Properties of Dental Materials by Dr Rashid Hassan

PROPERTIES OF

DENTAL

MATERIALS

By

Dr. Rashid Hassan

PROPERTIES OF DENTAL

MATERIALS

PROPERTIES OF DENTAL MATERIALS

Properties can be Categorized into:

1) Properties of dental material before mixing.

2) Properties of dental materials during mixing & setting.

3) Properties of dental materials after setting.

Properties Of Dental Materials Before Mixing

1) Shelf life.

2) Storage

PROPERTIES OF DENTAL MATERIALSBEFORE MIXING (UNMIXED STATE)

In unmixed state dental materials posses the following properties

1) SHELF LIFE: It is the length of time a dental

material can be stored without deterioration.

2) STORAGE: Must be stored as per

manufacturer’s instructions.

Over storage should be avoided.

Properties Of Dental Materials During Mixing & Setting

1) Mixing time

2) Working time

3) Setting time

PROPERTIES OF DENTAL MATERIALSDURING MIXING & SETTING

1) MIXING TIME:

The recommended time for mixing a material until the required consistency / homogenous mixture is achieved.

It’s the time from the addition of powder to water until mixing is completed.


2) WORKING TIME:

It’s the time during which a material can be mixed/manipulated ideally with no thickening effect.

Time available to use a workable mix.


3) SETTING TIME:

Time required for a material to reach a certain level of rigidity/elasticity.

Starts when the mixing begins until the material hardens (Setting reaction is complete)

Properties Of Dental Materials

After Setting

1) Biological properties.

2) Chemical properties.

3) Electrical properties.

4) Thermal properties.

5) Physical properties.

6) Mechanical properties.

Properties Of Dental Materials After Setting

BIOLOGICAL PROPERTIES Dental material should

be non toxic, non irritant, non allergic, non carcinogenic & non mutagenic to oro-dental tissues.

Primarily the dental material must be harmless to manufacturer, assistant, dental surgeon, patient and any other relevant person.

Properties Of Dental Materials After Setting

BIOLOGICAL PROPERTIES

• The material is said to be “biocompatible” when it possesses the property of being non destructive (non toxic/non irritant) in a biological system.

• Biocompatibility id defined as,“The ability of a material to elicit an appropriate biological response in a given application in the body.”

12

13

TESTS FOR MEASURING

BIOCOMPATIBILITY

1. IN VITRO TEST

• Performed outside the organism.

• First screening test to evaluate biological response of a new material.

• Conducted in test tube, cell culture dish, flask or other container.

• Contact may be direct/indirect.

14

15

1. IN VITRO TESTADVANTAGES

• Relatively fast.• Inexpensive.• Controlled

conditions.• Large scale

testing.

DISADVANTAGES• Lack of relevance

to in vivo conditions.

• Lack of complex systemic co-ordinations.

2. THE ANIMAL TEST

• Material placed into an intact organism.

• Commonly used animals are mice, rats, hamsters or guinea pigs.

• Other animals used are cats, sheep's, dogs, monkeys and baboons 16

17

2. THE ANIMAL TEST

ADVANTAGES

• Intact biological system.

• Act as a bridge between in vitro and usage test.

DISADVANTAGES

• Expansive.

• Difficult to control.

• May take months or years.

• Ethical concerns.

18

3. USAGE TEST

Performed in animals or humans.

In case of humans it is called as clinical trial.

Choice of animal is limited.

Animals used should be large, with anatomy similar to the humans.

Human clinical trial is the “gold standard”.

19

3.USAGE TESTDISADVANTAGES:

• Very costly.• Ethical permission required (In case

of humans)• Prolonged time required for

results.• Legal liabilities.

20

TESTS USED TOGETHER

Usage

SECONDARY

PRIMARY

Progress of testing

Number of materials

CHEMICAL PROPERTIES OF

DENTAL MATERIALS

1) Sorption.

2) Adsorption.

3) Absorption.

4) Diffusion.

5) Osmosis.

6) Solubility.

7) Erosion.

8) Adhesion.

9) Cohesion.

10) Surface energy.

11) Wetting.

12) Contact angle of wetting.

13) Leaching.

CHEMICAL PROPERTIES OF DENTAL MATERIALS

1) SORPTION: The taking up and holding of o

ne substance by another. Sorption is used especially as a general term for absorption and adsorption

2) ADSORPTION: The process in which liquid or

gas molecules adheres firmly to the surface of solid or liquid.

3) ABSORPTION: The process in which a liquid

or gas molecules penetrate into the solid material.


4) DIFFUSION:

The process by which molecules intermingle as a result of their kinetic energy of random motion.

5) OSMOSIS:

The diffusion of solute from the region of low concentration to the region of high concentration through semi permeable membrane is called osmosis.


6) SOLUBILITY:

It is the measurement of the extent to which a material will dissolve in a given fluid. e.g. saliva or water.


7) EROSSION:

The process which combines the chemical process of dissolution with a mild mechanical action.

In dentistry erosion is used to describe the destruction of natural hard tissue by acids (either occurring naturally or present in food/drinks)


8) ADHESION:

Force of attraction between molecules of different substances.

9) COHESION:

Force of attraction between molecules of same substance.

10) SURFACE ENERGY/SURFACE TENSION:

The increase in energy per unit area is called as surface energy or surface tension.

Interfacial tension that exists between the two surfaces due to unbalanced intermolecular forces.

For adhesion to exist the surfaces must be attracted to one another at their interface. 27



11) WETTING/WETABILITY:

Interfacial tension between a liquid and a solid resulting in a contact angle of less than 90°.

Adhesion is negligible when the surface molecules of the two materials are separated by a distance greater than 0.7nm.

28


12) CONTACT ANGLE OF WETTING: The extent to which an

adhesive can wet the surface of adherend can be determined by measuring the contact angle between the adhesive and the adherend.

If the molecules of adhesive are attracted more to the molecules of the surface, the adhesive will spread completely over the surface of the solid.

29

Complete wetting occurs

at 0° and no wetting

occurs at 180°.

ELECTRICAL PROPERTIES OF DENTAL MATERIALS

1) Conductor

2) Insulator

3) Electric conductivity

4) Galvanism


1) CONDUCTOR:

A conductor is a material that allow the flow of electrical currentin one or more directions. A metal wire is a common electrical conductor.

2) INSULATOR:

The materials that offer high resistance to the flow of electric current. Are called insulators.


3) ELECTRICAL CONDUCTIVITY:

The ability of a material to conduct an electric current is called electric conductivity.4) GALVANISM:

Galvanic action occurs when two electrochemically dissimilar metals are in contact and a conductive path occurs for electrons and ions to move from one metal to the other.

THERMAL PROPERTIES OF DENTAL MATERIALS

1) Boiling point.2) Melting point.3) Freezing point.

4) Dew point.5) Heat of fusion.

6) Heat of vaporization.7) Thermal conductivity.

8) Thermal diffusivity.9) Specific heat.

10)Coefficient of thermal expansion.


BOILING POINT:

The temperature at which a liquid boils and turns to vapour..

MELTING POINT:

The temperature at which a given solid will melt.

FREEZING POINT:

The temperature at which a liquid turns into a solid when cooled.


DEW POINT: The atmospheric temperature

(varying according to pressure and humidity) below which water droplets begin to condense and dew can form.

A higher dew point indicates more moisture in the air.

Dew point greater than 20 °C (68 °F) is considered uncomfortable and greater than 22 °C (72 °F) is considered to be extremely humid.

Frost point is the dew point when temperatures are below

freezing.


HEAT OF FUSION:Heat of fusion is the energy required to change a gram of a substance from the solid to the liquid state at melting temperature.

HEAT OF VAPOURIZATION:Heat of vaporization is the energy required to change a gram of a liquid into the gaseous state at the boiling point Is called the "heat of vaporization"


THERMAL CONDUCTIVITY:It is the measure of the ability of a material to allow the flow of heat.THERMAL DIFFUSIVITY:Thermal conductivity of a substance divided by the product of its density and its specific heat capacity.SPECIFIC HEAT:The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius.

COEFFICIENT OF THERMAL EXPANSION:Change in length per unit original length per degree rise in temperature is called coefficient of thermal expansion.

PHYSICAL PROPERTIES OF DENTAL MATERIALS

A. OPTICAL PROPERTIES

1) Colour.

2) Hue.

3) Value.

4) Chroma.

5) Transparency.

6) Translucency.

7) Opacity.

8) Fluorescence.

B. RHEOLOGICAL PROPERTIES

1) Viscosity.

2) Creep & Flow.

3) Viscoelasticity.

4) Newtonian behavior.

5) Pseudoplastic behavior.

6) Thixotrophic behavior.

7) Dilatant


A. OPTICAL PROPERTIESCOLOUR:Combined intensities of the wavelengths present in the beam of light determine the property calour.HUE:The property associated with colour of an object (i.e. Red, Green, Blue)VALUE:The amount of lightness or darkness of a colour is called value (i.e. from bright to dull)

CHROMA:

Degree of saturation of a particular hue.Highe the chroma = more intense the colour.


A. OPTICAL PROPERTIESTRANSPARENCY:

The property of a material that allows the passage of light in such a manner that the object may be clearly seen through.

TRANSLUCENCY:

The property of a material that permits passage of light but disperses the light so the object cannot be seen through.


A. OPTICAL PROPERTIESOPACITY:

The property of a material that prevents the passage of light.

FLUORESCENCE:

The phenomenon of emission of light by a substance that has absorbed light or other electromagnetic radiations.



The study of flow of a material is

called “Rheology”

VISCOSITYResistance of a liquid to flow

43

Viscosity is dependent upon interatomic bonding

Water molecules has weak interatomic bonding thus flows easily as compared to oil/honey



CREEP AND FLOW“Creep is defined as the time dependent plastic strain of a material under static load or constant stress”

“Flow describes the rheology of the amorphous materials in dentistry” 44


B. RHEOLOGICAL PROPERTIESVISCOELASTICITYThe behavior that is intermediate between viscous liquid and elastic solid.e.g. Elastomeric impression materials.The more rapidly the material is loaded or unloaded the more elastically the material will behave.

45


B. RHEOLOGICAL PROPERTIESNEWTONIAN BEHAVIOR

When shear strain rate is proportional to shear stress, the behavior is called Newtonian behavior.

PSEUDOPLASTIC BEHAVIOR

Material is called pseudoplastic when viscosity decreases with increase in shear rate.

THIXOTROPIC BEHAVIOR

Material is called thixotropic when it exhibit a different viscosity after deformation.

DILATANT

Behavior seen in liquids that show higher viscosity with increase in shear rate.

MECHANICAL PROPERTIES OF

DENTAL MATERIALS1) Stress.2) Strain.

3) Proportional limit.4) Yield point.

5) Modulus of elasticity.6) Poision’s ratio.

7) Toughness.8) Brittleness.9) Strength.

10) Resilience.11) Hardness.

12) Relaxation.13) Permanent deformation.

14) Ductility.15) Maleability.

MECHANICAL PROPERTIES OF DENTAL MATERIALS

• Defined by the laws of mechanics.

• The physical science that deals with energy and forces and their effects on the bodies.

• Mechanical properties need to be considered collectively.

• Intended application of a material is important.

Failure or success potential of any prosthesis / restoration is dependent upon the mechanical properties of the material.

The material response may be,

1. Elastic …. reversible on force removal.

2. Plastic …… Irreversible / non-elastic.


Generally, the force applied may be

1. Axial (tensile or compressive)

2. Shear (sliding, rubbing)

3. Tortional (twisting movement)

4. Bending (bending movement)

TENSION

• Tension results when a body is subjected to two sets of forces directed away from each other in a straight line.

• Causes the material to elongate/Stretch.


COMPRESSION

• Compression results when the body is subjected to two sets of forces directed towards each other in a straight line.

• Causes the material to shorten.

SHEAR FORCE

• Shear is a result of two sets of forces directed parallel to each other , but not along the same straight line.

MECHANICAL PROPERTIES OF DENTAL MATERIALSTORSION

Torsion results from the twisting of the body.

BENDING

• Bending results by applying bending movement.

MECHANICAL PROPERTIES OF DENTAL MATERIALSSTRESS

• When a force acts on the body, a resistance is developed to the external force applied which is equal in magnitude/intensity and opposite in direction to the applied force and is called as “STRESS”

• Denoted by “S” or “σ”

• Designated as force per unit area (σ=N/m²)

• Pascal = 1 N / m².

STRAIN

• Relative deformation of an object due to stress.

• It is change in length per unit length.

• It may be elastic, plastic or both elastic and plastic.

• It is denoted by “ε”

• Designated as ∆L / L.


PROPORTIONAL LIMIT

• It is the maximum stress at which the stress is equivalent/proportional to strain and above this limit the plastic deformation of a material occurs.

• The material may be subjected to any type of applied force.

53


YIELD POINT

The point beyond which stress causes a material to undergo permanent deformation.

Yield point is always slightly higher than proportional limit.

Eg: Gold alloy

(Proportional limit = 276 Mpa& Yield point = 324 Mpa)

Material does not recover elastically when stress is removed.


MODULUS OF ELASTICITY

• It is relative stiffness or rigidity of a material.

• Measured by the slope of the elastic region Also called as Young ‘s modulus.

• It is measured by the slope of stress strain curve.

• If a tensile or compressive stress (below the proportional limit) is divided by corresponding strain value, a constant of proportionality will be obtained.

• Unaffected by the amount of elastic or plastic stress induced in the material.

• Independent of ductility of a material.

• The lower the strain for a given stress, greater will be the elastic modulus.

• E.g. two wires of same shape and size.

• Polyether impression materials.

• Unit is Giganewtons/m² (GPa).

55

STRESS STRAIN CURVE

MECHANICAL PROPERTIES OF

DENTAL MATERIALS

56

For materials in which strain is independent of the length of time that a load is applied

“ STRESS STRAIN CURVES“ are important.


STIFFNESS & FLEXIBILITY

1) If longitudinal portion of the curve is closer to the long axis the material is stiff & not flexible.

2) If it is away from the long axis the material is flexible.

TOUGHNESS & BRITTLENESS

1) If material fractures after a long concave portion of the curve, it donates that the material is tough & ductile.

2) If elastic portion of the curve is minimal, it shows the brittleness of the material 57

ANALYSIS FOR A STRESS STRAIN CURVE


STRNGTH & WEAKNESS

• If longitudinal portion of curve is longer, means that the material is strong.

• If longitudinal portion is short the material is weak.

HENCE FROM THE ANALYSIS OF THE STRESS STRAIN

CURVE IT IS POSSIBLE TO HAVE AN IDEA ABOUT THE

PROPERTIES OF A MATERIAL.

58





POISION’S RATIO

• If a cylinder is subjected to a tensile or compressive stress, there will be simultaneously an axial or lateral strain.

• Within the elastic range, the ratio of letaral to axial strain is known as Poision’sRatio.

60


STRENGTH

Strength is the maximum stress that a material can withstand without sustaining a specific amount of plastic strain.

OR

Stress at the point of fracture.

SHEAR STRENGTH:Maximum shear stress at the point of fracture.

FLEXURAL STRENGTH:Defined as “force per unit area at the point of fracture when a material is subjected to flexural loading”Also called as “BENDING STRENGTH” or “MODULUS OF RUPTURE”

61


FATIGUE STRENGTH• Determined by subjecting

a material to cyclic stress of maximum known value and determining the number of cycles required to cause failure of the material.

• Maximum service stress (endurance limit) can be maintained without failure over an infinite number of cycles.

• Endurance limit is lower for materials with brittle and rough surface.

IMPACT STRENGTH• Impact is the reaction of a

stationary object to a collusion with a moving body.

• Impact strength is defined as energy required to fracture a material under an impact force.

• The energy units are joules.


TOUGHNESS

The energy required to fracture a material is called toughness.

Also determined by the total area under stress strain curve.

Toughness describes how difficult it is to break a material.


BRITTLENESS

• It is opposite of toughness.

• When a material fractures at or near its proportional limit.

• Should not be confused with the lack of strength.

• Porcelain, Dental stone & Cements are examples of a brittle material.


RESILIENCE(Springiness)

• It is the amount of energy absorbed by a material when it is stressed not to exceed its proportional limit.

• Measured in terms of modulus of resilience (amount of energy stored in the body)

• Modulus of resilience=Proportional limit/Modulus of elasticity


CLASSIFICATION OF HARDNESS TESTS

HARDNESS

• In mineralogy, relative hardness of a substance is based upon its ability to resist scratching.

• In metallurgy and mostly in all other disciplines, hardness is defined as resistance to indentation.

• Higher hardness number = more hardness.

66



68

KNOOP HARDNESS TEST

BRINELL & ROCKWELL

HARDNESS TEST

VICKERS HARDNESS

TEST

Shapes produced by indentersOn materials


RELAXATION

Change in shape due to release of stresses is referred as relaxation.

Example:

Dental waxes & other thermoplastic materials.


PERMANENT DEFORMATION

• After crossing the elastic limit with continuous stress the resulting change in strain (dimension) is permanent.

For example:

Elastic impression materials


DUCTILITY• Ability of a material to

deform plastically under a tensile stress before fracture. e.g. metal drawn readily into long thin wires.

MALLEABILITY• The ability of a material

to sustain plastic deformation, without fracture under compression.

• Gold is the most ductile and malleable pure metal, followed by silver.

• Platinum is ranked third in ductility.

• Copper ranks third in malleability.

71

THANK YOU

Science

Properties of Dental Materials by Dr Rashid Hassan