B.D.S. M.Sc. Prosth.

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 HO 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).

T weak

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 1C.

It is the quantity of heat needed to raise the temperature of 1 g of the

substance 1C.

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 1C 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 120C (dry calcination). The

hemihydrate produced is called -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 125C (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

-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 (100C), 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

100C 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) are 1-

effective accelerators.

Sodium chloride in small amounts shortens the setting reaction but 2-

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 the 4-

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 100C 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 m in width, whereas types III, IV, and V reproduce a groove

50 m 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 and 2-

provide enough strength at higher temperatures to withstand the impact

force of the molten metal.

Stability at higher temperatures: Investment must not decompose to give 3-

off gases that could damage the surface of the alloy.

Sufficient expansion: Enough to compensate for shrinkage of the wax 4-

pattern and metal that takes place during the casting procedure.

Beneficial casting temperatures: Preferably the thermal expansion versus 5-

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 divestment 8-

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

-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 700C. The calcium sulfate portion of

the investment decomposes into sulfur dioxide and sulfur trioxide at

temperatures over 700C, 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 100.3C, 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 74C 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.

Impression plaster.

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.

Impression plaster.

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 -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 37C 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 (100C).

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.

65-68C temperature is ideal when agar can be stored in the sol

condition till needed.

46C 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.

100C Liquefaction

section (10 minutes)

65-68C Storage section

(10 minutes)

46C 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 18C to 21C 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 18-20C)

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.

3- Excellent reproduction of surface details.

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.

1- Pleasant color and odor.

2- Direct skin contact should be avoided to prevent any allergic

reactions.

3- Excellent reproduction of surface details.

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).

1- Pleasant color and odor.

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 b