Terminology

This chapter provides brief descriptions of physical and chemical properties as background and preparation for more detailed discussion in later chapters in which these properties are used to describe the characteristics of specific dental materials.

Physical properties are based on the laws of mechanics, acoustics, optics, thermodynamics, electricity, magnetism, radiation, atomic structure, and nuclear phenomena. Hue, value, and chroma relate to color and perception and are physical properties based on the laws of optics, which is the science that deals with the phenomena of light, vision, and sight. Thermal conductivity, diffusivity, and expansion are physical properties based on the laws of thermodynamics. Mechanical properties, a subset of physical properties based on the laws of mechanics, are discussed in Chapter 4.

Chemical properties are based on the ways in which substances interact, combine, and change, as governed by their outer orbital electrons. The outer electrons are responsible for binding atoms together in molecules and for the electrical, thermal, optical, and magnetic properties of solids.

As examples, the physical properties of color and thermal expansion are of particular importance to the performance of dental ceramics (Chapter 18). Flow and viscosity (the resistance of a fluid to flow) are essential properties of impression materials (Chapter 8). Creep (slow deformation under a static load) is relevant to the clinical performance of amalgam (Chapter 15). Tarnish and corrosion are electrochemical properties that strongly affect the performance of metals and their alloys.

Rheology

Rheology is the study of the deformation and flow characteristics of matter, whether liquid or solid. Viscosity is the resistance of a fluid to flow. Dental professionals must manipulate a wide variety of dental materials in a fluid state in order to achieve successful clinical outcomes. Moreover, the success or failure of a given material may be as dependent on its manipulation and handling properties in the liquid state as it is on its performance properties as a solid. Most dental materials are initially in a fluid state so that they can be placed and shaped as required; then they undergo transformation to a solid state, in which they are durable and perform their function. Cements and impression materials undergo a fluid-to-solid transformation in the mouth. Gypsum products used in the fabrication of models and dies are transformed extraorally from fluid slurries into solids (see Chapter 9). Amorphous materials such as waxes and resins appear solid but actually are supercooled liquids that can flow plastically (irreversibly) under sustained loading or deform elastically (reversibly) under small stresses (see Chapters 6 and 10). The ways in which these materials flow or deform when subjected to stress are important to their use in dentistry. The study of flow characteristics of materials is the basis for the science of rheology (also see Chapter 6).

Although a liquid at rest cannot support a shear stress (shearing force per unit shearing area), most liquids, when placed in motion, resist imposed forces that cause them to move. This resistance to fluid flow (viscosity) is controlled by internal frictional forces within the liquid. Thus viscosity is a measure of the consistency of a fluid and its resistance to flow. A highly viscous fluid flows slowly. Dental materials have different viscosities depending on their intended clinical application. Dental assistants, dentists, and dental students who have observed the more viscous nature of zinc polycarboxylate and resin cements compared with zinc phosphate cement when these materials have been properly mixed as luting cements, are familiar with these viscosity differences. In everyday life we find wide differences in viscosities among such fluids as water, syrup, ketchup, moisturizing cream, and toothpaste.

This concept is illustrated in Figure 3-1. A liquid occupies the space between two flat surfaces, as, for example when a spatula is moved through a pasty fluid such as a dental cement to blend two components on a mixing pad. The mixing surface is fixed, and the upper surface (e.g., a spatula blade) moves to the right at a given velocity (V). A force (F) is required to overcome the frictional resistance within the fluid (i.e., the viscosity) and cause the fluid to flow. As discussed in Chapter 4, stress is the force per unit area that develops within a structure when an external force is applied. This stress causes a deformation, or strain, to develop. Strain is calculated as a change in length divided by the initial reference length. If the two surfaces have an area (A) in contact with the liquid, a shear stress (τ) can be defined as τ = F/A. The shear strain rate, or rate of change of deformation, is ε = V/d, where d is the shear distance of the upper surface relative to the fixed lower surface and V is the velocity of the moving surface. As the shear force F increases, V increases, and a curve can be obtained for force versus velocity. This type of curve is analogous to the load-versus-displacement curves discussed in Chapter 4, which are derived from static measurements on solids.

Curves depicting shear stress versus shear strain rate are used to characterize the viscous behavior of fluids. The rheologic behaviors of four types of fluids are shown in Figure 3-2. An “ideal” fluid produces a shear stress proportional to the strain rate. That is, the greater the force applied, the faster the fluid flows and the plot is a straight line. This is known as Newtonian viscosity. Because viscosity (h) is defined as the shear stress divided by the strain rate, τ/ε, a Newtonian fluid has a constant viscosity and exhibits a constant slope of shear stress plotted against strain rate, as seen in Figure 3-2. The plot is a straight line and resembles the elastic portion of a stress-strain curve (see Chapter 4), with viscosity the analog of the elastic modulus (elastic stress divided by elastic strain). Viscosity is measured in units of megapascals (MPa) per second, or centipoise (cP). Pure water at 20°C has a viscosity of 1.0 cP, whereas the viscosity of molasses is approximately 300,000 cP. This value is similar to that of tempered agar hydrocolloid impression material (281,000 cP at 45 °C). Of the elastomeric impression materials, light-body (“thin” consistency) polysulfide has a viscosity of 109,000 cP, compared with 1,360,000 cP for heavy-body (“thick” consistency) polysulfide at 36 °C.

The viscosity of many dental materials decreases with increasing strain rate until it reaches a nearly constant value. That is, the faster they are stirred, forced through a syringe, or squeezed, the less viscous and more fluid they become. This is pseudoplastic viscosity and is illustrated by the change in slope of the plot in Figure 3-2. Liquids that show the opposite behavior are dilatant and become more rigid as the rate of deformation (shear strain rate) increases. That is, the faster they are stirred, etc., the more viscous and resistant to flow they become.

Finally, some classes of materials behave like a rigid body until some minimum value of shear stress is reached. This is represented by the offset along the shear-stress axis. These fluids, which exhibit rigid behavior initially and then attain constant viscosity, are referred to as “plastic.” Ketchup is a familiar example—a sharp blow to the bottle is usually required to produce an initial flow.

The viscosity of most fluids decreases rapidly with increasing temperature. Viscosity may also depend on previous deformation of the liquid. Such fluids become less viscous and more flowable upon repeated applications of pressure and are termed thixotropic. Dental prophylaxis pastes, plaster of Paris, resin cements, and some impression materials are thixotropic. The thixotropic nature of impression materials is beneficial because the material does not flow out of a mandibular impression tray until it is placed over dental tissues, and a prophylaxis paste does not flow out of a rubber cup until it is rotated against the teeth to be cleaned. If these materials are stirred rapidly and the viscosity is measured, a value is obtained that is lower than the value for a sample that has been left undisturbed.

The viscosity of a dental material may determine its suitability for a given application. Likewise, the nature of the curve representing shear stress versus shear strain rate can be important in determining the best way to manipulate a material. As explained in more detail later, the viscosity as a function of time can also be used to measure the working time of a material that undergoes a liquid-to-solid transformation.

After a substance has been permanently deformed (plastic deformation), there are trapped internal stresses. For example, in a crystalline substance such as a metal, the atoms in the crystal structure are displaced and the system is not in equilibrium. Similarly, in amorphous structures, some molecules are too close together and others too far apart when the substance is permanently deformed.

The displaced atoms are not in equilibrium positions and are therefore unstable. Through a solid-state diffusion process driven by thermal energy, the atoms can slowly return to their equilibrium positions. The result is a change in the shape or contour of the solid as the atoms or molecules change positions. The material warps or distorts. Such stress relaxation is a problem with elastomeric impression materials and can lead to distortions in the impression and subsequent lack of fit, as discussed in Chapter 8.

The rate of relaxation increases with an increase in temperature. For example, if a wire is bent, it may tend to straighten out if it is heated to a high temperature. At room temperature, any such relaxation caused by rearrangement of metal atoms may be negligible. On the other hand, there are many noncrystalline dental materials (such as waxes, resins, and gels) that, when manipulated and cooled, can then undergo relaxation (with consequent distortion) at an elevated temperature. Considerable attention is given to this phenomenon in succeeding chapters because such dimensional changes by relaxation may result in an inaccurate fit of dental appliances.

Color and Optical Effects

The preceding sections have focused on those properties that are necessary to permit a material to restore the function of damaged or missing natural tissues. Another important goal of dentistry is to restore or improve esthetics—the color and appearance of natural dentition. Esthetic considerations in restorative and prosthetic dentistry have received increasingly greater emphasis in recent decades, and the challenges have grown even greater in the past few years following the widespread use of bleaching and whitening technologies. Thus, the development of a general-purpose, technique-insensitive, direct-filling, tooth-colored, color-stable restorative material remains one of the more serious challenges of current dental materials research.

Since esthetic dentistry imposes severe demands on the artistic abilities of the dentist and technician, knowledge of the underlying scientific principles of color and other optical effects is essential. This is especially true for the increasingly popular restorations that involve ceramic materials (see Chapter 18). More comprehensive treatments of this subject can be found in the “Selected Readings” at the end of this chapter.

Nature of Light and the Role of Human Vision

Light is electromagnetic radiation that can be detected by the human eye. The eye is sensitive to wavelengths from approximately 400 nm (violet) to 700 nm (dark red), as shown in Figures 3-3 and 3-4. The reflected light intensity and the combined intensities of the wavelengths present in incident and reflected light determine the appearance properties of hue, value, and chroma, which are shown in Figure 3-5 and discussed in the next section. For an object to be visible, it must reflect or transmit light incident on it from an external source. The incident light is usually polychromatic; that is, a mixture of the various wavelengths, commonly known as “white” light. Incident light is selectively absorbed or scattered (or both) at certain wavelengths. The spectral distribution of the transmitted or reflected light resembles that of the incident light, although certain wavelengths are reduced in magnitude.

The phenomenon of vision, and certain related terminology, can be illustrated by considering the response of the human eye to light reflected from an object. Light from an object that is incident on the eye is focused in the retina and is converted into nerve impulses, which are transmitted to the brain. Cone-shaped cells in the retina are responsible for color vision. These cells have a threshold intensity required for color vision and respond to wavelengths as shown in Figure 3-4 for both normal color vision and color-deficient vision. Someone with normal vision has maximum sensitivity in the green-yellow region at about 550 nm and is least sensitive in the red and blue-violet regions of the spectrum.

The signals from the retina are processed by the brain to produce the psychophysiological perception of color. Because a neural response is involved in color vision, constant stimulation by a single color may result in color fatigue and a decrease in the eye’s response. Defects in certain portions of the color-sensing receptors result in the different types of color blindness; thus humans vary greatly in their ability to distinguish colors. In a scientific sense, one might liken the normal human eye to an exceptionally sensitive differential colorimeter, a scientific instrument that measures the intensities and wavelengths of light. Although colorimeters are more precise than the human eye in measuring slight differences in colored objects, they are inaccurate for rough or curved surfaces. The eye is highly sensitive in comparing two colors seen side by side, whether on rough, smooth, flat, or curved surfaces.

The Nature of the Object under View

Esthetics is critically important in dealing with dental restorative materials. For good esthetics, the interaction of light with restorative materials must mimic the interaction of light with natural teeth. The nature of the restorative material, or that of any object under view, determines how that object will appear. Electromagnetic radiation in the visible region interacts with an object through reflection from its surface, absorption, refraction, or transmission (i.e, by passing through unchanged.) These phenomena determine the opacity, translucency, or transparency of an object. Light reflected from rough surfaces scatters in many directions because it is reflected at many angles by the uneven surface. This leads to an appearance that ranges from mirrorlike for a perfectly smooth surface (termed specular reflectance) to the flat, dull appearance (termed diffuse reflectance) of a surface such as chalk.

The opacity of a material is related to the amount of light it can absorb and/or scatter. The opposite of opacity is translucency. For example, if 1-mm thicknesses of each of two materials absorb 20% and 50%, respectively, of the light passing through them, the former is less opaque or more translucent than the latter. Transparent materials are at the far end of the translucency scale, absorb no light, and transmit 100% of the light that passes through them.

Enamel is a composite material consisting of hydroxyapatite crystals in a protein matrix. When light strikes enamel, some is reflected, some refracted, some absorbed, and some transmitted. Enamel has a refractive index of 1.65 and is translucent.

 Critical Question

How is color described objectively and quantitatively?

Three Dimensions of Color

Verbal descriptions of color are not precise enough to describe the appearance of teeth. For example, the definition of puce is “a brilliant purplish-red color,” according to Microsoft’s Encarta World English Dictionary, while Webster’s Third New International Dictionary defines it as “a dark red that is yellower and less strong than cranberry, paler and slightly yellower than average garnet, bluer, less strong, and slightly lighter than pomegranate, and bluer and paler than average wine.” These definitions are too variable, complex, and imprecise to describe a desired color of a dental crown to a laboratory technician. Such a written description is subjective and does not clearly and unambiguously allow one to perceive the color. To overcome this problem, color perception is described by three objective variables: hue, value, and chroma. These three parameters constitute the three dimensions of “color space,” as shown in Figure 3-5.

• Hue: The dominant color of an object, for example red, green, or blue. This refers to the dominant wavelengths present in the spectral distribution. The continuum of these hues creates the 3-D color solid shown in Figure 3-5.

• Value: Value is also known as the gray scale. It is the vertical, or Z-axis, of Figure 3-5. Value increases toward the high end (lighter) and decreases toward the low end (darker). Value is also expressed by the “lightness” factor (L* in Figure 3-6), with varying levels of gray between the extremes of white and black. Teeth and other objects can be separated into lighter shades (higher value) and darker shades (lower value). For example, the yellow of a lemon is lighter than the red of a cherry. For a light-diffusing and light-reflecting object such as a tooth or dental crown, value identifies the lightness or darkness of a color, which can be measured independently of the hue.

The components of a color space can be more easily visualized by its individual parts in Figure 3-5, B, here seen as discs stacked along the value axis (lightness, L*) on a scale of 0 to 10 from black to white. Around the periphery are 10 basic hues (dominant wavelength/color). Chroma (strength, saturation) radiates out from the value axis like the spokes of a wheel (illustrated by green).

Figure 3-6 represents a horizontal plane, perpendicular to the L* (value) axis, through the color solid in Figure 3-5. This color chart is based on the Commission Internationale de l’Éclairage L*a*b* color space, in which L* represents the value of an object, a* is the measurement along the red-green axis, and b* is the measurement along the yellow-blue axis. The color of a red apple is shown by the letter A in the upper and lower charts. Its color appearance can be expressed by L* = 42.83, a* = 45.04, and b* = 9.52. In comparison, a dental body (gingival) porcelain of shade A2 can be described by a higher (lighter) L* of 72.99, a lower a* of 1.00, and a higher b* of 14.41.

• Chroma: Chroma is the degree of saturation of a particular hue. For example, red can vary from “scarlet” to light pink, where scarlet has a high saturation and pink has a low saturation. The yellow color of a lemon is a more saturated, “vivid,” color than that of a banana, which is a less saturated, “dull” yellow. Chroma varies radially, perpendicular to the value/L* axis (see Figure 3-6, A, near the bottom right). Colors in the center are dull (gray). In other words, the higher the chroma, the more intense the color. Chroma is not considered separately in dentistry. It is always associated with the hue and value of dental tissues, restorations, and prostheses. One can see the relationship among these dimensions of color in the adjustments on a color television set, which use the same principles of hue, value, and chroma.