Mujtaba Ashraf

PHYSICAL PROPERTIES OF DENTAL MATERIALS

Presented byMujtaba AshrafMDS JR-1Dept. of Prosthodontics

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Introduction DefinitionThe various physical properties include:Hardness.Concept of Stress and Strain.

ViscosityCreep and flow.

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Contents

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• Color and color perception.• Thermophysical properties.• Introduction to tarnish & corrosion• Types of corrosion

Conclusion

References3

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Dental materials are the very basic of modern dentistry.

They form the vital integral upon which all of dental procedures are formulated and executed.

Hence, it becomes imperative that every dentist is equipped with a sound knowledge of every aspect of these fascinating materials.

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Introduction

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Physical properties are based on the laws of mechanics, acoustics, optics, thermodynamics, elasticity, magnetism, radiation, atomic structure, or nuclear phenomena.

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What are Physical Properties?

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Any appliance/ prosthesis / restoration undergoes deformation due to various forces of mastication

Complete knowledge of physical properties of material enhances:-

- effectiveness of prosthesis- safety limitation- longitivity of prosthesis

In dentistry?

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• Property of being difficult to indent, cut or scratch.

• Simply resistance to plastic deformation or permanent deformation of material.

• Used as an index of the ability of a material to resist abrasion or wear.

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1. Hardness

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The surface hardness tests commonly used in dentistry are :•BRINELL•KNOOP•VICKERS•ROCKWELL•BARCOL•SHORE

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Brinell hardness test Oldest methods used to test metals and

alloys Small steel or tungsten carbide ball,

typically 1.6 mm in diameter, subjected to a weight of 123 N and it remain in contact to specimen for 30 sec

Smaller the area of indentation, harder the material and larger the BHN value.

Ratio of the load applied to the area of the indentation produced.

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Also known as 136 degree diamond pyramid test.

The method is similar to BHN except that a 136 degree diamond pyramid shaped indenter is forced into the material with a definite load application.

The indenter produces a square indentation,

the diagonals of which are measured.

Vickers Hardness Test

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It is suitable for brittle materials so it is used for the measurement of hardness of tooth structures and also of dental casting gold alloys.

A significant advantage of this test is that it can be used for testing very small specimens because the indenter tip is small.

Also, because the load applied to the specimen can be varied, the test can be used on materials that are soft as well as on those that are hard.

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Knoop hardness test Diamond is used as indenter having extended

pyramidal shape with the length to width ratio being 7:1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge.

Materials with KN Dentin-68 Gold Foil-69 Tooth Enamel-343

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•Used for thin plastic or metal sheets

•Advantage of this method is that materials with a great range of hardness can be tested by varying the test load.

•Disadvantage is that it needs very high polished and flat surface.

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ROCKWELL HARDNESS TEST

This was developed as a rapid method for hardness determination.

A ball or metal cone indenter is normally used and the depth of the indentation is measured with a sensitive dial micrometer.

The indenter balls or cones are of different

diameters and load applications ( 60 to 150 kg)

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The advantages of Rockwell system is that hardness is read directly and it is good for testing viscoelastic materials.

The disadvantages are that a preload is needed, greater time required and the indentation may disappear immediately on removal of the load.

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BARCOL HARDNESS TEST This method is used to study the depth of cure of

resin composites. The Barcol indenter is a spring loaded needle with

a diameter of 1 mm that is pressed against the surface to be tested.

If no penetration of the needle into the surface occurs the scale reads 100.

The reading on the scale decreases as the indenter penetrates the surface.

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Depth of cure of a resin composite is tested by preparing specimens varying in thickness from 0.5 to 6.0 mm.

Then the top surface of a specimen is activated by a light curing unit.

The Barcol hardness of the top surface is compared with that of the bottom surface.

10% decrease in Barcol hardness of a resin composite results in a 20% decrease in the flexural strength.

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This method is used to determine the hardness of rubbers because the indentation disappears after removal of load.

An instrument called Shore Durometer is used in the rubber industry to determine the relative hardness of elastomers.

The indenter is attached by a lever to a scale that is graduated from 0 to 100 units.

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Shore Hardness Test

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If the indentor completely penetrates the specimen, a reading of 0 is obtained, and if no penetration occurs, a reading of 100 units results.

The usual method is to press down firmly and quickly on the indenter and record the maximum reading as the shore hardness.

The test has been used to evaluate soft denture liners, mouth protectors and maxillofacial elastomers.

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Knoop and Vickers test are called microhardness tests.

The Brinell and Rockwell are macrohardness test.

Knoop & Vicker’s tests used loads less than 9.8N. The indentations are small and are limited to a depth of less than 19 mm.

They are capable of measuring hardness in small regions of thin objects

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• When force applied on a body, internal resistance to load is stress.

• The strain may be elastic /reverts back to original form or plastic/ permanent deformation.

• The value of stress which causes an object to give way or break is called strength of material.

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2. Concept of Stress and Strain

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Force induced by or resisting an external force.

Stress= Force per unit area

Stress is equal and opposite in direction to the load or external force.

Stress

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Types of Stress

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Axial

Tensile

Compressive

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Non Axial Shear

Torsion

Bending

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Can be defined as change in length per unit length of the body when subjected to stress.

Has no definite unit.

Strain

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Strain can either be elastic or plastic

Elastic strain is strain that totally disappears once the external load that caused it is removed.

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Plastic strain is strain that permanently remains once the external load that caused it is removed.

It occurs when the force applied to the

atoms moves them so far from their equilibrium position that they do not return to it once the force is removed.

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Stress vs. Strain Graph

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3. Viscosity Resistance of a liquid to flow

Viscosity is controlled by internal frictional forces and is measured in poise (Mpa/sec) or centipoise.

Study of flow characteristics of materials and deformation is rheology.

The term rheology was coined by Eugene C. Bingham.

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•The success or failure of a given material may be as dependent on its properties in the liquid state as it is on its properties as a solid.

•Materials like cements and impression materials undergo a liquid-to-solid transformation in the mouth.

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•Gypsum products used in the fabrication of models and dies are transformed from slurries into solid structures

•Amorphous materials such as waxes and resins appear solid but actually are supercooled liquids that can flow plastically under sustained loading or deform elastically under small stresses.

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To explain viscous nature of some materials, shear stress v/s shear strain rate curve can be plotted.

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NEWTONIAN FLUID:- an ideal fluid- Shear stress proportional to strain rate-Straight line on curve-Viscosity(η)= shear stress(τ)/strain rate (ε)-Constant velocity.

PSEUDOPLASTIC FLUID:-viscosity decreases with increasing strain rate, until it reaches a nearly constant value.e.g. Ketchup, blood, nail-polish

DILATENT FLUID Viscosity increase with increasing stress.The material become more rigid under stress(disadvantage)e.g.-Acrylic denture base material, sand in water

PLASTIC FLUIDMaterial behaves rigid until a minimum of stress is applied ,then it starts behaving like Newtonian fluid.e.g.- clay suspension, composite material

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4.CREEP AND FLOW

The term creep implies a relatively small deformation produced by a relatively large stress over a long period of time whereas flow implies a greaterdeformation produced more rapidly with a smaller applied stress.

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•Creep is defined as a time dependent plastic strain of a material under a static load or constant stress.

•Because of its low melting range, dental amalgam can slowly creep from a restored tooth site under periodic sustained stress, such as would be imposed by patients who clench their teeth.

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•The term flow, rather than creep, has generally been used in dentistry to describe the rheology of amorphous materials such as waxes.

•The flow of wax is a measure of its potential to deform under a small static load even that associated with its own mass.

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What is Color?

Sensation induced from light of varying wavelengths reaching eye.

Cone cells of retina are responsible for spectral sensitivity.

Determined visually by measurement of hue, chroma, and value.

5.Color and Color Perception

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HueHue is defined as the particular variety ofa color, shade, or tint.

E.g. RED, GREEN YELLOW

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Chroma The intensity of the color, degree of saturation of a particular hue.

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Value Value is defined as the relative lightnessor darkness of a color or the brightness of an object.

• Value of 0 = black• Value of 10 = white

Higher value = lighter shade

Lower Value = darker shade

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The Munsell Color System•Professor Albert H. Munsell•Basic principles first published in 1905•Hues are divide into 10 gradations: yellow, yellow-red, red, red-purple, purple, purple-blue, blue, blue-green, green

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The Commission Internationale de l’Eclairage (CIE),

an international color research group published the CIELAB color system in 1976.

CIELAB Color System

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•3-dimensional color system• L* refers to brightness (0 to 100)• a* represents red (+a*) vs. green (-a*)• b* indicates yellow (+b*) vs. blue (-b*).•When a* and b* are zero, the L value represents the continuum of black to white

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Other Color Terms

• Opacity is the measure of impenetrability of visible light.

•Transparency- capable of transmitting light

•Translucency-Diffused transmission of visible light

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•Metamerism-Phenomenonin which the color ofan object under one type oflight appears to changewhen illuminated by differentlight source.

Different light sources interact differently with different materials.

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•Fluorescence is the absorption of light by a material and the spontaneous emission of light in a longer wavelength.

•In a natural tooth, it primarily occurs in the dentin because of the higher amount of organic material.

•UV light is absorbed and fluoresced back as light primarily in the blue end of the spectrum.

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The color of teeth encompasses only a small portion of the total color space.

The color ranges of human teeth have been measured by different researchers at different times and using different methods and color notation systems.

All of the studies indicate that human teeth are in the yellow-red to yellow portion of the spectrum, they are relatively high in Value (light or bright), and they have a relatively low Chroma (not too much color intensity).

Color of Human Teeth

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Shade guides are used in determining the color of natural teeth so that artificial substitute restorations will possess similar color and esthetics.

Clinical shade selection involves direct visual comparison of the different color samples that are present in a shade guide with the natural teeth and determination of which one best matches the teeth.

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Dental Shade Guides

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The VITAPAN Classical shade guide introduced in 1956 still is widely used for shade matching in dentistry.It has16 shade samples

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•VITA SYSTEM 3D-MASTER introduced in 1998.•It has 26 shades, divided into group 1to5.•Tabs are marked using a number-letter-number combination.•First number i.e. 1-5 represent Value •Letter L, M, R represent Hue from yellowish to reddish•The second number designates Chroma

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Tips for shade-taking:• Ensure the tooth condition is appropriate (e.g. clean, hydrated) for matching

• Tooth shades should be determined in daylight or under standardized daylight lamps and not under operation lamps.

• Because eyes usually tire after 5 -7 seconds, it is recommended to make a selection quickly.

• Avoid bright colors in the shade-taking environment, i.e. no lipstick, tinted eyeglasses, no bright-colored clothes.

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•Consider the selection distance. A selection made at one to three feet is generally more useful than one made in close proximity to the teeth.

•Evaluate the patient’s natural teeth to determine their color characteristics by looking at the cervical aspect of the teeth.

•Evaluate prospective shade guide specimens one at a time by holding them next to the tooth being matched .

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Thermal Conductivity : • Heat transfer through solid substances is most

commonly occur by means of conduction. • The conduction of heat through metals occurs

through the interactions of crystal lattice vibrations and by the motion of electrons and their interaction with atoms.

6.Thermal properties

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• The thermal conductivity, or coefficient of

thermal conductivity, is the quantity of heat

in calories per second that passes through a

specimen 1 cm thick having a cross-sectional

area of 1 cm2 when the temperature

difference between the surfaces

perpendicular to the heat flow of the

specimen is 1K.

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According to the second law of thermodynamics,

Heat flows from points of higher temperatureto points of lower temperature.

•Materials that have a high thermal conductivity are called conductors.

•Materials of low thermal conductivity are called insulators.

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• The International System (SI) unit or measure for thermal conductivity is watt per meter per second per degree Kelvin (W x m¯1 x s¯1 x K¯1).

• The higher the thermal conductivity, the greater is the ability of the substance to transmit thermal energy, and vice versa.

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• Compared with a resin-based composite that has a low thermal conductivity,

heat is transferred more rapidly away from the tooth when cold water contacts a

metallic restoration because of its higher thermal conductivity.

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•This increased conductivity of the metal compared with that of the resin composite induces greater pulpal sensitivity, which is experienced as a negligible, mild, moderate, or extreme discomfort, depending on previous tooth trauma and the pain response of the patient.

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The coefficient of thermal expansion describes how the size of an object changes with a change in temperature.

It measures of the fractional change in size per degree change in temperature at a constant pressure.

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Coefficient of Thermal expansion

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Clinical Significance

•Close matching of the coefficient of thermal expansion (α) is important between the tooth and the restorative materials to prevent marginal leakage.

•Opening and closing of gap results in breakage of marginal seal between the filling and the cavity wall, this breakage of seal leads to marginal leakage, discoloration & hypersensitivity.

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• It is a measure of the rate at which a body with a no uniform temperature reaches a state of thermal equilibrium.

• The thermal conductivity of zinc oxide-eugenol is slightly less than that of dentin, its thermal diffusivity is more than twice that of dentin.

Thermal Diffusivity

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•The square root of thermal diffusivity is indirectly proportional to the thermal insulation ability, whereas the thickness of the cement base is directly related to its benefit as an insulator.•Thus the thickness of the liner is a more important thermal insulation factor than the thermal diffusivity

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• Relevance of thermal diffusivity :

• In the oral environment, temperatures are not constant during the ingestion of foods and liquids. Under such conditions, thermal diffusivity is important.

• Typical values of thermal diffusivity in units of 10-4 cm2/sec are as follows:

• pure gold-11,800• Amalgam-960 • Composite- 19-73,

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• Water-14• Glass ionomer- 22• Dentin-18-26• Enamel- 47• zinc phosphate cement- 30

• Thus for a patient drinking ice water, the low specific heat of amalgam and its high thermal conductivity suggest that the higher thermal diffusivity favors a thermal shock situation more than that is likely to occur when only natural tooth structure is exposed to the cold liquid.

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• For any thermal insulator, tooth structure must be present in sufficient thickness for insulating dental cements to be effective. When the layer dentin between the bottom of the cavity floor and the pulp is too thin, the dentist should place an additional layer of an insulating base

• The low thermal conductivity of enamel and dentin aids in reducing thermal shock and pulpal pain when hot or cold foods are taken into the mouth.

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7.Tarnish• Tarnish is a surface discoloration of the

metal or even a slight loss of the luster.• This surface discoloration is due to either,

the formation of hard and soft deposits on the surface, or the formation of thin films of oxides, sulfides or chlorides.

• Tarnish is often the first step of corrosion

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• Stain or discoloration arises from pigment –producing bacteria, drugs containing chemicals such as iron or mercury & absorbed food debris.

• Although deposits are the main cause of the tarnish in the oral environment, surface discoloration may also arise on a metal from the formation of thin films, such as oxides, sulfides, or chlorides.

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• Corrosion is not only a surface discoloration but is a disintegration of a metal by reaction with its environment.

• Corrosion may occur due to moisture, atmosphere, acid or alkaline solutions, & certain chemicals.

• Common e.g. rusting of iron, a complex chemical reaction in which iron combines with oxygen in air & water to form hydrated oxide of iron.

Corrosion

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• Various causes of tarnish & corrosion• Eggs & other food contain significant amount of sulfur,

various sulfides (hydrogen or ammonium sulfide) corrode silver, copper, mercury & similar metals present in dental alloys.

• water, oxygen & chlorine ions present in saliva.

• various acidic solutions such as phosphoric , acetic, & lactic acids at proper concentration & pH promote corrosion.

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Classification of corrosionChemical / dry

corrosion

Metal and non-metals

Electrochemical /wet corrosion

Dissimilar metals

Heterogeneous surface composition

Stress corrosion

Concentration cell corrosion

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1) Non aqueous (dry) or chemical corrosion:•In which there is a direct combination of metallic and non-metallic elements•Electrolytes are absent•e.g. oxidation, halogenations, or sulfarization reaction. •This type of corrosion is less susceptible to occur in the mouth.•Example, oxidation of metal surface during soldering and heat treatment procedures.

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2) Aqueous (wet) or electrolytic corrosion:Corrosion occurs in presence of water or some other liquid electrolyte

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Types of electrolytic corrosion:

1) Galvanic cell corrosion.

2) Stress corrosion.

3) Concentration cell corrosion.

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Galvanic cell CorrosionGalvanism is results of difference in potential between dissimilar restoration in opposing or adjacent tooth.

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Galvanic ShockA pain sensation caused by electric current generated by a contact between two dissimilar metal forming a galvanic cell in oral environment.

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Stress Corrosion

Degradation by the combined effect of mechanical stress and corrosive environment, usually in form of cracking.

Example, a metal which has been stressed by burnishing produces the localized stress in some part of structureIf stressed and under stressed metals are in contact in an electrolyte the stressed area will become anode and will corrode

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Concentration cell corrosion (difference in composition of electrolyte)

A homogeneous metal or alloy can undergo electrolytic corrosion where there is a difference in electrolyte concentration across the specimen.Example; a metallic restoration which is partly covered by food debris will differ from that of saliva, and this can contribute to the corrosion of the restoration.

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Protection against corrosion• Certain metals develop a thin ,adherent , highly

protective film by reaction with the environment , such a metal is said to be passive.

• A thin surface oxide forms on chromium , a good e.g. of a passivating metal, & stainless steel contain sufficient amounts of chromium to passivate the alloy.

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• Avoid using dissimilar metals.

• Patients should be warned against using household bleaches for cleaning partial denture frameworks or removable orthodontic appliances that are alloyed with chromium.

• The surface of any dental restoration should be smooth and lustrous. A polished, smooth surface provides easier cleaning and prevents accumulation of debris.

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conclusion A proper knowledge of physical properties of

dental materials helps us in making correct choice for various clinical restorations. This in turn increases the durability and life span of the restoration.

When tooth shade is selected using conventional means, knowledge & skill of practitioner comes into play.

Technique based system provide dentist with distinct advantage in creating highly esthetics , natural looking restoration

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References Phillips science of dental material

10th & 11th edition

RESTORATIVE DENTAL MATERIALS – CRAIG 13th edition

Dental Materials and Their Selection- 3rd edition by William J. O'Brien

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