PHYSICAL PROPERTIES

OF

DENTAL MATERIALS

1

Contents

• Introduction
• Rheological properties
• Thermal properties
• Electrical properties
• Chemical properties
• Optical properties
• Conclusion
• References

INTRODUCTION

• A physical property is any measurable parameter that describes the state of a physical system
• A thorough knowledge of fundamental principles of physical science serve to describe transformation of materials subjected to external influences such as temperature, force, pressure or light

Physical properties of Dental Materials are based on

The laws of mechanics

• Creep and flow
• Abrasion and abrasion resistance
• Viscosity

Thermodynamics

• Thermal conductivity
• Thermal difussivity
• Co efficient of thermal expansion

Electrical property

• Galvanism

Optical property

• Color
• Metamerism
• Fluroscence

Chemical property

• Tarnish and corrosion

• Study of deformation and flow characteristics of matter
• The term can be applied to both liquids and solids.
• Flow properties of materials
• Viscosity
• Viscoelasticity
• Creep
• Thixotropic property

VISCOSITY

• Resistance of a fluid to flow

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• Controlled by internal frictional forces within the liquid

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• It is the measure of the consistency of a liquid and resistence to flow

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• A highly viscous fluid flows slowly

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• Unit: megapascal per sec or centipoisec

THIXOTROPY

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• Thixotropy is a property of gels or other fluid to become less viscous and flow when shaken, stirred, patted or vibrated

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• Depend on the previous deformation.

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• Also known as shear thinning fluid

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• Occurs due to some molecular arrangement during mixing or lack of time for

molecule to return to normal arrangement before mixing again.

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• Longer the fluid is mixed at a given shear rate, lower the shear stress and hence velocity

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• When the shear force is decreased to zero, the viscosity increases to the original value.

• Eg : Plaster of paris, Fluoride gels, Resin cements, Prophylatic pastes

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• Material does not flow out of a mandibular impression tray until placed over tissues.

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• Prophylaxis paste does not flow out of the rubber cup unless rotated against the teeth

CLINICAL APPLICATION

• For dental materials ,Newtonian and pseudoplastic behaviour are commonly

encountered

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• Dilatancy is rare

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• Waxes are super cooled liquids that can flow plastically under sustained loading.

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• Gypsum products: pseudoplastic mix- on vibration or shaking become thinner and flow better and then transform from slurries into solid structures.

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• Pseudoplastic luting cements[znpo4,znoe]- when pressed hard against tooth become thinner and excess flow out

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• Casting alloy liquids: made thinner by adding iridium-to increase flow to avoid

incomplete casting

VISCOELASTICITY

• The property of materials that exhibit behavior of both viscous liquid and elastic solid

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• Materials having mechanical properties dependent on loading rate

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• Polymers like soft relining materials
• Denture base polymers
• Elastomeric impression- Agar-agar, Alginate
• Creep of dental amalgam

MECHANICAL MODELS OF SOLIDS AND FLUIDS

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• An elastic solid can be viewed as a spring and fluid as a dashpot

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• When a constant load is applied to a spring , an instant strain occurs and remain constant with time

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• When the load is removed , the strain instantaneously decrease to zero

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• When a constant load is applied to an ideal viscous element , strain increase linear with

time

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• When the load is removed , no further increase or decrease in strain occur

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• The elastic element reacts instantaneously to change in load

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• Viscous element react after a finite time

MECHANICAL MODELS OF VISCOELASTICITY

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• Viscoelastic material exhibit both characteristics of solid and fluid

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• Viscoelastic behavior explained in terms of combination of simple model a spring and dashpot

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• models

MAXWELL MODEL

CLINICAL IMPORTANCE

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• In case of elastic impression materials, they donot immediately lose theri strain

when load is removed

• Therefore on removal from mouth,these materials remain stressed, and thus time is required for materials to recover before a die can be poured

OPTICAL PROPERTIES

• Light is an electromagnetic radiation detected by the human eye
• The eye is sensitive to wavelength from 400 nm (Violet) to 700nm (Dark red)

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• Incident light is polychromatic
• For an object to be visible, it must reflect or transmit light incident

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• The reflected light intensities and combined intensities of WL present in incident and reflected light determine the appearance properties.

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• Human more sensitive to light in green- yellow region
• Least sensitive to red or blue

PROPERTIES OF MATERIALS IN RELATION TO LIGHT TRANSMISSION AND

ABSORPTION

Transparency

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• Property of a material, that allows the passage of light in such a manner that little distortion takes place
• Objects can be clearly seen through them
• e.g. glass, pure acrylic resin.

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Translucency

_▪▪ Property of the material, which allows the passage of some light and scatters or reflects the rest .

• In such manner, the object cannot be clearly seen through them.
• Translucency decreases with increasing the scattering centers.

• e.g. tooth enamel, porcelain, composite and pigmented acrylic resin natural teeth.

Opacity

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• Property of the material that prevents the passage of light.
• Opaque material absorbs all of the light.
• Objects cannot be seen through them.

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Eg. metal-ceramic restoration

• Black color materials absorb all light colors.
• White color materials reflect all light colors.
• Blue color materials absorb all light colors but reflect its color.

COLOR

• Sensation induced from light of varying wavelength reaching eye

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• The perception of the Color of an object is the result of a physiological response to a physical stimulus.

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• Albert Munsell described color as a three dimensional phenomenon

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• According to one of Grassman’s Laws, eye can distinguish differences in only three parameters of color

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Dimensions are:

• Hue
• Value
• Chroma

HUE

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• The dominant wavelength of a color

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• It represents the color of the material

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• Primary colors - Red, Blue and Green.

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• Any other color may be produced by proper combination of these colors.

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• This dimension does not tell whether the color is dark or light or strong or weak

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• Least important in shade selection

VALUE

• Value is the most dominant factor of the three color elements

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• Relative lightness or darkness of a color

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• Also known as grey scale

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• Value of 0 = Back
• Value of 10 = White

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• Value increases towards the high end and decreases

towards the low end

• Most relevant parameter in shade matching.

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• Teeth and other objects can be separated into lighter and darker shades

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• depends on

Incident light

Surface finish of object and the background

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• Value can be measured independently of hue.

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• Shade guide is arranged on the basis of the Value parameter.

CHROMA

• It represents the strength of the color or degree of saturation of the color

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• Higher chroma more intense the color

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• Low chroma appear dull

VALUE

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• The quality by which we distinguish a light color from a dark one

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• It is the vertical axis is to our circle of hue

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• Value ranges from zero to ten

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• Black at the lower pole and white at the top

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• This representing total absence of light in lower end and pure light in the upper end

CHROMA

• Measured radially outwards from the neutral vertical axis

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• Number of steps away from grey

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• Colors in centre are dull or grey

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• Range 0-10

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• Represented by horizontal bar

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• Increases along radius

HUE

• The quality by which we distinguish one color from another as, red from yellow , a green , a blue from purple

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• measured by degrees around horizontal circles

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• Measured on a scale from 2.5 to 10 in increments of 2.5 for each of the 10 color families

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• Divided into 10 gradations

Red Yellow red Yellow

Green- yellow Green

Blue- green Blue

Purple -blue Purple

Red- purple

SHADE SELECTION

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• Value is determined first by the selection of a tab that most nearly correspond with white or darkness of the color.

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• Ranges from 0 – 10

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• Chroma is determined next with tabs that are close to measured value , but are of increasing saturation

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• Ranges from achromatic or grey to highly saturated color

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• Hue is determined by matching with color tabs of value and chroma already determined

SHADE GUIDE

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• Shade guides are used in determining the color of natural teeth

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• So that the artificial teeth will posses the similar color

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• Shade selection involves “ direct visual comparison of different color samples present in shade guides with the natural teeth and determination of which one suits the teeth

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VITAPAN Classical shade guide

• Introduced in 1956
• Widely used in dentistry for color matching in dentistry
• It has 16 shade samples

VITA SYSTEM 3D MASTER

• It has 26 shades

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• Divided into group 1 to 5

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• Tabs are marked using a number letter number combination

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• First no. 1 -5 represents “Value”

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• Letter L ,M , R represents “Hue”

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• The second no represents “Chroma”

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NATURAL TEETH- DIMENSIONS OF COLOR

• HUE: yellow red to yellow portion of the spectrum 566-586 nm

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• CHROMA: low [ .35-.4]

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• VALUE: high [36-45]

Tips for selecting shade

• Ensure the tooth condition is appropriate for matching

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• Tooth shade should be determined under daylight or under standardized daylight lamps

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• Not under operating lamps

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• Eyes usually tire after 5 -7 sec , it is recommended to select quickly

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• Avoid bright colors in shade taking environment. ie , lipstick, tinted eyeglasses, no bright

colored clothes.

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• Selection distance - selection made at 1 – 2 feet is generally more useful than one made in close proximity to the teeth

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• Evaluate prospective shade guide specimen one at a time by holding them net to the tooth being matched

FACTORS AFFECTING COLOR APPEARANCE AND SELECTION

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• Shade guide is used for color matching. So, it is important to match colors under appropriate conditions.

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Source

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• Different sources have different color content.
• i.e. Incandescent light has a color content different from that of fluorescent light.

Surrounding

• Colors of wall, lips or clothes of the patient modify the type of light reaching the object.

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Object

• Translucency:

It controls lightness or darkness of color.

High translucency gives a lighter color appearance. i.e more vital tooth appearance

• Surface texture

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Determines the relative amount of light reflected from the surface Smooth surface appears brighter than rough surface.

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• Presence of scattering centers as inclusions or voids

This increase opacity and lower the value (more dark)

Thickness

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• The thickness of a restoration can affect its appearance. Increase in thickness, increase opacity, and lower the value.

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Observer

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• Color response

Eye responds differently among individuals.

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• Color Vision

Some individuals may have color blindness and inability to distinguish certain colors.

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• Color Fatigue

Constant stimulus of one color decreases the response to that color.

Metamerism

• It is the change of color matching of two objects under different light sources.
• objects under one type of light appears to change when illuminated by different light source

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Fluorescence

• Absorption of light by a material and the spontaneous emission of light in a longer wavelength
• Thus tooth actually becomes a light source
• It makes the teeth bright and vital, as it increases the brightness.

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• In natural source, it primarily occurs in the dentine because of the higher amount of organic material
• Under some fluorescent light conditions artificial teeth or restorations without any fluorescence

completely black out.

• Anterior restorative materials have fluorescing agent[ rare earth elements] Dental porcelain-cerium oxide

THERMAL PROPERTIES

• Materials placed in oral environment are constantly subjected to change in temperature

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• The response a material to a source of heat depends on the ease with which heat is transferred through the material

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• when restorative materials are placed in deep cavities, heat transmitted to the vital pulp

must be limited to prevent thermal shock

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• Denture base in contact with mucosal surface – transmission of certain amount of heat is desirable for sensation of heat and cold

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• So it is necessary to understand the thermal properties of dental materials

THERMAL CONDUCTIVITY

• Physical property that governs the heat transfer through a material by flow

conductive

• Involves transfer of thermal energy from one part of material to another across a temperature gradient

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• It is defined as the quality of heat in calories per sec passing through a material 1cm thick with cross section of 1cm2 having a temperature difference of 1k

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• SI unit – watts per meter per kelvin

• According to 2^nd law of thermodynamics, heat flows from higher temperature point to lower temperature point

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• Materials with high thermal conductivity - Thermal conductors

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• Materials with low thermal conductivity - Thermal insulators

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• Higher thermal conductivity , greater ability to conduct heat

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• Lower thermal conductivity , less ability to conduct heat

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• Thermal conductivity increases in the order

Polymer < Ceramic < Metals

CLINICAL APPLICATION

• In oral environment , rapid change in temperature occurs during ingestion of hot and cold

foods and liquids

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• Such conditions thermal diffusivity play an important role

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• Enamel and dentine are effective thermal insulators

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• The thermal diffusivity and conductivity is comparable with cementing materials like GIC, composite, zinc phosphate

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• When the remaining dentine between the cavity and pulp is too thin , an insulating base material should be placed below the metallic restoration

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• Thermal conductivity of Zinc Oxide Eugenol is slightly less than dentin but its diffusivity

is more than twice that of dentin , therefore greater thickness is required on placement.

TARNISH

• Surface discoloration on a metal or even a slight loss or alteration of the surface finish or

lusture.

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• In the oral environment, tarnish often occurs from the formation of hard and soft deposits on the surface of the restoration.

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• Calculus is the principal hard deposit, and its color varies from light yellow to brown.

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• Also arise from the formation of thin films of sulfides , oxides or chlorides

CORROSION

• Corrosion is not a surface discoloration but actual deterioration of a metal by reaction with the

environment .

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• Cause severe disintegration of metals

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• Disintegration of a metal by the action of corrosion may occur through the action of moisture, atmosphere, acid or alkaline solutions and certain chemicals.

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• Corrosion often preceded by tarnish

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• Tarnish film accumulates components that chemically attack the metallic surface.

CLASSIFICATION

CHEMICAL CORROSION

• In this type of corrosion there is direct combination of metallic and non metallic

elements to yield a chemical compound through the processes

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• Oxidation
• Halogenation
• Sulferization

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• Also referred as dry corrosion

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• Eg. discoloration of silver by sulfide forms by chemical corrosion
• Oxidation of silver-copper alloy particles

ELECTROCHEMICAL CORROSION

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• Chemical corrosion is seldom isolated and is almost invariably accompanied by electrochemical corrosion.

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• Also referred as “wet corrosion” (requires the presence of water or some other

fluid electrolyte).

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• In order to continue the process it requires the pathway for the transport of electrons (electric current).

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• More important for dental restorations.

• Electrolytic corrosion may takes place in the mouth with saliva acting as an electrolyte , when the following condition are present in it:

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Different metals and alloys Heterogeneous composition of alloys Cold work metal

Difference in oxygen tension

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• E.g. anode can be dental amalgam, cathode may be gold alloy restoration and saliva as electrolyte.

GALVANIC CORROSION

• Galvanic corrosion occurs when dissimilar metals lie in direct physical contact with each other.

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• Eg- if a gold restoration comes in contact with an amalgam restoration , the amalgam forms the anode and starts corroding.

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• When two restoration touch causes sharp pain.

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• The best precaution is to avoid dissimilar metals in contact

STRESS CORROSION

• Since the imposition of stress increases the internal energy of an alloy

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• Either through the elastic displacement of atoms or the creation of microstrain fields associated with dislocations, the tendency to undergo corrosion will be increased.

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• For most metallic appliances, the deleterious effects of stress and corrosion, called

stress corrosion, are most likely to occur during fatigue or cyclic loading.

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• Electrochemical cells consisting of the more deformed metal regions(anodic),saliva and underformed or less deformed metal regions(cathodic) are created , and the deformed regions will experience corrosion attack.

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• This is one reason why excessive burnishing of the margins of metallic restorations is contraindicated.

CONCLUSION

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• Knowledge of properties and behavior of materials is essential for correct selection of material, manipulation techniques and precautions which serves in designing and constructing effective dental prosthesis