chapter 1 materials science

8/2/2019 Chapter 1 Materials Science

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CHAPTER I

M A T E R I A L S S C I E N C EOur dignity is not what we do, butwhat we understand.

George Santayana

The ideal tooth-colored restorative (ie, directrestorative) would have the capacity to adhere to enamel and dentin, maintain a smooth surface, maintain desired color, resist water (insolubility), resist wear, resist fracture, resemble tooth structure in stiffness, react to temperature change like other toothstructures, resist leakage, maintain marginal integrity, not irritate pulpal tissues, inhibit caries, place easily, and repair easily.

No available restorative can meet all of theserequirements; however, by matching the charac-teristics of various materials to the needs of a spe-cific tooth, it is possible to approach this ideal foreach restoration undertaken.Clinicians who un derstand the chemical natureand physical properties of a material are morelikely than those who do not to make good deci-sions concerning its use and app lication. This book

begins with a review of basic concepts in materialand restorative science to provide a foundation forimproved understanding of dental m aterials.C H E M IS T R Y O F T O O T H - C O L O R E DRESTORATIVESA direct restorative transforms in the mouth froma fluid or putty-like material into a tooth-like solid.There are three common mechanisms by which

direct tooth-colored restoratives undergo transfor-ma tion: acidbase rea ctions, polym erization reac-tions, and precipitation reactions.'"'Acid-base reactionsMany d ental m aterials und ergo an acid-ba se reac-tion when they set. The resulting material is chem-ically referred to as a salt. Examples of this reactionin everyday life are commonplace (eg, concrete).The first tooth-colored restoratives to undergo thistype of setting were the silicate cements. Allacid-base reactions are similar. Acid molecules (aconfiguration of atoms) have a shortage of elec-trons, and base molecules have an excess of elec-trons. When acids and bases react, they transferelectrons between them, creating a more stablecompound. This exchange of electrons results inheat generation during the setting reaction. Sincethe ions required to initiate a setting reaction existonly in w ater, all acid-base materials contain water.Once an acid-base reaction is complete, however,the resulting salt typically does not include water.

There are two types of acid-base reactions:those involving inorganic components and thoseinvolving organic components. Examples of aninorganic acid-base reaction are zinc phosphatecement and silicate cement. Examples of anorganic acid-base reaction are glass-ionomercement and polycarboxylate cement. Acid-basereactions that contain inorganic components aregenerally stable outside the mouth in the absenceof moisture, whereas those containing organiccomponents are generally not stable. For example,zinc phosphate cement is stable in a dry environ-ment whereas glass-ionomer cement is not.Polymerization reactions1 he most com mon type of setting reaction for directtooth-colored restoratives involves the formation of


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Tooth-Colored Restoratives

resin polymers. A polymer is a molecule or g roup ofmolecules m ade u p of repeating single units that arecovalently bonded. The individual units of a poly-mer are referred to as monomers. "Poly" meansmany, and "mono" means one. Hence, polymethylmethacrylate is a polymer made up of multiplemethacrylate monomers. Polymers form from oneor m ore types of monomer. Th e process of convert-ing mono mers into a polymer is called polymeriza-tion . If two or more different monom ers are poly-merized, the resulting material is a copolymer(Figure l-l). Combining different monomers cre-ates materials with unique properties that reflect thecharacteristics of the individual m onom ers.

All mono mers have at least one carbon -carbondouble bond (C^C) that becomes a s inglecarbon-carbon bond when they join to form apolymer. Monomers with two or more carbon-carbon dou ble bonds can transform to cross-linkedpolymers (Figure 1-2, A). Cross-linking usuallyresults in improved physical properties in dentalmaterials. Most dental restorative polymers containcross-linked copolymer components for durability(Figure 1-2, E).An initiation system starts the transformation ofmonomers into polymers and copolymers. The ini-tiation reaction creates a molecule wirh a free radi-cal (an unpaired electron). This unpaired electronmakes the radical highly reactive. When a free rad-ical collides with a m onomer's dou ble b ond, it pairswith one of the electrons of the double bond, leav-ing the other member of the pair free. Thus, themonomer itself becomes a free radical that can reactwith another monomer. Ideally, this process con-tinues until all of the monomers become polymer-

ized. Th e degree to which monom ers convert into apolymer is referred to as the degree of conversion.Th e most common polymers used in the dental fieldcontain methacrylates. Methacrylates with twodouble bonds are called dimethacrylates. The advan-

tage of a dimethacrylate is that it allows for cross-linking, as illustrated in Figure 1-2. Most resinsused in dentistry have a conversion of about 40 to60% when polymerized in the mouth and over60% to nearly 100% when cured in a laboratory.Polymerization shrinkage patternsAll polymerizing resins shrink during curing. C om -posite resin shrinkage is about 2 to 5% by volume,depending on the filler loading (filler particles donot shrink) and the percentage of conversion. Theless thefiller oading and th e higher the rate of con-version, the greater the shrinkage. In the laboratory,or when cured outside the m outh , chemically curedmaterials shrink toward their center, because theinitiators are mixed throughout the material (Fig-ure 1-3). Lighr-cured materials, on the other hand,shrink toward the source of initiation, which is thecuring light (Figure 1-4).

In clinical use, shrinkage patterns are muchmore complex. Active bonding agents placed on atooth usually start the initiation process when thecomposite contacts them. Some researchers alsobelieve a tooth's inherent heat causes curing tooccur along the tooth interface sooner than it doesalong cooler portions of composite away fromtooth structure. Thu s, compo sites generally start toshrink toward cured bonding agents, since thepolymerization process has already started there.When a composite is placed against a light-curedbonding agent and then light-cured, the compos-ite is initiated from two sides. However, the rate ofpolymerization is not equal on the two sides in tha tthe composite facing the light polymerizes morequickly and has larger effect on the direction ofcomposite shrinkage (see Figure 14).Precipitation reactionsPrecipitation reactions involve the loss of a solvent.In this case, the liquid materials comm only containresins diluted in an organic solvent. WTien exposed

BFigure 1-1. A, A linear polymer is made up of multiple units of one type ot monomer. B, A copolymer includes more than onetype of monomer.


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Materials Science

A BFigure 12. A, Cross-Unking of polymer. M onomers wich two carbon-carbo n double bonds make cross-linking possible, fi. Poly-mers with cross-linked components have better durability.

to air, the solvent evaporates and concentrates theresin into a solid. Examples ot these reactions are\vall pain t,fingernailpolish, dental varnish, etc. Thesetting reaction is referred to as drying. Presently,iew dental restoratives set through a precipitationreaction. Precipitation is used, however, in settingbond ing agents, cavity varnishes, and some surfacecoat ings. With bonding agents , the solventenhances the agent's penetration o fthe tooth . Evap-oration oi the solvent concentrates the monomerprior to polymerization and improves durability.Most materials that set by drying contain alarge molecule (resin) that is suspended in a volatilesolvent (thinner). During drying, the loss of thesolvent brings the component of greater molecularweight out of the solution and turns it into a solid(Figure 1-5). Precipitation materials used in themouth must be insoluble in water (at least theresulting resin) to avoid reversal of this process in

the oral fluids. Precipitation reactions result in theleast durable restoratives and are recommendedonly for temporary treatment of tooth structure.PHYSICAL PROPERTIES ANDDESTRUCTIVE FORCESKnowledge ofthe physical properties of restorativematerials can help predict their susceptibility tobreakage under occlusal function.Compressive strengthCompressive strength is a measure ofthe amountof force a material can support in a single impactbefore breaking (Figure 1-6). This physical prop-erty is one of the easiest to measure and is oftencited in advertisements for dental materials. Com-pressive strength is such a commonly used physi-cal property that it has acquired a greaterrespectability in the profession than is appropriate

1 - 3 . Autocured resin, polymerization shrinkage pa ttern. The left sphere represents the volume of an autocurcd materialprior to polymerization. The right sphere reptesents the volume and shrinkage pattern of the matetial after polymerization.


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4 Tooth-C olored Restoratives

Figure 1-4. The leh sphere represents the volume of light-cured composite prior to polymerization. The right sphere representsthe volume and shrinkage pattern ofthe material after polymerization when not attached to any surface. Note how the materialmoves toward the light of the curing tip at the right.

to its actual clinical relevance. There is no directcorrelation between compressive strength and clin-ical performance. However, compressive strengthdoes measure strength, and it gives an indication ofa material's resistance to creep and plasticity. Inconjunction with a sound understanding of theclinical purpose of a dental material, measurementof compressive strength is sometimes used as ascreening test in the development of new materials.Tensile strengthTensile strength is the amount of force that can beused to stretch a material in a single impact priorto breaking (Figure 1-7). This physical property ismore difficult to measure than compressivestrength. The tolerance of the measuring device iscritical. Materials must be pulled at an exact180-degree angle from each other to eliminate theinfluence of shear forces. The clinical relevance oftensile strength is limited.

Resin in Solvent Resin PrecipitantFigure 1-5. Precipitatio n reac tion. A volume of solventevaporates and leaves a solid behind.

Diametrical tensile strengthThis is a theoretical tensile strength measurementthat is calculated by measuring the compressivestrength of a disc of material (Figure 1-8). T his testis easier to perform and is more con sistent than thenormal tensile strength test.Shear strengthShear strength is the maximum shear stress that amaterial can absorb in one impact before failure

Figure 1-6. Compressive strength. The am ount of force amaterial can support in a single impact.


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Materials Scietice

Figure 1-7. Tensile strength. The a mo unt o f stretching forcea material can withstand.

(Figure 1-9). T he punch test is a comm on methodof measuring shear strength. In this test, shearstrength is calculated from the compressive forceapplied, the diameter ofthe punch, and the thick-

ness ofthe material tested. Shear strength has beenused to measure the bond strength between differ-ent materials. In this test, a disc of material isbonded to a surface, a chisel instrument is placedabove the disc, or a loop of wire is attached. Theforce required to shear the disc from the bondedsurface is the bond strength ofthe tested adhesive(Figure 1-10). Th is test is easier to perform than atensile test on two bonded materials.

Unfortunately, the punch test has no direct cor-relation to the clinical performance of a m aterial.Further, there is little agreement in the researchcomm unity on how toconduc t this test, althoughstandards are being developed. Shear strength datafrom different testing laboratories show extremelylarge variations are possible even when testing thesame materials with the same instrum ents.StiffnessStiffness is also called the modulus of elasticity,elastic modulus, or Young's modulus. Stiffnessdetermines resistance to flexure and deformation,or the amount of bending when loaded (Figure1-11). Th e m easure of stiffness has been related topredicting the potential results of cyclic loadingoutside the orai environment. Stiffness can bemeasured by placing a force on a material andmeasuring the deformation. It can be calculated ina nondestructive way by measuring the harm onicsof a material when vibrated.

Figure 1-8. The diametrical tensile strength test is used tocalculate tensile strength. Figure 1-9. Shear strength. The maximum shear stress amaterial can absorb in one impact.


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M aterials Science 7

materialsmetals, polymers, and cementsseemsTO fail by mechanisms specific to that group, mak-ing generalization difficult. The phenomenon iscalled fatigue because, unde r certain loading co ndi-tions, a com pon ent appears to tire, losing strengthover a period of time in service. Two types of load-

ing condition s can cause these sym ptom s: (1) cyclicloading and (2) steady loading. Both are more severein the presence of a chemically active agent. Theprogressive loss of strength that accompanies cyclicloading is attri buta ble to the gradual spread ofcracks. Cyclic loading is illustrated in Figure 1-1 3.

Figure 1-1.3. Th e progressive and cumulative damag e that occurs durin g cyclic loading. Th e restoration eventually fails.


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o Tooth-Colored Restoratives

The mouth is unique in that it combines cyclicloading with a chemically active environment.The most common chemically active agent indent i s t ry is sal iva, which contains varyingamounts of water and other components. Salivavaries from patient to patien t, and individual dif-ferences can explain some of che atypical resultsseen in some mouths. Apatient's diet may alsocontain substances that are chemically reactive toteeth and restorations.

Cyclic loading might appear less harmful torestorative materials than steady loading, becausethe average deflection (over the cyclic period) is lessthan the steady deflection. In practice, it is the cycli-cally loaded materials that break first from occlusalforces; statically loaded materials, such as thosemaintaining, for example, resting contact points,last considerably longer. Since the growth of a crackrequires plastic deform ation, cracking occurs m orerapidly in ductile materials, such as plastics. Stifferrestoratives are more resistant to fatigue, becausethey are under less strain when loaded.

The clinical effects of fatigue are important inall dental restoratives, because the force needed tocause failure decreases over time. The rate of weak-ening is thought to be related to the rate of crackpropagation in the material in response tostressabsorption over time. Fatigue explains why manydental restorations provide excellent service for anumb er of years and then suddenly break u nder arelatively minor load. Figure 1-14 illustrates therelation between stress and time in restorationbreakage, demonstrating the fatigue phenom enon.

tCOc di_

(A

Time ^Figure 1-14. The weakening of a material over time as aresult of cyclic loading. Clinically, this means th at materialsbecome more brittle and less durable over time.

Most restoration fractures occur in the marginalridge areas. These areas are the least supported andabsorb the most static and cyclic stress and strain;thus, they are the most inclined to fracture.Fracture toughnessFracture toughness is an important measure of amaterial's susceptibility to fatigue. Stress is theamount of force placed on an object, and strain isthe amount of deformation that occurs under thatstress. All materials underg o strain (such as a bend-ing force) when stressed. Figure 1-15 illustrates

Porcelains

Breakagepoint

t

Strain

Resins

Breakagepoint \

Strain

CO

Metals

- M

fBreakagepointStrain

Figure 1-15. Th e stress-strain curves of a material show the amount of flexure itproduces under agiven stress. At a criticalstress, the m aterial fractures, because its maximum am ount of deformation (elastic limit) has been exceeded.


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Materials Science

how different materials react to stress up to theirbreaking point. As shown, porcelain bends little,even w hen placed under considerable stress. Resins(plastics) are different in that they bend a lot evenunder tow stress. Metals can tolerate considerablestress and bending.

Fracture toughness is defined as the area underche curve when viewing a plot of the stress andstrain relation of a restorative material. It is a meas-ure ofthe total amo unt of stress a material can takebefore failing (Figure 1-16). It is related to theenergy needed for flexure to a breaking point,which is calledflexuralstrength. Flexural strength,bending strength, and fracture resistance arc termsused interchangeably. Owing to its ease of meas-urement, flexural strength is the physical propertymost commonly used to indicate the fracturetoughness of a material. However, many re-iearchers believe that fracture toughness is the bestphysical property to measure to predict the wearand fracture resistance of a restorative.

The graphs in Figure 116 indicate the differ-ences in fracture toughness among porcelains,resins, and metals. The clinical performance ofmetal restorations bears out their fracture resist-ance. Porcelain and resins used alone have a longhistory of breakage under stress; to extend theirlongevity, they are often supported with metal. Theway in which these materials are used toge ther canprofoundly affect the physical properties of theresulting restoration.

Surface hardnessSurface hardness is the resistance of a material todeformation from compressive contact with apredetermined object (Figure 1-17). There aremany ways to m easure hardness, depending on theshape ofthe object used to deform the surface ofthe material being tested.

Brinell hardness, one ofthe oldest hardness testmethods used in dentistry, measures resistance topenetration by a small steel ball, 1.6 mm (1/16inch) in diameter, when subjected to a force of27.7 po unds. The resulting number, known as theBrinell hardness number (BHN), is calculated bya formula that uses load, area, and indentation asvariables.Knoop hardness uses a specially made diamondindenting tool. The Knoop hardness number(KHN) is also calculated using the variables ofload, area, and indentation. The units of Knoophardness are measured in kilograms per squaremillimeter (Kg/mm-).Vickers hardness uses a 136-degree diamondpyramid; it is used in applied loads. It is commonlyused in dentistry and is about 2.45 N for enamel.Rockwell hardness is a rapid testing method inwhich an instrument applies a load to a materialand a dial quickly calculates a hardness number.

This method is commonly used with plastics, sincethe device can be kept on the material for varyingamounts of time to measure percent of recovery.

Porcelains

Fracturetoughness

tC O

C O

Resins

Fracturetoughnesst

MetalsFracturetoughness

Strain StrainFigure 1-16. Fracture toughness is related to the area under the stress-strain curve. Note that m etals are far superior ro porce-lains and resins.


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1 0 Tooth-C olored Restoratives

Figure 1-17. Surface hardness. Th e resistance of a materialto deformation during compression.

AbrasionAbrasion, or wear, is the progressive loss of materialfrom the surface because of relative motion. Wearis related to a material's coefficient of friction. Itexplains why m etals perform so well in high-stressareas whereas heterogen eous glass-containing plas-t ics and ionomers wear more rapidly.

One result of wear in a heterogeneous mater ialis roughness at the microscopic level. Because ofroughness, contact between the surfaces of twoobjects can result in frictional forces that micro-scopically fracture off pieces from the surface,resulting in material loss (Figure 1-18).

Th ere are four types of wear: adhesive wear, lossof material owing to contact between filler shearingpoints; abrasive wear, deformation of a softer ma-ter ial by a harder one; fat igue wear , breakingaway of material as a result of cyclic loading; andcorrosive wear, removal or chemical softening ofa surface.ErosionErosion is the loss of substance from a material bychemical means. In dentistry, acid from foods andgastr ic f luids (eg, bulimia) are the most commonsources of erosion (Figure 1-19).Roughness or smoothnessRoughness refers to the surface texture of a material.There are two types: the smoothness resulting froma finishing process, referred to as applied or acquiredsmoothness, and the smoothness of an unpolishedmaterial, referred to as inherent smoothness. Inher-ent smoothness depends on the filler particle size ofthe material. A finished material will always returnto its inherent smoothness. For example, if a ma-terial has filler particles of 1 to 10 \im, it will alwaysreturn to a smoothness of 10 |4.m; therefore, if it ispolished to 5 M-m, its roughness will double overtime (Figure 1-20). The clinical significance ofroughness is discussed in greater detail in C hap ter 9.Smoothness or roughness is measured in microns orin grit. A smoothness of less than 1 \im or a gritgreater than 600 is considered as smooth as enamel.

Figure 1-1 8. Wear and abrasion . Th e progressive loss ofmaterial from its surface as a result of relative motion. Figure 1-19. Erosion. The loss of substance from a materialby chemical means.


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M aterials Science 11

After wear

Appliedsmoothness InherentroughnessFigure 1-20. Roughnes s or smo othness is measured in gritand microns. Many materials can be poHshed to a high his-ter, referred to as applied polish or smoothness. After aperiod, each material reverts to an inherent polish based onthe size of its heterogeneous components.

Cutting instruments are commonly measured ingrit whereas polishing instruments are frequentlymeasured in microns.As a general rule for d ental uses, a roughness ofless than 300 grit is coarse, 300 to 600 grit is inter-mediate, and 600 to 1200 grit is smooth en ough fora final finish equal to or better than enamel.

Coefficient of thermal expansionThe coefficient of thermal expansion refers to theamount of expansion and contraction a material

undergoes in relation to temperature (Figure121). A tooth expands and contracts with thermalchanges. A high coefficient of thermal expansionindicates a relatively high degree of dimensionalchange in reaction to temperature (also referred toas a high coefficient). Studies show that there is adirect relation between marginal leakage and ther-mal changes. The greater the difference in thethermal coefficient between the tooth structureand the restorative, the greater the leakage.Water sorptionWater sorption is a critical physical property fordirect restoratives because increased absorption ofwater increases the volum e of a restorative (Figure1-22). This property has great clinical significancewhen polymers are used for buildups since,through water sorption, a polymer can enlarge inthe lapse time between impression-taking andcementation appointm ents. More important, wateris a softener of plastics and increases the deteriora-tion ofthe resin matrix. In addition, water sorptionusually decreases color stability sitice water-solublestains can penetrate th e restoration.Fluoride releaseFluoride release is an im port ant feature of a dentalrestorative. Fluoride release should not be confusedwith fluoride content. Many materials containfluoride but do not release it. The minimumam oun t of fluoride release necessary to effectivelyinhibit recurrent decay is unknow n, bu t it is pro b-ably over 20 ppm per day.

ExpansionContraction

Norm al Hot ColdFigure 1-21 . Coefficient of thermal expansion. Th e amou nt of expansion and contraction a material dem onstrates in relation totemperature.


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Dry In waterFigure 1-22. Wacer sorprion. The volum e of water a materialcan absorb.

The amount and duration of fluoride releasevaries greatly among dental materials. Mostfluoride-releasing materials that have demonstratedclinical effectiveness share common features: (1)the materials contain water, which is necessary to

transport the fluoride ions out ofthe material; (2)the fluoride is retained in the material in an inor-ganic state as a soluble salt; and (3) the materialsproduce some acidbase reactions that initiate therelease of the inorganic fluoride in an ionic state.Alternatively, in some cases, a rare earth, such asytterbium trifluoride (YbF,), is added to a com-posite resin as filler, resulting in short-term, low-level fluoride release. The caries-inhibiting effec-tiveness of this type of com posite material is not asgreat as that of aqueous ion-con taining systems.Peel strengthIn the peel strength test, all of the forces are placedat the end ofthe bonded specimen rather than inthe middle, such that the force is exerted on onearea at a time. By contrast, when testing compres-sive strength, the forces are equally spread overevery molecule used for attachment. Over time,peel stress separates substances with a relativelysmall am oun t of force ( 10% o r less of the required

Tensile Peeling Peel Failure

Bonded Materials(forces distributed) Peel Process{smali force over time) Material Separation(cohesive faliure)Figure 1-2 3. Peel versus tensile. With tensile force, che direction of force is always 90 degrees to the bond ed interface. Thisdistributes the stress over the entire adhesive surface, With peel force, the force is placed on one small area, which results in amore rapid failure of a portion of the bond. Once the bond is broken, the force moves along the surface and breaks anothersmall portion ofthe bond, and so on, until the entire system fails.


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M aterials Science 1 3

tensile force with adhesives), especially in the pres-ence of cyclic loading (Figure 1-23). Peel strengthfailure can be greatly reduced or eliminated by theuse of resistance form in a restoration, which pre-vents stress on the interface. Clinically, peel strengthhas a much more significant effect than tensilestrength.Peel energy: Boeing testPeel energy, otherwise known as the energy ofadherence, the wedge test, or the Boeing test(because of its use in testing aircraft structures) isthe force requ ired to sustain a peel motion {Figure124). This physical property is often used tomeasure the failure rate of air foils in aircraftdesigns. Although not commonly used in den-tistry, this physical property could provide mean-ingful information about an adhesive interface,because it contributes to cyclic fatigue.Contact angleContact angle is the measure of how well afluidwetsa solid. A smaller co ntact angle indicates that a liq-uid has good ability to penetrate the micromechan-ical porosities of a surface. The angle between anadhesive and a bondable substance is of enormoussignificance in determining micromechanical reten-tion and the potential for chemical adhesion. Themeasurement of contact angle and a demonstrationof its applied effects are shown in Figure 1-25.COHESIVE FORCESThe bonding forces that hold materials togetherare called cohesive forces. Generally, the atoms in

these materials have positive or negative chargesand are referred to as ions. A positive ion is shortone or more electrons, and a negative ion has oneor more extra electrons. Bonds are formed whenatoms com bine to reduce these charges.Primary bondsPrimary bonds are chemical in nature and areformed through the attraction of positive and neg-ative ions. Th ere are three types of primary b onds:1. Ionic bond s occur when atom s transfer electrons(eg, sodium chloride, Na*Ch). Materials thatresult from acid-base reactions are called salts(Figure 1-26).2. Covalent bonds occur when two atoms share elec-

trons (eg, polymers). These bonds often formbetween the carbon and hydrogen atoms found inmost organic materials (Figure 1-27).3. Metallic bonds occur when many atoms shareavailable electrons. These primary bonds areresponsible for the stre ngth, elasticity, and frac-ture toughness of the crystalline solids calledmetals (Figure 128).Secondary bondsSecondary bonds involve complex physical inter-actions between various kinds of molecules. Com-mon secondary bonds are van der Waals forces anddipole forces. These forces hold liquids and non-rigid solids (plastics) together and include attrac-tions between polar molecules (fluctuatingdipoles), hydrogen bonds (permanent dipoles),and other secondary molecular attractions. The

Adhesive

Figure 1-24. Peel energy. The energy of adherence (also called the wedge test and Boeing test) is the peel energy required tosustain a peel reaction over time.


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Contact angle Effect on resin penetrat ionMaximum resin penetration

Good wetting

: 90^ Minimum resin penetration

Minimal wetting

/ \

>9 0 No resin penetration

No wettingB

\

Figure 1-25. Effect of wetting on surface adhesion. A, The relation berween the contact angle and the wetting of a substance.B, Improved adhesive pene tration on a porous dencin surface as the contact angle of the adhesive decreases.

most common example of this bonding is water.Secondary bonds are responsible for viscosity andresistance to deformation (Figure 1-29).W H Y ME A S U R E P H Y S IC A LPROPERTIES?Physical prop erties are measures of a material. Th eseproperties have great significance in dental researchbecause they provide the information needed toassess the characteristics of and improvements inmaterials under development. The physical proper-ties of a tooth set the standard for materials attachedto a t ooth . The ory suggests that if a restorative canbe made to hold properties similar to those of nat-ural tooth structure, it should perform as well as anoriginal too th. In the field of civil engineering, cer-tainly, new designs are built and tested under con-

ditions that exactly m atch or well approximate thoseunder wh ich they are to perform. Unfortunately, indentistry, this type of testing is seldom d one. Few oftoday's newer materials have undergone long-termclinical testing prior to marketing. This makes thedental practitioner who purchases a new material.

Figure 1-26. Tight packing of molecules in acid-base reac-tions that form ionic bonds.


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Ma terials Science 15

Figure 1-27. Th e carbo n-carb on bond ing links formed inpolymer reactions create covalent bonds. Figure 1-29. Strong attraction between molecules of nega-tive and positive charge creates the secondary' bonds that holdtogether liquids including water.

and the patients who have the material placed intheir mouths, the actual test site.To the d entist, success is measured not by phys-

ical properties but by clinical performance. Amaterial that displays some good physical proper-ties during development is not necessarily a ma-terial that will perform well in the mouth. Theindividual properties that scientists measure usu-ally are not the cause of restoration failure in themo uth. T hus, the only real assurance a dentist hasof a material's safety and reliability is the test oftime. Generally, if a dental material has been on themarket unchanged for over 5 years, it has provenitself reasonably safe and reliable. In the absence ofmanu facturer-funded clinical testing, therefore, 5-year clinical reports from the field are a good wayof assessing which materials ate proving their value.Although commonly measured physical prop-erties are a poor predicto r of clinical success, theseproperties are useful in comparing materials toone another. Success in clinical dentistry is basedon a chain of events, from the material and itsplacement to the host response. Physical propertiesallow a dentist to measure a single link in this com-plex chain. T he measures of physical p roperties areuseful only if they m easure characteristics that aresignificant to the success of a single restoration in

Figure 1-28. The compacting and sharing of electroncharges that form in metals.

a single tooth in an individual. These variables areenormous, but useful trends do exist.

UNITS AN D CONVERSIONSBelow is a partial listing of the units of measureused to determine the physical properties, state,temperature, and size of dental systems. Eachdescription includes a small amo unt of history onthe unit and its most common conversions."*Weight unitsWeight units are the gravitational forces applied byan object irrespective ofthe area ofthe object.Grain . A unit of weight based on the weight of agrain of wheat taken as an average ofthe weight ofgrains from the middle of the ear and equal to0.0648 grams.Carat. A unit ot weight based on che weight offour grains. In the United States before 1913, onecarat was equal to 205.3 milligrams (mg). After1913, the international standard for the carat wasadopted, which was then standardized at 200 mg.This unit is mainly used to measure the weight ofprecious stones and pearls.Pound (Ib). A unit of weight equal to 12 troyounces {historically 1 pound = 5760 grains =0.3732 kilograms). Presently, English-speakingpeople use the avoirdupois pound, which is nowthe most commonly used: 1 pound equals 7000grains or 0.4536 kilograms.Ou nce (oz). A unit of weight that represents 1/16ofthe present-day pound (technically referred to asthe avoirdupois pound).Gram (g). A unit of weight based on the weight of1 cubic centimeter (cm^) of watet at its maximum


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density. This u nit is the standard for the m etric sys-tem of weights.ConversionsOne kilogram (kg) equals 1000 grams; 1milligram(mg) equals one tho usand th of a gram; 1 nanogram(ng) equals one billionth of a gram.Force unitsNewton (N). A force scale named after Britishmathem atician Sir Issac Newton in 1727. It is theun it of force req uired to give a free mass of 1 kilo-gram an acceleration of 1 meter per second persecond {second"). On a larger scale, 1 meganew-ton (MN) is 1 million (10*') newtons. On a smallerscale, 1 newton equals 1 million (10^) dynes.

The normal biting force generated on the firstmolars in a hum an mou th ranges from 400 to 800newtons, or 90 to 200 pounds (mean, 550 N or1251b).Dyne. The unit of force required to give a freemass of 1 gram an acceleration of 1 centimeter persecond per second (second-).ConversionsOne newton equals 0.223 pounds and 1 poundequals 4.44 newtons; 1 newton equals 1 milliondynes.Length unitsFoot (ft). A un it of length based on the length ofa British king's foot. One foot is equal to 0.3048meters, or 3.28 feet = 1 m eter.Inch (in). A unit of length equal to 1/12 of a foot.One inch is equal to 2.54 centimeters.Meter (m). A unit of length equal to the distancebetween two lines on a platinum-iridium bar keptat the International Bureau of Weights and Mea-sures near Paris. One meter is equal to 39.37 inchesand is equal to 1,650,763.73 wavelengths of theorange-red light of the excited krypton of massnumber 86. Use of this constant allows this unit oflength to be reproduced anywhere.ConversionsOne centimeter (cm) equals 1/100 of a meter; onemillimeter (mm) equals 1/1000 of a meter; onemicrometer (jim) equals one millionth of a meter;one nanom eter (nm) equals one billionth of a meter.

Stress unitsStress is the unit force applied per unit area. Stressunits are comm only used to m easure bond strengthsassociated with dental m aterials. Currently, the pre-ferred unit in dental science is the megapascal.Megapascal (MPa). A force scale named afterFrench scientist Blaise Pascal in 1662. It is a meas-ure of force over area. The megapascal is equal to1 meganewton (MN) per meter per meter (m^^),which is equivalent to 1 N/mm^.ConversionsOn e MPa equals 10.196 kg/cm^; 1 psi equals0.07032 kg/cm-; 1 kg/cm^ equals 14.22 psi; 1 MPaequals 145 psi; 1 psi equals 0.006 89 MP a;1 kg/cm^ equals 0.098077 MPa.Thermometric unitsFahrenheit {F). A thermometric scale namedafter German physicist Gabriel D. Fahrenheit in1736. In this thermometric scale, the boilingpoint of water is 212F, whereas the freezingpoint of water is 32F, under standard atmos-pheric pressure. The zero point of the Fahrenheitscale approximates the temperature produced bymixing equal quantities by weight of snow andcommon salt.Centigrade or Celsius {C). A thermometric scaleon which the interval between two standard points,the freezing and the boiling point of water, isdivided into 100 degrees, 0 representing the freez-ing point and 100 the boiling point.Conversionsx 'G equals xF - 32 x 5/9.Kelvin (K). A thermometric scale named afterBritish physicist William Lord Kelvin in 1907. Inthis thermometric scale, centigrade degrees arerelated to absolute zero (defined as 0K). 1 Kelvinis the equivalent of -2 76 .l6 C .

The Kelvin thermometric scale is used in pho-tography to measure the tem perature of light sourcesused to illuminate objects. Golor-corrected lighting,which is equivalent to average daylight, is 5400 K.ConversionsxK equals xC + 276.16 and x C equals xF - 32x 5 / 9 .


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M aterials Science I /

REFERENCES 3. O'Brien WJ- Denta l materials: properties and selec-tion. Chicago: Quintessence. 1989.1. Phillips RW. Skinners' science of dental materials.9da ed. Philadelphia: WB Saunders, 1991. ^- ^"^^ ^^ (after Webster N [1758-1843]). Webster'sthird new international dictionary of the English lan-2. Craig RG. Restorative dental materials. 8th ed. St. guage (unabridg ed). Springfield, MA: G& C Merriam,Louis: CV Mosby, 1989. 1902 to present.


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chapter 1 materials science

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