Сraig. Dental Materials

double bonds or from saturation by water. As discussed later, the value of the coefficient of thermal expansion of a polymer changes at T'.

HEAT OF FUSION

The heat of fusion, L, is the heat in calories, or joules, J , required to convert 1 g of a material from solid to liquid state at the melting temperature. The equation for the calculation of heat of fusion is L = Q/m, where Q is the total heat absorbed and m is the mass of the substance melted. Therefore in practical applications it is apparent that the larger the mass of material being melted, the more heat required to change the total mass to liquid. The heat of fusion is closely related to the melting or freezing point of the substance, because, when the change in state occurs, it is always necessary to apply additional heat to the mass to cause liquefaction, and as long as the mass remains molten, the heat of fusion is retained by the liquid. When the mass is frozen, or solidified, the heat that was retained in the liquid state is liberated. The difference in energy content is necessary to maintain the kinetic molecular motion, which is characteristic of the liquid state.

The values for heat of fusion of some common substances (given in round numbers) are listed in Table 3-3. It may be seen that the values for heat of fusion of gold and the metals used for dental gold alloys (silver and copper) are below those of many other metals and compounds. This is true also for the specific heat of gold and its alloys.

THERMAL CONDUCTIVITY

The thermal conductivity, K , of a substance is the quantity of heat in calories, or joules, per second passing through a body 1 cm thick with a cross section of 1cm2when the temperature difference is lo C. The units are cal/sec/cm2/(" C/cm). The conductivity of a material changes slightly as the surrounding temperature is altered, but generally the difference resulting from temperature changes is much less than the difference that exists between different types of materials.

Common experience indicates that metals are

For penodic updates,

better heat conductors than nonmetals. Several important applications of thermal conductivity exist in dental materials. For example, a large gold or amalgam filling or crown in proximity to the pulp may cause the patient considerable discomfort when hot or cold foods produce temperature changes; this effect is mitigated when adequate tooth tissue remains or nonmetallic substances are placed between the tooth and filling for insulation. Such filling materials as cements are relatively poor conductors and insulate the pulp area.

The difference in thermal conductivity of denture base materials likewise may cause differences in soft-tissue response. A metal base, a good conductor, causes a prompt tissue response, as shown in Fig. 3-11,whereas an acrylic denture base causes a more delayed response to thermal changes. Dental literature indicates that a good thermal conductor is preferred for denture bases to maintain good health in the supporting tissues by having the heat readily conducted to and from the tissue by the denture base. This concept has not been established on the basis of experimental data.

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(Adapted from Wehner PJ: Heat transfer properties of denture base materials, master's thesis,University of Michigan School of Dentistry, 1959.)

A better understanding of the conductivities of various restorative materials is desirable to develop an appropriate degree of insulation for the pulp tissue, comparable with that in the natural tooth, and to produce normal thermal stimulation in the supporting soft tissue under complete dentures. The conductivity of certain dental materials is listed in Table 3-4. Nonmetallic materials appear to be less effective as conductors than metals are, for which reason they serve as good insulators. Dental cements have a thermal conductivity similar to those of dentin and enamel. Note that the thickness of a cement base and its thermal conductivity are important in reducing the thermal transfer to the pulp, and remember that the temperature difference across an insulator depends on the extent of the heating or cooling period and the magnitude of the temperature difference.

SPECIFIC HEAT

The specific heat, Cp, of a substance is the quantity of heat needed to raise the temperature of one gram of the substance 1" C. Water is usually chosen as the standard substance and 1 g as the standard mass. The heat required to raise the temperature of 1 g of water from 15" to 16" C is 1 cal, which is used as the basis for the definition of the heat unit. Most substances are more readily heated, gram for gram, than water.

Thermal Conductivity

METALS

Silver

Copper

Gold

Platinum

Dental amalgam

Mercury

NONMETALS

Gypsum

Zinc phosphate cement

Resin composite Porcelain Enamel

Dentin Zinc oxide-

eugenol cement Acrylic resin Beeswax

Obviously the total heat required to raise the temperature of a substance loC depends on the total mass and the specific heat. For example, 100 g of water requires more calories than 50 g of water to raise the temperature 1" C. Likewise,

because of the difference in specific heat of water and alcohol, 100 g of water requires more heat than 100 g of alcohol to raise the temperature the same amount. In general, the specific heat of liquids is higher than those of solids. Some metals have specific heat values of less than 10% that of water.

During the melting and casting process, the specific heat of the metal or alloy is important because of the total amount of heat that must be applied to the mass to raise the temperature to the melting point. Fortunately, the specific heat of gold and the metals used in gold alloys is low, so prolonged heating is unnecessary. The specific heat of both enamel and dentin has been found to be higher than that of metals used for fillings, as shown in Table 3-5.

THERMAL DlFFUSlVlTY

The thermal diffusivity, A, is a measure of transient heat-flow and is defined as the thermal

The units of thermal diffusivity are mm2/sec. The thermal diffusivity describes the rate at

which a body with a nonuniform temperature approaches equilibrium. For a gold inlay or crown or a dental amalgam, the low specific heat combined with the high thermal conductivity creates a thermal shock more readily than normal tooth structure does. Values of thermal diffusivity of some materials are listed in Table 3-6. These values may vary somewhat with composition of the particular restorative material. For example, the thermal diffusivity of a zinc polyacrylate cement increases from 0.14 to 0.51 mm2/sec as the powder/liquid ratio (by weight) increases from 0.5 to 5.0.

As mentioned in the discussion of thermal conductivity, thickness of the material is important. A parameter governing lining efficiency (Zl

COEFFICIENT OF THERMAL EXPANSION

The change in length (l,,,, - loriginal)per unit length of a material for a 1" C change in temperature is called the linear coefficient of thermal expansion, a, and is calculated as follows:

The units are represented by the notation /" C, and because the values are usually small they are expressed in exponential form such as 22 x C. A less common practice is to report the change in parts per million (ppm) and the previous number would be expressed as 22 ppm. The linear coefficients of thermal expansion for some materials important in restorative dentistry are given in Table 3-7. Although the coefficient is a material constant, it does not remain constant over wide temperature ranges. For example, the linear coefficient of thermal expansion of a dental wax may have an average value

40 to 50" C. The coefficient of thermal expansion of a polymer changes as the polymer goes from a glassy state to a softer, rubbery material. This change in the coefficient corresponds to the glass transition temperature (T,).

Either the linear or volumetric coefficient of thermal expansion may be measured, and for most materials that function as isotropic solids, the volumetric thermal coefficient may be considered to be three times the linear thermal coefficient.

Both linear expansion and volume expansion are important in restorative materials and processes. It is obvious that with a reduction of temperature, or cooling, there is a contraction of the substance that is equal to the expansion that

results from heating. Accordingly, tooth structure and restorative materials in the mouth will expand when warmed by hot foods and beverages but will contract when exposed to cold substances. Such expansions and contractions may break the marginal seal of an inlay or other filling in the tooth, particularly if the difference between the coefficient of expansion of the tooth and the restorative material is great. The high coefficient of expansion of pattern waxes is an important factor in the construction of properly fitting restorations. The change in volume as a result of cooling is responsible for the shrinkage spots or surface cracks that often develop in gold alloy castings during solidification. Compensation for the contraction that occurs during the cooling of gold alloys must be made if accurate gold castings are to result. Therefore in some materials and in certain operations, the coefficient of thermal expansion may be equally as important as the strength, hardness, or esthetic appearance of the material. The values in Table 3-7 show that with comparable temperature

changes, n~aterialssuch as acrylic resin and amalgam expand more than tooth tissue, whereas ceramic expands less. The coefficient of inlay pattern wax is exceptionally high when compared with that of other materials.

Of particular importance in casting investments is the property of thermal expansion of three crystalline polymorphic forms of silica. As a principal ingredient in dental investments that are to be heated before a metal casting is made, the amount of expansion at various temperatures is critical and important. This quality of silica compounds in relation to use in casting investments was described in 1932. Curves in Fig. 3-12 illustrate the relative percentage of thermal expansion of the four forms of silica at different temperatures below about 800" C. Of the crystalline forms, cristobalite shows the greatest expansion at the lowest temperature and quartz requires a higher temperature to develop an equal amount of expansion as cristobalite. Fused silica has long been recognized as having an exceedingly low thermal expansion.

Fig. 3-12 Thermal expansion curves for four types of silica.

(Adapted from Volland RH, Paffenbarger GC: J Am

Dent Assoc 19:185, 1932.)

ELECTRICAL CONDUCTIVITY A N D RESISTIVITY

The ability of a material to conduct an electric current may be stated either as specific conductance or conductivity, or, conversely, as the specific resistance or resistivity. Resistivity is the more common term. The resistance of a homogeneous conductor of uniform cross section at a constant temperature varies directly with the length and inversely with the cross-sectional area of the specimen, according to the equation

in which Ris the resistance in ohms, p (rho) is the resistivity, 1 is the length, and A is the section area. The resistivity depends on the nature of the material. If a unit cube of 1-cm edge length is employed, the I and A are equal to unity, and in this case R = p. The resistivity is expressed as

ohm-centimeters where R is in ohms, 1 is in centimeters, and A is in square centimeters.

The change in electrical resistance has been used to study the alteration in internal structure of various alloys as a result of heat treatment. An early investigation of the gold-copper alloy system by electrical conductivity methods revealed a change in internal crystal structure with an accompanying change in conductivity. The correlation of these conductivity studies with related changes in other properties established the fundamental basis of structural changes associated with heat-treatment operations on dental gold alloys.

Values for the resistivity of human tooth structure are shown in Table 3-8. Resistivity is important in the investigation of the pain perception threshold resulting from applied electrical stimuli and of displacement of fluid in teeth caused by ionic movements. The electrical resistance of normal and carious teeth has been observed to differ, with less resistance offered by the carious tissue. Sound enamel is a relatively poor conductor of electricity, whereas dentin is somewhat better (see Table 3-81,

The conductivity of materials used to replace tooth tissue is of concern in restorative dentistry. The effectiveness of insulating cement bases and

other nonmetallic restorative materials is not yet established. Several studies have measured the resistivity of dental cements (see Table 3-8). The zinc oxide-eugenol cements have the highest resistivity, followed by the zinc polyacrylate and zinc phosphate cements. The glass ionomer cements are the most conductive of the cements and have values most similar to dentin.

DIELECTRIC CONSTANT

A material that provides electrical insulation is known as a dielectric. The dielectric constant or relative permittivity, E,, compares the permittivity, E, of the dielectric to the permittivity, E,,, of empty space:

E,. = &/&,

where E, of a vacuum is 8.854 x 10-l2 farad/m. The dielectric constant varies with the temperature, bonding, crystal structure, and structural defects of the dielectric.

Values of the dielectric constant for human dentin and several dental cements are listed in Table 3-9 The dielectric constant of a dental cement generally decreases as the material hardens. This decrease reflects a change from a paste that is relatively ionic and polar to one that is less so. As shown by the high values of permittivity of the glass ionomer and zinc polyacrylate cements in Table 3-9, these cements have a high ionic content and are quite polar compared with zinc oxide-eugenol cements and human dentin.

Dielectric measurements have been used to

study polymer-filler interactions in dental composites with and without silane coupling agents, and the effect of moisture on these interactions. The measurements showed that increasing the filler content restricted the mobility of the main polymer chains and that compatible silanes did not form a separate interphase at the polymerfiller interface. It was also shown that bulk water could exist at the interface if the filler was not silanated. Therefore correct silanation of fillers used in dental composites is essential for their successful application.

The problem of electrical insulation is made more complex by the presence of galvanic currents in the mouth, resulting from cells formed from metallic restorations. Recent studies indicate that a cement base does not effectively insulate the pulp from the electric current developed in a metallic restoration in the mouth. How much insulation is essential or how to effectively

restore the tooth to its original status of ecluilil>- rium is currently not known.

ELECTROMOTIVE FORCE

Working with metals and alloys for dental restorations or with instruments that are susceptible to corrosion necessitates some understanding of the relative position of the metal in the electromotive force series. The electroniotive series is a listing of electrode potentials of metals according to the order of their decreasing tendency to oxidize in solution. This serves as the basis of comparison of the tendency of metals to oxidize in air. Those metals with a large negative electrode potential are more resistant to tarnish than those with a high positive electrode potential. In general, the metals above copper in the series, such as aluminum, zinc, and nickel, tend to oxidize relatively easily, whereas those below copper, such

as silver, platinum, and gold, resist oxidation. A list of oxidation-reduction potentials for some common corrosion reactions in water and in salt water is given in Table 3-10. The values of electrode potential and the order of the series change when measured in a saline solution rather than water. The electrode potentials of some dental alloys measured in artificial saliva at 35' C are listed in Table 3-11.

Likewise, it is possible to determine from the electromotive force series that the reduction of the oxides of gold, platinum, and silver to pure metal can be accomplished more readily than with those metals that have a higher electromotive force value.

GALVANISM

The presence of metallic restorations in the mouth may cause a phenomenon called galvalzic action, or galvanism. This results from a difference in potential between dissimilar fillings in opposing or adjacent teeth. These fillings, in conjunction with saliva or bone fluids such as electrolytes, make up an electric cell. When two opposing fillings contact each other, the cell is

short-circuited, and if the flow of current occurs through the pulp, the patient experiences pain and the more anodic restoration may corrode. A single filling plus the saliva and bone fluid may also constitute a cell of liquid junction type. As shown in Fig. 3-13, ions capable of conducting electricity can easily migrate through dentin and around the margins of a restoration.

Studies have indicated that relatively large currents will flow through metallic fillings when they are brought into contact. The current rapidly falls off if the fillings are maintained in contact, probably as a result of polarization of the cell. The magnitude of the voltage, however, is not of primary importance, because indications support the fact that the sensitivity of the patient to the current has a greater influence on whether pain is felt. Although most patients feel pain at a value between 20 and 50 pamp, some may feel pain at 10 pamp, whereas others do not experience it

Modified from Arvidson K, Johansson EG: Scand J Dent

Res 85:485, 1977.

'Wigh positive sign indicates a strong tendency for the metal to go into solution.

Fig. 3-13Autoradiograph of a longitudinal section of a dog's permanent tooth in which the pulp has been capped with 45/20 Ca hydroxide. The dark areas in the tooth are evidence that migration of the Ca ions has occurred and that a circuit is possible.

(Courtesy Avery JK, University of Michigan School of Dentistry,

1958.)

until 110 vamp are developed. This is a possible explanation for the fact that some patients are bothered by galvanic action and others are not, despite similar conditions in the mouth.

The galvanic currents developed from the contact of two metallic restorations depend on their composition and surface area. An alloy of stainless steel develops a higher current density than either gold or cobalt-chromium alloys when in contact with an amalgam restoration. As the size of the cathode (such as a gold alloy) increases relative to that of the anode (such as an amalgam), the current density may increase. The larger cathode likewise can enhance the corrosion of the smaller anode. Current densities associated with non-y,-containing amalgams appear to be less than those associated with the ?,-containing amalgams.

ELECTROCHEMICAL CORROSION

The corrosion and electrochemical behavior of restorative materials have received new interest with the study of multiphase systems such as gold alloys and amalgam. For example, the corrosion of y, y,, and y, phases in amalgam has been studied by electrochemical means. Anodic and cathodic polarization measurements indicated no strongly passive behavior of these phases in artificial saliva. The dental amalgam specimens became pitted at the boundaries between the phases or in y, phase. Other studies, however, indicate that amalgam alloys exhibit decreasing electrochemical potentials, resulting in noble values when stored in neutral solutions. The addition of copper to amalgam alloys to form copper-tin compounds during hardening has improved the resistance of amalgam to chlo-

ride and galvanic corrosion. As shown in Fig. 3-14 the anodic activity of AgSn amalgam is quite different from AgSn + AgCu amalgams. The AgSn + AgCu amalgam remains passive under the testing conditions, whereas the AgSn amalgam does not.

Studies of corrosion of surgical stainless steel, stainless steel orthodontic brackets, and endodontic silver cones also have been reported. Corrosion of these alloys and others can result in decreased mechanical properties and the formation of corrosion products, which in some instances accumulate in the human organs. As shown previously in Table 3-10, corrosion can be affected by the environment, and certain metals such as cobalt and copper corrode more rapidly in a saline solution containing serum albumin and fibrinogen proteins.

ZETA-POTENTIAL

A charged particle suspended in an electrolytic solution attracts ions of opposite charge to those at its surface. The layer formed by these ions is called the Stern layer. To maintain the electrical balance of the suspending fluid, ions of opposite charge are attracted to the Stern layer. The potential at the surface of that part of the diffuse double layer of ions is called the electrokinetic or zeta-potential.

The zeta-potential, 5,of a solid as a porous plug can be determined by measuring the streaming potential, E, which is the electric field caused when a liquid is made to flow along a stationary charged surface. The relation is as follows:

Adapted from O'Brien WJ, editor: Dental materia1s:pmperties and selection, Chicago, 1989, Quintessence. "Measured in Hanks' balanced salt solution at 30" C.

and characterize particle surfaces. Effects of pH, surface-active agents, and enzymes on zetapotential are important. Zeta-potential may affect the near-surface mechanical properties (such as wear) of a material. The zeta-potentials of some materials are listed in Table 3-12.

OTHER PROPERTIES

Certain properties often are highly important in the selection and manipulation of materials for use either in the mouth or for laboratory applications. Five such properties are tarnish and discoloration, water sorption, solubility and disintegration, setting time, and shelf life.

TARNISH AND DISCOLORATION

5= (8.47 x ~ o ~ ) ( I ~ / E ) ( A E / A P )

( R O , l N KC1 'k ~ .Nl KC1IRp)

where q is viscosity in poises, E is in millivolts, Rp is resistance in ohms of the porous plug, k is in ohm-lcm-I, and P is pressure in cm Hg. Rk is a cell constant for 0.1 N KC1. The term E is the dielectric constant, which is 78.54 for water at 25' C.

Electrophoresis may be used to increase the stability of colloids, stimulate adsorption of ions,

Discoloration of a restorative material from any cause is a very troublesome quality. The tarnish of metal restorations from oxide, sulfide, or any other materials causing a surface reaction is a critical quality of metal restorations in the mouth and of laboratory and clinical instruments. The process of steam sterilization of surgical instruments has long presented a serious problem of tarnish and corrosion. Many nonmetallic materials such as cements and composite restorations have displayed a tendency to discolor in service