Friday, August 30, 2013
INDIAN DENTAL ACADEMY: STRUCTURE OF MATTER AND PRINCIPLES OF ADHESION: INTRODUCTION The principle goal of dentistry is to maintain or improve the oral health of the patient. A wide variet...
Thursday, August 29, 2013
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.
STRUCTURE OF MATTER AND PRINCIPLES OF ADHESION
To better understand the properties of a material it is essential to known them from the atomic point of view. All matter is made up of atoms and these atoms are further held together by atomic interactions to form larger particles called molecules.
Atom – Smallest particle of a chemical element.
Molecule – group of atoms.
Eg : When H2O vapor condenses to form a liquid, energy in the form of heat is released, known as the heat of vaporization. One can conclude that the gaseous state possesses more energy than does the liquid state. Although the molecule in the gaseous state exerts a certain amount of mutual attraction, they can diffuse readily and need to be confined in order to keep the gas intact.
Although the atoms may also diffuse in the liquid state, their mutual attractions are greater, and energy is required for separation as described. If the energy of the liquid decreases sufficiently by virtue of a decrease in temperature, a second transformation is state may occur and the liquid changes to a solid or freezes. Again energy is released in the form of heat. In this case the energy evolved is known as the latent heat of fusion. In as much energy is required from a change of solid to liquid one can conclude that the attraction between the atoms (or molecules) in the solid state is greater than liquid or gas. If this were not true the metal would deform readily and gasify at low temperature.
Change can also take place from a solid to a gas by a process known as sublimation, but this phenomenon is not likely to be of practical importance so far as the dental materials are concerned.
The forces that holds atoms together are of the cohesive type. These inter atomic bonds may be classified as
a) Hydrogen bonding
b) Van der waals forces
Ionic bonds : Are simple chemical type bonds resulting from mutual attraction of positive and negative charges. Classic eg. Na and Cl.
These type of bonds exist in certain crystalline phases of some dental materials such as gypsum and zinc phosphate cement.
Covalent bonds : In many chemical compounds, two valence electrons are shared. H2 is an example of this type of bond.
It occurs mainly in dental resins.
Metallic bonds : One of the chief characteristics of a metal is its ability to conduct heat and electricity. Such energy conduction is due to the mobility of the so – called free electrons present in metals. The outer valence shell can be removed easily from the metallic atom, leaving the balance of the electrons tied to the nucleons, thus forming a positive ion. The free valence electrons are able to move about in the metal space lattice to form what is sometimes described as an electron “cloud” or “gas”.
The electrostatic attraction between this electron cloud and the positive ions in the lattice provides the force that bonds the metal atoms together as a solid. The free electrons act as conductors of both thermal energy and electricity. They transfer energy by moving readily from areas of higher energy to those of lower energy, under the influence of either a thermal gradient or an electrical field.
Deformability is associated with slip along crystal planes, and thus the ability to easily regroup and still retain the cohesive nature of the metal as deformation occurs.
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.
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 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.
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 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 saliva and soft tissue.
Plaque and calculus to tooth……… adhesion.
When 2 subs are brought together into ultimate contact with each other the molecules of one sub adhere or are attracted of molecule of another.
Unlike molecule – adhesion
Like molecule – cohesion
Material or film that produces adhesion – adhesive
Material to which it is applied – adherend
Screws, bolts, undercut.
Acid etching – composite.
For adhesion to exist, the surfaces must be attracted to one another at their interface.
Energy at the surface is more than at the centre. Because at the outer surface the atoms are not equally attracted in all directions.
Increase in energy per unit are or surface is referred to as the surface energy or surface tension.
Eg. Molecules in the air may be attracted to the surface and become adsorbed on the material surface.
Silver, platinum and gold adsorb O2.
With gold bonding forces are 20 but in case of silver the attraction may be controlled by chemical or 10 bonding and silver oxide may form.
When 10 bonding is involved, the adhesion is termed chemisorption.
In short, greater the surface energy, greater the capacity for adhesion.
Strong attachments of two substances can also be accomplished simply by mechanical bonding or retention rather than molecular attraction. Even structural retention may be somewhat gross, as by screws, bolts and undercuts. It may also involve more subtle mechanisms as by penetration of the adhesive into microscopic or submicroscopic irregularities (eg. Revices and pores) in the surface of the substrate.
A fluid or semiviscous liquid adhesive is best suited for such a procedure, since it readily penetrates into these surface discrepancies. Upon hardening the multitude of adhesive projections embedded in the adherand surface provides the footholds for mechanical attachment.
Acid etching resin projections provide retention as it flows into the minute pores created by 37% phosphoric acid.
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 welting.
Ability of an adhesive to wet the surface is influenced by number of factors.
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.
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.
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 sentivity
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.
Greatest problem asso with bonding to tooth surfaces is water or saliva contamination. Inorganic components of tooth structure have a strong affinity for water. To remove the water, the enamel and dentin would have to be heated to increase temperature.
Most restorative materials must withstand forces, during either fabrication or mastication mechanical properties are therefore important in understanding and predicting a materials behavior under load. Because no single mechanical property can give a true measure of quality, understanding the principles involved in a variety of mechanical properties is essential to obtain the ‘Maximum service”.
An important factor in the design of a dental prosthesis is strength, a mechanical property of a material that ensures that the prosthesis serves its intend of firm a effectively safely and for a reasonable period.
It is gained thru one body pushing or pulling on another. Forces applied thru actual contact or at a distance.
The result of force is
(a) Change in position of body at rest
(b) Motion of the body.
If force applied to body results in no movement of body thru deformation results
Force is defined by 3 characters
a) Point of application
The unit of force is NEWTON (N)
Occlusal forces – Most important application of physics in dentistry is the study fo forces applied to teeth and dental restorations.
Biting forces in case of molars – incisors
Adults – 400-800N (molar)
Child – 235-494 with 22N yearly
We can surmise that the forces of occlusal and response of the underlying tissue change with anatomical location. Therefore a material or design sufficient to withstand the forces of occlusion on the incisor of a child may not be sufficient for the first molar of an adult who has a malocclusion or bridge.
When an external force acts upon a solid body, a reaction force results within the body that is equal is magnitude but opposite in direction to the external force. The external force will be called the “load” on the body.
The internal reaction is equal in intensity and opposite in direction to the applied external force, and is called stress.
Both the applied force and internal resistance (stress) are distributed over a given area of the body and so the stress in a structure is designated as the force per unit area in this respect stress resembles for
Stress = Force
Unit “Megapascals” – MPa
FORCES ON RESTORATIONS
Equally important to the study of forces on natural dentition is the measurement of force and stresses on restorations such as inlays, fixed bridges removable partial dentures and complete dentures.One of the first investigations of occlusal forces showed that the average biting force on patients who had a fixed bridge replacing a first molar was 250N on the restored side and 300 N on the opposite side, where they had natural dentition.
Force measurements on patients with removable partial dentures are in the range of 65 to 235 N for patients with complete dentures.
The average force on the molars and bicuspids was about 100 N whereas the forces on the incisors averaged 40 N. The wide range in results is possibly caused by age and gender variations in the patient populations. In general the biting force applied by women in 90 N less than that applied by men.
These studies indicate that
Chewing forces on the 1st molars of patients with fixed bridges is about 40% of the force exerted by patients with natural dentitions.
Decrease in force is obtained with CD or RPD. In such patients only 15% of force is applied.
We can therefore surprise that the forces of occlusion and the response of underlying tissue changes with anatomic location, age, malocclusion and placement of a restorative appliance.
Therefore a material or design sufficient to withstand the forces of occlusion on the incisor of a child may not be sufficient for the first molar of an adult with a malocclusion or bridge.
Internal resistance to force application is impractical to measure, the more convenient procedure is to measure external forces (F) applied to the cross sectional area (A), which can be described as the stress typically denoted as S or s. The unit of stress therefore is the unit of force (N) divided by a unit of area or length squared and is commonly expressed as Pascal.
1 Pa = 1N /m2 = 1 MN /mm2
Stress in a structure varies directly with force and inversely with area, it is therefore necessary to determine the area over which the force acts. Particularly true with dental restorations, as forces applied over small areas eg. clasps on RPD, orthodontic wires.
Stress is always stated as though the force were equivalent to that applied to 1m2 section, but a dental restoration obviously does not have a square meter of exposed occlusal surface area. A small occlusal pit restoration may have no more than 4mm2 of surface area, if it were assumed that the restoration were 2mm on a side. If a biting force of 440 N should be concentrated on this area, the stress developed would be 100MPa, therefore stresses equivalent to several hundreds of MPa occur in many types of restorations.
TYPES OF STRESS
A force can be directed to a body from any angle or direction and often several forces are combined to develop complex stresses in a structure. In general individually applied forces may be axial (tensile or comp), shear, bending or torsional. All stresses however can be combined into 2 basic types axial and shear.
Tension results in a body when it is subjected to two sets of forces directed away from each other in the same straight line.
Compression results when the body is subjected to two sets of forces directed towards from each other in the same straight line.
Shear results when two sets of forces are directed parallel to each other.
Torsion results from the twisting of a body. Bending results from an applied bending moment.
It is caused by a load that tends to stretch or elongate a body. It is always accompanied by tensile strain.
The deformation of a bridge and the diametral compressive loads of a cylinder represent samples of these complex stress situations.
If a body is placed under a load that tends to compress or shorten it, the internal resistance to such a load is called a compressive stress. A compressive stress is associated with compressive strain. To calculate either tensile stress or compressive stress, the applied force is divided by the cross-sectional area perpendicular to the force direction.
Although the shear bond strength of dental adhesive systems is often advertised, most dental prosthesis and restorations are not likely to fail because of pure shear stresses.
Shear stress tends to resist the sliding of one portion of a body over another. Shear stress can also be produced by twisting or torsional action on a material. For example, if a force is applied along the surface of a tooth enamel by a sharp – edged instrument parallel to the interface between the enamel and orthodontic bracket, the bracket may debond by shear stress failure of the resin luting agent. Shear stress is calculated by dividing the force by the area parallel to the force direction.
In the oral environment shear failure is unlikely to occur for many of the brittle material because restored tooth surfaces are generally rough in surface morphology and they are not planar.
The presence of chamfers, bevels, or changes in curvature of a bonded tooth surface would make shear failure of a bonded material highly unlikely. Further more to produce shear failure the applied force must be located immediately adjacent to the interface.
FLEXURAL STRESS (Bending)
Flexural stress is exhibited in a 3 unit bridge and a 2 - unit cantilever bridge. It is produced by bending force in dental appliances in one ways
1) By subjecting a structure such as a FPD to three point loading, where by the endpoints are fixed and a force is applied between these endpoints,
2) By subjecting a cantilevered structure that is supported at only one end to a load along any part of the unsupported section.
When patient bites into an apple the anterior teeth receive forces that are at an angle to their long axes, thereby creating flexural stresses within the teeth.
Tensile stress develops on the tissue side of the bridge and compressive stress develops on the occlusal side. Between these two areas is the neutral axis that represents a state of no tensile stress and no compressive stress.
For a canteliver bridge the maximum tensile stress develops on the occlusal surface or the surface that is becoming more convex.
In the discussion of force, it was pointed out that a body undergoes deformation when a force is applied to it. It is important to recognize that each types of stress is capable of producing a corresponding deformation in a body.
The deformation resulting from a tensile or pulling force is an elongation of a body in the direction of applied force, where as a compressive or pushing force causes compression or shortening of the body in the direction of loading.
Strain E is described as the change in length per unit length of the body when it is subjected to a stress. Strain has no unit of measurement but is represented a pure number obtained from the full equation.
Strain E Deformation = L – L0 = DL
Original length L0 L0
Regardless of the composition or nature of the material and type of stress applied to the material, deformation and strain result with each stress application.
Significance : A Restoration material such as a clasp or an orthodontic wire which can with stand a large amount of strain before failure can be bent and adjusted with less chance of fracturing.
STRESS STRAIN CURVES
Consider a bar of material subjected to an applied force F. We can measure the magnitude of the force and the resulting deformation.
If we next take another bar of the same material, but diff dimensions the force – deformation characteristic change.
However if we normalize the applied force by the cross sectional area A (stress) of the bar and neuralize the deformation by the original length (strain) of the bar, the resultant stress – strain curve now becomes independent of the geometry of the bar.
It is therefore preferential to report the stress – strain deformation characteristics. The stress – strain relationship of a dental material is studied by measuring the load and deformation and then calculating the corresponding stress and strain.
An s-s curve for a hypothetical material that was subjected to increase tensile stress until is show.
The stress is plotted vertically and the strain is plotted horizontally. As the stress is increase the strain is increases. In fact in the ventral portion of the curve from 0 to A, the strain is linearly proportional to the stress and as the stress is doubled, the amount of strain is also doubled when a stress that is higher than the value registered at A is achieved, the strain changes are no longer linearly proportional to the stress changes. Hence the value of the stress at A known as proportional limit.
PROPORTIONAL AND ELASTIC LIMITS
The proportional limit is defined as the greatest stress that a material will sustain without a deviation from the proportionality of stress to strain. Below the proportional limit, no permanent deformation occurs in a structure when stress removed it return to its original dimensions. Within this range of stress application, the material is elastic in nature, and if the material is stressed to a value below the proportional limit, an elastic or reversible strain will occur. The region of the stress strain curve below the proportional limit is called the elastic region. The application of a stress greater than the proportional limit results in a permanent or irreversible strain in the sample, and the region of the stress – strain curve beyond the proportional limit is called the plastic region.
The elastic limit is defined as the maximum stress that a material will withstand without permanent deformation. For all practical purposes, therefore, the proportional limit and elastic limit represent the same stress with in the structure, and the terms are often used interchangeably in referring to the stress involved.
The concepts of elastic and plastic behavior can be realized with a schematic model of the deformation of atoms in a solid under stress. The atoms are shown in (Fig A) with no stress applied, and in (Fig B) with an applied stress that is below the value of the proportional limit.
When the stress shown in B is removed, the atoms return to their positions shown in A. When a stress is applied that is greater than the proportional limit, the atoms move to a position as shown in (Fig C) and after removal of the stress, the atoms remain in this new position. The application of a stress greater than the proportional or elastic limit results in an irreversible or permanent strain in the sample.
YIELD STRENGTH / YIELD STRESS
It is the property that is used to describe the stress at which the material begins to function in a plastic manner. At this stress, a limited permanent strain has occurred in the material.
The yield it is defined as the stress at which a material exhibits a specified limiting deviation from proportionality of stress to strains.
When a structure is permanently deformed, even to a small degree, it does not return completely to its original dimensions when the stress is removed. Therefore prop limit, elastic limit, yield it of a maternal are among its most important properties.
Any dental structure that is permanently deformed through the forces of mastication is usually a functional failure to some degree.
For eg. bridge that is permanently deformed thorough the application of excessive biting forces would be shifted out of the proper occlusal relation for which it was originally designed.
The prosthesis becomes permanently deformed because a stress equal to or greater than the yield strength was developed.
Recall also that malocclusion changes the stresses placed on a restoration, a deformed prosthesis many therefore by subjected to greater stresses than originally intended. Usually a # does met occur under such conditions but rather only a permanent deformation results, which represents a destructive eg of deformation.
A constructive eg of permanent deformation and stresses in excess of the elastic limit is observed when an appliance or dental structure is adapted or adjusted for purposes of design for eg in the process of shaping an ortho appliance or RPD clamp it may be necessary to endure stress into the structure in excess of the yield at if the material is to be permanently bent or adapted.
The test specimen is subjected to its greatest stress at point C. the ultimate tensile strength or stress is defined as the maximum strength or stress a material can withstand before failure in tension.
The ultimate strength of an alloy is used in dentistry to give an indication of the size or cross section required for a given restoration. Note
Stress at which a material fracture
Note that a mat does not necessarily fracture at the point at which the maximum stress occurs. After a max stress is applied some materials begin to elongate excessively and the stress calculated from the force and the original cross sectional area may drop before final fracture occurs.
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
There are several important mechanical properties and parameters that are measures of the elastic or reversible deformation behavior of dental materials.
Elastic modulus / young’s modulus
Dynamic young’s modulus
The term describes the relative stiffness or rigidity of a material.
Here is a fig of a stress – strain graph for a stainless steel were that has been subjected to a tensile test ultimate tensile strength, yield, prop limit elastic modulus are shown.
This fig represents a plot of true stress versus strain because the force ahs been divides by the changing cross sectional area as the wire being stretched. The straight line region represents reversible elastic deformation, because the stress remains below the prop limit of 1020mpa and the curved region represents irreversible plastic deformation that is not recovered when the wire fractures at a stress of 1625 mpa. However the elastic deformation is fully recovered when the force is removed or when the wire fractures.
We can see this easily while bending a wire in our hands a slight amount and then reducing the force. It straightens back to its original shape as the force is decreases to zero and assuming that the induced stress has not exceeded the proportional limit.
This principle can be illustrated by demonstrating a burnishing procedure for an open metal margin, where a dental abrasive stone is shown rotating against the metal margin to close the marginal gap as a result of elastic plus plastic strain. However after the force is removed the margins springs back an amount equal to the total elastic strain. Only by removing the screws from a tooth or die can total closure be accomplished. Because we must provide at least 25mm of clearance for the cement, total burnishing on the tooth or die is usually adequate since the amount of elastic strain recovery is relatively small.
The term used to designate it “E” elastic modulus of a material is a constant and is unaffected by the amount of elastic or plastic stress that can be induced in a material.
Force per unit area / giganewtons per square meter. GN/m2 or giga pascals (GPA)
Dynamic Young’s Modulus : Elastic modulus can be measured by a dynamic method as well as the static techniques that were described in the previous section since the velocity at which sound travels through a solid can be readily measured by ultrasonic longitudinal and transverse wave transducers and appropriate receivers. Based on this velocity and the density of the material, the elastic modulus and poisson’s ratio can be determined. This method of determining dynamic elastic moduli is less complicated than conventional tensile or compressive tests, but the values are often found to be higher than the values obtained by static measurements. For most purposes, these values are acceptable.
If, instead of uniaxial tensile or compressive stress, a shear stress was induced, the resulting shear strain could be used to define a shear modulus for the material. The shear modulus (G) can be calculated from the elastic modulus (E) and Poisson’s ratio (v). It is determined by the equation,
DUCTILITY AND MALLEABILITY
Two very significant properties of metals and alloys. These properties cannot always be determined with certainly from a stress – strain curve.
Ductility is the ability of a material to be plastically deformed, and it is indicated by the plastic strain. A high degree of compression or elongation indicated a good malleability and ductility.
Ductility:- if a material represents its ability to be drawn into wire under a force of tension. The material is subjected to a permanent deformation. While being subjected to these tensile force. The malleability of a substance represents its ability to be hammered or rolled into thin sheets without fracturing.
Ductility is a property that has been related to the work ability of a material in the mouth. Ductility has also been related to burnishability of the margins of a casting.
Metals tend be ductile, whereas ceramics tend to be brittle
Resilience of a material to permanent deformation. It indicates the amount of energy necessary to deform the material to the proportional limit. This term is associated with springiness. The material with the larger elastic area has the higher resilience.
When a dental restoration is deformed during mastication, the chewing force acts on the tooth structure, the restoration, or both and the magnitude of the structure’s deformation is determined by the induced stress. In most dental restorations, large strains are precluded because of the proprioceptive response of neural receptors in the periodontium. The pain stimulus causes the force to be decreases and induced stress to be reduced, thereby preventing damage to the teeth or restorations.
Eg in an inlay (proximal) excessive movement of the adjacent tooth is seen if large proximal strains develop during compressive loading on the occlusal surface. Hence the restorative material should exhibit a moderately high elastic modulus and low resilience, thereby limiting the elastic strain that is produced.
Mn/m3 Mega newtons / cubic meter
Resilience has particular importance in the evaluation of orthodontic wires because the amount of work expected from a particular spring is having a tooth is of interest. There is also interest in the amount of stress and strain at the proportional limit because these factors determine the magnitude of the force that can be applied to the tooth and how for the tooth will have to move before the spring is no longer effective.
During axial loading in tension or compression there is a simultaneous axial and lateral strain.
Under tensile loading, as a material elongates in the direction of load, there is a reduction in cross section. Under compressive loading, there is an increase in the cross section.
Within the elastic range, the ratio of the lateral to the axial strain is called Poisson’s ration.
In tensile loading, the Poisson’s ratio indicates that the reduction in the cross section is proportional to the elongation during the elastic deformation. The reduction in cross section continues which the material is fractured.
Values of Poisson’s Ratio of some restorative dental materials
Brittle subs such as hard gold alloys and dental amalgam show little permanent reduction is cross section during a tensile test, whereas ductile materials such as soft gold alloys, which are high in gold contents show a high degree of reduction in cross section area.
It is defined as the amount of elastic and plastic deformation energy required to fracture a material and it is a measure of the resistance to fracture.
It can be measured as the total area under the stress-strain curve from zero stress to the fracture stress. Toughness depends on strength and ductility. The higher the strength and the higher the ductility, the greater the toughness. Thus it can be concluded that a tough material is generally strong, although a strong material is not necessary tough.
Units MN/m3 or Mpa /m
Therefore toughness is the energy required to stress that material to the point of fracture.
Mechanical property that describes the resistance of brittle materials to the catastrophic propagation of flows under an applied stress.
Fracture mechanics characterizes the behavior of materials with cracks or flows, which may arise naturally in a material or nucleate after a time in service. In either case, any defect generally weakens a material and sudden fractures can arise at stresses below the yield stress. Sudden catastrophic fractures typically occur in brittle materials that point.
Fracture toughess of selected dental mats.
Mpa m ½
1.5 – 2.1
0.8 – 2.2
0.6 – 1.8
We have the ability to plastically deform and redistribute stresses.
2 simple examples illustrate the significance of defects on the fracture of materials. If one takes a piece of paper and tries to tear it, grater effort is needed than if a tiny cut is made in the paper.
Similarly, it takes a considerable force to break a glass bar, however, if a small notch is placed on the surface of the glass bar less force is needed to cause fracture.
If the same experiment is performed on a ductile material, we find that a small surface notch has no effect on the force required to break the bar, and the ductile bar can be bent without fracturing for a brittle material, such as glass, no local plastic deformation is associated with fracture whereas for a ductile material, plastic deformation such as the ability to bend, occurs without fracture.
The ability to be plastically deformed without fracture or the amount of energy required for fracture is the fracture toughness.
Therefore larger flow lower stress needed to cause fracture. This is because the stresses which would normally be supported by material are not concentrated at the edge of flaw.
Presence of fillers in polymers substantially increases fracture toughness. 50 wt% zinconia to porcelain increases fracture toughness.
May be broadly defined as the resistance to permanent surface indentation or penetration.
Measure as a force per unit area of indentation and in mineralogy, the relative hardness of a substance is based on its ability to resist scratching. In metallurgy and in most other disciplines, the concept of hardness that is most generally accepted is the “resistance to indentation”. It is on this precept that most modern hardness tests are designed.
It is apparent that hardness is important. It is indicative of the case of finishing of a structure and its resistance to in-service scratching. Finishing or polishing a structure is important for esthetic purposes and as discusses previously scratches can compromise fatigue strength and lead to permanent failure. Some of the most common methods of testing the hardness of restorative are the
Knoop Micro hardness test
BRINELL HARDNESS TEST
It is among the oldest methods used to test metals and alloys used in dentistry. The method depends on the resistance toe the penetration of a small still or tungsten carbide ball typically 1.6 nm in diameter, when subjected to a weight of 123M. in testing the Brinell hardness of a material the penetration remains in contact with the sample used for a fixed time of 30 seconds. After which it is removed and the indentation diameter is carefully measured. Used to determined hardness of metals and metallic materials in dentistry. It is related to proportional limit and ultimte strength of dental gold and alloys.
L is the load in kg.
D is the diameter of the ball in millimeters
d is the diameter of the ball in indentation millimeter
Smaller the area of the indentation, the harder the material and the larger the BHN value.
Advantage – Test is good for determining average hardness values.
Disadvantage – poor for determining very localized values.
(PN) not suitable for brittle materials or dental elastic that exhibit elastic recovery.
KNOOP HARDNESS TEST
This test was developed to fulfill the needs o a microindentation test method. A load is applied to a carefully prepared diamond indenting tool with a pyramid shape, and the lengths of the diagonals of the resulting indentation in the material are measured. The shape of the indenter and the resulting indentation are measured.
KHN = L/I2Cp
L – load applied
l = length of the long diagonal of the indentation.
Cp = constant relating l to the projected area of the indentations.
Advantage : materials can be tested with a great range of hardness simply by varying the test load.
Disadvantage : high by polished and flat test samples time consuming.
VICKER’S HARDNESS TEST
The 136 degree diamond pyramid, or Vicker’s hardness test, is also suitable for testing the surface hardness of materials. It has been used to a limited degree as a means of testing the hardness of restorative dental materials. The method is similar in principle to the Knoop and Brinell tests except that a 136 degree diamond pyramid – shaped indenter is forced into the material with a definite load applications. The indenter produces a square indentation, the diagonals of which are measured as shown in pic previously.
Useful for brittle stuff therefore measure hardness of tooth.
ROCKWELL HARDNESS TEST
Was developed as a rapid method for hardness determinations. A ball or metal cone indenter is normally used and the depth of the indentation is measured with a sensitive deal micrometer. The indenter balls or cones are of several diff diameters, as well diff load applications (60-150) with each combination described as a special Rockwell scale.
“no suitable for brittle materials”
how hardness read directly.
Good for testing viscoelasticity of materials.
Disadvantage – preload needed increases time
Indentations may disappear immediate when the load is removed.
Is generally considered to be the opposite of toughness. For eg. glass is brittle at room temp, it will not bend appreciably without breaking. In other words, a brittle material is apt to fracture at or near its proportional limit.
However a brittle material is not necessarily lacking in strength. For eg. shear strenght of glass is low, but its tensile strength is very high.
“it is the relative inability of a material to sustain plastic deformation before fracture of a material occurs.
Eg. amalgams, ceramics and composite are brittle at oral temps (5-550C) they sustain little or no plastic strain before they fracture. Therefore a brittle material fractures at or near its proportional limit.
Therefore amalgam nonresin luting agents will have little or no burnishability because they have no plastic deformation potential.
ABRASION, FRICTION AND WEAR
Friction is the resistance to motion of one material body over another. If an attempt is made to move one body over the surface of another a restraining force to resist motion is produced. This restraining force is the (static) frictional force and result from the molecules of the two objects bonding where their surfaces are in close contact. Frictional force, Fs is proportional to the normal force (F^) between the surfaces and the (static coefficient of friction (ms).
Similar materials have a greater coefficient of friction and if a lubricating medium exists at the interface, the coefficient of friction is reduced.
Frictional behavior therefore arises from surfaces that, because of microroughness, have a small real contact area.
An example of the importance of friction …dental implant – surface roughed to reduce motion between implant and adjacent tissue. It is percieved that a rough surface and resultant less motion will provide better osseointegration.
Is a loss of material resulting from removal and relocation of materials through the contact of two or more materials. When 2 solid materials are in contact, they only touch at the tips of their highest asperities.
Wear is usually undesirable but during finishing and polishing wear is beneficial.
4 types of wear
Abrasive wear involves soft surface in contact with a harden surface. In this type of wear, particles are pulled off of one surface and adhere to the other during sliding.
Corrosive - 20 to physical removal of a layer therefore related to chemical activity.
Metals – adhesive wear
Polymers – abrasive and fatigue over.
In case of dental appliances ad restorations a high value for the elastic limit is a necessary requirement of the materials from which they are fabricated, because the structure is expected to return to its original shape often it has been stressed. Usually a moderately high modulus of elasticity is also desirably because only a small deformation will develop under a considerable stress, such as in the case of an inlay.
There are instances in which a larger strain or deformation may be needed with a moderate or slight stress. For example, in an orthodontic appliance, a spring is after bent a considerable distance under the influence of a small stress. In such a case, the structure is said to be flexible and it possesses the property of flexibility. Maximum flexibility is defined as the strain that occurs when the material is stressed to its proportional limit.
In the previous discussions of the relationship between stress and strain, the effect of load application rate was not considered. In many metals and brittle materials, the effect is rather small. However the rate of loading is important in many materials, particularly polymers and soft tissues.
The mechanical properties of many dental materials, such as agar, alginate, elastomeric, impression materials and waxes, amalgam and plastics, dentin, oral mucosa and pdl are dependent on how fast they are stressed, for these materials increasing the loading (strain) rate produces a different stress -–strain curve with higher rates giving higher values for the elastic modulus, proportional limit and ultimate strength. Materials that have mechanical properties dependent on loading rate termed elastic. Materials that have mechanical properties dependent on loading rate are termed viscoealstic. In other words these materials have characteristics of an elastic solid or a viscous fluid.
FLUID BEHAVIOR AND VISCOCITY
In addition to the many solid dental materials that exhibit some fluid characteristics, many dental materials, such as cements and impression materials, are in the fluid state when formed. Therefore (viscous) fluid phenomena are important. Viscosity (n) is the resistance of a fluid to flow and is equal to the shear stress divided by the shear strain rate.
When a cement or impression material sets, the viscosity increases, making it less viscous and more solid like
The unit of viscosity are POISE
Or centipoise “cp”
The behavior of elastic solids and viscous fluids can be understood from simple mechanical models. An elastic solid can be viewed as a spring when the spring is stretched by a force “F” it displaced a distance c. the applied force and resultant displacement are proportional and the constant of proportionality is the spring constant R . Therefore
F = R x X
Note that the model of an elastic element does not involve time. The spring acts instantaneously when stretched therefore an elastic solid is nondependent of loading rate.
Although the viscosity of fluid is proportional to the shear rate, the proportionality differs for different fluids. Fluids may be classified as
Dilatant depending on how their viscosity varies with shear shear rate certain dental cements and impression materials are Newtonian. The viscosity of a N liquid is constant an independent of shear rate. The viscosity of a pseudoplastic liquid decreases with increasing shear rate. Several endodontic cements are pseudoplastic, as are monophase rubber impression materials.
When subjected to low shear rate during spatulation or while an impression is made in a tray, these impression materials have a high viscosity and possess body in the tray. These materials, however can also be used in a syringe, because at the higher shear rates encountered as they pass through the syringe tip, the viscosity decreases as much as tenfold the viscosity of a dilatant liquid increases with increasing shear rate.
Eg of dilatant liq ® fluid – denture base resins.
Two additional factors that influence the viscosity of a material are time and temp.
The viscosity of a non setting liquid is typically independent of time and decreases with increasing temperature. Most dental materials, however, begin to set after the components have been mixed and their viscosity increases with time, as evidenced by most dental cements and impression materials.
A notable exception is ZnO that requires 2% of moisture to sit on the mixing pad then materials maintain a constant viscosity that is described clinically as a ling working time once placed in the mouth however the ZnO materials show rapid increases in viscosity because exposure to heat and humidity accelerate the setting reaction.
In general for a material that sets, viscosity increases with increasing temperature. However the effect of heat on the viscosity of a material that sets depend on the nature of the setting reaction.
For eg. Zn phosphicum, Zn polycarb
The setting reaction of A is highly exothermic, and miningat reduced temp results in a lower viscosity than when mixed at high time. The setting reaction of B is less affected by temp. addi working time is achieved by axis a cool or frozen mixing slab.
After a substance has been permanently deformed, there are trapped internal stresses. For eg in a crystalline substance the atoms in the space lattice are displaced and the system is not in equilibrium.
It is understandable that such a situation is not very stable. The displaced atoms may be said to be uncomfortable and wish to return to normal regular positions given time by diffusion they will move back. The result is a change in the shape or contain of the solid as a gross manifestation of the rearrangement is atomer or molecular positions. The material is said to warp or distort. Such a relief of stress is known as relaxation.
Rate of relaxation will increase with an increase in temperature. For example if a wire is bent, it may tend to straighter out if it is heated to a high temp. At room temp any such relaxation or diffusion may be negligible. On the other hand, there are many noncrystalline dental materials eg waxes, resins, gels that can relax during storage at room temp after being bent or molded.
Physical properties are based on the laws of mechanics, acountics, optics, thermodynamics, electricity, magnetism radiation, atomic structure, or nuclear phenomenon. Hue, Chroma and Value and translucency are physical properties that are based on the laws of optics, which is the science that deals with phenomena of light, vision, and light. Thermal conductivity and coefficient of thermal expansion are physical properties based on the laws of thermodynamics.
ABRASION AND ABRASION RESISTANCE
Hardness has often been used as in index of the ability of a material to resist abrasion and wear. The ability of enamel by ceramic and other restorative material is well known. Along with hardness of material other factors affecting enamel wear are biting force, frequency of chewing, abrasiveness of the diet, composition of liquids, temperature changes, physical properties of the material and surface irregularities of the material. Although dentists cannot control the biting force, they can polish the abrading ceramic surface to reduce the rate of destructive enamel wear.
VISIOSITY : The resistance of liquid to motion is called viscosity and it is controlled by internal frictional forces within the liquid. Viscosity is the measure of the consistency of a fluid and its inability to flow.
An ‘ideal fluid’ has shear stress that is proportional to strain rate and the plot is a straight line in the graph . Such behaviors is called Newtonian. A Newtonian fluid has a constant viscosity and straight like resembles elastic portion of a stress-strain curve.
Viscosity is measured in units of MPa/sec. Or POISE. Higher the value, the more viscous is the material.
Eg. Pure water at 200C – viscosity = 1.0 centipoise. (cP)
Agar hydrocolloid impression – viscosity = 281, 100 cP
Material at 450C
Light body polysulfide – viscosity = 109,000 cP
Heavy body polysulfide – viscosity = 1,360,000 cP
Pseudoplastic : For many dental material viscosity decreases with increasing shear rate until it reaches a constant value. E.g. Polysilicon pseudoplastic material, cements like zinc phosphate, zinc oxide Eugenol.
Dilatant : These liquids become more rigid as the rate of deformation increases. E.g. cold cure resin dough.
Plastic : Some classes of material behave like a rigid body until some minimum value of shear stress is reached. E.g. catsup. (a sharp blow to the bottle is required to produce initial flow)
- Viscosity of most liquids decrease rapidly with increasing temperature.
- A liquid that becomes less viscous and more fluid under pressure is referred to as thixotropic. E.g. Dental prophylaxis paste, plaster, resin cements, agar.
CREEP AND FLOW
- Creep is defined as the time dependent plastic strain (deformation)of a material under static load or constant stress.
- Metal creep usually occurs as the temperature approaches within a few hundred degrees of the melting range. Metals used in dentistry for cast restorations or substrates for porcelain veneers have melting points much higher than mouth temperature and thus are not susceptible to creep deformation except when they are heated to very high temperature.
- The most important exception is dental amalgam, which has components with melting points slightly above room temperature. Because of low melting range, dental amalgam can slowly creep from a restored tooth under stress as produced by patients who clench their teeth.
- According to American Dental Association specification creep must be <8%.
Following are the approximate value for various types of alloys :
1) Low copper lathe cut – 2%
2) Low copper spherical – 1%
3) High copper admix – 0.5%
4) High copper single composition – 0.05 – 0.1%
FLOW : Is the time dependent deforming property of amorphous material such as waxes to deform under a small static load or even load associated with its own mass.
Static creep : Is the time dependent deformation produced in a completely set solid subjected to a constant load.
Dynamic creep : Refers to this phenomenon when the applied stress is fluctuating such as fatigue type test.
Another important goal of dentistry is to restore the colour and appearance of natural dentition. Aesthetic considerations in restorative and prosthetic dentistry have assumed a high priority within past several decades. For e.g. the search for an ideal general purpose, direct filling ‘tooth coloured’ restorative material is one of the challenges of present dental material research.
Light is electromagnetic radiation that can be detected by the human eye. The eye is sensitive to wavelengths from approximate 400mnm (violet) to 700nm (dark-red) (fig)
The reflected light intensity and the combined intensities of the wavelength present in a beam of light determines the appearance properties (hue, value and chroma). For an object to the visible, it must reflect or transmit light incident on it from an external source. The latter is the case for objects that are of dental interest. The incident light is polychromatic, i.e. mixture of various wavelength.
The eye is most sensitive to light in the green-yellow region (wavelength 550 nm) and least sensitive at either extreme i.e. red or blue.
Three dimensions of colour : Verbal description of colour are not precise enough to describe the appearance of teeth or restoration surface. To accurately describe our perception of a beam of light reflected from a tooth or restoration surface, three variables must be measured. Quantitatively, the colour and appearance must be described in three dimensional colour space by measurement of hue, value and chroma.
Hue : Describes the dominant colour of an object. E.g. red, green or blue. This refers to the dominant wavelength present in the spectral distribution.
Value : Is the lightness or darkness of a colour, which can be measured independently of the hue. Teeth or other object can be separated into lighter shades (higher value) and darker shades (lower value).
Chroma : Represents the degree of saturation of a particular bone. The higher the chroma, more intense is the colour. Chroma is always associated with hue and value.
In dental operatory, colour matching is usually done by the use of shade guide to select the colour of ceramic veneers, inlays or crowns. (fig)
One of the common method to define and measure colour quantitatively is Mullur system. This system is viewed as cylinder. Hues are arranged sequentially around the perimeter of the cylinder Chroma. Increases along a radius out from the axis. Value varies along the length of the cylinder from black at bottom, to neutral gray at the centre, to white at the top.
Because, spectral distribution of light reflected from or transmitted through an object is dependent on the spectral content of the incident light, the appearance of an object is quite dependent on the nature of the light by which object is viewed. Daylight, incandescent and fluorescent lamps and common sources of light in dental operatory and they have different spectral distributions. Objects that appear to be colour matched under one type of light may appear different under another light source. This phenomenon is called METAMERISM. If possible, colour matching should be done under two or more different lights and one being daylight.
Sometimes, natural tooth absorbs light at wavelengths too short to be visible to human eyes ie. between 300 –400 nm called as near – ultraviolet radiation. The energy absorbed is converted into light with longer wavelengths and tooth actually becomes a light source. This phenomenon is called FLUORESCENCE. The emitted light, blue – white colour, is primarily in the 400 –450 nm range. Fluorescence makes a definite contribution to the brightness and vital appearance of human tooth. A person with ceramic crowns are composite restorations that lacks fluorescing agent appears to be missing teeth when, viewed under a black light in a night club.
Thermal Conductivity : Heat transfer through solids most commonly occurs by means of conduction. It is the thermophysical measure of how well heat is transferred through a material by the conductive flow.
Thermal conductivity or co efficient of thermal conductivity is the quantity of heat in calories per second that passes through a specimen 1 cm thick having a cross – sectional area of 1 cm2 when temperature differential between the surface perpendicular to the heat flow of specimen is 10C.
According to IInd law of thermodynamics, heat flows from points of higher temperature to points of lower.
Material having high thermal conductivity are called conductors. Whereas of low thermal conductivity are called insulators (higher the value, greater is the ability to transmit thermal energy).
Unit – W/m/0k
- Silver - 385 W/m/0k
- Copper – 370 W/m/0k
Thermal diffusivity : It is the measure of the rate at which a body with a non – uniform temperature reaches state of thermal equilibrium.
Although thermal conductivity of ZnOE is slightly less than dentin, its diffusivity is more than twice of dentin.
Mathematically, thermal diffusivity (h) is related to thermal conductivity (k) as :
H = k
Where cp = temperature dependent specific heat capacity.
P = temperature dependent density.
- Silver ® 1.64 cm2 /sec.
- Copper ® 1.14 cm2 /sec.
Linear coefficient of thermal expansion : Defined as change in length per unit original length of a material. When its temperature is raised 10C.
- Polymethyl metha – acrylate ®81 x 10-6/0c
- Dentin ® 8.3 x 10-6 /0c
- Enamel ® 11.4 x 10-6/0c
‘Little knowledge is dangerous’ as rightly said, thus a thorough understanding of properties of dental materials enables a professional to ensure the eventual success of the treatment. It is a must for every dentist that they should posses sufficient knowledge of properties so that they can exercise the best judgement possible in selection of an appropriate material right from the impression procedures to the fabrication of the prosthesis.The efficacy of the end product depends on the type of material used and in turn its proper handling.
1. Science of Dental Materials : by Anusavice (Skinners), 11th Edn.
2. Restorative Dental Materials : by Robert G. Craig, 9th Edn.
3. Elements of Dental Materials : by Ralph W. Phillips, 4th Edn.
4. Notes on Dental Materials : by E.C. Combe, 5th Edn.
5. Applied Dental materials : by John F. McCabe, 7th Edn.
Ø Structure of matter and principles of adhesion
Ø Interatomic bonds
Ø Crystalline structure
Ø Noncrystalline structure
Ø Adhesion and bonding
Ø Adhesion to tooth structure
Ø Mechanical property
o Proportion and Elastic limit
o Yield stress and yield strain
Ø Mechanical property based on elastic deformation
o Elastic modulus
o Dynamic young’s modulus
o Ductility and Malleability
o Abrasion and Friction wear
o Fluid behaviour and viscosity
Ø Physical property
o Abrasion and abrasion resistance
o Creep and flow
Ø Thermophysical Properties
o Thermal conductivity
o Thermal diffusivity