Physical properties of dental materials /certified fixed orthodontic courses by Indian dental academy

INDIAN DENTAL ACADEMY
Leader in continuing dental education

www.indiandentalacademy.com

 Dentistry is mainly a material science branch
 To fabricate any prosthesis we MUST know
advantages and limitations of those material
 Proper selection of material
 So to know the things what we use in routine
practice is essential


 what are physical properties?
Abrasion and abrasion resistance.
Viscosity.
Structural and stress relaxation.
Creep and flow.
Color and color perception.
Tarnish and corrosion.


 Properties based on laws of,
-machanics, acoustics, optics, thermodynamics,
electricity, magnetism, radiation, atomic structure
or nuclear phenomena.

Hue,value,chroma and translucency – based on
laws of OPTICS

thermal conductivity and coefficient Of thermal
expansion – based on laws of THERMODYNAMICS

viscosity – related to MATERIAL SCIENCE AND
MECHANICS

 Hardness: index of ability of a
material to resist wear or abrasion.

In oral cavity, abrasion is a complex
mechanism, with interaction of
numerous factors.

So, hardness can be used to compare
similar materials(e.g:one brand of
metals with other) but invalid for
dissimilar materials(e.g:resins and
metals)

 A reliable invitro test for abrasion resistance should
simulate particular abrasion in vivo

due to complex clinical environment, invitro and in vivo
tests will differ.

For e.g; abrasion of enamel of atooth opposing ceramic
crown is affected by;

1.bite force 2.frequency of chewing
3.abrasiveness of diet 4.composition of liquids
5.temperature changes 6.surface roughness
7.phy.props. Of materials 8.surface irregularities


 Resistance of a liquid to flow.

Success of given dental material depends on its
properties in liquid state as much as it is in solid
state.

Dentist has to manipulate many materials in
liquid state to achieve successful clinical out
comes.

E.g; -cements, impression materials, gypsum

Some amorphous materials such as waxes and
resins appear solid but they are supercooled liquids
that can flow,

- plastically(irreversibly) with sustained loading or
-elastically(reversibly) with small stresses.

Rheology :Is the study of deformation and flow
characterstics of matter.


 Most liquids when placed in motion, resists
imposed forces that cause them to moove
because of internal frictional forces with in the
liquid.

Thus viscosity is a measure of consistency of a
fluid and its inability to flow.

Highly viscous liquids flows slowly compared to
less viscous liquids.

E.g;zinc poly carboxylate cement and resin
cements compared with zn. phosphate cement

 Liquid occupying space between two plates –
lower plate fixed and upper being mooved to
right with a velocity V and a force F is required
to overcome viscosity.


 Stress: force per unit area that develops with
in a structure when external force is applied.

Shear Stress = F/A
A – area of plates in contact with liquid.

Strain: deformation caused by stress.

Strain rate = V/D
D – distance moved by upper plate relative
to lower plate

viscosity = shear stress/shear strain
rate

 Units of measurement: measured in units of
Mpa/sec or centipoise(cp)

 e.g; viscosity of pure water at 20◦C – 1 cp

tempered agar(45◦C) – 281,000cp
elastomeric impression materials

light body(109,000)
putty(1,360,000)

 Used to explain viscous nature of some materials.

Based on rheologic behavior, fluids can be
classified into;
-newtonian,
-pseudoplastic,
-dilatant,
-plastics

Nature of this curve for a given material is
important in determining best way to manipulate
that material.

Similarly viscosity is plotted against time can be
used to determine working time of a material that
undergoes liquid – solid transformation

 Ideal fluid.

Demonstrates - stress α strain.

Plot - straight line

exhibits constant viscosity.

Slope is constant.


 Viscosity decreases with increase in strain
rate till it reaches constant value.

With increase in strain rate, shear stress
rate increases to reach a constant value.


 Opposite to pseudoplastic behavior.

Viscosity increases with increase in shear
strain rate.


 They behave like rigid body until some
minimum value of shear stress is reached.

This minimum value is called OFFSET.

Exhibits rigid behavior initially, and then
attains constant viscosity.
E.g; ketchup in bottle.


 Viscosity of most liquids decreases rapidly
with increase in temperature and it also
depends on previous deformation of liquid.

A liquid that becomes less viscous and
more fluid under repeated application of
pressure is referred to as thixotrophic.

E.g; dental prophylaxis pastes, plaster of
paris, some impression materials.

 Thixotrophic property often confused with
pseudoplasticity.

A thixotrophic material does not flow until
sufficient energy in the form of impact force is
applied to overcome its yield stress.

Beyond this point , the material becomes very
fluid.


 Dental prophylactic pastes - they will not
flow out of a rubber cup until it is rotated
against the teeth to be cleaned.

Impression materials - does not flow out of
an impression tray until placed over dental
tissues which is benificial for mandibular
impression .

Plaster of paris - if stirred rapidly and
viscosity is measured, the value is lower than
the value for a sample that left undisturbed
due to thixotrophic property.

 A stable substance
application of stress
displacement of atoms from equilibrium

trapped internal stresses

permanent deformation
(plastic deformation)

change in shape and contour of solid

www.indiandentalacademy.com warps
material or distorts

 The rate of relaxation increases with increase
in temperature.
E.g;if a wire is bent, it may tend to
straighten out if heated to high temperature.

 At room temperature such relaxation –
negligible.

Many non-crystalline dental materials (such as
waxes,resins,and gels) that when manipulated
and cooled can then undergo
relaxation(distortion) at elevated
temperature.

results in inaccurate
www.indiandentalacademy.com fit of dental appliances

 CREEP - def: time dependent plastic strain of
a material under a static load or constant
stress.

A metal held at a temperature near its melting
point - subjected to constant stress -
increase in strain over time.

Metal creep usually occurs as the temperature
increases to with in a few hundred degrees of
melting range.
E.g;creep of amalgam

 Amalgam creep –

dental amalgam usually contains 42-52% of Hg
and begin melting at a temperature only slightly
above room temperature.

Restored tooth with amalgam

clenching and biting

periodic sustained stress

destruction to dental prosthesis


 Flow :
is generally used in dentistry to describe the
rheology of amorphous materials such as waxes.

Flow of a wax is a measure of its potential to
deform under a small static load ,even that
associated with its own weight.


 Although creep or flow can be measured
under any type of stress, compressive
stress is used in testing dental materials.

Cylinder of particular dimension subjected
to given compressive stress for a specific
time and temperature, % decrease in
length gives creep or flow.


 Light: electromagnetic radiation that can
be detected by human eye.

Eye is sensitive to -λ of 400nm(violet)-
700nm(red).

The reflected light intensity and the
combined intensities of λ’s present in
incident and reflected light determines the
appearance properties i.e hue, value ,and
chroma.

 Cone shaped cells are responsible for color
vision.
 Optic nerve
 Image on retina is focused, then energy in
visual spectrum is converted to electric
potential by rods and cones through chemical
reaction.




 eye is most sensitive to light in green-yellow
region(550nm) and least sensitive at red or blue
regions of color spectrum.

 Color fatigue: decrease in eyes response to
color because of constant stimulation by
single color.

Color blindness: defect in certain portion of
color sensing receptors.

Thus human observers greatly differs in their
ability to distinguish colors.


 Colorimeter : exceptionally sensitive scientific
instrument that measures intensity and wave
length of light.

Advantages : colorimeter is more precise than
human eye in measuring slight differences in
colored objects.

Disadvantages: it is extremely in accurate when
used on rough or curved surfaces .

eye can differentiate colors seen side by side on
smooth or irregular surfaces, whether curved or
flat.


 Verbal description of color - not precise to
describe appearance of teeth.

So three variables must be measured to
accurately describe color of tooth or
restoration. They are,

1. HUE.
2.VALUE.
3.CHROMA.

 Defined as particular variety of a color,
shade or tint.

Describes the dominant color of an object.
E.g: red, green or blue.

Refers to dominant wavelengths present in
spectral distribution.


 Identifies the lightness or darkness of color
which can be measured independently of
hue.

E.g:yellow of a lemon is lighter than is the
red of a cherry.

Teeth can be separated into lighter
shades(high value), and darker
shades(lower value).

 Degree of saturation of a particular hue.

This is difference in color intensity or
concentration of pigment.

E.g:yellow of lemon is more vivid than
that of banana which is dull yellow.

Higher the chroma, more intense the color.

In dentistry ,chroma is always associated
with hue and value of dental tissues.

 Hue changes occur in circumferential direction.

Value varies vertically.

increases towards the top(whiter) and decreases
towards bottom(darker or more black).

Chroma varies radially.

Increases from center outwards.


 used to peform color matching in dental
operatory or laboratory to select color of crowns,
inlays, veneers etc.

Usually neck region is grinded away, because
correct shade is determined from gingival half of
the tab not from the neck.


 Objects that appeared to be color
matched under one type of light may
appear different under other light source.

Spectral distribution of light reflected from
or transmitted through depends on spectral
content of incident light.

Common sources – day light, incandescent
and fluorescent.

 The energy that the tooth absorbs is
converted into alight with longer
wavelengths, in which tooth actually
becomes a light source.

Natural teeth absorbs wavelength too short
to be visible to human eye referred as near
U.V.radiation(300-400nm).

Emitted color is bluish white with wave
length 400-450nm range contributing to
brightness and vital appearance of human
eye.

 Thermal conductivity Is a thermophysical measure
that how well heat is transferred through a
material by conductive flow.

Conduction through metal – crystal lattice
vibrations, motion of electrons ,and their
interaction with atoms.

Measured under steady state conditions (constant
T ).

Thermal conductivity  area(┴to heat flow ) and
temperature gradient across

 Coefficient of thermal conductivity: is the
quantity of heat in calories per second, that
passes through a specimen 1cm thick having a
cross sectional area of 1 cm sq. when the
temperature difference between the surfaces
perpendicular to heat flow is 1 k.

High conductivity - conductors
low conductivity - insulators

units of measurement–watt/meter/sec/kelvin.


 Is a measure of the rate at which a body with a
non uniform temperature reaches a state of
thermal equilibrium.

For e.g: the thermal conductivity of
ZOE <dentin but thermal diffusivity is twice that
of dentin.

Thickness of cement base directly related to
benefit as insulator.


 Applied e.g’s:

- importance of thickness of cement base as
thermal insulator.

- Difference in thermal conductivity of
metallic and resin denture bases.


 Change in length per unit length of a
material when its temperature is raised 1˚k.
(μm/m˚k)
 Coefficient of thermal expansion for ENAMEL
is 1


 Other applications of 

- inlay wax removed from tooth or die in
warmer area and stored in cooler area.

- Denture teeth arranged in wax in warm
area and stored in cooler area causing shift in
positions of teeth
- stresses produced from metal ceramic
restorations when porcelain veneer is fired to
a metal substrate.


 Tarnish: the process by which a metal surface is
dulled or discolored when a reaction with a
sulfide ,oxide, chloride, or other chemical causes
a thin film to form.
(or)
tarnish is observable as a surface discoloration on
a metal , or as a slight loss or alteration of the
surface finish or luster.
‘Tarnish is often a forerunner of corrosion’


 Often occurs from formation of hard and
soft deposits on the surface of restoration.

Soft deposits - plaque, films of
bacteria,mucin, stains from pigment
producing bacteria, drugs containing iron
or mercury, adsorbed food debris

hard deposits - calculus.

Also from formation of thin films such as
oxides, sulfides, chlorides which is an
early indication of corrosion

 Corrosion: chemical or electrochemical
process in which a solid usually a metal,is
attacked by an environmental agent,
resulting in partial or complete dissolution.

(or)

it is a process in which deterioration of a
metal is caused by reaction with its
environment and is not merely a surface
deposit.

 Disintegration of a metal by corrosion may
occur in mouth because of

- warmness and moistness .

- fluctuations in temperature.

- ingested foods with wide range of PH

- acids liberated from localized attachment of

debris.

 A tarnish film may intime accumulate
elements and compounds that chemically
attack metal.

For e.g; egg and certain foods
containing sulfur .

sulfides such as hydrogen, or ammonium
corrode Ag,Cu,Hg and similar metals present
in dental alloys and amalgam.

Specific ions play role in corrosion of certain
alloys .
For e.g; Oxygen and chloride in
corrosion of amalgam

 Broadly classified into,

1. chemical /dry corrosion.
2.electrochemical/wet corrosion
- galvanic corrosion
- stress corrosion
- concentration cell corrosion


 Requires presence of water/fluid
electrolyte and also a pathway for
transport of electrons.

Electrochemical cell:

composed of 3 components
1. anode.(e.g; dental amalgam)
2 cathode.(e.g; gold alloy restoration)
3. electrolyte.(e.g; saliva)

 Anode is a surface or site, on a surface where,
+ve ions are formed (surface undergoing
oxidation and corroding.) with the production of
free electrons.
0 + _

M M + e

at cathode, reduction reaction occurs that
consumes free electrons produced at anode.
+ _ 0 + _
M + e M / 2H + 2e H2 /
_
_ _
2H2O + O2 + 4e 4(OH)


 Electrolyte supplies the ions needed at
cathode ,and carries away the corrosion products
at the anode.

External circuit – path to carry electrons from
anode to cathode.

For corrosion to be an ongoing process, -
production of e at anode must be balanced by
consumption in reduction reaction at cathode.

Cathodic reaction - primary driving force for
electro-chemical corrosion – an important
consideration in determining rate of corrosion.

 Classification of metals by their equilibrium
values, of electrode potential there by
arranging them in the order of their
dissolution tendencies in water.

Potential value –calculated for a standard
state, consisting of one atomic wt. of ions in
100ml of water at 25˚c.

The half cell potentials considered as voltage
of an E-C cell in which one electrode is
hydrogen electrode designated as 0 potential
www.indiandentalacademy.com of choice.
and other is element

 If two metals are immersed in an electrolyte,
connected by an electrical conductor,
- metal with low V - anode
- metal with high V – cathode

for e.g; E-C cell with Cu and Zn electrodes in
aqueous acidic solution Zn becomes anode and
undergoes surface dissolution.

Magnitude of resulting corrosion influenced by
salivary,
- concentration of its components
- pH
- surface tension
- buffering capacity

 corrosion

increase in metal ion
content in environment

saturation of ions in electrolyte

prevent further corrosion

metal ceases corroding
usually, the dissolved ions from dental restorations
removed by food, fluids and brushing, there by
corrosion continues.

 An important type of E-C corrosion occurring
when combinations of dissimilar metals are in
direct physical contact.


 Amalgam restoration on lower tooth opposing
upper tooth with gold inlay

electric circuit(because of saliva) with
potential difference between metals

when teeth brought into contact

short circuit through alloys

sharp
www.indiandentalacademy.com pain

 When teeth are not in contact - still electric
circuit is present due to potential difference
between metals. Saliva forms electrolyte and
hard and soft tissues forms external circuit.

Electric current generated between gold and
amalgam restorations, when they are not in
contact -
0.5 – 1 μA with potential difference -500mV.

Coating with varnish tends to eliminate galvanic
shock.


 Single metallic restoration:
current also associated with single isolated
metallic restoration.

Electric cell is generated as a result of potential
difference created by two electrolytes -
- saliva and tissue fluid.

(tissue fluid : used to denote , dentinal fluid, soft
tissue fluid, blood that provides a means of
external circuit.)

because chloride concentration several times
higher than saliva, - interior surface of restoration
exposed to dentinal fluid will have a more active
electric potential.

 Associated with heterogeneous
composition of dental alloys.

Corrosion resistance of multiphase alloys is
generally less than that of single phase
solid solution.

E.g: when alloy containing a two phase
micro structural constituents immersed in
electrolyte, the lamellae of phase with
www.indiandentalacademy.com attacked and corrosion
lower potential are
results.

 Solder joints - corrode because of difference
in composition of alloy and solder.

Impurities - contaminating metals cause
corrosion


 Imposition of stresses increases internal
energy of an alloy through elastic displacement
of atoms or creation of micro strained fields
associated with dislocation then the tendency
to undergo corrosion increases called stress
corrosion.

likely to occur during fatigue or cyclic loading.

Electro-chemical cell forms with more
deformed regions (anode), less deformed
regions(cathode), and saliva.
E.g:failure of RPD framework due to cyclic
stress

 Occurs whenever there are variations in
the electrolytes or in the composition of
the given electrolyte with in system.

E.g : difference in electrolyte compositon
contacting restoration on occlusal and
proximal surfaces.

Similar type of corrosion occurs due to
difference in oxygen concentration
between parts of same restoration.
E.g : pits in restorations.

 Deepest portion of pit - low o2 concentration
because of debris - anode.

Alloy surface around rim of the pit -
cathode.

Crevice corrosion:
corrosion at the junction of tooth and
restoration because of presence of food
debris causing changes in o2 concentration
and change in electrolyte.


 Metallic and non metallic coatings over
gold alloy restorations are ineffective
because,
-were too thin,
-were incomplete,
-did not adhere to metal,
-were readily scratched,
-were attacked by oral fluids.

When dissimilar metals in contact, -
painting a non conductive film.

 Certain metals develop a thin ,adherent, highly
protective film by reaction with environment
called passivation.
E.g : passivation of iron with chromium and
passivation by titanium due to titanium oxide
formation
Chromium passivated metals – susceptible to
stress corrosion and pitting corrosion and
certain ions such as chloride will disrupt
protective layer.


 Corrosion resistance is very important
consideration dental alloys because release
corrosion products affect biocompatibility.
A guideline that has been employed by
manufacturers for many dental alloys is atleast
50% atoms should be noble metals.
Palladium – prevents sulfur tarnishing of silver
alloys.
Base metal alloys susceptible to tarnish with
chloride.

 Many base materials below restorations lose
the property of insulation when they becomes
wet through microleakage or dentinal fluid.

Practical method of eliminating galvanic
currents - application of varnish.

It has been suggested that Galvanic currents
may account for many types of dyscrasias
such as lichenoid reactions, ulcers,
leukoplakia, cancer, and kidney disorders but
research has failed to find correlation.

 With the latest advance in material aspect,
there is emergence of many materials in
dentistry.
 the complete understanding of the various
properties of the materials can make out the
suitable material of choice.
 And this will definitely lead to better quality
of treatment.


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Physical properties of dental materials /certified fixed orthodontic courses by Indian dental academy