Contents:
1. Introduction.
2. Matter
3. Interatomic bonding
a). Primary bonding
Ionic
Covalent
Metallic
b). Secondary bonding
Hydrogen bonds
Vanderwaals
forces
4. Interatomic bond distance &
Bond energy.
5. Crystalline structure – Lattice
types.
6. Diffusion.
7. Adhesion
& bonding.
8. Factors influencing adhesion.
a). Wetting.
b). Surface energy.
c). Contact angle.
9. Solubility and sorption.
10. Adhesion to tooth structure.
11. References.
INTRODUCTION:
An
object can occupy one of the three different states of matter, such as solid,
liquid, gas.
In dentistry we make use of all of
them although dental materials exist primarily as solids or liquids.
The state of a material is a function
of temperature. The more energy that is put into a material by increasing its
temperature, the more difficult it is to keep the atoms (or) molecules in close
proximity to one another. Thus, the atoms (or) molecules tend to move apart and
expand as heat is applied. Therefore increasing the energy within a given material
through the application of heat can have a destabilizing effect on both its
structure and dimensions. The structure of a material can be described on both
a microscopic and macroscopic level.
On the microscopic level, we
experience the material through the arrangement of its atoms and their bonding
schemes. On the macroscopic level we see a material as a solid liquid or gas.
The
principle goal of dentistry is to maintain or improve the oral health of the
patient. A wide variety of dental materials are involved in the clinical
application. Material should be carefully selected. Through understanding and
experimentation it is possible to maximize any one property, but in no
application is it possible to select a material for one property above. It is
precisely in the balance of one factor against another that the materials are
used successfully. Hence it is essential to know, the properties of the dental
materials, to be able to understand the properties and reactions of the
material and predict the outcome.
MATTER:
It is the substance from which all
physical things are made. Matter is any thing that occupies space and has
weight. Every thing that exists in the universe (seen or unseen) is matter and
it is composed of millions of tiny particles called molecules.
Molecules are composed of still
smaller units called atoms and atoms themselves are made up of even tinier
particles called protons, neutrons and electrons.
Properties of
matter:
1) All matter
has mass and weight. Mass is the measure of the quantity (or) amount of matter,
and this remains same always. Weight of an object is the force of attraction
exerted on the object by gravitation, and this varies at different places.
For Ex: An
object weighing one pound on earth would weigh only a few ounces on the moon,
which has a weaker gravitational pull, but its mass would not change as in the
case of space travelers who
float in the space.
2) All matter
has inertia : That means, matter has a tendency to remain at rest if at rest, or
to continue moving in a straight path with constant speed if in motion. Inertia
of matter depends on its mass-greater the mass of an object, greater is its
inertia.
3) Density: All
matter has density.
This is the mass per unit volume of
a substance.
4) Matter can
conduct heat and electricity.
5) There are
three states of matter.
Solid, liquid and gas.
Molecule:
It is
the smallest particle of a substance, which can exist on its own, and retains
the properties of that substance.
Molecular
weight: is the weight of a molecule relative to
that of an atom of carbon 12 taken as 12. This is expressed in gram molecule.
Atom: Is the smallest particle of any element that shows the chemical
behaviour of that element. Atom cannot be divided as it is the smallest. There
are many kinds of atoms as there are elements. An element is made up of only
one kind of atoms.
Element: Is a basic substance that cannot be separated into different
substances.
Atom is the basic unit from which
molecules and aggregates of molecules, which represent particular matter are
built. Atom is made up of a 3 types of fundamental particles like protons,
neutrons and electrons. Number of protons and number of neutrons, gives the
mass to the nucleus and also the atomic weight.
INTERATOMIC
BONDING:
Atoms do not exist singly, instead
are joined with other atoms of the same kind, to form molecules. The number of
atoms joining together may be two or in thousands. When such a thing happens
there exists a real thing or substance. The mechanism of atoms coming together
is through forces of attraction and the atoms going away from each other is
through forces of repulsion.
Forces of attraction that make the
atoms to come together are called interatomic bonds.
These bonds are
classified as
1) Primary bonds
– Chemical in nature, permanent and strong
a)
Ionic bond
b)
Covalent bond
c)
Metallic bond
2) Secondary
bonds – Physical in nature, weak bonds also called Vander-walls forces.
PRIMARY BOND:
Ionic bond: Normally atom is electrically neutral because of balance of
positively charged proton and negatively charged electron. An atom can become
ion if it loses or gains one of its outermost circulating electron. This can be
a positive ion or negative ion. Atom can be a positive ion or a negative ion.
Such differently charged two atoms will be attracted to each other because of
differing polarity, and a bond takes place between atoms.
Ex: Such a bond
is seen in sodium chloride molecule.
When such 2 atoms come together
sodium atom loses its one outermost electron and becomes positive ion. Chlorine
atom takes one more electron from sodium and becomes negative ion. Thus +ve Na
and –ve Cl attract each other and stay together to form an NaCl molecule.
Ex: Such bonding
takes place with glass ionomer or polycarboxylate cement and tooth enamel. The
bond in this case is between negatively charged atoms in the cement and
positively charged atoms in the tooth enamel.
This is called as true form of
chemical adhesion.
Covalent bond:
This kind of bond, takes place by
sharing of electron between 2 atoms.
Ex: Such a bond is seen when 2 hydrogen atoms come together each
hydrogen atom has only one electron surrounding it when 2 hydrogen atoms come
together one electron of each atom is shared by both atoms.
Similarly
chlorine atom will join with another chlorine atom and share the outer
electrons by covalent bond to form Cl2 covalent bond is very strong
and stable.
This kind of bond is seen with
carbon-carbon bond that takes place in denture base resin (acrylic) or dental
composite resin. Also seen in silicon-oxygen bond of dental ceramics.
Metallic
bond:
This
is seen in metallic elements. The atoms of the metal are arranged in orderly
rows. The atoms lose their outermost valence electrons to form metal ions with
a net positive charge called cations .
The
freed valence electrons roam about together like a gaseous cloud in the
interstices formed by the arrangement of solid spheres. This electron cloud
acts like a glue that holds together different atoms. This is called metallic
bond, which is responsible for the strength, conductivity, of heat and
electricity of metals. This bond is strong and stable.
INTER ATOMIC SECONDARY BONDS
In contrast to primary bonds secondary bonds do not share electrons. Instead, charge variations among molecules or atomic groups include polar forces that attract the molecules.
Hydrogen bonding
This bond can be understood by studying a water molecule. Attached to the oxygen atom are two hydrogen atoms. These bonds are covalent because the oxygen and hydrogen atoms share electrons.
As a result the protons of the hydrogen atoms pointing away from the oxygen atoms are not shielded effectively by the electrons. Thus the proton side of the water molecule becomes positively charged. On the opposite side of the water molecule, the electrons that fill the outer orbit of the oxygen provide a negative charge. Thus a permanent dipole exists that represents an asymmetric molecule. H2 bond, associated with the positive charge of hydrogen caused by polarization is an important example of this type of secondary bonding.
When a H2O molecule intermingles with other water molecules, the hydrogen (+ve) portion of one molecule is attracted to the oxygen portion of its neighboring molecule, and the hydrogen bridge is are formed.
VAN DER WAALS FORCES
It
is a more a physical than chemical bond. These forces form the bases of a
dipole attraction. Eg : in an inert gas, the electron field is constantly
fluctuating. Normally the electrons of the atoms are distributed equally round
the nucleus and produce an electrostatic field around the atom. However this
field may fluctuate so that its charge becomes momentarily positive and
negative. A fluctuating dipole is thus created that will attract other similar
dipoles. Such interatomic forces are quiet weak.
Inter atomic bond distance and bonding energy:
Regardless of the type of matter, there is a limiting factor that
prevents the atoms or molecules from approaching each other too closely, that
is the distances between the center of an atom and that of its neighbor is
limited to the diameter of the atoms involved.
If
the atoms approach too closely, they are repelled from each other by their
electron charges. On the other hand, forces of attraction tend to draw the
atoms together. The position at which these forces of repulsion and attraction
become equal in magnitude is the normal or equilibrium position of the atoms.
Thermal energy
Thermal energy is accounted for by the kinetic energy of the atoms
or molecules at a given temperature. The atoms in a crystal at temperatures
above absolute zero temperature are in a constant state of vibration and the
average amplitude will be dependent on the temperature, the higher the
temperature the greater the amplitude, and consequently, the greater the
kinetic or internal energy. The overall effect represents the phenomenon known
as thermal expansion.
If
the temperature continues to increase the interatomic spacing will increase,
and eventually a change of state will occur.
The
thermal conductivity depends mainly on the number of free electrons in the
material.
As
metallic structures contain many free electrons and most metals are good
conductors of heat as well as electricity, whereas non-metallic materials do
not include many free electrons and consequently they are generally poor
thermal and electrical conductors.
CRYSTALLINE STRUCTURE:
Dental
materials consist of many millions of atoms or molecules. They are arranged in
a particular configuration.
In
1665 Robert Hooke simulated the characteristic shapes of crystals by stacking
musket balls in piles.
The
atoms are bonded by either primary or secondary forces. In solid state they
combine in the manner that will ensure a minimal internal energy.
For
eg. Sodium and chlorine share one electron as described previously. In the
solid state, however they do note simply pair together but rather all of the
positively charged sodium ions attract all of the negative chlorine ions, with
the result that they form a regularly spaced configuration known as space
lattice or crystal, here every atom is spaced equally from every other atom.
There
are 14 possible lattice types, but many of the metals used in dentistry belong
to the cubic system.
Non
crystalline structure eg. Glass and waxes structures other than the crystalline
form that occur in the solid state eg. Glass and waxes.
Waxes
– solidify as amorphous materials meaning that the molecules are distributed at
random. Though there may be a tendency for the arrangement to be regular.
Glass
is considered to be a noncrystalline solid, yet its atoms tend to forma short –
range order lattice instead of the long-range order lattice characteristic of
crystalline solids. In other words, the ordered arrangement of the glass is
more or less localized with a considerable number of disordered units between
them.
Such
an arrangement is also typical of liquids such solids are sometimes called
supercooled liquids.
Non
crystalline solids do not have a definite melting temperature but rather they
gradually softer as the temperature is raised and gradually hardens as they
cool. The temperature at which there is an abrupt decrease in the thermal
expansion cuff, is called the glass transition temperature or glass
temperature.
Below
Tg a glass loses its fluid characteristics and has significant resistance to
deformation.
Eg : synthetic dental resins.
DIFFUSION
Diffusion of molecules in gases and liquids is not known. However
molecules and atoms diffuse in the solid state as well.
At
any temperature above absolute zero, the atoms of a solid possess some amount
of kinetic energy as previously discussed. However the fact is that all the
atoms do not possess the same amount of energy, these energies vary from very
small to quiet large. With the average energy related to the absolute
temperature. Even at very low temperatures some atoms will have large energies.
If the energy of a particular atom exceeds the bonding energy, it can, move to
another position is the lattice.
Atoms
change position in pure solids, even under equilibrium conditions, this is
known as self diffusion.
Increase
temperature greater the rate of diffusion .The diffusion rate will however vary
with the atom size, interatomic or intermolecular bonding, lattice.
ADHESION AND BONDING
Adhesion
is a phenomenon involved in many situations in dentistry.
Eg.
Leakage adjacent to dental restorative material is affected by the adhesion
process. The retension of artificial dentures is probably dependent, to some
extent on the adhesion between denture and saliva and between
Three states of matter:
Three states
of matter are solids, liquid and gas.
Solid:
Solid
has definite shape and volume. Ex: Block of wood, stone work is necessary to
change the shape of solid. They have definite atomic arrangement and are
resistant to deformation. The molecules of the solid are moving about in a
restricted area, atoms of solid have high attraction force between them and low kinetic energy.
Crystalline solid:
Crystalline
solid is one in which molecules are arranged in a definite geometric pattern
Ex: metals.
Amorphous solid:
Amorphous
solid is one in which molecular arrangement is irregular ex: Glass, wax, dental
resin.
Glass
if also called as supercooled liquid, because it is basically a liquid, at a
low temperature which can be rigid or semirigid. It is highly viscous and has
little resistance to deformation.
Glass
has no fixed melting or solidifying
point, it gradually changes to liquid and in reverse gradually solidifies. The
temperature at which this change occur is known a glass transition temperature
which is designated as Tg.
Liquid:
Liquid
has a definite volume, but not a definite shape. Liquid takes the shape of the
container in which it is kept. The molecules in a liquid are free to move
about. The molecules are sufficiently close to each other to have mutual
attraction on each other and on this depends the liquids fluidity or viscosity.
Gas:
Gas has molecules which have freedom of movement and are not
restricted to a given area. Gas has no definite shape and no definite volume.
It expands to fill its container.
Change of state: When a solid is heated, the molecules acquire more kinetic energy
and vibrate more rapidly. They acquire enough energy to breakout of their
positions and move about among the other molecules. When this happens solid
turns into liquid and the process is called melting.
When
a liquid is heated, the molecules gain more KE
and some gain enough energy to break away from the surface of the liquid
and become gas. This is evaporation when bubbles are formed during this change
it is called boiling.
Change
of state is basically controlled by proper temperature and pressure on the
mass.
Crystal – Crystal is any solid whose atoms are arranged in an orderly and repeated
pattern ex: Crystals
of quartz metals.
Uses of crystals: In jewellary, in watches, hearing aids, microphones mica crystals
are used as insulators in electrical equipments.
PROPERTIES:
Physical Properties: most crystals have sharp melting point ex: Metal many crystals
cleave or split along change planes, which are planes of weak bonding that run
parallel to one another through out the crystal. This is due to the
orderly and repeatition of atomic
structure. Crystals
conduct heat and electricity in one direction but act as insulator in other
direction.
Optical properties: light is refracted in crystals.
X-ray diffraction – breaking up spread
The structure of the crystals can be
studied by X-ray diffraction.
Classification of crystals: Crystals are classified by the form of the unit cells.
·
The unit cell is the basic 3
dimensional repeating structure of which crystals are composed.
·
The unit cell may grow in 7
basic different shapes, determined by the relative lengths of their axis and
the angle that these axes make with each other.
·
The seven forms of unit cell
define the crystal systems and every crystal is classified by the position of
the atoms or ions in these cells.
·
In a simple cubic type, only
the corners of the cells are occupied by atoms.
·
In body centered cells, the
corners and the centers of the cell are occupied by atoms.
·
In face centered cells, the
corners and the centers of the faces are occupied by atoms.
·
There are 14 different
combinations of cell structures and atomic arrangement. These combinations are
called space lattices rocks, metals and ice are not single crystals, but are
composed of many small crystalline material.
·
Substances composed of single
crystal are called monocrystalline unit crystal cell is the smallest repeating
unit of atoms.
Diffusion of atoms in solids: Diffuse = Spread out
Atoms
have energy particles – means they have internal energy, some atoms have high
energy level than others. High energy atom can move or change its place and
occupy another position in the material.
Rate
of diffusion depends on temperature, higher the temperature greater will be the
diffusion.
Consequences of diffusion of atoms:
1) Solid state reactions: Number of solid state reactions occur in a solid substance and it is
due to the diffusion of atoms.
For ex: In gold-copper alloys, gold and
copper atoms may be distributed randomly in the space lattices leading to
disordered lattice. Due to heat treatment this pattern may change in which gold
atoms occupy the center of the cubic type of space lattice and copper atoms
occupy the corner positions in the space lattice.
This
will change the physical properties of hardness, brittleness strength and
conductivity of a material.
2) Diffusion can bring about change in
the shape or contour of a material known as warpage or distortion.
1) ADHESION:
Adhesion
is attraction between unlike molecules. i.e., if two different substances are
made to come in contact with each other at their inter surfaces and if the two
surfaces are stuck together it is said that adhesion has taken place.
For example gum and paper.
Gum
and paper are two different types of materials having different types of
molecules. When gum is applied to the surface of the paper, the attraction
between the molecules of gum and molecules of paper takes place and it is
adhesion.
2) COHESION:
Cohesion
is attraction between like molecules. In this case only one materials is
involved. For example water.
It
is mad up of molecules of hydrogen and oxygen. The two together form water
molecule such as H2O. There are innumerable numbers of water
molecules in water. Water remains as
water as long as there is attraction between one water molecule and another
molecule. This attraction between similar type of molecules is cohesion.
Adhesion
and cohesion help in the retention of complete dentures as follows.
When
the denture is placed in the mouth in contact with mucous membrane, adhesion
and cohesion both play a role.
In
this three materials (or) three types of molecules are involved, they are –
Denture
material
Saliva
Mucous
membrane.
Fitting
surface of denture has saliva layer, and saliva is in contact with mucous
membrane. Thus, there is adhesion between denture surface molecules and saliva
molecules. There is cohesion between saliva molecules and saliva molecules.
There is adhesion between saliva molecules and mucous membrane surface.
In dentistry,
adhesion is an important requirement of any restorative materials so that there
is a bond between tooth enamel and artificial restorative materials, which
helps in retention of the restoration in the tooth.
Adhesive – is a material (or) film of
material used to produce adhesion.
Adherand – is the materials or surface of
a material to which adhesive is applied.
For ex: Gum is an adhesive, paper on
which it is applied is adherand. Attraction between molecules of gum and paper
is adhesion.
Factors influencing adhesion:
These are
1)
Wetting
2)
Surface energy
3)
Contact angle.
1) WETTING:
It is the
ability of a liquid (adhesive) to flow and adapt to the surface of a solid.
It
is mainly dependent on surface tension. Surface tension is the molecular
attraction at the surface of liquids and so the surface of a liquid is actually
in a state of tension as if it were being pulled tight. This property makes the
surface of a liquid to behave like thinly stretched rubber sheets. Such
surfaces tend to become as small as possible.
For example: A drop of water is round in
shape, mercury also forms a droplet of round shape. But water as such has low
surface tension, where as mercury has high surface tension.
Therefore
a drop of water may be round in shape but spreads immediately and flows on the
surface. On the other hand drop of mercury remains same and does not spread
out, but will wet the surface only if the surface is clean.
It is difficult to force two solid
surface to adhere.
When
placed in apposition only high spots are in contract. Because these areas
usually constitute only a small percentage of the total surface, no perceptible
adhesion takes place. The attraction is generally neglible when the surface
molecules of the attracting substances are separated by distances greater than
0.7 nm.
One
method of overcoming this difficulty is to use a fluid that flows into these
irregularities and thus provides contact over a greater part of the surfaces of
the solid.
To
produce adhesion in this manner, the liquid must flow easily over the entire
surfaces and adheres to the solid. This characteristic is referred to as
wetting.
Ability
of an adhesive to wet the surface is influenced by number of factors.
Cleanliness
Eg.
Oxide film on metallic surfaces.
Some
substances have ¯ surface energy hence only a few
liquids wet their surface.
Close
packing of the structural organic groups and the presence of halogens may
prevent wetting.
Metals
interact vigorously with liquid adhesive because of increase surface energy.
2) SURFACE ENERGY:
For adhesion
to exist, the surfaces must be attracted to one another at their interface. The
energy at the surface of a solid is greater than in its interior inside the
lattice all of the atoms are equally attracted to each other. The interatomic
distances are equal and the energy is minimal.
At
the surface of the lattice the energy is greater because the outermost atoms
are not equally attracted in all directions.
The
increase in energy per unit area of surface is referred to as the surface
energy. In liquids the surface energy is known as surface tension.
3) CONTACT ANGLE:
The contact
angle is the angle formed by the adhesive with the adherend at their interface.
The extent to which an adhesive will meet the surface of an adherend may be
determined by measuring the contact angle, between the adhesive and the
adherend.
The
greater the tendency to wet the surface, the lower the contact angle, until
complete wetting occurs at an angle equal to zero.
Capillary rise : The penetration of liquids into narrow crevices is known as
capillary action.
This equation
relates the differential capillary pressure developed when a small tube of
radius r is inserted in a liquid of surface tension g (usually expressed in dynes / cm) and with a contact angle q.
If the contact
angle of the liquid on the solid is less than 900 DP will be negative and the liquid will be depressed.
CONTACT ANGLE OF WETTING
The
extend to which an adhesive wets the surface of an adherand may be determined
by measuring the contact angle between the adhesive and adherand.
The
contact angle is the angle formed by the adhesive with the adherend at their
interface. If the molecules of the adhesive are attracted to the molecules of
the adherend as much as or more than they are to themselves, the liquid
adhesive will spread completely over the surface of the solid, and no angle (q = 0 degrees) will be formed. Thus the forces of adhesion are
stronger than the cohesive forces holding the molecules of the adhesive
together.
Tendency
of liquid to spread increases with decrease in contact angle. Therefore contact
angle is the indication of spreadability or wettability. Thus the smaller the contact angle between an
adhesive and an adherend, the better the ability of the adhesive to fill in
irregularities on the surface of the adherend. Also the fluidity of the
adhesive influences the extent to which these voids or irregularities are
fitted.
SOLUBILITY AND SORPTION:
One of the
required of a dental restorative material is that, it should be stable in the
oral environment.
It should
undergo a minimal amount of dimensional change and chemical alteration.
All dental
materials are soluble to some extent and dissolve in water.
The least
soluble of dental materials are the porcelains and ceramics.
In polymers the
unreacted molecules may be readily extracted or dissolved into oral fluids.
The loss of
small organic molecules from soft tissue conditioners and denture liners is
responsible for them hardening in the mouth and becoming irritating.
Metallic ions
are slowly released from cast restorations and amalgams.
Sorption is
the uptake of fluids or substances by a material.
This process
is usually confined to polymeric materials and can also occur at the union
between 2 materials, such as porcelains and a metal interface as a PFM
restoration. Sorption may lead to subtle discoloration of the porcelain. The
result of sorption in a polymeric material is often a swelling or increase in
dimension.
The uptake of
foreign materials can lead to chemical disintegration that occurring in dental
cements at the margin between the cast restoration and the tooth. This results
in loosening of restoration and decay of possible tooth structure.
ADHESION TO TOOTH STRUCTURE:
Associated
principles of adhesion can be readily related to dental situations. For eg.
when contact angle measurements are used to study the wettability of enamel and
dentin. It is found that the wettability of these surfaces is markedly reduced
after the topical appreciation of an aqueous fluoride solution.
Thus
fluoride treated enamel surface retains less plaque over a given period,
presumably because of a decrease in surface energy. Therefore decreases in
dental caries.
Higher
surface energy of many restorative materials compound with that of the tooth,
there is great tendency for the surface and margins of the restoration to
accumulate debris. Therefore increases marginal caries.
Under certain instances,
1)
Recurrent caries
2)
Pulpal sensitivity
3)
Deterioration of the margins of
restoration can be associated with a lack of adhesion between restoration.
Enamel and
dentin of tooth have varying amounts of organic and inorganic components. A
material that can adhere to the organic components may not adhere to the
inorganic components, and an adhesive that bonds to enamel may not adhere to
dentin to the same extent.
After cavity
preparation, tenacious microscopic debris covers the enamel and dentin
surfaces. This surface contamination called the smear layer, reduces wetting.
REFERENCES
1. Anusavice K.J.-“Phillips’ Science of
Dental materials” 11th edition , 2003
2. Combe E.C. – “Notes on Dental Materials”6th edition , 1992
3. Craig’s R.G., Powers J.M. – “Restorative
Dental Materials” 11thedition, 2002
4.Gladwin M,
Bagby M – “Clinical Aspects of Dental Materials” 2nd edition,
2004
5. Mc Cabe J.F. – “Applied Dental Materials”
7th edition , 1992
6. Phillips R.W.-“Skinner’s Science of Dental
Materials”9th edition , 1992
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