PHYSICAL PROPERTIES OF DENTAL MATERIALS.

Presented by: - Under Kind Guidance of:-

Dr. Prasad Rane Dr. W.N.Ghonmode

Dr. M.G. Chandak

CONTENTS

1) INTRODUCTION.

2) RHEOLOGICAL PROPERTIES.I. VISCOSITY

II. VISCOELASICITYIII. CREEP

3) THERMAL PROPERTIES.I. THERMAL CONDUCTIVITY

II. THERMAL DIFFUSIVITY.III. EXOTHERMIC REACTIONS.IV. COEFFICIENT OF THERMAL EXPANSION.

4) MECHANICAL PROPERTIES.I. STRESS STRAIN RELATION

II. MODULUS OF ELASTICITY.III. RESILIENCY.IV. TOUGHNESS. V. FATIGUE PROPERTY.

VI. FLEXIBILITYVII. ABRASION RESISTANCE.

VIII. HARDNESSIX. DUCTILITYX. MALLEABILTY.

5) ELECTRICAL PROPERTIES.I. GALVANISM.

6) CHEMICAL PROPERTIES.I. TARNISH

II. CORROSION.

7) OPTICAL PROPERTIES.I. COLOR

II. TRANSLUCENCYIII. SURFACE TEXTURE.

8) ADHESION.

9) CONCLUSION.

10) REFERENCES.

INTRODUCTION:-

Many factors must be taken in to account when considering the properties which are relevant to the successful performance of material used in dentistry. The situation in which the material is to be used and the recommended technique for its manipulation define the properties which characterize the material.

Many tests are used to evaluate dental materials and their properties.

Material properties:-A. During storage:-

i. Shelf lifeB. During mixing:-

i. Method of dispensing ii. Mixing time.

iii. Viscosity C. During setting:-

i. Rate of set.ii. Working time.

iii. Setting time.iv. Dimensional changes.v. Temperature rise on setting.

D. Set material:-i. Physical properties

ii. Mechanical properties.iii. Thermal properties.iv. Chemical properties.v. Biological properties.

The acceptance of any product by the dentist depends on the properties of the unmixed material, properties during mixing and setting of material and properties during set of the material.

Physical properties are based on laws of mechanics ,optic, thermodynamics ,electricity, radiation, atomic structure or nuclear phenomena.

PROPERTIES BASED ON LAWS OF FLOW CHS:-( RHEOLOGICAL PROPERTIES).

RHEOLOGY:- Study of matter of flow {of characteristics}.

VISCOSITY:- Most liquids when placed in motions resist imposed forces

that cause them to move . this resistance to motion is called viscosity.Controlled by internal frictional forces in liquids.

Viscosity is a measure of consistency of a fluid and its inability to flow. Viscosity is measured in units of MPa per second.

n = shear stress( shear rate (E)

shear stress = k(shear rate) n . { k = flow index}

Plate moving under influence of shearing forces.Shear stress(T) = F/AShear strain rate (rate of change of deformation)

E=V/d

For each new value of F we gwt new value of V.

In ideal fluids shear stress is proportional to the strain rate . thus plot is straight line.[ Newtonian behaviour]

PSEUDOPLASTIC PROPERTIES:-viscosity decreases with increasing shear rate untill it reaches a

constant value.

DILATANT BEHAVIOUR:-Viscosity increases as the rate of deformation is increased and

liquids become more rigid.

PLASTIC BEHAVIOUR:-Materials behave as rigid body until some minimum value of

shear stress is developed.

THIXOTROPIC BEHAVIOUR:-Property of liquid to become more fluid and more viscous ,

decreased pressure. e.g. Dental prophylaxis paste . plasters ,resin cements, impression materials.

As flow of liquids is measured by viscosity for solids it can be done by creep and viscoelasticity.

CREEP:-It is time dependantplastic deformation of a material under a

static load or constant stress.

Sagging:- If a metal is held at a temperature near it smelting point and is subjected to a constant applied stress results in strain.

Creep:- Occurs in dental amalgam .[componenta with melting points slightly above room temperature.]

Putty consistency:- fractures on rapid stretching.flatens out under its own weight. Creep measured as percentage of shortening in length (compression employed in testing dental materials).

VISCOELASTICITY:-Property of being viscous and elastic .viscosity of liquids and

elasticity of solids . simple and effective way is by models based on springs and a dashpot.

ELASTOMERIC IMPRESSION MATERIALS.

ELASTOMERIC IMPRESSION MATERIALS:-To avoid excessive permanent deformation , materials should

not be loaded for any longer than necessary [removed from mouth with short sharp pull].

[rapid loading and unloading →more elastic response]

THERMAL PROPERTIES:- Wide temperature fluctuations in oral cavity due to ingestion

of hot or cold food and drink Exothermic setting reactions of materials. Dimensional changes associated with temperature difference

in restorative materials.

I. THERMAL CONDUCTIVITY:- “The rate of heat flow per unit temperature gradient”.

Materials conducting heat are called “conductors”. Materials with low heat conductivity are called “insulators”. Thermal conductivity is a equilibrium property.

MATERIALS THERMAL CONDUCTIVITY

Enamel 0.92Dentin 0.63

Acrylic resin 0.21Dental amalgam 23.02

Zinc phosphate cement 1.17Zinc oxide eugenol cement 0.46

Silicate 0.75Porcelain 1.05

II. THERMAL DIFFUSIBILITY:- “It is a rate at which a body with a nonuniform

temperature reaches a state of thermal equilibrium”.T.D = Thermal conductivity (k) . Heat capacity(Cp) X Density(p)

Better indication of the way in which a material responds to transient thermal stimuli.

Diffusibility allows calculations of the temperature change in pulp.

Naturally this should be as small as possible.

Measure by thermocouple in a specimen material.

Low value of diffusibility is indicated in many cases.

High value of diffusibility is preferred(indicated) in denture base materials. (patient retains satisfactory response to hot and cold).

III. EXOTHERMIC REACTIONS:- { need for pulp protection}.

MATERIALS TEMPERATURE RISE (C)

Zinc oxide eugenol cement 0.2Zinc phosphate cement 1.9

Acrylic resin 9.6Composite resin 4.0

GIC 1.0

Considerations of the temperatre against time curve which results from experiments produces a convenient method for measuring working time and setting time.

IV. COEFFICIENT OF THERMAL EXPANSION:-

Fractional increase in length of the body for each degree centigrade increase in temperature.

ά = ∆ L / L ∆T

Property important for filling materials.

MATERIALS COEFFICIENT OF THERMAL

EXPANSION (ppm C)

Enamel 11.8Dentin 8.0

Acrylic resin 90Porcelain 4Amalgam 25

Composite resin 25-60Silicate cements 10

Value of coefficient of thermal expansion should be no close to that of tooth structure.

`MECHANICAL PROPERTIES:-

The physical science that deals with enery and forces and their effects on bodies. Mechanical properties are measured , responses both elastic (reversible on force removal) and plastic (irreversible on force removal) of materials under an applied force or distribution of forces.

1) Elastic(reversible) deformation i.e proportional limit, resilience, modulus of elasticity.

2) Plastic (irreversible) deformation i.e percentage elongation .3) Combination of elastic and plastic deformation i.e

toughness or yield strength.

STRESS:- Force per unit area.MPa unit preferred as it is consistent with SI system or units.

STRAIN:- Change in lenht per unit original length.

TYPES OF STRESS1) TENSILE STRESS :- Stress is caused by a load that tends

to strech or elongate a body.2) COMPRESSIVE STRESS :- Body is placed under load

that tends to compress or shorten it. Internal resistance to such a load is calle compressive stress.

3) SHEAR STRESS :- Tends to resist the sliding of one portion of a body over another. E.g Debonding of orthodontic brackets . shear stress is calculated by dividing the force by the area parallel to the force direction.

Presence of chambers , bevels or changes in the curvature of bonded tooth surface would make shear failure of a bonded material highly unlikely.

4) FLEXURAL STRESS(BENDING STRESS):-

RELATIONSHIP BETWEEN STRESS AND STRAIN:-Stress and strain are closely related as ‘cause’ and ‘effect’. This

relationship is used to characterize the mechanical properties of materials.

PROPORTIONAL LIMIT (P):- The value of stress which corresponds to the limit or proportionality is referred as proportional limit.

YIELD STRESS (δ ):- it is the stress at which plastic deformation begins . in day to day practice it is very difficult to detect yield strss as there is gradual transition of slope rather than a rapid change . in 3unit bridges , if tensile stress exceeds the yield stress , the restoration will deform permenently.

PROFF STRESS:- it is defined as stresss required to produce certain amount of plastic strain(0.2%) . It is used as a measure of the onset of yielding of the material .

ELASTIC LIMIT(E):- Stress beyond which strain are not fully recovered . Proportional limit and elastic limits are often given in approximations.

High proportional limits are required by connectors or partial dentures .

MODULUS OF ELASTICITY (YOUNGS MODULUS / ELASTIC MODULUS ):- It is the constant that relates stress strain in linear elastic region and is a measure of stiffness of a material .

it is often very large for real materials so it is expressed in values of Gigapascals (GPa).(1GPa = 10 N/m).

Slope of straight line of stress strain graph gives measure of modulus of elasticity.

It is a unit of stress.

Modulus of elasticity = Stress Strain

Choice of nomenclature for this property is somewhat unfortunate, since it in fact gives an indication of the rigidity of a material and not elasticity.

Steep slope is seen in rigid materials (restorative materials)Shallow slope is seen flexible materials (impression

materials).Value of strain recorded between point E to T indicates

permanent deformation which can be imported to a material upto point of fracture.

For tensile tests it gives ductility.For compressive tests it gives malleability.

RESILIENCY:- Energy absorbed by material in undergoing elastic deformation upto the elastic limit.(no permanent deformation).

High value is one of the parameters for elastomers.

TOUGHNESS:- Total amount of energy which a material can absorb upto the point of fracture.

Material capable of absorbing large amount of energy are called tough materials .

Opposite to toughness is brittleness .

FATIGUE PROPERTY:- Stress values well below the ultimate tensile strength can produce premature fracture of dental prosthesis because microscopic flaws grow slowly over many cycles of stress.

FLEXIBILITY:- Strain that occurs when the material is stressed within proportional limt.

ABRASION RESISTANCE:- Abrasion caused by tooth brushing . intermittent contact with antagonist tooth with restoration in centric and eccentric movements leads to abrasion . it can be reduced if modulus is high.Goldilocks principle:-

HARDNESS:- “Ability of the substance to resist the indentation.”

Strength and proportional limits related.Brinell and Rockwell tests – macrohardness , strength used is

more than 9.8 N/m.Knoop and Vickers tests – microhardness , strength used is

less than 9.8 N/m.

Hard materials are more difficult to polish by mechanical means.

DUCTILITY AND MALLEABILITY:- DUCTILITY:- ability of a material to sustain a large

permanent deformation under a tensile load without fracture.Ductility is measured by:-

1. Percentage elongation after fracture.2. Reduction in area of fracture d segment.3. Cold bend test.

MALLEABILITY:- Ability of a material to sustain considerable permanent deformation without rupture under compression as in hammering or rolling into a sheet.

ELECTRICAL PROPERTIES:-

GALVANISM:- It is the generation of electric current that the patient can feel .

Results from dissimilar metals in mouth.Metals go into dissolution when placed in electrolyte[ saliva

acts as electrolyte]. Patient feels pain and complains pf metallic taste.Temporary plastic crowns are used to prevent this problem as

they are poor conductors .

CHEMICAL PROPERTIES:-