YAWAR 1

U L F A TU L F A T

YAWAR 2

PROPERTIES OF DENTAL PROPERTIES OF DENTAL

MATERIALSMATERIALS PROPERTIES OF DENTAL PROPERTIES OF DENTAL

MATERIALSMATERIALS ByBy

DR. YAWAR HAYAT KHANDR. YAWAR HAYAT KHAN BDS (Pb), M.Sc. (London)BDS (Pb), M.Sc. (London)

Assistant Professor/Head of Department Assistant Professor/Head of Department (Dental Materials)(Dental Materials)

ISLAMIC INTERNATIONAL MEDICAL COLLEGE (DENTAL SECTION)ISLAMIC INTERNATIONAL MEDICAL COLLEGE (DENTAL SECTION)

YAWAR 3

PROPERTIES OF DENTAL PROPERTIES OF DENTAL MATERIALSMATERIALS

PROPERTIES OF DENTAL PROPERTIES OF DENTAL MATERIALSMATERIALS

Many factors must be taken into account Many factors must be taken into account when considering which properties are when considering which properties are relevant to the successful performance of a relevant to the successful performance of a material.material.

Several studies evaluate distinct properties Several studies evaluate distinct properties of dental materials which can influence and of dental materials which can influence and predict their performance.predict their performance.

Dental products have been developed very Dental products have been developed very rapidly and, consequently no. of studies rapidly and, consequently no. of studies designed to evaluate their properties is also designed to evaluate their properties is also increasing.increasing.

YAWAR 4

PROPERTIES OF DENTAL PROPERTIES OF DENTAL MATERIALSMATERIALS

PROPERTIES OF DENTAL PROPERTIES OF DENTAL MATERIALSMATERIALS

No single property can be used to define the No single property can be used to define the

quality of a material.quality of a material.

Several combined properties determined Several combined properties determined

from standardized laboratory and clinical from standardized laboratory and clinical

tests, are employed to give a measure of tests, are employed to give a measure of

quality of a material.quality of a material.

YAWAR 5

SCHEMATIC REPRESENTATION OF SCHEMATIC REPRESENTATION OF PROPERTIES USED TO CHARACTERIZE PROPERTIES USED TO CHARACTERIZE

DENTAL MATERIALSDENTAL MATERIALS

SCHEMATIC REPRESENTATION OF SCHEMATIC REPRESENTATION OF PROPERTIES USED TO CHARACTERIZE PROPERTIES USED TO CHARACTERIZE

DENTAL MATERIALSDENTAL MATERIALS

MATERIAL PROPERTIES

DURING STORAGE(before use)•Shelf life

DURING MIXING

•Methods of dispensation•Mixing time•viscosity

DURING SETTING•Rate of set•Working time•Setting time•Temperature rise on setting•Dimensional changes

SET MATERIAL•Physical•Chemical •Biological•Mechanical

YAWAR 6

PROPERTIES OF SET PROPERTIES OF SET MATERIALMATERIAL

PROPERTIES OF SET PROPERTIES OF SET MATERIALMATERIAL

1.1. PHYSICAL PROPERTIESPHYSICAL PROPERTIES

2.2. CHEMICAL PROPERTIESCHEMICAL PROPERTIES

3.3. BIOLOGICAL PROPERTIESBIOLOGICAL PROPERTIES

4.4. MECHANICAL PROPERTIESMECHANICAL PROPERTIES

YAWAR 7

1.1. PHYSICAL PROPERTIESPHYSICAL PROPERTIES1.1. PHYSICAL PROPERTIESPHYSICAL PROPERTIES

A.A. OPTICAL PROPERTIESOPTICAL PROPERTIES

B.B. THERMAL PROPERTIESTHERMAL PROPERTIES

C.C. ELECTRICAL PROPERTIESELECTRICAL PROPERTIES

D.D. RHEOLOGICAL PROPERTIESRHEOLOGICAL PROPERTIES

E.E. MISCELLANEOUS PROPERTIESMISCELLANEOUS PROPERTIES

YAWAR 8

A.A. OPTICAL PROPERTIESOPTICAL PROPERTIESA.A. OPTICAL PROPERTIESOPTICAL PROPERTIES

i.i. COLOURCOLOUR

ii.ii. TRANSLUCENCYTRANSLUCENCY

iii.iii. SURFACE TEXTURESURFACE TEXTURE

YAWAR 9

i.i. COLOURCOLOURi.i. COLOURCOLOUR• Colour selection is very importantColour selection is very important• Depends upon no. of physical factorsDepends upon no. of physical factors• Important parameters used to define colour are Important parameters used to define colour are

hue, chroma and valuehue, chroma and value• HueHue: Basic colour depending on the wavelength it : Basic colour depending on the wavelength it

reflects e.g. blue & greenreflects e.g. blue & green• ChromaChroma: Measures the intensity of the colour e.g. : Measures the intensity of the colour e.g.

low value of chroma indicates a weak colourlow value of chroma indicates a weak colour• ValueValue: value is darkness or lightness of the colour: value is darkness or lightness of the colour• Dental applications: Aesthetics , shade guides & Dental applications: Aesthetics , shade guides &

fluorescent agentsfluorescent agents

YAWAR 10

ii.ii. TRANSLUCENCYTRANSLUCENCYii.ii. TRANSLUCENCYTRANSLUCENCY

• TransparentTransparent: Allows the light to pass : Allows the light to pass

through itthrough it

• TranslucentTranslucent: Allows some light to pass : Allows some light to pass

through itthrough it

• OpaqueOpaque: Does not allow light to pass : Does not allow light to pass

through itthrough it

YAWAR 11

iii.iii. SURFACE TEXTURESURFACE TEXTUREiii.iii. SURFACE TEXTURESURFACE TEXTURE• Whether the material is shiny or matt the Whether the material is shiny or matt the

surface texture indicates the smoothness surface texture indicates the smoothness

of the surfaceof the surface

• Enamel has a shiny surface hence it is Enamel has a shiny surface hence it is

smoothsmooth

• As the surface gets rough, it becomes As the surface gets rough, it becomes

mattmatt

YAWAR 12

B. B. THERMAL PROPERTIESTHERMAL PROPERTIESB. B. THERMAL PROPERTIESTHERMAL PROPERTIES• Materials placed in the oral environment are Materials placed in the oral environment are

constantly subjected to change in temperatureconstantly subjected to change in temperature

• Further classified into Further classified into

i.i. TemperatureTemperature

ii.ii. Heat of fusionHeat of fusion

iii.iii. Specific heatSpecific heat

iv.iv. Thermal conductivityThermal conductivity

v.v. Thermal diffusivityThermal diffusivity

vi.vi. Coefficient of thermal expansionCoefficient of thermal expansion

YAWAR 13

i.i. TEMPERATURETEMPERATURE• Temperature of a food affects oral temperatureTemperature of a food affects oral temperature

ii.ii. HEAT OF FUSIONHEAT OF FUSION• Heat in calories or joules required to convert 1 gm of a Heat in calories or joules required to convert 1 gm of a

material from solid to liquid state at the melting material from solid to liquid state at the melting

temperaturetemperature

iii.iii. SPECIFIC HEATSPECIFIC HEAT

• Specific heat (CSpecific heat (Cpp) of a substance is the quantity of heat ) of a substance is the quantity of heat

needed to raise the temperature of a unit mass of a needed to raise the temperature of a unit mass of a

substance by 1ºC.substance by 1ºC.

• CCpp of metals and alloys should be known (temperature at of metals and alloys should be known (temperature at

which it should be melted)which it should be melted)

• When metallic fillings are used, the CWhen metallic fillings are used, the Cpp of the materials of the materials

should be low, this reduces heat conductionshould be low, this reduces heat conduction

YAWAR 14

iv.iv. THERMAL CONDUCTIVITYTHERMAL CONDUCTIVITY• The factor which determines the ease with which the heat The factor which determines the ease with which the heat

is transferred to a material is its thermal conductivity (K)is transferred to a material is its thermal conductivity (K)• It is defined as rate of heat flow per unit temperature It is defined as rate of heat flow per unit temperature

gradient. Units are cal/ sec/ cmgradient. Units are cal/ sec/ cm22 • Different materials have different values of KDifferent materials have different values of K• Materials with high K Materials with high K →→ good conductors of heat and cold good conductors of heat and cold• Materials with low K Materials with low K →→ bad conductors of heat and cold bad conductors of heat and cold• Metals are good conductors as compared to plastics and Metals are good conductors as compared to plastics and

ceramicsceramics• Composites and zinc phosphates have same values of K as Composites and zinc phosphates have same values of K as

that of enamel and dentinethat of enamel and dentine• Dental considerationsDental considerations

• A metallic restoration conducts heat A metallic restoration conducts heat →→ transferred to pulp transferred to pulp →→ pain and discomfort pain and discomfort →→ can be avoided by placing insulating can be avoided by placing insulating bases (cements) as they are poor conductorsbases (cements) as they are poor conductors

• Denture bases can be made up of metals or resins. Metallic is Denture bases can be made up of metals or resins. Metallic is better as it conducts heat and maintains tissue health.better as it conducts heat and maintains tissue health.

YAWAR 15

v.v. THERMAL DIFFUSIVITYTHERMAL DIFFUSIVITY• It is a measure of transient heat flowIt is a measure of transient heat flow• Thermal diffusivity =Thermal diffusivity = Thermal ConductivityThermal Conductivity

Density x specific heatDensity x specific heat• ΔΔ = = KK

ρρ x C x CPP

• Units are mmUnits are mm22/S/S• It is related to the thickness of material, e.g., liners should It is related to the thickness of material, e.g., liners should

have adequate thickness when used below restorationshave adequate thickness when used below restorations• In case of metallic restorations, high thermal diffusivity In case of metallic restorations, high thermal diffusivity

than the tooth structure results in thermal sensitivity, e.g., than the tooth structure results in thermal sensitivity, e.g., amalgam fillings, gold inlays and crownsamalgam fillings, gold inlays and crowns

YAWAR 16

vi.vi. COEFFICIENT OF THERMAL EXPANSIONCOEFFICIENT OF THERMAL EXPANSION

• When a materials is heated the extra energy it When a materials is heated the extra energy it absorbs causes the atoms and molecules to vibrate absorbs causes the atoms and molecules to vibrate with an increased amplitude. As a result the material with an increased amplitude. As a result the material expands.expands.

• Most common way of measuring this expansion is by Most common way of measuring this expansion is by taking the length of the material, heating it to a taking the length of the material, heating it to a certain temperature and then measure resultant certain temperature and then measure resultant change in lengthchange in length

• Most often linear thermal expansion is expressed as Most often linear thermal expansion is expressed as coefficient of thermal expansion (coefficient of thermal expansion (άά).).

• Defined as change in length per unit length of the material for Defined as change in length per unit length of the material for

1 ºC change in temperature. Units are ppm/ ºC.1 ºC change in temperature. Units are ppm/ ºC.

• For restorative materials it should be similar or close to the For restorative materials it should be similar or close to the

tooth structure, otherwise thermal mismatch can give rise to tooth structure, otherwise thermal mismatch can give rise to

marginal gap formation, breakdown of adhesive bonds and marginal gap formation, breakdown of adhesive bonds and

finally failure of restoration.finally failure of restoration.

YAWAR 17

C.C. ELECTRICAL ELECTRICAL PROPERTIESPROPERTIES

i. Resistivity & conductivityii. Dielectric Constantiii. Galvanismiv. Electrochemical Corrosion

YAWAR 18

i.i. Resistivity & Resistivity & ConductivityConductivity• The ability to conduct electric current may be stated

either as “specific conductance” or “conductivity” and conversely as the “specific Resistance” or “resistivity”.

• The sensitivity of the tooth structure depends upon the electrical resistance of the tooth structure. Carious teeth shows less resistance than normal.

• Zinc Oxide Eugenol has the highest electrical resistivity.

• Conductivity is important in case of restorative materials with GIC being the most conductive.

YAWAR 19

ii.ii. Dielectric ConstantDielectric Constant

• A material that provides electrical insulation is known as dielectric constant.

• The value of dielectric of a dental cement decreases as it hardens.

• Electrical insulation by cement bases is especially important in case of galvanism.

YAWAR 20

iii.iii. GalvanismGalvanism• The presence of metallic restoration in mouth may cause a phenomenon called

as Galvanic action or galvanism.• It is the generation of electrical current that the patient can feel• It results from difference in potential between dissimilar materials in opposing

or adjacent teeth• Examples:

• Metals placed in an electrolyte (liquid containing ions) have various tendencies to go into solution. Aluminium the alloys of which are used as temporary crowns has great tendency to go into solution and has electrode potential of +1.33 volts.

• Gold on the other hand has little tendency to go into the solution as indicated by the electrode potential of -1.36 volts.

YAWAR 21

iii.iii. GalvanismGalvanism• In this situation, two opposing teeth where oral

fluids acts as electrolytes. The system is similar to that of an electrical cell. When the two restorations touch the current flows because the potential difference is 2.69 volts and the patient feels pain, discomfort and metallic taste.

• The same effect is experienced if an aluminium foil from a baked potato becomes wedged between two teeth and contacts a gold restoration. Temporary plastic crowns are used to prevent this problem since they are poor electrical conductors.

YAWAR 22

iv.iv. Electro Chemical Electro Chemical CorrosionCorrosion

• Corrosion is dissolution of metals in the mouth.• As a result of galvanic action material goes into

solution and roughness & pitting occurs.• May also be observed when a gold alloy is

contaminated with iron during handling in the dental laboratory

YAWAR 23

D.D. RHEOLOGICAL RHEOLOGICAL PROPERTIESPROPERTIES

• They are important as they have a major influence on the handling characteristics of the material

i. Viscosityii. Viscoelasticityiii. Time - dependent properties

a) Creep and flowb)Stress relaxation

YAWAR 24

i.i. ViscosityViscosity• The viscosity of a fluid is its resistance to flow and is

equal to the shear stress divided by the strain rate.• η = shear stress/ strain• Units are Pascal seconds (Pas)• The viscosity of a fluid depends upon number of factors

• Nature of substance• Temperature• Pressure• Shear rate

• Two types of flow behaviour: Newtonian & Non-Newtonian

YAWAR 25

i.i. ViscosityViscosity• Newtonian Flow Behaviour

• Fluids which depends upon shear stress• Only 10% of liquids are Newtonian

• Non - Newtonian Flow Behaviour• When it does not follow the Newton’s law• It has the following types

• Bingham flow e.g. composite filling materials• Plastic Flow• Pseudoplastic or dilatent e.g.

• Polymers• Natural resins• Blood• Impression materials• Fluorides gels

• Thixotropy when some fluids are sheared at a steady rate viscosity decreases with time, on standing the fluid regain its original viscosity. It occurs due to structural breakdown and reformation. It is exhibited by many dental materials extremely useful in impression materials

YAWAR 26

i.i. ViscosityViscosity• Dental applications

• Materials should have good viscosity and flow to be easily manipulated• Material has poor rheological properties when there is no well defined

working time• Material has ideal rheological properties when it has well defined

working time followed by sudden and complete setting• Measurement of Viscosity

• Capillary viscometers• Falling body viscometers• Rotational viscometers• Parallel plate plastometers

YAWAR 27

ii.ii. ViscoelasticityViscoelasticity• Materials showing combination of viscous & elastic properties are called as

viscoelastic• It can be explained by models

• Spring• Dashpot• Maxwell (combination)

• Dental applications• Polymers like soft relining materials & denture base polymers exhibit

viscoelastic behaviour• Elastomeric impression materials agar-agar; alginate• Creep of dental amalgam

YAWAR 28

iii.iii. Time Dependent PropertiesTime Dependent Properties• Creep and Flow

• Creep is a time dependent plastic deformation. It is of two types- static and dynamic, e.g., dental amalgam

• Flow similar term to creep but deformation which occurs at a smaller stress, e.g., amorphous materials like waxes and resins

• Stress Relaxation • It is a slow change in shape with a decrease in force overtime• Examples:

• When orthodontic arch wire is bent, it can alter tooth position due to stress relaxation as it regain its original contour

• Orthodontic elastics have to be frequently changed because loss of force occurs due to stress relaxation

• Waxes like inlay wax or impression wax when left for a long time undergo distortion due to stress relaxation

YAWAR 29

E.E. MISCELLANEOUS PROPERTIESMISCELLANEOUS PROPERTIESi. Dimensional changes

ii. Density

i. Dimensional changes :• Dimensional change is the percentage of shrinkage or expansion of a

material

• Maintaining dimensional changes during dental procedures such as taking impressions and pouring models is highly important in accuracy of dental restorations

• Dimensional changes may occur during setting as a result of chemical reaction, as with rubber impression materials or composites and from cooling of wax patterns, gold restorations during fabrication

• Ideally there should be no dimensional changes on setting

YAWAR 30

E.E. MISCELLANEOUS PROPERTIESMISCELLANEOUS PROPERTIESii. Density:

• Fundamental property which affects design aspect of dental appliances

• it is necessary to consider density for choosing an alloy with which to construct components of an upper denture

• A bulky design in a heavy alloy would result in large displacing forces making retention difficult

• In order to reduce such destabilizing forces one could choose to use a lower density alloy and to keep the alloy bulk to a minimum.

YAWAR 31

To be continued ….