Properties of Materials
Properties of Materials
Chapter Overview
This chapter discusses the physical, electrical, and mechanical properties of materials used in dentistry, emphasizing their significance due to exposure to oral environments and biting forces.
Standards for dental materials are established by the American National Standards Institute (ANSI) and the American Dental Association (ADA).
Learning Objectives
Lesson 2.1: Properties of Materials
Dimensional Change
Define dimensional change and the linear coefficient of thermal expansion, including examples of their relevance in clinical dentistry.
Thermal and Electrical Properties
Illustrate instances where thermal and electrical properties of restorative materials impact clinical dentistry.
Solubility and Water Sorption
Provide examples of how solubility and water sorption relate to the success of dental restorative materials.
Wettability
Describe the clinical importance of wettability concerning tooth structure or dental materials.
Stress and Strain
Define stress and strain, elucidating their differences and importance in dental material selection.
Mechanical Properties
Describe concepts such as elastic modulus, proportional limit, yield strength, ultimate strength, and elongation/compression concerning dental materials.
Resilience vs. Toughness
Differentiate between resilience and toughness and how they relate to strength properties.
Hardness of Materials
Rank the hardness of dentin, enamel, and common dental restorative materials, noting caution in comparing Knoop and nano-hardness values.
Strain-Time Curve Importance
Explain why for certain materials, a strain-time curve is more informative than a stress-strain curve.
Dimensional Change
Importance: Maintaining dimensions during the preparation of impressions and models is crucial for accuracy.
Origin of Changes: May arise from chemical reactions during material setting or cooling; is typically expressed as a percentage of the original dimension (length/volume).
Volumetric Change: More challenging to measure compared to linear change.
Thermal Dimensional Change
Effects of Temperature: Temperature fluctuations in the oral cavity can induce dimensional changes in materials and neighboring tooth structures.
Expansion Mismatch: The thermal expansion of restorative materials usually does not align with that of human teeth, potentially causing leakage of oral fluids between restoration and tooth if expansion differential occurs.
Coefficient of Thermal Expansion: This is expressed as a linear thermal expansion coefficient, indicating the change in dimensions per degree change in temperature.
Characteristics:
Not uniform over the entire temperature range.
Typically greater for liquids than solids.
For solids, it shows increased expansion at certain temperatures.
Specific Dimensions
Comparison: Values for amalgam and composite materials are about three to five times those of human teeth.
Behavior on Cooling: Restorations tend to contract more than teeth when cooled.
Fluid Penetration: Oral fluids may infiltrate the space between materials upon cooling, and this fluid is expelled when temperatures normalize.
Percolation
Risks: Percolation leads to potential irritation of dental pulps and chances of recurrent decay. Over time, percolation tends to decrease, especially after the insertion of amalgam.
Thermal Conductivity
Characteristics of Materials: Different materials have varying rates of thermal conductivity:
Metals: Higher thermal conductivities compared to polymers or ceramics.
Poor Conductors: Enamel and dentin are poor thermal conductors in comparison to gold alloys and dental amalgam.
Composite restorations exhibit thermal conductivities comparable to tooth structure while glass ionomer cement bases function similarly.
Thin layers of cavity varnishes are ineffective thermal insulators.
Electrical Properties
Galvanism: Defined as the generation of electrical currents that patients can feel when two dissimilar metals interact in the mouth, often causing pain or a metallic taste.
Corrosion: Refers to the dissolution of metals in the mouth, exacerbated by the presence of dissimilar metals adjacent to one another. Roughness and pitting may result from corrosion, while tarnishing is a surface reaction triggered by food components or saliva.
Solubility and Sorption
Importance: Solubility and sorption of materials when exposed to oral fluids are critical for selection.
Laboratory Evaluation: Clinical evaluations typically assess materials in distilled water, not accounting for real-world conditions involving plaque and acids.
Measurement Metrics
Reporting Methods:
As a weight percentage of soluble or sorbed material.
As weight of dissolved or sorbed material per unit surface area.
Absorption vs. Adsorption:
Absorption: Uptake of liquid by the bulk solid;
Adsorption: Molecule concentration at the surface of solids/liquids.
Wettability
Observation Method: The wettability of materials can be identified by the shape of a liquid drop on a solid surface.
Hydrophilic vs. Hydrophobic: A lower contact angle indicates good wetting (hydrophilic), while a contact angle exceeding 90 degrees suggests poor wetting (hydrophobic).
Influencing Factors: The degree of wettability is affected by the relative surface energies of the solid and liquid and their intermolecular attractions.
Higher energy solids and lower energy liquids encourage good wetting; low-energy solids (e.g. wax, Teflon) cause liquids to bead up.
Mechanical Properties
Stress
Definition: Stress is characterized as force per unit area and is quantified by the formula: \text{Stress} = \frac{\text{Force}}{\text{Area}}.
Stress is a response of materials to external force, where a higher stress value results from a smaller area of application.
Types of Stress
Compressive Stress: When the material is squeezed.
Tensile Stress: When the material is pulled apart.
Shear Stress: Causes one portion to slide over another.
Strain
Definition: Strain refers to the change in length (deformation) per unit length when a material experiences applied force, described by:
\text{Strain} = \frac{\text{Deformation}}{\text{Length}}.Characteristics: Strain is dimensionless, with different dental materials exhibiting varying degrees of strain when stress is applied.
Stress-Strain Curves
These curves allow comparison of mechanical properties by plotting various forces against corresponding stress and strain values, which could be gathered under compression, tension, or shear forces.
Elastic Modulus
Definition: Refers to the ratio of stress to strain within the linear portion of a stress-strain curve, calculated as:
\text{Elastic Modulus} = \frac{\text{Stress}}{\text{Strain}}.Interpretation: It is an indicator of material stiffness, with gold alloys being similar in stiffness to enamel, whereas unfilled resins are more flexible, and elastic impression materials have low elastic moduli.
Proportional Limit and Yield Strength
Definition: These are values that measure the maximum stress allowed before permanent deformation occurs.
The proportional limit signifies the stress level where linear deformation ceases.
Yield strength is the stress at an arbitrarily set level of permanent strain and is typically slightly higher than the proportional limit.
Material Behavior: Materials remain elastic below the proportional limit and exhibit plastic characteristics beyond this limit. Unfilled acrylic plastics show permanent deformation at relatively lower stress than composites.
Ultimate Strength
Defined as the maximum stress that a material can withstand before fracturing, with tensile and compressive strengths potentially differing significantly. The bonding between materials can be assessed in tension or shear, which may be chemical, mechanical, or both.
Elongation and Compression
Evaluation: Indicators of how much deformation a material can sustain before rupture is measured as a percent of elongation under tensile stress or compression under compressive stress.
Ductility and Malleability: Elongation indicates ductility, while compression indicates malleability, particularly how much plastic strain occurs before fractures. For instance, gold is ductile while composites are more brittle, failing within 2-3% compression.
Resilience and Toughness
Resilience: Energy absorbed to permanently deform a material, indicated on the stress-strain curve up to the elastic limit.
Toughness: The total energy required to fracture a material, reflected by the area under the stress-strain curve until the point of fracture.
Hardness
Hardness indicates a material's resistance to indentation and is complex, without direct correlation to yield strength or wear resistance, chiefly reported in Knoop hardness values defined via diamond indentation metrics.
Knoop Hardness Calculation: Derived from the length of the long diagonal of the indentation created by a diamond under a known force over a 1 mm² area; larger indentations result in smaller values.
Nano-indentation Method: Allows for measurement of hardness in smaller areas and was utilized in studying the resin-dentin bonding in dental composites.
Strain-Time Curves
Overview: Some materials exhibit time-dependency in their strain response when loads are applied, characterized by viscoelastic strain (recoverable) and viscous flow (not recoverable).
Impact of Load Rate: The tensile strength tends to increase with the rapid application of load, denoting behavior that varies with speed.
Dynamic Properties
High-Rate Loading: Materials respond differently under conditions such as impact or extreme loading.
Measurement: Dynamic modulus assesses stiffness during high strain rates, while dynamic resilience measures the energy absorbed under such conditions.
Questions
An open invitation for further inquiries or clarifications related to the material discussed.