Basic Property Onby Abbbe

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Matteranything that has weight and

occupies space

Types of matter Solid

Liquid

Gas Energythe ability to do work

11/5/2010 2Abebe teka, DMD.



Types of materials Non-crystalline or amorphousno definite

atomic structure. Crystalline or space latticea definite atomic

structure Atom Element Compound Molecule Alloy




Define dimensional change and linear coefficientof thermal expansion and give examples of theirimportance to clinical dentistry

Give examples of where thermal and electricalproperties of restorative materials are importantin clinical dentistry

Define stress and strain and illustrate how theydiffer

List example of where solubility and watersorption are important in success of dentalrestorative materials




1st materials used to replace missing portion

of teeth are exposed to attack by the oral

environment and subjected to biting force 2nd restorative material are cleansed and

polished by various prophylactic procedures

as a result their properties are the basis forthe selection of materials to be used inparticular dental procedure and restoration




The establishment of critical physicalproperties for various types of dental

materials has led to the development of minimum standard or specifications Selection of materials should be influenced

by their effect on the oral tissue and by

possible toxic effect if ingested. the colourand optical qualities of materials also areimportant in selection of restorativematerials




Is the percent shrinkage of expansion of

materials

Maintaining dimension during dentalprocedures such as preparing impression and

models is important in accuracy of dentalrestorations. Dimensional change may occur

during setting as a result of a chemicalreaction .




To compare materials easily the dimensional

change expressed as a percentage of original

length or volumeThermal dimensional change; restorative

material is subjected to temperature changein the mouth this change result in

dimensional change




Because of the thermal expansion of

restorative materials usually does not much

that of the tooth structure a differentialexpansion occurs that may result in leakage

of oral fluids b/n the restoration and the tooth

The linear thermal expansion of a materials is

expressed as coefficient of thermal expansionis a measure of how much it expands per unit

length if heated 1 degree higher




A clinical effect of this difference of thermalexpansion e.g. the tooth contained a poorly

bounded composite restoration that cooledby the drinking of cold liquid the restorationwould contract more than the tooth and asmall gap would result at junction b/n the two

materials oral fluid penetrate this space whentemperature return to normal this fluid isforced out of the space this phenomenon iscalled percolation




Percolation is undesirable because of possibleirritation of pulp ands recurrent caries

Thermal conductivity; is a measure of heattransferred and defined as the no. of caloriesper second flowing through an area of 1cm2in which the temperature drop along the

length of the spacemen is 1 degreecentigrade per centimetre.




Human enamel and dentin are poor thermal

conductors compared with gold alloys and

dental amalgam




Two electrical properties of interest are

galvanism and corrosion

Galvanism is the generation of electricalcurrents that the patient can feel result from

the presence of dissimilar metals in themouth. Metals placed in an electrolyte (a

liquid that contains ions)have varioustendencies to go in a solution




When the two metallic restoration touchcurrent flows and there will be a potential

difference .the patient may experience painand frequently complain of metallic taste Corrosion also can result from the same

condition when adjacent restoration of

dissimilar metals that result is dissolution of metals in the mouth as result of galvanicaction materials goes in to solution androughness and pitting occur




Corrosion also may result from chemical

attack of metals by components in food and

saliva e.g. dental amalgam react withsulphides and chemicals in the mouth this

effect is referred to as tarnish




The solubility of materials in mouth andsorption (adsorption plus absorption )of oral

fluids by the material are important criteria intheir selection Solubility and sorption are reported in two

ways ;1 in weight percent of soluble or sorbed

material and (2) as the weight of dissolved orsorbed material per unit of surface area




Absorption refers to the uptake of liquid by

the bulky solid e.g. the equilibrium

absorption of water by acrylic polymer is inrange of 2%.

Adsorption indicates concentration of molecules at the surface of a solid or liquid

e.g. adsorption of component of saliva at thesurface of tooth structure or of detergent

adsorbed on the surface of a wax pattern




Is a measure of the affinity of a liquid for asolid as indicated by spreading of a drop.

The wet ability of a solid by a liquid can beobserved by shape of a drop of a liquid on thesolid surface the shape of a solid is identifiedby the contact angle if a low contact angleoccurs the solid is wetted readily by the liquid

(hydrophilic, if the liquid is water) if thecontact angle is greater than 90 degree poorwetting occurs (hydrophobic if the liquid iswater)




G. V. Black is responsible for many of the

theories used in testing dental materials for

the mouth still used today.It includes both the physical and chemical

changes




Biting forces

The average biting force of a person with

natural dentition is approximately 150pounds . In the posterior , that becomes

approximately 30,000 psi of pressure on asingle cusp of a molar.

Materials considered for use in restoring theocclusal surfaces must have sufficient

strength to withstand these forces.




STRESSTHE INTERNAL FORCE THAT

RESISTS ANOTHER FORCE EXTERNALLY

APPLIED. STRAIN IS THE AMOUNT OF MOVEMENT

OR CHANGE OF SHAPE CREATED BY AFORCE.




ANY PUSHOR PULL UPON MATTER

STRESS AND STRAIN IS A RESPONSE TO

FORCE.




COMPRESSIVEPUSHES THINGS

TOGETHER

TENSILEPULLS AND STRETCHES SHEATSLICES APART




THE ABILITY TO REGAIN SHAPEWHEN STRESSIS REMOVED

TYPES ELASTICELASTICITY INELASTICMATERIAL REMAINSPERMANENTLY DEFORMED

ELASTIC LIMITTHE MAXIMUM STRESS AMATERIAL CANWITHSTANDWITHOUT BEINGDEFORMED PERMANENTLY

MODULUS OF ELASTICITY (YOUNGSMODULUS)




ELASTIC LIMIT (HOOKS LAW)

YIELD STRENGTH

PERMANENT DISTORTION OCCURSBEYOND THE PROPORTIONAL LIMIT

ULTIMATE STRENGHTTHE MAXIMUMSTRENGHT OBTAINED BASED ON THE

ORIGINAL DIMENSION OF THE OBJECT

TOUGHNESSTHE ABILITY OF A MATERIAL

TO RESIST FRACTURE




DUCTILITYABILITY OF A MATERIAL TO

WITHSTAND PERMANENT DEFORMATION

UNDER TENSIL STRENGTH. MALLEABILITYHAMMERED OR ROLLED

INTO SHAPE

FLOW, CREEPOR SLUMP DEFORM

PERMANENTLY UNDER A CONSTANTLOAD.




HARDNESSRESISTANCE TO

PENETRATION BRINELHARDNESS

NUMBER BHN ISHIGH THE MATERIAL IS SOFT

KNOOP HARDNESS NUMBER

MOHSALSO KNOWN AS THE STRATCH

HARDNESS DISTORTION




THE TOOTH IS AN EXCELLENT HEAT

INSULATORWHICH MEANS THAT IT HAS A

LOWTHERMAL CONDUCTIVITY VALUE THERMAL CONDUCTIVITY

THERMAL EXPANSION




FORCE THAT CAUSES UNLIKE MOLECULES

TO ATTRACT

ADHESION ACTION OF A LIQUID VISCOSITY

WETTING

FILM THICKNESS

SURFACE TENSION ENERGY




TOXIC EFFECTS OF MATERIALS

ACIDITY/ALKALINITY

TEMPERATURE CHANGES MICROLEAKAGE




MOISTURE

TEMPERATURE CHANGES

INERT AESTHETIC FACTORS

RETENTION

GALVINISM




THE ABILITY TO REGAIN SHAPEWHEN STRESSIS REMOVED

TYPES ELASTICELASTICITY INELASTICMATERIAL REMAINSPERMANENTLY DEFORMED

ELASTIC LIMITTHE MAXIMUM STRESS AMATERIAL CANWITHSTANDWITHOUT BEINGDEFORMED PERMANENTLY

MODULUS OF ELASTICITY (YOUNGSMODULUS)




is the force with which a structure resists anexternal load placed on it. It is the internalreaction to an externally applied load and is

equal in magnitude but opposite in direction tothe external load; although technically theinternal force, this is difficult to measure and sothe accepted way of measuring stress is tomeasure the external load applied to the crosssectional area; measured in force per area unitssuch as kg/cm2, MPa (MN/m2), or psi; isrepresented by the Greek letter, sigma. Stress =Force/Area






is the change in length per unit length that a

material undergoes when a force is applied to

it; it is dimensionless because it has lengthper length units of measurement; is often

expressed as a percentage; is represented bythe Greek letter, epsilon.




Strain = Change in Length/Original Length Strain can either be elastic or plastic. Elastic

strain is strain that totally disappears once theexternal load that caused it is removed. Elasticstrain is based upon the fact that a net force of zero exists between two atoms when they are atequilibrium.

If a compressive or tensile force is exerted on theatoms, an opposite force will attempt to movethem back to their equilibrium position.




Plastic strain is strain that permanently

remains once the external load that caused it

is removed. It occurs when the force appliedto the atoms moves them so far from their

equilibrium position that they do not returnto it once the force is removed.






Many of the basic physical properties of

dental materials can be represented on a

stress-strain diagram. For example: the straight part of the line represents the

region of elastic deformation

--the curved part of the line represents the

region of elastic and plastic deformation

--the slope of the straight part of the line

represents modulus of elasticity




--the length of the curved part of the line

represents ductility

--the area under the straight part of the linerepresents resilience

--the area under the entire line representstoughness




is a measure of the relative stiffness or rigidity of a material. The unit values are those of force per

area because Modulus of Elasticity =Stress/Strain Note, however, that this only applies to the

elastic portion of the stress-strain diagram. On

the stress-strain diagram, the modulus isindicated by the slope of the linear part of theline.

Therefore, a material with a steep line will havea higher




modulus and be more rigid than a material

with a flatter line.

Modulus is a reflection of the strength of theinteratomic or intermolecular bonds. It is

unrelated to strength and to proportionallimit and is unaffected by age hardening heat

treatment and by cold working.




is the amount of stress required to producepermanent deformation of a material; canalternatively be defined as the limit of proportionality

of stress to strain; is represented on the stress-straindiagram as the point where the plotting converts froma straight line to a curve. Below the proportional limit,stress is proportional to strain. Stresses below theproportional limit cause elastic (non-permanent)déformation and those above it cause elastic andplastic (permanent) deformation.

A high proportional limit is desirable for a restorativematerial.




is the maximum amount of stress that a

structure can withstand and still return to its

pre-stressed dimensions; it is, for all practicalpurposes, the same as the proportional limit.




is the point of first marked deviation from

proportionality of stress to strain on the

stress-strain diagram; it indicates that thestructure is undergoing a pronounced degree

of deformation with little additionally appliedstress.




is the amount of stress required to produce apredetermined amount of permanent strain (usually0.1% or 0.2% which is called the percent offset).Although many feel it is equivalent to proportionallimit, it is a useful property because it is easier tomeasure than the proportional limit. This is becauseyou are already a certain way out on the stress-straincurve and are not attempting to measure the exactpoint where proportionality of stress to strain ends. Itis measured using the stress-strain diagram bylocating the point 0.1% or 0.2% out on the strain axisand drawing a line up to the curve which is parallel tothe line found in the elastic region.




is the maximum amount of stress that a

material can withstand without undergoingfracture or rupture. It can be applied to

compressive, tensile, or shear stresses (i.e.,compressive strength is the maximum

amount of stress that a material canwithstand without undergoing fracture or

rupture in compression)




is the amount of stress required to produce

fracture or rupture.

DUCTILITY is the ability of a material to undergo

permanent tensile deformation withoutfracture or rupture, or the degree to which

you can permanently deform a structureusing a tensile force without it undergoing

fracture or rupture.




An increase in temperature decreases ductilitybecause a material's strength generallydecreases with an increase in temperature.

Ductility is unrelated to proportional limit. Oneway that ductility is used in dentistry is as ameasure of burnish ability. The burnish abilityindex is defined as the percentage elongation

divided by the yield strength. Therefore, thegreater the ductility and the lower the yieldstrength, the greater the burnish ability.




is the ability of a material to undergo permanentcompressive deformation without fracture orrupture or the degree to which you can

permanently deform a structure using acompressive force without it undergoingfracture or rupture.

An increase in temperature generally results in aincrease in malleability because malleability isdependent upon dislocation movement, anddislocations generally move more easily at ahigher temperature.




is the material behavior where a materialundergoes fracture or rupture with little or noprior permanent deformation. Materials that are

brittle usually have a very ordered atomicstructure which does not permit the easymovement of dislocations. A good example isthe class of materials known as ceramics. Their

ordered atomic structure does not permit easydislocation movement, and hence, they arebrittle.




is the resistance of a material to permanentdeformation under sudden impact; may also bedefined as the amount of energy absorbed by a

material when it is stressed to a point just shy of its proportional limit.

A high modulus of resilience is desirable in arestorative dental material. For orthodontic

wires, it means that they are capable of storingenergy which may then be delivered over anextended period of time.






A great deal of effort has been expended in

dental materials research in an attempt to

find ways of increasing toughness.




Wear is the loss of material from one or both of two contacting surfaces because of themechanical activity between them. It is a

complicated process and is affected byproperties such as ductility, hardness, andultimate strength.

F our Types of Wear: 1. Abrasive 2. Adhesive 3. Fatigue 4. Corrosive






Fatigue

occurs when fatigue from cyclic loading

causes cracks to develop under thecontacting surfaces; the sliding action then

causes the surfaces to be lost.

Corrosive occurs when two contacting

surfaces corrode and the sliding action causesthe corrosion by-products to be worn away.




Adhesive occurs when asperities

(microscopic projections) from the two

contacting surfaces adhere or cohere to eachother and fragment as the surfaces move.

This is the most common type of wear andthe most difficult one to prevent because

even the most highly polished surfacesexhibit asperities.




Porcelain is a good example of the fact that

intraoral wear must always be considered to

be a coupled phenomenon; not just theinvolved material should be examined for

wear, but also the opposing dentition orrestorative material.








describes materials that exhibit

characteristics of both a viscous liquid and an

elastic solid. These types of materials haveproperties that vary with rate of loading.

Viscoelastic materials, like alginates, showlittle permanent deformation when loaded

quickly but exhibit a great deal of permanentdeformation if loaded slowly.




1. Newtonian (ideal) Liquids are liquids with a

constant viscosity regardless of shear rate.

Examples include water and newly mixed zincphosphat cement




2. Plastic Liquids are rigid until a certain yield

stress is applied to them.

The application of this yield stress to causeflow in a plastic material is called the

Bingham characteristic. A non-dentalexample is catsup.




3. Pseudoplastic (shear-thinning) Liquids show

a decrease in viscosity with an increase in

shear rate. Examples include thepolycarboxylate cements and non-water

mixed glass ionomer cements.




4. Dilatant (shear-thickening) Liquids show an

increase in viscosity with an increase in shear

rate. Traditional (conventional) resincomposites are a good example.




5. Thixotropic Liquids are liquids whose viscosity

depends upon their previous shear history. In

other words, if you shear them, their viscositywill decrease; if you then allow them to

remain undisturbed, their viscosity willincrease to pre stressed levels. Examples

include prophy gels pastes and topicalfluoride gels. The opposite of thixotropic is

rheopexic.








The reason many highly viscous cements

(e.g., polycarboxylates, non-water mixed

glass-ionomer cements) form thin filmthickness layers is because they are pseudo

plastic.




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Basic Property Onby Abbbe