of accuracy.
No release of gas during the setting of the impression or cast and die
materials.

They cannot engage undercuts, so their use is restricted to edentulous
patient without undercut.
a- Impression plaster.
b- Impression compound.
c- Zinc oxide eugenol.
d- Impression wax.
They can engage undercuts, and they may be used in edentulous,
partially dentate, and fully dentate patients.
:
a- Reversible hydrocolloid (agar-agar).
b- Irreversible hydrocolloid (alginate).
a- Polysulfide.
b- Silicone:
- Condensation polymerizing silicone.
- Addition polymerizing silicone.
c- Polyether.

 Impression plaster.
 Zinc oxide eugenol.
 Alginate.
 Polysulfide.
 Polyether.
 Silicones.
 Impression compound.
 Impression wax.
 Agar-agar.
It is not compress tissue during seating of the impression.
 Zinc oxide eugenol.
 Alginate.
 Agar-agar.
2-
It compresses tissue during seating of impression, the material more
viscous.
 Impression compound.
Material fairly viscous whilst under low stress conditions may
become fluid during recording of impression.
 Polysulfide.

The material is compatible with moisture and saliva.
 Alginate.
 Addition polymerizing silicone.
 Polyether.
Ability of material to repel saliva, a dry field is essential for such
materials.
 Polysulfide.
 Condensation polymerizing silicone.
It presents as powder mixed with water in water/powder ratio
(W/P= 0.60), 100 g powder/60 ml water.
1- Calcium sulfate β-hemihydrate.
2- Potassium sulfate: to reduce expansion, and to accelerate the setting
reaction.
3- Borax: to reduce the rate of setting.
4- Starch: to help disintegration of impression on separation from the
plaster or stone cast.

After cast hardening, the impression and cast are put in hot water. The
starch swells and the impression disintegrates, making it easy to separate
the cast from the impression.
1- Setting time (5 minutes).
2- The mixed material has a very low viscosity, so it is mucostatic.
3- It is hydrophilic.
4- It adapts to the soft tissue and recording their surface detail with great
accuracy.
5- The dimensional stability is very good (a dimensional change during
setting is 0.06 %).
6- A separating medium must be used between the impression plaster
and the pouring plaster or stone.
7- The material is rigid once set, and thus unable to record undercuts.
8- Patient complains very dry sensation after having impression
recorded because of water absorbing nature of this material.
9- The material is best used in a special tray, made from acrylic (1.5 mm
spacer).
1- Final impression for completely edentulous arch.
2- Occlusal bite registration.

Impression compound is described as a rigid, reversible impression
material which sets by physical changes. On applying heat, it softens and
on cooling it hardens.
They supplied as sheet, stick, and cake.
Figure (4-1): (A) This shows examples of dental compound in the form
of either cake or sheet or in the form of sticks. The slabs are used to
make impressions of edentulous areas in the mouth whilst the sticks are
used as tray extension materials or for extending special trays. (B) This
shows a typical edentulous impression recorded in impression
compound. Note the lack of any fine detail in this impression due to the
very high viscosity of the material.

1- Thermoplastic resins.
2- Wax.
3- Plasticizer: stearic acid: addition of plasticizer to overcome brittleness.
4- Filler: talc, calcium carbonate added to:
a- Overcome tackiness.
b- Control degree of flow.
c- Minimize shrinkage due to thermal contraction.
d- Improve rigidity of impression material.
 Sheet form material: it is softened using water bath, a temperature in
range (55-60 °C), knead the material after it has been heated in water
to ensure its being at a uniform temperature. Storage in hot water
should not be long that important constituents such as stearic acid may
be leached out. Overheating make the compound sticky and difficult
to handle.
 Stick form material: it is softened over a flame. The compound should
not be allowed to boil; otherwise, the plasticizers are volatilized.
It is used to prepare a tray for making an impression. It is generally
stiffer and has less flow than regular impression compound.
1- It is mucocompressive.
2- Because of high viscosity and low flow; therefore, the reproduction of
surface detail is not very good.
3- It is not used to record the undercut, because it is rigid once cooled.
4- Poor dimensional stability. It has high value of coefficient of thermal
expansion and undergoes considerable shrinkage on removal from the
mouth. Also because pressure is applied during formation of an impression
(mucocompressive), residual stress exists in cool impression, the gradual
relief of internal stresses may cause distortion of impression (the cast
should be poured as soon as possible or at least within the hour).

5- Impression compound has low thermal conductivity, therefore, time must
be allowed during heating or cooling to allow impression compound to
come to uniform softening.
6- This material can be reused a number of times for the same patient only, in
case of errors.
7- The material has sufficient body to support itself to an extent especially in
the peripheral portions.
1- Difficult to record details because of its high viscosity.
2- Compress soft tissues while making impression.
3- Distortion due to its poor dimensional stability.
4- Difficult to remove it if there are severe undercuts.
5- There is always the possibility of overextension especially in the peripheral
portions.
1- Type I sheet form: It is used for recording primary impression of
edentulous ridges using stock tray.
2- Type I stick form: It is used for border molding of an acrylic special tray
during fitting of the tray.
3- Type II tray compound: It is used to make a special tray (now largely
replaced by acrylic tray).

1- Cementing and insulating medium.
2- Temporary filling.
3- Root canal filling material.
4- Surgical pack in periodontal surgical procedures.
5- Bite registration paste.
6- Temporary relining material for dentures.
7- Impression material for edentulous area.
1- Type I (Hard).
2- Type II (Soft).
1- Base paste (white in color).
2- Accelerator or reactor or catalyst paste (red in color).
.
Figure (4-2): This shows a
typical example of impression
paste materials. They consist
of two pastes which are
extruded out onto the mixing
slab and mixed together by
hand using a spatula. The
main active ingredient of one
paste is zinc oxide whilst the
main active ingredient of the
other paste is eugenol.

Zinc oxide (reactive component)
(87%).
Fixed vegetable or mineral oil
(act as plasticizer, and aids in
masking the action of eugenol as
an irritant) (13%)
Oil of cloves or eugenol (reactive
component) (12%).
Gum (speed the reaction) (50%).
Filler (20%).
Lanolin (3%).
Resinous Balsam (improve flow
and mixing properties) (10%).
CaCl2 (accelerator solution) and
coloring agent (5%).
The setting reaction is a typical acid-base reaction to form a chelate. This
reaction called chelation and the product is called zinc eugenolate.
1- ZnO + H2O Zn(OH)2
2- Zn(OH)2 + 2HE ZnE2 + 2H2O
The set material consists of a matrix of amorphous zinc eugenolate
surrounding and holds the unreacted zinc oxide particles.
Initial setting time Final setting time
Type I (Hard) 3-6 minutes 10 minutes
Type (Soft) 3-6 minutes 15 minutes

a- Particles size of zinc oxide powder: if the particle size is small, the
setting time is less.
b- By varying the lengths of the two pastes.
c- By adding a drop of water, the setting time can be decreased.
d- Longer the mixing time, shorter is the setting time.
e- High atmospheric temperature and humidity decrease the setting time.
f- Cooling the mixing slab, spatula increase the setting time.
g- By adding a drop of oil or wax, the setting time can be increased.
2- It registers surface details accurately due to its good flow.
3- The material has mucostatic properties.
4- The material is rigid once set and cannot be used for making
impression of teeth and undercut areas.
5- It requires a special tray for impression making; it has adequate
adhesion to acrylic tray.
6- It is dimensionally stable, a negligible shrinkage (less than 0.1%) may
occur during hardening.
7- No separating medium is required before the cast is poured because it
does not stick to the cast material.
8- The paste tends to adhere to the skin, so the skin around the lips
should be protected with Vaseline to make the cleaning process much
easier.
9- Eugenol can cause burning sensation and tissue irritation. Non
eugenol paste were developed, here the zinc oxide is reacted with a
carboxylic acid.
10- It can be checked in the mouth repeatedly, and minor defects can be
corrected locally without discarding a good impression.
The mixing is done on oil impervious or glass slab. Equal length of base
paste and catalyst paste squeezed on to mixing slab and mixed until a
uniform color is observed. The mixing time is 1 minute.

1- Final impression of edentulous ridge.
2- Occlusal bite registration.
Impression waxes are rarely used to record complete impression but are
used to correct small imperfection in other impression. Waxes are
generally used in combination with other impression materials
These materials consist of a mixture of low melting paraffin wax and
beeswax in ratio about 3:1. It may also contain metal particles. The flow
at 37°C is 100 %. These waxes are subjected to distortion during removal
from the mouth. They should be poured immediately.
Waxes have larger coefficient of thermal expansion of any material used
in restorative dentistry.
1- To make functional impression of free end saddles (class I and class II
removable partial dentures).
2- To record posterior palatal seal in dentures.
3- Functional impression for obturators.
Figure (4-3): This shows the
two pastes of zinc oxide and
eugenol being mixed
together. Here we see the
advantage of using pastes of
different colors since it is
possible to tell when proper
mixing has been achieved. In
this case there are still
obvious streaks of the two
individual pastes showing
that mixing is incomplete

The colloids are often classed as the fourth state of matter known as
colloidal state, they can exist in the form of viscous liquid known as a sol,
or a jelly like elastic semi-solid described as a gel.
If the particles are suspended in water, the suspension is called
hydrocolloid.
Hydrocolloid impression materials are based on the colloidal suspension
of polysaccharide in water.
 In sol form: There is random arrangement of polysaccharide chain.
 In gel form: The long polysaccharide chains become aligned and material
becomes viscous and develops elastic properties.
Gelation: It is conversion of sol to gel.
Based on the mode of gelation, they are classified as:
1-
Set by lowering the temperature e.g. Agar. This makes them reusable.
2-
Set by a chemical reaction. Once set it is usually permanent e.g. Alginate.
Agar hydrocolloid was the first successful elastic
impression material to be used in dentistry. It is an
organic hydrophilic colloid extracted from certain
types of seaweed. Although it is an excellent
impression material and yields accurate
impressions, presently it has been largely replaced
by alginate hydrocolloid and rubber impression
materials.

1- For cast duplication (during fabrication of cast metal removable
partial denture).
2- For full mouth impressions without deep undercuts.
3- For crown and bridge impressions before elastomers came to the
market.
4- As tissue conditioner.
1- Gel in collapsible tubes (for impressions with water cooled tray).
2- A number of cylinders in a glass jar (syringe material).
3- In bulk containers (for duplication).
 Agar (12%)
 Water (85%)
 Borates (0.2%)
 Potassium sulfate (1-2%)
 Alkyl benzoate (0.1%)
 Glycerin
 Coloring and flavoring agents
(traces)
Colloid
It acts as dispersion medium.
To improve the strength of gel.
To ensure proper setting of gypsum
cast against agar (accelerator for
cast material)
Preservative.
Thixotropic material (it acts as
plasticizer).
Agar hydrocolloid requires special equipment:
1- Hydrocolloid conditioner.
2- Water cooled rim lock tray.
Agar is normally conditioned prior to use by specially designed
conditioning bath (temperature controlled water bath). The conditioning
bath consist of three compartments each hold at different temperature.

 The tube of the gel converted to viscous liquid after 10 minutes in
boiling water (100°C).
 The sol should be homogenous and free of lumps.
 Every time the material is reliquefied, 3 minutes should be added.
This because it is more difficult to break down the agar brush heap
structure after a previous use.
 It should not be reheated more than 4 times.
 65-68°C temperature is ideal when agar can be stored in the sol
condition till needed.
 46°C for about 2 minutes with material loaded in the tray, this is
done to reduce the temperature so that it can be tolerated by the
sensitive oral tissue. It also makes the material viscous.
100°C
Liquefaction
section
(10 minutes)
65-68°C
Storage
section
(10 minutes)
46°C
Tempering
section
(2 minutes)

The tray containing the tempered material is removed from the bath. The
outer surface of the agar sol is scraped off, then the water supply is
connected to the tray and the tray is positioned in the mouth. Water is
circulated at 18°C to 21°C through the tray until gelation occur, rapid
cooling (ice cold water) is not recommended as it can induce distortion.

Alginate was developed as a substitute for agar when it became scarce
due to World War II (Japan was a prime source of agar). Currently,
alginate is more popular than agar for dental impression, because it has
many advantages.
1- Fast setting.
2- Normal setting.
A powder that is packed in bulk container (sachets), a plastic scoop is
supplied for dispensing the bulk powder, and a plastic cylinder, is
supplied for measuring the water.
1- It is used for impression making.
 When there are undercuts.
 In mouth with excessive flow of saliva.
 For partial dentures with clasps.
2- For making preliminary impression for complete denture.
3- For impression to make study models and working casts.
4- For duplicating models.

1- Sodium or potassium or
triethanolamine alginate.
2- Calcium sulfate (reactor).
3- Zinc oxide.
4- Potassium titanium fluoride.
5- Diatomaceous earth.
6- Sodium phosphate (retarder).
7- Coloring and flavoring
agents.
15 %
16 %
4 %
3 %
60 %
2 %
traces
Dissolves in water and reacts with
calcium ions.
Reacts with potassium alginate
and forms insoluble calcium
alginate.
Acts as filler.
Gypsum hardener.
Acts as filler.
Reacts preferentially with calcium
sulfate.
e.g. wintergreen, peppermint and
anice, orange etc.
Sodium alginate powder (soluble) dissolves in water to form a sol, that
react with calcium sulfate (reactor) to form calcium alginate (insoluble
gel); this reaction is too fast, there is not enough working time, so the
reaction is delayed by addition of a retarder (sodium phosphate).
Calcium sulfate reacts with the retarder
first, after the supply of the retarder is over does
calcium sulfate reacts with sodium alginate, this
delays the reaction and ensures adequate
working time for the dentist.

1- Alginate has a pleasant taste and small.
2- Its flexibility is about 14 % at a stress of 1000 gm/cm2
; lower W/P
ratio (thick mixes) results in lower flexibility.
3- Alginate is highly elastic, but less than agar.
4- The elastic recovery is 97.3 %, permanent deformation is less if the set
impression is removed from the mouth quickly.
5- Detail reproduction is also lower when compared to agar.
6- Compressive strength is 5000-8000 gm/cm2
.
7- Tear strength is 350-700 mg/cm2
.
 W/P ratio, too much or too little water reduces strength.
 Mixing time, over and under mixing both reduce strength.
 Time of removal of impression, strength increases if the time of
removal is delayed for few minutes after setting.
8- Set alginate has poor dimensional stability due to evaporation,
syneresis, and imbibition. The alginate impression should be poured
immediately. If storage is unavoidable, keeping in a humid atmosphere
of 100 % relative humidity (wrap with wet paper towel). Even under
these conditions storage should not be done for more than 1 hour.
9- Alginate does not adhere well to the tray. Retention to the tray is
achieved by mechanical locking in the tray (rim lock, perforated tray)
or by adhesive.
10- The silica particles present in the dust of alginate powder are health
hazard.
11- Shelf life and storage: Alginate material deteriorates rapidly at elevated
temperature and humid environment.

Mixing time: 45-60 seconds.
Working time: 1-2 minutes.
Setting time (gelation time): 2-4 minutes.
 Control gelation time
1- Gelation is best controlled by adding retarders. (Manufacturer's
hands).
2- The dentist can best control the setting time by altering the
temperature of the water; colder the water, longer is the setting
time, even the mixing bowl and spatula can be cooled.
 Test for set
The alginate loses its tackiness and rebound fully when prodded
with a blunt instrument; some alginate are available with (color
indicator), which on mixing is one color and on setting change to a
different color.
1- It is easy to mix and manipulate and need minimum equipment.
2- Flexibility of the set impression.
3- If properly handled, it gives accuracy and good surface details even in
presence of saliva.
4- Low cost.
5- Comfortable to the patient.
6- It is hygienic.
1- It cannot be corrected.
2- Poor tear strength.
3- Distortion may occur without it being obvious if the material is not
held steady while it is setting.
4- It cannot be stored for long time.
5- Because of the above drawbacks and because of availability of better
materials it is not recommended when high level of accuracy is
required e.g. cobalt chromium RPD, crown and bridge, etc.

Figure (4-6): Sketch of tear strength specimen with load
applied in the directions of the arrows; the specimen tear at
the V-notch.
TECHNICAL CONSIDERATIONS OF ALGINATE
1. Impression should not be exposed to air because some dehydration will
occur and result in shrinkage.
2. Impression should be protected from dehydration by placing it in a humid
atmosphere or wrapping it in a damp paper towel until a cast can be
poured. To prevent volume change, this should be done within 15 minutes
after removal of the impression from the mouth.
3. Impression should not be immersed in water or disinfectants, because
some imbibition will occur, and result in expansion.
4. Exudate from hydrocolloid has a retarding effect on the chemical reaction
of gypsum products and results in a chalky cast surface. This can be
prevented by pouring the cast immediately.
5. When alginate is used, place the measured amount of water (at 18-20°C)
in a clean, dry, rubber mixing bowl. Add the correct measure of powder.
Stir rapidly against the side of the bowl with a short, stiff spatula. This
should be accomplished in less than (1 minute). The patient should rinse
his or her mouth with cool water to eliminate excess saliva while the
impression material is being mixed and the tray is being loaded.
6- To prevent internal stresses in the finished impression, do not allow the
tray to move during gelation (hold the tray immobile for 3 minutes). Do
not remove the impression from the mouth until the impression material
has completely set (releasing the surface tension).
The stone cast should not be separated for at least 45 minutes; the cast
should not be left in the alginate impression for too long a period because:
1- After setting the alginate can act as sponge, deprive stone from water
result in a rough chalky surface.
2- Dried alginate becomes stiff, so removal of cast can break the teeth.

In addition to the hydrocolloids there is another group of elastic
impression materials, they are soft rubber like and are known as
elastomers, or synthetic rubbers, or rubber base, or rubber impression
materials, or elastomeric impression materials.
They are non-aqueous elastomeric dental impression materials.
1- Polysulfide.
2- Poly ether.
3- Silicon.
a- Condensation polymerizing.
b-Addition polymerizing.
1- Light body.
2- Medium or regular body.
3- Heavy body or tray consistency.
4- Very heavy or putty consistency.
1- Impressions of prepared teeth for fixed partial dentures.
2- Impression for removable partial dentures.
3- Impression of edentulous mouth for complete dentures.
4- Polyether is used for border molding of special tray.
5- For bite registration.
6- Silicon duplicating material is used for making refractory cast.
 Regardless of type all elastomeric impression materials are supplied
as two paste system (base and catalyst) in collapsible tubes.
 Putty consistency is supplied in jar.

This was first elastomeric impression material to be introduced. It is also
known as Mercaptan or Thiokol.
1- Light body.
2- Medium body.
3- Heavy body.
1- Liquid polysulfide polymer. (80-85 %).
2- Inert fillers (titanium dioxide, zinc sulfate, copper carbonate, or
silica). (16-18 %).
1- Lead dioxide. (60-68 %).
2- Dibutyl phthalate (30- 35 %).
3- Sulfur. (3 %).
4- Other substances like (deodorant, and magnesium stearate (retarder) (2 %).
Figure (4-7): Polysulfide
impression material.
The two pastes with
contrasting colors are
mixed together on a
mixing pad with a metal
spatula.

1- Unpleasant odor and color.
2- It is extremely viscous and sticky, mixing is difficult. However, they
exhibit pseudoplasticity.
3- It has long setting time (12 minutes). Heat and moisture accelerate
the setting time.
4- Excellent reproduction of surface details.
5- It has highest permanent deformation (3-5 %) among the elastomers,
so pouring of the cast should be delayed by half an hour. Further
delay is avoided to minimize curing shrinkage, and shrinkage from
loss of by-product (water).
6- It has high tear strength (4000 gm/cm2
).
7- It has good flexibility and low hardness.
8- It is hydrophobic so the mouth should be dried thoroughly before
making an impression.
1- Unpleasant odor.
2- Dirty staining.
3- High amount of effort required for mixing.
4- Long setting time.
5- High shrinkage on setting.
6- High permanent deformation.

These materials were developed to overcome some of the disadvantages
of polysulfide.
This was the earlier of the two silicone impression materials. It is also
known as conventional silicone.
1- Light body.
2- Putty consistency.
1- Polydimethyl siloxane.
2- Colloidal silica or metal oxide fillers (35-75 %) depending on viscosity.
3- Color pigments.
1- Stannous octoate (catalyst).
2- Orthoethyl silicate (cross linking agent).
1- Pleasant color and odor.
2- Setting time is 8-9 minutes.
4- Dimensional stability is comparatively less because of the high
polymerizing shrinkage, and shrinkage from loss of by-product (ethyl
alcohol). The cast should be poured immediately, the permanent
deformation is also high (1-3 %).
5- The tear strength is lower than polysulfide (3000 gm/cm2
).
6- It is stiffer and harder than polysulfide, care should be taken while
removing the stone cast from the impression to avoid any breakage.
7- It is hydrophobic.
8- Direct skin contact should be avoided to prevent any allergic
reactions.

They were introduced later. It has better properties than condensation
silicone. It is also known as polyvinyl siloxane.
1- Light body.
2- Medium body.
3- Heavy body.
4- Putty consistency.
1- Poly methyl hydrogen siloxane.
2- Other siloxane prepolymers.
3- Fillers.
1- Divinyl polysiloxane.
2- Other siloxane prepolymers.
3- Platinum salt (catalyst).
4- Palladium (hydrogen absorber).
5- Retarders.
6- Fillers.
2- Direct skin contact should be avoided to prevent any allergic
reactions.
4- Setting time is 5-9 minutes.
5- It has the best dimensional stability among the elastomers. It has low
polymerizing shrinkage, and the lowest permanent deformation (0.05-
0.3 %). The cast pouring should be delayed by 1-2 hours; because of
hydrogen gas is liberated during polymerization, air bubbles will
result.

6- It hydrophobic, so similar care should be taken while making the
impression and pouring the wet stone. Some manufactures add a
surfactant (detergent) to make it more hydrophilic.
7- It has low flexibility and it harder than polysulfide; care should be
taken while removing the stone cast from the impression to avoid any
breakage.
Polyether was introduced in the 1970. It has good mechanical properties
and dimensional stability.
1- Light body.
2- Medium body.
3- Heavy body.
Figure (4-8): Section of an
impression in which heavy
body (A), and light body
(B) materials have been
used to obtain optimal
accuracy and dimensional
stability.
Figure (7-9): Polyether impression material. The two pastes have been
extruded on to the mixing pad ready for mixing using a metal blade spatula.

1- Polyether polymer.
2- Colloidal silica (filler).
3- Glycol ether or phthalate (plasticizer).
1- Aromatic sulfonate ester (cross-linking agent).
2- Colloidal silica (filler).
3- Phthalate or glycolether (plasticizer).
2- The sulfonic ester may cause skin reaction; direct skin contact
should be avoided.
3- Setting time is around (8 minutes), heat decrease setting time.
4- Dimensional stability is very good. Polymerizing shrinkage is low.
The permanent deformation is low (1-2 %). The impression should
not be stored in water or in humid climate, because polyethers
absorb water and can change dimension.
5- It is extremely stiff (flexibility 3 %). Its hardness is higher than
polysulfide and increase with time; care should be taken while
removing the stone cast from the impression to avoid any breakage.
6- The tear strength is good (3000 gm/cm2
).
7- It is hydrophilic, so moisture in the impression field is not so
critical. It has the best compatibility with stone.
1- The working time was short.
2- The material was very stiff.
3- It is expensive.

1- Impressions are usually made in special trays. Perforated stock trays are
used only for making impression in putty consistency.
2- The spacing given is between 2-4 mm.
3- Elastomers do not adhere well to the tray. An adhesive should be
applied onto the tray and allowed to dry before making impression.
4- The bulk of the impression should be made with a heavier consistency
(to reduce shrinkage), light body should only be used in a thin layer as a
wash impression.
 Tray used: spaced special tray.
 Viscosity used: regular body only.
Method
The paste is mixed and material is loaded onto the tray, the tray with
material is seated over the impression area, the material is allowed to set.
 Tray used: spaced special tray.
 Viscosity used: (a) heavy body and (b) light body.
Method
The two viscosities are mixed simultaneously but on separate pads. The
heavy body is loaded onto the tray while the light body is loaded into the
syringe. The syringe material is injected onto the area of impression. The
tray containing the heavy body if then seated over it. Both materials set
together to produce a single impression.

 Tray used: perforated stock tray.
 Viscosity used: (a) putty (b) light body.
Method
First a primary impression is made with putty in the stock tray. After
setting it is kept aside. Light body is mixed and spread into the putty
impression. The primary impression is then seated over the impression
area and held till it is set.
1- More uniform mix.
2- Less air bubbles incorporated in mix.
3- Reduced working time.
Figure (4-10): Addition silicone impression materials packaged with auto-mixed
cartridges, mixing gun, and static mixing tips, and dynamic mechanical mixer.

Figure (4-11): The bulk
packaging of an elastomeric
impression material. The
pastes are extruded through
the mixing nozzle using an
electrically powered motor
inside the device. The mixed
material can be extruded
directly into an impression
tray which is held underneath
the nozzle. The nozzle itself is
disposable and is replaced
with a fresh nozzle for each
individual mix.
Figure (4-12): Top left,
impression tray containing
elastomeric impression is
seated too late as elasticity
starts to develop. Top right,
increased seating pressure is
applied to overcome the
stiffness of impression
material. Lower left,
distortion develops because
of recovery of excessive
elastic deformation. Lower
right, the die produced in the
distorted (inaccurate)
impression is too narrow and
too short.

It is a metal containing two or more elements, at least one of which is
metal, and all of which are mutually soluble in the molten state.
: They are materials resist corrosion in the mouth. (gold,
platinum, palladium, silver, rhodium, ruthenium, iridium, osmium).
: This term indicates the intrinsic value of the metal.
The eight noble metals are also precious metal, but all precious metals are
not noble.
: Pure gold is soft, ductile, yellow hue. The density is 19.3
gm/cm3
, melting point is 1063°C, good chemical stability, not corrode
and not tarnish.
: Whitest metal, its density is 10.4 gm/cm3
, melting point is
961°C.
: Its density is 12.02 gm/cm3
, melting point is 1552°C.
: its density is 21.65 gm/cm3
, melting point is 1769°C.
: These are not noble metals, (chromium, cobalt, nickel,
copper,…..etc).
They are important components of dental casting alloy because:
a- Their influence on physical properties.
b-Control of the amount and type of oxidation.
c- Their strengthening effect.

 Type I: soft.
 Type II: medium.
 Type III: hard.
 Type IV: extra-hard.
1- Binary (2 elements).
2- Ternary (3 elements).
3- Quaternary (4 elements).
1- Alloys for all metal; metal with resin veneer restorations.
2- Alloys for metal ceramic restorations.
3- Alloy for removable dentures.
1- It should not tarnish and corrode in the mouth.
2- It should strong.
3- Biocompatible (non-toxic, non-allergic).
4- It should be easy to fabricate (melt, cast, cut, and grind).
5- It should flow well, and duplicate fine details during casting.
6- It should have minimal shrinkage on cooling after casting.
7- It should easy to solder.

Alloys for all metal restorations
1- Gold alloys (composed of gold, copper, silver, platinum, palladium, and
other additives).
2- Silver-palladium alloys.
3- Nickel-chromium alloys.
4- Cobalt-chromium alloys.
Alloys for metal ceramic restorations
1- Gold-palladium-platinum alloys.
2- Palladium-silver alloys. Cheap
3- Nickel-chromium alloys.
Figure (5-1):
Cutaways of all-
ceramic crown
(left) and
porcelain fused to
metal crown
(right).

HOW DOES PORCELAIN BOND TO THE ALLOY?
Ceramic adheres to metal primarily by chemical bond. A covalent bond is
established by sharing 02 in the elements in the porcelain and the metal
alloy.
These elements include silicon dioxide (Si02 in the porcelain and
oxidizing elements such as silicon, indium, and iridium in the metal alloy.
Alloys for removable dentures
8- It should be light weight.
9- It should have high stiffness (to make the framework thin).
10- It should have good fatigue resistance.
11- It should not react to commercial denture cleanser.
12- Economical consideration.
Alloy used:
a- Cobalt (to give hardness, strength, rigidity).
b- Chromium (to ensure corrosion resistance by passivating effect).
c- Nickel (to decrease fusion temperature and increase ductility).
d- Molybdenum or tungsten (to increase hardness).
e- Iron and copper (to increase hardness).
f- Manganese and silicon (to prevent oxidation).
g- Boron (to increase hardness and deoxidizer).
h- Carbon (to strengthen the alloy).
a- Nickel.
b- Chromium.
c- Molybdenum.
d- Other minor additions like aluminum, iron, silicon, copper, manganese, tin.
The function of each ingredient is discussed previously

Filling materials are used to replace missing parts of the tooth.
1- Dental caries.
2- Trauma.
3- Abrasion.
Parts of teeth which require replacement by restorative materials vary in size
of cavity, shape, and location in the mouth; no single restorative material is
suitable for all cases. For some situations, the strength and abrasion
resistance of material may be the prime consideration, in other situation
appearance and adhesive properties may become more important.
1- Working time should be sufficiently long, to enable manipulation and
placement of material before setting.
2- Setting time should be short for comfort of both the patient and clinician.
3- The material must withstand large variation in pH and a variety of solvents
which may be taken into mouth in drink food stuffs and medicaments.
4- Metallic materials should not undergo excessive corrosion, or be involve in
the development of electrical currents which may cause "Galvanic pain".
5- Filling should be good thermal insulator, protecting the dental pulp from the
harmful effect of the hot and cold stimuli (low thermal diffusivity).
6- Materials should have values of coefficient of thermal expansion similar to
those of enamel and dentin.
7- Materials should have satisfactory mechanical properties to withstand the
force applied, e.g. abrasion resistance, compression and tensile strength,
modulus of elasticity.
8- They should adhere well to the tooth walls and seal the margin prevent
ingress of fluid and bacteria. Also reduces the amount of cavity preparation
required in order to achieve retention of the filling.
9- They should be harmless to the operator and to the patient and should not
irritant to dental pulp and soft tissue.
10- Easily polished.
11- Should be bacteriostatic and anticariogenic.
12- It should be radiopaque to diagnose the marginal caries.

 Amalgam.
 Direct gold filling.
 Polymeric
o Filled resin (composite).
o Unfilled resin (acrylic).
 Non-polymeric:
o Silicate cement.
o Glass ionomer cement.
 Silicate cement.
 Acrylic.
 Composite.
It is a special type of alloy in which mercury is one of the components.
Mercury is able to react with other metals to form a plastic mass, which is
conveniently packed into a prepared cavity in a tooth, and then this mass
is hardened. It is the most widely used filling material for posterior teeth.

Figure (6-1): Amalgam restorations.
1- As a permanent filling material in:
a- Class I and class II cavities.
b- Class V cavities where esthetic is not important.
2- In combination with retentive pins to restore a crown.
3- For making a die.
4- In retrograde root canal fillings.
5- As a core material.
I- Based on copper content
1- Low copper alloys: contain less than 6 % copper (conventional alloy).
2- High copper alloys: contain more than 6 % copper.
II- Based on shape of alloy particles
1- Lathe-cut alloys: (irregular shape often needle-like either coarse
grain or fine grain which is preferred because ease of carving).
2- Spherical alloys.
3- Blend of lathe-cut and spherical particles.
III- Based on size of alloy particles
1- Microcut.
2- Macrocut.

Figure (6-2): Dental amalgam alloys (Lathe-cut alloy particles).
Figure (6-3): Spherical alloy particles.
Figure (6-4): Lathe-cut particles of conventional alloy and spherical particles.

Bulk powder and mercury.
Alloy and mercury in disposable capsules mixed by amalgamator
machine; figure (6-5).
(It is the major element in the reaction).
Whitens the alloy.
Decrease the creep.
Increase the strength.
Increase the expansion on setting.
Increase the tarnish resistance in the amalgam filling.
Control the reaction between silver and mercury, without tin the
reaction is too fast and the setting expansion is unacceptable, but it
decrease strength and hardness, and reduce tarnish and corrosion
resistance, so the amount of tin should be controlled.
Figure (6-5)

Increase hardness and strength.
Increase setting expansion.
It is not affect the reaction and properties, but it is added in small
amount to act as deoxidizer thus prevents oxidation of major elements
during manufacturing.
When alloy powder and mercury are triturated, the silver and tin in the
outer portion of the particles dissolve into the mercury. At the same time
mercury diffuses into the alloy particles and starts reacting with silver and
tin present in it, forming (silver-mercury) and (tin-mercury) compounds.
 The silver-tin compound (unreacted alloy powders) known as gamma
phase (γ).
 The silver-mercury compound is known as gamma one phase (γ 1).
 The tin-mercury compound is known as gamma two phase (γ 2).
Gamma
(Unreacted particle)
Gamma 2 (Tin-Mercury)
Gamma 1 (Matrix)

Microleakage:
With age the amalgam has self-sealing property that decreases the
microleakage due to the corrosion products that forms in the tooth-
restoration interface.
Effect of moisture contamination (delayed expansion):
If a zinc-containing amalgam is contaminated by moisture during
condensation large expansion can take place. It usually starts after 3-5
days and may continue for months. It may reach 4 % that produce
pressure on the pulp and cause post-operative sensitivity.
Figure (6-7): An occlusal amalgam filling which has caused the tooth to
crack. The most likely cause of this cracking is the expansion of the
amalgam during or shortly after setting.
Effect of trituration:
Under- and over-trituration will decrease the strength.

Effect of mercury content:
 Low mercury in mixing lead to dry, granular mix resulting in rough
pitted surface that invites corrosion.
 High mercury can produce a marked reduction in strength.
Effect of condensation:
Higher condensation pressure results in higher compressive strength
this happen only in lathe-cut alloys. The condensation will decrease
porosity, and remove excess mercury from lathe-cut amalgam. If
heavy pressure is used in spherical amalgam, the condensation will
punch through. However, spherical amalgam condensed with lighter
pressure produces adequate strength.
Effect of cavity design:
 Should be designed to reduce tensile stresses.
 The cavity should have adequate depth, because amalgam has
strength in bulk.
Creep value:
Creep is related to marginal breakdown.
 Low-copper amalgam 0.8-8 %.
 High- copper amalgam 0.4-1 %.
Figure (6-8): Creep of amalgam causes the formation of unsupported
edges which can fracture.
Amalgam does not adhere to tooth structure, so retention is obtained
through mechanical locking.

Amalgam restorations often tarnish and corrode in the mouth. This
corrosion can be reduced by:
 Smoothing and polishing the restoration.
 Correct Hg/alloy ratio and proper manipulation.
 Avoid dissimilar metals including mixing of high, and low copper
amalgams.
Reasonably easy to insert.
Maintains anatomic form well.
Has adequate resistance to fracture.
After a period of time prevents marginal leakage.
Cheap.
Have long service life.
The color does not match tooth structure.
Brittle.
Corrosion and galvanic action.
They eventually show marginal breakdown.
They do not bond to tooth structure.
Risk of mercury toxicity.
Mercury is toxic, free mercury should not be sprayed or exposed to the
atmosphere. This hazard can arise during trituration, condensation, and
finishing, and also during the removal of old restorations at high speed.
Avoid mercury vapors inhalation and skin contact with mercury as it can
be absorbed.

Approximately 80 % of the mercury vapor will be absorbed in the lungs,
and 5–10 % of the mercury (saliva) will be resorbed in the
gastrointestinal tract. The hypothesized intake of mercury via oral mucosa
or dental pulp, however, seems to be negligible.
There are no scientific studies that show that having dental amalgams is
harmful, or that removing your amalgam fillings will improve your
health. (U.S. Food and Drug Administration, consumer information, October 2006)
It has been determined that the dental amalgam fillings do not pose a
health risk, although they do account for some mercury exposure to those
having such fillings.
Mercury has a cumulative toxic effect. Dentists are at high risk. Through
it can be absorbed by skin or by ingestion; the primary risk is from
inhalation.
Mercury accumulates in the kidneys. If the dose exceeds the capacity
limit, direct toxic damage of the proximal renal tubules.
The target organ of prime concern is the central nervous system. Tremor
and psychological disturbances (erethism) are classical symptoms of a
chronic mercury intoxication caused by extensive occupational exposure.
Erethism is characterized by acute irritability, abnormal shyness, timidity,
and overreaction to criticism. Disturbance of memory, loss of appetite,
depression, fatigue, and weakness may also occur. Further symptoms of
chronic intoxication with inorganic mercury are decreased nerve
conduction velocity and gastrointestinal disturbances. Oral symptoms,
including metallic taste, swollen salivary glands, disturbed salivation,
severe gingivitis, mucosal ulcerations, necrosis, and even tooth loss have
also been reported. Clinical symptoms of mercury poisoning that may be
found in heavily exposed persons.

The clinic should be well ventilated.
The mercury should be stored in well-sealed container.
Amalgam scrap and materials contaminated with mercury or amalgam
should not be subject to heat sterilization.
Vacuum cleansers are not used because they disperse the mercury.
Skin contacted with mercury should be washed with soap and water.
While removing the old fillings, a water spray, mouth mask, and suction
should be used.
The use of ultrasonic amalgam condenser is not recommended as a spray of
small mercury droplets is observed.
If the mercury contact the gold jewelry the mercury bonds permanently to
the gold and ruins, but boiling it in coconut oil can fix it.
Annually, a (programme for handling toxic materials) is monitored for actual
exposure levels.
It is pulpal irritation due to low pH (5-3.5).
Brittle and has weak mechanical properties.
Shrinkage on setting.
High solubility and disintegration.
Unfilled acrylic polymer where introduced about 1945 and were improved
so that they were in moderate usage in the 1960s. The unfilled acrylic
material possessed improved resistance to solubility and has no problems
with dehydration, although staining was a problem. The undesirable qualities
of unfilled acrylics were large dimensional change on setting and with
temperature, resulting in percolation of saliva at margins; low mechanical
strength and stiffness; low resistance to wear; and recurrent decay.

The term composite material may be defined as a compound of two or
more distinctly different materials with properties that are superior or
intermediate to those of the individual constituents.
Composite is polymeric filling material reinforced with filler particles.
It was developed in 1962s to overcome the disadvantages in physical and
mechanical properties of acrylic filling and of silicate cement. It is most
popular anterior filling material. Nowadays, composite is used as anterior
and posterior filling materials.
Bisphenol -A- glycidyl methacrylate monomer (Bis-GMA) or urethane
dimethacrylate. Bis-GMA monomer is most commonly used. Its
properties were superior to those of acrylic resins. It has a high viscosity
which required the use of diluent monomers. The commonly used
diluents monomer is tetraethyl glycol dimethacrylate (TEGDMA).
Types of filler
They are obtained by grinding or milling the quartz. They are mainly
used in conventional composites. They are chemically inert and very
hard. This make restoration more difficult to polish and can cause
abrasion of opposing teeth and restoration.
They are microfiller; added in small amount (5 wt %) to modify the
paste viscosity. Colloidal silica particles have large surface area thus
even small amount of microfiller thicken the resin. In microfilled
composites, it is only inorganic filler used.

These filler provide radiopacity to resin restoration. Its refractive
index is 1.5 e.g. barium, zirconium, and strontium glasses. The most
commonly used is barium glass. It is not as inert as quartz some
barium may leach out.
As less resin is present, the curing shrinkage is reduced.
Reduced water sorption and coefficient of thermal expansion.
Improves mechanical properties like strength, stiffness, hardness, and
abrasion resistance.
Amount of filler added.a-
Size of particles and its distributionb-
In order to increase the amount of filler in the resin, it is necessary to
add the filler in a range of particles size. If a single particle size is
used, a space will exist between particles, smaller particles can then
fill up these spaces.
Index of refractionc-
For esthetic, the filler should have a translucency similar to tooth
structure. To achieve this, the refractive index of filler should closely
match that of the resin. Most glass and quartz filler have a refractive
index 1.5, which much than that of bis-GMA.
Its hardnessd-
Radiopacitye-
Coupling agents bond the filler particles to the resin matrix. This allows
the more plastic resin matrix to transfer stress to stiffer filler particles.
The most commonly used coupling agent is organosilane.

They improve the physical and mechanical properties of resin.
Prevent the filler from being dislodged from the resin matrix.
They prevent water from penetrating the filler-resin interface,
microleakage of fluids into filler-resin interface led to surface staining.
Hydroquinone acts as inhibitor to prevent premature polymerization.
UV-absorber adds to improve color stability.
Opacifiers like titanium dioxide and aluminum oxide.
Color pigments add to match tooth color.
This is two paste systems
 : contains benzoyl peroxide initiator.
 : tertiary amine activator.
When two pastes are spatulated the amine reacts with the benzoyl
peroxide to form free radical which starts the polymerization.
Figure (6-9): Atypical two paste
composite material. Approximately
equal amounts of two pastes are
taken out of the containers using a
plastic spatula and then they are
mixed together on a paper pad.

The science of dental materials

The science of dental materials

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