Chapter 3: Physical and Chemical Properties of Solids – Comprehensive Study Notes

Rheology

  • Rheology: study of deformation and flow characteristics of matter (liquids and solids); viscosity is a key measure of flow resistance.
  • Key terms:
    • Dilatant: Resistance to flow increases as deformation rate increases; faster stirring makes fluid more viscous.
    • Pseudoplastic: Viscosity decreases with increasing strain rate; fluids flow more easily when stressed (shear-thinning).
    • Thixotropic: Time-dependent decrease in viscosity under steady shear; materials regain viscosity when shear stops (also called shear-thinning in some contexts).
    • Viscosity: Resistance of a fluid to flow; viscosity η is the ratio of shear stress τ to shear rate ε̇ (τ/ε̇).
  • Newtonian fluids: τ is proportional to ε̇; constant viscosity; straight-line τ vs ε̇ plot; viscosity is constant.
  • Non-Newtonian behaviors: different rheologic curves (dilatant, pseudoplastic, plastic) shown graphically; important for handling dental materials.
  • Practical notes: Dental materials are often handled in fluid state and transform to solids in the mouth; flow and viscosity affect handling, working time, and final fit. Examples: zinc polycarboxylate and resin cements (luting), zinc phosphate cement, impression materials, prophylaxis pastes.
  • Key measurements and relationships:
    • Shear stress: τ = F / A where F is shear force and A is contact area.
    • Shear strain rate: ε̇ = V / d where V is velocity of moving surface and d is shear distance.
    • Newtonian viscosity: a constant slope in τ vs ε̇; viscosity is the slope (τ/ε̇).
    • Figure references: Figure 3-1 illustrates shear strain between plates; Figure 3-2 shows τ vs ε̇ for Plastic, Dilatant, Newtonian, and Pseudoplastic fluids.
  • Practical viscosity data (typical ranges):
    • Water at 20°C: ~1 cP (1 mPa·s).
    • Molasses: ~3×10^5 cP.
    • Agar hydrocolloid impression material (thermally set): ~2.81×10^5 cP at 45°C.
    • Light-body elastomeric impression materials (polysulfide): ~1.09×10^5 cP; heavy-body polysulfide ~1.36×10^6 cP at related temperatures.
  • Temperature and history effects:
    • Viscosity generally decreases with increasing temperature.
    • Some materials are thixotropic: viscosity lowers with agitation; can measure working time via viscosity-time curves.
  • Structural relaxation is connected to rheology (see Structural Relaxation).

Structural Relaxation

  • Post-deformation rearrangement: after permanent plastic deformation, atoms may be displaced from equilibrium; solid-state diffusion driven by thermal energy allows atoms to slowly return toward equilibrium, altering shape/contour.
  • Consequences: distortion or warping can occur in elastomeric impression materials and other noncrystalline dental materials during manipulation and cooling; may lead to inaccurate fits of dental appliances.
  • Temperature dependence: rate of relaxation increases with temperature; room-temperature relaxation may be negligible for metals but can occur in noncrystalline materials (wax, resins, gels).
  • Implications for dental practice: dimensional changes due to relaxation can affect the accuracy of fixed dental prostheses and other appliances; subsequent chapters discuss these consequences in detail.
  • Creep (a related concept): slow, time-dependent plastic strain under a static load; discussed in the context of amalgam and alloy behavior; related to sag in bridge frameworks at porcelain-firing temperatures (porcelain veneering temperatures ~1000°C).
  • Creep testing and standards: creep tests are specified in ANSI/ADA Spec No. 1 for amalgam alloy and ISO No. 1559; the term “flow” is widely used for amorphous materials in dentistry.
  • Numerical examples of creep/flow rates and values are used to compare materials (e.g., water, agar, dental elastomers).

Color and Optical Effects

  • Color perception is a function of light interaction with matter and human vision; esthetics require color that mimics natural dentition.
  • Nature of light and human vision:
    • Visible light range: approximately 400–700 nm; hue, value, and chroma describe color appearance.
    • Light interaction mechanisms: reflection, absorption, refraction, transmission; these determine opacity, translucency, and transparency.
    • The eye’s color perception arises from cone receptors; normal vision has peak sensitivity in the green-yellow region (~550 nm).
    • Color-deficient vision (color blindness) varies; prevalence ~8% in men, ~0.5% in women for red-green deficiency. See Figure 3-4.
  • The nature of the object under view:
    • Enamel composition: hydroxyapatite crystals in a protein matrix; refractive index n ≈ 1.65; enamel is translucent.
    • Light interactions produce a spectrum that resembles incident light but with wavelength-dependent absorption and scattering.
  • The three dimensions of color space (Hue, Value, Chroma):
    • Hue: dominant color (red, green, blue, etc.).
    • Value: lightness from black (low) to white (high); also described by L* in CIELAB space.
    • Chroma: saturation or intensity of the hue; radiates from the value axis in a color-space model.
    • Visualizing color space: Lab* model (CIELAB): L* = lightness, a* = red-green axis, b* = yellow-blue axis.
    • Example color coordinates: apple red: L* = 42.83, a* = 45.04, b* = 9.52; dental body porcelain shade A2: L* = 72.99, a* = 1.00, b* = 14.41.
  • Color space representations:
    • Figure 3-5 (A) shows a 3-D color solid (Lab). Hue directions around periphery; chroma radiates outward from the central L axis.
    • Figure 3-6 (B) shows a horizontal plane through L* with corresponding a, b axes; color solid is described by L, a, b* values.
  • Chroma and hue examples:
    • Hue categories: A (red-brown), B (red-yellow), C (gray), D (red-gray).
    • Higher chroma means more vivid color; dull colors have low chroma.
  • Color matching in dentistry:
    • Shade guides (e.g., Vitapan) are used to select ceramic veneer/crown shades; tabs are grouped by hue and value.
    • Shade guides may be arranged by hue groups or by value order (lightest to darkest): B1, A1, B2, D2, A2, C1, C2, D4, A3, D3, B3, A3.5, B4, C3, A4, C4 (typical modern practice).
    • Shade matching is aided by photographs and drawings; the technician should see patient’s natural teeth when possible.
    • Patient preferences: patients may prefer lighter shades than natural teeth; exact matches are often impractical.
  • The effect of the observer:
    • Color signals are transmitted through three cone types (red, green, blue) with overlapping sensitivities; receptor fatigue can affect color perception.
    • Age, gender, and clinical experience may have little effect on shade matching accuracy, though color-deficient individuals may require assistance.
  • Metamerism (critical concept):
    • Object color appearance can change with different light sources; daylight vs artificial light can yield different matches.
    • To mitigate metamerism, color matching should be performed under multiple light sources, including daylight, and shade-matching should be tested under similar lighting conditions.
  • Fluorescence and radiopacity:
    • Fluorescence: natural teeth absorb near-UV (300–400 nm) and emit light at longer wavelengths (blue-white, ~400–450 nm); fluorescence enhances brightness/vital appearance.
    • Fluorescent agents in ceramics/composites affect brightness; restorations lacking fluorescence may appear less natural under certain lighting.
    • Radiopacity: X-ray contrast depends on density and atomic number; polymers/resins are radiolucent, metals are radiopaque.
    • To be detectable radiographically, composites must have radiopacity at least equal to aluminum; i.e., a radiopacity threshold relative to aluminum is used for ADA requirements.
    • Dentin radiodensity is similar to aluminum; enamel approx. twice that of dentin; radiopacity of restorations should be optimized to distinguish from adjacent tissues.
    • Radiopacity in denture resins often achieved with strontium or barium-containing fillers; some resins use barium sulfate to increase radiopacity.
  • The effect of light source on color perception (metamerism) and the observer's perception:
    • Daylight, incandescent, and fluorescent lamps have different spectral distributions; color matching may appear different under these sources.
    • Practical guidance: color matching should be performed under daylight and replicated in lab lighting conditions; eye perception is more critical than instrument measurements for esthetics in many cases.
  • Critical questions:
    • How is color described objectively and quantitatively? (Introduction to color space variables Hue, Value, Chroma and the Lab* system.)
    • How do lighting conditions cause metamerism in shade matching? (Different light sources alter perceived color; match under daylight and lab lighting.)

Thermal Properties

  • Key thermal properties:
    • Coefficient of thermal expansion (linear): α = (ΔL) / (L ΔT); change in length per unit original length per 1 K temperature rise.
    • Thermal conductivity (κ): heat transfer through a material per unit thickness and area with a 1 K temperature difference, under steady-state conditions.
    • Thermal diffusivity (often denoted α or sometimes a): speed at which a temperature change propagates through a material; αth = κ / (ρ cp).
  • Temperature scales:
    • Kelvin: 1 K = 1 °C; 0 K = −273.15 °C; K scale is absolute; no degree symbol; 1 K equals 1 °C increase.
  • Important distinctions:
    • Thermal conductivity κ: conductivity of heat flow through a material; higher κ means better heat transfer.
    • Thermal diffusivity αth: how quickly a temperature change spreads; high αth implies rapid temperature change within the material.
    • In dentistry, both κ and α_th matter for predicting heat transfer during procedures and thermal shock risk to the pulp.
  • Practical implications for dental materials:
    • Enamel and dentin are effective thermal insulators (relatively low κ and α_th) compared with many restorative materials.
    • Materials like glass ionomer, zinc phosphate, and composite have κ values closer to dentin, offering better thermal protection than metals but higher than enamel/dentin, influencing the need for insulating bases when thin dentin is present.
    • The expansion/contraction coefficients matter for margins and seal: mismatches can cause microleakage and debonding.
  • Representative data (α values, ppm K⁻¹ or dimensionless ratio relative to enamel):
    • Aluminous porcelain: α ≈ 6.6 (relative to enamel ≈ 0.58)
    • Dentin: α ≈ 8.3 (relative ≈ 0.75)
    • Commercially pure titanium: α ≈ 8.5 (relative ≈ 0.77)
    • Type II glass ionomer: α ≈ 11.0 (relative ≈ 0.96)
    • Enamel: α ≈ 11.4 (reference = 1.00)
    • Gold-palladium alloy: α ≈ 13.5 (relative ≈ 1.18)
    • Gold (pure): α ≈ 14.0 (relative ≈ 1.23)
    • Palladium-silver alloy: α ≈ 14.8 (relative ≈ 1.30)
    • Amalgam: α ≈ 25.0 (relative ≈ 2.19)
    • Composite: α ≈ 14–50 (relative ≈ 1.2–4.4)
    • Denture resin: α ≈ 81.0 (relative ≈ 7.11)
    • Pit and fissure sealant: α ≈ 85.0 (relative ≈ 7.46)
    • Inlay wax: α ≈ 400.0 (relative ≈ 35.1)
  • Practical guidance: Direct restorative materials can change size up to about 4.4 times more than enamel per 1°C change; wax patterns can contract significantly during cooling and storage, affecting crown casting accuracy.
  • Thermal considerations in practice:
    • For metal-ceramic restorations, mismatch in thermal expansion between porcelain and coping can generate tensile stresses during cooling; matching α within about 4% minimizes crack formation in porcelain.
    • Use insulating bases when remaining dentin is thin to limit thermal insult to the pulp.

Electrochemical Properties

  • Core concepts:
    • Corrosion is an electrochemical process where metal surfaces are attacked by environmental species, resulting in dissolution or altered surface properties.
    • Tarnish is surface discoloration or dulling due to thin oxide/sulfide/chloride films; tarnish often precedes corrosion.
    • Gold is resistant to corrosion; many ceramics do not corrode due to being fully oxidized; glasses and glazes can dissolve under certain media.
  • Electrochemical corrosion vs chemical (dry) corrosion:
    • Chemical corrosion: direct chemical reaction without water (e.g., silver sulfide formation in sulfur-rich environments).
    • Electrochemical (wet) corrosion: requires a conductive electrolyte (e.g., saliva) and an electron pathway; most important in dentistry.
  • Electrochemical mechanism (galvanic cells):
    • In an electrolyte, metal oxidation at the anode releases electrons and metal ions; the cathode consumes electrons via a reduction reaction.
    • Anode: region where oxidation occurs (M → M^n+ + ne⁻).
    • Cathode: region where reduction occurs (M^n+ + ne⁻ → M) or other reductions (e.g., 2 H⁺ + 2 e⁻ → H₂).
    • The external circuit (saliva/tissue fluids) completes the current; EMF is the potential difference between anode and cathode.
  • Electromotive series (Table 3-3) lists electrode potentials (V) for various metals in aqueous solution at 25°C; more positive potentials indicate greater resistance to oxidation/corrosion. Selected values:
    • Au+ → +1.50 V
    • Au3+ → +1.36 V
    • Pt2+ → +0.86 V
    • Pd2+ → +0.82 V
    • Hg2+ → +0.80 V
    • Ag+ → +0.80 V
    • Cu+ → +0.47 V
    • Bi3+ → +0.23 V
    • Sb3+ → +0.10 V
    • H+ → 0.00 V
    • Pb2+ → −0.12 V
    • Sn2+ → −0.14 V
    • Ni2+ → −0.23 V
    • Cd2+ → −0.40 V
    • Fe2+ → −0.44 V
    • Cr2+ → −0.56 V
    • Zn2+ → −0.76 V
    • Al3+ → −1.70 V
    • Na+ → −2.71 V
    • Ca2+ → −2.87 V
    • K+ → −2.92 V
  • Galvanic currents in the mouth:
    • When two different metals contact saliva, a galvanic cell can form; typical current between a gold crown and amalgam restoration can be ~0.5–1 μA with ≈500 mV potential difference; currents are larger with dissimilar alloys.
    • Even identical alloys have minor potential differences due to surface composition and microstructure.
    • In the absence of direct contact, currents can still occur between an isolated restoration and tissue fluids due to electrochemical potential differences across the electrolyte environment.
  • Dissimilar metals and corrosion paths:
    • Dissimilar metal contact (galvanic corrosion) can occur in interproximal areas or opposing restorations (e.g., amalgam opposing a gold inlay).
    • The more active metal (anode) dissolves preferentially, and the cathodic regions accumulate corrosion products; crevice and concentration-cell corrosion can exacerbate attack in crevices and marginal gaps.
  • Stress corrosion and crevice corrosion:
    • Stress increases corrosion risk; localized corrosion occurs at notches/pits with accelerated attack due to dislocation and microstrain.
    • Crevice corrosion occurs under deposits, cracks, or marginal gaps causing differential oxygen concentration and localized attack.
  • Surface heterogeneity and passive films:
    • Alloys with multiple elements can have heterogeneous microstructures that form galvanic couples; grain boundaries and second-phase particles can become anodic relative to grains.
    • Passive films (e.g., chromium oxide on stainless steel; protective oxide films on titanium) can protect metals, but can be disrupted by tensile stress, chloride ions, or mechanical damage.
  • Protection strategies:
    • Use varnishes to insulate surfaces against saliva and suppress galvanic currents.
    • Passivation (e.g., stainless steel with chromium-rich oxide layer; titanium with TiO₂ layer) reduces corrosion propensity but can be compromised by stress and halides.
    • Copper- or nickel-containing coatings may fail if integrity is compromised.
    • Noble metals (gold, platinum, palladium) resist corrosion; major design guideline is to ensure sufficient noble metal content to minimize corrosion (e.g., at least half the atoms noble metal in some batches).
  • Special considerations:
    • Pitting corrosion often initiates at pits and defects; pits lower oxygen concentration at bottom become anodic; margins around pits become cathodic, accelerating localized attack.
    • Solder joints and interfaces may be susceptible to corrosion due to composition differences.
    • Mercury interactions with other metals in amalgams can influence corrosion and setting reactions; proper storage and handling reduce unintended reactions.
  • Clinical significance of galvanic currents:
    • Generally discomfort is rare and transient; varnish coatings reduce risk; avoid placing new amalgam restorations directly against gold crowns if possible.
    • Corrosion products and metallic ions can cause aesthetic changes (discoloration) and metallic taste.

Magnetic Materials

  • Magnetic materials and their relevance in dentistry:
    • Used for retention of implant-borne prostheses and orthodontic tooth movement; magnets generate a magnetic force that can be directed through soft tissues.
  • Physical basis:
    • Magnetism arises from circulating electrons; aligned spins produce a net magnetic field with North/South poles.
    • Magnetic flux density is measured in Tesla (T); field strength scales with magnet size and material.
  • Practical considerations:
    • Magnets can lose flux over time or when heated; exposure to high temperatures (e.g., 80–90°C during resin curing) can reduce magnetic strength.
    • Biocompatibility thresholds: exposure to static magnetic fields less than 40 mT is generally considered safe; higher fields (e.g., MRI) can cause vertigo, nausea, or other vestibular disturbances in certain people.
  • Safety and clinical use:
    • Magnets used inside dental devices must be plated or coated to resist corrosion and minimize corrosion product release.
    • Magnetic attachments in removable partial dentures have field strengths typically in the 10–15 mT range, well below the safety threshold cited in guidelines.
  • Important cautions:
    • MRI exposure can pose risks; avoid configuring dental devices with strong ferromagnetic components near strong external magnets during imaging.

Selected Readings

  • O’Brien WJ: Biomaterials Properties Database – online resource for dental material properties (bond strengths, friction, hardness, expansion, optical properties, density, electrical properties).
  • Key texts and articles cited in the chapter include: McLean JW (Dental ceramics), Miller LL (shade matching), Paravina RD (Dental Color Matcher), Parker RM (Shade matching for indirect restorations), Yuan JC-C et al. (Natural tooth color space). Note: shade guides and color matching literature emphasize real-world esthetic outcomes and the limitations of subjective matching.
  • Electrochemical properties: Berzins DW et al. (Pd-alloy electrochemistry), Chaturvedi TP (corrosion in dental implants), Marek M (amalgam-environment interactions), Meyer J-M and Reclaru L (corrosion resistance of noble casting alloys), Upadhyay D et al. (corrosion of dental alloys), Walker RS et al. (galvanic interaction gold-ampal), Haidary A et al. (swallowed dental appliance case reports).
  • Color and optics literature includes: American Dental Association (shade guides/esthetic dentistry), Antonson and Anusavice (contrast ratios in veneering/core ceramics), Barna et al. (influence of light intensity on color perception), Johnston and Kao (appearance assessment by colorimetry).
  • The Selected Readings section provides additional in-depth coverage on color science, shade matching, and corrosion science as it relates to dental materials.

Equations and Key Formulas (LaTeX)

  • Shear stress and strain rate in a viscous fluid:
    • τ=FA\tau = \frac{F}{A}
    • ε˙=Vd\dot{\varepsilon} = \frac{V}{d}
  • Viscosity relation for Newtonian fluid:
    • η=τε˙\eta = \frac{\tau}{\dot{\varepsilon}}
  • Coefficient of thermal expansion:
    • α=ΔLLΔT\alpha = \frac{\Delta L}{L \Delta T}
  • Thermal diffusivity (speed of temperature change through a material):
    • α<em>th=κρc</em>p\alpha<em>{th} = \frac{\kappa}{\rho c</em>p}
  • Relative time-dependent deformation concepts:
    • Creep: time-dependent plastic strain under static load (no single formula given in the text; concept described).
  • Electromotive force and galvanic cell basics:
    • Anodic oxidation (example form): M→Mn++ne−M \rightarrow M^{n+} + ne^-
    • Cathodic reduction (example forms): Mn++ne−→MM^{n+} + ne^- \rightarrow M, 2H++2e−→H<em>22H^+ + 2e^- \rightarrow H<em>2, 2H</em>2O+2e−→H2+2OH−2H</em>2O + 2e^- \rightarrow H_2 + 2OH^-
  • Electromotive series (potentials) – representative values (V at 25°C):
    • Au+, +1.50; Au3+, +1.36; Pt2+, +0.86; Pd2+, +0.82; Hg2+, +0.80; Ag+, +0.80; Cu+, +0.47; Bi3+, +0.23; Sb3+, +0.10; H+, 0.00; Pb2+, −0.12; Sn2+, −0.14; Ni2+, −0.23; Cd2+, −0.40; Fe2+, −0.44; Cr2+, −0.56; Zn2+, −0.76; Al3+, −1.70; Na+, −2.71; Ca2+, −2.87; K+, −2.92.
  • Metamerism concept:
    • Color appearance depends on the spectral distribution of the light source; different light sources can yield different color matches for the same object.
  • Radiopacity standards:
    • Dental resins should have radiopacity at least equal to aluminum for detectability on radiographs; polymers are radiolucent; metals are radiopaque.
  • Representative data on densities and thermal properties (Table 3-1):
    • Water: density 1.00 g/cm³; c_p 1.00 cal/g·K; κ 0.44 W/m·K; diffusivity 0.0014 cm²/s
    • Enamel: density 2.97 g/cm³; c_p 0.18 cal/g·K; κ 0.93 W/m·K; diffusivity 0.0047 cm²/s
    • Dentin: density 2.14 g/cm³; c_p 0.30 cal/g·K; κ 0.57 W/m·K; diffusivity 0.0018–0.0026 cm²/s
    • Amalgam: density 11.6 g/cm³; c_p 0.005 cal/g·K; κ 22.6 W/m·K; diffusivity 0.96 cm²/s
    • Pure gold: density 19.3 g/cm³; c_p 0.03 cal/g·K; κ 297 W/m·K; diffusivity 1.18 cm²/s
    • (Tables include other materials like glass ionomer, composite, zinc phosphate, etc.)

Key Concepts and Takeaways (Connections and Implications)

  • Rheology guides handling of every phase from fluid to solid in dentistry; manipulating viscosity affects working time, flow, and the final fit of restorations.
  • Structural relaxation and creep emphasize that even after shaping, some materials may distort with time or temperature, affecting fit and marginal integrity.
  • Color and optical effects underpin esthetic dentistry: matching hue, value, and chroma under realistic lighting; understanding color spaces (Lab*, Munsell) and the limitations of shade guides; metamerism emphasizes the need to evaluate color under multiple lighting conditions.
  • Thermal properties warn that mismatches in thermal expansion between restorative components can cause marginal leakage and porcelain cracking; appropriate insulating bases are important in thin dentin scenarios.
  • Electrochemical properties highlight the oral environment as a galvanic cell: dissimilar metals, saliva as electrolyte, and the constant risk of corrosion and galvanic shock; protective strategies include varnishes, passivation, and avoiding unnecessary contact between dissimilar metals.
  • Magnetic materials play a role in implant retention and orthodontics, but safety considerations (field strength, MRI exposure) are essential.
  • The Selected Readings provide clinically relevant sources for deeper dives into color science, shade matching, shading guides, and corrosion behavior.

Notes on Formulation and Calculations

  • Use the given equations to estimate material behavior during processing and clinical use:
    • Mechanical stress and strain relationships (τ, ε̇) for handling fluids and impression materials.
    • Thermal behavior (α, κ, c_p, ρ) to predict temperature changes in restorations during thermal challenges.
    • Electrochemical potentials (EMF) to assess corrosion tendency and galvanic interactions in mixed-metal restorations.
  • When communicating results or planning treatments, consider:
    • Matching thermal expansion to reduce marginal leakage.
    • Evaluating radiopacity and fluorescence of materials for proper radiographic visibility and natural appearance.
    • Assessing shade matching under realistic lighting and considering patient preferences.