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By
Head of Department
Dr. Rashid Hassan
Assistant Professor
Science of Dental Materials Department
RAWAL INSTITUTE OF HEALTH SCIENCES
RAWAL COLLEGE OF DENTISTRY
ISLAMABAD

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Slide 2

 Atoms

are bonded to each other by either
primary or secondary bondS By attraction of
positive and negative charges of ions.
 Thus they form a regularly spaced
configuration , known a s space lattice or
crystal.
 There are 14 possible lattice shapes and
forms.

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 Triclinic

simple.
 Monoclinic simple.
 Monoclinic body
centered.
 Orthorhombic
simple.
 Orthorhombic base
centered.
 Orthorhombic body
centered.

 Orthorhombic

face

centered.
 Hexagonal.
 Rhombohedral.
 Tetragonal simple.
 Tetragonal body
centered.
 Cubic simple.

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SIMPLE
CUBIC

BODY
CENTERED
CUBIC


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FACE CENTERED
CUBIC

HEXAGONAL
CLOSE PACKED

ORTHORHOMBIC
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Slide 6

Structures

other than crystalline
forms can also occur in solid
state.
Molecules are distributed in a
random pattern.
Dental waxes are an example of
amorphous structures.
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Slide 7









The increase in energy per
unit area is called as
surface energy or surface
tension.
Interfacial tension that
exists between the two
surfaces due to unbalanced
intermolecular forces.
For adhesion to exist the
surfaces must be attracted
to one another at their
interface.
The energy at the surface is
greater than that of its
interior.
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Slide 8







At the surface the energy is
greater because the
outermost atoms are not
equally attracted in all
directions.
The surface atoms tend to
form bond to other atoms
in close proximity in order
to reduce the surface
energy.
When primary bonding is
involved, the adhesion is
called CHEMISORPTION

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 Interfacial

tension between a liquid and a
solid resulting in a contact angle of less than
90°.
 Adhesion is negligible when the surface
molecules of the two materials are separated
by a distance greater than 0.7nm.
 In case of true wetting of the surface,
adhesion failure should not occur.

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 Cleanliness

of the surface is very important to
better adhesion.
 Surface energy is very crucial.
 Surface energy of waxes is very low that few if
any liquid can wet their surface.
 Sometimes a synthetic resin is used where
adhesion is not desired.

e.g. Teflon (Polytetraflouroethylene)

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Slide 11

 The

extent to which an adhesive can wet the
surface of adherend can be determined by
measuring the contact angle between the
adhesive and the adherend.
 Contact angle is the angle formed at the
interface of the adhesive and the adherend.
 If the molecules of adhesive are attracted
more to the molecules of the surface, the
adhesive will spread completely over the
surface of the solid.
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Slide 12

 Complete

wetting occurs at 0° and no wetting
occurs at 180°.
 Entrapment of air bubbles during adhesive
application leads to voids and pores.
 When a thermal energy is applied to such
adhesive, stress concentrations develop.
 These voids may lead to crack propagation.

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 Tooth

is a composite material.
 The composition of both enamel and dentin
varies.
 The materials that adhere to the organic
component of the tooth may not adhere to
the inorganic component.
 Surface contamination (smear layer) reduces
the adhesion in cavity preparation.
 Inorganic component has strong affinity for
water.
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Slide 17

QUESTIONS???

Questions ???
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